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Effects of Processing Variables on Microstructure and Yield Ratio of High Strength Constructional Steels
JOURNALOFIRON ANDSTEELRESEARCH,INTERNATIONAL２ ０ １ ６,２ ３( ８):８ １ ５ Ｇ ８ ２ １
Ef f e c t so fPr o c e s s i ngVa r i ab l e son Mi c r o s t r u c t u r eandYi e l dRa t i oo f Hi t r e ng t hCon s t r u c t i ona lS t e e l s ghS J i anKANG１ , Cheng Ｇ n i ngLI１ , Xi ao Ｇ l e iLI１ ２ , J i n ＧhuaZHAO１ , １ １ GuoYUAN , Guo Ｇ dong WANG ,
( １． S t a t eKeyLabo r a t o r fRo l l i ngandAu t oma t i on,No r t he a s t e r nUn i ve r s i t i aon i ng,Ch i na; yo y,Shenyang１１０８１９,L ,Lt ,Qi ２． ShougangQi an ′ anI r onandS t e e lCo． d． an ′ an０６４４０４,Hebe i,Ch i na) Ab s t r a c t:Thep r o c e s so f“ c on t r o l l edr o l l i ng＋r e l axa t i on＋u l t r af a s tc oo l i ng ( UFC)”f o rh i t r eng t hc ons t r uc t i ona l ghs s t e e lwi t hl owy i e l dr a t i owa sp r e s en t ed．Mi c r o s t r uc t u r eandc o r r e spond i ngr e l a t i onsh i t hl owy i e l dr a t i owe r ei n Ｇ pwi ve s t i t ed．Ther e su l t sshowedt ha tt hec ons t r uc t i ona ls t e e l swi t hmu l t i s emi c r o s t r uc t u r eo fba i n i t i cf e r r i t e,ma r Ｇ ga pha t ens i t e Ｇ aus t en i t e (MＧA)andl a t hba i n i t ewe r eob t a i nedt hr ought hec r e a t i vep r o c e s s．Theg r a i ns i z ede c r e a s ed wi t h t hede c r e a s ei nf i n i shr o l l i ngt empe r a t u r e,wh i chenhanc edt hes t r eng t hbyt heg r a i nr e f i nemen ts t r eng t hen i ng．The r ope rr e l axa t i ont r e a tmen tp r omo t edt heba i n i t i cf e r r i t el a t hwi d t handt hef o rma t i ono fb l o cky MＧAc ons t i t uen t． I n p add i t i on,bo t ht het ens i l eandy i e l ds t r eng t hi nc r e a s edwi t ht hede c r e a s ei nf i n i shr o l l i ngt empe r a t u r eand UFCf i na l t empe r a t u r e,bu tt hey i e l ds t r eng t hi nc r e a s ed mo r es i i f i c an t l hant ens i l es t r eng t h,wh i chc aus edt hei nc r e a s ei n gn yt i e l dr a t i o．Byus i ngt hep r o c e s so f“ c on t r o l l edr o l l i ng＋r e l axa t i on＋u l t r af a s tc oo l i ng”,t heexc e l l en tc omp r ehens i ve y me chan i c a lp r ope r t i e so f７８０ MPag r adec ons t r uc t i ona ls t e e l so f１２－４０ mmi nt h i ckne s swe r ea ch i e ved． Ke r d s:c ons t r uc t i ona ls t e e l;l owy i e l dr a t i o;h i t r eng t h;u l t r af a s tc oo l i ng;mu l t i s emi c r o s t r uc t u r e ghs pha ywo
Wi t ht her ap i d deve l opmen to fs t e e lf r amed bu i l d i ngs,t heupda t eandupg r ade p r o c e s so fc on Ｇ s t ruc t i ona ls t e e l si simpe r a t i vei no r de rt oove r c ome r ob l emssucha sr e s ou r c e,env i r onmen tandene r Ｇ p t c． I ti sve r c e s s a r odeve l oph i t r eng t h gy,e yne yt ghs c ons t ruc t i ona ls t e e l s wi t h exc e l l en tc omp r ehens i ve , , r ope r t i e sf o rexamp l et heh i t r eng t ha s e i smi c p ghs [] s t e e lwi t ht ens i l es t r eng t ho f７８０ MPa１ ．Mo r eove r,
s t r eng t handl ow y i e l dr a t i ocons t ruc t i ona ls t e e lwa s t oi n t r oduc et hei n t e r c r i t i c a lhe a tt r e a tmen t( IHT) [ ] i n t oo f f Ｇ l i ne quench i ng andt empe r i ng p r oc e s s７Ｇ９ ． Thedua l Ｇ s e mi c r os t ruc t ur ecompos edo ff e r r i t e pha andt empe r ed ma r t ens i t ec anbeob t a i neda f t e rt he he a tt r e a tmen tp r oc e s si nc l ud i ng IHT．Mo r eove r,
h i t r eng t hcons t ruc t i ona ls t e e lp l a t e so f７８０ MPa ghs i nt ens i l es t r eng t h we r ea l sodeve l opedv i at hehe a t ( ) t het e chno l og i c a li nnova t i oni sa l s oo fs i n i f i c a n t i m Ｇ t r e a t m e n to n Ｇ l i n e r o c e s s HO P f o l l o w i n t h ea c Ｇ g p g [, ] r t anc et o me e tc on t i nuous l nc r e a s i ng demands． c e l e r a t edcoo l i ng１ １０ ．The s edeve l opeds t e e lp l a t e s po yi They i e l dr a t i o( i e l d Ｇ t o Ｇ t ens i l es t r eng t hr a t i o)i san we r e cha r a c t e r i z ed by t he mi c r os t ruc t ur eo ff i ne y impo r t an tpe r f o rmanc ei ndexf o re s t ima t i ngh i Ｇ ma r t ens i t e Ｇ aus t en i t e (MＧA)cons t i t uen td i spe r s edi n ghde f o rma t i onr e s i s t anc e．Thel owe ry i e l dr a t i or e f l e c t s t heba i n i t i cf e r r i t e( BF)ma t r i x,wh i chcon t r i bu t ed t heh i rr e s i s t anc et ode f o rma t i onf r omy i e l d i ngt o t ot heh i t r eng t handl ow y i e l dr a t i o． ghe ghs , , l a s t i ci ns t ab i l i t even t i ngt hesuddende c r e a s ei n I nt he p r e s en t wo rk a nove lt e chno l og i c a l p y pr [ ] s t r eng t h２Ｇ６ ．The r e f o r e,i ti se s s en t i a lt os t udyt he p r oc e s sf o r７８０ MPag r adel ow y i e l dr a t i ocons t ruc Ｇ e f f e c t so fpr o c e s s i ngr ou t e son mi c r o s t ruc t u r e,me Ｇ t i ona ls t e e lwa sp r e s en t edc r e a t i ve l i ngt r ans Ｇ ybyus , chan i c a l pr ope r t i e s and t he r e l a t i onsh i twe en f o rma t i oncon t r o lt oob t a i np r ope r mu l t i Ｇ s e mi Ｇ p be pha mi c r os t ruc t u r a lcha r a c t e r i s t i c sandy i e l dr a t i o． c r os t ruc t ur e． Fur t he rmo r e,t her e l axa t i onwa si n t r o Ｇ Conv e n t i on a lt e c hno l ogyf o r ma nu f a c t u r i ng h i h d u c e di n t oc o n v e n t i o n a lt h e r m o Ｇ m e c h a n i c a lc o n t r o l g Founda t i onI t em: I t em Spons o r edbyNa t i ona lNa t ur a lSc i enc eFounda t i ono fCh i na ( ５１５０４０６３) B i o r aphy: J i anKANG,Do c t o r; E Ｇma i l: wwwkang i an＠１２６ c om; Re c e i v e dDa t e:Novembe r１６,２０１５ j g Co r r e s i ngAu t ho r:GuoYUAN,Do c t o r,Pr o f e s s o r; E Ｇma i l:yuanguo r a l＠s i na c om pond
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２３
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r oc e s s( TMCP)．Theu l t r af a s tc oo l i ng ( UFC)s Ｇ p ys t em ( deve l opedbyS t a t eKey Labo r a t o r fRo l l i ng yo andAu t oma t i ono fNo r t he a s t e rn Un i ve r s i t he yandt UFCs t em wi t hah i oo l i ngc apa c i t sbe en ys ghc y ha app l i edt oi ndus t r i a lpr oduc t i oni n Ch i na)wa sa l s o adop t eda f t e rr e l axa t i ont oexp l o r et he mi c r o s t ruc Ｇ t u r a lcon t r o lo fc ons t ruc t i ons t e e l．Compa r ed wi t h t heconven t i ona la c c e l e r a t edc oo l i ng,t he UFC ap Ｇ l i e s t h eh i h e r r e s s u r e c o o l i n w a t e r t oa c h i e v e t h e p g p g h i oo l i ngr a t e． Th ec o o l i ngr a t eo fUFCi s２－５t ime s ghc h i rt ha nt ha to fa c c e l e r a t edc oo l i ng．The mi c r o Ｇ ghe s t ruc t ur a lcha r a c t e r i s t i c s by t h i sp r o c e s so f“ c on Ｇ t r o l l e dr o l l i ng＋r e l a x a t i on＋ UFC”a ndi t se f f e c t son i e l dr a t i owe r et h ef o cuso ft h i si nve s t i t i on． y ga
１ Ma t e r i a l sand Me t hod s
Thechemi c a lc ompo s i t i onso ft heexpe r imen t a l s t e e l sa r eshowni nTab l e１．Thes t e e l swe r ec a s ti n a１５０kgva cuumi nduc t i onf u rna c e,f o r t gedandcu i n t os r ei ngo t so f８０mmi nt h i ckne s s,１００mmi n qua wi d t hand１５０mmi nl e ng t hf o rs t e e lA ( A１,A２,A３,
A４)and１５０ mmi nt h i c kn e s s,１００ mmi n wi d t hand １５０ mmi nl eng t hf o rs t e e lB,r e spe c t i ve l n Ｇ y．Thei o t sw e r es o a k e da t１ ２ ０ ０ ℃ f o ra b o u t１ ２ ０ m i na n d g r o l l edi n t o１２ mm ( s t e e lA)and４０ mm ( s t e e lB) t h i ckp l a t e sbyaϕ４５０mmdoub l er o l lr eve r s i ngho t Ｇ r o l l i ng mi l l． I ns t e e lA,t her e du c t i onl a r e rt h a n１ ８ ％ g o fe a chpa s si naus t en i t er e c r t a l l i z a t i onr eg i onand ys t het o t a lr educ t i ono fove r５８％i nnon Ｇ r e c r t a l l i z a Ｇ ys t i onr eg i on we r eapp l i eddur i ngt hecont r o l l edr o l l Ｇ , i ngpr oc e s s andt hef i na lt h i ckne s swa s１２ mma f Ｇ t e re i s s e s．I ns t e e lB,t her educ t i ons menＧ ghtpa t i onedabovei naus t en i t er e c r t a l l i z a t i onandnon Ｇ ys r e c r t a l l i z a t i onr eg i on we r e mo r et han１５％ and ys ５２％ r e spe c t i ve l e e lB wa sr o l l edi nt ot hes t e e l y．St l a t ewi t ht h i ckne s so f４０ mm．Af t e rf i n i shr o l l Ｇ p i ng,t hep l a t e swe r er e l axedf o raf i xedt ime,imＧ med i a t e l t e rcoo l edt ova r i oust empe r a t ur e sby y wa , UFCsys t em andf i na l l i rcoo l edt or oom t emＧ ya r a t ur e．Thes chema t i cd i ag r am o ft hepr oc e s s i ng pe r out eand t he pr oc edur e de t a i l sa r e pr e s ent edi n F i １andTab l e２． g
