JP2012214859A - Spring, and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a spring that has a high compression residual stress layer having improved fatigue resistance and that has increased simplicity of the production process and reduced materials cost, and to provide a method for producing the spring.SOLUTION: A steel material contains 0.5-0.7 mass% C, 1.0-2.0 mass% Si, 0.1-1.0 mass% Mn, 0.1-1.0 mass% Cr, ≤0.035 mass% P, ≤0.035 mass% S, and the remainder comprising iron and inevitable impurities. The steel material is processed by the following steps in sequence: a forming step of forming the steel material into spring shape; a heat treatment step of austenitizing it at a temperature in a range of (a temperature of Ac3 point) to (the temperature of Ac3 point +250°C), cooling it at a speed of ≥20°C/sec, isothermally holding it for 400 seconds or more at a temperature in a range of (Ms point -20°C) to (Ms point +60°C), and cooling it to a room temperature at cooling speed of ≥20°C/sec; a setting step of giving a permanent strain to it; and a shot peening step of impacting it with shot.

Description

  The present invention relates to a spring having a high compressive residual stress layer and a manufacturing method thereof.

  The valve spring material for automobile engines includes carbon steel oil temper wire (SWO-V), Cr-V steel oil temper wire (SWOCV-V), Si-Cr steel oil temper wire (SWOSC-V), etc. in JIS standards. In the past, 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 lighter valve springs in order to improve the fuel efficiency of automobiles, and there is a tendency to increase the tensile strength of the strands in order to increase the design stress of the springs. However, along with the increase in strength, JIS standard oil tempered wires have significantly increased notch susceptibility to defects such as wrinkles or inclusions, which can cause breakage during cold spring forming (coiling) and brittleness during use. The tendency to show a typical form of destruction is a problem. In addition, the spring has a tensile residual stress after coiling in the direction of compressive external force during coiling, and a compressive residual stress after coiling in the direction of tensile external force during coiling. These residual stress values tend to increase. Furthermore, it is known that when a coil spring is compressed and deformed, the highest tensile stress is applied to the inner surface of the coil. Therefore, when compressively deforming a cold-formed coil spring, a high tensile stress at the time of spring compression is superimposed on the inside of the coil in addition to the tensile residual stress after coiling, often resulting in a decrease in fatigue strength.

  One means for this is to apply a compressive residual stress that is high and deep inside the wire surface. For example, fatigue resistance is widely improved by applying compressive residual stress to the surface layer of the wire by shot peening. Further, by increasing the compressive residual stress of the surface layer by shot peening, it is possible to reduce early breakage starting from the surface. However, since the yield strength of the strand increases with increasing hardness, the amount of plastic strain on the surface layer given by shot peening decreases, and the region where compressive residual stress remains (the compressive residual stress becomes zero from the surface of the strand) It is difficult to form a thick (distance in the depth direction to the position). In order to solve these problems, the following methods have been proposed.

  Patent Document 1 describes a spring having excellent fatigue resistance manufactured using an oil temper steel material in which an element such as V is added to a chemical component of JIS standard steel. However, these additive elements increase the toughness of the steel material by refining crystal grains and contribute to the improvement of fatigue resistance, but the material cost increases.

  Patent Document 2 describes a spring in which the fatigue strength is improved by adjusting the chemical composition of steel, reducing the size of inclusions that become fatigue starting points, and reducing the crystal grain size. Although this spring shows an improvement in fatigue strength, its fatigue strength level (maximum shear stress τmax = about 1200 MPa) is compared with the practical strength (τmax = about 1300 to 1400 MPa) required for a light weight high strength valve spring in recent years. And low. In Patent Document 2, nitriding treatment is added to obtain higher fatigue strength. However, although nitriding is expected to improve fatigue resistance due to an increase in surface hardness, it is necessary to completely remove iron nitride on the surface layer that can cause a decrease in fatigue strength after nitriding, so the manufacturing process becomes complicated, In addition, the nitriding cost is high, resulting in high cost.

JP-A-64-83644 JP 2005-120479 A

  It is an object of the present invention to provide a spring having a high compressive residual stress layer with improved fatigue resistance and a method for manufacturing the same while reducing the material cost and simplifying the manufacturing process.

