Effects of Nb content and deformation on reheated austenite grain size and distribution of 800 MPa high strength steel for hydropower station before quenching

doi:10.13251/j.issn.0254-6051.2023.04.001

Abstract

Abstract: Effects of Nb content and hot deformation amount on austenite grain size and distribution of 800 MPa grade high strength tested steel for hydropower station during reheating for quenching were studied by using a Gleeble 3500 testing machine to carry out the combined simulation test of two-stage deformation at 1000 ℃+820 ℃ and subsequent reheating at 900 ℃ for different time, and the particle size and distribution of deformation-induced Nb(C,N) precipitates were observed by transmission electron microscopy. The results show that the grain size of as-deformed austenite (D) has an important heredity effect on the grain size of as-reheated austenite (D′), and there is a functional relationship between the two and the reheated holding time (t) at 900 ℃: D′=(1.0057D-6.9785)×(t/300)^0.215, which can be used to predict the grain size (D′) for the 800 MPa high-strength steel with high accuracy. The increase of Nb content reduces both the grain sizes D and D′, and also improves grain size distributions and reduces the probability of individual coarse grains. Under the common industrial quenching heating system, adding 0.03%Nb and 0.05%Nb have very similar grain refining effect, then considering its economy, adding 0.03%Nb is preferred. The TEM observation results show that in the as-deformed specimens, a large number of Nb(C,N) particles with the size of 10-30 nm are observed, the number and density of particles increase with the increase of Nb content, but the particle size does not increase significantly. Based on thermodynamic calculation and comprehensive consideration of particle size and formation time, it is inferred that these particles are deformation-inducted Nb(C,N) precipitates formed at 1000 ℃.

Key words: high strength steel for hydropower station, Nb microalloying, hot deformation, reheating for quenching, grain refining

CLC Number:

TG156.1

Zou Yang, Zhang Suyuan, Zhang Xuefeng, Zhang Yuefei, Wang Kun, Liu Guoquan. Effects of Nb content and deformation on reheated austenite grain size and distribution of 800 MPa high strength steel for hydropower station before quenching[J]. Heat Treatment of Metals, 2023, 48(4): 1-9.

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