MICROSTRUCTURE EVOLUTION OF HYPEREUTEC- TOID STEELS DURING WARM DEFORMATION II. Cementite Spheroidization and Effects of Al

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1 45 2 Vol.45 No ACTA METALLURGICA SINICA Feb. 29 pp Ï ÄÑ ÇÚÉÛØÒÄ II. Ë «Ï Al ÏÙ Ü Đ 1) ÞßÝ 1) 2) Ð 1) 1) 1) Ô Ã ÄÏÒ, 183 2) Ô Ã Ô Å, 183 Ö Ô ÞÜ Gleeble 15» ÌÅ¼ Ð, Ù Al Æ Ð Ê Ì Ì½» (α+θ) Û Ú. Æ :, Al Æ Ð ÌÃ» Ì ÒÐ Ã Ù Ð Û ; Ê, Ù Al Å Fe C ÚÂ, ±µì Ì Ð, Ò Ì Ð Ù, Ì½» (α+θ) Û. È Ð, Ê, Ì,, Al Ù ËÎ TG142.1 ÐÅ A Ã (29) MICROSTRUCTURE EVOLUTION OF HYPEREUTEC- TOID STEELS DURING WARM DEFORMATION II. Cementite Spheroidization and Effects of Al CHEN Wei 1), LI Longfei 1), YANG Wangyue 2), SUN Zuqing 1), ZHANG Yan 1) 1) State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 183 2) School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 183 Correspondent: YANG Wangyue, professor, Tel: (1) , wyyang@mater.ustb.edu.cn Supported by National Natural Science Foundation of China (No ) and Doctoral Fund of Ministry of Education of China (No.25817) Manuscript received , in revised form ABSTRACT The effects of Al on the spheroidization of cementite and the formation of ultrafine (α+θ) microstructure of hypereutectoid steel during isothermal spheroidizing and warm deformation were investigated by uniaxial hot compression simulation experiment. The results indicate that during isothermal spheroidizing, the fine grained cementite particles and ferrite are obtained by addition of Al. Ultrafine (α+θ) microduplex microstructures can be formed by warm deformation in a very short time. During warm deformation, the diffusion of carbon and iron atoms are impeded with the addition of Al, the spheroidization of lamellar cementite and the coarsening of cementite particles are retarded. And the re precipitation of cementite particles in ferrite matrix is also restricted, resulting in a more ultrafine (α+θ) microduplex microstructure. KEY WORDS hypereutectoid steel, warm deformation, cementite, spheroidization, Al ¹ ¾¼ µ, Ý ¼Ã Ü Ñ Í¾¼ (α+θ) Ü» ÕÔ [1 6]. ¼Ã Ü µ, ßĐË ÂÝ ² Í, Í Ë ¾ Û µ * Õ ¹ , Â ¹ Å Ó ¹ ÐÉ : , ÐÉÏÞ : Á Ì :,, 1979 Î, Î Đ [7], «Ë ² Í ¼± ÄÑ Ë [7 11]., ÐĐ ¹, Ó± Ê ¾ ¹Í Ë, µà Ú Al [12 15], Al ÀÐ Ë¾¼ Þ Î ßÅ ÂÝ,», µ Al ÀÐ¾ÜÎ ÛÕ [9,16,17], Ò Ç «Đ Al Ð ¹ µ Ë µ² Í Í È Æ¹ ÆÆ. [18] ºÆ, º Al Ð ¹ µ Ð µ Ë µ²

