High-strength steel sheet having superior workability and manufacturing method therefor

Abstract

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

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, ferrite which is a soft structure, and tempered martensite, bainite, and retained austenite which are hard structures,   wherein the high-strength steel sheet satisfies the following [Relational Expression 1]
   0.4≤[ H ] F /[ H ] TM+B+γ ≤0.9  [Relational Expression 1]
 
   in the above Relational Expression 1, [H] F  and [H] TM+B+γ  are nanohardness values measured using a nanoindenter, [H] F  is an average nanohardness value Hv of the ferrite which is the soft structure, [H] TM+B+γ  is the average nanohardness value Hv of the tempered martensite, the bainite, and the retained austenite which are the hard structures.   
     
     
         2 . The high-strength steel sheet of  claim 1 , including 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, by volume fraction, 30 to 70% of tempered martensite, 10 to 45% of bainite, 10 to 40% of retained austenite, 3 to 20% of ferrite, and unavoidable microstructure components. 
     
     
         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 2] 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 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)]*[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.   
     
     
         6 . A manufacturing method for high-strength steel sheet having superior workability, 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;   coiling up the hot-rolled steel sheet into a coil;   performing a 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 (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 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 (secondary heating) the primarily cooled steel sheet to a temperature within a range of 300 to 500° C. at an average temperature increase rate of 5° C./s or more, and maintaining (secondarily maintaining) the primarily cooled steel sheet for 50 seconds or more; and   cooling (secondarily cooling) the secondarily heated steel sheet to room temperature.   
     
     
         7 . The manufacturing method of  claim 6 , 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%.   
     
     
         8 . The manufacturing method of  claim 6 , wherein a total content (Si+Al) of Si and Al included in the steel slab is 1.0 to 6.0 wt %. 
     
     
         9 . The manufacturing method of  claim 6 , 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. 
     
     
         10 . The manufacturing method of  claim 6 , wherein the hot-rolled steel sheet is coiled at a temperature within a range of 300 to 600° C. 
     
     
         11 . The manufacturing method of  claim 6 , wherein a reduction ratio of the cold rolling is 30 to 90%. 
     
     
         12 . The manufacturing method of  claim 6 , wherein a cooling rate of the secondary cooling is 1° C./s or more.