Effects of overaging temperature on the microstructure and properties of 600 MPa cold-rolled dual-phase steel

C-Mn steels prepared by annealing at 800A degrees C for 120 s and overaging at 250-400A degrees C were subjected to pre-straining (2%) and baking treatments (170A degrees C for 20 min) to measure their bake-hardening (BH2) values. The effects of overaging temperature on the microstructure, mechanical properties, and BH2 behavior of 600 MPa cold-rolled dual-phase (DP) steel were investigated by optical microscopy, scanning electron microscopy, and tensile tests. The results indicated that the martensite morphology exhibited less variation when the DP steel was overaged at 250-350A degrees C. However, when the DP steel was overaged at 400A degrees C, numerous non-martensite and carbide particles formed and yield-point elongation was observed in the tensile curve. When the overaging temperature was increased from 250 to 400A degrees C, the yield strength increased from 272 to 317 MPa, the tensile strength decreased from 643 to 574 MPa, and the elongation increased from 27.8% to 30.6%. Furthermore, with an increase in overaging temperature from 250 to 400A degrees C, the BH2 value initially increases and then decreases. The maximum BH2 value of 83 MPa was observed for the specimen overaged at 350A degrees C.