WO2022255727A1 - Wire rod and steel wire for spring, spring with improved strength and fatigue limit, and method for manufacturing same - Google Patents

Abstract

Disclosed are a wire rod and a steel wire for a spring, a spring with improved strength and fatigue limit, and a method for manufacturing same. The disclosed wire rod for a spring with improved strength and fatigue limit, according to one embodiment, comprises, in percentage by weight: 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 balance consisting of Fe and unavoidable impurities, wherein Mn+Cr≤1.8% may be satisfied, and 0.05 at%≤Mo+W≤0.15 at% may be satisfied.

Description

Spring wire, steel wire, spring with improved strength and fatigue limit, and manufacturing method thereof

The present invention relates to a spring wire rod, steel wire, spring with improved strength and fatigue limit, and a method for manufacturing the same, and more particularly, to a 2,200 MPa class ultra-high strength spring steel, which has excellent strength and workability and can be nitriding even at high temperatures. It relates to wire rods, steel wires, springs with improved nitriding treatment characteristics and fatigue limit, and a manufacturing method thereof.

Due to the continuous demand for weight reduction of automobile parts due to the weight reduction of vehicles, springs used in automobile transmissions and engine valves also require continuous high strength. However, as the wire diameter becomes thinner due to the high strength of the spring material and the sensitivity to the inclusions increases, the fatigue limit decreases. That is, there is a limit to improving the fatigue limit through strength improvement. To overcome this, spring manufacturers tried to increase the fatigue limit of spring materials by maintaining strength and improving surface hardness through nitriding treatment.

Normally, nitriding treatment is performed at 500 ° C or higher for other parts, but in the case of spring steel, nitriding treatment is performed at 420 ~ 460 ° C to prevent deterioration in strength, and for a long time of 10 hours or more to ensure sufficient nitrogen penetration depth. heat treatment is carried out.

Since the tempering heat treatment temperature of normal spring steel is below 450℃, most spring steels lose strength greatly when heat treated for a long time at 420~450℃. materials must be used. However, when a large amount of components such as Mo and V, which are carbide-forming elements, are added, the decrease in strength during nitriding treatment can be suppressed, but a low-temperature structure can be formed due to segregation in the center, and a problem in that the cross-section reduction rate can occur .

In addition, since the high-temperature heat treatment process is repeated in the process of the spring material, prior austenite grain size (PAGS) control is a problem, and carbide control technology during the heat treatment process is also required.

On the other hand, spring manufacturers want to shorten the process time by performing nitriding treatment at as high a temperature as possible in order to shorten the nitriding treatment time, and at the same time, they require high-strength wire rods that do not cause problems in on-site productivity.

Therefore, it is required to develop a wire rod and a steel wire having excellent quality such as strength and workability, as well as improved nitriding characteristics and fatigue limit.

(Patent Document 0001) Korean Patent Publication No. 10-2000-0043776 (published on July 15, 2000)

In order to solve the above problems, the present invention is intended to provide a wire rod, steel wire, spring, and a method for manufacturing the same, which are excellent in strength and workability, are easily nitrided even at high temperatures, and have improved nitriding characteristics and fatigue limit.

In order to achieve the above object, the spring wire rod having improved strength and fatigue limit according to the present invention contains, by weight, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb: 0.05% or less, including Fe and other unavoidable impurities, satisfying Mn+Cr≤1.8%, satisfying 0.05at%≤Mo+W≤0.15at%, 1mm in the center of the section perpendicular to the longitudinal direction In the area of ² , the ratio of the area satisfying at least one of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% in weight% is 10% or less.

Here, the wire rod may include 80% or more of pearlite structure and the remaining bainite structure or martensite structure in area fraction.

Here, the wire rod may have an average particle diameter of prior austenite of 20 μm or less.

Here, in the wire rod, carbonitrides having a maximum diameter of 15 μm or more may be distributed in less than 2/cm ² in a longitudinal and horizontal cross section within a surface depth of 1 mm.

Here, the wire rod may have a tensile strength of 1,400 MPa or less and a cross-sectional reduction ratio of 35% or more.

In addition, the method for manufacturing a wire rod for a spring having improved strength and fatigue limit according to the present invention for achieving the above object is, in weight%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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 % to 0.2% or less, Nb: 0.05% or less, preparing a bloom by continuously casting molten steel containing the remaining Fe and other unavoidable impurities; Heating the bloom to a temperature of 1,200 ° C. or higher and then rolling it into a billet; heat-treating the billet at 1,030° C. or higher and then rolling it into a wire rod at a temperature of 1,000° C. or lower; winding the wire rod at a temperature of 800 to 900° C.; and cooling the wound wire rod at a rate of 0.5 to 2° C./s.

Here, the continuous casting step may include light reduction with a total reduction of 20 mm or more.

Here, the light pressure is rolled to 4 mm or less for each rolling roll, and the cumulative reduction may be 60% or more when the solidification fraction is 0.6 or more.

In addition, the steel wire for a spring with improved strength and fatigue limit according to the present invention for achieving the above object, in weight%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% Hereinafter, Nb: 0.05% or less, including the remainder Fe and other unavoidable impurities, satisfying Mn + Cr ≤ 1.8%, satisfying 0.05at% ≤ Mo + W ≤ 0.15at%, by area fraction, tempered martens It contains more than 85% of the site structure and the remaining austenite structure.

Here, the steel wire may have a prior austenite average particle diameter of 15 μm or less.

Here, in the steel wire, carbonitrides having a maximum diameter of 15 μm or more may be distributed in less than 2 pieces/cm ² in a longitudinal and horizontal cross section within a surface depth of 1 mm.

Here, in an area of 100 μm ² , the number of carbides is 10 to 50, the maximum diameter of the carbides is 5 to 50 nm, and the V or Nb content may be 10 at% or more.