  The inventors of the present invention conducted intensive research on 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 steel composition and subsequent annealing conditions, but it is fundamentally difficult to eliminate the effect on fatigue strength while maintaining the high strength of the steel. It came to the idea that there was. Therefore, it has been considered that it is effective to substantially reduce the residual stress generated by coiling by heating the coiled spring to the austenitizing temperature. In addition, it has been found that the fatigue resistance of the base metal itself is improved by performing austempering treatment on the spring heated to the austenitizing temperature and making the structure excellent in balance between strength, ductility and toughness. .

  Furthermore, the retained austenite on the wire surface layer is transformed into martensite by processing-induced transformation by subsequent setting and shot peening. At this time, it was found that a compressive residual stress layer reaching deep inside was formed due to volume expansion, and the progress of fatigue cracks could be suppressed and fatigue resistance could be improved. In particular, by setting the shear strain during setting to 0.0190 to 0.0220, compressive residual stress can be applied to the deeper interior than shot peening, preventing fatigue failure starting from the interior, and fatigue resistance. It was found that can be improved.

  In addition, since the strand surface yields and deforms by setting, the compressive residual stress of the strand surface is impaired. However, it has been found that the compressive residual stress on the surface of the strand can be recovered by performing shot peening after the usual setting at the end of the spring manufacturing process. In addition, this shot peening can remove the microcracks that have occurred in the previous process and improve the surface properties, thus preventing fatigue failure starting from microcracks on the surface and improving fatigue resistance. I found out that I could do it.

  In the present invention, a low-priced material such as a JIS standard oil tempered wire or a hard-drawn wire having the same composition can be used as a material before coiling, and no complicated heat treatment such as quenching and tempering is used, and normal setting and shot peening are performed. It has been found that a spring having a high compressive residual stress layer can be produced by using it. In addition, the manufacturing cost can be reduced by omitting the conventional nitriding treatment.

  That is, the manufacturing method of the spring of the present invention is, 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 forming step of forming a spring material into a steel material having a component composed of iron and inevitable impurities, and Ac3 After austenitizing at a temperature of point to (Ac3 point + 250 ° C.), it is cooled at a rate of 20 ° C./second or more, and is kept isothermal at a temperature of (Ms point−20 ° C.) to (Ms point + 60 ° C.) for 400 seconds or more. Next, a heat treatment step for cooling to room temperature at a cooling rate of 20 ° C./second or more, a setting step for imparting permanent strain, and a shot peening step for projecting shots are performed in order.

  Moreover, 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 ~ 1.0%, P: 0.035% or less, S: 0.035% or less, the remainder having a component consisting of iron and inevitable impurities, and in any cross section, bainite is 65% or more in area ratio, 4-13% of retained austenite, the balance (including 0%) has a structure that is martensite, the average carbon concentration in the retained austenite is 0.65-1.7%, The compressive residual stress of the surface is 900 to 2000 MPa, the compressive residual stress at a position of 0.3 mm from the surface is 200 MPa or more, and the average Vickers hardness of the core is 550 to 650 HV in an arbitrary cross section. It is characterized by. Here, it is preferable that the equivalent circle diameter of the cross section of the strand is 1.5 to 5.0 mm. The spring of the present invention is preferably a coil spring. In addition, you may use the spring of this invention for a stabilizer, a leaf | plate spring, a tension rod, a disc spring, etc.

  According to the present invention, a steel wire having a spring steel composition of JIS standard that does not contain an expensive alloying element and is easily available can be highly compressed on the surface layer of the wire without complicated heat treatment and surface hardening treatment. A spring having a residual stress layer and excellent in fatigue resistance can be obtained. In addition, according to the spring of the present invention, the amount of alloying elements is small, the recyclability is excellent, the manufacturing process can be simplified, and the productivity can be improved and the energy can be saved by shortening the processing time.

  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: 0.5-0.7%
C is an important element for securing a desired high strength and for obtaining a desired retained austenite ratio at room temperature, and it is necessary to contain 0.5% or more. However, if the C concentration is excessive, the ratio of retained austenite, which is a soft phase, is excessively increased and it becomes difficult to obtain a desired strength. For this reason, the C content is set to 0.5 to 0.7%.

