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production of low phosphor stainless steel
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Production of Low Phosphorous Stainless Steel by the
Reducing Dephosphorization Process*
By Kaxuo KITAMURA,** Mitsunori FUNAZAKI,*** Yoshiyuki IWANAMI***
and Tomoo TAKENOUCHI**
Synopsis In order to produce stainless steel with low phosphorous content below 0.01 %, experiments were carried out to find optimum conditions for the dephosphorization by metallic Ca and CaC2. The results are summarized as follows :
(1) Dephosphorization by metallic Ca should be done at temperatures below the boiling point of Ca and the composition of the steel has to be determined not to precipitate CaC2.
(2) On the other hand, dephosphorization by CaC2 is effective at high temperatures, where the composition of the steel has to be selected to keep the activity of carbon in the range 0.020.3 (at 1 600 °C).
(3) Based upon the experiments, a new process of mixing two melts, one of which is Cr free steel dephosphorized by conventional oxidizing meth-od and the other is Cr containing steel dephosphorized by Ca or CaC2, has been developed to attain P content less than 0.01 %.
I. Introduction
Though low phosphorous content of stainless steel below 0.01 % is known to be effective to stress cor-rosion cracking,l~ weld hot cracking2~ and so on, it is difficult to decrease the content by conventional oxidizing refining because of preferential oxidation of chromium. Therefore, special dephosphorization methods have been studied and these can be classified into following two methods. One is soft oxidizing dephosphorization by the slag containing Li2CO3,3~ Na2C034> or Ba05~ as principal elements which are more basic than Ca0 and have low melting point, and the other is reducing dephosphorization by Ca-CaF26~ and CaC2-CaF2.7~ Each of them, however, has demerits. The former needs a high flux ratio and can only achieve high degree of dephosphorization in high chromium steels below 20 % Cr. On the other hand, the latter requires high cost and leads to low productivity because of the necessity to use of ESR in the case of Ca-CaF2, and shows low efficiency of the reaction due to the errosion of refractories in the case of CaC2-CaF2.
In the course of the experimental study to settle these demerits, it was made clear that reducing de-phosphorization could be effectively achieved by add-ing metallic Ca at lower temperatures below the boil-ing point and sole CaC2 at higher temperatures to the molten high chromium steel. However, it was found to be difficult to reduce phosphorous content indus-trially below 0.01 % by these methods in case of 18 % Cr steel. Then a new process was developed to achieve low phosphorous level, where two melts were mixed. One was chromium free melt dephosphorized by conventional oxidizing method and the other was
high chromium melt dephosphorized by above men-tioned reducing method. The results of the experi-mental and industrial tests will be described in the following.
II. Experimentals
The experimental apparatus is shown in Fig. 1. 250 g of mother alloy was melted in a fused Mg0 crucible (30 mmrn X 100 mm') placed in a high fre-quency induction furnace of 10 kVA with a graphite sleeve (37 mmID X 50 mm01 X 150 mmrI). After holding the molten steel at selected temperatures, either Ca or CaC2 was added. Using silica tubes, about 5 g of sample were taken at the position of 5 N 10 mm above the bottom of the crucible at desired times. The atmosphere in the furnace was controlled by blowing argon gas at 1 000 cumin. As the slag got hardened by the addition of CaC2, it was stirred by tungsten rod before sampling.
The phosphorous content of samples was analyzed by the molybdenum blue absorptiometric method. The purity of Ca is 99 %, and the composition of CaC2 is shown in Table 1. The chemical composi-tion of mother alloys is shown in Table 2, where Ca was added to alloys A' -.G and CaC2 to alloys H' -.Q.
Table
Fig. 1. Experimental apparatus.
1. Composition of calcium carbide. (wt%)
Presented to the 99th ISIJ Meeting, April 1980, 5227 and the 105th ISIJ Meeting, April 1983, 5257, both Tokyo in Tokyo. Manuscript received September 20, 1983. © 1984 ISIJ Research Laboratory, Muroran Plant, The Japan Steel Works, Ltd., Chatsumachi, Muroran 051. Melting Shop, Muroran Plant, The Japan Steel Works, Ltd.
at The University of
Research Article ( 631)
( 632 ) Transactions ISIJ, Vol. 24, 1984
III. Experimental Results
1. Dephosphorization by Ca
As Ca becomes gaseous at conventional steelmaking temperatures because of its low boiling point of 1 492 °C
, dephosphorization is expected to be influenced significantly by the temperature. Therefore, 1 % of Ca was added to alloy A kept at 1 36O'' 1 550 °C. The change of P content with time is shown in Fig. 2. The duration time of the reaction was longer at lower temperatures and lower P content was obtained. A slight rephosphorization was observed when the melt was kept over the boiling point of Ca. Other impurities such as S, Sn, Sb, As also decrease with time, and the degree of removal of each impurity, ~J={[%i]o- [%i]t}/[%i]o X 100] at 20 min after Ca addition is shown in Fig. 3. Here, [%i]0 and [%]t are the contents of impurity i at t = 0 and t = t. It is seen from the figure that the degree of impurity re-moval changes suddenly at the boiling point of Ca and the change of P and Sn are especially remarkable.
