Effect Of Asymmetrical Rolling On Austenite Transformation And Microstructure Evolution In 22MnB5 Steel

In recent years,the continuous development of automobile industry,the safety and environmental protection requirements of the automobile are increasing,and the vehicle body lightweight has become a hot spot in the industry research.Hot forming ultra high strength steel has been widely used in automobile structural parts due to its high strength,but its low plasticity restricts its further development and application.Therefore,in order to improve the comprehensive properties of ultra high strength steel,it is necessary to improve the microstructure.In this paper,22MnB5 steel is taken as the research object.Through the predeformation treatment of different deformation quantity and speed ratio at room temperature and cryogenic condition,the behavior of austenite transformation and microstructure evolution in the heating process are explored,and the evolution behavior of the typical microstructure and texture orientation is analyzed in the process of one heating and compound heating.The experimental results show that:(1)At room temperature rolling,the deformation degree of asymmetrical rolling is larger than that of symmetrical rolling,and the microstructure is refined.When the deformation is 70%,the average grain size is about 3 ?m.During the process of rolling,a large number of dislocations are produced in the microstructure.The dislocation density of the surface layer is higher than that of the core part,and the proportion of the small angle grain boundary is above 50%,which indicates that the internal dislocation and other defects in the sheet are increased,and the texture components are concentrated near {110}<110> and {111}<110>,and the maximum texture strength is 7.238.(2)In cryogenic rolling,the deformation degree of tissue is more intense than that at room temperature.Cryogenic rolling can inhibit the dislocation recovery,the dislocation proliferation speed is fast,the intertwined entanglement is easy to form the dislocation plug,and the lamellar microstrip substructure appears.After cryogenic rolling,the proportion of grain boundary in small angle will also increase.Besides {110}<110> and {111}<110> texture components,there is a strong{112}<110> texture,and the maximum texture strength is 12.044.(3)After the rolling predeformation treatment,the heat treatment experimentswere carried out on the plate with one time heating and compound heating.The results showed that the asymmetrical rolling could promote the austenite transformation more than the symmetrical rolling;And the cryogenic rolling was more advantageous to the austenite phase transformation than the room temperature rolling;And compound heating can accelerate austenite formation and refine grain size more than once heating.The above two processes are verified by salt bath quenching,at 750 ?,the ferrite was recrystallized.With the increase of the deformation and speed ratio,the equiaxed grain could be obtained,which was stronger in the vicinity of the {111}<110> and {110}<110> texture components.When the temperature is 800? and 850?,the martensite is gradually formed and austenitizing is achieved finally,and the martensite structure is obtained.Therefore,asymmetrical rolling can accumulate deformable energy and defect density more than symmetrical rolling,which helps to maximize the nucleation density of the austenite deformation,and has a beneficial effect on the subsequent ferrite recrystallization type,austenite transformation behavior and quenched martensite through the tissue genetic effect,and lays a foundation for the subsequent hot stamping process.The foundation helps to improve the comprehensive performance of final forming parts.