Advanced Thermo-mechanical Process for Homogenous Hierarchical Microstructures in HSLA Steels

Engineering microstructures in high-strength low-alloy steels via advanced thermo-mechanical processing is a promising approach to overcome challenges around low work hardenability and toughness in ultrafine-grained mild steels. Recently, multiscale-hierarchical microstructures with ultrafine grains and two populations of precipitates decorating high-angle grain boundaries and dislocation structures were achieved by some of the current authors in a modern Ti-Mo-Nb high-strength low-alloy steel. However, the high-strain rate of 10 s⁻¹ during single-pass plane-strain compression at 600 °C led to the formation of macroscopic shear bands. Here, we propose an optimized advanced multi-hit thermo-mechanical process for achieving homogenous hierarchical microstructures in the same steel without strain localization. This is verified via microscopy and thermo-kinetic modelling, using the software MatCalc. A typical body-centred-cubic rolling texture is achieved in contrast to previous process design. Ultrafine crystallites confined by a mixture of high-angle gain and subgrain boundaries are formed, decorated by two types of precipitates. Large FeMnC-rich cementite particles are found on grain boundaries and smaller TiNbC-rich precipitates on dislocations and subgrain boundaries. It is shown that TiNbC particles transform to a core-shell structure when subjected to direct aging. Thermo-kinetic modelling underpins experimental results concerning the detailed evolution of crystallite size, precipitate morphology and composition, enabling a through-process description of the microstructural evolution.