UDC 669.046.545.2:669.046.546.2:661.321.1

Dephosphorization and Desulfurization

Pig Iron by Na2CO3*

of Molten

By Takaharu MORI YA* * and Masanobu F UJII * *

Synopsis

Dephosphorization and desulfurization of hot metal by adding Na2C03 was studied experimentally. The results obtained are as follows:

(1) The dephosphorization ratio was improved with lower tempera-ture of hot metal, lower Si content and greater quantity of Na2C03 added.

(2) The desulfurization ratio was slightly improved with lower temperature of hot metal. No effect on the ratio was observed with varia-tions in the Si content and quantity of Na2C03 added.

(3) Though the dephosphorization was attained in 3- 6 min after addition of Na2C03 under high Si content, rephosphorization was ob-served subsequently.

(4) The gas given off after addition of Na2C03 was CO. The dust mainly consisted of Na2C03.

(5) The main interactions between Na2C03 and molten pig iron are assumed to be as follows:

(a) Volatilization of Na2C03

(Na2C03) = Na2C03 (g)

(b) Reduction by C

(Na2C03)+C = (Na20)+2C0 (g)

(c) Desiliconization

(Na2C03)+Si = (Na20.Si02)+C

(d) Dephosphorization

5(Na2C03)+4P = 5(Na20)+2(P205)+5C

(e) Rephosphorization 5(Na20)+2(P205)+5Si = 5(Na20.Si02)+4P

I. Introduction

In recent years, the P content and S content of molten pig iron have tended to increase due to changes

in the raw materials used in blast furnaces, and it has become difficult to cope with this problem through

a BOF process to produce low phosphorous and low

sulfur steel. For this reason, the need for preliminary dephosphorization and desulfurization of molten pig iron has increased. With respect to desulfurization,

a variety of preliminary desulfurization processes

have already been developed and put into practice. On the other hand, even though various investiga-

tionsl-3~ have been done on dephosphorization, prob-lems on productivity, treatment cost, and yield rate

remain unsolved. The authors have paid attention to the fact that

dephosphorization simultaneously with desulfuriza-tion can be attained by adding a great quantity of Na2C03, which is known as a desulfurization agent,

to the molten pig iron, and have investigated the pos-

sibility of simultaneous desulfurization and dephos-

phorization of molten pig iron using Na2C03. It has been known for some time that dephosphori-

zation, simultaneously with desulfurization, can be

attained by oxidation refining using a Na2C03 or Na20 system slag,4-7) but it can hardly be said that the basic investigations are satisfactory. In the

present report, we investigated factors which affect dephosphorization, and desulfurization of molten pig iron by Na2C03 using a high frequency induction furnace in the laboratory, and also examined the relations between Na2C03 and constituents of molten

pig iron.

II. Investigation into Factors Which Have Effect on Dephosphorization and Desulfurization

With regard to dephosphorization of molten pig iron by a Na2C03 or Na20 system slag, Maddocks and Turkdogan,4,5~ Charter and Charles,s~ and Oelsen7~ have reported on this problem, but in these reports it is assumed that oxidizing agents such as FeO, 02(g), and Na2SO4 are jointly used, and Na2C03 acts as a fixing agent of P205.

Since the authors confirmed through preliminary tests that Na2C03 had enough dephosphorization effect even with a single application all the tests mentioned below were carried out by a single addition of Na2C03. We made investigation into the effects of the tempera-ture of molten iron, Si content of molten iron, and

quantity of Na2C03 added on dephosphorization and desulfurization.

1. Test Procedures

Figure 1 illustrates an outline of the testing ap-

paratus, and Table 1 shows the testing conditions. In the tests, a MgO crucible was used for the high frequency induction furnace, in which 7 kg of pig iron was melted, and a predetermined quantity of Na2C03 was added at one time after a fixed temperature was reached. This condition was maintained for 15 min. The metal and slag samples were taken every 3 min after addition, and the change of concentration of its constituents was investigated.

The composition of molten pig iron was C = 3.5%, Mn=0.70%, P=0.250% and 5=0.030%, and Si was varied within the range of O'..-O.60%. Also the tem-

perature of molten iron was in three levels of 1250°, 1350°, 1450°C, and the quantity of Na2C03 added was varied within the range of l0-.40 g/kg-HM. Since the temperature of molten iron fell fairly simul-taneously with the addition of Na2C03, careful con-sideration was given to keeping the temperature of molten iron constant after addition, by making the temperature before addition higher allowing for its

*

**

Originally published

Kure Works, Nisshin

in .Nisshin Steel Technical Report, (1980), No. 41, Steel Co., Ltd., Showadori, Kure 737.

