Patent Publication Number: US-5156694-A

Title: Method of producing formable thin steel sheets

Description:
This is a divisional of application Ser. No. 449,724, filed on Dec. 12, 1989, now U.S. Pat. No. 5,053,194. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to hot rolled steel sheets, cold rolled steel sheets and surface treated steel sheets having not only improved formability for press forming, deep drawing or the like but also improved fatigue resistance at a welded joint. 
     2. Related Art Statement 
     In general, the thin steel sheets are widely used for press forming, deep drawing and the like. However, it is required to have properties in accordance with use purposes in addition to the above formability. For example, the thin steel sheets are frequently subjected to a welding, particularly, spot welding irrespective of cold rolled sheets, hot rolled sheets and surface treated sheets. 
     Particularly, the thin steel sheet is used for automobiles. In this case, the spot number in the spot welding per one vehicle amounts to several thousand points and also stress concentration is apt to caused in the welded joint portion when a load is applied from exterior. That is, the fatigue breakage through the repetition of such a stress concentration during the running of the vehicle is caused in the welded joint portion, resulting in the occurrence of serious accidents. In the formable thin steel sheet, therefore, the fatigue resistance of the welded joint is a very important characteristic. 
     On the other hand, extreme-low carbon steels having a formability higher than that of the conventional low carbon steel are frequently used for the thin steel sheet. However, the fatigue strength of the extreme-low carbon steel may be lowered due to poor texture of heat-affected zone in the welded joint in accordance with the conditions. 
     Moreover, it is demanded to more improve the safety of machines and structures such as automobiles and the like as a worldwide theme, and consequently it becomes significant to enhance the fatigue strength of the welded joint as compared with the case of using the conventional steel sheets. 
     In this connection, there are proposed various steel sheets in Japanese Patent laid open No. 54-135616, No. 53-52222, No. 61-246344, No. 58-25436, No. 53-137021, No. 58-110659 and the like. However, all of these techniques disclose the mechanical properties of the cold rolled steel sheet but are silent in the fatigue strength of the welded joint. 
     Furthermore, Japanese Patent laid open No. 63-317625 discloses a method of controlling amounts of Ti, Nb and B to particular ranges for improving the fatigue resistance of the welded joint in the steel sheet. In this method, however, the tensile shear fatigue properties in the spot welded zone are considered, but there is no consideration on the cross tensile fatigue properties. Moreover, Japanese Patent laid open No. 225748 discloses cold rolled steel sheets having excellent fatigue properties, but in this case the fatigue properties of the sheet itself are merely improved. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide thin steel sheets having not only an improved formability for press forming, deep drawing or the like but also excellent fatigue resistance at welded joints, particularly fatigue resistance in spot welding. 
     According to a first aspect of the invention, there is the provision of a formable thin steel sheet having an improved fatigue resistance at welded joints, comprising not more than 0.003 wt % of C, not more than 1.0 wt % of Si, not more than 1.0 wt % of Mn, not more than 0.15 wt % of P, not more than 0.020 wt % of S, not more than 0.0045 wt % of O, not more than 0.0020 wt % of N, not more than 0.15 wt % of Al provided that a ratio of Al/N is not less than 30, and the balance being Fe and inevitable impurities. 
     In a preferred embodiment of the first invention, the steel sheet contains at least one of 0.001-0.025 wt % of Nb and 0.0002-0.0020 wt % of B, or further contains at least one of not more than 0.10 wt % of Ti, not more than 0.10 wt % of V, not more than 0.10 wt % of Zr, not more than 0.10 wt % of Ca, not more than 1.0 wt % of Cr, not more than 1.0 wt % of Cu and not more than 1.0 wt % of Ni. 
     According to a second aspect of the invention, there is the provision of a method of producing formable thin steel sheets having an improved fatigue resistance at welded joints, which comprises hot rolling a sheet of steel comprising not more than 0.003 wt % of C, not more than 1.0 wt % of Si, not more than 1.0 wt % of Mn, not more than 0.15 wt % of P, not more than 0.020 wt % of S, not more than 0.0045 wt % of O, not more than 0.0020 wt % of N, not more than 0.15 wt % of Al provided that a ratio of Al/N is not less than 30, and the balance being Fe and inevitable impurities at a finish temperature of not lower than 600° C., cold rolling the hot rolled sheet at a rolling reduction of not less than 60% and then subjecting the cold rolled sheet to a recrystallization annealing at a temperature of not higher than A C3  transformation point. 
     In preferred embodiments of the second invention, the hot rolled sheet is coiled at a coiling temperature of not lower than 200° C. after the hot rolling, and the resulting thin steel sheet is subjected to a galvanizing or electroplating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein: 
     FIG. 1 is a graph showing influence of oxygen amount and Al/N ratio upon the value of tensile shear fatigue limit in the spot welded joint of the cold rolled steel sheet; 
     FIG. 2 is a graph showing influences of oxygen amount and Al/N ratio upon the value of tensile shear fatigue limit in the spot welded joint of the hot rolled steel sheet; 
     FIG. 3 is a graph showing an influence of oxygen amount upon the value of tensile shear fatigue limit in the spot welded joint when Al/N ratio of the hot rolled steel sheet is about 37; 
     FIG. 4 is a schematically sectional view of a specimen used for tensile shear fatigue test of spot welded joint showing a position of crack produced in the fatigue test; 
     FIG. 5 is a graph showing influences of oxygen amount and Al/N ratio upon the value of cross tensile fatigue limit in the spot welded joint; 
     FIGS. 6a and 6b are graphs showing an influence of Al/N ratio upon values of cross tensile fatigue limit and tensile shear fatigue limit in the spot welded joint when oxygen amount is about 0.0030 wt %; and 
     FIGS. 7a and 7b are schematic views showing modes of spot welded specimen in the tensile shear fatigue test and cross tensile fatigue test, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The inventors have aimed at a point that there are less reports on the influence of steel component upon the fatigue properties though the fatigue properties of welded joints in the thin steel sheet are very important even in articles using such steel sheet and made various studies with respect to the influence of steel components on the fatigue properties of the welded joint, particularly fatigue properties of the spot welded joint, and found out the following knowledges. 
     At first, the invention is described with respect to experimental results leading in the success of the invention. Moreover, the fatigue test for the spot welded joint is carried out by a fatigue test method of the spot welded joint according to JIS Z3138, and the fatigue limit value means an upper limit of loading range when a repeat number of loading applied to the test specimen is 10,000,000 times. 
     In FIG. 1 are shown a relationship among oxygen amount, Al/N ratio and tensile shear fatigue limit value at the spot welded joint in a cold rolled steel sheet of 0.8 mm in thickness. The chemical composition of steels used in the fatigue test is shown in the following Table 1, and the conditions of the spot welding are shown in the following Table 2. Moreover, the steel sheet was hot rolled at a finish temperature of about 900° C., cold rolled at a rolling reduction of 75-80% and continuously annealed at a temperature of 820°-840° C. 
     In FIG. 1, a shadowed area shows a region that the fatigue limit value is higher by 10% or more than that of the conventional low carbon aluminum killed and box annealed steel sheet (tensile shear fatigue limit: 82 kgf), which corresponds to a region that the oxygen amount is not more than 0.0045 wt % and the Al/N ratio is not less than 30. 
     
