Effects of process parameters on the microstructure and mechanical properties of medium-carbon steels for improving high-strain rate properties

Abstract

The effects of accelerated continuous cooling and isothermal heat treatment process on the microstructure and mechanical properties of medium carbon bainitic steels was investigated to find an alternative process of the conventional austempering treatment process. Two kinds of medium carbon steel billets melted by Vacuum Induction Melting (VIM) were hot rolled to the thickness of 6∼12mm plates and accelerated-cooled to predetermined temperatures and then isothermal heat treatment was conducted immediately. For quantitative analysis of microstructures, the fraction of constituent phases such as retained austenite, martensite, and bainite was measured by XRD and dilatometry. A series of hardness, tensile, and impact tests was conducted to evaluate the mechanical properties of the plates. As the finish cooling temperature increased, the fraction of retained austenite increased and the fraction of martensite and bainite decreased due to the differences in carbon diffusion and transformation kinetics during isothermal heat treatment process. Microstructural observation results showed that the lath/block thickness and fraction of bainite and martensite were mainly dependent on austempering temperature and time. The thickness of bainite and the fraction of martensite were found to be the major factors influencing mechanical properties of the present medium-carbon nanostructured bainitic steels.