Here, the steel wire may have a tensile strength of 2,100 MPa or more and a cross-sectional reduction ratio of 45% or more.

In addition, a method for manufacturing a steel wire for a spring having improved strength and fatigue limit according to the present invention for achieving the above object includes LP heat treatment of the wire rod; preparing a steel wire by drawing the wire rod subjected to the LP heat treatment; and subjecting the steel wire to QT heat treatment, wherein the LP heat treatment includes: a first austenitizing step of heating to 950 to 1100° C. within 3 minutes and then maintaining the temperature for 3 minutes or less; and passing the first austenitized wire rod through a lead bath at 650 to 700° C. within 3 minutes.

Here, the pearlite transformation completion time in the LP heat treatment step may be less than 130 seconds.

Here, prior to the LP heat treatment step, the step of LA heat treatment of the wire rod is further included, and the step of LA heat treatment includes heat treatment at 650 to 750° C.; and pickling; may further include.

Here, the QT heat treatment step is a second austenitizing step of heating to 900 to 1000 ° C. within 3 minutes and then maintaining it for 3 minutes or less; First oil quenching at 70° C. or lower; Tempering step of heating to 450 to 550 ° C. within 3 minutes and then maintaining it for 3 minutes or less; and performing a second oil quenching at 70° C. or less.

In addition, the spring with improved strength and fatigue limit according to the present invention for achieving the above object, in weight%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb: 0.05% or less, including the remainder Fe and other unavoidable impurities, satisfies Mn+Cr≤1.8%, satisfies 0.05at%≤Mo+W≤0.15at%, and can withstand 10 million repeated stresses. degree is 700 MPa or more.

In addition, a method for manufacturing a spring having improved strength and fatigue limit according to the present invention for achieving the above object includes the steps of cold forming the steel wire into a spring shape; subjecting the molded spring to stress relief heat treatment; and nitriding at a temperature of 420 to 450° C. for 10 hours or more.

In addition, in the method of manufacturing a spring with improved strength and fatigue limit according to the present invention for achieving the above object, the fatigue limit after the nitriding treatment can be increased by 10% or more.

According to one aspect of the present invention, to provide a wire rod, a steel wire, a spring, and a method for manufacturing the same, which can suppress the generation of cold tissue in the center by reducing segregation in the center, secure an excellent cross-section reduction rate, and at the same time secure a tensile strength of 2,200 MPa or more. can

According to another aspect of the present invention, it is possible to provide a wire rod, a steel wire, a spring, and a manufacturing method thereof having improved nitriding characteristics and fatigue limit by controlling the size of crystal grains and the number of precipitates.

The wire rod for a spring with improved strength and fatigue limit according to the present invention contains, by weight, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb: 0.05% or less, balance Fe and other unavoidable impurities, satisfies Mn + Cr ≤ 1.8%, satisfies 0.05 at% ≤ Mo + W ≤ 0.15 at%, in the center 1 mm ² area of the cross section perpendicular to the longitudinal direction, in weight%, A ratio of an area satisfying at least one of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% is 10% or less.

Preferred embodiments of the present invention are described below. However, the embodiments of the present invention can be modified in many different forms, and the technical spirit of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Terms used in this application are only used to describe specific examples. Therefore, for example, expressions in the singular number include plural expressions unless the context clearly requires them to be singular. In addition, the terms "include" or "have" used in this application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, but other features It should be noted that it is not intended to be used to preliminarily exclude the presence of any steps, functions, components, or combinations thereof.

Meanwhile, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Accordingly, certain terms should not be interpreted in an overly idealistic or formal sense unless clearly defined herein. For example, in this specification, a singular expression includes a plurality of expressions unless there is a clear exception from the context.

In addition, "about", "substantially", etc. in this specification are used at or in the sense of or close to the value when manufacturing and material tolerances inherent in the stated meaning are presented, and are accurate to aid in understanding the present invention. or absolute numbers are used to prevent unfair use by unscrupulous infringers of the stated disclosure.

In the wire rod for a spring having improved strength and fatigue limit according to an embodiment of the present invention, in weight%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb: 0.05% Hereinafter, the remaining Fe and other unavoidable impurities are included.

Hereinafter, the reason for limiting the composition range of each alloy element will be described. Hereinafter, unless otherwise specified, units are % by weight.

The content of C is 0.6 to 0.7%.

C is an element that improves the strength of the material, and may be added in an amount of 0.6% or more to ensure sufficient strength of the material. However, if the C content is excessive, the impact characteristics significantly decrease after QT (Quenching & Tempering) heat treatment, and the possibility of low-temperature texture greatly increases during wire production, resulting in inferior wire quality. In addition, when the C content is excessive, the LP heat treatment time, which is one of the steel wire manufacturing processes, is greatly increased, and productivity may be lowered. Considering this, the upper limit of the C content may be limited to 0.7%.

The content of Si is 2.0 to 2.5%.

Si is not only used for deoxidation of steel, but also is an element that is advantageous for securing strength through solid solution strengthening, and may be added in an amount of 2.0% or more to suppress a decrease in strength during nitriding and improve deformation resistance of a spring. However, when the Si content is excessive, surface decarburization may be caused, and workability of the material may be deteriorated. Considering this, the upper limit of the Si content may be limited to 2.5%.

The content of Mn is 0.2 to 0.7%.

Mn is a hardenability improving element, and may be added in an amount of 0.2% or more to secure hardenability and high-strength tempered martensitic structure of the material, and fix S as Mn to make it harmless. However, when the Mn content is excessive, the quality may deteriorate due to segregation. Considering this, the upper limit of the Mn content may be limited to 0.7%.

The content of Cr is 0.9 to 1.5%.

Cr is a hardenability improving element together with Mn, and may be added in an amount of 0.9% or more to improve the softening resistance of steel during nitriding treatment. However, if the Cr content is excessive, the toughness of the steel wire is greatly reduced and the generation of a low-temperature structure is promoted during cooling of the wire rod. Considering this, the upper limit of the Cr content may be limited to 1.5%.

