Toughening of martensite matrix in high strength low alloy steel: Regulation of variant pairs

The study focuses on the significant effects of carbon and alloying elements on variant selection during martensite transformation and its influence on toughness. 0.17C steel with low content of alloying elements and 0.13C steel with relatively higher alloying content were studied. The strength of quenched 0.13C steel was inferior to 0.17C steel, and their yield strengths were 894 and 999 MPa, respectively. However, -40 degrees C Charpy impact energy of 0.17C and 0.13C steels was 40 and 198 J, respectively. An analysis by electron backscattered diffraction (EBSD) underscored that a larger proportion of V1/V2 variant pairs and a lower content of V1/V4 variant pairs in 0.13C steel reduced the residual stress, thereby significantly enhancing the toughness of steel. In addition, the high density of high angle grain boundaries (HAGBs, especially V1/V2 variant pairs) had the ability to hinder crack propagation, which further toughens the matrix.