Hot-rolled steel and its manufacturing method

By using Cu and Ni to form enriched regions and optimizing heating conditions, the method addresses the issue of thick decarburized layers in hot-rolled steel, enhancing cold workability and fatigue properties while avoiding toxic elements and improving productivity.

JP7768362B2Active Publication Date: 2025-11-12JFE STEEL CORP
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Patent Information

Application Number
JP2024519955
Authority / Receiving Office
JP · JP
Patent Type
Patents
Current Assignee / Owner
Priority Date
2023-01-30
Filing Date
2023-12-06
Publication Date
2025-11-12
Estimated Expiration
2043-12-06

AI Technical Summary

Technical Problem

Existing hot-rolled steel materials face issues with excessively thick decarburized layers, leading to poor strength, fatigue properties, and reduced cold workability due to non-uniform austenite grain size and carbon diffusion, which are exacerbated by the use of toxic and difficult-to-handle elements like Te and Se, and require precise temperature control, affecting productivity.

Method used

Incorporating Cu and Ni into the steel composition, optimizing their balance, and controlling heating parameters to form enriched regions that suppress decarburization, while adding Sn to enhance decarburization suppression and limiting Sn concentration to prevent grain boundary embrittlement, thereby controlling the decarburized layer thickness and improving cold workability.

Benefits of technology

The method results in a hot-rolled steel material with a sufficiently suppressed decarburized layer and excellent cold workability, ensuring uniform grain size and improved fatigue properties without the need for toxic elements and precise temperature control.

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Abstract

Provided is a hot-rolled steel material in which the thickness of a decarburized layer is sufficiently inhibited and that has excellent cold workability. A hot-rolled steel material according to the present invention is characterized by having a base iron having a predetermined component composition, and a decarburized layer formed on the surface of the base iron, and is characterized in that: in the decarburized layer, an enriched region in which at least one of Cu and Ni is enriched exists; the coverage of the enriched region with respect to the surface of the decarburized layer is at least 50%; the greatest depth of the enriched region in the decarburized layer is 1-150 μm; the total decarburization depth (DM-T) of the decarburized layer as determined in accordance with JIS G 0558 is at most 0.80 mm; the total area ratio of ferrite and pearlite in the base iron is at least 90.0%; and the average Vickers hardness of the base iron is at most 250 HV.
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Citation Information

Patent Citations

  • Steel for universal joint cross shaft for cold extrusion and manufacturing method thereof

    CN114959442A

  • Steel for machine structural having excellent cold workability and low decarburizing property, and production method therefor

    JP2004250768A

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    JP2010132943A

  • Case hardened steel and method for manufacturing the same

    JP2012072427A

  • Hot-rolled steel bar or wire rod for cold forging

    JP2013234354A