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Effect of Boron on CGHAZ Microstructure and Toughness of High Strength Low Alloy Steels
JOURNALOFIRON ANDSTEELRESEARCH,INTERNATIONAL２ ０ １ ４,２ １( ８):７ ８ ７ Ｇ ７ ９ ２
Ef f e c to fBo r ononCGHAZ Mi c r o s t r u c t u r eandToughne s so f Hi t r e ng t hLowAl l o t e e l s ghS yS HanYANG, Xi Ｇx i a WANG, J i n ＧboQU
( I ns t i t u t eo fRe s e a r cho fI r onandS t e e l,Sha Ｇ s t e e l,Zhang i agang２１５６２５,J i angsu,Ch i na) j Ab s t r a c t:Ef f e c to fbo r on ont he mi c r o s t r uc t u r eandimpa c tt oughne s si nt hec oa r s e Ｇ r a i ned he a t Ｇ a f f e c t edz one g ( CGHAZ)o ftwoh i t r eng t hl owa l l oys t e e l s,bo r on Ｇ f r e eandbo r on Ｇ c on t a i n i ng,wa si nve s t i t edby me anso f ghs ga we l dt he rma ls imu l a t i ont e s t．Ther e su l tshowst ha t,f o rt hebo r on Ｇ f r e es t e e l,a mi c r o s t r uc t u r ec ons i s t i ngo fg r a i n bounda r e r r i t edegene r a t e spe a r l i t eandg r anu l a rba i n i t ef o rl onge rt８/５ ( t hec oo l i ngt imef r om ８００t o５００ ℃ ), yf , rt hebo r on Ｇ c on t a i n i ngs t e e l g r anu l a rba i n i t ei sdomi nan tf o rawi der angeo f wh i l el a t hba i n i t ef o rsho r t e rt８/５ ．Fo t i nuousc oo l i ngt r ans f o rma t i on ( CCT)s t udyont heCGHAZi nd i c a t e st ha tt het r ans f o rma t i ons t a r tt empe r a Ｇ t８/５ ．Con t u r ede c r e a s e sbyabou t５０－１００ ℃ unde rd i f f e r en tt８/５ ,f o rt hebo r on Ｇ c on t a i n i ngs t e e lc ompa r edwi t ht hebo r on Ｇ f r e e s t e e l．Thep r e s enc eo fbo r onsupp r e s s e st henuc l e a t i ono ff e r r i t ea tp r i o raus t en i t eg r a i nbounda r i e sandhenc een l a r Ｇ o rg r anu l a rba i n i t et r ans f o rma t i on．Howe ve r,t headd i t i ono fbo r onde t e r i o r a t e st heimpa c t st her angeo ft８/５ f ge , t oughne s so fCGHAZ wh i ch maybeduet oa ma r ked l nc r e a s edf r a c t i ono fma r t ens i t e Ｇ aus t en i t e (MＧA)c ons t i t u Ｇ yi en t sandde c r e a s edf r a c t i ono fh i l eg r a i nbounda r i e s． ghang Ke r d s:bo r on;h i t r eng t hl owa l l oys t e e l;c oa r s e Ｇ r a i nedhe a t Ｇ a f f e c t edz one;c on t i nuousc oo l i ngt r ans f o rma Ｇ ghs g ywo ; t i on impa c tt oughne s s DOI:１０ １３２２８/ boyuan i s sn１００６ Ｇ ７０６x ２０１３０１４１ j
Hi t r e ng t hl owa l l o t e e l s,w i t hy i e l ds t r e ng t h ghs ys t i c a l l nt her angeo f４６０－５５０ MPa,havebe en yp yi wi de l edi nc ons t ruc t i on o fl a r c a l e we l ded y us ges s t ruc t u r e s,sucha sp r e s su r eve s s e l s,p i l i ne s,o f f Ｇ pe sho r ep l a t f o rms,c ons t ruc t i onandmi n i ng ma ch i ne r Ｇ i e s．Toa ch i eveh i t r eng t hwi t hou to rwi t hal i t t l e ghs compr omi s eono t he rp r ope r t i e s,pa r t i cu l a r l oughＧ yt ne s s,ap r ope rde s i fchemi c a lc ompo s i t i oncou Ｇ gno l ed wi t hadvanc ed p r o c e s s i ngt e chn i s,sucha s p que t he rmo Ｇme chan i c a lc on t r o lp r o c e s s( TMCP), i sne e Ｇ , ded．Un f o r t una t e l he ba l anc eo fh i t r eng t h y t gh s andgoodt oughne s sc anbee a s i l i s rup t edbyt he yd t he rma lc l e sexpe r i enc eddu r i ng we l d i ng,p r odu Ｇ yc c i nga r e a so fpoo rt oughne s sknowna she a t Ｇ a f f e c t ed z one s (HAZs)．Oneo ft he we ake s tpa r t so ft he s e HAZsi nh i t r eng t hs t e e l sha sbe enf oundt obe ghs t hec oa r s e Ｇ r a i nedhe a t Ｇ a f f e c t edz one ( CGHAZ)． g Bo r oni soneo ft he mo s te f f e c t i vee l emen t st o [ ] i nc r e a s eha r denab i l i t fs t e e l s１－３ ,wh i ch make si t yo o s s i b l e t o r e d u c e t h e c o n t e n t so f c o s t l a l l o i n e l e Ｇ p y y g men t s,sucha s Mo,NiandNb,wi t hasma l ladd i Ｇ
t i ono fbo r on． I ti s we l le s t ab l i shed４－６ t ha tbo r on i nc r e a s e sha rdenab i l i t nh i b i t i ngt henuc l e a t i on y byi o fp r oeu t e c t o i df e r r i t eandp r omo t i ngt het r an s f o rma Ｇ /o t i ono fb a i n i t eand rma r t e n s i t e．Howe v e r,i ta e a r s pp t ha tt hee f f e c to fbo r onont hemi c r o s t r u c t u r eandp r op Ｇ e r t i e so f HAZ,i n pa r t i cu l a rt hecon t i nuouscoo l i ng t r ans f o rma t i on ( CCT)behav i o ro fCGHAZ,ha sno t be enadequa t e l t ud i ed．Toeva l ua t et hewe l dab i l i t ys y o fh i t r eng t hs t e e l s,we l dt he rma lc l es imu l a Ｇ ghs yc t i ont e chn i sa r egoodapp r oa che st o me a sur et he que HAZt oughne s sandana l z et hemi c r ome chan i smso f y f a i l ur e． I tha sbe enshownt ha ts imu l a t edHAZt e s t s a r ee conomi c a landr ep r oduc i b l e,andg i vet hes imi Ｇ l a rt oughne s sr ank i nga st hos eona c t ua lwe l dHAZs． I nt h i ss t udy,t hemi c r os t ruc t ur eandimpa c tt oughＧ ne s si nt heCGHAZo fh i t r eng t hs t e e l swe r ei n Ｇ ghs ve s t i t edby we l dt he rma ls imu l a t i ont e s t,wi t han ga empha s i sont hee f f e c to fbo r on． [
