Effect of cooling rate on the microstructure and mechanical properties of microalloyed forging steel

The effect of cooling rate after hot deformation on the microstructure and mechanical properties of a commercial microalloyed forging steel (30MSV6 grade) was investigated using the hot compression test. Hot compression test was performed at 1150 and 925 °C followed by air cooling at different rates. Final microstructures and mechanical properties were evaluated by optical microscopy and shear punch test (SPT), respectively. The results indicated that by increasing the cooling rate, the as-received ferritic-pearlitic microstructure changes to the acicular ferrite, bainite or martensite. It is shown that both yield and ultimate strength increase but the ductility decreases significantly. At a constant cooling rate by decreasing the deformation temperature, the volume fraction of acicular ferrite and the mean length of ferrite plates decrease due to the formation of a layer of grain boundary ferrite, which leads to the improvement of toughness.