Spring steel wire

Abstract

A spring steel wire has: a main body portion made of steel and having a wire shape; and an oxide layer covering an outer peripheral surface of the main body portion. The steel constituting the main body portion contains 0.6 mass% or more and 0.7 mass% or less of C, 1.7 mass% or more and 2.5 mass% or less of Si, 0.2 mass% or more and 1 mass% or less of Mn, 0.6 mass% or more and 2 mass% or less of Cr, and 0.08 mass% or more and 0.25 mass% or less of V, with the remainder consisting of Fe and inevitable impurities. The structure of the steel constituting the main body portion is a tempered martensite structure. The oxide layer includes a high-Si concentration layer in which the maximum concentration of Si is 2.5 times or more and 5.5 times or less of that of the main body portion. The main body portion includes a grain boundary oxide layer configured to constitute the outer peripheral surface and having a thickness of 0.5 μm or more and 2.5 μm or less.

Description

Technical Field

[0001] This disclosure relates to steel wire for springs.

[0002] This application claims priority based on Japanese Application No. 2021-128969, filed on August 5, 2021, and invokes the entire contents of the stated Japanese application. Background Technology

[0003] Various oil-tempered steel wires (spring wires) are known for improving the workability of springs (for example, see Japanese Patent Application Publication No. 2004-115859 (Patent Document 1), Japanese Patent Application Publication No. 2018-12868 (Patent Document 2), and Japanese Patent Application Publication No. 2017-115228 (Patent Document 3)). Furthermore, oil-tempered steel wires are known for improving the fatigue strength of springs (for example, see Japanese Patent Application Publication No. 2004-315968 (Patent Document 4), Japanese Patent Application Publication No. 2006-183136 (Patent Document 5), Japanese Patent Application Publication No. 2008-266725 (Patent Document 6), International Publication No. 2013 / 024876 (Patent Document 7), Japanese Patent Application Publication No. 2012-077367 (Patent Document 8), and International Publication No. 2015 / 115574 (Patent Document 9)).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2004-115859

[0007] Patent Document 2: Japanese Patent Application Publication No. 2018-12868

[0008] Patent Document 3: Japanese Patent Application Publication No. 2017-115228

[0009] Patent Document 4: Japanese Patent Application Publication No. 2004-315968

[0010] Patent Document 5: Japanese Patent Application Publication No. 2006-183136

[0011] Patent Document 6: Japanese Patent Application Publication No. 2008-266725

[0012] Patent Document 7: International Publication No. 2013 / 024876

[0013] Patent Document 8: Japanese Patent Application Publication No. 2012-077367

[0014] Patent Document 9: International Publication No. 2015 / 115574 Summary of the Invention

[0015] The spring steel wire according to this disclosure comprises: a steel body having a linear shape; and an oxide layer covering the outer peripheral surface of the body. The steel constituting the body contains 0.6% to 0.7% by mass of C (carbon), 1.7% to 2.5% by mass of Si (silicon), 0.2% to 1% by mass of Mn (manganese), 0.6% to 2% by mass of Cr (chromium), and 0.08% to 0.25% by mass of V (vanadium), with the remainder consisting of Fe (iron) and unavoidable impurities. The microstructure of the steel constituting the body is tempered martensitic. The oxide layer comprises a high Si concentration layer, with a maximum Si concentration of 2.5 to 5.5 times that of the body. The body includes a grain boundary oxide layer configured to form the outer peripheral surface and having a thickness of 0.5 μm to 2.5 μm. Attached Figure Description

[0016] Figure 1 It is a schematic diagram showing the structure of the steel wire used for springs.

[0017] Figure 2 This is a schematic cross-sectional view showing the structure of the steel wire used for the spring.

[0018] Figure 3 This is a schematic cross-sectional view showing the structure near the boundary between the main body of the spring wire and the oxide layer.

[0019] Figure 4 This is a flowchart outlining the manufacturing process of steel wire for springs. Detailed Implementation

[0020] [The problem this disclosure aims to solve]

[0021] As disclosed in Patent Documents 1-3, an oxide layer is sometimes formed on the surface of spring steel wire to improve the workability of the spring. Furthermore, as disclosed in Patent Documents 4-9, spring steel wire that can improve the fatigue strength of the spring is required. As one measure to improve the fatigue strength of the spring, nitriding is sometimes performed.

