Study on the relationship between the refined hierarchical microstructure, yield strength and impact toughness of low-carbon martensitic steel at different quenching temperatures

In this work, a new equation to evaluate the yield strength change caused by grain refinement was proposed in low-carbon martensitic steel, based on the Hall-Petch formula. MATLAB analysis showed that martensite and the prior austenite grain (parent phase) were more consistent with the Kurdjumov and Sachs orientation relationship in the martensitic transformation process. The martensite variant selectivity became significant and was dominated by the CP group, among which the V1, V3, V4, V7, V11, and V12 variants were higher in content. Using OM, SEM, EBSD, and TEM characterized the sample at different quenching temperatures. Then, the relationship between the refined hierarchical microstructure and both yield strength (YS) and impact toughness (IT) was carefully analyzed. Researched results found that with grain refinement, the YS first increased rapidly and then slowly increased. It indicated the Hall-Petch relationship was a discontinuous formula. Utilizing the new equation, we concluded that martensite laths and prior austenite grains were the microscopic units that determined the YS of the steel. Martensite packets and blocks contributed nearly same to the strength of the steel, while the growth of martensite packets into square shapes would led to a decline in the strength of the steel. Additionally, the IT of the steel was determined by the overall grain boundary length of high strain gradient plastic zone and martensite laths width, in which high-angle grain boundaries contributed more to IT than lowangle grain boundaries.