JP2013036087A - Material for spring and manufacturing method thereof, and spring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material for spring having high strength, such as ≥1,900 MPa tensile strength and high ductility, and a manufacturing method thereof.SOLUTION: The material for spring is composed of an iron-based alloy containing prescibed components, and has the area-ratio of the internal structure in any cross section, whose tempered martensite is 30-80%, lower bainite is 5-70% and retained austenite is 8-15%, and the average carbon concentration in the retained austenite is 1.0-2.0 wt.%. The manufacturing method has the following steps in order in which an austenizing step with the temperature of >Acpoint to ≤(Acpoint+250°C), a quenching step of cooling at ≥20°C/sec cooling rate and holding at (Ms point-200°C) to ≤Ms point during 10-60 sec, an isothermal transformation step of heating at ≥10°C/sec rate and holding at >Ms point to ≤(Ms point+70°C) during 90 to 3,600sec, and a cooling step of cooling down to the room-temperature.

Description

  The present invention relates to a spring material and a spring having an excellent balance between strength and ductility, and more particularly to a spring material in which a decrease in ductility, which is a problem with a spring material having a tensile strength of 1900 MPa or more, is suppressed, and a method for producing the same. Regarding springs.

  The suspension springs for automobiles and the valve springs for automobile engines are required to be further lightened in order to improve the fuel efficiency of automobiles, and in recent years, springs having a tensile strength of 1900 MPa or more have been realized. However, when the steel structure is composed only of tempered martensite and retained austenite, the notch sensitivity generally increases as the strength of the steel increases, so pits, surface flaws, inclusions, etc. generated in a corrosive environment are removed. There is a concern that the resulting crack is likely to propagate and adversely affect the spring characteristics. For this reason, there is a need for a steel material that has both high strength and high ductility and is unlikely to propagate cracks.

  In order to solve such problems, the present inventors have proposed a high strength and high ductility spring steel having a tensile strength of 1800 MPa or more mainly composed of lower bainite and having a structure having a high average carbon concentration in retained austenite (patent). Reference 1). Here, the lower bainite is composed of lath-shaped bainitic ferrite and retained austenite and / or iron carbide existing between and inside bainitic ferrite, and iron carbide regularly arranged inside bainitic ferrite. It is the feature that exists. However, there is a need for a high-strength, high-ductility spring steel with further improved tensile strength for further weight reduction.

  Also proposed is a steel sheet having a tensile strength of 980 MPa or more, which is excellent in ductility and stretch flangeability, which defines the area ratio of the martensite, tempered martensite, upper bainite and retained austenite to the entire structure and the carbon concentration in the retained austenite. (Patent Document 2). Here, the upper bainite is composed of lath-shaped bainitic ferrite and residual austenite and / or iron carbide existing between bainitic ferrite, and there is iron carbide regularly arranged inside bainitic ferrite. The feature is not to. However, since it contains soft upper bainite, it is difficult for the tensile strength to exceed 1900 MPa. In the example of Patent Document 2, a steel sheet having a tensile strength of 2234 MPa and a breaking elongation of 8% is shown as a comparative example, but the martensite ratio is high and the ductility is poor. That is, with the structure described in Patent Document 2, it is considered difficult to obtain a steel sheet having a tensile strength of 1900 MPa or more and excellent ductility.

JP2010-222671 JP 2010-090475 PR

  An object of the present invention is to provide a spring material having a high tensile strength of 1900 MPa or more and a high ductility, a manufacturing method thereof, and a spring.

  As a result of earnest research on the method for improving the ductility of the spring material, the present inventors have found that a structure mainly composed of tempered martensite and lower bainite and containing residual austenite having a high average carbon concentration has a high tensile strength. Even if it has, the knowledge that it shows an excellent strength-ductility balance was obtained, and the present invention was achieved.

  That is, the spring material of the present invention is made of an iron-based alloy, and is an area ratio of an internal structure in an arbitrary cross section, with tempered martensite being 30 to 80%, lower bainite being 5 to 70%, and retained austenite being 8 to 15 %, And the average carbon concentration in the retained austenite is 1.0 to 2.0 wt%.

  Another spring material of the present invention is made of an iron-based alloy, and is an area ratio of an internal structure in an arbitrary cross section, with tempered martensite being 30 to 80%, lower bainite being 5 to 70%, and martensite being 0. More than 15% and 15% or less, the retained austenite is 8 to 15%, and the average carbon concentration in the retained austenite is 1.0 to 2.0 wt%.

