JP2021530623A - Spring steel with excellent fatigue life and its manufacturing method - Google Patents

Abstract

A spring steel having an excellent fatigue life and a method for producing the same, the weight percentage of the chemical composition thereof is: C: 0.52-0.62%; Si: 1.20-1.45%; Mn: 0. 25-0.75%; Cr: 0.30-0.80%; V: 0.01-0.15%; Nb: 0.001-0.05%; N: 0.001-0.009% O: 0.0005-0.0040%; P: ≦ 0.015%; S: ≦ 0.015%; Al: ≦ 0.0045%; the balance is Fe and unavoidable impurities, and 0.02 ≦ Satisfy (2Nb + V) / (20N + C) ≦ 0.40. The fine structure of the spring steel according to the present invention is a tempered troostite + sorbite structure, the original austenite crystal grain size is less than 80 um, the alloy nitrogen / carbon precipitate size is 5 to 60 nm, and the single particle inclusions are the most. It is significantly less than 30 um. The above spring steel has a machining strength of 2020 MPa or more, and at the same time has good plastic toughness (surface reduction rate ≧ 40%) and fatigue life of ≧ 800,000 times, and is highly stressed in industries such as automobiles and machinery. Can meet the requirements of springs.

Description

The present invention relates to spring steel and its manufacturing method, particularly for vehicles having a processing strength of 2020 MPa or more, a surface reduction rate of ≥40%, a finely structured structure, high steel purity, low cost and excellent fatigue life. Involved in spring steels with excellent fatigue life and methods of manufacturing them that can be used to process springs.

As an important shock absorber and functional component, springs are widely used in various aspects of social production and people's lives, such as transportation, machinery manufacturing, automobile industry, military industry and daily life. Steel wires are desired to have high elastic limits, yield strength, and tensile strength, as springs are used within the elastic range, should return to their original position after unloading, and the smaller the plastic deformation, the better. .. The higher the yield ratio, the closer the elastic limit is to the tensile strength, the higher the strength utilization, and the stronger the elasticity of the manufactured spring. Since the spring absorbs impact energy depending on the elastic deformation, the spring steel wire does not need to be highly plastic, but at least it has enough plasticity to withstand the formation of the spring and sufficient toughness to withstand the impact energy. There is a need. Since springs normally operate for a long time under alternating stress, they need to have a high fatigue limit, excellent creep resistance, and excellent anti-elasticity reduction property.

With technological advances in the automobile and machinery industries, the demand for strength and fatigue life of spring parts is increasing, and advanced steel companies in each country are paying attention to the development of spring manufacturing materials with excellent strength, plasticity, and fatigue resistance. Has been done.

Currently, conventional Cr-V-based, Cr-Mn-based, and Si-Mn-based spring steel materials cannot meet the manufacturing requirements for high-strength springs and generally have higher strength and better yield ratio. The Si—Cr spring steel used has also reached the limits of strength and fatigue life.

The alloy composition of high-strength spring steel disclosed in Chinese Patent CN101787493B is 0.56% to 0.64% C, 0.80% to 1.10% Si, 0.80% to 1.20% Mn, P. ≤0.035%, S≤0.03%, 0.80% to 1.20% Cr, 0.60% to 1.00% Mo, 0.20% to 0.30% V, 0.05% ~ 0.12% Nb, 0.01% to 0.060% N, 0.02% to 0.07% RE, and the rest is Fe. A large amount of Mn, Cr, and Mo alloy elements are added to the design material, but Mo is mainly used to improve tempering stability, sustained creep resistance, and heat resistance of steel.

The composition of the spring steel alloy disclosed in Chinese patent CN100455691C is 0.4-0.6% C, 1.7-2.5% Si, 0.1-0.4% Mn, 0.5-2. It is 0.0% Cr, 0-0.006% N, and 0.021-0.07% Al. However, since the design route of high carbon, high silicon, low manganese alloy is adopted, and mainly by controlling the amount and size of the remaining austenite, it is considered to enhance the hydrogen embrittlement resistance of the steel. At the same time as the demand for quenching and tempering processes increases, the content of alloy Al is high, which makes it difficult to control inclusions in the smelting process, and hard and brittle alumina tends to reduce the fatigue life of the spring. ..

