Study On Microstructure And Property Of Cold Rolled And Continuous Annealled DP Steel With Low C-Si-Mn System

In order to satisfy reduced steel rolling technology and automobile steel development, and accelerate domestic dual phase steel developing and using, combining with metallogoraphic analysis and mechanical property testing, the effect of initial structure, intercritical reheating temperature and holding time, quenching temperature, overaging temperature and holding time during continuous annealing process on microstructures and properties of low C-Si-Mn system DP steels was researched in this thesis, applying thermal simulator, resistance furnace and salt bath furnace. Meanwhile industrial was intruduced and measures of improving tensile strength of trial-producing steel sheet were proposed. The main works and achievements were shown as following:(1) The effect mechanism of initial grain size and matrix structure on microstrctures and properties of tested steels was clarified. It's found that initial fine grain structure and annealing martensite matrix contribute to obtaining better properties for tested steels.Using high-temperature resistance furnace and low-temperature salt bath furnace to simulateγ+αdual phase region temperature and overaging temperature respectively, the effect of different initial grain size and matrix structure on microstructures and properties of tested steels was researched. The results show that the initial grain size is smaller, the martensitic volume fraction is more, the diffusion distance of alloying elements is shorter, austenitic composition is more homogeneous and the time for accomplishing microstructure and property stablization is earlier. After undergoing continuous annealing process, fine and fibrous second phase exists in bainite and annealing martensite matrix, while coarse and network second phase exists in ferrite and pearlite matrix. Plasticity of the former is superior to the latter which attributes to the former owing excellent combination of phase interface for reheating cavity forming during the deformation. But plasticity of the former is inferior to the latter when overaging at 350℃and 450℃, which attribute to weak combination between brittle carbide and matrix, and more dispersed carbide leading to improve cavity forming probability in the former microstructure.(2) The effect mechanism of intercritical reheating temperature and holding time on microstrctures and properties of tested steels was clarified. Reducing radius method as an aided analysis method was applied.Using MMS-200 simulator, bright and continuous annealing simulator andresistance furnace, the effect of intercritical temperature and holding time on microstructures and properties of tested steels was researched. Reducing radius method is first time applied to test tensile strength in MMS-200 experiment. It is found that with intercritical temperature increasing, austenitic volume fraction increases, and austenitic average carbon content decreases which results in more bainite transformation due to austenitic hardenability reducing during air-jet cooling. After reheating to intercritical temperature, austenitic volume fraction increases slowly with holding time. Mechanical properties become stable resulting from austenitic composition homogenization with holding time, due to carbon and manganese constantly diffusing.(3) The effect mechanism of solw-cooling temperature on microstrctures and properties of tested steels was clarified. The quenching temperature for obtaining optimal comprehensive mechanical property was put forward.The effect of solw-cooling temperature on microstructures and properties of tested steels was researched through simulating slow cooling stage of continuous annealing process by furnace cooling. The results show that austenite patially transformate epitaxial ferrite during slow cooling. Martensitic amount decreases while ferrite and pearlite amount increases, and tensile strength decreases while elongation ratio improves with decreasing solw-cooling temperature. 0.08C steel and 0.17C steel has optimal comprehensive mechanical property when solw-cooling temperature is 720℃and 700℃respectively.(4) The effect mechanism of overaging temperature and holding time on microstrctures and properties of tested steels was clarified. Martensitic variation characteristic with overaging time prolonging was analysed by C-J method as a indirect and effective analysis method.The effect of overaging temperature and overaging time on microstructures and properties of tested steels was researched by bright and continuous annealing simulator, resistance furnace and salt bath furnace. C-J analysis method was applied for expaining impact factor of overaging time when overaging at 200℃. Results show that austenitic transformation product is from granular bainite to upper bainite, or is from bainite to martensite with decreasing overaging temperature. With decreasing overaging temperature, tensile strength of tested steel improves while elongation and yield point elongation decrease. When overaging at 450℃, yield strength of tested steel increases with overaging time, resulting from that cottrell atmosphere nails dislocation due to interstitial atoms segregating near dislocation line. When overaging at 200℃, strength and initial work hardening rate of tested steel decrease while elongation increases with overaging time, which attribute to martensitic decomposition and recovery leading to martensitic strength and hardness reducing.(5) Industrial trial-producing process of DP590 steel sheet was introduced. The measures that improving rapid cooling rate and decreasing overaging temperature were proposed for resolving non-reaching standard problem of tensile strength of trial-producing steel sheet, through experimental study performed on bright and continuous annealing simulator.