Wire rod and steel wire for spring, spring with improved strength and fatigue limit, and the method for manufacturing the same

Abstract

A wire rod for springs, a steel wire, a spring, and a method for manufacturing the same are disclosed, with improved strength and fatigue limit. According to one embodiment of the disclosed wire for springs with improved strength and fatigue limit, in weight%, it comprises 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, and the remainder being Fe and other unavoidable impurities, satisfying Mn+Cr≤1.8% and satisfying 0.05at%≤Mo+W≤0.15at%.

Description

Technology Field

[0001] The present invention relates to a spring wire, steel wire, spring, and a method for manufacturing the same, with improved strength and fatigue limit. More specifically, it relates to a 2,200 MPa ultra-high strength spring steel, which has excellent strength and processability, is easy to nitrid at high temperatures, and has improved nitriding characteristics and fatigue limit, and a method for manufacturing the same. Background Technology

[0002] Due to the lightweighting of vehicles and the continuous demand for lighter automotive parts, springs used in automotive transmissions and engine valves are also required to have continuously higher strength. However, as the wire diameter becomes thinner due to the increased strength of spring materials, sensitivity to inclusions increases, leading to a decrease in the fatigue limit. In other words, there is a limit to improving the fatigue limit through strength enhancement. To overcome this, spring manufacturers have sought to increase the fatigue limit of spring materials by maintaining strength while improving surface hardness through nitriding treatment.

[0003] Typically, nitriding is performed at temperatures above 500°C for other parts, but in the case of spring steel, nitriding is performed at 420 to 460°C to prevent a decrease in strength, and a long heat treatment of more than 10 hours is performed to ensure sufficient nitrogen penetration depth.

[0004] Since the tempering heat treatment temperature of conventional spring steel is 450°C or lower, most spring steels lose strength significantly when heat-treated for a long time at 420 to 450°C. Therefore, high-alloy materials containing elements that can improve softening resistance by forming carbides must be utilized. However, while adding large amounts of carbide-forming elements such as Mo and V can suppress strength reduction during nitriding treatment, problems may arise such as the formation of a low-temperature structure due to central segregation and a decrease in the reduction of area.

[0005] Furthermore, since spring materials undergo repeated high-temperature heat treatments during the manufacturing process, controlling the Prior Austenite Grain Size (PAGS) is a challenge, and carbide control technology during the heat treatment process is also required.

[0006] Meanwhile, spring manufacturers want to shorten process time by performing nitriding at the highest possible temperature to reduce the treatment time, and at the same time, they require high-strength wire rods that do not cause issues with on-site productivity.

[0007] Therefore, there is a need to develop wire rods and steel wires that possess excellent quality, such as strength and processability, while simultaneously having improved nitriding characteristics and fatigue limits. Prior art literature

[0008] Korean Published Patent Application No. 10-2000-0043776 (Date of publication: July 15, 2000) The problem to be solved

[0009] To solve the aforementioned problems, the present invention aims to provide a wire rod, steel wire, spring, and a method for manufacturing the same, which have excellent strength and processability, are easy to nitrid even at high temperatures, and have improved nitriding characteristics and fatigue limit. means of solving the problem

[0010] A spring wire with improved strength and fatigue limit according to the present invention for achieving the above-mentioned purpose comprises, 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, and the remainder being Fe and other unavoidable impurities, satisfying Mn+Cr≤1.8% and satisfying 0.05at%≤Mo+W≤0.15at%, and a 1mm section at the center of a cross-section perpendicular to the longitudinal direction.2 In terms of area, the proportion of the area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% in weight% is 10% or less.

[0011] Here, the wire may contain at least 80% of a pearlite structure in terms of area fraction, and the remainder being a bainite structure or a martensite structure.

[0012] Here, the above wire may have an average austenite particle size of 20㎛ or less.

[0013] Here, the wire contains 2 carbonitrides / cm² with a maximum diameter of 15㎛ or more in a longitudinal cross-section within a surface depth of 1mm. 2 It can be distributed as less than

[0014] Here, the above wire may have a tensile strength of 1,400 MPa or less and a cross-sectional reduction rate of 35% or more.

[0015] In addition, a method for manufacturing a spring wire rod with improved strength and fatigue limit according to the present invention for achieving the above-mentioned purpose comprises the steps of: continuously casting molten steel containing, 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, and the remainder being Fe and other unavoidable impurities to produce a bloom; and heating the bloom to a temperature of 1,200°C or higher and then rolling it into a billet. The method comprises the steps of: 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; coiling the wire rod at a temperature of 800 to 900°C; and cooling the coiled wire rod at a rate of 0.5 to 2°C / s.

[0016] Here, the continuous casting step may include light reduction of a total reduction amount of 20 mm or more.

[0017] Here, the above-mentioned light-pressure rolling is performed by rolling to a thickness of 4 mm or less per rolling roll, and when the solidification fraction is 0.6 or more, the cumulative reduction amount may be 60% or more.

[0018] In addition, the spring steel wire according to the present invention, which has improved strength and fatigue limit to achieve the above-mentioned purpose, comprises, 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, and the remainder being Fe and other unavoidable impurities, satisfying Mn+Cr≤1.8% and satisfying 0.05at%≤Mo+W≤0.15at%, and in area fraction, comprises 85% or more of a tempered martensite structure and the remainder being an austenite structure.

[0019] Here, the above steel wire may have an average austenite grain size of 15㎛ or less.

[0020] Here, in a cross-section parallel to the longitudinal direction within a surface depth of 1 mm, the steel wire contains 2 carbonitrides / cm² with a maximum diameter of 15 µm or more. 2 It can be distributed as less than

[0021] Here, 100㎛ 2 In terms of area, the number of carbides is 10 to 50, the maximum diameter of the carbides is 5 to 50 nm, and the content of V or Nb may be 10 at% or more.

