| Equal channel angular pressing (ECAP) is an important technique for producing ultrafine-grained (UFG) metals. During the processes of ECAP, metals with different stacking fault energy (SFE) exhibit different microstructural evolution. In this paper, the new automotive steel-TWIP steel (30Mn-3Si-3Al) was chosen as the starting metals to carry out ECAP experiment at room temperature, and to observe recrystallization annealing of ECAPed samples. At the same time, with the help of optical microscope (OM) and transmission electronic microscope (TEM), the TWIP steel by tension at different strain were observed. Microstructure evolutions during ECAP were investigated in detail; Mechanical behaviors, effect of annealing methods on both structures and properties of ECAPed samples, mechanisms of both recovery and recrystallization and the TWIP effect were also investigated.The TWIP steel are ECAPed for 1-4 passes at room temperature. After one pass, a large number of deformation twins with band widths of 10～40 nm were produced. At the same time, these deformation twins were cut down by microshear bands. With increase of pressing passes, more twinning systems were activated and twin bands intersected each other, curved and fragmented finally. Also, both the number and the width of shear bands were increased, which cut twinning bands and refine more area of microstructure. After 4 pressing passes, ultrafine grains with sizes of 40～120 nm were obtained. While several areas within which deformation twins were not cut down were distributed in the matrix of ultrafine grains. The width of these twin bands was decreased to 5～20 nm.we carried out a comparative study on microstructures and the mechanical properties of the original state, the one pass pressing state, the ECAP-1P+85O℃×1h/A.C state and the ECAP-1P+1000℃×1h/A.C state. The results of the experiment reveal that:during the deformation, because of the large amount of obstruction of mutual deformation resistance forces and dislocations of deformation twins in the deformation area, the TWIP steel showed a high degree of work harding for processing. After the air-cooling treatment of ECAP-1P+850℃×1h, the plasticity had been slightly decreased compared to the original state while the strength had been increased. After the recrystallization, the sizes of grains were comparatively small, therefore dynamic recovery had been reduced effectively and the work harding ability for processing had been improved. After the air-cooling treatment of ECAP-1P+1000℃×1h, the plasticity had been improved compared to the original state under the same strength. At the same time, the growing of grains had increased the length of shear channel of dislocation to the direction of the area of twins, and strengthened the dynamic recovery, thus showed an even higher degree of plasticity and the even lower degree of work harding ability during processing.With continue observing the microstructural of TWIP steel at different tension, we find that:lots of dislocation can been seen in crystal grains and twin can't be found atε=5%; when the tension is up to 10%, lots of flakes like twins can be seen in crystal grain by optical microscope, and more higher density can been seen by transmission electronic microscope; in the larger tension, lots of deformation twins were produced, and more twinning systems were activated and twin bands intersected each other. Crystal grain is cut to cell by twins.The TWIP effect were found during tension:(a) dislocation were against by twins, work handing were raised in this part so made the tension in other parts. (b) The twins which were produced in the fist were against the twins which were produced as follow, and the more twin boundaries also against dislocation to move, so work handing continue raised. (c) The process of twins have a lot of deformation. (d) shear bands can change the direction of twins, which increase deformation. |
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