Effects of Different Austenitising Conditions on the Strength–Ductility Balance in a High-Strength Low-Alloy Steel

Abstract

With the addition of microalloy elements to a high-strength low-alloy (HSLA) steel, various fine particles of carbides and nitrides are formed, which increase the matrix strength. These precipitates play a crucial role in precipitation strengthening. However, the role of precipitates in microstructural refinement is frequently overlooked. In this study, a series of hot-rolled HSLA steel samples were reheated to different temperatures above the austenite transformation point for a specified period to refine austenite grains via precipitation, then cooled to a dual-phase (austenitic/ferritic) region, and finally air-cooled to room temperature. The influences of different austenitising conditions on the microstructure and mechanical properties of the HSLA steel were examined. When a hot-rolled sample was reheated to 15 °C above the austenitic transition temperature for 20 min and then cooled to 25 °C below the austenitic transition temperature for 25 min, the most low-angle boundaries were formed, and the smallest effective grain size was achieved. Meanwhile, compared with the hot-rolled sample, the tensile and yield strengths of the reheated sample increased by 12.3% and 3.4%, respectively, while the elongation increased by 162.5%, exhibiting a good strength–ductility balance. By adopting an appropriate austenitising process, precipitates can refine the crystalline grains during austenitisation, thereby enhancing the comprehensive mechanical properties of the steel. Meanwhile, excessively high austenitising temperatures lead to the coarsening of the steel microstructure, decreasing the microstructural refinement efficiency via precipitation and consequently degrading the comprehensive mechanical properties of the steel. The findings provide valuable insights into the preparation process design of such steels or other steels with similar microstructures.