High strength steel sheet having excellent workability and method for manufacturing same
Provided is a steel sheet that may be used for automobile parts and the like, and to a steel sheet having an excellent balance of strength and ductility, an excellent balance of strength and hole expansion ratio, and an excellent yield ratio evaluation index, and a method for manufacturing the same. 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
includes, as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures,
wherein the high strength steel sheet satisfies the following relational expressions 1 to 4:
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;
V (1.2 μm,γ)/ V (γ)≥0.12 [Relational Expression 2]
where V(1.2 μm, γ) is a fraction (vol %) of the retained austenite having an average grain size of 1.2 μm or more, and V(γ) is a fraction (vol %) of the retained austenite of the steel sheet;
V (lath,γ)/ V (γ)≥0.5 [Relational Expression 3]
where V(lath, γ) is a fraction (vol %) of the retained austenite in lath form, and V(γ) is a fraction (vol %) of the retained austenite of the steel sheet; and
T (γ)/ V (γ)≥0.08 [Relational Expression 4]
where T(γ) is a fraction (vol %) of 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 having excellent workability 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 having excellent workability of claim 1 , wherein the microstructures of the steel sheet 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 having excellent workability of claim 1 , wherein a balance (B TE ) of tensile strength and elongation expressed by the following relational expression 5 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 6 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 7 satisfies 0.15 to 0.42:
B TE =[Tensile Strength ( TS,MPa )] 2 *[Elongation ( El ,%)] 1/2; [Relational Expression 5]
B TH =[Tensile Strength ( TS,MPa )] 2 *[Hole Expansion Ratio ( HER ,%)] 1/2 ; and [Relational Expression 6]
I YR =1−[Yield Ratio ( YR )]. [Relational Expression 7]
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 (primary heating) the cold-rolled steel sheet to a temperature of 700° C. at an average heating rate of 5° C./s or more, heating (secondary 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 (primary maintaining) the secondarily heated steel sheet for 50 to 1200 seconds;
cooling (primary cooling) the primarily maintained steel sheet to a temperature within a range of 400 to 600° C. at an average cooling rate of 2 to 100° C./s, and then maintaining (secondary maintaining) the primarily cooled steel sheet for 5 to 600 seconds;
cooling (secondary cooling) the secondarily maintained steel sheet to a temperature within a range of 300 to 500° C. at an average cooling rate of 1 to 100° C./s, and then maintaining (tertiary maintaining) the secondarily cooled steel sheet for 5 to 600 seconds;
cooling (tertiary cooling) the tertiarily maintained steel sheet to a temperature within a range of 200 to 400° C. at an average cooling rate of 2 to 100° C./s;
heating (tertiary heating) the tertiarily cooled steel sheet to a temperature within a range of 400 to 600° C. at an average heating rate of 5 to 100° C./s, and then maintaining (quaternary maintaining) the tertiarily heated steel sheet for 10 to 1800 seconds;
cooling (quaternary cooling) the quaternarily maintained steel sheet to a temperature within a range of 300 to 500° C. at an average cooling rate of 1 to 100° C./s, and then maintaining (quintically maintaining) the quaternarily cooled steel sheet for 10 to 1800 seconds; and
cooling (quintically cooling) the quintically maintained steel sheet to room temperature at an average cooling rate of 1° C./s or more.
6 . The method for manufacturing a high strength steel sheet having excellent workability 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 for manufacturing a high strength steel sheet having excellent workability 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%.
8 . The method for manufacturing a high strength steel sheet having excellent workability of claim 5 , wherein the cooling rate (Vc1) of the primary cooling and the cooling rate (Vc4) of the quaternary cooling satisfies a relation of Vc1>Vc4.