Effect of final cooling temperature on microstructure and mechanical properties of low yield ratio composite precipitation strengthening steel

doi:10.13251/j.issn.0254-6051.2021.04.023

Abstract

Abstract: By using thermo-mechanical simulation, the microstructure of a composite precipitation strengthening steel with low yield ratio under various cooling rates and the corresponding dynamic continuous cooling transformation (CCT) curves were determined. Then the effects of different final cooling temperature on mechanical properties were studied by property comparison, and that on microstructure by means of OM, SEM and TEM. The results show that the dominant microstructure changes from granular bainite into lath bainite with the increase of cooling rate and no ferrite is observed, which shows a good hardenability of the steel. As the final cooling temperature increases from 400 ℃ to 450 ℃, the lath substructure coarsens and the dislocation density decreases, while accelerated diffusion of alloy elements leads to higher number of Cu-rich particles and Nb/Ti carbides, resulting in enhanced precipitation strengthening effect. The optimized final cooling temperature is 450 ℃, at which the steel has a larger proportion of granular bainite, thus resulting a good match of high strength and low yield ratio.

Key words: final cooling temperature, Cu-rich phase, carbides, low yield ratio, lath substructure

CLC Number:

TG355

Ma Changwen, Ma Longteng, Wang Yanfeng, Yang Yongda, Han Chengliang. Effect of final cooling temperature on microstructure and mechanical properties of low yield ratio composite precipitation strengthening steel[J]. Heat Treatment of Metals, 2021, 46(4): 131-137.

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