The damage behavior of high-strength medium Mn steels

Abstract

Although various structural materials have been developed and widely used in modern civilization, metals and alloys are still the primary workhorse in applications such as automotive and infrastructure. Among numerous metallic materials, high-strength medium manganese (Mn) steels, the latest generation of advanced high-strength steels, have been intensely studied. Compared to other metallic materials, medium Mn steels are characterized by their multi-phase structures, fine/ultrafine grain size, complex deformation mechanisms and excellent mechanical properties. The most pronounced plastic deformation mechanism of medium Mn steels is martensitic transformation, which is widely reported to be beneficial for tensile strength and ductility. However, the role of martensitic transformation in other loading scenarios, such as fracture or fatigue tests, remains unclear. Therefore, this thesis aims to probe the damage behavior of medium Mn steels and the respective role of martensitic transformation. The first investigation examines the double-edged sword impact of martensitic transformation on the ductility and fracture toughness of medium Mn steels and proposes, for the first time, the dual role of martensitic transformation. Despite its pronounced strengthening effect, this chapter underscores avoiding massive martensitic transformation in designing tough medium Mn steels. To deal with this dilemma and achieve strength-toughness synergy, the subsequent chapter introduces a novel medium Mn steel produced through the warm-rolling (WR) route and explores how it effectively overcomes the adverse effects of martensitic transformation on fracture toughness. The WR steel demonstrates that the nanoscale Mn segregation at phase boundary can significantly enhance the mechanical stability of local austenite. Compared to its counterpart steel without chemical segregation, the WR steel exhibits similar tensile properties but notably improved fracture toughness due to the stabilized interfacial austenite, which acts as a deformation buffer layer between the fresh martensite and ferrite to enhance their co-deformation capabilities. The third investigation builds on the previous work and examines the mechanical properties of the WR steel at cryogenic temperatures. Tensile tests reveal an exceptionally high tensile strength (exceeding 2.1 GPa) coupled with an excellent ductility of 30%. Fracture tests demonstrate a remarkable crack-initiation toughness of 151 MPa×m1/2, surpassing most high-strength steels at room temperature. Multiscale advanced characterization confirms the existence of nanoscale austenite at phase boundaries with a high density of deformation twinning, similar to the phenomenon observed at room temperature. The concluding chapter investigates the fatigue properties of WR steel. Fatigue crack growth rates are measured, and the WR steel exhibits superior fatigue performance compared to the cold-rolled steel in both near-threshold and Paris law regions. Post-fatigue characterization reveals co-deformation of ferrite, martensite and nanoscale austenite, with no sign of strain localization in the WR material. Furthermore, it is confirmed that the Mn-segregated interface can effectively sustain significant plastic deformation and delay fatigue crack growth.