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KTH - School of Industrial Engineering and Management
Evaluation of Ce Addition by Different Wire in Liquid 316 Stainless Steel
MH250X
Master of Science Thesis
by
Oscar Juneblad
Division of Applied Process Metallurgy
Department of Material Science and Engineering
KTH Royal Institute of Technology
Stockholm, Sweden
2015-06-26
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Preface/Acknowledgments The following work has been carried out at the Department of Materials Technology, Royal institute of Technology (KTH) from February to July 2015. This master thesis work is part of “phase 1 – JK24058” in a long-‐term collaboration between KTH, Ferrox and Sandvik Materials Technology (SMT) to evaluate the possibility of enabling alloying with REM in the Cu-‐mold in the Continuous casting machine.
Professor Andrey Karasev have been the supervisor, along with Pär Jönsson, Bo Rågberg (SMT) and Olle Sundqvist (SMT) as co-‐supervisors.
I would like to thank Andrey Karasev for interesting discussions and helping out with the lab-‐scale experiments and SEM examination. Furthermore I would like to thank Pär Jönsson for introducing me to this topic as well as for the support on the way. Also I would like to thank Olle and Bo from SMT for willingly sharing their precious time to answer questions and inviting me up to Sandviken for a personal field trip. Lastly, I would like to thank my family who has supported me throughout the entire process. I will be forever grateful.
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Abstract It is well known that REMs are strong oxide and sulphide formers that can easily form large clusters which have harmful effect on the casting process as well as the quality of the final steel product. By adding these elements right before casting, the number of narrow transfer parts are eliminated (compared to if added in ladle) Also, the REM-‐inclusions has less time to sinter together to form large clusters, preventing clogging.
The general idea behind this alloying method in the continuous casting machine is to feed a wire of FeSiRE powder blend, coated with a metal strip, into the melt in the chilled Cu-‐mold (CC-‐mold) Adding REMs to the steel, in particular Ce, can increase the resistance to oxidation at high temperatures by improving the properties of the chromia layer. This is of big interest for SANDVIK as it can improve their corrosion resistant grades and may also, in the future, enable alloying in with other volatile elements such as Zr.
This master thesis has the objective to find out the dissolution time of the wire, coated with three different metal-‐strips; Steel, Cu and Al. The experiments were performed with steel grade 316L, provided by SMT, in a 2kg melt in a lab-‐scaled induction furnace at 1500oC, 1510oC and 1530oC. The operations were performed both with and without FeSiRE-‐powder inside. The results obtained with powder inside at 1500oC showed that the Al-‐wire experienced the shortest dissolution time ( 0,5 to 1s) followed by Cu (≤10s) and Steel (18 to 20s). In addition to this, sampling procedures was implemented (-‐1, 1, 3, 5 10 and 29mins after wire addition) in a depth of 40mm for each wire. Here, the yield of Ce 1 minute after wire addition was highest for the Steel wire (41.9%) followed by Cu-‐wire (25%) and Al-‐wire (<14.8%). From samples taken 1 and 5mins after wire addition in the Al-‐wire experiment, inclusions were extracted and collected on a film filter after electrolytic extraction and filtration. The film filter was observed in SEM. The morphology and compositions were analysed and compared. It was found that Ce and La was present as Ce-‐La-‐oxy-‐sulfides both individually and on Al-‐Mg-‐O clusters.
KEY WORDS: Stainless steel; Continuous Casting; Mold Metallurgy; REM-‐clusters; Electrolytic Extraction.
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Table of Contents
Preface/Acknowledgments ....................................................................................................... 2
Abstract .................................................................................................................................... 3
1. Introduction .......................................................................................................................... 5 1.1 Background ................................................................................................................................... 5 1.2 316 Stainless steel ......................................................................................................................... 6 1.3 Steel production line ..................................................................................................................... 6 1.4 Principles of conventional continuous casting ............................................................................... 7 1.5 Mold metallurgy ........................................................................................................................... 9
1.5.1 The CC-‐mold ................................................................................................................................... 9 1.5.2 Melt flow ...................................................................................................................................... 10 1.5.3 Solidification process ................................................................................................................... 10
1.6 Alloying by Ce ............................................................................................................................. 11 1.6.1 REM’s role in steel ........................................................................................................................ 11 1.6.2 Formation of REM clusters ........................................................................................................... 12 1.6.3 Clogging ........................................................................................................................................ 13 1.6.4 Ce effect on macrostructure and properties ................................................................................ 14
1.7 Methods of addition ................................................................................................................... 16 1.8 Purpose of study ......................................................................................................................... 18
2. Experimental ....................................................................................................................... 19 2.1 Setup .......................................................................................................................................... 19 2.2 Method ....................................................................................................................................... 21
2.2.1 Sample preparation ...................................................................................................................... 21 2.2.2 The EE-‐process ............................................................................................................................. 21 2.2.3 SEM investigation of inclusions .................................................................................................... 22 2.2.4 Dissolution of wire and sampling ................................................................................................. 22
3. Results and Discussion ........................................................................................................ 25 3.1 FeSiRE particle size distribution ................................................................................................... 25 3.2 Dissolution of wire ...................................................................................................................... 26
3.2.1 Wire without powder ................................................................................................................... 26 3.2.2 Wire with powder ........................................................................................................................ 28
3.3 Composition analysis .................................................................................................................. 30 3.4 Inclusion formation ..................................................................................................................... 33
4. Conclusions ......................................................................................................................... 35
5. Future work ........................................................................................................................ 35
6. Bibliography ........................................................................................................................ 36
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1. Introduction
1.1 Background It is well known today that many elements with a strong affinity to oxygen are very useful deoxidizers as well as contributing to increased mechanical properties of the final steel. Ce for example, is one of the rare earth metals (REM) and can be added in small amounts to (i) promote an increasing high temperature resistance (ii), control the shape of inclusions and (iii) refining the microstructure by increasing the equiaxed zone. However, since it is hard to add this element to the steel due to that clusters and inclusion networks are easily formed, reducing the castability, it is not used to a very large extent today. The addition of such active elements during ladle treatment promotes formation of non-‐metallic inclusions in the liquid steel at an early stage of the process. These inclusions then have a possibility to grow longer and to form large clusters, causing clogging, as they are present in the steel during ladle treatment, transport and final casting.
Clogging occurs in narrow transfer parts and is a huge problem in the continuous casting industry and can in worst case scenarios cause a complete shutdown of the process, rendering a large economic loss to the company if a large amount of steel is left to cast in the ladle or tundish [1]. Especially vulnerable is the outlet of the tundish in the SEN part. Earlier studies has shown that nozzle clogging increases drastically with increasing concentrations of Ce and that the concentration of insoluble Ce determines the amount of inclusions and clusters formed in the steel. It has also been concluded that clusters containing Ce and La oxides are primarily the main cause of clogging for the steel grades studied [2].
Prevention or minimization of clogging is normally done by a modification of the inclusions in the steel to create inclusions that are less likely to clog, for example with Ca-‐modification, or by replacing the continuous casting process with ingot casting (which is the case for stainless steel grades with Ce addition today). The continuous casting process is, however, a very effective bulk process for manufacturing of semi-‐finished products of standards forms in large series [3]. Between 2010 and 2012, this process accounted for 95.6% of the total steel production in the world and is a big producer of both ordinary steel grades as well as stainless steels [4]. The continuous casting machine at SANDVIK in Sandviken, SWEDEN, has a capacity of producing 310 000 ton/year and accounts for 90% on the total bulk production, so there is a big ambition of enabling Ce addition to this process. [5]
The present study is focused on the high temperature stainless steel grade 316, which is manufactured by continuous casting by SANDVIK. The desire from SANDVIK is to increase the high temperature resistance of this steel grade by alloying with Ce, fed as wire into the melt in the CC-‐mold. By adding Ce late in the process, it would decrease the time for Ce to form a large number of oxide inclusions, which can sinter together and form clusters causing clogging. The wire contains a FESiRE powder blend and is coated with three different strips; steel, Cu and Al. The addition is done in a 2kg melt in a laboratory scaled induction furnace at 1500oC, 1510oC and 1530oC for all three wires to estimate the dissolution rates of each wire. Sampling procedures are implemented after each type of wire-‐addition in order to analyse the chemical composition of the melt, over time, as well as inclusions characteristics in high Ce content zones. More specifically, the characteristics of Ce inclusions are analyzed on a film filter after Electrolytic extraction (EE) together with Scanning Electron Microscope (SEM).
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1.2 316 Stainless steel Grade 316 is the standard Mo-‐bearing grade, second in importance to 304 amongst the austenitic stainless steels. It is an austenitic stainless Ni-‐Cr steel with additions of Mo. The austenitic stainless steels have excellent toughness, even at cryogenic temperatures and provide great form-‐ and weldability as well as resistance to corrosion. The addition of Mo increases the general corrosion resistance, improves resistance to pitting from chloride ion solutions and provides increased strength at elevated temperatures.
Some typical applications are:
• Food processing equipment • Boat fittings • Mining screens • Nuts and bolts • Springs • Medical implants • Laboratory benches & equipment • Heat exchangers
The 316 grade is also available in high (316H) and low (316L) carbon variants with its corresponding composition presented in table 1. The 316L is immune to grain boundary carbide precipitation and is therefore suitable to use in heavy gauge welded components. The 316H is advantageous for applications in elevated temperatures where the increased carbon content delivers a greater tensile and yield strength [6].
Table 1: Typical chemical composition of the steel grade 316 [6].
Elements [wt%] C Mn Si P S Cr Ni N 316 0.08 max 2.0 0.75 0.045 0.03 16-‐18 10-‐14 0.1 316L 0.03 max 2.0 0.75 0.045 0.03 16-‐18 10-‐14 0.1 316H 0.1 max 2.0 0.75 0.045 0.03 16-‐18 10-‐14 -‐
1.3 Steel production line There are two different production routes for steel production (i) ore-‐based with reduction on the blast furnace and (ii) scrap based with melting of scrap in electric arc furnace (EAF).
