Study On The Microstructure And Properties Of Rapid Continuous Heat Treatment Process Of DP780 Steel

Dual-phase steel has excellent mechanical properties and molding properties as perfect high-strength steel plate. High-strength dual phase steel is one of the most widely used variety of automobile structure. At present, international supplies of cold rolled dual phase steel sheet is on the highest level of 1180 MPa, but only 780 MPa in China. Comparing with the world’s advanced level, there is a large gap in China. So the development of high-level dual-phase steel is imminent. This paper aims to study the effect of rapid continuous heat treatment on the microstructure and mechanical properties of DP780 steel.In this paper, an automatic phase-change instrument was employed, and the Ac1 and Ac3 of DP780 hot rolled dual-phase steel at different heating rates were determined through the expansion curve. Then, the CHT curve of DP780 dual-phase steel was drawn depending on the phase transition point. Still, the Ar3 and Ar1 at the different cooling rates were determined through combinations of metallographic method and hardness mearsurement.Using single factor variables, rapid continuous heat treatment was simulated in automatic phase-change instrument. The best heat treatment process was found by changing heating rate, heating temperature, holding time and cooling rate. In the process of heat treatment, when heating rate was 100 ℃·s-1 and just the heating rate was changed, volume fraction of martensite in the organization was 48% and maximum hardness value was 369 HV. When heating temperature was 820℃ and just heating temperature was changed, volume fraction of martensite in the organization was 52% and maximum hardness value was 424 HV. When holding time was 80 s and just holding time was changed, volume fraction of martensite in the organization was 53% and maximum hardness value was 319 HV. When cooling rate was 200 ℃·s-1 and just cooling rate was changed, volume fraction of martensite in the organization was 57% and maximum hardness value was 303 HV.DP780 steel had been treated at the temperature of 740,750,760,770,790 and 820℃ for 60 and 90 s, respectively, quickly taken out into water for quenching. The microstructure was observed, and the mechanical properties were measured. It showed that when quenching time was 60 s, tensile strength increased and elongation reduced with the increasing of quenching temperature. When quenching time was 90 s, with quenching temperature increasing, tensile strength increased firstly, then decreased, and then increased again, while elongation decreased firstly, then increased, and then decreased again. The best quenching process was that quenching time was 90 s at 760℃. The tensile strength and the elongation reached 1141 MPa and 11%, respectively. Island martensites were interconnected to each other, presenting obvious continuity. The volume fraction of martensite in the organization was 32%.