Nippon Steel Corporation

Takayoshi Matsui

Koichi Yoshimura

Koji Takano

Wataru Murata

Satoshi Araki

Mizuo Sakakibara

The present invention makes the best use of a low-cost chemical composition in providing a high strength and high corrosion resistance stainless steel, which has improved delayed fracture resistance and toughness in particular, for building and construction uses, and as, for example, a stainless steel tapping screw. The present invention is, specifically, a stainless steel and a stainless steel screw with high strength and high toughness and excellent in delayed fracture resistance, characterized by: comprising, by mass, 0.01 to 0.25% of C, 0.05 to 1.0% of Si, 0.1 to 2.0% of Mn, 0.1 to 3.0% of Ni, 11.0 to 16.0% of Cr, 0.01 to 0.15% of N, and 0.01 to 3.0% of Mo; containing, optionally, 0.001 to 0.005% of B and/or one or more of 0.05 to 0.5% of Ti, 0.05 to 0.5% of Nb, and 0.05 to 0.5% of W; having less than 10% of ferrite in the center portion of the material; and having a mixed structure of martensite and 3 to 30% of austenite in the surface layer from the outermost surface to the depth of at least 1 &mgr;m, and a method to produce the same.

High-strength, high-toughness stainless steel excellent in resistance to delayed fracture

High strength martensitic stainless steel having high rusting resistance which comprises, by weight, 0.13 to 0.20% of C, 0.5 or less of Si, 2.0% or less of Mn, 1.0 to 2.5% of Ni, 12.0 to 16.0% of Cr, 1.3 to 3.5% of Mo, 0.06 to 0.13% of N, if necessary, 0.001 to 0.010% of B, or 0.05 to 1.0% of Ti, 0.05 to 1.0% of Nb, which satisfies 16 to 21% of ARI value for a rusting resistance index (Formula (1)), less than 0% of DI value for a .delta.-ferrite content index (Formula (2)), less than 0% of MI value for martensite content index (Formula (3)), less than 260% of W.sub.1 or W.sub.2 value for a cold workability index (Formulas (4) or (5)), with the balance comprising substantially Fe and inevitable impurities, said steel being characterized in that the martensite structure or the tempered martensite structure is contained, in which a Cr carbide of 0.2 .mu.m or less in grain size is deposited, especially enabling to produce a self drilling-tapping screw superior in screwing ability and rusting resistance, a nail superior in driving ability and rusting resistance, a cutter having high rusting resistance, a high strength spring superior in rusting resistance, etc.

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Tokyo, Japan

Takayoshi Matsui