  First, the reason for limiting the area ratio of the internal tissue in an arbitrary cross section will be described together with the operation of the present invention.

Tempered martensite: 30-80%
Tempered martensite has a high hardness and is excellent in ductility, and thus is a structure necessary for improving the strength-ductility balance of the material. Tempered martensite is obtained by austenitizing the material, rapidly cooling to produce martensite, and further tempering at a predetermined temperature. If the area ratio of the tempered martensite is less than 30%, the area ratio of the as-quenched martensite is high, so that the tensile strength is high but the ductility is poor. On the other hand, if the area ratio of the tempered martensite is too small, the area ratio of the lower bainite becomes too high, making it difficult to obtain a desired tensile strength. On the other hand, if the area ratio of tempered martensite exceeds 80%, the area ratio of lower bainite and retained austenite decreases, so that the ductility becomes poor as described below.

Lower bainite: 5-70%
Lower bainite is a metal structure obtained by isothermal transformation (bainite transformation) of austenitized material at a low temperature in a metal bath or salt bath, and then cooling to room temperature. It consists of bainitic ferrite and iron carbide. Is done. The base bainitic ferrite has a high dislocation density, and the iron carbide has a precipitation strengthening effect, so that the tensile strength can be improved. Further, in a normal quenching and tempering material, iron carbide precipitates at grain boundaries such as prior austenite and martensite blocks, and the grain boundary strength is lowered, so that ductility is likely to be lowered. On the other hand, in the lower bainite structure, iron carbide is finely precipitated on the bainitic ferrite matrix, so that the decrease in grain boundary strength is small, and the decrease in ductility can be prevented. Furthermore, in the process of forming the lower bainite, carbon is discharged from bainitic ferrite to the surrounding supercooled austenite, and the presence of Si suppresses the formation of iron carbide, so the carbon concentration in the supercooled austenite Higher than the average carbon concentration. A part of the supercooled austenite whose carbon concentration is increased and chemically stabilized becomes residual austenite by the subsequent cooling.

  Thus, the lower bainite is an indispensable structure for obtaining high strength and high ductility, and the area ratio is set to 5 to 70%. When the area ratio of the lower bainite is less than 5%, retained austenite having a desired average carbon concentration cannot be obtained. On the other hand, when the area ratio of the lower bainite exceeds 70%, the area ratio of tempered martensite or martensite is small. Therefore, it becomes difficult to obtain a desired tensile strength.

Residual austenite: 8-15%
Residual austenite is effective in increasing ductility using the TRIP (Transformation-Induced Plasticity) phenomenon and improving tensile strength by strain hardening. In order to obtain high ductility, 8% or more of retained austenite is necessary. However, since retained austenite is soft, excessive strength causes a decrease in tensile strength. For this reason, retained austenite is suppressed to 15% or less.

Average carbon concentration in retained austenite: 1.0 to 2.0 wt%
In order to obtain high strength and high ductility, a high average carbon concentration in the retained austenite is an indispensable condition. Since the average carbon concentration in retained austenite increases as carbon is discharged from bainitic ferrite to the surrounding supercooled austenite when austenite transforms into bainite, locally the carbon concentration of individual retained austenite Are considered different. Residual austenite is more stable against deformation as its carbon concentration is higher, and tends to hardly transform into a work-induced martensite phase. Therefore, in the early stage of plastic deformation, the retained austenite having a relatively low carbon concentration is hardened while undergoing martensitic transformation by TRIP to improve ductility, and when plastic deformation proceeds, the retained austenite having a high carbon concentration undergoes martensitic transformation. As a result, high ductility is exhibited. In order to obtain the desired high strength and high ductility, the average carbon concentration in the retained austenite needs to be 1.0 wt% or more. When the average carbon concentration in the retained austenite is less than 1.0 wt%, most of the retained austenite is dominated by transformation hardening by TRIP in the early stage of plastic deformation, so that further improvement of ductility cannot be obtained when plastic deformation progresses. As a result, the desired high strength and high ductility cannot be obtained. However, when the average carbon concentration in the retained austenite exceeds 2.0 wt%, the TRIP phenomenon does not appear or is slight even when plastic working, and it is difficult to obtain a desired tensile strength.

