MICROSEGREGATION OF SOLUTE ELEMENTS IN SOLIDIFYING MUSHY ZONE OF STEEL AND ITS EFFECT ON LONGITUDINAL SURFACE CRACKS OF CONTINUOUS CASTING STRAND

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1 45 8 Vol.45 No.8 29 Ê 8 Ì µ ACTA METAURGICA SINICA Aug. 29 pp ß Ø Ç ÐÍ ÐÜ Ê Ì» É±Ã ÚÒ (Ò Ï À³Ö Ë, 114) º Ueshima Æ ÒÜ È,» Í µ δ/γ Ö ± Ó, Þ Ï Ü 1 /s Í È Ø Ë ÕÖ, Þ Ó Æ ÏÅÕÖÆ ÓÀ ÂÚË ÕÖ θ B ÁÐ ², Ð²É ³Ì P, S ²ØË ÕÖÐ C ² ², P, S ² Î Å ÁÜ ¹ Ç Â. µ Å, ± Ó, ¹, Ë ÕÖ, Ð Đ ½ TG777 Â A (29) MICROSEGREGATION OF SOUTE EEMENTS IN SOIDIFYING MUSHY ZONE OF STEE AND ITS EFFECT ON ONGITUDINA SURFACE CRACKS OF CONTINUOUS CASTING STRAND CAI Zhaozhen, ZHU Miaoyong School of Materials and Metallurgy, Northeastern University, Shenyang 114 Correspondent: ZHU Miaoyong, professor, Tel: (24) , myzhu@mail.neu.edu.cn Supported by Program for New century Excellent Talents in University (No. NCET 4 285) Manuscript received , in revised form ABSTRACT The solidification of molten steel in continuous casting mold is a complicated nonequilibrium process with high cooling rate of 1 1 /s. At such a cooling rate, the segregation of the solute elements such as C, Si, Mn, P and S in brittle temperature range ( θ B ) will vary with their initial contents and influence on the thermal strain significantly which could greatly increase the incidence ourface defects otrand. In this paper, a microsegregation model oolute elements in mushy zone with δ/γ transformation during solidification was established based on the regular hexagon transverse cross section of dendrite shape proposed by Ueshima by finite difference method under the non-equilibrium solidification condition at 1 /s of cooling rate and the brittle temperature range θ B was determined. The distribution characteristics oolute elements and the effect of their segregations on θ B and thermal strain were investigated. The results show that both P and S are the most serious segregation elements in final stage oolidification and affect on θ B significantly together with carbon content in molten steel. The mechanism that increasing contents of P and S may increase the probability of longitudinal surface crack for continuous casting strand was presented by calculating the change law of thermal strain with carbon content under different of P and S contents. KEY WORDS continuous casting, microsegregation, longitudinal surface cracks, brittle temperature range, thermal strain «Ûß «¼ Ð³, * Å Í Đ¹Ô É Ã NCET µ : , µ : Ô : ¾, Å, 1982 Ë, ± º Ý [1]. Ü «º ¾Ü, É Î Ö, ÐÇ ÑÝ ÇÀ ÝÓÅß Ô, Î Ð ÓÝ À, ÉÝ µà

1 ÅÔ 1.1 È ÑÎ Ë ÆÕ 1.1.1 ÅÆ «ÓÝ Ø Ï 1 Ð, ÒÓÝ Ø Î Ù Ù Ð Ø, ØÛÂ ¼ÓÝ ÄÝÎ ÚÎ ; ÓÝ Ð ÓÝ, ÐÇ Ç Í µ, Ó Å µ ; ÑÝ Ó¼ Ó Ö Ó, ÅÓÝ Ø Ö µ Í, Ø Í Ù ; Î, ± δ, γ, «± Í, Î Ð Â δ/γ Ó ¹ Ó Û Ó Å ÐÏÖ¼ ÐÓÝ Ö Í Ñ [5]. 1.1.2 ÅÆ Ð Û, Ù «ßÛ ( Ï 1d ), ÐÇ x Ä.

