High-strength coil spring and method of producing same
This application is a continuation-in-part of now abandoned application, Ser. No. 07/433,207 filed on Nov. 8, 1989 now abandoned. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high-strength coil spring and a method of producing the same. The coil spring according to the present invention is effectively used as a high-strength spring for an engine and other high-strength springs requiring a high fatigue resistance. 
     2. Prior Art 
     In general, a higher tensile strength is desired for spring materials but it has been known that if a tensile strength exceeds a certain limit, the toughness and a fatigue resistance are correspondingly reduced. 
     In addition, a coil spring has been used after forming which is then subjected to a quenching treatment followed by being subjected to a shot peening treatment to add a compressive residual stress to a surface thereof, but an effective shot peening treatment gives a surface roughness Rmax of 6 to 20 μm, so that not only it has been impossible to remove surface defects having a surface roughness of 6 to 20 μm or less but also impressions due to the shot peening have covered the surface defects to be turned into damaged portions and fatigue nuclei in many cases. It goes without saying that the Rmax can be reduced by the subsequent various kinds of polishing treatment but since a surface layer is removed, portions, to which a compressive residual stress has been applied, of an outer layer introduced with much trouble is lost, whereby the fatigue resistance is reduced. 
     PROBLEMS TO BE SOLVED BY THE INVENTION 
     It is expected that if clean steels, of which the concentration of nonmetallic inclusions has been reduced, such as chromium-vanadium steel and chromium-silicon steel, are used, also the conditions for drawing forth the highest fatigue resistance as a spring are different from the conventional ones. That is to say, the tensile strength of the present chromium-vanadium steel and chromium-silicon steel is set so that the best fatigue properties may be obtained with a level of inclusions and surface defects in the conventional materials as the base but it can be expected that if merely the problems of surface defects are solved for the clean steels, the fatigue resistance can be improved by still further heightening the tensile strength. 
     MEASURES FOR SOLVING THE PROBLEMS 
     In view of the above description, the present invention has found a high-strength coil spring with high fatigue resistance using a clean steel wire, such as a chromium-vanadium steel wire and a chromium-silicon steel wire, by forming it in the shape of a spring, quenching and tempering at lower temperatures to heighten the tensile strength, and subjecting it to a shot peening treatment followed by subjecting it to an electrolytic polishing treatment. This procedure does not exert a bad influence on fatigue resistance and removes surface defects. 
     That is to say, the present invention provides 
     (1) A high-strength coil spring, characterized in that its surface roughness Rmax is made 5 μm or less by coiling a steel wire formed of steels comprising C of 0.4 to 1.0% by weight, Si of 0.1 to 2.0% by weight, Mn of 0.4 to 1.2% by weight, Cr of 0.3 to 1.5% by weight, V of 0.001 to 0.3% by weight and the remainder Fe and inevitable impurities, of which the cleanness is 0.01% or less, and then subjecting the coiled steel wire to a quenching treatment and a tempering treatment to regulate its tensile strength followed by subjecting to a shot peening treatment and a polishing treatment. The cleanliness of the steel is measured by the so-called &#34;index of cleanliness&#34; as described in JIS G 0555. This index of cleanliness is designed to measure the amount of nonmetallic inclusions in the steel and other impurities according to the formula: ##EQU1##  where, P: Total number of grating points on the glass plate in the visual field 
     f: Number of the visual fields 
     n: Number of grating points occupied by the inclusions through the visual fields numbering f. 
     (2) A method of producing a high-strength coil spring, characterized in that its surface roughness Rmax is made 5 μm or less by coiling a steel wire formed of steels comprising C of 0.4 to 1.0% by weight, Si of 0.1 to 2.0% by weight, Mn of 0.4 to 1.2% by weight, Cr of 0.3 to 1.5% by weight, V of 0.001 to 0.3% by weight and the remainder Fe and inevitable impurities, of which cleanness is prepared at 0.01% or less, and then subjecting the coiled steel wire to a quenching treatment and a tempering treatment to regulate its tensile strength followed by subjecting to a shot peening treatment and a polishing treatment. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1(A) to (D) is a graph showing the relation between the tempering temperatures and mechanical properties of a chromium-silicon steel wire quenched in oil, in which 
     FIG. 1(A) shows the relation between the tempering temperature and the hardness; 
     FIG. 1(B) shows the relation between the tempering temperature and the tensile strength; 
     FIG. 1(C) shows the relation between the tempering temperature and the reduction of area; and 
     FIG. 1(D) shows the relation between a tempering temperature and the fatigue strength. 
     FIG. 2 is a graph which shows the distribution of the residual stress in the direction of the depth of the steel wire after the quenching treatment and the tempering treatment by the relationship between the distance from the surface and the longitudinal residual stress. 
     FIG. 3(A) and (B) is a graph showing the distribution of the residual stress on the inner side of a coil spring in a process (F-1) of the present invention and the conventional process (F-7). 
    
