Abstract: Twining-induced plastic deformation (TWIP) steel has low yield strength and high plasticity, which is not conducive to its popularization and application. In order to improve the comprehensive mechanical properties of the TWIP steel, the surface mechanical rolling treatment (SMRT) is used to introduce a strengthened layer with certain thickness on the surface of annealed low-stacking faults energy single-phase austenitic TWIP steel. The surface feature, microstructure and fracture morphologies were investigated by means of laser scanning confocal microscope, optical microscope and scanning electron microscope respectively, and the hardness gradient and tensile properties were evaluated by Vickers hardness tester and MTS universal testing system. The results show that after the SMRT, the TWIP steel surface forms a strengthened layer of about 1.4 mm in thickness, and with good surface integrity. The microstructure analysis shows that the surface grains are refined, and the dislocation density and deformation twin density are increased. The mechanical property test results show that the surface hardness of the TWIP steel after SMRT reaches 647.5 HV0.1, which is about 2 times higher than that of the as-annealed state, though it decreases gradually from the surface to the as-annealed hardness of 230 HV0.1. The yield strength, tensile strength, and percentage total extension at fracture of the as-annealed TWIP steel are about 270 MPa, 878 MPa, and 73%, respectively. After the SMRT, the yield strength of the TWIP steel is increased to 1008 MPa, which is increased to about 3.7 times that of the annealed; the tensile strength and percentage total extension at fracture are increased to 1142 MPa and to about 20.5%, respectively, showing excellent comprehensive mechanical properties.

Key words: TWIP steel, annealing, surface mechanical rolling treatment (SMRT), microstructure, mechanical properties