[0022] Here, the steel wire may have a tensile strength of 2,100 MPa or more and a cross-sectional reduction rate of 45% or more.

[0023] In addition, a method for manufacturing a steel wire for springs with improved strength and fatigue limit according to the present invention for achieving the above-mentioned purpose comprises: a step of heat-treating the wire rod at LP; a step of drawing the wire rod heat-treated at LP to prepare a steel wire; and a step of heat-treating the steel wire at QT; wherein the step of heat-treating at LP includes a first austenitizing step of heating to 950 to 1100°C within 3 minutes and maintaining for 3 minutes or less; and a step of passing the wire rod heat-treated at 650 to 700°C through a lead bath within 3 minutes.

[0024] Here, the pearlite transformation completion time in the above LP heat treatment step may be less than 130 seconds.

[0025] Here, prior to the step of LP heat treatment, the method further includes a step of LA heat treatment of the wire rod, and the LA heat treatment step may further include a step of heat treatment at 650 to 750°C; and a step of pickling.

[0026] Here, the QT heat treatment step may include: a second austenitizing step of heating to 900 to 1000°C within 3 minutes and maintaining for 3 minutes or less; a first oil quenching step at 70°C or less; a tempering step of heating to 450 to 550°C within 3 minutes and maintaining for 3 minutes or less; and a second oil quenching step at 70°C or less.

[0027] In addition, the spring with improved strength and fatigue limit according to the present invention for achieving the above-mentioned purpose comprises, 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, and the remainder being Fe and other unavoidable impurities, satisfying Mn+Cr≤1.8% and 0.05at%≤Mo+W≤0.15at%, and has a fatigue limit of 700 MPa or more capable of withstanding ten million cycles of repetitive stress.

[0028] In addition, a method for manufacturing a spring with improved strength and fatigue limit according to the present invention for achieving the above-mentioned purpose comprises: a step of cold forming the steel wire into the shape of a spring; a step of stress relief heat treatment of the formed spring; and a step of nitriding treatment at a temperature of 420 to 450°C for 10 hours or more.

[0029] Here, the fatigue limit may increase by more than 10% after the above nitriding treatment. Effects of the invention

[0030] According to one aspect of the present invention, a wire rod, steel wire, spring, and a method for manufacturing the same can be provided, which can suppress the occurrence of low-temperature structures in the center by reducing segregation in the center and secure an excellent cross-sectional reduction rate while simultaneously securing a tensile strength of 2,200 MPa or more.

[0031] According to another aspect of the present invention, by controlling the grain size and the number of precipitates, it is possible to provide a wire rod, a steel wire, a spring, and a method for manufacturing the same, with improved nitriding characteristics and fatigue limit. Specific details for implementing the invention

[0032] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0033] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0034] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense. For instance, singular expressions in this specification include plural expressions unless the context clearly indicates an exception.

[0035] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0036] A spring wire with improved strength and fatigue limit according to one embodiment of the present invention comprises, 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, and the remainder being Fe and other unavoidable impurities.

[0037] The reasons for limiting the compositional range of each alloying element are explained below. Unless otherwise noted, units are weight percent.

[0038] The content of C is 0.6 to 0.7%.

[0039] C is an element that improves the strength of the material, and can be added in an amount of 0.6% or more to ensure sufficient strength of the material. However, if the C content is excessive, impact properties may significantly decrease after QT (Quenching & Tempering) heat treatment, and the possibility of low-temperature microstructure formation during wire rod production may significantly increase, potentially leading to a decline in wire rod quality. Additionally, if the C content is excessive, the time required for LP heat treatment, which is one of the steel wire manufacturing processes, may increase significantly, potentially reducing productivity. Considering this, the upper limit of the C content may be restricted to 0.7%.

[0040] The Si content is 2.0 to 2.5%.

[0041] Silicon is used not only for deoxidizing steel but is also an element advantageous for securing strength through solid solution strengthening; therefore, it can be added in an amount of 2.0% or more to suppress strength reduction during nitriding treatment and to improve the deformation resistance of springs. However, if the Si content is excessive, it may cause surface decarburization and may impair the machinability of the material. Considering this, the upper limit of the Si content may be limited to 2.5%.

[0042] The Mn content is 0.2 to 0.7%.

[0043] Mn is a hardenability-enhancing element and can be added in an amount of 0.2% or more to secure the hardenability of the material and a high-strength tempered martensite structure, and to fix S as Mn to render it harmless. However, if the Mn content is excessive, the quality may be degraded due to segregation. Considering this, the upper limit of the Mn content may be limited to 0.7%.

[0044] The Cr content is 0.9 to 1.5%.

[0045] Cr, along with Mn, is a hardenable-enhancing element and can 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, it significantly reduces the toughness of the steel wire and promotes the formation of low-temperature structures during cooling of the wire rod. Considering this, the upper limit of the Cr content may be limited to 1.5%.

[0046] The P content is 0.015% or less.

[0047] Since P is an element that segregates at grain boundaries, lowering the toughness of the material and reducing resistance to hydrogen delayed fracture, it is desirable to exclude it from steel materials as much as possible. Considering this, the upper limit of the P content can be limited to 0.015%.

[0048] The S content is 0.01% or less.

[0049] Like P, S can segregate at grain boundaries to reduce toughness, and also form MnS, which can reduce resistance to hydrogen delayed fracture. Considering this, the upper limit of the S content can be limited to 0.01%.

[0050] The Al content is 0.01% or less.

[0051] Al is a powerful deoxidizing element that can remove oxygen from steel to increase cleanliness, but it can form Al2O3 inclusions, which can reduce fatigue resistance. Considering this, the upper limit of the Al content can be limited to 0.01%.