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１ Exp e r imen t a l
Thechemi c a lcompos i t i ono ft het e s t eds t e e l si s
B i o r aphy: HanYANG,Ma s t e r,Eng i ne e r; E Ｇma i l: du＠１６３ c om; Re c e i v e dDa t e: May３１,２０１３ yanghans 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 ２１
７８８
i v e ni nT a b l e１． S t e e lA ( bo r on Ｇ f r e e)a nds t e e lB ( bo Ｇ g ) r on Ｇ c on t a i n i ng have ne a r l hes amec ompos i t i on, yt exc ep tbo r on．The s es t e e l swe r e me l t edi na１５０kg ( : va cuumi nduc t i on me l t e r mode l Cons a r c,USA), c a s tandcu ti n t os l abso f１２５ mmi nt h i ckne s s．The s l abswe r eaus t en i t i z eda t１２００ ℃ f o r１２０ mi n,and t henho tr o l l edi n t o１５mmＧ t h i ckp l a t e s,f r omas t a r t
t empe r a t ur eo f１０５０ ℃ t oaf i n i sht empe r a t ur eo f , ８３０ ℃ onap i l o tho tr o l l i ngmi l l．Theho tr o l l e dp l a t e s we r er ehe a t edandquencheda t９２０ ℃ andt hent emＧ r eda t６２０ ℃．F i １showst hemi c r os t ruc t ur e so f pe g s t e e l sAandBi nt hequenchedandt empe r ed ( QT) cond i t i on． The rma lc l e ss imu l a t i ngt heh i s t o r fCGHAZ yc yo
Tab l e１ Ch emi c a lc ompo s i t i ono ft h et e s t e ds t e e l s ma s s％
S t e e l A B
C
０ ０６ ０ ０６
S i
０ １０ ０ １０
Mn
１ ４７ １ ４９
P
０ ００８１ ０ ００７２
S
０ ００４２ ０ ００３８
Nb
０ ０１０ ０ ０１１
V
０ ０３７ ０ ０４２
Ti
Ni＋Cr＋Mo
０ ０１４
０ ６１
０ ０１１
０ ６５
B
－
０ ００１１
F i １ Mi c r o s t r u c t u r e so fs t e e lA ( a)ands t e e lB ( b)i nt h eQTc ond i t i on g
du r i ng we l d i ng we r epe r f o rmedonat h e rmome c han i Ｇ c a ls imu l a t o r (mod e l:Gl e e b l e Ｇ ３８００)．Th e r ea r ef ou r s amp l e sf o re a cht８/５ ( t hec oo l i ngt imef r om ８００t o ５００ ℃ ),onef o rt heCGHAZ CCTs t udy ( wi t hd i Ｇ me n s i on so fϕ１０ mm×８０ mm)a ndt h eo t h e r sf o rt he CGHAZimp a c tt oughn e s se x ami n a t i on (w i t hd ime n Ｇ ) s i on so f１０mm×１０ mm×８０ mm ．Th es amp l e swe r e c u ta nd ma c h i n e df r om t h e qu e n c h e da nd t emp e r e d l a t e si nt h et r a n s v e r s ed i r e c t i on．Th ep e a kt emp e r a t u r e p du r i ngt h et h e rma lc c l e swa sc ho s e na s１３００ ℃ and y he l df o r０ ５s． t８/５ wa sv a r i e df r om５t o５００s． Mi c r o s t ruc t u r e so ft hes amp l e s we r er eve a l ed bye t ch i ng wi t h４％ Ni t a lo rt heLePe r a ′sso l u t i on andobs e r vedonanop t i c a lmi c r o s c ope (mode l:Ca r l Ze i s sAx i oImage rA１m)andaf i e l demi s s i ons c an Ｇ n i ng e l e c t r on mi c r o s c ope ( FESEM, mode l:JSMＧ ７００１F)equ i t hane l e c t r on ba cks c a t t e rd i f Ｇ pped wi ( , : ) f r a c t i on EBSD mode l Channe l５ s t em．The ys s i z eandd i s t r i bu t i ono fma r t ens i t e Ｇ aus t en i t e (MＧA) cons t i t uen t swe r eme a su r edbyanimageana l z e ri n Ｇ y s t a l l edwi t ht heImage ＧPr oP l uss o f twa r e．Animpa c t t e s twa s pe r f o rmedi na c c o r danc e wi t h GB/T ２２９ wi t hs t anda r dVＧ no t c hs e c ime n s( １０ mm×１０ mm× p ) ５５ mm a t －２０ ℃ byus i nga４５０Jimpa c tt e s t e r ( mode l:I ns t r on S I Ｇ １M )．Ha r dne s st e s ti na c co rd Ｇ
anc ewi t hGB/T４３４０ １wa sconduc t edonaVi cke r s ha rdne s st e s t e r( mode l:Tukon２１００B)unde r１０kg l oad．Thef r a c t ur esur f a c e so ft heCha r c imens pyspe we r ea l soobs e r vedont heFESEM．
２ Re s u l t sandDi s c u s s i on
２ １ Mi c r o s t r u c t u r e F i ２showst heop t i c a landSEM mi c r og r aphs g o ft heCGHAZso fs t e e lA．As i i f i c an tchangei n gn t heCGHAZ mi c r os t ruc t ur ewi t ht８/５ wa sobs e r ved． Whent het８/５ i ssho r t e r( ５s),t he mi c r os t ruc t ur e ma i n l c o n s i s t so f l a t hb a i n i t ew i t has m a l la m o u n t y ( ( ) ( ) ) , o fg r anu l a rba i n i t e F i ２ a and２ d a st he gs coo l i ngr a t ei sh i randt hedwe l l i ngt imei nt he ghe aus t en i t er eg i oni ssho r t e r．Whent het８/５i si nc r e a s ed t o４０s,g r anu l a rba i n i t ei sdomi nan ti nt he mi c r o Ｇ s t ruc t ur ea sshowni nF i ２( b)and２( e)．Gr anu l a r gs ba i n i t ei sani n t e rmed i a t et empe r a t ur et r ans f o rma Ｇ t i onpr oduc t,cha r a c t e r i z edbya mi x t ur eo fba i n i t i c f e r r i t eandMＧAcons t i t uen t． Dur i ngcoo l i ngf r omt he akt empe r a t ur e( １３００ ℃ ),aus t en i t es t a r t st o pe t r ans f o rmt oba i n i t i cf e r r i t e,andt henc a rbond i f f u Ｇ /aus s e sf r om t he ba i n i t i cf e r r i t e t en i t ei n t e r f a c et o t heun t r ans f o rmedaus t en i t e． S t ab i l i z e dbyt hee n r i c h Ｇ me n to fc a r bon,apa r to ft heaus t en i t et r ans f o rmst o
