BEHAVIOR OF MARTENSITE REVERSE TRANSFORMA- TION IN 18Mn TRIP STEEL DURING WARM DEFORMATION

Ð 46 Ð 10 Vol.46 No.10 2010 10 Þ Ð 1153 1160 Ì ACTA METALLURGICA SINICA Oct. 2010 pp.1153 1160 18Mn TRIP Â«É ÓÙÞÔ Â ( «Õ² Û, «100083) Ñ Ò Ê ¼ XRD «EBSD À Æ ³Â «18Mn 100 500 Ð Ä Â ß. Ð Ï, 300 Ï, TRIP,  Ò, Á ; Æ ± ¼, bcc Â Ì Ä ± «, «² ȵº ; ¼«Á Ä Á Í, ÍÀ  ; Ï, À¾  Á {110} {100}. bcc Â Ü ¼ hcp Â. «ß, hcp  Þ. Ê 18Mn,, TRIP,  РРTG111.5, TG142.33 ÄØÒ A Á 0412 1961(2010)10 1153 08 BEHAVIOR OF MARTENSITE REVERSE TRANSFORMA- TION IN 18Mn TRIP STEEL DURING WARM DEFORMATION LU Fayun, YANG Ping, MENG Li, MAO Weimin School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 Correspondent: YANG Ping, professor, Tel: (010)82376968, E-mail: yangp@mater.ustb.edu.cn Supported by National Natural Science Foundation of China (No.50771019) and Specialized Research Fund for the Doctoral Program of Higher Education (No.20090006110013) Manuscript received 2010 06 13, in revised form 2010 08 12 ABSTRACT High manganese steels show significant potential for industrial application due to their remarkable TRIP/TWIP effects at room temperature. The study on the TRIP behavior during warm deformation is important in controlling microstructures and properties of high manganese steels. In this paper, the microstructures, phase structures and reverse transformation of martensites to austenite in a high manganese steel which is composed of two types of martensites and austenite were investigated under warm deformation (100 500 ) by means of the determination of transformation temperature, calculation of phase diagram, microstructure observation, XRD analysis and EBSD orientation imaging technique. Results show that during compression above 300, TRIP effect disappeared and reverse transformation from martensite to austenite was enhanced. The transformation from bcc martensite to austenite was determined to be diffusive and no nucleation of austenite was needed. The warm deformation of austenite leads to the formation of coarse deformation twins and the mechanical stabilization of austenite, which suppressed the subsequent martensitic