Carbide characterization in a Nb-microalloyed advanced ultrahigh strength steel after quenching–partitioning–tempering process

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

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Carbide characterization in a Nb-microalloyed advanced ultrahigh strength steel after quenching–partitioning–tempering process X.D. Wang a , W.Z. Xu a , Z.H. Guo a,∗ , L. Wang b , Y.H. Rong a a b

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

a r t i c l e

i n f o

Article history: Received 10 November 2009 Received in revised form 18 January 2010 Accepted 9 February 2010

Keywords: Advanced ultrahigh strength steel Quenching–partitioning–tempering process Carbide Characterization

a b s t r a c t Based on the observations of scanning electron microscopy and transmission electron microscopy, four kinds of carbides were identified in a Nb-microalloyed steel after quenching–partitioning–tempering treatment. In addition to transitional epsilon carbide that usually forms in silicon-free carbon steel, other three types of niobium carbides (NbC) formed at various treatment stages respectively. They are incoherent NbC inclusion that nucleated at solidification mainly, fine NbC that nucleated in lath martensite at tempering stage and regular polygonal NbC that nucleated in austenite before quenching. Their formation mechanisms on steel were discussed briefly based on thermodynamics. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Quenching and partitioning (Q&P) treatment of steel, proposed originally by Speer et al. [1,2], was used to obtain stable dual-phase structure of martensite plus retained austenite. The Q&P treatment, constrained carbon equilibrium (CCE) model, refers to carbon diffusion (or partition) from supersaturated martensite (formed during quenching) into retained austenite at a given temperature [3–5]. After partitioning, the carbon-enriched austenite becomes stable at room temperature. In order to ensure the best effect of austenite stabilization due to carbon enrichment, carbide formation elements, such as niobium and vanadium, are eliminated from steel. On the other hand, carbide suppression elements, such as silicon and aluminum, are added into steel [6–8]. However, although the desired strength–ductility combination of steels that subjected to Q&P treatment can be reached by adjusting martensite fraction, this treatment excludes potential strengthening manners of grain refinement and carbide precipitation through the addition of micro-alloying elements. In the present study, different from the Q&P treatment, carbide formation element Nb was added into steels instead of eliminating it. Meanwhile, the carbon content in steels was increased to compensate its possible depletion due to carbide formation. This arrangement results in a novel heat treatment

∗ Corresponding author. Tel.: +86 21 54745567; fax: +86 21 54745560. E-mail address: [email protected] (Z.H. Guo). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.02.026

manner: quenching–partitioning–tempering (Q–P–T) process, i.e., additional tempering process for carbide precipitation is carried out after traditional Q&P treatment [9]. Earlier experimental results indicate that steels containing carbide formation elements show better mechanical properties after Q–P–T treatment compared with those only after Q&P treatment [10]. The contribution of tempering stage is thus significant [10,11]. To understand the strengthening mechanism at tempering stage and to control mechanical property of steels through Q–P–T treatment, the mechanism 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 slabs with thickness 35 mm, provided by Technical Center of Shanghai Baosteel, were heated up to 1250 ◦ C for 1 h, and then hot rolled into a thickness of 3 mm with the finishing temperature of 860 ◦ C. Finally, these sheets were rolled to a thickness of 1 mm at room temperature. The as-rolled sheets were subjected to Q–P–T process, that is, austenitized at 800 ◦ C for 300 s, followed by quenching into salt bath 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) observation were etched by 2% nital, and were observed by a FEI SIRION 200 field emission microscope and a JSM-6460 microscope equipped

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Fig. 1. (a) SEM image of incoherent NbC inclusions in sample I, and (b) EDS profile of a carbide designated by “+” in (a).

