Effects of deformation parameters on microstructure and mechanical properties of a 1300 MPa grade steel for engineering machinery

A 1300 MPa grade engineering steel was processed by a two-stage deformation schedule to investigate the effects of second-stage deformation temperature and strain on transformation behavior, final microstructure, and tensile response. After austenitization, the second deformation step was conducted at 850 or 920 °C with strains of 30% or 50%, followed by controlled cooling. The results show that the influence of temperature depends strongly on strain level. At 30% strain, the higher deformation temperature is associated with recovery-related structural adjustment and lower defect density, which leads to lower strength but higher ductility. Under the 50% strain condition, the 920 °C produces more pronounced microstructural refinement and a higher fraction of high-angle grain boundaries, which contributes to the increase in strength. The strength difference among the tested conditions is mainly related to the combined contributions of grain boundary strengthening and dislocation strengthening, while the stored deformation energy also affects microstructural recovery. Martensite variant analysis reveals that deformation temperature and strain change the hierarchical crystallographic organization of martensite, including variant arrangement, packet/block-related grouping, and Bain group characteristics. Among the investigated conditions, the 920 °C-50% sample exhibits the highest strength, while the 920 °C-30% sample shows the best strength–ductility balance, which is related to its moderate strengthening level, relatively low defect storage state, and characteristic transformed substructure.