STEEL WIRE FOR SPRINGS

A steel wire for springs includes a main steel body and having a line shape, and an oxidized layer covering an outer peripheral surface of the main body. The steel constituting the main body contains between 0.6 and 0.7 mass % C, between 1.7 and 2.5 mass % Si, between 0.2 and 1 mass % Mn, between 0.6 and 2 mass % Cr, and between 0.08 and 0.25 mass % V, with the balance consisting of Fe and unavoidable impurities. The steel constituting the main body has a tempered martensitic structure. The oxidized layer includes a high Si concentration layer having a maximum concentration of Si of not less than 2.5 times and not more than 5.5 times that of the main body. The main body includes an intergranular oxidized layer arranged to constitute the outer peripheral surface and having a thickness of not less than 0.5 μm and not more than 2.5 μm.

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Description

The present disclosure relates to a steel wire for springs.

The present application claims priority based on Japanese Patent Application No. 2021-128969 filed on Aug. 5, 2021, the entire contents of which are incorporated herein by reference.

BACKGROUND ART

A variety of oil quenched and tempered wires (steel wires for springs) with improved workability into springs are known (see, for example, Japanese Patent Application Laid-Open No. 2004-115859 (Patent Literature 1), Japanese Patent Application Laid-Open No. 2018-12868 (Patent Literature 2), and Japanese Patent Application Laid-Open No. 2017-115228 (Patent Literature 3)). Oil quenched and tempered wires with improved fatigue strength of springs are also known (see, for example, Japanese Patent Application Laid-Open No. 2004-315968 (Patent Literature 4), Japanese Patent Application Laid-Open No. 2006-183136 (Patent Literature 5), Japanese Patent Application Laid-Open No. 2008-266725 (Patent Literature 6), International Publication WO 2013/024876 (Patent Literature 7), Japanese Patent Application Laid-Open No. 2012-077367 (Patent Literature 8), and International Publication WO 2015/115574 (Patent Literature 9)).

CITATION LIST Patent Literature

  • Patent Literature 1: Japanese Patent Application Laid-Open No. 2004-115859
  • Patent Literature 2: Japanese Patent Application Laid-Open No. 2018-12868
  • Patent Literature 3: Japanese Patent Application Laid-Open No. 2017-115228
  • Patent Literature 4: Japanese Patent Application Laid-Open No. 2004-315968
  • Patent Literature 5: Japanese Patent Application Laid-Open No. 2006-183136
  • Patent Literature 6: Japanese Patent Application Laid-Open No. 2008-266725
  • Patent Literature 7: International Publication WO 2013/024876
  • Patent Literature 8: Japanese Patent Application Laid-Open No. 2012-077367
  • Patent Literature 9: International Publication WO 2015/115574

SUMMARY OF INVENTION

A steel wire for springs according to the present disclosure includes: a main body made of a steel and having a line shape; and an oxidized layer covering an outer peripheral surface of the main body. The steel constituting the main body contains not less than 0.6 mass % and not more than 0.7 mass % carbon (C), not less than 1.7 mass % and not more than 2.5 mass % silicon (Si), not less than 0.2 mass % and not more than 1 mass % manganese (Mn), not less than 0.6 mass % and not more than 2 mass % chromium (Cr), and not less than 0.08 mass % and not more than 0.25 mass % vanadium (V), with the balance consisting of iron (Fe) and unavoidable impurities. The steel constituting the main body has a tempered martensitic structure. The oxidized layer includes a high Si concentration layer having a maximum concentration of Si of not less than 2.5 times and not more than 5.5 times that of the main body. The main body includes an intergranular oxidized layer arranged to constitute the outer peripheral surface and having a thickness of not less than 0.5 μm and not more than 2.5 μm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating the structure of a steel wire for springs.

FIG. 2 is a schematic cross-sectional view illustrating the structure of the steel wire for springs.

FIG. 3 is a schematic cross-sectional view illustrating the structure of an area around the boundary between a main body and an oxidized layer of the steel wire for springs.

FIG. 4 is a flowchart schematically illustrating a method of producing the steel wire for springs.

 DESCRIPTION OF EMBODIMENTS Problems to be Solved by Present Disclosure

As disclosed in Patent Literatures 1 to 3, an oxidized layer may be formed on a surface of a steel wire for springs for the purpose of improving workability into springs. Further, as disclosed in Patent Literatures 4 to 9, there is a need for a steel wire for springs that is capable of improving the fatigue strength of springs. Nitriding treatment may be conducted as one of the measures for improving the fatigue strength of springs.