Martensite: more than 0% and not more than 15% If the spring material of the present invention further contains more than 0% martensite, the strength can be further improved. Martensite is a structure with poor ductility, but its hardness is very high. When the area ratio of martensite is more than 0% and 15% or less, the tensile strength can be increased without a significant decrease in ductility. However, when the area ratio of martensite exceeds 15%, the ductility is remarkably lowered, the tensile strength is not increased, and the balance between strength and ductility is remarkably lowered. For this reason, in another spring material of the present invention, in order to further improve the strength, martensite is contained more than 0% and 15% or less.

  In the spring material of the present invention, the above iron-based alloy is, in mass%, C: 0.45 to 0.65%, Si: 1.0 to 2.5%, Mn: 0.1 to 1.0. %, Cr: 0.1 to 1.0%, P: 0.035% or less, S: 0.035% or less, and the balance is preferably made of iron and inevitable impurities. Below, the reason for limitation of the average component of these elements is demonstrated.

C: 0.45-0.65%
C is an element effective for securing a tensile strength of 1900 MPa or more and a desired retained austenite area ratio, and is preferably contained in an amount of 0.45% or more. However, when the C content is excessive, the area ratio of the relatively soft retained austenite is excessively increased and it becomes difficult to obtain a desired tensile strength. Therefore, the C content is preferably suppressed to 0.65% or less. .

Si: 1.0-2.5%
Si has an action of suppressing precipitation of iron carbide when carbon is discharged from bainitic ferrite to surrounding supercooled austenite when bainitic ferrite, which is a base of lower bainite, is generated. That is, since Si is hardly dissolved in iron carbide, iron carbide is deposited avoiding Si. However, since precipitation takes a very long time, precipitation of iron carbide is suppressed. Therefore, it is an effective element for obtaining a desired ratio of retained austenite having a high average carbon concentration by dissolving a large amount of C in austenite. Si is a solid solution strengthening element and is an effective element for obtaining high strength. For this reason, it is preferable that content of Si is 1.0% or more. However, if the Si content is excessive, the area ratio of the soft retained austenite becomes too high and the strength is lowered, so the Si content is preferably suppressed to 2.5% or less.

Mn: 0.1 to 1.0%
Mn is added as a deoxidizing element, but is also an element that stabilizes austenite. Therefore, it is preferable to add 0.1% or more in order to obtain a desired amount of retained austenite. On the other hand, if the Mn content is excessive, segregation of Mn occurs and the workability is liable to decrease, so the Mn content is preferably suppressed to 1.0% or less.

Cr: 0.1 to 1.0%
Cr is an element that improves the hardenability of the material and improves the strength. In addition, there is also an effect of delaying pearlite transformation in an isothermal transformation curve (TTT diagram; Time Temperature Transformation Diagram), and since a tempered martensite structure and a lower bainite structure can be stably obtained, 0.1% or more is added. It is preferable. However, if adding over 1.0%, the workability tends to decrease, so the Cr content is preferably suppressed to 1.0%.

P: 0.035% or less, S: 0.035% or less P and S are elements that promote intergranular fracture due to grain boundary segregation, so a lower content is desirable, and 0.035% or less. It is preferable. More preferably, it is 0.01% or less.

In the spring material of the present invention, the tensile strength is desirably 1900 MPa or more in order to reduce the weight of the spring. In general, the tensile strength and the elongation at break, which is one of representative characteristic values representing ductility, are in a trade-off relationship. When the tensile strength is 1900 MPa or more, the parameter Z defined below is 20000 or more. It is desirable to be.

  The spring material of the present invention is mainly used for suspension springs and valve springs for automobiles. In order to satisfy the required specifications, the equivalent circle diameter of the spring material is preferably 1.5 to 15 mm.

  Moreover, the manufacturing method of the spring material of the present invention uses a material made of an iron-based alloy and austenitizes at a temperature exceeding the Ac3 point (Ac3 point + 250 ° C) and below, and at a rate of 20 ° C / second or more. A quenching step of cooling and holding at a temperature of (Ms−200 ° C.) or higher and lower than or equal to Ms point for 10 to 60 seconds and heating at a rate of 10 ° C./second or higher and a temperature exceeding the Ms point (Ms point + 70 ° C.) or lower. The isothermal transformation step for 90 to 3600 seconds and the cooling step for cooling to room temperature are sequentially performed. Here, the Ac3 point is the boundary temperature between the austenite single-phase region and the ferrite + austenite two-phase region observed during heating, and the Ms point starts martensite generation from supercooled austenite during cooling. Temperature.