[0022] However, according to the research conducted by the present invention, the following tendency has been found: if an oxide layer is formed on the surface for the purpose of improving the workability of the spring, the fatigue strength of the spring will not increase sufficiently even when nitriding treatment is performed. Therefore, one of the objects of this disclosure is to provide a spring steel wire that can simultaneously improve the workability of the spring and improve the fatigue strength of the spring.

[0023] [The Effects of This Disclosure]

[0024] Based on the above-mentioned steel wire for springs, both the machinability of the springs and the fatigue strength of the springs can be improved.

[0025] [Description of embodiments of this disclosure]

[0026] First, embodiments of this disclosure will be described. The spring steel wire of this disclosure comprises: a steel body having a linear shape; and an oxide layer covering the outer peripheral surface of the body. The steel constituting the body contains 0.6% by mass or more and 0.7% by mass of C, 1.7% by mass or more and 2.5% by mass of Si, 0.2% by mass or more and 1% by mass of Mn, 0.6% by mass or more and 2% by mass of Cr, and 0.08% by mass or more and 0.25% by mass of V, with the remainder consisting of Fe and unavoidable impurities. The microstructure of the steel constituting the body is tempered martensitic. The oxide layer comprises a high Si concentration layer, the maximum Si concentration of which is 2.5 times or more and 5.5 times that of the body. The body includes a grain boundary oxide layer configured to form the outer peripheral surface and having a thickness of 0.5 μm or more and 2.5 μm or less.

[0027] The inventors investigated the cause of the following phenomenon: if an oxide layer is formed on the surface with the aim of improving the workability of the spring, the fatigue strength of the spring will not increase sufficiently even after nitriding treatment. As a result, it was found that the diffusion of Si affects the nitriding process, which led to the invention.

[0028] Specifically, when the surface of the spring steel wire is oxidized, an oxide layer composed of Fe oxides is formed on the surface of the spring steel wire. At this time, although Si and Cr contained in the steel constituting the spring steel wire have a high affinity for oxygen, just like Fe, the diffusion rate of Si and Cr is lower than that of Fe, so they cannot reach the oxide layer and remain near the outer peripheral surface of the main body. As a result, a layer with a high concentration of Si and Cr is formed near the outer peripheral surface of the main body. Si and Cr also have a high affinity for N (nitrogen). Therefore, when the spring steel wire is processed into the shape of a spring, the oxide layer is removed, and further nitriding is performed, the N penetrating from the surface will form compounds with Si and Cr and be captured near the surface, thus hindering the penetration of N into the interior. As a result, the thickness of the nitriding layer, which contributes to the improvement of fatigue strength, becomes smaller, and thus the fatigue strength is not sufficiently improved.

[0029] On the other hand, when oxidation is further carried out during oxide layer formation, Si near the outer peripheral surface of the main body diffuses into the oxide layer, forming a high Si concentration layer within the oxide layer. This reduces the Si concentration near the outer peripheral surface of the main body, and a grain boundary oxide layer is formed on the main body in a manner that constitutes the outer peripheral surface of the main body. The grain boundary oxide layer is a layer formed by the intrusion of oxygen along the grain boundaries of old austenite grains, where the diffusion rate of elements is faster than in other parts. According to the present invention, by including a high Si concentration layer in the oxide layer, where the maximum Si concentration is 2.5 times that of the main body and less than 5.5 times, and by proceeding oxidation to the point where the thickness of the grain boundary oxide layer is 0.5 μm or more and less than 2.5 μm, Si that traps N near the surface diffuses into the oxide layer, thereby sufficiently reducing the Si concentration near the surface. As a result, the thickness of the nitrided layer formed during the nitriding treatment increases, thereby improving the fatigue strength of the spring. When the maximum Si concentration in the oxide layer is less than 2.5 times that of the main body and the thickness of the grain boundary oxide layer is less than 0.5 μm, the diffusion of Si into the oxide layer becomes insufficient, and the thickness of the nitrided layer does not increase sufficiently. When the maximum concentration of Si in the oxide layer exceeds 5.5 times that of the main body and the thickness of the grain boundary oxide layer exceeds 2.5 μm, Cr, V and other substances that contribute to the increase of the hardness of the main body will diffuse into the oxide layer, thereby reducing the hardness of the main body and the fatigue strength of the spring.