The content of P is 0.015% or less.

Since P is an element that is segregated at the grain boundaries to reduce the toughness of the material and the resistance to delayed hydrogen fracture, it is desirable to exclude it from the steel material as much as possible. Considering this, the upper limit of the P content may be limited to 0.015%.

The content of S is 0.01% or less.

S, like P, is segregated at grain boundaries to reduce toughness, and also to form MnS to reduce delayed hydrogen fracture resistance. Considering this, the upper limit of the S content may be limited to 0.01%.

The content of Al is 0.01% or less.

Al is a strong deoxidizing element that can remove oxygen in steel to increase cleanliness, but it can form Al ₂ O ₃ inclusions and reduce fatigue resistance. In consideration of this, the upper limit of the Al content may be limited to 0.01%.

The content of N is 0.01% or less.

N is an impurity, but combines with Al or V to form coarse AlN or VN precipitates that do not dissolve during heat treatment. Considering this, the upper limit of the N content may be limited to 0.01%.

The content of Mo is 0.25% or less.

Mo is an element that improves softening resistance in materials for nitriding treatment and increases strength during tempering by forming carbides together with V. In addition, Mo is an element that forms MC carbide to maintain the strength of the material even during long-term heat treatment. However, when the Mo content is excessive, the formation of a pearlite structure is suppressed, and the quality of the wire rod may be deteriorated due to the formation of a low-temperature structure after rolling the wire rod. In addition, when the Mo content is excessive, pearlite transformation is suppressed even during LP heat treatment before wire drawing, so that the pearlite transformation time increases, resulting in a significant decrease in productivity. Considering this, the upper limit of the Mo content may be limited to 0.25%.

The content of W is 0.25% or less.

W, together with Mo, is an element that can improve softening resistance in nitriding materials, and like Mo, it can form MC carbide to maintain the strength of the material even during long-term heat treatment. However, when the W content is excessive, the formation of pearlite may be suppressed and the formation of a low-temperature structure may be promoted in the wire rod. Considering this, the upper limit of the W content may be limited to 0.25%.

The content of V is 0.05 to 0.2%.

V, together with Mo, is an element that improves softening resistance in the material for nitriding treatment, and increases strength during tempering by forming carbides, and can maintain strength even during long-term nitriding treatment. In addition, V, unlike Mo and W, has a high solid solution temperature of carbides and serves to maintain the size of old austenite grains. In addition, since V accelerates pearlite transformation, low-temperature structure can be suppressed during wire rod production, and the constant temperature transformation time during LP heat treatment can be shortened, so productivity can be improved during the steel wire manufacturing process, so 0.05% or more can be added. However, if the content of V is excessive, coarse carbonitrides may be formed in the process of producing the wire rod, and the temperature of the heating furnace should be increased during wire rod rolling. Considering this, the upper limit of the V content may be limited to 0.2%.

The content of Nb is 0.05% or less.

Nb is a carbonitride forming element, and has a higher solid solution temperature than V, so the control effect of the grain size of prior austenite compared to V is excellent. However, when the content of Nb is excessive, a problem of coarsening the grain size of prior austenite may occur. In consideration of this, the upper limit of the Nb content may be limited to 0.05%, and when the grain size of prior austenite is controlled through the manufacturing process, the addition of Nb may be omitted.

In addition to the above composition, the remaining components are iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

Meanwhile, the wire rod having improved strength and fatigue limit according to an embodiment of the present invention may satisfy Mn+Cr≤1.8% in terms of weight%.

When the sum of Mn and Cr exceeds 1.8%, a low-temperature structure such as bainite or martensite may be generated during the wire rod cooling process, and the time to complete pearlite transformation may increase during LP heat treatment. In addition, when the sum of Mn and Cr exceeds 1.8%, the carbon equivalent (Ceq) increases significantly and the amount of W and Mo added is limited, so it is impossible to prevent a decrease in the strength of the material during nitriding treatment. . In addition, when the carbon equivalent increases, the pearlite transformation time becomes longer, so that a complete pearlite structure cannot be secured during the wire rod cooling process, and productivity decreases as the LP heat treatment time becomes longer.

In addition, the wire rod having improved strength and fatigue limit according to an embodiment of the present invention may satisfy 0.05at%≤Mo+W≤0.15at%. Here, at% means atomic weight %.

When the sum of at% of Mo and W is less than 0.05 at%, it is difficult to use the steel as nitriding steel because the decrease in strength cannot be suppressed during nitriding treatment. On the other hand, when the sum of at% of Mo and W exceeds 0.15 at%, the carbon equivalent increases and the pearlite transformation time is delayed, resulting in a decrease in productivity.

On the other hand, the reason for controlling by at% is to match Mo and W with carbides in a 1:1 ratio, since Mo and W contribute to strength improvement by making carbides in the form of MC (M=Mo or W, C=carbon) to be.

In addition, the wire rod according to an embodiment of the present invention can secure a pearlite transformation completion time of less than 130 seconds during LP (Lead Patenting) heat treatment. Here, the LP heat treatment process may include heating at 950 to 1100 °C and then rapidly cooling to 650 to 750 °C. When the pearlite transformation completion time exceeds 130 seconds during the LP heat treatment, a problem of reduced productivity occurs.

In addition, the wire rod having improved strength and fatigue limit according to an embodiment of the present invention may include a pearlite structure of 80% or more in area fraction.

In addition, the wire rod having improved strength and fatigue limit according to an embodiment of the present invention may have an average particle diameter of prior austenite of 20 μm or less. When the prior austenite average particle diameter exceeds 20 μm, the time of the LP heat treatment process increases and the workability of the wire rod deteriorates.