                                           TABLE 1                                 
__________________________________________________________________________
(wt %)                                                                    
Kind of                                                                   
steel C       Si Mn P  S   Nb     B                                       
__________________________________________________________________________
Nb, B 0.0009˜0.0014                                                 
              0.01                                                        
                 0.1                                                      
                    0.015                                                 
                       0.01                                               
                           --     --                                      
not added                                                                 
Nb, B 0.0008˜0.0013                                                 
              0.01                                                        
                 0.1                                                      
                    0.015                                                 
                       0.01                                               
                           0.003˜0.006                              
                                  0˜0.0008                          
added                                                                     
low carbon                                                                
      0.038   0.02                                                        
                  0.22                                                    
                    0.018                                                 
                        0.013                                             
                           --     --                                      
steel*                                                                    
__________________________________________________________________________
 *comparative steel                                                       
 
    
     
                       TABLE 2                                                     
______________________________________                                    
                                Average                                   
Sample size                                                               
          Welding conditions    nugget                                    
width length             welding                                          
                                welding diameter                          
(mm)  (mm)    chip       force  current (mm)                              
______________________________________                                    
40    150     Cr--Cu,    200 kgf                                          
                                8.5˜9.5 kA                          
                                        5.0                               
              4.8 φmm,                                                
              CF model                                                    
______________________________________                                    
 
    
     In FIG. 2 is shown a relationship among oxygen amount, Al/N ratio and tensile shear fatigue limit value at the spot welded joint in a hot rolled steel sheet of 2.6 mm in thickness. The chemical composition of steels used in the fatigue test is shown in the following Table 3, and the conditions of the spot welding are shown in the following Table 4. Moreover, the steel sheet was hot rolled at a finish temperature of about 900° C. and coiled at a coiling temperature of 550° C. 
     In FIG. 2, a shadowed area shows a region that the fatigue limit value is higher by 10% or more than that of the conventional low carbon aluminum killed and hot rolled steel sheet (tensile shear fatigue limit: 168 kgf), which corresponds to a region that the oxygen amount is not more than 0.0045 wt % and the Al/N ratio is not less than 30 likewise the case of the cold rolled sheet. 
     
                                           TABLE 3                                 
__________________________________________________________________________
(wt %)                                                                    
Kind of                                                                   
steel C       Si Mn P  S   Nb     B                                       
__________________________________________________________________________
Nb, B 0.0009˜0.0015                                                 
              0.01                                                        
                 0.1                                                      
                    0.015                                                 
                       0.01                                               
                           --     --                                      
not added                                                                 
Nb, B 0.0007˜0.0013                                                 
              0.01                                                        
                 0.1                                                      
                    0.015                                                 
                       0.01                                               
                           0.003˜0.008                              
                                  0˜0.0010                          
added                                                                     
low carbon                                                                
      0.032   0.02                                                        
                  0.25                                                    
                    0.018                                                 
                        0.013                                             
                           --     --                                      
steel*                                                                    
__________________________________________________________________________
 *comparative steel                                                       
 
    
     
                       TABLE 4                                                     
______________________________________                                    
                                Average                                   
Sample size                                                               
          Welding conditions    nugget                                    
width length             welding                                          
                                welding diameter                          
(mm)  (mm)    chip       force  current (mm)                              
______________________________________                                    
50    180     Cr--Cu,    650 kgf                                          
                                12˜14 kA                            
                                        10.0                              
              9 φmm,                                                  
              CF model                                                    
______________________________________                                    
 
    
     In FIG. 3 is shown a relationship between tensile shear fatigue limit value and oxygen amount when the Al/N ratio is about 37, from which it is clear that the fatigue limit value higher than the conventional low carbon aluminum killed and hot rolled steel sheet (tensile shear fatigue limit: 168 kgf) is obtained when the O amount is not more than 0.0045 wt %. 
     In these tests, the breakage due to the fatigue results from the occurrence of cracks generated at heat-affected zone as shown in FIG. 4, in which letter A is a position of crack generated, letter B a nugget portion, letter C a heat-affected zone and letter D a thin steel sheet. 
     In order to elucidate these reasons, the inventors have investigated a hardness distribution in a section of a welded zone on a specimen having a high fatigue limit value and found that the hardness difference ranging from the fused zone to the heat-affected zone is small as compared with the steel sheet having a low fatigue limit value and is smooth in the distribution. From this fact, it is considered that such a small hardness difference effectively acts to the occurrence of fatigue cracks and the propagation thereof due to stress concentration in the welded joint portion under stress loading. 
     Furthermore, it has been found from FIGS. 1-3 that the fatigue limit value becomes higher in steel sheets containing at least one of Nb and B within a proper amount. 
     On the other hand, a cold rolled Ti-containing steel sheet of 0.7 mm in thickness having a chemical composition as shown in the following Table 5 was welded under spot welding conditions as shown in the following Table 6, and then a cross tensile fatigue test was made thereto. In this case, the steel sheet was hot rolled at a finish temperature of about 900° C., cold rolled at a rolling reduction of 75-80% and continuously annealed at a temperature of 820°-840° C. 
     
                       TABLE 5                                                     
______________________________________                                    
(wt %)                                                                    
Kind                                                                      
of                                                                        
steel C       Si     Mn   P    S     Ti   Nb   B                          
______________________________________                                    
not   0.0009  0.01   0.1  0.015                                           
                               0.01  --   --   --                         
added                                                                     
      0.0018                                                              
Ti    0.0008  0.01   0.1  0.015                                           
                               0.01  0.026                                
                                          --   --                         
added                                                                     
steel 0.0015                         0.052                                
Ti, Nb,                                                                   
      0.0006  0.01   0.1  0.015                                           
                               0.01  0.022                                
                                          0.003                           
                                               0                          
added 0.0014                         0.048                                
                                          0.018                           
                                               0.0012                     
steel                                                                     
low   0.032   0.02    0.25                                                
                          0.018                                           
                                0.013                                     
                                     --   --   --                         
carbon                                                                    
steel*                                                                    
______________________________________                                    
 *comparative steel                                                       
 
    
     
                       TABLE 6                                                     
______________________________________                                    
                                Average                                   
Sample size                                                               
          Welding conditions    nugget                                    
width length             welding                                          
                                welding diameter                          
(mm)  (mm)    chip       force  current (mm)                              
______________________________________                                    
50    150     Cr--Cu,    165 kgf                                          
                                7.2˜7.9 kA                          
                                        4.0                               
              4.5 φmm,                                                
              CF model                                                    
______________________________________                                    
 
