Development of Ti-V-Mo Complex Microalloyed Hot-Rolled 900-MPa-Grade High-Strength Steel

doi:10.1007/s40195-015-0243-7

Abstract

Abstract:

A new Ti-V-Mo complex microalloyed hot-rolled high-strength steel sheet was developed by controlling a thermo-mechanical controlled processing (TMCP) schedule, in particular with variants in coiling temperature. The effects of coiling temperature (CT) on various hardening mechanisms and mechanical properties of Ti-V-Mo complex microalloyed high-strength low-alloy steels were investigated. The results revealed that the steels are mainly strengthened by a combined effect of ferrite grain refinement hardening and precipitation hardening. The variation in simulated coiling temperature causes a significant difference in strength, which is mainly attributed to different precipitation hardening increment contributions. When the CT is 600 °C, the experimental steel has the best mechanical properties: ultimate tensile strength (UTS) 1000 MPa, yield strength (YS) 955 MPa and elongation (EL) 17%. Moreover, about 82 wt% of the total precipitates are nano-sized carbide particles with diameter of 1-10 nm, which is randomly dispersed in the ferrite matrix. The nano-sized carbide particles led to a strong precipitation hardening increment up to 310 MPa.

Key words: Hot-rolled high-strength steel, Strengthening mechanism, Nano-sized carbide, Precipitation hardening, Coiling temperature

Ke Zhang, Zhao-Dong Li, Xin-Jun Sun, Qi-Long Yong, Jun-Wei Yang, Yuan-Mei Li, Pei-Lin Zhao. Development of Ti-V-Mo Complex Microalloyed Hot-Rolled 900-MPa-Grade High-Strength Steel[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(5): 641-648.

Figures/Tables 9

Fig. 1 Schematic illustration describing the TMCP schedule

Fig. 2 SEM images showing microstructures of the samples after coiling simulation at different CTs: a 550 °C; b 600 °C; c 650 °C

Fig. 3 Microstructures of the samples after coiling simulation at 550 °C a, 600 °C b, 650 °C c, where black and red lines indicate the high misorientation angle boundaries (θ ≥ 15°) and low misorientation angle boundaries (2° ≤ θ < 15°), respectively

Table 1 Mechanical properties, AVGS, and DSDs of the steel at different CTs

CT (°C)	YS (MPa)	UTS (MPa)	EL (%)	AVGS (μm)	DSD (m^-2)
550	815	930	17.8	2.60 (0.9)	2.0054 × 10¹³
600	955	1000	17.0	2.32 (0.5)	1.5293 × 10¹³
650	885	959	18.5	2.50 (0.7)	1.5113 × 10¹³

Fig. 4 a TEM image shows the supersaturated precipitations of carbides in the sample coiled at 600 °C; b TEM image shows the grain boundary with carbides in the sample coiled at 600 °C; c EDS spectrum of a precipitate in a; d TEM image shows precipitates randomly in the ferrite matrix of the sample coiled at 550 °C; e TEM image shows precipitates randomly in the ferrite matrix of the sample coiled at 600 °C; f TEM image shows precipitates randomly in the ferrite matrix of the sample coiled at 650 °C

Table 2 Quantitative analysis results of precipitates coiling at different CTs (in wt%)

CT (°C)	MC					M₃C
CT (°C)	Ti	Mo	V	C	∑	Fe	Mn	Cr	Mo	V	C	∑
550	0.034	0.041	0.023	0.019	0.117	0.378	0.054	0.021	0.042	0.012	0.035	0.542
600	0.064	0.085	0.060	0.041	0.250	0.079	0.010	0.013	0.025	0.009	0.030	0.166
650	0.062	0.130	0.095	0.054	0.341	0.168	0.030	0.010	0.029	0.010	0.017	0.264

Fig. 5 Mass fraction of MC particles distributed in different size intervals

Table 3 Comparison of YS between experimental results and the predictions with Eq. (1) (in MPa)

CT (°C) name	σ₀	σ_s	σ_g	σ_p	σ_d	σ_cal	σ_exp
550	48	221	341	124	89	823	815
600	48	141	361	310	78	938	955
650	48	140	347	263	77	875	885

Fig. 6 Comparison of YS between the experimental results and the predictions with Eq. (1) for the steel at different CTs

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