Excellent Cold-Workability Exhibiting High-Strength Steel Wire or Steel Bar or High-Strength Shaped Article, and Process for Producing Them

Abstract

There are provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, or high-strength shaped article and a process for producing them. In particular, there is provided a process comprising carrying out hot working at 350 to 800° C. of a steel ingot, cast slab, steel slab or steel semifinished product having a C content of not greater than the solid solution limit of carbon of ferrite phase at Ael point and not greater than 0.010 mass % and being free of any cementite, or having a C content of >0.01 to 0.45 mass % to thereby obtain a material whose average crystal grain diameter in a cross section perpendicular to the longitudinal direction is ≦3 μm, and thereafter carrying out cold working of the material to thereby attain formation of a ferrite structure whose average crystal grain diameter in a cross section perpendicular to the longitudinal direction is ≦500 nm.

Description

TECHNICAL FIELD [0001]The present invention relates to an excellent cold-workability exhibiting high-strength steel wire or steel bar or a high-strength shaped article such as a screw and a bolt utilizing those characteristics, and a process for producing those steel wire or steel bar and high-strength shaped article.BACKGROUND ART[0002]Conventionally, regarding a screw or a bolt produced by shaping a steel wire or a steel bar by cold working such as cold heading, and rolling and / or machiening, and other high-strength parts for machine structural use, a steel wire rod produced by hot processing is processed into a steel wire having the desired wire diameter by cold working, the steel wire obtained is subjected to a so-called spheroidizing annealing treatment that makes a cementite in a metal structure spherical, by heating the steel wire at a temperature of about 700° C. for a long period of time of from about ten-odd hours to on day and night to soften the material, thereby improvi...

AI Technical Summary

Benefits of technology

The invention provides an excellent cold-workability exhibiting high-strength steel wire or steel bar with fine grain sizes and no cementite. The steel has a ferrite structure with an average grain size in a cross section perpendicular to a longitudinal direction of 500 nm or less. The steel has a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ael point. The invention also provides a high-strength shaped article with the same excellent cold-workability. The process for producing the steel involves warm working and cold working. The technical effects of the invention include improved cold-workability, high-strength, and excellent shaped article production.

Problems solved by technology

However, because the shaped article thus processed does not meet strength necessary as a final product by the above softening treatment, it is required that the shaped article is subjected to thermal refining such as quenching / tempering.

Thus, in the production steps of the conventional high-strength parts for machine structural use, because of a previous softening treatment to a raw material and thermal refining to a shaped article after cold working, it requires long time, and at the same time, is complicated, loss of heat energy is large and productivity is low.

Thus, there were problems in the points of increase in heat treatment cost and administration of delivery time.

Further, the problem remains such that in order to form C in the steel as a carbide other than Fe3C, V which is relatively expensive metallic element is added, and this incurs increase in costs.

However, to make this actually possible, it is necessary to melt-produce a steel such that a cementite is not substantially formed in a standard structure of a steel, as a chemical component composition.

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