    
     In this test, a relation of oxygen amount and Al/N ratio to the cross tensile fatigue limit value is shown in FIG. 5. From FIG. 5, it has been found that the cross tensile fatigue limit value becomes considerably high when the oxygen amount and Al/N ratio in the Ti-containing steel and Ti, Nb and B containing steel are within ranges shown by a shadowed region, that is, the oxygen amount is not more than 0.0045 wt % and the Al/N ratio is not less than 30. 
     In FIG. 6a is shown a relationship between cross tensile fatigue limit and Al/N ratio when the oxygen amount is 0.0030 wt %. As seen from FIG. 6a, in the Ti-containing steel and Ti-Nb-B containing steel, the high fatigue limit value is obtained when the Al/N ratio is not less than 30. Furthermore, it is understood from the simultaneously conducted tensile shear fatigue test that the addition of Ti or Ti-Nb-B does not affect the fatigue limit as shown in FIG. 6b. 
     Moreover, similar results are obtained in the hot rolled steel sheets. 
     The reason why the excellent cross tensile fatigue limit value is obtained under the above conditions is considered as follows. That is, the breakage due to fatigue is led from the cracks generated at the heat-affected zone even in the cross tensile fatigue test. In case of Ti-containing steel, it is considered that the solid soluted Ti or Ti series precipitate acts to improve the toughness of the heat-affected zone, whereby the cross tensile fatigue properties are improved. 
     And also, it has been found that the similar effect is obtained by adding at least two of Ti, V, Zr, Ca, Cr, Cu and Ni within proper ranges in addition to the steel containing only Ti. 
     For the reference, the methods of tensile shear and cross tensile fatigue tests using spot welded specimens are schematically shown in FIGS. 7a and 7b, respectively. As seen from FIGS. 7a and 7b, the deformation mode is largely different between both the test methods. 
     The reason why the chemical composition of the steel used in the invention is limited to the above range will be described below. 
     C: The C amount should be considerably lower than that of the conventional low carbon steel in order to obtain steels having good elongation and r-value. Furthermore, the fatigue resistance becomes advantageously improved as the C amount reduces in the steel according to the invention. Therefore, the C amount is not more than 0.003 wt %, preferably not more than 0.0015 wt %. 
     Si: The Si amount should be not more than 1.0 wt % because when the amount exceeds 1.0 wt %, the elongation and drawability of the steel sheet are degraded. 
     Mn: The excessive addition of Mn degrades the elongation and drawability of the steel sheet likewise Si, so that the Mn amount should be not more than 1.0 wt %. 
     P: When the P amount exceeds 0.15 wt %, P segregates into the grain boundary to cause brittleness, so that it should be not more than 0.15 wt %. 
     S: When the S amount is too small, the descaling property is degraded to make the surface properties bad, so that the lower limit is 0.0035 wt %. While, when the amount exceeds 0.020 wt %, the corrosion resistance is considerably degraded, so that the upper limit is 0.020 wt %. 
     O: The O amount is particularly important in the invention because it is considered that O at solid soluted state or in form of oxide affects the occurrence and propagation of cracks. Therefore, in order to obtain the fatigue properties higher than those of the conventional low carbon steel sheet, the O amount is necessary to be not more than 0.0045 wt %. Preferably, it is not more than 0.0035 wt %. 
     N: As the N amount becomes larger, the Al amount required becomes excessive to degrade the surface properties as mentioned later. Therefore, the N amount is not more than 0.0020 wt %, preferably not more than 0.0017 wt %. 
     Al: The Al amount is also important in the invention because it is considered that the fatigue properties are improved by an influence of distribution state of solid soluted Al or AlN precipitate upon the structure of the heat-affected zone. Therefore, it is closely related to the N amount. In order to improve the fatigue properties of the welded joint, it is required to have Al (wt %)/N (wt %) ratio of not less than 30. Moreover, when the Al amount is too large, the surface properties are degraded, so that the upper limit is 0.15 wt %. 
     Nb, B: These elements are effective for the improvement of fatigue properties, but when the amount to be added becomes excessive, the recrystallization temperature undesirably rises. Therefore, at least one of Nb and B may be added within ranges of 0.001 wt %≦Nb≦0.025 wt % and 0.0002 wt %≦B≦0.0020 wt %, respectively, for improving the fatigue properties. 
     Ti, V, Zr, Ca, Cr, Cu, Ni: It is considered that each of these elements affects the structure of the heat-affected zone at a solid solution state or a precipitate state to enhance the fatigue properties. However, the excessive addition degrades the quality of the steel sheet. Therefore, at least one of Ti, V, Zr, Ca, Cr, Cu and Ni may be added within ranges of not more than 0.10 wt % in each of Ti, V, Zr and Ca and not more than 1.0 wt % in each of Cr, Cu and Ni, respectively, for particularly improving the cross tensile fatigue properties. 
     The invention will be described below with respect to preferable conditions in the production of formable thin steel sheets using the above chemical composition of steel as a starting material. 
     In the production of hot rolled steel sheets, the finish temperature is limited to not lower than 600° C. because when the finish temperature in the hot rolling is lower than 600° C., the deep drawability is degraded. Furthermore, the coiling temperature is limited to not lower than 200° C. because when the coiling temperature is lower than 200° C., the quality is degraded. 
     In the production of cold rolled steel sheets, the finish temperature at the hot rolling step is not lower than 600° C., preferably not lower than 800° C. because when it is lower than 600° C., the deep drawability is degraded. Furthermore, the rolling reduction at the cold rolling step is not less than 60% in order to obtain a satisfactory formability. Moreover, the annealing temperature at the continuous annealing step after the cold rolling is not higher than A C3  point because when it is higher than A C3  point, the crystal grains become coarse. Particularly, the lower limit of the annealing temperature is not critical, but it is preferably higher by 30° C. than the recrystallization temperature. As the annealing method, a box annealing may be used. 
     Of course, these thin steel sheets may be subjected to a skin pass rolling within a usual range, i.e. about few percent of the sheet gauge (mm) for correcting the sheet shape and the like. 
     Even if the thin steel sheet is subjected to a galvanizing or an electroplating, the breakage in the fatigue test is generated from the heat-affected zone, so that according to the invention, the thin steel sheet may be subsequently subjected to a surface treatment such as galvanizing, electroplating or the like. 
     As the welding method, the fatigue strength in the heat-affected zone comes into problem in MIG method, TIG method and the like in addition to the spot welding, so that the invention is effective for improving the fatigue strength of welded joint even in these welding methods. 
     The following examples are given in illustration of the invention and are not intended as limitations thereof. 
     EXAMPLE 1 
     A steel having a chemical composition as shown in the following Table 7 was melted to form a slab, which was hot rolled at a finish temperature of 850°-900° C., cold rolled at a rolling reduction of 71-78% and continuously annealed at an annealing temperature of 790°-830° C. to obtain a cold rolled steel sheet of 0.8 mm in thickness. Moreover, the steel No. 18 was the conventional low carbon aluminum killed steel and was produced by box annealing. 
     The steel Nos. 1-9 were acceptable in the invention, among which the steel Nos. 1 and 8 were subjected to a galvanizing and electroplating, respectively. 
     The steel Nos. 10-17 were comparative examples, whose chemical compositions were outside the range of the invention. 
     The mechanical properties and tensile shear fatigue limit value at spot welded joint portion (upper limit of loading range when the repeat number of tensile loading was 10,000,000) were measured with respect to these cold rolled steel sheets to obtain results as shown in the following Table 8. 
     Moreover, a specimen of JIS Z2201 No. 5 was used in the tensile test, and the spot welding conditions and tensile shear fatigue test conditions were the same as in Table 2. 
     
                                           TABLE 7                                 
__________________________________________________________________________
Chemical composition (wt %)                                               
No.                                                                       
   C   Si Mn P  S  N   Al O   others                                      
                                   Remarks                                
__________________________________________________________________________
 1 0.0007                                                                 
       0.01                                                               
          0.21                                                            
             0.015                                                        
                0.008                                                     
                   0.0013                                                 
                       0.051                                              
                          0.0018   acceptable                             
                                   example                                
 2 0.0021                                                                 
       0.02                                                               
          0.26                                                            
             0.021                                                        
                0.015                                                     
                   0.0007                                                 
                       0.075                                              
                          0.0023   acceptable                             
                                   example                                
 3 0.0015                                                                 
       0.6                                                                
          0.18                                                            
             0.016                                                        
                0.012                                                     
                   0.0011                                                 
                       0.066                                              
                          0.0028   acceptable                             
                                   example                                
 4 0.0018                                                                 
       0.03                                                               
          0.55                                                            
             0.069                                                        
                0.005                                                     
                   0.0016                                                 
                       0.062                                              
                          0.0029   acceptable                             
                                   example                                
 5 0.0005                                                                 
       0.01                                                               
          0.12                                                            
             0.015                                                        
                0.007                                                     
                   0.0015                                                 
                       0.055                                              
                          0.0033                                          
                              Nb: 0.005                                   
                                   acceptable                             
                                   example                                
 6 0.0008                                                                 
       0.01                                                               
          0.12                                                            
             0.017                                                        
                0.016                                                     
                   0.0014                                                 
                       0.045                                              
                          0.0021                                          
                              Nb: 0.016                                   
                                   acceptable                             
                                   example                                
 7 0.0009                                                                 
       0.02                                                               
          0.20                                                            
             0.005                                                        
                0.009                                                     
                   0.0009                                                 
                       0.041                                              
                          0.0012                                          
                              B: 0.0006                                   
                                   acceptable                             
                                   example                                
 8 0.0018                                                                 
       0.01                                                               
          0.35                                                            
             0.025                                                        
                0.011                                                     
                   0.0012                                                 
                       0.038                                              
                          0.0022                                          
                              Nb: 0.007                                   
                                   acceptable                             
                              B: 0.0005                                   
                                   example                                
 9 0.0008                                                                 
       0.02                                                               
          0.26                                                            
             0.022                                                        
                0.018                                                     
                   0.0016                                                 
                       0.062                                              
                          0.0032                                          
                              Nb: 0.018                                   
                                   acceptable                             
                              B: 0.0017                                   
                                   example                                
10 0.0022                                                                 
       0.01                                                               
          0.15                                                            
             0.012                                                        
                0.009                                                     
                   0.0017                                                 
                       0.023                                              
                          0.0034   comparative                            
                                   example                                
11 0.0014                                                                 
       0.01                                                               
          0.16                                                            
             0.013                                                        
                0.011                                                     
                   0.0022                                                 
                       0.070                                              
                          0.0029   comparative                            
                                   example                                
12 0.0016                                                                 
       0.02                                                               
          0.14                                                            
             0.015                                                        
                0.012                                                     
                   0.0014                                                 
                       0.048                                              
                          0.0053   comparative                            
                                   example                                
13 0.0033                                                                 
       0.03                                                               
          0.23                                                            
             0.015                                                        
                0.004                                                     
                   0.0012                                                 
                       0.080                                              
                          0.0042   comparative                            
                                   example                                
14 0.0016                                                                 
       1.12                                                               
          0.10                                                            
             0.022                                                        
                0.004                                                     
                   0.0015                                                 
                       0.075                                              
                          0.0036   comparative                            
                                   example                                
15 0.0022                                                                 
       0.02                                                               
          1.21                                                            
             0.026                                                        
                0.006                                                     
                   0.0019                                                 
                       0.069                                              
                          0.0029   comparative                            
                                   example                                
16 0.0022                                                                 
       0.01                                                               
          0.23                                                            
             0.022                                                        
                0.009                                                     
                   0.0015                                                 
                       0.081                                              
                          0.0019                                          
                              Nb: 0.027                                   
                                   comparative                            
                                   example                                
17 0.0015                                                                 
       0.01                                                               
          0.16                                                            
             0.009                                                        
                0.006                                                     
                   0.0016                                                 
                       0.062                                              
                          0.0035                                          
                              B: 0.0026                                   
                                   comparative                            
                                   example                                
18 0.0350                                                                 
       0.01                                                               
          0.16                                                            
             0.015                                                        
                0.016                                                     
                   0.0042                                                 
                       0.035                                              
                          0.0056   conventional                           
                                   example                                
__________________________________________________________________________
 
