Effect of Carbon on Nitrogen Solubility and AlN Formation in High Al Alloyed Liquid Steels

Transcription

1 ISIJ Internatonal, Vol. 54 (2014), o. 7, pp Effect of arbon on trogen Solublty and Formaton n Hgh loyed Lqud Steels Jung-Mock JAG, Seok-Hyo SEO, Young-Dae KIM, Hyo-Jong A and Jong-Jn PAK* Department of Materals Engneerng, Hanyang Unversty, Ansan, Korea. (Receved on February 8, 2014; accepted on Aprl 10, 2014) The ntrogen solublty and the solublty product n lqud Fe alloys contanng a wde composton range of carbon and alumnum were measured n the temperature range of K. umnum and carbon both decreased the ntrogen solublty n lqud ron. The smultaneous effect of alumnum and carbon on ntrogen solublty n lqud ron was determned. The solublty product n lqud ron decreased sgnfcantly wth carbon addton prmarly due to the large effect of carbon on ntrogen solublty. Usng the Wagner s formalsm, the expermental results were thermodynamcally analyzed to determne the nteracton parameters among carbon, alumnum and ntrogen n lqud ron as follows: e, r ( ) = ± [ ]: mass%, [] mass%, K = ± , r = 0 [ ] 3. 9 mass%, []: mass%, K ( ) KEY WORDS: TRIP steels; alumnum; carbon; ntrogen solublty; nteracton parameter. 1. Introducton * orrespondng author: E-mal: jjpak@hanyang.ac.kr DOI: Recently, a seres of new advanced hgh strength steels (AHSS) have been developed for the future automotve ndustry. Among these steel grades, transformaton nduced plastcty (TRIP) and twnng nduced plastcty (TWIP) steels offer an outstandng combnaton of strength and ductlty wth very hgh alumnum (1.5 2 mass%) and medum carbon composton range. 1 7) However, these alumnumrch steel grades placed great challenges to steelmakng and castng processes because can be easly formed n lqud steel at such hgh alumnum content. 8) s consdered as a detrmental phase for the hot ductlty of the steels. 9) The thermodynamcs of alumnum, ntrogen and formaton n lqud Fe alloy s avalable n the authors recent studes, 10,11) and the effect of carbon on the ntrogen solublty n lqud Fe alloy s extensvely studed by varous authors ) However, the effect of carbon on formaton and the smultaneous effect of carbon and alumnum on ntrogen solublty n lqud Fe alloy are not known yet. In the present study, the effect of carbon on ntrogen solublty and solublty product n lqud Fe alloys were determned over a wde range of carbon content n the temperature range from to K. The expermental results were thermodynamcally analyzed to determne the frst- and the second-order nteracton parameters among carbon, alumnum and ntrogen usng avalable thermodynamc data of Fe and Fe systems. 11,12) 2. Expermental The ntrogen solublty and the solublty product n lqud Fe alloys were determned usng the metalgas and metal-ntrde-gas equlbraton experments. Detaled descrptons of the expermental apparatus and procedure are avalable n the authors recent studes ) Fve hundred grams of hgh purty electrolytc ron or Fe alloys contaned n an 2 O 3 crucble (outer dameter (OD): 56 mm, nner dameter (ID): 50 mm, heght (H): 96 mm) was melted n the temperature range from to K by a 15 kw/30 khz hgh frequency nducton furnace. After meltng the ron, the melt temperature was drectly measured by a Pt/Pt-13 mass%rh thermocouple sheathed wth an 8 mm OD alumna tube mmersed n the melt. The temperature fluctuaton of ron melt could be controlled wthn 2 K durng experment by the PID controller of the nducton furnace. After the melt temperature was reached at a desred value, the melt was deoxdzed by blowng an Ar-10 %H 2 gas onto the melt surface at a hgh flow rate of ~5 000 ml/mn for 2 h. The oxygen content n the melt decreased to a value less than 20 mass ppm. Then, the gas was swtched to a mxture of Ar-10 %H 2 and 2 gases to have a desred ntrogen partal pressure. The flow rate of the gas mxture was controlled by a mass flow controller n the range from to ml/mn dependng on ntrogen partal pressures n the gas. Strong agtaton of the melt by an nducton furnace resulted n a fast attanment of equlbrum ntrogen solublty n lqud 2014 ISIJ 1578