In addition, the wire rod having improved strength and fatigue limit according to an embodiment of the present invention has, in weight%, C > 0.85%, Si > 3.0%, Mn > 0.8% in the center 1 mm ² area of the cross section perpendicular to the longitudinal direction. , Cr > 2.0% may be less than 10%.

When the above-mentioned area ratio exceeds 10%, the quality of the material is deteriorated, such as a low-temperature structure caused by segregation in the center, and the reduction of area (RA) after manufacturing the steel wire is inferior, resulting in poor workability There is a problem that the breakage frequency increases during spring processing. In addition, when the above-described area exceeds 10%, the carbide effect may be reduced due to the concentration of carbide-forming elements in the center.

In addition, in the wire rod having improved strength and fatigue limit according to an embodiment of the present invention, carbonitrides having a maximum diameter of 15 μm or more are distributed in less than 2/cm ² in a cross section horizontal to the longitudinal direction within a surface depth of 1 mm. can

When carbonitrides of 15 μm or more are present on the surface of the wire rod, fatigue fracture may occur in the material. Therefore, it is preferable that carbonitrides having a maximum diameter of 15 µm or more exist in an amount of less than 2/cm ² in a longitudinal and horizontal cross section within a surface depth of 1 mm.

In addition, the wire rod having improved strength and fatigue limit according to an embodiment of the present invention may have a tensile strength of 1,400 MPa or less and an area reduction ratio (RA) of 35% or more.

Next, a method for manufacturing a wire rod for a spring having improved strength and fatigue limit according to an embodiment of the present invention will be described.

According to an embodiment of the present invention, a method for manufacturing a wire rod for a spring having improved strength and fatigue limit, in weight%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb : preparing a bloom by continuously casting molten steel containing 0.05% or less of Fe and other unavoidable impurities; Heating the bloom to a temperature of 1,200 ° C. or higher and then rolling it into a billet; heat-treating the billet at 1,030° C. or higher and then rolling it into a wire rod at a temperature of 1,000° C. or lower; winding the rolled wire at a temperature of 800 to 900° C.; and cooling the wound wire rod at a rate of 0.5 to 2° C./sec.

The reason for limiting the composition range of each alloy element is as described above, and each manufacturing step will be described in detail below.

According to one embodiment of the present invention, the continuous casting step may include light reduction with a total reduction of 20 mm or more.

In the continuous casting machine, a cast steel having an unsolidified layer at the end of solidification is cast while being gradually reduced by a collection of reduction rolls at a total reduction amount and reduction rate equivalent to the sum of the solidification shrinkage amount and the heat shrinkage amount. It tells you how to do it. Here, the total amount of reduction is the amount of reduction from the start of reduction to the end of reduction. When the total reduction is less than 20 mm, it is difficult to secure the effect of removing segregation under light pressure, so the total reduction under light pressure can be controlled to 20 mm or more to minimize segregation of the wire rod.

In addition, according to one embodiment of the present invention, each of the light pressure rolling rolls may be rolled to 4 mm or less, and the cumulative reduction may be 60% or more when the solidification fraction is 0.6 or more. The solidification fraction means the ratio of the weight of molten steel that has become a solid phase to the weight of all molten steel.

On the other hand, if the casting speed is too slow, solidification is completed before light pressure, and the ratio of the liquid phase to the solid phase is too low, so it is difficult to secure the effect of removing segregation under light pressure. On the other hand, if the casting speed is too fast, the ratio of the liquid phase to the solid phase is too high, and segregation due to solidification shrinkage is generated, which is not preferable. Therefore, it is necessary to control the casting speed so that the reduction amount is 60% or more when the solidification fraction is 0.6 or more.

The amount of cooling water is appropriately adjusted so that solidification can be completed to the point where light pressure reduction is completed. Depending on the facility, Mold-EMS (Mold Electro Magnetic Stirrer) and Strand-EMS can follow the conditions of the existing spring steel or can be set arbitrarily.

On the other hand, unlike normal wire rods for springs, spring steel for nitriding treatment needs to control internal carbonitrides because many high-alloy components are added. Therefore, according to one embodiment of the present invention, the prepared bloom can be heated to a temperature of 1,200 ° C. or higher and then rolled into a billet to minimize internal carbonitrides.

Thereafter, the billet may be heat treated at 1,030 ° C or higher and then rolled into a wire rod at a temperature of 1,000 ° C or lower.

When the heat treatment temperature of the billet is less than 1030 ° C., the V component in the material is not sufficiently melted to dissolve carbides, resulting in a decrease in softening resistance in the final product. The rolling with a wire rod may be performed at a temperature of 1000° C. or less so that the coiling temperature may be performed at 900° C. or less.

Thereafter, the rolled wire may be wound at a temperature of 800 to 900 °C.

If the temperature difference between the rolling temperature and the winding temperature of the wire rod is large, F decarburization due to local supercooling may occur severely. In consideration of this, the step of winding the rolled wire rod may be performed at a temperature of 800 to 900 °C.

Thereafter, the wound wire may be cooled at a rate of 0.5 to 2° C./s.

Unlike normal wire rods for springs, spring steel for nitriding treatment needs to suppress the low-temperature structure because a lot of high-alloy components are added. Decarburization may occur when the wound wire is cooled at a rate of less than 0.5° C./s. On the other hand, when the cooling rate exceeds 2 ° C. / s, fracture may occur in the material due to the low-temperature structure.

Next, a steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present invention will be described.

In the spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention, in weight%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb: 0.05% Hereinafter, the remaining Fe and other unavoidable impurities are included.

In addition, the spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention may satisfy Mn+Cr≤1.8%.

In addition, the spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention may satisfy 0.05at%≤Mo+W≤0.15at%.

The reason for limiting the composition range of each alloy element is as described above.

In addition, the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present invention may include, in area fraction, 85% or more of a tempered martensite structure and the remaining austenite structure.

