[Document Type]Speci6cation
[
[00(M]
ln addition, a6ter being mounted in cars ascomponents, 山 esteelsheets need to
haveaprope"ythatmal 【esl山esteelsheetsbenoteasilybrokenevenwhen impacted by
collisionorthelike(impactresistance). Pz旧 icularly,incoldclimateswhere the stee1
5 sheets are used at lowtemperatures,members arelikelytoembrittle,and 山 us 山 ere is a
need For improvingthe low temperature toughness ofl山esteelsheets in orderto ensme
山 e impact resistanceofthecomponents. Thatis, 山 insteelsheetsthata re used For
componentsoftheabove-describedmembers m erequiredtohavenoton1yexceptiona1
workability but 副 so low temperaturetoughnessasanex 立emelyimpon:ant propel直y For
lo ensuring 山 eimpactresistance. Thelowtemperaturetoughness is aprope"ythatis
speci行edbyv 画rs(Chi組rpyfractureappe そばance 立ansitiontemperat 山re)orthelike.
[0005]
As a steel sheet from which e×ceptionalductilityczmbeobtained,aDuaI Phase
steelsheet (hereinaaer,"DPsteer')composedofacompositest ・tlctureofso 負たrrite md
15 6u11 h;廻rdma ーtensiteis]b1own. However,DPstee1has exceptiona1ducti1ity, but is poor
in terms ofhole expzu・sibilityin somecases slncevoidsaregeneratedin 山 einterface
between 定rriteandmaltensite,whichhavesignil 日cantlydil帯erenth 組rdness, tocause
cracking.
[0006]
20 PatentDocument l proposes ahigh stren蝉 lhotrolledsteelsheethavinga
tensile streng;hof980 ハ4Paormoreandbeingexceptionalintermsofshape 行xability,
hole expansibility and bendability,inwhich 山 em ea 丘actions ofbainitic た rrite,
ma1tensite and bainite are each set to 90%ormore,5%orless,and 5% or less
However, in the invention describedinPatentDocument l,thereaiecases where
25 sufficientelongationcannotbeobtainedsincebainitic6erriteis amaincomponent.
2
[0007]
PatentDocument2proposes ahigh stren蝉 lhotrolledsteelsheet having a
tensilestren 停 hof980M[Paormore,inwhich90%ormoreofbainitein terms ofi紅ea
fraction is included as amainphase, 山 eremainderiscomposedofoneormore stmctures
5 selected frommartensite,austenite; 掴ld 免 rrite,and 山 eamountand average grain
diameter ofcementite matis dispersedin 山 esbnicturearecon 立olled,merebyimproving
山 e hole expansibility. However,in 山 einventiondescribedinPatent Document 2, since
中 e hot‐rolled steel sheet is coiledat3300Cto4700C,whichisatramsitionboilingreglon,
山 ere 嶺 e cases where propeinyunevenness is caused due to temper 砥 uureunevenness ・n
lo me sheetsturface.
[0008]
Patent Document 3 proposes ahot‐rolledsteelsheethaving exceptional fatigt1e
propenies,inwhich 山 e 免 rritefractionis50%to95%, 出 efractionofa6ull ‐hard second
phase consistingofmartensite andresidual austeniteis5%to50%, 山 einterrelationshiP
15 between theamounts ofcarbide ‐Formingelements ormerelationship between the
ci虹bide- ormingelement そmdmeCcontentissettoapredetermined range, alld山 en 山 e
average grmn dialneterofprecipitates and 山 e 丘actionof 山 eprecipit 紙es 嶺 especil日ed.
However, in the invention described in Patent Document3,sinceso 負 6erriteiscontained
as 山l e maincomponent,and 中 es 立eni夢hisens 皿 ed by me precipitation h;虹deningofa
20 6necarbide, 山 erearecaseswheresufficientlowtemperaturetoughness cannotbe
obtained
[0009]
Patent Document4 proposes ahot‐rolledsteelsheetbeing exceptional in terms
ofb 水 ehardenabilityandlowtemperaturetoughnessandhavingatensiles 立eng曲 of980
25 ハ4Paormore,inwhichthetotalofthevolume 丘actions ofanyoneorbo 山 oftempered
3
maltensite and lower bainite is 90%olmole,a lldtheaveragedislocation density in 山 e
temperedmaltensiteand 山 elowerbainiteis speci行ed. lnaddition,Patent Document 4
discloses 山紙 山 e lowtemperaturetoughnessis 6uばtherimprovedbysettingthe number
density ofiul iron‐basedc そばbidein 山 etemperedmaltensiteand 山 elower bainite and 山 e
5 efFective grain size andaspectratioofl 山etemperedmaltensiteand 山 elower bainite to
predeterminedra ぱ1ges. Ho wever,inPatentDocument4,hole e×pansibilitythat is
considered tobenecessaryForthepress ‐Formingofsuspensionmembersis nottaken into
account.
[CitationList]
10 [Patent Document]
[0010]
[Patent Document l]
Japanese Unexannined p副[entApp.ication,FirstPublicationNo.2008-255484
[PatentDocument2]
15 Japanese Unexannined p副[entApp.ication,FirstPublicationNo.2014-205890
[PatentDocument3]
Japanese Unexannined p副[entApp.ication,FirstPublicationNo.2009-84648
[PatentDocument4]
PTC 1nternautionaIPublication Jo.＼スバ)2014/132968
20 [NonPatentDocument]
[0011]
[Non Patent Document l]
Tsuchiyama, T(2002), ‘Physic 副 MemQingofTemperingPi 組rameter and lts
Applic 紙ion For Continuous Heating or Cooling HeatTreatmentProcess',He 紙 Tremment
25 (3rded.,uol.42)pp.163 ‐168
4
[Summary of 山 elnvention]
[Problems to be Solved bythe lnvention]
[0012]
As described above, steel sheetshavingahighs 口engdlandbeing exceptional in
5 termsofelongation,holeexpansibilityandlowtemperaturetoughnesshavenot yet been
proposed. The present invention has been madeinconsiderationof 山 eabove ‐described
problems,andano 覇 ectiveof 山 epresentinventionistoprovideasteelsheethaving a
high streng山 and being exceptionalintermsofelong 紙ion(pa 旧 icularly,uniform
elongation),holeexpansibilityandloWtemper 紙u止etoughnessandamanuf:acturing
lo me 山 odthereof
[Meをms forsolvingtheProblem]
[0013]
The present inventors Found 山 atitispossibletomanufactureasteel sheet
havingahighstrengdlandbeingexceptionalintermsofuniform elongation, hole
15 expansibility and low temperaturetoughnessbycon 立olling 山 etexture and
microsbmct 皿 eofthesteelsheetl 山rough 山 eoptimizationofl 山echemicalcomposition
and manufacturing conditions ofthesteelsheet.
[0014]
Thegistofthepresentinventionis asdescribed below
20 [1]Asteelsheetaccordingtooneaspectofthepresentinventionhas a chemical
compositioncontaining,bymass%,C:0. α}0%too.180%,Si:0.005%to2.00%,M[n
l.00%to3.00%,Ti:more 山 ano.200%ando.400%or less,sol.AI:0.001%to l.000%,
N:0.0010%to o.0100%,P:0.100%or less,S:0.0100%orless,Nb:0%too.100%,V:
0%too.500%,M[o:0%too.500%,Cu:0%to l.00%,Ni:0%to l.00%,Cr:0%to2.00%,
25 B:0%too.0030%,Ca:0%too.0100%,M[g:0%too.0100%,RE]M:0%too.0100%and
5
Bi:0%too.0200%wi 山 aremainder ofFe and impurities,atadepth position of l/4 ofa
sheetl山icblessfromasurface,a nareafractionofGAMo.5-I.7thatiscryst 副 grains having
aGAMofmore 山 ano.50and l.70 orlessis 50%ormoreand loo%orless, 山 eGAM
representinganaverageofcrystalorientautiondilferencesinonecrystalgrain that are
5 obtained byEBSDan 副ysis,anareafractionofGAM 〉I.7山 atiscrystalgrains having 山 e
GAMofmore 山 an l.70 iso%ormorei 掴ld20%orless,a nareaf raction ofGAM 〈o.5山 at
is crystal grains havingtheGAMofo.50 orlessiso%ormoreandlessthan 50%, an area
fraction ofresidualausteniteiso%ormoreandlessthan4%,atotala rea fraction of 山 e
residualaustenite,f reshmartensite,cementiteandpe そ虹liteiso%or more and lo% or
lo less, anaveragegrainsizeis15.0 一morless,anaveragedislocaution density is l.o x lo14
/m2ormoreand4.ox lo15/m2orless,atot …uofpoledensitiesof{211}<011>and
{332}<113>ina 山 ic如 ess middleponion is 12.oorless,a nd atensilestrengdbis980
ハ4Paorrnore.
[2]The steel sheet according to[1],inwhich 山 echemicalcomposition may
15 contain,bymass%,oneormoreselectedfrom 山 egroupconsisting ofNb:0.001%to
o.100%,V:0.005%too.500%,M[o:0.001%too.500%,Cu:0.02 ‐%to l.00%,Ni:0.02%
to l.00%,Cr:0.02%to2.00%,B:0.0001%too.0030%,Ca:0.0002%too.0100%,M[g:
0.0002‐%too.0100%,REM:0.0002 ‐%too.0100%and Bi:0.0001%too.0200%
[3]The steel sheet according to[1]or[2],inwhichaplatinglayer may be
20 Formedon 山 esurface.
[4]The steel sheet according to[3],inw hichtheplatinglayer may be ahot‐dip
galvanizedlayer
[5] The steel sheetaccordingto[3],inwhichtheplatinglayer may be ahot‐dip
galvannealedlayer
25 [6]Asteel sheetm 瀧lu1陸ctwingmelhodaccordingto 嶺lol山eraspect ofthe
6
Present invention is a manu 魚 cturing method ofthesteelsheetaccordingto[1],having a
heating steP ofheating a slab or steeIPiece having the chemicalcom ーPositionaccordingto
[1], ahotrolling stePofPerformingmulti ‐passhotrollingontheslabor 山 esteeI Piece
aaer 山 e hei紙ing steP using aP1urality ofrolling standstoobti 額nahot ‐rolledsteel sheet, a
5 coiling steP ofcoiling 山 ehot‐rolledsteelsheetandahe 孤 立÷eatmentstePofPerForming a
heat ÷u e砥mentonthehot ‐rolledsteelsheeta コ6ter山ーecoilingsteP,inw hich,in 山ーeheating
steP, ahei紙ing temPerature is set to 12800C orhigher andatemper 須田 eSRT(OC)
rePresented bythe following formula(1)orhigher,in 山 ehotrollingsteP,when a 行nish
temPerature is rePtesented byFT in aunitofo.C,atotalrollingreduction in atemPerature
lo rangeofhigher 故 anFT 十 500.CandFT 十 1500.Corloweris setto50% or more, atotal
rolling reductioninatemPeri 紅uurerangeofFTtoFT 十 50O.Cis setto40% to 80%, atime
necessそばy fiorro11inginthetemPeri 紅urerangeofFTtoFT 十 50Ocis setto0.5to10. ○
seconds, two or more Passes ofrolling 組 ePerFormedineachof 山 etemPeratuurerange of
higher 山 an FT 十 500.C andFT 十 1500.Corlowerand 山 etemPeraturerangeofFTtoFT+
15 500.C, an average cooling ri=[einatemPeraturerangeofFTtoFT 十 1000・C is set to
6.0 0.C/secorfasterand40.00.C/secorslower,FTis settoequaltoorhigher 山 anAJ3 山 at
is obtained from me followingformula(2),equaltoorhigherthanTR(0C) 山 atis
obtainedfrommefollowingformula(3)andlloo0Corlower,watercoolingisinitiated
within3.oseconds 丘omcomP1etionof 行nishrolling,anaveragecoolingrate in a
20 temPerature rangeofFTto7500 ・Cis setto300.C/secorfaster,theslabormesteeIPiece
is dwe1t in atemPera 如 Irerangeof750Octo620O.cFor20seconds or shoder and then
cooledtoacoolingstoPtemper 砥uurematis 6200Cto5700 ・C suchthatanaverage cooling
rate in atemPerature rangeof 中 ecoolingstoPtemperaturereaches300.C/secorfaster, in
中 e coiling steP, me hot‐rolledsteelsheetiscoiledat5700.Corlower, in the heat
25 せeatment step, a ma ×imumattainmenttemPeratureTma ×is setto5500Corhigher and
7
7200C or lower, and atempering parameter Ps is setto14000to18000.
SRT(oC)=1630 十9o x ln([C]x[Ti])… (1)
AJ3 C)=901 ‐325x[C]+33x[Si] ‐92x[M[n]+287x[P]+40x[sol.AJ]
(2)
5 TR(oC)=800 十70ox[Ti]+looo x[Nb](3)
Here,[element symbol]in 山 e 貴ゞ) mulae(1),(2)and(3)indicatesthea lnount of
each elementbymass%,a ーl dzeroisassignedinacasewherethe element is not
contained.
[7]The steel sheet m 瀧lu1陸cturingmelhodaccordingto[6],inwhich, in山 e hot
lo rolling step, the water coolingmaybeinitiatedwithino.3 secondsf rom completion ofl山e
行nishrolling,a11dcoolinginwhichanaveragecoolingrateinatemperatwre ramge ofFT
to FT ‐40oCis loooC/secorfastermaybeperFormed.
[EfFects of 山 elnvention]
[0015]
15 Accordingto 山 e above ‐describedaspectsofl 山epresentinvention,itispossible
to provide a steel sheet havingahighsl に÷eng 山 andbeingexception 副 interms ofuniForm
elongation, hole expansibilityandlowtemper 紙uuretoughness and a manufacturing
melhodthereof. The"steelsheer'ofthepresentinventionincludes "plをuedsteelsheer'
having aplating layer on 山 esuurface.
20 VVhen 山 e steel sheet according to the present invention is usedas amm;erial ofa
c;虹 suspenslon component,itiseasytowork 山 esteelsheetintoacomponent shape, and
山 e steel sheetis capableofwi 山 standing 山 euseinanex せemelycoldclimate,and 山 us
industrial contribution is extremely signi6cant.
[Embodiment for implementing 山 elnvention]
25 [0016]
8
Hereinafter,asteelsheetaccordingtoanaspectofthepresentinvention (山 e
steelsheetaccordingtomepresentembodiment mdasteelsheetmanu 魚 ctwing melhod
accordingto 山 e present embodiment willbedescribedindetail. First,山 echemical
composition ofthe steel sheet accordingto 山 epresent embodiment(inthecaseofa
platedsteelsheet, 山 echemicalcompositionofabasesteelsheetexcludingaplating
layer) willbedescribed. However,thepresentinventionisnotlimited only to a
con 行guration disclosedin 山 epresentembodimentandcanbemodi 行edinavariety of
manners within 山 escopeof 山 egistof 値 epresentinvention.
