Influence of intercritical annealing temperature on formability and mechanical properties of medium-manganese-steel in press hardening

Abstract

Abstract. Ultra-high-strength-steel parts produced by press hardening are widely used in the automotive sector for lightweight construction and passenger safety applications. Medium-manganese-steels (MMnS) are currently investigated as an alternative to boron-manganese steels. Their favorable mechanical properties of high strength and high ductility after quenching are often based on rather complex heat treatment strategies resulting in a multiphase microstructure. One possible way such a microstructure can be obtained, is intercritical annealing and subsequent quenching during the press hardening process. For this processing route formability during hot stamping and final properties of the quenched material are dependent on the annealing temperature. For a successful part production, an annealing temperature that satisfies both, in-process properties, as well as final mechanical properties is mandatory. In this paper, the suitability of the intercritical annealing process route for press hardening of MMnS is investigated for a specific MMnS alloy. Formability in hot stamping conditions with respect to different annealing temperatures (range 700°C – 800°C) are examined by performing hot tensile tests, as well as experimental trials using a hot stamping tool. Final properties are analyzed by tensile tests. The formability during hot tensile tests and hot stamping show a strong positive correlation to the annealing temperature, while the values for uniform and ultimate elongation of the quenched material shows a strong negative correlation to the annealing temperature. The tensile strength of the quenched material shows a low sensitivity to the annealing temperature. No annealing temperature that satisfies both, in-process and final properties, could be found for the investigated MMnS alloy.