Excellent strength-toughness balance of low-carbon low-alloy steel manufactured by a three-stage rolling coupling inter-pass reheating process

Abstract

In this study, a low-carbon low-alloy (LCLA) steel with ferrite and bainite mixed structure was successfully prepared by a novel three-stage rolling coupling inter-pass reheating process (TPS). The plate was compared to a plate prepared by conventional thermo-mechanical processing (CPS). The study on microstructure evolution, quasi-static tensile properties, dynamic Charpy impact toughness and fracture behavior of the experimental steels were deeply investigated. The results manifests that the TPS technology effectively refine the grain size of austenite grain and optimizes the distribution of the driving force behind phase transitions, thereby facilitating the development of a ferrite-bainite mixed microstructure (83.4 ± 6 % GB + 15.2 ± 3 % PF) in TPS steel. In contrast to CPS steel, the newly developed TPS steel significantly enhances low-temperature impact toughness while maintaining high strength, namely, a yield strength of 532 ± 7 MPa, a total elongation of 21.3 %, and the Charpy impact energy of 137 J at −60 °C. The refined grains and synergistic deformation of mixed microstructure can contribute to the excellent ductility without compromising strength in TPS steel. The excellent impact toughness of TPS steel is induced by multi-factor coupling. Finer initial effective grain size, higher proportion of high angle grain boundarie, higher intensity of the desired texture {332} 〈113〉, and significant hetero-deformation induced stress are all factors affecting toughness. Additionally, benefitted to the “weak interface” between ferrite and bainite, the delamination toughening is fully activated (extrinsic toughening), which significantly improves the overall fracture resistance. Based on the performance optimization results, the feasibility of preparing the LCLA steel with excellent combination of strength, ductility and toughness using three-stage rolling coupling inter-pass reheating process was verified. This innovative approach offers a pathway for the development of superior steel suitable for engineering applications.