[0030] In the spring steel wire of this disclosure, the content of each constituent element of the steel constituting the main body is appropriately set, and the steel constituting the main body has a tempered martensitic structure. Furthermore, the main body is covered with an oxide layer that contributes to improving the workability of the spring. The oxide layer includes a high Si concentration layer, where the maximum Si concentration is 2.5 times or more and 5.5 times that of the main body, and the oxidation proceeds to the point where the thickness of the grain boundary oxide layer is 0.5 μm or more and 2.5 μm or less. This allows for improved workability of the spring due to the formation of the oxide layer, while suppressing the hindrance of Si to the formation of the nitride layer, thereby improving the fatigue strength of the spring. Thus, the spring steel wire of this disclosure achieves both improved workability and improved fatigue strength of the spring.

[0031] The reasons why the composition of the steel constituting the main body should be set within the above range will be explained below.

[0032] Carbon (C): ≥0.6% by mass and ≤0.7% by mass

[0033] Carbon (C) is an element that significantly affects the strength of steel with a tempered martensitic structure. For spring steel wire, from the viewpoint of obtaining sufficient strength, the C content needs to be set at 0.6% by mass or higher. On the other hand, when the C content increases, the toughness decreases, and processing may become difficult. From the viewpoint of ensuring sufficient toughness, the C content needs to be set at 0.7% by mass or lower.

[0034] Silicon (Si): ≥1.7% by mass and ≤2.5% by mass

[0035] Si possesses properties that inhibit softening caused by heating (softening resistance). Furthermore, Si increases the hardness of the steel in the area (interior) outside the nitrided layer formed after processing into the spring. From the viewpoint of inhibiting softening caused by heating during spring steel wire processing and spring use, and increasing the hardness of the steel, thereby increasing the fatigue strength of the spring, the Si content needs to be set at 1.7% by mass or more, or even 1.8% by mass or more. On the other hand, excessive addition of Si will decrease toughness. From the viewpoint of ensuring sufficient toughness, the Si content needs to be set at 2.5% by mass or less. From the viewpoint of emphasizing toughness, the Si content can also be set at 2.0% by mass or less.

[0036] Manganese (Mn): ≥0.2% by mass and ≤1% by mass

[0037] Mn is an element added as a deoxidizer in steel refining. To function effectively as a deoxidizer, the Mn content needs to be at least 0.2% by mass, preferably at least 0.3% by mass. On the other hand, excessive addition of Mn will decrease toughness. Therefore, the Mn content needs to be at least 1% by mass, or even less than 0.5% by mass.

[0038] Chromium (Cr): ≥0.6% by mass and ≤2% by mass

[0039] Cr improves the hardenability of steel. Furthermore, Cr functions as a carbide-forming element in steel, contributing to the refinement of the metal structure through the formation of fine carbides and suppressing softening during heating. From the viewpoint of reliably achieving these effects, Cr needs to be added at least 0.6% by mass, preferably at least 1.7% by mass. On the other hand, excessive Cr addition leads to a decrease in toughness. Therefore, the amount of Cr added should be 2% by mass or less, preferably 1.9% by mass or less.

[0040] Vanadium (V): ≥0.08% by mass and ≤0.25% by mass

[0041] V also functions as a carbide-forming element in steel, contributing to the refinement of the metal structure through the formation of fine carbides and suppressing softening during heating. Due to its high solution temperature, V carbides exist insolublely during the quenching and tempering of steel, which is particularly beneficial for the refinement of the metal structure (grain refinement). Furthermore, through nitriding treatment performed after spring machining, V becomes nitride, which can suppress slippage within the crystals under repeated stress loading on the spring, thereby contributing to improved fatigue strength. From the viewpoint of reliably achieving such effects, V needs to be added at least 0.08% by mass, preferably at least 0.1% by mass. On the other hand, excessive addition of V can lead to a decrease in toughness. Therefore, the amount of V added should be set to 0.25% by mass or less, or even 0.2% by mass or less.