However, according to the studies of the present inventors, when an oxidized layer is formed on the surface for the purpose of improving the workability into springs, the fatigue strength of the springs tends not to be sufficiently increased even if the nitriding treatment is conducted. Thus, one of the objects is to provide a steel wire for springs that can improve both the workability into springs and the fatigue strength of springs.

Advantageous Effects of Present Disclosure

The above-described steel wire for springs is capable of improving both the workability into springs and the fatigue strength of springs.

Description of Embodiments of Present Disclosure

Embodiments of the present disclosure will be listed and described first. The steel wire for springs of the present disclosure includes: a main body made of a steel and having a line shape; and an oxidized layer covering an outer peripheral surface of the main body. The steel constituting the main body contains not less than 0.6 mass % and not more than 0.7 mass % C, not less than 1.7 mass % and not more than 2.5 mass % Si, not less than 0.2 mass % and not more than 1 mass % Mn, not less than 0.6 mass % and not more than 2 mass % Cr, and not less than 0.08 mass % and not more than 0.25 mass % V, with the balance consisting of Fe and unavoidable impurities. The steel constituting the main body has a tempered martensitic structure. The oxidized layer includes a high Si concentration layer having a maximum concentration of Si of not less than 2.5 times and not more than 5.5 times that of the main body. The main body includes an intergranular oxidized layer arranged to constitute the outer peripheral surface and having a thickness of not less than 0.5 μm and not more than 2.5 μm.

The present inventors investigated the reasons why the fatigue strength of springs is not sufficiently increased, even if nitriding treatment is conducted, when an oxidized layer is formed on the surface for the purpose of improving the workability into springs. As a result, the inventors have found that the diffusion of Si affects the progress of nitriding, and arrived at the present invention.

Specifically, when a surface of a steel wire for springs is oxidized, an oxidized layer composed of Fe oxides is formed on the surface of the steel wire. At this time, although Si and Cr contained in the steel constituting the steel wire for springs have a high affinity for oxygen like Fe, their diffusion rates are smaller than that of Fe, so Si and Cr cannot reach the oxidized layer and remain in the vicinity of the outer peripheral surface of the main body. This results in the formation of a layer with high concentrations of Si and Cr in the vicinity of the outer peripheral surface of the main body. Si and Cr also have a high affinity for nitrogen (N). Therefore, after the steel wire for springs is worked into a spring shape, when the oxidized layer is removed and further the nitriding treatment is conducted, N that has penetrated from the surface forms compounds with Si and Cr, and thus is trapped in the vicinity of the surface and prevented from penetrating into the interior. As a result, a nitrided layer contributing to improved fatigue strength becomes small in thickness, hindering sufficient improvement of the fatigue strength.

On the other hand, if the oxidization is made further progress during the formation of the oxidized layer, Si in the vicinity of the outer peripheral surface of the main body diffuses into the oxidized layer. In the oxidized layer, a high Si concentration layer is formed, and the amount of Si in the vicinity of the outer peripheral surface of the main body is decreased. In the main body, an intergranular oxidized layer is formed, constituting the outer peripheral surface of the main body. The intergranular oxidized layer is a layer in which oxygen has penetrated along the prior austenite grain boundaries where the diffusion of elements is faster than in the other portions. According to the investigations of the present inventors, making the oxidization progress to the extent that the oxidized layer includes a high Si concentration layer having a maximum concentration of Si of not less than 2.5 times and not more than 5.5 times that of the main body and the intergranular oxidized layer has a thickness of not less than 0.5 μm and not more than 2.5 μm allows Si, which would trap N in the vicinity of the surface, to diffuse into the oxidized layer, and the amount of Si in the vicinity of the surface is sufficiently decreased. As a result, the nitrided layer formed with the nitriding treatment becomes greater in thickness, leading to improved fatigue strength of springs. If the maximum concentration of Si in the oxidized layer is less than 2.5 times that of the main body, or if the thickness of the intergranular oxidized layer is less than 0.5 μm, the diffusion of Si into the oxidized layer becomes insufficient, leading to an insufficient thickness of the nitrided layer. If the maximum concentration of Si in the oxidized layer exceeds 5.5 times that of the main body, or if the thickness of the intergranular oxidized layer exceeds 2.5 μm, Cr and V, which contribute to increased hardness of the main body, diffuse into the oxidized layer, leading to decreased hardness of the main body and reduced fatigue strength of springs.

In the steel wire for springs of the present disclosure, the contents of the constituent elements of the steel constituting the main body are appropriately set, and the steel constituting the main body has the tempered martensitic structure. The main body is covered with the oxidized layer, which contributes to improved workability into springs. The oxidization has progressed to the extent that the oxidized layer includes the high Si concentration layer having the maximum concentration of Si of not less than 2.5 times and not more than 5.5 times that of the main body and the thickness of the intergranular oxidized layer is not less than 0.5 μm and not more than 2.5 μm. With the above, it is possible to suppress the inhibition of the formation of the nitrided layer by Si to thereby achieve an improvement in the fatigue strength of springs, while achieving an improvement in the workability into springs through the formation of the oxidized layer. Thus, the steel wire for springs of the present disclosure is capable of improving both the workability into springs and the fatigue strength of springs.