In ore-‐based steel production, pig iron is extracted from iron ore using coke. The iron from the BF is then further refined and converted to steel in the basic oxygen furnace (BOF) where scrap is added, about 20% of the total charge in order to regulate the temperature and for alloying. The carbon content is reduced by refining with oxygen in the BOF causing a lot of exothermic reactions which is necessary for the temperature increase as the scrap is added. The melt in then transported for ladle treatment where the final alloying, refining and temperature is adjusted.
The scrap based production is entirely based on scrap. The scrap is melted by generating an electric arc between three electrodes together with extensive oxy fuel burners. The arc temperature can reach 10.000-‐15.000 oC and is necessary in the beginning of the melting process because of the high melting temperature of steel. The stainless steel production uses an AOD-‐process for decrease of carbon content
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before the melt is transported for ladle treatment where composition and temperature is adjusted depending on requirements of the specific steel grade [7].
There are two casting methods used in the steel industry; ingot casting and continuous casting. Continuous casting is the most efficient method when time and economics is considered and the product will also be more homogenous and contain fewer casting defects. However, the ingot casting can produce larger volumes of a cast object and shorter series of products and the casting procedure is also less complicated. Some steel grades with certain types of alloying elements, such as REM, can only be casted with ingot casting due to production problems such as clogging.
Theoretically, all steel can be produced using the EAF route today. However, at present there is not sufficient amount of scrap available in the word to fully meet today’s demand of steel. In 2013, around 40% of all steel produced in Europe was from using the EAF route and in Sweden this number was around 30% [4]. Highly alloyed steels, such as stainless steels, are most often manufactured by the EAF process since a great discount can be made from alloying with elements, such as Cr and Ni, from scrap instead of using primary alloying. The stainless steel 316 for example, is manufactured via the scrap-‐based route and is casted as typically square shaped cross-‐sections in the continuous casting machine.
1.4 Principles of conventional continuous casting In the continuous casting process, molten metal is poured from a ladle down into a tundish and then through a submerged entry nozzle into a mold cavity, figure 1. The tundish acts as a buffer in order to maintain a continuous flow of melt down into the CC-‐mold during ladle exchange and attains most often a rectangular shape. Nozzles are located along its bottom to distribute liquid steel into the CC-‐mold. The tundish also serves several other functions such as to enhance oxide inclusion refinement by slag entrapment, and maintains a steady metal height above the nozzles, keeping the steel flow uniform and provide a more stable stream of melt into the mold [3].
The CC-‐mold is water-‐cooled and fabricated from a high purity copper alloy with an inner face plated with Cr or Ni. When the melt enters the mold, a thin solidifying shell is established along the mold walls with a thickness strong enough to maintain the strand shape as it passes into a series of secondary cooling zones. Right below the mold, through which the strand, consisting mostly of a liquid core, the strand is sprayed heavily with water or water and air to further solidify. Oscillation of the CC mold is done throughout the entire process in order to minimize friction and sticking of solidifying shell [8]. Without doing this, there is a risk of shell tearing and liquid steel breakouts which can damage the equipment and cause complete shutdown of the process due to clean up and repairs.
Rollers are used to move the strand along at a constant rate once it leaves the mold. These rollers help to bend the strand in the correct direction along its given path through the cooling zones. The amount of water per unit time in the cooling zones often decreases with the distance from the CC-‐mold [9]. Another set of rollers will later be used to straighten the strand and will change the direction of flow of metal from vertical to horizontal. During unbending of the strand, the solid shell outer radius is under tension while the inner radius is under compression. If the strain is excessive it can cause cracking, which can seriously affect the mechanical properties of the steel. These excessive strains are minimized by progressively increasing the radius in order to gradually straighten out the product into the horizontal plane [10]. The strand gradually solidifies throughout this transportation and is fully solidified when it reaches the “metallurgical length”, which is the distance from the mold to where the strand becomes fully solid. For stainless steel grade 316, this length is about 23m.
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Once the strand in straightened it is transferred on roller tables to a cutting machine. The cutting machine cuts the product into ordered lengths, either by torches or mechanical shears, and is then transported to further processing.
Figure 1: Schematic illustration of the continuous casting process [30].
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1.5 Mold metallurgy
1.5.1 The CC-‐mold The CC-‐mold and the process in the CC-‐mold are very important for the final result of the continuous casting product. The mold has basically two purposes; it should (i) define the shape and cross section of the casting and remove heat, and (ii) facilitate formation of a solid shell.
CC-‐molds are typically made of pure Cu because of its good thermal conductivity (401W/mK), or precipitation-‐hardened Cu alloyed with Cr, since a very high heat flux is needed in order to quickly start to solidify the outer shell of the casting [11]. The surface is coated electrolytically with a thin layer of Ni in order to increase the wear resistance. The mold consists of water-‐cooling plates where the water circulates through slots in the mold to extract heat from the liquid steel. The upper part is covered with burnt brick or plating which protects the mold from damage and the liquid melt is protected by casting powder. There are generally two types of CC-‐molds; tube CC-‐ molds and block or plate CC-‐molds. The block CC-‐molds are used for larger square (>200cm2)-‐ or rectangular cross sections and is illustrated in figure 2 [9].
SANDVIK uses block or plate CC-‐molds in their three-‐stringed casting machine in Sandviken which basically attends an open-‐ended box structure with a length of 700mm and cast dimensions of either 365 x 365 or 265 x 265 mm. Table 2 shows some basic casting parameters about the CC-‐mold when casting grade 316L. The casting nozzles are made out of graphitized alumina with a immersion depth of 215-‐80=135mm and four outlet holes directed diagonally upwards [12]. (The outlet holes are placed 80mm above the bottom of the casting nozzle). Electromagnetic coils are positioned 0.4 meters under each mold to provide stirring of the melt, parallel to the casting direction. The reason for this is to destroy the dendritic structure that forms during the cooling and thereby increasing the equiaxed zone at the middle of the cast strand. [5]
Table 2: Casting parameters for grade 316L [13].
Continuous caster
Mold width (mm) 265 Mold length (mm) 365
Steel density (kg/dm3) 7,7 Casting speed (m/min) 0,8
Flow rate (kg/min) 596
Figure 2: Block CC-‐mold [9].
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1.5.2 Melt flow The melt flow in the mold is greatly influenced by the jets from the casting tube, which in turn is dependant on the geometry, depths and dimensions of the nozzle exit. The jet causes forced convection in the mold together with natural convection, which is always present during the solidification process. The casting nozzles are immersed at a moderate depth and are often designed with exits on the sides at some angle between 0-‐90degrees, relative to the vertical axis, in order to avoid inclusion of trapped particles at the solidification front. When the melt, which is superheated by about 30oC, enters the mold it is split into two strongly circulation flows directed upwards and downwards [9].This causes violent turbulent motion of the melt and contributes strongly to a homogenous melt in terms of temperature and composition.
Sandvik Steels has developed a water model of Sandviks CC-‐mold in order to simulate flow patterns in the mold during casting. Figure 3 presents the flow conditions obtained in the study using a immersion depth of 265mm, casting speed of 1,2m/min and casting nozzles directed horisontally towards the walls. As the water leaves the holes and hits the walls the liquid is slowed down and is directed either upwards or downwards. The water directed downwards creates a downward stream. According to the study, the downward stream between the outlet holes increases as the immersion depth increases. [12]
1.5.3 Solidification process During the short period of time the melt stays in the CC-‐mold, the strand has to solidify rapidly on the surface to get such thickness that it can resist the ferrostatic pressure from the melt in the interior and be drawn out of the mold for further solidification inside. This solidification process is illustrated in figure 4. The solidification starts very close to the mold at the upper part of the vertical surface of the melt. At first, when the melt gets in contact with the mold wall, the thickness of the solidifying shell grows rapidly as the heat transfer rate is very high. As the shell thickness now grows continuously down the vertical mold wall its power to resist the ferrostatic pressure from the core increases with time. As soon as the melt start to solidify, cooling shrinkage occurs and the shell contracts and start losing contact with the mold wall, this happen initially at the corners where the cooling is strongest, figure 4. Finally the contact between the mold wall and shell is lost and an air gap is formed. This air gap decreases the heat transport strongly since the metal is no longer in contact with the mold wall and so the solidification rate decreases
Figure 3: Flow pattern in CC-‐mold according to study [12].
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since heat can no longer be removed at the same rate as before. As the shell moves further down and reaches outside the mold it is strongly cooled by water.
The mold is made in a somewhat conical shape (upside down cone) in order to compensate for the solidification and cooling shrinkage of the casting strand [9]. In this way a constant air gap between the mold wall and the strand can be achieved instead of an increasing one.
Figure 4: Shell growth in the melt close to the CC-‐mold in (a) seen from the side, and in (b) seen from above. In region 1 with close contact between the shell and mold, In region 2 with an air gap between shell and mold and in region 3 outside the mold where the shell is strongly cooled by water [9].
1.6 Alloying by Ce
1.6.1 REM’s role in steel Typical for rare earth elements is their high reactivity and strong attraction for oxygen, carbon, nitrogen and sulfur which makes them interesting in steel making. REM are typically found associated together and it is both difficult and costly to separate the oxides in order to produce a pure metal of any of the rare earths. Therefore what is used most widely today in steel making is to add REM to the steel either as (i) mischmetal or as (ii) REM silicides. A standard composition of mischmetal is; 50% Ce, 25% La, 15% Nd and 6% Pd and the rest is iron and residual impurities such as Al, Ca Mg, O and Si [14]. REM silicides, which are cheaper to produce than mischmetal, consist of equal portions of REMs, Si and Fe [15].
The REMs strong affinity to, mostly, oxygen and sulphur makes them potent deoxidizers and desulphurizers, forming compounds according to reaction 1-‐3. However, they are practically never used for this reason since alternate alloys are more economical. The main reason for alloying with REM is to modify the shape of non-‐metallic inclusions in order to improve the mechanical properties of the steel, so called “inclusions shape control”. In cast steels, the negative effect of sulphur on the mechanical properties is well known. The sulfides that form are concentrated at the grain boundaries during solidification. This will lower the ductility of the steel making it more brittle and thereby increasing the risk of fracture on working. Sulphides (in particular MnS), oxide and silicates can be soft enough to deform during hot working temperatures and are deformed into elongated shapes parallel to the rolling direction. These elongated inclusions are especially harmful to transverse impact strength and ductility because of their sharp ends that acts as crack initiators.
