HIGH-STRENGTH STEEL EXCELLENT IN DELAYED BREAKAGE RESISTANCE, AND HIGH-STRENGTH BOLT

摘要

PROBLEM TO BE SOLVED: To provide high-strength steel excellent in delayed breakage resistance which can further suppress hydrogen embrittlement represented by a delayed breakage phenomenon which appears accompanied by the high-strengthening of a steel material, and a high-strength bolt composed of the high-strength steel.

SOLUTION: The high-strength steel contains, by mass, C: 0.20 to 1.50%, Si: 0.20 to 5.00%, Mn: 0.10 to 3.00%, P: 0.0005 to 0.1000%, S: 0.0005 to 0.2000% and N: 0.0020 to 0.0200%, and also contains, if necessary, one or two kinds of Cr: 0.01 to 5.00% and Mo: 0.01 to 1.00%. A remaining part of the steel is composed of Fe and inevitable impurities, a steel tissue is a quenched martensite tissue in which Fe-based carbide is finely dispersed, furthermore, the size of the Fe-based carbide ε in quenched martensite is 1 to 20 nm, and its area ratio is not smaller than 1.0%.

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