The effect of the amount of Ca addition on the
degree of removal of P and the other impurities at 20 min after Ca addition is shown in Fig. 4. It is seen from the figure that ~2 increases with increasing the amount of Ca addition and that more than 65 % of each impurity can be removed by the addition of Ca over 1.0 %.
As mentioned above lower temperatures are fa-vorable for the dephosphorization by Ca, so that car-bon content should be raised to drop the melting point of the steel. In that case, however, suppression of dephosphorization might be expected due to the for-mation of CaC2. Therefore, experiments were also made at 1 420 and 1 480 °C using alloys B'-G. Each value of ?P and ac at 20 min after Ca addition is shown in Table 3. Where, ac was calculated from Eq. (1).
ac = f0[%C}
log f c = ecc) sl [%CJ +e G ~) 91 {%Si] +eGC'I iol {%Cr] = 0.19[%C]-}-0.106[%Si]-0.036[%Cr]
...........................(1)
Fig. 2. Effect of bath temperature on the
content with time.
change of P Fig . 3. Effect of temperature on the
Sn and Sb.
removal of P, S,
Table 2. Chemical composition of mother alloys. %)
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Transactions ISIJ, Vol. 24, 1984 (633)
It can be seen from the table that remarkable increase
of aC results in the suppression of dephosphorization.
2. Dephosphorization by CaC2
It is known that CaC2 decomposes through contact
with melt at steelmaking temperature of 1400 N 1 700 °C and supplies Ca in spite of its high melting point
of 2 300 °C. Since produced Ca vaporizes remark-
ably at higher temperature over the boiling point, the control of decomposition rate of CaC2 is important to increase the efficiency of the reaction.
It has been reported' that CaC2 has dephosphori-zation potential because of the formation of Ca3P2 by
the reaction between Ca dissolved in CaF2 and P. The amount of CaF2 added to furnace or ladle, how-
ever, should be limited to prevent the errosion of linings. Therefore, the effect of CaF2 was examined
as follows. The effect of CaF2 addition to CaC2 on the phosphorous change with time is shown in Fig. 5. It can be seen from this figure that though higher
dephosphorization can be obtained at the beginning
of the reaction by the addition of CaF2, it decreases as time goes because of rephosphorization, and that the lowest P content is obtained at the final stage of
reaction in the case of without CaF2 addition. During the experiments, following phenomena such as rapid
dissolution of carbon, generation of Ca flame and er-rosion of crucible were observed with increasing CaF2.
From these facts it was considered that the efficiency of dephosphorization decreased with increasing CaF2
due to the vaporization loss of Ca at early stage and
the reaction with MgO which composed crucible. Ac-cordingly, the sole addition of CaC2 was considered to be suitable for ladle treatment and the following ex-
periments were carried out using sole CaC2. The change of P and C content with time is shown
in Fig. 6 forr the experiments using alloy H and 8 % of CaC2 at 1 500'- 1 600 °C. It is seen from the fig-ure that high dephosphorization rates, corresponding to rapid decomposition of CaC2 were obtained at higher temperatures. The effect of the amount of
Fig . 4. Effect of Ca-amount added in melt on the removal
of P, S, As, Sn and Sb.
Table 3. rp obtained by Ca dephos phorization.
Fig. 5. Effect of flux composition on the
content with time.
change of P
Fig . 6. Effect of bath temperature on
C content with time.
the change of P and
Fig. 7. Effect of CaC2-amount added in melt on the degree
of dephosphorization.
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(634 ) Transactions ISIJ, Vol. 24, 1984
CaC2 added on the degree of dephosphorization after 20 min is shown in Fig. 7. In this way the degree of dephosphorization saturates over 4 % of CaC2 addi-tion. The change of P content with time is shown in Fig. 8 for the experiments using 45%Cr-0.54%C and 4 % of CaG2 at 1 600 °C. When experiment was carried out with low initial carbon content, dephos-phorization proceeded rapidly at the beginning due to the rapid decomposition of CaG2. After that, how-ever, rephosphorization took place and the final degree of dephosphorization was decreased. Accordingly, the optimum C content is determined to be 1 2 % C for 45 % Cr steel.