The reasons for limiting the component composition of the steel constituting the main body to the above-described ranges will now be described.

Carbon (C): Not Less than 0.6 Mass % and not More than 0.7 Mass %

C is an element that greatly affects the strength of a steel having a tempered martensitic structure. For achieving sufficient strength as a steel wire for springs, the C content is required to be 0.6 mass % or more. On the other hand, an increased C content may reduce toughness, leading to difficulty in working. For ensuring sufficient toughness, the C content is required to be 0.7 mass % or less.

Silicon (Si): Not Less than 1.7 Mass % and not More than 2.5 Mass %

Si has the property of suppressing softening due to heating (resistance to softening). Si also increases the hardness of the steel in an area (the interior) other than a nitrided layer that is formed after working of wire into a spring. For suppressing the softening due to heating at the time of working the steel wire into a spring and at the time of using the spring, and for increasing the hardness of the steel to thereby increase the fatigue strength of the spring, the Si content is required to be 1.7 mass % or more, and it may be 1.8 mass % or more. On the other hand, Si added in an excessive amount will reduce toughness. For ensuring sufficient toughness, the Si content is required to be 2.5 mass % or less. From the standpoint of focusing on the toughness, the Si content may be 2.0 mass % or less.

Manganese (Mn): Not Less than 0.2 Mass % and not More than 1 Mass %

Mn is an element added as a deoxidizing agent in steel refining. To achieve the function as the deoxidizing agent, the Mn content is required to be 0.2 mass % or more, and preferably 0.3 mass % or more. On the other hand, Mn added in an excessive amount will reduce toughness. Therefore, the Mn content is required to be 1 mass % or less, and may be 0.5 mass % or less.

Chromium (Cr): Not Less than 0.6 Mass % and not More than 2 Mass %

Cr has an effect of improving the hardenability of steel. Further, Cr functions as a carbide-forming element in the steel and contributes, through the formation of fine carbides, to the refinement of the metal structure and the suppression of softening during heating. To ensure that these effects are achieved, Cr is required to be added in an amount of 0.6 mass % or more, and preferably added in an amount of 1.7 mass % or more. On the other hand, Cr added in an excessive amount will cause degradation of toughness. Thus, the amount of Cr added is required to be 2 mass % or less, and preferably 1.9 mass % or less.

Vanadium (V): Not Less than 0.08 Mass % and not More than 0.25 Mass %

V also functions as a carbide-forming element in the steel and contributes, through the formation of fine carbides, to the refinement of the metal structure and the suppression of softening during heating. V carbides, having a high dissolution temperature, are present without being dissolved during quenching and tempering of the steel, so they contribute particularly greatly to the refinement of the metal structure (refinement of crystal grains). Further, the nitriding treatment performed after the working of wire into a spring forms V nitrides, which may suppress the occurrence of slippage in crystals when repeated stress is applied to the spring, thereby contributing to the improvement in fatigue strength. To ensure that these effects are achieved, V is required to be added in an amount of 0.08 mass % or more, and preferably added in an amount of 0.1 mass % or more. On the other hand, V added in an excessive amount will cause degradation of toughness. Thus, the amount of V added is required to be 0.25 mass % or less, and may be 0.2 mass % or less.

Unavoidable Impurities

During the process of producing the steel constituting a steel wire for springs, phosphorus (P), sulfur(S), and the like are inevitably mixed into the steel. Phosphorus and sulfur contained in excessive amounts will cause grain boundary segregation and produce inclusions, thereby deteriorating the properties of the steel. Therefore, the phosphorus content and sulfur content are each preferably 0.025 mass % or less. Nickel (Ni) and cobalt (Co), which are austenite-forming elements, tend to form residual austenite during quenching. In the residual austenite, C may be dissolved in a large amount, which decreases the amount of carbon in the martensite, probably causing a reduction in hardness of the steel constituting the main body. The reduced hardness leads to a reduced fatigue strength. Therefore, Ni and Co are contained in an amount present as unavoidable impurities, without being added intentionally. Further, titanium (Ti), niobium (Nb), and molybdenum (Mo), which are carbide-forming elements, prolong the time required for pearlite transformation in the patenting treatment performed before wire drawing, thereby reducing the efficiency of steel wire production. Therefore, Ti, Ni, and Mo are contained in an amount present as unavoidable impurities, without being added intentionally. The content of Ni as an unavoidable impurity is 0.1 mass % or less, for example. The content of Co as an unavoidable impurity is 0.1 mass % or less, for example. The content of Ti as an unavoidable impurity is 0.005 mass % or less, for example. The content of Nb as an unavoidable impurity is 0.05 mass % or less, for example. The content of Mo as an unavoidable impurity is 0.05 mass % or less, for example.

