Quenchable steel sheet having high hot press workability and method of manufacturing the same

Abstract

The quenchable steel sheet has an alloy composition including carbon (C) in an amount of 0.15 to 0.30 wt %, silicon (Si) in an amount of 0.05 to 0.5 wt %, manganese (Mn) in an amount of 1.0 to 2.0 wt %, boron (B) in an amount of 0.0005 to 0.0040 wt %, sulfur (S) in an amount of 0.003 wt % or less, phosphorus (P) in an amount of 0.012 wt % or less, one or more selected from among calcium (Ca) in an amount of 0.0010 to 0.0040 wt % and copper (Cu) in an amount of 0.05 to 1.0 wt %, two or more selected from among cobalt (Co), zirconium (Zr) and antimony (Sb), and iron (Fe). Alloy elements are controlled to increasing hot ductility and enabling pressing at 600 to 900° C. so that a tensile strength of 1400 MPa or more and an elongation of 8% or more are obtained after pressing.

1 . A method of making an article using a quenchable steel sheet, comprising:
 (a) making a hot-rolled steel sheet, which comprises:
 heating a steel slab to a temperature of at 1100° C. or higher, the steel slab comprising carbon (C) in an amount of 0.15 to 0.3 wt %, silicon (Si) in an amount of 0.05 to 0.5 wt %, manganese (Mn) in an amount of 1.0 to 2.0 wt %, boron (B) in an amount of 0.0005 to 0.004 wt %, sulfur (S) in an amount of 0.003 wt % or less, phosphorus (P) in an amount of 0.012 wt % or less, iron (Fe) and other inevitable impurities, 
 hot-rolling the steel slab at a temperature of Ar 3  to Ar 3 +50° C.; and 
 winding the hot-rolled steel sheet into a roll; 
   (b) washing the hot-rolled steel sheet using acid, cold-rolling the steel sheet, and plating the steel sheet to inhibit oxide scale from being generated during hot-pressing process;   (c) hot pressing the cold-rolled, plated steel sheet at a temperature of at 600 to 900° C. to make an article having a desired shape; and   (d) cooling the hot pressed article, wherein that the article has a tensile strength of 1400 MPa or more and an elongation of 8% or more.   
     
     
         2 . The method of  claim 1 , wherein the plated steel sheet comprises an Al—Si plated layer. 
     
     
         3 . The method of  claim 1 , wherein the plated steel sheet comprises a plated layer with a thickness of 10 μm to 30 μm. 
     
     
         4 . The method of  claim 1 , wherein, in the step (b), the cold-rolling is performed at a reduction ratio of about 50%. 
     
     
         5 . The method of  claim 1 , wherein, in the step (c), the hot-pressing comprises:
 heating the plated steel sheet to a temperature of 700° C. or higher and hot-pressing the plated steel, and   cooling the hot-pressed article within a mold.