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Evolution of Surface Transversal Crack in Intermediate Slabs during Hot Rolling Deformation
JOURNALOFIRON ANDSTEELRESEARCH,INTERNATIONAL２ ０ １ ４,２ １( ４):４ ７ ６ Ｇ ４ ８ ０
Ev o l u t i ono fSu r f a c eTr an s v e r s a lCr a cki nI n t e rme d i a t e S l ab sdu r i ngHo tRo l l i ngDe f o rma t i on Yan Ｇ zhaoLUO, J i ong Ｇmi ngZHANG, Qi angLIU, Mao ＧkangLI, Yi Ｇ t ong HAN, Yi Ｇ f e iCAO
( Schoo lo fMe t a l l u r i c a landEc o l og i c a lEng i ne e r i ng,Un i ve r s i t fSc i enc eandTe chno l ogyBe i i ng,Be i i ng１０００８３,Ch i na) g yo j j Ab s t r a c t:I nt hec on t i nuousc a s t i ngo fs t e e l s,su r f a c et r ans ve r s a lc r a cksa r eo f t enf ound．The s ede f e c t smays e ve r e l y i n f l uenc et hef i na lqua l i t ft hep r oduc t s．Thee vo l u t i ono ft r ans ve r s a lVＧ shapedc r a ckswi t hd i f f e r en tdep t hont he yo su r f a c eo fac on t i nuous l a s ts t e e ls l abdu r i ngho tr o l l i ngwa ss t ud i ed．Thea r t i f i c i a lVＧ shapedc r a ckswe r emadeon yc t hesu r f a c e．Ther o l l i ngp r o c e s spa r ame t e r sf r omani ndus t r i a lr o l l i ngmi l lhavebe enus eda sar e f e r enc e．Thespe c i Ｇ menso fr o l l edwo r kp i e c ei ni n t e rmed i a t es l abswe r eob t a i neda f t e rd i f f e r en tr o l l i ngpa s s e s．Themo r l ogyo fsu r Ｇ pho f a c ec r a ckand mi c r o s t r uc t u r ee vo l u t i oni nt her o l l i ngp r o c e s swe r ei nve s t i t edbyop t i c a lmi c r o s c opy．Ther e su l t s ga showt ha tt hedep t ho fsu r f a c et r ans ve r s a lc r a ckg r adua l l c r e a s edwi t ht hei nc r e a s eo fr o l l i ngpa s s e s．Theg r a i ns i z e yde o ff e r r i t eandpe a r l i t eont hes amp l esu r f a c ea l s og r adua l l educ ed．Themi c r o s t r uc t u r e sa r oundc r a ckswi t ht hed i f Ｇ yr f e r en tdep t ha r ea lmo s ti den t i c a l,wi t hou td i r e c tc o r r e l a t i onwi t ht hei n i t i a lc r a ckdep t h． Ke r d s:i n t e rmed i a t es l ab;su r f a c et r ans ve r s a lc r a ck;mi c r o s t r uc t u r e;ho tr o l l i ng ywo
I nt hec on t i nuousc a s t i ng o fs t e e l s,su r f a c e t r ansve r s a lc r a cksa r eo f t enf oundduet ochemi c a l , compo s i t i ono ft hema t e r i a lt he rma ls t r e s sando t hＧ [] e rf a c t o r s１ ．I ft he s ec r a cksc anno tbee l imi na t ed dur i ngsubs equen tho tr o l l i ngp r o c e s s,t he s ec r a cks appe a r i ngonsu r f a c es l ab mayc aus es e r i ousqua l i t y r ob l emi nf i na lp r oduc t s． I ti sobv i oust ha te l imi na Ｇ p t i ono ft hepo s s i b l ec aus e so ft hede f e c twe r eane f Ｇ f e c t i veme t hodf o reng i ne e r sandp r oduc e r s．Sot he i nve s t i t i ono ft heevo l u t i ono ft hesu r f a c ec r a cki n ga i n t e rmed i a t es l absi so ft heo r e t i c a ls i n i f i c a n c et o g op t imi z et her o l l i ngp r o c e s sandt oimp r ovet hef i na l [] l i t fp r oduc t s２ ．Thei n t e rmed i a t es l absa r e qua yo comi ngo f ft h ep r odu c t i onl i n ea f t e re a c hr o l l i ngp a s s． Mo s to fr e s e a r che r sf o cus on t he c aus e so f c r a cksandf i nd i ngawayt oavo i dt hem．Ther e a sons o ft hec r a ckss e emt obec l e a r,sucha st henon Ｇme Ｇ , , t a l l i ci nc l us i ons e l emen ts eg r ega t i on unevenhe a t Ｇ i ngands oon．Howeve r, i ti ss e l domr e s e a r chedt ha t t heevo l u t i ono fc r a cksi nt heho tr o l l i ngp r o c e s s,e s Ｇ c i a l l abo r a t o r t udy on evo l u t i on o f su r f a c e pe yl ys [] t r ansve r s a lc r a cks e emseven muchl e s s３ ．
I nr e c en tye a r s,wi t ht her ap i ddeve l opmen to f t hecompu t e rha rdwa r eandso f twa r e,cons i de r ab l e i nve s t i t i onshavebe enc a r r i edont heevo l u t i onbe Ｇ ga hav i o ro fc r a cksdur i ngr o l l i ngp r oc e s s,andt hef i Ｇ n i t ee l emen t me t hod ( FEM )wa scons i de r eda sa r ep r e s en t a t i ve nume r i c a l ana l i st oo l．Many r e Ｇ ys s e a r che r sus et hef i n i t ee l emen tme t hodt os t udyt he c l osur eandg r owt hbehav i o ro fsur f a c ec r a cki nr o l l Ｇ i ngp r oc e s s．Pr oc e s s i ng mapand２Df i n i t ee l emen t [] [] [] me t hodwe r eapp l i edby Awa i s４ ,Son５ andKim ６ t os imu l a t et hei n f l uenc eo fr o l l i ngt empe r a t ur eon [ ７, ８] sur f a c ec r a cko fba r s．Er va s t iandS t åh l be r Ｇ g ana l z edt hebehav i o ro fl ong i t ud i na lc r a cksandt r ans Ｇ y ve r s a lc r a cksi nt heho tr o l l i ngp r oc e s s．Yukawae t [ ９] a l s imu l a t edt hebehav i o ro fr e c t ang l ec r a ckson t hes t r i f a c e by ２D r i i d Ｇ l a s t i c FEM．Yu e t p sur g p [ , ] a l １０ １１ s imu l a t edt he behav i o ro ft het r ansve r s a l c r a cksons l abco rne randt hel ong i t ud i na lc r a cksa t s l ab edge i n VＧH (ve r t i c a l Ｇho r i z on t a l) r o l l i ng [ ] r oc e s sbyt he３D FEM １２ ．The i r wo rksp r ov i dea p wayt oana l z et hec l osur eandg r owt ho ft r ansve r s a l y c r a cksdur i ng mu l t i Ｇ s s VＧH r o l l i ng by FEM and pa