2 95 Õ ¹ 45 Å È, ² º Ç [2,3]. Kobayashi [4] Ë ¾ Ôµ.5 /s Ý Ù Æ δ/γ Î ÓÝÇ C, Si, Mn, P, S ÐÇ Î ³, ÃÂµ Î É Ù Fe C Ï. Ueshima [5] ÓÝ Ø Ð, ÔµÆ δ/γ Î ÓÝÇ Ð µ. Kim [6]» Ueshima Ã, Ôµ Ý.17 /s, ÓÝ 1 µm É Ù Ð ÔÛ«º ¼Ç º. «º Ê ÑÝ, ÑÝ ÇÀ Î Ý 1 1 /s [7,8], ÈÓÝ 2 µm» Ç [9], ÝÒÍ Î Ý Í, Í ÖÙ ¾¼ Ð Ô Ö Í [1]. ÄÐ, Û «ÑÝ Ý É Ù Ð Ô³ ß ¾.» Ueshima ÃÂÓÝ Ã, ««Ë ÝÉ, ß Ôµ «Ð Ç Ì Ö Ç ÔÁ Ã Û «º ³. 1 ÅÔ 1.1 È ÑÎ Ë ÆÕ ÅÆ «ÓÝ Ø Ï 1 Ð, ÒÓÝ Ø Î Ù Ù Ð Ø, ØÛÂ ¼ÓÝ ÄÝÎ ÚÎ ; ÓÝ Ð ÓÝ, ÐÇ Ç Í µ, Ó Å µ ; ÑÝ Ó¼ Ó Ö Ó, ÅÓÝ Ø Ö µ Í, Ø Í Ù ; Î, ± δ, γ, «± Í, Î Ð Â δ/γ Ó ¹ Ó Û Ó Å ÐÏÖ¼ ÐÓÝ Ö Í Ñ [5] ÅÆ Ð Û, Ù «ßÛ ( Ï 1d ), ÐÇ x Ä. Í, «Æ δ/γ C, Si, Mn, P, S Ñ ÐÇ Î Â µ. ÐÇ Ç Fick Ñ Þ, / Ó Ð³ Ñ ¼ ÐÇ Ó Å Ö¾ Í Ñ, Þ [5]. Ç Ö¾ K δ/, K γ/ ¼ K φ/ Ã Ö¾ D δ ¼ D r 1. Ë «ÓÝ Â Ý ¼ C ³. ÓÝ E Bealy ¼ Thomas ÈÓÝ [11] Ê : λ = Kv m wc n (1), λ ÓÝ, µm; K, m ¾, ¼.26278; v Ý, /s; C ³ (Ð³ «¾); n ÙÙÙ C ³,.15, n= ,.15 1., n= ÄÐ «Ü¼ [7], Ù Ý 1 /s. Ó ¹ÓÉ : Î Ð Ç ÐÇ ÏÖ Ð Ö θ, δ/γ Ó Å ÐÇ ÏÖ Ð Ö θ Ar4 ÓÝ Ð Ö, Û Ó ß ¹. θ ¼ θ Ar4 Ê ¼ (2) [12] ¼ (3) [5] : 1 Å Í µ Â ¾Î Fig.1 Schematic diagrams oolidification of continuous casting process and model schematic diagram of continuous casting process schematic diagram showing the morphology of dendrites (c) transverse cross sections of dendrites (d) part of transverse cross sections of dendrite to be analyzed

