High-strength steel strip having excellent workability, and method for manufacturing same

Abstract

Provided is a high-strength steel strip having excellent workability, and a method for manufacturing the same, and more specifically, to: a high-strength steel strip has excellent yield strength and elongation rate due to having a uniform microstructure, and thus does not suffer cracking when worked; and a method for manufacturing same.

1 . A steel strip comprising, by wt %, 0.05 to 0.15% of C, 0.01 to 1.0% of Si, 1.0 to 2.0% of Mn, 0.005 to 1.0% of Cr, 0.01 to 0.10% of Al, 0.001 to 0.02% of P, 0.001 to 0.01% of S, 0.001 to 0.01% of N, 0.005 to 0.11% of Ti, 0.005 to 0.07% of Nb, and a balance of Fe and unavoidable impurities,
 wherein an R value defined in the following Relational Expression 1 satisfies 0.3 to 1.0,   a surface portion (where t represents a thickness of the steel strip in a range of 0 to t/4 and a central portion (not including t/4) in a range of t/4 to t/2 based on a cross section each contain, by area %, 90% or more of ferrite and bainite in total, less than 5% of pearlite and carbides having a diameter of 0.5 μm or more, and less than 5% of a martensite and austenite (MA) phase, as a microstructure,   a product (YSxT-El) of a yield strength and an elongation of the steel strip is 16,000 MPa·% or more, and   a thickness of the steel strip is 10 mm or more,
   R=[C]*+0.7x[Mn]+8.5×[P]+7.5×[S]−0.9×[Si]−1.5×[Nb]
 
   [C]*=[C]−[C]×Q
 
   Q=([Nb]/93+[Ti]/48)/([C]/12) 
   ([C], [Mn], [P], [S], [Si], [Nb], and [Ti] in Relational Expression 1 represent wt % of the corresponding alloying elements, respectively).   
     
     
         2 . The steel strip of  claim 1 , wherein the thickness of the steel strip is 15 mm or more. 
     
     
         3 . The steel strip of  claim 1 , wherein the pearlite and the carbides having a diameter of 0.5 μm or more is 3% or less and the MA phase is 3% or less, in terms of area % in the central portion of the steel strip. 
     
     
         4 . The steel strip of  claim 1 , wherein the bainite is 20% or less, the pearlite and the carbides having a diameter of 0.5 μm or more is less than 2%, and the MA phase is 3% or less, in terms of area % in the surface portion of the steel strip. 
     
     
         5 . The steel strip of  claim 1 , wherein a difference between an average hardness value and a maximum hardness value of hardness values measured at intervals of 0.5 mm from a point located at 0.5 mm directly below a surface of a specimen to a point located at 0.5 mm directly below a back surface based on an arbitrary line perpendicular to a thickness cross section of the steel strip is 20 Hv or less. 
     
     
         6 . A method for manufacturing a steel strip, the method comprising:
 reheating a steel slab containing, by wt %, 0.05 to 0.15% of C, 0.01 to 1.0% of Si, 1.0 to 2.0% of Mn, 0.005 to 1.0% of Cr, 0.01 to 0.1% of Al, 0.001 to 0.02% of P, 0.001 to 0.01% of S, 0.001 to 0.01% of N, 0.005 to 0.11% of Ti, 0.005 to 0.07% of Nb, and a balance of Fe and unavoidable impurities, and satisfying an R value defined in the following Relational Expression 1 of 0.3 to 1.0;   hot rolling the reheated steel slab in a temperature range of 800 to 1,150° C. at a reduction ratio of 20 to 50% so as to have a thickness of 10 mm or more and performing hot rolling which is finished in a temperature range of Tn−50 to Tn defined in the following Relational Expression 2;   performing first cooling on the hot-rolled steel strip to a temperature range of 450 to 550° C. at a cooling rate equal to or higher than CR Min  defined in the following Relational Expression 3 and then coiling the cooled hot-rolled steel strip; and   performing second cooling on the coiled steel strip,
   R=[C]*+0.7×[Mn]+8.5×[P]+7.5×[S]−0.9×[Si]−1.5×[Nb]
 
   [C]*=[C]−[C]×Q
 
   Q=([Nb]/93+[Ti]/48)/([C]/12)  [Relational Expression 1]
 
   ([C], [Mn], [P], [S], [Si], [Nb], and [Ti] in Relational Expression 1 represent wt % of the corresponding alloying elements, respectively)
   Tn=730+92×[C]+70×[Mn]+45×[Cr]+650×[Nb]+410×[Ti]−80×[Si]−1.4×( t− 8)  [Relational Expression 2]
 
   (in Relational Expression 2, a unit of Tn is ° C., and [C], [Mn], [Cr], [Nb], [Ti], and [Si] represent wt % of the corresponding alloying elements, respectively)   (t in Relational Expression 2 is a thickness (mm) of a final rolled strip)
   CR Min  =76.6−157×[C]−25.2×[Si]−14.1×[Mn]−27.3×[Cr]+61×[Ti]+448×[Nb]  [Relational Expression 3]
 
   (in Relational Expression 3, a unit of CR Min  is ° C./s, and [C], [Si], [Mn], [Cr], [Ti], and [Nb] represent wt % of the corresponding alloying elements, respectively).   
     
     
         7 . The method for manufacturing a steel strip of  claim 6 , wherein the reheating is performed in a temperature range of 1,200 to 1,350° C. 
     
     
         8 . The method for manufacturing a steel strip of  claim 6 , wherein during the first cooling, the cooling rate is 80° C./sec or less. 
     
     
         9 . The method for manufacturing a steel strip of  claim 6 , wherein during the second cooling, air cooling or water cooling is performed to a temperature range of room temperature to 200° C.