Tab l e１ Ch emi c a lc ompo s i t i on so fe xp e r imen t a ls t e e l s ma s s％
S t e e l A B
C
０ ０６０ ０ ０６７
S i
０ ２５ ０ ２７
Mn
１ ９０ １ ８９
Nb
０ ０２３ ０ ０３８
Ti
０ ０１１ ０ ０１８
Ni
０ ２３ ０ ３１
Cr
０ ２０ ０ １９
Mo
０ ３１ ０ ２７
Cu
０ ２６ ０ ２６
Al
０ ０２ ０ ０３
P
０ ００３ ０ ００５
S
０ ００２ ０ ００３
Me t a l l og r aph i c spe c imens we r er e spe c t i ve l y e t chedby４vo l ％ n i t a lwi t h１０sandLepe r aso l u Ｇ t i on ( ４vo l ％ p i c r a l＋１vo l ％ sod i um me t ab i su l f i t e so l u t i on)wi t h６０s．The mi c r os t ruc t ur e swe r ean a Ｇ l z e du s i ngt heLe i c aDMIRMop t i c a lmi c r o s c op e( OM) y andt heFEIQUANTA６００s c ann i nge l e c t r onmi c r o Ｇ ( ) ( s cope SEM ．Spe c imenso fEBSD e l e c t r onba ck Ｇ s c a t t e r edd i f f r a c t i on)we r epo l i shedi nt hee l e c t r o l t e y c on s i s t i ngo f１２ ５vo l ％ p e r c h l o r i ca c i d,８２ ５vo l ％ abso l u t ee t hy la l coho land５ vo l ％ wa t e rus i ng a vo l t ageo f２８Vf o r２０s．EBSDana l i swa sconduc Ｇ ys t edont heSEM wi t ht hes t eps i z eo f０５μm．Th i n
F i １ S ch ema t i ci l l u s t r a t i ono fp r o c e s s i ngr ou t e g
S t e e l A１ A２
A３ A４ B
/ Th i ckne s s mm １２ １２ １２ １２
４０
Tab l e２ Pr o c e s s i ngp r o c e du r e so fe xp e r imen t a ls t e e l s F i n i shr o l l i ng /℃ t empe r a t ur e
UFCi n i t i a lc oo l i ng /℃ t empe r a t ur e
UFCf i n i shc oo l i ng /℃ t empe r a t ur e
/ Coo l i ngr a t e (℃ s－１)
８１６
７７８
４６０
６２ ５
８５７
７７８ ８１８
８３２
f o i l swe r epr epa r edf o rt hef u r t he ri nve s t i t i ono f ga mi c r os t ruc t u r eandobs e r vedbyaFEITECNAIG２
７８０ ７７５ ７７７
７９４
４７１ ４５２ ５９０
３９５
６７ ２ ６９ ４ ６７ ８
１７ ６
２０t r ansmi s s i on e l e c t r on mi c r os cope (TEM )．The r e t a i nedaus t en i t ef r a c t i on wa s me a sur ed by XＧ r ay
I s sue８ Ef f e c t so fPr o c e s s i ngVa r i ab l e son Mi c r o s t r uc t u r eandYi e l dRa t i oo fHi t r eng t hCons t r uc t i ona lS t e e l s ghS
d i f f r a c t i on ( XRD)．Spe c imenswe r ee l e c t r o Ｇ l i shed po ands c annedf r om４０ °t o１１０ °f o r２ θ．Theda t awe r e [ １１, １２] r o c e s s e da n dc a l c u l a t e da s r e v i o u ss t u d i e s ． p p Thes t anda r dt ens i l et e s ts amp l e swe r ep r epa r ed f r omt her o l l edp l a t e si nt het r ans ve r s ed i r e c t i onand t e s t eda tr oomt empe r a t u r eonanINSTRONun i ve r Ｇ / s a lt e s t i ng ma ch i nea tac r o s s Ｇ he adspe edo f３ mm mi n．Cha r no t chedimpa c tspe c imenswe r ea l s o py VＧ cu ta l ongt het r ans ve r s ed i r e c t i on．Theimpa c tspe c i Ｇ menswe r et e s t e da t－４０ ℃ a f t e rs o a k e df o r１５mi ni n t h eb a t ho fa l c oho lc on t r o l l edbyar e f r i r a t i onp l an t． ge
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２ Re s u l t sandDi s c u s s i on Op t i c a lmi c r og r aphso fs t e e l s A１,A２and A３ a r epr e s en t edi nF i ２． I tc anbes e ent ha tt hemi c r o Ｇ g s t ruc t ur e so fs t e e l sA１andA２wi t ht hef i n i shr o l l i ng t empe r a t ur e so f８５７ ℃ and８１６ ℃ r e spe c t i ve l r e ya s imi l a r．Theya r ebo t hcompos edo ft he mu l t i s e pha cons i s t i ngo fl a t hba i n i t e( LB)andg r anu l a rba i n i t e ( ( ) ( ) GB),a sshowni nF i s ． ２ a a n d２ b ． F o rs t e e l g A３t ha twa sf i n i shr o l l eda t７７８ ℃ andcoo l edby UFC wi t hou tr e l axa t i ona f t e rho tr o l l i ng,t hemi c r o Ｇ
( a)S t e e lA１; ( b)S t e e lA２; ( c)S t e e lA３．
F i ２ OM mi c r o s t r u c t u r e so fs t e e l sA ( e t ch e dby４v o l ％ n i t a l)und e rd i f f e r en tp r o c e s s i ngr ou t e s g
s t ruc t ur ewa sr e f i nedduet ot hel owe rf i n i shr o l l i ng , ( ) t empe r a t u r ea sshowni nF i ２ c ． g F i ３ shows t he d i ame t e rd i s t r i bu t i ons o f g ( r a i n sw i t hh i ha n l e r a i nb o u n d a r i e s l a r e r t h a n g g g g g １５ °)me a su r edbyEBSD． I tc anbes e ent ha tt hef r a c Ｇ t i ono fg r a i nswi t ht hed i ame t e rove r１５μmi sl imi t Ｇ edi na l lt hr e es t e e l s,andt hed i ame t e ro f mo s to f t hemr ange sf r om１t o１５μm．Thee f f e c t i veg r a i nd i Ｇ ame t e r s,i e ,t heave r age g r a i ns i z e s,a r e８ ３８, ５ ９５and４ ９３μmi nt hes t e e l sA１,A２andA３,r e Ｇ , spe c t i ve l I ns t e e lA１ f u r t he rmo r e,t hef r a c t i ono f y．
cha r a c t e r i z edbyt hei nc r e a s ei ng r a i nbounda r i e sand d i s l oc a t i ons．Wi t ht hede c r e a s ei nf i n i shr o l l i ngt emＧ r a t ur e,t hede f o rma t i onso faus t en i t e,d i s l oc a t i ons pe and d i s l oc a t i on c e l l sa r ei nc r e a s ed, wh i ch supp l y / ( / mo r equan t i t i e so fnuc l e a t i onf o rγ α aus t en i t ef e r Ｇ r i t e)t r ans f o rma t i on．Add i t i on a l l hed e c r e a s eo ff i n Ｇ y,t i s hr o l l i ngt emp e r a t u r ec anp r ov i d eg r e a t e rd e f o rma t i on [ , ] s t o r e de n e r ha tc ani n c r e a s et henu c l e a t i onr a t i o１３ １４ ． gyt The r e f o r e,t her educ t i ono ff i n i shr o l l i ngt empe r a Ｇ t ur ec anp r omo t et heg r a i nr e f i nemen tanda f f e c tt he mi c r os t ruc t ur a levo l u t i on dur i ngr e l axa t i onf o l l owＧ r a i ns wi t ht hed i ame t e rove r１５ μm i s１４ ７５％． i ngt hecon t r o l l edr o l l i ngp r oc e s s． g Th i sf r a c t i oni ns t e e lA２de c r e a s e st o８ ５９％ duet o Thesubs t ruc t ur e so fs t e e l s A２and A３ we r e t hede c r e a s eo ff i n i shr o l l i ngt empe r a t u r e．F i ３( c) f ur t he ri nve s t i t ed by SEM．SEM obs e r va t i ons g ga ( i nd i c a t e st ha tt heg r a i nd i ame t e ri ns t e e lA３i sa l l F i ４)i nd i c a t et ha tt hes t e e lpos s e s s e st he mu l t i Ｇ g d i s t r i bu t edbe l ow１５μm．Af t e rho tr o l l i ngi nnon Ｇ r e Ｇ pha s e mi c r os t ruc t ur e s cons i s t i ng o f LB and GB, c r t a l l i z a t i onr eg i on,t heaus t en i t ei sde f o rmeda s bo t ho fwh i chcomp r i s edBFma t r i xandMＧAi s l ands ys “ akes t ruc t u r e”．Th i s“ akes t ruc t u r e”i s d i s t r i but i ngi ntheBF ma t r i x． I twasLBwhent he panc panc
( a)S t e e lA１; ( b)S t e e lA２; ( c)S t e e lA３．
F i ３ Gr a i nd i ame t e rd i s t r i bu t i on s( ana l z e dbyEBSD)o fs t e e l sAt r e a t e da td i f f e r en tf i n i s hr o l l i ngt emp e r a t u r e s g y
８１８
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２３
F i ４ SEM mi c r o s t r u c t u r e so fs t e e l sA２ ( a)andA３ ( b)andOM mi c r o s t r u c t u r e so fs t e e lA３e t ch e dbyLePe r a( c) g
