A Novel High-Strength Oxidization-Resistant Press Hardening Steel Sheet Requiring No Al–Si Coating

Abstract

Press-hardening steels (such as 22MnB5) have been widely used in automobile structure for light-weighting. However, these steels have relatively low elongation (< 8%), and particularly are severely oxidized if an Al–Si coating is not applied during the hot forming process; while the addition of Al–Si coating causes a high cost for paying the intellectual property. In this paper, we present a new medium-Mn TRIP steel, which not only has better mechanical properties but also higher resistance to oxidation than 22MnB5 so that requires no Al–Si coating for protection. A relatively low austenitization temperature was employed, leading to the formation of very fine prior austenite grains (< 1 μm), and the final microstructure composed of martensite as matrix, carbide precipitates and about 11% retained austenite. As a consequence, this newly designed steel possesses more than 1400 MPa yield strength, 1700 MPa ultra-tensile strength, and about 12% total elongation after press hardening. Furthermore, the thickness of oxidation layer is less than 3 μm, which is much smaller than the one for 22MnB5 steel.