Steel plate, member and manufacturing method thereof

By controlling the temperature during winding and gradually cooling the annealing process, the solution addresses the challenge of maintaining uniform mechanical properties in high-strength steel sheets, resulting in improved ductility, hole spreadability, and press moldability.

JP7655450B2Active Publication Date: 2025-04-02JFE STEEL CORP
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Patent Information

Application Number
JP2024536978
Authority / Receiving Office
JP · JP
Patent Type
Patents
Current Assignee / Owner
Priority Date
2022-09-30
Filing Date
2023-09-21
Publication Date
2025-04-02
Estimated Expiration
2043-09-21

AI Technical Summary

Technical Problem

Existing high-strength steel sheets with tensile strength of 1180 MPa or higher face challenges in maintaining uniform mechanical properties in the longitudinal direction of the coil, leading to variations in press moldability due to high cooling speeds and unstable cooling stop temperatures during annealing.

Method used

The solution involves controlling the temperature during winding of hot rolled sheets to homogenize the structure in the longitudinal direction of the coil. This is achieved by gradually cooling the temperature from the near Ms point to the cooling stop temperature in the annealing process, ensuring a consistent cooling stop temperature and reducing variations in mechanical properties.

Benefits of technology

This approach results in high-strength steel sheets with excellent ductility and hole spreadability, while maintaining excellent stability of mechanical properties in the longitudinal direction of the coil, thereby enhancing press moldability.

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Abstract

Provided are: a steel sheet having high strength, excellent in terms of ductility and hole expansibility, and having highly stable mechanical properties along the longitudinal direction of the coil; and a method for producing the steel sheet.　The steel sheet has a composition containing, in terms of mass%, 0.08-0.35% C, 0.4-3.0% Si, 1.5-3.5% Mn, up to 0.02% P, up to 0.01% S, up to 1.0% sol. Al, and up to 0.015% N, the remainder comprising Fe and unavoidable impurities, and has a steel structure in which the areal content of ferrite is 5% or less (including 0%), the total areal content of tempered martensite and lower bainite is 70% or higher, the volume content of retained austenite is 5-15%, and the areal content of fresh martensite is 10% or less (including 0%). The steel sheet has a standard deviation of coil-longitudinal-direction tensile strength (TS) of 30 MPa or less.
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