Spring wire rod and steel wire, spring, and manufacturing method thereof

Abstract

Provided are wire rods and steel wires for springs with improved resistance to permanent deformation, springs, and methods for manufacturing these. [Solution] The spring of the present invention includes a tempered martensite structure, and among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides having a thickness of 100 nm or less is 5 nm to 12 nm, and the tempered martensite structure is contained at an area fraction of 90% or more, and the density of the carbides having a thickness of 100 nm or less is 20 particles / μm 2 More than 40 / μm, preferably 2 More than 64 pieces / μm 2 and the dislocation density is 6.0×10 13 / m 2 That's it, 3.0 x 10 14 / m 2 The weight percentages are C: 0.48 to 0.62%, Si: 1.5 to 2.0%, Mn: 0.2 to 0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5 to 0.8%, N: 0.003 to 0.015%, and the remainder is Fe and unavoidable impurities.

Description

[Technical Field]

[0001] The present invention relates to wire rods and steel wires for springs, springs, and methods for manufacturing the same. [Background technology]

[0002] Suspension springs are designed to withstand repeated fatigue loads in the elastic range below the yield strength, but plastic deformation occurs in localized areas, resulting in fatigue failure. Furthermore, suspension springs are constantly subjected to compressive stress due to the weight of the vehicle when empty, resulting in localized permanent deformation. The cause of this localized plastic deformation is the movement of dislocations due to repeated fatigue loads and static loads resulting from stresses below the yield strength.

[0003] Traditionally, carbide elements such as Mo, V, and W have been added to tempered martensitic steel to improve its fatigue strength and permanent deformation resistance. However, adding large amounts of relatively expensive carbide elements reduces price competitiveness. Therefore, it is necessary to develop technology that can optimize the amount of carbide elements and provide manufacturing methods that accommodate this. Summary of the Invention [Problem to be solved by the invention]

[0004] SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, an object of the disclosed invention is to provide wire rods and steel wires for springs, springs, and methods for manufacturing the same, which have improved resistance to permanent deformation. [Means for solving the problem]

[0005] A spring according to one embodiment of the present invention includes a tempered martensite structure, and is characterized in that the Fe carbides precipitated within the tempered martensite structure have an average thickness of 5 nm to 12 nm, and the thickness of the carbides is 100 nm or less.

[0006] Furthermore, the spring according to one embodiment of the present invention may contain the tempered martensite structure in an area fraction of 90% or more.

[0007] In addition, in the spring according to an embodiment of the present invention, the density of the carbide particles having a thickness of 100 nm or less is 20 particles / μm 2 It may be more than that.

[0008] In addition, in the spring according to an embodiment of the present invention, the density of the carbide particles having a thickness of 100 nm or less is 40 particles / μm 2 It may be more than that.

[0009] In addition, in the spring according to one embodiment of the present invention, the density of the carbide particles having a thickness of 100 nm or less is 64 particles / μm 2 It may be the following:

[0010] Furthermore, the spring according to one embodiment of the present invention has a dislocation density of 6.0×10 13 / m 2 It may be more than that.

[0011] Furthermore, the spring according to one embodiment of the present invention has a dislocation density of 3.0×10 14 / m 2 It may be the following:

[0012] Furthermore, the spring according to one embodiment of the present invention may contain, by weight %, C: 0.48 to 0.62% and Si: 1.5 to 2.0%.

[0013] Furthermore, the spring according to one embodiment of the present invention contains, by weight, Mn: 0.2 to 0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5 to 0.8%, N: 0.003 to 0.015%, and the remainder being Fe and unavoidable impurities.

[0014] In the spring according to one embodiment of the present invention, the carbide having a thickness of 100 nm or less may have an average thickness of 6 nm to 10 nm.

[0015] Furthermore, the spring according to one embodiment of the present invention may have a minimum outer diameter (D) relative to the diameter (d) of 4 or more (D / d≧4).

[0016] Furthermore, the spring according to one embodiment of the present invention may have a tensile strength of 2000 MPa or more.

[0017] Furthermore, the spring according to one embodiment of the present invention may have a permanent deformation, which is the difference in height before and after compression, of 3.0 mm or less when compressed for 48 hours.

[0018] A spring steel wire according to another embodiment of the present invention includes a tempered martensite structure having an area fraction of 90% or more, and among the Fe carbides precipitated in the tempered martensite structure, the carbides having a thickness of 100 nm or less have an average thickness of 5 nm to 12 nm.

[0019] Furthermore, the spring steel wire according to one embodiment of the present invention may contain, by weight percent, C: 0.48 to 0.62% and Si: 1.5 to 2.0%.

[0020] Furthermore, the spring steel wire according to one embodiment of the present invention contains, by weight, Mn: 0.2 to 0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5 to 0.8%, N: 0.003 to 0.015%, and the remainder being Fe and unavoidable impurities.

[0021] In the spring steel wire according to one embodiment of the present invention, the carbides having a thickness of 100 nm or less may have an average thickness of 6 nm to 10 nm.