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Adding REM to the steel captures the sulphur content into stable compounds according to reaction 2 and 3. These compounds tend to form a more globular or spherical shape that do not locate at the grain boundaries, thus enhancing the ductility greatly in comparison to a steel that has not been REM treated [16].
2𝑅𝑒 + 3𝑂 → 𝑅𝑒!𝑂! 1
𝑥𝑅𝑒 + 𝑦𝑆 → 𝑅𝑒!𝑆! (2)
2𝑅𝑒 + 2𝑂 + 𝑆 → 𝑅𝑒!𝑂!𝑆 (3)
Although the addition of REM can increase the mechanical properties of the final steel, it also brings some problems with it. The various compounds formed by REM can easily attach to each other forming large clusters that are hard to get rid of. Ce for example has an atomic mass of almost three times the mass of iron so when Ce forms oxides and sulphides, these molecules are hard to separate from the liquid melt by floating up to the surface due of their relatively high weight compared to the melt [12]. Another drawback is that soluable Ce in the melt can easily react with the refractory according to reaction 4. The Ce is reducing the alumina and forming Ce-‐oxide inclusions and precipitation of REM-‐oxides on the furnace walls, wearing the refractory wall [2]. Because of its high affinity to oxygen, Ce can also react with slag and/or air to further lower the yield of the metal.
2𝐶𝑒 + 𝐴𝑙!𝑂! = 𝐶𝑒!𝑂! + 2𝐴𝑙 (4)
1.6.2 Formation of REM clusters As for REM clusters, the compounds that will form in the liquid steel depend on the concentrations of Ce, La, Al, O and S dissolved in the steel. The high melting temperature and contact angle between oxide and melt promotes these inclusions to form large clusters. Characterisation of REM clusters was studied by A. Karasev, Y.Bi and P.G. Jönsson in “Three dimensional Evaluations of REM Clusters in Stainless Steel”. In this study a pilot trial (250kg) as well as industrial heats (100t) were carried out. For the pilot trial (PT), 250kg of melt were melted at 1470oC in an alumina crucible in an induction furnace for a REM alloyed stainless steel grade similar to the 316. Samples were taken at different holding times after mischmetal addition and after 10 minutes of holding the melt was started to be cast. A complete nozzle clogging occurred after about 15 minutes after the mischmetal addition. Two types of morphologies of clusters were found in the samples taken from the PT, named type A (1.5 to 20μm) and type B (2.0 to 8.0μm), table 3. Type A clusters consisted of big size (1 to 3.6μm) irregular and regular inclusions and small size (≤0.5μm) spherical inclusions. The spherical inclusions were located mainly at the edge of the irregular and regular inclusions. The type B clusters consisted of irregular inclusions only. The Ce and La content of all different REM clusters was shown to be almost stable by time where the La content was larger than the Ce content in all samples taken and there were no obvious difference in composition between type A and B clusters [17].
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Table 3: Inclusion characteristics found in the study [17].
1.6.3 Clogging Clogging is the build-‐up of material in narrow transfer parts where melt is transported through and is a huge problem in the continuous casting process. The clogged material can be either oxide particles from the melt, or material that are a product from reactions between the steel and the ceramic lining of the nozzle. The material that attach to the wall is most often solid particles with high contents of Al, Si, Ti or REM (in particular Ce). In general, the formation of clogging can be assumed to follow three steps; (i) transportation to the nozzle wall, (ii) adhesion to the nozzle wall and (iii) accumulation of a clogging network [2].
The transportation of particles to the nozzle wall depends on the fluid flow. The melt flow at a nozzle is turbulent causing recirculation zones because of the shape of the nozzle. Within a recirculation zone, turbulent velocity fluctuations oriented in all directions are present which will enhance the transportation of particles to the nozzle wall. The particle will, however, only stick to the wall if it has correct adhesion properties in comparison to the wall properties. The particles are attached to the nozzle wall by surface tension. The surface tension of the steel creates a void, which leads to an attractive force between the particle and the wall. Thereafter, new particles will attach to the first particles and a network is built up. As the network grows the attachment of particles will be further enhanced by sintering bounds between each other and after a sufficient amount of time this network will be big enough to clog the nozzle [18] [2]. Figure 5 shows a light optical microscope (LOM) image of a typical clogging network starting at the nozzle wall, growing inwards. There are clear differences in the areas depending on the distance from the wall. Image A has a denser network whereas C shows more defined clusters.
Heat Type Type A Type B
PT
Typical photo
Size range (μm) 1.5-‐20.0 2.0-‐8.0
(a) (b) (c)
Figure 5: Image of three layers visible in the clogged material in a nozzle from plant trials. The nozzle wall is to the left layer of image A [2].
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There are many methods and theories today of how to avoid clogging. The most obvious one is of course to decrease the concentration of deoxidation products and prevent formation of reoxidation product (this is the main idea behind the wire-‐feeding process suggested by Sandvik). Solid particles can also be transformed to liquid inclusions by Ca treatment. Adding Ca to the melt prevents formation of solid alumnia and thus reduces their ability to stick to the nozzle wall or get caught in an already built up network. However, Ca cannot be added to all steel grades due to limitations in the alloy composition.
It is also possible to modify the material and geometry of the nozzle in various ways. But since the influence of nozzle material and geometry on the clogging rate has not been part of this work as this work is founded on the fact that the metallurgy and deoxidation of the melt is the main source of clogging, this theory has been excluded.
1.6.4 Ce effect on macrostructure and properties There are several benefits of adding Ce to steel. By alloying with Ce, it is possible to modify the macrostructure of a casting. A standard macrostructure of a continuously cast stainless steel bloom is shown in figure 6 and is typically divided into three zones; Chill zone, Columnar zone and Equiaxed zone [9]:
.
The formed Ce inclusions can act as heterogeneous nucleation during casting refining the microstructure. Ce inclusions form during low undercooling making it possible to grow equiaxed grains, ahead of the solidifying front, reducing the area of the columnar zone. By reducing this columnar zone, the solidified steel will have larger zone of equiaxed smaller grains improving the mechanical properties of the steel. Studies has shown that when adding 0.05, 0.075 and 0.1 wt% Ce to a 120 kg casts at 1525oC and 1540oC, the columnar zone was reduced from 22mm to beeing absent and a linear increase in both yield and ultimate tensile strength was increased with increasing amounts of Ce for both temperatures [19]. This grain refinement has also been achieved when adding Ce together with Al, in an austenitic high manganese steel, and the types of Ce containing particles formed was shown to depend on the amount of Ce added [20] A substantial reduction in the dendrite arm spacing has also been achieved when adding Ce via the master alloy Fe-‐Cr-‐Si-‐Ce to a S254-‐SMO austenitic stainless steel grade. This Ce addition was promoting the formation of Ce-‐Al-‐oxide inclusions in the liquid steel prior to solidification [21].
Figure 6: Macrostructure of a continuously cast stainless steel bloom consisting of three zones; chilled zone, columnar zone and equiaxed zone [3].
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Ce can also be added in small amounts to certain heat resisting grades in order to increase the resistance to oxidation at high temperatures. This is the main reason for the 316L stainless steel grade. By Micro-‐alloying (MA) with Ce and Si, it is possible to improve the properties of the chromia (Cr2O3) scale layer that provides the basis of oxidation-‐ and corrosion resistance. At temperatures above 1000oC this chromia layer becomes unstable and start to crumble. The MA promotes a thinner, tougher and more adherent oxide [22]. The effect of REM in stainless steels was studied where small amounts of REM additions to Fe-‐10Cr and Fe-‐20Cr alloys was shown to significantly improve the steel’s resistance to high temperature oxidation at 1000oC, figure 7. [23]
There are various mechanisms proposed to explain this improvement in oxidation resistance when adding Ce to stainless steels [23]:
• Ce help to increase the attachment of the created chromia layer to the surface of the steel by creating a phase between the steel and the oxide layer.
• Ce and Ce-‐oxides can act as nucleation sites on the surface of the steel to promote growth of the chromia oxide layer.
• Ce increases the strength of the chromia layer.
Figure 7: The effects of various amounts of Ce in both metal and oxide form on the high temperature oxidation of Fe-‐20Cr at 1000oC [23].
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1.7 Methods of addition Ingot casting In ingot casting, REM can be added to the steel either as REM silicides or as a mischmetal alloy. Mischmetal can be alloyed to the molten steel by plunging precast canisters of appropriate weight and attach these to the end of a billet to provide deep immersion in the ladle. REM silicides are less reactive than mischmetal and are instead added to the second pouring ladle when reladling a heat, or into the ingot mold. This addition is done right into the pouring stream to insure adequate dispersion and mixing as the mold is filled with melt [24] [15].
Continuous casting (Wire feeding in CC-‐mold) Today, addition of REM is not done in large industrial scales because of the metals strong affinity to oxygen and thereby easily form inclusions and clusters causing clogging in the various transfer parts in the CC-‐machine. A solution to this problem, suggested by Sandvik, is to add Ce as a wire into the melt in the CC-‐mold. The reason for this is to avoid reaction with oxygen to form oxide inclusions, and also to give less time for the inclusions to form large clusters before the strand is fully solidified. Less time to form clusters will reduce clogging during the transfer of the melt. Adding Ce late in the process eliminates transfer parts where clogging may occur and will at the same time increase the yield, compared to if added in the ladle. This would also result in a more cost-‐effective steel production, compared to ingot casting, in terms of less scrapping of steel, which especially in stainless steel making, is very costly. Less scrapping results in lower energy consumption and use of material resources.
Apart from process-‐related benefits, it should be mentioned that this alloying method also comes with other benefits. “inclusion shape control” and an increased zone of equiaxed crystals are two factors that Ce can contribute with, as explained earlier, and can have big impact on the final mechanical properties of the steel.
The general idea is to feed a wire of FeSiRE powder blend, coated with a metal strip, into the melt from the side, figure 8. The wire is fed through a supply tube with a predetermined speed set by the feeder. During a short period of time, the strip has to melt easily and not leave any alloying effects on the casted material and the powder has to dissolve completely in the melt and spread out homogeneously in the casted cross-‐section of the strand.
