Effects of Pre-strain and Baking Temperature on the Microstructure and Bake-hardening Behavior of Mild Steel and Dual Phase Steel

In a typical process,mild steel and dual phase steel were respectively annealed at 800 ℃ for 90 s,cooled slowly to 680 ℃ and then cooled rapidly to 300 ℃ to carry out the overaging treatment.As a result,the ferrite-cementite mild steel and ferrite-martensite dual phase steel were obtained after the continuous annealing process.After pre-straining,the specimens were baked to measure the bake-hardening（BH） values.The influences of pre-strain and baking temperature on the microstructure evolution and bake-hardening behavior of mild steel and dual phase steel were investigated.It was found out that the bake-hardening（BH） value gradually increased with an increase in pre-strain from 0% to 8%.The BH value reached a maximum value of 65 MPa at the pre-strain of 8% in the mild steel.In the dual phase steel,the BH value apparently increased with an increase in pre-strain from 0% to 1%; however,increasing pre-strain from 1% to 8% led to a decrease in the BH value.The BH value reached a maximum value of 79 MPa at the pre-strain of 1%.Furthermore,an increase in the baking temperature from 140 ℃ to 300 ℃ favored a gradual improvement in the BH value in both of mild steel and dual phase steel.And the maximum BH values at the baking temperature of 300 ℃ were 71 MPa and 148 MPa in the mild steel and dual phase steel,respectively.The yielding platform appeared in the engineering stress-strain curve in the mild steel after baking.While the yielding platform was not observed in the engineering stress-strain curves of dual phase steel after baking at 140～170 ℃.However,the yielding platform appeared in the engineering stress-strain curve due to the martensite tempering in the dual phase steel baked at 210～300 ℃.