    
     
                                           TABLE 8                                 
__________________________________________________________________________
   Surface                                                                
          Y.S. T.S. El.    S-FL                                           
No.                                                                       
   treatment                                                              
          kg/mm.sup.2                                                     
               kg/mm.sup.2                                                
                    %  r-value                                            
                           kgf                                            
                              Remarks                                     
__________________________________________________________________________
 1a                                                                       
   none   16.2 30.5 51.0                                                  
                       2.02                                               
                           125.6                                          
                              acceptable                                  
                              example                                     
 1b                                                                       
   galvanizing                                                            
          17.8 32.0 49.2                                                  
                       1.90                                               
                           116.7                                          
                              acceptable                                  
                              example                                     
 1c                                                                       
   zinc   17.5 31.4 49.8                                                  
                       1.95                                               
                           126.5                                          
                              acceptable                                  
   electroplating             example                                     
 2 none   17.2 31.5 49.0                                                  
                       1.91                                               
                           114.0                                          
                              acceptable                                  
                              example                                     
 3 &#34;      19.6 32.9 46.5                                                  
                       1.92                                               
                           105.8                                          
                              acceptable                                  
                              example                                     
 4 &#34;      19.2 36.6 44.1                                                  
                       1.91                                               
                           122.5                                          
                              acceptable                                  
                              example                                     
 5 &#34;      15.6 29.8 53.0                                                  
                       2.21                                               
                           135.5                                          
                              acceptable                                  
                              example                                     
 6 &#34;      15.9 30.2 52.5                                                  
                       2.16                                               
                           132.2                                          
                              acceptable                                  
                              example                                     
 7 &#34;      16.8 31.0 52.2                                                  
                       2.18                                               
                           130.5                                          
                              acceptable                                  
                              example                                     
 8a                                                                       
   &#34;      17.5 30.2 51.6                                                  
                       2.06                                               
                           154.5                                          
                              acceptable                                  
                              example                                     
 8b                                                                       
   galvanizing                                                            
          18.7 31.8 50.2                                                  
                       1.92                                               
                           142.5                                          
                              acceptable                                  
                              example                                     
 8c                                                                       
   zinc   18.3 31.5 50.8                                                  
                       1.95                                               
                           147.0                                          
                              acceptable                                  
   electroplating             example                                     
 9 none   20.1 31.6 48.1                                                  
                       1.86                                               
                           145.4                                          
                              acceptable                                  
                              example                                     
10 &#34;      18.7 30.2 45.8                                                  
                       1.71                                               
                            78.8                                          
                              comparative                                 
                              example                                     
11 &#34;      17.5 31.2 47.2                                                  
                       1.72                                               
                            82.6                                          
                              comparative                                 
                              example                                     
12 &#34;      19.2 31.0 47.0                                                  
                       1.67                                               
                            83.2                                          
                              comparative                                 
                              example                                     
13 &#34;      21.2 32.1 45.0                                                  
                       1.60                                               
                            86.5                                          
                              comparative                                 
                              example                                     
14 &#34;      26.0 37.2 37.0                                                  
                       1.38                                               
                            82.5                                          
                              comparative                                 
                              example                                     
15 &#34;      23.9 36.2 38.1                                                  
                       1.52                                               
                            80.5                                          
                              comparative                                 
                              example                                     
16 &#34;      21.2 32.1 45.0                                                  
                       1.60                                               
                            83.4                                          
                              comparative                                 
                              example                                     
17 &#34;      22.0 31.8 44.0                                                  
                       1.38                                               
                            85.4                                          
                              comparative                                 
                              example                                     
18 &#34;      18.8 31.9 45.0                                                  
                       1.72                                               
                            82.2                                          
                              conventional                                
                              example                                     
__________________________________________________________________________
 S-FL: tensile shear fatigue limit                                        
 
    
     As seen from Table 8, all of the steels according to the invention exhibit good mechanical properties and tensile shear fatigue limit value, while the comparative steels and the conventional steel are poor in either the mechanical properties or the tensile shear fatigue limit value. 
     Furthermore, the surface treated steels according to the invention are naturally excellent in the properties as compared with the comparative and conventional steels because the breakage in the fatigue test is generated from the heat-affected zone. 
     Moreover, in the steel Nos. 5-9 containing either Nb or B or both, the fatigue resistance at the heat-affected zone is further improved, so that they exhibit a higher tensile shear fatigue limit value among the steels according to the invention. 
     EXAMPLE 2 
     A steel having a chemical composition as shown in the following Table 9 was melted to form a slab, which was hot rolled at a finish temperature of 830°-900° C. and would at a coiling temperature of 550°-650° C. to obtain a hot rolled steel sheet of 2.6 mm in thickness. 
     The steel Nos. 1-9 were acceptable in the invention, among which the steel Nos. 2 and 8 were subjected to a galvanizing and electroplating, respectively. 
     The steel Nos. 10-17 were comparative examples, whose chemical compositions were outside the range of the invention, and the steel No. 18 was the conventional low carbon aluminum killed steel. 
     The mechanical properties and tensile shear fatigue limit value at spot welded joint portion (upper limit of loading range when the repeat number of tensile loading was 10,000,000) were measured with respect to these hot rolled steel sheets to obtain results as shown in the following Table 10. 
     Moreover, a specimen of JIS Z2201 No. 5 was used in the tensile test, and the spot welding conditions and tensile shear fatigue test conditions were the same as in Table 4. 
     
