The effect of thermomechanical process on the nanobainitic medium carbon steels

The austenite grain size has a significant influence on phase transformation and mechanical properties of bainitic steels. The martensite treatment including cold rolling and subsequent annealing of microstructures with high percentage of martensite is one of the most effective thermo-mechanical methods for the grain refinement in steels. The present study aims to investigate the effects of thermomechanical and austempering processes on the microstructure and mechanical properties of the medium carbon high aluminium nanobainitic steel. In this regard, nanostructured bainitic steels with lower carbon content (0.45 wt%) than conventional supper bainitic steel were cast and hot rolled. To obtain the bainitic structure the specimens were austenitized at 1030 °C for 20 min and quenched in water. The specimens were subjected to 20% reduction in thickness by cold rolling and subsequent annealing at 600 °C for 20 min. The specimens were austenitized and transformed isothermally at 340 °C and finally were quenched in water. The microstructure was characterized by X-ray diffraction (XRD), optical (OM) and field emission scanning electron microscopy (FESEM) as well as tensile test for mechanical properties. The results showed that no significant changes were obtained in properties due to the increase in the rate of bainitic transformation and low changes in the austenite grain size. For specimens with a coarse austenite grain size (47 µm) the strength of 1279 MPa and elongation of 23% and for specimens with a fine austenite size (33 µm) the strength of 1231 MPa with a elongation of 19%, were obtained.