Steel material

The use of a steel material with a tailored chemical composition and microstructure in bolts addresses the issue of hydrogen embrittlement susceptibility, enhancing their strength and durability by dispersing fine MC-type carbides and optimizing cementite particle distribution, thereby reducing the risk of delayed fracture.

US20250101556A1Pending Publication Date: 2025-03-27NIPPON STEEL CORPORATION
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Patent Information

Application Number
US18/730067
Authority / Receiving Office
US · United States
Patent Type
Applications(United States)
Current Assignee / Owner
Priority Date
2022-03-04
Filing Date
2023-03-03
Publication Date
2025-03-27

AI Technical Summary

Technical Problem

Bolts with high strength often have increased susceptibility to hydrogen embrittlement, leading to delayed fracture, and existing steel materials do not adequately address this issue.

Method used

A steel material with a specific chemical composition and microstructure, including C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.80 to 1.50%, Mo: 0.35 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0300%, and a microstructure with an area fraction of hard structure of 90% or more and Vickers hardness of 220 to 400 HV, along with a high number density and specific distribution of cementite particles, is used to produce bolts with reduced hydrogen embrittlement susceptibility.

Benefits of technology

The proposed steel material effectively lowers the hydrogen embrittlement susceptibility of bolts, ensuring they can withstand high tensile strengths without premature fracture, making them suitable for demanding applications.

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Abstract

A steel material according to the present disclosure consists of, by mass %, C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: more than 0.80 to 1.50%, Mo: 0.35 to 1.50%, V: 0.01 to 0.50%, Al: 0.005 to 0.100%, and N: 0.0010 to 0.0300%, with the balance being Fe and impurities. The area fraction of hard structure is 90% or more and the Vickers hardness is 220 to 400 HV. The number density of cementite particles in the hard structure is 4.0 pieces / μm2 or more. The number ratio of cementite particles having an area of 0.0005 to 0.0100 μm2 among a plurality of the cementite particles in the hard structure is 50.0% or more. The sample standard deviation of the areas of a plurality of the cementite particles in the hard structure is 0.070 μm2 or less.
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