  Hereinafter, the manufacturing method of the spring material of this invention is demonstrated. A material made of an iron-based alloy is used, but the structure of the material before austenitization is not particularly limited. For example, it is possible to use a hot-forged or drawn steel bar material as a raw material.

Austenitizing process The temperature of austenitizing needs to exceed Ac3 point (Ac3 point + 250 ° C.) or less. Below the Ac3 point, the material does not become austenite, and a desired structure cannot be obtained. Moreover, when it exceeds (Ac3 point +250 degreeC), a prior-austenite particle size will become coarse easily and there exists a possibility that ductility may fall.

Quenching step Cooling is performed at a rate of 20 ° C./second or more from the austenitizing temperature, and the material is quenched by holding at a temperature of (Ms−200 ° C.) or more and Ms point or less for 10 to 60 seconds. Thereby, a part of supercooled austenite undergoes martensitic transformation. This martensite becomes tempered martensite after the isothermal transformation process described later. The faster the cooling rate, the better. If it is less than 20 ° C./second, soft ferrite or pearlite is generated during cooling, and the desired structure cannot be obtained. When the quenching temperature is less than (Ms−200 ° C.), martensite is excessively generated, and therefore, lower bainite and residual austenite are hardly obtained in the subsequent steps. On the other hand, if the Ms point is exceeded, martensite itself cannot be obtained. Further, if the holding time is less than 10 seconds, martensite is not uniformly generated up to the inside of the material, so that a desired structure cannot be obtained. On the other hand, when the retention time exceeds 60 seconds, the area ratio of the martensite to be generated is saturated, so the upper limit is substantially 60 seconds.

Isothermal transformation process The material is heated at a rate of 10 ° C./second or more from the quenched temperature and held at a temperature exceeding the Ms point (Ms point + 70 ° C.) and below for 90 to 3600 seconds. Thereby, a part of austenite is transformed into lower bainite, and part or all of martensite becomes tempered martensite. The higher the rate of temperature rise, the better. If it is less than 10 ° C./second, a homogeneous lower bainite cannot be obtained, and it takes much time to start producing the lower bainite, which is uneconomical. If the transformation temperature is equal to or lower than the Ms point, it is very difficult to obtain a desired area ratio of the lower bainite. On the other hand, when (Ms point + 70 ° C.) is exceeded, soft upper bainite is generated, so that the tensile strength decreases. Further, if the holding time is less than 90 seconds, the amount of lower bainite produced is small and a desired structure cannot be obtained. On the other hand, even if the holding time exceeds 3600 seconds, the area ratio of each tissue does not substantially change, so the upper limit is 3600 seconds in consideration of production efficiency and cost. In addition, by adjusting the transformation temperature and the holding time, the area ratio of the martensite structure can be maintained at more than 0% and 15% or less, and a higher strength spring material can be obtained.

Cooling step After the isothermal transformation step, the material is cooled to room temperature. Although the cooling rate is not particularly specified, water cooling or air cooling is preferable in consideration of production efficiency and cost.

  In the method for producing a spring material of the present invention, the iron-based alloy is in mass%, C: 0.45 to 0.65%, Si: 1.0 to 2.5%, Mn: 0.1 to 1.%. It is preferable that 0%, Cr: 0.1 to 1.0%, P: 0.035% or less, and S: 0.035% or less are satisfied, and the balance is made of iron and inevitable impurities.

  Furthermore, the spring of the present invention is characterized by being made of the above-mentioned spring material and manufactured by the above manufacturing method, and it is desirable to perform shot peening and setting as necessary. By performing shot peening, compressive residual stress can be imparted to the spring surface and fatigue resistance can be improved. Moreover, the sag resistance can be improved by performing setting.

  According to the present invention, it is possible to use a standard spring material such as JIS or SAE that does not contain an expensive alloy element and is easily available, and does not require a complicated heat treatment, and has a high strength and high ductility. Materials, manufacturing methods thereof, and springs can be provided. Moreover, since the spring material and spring of the present invention can use an alloy having a small amount of additive elements, it is excellent in recyclability. Furthermore, since the spring material and the spring of the present invention can simplify and shorten the manufacturing process as compared with a quenching and tempering treatment material that has been widely used conventionally, energy saving can be achieved.

