Manganese Steel In The Cold Rolling Forming Performance Study

Developing high strength, high ductility and low cost steels is an inevitable for auto steel materials, because the energy saving and exhaust emission reduction becomes the main trend of the automobile industry, which could be solved effectively and feasibly by weightlightening without losing of the safety. However, with the increasing of the strength of steels, their formidability would decrease significantly, thus it is necessary to develop automobile steels with very good forming properties. In light of the idea of the microstructural control of Multi-scale, Multiphase and Metastable (simply being called "M3"), the third generation automobile steels with large fraction of austenite embedded in the ferritic matrix were successfully developed in Central Iron and Research Institute(CISRI), which gives the products of strength to ductility no less than30GPa%. The formability of this third generation automobile steels has been studied in this research.The chemical composition of the steels applied in this research is (wt,%) C0.1, Mn5.0, P0.008, S0.002and N0.003, which was industry tried in Taiyuan steels company (TISCO) through the production schedual of hot rolling, box annealing and cold rolling, with final thickness of the cold rolled steel sheets about1.8mm. In order to obtain different properties of strength and ductility, the cold rolled sheets were intercritically annealed, which results in duplex structure of ferritic matrix and large fractioned austenite. The microstructures were examined by optical microscopy, scanning electron microscopy, electron back scattered diffraction and x-rays. The mechanical properties were measured by uniaxile tensle test. The formabilities were evaluated by Hole Expansion test, Deep drawing, Erichsen test and Bake hardening test, which were simulated and analyzed using the CAE software. The results indicated that the microstructure and the austenite volume fraction could be controlled by the changing of the austenite reverted transformation temperature and time. After ART-annealing, the duplex structure with large fractioned austenite and ultrafine grained ferrite (0.3-0.6m) could be obtained, which gives an excellent mechanical properties with tensile strength about804.5-1275MPa, total elongation about25-41.5%and the product of strength to ductility no less than30GPa%. The forming properties were obtained in these steels, for example, the hole expansion ratio(HE) about83%, the limited drawing ratio(LDR) about2.05, Cupping value about10.218, qualified bake hardening (BH) value about50MPa. The simulation of the forming properties indicates that the simulation could be applied to predict the real forming property of the studied steels, which could be used for the industrial application. Based on the microstructure observation and properties analysis, it is believed that the excellent mechanical properties could be attributed to the TRIP effects of the large fractioned austenite and the ultrafine grained matrix of the ferrite, which results in not only high uniform elongation but also non-uniform elongation. However, the forming properties could be found in the steels with austenite fraction of about6.7-12%, which is mainly corresponding to non-uniform elongation. At the same time, the deep drawing property could be obtained when the austenite fraction is about12%, in which the parameter of the texture inhomogeneity (r) could be increased due to the increased-fiber texture component. The bulging performance is researched when the austenite fraction is about12-18%, the Erichsen test indicates that the high post-uniform elongation gives high Cupping value of the studied steels. The simulation results show a similar trend of the forming behaviors with experimental results. Both the experimental results and softwear simulation indicate that the new developed steels assume an excellent forming performance comparing with conventional TRIP and DP steels. Thus it could be concluded that the new developed medium steel is a promising materials for the automobile design and application based on this research.