Steel

EP4682285A4Pending Publication Date: 2026-06-24NIPPON STEEL CORPORATION
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Patent Information

Authority / Receiving Office
EP · EP
Patent Type
Applications
Current Assignee / Owner
NIPPON STEEL CORPORATION
Filing Date
2024-03-15
Publication Date
2026-06-24

AI Technical Summary

Technical Problem

Existing steel products used in cryogenic pressure vessels face challenges in maintaining high strength and low-temperature toughness, especially after post weld heat treatment (PWHT), which is essential for applications in low-temperature environments.

Method used

A steel product with a specific chemical composition and microstructure, including 0.03% to 0.20% C, 0.01% to 0.50% Si, 0.10% to 2.00% Mn, 4.51% to 6.10% Ni, and controlled microstructural features such as lower bainite and martensite, ensures high strength and toughness before and after PWHT, with a Charpy impact absorption energy of 150 J or more at -110°C.

Benefits of technology

The steel product maintains excellent low-temperature toughness and strength regardless of PWHT, enhancing its suitability for cryogenic applications by optimizing the chemical composition and microstructure to improve toughness and strength.

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Abstract

A steel product having a specific chemical composition, in which α expressed by a following formula is 5.0 or more and 16.0 or less; having a tensile strength of 615 MPa or more and 930 MPa or less; and having a microstructure in a region that is 1 / 4 of a thickness from a surface that contains lower bainite and martensite, with a total of area ratios of the lower bainite and the martensite being 15.0% or more, a total of area ratios of upper bainite, the lower bainite, and the martensite being 90.0% or more, and an area ratio of residual austenite being less than 1.7%. α = 0.50 × √[C] × (1+0.64[Si]) × (1+4.10[Mn]) × (1+0.27[Cu]) × (1+0.52[Ni]) × (1+2.33[Cr]) × (1+3.14[Mo])
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