JP5653020B2 - Spring steel and springs with excellent corrosion fatigue strength - Google Patents

Description

  The present invention relates to a spring steel and a spring, and more particularly to a spring steel and a spring excellent in corrosion fatigue strength.

  In recent years, higher strength steels for springs and springs have been required due to various demands. In a high-strength spring, if the hardness is increased to ensure sag resistance, impact resistance, toughness, and corrosion fatigue strength tend to decrease. Various materials have been studied from this viewpoint (Patent Document 1).

International Publication WO2006 / 022009 Pamphlet

  However, even with the composition described in the above-mentioned patent document, it has been difficult to satisfy these requirements at a preferable level and at a low cost. Therefore, an object of the present disclosure is to provide a spring steel and a spring that are excellent in corrosion fatigue strength even if the strength is high.

  As a result of various studies on the alloy composition of the spring steel, the present inventors have found an alloy composition of the spring steel that exhibits good corrosion fatigue strength while ensuring high strength, and completed the present invention. According to the present invention, the following means are provided.

According to the disclosure of the present specification, by mass%, C: 0.35% to 0.55%, Si: 1.60% to 3.00%, Mn: 0.20% to 1.50% Hereinafter, Cr: 0.10% or more and 1.50% or less, Ni: 0.40% or more and 3.00% or less, Mo: 0.05% or more and 0.50% or less, and V: 0.05 % Containing at least one element selected from the group consisting of not less than 0.50% and not less than 0.50%, with the balance being Fe and inevitable impurities,
The following formulas (1) and (2):
0.400% ≦ C (%) + Mn (%) / 6 + Si (%) / 24 + Ni (%) / 40 + Cr (%) / 5 + Mo (%) / 4 + V (%) / 14 ≦ 0.800% Formula ( 1)
0.540% ≦ C (%) + Mn (%) / 6 + Si (%) / 24 ≦ 0.670% Formula (2)
A spring steel characterized by satisfying the above is provided. The spring steel preferably satisfies the formula (3) in which the lower limit is 1.35%.

The spring steel further has the following formula (3):
1.20% ≦ Si (%) − 0.46 C (%) − 1.08 Mn (%) (3)
May be satisfied. Further, the spring steel further comprises the following formulas (4) and (5):
159C (%)-Si (%) + 8 Mn (%) + 12 Cr (%) ≦ 84% (4)
6.0C (%)-Si (%)-0.3Mn (%) + 1.4Cr (%) ≦ 1.2% Formula (5)
Either of the above may be satisfied.

  Further, the spring steel has C of 0.45% to 0.50%, Si of 2.00% to 2.50%, and Mn of 0.40% to 0.50%. Good. Further, the spring steel may contain Ni, Mo and V. Preferably, C is 0.46% or more and 0.49% or less.

Moreover, this steel for springs is the 1 type, or 2 or more types of characteristics selected from the following (1)-(3) after quenching and tempering processing:
(1) Corrosion durability is 40000 times or more (preferably 45000 times or more)
(2) Charpy impact value is 70 J / cm ² or more (preferably 80 J / cm ² or more)
(3) Delayed fracture strength is 800 MPa or more (preferably 910 MPa or more)
May be satisfied. Further, the steel for spring preferably has a Rockwell hardness of HRC53 or more and HRC56 or less after quenching and tempering.

  According to the disclosure of the present specification, a spring that is manufactured from any one of the above-described spring steels and that is HRC53 or more and HRC56 or less is also provided.

  According to the spring steel disclosed in this specification, a spring having good durability such as corrosion fatigue strength can be produced by satisfying the above composition and the above formulas (1) and (2). . The alloy component composition in the spring steel disclosed herein contributes to good strength and corrosion fatigue strength, and equation (1) relates to the carbon equivalent. When the contents of these elements, that is, C, Si, Mn, Cr, Mo, and V satisfy the formula (1), a desired strength, typically about HRC53 to HRC56, is obtained by quenching and tempering. Strength spring steel can be easily realized. Further, by satisfying the formula (2), it becomes possible to manufacture spring steel having durability such as good corrosion fatigue strength. Hereinafter, embodiments disclosed in the present specification will be described in detail.