BFand MＧAi s l andsexh i b i t edl a t h Ｇ l i ke,wh i l e GB wa st heBF wi t h MＧAi s l andsd i s t r i bu t i ngi nt hem． Bu ti ti sf oundt ha tt he r ea r ed i f f e r enc e si ns i z e, mo r l ogyandd i s t r i bu t i ono fMＧAc ons t i t uen tbe Ｇ pho twe ens t e e l sA２andA３． Fo rs t e e lA２,t heMＧAc on Ｇ s t i t uen t s( F i ４( a))appe a ra si r r egu l a rs c a t t e r ed g i s l ands．Thes t ruc t u r eo fba i n i t ei sr e l a t i ve l oa r s Ｇ yc ened and chao t i c．Fu r t he rmo r e,t i c a ll a t h Ｇ l i k e yp s t ruc t ur eo fBFi n GBi sd i f f i cu l tt o deve l op．Fo r s t e e lA３,howe v e r,mo s to ft h e MＧAc on s t i t u e n t sa r e ( ( ) ) f i neandl ong F i ４ c ．Thecha r a c t e r i s t i ce l onga Ｇ g t edBF mi c r o s t ruc t u r ei ss e a r a t e dbye l ong a t e d MＧA p c ons t i t uen t s( F i ４( b))． g Dur i ngt hec oo l i ngp r o c e s sa f t e rr o l l i ng,BF mi Ｇ c r os t ruc t u r es t a r t st of o rm whent het empe r a t u r e de c r e a s e st ot het r ans f o rma t i ons t a r tt empe r a t u r eo f ba i n i t e．Ca rbonc anbepa r t i t i onedf r om BFi n t ot he un t r ans f o rmedaus t en i t e,wh i chimp r ove st hes t ab i l Ｇ i t faus t en i t e．Thec a rbon Ｇ r i chun t r ans f o rmedaus Ｇ yo t en i t ewi t hsu f f i c i en ts t ab i l i t anbet he rmodynami Ｇ yc c a l l s s i b l ef o rf u r t he rba i n i t et r ans f o rma t i on, yimpo t hent r ans f o rmst o ma r t ens i t eo rpa r t i a l l ema i ns yr s t ab l ea tr oom t empe r a t u r e．Gene r a l l hef o rma Ｇ y,t t i ono f MＧA c ons t i t uen tc an bede s c r i beda s men Ｇ [ ] [ , ] t i oned above１５Ｇ１７ ．Some s t ud i e s１８ １９ have shown t ha tt hec a rbonc onc en t r a t i oni n MＧAc ons t i t uen ti s h i rt hant ha ti nBFandt heave r agec a rbonc on Ｇ ghe c en t r a t i ono fs t e e l．Th i sc anbea t t r i bu t edt or e Ｇ r Ｇ pa t i t i ono fc a rbondu r i ngt hec oo l i ngp r o c e s s． Fo rs t e e lA３,l a r f e c tdens i t si n t r o Ｇ gede y wa duc edi np r i o raus t en i t eby ho tr o l l i ng de f o rma t i on a tl owt emp e r a t u r e． I na dd i t i on,t h e r ewa sno te nough t imef o rt her e c ove r fde f o rmedaus t en i t eduet o yo t heabs enc eo fr e l axa t i on．Thede f o rmedaus t en i t e wi t hou tr e c ove r f f e c tpo s s e s s edh i r denab i l i Ｇ ye ghha t i chhads i i f i c an te f f e c tont het r ans f o rma Ｇ y,wh gn , , t i onbehav i o ro fba i n i t ei e t her e f i nemen to fba i Ｇ n i t i cs t ruc t u r e．De f e c t sp r oduc edby ho tr o l l i ngde Ｇ
f o rma t i oncou l da c ta st hee f f e c t i venuc l e a t i ons i t e s f o rba i n i t et r ans f o rma t i on,andt heg r owt ho fba i n i t e subun i tsucha sBFwa sr e s t r i c t edbye a cho t he r．On t heo t he rhand,mo r l og i c a lcha r a c t e r i s t i co fMＧA pho cons t i t uen twa sc l os e l e l a t edt ot hec a rbonenr i chＧ yr , men t wh i chwa sf ur t he rmo r eg r e a t l n f l uenc edby yi t hebehav i o ro fc a rbond i f f us i onandd i sp l a c i vet r ans Ｇ f o rma t i ondur i ngt hef o rma t i ono f GB．Be c aus eo f t hef a s tcoo l i ngimmed i a t e l f t e rr o l l i ng de f o rma Ｇ ya t i onf o rs t e e lA３,t hel ongr anged i f f us i ono fc a rbon ; , wa ssupp r e s s eds i i f i c an t l sacons equenc et he gn y a f o rma t i ono fb l ockyc a rbon Ｇ r i cha r e a swi t hl a r i z e ges i naus t en i t e wa si nh i b i t ed．The r e f o r e,i ns t e e lA３,
MＧAcons t i t uen t sd i s t r i bu t edbe twe enBFl a t hswe r e mos t l i neandl ong,andappe a r edi nt heshapeo f yf , , f i lm．Compa r ed wi t hs t e e lA３ howeve r s t e e lA２ wa scha r a c t e r i z edbyi n t r oduc i ng ar e l axa t i ona f t e r r o l l i ng wi t hi nc r e a s i ng t hef i n i shr o l l i ng t empe r a Ｇ t ur e．Theha rdenab i l i t fp r i o rde f o rmedaus t en i t e yo wa sr educ edwi t ht hede c r e a s ei nde f e c tdens i t ydue t or e cove r f f e c t dur i ng t he r e l axa t i on p r oc e s s． ye Cons equen t l he coa r s en i ng t endenc f ba i n i t e y,t yo subun i t,sucha sBFl a t hs,cou l dbeusua l l ound, yf a sshowni nF i ４( a)．Add i t i ona l l tc anbede Ｇ g y,i
duc edf r omt h i sr e su l tt ha tp r ope rr e l axa t i ont r e a t Ｇ men twou l dp r omo t et heBFl a t hwi d t handt hef o r Ｇ ma t i ono fb l ocky MＧAcons t i t uen t．Th i smaybea s Ｇ soc i a t ed wi t ht heevo l u t i ono fsubs t ruc t ur edur i ng r e l axa t i ona f t e rr o l l i ngandi t se f f e c tonr e Ｇ r t i t i on pa o fc a rbondur i ngba i n i t et r ans f o rma t i on,andf ur t he r i nve s t i a t i o nf o r t h i s i ss t i l ln e e d e d ． g F i ５ showst he b l ocky MＧA cons t i t uen ti n g s t e e lA２obs e r vedbyTEM．Themax imum Fe r e td i Ｇ ( ) , ame t e rb o fMＧAcons t i t uen ti s０ ８３μm andt he a spe c tr a t i o( t he max imumＧ t o Ｇmi n imum Fe r e t( a) d i ame t e rr a t i o)i s１ ６． I tc anbef oundt ha tt heBF ma t r i xad a c en tt o MＧA cons t i t uen tcon t a i nssome j d i s l oc a t i ons,wh i ch mayr e su l tf r om uneven pha s e
I s sue８ Ef f e c t so fPr o c e s s i ngVa r i ab l e son Mi c r o s t r uc t u r eandYi e l dRa t i oo fHi t r eng t hCons t r uc t i ona lS t e e l s ghS
t r ans f o rma t i on by ashe a r me chan i sm．Mo r eove r, t hec en t r a lz one si n MＧA c ons t i t uen t si nd i c a t edby t hec i r c l e sa r ei den t i f i eda s ma r t ens i t ef r omt hes e Ｇ ( ) ( ) l e c t eda r e ad i f f r a c t i on SAD pa t t e rnsi nF i ５ c ． g I twa sa l s od i f f i cu l tt of i nd gammapha s e( f c cl a t Ｇ t i c e) i no t he rz one swi t h i nMＧAc ons t i t uen tf r omt he
８１９
SADana l i s．XRDr e s u l ts howe dt ha tt het o t a lvo l ume ys f r a c t i ono fau s t e n i t ewa son l t２ ４vo l ％ ;t hus, yabou t her e t a i nedaus t en i t ei sha rd l oundi nt hes t e e l． yf , The r e f o r ei tc anbecon f i rmedt ha tt heMＧAcons t i t Ｇ uen t scons i s t ma i n l f ma r t ens i t e,wh i chensur e s yo t heh i i e l ds t r e s s． ghy
( a)Br i tf i e l dimage; ( b)Da rkf i e l dimage; ( c)SADpa t t e r no fMＧAc ons t i t uen ta si nd i c a t edbyt hec i r c l ei n( a)． gh
F i ５ TEM mi c r o r aph so fMＧAc on s t i t u en t si ns t e e lA２ g g
F i ６showst hee f f e c to f UFCf i n i shc oo l i ng g t empe r a t u r eon mi c r o s t ruc t u r eo ft hes t e e l s．When t heUFCf i na lt empe r a t u r ei nc r e a s e st o５９０ ℃ ,s t e e l A４ha st hemi c r o s t ruc t u r eo fGB,bu twi t hou tLB． I t i ss e enf r om F i ６( a)t ha tmo s to fBFpha s e sp r e s Ｇ g en tb l ockyand MＧA c ons t i t uen t s wi t hi r r egu l a ri s Ｇ l andshapea r ed i s o r de r l i s t r i bu t edi n BF ma t r i x． yd [ ] Ba s edont hep r ev i ous l l i shedequa t i on１７ ,t he ypub Bs )wa s ba i n i t et r ans f o rma t i ons t a r tt empe r a t u r e( c a l cu l a t eda s５８１ ℃．The r e f o r e,a th i h e rU F Cf i n a l g , t empe r a t u r eo f５９０ ℃ t heba i n i t et r ans f o rma t i oni n s t e e lA４ ma i n l c cu r r eddu r i nga i rc oo l i ng．F i ６ yo gs ( b)and６( c)showt hecha r a c t e r i s t i c so f MＧAc on Ｇ , s t i t uen t si ns t e e l sA４andA２ r e spe c t i ve l l Ｇ y．Thevo umef r a c t i ono fMＧAc ons t i t uen tr e a che s１４ ７vo l ％ i ns t e e lA４,wh i chi sl a r rt han７ ８vo l ％i ns t e e l ge , A２ i nd i c a t i ngt ha tt hevo l umef r a c t i onandt hes i z e o f MＧA c ons t i t uen t sf o rmed du r i ng a i rc oo l i ng a t
h i rUFCf i na lt empe r a t ur ea r el a r rand mo r e ghe ge coa r s et hant hos eo fone sf o rmedma i n l i ngUFC ydur coo l i ng．Thesu f f i c i en tc a rbon d i f f us i on dur i ng a i r coo l i ngi ns t e e lA４c aus edsomel a r a rbon Ｇ r i cha r Ｇ gec e a s．Mo r e ov e r,t hes t ab i l i t ft he s ec a r bon Ｇ r i c hau s t e n Ｇ yo i t ea r e a sc anbeimpr oved,l e ad i ngt ot hef o rma t i on o fcoa r s eMＧAcons t i t uen t swi t hh i a rbonconc en Ｇ ghc , t r a t i on．Howeve rt hef o rma t i ono fl a r l ockyc a r Ｇ geb bon Ｇ r i chaus t en i t ei ns t e e lA２cou l dbesupp r e s s ed duet ot hef a s tcoo l i ngandl ow UFCf i n a lt emp e r a t u r e． I ti sc on c l ud e dt ha tt hecoa r s eMＧAcons t i t uen t swi t h i s l andshapecou l dbeavo i dedbyus i ng UFC． Ther o l l i ngandcoo l i ngcond i t i onsc ans i i f i Ｇ gn c an t l n f l uenc et heme chan i c a lpr ope r t i e s．Asl i s t ed yi i n Tab l e ３,bo t ht he t ens i l es t r eng t h and y i e l d s t r eng t hi nc r e a s ewi t ht hede c r e a s ei nf i n i shr o l l i ng t empe r a t ur eand UFCf i na lt empe r a t ur e,wh i l et he i e l ds t r eng t hi n c r e a s e smo r es i i f i c an t l hant ens i l e y gn yt