2 ISIJ Internatonal, Vol. 54 (2014), o. 7 ron under a ntrogen partal pressure wthn 1 h. In case of the ntrogen solublty measurement n Fe and Fe melts, alumnum (99.9% purty) was added through an 18 mm OD quartz tube after confrmng the equlbrum ntrogen solublty n pure lqud ron or Fe alloy melts under a ntrogen partal pressure. In one set of ntrogen solublty measurement experments at K, alumnum and carbon addtons (99.99% purty graphte) were made alternately. After each alumnum and carbon addton n lqud ron, the new equlbrum ntrogen solublty was attaned wthn 30 mnutes. Ths was confrmed by samplng and n stu ntrogen analyss wth tme after the addtons. In order to determne the effect of carbon on the solublty product n lqud ron, alumnum was added n pure lqud ron or Fe alloy melts untl an layer was formed on the surface of the melts under a gven ntrogen partal pressure, and then carbon was added up to 3.9 mass%. The formaton of n the melts could be also confrmed by a sharp decrease n ntrogen content checked by the analyss of metal samples durng the experment. After each carbon addton, a new solublty equlbrum was attaned wthn 1 h. The metal sample of about 10 g was extracted by a 4 mm ID quartz tube connected to a syrnge (10 ml), and t was quenched rapdly n water wthn 2 s. The metal samples were carefully cut for the chemcal analyss. The ntrogen and oxygen contents n the metal sample were analyzed by the ntrogen/oxygen analyzer (LEO T-600 apparatus; LEO orporaton, St. Joseph, MI) wth an accuracy of ±2 mass ppm, and the carbon content was analyzed by the carbon/sulfur analyzer (S-800, Eltra, euss, Germany) wthn an error range of 1%. umnum n the metal sample was analyzed by the nductvely coupled plasma atomc emsson spectroscopy (IP-AES, SPETRO AROS apparatus, manufactured by Spectro Analytcal Instruments, Kleve, Germany) usng approprate standard solutons contanng the same amount of Fe (2 000 mass ppm) as the sample solutons. The analytcal lmt for alumnum n the metal sample was 5±1 mass ppm. After each solublty experment, the melt remaned n the crucble was quenched by blowng helum gas onto the melt surface. In order to check the presence of nclusons ncludng n the quenched melts, about 10 g of metal sampled near the upper surface of the melt was dssolved n dlute Hl (1+1) soluton heated n a water bath for 72 h. After the complete dssoluton of metallc porton, the resdue was fltrated and analyzed by the X-ray dffracton analyss (XRD, Hgh power X-ray Dffractometer System, Rgaku D/ MAX-2500/P). 3. Results and Dscusson 3.1. Effect of arbon and umnum on trogen Solublty n Lqud Fe Melt The expermental results of ntrogen solublty and solublty measurements for Fe and Fe melts are summarzed n Table 1. Fgure 1 shows the varatons of equlbrum ntrogen solublty n Fe and Fe melts wth alumnum addtons under varous ntrogen partal pressures of atm n the temperature Table 1. Temp. ( K) Expermental results of ntrogen solublty and solublty measurements for Fe and Fe melts. P 2 (atm) [% ] [% ] Observed [% ] alculated [%] saturaton sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat sat ISIJ

