Effect of Step-Cooling Process on Structures and Properties of Microalloy Ferrite-Bainite Dual-Phase Steels

The continuous cooling transformation diagram of microalloyed ferrite-bainite dual-phase steels after hot deformation was tested by using Gleeble-1500 hot simulator. In the light of the phase transformation rule of test steels, the effects of holding time and holding temperature on the microstructure and mechanical properties of test steels were investigated in step-cooling process after one-pass deformation. Results show that the ferrite grain size slightly increased with the holding temperature increased from 630 ℃ to 700 ℃, while the strength and elongation increased first and then decreased. The optimized holding temperature was 680 ℃. With prolongation of the holding time from 5 s to 8 s at 680 ℃, the ferrite grain increased gradually in size and was equiaxed, while the strength and elongation of test steels declined. And the optimized combined properties can be given when holding time was 5 s.