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Materials Science and Engineering A 527 (2010) 3373–3378

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

arbide characterization in a Nb-microalloyed advanced ultrahigh strength steelfter quenching–partitioning–tempering process

.D. Wanga, W.Z. Xua, Z.H. Guoa,∗, L. Wangb, Y.H. Ronga

School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, ChinaBaosteel Research and Development Technology Center, Shanghai 201900, China

r t i c l e i n f o

rticle history:eceived 10 November 2009eceived in revised form 18 January 2010ccepted 9 February 2010

a b s t r a c t

Based on the observations of scanning electron microscopy and transmission electron microscopy, fourkinds of carbides were identified in a Nb-microalloyed steel after quenching–partitioning–temperingtreatment. In addition to transitional epsilon carbide that usually forms in silicon-free carbon steel, otherthree types of niobium carbides (NbC) formed at various treatment stages respectively. They are inco-

eywords:dvanced ultrahigh strength steeluenching–partitioning–temperingrocess

herent NbC inclusion that nucleated at solidification mainly, fine NbC that nucleated in lath martensite attempering stage and regular polygonal NbC that nucleated in austenite before quenching. Their formationmechanisms on steel were discussed briefly based on thermodynamics.

© 2010 Elsevier B.V. All rights reserved.

arbideharacterization

. Introduction

Quenching and partitioning (Q&P) treatment of steel, proposedriginally by Speer et al. [1,2], was used to obtain stable dual-phasetructure of martensite plus retained austenite. The Q&P treat-ent, constrained carbon equilibrium (CCE) model, refers to carbon

iffusion (or partition) from supersaturated martensite (formeduring quenching) into retained austenite at a given temperature3–5]. After partitioning, the carbon-enriched austenite becomestable at room temperature. In order to ensure the best effect ofustenite stabilization due to carbon enrichment, carbide forma-ion elements, such as niobium and vanadium, are eliminated fromteel. On the other hand, carbide suppression elements, such as sil-con and aluminum, are added into steel [6–8]. However, althoughhe desired strength–ductility combination of steels that subjectedo Q&P treatment can be reached by adjusting martensite frac-ion, this treatment excludes potential strengthening manners ofrain refinement and carbide precipitation through the addition oficro-alloying elements.In the present study, different from the Q&P treatment, car-

ide formation element Nb was added into steels instead ofliminating it. Meanwhile, the carbon content in steels wasncreased to compensate its possible depletion due to carbideormation. This arrangement results in a novel heat treatment

∗ Corresponding author. Tel.: +86 21 54745567; fax: +86 21 54745560.E-mail address: zhenghongguo@sjtu.edu.cn (Z.H. Guo).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.02.026

manner: quenching–partitioning–tempering (Q–P–T) process, i.e.,additional tempering process for carbide precipitation is car-ried out after traditional Q&P treatment [9]. Earlier experimentalresults indicate that steels containing carbide formation elementsshow better mechanical properties after Q–P–T treatment com-pared with those only after Q&P treatment [10]. The contributionof tempering stage is thus significant [10,11]. To understandthe strengthening mechanism at tempering stage and to controlmechanical property of steels through Q–P–T treatment, the mech-anism of carbide precipitation should be investigated.

2. Materials and methods

A medium carbon steel with Nb addition, Fe–0.485C–1.195Mn–1.185Si–0.98Ni–0.21Nb, is used in this study. The slabswith thickness 35 mm, provided by Technical Center of ShanghaiBaosteel, were heated up to 1250 ◦C for 1 h, and then hot rolledinto a thickness of 3 mm with the finishing temperature of 860 ◦C.Finally, these sheets were rolled to a thickness of 1 mm at roomtemperature.

The as-rolled sheets were subjected to Q–P–T process, that is,austenitized at 800 ◦C for 300 s, followed by quenching into saltbath at 170 ◦C for 10 s, then by partitioning and tempering at 400 ◦C

for 10 s (sample I) or 1800 s (sample II) in molten salt respectively,and finally water quenched to room temperature.

