High-strength and high-formability steel sheet, and method for manufacturing same

Abstract

Provided is a high-strength and high-formability steel sheet and a method of manufacturing the same. The high-strength and high-formability steel sheet according to an embodiment of the present disclosure includes carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities, wherein contents of C, Mn, and Si meet a relationship of XC, wt %+0.066×XSi, wt %+0.043×XMn, wt %≤0.4, and wherein the high-strength and high-formability steel sheet meets a yield strength (YS): 500 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T.EL): 23% or more, and a product of tensile strength and elongation: 23,000 MPa % or more.

1 . A high-strength and high-formability steel sheet comprising carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities,
 wherein contents of C, Mn, and Si meet a relationship of X C, wt % +0.066×X Si, wt % +0.043×X Mn, wt % ≤0.4, and   wherein the high-strength and high-formability steel sheet meets a yield strength (YS): 500 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T.EL): 23% or more, and a product of tensile strength and elongation: 23,000 MPa % or more.   
     
     
         2 . The high-strength and high-formability steel sheet of  claim 1 , wherein the high-strength and high-formability steel sheet has a mixed structure of retained austenite, ferrite, and martensite/tempered martensite,
 wherein an area fraction of ferrite is 20% to 50%,   wherein an area fraction of retained austenite is 5% to 20%, and   wherein an area fraction of martensite/tempered martensite is a remaining area fraction.   
     
     
         3 . The high-strength and high-formability steel sheet of  claim 1 , wherein the high-strength and high-formability steel sheet has a uniform elongation/total elongation ratio of 0.7 or more and less than 1. 
     
     
         4 . The high-strength and high-formability steel sheet of  claim 1 , wherein, when 5% plastic deformation is applied in a direction perpendicular to a rolling direction of the high-strength and high-formability steel sheet, a reduction rate in an area fraction of retained austenite of the high-strength and high-formability steel sheet before and after the application is more than 0% and no more than 50%. 
     
     
         5 . The high-strength and high-formability steel sheet of  claim 1 , further comprising a combination of titanium (Ti), niobium (Nb), and vanadium (V): more than 0 wt % and up to 0.05 wt %. 
     
     
         6 . A method of manufacturing a high-strength and high-formability steel sheet, the method comprising:
 producing a hot-rolled steel sheet by hot rolling a steel material comprising carbon (C): 0.1 wt % to 0.3 wt %, silicon (Si): 1.0 wt % to 2.0 wt %, manganese (Mn): 1.5 wt % to 3.0 wt %, aluminum (Al): more than 0 wt % and up to 0.05 wt %, phosphorus (P): more than 0 wt % and up to 0.02 wt %, sulfur (S): more than 0 wt % and up to 0.005 wt %, nitrogen (N): more than 0 wt % and up to 0.006 wt %, and a balance of iron (Fe) and other unavoidable impurities;   producing a cold-rolled steel sheet by cold rolling the hot-rolled steel sheet;   performing annealing by heating the cold-rolled steel sheet at a heating rate of 1° C./s to 10° C./s to a temperature higher than 780° C. and lower than 840° C., and holding for 50 sec. to 110 sec.;   multi-stage cooling the cold-rolled steel sheet; and   performing post-annealing by heating the cold-rolled steel sheet at a heating rate of 20° C./s or more to a temperature higher than 380° C. and lower than 450° C., and holding for 10 sec. to 240 sec.,   wherein contents of C, Mn, and Si meet a relationship of X C, wt % +0.066×X Si, wt % +0.043×X Mn, wt % ≤0.4.   
     
     
         7 . The method of  claim 6 , wherein the producing of the hot-rolled steel sheet comprises:
 reheating a steel material with the alloy composition at 1,150° C. to 1,250° C.;   producing a hot-rolled steel sheet by hot rolling the reheated steel material at a finishing delivery temperature (FDT) of 850° C. to 1,000° C. with a cumulative reduction ratio of 70% or more and 90% or less;   cooling the hot-rolled steel sheet at a cooling rate of 10° C./s to 30° C./s to 500° C. to 700° C.; and   coiling the hot-rolled steel sheet at 500° C. to 700° C., and   wherein the hot-rolled steel sheet has a mixed structure of ferrite and pearlite, with an area fraction of ferrite being 20% to 50%, and an area fraction of pearlite being a remaining area fraction.   
     
     
         8 . The method of  claim 6 , wherein the multi-stage cooling comprises:
 primarily cooling the cold-rolled steel sheet at a cooling rate of 1° C./s to 10° C./s to 550° C. to 750° C.; and   secondarily cooling the cold-rolled steel sheet at a cooling rate of 50° C./s or more to a temperature higher than 180° C. and lower than 240° C., and holding for 5 sec. to 20 sec.   
     
     
         9 . The method of  claim 6 , wherein the high-strength and high-formability steel sheet manufactured by performing the method:
 meets a yield strength (YS): 500 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T.EL): 23% or more, and a product of tensile strength and elongation: 23,000 MPa % or more;   has a mixed structure of retained austenite, ferrite, and martensite/tempered martensite, with an area fraction of ferrite being 20% to 50%, an area fraction of retained austenite being 5% to 20%, and an area fraction of martensite/tempered martensite being a remaining area fraction; and   has a uniform elongation/total elongation ratio of 0.7 or more and less than 1, and   wherein, when 5% plastic deformation is applied in a direction perpendicular to a rolling direction of the high-strength and high-formability steel sheet, a reduction rate in the area fraction of retained austenite of the high-strength and high-formability steel sheet before and after the application is more than 0% and no more than 50%.