組成変調型積層Co-Ni合金めつきの製鋼用連続鋳造鋳型への適用

摘要

製鉄における連続铸造は，铸型（モールド）に溶鋼を注ぎ，側面が凝固したものをモールドの底から連続的に引き出し，直接一定の形の鋼片を作る重要な工程である。モールドには,冷却効果を高めるために銅合金が用いられる。その使用状況 は，高温，熱衝撃，铸片との摩耗，却水による腐食など，非常に過酷であるため，モールドの表面保護を目的として，クロムめつき，ニッケルめっき，Ni-P合金めつき，溶射皮膜など，種々の表面処理が用いられてきた。近年では，耐熱・耐酸化性に優れるコバルト-ニッケル（Co-Ni)合金めつきが用いられ，モールドの長寿命化と大幅な製造コスト削減に貢献している。%With the aim of extending the lifetime of molds used for continuous steel casting, we developed a composition-modulated Co-Ni alloy plating with alternately laminated layers of different compositions. Alloy films with laminated structures comprising few-micrometer-thick layers were formed using a single plating bath by periodically altering the agitation intensity in the bath during plating. The hardness of the as-plated composition-modulated Co-Ni alloy film was 350 HV, which was higher than that of a conventional non-laminated Co-Ni alloy plating film. Heat treatment slightly decreased the hardness. Nevertheless, the composition-modulated film exhibited higher tensile strength and higher hardness than the conventional alloy film after heat treatment at temperatures of 200-700℃. The breaking elongation of the composition-modulated film increased to 30% after heat treatment at 700℃. Finally, the composition-modulated Co-Ni alloy plating was applied to a mold surface, which was then used for continuous steel casting. It exhibited excellent resistance against crack formation by thermal shock resulting from contact with molten steel during continuous casting.