JP5941439B2 - Coil spring and manufacturing method thereof - Google Patents

Description

  The present invention relates to a coil spring and a manufacturing method thereof, and more particularly to a coil spring excellent in fatigue resistance and a manufacturing method thereof.

  Coil springs are used as valve springs, clutch springs, suspension springs and the like in automobile engines, clutches, suspensions and the like. Since the coil spring is repeatedly used with a high stress over a long period of time, a high level of fatigue resistance is required.

  For example, as a wire for a valve spring in an engine, JIS includes an oil temper wire for a valve spring (SWO-V: JIS G 3561), a chrome vanadium steel oil temper wire for a valve spring (SWOCV-V: JIS G 3565), and Silicon chrome steel oil tempered wires (SWOSC-V: JIS G 3566) for valve springs are defined, and conventionally, SWOSC-V having excellent fatigue strength has been mainly used.

  These wire rods are subjected to quenching and tempering treatment after drawing the rolled material, and are made to have the required strength, and after using this, coiling into a spring of the required shape, nitriding, shot peening, temper, A general manufacturing method of a valve spring is to obtain a spring having excellent fatigue resistance by performing a treatment such as setting.

  From the viewpoint of environmental protection and resource protection, there is a high demand for purifying exhaust and improving fuel efficiency for automobiles, but the major contribution to these is the weight reduction of the vehicle, and the parts that make up the body are also lightweight. Efforts to make it happen are constantly being made.

  As for the valve spring, it is possible to make the valve spring more compact by further improving its fatigue resistance, and it is possible to contribute to the weight reduction of the engine. Therefore, proposals have been made to improve the fatigue resistance of valve springs.

  For example, Patent Document 1 has a predetermined component composition, has a carburized hardened layer (0.05 to 1.00 mm) on the surface, and has a hardness at a position of 0.02 mm from the surface within a predetermined range ( 650-1000 Hv), a technique for improving fatigue resistance is disclosed.

JP 2012-77367 A

  Although the fatigue resistance of the above-mentioned Patent Document 1 is about 50 million times, recent automobiles have been further reduced in weight and output, and accordingly, a coil spring having further excellent fatigue resistance has been developed. It is requested.

  The present invention has been made paying attention to the above circumstances, and an object thereof is to provide a coil spring excellent in fatigue resistance and a method of manufacturing such a coil spring excellent in fatigue resistance. There is.

  The present invention that has solved the above problems is C: 0.40 to 0.70% (% means “mass%”, the same applies to the chemical composition), Si: 1.50 to 3.50% , Mn: 0.30 to 1.50%, Cr: 0.10 to 1.50%, V: 0.50 to 1.00%, Al: 0.01% or less (not including 0%) The balance is made of iron and steel, which is an inevitable impurity, and the average grain size number of the prior austenite crystals at a depth of 0.3 mm from the surface layer is 11.0 or more, and the grain size number difference of the prior austenite crystals is It is within the range of less than 3 compared to the maximum frequency particle size number, and is provided with a carburized hardened layer having a depth of 0.30 to 1.00 mm from the surface layer, and the depth direction (1/4) × diameter from the surface layer. Summary that the average value of Vickers hardness at the position is 600 or more A.

  Furthermore, it is also a preferred embodiment that the chemical component composition of the coil spring includes Ni: 1.50% or less (not including 0%) and / or Nb: 0.50% or less (not including 0%). .

  In manufacturing the coil spring having excellent fatigue resistance as described above, it is recommended that the vacuum carburizing treatment be performed at 1000 ° C. or higher.

  According to the present invention, the chemical composition and the prior austenite grain size are appropriately controlled, and the depth of the carburized hardened layer from the coil spring surface layer and the Vickers hardness are appropriately controlled, thereby improving fatigue resistance. An excellent coil spring can be provided. Moreover, according to the method of the present invention, a coil spring having excellent fatigue resistance can be provided.

FIG. 1 is a schematic explanatory view of a carburized hardened layer measurement position of a coil spring and a Vickers hardness measurement position of a 1/4 × diameter position. FIG. 2 is a schematic explanatory view of the measurement position of the prior austenite grain size of the coil spring.

  In order to provide a coil spring having excellent fatigue resistance, the fatigue resistance of the present invention is further improved than before, and results in exceeding 60 million times in the fracture life test in the examples described below. We examined from various angles. In Patent Document 1, while increasing the amount of C added and controlling the metallographic structure, it was not possible to obtain a fracture life of 60 million times (Example 4 of Patent Document 1 and the Example). No. 8 in Table 2 that simulates the above.