I s sue８ Ef f e c to fBo r ononCGHAZ Mi c r o s t r uc t u r eandToughne s so fHi t r eng t hLow Al l oyS t e e l s ghS
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( b),( e)t８/５＝４０s; ( c),( f)t８/５＝１６０s． a),( d)t８/５＝５s; (
F i ２ Op t i c a landSEM mi c r o r aph so ft h eCGHAZi ns t e e lA g g
ma r t ens i t ea tl owe rt empe r a t u r e so rr ema i ns un Ｇ , t r ans f o rmeddownt ot her oom t empe r a t u r e f o rmＧ i ngt heMＧAc ons t i t uen t． Whent het８/５ i sf u r t he ri nc r e a s edt o１６０ s,
o fGBFandt hec emen t i t eex i s t si nt hef o rmo fsma l l i e c e so rg r anu l e s． p F i ３s howst heSEM mi c r og r a so ft heCGHAZs g ph o fs t e e lB,wh i chi smuchd i f f e r en tf r omt ha to fs t e e l A．A mi c r os t ruc t ur econ t a i n i ngl a t hba i n i t eandma r Ｇ t ens i t ei sob t a i nedf o rsho r t e rt８/５ ( ５s,F i ３( a)), g
r a i nbounda r e r r i t eanddegene r a t epe a r l i t eappe a r g yf i nt hemi c r o s t ruc t u r eo fg r a nu l a rb a i n i t edu et oal on Ｇ e rd i f f us i ont ime,a sshowni nF i ２( c)and２( f)． wh i l eg r anu l a rba i n i t ef o rawi der angeo fl onge rt８/５ g gs ( , ( ) ( ) ) , Gr a i nbounda r e r r i t enuc l e a t e son p r i o raus t en i t e ４０and１６０s F i ３ b and３ c ．Howeve r wi t h yf gs r a i nbounda r i e sandg r owsa l ongt hebounda r i e s( i n t hei nc r e a s eo ft８/５ ,t he g r anu l a rba i n i t e be come s g l eng t h)andi n t ot heg r a i ni n t e r i o r s( i n wi d t h)．De Ｇ coa r s e randt hes i z eo f MＧA cons t i t uen t si nc r e a s e s． r a t epe a r l i t egene r a l l o rmsa tt hebounda r i e s Nog r a i nbounda r e r r i t eanddegene r a t epe a r l i t ea r e gene yf yf
( a)t８/５＝５s; ( b)t８/５＝４０s; ( c)t８/５＝１６０s．
F i ３ SEM mi c r o r aph so ft h eCGHAZi ns t e e lB g g
f oundf o rs t e e lB． ２ ２ CGHAZCCTd i a r am g CGHAZ CCT d i ag r amso fs t e e l s A andBa r e ( ) ( ) , showni nF i ４ a and ４ b r e spe c t i ve l r gs y．Fo bo t hs t e e l s,t het r ans f o rma t i ons t a r tt empe r a t ur ei n Ｇ c r e a s e sa st het８/５i si nc r e a s ed．Al onge rt８/５ ,l e ad i ng t oal onge rdwe l l i ngt imea th i empe r a t ur er e Ｇ ght i on,i sf avo r ab l et ot hed i f f us i vepha s et r ans f o rma Ｇ g , , t i on．Ont hec on t r a r r t e rt８/５ l e ad i ngt oa y asho
h i rcoo l i ng r a t e,i sf avo r ab l et ot he d i sp l a c i ve ghe s et r ans f o rma t i on．A b i i f f e r enc ebe twe ent he pha gd twos t e e l si st ha tt het r ans f o rma t i ons t a r tt empe r a Ｇ t ur eo fs t e e lBi sabou t５０－１００ ℃ l owe rt hant ha t o fs t e e lA unde rd i f f e r en tt８/５ ．Ther angeo ft８/５f o r t heha rde rpha s e s,i e LBand M,i ns t e e lBi sa l so wi de rt hant ha ti ns t e e lA． I na c co rdanc e wi t h mi c r os t ruc t ur e,t heha rd Ｇ ne s so fCGHAZde c r e a s e swi t ht hei nc r e a s eo ft８/５ ． Unde rt hes amet８/５ ,t heha rdne s so ft hebo r on Ｇ con Ｇ
７９０
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
M— Ma r t ens i t e; B—Ba i n i t e; GB—Gr anu l a rba i n i t e; GBF—Gr a i nbounda r e r r i t e; DP—Degene r a t epe a r l i t e． yf
F i ４ CGHAZCCTd i a r amso fs t e e lA ( a)ands t e e lB ( b) g g
t a i n i ngs t e e li sh i rt hant ha to ft hebo r on Ｇ f r e e ghe s t e e l,andt heb i s td i f f e r enc ebe twe ent hetwo gge s t e e l sappe a r sa tt８/５ o f５s． I tc ana l s obes e ent ha t t her a t eo fde c r e a s ei nha r dne s sde c r e a s e s wi t hi nＧ c r e a s i ngt８/５ ,ands t a r t st ol eve lo f fa tapp r ox ima t e l y ６０s．Thes i n i f i c a n td i f f e r e n c e i nt h em i c r o s t r u c t u r e g andha r dne s so ft hetwos t e e l si sbe l i evedt obea t Ｇ t r i bu t edt ot headd i t i on o fbo r on．Bo r ont endst o s eg r ega t es t r ong l tg r a i nbounda r i e sandi s mos t ya e f f e c t i vei nsupp r e s s i ngt hef o rma t i ono fpr oeu t e c Ｇ t o i df e r r i t eand p r omo t i ngt heba i n i t eand/o r ma r Ｇ t ens i t et r ans f o rma t i on． ２ ３ Impa c tt oughne s s F i ５showst hee f f e c to ft８/５ ont heimpa c t g t oughne s so fs t e e l s A and B．The r ei sv i r t ua l l y no changei nt het oughne s sf o rs t e e lA,wh i l et oughＧ ne s sde c r e a s e sg r adua l l t ht８/５ f o rs t e e lB ( wi t h y wi anexc ep t i ono f４０s)．Fo ra l lt het８/５ t e s t ed ( ４０－ ２４０s),s t e e l A ha s much h i rt oughne s st han ghe s t e e lB．Oner e a s onf o rt ha t maybeduet ot hei n Ｇ c r e a s ei nt hevo l umef r a c t i ono ft hes e c ond pha s e, i e MＧAc ons t i t uen t,i ns t e e lB．MＧAc ons t i t uen t s , a r eha r dandb r i t t l epa r t i c l e sa c t i nga st hepo t en t i a l sou r c e sf o rc r a cki n i t i a t i on,andgene r a l l e ad i ngt o yl t hede t e r i o r a t i on o fimpa c tt oughne s si nt he we l d [ ７－９] HAZ ．F i ６showst hec ompa r i s ono fmo r l Ｇ g pho ogy,s i z eand d i s t r i bu t i on o f MＧA c ons t i t uen t si n s t e e l sAandB．Thenumbe rands i z eo fMＧAcons t i t Ｇ uen ti nc r e a s e ma rked l t ht headd i t i ono fbo r on y wi ( c ompa r i ngF i ６( b)and６( d)wi t hF i ６( a)and gs gs ６( c))．Spe c i f i c a l l henumbe ro fl a r Ｇ y,t ge MＧA con ２ , s t i t uen t swi t hana r e ag r e a t e rt han０ ６μm wh i ch [] a r emo r el i ke l oi n i t i a t ec r a ck i ng４ ,i smuchh i yt ghＧ e ri ns t e e lBt hant ha ti ns t e e lA． Thef r a c t i ono fh i l eg r a i nbounda r i e smay ghang