The change of P content with time is shown in Fig. 9 for the experiments using O'45%Cr-1 %C steel and 4 % of CaC2 at 1 600 °C. It was found that the higher degree of dephosphorization was obtained with the higher Cr content. Figure 10 shows the relation between 4P(=[%P]o-[%P]t) and 4C(=[%C]t-
[%C]0) at the early stage of the reaction in the case of Fig. 9. As seen from the figure, 4P is directly pro-
portional to 4C and the relation is expressed by Eq. (2). 4P = 0.0254C ........................(2)
On the other hand, dephosphorization reaction is considered to be expressed by Eq. (11) as will be mentioned later, so that Eq. (3) should be given stoi-chiometorically.
4P = 0.864C ........................(3)
The comparison between Eq. (2) and Eq. (3) reveals that only 2.9 % of Ca produced by the decomposition of CaC2 is used for the dephosphorization.
Iv. Discussion
1. Partial Pressure of Oxygen at Reaction Interface
Recently, the effect of partial pressure of oxygen on the form of phosphorus in the Ca0-A1203 slag was studied.11~ From the results it was made clear that, when Pot was lower than about 10-18 atm, phos-
phide (P3-) was predominent in the melt, and that reducing dephosphorization took place. Therefore,
partial pressure of oxygen in this experimentals was calculated. When metallic Ca is added to the melt kept at lower temperatures below the boiling point, the par-tial pressure of oxygen at the interface can be calcu-lated by Eq. (4), where liquid Ca is supposed to be existing on the melt surface for a limited time.
2Ca0 (s) = 2Ca (1)+02 (g)
4G4) = 307100-51.28 T12)
0
t K G(4) _ -log a°-aP-°2 = o log Pa g = c4~ - 4.576T ^ ` acao 2 ...........................(4)
Calculating Pot in the temperature range of 1 350 1 492 °C from Eq. (4), 10-31~ 10-27 atm are obtained under the condition that CaC2 does not precipitate.
On the other hand, when CaC2 is added to the melt kept at higher temperatures over the boiling point of Ca, Ca formed by the decomposition is considered to exist as gas phase at the interface. Therefore, the
partial pressure of oxygen is calculated from Eq. (5).
2Ca0 (s) = 2Ca (g)+02 (g)
4G~5) = 380 200-93.24T12)
0
P log K(5) = --4.576T = log
2 log Pca+log Pot
2 pu ca' - 02
2 acao
l
1
.(5)
Fig . 8. Effect of initial carbon
P content with time.
content on the change of
Fig. 9. Effect of chromium
content with time.
content on the change of P
Fig. 10. Relation between
results in Fig. 9.
4P and 4C calculated from the
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Transactions ISIJ, Vol. 24, 1984 (635)
The partial pressure of Ca in Eq. (5) is calculated from Eq. (6).
C CaC2 (s) = Ca(g)+2
4G~6) - 61410-32.3 T
4G° P a ......(6) • 2 log K(6) _ - 4.576T _ log a2 c CaC2
logPCa+2 log ac
Where, Eqs. (7) and (8) are used to calculate 4G~6)
CaC2 (s) = Ca (g)+2C (s)
4G)=51210-12.3T'2 C(s)=C J
............(8) 4G)=5 C8100-10.OT13)
From Eqs. (5) and (6), the partial pressure of oxygen is expressed by the following equation.
log P, - log K(5) -2 log K(6) +4 log ac ...... 56 270 = - T --+6.28+4 log ac I
Therefore, when CaC2 is added to the melt, Pot is calculated by Eq. (9) as functions of temperature and ac, and the relation is shown in Fig. 11. It is seen from the figure that Pot decreases remarkably with decreasing temperature and ac. As will be described later, suitable activity of initial carbon is 0.02 to 0.3 in the case of CaC2 addition and then the Pot is cal-culated to be lower than 10-25 atm.
Therefore, partial pressure of oxygen at the inter-face between molten steel and Ca or CaC2 is far less than 10-18 atm, and from this facts reducing dephos-
phorization is considered to proceed.
2. Equilibrium of Dephosphorization
Dephosphorization by metallic Ca at lower tem-
peratures or by CaC2 at higher temperatures are con-sidered to be expressed by Eq. (10) or Eq. (11) as total reaction.