  Therefore, as a result of studying the chemical composition, the metal composition, and the like in order to achieve better fatigue resistance, the present inventors have found that the toughness and strength of the coil spring have an effect on fatigue breakage during use of the coil spring. We obtained knowledge that fatigue resistance can be greatly improved by appropriately controlling these.

  First, in order to increase the strength of the coil spring, the depth of the carburized hardened layer from the steel surface layer (hereinafter simply referred to as “surface layer”) forming the coil spring and the inside of the steel (1/4 × coil spring) It is necessary to sufficiently secure the Vickers hardness of the steel wire forming the diameter D, which may be hereinafter referred to as “1/4 × D”. In order to sufficiently secure the depth of the carburized hardened layer and the Vickers hardness, it is necessary to increase the temperature during the carburizing process. However, the fracture life cannot be improved only by carburizing at a high temperature. The reason for this is that when carburizing at a high temperature, the crystal grains of the prior austenite become coarse or the crystal grain size of the prior austenite varies (there is a difference in grain size number: hereinafter referred to as “mixed grain”). This is because the toughness of the coil spring is remarkably lowered and the fracture life is deteriorated.

  As a result of intensive studies by the present inventors on such problems, it has been found that the above problems can be solved by appropriately controlling the chemical composition of the steel. In particular, it was found that by increasing the amount of V in the chemical component composition, coarsening of the crystal grain size of the prior austenite can be suppressed even when carburizing at high temperature, and mixed grains can also be suppressed.

  And on the premise that the following chemical composition is satisfied in the present invention, the balance between strength and toughness required for improving fatigue resistance is further achieved by appropriately controlling the carburized layer depth, Vickers hardness, and prior austenite grain size. As a result, the present inventors have found that a coil spring having excellent fatigue resistance can be provided.

  Hereinafter, the chemical component composition of the coil spring of the present invention will be described.

C: 0.40 to 0.70%
C is an element useful for ensuring the strength of a coil spring to which a high stress is applied and the Vickers hardness at the 1/4 × D position of the coil spring. In order to exert such an effect, the C content is 0.40% or more, preferably 0.45% or more, more preferably 0.50% or more. However, when the C content is excessive, the toughness is lowered and the surface flaw of the coil spring is increased to reduce the fatigue resistance. Therefore, the C content is 0.70% or less, preferably 0.65% or less, more preferably 0.60% or less.

Si: 1.50 to 3.50%
Si, like C, is an element useful for ensuring Vickers hardness, and is an element effective for improving the strength of the coil spring and improving fatigue resistance and sag resistance. In order to exert such an effect, the Si content is 1.50% or more, preferably 1.80% or more, more preferably 2.10% or more. However, when the Si content is excessive, the toughness is deteriorated, the cold workability and hot workability in the manufacturing process of the coil spring are lowered, the product yield is deteriorated, and the decarburization by the heat treatment is promoted, and the fatigue resistance is increased. Sex is reduced. Therefore, the Si content is 3.50% or less, preferably 3.30% or less, more preferably 3.10% or less.

Mn: 0.30 to 1.50%
Mn is an element effective for enhancing the hardenability and improving the strength of the coil spring. Moreover, it has the effect | action which fixes S in steel harmful to fatigue resistance as MnS, and reduces the damage. In order to exert such an effect, the Mn content is 0.30% or more, preferably 0.40% or more, more preferably 0.50% or more. However, when the Mn content is excessive, not only the toughness is deteriorated but also cold workability and fatigue strength are lowered. Therefore, the Mn content is 1.50% or less, preferably 1.20% or less, more preferably 0.90% or less.

Cr: 0.10 to 1.50%
Cr, like Mn, is an element effective for improving the hardenability and improving the strength of the coil spring. Cr also has the effect of reducing the activity of C and preventing decarburization during hot rolling or heat treatment. In order to exhibit such an effect, the Cr content is 0.10% or more, preferably 0.15% or more, more preferably 0.20% or more. However, when the Cr content is excessive, the C diffusion coefficient in the vacuum carburizing process is remarkably lowered, so that it becomes difficult to form a desired carburized hardened layer and the fatigue resistance is lowered. Further, when the carburizing temperature is increased in order to secure a desired carburized hardened layer, the prior austenite crystal is coarsened and mixed grains are produced, resulting in deterioration of fatigue resistance. Therefore, the Cr content is 1.50% or less, preferably 1.20% or less, more preferably 0.90% or less.