3 ISIJ Internatonal, Vol. 54 (2014), o. 7 Table 1. ontnued. Temp. ( K) P 2 (atm) [% ] [% ] Observed [% ] alculated [% ] saturaton sat sat sat sat sat sat sat sat sat sat sat sat sat. range from to K. The ntrogen solublty decreases lnearly as the alumnum content ncreases n lqud ron when the melt s not saturated wth as shown as open symbols n Fg. 1. When s formed at hgher contents, the ntrogen solublty decreased along the solublty product lnes for the reacton equlbrum of (s) = + 11) as shown as sold symbols n the fgure. In an experment at K under a ntrogen partal pressure of 0.19 atm, alumnum and carbon addtons were made alternately. As shown n Fg. 1(b), alumnum was frst added up to 0.58 mass% n Fe-0.39 % melt, then carbon was ncreased to 2.03 mass% and alumnum was added agan untl an layer was formed on the surface of the melt. The dssoluton of ntrogen n lqud ron alloys can be expressed as 1 2( g) = 2... (1) o ΔG = T J/g atom 21) 1 K f [% ] = 12 /... (2) P where K s the equlbrum constant for Reacton (1), [% ] s the equlbrum ntrogen content n mass% and, f s the Henran actvty coeffcent of ntrogen, for whch the reference state s the nfntely dlute soluton,.e., f 1 when [% ] 0. P 2 s the ntrogen partal pressure n atm over the melt surface. 2 Fg. 1. Equlbrum relatons between [%] and [%] n Fe and Fe melts: (a) K, (b) K and (c) 1773 K. In Fe alloy melts, the actvty coeffcent of ntrogen, f can be determned from the followng equaton. e 2 log f = e [% ] + r [% ] + 2, e [% ] + r [% ] + r [% ][% ] r... (3) where and are the frst- and the second-order nteracton parameters of elements on ntrogen n lqud ron. In the authors recent studes, the thermodynamcs of ntrogen and formaton n Fe melt, 11) and the nteracton parameter of carbon on ntrogen n Fe melt 12) were determned. Thermodynamc parameters used n the present study are summarzed n Table 2. As mentoned earler, the oxygen content was less than 20 mass ppm n the melt. Therefore, the effect of oxygen on ntrogen was assumed to 2014 ISIJ 1580

4 ISIJ Internatonal, Vol. 54 (2014), o. 7 Table 2. Thermodynamc parameters used n the present study. e r e r log K K Temp. range (K) omp. range Ref [% ] [% ] [% ] [% ] [% ] [% ] [% ] [% ] ( /T+7.03) Fg. 3. XRD patterns for (a) nclusons and (b) stochometrc powder., Fg. 2. Relaton of Eq. (3) to determne the value of r n Fe melts. ΔG = T J/mol 11)... (5) be neglgble. Fgure 2 shows the relaton to determne the value of, r n Eq. (3) usng the ntrogen solublty data for Fe melts not saturated wth n Table 1. The value of, r can be determned as 0.038± from the slope of a straght lne obtaned by the regresson analyss of the data, n Fg. 2. The temperature dependence of r value was neglgble n the temperature range from to K Effect of arbon on Solublty Product n Lqud Iron The expermental results of solublty measurement wth carbon addtons n Fe melts are also summarzed n Table 1. The ntrde formed n the melt durng the metal-ntrde-gas equlbraton experments were dentfed as a pure sold. The XRD data on the fltrated resdue from the dssolved metal samples was compared wth the XRD pattern of the stochometrc powder (99% purty, <10 μm, drch hemcal o.) as shown n Fg. 3. By comparng 2θ values of the dffracton peaks, the alumnum ntrde formed n Fe melt n the present study can be consdered as a pure sold stochometrc. The reacton equlbrum for the dssoluton of pure sold n lqud ron can be wrtten as ()= s +... (4) K hh = = f f [% ][% ]... (6) a where K s the equlbrum constant for Reacton (4), and h and h are the Henran actvtes of alumnum and ntrogen relatve to the 1 mass% standard state n lqud ron, and f and f are the actvty coeffcents of alumnum and ntrogen, respectvely. The actvty of n Eq. (6) s unty n the present study. Fgure 4 shows