Here, the maximum concentration of Si in the high Si concentration layer included in the oxidized layer can be measured, for example, by linear analysis using energy dispersive X-ray spectroscopy (EDX). Specifically, the steel wire for springs is first cut in a cross section perpendicular to the longitudinal direction. The concentration of Si in the oxidized layer in the cross section is examined by conducting linear analysis from the interface between the main body and the oxidized layer toward the oxidized layer, in the direction perpendicular to the interface. Then, the ratio of the resultant concentration to the Si concentration in the main body is calculated. This can be repeated three times, for example, to calculate the average thereof as the maximum concentration of Si. For the thickness of the intergranular oxidized layer, the area around the interface between the main body and the oxidized layer in the same cross section as above is observed using a scanning electron microscope (SEM), to measure a maximum thickness of the intergranular oxidized layer in three fields of view, for example. Then, the average thereof can be calculated as the thickness of the intergranular oxidized layer in the steel wire for springs.

In the above-described steel wire for springs, the oxidized layer may have a thickness of not less than 2 μm and not more than 5 μm. Setting the thickness of the oxidized layer to 2 μm or more facilitates achieving the structure including the high Si concentration layer and the intergranular oxidized layer as described above. Setting the thickness of the oxidized layer to 5 μm or less can avoid the increase in production cost due to the formation of an oxidized layer more than required.

In the above-described steel wire for springs, the oxidized layer may contain not less than 80 mass % triiron tetraoxide (Fe3O4). With this configuration, an oxidized layer more effective in improving the workability into springs can be obtained.

Details of Embodiments of Present Disclosure

An embodiment of the steel wire for springs of the present disclosure will be described below with reference to the drawings. In the drawings referenced below, the same or corresponding portions are denoted by the same reference numerals and the description thereof will not be repeated.

FIG. 1 is a schematic view illustrating the structure of a steel wire for springs. FIG. 2 is a schematic cross-sectional view illustrating the structure of the steel wire for springs. FIG. 2 is a cross-sectional view in a plane perpendicular to the longitudinal direction of the steel wire for springs.

Referring to FIGS. 1 and 2, the steel wire 1 for springs in the present embodiment includes a main body 10 made of a steel and having a line shape, and an oxidized layer 20 covering an outer peripheral surface 10A of the main body 10. An outer peripheral surface 20A of the oxidized layer 20 is the outer peripheral surface of the steel wire 1 for springs. Referring to FIG. 2, the steel wire 1 for springs has a diameter ϕ of, for example, not less than 1.5 mm and not more than 8.0 mm. The oxidized layer 20 has a thickness t of, for example, not less than 2 μm and not more than 5 μm. The oxidized layer 20 contains not less than 80 mass % Fe3O4.

The steel constituting the main body 10 contains not less than 0.6 mass % and not more than 0.7 mass % C, not less than 1.7 mass % and not more than 2.5 mass % Si, not less than 0.2 mass % and not more than 1 mass % Mn, not less than 0.6 mass % and not more than 2 mass % Cr, and not less than 0.08 mass % and not more than 0.25 mass % V, with the balance consisting of Fe and unavoidable impurities. The steel constituting the main body 10 has a tempered martensitic structure. The steel wire 1 for springs in the present embodiment is an oil quenched and tempered wire.

FIG. 3 is a schematic cross-sectional view illustrating the structure around the boundary between the main body and the oxidized layer of the steel wire for springs. Referring to FIG. 3, the oxidized layer 20 includes a high Si concentration layer 21 having a maximum concentration of Si of not less than 2.5 times and not more than 5.5 times that of the main body 10. The main body 10 includes an intergranular oxidized layer 11 arranged to constitute the outer peripheral surface 10A and having a thickness of not less than 0.5 μm and not more than 2.5 μm.

In the steel wire 1 for springs of the present embodiment, the contents of the constituent elements of the steel constituting the main body 10 are appropriately set, and the steel constituting the main body 10 has a tempered martensitic structure. The main body 10 is covered with the oxidized layer 20, which contributes to improved workability into springs. Further, the oxidation has progressed to the extent that the oxidized layer 20 includes the high Si concentration layer 21 having the maximum concentration of Si of not less than 2.5 times and not more than 5.5 times that of the main body and the intergranular oxidized layer 11 has the thickness of not less than 0.5 μm and not more than 2.5 μm. With the above, it is possible to suppress the inhibition of the formation of the nitrided layer by Si to thereby achieve an improvement in the fatigue strength of springs, while achieving an improvement in the workability into springs through the formation of the oxidized layer 20. Thus, the steel wire 1 for springs of the present embodiment is a steel wire for springs that is capable of improving both the workability into springs and the fatigue strength of springs.

