AUTHORS: Manfred Bock and Christian OehlerSaar-Metallwerke GmbH

SLAG COMPOSITION

A lime-saturated slag is important both for steel-making metallurgy and to prevent excessive wear of

the converter lining. Lime added before and during theblow should ensure a saturated slag at the end of theblowing process. Converter slags can be described withinthe basic ternary system FeO-CaO-SiO2 (see Figure 1,upper part). In this system the so-called C2S (dicalciumsilicate) nose and C2S saturation line extend far into theinner area of this ternary system. C3S (tricalcium silicate)saturated slags below the C2S area occur only within asmall band of between 10 and 30% SiO2 and only slagson the right-hand side of the C3S area, between 0 and10% SiO2, are in equilibrium with pure solid lime. Thisline is called the CaO saturation line.

In converter slags, the components CaO, FeO and SiO2make up typically 80–85% of the total slag constituents.Additions of MnO, MgO, P2O5 and Al2O3 push the C2Sand C3S saturation lines towards the binary CaO-SiO2boundary system and CaO corner. The lime saturation lineof commercial slags shown in the lower part of Figure 1extends beyond 20% SiO2 and is not comparable to thepure ternary system.

To convert the analysis results of commercial slags for usein the pure ternary system, the data for CaO, FeO and SiO2(say, 85% total), each individual content figure must bemultiplied by the factor 100/85 and in the diagram thesefigures are marked CaO’, FeO’ SiO2’. The figure for the FeOcontent is calculated from the figure for Fe total (Fe x 1.28).

The slag development path for different hot metal Sicontents is shown in Figure 2. According to this diagram,the general slag development path is as follows: startingfrom the high FeO’ corner, the SiO2’ and CaO’ contentsrise, as a result of increasing silicon oxidation and lime

Good control of slag development, oxygen flowand lance practice, and use of bottom stirringand re-blow practice are key aspects of themetallurgical control of steelmaking.Knowledge of interactions between processchemistry, blowing dynamics and converterlining wear can achieve both efficientsteelmaking and long converter lining life.

Metallurgy and lining life in basic oxygen converters

STEELMAKING

r Fig.1 CaO-FeO-SiO2 ternary system

r Fig.2 Slag development paths

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dissolution. The higher the initial hot metal Si content, thehigher the SiO2 content early on in the blowing process.

At the end of the blow slags should be slightly limesupersaturated in order to avoid excessive refractory wear,ie, they should be within the lower diagram of Figure 1. Inorder to achieve this goal a lime addition rate is necessarywhich is adapted to the Si content in the hot metal andto the target slag Fe content.

In Figure 2 lines from the CaO’-SiO2’ boundary system,which are linked to the FeO’ corner are shown. Each ofthese lines represents a certain basicity ratio: CaO/SiO2.Lines of equal basicity are cut by the lime saturation line.Higher basicity equates to higher Fe (FeO) content in theequilibrium condition. Slags with a composition located onthe saturation line are lime saturated. Slags below this linecontain excess free lime as it cannot be fully dissolved andthe FeO content in the heat may be too low. Slags abovethis line are lime under-saturated and do not containenough lime, and the FeO content maybe too high.

Figure 3 is derived from Figure 2 by using theintersection points between the lines of equal basicity andthe saturation line, and defines the lime saturation line ofa commercial slag. Slags below the line are lime under-saturated and the slags above the line contain excess lime.

The requirement of a lime-saturated slag at the end of the blow in order to avoid excessive refractory wear is valid both for pure dolomite and magnesite linings.Figure 4 documents results from the Didier researchdepartment and illustrates increased slag MgO solubilityas the degree of CaO under-saturation increases.

The practice of working with lime under-saturated slagsis a result of an incorrect assumption that CaO will bereplaced by an equivalent quantity of MgO (contained inthe dolomite). A reduction in the amount of lime added

p Fig.3Relationship

between slagbasicity and

Fe contentalong the limesaturation line

r Fig.4 Relationship between lime saturation index and MgO content

r Fig.5 MgO saturation in CaO-FeO-SiO2-MgO system

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automatically leads to low basicity figures. There are steelplants working with basicities below 2–2.5, however, inorder to stay on the lime saturation line the slag Fecontents must be kept below 10%. This is metallurgicallyundesirable if dephosphorisation is required. An Fecontent of 17–18% is more practical.

