JP2018176268A - Manufacturing method for compression coil spring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compensation coil spring which is high in durability and high in settling resistance by forming a C-enrichment layer having a uniform thickness on a surface of a wire rod, and imparting a proper compression residual stress distribution to the molded wire rod.SOLUTION: A manufacturing method for a compression coil spring comprises: a coiling step of hot-molding a steel wire rod by using a coil spring molding machine; a quenching step of quenching a coil as it is which is cut after being coiled, and whose temperature remains in an austenite area; an annealing step of refining the quenched coil; and a shot peening step of imparting compression residual stress to a surface of the wire rod. The coil spring molding machine 1 has a high-frequency heating coil 40 for raising a temperature of the steel wire rod M up to the austenite area between an outlet of a feed roller 10 and a coiling tool 20. A member 50 for covering an external periphery of the steel wire rod is arranged in a part or the whole area reaching the coiling tool 20 from an inlet 50a side of the steel wire rod in the high-frequency heat coil 40, and gas supply means 60 for supplying carbonized hydrogen-system gas into an enclosure member 50 is arranged.SELECTED DRAWING: Figure 2

Description

The present invention relates to a method of manufacturing a compression coil spring used, for example, in an automobile engine or clutch, and in particular, to a compression coil spring having excellent fatigue resistance and fatigue resistance even in a high stress operating environment. On the way.

  In recent years, with the background of environmental problems, the demand for lower fuel consumption of automobiles has become more severe year by year, and there is a strong demand for smaller and lighter automobile parts than ever before. In response to the demand for smaller size and lighter weight, compression coil spring parts such as a valve spring used in an engine and a clutch torsion spring used in a clutch are made by strengthening the material and surface treatment Research on surface strengthening is active, and the results are used to improve fatigue resistance and fatigue resistance, which are important as the characteristics of coil springs.

  Generally, methods of manufacturing a coil spring are roughly classified into a hot forming method and a cold forming method. The hot forming method is a coil spring in which cold forming is difficult due to its poor formability such as a large wire diameter d and a small spring index D / d which is a ratio of the coil average diameter D to the wire diameter d. It is used for forming, and carbon steel and spring steel are used as a coil spring wire material. In the hot forming method, the wire is heated to a high temperature so as to be easily processed, wound around a core metal and coiled in a coil spring shape, and after quenching and tempering, shot peening and setting are further performed to obtain main performance as a coil spring performance. Fatigue resistance and fatigue resistance are obtained. In the hot forming method, coiling with a coreless metal is technically very difficult, so it has not been put to practical use until now. Therefore, it is essential in the conventional art to use a core metal in the hot forming method, and as a coil spring that can be formed, the degree of freedom in shape is lower as compared with a cold forming method that can be coiled with a coreless metal.

  On the other hand, the compression coil spring of the valve spring and the clutch torsion spring class can be cold-formed because the wire diameter is relatively thin. And since high dimensional accuracy can be easily obtained because there is no transformation or thermal expansion and contraction due to heating, and mass productivity (tact, cost) due to processing speed, equipment cost, etc. is also high, this class of compression coil spring is manufactured. The cold forming method has conventionally been adopted for. In addition, for this cold forming method, a forming technique using a coreless metal has been established, and the high degree of freedom in shape of the coil spring is also a major cause of using the cold forming method. The manufacturing technology of the compression coil spring of the valve spring and the clutch torsion spring class by the above has not been put to practical use so far. In the cold forming method, hard drawn wires such as carbon steel wire, hard steel wire, piano wire, and spring steel wire have been conventionally used as the coil spring wire. However, in recent years, it has been required to increase the strength of materials from the viewpoint of weight reduction, and expensive oil-tempered wires are widely used.

  In the cold forming method, as shown in FIG. 1 (C), the wire is cold coiled in a coil spring shape, and after annealing, nitriding treatment, shot peening and setting are applied as needed. Here, the purpose of annealing is to remove the residual stress generated by processing that is an impediment to improving the fatigue resistance of the coil spring, and combined with the application of compressive residual stress to the surface by shot peening, It contributes to the improvement of fatigue resistance. In the case of a coil spring used under high load stress such as a valve spring or a clutch torsion spring, a surface hardening treatment by a nitriding treatment is applied before the shot peening as needed.

  Research aimed at further improving fatigue resistance is actively conducted. For example, Patent Document 1 describes an oil-tempered wire for cold forming, and discloses a technique for improving fatigue resistance by utilizing process-induced transformation of retained austenite. Further, Patent Document 2 discloses means for directly spraying hydrocarbon gas from one nozzle onto the surface of a steel wire rod during heating and quenching, and forming a C-rich layer on the surface of the steel wire rod. It is done. Patent Document 3 discloses a technique for imparting a large compressive residual stress by performing multi-stage shot peening at different projection speeds on the surface of a wire material subjected to a nitriding treatment to improve the fatigue resistance.

  In Patent Document 1, residual stress occurs in the coil spring after coiling. The residual stress, in particular, the tensile residual stress in the direction of the linear axis generated on the inner surface of the coil is a factor inhibiting the improvement of the fatigue resistance as a coil spring. And although annealing is usually performed in order to remove the residual stress by this processing, even with the wire with high softening resistance described in Patent Document 1, the desired strength of the wire is maintained and this residual stress is completely eliminated. The difficulty of removing it is easy to estimate and is well known to those skilled in the art. Therefore, when shot peening is applied thereafter, it is difficult to provide sufficient compressive residual stress to the wire surface due to the influence of tensile residual stress remaining on the inner diameter side of the coil due to processing, and sufficient fatigue resistance as a coil spring Can not get.

  Further, according to Patent Document 2, when coiling processing is performed in a state where the steel wire is heated to the austenite region, carburizing treatment is simultaneously applied to the steel wire, thereby eliminating the generation of residual stress due to the processing and It is disclosed that the formation of a C-enriched layer to efficiently obtain the effects of shot peening and setting performed later. In this case, a hydrocarbon-based gas is directly sprayed onto the surface of the steel wire from one nozzle during heating and hardening, and a C-rich layer is formed on the surface of the steel wire. However, in this method, it is easily estimated that variations occur in the thickness and surface C concentration of the C-rich layer in the wire circumferential direction. And, the variation forms a portion of excessive C-concentrated layer thickness and C concentration, and a thin C-concentrated layer thickness and C concentration, with respect to the desired C-concentrated layer thickness and C concentration. In the part where the C concentration is high, the transformation from austenite to martensite is inhibited, resulting in an increase in retained austenite phase. As a result, improvement in fatigue resistance is expected, but deterioration in fatigue resistance is inevitable. The magnitude of the compressive residual stress in the vicinity of the surface introduced by shot peening is proportional to the yield stress in the vicinity of the surface affected by the shot peening in the steel wire, that is, the C concentration. Therefore, in the dilute C-enriched layer, compressive residual stress in the vicinity of the surface introduced by shot peening does not reach a desired magnitude, and fatigue cracks originating from the vicinity of the surface (including the outermost surface) are generated Preventive effect is not enough. In addition, since the surface hardness rise is also small, it is not possible to prevent wear at the inter-line portion where contact is repeated during operation, and there is a possibility of causing early breakage starting from the wear portion. From these facts, if a dilute C-rich layer is present, improvement in fatigue resistance can not be expected.