An example of a method of producing the steel wire 1 for springs will now be described with reference to FIG. 4. FIG. 4 is a flowchart schematically illustrating the method of producing the steel wire 1 for springs of the present embodiment. Referring to FIG. 4, in the method of producing the steel wire 1 for springs of the present embodiment, a wire preparing step is first performed as a step S10. In this step S10, a wire composed of a steel containing not less than 0.6 mass % and not more than 0.7 mass % C, not less than 1.7 mass % and not more than 2.5 mass % Si, not less than 0.2 mass % and not more than 1 mass % Mn, not less than 0.6 mass % and not more than 2 mass % Cr, and not less than 0.08 mass % and not more than 0.25 mass % V, with the balance consisting of Fe and unavoidable impurities, is prepared.

Next, referring to FIG. 4, a patenting step is performed as a step S20. In this step S20, the wire prepared in step S10 is subjected to patenting. Specifically, heat treatment is carried out in which the wire is heated to a temperature range not lower than the austenitizing temperature (A1 point), then rapidly cooled to a temperature range higher than the martensitic transformation starting temperature (Ms point), and held in that temperature range. With this, the structure of the wire becomes a fine pearlite structure with small lamellar spacing. In this patenting treatment, the heating of the wire to the temperature range not lower than the A1 point is preferably performed in an inert gas atmosphere from the standpoint of suppressing the occurrence of decarburization.

Next, referring to FIG. 4, a surface layer removing step is performed as a step S30. In this step S30, the surface layer of the wire that has undergone the patenting in step S20 is removed. Specifically, the wire is passed through a shaving die, for example, whereby a decarburized layer or the like on the surface formed through the patenting is removed. Although this step is not an indispensable step, even if a decarburized layer or the like is formed on the surface due to patenting, such a layer can be removed by performing this step.

Next, an annealing step is performed as a step S40. In this step S40, the wire with its surface layer removed in step S30 is subjected to annealing. Annealing is a heat treatment that is carried out for softening the wire. In the present embodiment, in addition to this, the formation of the oxidized layer 20 and the intergranular oxidized layer 11, and the adjustment of the maximum concentration of Si in the high Si concentration layer 21 in the oxidized layer 20 and the adjustment of the thickness of the intergranular oxidized layer 11 are carried out in this step S40.

In the step S40, it is necessary to make the oxidation of the wire progress to exceed the state where a layer having high concentrations of Si and Cr is formed in the vicinity of the outer peripheral surface 10A of the main body 10, and to reach the state where the high Si concentration layer 21 and the intergranular oxidized layer 11 are formed. Further, it is necessary to adjust the maximum concentration of Si in the high Si concentration layer 21 to fall within a narrow range of 2.5 times or more and 5.5 times or less of that of the main body 10, and also to adjust the thickness of the intergranular oxidized layer 11 to fall within a narrow range of 0.5 μm or more and 2.5 μm or less. An annealing step is generally carried out in an inert gas atmosphere of N, argon (Ar), or the like. However, from the standpoint of forming the oxidized layer 20 and the intergranular oxidized layer 11 simultaneously with the annealing as described above, the step S40 is carried out in an oxidizing atmosphere. Furthermore, it is important to select the atmosphere, temperature, and time from the standpoint of making the oxidation progress to the state where the high Si concentration layer 21 and the intergranular oxidized layer 11 are formed, and also from the standpoint of strictly adjusting the maximum concentration of Si in the high Si concentration layer 21 and the thickness of the intergranular oxidized layer 11 as described above. Specifically, it is suitable to impart appropriate oxidizing ability to the atmosphere and to conduct the treatment at a high temperature. For example, an atmosphere with water vapor intentionally mixed into an inert gas is adopted, and heat treatment of maintaining the wire at a temperature of not less than 650° C. and not more than 700° C. for not less than one hour and not more than three hours is carried out. The concentration of water vapor may be set such that, for example, water vapor of 2 L or more and 3 L or less in terms of water in liquid state is contained per 1 m3 of the volume of a furnace used for performing the annealing treatment. The pressure in the furnace may be, for example, an atmospheric pressure (a pressure of one atmosphere).