$^2* 3 #Z : + ' Q 7'VE a 9U 7!U"Yw. 1 otr _ey + -~Y!\5t (& tr, %) ": 1 ~ () T1 ~), C.97, Si.26, Mn.31, P.56, S.4, Fe ; 2 ~ ( 1 ~Y. N.95% Y Al), C 1., Si.27, Mn.31, P.56, S.4, Al.95, Fe.$ $S7L & Image Tool Ct -IH, ym9p( v ^~;I U o~l H <U ~=r. o^z 2 + - ~ a g!, 65 \ ) 5 min U G 4ge2 (F 1). -), 1 ~Y "2 ` g ^R N Y 2-9U, G 1a ) G ; n 2 ~ ){ Al Y H, o% 9U YS5 (F 1b ) G ),!$ $52±.22) µm, v. ^~ EL; n 2 ~ ){ Y v Al Y HD U o~ye!,! U ~=;I, 'k;i" (.46±.19) µm( 2b), Y L H Y v ^~ G. \)7!;, ) S 3F[2 Y7!. C *5 [18] YO r, +~ 65,.1 s H1 s! 1.61 ( " 16 s) Y, 9,U7!Y2 (α+θ) x ;G.$

2 ^2* 3 #Z : + ' Q 7'VE a 9U 7!U"Yw. 1 otr _ey + -~Y!\5t (& tr, %) ": 1 ~ () T1 ~), C.97, Si.26, Mn.31, P.56, S.4, Fe ; 2 ~ ( 1 ~Y. N.95% Y Al), C 1., Si.27, Mn.31, P.56, S.4, Al.95, Fe. ) S m *5 [18] ) p ;. " ) SY&*U Y7! 9l, kf bd )[U \)7! *, \))h" 65, \) YB" 5 min 2 h. ) S8\)7! * Ykf k `)O1 q D( l, a K_ y 3% 4% Fy f ` 1Z, { y ZEISS Supra 55.o QL b b (SEM) [U3 G +. S7L & Image Tool Ct -IH, ym9p( v ^~;I U o~l H <U ~=r. o^z ~ a g!, 65 \ ) 5 min U G 4ge2 (F 1). -), 1 ~Y "2 ` g ^R N Y 2-9U, G 1a ) G ; n 2 ~ ){ Al Y H, o% 9U YS5 (F 1b ) G ),! 2 &*Bi, G "2 ` g ^R N Y 2 - v. -), 1 2 ~2 &*Bit " (.32±.6) (.27±.5) µm. { &*2 \V,, /'y7! * m _-7!, {M~%Y \V.?n) 7! * m Xg Y Y B, G, l d y + ~, 65 \ ) 7 h &, U. 7! Y G ( 2). 7! G ), 1 ~Y U o~ ;I L " EL ( 2a), ' k ;I " (.52±.22) µm, v. ^~ EL; n 2 ~ ){ Y v Al Y HD U o~ye!,! U ~=;I, 'k;i" (.46±.19) µm( 2b), Y L H Y v ^~ G. \)7!;, ) S 3F[2 Y7!. C *5 [18] YO r, +~ 65,.1 s H1 s! 1.61 ( " 16 s) Y, 9,U7!Y2 (α+θ) x ;G. Ux ;G ), 1 ~Y v ^~ ' k ; I" (.61±.23) µm, n 2 ~" (.44±.14) µm; +~YU o~;ik6sut (F 3). -), j +U o~t v ^RN, 4ge{&*U E j 7! UR; j+ t v ^, 4ge{ S 9 )F P Y U ~ =Æ-?UR. (U 2 ~)^R ^ U o~'km t " (.14±.5) (.7±.2) µm, k H 1 ~ ^R ^ U o ~ ' km (t " (.19±.8) (.8±.2) µm); n, 2 ~YN & 2 U ~ =Ar " 15.1 [12 15] [12,13] II. R V4 5 Al Vt= 157 g 1 *,} 