Microsegregation and high temperature properties of high manganese steels

Abstract

High-manganese steels containing 10 to 30% Mn exhibit high strength and exceptional plasticity due to TWIP effect (TWinning Induced Plasticity) or via multiple martensitic transformations TRIP effect (TRansformation Induced Plasticity). High Mn steel also has low density because of Al addition and features high specific energy absorption. Therefore, it is a very promising material for the automotive industry. But it is hard to cast high Mn steel by the continuous casting process, and even by Horizontal Single-Belt Casting, as the slab is susceptible to cracking. This is because high Mn steel has poor hot ductility. In order to design alloys with good ductility, a new shear test equipment was developed in the context of this work. It can test high temperature mechanical properties in as cast state solidified directly from the melt. In this way, the high Mn steels with good hot ductility can be selected and subsequently produced in the steel industry. The reasons why some high Mn steels are prone to cracking are complex. High Mn steel with high carbon content has severe microsegregation, especially regarding Si. Furthermore with carbon content increasing, carbide formation increases. Engineering stress and true stress of high Mn steel at 1000 ℃ were investigated. It has to be point out that high elongation does not necessarily mean good hot ductility. In the experiments, samples that have cracks can also exhibit high elongation at high temperature. After cooling down to room temperature and subsequently annealing for 1 hour at 1020℃ in the muffle furnace, hot ductility of high Mn steel becomes much better. For the Horizontal Single-Belt Casting process, casting of high Mn high carbon steels can be processed, because the strip has no bending during the casting. After the casting, the strip should not be inline-rolled directly, but should first be reheated, and then rolled. For the traditional continuous casting process, it is not recommended to cast high Mn high C full austenite steels. If these are needed, the possible process is to cast ferritic+austenitic steel firstly, and subsequently anneal it in order to remove the ferrite phase.