[0022] In addition, in the spring steel wire according to one embodiment of the present invention, the density of the carbides having a thickness of 100 nm or less is 40 particles / μm 2 ~64 pieces / μm 2 may be.

[0023] Furthermore, the spring steel wire according to one embodiment of the present invention may have a fatigue strength ratio of 0.45 or more.

[0024] A spring wire rod according to another embodiment of the present invention is characterized by the following weight percentages: C: 0.48-0.62%, Si: 1.5-2.0%, Mn: 0.2-0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5-0.8%, N: 0.003-0.015%, and the remainder being Fe and unavoidable impurities.

[0025] In addition, in the spring wire rod according to one embodiment of the present invention, the thickness of the ferrite decarburized layer may be 0.03 mm or less.

[0026] A method for manufacturing a spring wire rod according to another embodiment of the present invention includes the steps of: manufacturing a billet consisting of, by weight, C: 0.48-0.62%, Si: 1.5-2.0%, Mn: 0.2-0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5-0.8%, N: 0.003-0.015%, and the remainder being Fe and unavoidable impurities; finish-rolling the billet at 930°C to 1010°C to manufacture a wire rod; and cooling the wire rod after finish-rolling to 730°C at a cooling rate of more than 10°C / s.

[0027] A method for manufacturing a spring steel wire according to another embodiment of the present invention includes the steps of: manufacturing a wire rod consisting of, by weight%, 0.48-0.62% C, 1.5-2.0% Si, 0.2-0.55% Mn, 0.015% or less P, 0.020% or less S, 0.5-0.8% Cr, 0.003-0.015% N, and the remainder being Fe and unavoidable impurities; reheating the wire rod at 900°C to 990°C and then quenching the wire rod; and tempering the quenched wire rod at 415°C to 465°C.

[0028] Also, the method for manufacturing a spring steel wire according to an embodiment of the present invention may further include a step of performing a secondary tempering at 300°C to 400°C after the tempering step.

[0029] In addition, a method for manufacturing a spring steel wire according to an embodiment of the present invention includes a tempered martensite structure having an area fraction of 90% or more, and among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides having a thickness of 100 nm or less is 5 nm to 12 nm, and the carbide density is 20 particles / μm 2 ~64 pieces / μm 2 and the dislocation density is 6.0×10 13 / m 2 ~3.0×10 14 / m 2 may be.

[0030] A method for manufacturing a spring according to another embodiment of the present invention includes cold forming a compression coil spring manufactured so that the minimum outer diameter (D) of the spring product relative to the diameter (d) of the steel wire is 4 or more (D / d≧4), and performing a stress relief heat treatment at 190°C to 290°C.

[0031] In addition, in a method for manufacturing a spring according to an embodiment of the present invention, the spring includes a tempered martensite structure with an area fraction of 90% or more, and among the Fe carbides precipitated in the tempered martensite structure, the average thickness of carbides having a thickness of 100 nm or less is 5 nm to 12 nm, and the carbide density is 20 particles / μm 2 ~64 pieces / μm 2 and the dislocation density is 6.0×10 13 / m 2 ~3.0×10 14 / m 2 may be. [Effects of the Invention]

[0032] According to one embodiment of the disclosed invention, by controlling the alloy components and the manufacturing method, it is possible to provide a spring wire rod, a steel wire, a spring, and a manufacturing method thereof, which have improved permanent deformation resistance and excellent tensile strength and fatigue strength ratio.

[0033] The effects that can be obtained in this specification are not limited to the effects described above, and other effects not mentioned here will be clearly understood by those having ordinary skill in the art to which the present invention pertains from the following description. [Brief explanation of the drawings]

[0034] [Figure 1] FIG. 2 is a diagram showing the pattern and minor axis of unit carbides observed by a TEM (transmission electron microscope) in one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0035] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are presented to fully convey the spirit of the disclosed invention to those skilled in the art to which the disclosed invention pertains. The disclosed invention is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, parts irrelevant to the description may be omitted to clarify the disclosed invention, and the sizes of components may be somewhat exaggerated to facilitate understanding.

[0036] Throughout the specification, when a part is said to "comprise" certain elements, this means that it may further include other elements, rather than excluding other elements, unless specifically stated to the contrary.

[0037] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0038] A spring according to one embodiment of the present invention will now be described.

[0039] A spring according to one embodiment of the present invention includes a tempered martensite structure. Among the Fe carbides precipitated within the tempered martensite structure, the average thickness of the Fe carbides with a thickness of 100 nm or less is 5 nm to 12 nm. Here, the "average thickness" of the carbides refers to the average value of the thicknesses of a plurality of unit carbides. The thickness of the unit carbide used in calculating the average thickness refers to the length of the minor axis in the cross section of the carbide. It is not limited to the direction of the cross section of the carbide, and may be, for example, the C-section or L-section direction. Here, the "minor axis" refers to the largest width perpendicular to the major axis, and the "major axis" refers to the longest diameter of the crystal grain. Figure 1 shows the pattern and minor axis of a unit carbide observed using a transmission electron microscope (TEM) in the present invention. The unit carbide may have an elliptical or near-elliptical structure, as shown in (a) or (b). Generally, a structure like (a) or (b) is observed during TEM measurement, although a pattern of overlapping carbides, as shown in (c) or (d), may also be observed. Referring to FIG. 1, in cases such as (a) or (b), the thickness of a unit carbide refers to the minor axis as shown, and in cases such as (c) or (d), the thickness of a unit carbide refers to the length of the longest minor axis of the carbide.

