JP5523241B2 - Spring and manufacturing method thereof - Google Patents

Description

  The present invention relates to a spring excellent in fatigue resistance and a method for manufacturing the same.

  For example, the material of a valve spring for an automobile engine is JIS standard carbon steel oil temper wire (SWO-V), Cr-V steel oil temper wire (SWOCV-V), Si-Cr steel oil temper wire (SWOSC-V). Conventionally, Si—Cr steel oil tempered wires have been widely used from the viewpoint of fatigue resistance and sag resistance. In recent years, there has been a strong demand for weight reduction of valve springs in order to improve the fuel efficiency of automobiles, and the tensile strength of strands tends to increase in order to increase the design stress of the springs. However, when the metal structure is tempered martensite, such as JIS standard oil tempered wire, the notch susceptibility to defects such as wrinkles or inclusions increases remarkably as the strength of the wire increases. There has been a problem that breakage during (coiling) and a tendency to show a brittle fracture form during use become strong.

  Also, in coil springs, tensile residual stress is generated after coiling in the direction of compressive external force during coiling, and compressive residual stress is generated after coiling in the direction of tensile external force during coiling. These residual stress values tended to increase as the strength increased. Furthermore, it is known that when the coil spring is compressed and deformed, the highest tensile stress is applied to the inside of the coil on the surface. Therefore, when compressively deforming a cold-formed coil spring, the inside of the coil is often subjected to high tensile stress during spring compression in addition to the tensile residual stress after coiling, and often becomes the starting point of fatigue failure.

  Therefore, it is necessary to maintain the fatigue resistance of the coil spring against a high acting stress, and a countermeasure for this is to form a high compressive residual stress in the surface layer of the wire over a deep range. For example, it has been widely practiced to improve the fatigue resistance of a spring by forming a compressive residual stress in the surface layer of the wire by shot peening.

  However, in recent years, since the yield strength of strands has increased with the increase in hardness, the amount of plastic strain on the surface layer given by shot peening has decreased, and the compressive residual stress layer (position where the compressive residual stress is zero from the surface) It is becoming difficult to form a deep distance (hereinafter the same).

  Also, by increasing the compressive residual stress of the outermost layer by shot peening, early breakage starting from the surface is suppressed, but the combined stress (net stress received inside the material) distribution of acting stress and residual stress is The maximum depth in the radial direction is a region of about 200 to 600 μm from the surface, although it depends on the wire diameter and the acting stress due to the recent increase in design stress. If inclusions of about 20 μm are present in the range, stress concentration is generated in the inclusions so as to exceed the fatigue strength of the material and become a breakage starting point. In order to solve these problems, the following methods have been proposed.

  Patent Document 1 discloses a spring having excellent fatigue resistance manufactured by using an oil tempered wire in which an element such as V is added to a chemical component of JIS standard steel. However, such an additive element contributes to the improvement of fatigue resistance by increasing the toughness of the steel material by refining crystal grains, but there is a problem that the material cost becomes high.

  Patent Document 2 discloses a Si-killed steel wire spring having excellent fatigue characteristics formed by using a steel material in which the amount of addition of Ba, Al, Si, Mg or Ca is adjusted. However, in order to contain these additive elements in a well-balanced manner, it is presumed that management in the steel refining process becomes extremely difficult, resulting in high costs.

  Patent Document 3 discloses a spring having excellent fatigue strength by adjusting the chemical composition of steel to reduce the size of inclusions that become fatigue starting points and to reduce the crystal grain size. In such a spring, an increase in fatigue strength is observed, but the fatigue strength level (maximum shear stress τmax = about 1200 MPa) is a practical strength (τmax = about 1300 to 1400 MPa) required for a light weight high strength valve spring in recent years. ) Is low.

  Patent Document 3 describes that a nitriding treatment is added to obtain higher fatigue strength. However, although nitriding is expected to improve fatigue resistance by increasing the surface hardness, it is necessary to completely remove iron nitride on the surface layer, which can cause a decrease in fatigue strength after nitriding, and the manufacturing process is complicated. And the cost of nitriding treatment is high, resulting in high cost.

