High strength steel sheet having superior workability and method for manufacturing same

Abstract

Provided is a steel sheet which can be used for automobile parts and the like, and relates to a steel sheet having a superior balance of strength and ductility and strength and hole expansion ratio and superior bending formability, and a method for manufacturing same.

1 . A high strength steel sheet having superior workability, comprising:
 by wt %, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, a balance of Fe, and unavoidable impurities; and   as microstructures, 30 to 70 vol % of tempered martensite, 10 to 45 vol % of bainite, 10 to 40 vol % of retained austenite, 3 to 20 vol % of ferrite, and an unavoidable structures,   wherein the high-strength steel sheet satisfies the following [Relational Expression 1]
   1.02≤[Si+Al] F /[Si+Al] av ≤1.45  [Relational Expression 1]
 
   where [Si+Al] F  is an average total content (wt %) of Si and Al included in the ferrite, and [Si+Al] av  is an average total content (wt %) of Si and Al included in the retained austenite.   
     
     
         2 . The high strength steel sheet of  claim 1 , further comprising:
 one or more of the following (1) to (9):   (1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5%;   (2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%;   (3) one or more of Cu: 0 to 4.5% and Ni: 0 to 4.5%;   (4) B: 0 to 0.005%;   (5) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y, and Mg: 0 to 0.05%;   (6) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%;   (7) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%;   (8) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and   (9) Co: 0 to 1.5%.   
     
     
         3 . The high strength steel sheet of  claim 1 ,
 wherein a total content (Si+Al) of Si and Al is 1.0 to 6.0 wt %.   
     
     
         4 . The high strength steel sheet of  claim 1 , wherein a balance B T·E  of tensile strength and elongation expressed by the following [Relational Expression 2] is 22,000 (MPa %) or more, a balance B T·H  of tensile strength and hole expansion ratio expressed by the following [Relational Expression 3] is 7*10 6  (MPa 2 % 1/2 ) or more, and bendability B R  expressed by the following [Relational Expression 4] is 0.5 to 3.0,
     B   T·E =[Tensile Strength(TS, MPa)] 2 *[Elongation(El, %)]   [Relational Expression 2]
       B   T·H =[Tensile Strength(TS, MPa)] 2 *[Hole Expansion Ratio(HER, %)] 1/2   [Relational Expression 3]
       B   R   =R/t   [Relational Expression 4]
   where R is a minimum bending radius (mm) at which cracks do not occur after a 90° bending test, and t is a thickness (mm) of the steel sheet.   
     
     
         5 . A method for manufacturing a high strength steel sheet having superior workability, the method comprising: heating and hot rolling a steel slab including, by wt %, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, a balance of Fe, and unavoidable impurities;
 coiling the hot-rolled steel sheet;   performing hot-rolled annealing heat treatment on the coiled steel sheet in a temperature within a range of 650 to 850° C. for 600 to 1700 seconds;   cold rolling the hot-rolled annealing heat-treated steel sheet;   heating (primarily heating) the cold-rolled steel sheet to a temperature within a range of Ac1 or higher and less than Ac3 at an average temperature increase rate of 5° C./s or more, and maintaining (primarily maintaining) the primarily heated steel sheet for 50 seconds or more;   cooling (primarily cooling) the primarily heated steel sheet to a temperature within a range of 100 to 300° C. at an average cooling rate of 1° C./s or more;   heating (secondarily heating) the primarily cooled steel sheet to a temperature within a range of 300 to 500° C., and maintaining (secondarily maintaining) the primarily cooled steel sheet for 50 seconds or more; and   cooling (secondarily cooling) the primarily cooled steel sheet to room temperature.   
     
     
         6 . The method of  claim 5 , wherein the steel slab further includes one or more of the following (1) to (9):
 (1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5%;   (2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%;   (3) one or more of Cu: 0 to 4.5% and Ni: 0 to 4.5%;   (4) B: 0 to 0.005%;   (5) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y, and Mg: 0 to 0.05%;   (6) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%;   (7) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%;   (8) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and   (9) Co: 0 to 1.5%.   
     
     
         7 . The method of  claim 5 , wherein a total content (Si+Al) of Si and Al included in the steel slab is 1.0 to 6.0 wt %. 
     
     
         8 . The method of  claim 5 , wherein the steel slab is heated to a temperature within a range of 1000 to 1350° C., and is subjected to finish hot rolling in a temperature within a range of 800 to 1000° C. 
     
     
         9 . The method of  claim 5 , wherein the hot-rolled steel sheet is coiled in a temperature within a range of 300 to 600° C. 
     
     
         10 . The method of  claim 5 , wherein a reduction ratio of the cold rolling is 30 to 90%. 
     
     
         11 . The method of  claim 5 , wherein a cooling rate of the secondary cooling is 1° C./s or more.