Multiscale Modeling of Stress–Strain Localization Induced by Representative Three‐Dimensional MnS Inclusions in Free‐Cutting Steel

The MnS inclusions in a sulfur‐bearing free‐cutting steel were characterized using X‐ray micro‐CT to obtain statistical data on inclusion size, aspect ratio (AR), and spatial distribution. Based on representative three‐dimensional MnS inclusion geometries derived from micro‐CT statistics, a multiscale elastoplastic finite element model was developed in ABAQUS. The mechanical parameters of MnS inclusions were determined through first‐principles calculations and molecular dynamics simulations, while the constitutive behavior of the steel matrix was calibrated using quasi‐static tensile tests. Material parameters were transferred from the atomic scale to the continuum scale through a multiscale modeling framework. The results show that MnS inclusions are predominantly elongated, with ARs mainly ranging from 2.0 to 7.5. When the AR exceeds approximately 3, stress concentration at the inclusion tips and the inclusion–matrix interface increases markedly. At a fixed AR, stress concentration depends non‐monotonically on inclusion size, reaching a minimum at an intermediate size under the same loading condition; excessively large inclusions, however, cause pronounced stress peaks. Plastic strain initiates preferentially at the axial ends of MnS inclusions and propagates into the surrounding matrix, forming characteristic X‐shaped high‐strain bands. In tangential or intersecting inclusion arrangements, stress field superposition further intensifies strain localization and local damage.