Revisiting the effect of austenite grain size on hierarchical structure and mechanical properties of high-strength steel
Zhan Gao, Jingxiao Zhao, Jieru Yu, Kaijie Yuan, Xiaoming Dong, Yishuang Yu, Shilong Liu, Xuejun Jin
Abstract
Austenite grain refinement enhances strength and toughness by producing refined hierarchical structure with dense high-angle grain boundaries. Yet, the contributions of distinct types of boundaries remain unclear. This study revisits the influence of austenite grain size on hierarchical structure and mechanical properties using two high-strength steels with differing prior austenite grains (PAGs) produced by single or double quenching and tempering (SQT or DQT). Both steels exhibit a tempered martensite matrix with fine carbide precipitates. However, DQT steel shows finer PAGs and packets but coarser blocks than SQT steel, resulting in simultaneous increases in yield strength and impact energy. In DQT steel, refined austenite grains reduce γ-pocket (later-transformed austenite) size and thus yield finer packets, whereas the higher martensite start temperature promotes variant selection and the formation of coarser blocks. Within the refined hierarchical structure, the yield strength enhancement primarily stems from grain boundary strengthening, as packet boundaries pose stronger barriers to dislocation slip. The toughness improvement results from effective dislocation pile-up and efficient deflection of cleavage cracks provided by PAG and packet boundaries, which synergistically enhance the impact fracture resistance. These results validate and strengthen the basis for designing high-strength steels through grain boundary engineering.