Figure 8: Schematic illustration of the wire feeding process by Sandvik [25].
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There has been previous trials of adding REM into the CC-‐mold by companies in China; Shanghai no.1 steel plant in 1992, Shoudu iron steel company in 1981 and Daye special steel company in 1995. Even though the conditions of these plant trials were somewhat different in terms of casting temperature, composition of steel, casting speed, and strip material common results were presented regarding homogenous distribution of Ce in the cross-‐section of the casted strand. In addition, the dendrite arms spacing was reduced by 13% in the Shoudu iron steel company trials and by 20-‐40% in the Daye Special steel company, respectively [12].
More recent trials has been carried out by Sandvik, going all the way back to -‐98. These attempts involved two with Ce additions and one with Ca addition in the CC-‐mold. For Ce, the overall results show a yield of Ce of 50-‐ and 70% and good evenness in the cross-‐section as well as surface finish of the strand [25].
The wire feeding rate needed in order to end up with a final composition in the strand of 0.05wt% can be theoretically calculated with data from wires, FeSiRE powder and casting parameters from Sandviks continuous caster in table 4, 7 and 2, respectively. By assuming a metallurgical yield of Ce of 60%.
Table 4: Data of wires used in industrial trials by SANDVIK [13].
The feeding rate of the wire is calculated according to equation 5 and gives a feeding rate of the wire of 14.78m/min.
(𝐶𝑒 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑡𝑎𝑟𝑔𝑒𝑡𝑒𝑑𝑀𝑒𝑡𝑎𝑙𝑙𝑢𝑟𝑔𝑖𝑐𝑎𝑙 𝑦𝑖𝑒𝑙𝑑 )
𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝐶𝑒 𝑖𝑛 𝐹𝑒𝑆𝑖𝑅𝐸𝑝𝑜𝑤𝑑𝑒𝑟∗
𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 ∗ 1000𝑃𝑜𝑤𝑑𝑒𝑟 𝑤𝑒𝑖𝑔ℎ𝑡
= 𝐹𝑒𝑒𝑑𝑖𝑛𝑔 𝑟𝑎𝑡𝑒!"!!"#$ (5)
(!.!!!"!.! )
!.!!∗ !"#∗!"""
!"#= 14.78𝑚/𝑚𝑖𝑛 (6)
The wire has to be fully dissolved at an immersion depth of 0.15m in the CC-‐mold for good homogenization from the turbulence in the melt flow [5]. This gives a dissolution time of the wire of ≈0.6s according to equation 7.
𝐼𝑚𝑚𝑒𝑟𝑠𝑖𝑜𝑛 𝑑𝑒𝑝𝑡ℎ𝐹𝑒𝑒𝑑𝑖𝑛𝑔 𝑟𝑎𝑡𝑒
= 0.15𝑚0.25𝑚/𝑠
= 0.6𝑠 (7)
Cored wire addition
Wire diameter (mm) 13,6 13,6 13,6
Type of strip steel copper aluminium
Powder weight per meter (g/m) 210 210 210 Strip weight per meter (g/m) 168 195 60
Strip thickness (mm) 0,4 0,4 0,4
18
1.8 Purpose of study Dissolution rate of coated wires (Steel, Cu and Al) containing FeSiRE-‐powder. The wire has to be dissolved at a level where the powder can be quickly distributed by the flow in the CC-‐mold before casting. If the wire dissolves too fast it could mean that alloying elements end up in the slag or atmosphere instead of the melt, lowering the yield of the elements added. Too long dissolution time means lack of alloying elements in the casted product and possible breakout since the cooling effect of the wire on the melt will be increased. Inclusion characteristics over time in high content Ce zones. The formation of REM inclusions and clusters over time is important for understanding of inclusions characteristics that are formed after wire-‐addition. How the morphology, composition and size of these inclusions change as a function of time and how they may affect the castablility and properties of the final steel.
19
2. Experimental
2.1 Setup A vertical, induction furnace is to be used as heat source for four lab-‐scale experiments in order to evaluate the dissolution of the three different strips of wire (Steel, Cu and Al) as well as the FeSiRE powder added to the melt, table 5. About 2 kg of solid 316 stainless steel, taken from two charges, is placed in a alumina crucible with high purity (Al2O3 > 99.5% ; Fe2O3 < 0.01%) to fill about 2/3 of the total height, appendix-‐1. The crucible is lowered down into the bottom of an induction furnace. A lid closes the furnace once the melting starts and an Ar nozzle is placed through the lid directed towards the surface of the melt in order to add Ar, with 0.5bar, continuously to the melt surface to prevent reoxidation from air. A thermocouple (Al2O3 tube, max 1600oC), connected to a live temperature display, is placed in between the crucible and the furnace to symbolize the temperature of the melt. The setup is schematically illustrated in figure 9. The heat rate was set to 4oC/min from 25oC to 600oC and then 6oC/min from 600oC to 1500oC/1510oC/1530oC (casting temperature of 316L is ≈1500oC). When the melt reached the final temperature it is isothermally held for about 30 minutes to make sure the temperature is stable and that all metal is melted before the experiments can start. The extra degrees are necessary for a start since the thermocouple is placed outside of the crucible and might therefore show a higher temperature than in the melt. Also the volume of the melt compared to the wire is relatively low compared to industry and there is no continuous flow of melt, which means that the wire will have a greater cooling effect on the surrounding melt.
Table 5: Setup information for all four lab-‐scale experiments.
Exp. Number Initial weight of melt (g)
Temperature of melt (oC) Holding time (min) Charge nr.
1 2128.1 1530 20 540676 2 2083.6 1530 24 540676 3 2028.9 1500 30 540676 4 1997.7 1510 78* 542169
* The steel in experiment no. 4 was not melted during 60min at 1500oC and thus the temperature and holding time had to be increased.
Figure 9: Schemtic illustration of the experimental setup.
20
The composition of the three different wire strips and the charges can be seen in table 6. The wires (!13.6mm, wall thickness 0.4mm) contain the same type of FeSiRE powder blend with its composition presented in table 7.
Table 6: Composition of wire-‐strips and charge 316L taken from two different heats [26].
Table 7: Composition of powder blend [26].
Strip 316L charge nr. Element Aluminum Copper Steel 540676 542169 Fe(wt%) 0.27 99.682 C(wt%) 0.037 0.012 0.014 Ni(wt%) ≤0.02 11.20 11.17 Cr(wt%) ≤0.02 16.68 16.69 Cu(wt%) 0.015 99.9 0.3 0.3 Al(wt%) 96.4 0.026 0.003 0.003 Si(wt%) 0.15 0.01 0.62 0.57 Mn(wt%) 0.29 0.23 1.84 1.77 P(wt%) 0.01 0.01 0.029 0.028 S(wt%) 0.005 0.023 0.024 Ti(wt%) 0.025 <0.003 <0.003 Zn(wt%) ≤0.02 Mg(wt%) 2.74 Sn(wt%) ≤0.04 Pb(wt%) ≤0.01 Mo(wt%) 2.05 2.02 N(wt%) 0.04 0.033 Ca(wt%) 0.0022 0.0028 W(wt%) 0.03 Co(wt%) 0.09 V(wt%) 0.052 Nb(wt%) 0.01 B(wt%) 0.0006
Powder Weight/m of wire
REM(wt%) Si(wt%) Al(wt%) Ca(wt%) Ti(wt%) B(wt%)
FeSiRE 430g 25.37 31.45 0.3 0.25 <0.5 <0.05 (62.45% Ce
32.05% La 5.54% Pr, Nd, Sm)
21
2.2 Method
2.2.1 Sample preparation Sampling procedures was implemented manually with quartz tubes for each type of wire (Exp. 2.0-‐Steel, Exp. 3.0-‐Cu and Exp. 4.0-‐Al) in order to analyse the composition of elements; Ce, S, Al, O and Cu over time as well as inclusion characteristics for the Al-‐wire in Exp. 4.0. All samples taken (appendix-‐8) were sent to SANDVIK for composition analysis using HFIR, ICP and EXTR techniques. Samples A1 and A5 were also used for extraction and 3D investigations of non-‐metallic inclusions (NMI) in Electrolytic extraction (EE) after addition of Al-‐wire to the melt. About 20mm was cut off from the 6mm thick cylindrical samples and was grinded along its long side down to the centre surface, which was to be extracted in the EE process.
2.2.2 The EE-‐process EE was applied for investigation of inclusion characteristics (such as size-‐range, morphology and composition) for sample A1 and A5. In EE, inclusions are extracted from the metal surface by dissolving a small layer of the metal matrix in an electrolyte with presence of electric current. Non-‐metallic inclusions, which are not soluble in the electrolyte, remains in the solution and is collected on a film filter after filtration. Figure 10 shows a schematic illustration of the EE process.
All three samples went through similar procedure; First off, the sample surface was grinded in order to remove impurities, such as dust, oxides etc. The sample was then very carefully weighed before lowered down into the jar of electrolyte until it was fully underneath the surface. Two wires were connected to a potentiostat where current, voltage and electric charge could be regulated. To stop the process, a timer was set to terminate the experiment when 500 coulombs was reached. When the experiment was stopped, the solution was poured through the filtration system. The film filter, now containing inclusions from the solution, was removed and ready for SEM observation. The sample was weighed again in order to calculate the dissolved weight from the extraction process.
This EE was carried out using 250ml of 2% TEA solution. The current density during the extraction process was 15-‐50mA/cm2. The weight of the dissolved metal during the extraction was 0.08-‐0.1 grams. The solution containing NMI after the EE was filtrated through a polycarbonate film filter with a pore-‐size of 0.4μm. Inclusions were analysed on the surface of the film filter using a SEM.
Figure 10: Schematic illustration of the EE setup and filtration [31].