F i ５ Va r i a t i ono fimpa c tt oughn e s swi t ht８/５ g
beano t he rf a c t o ri n f l uenc i ngimpa c tt oughne s s．Ac Ｇ , co rd i ngt ot heEBSDr e su l tt hef r a c t i ono fh i Ｇ ghan l eg r a i nbounda r i e so fs t e e lAi sh i rt hant ha to f g ghe , / s t e e lB,a sshowni nF i ７． Wh e nt h e t i s１ ６ ０s g ８５ t hef r a c t i ons o fh i l eg r a i n bounda r i e sf o r gh ang s t e e l sA andBa r e０ ３３０and０ ２１６,r e spe c t i ve l y,
i nd i c a t i ngt ha tbo r onde c r e a s e st hef r a c t i ono fh i gh ang l eg r a i nbounda r i e s．Hi l eg r a i nbounda r ghang y c an ber ega rded a san e f f e c t i ve obs t a c l et oc r a ck [ ] r opaga t i on８－１１ ,f o r c i ngac l e avagec r a ckt ochange p , i t sp r opaga t i onpa t h andhenc ei sbene f i c i a lt oimＧ r ov i ngt heCGHAZt oughne s s． p F i ８showst hef r a c t ur esur f a c e so fs t e e l sA g andBa tt８/５ o f１６０s．Bo t ht hef r a c t ur esur f a c e sa r e o ft hequa s i Ｇ c l e avaget n wh i cht her i ve rpa t Ｇ ype,i t e rnsc an bes e en．Cl e avagef a c e t so fs t e e lA a r e sma l l e r,andt he r ea r educ t i l ef r a c t ur ebandsa si nd i Ｇ c a t edbyt hea r r owi nF i ８( a),wh i chl a r l Ｇ g ge ycon t r i bu t e st ot hed i s s i t eene r i ngc r a ckp r opa Ｇ pa gydur [ ] t i ona tc r a cka r r e s t１２ ． I ncon t r a s t,s t e e lBha sa ga comp l e t ec l e avagef r a c t ur ewi t hl a r l e avagef a c e t s gec ( ( ) ) , F i ８ b r e su l t i ngi n al ow ene r o rc r a ck g gy f [] r opaga t i on８ ． p
I s sue８ Ef f e c to fBo r ononCGHAZ Mi c r o s t r uc t u r eandToughne s so fHi t r eng t hLow Al l oyS t e e l s ghS
( t８/５＝１６０s; ( t８/５＝１６０s． a),( c)S t e e lA, b),( d)S t e e lB,
F i ６ Mo rpho l o i z ed i s t r i bu t i ono fMＧAc on s t i t u en t s g g yands
F i ７ Gr a i nbounda r s o r i en t a t i ond i s t r i bu t i on so fs t e e lA ( a)ands t e e lB ( b)a tt８/５ ＝１６０s g ymi
F i ８ SEMf r a c t o r aph so ft h eimpa c ts e c imen so fs t e e lA ( a)ands t e e lB ( b)a tt８/５ ＝１６０s g g p
７９１
７９２
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
３ Conc l u s i on s
( １)TheCGHAZ mi c r o s t ruc t u r eo ft hebo r on Ｇ f r e es t e e lma i n l ons i s t so fg r a i nbounda r e r r i t e, yc yf
degene r a t epe a r l i t eand g r anu l a rba i n i t ef o rl onge r , i l et ha to ft he bo r on Ｇ c on t a i n i ng s t e e li s t８/５ wh r anu l a rba i n i t edomi nan tf o rawi de rr angeo ft８/５ ． g ( ２)Theadd i t i ono fbo r onsupp r e s s e st hef o r Ｇ ma t i on o fg r a i n bounda r e r r i t e and l owe r st he yf t r ans f o rma t i ons t a r tt empe r a t u r e,henc ep r omo t i ng t hef o rma t i ono fha r dpha s e s,i e l a t hba i n i t eand ma r t ens i t ei nt hebo r on Ｇ c on t a i n i ngs t e e l． ( ３)Bo r onde t e r i o r a t e st heimpa c tt oughne s s , , o fCGHAZ wh i ch maybeduet o apa r tf r om t he changei nt he ba s e mi c r o s t ruc t u r e,an i nc r e a s ed f r a c t i ono fMＧAc ons t i t uen t sandde c r e a s edf r a c t i on o fh i l eg r a i nbounda r i e s． ghang Re f e r enc e s:
[ １] Ph．Ma i t r ep i e r r e,D．Th i ve l l i e r,R．Tr i eo t,Me t a l l．Tr ans．A６
( １９７５)２８７ Ｇ ３０１． [ ２] B．Hwang,D W．Suh,S J．Kim,Sc r i t a Ma t e r．６４ ( ２０１１) p No １２,１１１８ Ｇ １１２０． [ ３] L Y．Lan,C L．Qi u,D W．Zhao,YF．J i ang,X H．Gao, L X．Du,J． I r onS t e e lRe s． ２３ ( ２０１１)No ６,１３ Ｇ １７． [ ４] S．Kim,Y．Kang,C． Le e,Ma t e r． Sc i． Eng．A５５９ ( ２０１３)１７８ Ｇ １８６． [ ５] G． Sh i s a t o,T．Fu i sh i r o,T．Ha r a,Ma t e r．Sc i．Eng．A５５６ ge j ( ２０１２)３５８ Ｇ ３６５． [ ６] X M．Wang,X L．He,I S I JI n t． ４２ ( ２００２)Supp l emen t,S３８ Ｇ S４６． [ ７] E．Bonnev i e,G．Fe r r i è r e,A． I kh l e f,D．Kap l an,J M．Or a i n, Ma t e r． Sc i．Eng．A３８５ ( ２００４)３５２ Ｇ ３５８． [ ８] L Y．Lan,C L．Qi u,D W． Zhao,X H．Gao,L X．Du,Ma Ｇ t e r． Sc i．Eng．A５２９ ( ２０１１)１９２ Ｇ ２００． [ ９] A． Lambe r t ＧPe r l ade,AF．Gour s,J． Be s s on,T． S t ur e l,A． gue P i ne au,Me t a l l．Ma t e r．Tr ans．A３５ ( ２００４)１０３９ Ｇ １０５３． [ １０] M．Dí a z ＧFuen t e s,A． IZAＧMend i a,I．Gu t i é r r e z,Me t a l l．Ma Ｇ t e r．Tr ans．A３４ ( ２００３)２５０５ Ｇ ２５１６． [ １１] WY．Lu,J．Ch e n,S S．Ch ak r a v a r t hu l a,Y．Qi a o,Eng．Fr a c t． Me ch． ７７ ( ２０１０)７６８ Ｇ ７７５． [ １２] E．Bouyne,HM．F l owe r,T C．L i nd l ey,A． P i ne au,Sc r i t a p Ma t e r． ３９ ( １９９８)２９５ Ｇ ３００．
