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The tempering behavior of granular structure in a Mn-series low carbon steel
Materials Science and Engineering A 528 (2011) 920–924
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The tempering behavior of granular structure in a Mn-series low carbon steel Han Zhang ∗ , Xiaole Cheng, Bingzhe Bai, Hongsheng Fang Key Laboratory for Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Qinghuayuan Street, 100084 Beijing, China
a r t i c l e
i n f o
Article history: Received 23 March 2010 Received in revised form 1 September 2010 Accepted 27 October 2010
Keywords: Granular structure Temper brittleness Toughness Carbides Ferrite
a b s t r a c t The granular structure in a Mn-series low carbon steel composed of ferrite matrix and martensite–austenite islands does not exhibit temper brittleness which is quite different from common microstructures in steels. This characteristic facilitates the performance optimization through adjusting tempering temperature. A good combination of tensile strength (750–1000 MPa) and impact toughness (Aku, 138–154 J) can be obtained after quenching and tempering at 400 ◦ C for a round billet with 250 mm in diameter. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the late 1950’s, Habraken [1] firstly found the existence of a special microstructure (named granular bainite) in low and medium carbon alloyed steels, which consists of bainitic ferrite matrix and island constituents. Previous investigations suggest that the islands in granular bainite are originally carbon-rich austenite and transform partially to martensite during subsequent cooling process. Therefore, the islands are composed of martensite and retained austenite when cooled to room temperature (generally called martensite–austenite island) [2]. In recent years, many steels with granular bainite as the dominant phase have been developed [3,4]. Besides, a granular structure composed of pro-eutectoid ferrite and martensite–austenite island was firstly found by Fang et al. [5]. Subsequently, the transformation of granular structure as well as the relationship between the granular structure and its mechanical properties has been studied in detail [6,7]. On the other hand, the microstructure of carbide-free bainite/martensite (CFB/M) was found to possess an excellent combination of strength and toughness [8,9]. It is known that the CFB lath is composed of bainitic ferrite and filmy carbon-enriched retained austenite. Although the retained austenite located inside the CFB laths was beneficial for improving the temper resistance, the temper brittleness of the CFB/M microstructure could not be avoided after tempering at certain temperatures [10]. In fact, temper brittleness has long been a troublesome problem in steels.
∗ Corresponding author. Fax: +86 010 62771160. E-mail address: [email protected] (H. Zhang). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.097
Until now, little has been reported about the impact toughness of the granular structure after tempering. So the present work will lay emphasis on the impact toughness of the granular structure after tempering at different temperature and further put forward a novel granular structure without temper brittleness. Besides, special attention is paid to the evolution of the granular structure during tempering.
2. Experimental A steel round billet of Ф250 mm × 400 mm (i.e. 0.1C–1.9Mn– 0.8Si–0.5Cr–0.015P–0.009S steel, wt%) was provided by Xining Iron&Steel Co., Ltd of China after austenitizing at 90 ◦ C and oil quenching. Granular structure and CFB/M microstructure were obtained from the central part and the near surface part of the round billet. The two different microstructures resulted from different cooling rates (i.e. about 1 ◦ C/s and 20 ◦ C/s, respectively) during oil quenching. Specimens of Ф13 mm × 90 mm and 10.5 mm × 10.5 mm × 55 mm cut from the round billet were used to undergo tempering. After tempering (tempering time all set as 2 h), standard short gauge tensile specimens (gauge length 40 mm; gauge diameter 8 mm) and Charpy U-notch impact specimens (10 mm × 10 mm × 55 mm) were machined from the tempered specimens for testing of mechanical properties. Microstructural observation was conducted using transmission electron microscopy (TEM, JEOL JEM-2011) and scanning electron microscopy (SEM, JEOL JSM-6301F). The amount of retained austenite was measured by X-ray diffractometer (XRD, TTR–3). CCT curve of the experimental steel was determined by Formastor-D automatic phase transformation tester.
