HELICAL COMPRESSION SPRING AND METHOD FOR PRODUCING SAME

Abstract

A compression coil spring includes a steel wire material containing, hereinafter in weight %, 0.5 to 0.7% of C, 1.2 to 3.0% of Si, 0.3 to 1.2% of Mn, 0.5 to 1.9% of Cr and 0.05 to 0.5% of V as necessary components, one or more kinds selected from not more than 1.5% of Ni, not more than 1.5% of Mo and not more than 0.5% of W as freely selected components, and iron and inevitable impurities as the remainder; the C-condensed layer which exceeds the average concentration of C contained in the steel wire material exists at a surface layer part, and the thickness of the C-condensed layer is within 0.01 to 0.05 mm along the entire circumference of the steel wire material.

Claims

1. A compression coil spring, comprising a steel wire material containing, hereinafter in weight %, 0.5 to 0.7% of C, 1.2 to 3.0% of Si, 0.3 to 1.2% of Mn, 0.5 to 1.9% of Cr and 0.05 to 0.5% of V as necessary components, one or more kinds selected from not more than 1.5% of Ni, not more than 1.5% of Mo and not more than 0.5% of W as optional components, and iron and inevitable impurities as the remainder, wherein a C-condensed layer which exceeds average concentration of C contained in the steel wire material exists at a surface layer part, and thickness of the C-condensed layer is within 0.01 to 0.05 mm along the entire circumference of the steel wire material.

2. The compression coil spring according to claim 1, wherein internal hardness at a freely selected cross section of the steel wire material is in a range of 600 to 710 HV, and maximum hardness of the C-condensed layer is not less than 30 HV greater than the internal hardness.

3. The compression coil spring according to claim 1, wherein average crystal grain diameter, an interface of direction angle difference of not less than 5 defined as a grain interface, measured by the SEM/EBSD method, is not greater than 1.3 m.

4. The compression coil spring according to claim 1, wherein in a maximum principal stress direction of an inner diameter side of the coil spring generated when a compressive load is loaded on the coil spring, when defining a depth from a surface of the wire material at which value of unloaded compressive residual stress is zero as a crossing point, and when defining a value of an integral from a surface to the crossing point in a residual stress distribution curve having residual stress on the vertical axis and depth from the surface on the horizontal axis as I.sub.R, the I.sub.R is not less than 150 MPa.Math.mm.

5. The compression coil spring according to claim 1, wherein with respect to residual austenite volume ratio R measured by X-ray diffractometry, when defining a value of an integral from a surface to a depth of 0.5 mm in a residual austenite distribution curve having residual austenite volume ratio on the vertical axis and the depth from the surface on the horizontal axis as I.sub.R, the I.sub.R is not more than 3.4%-mm.

6. The compression coil spring according to claim 1, wherein surface roughness Rz (maximum height) is not more than 20 m.

7. A method for production of a compression coil spring, comprising: a coiling process in which steel wire material is hot-formed by a coil spring forming apparatus, a quenching process in which a coil which is coiled and cut off and is still at an austenite temperature range is quenched as it is, a tempering process in which the quenched coil is thermally refined, and a shotpeening process in which compressive residual stress is imparted to a wire material surface, wherein heating, carburizing and hot-forming are performed in the coiling process and the coil spring forming apparatus comprises a feed roller continuously supplying the steel wire material, a coiling part coiling the steel wire material in a coil shape, and a cutting means for cutting the steel wire material which is continuously supplied from upstream after the steel wire material is coiled at a predetermined number of windings, the coiling part comprises a wire guide for introducing the steel wire material supplied by the feed roller to an appropriate position in a processing part, a coiling tool including a coiling pin or coiling roller for processing the steel wire material supplied via the wire guide into a coil shape, and a pitch tool for imparting pitch, the coil spring forming apparatus further comprises a heating means in which the steel wire material is heated to an austenite temperature region between an outlet of the feed roller and the coiling tool, a covering member covering outer circumference of the steel wire material is arranged along a part of or along the entirety of the region between steel wire material inlet side in the heating means and the coiling tool, and a gas supplying means supplying hydrocarbon gas in the covering member is arranged.