transformation during quenching. The austenitic grains in which reverse martensitic transformation completed at the latest, show mainly {110} and {100} orientations. In addition, hcp martensite could hardly be detected around bcc martensite, and the transformation of hcp martensite into austenite is regarded to be reversible and diffusionless. KEY WORDS 18Mn steel, warm deformation, TRIP effect, reverse transformation of martensite * Å Î 50771019 ³± Ñ ÀÎ 20090006110013 ¾ Ø : 2010 06 13, Ø : 2010 08 12 «Ð :,, 1985 ±, ± ± DOI: 10.3724/SP.J.1037.2010.00283 Ê TRIP. ¹, TRIP Ë, ¹ ² ¼ TRIP [1] ; ¹, ¹ Ã, È ¹ µô TRIP [2,3]. Ó½

1154 Ð 46 TRIP Ñ ² à ¾Ò Ü M s à ¾Ò Ü M d. Î Ü, TRIP µ ٺر, ± ÎÆ» [4,5]. Ì 2 à ¹, hcp Ã, ; α à Å, ³. Á [6,7] Fe Mn Si ± Ñ ε à ². ÆÖÆ [8 10], 304 ³ ± Ñ Ã 400 Ð ²Ã., ß ¹ à Á. Á à ¼ TRIP µ 18Mn 100 500 ¹, È ¹ Ü Ý Ü; ε α à ; TRIP ± ±, Ò ; ¹ Ò». 1 Ï Ê 18Mn, Ù ( À, %) : Fe 17.48Mn 0.0045C 3.04Si 1.80Al 0.0058S. Ù Ö 1050, 0.5 h Á Þ, ¾ Þ Ü 1050, ²Þ Ü 800, ÞÁ. «ÁÙ 18Mn ÆÐ 6 mm, 10 mm Ú½ Ç, 1100 1 h ÁÁË. Á, 100 500 ºÅ. CMT4305 Ô Ù Å, 10% 30%, Ç 10 2 s 1. Gleeble 1500 10 /s Ç 100, 300 500 Á 10%, 20%, 30% Á, ÁË, Ç 10 2 s 1. ÈÚ½ÇÔ Í½ Å. º ËÊ DIL805A È ± Ò. D/MAX RB Æ X à (XRD) Ç Ô± Å; «È D5000 X à ÇÔ Å. Thermo Cal ¾ Ë Ó. 5%( À) É ÍÔĐ ¾Á, Leo 1450 Zeiss SUPRA 55 ÎÔÆ (SEM) HKL Channel 5 Ô Ã (EBSD) ß ÇÔ Å. 2 2.1 18Mn Ú Ä, É ØÙ Ò. Ó 1a 18Mn 1100 Á ËÁ. ÕÌ Ã, Ò±Å Æ hcp Ã, ÍÝÐÆ bcc Ã, ÑÚ. Ó 1b 30% Á, Õà Ð, Ã. EBSD ÔØ 3 Ì S N» {111} γ /{0001}ε, 1 10 / 11 20 ε Burgers» {0001} ε /{110} α, 11 20 ε / 1 11 α, Ú³ K S» [11]. 2.2 ÅÃÌÇÍ ÅĐ Û Ó ÜÁ ( ³ Ü), Æ É Đ Å. Ø 18Mn ¹ Ò Ë Õ Ó, ÕÓ 2 Ó 3. Ó 2 ÞÁ ÇÔ» ¹. Õ 150 200 ¹, ÇÔ ², ß Ô bcc Ã Í Å ; 600 700 ², ß Ã Í Å. ¹, Ç ÅÒ, Ý 1 18Mn Æ Fig.1 Microstructures of 18Mn steel at room temperature Relative length change (a) undeformed (b) compressed by 30% 0.025 0.020 0.015 0.010 0.005 0.000 1 Heating 0 200 400 600 800 1000 Temperature, o C 2 18Mn ³Ú Í Ä 2 Cooling Fig.2 Length change of 18Mn steel during heating and cooling (heating at 10 /s and cooling at 5 /s)