[0040] Carbides are obstacles that hinder the movement of dislocations, and larger carbides have less dispersion effect. Therefore, in the present invention, carbides of 100 nm or less are used as the standard for effective carbides. This is because nano-carbides of 100 nm or less play a role in strengthening strength by hindering the movement of dislocations, while coarse carbides of micron size that do not affect the improvement of permanent deformation resistance are excluded to eliminate average errors.

[0041] That is, the average thickness of carbides in the tempered martensite structure that are 100 nm or less is controlled to 5 nm to 12 nm. That is, if the average thickness of carbides that are 100 nm or less is less than 5 nm, dislocations will cut through the carbides and move, and dislocation movement cannot be prevented. If it exceeds 12 nm, the fatigue strength and permanent deformation resistance required by the present invention will not be satisfied due to the coarse carbides. Preferably, the average thickness of carbides that are 100 nm or less can be 6 nm to 10 nm.

[0042] Permanent deformation occurs when microstructural defects, which act as strengthening mechanisms in springs and steels, fail to function. The strengthening mechanisms of tempered martensite structures can be dislocations, precipitates, and solute elements (such as supersaturated [C] and solute [Mn]). Dislocations combine and disappear, precipitates combine and coarsen, and supersaturated [C] precipitates, causing a decrease in strength and resulting in permanent deformation. The purpose of the present invention is to prevent softening due to dislocations, which is one of the strengthening mechanisms in the operating environment.

[0043] In addition, the spring according to an embodiment of the present invention may contain the tempered martensite structure in an area fraction of 90% or more.

[0044] In addition, in the spring according to an embodiment of the present invention, the density of the carbide particles having a thickness of 100 nm or less is 20 particles / μm 2 or more, preferably 40 / μm 2 More preferably, 64 particles / μm 2 The density of fine carbides can be less than μm 2 When the number of dislocations is less than 20 per μm, the dislocations move freely, so they meet and disappear, causing a softening of the material strength. 2 When the number of particles per unit is 40 or more, the balance between strength and softening resistance is optimal. 2 To exceed 64 particles per μm, the carbon content in the steel must be high, but this increases the strength and reduces formability. Therefore, the density of carbides with a thickness of 100 nm or less is 64 particles / μm. 2Controlled as follows:

[0045] Furthermore, the spring according to one embodiment of the present invention has a dislocation density of 6.0×10 13 / m 2 or more, 3.0 x 10 14 / m 2 The dislocation density can be 3.0×10 or less. 14 / m 2 If the carbon content exceeds 6.0×10, dislocation annihilation occurs easily, resulting in poor fatigue strength and permanent deformation resistance. In addition, if the carbon content is 0.48% or more, in order to achieve a strength of 2,000 MPa or more, 13 / m 2 In this way, the dislocation density can be controlled. In particular, dislocations are formed when the material transforms into martensite during quenching after heating, and some of them disappear during tempering. In contrast, in the present invention, the dislocation density is kept low by increasing the tempering temperature through the addition of Si, and the solid solution strengthening effect of the addition of Si and the precipitation strengthening effect of fine carbides can be controlled in a well-balanced manner.

[0046] Furthermore, a spring according to one embodiment of the present invention may contain, by weight, C: 0.48-0.62% and Si: 1.5-2.0%, Mn: 0.2-0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5-0.8%, N: 0.003-0.015%, with the remainder being Fe and unavoidable impurities.

[0047] The reasons for limiting the alloying element contents in the examples of the present invention will be explained below. Unless otherwise specified, the units are % by weight.

[0048] The C (carbon) content may preferably be 0.48 to 0.62%.

[0049] C is an element that is effective in increasing strength through solid solution strengthening, precipitation strengthening, and martensite strengthening. A carbon content of 0.18% to 0.70% can realize a tempered martensite structure. In particular, to ensure formability as a spring with a strength of 2000 MPa or more, which is required for spring products, a carbon content of 0.48 to 0.62% is preferred. In particular, excessive C content can reduce toughness and cause sudden brittle fracture. Taking this into consideration, a C content of 0.48 to 0.62% is preferred.

[0050] The Si (silicon) content may preferably be 1.5 to 2.0%.