8. The method for production of a compression coil spring according to claim 7, wherein the heating means is a high-frequency heating apparatus, and a high-frequency heating coil is arranged so as to be coaxial with the steel wire material on a route passing the steel wire material in the wire guide, or on a route of passing the steel wire material in a space between an end of a steel wire material outlet side of the wire guide and the coiling tool.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0057] FIG. 1 is a diagram showing an example of a method for production of a coil spring.

[0058] FIG. 2 is a conceptual diagram showing a forming part in the coiling apparatus in the Embodiment of the present invention.

[0059] FIG. 3 is a graph showing the residual stress distribution of the coil spring used in the Example.

[0060] FIG. 4 is a graph showing the residual austenite distribution of the coil spring used in the Example.

EXPLANATION OF REFERENCE SYMBOLS

[0061] 1: Coiling apparatus forming part, 10: feed roller, 20: coiling part, 21: wire guide, 22: coiling tool, 22a: coiling pin, 30: cutting means, 30a: cutting blade, 30b: inner mold, 40: high frequency heating coil, 50: covering member, 50a: steel wire material inlet of covering member, 50b: steel wire material outlet of covering member, 60: gas supplying part (gas supplying means), M: steel wire material.

BEST MODE FOR CARRYING OUT THE INVENTION

[0062] Hereinafter, Embodiments of the present invention are explained in detail. FIG. 1 shows each of the processes for production. Process (A) is the process for production of the compression coil spring of the present invention, and the other processes are conventional examples. The process for production shown in Process (A) is a hot forming method by the following coiling apparatus, and the processes for production shown in Processes (B) and (C) are cold forming methods by a freely selected coiling apparatus.

[0063] FIG. 2 shows a conceptual diagram of the forming part 1 of the coiling apparatus used in the processes for production (A) shown in FIG. 1. As shown in FIG. 2, the coiling apparatus forming part 1 includes a feed roller 10 continuously supplying the steel wire material M, and a coiling part 20 coiling the steel wire material M in a coil shape. The coiling part 20 includes a wire guide 21 for introducing the steel wire material M supplied by the feed roller 10 to an appropriate position, the coiling tool 22 having a coiling pin (or a coiling roller) 22a for processing the steel wire material M supplied via the wire guide 21 into a coil shape, and a pitch tool (not shown) for imparting pitch. Furthermore, the coiling apparatus forming part 1 includes a cutting means 30 having a cutting blade 30a for cutting the steel wire material M which is continuously supplied from upstream after the steel wire material M is coiled at a predetermined number of windings and an inner mold 30b, and a high-frequency heating coil 40 heating the steel wire material M between outlet of the feed roller 10 and a coiling tool 22.

[0064] A covering member 50 consisting of ceramic, for example, is arranged inside the high-frequency heating coil 40. The covering member 50 includes a steel wire material inlet 50a and a steel wire material outlet 50b of smaller diameter at both ends thereof. A gas supplying part (gas supplying means) 60 supplying hydrocarbon gas to the covering member 50 is arranged in a vicinity of the steel wire material inlet 50a of the covering member 50. The gas supplying part 60 supplies hydrocarbon gas from the steel wire material inlet 50a, for example, of the covering member 50, to the inside. It should be noted that hydrocarbon gas may be supplied from the steel wire material outlet 50b.

[0065] Rapid heating in the coiling apparatus forming part 1 is performed by the high-frequency heating coil 40, and the steel wire material is heated up to an austenite temperature region within 2.5 seconds. The location of arrangement of the high-frequency heating coil 40 is shown in FIG. 2. The high-frequency heating coil 40 is arranged at an outer circumference of the covering member 50. The steel wire material M passing through the inside of the covering member 50 is heated by the high-frequency heating coil 40, and is carburized by hydrocarbon gas filled in the covering member 50. The gas supplying part supplies hydrocarbon gas into the covering member 50 in an amount considering the density and the flow rate of hydrocarbon gas in the covering member 50, which contributes to carburizing property.

[0066] The high-frequency heating coil 40 is arranged in the vicinity of the wire guide 21, and the coiling part 20 is arranged in order to form immediately after heating the steel wire material M. In the coiling part 20, the steel wire material M, which goes through the wire guide 21, is contacted to the coiling pin 22a and is bent at a predetermined curvature, and furthermore, is contacted to a coiling pin 22a downstream and is bent at a predetermined curvature. Furthermore, the steel wire material M contacts the pitch tool so as to impart a pitch to obtain required coil shape. When the wire material is wound at a predetermined number of windings, the wire material is cut by shearing between the inner mold 30b and a linear part by the cutting blade 30a of the cutting means 30, so that the steel wire material M which is supplied from upstream and the steel wire material M which is formed in the spring shape are cut off.