1, in Japanese. Engl ish version received Octob er 13, 1980.

( 732 ) Research Article

Transactions ISIJ, Vol. 21, 1981 (733)

temperature drop. The temperature of molten iron was measured continuously by a thermocouple at-tached on the bottom of the crucible.

Metal samples were sucked using a silica tube. C was analyzed by the combustion method, and other elements by fluorescent X-rays.

Slag samples were taken by allowing them to stick to a steel bar, and analyzed by chemical analysis method.

2. The Effect of Temperature of Molten Iron

The tests were performed at temperatures of 1250°, 1350°, and 1450°C. The quantity of Na2-C03 added was 40 g/kg-HM and Si content of molten iron before treatment (hereinafter referred to as SiI) was 0% and 0.60% (two cases were tested).

According to observations in testing, Na2C03 melted, producing a strong flare and dust, simul-taneously with its addition, and it took about 2 min until the foaming of slag ceased and reached a steady state. The flare and dust increased with higher temperature of molten iron and smaller SiI content. Moreover, in the case of SiI = 0 % the slag (Na2C03 added) on the surface of molten metal was lost through volatilization in 4 to 6 min after addition of Na2C03, and some sticking of the slag on the side wall of the crucible could be slightly observed. On the other hand, in the case of SiI=0.60%, the slag which re-mained fluid more than 15 min covered the molten metal surface.

Figure 2 shows the variations in P % and S % in the case of SiI = 0 %. The dephosphorization and desulfurization rates at each temperature are very high. The dephosphorization and desulfurization reactions finished in 3 to 6 min after the addition of Na2C03, after which i% and S% became constant. The lower the temperature of the molten metal, the

greater becomes the dephosphorization and desulfuri-zation effects. Especially, dephosphorization is im-

proved greatly between 1 350° and 1250°C. Data on C% and Mn% are omitted because they change little with addition of Na2C03. Table 2 shows composition of slag stuck to the crucible after 15 min. The slag stuck to the crucible contained more than 19% (P205) and more than 1.5 % (5).

In the case of 5i1=0%, slag on the molten metal surface was lost after 4 to 6 min, and this might be attributable to the volatilization loss of Na2C03. Dephosphorization and desulfurization products are condensed into slag which tends to have a higher melting point and solidifies on the crucible walls.

Figure 3 shows variations in P %, S %, and j % at SiI = 0.6 %. The variation in P % is such that de-

phosphorization occurs rapidly for 3 min after addi-tion of Na2C03 in the cases of 1350° and 1450°C, but after that rephosphorization occurs. Although rephosphorization cannot be observed at 1 250°C, the dephosphorization effect is small compared with the case of SiI=0%. The desulfurization behavior is almost similar to that at SiI=0%, and the lower the temperature of molten iron is, the higher becomes the desulfurization effect. Also, Si % is greatly de-creased by adding Na2C03.

Figure 4 illustrates variations in slag composition.

(P205)% becomes a maximum in 3 to 6 min at

Fig. 1. Outline of testing apparatus.

Table 1. Testing conditions.

Fig. 2. Relationship between temperature of molten iron and dephosphorization/desulfurization behavior.

(SCI=O%)

(734) Transactions ISIJ, Vol. 21, 1981

1350° and 1450°C, and after that it decreases again, but at 1250°C the decrease in (P2O5)% is not ob-served beyond 3 min. This corresponds to the variations in P % shown in Fig. 3. The rate of decrease in (Na2CO3)% becomes greater with in-creasing temperature of molten iron, and at 1350° and 1450° C it drops to 10% or below after 15 min.

