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

Abstract

The present invention provides a wire rod, steel wire, and spring which are excellent in strength and workability, can be easily nitrided even at high temperatures, and have improved nitriding properties and fatigue limit, and a manufacturing method thereof. [Solution] The alloy is composed of, by weight percent, C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities, Mn+Cr≦1.8%, 0.05at%≦Mo+W≦0.15at%, and 1mm in the center of a cross section perpendicular to the length direction 2 The alloy is characterized in that the ratio of the area that satisfies one or more of C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0% by weight is 10% or less.

Description

[Technical Field]

[0001] The present invention relates to spring wires, steel wires, springs, and methods for manufacturing the same, which are ultra-high-strength spring steels of the 2,200 MPa class, which have excellent strength and workability, and which are easily subjected to nitriding treatment even at high temperatures, and which have improved nitriding treatment characteristics and fatigue limits, as well as methods for manufacturing the same. [Background technology]

[0002] The increasing demand for lighter vehicles has led to a sustained need for weight reduction in automotive parts, and consequently, the springs used in automotive transmissions and engine valves are also required to maintain sustained high strength. However, increasing the strength of spring materials results in thinner wire diameters and increased sensitivity to inclusions, which lowers the fatigue limit. In other words, there is a limit to how much the fatigue limit can be improved through increased strength. To overcome this, spring manufacturers have attempted to increase the fatigue limit of spring materials by maintaining strength and improving surface hardness through nitriding treatment.

[0003] While nitriding is typically performed at temperatures above 500°C for other parts, in the case of spring steel, nitriding is performed at 420-460°C to prevent a decrease in strength, and the heat treatment is carried out for a long period of 10 hours or more to ensure sufficient nitrogen penetration depth. Since the tempering heat treatment temperature for typical spring steel is 450°C or lower, if heat treatment is performed for a long time at 420-450°C, the strength of many spring steels will decrease significantly. Therefore, it is necessary to use high-alloy materials to which elements that can form carbides and improve softening resistance are added. However, when large amounts of carbide-forming elements such as Mo and V are added, the decrease in strength during nitriding can be suppressed, but a low-temperature structure may be formed due to segregation in the center, which may cause a problem in which the cross-sectional reduction rate decreases.

[0004] Furthermore, since the spring material undergoes repeated high-temperature heat treatment processes during manufacturing, controlling the prior austenite grain size (PAGS) becomes a challenge, and carbide control technology during the heat treatment process is also necessary. Furthermore, the spring manufacturing company desires to shorten the nitriding process time by performing the nitriding treatment at the highest possible temperature, while simultaneously requiring high-strength wire that does not pose a problem to on-site productivity. Therefore, there is a need for the development of wires and steel wires that are superior in quality, such as strength and workability, while also having improved nitriding characteristics and fatigue limits. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Korean Published Patent No. 10-2000-0043776 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The present invention has been made to solve the above problems, and its objective is to provide wires, steel wires, springs, and methods for manufacturing the same that are excellent in strength and processability, easy to perform nitriding treatment even at high temperatures, and have improved nitriding characteristics and fatigue limits. [Means for solving the problem]

[0007] To achieve the above objectives, the spring wire according to the present invention, which has improved strength and fatigue limit, consists of, 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, the remaining Fe and other unavoidable impurities, satisfying Mn+Cr≦1.8% and 0.05at%≦Mo+W≦0.15at%, with a thickness of 1 mm in the center of the cross-section perpendicular to the length direction. 2 In terms of area, the proportion of the area that satisfies one or more of the following conditions in weight percent is 10% or less: C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0%.

[0008] The aforementioned wire preferably contains 80% or more of pearlite tissue by area fraction, with the remainder being bainite or martensite tissue. The wire material may have an average prior austenite particle size of 20 μm or less. The aforementioned wire has a cross-section horizontal to the length direction with a surface depth of 1 mm or less, containing 2 carbonitrides per cm with a maximum diameter of 15 μm or more. 2 It is preferable that the distribution be less than [a certain value]. The aforementioned wire material should have a tensile strength of 1,400 MPa or less and a cross-sectional reduction rate of 35% or more.

[0009] To achieve the above objective, the present invention provides a method for manufacturing spring wire with improved strength and fatigue limit, comprising, 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 remaining Fe and The process is characterized by comprising the steps of: continuously casting molten steel consisting of molten steel and other unavoidable impurities to prepare a bloom; heating the bloom to a temperature of 1,200°C or higher and then rolling it into a billet; heat treating the billet to 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.

[0010] The continuous casting step preferably includes light reduction with a total reduction of 20 mm or more. The aforementioned light reduction involves rolling each rolling roll to 4 mm or less, and when the solidification fraction is 0.6 or higher, the cumulative reduction amount can be 60% or more.

[0011] To achieve the above objective, the spring steel wire according to the present invention, which has improved strength and fatigue limit, is characterized by comprising, 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, the remaining Fe and other unavoidable impurities, satisfying Mn+Cr≦1.8%, satisfying 0.05at%≦Mo+W≦0.15at%, and containing, by area fraction, 85% or more of tempered martensite structure and the remaining austenite structure.

