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

Abstract

The present invention relates to a high-strength plated steel sheet having a plated layer on a surface of a base steel sheet, contains predetermined steel compositions, and includes, in order from a base steel sheet side of an interface between the base steel sheet and the plated layer: a soft layer having a Vickers hardness of 90% or less of a Vickers hardness of a t/4 portion of the base steel sheet, where t is a thickness of the base steel sheet, and a hard layer having a structure containing martensite and bainite as main components. The soft layer has an average depth D of 20 μm or more, an internal oxide layer has an average depth d of 4 μm or more and less than D.

1 . A high-strength plated steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on a surface of a base steel sheet, wherein
 (1) the base steel sheet contains, by mass %,
 C: 0.05 to 0.25%, 
 Si: 0.5 to 2.5%, 
 Mn: 2.0 to 4%, 
 P: greater than 0% and less than or equal to 0.1%, 
 S: greater than 0% and less than or equal to 0.05%, 
 Al: 0.01 to 0.1%, and 
 N: greater than 0% and less than or equal to 0.01%, 
   a balance of the base steel sheet consists of an iron and inevitable impurities,   (2) the base steel sheet includes, in order from a base steel sheet side of an interface between the base steel sheet and the plated layer,
 an internal oxide layer containing at least an oxide selected from the group consisting of Si and Mn, 
 a soft layer including the internal oxide layer and having a Vickers hardness of 90% or less of a Vickers hardness of a t/4 portion of the base steel sheet, where t is a thickness of the base steel sheet, and 
 a hard layer having a structure containing martensite and bainite as main components, wherein 
   the soft layer has an average depth D of 20 μm or more,   the internal oxide layer has an average depth d of 4 μm or more and less than D, and   the plated steel sheet has a tensile strength of 980 MPa or more.   
     
     
         2 . A high-strength plated steel sheet according to  claim 1 , wherein
 the base steel sheet further contains, by mass %, at least a material selected from the group consisting of
 Cr: greater than 0% and less than or equal to 1%, 
 Mo: greater than 0% and less than or equal to 1%, 
 B: greater than 0% and less than or equal to 0.01%, 
 Ti: greater than 0% and less than or equal to 0.2%, 
 Nb: greater than 0% and less than or equal to 0.2%, 
 V: greater than 0% and less than or equal to 0.2%, 
 Cu: greater than 0% and less than or equal to 1%, and 
 Ni: greater than 0% and less than or equal to 1%. 
   
     
     
         3 . A 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 a relationship of D>2d. 
     
     
         4 . A method for producing the high-strength plated steel sheet according to  claim 1 , the method comprising:
 a hot rolling step of coiling a steel sheet satisfying steel compositions in said base steel sheet at 600° C. or more;   a step of pickling and cold rolling the steel sheet to cause a remaining internal oxide layer to have an average depth d of 4 μm or more;   a step of oxidizing the steel sheet at an air-fuel ratio of 0.9 to 1.4 in an oxidizing zone; and   a step of soaking the steel sheet in a range from point Ac 3  to point Ac 3 +100° C. in a reducing zone,   wherein these steps are performed in this order.   
     
     
         5 . A method for producing the high-strength plated steel sheet according to  claim 1 , the method comprising:
 a hot rolling step of coiling a steel sheet satisfying steel compositions in said base steel sheet at 500° C. or more;   a step of keeping a temperature of the steel sheet at 500° C. or more for 80 minutes or longer;   a step of pickling and cold rolling the steel sheet to cause a remaining internal oxide layer to have an average depth d of 4 μm or more;   a step of oxidizing the steel sheet at an air-fuel ratio of 0.9 to 1.4 in an oxidizing zone; and   a step of soaking the steel sheet in a range from point Ac 3  to point Ac 3 +100° C. in a reducing zone,   wherein these steps are performed in this order.