The composition design of the spring steel alloy disclosed in Chinese patent CN1279204C is: 0.30-0.50% C, 0.80-2.0% Si, 0.50-1.0% Mn, 0.40-1. It is 0% Cr, 0.01-0.5% W, 0.08-0.30% V, 0.005-0.25% rare earth element, and may contain 0.001-0.10% B. The alloy mainly adopts a low carbon design and increases the strength by increasing the content of Si element, and at the same time, the addition of element W improves the hardenability of steel and improves the deformation resistance. Although it prevents decarburization, it is difficult to control the refining and heat treatment of W and rare earth elements.

Chinese patent CN1039725C discloses spring steel with low decarburization and high toughness for vehicle suspension springs. In such steels, the Si content of the element increases without reducing the C content, 0.5-0.7% C, 1.0-3.5% Si, 0.3-1. 5% Mn, 0.3-1.0% Cr, 0.05-0.5% V and / or Nb, P less than 0.02%, S less than 0.02%, 0.5-5. It contains 0% Ni and other unavoidable impurities, with the balance being Fe. In order to solve the decarburization problem and improve the toughness of the material, a large amount of Ni elements are added to the material, and the cost of the alloy increases.

In existing alloys, the strength of the material is improved mainly by adjusting the elements of C, Si, and Mn, but if the Si content is too small, the elastic limit of the material is lowered and the antielasticity lowering property is lowered. On the other hand, if the Si content is too high, the plasticity of the material is lowered and at the same time, decarburization control becomes difficult, which affects the fatigue life of the spring. Excessive addition of alloying elements increases the material cost and at the same time affects the size of the precipitate and reduces the fatigue performance of the material. The design strength of the material is still low and the fatigue life of the spring is not considered very much.

Although the strength of spring materials is continuously improving due to the weight reduction of automobiles and technological progress of the mechanical industry, the Cr-V type, Cr-Mn type, Si-Mn type, which are generally used at present, are used. Cr-Si based spring steel has already reached the limit of materials.

An object of the present invention is to provide a spring steel having an excellent fatigue life and a method for producing the same, and the above-mentioned spring steel has a processing strength of 2020 MPa or more and at the same time has good plastic toughness (surface reduction rate ≥ 40%). And with a fatigue life of ≧ 800,000 times, it can meet the demand for high stress springs in industries such as automobiles and machinery.

In order to achieve the above object, the technical plan of the present invention is:
A spring steel with excellent fatigue life, the weight percentage of its chemical composition is C: 0.52-0.62%;
Si: 1.20-1.45%;
Mn: 0.25-0.75%;
Cr: 0.30-0.80%;
V: 0.01-0.15%;
Nb: 0.001-0.05%;
N: 0.001-0.009%;
O: 0.0005-0.0040%;
P: ≤0.015%;
S: ≤0.015%;
Al: ≤0.0045%;
The balance is Fe and unavoidable impurities, and satisfies 0.02 ≦ (2Nb + V) / (20N + C) ≦ 0.40.

The fine structure of the spring steel according to the present invention is a tempered troostite + sorbite structure, the original austenite crystal grain size ≤ 80 um, the alloy nitrogen / carbon precipitate size is 5-60 nm, and the single particle inclusions are the largest. Significantly ≤30 um.

In the component design of spring steel according to the present invention:
C is a component necessary to ensure the room temperature strength and hardenability of the spring steel, and is also an element for the spring steel to reach a high elastic limit and good anti-elasticity lowering property; C content is 0. If it is less than 52%, it cannot be guaranteed that the strength of the alloy spring steel will be 2020 MPa or more, which is disadvantageous for the precipitation of carbon nitride of trace alloy elements. At the same time as the size of the carbide becomes too large, the plasticity of the material decreases, which is disadvantageous for high strength and good plastic toughness and affects the fatigue life of the material, so the C element content is less than 0.62%. There must be.

Si is a non-carbohydrate forming element, which mainly dissolves in the ferrite phase and plays a role of strengthening. Increasing the content of alloyed silicon contributes to the improvement of the elastic limit and anti-elasticity lowering property of the material, and the spring performance is improved. Optimized, but too high a Si content reduces the plasticity of the material, which is detrimental to the molding of the spring and affects the life of the manufactured spring, while at the same time a high Si content of the material The tendency of decarburization increases during manufacturing and heat treatment, and the processing cost increases; and the Si content of the material is controlled in the range of 1.2 to 1.45%.

Mn is an element commonly added to steel that can effectively improve hardenability and strength and has little effect on the plasticity of steel; ensuring the strength and hardenability of the alloy. Therefore, the Mn content needs to be 0.25% or more. If the Mn content is too high, it causes serious segregation and at the same time causes the growth of crystal grains. Therefore, it is necessary to control Mn in the steel, and the allowable range is 0.25 to 0.75%.