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 ( ５０２７１０６０) B i o r aphy: Yan Ｇ zhaoLUO,Do c t o r; E Ｇma i l: l uoyanzhao＠ho tma i l c om; Re c e i v e dDa t e:Ap r i l３,２０１３ g Co r r e s i ngAu t ho r:J i ong Ｇmi ngZHANG,Do c t o r,Pr o f e s s o r; E Ｇma i l:jmz ２０３＠s i na c om pond
I s sue４ Evo l u t i ono fSu r f a c eTr ans ve r s a lCr a cki nI n t e rmed i a t eS l absdu r i ng Ho tRo l l i ngDe f o rma t i on
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expe r imen t．Howeve r,t he p r opaga t i on and d i s ap Ｇ a r anc eo ft he s esu r f a c ede f e c t si nho tr o l l i nga r e pe no tye tf u l l i r edou t,andt hep r even t i ve me a s Ｇ yf gu ur e sa r ea l s ono t we l ldeve l oped．Thea im o ft h i s s t udyi st oi nve s t i t et heevo l u t i onbehav i o ro ft he ga sur f a c ede f e c t swi t haVshapeont hes l absi nmu l t i Ｇ s sho tr o l l i ngp r o c e s sandt hepo s s i b i l i t fmi n i Ｇ pa yo mi z i ng．The pr e s en tr e s e a r chi she l f u lf o runde r Ｇ p s t and i ngt heevo l u t i ono fsu r f a c et r ans ve r s a lc r a ck dur i ngho tr o l l i ngandgu i d i ngt hep r oduc t i on．
１ Exp e r imen t a lMa t e r i a l sand Me t hod s
Thea r t i f i c i a lVＧ shapedc r a ckswe r emadet oan Ｇ a l z et heevo l u t i ono fsu r f a c et r ans ve r s a lc r a cks．The y chemi c a lc ompo s i t i ono ft hewo rkp i e c eexami nedi n t h i ss t udyi sg i veni nTab l e１． Tab l e１ Ch emi c a lc ompo s i t i ono ft h ewo rkp i e c ema t e r i a l ma s s％ C
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Theexpe r imen twa sc a r r i edou tont hel abo r a Ｇ t o r l li nt heUn i ve r s i t fSc i enc eandTe chno l o Ｇ y mi yo i i ng．Theh i s the a t i ngt empe r a t u r eo ft he gyBe j ghe , r ehe a t i ngdev i c ei s１３００℃ wh i chc an me e tt her e Ｇ i r emen to fexpe r imen t s．Thed i ame t e ro fwo rkr o l l qu ontwor eve r s i b l eho tr o l l i ng mi l li s３５０ mm,and t he max imum r o l l i ng p r e s su r ei s１２００ kN,wh i ch c anad us tt hepr e s su r er educ t i onau t oma t i c a l l j y．The s amp l e swe r ecu tf r oms l absa sshowni nF i １．The g ( ) s i z eo ft hes amp l e si s９０ mm t h i ckne s s ×７５ mm ( wi d t h)×２５０ mm ( l eng t h)．Toexami net hee f f e c t o ft hei n i t i a ldep t h ont hec l o su r eand g r owt ho f , c r a ck f ou rVＧ shapedt r ans ve r s a lc r a ckswe r ec a r ved ont het opsu r f a c eo fs amp l e sr e spe c t i ve l os imu Ｇ yt l a t et her e a lc r a cks．Thewi d t ho fc r a cksi s１ mma t , , , , , t het op andt hedep t hi s３ ４ ５ and６ mm r e Ｇ spe c t i ve l i ２s howst h e mo r l ogyo fc r a c k si n y．F g pho t hewo r kp i e c eb e f o r ephy s i c a ls imu l a t i on．Th eVＧ s ha e d p c r a cksc r o s s edt hewho l ewo rkp i e c e． Ea chwo rkp i e c ewa sh e a t e dupt os e c i f i ct emp e r a Ｇ p t u r e１３００℃i nt h er e h e a t i ngf u r n a c ea ndh e l df o r６０mi n
F i １ Th es k e t chmapo fs amp l e g
F i ２ Lo c a t i ono fc r a ck si nt h es e c imen g p b e f o r et h ephy s i c a ls imu l a t i on
t oensur eaun i f o rmt empe r a t ur ebe f o r es t a r t i ngt he r oughr o l l i ng．Thewo rkp i e c e st akenf r omt her ehe a Ｇ t i ngf urna c ewe r ede s c a l edandt henr o l l edi nt hel a Ｇ bo r a t o r o l l i ng mi l l．Thewo rkp i e c ewa so f ft hel i ne yr a f t e rt hef i r s t,t h i rd,f i f t h,ands even t hr o l l i ngpa s Ｇ s e s．The s amp l e wa sg r ound and po l i shed us i ng s t anda rdme t a l l og r aph i cp r oc edur e sandt hene t ched wi t h４％ n i t a lt or eve a lt hemi c r os t ruc t ur e．Themi Ｇ c r os t ruc t ur ewa sobs e r ved wi t hop t i c a lmi c r os copy． Ther o l l i ng p r oc e s s pa r ame t e r sf r om ani ndus t r i a l r o l l i ng mi l lhavebe enus eda sar e f e r enc e,andde Ｇ t a i l so fr o l l i ngcond i t i onsa r eshowni nTab l e２． Tab l e２ Ro l l i ngs ch e du l e sf o rana l s i so ft h e y c r a ckd ep t he v o l u t i on Ro l l i ng s s pa
Reduc t i on r a t i o/％
Cumu l a t i ve r educ t i on/mm
Th i ckne s sa f t e r r o l l i ng/mm
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２ Re s u l t sandDi s c u s s i on ２ １ Mo r l o fc r a ck si ni n t e rme d i a t es l ab s pho gyo The quan t i t a t i ve de s c r i t i o n s o ft h e d e f e c t s p we r ehe l f u lt ogu i det hei ndus t r i a lp r oduc t i on．The p de f o rma t i onbehav i o ro fc r a ckc anbeana l z edba s ed y ont he e vo l u t i on o ft he c r a c kd e t h d u r i n r o l l i ng p g , , , r o c e s s．Thec r a c k swi t hd e t ho f３ ４ ５ and６ mm p p havebe enana l z ed．Theevo l u t i ono ft hec r a cksha s y be ens t ud i eddur i ngd i f f e r e n tp a s s e si nF i s ３ － ６ ． g I ft her e l a t i onbe twe ent heshapeo ft hede f e c t be f o r eanda f t e rr o l l i ngi sc l e a r,i tc anp r ov i d et hee f Ｇ f e c t i v e me t hodt op r ed i c tt heevo l u t i on o fc r a cks． , Fr om F i ３t o６ t her e l a t i onsh i twe en t he gs ps be shape so fsur f a c ede f e c ta f t e rr o l l i ng we r eob t a i ned． Thec r a ckdep t hd e c r e a s e dwhe nc r a c k sc amei n t oc on Ｇ