[0052] The content of N is 0.01% or less.

[0053] N is an impurity, or 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 can be limited to 0.01%.

[0054] The Mo content is 0.25% or less.

[0055] Mo is an element that improves softening resistance in materials for nitriding treatment and increases strength during tempering by forming carbides together with V. Additionally, Mo forms MC carbides, which maintains the strength of the material even during prolonged heat treatment. However, if the Mo content is excessive, it inhibits the formation of pearlite structure, which can lead to a decline in wire rod quality due to the formation of a low-temperature structure after wire rod rolling. Furthermore, if the Mo content is excessive, it inhibits pearlite transformation even during LP heat treatment prior to drawing, resulting in an increased pearlite transformation time and significantly reduced productivity. Considering this, the upper limit of the Mo content can be restricted to 0.25%.

[0056] The W content is 0.25% or less.

[0057] W is an element that, along with Mo, can improve softening resistance in materials for nitriding treatment. Like Mo, it forms MC carbides, which can maintain the strength of the material even during long-term heat treatment. However, if the W content is excessive, it can inhibit pearlite formation and promote the formation of a low-temperature structure in the wire rod. Considering this, the upper limit of the W content can be limited to 0.25%.

[0058] The V content is 0.05 to 0.2%.

[0059] V, along with Mo, is an element that improves softening resistance in materials for nitriding treatment. It forms carbides to increase strength during tempering and can maintain strength even during prolonged nitriding treatment. Furthermore, unlike Mo and W, V has a high solid solution temperature for carbides, which helps maintain the pre-austenite grain size. Additionally, since V accelerates pearlite transformation, it can suppress low-temperature structures during wire rod production and shorten the isothermal transformation time during LP heat treatment, thereby improving productivity in the steel wire manufacturing process; thus, it can be added at a level of 0.05% or higher. However, if the V content is excessive, coarse carbonitrides may form during the wire rod production process, and the furnace temperature must be increased during wire rod rolling. Considering this, the upper limit of the V content may be restricted to 0.2%.

[0060] The Nb content is 0.05% or less.

[0061] Nb is a carbonitride-forming element and has a higher solid solution temperature than V, so it has a superior effect in controlling the pre-austenite grain size compared to V. However, if the Nb content is excessive, a problem may arise where the pre-austenite grain size becomes coarsened. Considering this, the upper limit of the Nb content can be limited to 0.05%, and the addition of Nb can be omitted if the pre-austenite grain size is controlled through the manufacturing process.

[0062] In addition to the composition mentioned above, the remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0063] Meanwhile, the wire rod with improved strength and fatigue limit according to one embodiment of the present invention can satisfy Mn+Cr≤1.8% in weight%.

[0064] If the sum of Mn and Cr exceeds 1.8%, low-temperature structures such as bainite or martensite may be formed during the wire rod cooling process, and the completion time for pearlite transformation during LP heat treatment may be prolonged. Furthermore, if the sum of Mn and Cr exceeds 1.8%, the carbon equivalent (Ceq) increases significantly, limiting the addition amounts of W and Mo; consequently, it becomes impossible to prevent a decrease in material strength during nitriding treatment. Additionally, an increase in carbon equivalent prolongs the pearlite transformation time, making it difficult to secure an intact pearlite structure during the wire rod cooling process. This leads to a problem of reduced productivity as the LP heat treatment time is extended.

[0065] In addition, the wire rod with improved strength and fatigue limit according to one embodiment of the present invention can satisfy 0.05at%≤Mo+W≤0.15at%. Here, at% means atomic weight%.

[0066] When the sum of the at%s of Mo and W is less than 0.05 at%, it is difficult to use as nitrided steel because the reduction in strength during nitriding treatment cannot be suppressed. On the other hand, when the sum of the at%s of Mo and W exceeds 0.15 at%, the carbon equivalent increases, and as the pearlite transformation time is delayed, a problem of reduced productivity occurs.

[0067] Meanwhile, the reason for controlling at% is to ensure a 1:1 correspondence between Mo and W and the carbides, as Mo and W contribute to strength improvement by forming carbides in the form of MC (M=Mo or W, C=carbon).

[0068] In addition, the wire rod according to one 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 a step of heating at 950 to 1100°C and then rapidly cooling to 650 to 750°C. If the pearlite transformation completion time during LP heat treatment exceeds 130 seconds, a problem of reduced productivity occurs.

[0069] In addition, the wire rod with improved strength and fatigue limit according to one embodiment of the present invention may include a pearlite structure of 80% or more in area fraction.

[0070] In addition, the wire rod with improved strength and fatigue limit according to one embodiment of the present invention may have an average austenite grain size of 20㎛ or less. If the average austenite grain size exceeds 20㎛, the time of the LP heat treatment process increases, and a problem arises in which the processability of the wire rod deteriorates.

[0071] In addition, the wire rod with improved strength and fatigue limit according to one embodiment of the present invention has a center of 1 mm in a cross-section perpendicular to the longitudinal direction. 2 In terms of area, the proportion of the area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% in weight% may be 10% or less.

[0072] If the aforementioned area ratio exceeds 10%, the quality of the material deteriorates, such as the formation of a low-temperature structure due to segregation in the center, and the reduction of area (RA) after steel wire manufacturing deteriorates, resulting in reduced machinability and a higher frequency of breakage during spring processing. Additionally, if the aforementioned area exceeds 10%, the carbide effect may be reduced due to the concentration of carbide-forming elements in the center.