[0042] Unavoidable impurities

[0043] In the manufacturing process of steel used for spring wire, phosphorus (P) and sulfur (S) inevitably mix into the steel. When phosphorus and sulfur are present in excess, grain boundary segregation or inclusions can occur, thus deteriorating the properties of the steel. Therefore, the content of phosphorus and sulfur is preferably set to 0.025% by mass or less, respectively. Furthermore, nickel (Ni) and cobalt (Co), as austenite-forming elements, tend to form retained austenite during quenching. Since a large amount of carbon can be dissolved in retained austenite, the carbon content in martensite will decrease, potentially leading to a decrease in the hardness of the steel constituting the main body. A decrease in hardness leads to a decrease in fatigue strength. Therefore, Ni and Co are set at contents where they are present as unavoidable impurities rather than intentionally added. In addition, titanium (Ti), niobium (Nb), and molybdenum (Mo), as carbide-forming elements, prolong the time required for pearlite phase transformation during the sorbitization treatment performed before wire drawing, thus leading to a decrease in the manufacturing efficiency of the steel wire. Therefore, Ti, Nb, and Mo are set at contents where they are present as unavoidable impurities rather than intentionally added. The content of Ni, an unavoidable impurity, is, for example, 0.1% by mass or less. The content of Co, an unavoidable impurity, is, for example, 0.1% by mass or less. The content of Ti, an unavoidable impurity, is, for example, 0.005% by mass or less. The content of Nb, an unavoidable impurity, is, for example, 0.05% by mass or less. The content of Mo, an unavoidable impurity, is, for example, 0.05% by mass or less.

[0044] Here, the maximum concentration of Si in the high-Si-concentration layer included in the oxide layer can be determined, for example, by line analysis using EDX (Energy Dispersive X-ray Spectroscopy). Specifically, first, the spring steel wire is cut with a cross-section perpendicular to the longitudinal direction. For the Si concentration in the oxide layer of this cross-section, line analysis is performed from the interface between the main body and the oxide layer toward the oxide layer side in a direction perpendicular to the interface. Then, the ratio to the Si concentration in the main body is calculated. For example, the above process can be repeated three times, and the average value is calculated as the maximum Si concentration. Furthermore, regarding the thickness of the grain boundary oxide layer, the area near the interface between the main body and the oxide layer in the same cross-section is observed using SEM (Scanning Electron Microscope), and for example, the maximum value of the grain boundary oxide layer thickness in three fields of view is measured. Then, their average value can be calculated as the thickness of the grain boundary oxide layer of the spring steel wire.

[0045] In the aforementioned spring steel wire, the oxide layer thickness can be 2 μm or more and 5 μm or less. By setting the oxide layer thickness to 2 μm or more, it is easy to achieve the structure described above, which includes a high Si concentration layer and a grain boundary oxide layer. By setting the oxide layer thickness to 5 μm or less, the increase in manufacturing costs caused by the formation of unnecessary oxide layers can be avoided.

[0046] In the aforementioned steel wire for springs, the oxide layer may contain 80% by mass or more Fe3O4 (iron tetroxide). Based on this configuration, a more effective oxide layer can be obtained that improves the processability of the opposing spring.

[0047] [Details of the embodiments of this disclosure]

[0048] Next, embodiments of the spring wire of this disclosure will be described with reference to the accompanying drawings. It should be noted that in the following drawings, the same or equivalent parts will be labeled with the same reference numerals, and their descriptions will not be repeated.

[0049] Figure 1 It is a schematic diagram showing the structure of the steel wire used for springs. Figure 2 This is a schematic cross-sectional view showing the structure of the steel wire used for the spring. Figure 2 It is a cross-sectional view of the steel wire for springs on a plane perpendicular to the length direction.

[0050] Reference Figure 1 and Figure 2 The spring wire 1 in this embodiment includes: a steel main body 10 having a wire-like shape; and an oxide layer 20 covering the outer peripheral surface 10A of the main body 10. The outer peripheral surface 20A of the oxide layer 20 is the outer peripheral surface of the spring wire 1. (Refer to...) Figure 2 The diameter φ of the steel wire 1 for the spring is, for example, 1.5 mm or more and 8.0 mm or less. The thickness t of the oxide layer 20 is, for example, 2 μm or more and 5 μm or less. The oxide layer 20 contains 80% by mass or more of Fe3O4.