・ Si: 1.0-2.0%
After the steel material is austenitized, bainitic ferrite is generated by isothermal holding, and C is discharged into the austenite. Thereby, a retained austenite having a high C concentration can be obtained. Si has an effect of suppressing the formation of carbides when C is discharged from bainitic ferrite into austenite, and is an indispensable element for obtaining high C concentration retained austenite as defined in the present invention. Si is an element contributing to solid solution strengthening, and is an effective element for obtaining high strength. In order to obtain these effects, Si is contained by 1.0% or more. However, if the amount of Si is excessive, the ratio of soft retained austenite is increased, and conversely, the strength is decreased, so the content is suppressed to 2.0% or less. For this reason, content of Si shall be 1.0-2.0%.

・ Mn: 0.1-1.0%
Mn is added as a deoxidizing element, but is also an element that stabilizes austenite. It is desirable to contain 0.1% or more in order to obtain retained austenite specified in the present invention. On the other hand, if the Mn content is excessive, segregation occurs and the workability is liable to decrease, so the content is suppressed to 1.0% or less. For this reason, content of Mn shall be 0.1-1.0%.

・ Cr: 0.1-1.0%
Cr is an element that can enhance the hardenability of the steel material and facilitate high strength. Moreover, since it also has an effect of delaying pearlite transformation and can stably obtain a bainite structure (suppress pearlite structure) at the time of cooling after austenitizing heating, it is contained in an amount of 0.1% or more. However, if it is excessively contained exceeding 1.0%, iron carbide tends to be generated, and retained austenite is hardly generated, so that it is suppressed to 1.0%. For this reason, the Cr content is set to 0.1 to 1.0%.

P: 0.035% or less and S: 0.035% or less Since P and S are elements that promote grain boundary segregation due to grain boundary segregation, the content is preferably low, and the upper limit is 0.035. %. Preferably, it is 0.01% or less.

Next, the manufacturing method of the spring of this invention is demonstrated.
-Forming process The forming process is a process of forming a steel material into a desired shape, and is preferably coiling. Although the temperature of the steel material at the time of shaping | molding is not specifically limited, In order to suppress manufacturing cost, the cold forming normally performed is preferable. 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.

-Seat surface grinding process This process is performed as necessary, and both end surfaces of the spring-shaped steel material are ground so as to be a plane perpendicular to the axis of the spring.

-Heat treatment process The formed steel is austenitized and kept isothermal, and then cooled. The structure of the steel material before austenitization is not particularly limited. For example, it is possible to use a strip steel material that has been hot forged or drawn. The austenitizing temperature needs to be from Ac3 point to (Ac3 point + 250 ° C.). If it is less than Ac3 point, it does not become austenite, and it becomes difficult to obtain a desired bainite ratio. Moreover, when it exceeds (Ac3 point +250 degreeC), a prior-austenite particle size will become easy to coarsen and there exists a possibility of causing a fall of ductility. The faster the cooling rate to the temperature at which the temperature is maintained isothermally after austenite is better, it is necessary to carry out at a cooling rate of 20 ° C./s or higher, preferably 50 ° C./s or higher. When the cooling rate is less than 20 ° C./s, it becomes difficult to obtain a desired structure because pearlite is generated during cooling.

  The isothermal holding temperature needs to be (Ms point−20 ° C.) to (Ms point + 60 ° C.), which is a very important control factor for obtaining a desired tissue. If it is less than (Ms point −20 ° C.), a lot of martensite is generated in the early stage of transformation, so that the improvement of ductility is hindered and it becomes difficult to obtain a desired bainite ratio. On the other hand, when it exceeds (Ms point + 60 ° C.), the tensile strength is lowered, so that the strength is lowered. In addition, the time for performing isothermal holding needs to be 400 seconds or more, which is also a very important control factor. When the isothermal holding time is less than 400 seconds, the bainite transformation hardly progresses, so the bainite ratio becomes small. On the other hand, even if the isothermal holding time is too long, the amount of bainite produced is saturated and causes an increase in production cost. The cooling rate after isothermal holding is preferably as fast as possible in order to obtain a uniform structure. A cooling rate of 20 ° C./s or higher is required, and preferably 50 ° C./s or higher. Specifically, oil cooling or water cooling is good. On the other hand, when the cooling rate is less than 20 ° C./s, a structure (such as pearlite) other than the structure defined in the present invention is generated.

