Development of a Novel Technology for Rapidly Austenitizing Usibor® 1500P Steel

摘要

Hot forming die quenching (HFDQ) is a relatively new process in the field of automotive manufacturing. It involves slowly heating Usibor® 1500P steel (a 22MnB5 steel substrate with a protective Al-Si coating) sheets to temperatures exceeding 880 °C, and holding it for a fixed amount of time to ensure the crystallographic structure has become fully austenitic. Once austenitized, the steel is then simultaneously formed and quenched into complex geometries in a single stroke. The quenching stage is critical, allowing for the formation of a hard and strong martensitic crystal structure that would not otherwise be formable at room temperature. These high-strength steel members allow automotive manufacturers to use thinner material cross sections in structural members, thereby reducing the net vehicle weight and improving fuel efficiency without compromising crash performance. Most HFDQ lines use roller hearth furnaces to austenitize ultra high strength steel blanks. While this process is a capable, proven industrial technology, it is limited to relatively inefficient modes of heat transfer, and thus requires long heating times, large furnaces, and considerable energy to maintain temperature. It is the purpose of this work to describe an alternative heating technology in which Usibor® 1500P coupons are austenitized by bringing them into contact with an electrically-heated monolith. In a laboratory-scale prototype, Usibor® coupons were austenitized in less than 25 seconds; subsequent material characterization and dilatometry investigations confirm that a fully martensitic structure is formed, and that the hardness and yield strength are comparable to furnace-treated samples. Tailoring material properties in HFDQ structural members (body-in-white components) introduces a combination of strong martensitic structures and other softer, more ductile daughter phases such as bainite, ferrite, and pearlite. Most tailoring efforts have focused on controlling the localized quenching rate during the forming stage through selective heating and cooling of the forming dies. This work presents an alternative, based on direct contact heating, in which tailoring is achieved through non-uniform austenitization during the heating stage of HFDQ. Experiments carried out on Usibor® 1500P coupons show that it is possible to create a fully-hardened zone within the coupon that transitions from martensite to softer phases over a relatively short distance.