H. Zhang et al. / Materials Science and Engineering A 528 (2011) 920–924
3. Results and discussion 3.1. Granular structure and CFB/M microstructure Fig. 1(a) and (b) shows the granular structure and CFB/M microstructure, respectively. The granular structure contains ferrite matrix and some martensite–austenite islands while the CFB/M microstructure is composed of martensite matrix and a small amount of CFB. The TEM micrograph of the granular structure is shown in Fig. 2(a). The central dark zone represents a martensite–austenite island which is identified as martensite from the diffraction spots in Fig. 2(b). The diffraction ring pattern of martensite could be seen. Notably, retained austenite is not found in the martensite–austenite island through TEM. The quantitative analysis on retained austenite by X-ray diffractometer suggests the amount of retained austenite in the martensite–austenite islands is quite small (<1%). The light zone in Fig. 2(a) is the ferrite matrix of the granular structure whose diffraction spots are shown in Fig. 2(c). Fig. 3 shows CCT curve of the experimental steel. Dur-
921
ing cooling at 1 ◦ C/s, austenite firstly transforms to ferrite within 700–580 ◦ C and then transforms to bainite within 560–370 ◦ C, leading to the formation of the granular structure. During cooling at 20 ◦ C/s, the austenite–bainite transformation (denoted by dash line) can not be detected by the phase transformation tester since only a small amount of bainite is formed during cooling. This corresponds with the microstructure in Fig. 1(b), which mostly contains martensite. 3.2. Effect of tempering temperature on mechanical properties Fig. 4(a) shows the impact toughness of the granular structure and the CFB/M microstructure after tempering, from which it can be seen that the granular structure does not exhibit temper brittleness while the CFB/M microstructure has two temper brittleness zones at around 350 ◦ C and 500 ◦ C. It is generally considered that the temper brittleness of the CFB/M microstructure is mainly caused by the precipitation of plate-shape carbide at around 350 ◦ C [11] or the segregation of sulfur (or phosphorus) at around
Fig. 1. SEM micrographs of (a) granular structure and (b) CFB/M microstructure.
Fig. 2. (a) TEM micrograph of the granular structure; (b) diffraction spots of the central dark zone in (a); (c) diffraction spots of the light zone in (a).
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with the increase of tempering temperature above 500 ◦ C, compared with below 500 ◦ C. This is because ferrite recrystallization and carbide coarsening count a lot during tempering above 500 ◦ C. On the other hand, the tensile strength of the CFB/M microstructure decreases obviously with the increase of tempering temperature within the tempering temperature range of 350–450 ◦ C, which indicates the precipitation, spheroidization and coarsening of carbides affect the tensile strength of the CFB/M microstructure greatly. 3.3. Effect of tempering temperature on granular structure
Fig. 3. CCT curve of the experimental steel (A: austenite; F: ferrite; B: bainite).