(1) Production Process (A)

[0067] The process (A) in FIG. 1 shows the process for production of the first Embodiment. First, a steel wire material M containing, hereinafter in weight %, 0.5 to 0.7% of C, 1.2 to 3.0% of Si, 0.3 to 1.2% of Mn, 0.5 to 1.9% of Cr and 0.05 to 0.5% of V as necessary components, one or more kinds selected from not more than 1.5% of Ni, not more than 1.5% of Mo and not more than 0.5% of W as freely selected components, and iron and inevitable impurities as the remainder, and having a circle equivalent diameter of 1.5 mm to 10 mm is prepared. This steel wire material M is supplied by a wire supplying apparatus (not shown) to the feed roller 10, is heated to an austenite region within 2.5 seconds by the high-frequency heating coil 40, and is then coiled in the coiling part 20 (coiling process).

[0068] In this process, carburization treatment of the steel wire material M in the covering member 50 is performed simultaneously. Carburizing is performed at the wire material temperature of 850 to 1150 C., so that the C-condensed layer having a maximum concentration of C of 0.7 to 1.2% and thickness of 0.01 to 0.05 mm is formed on the surface of the steel wire material M. In this way, a surface part can be obtained in which hardness is not less than 30 HV higher than that of the internal hardness of the wire material.

[0069] Next, the coil, which is cut off after coiling and still has a temperature in an austenite region, is quenched as it is in a quenching vessel (not shown) (quenching process, performed in a quenching liquid of oil at about 60 C., for example). Furthermore, tempering is performed (tempering process, performed at 150 to 500 C., for example). By quenching, a high hardness structure including martensite structure can be obtained, and furthermore, by tempering, a tempered martensite structure having superior toughness can be obtained. Here, as the quenching and tempering treatments, a typical method can be employed. Heating temperature of the wire material before quenching, kind and temperature of the quenching liquid, and temperature and time of tempering are appropriately set depending on material of the steel wire material M.

[0070] Furthermore, by performing shotpeening treatment on the steel wire material M (shotpeening process) and setting treatment (setting process), a required fatigue resistance can be obtained. Since coiling is performed in conditions heated to an austenite region, generation of residual stress by processing can be prevented. Therefore, compressive residual stress can be more easily imparted by shotpeening compared to a cold forming method in which tensile residual stress is generated at a surface of the inner diameter side of a coil by processing, and compressive residual stress which is deep from the surface and large can be effectively imparted at the inner diameter side of the spring at which large stress are applied. Furthermore, by performing setting treatment, a further deeper compressive residual stress distribution is formed in the maximum principal stress direction when used as a spring, and fatigue resistance can be improved.

[0071] In this Embodiment, a multi-step shotpeening treatment including a first shotpeening treatment by shot having particle diameters of 0.6 to 1.2 mm, a second shotpeening treatment by shot having particle diameters of 0.2 to 0.8 mm, and a third shotpeening treatment by shot having particle diameters of 0.02 to 0.30 mm is performed. Since smaller shot is used in a later shotpeening treatment than in an earlier shotpeening process, surface roughness of the wire material can be even.

[0072] As the shot used in the shotpeening, a high hardness particle such as steel cut wire, steel beads and of the FeCrB type can be used. Furthermore, compressive residual stress can be controlled by sphere equivalent diameter, projection rate, projection time of the shot, or projection method in multiple steps.

[0073] Furthermore, in this Embodiment, hot setting is performed as the setting treatment, heating is performed to 100 to 300 C., and plastic strain is imparted to the steel material having a spring shape so that shear strain amount acting at the surface of the wire material is not less than shear strain amount in action stress in a case in which it is used as a practical spring.