(Spring steel)
The spring steel of the present invention is mass%, C: 0.35% to 0.55%, Si: 1.60% to 3.00%, Mn: 0.20% to 1.50%. Cr: 0.10% or more and 1.50% or less.

(C: carbon)
C is preferably contained in an amount of 0.35% to 0.55%. In the present spring steel, when C is within this range, a spring steel with good strength can be obtained by quenching and tempering. If it is less than 0.35%, spring steel with good strength cannot be obtained by quenching and tempering. On the other hand, if it exceeds 0.55%, the toughness is lowered, and there is a possibility that quench cracking occurs during water quenching. Furthermore, there is a risk that fatigue strength and corrosion fatigue strength may be reduced. Although there is a relationship with other alloy components, C is preferably 0.45% or more and 0.50% or less. Within this range, it is easy to achieve good strength, and in relation to other alloy components, one or more of the formulas (1) to (5) are appropriately satisfied, and the corrosion fatigue strength is initially set. It is easy to obtain good durability. More preferably, the upper limit is 0.49%, and further preferably 0.48%. The lower limit is preferably 0.46%, more preferably 0.47%.

(Si: silicon)
Si is preferably 1.60% or more and 3.00% or less. In the spring steel, when Si is within this range, it is effective for improving sag resistance, tempering characteristics and corrosion fatigue strength. In this range, the corrosion fatigue strength is improved as the Si content increases. If it is less than 1.60%, it is difficult to obtain sufficient effects, and if it exceeds 3.00%, the toughness tends to be lowered, and decarburization during rolling and heat treatment (quenching) is promoted. In the spring steel, from the viewpoint of corrosion fatigue strength and the like, the lower limit is preferably 2.00%, more preferably 2.10%. Preferably, the upper limit is 2.50%. More preferably, it is 2.40%.

(Mn: Manganese)
Mn is preferably 0.20% or more and 1.50% or less. In the spring steel, when Mn is within this range, good corrosion fatigue strength can be obtained. If Mn exceeds 1.50%, the corrosion fatigue strength tends to decrease, and if Mn is less than 0.20%, the strength and hardenability tend to be insufficient, and cracking tends to occur during rolling. . In the present spring steel, from these viewpoints, the upper limit is more preferably 0.70%, and further preferably 0.50% or less. More preferably, the lower limit is 0.40%.

(Cr: Chrome)
Cr is preferably 0.10% or more and 1.50% or less. In this spring steel, when Cr is within this range, it is useful for securing strength and improving hardenability. If Cr is less than 0.10%, such effects are insufficient, and if it exceeds 1.50%, the tempered structure becomes non-uniform and the sag resistance tends to be impaired. Preferably, the upper limit is 0.30%. Preferably, the lower limit is 0.20%.

  The spring steel of the present invention is made of Ni: 0.40% to 3.00%, Mo: 0.05% to 0.50% and V: 0.05% to 0.50%. It is preferable that one or more selected elements are contained in the concentration. The spring steel preferably contains all elements at the aforementioned concentrations. By doing so, not only good toughness can be obtained, but also good corrosion fatigue strength can be obtained.

(Ni: nickel)
Ni is preferably 0.40% or more and 3.00% or less. In this spring steel, if Ni is within this range, the corrosion resistance is improved. If it is less than 0.40%, the effect is insufficient, and if it exceeds 3.00%, the corrosion resistance improving effect tends to be saturated. More preferably, the upper limit is 1.00% or less, and still more preferably 0.60% or less. The spring steel preferably contains at least Ni among Ni, Mo and V.

(Mo: Molybdenum)
Mo is preferably 0.05% or more and 0.50% or less. In this spring steel, when the Mo is within this range, the corrosion fatigue strength can be improved. If it is less than 0.05%, this effect is insufficient, and even if it exceeds 0.50%, the effect tends to be saturated. Preferably, it is 0.20% or less. More preferably, it is 0.10% or less.

