Thermodynamic Interactions of Nb and Mo on Ti in Liquid Iron

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1 Materials Transactions, Vol. 49, o. 4 (28) pp. 854 to 859 #28 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Thermodynamic Interactions of and Mo on in Liquid Iron Tae-In Chung* 1, Joong-Beom Lee* 2, Jin-Goo Kang, Jong-Oh Jo, Bo-Ho Kim and Jong-Jin Pak* 3 Department of Metallurgical and Materials Engineering, Hanyang University, Ansan , Korea Thermodynamic interactions of niobium and molybdenum on titanium in liquid iron were studied in the temperature range of K by measuring the effects of niobium and molybdenum on the solubility product for formation in liquid iron using the metalnitride-gas equilibration technique. The experimental results were thermodynamically analyzed using Wagner s interaction parameter formalism to determine the first-order interaction parameters of niobium and molybdenum on titanium in liquid iron as :495 :49 and :1 :26, respectively, at 1873 K. Temperature dependence of these parameters was also determined as follows. [doi:1.232/matertrans.mer27286] e ¼ 116=T :57; e Mo ¼ :1 ð KÞ (Received ovember 12, 27; Accepted January 25, 28; Published March 19, 28) Keywords: liquid iron, titanium, nitrogen, titanium nitride, niobium, molybdenum, interaction parameter 1. Introduction tanium, niobium and molybdenum are important alloying elements in ferritic stainless steels to improve corrosion resistance and mechanical properties. They are often added together to meet product specifications. tanium is a strong nitride former in liquid steel. Recently, the formation of during cooling or solidification of liquid stainless steels is of great interest due to its catalytic effect on the formation of equi-axed cast structure. 1 6) Another important issue with these steel grades is how to control the formation of and titanium oxides inclusions which can cause a nozzle clogging during continuous casting and various defects in final products. 7,8) In spite of its importance, thermodynamic relations among titanium, niobium and molybdenum in liquid stainless steels are very limited. In author s recent studies, 9 13) thermodynamics of Fe-i- - (i ¼ Cr, i, Si and Al) melts were studied using the metal-nitride-gas equilibration technique. In the present study, thermodynamic interactions of niobium and molybdenum on titanium in liquid iron were studied by measuring the effect of niobium and molybdenum on the solubility product of in liquid iron utilizing a high frequency induction furnace in the temperature range of K. Using Wagner s formalism, 14) the interaction parameters of niobium and molybdenum on titanium were determined from the experimental results. 2. Experimental Procedure A 15 kw/3 khz high frequency induction furnace was used in the present study as shown in Fig. 1. Descriptions of experimental apparatus and procedure are available elsewhere. 12,13) Five hundred grams of high purity electrolytic * 1 Graduate Student, Hanyang University, Present address: Technical Research Laboratories, POSCO, Pohang 79-69, Korea * 2 Graduate Student, Hanyang University, Present address: Steelmaking Research Team, Hyundai Steel Co., Ltd., Dang Jin , Korea * 3 Corresponding author, jjpark@hanyang.ac.kr Fig. 1 A schematic diagram of experimental system. iron (99.95 mass% purity, 2 mass ppm O, <5 mass ppm, 18 mass ppm C, <1 mass ppm Si) contained in an Al 2 O 3 crucible (OD: 56 mm, ID: 5 mm, H: 96 mm) was melted in the temperature range of K. After melting the iron, the temperature of the melt was monitored by a Pt/Pt-13 mass% Rh thermocouple sheathed with an 8 mm OD alumina tube immersed in the melt. Any possible influence of high frequency noise on the temperature reading was avoided by grounding the circuit of the thermocouple. Preliminary trials confirmed that no significant noise was detected. The temperature fluctuation of iron melt could be controlled within 2 K during experiment by the PID controller of the induction furnace. The temperature reading of the PID controller was calibrated by the sourcing DC voltage calibrator for the thermal EMF of R-type thermocouple. After the temperature of iron melt reached a desired value, an Ar-1%H 2 gas was blown onto the melt surface at a high flow rate of 5 L/min for 2 h to deoxidize the melt. The oxygen content in the iron melt after this procedure was in the

