REGULATION OF RETAINED AUSTENITE AND ITS EFFECT ON THE MECHANICAL PROPERTIES OF LOW CARBON STEEL

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1 Λ 48 E Λ 9 2 Vol.48 No ! 9 K Λ ß ACTA METALLURICA SINICA Sept pp cξ$ ρ`e)lf.rsqy) p ffl»νfl Ωfi ( I ) w 14:Nbx + t ENμT bx & () TWeD$ν 8/rWe zvxb 8 F0ff- ΦΠ-"ΩS%);F Π-4( (}Wy5> Π-}HDyß l. h4 SEM XRD "Ω EBSD }ν'4twe# 8y>O l/rx"d. %Xhy 3ß l g8v5> Π-yzv ffi_;"stπ: fi t ENμT# χ ffi Π- lvy0 Mn 0 C fl bx &# 5~ Π-P&ZIvyfl]0 C 2Zf yt+ Dff0 WeQ" 3 &8P FSzv@Z 10% ]vy5> Π- ff5~ Π-P&ZIvyfl]0 C Uν3b8v 5> Π-ypDΩ4P FSyHffs7sxqM yν4. ;SA)y5> Π-PpffiZIvy TRIP i v 3b8zP nffipzivzvxk yμd.sfim _ffywx=ffiaffiuyr`%g. 7Ψ%X" 8y=ffiΞ@ Zx 2 Pa % J-=ffil; 00 MPa K =ffi@z 900 MPa LffnP 1% "f =FfflkO}Vuisx 39 J. ;H' &8 5> Π- ΦΠ- F0ff- TRIPi Λh03O@ T113 koffi_x A k fi@ (2012) REULATION OF RETAINED AUSTENITE AND ITS EFFECT ON THE MECHANICAL PROPERTIES OF LOW CARBON STEEL REN Yongqiang XIE Zhenjia SHAN Chengjia School of Materials Science and Engineering University of Science and Technology Beijing Beijing Correspondent: SHAN Chengjia professor Tel: (010) cjshang@ustb.edu.cn Supported by National Basic Research Program of China (No.2010CB30801) Manuscript received in revised form ABSTRACT The development of high performance steels needs to realize the combination of high strength high plasticity and high toughness. Multiphase microstructure which contains a specific proportion of retained austenite is conductive to enhance the toughness and plasticity of the steel. Making use of the quenching+intercritical reheating quenching and partitioning () process a multiphase microstructure which was composed of intercritical ferrite martensite and well distributed retained austenite (primarily distributed in the prior austenitic grain boundary and the phase boundary) can be obtained in the steel. By means of SEM XRD and EBSD microstructures of the steel in different heat treatment stages were characterized. The results indicated that the obtention of the retained austenite was mainly dependent on the following two stages: the first one is the enrichment of the carbon and manganese in the reversed austenite during the intercritical reheating process; the second stage is the secondary enrichment of carbon in retained austenite during the following quenching and partitioning process. After the two stages of element enrichment treatment more than 10% volume fraction of retained austenite was obtained and the second stage of treatment plays an important role in the formation and stabilization of the metastable austenite. Due to the strengthening and toughening effect of the widely distrubuted retained austenite this kind of steel obtained a continuous work hardening ability and thus achieved a good combination of strength and plasticity. Test results W Π VΨ;χY rflπaν 2010CB30801 ffsr< 3 : ffsy5< 3 : ν^ρ : Z3= ß 1983 "r & r DOI: /SP.J

