Effect of cooling rate on the evolution of microstructure and mechanical properties of nonisothermally partitioned steels

Abstract

In the present investigation, multiphase microstructures containing a combination of ferrite, martensite, retained austenite and carbides have been produced by altering the cooling rate in low alloy steels. The mechanical properties have been evaluated and correlated with ensuing microstructural features. The as-cast alloys were austenitized, hot rolled to about 93% reduction in thickness, followed by cooling to 200 °C on the run-out table. The cooling rates, namely 50 and 70 °C/s, were employed for this study. The steel plates were then cooled slowly to room temperature in a furnace to simulate the nonisothermal partitioning, similar to the hot-rolled coil cooling. The results show that the alloy with lower carbon and Mn content (Alloy-1) reveal ferrite formation (35.4 ± 4.1 vol%) at the cooling rate of 50 °C/s. However, at a higher cooling rate of 70 °C/s, ferrite formation was circumvented and the presence of martensite, retained austenite (6.3 ± 0.13 vol%) and carbides were observed. Although no significant difference was observed in the hardness and strength values for these two cooling rates, the presence of retained austenite at a higher cooling rate (i.e. 70 °C/s) led to better ductility and impact toughness. In the other alloy, with higher carbon and Mn addition (Alloy-2), the ferrite formation was considerably reduced even for the cooling rate of 50 °C/s. As a result, it showed higher hardness and strength (~1.5–2.0 times), with a concurrent decrease in the ductility and impact toughness, in comparison to Alloy-1.