2019 年 105 巻 7 号 p. 742-751
We examined the effect of boron (B) addition (up to 10 ppm) on the liquid zinc (Zn) embrittlement of 490 MPa grade steels (SN490B) containing a trace amount (approximately 70 ppm) of nitrogen (N) via notched-bar tensile (NBT) tests. The studied steels were heat-treated in order to reproduce the microstructure (martensite structure) of the heat -affected zones in the building structural steels. The NBT tests indicate a slight effect of B addition on the liquid Zn embrittlement of the studied steels containing trace N; this is different from the results of previous studies on the liquid Zn embrittlement of N-free steels. However, the addition of titanium (Ti) into the studied steels induces remarkable liquid Zn embrittlement. The NBT-tested specimens of the Ti-added steels exhibit a brittle fracture surface, indicating intergranular fracture induced by liquid Zn. Its associated cracks preferentially propagate along prior austenitic boundaries, and Zn is enriched at these crack tips. Minute chemical analysis reveals a significant segregation of B at prior austenitic boundaries in the Ti-added steels. These results indicate that the segregated B would facilitate crack propagation along the prior austenitic boundaries induced by liquid Zn, resulting in enhanced liquid Zn embrittlement. The effects of N and Ti on B segregation and the related liquid Zn embrittlement are discussed.