In addition, the spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention may have an average particle diameter of prior austenite of 15 μm or less.

In addition, in the spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention, in the center 1 mm ² area of the cross section perpendicular to the longitudinal direction, in weight%, C > 0.85%, Si > 3.0%, Mn > A ratio of an area satisfying at least one of 0.8% and Cr > 2.0% may be 10% or less.

When the above-mentioned area ratio exceeds 10%, the quality of the material is deteriorated, such as a low-temperature structure due to central segregation, and workability is lowered, resulting in an increase in breakage frequency during spring processing of the steel wire. In addition, when the above-described area exceeds 10%, the carbide effect may be reduced due to the concentration of carbide-forming elements in the center.

In addition, in the spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention, the number of carbonitrides having a maximum diameter of 15 μm or more is 2 per 100 mm length in a cross section horizontal to the longitudinal direction within a surface depth of 1 mm. may be less than

In the case where carbonitrides of 15 μm or more are present on the surface of the steel wire, fatigue fracture may occur in the material. It is preferable that less than two exist per 100 mm length in the cross section horizontal to the longitudinal direction within a surface depth of 1 mm.

In addition, in the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present invention, the number of carbides is 10 to 50 in an area of 100 μm ² , the carbide has a maximum diameter of 5 to 50 nm, and V or The Nb content may be 10 at% or more.

In the case of carbide containing V or Nb, when it starts to grow larger than 10 nm, it grows with not only V but also other carbide-forming elements such as Cr and Mo, so it is a carbide-forming element to be used for suppressing the growth of former austenite grains and for precipitation hardening. should be properly distributed.

When the number of carbides having a maximum diameter of 5 to 50 nm is less than 10, it is difficult to control the grain size of prior austenite. On the other hand, when the number of carbides having a maximum diameter of 5 to 50 nm exceeds 50, the amount to be utilized for precipitation hardening of 5 nm or less may be reduced, and the tensile strength of the steel wire may be lowered.

In addition, the steel wire for a spring having improved strength and fatigue limit according to an embodiment of the present invention may have a tensile strength of 2,100 MPa or more and a section reduction ratio (RA) of 45% or more.

Next, a method for manufacturing a spring steel wire having improved strength and fatigue limit according to an embodiment of the present invention will be described.

A method for manufacturing a steel wire for a spring according to an embodiment of the present invention includes LA heat treatment of a wire rod according to an embodiment of the present invention; LP heat treatment; and preparing a steel wire by drawing the wire rod. and QT heat-treating the steel wire.

First, low temperature annealing (LA) may be performed on the wire rod according to an embodiment of the present invention at 650 to 750 ° C.

Although not limited thereto, the LA heat treatment step is preferably performed within 2 hours because carbides coarsen as the process time increases, making it difficult to control carbides in subsequent processes. Through the LA heat treatment, the strength of the wire rod can be lowered to 1,200 MPa or less, and the LA heat treatment step can be omitted if necessary.

Thereafter, after pickling the wire rod subjected to the LA heat treatment, LP (Lead Patenting, LP) heat treatment may be performed.

The LP heat treatment is a first austenitizing step of heating to 950 to 1100 ° C within 3 minutes and then maintaining it for 3 minutes or less, and the first austenitized wire rod in a lead bath of 650 to 700 ° C within 3 minutes It may include a passing step.

By heating to 950 to 1100 ° C within 3 minutes and then holding it for 3 minutes or less, an austenitizing process is performed to secure the austenite structure and at the same time to re-dissolve the coarsened carbides in the LA process. have.

Subsequently, the first austenized wire rod is rapidly cooled by passing through a lead bath at 650 to 750 ° C. within 3 minutes to undergo constant temperature transformation, and a pearlite structure can be secured. When the temperature of the lead bath is less than 650 ° C, a low-temperature structure may be formed. On the other hand, when the lead bath temperature exceeds 750 ° C., the carbide may be coarsened and the strength may be reduced.

Thereafter, a steel wire may be prepared by drawing the wire rod subjected to the LP heat treatment. At this time, the wire diameter of the prepared steel wire may be 5 mm, and LP heat treatment may be performed again to secure the wire diameter of the steel wire to 2 mm or less.

Thereafter, in order to secure a tempered martensitic structure, the prepared steel wire may be subjected to a QT heat treatment process.

According to one embodiment of the present invention, the QT heat treatment step, a second austenitizing step of heating to 900 to 1000 ° C. within 3 minutes and then maintaining it for 3 minutes or less; First oil quenching at 70° C. or lower; Tempering step of heating to 450 to 550 ° C. within 3 minutes and then maintaining it for 3 minutes or less; and performing a second oil quenching at 70° C. or lower.

In the QT heat treatment step, the austenitizing temperature may be performed at 900 to 1000 ° C. to maintain fine carbides precipitated during LP heat treatment. Although not limited thereto, the austenitizing process in the QT heat treatment step is preferably performed for 6 minutes or less.

In the QT heat treatment step, when the tempering temperature is less than 450° C., the nitriding treatment temperature is lowered, additional carbide formation cannot be induced, and toughness deteriorates. On the other hand, when the tempering temperature exceeds 550 ° C. in the QT heat treatment step, sufficient strength cannot be secured.

Next, a spring with improved strength and fatigue limit according to an embodiment of the present invention will be described.

In the spring with improved strength and fatigue limit according to an embodiment of the present invention, in weight%, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb: 0.05% or less, It contains the remaining Fe and other unavoidable impurities, satisfies Mn+Cr≤1.8%, and satisfies 0.05at%≤Mo+W≤0.15at%.

The reason for limiting the composition range of each alloy element is as described above.

In addition, the fatigue limit of the spring according to an embodiment of the present invention increases by 10% or more after nitriding. Here, the fatigue limit means a limit that can withstand more than 10 million repeated loads during a fatigue test after designing a spring.