[0073] In addition, the wire rod with improved strength and fatigue limit according to one embodiment of the present invention comprises 2 carbonitrides / cm² with a maximum diameter of 15㎛ or more in a cross-section parallel to the longitudinal direction within a surface depth of 1mm. 2 It can be distributed as less than

[0074] If carbonitrides larger than 15 µm are present on the surface of the wire rod, fatigue fracture may occur in the material. Therefore, in a longitudinal and horizontal cross-section within a surface depth of 1 mm, 2 carbonitrides with a maximum diameter of 15 µm or more per cm2 It is desirable for it to exist at a level less than [value].

[0075] In addition, the wire rod with improved strength and fatigue limit according to one embodiment of the present invention may have a tensile strength of 1,400 MPa or less and a reduction in area (RA) of 35% or more.

[0076] Next, a method for manufacturing a spring wire with improved strength and fatigue limit according to one embodiment of the present invention will be described.

[0077] A method for manufacturing a spring wire rod with improved strength and fatigue limit according to one embodiment of the present invention comprises the steps of: continuously casting molten steel containing, 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, and the remainder being Fe and other unavoidable impurities to produce a bloom; and heating the bloom to a temperature of 1,200°C or higher and then rolling it into a billet. The method comprises the steps of: 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; coiling the rolled wire rod at a temperature of 800 to 900°C; and cooling the coiled wire rod at a rate of 0.5 to 2°C / sec.

[0078] The reason for limiting the compositional range of each alloying element is as described above, and each manufacturing step is explained in more detail below.

[0079] According to one embodiment of the present invention, the continuous casting step may include light reduction of a total reduction amount of 20 mm or more.

[0080] A method of casting a billet at the end of solidification having an unsolidified layer within a continuous casting machine by slowly reducing it using a collection of reduction rolls with a total reduction amount and a reduction rate equivalent to the sum of the solidification shrinkage and the thermal shrinkage is called light reduction. Here, the total reduction amount refers to the reduction amount from the start of reduction to the end of reduction. When the total reduction amount is less than 20 mm, there is a problem in that it is difficult to secure the segregation removal effect by light reduction, so the total reduction amount of light reduction can be controlled to 20 mm or more to minimize segregation of the wire rod.

[0081] In addition, according to one embodiment of the present invention, rolling is performed to a thickness of 4 mm or less for each rolling roll that applies light pressure, and the cumulative reduction amount can be 60% or more when the solidification fraction is 0.6 or more. The solidification fraction refers to the ratio of the weight of the molten steel that has become a solid phase to the total weight of the molten steel.

[0082] Meanwhile, if the casting speed is too slow, solidification is completed before light reduction, and the ratio of the liquid phase to the solid phase is too low, making it difficult to secure the effect of removing segregation by light reduction. On the other hand, if the casting speed is too fast, the ratio of the liquid phase to the solid phase is too high, which is undesirable as segregation is generated due to solidification shrinkage. 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 higher.

[0083] The amount of cooling water is appropriately adjusted so that solidification is completed up to the point where light compression is finished. Depending on the equipment, Mold-EMS (Mold Electro Magnetic Stirrer) and Strand-EMS may follow the conditions of conventional spring steel or be set arbitrarily.

[0084] Meanwhile, unlike conventional spring wire rods, spring steel for nitriding treatment contains a large amount of high-alloy components, so it is necessary to control internal carbonitrides. Accordingly, 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.

[0085] Afterwards, the billet can 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.

[0086] If the heat treatment temperature of the billet is below 1030℃, the V component in the material does not melt sufficiently, so carbides cannot be dissolved, resulting in a problem where the softening resistance of the final product is reduced. The step of rolling into wire rod can be performed at a temperature of 1000℃ or lower so that the coiling temperature can be performed at 900℃ or lower.

[0087] Afterwards, the rolled wire can be wound at a temperature of 800 to 900°C.

[0088] If there is a large difference between the temperature of the wire rolling stage and the temperature of the coiling stage, severe decarburization due to localized supercooling may occur. Taking this into consideration, the coiling stage of the rolled wire can be performed at a temperature of 800 to 900°C.

[0089] Afterwards, the wound wire can be cooled at a rate of 0.5 to 2℃ / s.

[0090] Unlike ordinary spring wires, spring steel for nitriding treatment contains a large amount of high-alloy components, so it is necessary to suppress the low-temperature structure. If the wound wire is cooled at a rate of less than 0.5℃ / s, decarburization may occur. On the other hand, if the cooling rate exceeds 2℃ / s, fracture may occur in the material due to the low-temperature structure.

[0091] Next, a steel wire for springs with improved strength and fatigue limit according to one embodiment of the present invention will be described.

[0092] A steel wire for springs with improved strength and fatigue limit according to one embodiment of the present invention comprises, 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, and the remainder being Fe and other unavoidable impurities.

[0093] In addition, the spring wire with improved strength and fatigue limit according to one embodiment of the present invention can satisfy Mn+Cr≤1.8%.

[0094] In addition, the spring wire with improved strength and fatigue limit according to one embodiment of the present invention can satisfy 0.05at%≤Mo+W≤0.15at%.

[0095] The reason for limiting the compositional range of each alloying element is as described above.

[0096] In addition, the spring wire with improved strength and fatigue limit according to one embodiment of the present invention may include, in terms of area fraction, 85% or more of a tempered martensite structure and the remainder being an austenite structure.

[0097] In addition, the spring wire with improved strength and fatigue limit according to one embodiment of the present invention may have an average austenite particle size of 15 μm or less.

[0098] In addition, the steel wire for a spring with improved strength and fatigue limit according to one embodiment of the present invention has a center of 1 mm in a cross-section perpendicular to the longitudinal direction. 2 In terms of area, the proportion of the area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% in weight% may be 10% or less.

[0099] If the aforementioned area ratio exceeds 10%, the quality of the material deteriorates, such as the formation of a low-temperature structure due to segregation in the center, and the workability is reduced, leading to a problem where the frequency of breakage increases when steel wire is processed into springs. In addition, if the aforementioned area exceeds 10%, the carbide effect may be reduced due to the concentration of carbide-forming elements in the center.

