5th International High Manganese Steel Conference - HMNS 2022

This paper presents the results of recent investigations on the influence of the hot press forming (HPF) temperature cycles for a novel medium manganese, high ductility 1000MPa zinc coated steel (HQ1000-GI), with specific emphasis on the risks of liquid metal embrittlement (LME) induced cracking during resistance spot welding (RSW).

In this study, a zinc coated 7Mn steel was hot formed using several temperature cycles. Subsequently, the LME sensitivity of the hot formed parts was investigated using both dedicated resistance spot welding experiments as well as high temperature tensile testing. Tested samples were subjected to visual and microstructural investigations to reveal the occurrence of LME cracks and their relation with the substrate and coating conditions after hot forming.

For low temperature HPF cycles, and subsequently welded under very high heat input weld conditions, it is observed that the Medium manganese HQ1000-GI can be moderately sensitive to LME induced cracking in the heat affected zone of the resistance spot welds. At the same time, it is shown that the sensitivity to LME during welding is significantly reduced after hot forming at higher temperatures. A similar trend is obtained using high temperature tensile tests for time-temperature loads relevant for welding.

Analyses of the substrate coating interface shows that the heat cycle applied in the hot forming process affects both the substrate and the coating structure, which both add to the reduced LME sensitivity.

Combined with other recent application studies on the HQ1000-GI steel, it is shown that a hot forming process window can be defined for the that provides hot formed parts with excellent strength-ductility properties, low springback, good corrosion properties and with good weldability.