Pneumatic tire
This application is a continuation of now abandoned application Ser. No. 461,976 filed Jan. 26, 1983. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pneumatic tire reinforced with steel cords. 
     Pneumatic tires reinforced with steel cords have been used preferably for high speed and heavy load vehicles for transportation as well as construction vehicles. But due to the heavy specific gravity of steel cords, the weight of tires becomes heavy so that a large amount of fuel has been consumed. 
     2. Prior Art 
     In order to reduce the tire weight, it has been attempted to use a specially compounded rubber or to reduce the thickness of rubber in some portions, to the extent that reduced faults occur in the tire functions. But the amount of steel cords has not been reduced because if the amount of steel cords is reduced, the safety ratio of tires is lowered. 
     Besides, in a pneumatic radical tire reinforced with steel cords, concentration of stress usually occurs at the interface between rigid steel cords and flexible rubber, thereby readily causing rubber separation from the end portions of steel cords or rubber separation in belt plies in a radial tire. Moreover, a conventional pneumatic tire reinforced with steel cords caused a separation failure due to detachment of a surface metal plating induced by water penetration into an interface between an iron body and a surface metal plating of steel cords when being exposed to high temperature and high humidity for a long time. 
     The inventors have conducted various investigations and experiments in order to reduce the amount of steel cords in order to lighten tire weight and thereby decreasing fuel consumption and diminishing separation failures in a pneumatic tire. As a result, in Japanese Patent Application No. 55-125625 laid open on Mar. 26, 1982 under No. 57-51502 the inventors proposed a pneumatic tire reinforced at least partially with steel cords in which most of steel filaments of steel cords comprise an iron material containing carbon at 0.75% to 0.85% by weight and the steel cords have the tensile strength calculated by the following formulas; ##EQU2## In the above formulas, D is the diameter (mm) of steel filaments of a steel cord. N is the number of steel filaments of a steel cord. W is the weight (g) of a steel cord per 1 m. TS is tensile strength (kg) of a steel cord. The denominator, 7.86 shows the specific gravity of iron. The above formula (1) and (2) are applied in the range of N being 3 to 50 and D being 0.15 to 0.40. If D is less than 0.15 mm, productivity is poor and production is not practical because of high cost. If D is more than 0.40 mm, the rigidity of a steel cord becomes too high so that the tire easily undergoes separation, and fatigue resistance is reduced. In respect to N, when the diameters of the core filaments are different from each other and outer filaments, the number, m, of core filaments shall be taken as 1 to be added to the number, n, of outer filaments. Namely, N is taken as n+1. 
     On the other hand, the tensile strength of conventional steel cords used for tires can be calculated by the formula; ##EQU3## 
     In order to obtain a steel cord having the tensile strength satisfying TS value according to the above formula 1, the carbon content in an iron material for steel filaments should be in the range of 0.75 wt% to 0.85 wt% which is more than the carbon content 0.69 wt% to 0.73 wt% in conventional steel filaments. In case of a carbon content less than 0.75 wt%, the tensile strength is low. A carbon content more than 0.85 wt% makes heat treatment of the steel filaments very difficult and results in a deficiency in tenacity of the steel cord obtained. As a result, when a pneumatic tire employing such steel cords treads on projections and the like, they are readily broken down. 
     The steel filaments containing the above mentioned amount of carbon are brass-plated in order to increase the adhesiveness thereof to rubber. The preferable composition ratio of the brass is 60 wt% to 70 wt% copper and 30 wt% to 40 wt% zinc. If copper contained in brass is less than 60 wt%, the brass has a β phase so that such brass-plated irons are impossible to be drawn into steel filaments in cooperation with the high hardness of iron materials. If copper contained in brass is more than 70 wt%, pin holes are formed on the brass when steel filaments are drawn. One of the special findings in said invention is that no breakage occurs between the steel filaments and the brass when a tire is exposed to high temperature and high humidity, by virtue of firm bonding between steel filaments having high carbon content and plated brass. This is because higher pressure is applicable to materials in a wet-drawing process using a lubricant than in a drawing process for conventional materials containing low carbon, so that brass is pressed on and bonded to iron cores when the materials pass through drawing dies. Assuming that the strength of tires is uniform, the reinforcing effect of a steel cord is small if the tensile strength of a steel cord is smaller than the value calculated from said formulae 1 and 2. Accordingly, the amount of steel cords can not be reduced. As a result, tire weight and fuel consumption are not so different from those of tires employing conventional steel cords. If on the contrary the tensile strength of a steel cord is bigger than the value calculated from said formula 1 and 2, the amount of steel cords corresponding to the increased amount of the tensile strength can be reduced in order to maintain the same strength as that of a tire comprising conventional steel cords. But the rigidity of a tire and the rigidity of the composite of rubber and steel cords become small so that the wear resistance of a tire diminishes. 