5
Numerical limitingranges to be expressed below using"to “include numerical
lo v副ues at bomends in山 erangesasthelowerlimitand 山 eupperlimit.However, in 山 e
case ofbeing expressedas “less 山 ad'or “more 山 an “,v 副 ues arenotincluded in 山 e
numerical 【嶺1ges. ln 山 eFollowingdescription, “% “【eg嶺 ding 山 echemical
composition ofstee1 indicates "mass%''in a11 cases
[0017]
15
(C:0.040%too.180%)
C is an e爺ective element Forthe Formation ofa low temperaturet 【amsFormation
phase such as martensiteorbainite 山 atcon 立ibutestoimprovement in st【engh. ln
addition,Cisalsoanelement 山 atbondstoTior 山 eliketoForm a carbide, 山 e【eby
20 1ncreasing 中 es 立engl山 ofsteel. Wrhen 山 eCcontentisless 山 ano.040%,itisdifficultto
obtain atensiles 立en停 hof980M[Paormore. Therelbre,theCcontentissettoo. ○40%
or more. Theccontentispre た lab1y 〇・〇6〇%ormore.
on me otherhand, when the Ccontentismore 山 ano.180%, 山 eweldability
deteriorates. ThereFore, 山 eCcontentis settoo.180%orless. TheCcontent is
25 pre先【ablyo.170%orLess,morepreferablyo.160%orless,andstillmore pre定【ably
9
0.140%or less
[0018]
(Si:0.005%to2.00%)
Si is an elementhaving an action ofincreasing 山 es 口en蝉 lofsteel by solid
5 solutions 口engdbeningand 山 eenhzmcementofhardenability lnaddition,Si is副 so an
element having an action ofsuPPressing 山 ePreciPitationofcementite. VVhen the Si
content is lessthano.005%,itbecomes difficulttoexhibit 山 eabove ‐describedactions.
ThereFore, 山 e si contentis settoo.005%ormore. Thesicontentispre 免rablyo.01%
or more, more Preferablyo.03%ormore,stillmorePre 免 rablyo.05%or more and far
lo stillmorePre 免rablyo.10% or more
on 山 e otherhand, when thesicontentis morethan2.oo%, 山 e smface
propejn[ies ofthesteelsheetsignil 丘cantlydeteriorateduetosurface oxidation in ahot
rollingstep. ThereFore,thesicontentis setto2.oo%orless. Thesicontent is
pre免 rably l.50%orless andmorepre 免 rably l.30%or less
15 [ool9]
(Mn:1.oo%to3.oo%)
Mn is an elementhaving an action ofincreasing 山 es 口en蝉 lofsteelbysolid
solutions せengdbeningand 山 eenhmcementofhardenability When 山 eamount ofMn
is less山 an l.oo%, itbecomes difficulttoobtainatensiles 立eng 山 ofthesteel sheet of
20 980 M[Pa ormore. ThereFore,the:Mncontentis setto1.oo%ormore.The :Mn
content is pre免rably l.20% or more
on 山 e otherhand, whenthe:Mncontentbecomesexcessive, 山 eplastic
anisotropy increases, and 山 eholeexpansibilitydeteriori 紙es insomecases.The reason
山 ereFor is notclear,butis assumedtoresult 丘om 山 eprecipitationofa12urge amount of
25 ハ4ns attributedtoa1a ばgeamountof ハ4ncontainedandrecryst 副1izationduringhotro11ing
lo
atbごibiuted toD4n segregmion ol a local unevenness in 化 rriticu÷ansFormmion a賃二er 行nish
rolling. ThereFore,the ハ4ncontentis setto3.00%orless. The ハ4ncontent is
Prelたrably2.80%orless,morePreferably2.60%orless,a lldstillmorePre 定 rably2.40%
orless
5 [0020]
(Ti:more 山 ano.200%ando.400%or less)
Tiis an elementthat bonds to c to Forms aTi‐basedc 嶺 bide;掴Idcontributes to
imProvement in the tensilestrengl山 ofl山esteelsheet. lnaddition,TiisalsoanLelement
having an action ofre 行ning 山 emetallograPhicstmct 班 ebyForming aTi nitride山 at
lo suPPresses 山 e co嶺 seningofausteniteduring 山 eheatingofslabsandduringhotrolling.
VVhen 山 e Ti content is o.200%orless,itbecomes dilficulttoobt 額ndesiredProPen[ies
due to an insuaEicient amount ofPreciPitmionh 棚rdening. ThereFore,theTicontent is set
to more man o.200%. TheTicontentisPre 免rablyo.210%ormore,more Pre免rably
o.215%ormore,andstillmorePre 免rablyo.220%ormore.
15 on me otherhand, when the Ti content becomes excessive,acoarse Ti‐based
cmbide remaining in austenite in a non‐solidsolutionstatePromotestheFormation of
voids tha直aieiniti紙edfromTi ‐basedinclusionsinanon ‐solidsolutionstate during
w orlding and degrades theuniFormelongation. ThereFore,theTicontent is setto
o.400%orless. TheTiContentisPrel たrablyo.350%orless 虹i ld more Prel危rably
20 0.300% or less
The Ti‐basedcmbiderel たrstoacz 虹bidehavingaNaCI-tyPecrystal sbmcture
containing Ti. Cmbides containingasmallaunountofadifFerentcz 虹bide‐Forming 副loy
element, ForexamP1e,M[o,Nb,V,Cror 「Wwithin 山 eraは1geofthechemicalcomPosition
sPeci行ed by 山 e Present embodiment canalsoberegardedas 山 eTi‐basedcarbide as long
25 as 中 e carbidescontainTi. lnaddition,carboni 立idesinwhichsomeofcarbon is
ll
substituted by ni立ogen are aIS0regardedastheTi ‐basedcarbide.
[0021]
(sol.AI:0.001%tol.000%)
A1isanelementhavinganactionofcleaning steel by deoxidation in a
5 steelm 副kingstage(suPPressingtheinitiationofade6ectsuchasablowholein steel)and
promoting たrritic立ansFormation. W/hen 山 esol.A1contentisless 山 an o.001%, it
becomes di任iculttoe ×hibit山ーeabove ‐describedaction. ThereFore,thesol.A1 content is
setto 〇0 〇1%ormore. Theso1.A1contentispre たrab1y 〇・〇1〇%or more and more
pre免rablyo.020%ormore.
10 lncident 副ly,even when 山 e sol.A1contentis settomore 中 anL I.000%, 山 eefFect
ofthe above ‐described action is satura直ed,andthere6ningcostincreases. ThereFore,
山 e sol.A1 content is set to l.000%orless. Thesol.A1contentispreferably o.800% or
lessandmorePre 免 rablyo.600%orless. “SOI.AI''means acid‐solubleAI.
[0022]
15 (N:0.0010%too.0100%)
N is im element having an action ofre 行ning 山 emicrosbmcturebyForming aTi
ni立ide that suppressesl 山ecoi狂seningofaustenitedl 虹ing 山 eheatingofslabsandduring
hot rolling. W〆hen theNcontentisless 山 ano.0010%,itbecomesdifficulttoexhibit 山l e
above ‐describedaction. ThereFore,theNcontentis settoo.0010% or more. The N
20 contentispre 定 rablyo.0015%ormorealldmorepre 定 rablyo.0020%ormore.
on 山 e otherhand, when the N contentis morethano.0100%,acoarse Ti ni立ide
is Formedtodegrade 山 eholeexpansibilityof 山 esteelsheet. ThereFore, 山 eNcontent
is set to o.0100%orless. TheNcontentispre 免 rablyo.0060% orless and more
pre免 rablyo.0050%or less
25 [0023]
12
(P:0.100%or less)
Pis an elementthat is contained in steel as;紅1imp 皿 itya11dis;anelementthat
degrades the hole e×PansibilityorlowtemPeraturetoughnessof 山ーesteelsheet.
ThereFore, 山 e P contentis settoo.100%orless. ThePcontentispre 先rably o.α~0% oぜ
5 1ess, more Pre定 rablyo.040%orless imdstillmorePre 定 rablyo.020%or less. Pis
incorPorated from araw material asanimPurity,andthelowerlimit mereofdoes not
need to be Paulicularlylimited,butthePcontentisPrel たrablyas smallasPossible to
ensure me hole exPansibility orthe low temPeraturetoughness. However,when the p
content is excessively decreased,themanufacturingcostincreases. From me viewPoint
lo ofmemanu 云acturingcost, 山 ePcontentisPrel たrablyo.001%or more 掴i・d more
Pre免rablyo.005%ormore.
[0024]
(S:0.0100%or less)
S is an element 中 at is containedasanimPI 虹ityandisanelementmat degrades
15 伍 e workability ofmesteelsheet. ThereFore,thes contentis settoo.0100%orless.
The S content is Preferably o.0080%orless,morePre 定 rablyo.0060%or less 虹i ld still
morePre6erablyo.0030%orless. S is incorPoria直edfromtherawmmerial as an
ImPurit ヌ and the lower limit thereofdoes not need to be Pauticularlylimited,butthe S
content is Preた rablyas smallasPossible 丘ommeviewPointofensuring 批ーeworkability
20 However, when me s content is excessivelydecreased,themanufacturingcost increases
Fromthe viewPoint ofme manufacturingcost,mes contentisPre 免rably o.0001% or
more, more Pre免 rablyo.0005%ormoreandstillmorePre 免 rablyo.0010%or more
[0025]
TheremainderofmechemicalcomPositionofmesteelsheet according to the
25 Present embodimentmaybeFeandimP ー虹ities. lnmePresent embodiment,me
13
lmpurity meをmsanelementlmatisallowedaslongasmesteelsheet accordingto me
present embodimentisnotadverselya 節ected.
lncidentally, 山 e steel sheetaccordingtomepresent embodimentmaycontain,
instead of some ofFe, one or more ofthe followingoptionalelements(Nb,VM0,Cu,
5 Ni, Cr, B,Ca,Mg,RE;M and Bi). Thatis,thechemicalcompositionmay contain the
above ‐described elements;mdmeoptionalelementstobedescribed below, and 山 e
remaindermaybeFeandimp1 虹ities. Since 山 esteelsheetaccordingtomepresent
embodiment is capable ofachieving 山 eo ectiveevenwhennotcontainingthe optional
elements, 山 e lowerlimitoftheamountof 山 eoptionalelementsiso%.
10 [0026]
(Nb:0%too.100%)
Nb is an optional element. NbisanelementhavingefFectsofsuppressing 山 e
coaisening ofthe grain sizes ofthesteelsheet,re 行ning 6erritegr額n dianneters,and
1ncreasing 山 es 立eng 山 ofmesteelsheetby 山 eprecipitationhardening ofNbC. ln 山 e
15 case ofreliablyobtainingmesee 爺ects,theNbcontentispre 化 rablyset to o.001%or
more. The Nb content is morepre 免 rablyo.005%ormore.
on me otherhand, when the Nbcontentismore 中 ano.100%,theabovedescribed e1ぼects are saturated,andtherearecaseswhere 中 erollingForce during 行nish
rolling is increased. ThereFore,inacasewhereNbiscontained, 山 eNbcontent is setto
20 0.100%orless. TheNbcontentispre 免 rablyo.070%or less and more pre免 rably
o.050%or less
[0027]
(V:0%too.500%)
V is an oPtional element. Vis anLelementhavinge1 ぼects ofincreasingthe
25 sせen蝉 lof 山 esteelsheetbyformingasolidsolutioninsteelandalso improving 中 e
14
sせengdlof 山 esteelsheet 山 roughprecipitationhardeningbybeing precipitated in steel
asacarbide,ani 立ide,acarbonitride,or 山 elike. ln 山 ecaseofreliably obtainingthese
efaects,thevcontentispre たrab1ysetto 〇0 〇5%orr][lore. Thevcontent is more
pre免rablyo.010%ormore.
5 0n 山 e otherhand, w hentheVcontente ×ceeds o.500%,there 嶺 ecases w here
山 e toughnessof 山 esteelsheetdeteriori 紙es. ThereFore,inacasew hereVis contained,
山 e V content is setto o.500%orless. TheVcontentispre 免 rablyo.300%or less
[0028]
(Mo:0%too.500%)
1O N[o is an optional element. M[ois imelementhavingefFects ofincreasing the
hi嶺denabilityofsteel 嶺ldachieving 山 ehigh-strengl 山eningofl 山esteelsheetbyForming a
cmbide or a cz虹bonitride. ln 山 ecaseofreliablyobtaining 山 eseefFects, 山 e:Mocontent
is pre定 rablysettoo.001%ormore. The:Mocontentismore preferably o.005%or
rnore.
15 0n 山 e otherhand, whenthe:Mocontentexceedso.500%, 山 erea re cases where
山 e cracking susceptibility of slabs isenhanced. ThereFore,inacasew here ハ4o is
contained, theAdocontentis setto 〇.5〇〇%or1ess. TheAdo content is preたrab1y
o.300%or less
[0029]
20 (Cu:0%tol.00%)
Cu is ian option 副 element. Cuis imelementhavinganefFectofimproving 山 e
toughness ofsteel and an efFectofincreasing 山 es 立ength.ln 山 ecaseofreliably
obtmning theseefaects,thecucontentispreferab1y setto 〇・〇2% or more. Thecu
contentismorepre 免 rablyo.08%ormore.
25 0n the otherhand, when Cu is excessivelycontained,thereaiecases where the
15
weldability of 山ーe steel sheetdeteriorates. ThereFore,inacasew hereCuiscontmned,
山 e Cu content is set to l.00%orless. TheCucontentisPre たrablyo.50% or less and
morePre 免 rablyo.30%or less
[0030]
5 (Ni:0%tol.00%)
Ni is an optional element. Niisanelementhavinganemectofimproving 山 e
toughness ofsteel and an efFect ofincreasing 山 estrength. lnthecaseofreliably
obtmning theseefaects,the Jicontentispre た rab1ysetto 〇・〇2%ormore.The Ji
content is more pre 免 rablyo.10%ormore.
lo on 山 e otherhand, when Ni is excessivelycontained,notonlydoes the 副loying
cost increase,butl 山 ereare 副 socaseswhere 山 etoughnessofaweldedheat-a1 ぼected zone
in the steel sheet deteriorates. ThereFore,inacasewhereNiis contained, 山 eNicontent
is set to l.oo%orless. TheNicontentispre 定 rablyo.50%or less and more pre定 rably
o.30%or less
15 [oo31]
(Cr:o%to2.oo%)
Cr is an optional element. Crisanelementhavinganemectofpromoting 山 e
Formation offresh martensiteor 山 elikebyenhzulcing 山 ehi嶺denabilityofsteel. ln 山 e
case ofreliably obtaining 山 is emect, 山 eCrcontentispre 免rablysettoo.o2%ormore.