2 Λ 9 2 Y2< : %7u4=±ΞxΦTΨμl tffx_ 1075 indicated that steel treated by the process shows excellent comprehensive mechanical properties: the product of strength and elongation is greater than 2 Pa % the yield strength and tensile stength is more than 00 and 900 MPa respectively the uniform elongationg is above 1% and the half thickness size impact toughness at room temperature reaches to 39 J. KEY WORDS low carbon steel retained austenite martensite intercritical ferrite TRIP effect ;vfl91/ zψzο!λ;>ffl ;Wvd;χ v. flfffifl ;9/>ffldWvz 9j'!ΛR5 >tb]χ>t. 1#^ Q;9/χv' UoψZ y DP(ν]) [12] TRIP(]χ:rχv) [3 ] TWIP(± 5:rχv) [7 9] #ff Q&P(cy +') [1011] ~ρ UXfE%. Ω< [1213] #» TRIPj+Λ;91/ χvz9j'!. 25(5zE+m #ffffi fi» ο9wz?φ±.0s»mψuebx.zω<u± ψzyc9 TRIP j+z ρmo9 `1fi. ο ±.9 [1415] Ψ±.9 (Q&P 9) [ ] ο±. 9 [21] #ffwjψ±.9 [22] ~. Qi5 TRIP j+w bnj+q<`f0flq@ c.wwtfficfiifiz?φ±. 5wομßLχ 9wz C ^w# i5wvdr_v. VS 25f E+m `fψu.wzρ]»m #ff`f0fl Q.wwtfficfiIfiz?Φ±.Λ5wzΩ<} ± [ ]. ρeο=^ C wb 0.23%(tw+ ) z Mn Si 9 w?φ±.z»mψue%ff5nλsb0sy(# dω<. 25u]FOνUE%ΨU9wψΦχΦ±. z C $ffl #D w>-1 C z:l. 0fi3[c y +'Xf ywt7φ1fi. Ψ±.# ffffic+z?φ±.iezρ]»m. i5d : (SEM) X k[fflk (XRD) #ff ffiνckfflk (EBSD) ~ρ fi»ο4c9(5uxf$»z?p» m0sy#e 5w=9w?Φ±.οnΛvflz-` 0sy!. 1 ^v"ua1/ z kg bs + νq tλe+ (tw+ %)B: C 0.23 Mn 1.80 Si 1.35 S P Fe?w. ffi±ω SE 0 mm 80 mm 240 mm z?') }mνut 1250 Ξ HMt 1 h 1100 ΞIX 8[?u^UXqE mm lz9fi. yxfflb 80 Ξ XoSdrΦ. 25U (!8w4c9z A C3 =84 Ξ A C1 =742 Ξ M s = 338 Ξ M f =28 Ξ (5w A C3 ΛνUwUN1fi. O-ΦχEΦ±.zyyffl A C1 ΛνUw R.dΦ ±.ΦχzI~ffl M s ΛdQwΨ±.Φχz8~ ffl M f BdQwΨ±.Φχzyyffl). nλvflz $zzω B`9figffiXd?=E z mm 20 mm 180 mm ff). #z 8fiz A C3 i 1 8yTWeD$ I Fig.1 Schematics of Q (a) and Q IQ IQ&T (b) treatments applied to the steel (T 1 =800 Λ T 2 =750 Λ T 3 =320 Λ T 4 =280 Λ T 5 =770 Λ T =300 Λ T 7 =300 Λ t 5 =15 min t 7 =15 min) A C1 M s d M f fflb3m 25`7 1a $ E%0 suxf (). flu ffiω Q^ wνut 900 Ξ 30 min }k;oφ±.t oοcrφ wtoψ±.»m (Q E%); ffi O^νUt Ξzu]Fρzμflffl (ρz wm 770 ΞOν U 15 min) Q4fflT}ksE)fiffjzΦ1fi. #ffψφχφ±. %3[ }ψφχφ±.1 Mn d 1 C "oοcr ΞkοfflFzμfl)fi ffl (ρz wmcyr 300 Ξ By3Rflffgz M: Qcy5οwνay)fiffjz?!P[ UscyfflQ Ξkοz/HdflB 100 Ξ/s 25 # w s cyomt 5hoz8;8) OΠfl( a^ w 300 Ξ~Xf 15 min osdrφ ( E%). Byfl<ο4c9@$»wz»mχt UΞffnΛsB0sΩ< 4Q Ω 5 μy 900 Ξ

3 (107 S([(V(Q A 30 min 102 (Q)900 C C> /02 (IQ) 900 C C> / C? A S 2 (IQ&T) R )M 1b 7 JE. 4T?5RID L QT+U 3 50 mm + FVPLQ 5 mm 10 mm 55 mm + USTUM Charpy V QWVT (Charpy impact test) L Q. A VP8HWW B/T VP5. X R s 1 ; A VT8HW W B/T HVPLQXXUVWW AIQF I4STO 3% \] SNUXYV <E Z 9. 5%Y Oxford EBSD ^Z_ ZEISS ULTRA 55 Z75FA (SEM/EBSD) 7 )5 - &2.B C H 9 F W @12H[3F EBSD 5\B D DMAX RB 12KV ` Y[ X 7 B E7Z (XRD) 7) =:3 H 5 F[ 40 kv F X 150 ma \\ a \ E7X1- $ E7X 4]]7K/7B; XRD = ^1 20b [28] _+2;$71 (α) 801 (γ) > YZ= Kα Kγ 5D (1) 0bY+LE7X `3= BJ;788011`3=3< LX `3=L^ ) ^ 48 Vi = 1 1+ (1) Ia K γ Iγ K α D7 Vi %<LX `3=; Iα Iγ 3 ]3)7$71801)#E7Xa` + ;14 2`Z/ [28] 0b;O (2) 120b [29]: ω(c)γ = (aγ )/0.004 (2) ) D7 ω(c)γ % C?:3 = %; aγ % 78801)Z^= nm ;14 Cohen U[_ / [30] b\801<le7x;. /`KS]_O % C: Si Mn 7: )Z^ = D K : C <U;4 ]/D7`5KZ Si Mn D [29] /:; <=>?@ABCDEF 3P:7)/ 53<3[1B79 SEM P 01 (IQ) +770 A (IQ&T) 3 2 M 2 JE. M a0ro1 2 4/ 4 7 < (Q) C> / C L5 532 > / C? S2.1QF 9 F W1 3F ) C ; c ID]]022.)a4OFc 01 9b E S F (M 2a); ID1 d > / 15 min A?&10 2-B;LQ Cb DA$71 [31] (IF) (M) (M 2b); M 2c 3M 2b J75LQ e 2 Q IQ IQ&T SEM Fig.2 SEM images of the steel after treatments of Q (a) IQ (b) IQ&T (c) and enlarged image of the square in Fig.2c (d) (IF intercritical ferrite M martensite TM tempered martensite)