[0100] In addition, the spring steel wire with improved strength and fatigue limit according to one embodiment of the present invention may have fewer than 2 carbonitrides with a maximum diameter of 15 μm or more per 100 mm length in a cross-section parallel to the longitudinal direction within a surface depth of 1 mm.

[0101] If carbonitrides of 15㎛ or more are present on the surface of the steel wire, fatigue fracture may occur in the material. It is desirable that there be fewer than 2 per 100mm length in a cross-section parallel to the longitudinal direction within a surface depth of 1mm.

[0102] In addition, the steel wire for a spring with improved strength and fatigue limit according to one embodiment of the present invention is 100㎛ 2 In terms of area, the number of carbides is 10 to 50, the maximum diameter of the carbides is 5 to 50 nm, and the content of V or Nb may be 10 at% or more.

[0103] In the case of carbides containing V or Nb, when they start to grow larger than 10 nm, they grow by including not only V but also other carbide-forming elements such as Cr and Mo, so the distribution of carbide-forming elements to be utilized for inhibiting the growth of old austenite grains and precipitation hardening must be properly carried out.

[0104] When the number of carbides with a maximum diameter of 5 to 50 nm is less than 10, there is a problem in controlling the pre-austenite grain size. On the other hand, when the number of carbides with a maximum diameter of 5 to 50 nm exceeds 50, the amount available for precipitation hardening of 5 nm or less is reduced, which may lower the tensile strength of the steel wire.

[0105] In addition, the spring wire with improved strength and fatigue limit according to one embodiment of the present invention may have a tensile strength of 2,100 MPa or more and a reduction in area (RA) of 45% or more.

[0106] Next, a method for manufacturing a steel wire for springs with improved strength and fatigue limit according to one embodiment of the present invention will be described.

[0107] A method for manufacturing a steel wire for a spring according to one embodiment of the present invention comprises: a step of LA heat treating a wire rod according to one embodiment of the present invention; a step of LP heat treating; a step of drawing the wire rod to prepare a steel wire; and a step of QT heat treating the steel wire.

[0108] First, a wire rod according to one embodiment of the present invention can be subjected to low temperature annealing (LA) at 650 to 750°C.

[0109] Although not limited to this, it is desirable to perform the LA heat treatment step within 2 hours, as longer process times lead to coarsening of carbides, making it difficult to control carbides in subsequent processes. Through LA heat treatment, the strength of the wire rod can be reduced to 1,200 MPa or less, and the LA heat treatment step may be omitted if necessary.

[0110] Afterwards, the LA heat-treated wire rod can be pickled, and then LP (Lead Patenting, LP) heat treatment can be performed.

[0111] The above LP heat treatment may include a first austenitizing step of heating to 950 to 1100°C within 3 minutes and maintaining for 3 minutes or less, and a step of passing the first austenitized wire rod through a lead bath at 650 to 700°C within 3 minutes.

[0112] By performing an austenitizing process that involves heating to 950 to 1100°C within 3 minutes and maintaining it for 3 minutes or less, an austenite structure can be secured while simultaneously resolving the coarse carbides from the LA process.

[0113] Next, the first austenitized wire rod is passed through a lead bath at 650 to 750°C for less than 3 minutes to rapidly cool and isothermally transform, thereby securing a pearlite structure. If the lead bath temperature is below 650°C, a low-temperature structure may be formed. On the other hand, if the lead bath temperature exceeds 750°C, carbides may coarsen and strength may decrease.

[0114] Afterwards, the LP heat-treated wire can be drawn to produce steel wire. At this time, the diameter of the produced steel wire may be 5 mm, and LP heat treatment may be performed again to ensure the diameter of the steel wire is 2 mm or less.

[0115] Afterwards, in order to obtain a tempered martensite structure, the steel wire prepared above can be subjected to a QT heat treatment process.

[0116] According to one embodiment of the present invention, the QT heat treatment step may include: a second austenitizing step of heating to 900 to 1000°C within 3 minutes and maintaining for 3 minutes or less; a first oil quenching step of heating to 450 to 550°C within 3 minutes and maintaining for 3 minutes or less; and a second oil quenching step of heating to 450 to 550°C within 3 minutes and maintaining for 3 minutes or less; and a second oil quenching step of 70°C or less.

[0117] In the QT heat treatment step, the austenitizing temperature can be performed at 900 to 1000°C to maintain the fine carbides precipitated during the LP heat treatment. Although not limited to this, it is preferable to perform the austenitizing process in the QT heat treatment step for 6 minutes or less.

[0118] If the tempering temperature is below 450°C during the QT heat treatment step, the nitriding temperature is lowered, additional carbide formation cannot be induced, and a problem of reduced toughness occurs. On the other hand, if the tempering temperature exceeds 550°C during the QT heat treatment step, sufficient strength cannot be secured.

[0119] Next, a spring with improved strength and fatigue limit according to one embodiment of the present invention will be described.

[0120] A spring with improved strength and fatigue limit according to one embodiment of the present invention comprises, 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, and the remainder being Fe and other unavoidable impurities, satisfying Mn+Cr≤1.8% and satisfying 0.05at%≤Mo+W≤0.15at%.

[0121] The reason for limiting the compositional range of each alloying element is as described above.

[0122] In addition, the spring according to one embodiment of the present invention has a fatigue limit that increases by 10% or more after nitriding treatment. Here, the fatigue limit refers to the limit at which the spring can withstand a repetitive load of 10 million or more cycles during a fatigue test after designing the spring.

[0123] In addition, a spring according to one embodiment of the present invention may have a fatigue limit of 700 MPa or more capable of withstanding ten million cycles of repetitive stress.