                                           TABLE 9                                 
__________________________________________________________________________
Chemical composition (wt %)                                               
No.                                                                       
   C   Si Mn P  S  N   Al O   others                                      
                                   Al/N                                   
                                       Remarks                            
__________________________________________________________________________
 1 0.0008                                                                 
       0.01                                                               
          0.20                                                            
             0.015                                                        
                0.008                                                     
                   0.0012                                                 
                       0.050                                              
                          0.0016   41.7                                   
                                       acceptable                         
                                       example                            
 2 0.0013                                                                 
       0.02                                                               
          0.21                                                            
             0.020                                                        
                0.015                                                     
                   0.0009                                                 
                       0.070                                              
                          0.0023   77.8                                   
                                       acceptable                         
                                       example                            
 3 0.0015                                                                 
       0.50                                                               
          0.26                                                            
             0.016                                                        
                0.010                                                     
                   0.0014                                                 
                       0.066                                              
                          0.0023   47.1                                   
                                       acceptable                         
                                       example                            
 4 0.0010                                                                 
       0.03                                                               
          0.60                                                            
             0.056                                                        
                0.005                                                     
                   0.0015                                                 
                       0.060                                              
                          0.0030   40.0                                   
                                       acceptable                         
                                       example                            
 5 0.0006                                                                 
       0.02                                                               
          0.12                                                            
             0.015                                                        
                0.007                                                     
                   0.0015                                                 
                       0.055                                              
                          0.0020                                          
                              Nb: 0.006                                   
                                   36.7                                   
                                       acceptable                         
                                       example                            
 6 0.0025                                                                 
       0.01                                                               
          0.12                                                            
             0.017                                                        
                0.016                                                     
                   0.0014                                                 
                       0.045                                              
                          0.0033                                          
                              Nb: 0.013                                   
                                   32.1                                   
                                       acceptable                         
                                       example                            
 7 0.0009                                                                 
       0.02                                                               
          0.20                                                            
             0.005                                                        
                0.009                                                     
                   0.0009                                                 
                       0.041                                              
                          0.0012                                          
                              B: 0.0005                                   
                                   45.6                                   
                                       acceptable                         
                                       example                            
 8 0.0013                                                                 
       0.01                                                               
          0.35                                                            
             0.025                                                        
                0.011                                                     
                   0.0012                                                 
                       0.038                                              
                          0.0022                                          
                              Nb: 0.008                                   
                                   31.7                                   
                                       acceptable                         
                              B: 0.0005                                   
                                       example                            
 9 0.0008                                                                 
       0.02                                                               
          0.26                                                            
             0.022                                                        
                0.010                                                     
                   0.0014                                                 
                       0.056                                              
                          0.0022                                          
                              Nb: 0.018                                   
                                   40.0                                   
                                       acceptable                         
                              B: 0.0017                                   
                                       example                            
10 0.0012                                                                 
       0.01                                                               
          0.15                                                            
             0.012                                                        
                0.009                                                     
                   0.0017                                                 
                       0.020                                              
                          0.0034   11.8                                   
                                       comparative                        
                                       example                            
11 0.0014                                                                 
       0.01                                                               
          0.10                                                            
             0.014                                                        
                0.011                                                     
                   0.0022                                                 
                       0.070                                              
                          0.0029   31.8                                   
                                       comparative                        
                                       example                            
12 0.0016                                                                 
       0.02                                                               
          0.14                                                            
             0.015                                                        
                0.015                                                     
                   0.0014                                                 
                       0.048                                              
                          0.0055   34.3                                   
                                       comparative                        
                                       example                            
13 0.0035                                                                 
       0.03                                                               
          0.23                                                            
             0.015                                                        
                0.016                                                     
                   0.0012                                                 
                       0.080                                              
                          0.0040   66.7                                   
                                       comparative                        
                                       example                            
14 0.0016                                                                 
       1.10                                                               
          0.10                                                            
             0.022                                                        
                0.013                                                     
                   0.0015                                                 
                       0.075                                              
                          0.0023   50.0                                   
                                       comparative                        
                                       example                            
15 0.0013                                                                 
       0.02                                                               
          1.25                                                            
             0.026                                                        
                0.006                                                     
                   0.0019                                                 
                       0.069                                              
                          0.0029   36.3                                   
                                       comparative                        
                                       example                            
16 0.0012                                                                 
       0.01                                                               
          0.23                                                            
             0.022                                                        
                0.009                                                     
                   0.0015                                                 
                       0.081                                              
                          0.0019                                          
                              Nb: 0.028                                   
                                   54.0                                   
                                       comparative                        
                                       example                            
17 0.0008                                                                 
       0.01                                                               
          0.16                                                            
             0.009                                                        
                0.006                                                     
                   0.0016                                                 
                       0.062                                              
                          0.0025                                          
                              B: 0.0026                                   
                                   38.8                                   
                                       comparative                        
                                       example                            
18 0.036                                                                  
       0.01                                                               
          0.26                                                            
             0.018                                                        
                0.016                                                     
                   0.0050                                                 
                       0.035                                              
                          0.0056    7.0                                   
                                       comventional                       
                                       example                            
__________________________________________________________________________
 
    
     
                       TABLE 10                                                    
______________________________________                                    
     Surface  Y.S.     T.S.   El.  S-FL                                   
No.  treatment                                                            
              kg/mm.sup.2                                                 
                       kg/mm.sup.2                                        
                              %    kgf  Remarks                           
______________________________________                                    
 1   none     16.8     31.5   54.0 208  acceptable                        
                                        example                           
 2a  &#34;        16.5     30.2   54.7 210  acceptable                        
                                        example                           
 2b  galvan-  17.5     31.8   52.0 204  acceptable                        
     izing                              example                           
 2c  zinc     17.1     31.5   52.7 208  acceptable                        
     electro-                           example                           
     plating                                                              
 3   none     19.2     33.9   50.5 218  acceptable                        
                                        example                           
 4   &#34;        20.5     37.8   48.1 210  acceptable                        
                                        example                           
 5   &#34;        15.0     29.6   56.0 232  acceptable                        
                                        example                           
 6   &#34;        15.7     31.2   56.3 228  acceptable                        
                                        example                           
 7   &#34;        16.8     32.0   54.5 220  acceptable                        
                                        example                           
 8a  &#34;        18.5     31.7   54.6 236  acceptable                        
                                        example                           
 8b  galvan-  20.1     32.5   52.2 223  acceptable                        
     izing                              example                           
 8c  zinc     19.7     31.9   52.8 238  acceptable                        
     electro-                           example                           
     plating                                                              
 9   none     20.4     32.2   50.6 220  acceptable                        
                                        example                           
10   &#34;        18.8     30.8   49.7 160  comparative                       
                                        example                           
11   &#34;        18.5     32.5   50.2 172  comparative                       
                                        example                           
12   &#34;        19.6     31.6   51.0 166  comparative                       
                                        example                           
13   &#34;        21.8     33.4   48.8 178  comparative                       
                                        example                           
14   &#34;        26.0     37.8   43.2 176  comparative                       
                                        example                           
15   &#34;        24.9     36.9   45.1 181  comparative                       
                                        example                           
16   &#34;        23.2     32.8   49.1 166  comparative                       
                                        example                           
17   &#34;        23.5     32.8   48.0 172  comparative                       
                                        example                           
18   &#34;        20.6     32.9   51.1 175  conventional                      
                                        example                           
______________________________________                                    
 S-FL: tensile shear fatigue limit                                        
 
    
     As seen from Table 10, all of the steels according to the invention exhibit good mechanical properties and tensile shear fatigue limit value, while the comparative steels and the conventional steel are poor in either the mechanical properties or the tensile shear fatigue limit value. 
     Furthermore, the surface treated steels according to the invention are naturally excellent in the properties as compared with the comparative and conventional steels because the breakage in the fatigue test is generated from the heat-affected zone. 
     Moreover, in the steel Nos. 5-9 containing either Nb or B or both, the fatigue resistance at the heat-affected zone is further improved, so that they exhibit a higher tensile shear fatigue limit value among the steels according to the invention. 
     EXAMPLE 3 
     A steel having a chemical composition as shown in the following Table 11 was melted to form a slab, which was subjected to the following treatments under production conditions as shown in the following Table 12. 
     The hot rolled steel sheet of 2.6 mm in thickness was produced by subjecting the slab at a finish temperature of 830°-900° C. and winding at a coiling temperature of 550°-650° C. 
     On the other hand, the slab was hot rolled at a finish temperature of 830°-920° C. and coiled at a coiling temperature of 550°-650° C. to obtain a hot rolled sheet of 3.2 mm in thickness. Then, the hot rolled sheet was cold rolled to a thickness of 0.7 mm at a rolling reduction of 78%, annealed at 750°-880° C. and further subjected to a skin pass rolling at 0.7%. 
     Furthermore, a part of the hot rolled steel sheets and cold rolled steel sheets was subjected to a galvanizing or electroplating. 
     The steel Nos. 1-14 and Nos. 26-36 were acceptable in the invention, and the steel Nos. 15-24 and Nos. 37-43 were comparative examples, whose chemical compositions were outside the range of the invention. Moreover, the steel Nos. 25 and 44 were the conventional low carbon aluminum killed steel, in which the steel No. 25 was produced by box annealing. 
     The mechanical properties and cross tensile fatigue limit value at spot welded joint portion (upper limit of loading range when the repeat number of tensile loading was 10,000,000) were measured with respect to these thin steel sheets to obtain results as shown in Table 12. 
     Moreover, a specimen of JIS Z2201 No. 5 was used in the tensile test, and the spot welding conditions and cross tensile fatigue test conditions were the same as in Table 6 in case of the cold rolled steel sheets and were carried out under conditions as shown in the following Table 13 in case of the hot rolled steel sheets. 
     
