Individual mechanical properties of ferrite and martensite in Fe–0.16mass% C–1.0mass% Si–1.5mass% Mn steel

Abstract

Abstract The dual-phase steel generally contains a ferrite matrix and an appropriate amount of martensitic phase. Strength and deformability of the dual-phase steel are related to a difference in mechanical property between ferrite and martensite. Reliable experimental information of the above relationship is, however, rather limited. In order to examine individual mechanical properties of ferrite and martensite in the dual-phase steel, the nanoindentation technique was applied focusing on the microstructure. The chemical composition of the steel was 0.16 mass% C, 1.0 mass% Si, 1.5 mass% Mn, 0.01 mass% P, 0.003 mass% S and balance Fe. The sheet specimens were isothermally annealed at 1048 K for 600 s, water-quenched and tempered in a temperature range between 473 and 923 K for 300 s, to obtain a dual-phase steel with 65 vol.% of ferrite. Tensile tests of the heat-treated specimens were carried out. Nanoindentation experiments were performed with the peak load of 1000 μN, using a cube corner type as an indenter. Although almost all the peak loads in this study are the same, indentation sizes in ferrite grains are larger than those in martensitic grains. This implies ferrite has lower deformation resistance than martensite in the dual-phase steel. The nanohardness for ferrite and martensite is 2.8 GPa and 7.2 GPa in the specimen tempered at 473 K, respectively. The nanohardness for martensite decreases with increasing the tempering temperature, which is similar to the temper softening of the tensile strength. On the other hand, the yield stress is almost constant with tempering temperatures below 473 K and significantly increases in the temperature range between 473 and 623 K. These results mean that the mixing rule cannot be applied to the yield strength, but it is applicable to tensile strength.