Electric Furnace 1966 - The Problems of High Oxygen Content in Liquid Steel …library.aimehq.org/library/books/Electric Furnace 1966... · · 2010-02-08in Liquid Steel And Possible

2 Electric Furnace Proceedings, 1966

The Problems of High Oxygen Content in Liquid Steel And Possible.

Methods of Control

by B. A. Strathdee

The title of this paper, chosen several months ago when the work was in the initial production stage, is sufficiently general to accommodate any changes that might have oc- curred. Fortunately, few changes were encountered and the title is still appropriate. The problems re- ferred to are those 'associated with rapid sampling and analyzing for high oxygen content i n molten steel. Such an analysis is the initial phase of deoxidation control.

INTRODUCTION

Steelmaking is a process of pref- erential oxidation regardless of whether it occurs in the open hearth or in the basic oxygen furnace. The impurities in the charge are oxidized in preference to the iron, and, since these oxides are insoluble in steel, they are incorporated in the slag, or removed as gases. The process is not perfectly selective and some of the iron is also oxidized to form iron oxide which may either enter the slag or remain in solution in the molten steel. The amount of oxygen dissolved in the steel at the end of refining is extremely important since this oxygen governs the structure and the cleanliness of the resulting product.

Typical cross sections of four ingots are illustrated in Fig. 1. They are arranged in order of increasing amounts of oxygen in the liquid steel at the time of solidification. The first ingot has had all the oxygen removed or tied up ' as oxide prior to solidification and no gas is evolved in the mold. The structure shows the shrinkage and bridging, that occurs with most killed steel ingots.

The second ingot, which is semi- killed, has a small amount of gas evolved during solidification, just sufficient to compensate for the shrinkage that normally occurs. The third ingot evolves an appreciable amount of oxygen in the form of CO. The bubbles of CO, rising in front of the solid-liquid interface generate a rimming action in the

B. A. STRATHDEE is a research metal- lurgist with Dominion Foundries and Steel, Ltd., Hamilton, Ont., Canada.

liquid steel in the mold. Once a suitably 'thick shell of dense steel forms, gas evolution decreases sufficiently to cause the steel to rise and solidify- against a metal cap. This prevents. further evolution of gas and the core of the ingot solidifies quiescently.

The fourth ingot structure is typical of the fully rimmed steel. Copi- ous amounts of gas are evolved and a vigorous rimming action takes place during the solidification. There is insufficient gas remaining in the latter stages of solidification to offset the shrinkage that occurs.

Cleanliness is also related to the oxygen content. In the case of the rimmed capped steel, the oxygen precipitates during solidification and is available to react with the carbon, manganese, aluminum, etc., con- centrated in front of the solid-liquid interface. The CO that forms con- tributes to the rimming action rising in the mold and escaping. The remaining available oxygen reacts with the mold deoxidizers. Separa- tion of the products of mold deoxidation is thought to be less than for

the products of ladle deoxidation. Conseauentlv. the cleanliness. as well as the skucture of the ingot, is related to the oxygen content of the liquid steel prior to teeming.

This phenomena of gas evolution is the general demarkation between low-oxygen and high-oxygen steels. Possibly, the most difficult to make from the standpoint of oxygen control is the rimmed capped steel. To leave an excess of oxygen is not too drastic since mold additions of a deoxidizer can be made to reduce the oxygen to the proper level. To remove too much oxygen is a cardi- nal error.' It is practically impossi- ble to properly reoxidize the steel to the required level in the mold.

Consequently, a rapid method of oxygen determination is an essential tool for properly controlling deoxidation, particularly at the high levels of oxygen content associated with rimming steels.

PREVIOUS PRACTICE

In the past, the oxygen content has been estimated from the carbon-oxygen relationships, assuming

Fig. 1-Ingot structures of killed, semi-killed, capped, and rimmed steels.