4 9 & # : $ %'%(& '&%&''! ')" / 300 C?AS21 9FW M 2d 3M 2c 7b\.d/cS2 01 9Qc ^ C M 2d ;c / 300 C7 ) min _+S ?dS2-.;e`V <4O-7Æ HOf0g. M 3 3 I 532. (M 1a) C 1.8Mn 1.35Si ) <C 9. C M 3a ;c df8 0 1 )e h ID 2.1)7 1 9/ X IQ 2.1)b DA$71F W (M 2b). b\ M 3b c 1- M 2 ;c.@ i 3 2) K ) 7dD A$71 [31] (IF) - f 0 gh OS2 0 1 [232] (TM - H O f 0 g be -M 2d 7 j) R h ao 4 +=KL (RA) bg k S2 0 1 [32] (UM ;e[e01 [2433] ). )M 3c 7JE H I ( 1077 )a4o< 4 4S+ 9bESF. Kl'% Nc@D SEM W]a5 P<'^d 3FO@i @:--3@SF -Y 5 _K@D1+\ XRD 1- EBSD 1/+12 =3F fcm 2 M 3 SEM ;1`m dgao&2.<b7 J ; 9. ;c (e%b3'<. J )= 3 1 foi C3;c )K IQ&T ) +? fl ipr1-v1pbr>;<'( j;he )/VT!# &`.## gk )^AU MST (5 mm 10 mm 55 mm) VT!#i< 39 J fr3g'( 24 J. M 4 3 ID IQ&T 2.1)I5= I5.c B. C M 7;c / 0 8.1% I5.h n IQ&T )I5=a 4 OX d d( S 9 - ) I5=X d d(l ' (<I % i\ hn jl'1gt5.m?-jjxd?5n0 = <o :3g. 1 gj IQ&T k lpkhm nliqoe (frmogopnpopshlqj) P Table 1 Mechanical properties of the steel after IQ&T and treatments (Impact toughness values of the IQ&T and steels are obtained by the half thickness size samples) Rm Rp0.2 Agt A Rm A Akv MPa MPa % % Pa % J IQ&T Process Note: Rm tensile strength Rp0.2 yield strength Agt uniform elongation A total elongation Rm A products of tensile strength to total elongation Akv V type gap Charpy impact toughness 3 SEM Fig.3 SEM images of the steel after treatment (a) and enlarged images of the square I (b) and square II (c) in Fig.3a (RA/UM retained austenite or untempered martensite) e ^ t ir t 4 IQ&T Fig.4 True tensile stress true strain curves of the samples treated by the IQ&T and process

5 ( S([(V(Q QRSTU>?VWX CDYZ[\] ^_` abcd[\] efd^_ 7I C IQ IQ&T XRD 1QF788 2 =. Y XRD j )M 5 JE. CM7;c I IQ 532.QF7q/ ' < X. QF/ T C 15 min?as2 (IQ&T) 2.1 k =kf3'<7880 1X. jl'id>/a?&jjj? f(;=# _EID1dfB+M? A S 22. ;5leJ3l. 7: XRD j;1i? < dh[<8 01 X s C- += :;O @:3 14%. M 3ID IQ IQ&T QF `3 = C M;c 7:I IQ 2.J;) IDT1 300 C 15 min? A S2 (IQ&T) 2. )7788 I 48 O4'<R9 jl' 300 C_+S2/H2r+@u? l; ` h o L@ C :- < d( (C 1.45%). ;c I 300 C_+S21 IQ )7 J =k i(@ C : j3'@ C (78801S2m =; <(:@ C? ID 2.J ;) `3= (14%) (1.4%) % 3 )7 ( j;m]3' 02 3% (Q&P) B7k8 01 /3%Dr7[ EC C 7)7D;801F--/ AR ; = q/?<ny><. I ) ghik C cd[\]djk l mbnocpqrsrtnduv 4 7 )VP @ n1 2 3F ) M JE Kl ' % C 3.2 EBSD ^ m 7. : Æ EBSD 5\B t T / M 7 u /+=OST/ 0.12 μm 1 R d` M 7 J qvo :<?:8oQF7788 H M 7a 7 ;1p< ID 2.R1QF / w 4 Æ3@ S F; / h H@ j v ww ap bg_b ` h /f8 01 )e21-) + v x n a^^æj q bgb q\? Kikuchi st+ BC ^/c jvæjq\t+ /c-; %[E01bgS201 [230]. M 7b w 5 IQ IQ&T XRD Fig.5 XRD patterns of the steel treated by IQ IQ&T and process ^ n C IQ IQ&T Fig. Volume fraction and carbon content of retained austenite in steel treated by IQ IQ&T and process x rr r 7 EBSD Fig.7 EBSD analysis of retained austenite distribution in the steel before (a) and after (b) stretching (white corresponds to fcc lattice retained austenite)

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