3Ca (1)+2P = Ca3P2 (s) ..................(10)
3CaC2+2P = Ca3P2 (s)+6C ............(11)
Where, standard free energy change of each reaction expressed by Eqs. (16) and (17) can be derived from Eqs. (12)N(15).
3Ca (s)+P2 (g) = Ca3P2
4G2)=-142800+22.5T'4 1/2P2(g)=P J ..
4G«3) _ -29 200-4.6T15) Ca(s)=Ca(l)
4G«4) = 2 070-1.85 T12) Ca (l)+2C = CaC2 (s) 15
4G~15) _ -23 900+13.2T'2"3 ) J ...... 4G0)= -90610+37.25T...............(16)
4G1)= -18910-2.42T ...............(17)
In the case of CaC2 dephosphorization, the equilib-rium P content, [P] e, can be calculated from Eq. (18), which was derived from Eqs. (11) and (17), assuming that the activity of Ca3P2 is unity because CaC2 exists as solid in the slag phase.
°
toP - 1 _ G~11' - logf ~+3 to g[ ]e_ 2 4.576T - gfc
+3 log [%C] ..............................(18)
Where, fp is calculated by Eq. (19), though consensus has not been obtained for ePCr) values reported up to this time.
log f p = log f PC)+ log fl °+ °+ log f r) = 0.24[%C]+0.118[%Si]+ep[%Cr]
........................(19)
For example, -0.037 is proposed in 11 N30%Cr-P--Fe alloy by Schenck et al.,ls~ -0.025 by Frohberg et a1.17) and -0.030 by Hadrys et a1.'° ) Therefore, the value ePCr> was calculated from Eqs. (18) and (19) by using the data which were thought to have suf-ficiently arrived at the equilibrium in Figs. 6 and 8. The obtained values for ePCr) are shown in Table 4 and are in the range of -0.040 -0.044, which is close to Schenck's value. This indicated that Cr re-markably decreases the activity of P in the melt.
3. Optimum Conditions for Dephosphorization Though Cr decreases the activity of P as men-
tioned above, high Cr content is found to be fovorable for dephosphorization. That is considered to be due to the effect of Cr which reduces the activity of C.
The effect of ac on the dephosphorization by metal-lic Ca at lower temperatures is shown in Fig. 12. The value of each plot is already listed in Table 3. The critical activity of carbon, ac, for the precipita-tion of CaC2 is calculated at 1 420 and 1 480 °C by assuming aca(1) =1 and acac2=1 in Eq. (15). Cal-culated values of ac are 0.79 and 0.89, respectively, and
Fig. 11. Relation between the partial pressure of oxygen, p;2 and aC in the case of CaC2 addition.
Table 4. Calculated ePCr) from and 8.
the data of Figs. 6
(636) Transactions ISIJ, Vol. 24, 1984
are shown in the figure. It is seen from the figure that the degree of dephosphorization decreases in the region of CaC2 precipitation, so the composition of the steel should be adjusted to hold ac<ac. On the other hand, the melting point of the steel should be below 1 400 °C, taking the temperature drop during practical operation into consideration. Accordingly, the region of effective dephosphorization by Ca is illustrated as a shadowed portion in Fe-C-Cr phase diagram'8) of Fig. 13.
For dephosphorization by CaC2 at higher tempera-tures, aC before treatment should be adjusted low be-cause it is increased by the pick up of carbon during the treatment. The relation between the degree of dephosphorization which is obtained from the reached values in Figs. 8 and 9 and ac before CaC2 addition is shown in Fig. 14. It is seen from the figure that high degrees of dephosphorization over 50 % can be obtained in the region of a0=0.02'-O.3. In the re-
gion of a0<0.02, however, the degree of dephosphori-zation decreases by the vaporization loss of Ca due to the rapid decomposition of CaC2 and it also decreases in the region of aC>0.3 due to the suppressed decom-
position of CaC2.
V. Industrial Scale Tests
From the experimental tests it was found that de-
phosphorization of high Cr steels can be achieved by reducing dephosphorization method. However, it is
difficult to get low phosphorous content less than
0.01 % only by the reducing dephosphorizationn under unfavorable condition of industrial scale. A new pro-cess, therefore, was developed taking advantage of ef-ficient dephosphorization in high Cr steel. The pro-cess is illustrated in Fig. 15. It is seen from the figure that high Cr steel melted in EAF(A) is decarburized to the desired carbon content and tapped into the ladle and then dephosphorized by adding Ca or CaC2 under the reducing condition with stirring. After de-
phosphorization, the high Cr steel melt is also de-carburized to the desired carbon content by mainly VOD. On the other hand, low alloy steel melt with-out Cr is dephosphorized by conventional oxidizing method in EAF(B). After that both melts are mixed in the LRF (Ladle Refining Furnace), where final composition and temperature are adjusted.
