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Nb-microalloyed advanced high-strength steel treated by quenching–partitioning–tempering process

Nb-microalloyed advanced high-strength steel treated by quenching–partitioning–tempering process
Nb-microalloyed advanced high-strength steel treated by quenching–partitioning–tempering process

Materials Science and Engineering A 506(2009)111–116

Contents lists available at ScienceDirect

Materials Science and Engineering

A

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m s e

a

Enhancement of the mechanical properties of a Nb-microalloyed advanced high-strength steel treated by quenching–partitioning–tempering process

N.Zhong a ,X.D.Wang a ,L.Wang b ,Y.H.Rong a ,?

a School of Materials Science and Engineering,Shanghai Jiao Tong University,Shanghai 200030,China b

Baosteel Research and Development Technology Center,Shanghai 201900,China

a r t i c l e i n f o Article history:

Received 13September 2008Received in revised form 12November 2008

Accepted 12November 2008

Keywords:

Advanced high-strength steel Microstructure

Quenching–partitioning–tempering process Mechanical property

a b s t r a c t

There is an urgent requirement for better combination of high strength and adequate tough-ness of steels to meet the demand of industrial applications.The present study shows that an Fe–0.2C–1.5Mn–1.5Si–0.05Nb–0.13Mo steel subjected to quenching–partitioning–tempering (Q–P–T)process based upon modifying the recently developed Q&P treatment by Speer et al.exhibits high-tensile strength (1500MPa)and relatively high elongation (15%),therefore,Q–P–T steels have become a novel group of advanced high-strength steels.The origin of such excellent mechanical properties was revealed by characterization of microstructure.

?2008Elsevier B.V.All rights reserved.

1.Introduction

Over the years,advanced high-strength steels (AHSS)steels have been investigated intensively for seeking the better combination of high strength and adequate toughness [1,2],because of the acute concern on decreasing weight of steel parts so as to save energy and raw materials as well as on environment protection,especially in automobile industry.Several sorts of AHSS steels,such as dual-phase (DP)steels [3]and transformation-induced plasticity (TRIP)steels [4]have been developed,but the strength of the above AHSS steels with the carbon content of 0.05–0.2wt%is staying within a range of 500–1000MPa [1,4,5].In order to gain higher strength,recently,Speer et al.[6–8]proposed a new heat treatment for pro-ducing martensitic steel:quenching and partitioning (Q&P)process.The Q&P heat treatment [9]sequence involves quenching from austenite-?eld temperature (AT)to a temperature (QT)between the martensite-start (M s )and martensite-?nish (M f )temperatures,followed by a ‘partitioning’treatment either at (called 1-step Q&P),or above (called 2-step Q&P)the initial quench temperature,and at this temperature (PT)carbon can partition from the supersaturated martensite phase to the untransformed austenite phase in enough time (t ),thereby stabilizing retained austenite phase to room tem-perature.The schematic Q&P heat treatment is showed in Fig.1[9].The Q&P steels generally consist of carbon-depleted marten-

?Corresponding author.Tel.:+862154745567;fax:+862154745560.E-mail address:yhrong@https://www.sodocs.net/doc/7b12649794.html, (Y.H.Rong).site and carbon-enriched stabilized austenite,which has the direct attraction of achieving higher strength levels.Speer et al.[6]also proposed the corresponding thermodynamic model for the Q&P heat treatment.First,they named the model Constrained Paraequi-librium (CPE)[6],and after discussion with Hillert et al.[10,11],they ?nally called the thermodynamic condition as “Constrained Carbon Paraequilibrium”(CCE)[9,12].

