Han Dong

Effect of Quenching and Partitioning with Hot Stamping on Martensite Transformation and Mechanical Properties of AHSS

Two-step quenching and partitioning treatment with hot stamping was applied to advanced high-strength steel (AHSS). The newly treated steel possesses a fine microstructure and typically curved micromorphology. The martensite start temperature of the newly treated steel is increased through the effect of plastic deformation on austenitic microstructure. However, the martensite volume fraction of this steel is deceased because of the enhanced stability of the untransformed austenite after plastic deformation. Consequently, the fraction of retained austenite is increased. The newly treated steel also shows excellent mechanical properties. The volume fraction of retained austenite reaches the highest value of 17.2% when hot stamping is performed at 750xa0°C. Hence, the steel displays favorable plasticity with an elongation of 14.5%. Moreover, the highest hardness value of 426xa0HV is obtained when hot stamping is performed at 650xa0°C. The newly developed process may be employed to develop a new generation of AHSSs.

A Comparative Study on Formability of the Third-Generation Automotive Medium-Mn Steel and 22MnB5 Steel

Third-generation advanced automotive medium-Mn steel, which can replace 22MnB5 steel, was newly developed to improve the lightweight and crashworthiness ofxa0automobile. Studies on the formability and simulation method of medium-Mn steel have just been initiated. In this study, finite element simulation models of square-cup deep drawing were established based on various material property experiments and validated by experiments. The effects of blank holder force (BHF), fillet radii of tools (die and punch) on the maximum drawing depth (MDD), thickness distribution of the formed products, and the microstructure before and after forming were investigated and compared with those on 22MnB5 steel. Results show that the MDD of the two steels decreased with increased BHF but increased with the fillet radius of punch; however, the fillet radius of die showed no significant effect on the MDD for both steels. Compared with hot-formed 22MnB5 steel, the martensitic transformation of the hot-formed medium-Mn steel is rarely influenced by the process parameters; thus, it holds the complete, fine-grained, and uniform martensitic microstructure. Moreover, the medium-Mn has better formability, lower initial blank temperature, and smaller impact of BHF and fillet radius of tools on the hot-formed product. Thus, a theoretical basis for the replacement of 22MnB5 steel by medium-Mn steel in hot forming process is provided.