    
     OPERATION 
     When a steel wire formed of steels comprising C of 0.4 to 1.0% by weight, Si of 0.1 to 2.0% by weight, Mn of 0.4 to 1.2% by weight, Cr of 0.3 to 1.5% by weight, V of 0.001 to 0.3% by weight and the remainder Fe and inevitable impurities is used as a material in the present invention, the reason why (i) the cleanness is prepared at 0.01% or less is so that the fatigue fracture due to the non-metallic inclusions contained in the steel wire having the above described chemical composition is avoided. This can be achieved by devising the deoxidation method such as to optimize the vacuum degassing and a refining slag conditions. 
     In addition, the reason why (ii) the quenching treatment and the tempering treatment are carried out after the coiling is that if such procedures are carried out before the coiling, the high-strength material according to the present invention is apt to be insufficient in toughness and also its sensitivity to a surface defects is strong, so that the probability of breakage during coiling would increase. 
     Furthermore, the reason why (iii) the tensile strength of the chromium-vanadium steel wire quenched in oil for use in the valve-spring by the present invention is increased by 10% in comparison with the values provided in Table 3-2 of JIS G-3565 and the tensile strength of the chromium-silicon steel wire quenched in oil for use in the valve-spring by the present invention is increased by 10% in comparison with the values provided in Table 3-1 of JIS G-3566 is because the surface defects and inclusions are removed, which means that the matrix has a sufficient toughness and also the fatigue strength can be enhanced. 
     
                       TABLE 3-2                                                   
______________________________________                                    
(Tensile strength as described in JIS G 3566)                             
Standard wire  Tensile strength                                           
diameter.sup.(1) mm                                                       
               N/mm.sup.2                                                 
______________________________________                                    
1.60           1960 to 2110                                               
1.80           1960 to 2110                                               
2.00           1910 to 2060                                               
2.30           1910 to 2060                                               
2.60           1910 to 2060                                               
2.90           1910 to 2060                                               
3.20           1860 to 2010                                               
3.50           1860 to 2010                                               
4.00           1810 to 1960                                               
4.50           1810 to 1960                                               
5.00           1760 to 1910                                               
5.50           1760 to 1910                                               
6.00           1710 to 1860                                               
6.50           1710 to 1860                                               
7.00           1660 to 1810                                               
8.00           1660 to 1810                                               
______________________________________                                    
 Note .sup.(1) The standard wire diameters shall be as specified in 5.1 of
 JIS G 3566.                                                              
 Remarks: For an intermediate diameter, the tensile strength specified of 
 the nearest larger wire diameter shall be applied.                       
 
