Effects Of Cerium On Microstructure And Mechanical Properties Of Hot-rolled TWIP Steel

TWIP steel is the second generation of advanced high strength steel and has become the main candidate material for automotive lightweight due to its excellent mechanical properties.In this paper,the effects of Ce on microstructure,inclusions and mechanical properties of TWIP steel were systematically studied by scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM)and tensile testing machine.The microalloying mechanism of Ce on TWIP steel was revealed.The influence and mechanism of different Ce content on plastic deformation mechanism and anisotropy of impact toughness of TWIP steel were clarified.The ductility of TWIP steel can be significantly improved by adding Ce element.When Ce was 0.015 wt.%,the total elongation of TWIP steel increased significantly.The tensile strength and yield strength of TWIP steel were improved when Ce was 0.045 wt.%.The effect mechanism of Ce on strain hardening behavior of TWIP steel has two reasons.Firstly,the addition of Ce reduces the stacking fault energy of TWIP steel,which improves the number of annealing twins of TWIP steel and formation rate of deformation twins.The addition of Ce has a significant effect on the interaction mode between deformation twins.In the test samples,the deformation twins of 0Ce samples are T-shaped,while the deformation twins of 0.015 Ce and0.045 Ce samples are X-shaped.This is mainly because Ce promotes dislocation near grain boundaries.Therefore,the addition of Ce is beneficial to improve the strain hardening capacity and ductility of TWIP steel.In hydrogen-rich environment,the comprehensive mechanical properties of 0.015 Ce sample are higher.The tensile strength and elongation of the three TWIP steel samples decreased significantly in the hydrogen-rich environment compared with the atmospheric environment.The mechanical properties of 0.045 Ce decreased significantly,which was attributed to the fact that both H and Ce atoms reduced the stacking fault energy of TWIP steel,therefore 0.045 Ce steel more prone to the martensitic transformation in a hydrogen-rich environment,which promoted the propagation of cracks,leading to premature fracture of0.045 Ce steel under stress.The mechanical properties of TWIP steel are greatly reduced.Adding 0.045 wt.% Ce can significantly reduce the anisotropy of TWIP steel.The TWIP steel without Ce has significant notched impact toughness anisotropy,and the ratio of impact absorption energy between transverse and rolling direction of 0Ce steel is 0.741.When the addition of Ce is 0.045 wt.%,the ratio of impact absorption energy between transverse direction and rolling direction is 0.925.This is mainly attributed to spheroidization of elongated inclusions by Ce.The elongated inclusions increase the stress concentration of TWIP steel under impact load and promote crack formation,resulting in long dimples in lateral impact fracture of 0Ce and 0.015 Ce samples.When Ce content is 0.15 wt.%,the spheroidization of inclusion is not sufficient,which has no beneficial effect on anisotropy of TWIP steel.