500 ◦ C during tempering. Interestingly, the impact toughness of the granular structure discussed in the present work almost remains stable within the tempering temperature range of 300–550 ◦ C. This may be attributed to the characteristic of the granular structure which contains soft phase (i.e. ferrite matrix) and hard phase (i.e. martensite–austenite islands). The carbide precipitation and spheroidization that take place in the martensite–austenite islands during tempering (as addressed in the following section) are considered to have little effect on impact toughness due to the following reasons: (1) martensite–austenite islands distribute discontinuously in the ferrite matrix; (2) the carbides precipitating in martensite–austenite islands during tempering are accordingly discontinuous and in a smaller amount. As a result, coarse plate-shape carbides detrimental to impact toughness are avoided. Besides, the discontinuous islands of granular structure might also relieve the effect of sulfur (or phosphorus) segregation on temper brittleness, which could be suggested from the obvious difference of impact toughness between CFB/M microstructure and granular structure after tempering at 500 ◦ C. When the tempering temperature is higher than 550 ◦ C, the impact toughness increases obviously with the increase of tempering temperature while the tensile strength decreases at the same time. Notably, the change tendency is almost the same for the granular structure and CFB/M microstructure, implying the similarity of the microstructure evolution during tempering above 550 ◦ C. In addition, it can be seen from Fig. 4(b) that the tensile strength of the granular structure decreases more greatly
Fig. 5(a–c) shows the granular structure after tempering at 350 ◦ C, 500 ◦ C and 620 ◦ C, respectively. After tempered at 350 ◦ C, the internal stress might be relieved to some extent and the morphology of the martensite–austenite islands and ferrite matrix change very little. It is well known that plate-shape carbides precipitate in martensite during tempering around 200–300 ◦ C. Although carbides are not seen in Fig. 5(a) through SEM, it can be observed through TEM in Fig. 6(a) that very fine plate-shape carbides (shown by white arrows) precipitate on the islands. If the tempering temperature is set higher, the plate-shape carbides may spheroidize and grow. It can be seen from Fig. 5(b) that spherical carbides are dispersed in martensite–austenite islands. Besides, the evidence of ferrite recovery and recrystallization can be found on the ferrite matrix, which is denoted by black arrows in Fig. 5(b). When the tempering temperature is elevated to 620 ◦ C, the spherical carbides grow coarser (as shown by white arrows in Fig. 5(c)) and the boundary between martensite–austenite islands and ferrite matrix becomes unclear. Fig. 6(b) shows the spherical carbides on the islands (shown by black arrows) after tempering at 620 ◦ C are in the size of 50–250 nm, indicating the growth of spherical carbides during tempering at 620 ◦ C is not fast. In the present work, the impact toughness and tensile strength of the granular structure do not change greatly after tempered below 500 ◦ C. This suggests that the temper stability of the granular structure is high. In addition, the black arrow in Fig. 5(c) shows a crystallized ferrite grain with clear grain boundary, which indicates that the ferrite recrystallization takes place more completely. 3.4. Optimization of the tempering temperature It can be seen from the above that the granular structure in the Mn-series low carbon steel is stable. Once the steel composition and heat treatment are reasonably designed, granular structure can be easily obtained. Due to the inexistence of the temper brittleness within the whole range of tempering temperature, there is no need to avoid temper brittleness during tempering of the granular
Fig. 4. (a) Impact toughness and (b) tensile strength of the granular structure and the CFB/M microstructure after tempering.
H. Zhang et al. / Materials Science and Engineering A 528 (2011) 920–924
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Fig. 5. SEM micrographs of the granular structure after tempering at (a) 350 ◦ C, (b) 500 ◦ C and (c) 620 ◦ C.
Fig. 6. TEM micrograph of the martensite–austenite island after tempering at (a) 350 ◦ C and (b) 620 ◦ C.
structure. Therefore, the performance optimization for the granular structure through tempering is simplified. As for the granular structure in this study, the optimal tempering temperature is located at 300–500 ◦ C because the combination of strength and toughness is the best within this temperature range. Most importantly, when a large engineering component (e.g. round billet with 250 mm in diameter) is produced using the steel in this study, tempering at 400 ◦ C after quenching may ensure excellent mechanical properties for the whole body (from the central part to the near surface part): tensile strength 750–1000 MPa; impact toughness (Aku) 138–154 J. Accordingly, the granular structure can be considered as an excellent microstructure in steels and should be paid attention to during the design of steels in future.
4. Conclusion 1. The granular structure in the Mn-series low carbon steel used in this study does not exhibit temper brittleness which is quite different from common microstructures in steels (e.g. CFB/M and martensite microstructure). 2. The impact toughness of the granular structure remains stable within the tempering temperature range of 300–550 ◦ C. The first kind (i.e. at ∼350 ◦ C) and second kind (i.e. at ∼500 ◦ C) of temper brittleness have been avoided for the granular structure composed of ferrite matrix and martensite– austenite islands. 3. The granular structure has high temper stability which could be favorable for avoiding temper brittleness.