    
     FIG. 1(A) to (D) are graphs showing the influences of the lowering of the tempering temperature for a chromium-silicon steel wire having a diameter of 4.0 mm quenched in oil as opposed to conventional materials (tempered at 400° C. for obtaining a tensile strength corresponding to JIS G-3566) upon the mechanical properties of the steel wire such as hardness, the tensile strength of the wire reduction in area and fatigue strength. 
     It is natural that if the tempering temperature is lowered, as shown in FIG. 1(A), the hardness is increased. 
     The tensile strength and the fatigue strength (by the rotating bending test) are correspondingly reduced, as shown by (b) in FIG. 1(B) and (D). However, in the case where the surface is subjected to these properties are electrolytic polishing, they are contrarily increased until a certain temperature (250° C. as for the tensile strength and 350° C. as for the fatigue strength) with the reduction of the tempering temperature, as shown by (a) in FIG. 1(B) and (D). That is to say, it was found that according to the conventional method, the strength of the matrix itself is not sufficiently exhibited due to the surface defects. 
     It can be found from the above description that even though the tensile strength after the quenching and the tempering treatment is increased over conventional materials, superior performances can be obtained by reducing the surface defects. 
     FIG. 1(C) is a graph showing the comparison of the steel wire (b) as heat treated with the steel wire (a) electrolytic polished after heat treatment as to the reduction of area. 
     (iv) The reason why the polishing treatment is carried out after the shot peening treatment is that a zone having the largest compressive residual stress exists at a depth of 100 to 150 μm from the surface, as shown by FIG. 2 which is a graph showing the distribution of the residual stress in the direction of depth of a steel elementary wire after the quenching treatment and the tempering treatment. Accordingly, it is thought that if a thickness of a portion to be removed by the polishing treatment after the shot peening treatment is 100 μm or less, the compressive residual stress of the uppermost surface is rather increased, so that no bad influence is exerted on the fatigue characteristics. 
     The steel wire used in the present invention comprises C, Si, Mn, Cr, V, Fe and inevitable impurities but the content of C is limited within a range of 0.4 to 1.0% by weight, Si 0.1 to 2.0% by weight, Mn 0.4 to 1.2% by weight, Cr 0.3 to 1.5% by weight and V 0.001 to 0.3% by weight for the following reasons. 
     That is to say, if the content of C is less than 0.4% by weight, sufficient strength is not obtained and if content of C exceeds 1.0% by weight, shrink crackings are apt to be brought about during the quenching treatment. 
     If the content of Si is less than 0.1% by weight, the heat resistance is deteriorated and if the content of Si exceeds 2.0% by weight, cracks are apt to be brought about on the surface during the hot rolling. 
     If the content of Mn is less than 0.4% by weight, the quenchability is deteriorated to lead to an insufficient strength and if the content of Mn exceeds 1.2% by weight, the workability is deteriorated. 
     The content of Cr within the range of 0.3 to 1.5% by weight is effective for the obtainment of the superior hardenability and heat resistance. 
     The content of V within the range of 0.001 to 0.3% by weight is preferable in view of the preservation of the superior micronization of crystalline particles and hardenability. 
     PREFERRED EMBODIMENTS 
     The present invention will be below described in detail with reference to the preferred embodiments. 
     EXAMPLE 1 
     A steel wire with a diameter of 4.0 mm and chemical compositions and a cleanness characteristics shown in Table 1 was produced and springs of which dimensions is shown in Table 3, was produced by the manufacturing processes shown in Table 2 from this steel wire. And, the mechanical properties after the quenching treatment and the tempering treatment and the a number of cycles to fracture when the fatigue test was carried out at a mean clamping stress τm of 60 kg/mm 2  and an amplitude stress τa of 45 kg/mm 2  are shown in Table 4. 
     In addition, the mechanical properties of a sample obtained by coiling followed by being subjected to a quenching treatment and a tempering treatment in the manufacturing process shown in Table 2 are difficult to measure, so that the mechanical properties of this sample were substituted by the characteristic values of a sample obtained by subjecting an elementary wire, which had not been subjected to the coiling, to the same subsequent treatments. In addition, the results of the fatigue tests are the average values for n=4 to 11. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Chemical Composition and Cleanness of Steel                               
Wires to be Tested                                                        
                                  Clean-                                  
C     Si  Mn  P   S   Cr  V   Fe  ness                                    
(wt %)                                                                    
      (wt %)                                                              
          (wt %)                                                          
              (wt %)                                                      
                  (wt %)                                                  
                      (wt %)                                              
                          (wt %)                                          
                              (wt %)                                      
                                  (%)                                     
__________________________________________________________________________
A 0.51                                                                    
      0.25                                                                
          0.78                                                            
              0.009                                                       
                  0.008                                                   
                      1.02                                                
                          0.22                                            
                              Rest                                        
                                  0.003                                   
B 0.46                                                                    
      0.34                                                                
          0.50                                                            
              0.008                                                       
                  0.010                                                   
                      1.2 0.25                                            
                              Rest                                        
                                  0.005                                   
C 0.64                                                                    
      0.13                                                                
          0.94                                                            
              0.010                                                       
                  0.005                                                   
                      0.81                                                
                          0.16                                            
                              Rest                                        
                                  0.003                                   
D 0.59                                                                    
      0.20                                                                
          0.48                                                            
              0.007                                                       
                  0.006                                                   
                      1.10                                                
                          0.20                                            
                              Rest                                        
                                  0.042                                   
E 0.58                                                                    
      0.22                                                                
          0.70                                                            
              0.006                                                       
                  0.007                                                   
                      0.96                                                
                          0.23                                            
                              Rest                                        
                                  0.078                                   
__________________________________________________________________________
 
