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JP-7856676-B2 - Spring wire, steel wire, spring, and method for manufacturing the same, with improved strength and fatigue limit.

JP7856676B2JP 7856676 B2JP7856676 B2JP 7856676B2JP-7856676-B2

Inventors

  • イ，ジュンモ
  • チェ，ソクファン
  • チェ，ミョンス

Assignees

  • ポスコ カンパニー リミテッド

Dates

Publication Date
20260511
Application Date
20220526
Priority Date
20210602

Claims (11)

  1. In weight percent, it consists of C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2%, Nb: 0.05% or less, and the remainder being Fe and other unavoidable impurities. Satisfying Mn + Cr ≤ 1.8%, The condition 0.05 at% ≤ Mo + W ≤ 0.15 at% is satisfied. A spring wire material with improved strength and fatigue limit, characterized in that, in the central 1 mm² area of a cross-section perpendicular to the length direction, the proportion of areas satisfying one or more of the following by weight percentages is 10% or less: C > 0.85%, Si > 3.0%, Mn > 0.8%, Cr > 2.0%.
  2. A spring wire material with improved strength and fatigue limit, as described in claim 1, characterized by containing 80% or more pearlite structure by area fraction, with the remainder being bainite or martensitic structure.
  3. The spring wire material described in claim 1, characterized in that the average particle size of the prior austenite is 20 μm or less, exhibiting improved strength and fatigue limit.
  4. The spring wire material described in claim 1, characterized in that carbonitrides with a maximum diameter of 15 μm or more are distributed at a rate of less than 2 per cm² in a cross section horizontal to the length direction with a surface depth of 1 mm or less.
  5. A spring wire material with improved strength and fatigue limit, as described in claim 1, characterized by having a tensile strength of 1,400 MPa or less and a cross-sectional reduction rate of 35% or more.
  6. This stage involves continuously casting molten steel, consisting of, by weight percent, C: 0.6–0.7%, Si: 2.0–2.5%, Mn: 0.2–0.7%, Cr: 0.9–1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%–0.2%, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities, to prepare a bloom; The step of heating the bloom at a temperature of 1,200°C or higher, and then rolling it into a billet; The step of heat-treating the billet at 1,030°C or higher, and then rolling it into wire at a temperature of 1,000°C or lower; A method for manufacturing a spring wire with improved strength and fatigue limit, as described in any one of claims 1 to 5, comprising the steps of: winding the wire at a temperature of 800 to 900°C; and cooling the wound wire at a rate of 0.5 to 2°C/s.
  7. The method for manufacturing spring wire according to claim 6, characterized in that the continuous casting step includes light reduction with a total reduction of 20 mm or more.
  8. In weight percent, it consists of C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2% or less, Nb: 0.05% or less, and the remainder being Fe and other unavoidable impurities. Satisfying Mn + Cr ≤ 1.8%, The condition 0.05 at% ≤ Mo + W ≤ 0.15 at% is satisfied. In terms of area fraction, it contains 85% or more tempered martensite and the remainder austenite. A spring steel wire with improved strength and fatigue limit, characterized in that, in an area of 100 μm² , the number of carbides is 10 to 50, the carbides have a maximum diameter of 5 to 50 nm, and the V or Nb content is 10 at% or more.
  9. The spring steel wire described in 8 , characterized in that it has a tensile strength of 2,100 MPa or more and a cross-sectional reduction rate of 45% or more, thereby improving strength and fatigue limit.
  10. A step of LP heat treatment of the wire according to any one of claims 1 to 5; The steps include: drawing the LP heat-treated wire to prepare a steel wire; and performing QT heat treatment on the steel wire; The aforementioned LP heat treatment step is, A method for manufacturing spring steel wire with improved strength and fatigue limit, comprising: a first austenitization step of heating to 950-1100°C within 3 minutes and maintaining the temperature within 3 minutes; and a step of passing the first austenitized wire through a lead bath at 650-700°C within 3 minutes.
  11. In the LP heat treatment stage, A method for manufacturing spring steel wire with improved strength and fatigue limit, as described in claim 10 , characterized in that the time required for the completion of perlite transformation is less than 130 seconds by satisfying the composition range conditions described in claim 1.

Description

This invention relates to spring wire, steel wire, spring, and a method for manufacturing the same, which are ultra-high-strength spring steels of the 2,200 MPa class, possessing excellent strength and workability, and being easily subjected to nitriding treatment even at high temperatures, thereby improving nitriding characteristics and fatigue limit, as well as a method for manufacturing the same. The increasing weight reduction of vehicles has led to a sustained demand for weight reduction in automotive components, and consequently, the springs used in automotive transmissions and engine valves are also required to maintain sustained high strength. However, increasing the strength of spring materials results in thinner wire diameters and increased sensitivity to inclusions, which lowers the fatigue limit. In other words, there is a limit to improving the fatigue limit through increased strength alone. To overcome this, spring manufacturers have attempted to increase the fatigue limit of spring materials by maintaining strength and improving surface hardness through nitriding treatment. While nitriding is typically performed at temperatures above 500°C for other parts, in the case of spring steel, nitriding is performed at 420-460°C to prevent a decrease in strength, and the heat treatment is carried out for a long period of 10 hours or more to ensure sufficient nitrogen penetration depth. Since the tempering heat treatment temperature for ordinary spring steel is 450°C or lower, if heat treatment is performed for a long time at 420-450°C, the strength of many spring steels will decrease significantly. Therefore, it is necessary to use high-alloy materials to which elements that can form carbides and improve softening resistance are added. However, when large amounts of carbide-forming elements such as Mo and V are added, the decrease in strength during nitriding can be suppressed, but a low-temperature structure may be formed due to segregation in the center, which may cause a problem in which the cross-sectional reduction rate decreases. Furthermore, since the spring material undergoes repeated high-temperature heat treatment processes during manufacturing, controlling the prior austenite grain size (PAGS) becomes a challenge, and carbide control technology during the heat treatment process is also necessary. Furthermore, the spring manufacturing company desires to shorten the nitriding process time by performing the nitriding treatment at the highest possible temperature, while simultaneously requiring high-strength wire that does not pose a problem to on-site productivity. Therefore, there is a need for the development of wires and steel wires that are superior in quality, such as strength and workability, while also having improved nitriding characteristics and fatigue limits. Korean Published Patent No. 10-2000-0043776 The spring wire material with improved strength and fatigue limit according to the present invention consists of, by weight percent, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2% or less, Nb: 0.05% or less, the remaining Fe and other unavoidable impurities, satisfying Mn + Cr ≤ 1.8%, and satisfying 0.05 at% ≤ Mo + W ≤ 0.15 at%, with the central 1 mm of the cross-section perpendicular to the length direction being the most efficient. In the two areas, the proportion of areas satisfying one or more of the following weight percentages is 10% or less: C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0%. Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified into various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, embodiments of the present invention are provided to give a more complete explanation of the present invention to a person with average skill in the art. The terms used in this application are solely for illustrative purposes. Therefore, singular expressions include plural expressions unless the context clearly requires them to be singular. Furthermore, it should be noted that terms such as “includes” or “comprising” used in this application are used to explicitly indicate the presence of features, stages, functions, components, or combinations thereof described in the specification, and not to provisionally exclude the presence of other features, stages, functions, components, or combinations thereof. Unless otherwise defined, all terms used herein should be considered to have the same meaning as that generally understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, no particular term should be interpreted in an overly idealistic or formal sense. For example, in this specification, singular expressions include plura