Deoxidation, Theory, and Practice 3

.O1 -

. , , I . . , . a , , , , 1 1

.02 . 0 4 .08 .08 .10 . 12 . 14

CARBON %

Fig. 2-Carbon-oxygen equilibrium and practical relationship.

equilibrium, or from the slag FeO, supposing that this in equilibrium with the oxygen in the metal bath. Such equilibrium conditions are rarely encountered in steelmaking.

As an illustration, Fig. 2 shows the familiar carbon-oxygen equilibrium curve. In this case, at 160O0C, % C x 7% 0 = 0.002. Above this is plotted a band representing actual results obtained by various sampling and analytical techniques. The band is wide, and the width increases with increasing oxygen content and decreasing carbon content. For example, if the bath carbon were analyzed a t 0.05 7% a t the end of refining, the carbon-oxygen equilibrium curve would indicate a bath oxygen content of 0.04 %, but the actual measurements show that the oxygen content can vary from 0.05 to 0.10 %. Obviously, carbon analysis alone is an unsatisfactory and mis- leading indication of oxygen content. Some improvement can be re- alized by considering the effect of manganese, but such estimates are no substitute for an actual analysis of oxygen.

loss, there is also the possibility of oxygen pickup when the steel is exposed to the atmosphere. These ef- fects must be prevented or minimized in designing a suitable sampling technique.

Immersion Sampl ing

Direct sampling, by immersing a suitable device into the liquid steel, is logically desirable providing that it copes with the elusive behaviour of oxygen.

1. Bomb-Sampling. The most com- mon technique is bomb-sampling. This is schematically represented in Fig. 3. A sample mold, mounted on the end of a lance, contains sufficient aluminum for deoxidation. Slag pickup is minimized by a suitable cap which melts off once the sampler is in the molten metal. In effect, a small ingot of steel is obtained. It differs from regular ingots mainly in size, but it incorporates the same drawbacks as a representative sample for oxygen. The sampler has a relatively large thermal mass and may cause localized chilling and precipitation of oxygen in the bath. Consequently, some depletion of oxygen occurs even before the liquid steel enters the mold. The mold cavity contains approximately 1 cu in. of steel. This large sample does not freeze immediately and segregation of oxide occurs. Since the resulting sample is nonhomogeneous and too large to be analysed in one portion, problems in sample preparation are encountered. At best, the bomb sampler produces dubious results and the time required is too long for production control.

2. Electronite Sampler. Of the improved immersion samplers tried, including some developed a t Do- fasco, the most successful was a

device marketed by the Electronite Engineering Co. A cutaway view of this sampler is shown in Fig. 4. An evacuated glass tube with a thin pyrex "window" at one end and containing aluminum as a deoxidizer is mounted in a refractory material in a conventional cardboard dunker tube. Not shown is a thin-walled steel cap which protects the exposed end of the evacuated tube as it passes through the slag. When in contact with molten steel, the thin glass window immediately opens, al- lowing the steel to enter the tube where it is deoxidized and quickly solidified.

The main drawback to this device is that a good solid pin is not obtained in every attempt. The pins cannot be immediately extracted from the holder and examined in order that another sample be taken if the first is unsatisfactory. As will be discussed later, at least three suitable pins are required for analysis, and to insure this, five or six must be attempted.

The electronite samplers perform better in obtaining samples from the ladle where the oxygen content is lower than in the bath.

Spoon Sampl ing

The standard procedure for obtaining pin samples for spectrographic analysis of manganese, cop- per, nickel, etc., is to obtain a large spoon sample of approximately 3-5 lb directly from the bath, deoxidize it with aluminum wire, then suck a portion of this sample into an evacuated pyrex tube. Normally a solid clean pin suitable for spectrographic analysis is the result. Variations on this technique have been tried in order to obtain a sample for oxygen analysis. Usually the drawbacks as-

S A M P L I N G

A rapid determination of oxygen in liquid steel is primarily depen- - - dent on the sampling technique used. The problems associated with 'BOMB* SAMPLER sampling are illustrated in Fig. 1. The ingots are actual s a m ~ l e s of the steel in the ladle from which they originated. But are they representative samples with respect to oxygen?