When using a magnesite lining, in order to minimiselining wear the slag should be both CaO and MgOsaturated. The maximum soluble MgO contents in the FeO-CaO-SiO2-MgO system are illustrated in Figure 5.According to this figure the MgO solubility increases withincreasing SiO2 content. Figure 6 shows the maximumsoluble MgO content along the lime saturation line in thisquaternary system. Slags with low basicity, equivalent tolow FeO content in the slag according to Figure 3, have thehighest MgO solubility, therefore a magnesite lining is mostheavily attacked early in the blow when the slag basicity isstill low. MgO solubility decreases with increasing basicityand FeO. Figure 6 also indicates that even for a low slagbasicity of 2.0, 8% MgO can be dissolved in the final slag,whereas it is 6% for a normal basicity of 3.5.

Above the saturation line all the MgO cannot be molten,thus for instance, with an MgO-saturated slag, a furtherincrease in slag basicity will cause MgO to be precipitatedand increase the viscosity of the slag, with the result thatheavy build-ups on the converter bottom and walls canoccur (bottom build-ups of 2.5m are possible). These build-ups prolong lining life (a new record in the USA: 70,000heats!), but the metallurgy is also influenced by changesin the bath level and the inner converter diameter in thebath level area. This can be seen in Figure 7, illustrating thechange in jet penetration depth, L, on slag FeO. If the bathdepth, Lo, reduces for a constant penetration depth of the

oxygen jets, the FeO content in the slag will increase. Thismeans that deeper baths require deeper oxygen jetpenetration, ie, harder blowing lances.

Lining life is influenced by the slag analysis throughoutthe blow. In the boundary system FeO-SiO2, shown inFigure 1, there is a compound fayalith (2FeO.SiO2) with avery low melting point of 1,205°C. The higher the hotmetal Si content the longer the time period that isrequired to pass through the area of fayalith-containingslags. This area, together with the high MgO solubility atthe low basicities that exist at this part of the blow, havea very unfavourable influence on lining life. Therefore, itis very important that the added lime dissolves quickly inorder to raise the slag basicity as early as possible.

The use of soft burnt lime and a sufficiently large lancedistance to the metal bath at the beginning of the blow(which enhances Fe oxidation and therefore limedissolution), are favourable to achieving this aim. Also tofacilitate early lime solution the lime addition should becomplete within three to four minutes of blow start.

In some plants fluorspar (CaF2) is added to flux the lime,especially when high basicities (up to 6) are required, orwith low FeO slags, otherwise the amounts of undissolvedlime (from C2S precipitation) would be too high. Fluorsparlowers the melting point of CaO and C2S and the viscosityof the converter slag. Additions, however, should beminimised because the volatile and aggressive compoundSiF4 is formed, which heavily attacks the steel pipes of thewaste gas system above the converter. The fluorsparaddition also reduces the melting point of the refractory ofthe converter lining. Typical plant consumptions are1–3kg/t liquid steel. Alumina (Al2O3) has a similarmelting point lowering effect but reacts more slowly.

PHOSPHORUS REMOVALThe achievable steel phosphorus level is affected by hotmetal P%, slag basicity and FeO. Figure 8 shows the a

r Fig.6 Relationship between slag basicity andMgO saturation

r Fig.7 Effect of bath penetration on slag Fe content

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calculated required lime addition rates and influence ofslag Fe content for a hot metal with 0.09% phosphorous.Phosphorus removal is also inversely related to steeltemperature.

LIME QUALITYLime is not pure CaO. It contains impurities such as SiO2and Al2O3 which must be compensated for in theadditions calculations. Also, its metallurgical efficiency isaffected by the particle size and reactivity (or degree ofburning). The ideal particle size is 10–50mm, as particlesbelow 6mm are extracted from the converter, together

with the waste gas; up to 30% in some cases. If thishappens the slag produced can be under-saturated,causing additional converter lining wear.