( a)S t e e lA４ ( e t chedby４vo l ％ n i t a l); ( b)S t e e lA４ ( e t chedbyLePe r a); ( c)S t e e lA２ ( e t chedbyLePe r a)．
F i ６ OM mi c r o s t r u c t u r e so fs t e e l sAund e rd i f f e r en tp r o c e s s i ngr ou t e s g
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２３
８２０
Tab l e３ Me chan i c a lp r op e r t i e so fe xp e r imen t a ls t e e l s Rp０２/
Rm/
Rp０２/
A１
６３５
８５５
０ ７４
１７ ５
１６７
A３
７５５
９００
０ ８４
１８ ８
２１４
S t e e l A２ A４ B
MPa
６７０ ５７０
６３５
MPa
８９５ ８２５
８３５
Rm
A/％
０ ７５
１８ ３
０ ６９
０ ７６
１９ ８
２１ ０
Akv(－４０ ℃ )/ J
１８６ １５７
１５５
No t e:Rp０２—Yi e l ds t r eng t h;Rm —Tens i l es t r eng t h; A —El onga t i on;Akv(－４０ ℃ )—Impa c tt oughne s sa t－４０ ℃．
s t r eng t h,wh i chr e su l t si nt hei nc r e a s i ngt r endf o r t he y i e l dr a t i o．S t e e l A２ ha sexc e l l en t me chan i c a l ope r t i e si nc l ud i ngh i t r e ng t ha ndl ow y i e l dr a t i o pr ghs ( b e l ow ０ ８),me e t i ngt h er e i r eme n to ft h es t r a i n Ｇ qu b a s e dd e s i Th ep e r f o rma n c er e i r eme n t so f７８０MPa gn． qu r adecons t ruc t i ona ls t e e l wi t hl ow y i e l dr a t i of o r g , h i Ｇ r i s ebu i l d i ngs a c c o r d i ngt ot heJ apane s er e l a t Ｇ gh [] edda t a１ ,a r ea sf o l l ows:y i e l dr a t i o ≤０ ８５,y i e l d s t r eng t h ≥６３０ MPa,t ens i l es t r eng t h ≥７８０ MPa, , ande l onga t i on ≥１６％．Obv i ous l hegoodba l anc e yt
o fme chan i c a lp r ope r t i e sc anbeob t a i ned whent he f i n i shr o l l i ngt empe r a t ur eand UFCf i na lt empe r a Ｇ t ur ea r e８１６ ℃ and４６０ ℃ ,r e spe c t i ve l ． U n d e r t h e y op t imi z edpr oc e s s i ngc ond i t i on ss howni nT ab l e２f o r t h i c kn e s so f４０mm,a l lt hep e r f o rman c ei nd e x e so fs t e e l Bc omeupt ot her e l evan ts t anda rdsa sp r e s en t edi n Tab l e３． F i ７and８p r e s en tt hemi c r os t ruc t ur eand gs ha rdne s sa td i f f e r en tpos i t i onsa l ongt het h i ckne s s d i r e c t i on, wh i ch i nd i c a t et he mi c r os t ruc t ur e un i Ｇ f o rmi t l ongt het h i ckne s sd i r e c t i oni ns t e e lB． ya Theme c han i c a lp r op e r t i e so fs t e e l sa r ed e t e rmi n e d byt hemi c r o s t r u c t u r e．Pr e v i ou ss t ud i e si nd i c a t edt ha t t heapp r opr i a t er egu l a t i ono fmu l t i s emi c r os t ruc Ｇ pha t ur econ t a i n i ngha rdandso f tpha s e swa sc r i t i c a lt o r educ et he y i e l dr a t i o cons i de r i ng h i t r eng t h, gh s t hr ought hecon t r o lo fcons t i t uen t pha s espe c i e s, d i s t r i bu t i onandp r opo r t i on２０ ２１ ．Thecompos i t emi Ｇ , c r os t ruc t ur eu s u a l l n c l ud e sf e r r i t e＋b a i n i t ef e r r i t e＋ yi [ , ] ma r t e n s i t e,ba i n i t e＋ma r t ens i t e,andsoon２２ ２３ ． I nt h i ss t udy,s t e e lA２ha sexc e l l en tme chan i c a l [ , ]
( a)Ne a rt heuppe rsur f a c e; ( b)１/４; ( c)Cen t e r; ( d)３/４; ( e)Ne a rt hel owe rsur f a c e．
F i ７ Mi c r o s t r u c t u r e so fs t e e lBt h r ought h et h i ckn e s s g
F i ８ Typ i c a lha r dn e s sd i s t r i bu t i oni ns t e e lB g t h r ought h et h i ckn e s s
t i ngandha rden i ngt ot r ans f e rl oad,y i e l dde f o rma Ｇ [ ] t i onb e i n st oo c c u ri nt heha r dpha s e２０ ． I nt h i sc a s e, g t hef r a c t u r ec au s e dbye x c e s ss t r e s sc on c e n t r a t i onc anb e , de l ayed．Cons equen t l he mu l t i s e mi c r os t ruc Ｇ y t pha t ur ei ns t e e lA２ha sl owe ry i e l ds t r eng t h( i nduc edby BF)andh i rt ens i l es t r eng t h (MＧAcons t i t uen t), ghe wh i chr e su l t si nt hel ow y i e l dr a t i oo f０ ７５． Cons equen t l her e f i ned mi c r os t ruc t ur e sc an y,t b eob t a i n e dund e rt heh i n t e n s i t oo l i ngby UFC, ghi yc l e ad i ngt ot heexc e l l en t me chan i c a lp r ope r t i e s．By app l i ngt h i snew p r oc e s s,c omp r e he n s i v eme c han i c a l y r op e r t i e so ft hes t e e l swi t ht h i c kn e s so f１２－４０ mm p cou l dme e tt her e l a t eds t anda rdcomp l e t e l y．
r ope r t i e s,i nwh i cht heBF ma t r i xa c t sa st hes o f t p s eand MＧAc ons t i t uen ta c t sa st heha r dpha s et o pha ensur eh i t r eng t h．Wi t hi nc r e a s i ngapp l i edl oad, ghs ３ Conc l u s i on s l a s t i cde f o rma t i onf i r s t l c cu r si nt hes o f tpha s e． p yo ( １)The mu l t i s e mi c r os t ruc t ur econs i s t i ng Whent hes o f tpha s eha ssu f f i c i en ts t r a i na c cumu l a Ｇ pha
I s sue８ Ef f e c t so fPr o c e s s i ngVa r i ab l e son Mi c r o s t r uc t u r eandYi e l dRa t i oo fHi t r eng t hCons t r uc t i ona lS t e e l s ghS
o fLBandGB wa sob t a i nedbyt hep r o c e s so f“ c on Ｇ t r o l l edr o l l i ng＋r e l axa t i on＋UFC”f o rh i t r eng t h ghs c ons t ruc t i ona ls t e e lw i t hl owy i e l dr a t i o．Wh e nt h ef i n Ｇ i s hr o l l i ngt emp e r a t u r ed e c r e a s e sf r om ８５７t o７７８ ℃ , t heg r a i ns i z e wa sr educ edr ema rkab l r ope r y．Thep r e l axa t i onpr o c e s sp r omo t edt he wi d t ho fBFl a t hs andt hef o rma t i ono fb l o cky MＧAc ons t i t uen t,l e ad Ｇ i ngt ot hede c r e a s ei ny i e l dr a t i o． ( ２)When UFCf i na lt empe r a t u r ei nc r e a s edt o , ５９０ ℃ t he mi c r o s t ruc t u r eo ft hes t e e l wa sc omＧ ed o f GB,i n wh i ch mo s to f BF showed t he pos b l ocky mo r l ogy and MＧA c ons t i t uen t s wi t hi r Ｇ pho r egu l a ri s l andshape we r ed i s o r de r l i s t r i bu t edi n yd BF ma t r i x． I nt h i sc a s e,t hes t e e lpo s s e s s edt hel owＧ , e s ty i e l dr a t i o bu tt hel owe s ty i e l ds t r eng t h and t ens i l es t r eng t h． ( ３)Theexc e l l en tc omp r ehens i ve me chan i c a l r ope r t i e sc an beob t a i ned whent hef i n i shr o l l i ng p t empe r a t u r eand UFCf i na lt empe r a t u r ef o rt h i ck Ｇ ne s so f１２mm we r e８１６ ℃ and４６０ ℃ r e spe c t i ve l y, i e ,y i e l ds t r eng t ho f６７０ MPa,t ens i l es t r eng t ho f ８９５ MPa,y i e l dr a t i oo f０ ７５andpe r c en t agee l onga Ｇ t i ono f１８ ３％ ,wi t ht he Cha r c tene r f pyimpa gy o １８６Ja t－４０℃． Anda l lp e r f o rma n c ei nd e x e sf o rt h i c k Ｇ n e s so f１２－４０mmc ana l s ome e tt her equ i r emen t so f r e l evan ts t anda r ds． Re f e r enc e s:
[ １] K．Ueda,S．Endo,T． I t o,JFE Te ch．Rep． １８ ( ２００７)２３ Ｇ ２８． [ ２] S．Go t o,C．Kami,S．Kawamu r a,Ma t e r． S c i．Eng．A６４８ ( ２０１５)
８２１
４３６ Ｇ ４４２． [ ３] Y．Fukumo t o,Eng． S t r uc t． １８ ( １９９６)７８６ Ｇ ７９１． [ ４] B．Ka t o,I S I JI n t． ７６ ( １９９０)１２０３ Ｇ １２１０． [ ５] B．Ka t o,I S I JI n t． ７４ ( １９８８)９５１ Ｇ ９６１． [ ６] S K．Kim,YM．Kim,YJ． L im,NJ．Kim,Me t．Ma t e r． I n t． １２ ( ２００６)１３１ Ｇ １３５． [ ７] J．Ka ng,C．Wa ng,G D．Wa ng,Ma t e r．S c i．Eng．A５５３ ( ２０１２) ９６ Ｇ １０４． [ ８] G．Ro s enbe r S i na i ová,L ′． Juha r,Ma t e r．Sc i．Eng．A５８２ g,I． ( ２０１３)３４７ Ｇ ３５８． [ ９] P．T a o,H．Yu,Y． Fa n,Y． Fu,Ma t e r．De s． ５４ ( ２０１４)９１４ Ｇ ９２３． [ １０] K．Ueda,S．Suzuk i,S．Mi t ao,Ma t e r．Sc i．Fo r um ６３８ Ｇ ６４２ ( ２０１０)３５５５ Ｇ ３５６０． [ １１] X D．Tan,YB．Xu,X L．Yang,D．Wu,Ma t e r． Sc i． Eng．A ５８９ ( ２０１４)１０１ Ｇ １１１． [ １２] K．Sug imo t o,N．Usu i,M．Kobaya sh i,S．Ha sh imo t o,I S I J I n t． ３２ ( １９９２)１３１１ Ｇ １３１８． [ １３] Z L．Mi,HT． J i ang,Z C．L i,MF．Chen,Z G．Wang,J． I r onS t e e lRe s． I n t． ２０ ( ２０１３)No １０,７５ Ｇ ８０． [ S I JI n t． １４] F Q． J i,C N．L i,S．Tang,G．Yuan,GD．Wang,I ５６ ( ２０１６)６０２ Ｇ ６０９． [ １５] J．Hu,L X．Du,J J．Wang．Ma t e r．Sc i．Eng．A５５４ ( ２０１２)
７９ Ｇ ８５． [ １６] HF．Lan,L X．Du,R D K．Mi s r a,Ma t e r． Sc i．Eng．A６１１ ( ２０１４)１９４ Ｇ ２００． [ １７] S．Tsuyama,H．Nakami ch i,K．Yamada,S．Endo,I S I JI n t． ５３ ( ２０１３)３１７ Ｇ ３２２． [ L．Gu i,Z L．Tan,B Z． Ba i,YQ． １８] GH．Gao,H． Zhang,X t a Ma t e r． １０１ ( ２０１５)３１ Ｇ ３９． Weng,Ac [ t e r． Sc i． Eng．A Y． L i u,GD．Wang,Ma １９] J．Chen,S．Tang,Z ５５９ ( ２０１３)２４１ Ｇ ２４９． [ ２０] Z．L i,D．Wu,Ma t e r． Sc i．Eng．A４５２ ( ２００７)１４２ Ｇ １４８． [ ２１] C．Ouch i,I S I JI n t． ４１ ( ２００１)５４２ Ｇ ５５３． [ r t he a s t．Un i v． ２７ H．L i u,GD．Wang,J．No ２２] P J．Zhang,X ( ) ２００６ ４１４ Ｇ ４１７． [ ２３] AJ．De a r do,I S I JI n t． ３５ ( １９９５)９４６ Ｇ ９５５．