Cr can improve the hardenability of spring steel and at the same time precipitate alloy cementite in the tempering process to improve the material strength; the Cr element also has the effect of refining the structure of this material. In the design of Cr, in order to exert the role of strengthening solid-liquid and precipitation, and at the same time to improve the material structure, it is necessary to control the content to 0.30 to 0.80%.

The V and Nb elements are generally used as trace alloy elements to be added to steel, and these two elements have a strong tendency to form nitrides and carbides, and the precipitation and nucleation of carbonitrides during the tempering process. The formation rate can be increased and the structure can be made finer. The carbonitrides of V and Nb precipitate during the rolling process of the wire coil, which contributes to a smaller particle size of austenite in the material and improved strength and plasticity of the material. Nanoscale precipitates help improve material strength, plasticity, and fatigue life, but too high a content of V and Nb in the alloy increases the size of the precipitate; the interaction between the two elements. In consideration of the above, if the V addition amount is controlled to 0.01 to 0.15% and the Nb content is controlled to 0.001 to 0.05% after a plurality of verifications, a better effect can be obtained. Increasing the N content increases the brittleness of the material, and considering the effect of N on the precipitation of alloying elements, it is necessary to control N in steel to 0.001 to 0.009%; at the same time, the precipitate is fine. (2Nb + V) / (20N + C) in steel is controlled in the range of 0.02 to 0.40, preferably 0.045 to 0.37; in some embodiments, (2Nb + V) in steel. / (20N + C) is in the range of 0.15 to 0.37. In order to achieve high strength, excellent plasticity and high fatigue life of the manufactured spring, the original austenite particle size of the material after tempering is controlled in the range of ≤80 um and the size of precipitation in steel is controlled in the range of 5-60 nm. Will be done.

In steel, Al mainly exerts a deoxidizing effect, but alumina deoxidized by Al is a hard and brittle phase, which has a great influence on the fatigue life of the spring, and a large brittle inclusion is an abnormal spring. It is one of the main factors that cause destruction. To effectively control the alumina inclusions in the steel, Al in the steel is ≤0.0045% and the oxygen content is controlled in the range 0.0005-0.0040%; under high strength. In order to improve the fatigue life of the spring, it is necessary to control the width of the single particle inclusions in the steel to ≦ 30um.

In order to ensure the toughness of the material and prevent defects such as high temperature brittleness and low temperature brittleness in the manufacturing process, the content of harmful P and S elements in steel is controlled to 0.015% and 0.015% or less, respectively. , Improve the purity of steel.

The method for producing a spring steel having an excellent fatigue life according to the present invention includes smelting, continuous casting, rough rolling, high-speed wire rolling, stealmore cooling, wire coil wire drawing, and tempering treatment.

The above smelting employs an electric furnace or converter, and after smelting, refinement is carried out outside the furnace using an LF furnace and VD or RH degassing treatment; during the LF refining process, the composition and alkalinity of the synthetic slag. The content of P and S elements in the steel was controlled to less than 0.015% and 0.015%, and argon stirring was performed so that the refined slag completely reacted with the inclusions in the molten steel. The vacuum degassing time of VD or RH must be at least 30 minutes to ensure sufficient degassing, and finally the O content is 0.0005 to 0.0040. %, N content is controlled to 0.0010 to 0.0090%, H content is controlled to less than 2 ppm; after purification is complete, the smelting time of the ladle is to promote the uplift of large particle inclusions. By making it longer than 15 minutes, the size of inclusions in the molten steel can be controlled to ≦ 30 um.

In high speed wire rolling, the heating of the heating furnace is controlled to 920 to 1150 ° C. and the holding time is 1.0 to 3.0 h; in the wire coil high speed wire rolling process, the rolling speed is controlled to 15 to 115 m / s. A preferred scheme for online temperature control is to set the inlet temperature of the finish rolling mill unit to 880-1050 ° C, the inlet temperature of the drawroller / straightening machine unit to 840-970 ° C, and the spinning temperature to 800-950 ° C.

Preferably, to cast a 320-500 mm round or square billet, a continuous casting machine is used and the tensile speed in the continuous casting process is adjusted to the range of 0.5-0.8 m / min. By increasing the amount under light pressure at the end of the period to less than 1.08, the carbon segregation in the core of the billet is controlled to less than 1.08; this prevents secondary oxidation in the casting process of molten steel and at the same time exceeds 30 um. Floating and removing inclusions are facilitated.

