Effect of Strain Applied During Thermomechanical Processing on Austenite Grain Size After Carburizing in Microalloyed Steels

Abstract Thermomechanical processing (TMP) in the warm work temperature regime, coupled with microalloying, can produce fine austenite grains that resist growth during carburizing, which is an essential requirement for electric vehicle drivetrain components. This study examines the effects of Al, V, and Nb microalloy additions and TMP strain on prior austenite grain (PAG) size after TMP and subsequent pseudo-carburizing. To simulate TMP, hot torsion tests were performed on microalloyed 1045 steel bars using a Gleeble 3500 thermomechanical simulator at 800 °C, creating a strain gradient across the cylindrical cross-section. This approach allowed for the evaluation of PAG size, after pseudocarburizing at a temperature of 950 °C, at different TMP strain levels within a single sample. Greater TMP strain led to increased grain refinement in all steels after pseudocarburizing. The finest PAG sizes were observed in the Nb-V steel, followed by the V only and Al only steels. However, the V steel exhibited a higher tendency for abnormal grain growth when subjected to intermediate strain levels. These findings suggest that optimizing warm-working strain and employing multicomponent microalloying can effectively achieve finer post-carburized PAG sizes, enabling the performance necessary for the next generation of automotive gear steels.