22
2.2.3 SEM investigation of inclusions Characterisation of inclusions was determined by observing the printed SEM-‐pictures. A composition analysis with Eds and a size range was implemented for five types of main morphologies that were found, figure 11. The size of each inclusion was measured by measuring the longest length. The contrast of inclusions in the pictures can indicate which main elements that are more dominant; white color is an indication of Ce and La oxides, light grey is intermetallic phase and grey color is Al or Mg oxides. However, the contrast between these different types of compounds was shown to be very vague and it was thus not possible to estimate frequency in a good way only by looking at the printed pictures. Also, the inclusions found was shown to be very complex, consisting of many different compounds covering each other.
2.2.4 Dissolution of wire and sampling Experiment 1 100mm of wire, without any powder inside, was lowered down into the melt to evaluate the dissolution time of the three different strip-‐materials (Steel, Cu and Al). The strip is put down into the melt 40mm below the melt surface and is held constant for a predetermined time. After that the strip is taken up from the melt and is observed to see if fully dissolved or not. A new unexposed strip of same material is put down into the melt, this time for longer or shorter time depending on the previous observation. The strip is then taken up again and is observed if fully dissolved or not. This cycle is done repeatedly with different time intervals for all three strip-‐materials individually until an as close dissolution time as possible has been found. A schematic illustration of the experimental procedure can be seen in figure 12.
Figure 11: Pictures taken in SEM showing (a) even distribution of typical inclusions found on film filter and (b) Rem cluster with presence of Al2O3.
L
23
Experiment 2 A sampling procedure was implemented according to figure 13 for trial (2.1.t1), which was done in a clean and unexposed melt, before dissolution rate experiments, figure 14. Same procedure was implemented as in experiment 1 but this time the strips contains FeSiRE powder and thus the holding times are different. The ends of the wires are sealed by two different methods in order to keep the powder inside the strips; (*) “standard” closing with a polymer and, (**) “para-‐film” with aluminium foil. The “para-‐film” method was, unfortunately, proven to be unsuccessful due to fire and thus those trials are not included in the results. When the experimental procedure was finished the furnace was shut off and a cooling rate was registered, appendix-‐7.
Figure 12: Holding times of wires without powder inside at 1530oC. The numbers inside the brackets are aimed times but due to freezing of metal the strips were unable to get out from the holes of the lid and thus the holding times were delayed.
Figure 14: Holding time of wires with powder inside at 1530oC. The numbers inside the brackets are aimed times but due to freezing of metal and fire from the para-‐film sealing the wires were unable to get out from the holes of the lid and thus the holding times were delayed.
Figure 13: Sampling procedure for steel-‐wire at 1530oC.
24
Experiment 3 A sampling procedure was implemented according to figure 15 for trial (3.1.t1), which was done in a clean and unexposed melt, followed up by dissolution rate experiments, figure 16. Same procedure is implemented as in experiment 2 but has been somewhat optimized in terms of; (i) only standard sealing is used and (ii) wires with a length of 240mm instead of 100mm was used in order to prevent the upper edge of the wire from getting stuck in the lid when taking the wire up from the melt, figure 1. A temperature of 1500oC in the melt was set since the thermocouple, from experiment number 2, was shown to symbolize the real temperature of the melt better than expected.
Figure 15: Sampling procedure for Cu-‐wire at 1500oC.
Experiment 4 A sampling procedure was implemented according to figure 17. After the last sample was taken the melt was cooled down with a cooling rate of 4oC/min, which corresponds to the actual average cooling rate of a 316L strand in Sandvik’s continuous casting machine, appendix-‐2. Inclusion characteristics are to be analysed in samples A1 and A5.
Figure 16: Holding times of wires with powder inside at 1500oC. The numbers inside the brackets are aimed times but due to freezing of metal the wire were unable to get out from the holes of the lid and thus the holding times were delayed.
Figure 17: Sampling procedure for Al-‐wire at 1510oC.
25
3. Results and Discussion
3.1 FeSiRE particle size distribution
110.87grams of FeSiRE powder, from wire, was sieved through three different levels (0.5mm, 1mm, and 1.4mm) in a sieving machine for 9 minutes. From figure 18 it is clear that >55wt% of all powder has a size smaller than 0,5mm. This means that the powder has a large overall reaction surface and thereby also a relatively high dissolution rate, since smaller particles can dissolve much faster than larger. This is a good thing because the powder needs to be dissolved and homogenously distributed during a very short period of time in the CC-‐mold. However, a large reaction surface also means a large exposed area for the powder to be oxidized on, which is particularly critical in this case since the powder contains many elements with a strong affinity to oxygen. This can, in turn, bring some unwanted oxides into the melt during casting that can be harmful for the mechanical properties of the final steel.
0
10
20
30
40
50
60
wt(%)
Paracle size (mm)
Paracle size distribuaon
0 0.5 1 1.4
Figure 18: Particle size distribution for FeSiRE-‐powder,
26
3.2 Dissolution of wire
3.2.1 Wire without powder The results obtained from the dissolution of wires, without any powder inside, are presented in figure 19. Pictures of the dissolved strips can be seen in appendix-‐4.
All strips were lowered down 40mm below the surface of the melt and were held constant for the given time. Strip number 3 for steel and 4 for Cu was accidently put down 50mm below the melt surface. This minor deviation does however play less of a role and should not have any effect on the out coming results.
For steel All trials, except for trial 1.1.t1, experienced freezing of metal on the outside of the wire, making it impossible to take the wire up through the furnace lid at the aimed times and the holding times were therefore extended. This could be somewhat expected since the steel-‐strip is a low-‐alloyed steel with a carbon content of 0.037wt% and has a melting point close to 1500oC. For trial 1.1.t3 and 1.1.t4 one can notice some build up of solidified melt near the dissolved edge when in contact with the melt which confirms this phenomenon, appendix-‐3.
For Cu The results show that the strips below the melt surface dissolved completely for all holding times. In addition, the part above the surface remained solid in all trials except for except for 1.2.t1 where the long holding time dissolved an additional 20mm above the melt surface, figure 20. The strips for trial 1.2.t2, 1.2.t3 and 1.2.t4 has all very distinct edges at the dissolved ends, indicating that the dissolution of the strip only occurs when Cu is in contact with the melt and not above.
Figure 19: Dissolved lengths of strip-‐materials during different holding times for Exp.1.1-‐1.3 at 1500oC.
27
For Al Only 2 trials were performed since the strip was shown to dissolve very fast. This can be explained by the fact the melting point of aluminium is less than half of the temperature of the melt. It is clear that for both trials a significant length above the melt surface has dissolved almost completely, appendix-‐4.
The trials for Al gives interesting information for the wire feeding process regarding the yield of Ce added. If the strip melts 20-‐30mm above the melt surface during such short time intervals there is a great risk of FeSiRE powder ending up in the casting powder or atmosphere, instead of in the melt, lowering the yield of Ce added. Cu however shows more promising results because of the distinct edges at the dissolved ends for short holding times. This indicates that the FeSiRE powder will stay inside the strip above the melt surface, unlike with Al whereas the powder might escape before entering the melt. However, since this experiment was executed without powder inside the wire might behave differently in terms of dissolution time compared to with powder inside, which is evaluated in upcoming experiment 2,3 and 4.
When the experiment was finished the furnace was shut off and a cooling rate was registered manually and is plotted in appendix-‐9. It is shown that at TS, which is around 1390oC for grade 316 (Thermo-‐calc), the cooling rate drastically decreases due to that the metal is fully solidified. This indicates that the temperature registered by the thermocouple corresponds very well to the actual temperature inside of the melt.
Figure 20: Dissolution of Exp.1.2.t1-‐1.2.t4 at 1530°C. Trial 1.2.t4 was accidentily put down 50mm below the melt surface.
28
3.2.2 Wire with powder The results obtained from the dissolution of wire-‐strips, with powder inside, are presented in figure 21. Pictures of the dissolved wires and samples taken can be seen in appendix-‐5, appendix-‐6 and appendix-‐7. All wires were lowered down 40mm below the melt surface and were held constant for a specific amount of time. For experiment 2 the wires that were closed with the “para-‐film” method was fired immediately when it got in contact with the melt and thus these trials failed and are not included in the results.
For steel The trials done at 1530oC shows that the dissolution time of the Steel-‐wire is between 5 and 15s. A more exact dissolution time was not able to get because of the fire from the para-‐film sealing. At 1500oC there is a slight increase in dissolution time to between 18 and 20s, which is to be expected because of the lower temperature of the melt.
For Cu The trials done at 1530oC shows that the dissolution time of the Cu-‐wire is between 10 and 11s. For 1500 and 1530oC holding times of 2, 5 and 10s was set but due to freezing of metal on the outside of the wire it was not able to get the wire up through the furnace lid at the aimed time and thus the holding times were extended to 20, 10 and 11s respectively. There seem to be no significant difference in dissolution time when increasing the temperature 30oC.
For Al Only one trial was done at 1530oC and it was shown to fully dissolved 10mm above the melt surface after only 2 seconds. Based on these results, holding times of 1s and less was executed at 1500oC to try to estimate a more exact dissolution time, figure 22. The results show that a dissolution time between 0.5 and 1s is needed in order for the Al-‐wire to be fully dissolved below the melt surface. These results
Figure 21: Dissolved lengths of wires at different holding times for Exp. 2.1-‐2.3, 3.1-‐3.3 and 4.0 at 1530, 1500 and 1510oC, respectively.
29
correspond very well to aimed dissolution time of 0.6s with a feeding rate of 0.25m/s, equation 7, set by SANDVIK. One can also conclude that the feeding rate should by no means be decreased since this will result in that the wire will dissolve above the melt surface. By doing that there is a great risk of FeSiRE powder ending up in the slag or atmosphere, instead of the melt, lowering the yield of the elements added. It should also be noted that even though the Al-‐strip has not dissolved completely for trial 3.3.t3, all the 40mm of powder inside has still been able to escape through local melted zones of the Al-‐strip and the melted edge of the wire is not as sharp as for Cu and Steel, Appendix-‐6.
The behaviour of how the wires and powder inside dissolve is therefore also an important factor to consider and is most likely different when the wire is fed (as in the industry) compared to when held constant. It should also be pointed out that the holding times less than 1s is not very exact since the timer and handling of wire was done manually, by hand.
