Abstract: With the global energy crisis and increasingly serious environmental problems, the automotive industry has the trend towards lightweight design to meet thedemands of energy saving, consumption reduction, environmental protection and safety. The main approaches to implement automotive lightweighting are as follow. Ⅰ. Employ the advanced materials and processes, that is, adopt lightweight materials or forming technology to achieve the weight reduction goal. Ⅱ. Optimize the structural design, including thin wall, cavity, miniaturization, compounding and structural and technological improvement of body parts., The application of high strength steel as the main material of the automobile can not only contribute to reducing of the body thickness and weight, but also improve the safety performance. At present, the development of high strength steel has experienced three stages.As regard to microstructure, the first generation advanced high strength steel consists of ferrite (BCC) as matrix, the second generation consists of austenite (FCC), and the third generation consists of ferrite and retained austenite. Although the strength of the first generation advanced high strength steel is relatively high, its plasticity is limited. For the sake of solving this problem, a large amount of alloying elements, such as Cr, Ni, mn, Si and Al, are added in the second generation advanced high strength steel. Nevertheless, this approach increases the production cost, and also causes some problems in the subsequent processing, which blocks its widespread application in large-scale production. Accordingly, the content of alloying elements has been reduced in the third ge-neration advanced high strength steel, and the main alloying elements are composed of C, mn, Al, and Si. In this case, high strength and high plasticity can be achieved at a lower cost, as well as an excellent combination of elongation and tensile strength. As a representative of the third generation advanced high strength steel, medium manganese high strength steel can achieveimproved plasticity and strength through TRIP or TWIP effects of retained austenite during deformation. The medium manganese high strength steel consists of sub-micron austenite and ferrite which are formed by the austenite reverse transformation in the annealing process of hot or cold rolled sheets. In recent years, most researches have focused on how to obtain appropriate fraction of stable retained austenite by optimizing process parameters, such as heating rate, austenitizing temperature, annealing temperature, annealing time and cooling rate, and fruitful results have been achieved. However, the study on its mechanical properties is limited to tensile properties. There is a lack of research on subsequent forming properties and fracture mechanism of medium manganese steel. In this article, the development history and recent research situation of medium manganese high strength steel for automotive application are summarized. The advantages of medium manganese steel in material cost and mechanical properties are emphasized. Firstly, chemical composition design of medium manganese steel is introduced, as well as the effects of alloy element on the structures and properties of medium manganese steel. Secondly, the effects of intercritical annealing process parameters, including austenitization temperature, annealing temperature, annealing time, heating rate and cooling rate, on adjusting retained austenite are analyzed. Finally, the enhanced strength and toughness of me-dium manganese steel are revealed due to the TRIP and TWIP effects of retained austenite during the deformation process. The nucleation and growth of microvoid and fracture mechanism are explained. The cause of the lüders deformation behavior during stretching is studied. The application of medium manganese steel in hot forming process is introduced. In addition, the future development trend of medium manganese steel is predicted. It is believed that the industrial production and practical application of medium manganese steel can be realized in near future.
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