[0051] Silicon (Si) can be used to deoxidize steel and inhibits carbide growth during tempering, allowing for higher tempering temperatures. It can be added at 1.2–2.5%. As the tempering temperature increases, dislocation annihilation reduces the dislocation strengthening effect, the amount of solute carbon precipitates increases, reducing the solid solution strengthening effect, and the precipitated carbides grow, reducing the precipitation strengthening effect. Si counters the reduced strength due to the reduced dislocation strengthening effect and the reduced solid solution strengthening effect of carbon by slowing carbide growth and enhancing the precipitation strengthening effect, thereby increasing the solid solution strengthening effect of silicon, thereby achieving the target strength. Therefore, the formation of ultrafine carbides increases the precipitation strengthening effect at the target strength, and the annihilation of dislocations reduces the dislocation strengthening effect. Therefore, the addition of Si minimizes the annihilation of mobile dislocations during cyclic fatigue and improves permanent deformation resistance. Taking this into consideration, Si can be added at 1.5% or more. However, if the Si content exceeds 2.0%, the activity of carbon increases when added during the wire manufacturing process, accelerating the decarburization phenomenon, making it difficult to control decarburization in conventional wire manufacturing equipment. Considering this, the Si content is preferably 1.5 to 2.0%.

[0052] The Mn (manganese) content may preferably be 0.2 to 0.55%.

[0053] Mn is an element that improves hardenability and can be contained in a content of 1.2% or less. Adding Mn causes the S impurity in steel to precipitate as MnS, thereby preventing surface defects caused by low-melting-point emulsions. Taking this into consideration, Mn can be added in an amount of 0.2% or more. However, if the Mn content is excessive, the dislocation density during quenching heat treatment may increase, potentially resulting in poor permanent deformation resistance. Taking this into consideration, the Mn content is preferably 0.2 to 0.55%.

[0054] The P (phosphorus) content may be 0.015% or less.

[0055] P is an element that can segregate at grain boundaries and reduce impact toughness, so the P content can be limited to 0.015% or less.

[0056] The content of S (sulfur) may be 0.020% or less.

[0057] S is an element that not only segregates at grain boundaries to reduce toughness like P, but also forms low-melting point emulsions to hinder hot rolling. In consideration of this, the S content may be 0.020% or less.

[0058] The Cr (chromium) content may preferably be 0.5 to 0.8%.

[0059] Cr is an effective element for improving hardenability and ensuring strength, and can be added at 1.2% or less. Like Si, Cr also plays a role in suppressing carbide growth during tempering. Taking this into consideration, Cr content can be added at 0.5% or more. However, excessive Cr content can form a chromium oxide layer on the surface, increasing the C / A ratio of corrosion pits and causing the notch effect, which is undesirable from the perspective of corrosion fatigue durability. Taking this into consideration, a Cr content of 0.5-0.8% is preferred.

[0060] The N (nitrogen) content may be 0.003 to 0.015%.

[0061] N can combine with Ti added to steel to form nitrides. Manufacturing costs may increase if the N content is kept below 0.003%. However, if the N content exceeds 0.015%, coarse nitrides may be formed, potentially resulting in poor impact toughness.

[0062] The content of V, W, Mo, Ti, or Nb may each be 0.05% or less.

[0063] Since V, W, Mo, Ti, or Nb forms coarse carbonitrides and prevents the formation of fine carbides, the content of V, W, Mo, Ti, or Nb is controlled to 0.05% or less in the present invention.

[0064] The remaining component of the disclosed invention is iron (Fe). However, it cannot be excluded that unintended impurities may inevitably be introduced from raw materials or the surrounding environment during normal manufacturing processes. These impurities are known to anyone skilled in normal manufacturing processes, and therefore not all of them are specifically mentioned herein.

[0065] Furthermore, the spring according to one embodiment of the present invention may have a minimum outer diameter (D) relative to the diameter (d) of 4 or more (D / d≧4).

[0066] Furthermore, the spring according to one embodiment of the present invention may have a tensile strength of 2000 MPa or more.

[0067] Furthermore, the spring according to one embodiment of the present invention may have a permanent deformation, which is the difference in height before and after compression, of 3.0 mm or less when compressed under a stress of 1274 MPa for 48 hours.

[0068] Hereinafter, a spring steel wire according to one embodiment of the present invention will be described.

[0069] According to another embodiment of the present invention, a spring steel wire includes a tempered martensite structure with an area fraction of 90% or more, and among the Fe carbides precipitated within the tempered martensite structure, the average thickness of the carbides with a thickness of 100 nm or less is 5 nm to 12 nm. The tempered martensite structure can improve fatigue strength and permanent deformation resistance by restricting dislocation movement due to the carbides formed during tempering. In addition to the tempered martensite structure, retained austenite can be included. If the retained austenite fraction exceeds 10%, the retained austenite will transform into martensite during spring forming, resulting in poor formability. In particular, even if no damage occurs during forming, the martensite transformed during forming has a high dislocation density and does not precipitate carbides, accelerating dislocation annihilation and facilitating permanent deformation. Therefore, the retained austenite fraction is controlled to 10% or less.

[0070] If the average thickness of the carbides having a thickness of 100 nm or less is less than 5 nm, dislocations will cut through the carbides and move, making it impossible to prevent dislocation movement, and if it exceeds 12 nm, the fatigue strength and permanent deformation resistance required by the present invention will not be satisfied due to the coarse carbides. Preferably, the average thickness of the carbides having a thickness of 100 nm or less can be 6 nm to 10 nm.