The results at 1500oC confirm that the use of Al-‐wire is probably the best choice regarding the dissolution time set by SANDVIK with the given diameter of the wire of 13.6mm. However, an alternative could be to use the Cu-‐wire instead because of the finer melting at the edges and less escape of powder from the not fully dissolved lengths of the wire. In this case the diameter of the wire has to be increased and the feeding rate has to be decreased in order to end up with the same amount of FeSiRE-‐powder added into the melt. Based on the results the dissolution time of the Cu wire is around 10s. By using the same immersion depth of 0.15m, a new feeding rate was calculated according to equation 2. By knowing the feeding rate for both Al and Cu wire, and the dimensions of the Al wire, it is possible to calculate the diameter of the Cu wire by looking at the individual volume flows, equation 9 and 10.
𝐹𝑒𝑒𝑑𝑖𝑛𝑔 𝑟𝑎𝑡𝑒!"!!"#$ =𝐼𝑚𝑚𝑒𝑟𝑠𝑖𝑜𝑛 𝑑𝑒𝑝𝑡ℎ𝐷𝑖𝑠𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒
=0.15𝑚10𝑠
= 0.015𝑚/𝑠 (8)
𝑉𝑜𝑙𝑢𝑚𝑒 𝑓𝑙𝑜𝑤 = (𝑟! ∗ 𝜋 ∗ ℎ)/𝑠 (9)
𝑉𝑜𝑙𝑢𝑚𝑒 𝑓𝑙𝑜𝑤!"!!"#$ = 𝑉𝑜𝑙𝑢𝑚𝑒 𝑓𝑙𝑜𝑤!"!!"#$ (10)
0.0068! ∗ π ∗ 0.25 = 𝑟!"!!"#$! ∗ π ∗ 0.015 è 𝑑!"!!"#$ = 0.0304𝑚 (11)
Figure 22: Dissolved lengths of Exp. 3.1 at 1500oC.
30
According to the calculations it is possible to use the Cu-‐wire instead of the Al-‐wire but that would demand a feeding rate of 0.015m/s and a diameter of 30.4mm. One thing to consider though is the alloying effect of the Cu-‐strip on the mechanical properties of the final steel. Cu promotes an austenitic microstructure and is added to in the stainless steel grade 316 (0.3wt%) in order to enhance the corrosion resistance in certain acids and to decrease work hardening for improved machine –and formability [27]. However, the presence of more than 0.2wt% Cu in the steel can leave a Cu-‐enriched zone containing a low melting phase located on the grain boundaries [28]. During forging and hot rolling over 1090oC this phase can in severe cases melt, leaving cavities in the steel making it unworkable. Fortunately, the degree of this effect can be decreased by preheating the melt in a non-‐oxidizing atmosphere and/or with increased Ni-‐content in the steel and should thereby not be a big problem when casting the 316L steel grade in the CC-‐machine.
This type of calculation can of course be implemented for steel-‐wire as well but because of its relatively high dissolution time this would demand a very slow feeding rate with large diameter. However, the steel wire is perhaps most convenient when looking at alloying effects the wire has on the final steel and there are more parameters that can be changed to optimize this feeding process. For example by decreasing the wall thickness of the Steel-‐wire and/or increasing the amount of Ce in the powder.
3.3 Composition analysis The results from the composition analysis by Sandvik from samples taken in the melt over time for Steel, Cu and Al-‐wires are presented in figure 23.
Figure 23: Composition analysis from samples taken in melt at -‐1, 1, 3, 5, 10 and 29minutes after wire addition.
0
0,02
0,04
0,06
0,08
0,1
-‐1 4 9 14 19 24 29
wt(%)
ame (min)
Exp.2.0 Steel-‐wire
0
0,02
0,04
0,06
0,08
0,1
-‐1 4 9 14 19 24 29
wt(%)
ame (min)
Exp.3.0 Cu-‐wire
0
0,02
0,04
0,06
0,08
0,1
-‐1 4 9 14 19 24 29
wt(%)
ame (min)
Exp.4.0 Al-‐wire Ce
S
Al
O
31
40mm of wire (2.73g of Ce) was added into the melt for all three experiments. The amount of Ce in the melt one minute after wire addition was shown to be significantly low in the Al-‐wire case (<0.02wt%) compared to the Cu-‐wire (0.033wt%) and Steel-‐wire (0.052wt%). This gives a yield of Ce of 41.9% for the steel wire, followed by 25% for the Cu-‐wire and <14.8% for the Al-‐wire, calculated according to appendix-‐3. This behaviour could be explained by the sampling procedure and the dissolution rate of the wires. All samples were taken at 40mm immersion depth (10mm from the bottom of the crucible). Since the Al-‐wire was shown in the dissolution rate experiments to dissolve very fast (0.5-‐1s at 1500oC, figure 21) the powder might have escaped through the strip just above or below the melt level. This means that the powder is not immersed as deep into the melt before it start to dissolve, resulting in an uneven distribution of Ce. The same goes for the Cu-‐wire, but since the dissolution time is longer than for Al, a higher yield of Ce is obtained at the sampling level. The steel-‐wire has the longest dissolution time resulting in highest yield of Ce when comparing the three wires.
For the Al-‐wire, the high peak of Al when wire is added can be explained by the wire-‐strip. As the Al is added into the melt the amount of total Al will increase. Directly after this addition, Al will start to react with O to form inclusions and clusters that float up to the surface and thereby decrease the Al content continuously throughout the experiment.
Figure 24 shows the behaviour of Cu in Exp.3.0 and is the same plot as in figure 23 but with extended vertical axis. It is clear that the Cu-‐content increases drastically after the first minute after wire addition from 0.3-‐0.9wt% and is then more or less kept constant throughout the whole sampling procedure and does not seem to react with any other element.
0
0,2
0,4
0,6
0,8
1
-‐1 4 9 14 19 24 29
wt(%)
ame (min)
Exp.3.0 Cu-‐wire
Ce
S
Al
O
Cu
Figure 24: Composition analysis from samples taken in melt at -‐1, 1, 3, 5, 10 and 29minutes after Cu-‐wire addition.
32
Regarding the O content, it decreases rapidly in the start and then increases slightly until it seems to reach almost constant values in all experiments. The reason for this increase in O content is most likely due to reoxidation. Another interesting aspect in to look at the oxygen level before and 1 minute after wire addition to see if the FeSiRE-‐powder is oxidized or not before added to the melt. A normal oxygen level in the 316L steel is 50-‐60ppm but due to remelting the initial oxygen content in these experiments are a few times higher. By calculating the difference in O content before and after wire addition according to equation 12, it is clear that the values are positive in all three experiments. This gives an indication to that the FeSiRE-‐powder blend is not very oxidized before added to the melt and also that the reoxidation-‐conditions in the melt are good. The difference in initial oxygen content for the three experiments can be explained by the difference in holding time of melt, table 5. Longer holding time means that more oxygen and air can come to the surface and, since the Ar protection is not ideal, the melt is therefore more oxidized.
𝑂!"!#!$% –𝑂! !"#$%& (12) 𝑆𝑡𝑒𝑒𝑙 − 𝑤𝑖𝑟𝑒: 209𝑝𝑝𝑚 − 199𝑝𝑝𝑚 = 10𝑝𝑝𝑚 (13)
𝐶𝑢 − 𝑤𝑖𝑟𝑒: 334𝑝𝑝𝑚 − 170𝑝𝑝𝑚 = 164𝑝𝑝𝑚 (14)
𝐴𝑙 − 𝑤𝑖𝑟𝑒: 444𝑝𝑝𝑚 − 169𝑝𝑝𝑚 = 275𝑝𝑝𝑚 (15)
33
3.4 Inclusion formation A total of 30 spectrum points were analysed on typical morphologies found in sample A1 and A5 from SEM observation. Inclusion characteristics (morphology and composition) is presented in table 8. As can be seen in the table, a lot of complex inclusions were found with similar contrasts and a wide range in composition, which made it hard to estimate frequency and number of inclusion morphologies only by observing the SEM pictures. In order to get more accurate information about this, more time is needed in the SEM to analyse a higher number of inclusions and check the composition on every single one of them. It should also be noted that SEM spectrum analyses a depth of 1µm, which means that in some cases the film filter contributes to an increased O and C content in spectrum point analysis. In order to eliminate this, the values of each element were normalized with respect to the composition of the film filter before printed in table 8.
Table 8: Inclusion characteristics from sample A1 and A5.
Type 1 2 3 4 5
Morphology
Size range (μm) 1 6.74 – 8.85 2.1 – 11.8 1 -‐ 5.4 2 -‐ 4
Main elements (Ce-‐La-‐S-‐O) (Ce-‐La-‐S-‐O)
(MgO-‐Al2O3)
(MnS(Cu))
(Fe-‐Cr-‐Mo-‐Ni) (MnS(Cu))
( Al2O3)
( Mn-‐Si)
Compo
sitio
n rang
e of non
-‐metallic in
clusions
(wt%
)
Ce: ~ 59
La: ~ 12
S: ~ 14
O: ~ 8
Ce: 26 ~ 28
La: 15 ~ 16
S: 1~10
Mn: 1 ~ 5
S: 1 ~ 3
Cu: 1 ~ 5
Mn: 22 ~ 37
S: ~25
Cu: 6 ~ 37
Al: 17 ~ 31
O: 52 ~ 76
Mg: 1 ~ 9
O: 24 ~ 70
Al: 7 ~ 17
Fe: 41 ~ 66
Ni: 7 ~ 10
O: 0 ~ 2
Mo: 3 ~ 16
Cr: 17 ~ 26
Mn: 0 ~ 13
Si: 0 ~ 9
34
Ce and La was found presented in spherical and cluster-‐shaped morphologies (type 1 and 2), which is confirmed by previous studies. From observation in SEM the clusters was shown to consist of moslty Ce-‐La oxides and Ce-‐La-‐oxy-‐sulphides with the presence of Al and Mg-‐oxides. This was to be expected since Ce and La has a high affinity to both O and S and thereby forms inclusions in forms of Ce-‐La-‐oxy-‐suplhides from the Ce, La S and O dissolved in the melt. Ce also reduces the already present aluminium and Mg-‐ oxide in the melt, and is thereby present on Al-‐Mg-‐O clusters. The composition analysis shows that type 1 and 2 was only found in sample A1 and A5, respectively. This could be an indication to that the small size REM inclusions have, by time, agglomerated to larger clusters from the natural convection in the melt. However, the REM inclusions found were not as frequent as expected, considering the relatively high amount of Ce added to the melt from wire. But, due to the low yield of Ce from adding the Al-‐wire, presented in chapter 3.3, this has its explanation.