Specimens for scanning electron microscope (SEM) observationwere etched by 2% nital, and were observed by a FEI SIRION 200field emission microscope and a JSM-6460 microscope equipped

3374 X.D. Wang et al. / Materials Science and Engineering A 527 (2010) 3373–3378

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Fig. 1. (a) SEM image of incoherent NbC inclusions in sam

ith an electron energy dispersive spectrometry (EDS). Specimensor transmission electron microscope (TEM) observation were pre-ared by mechanical polishing and electropolishing in a twin-jetolisher using 4% perchloric acid solution. TEM investigation wasarried out in JEM-100CX and JEM 2010 microscopes equipped withDS operated at 200 kV.

. Results

.1. SEM characterization

Under SEM observation, the basic microstructure of specimens martensite mainly. This is consistent with the estimation resultf Koistinen–Marburger equation [12], i.e., ∼67 vol% constitution inhe steel is martensite after present Q–P–T treatment. Some parti-les with spontaneous morphology are visible within both sample

and sample II, respectively. Fig. 1(sample I) shows an example.s arrow indicated in Fig. 1(a), some spontaneous particles dis-

ribute in martensitic matrix randomly. Their size is in the rangef microns. EDS analysis (e.g., “+” in Fig. 1(a)) indicates that theyre NbC mainly (Fig. 1(b)). Since similar microstructure was also

ig. 2. SEM images of microstructure in the specimen after Q–P–T treatment. Gray band isagnification, (c) sample II, low magnification, (d) sample II, high magnification.

, and (b) EDS profile of a carbide designated by “+” in (a).

found in sample II, formation of these NbC would have no rela-tionship with respect to tempering temperature and time. Besidesthese structural features, careful examination was carried out to seeif there exist other phases in steel. In Fig. 2, another two kinds ofphases were found. The first one is gray bands with length <0.5 �m(Fig. 2(b)). These gray bands distributed regularly within marten-site under short partitioning and tempering time (sample I). Whentempering time becomes longer, they disappear again (sample II,Fig. 2(d)). The second one is white spots. In Fig. 2(a) and (c), it ishardly to see these white spots because they are too small. How-ever, under high magnification (Fig. 2(d)), these white spots can befound with size in nano-scale, and may be the carbide precipitatesin martensite matrix.

According to above observation, it is indicated that (1) the nano-sized white spots in Fig. 2 are the carbide precipitates which formsat partitioning and tempering stages; (2) spontaneous NbC in big

size in Fig. 1, different from white spots (carbide) in small size inFig. 2, should nucleate at higher temperature before Q–P–T treat-ment; (3) in Fig. 2, the appearance and disappearance of gray bandsmay be related to evolution of transitional phase after quenching.Detailed mechanism needs to be investigated further.

shown as arrow indicated in (b). (a) Sample I, low magnification, (b) sample I, high

X.D. Wang et al. / Materials Science and Engineering A 527 (2010) 3373–3378 3375

Fig. 3. TEM microstructure of incoherent NbC inclusions in sample I. (a) Bright field image, (b) dark field image with g = 0 4 2MC reflection, and (c) corresponding diffractionpattern.

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ig. 4. TEM microstructure of NbC precipitates in sample I. (a) Bright field image, (b

.2. TEM characterization

Under TEM observation (Fig. 3), two-dimensional image of spon-aneous NbC in Fig. 1 shows the polygonal boundary. Their averageize is about 1–2 �m; their structure, based on selected-area elec-ron diffraction (SAED) analysis (Fig. 3(b) and (c)), is determined as1(NaCl)-typed carbide. Specific orientation relationship (OR) wasot found between them and martensite matrix. Considering theirize and topography, it is believed that these NbC particles form inhe liquid and are embedded as incoherent inclusions in austen-te during solidification; or they could precipitate at hot rollingtage (1250 ◦C) before Q–P–T treatment, but OR was destroyed by

ollowing deformation.

On the other hand, the carbides observed in Fig. 2 may be relatedo the precipitation occurring at lower temperature. In Fig. 4, theize of white spot (NbC) is about 5–20 nm (the lower limit 5 ± 3 nms dominant) after partitioning and tempering for 10 s; while in

ig. 5. TEM microstructure of NbC precipitates in sample II. (a) Bright field image, (b) dar

field image with g = 111̄MC reflection, and (c) corresponding diffraction pattern.

