Microstructure and Properties of Low- and Medium-C Press-Hardened Steels During Hot Stamping with Intermediate Pre-cooling Stage Tailored Process

Abstract High-strength steels contribute to meeting green transition goals by enabling lightweighting of vehicles. However, sufficient ductility, which is typically compromised by high strength, is a prerequisite for the use of a material in structural components. Tailored processes address this challenge by improving ductility in critical areas by applying locally a softening heat treatment. Such processes are established based on detailed time-temperature transformation information that is often not available for newly developed grades. In this study, by coupling physical simulation informed by finite element modeling of process conditions with tensile testing and optical and scanning electron microscopy, we investigate the microstructural evolution and resulting mechanical properties of a recently introduced high-strength B-containing steel (37MnB4) for automotive applications. The investigation focuses on an advanced press-hardening process with an intermediate pre-cooling step prior to stamping and quenching operations. The results are compared with those obtained with the established grade 22MnB5, where the process has previously been shown to be successfully applicable, in terms of their response to this tailored process. The data indicate that the higher hardenability of 37MnB4 compared to 22MnB5 dictates the need for a process modification. Furthermore, it is shown that extending the pre-cooling phase, the simplest adaptation of the process to the new scenario, produces an unsuitable ferritic-martensitic microstructure, resulting in both reduced strength and reduced ductility.