In addition, the spring according to an embodiment of the present invention may have a fatigue limit of 700 MPa or more capable of withstanding stress repeated 10 million times.

In addition, in the spring according to an embodiment of the present invention, the change in strength before and after nitriding treatment is 15% or less, and the nitriding treatment temperature may be 430 ° C. or more.

Next, a method for manufacturing a spring having improved strength and fatigue limit according to an embodiment of the present invention will be described.

A method for manufacturing a spring having improved strength and fatigue limit according to an embodiment of the present invention includes cold forming a steel wire according to an embodiment of the present invention to form a spring; subjecting the molded spring to stress relief heat treatment; and nitriding.

The steel wire according to an embodiment of the present invention can improve the fatigue limit through nitriding before the shot peening step in the spring manufacturing process. At this time, if the nitriding treatment temperature is too low, nitrogen cannot properly penetrate the surface, and if the nitriding treatment temperature is too high, the hardness of the center of the material decreases, making it impossible to secure the desired strength of the material. In consideration of this, the nitriding process may be performed at a temperature of 420 to 450° C. for 10 hours or more.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for exemplifying the practice of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

{Example}

For various alloy composition ranges shown in Table 1 below, blooms were prepared by performing a casting process at a total diameter reduction of 10 to 25 mm. The prepared bloom was subjected to homogenization heat treatment at 1,200 ° C, heat treatment at 1050 ° C, and then hot-rolled to a final wire diameter of 6.5 mm while lowering the temperature to 850 ° C, to prepare a wire rod having a final wire diameter of 6.5 mm. Thereafter, the rolled wire rod was wound at 800 to 900 ° C and then cooled at a rate of 1 ° C / s.

	Alloy element (% by weight)
	C	Si	Mn	Cr	P	S	Mo	V	Al	Nb	W
Example 1	0.63	2.2	0.3	1.2	0.009	0.005	0.2	0.15	<0.003	0.02
Example 2	0.63	2.2	0.3	1.2	0.011	0.005	0.15	0.15	<0.003		0.1
Comparative Example 1	0.63	2.2	0.3	1.2	0.009	0.005	0.2	0.15	<0.003	0.02
Comparative Example 2	0.63	2.2	0.3	1.2	0.011	0.005	0.15	0.02	<0.003		0.1
Comparative Example 3	0.63	2.2	0.3	1.2	0.009	0.005	0.2	0.15	<0.003	0.02	0.2
Comparative Example 4	0.63	2.2	0.3	1.2	0.009	0.005	0.2	0.15	<0.003	0.02

Table 2 below shows the at% content of W + Mo and the total reduction in pressure in Examples and Comparative Examples. The segregation area in Table 2 below was derived by analyzing the center 1 mm ² of the cross section perpendicular to the longitudinal direction of the manufactured wire rod. The 'C segregation area' in Table 2 means the ratio of the area satisfying C > 0.85% by weight in the area of 1 mm ² in the center of the section perpendicular to the longitudinal direction. 'Si segregation area' means the ratio of the area satisfying Si > 3.0% by weight in the central 1 mm ² area of the cross section perpendicular to the longitudinal direction. 'Mn segregation area' means the ratio of the area satisfying Mn > 0.8% by weight in the center 1 mm ² area of the cross section perpendicular to the longitudinal direction. 'Cr segregation area' means the ratio of the area satisfying Cr > 2.0% by weight in the center 1 mm ² area of the cross section perpendicular to the longitudinal direction. The segregation area was measured using an electron probe X-ray Micro Analyzer (EPMA) having a model name of EMPA-1600.

	W+Mo (at%)	total pressure load (mm)	C Segregation area (%)	Si Segregation area (%)	Mn Segregation area (%)	Cr Segregation area (%)	Sum of C, Si, Mn, Cr segregation area (%)
Example 1	0.11	25 mm	<1%	2.5	<1%	2.5	<7%
Example 2	0.08	25mm	<1%	4.3	<1%	2.3	<8.6%
Comparative Example 1	0.11	10mm	5.5	11.2	3.1%	10.2	30%
Comparative Example 2	0.08	25mm	<1%	4.5	<1%	2.2	<8.7%
Comparative Example 3	0.17	25mm	<1%	3.2	<1%	3.4	<8.6%
Comparative Example 4	0.11	25 mm	<1%	4.2	<1%	2.4	<8.6%

Referring to Table 2, Examples 1 and 2 were satisfied with the alloy composition and total light reduction presented by the present invention, so that the sum of the segregated areas of C, Si, Mn, and Cr was less than 10%. In comparison, in Comparative Example 1, as a result of the total hardness being less than 20 mm of 10 mm, the sum of the segregated areas of C, Si, Mn, and Cr reached 30%.

Table 3 below shows the tensile strength, area reduction ratio (RA), center low-temperature structure, prior austenite average particle diameter, pearlite structure, and number of carbonitrides of the prepared wire rod. The average particle diameter of old austenite, the pearlite structure, and the number of carbonitrides were measured using a scanning electron microscope (SEM) with model name JEOL, JSM-6610LV.

'O' in Table 3 below means the case where the low-temperature structure exceeds 20% in area fraction, and 'X' means 'X' when the area fraction of the low-temperature structure is 20% or less.

For the pearlite structure in Table 3 below, 8 specimens were prepared by dividing a 3m wire rod into 8 parts, and then when the microstructure of the cross section perpendicular to the longitudinal direction of each specimen was measured, the pearlite structure was detected as an area fraction of 80% or more means the number of specimens.

The number of carbonitrides in Table 3 below is the maximum diameter of 15 when the microstructure of the longitudinal and horizontal cross-section is measured after preparing 10 specimens with a length of 1 cm by dividing a 10 cm-long wire rod into 10 equal parts, and then measuring the microstructure of the longitudinal direction and the horizontal cross section within 1 mm of the surface depth. It is the number of carbonitrides larger than ㎛.