[0012] The steel wire should preferably have an average grain size of prior austenite of 15 μm or less. The steel wire has a cross-section within 1 mm of the surface that is parallel to the length direction, containing 2 carbonitrides per centimeter with a maximum diameter of 15 μm or more. 2 It can be distributed between values ​​less than 100 μm 2 In terms of area, the number of carbides is preferably 10 to 50, the carbides have a maximum diameter of 5 to 50 nm, and the V or Nb content is preferably 10 at% or more. The steel wire may have a tensile strength of 2,100 MPa or more and a cross-sectional reduction ratio of 45% or more.

[0013] The present invention provides a method for manufacturing a spring steel wire with improved strength and fatigue limit to achieve the above objective, comprising the steps of: LP heat treatment of the wire material; drawing the LP heat-treated wire material to prepare a steel wire; and QT heat treatment of the steel wire, wherein the LP heat treatment step includes a first austenitization step of heating to 950-1100°C within 3 minutes and maintaining the temperature within 3 minutes; and passing the first austenitized wire material through a lead bath at 650-700°C within 3 minutes.

[0014] The time required to complete the pearlite transformation during the LP heat treatment stage should preferably be less than 130 seconds. Prior to the LP heat treatment step, the process further includes a step of LA heat treatment of the wire, and the LA heat treatment step may further include a step of heat treatment at 650 to 750°C; and a step of pickling. The QT heat treatment step may include a second austenitizing step of heating to 900-1000°C within 3 minutes and maintaining the temperature for within 3 minutes; a first oil quench step at 70°C or below; a tempering step of heating to 450-550°C within 3 minutes and maintaining the temperature for within 3 minutes; and a second oil quench step at 70°C or below.

[0015] The spring according to the present invention with improved strength and fatigue limit for achieving the above object consists of, 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, and the balance Fe and other inevitable impurities, satisfying Mn + Cr ≦ 1.8%, 0.05 at% ≦ Mo + W ≦ 0.15 at%, and having a fatigue limit of 700 MPa or more that can withstand repeated stress of ten million times.

[0016] The method for manufacturing a spring according to the present invention with improved strength and fatigue limit for achieving the above object includes a step of cold-forming the steel wire into the shape of a spring; a step of stress-relieving 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. The method for manufacturing a spring according to the present invention with improved strength and fatigue limit for achieving the above object is characterized in that the fatigue limit after the nitriding treatment increases by 10% or more.

Effects of the Invention

[0017] According to one aspect of the present invention, it is possible to provide a wire rod, steel wire, spring, and a manufacturing method thereof that suppress the generation of a low-temperature structure in the central portion by reducing central segregation, ensure an excellent cross-sectional reduction rate, and at the same time ensure a tensile strength of 2,200 MPa or more. According to another aspect of the present invention, it is possible to provide a wire rod, steel wire, spring, and a manufacturing method thereof with improved nitriding treatment characteristics and fatigue limit by controlling the crystal grain size and the number of precipitates. The various and beneficial advantages and effects of the present invention are not limited to the above-described content and can be more easily understood in the process of explaining specific embodiments of the present invention.

Modes for Carrying Out the Invention

[0018] The spring wire according to the present invention, which has improved strength and fatigue limit, consists of, 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, with the remaining Fe and other unavoidable impurities, satisfying Mn+Cr≦1.8% and 0.05at%≦Mo+W≦0.15at%, with a cross-sectional area perpendicular to the length, measuring 1 mm in the center. 2 In terms of area, the proportion of areas satisfying one or more of the following conditions in weight percent is 10% or less: C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0%.

[0019] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified into various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, embodiments of the present invention are provided to give a more complete explanation of the present invention to a person with average skill in the art. The terms used in this application are solely for illustrative purposes. Therefore, singular expressions include plural expressions unless the context clearly requires them to be singular. Furthermore, it should be noted that terms such as “includes” or “comprising” used in this application are used to explicitly indicate the presence of features, stages, functions, components, or combinations thereof described in the specification, and not to provisionally exclude the presence of other features, stages, functions, components, or combinations thereof.

[0020] Unless otherwise defined, all terms used herein should be considered to have the same meaning as that generally understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, no particular term should be interpreted in an overly idealistic or formal sense. For example, in this specification, singular expressions include plural expressions unless the context clearly indicates otherwise. Furthermore, the terms “about,” “substantially,” etc., as used herein, are used in the sense of the numerical value or an approximate value when manufacturing and material tolerances specific to the meaning referred to are presented, and are used to prevent unscrupulous infringers from unfairly exploiting disclosures that refer to precise or absolute numerical values ​​to aid in understanding the invention.

[0021] A spring wire with improved strength and fatigue limit according to one embodiment of the present invention consists of, by weight percent, C: 0.6~0.7%, Si: 2.0~2.5%, Mn: 0.2~0.7%, Cr: 0.9~1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%~0.2% or less, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities. The following explains the reasons for limiting the component range for each alloying element. Unless otherwise specified, units are in weight percent.

[0022] The C content is 0.6-0.7%. Carbon (C) is an element that improves the strength of a material, and it can be added in amounts of 0.6% or more to ensure sufficient strength. However, if the carbon content is excessive, the impact properties will decrease significantly after QT (Quenching & Tempering) heat treatment, and the possibility of low-temperature structures occurring during wire production will greatly increase, potentially degrading the quality of the wire. In addition, if the carbon content is excessive, the LP heat treatment time, one of the processes in steel wire manufacturing, will increase significantly, reducing productivity. Considering this, it is best to limit the upper limit of the carbon content to 0.7%.

