Precipitation Behavior and Microstructural Evolution of Ferritic Ti-V-Mo Complex Microalloyed Steel

doi:10.1007/s40195-018-0726-4

Abstract

Abstract:

Precipitation behavior of (Ti, V, Mo)C and microstructural evolution of the ferritic Ti-V-Mo complex microalloyed steel were investigated through changing coiling temperature (CT). It is demonstrated that the strength of the Ti-V-Mo microalloyed steel can be ascribed to the combination of grain refinement hardening and precipitation hardening. The variation of hardness (from 318 to 415 HV, then to 327 HV) with CT (from 500 to 600-625 °C, then to 700 °C) was attributed to the changes of volume fraction and particle size of (Ti, V, Mo)C precipitates. The optimum CT was considered as 600-625 °C, at which the maximum hardness value (415 HV) can be obtained. It was found that the atomic ratios of Ti, V and Mo in (Ti, V, Mo)C carbides were changed as the CT increased. The precipitates with the size of<10 nm were the V-rich particles at higher CT of 600 and 650 °C, while the Ti-rich particles were observed at lower CT of 500 and 550 °C. Theoretical calculations indicated that the maximum nucleation rate of (Ti, V, Mo)C in ferrite matrix occurred around 630 °C, which was consistent with the 625 °C obtained from experiment results.

Key words: Nano-sized precipitates, Precipitation hardening, Ti-V-Mo, Coiling temperature, Ferrite

Ke Zhang, Hui Wang, Xin-Jun Sun, Feng-Li Sui, Zhao-Dong Li, En-Xiang Pu, Zheng-Hai Zhu, Zhen-Yi Huang, Hong-Bo Pan, Qi-Long Yong. Precipitation Behavior and Microstructural Evolution of Ferritic Ti-V-Mo Complex Microalloyed Steel[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(9): 997-1005.

Figures/Tables 10

Fig. 1 Schematic illustration describing TMCP schedule.

Fig. 2 OM images of Ti-V-Mo steels at CTs of 500 °C a, 525 °C b, 550 °C c, 575 °C d, 600 °C e, 625 °C f, 650 °C g, 675 °C h, 700 °C i.

Fig. 3 SEM images of Ti-V-Mo steels at CTs of 500 °C a, 550 °C b, 600 °C c, 650 °C d.

Fig. 4 EBSD images of Ti-V-Mo steels at CTs of 500 °C a, 550 °C b, 600 °C c, 650 °C d, 700 °C e.

Fig. 5 TEM images of nano-scale particle precipitation at CTs of 500 °C a, 550 °C b, 600 °C c, 650 °C d, 700 °C e, particle size distribution of different CTs f.

Fig. 6 Atom ratios of Ti, V or Mo in (Ti, V, Mo)C particles at different CTs.

Fig. 7 HRTEM image of a particle of ferrite matrix at CT of 600 °C a, magnified HRTEM image of a, b, corresponding fast Fourier transformed diffractogram c, EDS analysis of particle in a, d.

Fig. 8 Variations of hardness as a function of different CTs.

Fig. 9 a PTT, b NrT curves of (Ti, V, Mo)C precipitated from ferrite (t0.05da: time used to complete volume fraction 5% of the total (Ti, V, Mo)C precipitated; t0da: beginning time of (Ti, V, Mo)C precipitated; lg(t0.05da/t0da) refers to the relative beginning time of (Ti, V, Mo)C precipitated; I nucleation rate, K constant independent of temperature) .

Fig. 10 Coarsening kinetics of (Ti, V, Mo)C in ferrite.

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