Hot V-Bend Formability of Galvannealed Boron Steel for Hot Stamping

Abstract

AbstractThe hot stamping process has been widely applied to the production of automotive body parts because the stamped panels with both high strength and good shape-accuracy are easily obtained. For the purpose of omitting the de-scalling treatment, the galvannealed (GA) boron steel for hot stamping was developed. When the steel is hot formed with the existence of liquid zinc, brittle fracture occurs by the infiltration of liquid zinc into the solid metal. This phenomenon is well known as Liquid Metal Embrittlement (LME). In this study hot V-bend test was carried out to estimate LME behaviour for GA boron steel in terms of suitable conditions of both heating and forming processes. First, LME disappeared with an increase of dwell time in gas furnace. In this heating condition, the dwell time that LME disappeared was 225 s. And the depth of LME crack became small as dwell time, and LME crack of 10 micrometer size was also observed in the dwell time of 210 s just before liquid zinc would be completely depleted from the Г contents index IГ based on X-ray diffraction pattern analysis for Г-phase of V-bent test specimen. In addition, in such condition, LME did not occur when a bending strain was small of 0.05 true strain. The overbend crush test for V-bent test specimen was carried out to investigate the influence of LME crack on the crash performance. Consequently it turned out that LME crack of about 10 micrometer did not have much influence on the crash performance. And this method is supposed to be very simple and valuable as a quality verification test for LME of the hot stamped parts in production.KeywordsGalvannealed boron steelLiquid metal embrittlementDwell time in combustion gas furnaceBending strainCrash performance