JOURNALOFIRON ANDSTEELRESEARCH,INTERNATIONAL２ ０ １ ６,２ ３( ８):８ １ ５ Ｇ ８ ２ １
Ef f e c t so fPr o c e s s i ngVa r i ab l e son Mi c r o s t r u c t u r eandYi e l dRa t i oo f Hi t r e ng t hCon s t r u c t i ona lS t e e l s ghS J i anKANG１ , Cheng Ｇ n i ngLI１ , Xi ao Ｇ l e iLI１ ２ , J i n ＧhuaZHAO１ , １ １ GuoYUAN , Guo Ｇ dong WANG ,
( １． S t a t eKeyLabo r a t o r fRo l l i ngandAu t oma t i on,No r t he a s t e r nUn i ve r s i t i aon i ng,Ch i na; yo y,Shenyang１１０８１９,L ,Lt ,Qi ２． ShougangQi an ′ anI r onandS t e e lCo． d． an ′ an０６４４０４,Hebe i,Ch i na) Ab s t r a c t:Thep r o c e s so f“ c on t r o l l edr o l l i ng＋r e l axa t i on＋u l t r af a s tc oo l i ng ( UFC)”f o rh i t r eng t hc ons t r uc t i ona l ghs s t e e lwi t hl owy i e l dr a t i owa sp r e s en t ed．Mi c r o s t r uc t u r eandc o r r e spond i ngr e l a t i onsh i t hl owy i e l dr a t i owe r ei n Ｇ pwi ve s t i t ed．Ther e su l t sshowedt ha tt hec ons t r uc t i ona ls t e e l swi t hmu l t i s emi c r o s t r uc t u r eo fba i n i t i cf e r r i t e,ma r Ｇ ga pha t ens i t e Ｇ aus t en i t e (MＧA)andl a t hba i n i t ewe r eob t a i nedt hr ought hec r e a t i vep r o c e s s．Theg r a i ns i z ede c r e a s ed wi t h t hede c r e a s ei nf i n i shr o l l i ngt empe r a t u r e,wh i chenhanc edt hes t r eng t hbyt heg r a i nr e f i nemen ts t r eng t hen i ng．The r ope rr e l axa t i ont r e a tmen tp r omo t edt heba i n i t i cf e r r i t el a t hwi d t handt hef o rma t i ono fb l o cky MＧAc ons t i t uen t． I n p add i t i on,bo t ht het ens i l eandy i e l ds t r eng t hi nc r e a s edwi t ht hede c r e a s ei nf i n i shr o l l i ngt empe r a t u r eand UFCf i na l t empe r a t u r e,bu tt hey i e l ds t r eng t hi nc r e a s ed mo r es i i f i c an t l hant ens i l es t r eng t h,wh i chc aus edt hei nc r e a s ei n gn yt i e l dr a t i o．Byus i ngt hep r o c e s so f“ c on t r o l l edr o l l i ng＋r e l axa t i on＋u l t r af a s tc oo l i ng”,t heexc e l l en tc omp r ehens i ve y me chan i c a lp r ope r t i e so f７８０ MPag r adec ons t r uc t i ona ls t e e l so f１２－４０ mmi nt h i ckne s swe r ea ch i e ved． Ke r d s:c ons t r uc t i ona ls t e e l;l owy i e l dr a t i o;h i t r eng t h;u l t r af a s tc oo l i ng;mu l t i s emi c r o s t r uc t u r e ghs pha ywo
Wi t ht her ap i d deve l opmen to fs t e e lf r amed bu i l d i ngs,t heupda t eandupg r ade p r o c e s so fc on Ｇ s t ruc t i ona ls t e e l si simpe r a t i vei no r de rt oove r c ome r ob l emssucha sr e s ou r c e,env i r onmen tandene r Ｇ p t c． I ti sve r c e s s a r odeve l oph i t r eng t h gy,e yne yt ghs c ons t ruc t i ona ls t e e l s wi t h exc e l l en tc omp r ehens i ve , , r ope r t i e sf o rexamp l et heh i t r eng t ha s e i smi c p ghs [] s t e e lwi t ht ens i l es t r eng t ho f７８０ MPa１ ．Mo r eove r,
s t r eng t handl ow y i e l dr a t i ocons t ruc t i ona ls t e e lwa s t oi n t r oduc et hei n t e r c r i t i c a lhe a tt r e a tmen t( IHT) [ ] i n t oo f f Ｇ l i ne quench i ng andt empe r i ng p r oc e s s７Ｇ９ ． Thedua l Ｇ s e mi c r os t ruc t ur ecompos edo ff e r r i t e pha andt empe r ed ma r t ens i t ec anbeob t a i neda f t e rt he he a tt r e a tmen tp r oc e s si nc l ud i ng IHT．Mo r eove r,
h i t r eng t hcons t ruc t i ona ls t e e lp l a t e so f７８０ MPa ghs i nt ens i l es t r eng t h we r ea l sodeve l opedv i at hehe a t ( ) t het e chno l og i c a li nnova t i oni sa l s oo fs i n i f i c a n t i m Ｇ t r e a t m e n to n Ｇ l i n e r o c e s s HO P f o l l o w i n t h ea c Ｇ g p g [, ] r t anc et o me e tc on t i nuous l nc r e a s i ng demands． c e l e r a t edcoo l i ng１ １０ ．The s edeve l opeds t e e lp l a t e s po yi They i e l dr a t i o( i e l d Ｇ t o Ｇ t ens i l es t r eng t hr a t i o)i san we r e cha r a c t e r i z ed by t he mi c r os t ruc t ur eo ff i ne y impo r t an tpe r f o rmanc ei ndexf o re s t ima t i ngh i Ｇ ma r t ens i t e Ｇ aus t en i t e (MＧA)cons t i t uen td i spe r s edi n ghde f o rma t i onr e s i s t anc e．Thel owe ry i e l dr a t i or e f l e c t s t heba i n i t i cf e r r i t e( BF)ma t r i x,wh i chcon t r i bu t ed t heh i rr e s i s t anc et ode f o rma t i onf r omy i e l d i ngt o t ot heh i t r eng t handl ow y i e l dr a t i o． ghe ghs , , l a s t i ci ns t ab i l i t even t i ngt hesuddende c r e a s ei n I nt he p r e s en t wo rk a nove lt e chno l og i c a l p y pr [ ] s t r eng t h２Ｇ６ ．The r e f o r e,i ti se s s en t i a lt os t udyt he p r oc e s sf o r７８０ MPag r adel ow y i e l dr a t i ocons t ruc Ｇ e f f e c t so fpr o c e s s i ngr ou t e son mi c r o s t ruc t u r e,me Ｇ t i ona ls t e e lwa sp r e s en t edc r e a t i ve l i ngt r ans Ｇ ybyus , chan i c a l pr ope r t i e s and t he r e l a t i onsh i twe en f o rma t i oncon t r o lt oob t a i np r ope r mu l t i Ｇ s e mi Ｇ p be pha mi c r os t ruc t u r a lcha r a c t e r i s t i c sandy i e l dr a t i o． c r os t ruc t ur e． Fur t he rmo r e,t her e l axa t i onwa si n t r o Ｇ Conv e n t i on a lt e c hno l ogyf o r ma nu f a c t u r i ng h i h d u c e di n t oc o n v e n t i o n a lt h e r m o Ｇ m e c h a n i c a lc o n t r o l g Founda t i onI t em: I t em Spons o r edbyNa t i ona lNa t ur a lSc i enc eFounda t i ono fCh i na ( ５１５０４０６３) B i o r aphy: J i anKANG,Do c t o r; E Ｇma i l: wwwkang i an＠１２６ c om; Re c e i v e dDa t e:Novembe r１６,２０１５ j g Co r r e s i ngAu t ho r:GuoYUAN,Do c t o r,Pr o f e s s o r; E Ｇma i l:yuanguo r a l＠s i na c om pond
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２３
８１６
r oc e s s( TMCP)．Theu l t r af a s tc oo l i ng ( UFC)s Ｇ p ys t em ( deve l opedbyS t a t eKey Labo r a t o r fRo l l i ng yo andAu t oma t i ono fNo r t he a s t e rn Un i ve r s i t he yandt UFCs t em wi t hah i oo l i ngc apa c i t sbe en ys ghc y ha app l i edt oi ndus t r i a lpr oduc t i oni n Ch i na)wa sa l s o adop t eda f t e rr e l axa t i ont oexp l o r et he mi c r o s t ruc Ｇ t u r a lcon t r o lo fc ons t ruc t i ons t e e l．Compa r ed wi t h t heconven t i ona la c c e l e r a t edc oo l i ng,t he UFC ap Ｇ l i e s t h eh i h e r r e s s u r e c o o l i n w a t e r t oa c h i e v e t h e p g p g h i oo l i ngr a t e． Th ec o o l i ngr a t eo fUFCi s２－５t ime s ghc h i rt ha nt ha to fa c c e l e r a t edc oo l i ng．The mi c r o Ｇ ghe s t ruc t ur a lcha r a c t e r i s t i c s by t h i sp r o c e s so f“ c on Ｇ t r o l l e dr o l l i ng＋r e l a x a t i on＋ UFC”a ndi t se f f e c t son i e l dr a t i owe r et h ef o cuso ft h i si nve s t i t i on． y ga
１ Ma t e r i a l sand Me t hod s
Thechemi c a lc ompo s i t i onso ft heexpe r imen t a l s t e e l sa r eshowni nTab l e１．Thes t e e l swe r ec a s ti n a１５０kgva cuumi nduc t i onf u rna c e,f o r t gedandcu i n t os r ei ngo t so f８０mmi nt h i ckne s s,１００mmi n qua wi d t hand１５０mmi nl e ng t hf o rs t e e lA ( A１,A２,A３,
A４)and１５０ mmi nt h i c kn e s s,１００ mmi n wi d t hand １５０ mmi nl eng t hf o rs t e e lB,r e spe c t i ve l n Ｇ y．Thei o t sw e r es o a k e da t１ ２ ０ ０ ℃ f o ra b o u t１ ２ ０ m i na n d g r o l l edi n t o１２ mm ( s t e e lA)and４０ mm ( s t e e lB) t h i ckp l a t e sbyaϕ４５０mmdoub l er o l lr eve r s i ngho t Ｇ r o l l i ng mi l l． I ns t e e lA,t her e du c t i onl a r e rt h a n１ ８ ％ g o fe a chpa s si naus t en i t er e c r t a l l i z a t i onr eg i onand ys t het o t a lr educ t i ono fove r５８％i nnon Ｇ r e c r t a l l i z a Ｇ ys t i onr eg i on we r eapp l i eddur i ngt hecont r o l l edr o l l Ｇ , i ngpr oc e s s andt hef i na lt h i ckne s swa s１２ mma f Ｇ t e re i s s e s．I ns t e e lB,t her educ t i ons menＧ ghtpa t i onedabovei naus t en i t er e c r t a l l i z a t i onandnon Ｇ ys r e c r t a l l i z a t i onr eg i on we r e mo r et han１５％ and ys ５２％ r e spe c t i ve l e e lB wa sr o l l edi nt ot hes t e e l y．St l a t ewi t ht h i ckne s so f４０ mm．Af t e rf i n i shr o l l Ｇ p i ng,t hep l a t e swe r er e l axedf o raf i xedt ime,imＧ med i a t e l t e rcoo l edt ova r i oust empe r a t ur e sby y wa , UFCsys t em andf i na l l i rcoo l edt or oom t emＧ ya r a t ur e．Thes chema t i cd i ag r am o ft hepr oc e s s i ng pe r out eand t he pr oc edur e de t a i l sa r e pr e s ent edi n F i １andTab l e２． g
