A Novel 1000 MPa Grade Ultrafine-Grained Dual-Phase Press Hardening Steel with Superior Oxidation Resistance and High Ductility

Abstract

1000 MPa grade low-carbon martensite press hardening steels (PHS) are widely used in energy-absorbing domains of automotive parts, such as the bottom of a B-pillar. To prevent oxide scale formation during hot forming, this PHS is often required to be protected by an additional Al–Si coating. In addition, although the low carbon martensitic microstructure grants it excellent bending toughness, the ductility tends to be limited. In this study, a novel 1000 MPa grade ultrafine-grained (UFG) martensite–ferrite (F–M) dual-phase (DP) PHS with superior oxidation resistance was designed using tailored additions of Cr, Mn, and Si, and refining the initial microstructure. Only 0.55 ± 0.18 μm thick oxide film is formed in the designed steel during austenitizing heating and stamping, which is significantly lower than the 24.6 ± 3.1 μm thick oxide film formed in conventional 1000 MPa grade low-carbon martensite PHS under the identical condition. The superior oxidation resistance of designed steel can be attributed to the rapid formation of the protective Si-rich, Cr-rich, and Mn-rich oxide layers during annealing. Moreover, due to the presence of ferrite and ultrafine microstructure, the designed steel also shows a significant improvement in ductility from 8.5% to 16.8% without sacrificing strength and bending toughness compared with conventional 1000 MPa grade low-carbon martensite PHS.

A Hot‐Stamped Medium‐Mn Steel with Superior Strength–Ductility–Bendability Combination and Thin Oxidation Layer

“…In general, the oxides formed by the elements of Al, Si, or Mn are usually more denser than Fe oxides. [38] Due to the (Al, Si)-O oxide band formed at the bottom of the oxidation layer, further oxidation of the investigated medium-Mn steel matrix is retarded because iron and oxygen ions cannot easily diffuse through the (Al, Si)-O oxide band. [21] Meanwhile, the formation of Mn-O oxide band in the outer oxidation layer can prevent the inward diffusion of oxygen ions.…”