CCT curve and phase transformation behavior of 80QL steel

doi:10.13251/j.issn.0254-6051.2025.05.015

Abstract

Abstract: Continuous cooling transformation test of 80QL steel at deformation temperature of 950 ℃, deformation rate of 10 s^-1, deformation amount of 40%, and cooling rate ranging from 0.5 ℃/s to 80 ℃/s was carried out by using Gleeble-3500 thermal simulation machine. The transformation point temperature was measured by dilatometry method and the CCT curve was plotted. The microstructure evolution under different cooling rates was investigated by using SEM and EBSD, and Rockwell hardness was measured. The results show that the Ac₁ temperature of the 80QL steel is 729 ℃, and the Ac₃ temperature is 886 ℃. When the cooling rate is in the range of 0.5-5 ℃/s, only pearlite transformation occurs. With the increase of cooling rate, the lamellar spacing of pearlite decreases from 138 nm to 127 nm, and the grain size decreases from 9.15 μm to 6.64 μm. When the cooling rate is within the range of 10-20 ℃/s, both pearlite and bainite phase transformations occur simultaneously, and with the increase of cooling rate, the bainite content increases. When the cooling rate reaches 30 ℃/s, martensitic transformation occurs, and the martensite content gradually increases with the further increase of cooling rate. When the cooling rate is 80 ℃/s, it is all martensite and the hardness reaches 64 HRC. According to the EBSD test results, as the cooling rate increases, the grain size decreases and the dislocation density increases, which is also the reason for the increase in hardness.

Key words: 80QL steel, continuous cooling transformation, phase transformation behavior, grain size, dislocation density

CLC Number:

TG142.1

Wang Hongfeng, Han Shudong, Wang Qingjuan, Wang Qingren, Xu Rong, Zhang Wei, Hu Nan. CCT curve and phase transformation behavior of 80QL steel[J]. Heat Treatment of Metals, 2025, 50(5): 94-100.

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