Ultra-high strength steel sheet having excellent phosphatability and hole expandability and method for manufacturing same

Abstract

Provided is an ultra-high-strength steel sheet having high phosphatability and hole expandability, including, by wt %, carbon (C): 0.08% to 0.2%, silicon (Si): 0.05% to 1.3%, manganese (Mn): 2.0% to 3.0%, phosphorus (P): 0.001% to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.3% to 1.2%, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.01% to 0.05%, nitrogen (N): 0.001% to 0.01%, and a balance of iron (Fe) and inevitable impurities, satisfying 3.4≤Ti/N≤10, 1.0≤Mn/(Si+Cr), and 0.7≤Mn*/(Si*+Cr*)≤Mn/(Si+Cr) where Ti, N, Mn, Si, and Cr refer to a weight percent (wt %), and Mn*, Si*, and Cr* refer to an average of values obtained by GDS component analysis from the surface to the 0.1 μm position in the thickness direction.

1 . An ultra-high-strength steel sheet having high phosphatability and hole expandability, comprising, by wt %, carbon (C): 0.08% to 0.2%, silicon (Si): 0.05% to 1.3%, manganese (Mn): 2.0% to 3.0%, phosphorus (P): 0.001% to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.3% to 1.2%, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.01% to 0.05%, nitrogen (N): 0.001% to 0.01%, and a balance of iron (Fe) and inevitable impurities,
 wherein titanium (Ti) and nitrogen (N) satisfy Formula 1 below, manganese (Mn), silicon (Si), and chromium (Cr) satisfy Formula 2 below, contents of manganese (Mn), silicon (Si), and chromium (Cr) in a surface layer ranging from a surface to a 0.1 μm position in a thickness direction satisfy Formula 3 below, and the ultra-high-strength steel sheet has a yield ratio of 0.8 or greater,
   3.4≤Ti/N≤10  [Formula 1]
 
   1.0≤Mn/(Si+Cr)  [Formula 2]
 
   0.7≤Mn*/(Si*+Cr*)≤Mn/(Si+Cr)  [Formula 3]
 
   where in Formulas 1 and 3, each of Ti, N, Mn, Si, and Cr refers to a weight percent (wt %) of the element, and in Formula 3, each of Mn*, Si*, and Cr* refers to an average of values obtained by GDS component analysis from the surface to the 0.1 μm position in the thickness direction.   
     
     
         2 . The ultra-high-strength steel sheet of  claim 1 , further comprising, by wt %, at least one of niobium (Nb): 0.01% to 0.05%, molybdenum (Mo): 0.01% to 0.20%, vanadium (V): 0.01% to 0.20%, and tungsten (W): 0.01% to 0.20%, wherein the ultra-high-strength steel sheet satisfies Formula 4 below,
   0.01≤Nb+0.2(Mo+V+W)≤0.05  [Formula 4]
   where each of Nb, Mo, V, and W refers to a weight percent (wt %) of the element.   
     
     
         3 . The ultra-high-strength steel sheet of  claim 1 , wherein the ultra-high-strength steel sheet has a microstructure comprising martensite or tempered martensite in an area fraction of 50% to 80%, bainite in an area fraction of 10% to 30%, retained austenite in an area fraction of less than 5%, and a balance of ferrite. 
     
     
         4 . The ultra-high-strength steel sheet of  claim 1 , wherein the ultra-high-strength steel sheet has a value of tensile strength (MPa)×hole expanding ratio (HER) within a range of 40000 or greater. 
     
     
         5 . The ultra-high-strength steel sheet of  claim 1 , wherein the ultra-high-strength steel sheet is a cold-rolled steel sheet or a hot-dip galvanized steel sheet. 
     
     
         6 . A method for manufacturing an ultra-high-strength steel sheet having high phosphatability and hole expandability, the method comprising:
 preparing a steel material comprising, by wt %, carbon (C): 0.08% to 0.2%, silicon (Si): 0.05% to 1.3%, manganese (Mn): 2.0% to 3.0%, phosphorus (P): 0.001% to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.3% to 1.2%, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.01% to 0.05%, nitrogen (N): 0.001% to 0.01%, and a balance of iron (Fe) and inevitable impurities, wherein titanium (Ti) and nitrogen (N) satisfy Formula 1 below, and manganese (Mn), silicon (Si), and chromium (Cr) satisfy Formula 2 below;   hot rolling and cold rolling the steel material to manufacture a cold-rolled steel sheet;   annealing the cold-rolled steel sheet at 800° C. to 850° C.;   rapidly cooling the annealed cold-rolled steel sheet to a temperature range of a martensite start temperature (Ms) to a bainite start temperature (Bs) and then maintaining the cold-rolled steel sheet;   after the maintaining, cooling the cold-rolled steel sheet at a rate of 10° C./min to 50° C./min; and   after the cooling, removing surface oxides from the cold-rolled steel sheet,   wherein the maintaining of the cold-rolled steel sheet is performed for a period of time satisfying Formula below, and the removing of the surface oxides is performed in conditions satisfying Formula 6 below,
   3.4≤Ti/N≤10  [Formula 1]
 
   1.0≤Mn/(Si+Cr)  [Formula 2]
 
   300<4729+71C+25Mn−16Si+117Cr−20.1T+0.01991 2 ≤500  [Formula 5]
 
   (HCl concentration×HCl temperature)/(1.33+Mn+7.4Si+0.8Cr)×(47+2.1Mn+13.9Si+4.3Cr)≥1  [Formula 6]
 
   where in Formulas 1 to 6, each of Ti, N, Mn, Si, Cr, and C refers to a weight percent (wt %) of the element, and T in Formula 5 refers to a rapid cooling stop temperature (° C.), and a value calculated by Formula 5 is in seconds.   
     
     
         7 . The method of  claim 6 , wherein the steel material further comprising, by wt %, at least one of niobium (Nb): 0.01% to 0.05%, molybdenum (Mo): 0.01% to 0.20%, vanadium (V): 0.01% to 0.20%, and tungsten (W): 0.01% to 0.20%, and the steel material satisfies Formula 4 below,
   0.01≤Nb+0.2(Mo+V+W)≤0.05  [Formula 4]
   where each of Nb, Mo, V, and W refers to a weight percent (wt %) of the element.   
     
     
         8 . The method of  claim 6 , wherein the annealing of the cold-rolled steel sheet is performed in an environment having a dew point of −35° C. to −50° C. 
     
     
         9 . The method of  claim 6 , wherein the rapid cooling of the annealed cold-rolled steel sheet is performed at a cooling rate of 100° C./min to 600° C./min. 
     
     
         10 . The method of  claim 6 , wherein contents of manganese (Mn), silicon (Si), chromium (Cr) in a surface layer ranging from a surface to a 0.1 μm position in a thickness direction of the ultra-high-strength steel sheet satisfy Formula 3 below,
   0.7≤Mn*/(Si*+Cr*)≤Mn/(Si+Cr)  [Formula 3]
 
 where in Formula 3, each of Mn, Si, and Cr refers to a weight percent (wt %) of the element, and each of Mn*, Si*, and Cr* refers to an average of values obtained by GDS component analysis from the surface to the 0.1 μm position in the thickness direction.