(SiO2)% and (Na2O)% increase rapidly up to 5 to 6 min, after that (SiO2) becomes nearly constant at about 40%, and (Na2O) at about 50%. Since the

(T.Fe), (MnO), and (S) components of the slag were nearly constant at 1 % or under those data are omitted. Figure 5 illustrates the relationship between the dephosphorization ratio or desulfurization ratio, and the temperature of molten iron at 15 min after addi-

tion of Na2CO3. Both the dephosphorization ratio and desulfurization ratio increased with lower tem-perature of molten iron. Sij% has little effect on the desulfurization ratio, but the dephosphorization ratio increases with lower Sij content. At 1250°C and Sii = 0%, higher dephosphorization ratio and desul-furization ratio, up to 96% and 86%, respectively, were obtained. Thus, the temperature of molten iron and Sij % have a significant effect on the de-

phosphorization ratio, and it is considered that the dephosphorization ratio decreases due to consump-tion of Na2CO3 by a desiliconization reaction and the volatilization of Na2CO3.

3. Effect of Quantity of Na2CO3 Added Figure 6 shows the behavior of P % and S % when

various quantities of Na2CO3 are added. The quan-tities added were 10, 20, 30 and 40 g/kg-HM with the temperature of molten iron at 1250°C and at Sij=0.6%. The dephosphorization effect decreases

Table 2. Composition of slag stuck to crucible (after 15 min).

Fig. 3. Relationship between temperature of molten iron and dephosphorization/desulfurization behavior.

(Sij; 0.60%)

Fig. 4. Variations in 40 g/kg-HM).

slag composition (Na2C03

Transactions ISI1, Vol. 21, 1981 (735)

with decreasing quantity of Na2C03 added, and in the range of 30 g/kg-HM or less, rephosphorization occurs. No influence of added quantity on desul-furization effect was observed when the quantity of Na2C03 added was 10 g/kg-HM or over.

4. Effect of Sii%

Figure 7 illustrates variations in i% and S% with respect to SiI=O, 0.10, 0.30, 0.60% at 1250°C and 20 g/kg-HM in quantity of Na2C03 added. The dephosphorization effect decreases with increasing Sii %, and at SiI = 0.30 % or over rephosphorization occurs. No effect of SiI% on the desulfurization was observed. Based on the above results, the relationships be-tween dephosphorization ratio/desulfurization ratio and SiI%/quantity of Na2C03 added at 1250°C tem-

perature of molten iron at 15 min of treatment time are summarized in Fig. 8. The desulfurization ratio becomes nearly constant at 70 to 80% when Nat-C03 is greater than 10 g/kg-HM, and is not affected by SiI% or the quantity of Na2C03 added. The dephosphorization ratio was increased with larger

quantity of Na2C03 added and lower SiI content, and in the case of SiI = 0%, a dephosphorization of 40% was obtained with 10 g/kg-HM of Na2C03 added.

5. Relationship between Rephosphorization Behavior and Si Content

According to the above result, a small effect of temperature of molten iron on desulfurization was observed, but no effect on the ratio was observed

Fig. 5. Relationship between temperature of molten iron and dephosphorization ratio/desulfurization ratio.

(after 15 min)

Fig. 6. Relationship between quantity of Na2C03 added and dephosphorization/desulfurization behavior.

(Sli=; 0.60%)

Fig. 7. Relationship between SiI content and dephosphori- zation/desulfurization behavior. (1250°C)

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( 736 ) Transactions ISIJ, Vol. 21, 1981

with variations in the SiI content and the quantity of Na2CO3 added. On the other hand, the tempera-ture of molten iron, SiI content, and quantity of Na2CO3 added have significant effects on dephos-

phorization. Especially, in the case of higher SiI content, rephosphorization occurs after 3 min after addition of Na2CO3, with decreasing greatly the ratio. As is evident from Figs. 3, 6 and 7, for a charge at which rephosphorization occurs, Si of 0.10% or more remains in metal obtained immediately after initial dephosphorization (3 to 6 min after addition of Na2CO3). The desiliconization reaction proceeds together with the rephosphorization reaction. As slag composition varies (Fig. 4), (P205) % increases during the initial dephosphorization period, then decreases again with decreasing (Na2CO3) %.

To study the rephosphorized charge after 15 min, the relationships between (P205)%/P% and (Na2-CO3)%/Si% were obtained (Fig. 9). These are linear relations, in which (P205)/P% increases as

(Na2CO3)%/Si% increases. Moreover, in this rela-tion (P205)%/P% increases with lower temperature. From these results, it is clear that the rephosphoriza-tion reaction has a strong relation with the desili-conization reaction. It is assumed that a reduction of the dephosphorization products by j% leads to rephosphorization, in the case that added Na2CO3 is consumed through volatilization loss to decrease

(Na2CO3)% in the slag, resulting in a shortage of oxidizing agent needed for the desiliconization reac-tion. As described later, the dephosphorization prod-uct may be considered to be 5 (Na20)+2 (P205); consequently it is assumed that rephosphorization reaction occurs in accordance with Eq. (1).