2 Thermodynamic Interactions of and Mo on in Liquid Iron Melt Intensity KeV Fig. 2 SEM-EDS analysis result on precipitates formed in the melt. range of 152 mass ppm. Then the gas was switched to a mixture of Ar-1%H 2 and 2 gases (P 2 ¼ 1132:5 Pa). The flow rate of each gas was controlled by a mass flow controller at a total flow rate of 2 L/min. Strong agitation of melt by an induction furnace resulted in a fast attainment of equilibrium nitrogen solubility in liquid iron within 1 h. After confirming the saturation of nitrogen in liquid iron by sampling and analysis, pellets of sponge titanium (99.5 mass% purity) were dropped into liquid iron through an 18 mm ID quartz tube. After the predetermined equilibration time of 1 h, a metal sample of about 1 g was extracted by a 4 mm ID quartz tube connected to a syringe (1 ml) and it was quenched rapidly in water within 2 s. tanium additions and sampling were repeated until a stable layer was formed on the surface of the iron melt. The formation of in iron melt could be also confirmed by a sharp decrease in nitrogen content checked by the analysis of metal samples during experiment. After the saturation of in liquid iron, pellets of niobium (99.8% purity) or molybdenum (99.8% purity) additions and samplings were carried out with the predetermined equilibration time of 2 h. The amount of alloy additions was up to 2.5 mass% for niobium an 3. mass% for molybdenum in liquid iron. In order to insure the saturation of in liquid iron during alloy additions, about 1 g of pellets were added onto the melt. pellets were sintered from the stoichiometric powder (99% purity, <1 mm, Aldrich Chemical Co.) in a pure nitrogen atmosphere at 1923 K for 12 h. The metal samples extracted by a quartz tube during experiment were carefully cut for the chemical analysis. Four specimen of each metal sample were prepared for the analysis of nitrogen and oxygen by the inert gas fusioninfrared absorptiometry. For the analysis of titanium, niobium and molybdenum, the metal sample (.2 g) was dissolved in 2 ml of HCl (1+1) in a glass beaker of 5 ml capacity heated in a water bath for 2 h. The leaching test of powder (99% purity, <1 mm, Aldrich Chemical Co.) indicated that was nearly insoluble in dilute HCl(1+1) solution heated in a water bath up to 6 h. In author s previous studies, 9 13) the detailed procedure for chemical analysis is available. The analytical limits for titanium, niobium and molybdenum in metal sample were 5 1 mass ppm, 1 2 mass ppm and 5 1 mass ppm, respectively. 3. Result and Discussion 3.1 Inclusion identification After each experiment, the melt remained in an Al 2 O 3 crucible was quenched by blowing helium gas onto the melt surface. The quenched metal was cross-sectioned and examined with an optical microscope for the presence of any inclusions including. The center part of metal sample was virtually clean without any noticeable inclusions. However, a layer of inclusions was observed on the melt surface by SEM-EDS analysis (FE-SEM, Hitachi S48) as shown in Fig. 2. The layer was virtually a pure titanium nitride phase free from other elements such as niobium and molybdenum. There was no other nitride inclusions observed in the melt under the present experimental condition. In order to check the stoichiometry of titanium nitride phase formed in the melt, the inclusions formed in the melt were identified and characterized by X-ray diffraction analysis (XRD, High power X-ray Diffractometer System, Rigaku D/MAX-25/PC). About 1 g of metal sample was dissolved in dilute HCl(1+1) solution heated in a water bath for 72 h. After filtration, the residue was analyzed by the XRD and compared with the XRD pattern of the stoichiometric powder (99% purity, <1 mm, Aldrich Chemical Co.) as shown in Fig. 3. The residue was identified as a mixture of, O 2 and Al 2 O 3 phases as shown in Fig. 3(a). By comparing 2 values of the diffraction peaks of the stoichiometric powder shown in Fig. 3(b), titanium nitride in the residue can be considered as the stoichiometric. Therefore, considering the results in Figs. 2 and 3, it can be concluded that pure solid is the equilibrium phase under the present experimental condition. 3.2 Effect of on the solubility product of in liquid iron Figure 4 shows the variation of equilibrium nitrogen solubility with titanium additions in Fe-- melt under a nitrogen partial pressure of Pa at 1873 K. A reduced nitrogen pressure was used to avoid an excessive formation of on titanium additions. As shown as open symbols in the figure, the nitrogen solubility increases linearly as the titanium content increases in liquid iron. When the titanium content exceeds a critical value, the nitrogen solubility sharply decreases due to the formation of in the melt