Preferably, the above rough rolling employs a twice-heating production process and blooms the cast billets into 115-170 mm round or square billets at a temperature of 1050-1270 ° C. It is lump-rolled and the total rolling reduction rate exceeds 40%.

Preferably, the wire drawing speed does not exceed 3.5 m / min when the wire rod coil is drawn.
Preferably, in the above tempering treatment, before the tempering treatment, the heating temperature of the drawn steel wire is controlled in the range of 850 to 1100 ° C., the quenching medium is oil or water, and the temperature of the quenching medium is 15 to 40. The temperature is controlled to ℃, the tempering temperature is controlled to 370 to 550 ° C, and the size of nitrogen and carbon precipitates of the produced steel wire is controlled in the range of 5 to 60 nm.

Preferably, in the above Stealmore cooling, the airflow of the 14 blowers in the Stealmore line is adjusted as follows: F1-F7 blower airflow is 10-100%, F8-F12 blower airflow is 0-50%. , F13-F14 Blower Air volume is set to 0 to 50%.

In the method for producing spring steel according to the present invention:
The above smelting employs an electric furnace or converter, and after smelting outside the furnace (preventing slag from entering the ladle when the electric furnace or converter is tapped), the LF furnace and VD or RH. Purification using degassing is performed; during the LF purification process, the composition and alkalinity of the synthetic slag is adjusted to control the content of P and S elements in the steel to less than 0.015% and 0.015%. Agitate the refined slag with argon to completely react with the inclusions in the molten steel to achieve modification and removal of the inclusions; to ensure sufficient gas removal, allow a vacuum degassing time of VD or RH. It needs to be 30 minutes or more, and finally, the O content is controlled to 0.0005 to 0.0040%, the N content is controlled to 0.0010 to 0.0090%, and the H content is controlled to less than 2 ppm. After the purification is complete, the sedation time of the ladle exceeds 15 minutes and the size of the inclusions in the molten steel is controlled to ≤30 um in order to promote the lifting of large particle inclusions.

Carbon segregation at the core of the billet by casting a 320-500 mm round or square billet using a continuous casting machine and adjusting the tensile speed and the amount of final light reduction as parameters in the continuous casting process. Is controlled to less than 1.08. Secondary oxidation in the casting process of molten steel is prevented, and at the same time, inclusions exceeding 30 um can be easily lifted and removed. Using a bi-igneous material process, continuously cast billets are bloomed and lump-rolled into round or square billets of 115-170 mm at a temperature of 1050 to 1270 ° C., and the total rolling reduction ratio exceeds 40%. Miniaturize the structure.

It is heated in a heating furnace and controlled at 920 to 1150 ° C., and the holding time is 1.0 to 3.0 hours. In the process of wire coil high-speed wire rolling, the rolling speed is controlled to 15-115 m / s. A preferred scheme for online temperature control is to set the inlet temperature of the finish rolling mill unit to 880-1050 ° C, the inlet temperature of the drawroller / straightening machine unit to 840-970 ° C, and the spinning temperature to 800-950 ° C. By adjusting the temperature of the rolling process and the spinning temperature, the original austenite grains of the material are refined to ≦ 80 um and the size of the precipitate is controlled to 5 to 60 nm.

The size of the wire coil after rolling is Ф5-28 mm, and after rolling the wire coil, the conversion of the wire coil structure is controlled by adjusting the air volume of the blower of the Stelmore line. Adjust the air volume of the 14 blowers of the Stelmore line as follows: F1-F7 blower air volume to 10-100%, F8-F12 blower air volume to 0-50%, F13-F14 blower air volume to 0- Set to 50%.

Before the heat treatment, it is necessary to perform the wire drawing process of the wire rod coil, and at the time of wire drawing, the wire drawing speed is controlled so as not to exceed 3.5 m / min. Before the tempering process, the heating temperature of the drawn steel wire is controlled in the range of 850 to 1100 ° C., the quenching medium is oil or water, the temperature of the quenching medium is controlled to 15 to 40 ° C., and the tempering temperature is adjusted. The temperature is controlled from 370 to 550 ° C., and the size of the precipitate of the produced steel wire is controlled in the range of 5 to 60 nm.