[0023] The Si content is 2.0-2.5%. Si is not only used for deoxidizing steel, but is also an advantageous element for ensuring strength through solid solution strengthening. It can be added at a concentration of 2.0% or more to suppress strength reduction during nitriding and improve the deformation resistance of springs. However, if the Si content is excessive, surface decarburization may be induced, potentially degrading the workability of the material. Considering this, it is best to limit the Si content to an upper limit of 2.5%.

[0024] The Mn content is 0.2-0.7%. Mn is an element that enhances hardening ability, and can be added in amounts of 0.2% or more to ensure the hardening ability of the material and a high-strength tempered martensite structure, and to fix sulfur as a Mn compound and render it harmless. However, if the Mn content is too high, there is a risk of quality deterioration due to segregation. Considering this, it is best to limit the upper limit of the Mn content to 0.7%.

[0025] The Cr content is 0.9-1.5%. Cr, along with Mn, is an element that improves hardening ability, and can be added in amounts of 0.9% or more to improve the softening resistance of steel during nitriding treatment. However, if the Cr content is excessive, it will significantly reduce the toughness of the steel wire and promote the formation of a low-temperature structure during the cooling of the wire. Considering this, it is best to limit the upper limit of the Cr content to 1.5%.

[0026] The P content is 0.015% or less. Since phosphorus (P) segregates at grain boundaries, reducing the toughness of the material and decreasing its resistance to hydrogen-delayed fracture, it is preferable to remove it from steel materials as much as possible. Taking this into consideration, it is preferable to limit the upper limit of the P content to 0.015%.

[0027] The sulfur content is 0.01% or less. Like phosphorus (P), sulfur (S) can segregate at grain boundaries, reducing toughness, and may also form MnS, potentially decreasing resistance to hydrogen-delayed fracture. Considering this, it is advisable to limit the S content to 0.01%.

[0028] The aluminum content is 0.01% or less. Al is a powerful deoxidizing element that can remove oxygen from steel and improve its cleanliness, but it can also form Al2O3 inclusions, potentially reducing fatigue resistance. Considering this, it is best to limit the Al content to 0.01%.

[0029] The N content is 0.01% or less. N combines with impurities or Al or V to form coarse AlN or VN precipitates that do not dissolve during heat treatment. Considering this, it is advisable to limit the N content to 0.01%.

[0030] The Mo content is 0.25% or less. Mo is an element that improves softening resistance in nitriding materials, forms carbides with V, and increases strength during tempering. Furthermore, Mo forms MC carbides, maintaining material strength even during prolonged heat treatment. However, excessive Mo content can suppress pearlite formation, potentially degrading wire quality after rolling due to the formation of a low-temperature structure. Additionally, excessive Mo content suppresses pearlite transformation during LP heat treatment before wire drawing, increasing the pearlite transformation time and significantly reducing productivity. Considering these factors, it is best to limit the Mo content to 0.25%.

[0031] The W content is 0.25% or less. W, along with Mo, is an element that can improve softening resistance as a material for nitriding treatment. Similar to Mo, it forms MC carbides, which can maintain the strength of the material even during prolonged heat treatment. However, if the W content is excessive, it may suppress pearlite formation and promote the formation of low-temperature structures in the wire. Considering this, it is best to limit the upper limit of the W content to 0.25%.

[0032] The V content is 0.05-0.2%. V, along with Mo, is an element that improves softening resistance in materials for nitriding treatment. It forms carbides, increasing strength during tempering and maintaining strength even during prolonged nitriding. Unlike Mo and W, V has a high solid solution temperature for its carbides, thus playing a role in maintaining the prior austenite grain size. V accelerates pearlite transformation, thus suppressing low-temperature structures during wire production. V also shortens the isothermal transformation time during LP heat treatment, improving productivity in the steel wire manufacturing process, and can therefore be added at concentrations of 0.05% or more. However, if the V content is excessive, there is a risk of forming coarse carbonitrides during the wire production process, requiring a higher furnace temperature during wire rolling. Considering this, it is best to limit the upper limit of V content to 0.2%.

[0033] The Nb content is 0.05% or less. Nb is a carbonitride-forming element, and because its solid solution temperature is even higher than that of V, it is superior to V in controlling the grain size of the prior austenite crystals. However, if the Nb content is excessive, there is a risk of the prior austenite crystal grain becoming coarser. Considering this, the upper limit of the Nb content is often limited to 0.05%, and if the prior austenite crystal grain size is controlled throughout the manufacturing process, the addition of Nb can be omitted.

[0034] The remaining component, other than the aforementioned composition, is iron (Fe). However, in the normal manufacturing process, unintended impurities from the raw materials or the surrounding environment may inevitably be introduced and cannot be eliminated. Since these impurities are known to any technician in the normal manufacturing process, their details are not specifically mentioned in this specification.

[0035] A wire with improved strength and fatigue limit according to one embodiment of the present invention can satisfy the weight percentage of Mn+Cr ≤ 1.8%. If the sum of Mn and Cr exceeds 1.8%, a low-temperature structure such as bainite or martensite will be formed during the cooling process of the wire, which may prolong the time required for pearlite transformation during nitriding. Furthermore, if 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, making it impossible to prevent a decrease in material strength during nitriding. In addition, an increase in carbon equivalent prolongs the pearlite transformation time, preventing the complete pearlite structure from being secured during the cooling process of the wire, resulting in a longer nitriding time and reduced productivity. Furthermore, a wire 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 %. If the sum of the at% of Mo and W is less than 0.05 at%, the reduction in strength during nitriding is not suppressed, making it difficult to use as nitrided steel. On the other hand, if the sum of the at% of Mo and W exceeds 0.15 at%, the carbon equivalent increases, and the pearlite transformation time is delayed, resulting in a problem of reduced productivity. The reason for controlling the ratio to at% is that Mo and W form carbides in the MC (M=Mo or W, C=carbon) form, which contributes to improved strength, and therefore, Mo and W are matched to the carbides in a 1:1 ratio.