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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 ２１
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ３ Cr a cks hap eo fd i f f e r en ti n i t i a ld ep t ha f t e rf i r s tr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ４ Cr a cks hap eo fd i f f e r en ti n i t i a ld ep t ha f t e rt h i r dr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ５ Cr a cks hap eo fd i f f e r en ti n i t i a ld ep t ha f t e rf i f t hr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ６ Cr a cks hap eo fd i f f e r en ti n i t i a ld ep t ha f t e rs e v en t hr o l l i ngpa s s g
t a c twi t hr o l la ssub e c t edt oc omp r e s s i ves t r e s s． j ２ ２ I n f l u e nc eo fd i f f e r e n ti n i t i a lc r a ckd e t honc l o Ｇ p s u r eandg r owt ho fc r a ck Wi t ht her o l l i ngp r o c e ed i ng,t hewo rkp i e c ede Ｇ f o rmsc on s t a n t l ndt h es h a eo fa r t i f i c i a lVＧ s ha e d y,a p p c r a ckswi l la l s ochange．Theevo l u t i onr egu l a r i t f yo t r ansve r s esu r f a c ec r a cki nr o l l i ngp r o c e s sande f f e c t o fd i f f e r en tc r a ck dep t h on c r a ck evo l u t i on we r e , s t ud i ed wh i chi so fs i i f i c anc et oc on t r o lt hesur Ｇ gn f a c equa l i t ft he wo rkp i e c e s．Thedep t ho fc r a ck yo wi t hd i f f e r en ti n i t i a la r t i f i c i a lc r a ckdep t ha f t e rr o l l Ｇ
i ng wa sshowni nTab l e３． Wi t ht hei nc r e a s eo fr o l l i ngpa s s e s,t hec r a ck dep t hde c r e a s ed．Thed i f f e r en tc r a ck dep t hsunde r t hes amer o l l i ngpa s sa r er e l evan tt ot hei n i t i a lc r a c k d e t h;a st h ei n i t i a lc r a c kd e t hwa s３,４,５,a nd６mm, p p Tab l e３ Cr a ckd ep t ha f t e rr o l l i ng mm Ro l l i ngpa s s １ ３ ５ ７
３ １ ０ ０ ８ ０ ４ ０ ２
I n i t i a lc r a ckdep t h ４ ２ ０ １ ６ １ １ ０ ７５
５ ２ ６ ２ １ １ ２ １ ０
６ ３ ０ ２ ８ １ ５ １ ２
I s sue４ Evo l u t i ono fSu r f a c eTr ans ve r s a lCr a cki nI n t e rmed i a t eS l absdu r i ng Ho tRo l l i ngDe f o rma t i on
a f t e rr o l l i ng,i twa s１ ０,２ ０,２ ６,and３ ０ mm, r e spe c t i ve l i ７showst heevo l u t i ono ft hec r a ck y．F g dep t ha f t e rt hef i r s t,t h i r d,f i f t h,ands even t hr o l l Ｇ i ngpa s s e s． Asshowni nF i ７,t hec r a ckdep t hde c r e a s ed g whenc r a cksc amei n t oc on t a c twi t hr o l landa r esub Ｇ e c t edt opa s sr o l l i ng．Thec r a ckdep t hr educ e sr ap i d Ｇ j l ompa r i ngade epe rc r a ck wi t hasha l l owone． ybyc Thed i f f e r en tdep t hc r a cksr educ edi nt hes ameway． Wi t ht her o l l i ngp r o c e ed i ng,t hec r a ckdep t hr educ ed mo r es l owl es imi l a rr e s u l t sa r eob t a i n e dbyEr v a Ｇ y．Th
F i ７ Ev o l u t i ono fc r a ckd ep t ha f t e rr o l l i ng g
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７８ s t iand S t åh l be r whoc a r r i edou tt henume r i c a l g , s imu l a t i on so ft heVＧ s ha e dc r a c k s anda r ei nag r e e Ｇ p men twi t ht hes imu l a t i ons． [ ,]
２ ３ Mi c r o s t r u c t u r eo fi n t e rme d i a t es l ab s I ti sava l i dwayt oa ch i eveexc e l l en tme chan i c a l r ope r t i e so fs t e e l s by op t imi z i ng mi c r os t ruc t ur e s p [ ] sub e c t ed t od i f f e r en tr o l l i ng１３－１５ ．The sur f a c e j c r a ckspe c imensa f t e rd i f f e r en tr o l l i ngpa s s e s we r e exami nedbyop t i c a lmi c r os copya sshowni nF i ８ gs t o１１． Asc anbes e enf r om F i ８t o１１,t he mi c r o Ｇ gs s t ruc t ur esur r ound i ngt hec r a cki si nt hef e r r i t eand a r l i t ef o rm．Thewh i t epa r ti sf e r r i t eandt heda rk pe i spe a r l i t e． S i n c et hec a r bonc on t e n ti nt hef e r r i t ei sl e s s t hant ha ti np e a r l i t e, t hep r opo r t i ono ff e r r i t ea r oundt he c r a cki sobv i ous l i rt hant ha ti nt he ma t r i x; yh ghe ox i da t i onl aye rex i s t sabou t１００μma r oundt hec r a c k ande x t e nd sou ta tbo t hends．Wi t ht hei nc r e a s eo f t hed i s t an c ef r omt hec r a c k, t hef e r r i t ec on t e n tg r a du a l l y de c r e a s ed．Asi ti sox i d i z i nga tmo s r ei nt her e he a t i ng phe , f u r n a c e oxygenha st hechanc et ocon t a c twi t ht he i n t e rna lma t r i xo ft hes amp l e,oxyg e np e rme a t e st os u r Ｇ
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ８ Mi c r o s t r u c t u r ea r oundc r a ckandma t r i xa f t e rt h ef i r s tr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ９ Mi c r o s t r u c t u r ea r oundc r a ckandma t r i xa f t e rt h et h i r dr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i １０ Mi c r o s t r u c t u r ea r oundc r a ckandma t r i xa f t e rt h ef i f t hr o l l i ngpa s s g