Tab l e１ Ch emi c a lc ompo s i t i on so fe xp e r imen t a ls t e e l s ma s s％
S t e e l A B
C
０ ０６０ ０ ０６７
S i
０ ２５ ０ ２７
Mn
１ ９０ １ ８９
Nb
０ ０２３ ０ ０３８
Ti
０ ０１１ ０ ０１８
Ni
０ ２３ ０ ３１
Cr
０ ２０ ０ １９
Mo
０ ３１ ０ ２７
Cu
０ ２６ ０ ２６
Al
０ ０２ ０ ０３
P
０ ００３ ０ ００５
S
０ ００２ ０ ００３
Me t a l l og r aph i c spe c imens we r er e spe c t i ve l y e t chedby４vo l ％ n i t a lwi t h１０sandLepe r aso l u Ｇ t i on ( ４vo l ％ p i c r a l＋１vo l ％ sod i um me t ab i su l f i t e so l u t i on)wi t h６０s．The mi c r os t ruc t ur e swe r ean a Ｇ l z e du s i ngt heLe i c aDMIRMop t i c a lmi c r o s c op e( OM) y andt heFEIQUANTA６００s c ann i nge l e c t r onmi c r o Ｇ ( ) ( s cope SEM ．Spe c imenso fEBSD e l e c t r onba ck Ｇ s c a t t e r edd i f f r a c t i on)we r epo l i shedi nt hee l e c t r o l t e y c on s i s t i ngo f１２ ５vo l ％ p e r c h l o r i ca c i d,８２ ５vo l ％ abso l u t ee t hy la l coho land５ vo l ％ wa t e rus i ng a vo l t ageo f２８Vf o r２０s．EBSDana l i swa sconduc Ｇ ys t edont heSEM wi t ht hes t eps i z eo f０５μm．Th i n
F i １ S ch ema t i ci l l u s t r a t i ono fp r o c e s s i ngr ou t e g
S t e e l A１ A２
A３ A４ B
/ Th i ckne s s mm １２ １２ １２ １２
４０
Tab l e２ Pr o c e s s i ngp r o c e du r e so fe xp e r imen t a ls t e e l s F i n i shr o l l i ng /℃ t empe r a t ur e
UFCi n i t i a lc oo l i ng /℃ t empe r a t ur e
UFCf i n i shc oo l i ng /℃ t empe r a t ur e
/ Coo l i ngr a t e (℃ s－１)
８１６
７７８
４６０
６２ ５
８５７
７７８ ８１８
８３２
f o i l swe r epr epa r edf o rt hef u r t he ri nve s t i t i ono f ga mi c r os t ruc t u r eandobs e r vedbyaFEITECNAIG２
７８０ ７７５ ７７７
７９４
４７１ ４５２ ５９０
３９５
６７ ２ ６９ ４ ６７ ８
１７ ６
２０t r ansmi s s i on e l e c t r on mi c r os cope (TEM )．The r e t a i nedaus t en i t ef r a c t i on wa s me a sur ed by XＧ r ay
I s sue８ Ef f e c t so fPr o c e s s i ngVa r i ab l e son Mi c r o s t r uc t u r eandYi e l dRa t i oo fHi t r eng t hCons t r uc t i ona lS t e e l s ghS
d i f f r a c t i on ( XRD)．Spe c imenswe r ee l e c t r o Ｇ l i shed po ands c annedf r om４０ °t o１１０ °f o r２ θ．Theda t awe r e [ １１, １２] r o c e s s e da n dc a l c u l a t e da s r e v i o u ss t u d i e s ． p p Thes t anda r dt ens i l et e s ts amp l e swe r ep r epa r ed f r omt her o l l edp l a t e si nt het r ans ve r s ed i r e c t i onand t e s t eda tr oomt empe r a t u r eonanINSTRONun i ve r Ｇ / s a lt e s t i ng ma ch i nea tac r o s s Ｇ he adspe edo f３ mm mi n．Cha r no t chedimpa c tspe c imenswe r ea l s o py VＧ cu ta l ongt het r ans ve r s ed i r e c t i on．Theimpa c tspe c i Ｇ menswe r et e s t e da t－４０ ℃ a f t e rs o a k e df o r１５mi ni n t h eb a t ho fa l c oho lc on t r o l l edbyar e f r i r a t i onp l an t． ge
８１７
２ Re s u l t sandDi s c u s s i on Op t i c a lmi c r og r aphso fs t e e l s A１,A２and A３ a r epr e s en t edi nF i ２． I tc anbes e ent ha tt hemi c r o Ｇ g s t ruc t ur e so fs t e e l sA１andA２wi t ht hef i n i shr o l l i ng t empe r a t ur e so f８５７ ℃ and８１６ ℃ r e spe c t i ve l r e ya s imi l a r．Theya r ebo t hcompos edo ft he mu l t i s e pha cons i s t i ngo fl a t hba i n i t e( LB)andg r anu l a rba i n i t e ( ( ) ( ) GB),a sshowni nF i s ． ２ a a n d２ b ． F o rs t e e l g A３t ha twa sf i n i shr o l l eda t７７８ ℃ andcoo l edby UFC wi t hou tr e l axa t i ona f t e rho tr o l l i ng,t hemi c r o Ｇ
( a)S t e e lA１; ( b)S t e e lA２; ( c)S t e e lA３．
F i ２ OM mi c r o s t r u c t u r e so fs t e e l sA ( e t ch e dby４v o l ％ n i t a l)und e rd i f f e r en tp r o c e s s i ngr ou t e s g
s t ruc t ur ewa sr e f i nedduet ot hel owe rf i n i shr o l l i ng , ( ) t empe r a t u r ea sshowni nF i ２ c ． g F i ３ shows t he d i ame t e rd i s t r i bu t i ons o f g ( r a i n sw i t hh i ha n l e r a i nb o u n d a r i e s l a r e r t h a n g g g g g １５ °)me a su r edbyEBSD． I tc anbes e ent ha tt hef r a c Ｇ t i ono fg r a i nswi t ht hed i ame t e rove r１５μmi sl imi t Ｇ edi na l lt hr e es t e e l s,andt hed i ame t e ro f mo s to f t hemr ange sf r om１t o１５μm．Thee f f e c t i veg r a i nd i Ｇ ame t e r s,i e ,t heave r age g r a i ns i z e s,a r e８ ３８, ５ ９５and４ ９３μmi nt hes t e e l sA１,A２andA３,r e Ｇ , spe c t i ve l I ns t e e lA１ f u r t he rmo r e,t hef r a c t i ono f y．
cha r a c t e r i z edbyt hei nc r e a s ei ng r a i nbounda r i e sand d i s l oc a t i ons．Wi t ht hede c r e a s ei nf i n i shr o l l i ngt emＧ r a t ur e,t hede f o rma t i onso faus t en i t e,d i s l oc a t i ons pe and d i s l oc a t i on c e l l sa r ei nc r e a s ed, wh i ch supp l y / ( / mo r equan t i t i e so fnuc l e a t i onf o rγ α aus t en i t ef e r Ｇ r i t e)t r ans f o rma t i on．Add i t i on a l l hed e c r e a s eo ff i n Ｇ y,t i s hr o l l i ngt emp e r a t u r ec anp r ov i d eg r e a t e rd e f o rma t i on [ , ] s t o r e de n e r ha tc ani n c r e a s et henu c l e a t i onr a t i o１３ １４ ． gyt The r e f o r e,t her educ t i ono ff i n i shr o l l i ngt empe r a Ｇ t ur ec anp r omo t et heg r a i nr e f i nemen tanda f f e c tt he mi c r os t ruc t ur a levo l u t i on dur i ngr e l axa t i onf o l l owＧ r a i ns wi t ht hed i ame t e rove r１５ μm i s１４ ７５％． i ngt hecon t r o l l edr o l l i ngp r oc e s s． g Th i sf r a c t i oni ns t e e lA２de c r e a s e st o８ ５９％ duet o Thesubs t ruc t ur e so fs t e e l s A２and A３ we r e t hede c r e a s eo ff i n i shr o l l i ngt empe r a t u r e．F i ３( c) f ur t he ri nve s t i t ed by SEM．SEM obs e r va t i ons g ga ( i nd i c a t e st ha tt heg r a i nd i ame t e ri ns t e e lA３i sa l l F i ４)i nd i c a t et ha tt hes t e e lpos s e s s e st he mu l t i Ｇ g d i s t r i bu t edbe l ow１５μm．Af t e rho tr o l l i ngi nnon Ｇ r e Ｇ pha s e mi c r os t ruc t ur e s cons i s t i ng o f LB and GB, c r t a l l i z a t i onr eg i on,t heaus t en i t ei sde f o rmeda s bo t ho fwh i chcomp r i s edBFma t r i xandMＧAi s l ands ys “ akes t ruc t u r e”．Th i s“ akes t ruc t u r e”i s d i s t r i but i ngi ntheBF ma t r i x． I twasLBwhent he panc panc
( a)S t e e lA１; ( b)S t e e lA２; ( c)S t e e lA３．
F i ３ Gr a i nd i ame t e rd i s t r i bu t i on s( ana l z e dbyEBSD)o fs t e e l sAt r e a t e da td i f f e r en tf i n i s hr o l l i ngt emp e r a t u r e s g y
８１８
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２３
F i ４ SEM mi c r o s t r u c t u r e so fs t e e l sA２ ( a)andA３ ( b)andOM mi c r o s t r u c t u r e so fs t e e lA３e t ch e dbyLePe r a( c) g
