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

Abstract

Provided is a steel sheet and a method for manufacturing the same, the steel sheet, which can be used for automobile parts and the like, having superb bendability, and excellent balance of strength and ductility and of strength and hole expansion ratio.

1 . A high strength steel sheet having excellent 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, tempered martensite, bainite, retained austenite, ferrite, and an unavoidable structure, wherein the high strength steel sheet satisfies the following [Relational Expression 1] and [Relational Expression 2].
   1.1≤[Si+Al] F /[Si+Al] γ ≤3.0  [Relational Expression 1]
 
   in the above Relational Expression 1, [Si+Al] F  is an average total content (wt %) of Si and Al included in the ferrite, and [Si+Al] γ  is an average total content (wt %) of Si and Al included in the retained austenite.
     T (γ)/ V (γ)≥0.1  [Relational Expression 2]
 
   in the above Relational Expression 2, T(γ) is a fraction (vol %) of the tempered retained austenite of the steel sheet, and V(γ) is a fraction (vol %) of the retained austenite of the steel sheet.   
     
     
         2 . The high strength steel sheet of  claim 1 , wherein 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 the microstructure of the steel sheet includes 30 to 70 vol % of tempered martensite, 10 to 45 vol % of bainite, 10 to 40 vol % of retained austenite, and 3 to 20 vol % of ferrite. 
     
     
         5 . 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 3] is 22,000 (MPa %) or more, a balance B T·H  of tensile strength and a hole expansion ratio expressed by the following [Relational Expression 4] is 7*10 6  (MPa 2 % 1/2 ) or more, and bendability B R  expressed by the following [Relational Expression 5] is 0.5 to 3.0.
     B   T·E =[Tensile Strength (TS,MPa)]*[Elongation (El, %)]  [Relational Expression 3]
       B   T·H =[Tensile Strength (TS,MPa)] 2 *[Hole Expansion Ratio(HER, %)] 1/2   [Relational Expression 4]
       B   R   =R/t   [Relational Expression 5]
   in the above Relational Expression 5, 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.   
     
     
         6 . A method for manufacturing a high strength steel sheet having excellent workability, comprising:
 providing a cold-rolled steel sheet 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;   heating (primary heating) the cold-rolled steel sheet to a temperature within a range of Ac1 or higher and less than Ac3, and maintaining (primary maintaining) the cold-rolled steel sheet for 50 seconds or more;   cooling (primary cooling) the cold-rolled steel sheet to a temperature within a range (primary cooling stop temperature) of 600 to 850° C. at an average cooling rate of 1° C./s or more;   cooling (secondary cooling) the cold-rolled steel sheet to a temperature within a range of 300 to 500° C. at an average cooling rate of 2° C./s or more, and maintaining (secondary maintaining) the cold-rolled steel sheet in the temperature within a range for 5 seconds or more;   cooling (tertiary cooling) the cold-rolled steel sheet to a temperature within a range (secondary cooling stop temperature) of 100 to 300° C. at an average cooling rate of 2° C./s or more;   heating (secondary heating) the cold-rolled steel sheet to a temperature within a range of 300 to 500° C., and maintaining (tertiary maintaining) the cold-rolled steel sheet in the temperature within a range for 50 seconds or more; and   cooling (quaternary cooling) the cold-rolled steel sheet to room temperature.   
     
     
         7 . The method of  claim 6 , wherein the cold-rolled steel sheet 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%.   
     
     
         8 . The method of  claim 6 , wherein a total content (Si+Al) of Si and Al included in the cold-rolled steel sheet is 1.0 to 6.0 wt %. 
     
     
         9 . The method of  claim 6 , wherein the cold-rolled steel sheet is provided by heating steel slab to 1000 to 1350° C.;
 performing finishing hot rolling in a temperature within a range of 800 to 1000° C.; 
 coiling the hot-rolled steel sheet in a temperature within a range of 300 to 600° C.; 
 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; and 
 cold rolling the hot-rolled annealing heat-treated steel sheet at a reduction ratio of 30 to 90%. 
 
     
     
         10 . The method of  claim 6 , wherein a cooling rate Vol of the primary cooling and a cooling rate Vc2 of the secondary cooling satisfy a relationship of Vol<Vc2.