-Setting process Setting is performed in order to significantly improve the elastic limit and reduce permanent deformation by applying plastic strain. Moreover, sag resistance can be further improved by performing setting at 200 to 300 ° C. (warm setting). In the setting step, it is preferable that the shear strain acting on the surface of the wire of the spring is 0.0190 to 0.0220. This is an important control factor for obtaining a thick and high compressive residual stress. If the shear strain acting on the surface of the strand during setting is less than 0.0190, a sufficient compressive residual stress layer cannot be obtained, and a compressive residual stress at a position of 0.3 mm from the surface defined in the present invention can be obtained. Can not. Further, if it exceeds 0.0220, a pre-crack is generated in the surface layer of the wire, the crack progresses at the time of use, and there is a high possibility that it breaks earlier than the desired life.

Shot peening process Shot peening is a process in which a shot made of metal or the like is collided with a spring to impart a compressive residual stress to the surface, thereby significantly improving the fatigue resistance of the spring. In the present invention, the residual stress generated by the molding is substantially zero after the heat treatment step, and a compressive residual stress layer that is high and deep inside the spring wire surface can be formed by shot peening.

  The shot peening process is preferably performed a plurality of times, and the sphere equivalent diameter of the shot used in the shot peening process performed later is preferably smaller than the sphere equivalent diameter of the shot used in the shot peening process performed earlier. Thereby, the surface roughness increased by the shot peening performed previously can be reduced. Moreover, it is preferable to perform one or more shot peening processes before the setting process.

  As the shot used in shot peening, high-hardness particles such as a cut wire, a steel ball, and a FeCrB system 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.

Next, the reason for limiting the area ratio in all tissues will be described.
-Bainite: 65% or more Bainite is a metal structure obtained by isothermal transformation of austenitic steel at a low temperature, and is composed of bainitic ferrite and iron carbide. Since the base bainitic ferrite has a high dislocation density and the iron carbide has a precipitation strengthening effect, the strength can be increased with a bainite structure. Furthermore, the bainite structure of the present invention is a structure in which iron carbide is finely precipitated on the bainitic ferrite matrix, and the decrease in grain boundary strength is small, and the decrease in ductility and toughness is small even when the strength is high. Thus, bainite is an indispensable structure for obtaining high strength and high ductility. And in order to obtain a desired high strength and high ductility, bainite needs 65% or more by area ratio.

  Usually, untransformed austenite in isothermal holding becomes martensite and retained austenite by cooling to room temperature thereafter. When the bainite area ratio is less than 65%, the C concentration in the untransformed austenite becomes small, so that a lot of martensite is generated by the subsequent cooling. When the martensite ratio is increased, high strength is obtained, but notch sensitivity is remarkably increased, so that fatigue strength is reduced. Also from this, bainite is made 65% or more by area ratio.

-Residual austenite: 4-13%
Residual austenite is effective in improving ductility and toughness and reducing notch sensitivity by utilizing the TRIP (Transformation-induced plasticity) phenomenon. The TRIP phenomenon is a phenomenon in which ductility and toughness of a material are remarkably improved by causing a processing-induced transformation accompanied by volume expansion by plastic deformation and generating processing-induced martensite. Residual austenite causes a work-induced martensitic transformation at the stress concentration portion at the crack tip, and the stress concentration can be reduced by its volume expansion. Further, since retained austenite undergoes work-induced martensitic transformation by shot peening, it is possible to form a compressive residual stress layer that is high in the surface and reaches deep inside by its volume expansion. For this reason, the retained austenite ratio is set to 4% or more, but if it is excessive, the material strength is remarkably lowered.

・ Martensite: balance (including 0%)
An appropriate amount of martensite may be contained depending on the case where a desired tensile strength is ensured. In addition, the case where the sum of bainite and retained austenite is 100% and martensite is 0% is included.