[0051] The steel constituting the main body 10 contains 0.6% to 0.7% by mass of C, 1.7% to 2.5% by mass of Si, 0.2% to 1% by mass of Mn, 0.6% to 2% by mass of Cr, and 0.08% to 0.25% by mass of V, with the remainder consisting of Fe and unavoidable impurities. The microstructure of the steel constituting the main body 10 is tempered martensitic. The spring wire 1 in this embodiment is oil-tempered steel wire.

[0052] Figure 3 This is a schematic cross-sectional view showing the structure near the boundary between the main body of the spring wire and the oxide layer. (See reference...) Figure 3 The oxide layer 20 includes a high Si concentration layer 21, where the maximum Si concentration is 2.5 times or more and 5.5 times or less than that of the main body 10. The main body 10 includes a grain boundary oxide layer 11, which is configured to form the outer peripheral surface 10A and has a thickness of 0.5 μm or more and 2.5 μm or less.

[0053] In the spring steel wire 1 of this embodiment, the content of each constituent element of the steel constituting the main body 10 is appropriately set, and the steel constituting the main body 10 has a tempered martensitic structure. Furthermore, the main body 10 is covered with an oxide layer 20 that helps improve the workability of the spring. The oxide layer 20 includes a high Si concentration layer 21, where the maximum Si concentration is 2.5 times or more and 5.5 times less than that of the main body, and oxidation is carried out to the point where the thickness of the grain boundary oxide layer 11 is 0.5 μm or more and 2.5 μm or less. Therefore, the formation of the oxide layer 20 improves the workability of the spring, and suppresses the hindrance of Si to the formation of the nitride layer, thereby improving the fatigue strength of the spring. Thus, the spring steel wire 1 of this embodiment is a spring steel wire that can simultaneously improve the workability of the spring and improve the fatigue strength of the spring.

[0054] Next, based on Figure 4 An example of a method for manufacturing steel wire 1 for springs will be described. Figure 4 This is a flowchart illustrating a general method for manufacturing the spring wire 1 according to this embodiment. (Refer to...) Figure 4In the manufacturing method of spring steel wire 1 according to this embodiment, the first step is to perform a wire preparation step (S10). In this step (S10), a steel wire containing 0.6% to 0.7% by mass of C, 1.7% to 2.5% by mass of Si, 0.2% to 1% by mass of Mn, 0.6% to 2% by mass of Cr, and 0.08% to 0.25% by mass of V, with the remainder consisting of Fe and unavoidable impurities, is prepared.

[0055] Next, refer to Figure 4 The process involves performing a sorbitizing treatment as step (S20). In this step (S20), the wire prepared in step (S10) undergoes sorbitizing treatment. Specifically, the heat treatment is performed as follows: after the wire is heated to a temperature range above the austenitizing temperature (A1 point), it is rapidly cooled to a temperature higher than the martensitic transformation initiation temperature (M). s The wire is heated to a temperature range above the A1 point and its temperature is maintained within this range. As a result, the wire's microstructure becomes a fine pearlitic structure with small interlamellar spacing. From the viewpoint of suppressing decarburization, the process of heating the wire to a temperature range above the A1 point in the above-described sorbitizing treatment is preferably carried out in an inert gas atmosphere.

[0056] Next, refer to Figure 4 A surface layer removal process is performed as step (S30). In this step (S30), the surface layer of the wire that underwent sorbitization treatment in step (S20) is removed. Specifically, for example, the wire passes through a trimming die, thereby removing the decarburized layer or the like formed on the surface due to the sorbitization treatment. This step is not mandatory, but by performing this step, even if a decarburized layer or the like has formed on the surface due to the sorbitization treatment, it can be removed.

[0057] Next, an annealing process is performed as process (S40). In this process (S40), the wire whose surface layer was removed in process (S30) is annealed. Annealing is a heat treatment performed to soften the wire. In this embodiment, in addition to the above, the following are also performed in this process (S40): formation of oxide layer 20 and grain boundary oxide layer 11, adjustment of the maximum concentration of Si in the high Si concentration layer 21 within oxide layer 20, and adjustment of the thickness of grain boundary oxide layer 11.

