Study On Microstructure And Mechanical Properties Of700MPa Grade Cold-rolled Weathering Dual Phase Steel

With the rapid development of national economy and high-speed, high-intensity light rail freight, as a kind of widely used steel, weathering steel not only need good corrosion resistance, but also need to improve their mechanical properties on the basis of lower costs. Cold-rolled weathering dual phase steel being applied in railway carriage, could reduce railway carriage weight and saving cost. Under the background of the State Key Laboratory of Rolling and Automation’s independent project, which is about the research and development of railway carriage board of700MPa grade cold-rolled weathering dual phase steel, this paper is to investigate the microstructure evolution during continuous process and continuous annealing process, to investigate the effect of annealing temperature, overaging temperature and slow-cooling process on the microstructure and mechanical properties of experimental steels, and to investigate corrosion resistance by dry/wet cyclic accelerated corrosion experiment. The main work is as follows:(1) The continuous cooling transformation (CCT) curves of experimental steel were determined by Formastor-F II in three different austenitizing temperatures. With the heating temperature decreasing, the ferrite transformation point was higher and the martensite transformation point was lower. The martensite transformation has happened with a low cooling velocity of0.5℃/s, while the pearlite is few. This is caused by the increasing hardenability of the steel for the high quantity of Cr and Mn.(2) The continuous annealing process of cold-rolled weathering dual phase steel was simulated by MMS-300thermomechanical sinulator and the microstructural evolution during continuous annealing process of the steel was studied. The results show that ferrite recrystallization, pearlite dissolution and austenite formed at initial period of continuous annealing of the experimental steel. The ferrite recrystallization is starting at640℃. The ferrite recrystallization and phase transformation occurs at the same time during the critical area. The volume fraction of austenite increased over time at the same heating temperature. With the slow-colding temperature rising, the volume fraction of martensite increased and the ferrite decreased. In the range of from350℃to450℃, the second phase were bainite and martensite. And the second phase mainly were the martensite when the tempting temperature below350℃.(3) Effect of annealing temperature, overaging temperature and slow-cooling process on the microstructure and mechanical properties of experimental steels had been studied by MMS-300thermomechanical simulator and continuous annealing simulator. It can be found that annealing temperature, overaging temperature and slow-cooling process have a major impact on the microstructure and mechanical properties of experimental steels. The strengh of experimental steel decreases and the elongation decreases when the annealing temperature increases.With decreasing overaging temperature ranging from350℃to200℃, tensile strength of experimental steel improves while elongation and yield point elongation decrease. The lower the slow-cooling temperature is, the lower strengh and the higher elongation in the same annealing temperature.(4) Dry/wet cyclic accelerated corrosion experiment was conducted by ESPECCORP.CSH-220constant temperature humidity chamber on the weathering steels as well as SPA-H steel and Q345steel, to investigate the corrosion resistance for weathering steels. Meanwhile, the difference of rust layer structure and morphology between weathering steels and carbon steel was analyzed.