  According to the present invention, a spring material having a tensile strength as high as 1900 MPa or more and high ductility can be obtained.

  Hereinafter, the present invention will be described in more detail using embodiments. First, in mass%, C: 0.45-0.65%, Si: 1.0-2.5%, Mn: 0.1-1.0%, Cr: 0.1-1.0%, A steel material satisfying P: 0.035% or less, S: 0.035% or less, the balance being iron and inevitable impurities, and a circle equivalent diameter of 1.5 to 15.0 mm is prepared. This steel is heated to austenitize in a metal bath or salt bath at a temperature exceeding the Ac3 point (Ac3 point + 250 ° C.) and below (austenitizing step). Next, it is cooled at a rate of 20 ° C./second or more, and is quenched using another metal bath or a salt bath (Ms-200 ° C.) and held at a temperature of the Ms point or less for 10 to 60 seconds (quenching step). Thereby, in addition to austenite, the martensite which a part of supercooled austenite transformed is obtained.

  Next, the steel material is heated at a rate of 10 ° C./second or more using another metal bath or salt bath, and is isothermally maintained at a temperature exceeding the Ms point (Ms point + 70 ° C.) and below for 90 to 3600 seconds (isothermal). Transformation process). Thereby, a part of austenite is transformed into lower bainite, and part or all of martensite becomes tempered martensite. At this time, in the formation process of the lower bainite, carbon is discharged from the bainitic ferrite to the surrounding supercooled austenite and the formation of iron carbide is suppressed by the presence of Si, so the carbon concentration in the supercooled austenite is increased. It can be a concentration. And the steel material after an isothermal transformation is cooled to room temperature by water cooling or air cooling (cooling process). Thereby, a retained austenite with a high average carbon concentration is obtained.

  The spring material obtained from such a manufacturing method is an area ratio of the internal structure in an arbitrary cross section, tempered martensite is 30 to 80%, lower bainite is 5 to 70%, and retained austenite is 8 to 15%. The average carbon concentration in the retained austenite is 1.0 to 2.0 wt%. Further, by adjusting the transformation temperature and holding time in the isothermal transformation step, it is possible to obtain a spring material with higher martensite content by more than 0% and not more than 15% and having higher strength. These spring materials have a tensile strength of 1900 MPa or more, a parameter Z defined in Equation 1 of 20000 or more, and are extremely excellent in strength and ductility.

  In addition, a spring may be manufactured by the above manufacturing method, and shot peening and setting may be performed on the spring to improve fatigue resistance and sag resistance. In shot peening and setting, various execution conditions are set according to the required performance.

  Commercially available spring steel SAE9254 (diameter 12 mm) comprising the average components described in Table 1 was prepared. In addition, the Ac3 point of this steel material measured using a fully automatic transformation point recording and measuring apparatus (former-F manufactured by Fuji Radio Engineering Co., Ltd.) was 796 ° C., and the Ms point was 268 ° C. This steel was heated in a salt bath at 850 ° C. for 10 minutes (austenitizing step), and then held at a temperature (T1) shown in Table 2 for a predetermined time (t1) using a different salt bath (quenching step). Further, the steel material was kept for a predetermined time (t2) at a temperature (T2) shown in Table 2 using another salt bath (isothermal transformation step), and then immersed in room temperature water and cooled (cooling step). The spring steel thus obtained was examined for the area ratio of various structures, tensile strength, and elongation at break in the following manner.

[Area ratio of various tissues]
The tempered martensite structure can be distinguished from an as-quenched martensite structure in which fine iron carbide is precipitated inside the martensite and no iron carbide is observed inside the martensite. The lower bainite structure is characterized by the presence of fine iron carbide regularly arranged inside acicular bainitic ferrite. These structures were mirror-polished on an arbitrary cross section of the spring steel, further corroded with a nitar, and then observed with an SEM (scanning electron microscope) to determine the area ratio. These results are also shown in Table 2.