                                           TABLE 11                                
__________________________________________________________________________
Chemical composition (wt %)                                               
No.                                                                       
   C   Si Mn P  S  Al N   O   others                Al/N                  
                                                        Remarks           
__________________________________________________________________________
 1 0.0008                                                                 
       0.01                                                               
          0.11                                                            
             0.012                                                        
                0.008                                                     
                   0.049                                                  
                      0.0014                                              
                          0.0023                                          
                              Ti: 0.031             35.0                  
                                                        acceptable        
                                                        example           
 2 0.0012                                                                 
       0.01                                                               
          0.08                                                            
             0.012                                                        
                0.010                                                     
                   0.062                                                  
                      0.0016                                              
                          0.0029                                          
                              Ti: 0.035             38.8                  
                                                        acceptable        
                                                        example           
 3 0.0011                                                                 
       0.01                                                               
          0.17                                                            
             0.010                                                        
                0.009                                                     
                   0.071                                                  
                      0.0020                                              
                          0.0032                                          
                              V: 0.063              35.5                  
                                                        acceptable        
                                                        example           
 4 0.0012                                                                 
       0.02                                                               
          0.22                                                            
             0.020                                                        
                0.009                                                     
                   0.038                                                  
                      0.0010                                              
                          0.0026                                          
                              Cr: 0.58              38.0                  
                                                        acceptable        
                                                        example           
 5 0.0015                                                                 
       0.01                                                               
          0.14                                                            
             0.018                                                        
                0.013                                                     
                   0.061                                                  
                      0.0018                                              
                          0.0032                                          
                              Cu: 0.83              33.9                  
                                                        acceptable        
                                                        example           
 6 0.0007                                                                 
       0.01                                                               
          0.13                                                            
             0.015                                                        
                0.012                                                     
                   0.047                                                  
                      0.0012                                              
                          0.0028                                          
                              Ti: 0.025, V: 0.016, Cr: 0.35               
                                                    39.2                  
                                                        acceptable        
                                                        example           
 7 0.0012                                                                 
       0.02                                                               
          0.15                                                            
             0.012                                                        
                0.015                                                     
                   0.063                                                  
                      0.0019                                              
                          0.0032                                          
                              Ti: 0.018, Zr: 0.041, Cu:                   
                                                    33.2                  
                                                        acceptable        
                                                        example           
 8 0.0013                                                                 
       0.01                                                               
          0.15                                                            
             0.018                                                        
                0.008                                                     
                   0.067                                                  
                      0.0020                                              
                          0.0035                                          
                              V: 0.042, Ca: 0.013, Cr: 0.31, Ni:          
                                                    33.5                  
                                                        acceptable        
                                                        example           
 9 0.0015                                                                 
       0.01                                                               
          0.11                                                            
             0.012                                                        
                0.010                                                     
                   0.059                                                  
                      0.0017                                              
                          0.0028                                          
                              Ti: 0.017, V: 0.031, Zr: 0.018, Cr:         
                                                    34.7,                 
                                                        acceptable        
                              Cu: 0.35                  example           
10 0.0009                                                                 
       0.01                                                               
          0.15                                                            
             0.011                                                        
                0.009                                                     
                   0.042                                                  
                      0.0011                                              
                          0.0025                                          
                              Ti: 0.028, Nb: 0.005  38.2                  
                                                        acceptable        
                                                        example           
11 0.0008                                                                 
       0.02                                                               
          0.18                                                            
             0.010                                                        
                0.007                                                     
                   0.058                                                  
                      0.0015                                              
                          0.0032                                          
                              Ti: 0.033, B: 0.0004  38.7                  
                                                        acceptable        
                                                        example           
12 0.0006                                                                 
       0.01                                                               
          0.15                                                            
             0.021                                                        
                0.009                                                     
                   0.068                                                  
                      0.0019                                              
                          0.0035                                          
                              Ti: 0.027, Nb: 0.003, B: 0.0003             
                                                    35.8                  
                                                        acceptable        
                                                        example           
13 0.0012                                                                 
       0.02                                                               
          0.15                                                            
             0.010                                                        
                0.011                                                     
                   0.050                                                  
                      0.0013                                              
                          0.0021                                          
                              V: 0.052, Nb: 0.012, B: 0.0005              
                                                    38.5                  
                                                        acceptable        
                                                        example           
14 0.0014                                                                 
       0.01                                                               
          0.14                                                            
             0.012                                                        
                0.008                                                     
                   0.061                                                  
                      0.0017                                              
                          0.0027                                          
                              Zr: 0.069, Cr: 0.37, Ni: 0.28, Nb:          
                                                    35.97                 
                                                        acceptable        
                                                        example           
15 0.0024                                                                 
       0.02                                                               
          0.20                                                            
             0.015                                                        
                0.010                                                     
                   0.055                                                  
                      0.0018                                              
                          0.0035                                          
                              Ti: 0.12              30.6                  
                                                        comparative       
                                                        example           
16 0.0013                                                                 
       0.01                                                               
          0.20                                                            
             0.018                                                        
                0.015                                                     
                   0.059                                                  
                      0.0018                                              
                          0.0079                                          
                              Ti: 0.024, B: 0.0007  32.8                  
                                                        comparative       
                                                        example           
17 0.0018                                                                 
       0.02                                                               
          0.14                                                            
             0.023                                                        
                0.012                                                     
                   0.038                                                  
                      0.0038                                              
                          0.0033                                          
                              Ti: 0.042             10.0                  
                                                        comparative       
                                                        example           
18 0.0025                                                                 
       0.01                                                               
          0.18                                                            
             0.018                                                        
                0.012                                                     
                   0.015                                                  
                      0.0016                                              
                          0.0035                                          
                              V: 0.023               9.4                  
                                                        comparative       
                                                        example           
19 0.0013                                                                 
       0.02                                                               
          0.12                                                            
             0.017                                                        
                0.012                                                     
                   0.060                                                  
                      0.0019                                              
                          0.0033                                          
                              Zr: 0.17              31.6                  
                                                        comparative       
                                                        example           
20 0.0010                                                                 
       0.02                                                               
          1.2                                                             
             0.010                                                        
                0.020                                                     
                   0.055                                                  
                      0.0018                                              
                          0.0030                                          
                              Ca: 0.089             30.6                  
                                                        comparative       
                                                        example           
21 0.0012                                                                 
       0.02                                                               
          0.13                                                            
             0.012                                                        
                0.010                                                     
                   0.081                                                  
                      0.0018                                              
                          0.0089                                          
                              Cu: 1.15              45.0                  
                                                        comparative       
                                                        example           
22 0.0048                                                                 
       0.01                                                               
          0.15                                                            
             0.012                                                        
                0.015                                                     
                   0.056                                                  
                      0.0017                                              
                          0.0028                                          
                              Ti: 0.037, Cr: 0.57, Ni: 0.42               
                                                    32.9                  