5(Na20)+2(P205)+5Si = 5(Na2O.Si02)+4P ...(1)

III. Gas and Dust When Adding Na2CO3

When adding Na2CO3 to molten iron, flare and dust are produced vigorously. Dust or flare causes much trouble in practical operation, but the factors which affect their formation have not been satisfac-

Fig. 8. Relationship between SiI content/quantity of Na2CO3 added and dephosphorization ratio/desul-

furization ratio. (1250°C)Fig. 9. Relationship between (P205)

and (Na2CO3)%/Si%.%/P%

Fig. 10. Outline of recovery test

device for gas and dust.

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Transactions ISIT, Vol. 21, 1981 (737)

torily clarified. We investigated into the factors which have an effect on the quantities of gas and dust

produced when adding Na2CO3 by gas recovery testing, and also examined the reactions between Na2CO3 and various constituents in molten iron.

1. Test Procedures

Figure 10 show the outline of the testing apparatus. The induction furnace and crucible were the same ones as used on the tests in the previous chapter, and the apparatus was constructed such that the furnace cover was installed on the top of the furnace body, so that the inside of the furnace was completely en-closed. The gas produced was captured in a recovery tank made of plastic through a conduit tube. Dust was captured by installing a stainless wire net in the conduit tube. The gas recovery tank was so con-structed as to recover the gas through water, having a maximum capacity of 701. The pressure of the recovered gas was +20 mmH2O.

The furnace cover was connected to the recovery unit after replacing the furnace atmosphere with Ar. Na2CO3 was added to molten iron, which was kept at a predetermined temperature for 15 min. The quantity of melt was 10 kg.

2. Factors Which Have an Effect on Gas and Dust

The relationships between quantity of gas and SiI % at 1350°C, with 40 g/kg-HM of Na2CO3 added, are shown in Fig. 11, and the data after 15 min treat-ment are given in Table 3. Production of gas occurs simultaneously with the addition of Na2CO3, and stops in 7 to 8 min. The gas is in greater quantity with lower SiI content. The gas given off, as shown in Table 3, contains more than 80% of CO, and 1 to 3% of CO2 and 02. Also, the dust is recovered in greater quantity with lower SiI content, and it consists of more than 70 %Na2CO3, 2 to 3%P205, and 1 to 5 %SiO2. It is assumed that the dust formed may be due mainly to the volatilization reaction of Na2CO3. Table 4 shows the dephosphorization ratio, desul-furization ratio, and desiliconization ratio after 15 min treatment, as compared with the results of an atmo-spheric treatment test. The desulfurization ratio is

nearly in same level as with the atmospheric treat-ment, but the dephosphorization ratio and desiliconi-zation ratio are improved with the gas recovery treat-ment. The reason may be that the quantity of Na2CO3 scattered is decreased, and the utilization rate of Na2CO3 is increased.

Table 5 shows the relationship between the quantity of gas given off and the temperature of molten iron at SiI = 0 % and 20 g/kg-HM for quantity of Na2CO3 added. The quantities of gas and dust produced increase rapidly with rising temperature of molten iron.

3. Reactions between Na2CO3 and Constituents in Molten Iron

From the afore-mentioned results, it is evident that the dust formed when adding Na2CO3 was mainly Na2CO3 which had been condensed after volatiliza-tion loss, and that the gas given off was CO.

In order to examine the details of the formation reactions of CO gas, and dephosphorization/desiliconi-zation, we investigated the behavior of gas and dust

given off for three constituent systems, i.e., the Fe-C system (C=, 3.5%), Fe-P system (P=12%), and Fe-Si system (Si~.19%), under the conditions of 1 350°C in temperature and 40 g/kg-HM in quantity of

Fig. 11. Relationship between quantity of gas given off

SiI content.

and

Table 3. Results of gas recovery test (after 15 min).

