Effects of Alloying and Step Cooling Conditions on the Microstructures and Mechanical Properties of 780Y Hot-Rolled Dual Phase Steels

摘要

Along with the intensifying of energy crisis and environmental problems, energy saving and safety have become the most important issues for the automobile industry. To meet these challenges, weight reduction is most effective, which has lead to the fast development and application of advanced high strength steels. Among these, dual phase (DP) steels have attracted much attention due to their superior properties. In this study, the effects of alloying and hot rolling conditions on the final microstructures and the related mechanical properties of hot-rolled DP780Y steels are discussed. Controlled rolling patterns, especially step cooling temperature and duration time, during the sheet traveling on a run-out table were simulated by a thermomechanical simulator to produce DP 780Y steels as a kind of low carbon steel containing small amounts of manganese (Mn) and phosphorus (P). The microstructures and quantitative analysis of the volume fractions and the fracture surface of the phases were characterized by optical Microscopy (OM), scanning electron microscopy (SEM), and an image analyzer, respectively. It was found that the alloy contents of the Mn and P elements not only affected the microstructure of the ferrite and martensite phase morphologies, proportions and formations, but also affected the performance of the final steel. In addition, increasing the cooling temperature and duration time decreased the yield and tensile strength, but increased the total elongation. The results showed that with the appropriate proportion of alloy addition and the cooling process control, an optimized balanced combination of high strength and good elongation can be obtained.