Method of producing cold rolled steel strip and production system for cold rolled steel strip

Abstract

A method of producing a cold rolled steel strip includes: subjecting a steel strip that has been cold rolled and subsequently continuously annealed to pickling by continuously feeding the steel strip into a mixed acid solution containing a first acid that is oxidizing and a second acid that is non-oxidizing to immerse the steel strip; and subsequently subjecting the steel strip to repickling by continuously feeding the steel strip into an acid solution containing a third acid that is non-oxidizing to immerse the steel strip. The concentration of the first acid in the mixed acid solution is lowered and the concentration of the second acid in the mixed acid solution is raised as the iron ion concentration in the mixed acid solution rises.

1 . A method of producing a cold rolled steel strip comprising:
 subjecting a steel strip that has been cold rolled and subsequently continuously annealed to pickling by continuously feeding the steel strip into a mixed acid solution containing a first acid that is oxidizing and a second acid that is non-oxidizing to immerse the steel strip; and   subsequently subjecting the steel strip to repickling by continuously feeding the steel strip into an acid solution containing a third acid that is non-oxidizing to immerse the steel strip, wherein   concentration of the first acid in the mixed acid solution is lowered and concentration of the second acid in the mixed acid solution is raised as iron ion concentration in the mixed acid solution rises.   
     
     
         2 . The method of producing a cold rolled steel strip according to  claim 1 , wherein
 the first acid is nitric acid.   
     
     
         3 . The method of producing a cold rolled steel strip according to  claim 1 , wherein
 at least one of the second acid and the third acid is at least one selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid.   
     
     
         4 . The method of producing a cold rolled steel strip according to  claim 1 , wherein
 the first acid is nitric acid, and the second acid and the third acid are hydrochloric acid.   
     
     
         5 . The method of producing a cold rolled steel strip according to  claim 4 , wherein
 concentration of the nitric acid in the mixed acid solution is set within a range of higher than 110 g/L and not higher than 188 g/L, and concentration of the hydrochloric acid in the mixed acid solution is set within a range of higher than 4.5 g/L and not higher than 12.5 g/L.   
     
     
         6 . The method of producing a cold rolled steel strip according to  claim 1 , further comprising immersing the steel strip in water after the pickling and before the repickling. 
     
     
         7 . The method of producing a cold rolled steel strip according to  claim 1 , wherein
 the picking and the repickling have a total pickling weight loss of 8 g/m 2  or more.   
     
     
         8 . The method of producing a cold rolled steel strip according to  claim 1 , wherein
 the steel strip contains 0.5 mass % to 3.0 mass % of Si.   
     
     
         9 . A production system for a cold rolled steel strip comprising:
 a first stock solution tank holding a stock solution of a first acid that is oxidizing, a second stock solution tank holding a stock solution of a second acid that is non-oxidizing, and a third stock solution tank holding a stock solution of a third acid that is non-oxidizing;   a first pipe extending from the first stock solution tank, a second pipe extending from the second stock solution tank, and a third pipe extending from the third stock solution tank;   a mixed acid solution circulation tank to which the first pipe and the second pipe are connected, and in which the first acid fed from the first stock solution tank and the second acid fed from the second stock solution tank are mixed and held;   a first valve disposed in the first pipe for adjusting a feed rate of the first acid from the first stock solution tank and a second valve disposed in the second pipe for adjusting a feed rate of the second acid from the second pipe;   an acid solution circulation tank to which the third pipe is connected and that holds the third acid fed from the third stock solution tank;   a mixed acid tank holding a mixed acid solution containing the first acid and the second acid;   an acid tank holding an acid solution containing the third acid;   at least two fourth pipes linking the mixed acid solution circulation tank and the mixed acid tank for circulating the mixed acid solution between the mixed acid solution circulation tank and the mixed acid tank;   at least two fifth pipes linking the acid solution circulation tank and the acid tank for circulating the acid solution between the acid solution circulation tank and the acid tank;   a sheet feeder continuously feeding a steel strip that has been cold rolled and subsequently continuously annealed, and immersing the steel strip in the mixed acid tank and the acid tank in this order;   a concentration meter measuring iron ion concentration in the mixed acid solution in the mixed acid tank; and   a controller controlling the first valve and the second valve based on output of the concentration meter such as to decrease the feed rate of the first acid from the first stock solution tank and increase the feed rate of the second acid from the second stock solution tank, and thereby to lower concentration of the first acid in the mixed acid solution and raise concentration of the second acid in the mixed acid solution as the iron ion concentration in the mixed acid solution rises.   
     
     
         10 . The production system for a cold rolled steel strip according to  claim 9 , further comprising
 a water tank that holds water and is positioned between the mixed acid tank and the acid tank, wherein   the sheet feeder continuously feeds the steel strip into the water tank after the steel strip exits the mixed acid tank, and subsequently continuously feeds the steel strip into the acid tank.   
     
     
         11 . The production system for a cold rolled steel strip according to  claim 9 , wherein
 the second acid and the third acid are the same type of acid, and the second stock solution tank and the third stock solution tank are the same tank.   
     
     
         12 . The method of producing a cold rolled steel strip according to  claim 2 , wherein
 at least one of the second acid and the third acid is at least one selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid.