Cold-rolled multiphase boron steels: microstructure and mechanical properties

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w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Original

Article

Cold-rolled

multiphase

boron

steels:

microstructure

and

mechanical

properties

Fábio

Dian

Murari

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_,

_André

_Luiz

_Vasconcelos

_da

_Costa

_e

_Silva

b

_,

Roberto

Ribeiro

de

Avillez

c,∗

a_Usiminas_Technology_Center,_Ipatinga,_MG,_Brazil

b_Escola_de_Engenharia_Industrial_{Metalúrgica,}_Universidade_Ferderal_Fluminense_(UFF),_Volta_Redonda,_RJ,_Brazil c_Pontifícia_Universidade_Católica_do_Rio_de_Janeiro_(PUC-Rio),_Rio_de_Janeiro,_RJ,_Brazil

a

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t

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Articlehistory:

Received15October2014

Accepted3December2014

Availableonline15January2015

Keywords: Multiphasesteels Boron Thermo-Calc® Dilatometry

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b

s

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The inﬂuence of the boron concentration on phase transformation characteristics,

microstructureandmechanicalpropertiesofmultiphasesteelswasinvestigatedusing

com-putationalthermodynamics(Thermo-Calc®_),_dilatometry,_quantitative_{metallography}_and

tensiletests.Pilotscale50kgsteelingotswerepreparedinaninductionfurnaceoperating

underanargongasatmospherewithboroncontentsbetween0and47ppm.Theingotswere

cutinto35mmthickblocks,whichwerereheatedto1250◦Cfor1handhotrolledforseven

passestoattainathicknessof7.0mm.Thehot-rolledsheetsweremachinedandthencold

rolledtoaﬁnalthicknessof1.2mm.ContinuousannealingcycleswereperformedinaBähr

dilatomerandinaGleeblemachine.Continuousannealinglaboratorysimulationsshowed

thatborondidnotsigniﬁcantlyinﬂuencetheamountofausteniteformedduringheating

andsoakingsteps.However,boroninﬂuencedaustenitetransformationduringthecooling

step,whichreducedtheamountofferriteandincreasedtheamountofbainite.Regarding

themechanicalproperties,addingboronincreasedstrengthanddecreasedductilityofthe

product.Thesteelswithboronconcentrationsupto27ppmexhibitedthegreatesteffect.

Theamountofaustenite,whichwascalculatedusingThermo-Calc®_,_was_slightly

overes-timatedcomparedwiththatobtainedbydilatometryandmetallography,particularlyfor

soakingtemperatureslowerthan800◦C.

EditoraLtda.Allrightsreserved.

1. Introduction

Advanced high-strength steels with multiphase

micro-structures,whichconsistofferrite,bainite, martensiteand

∗ _{Corresponding}_author.

E-mail:[email protected](R.R.deAvillez).

retainedaustenite,arebeingusedincreasinglymoreinmany

automotiveapplications;inparticular,thesesteelsare

replac-ing conventional HSLA steels due to their more desirable

strength–ductilityratio.Thesematerialsarecharacterizedby

aninterestingcombinationofhighstrength,good ductility,

http://dx.doi.org/10.1016/j.jmrt.2014.12.001

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Table1–Chemicalcompositionofthemultiphasesteelsusedinthestudy(inweightpercent). Steel C Mn Si P S Al B Nb+Ti N Ti/N B10 0.10 1.8 0.5 0.02 0.007 0.029 0.0013 >0.028 0.0052 4.4 B30 0.11 1.8 0.5 0.02 0.005 0.025 0.0027 0.0042 4.5 B50 0.09 1.8 0.5 0.02 0.007 0.034 0.0047 0.0051 4.7 Base 0.09 1.8 0.3 0.01 0.005 0.043 – – 0.0043 –

continuousyielding,highinitialworkhardeningrates(n val-ues)and alowyieldstress totensilestrength ratio(YS/TS) [1].

Inthe context ofmultiphasesteels, boronis anunique

alloyingelementinthatasolublecontentof0.0010–0.0030%

can provide a hardenability effect equivalent to adding

approximately0.5%ofother elements,suchasmanganese,

chromium ormolybdenum [2–6]. Thus,various approaches

havebeenattemptedtoproducemultiphasesteelswithboron

additionswiththeobjectiveofreducingcosts.

Itisgenerallyacceptedthatatthefullyaustenitized

con-dition,boronsegregationor theprecipitationofﬁne Fe23(C,

B)6 carbides during cooling at austenite grain boundaries

retardsthetransformationfromaustenitetoferriteby

imped-ingthenucleationofferrite,whichsubsequentlyimprovesthe

hardenabilityofsteel[7,8]. Intheproductionofmultiphase

steels,inhibitingferriteformationisexpectedtoincreasethe

amount of martensite or bainite. However, for multiphase

steels, ferrite grows during cooling from the (␣+␥) phase

1000 900 800 700 600 500 400 300 200 100 0 0 50 100 150 200 1000 900 800 700 600 500 400 300 200 100 0 Time (s) 0 50 100 150 200 250 Time (s) T e mper ature (ºC) T e mper ature (ºC) 760ºC 780ºC 800ºC 820ºC

a

b

Fig.1–(a)ThermalcyclesconductedintheBähr

dilatometertodeterminethevolumefractionofaustenite formedwithheatingsteps;(b)heatingwiththesoaking steps. 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 1.3 1.2 1.1 1.0 0.9 0.8 450 600 650 700 750 800 850 900 500 550 600 650 700 750 800 850 900 950 Temperature (ºC) Temperature (ºC) Δ L/L 0 (%) Δ L/L 0 (%) (α + Fe3C) (α + Fe3C) (α + Fe3C) (α + Fe3C) γ γ (γ) (γ) A A B C B C Ac₁ Ac₁ Ac₃ Ac₃

Fraction transformed = (AB/AC) x 100

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Fig.2–Exampleofapplicationoftheleverruleto

determinethefractionoftransformedaustenitebymeans ofdilatometry.(a)Continuousheating;(b)heatingwiththe soakingstep. 900 800 700 600 500 400 300 200 100 0 0 100 200 300 400 500 600 700 Time (s) T e mper ature (ºC) 780ºC 820ºC 57 s 650ºC 318 s 180ºC 5ºC/s 1.3 - 2.1ºC/s 5.9 - 7.7ºC/s 45 - 52ºC/s

Fig.3–Continuousannealingcyclesperformedinthe Gleeblesimulator.

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550 600 650 700 750 800 850 900 950 Temperature (ºC) 500 550 600 650 700 750 800 850 900 950 1000 Temperature (ºC) 500 550 600 650 700 750 800 850 900 950 1000 Temperature (ºC) 500 550 600 650 700 750 800 850 900 950 1000 Temperature (ºC) 1.0 0.8 0.6 0.4 0.2 0.0 A ustenite 1.0 0.8 0.6 0.4 0.2 0.0 A ustenite 1.0 0.8 0.6 0.4 0.2 0.0 A ustenite 1.0 0.8 0.6 0.4 0.2 0.0 A ustenite Base - TCW Base - DIL B10 - TCW B10 - DIL B30 - TCW B30 - DIL B50 - TCW B50 - DIL Lin. heating TCW 760ºC 780ºC 800ºC 820ºC Lin. heating TCW 760ºC 780ºC 800ºC 820ºC Lin. heating TCW 760ºC 780ºC 800ºC 820ºC

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Fig.4–VolumefractionofausteniteformedduringheatingandsoakingstepsdeterminedbyThermo-Calc®_(TCW)_and

dilatometry(DIL)fordifferentboronadditions.(a)Basemetal;(b)10ppmboron;(c)30ppmboron;(d)50ppmboron.

ﬁeldfromthealreadyexistingferrite.Nucleationofnew

fer-rite grains, which has been observed in several instances,

does not contribute signiﬁcantly to the kinetics of ferrite

formation while cooling the (␣+␥) mixture. Nevertheless,

because the energy of the ␣/␥ interface is comparable to

that of the ␥/␥ grain boundary, it could be expected that

boronredistributesfromprioraustenitegrainboundariesto

␣/␥ interfaces during intercritical annealing.If this occurs,

the boron would then interfere with the growth

mecha-nisms of ferrite into austenite during subsequent cooling

[9].

In the present work, the inﬂuence of boron content

on the phase transformation behavior,microstructure and

mechanical properties of multiphase steels, which were

intercritically annealed, was investigated by dilatometry,

quantitativemetallography,tensiletestsandcomputational

thermodynamics.

2. Experimental

setup

Forthisstudy,four50kgingotswerepreparedinavacuum

inductionfurnacewiththechemicalcompositionshownin

Table1.Theingotswerecutintoblocksthatwere35mmthick

andwerereheatedto1250◦Cfor1handhotrolledinseven

passestoobtain a7.0mmthickness. Thehot-rolledsheets

were machinedtoobtain ﬂat,clean surfacesandthencold

rolledtoaﬁnalthicknessof1.2mm.

Theformationofausteniteduringtheheatingstepinthe

continuousannealingprocesswasstudiedbymeansof

equi-librium computational thermodynamics (Thermo-Calc® _5.0

[10]andthethermodynamicdatabase,TCFE61_)._These_values

werecomparedwiththedilatometryresultsandthe

quantita-tivemetallographyresultsofthesamples,whereopticaland

scanningelectronmicroscopywereusedforthe

characteriza-tion.Forthedilatometry,theleverrulewasused,andcycles

bothwithandwithoutthesoakingstep(Figs.1and2)were

considered.Thecoolingrates obtainedbywaterquenching

werealwaysgreaterthan500◦C/stoensurethatthe

austen-itepresentatthehightemperaturewouldtransformintoonly

martensiteorbainiteandthatnopearlitewouldbepresent,

which meantthatthe volumefractionofausteniteformed

during heating/soakingcould be indirectlydetermined.For

thispurpose,thespecimenswereetchedwithLePera’sagent,

which left martensite and retained austenite (MA) white,

whereastheotherconstituentswerepaintedindifferent

col-ors.Thevolumefractionsoftheconstituentsweremeasured

byimageanalyzersoftware.Thesamplesforthetensiletests

hadagaugelengthof25mmthatwastransversaltotherolling

1 _Thermo-Calc_Software_Database_TCFE6_{Steels/Fe-alloys}

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1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 A ustenite/2nd constituent (%) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 A ustenite/2nd constituent (%) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 A ustenite/2nd constituent (%) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 A ustenite/2nd constituent (%) 10 20 30 40 50 Boron content (ppm) 10 20 30 40 50 Boron content (ppm) 10 20 30 40 50 Boron content (ppm) 10 20 30 40 50 Boron content (ppm) DIL MET TCW

a

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Fig.5–Resultsofthequantitativeanalysisoftheconstituents.Comparisonbetweenthedifferentmeasurementmethods andtheresultsobtainedbyThermo-Calc®_at_two_different_soaking_{temperatures.}_(a)₇₆₀◦_C;_(b)₇₈₀◦_C;_(c)₈₀₀◦_C;_(d)₈₂₀◦_C.

directionandwere machinedfrom thecold-rolled samples

toevaluatethemechanicalproperties.Tobettersimulatethe

actualcycles encounteredintheindustrialline of

continu-ousannealing,Gleebletestswerealsoperformed.Asshown

inFig.3,thecycleswerechosenbasedontypicalindustrial

lineparameters.Metallographicsamplesofthetensiletests

werepreparedfollowingstandardmetallographicprocedures

andexaminedbyopticalandscanningelectronmicroscopes.

To determine the volume fraction of polygonal ferrite (PF)

and the second constituent,a 4% Nitaletchant was used,

and LePera etchant was applied to highlight the MA

con-stituent.Thevolumefractionsofbainiteandtheundissolved

carbides(B+C)weredetermined bythedifference between

the volume fractions of the second constituent and that

ofMA.

3. Experimental

results

Fig. 4a shows the austenite fractions formed during

heat-ing,whichwereobtainedfromtwodifferentmethods:using

Thermo-Calc® _(TCW) _and _{experimentally} _determined _by

dilatometry(DIL).Thedilatometricexperimentsthatincluded

thesoakingstepareshowninFig.4b–dandcomparedwiththe

equilibriumcalculations.As seenfrom Fig.4a, theamount

ofaustenite calculated withthe Thermo-Calc® _for_a _given

intercritical temperature is greater than the experimental

valuedetermined bydilatometry. Thisresultwas expected

becausetheThermo-Calccalculationsconsideredthe

condi-tionofthermodynamicequilibrium.Therefore,itislikelythat

akineticeffectduringheatingslowsthegrowthofaustenite,

andequilibriumdidnotoccur.Regardingtheeffectofboron,

the addition ofthis element didnotsigniﬁcantlyinﬂuence

the amount ofaustenite formedduringheating.Thesmall

variationsthatdidoccuraremostlikelyduetodifferencesin

thelevelsofotherelements,suchascarbon,aluminumand

nitrogen.Furthermore,theamountofaustenitedetermined

bydilatometryat820◦Cwasequaltotheequilibriumvalue

calculated with Thermo-Calc®_. _This _result _shows _that _the

kineticsofaustenitegrowthoccursrapidlyandthus,isableto

reachequilibriumfortypicalindustrialheatingcycles.

There-fore,it ispossibletoinferthatthe stateofthermodynamic

equilibriumwasreachedfortemperaturesatapproximately

850◦Cforthepresentalloysduringtheuniformheatingcycle.