It is well known that the retention of austenite phase in the steels after heat treatment may improve the ductility and toughness [13].Conventional quenching to room temperature and temper-ing treatment may lead to formation of less amount of retained austenite and decomposition of austenite during tempering [14,15].While Q&P heat treatment can preserve certain amount of retained austenite by choice of a proper quenching temperature and enough partitioning time,and thus the ductility and toughness can be guar-anteed.However,formation of carbide is assumed to be avoided in Speer’model [6,7],because carbide formation consumes cer-tain amount of carbon in martensite,therefore there may not be enough carbon for the stabilization of retained austenite during cooling from partitioning temperature to room temperature.In order to make the most of carbon to stabilize the retained austen-ite,carbide forming elements (such as Nb,Mo,Ti)have not been considered as one of component of steels in Speer et al.’studies [16–20].It is known that the microalloying element [21,22],such as Nb and V,has both the effects of grain re?nement strength-ening and precipitation strengthening.In order to further raise the strength of steels by precipitation strengthening and maintain adequate ductility,a quenching–partitioning–tempering (Q–P–T)

0921-5093/$–see front matter ?2008Elsevier B.V.All rights reserved.doi:10.1016/j.msea.2008.11.014

112N.Zhong et al./Materials Science and Engineering A 506(2009)

111–116

Fig.1.Schematic Q&P heat treatment:austenitizing at AT is followed by quenching to a temperature (QT),where austenite transforms partially to martensite and then partitioning at an elevated temperature (PT)[9].

process is proposed by Hsu [23]to modify the Q&P technology.The design of Q–P–T approach is based on the following considerations:(1)carbide forming elements such as Nb,Mo and V leading to grain re?nement and formation of stable carbides are added to Q–P–T steel;(2)Si or/and Al are added for preventing cementite (Fe 3C)formation;(3)the carbon content of Q–P–T steel is properly higher to compensate the carbon consumption caused by carbide forma-tion in martensite;(4)both the partitioning temperature and time are limited based on the requirement of dispersive precipitation of carbides in martensite,namely,tempering includes partitioning.Therefore,the aim of the present work is to show the relationship between microstructure and mechanical property of a AHSS steel containing Nb and Mo subjected to Q–P–T process,and the origin of good combination of high-tensile strength and relatively high ductility is analyzed.2.Experimental

The designed chemical composition of the Q–P–T steel investi-gated in the present work is listed in Table 1.The M s of the present steel is calculated as 375?C by J-MatPro [24]and the QT is selected to be 220?C.

The steel was melted in a laboratory medium frequency furnace in the Technology Center of Baosteel Group.The ingot was forged to 35mm thick,and austenitized at 1250?C for 1h followed by hot rolling to 1mm.For Q–P–T heat treatment,the as-received sample was austenitized at 920?C for 300s,followed by quenching into a salt bath at 220?C.Then it was treated by partitioning–tempering at 400?C in molten salt bath for 10,20,40,180s,respectively,and ?nally water quenched to room temperature.It is worth to point out that the tempering temperature of 400?C is chosen to obtain

Table 1

Chemical composition of the Q–P–T steel sample (wt.%).C 0.2

Mn 1.5Si 1.5P 0.006S 0.005Nb 0.05Mo 0.13Al 0.044N

0.003

Fig.2.XRD spectra of the steel quenched to 220?C and tempered and partitioned at 400?C for 20s.

?ne carbides distributing dispersedly in matrix based on our previ-ous work [25],meanwhile,the selected temperature can satisfy the requirement of carbon partitioning from martensite to austenite [9].

The tensile specimens were machined with their axis oriented parallel to the rolling direction and cut into the gage length of 70mm,gage width of 20mm.They were tested on a Zwick T1-FR020TN A50machine at the strain rate of 10?3s ?1at room temperature.The microstructure of the steel was characterized by a JSM-6460(operated at 5kV with a working distance of 5mm)scanning electron microscope (SEM)after conventional Nital etch-ing.The TEM specimens were mechanically polished to a thickness of 70?m and electro polished in a twin-jet polisher using 4%per-chloric acid solution at ?20?C,then ?nally thinned by using a Gatan ion-beam milling machine at low angle of 2?so as to achieve large thin areas for the observation of precipitates.TEM investigation was carried out using a JEM-100CX microscope operated at 100kV.The amount of retained austenite is determined by a D/max-2550X-ray diffractometer (operated at 30kW,50mA)with Cu K ?radiation.Since the precise amount of retained-austenite present in the steel is dif?cult to determine exactly by means of XRD due to the strong texture developed during the rolling [26]and the strong texture can be clearly seen by comparing the change in the {111}fcc (nearly invisible and overlapping {110}bcc )and {220}fcc peak intensities (as shown in Fig.2),so the volume fraction of retained austenite was determined semi-quantitatively using a direct comparison method of comparing with the integrated intensity of the 220?and 211?peaks.