It should be noted that in the present embodiment, the oxidized layer 20 is formed in step S40 for simplification of the production process. However, the oxidized layer 20 may be formed in a step independent of step S40. In other words, the step S40 may be carried out in an inert gas atmosphere from the standpoint of performing annealing treatment alone, and the wire may be oxidized in another step. In this case, the strict selection of the atmosphere, temperature, and time as described above is required in the wire oxidizing step.

Next, a shot blasting step is performed as a step S50. In this step S50, the wire that has undergone the annealing treatment in step S40 and has the oxidized layer 20 formed thereon is subjected to shot blasting. Although this step is not an indispensable step, performing this can remove brittle Fe2O3 formed on the surface of the oxidized layer 20 and adjust the ratio of Fe3O4 in the oxidized layer 20.

Next, a wire drawing step is performed as a step S60. In this step S60, the wire that has undergone the shot blasting in step S50 is subjected to wire drawing (drawing process). The degree of working (reduction of area) in the wire drawing in step S60 can be set as appropriate, which can be, for example, not less than 50% and not more than 90%. Here, the “reduction of area” is a value related to the cross section perpendicular to the longitudinal direction of the wire, and obtained by dividing a difference between the cross-sectional areas before and after the wire drawing by the cross-sectional area before the wire drawing, expressed in percentage.

Next, a quenching step is performed as a step S70. In this step S70, the wire (steel wire) that has undergone the wire drawing in step S60 is subjected to quenching treatment in which the wire is heated to a temperature not lower than the A1 point of the steel and then rapidly cooled to a temperature not higher than the Ms point. More specifically, heat treatment is conducted in which, for example, the steel wire is heated to a temperature of not lower than 800° C. and not higher than 1000° C., and then rapidly cooled by being immersed in oil. With this, the steel constituting the main body attains a martensitic structure.

Next, a tempering step is performed as a step S80. In this step S80, the steel wire that has undergone the quenching treatment in step S70 is subjected to tempering treatment in which the wire is heated to a temperature lower than the A1 point of the steel and then cooled. The heating of the steel wire is conducted by immersing the steel wire in oil maintained at a predetermined temperature. More specifically, the heat treatment is conducted in which, for example, the steel wire is heated to a temperature of not lower than 400° C. and not higher than 700° C. and held for not less than 0.5 minutes and not more than 20 minutes, followed by cooling. With this, the steel constituting the main body 10 attains a tempered martensitic structure. The steel wire 1 for springs of the present embodiment can be produced through the above procedure.

EXAMPLES

Experiments to confirm the superiority of the steel wire for springs of the present disclosure were conducted in which steel wires for springs of the present disclosure were fabricated and worked into springs and the properties thereof were evaluated. The procedures and results of the experiments are as follows.

(1) Fabrication of Steel Wires for Springs

Steel wires having a diameter ¢ of 4 mm and the component compositions listed in Table 1 below were prepared, and subjected to the annealing step (S40) in the above embodiment to form an oxidized layer 20. The annealing was conducted under the conditions where the steel wire was heated to 675° C. in a furnace with a nitrogen atmosphere in which water vapor of 2.5 L in terms of water in liquid state per 1 m3 of the volume of the furnace was introduced. The pressure in the furnace was an atmospheric pressure (a pressure of one atmosphere). The holding time at 675° C. was varied in a range from 0.5 to 4 hours, to vary the degree of progress of oxidation. In Table 1, the numerical values indicate the mass percentages (mass %) of the respective components. Besides Fe, the elements other than C, Si, Mn, Cr, and V listed in Table 1 were not intentionally added; the balance is Fe and unavoidable impurities.

TABLE 1 (mass %) 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

Subsequently, all the steel wires were subjected to the quenching step (S70) and tempering step (S80) in the above embodiment under the same conditions, to obtain samples of oil quenched and tempered wires (steel wires for springs) with the oxidized layer 20 of 3.0±0.3 μm thickness. For the obtained samples, the maximum concentration of Si in the oxidized layer 20 (high Si concentration layer 21) and the thickness of the intergranular oxidized layer 11 were examined. The maximum concentration of Si was examined by linear analysis using Ultim Max170EDX manufactured by Oxford Instruments plc, attached to an SEM (GeminiSEM450) manufactured by Carl Zeiss AG. The concentration of Si at a location of 1.5 μm depth 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 oxidized layer 20 to this concentration was calculated. The linear analysis was conducted at three locations for each sample. The concentration of Si in the main body 10 and the maximum concentration of Si in the oxidized layer 20 were evaluated using the average of the three locations. The thickness of the intergranular oxidized layer 11 was evaluated using a maximum value in three fields of view of the SEM corresponding to the three locations where the linear analysis was conducted. The results are shown in Table 2. Referring to Table 2, Samples Nos. 1-3, 8-10, and 15-17 are of Inventive Examples that satisfy the conditions for the steel wire for springs of the present disclosure. Samples Nos. 4-7, 11-14, and 18-21 are of Comparative Examples that do not satisfy the conditions for the steel wire for springs of the present disclosure.

