Morphological Evolution of MnS During Hot Deformation and Isothermal Homogenization in Nonquenched and Tempered F40MnVS Grade Steel

Abstract

The control of MnS inclusions is crucial in developing high-quality nonquenched and tempered steel. Single-pass compression experiments are conducted on F40MnVS steel using a Gleeble-3800 thermomechanical simulation testing machine, and the hot deformation behaviors of MnS inclusions at temperatures of 950–1150 °C and strain rates of 0.01 s⁻¹ are investigated. Based on the experimental results of hot deformation, the steel is isothermally homogenized after forging to study the effect of holding time on the morphology and characteristics of MnS. Results indicate that at a lower deformation temperature of 950 °C and increased deformation, the relative plasticity of MnS diminishes, reducing the aspect ratio from 3.38 to 1.44 and primarily causing MnS fragmentation. At 1150 °C, as deformation increases, the relative plasticity of MnS also increases, with the aspect ratio rising from 1.46 to 2.01, leading to the growth of MnS. Under either low temperature and high deformation conditions or high temperature and low deformation, MnS fragmentation is more pronounced, resulting in more spherical MnS. With extended homogenization time, elongated MnS fractures, progressively transforming into spheroidal or ellipsoidal shapes before enlarging. The diffusion of S primarily controls the fracture and growth of MnS during isothermal heating.