Experiment on properties differentiation in tailor rolled blank of dual phase steel

The mechanical properties in tailor rolled blank of dual phase steel (DP-TRB) at different thickness zones present significant differences after annealing. In this paper, the differences in mechanical property of the tailor rolled blank were investigated. The work hardening behavior is analyzed by C-J analysis method. The results indicate that the microstructure of DP-TRB is composed of ferrite and martensite at 800–820 °C. It is interesting that the martensite islands gradually chang to equiaxed morphology from rod-like shape at annealing temperature of 800 °C. In addition, the differences in mechanical properties between different thicknesses zones become less with the increased intercritical temperature from 760 °C to 820 °C. It indicates that the difference in yield strength decreases from 230 MPa to 60 MPa with the intercritical temperature changing from 760 °C to 820 °C. Ferrite recrystallization or not is the main reason for the differential properties. The critical thickness for recrystallization is same ~ 1.6 mm, and the 1.6 mm zones is experimentally proved to possess the largest strength at 760 °C, 800 °C, and 820 °C. The samples in thinner zones are softened because of the recrystallization, while the samples in the larger thickness zone are not strong enough as 1.6 mm with less work-hardening. After the yield point, there is an extra work hardening stage in 1.6 mm and 1.8 mm samples relative to 1.0–1.4 mm samples, which can eliminate nonuniform deformation and prepare for the followed uniform deformation. Consequently, turning of this feature of properties differentiation rationally would play an important role in application of DP-TRB.