Research On Microstructure, Properties And Springback Of Cold-Rolled Dual Phase Steel

In order to adapt to the development of automobiles lightening, extend the application of dual phase steel in the automobile industry of our country, it’s of great significance to study the microstructure, property, forming limit and springback of dual phase steel which comes from continuous annealing of cold-rolled Q345. Combining with metallographic analysis and mechanical property testing, the effect of intercritical heating temperature, quenching temperature and overaging temperature on microstructure and properties were considered in this thesis. With the help of BCS universal sheet testing machine, the forming limit diagram was measured to study the forming ability of dual phase steel sheet. The U-shape sheet was also taken as an object to study the springback of tested dual phase steel. Combining with numerical simulation and physical experiment, the effects of virtual stamping speed, binder holding force and drawing depth on springback were considered in this thesis.The main works and achievements were stated as following:(1) The effect of intercritical heating temperature on microstructure and properties of dual phase steel was studied. As the intercritical heating temperature increased, the volume percentage of martensite increased, both yield strength and tensile strength of dual phase steel increased.(2) The effect of quenching temperature on microstructure and properties of dual phase steel was studied. As the quenching temperature decreased, austenite partially transform into epitaxial ferrite during slow cooling. In a certain temperature range, the decreasing of quenching temperature can increase the elongation and meanwhile decrease the yield strength of dual phase steel.(3) The effect of overaging temperature on microstructure and properties of dual phase steel was studied. With a decreasing overaging temperature, the elongation decreased and the tensile strength increased. With an increasing overaging temperature, a part of martensite resolved, the tensile strength decreased, and the yield platform appeared on the engineering strain-stress curve. (4) The forming limit diagram of dual phase steel was measured through the width changing of the specimen. The left part of the forming limit diagram was studied emphatically.(5) The effect of virtual stamping speed on the springback of dual phase steel sheet was studied. It was found that increasing the virtual stamping speed can effectively increase the simulation efficiency. It didn’t show a significant effect on the broadside springback angle and the flange springback angle when the virtual stamping speed was lower than5000mm/s.(6) Numerical simulation and physical experiment were taken to study the effect of binder holding force on the springback of dual phase steel sheet. It was found that as the binder holding force increased, the broadside springback angle and the flange springback angle effectively decreased, while the thickness reduction rate increased.(7) Numerical simulation and physical experiment were taken to study the effect of drawing depth on the springback of dual phase steel sheet. It was found that as the drawing depth increased, the broadside springback angle and the flange increased. The result of the numerical simulation shows the same tendency as the result of the physical experiment.