3 8 ½ : ÆÌ ĐÕ ßÅ ÒÁ Ù 951 ² 1 Æ Õ½» Õ½ [3,5] Table 1 Equilibrium distribution coefficients and diffusion coefficients oolute elements Solute element K δ/ K γ/ K δ/γ D δ, 1 4 µm 2 /s D γ, 1 4 µm 2 /s C exp( 81379/RT).761exp( /RT) Si exp( /RT).3exp( /RT) Mn exp( 22443/RT).55exp( /RT) Note: R=8.314 P exp( 2312/RT).1exp( /RT) S exp( /RT) 2.4exp( /RT) θ = w Si 4.9w Mn 3.44w P 38w S (2) θ Ar4 = w Si + 12w Mn 14w P 16w S (3), Ö¾ Û Fe X(X=C, Si, Mn, P, S) ÞÇ Ï, w X X Ç Ð³ ¾. ¼ Ð Ð, «C++ Ã Ù. 1 Ð, ÅÆ 2 Âµ ÉÊ ±. µ Ñ Ðß, ÚÊ µ * ± Ý.5 /s É Ù Ð Ô Fe C Ï, Ï 2. Ï, Ö, δ γ Ö, δ γ Ñ¼ Ö¼ Ö Ô Å C ³.1% Ê Â, Û ÓÝ ÂÝ Ð γ Ð Ð Ú γ ¼Ú Ð Ö; Kobayashi [4] «É ÙÛ Ø ¾.» Â, ÉÊ Ñ Â ¾ ½ ¹. Ë Â Fe C ÏÛ Ö, Ý, Â Î É ÙÓ Í: Ý ß Ö Ç, Òß «Û C ³ ß ¹ ( Ê É Ù.75.41%); ÐÇ Ô, ÝÓ À ÐÇ µ À, ÚÎ Ö Û Î É ² 2 Æ Table 2 Chemical composition oolute elements (mass fraction) Sample Si Mn P S Ref. * [4] A [13] B [14] C [13] A A2/B [15] A [6] A [6] B B B Si-2.Mn-.2P-.5S Nonequilibrium binary Fe-C phase diagram Equilibrium binary Fe-C phase diagram Experimental data [4] * º Fe C ÎÁ Fe C Î Fig.2 Comparison between non equilibrium pseuao binary Fe C phase diagram oample * carbon steel and equilibrium binary Fe C phase diagram Ö ºÎ Ù ÆÕ ½ÑÖ Ö ZST(zero strength temperature) ¼½ Ö ZDT(zero ductility temperature) Ë ² ³ ± È± ² ¾, ZST Â ZDT Ö Ç, Đ ÑÑÖ, Ð È, Ý ½ À Ý º. Ë Þ Ö Ñ Ì Ö. Û Ë «, Ì Ö Ö ε th ¼ ± [6] ε th = θ θ ref α dθ + ε δ γ (4), θ ref «Ö; α Ö¾; ε δ γ δ γ. θ Ö Æ ε th,θ ÂÖ Ö ε th,θ = 3 ρθref ρ θ 1 (5), ρ θref ¼ ρ θ «ÖÂ θ Ö ÆÛ Ö. ρ θ ÅÙ [16] Ê : ρ θ = f α ρ α,θ + f δ ρ δ,θ + f γ ρ γ,θ + f ρ,θ (6), f α, f δ f γ ¼ f θ Ö α δ γ ¼