Type 3, consisting of mostly Fe, Ni, Cr and MnS was often encountered in both samples. This “MnS-‐intermetallic phase” often attended a shape similar to the REM clusters, making it hard to separate them two without using composition analysis. The presence of Cu is because Cu is easily dissolved from the metal matrix during extraction and the dissolved Cu that ends up in the electrolyte start to precipitate on NMI. These covered NMI should therefore be analysed carefully and might not represent the actual composition of the entire inclusion.
The presence of the intermetallic phase is unknown but a theory could be that the phase has a higher melting temperature than the rest of the melt, which means that this phase start to solidify faster than the surrounding melt when the cooling starts.
The Al-‐oxides and Al-‐silicates found, type 5, can be explained by the Al and Si added to the melt (from the wire strip and FeSiRE-‐powder) and from the possible reduction of Al2O3 on the crucible wall caused by Ce, equation 4. As the Al2O3 from the crucible is reduced, Al reacts with the dissolved O in the melt and form aluminium oxides.
The presence of Mn on the Al-‐oxides is most likely due to precipitation during solidification. AlO has a higher melting temperature than Mn and is already presented in the liquid steel. As the steel start to solidify, Mn start to precipitate on and around the surface of the Al-‐oxides.
35
4. Conclusions • The majority of the weight (~55wt%) of the FeSiRE-‐powder has a particle size smaller than 0,5mm. • Dissolution rate of wires, with powder:
Steel-‐wire: 1530oC 5-‐15s 1500oC 18-‐20s Cu-‐wire: 1530oC <20(2)s 1500oC ≤10(5)s Al-‐wire: 1530oC <2s 1510oC <1s 1500oC 0,5-‐1s
• The yield of Ce 1 minute after wire addition was highest for steel-‐wire (41.9%) followed by Cu-‐wire (25%) and Al-‐wire (<14.8%)
• REM( Ce and La) was present as Ce-‐La-‐oxy-‐sulfides both individually (type 1) and on Al-‐Mg-‐O clusters (type 2)
5. Future work Regarding the dissolution rate experiments, the behaviour of the wires should be observed more in detail during dissolution in order to observe if the powder is immersed properly into the melt or is dropped on surface. This was not possible to do in this study because of a furnace lid blocking the sight of the melt surface. Sampling should be implemented at shorter times since the composition analysis results show that the content of Ce in the melt varies only the first three minutes. Some sort of stirring should also be implemented, in addition to the natural convection, in order get better homogenization of the melt during sampling.
As for the inclusion characterisation, larger number of inclusions needs to be analysed in order to get more accurate results regarding size range and composition. The inclusions formed were shown to be very complex and more time in the SEM is therefore needed for deeper understanding.
36
6. Bibliography [1] E Roos, A Karasev, and P G Jönsson, "Effect of Si and Ce Contents on the Nozzle Clogging in a REM Alloyed
Stainless Steel," Steel Research, vol. 85, no. 9999, pp. 1-‐10, 2015.
[2] E Roos, "A Study of Factors Influencing Nozzle Clogging of Special Steel Grades during Continuous Casting," KTH Royal Institute of Technology, Stockholm, Lic Thesis 2014.
[3] T R Vijayaram, "Metallurgy of Continuous Casting Technology," in Proc. of the Intl. Conf. on Advances in Civil, Structural and Mechanical Engineering -‐-‐ CSM 2013, Chennai, 2013, pp. 65-‐84.
[4] World Steel Association 2013, "World Steel In Figures," Beijing, 2013.
[5] B Rågberg, "Interview," February 2015.
[6] TechniCable. [Online]. http://www.tecni-‐cable.co.uk/s.nl/ctype.KB/it.I/id.138/KB.36576/.f
[8] B Kozak and J Dzierzawski. (2015) SteelWorks. [Online]. http://www.steel.org/Making%20Steel/How%20Its%20Made/Processes/Processes%20Info/Continuous%20Casting%20of%20Steel%20-‐%20Basic%20Principles.aspx
[7] Jernkontoret, Scrap-‐based Steelmaking, 2014.
[9] H Fredriksson and U Åkerlind, Materials Processing during Casting.: John Wiley & Sons, Ltd, 2006.
[10]
The Library Of Manufacturing. [Online]. http://thelibraryofmanufacturing.com/continuous_casting.html
[11]
The Engineering Toolbox. [Online]. http://www.engineeringtoolbox.com/thermal-‐conductivity-‐d_429.html
[12]
L Hennix, "Tillsats av Ce i Kokill vid Stränggjutning av Rostfria Stål," Sandviken, Master Thesis 2003.
[13]
SMT, "Excelark," 2014.
[14]
A R Jha, Rare Earth Materials: Properties and Applications.: CRC Press, 2014.
[15]
L A Luyckx, "The Rare Earth Metals in Steel," pp. 43-‐78, 1981.
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N Krishnamurthy and C Kumbar Gupta, Extractive Metallurgy of Rare Earths.: CRC Press, 2004.
[18]
K G Rackers and B G Thomas, "Clogging in Continuous Casting Nozzles," in 78th Steelmaking Conference Proceedings, vol. 78, Illinois, 1995, pp. 723-‐734.
37
[17]
Y Bi, A Karasev, and P G Jönsson, "Three Dimensional Evaluations of REM Clusters in Stainless Steel," ISIJ International, vol. 54, no. 6, pp. 1266-‐1273.
[19]
E S Dahle, "Grain Refinement of High Alloyed Steel With Cerium Addition," Materials Science and Engineering, Norweigan University of Science and Technology, Trondheim, Master Thesis 2011.
[20]
F Haakonsen, J K Solberg, O S Klevan, and C van der Eijk, "Grain Refinement of Austenitic Manganese Steels," in AISTech, vol. 2, Trondheim, 2011, pp. 763-‐771.
[21]
C van der Eijk, J Walmsley, Ö Grong, and O S Klevan, "Grain Refinement Of Fully Austenitic Stainless Steels Using A Fe-‐Cr-‐Si-‐Ce Master Alloy," in 59th Electric Furnace and 19th Process Technology Conferences, Phoenix, 2001.
[22]
Outokumpu. (2013, December) High Temperature Stainless Steels. [Online]. http://www.outokumpu.com/sitecollectiondocuments/austenitic-‐high-‐temperature-‐153ma-‐253ma-‐stainless-‐brochure.pdf
[23]
H J Grabke, T N Rhys-‐Jones, and H Kudielka, "The Effects of Various Amounts of Alloyed Cerium and Cerium Oxide on The High Temperature Oxidation of Fe-‐10Cr and Fe-‐20Cr Alloys," Pergamon Journals Ltd, vol. 27, no. 1, pp. 49-‐73, 1987.
[24]
(2009) AMG Vanadium. [Online]. http://www.metallurgvanadium.com/ceriumpage.html
[25]
O Sundqvist, "PPT -‐ Planeringsmöte Kokillmetallurgi -‐ SANDVIK," Feb. 2015.
[26]
S Gerardin, "Affival ," 2015.
[28]
MetallurgVanadium. [Online]. http://www.metallurgvanadium.com/copperpage.html
[27]
Outokumpu, Handbook of Stainless Steel. Avesta, Sweden: Outokumpu Oyj, 2013.
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J Elfsberg, "Oscillation Mark Formation in Continuous Casting Process," KTH, Royal Institute of Technology, Stockholm, Lic. Thesis 2003. [Online].
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Indiamart. [Online]. http://www.indiamart.com/gateway-‐international-‐indore/industrial-‐raw-‐materials.html
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H Doostmohammadi, "A Study of Slag/Metal Equilibrium and Inclusion Characteristics during Ladle Treatment and after Ingot Casting," Materials Science and Engineering, Royal Institute of Technology, KTH, Stockholm, PhD Thesis 2009.
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Appendix-‐1
From the dimensions given from figure A1, a total weight of the liquid metal required to fill 2/3 of the total height of the crucible was calculated. Equation A1 gives the general expression for a volume of a frustum, equation A2 gives the required volume of the melt and equation A3 gives the acquired amount, in weight, needed for each heat.
𝑉!"#$%#& =𝜋 ∗ ℎ3
∗ 𝑅! + 𝑅 ∗ 𝑟 + 𝑟! (𝐴1)
𝑉!"#$ =𝜋 ∗ 80 ∗ 23
3∗ 45! + 45 ∗ 34.5 + 34.5! = 285884.9𝑚𝑚! 𝐴2
𝑊!"#$ = 𝜌 ∗ 𝑉 = !.!!"!!!! ∗ 285884.9𝑚𝑚! = 2001𝑔 ≈ 2𝑘𝑔 (A3)
Appendix-‐2 In order to be able to simulate the formation of REM clusters in a solidified strand casted by Sandvik, a cooling rate was calculated from the temperature of the melt in the Cu-‐mould to the solidus temperature of the 316 stainless steel. The casting temperature of the 316 steel is 1500oC and the solidus temperature is 1390oC, according to Thermo Calc. This gives a temperature interval of 110oC. By knowing the metallurgical length (23m) and the casting speed (0,8m/min) of a 316 strand it is possible to calculate the maximum amount of time clusters can form, equation A4. In addition, a cooling rate can be calculated, equation A5.
𝑀𝑒𝑡𝑎𝑙𝑙𝑢𝑟𝑔𝑖𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ𝐶𝑎𝑠𝑡𝑖𝑛𝑔 𝑠𝑝𝑒𝑒𝑑
= 23𝑚
0.8𝑚/𝑚𝑖𝑛= 28.75min = 28min 45𝑠 (𝐴4)
𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙𝑡𝑖𝑚𝑒
=110!𝐶28.75
= 3.82!𝐶/min ≈ 4!𝐶/ 𝑚𝑖𝑛 (𝐴5)
Figure A1: Inner dimensions of crucible and outer diameter of wire, in mm. The thickness of the strip is 0.4mm.
