Effect of Nb on the Continuous Cooling Transformation Rule and Microstructure, Mechanical Properties of Ti-Mo Bearing Microalloyed Steel

doi:10.11900/0412.1961.2016.00437

Acta Metall Sin

2017, Vol. 53

Issue (6): 648-656 DOI: 10.11900/0412.1961.2016.00437

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Effect of Nb on the Continuous Cooling Transformation Rule and Microstructure, Mechanical Properties of Ti-Mo Bearing Microalloyed Steel

Xianling HE¹,Gengwei YANG¹(

),Xinping MAO^1,²,Chibin YU¹,Chuanli DA¹,Xiaolong GAN^1,²

1 State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
2 Research and Development Institute, Wuhan Iron and Steel (Group) Co., Ltd., Wuhan 430083, China

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Xianling HE,Gengwei YANG,Xinping MAO,Chibin YU,Chuanli DA,Xiaolong GAN. Effect of Nb on the Continuous Cooling Transformation Rule and Microstructure, Mechanical Properties of Ti-Mo Bearing Microalloyed Steel. Acta Metall Sin, 2017, 53(6): 648-656.

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Abstract

In recent years, with the fast development of automotive industry, more and more attention has been focused on developing high strength automobile steels with excellent formability. The microalloying elements, such as Nb, Ti, Mo, which can facilitate grain refinement and precipitation hardening, were added into steels to achieve high strength and good formability. The Ti-Mo and Ti-Mo-Nb microalloyed high strength ferritic steel were developed. In this work, the continuous cooling transformation curves (CCT) of Ti-Mo and Ti-Mo-Nb steels were obtained. And the effect of Nb on the microstructure and mechanical properties of Ti-Mo low carbon microalloyed steel was investigated by means of SEM, HRTEM and EDS. The results showed that Nb could raise the A_c1 and A_c3 temperatures, and restrain the ferrite-pearlite and bainite transformation. Moreover, Nb could also refine the microstructure and harden the matrix of steel which attributed to the strain-induced precipitation of nano-sized (Ti, Nb, Mo)C particles identified by HRTEM and EDS. It was also found that the strain-induced precipitation of (Ti, Mo)C was existed in the Ti-Mo steel. And both of (Ti, Mo)C and (Ti, Nb, Mo)C particles were NaCl type structure. The lattice constants/the average particle sizes of (Ti, Mo)C and (Ti, Nb, Mo)C were 0.432 nm and 0.436 nm / 12.11 nm and 8.69 nm, respectively.

Key words: Ti microalloyed steel CCT curve Nb nano-sized precipitation hardness

Received: 05 October 2016

Fund: Supported by China Postdoctoral Science Foundation (No.2014M562072)

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	Xianling HE
	Gengwei YANG
	Xinping MAO
	Chibin YU
	Chuanli DA
	Xiaolong GAN

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00437 OR https://www.ams.org.cn/EN/Y2017/V53/I6/648

Table 1 Chemical compositions of the microalloyed steels (mass fraction / %)

Fig.1 Schematic of dynamic continuous cooling transformation (CCT) curve

Fig.2 Temperature-expansion curves of Ti-Mo and Ti-Mo-Nb steels

Fig.3 SEM images of Ti-Mo steel cooled by different cooling rates (P—pearlite, PF—polygonal ferrite, GB—granular bainite, LB—lath bainite) (a) 0.5 ℃/s (b) 1 ℃/s (c) 5 ℃/s (d) 10 ℃/s (e) 20 ℃/s (f) 30 ℃/s (g) 50 ℃/s

Fig.4 SEM images of Ti-Mo-Nb steel cooled by different cooling rates(a) 0.5 ℃/s (b) 1 ℃/s (c) 5 ℃/s (d) 10 ℃/s (e) 20 ℃/s (f) 30 ℃/s (g) 50 ℃/s

Fig.5 Dynamic CCT curves of Ti-Mo (a) and Ti-Mo-Nb (b) steels (F—ferrite, B—bainite, CR—cooling rate, A_c1—start temperature of austenite formation during heating, A_c3—finish temperature of austenite formation during heating)

Fig.6 Hardness of Ti-Mo and Ti-Mo-Nb steels after cooling at different cooling rates

Fig.7 Changes of precipitate volume fraction with temperature in Ti-Mo and Ti-Mo-Nb steels

Fig.8 Morphologies (a, c) and EDS analyses (circles) (b, d) of the precipitates in Ti-Mo (a, b) and Ti-Mo-Nb (c, d) steels cooling at 50 ℃/s

Fig.9 Low (a, d) and high (b, e) magnified HRTEM images and corresponding fast Fourier transformation (FFT) diffractograms (c, f) of the interphase precipitation carbides in Ti-Mo (a~c) and Ti-Mo-Nb (d~e) steels (d—interplanar spacing)

Fig.10 Size percentage of the second particles of Ti-Mo and Ti-Mo-Nb steels at cooling rate of 50 ℃/s

Fig.11 Austenite grain boundary diagrams of Ti-Mo (a) and Ti-Mo-Nb (b) steels after deformation

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