(V: Vanadium)
V is preferably 0.05% or more and 0.50% or less. In this spring steel, when V is within this range, it is effective for refinement of crystal grains and precipitation hardening. If it is less than 0.05%, the effect is insufficient, and if it exceeds 0.50%, the carbide may form corrosion pits on the steel surface, which may be the starting point of crack fracture. In addition, toughness decreases. More preferably, it is 0.30% or less, More preferably, it is 0.20% or less. More preferably, it is 0.10% or less.

  Further, the spring steel can contain P (phosphorus). P tends to weaken the crystal grain boundary, so is preferably 0.010% or less, and more preferably 0.005% or less.

  Further, the steel for springs can contain S (sulfur). S, like P, has a tendency to weaken the crystal grain boundary, and is preferably 0.010% or less. More preferably, it is 0.005% or less.

  The spring steel can contain Cu (copper). In the spring steel, it is preferably 0.20% or less, and more preferably 0.05% or less.

  This spring steel can contain Ti (titanium) (preferably 0.005% or more and 0.030% or less) in addition to the alloy components described above. Further, B (boron) (preferably 0.0015% or more and 0.0025% or less) can be contained. The spring steel contains these alloy components, and the balance is Fe (iron) and consists of inevitable impurities.

  In addition to having such an alloy composition, the spring steel can satisfy the following formulas (1) to (5) as appropriate.

(Formula (1): 0.400% ≦ C (%) + Mn (%) / 6 + Si (%) / 24 + Ni (%) / 40 + Cr (%) / 5 + Mo (%) / 4 + V (%) / 14 ≦ 0.800 %)
The lower limit value of the formula (1) indicates a lower limit value for obtaining a desired strength by quenching and tempering, and if it is less than the numerical value, sufficient strength cannot be obtained. Further, the upper limit value of the formula (1) indicates an upper limit value for obtaining a desired strength. If the numerical value is exceeded, residual austenite at the time of quenching may increase, and transformation to martensite may be incomplete. There is a possibility that strength is not sufficient. In addition, there arises an inconvenience that dimensional changes are likely to occur. The range of Formula (1) is preferable for obtaining a range of Rockwell hardness HRC53 or more and HRC56 or less. The lower limit is preferably 0.700%, more preferably 0.730%. More preferably, it is 0.735%, and more preferably 0.740%. The upper limit is preferably 0.800%, more preferably 0.780%, and still more preferably 0.760%. In addition, it is preferable that the intensity | strength which Formula (1) intends is Rockwell hardness HRC.

  It is preferable that this steel for springs satisfy | fills the formula (2) mentioned later with this formula (1) at least. By satisfying these two formulas, a spring steel having high strength and excellent corrosion fatigue strength can be easily obtained.

(Formula (2): 0.540% ≦ C (%) + Mn (%) / 6 + Si (%) / 24 ≦ 0.670%)
Formula (2) is a formula for controlling the particle size of carbide and its distribution by selecting C, Mn (Mn / 6) and Si (Si / 24) from the carbon equivalent formula. It is considered that the balance between strength and toughness varies depending on the size and distribution of carbides, the delayed fracture characteristics (hydrogen sensitivity) change, and are strongly related to corrosion fatigue. The lower limit value and the upper limit value of the formula (2) are determined in relation to the corrosion fatigue strength. If the lower limit value is less than the above lower limit value, the corrosion fatigue strength decreases. Decreases. The lower limit is preferably 0.600%, more preferably 0.620%, still more preferably 0.630%, still more preferably 0.640%, and still more preferably 0. .650%. The upper limit is preferably 0.660%. In addition, it is preferable that the corrosion fatigue strength which Formula (2) intends is a thing obtained by the test method of the Example description described later.