     The above described pneumatic tire provided by the senior Japanese patent application is lighter in weight and smaller in rolling resistance and fuel consumption than a conventional tire and also it enjoys superior moisture heat coverage. But a separation failure has been experienced at a belt portion when said tire treads on a projection like a nail. Said separation results from the deterioration of strength of belt portion due to accidental cutting of steel cords contained therein. Our investigation reveals that it is attributed to the fact that the knot strength of steel cords is lowered with the increase in tensile strength thereof. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is the object of the present invention to improve the senior Japanese patent application and to provide a pneumatic tire reinforced by steel cords having the preferred tensile strength and having light weight, low rolling resistance and small fuel consumption and moreover, being free from separation failure by virtue of the increase of the knot strength of steel cords. 
     According to the present invention, a pneumatic tire at least partially reinforced by steel cords is constructed as follows. Namely, most of steel filaments of said steel cords comprise iron materials having carbon content of 0.75 wt% to 0.85 wt% and said steel cords have a tensile strength calculated from the following formula; ##EQU4## In the above formulae, D is the diameter (mm) of steel filaments of steel cords. N is the number of steel filaments of a steel cord. W is the weight (g) of a steel cord per 1 m. and TS is tensile strength (kg) of a steel cord. The denominator, 7.86 is the specific gravity of iron. Moreover, said steel cords have a maintenance rate of knot strength of more than 60%. 
     In order to maintain the knot strength of more than 60% while keeping the tensile strength at a preferred value, our investigation reveals that the reduction ratio from an iron material to a steel filament should be less than 97.3% and the size of non-metallic inclusion in an iron material such as non-ductile aluminum oxide, silica, etc. should be less than 10μ. The knot strength of a steel cord is measured according to JIS-L 1017. The reduction ratio (R) is calculated by the following formula. ##EQU5## 
     In the above formula, So is the sectional area of the iron material. S is the sectional area of the steel filament. The sizes of non-metallic inclusions are measured in accordance with JIS-G0555. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Represented in the drawing is a graph showing the relationship between the running distance and strength maintenance ratio relative to Embodiment 3 according to the present invention and comparison examples 7 to 9. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Foregoing description relative to the senior Japanese patent application is also applicable to the present invention. Accordingly, the description is hereinafter is only made with respect to an embodiment having a preferred tensile strength as well as a preferred maintenance ratio of the knot strength and with respect to comparison tires. 
     Embodiment 1 
     SWRS 82A specified by JIS-G3502 was chosen for the iron material of steel filaments according to the present invention. SWRS 72A was chosen for comparison purpose. Brass having the copper/zinc ratio of 67/33 was plated on both iron materials in the amount of 5.5 g/kg. 
     Steel filaments thus constructed were twisted with a double twister of the buncher type at the speed of 4,000 rpm to obtain steel cords of 1×4×0.22 mm having a twisting pitch of 10 mm. Radial tires 165SR13 were produced which comprised a belt layer containing said steel cords and a carcass layer containing polyester cords. The properties of the above steel filaments and of the steel cords are shown in Table 1. The properties of tires are shown in Table 2. 
     Rubber compositions used for making the tires in the Embodiments are as shown below. 
