The Effect Of Mn Pre-Partition On Microstructure And Mechanical Properties Of Medium Manganese Hot Stamping Steels

With the increasing attention of the society to the fuel efficiency automobile,light weight car bodies with improved passenger safety are of increasing importance for modern vehicles.Delivering high strength and high ductility alloys is critical for providing land vehicles with lightweighting options.Nowadays,the strength of advanced high strength steel(AHSS)is up to 2GPa,but the traditional cold stamping process can not meet the forming requirements when the strength of high strength steel is more than 1500MPa,hot stamping process is widely used to produce ultra-highstrength steel parts.In this research,the strength of 0.15C-7Mn medium-Mn steel can reach 1800MPa after direct hot stamping,while the elongation is poor,less than 5%,and the steel is sensitive to temperature,so it can not meet the requirements of industrial application.Based on the concept of Mn,the austenitic stable element,adding intercritical annealing(between Ac1 and Ac3)process before hot stamping process.Massive segregation of Mn in austenite phase.The effects of conventional austenitizing and rapid austenitizing on the microstructures and mechanical properties of medium manganese steels were investigated by comparing two kinds of Mn pre-partitioned and nonpartitioned microstructures.The main conclusions were summarized as follows:(1)After 0.15C-7Mn steels annealed in two-phase region,the microstructures of Mn-rich austenite and Mn-poor ferrite were obtained,and the initial microstructures with uneven partition of Mn were achieved.(2)Under conventional heating conditions,the pre-partition of Mn increased the content of retained austenite and elongation of 0.15C-7Mn hot-formed steel after stamping.Compared with conventional heating,rapid heating was beneficial to retain the non-uniform partition of Mn,and further improve the content of retained austenite and elongation after stamping.(3)The reason for fracture of quenched 0.15C-7Mn medium manganese steels was not the stability of austenite phase,but the poor toughness of quenched martensite.