JOURNALOFIRON ANDSTEELRESEARCH,INTERNATIONAL２ ０ １ ４,２ １( ８):７ ８ ７ Ｇ ７ ９ ２
Ef f e c to fBo r ononCGHAZ Mi c r o s t r u c t u r eandToughne s so f Hi t r e ng t hLowAl l o t e e l s ghS yS HanYANG, Xi Ｇx i a WANG, J i n ＧboQU
( I ns t i t u t eo fRe s e a r cho fI r onandS t e e l,Sha Ｇ s t e e l,Zhang i agang２１５６２５,J i angsu,Ch i na) j Ab s t r a c t:Ef f e c to fbo r on ont he mi c r o s t r uc t u r eandimpa c tt oughne s si nt hec oa r s e Ｇ r a i ned he a t Ｇ a f f e c t edz one g ( CGHAZ)o ftwoh i t r eng t hl owa l l oys t e e l s,bo r on Ｇ f r e eandbo r on Ｇ c on t a i n i ng,wa si nve s t i t edby me anso f ghs ga we l dt he rma ls imu l a t i ont e s t．Ther e su l tshowst ha t,f o rt hebo r on Ｇ f r e es t e e l,a mi c r o s t r uc t u r ec ons i s t i ngo fg r a i n bounda r e r r i t edegene r a t e spe a r l i t eandg r anu l a rba i n i t ef o rl onge rt８/５ ( t hec oo l i ngt imef r om ８００t o５００ ℃ ), yf , rt hebo r on Ｇ c on t a i n i ngs t e e l g r anu l a rba i n i t ei sdomi nan tf o rawi der angeo f wh i l el a t hba i n i t ef o rsho r t e rt８/５ ．Fo t i nuousc oo l i ngt r ans f o rma t i on ( CCT)s t udyont heCGHAZi nd i c a t e st ha tt het r ans f o rma t i ons t a r tt empe r a Ｇ t８/５ ．Con t u r ede c r e a s e sbyabou t５０－１００ ℃ unde rd i f f e r en tt８/５ ,f o rt hebo r on Ｇ c on t a i n i ngs t e e lc ompa r edwi t ht hebo r on Ｇ f r e e s t e e l．Thep r e s enc eo fbo r onsupp r e s s e st henuc l e a t i ono ff e r r i t ea tp r i o raus t en i t eg r a i nbounda r i e sandhenc een l a r Ｇ o rg r anu l a rba i n i t et r ans f o rma t i on．Howe ve r,t headd i t i ono fbo r onde t e r i o r a t e st heimpa c t st her angeo ft８/５ f ge , t oughne s so fCGHAZ wh i ch maybeduet oa ma r ked l nc r e a s edf r a c t i ono fma r t ens i t e Ｇ aus t en i t e (MＧA)c ons t i t u Ｇ yi en t sandde c r e a s edf r a c t i ono fh i l eg r a i nbounda r i e s． ghang Ke r d s:bo r on;h i t r eng t hl owa l l oys t e e l;c oa r s e Ｇ r a i nedhe a t Ｇ a f f e c t edz one;c on t i nuousc oo l i ngt r ans f o rma Ｇ ghs g ywo ; t i on impa c tt oughne s s DOI:１０ １３２２８/ boyuan i s sn１００６ Ｇ ７０６x ２０１３０１４１ j
Hi t r e ng t hl owa l l o t e e l s,w i t hy i e l ds t r e ng t h ghs ys t i c a l l nt her angeo f４６０－５５０ MPa,havebe en yp yi wi de l edi nc ons t ruc t i on o fl a r c a l e we l ded y us ges s t ruc t u r e s,sucha sp r e s su r eve s s e l s,p i l i ne s,o f f Ｇ pe sho r ep l a t f o rms,c ons t ruc t i onandmi n i ng ma ch i ne r Ｇ i e s．Toa ch i eveh i t r eng t hwi t hou to rwi t hal i t t l e ghs compr omi s eono t he rp r ope r t i e s,pa r t i cu l a r l oughＧ yt ne s s,ap r ope rde s i fchemi c a lc ompo s i t i oncou Ｇ gno l ed wi t hadvanc ed p r o c e s s i ngt e chn i s,sucha s p que t he rmo Ｇme chan i c a lc on t r o lp r o c e s s( TMCP), i sne e Ｇ , ded．Un f o r t una t e l he ba l anc eo fh i t r eng t h y t gh s andgoodt oughne s sc anbee a s i l i s rup t edbyt he yd t he rma lc l e sexpe r i enc eddu r i ng we l d i ng,p r odu Ｇ yc c i nga r e a so fpoo rt oughne s sknowna she a t Ｇ a f f e c t ed z one s (HAZs)．Oneo ft he we ake s tpa r t so ft he s e HAZsi nh i t r eng t hs t e e l sha sbe enf oundt obe ghs t hec oa r s e Ｇ r a i nedhe a t Ｇ a f f e c t edz one ( CGHAZ)． g Bo r oni soneo ft he mo s te f f e c t i vee l emen t st o [ ] i nc r e a s eha r denab i l i t fs t e e l s１－３ ,wh i ch make si t yo o s s i b l e t o r e d u c e t h e c o n t e n t so f c o s t l a l l o i n e l e Ｇ p y y g men t s,sucha s Mo,NiandNb,wi t hasma l ladd i Ｇ
t i ono fbo r on． I ti s we l le s t ab l i shed４－６ t ha tbo r on i nc r e a s e sha rdenab i l i t nh i b i t i ngt henuc l e a t i on y byi o fp r oeu t e c t o i df e r r i t eandp r omo t i ngt het r an s f o rma Ｇ /o t i ono fb a i n i t eand rma r t e n s i t e．Howe v e r,i ta e a r s pp t ha tt hee f f e c to fbo r onont hemi c r o s t r u c t u r eandp r op Ｇ e r t i e so f HAZ,i n pa r t i cu l a rt hecon t i nuouscoo l i ng t r ans f o rma t i on ( CCT)behav i o ro fCGHAZ,ha sno t be enadequa t e l t ud i ed．Toeva l ua t et hewe l dab i l i t ys y o fh i t r eng t hs t e e l s,we l dt he rma lc l es imu l a Ｇ ghs yc t i ont e chn i sa r egoodapp r oa che st o me a sur et he que HAZt oughne s sandana l z et hemi c r ome chan i smso f y f a i l ur e． I tha sbe enshownt ha ts imu l a t edHAZt e s t s a r ee conomi c a landr ep r oduc i b l e,andg i vet hes imi Ｇ l a rt oughne s sr ank i nga st hos eona c t ua lwe l dHAZs． I nt h i ss t udy,t hemi c r os t ruc t ur eandimpa c tt oughＧ ne s si nt heCGHAZo fh i t r eng t hs t e e l swe r ei n Ｇ ghs ve s t i t edby we l dt he rma ls imu l a t i ont e s t,wi t han ga empha s i sont hee f f e c to fbo r on． [