(Formula (3): 1.20% ≦ Si (%) − 0.46C (%) − 1.08Mn (%))
Expression (3) is an expression obtained by selecting C, Si and Mn from the alloy components of the spring steel and using the corrosion fatigue strength as a parameter, and is an equation for obtaining a spring steel having excellent corrosion fatigue strength. It is. When it is less than the lower limit, sufficient corrosion fatigue strength cannot be obtained. Preferably, the lower limit is 1.30%, more preferably 1.35%, still more preferably 1.40%, still more preferably 1.45%, and even more preferably 1 .50%. Moreover, although an upper limit is not prescribed | regulated in Formula (3), from a viewpoint of aiming at intensity | strength and decarburization suppression, Preferably it is 1.90%, More preferably, it is 1.70%. In addition, it is preferable that the corrosion fatigue strength which Formula (3) intends is a thing obtained by the test method as described in the Example mentioned later.

  Although this spring steel can satisfy only Formula (3), Preferably, Formula (3) is satisfied with Formula (1) and Formula (2). By doing so, it is possible to obtain a steel for springs having further excellent corrosion fatigue strength.

(Formula (4): 159C (%)-Si (%) + 8 Mn (%) + 12 Cr (%) ≦ 84%
Equation (4) is an equation obtained by selecting C, Si, Mn, and Cr from the alloy components of the spring steel and using the Charpy impact value as a parameter. This is a formula for obtaining spring steel. When the above upper limit is exceeded, a sufficient Charpy impact value (toughness) cannot be obtained. The upper limit is preferably 82%, more preferably 81%. In Formula (4), the lower limit is not particularly defined, but it is preferably 60%, more preferably 70%. In addition, it is preferable that the Charpy impact value which Formula (4) intends is a thing obtained by the test method of the Example description described later.

(Formula (5): 6.0C (%)-Si (%)-0.3Mn (%) + 1.4Cr (%) ≦ 1.2%)
Equation (5) is an equation obtained by selecting C, Si, Mn, and Cr from the alloy components of the spring steel and using delayed fracture strength as a parameter. Good delayed fracture strength related to durability. Is a formula for obtaining a spring steel having When the upper limit is exceeded, sufficient delayed fracture strength cannot be obtained. Preferably, the upper limit is 1.0%, more preferably 0.95%. In Formula (5), the lower limit is not particularly defined, but from the viewpoint of strength and decarburization suppression, it is preferably 0.65%, more preferably 0.68%, and even more preferably 0.70. %. In addition, it is preferable that the delayed fracture strength which Formula (5) intends is a thing obtained by the test method of the Example description described later.

The spring steel is preferably tempered to have a hardness of HRC53 to HRC56 after quenching and tempering. Within such a range, a lightweight and high strength spring can be obtained. Moreover, it is preferable that any of the corrosion fatigue strength, Charpy impact value, and delayed fracture strength of the spring steel satisfy the following characteristics after quenching and tempering treatment. That is, regarding the corrosion fatigue strength, it is preferable that the number of times of corrosion durability is 40000 times or more. More preferably, it is 45,000 times or more. The Charpy impact value is preferably 70 J / cm ² or more. More preferably, it is 80 J / cm ² or more, and further preferably, 85 J / cm ² or more. Furthermore, it is preferable that delayed fracture is 800 MPa or more. More preferably, it is 900 MPa or more, More preferably, it is 910 MPa or more, More preferably, it is 950 MPa.

  Next, a method of manufacturing a spring using such spring steel will be described. The spring steel disclosed in this specification can produce various springs by applying a known hot forming method, cold forming method, warm forming method and the like. For example, to manufacture a coil spring, it can be performed as follows. That is, the spring steel disclosed in the present specification is made into a round steel, a wire, a wire or a plate, etc., then formed into a coil shape, warm shot peening is performed on the formed coil, and warm shot peening is performed. A spring can be manufactured by performing hot setting on a later coil. By applying such a manufacturing method, an automobile suspension coil spring having excellent sag resistance and durability can be obtained. As a more specific embodiment, each process of coil forming, heat treatment, hot setting, warm shot peening, cold shot peening, and cold setting is performed using the spring steel disclosed in this specification. The form which manufactures the coil spring for automobile suspension is mentioned. The coil forming step may be performed hot (temperature higher than the recrystallization temperature of the wire), warm (temperature lower than the recrystallization temperature of the wire), or cold (room temperature). Moreover, the conventionally well-known various methods can be used for the method of shape | molding in coil shape, For example, you may shape | mold using a coiling machine and may shape | mold by the method of winding around a mandrel.

