Method of producing cold-rolled steel sheet as well as cold-rolled steel sheet and members for automobile

Abstract

In a method of producing a cold-rolled steel sheet being excellent in not only the phosphate treatability but also the corrosion resistance after coating under severe corrosion environment such as hot salt water immersion test or composite cycle corrosion test, a continuously annealed steel sheet after cold rolling preferably including 0.5-3.0 mass % of Si is pickled to remove a Si-containing oxide layer on a surface layer of the steel sheet and further repickled so that a surface covering ratio of an iron-based oxide on the surface of the steel sheet is not more than 40% and preferably a maximum thickness of the iron-based oxide is not more than 150 nm, as well as a cold-rolled steel sheet produced by this method and a member for automobile using the cold-rolled steel sheet.

1 . A method of producing a cold-rolled steel sheet, comprising steps of cold rolling a steel sheet, continuously annealing, pickling and further repickling it. 
     
     
         2 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the repickling uses a non-oxidizable acid different from an acid used in the pickling before repickling. 
     
     
         3 . A method of producing a cold-rolled steel sheet according to  claim 2 , wherein the non-oxidizable acid is any of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid and a mixed acid of two or more thereof. 
     
     
         4 . A method of producing a cold-rolled steel sheet according to  claim 2 , wherein the non-oxidizable acid is any of hydrochloric acid with a concentration of 0.1-50 g/L, sulfuric acid with a concentration of 0.1-150 g/L and a mixed acid of 0.1-20 g/L of hydrochloric acid and 0.1-60 g/L of sulfuric acid. 
     
     
         5 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the repickling is carried out at a temperature of a repickling solution of 20-70° C. for 1-30 seconds. 
     
     
         6 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the pickling is carried out with any of nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a mixed acid of two or more thereof. 
     
     
         7 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the pickling is carried out with any of a mixed acid of nitric acid and hydrochloric acid wherein a concentration of nitric acid is more than 50 g/L but not more than 200 g/L and a ratio (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration is 0.01-1.0, or a mixed acid of nitric acid and hydrofluoric acid wherein a concentration of nitric acid is more than 50 g/L but not more than 200 g/L and a ratio (HF/HNO 3 ) of hydrofluoric acid concentration to nitric acid concentration is 0.01-1.0. 
     
     
         8 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the steel sheet comprises 0.5-3.0 mass % of Si. 
     
     
         9 . A method of producing a cold-rolled steel sheet according to  claim 8 , wherein the steel sheet has a chemical composition comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass %, Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities. 
     
     
         10 . A method of producing a cold-rolled steel sheet according to  claim 8 , wherein the steel sheet contains, in addition to the chemical composition, one or more selected from Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N: not more than 0.008 mass %. 
     
     
         11 . A method of producing a cold-rolled steel sheet according to  claim 8 , wherein the steel sheet contains, in addition to the chemical composition, one or more selected from Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %. 
     
     
         12 . A cold-rolled steel sheet produced by a method as claimed in  claim 1 , characterized in that a Si-containing oxide layer is removed from the surface of the steel sheet by pickling after continuous annealing and a surface covering ratio of an iron-based oxide existing on the surface of the steel sheet after repickling is not more than 40%. 
     
     
         13 . A cold-rolled steel sheet according to  claim 12 , wherein a maximum thickness of the iron-based oxide existing on the surface of the steel sheet after repickling is not more than 150 nm. 
     
     
         14 . A member for automobile characterized by using a cold-rolled steel sheet as claimed in  claim 12   13 . 
     
     
         15 . A method of producing a cold-rolled steel sheet according to  claim 2 , wherein the repickling is carried out at a temperature of a repickling solution of 20-70° C. for 1-30 seconds. 
     
     
         16 . A method of producing a cold-rolled steel sheet according to  claim 3 , wherein the repickling is carried out at a temperature of a repickling solution of 20-70° C. for 1-30 seconds. 
     
     
         17 . A method of producing a cold-rolled steel sheet according to  claim 4 , wherein the repickling is carried out at a temperature of a repickling solution of 20-70° C. for 1-30 seconds. 
     
     
         18 . A method of producing a cold-rolled steel sheet according to  claim 2 , wherein the pickling is carried out with any of nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a mixed acid of two or more thereof. 
     
     
         19 . A method of producing a cold-rolled steel sheet according to  claim 3 , wherein the pickling is carried out with any of nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a mixed acid of two or more thereof. 
     
     
         20 . A method of producing a cold-rolled steel sheet according to  claim 4 , wherein the pickling is carried out with any of nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid and a mixed acid of two or more thereof.