4 952 Õ ¹ 45 Î Å ¾; ρ α,θ, ρ δ,θ, ρ γ,θ ¼ ρ,θ θ Ö α δ γ ¼ Ö. ± Ç Í α, (6) : ρ θ = f α ρ δ,θ + f δ ρ γ,θ + f ρ,θ (7), ρ δ,θ, ρ γ,θ Ã ρ,θ Ê Ù: ρ δ,θ = 1 (811.47θ)/[(1 )(1+.13 ) 3 ] (8) ρ γ,θ = 1 (816.51θ)/[(1 )(1+.8 ) 3 ] (9) ρ,θ = (.8.9 )(θ 155) (1) 2 ¾ 2.1 Ö ºÎ Ï¹ Clyne [17], Davies ¼ Shin [18], Kim [6] Ì Ö ³ ½ÑÖ Ö ZST Â½ Ö ZDT Ö ß, Ý µ Ö IT(iquid Impenetrable Temperature) Û«Ö Ù, Å ± Æ ¼º, ÖÓÐ Ñ ¾.9 Û Ö. Æ, Ã Æ ¼Þµº ; Ý º, Ê»ÓÝ µ Æ, º ÐÐÄ½, Ì. Matsumiya [19] «ÖÓÐÑ ¾.85 Û Ö. ½«Ý (1 /s) É Ù Ð ÔÛ «Î Ã, Ù =.884 Û Ö Ö. Ï 3 Ý 1 /s ÙÊ A, B, C «ÚÎ ( =1.) Û Ö, Û ZDT ¾ [13,14]. ÉÏ Â, ZDT Ê ÙÂ Ù ½ ¹. Ç, ¾ Û Ö ßÑ Ì Ö. 2.2 Ý Þ Ö ºÎ Ë Ï 4 A2 Î Ù ¾ =.884(Û Ö IT) ¼ =.98 (Û 99 Ð ) Ê C, Si, Mn, P, S Ñ ÐÇ Calculated ZDT, o C 152 Sample A Sample B Sample C Measured ZDT, o C 3 É ZDT Á ZDT Fig.3 Comparison between calculated ZDT (=1.) and measured ZDT [13,14] Ô Â C ³ Ö. w ¼ w ÐÇ Î Ð À ³¼Î Ù.884 Â.98 ³. ÉÏ» Â, S ¼ P Ô, Ò C ³ ÏÛ. C Ç Ô À C ³ Ï Ö, Û Ð Ö ( (2)), Û ZDT, Ç, ² Ô C, S, P «Ç Ì Ö ÔÃ ³ C ÉÏ 4 Â, ÐÇ C ³ Ï, Ô Û, ÒÔ± Ü. C ³ w 1 w 2 (ÂÝ Ý C ³), C ³ Ï ÚÎ, ÐÇ δ Ç, ÉÝ µ ÐÇ Ô Ö, Ô Ù. C ³Ì Ï, Î ÂÝ Ý, Si, P, S γ/ Ó Ö¾ K γ/ δ/ Ó Ö¾ K δ/, Ò γ Ö¾ δ ( 1), Ç Ô Ï, Ø C ³ Ï w 3 w 4 (ÂÝ γ C ³) ºÔ Ñ. C Ç δ/γ Ó Ö¾, «γ C Ù δ Ü, γ C ³. C ³ γ/ Ó ÐÞ, µ C Ô, Ý C w /w w /w 1 fs = w 1 + w 3 C Si Mn P S w 2 w 4 4 A2 Đ Æ Ï ½ =.884» =.98 Ó C ² Fig.4 Distributions oegregation ratio oolute element in residual liquid at =.884 and =.98 of A2 steel

5 8 ½ : ÆÌ ĐÕ ßÅ ÒÁ Ù 953 Ô C ³ Ï Ú Ù Ý ÑÔ±, ÒÙ Ö. ÉÏ Â, Mn ² Ô Û ÑÇ, Î δ/, γ/ ¼ δ/γ Ó Å Ö¾Ô Ú 1.,» Ô Û ¼, C ³ Ï Ñ. Ç,» : «, C ß Ñ» Ô, C ³ Û Si ¼ Mn, Û P ¼ S Û S S δ/ ¼ γ/ Ó Ö¾ K δ/ Â K γ/ Û È ÐÇ ¾³Æ, Ý Ô. Î Ø Ù, S ³Ä À, Ô, Ï 4. Ù, S Ô Ï ²À Î, Û A2 Ý, É Ï µ Ï 5 ² A1, A2, A3 ¼ A4 1 /s Ý Ù Fe C ÏÊ Ñ. ÉÏ» Â, À S ³ Ï, Ö, ÙÝ ZDT Û. ÉÏ Â, À S ³ Ï Ñ Ö Î ß µ, w 1 (δ Fe C ÒÖ) ¼ w 2 ( Ý C ³) ÂÝ ¹Ý, ÉÀ S ³.7%.1% ¼.2% À S ³.78%.6% ¼.1%, ÒÞ Ø. Ï 6a.14Si.36Mn.16P xs(ð³ ¾) S Û Ô³ (w S, - w S, ) Â À C ¼ S Si-.36Mn-.16P-.7S f w' s =.884 w' 1 2 =1. iquidus temperature 132 Complete solidified temperature Beginning of transformation End of transformation Si-.36Mn-.16P-.13S + =1. w' 1 w' 2 = Si-.36Mn-.16P-.39S (c) = w' 1 w' 2 = =1. w' 1 w' ³Ì S ² Fe C Î Fig.5 Non equilibrium pseuao binary Fe C phase diagrams oteels with different S content Si-.36Mn-.16P-.78S (d) = w S, -w S, w S w P, -w P, w P 6 P, S ²Ú Ú Ó² Fig.6 Effects of initial content of P and S on their absolute segregation amount