Ð 10 ß : 18Mn TRIP ¾ Á Þ 1155 Temperature, o C 1600 1400 1200 1000 800 600 bcc bcc+liquid bcc+fcc+liquid fcc+liquid fcc bcc+fcc bcc+cementite Liquid 400 bcc+m 5 C 2 bcc+cementite bcc+fcc+m 5 C 2 bcc+fcc+cementite 200 0 2 4 6 8 10 12 14 16 18 20 Mass fraction of Mn, % 3» Termo Calc Ê 18Mn Ò Fig.3 Phase diagram of 18Mn steel calculated by Termo Calc 600 ÑÅ, ² Å, ß hcp Ã Đ α Ã, ½ Ã, α Ã È hcp à ±² (Ó 1). ¹ Ó, ¹ hcp à ¹ ( Ò À Ë), Đ α à ¹Ì, Õ, Î ÜÛ ºØ, Æ ³ ½. ¹, Ô Ï, hcp Ã ß α Ã Í Å, ¼. 600 α à ŠÁ, «³ à Ò. Ó 3 ¾ Thermo Calc Ë 18Mn Õ Ó. Õ, 18Mn 700 ¾Â bcc, Å,  C, Mn 20% ¹, ³ Ø, 18Mn. Ó 3 Æ, 500 ³ Ø, Å Â, º, Å, ÂÀ³. ¹ 100 500 ¹, Ì Ã Í Å, Å Þ. 2.3 Å Î Ü Ã ËÅ È Ó 4a  18Mn ³ Ü Á Ø. Õ, Ì ε α à Ð, Ù±» (A); 30% Á (B ), Ó, α à Ç, Ï α à ; 18Mn 100, 300 500 30% Á,, α à Ó, (C, D E). 500 Á,», {110} γ à ÞÝ, Ø (E). Â, Þ Ü, Ã, TRIP µ Ó, Þ Ü ²Ã Intensity a.u. True stress, MPa (a) (100)hcp (111)fcc 1200 1000 (110)bcc (101)hcp (200)fcc (102)hcp (200)bcc (220)fcc (211)bcc (311)fcc (222)fcc E D B A 40 50 60 70 80 90 100 2, deg 800 600 400 200 (b) 0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 True strain 4 18Mn XRD Ö Æ ½ Æ Fig.4 XRD patterns (a) and true stress true strain curves (b) of 18Mn steel (A undeformed at room temperature; B compressed by 30% at room temperature; C compressed by 30% at 100 ; D compressed by 30% at 300 ; E compressed by 30% at 500 ). Ó 4b 18Mn ³ Ü ¾. Õ, Î Ü, 18Mn ÝÜ Å Û, Å ß, Ü, à Ó, TRIP µ ºØ±. 2.4 Å Î Üà ÖÀ Ë Ó 5  18Mn ³ Ü 30% Á. Õ, 100 30% ¹, à ÐÀ, Á ³ ; 300 ¹, à Ó, Ç TRIP µ, Â Þ Ü Ç Ã ; 500 30% Á, à ΠÓ, Ø ±, Æ α M Ì. Ó 5, α M ÝÙ³ ε (Ó 5c ±Ö ) Î. Æ Â, ¹, hcp à Ù. Ó 3, 500 fcc bcc Đ, ³É Î, bcc Ì, Ã É ÈÊÃ Í Å. C E D C B

1156 Ð 46 5 18Mn ² ÛÆ Ï 30% À Fig.5 Microstructures of 18Mn steel compressed by 30% at 100 (a), 300 (b) and 500 (c) Ó 6 18Mn 300, 400 500 30 min Á, ÁË Æ. ÇÔ, à Ð. Ó 6a 300 Á, α M, Ù³ à ± ²Ñ È Ê, Æ Ç Å. Ó 6b, 400 Á, ±Â Þ ÐÆ Ã (Ö ), à ÈÊÃ Í Å. Ó 6c, 500 Á, ± ÈÊÃ Í Å ± ( Ö ), ±Æ Þв α M( Ö ), ±» ĐÇÃ, 500 ¹, à ², ÈÊÃ Í Å, ±Ò» Ã, پà Æ. Â, 18Mn 300 ¹, à ² ; 300 Ñ, Ç ¼ ²Ã, 500 ². ¹, Ó Ì, 6 18Mn ² Û Fig.6 Microstructures of 18Mn steel heated for 30 min at 300 (a), 400 (b) and 500 (c) Á³É» à ; Á, ±Â Ã. Æ ±, Ê Ã ; ¹, ± Ì Ò Ã. 2.5 Õ ßÆ Ã Ñ Ã ÒÕ [12]. Ó 7 300 30% EBSD. Ó 7a, ż± µ е ÃÐÆ Þ α M. È α M Ó (Ó 7b c) Â, ż Ø {110}, µ {100} {111} 6 α M, Ù± {100} α M ¹Þ Ø, Ù ² Ù. α M Þ Ä,.  α M Ù³ à ± ³