20 The cr content is more preたrab1yo.o5%ormore.
on 山 e otherhand,whentheCrcontentbecomes excessive, 免 rritic
立ansFormationinacoolingprocessa 賃erhotrollingisexcessivelydelayed,and there are
cases where itbecomes difficultto obtain a desiredalnountof 免rrite.ThereFore, in a
case where Cr iscontained, 山 eCrcontentis setto2.oo%orless. TheCrcontent is
25 pre免 rably l.50% or less, morepreferably l.oo%orlessandstillmorepreferablyo.50%
16
orless
[0032]
(B:0%too.0030%)
B is ianoPtion 副 element. BisanelementhavinganactionofimProvingthe
5 peelingresistance by increasingthe grain boundarymengdbt 加 ough segregmion at grain
boundaries. ln 山 ecaseofobtainingthisefFect, 山 eBcontentispre 免 rably set to
o.0001% or more. TheBcontentismorepre 免rablyo.0002%ormore.
lncident 副ly,even when 山 eBcontentexceedso.0030%, 山 eabove ‐described
e1ぼect is saturated. lnaddition, 止lealloyingcostalsoincreases.ThereFore, in a case
lo whereBis contained, 山 eBcontentis settoo.0030%orless. TheBcontent is
pre免 rablyo.0025%orlessandmorepre 免 rablyo.0020%or less
[0033]
(Ca:0%too.0100%)
Cais an option 副 element. Cais anelementhavingane1 日当ect ofre 行ningthe
15 metallograPhicsb[uctureof 山 esteelsheetbydispersinganumberof 行neoxides in
molten steel. ln addition,caisanelementhavingane ぼectofimprovingthehole
expansibilityofthe steel sheetby 6xing s innookensteelas spheric副 Castosuppress
山 e Formation ofim elongated inclusionsuchas]M[ns. ln 山 ecaseofreliably obtaining
山 eseemects,theCacontentisprel 民キrablysettoo.0002%ormore. TheCacontent is
20 morepreIR}rablyo.0005%ormore.
0n 山 e otherhand, whenthe Cacontentexceedso.0100%,Ca0 in steel
increases, a11d 山 erearecaseswhere 山 etoughnessofl 山 esteelsheet deteriorates
ThereFore,inacasewherecaiscont 種ned,thecacontentis settoo.0100% or less
The Ca content is pre定 rablyo.0050%orless imdmorepreferablyo.0030%or less
25 [0034]
17
(Mg:0%too.0100%)
N[g is ianoption 副 element. SimilartoCa,M[gisanelementhaving e爺ects of
suppressingtheFormationofcomse:MnS byForminganoxideorasul 行deinmolten steel
and re行ning 山 e sb[ucture of 山 esteelsheetbydispersinganumberof 行neoxides. ln
5 山 e case ofreliably obtaining 山 eseefFects, 山 e:Mgcontentispre 定 rablysettoo.0002 ‐%
or more. The:Mg content is morepre 定 rablyo.0005%ormore.
0n 山 e otherhand, w henthe ハ4gcontentexceedso.0100%,anoxide in steel
lncreases, alld 山 etoughness ofl山 esteelsheetdeteriorates.ThereFore, in a case where
M[giscontained, 山 e:Mgcontentis settoo.0100%orless. The:Mgcontentis pre定 rably
lo o.0050% or lessandmorepreferablyo.0030%orless.
[0035]
(REM:0%too.0100%)
REM[ is an optional element. SimilartoCa,REM[is ianelementhaving el鎖ects
ofsuppressing 山 eFormationofco 嶺 se:Mns byForminganoxideorasul 行deinmolten
15 steel and re行ningthe stluctureofl 山esteelsheetbydispersinganumberof 行neoxides.
ln 山 e case ofobtaining 山 eseemects,theREM[contentispre 定 rablysetto o.0002‐% or
more. TheREM[contentismorepre 定rablyo.0005%ormore.
0n 山 e otherhand, whentheREM[contentexceedso.0100%,anoxide in steel
lncreases,a lld 山 erearecaseswhere 山 etoughness ofl山 esteelsheet deteriorates
20 ThereFore,inacasewhereRED4is contained,theRU 巳ハ4contentis setto ○・0100% or less
The REM[ content is pre定 rablyo.0050%orless andmorepreferablyo.0030%or less
日ere, REM (rz紅ee 鑓 thmetal)re 化rstoatotalof17elements consisting ofSc,Y
andla 皿山 anoids. ln 山 epresentembodiment,themnountofREM[re 化rsto 山 e total
amountofthese elements
25 [0036]
18
(Bi:0%too.0200%)
Bi is an optional element. Biis; 知LelementhavinganefFectofimproving 山 e
Formabilityofl 山esteelsheetbyre 行ning 山 esolidi 行cationsbnictme. lnordertoreliably
obtmn 山 is e僅ect,山 e Bicontentispre 定【ablysettoo.0001%ormore. TheBicontent is
5 morepre 免【ablyo.0005%ormore.
on me otherhand, when the Bi contentexceedso.0200%, 山 eabove ‐described
ei"「ect is satur砥ed, and thea11oyingcostincreases. ThereFore,inacasewhere Bi is
contained,theB contentis setto 〇.〇2〇〇%or1ess. TheBicontent is p『eた lab1y
o.0100% or lessandmorepreferablyo.0070%or less
lo [0037]
The above ‐described chemical compositionof 山 esteelsheetmay be measured
byano 【din嶺 yallalytic副 melhod. Forexample, 山 echemicalcomPosition maybe
measured using inductivelycoupledplasma ‐atomicemissionspect 【omel ぱy(ICP-AES).
C alld s may be measmed using an infrared abso【ptionmethodaaercombustion,andN
15 maybemeasmedusinganinertgasFusionthermalconductivitymelhod, ln a case
where aplating layer is providedonthesur 魚 ce,山 echemicalcomposition may be
analyzed a6ter山 e platinglayeron 山 esurfaceislemovedbymechanicalgrinding.
[00381
Next, the metallog 【卸 hicsbnjcture(microstmctme)ofl 山esteelsheet will be
20 described. ln thesteelsheetacco にdingtothepresentembodiment,inacasewhere the
average value ofthe averageorient 紙iondilfetencesinonecrystal gr額n 山 atcanbe
obtainedbyelec 立onbacksci 紙te【ingdi 節taction(EBSD malysis,which is o負en used 貴)【
crystalorient 砥ion 額1alysis,isde 行nedasthegrainaveragemisorientation (GAM), 山 ere
is aneedtocon せol 山 em ea 丘actionofcryst …Ugrmnshavingapredetermined GA 虹 ata
25 dep 値 positionofl/4ofthesheet せlickness 丘omthesurface. Speci 行cally,山 em ea
l9
fraction ofcryst 副 grainshavingaGAMof]mole 山 ano.50andl.7001less (GAMO.5 ‐I.7)is
50%or more and loo%orless,theareafractionofcrystalgrains having a GAM ofmore
山 an l.70(GAM 〉I.7)iso%ormorei 掴頑 20%orless,and 山 eareafractionofcrystal
grains having a GAM ofo.5o or less(GAM ≦o.5)iso%ormoreandlessthan50%
5 ln addition, in山 e steel sheet accordingto 山 epresent embodiment, in the
metallographic sbnictureatthedepthpositionofl/4ofl 山esheetthickness from 山 e
surface, 山 e area fractionofresidualausteniteiso%ormoreandless 山 an 4%, 山 e total
area fraction ofresidualaustenite,f reshmartensite,cementiteandpe そ虹liteis o % or more
and 1〇% or 1ess,theaveragegrainsizeis15 ・〇一mor1ess,andtheaveragedis1ocaution
lo densityis l.ox lo14/m2ormoreand4.ox lo15/m2orless.
ln addition, in山 e steel sheet accordingto 山 epresentembodiment,the total of
pole densitiesof{211}<011>and {332}<113>inthethicknessmiddlepod 〔ion is 12.oor
less
Hereinaaer,eachrequirementwillbedescribed in detail
15 [0039]
ln the steel sheet accordingto 山 epresentembodiment,atthedep 山 position of
l/4 ofl山 esheetl 山 icblessfrom 山 esurfaceofl 山 esteelsheet,GAMo.5 ‐I.7,GAM 〉I.7,
GAM ≦o.5,山 e area fraction ofresidual austenite, 山 etotal 嶺 eafractionof “residual
austenite,f reshmaltensite,cementiteandpe そ虹lite“,山 eaveragegrain size 嶺 ld 山 e
20 average dislocationdensity 組recon 立olledtobeinpredeterminedranges.Here, the
reason For specifying 山 esteelsumctuureat 山ーedepthpositionofl/ 4of 山 esheetthickness
丘om 山 esurfaceof 化lesteelsheetis 山 atthisdep 山 positionisthemiddlepoint between
山 e surface ofthesteelsheet そmd 山 esheetl 山icknessmiddle position alld山 esteel
sumctureat 山 epositionrepresents 山 esteelsumctureofl 山esteelsheet(indicates 山 e
25 average steel sumctureofl 山eoverallsteelsheet)except1 応r山 etexture. ln 山 e present
20
embodiment, the depth positionofl/4ofthesheet 山 icknessfromthe surface of 山 e steel
sheetmeansaroughly ±100 一mrangein 山 esheetl 山icbless directionfrom,asl 山ecenter,
山 e dep 山 position of l/4ofthesheet 山 icknessin 山 esheetthicknessdirection from 山 e
surface ofthesteelsheet.
5 [0040]
The present inventors Found 山 atas ローucturehavingdislocations or strmn to an
appropriate extent isexceptionalintermsof 山 ebalanceamongs せength,uniform
elongationandlowtemperaturetoughness andFoundthatsuch a shrlucturecan be de6ned
byG! 主人4tha直afeobtmnedbyEBSDanalysis
lo lnthepresent embodiment,GAM 〈o.5is 免 rrite,andGAM 〉I.7and GAMo.5 ‐I.7are
microsbnjctmes having a bcc crystal stmctureol 山 erl山 an 免 rrite,山 at is, one or more of
bainite,freshmaltensite,temperedbainite,temperedmartensite; 額ldpemlite.
[0041]
(Area fraction ofGAMo.5- ・.7:50%ormoreandloo%orless)
15 The GAM is the average value ofloc 副 cryst副 orientation diffierences in one
cryst…Ugri 額na11dis consideredtohaveacorrelationWiththedislocation density orthe
amountofe1asticsl 比÷aininthecrysta1grain. ordinari1y,anincreaseinthedis1ocation
density or elastic sせain in agrmnleadstoimprovementins 立engh,but degrades
workability. lnacrystalgrmnWheretheG! 主人4has beencon 立olledtomorethan o.50
20 and l.7o or less,itispossibletoimprove 山 es 立engthwi 山 outdegradingtheWorkability
ThereFore, in山 e steelsheetaccordingto 山 epresentembodiment,thea rea fraction of
GAMo.5-,.7iscon 立olledto50%ormore. Theareaf raction ofGAMo.5-,.7ispre 定 rably
60%ormoreandmorepre 免 rably70%ormoreandmaybe loo%
[0042]
25 (Area fraction ofGAM 〉・.7beingo%ormorei 額ld20%orless)
21
Acryst 副 grain wherethe GAMismore 山 anl.70(GAM 〉I.7)has ahigh
dislocmiondensityandelastics せain,and 山 es 口engdbincreases,but 山 eductilityispoor.
ThereFore, 山 e area fractionofGAM 〉I.7iscontrolledto20%orless. Thearea fraction
ofGAM 〉I.7ispre 定rably lo%orless 虹i ldmorepre 定rably5%orlessandmaybeo%
5 [0043]
(Area fraction ofGAM 〈o.5:0%ormoreandlessl 山an50%)
Acryst 副 grain wheretheGAMiso.50orless(GAM ≦o.5)has a low disloc砥ion
densityand elasticstrmnandis 山 use ぼectiveforimproving 山 eb 副ancebetween sせen帥
anduniFormelongation. ThereFore,suchacrystalgrmnmay be contmned. on the
lo ol山erhand,when 山 eareafractionofGAM 〈o.5becomes 50%ormore,itbecomes
dimcult to obtain a desiredstrengl 山. ThereFore, 山 eareaf raction ofGAM 〈o.5is setto
less than50%. Thea 工ea 丘actionofG! 主人4≦o.5ispre6erablylessthan40%. Sincethe
steel sheet accordingto 山 epresentembodimentiscapableofachieving theo 覇 ective
evenwhenGAM くo.5is notcontained,theareafractionofGAM ≦o.5maybeo%
15 [00く14]
(Area fraction ofresidual 紙lstenite:0%ormoreandlessthan4%)
(Total ;虹ea fraction ofresidual austenite,freshmaltensite,cementite and pe;刻rlite:0%or
moreandlo%orless)
Residual austenite, 丘eshmmtensite,cementiteandpe 測rliteactas crackinitiation
20 points and degrade 山 e hole e×pansibility orlowtemperaturetoughness of 山 esteelsheet.
ThereFore, 山 e total 嶺 ea fraction ofresidualaustenite,f reshmartensite,cementite 額i ld
pearlite is setto lo%orless. Thetotalareafractionispre 免 rably 8% or less and more
pre免 rably5%orless. Since 山 esteelsheetaccordingto 山 epresent embodiment is
capableofachieving 山 eo ectiveevenwhenthesemetallographic stmctures are not
25 contained, 山 etotal 嶺 eafractionofl 山esemetallographicsbmcturesmaybeo%
22
1n addition, alnongthese =leta11ographicstluctures,residua1austenitetransforms
into e×÷" elmely6ull-hmd 丘eshmmtensitebypreli ・nin嶺 yw orkingsuchaspunching and
山 ereby signilncantly degradestheholee ×pansibilityof 山 esteelsheet.ThereFore, the
area fraction ofresidual austenite is settoless 山 an4%. Theareafractionofresidua1
5 austeniteispre 免 rably3%orless,morepreferably2%orless,stillmorepre 免 rably less
山 an 2% and far stillmore preferably l%orless. Sincetheareafractionofresidual
austenite is pre免 rablyas smallaspossible, 山 eareafractionmaybeo%
ln addition, in山 e steel sheet accordingto 山 epresent embodiment, fresh
maltensiteispresentasamaltensite-austeniteconstituent(MA)
10 [0045]
(Average grain size:15.0 一morless)
ln the met 副lographicsbmctme,when 山 eaveragegrain size is coi虹se,山 e
丘actuu「e suurface unitduringnupturebecomeslaurge,andthelowtemperaturetoughness of
the steel sheetdeteriorad[es. ThereFore,theaveragegrainsizeis setto15.0 一m orless
15 The average grainsizeispre 免rably12.0 一morless,morepre 免rably lo.0 一morless,
and stillmore pre免 rably 7.0 一morless. Theaveragegrainsizeispre 免 rably as sm 副l as
possible, and thus the lower limit is notpauticularlylimited. However,itistechnically
difficulttore 行negrainsbynorm 副 hotrollingsuchthat 山 eaveragegrainsize becomes
smallerthan l.0 一m. ThereFore,theaveragegrainsizemaybe setto l.0 一m ormoreor
20 4.0 一mormore.
[0046]
The average grain size in山 epresentembodimentre 化rsto 山 eaverage value of
grmn slzes.nacasewherearegion 山 atis suurroundedbygrainboundmries having a
cryst≧U orient紙ion dimerence of 15O or moreandhasacircleequiv 副Lent diameter ofo.3
25 一m or more in the metallograPhic sbmcturehavingabcccryst 副 sbmcture,thatis, 定rrite,
23
bainite, fresh maltensite,temPeredbainite,temPeredmartensite 嶺ldPemlite is de行nedas
acryst 副 grain,and 山 egrainsizesofresidualausteniteandcementitearenotincluded in
山 e ci副Lculation ofthe averagegralnslze.