  In Patent Document 4, the difference (residual stress difference) between the coil inner side and the coil outer side after forming the coil spring (residual stress difference) is set to 500 MPa using a hard drawn wire ((ferrite + pearlite) structure or drawn wire with a pearlite structure). A hard spring having excellent fatigue strength, which is achieved by controlling the following, is disclosed. The technique disclosed in Patent Document 4 has an advantage of reducing the cost of quenching and tempering necessary for producing an oil tempered wire that has been widely used conventionally, but in order to reduce the residual stress difference to 500 MPa or less. In addition to adjusting the chemical composition of the steel, it is necessary to anneal at 400 ° C. or higher after forming the spring, which reduces the strength of the material and consequently makes it difficult to obtain a high-strength spring that can meet recent demands. Met.

  Patent Document 5 discloses a high fatigue strength spring steel wire excellent in cold formability by adding Mo, V or the like to the chemical components of JIS standard spring steel and performing austempering treatment. This technique aims to reduce the tensile residual stress inside the coil remaining after cold forming by setting the yield ratio (ratio of yield strength to tensile strength) to 0.85 or less. However, even if coiling is performed using a wire having a yield ratio of 0.85 or less, and annealing is performed after cold forming, it is difficult to sufficiently reduce the tensile residual stress generated after cold forming over the inside, Even after shot peening, it was difficult to form a deep compressive residual stress, and there was a limit to improving fatigue resistance. Moreover, patent document 5 is not describing about the structure and ratio of the structure | tissue of the steel wire for springs.

JP-A 64-83644 JP2008-163423 JP 2005-120479 A Japanese Patent No. 4330306 JP-A-2-57637

  The present invention has been made to solve the above-described problems of the prior art, and aims to reduce the material cost and simplify the manufacturing process, and provide a spring excellent in fatigue resistance and a method for manufacturing the spring. And

  The present inventors have intensively studied the fatigue strength of high-strength valve springs. As a result, the residual stress generated after coiling can be reduced to some extent by adjusting the spring components and annealing conditions after coiling, but the residual stress is made harmless against fatigue while maintaining high strength of steel. Therefore, it was considered effective to heat the spring to the austenitizing temperature after coiling so that the residual stress generated by coiling is substantially zero. In addition, the austenitizing temperature of the spring that has been heated to the austenitizing temperature is subsequently subjected to austempering treatment under specific conditions, and the fatigue resistance of the base material itself is improved by making the structure excellent in the balance between strength, ductility, and toughness. I found out. Furthermore, by performing shot peening, the retained austenite of the wire surface layer is transformed into martensite by the process-induced transformation, and this transformation is accompanied by volume expansion, resulting in the formation of high compressive residual stress in the surface layer and the development of fatigue cracks. It was found that this contributes to the improvement of fatigue resistance.

  The inventors of the present invention can use a low-priced material such as a JIS standard oil tempered wire or a hard-drawn wire having the same composition as the coil spring in which a high compressive residual stress is deeply formed on the surface layer. If an appropriate thermal history condition is selected to constitute a predetermined structure and satisfy the alloy element concentration condition, it can be manufactured by using ordinary shot peening in a later process without using a particularly complicated heat treatment process. I found it. Further, it has been found that even if the conventional nitriding treatment is omitted, it has high fatigue resistance according to the market demand, and the processing cost can be reduced and the process can be simplified.

  The spring of the present invention was made on the basis of the above knowledge, and in mass%, C: 0.5 to 0.7%, Si: 1.0 to 2.0%, Mn: 0.1 to 1. 0%, Cr: 0.1-1.0%, P: 0.035% or less, S: 0.035% or less, the balance having a component composed of iron and inevitable impurities, It has a structure containing 65% or more of bainite and 4 to 13% of retained austenite, and the average carbon concentration in the retained austenite is 0.65 to 1.7%. When the equivalent circle diameter is D (mm), the compressive residual stress layer is formed from the surface to a range of 0.35 mm to D / 4, and the maximum compressive residual stress is 800 to 2000 MPa. , The hardness at the center is 550 to 650 HV, and the depth from the surface In the range of .05～0.3Mm, characterized in that 50~500HV large high hardness layer than the hardness of the center is formed.

