Microstructure Evolution And Strength Of Mechanical Properties For Medium-Mn Steel Through Inter-critical Rolling

The objective of my study is high strength and high toughness of medium-Mn steelwhich microstructure will refine and have a certain volume of austenite throughinter-critical rolling. And stability of austenite and mechanism of TRIP effect will beexplained through tensile experiment with different temperature and different pre-tensiledeformation. Finally, before press forming stability of austenite and changing of strengthand toughness for inter-critical rolled steel will be analyzed through cold-rolling withdifferent temperature and different rolling deformation.(1) In the course of inter-critical rolling microstructure will change into duplex phasesof austenite and ferrite laths, which parallel with rolling direction and have few carbides.Till rolling deformation of82.5%thickness of laths is only0.12μm, and volume ofretained austenite keeps above30%. The yield strength and tensile strength increase withincrement of rolling deformation, and tensile strength reaches1300MPa under rollingdeformation of82.5%. Uniform elongation is unchanged with increment of rollingdeformation and the same as one of non-deformation. The product of tensile strength andelongation is about45GPa%which reaches the aim of third generation automobile steel.Although RT impact toughness declines, impact toughness at low temperature increasesduring inter-critical rolling which keeps25J at liquid nitrogen temperature.(2) After inter-critical rolling the yield strength, tensile strength and elongation of thestudied steel of77.5%rolling deformation have the trend of sine distribution at differenttensile temperature, which reach most remarkable effect of TRIP at RT. Luders band isfound in the curse of tensile, which is caused by non-uniform deformation when localretained austenite transforms into martensite under tensile stress. And this transformationcan delay necking in order to enhance elongation and strength of the studied steel.(3) After inter-critical rolling the studied steel is rolled at RT and373K. Microstructureremains duplex phases of austenite and ferrite laths, and with increment of cold-rollingdeformation volume of retained austenite declines. Disappeared austenite transforms intomartensite with precipitation of carbide between laths. A lot of austenite can be kept with increment of cold-rolling temperature, and after10%rolling deformation at373K volumeof retained austenite remains14%which can bring higher toughness than one after RTrolling. Yield strength and tensile strength increases through cold-rolling, and till10%rolling deformation at RT yield strength reaches1180MPa which is higher than one ofnon-deformation. This is caused by increment of dislocation after cold-rolling. Impacttoughness has a slight decline after RT rolling, but remains the same level ofnon-deformation after cold-rolling at373K, which indicated that higher rollingtemperature induces more retained austenite and suppresses increment of dislocationdensity.