[0036] A wire according to one embodiment of the present invention can ensure that the pearlite transformation is completed in less than 130 seconds during LP (Lead Patenting) heat treatment. Here, the LP heat treatment process includes heating to 950-1100°C followed by rapid cooling to 650-750°C. If the pearlite transformation is completed in more than 130 seconds during LP heat treatment, a problem of reduced productivity occurs. Furthermore, a wire with improved strength and fatigue limit according to one embodiment of the present invention may contain a pearlite structure of 80% or more by area fraction. In one embodiment of the present invention, the wire rod with improved strength and fatigue limit preferably has an average prior austenite grain size of 20 μm or less. If the average prior austenite grain size exceeds 20 μm, the LP heat treatment process time increases, and the processability of the wire rod deteriorates.

[0037] A wire with improved strength and fatigue limit according to one embodiment of the present invention has a cross section perpendicular to the length, with a central 1 mm 2 In terms of area, it is desirable that the proportion of area satisfying one or more of the following conditions in weight percent—C>0.85%, Si>3.0%, Mn>0.8%, and Cr>2.0%—be 10% or less. If the above area ratio exceeds 10%, the quality of the material deteriorates due to the formation of a low-temperature structure through central segregation, and the reduction of area (RA) deteriorates after the manufacture of the steel wire. This leads to a decrease in workability and a higher frequency of breakage during spring processing. Furthermore, if the above area exceeds 10%, there is a risk that the carbide effect will decrease due to the concentration of carbide-forming elements in the center. A wire with improved strength and fatigue limit according to one embodiment of the present invention has a cross section horizontal to the length and surface depth of 1 mm or less containing 2 carbonitrides with a maximum diameter of 15 μm or more per cm. 2 The distribution may also be less than the specified value. If carbonitrides larger than 15 μm are present on the surface of the wire, fatigue failure may occur in the material. Therefore, in a cross section horizontal to the length and with a surface depth of 1 mm or less, there should be at least 2 carbonitrides per cm with a maximum diameter of 15 μm or more. 2 It is preferable that it exists at a level less than [value]. A wire rod with improved strength and fatigue limit according to one embodiment of the present invention preferably has a tensile strength of 1,400 MPa or less and a cross-sectional reduction ratio (RA) of 35% or more.

[0038] Next, a method for manufacturing spring wire with improved strength and fatigue limit according to one embodiment of the present invention will be described. A method for manufacturing spring wire with improved strength and fatigue limit according to one embodiment of the present invention involves, 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 remaining Fe and so The process includes: continuously casting molten steel consisting of unavoidable impurities to prepare a bloom; heating the bloom to a temperature of 1,200°C or higher and then rolling it into a billet; heat treating the billet to 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 rod 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 alloying element is as described above, and each manufacturing stage will be explained in more detail below.

[0039] According to one embodiment of the present invention, the continuous casting step includes light reduction with a total reduction of 20 mm or more. Light reduction is a casting method in which a cast slab in the final stages of solidification with an unsolidified layer is gradually reduced by a collection of reduction rolls in a continuous casting machine at a total reduction amount and reduction rate equivalent to the sum of solidification shrinkage and thermal shrinkage. Here, the total reduction amount is the amount reduced from the start to the end of reduction. When the total reduction amount is less than 20 mm, it is difficult to ensure the segregation removal effect by light reduction, so it is preferable to control the total reduction amount of light reduction to 20 mm or more in order to minimize wire segregation. Furthermore, according to one embodiment of the present invention, the light reduction is preferably carried out by rolling each rolling roll to 4 mm or less, and when the solidification fraction is 0.6 or more, the cumulative reduction amount is 60% or more. The solidification fraction refers to the ratio of the weight of molten steel that has become solid phase to the total weight of molten steel. If the casting speed is too slow, solidification will be completed before light reduction, and the proportion of the liquid phase will be extremely small compared to the solid phase, making it difficult to ensure the segregation removal effect of light reduction. On the other hand, if the casting speed is too fast, the proportion of the liquid phase will be extremely large compared to the solid phase, resulting in segregation due to solidification shrinkage, which is undesirable. Therefore, when the solidification fraction is 0.6 or higher, it is necessary to control the casting speed so that the reduction amount is 60% or more.

[0040] The amount of cooling water used should be appropriately adjusted so that solidification is completed by the time light pressure is applied. Mold-EMS (Mold Electro Magnetic Stirrer) and Strand-EMS can be set according to the conditions for conventional spring steel or arbitrarily, depending on the equipment. Unlike ordinary spring wire, nitrided spring steel has a high proportion of high-alloy components added, so it is necessary to control the carbonitrides inside. According to one embodiment of the present invention, the bloom prepared above can be heated to a temperature of 1,200°C or higher and then rolled into a billet to minimize the carbonitrides inside. Afterward, the billet can be heat-treated at 1,030°C or higher, and then rolled into wire at a temperature of 1,000°C or lower.