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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 ２１
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i １１ Mi c r o s t r u c t u r ea r oundc r a ckandma t r i xa f t e rt h es e v en t hr o l l i ngpa s s g
f a c ef i r s t l hen g r adua l l t r a t e si n t ot he y,andt ypene ma t r i xwi t ht het imego i ngon．Anda c c o r d i ngt ot he d i f f us i on me chan i sm, t he oxygen c onc en t r a t i on r adua l l c r e a s edf r omt hesu r f a c et ot hei n t e rna l g yde ma t r i x,s ot hef e r r i t ec on t en ta r oundc r a cki sh i r ghe t hant ha ti nt hei n t e rna lma t r i xandabou t１００ μm ox i da t i onl aye ra r er e l a t i vet ooxygend i f f us i oncha r Ｇ [ , ] a c t e r i s t i c s１６ １７ ． ２ ４ Ana l s i so fc r a ckb e ha v i o rdu r i ngr o l l i ng y Asc anbes e enf r om F i ３t o６,t her e su l t so f gs d i f f e r en tc r a ck dep t ha f t e rf i r s t,t h i r d,f i f t h,and s even t hpa s s e sr o l l i nga r eana l z ed．Af t e rs evenpa s Ｇ y s e sr o l l i ng,t h ed e t hi s０ ２,０ ７５,１ ０,a nd１ ２mm, p
r e spe c t i ve l r a ckdep t hg r adua l l c r e a s eda f Ｇ y．Thec yde t e rsubs equen tr o l l i ngpa s s e s． I nt hei ndus t r i a lp r o Ｇ duc t i on pr o c e s s,t he su r f a c ec r a ck o fc on t i nuous c a s t i ng s l ab c ou l dd i s appe a ra f t e rr o l l i ng p r o c e s s． Whenr o l l i ng wa sc a r r i edon,t hetwosu r f a c e so f c r a ck wi l lc on t a c t wi t he a ch o t he r,b r i ng i ng t he r e s su r eontwoc r a cksu r f a c e swh i chi snamedwi t h p t hecon t a c tpr e s su r e．Thec omp r e s s i ves t r e s s e sa r e t hes t r onge s ti nt het h i ckne s sd i r e c t i on,wh i ch wi l l , maket hesu r f a c ec r a ckc on t a c te a cho t he rt hec r a ck dep t hde c r e a s edg r adua l l ti she l f u lf o rc r a ck yandi p c l osur e．Wi t ht her o l l i ngp r o c e ed i ng,t heme t a l l ur i Ｇ g , c a lcomb i na t i onhappensa tt hesu r f a c ec r a ck and [ ] t hec r a ckc anc l o s ea tl a s t１８ ．
３ Conc l u s i on s
( １)Thea r t i f i c i a lVＧ shapedc r a ckswe r e made t od i s c l o s et he mi c r o s t ruc t u r eandt heevo l u t i ono f t hec r a ckdep t hi nt her o l l i ngp r o c e s s．Wi t ht hei n Ｇ c r e a s eo fr o l l i ngpa s s e s,t hedep t ho fc r a ckwi t hd i f Ｇ f e r en ti n i t i a ldep t ha l lr educ ed．Thec r a ckdep t ha f t e r r o l l i ng ha sc l o s ec o r r e l a t i on wi t ht hei n i t i a lc r a ck dep t ha tt hes amer o l l i ngpa s s． ( ２)Themi c r o s t ruc t u r ea r oundc r a ckwi t ht he d i f f e r en tdep t ha r ea lmo s ti den t i c a l,wi t hou tobv i ous
co r r e l a t i onwi t ht hei n i t i a lc r a ckdep t h．The mi c r o Ｇ s t ruc t ur ea r ea l li nf e r r i t eandpe a r l i t ef o rm,andf e r Ｇ r i t econ t en ta r oundt hec r a ck wa sobv i ous l i r yh ghe ; t hant ha ti nt hema t r i x ox i da t i onl aye rex i s t sabout １００μma r oundt hec r a c kande x t e nd sou ta tbo t he nd s． ( ３)Thecomp r e s s i ves t r e s s e sa r et hes t r onge s t , i nt het h i ckne s sd i r e c t i on br i ng i ngt hep r e s sur eon twoc r a cksur f a c e s．Wi t ht her o l l i ngp r oc e ed i ng,t he c r a ckdep t hg r adua l l d e c r e a s e da f t e r r o l l i n a s s e y gp s, andt hedep t ho fc r a ckwi t hi n i t i a ldep t ho f３ mmi s on l ２ mma f t e r７r o l l i ngpa s s e s． y０ Re f e r enc e s:
[ １] K． Schwe r d t f ege r,I S I JI n t． ５１ ( ２０１１)１１９７ Ｇ １１９９． [ ２] HL．Yu,X H．L i u,J． I r onS t e e lRe s． ２１ ( ２００９)No ９,２３ Ｇ ２７． [ ３] J K．Br ima c ombe,K．So r ima ch i,Me t a l．Tr ans．B８ ( １９７７)
４８９ Ｇ ５０５． [ ４] M．Awa i s,HW． Le e,YT． Im,HC．Kwon,S． Byon,HD． Pa rk,J．Ma t e r．Pr o c e s s．Te chno l． ２０１ ( ２００８)７３ Ｇ ７８． [ ５] I H． Son,HD．Le e,S．Cho i,D L．Le e,YH． Im,J．Ma t e r． Pr o c e s s．Te chno l． ２０１ ( ２００８)９１ Ｇ ９６． [ ６] HY．Kim,HC．Kwon,HW．Le e,YH． Im,S M．Byon, HD．Pa rk,J．Ma t e r．Pr o c e s s．Te chno l． ２０５ ( ２００８)７０ Ｇ ８０． [ t e r．Pr o c e s s．Te chno l．９４ ７] Es a Er va s t i,Ui fS t åh l be r g,J．Ma ( １９９９)１４１ Ｇ １５０． [ t e r．Pr o c e s s．Te chno l．１０１ ８] Ea sEr va s t i,Ui fS t åh l be r g,J．Ma ( ２０００)３１２ Ｇ ３２１． [ ９] N．Yukawa,T． I sh i kawa,Y．Yo sh i da,A．Koya ch i,Te t su Ｇ t o Ｇ Hagane９１ ( ２００５)８６１ Ｇ ８６７． [ １０] HL．Yu,X H．L i u,GD．Wang,J． I r onS t e e lRe s． I n t． １５ ( ２００８)No ３,１９ Ｇ ２６． [ １１] HL．Yu,X H．L i u,C S．L i,L Q．Chen,Ma t e r．Sc i．Fo Ｇ r um５７５ Ｇ ５７８ ( ２００８)２４３ Ｇ ２４８． [ １２] N．Yukawa,Y．Yo sh i da,T． I sh i kawa,Te t su Ｇ t o ＧHagane,９２ ( ２００６)１９ Ｇ ２４． [ １３] L Y．Lan,C L．Qi u,D W．Zhao,X H．Gao,J． I r onS t e e l Re s． I n t． １８ ( ２０１１)No ２,５７ Ｇ ６３． [ １４] F R．Xi ao,B． L i ao,YY． Shan,GY．Qi ao,Y． Zhong,C L． Zhang,K．Yang,Ma t e r． Sc i．Eng．A４３１ ( ２００６)４１ Ｇ ５２． [ １５] W．Wang,W．Yan,L．Zhu,P．Hu,YY．Shan,K．Yang, Ma t e r．De s． ３０ ( ２００９)３４３６ Ｇ ３４４３． [ １６] YF．Gong,De B C．Cooman,I S I JI n t． ５１ ( ２０１１)６３０ Ｇ ６３７． [ １７] S． Jung,MS．Kwon,J．Pa rk,S B．Kim,Y．Huh,I S I JI n t． ５１ ( ２０１１)１１６３ Ｇ １１６８． [ １８] X H．L i u,HL．Yu,J． I r onS t e e lRe s． ２１ ( ２００９)No ６,３６ Ｇ ３９．