3 856 T.-I. Chung et al. (a) (b) O 2 Al 2 O 3 Table 1 The experimental results of, and equilibration in liquid iron under P 2 ¼ 1132:5 Pa at K. Temp.(K) [mass% ] [mass% ] [mass%] Saturation Fig. 3 [%] θ XRD patterns for (a) inclusions and (b) stoichiometric powder [%] P 2 =1132.5Pa unsaturated saturated Kim et al. 12) Fig. 4 Equilibrium relation between [%] and [%] in Fe-- melt under P 2 ¼ 1132:5 Pa at 1873 K. as shown as solid symbols in the figure. The solid line in Fig. 4 is the equilibrium solubility product of titanium and nitrogen for formation calculated using the thermodynamic data determined in author s previous study on formation in Fe-- melt. 12) The predicted line for the solubility product of at 1873 K is in excellent agreement with experimental data. In the present study, the effect of niobium on the solubility product of in liquid iron was measured as a function of temperature. The experimental results are summarized in Table 1. As the niobium content increases in the melt saturated with, the nitrogen solubility increases significantly while the titanium content decreases. Figure 5 shows the effect of niobium additions on the solubility product of titanium and nitrogen, log½%š½%š for saturation at 1873, 1923 and 1973 K. The solubility product increases with melt temperature, and it also increases with niobium additions. The reaction equilibrium for the dissolution of pure solid in liquid iron can be written as ðsþ ¼ þ ð1þ G o 1 ¼ :8T J/mol12Þ ð2þ K 1 ¼ h h ¼ f f ½%Š½%Š ð3þ a where K 1 is the equilibrium constant for Reaction (1) and, h and h are the Henrian activities of titanium and nitrogen relative to 1 mass% standard state in liquid iron, and f and f are the activity coefficients of titanium and nitrogen, respectively. Under the present experimental condition, the activity of is unity. The equilibrium constant, K 1 can be rewritten as the following relation for Fe--- alloy using Wagner s formalism: 14,15)

4 Thermodynamic Interactions of and Mo on in Liquid Iron P 2 = Pa.5.25 Fe--- melt P 2 =1132.5Pa Log[%][%] logf -.25 e = [%] Pa Fig. 5 Effect of additions on the solubility product of in Fe--- melt. logf e = Table 2 Thermodynamic parameters used in the present study. Parameter at 1873 K Temp. Dependency References log K 1 2: =T þ 5:63 12 e :36 857:2=T þ 4: e 1: =T þ 14: Pa e K1973 K 12 e :68 28=T þ :816 2 e Mo : K 2 logf e = log K 1 ¼ log f þ log f þ log½%š½%š ¼ e ½%Šþe ½%Šþe ½%Š þ e ½%Šþe ½%Šþlog½%Š½%Š ð4þ where the first-order interaction parameter values of e, e and e were reported in author s previous study12) as shown in Table 2. As shown in the table, the interaction between titanium and nitrogen in liquid iron is very strong. Therefore, a reduced nitrogen pressure of Pa was used to minimize the interaction between titanium and nitrogen in Fe-- melts containing niobium or molybdenum. In the present study, the oxygen content in the metal samples was analyzed to be less than 5 mass ppm, and its effect on titanium and nitrogen was assumed to be negligible. The e values were reported by many investigators by measuring the effect of niobium on nitrogen solubility in liquid iron. 16 2) The e values measured at niobium content in liquid iron below 1 mass% show little discrepancy in the range of :67 : ) The recommended value of the Japan Society for the Promotion of Science (JSPS) for e is :68 at 1873 K and 28=T þ :816 in the temperature range of K at niobium content below 9.7 mass% in liquid iron. 21,22) In the present study, the JSPS recommended value for e was used. Therefore, the first-order interaction parameter of niobium on titanium in liquid iron, e, can be determined from the present experimental results of solubility product of Fig with niobium additions and available thermodynamic parameters shown in Table 2. 12,21,22) Then eq. (4) can be rearranged as log f.5 1. Relations between log f ¼ e ½%Š [%] ¼ log K 1 e ½%Š e ½%Š e ½%Š e ½%Š log½%š½%š ð5þ where f is the interaction coefficient of niobium on titanium in liquid iron. Figure 6 shows the values of log f plotted vs. percent niobium in Fe--- melts containing up to 2.22 mass% in the temperature range of K using the relation expressed by eq. (5). At all temperatures, the data show excellent linear relationships. The first-order interaction parameter, e can be determined by a linear regression analysis of data in Fig. 6 as :495 :49, :334 :65 and :183 :76 at 1873, 1923 and 1973 K, respectively. The temperature dependence of e value can then be expressed as 116=T :57 as shown in Fig and [%] in Fe--- melts. 3.