  Moreover, the manufacturing method of the spring of this invention is the mass%, C: 0.5-0.7%, Si: 1.0-2.0%, Mn: 0.1-1.0%, Cr: 0.1 to 1.0%, P: 0.035% or less, S: 0.035% or less, forming step of forming a wire having a component composed of iron and inevitable impurities into a spring shape, and Ac3 point After austenitizing at a temperature of ~ (Ac3 point + 250 ° C), cool at a rate of 20 ° C / second or more, hold at a temperature of Ms point to (Ms point + 60 ° C) for 400 seconds or more, and then 20 ° C / second or more And a shot peening step of projecting shots onto the spring after heat treatment.

  Here, the Ac3 point is a boundary temperature at which the material shifts from a ferrite + austenite two-phase region to an austenite single-phase region during heating, and the Ms point is a temperature at which martensite starts to form during cooling. The “center” in the present invention means the center of a circle if the cross section is circular, but means the center of gravity if it is not circular such as a rectangle or an ellipse.

  According to the present invention, a strand of a spring steel composition of JIS standard that does not contain an expensive alloy element and is easily available, and does not require complicated heat treatment or surface hardening treatment, and has a high hardness layer on the surface of the strand. Thus, a spring having a thick high compressive residual stress layer and excellent fatigue resistance can be obtained. In addition, the spring of the present invention has a small amount of alloying elements and is excellent in recyclability, and can simplify the manufacturing process and improve productivity and save energy by shortening the processing time.

In the Example of this invention, the organization chart which shows the observation result (a) by the backscattered electron image in the same area of a strand cross section, the measurement result (b) by a C element map, and the measurement result (c) by a crystal structure (phase) map It is. It is a graph which shows the C density | concentration analysis result on the I-ll line | wire of FIG.1 (b).

  First, the reasons for limiting the chemical components of the steel used in the present invention will be described. In the following description, “%” means “mass%”.

C: 0.5 to 0.7%
C is an important element for securing a high strength of 1800 MPa or more and for obtaining a desired retained austenite ratio at room temperature. In order to obtain such an effect, it is necessary to contain 0.5% or more. is there. However, when the concentration of C is excessive, the ratio of retained austenite, which is a soft phase, is excessively increased and it becomes difficult to obtain a desired strength.

Si: 1.0-2.0%
Si has the effect of suppressing the formation of carbides from austenite when C is discharged from bainitic ferrite, which is a constituent of bainite, to austenite, and C, which is a requirement of the present invention, is dissolved in a high concentration. It is an indispensable element to obtain retained austenite. It is an element that contributes to solid solution strengthening and is an effective element for obtaining high strength. In order to obtain such an effect, the Si concentration needs to be 1.0% or more. However, if the Si concentration is excessive, the soft retained austenite ratio increases and conversely causes a decrease in strength, so it is suppressed to 2.0% or less.

Mn: 0.1 to 1.0%
Mn is added as a deoxidizing element during refining, but is also an element that stabilizes austenite. Therefore, in order to obtain retained austenite specified in the present invention, Mn is preferably contained in an amount of 0.1% or more. . On the other hand, when the concentration of Mn is excessive, segregation occurs and the workability is liable to be lowered.

Cr: 0.1 to 1.0%
Cr is an element that enhances the hardenability of the steel material and promotes higher strength. In addition, Cr also has an effect of delaying pearlite transformation, and can stably obtain a bainite structure (suppress pearlite structure) during cooling after austenitizing heating, so Cr should be contained by 0.1% or more. Is desirable. However, if the content exceeds 1.0%, iron carbide tends to be generated, and retained austenite is less likely to be generated.

P, S: 0.035% or less P and S are elements that promote grain boundary fracture due to grain boundary segregation, so a lower value is desirable, and the upper limit is set to 0.35%. The concentration of P and S is preferably 0.01% or less.

  Next, the reason for limiting the area ratio of the tissue in an arbitrary cross section will be described. The “transverse section” refers to a cross section orthogonal to the longitudinal direction of the spring wire.