    
     
                       TABLE 2                                                     
______________________________________                                    
Manufacturing Processes of Spring                                         
Manufacturing Process                                                     
______________________________________                                    
A-1   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax = 4μ)         
A-2   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax = 3μ)         
A-3   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax = 7μ)         
A-4   Coiling → Quenching, Tempering → Shot peening         
      →                                                            
      Electrolytic polishing (Rmax = 3μ)                               
A-5   Coiling → Quenching, Cryogenic tempering → Shot       
      peening → Electrolytic polishing (Rmax = 4μ)              
A-6   Coiling → Quenching, Cryogenic tempering → Shot       
      peening → Electrolytic polishing (Rmax = 2μ)              
A-7   Quenching, Tempering → Coiling → Low-temperature      
      annealing (400° C. × 15 min) → Shot peening     
A-8   Quenching, Tempering → Coiling → Low-temperature      
      annealing (400° C. × 15 min) → Shot peening     
      →                                                            
      Electrolytic polishing (Rmax = 2μ)                               
A-9   Quenching, Low-temperature tempering →  Coiling →     
      Low-temperature annealing → Shot peening →            
      Electrolytic polishing (Rmax = 4μ)                               
B-1   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax = 3μ)         
B-2   Hot coiling followed by cooling → Quenching, Low-            
      temperature tempering → Shot peening → Electrolytic   
      polishing (Rmax = 4μ)                                            
B-3   Hot coiling at 870° C. followed by quenching as it           
      is → Low-temperature tempering → Shot peening         
      →                                                            
      Electrolytic polishing (Rmax = 3μ)                               
C-1   Hot coiling at 870° C. followed by quenching as it           
      is → Low-temperature tempering → Shot peening         
      →                                                            
      Electrolytic polishing (Rmax = 4μ)                               
C-2   Coiling at 870° C. followed by quenching as it is →   
      Low-temperature tempering → Shot peening →            
      Mechanical polishing (Rmax = 3μ)                                 
D-1   Coiling at 870° C. followed by quenching as it is →   
      Low-temperature tempering → Shot peening →            
      Mechanical polishing (Rmax = 3μ)                                 
D-2   Coiling → Quenching, Tempering → Shot peening         
D-3   Quenching, Tempering → Coiling → Low-temperature      
      annealing → Shot peening                                     
D-4   Quenching, Tempering → Electrolytic polishing                
      (Rmax = 4μ)                                                      
D-5   Quenching, Low-temperature tempering → Low-                  
      temperature annealing → Shot peening → Electrolytic   
      polishing (Rmax = 3μ)                                            
E-1   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax                  
______________________________________                                    
      = 2μ)                                                            
 
    
     
                       TABLE 3                                                     
______________________________________                                    
Dimensions of Coil Spring                                                 
______________________________________                                    
Diameter of elementary wire                                               
                        4     mm                                          
Average coil diameter   24    mm                                          
Free height             55    mm                                          
Total number of turns   6.5                                               
Effective number of turns                                                 
                        4.5                                               
______________________________________                                    
 
    
     
                       TABLE 4                                                     
______________________________________                                    
Mechanical Properties and Fatigue                                         
Properties of Spring                                                      
         Tensile     Reduction Number of cycles                           
         strength    of area   at τ = 60 ±                         
Type     (kg/mm.sup.2)                                                    
                     (%)       45 kg/mm.sup.2                             
______________________________________                                    
A-1 (**1)                                                                 
         197         44        10.sup.8 or more                           
A-2 (**1)                                                                 
         196         46        10.sup.8 or more                           
A-3 (**2)                                                                 
         198         32        9.5 × 10.sup.6                       
A-4 (**2)                                                                 
         165         50        4.6 × 10.sup.6                       
A-5 (**2)                                                                 
         219          0        8.2 × 10.sup.5                       
A-6 (**2)                                                                 
         219          0        9.6 × 10.sup.5                       
A-7 (**3)                                                                 
         165         50        8.2 × 10.sup.6                       
A-8 (**2)                                                                 
         165         50        5.5 × 10.sup.6                       
A-9 (**2)                                                                 
         210         35        1.2 × 10.sup.7                       
B-1 (**1)                                                                 
         191         46        7.6 × 10.sup.7                       
                               (2/5 not broken)*                          
B-2 (**1)                                                                 
         189         50        6.2 × 10.sup.7                       
B-3 (**1)                                                                 
         187         49        5.8 × 10.sup.7                       
C-1 (**1)                                                                 
         184         46        8.2 × 10.sup.7                       
                               (3/5 not broken)*                          
C-2 (**1)                                                                 
         185         43        6.9 × 10.sup.7                       
                               (1/5 not broken)*                          
D-1 (**2)                                                                 
         194         35        8.9 ×  10.sup.6                      
D-2 (**2)                                                                 
         168         44        1.2 × 10.sup.6                       
D-3 (**3)                                                                 
         168         44        1.9 × 10.sup.6                       
D-4 (**2)                                                                 
         166         46        7.2 × 10.sup.5                       
D-5 (**2)                                                                 
         192          0        9.5 × 10.sup.5                       
E-1 (**2)                                                                 
         194          0        2.2 × 10.sup.5                       
______________________________________                                    
 Note:                                                                    
 **1 indicates a preferred embodiment of the present invention,           
 **2 indicating a comparative example, and                                
 **3 indicating the conventional example.                                 
 *In the case where the breakage does not occur at the number of repeated 
 times of 10.sup.8, an average value was calculated on the basis of       
 10.sup.8.                                                                
 