l 10 s ~7 18Mn {g : 18Mn TRIP 27B#I8`b 5z 300 1157 EW 3 30% e EBSD? Fig.7 EBSD orientation maps of 18Mn steel compressed by 30% at 300 (a) orientation map (grey denotes α M, red denotes ε M, other colors denote austenite) (b) pole figures of austenite (c) pole figures of α M f, ~' 7 4 I+ 9 D %* /4 f. 18Mn 300 4 -, x (, 6W ` (, γ α M 8 $^, P~ & α γ f8, $ k+ :b 0%3>"6 α M. o 8 18Mn 500 F X 30% I f EBSD!, q α M b k T C } EBSD M, J X U e C Z. o 1a < B, " α bb f [ ue 7 ε M, v 7 s Z i f :b!,.o 5c 1 8f ~6. < r, 1 8 ZFVf 4 G (o 8b % f O G, o 8c [ oh ), 7 < 4 -~ 1 8 f. G _ k " 6 f hcp K :b (q o 8b % f G & n), G * i f {111}[ 7 hcp K :b {0001}f. N r, " K :b b 4 N Z b O k, Z E o emb (U PRK :b). z f :b _ f! M Z o z g f K :b, J X 4 / Q & :b _. $ > Z 6 α M f :b j Miller H! (Z y 7 {103}), s u G / f {110}! :b _O k K :b T. = ; _ / Q k 1! hcp K :b b; k 4 ( α M b (o 8b? e), " {100}, {111}? 2 ( H!, jo * K :b7 u + - 4 f, ~8? 18Mn 500 EW 30% He va \ EBSD Fig.8 Microstructure and EBSD orientation maps of 18Mn steel compressed by 30% at 500 (a) microstructure of the steel (b) orientation map (red denotes ε M, grey denotes α M, other colors denote austenite, yellow lines denote twin boundaries) (c) pole figures of austenite (d) pole figures of ε M (e) pole figures of α M - k% + f _. P o 8b % } X &, ~!', ~ α M! :bf8 7& 9D!f&>9D, ~Ok/ :bf4=. α M

D o 9 18Mn 500 F X 30% I< V t f!, U ST CZf EBSD M (q o 9b), J X e C Z. o 9b E x :b 0 K {123}!, _ + K :b C "; l x K {317}! f 1158 ~9 500 EW 30% He va \ EBSD? G 18Mn Fig.9 Microstructure and EBSD orientation maps of 18Mn steel compressed by 30% at 500 (a) microstructure (b) orientation map (red denotes ε M, blue denotes α M, other colors denote austenite) (c) pole figures of austenite (d) pole figures of ε M (e) pole figures of α M l 46 :b _K :b z } (o 9b? e), D ^ % +. [13] b S! X, {100}! f :b Fe 2 f G i _ 4 K:b. " d5`f7, sk:b Vvf : b ({223}) 7u :b ({317}) Z fg!, JXu j :b8 α M -G&f*Q8, -S7d8 fu :b!.! EBSD! %, vo k18 VY!f&i- :b, 4 f :bsuz :b 9 b 'Z, N Æ O k J 4 =9 D, ~ 7 J 9K:bUeMbfd!