[0124] In addition, a spring according to one embodiment of the present invention has a strength change of 15% or less before and after nitriding treatment, and the nitriding treatment temperature may be 430℃ or higher.

[0125] Next, a method for manufacturing a spring with improved strength and fatigue limit according to one embodiment of the present invention will be described.

[0126] A method for manufacturing a spring with improved strength and fatigue limit according to one embodiment of the present invention comprises: a step of cold forming a steel wire according to one embodiment of the present invention into a spring; a step of stress relief heat treatment of the formed spring; and a step of nitriding treatment.

[0127] A steel wire according to one embodiment of the present invention can improve its fatigue limit by nitriding treatment before the shot peening step during 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 core of the material decreases, making it impossible to secure the desired material strength. Taking this into consideration, the nitriding treatment process can be performed at a temperature of 420 to 450°C for 10 hours or more.

[0128] The present invention will be explained in more detail below through examples. However, the description of these examples is merely for illustrating the implementation of the present invention and does not limit the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0129] {Example}

[0130] For various alloy composition ranges shown in Table 1 below, a bloom was produced by performing a continuous casting process with a total diameter reduction of 10 to 25 mm. The produced bloom was subjected to homogenization heat treatment at 1,200°C, heat treatment at 1,050°C, and then hot-rolled to a final diameter of 6.5 mm while lowering the temperature to 850°C to produce a wire rod with a final diameter of 6.5 mm. Subsequently, the rolled wire rod was coiled at 800 to 900°C and then cooled at a rate of 1°C / s.

[0131] Alloying elements (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

[0133] Table 2 below shows the at% content of W+Mo and the total light reduction of the Examples and Comparative Examples. The segregation area in Table 2 below is the center of the cross-section perpendicular to the longitudinal direction of the manufactured wire rod, 1 mm 2 It was derived by analyzing.

[0134] 'C Segregation Area' in Table 2 is the center of the cross-section perpendicular to the longitudinal direction, 1 mm 2 In terms of area, it refers to the ratio of the area satisfying C > 0.85 wt%. The 'Si segregation area' is the center of the cross-section perpendicular to the longitudinal direction, 1 mm 2 In terms of area, it refers to the ratio of the area satisfying Si > 3.0 wt%. The 'Mn segregation area' is the center of the cross-section perpendicular to the longitudinal direction, 1 mm 2 In terms of area, it refers to the ratio of the area satisfying Mn > 0.8 wt%. The 'Cr segregation area' is the 1 mm center of the cross-section perpendicular to the longitudinal direction. 2 In terms of area, it refers to the ratio of the area satisfying Cr > 2.0 wt%. The segregation area was measured using an Electron Probe X-ray Micro Analyzer (EPMA) with the model name EMPA-1600.

[0135] W+Mo(at%) Total reduction in pressure (mm) C Segregation Area (%) Si Segregation Area (%) Mn segregation area (%) Cr segregation area (%) Sum of segregation areas of C, Si, Mn, and Cr (%) Example 1 0.11 25 mm <1% 2.5 <1% 2.5 <7% Example 2 0.08 25 mm <1% 4.3 <1% 2.3 <8.6% Comparative Example 1 0.11 10 mm 5.5 11.2 3.1% 10.2 30% Comparative Example 2 0.08 25 mm <1% 4.5 <1% 2.2 <8.7% Comparative Example 3 0.17 25 mm <1% 3.2 <1% 3.4 <8.6% Comparative Example 4 0.11 25 mm <1% 4.2 <1% 2.4 <8.6%

[0137] Looking at Table 2 above, Examples 1 and 2 satisfied the alloy composition and total reduction amount presented in the present invention, and as a result, the sum of the segregation areas of C, Si, Mn, and Cr could be formed to be 10% or less. In contrast, Comparative Example 1 had a total reduction amount of 10mm, which is less than 20mm, and the sum of the segregation areas of C, Si, Mn, and Cr reached 30%.

[0138] Table 3 below shows the tensile strength, reduction in area (RA), core low-temperature structure, average austenite grain size, pearlite structure, and number of carbonitrides of the wire rod manufactured above. The average austenite grain size, pearlite structure, and number of carbonitrides were measured using a scanning electron microscope (SEM) with model names JEOL, JSM-6610LV.

[0139] In Table 3 below, 'O' indicates a case where the low-temperature tissue area fraction exceeds 20%, and 'X' indicates a case where the low-temperature tissue area fraction is 20% or less.

[0140] The pearlite structure in Table 3 below refers to the number of specimens in which the pearlite structure was detected in an area fraction of 80% or more when the microstructure of the cross-section perpendicular to the longitudinal direction of each specimen was measured after dividing a 3m wire rod into 8 equal parts to prepare 8 specimens.

[0141] The number of carbonitrides in Table 3 below is the number of carbonitrides with a maximum diameter of 15㎛ or more when a 10cm long wire is divided into 10 equal parts to prepare 10 specimens of 1cm in length, and the microstructure of the cross-section parallel to the longitudinal direction is measured within a surface depth of 1mm.

[0142] Tensile strength (MPa) Wire cross-sectional reduction rate (%) Core cryoscopic tissue Average grain size of the austenite (㎛) Pearlite structure 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

[0144] Looking at Table 3 above, Examples 1 and 2 did not form a low-temperature structure in the center, and the average grain size of the prior austenite was 20 μm or less. In addition, among the 8 specimens, at least 6 specimens in Examples 1 and 2 had a pearlite structure of 80% or more, and the machinability was excellent with a tensile strength of 1400 MPa or less. Furthermore, no carbonitrides were formed on the surface of Examples 1 and 2.