3.Results and discussion

3.1.Mechanical properties of Q–P–T steels

The dependence of ultimate tensile strength (Rm)and total elongation (EI)of Q–P–T steel on partitioning–tempering time are shown in Fig.3.Fig.3shows that the tensile strength of the as-quenched sample is the highest,about 1750MPa,but the elongation is only 7%.For the sample partition–tempered for 20s,an almost 200MPa decrease in Rm occurred comparing to that in as-quenched state,but the elongation sharply increases and reaches to a peak value of 17%.Then it monotonically decreases down to about 13%with increasing partitioning–tempering time till 180s.The tensile strength slightly decreases when partitioning–tempering from 10to 20s,then rises and reaches a maximum value of 1550MPa for the partitioning–tempering time up to 40s,indicating that there

N.Zhong et al./Materials Science and Engineering A506(2009)111–116

113

Fig.3.Mechanical properties of Q–P–T steel as a function of partitioning–tempering time,where Rm represents tensile strength and El represents total elongation.

is the most signi?cant effect of carbide precipitation strengthen-ing in the Q–P–T steels,which is con?rmed by TEM investigation below.As shown above,the overall mechanical property of the Q–P–T steel can be described as an ultra-high-tensile strength (above1500MPa)with relatively high ductility(15%).In addi-tion,a Q–P–T treated Fe–0.485C–1.195Mn–1.185Si–0.98Ni–0.21Nb steel with the tensile strength over2000MPa and the total elongation over10%has been developed[25].By the following comparing Q–P–T process with TRIP or Q&P process,the fea-ture of Q–P–T process in increasing strength will be emphasized.

A recent study[22]shows that the steel with similar chemical composition(Fe–0.18C–1.50Mn–1.50Si–0.05Nb–0.13Mo–0.044Al, in wt%)treated by intercritical annealing at800?C for70s and subsequent isothermal holding at400?C(a typical TRIP heat treatment)shows the tensile strength of1000MPa with a total elongation of20%.Another study[27]shows that the steel(Fe–0.20C–1.470Mn–1.51Si–0.047Nb–0.20Mo–0.028Al,in wt%)coiling at350?C displays tensile strength of1000MPa with a total elongation of20%.Other literatures have reported that the ten-sile strength and total elongation of a Q&P heated TRIP steel without any carbide forming element:Fe–0.2C–1.5Si–1.67Mn–0.046Al are 1100MPa and21%,respectively[28];while those of an Fe–0.19 C–1.63Si–1.59Mn–0.036Al–0.03Cr are1400MPa and9%,respec-tively[8].Comparing to the AHSS steels reported so far,a new family of steels with higher strength and relatively high ductility is pro-posed,which is termed as“Q–P–T”steels,drawn roughly in Fig.4 (the circle in bold black).Fig.4shows that the Q–P–T steels

have Fig.4.A schematic comparison of ultimate tensile strength and total elongation for dual phase(DP),TRIP,martensitic(M),Q&P,and Nb-microalloyed Q–P–T steels.

the highest strength with adequate ductility to meet the need of the high-strength parts in industry.

3.2.Microstructural characterization

3.2.1.The volume fraction of retained austenite of Q–P–T steel

The volume fraction of retained austenite in samples partition–tempered for10,20,40,80and180s are deter-mined semi-quantitatively by XRD as4.5,7.0,5.4,5.2and5.8%, respectively,while the diffraction peaks of austenite could not be observed in the as quenched sample.The above volume fraction of retained austenite is much lower than12%theoretically predicted basing upon the Koistinen–Marburger equation[29]since the equation is only suitable for Fe–Mn–C system and assumes“fully”partitioning of carbon from martensite to austenite while the Q–P–T steel has Nb and Mo alloying elements as well as the precipitation of carbides.The present study shows that the sample partition–tempered for10s possesses the fewest amount,4.5%, of retained-austenite,and the sample partition–tempered for 20s has the largest amount,7.0%,of retained austenite,and the sample partition–tempered for longer times(>40s),the amount of retained austenite keeps about5%.The early works by Speer et al.[7,8]shows that the carbon depletion in martensite of a 0.19C–1.59Mn–1.63Si steel partitioned at400?C could occur in a very short time less than0.1s while the homogenization of carbon concentration in austenite takes about10s.In the present

