鉄と鋼
Online ISSN : 1883-2954
Print ISSN : 0021-1575
ISSN-L : 0021-1575
論文
490MPa級鋼材の溶接熱影響部における溶融亜鉛脆化に及ぼす微量ボロン添加の影響
山本 雅之村山 敬司李 鴻美高田 尚記
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2019 年 105 巻 7 号 p. 742-751

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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.

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