[0041] If the heat treatment temperature of the billet is below 1030°C, the V component in the material will not dissolve sufficiently, preventing the carbides from being solid-solved, which leads to a problem of reduced softening resistance in the final product. The rolling stage into wire rod should be performed at a temperature of 1000°C or lower so that the winding temperature can be 900°C or lower. Subsequently, the rolled wire can be wound up at a temperature of 800-900°C. If there is a large temperature difference between the rolling stage and the winding stage of the wire rod, there is a risk of severe decarburization due to localized excessive cooling. Taking this into consideration, the winding stage of the rolled wire rod should ideally be performed at a temperature of 800-900°C. This allows the wound wire to be cooled at a rate of 0.5 to 2°C / s.

[0042] Generally, unlike spring wire materials, since a large amount of high alloy components are added to spring steel for nitriding treatment, it is necessary to suppress the low-temperature structure. When the coiled wire is cooled at a rate of less than 0.5 °C / s, decarburization may occur. On the other hand, when the cooling rate exceeds 2 °C / s, the material may break due to the low-temperature structure. Next, a spring steel wire with improved strength and fatigue limit according to an embodiment of the present invention will be described.

[0043] The spring steel wire with improved strength and fatigue limit according to an embodiment of the present invention consists of, 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 balance Fe and other inevitable impurities. The spring steel wire with improved strength and fatigue limit according to an embodiment of the present invention preferably satisfies Mn + Cr ≤ 1.8%. The spring steel wire with improved strength and fatigue limit according to an embodiment of the present invention can satisfy 0.05 at% ≤ Mo + W ≤ 0.15 at%. The reason for limiting the component ranges of each alloy element is as described above.

[0044] Also, the spring steel wire with improved strength and fatigue limit according to an embodiment of the present invention can contain, in area fraction, 85% or more of tempered martensite structure and the remaining austenite structure. The spring steel wire with improved strength and fatigue limit according to an embodiment of the present invention preferably has an average grain size of prior austenite of 15 μm or less. The spring steel wire with improved strength and fatigue limit according to an embodiment of the present invention has, in the central part of 1 mm of a cross-section perpendicular to the length direction 2 In terms of area, it is preferable that 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 % is 10% or less.

[0045] If the aforementioned area ratio exceeds 10%, the quality of the material deteriorates due to segregation in the center, resulting in the formation of a low-temperature structure, reduced workability, and an increased frequency of breakage when steel wire is processed into springs. When the aforementioned area exceeds 10%, the concentration of carbide-forming elements in the center reduces the carbide effect. In one embodiment of the present invention, a spring steel wire with improved strength and fatigue limit is preferable in which the number of carbonitrides with a maximum diameter of 15 μm or more in a cross section horizontal to the length direction with a surface depth of 1 mm or less is less than 2 per 100 mm length. If carbonitrides larger than 15 μm are present on the surface of the steel wire, fatigue failure may occur in the material. It is preferable that they be present in a lengthwise and horizontal cross-section with a surface depth of 1 mm or less, with fewer than two per 100 mm length.

[0046] A spring steel wire with improved strength and fatigue limit according to one embodiment of the present invention is 100 μm thick. 2 In terms of area, the number of carbides is preferably 10 to 50, the carbides have a maximum diameter of 5 to 50 nm, and the V or Nb content is preferably 10 at% or more. In the case of carbides containing V or Nb, once they begin to grow larger than 10 nm, they grow not only with V but also with other carbide-forming elements such as Cr and Mo. Therefore, the distribution of carbide-forming elements, which are used to suppress the growth of prior austenite grains and to promote precipitation hardening, must be appropriately controlled. When the number of carbides with a maximum diameter of 5-50 nm is less than 10, there is a problem in controlling the prior austenite grain size. On the other hand, when the number of carbides with a maximum diameter of 5-50 nm exceeds 50, the amount utilized for precipitation hardening of grains smaller than 5 nm decreases, which may reduce the tensile strength of the steel wire. A spring steel wire with improved strength and fatigue limit according to one embodiment of the present invention has a tensile strength of 2,100 MPa or more and a cross-sectional reduction ratio (RA) of 45% or more.

[0047] Next, a method for manufacturing spring steel wire with improved strength and fatigue limit according to one embodiment of the present invention will be described. A method for manufacturing spring steel wire according to one embodiment of the present invention includes the steps of: LA heat treatment of a wire rod according to one embodiment of the present invention; LP heat treatment; drawing the wire rod to prepare a steel wire; and QT heat treatment of the steel wire.

[0048] First, a wire according to one embodiment of the present invention is subjected to low-temperature annealing (LA) at 650-750°C. Although not limited to this, the LA heat treatment step is preferably performed within 2 hours, as a longer process time can cause the carbides to coarseen, making it difficult to control the carbides in subsequent processes. The strength of the wire is reduced to 1,200 MPa or less by the LA heat treatment, and the LA heat treatment step can be omitted if necessary.

[0049] Afterward, the LA heat-treated wire is pickled, followed by LP (Lead Patenting, LP) heat treatment. The LP heat treatment may include a first austenitizing step of heating to 950-1100°C within 3 minutes and maintaining that temperature for within 3 minutes, and a step of passing the first austenitized wire through a lead bath at 650-700°C within 3 minutes. By performing an austenitizing process, which involves heating to 950-1100°C within 3 minutes and maintaining that temperature within 3 minutes, an austenitizing structure can be secured while simultaneously further solid-solubilizing the carbides that were coarsened in the LA process.