with an electron energy dispersive spectrometry (EDS). Specimens for transmission electron microscope (TEM) observation were prepared by mechanical polishing and electropolishing in a twin-jet polisher using 4% perchloric acid solution. TEM investigation was carried out in JEM-100CX and JEM 2010 microscopes equipped with EDS operated at 200 kV. 3. Results 3.1. SEM characterization Under SEM observation, the basic microstructure of specimen is martensite mainly. This is consistent with the estimation result of Koistinen–Marburger equation [12], i.e., ∼67 vol% constitution in the steel is martensite after present Q–P–T treatment. Some particles with spontaneous morphology are visible within both sample I and sample II, respectively. Fig. 1(sample I) shows an example. As arrow indicated in Fig. 1(a), some spontaneous particles distribute in martensitic matrix randomly. Their size is in the range of microns. EDS analysis (e.g., “+” in Fig. 1(a)) indicates that they are NbC mainly (Fig. 1(b)). Since similar microstructure was also

found in sample II, formation of these NbC would have no relationship with respect to tempering temperature and time. Besides these structural features, careful examination was carried out to see if there exist other phases in steel. In Fig. 2, another two kinds of phases were found. The first one is gray bands with length <0.5 ␮m (Fig. 2(b)). These gray bands distributed regularly within martensite under short partitioning and tempering time (sample I). When tempering time becomes longer, they disappear again (sample II, Fig. 2(d)). The second one is white spots. In Fig. 2(a) and (c), it is hardly to see these white spots because they are too small. However, under high magnification (Fig. 2(d)), these white spots can be found with size in nano-scale, and may be the carbide precipitates in martensite matrix. According to above observation, it is indicated that (1) the nanosized white spots in Fig. 2 are the carbide precipitates which forms at partitioning and tempering stages; (2) spontaneous NbC in big size in Fig. 1, different from white spots (carbide) in small size in Fig. 2, should nucleate at higher temperature before Q–P–T treatment; (3) in Fig. 2, the appearance and disappearance of gray bands may be related to evolution of transitional phase after quenching. Detailed mechanism needs to be investigated further.

Fig. 2. SEM images of microstructure in the specimen after Q–P–T treatment. Gray band is shown as arrow indicated in (b). (a) Sample I, low magnification, (b) sample I, high magnification, (c) sample II, low magnification, (d) sample II, high magnification.

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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 diffraction pattern.

Fig. 4. TEM microstructure of NbC precipitates in sample I. (a) Bright field image, (b) dark field image with g = 111¯ MC reflection, and (c) corresponding diffraction pattern.

3.2. TEM characterization Under TEM observation (Fig. 3), two-dimensional image of spontaneous NbC in Fig. 1 shows the polygonal boundary. Their average size is about 1–2 ␮m; their structure, based on selected-area electron diffraction (SAED) analysis (Fig. 3(b) and (c)), is determined as B1(NaCl)-typed carbide. Specific orientation relationship (OR) was not found between them and martensite matrix. Considering their size and topography, it is believed that these NbC particles form in the liquid and are embedded as incoherent inclusions in austenite during solidification; or they could precipitate at hot rolling stage (1250 ◦ C) before Q–P–T treatment, but OR was destroyed by following deformation. On the other hand, the carbides observed in Fig. 2 may be related to the precipitation occurring at lower temperature. In Fig. 4, the size of white spot (NbC) is about 5–20 nm (the lower limit 5 ± 3 nm is dominant) after partitioning and tempering for 10 s; while in

Fig. 5, significant inhomogeneity in spot size, with size covering from 5 to 30 nm (the upper limit 30 ± 10 nm is dominant), indicates the continuous nucleation and growth of these carbides during partitioning and tempering stages. Crystallographic identification in both cases obtains same specific OR between these carbides and martensite, i.e., (1 1 0)MC //(1 0 0)˛ and [1 1 0]MC //[0 1 1]˛ , or BakeNutting OR. Occasionally, isolated cubic-like NbC can be observed in the specimens (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 from Bake-Nutting OR (Fig. 6(d)). Based on stereographic projection, it is a Nishiyama–Wassermann (N–W) OR. Considering this feature and the processing history of specimen, the possible nucleation stage of cubic-like NbC would be in the austenization stage (800 ◦ C × 300 s) just before quenching. At this stage, B1(NaCl)-typed carbide shows cube–cube OR with respect to austenite [13]. Due to relatively high

Fig. 5. TEM microstructure of NbC precipitates in sample II. (a) Bright field image, (b) dark field image with g = 111¯ MC reflection, and (c) corresponding diffraction pattern.