Lime with a wide particle size range also separates whencharging into storage bunkers such that coarse materialtravels to the outer side of the cone shaped charging pilewith the fine grained material remaining in the inner area.Thus, when charging the lime from the bunker, lime qualitywill be variable, with negative consequences for steelchemistry, temperature control and converter lining life.

BATH MOVEMENT AND BOTTOM STIRRINGAnother important precondition to achieve consistencyand controllability of blowing behaviour, and a low rate ofvariation of the results after the end of the blow, is asufficient bath movement during blowing. During themain decarburisation phase there is good bath movementas a result of CO formation. With decreasing bathmovement at C contents below ~0.30% resulting fromreduced CO gas formation, bath movement decreasesconsiderably, therefore, during this phase of the blow, thistask has to be fulfilled by lowering the blowing lance.Although the stirring effect, induced by the lance, is muchless than with CO formation, it ensures that bath stirring ismaintained to the end of the blow. This is one reason whybottom stirring with inert gases was introduced into manysteel plants. Although the gas quantity blown through theconverter bottom via plugs (typically in the range0.01–0.05Nm3/t/min) is small compared to the topblown oxygen, its stirring effect has multiple benefits inensuring the slag and bath are in greater equilibrium andin producing lower and more controllable Fe levels inturndown slags (see Figure 9) which are beneficial forconverter lining life.

Plug position is important both for metallurgicaleffectiveness and lining wear (see Figure 10). If the plugsare positioned too far away from the centre two differentcurrents in the steel bath are formed which considerablyreduce mixing and therefore affect the metallurgicalequilibria. Stirring plugs located far away from the centreincrease the wear of the converter lining in the transitionarea between converter bottom and wall (knuckle area).

RE-BLOWINGRe-blowing for final adjustment of temperature oranalysis is often required, but at the expense of increasediron oxidation and hence higher refractory wear. Forinstance, a reblow of less than one minute will raise thetemperature 20°C, but will also increase the slag Fe by5%. Figures 11a and 11b illustrate greater uniformity inslag FeO when using bottom stirring.

Although theoretically lime should be added during re-blowing in order for it to stay on the saturation line (as a

r Fig.8 Some factors influencing steel P level

r Fig.9 Influence of bottom stirring on slag Fe content

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result of the Fe increase), in most cases this is not done,and especially not in cases when the re-blow is requiredto raise temperature, as the temperature increase by theFe oxidation would be compensated for, to a large extent,by the heat consumption for lime dissolution. Althoughunder-saturated slags with quite high Fe contents andtemperatures are acceptable for metallurgical reasons,they are extremely detrimental to lining life and thedamage is greater the longer the molten steel is kept inthe vessel between blow end and tapping.

USE OF IRON OREIron ore pellets, which are added to cool the steel bath,also have an influence on lining life due to the increase inmolten FeO. Excessive amounts of added ore should beavoided because the additional amount of oxygenintroduced by the ore leads to an uncontrollable blowingbehaviour. Ore addition should preferably be completedduring the main decarburisation period otherwise theremay be insufficient carbon available to reduce the meltedore. However, there are BOF shops that do add ore

STEELMAKING

q Fig.10 Effect of bottom plugposition on stirring efficiency

s Fig.11 Effect of bottomstirring on re-blow slagoxidation, a) without, b) with stirring.

A B

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towards the blow end as a way of achieving lower turndown phosphorous contents, as phosphorus removal isenhanced with higher FeO levels.

If ore is always charged to the same side of theconverter through the charging chute, the FeO-rich slagwhich is formed locally at the trunnion area causeslocalised lining wear. For this reason some BOF shops varythe ore addition side to the converter.

LANCE PATTERN AND LANCE TIP DESIGNDuring a typical 14–22 minute blowing time, the iron isrefined and many tonnes of scrap and slag forming agentsare melted. During this period the dynamics of slagformation have to be controlled in a way that achieves anoptimum blowing process and the desired final turn downtemperature and steel composition. One of the tools forcontrol of slag formation is the blowing lance.

