Influence of process parameters on the hot stamping of carbon-martensitic chromium steel sheets

Abstract

Hot-stamped components are an important part in steel-intensive automobile lightweight design in which the use of the heat-treatable steel 22MnB5 has been established. However, the field of application is limited by the low elongation at break in the hardened state. In order to improve ductility and consequently the crash-performance, the formation of martensite can be locally suppressed. This process known as tailored tempering is accompanied by decreasing tensile strength. Regarding its lightweight potential, 22MnB5 is reaching its limits and new materials come into focus. Promising potential to fulfil the mentioned demands is offered by carbon-martensitic chromium steels. Besides improved tensile strength and elongation at break, this material series has further advantages. Due to its low critical cooling rate, the formation of martensite is achieved in the hot stamping process as well as by cooling at ambient temperature. Moreover, the low martensite-start-temperature allows the use of thin material sheets. However, the process management required to achieve the respective demands for automobile applications is not trivial. Considering the materials X20Cr13 and X46Cr13, this study investigates the influence of varying process parameters on mechanical properties. In order to comprehend the relationship between solution annealing and tempering parameters, a design of experiments has been performed by means of tensile tests. In addition, miniature tensile tests were conducted to obtain flow curves at varying forming temperatures and to examine the effects of varying cooling and strain rates.