Method of producing high-strength hot-dip galvanized steel sheet and method of producing high-strength galvannealed steel sheet

Abstract

A high-strength hot-dip galvanized steel sheet is produced by a method that includes a first heating step of holding a steel sheet at a temperature for a time in an atmosphere having a H 2 concentration and a dew point, a cooling step of cooling the steel sheet after the first heating step, an electrolytic treatment step of subjecting the steel sheet after the cooling step to electrolytic treatment under specific conditions, a second heating step of holding the steel sheet after the electrolytic treatment step at a temperature for a time in an atmosphere having a H 2 concentration and a dew point, and a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.

1 .- 7 . (canceled) 
     
     
         8 . A method of producing a high-strength hot-dip galvanized steel sheet comprising:
 a first heating step of holding a steel sheet at a temperature of 700° C. to 900° C. or 700° C. to 900° C. for 20 to 600 seconds in an atmosphere having a H 2  concentration of 0.05% to 25.0% by volume and a dew point of −45° C. to 0° C., the steel sheet having a composition of C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 5.00% or less, P: 0.100% or less, S: 0100% or less, and Al: 0.100% or less on a mass percent basis, the remainder being Fe and incidental impurities;   an electrolytic treatment step of subjecting the steel sheet after the first heating step to electrolytic treatment in an alkaline aqueous solution at a charge density of 1.0 to 400 C/dm 2 , the steel sheet acting as an anode;   a second heating step of holding the steel sheet after the electrolytic treatment step at a temperature of 650° C. to 900° C. or 650° C. to 900° C. for 15 to 300 seconds in an atmosphere having a H 2  concentration of 0.05% to 25.0% by volume and a dew point of 0° C. or less; and   a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.   
     
     
         9 . A method of producing a high-strength hot-dip galvanized steel sheet comprising:
 a first heating step of holding a steel sheet at a temperature of 700° C. to 900° C. or 700° C. to 900° C. for 20 to 600 seconds in an atmosphere having a H 2  concentration of 0.05% to 25.0% by volume and a dew point of −45° C. to 0° C., the steel sheet having a composition of C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 2.00%, Mn: 8.00% or less, P: 0.100% or less, S: 0100% or less, and Al: 0.100% or less on a mass percent basis, the remainder being Fe and incidental impurities;   an electrolytic treatment step of subjecting the steel sheet after the first heating step to electrolytic treatment in an alkaline aqueous solution at a charge density of 1.0 to 400 C/dm 2 , the steel sheet acting as an anode;   a pickling step of pickling the steel sheet after the electrolytic treatment such that a pickling weight loss is 0.05 to 5 g/m 2  on an Fe basis;   a second heating step of holding the steel sheet after the pickling step at a temperature of 650° C. to 900° C. or 650° C. to 900° C. for 15 to 300 seconds in an atmosphere having a H 2  concentration of 0.05% to 25.0% by volume and a dew point of 0° C. or less; and   a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.   
     
     
         10 . A method of producing a high-strength hot-dip galvanized steel sheet comprising:
 a first heating step of holding a steel sheet at a temperature of 700° C. to 900° C. or 700° C. to 900° C. for 20 to 600 seconds in an atmosphere having a H 2  concentration of 0.05% to 25.0% by volume and a dew point of −45° C. to 0° C., the steel sheet having a composition of C: 0.040% to 0.500%, Si: 1.00% or less, Cr: 0.10% to 3.00%, Mn: 8.00% or less, P: 0.100% or less, S: 0100% or less, and Al: 0.100% or less on a mass percent basis, the remainder being Fe and incidental impurities;   a pickling step of pickling the steel sheet after the first heating step such that a pickling weight loss is 0.05 to 5 g/m 2  on an Fe basis;   an electrolytic treatment step of subjecting the steel sheet after the pickling step to electrolytic treatment in an alkaline aqueous solution at a charge density of 1.0 to 400 C/dm 2 , the steel sheet acting as an anode;   a second heating step of holding the steel sheet after the electrolytic treatment step at a temperature of 650° C. to 900° C. or 650° C. to 900° C. for 15 to 300 seconds in an atmosphere having a H 2  concentration of 0.05% to 25.0% by volume and a dew point of 0° C. or less; and   a coating treatment step of subjecting the steel sheet after the second heating step to hot-dip galvanizing treatment.   
     
     
         11 . The method according to  claim 8 , wherein the electrolytic treatment in the alkaline aqueous solution in the electrolytic treatment step is performed for 2 seconds or more. 
     
     
         12 . The method according to  claim 8 , wherein the composition further includes at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. 
     
     
         13 . The method according  claim 8 , wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis. 
     
     
         14 . A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to  claim 8 . 
     
     
         15 . The method according to  claim 9 , wherein the electrolytic treatment in the alkaline aqueous solution in the electrolytic treatment step is performed for 2 seconds or more. 
     
     
         16 . The method according to  claim 10 , wherein the electrolytic treatment in the alkaline aqueous solution in the electrolytic treatment step is performed for 2 seconds or more. 
     
     
         17 . The method according to  claim 9 , wherein the composition further includes at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. 
     
     
         18 . The method according to  claim 10 , wherein the composition further includes at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. 
     
     
         19 . The method according to  claim 11 , wherein the composition further includes at least one element selected from Mo: 0.01% to 0.50%, Nb: 0.010% to 0.100%, B: 0001% to 0.0050%, and Ti: 0.010% to 0.100% on a mass percent basis. 
     
     
         20 . The method according  claim 9 , wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis. 
     
     
         21 . The method according  claim 10 , wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis. 
     
     
         22 . The method according  claim 11 , wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis. 
     
     
         23 . The method according  claim 12 , wherein the composition further includes at least one element selected from Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50 or less, N: 0100% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0100% or less, and REM: 0.010% or less on a mass percent basis. 
     
     
         24 . A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to  claim 9 . 
     
     
         25 . A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to  claim 10 . 
     
     
         26 . A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to  claim 11 . 
     
     
         27 . A method of producing a high-strength galvannealed steel sheet, comprising subjecting a high-strength hot-dip galvanized steel sheet to alloying treatment, wherein the high-strength hot-dip galvanized steel sheet is produced by the method according to  claim 12 .