     
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                  (percent by weight)                                     
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Natural rubber      100                                                   
Carbon black (HAF)  55                                                    
ZnO                 7                                                     
Stearic acid        1                                                     
Trimethyl-dihydroquinone polymer                                          
                    2                                                     
SiO.sub.2           8                                                     
Resorcin            2.5                                                   
Melamine derivative 2.5                                                   
Cobalt naphthenate  2.5                                                   
Sulfur              4                                                     
Dicyclohexyl-benzthiazyl-sulfenamide                                      
                    0.8                                                   
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                       TABLE 1                                                     
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              Embodi-                                                     
              ment   Comparison                                           
              1      1       2       3                                    
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Steel                                                                     
     Material       SWRS     SWRH  SWRS  SWRS                             
fila-               82A      82A   82A   72A                              
ment Carbon         0.81     0.81  0.81  0.70                             
     content (%)                                                          
     Largest size of                                                      
                    5        57    6     6                                
     non-metallic                                                         
     inclusion (μ)                                                     
     Reduction      96.5     96.5  97.5  96.5                             
     ratio (%)                                                            
     Diameter (mm)  0.219    0.220 0.220 0.219                            
Steel                                                                     
     Weight (g/m)   1.20     1.20  1.20  1.20                             
cord Tensile        51.1     51.3  54.2  44.0                             
     strength (kg)                                                        
Calculation                                                               
           maximum  45.7     45.7  45.7  36.5                             
value (kg) minimum  53.3     53.3  53.3  44.1                             
Maintenance     63       54      57    69                                 
ratio of knot                                                             
strength (%)                                                              
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                       TABLE 2                                                     
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           Embodi-                                                        
           ment   Comparison                                              
           1      1        2        3                                     
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Belt ply     2        2        2      2                                   
End count per 25 mm                                                       
             19.3     19.2     18.2   22                                  
Belt strength                                                             
             1973     1970     1973   1971                                
(kg/25 mm)                                                                
Steel cord amount                                                         
             402      401      380    460                                 
(g) per tire                                                              
Tire weight (kg)                                                          
             7.41     7.41     7.39   7.50                                
High speed durability                                                     
             pass     pass     pass   pass                                
Rolling resistance                                                        
             110      111      110    100                                 
Fuel consumption                                                          
             98       97       98     100                                 
Moisture heat                                                             
             95       93       98     70                                  
coverage (%)                                                              
Cut number of steel                                                       
             15       243      102    10                                  
cords per tire                                                            
Wear Resistance                                                           
             100      100      88     100                                 
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     High speed durability shown in Table 2 was measured in accordance with the drum test specified in U.S. FMVSS No. 109. The rolling resistance was measured as follows. 
     Test tires were mounted to a 1600 cc passenger car and a running distance by inertia was evaluated after releasing a clutch at a prescribed point with a running speed of 60 km/h. 
     Respective values are indicated in the form of index by setting the running distance of the comparison 3 to be 100. 
     The fuel comparison was measured after running the distance of 40 km and the values were indicated in the form of the index by setting the fuel consumption of the comparison 3 to be 100. Moisture heat coverage shows the rubber amount by percentage which adheres to the steel cords when the tires are disassembled to separate belt plies from each other after being exposed to the environment of 70° C., 95% RH for two weeks. 
     The number of the steel cords broken were measured by X-ray photographing the steel cords disposed at the belt portion of test tires. Test tires were mounted to a 1600 cc passenger car and ran the distance of 5,000 km at 30 km/h on a test track on which macadams having a size of 3 cm 3  in average were scattered all over the surface. 
     As apparent from the test results shown in Table 2, the number of the broken steel cords of the Embodiment 1 is smaller than those of the comparisons 2 and 3 whose knot strengths are small and not so different from that of comparison 3 relating to a prior art embodiment having small tensile strength. It shall be also understood that the comparison 2 including steel cords of higher tensile strength in comparison to Embodiment 1, is inferior in wear resistance due to low rigidity of the tread portion. 
     Embodiment 2 
     The same steel filaments as in Table 1 were twisted in the same manner and the steel cords of 3×0.20 mm+6×0.35 mm were obtained. Radial tires 1000R-2014 PR were produced which comprised a belt layer including said steel cords and a carcass layer of four plies made of polyester cords. The brass plate used for the steel filaments was a copper/zinc ratio of 63/37 and the amount was 5 g/kg. 
