Steel wire rod or bar steel with excellent cold forgeability

Abstract

A steel wire rod or steel bar as hot-rolled, including: by mass %: C: 0.1 to 0.6%, Si: 0.01 to 1.5%, Mn: 0.05 to 2.5%, Al: 0.015 to 0.3%, and N: 0.0040 to 0.0150%, and P: limited to 0.035% or less and S: limited to 0.025% or less, and the balance substantially consisting of iron and unavoidable impurities, wherein a depth of d (mm) from the surface of the surface layer region with 20 HV 0.2 or more higher, relative to HV 0.2 that is the average hardness in the region where the depth from the surface is from sectional radius R×0.5 (mm) to the center satisfies the formula (1); the steel structure of the surface layer region has a ferrite fraction of 10% or less by area ratio, with the balance being one or two or more of martensite, bainite and pearlite; the steel structure where the depth from the surface is from the sectional radius R×0.5 (mm) to the center is ferrite-pearlite or ferrite-bainite; and the surface roughness Ra in the circumferential direction when scales adhering to the surface have been removed is 4 μm or less.

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.

Images