Development of High Performance Press Hardening Steel Through Microstructure Engineering

Abstract

Microstructure engineering determines predominantly the properties and applications of steel. Martensite provides the highest strength compared to any other microstructural constitutes, thus press hardening steel with martensite microstructure has been widely used in vehicles' body structure to enhance the passive safety in the event of a crash. However, for such applications there are also potential problems related to martensite microstructure, such as limited ductility (bendability), low impact toughness (low energy absorption and low fracture resistance) and low resistance to hydrogen induced cracking. Usually these properties dominate the fracture behaviour of martensitic microstructure and consequently the crash-resistance performance of press hardened components. Therefore, in order to achieve high performance, it is necessary to optimize the steel microstructure during entire metallurgical processes. This paper introduces metallurgical solutions of microstructure engineering based on niobium metallurgy. The key point of microstructure engineering is to tailor and to fine-tune the microstructure details towards targeted applications with aims for high performance. The paper describes how to implement microstructure engineering in the press hardening steel by refining and homogenising the martensite matrix, strengthening and facilitating high angle grain boundaries in the martensite substructure, integrating a high density of homogenously distributed nano-sized NbC precipitates in the martensite matrix and finally creating a layer of soft microstructure near the surface area through controlled decarburization process during press hardening. The results and benefits of microstructure engineering to press hardening steel (22MnB5) are explained in detail.