Methods for producing a hot-dip galvanized steel sheet having excellent press formability

Abstract

A method for producing a hot-dip galvanized steel sheet which includes a hot-dip galvanization step, a temper rolling step and an oxidation step.

1 . A method for producing a hot-dip galvanized steel sheet comprising the steps of:
 (a) hot-dip-galvanizing a steel sheet to form a hot-dip galvanized layer;   (b) temper-rolling the steel sheet provided with the hot-dip galvanized layer from step (a); and   (c) subjecting the temper-rolled steel sheet from step (b) to an oxidation treatment by bringing the temper-rolled steel sheet into contact with an acidic solution having a pH buffering effect, and retaining the temper-rolled steel sheet in the solution for 1 to 30 seconds before washing with water.   
     
     
         2 . The method according to  claim 1 , further comprising an activation step of activating a surface of the steel sheet before or after the temper rolling step (b). 
     
     
         3 . The method according to  claim 2 , wherein the activation step further comprises controlling an Al-based oxide content in a surface oxide layer of the steel sheet before the oxidation step (c) so that an Al concentration is less than 20 atomic percent. 
     
     
         4 . The method according to  claim 2 , wherein the activation step comprises bringing the steel sheet into contact with an alkaline solution having a pH of 11 or more at 50° C. or more for 1 second or more. 
     
     
         5 . The method according to  claim 2 , wherein the activation step is performed before the temper rolling step (b). 
     
     
         6 . The method according to  claim 1 , wherein the acidic solution contains 1 to 200 g/l of Fe ions. 
     
     
         7 . A method for producing a hot-dip galvanized steel sheet comprising the steps of:
 (a) hot-dip-galvanizing a steel sheet to form a hot-dip galvanized layer;   (b) temper-rolling the steel sheet provided with the hot-dip galvanized layer from step (a);   (c) activating a surface of the steel sheet before or after the temper rolling step (b) and then;   (d) subjecting the temper-rolled steel sheet from step (b) to an oxidation treatment by bringing the temper-rolled steel sheet into contact with an acidic solution having a pH buffering effect, said acidic solution having a pH of 1 to 3 and containing 5 to 200 g/l of Fe ions, and retaining the temper-rolled steel sheet in the solution for 1 to 30 seconds before washing with water.   
     
     
         8 . A method for producing a hot-dip galvanized steel sheet comprising the steps of:
 (a) hot-dip-galvanizing a steel sheet to form a hot-dip galvanized layer;   (b) temper-rolling the steel sheet provided with the hot-dip galvanized layer from step (a);   (c) activating a surface of the steel sheet before or after the temper rolling step (b) and then;   (d) subjecting the temper-rolled steel sheet from step (b) to an oxidation treatment by bringing the temper-rolled steel sheet into contact with an acidic solution having a pH buffering effect, said acidic solution having a pH of 1 to 5, and retaining the temper-rolled steel sheet in the solution for 1 to 30 seconds before washing with water.   
     
     
         9 . The method according to  claim 7 , wherein the activation step comprises bringing the steel into contact with an alkaline solution with a pH of 11 or more at 50° C. or more for 1 second or more. 
     
     
         10 . The method according to  claim 8 , wherein the activation step comprises bringing the steel into contact with an alkaline solution with a pH of 11 or more at 50° C. or more for 1 second or more. 
     
     
         11 . The method according to  claim 8 , wherein the acidic solution contains 1 to 200 g/l of Fe ions.