[0058] In process (S40), the oxidation of the wire needs to exceed the state of forming a layer with high concentrations of Si and Cr near the outer peripheral surface 10A of the main body 10, and proceed to the state of forming a high Si concentration layer 21 and a grain boundary oxide layer 11. Furthermore, it is necessary to adjust the maximum Si concentration in the high Si concentration layer 21 to a narrow range of 2.5 times to 5.5 times that of the main body 10, and to adjust the thickness of the grain boundary oxide layer 11 to a narrow range of 0.5 μm to 2.5 μm. A typical annealing process is performed in an inert gas atmosphere such as N or Ar. However, from the viewpoint of simultaneously forming the oxide layer 20 and the grain boundary oxide layer 11 with annealing as described above, process (S40) is performed in an oxidizing atmosphere. Moreover, from the viewpoint of proceeding oxidation to the state of forming a high Si concentration layer 21 and a grain boundary oxide layer 11, and from the viewpoint of strictly adjusting the maximum Si concentration in the high Si concentration layer 21 and the thickness of the grain boundary oxide layer 11 as described above, the selection of atmosphere, temperature, and time is crucial. Specifically, it is preferable to impart a suitable oxidizing atmosphere and perform treatment at high temperatures. For example, a heat treatment is carried out using an atmosphere in which water vapor is intentionally mixed into an inert gas, and held at a temperature above 650°C and below 700°C for more than 1 hour and less than 3 hours. The concentration of water vapor can also be set, for example, per 1 m³ of the furnace used for performing the annealing treatment. 3 The volume includes a water vapor concentration of 2L to 3L when converted to liquid form. The pressure inside the furnace can be set to, for example, atmospheric pressure (1 atmosphere).

[0059] It should be noted that, in this embodiment, from the viewpoint of simplifying the manufacturing process, the oxide layer 20 is formed in step (S40). However, the oxide layer 20 can also be formed in a separate step distinct from step (S40). That is, from the viewpoint of performing only the annealing treatment, step (S40) can be carried out in an inert gas atmosphere, while the wire is oxidized in another step. In this case, the aforementioned strict selection of atmosphere, temperature, and time is required in the wire oxidation step.

[0060] Next, a shot peening process is performed as process (S50). In this process (S50), the wire that has undergone annealing treatment in process (S40) and formed an oxide layer 20 is shot peened. This process is not mandatory, but by performing this process, the brittle Fe2O3 formed on the surface of the oxide layer 20 can be removed, and the proportion of Fe3O4 in the oxide layer 20 can be adjusted.

[0061] Next, a wire drawing process is performed as process (S60). In this process (S60), the wire that has been shot-blasted in process (S50) is subjected to wire drawing (stretching). The degree of reduction of area (RR) in the wire drawing process (S60) can be appropriately set, for example, it can be set to 50% or more and 90% or less. Here, "RR" refers to the value obtained by dividing the difference between the cross-sectional area before wire drawing and the cross-sectional area after wire drawing by the cross-sectional area before wire drawing, expressed as a percentage, with respect to the cross-section perpendicular to the length direction of the wire.

[0062] Next, a quenching process is performed as process (S70). In this process (S70), the wire (steel wire) that has undergone wire drawing in process (S60) is heated to a temperature above the A1 point of the steel and then rapidly cooled to M. S Quenching treatment at temperatures below a certain point. More specifically, for example, heat treatment of steel wire by immersion in oil and rapid cooling after heating to a temperature above 800°C and below 1000°C. As a result, the microstructure of the steel constituting the main body becomes martensitic.

[0063] Next, a tempering process is performed as process (S80). In this process (S80), the steel wire that has undergone quenching treatment in process (S70) is subjected to a tempering treatment, which involves heating it to a temperature lower than the steel's A1 point and then cooling it. The heating of the steel wire is performed by immersing the steel wire in oil maintained at a specified temperature. More specifically, for example, the steel wire is heated to a temperature of 400°C or higher and 700°C or lower, and then cooled after being held for a time of 0.5 minutes or more and 20 minutes or less. As a result, the microstructure of the steel constituting the main body 10 becomes a tempered martensitic microstructure. Through the above process, the spring steel wire 1 of this embodiment can be manufactured.

