2018 年 104 巻 4 号 p. 218-226
SEM observation was conducted on cross-sections of galvannealed (GA) boron-steel sheet specimens, subjected to direct hot-stamping tests (V-bending), to study liquid metal embrittlement (LME), caused by liquid zinc in the coating. Specimens were heated to 1173 K (900°C) in a combustion gas furnace, and subsequently hot stamped in a cooled, V-shaped die. The locus of intersections between the Fe-Zn ferrite grain-boundaries (expected to be filled with liquid zinc) of the coating layer, prior austenitic grain boundaries of the steel substrate, and the coating interface, were examined. Specimen cracking originated at the coating/steel interface, and propagated along prior austenitic grain boundaries, where liquid zinc directly contacted the steel substrate. These prior austenitic grain boundaries were considered to be “geometrically favored” sites for initiating LME cracking. Cracking did not occur at sites where direct contact with liquid zinc was not established. There were numerous sites where cracking did not occur despite contact between liquid zinc and prior austenitic grain boundaries, at the coating/steel interface. In heavily cracked specimens, there were 4.7 to 5.9 cracks per mm of coating interface. Cracking occurred in only 23 to 36% of the “geometrically favored” sites at the coating interface. At the bottom of large cracks, cracks were round-bottomed. Vickers hardness at the bottom was lower than that at the sidewall. Therefore, ferrite or bainite transformation, enhanced by plastic deformation, was indicated. This suggested an absence of zinc propagation at deeper austenitic grain boundaries, terminating crack propagation despite the plastic deformation.