  Further, the retained austenite was determined for the area ratio by X-ray diffraction after mirror polishing in an arbitrary cross section of spring steel. The area ratio of martensite was determined by subtracting the total area ratio (%) of tempered martensite, lower bainite, and retained austenite from the area ratio 100%. This is because it is difficult to distinguish martensite and retained austenite in the SEM observation.

The average carbon concentration ([C] (mass%)) in the retained austenite was determined from the diffraction peak angles of (111), (200), (220) and (311) of austenite by X-ray diffraction. Using the constant a (nm), calculation was made according to the following relational expression. The results are also shown in Table 2.

[Tensile strength and elongation at break]
A round bar-shaped test piece (JIS 14A) having a diameter of 6 mm in parallel and a distance between gauge points of 30 mm was prepared by cutting, and a tensile test was performed on the test piece to obtain a tensile strength. Further, after the tensile test, the fracture surfaces were matched, and the elongation at break was determined from the increase in the distance between the gauge points relative to the distance between the original gauge points. These results are also shown in Table 2.

  As is apparent from Table 2, the temperature and holding time in the quenching process and the isothermal transformation process were within the range defined by the present invention. In the test pieces of 2 to 7, 9 to 13, and 15 to 17, a desired structure is obtained, the tensile strength is 1900 MPa or more, and the parameter Z is 20000 or more and exhibits high strength and high ductility (example of the present invention). .

  On the other hand, No. in which the holding time in the isothermal transformation process is outside the range defined in the present invention. The test pieces 1, 8, and 14 have the following problems (comparative example). That is, no. Since the specimens 1 and 8 have a short holding time in the isothermal transformation step, the ratio of martensite, retained austenite, and the average carbon concentration in retained austenite do not satisfy the requirements of the present invention, and as a result, the elongation at break decreases. Therefore, the parameter Z defined by the present invention cannot be secured. No. In the 14 specimens, the ratio of lower bainite, martensite, retained austenite, and the average carbon concentration in retained austenite do not satisfy the requirements of the present invention, and as a result, the elongation at break becomes small. Is not secured.

Claims (10)

  It is made of an iron-based alloy, and is an area ratio of an internal structure in an arbitrary cross section, tempered martensite is 30 to 80%, lower bainite is 5 to 70%, residual austenite is 8 to 15%, and average carbon in the residual austenite A spring material having a concentration of 1.0 to 2.0 wt%.   It is made of an iron-based alloy, and is an area ratio of an internal structure in an arbitrary cross section. A spring material characterized by being 15% and having an average carbon concentration in the retained austenite of 1.0 to 2.0 wt%.   The iron-based alloy is, in mass%, C: 0.45 to 0.65%, Si: 1.0 to 2.5%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1 The spring material according to claim 1 or 2, characterized by satisfying 0.0%, P: 0.035% or less, S: 0.035% or less, and the balance comprising iron and inevitable impurities. The spring material according to any one of claims 1 to 3, wherein the tensile strength is 1900 MPa or more, and a parameter Z defined below is 20000 or more.
Parameter Z = (Tensile strength (MPa)) × (Elongation at break (%))   5. The spring material according to claim 1, wherein the wire has a circle-equivalent diameter of 1.5 to 15.0 mm.   A spring made of the spring material according to claim 1. Using materials made of iron-based alloys,
When the boundary temperature between the austenite single-phase region observed during heating and the two-phase region of ferrite + austenite is Ac3 point, and the temperature at which martensite starts to form from supercooled austenite during cooling is Ms point, The material is austenitized at a temperature exceeding the Ac3 point (Ac3 point + 250 ° C.) and below, cooling at a rate of 20 ° C./second or more, and (Ms−200 ° C.) at a temperature not lower than the Ms point. A quenching process for ˜60 seconds, heating at a rate of 10 ° C./second or more, an isothermal transformation process for 90 to 3600 seconds at a temperature exceeding the Ms point (Ms point + 70 ° C.) and below, and cooling to cool to room temperature And a step of sequentially performing the steps.   The iron-based alloy is, in mass%, C: 0.45 to 0.65%, Si: 1.0 to 2.5%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1 The method for manufacturing a spring material according to claim 7, wherein 0.0%, P: 0.035% or less, and S: 0.035% or less are satisfied, and the balance is made of iron and inevitable impurities.   A spring manufactured by the manufacturing method according to claim 7 or 8.   The spring according to claim 9, wherein shot peening and setting are performed.