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

In the paper, C-Si-Mn steels were soaked at 800℃ for 120 s, and then rapidly cooled to 350℃ and held for 100~1 000 s to simulate the bainite isothermal transformation process. The influence of bainite isothermal time 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 com-posed of ferrite, bainite, martensite and retained austenite. The bainite and retained austenite formed in the isothermal stage, and the martensite isl and formed in the final cooling stage after isothermal process.With an increase in the bainite isothermal time, the solute C atoms fully diffused into the retained austen-ite due to the transformation of the supercooled austenite into bainite. Therefore, the average solute C content in the austenite increased due to the enrichment of solute C atoms and the decrease of the austen-ite volume fraction. And the martensite volume fraction reduced, and the retained austenite volume frac-tion increased. As a result, with an increase in the isothermal time from 100 s to 1 000 s, the yield strength increased from 596 MPa to 692 MPa, the tensile strength decreased from 1 455 MPa to 1 138 MPa, the YS/UTS ratio increased from 0.41 to 0.60, and the elongation decreased from 6.2% to 18.9%.The cold-rolled ultra-high strength TRIP steel with excellent comprehensive mechanical properties(R_m×A=21 510 MPa·%) was obtained at 1 000 s of bainitic isothermal holding.