  On the other hand, vacuum carburizing by batch processing has been conventionally performed. The carburized spring subjected to this treatment can improve the durability by obtaining deep and large compressive residual stress, but it is difficult to control the amount of carburized due to the system configuration, and the carburized depth obtained is a desired depth. It will be more than In particular, for the valve spring, it becomes excessive carburization, and with the increase of the retained austenite phase formed by the excessive C-concentrated layer thickness, the sag resistance is significantly reduced.

  Further, in Patent Document 3, the compressive residual stress in the vicinity of the wire surface (hereinafter referred to as the “surface”) of the coil spring is about 1400 MPa, and as a coil spring used under high load stress of valve spring and clutch torsion spring class, Its compressive residual stress is sufficient to suppress crack initiation on the surface. However, as the compressive residual stress on the surface is improved, the compressive residual stress in the wire becomes smaller, and the effect of the compressive residual stress on cracking in the wire originating from inclusions etc. Become. That is, in the method according to Patent Document 3, since the energy given by shot peening is limited, that is, although the change of the compressive residual stress distribution is given, it is difficult to greatly improve the total of the compressive residual stress. It is not considered to eliminate the influence of the residual stress due to the above-mentioned processing, and therefore the improvement in the fatigue resistance of the wire having the same strength is poor.

  Although various means for improving surface compressive residual stress have been put to practical use, as a result, for example, in a coil spring with a wire diameter of about 1.5 to 10 mm, the depth from the wire surface is 0.1 to 0.4 mm The maximum value of the combined stress, which is the sum of the applied stress and the residual stress due to the external load, exists in the range, and the highest part of the combined stress is the starting point of the failure. Therefore, securing a large compressive residual stress in the depth range of 0.1 to 0.4 mm is important for fatigue resistance.

Patent No. 3595901 JP, 2014-055343, A JP, 2009-226523, A

  As described above, in the conventional manufacturing method and Patent Documents 1 to 3 and the like, in response to the demand for coexistence of further improvement of fatigue resistance and fatigue resistance under high stress and cost reduction in recent years, Dealings are difficult. Moreover, since the residual stress by processing can not be completely eliminated by the annealing treatment after shaping | molding, the performance of the wire can not fully be utilized.

Under such a background, the present invention eliminates tensile residual stress due to coiling and forms a C 2 -rich layer uniformly on the surface of the wire within an appropriate C concentration and an appropriate thickness range, and after forming It is an object of the present invention to provide a method of manufacturing a compression coil spring having high durability and high sag resistance by giving an optimum compression residual stress distribution to the wire rod.

  The present inventors conducted intensive studies on the fatigue resistance and the sag resistance of a coil spring. Then, it has been conceived to form a thin and uniform carburized layer (hereinafter referred to as "C-enriched layer") on the surface of the steel wire. As a result, the residual austenite phase can be reduced and the sag resistance can be improved, the yield stress can be improved by making the vicinity of the surface high in hardness, and the effect of the shot peening to be performed later can be efficiently obtained. Can be improved.

The method for manufacturing a compression coil spring according to the present invention includes a coiling step of hot forming a steel wire rod by a coil spring forming machine, a hardening step of hardening a coil which is separated after coiling and whose temperature is still in the austenite range, and hardening In the method of manufacturing a compression coil spring including a tempering step of tempering the coil and a shot peening step of applying a compressive residual stress to the surface of the wire, the coiling step performs heating, carburizing and hot forming, The coil spring forming machine is continuously supplied from the rear after coiling a feed roller for continuously supplying a steel wire, a coiling portion for forming the steel wire into a coil shape, and a predetermined number of turns of the steel wire. And a cutting means for cutting the steel wire rod, and the coiling portion adds the steel wire rod supplied by the feed roller. Wire guide for guiding to the appropriate position of the part, coiling tool consisting of coiling pin or coiling roller for processing the steel wire supplied through the wire guide into a coil shape, and pitch for pitching And the coil spring forming machine further includes heating means for raising the temperature of the steel wire to the austenite region between the exit of the feed roller and the coiling tool, and the coiling tool from the steel wire inlet side of the heating means An enclosure member covering the outer periphery of the steel wire rod is disposed in a part or the entire area between the two, and has a gas supply means for supplying hydrocarbon-based gas in the enclosure member , the heating means is a high frequency heating device, steel wire rod The high frequency heating coil is arranged on the passage of the pipe so as to be concentric with the steel wire, and the enclosing member is arranged inside the high frequency heating coil. The high-frequency heating coil is characterized by heating the steel wire rod directly.

  In the method for manufacturing a compression coil spring of the present invention, it is preferable that the steel wire surface temperature at the time of contacting with the hydrocarbon-based gas is 850 to 1150 ° C. According to this carburizing condition, carburization can be efficiently performed in a short time while preventing significant coarsening of crystal grains of the wire rod. Further, in the method for manufacturing a compression coil spring of the present invention, it is preferable that the main component of the hydrocarbon-based gas is any one of methane, butane, propane and acetylene.

  In the above manufacturing method, the tempering step is performed to refine the coil spring hardened by the hardening step into a coil spring having an appropriate hardness and toughness. Therefore, the tempering step may be omitted if desired hardness and toughness can be obtained as it is quenched. In the shot peening step, multistage shot peening may be performed, and further, a low temperature aging treatment for the purpose of recovering the elastic limit may be combined as necessary. Here, the low temperature aging treatment can be performed after the shot peening step or between each stage of multistage shot peening, and shot peening with a shot having a particle diameter of 0.02 to 0.30 mm as a final stage in multistage shot peening When it is applied, it is preferable to carry out as the pretreatment in order to further increase the compressive residual stress on the outermost surface. The setting step is a treatment to increase the yield stress of the steel wire to improve the sett resistance, and there are various methods such as cold setting and hot setting as the setting applied to the coil spring, but depending on the desired characteristics select.

  According to the method of manufacturing the compression coil spring of the present invention, for example, a cylindrical enclosing member covering the outer periphery of the steel wire rod is disposed in a part or the entire area from the steel wire rod inlet side to the coiling tool in the heating means Since the hydrocarbon-based gas is supplied to the inside, the hydrocarbon-based gas concentration in the enclosure member can be easily controlled by controlling the supply amount of the hydrocarbon-based gas. Further, since the hydrocarbon-based gas uniformly surrounds the steel wire rod, the thickness of the C-concentrated layer formed by carburizing can be made uniform. That is, a C-concentrated layer having a thickness of 0.01 to 0.05 mm can be formed all around the steel wire rod.

