High-hardness steel sheet, and manufacturing method thereof

Abstract

(wherein Mn, Si, Cr, Ni and Mo are a value representing the content of each element by wt %, and C.R. is a value represent cooling rate during cooling a hot rolled steel sheet and the unit thereof is ° C./sec)

1 . A high-hardness steel sheet, the steel sheet provided by cooling a hot rolled steel sheet, comprising:
 carbon (C): 0.05 wt % to 0.3 wt %, silicon (Si): 0.5 wt % or less (excluding 0%), manganese (Mn): 2.5 wt % or less (excluding 0%), chrome (Cr): 1.5 wt % or less (excluding 0%), molybdenum (Mo): 1.0 wt % or less (excluding 0%), nickel (Ni): 1.0 wt % or less (excluding 0%), niobium (Nb): 0.1 wt % or less (excluding 0%), titanium (Ti): 0.1 wt % or less (excluding 0%), vanadium (V): 0.1 wt % or less (excluding 0%), boron (B): 0.01 wt % or less (excluding 0%), aluminum (Al): 0.1 wt % or less (excluding 0%), a balance of iron (Fe) and other unavoidable impurities;   having a minimum content of carbon (C) satisfying Relation (1);   having a microstructure comprising 95 vol. % or more of a martensite phase; and   having a Brinell hardness of 500 HB or more,
   C (a minimum content of carbon (C))≥0.481-0.104Mn-0.035Si-0.088Cr-0.054Ni-0.035Mo-0.0003C.R.  [Relation 1]
 
   where Mn, Si, Cr, Ni, and Mo are values representing the content of each element by wt %, C.R. is a value representing a cooling rate during the cooling a hot-rolled steel sheet, and the unit thereof is ° C./sec.   
     
     
         2 . The high-hardness steel sheet of  claim 1 , wherein the microstructure includes one or two of ferrite and bainite, in an amount of less than 5.0 vol. %, as a second phase structure, other than martensite. 
     
     
         3 . The high-hardness steel sheet of  claim 1 , wherein Relation (1) is derived from Relation (3),
   HB (Brinell hardness)=100.4+830.5*C+86.5*Mn+28.8*Si+73.4*Cr+44.5*Ni+28.8*Mo+0.252*C.R.  [Relation 3]
   where C, Mn, Si, Cr, Ni, and Mo are values representing the content of each element by wt %, C.R. is a value representing a cooling rate during the cooling a hot-rolled steel sheet, and the unit thereof is ° C./sec.   
     
     
         4 . The high-hardness steel sheet of  claim 1 , wherein the content of carbon (C) is 0.19 wt % to 0.3 wt %. 
     
     
         5 . The high-hardness steel sheet of  claim 1 , wherein the content of silicon (Si) is 0.21 wt % to 0.5 wt %. 
     
     
         6 . The high-hardness steel sheet of  claim 1 , wherein the content of manganese is 1.4 wt % to 2.5 wt %. 
     
     
         7 . A method of manufacturing a high-hardness steel sheet, the method of manufacturing a steel sheet, having a microstructure comprising 95 vol. % or more of a martensite phase and a Brinell hardness of 500 HB or more, comprising: hot-rolling and cooling a steel slab including carbon (C): 0.05 wt % to 0.3 wt %, silicon (Si): 0.5 wt % or less (excluding 0%), manganese (Mn): 2.5 wt % or less (excluding 0%), chrome (Cr): 1.5 wt % or less (excluding 0%), molybdenum (Mo): 1.0 wt % or less (excluding 0%), nickel (Ni): 1.0 wt % or less (excluding 0%), niobium (Nb): 0.1 wt % or less (excluding 0%), titanium (Ti): 0.1 wt % or less (excluding 0%), vanadium (V): 0.1 wt % or less (excluding 0%), boron (B): 0.01 wt % or less (excluding 0%), aluminum (Al): 0.1 wt % or less (excluding 0%), a balance of iron (Fe) and other unavoidable impurities, as a hot-rolled steel sheet, wherein a minimum content of carbon (C) satisfies Relation (1),
   C (a minimum content of carbon (C))≥0.481-0.104Mn-0.035Si-0.088Cr-0.054Ni-0.035Mo-0.0003C.R.  [Relation 1]
   where Mn, Si, Cr, Ni, and Mo are values representing the content of each element by wt %, C.R. is a value representing a cooling rate during cooling a hot-rolled steel sheet, and the unit thereof is ° C./sec.   
     
     
         8 . The method of manufacturing a high-hardness steel sheet of  claim 7 , wherein a cooling rate during the cooling the hot-rolled steel sheet is 20° C./sec to 150° C./sec. 
     
     
         9 . The method of manufacturing a high-hardness steel sheet of  claim 7 , wherein a cooling end temperature during the cooling the hot-rolled steel sheet is the Ms point (a martensite transformation start temperature) or below. 
     
     
         10 . The method of manufacturing a high-hardness steel sheet of  claim 7 , wherein the content of carbon (C) is 0.19% to 0.3%. 
     
     
         11 . The method of manufacturing a high-hardness steel sheet of  claim 7 , wherein the content of silicon (Si) is 0.21% to 0.5%. 
     
     
         12 . The method of manufacturing a high-hardness steel sheet of  claim 7 , wherein the content of manganese is 1.4% to 2.5%.