The Fundamengtal Research Of Controlled Rolling On Low Carbon Mn-Cr-Ni-Mo High Strength Steel

The aim of the present dissertation is to investigate the fundamental process ofcontrolled rolling on low carbon Mn-Cr-Ni-Mo high strength steel and study the deformedaustenitic recrystallization behavior in the controlled rolling process. In order to providethe foregoing foundation for the process of controlled cooling.The deformed austenite of low carbon Mn-Cr-Ni-Mo high strength steel was cooledto room temperature by the different cooling rates performed on a Gleeble-3500thermomechanical simulator equipped. The microstructure features were examined byoptical microscopy and transmission electron microscopy and the Vickers hardness wasmeasured on a hardometer, then obtained the continuous cooling transformation curve ofthis steel. With the assistance of Gleeble-3500thermomechanical simulator equipped tosimulate a single pass and double pass compression process respectively, to analyse therules of deformed austenitic dynamic recrystallization and static recrystallization.The results show that at the lower cooling rate0.5oC/s and1oC/s, there are theexistence of polygonal ferrite and quasi-polygonal ferrite in the microstructure. With thecooling rate increasing, when the cooling rate reached3oC/s, the main microstructure isgranular bainite. The proportion of the granular bainite reduced while the proportion of thelath bainite increased with the cooling rate increasing. As the cooling rate reached25oC/s,the major microstructure is lath bainite. With the cooling rate increasing, the hardness ofthe microstructure increased, the hardness of polygonal ferrite and quasi-polygonal ferriteis small and the hardness of lath bainite is higher than granular bainite.The microstructure of granular bainite and lath bainite is quite different. The M/Acomponent of granular bainite has the shape of elongated strip and mass existed at theclearance of the lath and prior austenite grain boundaries while the M/A component of lathbainite has the shape of pellicle and acicular existed at the clearance of the lath. Thedislocation density in the lath of lath bainite is higher than the lath of granular bainite.With the higher temperature and lower strain rate, the dynamic recrystallization ofthis steel is easy to take place, and the activity energy of dynamic recrystallization is381kJ/mol. The volume fraction of static recrystallization is enhanced with the higher deformation temperature and the longer interval time between deformation passes, theactivity energy of static recrystallization is446kJ/mol.