Microstructure Control And Mechanical Properties Of High Mn TRIP Steels

The microstructural evolution and corresponding mechanisms during warm deformation of martensite and subsequent intercritical annealing of high Mn TRIP steels (4.0 wt.%-8.0 wt.%Mn) were investigated using Gleeble hot simulation experiments and annealing experiments, and high Mn TRIP steels with a multiphase micro structure consisting of ultrafine-grained fcrrite matrix and M/A (martensite/retained austenite) islands were prepared. The relationships between the microstructural evolution and the room-temperature macroscopic mechanical properties of high Mn TRIP steels, as well as the influences of processing parameters and alloy elements, were investigated using scanning electron microscopy (SEM), electron back scattered diffraction (EBSD), transmission electron microscopy (TEM), and tensile tests. The microscopic stress/strain partitioning behavior among the constitute phases in high Mn TRIP steels during plastic deformation was studied using in-situ high-energy X-ray diffraction (In-situ HEXRD) based on synchrotron radiation. The results were as following:Martensitic structure could be obtained through air cooling after the austenization of high Mn TRIP steels. Warm deformation of martensite promoted the occurrence of the decomposition of martensite into ferrite and carbide particles, the dynamic recrystallization of ferrite, the reversed transformation of austenite and the dissolution of carbide particles. During subsequent intercritical annealing, static recrystallization of ferrite resulted in the increase in the fraction of equiaxed ferrite grains, and austenite was formed gradually with the continuous dissolution of carbide particles. By warm deformation of martensite and subsequent intercritical annealing, multiphase microstructures of high Mn TRIP steels could be obtained with smaller strains and shorter annealing times, which consisted of submicron-grained ferrite, martensite and retained austenite (RA). The comprehensive mechanical properties, i.e. the products of tensile strength and total elongation, of high Mn TRIP steels based on warm deformation of martensite were comparable with those with similar compositions subjected to long-time intercritical annealing after cold rolling of martensite.The yielding strength of high Mn TRIP steels increased with the increasing in the strain of warm deformation, and decreased with the increasing in the annealing time. With the increasing in the strain of warm deformation or the annealing time, the stability of RA was decreased, resulting in the higher tensile strength and the lower elongation of high Mn TRIP steels. The increasing in the deformation temperature or the decreasing in the strain rate during warm deformation of martensite, as well as the increasing in the annealing temperature, could lead to the more uniform multiphase microstructures with more RA of high Mn TRIP steels, resulting in the improvement in the work-hardening capability and the balance of tensile strength and total elongation of high Mn TRIP steels.During warm deformation of martensite, the deformation activation energy was increased by the increasing in Mn content, and decreased by the increasing in C content. The increase in Si content or Al content showed slight influence on the deformation activation energy. The decomposition of martensite was promoted by the increasing in C content, and retarded by the increasing in Mn content, Si content or Al content. Dynamic recrystallization of ferrite was accelerated by the increasing in C content, and retarded by the increasing in Mn content or Si content. Meanwhile, the reversed transformation of austenite was promoted by the increasing in Mn content, and retarded by the increasing in C content, Si content or Al content. The increasing in Mn content, leading to the increase in the volume fraction of RA with lower stability in the final multiphase microstructure, resulted in the improvement in the tensile strength of high Mn TRIP steels, but with lower elongation. The increasing in C content or Si content, leading to more RA with higher stability in the final microstructures, resulted in higher tensile strength of high Mn TRIP steels, while maintaining higher elongation. The increasing in Al content, leading to more RA with much higher stability in the final microstructures, resulted in the lower work-hardening capability and tensile strength of high Mn TRIP steels, with higher uniform elongation and total elongation.The microscopic stress/strain partitioning behavior among the constituent phases and the effect of the RA transformation dynamics on the work-hardening capability of high Mn TRIP steels prepared by two processing conditions during plastic deformation were investigated using in-situ HEXRD experiments based on synchrotron radiation, and the constitutive model of the stress-strain relationship of high Mn TRIP steels was established preliminarily based on Gladman-type mixture law.