In the industrial scale tests, high Cr steels were melted in 25 t EAF, and Ca or CaC2 were added to the bath surface. The bath temperature just before the addition was 1 480 °C for Ca and 1 575 1 680 °C for CaC
2, respectively. The results of the tests are summarized in Table 5. As shown in the table, high degree of dephosphorization, 42'.'66 %, could be achieved in every run. Furthermore, heats Nos. 4, 6 and 7 using CaC2 were proceeded to the final stage of the process shown in Fig. 15, and about 90 t of 18-8 stainless steel was produced in each run. In this case, Cr free steels with high Ni content were melted in 100 t EAF, and the results are shown in Fig. 16. Though a little pick up of P occurred due to the sup-
plemental addition of ferro-Cr during the process, low P content less than 0.01 % could be achieved.
Fig. 12. Effect
by Ca.
of ac on the degree of dephosphorization
Fig. 13. Region of e ffective dephosphorization by Ca.
Fig. 14. Effect of aC on the degree of de
by CaC2.phosphorization
Fig. 15. A new process for the
phorous stainless steel.
production of low phos-
Research Article
Transactions ISIJ, Vol. 24, 1984 (637)
Though the production of low P stainless steel has been made possible, by this process, but some prob-lems still remain. For instance, the yield of CaC2 for heat Nos. 4~ 7 were calculated and listed in Table 6. It is found from the table that the amount of CaC2 used for dephosphorization is only about 1.5 % and, even if the amount used for desulphurization is counted, only 2.3-3.5 % of CaC2 is effectively used. Furthermore, the relation between dC and dP for heat Nos. 4~ 7 is shown in Fig. 17. As shown in the figure, the dephosphorization ratio (4Pf4C) is ap-
proximately one-half of the experimental value. Then, equilibrium P content of these heats was calculated by Eq. (18) using epCr~ = '-' 0.040 N -0.044. The calcu-lated values are in the range 0.001 -O.003 % P, which is one order of magnitude smaller than the achieved values. So that very small portion of the total cal-cium used effectively and that is considered to be due to the following reasons:
(a) The slag off before treatment was not suffi-cient.
(b) Calcium reacted with oxygen or nitrogen when added in the air, and others. Accordingly, methods to control the atmosphere of conditions of injection has to be studied, which is especially effec-tive to (b).
VI. Conclusions
In order to decrease the P content of high Cr steel, reducing dephosphorization using metallic Ca or CaC2 were studied in laboratory experiments. Based on the experiments, a new process was developed to achieve low P content below 0.01 % P. The results are summarized as follows :
(1) The optimum conditions for dephosphoriza-
tion by Ca are, as shown in Fig. 13, low temperature below its boiling point and ac below ac, above which CaC2 begins to form.
(2) The sole addition of CaC2 is suitable for de-phosphorization at higher temperatures to avoid the vaporization loss of Ca and the errosion of crucible due to CaF2 addition. High degrees of dephosphori-zation over 50 % is obtained by controlling the com-
position of melts before CaC2 treatment in the region of aC=0.02N0.3 at 1 600 °C.
(3) In industrial scale tests, two melts were mixed; one was chromium free melt dephosphorized below 0.002 % P by conventional oxidizing method and the other was high chromium melt dephosphorized below 0.017 % by reducing method using CaC2. As results,
production of low phosphorus 18-8 stainless steel be-low 0.01 % P (90 t/ l heat) has been achieved.
Acknowledgements
Sincere gratitude and appreciation are due to Dr.
Table 5. Results of in dustrial scale tests.
Fig. 16. Change in P content during industrial scale tests.
Table 6. Yield of CaC2 for heat Nos. 4 to 7. %)
Fig. 17. Relation between JP and 4C during
ment of heat Nos. 47.
.
CaC2 treat-
(638) Transactions ISIJ, Vol. 24, 1984
Saburo Kawaguchi, Plant Manager and Managing Director of Muroran Plant, The Japan Steel Works, Ltd., who permitted the authors to publish the results. The authors wish to express their thanks to Dr. Kore-aki Suzuki, General Manager of Research Laboratory, Muroran Plant, for his advice and direction. Sincere appreciations are also due to Mr. Masaru Inui for his assistance to carrying out the experiments.
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