[0064] Example

[0065] The following experiments were conducted: the steel wire for springs of this disclosure was fabricated, processed into springs, and its properties were evaluated, thereby confirming the superiority of the steel wire for springs of this disclosure. The experimental process and results are as follows.

[0066] (1) Fabrication of steel wire for springs

[0067] A steel wire with a diameter φ of 4 mm and having the composition shown in Table 1 was prepared, and an oxide layer 20 was formed by performing the annealing process (S40) of the above embodiment. Annealing was carried out in the furnace for every 1 m... 3The oxidation process was carried out in a nitrogen atmosphere furnace with a volume of 2.5 L of water vapor (equivalent to liquid water) to heat the steel wire to 675°C. The furnace pressure was set to atmospheric pressure (1 atmosphere). The degree of oxidation was varied by changing the holding time at 675°C from 0.5 hours to 4 hours. In Table 1, the values ​​represent the mass percentage (mass%) of each component. Except for Fe, no elements other than C, Si, Mn, Cr, and V shown in Table 1 were intentionally added; the remainder consisted of Fe and unavoidable impurities.

[0068] [Table 1]

[0069] (quality%)

[0070] C Si Mn Cr V Steel wire A 0.65 1.9 0.4 1.8 0.20 Steel wire B 0.63 2.0 0.5 0.7 0.10 Steel wire C 0.67 2.3 0.9 1.3 0.15

[0071] Subsequently, the quenching process (S70) and tempering process (S80) of the above-described embodiment were performed on all steel wires under the same conditions, resulting in oil-tempered steel wire (spring wire) samples with an oxide layer 20 thickness of 3.0 ± 0.3 μm. The maximum concentration of Si in the oxide layer 20 (high Si concentration layer 21) and the thickness of the grain boundary oxide layer 11 were investigated for the obtained samples. The maximum concentration of Si was investigated using line analysis with an Oxford ULTIM MAX170EDX attached to a Carl Zeiss SEM (GeminiSEM450). Then, the concentration of Si at a depth of 1.5 μm from the outer peripheral surface 10A of the main body 10 was measured as the concentration of Si in the main body 10, and the ratio of the maximum concentration of Si in the oxide layer 20 to this concentration was calculated. Line analysis was performed at three locations for each sample. The concentration of Si in the main body 10 and the maximum concentration of Si in the oxide layer 20 were evaluated using the average values ​​at these three locations. Furthermore, the thickness of the grain boundary oxide layer 11 was evaluated using the maximum value from the three fields of view of the SEM corresponding to the three locations where line analysis was performed. The results are shown in Table 2. Referring to Table 2, samples No. 1-3, 8-10, and 15-17 are samples of embodiments that meet the conditions for spring steel wire of this disclosure. Samples No. 4-7, 11-14, and 18-21 are comparative examples that do not meet the conditions for spring steel wire of this disclosure.

[0072] [Table 2]

[0073]

[0074] (2) Investigation of hardness distribution

[0075] The samples listed in Table 2 were subjected to processing to achieve the shape of a compression spring, stress-relieving annealing, removal of oxide layer 20, nitriding, shot peening, and stabilization treatment to obtain compression springs corresponding to each sample. The nitriding treatment was carried out in a nitriding atmosphere at 440°C for 5 hours. Then, the hardness distribution from the surface to a depth of 120 μm was measured using a Vickers hardness tester. The measurement results are shown in Table 3.

[0076] [Table 3]

[0077]

[0078] Referring to Table 3, it can be seen that in samples No. 1-3, 8-10, and 15-17, which are examples, the hardness inside the steel wire, particularly at a depth of 80-100 μm near the maximum depth affected by nitriding, is higher than that in samples No. 4-7, 11-14, and 18-21, which are comparative examples. It can be seen that in samples 6, 7, 13, 14, 20, and 21, which are comparative examples with a high ratio of maximum Si concentration and a large grain boundary oxide layer thickness, the hardening achieved by nitriding, including near the surface, is insufficient. This can be attributed to excessive oxidation, causing Cr, V, and other elements that contribute to increased hardness to diffuse into the oxide layer, thereby reducing the hardness near the surface of the main body (near the surface of the spring). On the other hand, it can be seen that in samples 4, 5, 11, 12, 18, and 19, which are comparative examples with a low ratio of maximum Si concentration and a small grain boundary oxide layer thickness, although the hardness near the surface is sufficient, the hardening inside is insufficient. This is believed to be due to insufficient oxidation, resulting in the formation of a layer with high concentrations of Si and Cr with high affinity for N near the surface of the main body (near the surface of the spring). As a result, N that penetrates from the surface during the nitriding process is captured near the surface, and the thickness of the nitriding layer (the depth to which nitrogen reaches) becomes smaller.

