High-strength cold-rolled steel sheet, high-strength plated steel sheet, and methods for their manufacture

Abstract

A high-strength cold-rolled steel sheet providing a product with a good surface condition after press forming, having excellent bake hardenability and anti room temperature aging property, and having a dual phase structure with a tensile strength of at least 340 MPa is provided. A high-strength cold-rolled steel sheet has a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite and has a hardness distribution of the ferrite phase in an arbitrary cross section having a length of 10 mm in the widthwise direction of the sheet which satisfies the relationship prescribed by (Hv (max) −Hv (ave) )<0.5×(Hv (ave) . Hv (max) is the maximum Vickers hardness of ferrite grains in a region at a distance of from (⅛)t to (¼)t in the thickness direction from the surface when the thickness of the high-strength cold-rolled steel sheet is t, and Hv (ave) is the average Vickers hardness of ferrite grains in this region.

1 . A high-strength cold-rolled steel sheet characterized by having a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite, with the hardness distribution of the ferrite phase in an arbitrary cross section having a length of 10 mm in the widthwise direction of the sheet satisfying the following Equation (1):
     Hv   (max)   −Hv   (ave) <0.5 ×Hv   (ave)   (1)   wherein Hv (max)  is the maximum Vickers hardness of ferrite grains in a region where the distance from the surface in the depth direction is at least (⅛)t and at most (¼)t when the thickness of the high-strength cold-rolled steel sheet is t, and Hv (ave)  is the average Vickers hardness of ferrite grains in this region.   
     
     
         2 . A high-strength cold-rolled steel sheet as set forth in  claim 1  having a steel composition comprising, in mass percent, C: at least 0.0025% and less than 0.04%, Si: at most 0.5%, Mn: at least 0.5% and at most 2.5%, P: at most 0.05%, S: at most 0.01%, sol. Al: at most 0.15%, N: less than 0.008%, Cr: at least 0.02% and at most 2.0%, and a remainder of Fe and impurities. 
     
     
         3 . A high-strength cold-rolled steel sheet as set forth in  claim 2  wherein the steel composition further contains, in mass percent, B: at most 0.003% and/or Mo: at most 1.0%. 
     
     
         4 . A high-strength cold-rolled steel sheet as set forth in  claim 2  wherein the steel composition further contains, in mass percent, Ti: at most 0.1%. 
     
     
         5 . A high-strength plated steel sheet characterized by having a plating layer on the surface of a high-strength cold-rolled steel sheet as set forth in  claim 1 . 
     
     
         6 . A method of manufacturing a high-strength cold-rolled steel sheet having a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite, characterized by comprising the following steps (A) and (B):
 (A) a step of making a steel sheet by subjecting a steel ingot or slab having a steel composition as set forth in  claim 2  to hot rolling and cold rolling; and   (B) a step of subjecting the steel sheet to continuous annealing in which the steel sheet is soaked at a temperature of at least the Ac 1  transformation point and lower than the Ac 3  transformation point, and then it is cooled at a cooling rate of 15-200° C./sec in the temperature range from 650° C. to 450° C. and then cooled at a cooling rate of less than 10° C./sec in the temperature range from temperature T 1  (° C.) given by the following Equation (2) to temperature T 2  (° C.) given by the following Equation (3):
     T   1 (° C.)=445+200×C−50×(Mn+Cr)  (2) 
     T   2 (° C.)=330−2000×C−30×(Mn+Cr)  (3) 
   wherein the symbols for elements in Equation (2) and Equation (3) indicate the contents (in mass percent) of the respective elements in the steel.   
     
     
         7 . A method of manufacturing a high-strength cold-rolled steel sheet having a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite, characterized by comprising the following steps (A) and (C):
 (A) a step of making a steel sheet by subjecting a steel ingot or slab having a steel composition as set forth in  claim 2  to hot rolling and cold rolling; and   (C) a step of subjecting the steel sheet to continuous annealing in which the steel sheet is soaked at a temperature of at least the Ac 3  transformation point and lower than (Ac 3  transformation point+100° C.), and then it is cooled at a cooling rate of 15-200° C./sec in the temperature range from 650° C. to 450° C. and then cooled at a cooling rate of less than 10° C./sec in the temperature range from temperature T 1  (° C.) given by the following Equation (2) to temperature T 2  (° C.) given by the following Equation (3):
     T   1 (° C.)=445+200×C−50×(Mn+Cr)  (2) 
     T   2 (° C.)=330−2000×C−30×(Mn+Cr)  (3) 
   wherein the symbols for elements in Equation (2) and Equation (3) indicate the contents (in mass percent) of the respective elements in the steel.   
     
     
         8 . A method of manufacturing a high-strength plated steel sheet characterized by carrying out plating treatment on a high-strength cold-rolled steel sheet manufactured by a method as set forth in  claim 6 . 
     
     
         9 . A high-strength cold-rolled steel sheet as set forth in  claim 3  wherein the steel composition further contains, in mass percent, Ti: at most 0.1%. 
     
