High-strength ultrathin steel wire and method of manufacturing the same

Abstract

The present invention provides a steel wire, including chemical components of: C: 0.7-1.2 mass %; Si: 0.05-2.0 mass %; and Mn: 0.2-2.0 mass %, with a balance including Fe and inevitable impurities, in which the steel wire has a pearlite structure, the average C concentration at a center portion of a ferrite phase in an outermost layer of the steel wire is 0.2 mass % or lower, and a residual compressive stress in the longitudinal direction of the steel wire in the outermost layer is 600 MPa or more.

Description

TECHNICAL FIELD[0001]The present invention relates to a high-strength steel wire used for a steel cord for a vehicle tire, a saw wire and the like, and a method of manufacturing the same. More specifically, the present invention relates to an ultrathin steel wire that is strengthened through a wire-drawing working at a cold temperature using a die and that has a wire diameter of 0.04-0.4 mm and the strength of 4500 MPa grade or more.[0002]This application claims priority on Japanese Patent Application No. 2009-148051 filed on Jun. 22, 2009, the content of which is incorporated herein by reference.BACKGROUND ART[0003]With the increasing demand for reducing the weight of a tire, there exist increasing needs for further improving a tensile strength of a steel wire for a steel cord in a vehicle tire. Similarly, there exist increasing needs for further improving the tensile strength of a steel wire for a wire saw for precisely cutting a sapphire crystal, SiC crystal and the like. In resp...

AI Technical Summary

Benefits of technology

The present invention relates to a high-strength steel wire used for a steel cord in a vehicle tire and a method of manufacturing the same. The technical effect of the invention is to provide a highly ductile ultrathin steel wire with a tensile strength of 4500 MPa or greater, which has a high degree of ductility and favorable properties even after aging at room temperature for several days to several years. The invention also addresses the issue of decreased ductility when strengthening the steel wire through wire-thawing working using a die. Previous methods have only achieved limited strengthening of the steel wire without compromising ductility.

Problems solved by technology

Further, high-strength steel wire deteriorates its properties through aging at room temperature (20-40° C., several days to several years), and hence, it is also important not to substantially decrease favorable ductility through aging.

Recently, it has been found that the cementite in the pearlite structure is decomposed into fine pieces when strain on the wire excessively increases.

However, because of the extremely fine structure, the relationship between mechanical properties and the location and the state in which the carbon exists has not yet been uncovered, and in particular, many unclear points concerning the cause of the deterioration of ductility still exist.

In the actual high-strength steel wire, it is believed that the surface region and the central region in the steel wire do not always exhibit the same structure and the same amount of local strain, and this affects the properties of the steel wire.

However, in the past, even if the ultrathin steel wire was highly strengthened by increasing the strength of the element wire after final patenting treatment or the strain of the wire-drawing working, the ductility thereof largely decreased when the strength exceeded 4500 MPa, which made it difficult to put it to practical use.

However, as can be understood from Examples disclosed in these Patent Documents, the maximum tensile strength of the steel wire is 3500-3600 MPa, and there exists a limitation on highly strengthening the steel wire.

However, according to these techniques, it was impossible to realize the high-ductility ultrathin steel wire having a tensile strength of 4500 MPa or greater.

This is because the previous techniques were not able to perform measurement (and control).

However, these Patent Documents only refer to the entire variations, and do not define the carbon concentration at a specific location.

However, although Patent Document 12 and Patent Document 13 describe that the residual compressive stress is favorable, an absolute value thereof is small, and the range for keeping an extremely excellent ductility and strength in balance is not defined.

Further, there has not existed any example that discloses the relationship with the carbon condition in the outermost layer.

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