Effect of V-Ti Addition on Microstructure Evolution and Mechanical Properties of Hot-Rolled Transformation-Induced Plasticity Steel

doi:10.1007/s40195-018-0796-3

Abstract

Abstract:

In order to reveal the effect of V-Ti addition on the microstructure evolution and the mechanical properties of hot-rolled transformation-induced plasticity (TRIP) steel, two steels with 0.072V-0.051Ti steel (Bear-V-Ti steel) and 0.001V-0.001Ti steel (Free-V-Ti steel) were designed, respectively, and the comparison analyses were carried out by performing thermodynamic calculation and an experiment. With the thermodynamic calculation, the critical annealing temperature of a large fraction of retained austenite (~ 51%) obtained via solute enrichment was determined, and an optimized quenching at 650 °C and tempering at 200 °C adopted on the as-hot-rolled steel. The results show that the V-Ti TRIP steel displays more optimum mechanical stability during the tensile deformation, since the fraction and the mechanical stability of retained austenite are improved and the microstructure is also ultrarefined by V-Ti alloy precipitation. The yield strength of Bear-V-Ti steel increases from 650 to 800 MPa, and the ductility reaches 37%, showing that the comprehensive mechanical properties are greatly improved.

Key words: Medium manganese steel, Microstructure evolution, Mechanical properties, V-Ti

Hai-Cun Yu, Zhao-Zhen Cai, Gui-Qin Fu, Miao-Yong Zhu. Effect of V-Ti Addition on Microstructure Evolution and Mechanical Properties of Hot-Rolled Transformation-Induced Plasticity Steel[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(3): 352-360.

Figures/Tables 10

Table 1 Chemical composition of two experimental steels (wt%)

Steel	C	Mn	Al	V	Ti	P	S	N
Free-V-Ti	0.13	6.22	1.38	-	-	≤ 0.04	≤ 0.04	0.004
Bear-V-Ti	0.13	6.24	1.37	0.072	0.051	≤ 0.04	≤ 0.04	0.004

Fig. 1 Phase fraction of experimental steel predicted by Thermo-Calc software a and schematic illustration of heat treatment process b

Fig. 2 Thermodynamics calculations for Free-V-Ti steel and Bear-V-Ti steel based on TCFE7 database of Thermo-Calc: a C content in austenite; b Mn content in austenite; c Al content in austenite; d predicted fraction of retained austenite as a function of temperature

Fig. 3 SEM micrographs of hot-rolled Free-V-Ti steel samples after quenching at different temperatures: a 600 °C; b 650 °C; c 700 °C; d 750 °C

Fig. 4 SEM micrographs of hot-rolled Bear-V-Ti steel samples after quenching at different temperatures: a 600 °C; b 650 °C; c 700 °C; d 750 °C

Fig. 5 TEM micrographs of Bear-V-Ti after heat-treated at 650 °C, followed by tempering at 200 °C: a bright-field image; b dark-field image; c morphologies of precipitates; d EDS analysis results of small precipitates

Fig. 6 Mechanical properties of hot-rolled samples: a true strain-stress curves of Free-V-Ti steel; b true strain-stress curves of Bear-V-Ti steel; c UTS and TEL of Free-V-Ti steel; d UTS and TEL of Bear-V-Ti steel

Fig. 7 Measured austenite fractions of undeformed and fractured samples and austenite transformation ratio at 600 °C, 650 °C, 700 °C, and 750 °C: a XRD patterns of Bear-V-Ti steel at 650 °C; b XRD patterns of Free-V-Ti steel at 650 °C; c measured austenite fractions; d austenite transformation ratio

Fig. 8 Tensile properties and micrographs of hot-rolled samples: a yield strength of Free-V-Ti steel and Bear-V-Ti steel; b UTS × TEL of Free-V-Ti steel and Bear-V-Ti steel; c OM micrograph of Free-V-Ti steel at 650 °C; d OM micrograph of Bear-V-Ti steel at 650 °C

Fig. 9 True strain-stress plot a and work-hardening rate at 650 °C b of Bear-V-Ti and Free-V-Ti steels

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