5 858 T.-I. Chung et al C 165 C 16 C Table 3 The experimental results of, Mo and equilibration in saturated liquid iron under P 2 ¼ 1132:5 Pa at K. e Fig Effect of Mo on the solubility product of in liquid iron The experimental results for the effect of molybdenum on the solubility product of in liquid iron are summarized in Table 3. As the molybdenum content increases in the melt, the nitrogen solubility increases slightly while the titanium content decreases. Figure 8 shows the solubility product of as a function of molybdenum content in Fe-Mo-- melt at different temperatures. The solubility product did not change significantly with molybdenum additions at all temperatures. Using the similar thermodynamic relations described in the proceeding section, the first-order interaction parameter of molybdenum on titanium in liquid iron, e Mo, can be determined from the experimental results of solubility product of with molybdenum additions and available thermodynamic parameters shown in Table 2. 12,2) log f Mo 5.1 ¼ e Mo ½%MoŠ T -1 /1-4 K e =116/ T ¼ log K 1 e ½%Š e ½%Š e ½%Š e Mo ½%MoŠ log½%š½%š ð6þ where f Mo is the interaction coefficient of molybdenum on titanium in liquid iron. The e Mo values reported are in good agreement in the range of :9 :13 at 1873 K at molybdenum content up to 1 mass% ) Also, no significant temperature dependence of e Mo value was observed by Ishii et al.26) In the present study, the JSPS recommended value 21,22) of :11 at 1873 K was used assuming that the temperature dependency was negligible. Figure 9 shows the values of log f Mo plotted vs. percent molybdenum in Fe-Mo-- melts containing up to mass%mo using the relation expressed by eq. (6). The relations are temperature independent as shown in the figure. The first-order interaction parameter e Mo can be determined by a linear regression analysis of data in Fig. 9 as :1 :26 in the temperature range of K. Yoshikawa et al. 27) reported the e Mo value of :16 :19 at 1873 K by measuring the effect of molybdenum on the critical titanium concentration in liquid iron saturated with both 3 O 5 and 2 O 3 phases. The e Mo value determined 5.2 The temperature dependence of e 5.3 in liquid iron. 5.4 Temp.(K) [mass% ] [mass% ] [mass%mo] Log[%][%] [%Mo] P 2 = Pa 3 Fig. 8 Effect of Mo additions on the solubility product of in Fe-Mo- - melt. 4

6 Thermodynamic Interactions of and Mo on in Liquid Iron Fe---Mo Melt P 2 = Pa Technology of Materials funded by the Ministry of Commerce, Industry and Energy, Republic of Korea. REFERECES log f Mo Fig. 9 from a different experimental technique agrees fairly well with that by this work. 4. Conclusions Relations between log f Mo The effect of niobium and molybdenum on the solubility product for in liquid iron was determined using the metal-nitride-gas equilibration technique in the temperature range of K. As the niobium content increases in the melt, the nitrogen solubility increases significantly while the titanium content decreases. As the molybdenum