Effect of Ultra-Fast Cooling Rate on the Microstructure and Mechanical Properties of High Strength Cold-Rolled Sheet under Continuous Annealing

Abstract

The effect of different fast cooling rates on the microstructure and mechanical properties of the V and Ti microalloyed high strength cold-rolled sheet was studied under laboratory conditions. Five different fast cooling rates were set up as 20°C/s, 50°C/s, 200°C/s, 500°C/s and 1000°C/s, respectively. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the microstructure, and the mechanical properties were also tested. The results showed that with the increase of fast cooling rate from 20°C/s to 1000°C/s, the grains of martensite and ferrite were finer, and the average grain size of both martensite and ferrite decreased from 7.7μm to 3.9μm. The proportion of ferrite in the two phases decreased while that of the martensite increased from 25.7% to 62.1%. The morphology of martensite tended to be lath, and the density of dislocation in the ferrite grains nearby the martensite gradually increased. With cooling rate rising from 20°C/s to 1000°C/s, the yield strength of the experimental steel increased from 381MPa to 1074MPa, and the tensile strength increased from 887MPa to 1199MPa. And the elongation decreased from 14.2% to 7.2%, and the product of strength and elongation decreased from 12.6GPa·% to 8.6GPa·%.