]
１ Exp e r imen t a l
Thechemi c a lcompos i t i ono ft het e s t eds t e e l si s
B i o r aphy: HanYANG,Ma s t e r,Eng i ne e r; E Ｇma i l: du＠１６３ c om; Re c e i v e dDa t e: May３１,２０１３ yanghans 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 ２１
７８８
i v e ni nT a b l e１． S t e e lA ( bo r on Ｇ f r e e)a nds t e e lB ( bo Ｇ g ) r on Ｇ c on t a i n i ng have ne a r l hes amec ompos i t i on, yt exc ep tbo r on．The s es t e e l swe r e me l t edi na１５０kg ( : va cuumi nduc t i on me l t e r mode l Cons a r c,USA), c a s tandcu ti n t os l abso f１２５ mmi nt h i ckne s s．The s l abswe r eaus t en i t i z eda t１２００ ℃ f o r１２０ mi n,and t henho tr o l l edi n t o１５mmＧ t h i ckp l a t e s,f r omas t a r t
t empe r a t ur eo f１０５０ ℃ t oaf i n i sht empe r a t ur eo f , ８３０ ℃ onap i l o tho tr o l l i ngmi l l．Theho tr o l l e dp l a t e s we r er ehe a t edandquencheda t９２０ ℃ andt hent emＧ r eda t６２０ ℃．F i １showst hemi c r os t ruc t ur e so f pe g s t e e l sAandBi nt hequenchedandt empe r ed ( QT) cond i t i on． The rma lc l e ss imu l a t i ngt heh i s t o r fCGHAZ yc yo
Tab l e１ Ch emi c a lc ompo s i t i ono ft h et e s t e ds t e e l s ma s s％
S t e e l A B
C
０ ０６ ０ ０６
S i
０ １０ ０ １０
Mn
１ ４７ １ ４９
P
０ ００８１ ０ ００７２
S
０ ００４２ ０ ００３８
Nb
０ ０１０ ０ ０１１
V
０ ０３７ ０ ０４２
Ti
Ni＋Cr＋Mo
０ ０１４
０ ６１
０ ０１１
０ ６５
B
－
０ ００１１
F i １ Mi c r o s t r u c t u r e so fs t e e lA ( a)ands t e e lB ( b)i nt h eQTc ond i t i on g
du r i ng we l d i ng we r epe r f o rmedonat h e rmome c han i Ｇ c a ls imu l a t o r (mod e l:Gl e e b l e Ｇ ３８００)．Th e r ea r ef ou r s amp l e sf o re a cht８/５ ( t hec oo l i ngt imef r om ８００t o ５００ ℃ ),onef o rt heCGHAZ CCTs t udy ( wi t hd i Ｇ me n s i on so fϕ１０ mm×８０ mm)a ndt h eo t h e r sf o rt he CGHAZimp a c tt oughn e s se x ami n a t i on (w i t hd ime n Ｇ ) s i on so f１０mm×１０ mm×８０ mm ．Th es amp l e swe r e c u ta nd ma c h i n e df r om t h e qu e n c h e da nd t emp e r e d l a t e si nt h et r a n s v e r s ed i r e c t i on．Th ep e a kt emp e r a t u r e p du r i ngt h et h e rma lc c l e swa sc ho s e na s１３００ ℃ and y he l df o r０ ５s． t８/５ wa sv a r i e df r om５t o５００s． Mi c r o s t ruc t u r e so ft hes amp l e s we r er eve a l ed bye t ch i ng wi t h４％ Ni t a lo rt heLePe r a ′sso l u t i on andobs e r vedonanop t i c a lmi c r o s c ope (mode l:Ca r l Ze i s sAx i oImage rA１m)andaf i e l demi s s i ons c an Ｇ n i ng e l e c t r on mi c r o s c ope ( FESEM, mode l:JSMＧ ７００１F)equ i t hane l e c t r on ba cks c a t t e rd i f Ｇ pped wi ( , : ) f r a c t i on EBSD mode l Channe l５ s t em．The ys s i z eandd i s t r i bu t i ono fma r t ens i t e Ｇ aus t en i t e (MＧA) cons t i t uen t swe r eme a su r edbyanimageana l z e ri n Ｇ y s t a l l edwi t ht heImage ＧPr oP l uss o f twa r e．Animpa c t t e s twa s pe r f o rmedi na c c o r danc e wi t h GB/T ２２９ wi t hs t anda r dVＧ no t c hs e c ime n s( １０ mm×１０ mm× p ) ５５ mm a t －２０ ℃ byus i nga４５０Jimpa c tt e s t e r ( mode l:I ns t r on S I Ｇ １M )．Ha r dne s st e s ti na c co rd Ｇ
anc ewi t hGB/T４３４０ １wa sconduc t edonaVi cke r s ha rdne s st e s t e r( mode l:Tukon２１００B)unde r１０kg l oad．Thef r a c t ur esur f a c e so ft heCha r c imens pyspe we r ea l soobs e r vedont heFESEM．
２ Re s u l t sandDi s c u s s i on
２ １ Mi c r o s t r u c t u r e F i ２showst heop t i c a landSEM mi c r og r aphs g o ft heCGHAZso fs t e e lA．As i i f i c an tchangei n gn t heCGHAZ mi c r os t ruc t ur ewi t ht８/５ wa sobs e r ved． Whent het８/５ i ssho r t e r( ５s),t he mi c r os t ruc t ur e ma i n l c o n s i s t so f l a t hb a i n i t ew i t has m a l la m o u n t y ( ( ) ( ) ) , o fg r anu l a rba i n i t e F i ２ a and２ d a st he gs coo l i ngr a t ei sh i randt hedwe l l i ngt imei nt he ghe aus t en i t er eg i oni ssho r t e r．Whent het８/５i si nc r e a s ed t o４０s,g r anu l a rba i n i t ei sdomi nan ti nt he mi c r o Ｇ s t ruc t ur ea sshowni nF i ２( b)and２( e)．Gr anu l a r gs ba i n i t ei sani n t e rmed i a t et empe r a t ur et r ans f o rma Ｇ t i onpr oduc t,cha r a c t e r i z edbya mi x t ur eo fba i n i t i c f e r r i t eandMＧAcons t i t uen t． Dur i ngcoo l i ngf r omt he akt empe r a t ur e( １３００ ℃ ),aus t en i t es t a r t st o pe t r ans f o rmt oba i n i t i cf e r r i t e,andt henc a rbond i f f u Ｇ /aus s e sf r om t he ba i n i t i cf e r r i t e t en i t ei n t e r f a c et o t heun t r ans f o rmedaus t en i t e． S t ab i l i z e dbyt hee n r i c h Ｇ me n to fc a r bon,apa r to ft heaus t en i t et r ans f o rmst o
I s sue８ Ef f e c to fBo r ononCGHAZ Mi c r o s t r uc t u r eandToughne s so fHi t r eng t hLow Al l oyS t e e l s ghS