    
     EXAMPLE 2 
     A steel wire with a diameter of 4.0 mm and a chemical composition and a cleanness shown in Table 5 was produced and springs having the same dimensions as those shown in Table 3 of EXAMPLE 1 were produced by the manufacturing processes shown in Table 6 from this steel wire. And, the mechanical properties after the quenching treatment and the tempering treatment and a number of cycles to fracture when the fatigue test was carried out at a mean clamping stress τm of 60 kg/mm 2  and an amplitude stress τa of 50 kg/mm 2  were shown in Table 7. 
     In addition, the mechanical properties of a sample obtained by coiling followed by being subjected to a quenching treatment and a tempering treatment in the manufacturing process shown in Table 6 are difficult to measure, so that the mechanical properties of this sample were substituted by characteristic values as to a sample obtained by subjecting an elementary wire, which had not been subjected to the coiling, to the same subsequent treatments. In addition, the results of the fatigue tests are such that the average values for n=4 to 11. 
     
                                           TABLE 5                                 
__________________________________________________________________________
Chemical Compositions and Cleanness of Steel                              
Wires to be Tested                                                        
                                  Clean-                                  
C     Si  Mn  P   S   Cr  V   Fe  ness                                    
(wt %)                                                                    
      (wt %)                                                              
          (wt %)                                                          
              (wt %)                                                      
                  (wt %)                                                  
                      (wt %)                                              
                          (wt %)                                          
                              (wt %)                                      
                                  (%)                                     
__________________________________________________________________________
F 0.64                                                                    
      1.43                                                                
          0.68                                                            
              0.007                                                       
                  0.013                                                   
                      0.70                                                
                          0.002                                           
                              Rest                                        
                                  0.004                                   
G 0.50                                                                    
      1.21                                                                
          0.52                                                            
              0.006                                                       
                  0.009                                                   
                      0.54                                                
                          0.002                                           
                              Rest                                        
                                  0.003                                   
H 0.77                                                                    
      1.64                                                                
          0.80                                                            
              0.010                                                       
                  0.010                                                   
                      1.02                                                
                          0.003                                           
                              Rest                                        
                                  0.008                                   
I 0.62                                                                    
      1.47                                                                
          0.65                                                            
              0.009                                                       
                  0.015                                                   
                      0.69                                                
                          0.002                                           
                              Rest                                        
                                  0.026                                   
J 0.62                                                                    
      1.44                                                                
          0.68                                                            
              0.007                                                       
                  0.012                                                   
                      0.68                                                
                          0.004                                           
                              Rest                                        
                                  0.089                                   
__________________________________________________________________________
 
    
     
