12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

When heating steel blanks with different sheet thicknesses within prior to a hot press forming process, local heating rates may differ significantly. Therefore, the process window for heating the blanks is narrow, especially when aluminium-silicon (AlSi) coated material is used. The thicker part of the blank determines the minimum furnace dwell time for alloying of the coating to achieve sufficient paint adherence. The thinner part limits the maximum dwell time to ensure spot weldability. Additionally, when the heat treatment exceeds the process window, the amount of absorbed hydrogen from the furnace atmosphere may become detrimental. This study explores a modified AlSi coating concept for blanks with varying sheet thickness to minimise the temperature gradient within a blank and the hydrogen pickup during heating prior to press hardening. The approach used here is to form an iron rich layer on the AlSi coating surface by an adapted conversion treatment process. This results in elevated surface heat absorption and increased heating rates. Samples were produced by partly reducing the thickness of 22MnB5+AS150 blanks in a laboratory cold rolling process. The coating modification was applied before or after the cold rolling step. Additionally, a welded blank was produced in which only the thicker part was modified by a conversion treatment. The as produced samples were heated in a laboratory furnace, recording the heating rates of the thinner and thicker areas of each sample. Additionally, the amount of hydrogen pickup was analysed by thermal desorption analysis (TDA) from a separate set of samples. It has been found that the temperature gradient within a blank can be reduced significantly for all modified samples compared to the reference material without any surface modification. This is due to a greatly increased heating rate of the thicker part of the sample. Therefore, the minimum furnace dwell time can be reduced, enlarging the process window.