Bainite: 65% or more Bainite is a metal structure obtained by isothermally transforming an austenitic steel material at a temperature range of about 550 ° C. or less and exceeding the martensitic transformation start temperature. Bainitic ferrite and iron carbide Consists of. The base bainitic ferrite has a high dislocation density, and the iron carbide has a precipitation strengthening effect, so that the strength can be increased with a bainite structure. Furthermore, in the production method of the present invention, the use of steel with a high Si concentration and holding at the Ms point to (Ms point + 60 ° C.) suppresses the coarsening of the iron carbide. Has a finely precipitated structure on the bainitic ferrite ground, and the decrease in grain boundary strength is small and the decrease in ductility is small even at high strength. Thus, bainite is an indispensable structure for obtaining high strength and high ductility, and its area ratio is preferably as high as possible. In order to obtain high strength and high ductility as defined in the present invention, 65% or more is necessary. . On the other hand, the untransformed austenite during isothermal holding becomes martensite and retained austenite by cooling to room temperature thereafter. The condition that the bainite area ratio is less than 65% means that the isothermal holding time is short. Since the concentration of C in the untransformed austenite at that stage is small, the martensite ratio is increased by subsequent cooling. Therefore, when the bainite area ratio is less than 65%, the martensite ratio is high, so that high strength is obtained, but notch sensitivity is remarkably increased, so that excellent fatigue resistance cannot be obtained.

Residual austenite: 4% to 13%
Residual austenite is effective in reducing notch sensitivity due to increased ductility and toughness due to the TRIP (Transformation-induced plasticity) phenomenon. Residual austenite expands in volume due to deformation (strain) induced martensitic transformation at the stress concentration part at the crack tip, and compressive stress acts by the surrounding restraint force, stress concentration. It is thought that there is an effect that the degree of crack growth can be reduced and the crack growth rate is reduced. Further, the retained austenite is transformed into martensite by processing-induced transformation in the shot peening process. At this time, since volume expansion is accompanied, high compressive residual stress can be deeply formed on the surface layer. The retained austenite ratio is lower than the inside in the surface processed layer by shot peening, but 4% or more is required in an arbitrary cross section to exhibit the above-described effect of suppressing crack propagation, If there is, the material strength is remarkably lowered, so it is suppressed to 13% or less.

Average C concentration in retained austenite: 0.65% to 1.7%
Residual austenite has a higher tensile strain that initiates work-induced martensitic transformation as its C concentration increases, and consequently contributes to a reduction in notch sensitivity due to high ductility and toughness. In addition, the volume expansion coefficient in the processing-induced martensitic transformation of retained austenite increases as the C concentration of retained austenite increases, which promotes relaxation of stress concentration at the crack tip and generation of high deep compressive residual stress. It is thought that it is effective by improvement of. The average C concentration in the retained austenite needs to be 0.65% or more in order to obtain a compressive residual stress distribution (maximum compressive residual stress of 800 MPa or more) described later. On the other hand, if the C concentration in the retained austenite becomes too high, the retained austenite is remarkably stabilized, so that it acts only as a soft phase without any processing-induced transformation, so that the upper limit is 1.7%.

Next, the reasons for limiting the characteristics in the cross section of the spring will be described.
Compressive residual stress distribution on the surface layer The compressive residual stress on the surface layer is mainly given by shot peening. However, in the present invention, in addition to the compressive residual stress obtained by ordinary shot peening, a higher compressive residual stress is formed deeply by the processing-induced martensitic transformation of residual austenite present in the material. The depth of the compressive residual stress layer of the surface layer is set to 0.35 mm to D / 4 from the surface when the equivalent circle diameter of the cross section is D (mm). This is because, in the range of about 200 μm to D / 4 from the surface, for example, when the spring element wire diameter is in the range of 1.5 to 15 mm, the combined stress of the acting stress and the residual stress due to the external load is considered. Since this is a location that tends to be a starting point of fracture, if the thickness is less than 0.35 mm, it is insufficient to suppress fatigue fracture at the internal starting point. If the compressive residual stress layer is too thick, in order to maintain the stress balance of the entire steel material, there is a tensile residual stress that exists further inside than the depth (crossing point) where the compressive residual stress becomes zero. Since this tensile residual stress is added to the tensile stress generated in the spring element wire by an external load and promotes the generation of cracks, D / 4 is made the upper limit.