７８９
( b),( e)t８/５＝４０s; ( c),( f)t８/５＝１６０s． a),( d)t８/５＝５s; (
F i ２ Op t i c a landSEM mi c r o r aph so ft h eCGHAZi ns t e e lA g g
ma r t ens i t ea tl owe rt empe r a t u r e so rr ema i ns un Ｇ , t r ans f o rmeddownt ot her oom t empe r a t u r e f o rmＧ i ngt heMＧAc ons t i t uen t． Whent het８/５ i sf u r t he ri nc r e a s edt o１６０ s,
o fGBFandt hec emen t i t eex i s t si nt hef o rmo fsma l l i e c e so rg r anu l e s． p F i ３s howst heSEM mi c r og r a so ft heCGHAZs g ph o fs t e e lB,wh i chi smuchd i f f e r en tf r omt ha to fs t e e l A．A mi c r os t ruc t ur econ t a i n i ngl a t hba i n i t eandma r Ｇ t ens i t ei sob t a i nedf o rsho r t e rt８/５ ( ５s,F i ３( a)), g
r a i nbounda r e r r i t eanddegene r a t epe a r l i t eappe a r g yf i nt hemi c r o s t ruc t u r eo fg r a nu l a rb a i n i t edu et oal on Ｇ e rd i f f us i ont ime,a sshowni nF i ２( c)and２( f)． wh i l eg r anu l a rba i n i t ef o rawi der angeo fl onge rt８/５ g gs ( , ( ) ( ) ) , Gr a i nbounda r e r r i t enuc l e a t e son p r i o raus t en i t e ４０and１６０s F i ３ b and３ c ．Howeve r wi t h yf gs r a i nbounda r i e sandg r owsa l ongt hebounda r i e s( i n t hei nc r e a s eo ft８/５ ,t he g r anu l a rba i n i t e be come s g l eng t h)andi n t ot heg r a i ni n t e r i o r s( i n wi d t h)．De Ｇ coa r s e randt hes i z eo f MＧA cons t i t uen t si nc r e a s e s． r a t epe a r l i t egene r a l l o rmsa tt hebounda r i e s Nog r a i nbounda r e r r i t eanddegene r a t epe a r l i t ea r e gene yf yf
( a)t８/５＝５s; ( b)t８/５＝４０s; ( c)t８/５＝１６０s．
F i ３ SEM mi c r o r aph so ft h eCGHAZi ns t e e lB g g
f oundf o rs t e e lB． ２ ２ CGHAZCCTd i a r am g CGHAZ CCT d i ag r amso fs t e e l s A andBa r e ( ) ( ) , showni nF i ４ a and ４ b r e spe c t i ve l r gs y．Fo bo t hs t e e l s,t het r ans f o rma t i ons t a r tt empe r a t ur ei n Ｇ c r e a s e sa st het８/５i si nc r e a s ed．Al onge rt８/５ ,l e ad i ng t oal onge rdwe l l i ngt imea th i empe r a t ur er e Ｇ ght i on,i sf avo r ab l et ot hed i f f us i vepha s et r ans f o rma Ｇ g , , t i on．Ont hec on t r a r r t e rt８/５ l e ad i ngt oa y asho
h i rcoo l i ng r a t e,i sf avo r ab l et ot he d i sp l a c i ve ghe s et r ans f o rma t i on．A b i i f f e r enc ebe twe ent he pha gd twos t e e l si st ha tt het r ans f o rma t i ons t a r tt empe r a Ｇ t ur eo fs t e e lBi sabou t５０－１００ ℃ l owe rt hant ha t o fs t e e lA unde rd i f f e r en tt８/５ ．Ther angeo ft８/５f o r t heha rde rpha s e s,i e LBand M,i ns t e e lBi sa l so wi de rt hant ha ti ns t e e lA． I na c co rdanc e wi t h mi c r os t ruc t ur e,t heha rd Ｇ ne s so fCGHAZde c r e a s e swi t ht hei nc r e a s eo ft８/５ ． Unde rt hes amet８/５ ,t heha rdne s so ft hebo r on Ｇ con Ｇ
７９０
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
M— Ma r t ens i t e; B—Ba i n i t e; GB—Gr anu l a rba i n i t e; GBF—Gr a i nbounda r e r r i t e; DP—Degene r a t epe a r l i t e． yf
F i ４ CGHAZCCTd i a r amso fs t e e lA ( a)ands t e e lB ( b) g g
t a i n i ngs t e e li sh i rt hant ha to ft hebo r on Ｇ f r e e ghe s t e e l,andt heb i s td i f f e r enc ebe twe ent hetwo gge s t e e l sappe a r sa tt８/５ o f５s． I tc ana l s obes e ent ha t t her a t eo fde c r e a s ei nha r dne s sde c r e a s e s wi t hi nＧ c r e a s i ngt８/５ ,ands t a r t st ol eve lo f fa tapp r ox ima t e l y ６０s．Thes i n i f i c a n td i f f e r e n c e i nt h em i c r o s t r u c t u r e g andha r dne s so ft hetwos t e e l si sbe l i evedt obea t Ｇ t r i bu t edt ot headd i t i on o fbo r on．Bo r ont endst o s eg r ega t es t r ong l tg r a i nbounda r i e sandi s mos t ya e f f e c t i vei nsupp r e s s i ngt hef o rma t i ono fpr oeu t e c Ｇ t o i df e r r i t eand p r omo t i ngt heba i n i t eand/o r ma r Ｇ t ens i t et r ans f o rma t i on． ２ ３ Impa c tt oughne s s F i ５showst hee f f e c to ft８/５ ont heimpa c t g t oughne s so fs t e e l s A and B．The r ei sv i r t ua l l y no changei nt het oughne s sf o rs t e e lA,wh i l et oughＧ ne s sde c r e a s e sg r adua l l t ht８/５ f o rs t e e lB ( wi t h y wi anexc ep t i ono f４０s)．Fo ra l lt het８/５ t e s t ed ( ４０－ ２４０s),s t e e l A ha s much h i rt oughne s st han ghe s t e e lB．Oner e a s onf o rt ha t maybeduet ot hei n Ｇ c r e a s ei nt hevo l umef r a c t i ono ft hes e c ond pha s e, i e MＧAc ons t i t uen t,i ns t e e lB．MＧAc ons t i t uen t s , a r eha r dandb r i t t l epa r t i c l e sa c t i nga st hepo t en t i a l sou r c e sf o rc r a cki n i t i a t i on,andgene r a l l e ad i ngt o yl t hede t e r i o r a t i on o fimpa c tt oughne s si nt he we l d [ ７－９] HAZ ．F i ６showst hec ompa r i s ono fmo r l Ｇ g pho ogy,s i z eand d i s t r i bu t i on o f MＧA c ons t i t uen t si n s t e e l sAandB．Thenumbe rands i z eo fMＧAcons t i t Ｇ uen ti nc r e a s e ma rked l t ht headd i t i ono fbo r on y wi ( c ompa r i ngF i ６( b)and６( d)wi t hF i ６( a)and gs gs ６( c))．Spe c i f i c a l l henumbe ro fl a r Ｇ y,t ge MＧA con ２ , s t i t uen t swi t hana r e ag r e a t e rt han０ ６μm wh i ch [] a r emo r el i ke l oi n i t i a t ec r a ck i ng４ ,i smuchh i yt ghＧ e ri ns t e e lBt hant ha ti ns t e e lA． Thef r a c t i ono fh i l eg r a i nbounda r i e smay ghang