  In the present invention, since the hot coiling is performed by the coil spring manufacturing apparatus as described above, residual stress due to processing does not occur. And, since the temperature of the steel wire is raised to the austenite region within 2.5 seconds, coarsening of crystal grains can be prevented, and excellent fatigue resistance can be obtained. In addition, since the surface of the steel wire rod can be made to have high hardness in order to carry out carburizing treatment, compressive residual stress can be effectively imparted by shot peening to be performed later. In particular, in the method for manufacturing a compression coil spring according to the present invention, carburizing treatment can be performed efficiently by utilizing the heat at the time of hot coiling.

The compression coil spring manufactured according to the present invention has, for example, the following configuration. That is, by weight%, C 0.5 to 0.7%, Si 1.2 to 3.0%, Mn 0.3 to 1.2%, Cr 0.5 to 1.9%, As well as containing 0.05 to 0.5% of V, containing 1.5% or less of Ni, 1.5% or less of Mo, and 0.5% or less of W as an optional component, In a compression coil spring using a steel wire rod, the balance of which comprises iron and unavoidable impurities, the surface layer portion has a C-concentrated layer exceeding the average concentration of C contained in the steel wire rod, and extends over the entire circumference of the steel wire rod The thickness of the C-concentrated layer is in the range of 0.01 to 0.05 mm .

The reasons for limitation of the above numerical range are described below. First, the reasons for limitation of the chemical components of the steel wire rod used in the present invention will be described. In the above compression coil spring , 0.5% to 0.7% of C, 1.2 to 3.0% of Si, 0.3 to 1.2% of Mn, and 0.5 to Cr by weight% While containing 1.9% and 0.05 to 0.5% of V, Ni is not more than 1.5%, Mo is not more than 1.5%, and W is not more than 0.5% as an optional component Use a steel wire rod containing at least the species and the balance being iron and unavoidable impurities. In the following description, "%" means "% by weight".

(1) Material component C: 0.5 to 0.7%
C contributes to strength improvement. If the C content is less than 0.5%, the effect of improving the strength can not be sufficiently obtained, and therefore the fatigue resistance and the sag resistance become insufficient. On the other hand, when the content of C exceeds 0.7%, the toughness is lowered and the crack is easily generated. Therefore, the content of C is set to 0.5 to 0.7%.

Si: 1.2 to 3.0%
Si is effective for deoxidation of steel and contributes to the improvement of strength and the improvement of resistance to temper softening. If the content of Si is less than 1.2%, these effects can not be sufficiently obtained. On the other hand, if the content of Si exceeds 3.0%, decarburization is promoted and the surface strength of the wire is reduced, and the toughness is greatly reduced, which causes the occurrence of cracking when used as a coil spring. Therefore, the content of Si is set to 1.2 to 3.0%. On the other hand, although the effect on the performance of a coil spring is equivalent when the amount of Si is 2.4% to 3.0%, the increase of the Si content in this range increases the risk of cracking during casting in material production Therefore, the content of Si is preferably 2.4% or less.

Mn: 0.3 to 1.2%
Mn contributes to the improvement of the hardenability. When the content of Mn is less than 0.3%, it is difficult to secure sufficient hardenability, and the effect of the fixation of S (the formation of MnS) which is harmful to the ductility is also poor. On the other hand, when the content of Mn exceeds 1.2%, the ductility decreases, and cracking and surface flaws easily occur. Therefore, the content of Mn is set to 0.3 to 1.2%. On the other hand, although the effect on the performance of a coil spring is equivalent when the amount of Mn is 0.8% to 1.2%, the increase of the Mn content in this range is a risk of occurrence of breakage at the time of wire drawing in material production The content of Mn is preferably 0.8% or less.

Cr: 0.5 to 1.9%
Cr is effective in preventing decarburization, contributes to the improvement of strength and temper softening resistance, and is effective in the improvement of fatigue resistance. In addition, it is also effective for improving the resistance to warmth. Therefore, in the present invention, it is preferable to further contain 0.5 to 1.9% of Cr. If the content of Cr is less than 0.5%, these effects can not be obtained sufficiently. On the other hand, when the content of Cr exceeds 1.9%, the toughness is reduced, and cracking and surface flaws easily occur.

V: 0.05 to 0.5%
Since V is refined as fine carbides by heat treatment and is refined as crystal grains and improves strength without losing toughness, it is effective in improving fatigue resistance and improves sag resistance. V also contributes to the improvement of temper softening resistance. If the V content is less than 0.05%, such an effect can not be obtained. On the other hand, when V is contained in excess of 0.5%, a large amount of carbides are formed at the time of heating, leading to a decrease in toughness.

In the compression coil spring , one or more of Ni, Mo and W can be added as an optional component. As a result, it is also possible to produce a coil spring having higher performance or more suitable for the application.

Ni: 1.5% or less Ni contributes to the improvement of toughness and is effective in the improvement of fatigue resistance. In addition, Ni contributes to the improvement of the corrosion resistance. On the other hand, when the content of Ni exceeds 1.5%, the toughness is lowered.

Mo: 1.5% or less Mo contributes to the improvement of hardenability and toughness. Mo may be added instead of Mn contributing to the improvement of the hardenability, or Mo may be added together with Mn. Mo may be added instead of Ni which contributes to the improvement of toughness, and Mo may be added together with Ni. On the other hand, when the content of Mo exceeds 1.5%, a large amount of carbides are formed at the time of heating, leading to a decrease in toughness.

W: 0.5% or less W precipitates as fine carbides by heat treatment to refine the crystal grains and improve the strength without losing the toughness, which is effective in improving the fatigue resistance. Further, W improves the sag resistance and also contributes to the improvement of the temper softening resistance. On the other hand, if the content of W exceeds 0.5%, a large amount of carbides are formed at the time of heating, leading to a decrease in toughness.

  In the present invention, the following elements can be added in addition to the above optional elements of Ni, Mo and W.

B: 0.0003 to 0.003%
B improves the hardenability and has the effect of preventing low temperature brittleness. B also contributes to the improvement of the sag resistance. B may be added instead of Mn contributing to the improvement of the hardenability, or B may be added together with Mn. If the content of B is less than 0.0003%, such effects are scarce, and if it exceeds 0.003%, the effects may be saturated and the productivity and impact strength may be deteriorated.

Cu: more than 0% and 0.65% or less Cu is a metal element having a higher ionization tendency than iron electrochemically, and has an action of enhancing the corrosion resistance of steel, and thus is effective for improving the corrosion resistance. Cu may be added instead of Ni contributing to the improvement of the corrosion resistance, and may be added together with Ni. When the content of Cu exceeds 0.65%, cracking tends to occur during hot working.