Q–P–T Fig.5.SEM micrographs of the Q–P–T processed samples partition–tempered at400?C for(a)10s and(b)180s.

114N.Zhong et al./Materials Science and Engineering A506(2009)111–116

process,it seems that10s is not enough for the carbon“fully”partitioning from martensite to austenite,and it may takes about 20s for“fully”partitioning of carbon from martensite to austenite according to the highest amount of retained austensite because both alloying elements Nb and Mo consumed certain amount of carbon by carbide formation and retarded the partitioning of carbon from martensite to austenite.The later reason can be explained as that the af?nity between carbon atom and niobium atom is more attractive than that between carbon atom and iron atom[30,31].It is suggested that the process of partitioning of carbon from martensite to austenite should be more slower than that in the above0.19C–1.59Mn–1.63Si steel without Nb and Mo elements.On the other hand,interface migration[32–34] between martensite and austenite may play an important role for the decreasing of volume fraction of retained-austenite during partition–tempering process.We[32]have pointed out that the martensite/austenite interface may migrate during partitioning treatment for enough high temperature and long time based on the difference of chemical potential of iron atom between two phases, moreover,the migration direction of the martensite/austenite interface is dependent on the chemical potential of carbon in a steel.Speer et al.[33]also have reported that the marten-site/austenite interface may move in either direction,depending on the speci?c details of the phase fractions and compositions,and if interface migration occurs during carbon partitioning,the inter-face may move?rst in one direction and then to another.Recently, motion of the martensite/austenite interface coupling with car-bon diffusion in both phases has been modeled and

quantitatively

Fig.6.TEM bright-?eld image of the as quenched sample. analyzed by Santo?mia et al.[34].Their results of simulation show that bi-directional movement of the interface can occur in a 0.19C–1.59Mn–1.63Si steel,which may lead to net interface migra-tion of the order of0.05?m from the martensite phase to austenite phase in tens of seconds[34].Except for Nb and Mo,the com-position of the steel used in the present work is similar to that used by Speer et al.[16]and Santo?mia et al.[34],and it is sug-gested that the direction of motion of the martensite/austenite interface is from martensite phase to austenite phase,leading

to

Fig.7.TEM micrographs of the sample partitioning–tempering for40s:(a)bright-?eld image,(b)enlarged bright-?eld image of(a),showing the interlath retained austenite, (c)center dark-?eld image of retained austenite,g=(020),and(d)SEAD pattern of(b).

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Fig.8.TEM micrographs show niobium carbides precipitation in the martensite matrix of the sample partitioning–tempering for40s:(a)BF,(b)DF with SEAD pattern inserted.

the reduction of austenite volume after long time partitioning.This may be one important reason for decreasing of austenite volume fraction in samples partitioned for40s or longer.Moreover,the microalloying element Nb and Mo may have the effect of solute drag[35]on the interface,so it is reasonably to believe that the velocity of the interface migration will be more slower than that in0.19C–1.59Mn–1.63Si Q&P steel[34].Comparing to the marten-sitic steels with a little amount of retained austenite[1,5],the Q–P–T steel investigated in the present work holds a relatively large amount of retained austenite,and thus the ductility and toughness can be guaranteed.

3.2.2.SEM and TEM investigation

The SEM micrographs of the Q–P–T samples partition–tempered at400?C for10and180s are showed in Fig.5.It shows that the gen-eral feature of lath-like martensite con?guration does not change much even in the sample treated for longer time,but the retained austenite and carbide precipitates could not be discriminated from martensite in Fig.5because they are too?ne for SEM observation. Some literatures[7,36]report that there may be bainite formation during the partitioning treatment,especially for longer time,but there is no observation of bainite formation in the present study.It may attribute to both relatively short partitioning time(180s)and addition of Nb element which may retard bainite transformation [37].