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Fig. 6. TEM microstructure of cubic-like NbC in sample II. (a) Bright field image, (b) dark field image with g = 2 2 0MC reflection, (c) corresponding diffraction pattern, (d) indexing, and (e) EDS profile of NbC in (a).

carbon content in the present steel (medium carbon steel), subsequent quenching results in N–W OR between martensite and austenite, making this kind of NbC also shows N–W OR with respect to martensite. As mentioned above, the precipitation of coherent NbC involves the dissolution of gray band concurrently under the condition of prolonged partitioning and tempering times (Fig. 2). TEM observation also confirms that it is difficult to find gray band in sample II, while these gray bands distribute regularly at an angle about 30◦ along the martensite lath direction in sample I (Fig. 7). EDS analysis (see “+” in Fig. 7(a)) reveals the main compositions of gray band to be C and Fe. No trace of Nb was found (Fig. 7(e)). Structural identification by SAED (Fig. 7(b)–(d)) implies hcp feature, and thus they are determined as transitional epsilon (␧) carbides. Furthermore, a Jack-relationship between ␧ carbide and martensitic matrix: (1 0 1 0)ε //(0 1 1)˛ and (0 1 1 1)ε //(1 0 1)˛ , is found (Fig. 7(d)). Therefore, adding Si into the present steel did not hinder the nucleation of ␧ carbide from martensite. It is worth pointing out that ␧ carbide became unstable and dissolved again with the increase of tempering time, i.e., Nb exhibits stronger carbide formation capability than other elements in the present steel. With the increase of tempering time, some NbC particles may nucleate or grow in expense of ␧ carbides. 4. Discussion Present results confirm NbC can form at solidification, austenization and tempering stages respectively and also show different morphologies and crystallographic features. Thermodynamic analysis with Thermo-Calc software indicates that the critical formation temperature of NbC is about 1400 ◦ C for the present C and Nb contents. This temperature is in the range of austenite and liquid. NbC formed at this stage has no specific OR with respected to austenite. Based on solubility expression of NbC

in austenite [14]: lg[Nb][C] − 0.248[Mn] = 1.8 −

6770 T

(1)

The maximum solubility product of [Nb][C] (=0.0112) in austenite occurs at the melting point, 1400 ◦ C. At this temperature, the Nb content in austenite is about 0.0243 wt%, i.e., ∼88% Nb has finished reacting with C. The positive correspondence of [Nb][C] with T in Eq. (1) indicates the monotonic increase of NbC content with the decrease of temperature. This includes the growth of incoherent NbC inclusion that formed during solidification, and nucleation/growth of coherent NbC precipitates within austenite or martensite. It is seen that the size of incoherent NbC inclusions is significantly larger than those of precipitated under the condition of solid state, with the former may be in the range of microns while the later may be in the range of nanometers. After solidification, the following processing of steel results in two kinds of NbC precipitation, either from austenite or from martensite. Since the effective precipitation temperature of NbC is 1250 ◦ C (×3600 s) during hot rolling, the maximum NbC fraction formed from austenite, based on Eq. (1), is estimated as about 7%. Due to subsequent rolling, these NbC precipitates will lose their specific OR with respect to matrix, and therefore are difficult to be discriminated from incoherent NbC inclusion that forms previously. Another chance for NbC precipitation in austenite is in the stage of Q–P–T treatment, 800 ◦ C × 300 s for austenization. At this temperature with short time, low nucleation rate and limited size of NbC are predictable, leading to small fraction of NbC with cubic-like morphology [13]. After Q–P–T treatment, its size shall be smaller than those formed at 1250 ◦ C. Existence of N–W OR between these NbC and martensite only confirms cube–cube OR between them and prior austenite, but does not mean martensite is the parent matrix 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

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Fig. 7. TEM microstructure of ␧ carbides in sample I. (a) Bright field image, (b) dark field image with g = 1 01¯ 0 ε reflection, (c) corresponding diffraction pattern, (d) indexing, and (e) EDS profile of the area designated by “+” in (a).