an example for the effect of carbon addtons on alumnum and ntrogen contents for saturaton n lqud ron under a ntrogen partal pressure of 0.8 atm at K. As the carbon content ncreased up to 2.44 mass%, both alumnum and ntrogen contents decreased from 2.45 to 2.35 mass% and from to mass%, respectvely. The effect of carbon on ntrogen solublty s more sgnfcant than the effect of alumnum. Fgure 5 shows the effect of carbon addtons on the solublty product of, log [% ][% ], n the melts contanng alumnum n the range of mass% at K. The solublty product n Fe melts decreases wth carbon content prmarly due to the large effect of carbon on the ntrogen solublty. In order to determne the thermodynamc relaton between carbon and alumnum from the solublty product data n Fe melts, the equlbrum constant, K can be wrtten as the followng relaton usng the nter ISIJ

5 ISIJ Internatonal, Vol. 54 (2014), o. 7 Fg. 6. Relaton of log f vs [% ] n Fe melts. Fg. 4. Effect of addton on [% ] and [% ] n Fe melt for saturaton at K. log f = e [% ] + r [% ] 2 = log K e [% ] r [% ] e 2 [% ] 2 2 r [% ] e [% ] r [% ] e [% ] r 2, [% ] r [% ][% ] log[% ][% ]... (8) Fg. 5. acton parameters: Effect of addton on solublty product n Fe melt at dfferent melt temperatures. log K = log f + log f + log[% ][% ] 2 = e [% ] + r [% ] + e [% ] + r [% ] e e[% ] + r [% ] + e [% ] + r [% ], + r [% ][% ] + e [% ] + r [% ] + log[% ][% ] e r r... (7) where,, and are the frst- and the second-order nteracton parameters of elements on ntrogen and alumnum n Fe melts, respectvely, the value of r, was determned as n the precedng secton. The values of logk and the nteracton parameters used n Eq. (7) are summarzed n Table 2. As mentoned earler, the oxygen content was very low n the melt and the effect of oxygen on alumnum and ntrogen was assumed to be neglgble. Therefore, the specfc effect of carbon on alumnum can be determned from the solublty product data as a functon of carbon content n Fe melts. Eq. (7) can be rearranged as f where s the nteracton coeffcent of carbon on alumnum n lqud ron. Fgure 6 shows the values of log f plotted vs carbon content n Fe melts usng the relaton expressed by Eq. (8) and expermental data obtaned at varous ntrogen partal pressures and temperatures. The correlaton shows an excellent lnear relatonshp wth carbon content up to e r 3.9 mass%. Therefore, the values of and can be determned as 0.03± and 0, respectvely, by a lnear regresson analyss of the data. The temperature dependence of e and r values was neglgble n the temperature range from to K. The values of nteracton parameters determned n the present study are summarzed n Table 3. The e value determned n the present study usng the metal-ntrde-gas equlbraton technque n Fe melts s about three tmes lower than the e value of at K reported by Wlder and Ellott. 22) They estmated the ε value as 5.3 at K n lqud Fe alloy from the hpman and Flords alumnum dstrbuton data 23) between lqud Fe Ag alloys and Ag melts at K usng the extrapolated actvty data n lqud bnary Ag alloys at K wth an assumpton that logγ s nversely proportonal to absolute temperature. The e value of ε can be converted to the value as at K usng the Lups converson relatonshp. 24) Ths e value of at K s the only data compled as the recommended value by the Japan Socety for the Promoton of Scence (JSPS). 25) However, the relatvely hgh slver content up to 9.3 mass% n Fe Ag alloys equlbrated wth Ag alloys at K n hpman and Flords work does not warrant the accuracy of values for ε n Fe alloy system at K. The e and r values n alumnum-rch lqud steels are very mportant thermodynamc parameters to predct the formaton of nclusons lke and 2O 3 n TRIP and TWIP steel grades. In the present study, these values were accurately determned by consderng the frst- and the sec ISIJ 1582