     
                       TABLE 3                                                     
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              Embodi-                                                     
              ment   Comparison                                           
              2      4       5       6                                    
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Steel                                                                     
     Material       SWRS     SWRH  SWRS  SWRS                             
fila-               82A      82A   82A   72A                              
ment Reduction Core     96.5   96.5  97.5  96.0                           
     rate (%)  filaments                                                  
               Outer    96.0   96.0  97.4  95.7                           
               filaments                                                  
     Diameter  Core     0.20   0.20  0.20  0.20                           
     (mm)      filaments                                                  
               Outer    0.35   0.35  0.35  0.35                           
               filaments                                                  
Steel                                                                     
     Weight (g/m)   5.35     5.35  5.35  5.35                             
cord Tensile        197      196   220   164                              
     strength (kg)                                                        
Calculation                                                               
           maximum  184.6    184.6 184.6 143.7                            
value (kg) minimum  218.6    218.6 218.6 177.8                            
Maintenance     65       54      57    68                                 
ratio of knot                                                             
strength (%)                                                              
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                       TABLE 4                                                     
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           Embodi-                                                        
           ment   Comparison                                              
           2      4        5        6                                     
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Belt ply     4        4        4      4                                   
End count per 25 mm                                                       
             11.7     11.8     10.5   14                                  
Belt strength                                                             
             9220     9252     9240   9240                                
(kg/25 mm)                                                                
Steel cord amount                                                         
             4.25     4.30     3.81   5.10                                
(kg) per tire                                                             
Tire weight (kg)                                                          
             47.2     47.1     47.0   48.1                                
High speed durability                                                     
             pass     pass     failure                                    
                                      pass                                
Rolling resistance                                                        
             115      115      117    100                                 
Fuel consumption                                                          
             97       97       96     100                                 
Moisture heat                                                             
             95       93       96     68                                  
coverage (%)                                                              
Cut number of steel                                                       
             19       628      493    21                                  
cords per tire                                                            
Wear Resistance                                                           
             100      95       91     100                                 
XMA      rubber  10       13     9      64                                
analysis side                                                             
         cord    90       87     91     36                                
         side                                                             
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     Rolling resistance, fuel consumption and wear resistance in Table 4 are shown in the form of index value by setting the test value of comparison 6 to be 100. The figures for the cut number of steel cords and wear resistance are test values taken after a 6,500 km run along a bad road of a dumpcart, for transportation of macadams which was mounted with the respective tires according to Embodiment 2 and comparisons 4 to 6. Belt plies are subjected to great shock due to running along a bad road of a dumpcart so that fatigue resistance and tenacity are desired. The figures of XMA analysis are test values measured at broken portions of interface between steel cords and rubber by means of an X-ray micro-analyser after the measurement of moisture heat coverage. To be small in the value of cord side means that brass plated was taken off on the rubber side and that the bonding strength of brass with iron material was weak. 
     As shown in Tables 3 and 4, comparison 4 and comparison 5 employing steel cords having small knot strength are very large in the cut number of steel cords and inferior in wear resistance. 
     Embodiment 3 
     Steel cords of 3+9+15×0.175 mm+1 were prepared in the same manner as described in Embodiment 2. The same radial tires as described in Embodiment 2 were produced with the exception that one ply of carcass layer employing said steel cords was used instead of 4 plies of carcass layer employing polyester cords. 