-Average carbon concentration in retained austenite: 0.65 to 1.7%
Residual austenite has an effect of reducing notch sensitivity due to high ductility and toughness as a result of higher tensile strain at which processing-induced martensitic transformation starts as the C concentration increases. In addition, the volume expansion due to work-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 compressive residual stress that is high and deep inside the surface. Therefore, it is more effective in improving fatigue resistance. For these reasons, in order to obtain the desired excellent fatigue resistance, the average C concentration of retained austenite is required to be 0.65% or more. Residual austenite is remarkably stabilized when its C concentration becomes too high, and thus acts only as a soft phase, so the upper limit is 1.7%.

Next, the reason for limitation of various characteristics in an arbitrary cross section of the spring element wire will be described.
・ Compressive residual stress The compressive residual stress of the spring surface is given by setting and shot peening. However, in the present invention, in addition to the compressive residual stress obtained by normal setting and shot peening, a compressive residual stress layer that is higher and deeper on the surface is formed by processing-induced martensitic transformation of the retained austenite of the wire. . In the present invention, the compressive residual stress on the surface is set to 900 to 2000 MPa. If the surface compressive residual stress is less than 900 MPa, it is not possible to prevent crack growth of a microcrack existing on the spring surface, which is insufficient for suppressing fatigue fracture at the surface origin. On the other hand, if the maximum compressive residual stress on the surface is remarkably high, internal fracture tends to occur due to the tensile residual stress caused by the stress balance inside the strand, so 2000 MPa is set as the upper limit.

  Further, in consideration of the combined stress of the acting stress and the residual stress due to the external load in the wire diameter range in the present invention, the range of about 200 μm to D / 4 from the surface is likely to be the starting point of fatigue failure. When the compressive residual stress at a depth of 0.3 mm from the surface is less than 200 MPa, it is insufficient to suppress fatigue fracture at the internal origin. For this reason, the compressive residual stress at a depth of 0.3 mm from the surface is set to 200 MPa or more.

-Average Vickers hardness of core part The average Vickers hardness of a wire core part shall be 550HV or more in order to ensure the intensity | strength which can endure a required load. On the other hand, when the hardness is excessively high, the notch sensitivity of the steel material itself increases, and the fatigue strength may be reduced. Therefore, the Vickers hardness of the core portion is suppressed to 650 HV or less.

  According to the present invention, the material cost can be reduced and the manufacturing process can be simplified, and a spring having a high compressive residual stress layer with improved fatigue resistance can be obtained.

It is a figure which shows the manufacturing process of the spring of this invention, and the manufacturing process of the conventional spring.

  Hereinafter, the present invention will be specifically described. 1A to 1C show an example of a manufacturing process of the spring of the present invention. First, by 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%, Cold coiling is performed in a spring shape by a coiling machine using an oil temper steel material having a composition of P: 0.035% or less, S: 0.035% or less, and the balance being iron and inevitable impurities (forming step). And it austenitizes at the temperature of Ac3 point-(Ac3 point +250 degreeC) with respect to a spring-shaped steel material, Then, it cools at a speed | rate of 20 degrees C / sec or more, (Ms point-20 degreeC)-(Ms point +60 degreeC) ) At a temperature of 400 seconds or more, and then cooled to room temperature at a cooling rate of 20 ° C./second or more (heat treatment step). Thereby, the structure | tissue of this invention can be obtained. Moreover, the residual stress generated by coiling can be made substantially zero by heating the steel material to the austenitizing temperature.

  Next, a plastic strain is applied to the spring-shaped steel material so that the shear strain acting on the surface of the strand wire is heated to 200 to 300 ° C. and becomes 0.0190 to 0.0220 (setting step). At this time, the retained austenite of the wire surface layer is transformed into martensite by processing-induced transformation. Since this is accompanied by volume expansion, deep compressive residual stress is formed, fatigue crack growth can be suppressed, and fatigue resistance can be improved. Further, by giving plastic strain, the elastic limit can be remarkably improved, the amount of permanent deformation can be reduced, and the sag resistance can be improved.