                                                        comparative       
                                                        example           
23 0.0014                                                                 
       0.02                                                               
          0.12                                                            
             0.010                                                        
                0.018                                                     
                   0.11                                                   
                      0.0032                                              
                          0.0032                                          
                              V: 0.026, Ca: 0.020, Cr: 0.32, Ni:          
                                                    34.4                  
                                                        Comparative       
                                                        example           
24 0.0018                                                                 
       0.01                                                               
          0.21                                                            
             0.018                                                        
                0.012                                                     
                   0.068                                                  
                      0.0019                                              
                          0.0026                                          
                              Zr: 0.052, Ca: 0.041, Cr: 0.42, Cu: 0.41,   
                              Ni: 2.3               35.8                  
                                                        Comparative       
                                                        example           
25 0.036                                                                  
       0.01                                                               
          0.26                                                            
             0.018                                                        
                0.016                                                     
                   0.035                                                  
                      0.0050                                              
                          0.0056                     7.0                  
                                                        conventional      
                                                        example           
26 0.0006                                                                 
       0.01                                                               
          0.09                                                            
             0.012                                                        
                0.010                                                     
                   0.053                                                  
                      0.0015                                              
                          0.0032                                          
                              Ti: 0.035             35.3                  
                                                        acceptable        
                                                        example           
27 0.0007                                                                 
       0.02                                                               
          0.12                                                            
             0.015                                                        
                0.007                                                     
                   0.040                                                  
                      0.0011                                              
                          0.0027                                          
                              Zr: 0.085             36.4                  
                                                        acceptable        
                                                        example           
28 0.0013                                                                 
       0.02                                                               
          0.18                                                            
             0.025                                                        
                0.010                                                     
                   0.058                                                  
                      0.0015                                              
                          0.0031                                          
                              Ca: 0.027             38.7                  
                                                        acceptable        
                                                        example           
29 0.0014                                                                 
       0.02                                                               
          0.12                                                            
             0.015                                                        
                0.012                                                     
                   0.049                                                  
                      0.0012                                              
                          0.0030                                          
                              Ni: 0.33              40.8                  
                                                        acceptable        
                                                        example           
30 0.0008                                                                 
       0.01                                                               
          0.15                                                            
             0.012                                                        
                0.015                                                     
                   0.060                                                  
                      0.0017                                              
                          0.0025                                          
                              Ti: 0.028, V: 0.015, Cr: 0.38               
                                                    35.3                  
                                                        acceptable        
                                                        example           
31 0.0010                                                                 
       0.01                                                               
          0.15                                                            
             0.015                                                        
                0.010                                                     
                   0.056                                                  
                      0.0018                                              
                          0.0035                                          
                              Zr: 0.063, Cr: 0.33, Cu: 0.45               
                                                    31.1                  
                                                        acceptable        
                                                        example           
32 0.0009                                                                 
       0.02                                                               
          0.10                                                            
             0.010                                                        
                0.012                                                     
                   0.071                                                  
                      0.0018                                              
                          0.0012                                          
                              Ti: 0.025, Zr: 0.023, Ca: 0.018, Cr:        
                                                    39.4                  
                                                        acceptable        
                                                        example           
33 0.0013                                                                 
       0.01                                                               
          0.12                                                            
             0.012                                                        
                0.008                                                     
                   0.068                                                  
                      0.0020                                              
                          0.0028                                          
                              V: 0.045, Zr: 0.020, Ca: 0.027, Cu:         
                                                    34.0,                 
                                                        acceptable        
                              Ni: 0.43                  example           
34 0.0015                                                                 
       0.01                                                               
          0.15                                                            
             0.015                                                        
                0.012                                                     
                   0.047                                                  
                      0.0012                                              
                          0.0034                                          
                              Ti: 0.032, Cr: 0.30, Nb: 0.006              
                                                    39.2                  
                                                        acceptable        
                                                        example           
35 0.0010                                                                 
       0.02                                                               
          0.12                                                            
             0.012                                                        
                0.009                                                     
                   0.055                                                  
                      0.0017                                              
                          0.0030                                          
                              Ti: 0.033, Nb: 0.007, B: 0.0006             
                                                    32.4                  
                                                        acceptable        
                                                        example           
36 0.0009                                                                 
       0.01                                                               
          0.20                                                            
             0.010                                                        
                0.009                                                     
                   0.051                                                  
                      0.0015                                              
                          0.0028                                          
                              V: 0.042, Nb: 0.013, B: 0.0005              
                                                    34.0                  
                                                        acceptable        
                                                        example           
37 0.0041                                                                 
       0.02                                                               
          0.15                                                            
             0.010                                                        
                0.014                                                     
                   0.078                                                  
                      0.0020                                              
                          0.0029                                          
                              Ti: 0.015             39.0                  
                                                        comparative       
                                                        example           
38 0.0029                                                                 
       0.01                                                               
          0.17                                                            
             0.015                                                        
                0.010                                                     
                   0.062                                                  
                      0.0018                                              
                          0.0033                                          
                              Ti: 0.13              34.4                  
                                                        comparative       
                                                        example           
39 0.0015                                                                 
       0.03                                                               
          0.23                                                            
             0.013                                                        
                0.011                                                     
                   0.072                                                  
                      0.0015                                              
                          0.0033                                          
                              Cr: 2.2               48.0                  
                                                        comparative       
                                                        example           
40 0.0013                                                                 
       0.02                                                               
          0.10                                                            
             0.018                                                        
                0.010                                                     
                   0.17                                                   
                      0.0020                                              
                          0.0025                                          
                              Ni: 0.87              85.0                  
                                                        comparative       
                                                        example           
41 0.0015                                                                 
       0.01                                                               
          0.20                                                            
             0.012                                                        
                0.020                                                     
                   0.061                                                  
                      0.0078                                              
                          0.0032                                          
                              V: 0.042, Zr: 0.028, Cu: 0.37               
                                                     7.8                  
                                                        comparative       
                                                        example           
42 0.0011                                                                 
       0.02                                                               
          0.15                                                            
             0.010                                                        
                0.015                                                     
                   0.058                                                  
                      0.0018                                              
                          0.0072                                          
                              Zr: 0.067, Ca: 0.028, Cr: 0.41, Cu:         
                                                    32.2                  
                                                        comparative       
                                                        example           
43 0.0018                                                                 
       0.02                                                               
          0.10                                                            
             0.015                                                        
                0.012                                                     
                   0.071                                                  
                      0.0020                                              
                          0.0030                                          
                              Ti: 0.021, V: 0.015, Ca: 0.023, Cr: 1.8,    
                              Ni: 0.25              35.5                  
                                                        comparative       
                                                        example           
44 0.034                                                                  
       0.02                                                               
          0.22                                                            
             0.015                                                        
                0.018                                                     
                   0.032                                                  
                      0.0055                                              
                          0.0062                                          
                                --                   5.8                  
                                                        conventional      
                                                        example           
__________________________________________________________________________
 