[0050] Next, the first austenitized wire is rapidly cooled by passing it through a lead bath at 650-750°C for 3 minutes or less, thereby undergoing isothermal transformation and securing a pearlite structure. If the temperature of the lead bath is below 650°C, a low-temperature structure can be formed. On the other hand, if the temperature of the lead bath exceeds 750°C, the carbides become coarser, and the strength decreases. Subsequently, the LP heat-treated wire can be drawn to prepare steel wire. At this stage, the diameter of the prepared steel wire can be 5 mm, and further LP heat treatment is performed to ensure that the diameter of the steel wire is 2 mm or less. Subsequently, in order to secure a tempered martensite structure, the prepared steel wire can be subjected to a QT heat treatment process.

[0051] According to one embodiment of the present invention, the QT heat treatment step may include a second austenitizing step of heating to 900-1000°C within 3 minutes and maintaining the temperature for within 3 minutes; a first oil quench step of 70°C or below; a tempering step of heating to 450-550°C within 3 minutes and maintaining the temperature for within 3 minutes; and a second oil quench step of 70°C or below. During the QT heat treatment stage, the austenitization temperature can be set to 900-1000°C to maintain the fine carbides precipitated during the LP heat treatment. Although not limited to this, it is preferable that the austenitization process during the QT heat treatment be carried out within 6 minutes. If the tempering temperature during the QT heat treatment is below 450°C, the nitriding temperature will be too low, preventing further carbide formation and resulting in reduced toughness. On the other hand, if the tempering temperature during the QT heat treatment exceeds 550°C, sufficient strength cannot be ensured.

[0052] Next, we will describe a spring with improved strength and fatigue limit according to one embodiment of the present invention. A spring with improved strength and fatigue limit according to one embodiment of the present invention consists of, by weight percent, C: 0.6~0.7%, Si: 2.0~2.5%, Mn: 0.2~0.7%, Cr: 0.9~1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%~0.2% or less, Nb: 0.05% or less, the remaining Fe and other unavoidable impurities, satisfying Mn + Cr ≤ 1.8% and 0.05 at% ≤ Mo + W ≤ 0.15 at%. The reason for limiting the component range of each alloying element is as described above.

[0053] A spring according to one embodiment of the present invention exhibits a fatigue limit that increases by more than 10% after nitriding treatment. Here, the fatigue limit refers to the limit to which the spring can withstand more than 10 million repeated loads during fatigue testing after its design. A spring according to one embodiment of the present invention is preferable to have a fatigue limit of 700 MPa or more that can withstand repeated stress for 10 million cycles. In one embodiment of the present invention, the spring exhibits a change in strength of 15% or less before and after nitriding treatment, and the nitriding treatment temperature is preferably 430°C or higher. Next, a method for manufacturing a spring with improved strength and fatigue limit according to one embodiment of the present invention will be described. A spring manufacturing method with improved strength and fatigue limit according to one embodiment of the present invention includes the steps of: cold forming a steel wire into a spring according to one embodiment of the present invention; stress relief heat treatment of the formed spring; and nitriding treatment.

[0054] In one embodiment of the present invention, a steel wire can have its fatigue limit improved by nitriding treatment before the shot peening stage during the spring manufacturing process. However, if the nitriding temperature is too low, nitrogen cannot penetrate the surface accurately, while if the nitriding 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 is preferably carried out at a temperature of 420-450°C for 10 hours or more. The present invention will be described in more detail below based on examples. However, such examples are merely illustrative of how the present invention can be practiced, and the present invention is not limited by such examples. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom. {Example}

[0055] Blooms were produced by a continuous casting process with a total reduction of 10 to 25 mm for a range of diverse alloy compositions shown in Table 1 below. The produced blooms were homogenized by heat treatment at 1,200°C, then heat-treated at 1,050°C, and then hot-rolled to a final wire diameter of 6.5 mm while lowering the temperature to 850°C, thereby producing wire rods with a final wire diameter of 6.5 mm. After that, the rolled wire rods were wound at 800 to 900°C and then cooled at a rate of 1°C / s.

[0056] [Table 1]

[0057] Table 2 below shows the at% W+Mo content and total reduction amount for the examples and comparative examples. The segregation area in Table 2 below is the center of the 1 mm cross-section perpendicular to the length direction of the manufactured wire. 2 This was derived from the analysis. The "C segregation area" in Table 2 is the center of the 1 mm cross-section perpendicular to the length. 2 In terms of area, this refers to the proportion of the area where C > 0.85% by weight is satisfied. "Si segregation area" is the central 1 mm of the cross-section perpendicular to the length direction. 2 In terms of area, this refers to the proportion of the area where Si > 3.0 wt% is satisfied. "Mn segregation area" is the 1 mm center of the cross-section perpendicular to the length direction. 2 In terms of area, this refers to the proportion of the area satisfying Mn > 0.8% by weight. "Cr segregation area" is the central 1 mm of the cross-section perpendicular to the length direction. 2 In terms of area, this refers to the proportion of the area where Cr > 2.0% by weight is satisfied. The segregation area was measured using an electron microanalyzer (Electron Probe X-ray Micro Analyzer, EPMA) with the model name EMPA-1600.