For the killed steel, the oxygen present is in the form of oxide which is influenced by flotation and segregation during cooling and solidification. The overall average oxygen content may be representative, but the ingot is not homogeneous with respect to oxygen.

For the fully rimming ingot, the oxygen precipitates as CO when the steel chills and this continues throughout solidification. This ingot is severely depleted in oxygen and is not representative of the original ladle composition. These phenomena of precipitation and loss of oxygen as CO, and segregation of oxygen as oxides are encountered in all sampling methods whether they produce a 10-ton ingot or a 10-g analytical sample. As well as oxygen

mould

lance ring support

Fig. 3-Bomb-sampler.


Fig. GE lec t ron i te sampler.

sociated with bomb-sampling were encountered: chilling and precipitation of oxygen by the large thermal mass of the spoon; segregation of oxygen when the spoon sample was deoxidized; porous pins if no prior deoxidation was used; and large- diameter tubes produced slow cooling and segregation of aluminum if the deoxidizer was used in the evacuated tube and this resulted in nonhomogeneous pins.

A practical, but by no means per- fect, solution to this problem has been devised.

A suitable spoon sample of molten steel can be obtained with good technique and proper care. The spoon must be hot to prevent local chilling and precipitation of the oxygen, and well-slagged to minimize the nucleation of CO which would further accelerate the loss of oxygen. In practice, this can be achieved by taking slag, "Tectip" and spectrographic samples first. Then, when the spoon is hot and covered with a thin uniform coating of slag, a representative portion of liquid steel can be withdrawn from the furnace.

To extract a pin sample from this spoon a special evacuated tube is used. The technique is illustrated schematically in Fig. 5.

The tube is of Pyrex, 4 mm. ID, with a thin-walled bulb blown on one side near the end. A deoxidizer, in the form of aluminum shavings, is contained within the tube. To obtain a solid pin for analysis, the aluminum is shaken to the bottom and fills the bulb and lower end of the tube. The tube is quickly immersed in the spoon of liquid steel, bulb down. The thin wall of the bulb immediately melts and the liquid steel is deoxidized as it rushes into the glass tube. Since the diameter of the tube is small, the steel freezes immediately which prevents segregation or evolution of carbon monox- ide. Once the tube has filled, it is quenched in water. The result is a clean smooth solid pin suitable for oxygen analysis. A photograph of the evacuated sampling tube and the resulting pin is shown in Fig. 6.

The sampling procedure just described has proved to be the most successful in production trials. The

spoon /

tube

shredded aluminum h /

Fig. 5-Spoon-pin technique.

tg coating

!n steel

tubes are relatively inexpensive, and it is immediately obvious if a good or bad sample has been taken. The procedure is repeated until at least three pins suitable for analysis are obtained. Normally good pins are taken 70-80% of the time.

SAMPLE PREPARATION Normally the pins obtained by

either of the two sampling techniques described are clean and free of glass and scale. To remove any possible oxide formed during the cooling of the pins, they are washed in dilute hydrochloric acid, rinsed in water, and wiped dry. Each pin is sectioned by means of a hand shear as shown in Fig. 6. The sheared cross sections of the pins are then examined, and if gross porosity or inclu- sions are observed, the pin is dis- carded. The two portions from a pin are dried in acetone as added assur- ance against moisture. Then they are combined, weighed and analysed. This is repeated on two more pins so a total of three analyses are carried out.

ANALYSIS Several fast analytical techniques

are available. Rapid vacuum fusion equipment,

such as the Balzer's Exhalograph or the Heraeus-Engelhard Evolograph, will give an accurate oxygen analysis along with nitrogen and hydro- gen. However, an oxygen analysis requires about 5-7 min. When time is critical this is too slow.