V: 0.50 to 1.00%
V is an element effective for refining the prior austenite crystal grains. In particular, V is an element effective for suppressing coarsening of old austenite crystals and generation of mixed grains, which are problematic when the carburizing temperature is increased to secure a desired carburized hardened layer. In order to exert such an effect, the V content is 0.50% or more, preferably 0.53% or more, more preferably 0.56% or more. However, when the V content is excessive, a large amount of V carbide is formed, resulting in a decrease in ductility, and cold workability and fatigue resistance are deteriorated. Therefore, the V content is 1.00% or less, preferably 0.90% or less, more preferably 0.80% or less.

Al: 0.01% or less (excluding 0%)
Al is a deoxidizing element, but when it is contained in excess, inclusions such as AlN are formed. These inclusions significantly reduce the fatigue resistance of the coil spring. Therefore, the Al content needs to be reduced to 0.01% or less, preferably 0.008% or less, more preferably 0.006% or less.

  The basic chemical composition of the steel constituting the coil spring of the present invention is as described above, and the remaining component is substantially iron. Here, “substantially” does not impair the features of the present invention by mixing trace elements that are inevitably mixed in steel raw materials including scrap, iron making / steel making processes, and further steelmaking pretreatment processes. It means to allow in the range. For example, P (0.015% or less) and S (0.015% or less) are exemplified as inevitable impurities.

  In the present invention, as other elements, both Ni and Nb or one of them may be contained within the following range as required. Depending on the type of element to be contained, the characteristics of the coil spring are further improved.

Ni: 1.50% or less (excluding 0%)
Ni is an element effective for improving the toughness of the coil spring whose strength is increased by C. In order to exert such an effect, the Ni content is preferably 0.05% or more, more preferably 0.10% or more. However, when the Ni content is excessive, retained austenite is excessively generated and fatigue resistance is lowered. Therefore, the Ni content is preferably 1.50% or less, more preferably 1.20% or less, and still more preferably 0.90% or less.

Nb: 0.50% or less (excluding 0%)
Nb has an effect of refining crystal grains in hot rolling and quenching and tempering treatments, and is an element effective for improving ductility. In order to exert such an effect, the Nb content is preferably 0.01% or more, more preferably 0.02% or more. However, when the Nb content is excessive, V carbides are excessively generated to deteriorate ductility, and cold workability and fatigue strength are reduced. Therefore, the Nb content is preferably 0.50% or less, more preferably 0.40% or less, and still more preferably 0.30% or less.

  In order to improve fatigue resistance, not only the chemical composition is appropriately controlled as described above, but also the metal structure (control of old austenite crystals), carburized hardened layer, and Vickers hardness are appropriately controlled. is important.

Average grain size number of prior austenite crystals: 11.0 or more Fatigue resistance is greatly improved by refining the grain size number of prior austenite crystals at a depth of 0.3 mm from the surface layer of the coil spring and improving toughness. it can. In order to exert such an effect, the average grain size number of the prior austenite crystal is 11.0 or more, preferably 12.0 or more, more preferably 13.0 or more. On the other hand, from the viewpoint of improving toughness, the upper limit of the average grain size number of the prior austenite crystal is not particularly limited. However, in consideration of manufacturability and alloy cost, it is preferably approximately 15.0 or less, more preferably 14.0 or less. is there.

Difference in grain size number of prior austenite crystals: within a range of less than 3 compared to the most frequent grain size number When the variation in grain size number of the prior austenite crystals measured at a depth of 0.3 mm from the surface layer is large, the average grain size number Even if the above is satisfied, the toughness is remarkably lowered and the cold workability and fatigue resistance are deteriorated. Therefore, in the present invention, the measured grain size number of each prior austenite crystal needs to be further different from the maximum frequency grain size number by less than 3, preferably 2 or less, more preferably 1 or less. In the present invention, a case where the condition of such a particle number difference is satisfied is referred to as “no mixed particles”.

  In the present invention, fatigue resistance can be improved by satisfying the average grain size number of the austenite crystal grains and further suppressing mixed grains.