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4. The carbide precipitation and spheroidization in martensite–austenite islands have little effect on the impact toughness of the granular structure. 5. The inexistence of the temper brittleness facilitates the performance optimization through adjusting tempering temperature. When tempered at 400 ◦ C after quenching, a large engineering component produced using the Mn-series low carbon steel (e.g. round billet with 250 mm in diameter) may exhibit a good combination of tensile strength (750–1000 MPa) and impact toughness (Aku, 138–154 J) from the central part to the near surface part. Acknowledgements Electron microscopy analysis was supported by Beijing National Center for Electron Microscopy, Department of Materials Science
and Engineering, Tsinghua University. The authors would also like to express their appreciation to Xining Iron & Steel Co., Ltd of China for supplying the steel used in this study. References [1] L. Habraken, Proceedings of the Fourth International Conference on Electron Microscopy, Springer Verlag, Berlin, 1958. [2] V. Biss, R.L. Cryderman, Metall. Trans. 2 (1971) 2267–2276. [3] P.G. Xu, B.Z. Bai, F.X. Yin, et al., Mater. Sci. Eng. A 385 (2004) 65–73. [4] I.A. Yakubtsov, P. Poruks, J.D. Boyd, Mater. Sci. Eng. A 480 (2008) 109–116. [5] H.S. Fang, B.Z. Bai, X.H. Zheng, et al., Acta Metall. Sin. 22 (1986) 283–288. [6] Y.W. Wang, F.Y. Xu, X.X. Xu, et al., Acta Metall. Sin. 45 (2009) 559–565. [7] F.Y. Xu, B.Z. Bai, H.S. Fang, Acta Metall. Sin. 44 (2008) 1183–1187. [8] Y. Tomita, T. Okawa, Mater. Sci. Technol. 11 (1995) 245–251. [9] Y. Tomita, Mater. Sci. Technol. 11 (1995) 259–263. [10] D.Y. Liu, B.Z. Bai, H.S. Fang, et al., Mater. Sci. Eng. A 371 (2004) 40–44. [11] H.K.D.H. Bhadeshia, D.V. Edmonds, Met. Sci. 13 (1979) 325–334.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The tempering behavior of granular structure in a Mn-series low carbon steel Han Zhang ∗ , Xiaole Cheng, Bingzhe Bai, Hongsheng Fang Key Laboratory for Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Qinghuayuan Street, 100084 Beijing, China
a r t i c l e
i n f o
Article history: Received 23 March 2010 Received in revised form 1 September 2010 Accepted 27 October 2010
Keywords: Granular structure Temper brittleness Toughness Carbides Ferrite
a b s t r a c t The granular structure in a Mn-series low carbon steel composed of ferrite matrix and martensite–austenite islands does not exhibit temper brittleness which is quite different from common microstructures in steels. This characteristic facilitates the performance optimization through adjusting tempering temperature. A good combination of tensile strength (750–1000 MPa) and impact toughness (Aku, 138–154 J) can be obtained after quenching and tempering at 400 ◦ C for a round billet with 250 mm in diameter. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the late 1950’s, Habraken [1] firstly found the existence of a special microstructure (named granular bainite) in low and medium carbon alloyed steels, which consists of bainitic ferrite matrix and island constituents. Previous investigations suggest that the islands in granular bainite are originally carbon-rich austenite and transform partially to martensite during subsequent cooling process. Therefore, the islands are composed of martensite and retained austenite when cooled to room temperature (generally called martensite–austenite island) [2]. In recent years, many steels with granular bainite as the dominant phase have been developed [3,4]. Besides, a granular structure composed of pro-eutectoid ferrite and martensite–austenite island was firstly found by Fang et al. [5]. Subsequently, the transformation of granular structure as well as the relationship between the granular structure and its mechanical properties has been studied in detail [6,7]. On the other hand, the microstructure of carbide-free bainite/martensite (CFB/M) was found to possess an excellent combination of strength and toughness [8,9]. It is known that the CFB lath is composed of bainitic ferrite and filmy carbon-enriched retained austenite. Although the retained austenite located inside the CFB laths was beneficial for improving the temper resistance, the temper brittleness of the CFB/M microstructure could not be avoided after tempering at certain temperatures [10]. In fact, temper brittleness has long been a troublesome problem in steels.