  The maximum compressive residual stress of the compressive residual stress layer is 800 to 2000 MPa. The maximum compressive residual stress of the surface layer is desirably high in order to suppress the generation and propagation of fatigue cracks, and the maximum value needs to be 800 MPa or more in consideration of use at a high design stress. On the other hand, when the maximum compressive residual stress of the surface layer is remarkably high, the risk of internal fracture due to the tensile residual stress due to the stress balance inside deeper than the crossing point increases as described above, so 2000 MPa is set as the upper limit.

The hardness of the center (center of gravity) of the hardness distribution spring needs to be 550 HV or more in order to ensure the strength that can withstand the load required for the spring. On the other hand, when the hardness is excessively high, the elongation is generally reduced and the notch (crack) sensitivity of the steel material itself is increased, and the fatigue strength may be lowered. On the other hand, the high hardness layer on the surface layer of the spring is effective for suppressing the occurrence of cracks, and is required to be 50 HV or more larger than the hardness of the center (center of gravity). However, if the hardness of the high hardness layer is too high, it becomes brittle, so the upper limit of the increase width is 500 HV or less. Further, the thickness of the high hardness layer is required to be 0.05 mm or more in order to suppress the occurrence of cracks, but if it is too thick, the toughness of the steel material itself is reduced, so that it is suppressed to 0.3 mm or less.

  In the present invention, although not essential, martensite can be included, and when a desired tensile strength is ensured, the area ratio can be 5 to 30%. When the area ratio of martensite exceeds 30%, high strength can be obtained, but notch sensitivity becomes high, so that excellent fatigue resistance cannot be obtained.

  Next, a method for manufacturing the spring of the present invention will be described. In the present invention, the steel material having the above chemical composition is subjected to a coiling process, a seat polishing process for grinding both end faces of the spring, austenitized at a temperature of Ac3 point to (Ac3 point + 250 ° C.), and then 20 ° C./second. Cooling at the above speed, holding at a temperature of Ms point to (Ms point + 60 ° C.) for 400 seconds or longer, and then performing a shot peening step after a heat treatment step of cooling to room temperature at a cooling rate of 20 ° C./second or higher. Manufactured by. There is no particular limitation on the structure of the steel before heating to Ac3 point or higher. For example, a hot-forged or drawn steel strip can be used as the material. Hereinafter, each process will be described, and the reasons for limitation will be described as necessary.

Coiling process The coiling process is a process of cold forming into a desired coil shape. As a forming method, a method using a spring forming machine (coiling machine), a method using a cored bar, or the like may be used.

Seating surface polishing step This step is to grind both end surfaces of the spring so as to be a plane perpendicular to the axis of the spring, and is performed as necessary.

This is a treatment in which the spring after coiling is austenitized, cooled and kept isothermal, and then cooled. The isothermal holding can be performed, for example, by immersing a spring in a salt bath, but is not limited thereto, and any method such as using a lead bath can be applied. There are no particular restrictions on the structure of the steel before austenitization. For example, it is possible to use a hot-forged or drawn steel bar material as a raw material. The austenitizing temperature is from Ac3 point to (Ac3 point + 250 ° C.). If it is less than Ac3 point, it does not become austenite and a desired structure cannot be obtained. On the other hand, when the temperature exceeds (Ac3 point + 250 ° C.), the prior austenite grain size tends to be coarsened, which may cause a decrease in ductility.

  The faster the cooling rate to the temperature at which the temperature is maintained isothermally after austenite, the better. When the cooling rate is less than 20 ° C./second, pearlite is generated during the cooling, and the structure defined in the present invention cannot be obtained. The temperature for isothermal holding needs to be from Ms point to (Ms point + 60 ° C.), which is a very important control factor for the method of manufacturing the spring of the present invention. Below the Ms point, martensite generated in the early stage of transformation inhibits the improvement of ductility, and the bainite ratio defined in the present invention cannot be obtained. On the other hand, when the temperature to hold isothermally exceeds (Ms point + 60 ° C.), the retained austenite ratio is too high, the tensile strength is lowered, and it is impossible to obtain the strength to withstand the load as a spring. The isothermal holding time needs to be 400 seconds or more, which is also a very important control factor for the production method of the present invention. If it is less than 400 seconds, the bainite transformation hardly proceeds, so the martensite ratio becomes high and the bainite ratio is small, and the structure defined in the present invention cannot be obtained. Even if the isothermal holding time is too long, the amount of bainite produced is saturated and the production cost is increased.

  The cooling rate after isothermal holding is preferably as fast as possible to obtain a uniform structure. A cooling rate of 20 ° C./second or more is required, and preferably 50 ° C./second or more. Specifically, oil cooling or water cooling is good. On the other hand, if the cooling rate is less than 20 ° C./second, the structure tends to be non-uniform on the steel material surface and inside, and the structure defined in the present invention may not be obtained.

Shot peening process Shot peening is a process in which a shot made of metal or sand is made to collide with a spring and compressive residual stress is applied to the surface of the spring, thereby improving the fatigue resistance of the spring. In the present invention, in addition to the compressive residual stress obtained by ordinary shot peening, a higher compressive residual stress is deeply formed by the processing-induced martensitic transformation of the retained austenite. For shots used in shot peening, cut wires, steel balls, high hardness particles such as FeCrB, and the like can be used. Further, the compressive residual stress can be adjusted by a shot equivalent sphere diameter, a projection speed, a projection time, and a multi-stage projection method.

Setting process Setting is optionally performed in order to improve the elastic limit by applying plastic strain to the spring and to reduce the amount of sag during use (permanent deformation). Further, the retained austenite is transformed into work-induced martensite by setting. By performing setting at 200 to 300 ° C. (warm setting), it is possible to further improve the sag resistance.

  Using an austemper wire composed of the components shown in Table 1, after cold coiling into a predetermined shape by a coiling machine, heat treatment was performed under the conditions shown in Table 3. In the heat treatment, the spring was heated to Ac3 point to (Ac3 point + 250 ° C.) in a heating furnace to be austenitized, held in a salt bath maintained at a temperature T ° C. shown in Table 3 for a time t (seconds), and then cooled. Next, shot peening uses a round cut wire with a sphere equivalent diameter of 0.8 mm as the first stage, a round cut wire with a sphere equivalent diameter of 0.45 mm as the second stage, and a sphere equivalent diameter of 0.1 mm as the third stage. Of sand grains were used. Further, after the spring was heated to 230 ° C., setting corresponding to the maximum shear stress τ = 1473 MPa was performed. Table 2 shows the specifications of the spring thus manufactured. Various properties of the spring thus obtained were investigated as follows, and the results are shown in Table 3.

[Discrimination of structure phase and C concentration in retained austenite]
The tissue phase was distinguished by immersing a sample with a polished cross-section in a 3% nital solution for several seconds and using the subsequent tissue as follows. First, bainite is easily corroded by nitrite, so it looks black or gray in an optical micrograph, while martensite and residual austenite appear white in an optical microscope because of their high corrosion resistance to nital. Using this characteristic, the optical micrograph was subjected to image processing to determine the bainite (black and gray part) ratio and the total ratio of martensite and / or retained austenite (white part). The residual austenite ratio was determined by using an X-ray diffraction method for a buffed finish sample. The martensite ratio was determined by subtracting the retained austenite ratio determined by X-ray diffraction from the total ratio of martensite and retained austenite determined from the optical micrograph. The average C concentration in the retained austenite is the following using the lattice constant a (nm) obtained from each diffraction peak angle of (111), (200), (220) and (311) of austenite by X-ray diffraction. It calculated with the relational expression shown. These results are also shown in Table 3.
[Equation 1]
a (nm) = 0.3573 + 0.0033 x (mass% C)

  This validity was evaluated by other means. In FIG. 3, the backscattered electron image (SEM (Scanning Electron Microscopy) observation results (a), C element map (FE-EPMA (Field Emission) The measurement result (b) by Electron Probe Micro Analyze (b), the measurement result (c) by the crystal structure (phase) map (EBSD (Electron Backscatter Diffraction) are shown. FIG. 2 shows the I-ll line in FIG. 1 (b). Fig. 1 (b) shows the results of C concentration analysis for each of the remaining austenite, and the C concentration in the remaining austenite in each of the areas A and B in Fig. 1 (b) is about 1.2% to 1.5%. Since it is in the range of 1.3% to 1.7% and is almost equivalent to the average C concentration of 1.22% obtained from X-ray diffraction, a method for measuring the C concentration in residual austenite by X-ray diffraction is reasonable. Ah It can be judged.