Neutron-activation analysis is another technique which will provide an accurate analysis for oxygen in a short period of time. The possible safety hazard and complexities in- volved, as well as the relatively high cost, make it hard to justify this technique for a trial basis to check out sampling procedure. However, for routine analysis on a production scale, once standard deoxidation schedules have been developed, it should not be overlooked.

The third technique is inert gas fusion as illustrated by the LECO rapid oxygen analyser. This technique was chosen for the work described in this report because of its speed, simplicity, and relatively low cost. I t has proved to be a sat- isfactory device for obtaining rapid oxygen analysis both as an experi- mental trial tool and for routine production analyses.

ANALYTICAL RESULTS The analyses for oxygen along the

length of the pin indicated a fairly consistent value with the exception of the top one half in. and the lower 1 in. of the sample which gave ex- cessively high results. As shown in Fig. 6, two portions from the pin are combined and thus averaged for each analysis. Because of the differ- ence between the pins, three are analysed and the results are averaged. Obviously high values due to

Deoxidation, Theory, and Practice 5

D

CHECK

+ ANALYSE

Fig. &Pin tube and sample.

slag or glass entrapment are omit- ted. Because of this selection, the results tend to be biased on the low side of the actual oxygen content. This is an added safety factor to prevent overdeoxidation.

This combination of sampling and analysis for a rapid determination of oxygen in liquid steel is not highly accurate. The range of oxygen content encountered in making low- carbon rimming steels is from 300 to 1500 ppm (0.03 to 0.15%) as is shown later. An accuracy of + 10 ppm (& 0.6 to 3%) is not feasible but an accuracy of ? 10% is practically possible.

What is most important is the fact that the result obtained by this procedure is a far better rapid indication of the oxygen content of liquid steel than that obtained by any other method. Moreover, it is a simple technique that can be carried out by production personnel and is well suited for deoxidation control.

TIME REQUIRED The time from the end of refining

in the furnace to the start of tapping into the ladle is approximately 6 min. This is the limit imposed in obtaining a measure of the oxygen level in the bath in order for the results to be used by the melter to cal- culate the necessary addition of deoxidizer to the ladle. Using the techniques described, it is possible in routine production to provide oxygen analysis within this time limit. The time taken may be decreased by simplifying the sample preparation and by obtaining another oxygen analyser. (We presently have two LECO units.) I t is feasible that this technique may be shortened to approximately 4 min.

RESULTS I N PRODUCTION The procedure described for sam-

pling and analysis for oxygen has been in use for approximately one year, and in routine operation since early last summer. The initial results were obtained to determine what ox-

ygen levels were present in the bath at the end of refining and in the ladle prior to pouring. Fig. 7 com- bines three histograms to illustrate the variation in bath oxygen with bath carbon. This is in agreement with the band of practical data shown in Fig. 2. Both indicate a wide range of oxygen for a given carbon content, particularly at low values of carbon, and the average oxygen

40 I - 0.04 - 0.055% C n average = 770 ppm

101- 0.085 - 0.11% c n n average = 585 ppm.

40 Z

;

- . , , , . 200 400 600 800 1000 1200 1400

B A T H O2 (ppm. )

- 0. 08 - 0. 08% C average = 700 ppm.

2 . A

Fig. 7-Histogram bath oxygen distribution.

a

BC021 analysis (. 08% mnx. C) average = 535 ppm

BC031 & BC032 ( , 08/.10%C) average = 470 ppm

T C 21 analysis (. 08% m u . C) average = 520 ppm.

200 400 600 800 1000 1200 1400

LADLE O2 (ppm. )

Fig. &Histogram ladle oxygen distribution.

content increases with decreasing carbon content.

The same sampling and analytical techniques applied to steel in the ladle after deoxidation, but prior to pouring yielded the results shown in Fig. 8. Values for oxygen content for similar carbon analysis illustrate a wide spread similar to that from bath analyses. This further empha- sizes that a deoxidation schedule based on the carbon analysis alone is far from ideal since for these heats no attempt was made to relate the deoxidation practice to the bath oxygen content.