[0071] Furthermore, the spring steel wire according to one embodiment of the present invention may contain, by weight percent, C: 0.48 to 0.62% and Si: 1.5 to 2.0%.

[0072] Furthermore, the spring steel wire according to one embodiment of the present invention contains, by weight, Mn: 0.2 to 0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5 to 0.8%, N: 0.003 to 0.015%, and the remainder being Fe and unavoidable impurities.

[0073] In addition, in the spring steel wire according to one embodiment of the present invention, the density of the carbides having a thickness of 100 nm or less is 40 particles / μm 2 ~64 pieces / μm 2 It can be.

[0074] In addition, the spring steel wire according to the present invention may have a fatigue strength ratio (fatigue strength / tensile strength) of 0.45 or more. If the fatigue strength ratio is less than 0.45, the tempering heat treatment temperature must be lowered to increase the strength of the spring steel wire in order to achieve the required fatigue life of the spring, which increases the dislocation density and causes problems such as poor permanent deformation resistance.

[0075] Hereinafter, a spring wire rod according to one embodiment of the present invention will be described.

[0076] A spring wire rod according to another embodiment of the present invention is composed of, by weight, C: 0.48-0.62%, Si: 1.5-2.0%, Mn: 0.2-0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5-0.8%, N: 0.003-0.015%, and the remainder being Fe and unavoidable impurities.

[0077] In addition, the spring wire according to one embodiment of the present invention may have a ferrite decarburized layer thickness of 0.03 mm or less. During the process of reheating the wire and heat treating the steel wire, the austenizing heat treatment causes carbon inside the wire to diffuse to the surface, resulting in carbon recovery. However, since a decarburized layer with a thickness of 0.03 mm or less may result in recarburization, the thickness of the ferrite decarburized layer is controlled to 0.03 mm or less.

[0078] Next, a method for manufacturing a spring wire rod according to an embodiment of the present invention will be described.

[0079] A method for manufacturing a spring wire rod according to another embodiment of the present invention includes the steps of: preparing a billet containing, by weight, 0.48-0.62% C, 1.5-2.0% Si, 0.2-0.55% Mn, 0.015% or less P, 0.020% or less S, 0.5-0.8% Cr, 0.003-0.015% N, and the remainder being Fe and unavoidable impurities; finish-rolling the billet at 930°C to 1010°C to produce a wire rod; and cooling the wire rod after finish-rolling to 730°C at a cooling rate exceeding 10°C / s. If the finish-rolling temperature is too low, surface ferrite decarburization may occur. However, if the finish-rolling temperature is too high, a thick overall decarburized layer may form on the surface and the grain size may become coarse, making it difficult to achieve the desired permanent deformation resistance. In the stage of rapid cooling to 730°C at 10°C / s or more, rapid cooling is performed to the pearlite transformation region, and the surface decarburization temperature region in which the surface layer is induced by ferrite phase transformation can be avoided.

[0080] Next, a method for manufacturing a spring steel wire according to an embodiment of the present invention will be described.

[0081] A method for manufacturing a spring steel wire according to another embodiment of the present invention includes the steps of: manufacturing a wire rod containing, by weight, 0.48-0.62% C, 1.5-2.0% Si, 0.2-0.55% Mn, 0.015% or less P, 0.020% or less S, 0.5-0.8% Cr, 0.003-0.015% N, and the remainder being Fe and unavoidable impurities; reheating the wire rod at 900-990°C, followed by quenching; and tempering the quenched wire rod at 415-465°C. If the reheating temperature exceeds 990°C, the grain size may become coarse. However, if the reheating temperature is lower than 900°C, pearlite may not be sufficiently reverse-transformed, and residual pearlite may deteriorate the formability of the spring. Therefore, the reheating temperature is controlled to be between 900-990°C. Furthermore, if the tempering temperature exceeds 465°C, the target tensile strength may not be achieved, and if the tempering temperature is lower than 415°C, the dislocation density is high, which reduces fatigue strength due to the disappearance of mobile dislocations during fatigue testing, and permanent deformation may occur. Therefore, the tempering temperature is controlled to be between 415°C and 465°C.

[0082] Furthermore, a method for manufacturing a spring steel wire according to an embodiment of the present invention may further include a secondary tempering step at 300°C to 400°C after the tempering step. If the temperature is below 300°C, dislocation annihilation becomes ineffective, and as the heat treatment time increases, supersaturated carbon selectively precipitates from the prior austenite grain boundaries, which can induce temper martensite embrittlement. On the other hand, if the temperature exceeds 400°C, although dislocation annihilation is effective, the precipitates become coarse, making it difficult to control the desired size and density of carbides.

[0083] In addition, a method for manufacturing a spring steel wire according to an embodiment of the present invention includes a tempered martensite structure having an area fraction of 90% or more, and among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides having a thickness of 100 nm or less is 5 nm to 12 nm, and the carbide density is 20 particles / μm 2 ~64 pieces / μm 2 and the dislocation density is 6.0×10 13 / m 2 ~3.0×10 14 / m 2 It can be.

[0084] Next, a method for manufacturing a spring according to another embodiment of the present invention will be described.