[0047]
5 1n the present embodiment, 山 eaveragegrainsize; 額ld山 ea rea fraction ofeach
sbmcture are obtained by observing and measuringasbnictmeat 山 edepl 山 position of l/4
ofl山e sheet 山l icblessfromthesurfaceofl 山esteelsheetona cross section ofl山e steel
sheet pmralleltoarollingdirectionand 山 esheetthickness direction
Theaveragegrainsize,GAMo.5 ‐I.7,GAM 〉I.7,GAM 〈o.5…uld 山 ea rea fraction of
lo residual austenite aie obtainedbyscanningelectronmicroscope(SEM)observationand
EBSDani 副ysisusinganEBSDallalyzermadeupofathermalelecl ぱic行eldradiation
scannlng electron microscope and an EBSDdetector. Specil丘cally,inaregion that is
200 一m long in the rolling directionand loo-mlongin 山 esheet せlickness direction, ln
w hich 山 e dep 山 positionofl/ 4of 山 esheetthickness 丘om 山 esurfaceof 山 esteel sheet
15 is centered, cryst副 orient紙ioninFormationisobtainedato.2 いminterv 副Lsw hile
di爺erentiating 元 candbcc.Using 山 eaccompanyingsoaware( “○○虹 An 副ysis
(registered 立ademark)manu1 陵cturedbyAMETEK1nc. “)ofl山eEBSD 組lalyzer,
bound 嶺 ies where 山 e crystal orientation difFerence is 150ormoreareregi 虹dedascrystal
grain bound 嶺 ies, aregion 山 at is surroundedbycrystalgrainbound 嶺 ieshavingacrysta1
20 orientation difFerence of 150 or more and has a circleequiv 副entdiameter ofo.3 一m or
more is de6ned as a crystalgrain,andtheaveragegrainsizeofbccisobtained by a
me 山 od in which the Following mathematical expressionisused. ln 山 e ma 山 ematical
expression,Drepresents 山 eaveragegrainsize,Nrepresents 山 enumberofcryst 副 grains
山 at are included in the evaluation regionof 山 eaveragegrainsize,Airepresents the area
25 0fan i値 (i=1,2,..,N)crystalgrain,anddirepresentsmecircleequivalemdiameterof
24
山ーei伍 cryst…Ugr 額n.
[0048]
[Ma 山 .1]
D= i=I
i=I
5 [0049]
cryStaJ grain boundafieS havi]ngacrystalorientationdifference of 150 0r more
aremainly 免rritegrainboundaries or 山 eblockboundaries 。flow temperature
立ansFormationPhases,thatis,bainite,freshmaltensite, temPered bainite and temPered
maltensite. lnamelh 。dFormeasuring 山 e 免rritegraindimnetersaccording to JIS G
IO 0552:2013, there 嶺 e cases where agraindiameteriscomputedevenfiora 定 rritegrmn
having a cryst…Uorientationdi 畳当erence 。fless 山 an150,and,6uu 【thermore,the block of 山ーe
low temper 紙 uure ÷b ansFormationphaseisnotcomputed. ThereFore,asthe average gra1n
sizein 山 epresentembodiment,av 副ueobtainedbyEBSDanalysisasdescribed above is
adopted.
15 [0050]
The mrea 丘action ofresidual austenite is obtained by calculating 山 em ea 丘action
ofa metallograPhic stmcture determinedas 元 cbyEBSDanalysis
As 山l e area fractionofGAMo.5 ‐I.7,GAM 〉I.7orGAM 〈o.5,regions 山 m are
surrounded by bound 測rieshavingacrystalorientationdi 賃当erenceof5o or more and
20 having a circle equivalent diameterofo.3 一mormorearede 行nedas cryst副 grains, and
山 e m ea 丘action ofcrystal grains havingaGA 虹 v副ueineachrange,whichisavalue
25
obtained by grain average misorientation analysis equipped in o]M[Analysis(GAM
vi副ue), in bcc crystalgrains amongcrystalgrains is calculated.The reason 貴)rde 行ning
aboundafyhavingacrystalorientationdifferenceof500rmore as a crystaヱgrain
boundaryis 出獄 meremrecaseswhereitisnotpossibletodimerentiate diffierent
5 sbnictures Formed as vi虹 iantsclosetomesalmeprioraustenitegrainifnotso.
[0051]
The area 丘actionsofPez 虹litei額ldcementiteareobtainedbyobserving
metallographic sbmctures revealedby JitaletchingbySE ハ4observaution.
Theareafractionoffreshmaitensiteisobtainedbyobtainingmetotal mea
lo 丘action of 丘esh mmtensite and residual austenitebyobservingthema=[tensite-austenite
constituent (MA)revealedbyLeperacorrosionwi 山 anoptical microscope and
subtracting the a工ea 丘action ofresidualausteniteobtainedbytheabove ‐described
me 山 odfromthistotala rea fraction
The area fractionsofPez 虹lite,cementitei 額ldMAmay be obtained by image
15 analysis or may be obtained by apointcalculationmemod. Forexample,Forpearlite
and cementite, the area fractions maybeobtainedbyobserving 山 reeormorevisual
賃elds (100 一mx loo 一1n/visual行eld)atade 山 Positionofl/4ofl 山esheet 山l icbless
frommesurfaceof 山 esteelsheetatamagni 行cation 。floootimesandcalculating 山 e
area fraction byme point calculation memodwi 中 latticespacingsof5 一m. lnaddition,
20 me total嶺 ea fraction 。fM Amay be obtainedbyobservingtwo 。rmorevisual 行elds
(200 一m x200 一ロンvisual 行eld)inaregionatal 超 dePthpositionofmesheetthickness
fromme surface ofme steelsheetatamagni 行cationof500timesandcalculating me
area fraction byme point calculation memod at lattice spacingsof5 一m.
[0052]
25 (Average dislocation density:1.OX IO14/m2to4.OX IO15/m2)
26
ln the steel sheet accordingto 山ーePresentelm ーbodiment, 山ーeaveragedislocation
density in 山 esteelsheetst ・uctureat 山ーedePthPositionofl/ 4of 山 esheetthickness 丘om
山 e surfaceis setto4.ox lo15/m2orless. This isbecausedesireduniForm elongation is
obtmned. The average dislocationdensityispreIR;rably3.5 x lo15 /m2 or less and more
5 prelたrab1y3.OX1015/m2or1ess.
0n 山 e otherhand,w hentheaveragedislocmiondensityis less than l.o x lo14
/m2, itisdimculttoensure 中 es せength.Therefore, 山 eaveragedisloc 紙ion density is
setto l.o x lo14/m2 or more. The averagedislocationdensityispre6erably l.5 x lo14
/m2 or more;mdmorepre 免 rably2.ox lo14/m2ormore.
10 [0053]
Regarding 山 e average dislocationdensity,thehalfv 副uewidths of 山 e
difi ヨractionpeaksof(110),(211)and(220)atthedepthposition of・/4 ofthe sheet
山 icknessfromthesurfaceof 山 esteelsheetwereeachobtained using 山 e X-ray
difiractionmelhod,ands 立aineisobtainedbythe →Williamson ‐Hall method. The
15 average dislocation density is obtainedfromp=(14.462)/b2based on the obtmned s立ain e
and the Burgersvectorbofiron(b=0.25nm).
[0054]
The steel sheet according to 山 epresentembodiment speci行es thetot 副 of 山 e
poledensitiesof{211}<011>and{332}<113>atthethicknessmiddle pod[ion ofthe
20 steel sheet. Thethicknessmiddleponionin 山 epresentembodiment re化 rs toaramgeof
approximately200 一mineachof 山 efrontdirectionandtherear direction ofthe steel
sheetfrom 山 emiddlepositionin 山 esheetl 山icblessdirectionofl 山esteel sheet (the
depl山 positionofl/2ofthesheetl 山icblessfromthesur 魚 ceof 山 esteelsheet).
[0055]
27
The reason fiorsPecifying 山ーete×tureatthethicknessmiddle Pod[ion is that 山ーe
textureatthethickness ll"ddlepon[ionandmechanicalpropen[ies favorablycorrelate
with each other. Thereasonisnotclear,but 山 ePresentinventors assumeasdescribed
below. ln 山 e steel sheet,duetoGFictionbetweenarolland 山 esteelsheet during
5 rolling, shear deFormation occurs in oPP0sitedirections in山 efrontand rear of 山 e steel
sheet, andP1anes 立aindeFormationoccursin 山 e 山 icknessmiddlePonion.Since 山 e
texture of 山 e steelsheetchangesin 山 esheet 山 icknessdirectioninresPonse to 山 is
deFormation,and,since 山 edirectionsofthesheardeFormationareoPP0sitetoeach o山 er
in the front and rear ofthe steelsheet,symmetricorientationsdeveloPin 山 ef ront and
lo re嶺 in 山 etext 皿 e. ThereFore, 山 einnuences of 山 etextureonmechzmicaIProPenies are
o任set in 山 efrontandrear,and,consequently, 山 etextureatthethickness middle Ponion
andthemechanicaIProPenies favorablycorresPondtoeach other
[0056]
(TotalofPole densitiesof{211}<011>and{332}<113>at 山icblessmiddlePonion: 12.0
15 orless)
Due to the develoPment of{211}<011>and{332}<113>,P1astic anisoせoPy
lncreases, and 山 e hole exPansibilityofthesteelsheetsignil 日cantlydeteriorates
ThereFore, the totalofthePoledensitiesof{211} ÷<○11>and{332 ドー13>is set to 12.o or
less. The total ofthe Pole densitiesof{211}<011>and{332}<113>isPreIR;rably lo.0
20 or 1ess,morePreIR 〉rab1y 0or1ess,evenmorePrel 氏〉rab1y6.0or1ess and mostPrel たrab1y
5.oorless. Thetotalof 化lePoledensitiesisP1e た rablyassmallasPossible;however,
since 山 e Pole density ofeach orientation grouPis l.○inacasewherethesteel sheet does
not include any texture, 山 etotalofthePoledensitiesismorePre たrably set to a value
closeto2.0
25 ThePoledensitycanbeobtainedfromcrystalorientationinFormation bythe
28
EBSD melhod a11d is a synonym ofthe X-rayra ぱ1domintensityratio.
[0057]
{股 1} indicates a crystal plane parallel to arolled sm 魚 ceand 〈uvw 〉 indicates a
cryst副 orient砥ion paralleltotherollingdirection. Thatis,{hkl}indicates a
5 cryst副 inwhich {h盟 }isorientedinthesheets 皿魚 cenormallinedirection and is
oriented in the rollingdirection.
[0058]
ln addition, in山 e presentembodiment, 山 epoledensityofeachcrystal
orientation in山 e 山 ickness middle ponion is obtainedbymeasuring 山 eorientation
l。 inFormaution of l,0oo or more bcc crystalgrainswhiledilferentiating に c and bcc atthe
山 icknessmiddleponion(armgeof200 一mineachofl 山efrontdirection and 山 e re嶺
directionof 山 esteelsheetfrom 山 e 山 icknessmiddleposition(thedep 山 position of l/2
ofl山esheetl 山icblessfromthesur 魚 ceof 山 esteelsheet))byEBSDanalysis using a
deviceinwhichascanningelectronmicroscopeandanEBSDanaJyzeraiecombined and
15 01MAnalysis(registered 立ademark)manufacturedbyAM 旧 TEK1nc. and perft)rming
ODFallalysis inwhich 山 ehi虹monicseries expansion is used
[0059]

(発 nsiles せen帥 : 980Mpaormore)
20 The steel sheet according to 山 epresentembodimenthasahighs 立eni夢h alld is
exception 副 in terms oflow temperaturetoughness,elongation,and hole expansibility by
山 e con 立ol ofl山e chemicalcomposition, 山 emetallographicsbnictureandthetexture.
However, when 山 etensile s立eni夢hofl 山esteelsheetislow,anLefFectofvehicle body
weightreduction,stifl …nessimprovement,orthelikeisweak.ThereFore, 山 etensile
25 sせengdb(TS)of 山 esteelsheetaccordingto 山 epresentembodimentissetto980Mpaor
29
more. Thetensile streng;hispreIR}rablylloo ハ4PaormoreandmorePreIR;rably l180
ハ4Pa or more. Theupperlimitisnotpau[ticulaurlyspeci 行ed,butPressForming becomes
moredimcultas 山 es 口engdbincreases.Therefore,thetensiles 口en帥 ーmay be setto
180 0ハ4Paorless,1600 ハ4Paorless,or1400 ハ4Paorless.
5 [0060]
The steel sheet according to 山 ePresentembodimentmayhaveaplatinglayer on
山 esur 鍋ce (oneorbo 値 ). TheplatinglayerimProvesthecorrosion resistance. The
P1ating type isnotpau[ticu1mr1y1imitedandisordinmri1yZinc-basedP1atinginc1uding Z1nc
plating and zinc alloyP1ating. Asexamplesofl 山eplatedsteelsheet,anelectrolytic
lo zinc‐P1atedsteelsheet,anelec せolyticzinc-nickelalloy ‐P1atesteel sheet, ahot ‐dip
galvanizedsteelsheet,agalvannealedsteelsheet,ahot ‐dipzinc ‐aluminumalloy ‐P1ated
steel sheet,and 山 elike 嶺 eexempl そばyexalnples. Thep 夏江ingadhesionamount may be
anordin 組ryamount.
[0061]
15 1nthe steel sheet accordingto 山 ePresentembodiment,TS xE1,which is an
index of 山 e balancebetweentensiles 口engdl(TS)anduniformelongation (uEI), is
Pre定 rably6000M[Pa ・%ormore,morePreFefably7000M[Pa ・%ormoreandstill more
Pre定 rably8000M[Pa ・%ormore.
The elong例[ion of 山 esteelsheetisev 副uatedusingaNo.5test Piece ofJIS Z
20 2241:2011bythePercentageP1asticextensionatmaximum Force sPecil日edinJIS Z
2241:2011,thatis,uniformelongation(uEI).
ln the steel sheet accordingto 山 ePresentembodiment,Ts x 九,which is an
index of 山 e balancebetweentensiles せen帥 andholeexPansibility,ispre た rably40000
MPa ・% ormore andmorePre 定rably50000MPa ・%ormore.
25 The hole expzu・sibilityofl山esteelsheetis evaluatedby 山 elimitinghole
30
expansion r紙io )speci 行edinJ1SZ2256:2010.
ln addition, in me steel sheet accordingto 山 epresentembodiment, 山 ef racture
aPPe 硝縄1ceせansition te1mPerature(vlrs)intheCh: 組pyimpacttest as an inde× oflo w
temPeraturetoughnessisPre 免 rably-400Corlower.
5 The Charpy impact test is perFormed according to JIS Z 2242:2005.
[0062]
<]¥[anufacturing melhod>
Asteel sheet manufacturing me 山 odaccordingtothepresent embodiment is not
pzuticularlylimited,and 山 esteelsheetcanbeobtained by a manufacturing melhod
lo including 中 eFollowingsteps.
(1)Aheating step ofhe 測tingaslaborsteelpiecehaVingapredetermined
chemical composition.
(11)Ahotrolling step ofperforming multi‐passhotrollingon 中 eslab orthe
steeIPiece aa;er化le heating steP using aP1urality ofrollingstandstoobtainahot ‐rolled
15 steelsheet.
(111)Acoiling step ofcoiling 山 ehot‐rolledsteelsheet.