JOURNALOFIRON ANDSTEELRESEARCH,INTERNATIONAL２ ０ １ ４,２ １( ４):４ ７ ６ Ｇ ４ ８ ０
Ev o l u t i ono fSu r f a c eTr an s v e r s a lCr a cki nI n t e rme d i a t e S l ab sdu r i ngHo tRo l l i ngDe f o rma t i on Yan Ｇ zhaoLUO, J i ong Ｇmi ngZHANG, Qi angLIU, Mao ＧkangLI, Yi Ｇ t ong HAN, Yi Ｇ f e iCAO
( Schoo lo fMe t a l l u r i c a landEc o l og i c a lEng i ne e r i ng,Un i ve r s i t fSc i enc eandTe chno l ogyBe i i ng,Be i i ng１０００８３,Ch i na) g yo j j Ab s t r a c t:I nt hec on t i nuousc a s t i ngo fs t e e l s,su r f a c et r ans ve r s a lc r a cksa r eo f t enf ound．The s ede f e c t smays e ve r e l y i n f l uenc et hef i na lqua l i t ft hep r oduc t s．Thee vo l u t i ono ft r ans ve r s a lVＧ shapedc r a ckswi t hd i f f e r en tdep t hont he yo su r f a c eo fac on t i nuous l a s ts t e e ls l abdu r i ngho tr o l l i ngwa ss t ud i ed．Thea r t i f i c i a lVＧ shapedc r a ckswe r emadeon yc t hesu r f a c e．Ther o l l i ngp r o c e s spa r ame t e r sf r omani ndus t r i a lr o l l i ngmi l lhavebe enus eda sar e f e r enc e．Thespe c i Ｇ menso fr o l l edwo r kp i e c ei ni n t e rmed i a t es l abswe r eob t a i neda f t e rd i f f e r en tr o l l i ngpa s s e s．Themo r l ogyo fsu r Ｇ pho f a c ec r a ckand mi c r o s t r uc t u r ee vo l u t i oni nt her o l l i ngp r o c e s swe r ei nve s t i t edbyop t i c a lmi c r o s c opy．Ther e su l t s ga showt ha tt hedep t ho fsu r f a c et r ans ve r s a lc r a ckg r adua l l c r e a s edwi t ht hei nc r e a s eo fr o l l i ngpa s s e s．Theg r a i ns i z e yde o ff e r r i t eandpe a r l i t eont hes amp l esu r f a c ea l s og r adua l l educ ed．Themi c r o s t r uc t u r e sa r oundc r a ckswi t ht hed i f Ｇ yr f e r en tdep t ha r ea lmo s ti den t i c a l,wi t hou td i r e c tc o r r e l a t i onwi t ht hei n i t i a lc r a ckdep t h． Ke r d s:i n t e rmed i a t es l ab;su r f a c et r ans ve r s a lc r a ck;mi c r o s t r uc t u r e;ho tr o l l i ng ywo
I nt hec on t i nuousc a s t i ng o fs t e e l s,su r f a c e t r ansve r s a lc r a cksa r eo f t enf oundduet ochemi c a l , compo s i t i ono ft hema t e r i a lt he rma ls t r e s sando t hＧ [] e rf a c t o r s１ ．I ft he s ec r a cksc anno tbee l imi na t ed dur i ngsubs equen tho tr o l l i ngp r o c e s s,t he s ec r a cks appe a r i ngonsu r f a c es l ab mayc aus es e r i ousqua l i t y r ob l emi nf i na lp r oduc t s． I ti sobv i oust ha te l imi na Ｇ p t i ono ft hepo s s i b l ec aus e so ft hede f e c twe r eane f Ｇ f e c t i veme t hodf o reng i ne e r sandp r oduc e r s．Sot he i nve s t i t i ono ft heevo l u t i ono ft hesu r f a c ec r a cki n ga i n t e rmed i a t es l absi so ft heo r e t i c a ls i n i f i c a n c et o g op t imi z et her o l l i ngp r o c e s sandt oimp r ovet hef i na l [] l i t fp r oduc t s２ ．Thei n t e rmed i a t es l absa r e qua yo comi ngo f ft h ep r odu c t i onl i n ea f t e re a c hr o l l i ngp a s s． Mo s to fr e s e a r che r sf o cus on t he c aus e so f c r a cksandf i nd i ngawayt oavo i dt hem．Ther e a sons o ft hec r a ckss e emt obec l e a r,sucha st henon Ｇme Ｇ , , t a l l i ci nc l us i ons e l emen ts eg r ega t i on unevenhe a t Ｇ i ngands oon．Howeve r, i ti ss e l domr e s e a r chedt ha t t heevo l u t i ono fc r a cksi nt heho tr o l l i ngp r o c e s s,e s Ｇ c i a l l abo r a t o r t udy on evo l u t i on o f su r f a c e pe yl ys [] t r ansve r s a lc r a cks e emseven muchl e s s３ ．
I nr e c en tye a r s,wi t ht her ap i ddeve l opmen to f t hecompu t e rha rdwa r eandso f twa r e,cons i de r ab l e i nve s t i t i onshavebe enc a r r i edont heevo l u t i onbe Ｇ ga hav i o ro fc r a cksdur i ngr o l l i ngp r oc e s s,andt hef i Ｇ n i t ee l emen t me t hod ( FEM )wa scons i de r eda sa r ep r e s en t a t i ve nume r i c a l ana l i st oo l．Many r e Ｇ ys s e a r che r sus et hef i n i t ee l emen tme t hodt os t udyt he c l osur eandg r owt hbehav i o ro fsur f a c ec r a cki nr o l l Ｇ i ngp r oc e s s．Pr oc e s s i ng mapand２Df i n i t ee l emen t [] [] [] me t hodwe r eapp l i edby Awa i s４ ,Son５ andKim ６ t os imu l a t et hei n f l uenc eo fr o l l i ngt empe r a t ur eon [ ７, ８] sur f a c ec r a cko fba r s．Er va s t iandS t åh l be r Ｇ g ana l z edt hebehav i o ro fl ong i t ud i na lc r a cksandt r ans Ｇ y ve r s a lc r a cksi nt heho tr o l l i ngp r oc e s s．Yukawae t [ ９] a l s imu l a t edt hebehav i o ro fr e c t ang l ec r a ckson t hes t r i f a c e by ２D r i i d Ｇ l a s t i c FEM．Yu e t p sur g p [ , ] a l １０ １１ s imu l a t edt he behav i o ro ft het r ansve r s a l c r a cksons l abco rne randt hel ong i t ud i na lc r a cksa t s l ab edge i n VＧH (ve r t i c a l Ｇho r i z on t a l) r o l l i ng [ ] r oc e s sbyt he３D FEM １２ ．The i r wo rksp r ov i dea p wayt oana l z et hec l osur eandg r owt ho ft r ansve r s a l y c r a cksdur i ng mu l t i Ｇ s s VＧH r o l l i ng by FEM and pa
