Quenched steel sheet having excellent strength and ductility and method for manufacturing same

Abstract

Disclosed are a quenched steel sheet and a method for manufacturing the same. The quenched steel sheet according to an aspect of the present invention contains, in terms of wt %, C: 0.05˜0.25%, Si: 0.5% or less (excluding 0), Mn: 0.1˜2.0%, P: 0.05% or less, S: 0.03% or less, the remainder Fe, and other unavoidable impurities, wherein a refined structure of the steel sheet comprises 90 volume % or more of martensite with a first hardness and martensite with a second hardness.

1 . A method for manufacturing a quenched steel sheet comprising:
 cold rolling a steel plate comprising, by wt %, carbon (C): 0.05% to 0.25%, silicon (Si): 0.5% or less (with the exception of 0), manganese (Mn): 0.1% to 2.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, iron (Fe) as a residual component thereof, and other unavoidable impurities, and comprising ferrite and pearlite as a microstructure at a reduction ratio of 30% or more;   heating the cold-rolled steel plate to a heating temperature (T*) of Ar3° C. to Ar3+500° C.; and   cooling the heated steel plate,   wherein in the heating of the cold-rolled steel plate, a heating rate (v r , ° C./sec) satisfies relational expression 2, and in the cooling of the heated steel plate, a cooling rate (v c , ° C./sec) satisfies relational expression 3,
     v   r ≥( T*/ 110) 2   [Relational Expression 2]
 
     v   c ≥( T*/ 80) 2 .  [Relational Expression 3]
 
   
     
     
         2 . The method for manufacturing a quenched steel sheet of  claim 1 , wherein in the cooling of the heated steel plate, the high-temperature retention time (t m ) satisfies relational expression 4,
     t   m (8−0.006* T *) 2 .  [Relational Expression 4]
   
     
     
         3 . The method for manufacturing a quenched steel sheet of  claim 1 , wherein the microstructure of the steel plate is an austenite single phase structure having an average diameter of 20 μm or less.