     
         10 . A high-strength plated steel sheet characterized by having a plating layer on the surface of a high-strength cold-rolled steel sheet as set forth in  claim 2 . 
     
     
         11 . A high-strength plated steel sheet characterized by having a plating layer on the surface of a high-strength cold-rolled steel sheet as set forth in  claim 3 . 
     
     
         12 . A high-strength plated steel sheet characterized by having a plating layer on the surface of a high-strength cold-rolled steel sheet as set forth in  claim 4 . 
     
     
         13 . A method of manufacturing a high-strength cold-rolled steel sheet having a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite, characterized by comprising the following steps (A) and (B):
 (A) a step of making a steel sheet by subjecting a steel ingot or slab having a steel composition as set forth in  claim 3  to hot rolling and cold rolling; and   (B) a step of subjecting the steel sheet to continuous annealing in which the steel sheet is soaked at a temperature of at least the Ac 1  transformation point and lower than the Ac 3  transformation point, and then it is cooled at a cooling rate of 15-200° C./sec in the temperature range from 650° C. to 450° C. and then cooled at a cooling rate of less than 10° C./sec in the temperature range from temperature T 1  (° C.) given by the following Equation (2) to temperature T 2  (° C.) given by the following Equation (3):
     T   1 (° C.)=445+200×C−50×(Mn+Cr)  (2) 
     T   2 (° C.)=330−2000×C−30×(Mn+Cr)  (3) 
   wherein the symbols for elements in Equation (2) and Equation (3) indicate the contents (in mass percent) of the respective elements in the steel.   
     
     
         14 . A method of manufacturing a high-strength cold-rolled steel sheet having a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite, characterized by comprising the following steps (A) and (B):
 (A) a step of making a steel sheet by subjecting a steel ingot or slab having a steel composition as set forth in  claim 4  to hot rolling and cold rolling; and   (B) a step of subjecting the steel sheet to continuous annealing in which the steel sheet is soaked at a temperature of at least the Ac 1  transformation point and lower than the Ac 3  transformation point, and then it is cooled at a cooling rate of 15-200° C./sec in the temperature range from 650° C. to 450° C. and then cooled at a cooling rate of less than 10° C./sec in the temperature range from temperature T 1  (° C.) given by the following Equation (2) to temperature T 2  (° C.) given by the following Equation (3):
     T   1 (° C.)=445+200×C−50×(Mn+Cr)  (2) 
     T   2 (° C.)=330−2000×C−30×(Mn+Cr)  (3) 
   wherein the symbols for elements in Equation (2) and Equation (3) indicate the contents (in mass percent) of the respective elements in the steel.   
     
     
         15 . A method of manufacturing a high-strength cold-rolled steel sheet having a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite, characterized by comprising the following steps (A) and (C):
 (A) a step of making a steel sheet by subjecting a steel ingot or slab having a steel composition as set forth in  claim 3  to hot rolling and cold rolling; and   (C) a step of subjecting the steel sheet to continuous annealing in which the steel sheet is soaked at a temperature of at least the Ac 3  transformation point and lower than (Ac 3  transformation point+100° C.), and then it is cooled at a cooling rate of 15-200° C./sec in the temperature range from 650° C. to 450° C. and then cooled at a cooling rate of less than 10° C./sec in the temperature range from temperature T 1  (° C.) given by the following Equation (2) to temperature T 2  (° C.) given by the following Equation (3):
     T   1 (° C.)=445+200×C−50×(Mn+Cr)  (2) 
     T   2 (° C.)=330−2000×C−30×(Mn+Cr)  (3) 
   wherein the symbols for elements in Equation (2) and Equation (3) indicate the contents (in mass percent) of the respective elements in the steel.   A method of manufacturing a high-strength cold-rolled steel sheet having a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite, characterized by comprising the following steps (A) and (C):   (A) a step of making a steel sheet by subjecting a steel ingot or slab having a steel composition as set forth in  claim 4  to hot rolling and cold rolling; and   (C) a step of subjecting the steel sheet to continuous annealing in which the steel sheet is soaked at a temperature of at least the Ac 3  transformation point and lower than (Ac 3  transformation point+100° C.), and then it is cooled at a cooling rate of 15-200° C./sec in the temperature range from 650° C. to 450° C. and then cooled at a cooling rate of less than 10° C./sec in the temperature range from temperature T 1  (° C.) given by the following Equation (2) to temperature T 2  (° C.) given by the following Equation (3):
     T   1 (° C.)=445+200×C−50×(Mn+Cr)  (2) 
     T   2 (° C.)=330−2000×C−30×(Mn+Cr)  (3) 
   wherein the symbols for elements in Equation (2) and Equation (3) indicate the contents (in mass percent) of the respective elements in the steel.   
     
     
         16 . A method of manufacturing a high-strength plated steel sheet characterized by carrying out plating treatment on a high-strength cold-rolled steel sheet manufactured by a method as set forth in  claim 7 .