6 954 Õ ¹ 45 ³ ÖÏ. ÉÏ Â, Û ÍÀ C ³, À S ³.4%, À S ³Û S Û Ô ³, Ý.4%. ², À S ³.4%, ³ À S Û Ö, ÚÎ, S, Ñ Ö µ S À, S Û Ô³ P P Â S ±² ½ ÔÇ. S Ç, γ/ ¼ δ/ Ó Ö¾ K δ/ Â K γ/, Ý Ô Ö. ÉÏ 4» Â, P Ô Ô±Â S Û Í, C ³ Ï ß γ, P Ô Ñ C ³Đ Ç C ³ ÏÝÌ Ï. ÇÝ ²É ² P γ Ç Ö¾ S Ç ± ¾³Æ, P À γ ¾, P Ô S. ØÛ C ³ Ï ÓÝ ÚÎ, C ³ Ï Î Ý P ³ ÏÂ Ô³ Ï, P Ô ºÔ Ñ. Ï 6b.14Si.36Mn xp.13s P Û Ô³ÂÀ P ¼ C ³ Ö Ï.» Â, À P ³.64%, P À ³Û Û Ô³. ², «P ³Ù Î Ð Ö P Ô ÚÕ ; P Ç Ö¾ Ü. 2.3 Ý Ë ¼³Ä À ÛÓ Ï 7 A1, A2, A3, A4 ¼ B1, B2, B3, B4 Í P ¼ S À ³ÙÌ Ö θ B (=IT- ZDT) Â C ³ ÖÏ. Ï» Â, P, S À ³ Ï, Ï µ Î Á ÝÓÅ P, S Ô³, À µ ZDT, θ B Ï; À C ³ Ï, Ï µ P, S ÐÇ Ô, θ B Ù Ï. Ì Ö Ñ, (4) θ ref ¼ θ Ù IT ¼ ZDT, Ê : ε th = IT ZDT α dθ + ε δ γ (11)» Â, ε th θ B ÏÝ Ï. Ç, «, Ï C, P, S À ³ À Î Ð Ï, ÉÝ Ï «ÂÝ º È. A1, A2, A3, A4 ¼ B1, B2, B3, B4 Í P, S À ³ÙÌ Ö Ç ε th ÂÀ C ³ Ö Ï 8. ÉÏ Â, P, S ³ Ï, Ì Ö Ï, Ò C ³ Ý, Î ÂÝ Ý ÂÝ Ù. Ù ¼ÂÝ Ý P, S ³ Ý Û : P ³.8% Ï.64%, Ù B, o C B, o C x si-.36mn-xp-.13s 125 x.13si-.36mn-.16p-xs P» S ²Ú θ B Fig.7 Effects of initial content of P and S on θ B th, 1-3 th, W P W S P Â S ²ÚË ÕÖÆÐ Fig.8 Effects of P and S contents on thermal strain ε th in brittle temperature range , Ïµ 34.5%; S ³.7% Ï.78%, Ù , Ïµ 61.4%; ÙÂÝ Ý

8 ½ : ÆÌ ĐÕ ßÅ ÒÁ Ù 955 C ³ ¹. ÐÇ Ô, À Î Ð Å È ÝÓÅ ÀÝÄ Ù, ÝÓÅß º, ßº. Ç, P, S ³ Ï Ù Ï, Ï «Î Ð ºÈ. Saeki [15] A2/B1 «º Ú¾ÂÝ Ù Ý ÂÊ ÙÛ, Ù Ì Ö «º ÂÝÈ.

[1] Konishi J, Militzer M, Brimacombe J K, Samarasekera I V. Metall Mater Trans, 22; 33B: 413 [2] Thomas B G, Brimacombe J K, Samarasekera I V.

Trans Iron Steel Inst Jpn, 1988; 28: 543 [5] Ueshima Y, Mizoguchi S, Matsumiya T, Kajioka H. Metall Mater Trans, 1986; 17B: 845 [6] Kim K, Yeo T, Oh K H, ee D N.