TABLE 2 Concentration of Si Thickness of Annealing Conditions Main Ratio of Intergranular Sample Steel Temperature Time Body Maximum Oxidized Layer No. Wire (° C.) (hours) (mass %) Concentration (μm) 1 A 675 1.5 1.8 2.6 times 0.53 2 A 675 2.0 1.9 4.2 times 1.46 3 A 675 2.5 1.8 5.3 times 2.44 4 A 675 0.5 2.0 1.2 times 0.36 5 A 675 1.0 1.7 2.2 times 0.51 6 A 675 3.0 1.9 5.8 times 2.58 7 A 675 4.0 1.9 6.2 times 2.71 8 B 675 1.5 2.0 2.5 times 0.51 9 B 675 2.0 1.9 4.1 times 1.45 10 B 675 2.5 1.8 5.1 times 2.32 11 B 675 0.5 2.0 1.3 times 0.32 12 B 675 1.0 2.0 1.9 times 0.43 13 B 675 3.0 1.8 5.7 times 2.47 14 B 675 4.0 1.8 6.0 times 2.55 15 C 675 1.5 2.3 2.7 times 0.58 16 C 675 2.0 2.3 4.2 times 1.51 17 C 675 2.5 2.2 5.1 times 2.48 18 C 675 0.5 2.3 1.1 times 0.47 19 C 675 1.0 2.3 2.4 times 0.52 20 C 675 3.0 2.2 5.6 times 2.60 21 C 675 4.0 2.1 6.2 times 2.79

(2) Examination of Hardness Distribution

Each sample in Table 2 above was subjected to working into a compression spring shape, strain relieving annealing, removal of oxidized layer 20, nitriding, shot peening, and setting, to obtain a compression spring corresponding to each sample. The nitriding treatment was carried out under the conditions of heating to 440° C. in a nitriding atmosphere and holding for five hours. 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.

TABLE 3 Ratio of Thickness of Hardness Maximum Intergranular (HV) Sample Concentration Oxidized Layer 60 80 100 120 No. of Si (μm) Surface μm μm μm μm 1 2.6 times 0.53 963 666 644 628 616 2 4.2 times 1.46 965 683 661 640 621 3 5.3 times 2.44 969 664 656 636 620 4 1.2 times 0.36 970 625 618 619 616 5 2.2 times 0.51 969 631 617 612 613 6 5.8 times 2.58 943 641 610 607 609 7 6.2 times 2.71 938 628 608 608 607 8 2.5 times 0.51 947 658 634 618 616 9 4.1 times 1.45 955 679 650 631 612 10 5.1 times 2.32 957 657 646 634 611 11 1.3 times 0.32 948 621 609 610 610 12 1.9 times 0.43 943 628 606 604 606 13 5.7 times 2.47 932 632 603 601 602 14 6.0 times 2.55 934 625 601 601 599 15 2.7 times 0.58 959 662 639 624 613 16 4.2 times 1.51 962 678 656 636 616 17 5.1 times 2.48 964 660 652 634 615 18 1.1 times 0.47 965 620 612 611 611 19 2.4 times 0.52 963 625 615 602 607 20 5.6 times 2.60 941 637 608 604 606 21 6.2 times 2.79 935 624 602 606 601

Referring to Table 3, it can be seen that in Samples Nos. 1-3, 8-10, and 15-17 of Inventive Examples, the hardness in the interior of the steel wire, especially at a depth of 80 to 100 μm near the maximum depth affected by nitriding, is high compared to Samples Nos. 4-7, 11-14, and 18-21 of Comparative Examples. It can also be seen that in Samples 6, 7, 13, 14, 20, and 21, which are samples of Comparative Examples with large ratios of the maximum concentration of Si and large thicknesses of the intergranular oxidized layer, the hardening by nitriding is insufficient, including the area in the vicinity of the surface. This is conceivably because the oxidation has progressed excessively, and Cr, V and the like, which contribute to the increase in hardness, have diffused into the oxidized layer, resulting in a reduction in hardness in the vicinity of the surface of the main body (in the vicinity of 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 samples of Comparative Examples with small ratios of the maximum concentration of Si and small thicknesses of the intergranular oxidized layer, although the hardness in the vicinity of the surface is sufficient, the hardening in the interior is insufficient. This is conceivably because the progress of oxidation is insufficient, and due to a layer containing high concentrations of Si and Cr, having a high affinity for N, formed in the vicinity of the surface of the main body (in the vicinity of the surface of the spring), N that has entered from the surface during the nitriding treatment is trapped in the vicinity of the surface, resulting in a decrease in thickness of the nitrided layer (arrival depth of nitrogen).

