High strength steel sheet having excellent 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 and a method for manufacturing same, the steel sheet having an excellent balance between strength and ductility, an excellent balance between strength and hole expansion properties, and an excellent yield ratio evaluation score. The steel sheet includes: by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; and as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures.

1 . A high strength steel sheet having excellent workability, comprising:
 by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; and   as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures,   wherein the high strength steel sheet satisfies the following relational expression 1:
   0.03≤[ B]   FM   /[B]   TM ≤0.55  [Relational Expression 1]
 
   where [B] FM  is a content (wt %) of Boron (B) contained in the fresh martensite, and [B] TM  is a content (wt %) of Boron (B) contained in the tempered martensite.   
     
     
         2 . The high strength steel sheet of  claim 1 , further comprising:
 by wt %, one or more of the following (1) to (8):
 (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 4.0% and Ni: 0 to 4.0%; 
 (4) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y, and Mg: 0 to 0.05%; 
 (5) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%; 
 (6) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%; 
 (7) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and 
 (8) Co: 0 to 1.5%. 
   
     
     
         3 . The high strength steel sheet of  claim 1 , wherein the microstructures of the steel sheets include, by volume fraction, 10 to 30% of bainite, 50 to 70% of tempered martensite, 10 to 30% of fresh martensite, 2 to 10% of retained austenite, and 5% or less (including 0%) of ferrite. 
     
     
         4 . The high strength steel sheet of  claim 1 , wherein a balance (B TE ) of tensile strength and elongation expressed by the following relational expression 2 satisfies 3.0*10 6  to 6.2*10 6  (Mpa 2 % 1/2 ), a balance (B TH ) of tensile strength and a hole expansion ratio expressed by the following relational expression 3 satisfies 6.0*10 6  to 11.5*10 6  (Mpa 2 % 1/2 ), and a yield ratio evaluation index (I YR ) expressed by the following relational expression 4 satisfies 0.15 to 0.42:
     B   TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (El, %)] 1/2 ;  [Relational Expression 2]
       B   TH =[Tensile Strength (TS, MPa)] 2 *[Hole Expansion Ratio (HER, %)] 1/2 ; and  [Relational Expression 3]
       I   YR =1−[Yield Ratio (YR)].  [Relational Expression 4]
   
     
     
         5 . A method for manufacturing a high strength steel sheet having excellent workability, the method comprising:
 providing a cold-rolled steel sheet including, by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities;   heating (primarily heating) the cold-rolled steel sheet to a temperature of 700° C. at an average heating rate of 5° C./s or more, heating (secondarily heating) the primarily heated steel sheet to a temperature within a range of Ac3 to 920° C. at an average heating rate of 5° C./s or less, and then maintaining (primarily maintaining) the secondarily heated steel sheet for 50 to 1200 seconds;   cooling (primarily cooling) the primarily maintained steel sheet to a temperature within a range of 200 to 400° C. at an average cooling rate of 1° C./s or more;   heating (tertiarily heating) the primarily cooled steel sheet to a temperature within a range of 350 to 550° C. at an average heating rate of 5° C./s or more, and then maintaining (secondarily maintaining) the tertiarily heated steel sheet for 50 seconds or more; and   cooling (secondarily cooling) the secondarily maintained steel sheet to room temperature at an average cooling rate of 1° C./s or more.   
     
     
         6 . The method of  claim 5 , wherein the steel slab further includes one or more of the following (1) to (8):
 (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 4.0% and Ni: 0 to 4.0%;   (4) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y, and Mg: 0 to 0.05%;   (5) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%;   (6) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%;   (7) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%; and   (8) Co: 0 to 1.5%.   
     
     
         7 . The method of  claim 5 , wherein the cold-rolled steel sheet is provided by:
 heating steel slab to 1000 to 1350° C.;   performing finishing hot rolling at a temperature within a range of 800 to 1000° C.;   coiling the hot-rolled steel sheet at a temperature within a range of 350 to 600° C.;   pickling the coiled steel sheet; and   cold rolling the pickled steel sheet at a reduction ratio of 30 to 90%.