∗ Corresponding author. Fax: +86 010 62771160. E-mail address: [email protected] (H. Zhang). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.097
Until now, little has been reported about the impact toughness of the granular structure after tempering. So the present work will lay emphasis on the impact toughness of the granular structure after tempering at different temperature and further put forward a novel granular structure without temper brittleness. Besides, special attention is paid to the evolution of the granular structure during tempering.
2. Experimental A steel round billet of Ф250 mm × 400 mm (i.e. 0.1C–1.9Mn– 0.8Si–0.5Cr–0.015P–0.009S steel, wt%) was provided by Xining Iron&Steel Co., Ltd of China after austenitizing at 90 ◦ C and oil quenching. Granular structure and CFB/M microstructure were obtained from the central part and the near surface part of the round billet. The two different microstructures resulted from different cooling rates (i.e. about 1 ◦ C/s and 20 ◦ C/s, respectively) during oil quenching. Specimens of Ф13 mm × 90 mm and 10.5 mm × 10.5 mm × 55 mm cut from the round billet were used to undergo tempering. After tempering (tempering time all set as 2 h), standard short gauge tensile specimens (gauge length 40 mm; gauge diameter 8 mm) and Charpy U-notch impact specimens (10 mm × 10 mm × 55 mm) were machined from the tempered specimens for testing of mechanical properties. Microstructural observation was conducted using transmission electron microscopy (TEM, JEOL JEM-2011) and scanning electron microscopy (SEM, JEOL JSM-6301F). The amount of retained austenite was measured by X-ray diffractometer (XRD, TTR–3). CCT curve of the experimental steel was determined by Formastor-D automatic phase transformation tester.
H. Zhang et al. / Materials Science and Engineering A 528 (2011) 920–924
3. Results and discussion 3.1. Granular structure and CFB/M microstructure Fig. 1(a) and (b) shows the granular structure and CFB/M microstructure, respectively. The granular structure contains ferrite matrix and some martensite–austenite islands while the CFB/M microstructure is composed of martensite matrix and a small amount of CFB. The TEM micrograph of the granular structure is shown in Fig. 2(a). The central dark zone represents a martensite–austenite island which is identified as martensite from the diffraction spots in Fig. 2(b). The diffraction ring pattern of martensite could be seen. Notably, retained austenite is not found in the martensite–austenite island through TEM. The quantitative analysis on retained austenite by X-ray diffractometer suggests the amount of retained austenite in the martensite–austenite islands is quite small (<1%). The light zone in Fig. 2(a) is the ferrite matrix of the granular structure whose diffraction spots are shown in Fig. 2(c). Fig. 3 shows CCT curve of the experimental steel. Dur-
921
ing cooling at 1 ◦ C/s, austenite firstly transforms to ferrite within 700–580 ◦ C and then transforms to bainite within 560–370 ◦ C, leading to the formation of the granular structure. During cooling at 20 ◦ C/s, the austenite–bainite transformation (denoted by dash line) can not be detected by the phase transformation tester since only a small amount of bainite is formed during cooling. This corresponds with the microstructure in Fig. 1(b), which mostly contains martensite. 3.2. Effect of tempering temperature on mechanical properties Fig. 4(a) shows the impact toughness of the granular structure and the CFB/M microstructure after tempering, from which it can be seen that the granular structure does not exhibit temper brittleness while the CFB/M microstructure has two temper brittleness zones at around 350 ◦ C and 500 ◦ C. It is generally considered that the temper brittleness of the CFB/M microstructure is mainly caused by the precipitation of plate-shape carbide at around 350 ◦ C [11] or the segregation of sulfur (or phosphorus) at around
Fig. 1. SEM micrographs of (a) granular structure and (b) CFB/M microstructure.