[0074] The compression coil spring of the present invention produced by abovementioned process (A) has a C-condensed layer which has a concentration above the average concentration of C contained in the steel wire material at the surface layer part, and thickness of the C-condensed layer is within 0.01 to 0.05 mm along the entirety of the circumference of the steel wire material. In such a compression coil spring, since the thin and uniform C-condensed layer is formed at the surface of the steel wire material, not only is residual austenite phase low and sag resistance improved, but fatigue resistance can also be improved by rendering the vicinity of the surface high hardness so as to improve yield stress and by efficiently obtaining effects of shotpeening.

[0075] Next, in order to compare with the Embodiment of the present invention, processes (B) and (C) are explained.

[0076] In the process (B) of FIG. 1, cold coiling of the steel wire material M used in the process (A) is performed by a coiling apparatus (coiling process). Furthermore, temperature of the steel wire material after coiling is increased to an austenite region under a reduced pressure condition containing hydrocarbon gas so as to perform quenching (oil at about 60 C. for example is used as the quenching liquid) (carburizing + quenching process). Next, in a manner similar to the process (A), tempering process, shotpeening process and setting process are performed, in this order.

[0077] In the process (C), annealing and nitriding are performed instead of carburizing, quenching and tempering in the process (B).

EXAMPLES

1. Method for Production of Samples

[0078] Samples of coil spring were produced by each production process, and fatigue resistance of the samples were evaluated. First, oil-tempered wires having chemical compositions shown in Table 1 and iron and inevitable impurities as the remainder were prepared. Then, with respect to the oil-tempered wires, according to the production processes A to C shown in FIG. 1, coil springs with closed ends having a wire diameter of 4.1 mm, a spring index of 6, a total number of windings of 5.75, and a number of windings of valid part of 3.25 were produced by a hot forming method or cold forming method. It should be noted that OT wire in Table 1 means oil-tempered wire.

TABLE-US-00001 TABLE 1 (wt %) No. C Si Mn Cr V Ni Mo W Remarks A 0.65 1.30 0.60 0.50 0.08 OT wire B 0.56 1.99 0.80 1.01 0.09 0.21 OT wire C 0.60 2.35 0.30 1.85 0.35 0.30 OT wire D 0.73 2.16 0.70 1.00 0.10 0.15 0.10 OT wire

[0079] In the production process A, the steel wire was heated, carburized at treatment temperatures shown in Table 2 and coiled by a coiling apparatus having a high-frequency heating coil, covering member and gas supplying part (see FIG. 2), and quenched in an oil at a temperature of 60 C. In Table 2, the carburizing treatment temperature indicates the temperature of the surface of the steel wire. Subsequently, tempering treatment was performed in conditions shown in Table 2 (Examples 1 to 7, Comparative Examples 1 to 4).

[0080] The coiling + carburizing method in Table 2 means that the heated steel wire was carburized right before the coiling, A is a carburizing method in which the covering member and gas supplying part were used, and B is a carburizing method in which hydrocarbon gas was blown from a nozzle to the surface of the steel wire.

[0081] In the production process B, a cold coiling was performed by a freely selected coiling apparatus, the coiled steel wire material was heated to an austenite region under conditions of reduced pressure and containing hydrocarbon gas, quenching was performed in an oil at 60 C., and tempering was performed at 300 C. (Comparative Example 6). In the production process C, a cold coiling was performed, annealing was performed at 430 C., and nitriding was performed. In the nitriding treatment, a hardened layer having a depth of 0.04 mm was formed on the surface of the steel wire material (Comparative Examples 7 and 8).

[0082] Then, shotpeening treatment and setting treatment were performed for each sample. In the shotpeening treatment, a first shotpeening treatment by steel round cut wire having sphere equivalent diameter of 1.0 mm, a second shotpeening treatment by steel round cut wire having sphere equivalent diameter of 0.5 mm, and a third shotpeening treatment by steel beads having sphere equivalent diameter of 0.1 mm were performed, in this order. The setting was hot setting, which was performed at heating temperature of the coil spring at 200 C., and load stress of 1500 MPa.

