Effect of Bainite Isothermal Temperature on Microstructure and Mechanical Properties of Ultra-high Strength Cold Rolled Transformation Induced Plasticity Steel

In this paper,C-Si-Mn steels were soaked at 800 ℃ for 120 s,and then rapidly cooled to 350～410 ℃ and held for 600 s to simulate the bainite transformation process.The influence of bainite isothermal temperature on the microstructure and mechanical properties of ultra-high strength cold rolled transformation induced plasticity（TRIP）steel was investigated by means of scanning electron microscopy（SEM）and tensile test.The results showed that the microstructure of cold rolled TRIP steel was composed of ferrite,bainite,martensite and retained austenite.The bainite and retained austenite were formed in the isothermal stage,and the martensite island was formed in the final cooling stage after isothermal process.With an increase in the bainite isothermal temperature,the carbide precipitated fully due to the increase of diffusion coefficient.Therefore,the average solute C content in the supercooled austenite decreased due to the precipitation of solute C atoms.The martensite volume fraction increased,and the retained austenite volume fraction reduced.As a result,with an increase in the isothermal temperature from 350 ℃ to 410 ℃,the yield strength decreased from 720 MPa to 573 MPa,the tensile strength increased from 1 195 MPa to 1 312 MPa,and the elongation A₈₀decreased from 17.8% to 12.5%.Therefore,the cold rolled TRIP steel with excellent comprehensive mechanical properties(R_m×A₈₀=21 270 MPa·%)was obtained by bainite isothermal holding at 350 ℃.