BEHAVIOUR AND MECHANISM OF STRAIN HARDENING FOR DUAL PHASE STEEL DP1180 UNDER HIGH STRAIN RATE DEFORMATION

Abstract

Strain hardening behaviour and mechanism of a cold-rolled dual phase steel DP1180 under quasi-static tensile condition at a strain rate of 0.001 s~(-1) by electronic universal testing machine,and dynamic tensile condition at strain rates of 500 and 1750 s~(-1) by split Hopkinson tensile bar(SHTB) apparatus were systematically studied.According to the modified Swift true strain-stress model,the experimental data was regressed by using nonlinear fitting method,and strain hardening exponent in the modified Swift model was calculated by a modified Crussard-Jaoul method.The results revealed that there are two stage strain hardening characteristics of DP1180 steel at the strain rate range of 0.001—1750 s~(-1),the strain hardening ability of the stage I was enhanced with increase of strain rate,while the strain hardening ability of the stage II was weakened, and the transition strain was decreased.The ferrite near the martensite regions formed cell blocks with dislocation structures,with a size of 90 nm,due to the limit of deformation compatibility, and the existence of geometrically necessary boundary(GNB) made DP1180 steel not instantly damaged under deformation at high strain rates.In addition,the adiabatic temperature rise of△T= 103°C made martensite easy to have plastic deformation at a strain rate of 1750 s~(-1).