TABLE-US-00002 TABLE 2 Carburizing Coiling + Coiling treatment Tempering Annealing Production carburizing temperature temperature temperature temperature Sample Material process method ( C.) ( C.) ( C.) ( C.) Example 1 A A A 1100 C. 1100 C. 425 C. Example 2 B A A 1100 C. 1100 C. 425 C. Example 3 C A A 1100 C. 1100 C. 425 C. Example 4 C A A 1100 C. 1100 C. 445 C. Example 5 C A A 1000 C. 1000 C. 425 C. Example 6 D A A 1100 C. 1000 C. 425 C. Example 7 C A A 1100 C. 1100 C. 350 C. Comparative C A A 950 C. 950 C. 425 C. example 1 Comparative C A A 1100 C. 1100 C. 490 C. Example 2 Comparative C A A 1120 C. 1120 C. 300 C. example 3 Comparative C A A 1200 C. 1200 C. 425 C. example 4 Comparative C A B 1100 C. 1100 C. 425 C. example 5 Comparative C B 850 C. 300 C. Example 6 Comparative B C 430 C. example 7 Comparative C C 430 C. example 8

2. Method for Evaluation

[0083] Properties of these samples produced as mentioned above were examined as follows. The results are shown in Table 3.

(1) Hardness (HV)

[0084] Measurement of hardness was performed at a cross section of wire material of the coil spring using a Vickers hardness testing apparatus (trade name: FM-600, produced by Future Tech Corp.). The measured load was 25 gf from the surface to a depth of 0.02 mm (surface in Table 3), and 200 gf from the surface to depth of d (wire diameter)/4 mm (inside in Table 3). With respect to each of the depths, the measurement was performed at three freely selected concentric points and the average value thereof was calculated.

(2) Value of Integral of Compressive Residual Stress (I.SUB.R.), and Crossing Point (CP)

[0085] At a surface of the inner diameter side of the coil spring, compressive residual stress in the +45 direction with respect to the wire axis direction of the wire material (an approximate maximum principal stress direction when a compressive load is loaded to a spring) was measured using an X-ray diffraction type residual stress measuring apparatus (produced by Rigaku Corporation). The measurement was performed in conditions of tube: Cr, and collimator diameter: 0.5 mm. Furthermore, chemical polishing was performed on the entire surface of the wire material of the coil spring using hydrochloric acid, and then the measurement was performed. These processes were repeated so as to measure residual stress distribution along the depth direction, and according to the results, crossing point was measured. Furthermore, the value of the integral of the compressive residual stress was calculated by integrating compressive residual stress from the surface to the crossing point, in a diagram showing the relationship of depth and residual stress. It should be noted that residual stress distribution of Example 1 is shown in FIG. 3 as an example.

(3) Surface C Concentration (C.sub.C), and C-Condensed Layer Thickness (C.sub.t)

[0086] At the cross section of the wire material of the coil spring, by measuring at six points per every 60 degrees, the average value of the C concentration of the surface, the average value of thickness of the C-condensed layer, the maximum value, and the minimum value were measured. In the measurement, a line analysis was performed in conditions of beam diameter 1 m and measuring pitch 1 m using EPMA (trade name: EPMA-1600, produced by Shimadzu Corporation). The thickness of the C-condensed layer is defined as a depth from the surface at which the C concentration is the same as that inside of the wire material.

(4) Residual Austenite (I.SUB.R.)

[0087] At the cross section of the wire material of the coil spring, with respect to each of the measured depths from the outermost surface to 0.5 mm depth, the volume ratio of residual austenite was measured at six points per every 60 degrees, a residual austenite distribution curve having residual austenite volume ratio on the vertical axis and the radius direction of wire on the horizontal axis was obtained, and an integral value I.sub.R from the surface to 0.5 mm depth was calculated in the curve. The measurement was performed by using two-dimensional PSPC installed X-ray diffractometry apparatus (trade name: D8 DISCOVER, produced by Bruker). It should be noted that the residual austenite distribution of Example 1 is shown in FIG. 4 as an example.

(5) Surface Roughness (Rz (Maximum Height))

[0088] The surface roughness was measured using a non-contact three-dimensional shape measuring apparatus (trade name: NH-3, produced by MITAKA) according to JIS B0601. Conditions of the measurement were measuring magnification 100 times, measuring distance 4 mm, measuring pitch 0.002 mm, and cut-off value 0.8 mm.

(6) Average Crystal Grain Diameter (d.sub.GS)

[0089] The average crystal grain diameter was measured using JEOL JSM-7000F (OIM-Analysis Ver. 4.6, produced by TSL Solutions) according to SEM/EBSD (electron backscatter diffraction) method. Here, the measurement was performed at the one-quarter depth of the cross section of the coil spring (d/4) and in condition of observing magnification 5000 times, and an interface at which direction angle difference is 5 or more was defined as the grain boundary to calculate average crystal grain diameter.

