Researh On Microstructures,Mechanical Properties And Deformation Mechanisms Of High Manganese Steels With High Strength And Plasticity

With the prosperity of automobile industry, the conflicts between the automobiles and the environment and the energy are becoming more and more complicated. In this circumstance, light-weight products become a vital goal in the future development of automobile. At the same time, it is also very important to keep the security of automobiles in the lightening-weight process. Therefore, the application of high strength steels in automobile has been a trend. In recent years, the high manganese-containing steels demonstrating super-ductile and high-strength have received much interest.The microstructure, performance and deformation mechanisms of the high manganese steels with different stacking fault energy (SFE) values have been investigated in the present work. For high Mn steels with low SFE values, the effect of grain size on the mechanical properties and deformation mechanisms was analyzed. For high Mn steel with medium and high SFE values, the microstructural evolution was observed under different deformation amounts to further understand its deformation mechanisms. Meanwhile, the dislocation motion styles were predicted. The research results involve as following:1The deformation characteristics of the experimental steels with low stacking fault energy(1) Comparing the deformation behaviors of the steels with different grain size, it shows that in the experimental steel with large grain size (130μm) the twin bands intersect with each other in the plastic deformation stage, more exactly, in the true strain range of0.12-0.33, resulting in a slow increase in strain hardening rate; while in the steel of small grain size (13μm), there is no occurrence of twins intersection, leading to a slow decrease in the strain hardening rate. Therefore, it can be inferred that there are more twin systems which are due to more twin intersection in the steel with larger grain size, and hence the elongation is improved.(2)The stacking fault energy has impact on the strain hardening behavior of TWIP steels. For the01A1and03A1steels, in the initial stage of plastic deformation the dislocations tangles play a major role in the strain hardening behavior; when the strain reaches to30%, TWIP effect becomes the dominating deformation mechanism in the two experimental steels.The generation of deformation twins in the03A1steel is due to the monotonous decrease in strain hardening rate until it is fractured, and in the01A1steel with low stacking fault energy, the deformation twinning occurs simultaneously in two or more directions and intersects with each other, which increase the strain hardening rate. The01A1steel has the similar tensile strength to the03A1steel which has a higher yield strength.(3) When03Nb steel and OONb steel subjected to solution treatment at1000℃, the03Nb steel has a fine grain structure resulting from the effect of Nb. With the occurrence of TRIP and TWIP effect in the deformation, both the elongation and the strength are improved. The increases of stacking fault energy partially inhibited the TRIP effect. As the strengthening role of TRIP effect is better than TWIP effect, the tensile strength of the03Nb steel is lower than that of the OONb steel. Besides, small grains in the03Nb steel inhibit the TWIP effect, which reduces the plasticity of the material.2The deformation characteristics of the experimental steel with medium stacking fault energy(1) Compared with the microstructures of the06A1steel subjected to different strains, it is found that dislocation groups integrated high density dislocation walls (DDW),which are parallel to the slip direction. The micro-bands (MB) are formed when the steel are subjected to the strain of30%. In some regions with serious deformation, the already existing MB was deformed to S-shape, and it was called the S-Band.(2) The results of the ageing treatment for06A1steel shows that the strength is significantly improved after aging at550℃for10h, and the tensile strength and the yield strength are898MPa and482MPa respectively. The results of the microstructure show that, a short range ordered structure has been formed after ageing treatment. The plastic deformation is dependent on dislocation glide. When the dislocations meet the short range ordered structure, the activating stress must be increased so that the strength of the steel is increased.3The microstructure and deformation characteristics of the experimental steel with high stacking fault energy(1) After solution treatment, the microstructure of12A1steel contains austenite, ferrite and nano-sized κ paticles coherent to the austenite matrix. During the ageing process, κ phase has been corsened and the precipitated phases always grow along the orientation.(2) In the deformation of12A1steel, only planar slip occurs and there is no TWIP effect.4. The effect of Al on the deformation and strain hardening behaviors(1) Al content affects dislocation movement of the high-manganese steel. The movements of the dislocation in03Al experimental steel during deformation process are dislocation tangles, a few planar glides and deformation twins. For06A1experiment steel, the deformation behaviors during the plastic deformation process contain dislocation tangles, a large number of planar glides, the strengthening of micro bands and deformation twins. For12A1experiment steel, during the plastic deformation stage, the deformation behaviors are planer glides and refining of deformation bands.(2) According to the analysis of the strain hardening exponents of03Al,06A1and12A1steel, the strain hardening mechanisms of03A1are the formation of dislocation tangles and deformation twins; for06A1steel, the strain hardening of06A1rely mainly on the formation of dislocation tangles and micro-bands, which have a significant strengthening effect. For12A1steel, only the planar glide and the intersection of the slip band occur during deformation process, and the strengthening effect is poor.