                       TABLE 6                                                     
______________________________________                                    
Manufacturing Processes of Spring                                         
Manufacturing Process                                                     
______________________________________                                    
F-1   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax = 3μ)         
F-2   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax = 2μ)         
F-3   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax = 8μ)         
F-4   Coiling → Quenching, Tempering → Shot peening         
      →                                                            
      Electrolytic polishing (Rmax = 3μ)                               
F-5   Coiling → Quenching, Cryogenic tempering → Shot       
      peening → Electrolytic polishing (Rmax = 3μ)              
F-6   Coiling → Quenching, Cryogenic tempering → Shot       
      peening → Electrolytic polishing (Rmax = 2μ)              
F-7   Quenching, Tempering → Coiling → Low-temperature      
      annealing (400° C. × 15 min) → Shot peening     
F-8   Quenching, Tempering → Coiling → Low-temperature      
      annealing (400° C. × 15 min) → Shot peening     
      →                                                            
      Electrolytic polishing (Rmax = 3μ)                               
F-9   Quenching, Low-temperature tempering →  Coiling →     
      Low-temperature annealing → Shot peening →            
      Electrolytic polishing (Rmax = 3μ)                               
G-1   Coiling → Quenching, Low-temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax = 3μ)         
G-2   Hot coiling followed by cooling → Quenching, Low-            
      temperature tempering → Shot peening → Electrolytic   
      polishing (Rmax = 3μ)                                            
G-3   Hot coiling at 870° C. followed by quenching as it           
      is → Low-temperature tempering → Shot peening         
      →                                                            
      Electrolytic polishing (Rmax = 3μ)                               
H-1   Hot coiling at 870° C. followed by quenching as it           
      is → Low-temperature tempering → Shot peening         
      →                                                            
      Electrolytic polishing (Rmax = 4μ)                               
H-2   Hot coiling at 870° C. followed by quenching as it           
      is → Low-temperature tempering → Shot peening         
      →                                                            
      Mechanical polishing (Rmax = 4μ)                                 
H-3   Heating to 870° C. → Chilling to 500° C.,      
      Coiling at                                                          
      500° C. → Quenching, Low-temperature tempering        
      →                                                            
      Shot peening → Electrolytic polishing                        
I-1   Hot coiling at 870° C. followed by quenching as it           
      is → Low-temperature tempering → Shot peening         
      →                                                            
      Mechanical polishing (Rmax = 4μ)                                 
I-2   Coiling → Quenching, Tempering → Shot peening         
I-3   Quenching, Tempering → Coiling → Low-temperature      
      annealing → Shot peening                                     
I-4   Quenching, → Electrolytic polishing (Rmax = 3μ)           
I-5   Quenching, Low-temperature tempering → Coiling →      
      Low-temperature annealing → Shot peening →            
      Electrolytic polishing (Rmax = 3μ)                               
J-1   Coiling → Quenching, Low temperature tempering →      
      Shot peening → Electrolytic polishing (Rmax                  
______________________________________                                    
      = 3μ)                                                            
 
    
     
                       TABLE 7                                                     
______________________________________                                    
Mechanical Properties and Fatigue                                         
Properties of Spring                                                      
          Tensile   Reduction   Number of cycles                          
          strength  of area     at τ = 60 ±                        
Type      (kg/mm.sup.2)                                                   
                    (%)         50 kg/mm.sup.2                            
______________________________________                                    
F-1 (**1) 229       41          10.sup.8 or more                          
F-2 (**1) 228       42          10.sup.8 or more                          
F-3 (**2) 226       29          2.3 × 10.sup.7                      
F-4 (**2) 198       47          4.8 × 10.sup.6                      
F-5 (**2) 248       19          1.2 × 10.sup.6 #1                   
F-6 (**2) 248       20          1.7 × 10.sup.6 #1                   
F-7 (**3) 198       45          4.2 × 10.sup.6                      
F-8 (**2) 198       47          7.5 × 10.sup.6                      
F-9 (**2) 228       41          3.9 × 10.sup.7 #2                   
G-1 (**1) 219       46          10.sup.8 or more                          
G-2 (**1) 221       44          10.sup.8 or more                          
G-3 (**1) 215       41          8.5 × 10.sup.7 #3                   
H-1 (**1) 235       39          8.9 × 10.sup.7 #3                   
H-2 (**1) 235       39          10.sup.8 or more                          
H-3 (**1) 215       48          10.sup.8 or more                          
I-1 (**2) 227       32          1.2 × 10.sup.7                      
I-2 (**2) 199       39          2.1 × 10.sup.6                      
I-3 (**3) 199       39          1.5 × 10.sup.6                      
I-4 (**2) 199       41          2.0 × 10.sup.6                      
I-5 (**2) 227       22          1.5 × 10.sup.6                      
J-1 (**2) 227        0          5.1 × 10.sup.5                      
______________________________________                                    
 Note:                                                                    
 **1 indicates a preferred embodiment of the present invention,           
 **2 indicating a comparative example, and                                
 **3 indicating the conventional example.                                 
 #1 indicates that the fluctuation is large.                              
 #2 indicates that some pieces are broken during the coil forming thereof 
 and the fluctuation in shape is large.                                   
 #3 indicates that 2 pieces of 5 pieces are not broken and 10.sup.8 was   
 adopted for the calculation of an average value of the pieces which were 
 not broken.                                                              
 