  In the heat treatment step, heat treatment is performed on the coil formed into a coil shape by the above-described forming step. The heat treatment performed in this step differs depending on whether the above molding step is performed hot, warm or cold. That is, when the above molding process is performed hot, quenching and tempering are performed. Quenching and tempering imparts strength and toughness to the coil. On the other hand, when the above molding process is performed cold, low temperature annealing is performed. The low temperature annealing can remove harmful residual stress (residual tensile stress) inside and on the coil. The method of quenching and tempering the coil and low-temperature annealing of the coil can be performed by any conventionally known method.

  In the hot setting process, setting is performed in a state where the temperature of the coil is warm. By hot setting, a directional compressive residual stress is applied to the coil to improve durability, and a relatively large plastic deformation occurs in the coil, thereby improving the sag resistance of the coil. Here, the temperature at which hot setting is performed can be appropriately set within a temperature range that is equal to or lower than the recrystallization temperature of the wire and higher than room temperature. For example, the temperature of the coil can be set in a range of about 150 ° C. to 400 ° C. By performing setting in such a temperature range, the amount of plastic deformation imparted to the coil can be increased, and the sag resistance can be improved. Further, the settling allowance δh can be appropriately determined according to the total length L of the automobile suspension coil spring (the total length Ls at the time of setting). For the setting, various conventionally known methods can be used.

  In the warm shot peening process, the coil subjected to the above heat treatment is warm shot peened. Warm shot peening imparts a large compressive residual stress to the coil surface, improving the durability and corrosion fatigue resistance of the coil. Here, the temperature at which shot peening is performed can be appropriately set within a temperature range that is equal to or lower than the recrystallization temperature of the wire and higher than room temperature. For example, the coil temperature can be set to about 150 ° C. or higher and 400 ° C. or lower. Various conventionally known methods can be used for the steel ball shot method.

  In the cold shot peening process, shot peening is performed with the coil temperature at room temperature. By performing cold shot peening in addition to warm shot peening, the durability of the coil can be further improved. In addition, it is preferable to make the diameter of the steel ball used for cold shot peening smaller than the diameter of the steel ball used for warm shot peening. For example, when the diameter of the steel ball used for warm shot peening is 1.2 mm, the diameter of the steel ball used for cold shot peening is 0.8 mm. By performing warm shot peening and cold shot peening, a large compressive residual stress is imparted to the coil by the warm shot peening performed earlier, and the surface roughness of the coil is improved by the cold shot peening performed later, Coil durability and corrosion fatigue resistance are further improved. Various conventionally known methods can be used for the steel ball shot method.

  In the cold setting process, setting is performed in a state where the temperature of the coil is set to room temperature. By performing cold setting in addition to the hot setting, the sag resistance of the coil is further improved. The settling allowance δc of the cold setting can be appropriately determined according to the total length L of the automobile suspension coil spring (the total length Ls at the time of setting). Note that the cold setting sag δc is preferably smaller than the warm setting sag δh.

  In addition, each process of said cold shot peening and cold setting can be abbreviate | omitted, and only warm shot peening and hot setting can also be performed. Moreover, you may include other processes other than said each process. For example, a water cooling process may be performed after hot setting.

  As described above, according to the present invention, it is possible to obtain a spring steel and a spring having high strength and excellent durability such as corrosion fatigue strength. Such a spring is suitably used for a coil spring, a leaf spring, a torsion bar, a stabilizer and the like used for a vehicle suspension system or the like.