(IV)Aheat 立eatment stepofperformingaheat 立eatmenton 中 ehot‐rolled steel
sheeta6ter 山 ecoilingstep.
Hereinaaer, preIR}rable conditions For each stepwillbedescribed.
20 [0063]
[Heating step]
(Hei紙ing temperature: 1280oCorhigherandSRT(OC)orhigher)
Theheatingtemperat 班 eofaslabofsteelpiecethatistobesu 覇 ectedtohot
rollingis setto 1280oCorhigherandatemperatureSRT(oC)represented by me
25 倉)1lowing 恥 rmula(1)orhigher.Whentheheatingtemperatureislowerthan12800Cor
31
lower 中 anSRT(0C), 山 e solutionizingofTicarboni 立idebecomes insumcient, and 山 e
tensile strength oruniform elongation ofthesteelsheetdeterio ・砥es.Therefore, 山 e slab
or steeIPiecematis tobesu 覇 ectedtohotrollingisheatedsuchthat 山 etemperature
reaches 1280oC or higherandSRT(OC)orhigher. Here,‘せ letelmLPぼ 辻i 田 e of 化le sla心 or
5 steeIPiece is 12800C orhigherandSRT(OC)orhigher'means 山 atthe temPeratture of
me slaborsteelpieceisequ 副 toorhighermallthehighertemper 砥皿 eof 12800C alld
SRT(OC). ThehemingtemPeratureispre た rablyhigherman 13000C 瀧1d more
prel危rab1y1305olcorhigher.
0n me otherhand,Whentheheatingtemperatureishigher 中 an 14000C, mere
lo are cases where athick scale is Formedto decreasemeyieldorsigni 行cantdamage ls
caused inmeheating6 皿 naces. ThereFole,meheatingtemperat 班 eispre 定 rably 14000C
orlower
SRT C)=1630 十9oxln([C]x[Ti]) … (1)
Here, [element symbol]inmeformula(1)indicatesmeamnountofeachelement
15 bymass%.
The slab or steel piece 中 atistobeheatedmaybeaslaborsteelpiece obtained
by continuous casting or casting andbloomingormay 副 sobeaslab or steelpiece
obtained by additionallyperForminghotwor 超 ngorcoldworkingontheabove ‐described
slab or steelpiece.
20 [00(舛]
[Hotrolling step]
lnthehot rolling step,multi‐passhotrollingisperFormedonaslab or steel piece
having 山ーe above-describedchemicalcomPositionusingaplu ぱalityofrollingstands,
mereby manufacturing ahot-rolledsteelsheet. Themulti-passhot rolling cモm be
25 performed using areversemillorata はldemmill,and,f rommeviewPointofindustrial
32
Productivit ヌ at嶺ldem millis ple先 rablyusedinatleastseveralstagesfrom 山 eend.
ln the steel sheet mallufacturing melhod according to 山 ePresentembodiment,
山 e finish temperat 班 e FT and thetot 副 rollingreduction, 値 etimenecessaryForrolling,
山 e number ofpasses; 紅ld山 ecoolingconditionsineachtemperatmera は1ge based on 山 e
5 行nish temperat 皿 e FT in the hotrollingstepareas describedbelow.
Normally, 山 ese conditions arenotalls 立ictlycontrolled,but 山 e present
inventors Foundthat,w henthese 嶺 econ 立olledat 山 esametimeand,6u 【hermore, post
steps m e 副Lso con 立olled, itis possibletoobtainthesteelsheetaccordingto 山 e present
embodimenthavingahighstrengthandbeingexceptionalintermsofelongation, hole
lo expansibility and low temperaturetoughness.
[0065]
(Totalrollingreduction in temperature range ofhigherthan FT 十 500C and FT +150oC
orlower:50%ormore)
ln the steel sheet manufactuLring nlelhod accordingto 山 epresentembodiment,
15 w hen the 行nish temperature is representedbyFTin 山 eunitofo.C,recrystallized
austenite grains in 値 e steelsheetarerefinedbyincreasing 中 etotalrolling reduction of
hot rolling in atemperaturerangeofhigherthanFT 十 500CandFT+150oCorlower.
ln ordertore 行ne 山 erecrystallized austenitegrainsin 山 esteelsheet, 山 etot 副 rolling
reduction in the temperature range ofhigher 山 anFT 十 500・CandFT 十 1500.Corlower is
20 setto 50% ormore. W〆hen 山 etotalrollingreductionintheabove ‐described
temperature range is lessthan50%,austeniteisnotsuaEicientlyre 行ned. ln 山 is case,
metallograPhic sb[ucturesa 負erせansForm 砥ionbecomecomse,and recryst副lizi印[ion
between rollingpasses during rolling in a subsequent temperature rangeofFTtoFT+
50oC is delayed, which mal【esl山etexturea 負ertransFormationdevelop.The tota1
25 rolling reductionin 山 etemPeri 紅uurerangeofhigher 山 anFT 十 500.C and FT 十 1500Cor
33
lower is ple免 rably as highaspossible,butmaybe setto90%olless since 山 eindustrial
limit is approximately 90%
[0066]
(Totalrollingreduction in temperature range ofFTto FT+50oC:40%to80%)
5 (Time necessそばy 貴)rrollingintemper 須山rerangeofFTtoFT+500C: ○.5tolo. ○
seconds)
ln the steel sheet manufactuLring melhod accordingto 山 epresent embodiment,it
lspossibletoobtainasteelsheetbeingexceptionalinterms ofworkabilityand toughness
by appropriately con 立olling 山 etotalrollingreductioninatemper 砥uurerange ofFTto FT
IO 十 500C and 山 etimenecessaryforrolling(rollingtime)inassociationwi 山 中 e cooling
conditionsa 賃:erhotrollingtobedescribed below
vvhenthetotalrollingreductioninthetemperaturerange ofFTto FT 十 50 c is
less than40%, 山 esbnictmesa 負er 立ansFormationbecomecoi 虹 se,recrystallization
between 山 e rollingpassesandaaerrolling 行nishisdelayed,andthea lnount of
15 deFormation of 山 einsideof 山 esteelsheetbecomesuneven. ln 山 is case, a speci行c
orient紙iondevelopsa 賃eru÷ansFormation,whereby 山ーe hole expansibility of 山 e steel
sheet deteriorates. ThereFore,thetotalrollingreductioninthetemper 紙uurerange ofFT
to FT 十 500・Cis setto40%ormore. 0n 山 eo 山 erhand,whenthetotalrollingreduction
in the above ‐describedtemperaturerangeexceeds 80%,since 山 etexturesignil 日cantly
20 develops even aaerrecrystallizi印[ion,山 eholeexpansibilityof 山 esteelsheet deteriori紙es
ThereFore, 山 e totalrollingreductioninthetemper 砥uurerangeofFTtoFT 十 500・C is set
to80%or less
[0067]
ln the steel sheet mallufact 皿 ingmelhodaccordingto 山 epresentembodiment,
25 Guldlermore, itis alsoimpo"anttoappropriatelycon 立olthetimenecessaryForrollingin
34
山 e abovedescribed temperature rmge( 山 etemperat 班 eraは1geofFTtoFT+500C). ln
acase wherethetimenecess 宅uryForrollingintheabove-describedtemperaturerange ls
too shoht, recryst≧司L1ization does notproceedbetweenthepasses,rolling sせain is
excessivelyaccumul 紙ed,andaspeci 行corientationdeVelops,whichmakes itdifficultto
5 obtmn a desiredtexture. ThereFore, 山ーetimenecess そばyfiorrollingintheabovedescribed temperature range is set to o.5 secondsollonger. ThetimeispreIR}rably l.o
second or longer and morepreIR}rably2.oseconds orlonger. 0n 山 eother hand, in a
casewhere 値 etimenecessz 紅yForrollingin 山 eabove ‐describedtemperatwrerange is too
long, recrystallized grains grow between the rollingpasses,and 山 es ローuctures a負er
lo 立ansFormation becomecomse. ThereFore, 山 etimenecessz 紅yForrollingintheabovedescribed temperature range is setto 10.0secondsorshorter. Theti 菖leispreferab1y 8.0
seconds orsho"er,morepre 先 rably6.osecondsorsho"er,a lldstillmolepre 定 rably 5.o
seconds or shoner.
[0068]
15 wo or more passes ofrolling being perFormed in each oftemper 紙uurerangeofhigher
than FT 十 500.C and FT 十 1500.CorlowerandtemPer 紙u嶋 rangeofFT ぬ FT 十 500C)
ltis impo 質ant to rePeat worlding and recrystallizmioninboth 山ーerolling in山 e
temperature rangeofhigherl 山anFT 十 500CalldFT 十 150oCorlowerand 山 erolling in
the teー【ゴー:,per 紙urerangeofFTtoFT 十 500.C. ThereFore,tWoormorepassesofrolling 縄・e
20 PerFormed ineachof 山 etemPeratureranges.
[0069]
The totalrollingreduction in each temper 砥uurerangein 山 ehotrolling step
re化rs to the percentageof 山 etotalrollingreductioninapredetermined temperature
range with respecttomeinletsheetl 山icknessbeFore 山 einitialpassinthetemperatme
25 range (山 edimerencebetween 山 einletsheet 山 icblessbeforetheinitialpassofrolling in
35
山 is temperature range and 山 eoutletsheetl 山icblessa 賃erthe 行n副 pass ofrolling in this
temperaturerange).
[0070]
(Average cooling rate in temperature rmge ofFTto FT+1000C:6.00C/sec or 魚 ster and
5 40.o oC/secorslower)
The temperature of 山 esteelsheetduringthehotrollingchanges due to heat
generi紙ionbydefiormmionofrolling,relnovalofhei 紙 bycontactwitharoll,or 曲ーelike;
however, in 山 e present embodiment, 山 eaveragecoolingrateinatemperatuurerange of
FTto FT 十 1000.Cisappropri 紙elycon 立olledinordertocon 立oltheprecipitationofthe
lO Ti‐based そcばbideandthete ×ture. VVhen 山 esteelsheetisdweltFora long period oftime
ln a 1ow temperature region c1ose to the6nishtemperature,theTi-basedc …虹bide is
precipitated due to working,a lld山 estrengl 山 decreases. ontheol 山erhand,when the
rolling is completedwi 山 inashonperiodoftime, 山 etexturedevelops,and 山 e hole
expansibility deteriorates. ThereFore, 山 eaveragecoolingratein 山 etemperatwre ramge
15 ofFTtoFT 十 1oooCis setto6.ooC/secorfasterand40.ooC/secorslower. The
average cooling rate in this temperature range is pre6erably9.0 C/secor faster and
30.0 C/sec or slower and more pre6erably12.0 C/secor faster and 20.00.C/secor
slower
[0071]
20 The average cooling rate within 山 eabove ‐describedtemperaturerange is
con 立olled by con 立olling heat generationduetodeFormationand 山 e remov 副 ofheat by
山 econtactbetween 山 esteelsheet そmdarolll 山rough 山 econtrolofl 山erolling speed and
山 e rolling reduction. Furthermore,theaveragecoolingrateis con 立olledbyperForming
water cooling,inductionheating,or 山 elikeasnecess そばy. lnaddition,the average
25 coolingratein 山 eabove ‐describedtemperaturerangeisobtainedbymeasuringthe
36
surface temperature of 山 esteelsheetwi 山 aradiation-type 山 ermometeror 山 elike or by
simulation in a case wherethe measmementisdi 任icult.
[0072]
(FinishtemperatureFT C):Equ 副 toorhigherl 山anAJ3(0C)obtainedfromfonnula(2),
5 equalto or higherl山an TR(oC)obtainedfrom 貴)rmula(3), 瀧ldlloooCorlower)
AJ3 C)=901 ‐325x[C]+33x[Si] ‐92x[M[n]+287x[P]+40x[sol.AJ]
(2)
TR(oC)=800 十70ox[Ti] 十looo x[Nb](3)
Here,[element symbol]in 貴)rmulae(2)and(3)indic 砥esl山eamount ofeach
lo element by mass%, andzeroisassignedinacasewhere 山 eelementisnotcontained.
ln the steel sheet mallufact 皿 ingmelhodaccordingto 山 epresentembodiment,
山 ewor 超 ng and recrystallization ofaustenite are repeated during 行nishrolling, 山 ereby
re行ning 山 e met 副lographic sbmctmesandsuppressing 山 edevelopmentofthetexture.
Therelbre, 山 e 行nishtemperatureFTissettoequaltoorhigherthzmAr3 C) 山 atis
15 obtainedfrom 山 eformula(2 mdequaltoorhigher 山 anTR(0C) 山 atisobtained from
山 eformula(3). Here,the 行nishtemperatureFTre 免rstothesur 鏡 cetemperature ofthe
steel sheetimmediatelya 負er 行nalrolling.
[0073]
When FT is lower 山 anAr3(0C), 免 rritic立ansfbrmationproceedsduring 行nish
20 rolling, the coarse Ti‐basedcarbideisprecipitatedtogetherwi 山 the 化 rritic
立ansformation, alld山 e s立en停hofl 山esteelsheetdecreases. lnaddition,when 山 e FT is
lowerl 山anTR C), 紙・stenitea 負er 山 ehotrolling 縦ldbelbrecoolingbecomes
signil日cantlynat. ln 山 iscase,inthesteelsheetasa 行nalproduct, 山 e sb[luctureis
elongated in山 e rolling direction,andtheplasticanisotropybecomes laurge,which makes
25 山 e hole expansibility ofthe steel sheetdeteriori 紙e. VVhenFTis settoequaltoorhigher
37
山 anTR C),therecrystalliz 欲ionofwolkedm ・stenitebetween 山 erolling passeS 1s
appropr 淀江e1yacce1erated,whichmakesitPOSsib1etore6ne recryst副1iZed austenite
gr額ns. A 負:erthehotrolling,itispossibletoobtainasteelsheethavingsteel sb〔luctuures
andatextuuresuitableForlowtemperaturetoughnessandholeexpansibilityinassociation
5 with 山 e cooling conditions aaer 山 ehotrollingtobedescribedbelow.
0n 山 e otherhand,whenFTe ×ceeds llooo.C,thestlucturebecomes co 棚rse, and
山 e low temper 砥uuretoughnessofthesteelsheetdeteriorates.ThereFore, FT is setto
1100ocor1ower. TheFTisprel 民;rab1y1080ocor1owerandmore prelたrab1y 1060ocor
lower.
1O The temperature duringthe 6nish rolling reIR}rstothesurface temperature ofa
steelmaterial 嶺1dcanbemeas 皿 edwitharadiation ‐typethermometerorthelike.
