Microstructure, texture, and mechanical anisotropy of dual-phase steel: Effect of intercritical annealing temperature

Abstract

The effect of intercritical annealing temperature on the microstructure, texture, and mechanical anisotropic of dual-phase (DP) steel sheets has been investigated. It was observed that by the increment of intercritical annealing temperature, the fraction of martensite phase increases from 0.224 to 0.233 and finally to 0.315 in the 770-10, 800-10, and 830-10 samples, respectively. Eventually, a chain-like morphology of martensite islands was formed, which was beneficial for improving strength-ductility balance. The ferrite grain sizedid not show an explicit trend as it initially decreased from 10.7 to 7.2 μm and then increased to 8.5 μm by increasing the annealing temperature. Moreover, the martensite distribution was enhanced in both RD-ND and RD-TD planes as the annealing time was prolonged. The texture evolution was characterized by the gradual weakening of γ-fiber and decreasing the overall texture intensity. With regard to the Vickers hardness measurements, it exhibited a constant increase from 219.8 to 238.0 HV by the increment of martensite volume fraction. However, as the martensite carbon content was decreased gradually, the increase in hardness was not very sharp. The tensile curves of all DP samples revealed a low yield ratio and continuous yielding behavior. The typical rise in strength and fall in ductility was not readily observed as the annealing time increased, which was related to the different texture components, martensite morphology, and the distribution of martensite in different planes of DP samples. The isotropic behavior of the 830-10 sample was superior compared to other samples owing to the elimination of γ-fiber, the texture weakening, and the uniform distribution of martensite. The strain-hardening rate curves of DP samples revealed a high initial strain-hardening rate and a three-stage work-hardening behavior.