  And the shot peening process which projects a shot is performed. For example, as shown in FIG. 1A, the shot peening process is performed three times after the setting process. Further, as shown in FIGS. 1B and 1C, a shot peening process may be performed before the setting process. At this time, a round cut wire as a first step, a round cut wire with a sphere equivalent diameter smaller than the first step as a second step, and a sphere equivalent diameter smaller than the second step as a third step Use sand grains. In the heat treatment step, since the residual stress generated by coiling is made substantially zero, a compressive residual stress layer that is higher and deeper on the surface of the strand can be formed by this shot peening. And, by shot peening, the retained austenite of the wire surface layer is transformed into martensite by processing-induced transformation, so that deep compressive residual stress is formed, which suppresses the growth of fatigue cracks and further improves fatigue resistance. it can. In addition, since the wire surface yields and deforms due to setting, the compressive residual stress on the surface is impaired, but by applying shot peening after setting, the surface roughness can be improved while restoring the compressive residual stress on the wire surface, Fatigue fracture starting from the surface can be prevented, and fatigue resistance can be remarkably improved.

  The spring of the present invention produced by the process as described above has a structure in which an area ratio of 65% or more of bainite, 4 to 13% of retained austenite, and the balance (including 0%) is martensite in an arbitrary cross section. The average carbon concentration in the retained austenite is 0.65 to 1.7%, the compressive residual stress on the surface is 900 to 2000 MPa, and the position is 0.3 mm from the surface in any cross section. The compressive residual stress is 200 MPa or more, and the average Vickers hardness of the core is 550 to 650 HV in an arbitrary cross section. Therefore, the spring of the present invention is excellent in fatigue resistance.

  A spring was manufactured in accordance with each manufacturing process shown in FIGS. 1A to 1C show a manufacturing process of the spring of the present invention, and a shot peening process is performed after the setting process and before and after the setting process. FIG. 1D shows a conventional manufacturing process, in which a setting process is performed after the shot peening process. First, using an oil temper steel material SWOSC-V having chemical components shown in Table 1, a coil as shown in Table 2 was prepared after cold coiling into a predetermined shape by a coiling machine. And it heat-processed on the conditions of Table 3. In the setting, the shear strain shown in Table 3 was applied after heating at 230 ° C. for 10 minutes. The first shot peening uses a round cut wire with a sphere equivalent diameter of 0.8 mm, the second shot peening uses a round cut wire with a sphere equivalent diameter of 0.45 mm, and the third shot peening uses a sand grain with a sphere equivalent diameter of 0.1 mm. I went. Various properties of the spring thus obtained were investigated as follows. The results are also shown in Table 4.

-Average C density | concentration in a metal structure and a retained austenite A metal structure was performed as follows, after immersing a sample for several seconds in a 3% nital liquid. Because bainite is easily corroded by nital, it appears black or gray in optical micrographs. On the other hand, martensite and retained austenite appear white in the optical microscope because of their high corrosion resistance to nital. Utilizing 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 retained austenite (white part). The residual austenite ratio was determined using an X-ray diffraction method after buffing the 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 obtained by using a lattice constant a (nm) obtained from each diffraction peak angle of (111), (200), (220) and (311) of austenite by X-ray diffraction. It calculated using the relationship of the following formula | equation (1).
[Equation 1]
a (nm) = 0.3573 + 0.0033 × (mass% C) (1)

-Average Vickers hardness of core part In the cross section of a strand, five points of Vickers hardness in a center part were measured, and the average value was calculated | required.

・ Surface compressive residual stress and compressive residual stress at a position of 0.3 mm from the surface Compression on the inner diameter side surface of the coil spring in a direction that causes a tensile strain when inclined by 45 ° with respect to the axis of the wire and a spring pushing load is applied Residual stress was measured using X-ray diffraction. Further, the above measurement was performed after the entire surface of the coil spring was chemically polished, and the residual stress distribution in the depth direction was obtained by repeating this measurement, and the compressive residual stress at a position of 0.3 mm from the surface was obtained from the result. In the depth direction from the surface to a position of 0.3 mm, the compressive residual stress gradually decreased, and the compressive residual stress at a position of 0.3 mm from the surface was the lowest.

  No. satisfying the requirements of the present invention. Samples 1 to 3 had good surface compressive residual stress, and the compressive residual stress at a position of 0.3 mm from the surface was also a high value. This is because the compressive residual stress is temporarily lost because the surface of the element wire yields and deforms by setting, but the surface compressive residual stress can be recovered by performing shot peening after the setting step. On the other hand, No. which does not satisfy the conditions of the present invention. Samples 4 to 9 did not satisfy the predetermined compressive residual stress value for the following reason.