[0145] In contrast, Comparative Example 1 had a tensile strength exceeding 1400 MPa and a cross-sectional reduction rate of 35% or less, resulting in inferior processability, and a low-temperature structure was formed in the center. Additionally, among the 8 specimens of Comparative Example 1, only 5 had a pearlite structure of 80% or more, so a pearlite structure of 80% or more was not uniformly formed.

[0146] Comparative Example 2, when looking at the alloy composition in Table 1, was added at less than 0.05% V, so the average grain size of the austenite was coarsened to 24㎛, exceeding 20㎛.

[0147] Comparative Example 3 had a tensile strength of 1510 MPa and a reduction in area of ​​only 10%, which not only resulted in inferior machinability but also formed a low-temperature structure in the center. In addition, among the eight specimens of Comparative Example 3, only two had a pearlite structure of 80% or more, indicating that the pearlite structure was not sufficiently formed.

[0148] Next, the Examples and Comparative Examples were subjected to LA heat treatment at 720°C for 2 hours, followed by pickling and LP heat treatment. The LP heat treatment was performed by heating to the first austenitizing temperature within 3 minutes, and the remainder was carried out according to the conditions in Table 4 below. Additionally, Table 4 below shows the pearlite transformation times of the Examples and Comparative Examples during LP heat treatment. The pearlite transformation time was measured by deriving a Time-Temperature-Transformation (TTT) curve through dilatometry experiments.

[0149] LP heat treatment Pearlite transformation time (seconds) during LP heat treatment 1st austenitizing Napjo Temperature (°C) Duration (minutes) Temperature (°C) Passage time (minutes) 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

[0151] Examples 1 and 2 showed excellent productivity, with pearlite transformation times of less than 130 seconds, measured at 110 seconds and 105 seconds, respectively. In contrast, Comparative Example 3 showed inferior productivity, with a pearlite transformation time of 130 seconds, making on-site production difficult.

[0152] Next, the LP heat-treated examples and comparative examples were drawn to produce steel wires with a diameter of 3 mm. The produced steel wires were subjected to second austenitizing and first quenching, followed by tempering and second quenching to obtain QT steel wires. The wires were heated to the second austenitizing temperature within 3 minutes, and the first and second quenchings were performed in oil at 60°C. The remainder was carried out according to the conditions in Table 5 below.

[0153] QT heat treatment Second austenitizing Tempering Temperature (°C) Duration (minutes) Temperature (°C) Duration (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

[0155] Table 6 below shows the tensile strength, reduction in area (RA), and number of carbides of the manufactured QT steel wire. Here, the number of carbides is 100 µm 2 It refers to the number of carbides with a maximum diameter of 5–50 nm and a V or Nb content of 10 at% or more within the area. The number of carbides was determined by using a transmission electron microscope (TEM) at any 100 µm area on the wire rod surface. 2 After measuring 8 locations of the area, it is the average of the 8 measured values.

[0156] QT steel wire tensile strength (MPa) QT steel wire cross-sectional reduction rate (%) Number of carbides 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

[0158] Looking at Table 6 above, Examples 1 and 2 secured an excellent tensile strength of 2200 MPa or more, while also securing a cross-sectional reduction rate of 45% or more. In addition, Examples 1 and 2 were formed with 10 to 50 carbides.

[0159] In contrast, Comparative Example 1 had a cross-sectional reduction rate of only 32% and the number of carbides exceeded 50. Comparative Example 2 had a tensile strength of 2200 MPa or less, which was inferior, and the number of carbides was less than 10, resulting in a problem where it was difficult to control the average grain size of the prior austenite. Comparative Example 4 had a tensile strength of 2200 MPa or less, which was inferior, and the number of carbides exceeded 50.

[0160] Next, the above QT steel wire was cold-formed into the shape of a spring, the formed spring was heat-treated, and then nitrided at 420 to 450°C.

[0161] Table 7 below shows the failure during spring forming, fatigue limit, and fatigue limit values ​​after nitriding treatment.

[0162] The fatigue limit before and after nitriding treatment was measured under conditions of a stress ratio R (tensile capacity / compressive capacity) = -1 and a test speed of 30 to 60 Hz.

[0163] In Table 7 below, 'X' means that the spring was not broken during forming, and 'O' means that the spring was broken during forming.

[0164] Damage status Fatigue limit (MPa) before nitriding treatment Fatigue limit (MPa) after nitriding treatment 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

[0166] Examples 1 and 2 had excellent machinability and did not break, and the fatigue limit was measured to be 650 MPa or higher before nitriding treatment, indicating excellent fatigue limit. In addition, Examples 1 and 2 had a fatigue limit of 750 MPa or higher after nitriding treatment, and the fatigue limit increased by more than 10% after nitriding treatment compared to before nitriding treatment, indicating excellent nitriding treatment characteristics.

[0167] In contrast, Comparative Examples 1 and 2 were damaged due to inferior machinability, and the fatigue limit increased to less than 10% after nitriding treatment compared to before nitriding treatment.

[0168] Comparative Example 4 did not experience breakage during spring processing, but the fatigue limit after nitriding treatment did not increase by more than 10% compared to before nitriding treatment, so the nitriding treatment characteristics were inferior.

[0169] According to the disclosed embodiment, by optimizing the alloy composition and manufacturing conditions, excellent tensile strength and cross-sectional reduction are secured, while nitriding characteristics and fatigue limit are improved, making it possible to apply it as a material for automotive transmission gears and engine valves.