( 738 ) Transactions ISI1, Vol. 21, 1981

Na2CO3 added. Table 6 shows a list of experimental results. In

the case of the Fe-C system, the quantity of gas given off was 55 l and the quantity of dust produced was 275 g; both were significant. No slag was present on the surface of the molten metal after treatment. For the Fe-P system, the quantity of gas given off was 1.3 1, and the quantity of dust produced was 17 g; both gas and dust were small. For the Fe-Si system, the quantity of gas given off was 161, and the quantity of dust produced was 87 g; both were intermediate between the other systems. From these results, the main reactions which are responsible for gas produc-tion are assumed to be those between Na2CO3 and C in molten metal, in accordance with the following equation.

(Na2CO3)+C(Na2O)+2C0 (g) ............(2)

In the case of the Fe-C system, as against 400 g of quantity of Na2CO3 added, 275 g was discharged as dust; consequently, 125g of Na2CO3 took part in

gas production, and according to Eq. (2), 551 of gas

given off corresponded to 130 g of Na2CO3. More-over, the decrease in C is 0.15 %, and in practice is 0.33%. It is estimated from calculation that 76 g of (Na2O) may be formed, but no slag is present on the surface of the molten metal after treatment. It may be considered that it has stuck to the furnace wall or been absorbed into the crucible.

The Fe-P system has very little gas production, and it is considered that dephosphorization may be a reaction which does not accompany gas production. The slag after treatment is solid; it contains 37% (P2O5). A slight increase in C% is observed simul-taneously with dephosphorization, and it is assumed that C may be formed by the dephosphorization re-action. The ratio of the mol number of (Na2O) to

(P2O5) in 100 g of slag after treatment is about 2.5, and it is assumed that dephosphorization proceeds by a reaction as expressed in Eq. (3) :

NNa2O _ 0.635 _ 2.42 NP2O5 0.262

5(Na2CO3)+4P = 5(Na2O)+2(P2O5)+5C ...(3)

Photograph 1 shows the results of X-ray diffraction of slag in the molten Fe-P system after being treated

(hereinafter referred to as Fe-P system slag), and Fig. 12 shows Turkdogan and Maddocks8~ phase diagram of the Na2O-P205 system (converting Fe-P system slag into 2-element system of Na2O-P205 ; and their results are also shown in Fig. 12). According to the phase diagram, Fe-P system slag is anticipated to form 3Na2O . P2O5+2Na2O • P2O5 at room tempera-ture, but in X-ray diffraction, 3Na2O. P2O5 could be confirmed, but 2Na2O . P2O5 could not be detected. Supposing the dephosphorization to be as given by Eq. (3), in the case of Fe-P system tests, C% in-creases by 0.17% against dephosphorization of 0.35%, and the actual C% increase is 0.10%.

The Fe-Si system produced 161 of gas and 87 g of dust. As is seen from these values, the desiliconiza-tion reaction is accompanied with some CO produc-tion, but the amount is small. According to analysis of metal after treatment, a slight increase of C % was observed simultaneously with desiliconization, and the desiliconization reaction may be considered to occur in parallel with the formation reaction of C and CO. The ratio of the mol number of (Na2O) to (SiO2) in 100 g of slag after treatment, is about

Table 4. Comparison of

furization ratio

dephosphorization

and desiliconization

ratio,

ratio.

desul-

Table 5. Relationship between quantity of

duced and temperature.gas/dust pro-

Table 6. Quantity of gas/dust produced of Fe-C system, Fe-P system

Transactions ISIJ, Vol. 21, 1981 (739)

1.0, and it is assumed that the desiliconization reac-tion is mainly based on Eq. (4).

NNa20 _ 0.674 = 0.940 NSiO2 0.717

(Na2CO3) + Si = (Na20 . Si02) + C .......... 4)

Figure 13 illustrates Turkdogan et al.'s8 phase dia-

gram of Na20-Si02 and Photo. 2 shows the results of X-ray diffraction.

By X-ray diffraction, Na20 . Si02 could be clearly detected. On the other hand, the desiliconization reaction accompanying CO production may be con-sidered to proceed according to Eq. (5).

