| TWIP (twining induced plasticity) steels, which exhibit the high strength, excellent ductility, good forming property and high energy absorption capacity, have received more and more research attentions. The TWIP steels have advantages including low weight, high fuel efficiency and good safety due to its high energy absorption capacity. Therefore, they have great potential to be used as structural components to improve the security level of the automobile. Through the improvement of the production technology, the large scale production of TWIP steel sheets will come true, and the application prospect will be more promising. However, the research of TWIP steels has just been begun, and there are many technical problems to be solved. In the present thesis, the hot deformation behaviors, microstructure evolution of TWIP steel, the deformation mechanism and influence factors for cold rolled TWIP steels during tensile deformation were investigated. In addition, the effects of vanadium carbides (VC) on the delayed fractures and work hardening behavior were studied. Moreover, twin-roll casting TWIP steel strip process, microstructure, mechanical property and deformation mechanism were investigated. The main original works of this thesis are present as follows:(1) The hot deformation behavior and microstructure evolution of two TWIP steels with different chemical component were studied by the thermal simulation machine. The results show that vanadium (V) element in the solid solution has no significant effect on the dynamic recrystallization of TWIP steels. As decreasing the deformation temperature, the formation of VC precipitates inhibited the dynamic recrystallization of TWIP steels. The impact factors of high temperature deformation resistance for TWIP steels were analyzed, and a deformation resistance model was established. According to the theory of processing map, the processing map of TWIP steel has been established according to the dynamic material model. The results show that unsafe hot working regime exists in hot deformation processing at low temperatures and high strain rates. Therefore, the deformation in the rolling process should be carried out as much as possible at high temperature and low strain rate to produce a favorable deformation recrystallization microstructure to avoid thermal cracking.(2) The microstructure, mechanical properties and deformation mechanism of TWIP steel annealed at different temperature were investigated. The results show that the strain hardening behavior of TWIP steels can be divided into three stages during the tensile deformation. In stage I, dislocation gliding was the main deformation mechanism, and the strain hardening rate decreased and the strain hardening exponent was low. In stage II, primary deformation twinning has emerged, creating a strain hardening rate platform and strain hardening exponent increased due to the blockage of dislocations motion by deformation twin boundaries. In stage Ⅲ, the interactions of dislocation-twin and twin-twin were the main deformation mechanism and the strain hardening exponent increased to the highest value and thereafter declined. With the increasing of annealing temperature, the grains grew unceasingly, while the critical stress for the formation of deformation twinning went down. The TWIP effect was strengthened as the forming continually of deformation twinning in tensile deformation, and the total elongation increased either.(3) The deformation mechanism and the microstructure evolution of TWIP steel under different temperature and strain rate were investigated through tensile tests. It was found that with the increasing of deformation temperature, the deformation mechanism of TWIP steel transferred deformation twinning to crystallographic slip, according to the analysis of strain hardening rate-strain curves and microstructure observation. The stacking fault energy (SFE) of TWIP steels, Γ, at different temperatures was estimated by thermodynamic equations. The dependence of SFE on deformation mechanism was analyzed. It was pointed out that when 19mJ/m2≤Γ≤32mJ/m2, deformation mechanism was the deformation twinning; when 32mJ/m2<Γ<76mJ/m2, the governing deformation mechanisms were crystallographic slip and deformation twinning; when Γ≥76mJ/m2, the slipping was a predominant deformation mode. The effect of strain rate on deformation mechanism was studied. When 10-3s-1<ε<1s-1, the strain hardening rate decreased rapidly with the increasing of strain rate, because there was not enough time for piling up of dislocations to form deformation twins. Instead of decreasing, a strain hardening rate platform was formed with increasing the strain when the strain rate increased to 10 s-1. Due to the decreasing of critical stress during the deformation process at a higher strain rate, more grains began to form deformation twins which were weak to generate deformation twinning with original orientations. Therefore a platform of strain hardening rate was formed.(4) The effect of VC precipitates on the delayed crack and strain hardening behavior was investigated, and the interaction between VC precipitates and deformation twinning was discussed. It was indicated that VC precipitates were good options as hydrogen traps to improve the resistance against delayed fractures of TWIP steels. Through the study of strain hardening rate of two TWIP steels with similar grain sizes, it was found that the TWIP steel contained V had a shorter strain hardening rate platform, and entered the platform in a larger strain. The existence of VC precipitates will suppress the propagation of deformation twins. Through the observation of high resolution TEM, three typical interactions were observed, i.e., the twins can cut through the precipitates if their size was smaller than lOnm, they may partially cut through the precipitates with the size in the range from 10 to 50nm, and the precipitates can act as obstacles to totally block the propagation of deformation twins if their size larger than 50nm.(5) The post-treatment technology, microstructure, mechanical properties and deformation mechanism of TWIP steels fabricated by using twin-roll strip casting were investigated. By using strip casting process, the excellent surface quality was obtained. It was found that a good combination of strength and ductility can be achived, when the cast strip cold rolled directly after pickling and annealed at 800℃ for 10min. The yield strength and tensile strength were 420MPa and 1070MPa, respectively, and total elongation was 56%. Therefore, it was feasible and advantageous to produce TWIP steels by using the twin-roll strip casting process. Because of the fine grain size obtained in twin-roll cast specimen, the generation of deformation twinning was retarded, and lower quantity of deformation twins was obtained in the deformation process, resulting in lower strain hardening ability and elongation compared to conventional counterpart. |