65 [( 5 min X SEM B Fig.1 SEM images of hypereutectoid steels No.1 (a) and No.2 (b) after isothermal transformation at 65 for 5 min (arrows in Fig.1a indicating cementite laths at boundary of pearlite, arrows in Fig.1b indicating no cementite) 1 [19 22] g 2 *,} 65 [( 7 h X6 F Fig.2 SEM images showing spheroidized microstructures of hypereutectoid steels No.1 (a) and No.2 (b) after isothermal annealing at 65 for 7 h 1 ~Y mm. + ~ \ )7! ) S 9 ) Y 7! f \; 9G 4a b. \)7! 9), ;9kv S P 16 mm 2, 6 2

3 158 Î Ñ 45 Frequency, % Frequency, % (a) Cementite located at the grain boundaries, d gb Cementite at grain interiors, d gi d gb = (.19.8) m d gi = (.8.2) m Cementite particle size, nm (b) d gb = (.14.5) m d gi = (.7.2) m Cementite particle size, nm 3 Ð 65,.1 s 1, Ö ε=1.61 Ì ÒÐ À Fig.3 Distributions of the size of cementite in hypereutectoid steels No.1 (a) and No.2 (b) deformed at 65,.1 s 1 and ε=1.61 (dash dot line indicating the bimodal distribution of cementites) ÆÆ, 2 Û Ä, Ç µ 7 h. µ, µ Ë ² Í., 65,.1 s 1 À ½ Ë, 1 s ÀÐ Ë ¾ º, µ Ë ¾ Û µ Ø 4 Ö. µ Ë, 2 Û Ä. ß 4b Ö, µ µ, Æ² ¾ Í Ö ¾ Æ Ç, µ, ÓÏ Í ; Ò µ Ë µ, Æ² ¾ Í Ö ĐĐ µ. ß, 1 Æ² ¾ Í Ö ¾ À; Ç 2 Ë µ, Æ² ¾ Í Ö. Å, 4c µ ÖÐÑ, µ Ë 1 Ê Í Ö ½Á, ÓÏ ; Ò Ë µ, 2 Æ² ¾ Í Ö, ε=.96», Í Ö Đ 1, Ê Ö Đ 1 ; Ç ε=.96, Í Ö Ø Ö ĐĐ 1. Volume fraction of spheroidized pearlite, % N, 1 6 mm -2 N gb, N gi, 1 6 mm (a) 1 s 65 o C,.1 s o C, no deformed 7 h (b) (c) 65 o C,.1 s o C, no deformed o C,.1 s -1 2,, Ð Ê µ Đ Å ± ½ Ì Ð Õ (N) Å± ½ É Ì Ð Õ (N gb, N gi ) Þ Fig.4 Kinetics of spheroidization of pearlite (a), evolutions of total amounts of cementite particles per unit area (N) (b) and amounts of cementite particles at ferrite grain boundaries and in ferrite grains (N gb and N gi ) per unit area (c) for the two hypereutectoid steels N gi N gb during hot deformation and isothermal annealing 3 Í Õ ¾ ² Í Ú Ê, ß Gibbs Thomson Á Ö, À Í ² Ý Ú º C ĐĐÄ ² Ý Ú º µ C, ß Ë C, C ßĐ ÛÃ, Ò