The blowing process is strongly influenced by the lancetip design and the lance distance to the metal bath. Thebasis for the calculation of a lance design and thenumber of nozzles is the required oxygen flow rate perminute. The necessary blowing hardness is determinedby the size of the converter, the bath depth and by theinput materials to be processed. More slag, for instanceas a result of high hot metal Si content, requires higherblowing pressure. In general the blowing pressures arebetween 8 and 15 bar as measured right in front of thenozzles inside the lance tip. At the point of smallestnozzle diameter the blowing pressure is about half thepressure in front of the lance tip (the so-called Lavalpressure = 0.528 x pressure in front the nozzles insidethe tip). Maximum sonic speed is achieved as the nozzle

diverges in the flow and as the pressure in front of thenozzles increases. For blowing pressures of 8–15 bar, exitvelocities of Mach 1.95–2.36 are achieved. The length ofthe diffusor has to be specifically designed for theoxygen blowing pressure respective to the oxygenvelocity. If the diffuser is too short or too long, shockwaves are created. In Figure 12 different operating statesor conditions that can occur in a Laval nozzle areillustrated. When the oxygen exit pressure Pa does notcorrespond to the pressure for which the Laval nozzle iscalculated (the surrounding pressure in the vessel), therewill be a disturbed flow within or after the Laval nozzle.The recompression pushes whichoccur within the diffusoror within the free oxygen jet lead to pulsations of theoxygen jet and therefore to a loss of energy. Three typicalcases are possible:

` The exit pressure Pa is above the critical pressurePkrit

` The Laval nozzle behaves like a Venturi nozzle. Theflow is accelerated the converging part of the nozzle,then retarded in the diverging part. All velocities arebelow sonic speed

` The exit pressure Pa’ is below the pressure Pkrit, butabove the exit pressure Pa on which the calculationis based. In this case the gas has sonic speed at thesmallest nozzle diameter with recompression pushesin the diverging nozzle area. The oxygen jets comeaway from the diffusor walls inside the nozzle.

` The exit pressure Pa’ is below both the pressure Pkritand the exit pressure Pa on which the calculation isbased. After the oxygen jet leaves, the diffusor

r Fig.12 Different operating states for a Laval nozzle

After the nozzle exit the jet is bursting

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expansion occurs at a wider angle than the diffuserangle with recompression of the jets further awayfrom the nozzle.

All these non-design conditions reduce the kinetic energyof the oxygen jet and change the blowing characteristic ofthe lance. Such shock waves are also produced if thecalculated set oxygen flow rate is altered during use. For afixed lance distance, changes in the oxygen flow ratechange the impingement pressure of the oxygen jets at themetal bath surface so the blowing characteristic gets softeror harder. Both under-blown and over-blown nozzle tipshave a softer blowing characteristic because of the kineticenergy loss as result of the shock waves. In addition, under-blown nozzles damage the diffuser exits and thereforereduce the lance tip life. Blowing behaviour is bettercontrolled if the lance distance to the metal bath is alteredwhile working with a constant oxygen flow rate.

Many steel plants work with a fixed lance pattern duringblowing which must lead to variable results if there arevariations in the amounts and chemistry of the addedinput materials. Dynamic lance control which adapts todifferent conditions is of great benefit.

POST COMBUSTIONOxygen flow rate should preferably be constantthroughout the blow. Unfortunately, in many steel

plants the oxygen flow rate is reduced at times duringthe blow because of slopping or simply because it is laiddown in the blowing program. Depending on theduration and the amount of the reduced oxygen flowrate, this can have adverse consequences for converterlining life. An under-blown nozzle produces shockwaves, which lead to an intensive re-mixing with the up-streaming CO/CO2 gas, and therefore to additionalpost-combustion of CO to CO2.

In addition, the lance distance itself also has aninfluence on post-combustion. With increasing lancedistance, (softer blowing), the CO2 increases andalthough, theoretically, a 1% CO2 increase can melt anadditional 5–5.5kg scrap/t of steel, the practical resultis only a marginal increase at the expense of aconsiderable increase in lining wear, especially in theupper cone of the converter. The reason is that withincreasing post combustion the temperature of thewaste gas surrounding the oxygen jets increases andacquires a burner characteristic, able to reachtemperatures of 3,000°C. MS

Manfred Bock is Metallurgical Consultant and ChristianOehler is Managing Director, both at Saar-MetallwerkeGmbH, Saarbrücken, Germany

CONTACT: dr.c.oehler@saarmetall.de

STEELMAKING

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