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 ( ５０２７１０６０) B i o r aphy: Yan Ｇ zhaoLUO,Do c t o r; E Ｇma i l: l uoyanzhao＠ho tma i l c om; Re c e i v e dDa t e:Ap r i l３,２０１３ g Co r r e s i ngAu t ho r:J i ong Ｇmi ngZHANG,Do c t o r,Pr o f e s s o r; E Ｇma i l:jmz ２０３＠s i na c om pond
I s sue４ Evo l u t i ono fSu r f a c eTr ans ve r s a lCr a cki nI n t e rmed i a t eS l absdu r i ng Ho tRo l l i ngDe f o rma t i on
４７７
expe r imen t．Howeve r,t he p r opaga t i on and d i s ap Ｇ a r anc eo ft he s esu r f a c ede f e c t si nho tr o l l i nga r e pe no tye tf u l l i r edou t,andt hep r even t i ve me a s Ｇ yf gu ur e sa r ea l s ono t we l ldeve l oped．Thea im o ft h i s s t udyi st oi nve s t i t et heevo l u t i onbehav i o ro ft he ga sur f a c ede f e c t swi t haVshapeont hes l absi nmu l t i Ｇ s sho tr o l l i ngp r o c e s sandt hepo s s i b i l i t fmi n i Ｇ pa yo mi z i ng．The pr e s en tr e s e a r chi she l f u lf o runde r Ｇ p s t and i ngt heevo l u t i ono fsu r f a c et r ans ve r s a lc r a ck dur i ngho tr o l l i ngandgu i d i ngt hep r oduc t i on．
１ Exp e r imen t a lMa t e r i a l sand Me t hod s
Thea r t i f i c i a lVＧ shapedc r a ckswe r emadet oan Ｇ a l z et heevo l u t i ono fsu r f a c et r ans ve r s a lc r a cks．The y chemi c a lc ompo s i t i ono ft hewo rkp i e c eexami nedi n t h i ss t udyi sg i veni nTab l e１． Tab l e１ Ch emi c a lc ompo s i t i ono ft h ewo rkp i e c ema t e r i a l ma s s％ C
S i
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Theexpe r imen twa sc a r r i edou tont hel abo r a Ｇ t o r l li nt heUn i ve r s i t fSc i enc eandTe chno l o Ｇ y mi yo i i ng．Theh i s the a t i ngt empe r a t u r eo ft he gyBe j ghe , r ehe a t i ngdev i c ei s１３００℃ wh i chc an me e tt her e Ｇ i r emen to fexpe r imen t s．Thed i ame t e ro fwo rkr o l l qu ontwor eve r s i b l eho tr o l l i ng mi l li s３５０ mm,and t he max imum r o l l i ng p r e s su r ei s１２００ kN,wh i ch c anad us tt hepr e s su r er educ t i onau t oma t i c a l l j y．The s amp l e swe r ecu tf r oms l absa sshowni nF i １．The g ( ) s i z eo ft hes amp l e si s９０ mm t h i ckne s s ×７５ mm ( wi d t h)×２５０ mm ( l eng t h)．Toexami net hee f f e c t o ft hei n i t i a ldep t h ont hec l o su r eand g r owt ho f , c r a ck f ou rVＧ shapedt r ans ve r s a lc r a ckswe r ec a r ved ont het opsu r f a c eo fs amp l e sr e spe c t i ve l os imu Ｇ yt l a t et her e a lc r a cks．Thewi d t ho fc r a cksi s１ mma t , , , , , t het op andt hedep t hi s３ ４ ５ and６ mm r e Ｇ spe c t i ve l i ２s howst h e mo r l ogyo fc r a c k si n y．F g pho t hewo r kp i e c eb e f o r ephy s i c a ls imu l a t i on．Th eVＧ s ha e d p c r a cksc r o s s edt hewho l ewo rkp i e c e． Ea chwo rkp i e c ewa sh e a t e dupt os e c i f i ct emp e r a Ｇ p t u r e１３００℃i nt h er e h e a t i ngf u r n a c ea ndh e l df o r６０mi n
F i １ Th es k e t chmapo fs amp l e g
F i ２ Lo c a t i ono fc r a ck si nt h es e c imen g p b e f o r et h ephy s i c a ls imu l a t i on
t oensur eaun i f o rmt empe r a t ur ebe f o r es t a r t i ngt he r oughr o l l i ng．Thewo rkp i e c e st akenf r omt her ehe a Ｇ t i ngf urna c ewe r ede s c a l edandt henr o l l edi nt hel a Ｇ bo r a t o r o l l i ng mi l l．Thewo rkp i e c ewa so f ft hel i ne yr a f t e rt hef i r s t,t h i rd,f i f t h,ands even t hr o l l i ngpa s Ｇ s e s．The s amp l e wa sg r ound and po l i shed us i ng s t anda rdme t a l l og r aph i cp r oc edur e sandt hene t ched wi t h４％ n i t a lt or eve a lt hemi c r os t ruc t ur e．Themi Ｇ c r os t ruc t ur ewa sobs e r ved wi t hop t i c a lmi c r os copy． Ther o l l i ng p r oc e s s pa r ame t e r sf r om ani ndus t r i a l r o l l i ng mi l lhavebe enus eda sar e f e r enc e,andde Ｇ t a i l so fr o l l i ngcond i t i onsa r eshowni nTab l e２． Tab l e２ Ro l l i ngs ch e du l e sf o rana l s i so ft h e y c r a ckd ep t he v o l u t i on Ro l l i ng s s pa
Reduc t i on r a t i o/％
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Th i ckne s sa f t e r r o l l i ng/mm
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１６ ０ １６ ４ ２３ ８
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２ Re s u l t sandDi s c u s s i on ２ １ Mo r l o fc r a ck si ni n t e rme d i a t es l ab s pho gyo The quan t i t a t i ve de s c r i t i o n s o ft h e d e f e c t s p we r ehe l f u lt ogu i det hei ndus t r i a lp r oduc t i on．The p de f o rma t i onbehav i o ro fc r a ckc anbeana l z edba s ed y ont he e vo l u t i on o ft he c r a c kd e t h d u r i n r o l l i ng p g , , , r o c e s s．Thec r a c k swi t hd e t ho f３ ４ ５ and６ mm p p havebe enana l z ed．Theevo l u t i ono ft hec r a cksha s y be ens t ud i eddur i ngd i f f e r e n tp a s s e si nF i s ３ － ６ ． g I ft her e l a t i onbe twe ent heshapeo ft hede f e c t be f o r eanda f t e rr o l l i ngi sc l e a r,i tc anp r ov i d et hee f Ｇ f e c t i v e me t hodt op r ed i c tt heevo l u t i on o fc r a cks． , Fr om F i ３t o６ t her e l a t i onsh i twe en t he gs ps be shape so fsur f a c ede f e c ta f t e rr o l l i ng we r eob t a i ned． Thec r a ckdep t hd e c r e a s e dwhe nc r a c k sc amei n t oc on Ｇ