BFand MＧAi s l andsexh i b i t edl a t h Ｇ l i ke,wh i l e GB wa st heBF wi t h MＧAi s l andsd i s t r i bu t i ngi nt hem． Bu ti ti sf oundt ha tt he r ea r ed i f f e r enc e si ns i z e, mo r l ogyandd i s t r i bu t i ono fMＧAc ons t i t uen tbe Ｇ pho twe ens t e e l sA２andA３． Fo rs t e e lA２,t heMＧAc on Ｇ s t i t uen t s( F i ４( a))appe a ra si r r egu l a rs c a t t e r ed g i s l ands．Thes t ruc t u r eo fba i n i t ei sr e l a t i ve l oa r s Ｇ yc ened and chao t i c．Fu r t he rmo r e,t i c a ll a t h Ｇ l i k e yp s t ruc t ur eo fBFi n GBi sd i f f i cu l tt o deve l op．Fo r s t e e lA３,howe v e r,mo s to ft h e MＧAc on s t i t u e n t sa r e ( ( ) ) f i neandl ong F i ４ c ．Thecha r a c t e r i s t i ce l onga Ｇ g t edBF mi c r o s t ruc t u r ei ss e a r a t e dbye l ong a t e d MＧA p c ons t i t uen t s( F i ４( b))． g Dur i ngt hec oo l i ngp r o c e s sa f t e rr o l l i ng,BF mi Ｇ c r os t ruc t u r es t a r t st of o rm whent het empe r a t u r e de c r e a s e st ot het r ans f o rma t i ons t a r tt empe r a t u r eo f ba i n i t e．Ca rbonc anbepa r t i t i onedf r om BFi n t ot he un t r ans f o rmedaus t en i t e,wh i chimp r ove st hes t ab i l Ｇ i t faus t en i t e．Thec a rbon Ｇ r i chun t r ans f o rmedaus Ｇ yo t en i t ewi t hsu f f i c i en ts t ab i l i t anbet he rmodynami Ｇ yc c a l l s s i b l ef o rf u r t he rba i n i t et r ans f o rma t i on, yimpo t hent r ans f o rmst o ma r t ens i t eo rpa r t i a l l ema i ns yr s t ab l ea tr oom t empe r a t u r e．Gene r a l l hef o rma Ｇ y,t t i ono f MＧA c ons t i t uen tc an bede s c r i beda s men Ｇ [ ] [ , ] t i oned above１５Ｇ１７ ．Some s t ud i e s１８ １９ have shown t ha tt hec a rbonc onc en t r a t i oni n MＧAc ons t i t uen ti s h i rt hant ha ti nBFandt heave r agec a rbonc on Ｇ ghe c en t r a t i ono fs t e e l．Th i sc anbea t t r i bu t edt or e Ｇ r Ｇ pa t i t i ono fc a rbondu r i ngt hec oo l i ngp r o c e s s． Fo rs t e e lA３,l a r f e c tdens i t si n t r o Ｇ gede y wa duc edi np r i o raus t en i t eby ho tr o l l i ng de f o rma t i on a tl owt emp e r a t u r e． I na dd i t i on,t h e r ewa sno te nough t imef o rt her e c ove r fde f o rmedaus t en i t eduet o yo t heabs enc eo fr e l axa t i on．Thede f o rmedaus t en i t e wi t hou tr e c ove r f f e c tpo s s e s s edh i r denab i l i Ｇ ye ghha t i chhads i i f i c an te f f e c tont het r ans f o rma Ｇ y,wh gn , , t i onbehav i o ro fba i n i t ei e t her e f i nemen to fba i Ｇ n i t i cs t ruc t u r e．De f e c t sp r oduc edby ho tr o l l i ngde Ｇ
f o rma t i oncou l da c ta st hee f f e c t i venuc l e a t i ons i t e s f o rba i n i t et r ans f o rma t i on,andt heg r owt ho fba i n i t e subun i tsucha sBFwa sr e s t r i c t edbye a cho t he r．On t heo t he rhand,mo r l og i c a lcha r a c t e r i s t i co fMＧA pho cons t i t uen twa sc l os e l e l a t edt ot hec a rbonenr i chＧ yr , men t wh i chwa sf ur t he rmo r eg r e a t l n f l uenc edby yi t hebehav i o ro fc a rbond i f f us i onandd i sp l a c i vet r ans Ｇ f o rma t i ondur i ngt hef o rma t i ono f GB．Be c aus eo f t hef a s tcoo l i ngimmed i a t e l f t e rr o l l i ng de f o rma Ｇ ya t i onf o rs t e e lA３,t hel ongr anged i f f us i ono fc a rbon ; , wa ssupp r e s s eds i i f i c an t l sacons equenc et he gn y a f o rma t i ono fb l ockyc a rbon Ｇ r i cha r e a swi t hl a r i z e ges i naus t en i t e wa si nh i b i t ed．The r e f o r e,i ns t e e lA３,
MＧAcons t i t uen t sd i s t r i bu t edbe twe enBFl a t hswe r e mos t l i neandl ong,andappe a r edi nt heshapeo f yf , , f i lm．Compa r ed wi t hs t e e lA３ howeve r s t e e lA２ wa scha r a c t e r i z edbyi n t r oduc i ng ar e l axa t i ona f t e r r o l l i ng wi t hi nc r e a s i ng t hef i n i shr o l l i ng t empe r a Ｇ t ur e．Theha rdenab i l i t fp r i o rde f o rmedaus t en i t e yo wa sr educ edwi t ht hede c r e a s ei nde f e c tdens i t ydue t or e cove r f f e c t dur i ng t he r e l axa t i on p r oc e s s． ye Cons equen t l he coa r s en i ng t endenc f ba i n i t e y,t yo subun i t,sucha sBFl a t hs,cou l dbeusua l l ound, yf a sshowni nF i ４( a)．Add i t i ona l l tc anbede Ｇ g y,i
duc edf r omt h i sr e su l tt ha tp r ope rr e l axa t i ont r e a t Ｇ men twou l dp r omo t et heBFl a t hwi d t handt hef o r Ｇ ma t i ono fb l ocky MＧAcons t i t uen t．Th i smaybea s Ｇ soc i a t ed wi t ht heevo l u t i ono fsubs t ruc t ur edur i ng r e l axa t i ona f t e rr o l l i ngandi t se f f e c tonr e Ｇ r t i t i on pa o fc a rbondur i ngba i n i t et r ans f o rma t i on,andf ur t he r i nve s t i a t i o nf o r t h i s i ss t i l ln e e d e d ． g F i ５ showst he b l ocky MＧA cons t i t uen ti n g s t e e lA２obs e r vedbyTEM．Themax imum Fe r e td i Ｇ ( ) , ame t e rb o fMＧAcons t i t uen ti s０ ８３μm andt he a spe c tr a t i o( t he max imumＧ t o Ｇmi n imum Fe r e t( a) d i ame t e rr a t i o)i s１ ６． I tc anbef oundt ha tt heBF ma t r i xad a c en tt o MＧA cons t i t uen tcon t a i nssome j d i s l oc a t i ons,wh i ch mayr e su l tf r om uneven pha s e
I s sue８ Ef f e c t so fPr o c e s s i ngVa r i ab l e son Mi c r o s t r uc t u r eandYi e l dRa t i oo fHi t r eng t hCons t r uc t i ona lS t e e l s ghS
t r ans f o rma t i on by ashe a r me chan i sm．Mo r eove r, t hec en t r a lz one si n MＧA c ons t i t uen t si nd i c a t edby t hec i r c l e sa r ei den t i f i eda s ma r t ens i t ef r omt hes e Ｇ ( ) ( ) l e c t eda r e ad i f f r a c t i on SAD pa t t e rnsi nF i ５ c ． g I twa sa l s od i f f i cu l tt of i nd gammapha s e( f c cl a t Ｇ t i c e) i no t he rz one swi t h i nMＧAc ons t i t uen tf r omt he
８１９
SADana l i s．XRDr e s u l ts howe dt ha tt het o t a lvo l ume ys f r a c t i ono fau s t e n i t ewa son l t２ ４vo l ％ ;t hus, yabou t her e t a i nedaus t en i t ei sha rd l oundi nt hes t e e l． yf , The r e f o r ei tc anbecon f i rmedt ha tt heMＧAcons t i t Ｇ uen t scons i s t ma i n l f ma r t ens i t e,wh i chensur e s yo t heh i i e l ds t r e s s． ghy
( a)Br i tf i e l dimage; ( b)Da rkf i e l dimage; ( c)SADpa t t e r no fMＧAc ons t i t uen ta si nd i c a t edbyt hec i r c l ei n( a)． gh
F i ５ TEM mi c r o r aph so fMＧAc on s t i t u en t si ns t e e lA２ g g
F i ６showst hee f f e c to f UFCf i n i shc oo l i ng g t empe r a t u r eon mi c r o s t ruc t u r eo ft hes t e e l s．When t heUFCf i na lt empe r a t u r ei nc r e a s e st o５９０ ℃ ,s t e e l A４ha st hemi c r o s t ruc t u r eo fGB,bu twi t hou tLB． I t i ss e enf r om F i ６( a)t ha tmo s to fBFpha s e sp r e s Ｇ g en tb l ockyand MＧA c ons t i t uen t s wi t hi r r egu l a ri s Ｇ l andshapea r ed i s o r de r l i s t r i bu t edi n BF ma t r i x． yd [ ] Ba s edont hep r ev i ous l l i shedequa t i on１７ ,t he ypub Bs )wa s ba i n i t et r ans f o rma t i ons t a r tt empe r a t u r e( c a l cu l a t eda s５８１ ℃．The r e f o r e,a th i h e rU F Cf i n a l g , t empe r a t u r eo f５９０ ℃ t heba i n i t et r ans f o rma t i oni n s t e e lA４ ma i n l c cu r r eddu r i nga i rc oo l i ng．F i ６ yo gs ( b)and６( c)showt hecha r a c t e r i s t i c so f MＧAc on Ｇ , s t i t uen t si ns t e e l sA４andA２ r e spe c t i ve l l Ｇ y．Thevo umef r a c t i ono fMＧAc ons t i t uen tr e a che s１４ ７vo l ％ i ns t e e lA４,wh i chi sl a r rt han７ ８vo l ％i ns t e e l ge , A２ i nd i c a t i ngt ha tt hevo l umef r a c t i onandt hes i z e o f MＧA c ons t i t uen t sf o rmed du r i ng a i rc oo l i ng a t
h i rUFCf i na lt empe r a t ur ea r el a r rand mo r e ghe ge coa r s et hant hos eo fone sf o rmedma i n l i ngUFC ydur coo l i ng．Thesu f f i c i en tc a rbon d i f f us i on dur i ng a i r coo l i ngi ns t e e lA４c aus edsomel a r a rbon Ｇ r i cha r Ｇ gec e a s．Mo r e ov e r,t hes t ab i l i t ft he s ec a r bon Ｇ r i c hau s t e n Ｇ yo i t ea r e a sc anbeimpr oved,l e ad i ngt ot hef o rma t i on o fcoa r s eMＧAcons t i t uen t swi t hh i a rbonconc en Ｇ ghc , t r a t i on．Howeve rt hef o rma t i ono fl a r l ockyc a r Ｇ geb bon Ｇ r i chaus t en i t ei ns t e e lA２cou l dbesupp r e s s ed duet ot hef a s tcoo l i ngandl ow UFCf i n a lt emp e r a t u r e． I ti sc on c l ud e dt ha tt hecoa r s eMＧAcons t i t uen t swi t h i s l andshapecou l dbeavo i dedbyus i ng UFC． Ther o l l i ngandcoo l i ngcond i t i onsc ans i i f i Ｇ gn c an t l n f l uenc et heme chan i c a lpr ope r t i e s．Asl i s t ed yi i n Tab l e ３,bo t ht he t ens i l es t r eng t h and y i e l d s t r eng t hi nc r e a s ewi t ht hede c r e a s ei nf i n i shr o l l i ng t empe r a t ur eand UFCf i na lt empe r a t ur e,wh i l et he i e l ds t r eng t hi n c r e a s e smo r es i i f i c an t l hant ens i l e y gn yt
( a)S t e e lA４ ( e t chedby４vo l ％ n i t a l); ( b)S t e e lA４ ( e t chedbyLePe r a); ( c)S t e e lA２ ( e t chedbyLePe r a)．