[0085] According to another embodiment of the present invention, a method for manufacturing a spring includes cold-forming a compression coil spring manufactured to have a minimum outer diameter (D) of the spring product relative to the steel wire diameter (d) of 4 or more (D / d≧4), and then performing a stress relief heat treatment at 190°C to 290°C. If the outer diameter of the product is less than 4, the amount of processing during cold-forming is large, resulting in the generation of many moving dislocations. As a result, excessive dislocation annihilation during stress relief heat treatment to relieve residual stress makes it difficult to achieve the target tensile strength. Therefore, to achieve a tensile strength of 2000 MPa or more, the spring's outer diameter must be formed to be 4 or more times the diameter of the steel wire. If the stress relief heat treatment temperature is lower than 190°C, residual stress is not sufficiently relieved, resulting in delayed fracture in areas where residual hydrogen introduced during the manufacturing process is concentrated. If the heat treatment temperature exceeds 290°C, dislocations are annihilated and carbide growth occurs, preventing the target tensile strength from being achieved and making it difficult to achieve the target permanent deformation resistance.

[0086] In addition, in a method for manufacturing a spring according to an embodiment of the present invention, the spring includes a tempered martensite structure with an area fraction of 90% or more, and among the Fe carbides precipitated in the tempered martensite structure, the average thickness of carbides having a thickness of 100 nm or less is 5 nm to 12 nm, and the carbide density is 20 particles / μm 2 ~64 pieces / μm 2 and the dislocation density is 6.0×10 13 / m 2 ~3.0×10 14 / m 2 It can be.

[0087] The present invention will be described in more detail below through examples. However, the description of these examples is intended to illustrate the practice of the present invention, and the present invention is not limited by the description of these examples. The scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0088] {Example} For the various alloy composition ranges shown in Table 1 below, 40 to 60 kg ingots were produced in a vacuum induction melting furnace. The produced billets were solution heat treated for 4 hours in a heating furnace at 1200°C to remove the cast structure. Then, to produce wire from the billets, welding was performed on the rear end of a 10 m long billet. The welded billet was finish-rolled to produce a wire with a diameter of 15 mm. Next, it was quenched to 730°C at an average cooling rate of 12°C / s and then slowly cooled to room temperature. It was then drawn to a diameter of 13.8 mm, reheated using an induction heating heat treatment method, and quenched. The quenched wire was tempered to produce steel wire.

[0089] The finish rolling temperature, reheating temperature and tempering temperature are shown in Table 2.

[0090] [Table 1]

[0091] [Table 2]

[0092] Table 3 below shows the average carbide thickness, dislocation density, tensile strength, fatigue strength ratio, and permanent deformation. The average carbide thickness was measured using a transmission electron microscopy (TEM). Specifically, 10 random locations were measured at 160K magnification using a replica specimen. Then, component analysis was performed using energy dispersive X-ray spectroscopy (EDS). The average thickness of the carbide-confirmed areas was observed. The replica specimens were polished with sandpaper and then etched with a 10% AA solution (acetylaceton, 1% tetramethylammonium chloride, and methanol). The specimen surfaces were then carbon-coated using a JEOL Vacuum Evaporator JEE-420. The specimens were then mounted on Cu grids and their structures were observed using a TEM manufactured by FEI Tecnai OSIRIS. In parallel with this, 3 mm thin foil test pieces were prepared by polishing the sample to a thickness of 80 μm using sandpaper, punching a 3 mm circular test piece, and electrolytically etching it with 10% perchloric acid and 90% acetic acid.

[0093] Meanwhile, the average thickness of the carbides was measured and shown as the average thickness of the precipitated carbides having a thickness of 100 nm or less.

[0094] Carbide density was measured by creating replica specimens and 3mm thin foil specimens. Ten specimens were randomly selected and measured at 160,000x magnification. The carbide area was defined as the density. The carbide area was measured by printing a photograph of the specimen, coloring the carbide on a transparent film, scanning the film, and analyzing the observed area using the Image Analyzer program. For replica specimens, the samples were polished with sandpaper and then etched in a 10% AA solution (acetylaceton, 1% tetramethylammonium chloride, and methanol). The specimen surface was then carbon-coated using a JEOL Vacuum Evaporator JEE-420. The specimen was then mounted on a Cu grid and the structure was observed using a TEM (FEI Tecnai OSIRIS). In parallel with this, 3 mm thin foil test pieces were prepared by polishing the sample to a thickness of 80 μm using sandpaper, punching a 3 mm circular test piece, and electrolytically etching it with 10% perchloric acid and 90% acetic acid.

[0095] Dislocation density was measured using X-ray diffraction (XRD). CuKα radiation (40 kV 40 mA), 40° < 2θ < 100°, 0.02° / sec. After measurement, analysis was performed using Convolutional Multiple Whole Profile (CMWP) software. For details of the experimental method, please refer to K. Murasawa's Materials Transactions Vol. 59, 2018, p. 1135.

[0096] The tensile strength was measured based on JIS Z2241 standard by processing a JIS No. 4 subsize test piece into a diameter of 4 mm and conducting a tensile test at a strain rate of 0.01 / s.

