Effect of the Decarburized Area on Bendability in 1500-grade Hot-press-forming Steels

Abstract

The hot-press-forming (HPF) steels, mostly used in the automotive industry, have bendability issues due to safety of passengers. Weight reduction of automotive parts is important in the upcoming years to reduce the fuel consumption with respect to minimize the CO_2 air pollution. Hot press forming has therefore received much attention recently as an alternative technology to produce advanced high strength steels (AHSS) automotive parts. Due to the advantages of the overcoming the manufactural limit of high-strength steels, HPF process is essential in automotive industry. HPF Steels are mostly used in the production part directly connected to the passenger safety such as A pillar, B pillar, bumper beam, and roof reinforce etc. HPF is the process forming steels in high temperature and afterward quenching them to room temperature. The process enables to form products with a complex geometry without shape deviations. Therefore, in this study, we investigate the effect of the decarburized area on bendability in 1500MPa-grade full martensitic HPF steels. The decarburized area of Zn coating HPF steels is formed at the galvannealing process in an oxidizing atmosphere unlike typical HPF steels which annealed in a reducing atmosphere. Because Al-Si coating and Zn coating are commonly used in HPF steels, three sheets, Al-Si coated, Zn coated, and Zn coated accompanying the decarburized area, are used in this study. Bendability is one of the properties representing the car crash performance. Bending test can therefore be used to evaluate and improve HPF steels parts abilities. HPF steels, however, should secure high strength because they are used as automotive parts. Therefore, the most important thing is to have good bending properties while ensuring a certain level of strengths. Investigating the crack initiation of bending test is essential because bending properties are defined as bend angles at the maximum load point. Microstructure appearance before the crack initiation would be observed, dividing into small intervals of bend angles. Existence of the decarburized area in HPF steels improves the bending characteristics while maintaining strengths, so it will be addressed through the microstructure analysis in this study.