content increases in the melt, the nitrogen solubility increases slightly while the titanium content decreases. The experimental results were thermodynamically analyzed using Wagner s interaction parameter formalism to determine the first-order interaction parameters of niobium and molybdenum on titanium in liquid iron as :495 :49 and :1 :26, respectively, at 1873 K. Temperature dependence of these parameters was also determined as follows. e ¼ 116=T :57; e Mo ¼ :1 ð KÞ Acknowledgments 1 e Mo =.1 2 [%Mo] 26 and [%Mo] in Fe-Mo-- melts. The present work was financially supported by POSCO (Grant o.: 25Z24). This study was also supported by a grant from the Fundamental R&D Program for Core 3 4 1) K. akajima, H. Hasegawa, S. Khumkoa and M. Hayashi: ISIJ Int. 46 (26) ) K. akajima, H. Hasegawa, S. Khumkoa and S. Mizoguchi: Metall. Mater. Trans. B 34B (23) ) H. Fujimura, S. Tsuge, Y. Komizo and T. ishizawa: Tetsu-to-Hagane 87 (21) ) J. C. Villafuerte, H. W. Kerr and S. A. David: Mater. Sci. & Eng. A 194 (1995) ) T. Koseki and H. Inoue: J. Japan Inst. Metals 65 (21) ) H. Ohta and H. Suito: ISIJ Int. 47 (27) ) Y. Gao and K. Sorimachi: ISIJ Int. 33 (1993) ) D. S. Kim, B. D. You, Y. K. Shin, Y. Lee and B. H. Youn: J. of the Korean Inst. of Met. & Mater. 32 (1994) ) J. J. Pak, Y. S. Jeong, I. K. Hong, W. Y. Cha, D. S. Kim and Y. Y. Lee: ISIJ Int. 45 (25) ) J. J. Pak, Y. S. Jeong, S. J. Tae, D. S. Kim and Y. Y. Lee: Metall. Mater. Trans. B 36B (25) ) J. J. Pak, J. T. Yoo, Y. S. Jeong, S. J. Tae, S. M. Seo, D. S. Kim and Y. D. Lee: ISIJ Int. 45 (25) ) W. Y. Kim, J. O. Jo, T. I. Chung, D. S. Kim and J. J. Pak: ISIJ Int. 47 (27) ) W. Y. Kim, J. G. Kang, C. H. Park, J. B. Lee and J. J. Pak: ISIJ Int. 47 (27) ) C. Wagner: Thermodynamics of Alloys, (Addison-Wesley Press, Cambridge, Mass., 1952) p ) C. Wagner: Thermodynamics of Alloys, (Addison-Wesley Press, Cambridge, Mass., 1952) p ) D. B. Evans and R. D. Pehlke: Trans. Met. Soc. AIME 233 (1965) ) Z. Morita, K. Hachisuka, Y. Iwanage and A. Adachi: J. Japan Inst. Metals 35 (1971) ) R. D. Pehlke and J. F. Elliott: Trans. Met. Soc. AIME 218 (196) ) P. H. Turnock and R. D. Pehlke: Trans. Met. Soc. AIME 236 (1966) ) Z. Morita, T. Tanaka and T. Yanai: The 19th Comm. (Reaction), Japan Soc. Promotion Sci. (JSPS), Rep, o , (May, 1982). 21) Steelmaking Data Sourcebook, The Japan Society for Promotion of Science, The 19th Committee in Steelmaking, ew York, (Gordon and Breach Science Publishers, 1988) p ) Steelmaking Data Sourcebook, The Japan Society for Promotion of Science, The 19th Committee in Steelmaking, ew York, (Gordon and Breach Science Publishers, 1988) p ) V. Kashyap and. Parlee: Trans. Met. Soc. AIME 218 (196) ) Maekawa and akagawa: Testsu-to-Hangane 46 (196) ) H. Wada and R. D. Pehlke: Metall. Trans. B 8B (1977) ) F. Ishii and T. Fuwa: Testsu-to-Hangane 68 (1982) ) T. Yoshikawa and K. Morita: Proc. Sohn Intern. Sym. On Advanced Processing of Metals and Materials, Vol. 2-Thermo and Physicochemical Principles, (TMS, USA, 26),