(3) Examination of Fatigue Strength

Next, eight springs were fabricated for each of Samples Nos. 1-3 of Inventive Examples and Samples Nos. 4-7 of Comparative Examples, and subjected to a fatigue test. The fatigue test was conducted under the conditions of average stress of 686 MPa and stress amplitude of 630 MPa. The fatigue strength was evaluated according to the number of unbroken springs at 5.0×107 cycles and at 1.0×108 cycles of repetition. The results are shown in Table 4.

TABLE 4 Ratio of Number of Unbroken Maximum Thickness of Springs (pieces) Sample Concentration Intergranular 5.0 × 107 1.0 × 108 No. of Si Oxidized Layer (μm) cycles cycles 1 2.6 times 0.53 8 8 2 4.2 times 1.46 8 8 3 5.3 times 2.44 8 8 4 1.2 times 0.36 8 3 5 2.2 times 0.51 8 2 6 5.8 times 2.58 8 5 7 6.2 times 2.71 7 3

Referring to Table 4, it can be seen that Samples Nos. 1-3 of Inventive Examples all have high fatigue strength. As to Samples 4 and 5, the samples of Comparative Examples with small ratios of the maximum concentration of Si and small thicknesses of the intergranular oxidized layer, although they were not broken at 5.0×107 cycles, at least half of them were broken at 1.0×108 cycles. This is conceivably because, although the high hardness at the surface ensures fatigue strength of a certain level, the insufficient hardness in the interior makes them not strong enough to withstand the long-term fatigue of 1.0×108 cycles. As to Samples 6 and 7, the samples of Comparative Examples with large ratios of the maximum concentration of Si and large thicknesses of the intergranular oxidized layer, half of the samples were similarly broken at 1.0×108 cycles. In Sample 7 with a large ratio of the maximum concentration of Si and a large thickness of the intergranular oxidized layer, the breakage occurred even at 5.0×107 cycles. This is conceivably because, when the ratio of the maximum concentration of Si and the thickness of the intergranular oxidized layer are large, the hardness is insufficient not only in the interior but also at the surface.

(4) Summary of Experimental Results

The experimental results described above confirm that in the steel wire for springs of the present disclosure, even though the oxidized layer contributing to improved workability into springs has been formed on the surface, the inhibition of the formation of the nitrided layer by Si has been suppressed, thereby achieving an improvement in the fatigue strength of springs.

It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

    • 1: steel wire for springs; 10: main body; 10A: outer peripheral surface; 11: intergranular oxidized layer, 20: oxidized layer, 20A: outer peripheral surface; and 21: high Si concentration layer.

Claims

1. A steel wire for springs, comprising:

a main body made of a steel and having a line shape; and
an oxidized layer covering an outer peripheral surface of the main body;
the steel constituting the main body containing not less than 0.6 mass % and not more than 0.7 mass % carbon, not less than 1.7 mass % and not more than 2.5 mass % silicon, not less than 0.2 mass % and not more than 1 mass % manganese, not less than 0.6 mass % and not more than 2 mass % chromium, and not less than 0.08 mass % and not more than 0.25 mass % vanadium, with the balance consisting of iron and unavoidable impurities,
the steel constituting the main body having a tempered martensitic structure, the oxidized layer including a high silicon concentration layer having a maximum concentration of silicon of not less than 2.5 times and not more than 5.5 times that of the main body,
the main body including an intergranular oxidized layer arranged to constitute the outer peripheral surface and having a thickness of not less than 0.5 μm and not more than 2.5 μm.

2. The steel wire for springs according to claim 1, wherein the oxidized layer has a thickness of not less than 2 μm and not more than 5 μm.

3. The steel wire for springs according to claim 1, wherein the oxidized layer contains not less than 80 mass % triiron tetraoxide.

Patent History
Publication number: 20240344186
Type: Application
Filed: Mar 30, 2022
Publication Date: Oct 17, 2024
Applicant: SUMITOMO ELECTRIC INDUSTRIES, LTD. (Osaka-shi, Osaka)
Inventors: Hiromu IZUMIDA (Osaka-shi), Tetsuya NAKAJIMA (Osaka-shi), Hiroto KONTANI (Osaka-shi)
Application Number: 18/294,342
Classifications
International Classification: C22C 38/34 (20060101); C21D 1/18 (20060101); C21D 1/74 (20060101); C21D 6/00 (20060101); C21D 9/52 (20060101); C22C 38/02 (20060101); C22C 38/04 (20060101); C22C 38/24 (20060101);