Steel wire rod or steel bar having excellent cold forgeability

Abstract

This invention provides a steel wire rod / steel bar having excellent cold forgeability. This steel wire rod / steel bar is provided with a predetermined chemical component composition in the condition immediately after hot rolling, wherein the depth d (mm), from the surface, of a surface layer region having a mean hardness HV0.2 that is at least 20 higher than the mean hardness HV0.2 of the region from the cross-section radius (R)×0.5 (mm) to the center satisfies formula (1); the steel structure of the surface layer region comprises ferrite in a fraction of 10% or less by area ratio, with the balance being made up of one or more types of steel structure from amongst martensite, bainite, and pearlite; the steel structure from the cross-section radius (R)×0.5 (mm) to the center is a ferrite-pearlite or ferrite-bainite steel structure; and the surface roughness (Ra) in the circumferential direction when scales adhering to the surface have been removed is equal to or less than 4 mum.

Description

technical field [0001] The present invention relates to a hot-rolled steel wire rod or steel bar (including bar-in-coil; the same applies hereinafter) excellent in cold forgeability after spheroidizing annealing. This application claims priority based on Japanese Patent Application No. 2012-86844 for which it applied in Japan on April 5, 2012, and uses the content here. Background technique [0002] In recent years, there has been an increasing demand for cold forging that can reduce or omit machining such as cutting from the viewpoint of improving productivity. Compared with hot forging, cold forging has high deformation resistance and insufficient deformation capacity (ductility), so there is a problem that die cracking and steel cracking are prone to occur. [0003] Therefore, spheroidizing annealing for reducing deformation resistance and improving deformability is usually performed on steel materials for cold forging. Patent Document 1 discloses a wire rod / steel bar t...

AI Technical Summary

Problems solved by technology

Compared with hot forging, cold forging has high deformation resistance and insufficient deformation capacity (ductility), so there are problems that mold cracking and steel cracking are prone to occur.

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