# US 7,407,555 B2 · Oil tempered wire for cold forming coil springs

> Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt

- Number: US 7,407,555 B2
- Document ID: US7407555B2
- Status: Granted
- Kind: B2
- Application number: 11/086,410
- Filed: 2005-03-23
- Published: 2005-07-28
- Granted: 2008-08-05
- Earliest claimed: 2001-06-07
- Assignees: Chuo Hatsujo Kabushiki Kaisha
- Inventors: Hidetoshi Yoshikawa; Tomohiro Nakano; Takayuki Sakakibara; Masami Wakita
- Web page: https://patentgrep.com/patent/US7407555B2

## CPC

- C21D 8/06: during manufacturing of rods or wires
- C21D 9/525: {for wire, for rods (C21D9/54 takes precedence)}
- C21D 9/60: with induction heating
- C21D 9/02: for springs
- Y02P 10/25: Process efficiency
- C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B)
- Y02P: CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS

## Abstract

An oil-tempered wire for a cold-formed coil spring having a quality equivalent of or higher than a hot-formed coil spring is provided. A cold-formed coil spring made from the oil-tempered wire is also provided. Material used is the steel which contains, in weight percentage, 0.35 to 0.55% C, 1.8 to 3.0% Si, 0.5 to 1.5% Mn, 0.5 to 3.0% Ni, and 0.1 to 1.5% Cr. The ferrite fraction in the microscopic structure of this material is set to 50% or less. Hot rolled wire is cold drawn with a predetermined reduction of area, and a heat treatment using high frequency induction heating is conducted. It is preferable to set the maximum heating temperature between 900° C. to 1020° C. (favorably 950° C.) and holding time between 5 to 20 seconds. It is preferable to make the oil tempered material to have the grain size number of 9 or more, and the tensile strength from 1830 to 1980 MPa.

## Claims

### Claim 1

A method of producing a steel wire for a cold-formed coil spring comprising the steps of:
providing a steel material comprising, in weight percentage, 0.35 to 0.55% C, 1.8 to 3.0% Si, 0.5 to 1.5% Mn, 0.5 to 3.0% Ni, and 0.1 to 1.5% Cr;
hot-rolling the steel material to a wire with a predetermined diameter;
cold drawing the steel wire with a predetermined reduction of area;
heating the steel wire with a first high frequency induction heating, wherein a ferrite fraction of the cold-drawn wire's microscopic structure before the first high frequency induction heating is 40% or less to minimize deleterious effects of a ferrite component when the ferrite fraction is greater than 0% in austenization by the high frequency induction heating;
rapid cooling the steel wire; and
tempering the steel wire with a second high frequency induction heating.

### Claim 2 (depends on 1)

The method of producing a steel wire for cold-formed coil spring according to claim 1, wherein a maximum heating temperature in the first high frequency induction heating is 940° C. or higher and 1020° C. or lower.

### Claim 3 (depends on 1)

The method of producing a steel wire for cold-formed coil spring according to claim 1, wherein a maximum heating temperature in the first high frequency induction heating is 900° C. or higher and 950° C. or lower.

### Claim 4 (depends on 1)

The method of producing a steel wire for cold-formed coil spring according to claim 1, wherein a holding time at a maximum heating temperature in the first high frequency induction heating is 5 to 20 seconds.

### Claim 5 (depends on 1)

The method of producing a steel wire for cold-formed coil spring according to claim 1, wherein, after the first high frequency induction heating, the steel wire has a grain size number that is set to be 9 or more.

### Claim 6 (depends on 1)

The method of producing a steel wire for cold-formed coil spring according to claim 1, wherein a tensile strength of the steel wire after the tempering is set to be from 1830 to 1980 MPa.

### Claim 7 (depends on 1)

The method of producing a steel wire for cold-formed coil spring according to claim 1, wherein the material further comprises 0.01 to 0.025% N and 0.05 to 0.5% V, and the P content is restricted to 0.015% or less, and the S content is restricted to 0.01% or less.

### Claim 8 (depends on 1)

A cold-formed coil spring made from a steel wire produced by the method of producing a steel wire for cold-formed coil spring according to claim 1.