OXYGEN BALANCE The analysis for oxygen in the

ladle appeared to be higher than ex- pected, considering the amount of deoxidizers added to the ladle during tapping. As a check, an oxygen balance was calculated. This may be illustrated in Fig. 9. The total amount of oxygen in the bath should equal the total amount of oxygen in the ladle less the amount of oxygen combining with deoxidizers and forming part of the ladle slag. At best, this calculation is only a rough approximation since it depends on the accuracy of not only the bath and ladle oxygen analyses, but also on the carbon and manganese analyses and the weight of steel in the bath and ladle. The wide discrep- ancy in the oxygen balance was at- tributed to the oxygen pickup on tapping. Irregular opening of the furnace taphole caused fluctuations in tapping time and erratic configu- rations in the tap stream, which possibly affect the oxygen pickup. Fur- nace slag is also carried over into the ladle and this can be a rich and variable source of oxygen. Although im- provements have been made in min- imizing the cause of fluctuation, a variable amount of oxygen is still picked up during tapping and this must not be overlooked in designing a suitable ladle deoxidation schedule.

INITIAL DEOXlDATlON Now that a fast reliable oxygen

analysis was available, how could it be applied effectively to deoxidation? Any change introduced must be gradual, so that the acceptance and confidence of production personnel could be obtained. The initial deoxidation schedule for ladle deoxidation of low-carbon, bottom-poured, capped steel was derived by combining the previous A1 schedule based on carbon and the average amount of pouring Al, and relating this to bath oxygen. This is not a radical change from the previous deoxidation practice, but, since it relates to oxygen, the incidence of high-oxygen or "wild" heats has been re- duced. This timid approach to deoxidation based on oxygen rather than on carbon has shown favorable results.


1- -" ..- - ' O X Y G E N B A L A I

I I

I

4 in bath + 4 pickup on tapping 4 in ladle + O, in slag from deoxidation

Fig. 9-Oxygen "balance."

Just recently, a group of 50 top- ther conclusions would be premature poured, low-carbon, rimmed heats since all the ingots in this trial have were made with the ladle aluminum not been processed and inspected as addition calculated to leave 0.04 % final product. Oa in the steel, assuming only aluminum would deoxidize. On this basis, the amount of aluminum added to the ladle is greater (up to 10 times) than the previous deoxidation schedule based on carbon predicted. Con- siderably less aluminum (in some cases none) was added to the molds. With this bolder approach, the suc- cess obtained in producing satisfac- tory ingots is encouraging but fur-

WORK TO BE DONE The initial trials on deoxidation

based on oxygen analysis has opened the door to many problems. As ex- amples: - How much oxygen is required for

rimming? - Is oxygen the main factor, or

should % C x % 0 be the crite- rion?

- How efficient is Mn as a deoxidizer?

- How efficient is carbon as a deoxidizer?

- How much oxygen is picked up during teeming?

All of these questions, and more besides, must be evaluated before an ideal deoxidation schedule can be derived. But now we have an analytical technique that can assist in this.

SUMMARY Steelmaking is a process of "oxi-

dation" and the oxygen content is continually changing from the refining stage to the solidified ingot. Since oxygen is at least as important as carbon in steelmaking we cannot ex- ercise proper control over the process until we can regulate the level of oxygen.

This rapid technique for sampling and analyzing, as an indication of the oxygen content in liquid steel, has proven to be a practical production tool and an aid to attacking the problems of deoxidation.

Acknowledgment The author wishes to thank F. J.

McMulkin for his encouragement throughout this work; J. R. Atkin- son and production personnel for their cooperation during plant trials, and research personnel, in particular Gary Haughton, for their contribu- tion to the development of the sampling and analytical techniques. Thanks are also due to Dominion Foundries and Steel, Ltd. for the op- portunity to present this paper.

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Electric Furnace 1966 - The Problems of High Oxygen Content in Liquid Steel …library.aimehq.org/library/books/Electric Furnace 1966... · · 2010-02-08in Liquid Steel And Possible