Effect of Carbon Content on Microstructure and Deformation of Quenched and Tempered Martensite

Microstructure of martensite has variety in size of martensite blocks and mechanical properties, depending on transformation temperature in sequential martensite transformation. Transformation temperature and type of martensite (lath or butterfly) are known to depend on chemical composition, mainly carbon content of steels. In this paper, we used 0.2~0.8 mass%C steel to prepare two grade tensile strength steel (1500 MPa and 2000 MPa) after quenching and tempering. After tensile test of these steels, we investigated the microstructure, nano hardness distribution and deformation property by Kernel Average Misorientation (KAM) map of Electron Back Scattered Diffraction pattern analysis (EBSD). In microstructure, lath martensite size was refined by the increase of carbon content. But course butterfly martensite was also formed around the former austenite grain boundary in high carbon steel. Nano hardness was lower at the course lath martensite in low carbon steel and course butterfly martensite in high carbon steel. Deformation properties were evaluated by comparing KAM map at not deformed region and uniform elongation deformed region. In 1500 MPa grade steel, 0.2%C and 0.4%C steel deformed uniformly, but strain was concentrated at low KAM grains in 0.6%C carbon steel. In 2000 MPa grade steel, deformation was concentrated at low KAM grains even in 0.4%C steel. These results shows that mechanical properties and deformation properties of martensite is not uniform at the level of martensite blocks, even if macro strength is similar among steels with different chemical composition.