| Along with the development of lighter automobiles and more important in environment and resource problem, the steel industry would constantly make progress in high strength steels and even ultra high-strength steels to meet the needs for automotive industry. Microstructurc refinement is the only method that can increase both strength and toughness of steel materials in various methods, so that the microstructure refinement is an important direction in material research. This paper studies two aspects to the TWIP steel refinement: one is the equal channel angular pressing (ECAP) method, the other is the refining effect of Nb on TWIP steels.TWIP steels with different content of Mn, Al or with addition of Nb were designed, and the microstructural evolution during ECAP and the effects of Nb on TWIP steels and mechanical properties were studied by means of tensile tests and characterization through optical microscope (OM). X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). And the mechanisms of TWIP effect as well as the approaches of martensitic transformation were systematically investigated. The main results are described as follows.(1) Equal Channel Angular Pressing of the TWIP steel was successfully carried out at500℃or600℃with route C in this study. For23.8%Mn steel, the equiaxed submicrometer with high-angle boundaries were formed after four passes. The refining effect was the most significant after the first pass, then the angle of orientation was enlarged and grains with high-angle boundaries increased in the subsequent passes. The mechanical properties have been improved greatly after ECAP. The hardness increased from original172.3Hv to ECAPed300.91Hv. In31.1%Mn steel, the grain refining is not as obvious as the former one.(2) When solution temperature of this steel is during900℃～1100℃. the strength is getting lower while the elongation is larger with solution temperature increasing, but the plasticity is significantly low at1200℃. The large elongation and high tensile strength are due to twinning induced plasticity (TWIP) effect or transformation induced plasticity (TRIP) effect via multiple martensitic transformations.(3) The results show that the addition of Nb increases stacking fault energy and leads to superior ductility by favoring the formation of deformation twinning, i.e. TWIP effect. The microstructure characterization of TWIP steel containing Nb show that the addition of Nb greatly reduces the size of austensite grain, and it improves the strength of the steel. The comprehensive effects of Nb on grain refinement give rise to excellent mechanical properties.(4) SEM examination of fracture surfaces in tensile specimens exhibits all are toughness fractures. The results shows that the material has high-plasticity and its deformation and fracture mechanisms are the formation, growth and coalescence of micro-voids. No neck shrinkage appears during the test. |
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