39
Appendix-‐3 40mm of wire was dissolved in each of the three experiments. By knowing the powder weight and the amount of Ce in the FeSiRE-‐powder, table 7, the total amount of Ce added can be calculated according to equation A8 (following calculations is for Steel-‐wire, Exp.2.0, but same calculation procedure is implemented for Cu-‐wire, Exp.3.0, and Al-‐wire, Exp. 4.0, as well)
𝑃𝑜𝑤𝑑𝑒𝑟 𝑤𝑒𝑖𝑔ℎ𝑡: !"#!!
∗ 0.04𝑚 = 17.2𝑔 (A6)
𝑆𝑡𝑟𝑖𝑝 𝑤𝑒𝑖𝑔ℎ𝑡: !"#!!
∗ 0.04𝑚 = 6.72𝑔 (A7) 𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝐶𝑒 𝑎𝑑𝑑𝑒𝑑: 17.2𝑔 ∗ 0.2537 ∗ 0.6245 = 2.73𝑔 𝐶𝑒 (A8)
The total weight can be calculated from table 4 and 5.
𝑇𝑜𝑡𝑎𝑙 𝑤𝑒𝑖𝑔ℎ𝑡 𝑆𝑡𝑒𝑒𝑙 + 𝑤𝑖𝑟𝑒 = 2083.6𝑔 + 17.2𝑔 + 6.72𝑔 = 2107.52𝑔 (𝐴9)
The teoretical amount of Ce in the melt can now be calculated according to equation A10.
!.!"!!"#$.!"!
= 0.00129 = 0.129𝑤𝑡% (A10)
By knowing the amount of Ce 1 minute after wire addition (sample S1= 0.054wt% of Ce) the yield of Ce in the melt is thus:
!.!"#!.!"#
= 0.4186 ≈ 𝟒𝟏.𝟗% (A11)
40
Appendix-‐4 (Exp.1.1-‐1.3)
Figure A2: Dissolution rate experiments at 1530oC for (b) Steel-‐strip, (c) Cu-‐strip and (d) Al-‐strip.
(a)
41
Appendix-‐5 (Exp.2.1-‐2.3)
Figure A3: Dissolution rate experiments at 1530oC for (b) steel wire, (c) Cu-‐wire and (d) Al-‐wire. Trial 2.1.t2 and 2.2.t1 was sealed with para-‐film and are excluded from the results.
(a)
42
Appendix-‐6 (Exp.3.1-‐3.3)
(a)
43
Appendix-‐7 (Exp.4.0)
Figure A4: Dissolution rate experiments at 1500oC for (b) Steel-‐wire, (c) Cu-‐wire and (d) Al-‐wire.
Figure A5: Dissolution rate experiments at 1510oC for (a) Al-‐wire.
44
Appendix 8 (samples from Exp.2.0, 3.0 and 4.0)
Appendix-‐9
Figure A7: Cooling rate registered by thermocouple for all four experiments.
Figure A6: Samples taken according to the sample sceheme, for (a) steel wire: S0= 19,3g S1= 20,5g S3= 22,1g S5= 22,5g S10= 14,5g and S29= 20,8g, (b) Cu-‐wire: C0=24,82g C1= 17,56g C3= 21,06g C5= 21,3g C10= 24,77g C29=16,78g, and (c) Al-‐wire: A0=21,83g A1= 19,9g A3= 18,83g A5= 19,68g A10= 23,07g A29=21,22g.
(a) (b) (c)
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Trip Report 26/2 2015
Purpose The purpose of this trip was to get introduced to the main persons from Sandvik that is involved in this project and to get a deeper understanding of this new idea of alloying in the continuous casting machine. Another important reason for me personally was to be able to get in contact with the industry to get a new perspective and see how things are handled.
Content The trip started off at 12.00 with lunch where I was introduced to Bo Rågberg and Olle Sundqvist. Later I was given a tour around Sandviks research division FOU and sat down with Bo who presented some previous master-‐thesis work done in this field, going all the way back to -‐98. We also discussed my own work and I asked some questions regarding my own experiments and the amount of steel that I need. At 14.30 I was invited to a meeting regarding planning of future plant trials of wire feeding into the CC-‐mould. The meeting included a background about the project where some decisions were presented regarding a meeting in nov -‐14 with Affival+Ferrox. The conclusions from that meeting were that Affival delivers the wire and examines the possibility of delivering a feeder that can feed two wires at the same time. Affival is also expected to join the plant trials in 2015. After that, the meeting proceeded with some important discussions regarding the feeding process, such as;
• How and where the wire-‐feeding pipe should be mounted, either to the bottom of the tundish, or on a deck from a higher level.
• Dimensions of the wire-‐feeding pipe. • Where the feeder should be positioned depending on placing of wire-‐feeding pipe • How the wire-‐feeding pipe is going to be connected to the feeder • Where the wire-‐feeding pipe outlet is going to be positioned relative to the mould
Some questions where answered whereas most of them was still left to be answered during the practical trials to see what method work out the best. The meeting also included some practical matters of delegating tasks for the crew in order to move on and, hopefully, start doing some feeding experiments, without any melt, next Thursday (5th of March) when the continuous casting machine is under maintenance. This is provided that the feeder, which is borrowed, will arrive on time.
After the meeting I was given a personal tour in the steel plant from Bo Rågberg. The tour included the entire steel process route all the way from the EAF to finished casted slabs. Since the continuous casting process was under maintenance this day, we were able to take a very close look at some extra interesting and project-‐related process steps, such as the CC-‐mould and tundish, that normally is impossible to get close to when the process is on-‐going. Bo showed some potential placing’s of the feeder and how the feeding process briefly would be performed. We also had a look inside the cooling-‐room where the three cooling zones where positioned. Later we moved on and visited the area where the tundish and SEN-‐nozzles are preheated and also to the CC-‐mould workshop to see the shape of the mould and typical wear of the inner mould lining. The tour ended at 16.00 and that also wrapped up the visit for this time.
Summary This visit met the expectations and more. Except for getting introduced to the project and meet all the involved people. I also got a personal tour around the plant which was very interesting for me. Bo also presented an instrument called ”XXX” for analyzing of small samples that I could use to get the overall composition for my time dependant quartz-‐tube samples.
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Meeting 28/4 2015
Purpose The purpose of this meeting was to present work done by KTH and SMT this far in the project and also to inform and discuss about previous and future industrial trials.
Content The content of the meeting followd folowing agenda:
1. Minutes from the last meeting 2. Work done by KTH
• Ying Yang • Oscar Juneblad • Haji Muhammad
3. Work done by SMT 4. Hire of wire feeder 5. Planning of full scale project 6. Application for Vinnova 7. Miscellaneous
I held a presentation of 15minutes presenting my results this far and what work I have ahead of me in the near future before summer. After my presentation I was asked by Bo to recalculate a new diameter and feeding rate of the Cu-‐wire in order for it to be fully dissolved at an immersion depth of 15cm.
SMT presented the results from the first plant trial (15th of April), which was at the very end of a heat.
Some data for this trial: Steel grade: Duplex stainless steel Casting speed= 0,8m/min Wire feed= 13-‐14m/min
The result from this trial was a breakthrough after 78m of fed wire and the main cause of this problem was freezing of the meniscus. It was discussed how this problem arose and the main conclusion was due to a too low superheat of the melt before the wire was added. Samples were taken on the casted blooms and Ce, Al and Si was analysed at different locations in the cross-‐section of the bloom.
A desire is to have 0.05wt% of Ce in the finished cast product but however most of the samples showed too low levels of Ce i.e. low yield of Ce.
The hiring of wire-‐feeder could be prolonged to the 30th of May, which was good news in order to be able to perform upcoming trials:
Trial 2 (29th of April): Steel grade 316L Single casting
Trial 3 (end of May): All blooms of 2 strands in an entire heat
At the end of the meeting I was personally invited by Bo to come up to Sandviken for observation of trial 3. Lastly, a telephone meeting was arranged on the 6th of May at 13.00.
47
Summary It was nice for me personally to meet all persons involved in this project, both from KTH and from out in the industry. Also, the progress from both my colleagues at KTH and SMT was very interesting to see. Especially Ying Yang’s simulations since she had simulated same experiments as I had performed practically.
Telephone meeting 6/5 2015
Purpose The purpose for this meeting was how the project proposal should be carried out for application for future on going project to Vinnova and/or RFCS. Assuming that this proposal is successful, potential areas needs to be discussed regarding future phD work in this project. It was also informed about trial nr 2.
Content The meeting started of with a discussion regarding the application of this project that has a budget of 20-‐25milion SEK. SMT questioned KTH regarding if it was possible to write down a project proposal before the 2nd of June in order to send it in before summer and start the project the upcoming autumn. People at KTH however had opinions that the pre-‐study from previous plant trials done this year is not evaluated completely yet and this should be done before making an application. It was finally decided that the project should be divided into two steps:
Step 1 (send a application to Vinnova as a more limited project):
-‐ 1 steel grade -‐ only looking at one element (Ce) -‐ Finish the analysis -‐ Include pre-‐sudides done my previous master thesis worker showing great results of distribution
and composition of Ce in casted product -‐ 1year, 2 people, full time
Step 2 (send a more general application to RFCS):
-‐ More steel grades -‐ Invite more companies
At the end of the meeting it was decided that KTH and SMT should, individually, come up with workpackage of potential areas and questions that can be worked with in future as phD thesis etc. SMT will focus on more industrial questions and KTH on more scientifically. SMT mentioned one big question right away regarding analysing of mould flux composition when adding of Ce-‐wire in the CC-‐mould and how to modify the mould flux for this new mould metallurgy technique.
Trial nr.2 was successfully executed in terms of breakthrough or stopping of process but the casted strands had not yet been analysed.
Next telephone meeting was schedueled to the 21st of May at 14.00
Summary The project was decided to be divided into two seperate steps where the first step accounts for a more limited project in order to focus on one steel grade and element. Hopefully, this will get good enough results for applying for step number 2 to continue the project in a larger scale.
