For structural material, grain refinement is always the most effective way to improve the strength while not sacrificing the toughness. During the research on super-refined steel, deformation induced ferrite transformation (DIFT) was found as a more effective way than TMCP to refine steel grain, and thus plenty of efforts were put on this project, but there was no clear mechanism for DIFT yet. Based on the early research, this paper, therefore, has done a further work at a termperature range between Ae3-Ar3 in a low carbon steel aimed at studying DIFT.
Based on the powerful simulation function, a series of general simulations of deformation induced ferrite transformation (DIFT) at a temperature range between Ae3~Ar3 in Q235 low carbon steel were conducted on Gleeble3500 which could precisely record all of the parameters during the deformation. Stduy on stree-strain cruves during DIFT shows that the peaks exhibited in the curves are involved with deformation induced transformation which happened in inner grain or grain boundary. According to the stress-time curve and strain-time curve, strain dramatically postponed the strain induced transformation which lead to the start of the transformation began right ahead of the finish of deformation. And thus, the majority of the ferrite transformation process mainly happened after the deformation. Strictly, deformation induced transformation was a metadynamic transformation process with dynamic nucleation.
Obervations on microstructure by SEM and the measurement of carbon concentration by EPMA, the DIFT evolution was investigated combined with analysis on stress-time curve during the DIFT. The result indicates that the carbon concentration in DIF is far higher than that of conventional ferrite, deformation induced ferrite phase, therefore, is a quasi-stable transitional phase. A holding experiment after the deformation and a tempering treatment including a hardness measurement for the induced ferrite phase were conducted to study the deformation induced transformation and the thermal stability of the induced ferrite phase. Because of the thermal instability, an inverse transformation from DIF to austenite was found during the holding experiment after the deformation. What’s more, the super-saturated carbon would precipitate out from inner DIF, resulting in a transformation from DIF to conventional stable ferrite phase and a decrease in its Vickers-hardness during the process of tempering.
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