[0058] [Table 2]

[0059] Referring to Table 2 above, Examples 1 and 2, by satisfying the alloy composition and total reduction amount presented in the present invention, were able to form a mixture with a sum of C, Si, Mn, and Cr segregation area of ​​10% or less. In contrast, Comparative Example 1, with a total reduction amount of 10 mm (less than 20 mm), resulted in a sum of C, Si, Mn, and Cr segregation area reaching 30%.

[0060] Table 3 below shows the tensile strength, cross-sectional reduction rate (RA), core low-temperature structure, prior austenite average grain size, pearlite structure, and carbonitride count of the manufactured wire. The prior austenite average grain size, pearlite structure, and carbonitride count were measured using a scanning electron microscope (SEM) with model name JEOL, JSM-6610LV. In Table 3 below, "○" indicates that the cold tissue accounts for more than 20% of the area fraction, and "×" indicates that the cold tissue accounts for 20% or less of the area fraction. The pearlite structure in Table 3 below refers to the number of test specimens in which pearlite structure was detected in 80% or more area fraction when the microstructure of the cross-section perpendicular to the length of each test specimen was measured after dividing a 3m wire into 8 equal parts and producing 8 test specimens. The number of carbonitrides in Table 3 below is the number of carbonitrides with a maximum diameter of 15 μm or more, obtained by dividing a 10 cm long wire into 10 equal parts, producing 10 test pieces of 1 cm length, and then measuring the microstructure of the cross-section horizontal to the length within a surface depth of 1 mm.

[0061] [Table 3]

[0062] Referring to Table 3 above, Examples 1 and 2 were formed without the formation of a low-temperature structure in the center, and with an average prior austenite grain size of 20 μm or less. Furthermore, in Examples 1 and 2, 6 or more out of 8 test specimens had a pearlite structure of 80% or more, and exhibited excellent processability with a tensile strength of 1400 MPa or less. In addition, no carbonitrides were formed on the surface of Examples 1 and 2. In contrast, Comparative Example 1 had a tensile strength exceeding 1400 MPa, a cross-sectional reduction rate of 35% or less, deteriorated workability, and a low-temperature structure formed in the center. Furthermore, in Comparative Example 1, only 5 out of 8 test specimens had a pearlite structure of 80% or more, indicating that a uniform pearlite structure of 80% or more was not formed.

[0063] In Comparative Example 2, referring to the alloy composition in Table 1, the addition of V at a concentration of less than 0.05% resulted in the prior austenite average grain size becoming coarser, exceeding 20 μm to 24 μm. Comparative Example 3 had a tensile strength of 1510 MPa and a cross-sectional reduction rate of only 10%, indicating not only a deterioration in processability but also the formation of a low-temperature structure in the center. Furthermore, in Comparative Example 3, only two out of eight test specimens had a pearlite structure of 80% or more, indicating that the pearlite structure was not sufficiently formed. Next, the examples and comparative examples were subjected to LA heat treatment at 720°C for 2 hours, followed by pickling and then LP heat treatment. The LP heat treatment was carried out by heating to the first austenitization temperature within 3 minutes, and the remaining time was determined according to the conditions shown in Table 4 below. Table 4 also shows the pearlite transformation times for the examples and comparative examples during LP heat treatment. The pearlite transformation time was measured by deriving a TTT (Time-Temperature-Transformation) curve through dilatometry experiments.

[0064] [Table 4]

[0065] Examples 1 and 2 showed excellent productivity, with perlite transformation times measured at 110 seconds and 105 seconds, respectively, which is less than 130 seconds. In contrast, Comparative Example 3 showed a perlite transformation time of 130 seconds, resulting in such low productivity that on-site production was difficult. Next, the LP heat-treated examples and comparative examples were drawn to produce 3 mm diameter steel wires. The produced steel wires were subjected to secondary austenitization and first quenching, followed by tempering and second quenching to obtain QT steel wires. The wires were heated to the secondary austenitization temperature within 3 minutes, and the first and second quenching were performed in 60°C oil. The remaining steps were carried out according to the conditions in Table 5 below.

[0066] [Table 5]

[0067] Table 6 below shows the tensile strength, cross-sectional reduction rate (RA), and carbide count of the manufactured QT steel wire. Here, the carbide count is per 100 μm. 2 This 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 in the area. The number of carbides is measured on the wire surface at any 100 μm interval using an FEI Tecnai OSIRIS transmission electron microscope (TEM). 2 This value is the average of eight measurements taken at eight different points in the area.

[0068] [Table 6]

[0069] Referring to Table 6 above, Examples 1 and 2 secured excellent tensile strength of 2200 MPa or more, while simultaneously ensuring a cross-sectional reduction rate of 45% or more. Furthermore, Examples 1 and 2 were formed with 10 to 50 carbides. In contrast, Comparative Example 1 showed a cross-sectional reduction rate of only 32% and had more than 50 carbides. Comparative Example 2 deteriorated to a tensile strength of 2200 MPa or less, and was formed with fewer than 10 carbides, resulting in a problem where it was difficult to control the average grain size of the prior austenite. Comparative Example 4 also deteriorated to a tensile strength of 2200 MPa or less and had more than 50 carbides. Next, the QT steel wire was cold-formed into a spring shape, the formed spring was heat-treated, and then nitrided at 420-450°C.

[0070] Table 7 below shows the presence or absence of damage during spring molding, the fatigue limit, and the fatigue limit value after nitriding treatment. The fatigue limits before and after nitriding were measured under conditions of stress ratio R (tensile strength / compressive strength) = -1 and test speed of 30-60 Hz. In Table 7 below, "×" means that the spring was not damaged during molding, and "○" means that the spring was damaged during molding.