	Tensile strength (MPa)	Wire rod section reduction rate (%)	center cold tissue	Old austenite average grain size (μm)	perlite group	number of carbonitrides
Example 1	1221	42	X	14	7/8	0
Example 2	1231	35	X	18	8/8	0
Comparative Example 1	1455	25	O	20	5/8	2
Comparative Example 2	1253	35	X	24	7/8	0
Comparative Example 3	1510	10	O	15	2/8	0
Comparative Example 4	1233	42	X	16	8/8	0

Looking at Table 3, Examples 1 and 2 did not form a low-temperature structure in the center, and the average particle diameter of prior austenite was formed to 20 μm or less. In addition, Examples 1 and 2 had 6 or more specimens having a pearlite structure of 80% or more among 8 specimens, and had excellent workability with a tensile strength of 1400 MPa or less. In Examples 1 and 2, no carbonitride was formed on the surface.

In contrast, Comparative Example 1 had a tensile strength exceeding 1400 MPa and a cross-sectional reduction rate of 35% or less, so the processability was inferior, and a low-temperature structure was formed in the center. In addition, in Comparative Example 1, only 5 specimens having a pearlite structure of 80% or more among 8 specimens did not form a pearlite structure of 80% or more uniformly.

In Comparative Example 2, looking at the alloy components of Table 1, as V was added at less than 0.05%, the average particle diameter of prior austenite was coarsened to 24 μm exceeding 20 μm.

Comparative Example 3 had a tensile strength of 1510 MPa and a cross-sectional reduction rate of only 10%, resulting in poor processability and a low-temperature structure formed in the center. In addition, in Comparative Example 3, only two specimens having a pearlite structure of 80% or more among 8 specimens did not sufficiently form a pearlite structure.

Next, the Examples and Comparative Examples were pickled after LA heat treatment at 720 ° C. for 2 hours, and LP heat treatment was performed. The LP heat treatment was heated to the first austenitizing temperature within 3 minutes, and the rest was performed according to the conditions in Table 4 below. In addition, Table 4 below shows the pearlite transformation times of Examples and Comparative Examples during LP heat treatment. The pearlite transformation time was measured by deriving a TTT (Time-Temperature-Transformation) curve through a dilatometry experiment.

	LP heat treatment				Perlite transformation time (seconds) during LP heat treatment
	Primary austenitizing		lead
	temperature (℃)	holding time (minute)	temperature (℃)	transit time (minute)
Example 1	1000	3	675	2	110
Example 2	1000	3	675	2	105
Comparative Example 1	1000	3	675	2	110
Comparative Example 2	1000	3	675	2	112
Comparative Example 3	1000	3	675	2	130
Comparative Example 4	930	3	690	2	110

In Examples 1 and 2, the pearlite transformation times were measured as 110 seconds and 105 seconds, which are less than 130 seconds, respectively, and the productivity was excellent. In contrast, in Comparative Example 3, the pearlite transformation time was measured at 130 seconds, and productivity was inferior to the extent that on-site production was difficult.

Subsequently, the examples and comparative examples subjected to the LP heat treatment were wire-wired to produce a steel wire having a wire diameter of 3 mm. After the second austenizing and first quenching of the manufactured steel wire, tempering and second quenching were performed to obtain a QT steel wire. It was heated to the second austenitizing temperature within 3 minutes, and the first quenching and the second quenching were performed in oil at 60 °C. The rest was performed according to the conditions in Table 5 below.

	QT heat treatment
	Second austenitizing		Tempering
	Temperature (℃)	Hold time (minutes)	Temperature (℃)	Hold time (minutes)
Example 1	930	2	470	2
Example 2	930	2	470	2
Comparative Example 1	930	2	470	2
Comparative Example 2	930	2	470	2
Comparative Example 3	930	2	470	2
Comparative Example 4	930	2	470	2

Table 6 below shows the tensile strength, area reduction ratio (RA) and the number of carbides of the manufactured QT steel wire. Here, the number of carbides means the number of carbides having a maximum diameter of 5 to 50 nm and a V or Nb content of 10 at% or more in an area of 100 μm ² . The number of carbides is a value obtained by measuring 8 random points of 100 μm ² on the surface of the wire using a FEI Tecnai OSIRIS transmission electron microscope (TEM), and then averaging the measured values of the 8 points.

	QT steel wire tensile strength (MPa)	QT steel wire section reduction rate (%)	carbide count
Example 1	2242	51	31
Example 2	2232	49	23
Comparative Example 1	2232	32	65
Comparative Example 2	2180	44	2
Comparative Example 3	2352	44	24
Comparative Example 4	2120	46	61

Looking at Table 6, Examples 1 and 2 secured excellent tensile strength of 2200 MPa or more and at the same time secured a cross-sectional reduction rate of 45% or more. In Examples 1 and 2, the number of carbides was 10 to 50.

In contrast, in Comparative Example 1, the area reduction rate was only 32%, and the number of carbides exceeded 50. In Comparative Example 2, the tensile strength was inferior to 2200 MPa or less, and the number of carbides was formed to be less than 10, so that it was difficult to control the average grain diameter of prior austenite. Comparative Example 4 was inferior in tensile strength to 2200 MPa or less, and the number of carbides exceeded 50.

Next, after cold forming the QT steel wire into the shape of a spring, the formed spring was subjected to heat treatment and then nitriding at 420 to 450 ° C.

Table 7 below shows whether the spring was damaged during forming, the fatigue limit, and the fatigue limit value after nitriding treatment.

The fatigue limit before and after nitriding treatment was measured under the conditions of stress ratio R (tensile capacity/compressive capacity) = -1 and test speed of 30 to 60 Hz.

