Steel Plate

Abstract: The present invention provides a steel plate having both bending workability and hydrogen embrittlement resistance. This steel plate includes a plate thickness central section and a surface softness section formed on one or both sides of the plate thickness central section. The metallic structure of the plate thickness central section comprises, by volume, at least 60% annealed martensite, less than 30% each of ferrite, bainite, perlite, and residual austenite, and less than 5% quenched martensite. The surface softness section is more than 10 µm thick per side and no more than 15% as thick as the plate thickness central section, the average hardness of the surface softness section is no more than 0.90 times the average hardness of the plate thickness central section, the surface softness section includes carbides at a number density of at least 1×104/mm2, the average grain diameter of the carbides is 0.250 µm or less, and the logarithmic standard deviation of the grain diameter is 0.05 or less.

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