Metall Mater Trans, 1995; 26B: 361 [9] Zhu Z Y, Wang X H, Wang W J, Zhang J M. In: The Chinese Society for Metals ed.

Metall Mater Trans, 1996; 27B: 689 [12] Kawawa T. Tekko Binran (Handbook for Steel), Tokyo: ISIJ, 1981, 1: 25 [13] Schmidtmann E, Rakoski F.

7 8 ½ : ÆÌ ĐÕ ßÅ ÒÁ Ù 955 C ³ ¹. ÐÇ Ô, À Î Ð Å È ÝÓÅ ÀÝÄ Ù, ÝÓÅß º, ßº. Ç, P, S ³ Ï Ù Ï, Ï «Î Ð ºÈ. Saeki [15] A2/B1 «º Ú¾ÂÝ Ù Ý ÂÊ ÙÛ, Ù Ì Ö «º ÂÝÈ. 3 Á (1) «, C ß Ñ» Ô, C ³ Û Si, Mn Ô, Û P, S Û ; P, S ½ Ô, Ò Ô Ï ²À Î Á ; Ï P, S ³, δ Fe C ÒÖ, Ý Û C ³. (2) Ý ßÌ Ö ÂÝ Ù, Ï P, S ³ Ì Ö Ã Ù, «ÂÝº È Ï, «. Ï P, S ³ ÂÝ ÙÛ C ³. [1] Konishi J, Militzer M, Brimacombe J K, Samarasekera I V. Metall Mater Trans, 22; 33B: 413 [2] Thomas B G, Brimacombe J K, Samarasekera I V. Trans Iron Steel Soc AIME, 1986; 7: 21 [3] Kim K, Han H N, Yeo T, ee Y, Oh K H and ee D N. Ironmaking Steelmaking, 1997; 24: 249 [4] Kobayashi S, Nagamichi T, Gunji K. Trans Iron Steel Inst Jpn, 1988; 28: 543 [5] Ueshima Y, Mizoguchi S, Matsumiya T, Kajioka H. Metall Mater Trans, 1986; 17B: 845 [6] Kim K, Yeo T, Oh K H, ee D N. ISIJ Int, 1996; 36: 284 [7] Suzuki M, Yamaoka Y. Mater Trans, 23; 44: 836 [8] Muojekwu C A, Samarasekera I V, Brimacombe J K. Metall Mater Trans, 1995; 26B: 361 [9] Zhu Z Y, Wang X H, Wang W J, Zhang J M. In: The Chinese Society for Metals ed., Proceedings of Asia Steel International Conference, Beijing: Metallurgical Industry Press, 2: 358 [1] Suni J. PhD thesis, of Carnegie Mellon University, New York, 1991 [11] E Bealy M, Thomas B G. Metall Mater Trans, 1996; 27B: 689 [12] Kawawa T. Tekko Binran (Handbook for Steel), Tokyo: ISIJ, 1981, 1: 25 [13] Schmidtmann E, Rakoski F. Archiv Eisenhuttenwesen, 1983; 54: 357 [14] Shin G, Kajitani T. Suzuki T, Umeda T. Tetsu Hagané, 1992; 78: 587 (,, ¾Â, Ç. Ê, 1992; 78: 587) [15] Saeki T, Ooguchi S, Mizoguchi S, Yamamoto T, Mrsumi H, Tsuneoka, A. Tetsu Hagané, 1982; 68: 1173 ( ²,, ÆÅ, ÆÂ º, Å,. Ê, 1982; 68: 1173) [16] I C S, Thomas B G. Metall Mater Trans, 24; 35B: 1151 [17] Clyne T W, Wolf M, Kurz W. Metall Mater Trans, 1982; 13B: 259 [18] Davies G J, Shin Y. K. Solidification Technology in the Foundry and Cast House, ondon: the Metal Society, 1979: 517 [19] Matsumiya T, Saeki T, Tanaka J, Ariyoshi T. Tetsu Hagané, 1982; 68: 1782 (, ², ÇĐÆ,. Ê, 1982; 68: 1782)

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