COLD FORMABLE SPRING STEEL WIRE EXCELLENT IN COLD CUTTING CAPABILITY AND FATIGUE PROPERTIES AND MANUFACTURING PROCESS THEREOF

Abstract

Disclosed is a cold formable spring steel wire excellent in cold cutting capability and fatigue properties, in which the steel wire satisfies given composition, has an average globular carbide particle size [√(ab)]:1.0 μm or less with aspect ratio (a/b, a: major axis of carbide, b: minor axis of carbide) being 2 or less, a ratio (area %) of the globular carbide in the steel: (0.1 to 3)×amount (mass %) of C in the steel, an amount (mass %) of Cr in the globular carbide: [0.4×amount (mass %) of Cr in the steel] or less, hardenability factor (Dic): between 110 mm and 450 mm, and tensile stress of 2000 MPa or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above aspects and features of the present invention will be more apparent by describing certain embodiments of the present invention with reference to the accompanying drawings, in which:

FIG. 1 is a graph showing the relation between an average particle size of globular carbide particles and a crack generation rate during cold shear cutting;

FIG. 2 is a graph showing the relation between (a ratio of globular carbides in a steel/C content in a steel) and a crack generation rate during cold shear cutting;

FIG. 3 is a graph showing the relation between (a ratio of globular carbides in a steel/C content in a steel) and a burr generation rate during cold shear cutting;

FIG. 4 is a graph showing the relation between (Cr+Si) and tensile strength;

FIG. 5 is a graph showing the relation between (Cr/Si) and an average particle size of globular carbide particles; and

FIG. 6 is a graph showing the relation between (Cr/Si) and (a ratio of globular carbide in a steel/C content in a steel).

Claims

1. A spring steel wire, comprising: C: 0.45-0.70% (“%” herein means “mass %”),Si: 1.9-2.5%Mn: 0.15-1.0%, andCr: 0.7-2.0%,wherein:P: 0.015% or less (exclusive of 0%);S: 0.015% or less (exclusive of 0%);Cr+Si: 3.0% or more;Cr/Si: 0.95 or less;wherein the metallic structure of the steel satisfies:an average globular carbide particle size [√(ab)]: 1.0μm or less with aspect ratio (a/b, a: major axis of carbide, b: minor axis of carbide) being 2 or less;a ratio (area %) of the globular carbide in the steel: (0.1 to 3)×amount (mass %) of C in the steel;an amount (mass %) of Cr in the globular carbide: [0.4×amount (mass %) of Cr in the steel] or less;tensile stress: 2000 MPa or more; andhardenability factor (Dic) represented by the following formulas (1)-(3): 110 mm≦Dic≦450 mm,in the case where the C content is not less than 0.45% nor greater than 0.55%,

2. The spring steel wire as set forth in claim 1, further comprising (in mass %) at least one element selected from a group consisting of V: 0.4% or less, Ti: 0.1% or less and Nb: 0.1% or less.

3. The spring steel wire as set forth in claim 1, further comprising (in mass %) at least one element selected from a group consisting of Cu: 0.70% or less and Ni: 0.80% or less.

4. A manufacturing process of the spring steel wire as set forth in claim 1, wherein the process comprises the steps of: hot rolling a steel material that satisfies the composition set forth in claim 1;setting a cooling starting temperature after hot rolling to 900° C. or higher, and cooling the steel material from the cooling starting temperature down to 700° C. at a cooling rate of 10° C./sec or higher; andannealing the steel material at a temperature range of 550° C. to 700° C.