| 摘要 |
A screw or a tapping screw, comprising a hyperfine structure steel having ferrite grains of 3 mum or smaller in average grain size and a nitrided layer on a surface part, wherein a strength is increased, a surface hardness is increased, and the surface hardness and an inside hardness are kept in balance, whereby a novel tapping screw and novel general screws can be provided.
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