Rolled Rod for Cold-Forged Thermally Refined Article

Abstract

The rolled rod for cold-forged thermally refined articles according to the present invention has a chemical composition consisting, in mass %: C: 0.22 to 0.40%, Si: 0.35 to 1.5%, Mn: 0.20 to 0.40%, P: less than 0.020%, S: less than 0.015%, Cr: 0.70 to 1.45%, Al: 0.005 to 0.060%, Ti: 0.01 to 0.05%, B: 0.0003 to 0.0040%, N: 0.0020 to 0.0080%, and O: 0.0020% or less, with the balance being Fe and impurities, and that satisfies Formula (1) and Formula (2), wherein a total area fraction of pro-eutectoid ferrite and pearlite is 90% or more, an area fraction of the pro-eutectoid ferrite is 30% or more, and the rolled rod has a tensile strength of 700 MPa or less.0.50≤C+Si / 10+Mn / 5+5Cr / 22≤0.85  (1)Si / Mn>1.0  (2)

Description

TECHNICAL FIELD[0001]The present invention relates to a rolled rod that is a steel bar or a wire rod as is hot-rolled (as-rolled member), more specifically to a rolled rod for cold-forged thermally refined articles, which refers to a component subjected to cold forging and thermal refining.BACKGROUND ART[0002]In recent years, components such as shafts, bolts, and the like used for components in various kinds of industrial equipment, automobiles and building structures are in some cases produced by cold forging from the viewpoint of dimensional accuracy, yield, and producing cost. While such cold-forged articles are often used as is cold-worked, they are strengthened by thermal refining (quenching and tempering) when it is desired to provide higher strengths. Among such cold-forged articles, those subjected to the thermal refining are referred to as cold-forged thermally refined articles in the present specification.[0003]As a steel material to be subjected to thermal refining to pro...

AI Technical Summary

Benefits of technology

The present invention is a steel material that has excellent cold workability and can withstand cold forgings and thermal refining with high strength and resistance to hydrogen embrittlement. The steel meets certain content limitations for silicon, carbon, minerals, and chromium, which allow for solid-solution strengthening of silicon while ensuring hardenability and softening resistance after tempering. This steel material can lower material costs while maintaining its strength and performance characteristics.

Problems solved by technology

B increases hardenability of steel, as with the alloying elements such as Mo and V. However, when B containing steels are subjected to cold forging and thermal refining, so as to be used as cold-forged thermally refined articles (e.g., bolts) having tensile strengths of 1000 MPa or more, the B containing steels may have low hydrogen embrittlement resistances.

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