Fig. 2. (a) TEM micrograph of the granular structure; (b) diffraction spots of the central dark zone in (a); (c) diffraction spots of the light zone in (a).
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with the increase of tempering temperature above 500 ◦ C, compared with below 500 ◦ C. This is because ferrite recrystallization and carbide coarsening count a lot during tempering above 500 ◦ C. On the other hand, the tensile strength of the CFB/M microstructure decreases obviously with the increase of tempering temperature within the tempering temperature range of 350–450 ◦ C, which indicates the precipitation, spheroidization and coarsening of carbides affect the tensile strength of the CFB/M microstructure greatly. 3.3. Effect of tempering temperature on granular structure
Fig. 3. CCT curve of the experimental steel (A: austenite; F: ferrite; B: bainite).
500 ◦ C during tempering. Interestingly, the impact toughness of the granular structure discussed in the present work almost remains stable within the tempering temperature range of 300–550 ◦ C. This may be attributed to the characteristic of the granular structure which contains soft phase (i.e. ferrite matrix) and hard phase (i.e. martensite–austenite islands). The carbide precipitation and spheroidization that take place in the martensite–austenite islands during tempering (as addressed in the following section) are considered to have little effect on impact toughness due to the following reasons: (1) martensite–austenite islands distribute discontinuously in the ferrite matrix; (2) the carbides precipitating in martensite–austenite islands during tempering are accordingly discontinuous and in a smaller amount. As a result, coarse plate-shape carbides detrimental to impact toughness are avoided. Besides, the discontinuous islands of granular structure might also relieve the effect of sulfur (or phosphorus) segregation on temper brittleness, which could be suggested from the obvious difference of impact toughness between CFB/M microstructure and granular structure after tempering at 500 ◦ C. When the tempering temperature is higher than 550 ◦ C, the impact toughness increases obviously with the increase of tempering temperature while the tensile strength decreases at the same time. Notably, the change tendency is almost the same for the granular structure and CFB/M microstructure, implying the similarity of the microstructure evolution during tempering above 550 ◦ C. In addition, it can be seen from Fig. 4(b) that the tensile strength of the granular structure decreases more greatly
Fig. 5(a–c) shows the granular structure after tempering at 350 ◦ C, 500 ◦ C and 620 ◦ C, respectively. After tempered at 350 ◦ C, the internal stress might be relieved to some extent and the morphology of the martensite–austenite islands and ferrite matrix change very little. It is well known that plate-shape carbides precipitate in martensite during tempering around 200–300 ◦ C. Although carbides are not seen in Fig. 5(a) through SEM, it can be observed through TEM in Fig. 6(a) that very fine plate-shape carbides (shown by white arrows) precipitate on the islands. If the tempering temperature is set higher, the plate-shape carbides may spheroidize and grow. It can be seen from Fig. 5(b) that spherical carbides are dispersed in martensite–austenite islands. Besides, the evidence of ferrite recovery and recrystallization can be found on the ferrite matrix, which is denoted by black arrows in Fig. 5(b). When the tempering temperature is elevated to 620 ◦ C, the spherical carbides grow coarser (as shown by white arrows in Fig. 5(c)) and the boundary between martensite–austenite islands and ferrite matrix becomes unclear. Fig. 6(b) shows the spherical carbides on the islands (shown by black arrows) after tempering at 620 ◦ C are in the size of 50–250 nm, indicating the growth of spherical carbides during tempering at 620 ◦ C is not fast. In the present work, the impact toughness and tensile strength of the granular structure do not change greatly after tempered below 500 ◦ C. This suggests that the temper stability of the granular structure is high. In addition, the black arrow in Fig. 5(c) shows a crystallized ferrite grain with clear grain boundary, which indicates that the ferrite recrystallization takes place more completely. 3.4. Optimization of the tempering temperature It can be seen from the above that the granular structure in the Mn-series low carbon steel is stable. Once the steel composition and heat treatment are reasonably designed, granular structure can be easily obtained. Due to the inexistence of the temper brittleness within the whole range of tempering temperature, there is no need to avoid temper brittleness during tempering of the granular
Fig. 4. (a) Impact toughness and (b) tensile strength of the granular structure and the CFB/M microstructure after tempering.