Ti, Nb: 0.05 to 0.5%
Both Ti and Nb are elements having the same effect as V. If the content of these elements is less than 0.05%, such effects are poor, and if it exceeds 0.5%, a large amount of carbides are formed at the time of heating, leading to a decrease in toughness.

(2) C concentration distribution
In the above-mentioned compression coil spring , in order to increase the hardness of the surface of the wire and to improve the yield stress, a C-concentrated layer is formed on the surface portion of the wire by carburizing treatment. By improving the yield stress, it is possible to apply a large surface compressive residual stress by the subsequent shot peening. In addition, the surface roughness of the wire can be improved. Therefore, there is an effect of further improving the fatigue resistance. The C-concentrated layer contains C at a concentration exceeding the average concentration of C contained in the wire. In addition, in order to obtain these effects sufficiently, the maximum C concentration in the C-rich layer is 0.7 to 1.2%, and the C-rich layer (carburized depth) extends over the entire circumference of the steel wire rod. It forms in the range of the depth of 0.01-0.05 mm from the wire surface.

  When the maximum C concentration of the C-concentrated layer exceeds 1.2% or the thickness of the C-concentrated layer exceeds 0.05 mm, treatment must be performed at high temperature to efficiently perform the carburization reaction. Therefore, the grain size is deteriorated, and the fatigue resistance is easily reduced. In addition, when the C concentration exceeds 1.2%, a large amount of C which can not form a solid solution in the matrix precipitates as carbides at the grain boundaries to lower the toughness, which also tends to cause a decrease in the fatigue resistance. . Furthermore, when the thickness of the C-concentrated layer exceeds 0.05 mm, the proportion of retained austenite increases and the sag resistance deteriorates.

  On the other hand, when the maximum C concentration in the C-concentrated layer is less than 0.7% or the C-concentrated layer thickness is less than 0.01 mm from the surface of the wire, the following problems occur. That is, the magnitude of the compressive residual stress in the vicinity of the surface introduced by shot peening is proportional to the yield stress in the vicinity of the surface affected by the shot peening in the steel wire, that is, the C concentration. Therefore, in a dilute (concentration and depth) C-concentrated layer, compressive residual stress in the vicinity of the surface introduced by shot peening does not reach a desired magnitude, and fatigue originating from the vicinity of the surface (including the outermost surface) The effect of preventing cracking is not sufficient. In addition, since the surface hardness rise is also small, it is not possible to prevent wear at the inter-line portion where contact is repeated during operation, and there is a possibility of causing early breakage starting from the wear portion. From these facts, if a dilute C-rich layer is present, improvement in fatigue resistance can not be expected.

(3) Hardness distribution It is preferable that the internal hardness in arbitrary wire rod cross sections of a steel wire is 600-710 HV, and the highest hardness in a C concentration layer is 30 HV or more higher than internal hardness. This is because the C-concentrated layer on the surface of the wire is higher than the internal hardness, higher compressive residual stress can be obtained in the vicinity of the surface, and fatigue cracks originating from the vicinity of the surface (including the outermost surface) It is because it can prevent. If the above value is less than 30 HV, these effects are not noticeable.

(4) Crystal grain size It is preferable that the average crystal grain size (the boundary of the azimuth angle difference of 5 ° or more is a grain boundary) measured by SEM / EBSD (Electron Back Scatter Diffraction) method is 1.3 μm or less . When the average crystal grain size exceeds 1.3 μm, it is difficult to obtain sufficient fatigue resistance. And, the fact that the average grain size is small, that is, that the blocks and laths in the prior austenite grains are fine, is suitable for improving the fatigue resistance because the resistance to crack growth is large.

(5) Residual Stress Distribution The inventors of the present invention have found that the working stress required as a valve spring and a clutch torsion spring and various factors that can become fatigue fracture origins (eg, ductile toughness, nonmetallic inclusions, incompletely quenched structure, etc. From the fracture mechanics calculation in relation to the abnormal structure, surface roughness, surface flaw etc.) and verification by actual endurance test etc., we obtain the following conclusion about the compressive residual stress required near the wire surface of the coil spring The The compressive residual stress is in the direction of substantially maximum principal stress when a compressive load is applied to the spring, that is, in the + 45 ° direction with respect to the axial direction of the wire.

That is, in the above-mentioned compression coil spring , the value of compressive residual stress at no load becomes zero from the surface of the wire in the direction of maximum principal stress on the inner diameter side of the coil spring which occurs when a compression load is applied to the coil spring. the depth and crossing points, the residual longitudinal axis stress, when the integrated value of the surface in the residual stress distribution curves in which the horizontal axis and the depth from the surface to the crossing point was expressed as _I -σR, I -σR is 150 MPa · It is desirable to be at least mm. If it does not reach these figures, it is insufficient to suppress fatigue failure of the internal origin.

The compressive residual stress distribution is preferably formed by shot peening treatment or setting treatment. When multi-stage shot peening is performed in the shot peening process, the sphere equivalent diameter of a shot used for shot peening to be performed later is preferably smaller than the sphere equivalent diameter of a shot used for shot peening performed first. Specifically, the shot peening process includes a first shot peening process with a shot having a particle size of 0.6 to 1.2 mm and a second shot peening process with a shot having a particle size of 0.2 to 0.8 mm. It is preferable that it is a multistage shot peening process which consists of the 3rd shot peening process by the shot of particle diameter 0.02-0.30 mm. Thereby, it is possible to reduce the surface roughness increased by the previously implemented shot peening by the later implemented shot peening.

  The shot diameter and the number of steps in the shot peening treatment are not limited to the above, and the required residual stress distribution, surface roughness and the like may be obtained according to the required performance. Therefore, the shot diameter, the material, the number of steps, etc. are appropriately selected. Moreover, since the compressive residual stress distribution to be introduced also differs depending on the projection speed and the projection time, these are also appropriately set as needed.

(6) Retained austenite distribution Regarding retained austenite volume fraction γR measured using X-ray diffraction method, the retained austenite volume fraction on the vertical axis and the retained austenite distribution curve on the horizontal axis with depth from the surface 0.5 mm from the surface When the integral value up to the depth is expressed as I _γR , it is desirable that I _γR be 3.4% · mm or less. Thus, the sag resistance can be improved by limiting retained austenite.

(7) Surface Roughness As a valve spring, a clutch torsion spring, etc. used under high load stress, surface roughness is also important along with the above-mentioned compressive residual stress distribution in order to satisfy the required fatigue resistance. As a result of the inventors of the present invention carrying out fracture mechanics calculations and their verification experiments, the depth of surface flaws (ie, surface roughness Rz (maximum height)) is calculated with respect to the occurrence and development of cracks due to surface origin It has been found that the effect can be made harmless by setting it to 20 μm or less. Therefore, the surface roughness Rz is preferably 20 μm or less. When Rz exceeds 20 μm, the valleys on the surface become a stress concentration source, and the generation and propagation of cracks starting from the valleys are likely to occur, so it is easy to cause early breakage.

