Method for prevention of yellowing on surface of steel sheet after pickling

Abstract

In a method for prevention of yellowing on a surface of a steel sheet subjected to re-pickling, washing with water and drying after a surface of a continuously annealed steel sheet is pickled to remove Si-containing oxide layer from a surface layer of the steel sheet, the surface of the steel sheet is held at a wet state between the pickling and the re-pickling and between the re-pickling and the washing, and more preferably the washing is carried out with water having an iron ion concentration decreased to not more than 20 g/L, whereby the yellowing on the surface of the steel sheet after the pickling is prevented. Thus, cold rolled steel sheets being excellent in not only the appearance quality but also the phosphatability and corrosion resistance after painting are manufactured stably.

1 - 14 . (canceled) 
     
     
         15 . A method for prevention of yellowing on a surface of a steel sheet subjected to re-pickling, washing with water and drying after a surface of a continuously annealed steel sheet is pickled to remove Si-containing oxide layer from a surface layer of the steel sheet, characterized in that the surface of the steel sheet is held at a wet state between the pickling and the re-pickling and between the re-pickling and the washing. 
     
     
         16 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein the washing is carried out with washing water having an iron ion concentration decreased to not more than 20 g/L. 
     
     
         17 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 16 , wherein the washing water is further characterized by decreasing a concentration of nitric acid ion to not more than 10 g/L, a concentration of chloride ion to not more than 5 g/L, a concentration of fluoride ion to not more than 5 g/L and a concentration of sulfuric acid ion to not more than 5 g/L. 
     
     
         18 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein the washing is conducted by immersing into washing water at a temperature of not lower than 20° C. for not less than 3 seconds. 
     
     
         19 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 16 , wherein the washing is conducted by immersing into washing water at a temperature of not lower than 20° C. for not less than 3 seconds. 
     
     
         20 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein the re-pickling is conducted with a re-pickling solution having an iron ion concentration decreased to not more than 40 g/L. 
     
     
         21 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 20 , wherein the re-pickling solution is further characterized by decreasing a concentration of nitric acid ion to not more than 20 g/L. 
     
     
         22 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein the re-pickling is conducted with a non-oxidizing acid. 
     
     
         23 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 20 , wherein the re-pickling is conducted with a non-oxidizing acid. 
     
     
         24 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 22 , wherein the non-oxidizing acid is characterized to be any one of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid and a mixture of these two or more acids. 
     
     
         25 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 23 , wherein the non-oxidizing acid is characterized to be any one of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid and a mixture of these two or more acids. 
     
     
         26 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 22 , wherein the non-oxidizing acid is hydrochloric acid having a concentration of 0.1-50 g/L, sulfuric acid having a concentration of 0.1-150 g/L or a mixture of hydrochloric acid having a concentration of 0.1-20 g/L and sulfuric acid having a concentration of 0.1-60 g/L. 
     
     
         27 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 23 , wherein the non-oxidizing acid is hydrochloric acid having a concentration of 0.1-50 g/L, sulfuric acid having a concentration of 0.1-150 g/L or a mixture of hydrochloric acid having a concentration of 0.1-20 g/L and sulfuric acid having a concentration of 0.1-60 g/L. 
     
     
         28 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein the pickling is conducted with nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid or a mixture of these two or more acids as a pickling solution. 
     
     
         29 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein the pickling is conducted with a mixture of nitric acid and hydrochloric acid or a mixture of nitric acid and hydrofluoric acid as a pickling solution. 
     
     
         30 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein the steel sheet is a cold rolled steel sheet containing Si: 1.0-2.0 mass %. 
     
     
         31 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein the steel sheet contains C: 0.01-0.20 mass %, Mn: 1.0-3.0 mass %, P: not more than 0.05 mass %, S: not more than 0.005 mass % and Al: not more than 0.06 mass % in addition to Si: 1.0-2.0 mass %. 
     
     
         32 . The method for prevention of yellowing on a surface of a steel sheet according to  claim 15 , wherein a content ratio (Si/Mn) of Si to Mn exceeds 0.4.