| As the third generation of advanced high-strength steel,medium Mn steel has important application potential in the fields of automobile steel plate and offshore platform steel,which has received great attention,and important research has been carried out on the composition design,microstructure regulation,heat treatment process and phase transformation behavior of medium Mn steel.However,in view of the automobile steel plate steel,cold molding in different position,in different variables and also has different pressing rate.Likewise,the general automobile steel plate needs to undergo the paint-baking treatment after forming.At present,the effects of strain rate,pre-strain and baking treatment on the microstructure and mechanical properties of medium Mn steel and their mechanisms are not clear.This thesis takes a medium Mn TRIP steel with a small amount of rare earth cerium（Ce）as the research object.The heterostructure medium Mn steel was prepared by intercritical annealing plus low temperature tempering heat treatment.The effects of strain rate,pre-strain and baking treatment on microstructure,dislocation patterns,mechanical properties as well as the transformation mechanism of the above medium Mn steel were studied by in-situ EBSD,TEM,XRD and other advanced test technologies.The influence of the above process parameters on the austenite transformation behavior of medium Mn steel was revealed,and the suitable service conditions of medium Mn TRIP steel were obtained,which provided theoretical and experimental support for the development of the third generation of advanced high-strength steel.The main conclusions are as follows:（1）The influence of strain rates on the properties of medium Mn TRIP steel was studied.It was found that with the increase of strain rate,the yield strength decreased slightly,and the tensile strength and elongation showed a similar trend of first increasing and then decreasing.When the strain rate was 10-3s-1,the best combination of strength and plasticity was obtained.The ultimate tensile strength of the experimental steel was1293MPa,the total elongation was 53.3%,and the product of strength and elongation was68.9GPa·%,significantly exceeding the standard of 30GPa·%of the third generation of advanced high-strength steel.（2）The influence of pre-strains on the properties of medium Mn TRIP steel was studied.It was found that with the increase of pre-strains,the yield platform disappeared,and the yield strength and ultimate tensile strength showed an overall upward trend.In addition,the results showed that the elongation and the product of strength and elongation showed an overall downward trend,the austenite structure was transformed into M/A phase,and hetero-deformation-induced（HDI）stress significantly increased with the increase of pre-strain.（3）Through in-depth analysis of the strengthening and toughening mechanism of heterostructure medium Mn TRIP steel,it was found that the mechanical stability of austenite with different morphologies was dispersive,leading to the TRIP effect continuing to occur in a large strain range.In addition,HDI stress of the experimental steel could lead to the formation of a core-shell structure with austenite andα’-martensite,which improved the stability of the untransformed austenite,and thus significantly affected its TRIP effect,which increased continuously with the increase of true strain.（4）The influence of baking treatment on the properties of medium Mn TRIP steel with different pre-strains was studied.It was found that the yield platform of the experimental steel gradually disappeared with the increase of pre-strain.After baking treatment,the yield strength and ultimate tensile strength of the experimental steel increased significantly,and the bake hardening phenomenon continued to increase.When the pre-strain was 25%,the yield strength and tensile strength increased by 233MPa and152MPa respectively.（5）The effects of baking temperatures on the properties of medium Mn TRIP steel were studied.It was found the diffusion coefficient of the solid solution C atom of the experimental steel increased continuously with the increase of baking temperature,which led to the continuous increase of BH value;in addition,the product of strength and elongation reached more than 50GPa·%.When the baking temperature reached 190℃,the BH value changed relatively gently,and the baking temperature has no significant effect on the bake hardening effect of the studied medium Mn steel. |
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