High-strength plated steel sheet having excellent plating properties, workability, and delayed fracture resistance, and method for producing same

Abstract

The high-strength plated steel sheet of the present invention has a plated layer on the surface of a base steel sheet and contains predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/ 4 of the base steel sheet, where t is a sheet thickness of the base steel sheet: and a hard layer containing martensite, bainite, and ferrite in predetermined ranges. The average depth D of the soft layer is 20 μm or greater, and the average depth d of an internal oxide layer is 4 μm or greater and smaller than D.

1 . A high-strength plated steel sheet, comprising:
 a hot-dip galvanized or galvannealed layer on a surface of a base steel sheet,   wherein
 the base steel sheet comprises, in mass %, 
 C: 0.05 to 0.25%; 
 Si: 0.25 to 3%; 
 Mn: 1.5 to 4%; 
 P: more than 0% to 0.1% or less; 
 S: more than 0% to 0.05% or less; 
 Al: 0.005 to 1%; and 
 N: more than 0% to 0.01% or less, 
 the high-strength plated steel sheet sequentiaih has, from an interface of the base steel sheet and the hot-dip galvanized or galvannealed layer towards the base steel sheet: 
 an internal oxide layer containing at least one oxide selected from the group consisting of Si and Mn; 
 a soft layer including the internal oxide and having a Vickers hardness of 90% less of a Vickers hardness at a portion t/ 4  of the base steel sheet, where t is a sheet thickness of the base steel sheet; and 
 a hard layer containing martensite and bainite in a proportion of from 60 area % or more to less than 95 area %, and polygonal ferrite in a proportion of from more than 5 area % to 40 area % or less, 
   an average depth D of the soft layer is 20 μm or greater,   an average depth d of the internal oxide layer is 4 μm or greater and smaller than the D, and   the high strength plated steel sheet has a tensile strength of 980 MPa or higher.   
     
     
         2 . The high-strength plated steel sheet according to  claim 1 , wherein the base steel sheet further contains, in mass %, at least one of (a) to (c):
 (a) at least one 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 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 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 plated 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 D>2d. 
     
     
         4 . The high-strength plated 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 D>2d. 
     
     
         5 . A method for producing the high strength plated steel sheet according to  claim 1 , the method comprising:
 coiling, at a temperature of 600° C. or higher, a steel sheet containing, in mass %,
 C: 0.05 to 0.25%, 
 Si: 0.25 to 3%, 
 Mn: 1.5 to 4%, 
 P: more than 0% to 0.1% or less, 
 S: more than 0% to 0.05% or less, 
 Al: 0.005 to 1%, and 
 N: more than 0% to 0.01% or less; 
   subsequently pickling and cold rolling the steel sheet such that the internal oxide layer with the average depth d of 4 μm or more remains;   subsequently oxidizing the steel sheet at an air ratio of 0.9 to 1.4 in an oxidation zone;   subsequently soaking the steel sheet at a temperature of (Ac 1  point +50° C.) to (Ac 3  point 20° C.) in a reduction zone; and   subsequently cooling the steel sheet at an average cooling rate of 5° C./sec or higher down to a cooling stop temperature.   
     
     
         6 . A method for producing the high-strength plated steel sheet according to  claim 1 , the method comprising:
 coiling, at a temperature of 500° C. or higher, a steel sheet containing in mass %.
 C: 0.05 to 0.25%, 
 Si: 0.25 to 3%, 
 Mn: 1.5 to 4%, 
 P: more than 0% to 0.1% or less, 
 S: more than 0% to 0.05% or less, 
 Al: 0.005 to 1%, and 
 N: more than 0% to 0.01% or less: 
   subsequently keeping the steel sheet at a temperature of 500° C. or higher for 80 minutes or longer;   subsequently pickling and cold rolling the steel sheet such that the internal oxide layer with the average depth d of 4 μm or more remains;   subsequently oxidizing the steel sheet at an air ratio of 0.9 to 1.4 in an oxidation zone;   subsequently soaking the steel sheet at a temperature of (Ac 1  point +50° C.) to (Ac 3  point +20° C.) in a reduction zone; and   subsequently cooling the steel sheet at an average cooling rate of 5° C./sec or higher down to a cooling stop temperature.