2(Na2CO3)+Si = (2Na20.Si02)+2C0 (g) ...(5)

As discussed in the previous section, it is considered that the quantity of gas is decreased by increasing Sii content in normal molten pig iron, and judging from the aforementioned slag composition, the de-siliconization reaction proceeds mainly in accordance with Eq. (4). Then a weak reaction as expressed in Eq. (5) also occurs concurrently in the case that the initial addition of Na2CO3 is high and the Si content is high. According to the experimental data, the

quantity of dust produced tends to increase with greater quantity of gas, and the reason may be that the volatilization reaction of Na2CO3 is accelerated by strong agitation of the slag layer which is accom-

panied by gas production. From the above results, the main reactions caused

by addition of Na2CO3 may be considered to be as follows :

(1) Volatilization of Na2CO3

(Na2CO3) = Na2CO3 (g) ...............(6)

(2) Reduction by C

(Na2CO3)+C = (Na20)+2C0 (g) .........(7)

Photo. 1. Results o f X-ray diffraction of Fe-P system slag.

Fig. 12. Phase diagram of Na20-P205

position of Fe-P system slag.

system and corn-

(740) Transactions ISIJ, Vol. 21, 1981

(3) Desiliconization

(Na2C03) + Si = (Na20 . Si02) + C ......... (8)

(4) Dephosphorization

5(Na2C03)4P = 5(Na20)+2(P205)+5C ......(9)

(5) Rephosphorization

5(Na20)+2(P205)+5Si = 5(Na20.Si02)+4P ...(10)

4. Utilization Rate of Na2C03

Using the above-mentioned reaction types, the Na2C03 balance of the gas recovery test was calcu-lated. The ratio in terms of consumed Na2C03 is shown in Fig. 14. The utilization rate to the de-phosphorization reaction is about 20%, i.e., nearly constant, although slightly low at SiI =0.60 %. The rate to the desiliconization reaction increases with increasing SiI content, and reaches 52 % at Sit= 0.60%. Reaction with C, and dust, decrease greatly with increasing SiI content, and residual Na2C03 in slag is about l0%; i.e., nearly constant regardless of SiI content These results are obtained from data of

gas recovery tests, and since the amount of volatiliza-tion loss of Na2C03 increases in atmospheric treat-

ment, they cannot be considered identical to the

recovery test, but they are assumed to have a similar utilization rate in the dephosphorization period of

initial addition of Na2C03.

Iv. Conclusions

Dephosphorization and desulfurization of molten

pig iron by Na2C03 were examined in the laboratory using a high frequency induction furnace, and the following results were obtained.

(1) In the case that the quantity of Na2C03 added is more than 10 g/kg-HM, the desulfurization ratio was slightly increased at lower temperature of molten iron. No effect on the ratio was observed with variations in the SiI content and quantity of Na2C03 added. The ratio is nearly constant within the range of 70 to 80%.

(2 ) The dephosphorization ratio was increased with lower temperature of molten iron, lower SiI content and greater quantity of Na2C03 added.

(3) Though the dephosphorization was attained in 3 to 6 min after the addition of Na2C03 with high

Fig. 13. Phase diagram of Na20-Si02

position of Fe-Si system slag.

system and corn-

Fig. 14. Utilization rate of Na2C03.

Photo. 2. Results of X-ray diffraction of Fe-Si system slag.

Transactions ISIJ, Vol. 21, 1981 (741)

Sii content, rephosphorization was subsequently ob-served.

(4) The gas after addition of Na2C03 was CO. The dust mainly consisted of Na2C03.

(5) The quantity of gas and dust produced in-creases with increasing temperature of molten iron, and decreases with increasing Sii content.

(6) The main reactions between Na2C03 and molten pig iron are assumed to be as follows :

(a) Volatilization of Na2C03 (Na2C03) = Na2C03 (g)

(b) Reduction by C (Na2C03)+C = (Na20)+2C0

(c) Desiliconization (Na2C03)+Si = (Na20•Si02)+C

(d) Dephosphorization 5(Na2C03)+4P = 5(Na20)+2(P205)+5C

1)

2)

3)

4)

5)

6) 7) 8)

(e) Rephosphorization

5(Na20)+2(P2O5)+5J = 5(Na20•Si02)+4P

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and H. Tanaka : T etsu-to-Hagane, 62 (1976), A17. Y. Ito, T. Okajima, Y. Kochi and A. Koshida: Tetsu-to-

Hagane, 62 (1976), S78. W. R. Maddocks and E. T. Turkdogan : JISI, 162 (1949),

249. W. R. Maddocks and E. T. Turkdogan: JISI, 171 (1952),

128. W. J.B. Charter and J. A. Charles : JISI, 191 (1959), 319.

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1.