(7) Fatigue Resistance (Damage Ratio)

[0090] The fatigue test was performed in air at room temperature using a hydraulic servo type fatigue resistance testing apparatus (produced by Saginomiya Seisakusho, Inc.). The fatigue resistance was evaluated according to damage ratio (number of breaks/number of tests) when vibrated 20 million times in conditions of testing stress 735686 MPa, frequency 20 Hz, and number of test pieces 7 in each sample in components A and B in Table 1. The fatigue resistance was evaluated according to damage ratio (number of breaks/number of tests) when vibrated 20 million times in conditions of testing stress 760711 MPa, frequency 20 Hz, and number of test pieces 7 in each sample in component C.

(8) Sag Resistance (Residual Shear Strain Ratio )

[0091] A hot tightening test of the coil springs was performed. Conditions of the test were testing stress 1100 MPa, test temperature 120 C., and test time 48 hours. Using the following formula 1, residual shear strain ratio was calculated from load loss amount after the test compared to before the test.

=1 0 08DP/d .sup.3GFormula 1

[0092] (d: Wire diameter, D: Coil average diameter, P: Load loss amount after test compared to before test, G: Modules of rigidity

TABLE-US-00003 TABLE 3 C.sub.t I.sub.R HV L.sub. R C.sub.C (mm) (% .Math. mm) d.sub.GS Rz Damage Sample Surface inside (MPa .Math. mm) (%) Average Max Min Max (m) (m) (%) ratio Example 1 660 610 190 0.85 0.034 0.041 0.030 2.00 1.20 11.5 0.065 0/7 Example 2 670 615 192 0.88 0.035 0.043 0.033 2.10 1.10 11.0 0.060 0/7 Example 3 759 632 224 0.86 0.035 0.042 0.031 2.34 1.01 11.3 0.058 0/7 Example 4 680 600 237 1.10 0.036 0.040 0.035 0.60 1.07 10.3 0.055 1/7 Example 5 710 668 197 1.18 0.025 0.030 0.020 1.65 0.84 11.5 0.052 0/7 Example 6 770 650 240 0.89 0.036 0.043 0.031 2.20 1.00 10.2 0.050 0/7 Example 7 740 705 213 0.80 0.038 0.043 0.033 3.07 1.04 7.3 0.052 1/7 Comparative 685 663 159 0.78 0.013 0.015 0.010 0.37 0.64 7.8 0.047 4/7 example 1 Comparative 562 570 210 1.00 0.037 0.042 0.035 0.00 0.87 17.2 0.033 7/7 Example 2 Comparative 813 717 178 1.05 0.038 0.045 0.035 3.36 1.13 6.8 0.054 6/7 example 3 Comparative 760 650 220 1.10 0.043 0.045 0.038 3.40 1.35 11.0 0.080 6/7 example 4 Comparative 750 640 220 0.90 0.040 0.060 0.020 3.50 1.00 12.0 0.080 0/7 example 5 Comparative 753 698 174 1.10 0.090 0.100 0.080 3.55 0.73 6.9 0.093 1/7 Example 6 Comparative 780 590 143 0.24 0.70 4.2 0.042 4/7 example 7 Comparative 855 624 149 0.00 0.62 4.5 0.033 7/7 example 8

3. Results of Evaluation

(1) Hardness

[0093] As is obvious from Table 3, high fatigue resistance can be exhibited in the case in which inner hardness is in a range of 600 to 710 HV in Examples 1 to 7 in which the hot forming method of the process (A) was employed in the present invention. On the other hand, from results of Comparative Examples 2 and 3, sufficient fatigue resistance could not be obtained in a case in which hardness was less than 600 HV or not less than 710 HV even if the coil spring was produced by a hot forming method. Furthermore, in Examples 1 to 7, surface hardness was 30 HV or more greater than that of the inside by carburizing. High compression residual stress can be obtained in the vicinity of the surface in this way, and fatigue cracking can be prevented from occurring originating from the vicinity of the surface (including the outermost surface) (improvement of fatigue resistance). On the other hand, increase of surface hardness was less than 30 HV in Comparative Example 1, there was lots of abrasion at the middle wire part in which contact is repeated during action, early breakage occurred from the part, and sufficient fatigue resistance was not obtained.