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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 ２１
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ３ Cr a cks hap eo fd i f f e r en ti n i t i a ld ep t ha f t e rf i r s tr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ４ Cr a cks hap eo fd i f f e r en ti n i t i a ld ep t ha f t e rt h i r dr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ５ Cr a cks hap eo fd i f f e r en ti n i t i a ld ep t ha f t e rf i f t hr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ６ Cr a cks hap eo fd i f f e r en ti n i t i a ld ep t ha f t e rs e v en t hr o l l i ngpa s s g
t a c twi t hr o l la ssub e c t edt oc omp r e s s i ves t r e s s． j ２ ２ I n f l u e nc eo fd i f f e r e n ti n i t i a lc r a ckd e t honc l o Ｇ p s u r eandg r owt ho fc r a ck Wi t ht her o l l i ngp r o c e ed i ng,t hewo rkp i e c ede Ｇ f o rmsc on s t a n t l ndt h es h a eo fa r t i f i c i a lVＧ s ha e d y,a p p c r a ckswi l la l s ochange．Theevo l u t i onr egu l a r i t f yo t r ansve r s esu r f a c ec r a cki nr o l l i ngp r o c e s sande f f e c t o fd i f f e r en tc r a ck dep t h on c r a ck evo l u t i on we r e , s t ud i ed wh i chi so fs i i f i c anc et oc on t r o lt hesur Ｇ gn f a c equa l i t ft he wo rkp i e c e s．Thedep t ho fc r a ck yo wi t hd i f f e r en ti n i t i a la r t i f i c i a lc r a ckdep t ha f t e rr o l l Ｇ
i ng wa sshowni nTab l e３． Wi t ht hei nc r e a s eo fr o l l i ngpa s s e s,t hec r a ck dep t hde c r e a s ed．Thed i f f e r en tc r a ck dep t hsunde r t hes amer o l l i ngpa s sa r er e l evan tt ot hei n i t i a lc r a c k d e t h;a st h ei n i t i a lc r a c kd e t hwa s３,４,５,a nd６mm, p p Tab l e３ Cr a ckd ep t ha f t e rr o l l i ng mm Ro l l i ngpa s s １ ３ ５ ７
３ １ ０ ０ ８ ０ ４ ０ ２
I n i t i a lc r a ckdep t h ４ ２ ０ １ ６ １ １ ０ ７５
５ ２ ６ ２ １ １ ２ １ ０
６ ３ ０ ２ ８ １ ５ １ ２
I s sue４ Evo l u t i ono fSu r f a c eTr ans ve r s a lCr a cki nI n t e rmed i a t eS l absdu r i ng Ho tRo l l i ngDe f o rma t i on
a f t e rr o l l i ng,i twa s１ ０,２ ０,２ ６,and３ ０ mm, r e spe c t i ve l i ７showst heevo l u t i ono ft hec r a ck y．F g dep t ha f t e rt hef i r s t,t h i r d,f i f t h,ands even t hr o l l Ｇ i ngpa s s e s． Asshowni nF i ７,t hec r a ckdep t hde c r e a s ed g whenc r a cksc amei n t oc on t a c twi t hr o l landa r esub Ｇ e c t edt opa s sr o l l i ng．Thec r a ckdep t hr educ e sr ap i d Ｇ j l ompa r i ngade epe rc r a ck wi t hasha l l owone． ybyc Thed i f f e r en tdep t hc r a cksr educ edi nt hes ameway． Wi t ht her o l l i ngp r o c e ed i ng,t hec r a ckdep t hr educ ed mo r es l owl es imi l a rr e s u l t sa r eob t a i n e dbyEr v a Ｇ y．Th
F i ７ Ev o l u t i ono fc r a ckd ep t ha f t e rr o l l i ng g
４７９
７８ s t iand S t åh l be r whoc a r r i edou tt henume r i c a l g , s imu l a t i on so ft heVＧ s ha e dc r a c k s anda r ei nag r e e Ｇ p men twi t ht hes imu l a t i ons． [ ,]
２ ３ Mi c r o s t r u c t u r eo fi n t e rme d i a t es l ab s I ti sava l i dwayt oa ch i eveexc e l l en tme chan i c a l r ope r t i e so fs t e e l s by op t imi z i ng mi c r os t ruc t ur e s p [ ] sub e c t ed t od i f f e r en tr o l l i ng１３－１５ ．The sur f a c e j c r a ckspe c imensa f t e rd i f f e r en tr o l l i ngpa s s e s we r e exami nedbyop t i c a lmi c r os copya sshowni nF i ８ gs t o１１． Asc anbes e enf r om F i ８t o１１,t he mi c r o Ｇ gs s t ruc t ur esur r ound i ngt hec r a cki si nt hef e r r i t eand a r l i t ef o rm．Thewh i t epa r ti sf e r r i t eandt heda rk pe i spe a r l i t e． S i n c et hec a r bonc on t e n ti nt hef e r r i t ei sl e s s t hant ha ti np e a r l i t e, t hep r opo r t i ono ff e r r i t ea r oundt he c r a cki sobv i ous l i rt hant ha ti nt he ma t r i x; yh ghe ox i da t i onl aye rex i s t sabou t１００μma r oundt hec r a c k ande x t e nd sou ta tbo t hends．Wi t ht hei nc r e a s eo f t hed i s t an c ef r omt hec r a c k, t hef e r r i t ec on t e n tg r a du a l l y de c r e a s ed．Asi ti sox i d i z i nga tmo s r ei nt her e he a t i ng phe , f u r n a c e oxygenha st hechanc et ocon t a c twi t ht he i n t e rna lma t r i xo ft hes amp l e,oxyg e np e rme a t e st os u r Ｇ
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ８ Mi c r o s t r u c t u r ea r oundc r a ckandma t r i xa f t e rt h ef i r s tr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i ９ Mi c r o s t r u c t u r ea r oundc r a ckandma t r i xa f t e rt h et h i r dr o l l i ngpa s s g
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i １０ Mi c r o s t r u c t u r ea r oundc r a ckandma t r i xa f t e rt h ef i f t hr o l l i ngpa s s g
４８０
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 ２１