    
     
                                           TABLE 12                                
__________________________________________________________________________
Production conditions                                                     
                    Y.S. T.S. El      C-FL                                
No.                                                                       
   kind of steel                                                          
          surface treatment                                               
                    kgf/mm.sup.2                                          
                         kgf/mm.sup.2                                     
                              %  r-value                                  
                                      kgf Remarks                         
__________________________________________________________________________
 1a                                                                       
   cold rolled                                                            
          none      14.7 30.2 53.8                                        
                                 2.35 15.5                                
                                          acceptable                      
   steel sheet                            example                         
 1b                                                                       
   cold rolled                                                            
          galvanizing                                                     
                    16.2 31.0 52.7                                        
                                 2.20 15.0                                
                                          acceptable                      
   steel sheet                            example                         
 1c                                                                       
   cold rolled                                                            
          zinc electroplating                                             
                    16.2 30.8 52.9                                        
                                 2.28 15.5                                
                                          acceptable                      
   steel sheet                            example                         
 2 cold rolled                                                            
          none      15.1 31.0 53.2                                        
                                 2.41 15.0                                
                                          acceptable                      
   steel sheet                            example                         
 3 cold rolled                                                            
          none      16.2 31.8 53.0                                        
                                 2.38 15.0                                
                                          acceptable                      
   steel sheet                            example                         
 4 cold rolled                                                            
          none      20.0 33.2 51.8                                        
                                 2.13 15.5                                
                                          acceptable                      
   steel sheet                            example                         
 5 cold rolled                                                            
          none      20.8 33.5 52.6                                        
                                 2.22 14.5                                
                                          acceptable                      
   steel sheet                            example                         
 6 cold rolled                                                            
          none      18.5 32.1 53.5                                        
                                 2.30 15.0                                
                                          acceptable                      
   steel sheet                            example                         
 7 cold rolled                                                            
          galvanizing                                                     
                    19.8 33.0 51.9                                        
                                 2.28 14.5                                
                                          acceptable                      
   steel sheet                            example                         
 8 cold rolled                                                            
          none      21.0 33.8 52.3                                        
                                 2.17 15.0                                
                                          acceptable                      
   steel sheet                            example                         
 9 cold rolled                                                            
          zinc electroplating                                             
                    20.4 33.5 51.8                                        
                                 2.28 15.5                                
                                          acceptable                      
   steel sheet                            example                         
10 cold rolled                                                            
          gulvanizing                                                     
                    14.1 30.2 53.8                                        
                                 2.40 16.0                                
                                          acceptable                      
   steel sheet                            example                         
11 cold rolled                                                            
          none      13.3 29.1 55.4                                        
                                 2.47 16.0                                
                                          acceptable                      
   steel sheet                            example                         
12a                                                                       
   cold rolled                                                            
          none      14.7 31.0 54.2                                        
                                 2.53 17.0                                
                                          acceptable                      
   steel sheet                            example                         
12b                                                                       
   cold rolled                                                            
          gulvanizing                                                     
                    15.8 31.5 52.1                                        
                                 2.39 16.5                                
                                          acceptable                      
   steel sheet                            example                         
12c                                                                       
   cold rolled                                                            
          zinc electroplating                                             
                    15.0 31.9 52.5                                        
                                 2.45 17.0                                
                                          acceptable                      
   steel sheet                            example                         
13 cold rolled                                                            
          gulvanizing                                                     
                    16.0 31.2 54.5                                        
                                 2.50 16.5                                
                                          acceptable                      
   steel sheet                            example                         
14 cold rolled                                                            
          none      18.7 33.8 53.7                                        
                                 2.45 15.5                                
                                          acceptable                      
   steel sheet                            example                         
15 cold rolled                                                            
          none      20.4 30.2 48.2                                        
                                 1.47 11.0                                
                                          comparative                     
   steel sheet                            example                         
16 cold rolled                                                            
          none      18.1 31.0 47.0                                        
                                 1.98  8.5                                
                                          comparative                     
   steel sheet                            example                         
17 cold rolled                                                            
          gulvanizing                                                     
                    16.4 30.7 51.9                                        
                                 2.10  8.0                                
                                          comparative                     
   steel sheet                            example                         
18 cold rolled                                                            
          none      17.1 32.7 49.0                                        
                                 2.02  7.5                                
                                          comparative                     
   steel sheet                            example                         
19 cold rolled                                                            
          none      18.9 32.7 48.2                                        
                                 2.11 11.5                                
                                          comparative                     
   steel sheet                            example                         
20 cold rolled                                                            
          none      25.0 36.2 43.6                                        
                                 1.48 11.5                                
                                          comparative                     
   steel sheet                            example                         
21 cold rolled                                                            
          gulvanizing                                                     
                    22.3 33.9 51.4                                        
                                 1.55  8.0                                
                                          comparative                     
   steel sheet                            example                         
22 cold rolled                                                            
          none      22.5 34.5 44.1                                        
                                 1.43 11.0                                
                                          comparative                     
   steel sheet                            example                         
23 cold rolled                                                            
          none      21.8 35.3 45.7                                        
                                 1.57  8.0                                
                                          comparative                     
   steel sheet                            example                         
24 cold rolled                                                            
          zinc electroplating                                             
                    24.0 36.1 41.3                                        
                                 1.32 12.0                                
                                          comparative                     
   steel sheet                            example                         
25 cold rolled                                                            
          none      19.8 32.0 50.8                                        
                                 1.82  7.5                                
                                          conventional                    
   steel sheet                            example                         
26a                                                                       
   hot rolled                                                             
          none      15.6 29.8 54.2    150 acceptable example              
   steel sheet                                                            
26b                                                                       
   hot rolled                                                             
          gulvanizing                                                     
                    17.2 31.4 52.0    145 &#34;                               
   steel sheet                                                            
26c                                                                       
   hot rolled                                                             
          zinc electroplating                                             
                    16.5 31.0 52.8    145 &#34;                               
   steel sheet                                                            
27 hot rolled                                                             
          gulvanizing                                                     
                    18.3 33.8 52.4    135 &#34;                               
   steel sheet                                                            
28 hot rolled                                                             
          none      20.5 32.6 53.4    140 &#34;                               
   steel sheet                                                            
29 hot rolled                                                             
          none      21.8 33.0 52.2    130 &#34;                               
   steel sheet                                                            
30 hot rolled                                                             
          none      19.2 32.5 53.8    150 &#34;                               
   steel sheet                                                            
31 hot rolled                                                             
          zinc electroplating                                             
                    20.5 33.3 52.1    145 &#34;                               
   steel sheet                                                            
32 hot rolled                                                             
          none      19.8 32.8 53.5    140 &#34;                               
   steel sheet                                                            
33 hot rolled                                                             
          none      22.3 34.0 52.7    135 &#34;                               
   steel sheet                                                            
34 hot rolled                                                             
          gulvanizing                                                     
                    17.5 31.4 53.8    155 &#34;                               
   steel sheet                                                            
35a                                                                       
   hot rolled                                                             
          none      13.4 29.1 55.2    165 &#34;                               
   steel sheet                                                            
35b                                                                       
   hot rolled                                                             
          gulvanizing                                                     
                    14.5 30.9 52.1    150 &#34;                               
   steel sheet                                                            
35c                                                                       
   hot rolled                                                             
          zinc electroplating                                             
                    14.2 30.5 53.3    155 &#34;                               
   steel sheet                                                            
36 hot rolled                                                             
          none      16.1 30.2 54.1    160 &#34;                               
   steel sheet                                                            
37 hot rolled                                                             
          gulvanizing                                                     
                    21.6 33.8 46.8    105 comparative example             
   steel sheet                                                            
38 hot rolled                                                             
          none      17.2 32.1 46.0    105 &#34;                               
   steel sheet                                                            
39 hot rolled                                                             
          none      25.4 36.7 47.2    110 &#34;                               
   steel sheet                                                            
40 hot rolled                                                             
          none      23.3 34.9 45.1    100 &#34;                               
   steel sheet                                                            
41 hot rolled                                                             
          gulvanizing                                                     
                    19.7 33.0 49.5    85  &#34;                               
   steel sheet                                                            
42 hot rolled                                                             
          none      22.5 35.3 42.7    90  &#34;                               
   steel sheet                                                            
43 hot rolled                                                             
          zinc electroplating                                             
                    25.8 37.4 40.1    100 &#34;                               
   steel sheet                                                            
44 hot rolled                                                             
          none      20.3 33.6 49.3    75  conventional example            
   steel sheet                                                            
__________________________________________________________________________
 C-FL: cross tensile fatigue limit value                                  
 
    
     
                       TABLE 13                                                    
______________________________________                                    
                                Average                                   
Sample size                                                               
          Welding conditions    nugget                                    
width length             welding                                          
                                welding diameter                          
(mm)  (mm)    chip       force  current (mm)                              
______________________________________                                    
50    150     Cr--Cu,    650 kgf                                          
                                14˜17 kA                            
                                        7.8                               
              8.5 φmm,                                                
              CF model                                                    
______________________________________                                    
 
    
     As seen from Table 12, all of the steels according to the invention exhibit good mechanical properties and cross tensile fatigue limit value, while the comparative steels and the conventional steel are poor in either the mechanical properties or the cross tensile fatigue limit value. 
     Furthermore, the surface treated steels according to the invention are excellent in the properties as compared with the comparative and conventional steels because the breakage in the fatigue test is generated from the heat-affected zone. 
     Moreover, in the steel Nos. 10-14 and Nos. 34-36 containing either Nb or B or both, the fatigue resistance at the heat-affected zone is further improved, so that they exhibit a higher cross tensile fatigue limit value among the steels according to the invention. 
     As mentioned above, according to the invention, formable thin steel sheets having not only good formability for press forming, deep drawing or the like but also improved fatigue properties at welded joint are obtained, so that when they are applied to automobiles, structural members and the like, the prolongation of the life or the improvement of the safety is achieved.