The advantageous effects of the present invention are:
The strength of the spring steel produced by the steel composition and production method of the present invention can reach 2020 MPa or more, and the alloy has excellent plastic toughness while strengthening the material by nanoscale deposits at low cost. It has excellent spring forming properties, prevents machining cracks, and at the same time, the finer structure and control of the composition and size of inclusions increase the fatigue life of manufactured springs, reduce the weight of automobiles and in the mechanical industry. It can meet the demands of high strength and long life, contribute to the improvement of the technical level of the industry, and bring excellent economic benefits.

The chemical compositions of Examples A1-10 # and the three comparative steel types B1-3 # of the present invention are shown in Table 1 below, and the specific production methods are as follows:
Examples A1-5 # and the comparative steel types B1 and B2 alloys of the present invention are smelted by an electric furnace, and Examples A6 to 10 # and the comparative steel grade B3 alloy are smelted by a converter, and then the furnace. Purified outside; however, Examples A1-3 #, A6-8 #, and B1 alloys were purified by LF converter and VD and Examples A4-5 #, A9-10 #, B2, and B3 alloys. Was treated with LF and RH to optimize the texture and alkalinity of the synthetic slag; vacuum degassing time for A1-6 #, B1 was 30 minutes and vacuum degassing time for A7-10 #, B2, B3. Finally, the O content was controlled to 0.0005-0.0040%, the N content was controlled to N: 0.001-0.009%, and the H content was controlled to less than 2 ppm.

After the smelting is completed, A1-4 # and B1 are cast into a 300 mm round billet, A5-6 # is cast into a 450 mm round billet, and A7-9 # and B2 are cast into a 320 * 420 mm square billet. Casted, A10 # and B3 were cast into 500 mm square billets, and the casting process used a tundish cover and mold flux with excellent sealing. The blooming / slabbing rolling temperature of the A1-5 # and B1 continuously cast billets is 1050 ° C., and the end face size of the rolled square billets is 115 mm. The heating temperature of the square billets of A6 to 7 # and B2 is 1270 ° C., and the size of the rolled billet is 125 mm. The heating temperature of the square billets A8 to 107 # and B3 is 1100 ° C., and the size of the rolled billet is 170 mm.

The furnace temperature of the A1-4 # and B1 heating furnaces is controlled to 920 ° C. and the holding time is 1.0 hour, and the furnace temperature of the A5-10 #, B2 and B3 heating furnaces is controlled to 1150 ° C. and the holding time. Is 3.0 hours. In the process of wire coil high-speed wire rolling, the rolling speed was controlled to 15-115 m / s. Online temperature control scheme: A1-6 #, B1 alloy finish rolling mill unit inlet temperature 880-1050 ° C, drawing rolling mill / straightening machine unit inlet temperature 840-950 ° C, spinning temperature 800-950 ° C bottom. The inlet temperature of the finish rolling mill unit of A7 to 10 #, B2, and B3 alloy was set to 950 to 1050 ° C, the inlet temperature of the drawing rolling mill / straightening machine unit was set to 940 to 970 ° C, and the spinning temperature was set to 870 to 950 ° C.

However, the dimensions of the wire coil after rolling the A1 to 5 #, B1 and B2 alloys are Ф5 to 15 mm, respectively, and the dimensions of the wire coil after rolling the A6 to 10 # and B3 alloys are Ф16 to 28 mm. be. The Stealmore cooling process after rolling the A1-5 #, B1 alloys was: F1-F7 blower air volume was 40%, F8-F12 blower air volume was 5%, F13-F14 blower air volume was 40%. The Stelmore cooling process after rolling of A6-10 #, B2, B3 alloys is: F1-F4 blower air volume 50%, F5-F7 blower air volume 20%, F8-F12 blower air volume 15%, F13- The air volume of the F14 blower was set to 35%. The wire coil structure after Stelmore cooling is sorbite and a very small amount of ferrite.

Before the heat treatment, the wire coil is wire drawn, and the drawn steel wire is divided into three sets depending on the tempering treatment temperature, except that the A1 to 2 # and B1 heating temperatures are 850 ° C and the tempering temperature is 550. At ° C, the A3-7 #, B2 heating temperature is 980 ° C, the tempering temperature is 470 ° C, the A8-10 #, B3 heating temperature is 1100 ° C, and the tempering temperature is 370 ° C.

