Effect of Annealing Time on Microstructure and Properties of C-Mn-Nb-Ti Deep-Drawing Dual Phase Steel

The cold-rolled C-Mn-Nb-Ti deep-drawing dual phase steel was annealed at 860 °C. The effects of annealing time (180, 240, 300, 360 s) on the microstructure, texture characteristics, tensile properties and formability of the test steel were studied. The results show that the microstructure of the test steel was composed of ferrite, martensite and carbide under different annealing time periods. With the extension of annealing time, the content of martensite increased, and the grain size decreased first and then increased. When the annealing time exceeded 240 s, the ferrite grains were coarsened and showed polygonal shape. The test steel was mainly composed of α texture and γ texture. With the extension of annealing time, the orientation density of 111 γ texture increased first and then decreased, the yield strength and tensile strength of the test steel increased, the percentage elongation after fracture was basically unchanged first and then decreased significantly, and the plastic strain ratio and work hardening index increased first and then decreased. When the annealing time was 240 s, the plastic strain ratio and work hardening index reached the maximum values. After annealing at 860 °C for 240 s, the comprehensive mechanical properties and formability of the test steel were the best; the tensile strength was 526.1 MPa, the yield strength was 351.7 MPa, the percentage elongation after fracture was 30.77%, the plastic strain ratio was 0.21, and the work hardening index was 1.42.