Steel sheet for hot forming, hot-formed member, and method for manufacturing same

Abstract

Disclosed are a high-strength and non-plated steel sheet which is for hot forming and may be suitable for use in automotive structural members that require collision resistance characteristics, a hot-formed member, and a method for manufacturing same.A steel sheet for hot forming and a hot-formed member according to an embodiment of the present disclosure contain, in percent by weight (wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% of nitrogen (N), and 0.03 to 1.0% of niobium (Nb), with the remainder comprising iron (Fe) and inevitable impurities.

1 . A steel sheet for hot forming comprising, in percent by weight (wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb), and the remainder of iron (Fe) and inevitable impurities,
 wherein a microstructure comprises a ferrite phase and 20 vol % or less of a carbonitride.   
     
     
         2 . The steel sheet according to  claim 1 , wherein the ferrite phase has an average grain size of 100 μm or less. 
     
     
         3 . The steel sheet according to  claim 1 , wherein the steel sheet satisfies Expression (1) below:
   0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0   (1)
   (wherein Si, Cr, C, and Mn denote contents (wt %) of the elements, respectively).   
     
     
         4 . The steel sheet according to  claim 1 , wherein a content of Cr is from 3.5 to 5.5%. 
     
     
         5 . The steel sheet according to  claim 1 , further comprising less than 3.0% of nickel (Ni). 
     
     
         6 . The steel sheet according to  claim 1 , further comprising less than 0.1% of phosphorus (P) and less than 0.01% of sulfur (S). 
     
     
         7 . A hot-formed member comprising, in percent by weight (wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb), and the remainder of iron (Fe) and inevitable impurities. 
     
     
         8 . The hot-formed member according to  claim 7 , wherein the hot-formed member satisfies Expression (1) below:
   0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0   (1)
   (wherein Si, Cr, C, and Mn denote contents (wt %) of the elements, respectively).   
     
     
         9 . The hot-formed member according to  claim 7 , wherein an average oxygen content is 20 wt % or less at a point of 0.1 μm depth from the surface. 
     
     
         10 . The hot-formed member according to  claim 7 , wherein the hot-formed member has a yield strength of 1,100 MPa or more and a tensile strength of 1,500 MPa or more. 
     
     
         11 . The hot-formed member according to  claim 7 , wherein a content of Cr is from 3.5 to 5.5%. 
     
     
         12 . The hot-formed member according to  claim 7 , further comprising less than 3.0% of nickel (Ni). 
     
     
         13 . The hot-formed member according to  claim 7 , further comprising less than 0.1% of phosphorus (P) and less than 0.01% of sulfur (S). 
     
     
         14 . A method for manufacturing a hot-formed member, the method comprising:
 preparing a steel sheet for hot forming comprising, in percent by weight (wt %), 0.05 to 0.3% of carbon (C), 0.5 to 3.0% of silicon (Si), 0.1 to 2.0% of manganese (Mn), 3.0 to 9.0% of chromium (Cr), more than 0% and less than 0.2% of nitrogen (N), 0.03 to 1.0% of niobium (Nb), and the remainder of iron (Fe) and inevitable impurities;   heating the steel sheet at a rate of 1 to 1,000° C./sec to a temperature range of Ac3+50° C. to Ac3+200° C. and maintaining for 1 to 1,000 seconds; and   hot-forming the heated and maintained steel sheet and cooling the steel sheet at a rate of 1 to 1000° C./sec to a temperature below Mf.   
     
     
         15 . The method according to  claim 14 , wherein the steel sheet for hot forming satisfies Expression (1) below.
   0.80*Si+0.57*Cr−3.53*C−1.45*Mn−1.9>0   (1)
   (wherein Si, Cr, C, and Mn denote contents (wt %) of the elements, respectively).   
     
     
         16 . The method according to  claim 14 , wherein the steel sheet for hot forming comprises a microstructure comprising a ferrite phase and 20 vol % or less of a carbonitride,
 wherein an average grain size of the ferrite phase is 100 μm or less.   
     
     
         17 . The method according to  claim 14 , wherein a content of Cr in the steel sheet for hot forming is from 3.5 to 5.5%. 
     
     
         18 . The method according to  claim 14 , wherein the steel sheet for hot forming further comprises less than 3.0% of nickel (Ni). 
     
     
         19 . The method according to  claim 14 , wherein the steel sheet for hot forming further comprises less than 0.1% of phosphorus (P) and less than 0.01% of sulfur (S). 
     
     
         20 . The method according to  claim 14 , wherein the preparing of the steel sheet for hot forming comprises:
 reheating a slab in a temperature range of 1,000 to 1,300° C.;   preparing a hot-rolled steel sheet by finish-rolling the reheated slab in a temperature range higher than Ar3 and equal to or lower than 1,000° C.;   coiling the hot-rolled steel sheet in a temperature range higher than Ms and equal to or lower than 850° C.; and   acid-pickling the coiled, hot-rolled steel sheet.   
     
     
         21 . The method according to  claim 20 , further comprising:
 preparing a cold-rolled steel sheet by rolling the acid pickled, hot-rolled steel sheet with a reduction ratio of 30 to 80%; and   continuously annealing the cold-rolled steel sheet in a temperature range of 700 to 900° C.   
     
     
         22 . The method according to  claim 20 , further comprising batch-annealing the coiled, hot-rolled or acid-pickled steel sheet in a temperature range of 500 to 850° C. for 1 to 100 hours.