     
                       TABLE 5                                                     
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              Embodi-                                                     
              ment   Comparison                                           
              3      7       8       9                                    
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Steel                                                                     
     Material       SWRS     SWRH  SWRS  SWRS                             
fila-               82A      82A   82A   72A                              
ment Reduction      96.5     96.5  97.7  96.0                             
     ratio (%)                                                            
     Diameter (mm)  0.175    0.175 0.175 0.175                            
Steel                                                                     
     Weight (g/m)   5.22     5.22  5.22  5.22                             
cord Tensile        201.0    202.7 223.2 174.8                            
     strength (kg)                                                        
Calculation                                                               
           maximum  185.2    185.2 185.2 145.4                            
value (kg) minimum  218.4    218.4 218.4 178.6                            
Maintenance     62       55      56    68                                 
ratio of knot                                                             
strength (%)                                                              
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                       TABLE 6                                                     
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           Embodi-                                                        
           ment   Comparison                                              
           3      7        8        9                                     
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Belt ply     4        4        4      4                                   
End count per 25 mm                                                       
             11.3     11.2     10.2   13                                  
Carcass strength                                                          
             2271     2270     2278   2272                                
(kg/25 mm)                                                                
Steel cord amount for                                                     
             3.80     3.27     2.98   3.80                                
carcass (kg) per tire                                                     
Tire weight (kg)                                                          
             49.3     49.6     49.0   51.0                                
High speed durability                                                     
             pass     pass     pass   pass                                
Rolling resistance                                                        
             114      113      116    100                                 
Fuel consumption                                                          
             95       97       93     100                                 
Moisture heat                                                             
             100      95       100    68                                  
coverage (%)                                                              
Cut number of belt                                                        
             22       711      562    36                                  
cords per tire (kg)                                                       
Wear resistance                                                           
             105      97       87     100                                 
(100,000 km)                                                              
XMA      rubber  95       95     97     55                                
analysis side                                                             
         cord    5        5      3      45                                
         side                                                             
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     Rolling resistance and fuel consumption are shown in the form of index value by setting a test value of comparison 9 to be 100. Wear resistance was measured at 100,000 km running along a good road. 
     The accompanying FIGURE represents strength maintenance ratio or fatigue resistance at 50,000 km running and 100,000 km running. The Embodiment 3 and comparison 9 are superior in the strength maintenance ratio of fatigue resistance, but comparison 7 and comparison 8 are inferior. 
     Embodiment 4 
     Four steel filaments of Embodiment 1 were twisted at 3,000 rpm around 440 d (0.234 mm φ) of nylon monofilament by means of a tubular type twister to obtain steel cords of 1×0.234 mm+4×0.22 mm having a twisting pitch of 10 mm. Radial tires 165SR13 having a belt layer of said steel cords were produced as described in Embodiment 1. 
     
                       TABLE 7                                                     
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              Embodi-                                                     
              ment   Comparison                                           
              4      10      11      12                                   
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Steel                                                                     
     Material       SWRS     SWRH  SWRS  SWRS                             
fila-               82A      82A   82A   72A                              
ment Reduction      96.5     96.5  97.5  96.5                             
     ratio (%)                                                            
     Diameter (mm)  0.219    0.220 0.220 0.219                            
Steel                                                                     
     Weight (g/m)   1.22     1.22  1.22  1.22                             
cord Tensile        51.2     51.3  54.3  44.0                             
     strength (kg)                                                        
Calculation                                                               
           maximum  46.4     46.4  46.4  37.1                             
value (kg) minimum  54.2     54.2  54.2  44.9                             
Maintenance     70       53      57    69                                 
ratio of knot                                                             
strength (%)                                                              
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                       TABLE 8                                                     
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           Embodi-                                                        
           ment   Comparison                                              
           4      10       11       12                                    
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Belt ply     2        2        2      2                                   
End count per 25 mm                                                       
             18.9     18.9     17.8   22.0                                
Steel cord amount                                                         
             395      395      372    460                                 
(g) per tire                                                              
Tire weight (kg)                                                          
             7.42     7.42     7.40   7.50                                
High speed durability                                                     
             accepted accepted accepted                                   
                                      accepted                            
Rolling resistance                                                        
             111      111      110    100                                 
Fuel consumption                                                          
             98       98       98     100                                 
Moisture heat                                                             
             100      95       98     76                                  
coverage (%)                                                              
Cut number of steel                                                       
             2        268      126    7                                   
cords per tire (kg)                                                       
Wear Resistance                                                           
             100      98       77     100                                 
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     Rolling resistance, fuel consumption and wear resistance are shown in the form of index value by setting a test value of comparison 12 to be 100. 
     Summing up the test results described relative to Table 1 to Table 8, it can be said that tires employing steel cords including steel filament material SWRS82A are lighter in weight as the use amount of steel cords can be reduced and improved in rolling resistance and fuel consumption in comparison with conventional tires. But a large number of steel cords are broken during tire running when the maintenance ratio of knot strength is less than 60%. On the other hand, in the pneumatic tires of Embodiment 1 to Embodiment 4 according to the present invention, the number of broken steel cords during running is as small as in conventional tires or rather superior to that of conventional tires and besides, wear resistance is superior.