The TEM bright-?eld image of the as-quenched sample(Fig.6) displays that the lath martensite structures with a lot of dislocation tangles.

The bright-?eld image of Fig.7(a)shows dislocation-type lath martensite in the sample with partitioning–tempering for 40s.The dislocations can act as potential sites for nucleation of carbide precipitates marked by arrows in Fig.7(b).There exists retained austenite between martensite laths,which can be clearly seen in the enlarged bright-?eld image Fig.7(b)and is identi?ed by both the dark-?eld image(Fig.7(c))and the selected area electron diffraction(SAED)shown in Fig.7(d). Fig.7(b)and(c)show that the average width of martensite is less than200nm and the average thickness of?lm-like retained austenites is about20nm,respectively.The orientation relation-ships between martensite and retained austenite is identi?ed by SAED in Fig.7(d)as well-known Kurdjurmov–Sachs relationship: [ˉ1ˉ1ˉ1]?//[101]?,(1ˉ10)?//(ˉ111)?and Nishiyama–Wasserman relationship:[ˉ100]?//[101]?,(011)?//(ˉ111)?.Studies[1,38] show that microstructural re?nement can increases the strength and decreases the ductile–brittle transition temperature.As a result,such a?ne microstructure consisted of lath martensite and ?lm-like retained austenite leads to a good combination of high strength and ductility in the Q–P–T steels.

Fig.8(a)and(b)show that there are a large number of ?ne niobium carbides determined by SAED analysis precipi-tated dispersedly in the lath martensite matrix in the sample partitioning–tempering for40s.The diffraction spot of martensite overlapping that of niobium carbides was chosen for the dark-?eld imaging(b),and it is clear that very?ne niobium carbides with an average particle size of5nm precipitate uniformly and densely in the lath martensite matrix.The SAED analysis shows that the Baker–Nutting(B–N)orientation relationship[39]holds for the nio-bium carbides precipitate.Obviously,such a large number of?ne niobium carbides lead to a signi?cant precipitation strengthening effect in the Q–P–T steel.

4.Conclusion

The present study shows that good combination of high-tensile strength(above1500MPa)and relatively high ductility(15%)of a Nb-microalloyed low-carbon steel can be attained by applying the quenching–partitioning–tempering(Q–P–T)heat treatment.SEM and TEM investigations reveal that the Q–P–T steel consists of?ne martensite laths and dispersive carbide precipitates in the lath martensite matrix,which is mainly responsible for the ultra-high strength.Meanwhile,the?lm-like interlath retained austenite and the overall?ne structure of steel favors high ductility and tough-ness.As a result,it is supposed that the Q–P–T process may bring in a new group of AHSS steels comparing with other AHSS steels developed so far.

Acknowledgments

The present work is?nancially supported by the National Nat-ural Science Foundation of China(no.50771110)and Baosteel Co. Ltd.(Shanghai,China).

References

[1]G.Malakondaiah,M.Srinivas,P.R.Rao,Prog.Mater.Sci.42(1997)209.

[2]J.G.Speer,D.K.Matlock,J.Met.54(2002)19.

[3]M.S.Rashid,Ann.Rev.Mater.Sci.11(1981)245.

[4]L.Li,P.Wollants,Y.L.He,B.C.De Cooman,X.C.Wei,Z.Y.Xu,Acta Metall.Sin.16

(2003)457.

[5]C.Ouchi,ISIJ Int.41(2001)542.

[6]J.G.Speer,D.K.Matlock,B.C.De Cooman,J.G.Schroth,Acta Mater.51(2003)

2611.

[7]J.G.Speer,D.V.Edmonds,F.C.Rizzo,D.K.Matlock,Curr.Opin.Solid St.Mater.

Sci.8(2004)219.

[8]A.M.Streicher,J.G.Speer,D.K.Matlock,B.C.De Cooman,Proceedings of the Inter-

national Conference on Advanced High Strength Sheet Steels for Automotive Applications,2004,p.51.