[0097] Fatigue strength was measured using a JIS Z2274 No. 1 test piece with a gauge diameter of 5 mm and a rotating bending fatigue test in R=-1 mode. The fatigue strength was defined as the strength at which the fatigue life was 10 million cycles, and the fatigue strength ratio was defined as fatigue strength ÷ tensile strength.

[0098] The amount of permanent deformation was measured by compressing the test piece at 1274 MPa and maintaining the compression for 48 hours, and then measuring the difference in height before and after compression.

[0099] [Table 3]

[0100] Here, the TM structure fraction means the area fraction of tempered martensite. Referring to Table 3, Experimental Examples 1 to 22 satisfied the ranges of the alloy composition and manufacturing method of the present invention. Therefore, Experimental Examples 1 to 22 had a tempered martensite (TM) structure fraction of 90% or more and a dislocation density of 6.0 × 10 13 / mm 2 ~3.0×10 14 / mm 2 The average thickness of precipitated carbides, which are 100 nm or less, is 5 nm or more and 12 nm or less, the tensile strength is 2000 MPa or more, the fatigue strength ratio is 0.45 or more, and the permanent deformation amount is 3.0 mm or less. Therefore, the fatigue strength and permanent deformation resistance are excellent.

[0101] However, in Experimental Example 23, the carbon (C) content was less than 0.48%, so not only did the tensile strength not satisfy 2000 MPa, but the density of carbides was 20 particles / μm 2 The fatigue strength ratio was less than 0.45 and the permanent deformation exceeded 3.0 mm.

[0102] In Experimental Example 24, the carbon (C) content exceeded 0.62% and the carbide density was 64 particles / μm 2 The average thickness of the carbides exceeds 12 nm, and the dislocation density is 6.0 × 10 13 / m 2 The fatigue strength ratio was less than 0.45 and the permanent deformation exceeded 3.0 mm.

[0103] Experimental Example 25 corresponds to a silicon (Si) content of less than 1.5%, an average thickness of carbides exceeds 12 nm, and a dislocation density of 6.0 × 10 13 / m 2 The fatigue strength ratio was less than 0.45, the tensile strength was less than 2000 MPa, and the permanent deformation was more than 3.0 mm.

[0104] In Experimental Example 26, the silicon (Si) content exceeded 2.0%, and a decarburized layer was formed. In addition, the average thickness of the carbides was less than 5 nm, and the dislocation density was 6.0 × 10 13 / m 2 The fatigue strength ratio was less than 0.45 and the permanent deformation exceeded 3.0 mm.

[0105] Experimental Example 27 corresponds to a manganese (Mn) content of less than 0.2%, and the fatigue strength ratio was less than 0.45 due to MnS inclusions.

[0106] Experimental Example 28 has a manganese (Mn) content of more than 0.55% and a dislocation density of 3.0 × 10 14 / m 2 As a result, the fatigue strength ratio was less than 0.45 and the permanent deformation exceeded 3.0 mm.

[0107] Experimental Example 29 corresponds to a case where the chromium (Cr) content is less than 0.5% and the dislocation density is 3.0 × 10 14 / m 2 This resulted in a permanent deformation exceeding 3.0 mm.

[0108] In Experimental Example 30, the chromium (Cr) content was more than 0.8%, the average thickness of the carbides was less than 5 nm, and the carbide density was 64 particles / μm 2 The fatigue strength ratio was less than 0.45 and the permanent deformation exceeded 3.0 mm.

[0109] In Experimental Example 31, the finish rolling temperature was less than 930°C, a decarburized surface layer was formed, and the fatigue strength ratio was less than 0.45.

[0110] In Experimental Example 32, the reheating temperature was less than 900°C, and the average thickness of the carbides exceeded 12 nm due to the residual undissolved pearlite, the fatigue strength ratio was less than 0.45, and the permanent deformation exceeded 3.0 mm.

[0111] In Experimental Example 33, the tempering temperature was less than 415°C, the average thickness of the carbides was less than 5 nm, and the carbide density was 64 particles / μm 2 and the dislocation density also exceeds 3.0×10 14 / m 2 The fatigue strength ratio exceeded 0.45 and the permanent deformation exceeded 3.0 mm. The tempering temperature was low, the decomposition of retained austenite was small, and the temper martensite fraction was less than 90%.

[0112] In Experimental Example 34, the tempering temperature exceeded 465°C, the average thickness of the carbides exceeded 12 nm, and the carbide density was 20 particles / μm 2 The dislocation density is less than 6.0 × 10 13 / m 2 The tensile strength was less than 2000 MPa, and the permanent deformation resistance was more than 3.0 mm.

Claims

1. Contains tempered martensite structure, The spring is characterized in that the Fe carbides precipitated in the tempered martensite structure have a thickness of 100 nm or less and an average thickness of 5 nm to 12 nm.

2. 2. The spring according to claim 1, wherein the tempered martensite structure accounts for 90% or more of the area fraction.

