Effect Of Critical Annealing On Mechanical Properties Of Microalloyed Medium Manganese Steel And Its Mechanism

In recent years,high strength and light medium manganese steel has become the focus of the research and development of the third generation automotive steel because of its excellent strength and plasticity and taking into account the economy.How to further improve the mechanical properties of medium manganese steel is the focus of attention at present.In this paper,a small amount of V and Mo alloy elements are added to 11mn medium manganese steel to further refine the grain and realize fine grain strengthening.At the same time,combined with the critical annealing process,the stability of metastable austenite is adjusted to achieve effective trip effect and comprehensively improve the properties.In this paper,the effect of critical annealing process（annealing temperature and holding time）on the microstructure and properties of cold-rolled medium manganese steel is systematically studied,and the micro action mechanism is revealed.The research results are as follows:（1）The A_C1 and A_C3 temperatures of cold rolled microalloyed medium manganese steel are 450℃and 786℃respectively.After critical annealing at different temperatures（20℃interval）in the range of 610-670℃,the microstructure of the test steel is composed of ferrite,austenite,thermally induced martensite（produced by martensitic transformation during cooling after critical annealing）and residual martensite（inherited from hot rolling microstructure and not disappeared after critical annealing）.（2）With the increase of critical annealing temperature,the austenite content first increases and then decreases,and the stability continues to decrease,resulting in the strong plastic product（the product of tensile strength and fracture elongation）of the test steel first increases and then decreases;When the critical annealing temperature is 630℃and the annealing time is 30min,the comprehensive mechanical properties are the best,in which the yield strength is 1048mpa,the tensile strength is 1737mpa and the elongation is27.6%,that is,the strong plastic product is 37.8Gpa?%.Under this condition,since no obvious precipitates are found,the high yield strength mainly comes from fine grain strengthening.（3）In the range of 610-670℃critical annealing temperature,the austenite stacking fault energy changes with the increase of critical annealing temperature.Martensitic transformation occurs in metastable austenite during tensile deformation of the test steel at all temperatures,which induces trip effect to varying degrees,so that the work hardening rate of the test steel shows the multi-stage hardening characteristics of first decreasing,then increasing,then decreasing again until fracture.Based on the classical O-C model,a quantitative model of martensite transformation kinetics is established.（4）During the tensile deformation of metastable austenite,the martensite transformation is different from the conventional nucleation mode at the intersection of deformation bandsγ→α′.The direct nucleation is mainly due to the small grain size of metastable austenite,and the generation of deformation bands in different orientations is limited by the size effect in the deformation process.