Fig. 5, significant inhomogeneity in spot size, with size coveringfrom 5 to 30 nm (the upper limit 30 ± 10 nm is dominant), indicatesthe continuous nucleation and growth of these carbides during par-titioning and tempering stages. Crystallographic identification inboth cases obtains same specific OR between these carbides andmartensite, i.e., (1 1 0)MC//(1 0 0)˛ and [1 1 0]MC//[0 1 1]˛, or Bake-Nutting OR.

Occasionally, isolated cubic-like NbC can be observed in thespecimens (Fig. 6). Its size is larger than those in Figs. 4 and 5,but smaller than those in Fig. 3. SAED determination found(1 1 0)MC//(1 0 0)˛ and [1 1 0]MC//[0 1 3]˛, 26.6◦ deviation fromBake-Nutting OR (Fig. 6(d)). Based on stereographic projection, it is

a Nishiyama–Wassermann (N–W) OR. Considering this feature andthe processing history of specimen, the possible nucleation stage ofcubic-like NbC would be in the austenization stage (800 ◦C × 300 s)just before quenching. At this stage, B1(NaCl)-typed carbide showscube–cube OR with respect to austenite [13]. Due to relatively high

k field image with g = 111̄MC reflection, and (c) corresponding diffraction pattern.

3376 X.D. Wang et al. / Materials Science and Engineering A 527 (2010) 3373–3378

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ig. 6. TEM microstructure of cubic-like NbC in sample II. (a) Bright field image, (ndexing, and (e) EDS profile of NbC in (a).

arbon content in the present steel (medium carbon steel), sub-equent quenching results in N–W OR between martensite andustenite, making this kind of NbC also shows N–W OR with respecto martensite.

As mentioned above, the precipitation of coherent NbC involveshe dissolution of gray band concurrently under the conditionf prolonged partitioning and tempering times (Fig. 2). TEMbservation also confirms that it is difficult to find gray bandn sample II, while these gray bands distribute regularly at anngle about 30◦ along the martensite lath direction in sample(Fig. 7). EDS analysis (see “+” in Fig. 7(a)) reveals the main

ompositions of gray band to be C and Fe. No trace of Nb wasound (Fig. 7(e)). Structural identification by SAED (Fig. 7(b)–(d))mplies hcp feature, and thus they are determined as tran-itional epsilon (�) carbides. Furthermore, a Jack-relationshipetween � carbide and martensitic matrix: (1 0 1 0)ε//(0 1 1)˛ and0 1 1 1)ε//(1 0 1)˛, is found (Fig. 7(d)). Therefore, adding Si intohe present steel did not hinder the nucleation of � carbide from

artensite. It is worth pointing out that � carbide became unsta-le and dissolved again with the increase of tempering time,

.e., Nb exhibits stronger carbide formation capability than otherlements in the present steel. With the increase of temperingime, some NbC particles may nucleate or grow in expense of �arbides.

. Discussion

Present results confirm NbC can form at solidification, aust-nization and tempering stages respectively and also showifferent morphologies and crystallographic features. Thermo-

ynamic analysis with Thermo-Calc software indicates that theritical formation temperature of NbC is about 1400 ◦C for theresent C and Nb contents. This temperature is in the range ofustenite and liquid. NbC formed at this stage has no specific ORith respected to austenite. Based on solubility expression of NbC

k field image with g = 2 2 0MC reflection, (c) corresponding diffraction pattern, (d)

in austenite [14]:

lg[Nb][C] − 0.248[Mn] = 1.8 − 6770T

(1)

The maximum solubility product of [Nb][C] (=0.0112) in austen-ite occurs at the melting point, 1400 ◦C. At this temperature, theNb content in austenite is about 0.0243 wt%, i.e., ∼88% Nb hasfinished reacting with C. The positive correspondence of [Nb][C]with T in Eq. (1) indicates the monotonic increase of NbC con-tent with the decrease of temperature. This includes the growthof incoherent NbC inclusion that formed during solidification, andnucleation/growth of coherent NbC precipitates within austeniteor martensite. It is seen that the size of incoherent NbC inclusionsis significantly larger than those of precipitated under the condi-tion of solid state, with the former may be in the range of micronswhile the later may be in the range of nanometers.