F i ６ OM mi c r o s t r u c t u r e so fs t e e l sAund e rd i f f e r en tp r o c e s s i ngr ou t e s g
J ou r na lo fI r onandS t e e lRe s e a r ch,I n t e r na t i ona l Vo l ２３
８２０
Tab l e３ Me chan i c a lp r op e r t i e so fe xp e r imen t a ls t e e l s Rp０２/
Rm/
Rp０２/
A１
６３５
８５５
０ ７４
１７ ５
１６７
A３
７５５
９００
０ ８４
１８ ８
２１４
S t e e l A２ A４ B
MPa
６７０ ５７０
６３５
MPa
８９５ ８２５
８３５
Rm
A/％
０ ７５
１８ ３
０ ６９
０ ７６
１９ ８
２１ ０
Akv(－４０ ℃ )/ J
１８６ １５７
１５５
No t e:Rp０２—Yi e l ds t r eng t h;Rm —Tens i l es t r eng t h; A —El onga t i on;Akv(－４０ ℃ )—Impa c tt oughne s sa t－４０ ℃．
s t r eng t h,wh i chr e su l t si nt hei nc r e a s i ngt r endf o r t he y i e l dr a t i o．S t e e l A２ ha sexc e l l en t me chan i c a l ope r t i e si nc l ud i ngh i t r e ng t ha ndl ow y i e l dr a t i o pr ghs ( b e l ow ０ ８),me e t i ngt h er e i r eme n to ft h es t r a i n Ｇ qu b a s e dd e s i Th ep e r f o rma n c er e i r eme n t so f７８０MPa gn． qu r adecons t ruc t i ona ls t e e l wi t hl ow y i e l dr a t i of o r g , h i Ｇ r i s ebu i l d i ngs a c c o r d i ngt ot heJ apane s er e l a t Ｇ gh [] edda t a１ ,a r ea sf o l l ows:y i e l dr a t i o ≤０ ８５,y i e l d s t r eng t h ≥６３０ MPa,t ens i l es t r eng t h ≥７８０ MPa, , ande l onga t i on ≥１６％．Obv i ous l hegoodba l anc e yt
o fme chan i c a lp r ope r t i e sc anbeob t a i ned whent he f i n i shr o l l i ngt empe r a t ur eand UFCf i na lt empe r a Ｇ t ur ea r e８１６ ℃ and４６０ ℃ ,r e spe c t i ve l ． U n d e r t h e y op t imi z edpr oc e s s i ngc ond i t i on ss howni nT ab l e２f o r t h i c kn e s so f４０mm,a l lt hep e r f o rman c ei nd e x e so fs t e e l Bc omeupt ot her e l evan ts t anda rdsa sp r e s en t edi n Tab l e３． F i ７and８p r e s en tt hemi c r os t ruc t ur eand gs ha rdne s sa td i f f e r en tpos i t i onsa l ongt het h i ckne s s d i r e c t i on, wh i ch i nd i c a t et he mi c r os t ruc t ur e un i Ｇ f o rmi t l ongt het h i ckne s sd i r e c t i oni ns t e e lB． ya Theme c han i c a lp r op e r t i e so fs t e e l sa r ed e t e rmi n e d byt hemi c r o s t r u c t u r e．Pr e v i ou ss t ud i e si nd i c a t edt ha t t heapp r opr i a t er egu l a t i ono fmu l t i s emi c r os t ruc Ｇ pha t ur econ t a i n i ngha rdandso f tpha s e swa sc r i t i c a lt o r educ et he y i e l dr a t i o cons i de r i ng h i t r eng t h, gh s t hr ought hecon t r o lo fcons t i t uen t pha s espe c i e s, d i s t r i bu t i onandp r opo r t i on２０ ２１ ．Thecompos i t emi Ｇ , c r os t ruc t ur eu s u a l l n c l ud e sf e r r i t e＋b a i n i t ef e r r i t e＋ yi [ , ] ma r t e n s i t e,ba i n i t e＋ma r t ens i t e,andsoon２２ ２３ ． I nt h i ss t udy,s t e e lA２ha sexc e l l en tme chan i c a l [ , ]
( a)Ne a rt heuppe rsur f a c e; ( b)１/４; ( c)Cen t e r; ( d)３/４; ( e)Ne a rt hel owe rsur f a c e．
F i ７ Mi c r o s t r u c t u r e so fs t e e lBt h r ought h et h i ckn e s s g
F i ８ Typ i c a lha r dn e s sd i s t r i bu t i oni ns t e e lB g t h r ought h et h i ckn e s s
t i ngandha rden i ngt ot r ans f e rl oad,y i e l dde f o rma Ｇ [ ] t i onb e i n st oo c c u ri nt heha r dpha s e２０ ． I nt h i sc a s e, g t hef r a c t u r ec au s e dbye x c e s ss t r e s sc on c e n t r a t i onc anb e , de l ayed．Cons equen t l he mu l t i s e mi c r os t ruc Ｇ y t pha t ur ei ns t e e lA２ha sl owe ry i e l ds t r eng t h( i nduc edby BF)andh i rt ens i l es t r eng t h (MＧAcons t i t uen t), ghe wh i chr e su l t si nt hel ow y i e l dr a t i oo f０ ７５． Cons equen t l her e f i ned mi c r os t ruc t ur e sc an y,t b eob t a i n e dund e rt heh i n t e n s i t oo l i ngby UFC, ghi yc l e ad i ngt ot heexc e l l en t me chan i c a lp r ope r t i e s．By app l i ngt h i snew p r oc e s s,c omp r e he n s i v eme c han i c a l y r op e r t i e so ft hes t e e l swi t ht h i c kn e s so f１２－４０ mm p cou l dme e tt her e l a t eds t anda rdcomp l e t e l y．
r ope r t i e s,i nwh i cht heBF ma t r i xa c t sa st hes o f t p s eand MＧAc ons t i t uen ta c t sa st heha r dpha s et o pha ensur eh i t r eng t h．Wi t hi nc r e a s i ngapp l i edl oad, ghs ３ Conc l u s i on s l a s t i cde f o rma t i onf i r s t l c cu r si nt hes o f tpha s e． p yo ( １)The mu l t i s e mi c r os t ruc t ur econs i s t i ng Whent hes o f tpha s eha ssu f f i c i en ts t r a i na c cumu l a Ｇ pha
I s sue８ Ef f e c t so fPr o c e s s i ngVa r i ab l e son Mi c r o s t r uc t u r eandYi e l dRa t i oo fHi t r eng t hCons t r uc t i ona lS t e e l s ghS
o fLBandGB wa sob t a i nedbyt hep r o c e s so f“ c on Ｇ t r o l l edr o l l i ng＋r e l axa t i on＋UFC”f o rh i t r eng t h ghs c ons t ruc t i ona ls t e e lw i t hl owy i e l dr a t i o．Wh e nt h ef i n Ｇ i s hr o l l i ngt emp e r a t u r ed e c r e a s e sf r om ８５７t o７７８ ℃ , t heg r a i ns i z e wa sr educ edr ema rkab l r ope r y．Thep r e l axa t i onpr o c e s sp r omo t edt he wi d t ho fBFl a t hs andt hef o rma t i ono fb l o cky MＧAc ons t i t uen t,l e ad Ｇ i ngt ot hede c r e a s ei ny i e l dr a t i o． ( ２)When UFCf i na lt empe r a t u r ei nc r e a s edt o , ５９０ ℃ t he mi c r o s t ruc t u r eo ft hes t e e l wa sc omＧ ed o f GB,i n wh i ch mo s to f BF showed t he pos b l ocky mo r l ogy and MＧA c ons t i t uen t s wi t hi r Ｇ pho r egu l a ri s l andshape we r ed i s o r de r l i s t r i bu t edi n yd BF ma t r i x． I nt h i sc a s e,t hes t e e lpo s s e s s edt hel owＧ , e s ty i e l dr a t i o bu tt hel owe s ty i e l ds t r eng t h and t ens i l es t r eng t h． ( ３)Theexc e l l en tc omp r ehens i ve me chan i c a l r ope r t i e sc an beob t a i ned whent hef i n i shr o l l i ng p t empe r a t u r eand UFCf i na lt empe r a t u r ef o rt h i ck Ｇ ne s so f１２mm we r e８１６ ℃ and４６０ ℃ r e spe c t i ve l y, i e ,y i e l ds t r eng t ho f６７０ MPa,t ens i l es t r eng t ho f ８９５ MPa,y i e l dr a t i oo f０ ７５andpe r c en t agee l onga Ｇ t i ono f１８ ３％ ,wi t ht he Cha r c tene r f pyimpa gy o １８６Ja t－４０℃． Anda l lp e r f o rma n c ei nd e x e sf o rt h i c k Ｇ n e s so f１２－４０mmc ana l s ome e tt her equ i r emen t so f r e l evan ts t anda r ds． Re f e r enc e s:
[ １] K．Ueda,S．Endo,T． I t o,JFE Te ch．Rep． １８ ( ２００７)２３ Ｇ ２８． [ ２] S．Go t o,C．Kami,S．Kawamu r a,Ma t e r． S c i．Eng．A６４８ ( ２０１５)
８２１
４３６ Ｇ ４４２． [ ３] Y．Fukumo t o,Eng． S t r uc t． １８ ( １９９６)７８６ Ｇ ７９１． [ ４] B．Ka t o,I S I JI n t． ７６ ( １９９０)１２０３ Ｇ １２１０． [ ５] B．Ka t o,I S I JI n t． ７４ ( １９８８)９５１ Ｇ ９６１． [ ６] S K．Kim,YM．Kim,YJ． L im,NJ．Kim,Me t．Ma t e r． I n t． １２ ( ２００６)１３１ Ｇ １３５． [ ７] J．Ka ng,C．Wa ng,G D．Wa ng,Ma t e r．S c i．Eng．A５５３ ( ２０１２) ９６ Ｇ １０４． [ ８] G．Ro s enbe r S i na i ová,L ′． Juha r,Ma t e r．Sc i．Eng．A５８２ g,I． ( ２０１３)３４７ Ｇ ３５８． [ ９] P．T a o,H．Yu,Y． Fa n,Y． Fu,Ma t e r．De s． ５４ ( ２０１４)９１４ Ｇ ９２３． [ １０] K．Ueda,S．Suzuk i,S．Mi t ao,Ma t e r．Sc i．Fo r um ６３８ Ｇ ６４２ ( ２０１０)３５５５ Ｇ ３５６０． [ １１] X D．Tan,YB．Xu,X L．Yang,D．Wu,Ma t e r． Sc i． Eng．A ５８９ ( ２０１４)１０１ Ｇ １１１． [ １２] K．Sug imo t o,N．Usu i,M．Kobaya sh i,S．Ha sh imo t o,I S I J I n t． ３２ ( １９９２)１３１１ Ｇ １３１８． [ １３] Z L．Mi,HT． J i ang,Z C．L i,MF．Chen,Z G．Wang,J． I r onS t e e lRe s． I n t． ２０ ( ２０１３)No １０,７５ Ｇ ８０． [ S I JI n t． １４] F Q． J i,C N．L i,S．Tang,G．Yuan,GD．Wang,I ５６ ( ２０１６)６０２ Ｇ ６０９． [ １５] J．Hu,L X．Du,J J．Wang．Ma t e r．Sc i．Eng．A５５４ ( ２０１２)
７９ Ｇ ８５． [ １６] HF．Lan,L X．Du,R D K．Mi s r a,Ma t e r． Sc i．Eng．A６１１ ( ２０１４)１９４ Ｇ ２００． [ １７] S．Tsuyama,H．Nakami ch i,K．Yamada,S．Endo,I S I JI n t． ５３ ( ２０１３)３１７ Ｇ ３２２． [ L．Gu i,Z L．Tan,B Z． Ba i,YQ． １８] GH．Gao,H． Zhang,X t a Ma t e r． １０１ ( ２０１５)３１ Ｇ ３９． Weng,Ac [ t e r． Sc i． Eng．A Y． L i u,GD．Wang,Ma １９] J．Chen,S．Tang,Z ５５９ ( ２０１３)２４１ Ｇ ２４９． [ ２０] Z．L i,D．Wu,Ma t e r． Sc i．Eng．A４５２ ( ２００７)１４２ Ｇ １４８． [ ２１] C．Ouch i,I S I JI n t． ４１ ( ２００１)５４２ Ｇ ５５３． [ r t he a s t．Un i v． ２７ H．L i u,GD．Wang,J．No ２２] P J．Zhang,X ( ) ２００６ ４１４ Ｇ ４１７． [ ２３] AJ．De a r do,I S I JI n t． ３５ ( １９９５)９４６ Ｇ ９５５．