( a)３ mm; ( b)４ mm; ( c)５ mm; ( d)６ mm．
F i １１ Mi c r o s t r u c t u r ea r oundc r a ckandma t r i xa f t e rt h es e v en t hr o l l i ngpa s s g
f a c ef i r s t l hen g r adua l l t r a t e si n t ot he y,andt ypene ma t r i xwi t ht het imego i ngon．Anda c c o r d i ngt ot he d i f f us i on me chan i sm, t he oxygen c onc en t r a t i on r adua l l c r e a s edf r omt hesu r f a c et ot hei n t e rna l g yde ma t r i x,s ot hef e r r i t ec on t en ta r oundc r a cki sh i r ghe t hant ha ti nt hei n t e rna lma t r i xandabou t１００ μm ox i da t i onl aye ra r er e l a t i vet ooxygend i f f us i oncha r Ｇ [ , ] a c t e r i s t i c s１６ １７ ． ２ ４ Ana l s i so fc r a ckb e ha v i o rdu r i ngr o l l i ng y Asc anbes e enf r om F i ３t o６,t her e su l t so f gs d i f f e r en tc r a ck dep t ha f t e rf i r s t,t h i r d,f i f t h,and s even t hpa s s e sr o l l i nga r eana l z ed．Af t e rs evenpa s Ｇ y s e sr o l l i ng,t h ed e t hi s０ ２,０ ７５,１ ０,a nd１ ２mm, p
r e spe c t i ve l r a ckdep t hg r adua l l c r e a s eda f Ｇ y．Thec yde t e rsubs equen tr o l l i ngpa s s e s． I nt hei ndus t r i a lp r o Ｇ duc t i on pr o c e s s,t he su r f a c ec r a ck o fc on t i nuous c a s t i ng s l ab c ou l dd i s appe a ra f t e rr o l l i ng p r o c e s s． Whenr o l l i ng wa sc a r r i edon,t hetwosu r f a c e so f c r a ck wi l lc on t a c t wi t he a ch o t he r,b r i ng i ng t he r e s su r eontwoc r a cksu r f a c e swh i chi snamedwi t h p t hecon t a c tpr e s su r e．Thec omp r e s s i ves t r e s s e sa r e t hes t r onge s ti nt het h i ckne s sd i r e c t i on,wh i ch wi l l , maket hesu r f a c ec r a ckc on t a c te a cho t he rt hec r a ck dep t hde c r e a s edg r adua l l ti she l f u lf o rc r a ck yandi p c l osur e．Wi t ht her o l l i ngp r o c e ed i ng,t heme t a l l ur i Ｇ g , c a lcomb i na t i onhappensa tt hesu r f a c ec r a ck and [ ] t hec r a ckc anc l o s ea tl a s t１８ ．
３ Conc l u s i on s
( １)Thea r t i f i c i a lVＧ shapedc r a ckswe r e made t od i s c l o s et he mi c r o s t ruc t u r eandt heevo l u t i ono f t hec r a ckdep t hi nt her o l l i ngp r o c e s s．Wi t ht hei n Ｇ c r e a s eo fr o l l i ngpa s s e s,t hedep t ho fc r a ckwi t hd i f Ｇ f e r en ti n i t i a ldep t ha l lr educ ed．Thec r a ckdep t ha f t e r r o l l i ng ha sc l o s ec o r r e l a t i on wi t ht hei n i t i a lc r a ck dep t ha tt hes amer o l l i ngpa s s． ( ２)Themi c r o s t ruc t u r ea r oundc r a ckwi t ht he d i f f e r en tdep t ha r ea lmo s ti den t i c a l,wi t hou tobv i ous
co r r e l a t i onwi t ht hei n i t i a lc r a ckdep t h．The mi c r o Ｇ s t ruc t ur ea r ea l li nf e r r i t eandpe a r l i t ef o rm,andf e r Ｇ r i t econ t en ta r oundt hec r a ck wa sobv i ous l i r yh ghe ; t hant ha ti nt hema t r i x ox i da t i onl aye rex i s t sabout １００μma r oundt hec r a c kande x t e nd sou ta tbo t he nd s． ( ３)Thecomp r e s s i ves t r e s s e sa r et hes t r onge s t , i nt het h i ckne s sd i r e c t i on br i ng i ngt hep r e s sur eon twoc r a cksur f a c e s．Wi t ht her o l l i ngp r oc e ed i ng,t he c r a ckdep t hg r adua l l d e c r e a s e da f t e r r o l l i n a s s e y gp s, andt hedep t ho fc r a ckwi t hi n i t i a ldep t ho f３ mmi s on l ２ mma f t e r７r o l l i ngpa s s e s． y０ Re f e r enc e s:
[ １] K． Schwe r d t f ege r,I S I JI n t． ５１ ( ２０１１)１１９７ Ｇ １１９９． [ ２] HL．Yu,X H．L i u,J． I r onS t e e lRe s． ２１ ( ２００９)No ９,２３ Ｇ ２７． [ ３] J K．Br ima c ombe,K．So r ima ch i,Me t a l．Tr ans．B８ ( １９７７)
４８９ Ｇ ５０５． [ ４] M．Awa i s,HW． Le e,YT． Im,HC．Kwon,S． Byon,HD． Pa rk,J．Ma t e r．Pr o c e s s．Te chno l． ２０１ ( ２００８)７３ Ｇ ７８． [ ５] I H． Son,HD．Le e,S．Cho i,D L．Le e,YH． Im,J．Ma t e r． Pr o c e s s．Te chno l． ２０１ ( ２００８)９１ Ｇ ９６． [ ６] HY．Kim,HC．Kwon,HW．Le e,YH． Im,S M．Byon, HD．Pa rk,J．Ma t e r．Pr o c e s s．Te chno l． ２０５ ( ２００８)７０ Ｇ ８０． [ t e r．Pr o c e s s．Te chno l．９４ ７] Es a Er va s t i,Ui fS t åh l be r g,J．Ma ( １９９９)１４１ Ｇ １５０． [ t e r．Pr o c e s s．Te chno l．１０１ ８] Ea sEr va s t i,Ui fS t åh l be r g,J．Ma ( ２０００)３１２ Ｇ ３２１． [ ９] N．Yukawa,T． I sh i kawa,Y．Yo sh i da,A．Koya ch i,Te t su Ｇ t o Ｇ Hagane９１ ( ２００５)８６１ Ｇ ８６７． [ １０] HL．Yu,X H．L i u,GD．Wang,J． I r onS t e e lRe s． I n t． １５ ( ２００８)No ３,１９ Ｇ ２６． [ １１] HL．Yu,X H．L i u,C S．L i,L Q．Chen,Ma t e r．Sc i．Fo Ｇ r um５７５ Ｇ ５７８ ( ２００８)２４３ Ｇ ２４８． [ １２] N．Yukawa,Y．Yo sh i da,T． I sh i kawa,Te t su Ｇ t o ＧHagane,９２ ( ２００６)１９ Ｇ ２４． [ １３] L Y．Lan,C L．Qi u,D W．Zhao,X H．Gao,J． I r onS t e e l Re s． I n t． １８ ( ２０１１)No ２,５７ Ｇ ６３． [ １４] F R．Xi ao,B． L i ao,YY． Shan,GY．Qi ao,Y． Zhong,C L． Zhang,K．Yang,Ma t e r． Sc i．Eng．A４３１ ( ２００６)４１ Ｇ ５２． [ １５] W．Wang,W．Yan,L．Zhu,P．Hu,YY．Shan,K．Yang, Ma t e r．De s． ３０ ( ２００９)３４３６ Ｇ ３４４３． [ １６] YF．Gong,De B C．Cooman,I S I JI n t． ５１ ( ２０１１)６３０ Ｇ ６３７． [ １７] S． Jung,MS．Kwon,J．Pa rk,S B．Kim,Y．Huh,I S I JI n t． ５１ ( ２０１１)１１６３ Ｇ １１６８． [ １８] X H．L i u,HL．Yu,J． I r onS t e e lRe s． ２１ ( ２００９)No ６,３６ Ｇ ３９．