Improvement in the Strength–Ductility Balance of Tempered Martensite Steel by Controlling Cementite Particle Size Distribution

We attempted to improve the strength–ductility balance of tempered martensite steel by controlling cementite particle size distribution. Four types of samples of varying tempering temperatures were prepared: T973-60m and T823-60m samples were heated to 973 and 823 K and isothermally held for 1 h, while DT-60m and DT-15m samples were heated to 973 K and isothermally held for 1 h and 15 min and cooled to 823 K and isothermally held for 15 min and 1 h. As a result, the strength–ductility balance of DT-60m and DT-15m samples using two-stage tempering was superior to that of T973-60m and T823-60m samples. From microstructural observations, T973-60m sample mainly included large cementite particles in contrast to T823-60m sample which mainly included small cementite particles. DT-60m and DT-15m samples included both small and large cementite particles. The larger the area fraction of the small cementite particles, the higher was the tensile strength and the larger the area fraction of the large cementite particles, the higher was the total elongation. In the case of DT-60m and DT-15m samples, the nucleated voids were hard to coalesce due to the suppression of strain concentration at the interface between the matrix and coarse cementite particles. From these results, we concluded that strength–ductility balance could be improved by dispersing both small and large cementite particles.