Carburized hardened layer: 0.30 to 1.00mm deep from the surface layer of the coil spring
A suitable carburized hardened layer is effective in improving fatigue resistance. That is, by sufficiently curing the surface side of the coil spring, it is possible to suppress the occurrence of breakage starting from the spring surface when it is repeatedly used with a high load stress. In order to exert such an effect, it is necessary to form a carburized hardened layer having a depth of 0.30 mm or more, preferably 0.40 mm or more, more preferably 0.50 mm or more from the surface layer of the coil spring. However, when the carburized hardened layer becomes excessive, the carbide precipitates coarsely, so that the fatigue resistance deteriorates. Therefore, the carburized hardened layer needs to have a depth of 1.00 mm or less, preferably 0.90 mm or less, more preferably 0.80 mm or less from the surface layer of the coil spring.

Average value of Vickers hardness at position of depth direction (1/4) × diameter D from the surface layer: 600 or more Appropriate Vickers hardness (Hv) inside the steel of the coil spring is effective in improving fatigue resistance. In other words, if the internal hardness of the coil spring is low, when it is used repeatedly with high load stress, even if the stress applied to the spring is within the elastic limit, the coil spring will be plastically deformed and the required spring stress will not be exhibited. , Fatigue resistance decreases. Therefore, from the viewpoint of improving fatigue resistance, the average value of Vickers hardness at least in the depth direction (1/4) × D position from the surface layer of the coil spring is 600 or more, preferably 670 or more, more preferably 690 or more. is there. The upper limit of the average value of Vickers hardness is not particularly limited, but if it becomes too hard, the toughness may decrease and the fatigue resistance may decrease, so the average value of the Vickers hardness is preferably 750 or less, more preferably Is 730 or less.

  In manufacturing the coil spring having excellent fatigue resistance as described above, it is desirable to appropriately control the manufacturing conditions. In particular, in order to ensure the depth of the predetermined carburized hardened layer and the Vickers hardness (average value), it is effective to control the temperature during the vacuum carburizing process. Hereinafter, preferable conditions for manufacturing the coil spring of the present invention will be described.

  The coil spring of the present invention is made of steel satisfying the above-mentioned predetermined chemical composition, melted, hot forged, hot rolled to obtain a wire with a desired wire diameter, and is used for cutting, patenting, wire drawing, and oil tempering. Thereafter, it can be manufactured by forming it into a spring and subjecting it to a vacuum carburizing treatment. Thereafter, in order to further improve the fatigue characteristics, shot peening or setting may be performed as necessary.

  The melting, hot forging, and hot rolling conditions are not particularly limited, and general production conditions may be employed. For example, a steel ingot that satisfies the above-mentioned predetermined chemical composition is melted in a blast furnace, and then the ingot is rolled into a predetermined size to produce a billet of a predetermined size, which suppresses deformation resistance that affects workability and prior austenite crystals From the viewpoint of suppressing the coarsening of grains, for example, after heating to about 900 ° C. to 1100 ° C., hot rolling may be performed at a desired reduction rate to obtain a desired linear wire. Thereafter, the deoxidized layer on the surface of the wire rod is removed by skinning at a desired thickness, and the work hardened layer produced by the skinning treatment is removed and in order to obtain a structure excellent in wire drawing (for example, pearlite) Performs patenting treatment and softening annealing treatment in IH (high frequency heating) equipment.

  Then, after drawing to a desired wire diameter, an oil temper treatment is performed to obtain a spring wire. Then, it is formed into a spring with a desired coil diameter, free height, and number of turns. The reason why it is formed into a spring shape before carburizing treatment is that after carburizing and tempering to form a carburized hardened layer, the steel surface layer (carburized hardened layer) is hard and ductile, resulting in a coil spring. Because it is difficult to do.

  After forming into a spring shape, vacuum carburizing treatment is performed. In the present invention, in order to obtain the above-mentioned predetermined carburized hardened layer depth and Vickers hardness, it is necessary to perform vacuum carburizing treatment at a high temperature carburizing temperature of 1000 ° C. or higher. is there. When the carburizing temperature is lower than 1000 ° C., a desired carburized hardened layer and Vickers hardness cannot be obtained, and fatigue resistance is lowered. A preferable carburizing temperature is 1020 ° C. or higher, more preferably 1040 ° C. or higher. However, if the carburizing temperature is too high, carbides may precipitate coarsely or become too hard to reduce toughness and fatigue resistance. Therefore, the carburizing temperature is preferably 1100 ° C. or lower, more preferably 1080 ° C. or lower.

