Effects of Pre-strain and Baking Process on the Microstructure and Bake-hardening Behavior of 780 MPa Cold Rolled Dual-phase Steel

In a typical process,C-Mn steel was annealed at 800 ℃ for 60 s,and then cooled rapidly to 300℃ to carry out the over-aging treatment.The ferrite-martensite microstructure was obtained after the simulated continuous annealing process. After pre-straining and baking,the specimens were used to measure the BH₂ values.The influences of pre-strain,baking temperature and baking time on the microstructure evolution and bake-hardening behavior of the 780 MPa cold rolled dual-phase steel were investigated.It was found out that the BH value apparently increased with an increase of pre-strain in the range from 0% to 2%; however,increasing pre-strain from 2% to 8% led to a decrease in the BH₂ value.And the maximum of BH₂ value at the pre-straining of 2% is 51 MPa. Furthermore,the BH₂ value enhanced due to increaseing amount of Cottrell atmosphere in the ferrite grain interior and carbide in both of ferrite and martensite phases with increasing the baking temperature.The BH₂ value reached a maximum of 102 MPa at the baking temperature of 300 ℃.Moreover,with an increase in the baking time from 5 to 240 min,the BH₂ value enhanced gradually due to the Cottrell atmosphere formation and carbide precipitation in the ferrite.