F i ５ Va r i a t i ono fimpa c tt oughn e s swi t ht８/５ g
beano t he rf a c t o ri n f l uenc i ngimpa c tt oughne s s．Ac Ｇ , co rd i ngt ot heEBSDr e su l tt hef r a c t i ono fh i Ｇ ghan l eg r a i nbounda r i e so fs t e e lAi sh i rt hant ha to f g ghe , / s t e e lB,a sshowni nF i ７． Wh e nt h e t i s１ ６ ０s g ８５ t hef r a c t i ons o fh i l eg r a i n bounda r i e sf o r gh ang s t e e l sA andBa r e０ ３３０and０ ２１６,r e spe c t i ve l y,
i nd i c a t i ngt ha tbo r onde c r e a s e st hef r a c t i ono fh i gh ang l eg r a i nbounda r i e s．Hi l eg r a i nbounda r ghang y c an ber ega rded a san e f f e c t i ve obs t a c l et oc r a ck [ ] r opaga t i on８－１１ ,f o r c i ngac l e avagec r a ckt ochange p , i t sp r opaga t i onpa t h andhenc ei sbene f i c i a lt oimＧ r ov i ngt heCGHAZt oughne s s． p F i ８showst hef r a c t ur esur f a c e so fs t e e l sA g andBa tt８/５ o f１６０s．Bo t ht hef r a c t ur esur f a c e sa r e o ft hequa s i Ｇ c l e avaget n wh i cht her i ve rpa t Ｇ ype,i t e rnsc an bes e en．Cl e avagef a c e t so fs t e e lA a r e sma l l e r,andt he r ea r educ t i l ef r a c t ur ebandsa si nd i Ｇ c a t edbyt hea r r owi nF i ８( a),wh i chl a r l Ｇ g ge ycon t r i bu t e st ot hed i s s i t eene r i ngc r a ckp r opa Ｇ pa gydur [ ] t i ona tc r a cka r r e s t１２ ． I ncon t r a s t,s t e e lBha sa ga comp l e t ec l e avagef r a c t ur ewi t hl a r l e avagef a c e t s gec ( ( ) ) , F i ８ b r e su l t i ngi n al ow ene r o rc r a ck g gy f [] r opaga t i on８ ． p
I s sue８ Ef f e c to fBo r ononCGHAZ Mi c r o s t r uc t u r eandToughne s so fHi t r eng t hLow Al l oyS t e e l s ghS
( t８/５＝１６０s; ( t８/５＝１６０s． a),( c)S t e e lA, b),( d)S t e e lB,
F i ６ Mo rpho l o i z ed i s t r i bu t i ono fMＧAc on s t i t u en t s g g yands
F i ７ Gr a i nbounda r s o r i en t a t i ond i s t r i bu t i on so fs t e e lA ( a)ands t e e lB ( b)a tt８/５ ＝１６０s g ymi
F i ８ SEMf r a c t o r aph so ft h eimpa c ts e c imen so fs t e e lA ( a)ands t e e lB ( b)a tt８/５ ＝１６０s g g p
７９１
７９２
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
３ Conc l u s i on s
( １)TheCGHAZ mi c r o s t ruc t u r eo ft hebo r on Ｇ f r e es t e e lma i n l ons i s t so fg r a i nbounda r e r r i t e, yc yf
degene r a t epe a r l i t eand g r anu l a rba i n i t ef o rl onge r , i l et ha to ft he bo r on Ｇ c on t a i n i ng s t e e li s t８/５ wh r anu l a rba i n i t edomi nan tf o rawi de rr angeo ft８/５ ． g ( ２)Theadd i t i ono fbo r onsupp r e s s e st hef o r Ｇ ma t i on o fg r a i n bounda r e r r i t e and l owe r st he yf t r ans f o rma t i ons t a r tt empe r a t u r e,henc ep r omo t i ng t hef o rma t i ono fha r dpha s e s,i e l a t hba i n i t eand ma r t ens i t ei nt hebo r on Ｇ c on t a i n i ngs t e e l． ( ３)Bo r onde t e r i o r a t e st heimpa c tt oughne s s , , o fCGHAZ wh i ch maybeduet o apa r tf r om t he changei nt he ba s e mi c r o s t ruc t u r e,an i nc r e a s ed f r a c t i ono fMＧAc ons t i t uen t sandde c r e a s edf r a c t i on o fh i l eg r a i nbounda r i e s． ghang Re f e r enc e s:
[ １] Ph．Ma i t r ep i e r r e,D．Th i ve l l i e r,R．Tr i eo t,Me t a l l．Tr ans．A６
( １９７５)２８７ Ｇ ３０１． [ ２] B．Hwang,D W．Suh,S J．Kim,Sc r i t a Ma t e r．６４ ( ２０１１) p No １２,１１１８ Ｇ １１２０． [ ３] L Y．Lan,C L．Qi u,D W．Zhao,YF．J i ang,X H．Gao, L X．Du,J． I r onS t e e lRe s． ２３ ( ２０１１)No ６,１３ Ｇ １７． [ ４] S．Kim,Y．Kang,C． Le e,Ma t e r． Sc i． Eng．A５５９ ( ２０１３)１７８ Ｇ １８６． [ ５] G． Sh i s a t o,T．Fu i sh i r o,T．Ha r a,Ma t e r．Sc i．Eng．A５５６ ge j ( ２０１２)３５８ Ｇ ３６５． [ ６] X M．Wang,X L．He,I S I JI n t． ４２ ( ２００２)Supp l emen t,S３８ Ｇ S４６． [ ７] E．Bonnev i e,G．Fe r r i è r e,A． I kh l e f,D．Kap l an,J M．Or a i n, Ma t e r． Sc i．Eng．A３８５ ( ２００４)３５２ Ｇ ３５８． [ ８] L Y．Lan,C L．Qi u,D W． Zhao,X H．Gao,L X．Du,Ma Ｇ t e r． Sc i．Eng．A５２９ ( ２０１１)１９２ Ｇ ２００． [ ９] A． Lambe r t ＧPe r l ade,AF．Gour s,J． Be s s on,T． S t ur e l,A． gue P i ne au,Me t a l l．Ma t e r．Tr ans．A３５ ( ２００４)１０３９ Ｇ １０５３． [ １０] M．Dí a z ＧFuen t e s,A． IZAＧMend i a,I．Gu t i é r r e z,Me t a l l．Ma Ｇ t e r．Tr ans．A３４ ( ２００３)２５０５ Ｇ ２５１６． [ １１] WY．Lu,J．Ch e n,S S．Ch ak r a v a r t hu l a,Y．Qi a o,Eng．Fr a c t． Me ch． ７７ ( ２０１０)７６８ Ｇ ７７５． [ １２] E．Bouyne,HM．F l owe r,T C．L i nd l ey,A． P i ne au,Sc r i t a p Ma t e r． ３９ ( １９９８)２９５ Ｇ ３００．