H. Zhang et al. / Materials Science and Engineering A 528 (2011) 920–924
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Fig. 5. SEM micrographs of the granular structure after tempering at (a) 350 ◦ C, (b) 500 ◦ C and (c) 620 ◦ C.
Fig. 6. TEM micrograph of the martensite–austenite island after tempering at (a) 350 ◦ C and (b) 620 ◦ C.
structure. Therefore, the performance optimization for the granular structure through tempering is simplified. As for the granular structure in this study, the optimal tempering temperature is located at 300–500 ◦ C because the combination of strength and toughness is the best within this temperature range. Most importantly, when a large engineering component (e.g. round billet with 250 mm in diameter) is produced using the steel in this study, tempering at 400 ◦ C after quenching may ensure excellent mechanical properties for the whole body (from the central part to the near surface part): tensile strength 750–1000 MPa; impact toughness (Aku) 138–154 J. Accordingly, the granular structure can be considered as an excellent microstructure in steels and should be paid attention to during the design of steels in future.
4. Conclusion 1. The granular structure in the Mn-series low carbon steel used in this study does not exhibit temper brittleness which is quite different from common microstructures in steels (e.g. CFB/M and martensite microstructure). 2. The impact toughness of the granular structure remains stable within the tempering temperature range of 300–550 ◦ C. The first kind (i.e. at ∼350 ◦ C) and second kind (i.e. at ∼500 ◦ C) of temper brittleness have been avoided for the granular structure composed of ferrite matrix and martensite– austenite islands. 3. The granular structure has high temper stability which could be favorable for avoiding temper brittleness.
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4. The carbide precipitation and spheroidization in martensite–austenite islands have little effect on the impact toughness of the granular structure. 5. The inexistence of the temper brittleness facilitates the performance optimization through adjusting tempering temperature. When tempered at 400 ◦ C after quenching, a large engineering component produced using the Mn-series low carbon steel (e.g. round billet with 250 mm in diameter) may exhibit a good combination of tensile strength (750–1000 MPa) and impact toughness (Aku, 138–154 J) from the central part to the near surface part. Acknowledgements Electron microscopy analysis was supported by Beijing National Center for Electron Microscopy, Department of Materials Science
and Engineering, Tsinghua University. The authors would also like to express their appreciation to Xining Iron & Steel Co., Ltd of China for supplying the steel used in this study. References [1] L. Habraken, Proceedings of the Fourth International Conference on Electron Microscopy, Springer Verlag, Berlin, 1958. [2] V. Biss, R.L. Cryderman, Metall. Trans. 2 (1971) 2267–2276. [3] P.G. Xu, B.Z. Bai, F.X. Yin, et al., Mater. Sci. Eng. A 385 (2004) 65–73. [4] I.A. Yakubtsov, P. Poruks, J.D. Boyd, Mater. Sci. Eng. A 480 (2008) 109–116. [5] H.S. Fang, B.Z. Bai, X.H. Zheng, et al., Acta Metall. Sin. 22 (1986) 283–288. [6] Y.W. Wang, F.Y. Xu, X.X. Xu, et al., Acta Metall. Sin. 45 (2009) 559–565. [7] F.Y. Xu, B.Z. Bai, H.S. Fang, Acta Metall. Sin. 44 (2008) 1183–1187. [8] Y. Tomita, T. Okawa, Mater. Sci. Technol. 11 (1995) 245–251. [9] Y. Tomita, Mater. Sci. Technol. 11 (1995) 259–263. [10] D.Y. Liu, B.Z. Bai, H.S. Fang, et al., Mater. Sci. Eng. A 371 (2004) 40–44. [11] H.K.D.H. Bhadeshia, D.V. Edmonds, Met. Sci. 13 (1979) 325–334.