Effect of annealing temperature on microstructure and properties of a bimodal ultrafine-grained high-strength steel

doi:10.13251/j.issn.0254-6051.2025.05.024

Abstract

Abstract: A low carbon micro/nano-scale ultrafine-grained (UFG) steel with bimodal grain microstructure composed of ferrite and martensite was prepared from original coarse-grained steel by intercritical quenching from 780 ℃ and holding for 3 min. The effect of annealing temperature (400-650 ℃) on the microstructure and properties of the bimodal UFG steel was investigated. The results show that the so-prepared bimodal UFG steel with the ferrite/martensite structure exhibits tensile strengths of 773.47 to 1159.30 MPa and total elongations at fracture of 13.27% to 21.09%, representing a significant improvement in properties compared to the original coarse-grained steel. The optimal annealing temperature for the bimodal micro/nano-scale UFG steel is 600 ℃, at which the tensile strength is 891.33 MPa, the total elongation at fracture is 19.23%, and the product of strength and elongation (PSE) is 17.14 GPa·%. Compared to the original coarse-grained steel, the strength is nearly doubled with minimal sacrifice in elongation, and the PSE is significantly improved. In addition, as the annealing temperature increases, the bimodal distribution shifts towards larger sizes, with the peak corresponding to larger grains shifting upwards significantly. Consequently, the strength of the bimodal micro/nano-scale UFG steel gradually decreases, while the total elongation at fracture increases. The volume fractions of small-sized (<1 μm) and large-sized (>1 μm) ferrite grains influence the strength-ductility combination of the micro/nano UFG steel. Furthermore, the deformed ferrite preferentially undergo static recrystallization due to its faster recrystallization kinetics compared to quenched martensite. Additionally, the cementite particles precipitated from the martensite hinder the static recrystallization of the deformed martensite.

Key words: bimodal grain, ultrafine-grained (UFG) steel, annealing temperature, ferrite, recrystallization, mechanical properties

CLC Number:

TG142.1

Hua Xuebing, Yuan Qing, Zhang Qingxiao, Xiong Le. Effect of annealing temperature on microstructure and properties of a bimodal ultrafine-grained high-strength steel[J]. Heat Treatment of Metals, 2025, 50(5): 152-159.

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