[0074]
(Time from completion of 行nishrollingtoinitiationofwatercooling:Wi 山 in 3.o
seconds)
15 Aaer 山 e completion ofthe 行nish rolling,watercoolingisinitiatedwithin3. ○
seconds in orderto re負ne 山 e met 副lographicsbmctureusings せainaccumulated by 山 e
rolling. This water coolingmaybeperFormedinap.uuralityofdivided stages. vvhen
山 etimefrom 山 ecompletionof 山 e 行nishrollingto 山 einitiationof 山 ewatercooling
(山 etimeelapsingaaer 山 ecompletionofthe 行nishrollinguntil 山 ewatercoolingis
20 initiated) is longerthan3.oseconds,s せaininausteniteisrecovered,which makes it
dimcult to obtain a desiredstmcture. Thetimefrom 山 ecompletionofl 山e 行nishrolling
to the initiation ofthe water coo1ing is preferab1ywithin2. ○ seconds,more prel危rab1y
within l.osecondandstillmorepre 免 rablywi 山 ino.5 seconds. Thetime from 山 e
completionofthe 行nishrollingto 山 einitiationof 山 ewatercoolingispre 免 rably o.05
25 seconds or longer or Q1 seconds or longer in order to recrystaJL1izeaustenitea 負erthe
38
comーpletion ofthe 6nish rolling.
[0075]
(Average cooling rate in temperat 田 eraは1geofFTto7500C:300C/secor 魚 ster)
The con せol ofthe average cooling ratefrom 山 etemperatmeatwhich 山 e 行nish
5 rolling is completed(6nishtemperature:FT(oC))to750oCisimponmlttoobtain a
desired metallographic sbmctuure. Theaveragecoolingrateis ci副Lculated from a
temperaturech 嶺1geofFTto7500CandthetimetakenForthistemperatmechange, alld
山 is time includes 山 e timetakenfromthecompletionofl 山e 行nishrollingto 山 einitiation
ofl山e water cooling. W〆hen 山 e averagecoolingratein 山 eabove ‐described temperature
lo rangeis slower 山 an300C/sec,theFormationofa 行nestmcturebecomes difficult,allda
coi虹seTi‐basedc 嶺 bideisprecipitatedat 山 esametimewith 免rritictramsFormation in 山 e
coolingprocess,whichdegrades 山 es 立engthof 山 esteelsheet. Therefore, 山 eaverage
cooling rate in 山 e above ‐describedtemperaturerangeis setto300C/secorfaster. The
average cooling rateispre 定 rably400C/secorfasterimdmorepre 定 rably 50 0C/sec or
15 faster. The upper limit does not need tobepaulicularlylimited,butl 山eaverage cooling
rate ispre 定rably3000C/secorslower,morepreferably200 0C/secorslower,still more
pre先 rably1500C/secorslowerandfz 紅 stillmorepre 定 rably lloOC/secorslower from
山 e viewpoint of suppressing sheetwarpageat 立ibutedto 山 ermal s立ain
[0076]
20 Rapid cooling in ahigh temperature range aaer 山 eendofthe 行nishrolling in
山 e temperature range ofFTto7500.Cenablesthemet 副10graphic su〔lucture to be 6u【her
re行nedand 山 us6u 【herimproves 山 elowtemperaturetoughness of 山 esteel sheet. ln
order For 山 at,itis pre免 rabletoinitiatethewatercoolingwi 山 ino.3seconds fromthe
completionofthe 行nishrollingalldsettheaveragecoolingrate from FTto FT - 400C to
25 100 0C/secorfasterinadditiontosetting 山 eaveragecoolingrate from 山 eFTto7500C
39
to 30 0.C/sec or faster. ln this case, 山ーera dcoolingdoesnothinder 山ーewater cooling
to be perFormed in a step intended for rapid cooling in atemperature rangeofFTtoFT ‐
400.C and apluu「alityofcoolingstepsForperFormingsubsequentcooling. VVhen 山 e
average cooling ratefromFTtoFT-400Cis slowerthan looOC/sec,itbecomes difficult
5 toobtain 山 eabove ‐describedel 窟ect. Theaveragecoolingrate fromFTto FT - 400C is
pre定 rably1200C/secorfasterandmorepre 定 rably1500C/secorfaster. Regi虹ding 山 e
average cooling rate in 山 istemperaturerange, 山 eupperlimitdoesnot need to be
pzuticularly limited,butl 山eaveragecoolingrateispre 定 rably looooC/secorslower from
山 e viewpoint of suppressing atemperatme unevenness in 山 esteelsheet.
10 [0077]
The raPid cooling in山 e high temperaturerangeaaer 山 eendofthe 行nishrolling
(coolingfromFTtoFT- 400C)maybeperformednotonlya6ter 山 e 行nalstand ofthe
行nish rolling but also between the rollingstands. Thatis,inastand a負:ertherapid
cooling,rollingmaynotbeperFormedorrollingmaybeperFormedwitharolling
15 reduction of8% or lessfor 山 epurposeofshapecorrection,coolingcon せol,or 山 elike.
ln 山 is case, the rolling aGter山 erapidcoolingis notreg 嶺 dedasrollingina 行nishrolling
step.
[0078]
(Dwell time at 750oC to 620oC:20secondsorshoner)
20 Forthe puぱpose ofimprovlnguniForm elongation,6errite,which is a sbhucture
wheretheG! 主人4is 。・5o orless,maybeprecipita 直ed. lnorderForthat,adesired afea
fraction ofG M ≦o.5maybeobtainedbyadjusting 山 edwelltimeof 山 ehot‐rolled steel
sheet aGter山 e 行nish rolling in atemperaturerangeof750 Cto6200.C,where 先 rritic
立ansFormation becomesactive. W〆henthedwelltimein 山 eabove ‐described
25 temperature range exceeds20seconds, 定rriteisexcessivelyprecipitatedorpei 刻rliteor
40
cementite is excessivelyprecipitated,whichdecreases 山 es せength.Therefore, 山 e
dwe11time in the above ‐describedtemperaturerangeisprel たrab1ysetto20seconds or
shod二er. The dwe11 timeisprel 氏〉rab1y17 seconds orshon二er,morepreIR 〉rab1y14 seconds
or shon二er and stillmorepreIR}rably losecondsorshoner. Thelowerlimitmay be set to
5 one second in considerationoftheinst 副1ationcapacity.
[0079]
ln the present embodiment,thedwelltimeat7500Cto6200C re免rs to 山 e time
taken Forthetemperatureofl 山esteelsheet 面ter山 e 行nishrollingto reach 7500C and
山 en decreasetoreach6200.C,but 山 esteelsheetdoesnotnecess そばily need to be cooled
lo inthisrange 山 roughoutthistime.
[0080]
(AVerage cooling rate in temper 紙u止erangeof6200.Ctocoolingstop temperature of
570ocorlower:30Oc/secor 魚 ster)
ln orderto transFormuntransFormed austenite to 丘eshmmtensite or bainite, 山 e
15 steel sheet is cooledto 山 ecoolingstoptemperatuuresetto5700.Corlowersuch that 山 e
average cooling rate in atemperaturerange 丘om6200.Cto 山 ecoolingstop temperature
becomes 30 c/sec or faster. vvhentheaveragecoolingrateintheabove-described
temperature rangeis slower 山 an300C/sec,cementiteorpearliteisFormed during
cooling,whichmakes itdiぼiculttoobtainadesiredmetallographic sbmcture. The
20 average coolingratein 山 eabove ‐describedtemperaturerangeispreIR}rably 40 0.C/sec or
fasterandmorepreIR}rably500.C/secorfaster. Theupperlimitof 山 eaverage cooling
rateintheabove ‐describedtemper 須二urerangeisnotpa 」[ticu1aur1y1imited,but is prelたrab1y
3000C/secorslower,morepre 免rably2000C/secorslower,stillmore pre免rably
150 C/sec or slower and far stillmorepre6erably llo C/secor slower fromthe
25 viewpoint of suppressing sheet warpageattributedto 山 ermals せain.
41
[0081]
[Coiling step]
(Coiling Temperature:570oCorlower)
Aaer 山 e cooling, 山 e steel sheet is coiledat5700Co 【lower. ln a case where
山 e steel sheet has beencoiledathigher 山 an5700C, 山 erearecases where 山 e Ti‐based
cmbide is excessivelyprecipitatedatne 羅 山 ecoilingtemperat 皿 e due to slow cooling
aaer 山 ecoiling,whichdegrades 山 eholeexpansibilityor 山 etoughness.ThereFore, the
coiling temperature is set to 5700Corlower. Thecoilingtemperature is pre免 rably
5600Corlowerandmorepre 免rably5500Corlower. From 山 eviewpoint of
suppressingthe precipitationoftheTi ‐basedcarbideaaer 山 ecoiling, 山 ecoiling
temperature may be 570oC orlower,andthelowerlimitisnotlimited.
5
10
Thecoilingtemperatureandthecoolingstoptemperaturebecome almostthe
same temperatureinmanycases.
[0082]
15 [Heattreatment step]
(Mm 【imum attainment temperature Tma ×:5500Cto7200C)
ln the heattreatment step,freshmaltensiteandbainiteFormedinthehotrolling
step and the coiling step are temperedbyaheat 立eatment,a lld山 eTi‐basedcmbide is
precipit砥edwhileadjusting 山 edisloc 砥iondensity, 山 ere obtainingasteel sheet being
20 exception 副 intermsofs 立ength,uniformelongationandtoughness.When 山 e
maximum attainment temperat 田 eTmaxoftheheat 立eatmentstepis lowerl 山an5500C,
山 ere is a concern 山 at 山l e areafractionofGAM 〉I.7maybecomeexcessive due to a
delayed decreaseinelastics せaininthecrystalgrains. ThereFore,Tmax is set to 5500C
orhigher. Tmax ispre 免 rably5700Corhigherandmorepre 免 rably6000Corhigher.
25 0n 山 eo 山 er hand, whenTmaxishigher 山 an7200C,theTi ‐basedcarbidebecomes
42
coarse, which makes itdimculttoobtain 山 es 口ength.lnaddition, 山 ef ormation of
pearlite or coarse cementitedegrades 山 eholeexpansibilityortoughness ofthe steel
sheet. ThereFore,Tmaxis setto7200.Collower. Tmaxispre6erably7000.C ol lower
[0083]
5 (rllempering pafalneter ps:14000to18000)
ln the heattremment step,elasticstr額nanddislocationsinthecrystal grains
decreases,andtheTi-basedc 組rbideisprecipitated,wherebyastee1sheet having an
exception;副 b副allcebetweenstrengl 山 allduniEormelongationcanbeobtained. W〆hen
山 e temperingpi 虹ameterPs isless 中 anL14000,thedecreaseinelastic sせain alld
lo dislocations becomesinsu 任icient,a lld山 ee 爺ectofimproving 山 ebalance between
sせen帥 anduniformelongationcannotbeobtained.Therefore, 山 e Ps is setto 14000 or
more. on 山 eol山erhzmd,when 山 ePs ismore 山 an18000, 山 eTi‐based c嶺 bide
becomes coarse, and 山 es 口en 蝉 ldecreasesorpearliteorcoarsecementiteisformed,
which degrades the hole expansibility orthe toughness.ThereFore, 山 ePsis setto
15 1800o or less
[0084]
ln acase where 山 eheat 立eatmenttemperat 田 e is constamt, 山 eps czm be
obtainedfromPs=(T 十 273) × (20 十 log(t)). ln 山 eformula,Trepresents 山 e heat
立eatmenttemper 須山re(oC), 皿 dtrepresentsl 山ehe 孤 立eatmenttime(hours). However,
20 in acase where 山 e heat 立eatmenttemperatureisnotconstant,thatis,inacasewhere 山 e
temperature chzmges continuously as in continuous;mnealing,itispossibletocalculate
山 e temperingpi 虹ameter as imintegratedtemperingpi 虹ameterbyamethodinwhich the
heat 立eatmentstepistakenintoaccountas describedinNonPatent Document l. ln 山 e
present embodiment, 山 e integratedtemperingparametercalculatedbasedon 山 emethod
25 described in Non Patent Document l isusedas 山 etemperingpi 虹鑓neterPs.
43
[0085]
The tempering pi虹鑓neterPs is speci行c副lyobtainedby 山 eFollowingmethod.
The time taken from 山 e initiationofheatingto 山 eendofl 山eheatingisdivided
into atot 副 numberNofin 行nitelysmallunits oftime △t. Here,theaveragetemperature
5 inan (n-1)伍 sectionisrepresentedbyT 1 C),and 山 eaverage temperature in an n値
sectionisrepresentedbyTn(oc). Thetemperingp; 虹ameterP(1)corresponding to the
initialin行nitelysmallunitoftime(asectionin 山 ecaseofn=1)czmbeobtained from
山 e Following Formula. “log“indicatesacommonlogam 山 mwithbase lo.
P(1)=(TI 十273)x(20 十log(△t))
10 [0086]
P(1)canbeexpressedasavalueequivalenttopthat is ci副Lculated based on a
temperature T2 ;mdaheatingtimet2fromtheFollowingFormula.
(T,十273)x(20 十log(△t))=(T2 十273)x(20 十log(t2))
[0087]
15 T h e tim e t2 is a time taken(equivalenttime)toobtainpequivalent to the
integr…=;ed value ofP 山 at is calculatedbasedonheatinginthesectionbeFore 山 esecond
section (山 atis,山 e 行rstsection)at 山 etemperatureT2. Thehemingtimein 山 esecond
section (temperatureT2)isatimeobtainedbyaddingtheactualheating time △t to the
timet2. Therefore,anintegratedvalueP(2)ofpatapointintimewherethe heating in
20 山 e second section has been completed canbeobtained 丘om 山 eFollowingFormula.
P(2)=(T2 十273)x(20 十log(t2+△t))
[0088]
When 山 is恥 rmula is generalized,thefollowingformula(4)isobtained.
P(n)=(Tn 十273)x(20 十 log(tn十△t)) (4)
[0089]
The time tn is an equiv 副Lenttimefiorob 口紅ningPequivalenttotheintegrated
value ofP at aPoint in time where the heating in the(n-1) 伍 sectionhasbeen completed at
atemperatureTn. Thetimetncanbec 副culatedfrom 山 eformula(5).
5 10g(tn)=((T ・十273)/(Tn 十273))x(20 十log(tn-・))‐20 (5)
[0090]
The temperingpalrameterP(N), whichcanbeobtmnedbytheabovedescribed method,ismeintegratedvalueofPatapointintimewhere heating in the N 値
sectionhasbeencompleted,andthisisps.
10 [0091]
ln the present embodiment,thesteelsheetmaybemadeintoaplatedsteel sheet
byperFormingplatingin 山 eheattreatmentstep. Evenin 山 ecaseofperformingplating
a賃er山 e heat 立eatment, the platingmaybeperFormedwithin 山 escopeoftheabovedescribed heat 立eatment conditionsbyreg; 虹dingtheheattreatmentstepa11dtheplating
15 step as a continuous step. The platingmaybeanyofelectroplatingorhot ‐dipplating.
The plating type is also not p組ticulをUrlylimitedandis ordin測rilyzinc‐based p夏江ing
including zinc plating and zincalloyp 廟 [ing. Ase ×amples of 山ーepl 砥ed steel sheet, an
electrolyticzinc-platedsteelsheet,anelec せolyticzinc-nickel 副loy‐platesteel sheet, a
hot‐dip galvanizedsteelsheet,agalvannei 副Led steelsheet,ahot ‐dipzinc ‐aluminumalloy ‐
20 plated steel sheet,and 山 elike 組ree×emplaurye ×amples. Theplatingadhesionamount
may be an ordinaryamount. BeFore 山 eplating,Nior 山 elikemay be applied to 山 e
surfaceaspre-plating.
