Influence of bismuth on solid-state phase transformation and microstructure of ultra-low carbon steel during cooling

doi:10.13251/j.issn.0254-6051.2024.10.032

Abstract

Abstract: Influence of bismuth on solid-state phase transformation and microstructure of ultra-low carbon steel during cooling was studied by means of optical microscope (OM), scanning electron microscope (SEM-EDS) and differential scanning calorimetry (DSC). The results indicate that the addition of 0.06% bismuth to the tested ultra-low carbon steel delays the austenite-to-ferrite phase transformation during cooling and increases the activation energy of the transformation. The phase transformation start and end temperatures are decreased by 0-16 ℃ and 0-31 ℃, respectively, under different cooling rates, and the activation energy of the phase transformation is increasesd by 1364 kJ/mol. During cooling, bismuth tends to segregate at the austenite and ferrite grain boundaries, refining the austenite and ferrite grains. As the cooling rate increases, the ferrite grain size after transformation in the tested steel gradually decreases, with the average grain size reducing from 77.64 μm at 10 ℃/min to 66.03 μm at 20 ℃/min. Additionally, compared to the bismuth-free steel, the ultra-low carbon steel containing 0.06% bismuth has smaller ferrite grain size after transformation at the same cooling rates, with the average grain size reductions ranging from 3.28 μm to 13.74 μm.

Key words: bismuth, ultra-low carbon steel, solid-state phase transformation, microstructure, cooling process

CLC Number:

TG142.1⁺3

Wang Jie, Peng Hongbing, Wu Xianhui, Shi Hongyang, Zhu Mingchang, Wang Xiaoyong, Zhou Chaoyang. Influence of bismuth on solid-state phase transformation and microstructure of ultra-low carbon steel during cooling[J]. Heat Treatment of Metals, 2024, 49(10): 191-196.

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