The mechanical properties of the high-strength springs of Examples A1 to A10 and the comparative steel types B1 to B3 are shown in Table 2 below. From the table, it can be seen that the strength of each alloy reaches 2020 MPa or more, which is higher than that of the B1 to B3 samples of the comparative examples, and the material reduction rate can reach 40% or more, and good plastic toughness is obtained. Be prepared. The high-strength spring and the comparative alloy of the present invention were used as coil springs of the same model number, and a spring fatigue tester was used to test the fatigue life of the coil springs according to the GBT16947-2009 coil spring fatigue test standard. As the results are shown in Table 3, under the same conditions, the fatigue life of the high-strength spring steel of the present invention was superior to that of the comparative steel.

Claims (9)

A spring steel with excellent fatigue life, the weight percentage of its chemical composition is C: 0.52-0.62%;
Si: 1.20-1.45%;
Mn: 0.25-0.75%;
Cr: 0.30-0.80%;
V: 0.01-0.15%;
Nb: 0.001-0.05%;
N: 0.001-0.009%;
O: 0.0005-0.0040%;
P: ≤0.015%;
S: ≤0.015%;
Al: ≤0.0045%;
The balance is Fe and unavoidable impurities, and satisfies 0.02 ≦ (2Nb + V) / (20N + C) ≦ 0.40. The fine structure of the spring steel is tempered troostite + sorbite structure, the original austenite crystal grain size is ≤80 um, the alloy nitrogen / carbon precipitate size is 5-60 nm, and the maximum width of single particle inclusions is ≤30 um. The spring steel having an excellent fatigue life according to claim 1, wherein the spring steel is characterized by being. The spring steel having an excellent fatigue life according to claim 1 or 2, wherein the processing strength of the spring steel is ≧ 2020 MPa, the surface reduction rate is ≧ 40%, and the fatigue life is ≧ 800,000 times. .. The method for producing a spring steel having an excellent fatigue life according to claim 1 or 2 or 3, wherein smelting, continuous casting, rough rolling, high-speed wire rolling, stealmore cooling, wire coil wire drawing, and tempering treatment are performed. include.
The above smelting employs an electric furnace or converter, and after smelting, refinement is carried out outside the furnace using an LF furnace and VD or RH degassing treatment; during the LF refining process, the composition and alkalinity of the synthetic slag. The content of P and S elements in the steel was controlled to less than 0.015% and 0.015%, and argon stirring was performed so that the refined slag completely reacted with the inclusions in the molten steel. Smelting and removal of the , H content is controlled to less than 2 ppm; after the purification is completed, the stagnation time of the ladle is longer than 15 minutes to promote the uplifting of large particle inclusions and the size of the inclusions in the molten steel. Control to ≦ 30um.
In high speed wire rolling, the heating of the heating furnace is controlled to 920 to 1150 ° C. and the holding time is 1.0 to 3.0 h; in the wire coil high speed wire rolling process, the rolling speed is controlled to 15 to 115 m / s. The online temperature control scheme sets the inlet temperature of the finish rolling mill unit to 880-1050 ° C, the inlet temperature of the drawroller / straightening machine unit to 840-970 ° C, and the spinning temperature to 800-950 ° C. To cast 320-500 mm round or square billets, use a continuous casting machine, adjust the tensile speed in the continuous casting process to the range of 0.5-0.8 m / min, and under light pressure at the end of the period. The method for producing a spring steel having an excellent fatigue life according to claim 4, wherein the carbon segregation in the core of the billet is controlled to less than 1.08 by increasing the amount of the amount to less than 10 mm. For the above rough rolling, a dithermal material process is adopted, and the cast billet is bloomed and lump-rolled into a round or square billet of 115 to 170 mm at a temperature of 1050 to 1270 ° C. The method for producing a spring steel having an excellent fatigue life according to claim 4, wherein is more than 40%. The method for producing a spring steel having an excellent fatigue life according to claim 4, wherein the wire drawing speed does not exceed 3.5 m / min when the wire wire coil is drawn. In the above tempering treatment, before the tempering treatment, the heating temperature of the drawn steel wire is controlled in the range of 850 to 1100 ° C., the quenching medium is oil or water, and the temperature of the quenching medium is controlled to 15 to 40 ° C. The superiority according to claim 4, wherein the tempering temperature is controlled to 370 to 550 ° C., and the size of the nitrogen / carbon precipitate of the produced steel wire is controlled in the range of 5 to 60 nm. A method for producing spring steel having a fatigue life. In the above Stealmore cooling, the airflow of the 14 fans of the Stealmore line is adjusted as follows: F1-F7 fan airflow is 10-100%, F8-F12 fan airflow is 0-50%, F13- The method for producing a spring steel having an excellent fatigue life according to claim 4, wherein the F14 fan air volume is set to 0 to 50%.