Austenitization of Ultra High Strength Steel by Direct Contact Heating for Hot Forming Die Quenching

Abstract

Hot forming die quenching (HFDQ) is a heat treatment process used in automotive manufacturing to produce ultra-high strength steels. It involves heating steel sheets to a temperature above 1153 K and holding them for a period of time to ensure that the microstructure has fully transformed to austenite. Rapid quenching is subsequently performed to insure the austenite-martensite transformation of the steel crystallographic structure. The improved strength of these steels allows automotive manufacturers to use thinner sheets for structural members, resulting in reduced net vehicle weight and improved fuel efficiency without compromising safety performance. Austentization is mainly done using roller hearth furnaces, but the high energy requirements, slow heating time, and large production space has motivated investigation of direct contact heating as an alternative strategy. Previous work using a small scale prototype produced austenitized steel coupons in less than 25 seconds, and accomplished tailored microstructures within a coupon by use of different thermal effusivities in the die striking surface. Even though the striking surface is isothermal, the differing thermal properties of the steel and ceramic provide nonuniform heating over the blank and incomplete austenitization, leading to bainitic regions in the formed part. This method may be easier to control compared to other tailoring procedures, which focus on slower and nonuniform cooling during the quenching stage. This thesis builds upon the previous work and describes the design and fabrication of an electrically-fired, half scale industrial sized direct contact heating die with tailoring capabilities. The die was installed alongside a flat quenching die, with a transfer system between them, all encompassed within a 900 ton press. An FEM simulation was used to design the direct contact heating die. The model also included a constitutive metallurgical submodel, derived from Gleeble dilatometry measurements, that was used to predict heating and austenitization kinetics of the blank. Model predictions were confirmed using thermocouple measurements recorded during HFDQ, and metallurgical and microstructural analysis of formed blanks. The austenitization model allows for future optimization in the striking surface design as well as the performance of heat treated ultrahigh strength steels.