  Embodiments embodying the present invention will be described below. The following examples are specific examples for explaining the present invention, and do not limit the present invention.

Steels of reference examples, example samples, and comparative examples having chemical compositions shown in Table 1 below were produced by the following two types of manufacturing methods. That is, the steels used in Reference Examples 1 and 2, Example Samples 3 to 5 and Comparative Examples 1 to 7 used in the corrosion fatigue test are manufactured in the following (2), and the steel used in Comparative Example 8 is the following (1 ). Furthermore, the steel used in the Charpy impact value and delayed fracture test was manufactured by the following (2).
(1) A steel ingot obtained by melting steel in a blast furnace or an electric furnace for mass production was subjected to split rolling, and then wire rolling.
(2) After melting 2 tons of steel in a vacuum melting furnace, it was rolled into pieces and then rolled into a wire.

About these steel, the test of various items was done with the following method.
1. Corrosion fatigue test (1) Preparation of test piece The test piece was coiled by subjecting each steel wire to surface hardening, quenching and heating, then hot forming, quenching (oil cooling), and tempering. . The quenching heating condition was high-frequency induction heating of 990 ° C., and the spring hardness (hardness after tempering) was adjusted to HRC55. The outline of the obtained coil spring is shown in the following table.

(2) Test method A pit was artificially given to the obtained spring, and a fatigue test (JASOC604) was performed in a corrosive environment. The pit is masked with a small hole on the outer surface of the spring at the point where the main stress amplitude is maximum (3.1 turns from the coil end), and hemispherical hole (artificial pit) with a diameter of 600 μm and depth of 300 μm by electropolishing. Was granted. The stress concentration factor of normal stress (principal stress) in torsional load due to this pit is 2.2 according to the finite element method analysis. As the electrolytic solution, an aqueous ammonium chloride solution was used. The corrosive environment uses 5% NaCl aqueous solution as the corrosive solution, and after corroding only the artificial pit part with a spray device for 16 hours, cover the periphery of the artificial pit part with absorbent cotton soaked with 5% NaCl aqueous solution. Was wrapped in ethylene wrap to prevent drying. A fatigue test was performed in this state, and the number of repetitions until breakage was evaluated. In the fatigue test, the repetition rate was 2 Hz, and vibration was applied by parallel compression using a flat seat. The test height was such that the main stress condition in the absence of artificial pits at the artificial pit application position was 507 ± 196 MPa (height 220 mm at maximum load (4031 N), height 270 mm at minimum load (2079 N)). The results are also shown in Table 3. Table 3 also shows the numerical values of the formulas (1) to (5) based on the chemical composition and the sufficiency for each formula.

2. Charpy impact value (1) Preparation of test piece For each reference example, example sample and comparative example, a half-size test piece (5 × 10 mm cross section, length 55 mm 2 mm (width) × 2 mm (depth) at the center of a prism × R1 (U-shaped bottom) U notch formed).
(2) Quenching heating conditions:
Reference Examples 1 and 2 and Comparative Examples 1 and 2 were 900 ° C. × 30 minutes, Comparative Example 5 was high-frequency induction heating 960 ° C., and Example Samples 3 and 4 were high-frequency induction heating 990 ° C.
(3) Test piece hardness The hardness after tempering was adjusted to HRC55.
(4) Test method The test temperature was normal temperature, and the test was performed based on JIS Z2242. The results are shown in Table 3.

3. Delayed fracture test (Samples 3 and 4, Comparative Examples 5 and 8)
As the test piece, a tensile test piece (with a 1 mm depth annular notch) was used. The test method used was a constant load load-hydrogen charge method. Specifically, the time until the specimen was loaded with a constant load while being charged with hydrogen at a current density of 1.0 mA / cm ² in a pH 3 H ₂ SO ₄ solution was counted. The maximum load stress that did not break for 200 hours or more was evaluated as delayed fracture strength.
The quenching heating conditions for the test piece were as follows. That is, high frequency induction heating was set to 990 ° C. for Example Samples 3 and 4, and high frequency induction heating was set to 960 ° C. for Comparative Example 5. In Comparative Example 8, the temperature was 900 ° C. × 30 minutes. The results are also shown in Table 3.

