Study Of Rolling And Continuous Annealing Process Of Ultra-high Strength Dual Phase Steel

In order to reduce weight of the automobile, oil consumption and emissions, and improve security, the development of the high strength automobile steel has become an inevitable trend. In order to satisfy reduced steel rolling technology and automobile steel development, and accelerate domestic ultra-high strength dual phase steel developing and using, by the hot rolling experiments of the laboratory, ultra-high strength hot rolling ferritic/bainite dual plase steels was researched, and applying the continuous annealing simulator of the laboratory of simulating continuous annealing process, the effect of intercritical holding time, fast cooling rate and overaging temperature during continuous annealing process on microstructures and properties of ultra-high strength cold rolling dual phase steels was researched in this thesis.Under laboratory conditions, ultra-high strength dual plase steel above1000MPa grade was developed successfully by reasonable rolling system, cooling process and continuous annealing process scheme. The main works and achievements were shown as following:(1) Both phase transformation laws and recrystallization behavior of tested steels were investigated by thermal simulation experiment. Results showed that tested steels can get bainite and martensite organizations at slower cooling rate; it was more advantageous for austenite dynamic recrystalization at low strain rate and high deformation temperature; dynamic recrystallization start time is shortened with the increase of strain rate and the rise of temperature; static recrystallization softening fraction increases with temperature and holding time increasing.(2) The microstructures and properties of hot rolling tested steels were researched by the hot rolling experiments. Results showed that the ferrite grain of the tested steel get refined by reduce finishing temperature and increase rolling reduction. There is11.4%retain austenite that play a great contribution between good strong and plastic matching when tested steel cools in the air after laminar flow cooling to560℃; Cooling in the air or along with the furnace after laminar flow cooling to about600℃, it could get tensile strength≥1000MPa and elongation percentage≥16%good strong and plastic matching ferrite/bainite hot rolling dual phase steel.(3) The effect of intercritical holding time during continuous annealing process on microstructures and properties of tested steels was researched by simulating continuous annealing process. Results showed that the microstructures and properties of tested steel begin to stabilize at annealed180s above. when tested steel holds180s at820℃, there is7.5%retain austenite that play a great contribution between good strong and plastic match that tensile strength1350MPa and elongation percentage8.2%.(4) The effect of fast cooling rate during continuous annealing process on microstructures and properties of tested steels was researched by simulating continuous annealing process. Results showed that hardenability of tested steel is excellent, When cooling at5℃/s, martensite island distributed uniformly, retain austenite content is7.4%, and it could get tensile strength1360MPa and elongation percentage10.84%and have good strong plastic matching cold rolling dual phase steel.(5) The effect of overaging temperature during continuous annealing process on microstructures and properties of tested steels was researched by simulating continuous annealing process. Results showed that martensite islands are dispersed in the polygon ferritic matrix, retain austenite content is8.4%, it could get tensile strength1040MPa and elongation percentage15%and have good strong plastic matching cold rolling dual phase steel.