After solidification, the following processing of steel resultsin two kinds of NbC precipitation, either from austenite or frommartensite. Since the effective precipitation temperature of NbC is1250 ◦C (×3600 s) during hot rolling, the maximum NbC fractionformed from austenite, based on Eq. (1), is estimated as about 7%.Due to subsequent rolling, these NbC precipitates will lose theirspecific OR with respect to matrix, and therefore are difficult tobe discriminated from incoherent NbC inclusion that forms previ-ously. Another chance for NbC precipitation in austenite is in thestage of Q–P–T treatment, 800 ◦C × 300 s for austenization. At thistemperature with short time, low nucleation rate and limited size ofNbC are predictable, leading to small fraction of NbC with cubic-likemorphology [13]. After Q–P–T treatment, its size shall be smallerthan those formed at 1250 ◦C. Existence of N–W OR between these

NbC and martensite only confirms cube–cube OR between themand prior austenite, but does not mean martensite is the parentmatrix for their formation.

The last chance for NbC precipitation is at stage of tempering.Before tempering at 400 ◦C, only <0.01 wt% Nb is left for further

X.D. Wang et al. / Materials Science and Engineering A 527 (2010) 3373–3378 3377

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ig. 7. TEM microstructure of � carbides in sample I. (a) Bright field image, (b) dark find (e) EDS profile of the area designated by “+” in (a).

bC precipitation in theory, or less than 5% NbC precipitation inartensite can be used for strengthening of steel. However, as

eported in our previous paper [10], tempering at 400 ◦C enhancesechanical property of steel significantly. Furthermore, increase

f tempering time (from 10 s to 1800 s) results in the decrease ofensile strength (from ∼2100 MPa to ∼1700 MPa). This is anothervidence of precipitation strengthening because growth of NbC willeaken strengthening effect because of losing coherency betweenbC and martensite and the coalesce of NbC particles. Therefore,

he precipitation strengthening effect is obvious in the steels withmall addition of carbide promotion elements after the additionalempering process.

The formation of transitional � carbide is consistent with theesult of Grange et al. [15,16], i.e., adding Si only hinders furthervolution of transitional � carbide to stable cementite. Obviously,his is related to both thermodynamic and kinetic factors. Ther-

odynamically, only a small quantity of carbon was consumed forbC formation and thus strong super-saturation of carbon still exist

n martensite. This fact combining with high density of crystallineefects in martensite (Fig. 7) indicates a serious carbon segrega-ion/enrichment at local area would be possible. Under the properartitioning temperature, transitional � carbide can form immedi-tely after quenching. Since � carbide disappeared again with thencrease of tempering time, but did not transform to stable cemen-ite, which could be due to the inhibition effect of silicon. As pointedut in Ref. [17], cementite formation is controlled by the diffusionf silicon away from austenite/martensite interface, and diffusionate of silicon is low at 400 ◦C. On the other hand, carbon may beore preferable to diffuse to austenite during partitioning stage.

ecent experiment has revealed the depletion of carbon in marten-

ite is in an advanced stage during partitioning process [18]. Thisecreases the carbon supply for continuous formation of � carbide.ompared with Fe and Mn, Nb shows stronger carbon capture abil-

ty and thus, the nucleation and growth of NbC in martensite willnevitably promote dissolution of � carbide again.

age with g = 1 01̄ 0 ε reflection, (c) corresponding diffraction pattern, (d) indexing,

5. Conclusions

The carbides in a Nb-microalloyed steel after Q–P–T processhave been characterized by means of SEM and TEM combined withEDS analysis. The factors controlling evolution of these carbidesare analyzed, and the main results are presented in the followingpoints.

There exist four kinds of carbides with different crystallographicfeatures in the studied steel, that is, incoherent NbC inclusion whichforms during solidification of steel mainly, isolated cubic-like NbCprecipitated from austenite during austenitization at 800 ◦C, finespherical NbC formed from martensitic matrix during partitioningand tempering, and transitional � carbide generated from marten-site immediately after quenching.

Element Si cannot suppress the formation of transitional � car-bide and stable NbC, but it can inhibit further evolution of � carbideto cementite.

Fine spherical NbC continuously precipitates from martensiticmatrix during tempering. Therefore, they play an important role inthe significant strengthening of steel through Q–P–T treatment.

Acknowledgments

This work is financially supported by the National Natural Sci-ence Foundation of China (No. 50771110) and Baosteel Co. Ltd.(Shanghai, China).

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