Claims

Claim 1 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: 0.15 to 0.25%, W: 0.1 to 0.25%, V: 0.05% to 0.2% or less, Nb: 0.02 to 0.05%, and the remainder being Fe and other unavoidable impurities; optionally comprising either W or Nb; satisfying Mn+Cr≤1.8% and satisfying 0.05at%≤Mo+W≤0.15at%; and 1mm at the center of a cross-section perpendicular to the longitudinal direction. 2 A spring wire with improved strength and fatigue limit, wherein the ratio of the area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% in weight% of the area is 10% or less. Claim 2 A spring wire with improved strength and fatigue limit according to claim 1, comprising at least 80% of a pearlite structure by area fraction and the remainder being a bainite or martensite structure. Claim 3 In claim 1, a spring wire with improved strength and fatigue limit, having an average austenite grain size of 20㎛ or less. Claim 4 In claim 1, in a cross-section parallel to the longitudinal direction within a surface depth of 1 mm, 2 carbonitrides / cm² with a maximum diameter of 15 μm or more 2 Spring wire with improved strength and fatigue limit, distributed below [value]. Claim 5 In claim 1, a spring wire with improved strength and fatigue limit, having a tensile strength of 1,400 MPa or less and a cross-sectional reduction rate of 35% or more. Claim 6 A step of continuously casting molten steel, comprising, 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: 0.15 to 0.25%, W: 0.1 to 0.25%, V: 0.05% to 0.2% or less, Nb: 0.02 to 0.05%, and the remainder being Fe and other unavoidable impurities, and optionally including either W or Nb, by lightly reducing the total reduction amount to 20 mm or more to produce a bloom; a step of heating the bloom to a temperature of 1,200°C or higher and then rolling it into a billet; and at 1,030°C or higher A method for manufacturing a spring wire with improved strength and fatigue limit, comprising: a step of rolling the wire into a wire at a temperature of 1,000°C or lower after heat treatment; a step of winding the wire at a temperature of 800 to 900°C; and a step of cooling the wound wire at a rate of 0.5 to 2°C / s. Claim 7 delete Claim 8 A method for manufacturing a spring wire with improved strength and fatigue limit, wherein, in claim 6, the above-mentioned light-pressing is applied by rolling to a thickness of 4 mm or less per rolling roll, and the cumulative reduction amount is 60% or more when the solidification fraction is 0.6 or more. Claim 9 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: 0.15 to 0.25%, W: 0.1 to 0.25%, V: 0.05% to 0.2% or less, Nb: 0.02 to 0.05%, and the remainder being Fe and other unavoidable impurities; optionally comprising either W or Nb; satisfying Mn+Cr≤1.8% and satisfying 0.05at%≤Mo+W≤0.15at%; and 1mm at the center of a cross-section perpendicular to the longitudinal direction. 2 A steel wire for springs with improved strength and fatigue limit, wherein, in weight percent, the ratio of the area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% is 10% or less, and in area fraction, the tempered martensite structure is 85% or more and the remainder is an austenite structure. Claim 10 In claim 9, a steel wire for springs with improved strength and fatigue limit, having an average austenite grain size of 15㎛ or less. Claim 11 In claim 9, in a longitudinal section parallel to the surface depth of 1 mm or less, 2 carbonitrides / cm² with a maximum diameter of 15 µm or more 2 Steel wire for springs with improved strength and fatigue limit, distributed below [value]. Claim 12 In paragraph 9, 100㎛ 2 A steel wire for springs with improved strength and fatigue limit, having 10 to 50 carbides in area, the maximum diameter of the carbides being 5 to 50 nm, and a V or Nb content of 10 at% or more. Claim 13 In claim 9, a steel wire for springs with improved strength and fatigue limit, having a tensile strength of 2,100 MPa or more and a cross-sectional reduction rate of 45% or more. Claim 14 The method comprises: a step of LP heat treating a wire rod according to any one of claims 1 to 5; a step of drawing the LP heat-treated wire rod to prepare a steel wire; and a step of QT heat treating the steel wire; wherein the LP heat treatment step comprises: a first austenitizing step of heating to 950 to 1100°C within 3 minutes and maintaining for 3 minutes or less; and a step of passing the first austenitized wire rod through a lead bath at 650 to 700°C within 3 minutes, wherein the pearlite transformation completion time is less than 130 seconds; and the QT heat treatment step comprises: a second austenitizing step of heating to 900 to 1000°C within 3 minutes and maintaining for 3 minutes or less; a first oil quenching step at 70°C or lower; and a tempering step of heating to 450 to 550°C within 3 minutes and maintaining for 3 minutes or less. A method for manufacturing a steel wire for springs with improved strength and fatigue limit, comprising: a second oil quenching step at 70°C or lower. Claim 15 delete Claim 16 A method for manufacturing a steel wire for springs with improved strength and fatigue limit, wherein, in claim 14, prior to the step of LP heat treatment, the step of LA heat treatment of the wire rod is further included, and the step of LA heat treatment further includes: a step of heat treatment at 650 to 750℃; and a step of pickling. Claim 17 delete Claim 18 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: 0.15 to 0.25%, W: 0.1 to 0.25%, V: 0.05% to 0.2% or less, Nb: 0.02 to 0.05%, and the remainder being Fe and other unavoidable impurities; optionally comprising either W or Nb; satisfying Mn+Cr≤1.8% and satisfying 0.05at%≤Mo+W≤0.15at%; and 1mm at the center of a cross-section perpendicular to the longitudinal direction. 2 A spring with improved strength and fatigue limit, wherein the ratio of the area satisfying one or more of C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0% in weight% of the area is 10% or less, and the fatigue limit capable of withstanding ten million cycles of repetitive stress is 700 MPa or more. Claim 19 A method for manufacturing a spring with improved strength and fatigue limit, comprising: a step of cold forming a steel wire according to any one of claims 9 to 13 into the shape of a spring; a step of stress relief heat treatment of the formed spring; and a step of nitriding treatment at a temperature of 420 to 450°C for 10 hours or more. Claim 20 In claim 19, a method for manufacturing a spring with improved strength and fatigue limit, wherein the fatigue limit increases by 10% or more after the above nitriding treatment.