Twinning Induced Plasticity Steel Microstructure And Properties

As automobile industry has gradually become the major industry of China's national economy and along with the lighter of automobile, it is necessary to develop high strength and ultra high plasticity steels. A lot of new type steels are being developed in the world, such as hot stamping steel, DP steel, TRIP steel and TWIP steel. Both steel and automobile industries are interest in TWIP steel, because of its higher strength and higher plasticity.TWIP steel has high strength, high plasticity and high energy absorption, which attribute to the stabilization at room temperature and TWIP effect as deforming.In this study, the mechanical properties and microstructures of cold-rolled and annealed Fe-23Mn- 0.6C TWIP steel sheet were studied, which has low Al and Si contents.The main results are concluded as follows:1)With the OM and micro-hardness test, the recrystallization finishing temperature of the cold-rolled sheet was studied. The temperature is about 700℃when annealing for 1 to 2 minutes. The low recrystallization finishing temperature benefits the TWIP steel's commercial production on continuous annealing production line.2)With annealing temperature increasing, the strength of Fe-23Mn-0.6C TWIP steel sheet decreases, but the elongation almost keeps the constant. The strength-elongation product of annealed sheets reaches 80000MPa*% when annealing at 800℃.3)The effect of test conditions on TWIP steel was also studied. The tensile testing temperature and strain rate have effects on the mechanical properties of the TWIP steel, however the effect of strain rate is more pronounced. The strength-elongation product decreases with the increasing of the strain rate.4)Fe-23Mn-0.6C TWIP steel keeps a high strain hardening rate during tensile deformation. Its strain hardening rate-true strain curve has two or three peaks and the peak values declines slightly with the strain increasing. It is deduced that the peaks are associated with the formation of deformation twins.5) By using the OM experimental method, A full austenite micro structure is obtained for Fe-23Mn-0.6C TWIP steel when annealed at 700～950℃and water-quenched. The austenite grain size increases with annealing temperature increasing. By using the XRD test, The microstructures of fractured specimens under different tensile testing conditions are also with full austenite. By using the TEM and EBSD technology, microstructure of specimens with different strain capacity have been studied. Deformation twins form during tensile deformation. With strain increasing, the twins with different configurations and twin bundles are observed. The misorientation between the twin plane and the matrix is about 60°. With strain increasing, the misorientation of the intra-granular increases remarkably due to the formation of large amount of deformation twins, leading to grain refinement.