[9]D.V.Edmonds,K.He,F.C.Rizzo,Edmonds,B.C.De Cooman,D.K.Matlock,J.G.

Speer,Mater.Sci.Eng.A438–440(2006)25.

[10]M.Hillert,J.Agren,Scr.Mater.50(2004)697.

116N.Zhong et al./Materials Science and Engineering A506(2009)111–116

[11]M.Hillert,J.Agren,Scr.Mater.52(2005)87.

[12]J.G.Speer,D.K.Matlock,B.C.De Cooman,J.G.Schroth,Scr.Mater.52(2005)83.

[13]M.Sarikaya,A.K.Jhingan,G.Thomas,Metall.Trans.A14(1983)1121.

[14]F.Zia-Ebrahimi,G.Krauss,Metall.Trans.A14(1983)1109.

[15]F.Zia-Ebrahimi,G.Krauss,Acta Metall.32(1984)1767.

[16]F.C.Rizzo,D.V.Edmonds,K.He,J.G.Speer,D.K.Matlock,A.Clarke,Proceedings of

the International Conference on Solid–Solid Phase Transformations in Inorganic Materials,2005,p.535.

[17]A.Clarke,J.G.Speer,D.K.Matlock,F.C.Rizzo,D.V.Edmonds,K.He,Proceedings of

the International Conference on Solid–Solid Phase Transformations in Inorganic Materials,2005,p.99.

[18]F.L.H.Gerdemann,J.G.Speer,D.K.Matlock,Mater.Sci.For.480(2004)439.

[19]B.C.De Cooman,J.G.Speer,Steel Res.Int.77(2006)634.

[20]F.C.Rizzo,A.R.Martins,J.G.Speer,D.K.Matlock,A.Clarke,B.C.De Cooman,

Mater.Sci.For.539(2007)4476.

[21]J.W.Christian,The Theory of Transformations in Metals and Alloys,Oxford,UK.

[22]X.D.Wang,B.X.Huang,L.Wang,Y.H.Rong,Metall.Mater.Trans.A39(2008)1.

[23]T.Y.Hsu(Xu Zuyao),Mater.Sci.For.561–565(2007)2283.

[24]J-MatPro.

[25]X.D.Wang,L.Wang,Y.H.Rong,J.Mater.Res.,24,No.1,Jan2009,doi:10.1557/

JMR.2009.0029.[26]S.Vercammen,B.Blanpain,B.C.De Cooman,P.Wollants,Acta Mater.52(2004)

2005.

[27]S.Hashimoto,S.Ikeda,K.I.Sugimoto,S.Miyake,ISIJ Int.44(2004)1590.

[28]N.Zhong,X.D.Wang,B.X.Huang,Y.H.Rong,L.Wang,Proceedings of the3rd

International Conference on Advanced Structural Steels,2006,p.885.

[29]G.Krauss,Steels:Heat Treatment and Processing Principles,ASM International,

Metals Park,OH,USA,1990.

[30]K.Fujimura,M.Toshisada,T.Higashi,S.Urakawa,Tetsu-To-Hagane59(1973)

222.

[31]N.Shohoji,Mater.Sci.Technol.20(2004)301.

[32]N.Zhong,X.D.Wang,Y.H.Rong,L.Wang,J.Mater.Sci.Technol.22(2006)751.

[33]J.G.Speer,R.E.Hackenberg,B.C.De Cooman,D.K.Matlock,Philos.Mag.Lett.87

(2007)379.

[34]M.J.Santo?mia,L.Zhao,J.Sietsma,Scr.Mater.59(2008)159.

[35]M.Hillert,Metall.Trans.6A(1975)5.

[36]A.Clarke,PhD Thesis,Colorado School of Mines,Golden,CO,2006.

[37]Y.Jingsheng,Y.Zongsen,W.Chengjian,J.Met.40(1988)26.

[38]S.Morito,H.Yoshida,T.Maki,X.Huang,Mater.Sci.Eng.A438–440(2006)

237.

[39]R.G.Baker,J.Nutting,I.S.I.Special Report,No.64,1959.

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