(8) Coil Spring Shape The present invention is suitable for a compression coil spring having the following specifications, which has a large degree of processing at the time of coiling and requires high fatigue resistance. In the present invention, the equivalent circle diameter of the wire (including the diameter of a true circle calculated from the wire cross-sectional area, including non-circular cross sections including square and oval) is 1.5 to 10 mm, and the spring index is 3 to 3 20, which can be used for a generally cold-formed compression coil spring.

  Above all, the degree of processing at coiling is large (that is, in cold forming, the tensile residual stress generated on the inner diameter of the coil is large due to coiling), and valve springs and clutch torsion springs etc. that require high fatigue resistance Is suitable for a compression coil spring having a circle equivalent diameter of 1.5 to 9.0 mm and a spring index of 3 to 8.

Further, unlike the conventional hot forming method, the above-described compression coil spring is manufactured using the above-described coil spring forming machine, and therefore, no core metal is required at the time of coiling. Therefore, the degree of freedom of the spring shape that can be molded is high. That is, as a coil spring shape in the present invention, the present invention can be applied to coil springs having other shapes including a cylindrical shape having substantially no change in coil outer diameter in a typical full turn as a coil spring. For example, it is also possible to form a conical, bell-shaped, drum-shaped, barrel-shaped or other spring.

  Here, "cylindrical" is a spring with a constant coil diameter, and "conical" is a spring whose coil diameter changes conically from one end of the spring to the other end. The "bell-shaped" is a spring whose coil diameter is small at one end, is expanded toward the center, is a spring with the same diameter and reaches the other end, and is also referred to as "one-stop type". "Tick shaped" is a spring whose coil diameter is large at both ends and small at the center. A "barrel-shaped" is a spring whose coil diameter is small at both ends and large at the center, and is also referred to as "both-end throttled".

  The present invention uses carbon steel wire, hard steel wire, piano wire, spring steel wire, carbon steel oil temper wire, chromium vanadium steel oil temper wire, silicon chromium steel oil temper wire, silicon chromium vanadium steel oil temper wire used as a spring Application is possible to a line etc. Here, carbon steel wire, hard steel wire, piano wire, and spring steel wire are not subjected to heat treatment like oil temper wire, so they are less expensive as steel wire material than oil temper wire of the same composition. . In addition, because the heat treatment (quenching and tempering) is applied in the manufacturing method of the present invention, oil temper wire is used even if carbon steel wire, hard steel wire, piano wire and spring steel wire are used if the compositions are the same. However, it is possible to manufacture a compression coil spring having the same characteristics. Therefore, if the compositions are equal, using carbon steel wire, hard steel wire, piano wire, and spring steel wire can be inexpensively manufactured.

  According to the present invention, since a C-concentrated layer having a thin and uniform thickness is formed on the surface of a steel wire rod, the total amount of retained austenite phase can be small and sag resistance can be improved, and the vicinity of the surface has high hardness. The fatigue resistance can be improved by improving yield stress and efficiently obtaining the effect of shot peening.

It is a figure which shows an example of the manufacturing process of a coiled spring. It is the schematic of the shaping | molding part of the coiling machine in embodiment of this invention. It is a graph which shows the residual stress distribution of the coiled spring used in the Example. It is a graph which shows the retained austenite distribution of the coiled spring used in the Example.

  Hereinafter, embodiments of the present invention will be specifically described. Each manufacturing process is shown in FIG. FIG. 1A shows a method of manufacturing the compression coil spring of the present invention, and the other is a conventional example. The manufacturing process shown in FIG. 1 (A) is a hot forming method by the following coiling machine, and the manufacturing process shown in FIGS. 1 (B) and (C) is a cold forming method by any coiling machine is there.

  An outline of a coiling machine formed portion 1 used in the manufacturing process shown in FIG. 1 (A) is shown in FIG. As shown in FIG. 2, the coiling machine forming unit 1 includes a feed roller 10 for continuously supplying the steel wire M and a coiling unit 20 for forming the steel wire M in a coil shape. The coiling unit 20 has a wire guide 21 for guiding the steel wire rod M supplied by the feed roller 10 to an appropriate position, and a coil shape for processing the steel wire rod M supplied via the wire guide 21. A coiling tool 22 comprising a coiling pin (or coiling roller) 22a and a pitch tool (not shown) for pitching are provided. In addition, the coiling machine forming unit 1 includes a cutting means 30 having a cutting blade 30a and an inner die 30b for separating the steel wire rod M continuously supplied from the rear after coiling a predetermined number of turns, and the feed roller 10 The high frequency heating coil 40 which heats the steel wire rod M between an exit and the coiling tool 22 is provided.

  An enclosure member 50 made of, for example, a ceramic is disposed inside the high frequency heating coil 40. The enclosing member 50 is provided with a small diameter steel wire rod inlet 50a and a steel wire rod outlet 50b at its both ends. In the vicinity of the steel wire rod inlet 50 a of the enclosure member 50, a gas supply unit (gas supply means) 60 for supplying a hydrocarbon-based gas to the enclosure member 50 is provided. The gas supply unit 60 supplies a hydrocarbon-based gas to the inside from, for example, a steel wire rod inlet 50 a of the enclosing member 50. In addition, hydrocarbon type gas can also be supplied from the steel wire rod exit 50b.

  The rapid heating in the coiling machine forming portion 1 is performed by the high frequency heating coil 40, and the temperature of the steel wire is raised to the austenite region within 2.5 seconds. The installation position of the high frequency heating coil 40 is as shown in FIG. 2 and is disposed on the outer peripheral side of the enclosing member 50. The steel wire rod M passing through the inside of the enclosing member 50 is heated by the high frequency heating coil 40 and carburized by the hydrocarbon-based gas filling the enclosing member 50. The gas supply unit supplies the hydrocarbon-based gas in an amount in consideration of the density and flow velocity of the hydrocarbon-based gas in the enclosure member 50 contributing to the carburizing property into the enclosure member 50.

  The high frequency heating coil 40 is disposed in the vicinity of the wire guide 21, and a coiling portion 20 is provided so that the steel wire rod M can be formed immediately after heating. In the coiling portion 20, the steel wire M which has passed through the wire guide 21 is brought into contact with the coiling pin 22a and bent with a predetermined curvature, and is brought into contact with the downstream coiling pin 22a and bent with a predetermined curvature. Then, the steel wire rod M is brought into contact with the pitch tool, and the pitch is given so as to obtain a desired coil shape. At the desired number of turns, the cutting blade 30a of the cutting means 30 cuts by shear between the straight part of the inner mold 30b and separates the steel wire rod M supplied from the rear and the spring-shaped steel wire rod M .

