High-strength alloyed hot-dipped galvanized steel sheet having excellent workability and delayed fracture resistance, and method for producing same

Abstract

Disclosed herein is a high-strength galvannealed steel sheet having a galvannealed layer on a surface of a base steel sheet and containing predetermined steel components. The steel sheet sequentially has, from the interface of the base steel sheet and the galvannealed layer, towards the base steel sheet: an internal oxide layer and containing at least one oxide selected from the group consisting of Si and Mn; a soft layer including the internal oxide layer, and satisfying a predetermined Vickers hardness; and a hard layer made up of a structure mainly composed of martensite. The average depth D of the soft layer is 20 μm or greater, and the average depth d of the internal oxide layer is 4 μm or greater and smaller than D. A coefficient of variation of KAM of the base steel sheet at the portion t/4 is 0.66 or less.

1 : A high-strength galvannealed steel sheet, which has a galvannealed layer on the surface of a base steel sheet, wherein:
 (1) the base steel sheet comprises, in mass %
 C: 0.05 to 0.25%; 
 Si: 0.5 to 2.5%; 
 Mn: 2.0 to 4%; 
 P: more than 0% to 0.1% or less; 
 S: more than 0% to 0.05% or less; 
 Al: 0.01 to 0.1%; 
 N: more than 0% to 0.01% or less, and 
 iron and inevitable impurities; 
   (2) the high-strength steel sheet sequentially has, from an interface of the base steel sheet and the galvannealed layer, towards the base steel sheet:   an internal oxide layer comprising at least one oxide selected from the group consisting of Si and Mn;   a soft layer comprising the internal oxide layer and satisfying a Vickers hardness of 90% or less of a Vickers hardness at a portion t/4 of the base steel sheet, when t is a sheet thickness of the base steel sheet; and   a hard layer made up of a structure mainly comprising martensite, wherein the high-strength steel sheet satisfies:   an average depth D of the soft layer is 20 μm or greater; and   an average depth d of the internal oxide layer is 4 μm or greater and smaller than the D; and   a coefficient of variation of KAM (Kernel Average Misorientation) of the base steel sheet at the portion t/4 is 0.66 or less, a tensile strength being 1180 MPa or higher and a yield ratio YR being 73.0% or higher.   
     
     
         2 : The high-strength galvannealed steel sheet according to  claim 1 , wherein the base steel sheet further comprises, in mass %, at least one of (a) to (c) below:
 (a) at least one element selected from the group consisting of
 Cr: more than 0% to 1% or less, 
 Mo: more than 0% to 1% or less and 
 B: more than 0% to 0.01% or less; 
   (b) at least one element selected from the group consisting of
 Ti: more than 0% to 0.2% or less, 
 Nb: more than 0% to 0.2% or less and 
 V: more than 0% to 0.2% or less; and 
   (c) at least one element selected from the group consisting of
 Cu: more than 0% to 1% or less and 
 Ni: more than 0% to 1% or less. 
   
     
     
         3 : The high-strength galvannealed steel sheet according to  claim 1 , wherein the average depth d of the internal oxide layer and the average depth D of the soft layer satisfy the relationship D>2d. 
     
     
         4 : The high-strength galvannealed steel sheet according to  claim 2 , wherein the average depth d of the internal oxide layer and the average depth D of the soft layer satisfy the relationship D>2d. 
     
     
         5 : The high-strength galvannealed steel sheet according to  claim 1 , wherein the structure of the hard layer is
 ferrite: 0 area % to 5 area %, and   bainite: 0 area % to 10 area %,   
       with respect to the entire structure. 
     
     
         6 : The high-strength galvannealed steel sheet according to  claim 2 , wherein the structure of the hard layer is
 ferrite: 0 area % to 5 area %, and   bainite: 0 area % to 10 area %,   
       with respect to the entire structure. 
     
     
         7 : The high-strength galvannealed steel sheet according to  claim 3 , wherein the structure of the hard layer is
 ferrite: 0 area % to 5 area %, and   bainite: 0 area % to 10 area %,   
       with respect to the entire structure. 
     
     
         8 : The high-strength galvannealed steel sheet according to  claim 4 , wherein the structure of the hard layer is
 ferrite: 0 area % to 5 area %, and   bainite: 0 area % to 10 area %,   
       with respect to the entire structure. 
     
