Influence of cooling rate on the grain-refining effect of austenite deformation in a HSLA steel

Effective grain size is a direct indicator of the high angle grain boundary (HAGB) density of microstructures. A small effective grain size suggests a high density of HAGBs, which provide effective barriers to cleavage fracture. There have been many investigations concerning the effect of processing parameters on the effective grain sizes of steel microstructures. However, contradicting results were found for the influence of austenite deformation. In this research, to understand the influence of austenite deformation on effective grain size refinement, a low carbon Nb microalloyed steel was subjected to different austenite deformation conditions and was continuously cooled at a wide range of cooling rates (0.5–50 °C/s). Characteristics of transformed microstructures from recrystallised and deformed austenite were analysed through optical microscopy observation and EBSD mapping. In the whole cooling rate range adopted in this research, effective grain sizes were found to be refined by austenite deformation. However, with the rise of cooling rate higher than 10 °C/s. With the rise of the cooling rate higher than 10 °C/s, effective grain sizes are reduced for recrystallised austenite, while increasingly large effective grain sizes were found for deformed austenite. According to these experimental results, the influence of austenite deformation on effective grain size in a wider cooling rate range was proposed to be cooling rate dependent, and possible explanations for the contradicting results in the literature were discussed based on that.