(1) Manufacturing process (A)
Process (A) of FIG. 1 shows the manufacturing process of 1st Embodiment. First, by weight%, C 0.5 to 0.7%, Si 1.2 to 3.0%, Mn 0.3 to 1.2%, Cr 0.5 to 1.9%, As well as containing 0.05 to 0.5% of V, containing 1.5% or less of Ni, 1.5% or less of Mo, and 0.5% or less of W as an optional component, A steel wire rod M having a circle equivalent diameter of 1.5 to 10 mm, the balance of which is iron and unavoidable impurities, is prepared. The steel wire rod M is supplied to the feed roller 10 by a wire drawing machine (not shown), and the steel wire rod M is heated to the austenite region within 2.5 seconds by the high frequency heating coil 40 and coiling is performed in the coiling portion 20 (coiling) Process).

  At this time, carburizing treatment of the steel wire rod M in the enclosing member 50 is simultaneously performed. Carburizing treatment is performed at a wire temperature of 850 to 1150 ° C., and a C concentrated layer having a maximum C concentration of 0.7 to 1.2% and a thickness of 0.01 to 0.05 mm on the surface of the steel wire M Form. Thereby, the surface layer part which is 30 HV or more higher than the wire internal hardness can be obtained.

  Next, after coiling, a coil which is separated and whose temperature is still in the austenite region is quenched as it is in a quenching tank (not shown) (quenching solvent, for example, oil of about 60 ° C.) (quenching step) 150 to 500 ° C.) (tempering step). By hardening, it becomes a high hardness structure which consists of a martensitic structure, and by further tempering, it can be set as a tempered martensitic structure excellent in toughness. Here, the quenching and tempering treatment may be performed by a general method, and the heating temperature of the wire before quenching, the type and temperature of the quenching solvent, and the temperature and time of tempering are appropriately set according to the material of the steel wire M. .

  Furthermore, desired fatigue resistance can be obtained by subjecting the steel wire rod M to a shot peening treatment (shot peening step) and a setting treatment (setting step). Since coiling is performed in a state of being heated to the austenite region, generation of residual stress due to processing can be prevented. For this reason, compared with the cold forming method in which tensile residual stress is generated on the coil inner diameter side surface by processing, compressive residual stress is more easily given by shot peening, and the compression is deep and large compression from the surface on the inner diameter side of the spring which becomes high stress. Residual stress can be effectively applied. Furthermore, by performing the setting process, a deeper compressive residual stress distribution is formed in the direction of the maximum principal stress when used as a spring, and fatigue resistance can be improved.

  In the present embodiment, a first shot peening treatment with a shot with a particle diameter of 0.6 to 1.2 mm, a second shot peening treatment with a shot with a particle diameter of 0.2 to 0.8 mm, a particle diameter of 0. The multistage shot peening process which consists of the 3rd shot peening process by the shot of 02-0.30 mm is performed. In the shot peening treatment performed later, since the shot smaller than the shot peening treatment performed first is used, the surface roughness of the wire can be made smooth.

  Shots used for shot peening may be steel cut wire, steel beads, high hardness particles including FeCrB, and the like. In addition, the compressive residual stress can be adjusted by the ball equivalent diameter of the shot, the projection speed, the projection time, and the multistage projection method.

  Further, in the present embodiment, the hot setting is performed as the setting process, heating to 100 to 300 ° C., and the amount of shear strain acting on the surface of the wire is equal to or more than the amount of shear strain at the working stress in actual use as a spring. Apply plastic strain to the spring-shaped steel material.

  The compression coil spring of the present invention manufactured according to the above-described step (A) has a C-concentrated layer exceeding the average concentration of C contained in the steel wire rod in the surface layer portion, and the entire circumference of the steel wire rod is C It is a compression coiled spring whose thickness of a concentration layer enters into the range of 0.01-0.05 mm. In such a compression coil spring, since a C-concentrated layer having a thin and uniform thickness is formed on the surface of the steel wire rod, the residual austenite phase can be reduced and the sag resistance can be improved, and the vicinity of the surface is made high. The fatigue resistance can be improved by improving yield stress as hardness and efficiently obtaining the effect of shot peening.

Next, steps (B) and (C) will be described for comparison with the embodiment of the present invention.
In step (B) of FIG. 1, cold coiling is performed on the steel wire rod M used in step (A) by an arbitrary coiling machine (coiling step). Then, the temperature was raised to the austenite region under reduced pressure comprising a hydrocarbon gas to the steel wire material after coiling, (as a quenching agent, for example, 60 ° C. of about oil) quenching performing (carburizing + quenching process). Next, similarly to the step (A), the tempering step, the shot peening step, and the setting step are sequentially performed.

  In step (C), annealing and nitriding are performed without carburizing, quenching, and tempering in step (B).

1. Sample Preparation Method Samples of coil springs were prepared by each manufacturing process, and their fatigue resistance was evaluated. First, an oil-tempered wire having the chemical components shown in Table 1 and the balance of iron and unavoidable impurities was prepared. The wire diameter is 4.1 mm, the spring index is 6, the total number of turns is 5.75, and the effective number of turns for the oil-tempered wire according to the manufacturing steps A to C shown in FIG. A 3.25-turn, closed-end coil spring was produced. In Table 1, "OT wire" means oil-tempered wire.

  In the manufacturing process A, the steel wire is heated by a coiling machine (see FIG. 2) provided with a high frequency heating coil, an enclosure member, and a gas supply unit, and after carburizing treatment is performed at a treatment temperature shown in Table 2, coiling is performed, Hardened with oil at 60 ° C. In Table 2, the carburizing temperature is the surface temperature of the steel wire. Thereafter, tempering was performed under the conditions described in Table 2 (Inventive Examples 1 to 7, Comparative Examples 1 to 4).

  In Table 2, "coiling + carburizing method" indicates that carburizing is performed on a steel wire heated immediately before coiling, "A" is a carburizing method using an enclosure member and a gas supply unit, and "B" is It is a carburizing method which blows hydrocarbon type gas on the surface of a steel wire from one nozzle.

In manufacturing step B, after cold coiling by any coiling machine, the temperature was raised to the austenite region under reduced pressure comprising a hydrocarbon gas coiling steel wire rod, after quenching by 60 ° C. oil, 300 A tempering treatment was performed at ° C. (Comparative Example 6). In the manufacturing process C, after cold coiling, annealing treatment was performed at 430 ° C., and then nitriding treatment was performed. In the nitriding treatment, a hard layer having a depth of 0.04 mm was formed on the surface of the wire (Comparative Examples 7 and 8).

  Next, each sample was subjected to a shot peening treatment and a setting treatment. In the shot peening process, the first shot peening process using a steel round cut wire with a ball equivalent diameter of 1.0 mm, the second shot peening process using a steel round cut wire with a ball equivalent diameter of 0.5 mm, and the ball equivalent diameter The third shot peening treatment with 0.1 mm steel beads was sequentially performed. The setting was hot setting, and the heating temperature of the coil spring was 200 ° C., and the load stress was 1500 MPa.