4 2 : ³ É ³Î II. Ê ÅÎ Al ÎØ 159 Í ², ² Í Ð ² ² Í µ [8]. µ, ßĐ² Í µ Ç, Ð ² Ç, ÕÒÆ² ¾ Í Ö ÄÇ ( 4b), Ò¼ C Ê ÛÃ, Û [7]. µ Ë Õ ² Í Ð ² [9], Å Õ Đ Fe C ÛÃ [8], Ò ¾. µ, µ ËÍ Ê µí Ó Á ÙØ, ¹µØ Ú ÊÄ Ä Í Ó ß² Í Đ Í, Ò Ú ¼ÀÍ Ó ² Í, ÛÃ C Ú ÐÍ Ë Æ¹ Ë. µ ¹ µ Ë µ Ê [21,23,24]. µ Ë µ, Â Ú Í Ë, Ú µ¹ïä ² Â, C ØÛÛÃ Ë [23 25], Í µ C À²Â, ²Â» Ë Cottrell º, Á, È Í È [25] ; ß Ú ²Ü ÆÇ Í, Ú ²Â Á, C ÐÍ Ë Æ Ú º ºÆ¹ [23,24]. µ µ, ßĐ Ú µ²â, ³ÓÏ. 65,.1 s 1 À½ Ë 5 Đ. Ë, 2 «ĐĐ 1, Ç Al À Đ ± Ë «. ßÝ Ë µ Û À½Đ Í Ù, [26] ÍÈ ÕÔÍÊ µ 1 Ë kj/mol, Ú Ë Ë (3 kj/mol [26] ), Ò 2 Ë kj/mol., Ú Al À Đ Fe ÛÃ Á Fe Û ÃºÖ «[14,27]. Ò¼ Al À C Û Ã [28 3], Õ, Ë 2 µ² Í Û True stress, MPa o C,.1 s -1 Steel No.1 No True strain 5 Ð 65,.1 s 1 Ê ÖĐ Ö Fig.5 True stress strain curves of hypereutectoid steels during warm deformation at 65,.1 s 1. Õ Ú Æ¹ Í Ë Ú ²Ü ÆÇ ÓÏ «, 1, 2 Ë Đ, Ú ²Ü ÆÇ ÍÄ Ù, È Æ¹ Í Ö ÄÇ. Ç, ßĐ 2 «ĐĐ 1 ( 5), 2 Ú µ ²Â Đ, ßĐ Fe C ÛÃ, Õ Ë 2 ² Í ĐØ. Å Õ, Ë, 1 µ Í ÓÏ, Ú Ê Í Ö ½Á. Ò 2 µ Ð² Í ÈÆ¹, Í, Õ Ë 2 µ Ú Ê Í Ö Đ Đ 1. 2 µ ¾ ² ¼À, Í Ó ÁÄÀ, Æ² ¾ Í Ö Ñ, ± µ Ë µ Ú, 2 Í¼ À Ú. 4 (1) 65,.1 s 1 À½ Ë, µ, µ Ë ¹ ¾, Û Đ 4 Ö, ¼ Í¾¼ (α+θ) Ü. (2) Ú Al À Fe C ÛÃ, Đ ¹ Ë, ² Í Í Ú Æ¹, ¼Ó±Í, ¼ µ Ë Í ¾¼ (α+θ) Ü. (3) µ, Al À ¹ Ü µí Ó Ú Ä ¼À. Æ Ð [1] Sherby O D, Walser B, Young C M, Cady E M. Scr Metall, 1975; 9: 569 [2] Sherby O D, Oyama T, Kum D W, Walser B, Wadsworth J. J Met, 1985; 37(6): 5 [3] Oyama T, Sherby O D, Wadsworth J, Walser B. Scr Metall, 1984; 18: 799 [4] Furuhara T, Mizoguchi T, Maki T. ISIJ Int, 25; 45: 392 [5] Lesuer D R, Syn C K, Goldberg A, Wadsworth J, Sherby O D. JOM, 1993; 45(8): 4 [6] Syn C K, Lesuer D R, Goldberg A, Tsai H C, Sherby O D. Mater Sci Forum, 27; : 4844 [7] Chattopadhyay S, Sellars C M. Acta Metall, 1982; 3: 157 [8] Robbins J L, Shepard O C, Sherby O D. J Iron Steel Inst, 1964; 22: 84 [9] Harrigan M J, Sherby O D. Mater Sci Eng, 1971; 7: 177 [1] Paqueton H, Pineau A. J Iron Steel Inst, 1971; 29: 991 [11] Kaspar R, Kapellner W, Lang C. Steel Res, 1988; 59: 492 [12] Peng H F, Song X Y, Gao A G, Ma X L. Mater Lett, 25; 59: 333 [13] Lesuer D R, Syn C K, Whittenberger J D, Sherby O D. Metall Mater Trans, 1999; 3A: 1559 [14] Frommeyer G, Jimenez J A. Metall Mater Trans, 25; 36A: 295

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