Development of Advance High Strength Material with Improved Yield Ratio and Excellent Bake Hardening Property

摘要

Effect of various annealing parameters for mechanical property and microstructure for DP590,780 and 980 were investigated through Gieeble thermal simulation and Continuous Annealing Line (CAL) trials. Cold rolled full hard samples of Dual Phase (DP) grades were initially simulated in Gleeble for different soaking section, rapid cooling and over aging section temperatures. Mechanical properties along with microstructure were investigated for the same. Identical annealing cycles were performed in continuous annealing line and results were compared to the Gleeble simulation results. Significant improvement in tensile strength with a decrease in rapid cooling were observed in simulated samples and in CAL trial coils, whereas a decrease in tensile strength was observed due to decomposition of martensite at higher rapid cooling as well as over aging temperatures. There is a significant increase in yield strength observed after forming, baking and painting of annealed samples. YS to TS ratio of DP steel is observed less than 0.6 as compared to 0.8 in HSLA grades, indicating DP steels are more formable than HSLA grades at a similar strength level. It is also concluded that increasing annealing temperature beyond Ac3 does not increase the martensite fraction as well as tensile strength in continuous annealed steel.