Development of Low‐Carbon Low‐Alloy High‐Strength Carbide‐Free Bainitic Steel via Continuous Cooling for Automotive Applications

Medium‐carbon low‐alloy steels for automotive parts such as connecting rods, flange yokes, crankshafts, etc. are subjected to a rather long and complex quenching and tempering heat treatment cycle after forging. In this study, an effort has been made to explore the possibility of developing a low‐carbon carbide‐free bainitic steel where the required properties are obtained directly in air‐cooled condition after forging, eliminating the need for any post‐forging heat treatment. Existing computation tools were used to evaluate the effect of alloying elements on the parameters influencing the bainite transformation. Based on this analysis, an optimum steel composition was designed, and its properties were compared with those of the conventional quenched and tempered medium carbon steel (IS 37C15 steel). The designed alloy was processed through melting and casting, followed by hot forging and air cooling. The optimized composition showed bainite transformation in the cooling rate range of 0.5, to 5 °C/s with no evidence of any reconstructive transformation products. The microstructure consisted of fragmented bainitic ferrite, along with a small fraction of martensite–austenite constituents and carbon‐enriched retained austenite. The designed steel offers high strength with good ductility compared with 37C15 Q&T steels.