&>. Po 9e f[oh J, T 4 ( α M!, 2 ( {110}? 2 ( {100}. \ 2! EBSD H v! X, α K :b,y J Z 6 M, JX M4 6CZ, [Q7 hcp K:bN7d8 f :bk\ V*. H*Æ, hcp K:b! :b f 8 7 d f$ 7 d z", α K :b! hcp K :bu :b8 7' d f, ^& 9D!f. o 10! 5 9 18Mn ~ 4? 4 I f, L 9D. jo 18Mn u2. Op%7 :b, α M? ε M * T, N ^ \i X # 5 6F1,. o 10a ~ 10 18Mn } 3> 3 H 9a ( Fig.10 Austenitic orientation distribution in undeformed and warmly deformed 18Mn steel (a) austenite orientations in undeformed 18Mn, inverse pole figures (level max=1.2) (b) macrotexture of austenite after compresed at 500 by 30%, ODF φ2 =45 (c) orientations of austenite in 18Mn steel compressed at 500 by 30% inverse pole figures (level max=2.0)

Ð 10 ß : 18Mn TRIP ¾ Á Þ 1159 EBSD Ò Ø, 230 ż., Ù¾ 18Mn ± µ ÞÎ, Þ. 500 30% Á, Ï, Ã, Æ X à fcc, Ó 10b. Õ, Þ² ² Ø {110}, Æ Þ {113}, {110} Å. 500 ½ ÞÉ Å, È {110}. 18Mn 500 Á, Æ Å¼± Ã, ß Ç Å¼, Ù 10c. Õ, 500 Á à żÐß {110} {100}, {110} ÞÐ, ¹ Ø Î. Ù¾ ¹ Þ, 500, Ü, Ï, TRIP µ, ²Ã. ±»É ²Ã Æ ², ƹ ºØ {110} Ù, {100} ± ² ². ÜÍ Å, α M à ² ½, à ºØ Õ ²Í± ; ÈÙ {110} Å ÞÐ, ³ ² ÍÛ, È {110} ±Ã ³. 3 ÝÑ TRIP Á [8 16], Î ÜÍÇ ³, ß, Ë ², Ç ºÓß ½. Á, ºÅ 3 Ò³ : Î ßÓ Á ¹ Ã, Á ß. Đ ßÓ Á Î Ã, ε γ Í α γ, Á 2 Ã Ì ¹,» ± ε Ã. 18Mn ±, ¹ α È ε ±, ½ ε à ; ¹, Þ ½ ε Ã, ÙÙ ¹ ε Ã Þ ; Í ε à Þ, ¹ ², EBSD Å Ò, Ø. ¹ Ì Ï. ¼ Ø 18Mn ± α γ ² ½. Ì«Ã ÌÜ ², Þ Â ε Ã, ½ Á [6,7], ε Ã, ß ³ Ü ÇÔ,  ßà 3 Í : (1) É ² Å, ßß ½ ³º ; Ó ÜÞ, Ï, à Ð, ß ½. Þ Ü (300 ) ¹, bcc à ß, hcp à ¾Ü ÐÀ. Ê ² ½, ¹, 500 à º Ó. ½Ã Ó Ü 500 Û 300. (2) Á ¹ à ; ß 30 min Á, É ²» à (Ó 6). µ Î ßËʳ Ã È TRIP Ü ³. ĐØ, Ø,, ½ ¹ TRIP, Ñ È. (3) Ç ¹, ² ¼ Î Å. 18Mn Ï, ÜÌ Ï, ½ hcp à ÇÜ Ó Å ÇÜ; TRIP ±, Ì«Ó Å, Î ² à Â. Ü Á, ¹Îà ±, à ¼, ²³Đ Ã, ² ¾ Û,»É. Î Å Ë Î Ü Å. Î Å Í Å Mn TWIP (25 30Mn) ¹Ë Ç ½, Æ Ù ± ÔÊ Å» ³ Ã. ÅÆ, ÜÁ ¾Ü, Î ε Ã, µ γ ßßÄ α à ß, ½ ÁË ¹, α à ڳ È ßÅÂ, ε à ; Æ, µ α à ², Â Ê ² ½. 4 Ñ (1) Â, 18Mn 5 /s ¹ Ý 600 ² γ ε α Ã, Ò ²ÐÛ Å, ¹ 200 ² Ô bcc Ã Í Å, 600 ¹Â Å Í (ÈÊ) Ã, ² ¼ Å, α à ² ½. (2) γ, ε α Ì Ù¾, 100 ¹ TRIP µ ; 300 ¹, TRIP µ ±, à Ó. ² Å Ç ² ½ α /α γ, α ε Á ¹ γ ε α à ; ½ ² ¾Î Ü Û, à Ó,  ΠÅ. (3) Ã»É ε à ÈÙ, α à ÈÊÃ Í Å ÁĐ ² Ù, ³». ¹ Ï ³ ¼. 500 30% Á, Á α»é

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