Plated steel sheet having excellent surface quality, strength and ductility

Abstract

Disclosed are: a plated steel sheet having a plating film on a surface of a hot-rolled steel sheet; and a method for manufacturing the same, wherein the hot-rolled steel sheet comprises, by weight, 0.15 to 0.25% of C, 0.5% or less of Si, 0.5 to 2.0% of Mn, 0.03% or less of P, 0.015% or less of S, 0.05% or less of Al, 0.01% or less of N, 0.05% or less of Ti (excluding 0%), 0.01% or less of B (excluding 0%), a balance of Fe, and inevitable, impurities, satisfies the following relationship 1, and comprises, by area, 10 to 30% of ferrite, 20 to 40% of pearlite, and 35 to 55% of bainite, as a microstructure: 0.235 [C]+0.0158 [Mn]+0.0625 [Si]+0.0423 [Mo]+0.317 [Ti]+1.36 [Nb]≤0.075   [Relationship 1] Where [C], [Mn], [Si], [Mo], [Ti], and [Nb] represent the content (by wt %) of the corresponding elements contained in the steel sheet, respectively.

1 . A plated steel sheet having a plating film on a surface of a hot-rolled steel sheet,
 wherein the hot-rolled steel sheet comprises, by weight, 0.15 to 0.25% of C, 0.5% or less of Si, 0.5 to 2.0% of Mn, 0.03% or less of P, 0.015% or less of S, 0.05% or less of Al, 0.01% or less of N, 0.05% or less of Ti (excluding 0%), 0.01% or less of B (excluding 0%), a balance of Fe, and inevitable impurities, satisfies the following relationship 1, and comprises, by area, 10 to 30% of ferrite, 20 to 40% of pearlite, and 35 to 55% of bainite, as a microstructure:
   0.235 [C]+0.0158 [Mn]+0.0625 [Si]+0.0423 [Mo]+0.317 [Ti]+1.36 [Nb]≤0.075   [Relationship 1]
 
   
       Where [C], [Mn], [Si], [Mo], [Ti], and [Nb] represent the content (by wt %) of the corresponding elements contained in the steel sheet, respectively. 
     
     
         2 . The plated steel sheet according to  claim 1 , wherein a sum of the fractions of the ferrite, pearlite, and bainite is 90 area % or more. 
     
     
         3 . The plated steel sheet according to  claim 1 , wherein an average grain size of the ferrite is 20 μm or less (excluding 0 μm). 
     
     
         4 . The plated steel sheet according to  claim 1 , wherein an average colony size of the pearlite is 30 μm or less (excluding 0 μm). 
     
     
         5 . The plated steel sheet according to  claim 1 , wherein the plating film comprises, by weight, 10% or less of Mg (excluding 0%), 5% or less of Al (excluding 0%), a balance of Zn, and inevitable impurities. 
     
     
         6 . The plated steel sheet according to  claim 1 , having a yield strength of 450 to 600 MPa. 
     
     
         7 . The plated steel sheet according to  claim 1 , wherein a product of a yield strength and an elongation of the plated steel sheet is 8,500 MPa·% or more. 
     
     
         8 . The plated steel sheet according to  claim 1 , wherein the hot-rolled steel sheet is a thin material of less than 2.0 mm sheet thickness. 
     
     
         9 . A method for manufacturing a plated steel sheet, comprising:
 reheating a slab comprising, by weight, 0.15 to 0.25% of C, 0.5% or less of Si, 0.5 to 2.0% of Mn, 0.03% or less of P, 0.015% or less of S, 0.05% or less of Al, 0.01% or less of N, 0.05% or less of Ti (excluding 0%), 0.01% or less of B (excluding 0%), a balance of Fe, and inevitable impurities, and satisfying the following relationship 1, to 1100 to 1300° C.;   finish-rolling the reheated slab to a temperature of Ar3° C. or higher to obtain a hot-rolled steel sheet;   cooling the hot-rolled steel sheet at a rate of Vc to (Vc+30)° C./s defined by the following equation 1, and coiling the cooled hot-rolled steel sheet; and   hot-dip plating by dipping the coiled hot-rolled steel sheet in a hot-dip bath.
   0.235 [C]+0.0158 [Mn]+0.0625 [Si]+0.0423 [Mo]+0.317 [Ti]+1.36 [Nb]≤0.075   [Relationship 1]
 
   
       Where [C], [Mn], [Si], [Mo], [Ti], and [Nb] represent the content (by wt %) of the corresponding elements contained in the steel sheet, respectively.
   Vc=158.0−156.6 [C]+246.6 [Si]−40.32 [Mn]−25.74 [Cr]−73.26 [Ni]−8820 [B]−1483.2 [Ti]+1108.8 [Nb]−291.6 [Mo]−1092.6 [V]  [Equation 1]
 
 
       Where [C], [Si], [Mn], [Cr], [Ni], [B], [Ti], [Nb], [Mo], and [V] represent the content (by wt %) of the corresponding elements contained in the steel sheet, respectively. 
     
     
         10 . The method according to  claim 9 , wherein the finish-rolling is carried out in a range of (FDT−20)° C. to (FDT+20)° C. defined by the following equation (2):
   FDT (° C.)=1002.1−353 [C]+43.9 [Si]−74.1 [Mn]−20.4 [Cu]−19.9 [Cr]−45.6 [Ni]−80 [Mo]  [Equation 2]
 
 
       Where [C], [Si], [Mn], [Cu], [Cr], [Ni], and [Mo] represent the content (by wt %) of the corresponding elements contained in the steel sheet, respectively. 
     
     
         11 . The method according to  claim 9 , wherein the coiling is carried out in a range of (CT−20)° C. to (CT + 20)° C. defined by the following equation 3:
   CT=751.7−357.3 [C]−85.3 [Mn]−35 [Si]−73 [Cr]−36 [Ni]−84.4 [Mo]  [Equation 3]
 
 
       Where [C], [Mn], [Si], [Cr], [Ni], and [Mo] represent the content (by wt %) of the corresponding elements contained in the steel sheet, respectively. 
     
     
         12 . The method according to  claim 9 , wherein the hot-dip bath comprises, by weight, 10% or less of Mg (excluding 0%), 5% or less of Al (excluding 0%), a balance of Zn, and inevitable impurities.