Ultra-high strength hot-rolled steel sheet having excellent bending workability and method for manufacturing same

Abstract

Provided is an ultra-high strength hot-rolled steel sheet which is mainly used for components requiring high strength and excellent bending workability. The ultra-high strength hot-rolled steel sheet comprises: 0.1 to 0.25 wt % of C; 0.01 to 0.2 wt % of Si; 0.5 to 2.0 wt % of Mn; 0.005 to 0.02 wt % of P; 0.001 to 0.01 wt % of S; and a balance of Fe and other inevitable impurities. The ultra-high strength hot-rolled steel sheet further comprises 0.001 to 0.35 wt % of at least one element selected from the group consisting of Ti, Nb, Mo, Cr, and B, and the following Relational Expression 1 is satisfied, relational Expression 69.2-311.5[C]-0.1[Si]-4.0[Mn]-5.3[Cr]-2.6[Ni]-6.6[Ti]-660.6 [B]-39[P]≥0, where [C], [Si], [Mn], [Cr], [Ni], [Ti], [B], and [P] refer to wt % of the content of each element.

1 . An ultra-high strength hot-rolled steel sheet having excellent bending workability, comprising:
 0.1 to 0.25 wt % of C;   0.01 to 0.2 wt % of Si;   0.5 to 2.0 wt % of Mn;   0.005 to 0.02 wt % of P;   0.001 to 0.01 wt % of S; and   a balance of Fe and other inevitable impurities,   wherein the ultra-high strength hot-rolled steel sheet further comprises 0.001 to 0.35 wt % of at least one element selected from the group consisting of Ti, Nb, Mo, Cr, and B, and the following Relational Expression 1 is satisfied,
   69.2-311.5[C]-0.1[Si]-4.0[Mn]-5.3[Cr]-2.6[Ni]-6.6[Ti]-660.6[B]-39[P]≥0,  [Relational Expression 1]
 
   where [C], [Si], [Mn], [Cr], [Ni], [Ti], [B], and [P] refer to wt % of the content of each element.   
     
     
         2 . The ultra-high strength hot-rolled steel sheet of  claim 1 , wherein tensile strength of the ultra-high strength hot-rolled steel sheet is 1 GPa or higher, and tensile strength×elongation (TS×T-EL) is 10,000 or greater. 
     
     
         3 . The ultra-high strength hot-rolled steel sheet of  claim 1 , wherein bending workability (R/t) of the ultra-high strength hot-rolled steel sheet satisfies the following Relational Expression 2,
   bending workability( R/t )≤(tensile strength×0.00517−2.60345),  [Relational Expression 2]
   where R is the minimum bending radius at which cracking does not occur after 90° bending testing, and t is steel sheet thickness.   
     
     
         4 . The ultra-high strength hot-rolled steel sheet of  claim 1 , wherein the microstructure of the ultra-high strength hot-rolled steel sheet includes, by area fraction %, martensite of 95% or more, and a second phase of less than 5%. 
     
     
         5 . A method for manufacturing an ultra-high strength hot-rolled steel sheet having excellent bending workability, comprising:
 preparing a slab comprising 0.1 to 0.25 wt % of C, 0.01 to 0.2 wt % of Si, 0.5 to 2.0 wt % of Mn, 0.005 to 0.02 wt % of P, 0.001 to 0.01 wt % of S, and a balance of Fe and other inevitable impurities, wherein the slab further comprises 0.001 to 0.35 wt % of at least one element selected from the group consisting of Ti, Nb, Mo, Cr, and B, and the following Relational Expression 1 is satisfied;   reheating the slab at a temperature of 1,100-1,300° C.;   manufacturing a hot-rolled steel sheet by finish hot rolling the reheated slab at a finish hot rolling temperature of 850-1,000° C.;   cooling the hot-rolled steel sheet at a cooling rate of 100-300° C./s, wherein the following Relational Expression 3 is satisfied; and   coiling the cooled steel sheet at a coiling temperature of 350° C. or lower,
   69.2-311.5[C]-0.1[Si]-4.0[Mn]-5.3[Cr]-2.6[Ni]-6.6[Ti]-660.6[B]-39[P]≥0,  [Relational Expression 1]
 
   where [C], [Si], [Mn], [Cr], [Ni], [Ti], [B], and [P] refer to wt % of the content of each element,
   85.3-311.5[C]-0.1[Si]-4.0[Mn]-5.3[Cr]-2.6[Ni]-6.6[Ti]-660.6[B]-39[P]-6.9[cooling rate]≥0,  [Relational Expression 3]
 
   where [C], [Si], [Mn], [Cr], [Ni], [Ti], [B], and [P] refer to wt % of the content of each element, and units of the cooling rate are ° C./s, which refer to cooling rate from finish hot rolling temperature to coiling temperature.   
     
     
         6 . The method of  claim 5 , further comprising: after pickling the coiled hot-rolled steel sheet, reheating the hot-rolled steel sheet at a temperature of 450-480° C. and hot dip galvanizing the hot-rolled steel sheet, forming a galvanized layer on a surface thereof.