[0071] [Table 7]

[0072] Examples 1 and 2 exhibited excellent machinability, did not break, and measured a fatigue limit of 650 MPa or higher before nitriding, demonstrating superior fatigue limits. Furthermore, after nitriding, Examples 1 and 2 showed a fatigue limit of 750 MPa or higher, representing an increase of 10% or more in fatigue limits compared to before nitriding, demonstrating excellent nitriding treatment characteristics. In contrast, Comparative Examples 1 and 2 showed reduced machinability and fracture, and after nitriding treatment, the fatigue limit increased by less than 10% compared to before nitriding treatment. In Comparative Example 4, although no breakage occurred during the processing of the spring, the fatigue limit did not increase by more than 10% after nitriding compared to before nitriding, indicating a deterioration in the nitriding treatment characteristics.

[0073] According to the disclosed examples, by optimizing the alloy composition and manufacturing conditions, excellent tensile strength and cross-sectional reduction ratio are ensured, while nitriding properties and fatigue limits are improved, making it applicable to materials such as automotive transmission gears and engine valves. [Industrial applicability]

[0074] According to one example of the present invention, it is possible to provide spring wire, steel wire, springs, and methods for manufacturing the same, which have improved strength and fatigue limit.

Claims

1. In weight percent, it consists of C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2%, Nb: 0.05% or less, and the remainder being Fe and other unavoidable impurities. Satisfying Mn + Cr ≤ 1.8%, The condition 0.05 at% ≤ Mo + W ≤ 0.15 at% is satisfied. 1 mm in the center of the cross-section perpendicular to the length. ２ A spring wire material with improved strength and fatigue limit, characterized in that, in terms of area, the proportion of areas satisfying one or more of the following conditions in weight percent is 10% or less: C > 0.85%, Si > 3.0%, Mn > 0.8%, and Cr > 2.0%.

2. The spring wire material according to claim 1, characterized in that it contains 80% or more pearlite structure by area fraction, with the remainder being bainite or martensitic structure, thereby improving strength and fatigue limit.

3. The spring wire material described in claim 1, characterized in that the average particle size of the prior austenite is 20 μm or less, and has improved strength and fatigue limit.

4. Two carbonitrides with a maximum diameter of 15 μm or more are found per cm in the longitudinal and horizontal cross-sections within a surface depth of 1 mm. ２ A spring wire material with improved strength and fatigue limit as described in claim 1, characterized by being distributed at less than 1.

5. The spring wire material described in claim 1, characterized in that it has a tensile strength of 1,400 MPa or less and a cross-sectional reduction rate of 35% or more, thereby improving strength and fatigue limit.

6. This stage involves continuously casting molten steel, consisting of, by weight percent, C: 0.6–0.7%, Si: 2.0–2.5%, Mn: 0.2–0.7%, Cr: 0.9–1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%–0.2%, Nb: 0.05% or less, with the remainder being Fe and other unavoidable impurities, to prepare a bloom; The step of heating the bloom at a temperature of 1,200°C or higher, and then rolling it into a billet; The step of heat-treating the billet at 1,030°C or higher, and then rolling it into wire at a temperature of 1,000°C or lower; The step of winding the wire at a temperature of 800 to 900°C; and A method for manufacturing a spring wire with improved strength and fatigue limit, as described in any one of claims 1 to 5, comprising the step of cooling the wound wire at a speed of 0.5 to 2°C / s.

7. The method for manufacturing spring wire according to claim 6, characterized in that the continuous casting step includes light reduction with a total reduction of 20 mm or more.

8. In weight percent, it consists of C: 0.6-0.7%, Si: 2.0-2.5%, Mn: 0.2-0.7%, Cr: 0.9-1.5%, P: 0.015% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.01% or less, Mo: 0.25% or less, W: 0.25% or less, V: 0.05%-0.2% or less, Nb: 0.05% or less, and the remainder being Fe and other unavoidable impurities. Satisfying Mn + Cr ≤ 1.8%, The condition 0.05 at% ≤ Mo + W ≤ 0.15 at% is satisfied. In terms of area fraction, it contains 85% or more tempered martensite and the remainder austenite. 100 μm ２ A spring steel wire with improved strength and fatigue limit, characterized in that, in its area, the number of carbides is 10 to 50, the carbides have a maximum diameter of 5 to 50 nm, and the V or Nb content is 10 at% or more.

9. The spring steel wire described in 8, characterized in that it has a tensile strength of 2,100 MPa or more and a cross-sectional reduction rate of 45% or more, thereby improving strength and fatigue limit.

10. A step of LP heat treatment of the wire according to any one of claims 1 to 5; The steps of drawing the LP heat-treated wire to prepare steel wire; and The step of performing QT heat treatment on the steel wire; The aforementioned LP heat treatment step is, A first austenitization step in which the temperature is heated to 950-1100°C within 3 minutes and then maintained within 3 minutes; and A method for manufacturing spring steel wire with improved strength and fatigue limit, characterized by including a step of passing the first austenitized wire through a lead bath at 650 to 700°C for 3 minutes or less.

11. In the LP heat treatment stage, A method for manufacturing spring steel wire with improved strength and fatigue limit, as described in claim 10, characterized in that the time required for the completion of perlite transformation is less than 130 seconds by satisfying the composition range conditions described in claim 1.

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