Study On Interface Regulation And Deformation Mechanism Of TWIP/TRIP Multi-Layer Composite Steel

The use of high-strength steel sheets is one of the main means of light-weighting automobiles.In recent years,people have developed a series of advanced high-strength steel sheets.Among them,TWIP/TRIP steel has recived striking attetion with its excellent work hardening ability.The traditional TWIP/TRIP steel is in a completely recrystallized state and the microstructure is austenite.But its suffers from the low yield strength,which limits the further application of TWIP/TRIP steel.In this paper,TWIP steel and TRIP steel are hot rolled into multi-layer composite steel.We improve the yield strength with diffusion strengthening and pre-strain strengthening.The main research contents and results of this paper are as follows:In this paper,three different compositions of Fe-Mn-Si-Al based TWIP/TRIP steel were studied.We observed and analyzed the microstructure evolution and work hardening behavior during the stretching process.According to the fluctuation of the work hardening curve,it was divided into different stages.The effects of deformation twinning,stress induced ? martensite and dislocation slip on the work hardening at different strain conditions were analyzed.The TWIP/TRIP multi-layer composite steels were prepared by vacuum rolling technology with TWIP steel and TRIP steel which have excellent performance.The microstructure evolution and properties of the interface were studied.During the hot rolling process,the interface oxide layer becomes thinner,fractured and finally dispersed at the interface as the rolling reduction rate increases,and dynamic recrystallization occurs inside the matrix.The thicker oxide layer hinders the further growth of the crystal grains to refine the grains.When the oxidation layer has less hindrance,the grains cross the interface to the adjacent layers,so that the interfacial bonding strength is further improved.The effects of cold rolling pre-strain and heat treatment on the microstructure and properties of TWIP/TRIP multilayer composite steels were investigated.After pre-strain,sufficient and suitable deformation twins and ? martensite are produced,and then the pre-strained TWIP/TRIP multilayer composite steel was subjected to low temperature martensite recovery treatment.By using the reversibility of the martensitic transformation,the ? martensite was restored to a softer austenite matrix,and the deformation twins and dislocations have high thermodynamic stability at this temperature.In the subsequent stretching process,the yield strength of multi-layer composite steel was improved by deformation twinning and high-density dislocation.The softer austenite matrix induced ?martensitic transformation by re-deformation,ensuring good work hardening rate and plasticity.Different pre-strain rolling reductions and heat treatment process can achieve different strength and plasticity matching,realizing the asynchronous synergy of TWIP/TRIP effect and providing a new method for improving the yield strength of TWIP/TRIP multi-layer composite steel.