Abstract：Hardening and softening at high temperature can improve the microstructure and properties of the materials by enhancing the transformation nucleation of the steel. However，it is inevitable to soften between finishing deformation and ultra-fast cooling. The influence of delay time on ferrite and M/A（martensite/anstenite） phase in micro-alloyed low carbon steel（0.055C-1.47Mn-0.17Mo-0.054Nb-0.012Ti）was investigated through the Gleeble thermal simulation experiment，and the delay time is the time from the finishing deformation at 830 ℃ to ultra-fast cooling at the ratio of 80 ℃/s. The phase transformation ratio of M/A phase and the grain size were analyzed and measured by the scanning electron microscope and image analysis respectively. The results showed that in the time range of 2-7 s，the ferrite grain size increased firstly and then decreased with the increase of the delay time before the ultra-fast cooling. The maximum grain size was obtained at the delay time of 5 s while the minimum was 7 s. The volume ratio of M/A phase also showed a trend of decreasing first and then increasing with the delay time prolonging，the shape of M/A phase changed from point to block. Moreover the minimum M/A phase ratio was obtained at 5 s. Considering the influence of delay time on the ferrite and M/A grain size and the ratio of M/A phase，shortening the delay time was favorable to improve the toughness and strength of micro-alloy steel.
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GUO Jin，SHI Jia-xin，CHEN Yu-lai. Effect of delay time before rapid cooling on transformation of micro-alloyed steel. Iron and Steel, 2018, 53(5): 62-67.
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2018, Vol. 53 