2. Evaluation method Various properties were investigated as follows with respect to the sample obtained in this way. The results are shown in Table 3.

(1) Hardness (HV)
The hardness of the wire cross section of the coil spring was measured using a Vickers hardness tester (Future Tech FM-600). The measurement load is 25 gf at a position 0.02 mm deep from the surface ("surface" in Table 3), and 200 gf at a depth d (wire diameter) / 4 mm position ("inner" in Table 3). Measurement was made at any three concentric points, and the average value was calculated.

(2) Compression residual stress integral value (I- _{σ R} ), crossing point (CP)
X-ray diffraction type residual stress measuring device (made by RIGAKU) on the inside diameter side surface of the coil spring in the direction of + 45 ° (the direction of substantially maximum principal stress when a compressive load is applied to the spring) with respect to the wire axis direction of the wire ) Was used. The measurement was performed with a tube: Cr, a collimator diameter: 0.5 mm. The above measurement was performed after chemical polishing of the entire surface of the wire using hydrochloric acid with respect to a coil spring, and the above measurement was repeated to determine the residual stress distribution in the depth direction, and the crossing point was determined from the result. The compressive residual stress integral value was calculated by integrating the compressive residual stress from the surface to the crossing point in the relationship diagram between depth and residual stress. In addition, the residual stress distribution of the invention example 1 is shown in FIG. 3 as an example.

(3) Surface C concentration (C _C ), C-concentrated layer thickness (C _t )
The wire cross section of the coil spring was measured at six points every 60 °, and the average value of the surface C concentration, the average value of the thickness of the C-concentrated layer, the maximum value, and the minimum value were measured. For measurement, line analysis was performed using EPMA (Shimadzu EPMA-1600) with a beam diameter of 1 μm and a measurement pitch of 1 μm. The C-concentrated layer thickness was the depth from the surface to the same C concentration as the inside of the wire.

(4) Retained austenite (I _γR )
In the wire cross section of the coil spring, the volume fraction of six retained austenites is measured at every 60 ° for each measurement depth from the outermost surface to 0.5 mm, and the ordinate is the retained austenite volume fraction and the abscissa is the wire In the radial direction retained austenite distribution curve, an integral value I _γR from the surface to a depth of 0.5 mm was determined. For measurement, a two-dimensional PSPC mounted X-ray diffractometer (Bruker D8 DISCOVER) was used. In addition, the retained austenite distribution of the invention example 1 is shown in FIG. 4 as an example.

(5) Surface roughness (Rz (maximum height))
The surface roughness was measured according to JIS B0601 using a non-contact three-dimensional shape measuring apparatus (MITAKA NH-3). The measurement conditions were: measurement magnification: 100 times, measurement distance: 4 mm, measurement pitch: 0.002 mm, cutoff value: 0.8 mm.

(6) Average grain size (d _GS )
The average crystal grain size was measured by JEOL JSM-7000F (TSL Solutions OIM-Analysys Ver. 4.6) according to SEM / EBSD (Electron Back Scatter Diffraction) method. Here, the measurement was performed at a depth d / 4 of the cross section of the coil spring, and was performed at an observation magnification of 5000 times, and the average grain size was calculated with boundaries of 5 ° or more in azimuth angle as grain boundaries.

(7) Fatigue resistance (break rate)
A fatigue test was conducted at room temperature (in the air) using a hydraulic servo fatigue test machine (Kashimiya Seisakusho). For the components A and B in Table 1, test stress: 735 ± 686 MPa, frequency: 20 Hz, number of tests: 7 each, with a breakage rate after 20 million vibrations (number of breaks / number of tests) The fatigue resistance was evaluated. For component C, test stress: 760 ± 711 MPa, frequency: 20 Hz, number of tests: 7 for each, and fatigue resistance is evaluated by the breakage rate (number of breakages / number of tests) after 20 million vibrations. did.

(8) Settling resistance (residual shear strain rate Δγ)
A warm tightening test was performed on the coil spring. The conditions at that time are a test stress: 1100 MPa, a test temperature: 120 ° C., and a test time: 48 hours. Then, the residual shear strain rate Δγ was calculated from the amount of load loss after the test relative to before the test using the following equation 1.

3. Evaluation results (1) Hardness As can be seen from Table 3, in the invention examples 1 to 7 produced by the hot forming method of the step (A), the internal hardness is 600 to 710 HV and high fatigue resistance can be obtained . On the other hand, from the results of Comparative Examples 2 and 3, even with a coil spring manufactured by a hot forming method, sufficient fatigue resistance can not be obtained when the hardness is less than 600 HV or greater than 710 HV. Moreover, in the invention examples 1-7, the hardness of the surface is high 30 HV or more compared with the inside by carburizing. As a result, high compressive residual stress can be obtained in the vicinity of the surface, and the occurrence of fatigue cracks originating from the vicinity of the surface (including the outermost surface) can be prevented (improved fatigue resistance). On the other hand, in Comparative Example 1, the hardness increase of the surface is less than 30 HV, the wear in the inter-line portion where contact is repeated during operation is severe, and early breakage from the same portion is achieved, and sufficient fatigue resistance is obtained. Absent.

(2) Residual Stress Distribution In Inventive Examples 1 to 7, I _-σR is _{180 MPa} · mm or more, deep and large compressive residual stress can be obtained, and fatigue resistance is good. On the other hand, in Comparative Examples 7 and 8, I _{−σ R} is 150 MPa · mm or less, the compressive residual stress is shallow and small, and the fatigue resistance is lowered. The reason for this is that in the invention examples 1 to 7 produced in the step (A), tensile residual stress (remaining on the inner diameter side of the coil) generated in cold coiling hardly occurs in hot coiling, so tension is generated by cold coiling This is because compressive residual stress due to shot peening easily penetrates deeper from the surface as compared with Comparative Examples 7 and 8 in which residual stress is generated.

(3) Surface C concentration, C-concentrated layer thickness In the invention examples 1 to 7, surface C concentration is 0.7 to 1.2%, C-concentrated layer thickness (depth from the surface having the same C concentration as the inside of the wire) Carburization of 0.01 mm or more and 0.05 mm or less, and because the hardness near the surface is high, high compressive residual stress near the surface is obtained, and the surface roughness is also improved. High fatigue resistance can be obtained. On the other hand, in Comparative Example 5, the average C-thickened layer thickness is equivalent to that of Inventive Examples 1 to 7, but the carburizing method is different, so that the variation in C-thickened layer thickness is large. Therefore, the thickness exceeds 0.05 mm at a portion where the C-concentrated layer thickness is large, and excessive carburization causes an increase in retained austenite. (In relation diagram of a depth and gamma _R, the integrated value from the surface of gamma _R to 0.5mm depth) in the invention examples 1 to 7 I _{[gamma] R} is whereas the less 3.1% · mm, Comparative Example 5 In the case of the comparative example 5, as compared with the invention examples 1 to 7, the residual shear strain rate Δγ is as small as 0.050 to 0.065 and the sag resistance is good. The residual shear strain rate Δγ is as large as 0.080, and the sag resistance is lowered. In Comparative Example 6, the C concentration in the surface is 1.1%, and the C-concentrated layer thickness is 0.90 mm, and excessive carburization causes an increase in retained austenite, I _γR is as large as 3.55% · mm, and as a result, compared with Inventive Examples 1 to 7, the residual shear strain ratio Δγ is as low as 0.093 and the sag resistance is lowered.