In Table 7 below, 'X' means that the spring was not broken during molding, and 'O' means that the spring was broken during molding.

	Damage	Fatigue limit before nitriding (MPa)	Fatigue limit after nitriding (MPa)
Example 1	X	700	780
Example 2	X	710	780
Comparative Example 1	O	680	750
Comparative Example 2	O	650	700
Comparative Example 3	X	700	770
Comparative Example 4	X	660	710

Examples 1 and 2 had excellent processability and were not damaged, and the fatigue limit was measured at 650 MPa or more before nitriding treatment, so the fatigue limit was excellent. In addition, Examples 1 and 2 had a fatigue limit of 750 MPa or more after nitriding treatment, and the fatigue limit after nitriding treatment was increased by 10% or more compared to before nitriding treatment, so the nitriding treatment characteristics were excellent.

In contrast, Comparative Examples 1 and 2 were damaged due to poor workability, and after nitriding treatment, the fatigue limit increased to less than 10% compared to before nitriding treatment.

In Comparative Example 4, no breakage occurred during spring processing, but the fatigue limit after nitriding treatment did not increase to 10% or more compared to before nitriding treatment, and thus the nitriding treatment characteristics were inferior.

According to the disclosed embodiment, by optimizing the alloy composition and manufacturing conditions, excellent tensile strength and section reduction rate are secured, and at the same time, nitriding treatment characteristics and fatigue limit are improved, so that it can be applied to materials such as automobile transmission gears and engine valves. .

According to one embodiment of the present invention, it is possible to provide a spring wire rod, a steel wire, a spring with improved strength and fatigue limit, and a manufacturing method thereof.

Claims (15)

1. In weight percent, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb: 0.05% or less, remaining Fe and other unavoidable impurities,

   Satisfying Mn+Cr≤1.8%,

   Satisfying 0.05at%≤Mo+W≤0.15at%,

   In the center 1 mm ² area of the cross section perpendicular to the longitudinal direction, in weight%, the ratio of the area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, Cr > 2.0% is 10% or less, Spring wire with improved strength and fatigue limit.
2. According to claim 1,

   A wire rod for springs with improved strength and fatigue limit, containing 80% or more of pearlite structure by area fraction and remaining bainite structure or martensite structure.
3. According to claim 1,

   Wire rod for springs with improved strength and fatigue limit, with an average particle diameter of prior austenite of 20 μm or less.
4. According to claim 1,

   A spring wire with improved strength and fatigue limit, with less than 2/cm ² carbonitrides having a maximum diameter of 15㎛ or more distributed in a longitudinal and horizontal cross section within a surface depth of 1mm.
5. According to claim 1,

   A spring wire with improved strength and fatigue limit, with a tensile strength of 1,400 MPa or less and a cross section reduction of 35% or more.
6. In weight percent, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, Bloom by continuously casting molten steel containing N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05% to 0.2% or less, Nb: 0.05% or less, the remainder Fe and other unavoidable impurities ) preparing;

   Heating the bloom to a temperature of 1,200 ° C. or higher and then rolling it into a billet;

   heat-treating the billet at 1,030° C. or higher and then rolling it into a wire rod at a temperature of 1,000° C. or lower;

   winding the wire rod at a temperature of 800 to 900° C.; and

   A method for manufacturing a spring wire having improved strength and fatigue limit, comprising: cooling the wound wire at a rate of 0.5 to 2° C./s.
7. According to claim 6,

   The continuous casting step comprises lightly reducing the total reduction amount of 20 mm or more, a method for manufacturing a wire rod for a spring.
8. According to claim 7,

   the above pressure,

   A method of manufacturing a wire rod for a spring with improved strength and fatigue limit, wherein each rolling roll is rolled to a thickness of 4 mm or less, and the cumulative reduction is 60% or more when the solidification fraction is 0.6 or more.
9. In weight percent, C: 0.6 to 0.7%, Si: 2.0 to 2.5%, Mn: 0.2 to 0.7%, Cr: 0.9 to 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% to 0.2% or less, Nb: 0.05% or less, remaining Fe and other unavoidable impurities,

   Satisfying Mn+Cr≤1.8%,

   Satisfying 0.05at%≤Mo+W≤0.15at%,

   Steel wire for springs with improved strength and fatigue limit, containing at least 85% of the tempered martensitic structure and the remaining austenite structure by area fraction.
10. According to claim 9,

    Steel wire for springs with improved strength and fatigue limit with an average particle diameter of prior austenite of 15㎛ or less.
11. According to claim 9,

    Steel wire for springs with improved strength and fatigue limit, with carbonitrides having a maximum diameter of 15㎛ or more distributed at less than 2/cm ² in the lengthwise and horizontal cross section within a surface depth of 1mm.
12. According to claim 9,

    In an area of 100 μm ² , the number of carbides is 10 to 50,

    The carbide has a maximum diameter of 5 to 50 nm, and a V or Nb content of 10 at% or more, strength and fatigue limit improved spring steel wire.
13. According to claim 9,

    Steel wire for springs with improved strength and fatigue limit, with a tensile strength of 2,100 MPa or more and a section reduction of 45% or more.
14. LP heat treatment of the wire according to any one of claims 1 to 5;

    preparing a steel wire by drawing the wire rod subjected to the LP heat treatment; and

    Including; QT heat treatment of the steel wire;

    The LP heat treatment step,

    A first austenitizing step of heating to 950 to 1100 ° C within 3 minutes and then maintaining it for 3 minutes or less; and

    A method of manufacturing a steel wire for a spring with improved strength and fatigue limit, comprising the step of passing the first austenitized wire rod in a lead bath at 650 to 700 ° C within 3 minutes.
15. According to claim 14,

    In the LP heat treatment step,

    A method for manufacturing a steel wire for a spring with improved strength and fatigue limit, in which the pearlite transformation completion time is less than 130 seconds.