    
     It is found from the above described Table 4 of EXAMPLE 1 and Table 7 of EXAMPLE 2 that springs obtained by A-1, A-2, B-1, B-2, B-3, C-1, C-2, F-1, F-2, G-1, G-2, G-3, H-1, H-2 and H-3, which are the preferred embodiments of the present invention, are remarkably superior in fatigue useful life time. 
     Springs of D, E, I and J types inferior in cleanness, that is D-1, D-2, D-3, D-4, D-5, E-1, I-1, I-2, I-3, I-4, I-5 and J-1 are inferior in fatigue resistance. In addition, even in the case where steel wires containing the chemical compositions of A and F types are used, springs obtained by the manufacturing processes, in which the electrolytic polishing is not or insufficiently carried out, that is springs obtained by the processes of A-3, A-7, F-3 and F-7, are inferior in fatigue resistance. 
     Besides, also springs obtained by A-8 and F-8, which are the conventional manufacturing processes of A-7 and F-7 plus the electrolytic polishing process, are inferior to those obtained according to the preferred embodiments of the present invention in fatigue resistance. 
     Furthermore, springs obtained by A-4, A-5, A-6, F-4, F-5 and F-6, of which conditions are similar to those in the preferred embodiments of the present invention but the tempering conditions are not suitable, do not exhibit the sufficient fatigue resistance when they are too hard or soft. 
     Springs obtained by A-9 and F-9, of which treatment conditions in each process are same as those in the preferred embodiments of the present invention but the sequence of the processes are different, show problems in that they are inferior in fatigue resistance and difficult to be formed into springs. 
     Springs obtained by B-2 and G-2, in which the hot coiling is carried out, and springs obtained by B-3 and G-3, in which the hot coiling is carried out and then the quenching is carried out at that temperature, all exhibit superior fatigue resistance if the same low-temperature tempering process and subsequent processes as those in the preferred embodiments of the present invention are adopted. 
     It has been found from the above described EXAMPLE 1 and EXAMPLE 2 that a long useful life time of almost 10 8  as tested by the fatigue test at τ=60±45 kg/mm 2  (the fatigue test at τ=60±50 kg/mm 2  for chromium-silicon steel wire) if a chromium-vanadium steel wire or a chromium-silicon steel wire is subjected to the cold or hot coiling and then the quenching and tempering treatment to adjust its tensile strength larger than that of a chromium-vanadium steel oil-tempered wire for use in a valve spring according to JIS G-3565 by about 10% or the value larger than the tensile strength of a chromium-silicon steel oil-tempered wire for use in a valve spring according to JIS G-3566 by about 10% and the subsequent shot peening followed by the polishing treatment to give the surface roughness Rmax of 5 μm or less. 
     In addition, graphs showing the distribution of residual stress inside the coil after each process of F-1, which is the preferred embodiment of the present invention, and F-7, which is the conventional example, are shown in FIG. 3. In FIG. 3, a full line shows a longitudinal direction and a dotted line shows a tangential direction. 
     It is found from FIG. 3 that in F-1 the residual stress before the shot peening is about ±0 but in F-7 a residual tensile stress is remained in the longitudinal direction. 
     Accordingly, it seems that a compressive residual stress in the longitudinal direction after the shot peening in F-7 is reduced as much as that and the fatigue resistance is deteriorated. 
     On the other hand, it is found that in both F-1 and F-7 the compressive residual stress in a zone until a depth of 20 μm from the surface after the shot peening is smaller than that in a zone deeper than 20 μm. 
     Accordingly, it is found that the removal of the surfaces having the surface roughness of 20 μm or less by the polishing treatment has no bad influence upon the fatigue resistances on the whole. 
     In F-1 and H-1 in EXAMPLE 2 the thickness of the surface layer removed by the polishing treatment was 15 μm and that in H-2 was 12 μm. 
     EFFECTS OF THE INVENTION 
     As above described, the spring obtained by the present invention exhibits remarkably superior fatigue resistance, so that it is very useful for purposes, such as valve spring for use in car engine, requiring the reliability.