As shown in Table 3, it was found that Reference Examples 1 and 2 and Samples 3 to 5 which are examples of the present invention all had a good number of corrosion durability. All of these satisfied the formulas (1) to (5). In particular, Samples 3 to 5 in which the numerical value of the formula (3) is 1.35% or more had a high number of corrosion durability. Also, with respect to the Charpy impact value, Reference Examples 1 and 2 and Samples 3 and 4 satisfying the expressions (1) to (5) were good. From this, it can be seen that the other samples 5 similarly exhibit good Charpy impact values. Furthermore, regarding delayed fracture strength, Samples 3 and 4 satisfying the expressions (1) to (5) were good. From this, it can be seen that the other Reference Examples 1 and 2 and Sample 5 also exhibit good delayed fracture.

Claims (12)

In mass%, C: 0.35% to 0.55%, Si: 1.60% to 3.00%, Mn: 0.20% to 1.50%, Cr: 0.10% or more 1.50% or less , Ti: 0.005% or more and 0.030% or less, B: 0.0015% or more and 0.0025% or less ,
Furthermore, at least Ni selected from the group consisting of Ni: 0.40% to 3.00%, Mo: 0.05% to 0.50% and V: 0.05% to 0.50% Including 1 type or 2 types or more,
The balance consists of Fe and inevitable impurities,
The following formulas (1), (2) and (3):
0.730% ≦ C (%) + Mn (%) / 6 + Si (%) / 24 + Ni (%) / 40 + Cr (%) / 5 + Mo (%) / 4 + V (%) / 14 ≦ 0.780% Formula ( 1)
0.630% ≦ C (%) + Mn (%) / 6 + Si (%) / 24 ≦ 0.660% Formula (2)
1.40% ≦ Si (%) − 0.46C (%) − 1.08Mn (%) Formula (3)
A spring steel characterized by satisfying   The steel for springs according to claim 1 satisfying a formula (3) whose said lower limit is 1.45% or more.   The steel for springs according to claim 1 or 2 satisfying formula (3) whose upper limit is 1.90% or less. Furthermore, the following formulas (4) and (5);
159C (%)-Si (%) + 8 Mn (%) + 12 Cr (%) ≦ 84% (4)
6.0 C (%)-Si (%)-0.3 Mn (%) + 1.4 Cr (%) ≦ 1.2% Formula (5)
The spring steel according to any one of claims 1 to 3, wherein any one of the above is satisfied.   C is 0.45% or more and 0.50% or less, Si is 2.00% or more and 2.50% or less, and Mn is 0.40% or more and 0.50% or less. The spring steel described.   The spring steel according to claim 5, wherein C is 0.46% or more and 0.49% or less. Containing M o and V, the spring steel according to any one of claims 1 to 6.   C is 0.47% to 0.49%, Si is 2.10% to 2.40%, Mn is 0.40% to 0.50%, Cr is 0.20% to 0.30% The spring according to claim 6, wherein Ni is 0.40% or more and 0.60% or less, Mo is 0.05% or more and 0.20% or less, and V is 0.05% or more and 0.20% or less. Steel. After quenching and tempering, any of the following characteristics (1) to (3):
(1) Corrosion durability is 45,000 times or more. (2) Charpy impact value is 80 J / cm ² or more. (3) Delayed fracture strength satisfies 910 MPa or more.   The spring steel according to any one of claims 1 to 9, which has a Rockwell hardness of HRC53 or more and HRC56 or less. A spring manufactured by the spring steel according to claim 1, wherein the spring has a Rockwell hardness of HRC53 or more and HRC56 or less.   The spring according to claim 11, wherein the number of corrosion durability is 40000 times or more.