(2) Residual Stress Distribution

[0094] With respect to Examples 1 to 7, deep and large compressive residual stress, I.sub.R not less than 180 MPa, being good fatigue resistance, was obtained. On the other hand, in Comparative Examples 7 and 8, shallow and small compressive residual stress, I.sub.R not more than 150 MPa, being inferior fatigue resistance, was obtained. This reason is considered to be that in Examples 1 to 7, which were produced by the process (A), since there is little tensile residual stress (remaining in inner diameter side of coil) in a hot coiling than is generated in a cold-coiling, compressive residual stress by shotpeening easily enters deeper from the surface compared to cases of Comparative Examples 7 and 8 in which tensile residual stress was generated by cold coiling.

(3) Surface C Concentration and Thickness of C-Condensed Layer

[0095] In Examples 1 to 7, since surface C concentration was 0.7 to 1.2% and thickness of C-condensed layer (depth from the surface which is the same C concentration as inside the wire material) was 0.01 mm to 0.05 mm by carburizing and hardness at the vicinity of the surface was also high, high compressive residual stress can be obtained in the vicinity of the surface, surface roughness was improved, and high fatigue resistance could be obtained. On the other hand, in Comparative Example 5, although the average thickness of the C-condensed layer was similar to that in Examples 1 to 7, thickness of the C-condensed layer varied substantially because the carburizing method was different. Therefore, the thickness was over 0.05 mm at a part that had large thickness of the C-condensed layer, and thus, excess carburizing resulted in increase in residual austenite. I.sub.P (integral value of R from surface to 0.5 mm depth in relational diagram between depth and R) was not more than 3.1%-mm in Examples 1 to 7; in contrast, it was 3.5%-mm, which was large, in Comparative Example 5. As a result, residual shear strain ratio was 0.050 to 0.065, which was small, and this indicated good sag resistance in Examples 1 to 7; in contrast, residual shear strain ratio was 0.080, which was large, and this indicated inferior sag resistance in Comparative Example 5. Furthermore, C concentration at the surface was 1.1% and thickness of the C-condensed layer was 0.90 mm in Comparative Example 6, which indicates excess carburizing was performed, residual austenite was increased, and I.sub.R was 3.55%-mm, which was large. As a result, residual shear strain ratio was 0.093, indicating inferior sag resistance compared to Examples 1 to 7.

(5) Surface Roughness

[0096] With respect to Examples 1 to 7 in which high fatigue resistance was obtained, the surface roughness Rz (maximum height) was not more than 12.0 m, which was sufficient to satisfy the desirable surface roughness Rz of not greater than 20 m. Here, in a case in which Rz was greater than 20 m, a concave portion in the surface roughness may become a stress concentrating source, a break may be generated originating from the concave portion, propagate, and cause early breakage as a result. Furthermore, this surface roughness is formed by friction with tools during coiling or shotpeening treatment. The surface roughness formed by shotpeening is determined by a combination of hardness of the wire material and conditions such as particle diameter, hardness and projection rate of shot. Therefore, the conditions of shotpeening should be appropriately set so that Rz is not more than 20 m.

(6) Average Crystal Grain Diameter

[0097] In the Examples, average crystal grain diameter (d.sub.GS) was in a range of 0.84 to 1.30 m, having fine crystalline structure. This is because high frequency heating in a short time results in reduced coarseness of structure or results in reduction in size as mentioned above, and as a result, fine average crystal grain diameter was obtained and fatigue resistance was improved in Examples 1 to 7. In contrast, since coiling and carburizing temperature was high in Comparative Example 4 and average crystal grain diameter (d.sub.GS) was 1.35 m, which was large, compared to Examples. Therefore, sag resistance and fatigue resistance were deteriorated.

[0098] As mentioned above, according to the compression coil spring of the present invention, fatigue resistance and sag resistance can be greatly improved.

[0099] Since the present invention has high fatigue resistance and high sag resistance, it can be used for a valve spring, in particular a valve spring for a race car engine used under high stress conditions or as a clutch torsion spring used in a clutch.