  Next, carburizing treatment is performed. If the decarburization is large during the carburizing process and the variation in the processing temperature increases, the fatigue strength of the coil spring decreases. Therefore, in the present invention, vacuum carburization is performed from the viewpoint of suppressing decarburization and temperature variation. Moreover, a uniform carburized hardened layer can be formed with the said desired thickness by performing a vacuum carburizing process at the temperature of 1000 degreeC or more. The carburizing time and the diffusion time are not particularly limited, and may be such that the desired carburized hardened layer is formed. For example, the carburizing time may be 1 minute to 10 minutes, and the diffusion time may be 1 minute to 10 minutes.

After the carburization, gas cooled to a temperature below the A ₁ transformation point, or the oil quenching. Thereafter, it is also desirable to perform a reheating treatment (for example, at 830 ° C. to 850 ° C. for 10 to 30 minutes), whereby further refinement of the prior austenite crystal grains can be achieved.

  The obtained coil spring may be subjected to general shot peening or setting as necessary for the purpose of further improving fatigue resistance.

  In manufacturing the coil spring of the present invention, conditions other than those described above are not particularly limited, and general manufacturing conditions may be employed.

  The coil spring thus obtained can be used as a coil spring excellent in fatigue resistance in various applications such as an engine valve spring and a transmission spring as described above.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  A steel material was melted in a vacuum melting furnace so as to become steels A to H having chemical compositions shown in Table 1 (the balance was iron and inevitable impurities), and hot forged to produce a 155 mm square billet. The billet was heated to 1000 ° C. and hot-rolled to produce a spring wire having a diameter of 8.0 mm. The spring wire was soft annealed (held at 660 ° C. for 2 hours), and then the surface layer portion 0.15 mm of the spring wire was shaved to remove the decarburized layer. Thereafter, the spring wire was heated to a temperature of 900 ° C. or higher in a neutral gas atmosphere to be once austenite. Subsequently, lead patenting treatment (heating temperature: 980 ° C., lead furnace temperature: 620 ° C.) was performed on the wire for spring to transform the structure into pearlite. Thereafter, the wire for spring is cold-drawn to a wire diameter of 4.1 mm and treated with an oil temper under conditions suitable for each wire component (heating temperature: 900 ° C. to 1000 ° C., quenching oil temperature: 60 ° C., tempering) Temperature: 400 to 500 ° C.) to produce a spring wire. Using this spring wire, a cold spring was formed (coil average diameter 24.60 mm, free height 46.55 mm, effective number of turns 5.75) to obtain a spring.

Next, the obtained spring was heated to the “carburizing temperature” described in Table 2 below, and vacuum carburized (carburizing time 5 minutes, diffusion time 3 minutes). Thereafter, the spring was held at 950 ° C. for 15 minutes, then immersed in oil kept at 50 ° C. and quenched, and then tempered (350 ° C., 90 minutes). The obtained spring was subjected to three-stage shot peening (the particle diameter projected gradually from the first stage was reduced), and then hot setting (230 ° C., τ _max = 1600 MPa equivalent) was performed. The following measurements and tests were performed on the obtained coil springs (test materials No. 1 to 13).

(Depth of carburized hardened layer)
The depth of the carburized hardened layer was specified by measuring the carbon concentration of the coil spring. Specifically, as shown in FIG. 1, four lines are drawn at 90 ° intervals from the center point of the cross section of the steel wire forming the coil spring, and carbon (C)% concentration added on each line (C ) A depth equivalent to% was measured. The measured value was entered in the “depth of carburized hardened layer” column in the table. In this invention, the case where the depth of a carburized hardened layer exists in the range of 0.30 mm-1.00 mm was set as the pass.

(Vickers hardness at 1/4 x D position)
The hardness (Hv) of the coil spring was measured using a Vickers hardness tester. Specifically, as shown in FIG. 1, measurement is performed on four lines obtained by subtracting the position (d / 4) of ¼ × diameter D of the cross section of the steel wire forming the coil spring at intervals of 90 degrees ( The test load was 10 kgf), and the average value was obtained. The average value was described in the “Vickers hardness” column in the table. In the present invention, a Vickers hardness of 600 or more was considered acceptable.