Atthe time ofmanufacturingthe steel sheet accordingto 山 e present
embodiment,well ‐blowntemperrollingmaybeperFormed as appropriate For 山 e
25 pーmposeofshapecorrection.
45
[0092]
The sheetthickness of 山 e steelsheetaccordingto 山 epresent embodiment is not
ばticularlylimited,butisPrel たrably6.ommorless since,inacasewhere the sheet
山 icknessistoo 山 ick,stmctures 山 mareFormedin 山 esurfacelayerand 山 einside of 山 e
5 steel sheet becomesigni 行cantlydifFerent. on 山 eo 山 erhand,when 山 esheet 山 ickness
is too 山 in, since 山 readingduring 山 ehotrollingbecomesdimcult,ordinarily,the sheet
山 ickness is pre免rably l.ommormore. Thesheetthickness ismorepre 免rably l.2 mm
or rnore.
[Examples]
10 [0093]
Next,thee 畳当ects ofoneaspectofthepresentinVentionwillbemore specil丘cally
describedusingexamples,butconditionsin 山 eexamplesaresimply examples of 山 e
conditionsadoptedtocon 行rm 山 e た asibilityande1 日当ectofthepresentinvention,and the
present invention is not limited to these examples of 山 econditions.The present
15 invention is capable ofadopting a variety ofconditionswi 山 in山 escopeof 山 egistofthe
presentinvention as longastheo 覇 ectiveof 山 epresentinventionisachieved
[0094]
Steels having a chemical composition shown in Table l were melted,cast and
山 en made into 40 mm-"lick steelpieces byhotForging. Theobtainedsteelpieces were
20 heated, hotrolled byperformingapluralityoftimes(twotofompasses)ofrolling in a
small tandem milI Fortesting having aplurality ofrollingstandsinbo 山 ofatemperature
rangeofhigher 山 anFT 十 500.CandFT 十 1500.Corlowerandatemperaturerange ofFT
to FT 十 50O.C, thereby obti額ninghot ‐rolledsteelsheetshavingasheet せlickness of2.5 to
3.5 mm. These hot‐rolledsteelsheetswerecooledwi 山 water,coiledand 山 enheat25 せeated, 山 ereby obtaining steelsheets(hot-rolledsteelsheets).Each manu 魚 cturing
46
condition is shown in Table 2‐l andTable2 ‐2.
ln the heattreatment, 山 etimeintervalat 山 etimeofc 副culatingthetempering
parameter Ps was set toonesecond.
ln addition, plating was perFormedonsomeof 山 esteelsheets.
47
の寸
(Uo)
“ト
ゆの
トト
寸の
ゆゆ
のめ
0ゆ
ー;
のNOH
ONOー
トト
N
0ゆ
トゆ
トト
始め
ート
寸
ゆ
の0H
ゆ
(
ご
U
く
0のめ
Nのめ
ート
のめ
トト
Nのめ
トゆ
寸卜め
ヨト
めート
のめ
Nト
めート
mトゆ
寸のめ
ゆゆ
ー
のめト
寸寸ト
寸め
(U
ト幽の
Oめー
N寸的H
NOめー
Oトめー
Oーめー
象Nー
寸めNH
ーx)めー
Nー
ONー
ゆのNH
象Nー
ゆNH
ONー
Oーめー
Oーめー
トーめー
トNー
寸的H
トNー
のo豆旨五日ー℃日wo山uo℃□遍日リーバ求のめ三日)go三瀦oA日oo宝gローoAU
一円
NーOOバ)
】冨山〆
ーOOバ)
叩夏目
NーOOバ)
繍ひ
のHOO十0
酋
NーOO十O
・U
ゆmd
ーZ
Oめバ)
さU
「【)
o『と【
O「O
>
の寸N十○
PZ
ーめOバ)
の
OめOO十O
ーめOO十O
NめOO十O
OめOO十O
めOO十O
NNOO十O
ゆmoo」○
ゆmoo」○
のHOO十0
寸寸00十○
OめOO十O
のめ○0十0
NOO十O
めOO」()
96Od
めめOO十O
NOO十O
ーめOO十O
めOO」()
ーめOO十O
山
OO」()
ーO十O
ーーO十O
めーO十O
の00十0
OO」()
ョod
ョod
OーO十O
ーO十O
寸Ho十○
OO」()
NーO十O
OO」()
のHO十0
ーーO十O
ーO十O
ーO十O
OーO十O
ーO十O
Z
のめ00バ)
トーOOバ)
0寸00バ)
NOOバ)
ONOOバ)
めOOバ)
ーめOOバ)
ーめOOバ)
めNOOバ)
ーめOOバ)
NOOバ)
OめOOバ)
ゆmoo一ハ)
めOOバ)
N寸00バ)
OめOOバ)
NめOOバ)
OめOOバ)
ーNOOバ)
ーめOOバ)
、
一oの
ーO十O
OH寸十○
Oー十O
0寸的十○
ONめ十O
OH寸十○
00寸十○
00寸十○
0のめ十0
ONめ十O
ON」()
0ゆ的」○
OOめ十O
0のN十0
00寸十○
OH寸十○
0寸0十○
Oめ」()
0のめ十0
god
ート
ゆN一ハ)
めNバ)
始めN一ハ)
トめNバ)
のめN一ハ)
のNNバ)
ONバ)
Nめバ)
OトNバ)
めNバ)
トーNバ)
のNNバ)
のめNバ)
めNバ)
BNd
寸寸Nバ)
めーNバ)
ゆ的N一ハ)
N寸十○
ゆ的N一ハ)
口や′A
寸十N"
Oべu
Nーべu
ーOべu
ー.ー
寸十○
めNべu
Nべu
Nべu
寸N十N"
Nべu
寸0べu
のHべu
めべ
めべ
トめべ
のト十雨
トぬH
トの,H
H寸十N"
門の
O.ー
ゆod
十O
のd
N「O
Oー.ー
めN十O
Nめ十O
寸○,H
Nd
ーO.ー
ゆd
めd
ゆd
d
ゆd
の「H
ゆ0,H
d
N寸,H
U
ト。一十。
NゆHd
O」()
めN十O
ト。一十。
ーOー十O
寸の0十○
Nの0十0
寸0十○
OO」()
ーOー十O
ーOー十O
めOー十O
OO」()
Oーd
トーー十O
寸「0一
Oー十O
Oーd
Oー十O
ー忍○一の
く
酋
U
口
国
山
○
国
日
ー
図
一
一〆
Z
○
山
〇
“
の
ト
十日oロロリ〉【Hー【一日○の○要一○【ウ十一ooAooの○昌一oせ門の一戸oo一幅の○ロー冠〉嶋ー口署Y日oAの○十日oo-三口一○一槽wo堰Y【Hー[の○□ロー○0日-- 【OO】
[0096]
[Table 2‐1]
Steel sheet No. Steel
Heat"〕g Hot ro口ling
Heat"〕g temP ぼ atme
(oC)
Tot副 ro]血ng red川瀬 ollin
temP ぼ at皿 eraロー8e of
垣 gher 値狐 Fr 十 500C alld
Fr 十 1500Corlower
(%)
rlotalroDbing redIM泊 onin
temPeratmera ロー8e ofFTto
F1 +50OC
(%)
Ro]血ng 山血ein
temPeratureramgeofl 打rto
FT+ 5ooC
(sec)
Ayerage cool節 g ratein
temp ぼ at皿 eraα1ge ofl打rto
F1 十100OC
(OC/sec)
Fil立国ゴー1 te]mーPけ attlureFT
(oC)
I A 1350 60 79 3.2 15.I 997
2 A 1351 62 66 2.2 17.8 1003
3 A 1349 58 48 2.3 16.4 1002
4 A 1246 61 73 2.O 15.8 997
5 A 1352 墾 65 1.9 17.2 996
6 A 1354 54 堕 3.2 15.I 998
7 A 1351 63 72 魁 16.5 996
8 A 1352 65 67 3.5 艶 1001
9 A 1351 62 66 2.2 41.2 1015
10 A 1354 82 65 1.7 16.3 852
11 A 1349 62 45 2.3 16.5 1004
12 A 1350 64 45 2.6 15.8 1000
13 B 1353 81 66 1.9 16.9 996
14 C 1346 82 66 2.2 15.O 1001
15 皇 1384 71 70 2.O 15.I 983
16 E 1351 69 66 2.2 15.I 1006
17 E 1354 66 67 2.2 15.7 1002
18 E 1354 61 71 2.I 16.2 1007
19 E 1352 78 66 1.9 17.I 1001
20 E 1352 54 76 3.9 17.6 1006
21 E 1353 70 73 3.4 15.4 1007
22 E 1351 69 68 3.I 15.4 975
23 G 1306 78 66 1.9 17.I 1001
24 H 1347 66 66 2.2 16.8 1040
25 I 1343 69 66 1.8 17.8 1035
26 J 1345 66 65 1.7 16.7 994
27 K 1330 69 67 1.9 17.7 967
49
5
0
2
8 L
1330
7
9
66 1
“
^ノー 15.I
97
2
妙 M
1351
7
8
6
0
1.9 18.O
98
2
へ O
J
N
1354
6
7
66 1
“
^ノー 16.7 995
31 ○
1353
7
0
65 3.I 17.4 993
へ 2
J
P
1352 68 金 4
^ノー
" 3 卿
33
Q
1340 65 科 緬 崩 卿
科 R
1350 69 的 1
“
^ノー
" 4
98
4
35
S
ー
1382 68
7
0 郎 崩 靴
.
36
T
1345 科 総 4
^ノー
^
= 箆v
98
5
Under="]es indicate thatconespondbbog values afeout 縦de 値 escope of 値 e世 esent 血 ven 恒on.
[0097]
[Table 2‐2]
Steel sheet No.
HotroDbing Co=bing Heattreatment
T節PeelaPsing aller
fild曲 ro]血ng unl皿
Watercoo 五ng
i血 dadon
(sec)
Ay ぼ age coo 五ng
rateintemp ぼ at皿 e
ra口・geofFTtol 打r‐
40oC
(OC/sec)
Ay ぼ age coo 五ng
rateintemp ぼat皿 e
ra口1geofFTto
750oC
(OC/sec)
Dwen 山血eat
750oCto6200C
(sec)
Ayerage coolmg
rate賞om6200Cto
coolmgstep
temP ぼatme of
570oC orlower
(OC/sec)
Co幻丘11gtenIPぼ attue
(oC)
M血皿
attainuuent
temPerat 皿 eTmax
(oC)
rFemP ぼmg
PalmmeterPs
I 1.6 85 56 3 62 542 659 16228
2 0.I 165 56 2 61 506 611 15787
3 0.4 154 32 8 66 517 658 16294
4 0.3 106 96 2 62 533 640 16399
5 0.5 61 52 3 75 491 622 15992
6 0.4 69 41 4 69 466 634 16698
7 0.5 59 48 3 61 477 649 16873
8 1.I 37 40 5 62 515 628 166(“
9 0.I 152 60 3 71 523 625 15865
10 0.4 54 38 2 85 530 633 16674
11 0.I 167 81 3 64 35 685 17957
12 0.2 158 80 3 75 35 485 17056
13 0.8 65 55 5 60 532 642 17265
14 0.4 77 53 2 74 480 639 15839
15 0.I 152 82 3 53 481 630 15886
16 0.I 195 57 4 70 521 630 16609
17 0.I 201 堕 4 69 490 639 166414
18 0.I 178 83 翌 70 510 675 16600
19 0.I 178 61 3 66 525 684 18958
20 0.I 154 49 3 66 472 415 12154
21 0.I 156 55 3 62 487 765 17856
22 0.I 178 56 3 68 497 639 16764
23 0.I 178 61 I 66 517 617 16372
24 0.I 172 48 I 78 508 656 16575
25 0.I 179 47 3 75 528 673 16645
26 0.I 195 61 I 60 538 636 15965
51
27 0.I 165 45 2 71 530 637 16672
28 0.2 154 54 I 93 488 634 15803
29 0.I 187 50 I 76 524 656 16963
30 0.2 162 74 3 77 480 674 16887
31 0.3 168 50 3 76 542 648 16658
32 0.2 155 49 3 81 465 617 15978
33 0.I 172 80 3 65 511 578 15310
34 0.I 156 48 2 92 498 676 16779
35 0.I 174 46 2 84 465 673 17066
36 0.I 155 60 3 71 512 625 15821
Under="]es indicate thatcorrespondbbog values afe outside 値 escope of 値 e世 esent 血 ven 恒on.
52
[0098]
Regz 虹ding 中 e obtained steel sheets, 山 eareafractions ofGAMo.5 ‐I.7,GAM 〉I.7
and GAM 〈o.5,山 em eafractionofresidualaustenite(retained γ),thetotal 嶺 eaf raction of
residualaustenite,f reshmartensite,cementiteandpe そ虹lite,山 eaveragegrain size, 山 e
5 average dislocation densityof 山 emetallographicsumctureatthedep 山 position of l/4 of
山 e sheet せlickness 丘om 山 esurfaceofthesteelsheetand 山 epoledensityofeachcrystal
orientation in山 e 山 ickness middleponionwereobtainedby 山 eabove-described
me 山 ods. ln the measurementof 山 epoledensityof 山 ecryst 副 orientation in 山 e
山 icknessmiddleponion,inFormationofapproximately30oobcccrystal orientations was
lo measured.
[0099]
ln orderto evalua直ethemechanicalpropeniesoftheobtainedsteel sheets, the
tensile strengthTS(Mpa)and 山 euniformelongationuE1(%)wereev 副uatedwi 山 aNo.
5testpiece according to JIS Z 2241: 2011
15 The hole expansibility was evaluated bythe limitingholeexpansionratio 九 (%)
山 at was measmed according to JIS Z 2256:2010.
The low temperatme toughness was ev副uatedby 山 ef racture appez紅ance
立ansitiontemperat 山rev 画rs(OC mdevi 副uatedbyperft)rmingaChmrpyimpact testusing
aV ‐notch testpiece obtmnedbyworking 山 esteelsheetintoa2.5mmsub-size test piece
20 accordingtoJISZ2242:2005.
[0100]
Table 3‐l and Table 3-2 showthetestresultsof 山 emetallograPhicsb[uctures,
山 e textures and the mechanic 副 propenies. ln 山 e‘plating'columnofTable 3-2, “GI''
indic砥es ahot-dipgalvanizedlayerand"GA “indicatesahot-dipg 副vzu1ne副edlayer.
25 1n addition,regaidingtheexpressionoftheaveragedislocationdensityofTable 3‐1,f ior
53
exalnp1e, 8.3E 十 14 indicates 8.3 × 1014.