Fig. 4b–dshows theeffectofthe soakingtreatment, which

causestheamountofaustenitetoapproachitsequilibrium

valuesfor57sofsoakingat820◦C.Fig.5showstheamount

of austenite afterbeing water quenched from the soaking

temperaturedeterminedfrombothdilatometryand

quantita-tivemetallography(MET).Thevaluesobtainedbyquantitative

metallography were alwaysgreater than those obtainedby

dilatometry.However,thequantitativemetallographyresults

alsohadlargererrorsduetosurfacesamplingcomparedwith

those of the volumetric dilatometry measurement. As the

soaking temperature increased,the differencebetween the

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Fig.6–Microstructuralaspectofthestudiedsteels.Soakingtemperature:820◦C.Overagingtemperature:300◦C. Metallographicetching:Nital.Analysisperformedat1/4thickness.(a)Basemetal,opticalmicroscopy;(b)10ppmboron, opticalmicroscopy;(c)basemetal,scanningelectronmicroscopy;(d),10ppmboron,scanningelectronmicroscopy;(e) 30ppmboron,opticalmicroscopy;(f),50ppmboron,opticalmicroscopy;(g)30ppmboron,scanningelectronmicroscopy; (h),50ppmboron,scanningelectronmicroscopy.

doesnotrevealanyeffectthatcanbeascribedtotheboron

addition on the austenite growth while being heated or

soaked.

Fig.6showsthemicrostructureofthefoursteelsafter

sim-ulationof the cycles shown inFig. 3. Themore magniﬁed

micrographs(scanningelectronmicroscopy)inthisﬁgure

sug-gestthattheamountofconstituentsdoesnotseemtodepend

ontheboronamount.However,thequantitative

metallogra-phy results presented in Fig. 7 show that the increase in

boron (uptoapproximately 30ppm)initially decreasedthe

amountsoftheferriteandMAconstituents,whichresulted

inanincreaseintheamountofthesecondconstituents,in

particular, bainite.Hence, boronhas astrong effectduring

cooling.

Themechanical propertiesforthe as-annealedmaterial

shown in Fig. 8 follow the expected behavior due to the

microstructure.Theincreaseinboroncontentupto27ppm

causedanincreaseintheyieldstressandtensilestressand

decreasedthetotalelongation,whichisduetotheincreasein

theamountofthesecondconstituent.

30 20 10 0 0 3 6 9 12 15 MA (%) 780˚ C 820˚ C Boron content (ppm) 40 50 0 5 10 15 20 25 30 35 40 45 50 Boron content (ppm) 0 10 20 30 40 50 60 70 B + C (%) Boron content (ppm) 0 10 20 30 40 50 40 45 50 55 60 65 70 75 80 Ferrite (%) 780˚ C 820˚ C 780˚ C 820˚ C

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0 5 10 15 20 25 30 35 40 45 50

Boron content (ppm) Boron content (ppm)

Boron content (ppm) 10 5 20 30 40 50 0 10 15 20 25 30 35 TEL (%) 850 800 750 700 650 600 550 500 0 10 20 30 40 50 TS (MPa) YS (MPa) 500 450 400 350 300 250 780˚ C 820˚ C 780˚ C 820˚ C 780˚ C 820˚ C

Fig.8–Changesinmechanicalpropertieswithdifferentboronconcentrationsatdifferentsoakingtemperatures.

4. Conclusions

The effects of the austenitizing temperature on the ﬁnal

microstructureofindustrialsteelsmodiﬁedbyaddingboron

wereevaluatedbydilatometryand quantitativemicroscopy

andcomparedwiththermodynamicequilibriumcalculations.

Boronhadnomeasurableeffectonthebehaviorduringthe

austenitizationcycle. Equilibriumconditionswere obtained

aftersoaking for57sat820◦C.Therefore,equilibrium

ther-modynamicsmaybeusedtodeterminethe austenitization

temperatureandcouldbeusedduringthedevelopmentstage

ofalloydesign.

Atlower temperatures and shorteraustenization times,

the equilibrium calculations overestimated the amount of

austentitecomparedwiththevaluesobtainedby

dilatome-try.Therefore, anappropriatemodeltopredict the volume

fractionofausteniteduringheatingshouldconsiderthe

dif-fusionalgrowthofaustenite.

Inthe ﬁnal microstructure, the addition ofboron up to

27ppmreduced theamountofferrite,which increasedthe

amountofbainite.Themartensiteamountdecreasedasthe

boroncontent increased.Theincrease inthe soaking

tem-perature,from780◦Cto820◦C,alsoincreasedtheamountof

bainite.

Themechanical propertiesreﬂected the increase inthe

bainitefraction,whichwere determinedbymicrostructural

characterization;themechanicalresistanceincreasedandthe

ductilitydecreasedupto27ppmofboron.

Conﬂicts

of

interest

Theauthorsdeclarenoconﬂictsofinterest.

Acknowledgement

RobertoRibeirodeAvillezthanksCNPqforpartialfunding.

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