(Average grain size number of old austenite crystals)
The measuring method of the crystal grain size of the prior austenite crystal of the coil spring is as follows. Specifically, first, as shown in FIG. 2, a section divided into eight equal parts at intervals of 45 degrees from the center point of the cross section of the coil spring is determined. In each section, the grain size of the prior austenite crystal at a depth of 0.3 mm from the surface layer of the steel wire forming the coil spring toward the center is observed with an optical microscope (400 times magnification) based on JIS G 0551. (Size per field of view: 250 μm × 200 μm) was performed and measured. The average measured value was described in the column “average grain size number of old γ crystal” in the table. In the present invention, the average grain size number of the prior austenite crystals is 11.0 or more.

(Difference in grain number of old austenite crystals)
The method for judging the difference in the grain size numbers of the prior austenite crystals of the coil spring is as follows. Regarding the crystal grain size number of each of the prior austenite crystals measured above, the case where there was a crystal grain having a difference of 3 or more from the maximum frequency grain size number was judged to be mixed. In the “mixed grain” column of the table, “Yes” is described when mixed grains are present, and “None” is described when mixed grains are not present.

(Fatigue resistance: fatigue test)
Each test material obtained was subjected to a fatigue test up to 60 million times with a maximum shear stress (τ _max ) of 588 ± 441 MPa. When the shear stress could be applied to the test material up to 60 million times (that is, when it was not broken), it was determined as “A” (excellent in fatigue resistance), and described as “> 6000” in the table. If the test material could not be subjected to a shear stress of 60 million times (that is, it was broken in the middle), it was determined as “F” (inferior in fatigue resistance), and the number of times the fracture occurred in the table was determined. Described.

  These results can be considered as follows. No. Examples 1 to 7 are examples that satisfy the requirements defined in the present invention (chemical component composition, crystal grain size, carburized hardened layer depth, Vickers hardness). No. It can be seen that the coil springs 1 to 7 all have a long fracture life under high load stress (A judgment) and are excellent in fatigue resistance.

  In contrast, no. Nos. 8 to 13 did not satisfy the chemical composition defined in the present invention and preferred production conditions, and therefore the predetermined crystal grain size, the depth of the carburized hardened layer, the Vickers hardness, etc. could not be secured, and the fatigue resistance was poor. This is an example of (F determination).

  No. Nos. 8 and 9 are examples using the same steel type. This is an example of simulating No. 4 (steel type A, carburizing condition L of Patent Document 1). No. Nos. 8 and 9 are examples in which the V addition amount is small and the Cr addition amount is large, and the carburized hardened layer was shallow because the diffusion coefficient of C was remarkably lowered. In particular, no. Since the carburizing temperature of No. 8 was low, a sufficient depth of the carburized hardened layer could not be secured, and the fatigue resistance was poor. No. No. 9 was treated at the carburizing temperature recommended by the present invention, but since the amount of V added was small, the effect of refining the prior austenite crystal was not sufficiently obtained, and mixed grains were produced, resulting in poor fatigue resistance.

  No. No. 10 had a small amount of V added, so that when it was processed at a predetermined carburizing temperature, mixed grains were generated and fatigue resistance was inferior.

  No. 11 is an example in which the addition amount of C and Si is small and the carburizing temperature is low. In this example, the predetermined Vickers hardness was not obtained, and the fatigue resistance was inferior.

  No. Nos. 12 and 13 had a low carburizing temperature, so that a predetermined carburized hardened layer depth was not obtained, and fatigue resistance was inferior.

Claims (3)

C: 0.40 to 0.70% (% means “mass%”, the same applies to the chemical composition)
Si: 1.50 to 3.50%,
Mn: 0.30 to 1.50%,
Cr: 0.10 to 0 . 90 %,
V: 0.50 to 1.00%, and Al: 0.01% or less (not including 0%),
The balance consists of steel, which is iron and inevitable impurities,
The average grain size number of the prior austenite crystal at a depth of 0.3 mm from the surface layer is 11.0 or more, and the grain number difference of the prior austenite crystal is within a range of less than 3 compared to the maximum frequency grain number. There is a depth of 0. 0 from the surface. 5 provided with a carburized hardened layer of 0～1.00Mm, coil spring average value of Vickers hardness at the position in the depth direction (1/4) × diameter from the surface layer is characterized in that 600 or more. Furthermore,
The coil spring according to claim 1, comprising Ni: 1.50% or less (not including 0%) and / or Nb: 0.50% or less (not including 0%).   It is a manufacturing method of the coil spring of Claim 1 or 2, Comprising: A vacuum carburizing process is performed at 1000 degreeC or more, The manufacturing method of the coil spring characterized by the above-mentioned.