6 ISIJ Internatonal, Vol. 54 (2014), o. 7 Table 3., r e r ond-order effect of carbon and alumnum on ntrogen n Fe melts by the metal-gas and the metal-gas-ntrde equlbraton technque n the temperature range from to K. Usng those nteracton parameters, the crtcal ntrogen content to form n the melts contanng wde ranges of carbon and alumnum can be calculated from Eqs. (7) or (8) by the teraton method as shown n Table 1. The calculated solublty products of, log [% ][% ], are also shown as the sold lnes n Fg. 5. They are n excellent agreement wth the expermental data. 4. onclusons Thermodynamc parameters determned n the present study. Values Temp. range(k) omp. range Ref ± ± [% ]: [% ] 0.91 [% ] 3.9 [% ]: [% ]: [% ]: [% ] 3.9 [% ]: Present study Present study Thermodynamc relatons among carbon, alumnum and ntrogen were determned n lqud Fe alloy melts at K. The man fndngs of ths study can be summarzed as follow. (1) From the ntrogen solublty measurement n Fe melts, the second-order nteracton parameter of carbon and alumnum on ntrogen was determned as (2) From the solublty measurement n Fe melts, the frst- and second-order nteracton parameters between carbon and alumnum were determned as 22 Present study, r = ± [ ]: mass%,[] 0. 91mass%, K ( ) Acknowledgments Ths study was supported by the R&D enter for Valuable Recyclng (Global-Top R&BD Program) of the Mnstry of Envronment (Project o.: GT ). REFEREES 1) J. E. Jung, J. H. Park, J. S. Km, J. B. Jeon, S. K. Km and Y. W. hang: Met. Mater. Int., 20 (2014), 27. 2) S. B. Jeon, J. S. Km, K. W. Km, K. S. Son and D. G. Km: Korean J. Met. Mater., 52 (2014), 11. 3) W. Dabbouss and J. A. emes: ISIJ Int., 53 (2013), ) W. J. Dan and Z. G. Hu and W. G. Zhang: Met. Mater. Int., 19 (2013), ) J. E. Jn, M. S. Jung,. Y. Lee, J. Y. Jeong and Y. K. Lee: Met. Mater. Int., 18 (2012), ). S. Lm, H. S. Park, S. I. Km and. G. Park: Met. Mater. Int., 18 (2012), 647 7) L. Samek, E. De Moor, J. Pennng, J. G. Speer and B.. De ooman: Metall. Trans. A, 39A (2008), ) H. Yn: Proc. of Int. onf. of AISTech 2005, Vol. 2, AIST, Warrendale, PA, (2005), 89. 9) M. Vedan, D. Dellasega and A. Mannucc: ISIJ Int., 49 (2009), ) W. Y. Km, J. G. Kang,. H. Park, J. B. Lee and J. J. Pak: ISIJ Int., 47 (2007), ) M. K. Paek, J. M. Jang, M. Jang and J. J. Pak: ISIJ Int., 53 (2013), ) J. M. Jang, S. H. Seo, M. Jang, M. K. Paek and J. J. Pak: ISIJ Int., 54 (2014), ) R. D. Pehlke and J. F. Ellott: Trans. Met. Soc. AIME, 218 (1960), ) J. hpman: Metall. Mater. Trans. A., 180 (1955), ) V. H. Schenck, M. G. Frohberg and H. Graf: Arch. Esenhüttenwes., 30 (1959), ) S. Maekawa and Y. akagawa: Tetsu-to-Hagané, 46 (1960), ) F. Ish, S. Ban-ya and T. Fuwa: Tetsu-to-Hagané, 68 (1982), ) D. W. Gomersall, A. Mclean and R. G. Ward: Trans. Met. Soc. AIME, 242 (1968), ) W. I. Yavosky, A. G. Svyazhn and A. F. Vchkarev: Vzamobestve Metallov Gazov vstaleplavlnych Protsessakh, Trudy MISIS, Moscow, (1973), ) G. M. Grgoreneko, Y. M. Pomarn and W. I. Lakomsky: Problemy Spezelectrometallurg, aukova Dumka, Kev, (1973), ) R. D. Pehlke and J. F. Ellott: Trans. Met. Soc. AIME, 218 (1960), ) T.. Wlder and J. F. Ellott: J. Electrochem. Soc., 107 (1960), ) J. hpman and T. P. Flords: Acta Metall., 3 (1955), ). H. P. Lups and J. F. Ellott: Acta Metall., 14 (1966), ) The 19th ommttee n Steelmakng: Thermodynamc Data For Steelmakng, The Japan Socety for Promoton of Scence, Tohoku Unversty Press, Japan, (2010), 72. e = ± , r = 0 ([ ] 39. mass%,[]: mass%, K) ISIJ