Effect of Different Heat Treatment Processes on Microstructures and Mechanical Properties of DP Steels

The effects of two different heat-treatment processes on the microstructures and mechanical properties of dual-phase (DP) steels were investigated. The heat treatments were the traditional one-step heat-treatment process without quenching section (I) and the two-step heat-treatment process with a quenching section (II). The steels tested by heat-treatment process (II) favors the formation of a homogeneous and refined structure. The microstructure produced by heat-treatment process (II) contains intermediate hard-phase-tempered martensite, which reduces the hardness difference between the hard and soft phases and improves the homogeneity of the microstructure for DP steels. Compared with the steel produced by heat-treatment process (I), the steels tested by heat-treatment process (II) have a higher hole expansion property while maintaining good strength–elongation balance due to the low carbon content and dislocation density of the distribution of the tempered martensite grains, and the low hardness ratio of tempered martensite to matrix.