     
         9 : A method for producing the high-strength galvannealed steel sheet according to  claim 1 , the method comprising, in order:
 coiling, at a temperature of 600° C. or higher, a steel sheet satisfying steel components of the base steel sheet;   pickling-cold rolling such that there remain 4 μm or more of the average depth d of the internal oxide layer;   oxidizing at an air ratio in a range of 0.9 to 1.4 in an oxidation zone;   soaking within a range Ac 3  point to (Ac 3  point+100° C.), in a reduction zone;   after the soaking, performing cooling at an average cooling rate of 5° C./sec or higher over a range down to 600° C.;   setting to 20 seconds or less a keeping time in a temperature region of 480° C. or lower until immersion in a galvanizing bath;   after alloying, cooling at an average cooling rate of 10° C./sec or higher over a temperature region down to 300° C., and thereafter, cooling at an average cooling rate of 5° C./sec or lower over a temperature region from 300° C. to 150° C.   
     
     
         10 : A method for producing the high-strength galvannealed steel sheet according to  claim 1 , the method comprising, in order:
 coiling, at a temperature of 600° C. or higher, a steel sheet satisfying steel components of the base steel sheet;   pickling-cold rolling such that there remain 4 μm or more of the average depth d of the internal oxide layer;   oxidizing at an air ratio in a range of 0.9 to 1.4, in an oxidation zone;   soaking within a range Ac 3  point to (Ac 3  point+100° C.), in a reduction zone;   after the soaking, performing cooling at an average cooling rate of 5° C./sec or higher over a range down to 600° C.;   setting to 20 seconds or less a keeping time in a temperature region of 480° C. or lower until immersion in a galvanizing bath;   after alloying, performing cooling at an average cooling rate of 10° C./sec or higher over a temperature region down to 300° C.; and   performing tempering in such a manner that Expression (1) is satisfied:
   9000≦( A+ 273)×{log( B/ 3600)+20)}≦13500  Expression (1)
 
   
       wherein, A denotes tempering temperature (° C.), and B denotes tempering time (seconds). 
     
     
         11 : A method for producing the high-strength galvannealed steel sheet according to  claim 1 , the method comprising, in order:
 coiling, at a temperature of 500° C. or higher, a steel sheet satisfying the steel components of the base steel sheet;   keeping in a temperature region of 500° C. or higher for 80 minutes or more;   pickling-cold rolling such that there remain 4 μm or more of the average depth d of the internal oxide layer;   oxidizing at an air ratio in the range of 0.9 to 1.4, in an oxidation zone;   soaking within a range Ac 3  point to (Ac 3  point+100° C.), in a reduction zone;   after the soaking, performing cooling at an average cooling rate of 5° C./sec or higher over a range down to 600° C.;   setting to 20 seconds or less a keeping time in a temperature region of 480° C. or lower until immersion in a galvanizing bath;   after alloying, cooling at an average cooling rate of 10° C./sec or higher over a temperature region down to 300° C., and thereafter, cooling at an average cooling rate of 5° C./sec or lower over a temperature region from 300° C. to 150° C.   
     
     
         12 : A method for producing the high-strength galvannealed steel sheet according to  claim 1 , the method comprising, in order:
 coiling, at a temperature of 500° C. or higher, a steel sheet satisfying steel components of the base steel sheet;   keeping in a temperature region of 500° C. or higher for 80 minutes or more;   pickling-cold rolling such that there remain 4 μm or more of the average depth d of the internal oxide layer;   oxidizing at an air ratio in a range of 0.9 to 1.4, in an oxidation zone;   soaking within a range Ac 3  point to (Ac 3  point+100° C.), in a reduction zone;   after the soaking, performing cooling at an average cooling rate of 5° C./sec or higher over a range down to 600° C.;   setting to 20 seconds or less a keeping time in a temperature region of 480° C. or lower until immersion in a galvanizing bath;   after alloying, performing cooling at an average cooling rate of 10° C./sec or higher over a temperature region down to 300° C.; and   performing tempering in such a manner that Expression (1) is satisfied:
   9000≦( A+ 273)×{log( B/ 3600)+20)}≦13500  Expression (1)
 
   
       wherein A denotes tempering temperature (° C.), and B denotes tempering time (seconds).