[Center hardness]
In the cross section, five points of Vickers hardness were measured around the center of the cross section of the spring, and the average value was obtained as the center hardness.

[Thickness of high hardness layer]
In the cross section, the Vickers hardness was measured from the outer peripheral surface of the steel material toward the center, and the thickness from the surface was measured for a high hardness layer 50 to 500 HV larger than the hardness of the center.

[Residual stress distribution]
Residual stress was measured on the outer peripheral surface of the steel using an X-ray diffraction method. Moreover, the above-mentioned measurement was performed after the entire surface of the steel material was chemically polished, and the residual stress distribution in the depth direction was determined by repeating this measurement.

[Fatigue resistance]
A fatigue test was conducted with an average stress τm of 735 MPa and a stress amplitude τa of 637 MPa. The condition indicating the number of times of durability exceeding 1 × 107 times was excellent in fatigue resistance (O in Table 3). The fatigue properties were inferior (x in Table 3). Table 3 shows the survey results of various properties.

  No. satisfying the conditions defined in the present invention. 3 and 4 show excellent fatigue resistance. On the other hand, No. which does not satisfy the provisions of the present invention. Since 1-2 and 5 did not satisfy the following conditions, fatigue resistance was insufficient. That is, no. In No. 1, since the isothermal holding temperature in the heat treatment step is lower than the Ms point, the martensite generated at the early stage of transformation causes an excessive increase in the center hardness and inhibits the improvement in ductility.

  No. Since No. 2 had a short isothermal holding time in the heat treatment step, the martensite ratio was high, and as a result, the bainite ratio was small, resulting in an excessive increase in the center hardness. No. No. 5 has an isothermal holding temperature in the heat treatment step that is too high, so that the residual austenite ratio is too high, so that the center hardness is too low. Furthermore, even if the residual austenite is transformed into work-induced martensite, the surrounding restraint force is small due to the low hardness, the compressive residual stress is low and shallow.

  The present invention can be applied to a spring that requires fatigue resistance, such as a valve spring for an engine of an automobile.

Claims (5)

In mass%, C: 0.5 to 0.7%, Si: 1.0 to 2.0%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1.0%, P: 0.035% or less, S: 0.035% or less, with the balance being composed of iron and inevitable impurities,
In an arbitrary cross section, it has a structure containing 65% or more of bainite and 4 to 13% of retained austenite in an area ratio, and the average carbon concentration in the retained austenite is 0.65 to 1.7%.
In an arbitrary cross section, when the equivalent circle diameter of the cross section is D (mm), a compressive residual stress layer is formed in a range of 0.35 mm to D / 4 from the surface, and the maximum compressive residual stress is 800. ~ 2000 MPa,
In an arbitrary cross section, the center hardness is 550 to 650 HV, and a high hardness layer 50 to 500 HV larger than the center hardness is formed in the range of 0.05 to 0.3 mm deep from the surface. Spring characterized by.   The spring according to claim 1, wherein martensite is contained in an arbitrary cross section in an area ratio of 5 to 30%.   The spring according to claim 1 or 2, wherein the spring is formed of a wire having a diameter of 1.5 to 15 mm. In mass%, C: 0.5 to 0.7%, Si: 1.0 to 2.0%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1.0%, P: 0.035% or less, S: 0.035% or less, a molding step of molding a wire having a component made of iron and inevitable impurities into a spring shape,
After austenitizing at a temperature of Ac3 point to (Ac3 point + 250 ° C.), it is cooled at a rate of 20 ° C./second or more, held at a temperature of Ms point to (Ms point + 60 ° C.) for 400 seconds or more, and then 20 ° C. / A heat treatment step for cooling to room temperature at a cooling rate of at least 2 seconds;
A method for manufacturing a spring, comprising: a shot peening process for projecting a shot onto a spring after heat treatment. The method for manufacturing a spring according to claim 4, wherein a setting step for imparting permanent deformation to the spring is performed after the shot peening step.

Images