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 is pickled with a mixture of nitric acid and hydrochloric acid having a nitric acid concentration of more than 100 g/L but not more than 200 g/L and a ratio R (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration of 0.01-0.25 to remove Si-containing oxide formed on the steel sheet surface by continuous annealing, and a ratio of covering the surface of the steel sheet with an iron-based oxide formed by the pickling is not more than 85% and preferably a maximum thickness of the iron-based oxide existing on the surface of the steel sheet is not more than 200 nm.

1 . A method of producing a cold-rolled steel sheet having Si with a range of 0.5-3.0 mass %, characterized in that a continuously annealed steel sheet after cold rolling is pickled with a mixture of nitric acid and hydrochloric acid having a nitric acid concentration of more than 100 g/L but not more than 200 g/L and a ratio R (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration of 0.01-0.25. 
     
     
         2 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the mixture of nitric acid and hydrochloric acid has a nitric acid concentration of more than 110 g/L but not more than 140 g/L and a ratio R (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration of 0.03-0.25. 
     
     
         3 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the pickling is carried out at a temperature of a pickling solution of 20-70° C. for 3-30 seconds. 
     
     
         4 . A method of producing a cold-rolled steel sheet according to  claim 1 , 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 % and Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities. 
     
     
         5 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         6 . A method of producing a cold-rolled steel sheet according to  claim 1 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         7 . 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 ratio of covering the surface of the steel sheet with an iron-based oxide formed by the pickling is not more than 85%. 
     
     
         8 . A cold-rolled steel sheet according to  claim 7 , wherein a maximum thickness of the iron-based oxide existing on the surface of the steel sheet is not more than 200 nm. 
     
     
         9 . A member for automobile characterized by using a cold-rolled steel sheet as claimed in  claim 7 . 
     
     
         10 . A method of producing a cold-rolled steel sheet according to  claim 2 , wherein the pickling is carried out at a temperature of a pickling solution of 20-70° C. for 3-30 seconds. 
     
     
         11 . A method of producing a cold-rolled steel sheet according to  claim 2 , 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 % and Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities. 
     
     
         12 . A method of producing a cold-rolled steel sheet according to  claim 3 , 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 % and Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities. 
     
     
         13 . A method of producing a cold-rolled steel sheet according to  claim 2 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         14 . A method of producing a cold-rolled steel sheet according to  claim 3 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         15 . A method of producing a cold-rolled steel sheet according to  claim 4 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         16 . A method of producing a cold-rolled steel sheet according to  claim 2 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         17 . A method of producing a cold-rolled steel sheet according to  claim 3 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         18 . A method of producing a cold-rolled steel sheet according to  claim 4 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         19 . A method of producing a cold-rolled steel sheet according to  claim 5 , wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of 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 %. 
     
     
         20 . A cold-rolled steel sheet produced by a method as claimed in  claim 2 , characterized in that a Si-containing oxide layer is removed from the surface of the steel sheet by pickling after continuous annealing and a ratio of covering the surface of the steel sheet with an iron-based oxide formed by the pickling is not more than 85%.