3. The density of the carbides having a thickness of 100 nm or less is 20 / μm ２ The spring of claim 1 .

4. The density of the carbides having a thickness of 100 nm or less is 40 / μm ２ The spring of claim 1 .

5. The density of the carbides having a thickness of 100 nm or less is 64 / μm ２ 2. The spring of claim 1, wherein:

6. Dislocation density is 6.0 × 10 １３ / m ２ The spring of claim 1 .

7. Dislocation density is 3.0 × 10 １４ / m ２ 2. The spring of claim 1, wherein:

8. 2. The spring according to claim 1, comprising, by weight, C: 0.48 to 0.62%, and Si: 1.5 to 2.0%.

9. 9. The spring according to claim 8, comprising, by weight, Mn: 0.2 to 0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5 to 0.8%, N: 0.003 to 0.015%, and the remainder being Fe and unavoidable impurities.

10. 2. The spring according to claim 1, wherein the carbide having a thickness of 100 nm or less has an average thickness of 6 nm to 10 nm.

11. 2. The spring according to claim 1, wherein the ratio of the minimum outer diameter (D) to the diameter (d) is 4 or greater (D / d≧4).

12. 2. The spring of claim 1, wherein the spring has a tensile strength of 2000 MPa or more.

13. 2. The spring according to claim 1, wherein the permanent deformation, which is the difference in height before and after compression, is 3.0 mm or less when compressed at a stress of 1274 MPa for 48 hours.

14. Contains a tempered martensite structure with an area fraction of 90% or more, The spring steel wire is characterized in that, among the Fe carbides precipitated in the tempered martensite structure, the carbides having a thickness of 100 nm or less have an average thickness of 5 nm to 12 nm.

15. The spring steel wire according to claim 14, comprising, by weight%, C: 0.48 to 0.62%, and Si: 1.5 to 2.0%.

16. 16. The spring steel wire according to claim 15, comprising, by weight%, Mn: 0.2 to 0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5 to 0.8%, N: 0.003 to 0.015%, and the remainder being Fe and unavoidable impurities.

17. 15. The spring steel wire according to claim 14, wherein the carbides having a thickness of 100 nm or less have an average thickness of 6 nm to 10 nm.

18. The density of the carbides having a thickness of 100 nm or less is 40 / μm ２ ~64 pieces / μm ２ 15. The spring steel wire according to claim 14, wherein

19. 15. The spring steel wire according to claim 14, wherein the steel wire has a fatigue strength ratio of 0.45 or more.

20. A spring wire rod comprising, by weight, C: 0.48 to 0.62%, Si: 1.5 to 2.0%, Mn: 0.2 to 0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5 to 0.8%, N: 0.003 to 0.015%, and the remainder being Fe and unavoidable impurities.

21. 21. The spring wire according to claim 20, wherein the wire has a ferrite decarburized layer thickness of 0.03 mm or less.

22. producing a billet consisting of, in weight percent, 0.48 to 0.62% C, 1.5 to 2.0% Si, 0.2 to 0.55% Mn, 0.015% or less P, 0.020% or less S, 0.5 to 0.8% Cr, 0.003 to 0.015% N, and the remainder being Fe and unavoidable impurities; Finish rolling the billet at 930°C to 1010°C to produce a wire rod; and cooling the wire rod after finish rolling to 730°C at a cooling rate exceeding 10°C / s.

23. a step of manufacturing a wire rod consisting of, in weight percent, C: 0.48 to 0.62%, Si: 1.5 to 2.0%, Mn: 0.2 to 0.55%, P: 0.015% or less, S: 0.020% or less, Cr: 0.5 to 0.8%, N: 0.003 to 0.015%, and the remainder being Fe and unavoidable impurities; Reheating at 900°C to 990°C followed by quenching; and tempering the quenched wire at 415°C to 465°C.

24. The method for manufacturing a spring steel wire according to claim 23, further comprising a step of performing a secondary tempering at 300°C to 400°C after the tempering step.

25. The steel wire contains a tempered martensite structure in an area fraction of 90% or more, Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides having a thickness of 100 nm or less is 5 nm to 12 nm, The density of the carbide is 20 particles / μm ２ ~64 pieces / μm ２ and Dislocation density is 6.0 × 10 １３ / m ２ ~3.0 x 10 １４ / m ２ 24. The method for producing a spring steel wire according to claim 23, wherein

26. cold forming a compression coil spring manufactured by the manufacturing method according to claim 23, in which the ratio of the diameter (d) of the steel wire to the minimum outer diameter (D) of the spring product is 4 or more (D / d≧4); and performing a stress relief heat treatment at 190°C to 290°C.

27. The spring contains a tempered martensite structure at an area fraction of 90% or more, Among the Fe carbides precipitated in the tempered martensite structure, the average thickness of the carbides having a thickness of 100 nm or less is 5 nm to 12 nm, The density of the carbide is 20 particles / μm ２ ~64 pieces / μm ２ and Dislocation density is 6.0 × 10 １３ / m ２ ~3.0 x 10 １４ / m ２ 27. The method for manufacturing a spring according to claim 26, wherein:

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