[0101]
lnthe case of980MPaormore, 山 etensilestrengl 山 wasjudged asahigh
s口ength,andwhenv 野 s C)was ‐400Co ・lower, 山 esteelsheetwasjudged to be
5 excePtion;副 in ternIS 0flowtemPeraturetoughness. Theworkabilitywas evaluated by
山 e bal縄lcebetweenstrengl 山 anduniformelongation(Tsxl 正 1)and 山 ebalaulcebetween
sせen帥 andholeexpansibility(Tsx 九).lnacasewhereTSxuE1(Mpa ・%)was 6000
Mpa ・% ormore, 伍 e strengthwasjudgedtobehigh,and 位 euni あ rmelongationwas
judgedas"exceptional. “ lnacasewhereTsx 九 (Mpa%)was50000Mpa%ormore,
10 山 es 立ength wasjudgedto behigh,and 山 eholeexpansibilitywasjudged as
"exceptional. “
54
[0102]
[Table
3‐1]
Steel sheet No. Steel
Area
賞aJc樋onof
GAMo5
‐17
(%)
Area
賞aJc樋onof
GAM
〉17
(%)
Afea
賞action of
GAM
ゆ
5
(%)
Afea
賞action of
retained
γ
(%)
rlotal
afea
賞
総
樋on
ofreta
節 ed
仏
Peal
山 e,
賞esh
maften
縦te;
虹Id
cemen
記te
(%)
AyeragegrmmSIZe (凹 )
Ay
ぼ agedislocadon
den
縦ty
(/m2)
I
A 95
2
3
O
I 7.5 8.3E
十 14
2
A 88
5
7
O
I 6.4 1.IE
十 15
3
A 66
4 30
O
O 8.6 5.9E
十 14
4
A 92
6
2
O
I 7.O 8.8E
十 14
5
A 84
3 13
O
I 15.4 8.6E
十 14
6
A 71
5 24
O
I 7.9 7.6E
十 14
7
A 87
3 10
O
O 7.2 7.2E
十 14
8
A 61
4 35
O
O 8.4 7.3E
十 14
9
A 82
3 15
O
2 6.5 1.4E
十 15
10
A 63
2 35
O
O 3.7 6.7E
十 14
11
A 95
5
O
O
2 7.9 3.6E
十 15
12
A 73 26
O
I
I 7.O 3.3E
十 15
13
B 74 10 15
I
5 5.4 2.2E
十 15
14
C 86
6
8
O
2 5.O 7.4E
十 14
15 D. 77 13 10
O
旦 5.O 2.7E
十 15
16
E 79
5 16
O
I 5.2 1.OE
十 15
17
E 42
2 56
O
O 5.4 3.OE
十 14
18
E 10
3 87
O
2 7.O 7.8E
十 13
19
E 85
4 11
O
O 4.I 9.OE
十 14
20
E 56
墾 20
O
O 7.4 5.3E
十 15
21
E 78
6 16
O
2 5.3 1.2E
十 14
22
E 50
5 45
O
O 5.I 4.6E
十 14
23
G 93
3
4
O
O 4.O 9.OE
十 14
24
H 89
5
6
O
O 6.2 6.7E
十 14
25
I 87
O 13
O
O 5.3 4.6E
十 14
26
J 92
3
5
O
O 5.3 5.5E
十 14
27
K 87
I 12
O
O 5.O 8.3E
十 14
28
L 87
5
8
O
I 4.I 4.7E
十 14
55
29
M 92
I
7
O
O 4.2 5.9E
十 14
30
N 87
3 10
O
I 5.8 8.7E
十 14
31
○ 83
2 15
O
O 5.6 9.6E
十 14
32
P 84
4 12
O
2 5.9 9.OE
十 14
33
Q 79 15
I
≦
8 6.O 8.6E
十 14
34
R 84
3 13
O
O 5.2 7.6E
十 14
35
呈 85
I 14
O
O 4.9 9.7E
十 14
36
T 85
5 10
O
O 6.7 1.IE
十 15
Under="]es
indicate thatconespondbbog values
afe outside
値 escope of
値
e
世 esent
血 ven
恒on.
[0103]
[Table
3‐2]
Steelsheet
No.
Pole densitytot
…uof
{21
・km.
〉amd
{332}
〈113〉
TS
(MPa)
妃
I
(%)
Ts
x
ロEI
1pa・%)
入
(%)
TSX
入
⑪IPa・%)
vTrs
(oC) P1atiPg Remmk
I 6.1 1246 8.O 9959 62 77307
‐51 GI
〔= lvendonExmmp1e
2 3.2 1255 7.7 9608 80 100055
‐57 GI
〔= lvendonExmmp1e
3 3.8 1238 8.4 10382 76 94kキー
5
‐47 None
〔= lvendonExmmp1e
4 5.8 845 6.2 5239 64 54133
‐54 GI CompafadveExanople
5 11.5 1247 8.O 9920 33 41143
‐
9 GA CompafadveExanople
6 12.8 12糾 8.I 10062 20 24877
‐49 GA CompafadveExanople
7 12.3 1243 8.2 10130 23 28735
‐56 GA CompafadveExanople
8 3.4 968 8.I 7881 78 75980
‐48 GA CompafadveExanople
9 12.6 1258 6.2 9608 20 25160
‐55 GI CompafadveExanople
10 13.6 1241 8.2 10233 15 18612
‐76 GI CompafadveExanople
11 3.5 1325 6.3 8404 78 103550
‐46 GI
〔= lvendonExmmp1e
12 3.6 1368 4.2 5746 65 88920
‐50 GI CompafadveExanople
13 5.9 1161 6.9 7993 50 58030
‐42 GA
〔= lvendonExmmp1e
14 5.O 1188 8.I 9662 69 81373
‐63 GA
〔= lvendonExmmp1e
15 5.8 1206 6.6 8001 64 76847
‐25 GI CompafadveExanople
16 4.5 1179 7.8 9156 72 84987
‐65 GA
〔= lvendonExmmp1e
17 4.4 965 9.I 8823 73 70132
‐66 GA CompafadveExanople
18 3.4
盟 10.7 9892 79 72842
‐51 GA CompafadveExanople
19 4.7 970 7.9 7674 55 53350
‐28 GA CompafadveExanople
20 6.3 1282 4.2 5383 61 78
αア
8
‐55 None CompafadveExanople
21 6.O 924 6.6 6098 42 38808
‐26 GA CompafadveExanople
56
22 6.4 遊 8.7 8463 60 58800 ‐68 GA Compafa 樋veExamople
23 4.7 1161 7.9 9180 70 81609 ‐74 GA 〔= lvendonExmmp1e
24 4.5 1156 8.2 9520 72 83152 ‐61 GA 〔= lvendonExmmp1e
25 4.5 1009 8.7 8743 72 72587 ‐67 GA 〔= lvendonExmmp1e
26 5.O 1309 8.5 1"!ア8 69 90326 ‐67 GA 〔= lvendonExmmp1e
27 6.6 1257 8.O 10()L3 59 73512 ‐69 GA 〔= lvendonExmmp1e
28 6.6 1363 8.6 11774 59 80170 ‐71 GA 〔= lvendonExmmp1e
29 5.9 1120 8.4 9394 63 70960 ‐73 GI 〔= lvendonExmmp1e
30 6.3 1151 7.9 9136 61 69895 ‐61 GA 〔= lvendonExmmp1e
31 5.8 1174 7.8 9193 64 74630 ‐65 GI 〔= lvendonExmmp1e
32 5.9 1211 7.9 9571 63 76580 ‐57 GI 〔= lvendonExmmp1e
33 6.1 1624 8.O 12916 10 16242 ‐15 GA Compafa 樋veExamople
34 6.4 1228 8.I 9941 60 73798 ‐68 GA 〔= lvendonExmmp1e
35 12.3 1176 4.5 5292 23 27195 ‐69 GA Compafa 樋veExamople
36 5.2 1220 7.6 9272 65 79300 ‐55 GI 〔= lvendonExmmp1e
Under="]es indicate thatconespondbbog values afe outside 値 escope of 値 e世 esent 血 ven 恒on.
57
[01(M]
Accordingto Table 3‐l andTable3-2,itisFoundthat 山ーesteel sheets Nos.l to
3,11, 13,14,16,23to32,34and36,w hichwere 山ーeinventione ×alnP1es,hadahigh
sせen帥 andwereexceptionalintermsoflowtemperaturetoughness,uniform elongation
5 andholeexpansibility. on 山 eotherhand,itisfound 山須 山ーesteelsheetsNos.4to lo,
12, 15, 17 to 22, 33 and 35, which were thecompmrativeexamples,werepoorinterms of
any one ormoreof 山 epropeinies.
[lndusせi副 Applicability]
[0105]
1O Accordingto the above ‐describedaspectaccordingtothepresentinvention, itis
possibletoprovide a steel sheethavingahighs 口engdlandbeingexceptional in terms of
elongation,holeexpansibilityandlowtemperatmetoughnessandama11ufacturing
melhodthereof. W'henthesteelsheetaccordingtothepresent invention is used as a
material ofa componentForaninnerplatemember,astmcturalmember,asuspension
15 member, orthe likeofacar,itiseasytowork 山 esteelsheetintoacomponent shape, and
山 e steel sheetis capableofwi 山 standing 山 euseinanex せemelycoldclimate,and thus
industrial contribution is extremely signi6cant.

[CLAIMS]
W'hatisclaimed is
l. Asteel sheethaving achemical comPosition comPrlslng,bymass%
C:0. α10%too.180% 。;
5 Si:0.005%to2.00%;
Mm:1.00%to3.00%;
Ti:more than o.200% ando.400%orless;
sol.AI:0.001%to l.000%;
N:0.0010%too.0100%;
10 P:0.100%or less;
S:0.0100 o or less;
ト化 :0%too.100%;
V:0%too.500%;
N[o:0%too.500%;
15 Cu:0%tol.00 。;
Ni:0%to l.00%;
Cr:0%to2.00%;
B:0%too.0030%;
Ca:0%too.0100%;
20 M[g:0%too.0100%;
RE 1:0%too,0100%;mld
Bi:0%too.0200%,
wim aremainder ofFe and impl虹ities,
wherein, at a dePI山 Positionofl/4ofasheetl 山icknessfromasmface,
25 an area fraction ofGAMo.5-,.7 山 atis crystalgrainshavingaGAMofmorethan
59
0.50 and l.7o or lessis50%or]moreand loo%orless,theGAMrepresentinganaverage
ofcryst 副 orientation difFerences in one crystal grain that are obtainedbyEBSDanalysis,
皿 area fraction ofGAM 〉I.7that is crystal grainshavingtheGAMof]more 山 an
l.7o is o%ormoreand20%orless,
5 an area fraction ofGAM ≦o.5山 at iscrystalgrainshaving 山 eGAMofo.50 or less
is o% ormoreandless 山 an50%,
皿 area fraction ofresidual austenite is o%ormoreandless 山 an4%,
atotal 嶺 ea fraction ofl山eresidualaustenite,f reshmaltensite,cementite and
pearliteis o%ormoreand lo%orless,
10 an average grain size is 15.0 一morless,
an average dislocation density is l,ox lo14/m2ormoreand4,ox .o15/m2or
less,
atotal ofpole densitiesof{211}<011>and{332}<113>ina 山icbless middle
po 賞ionis12.o or less, aーl d
15 atensiles 立eni夢his980MPa or more
2. The steel sheet accordingtoclaim l,
wherein 山 e chemical composition contains,bymass%,oneormore selected
from 山 e groupconsistingof:
20 ト化 :0.001%too.100%;
V:0.005%too.500%;
N[o:0.001%too.500%;
Cu:0.02%to l.00%;
Ni:0.02%to l.00%;
25 Cr:0.02%to2.00%;
60
B:0.0001%too.0030%;
Ca:0.0002%too.0100%;
M[g:0.0002%too.0100%;
RE 1:0,0002%too,0100%;mld
5 Bi:0.0001%too.0200%
3. The steel sheet according to claim l o1 2,
w herein aplating layer is Formed on a surface.
10 4. The steel sheet according to claim 3,
w herein 山 eP1atinglayer is ahot-diP galvanized layer
5. Thesteelsheetaccordingtoclaim3,
wherein the plating layer is ahot‐dip ga1 vanneaJedlayer
15
6. A steel sheet manufact 皿 ing melhod thatisamallufacturingmelhodofl 山esteel sheet
accordingtoclaim l,comprising
aheating step ofhei印tingaslaborsteeIPiecehaving 山 echemicalcomposition
accordingtoclaim l;
20 ahot rolling step ofperForming multi‐passhotrollingon 山 eslab or 山 e steel
piece aaer 山 e heating stepusingaplu ぱalityofrollingstandstoobtainahot-rolled steel
sheet;
acoiling step ofcoilingthehot ‐rolledsteel sheet; and
aheattreatment step ofperFormingaheat 立eatmenton 山 ehot‐rolled steel sheet
25 aaer 山 ecoilingstep,
61
w herein, in 山ーe heatingsteP,
aheating te1mーper須田 e is set to 12800Corhigher;mdatemperatureSRT(OC)
represented bythe following formula(1)orhigher,
in山 e hot rolling step,
5 when a 6nish temperature is represented by FT in aunit ofoC,atotalrolling
reduction in atemperature rangeofhigherl 山anFT 十 500CandFT+1500Corlower is
setto50%ormore,
atotalrollingreduction in atemperature range ofFTto FT+50oCis setto40%
to 80%, atime necesszuy βorrollingin 山 etemperatmrerange ofFTto FT+500Cis setto
lo o.5to lo.oseconds,
two or more passes ofro11ing m eperFormedineachofthetemperaturerange of
higherl山an FT 十 500C andFT 十 1500Corlowerand 山 etemperaturerangeofFTtoFT+
50oC,
an average cooling ri=;einatemperaturera ぱ,geofFTtoFT 十 100OC is set to
15 6.o oC/sec or fasterand40.ooC/secorslower,
FT is set to equal to orhigherthanAJ3 that is obtained 丘omtheFollowing
formula(2), equaltoorhigher 山 anTR(0C)thatisobtainedfrom 山 ef ollowingformula
(3)andlloooCorlower,
water cooling is initiated wi山 in 3.osecondsfromcompletionof 行nishrolling,
20 anaveragecoolingrateinatemperaturera ローge ofFTto 7500C is set to 30 0C/sec
orfaster, 山 eslaborthesteelpieceis dweltinatemperaturerange of7500C to 6200C For
20secondsorshon:erand 山 encooledtoacoolingstop temper 砥uurethat is 6200C to
5700.C Such that an average coolingrateinatemperaturerangeof 山 ecooling stop
temperature reaches300.C/secorfaster,
25 in山 e coiling step,山 e hot‐rolled steel sheetis coiledat5700.Corlower,
62
in中 e heat 立eatmentstep,
amaximumattainmenttemPeratureTmaxis setto5500Corhigher and 7200C
orloWer,
瀧ーd atemperingp; 虹ameter Ps is setto14000to18000,
5 SRT(oC)=1630 十9o x ln([C]x[Ti])… (1)
AJ3 C)=901 ‐325x[C]+33x[Si] ‐92x[M[n]+287x[P]+40x[sol.AJ]
(2)
TR(oC)=800 十70ox[Ti] 十looo x[Nb](3)
here, [element symbol] inthe 貴)rmulae(1),(2) 瀧ld(3)indicatesl 山eamount of
lo each elementbymass%,a ーl dzeroisassignedinacasewherethe element is not
contained.
7. The steel sheet manu 魚cturing methodaccordingtoclaim6,
w herein, in 山 e hot rollingstep,
15 山 e water cooling is initiated within o.3 secondsf rom completion ofl山e 行nish
rolling,and
cooling in which an average cooling rate in atemperature range ofFTto FT ‐
40oC is loo oC/sec or fasterisperFormed.