NbC precipitation in theory, or less than 5% NbC precipitation in martensite can be used for strengthening of steel. However, as reported in our previous paper [10], tempering at 400 ◦ C enhances mechanical property of steel significantly. Furthermore, increase of tempering time (from 10 s to 1800 s) results in the decrease of tensile strength (from ∼2100 MPa to ∼1700 MPa). This is another evidence of precipitation strengthening because growth of NbC will weaken strengthening effect because of losing coherency between NbC and martensite and the coalesce of NbC particles. Therefore, the precipitation strengthening effect is obvious in the steels with small addition of carbide promotion elements after the additional tempering process. The formation of transitional ␧ carbide is consistent with the result of Grange et al. [15,16], i.e., adding Si only hinders further evolution of transitional ␧ carbide to stable cementite. Obviously, this is related to both thermodynamic and kinetic factors. Thermodynamically, only a small quantity of carbon was consumed for NbC formation and thus strong super-saturation of carbon still exist in martensite. This fact combining with high density of crystalline defects in martensite (Fig. 7) indicates a serious carbon segregation/enrichment at local area would be possible. Under the proper partitioning temperature, transitional ␧ carbide can form immediately after quenching. Since ␧ carbide disappeared again with the increase of tempering time, but did not transform to stable cementite, which could be due to the inhibition effect of silicon. As pointed out in Ref. [17], cementite formation is controlled by the diffusion of silicon away from austenite/martensite interface, and diffusion rate of silicon is low at 400 ◦ C. On the other hand, carbon may be more preferable to diffuse to austenite during partitioning stage. Recent experiment has revealed the depletion of carbon in martensite is in an advanced stage during partitioning process [18]. This decreases the carbon supply for continuous formation of ␧ carbide. Compared with Fe and Mn, Nb shows stronger carbon capture ability and thus, the nucleation and growth of NbC in martensite will inevitably promote dissolution of ␧ carbide again.

5. Conclusions The carbides in a Nb-microalloyed steel after Q–P–T process have been characterized by means of SEM and TEM combined with EDS analysis. The factors controlling evolution of these carbides are analyzed, and the main results are presented in the following points. There exist four kinds of carbides with different crystallographic features in the studied steel, that is, incoherent NbC inclusion which forms during solidification of steel mainly, isolated cubic-like NbC precipitated from austenite during austenitization at 800 ◦ C, fine spherical NbC formed from martensitic matrix during partitioning and tempering, and transitional ␧ carbide generated from martensite immediately after quenching. Element Si cannot suppress the formation of transitional ␧ carbide and stable NbC, but it can inhibit further evolution of ␧ carbide to cementite. Fine spherical NbC continuously precipitates from martensitic matrix during tempering. Therefore, they play an important role in the significant strengthening of steel through Q–P–T treatment. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (No. 50771110) and Baosteel Co. Ltd. (Shanghai, China). References [1] J.G. Speer, D.K. Matlock, B.C. De Cooman, J.G. Schroth, Acta Mater. 51 (2003) 2611–2622. [2] J.G. Speer, A.M. Streicher, D.K. Matlock, F.C. Rizzo, G. Krauss, in: E.B. Damm, M. Merwin (Eds.), Austenite Formation and Decomposition, TMS/ISS, Warrendale, PA, 2003, p.505–522. [3] D.V. Edmonds, K. He, F.C. Rizzo, B.C. De Cooman, D.K. Matlock, J.G. Speer, Mater. Sci. Eng. A 438–440 (2006) 25–34. [4] M. Hillert, J. Ågren, Scripta Mater. 50 (2004) 697–699.

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