Steel product excellent in corrosion resistance and corrosion fatigue resistance, and surface treatment method therefor

Abstract

The invention provides a steel product excellent in corrosion resistance and corrosion fatigue resistance, comprising a, coating film on the surface of the steel product, wherein a ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms and optionally added Si, Mn and Cr atoms is 0.5% or more in an average composition of the coating film, and the number of Al atoms in the average composition of the coating film is higher than the number of Al atoms in an average composition of the steel pro duct before surface treatment.

1 . A steel product excellent in corrosion resistance and corrosion fatigue resistance, comprising a coating film on the surface of the steel product, wherein a ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms and optionally added Si, Mn and Cr atoms is 0.5% or more in an average composition of the coating film, and the number of Al atoms in the average composition of the coating film is higher than the number of Al atoms in an average composition of the steel product before surface treatment.  
   
   
       2 . A steel product excellent in corrosion resistance and corrosion fatigue resistance, comprising a coating film on the surface of the steel product, wherein a ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms and optionally added Si, Mn and Cr atoms is 0.5% or more in an average composition in a region from the surface to a depth of 5 μm of the steel product, and the number of Al atoms in an average composition of the coating film is higher than the number of Al atoms in an average composition of the steel product before surface treatment.  
   
   
       3 . The steel product excellent in corrosion resistance and corrosion fatigue resistance according to  claim 2 , wherein the ratio of the number of Al atoms to the total number of Fe, C, Al, P and O atoms and optionally added Si, Mn and Cr atoms is 3% or more.  
   
   
       4 . A surface treatment method for a steel product, comprising: 
 rinsing the steel product after allowing an aqueous solution at room temperature having an acidity ratio in the range of 3.3 to 5.7 and containing Al ions to contact the surface of the steel product; and    drying the steel product.    
   
   
       5 . A surface treatment method for a steel product, comprising: 
 drying the steel product after allowing an aqueous solution at room temperature having an acidity ratio in the range of 3.3 to 5.7 and containing Al ions to contact the surface of the steel product; and    drying again the steel product after rinsing.    
   
   
       6 . The surface treatment method for a steel product, according to  claim 4 , comprising allowing an aqueous solution containing Al ions to contact the surface of the steel product at a temperature from 40 to 50° C. for 30 seconds or more.  
   
   
       7 . The surface treatment method for a steel product, according to  claim 5 , comprising allowing an aqueous solution containing Al ions to contact the surface of the steel product at a temperature from 40 to 50° C. for 30 seconds or more.  
   
   
       8 . The surface treatment method for a steel product, according to  claim 4 , comprising allowing an aqueous solution containing Al ions to contact the surface of the steel product at 30° C. for 180 seconds or more.  
   
   
       9 . The surface treatment method for a steel product, according to  claim 5 , comprising allowing an aqueous solution containing Al ions to contact the surface of the steel product at 30° C. for 180 seconds or more.  
   
   
       10 . The surface treatment method for a steel product, according to  claim 5 , comprising drying at 50° C. or less after allowing the steel product to contact the aqueous solution containing Al ions and before rinsing.  
   
   
       11 . The surface treatment method for a steel product, according to  claim 4 , wherein the acidity ratio of the aqueous solution containing Al ions is in the range of 3.8 to 5.4.  
   
   
       12 . The surface treatment method for a steel product, according to  claim 4 , wherein an aqueous solution having a concentration by weight of aluminum phosphate in the range of 1 to 10%, and having an acidity ratio in the range of 3.8 to 5.4 is used as a surface treatment solution.  
   
   
       13 . The surface treatment method for a steel product, according to  claim 5 , wherein the acidity ratio of the aqueous solution containing Al ions is in the range of 3.8 to 5.4.  
   
   
       14 . The surface treatment method for a steel product, according to  claim 5 , wherein an aqueous solution having a concentration by weight of aluminum phosphate in the range of 1 to 10%, and having an acidity ratio in the range of 3.8 to 5.4 is used as a surface treatment solution.  
   
   
       15 . The surface treatment method for a steel product, according to  claim 6 , wherein the acidity ratio of the aqueous solution containing Al ions is in the range of 3.8 to 5.4.  
   
   
       16 . The surface treatment method for a steel product, according to  claim 6 , wherein an aqueous solution having a concentration by weight of aluminum phosphate in the range of 1 to 10%, and having an acidity ratio in the range of 3.8 to 5.4 is used as a surface treatment solution.  
   
   
       17 . The surface treatment method for a steel product, according to  claim 7 , wherein the acidity ratio of the aqueous solution containing Al ions is in the range of 3.8 to 5.4.  
   
   
       18 . The surface treatment method for a steel product, according to  claim 7 , wherein an aqueous solution having a concentration by weight of aluminum phosphate in the range of 1 to 10%, and having an acidity ratio in the range of 3.8 to 5.4 is used as a surface treatment solution.  
   
   
       19 . The surface treatment method for a steel product, according to  claim 8 , wherein the acidity ratio of the aqueous solution containing Al ions is in the range of 3.8 to 5.4.  
   
   
       20 . The surface treatment method for a steel product, according to  claim 8 , wherein an aqueous solution having a concentration by weight of aluminum phosphate in the range of 1 to 10%, and having an acidity ratio in the range of 3.8 to 5.4 is used as a surface treatment solution.