(4) Surface Roughness In the invention examples 1 to 7 obtained with high fatigue resistance, the surface roughness Rz (maximum height) is 12.0 μm or less, and the desired surface roughness Rz of 20 μm or less is sufficiently satisfied. ing. Here, when Rz exceeds 20 μm, a valley in the surface roughness becomes a stress concentration source, and a crack is generated and developed starting from the valley, resulting in early breakage. Moreover, this surface roughness is formed by rubbing with tools at the time of coiling or by shot peening treatment. The surface roughness formed by the shot peening process is determined by the combination of the hardness of the wire and the conditions such as the particle size, hardness, and projection speed of the shot. Therefore, the conditions for shot peening need to be set appropriately so that Rz does not exceed 20 μm.

(5) Average Crystal Grain Size In the inventive example, the average crystal grain size (d _GS ) is 0.84 to 1.30 μm and has a fine crystal structure. This is because, as described above, performing heating in a short time by high frequency heating leads to suppression of coarsening of the structure or to miniaturization, and as a result, in the invention examples 1 to 7, the fine average crystal grain size is The resulting fatigue resistance is improved. On the other hand, in Comparative Example 4, the coiling and carburizing temperature is high, and the average grain size (d _GS ) is as large as 1.35 μm as compared with the invention example. Therefore, the fatigue resistance and fatigue resistance are reduced.

As mentioned above, according to the manufacturing method of the compression coiled spring of this invention, fatigue resistance and fatigue resistance can be improved significantly.

The compression coil spring manufactured according to the present invention is used in a valve spring, particularly a valve spring of a racing engine used under high stress, and a clutch because it has high fatigue resistance and high fatigue resistance. It can be used as a clutch torsion spring or the like.

  DESCRIPTION OF SYMBOLS 1 ... Coiling machine shaping part, 10 ... Feed roller, 20 ... Coiling part, 21 ... Wire guide, 22 ... Coiling tool, 22a ... Coiling pin, 30 ... Cutting means, 30a ... Cutting blade, 30b ... Inner type, 40 ... High frequency Heating coil, 50: enclosure member, 50a: enclosure member steel wire rod inlet, 50b: enclosure member steel wire rod outlet, 60: gas supply part (gas supply means), M: steel wire rod.

Claims (10)

A coiling step of hot forming a steel wire rod by a coil spring forming machine, a quenching step of quenching the coil whose coiling temperature is still in the austenite region after coiling, and a tempering step of tempering the quenched coil; In a method of manufacturing a compression coil spring including a shot peening step of applying a compressive residual stress to a wire surface, heating, carburizing and hot forming are performed in the coiling step, and the coil spring forming machine continuously performs steel A feed roller for supplying a wire, a coiling portion for forming the steel wire into a coil, and a cutting means for cutting the steel wire which is continuously supplied from the rear after coiling a predetermined number of turns of the steel wire; Have
The coiling unit includes a wire guide for guiding a steel wire supplied by the feed roller to an appropriate position of a processing unit, and a coil shape for processing the steel wire supplied via the wire guide. It has a coiling tool consisting of a coiling pin or coiling roller and a pitching tool for pitching.
The coil spring forming machine further includes heating means for raising the temperature of the steel wire to the austenite region between the coiling tool and the outlet of the feed roller, and the coiling tool is extended from the steel wire inlet side in the heating means. An enclosure member covering the outer periphery of the steel wire rod is disposed in a part or the whole of the space, and has a gas supply means for supplying a hydrocarbon-based gas into the enclosure member ,
The heating means is a high frequency heating device, and a high frequency heating coil is disposed on the passage path of the steel wire so as to be concentric with the steel wire.
The method for manufacturing a compression coil spring , wherein the surrounding member is disposed inside the high frequency heating coil, and the high frequency heating coil directly heats the steel wire . The method according to claim 1, wherein the enclosing member is made of a ceramic. The enclosure member is provided at both ends thereof with a steel wire rod inlet and a steel wire rod outlet each having a diameter smaller than that of the enclosure member, and the gas supply unit supplies hydrocarbon gas from the steel wire rod inlet of the enclosure member. The manufacturing method of the compression coiled spring of Claim 1 or 2 characterized by the above-mentioned. The enclosure member is provided at both ends thereof with a steel wire rod inlet and a steel wire rod outlet each having a diameter smaller than that of the enclosure member, and the gas supply unit supplies hydrocarbon gas from the steel wire rod outlet of the enclosure member. The manufacturing method of the compression coiled spring of Claim 1 or 2 characterized by the above-mentioned.   A C-concentrated layer exceeding the average concentration of C contained in the steel wire is formed in the surface layer portion of the steel wire, and the thickness of the C-concentrated layer is 0.01 to 0 over the entire circumference of the steel wire. The method for manufacturing a compression coil spring according to any one of claims 1 to 4, wherein the width is in the range of .05 mm.   The internal hardness in the arbitrary wire rod cross section of the above-mentioned steel wire rod shall be 600-710HV, and the highest hardness in said C concentration layer is made higher by 30 HV or more than internal hardness. The manufacturing method of the compression coiled spring as described in-.   7. The method according to claim 1, wherein an average crystal grain size (a boundary having an azimuthal angle difference of 5 ° or more is a grain boundary) measured using an SEM / EBSD method is 1.3 μm or less. Method of a compression coil spring. In the direction of maximum principal stress on the inner diameter side of the coil spring that occurs when a compressive load is applied to the coil spring, let the depth from the surface of the wire where the value of compressive residual stress at no load becomes zero be the crossing point. When the integral from the surface to the crossing point in the residual stress distribution curve where the horizontal axis is depth from the surface is expressed as I _-σR , I _-σR is 150 MPa · mm or more. The manufacturing method of the compression coiled spring in any one of Claims 1-7. Regarding retained austenite volume fraction γR measured using X-ray diffraction method, in the retained austenite distribution curve with the retained austenite volume fraction on the vertical axis and the depth from the surface on the horizontal axis, the integral value from the surface to 0.5 mm depth when expressed as I _{[gamma] R,} the manufacturing method of the compression coil spring according to claim 1, characterized in that the I _{[gamma] R} below 3.4% · mm.   The method for manufacturing a compression coil spring according to any one of claims 1 to 9, wherein the surface roughness Rz (maximum height) is 20 μm or less.

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