[0079] (3) Investigation of fatigue strength

[0080] Next, eight springs were fabricated for fatigue testing, one each for samples No. 1 to 3 (exemplary samples) and samples No. 4 to 7 (comparative samples). The fatigue tests were conducted under conditions of an average stress of 686 MPa and a stress amplitude of 630 MPa. Then, the results were measured based on 5.0 × 10⁻⁶ cycles. 7 The sum of 1.0 × 10⁻⁶ 8 The number of unbroken springs at each time point was used to evaluate fatigue strength. The results are shown in Table 4.

[0081] [Table 4]

[0082]

[0083] Referring to Table 4, samples No. 1 to 3, which are examples, all exhibit high fatigue strength. However, samples No. 4 and 5, which are comparative examples with lower Si concentration ratios and smaller grain boundary oxide layer thicknesses, show relatively high fatigue strength even after 5.0 × 10⁻⁶ cycles. 7 There was no loss at the time point of the repetition, but at a repetition count of 1.0 × 10⁻⁶. 8 More than half of the components failed at this time point. This is believed to be because, although the high surface hardness ensures a certain level of fatigue strength, the internal hardness is insufficient, thus affecting the 1.0 × 10⁻⁶ mm² fatigue strength. 8 For such long-term fatigue, the strength is insufficient. Samples 6 and 7, used as comparative examples with high Si concentration ratios and large grain boundary oxide layer thickness, also showed similar performance at a replication factor of 1.0 × 10⁻⁶. 8 At each time point, half of the samples were lost. Furthermore, in sample 7, which had the highest Si concentration ratio and the thickest grain boundary oxide layer, the loss was halved at 5.0 × 10⁻⁶ times. 7 The same time point also showed a loss. This is believed to be due to insufficient hardness not only internally but also on the surface, under conditions of maximum Si concentration ratio and large grain boundary oxide layer thickness.

[0084] (4) Summary of experimental results

[0085] Based on the above experimental results, it has been confirmed that in the spring steel wire disclosed herein, although an oxide layer is formed on the surface that helps improve the workability of the spring, it can also suppress the hindrance of Si to the formation of the nitride layer, thereby achieving an improvement in the fatigue strength of the spring.

[0086] It should be understood that the embodiments disclosed herein are illustrative in all respects and are not restrictive in any way. The scope of the invention is not defined by the foregoing description but by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0087] Explanation of reference numerals in the attached figures:

[0088] 1: Steel wire for springs; 10: Main body; 10A: Outer peripheral surface; 11: Grain boundary oxide layer; 20: Oxide layer; 20A: Outer peripheral surface; 21: High Si concentration layer.

Claims

1. A steel wire for springs, comprising: The main body is made of steel and has a linear shape; and An oxide layer covers the outer peripheral surface of the main body. The steel constituting the main body contains 0.6% to 0.7% by mass of carbon, 1.7% to 2.5% by mass of silicon, 0.2% to 1% by mass of manganese, 0.6% to 2% by mass of chromium, and 0.08% to 0.25% by mass of vanadium, with the remainder consisting of iron and unavoidable impurities. The steel constituting the main body has a tempered martensitic structure. The oxide layer includes a high silicon concentration layer with a maximum silicon concentration of 2.5 times to 5.5 times that of the main body portion. This high silicon concentration layer is formed within the oxide layer by silicon diffusion from near the outer peripheral surface of the main body portion. The main body includes a grain boundary oxide layer configured to form an outer peripheral surface and having a thickness of 0.5 μm or more and 2.5 μm or less.

2. The steel wire for springs according to claim 1, wherein, The thickness of the oxide layer is greater than 2 μm and less than 5 μm.

3. The steel wire for springs according to claim 1 or 2, wherein, The oxide layer contains more than 80% by mass of iron(III) oxide.

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