  That is, no. In the sample No. 4, shot peening was not performed after the setting step, and the surface of the strand was yielded and deformed by the setting, and the compressive residual stress remained impaired. No. In the sample No. 5, since the shear strain acting on the spring surface during setting is small, the surface compressive residual stress is No. 5. Although higher than the sample of 4, the compressive residual stress at a position of 0.3 mm from the surface was low. No. Since the sample No. 6 is shot peened after setting, the surface compressive residual stress is high. However, no. In the sample No. 6, since the shear strain acting on the spring surface during setting was small, a thick and high compressive residual stress could not be obtained, and the compressive residual stress at a position of 0.3 mm from the surface was low.

  No. In the sample No. 7, since the isothermal holding temperature is too low, martensite generated in the early stage of transformation excessively increases the core hardness, and the C concentration in the retained austenite decreases, so that the compressive residual stress is low. became. No. In the sample No. 8, since the isothermal holding time was short, the bainite transformation hardly progressed, and the bainite ratio was remarkably reduced. As a result, the martensite ratio was increased and the core hardness was excessively increased. Moreover, since the C concentration of the retained austenite was low, the compressive residual stress was low. Furthermore, no. In the sample No. 9, since the isothermal holding temperature was too high, the soft retained austenite ratio increased and the core hardness decreased. Even when the residual austenite was transformed into work-induced martensite, the hardness was low and the surrounding restraining force was small, and the surface compressive residual stress and the compressive residual stress at a position 0.3 mm from the surface were low.

  From these facts, it was confirmed that a spring excellent in fatigue resistance having a high compressive residual stress layer can be obtained by performing heat treatment under appropriate conditions and performing shot peening after setting.

Claims (8)

  In mass%, C: 0.5-0.7%, Si: 1.0-2.0%, Mn: 0.1-1.0%, Cr: 0.1-1.0%, P: 0.035% or less, S: 0.035% or less, with a forming step of forming into a spring shape for a steel material having the balance of iron and inevitable impurities, and a temperature of Ac3 point to (Ac3 point + 250 ° C.) After austenitization, it is cooled at a rate of 20 ° C./second or more, kept isothermally at a temperature of (Ms point−20 ° C.) to (Ms point + 60 ° C.) for 400 seconds or more, and then at a cooling rate of 20 ° C./second or more. A method for manufacturing a spring, comprising sequentially performing a heat treatment step for cooling to a temperature, a setting step for imparting permanent strain, and a shot peening step for projecting shots.   The method of manufacturing a spring according to claim 1, wherein the forming step is coiling.   The method for manufacturing a spring according to claim 1 or 2, wherein, in the setting step, a shear strain acting on a surface of the wire of the spring is 0.0190 to 0.0220.   The shot peening process is performed a plurality of times, and a sphere equivalent diameter of a shot used in a shot peening process performed later is smaller than a sphere equivalent diameter of a shot used in a shot peening process performed earlier. 4. A method for manufacturing a spring according to any one of 3 above.   The spring manufacturing method according to claim 1, wherein one or more shot peening processes are performed before the setting process.   In mass%, C: 0.5-0.7%, Si: 1.0-2.0%, Mn: 0.1-1.0%, Cr: 0.1-1.0%, P: 0.035% or less, S: 0.035% or less, with the balance being composed of iron and inevitable impurities, in any cross section, bainite is 65% or more in area ratio, residual austenite is 4 to 13%, The balance (including 0%) has a structure of martensite, the average carbon concentration in the retained austenite is 0.65 to 1.7%, and the surface compressive residual stress is 900 in any cross section. A spring characterized by having a compressive residual stress of 200 MPa or more at a position of 0.3 mm from the surface and having an average Vickers hardness of 550 to 650 HV in an arbitrary cross section.   The spring according to claim 6, wherein an equivalent circle diameter of the transverse section is 1.5 to 15.0 mm.   The spring according to claim 6 or 7, wherein the spring is a coil spring.

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