Effect of hot deformation on the evolution of microstructure, MnS inclusions and mechanical properties of non-quenched and tempered steel

Controlling the formation of small, dispersed spherical or spindle-shaped MnS has always been a key challenge in the development of high-quality non-quenched and tempered steel during the rolling. In this study, the Gleeble-3800 thermal simulator was first used to investigate the evolution of flow stress, microstructure, and MnS during the hot deformation of Ti-Zr-treated F38MnVS steel. A high-precision hot deformation constitutive equation and processing map were established, providing a theoretical basis for achieving the desired microstructure and MnS morphology during rolling. Subsequently, laboratory rolling experiments were conducted to study the effects of rolling temperature on MnS, microstructure, and mechanical properties of steel. The results indicate that hot deformation at 950–1050°C and a strain rate of 0.1–0.5 s −1 leads to a uniform dynamically recrystallized microstructure. Within this deformation range, MnS exhibits low relative plasticity, making it prone to fragmentation, thereby enabling the control of MnS size and aspect ratio. Low-temperature rolling is beneficial for grain refinement, MnS spheroidization and mechanical properties, while reducing anisotropy in plasticity and toughness of the steel. The average grain size of 950-R steel is 9.2 μm, the aspect ratio of MnS in the rolling and transverse directions was 2.7 and 1.9, respectively. The yield strength in the rolling direction was 875 MPa, and 869 MPa transversely; elongation was 28.3% and 27.1%; area reduction was 53.2% and 50.1%; and impact energies were 81.4 J and 78.4 J, respectively. The steel exhibited high strength, toughness, and reduced anisotropy.