Designing a novel isotropic high-strength dual-phase steel

Abstract

In this research, a novel isotropic high-strength dual-phase (DP) steel was fabricated by intermediate annealing and changing the strain path. The results showed that the deformation bands are formed in the unidirectional-rolled (UR) sample more than in the cross-rolled (CR) steel because the cross-slip is easy to occur in the CR sample. It was found that intermediate annealing (between cold rolling and intercritical annealing) increases the fraction and size of martensite islands. Also, intermediate annealing changed the morphology of martensite from plate to lath. It was very important to tune the ferrite grain size in a range that could reach the maximum fraction of martensite. The UR-A-IA DP sample has a much better martensite distribution compared to the CR-A-IA DP steel. The variations of hardness and strength were very similar to the variation of martensite fraction. According to the results of mechanical properties, the best samples were the CR-A-IA DP sample at the tensile direction of 0° and the UR-A-IA DP sample at the tensile direction of 90°, which indicates performing intermediate annealing before intercritical annealing can increase the strength of DP steel. The strain-hardening rate vs. true strain curves showed that the slope of the strain-hardening rate reduction is more severe in the DP samples without intermediate annealing due to the smaller fraction of martensite and the lower plasticity of plate martensite. Performing intermediate annealing before intercritical annealing reduced the anisotropy level and also decreased the difference between the strain-hardening behavior in the three tensile directions, which was owing to increasing the martensite fraction. By performing the intermediate annealing, the dominant nucleation mode of microvoids changed from ferrite-martensite decohesion to ferrite-ferrite, ferrite-martensite, and martensite-martensite decohesion on the fracture surfaces.

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