Steel sheet having excellent formability and strain hardening rate
Provided is a steel sheet suitably used for an automobile structural member and the like, and more particularly, to a steel sheet having high strength and excellent formability that may prevent processing defects such as cracks or wrinkles occurring during the press-forming, and a method for producing the same.
1 . A steel sheet comprising: by wt %, 0.10 to 0.16% of carbon (C), 1.0% or less (excluding 0%) of silicon (Si), 1.4 to 2.2% of manganese (Mn), 1.0% or less of chromium (Cr), 0.1% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur(S), 1.0% or less (excluding 0%) of aluminum (sol.Al), 0.01% or less (excluding 0%) of nitrogen (N), 0.05% or less (excluding 0%) of antimony (Sb), and a balance of Fe and unavoidable impurities; and
bainite having an area fraction of 5 to 25%, 3% or more of retained-austenite, and a balance of ferrite and martensite in a microstructure,
wherein the steel sheet satisfies the following Relational Expression 1:
{(C+Si+Al)/((10×(C+Ti+Nb))+(2×Si)+Mn+Cr)/(TS)}×1,000≥0.28[Relational Expression 1]
where each element represents a weight content, and TS represents a tensile strength (MPa).
2 . The steel sheet of claim 1 , wherein the number of retained-austenite having an average grain size of 2 μm or less adjacent to the bainite phase is 80% or more of the total number of retained-austenite.
3 . The steel sheet of claim 1 , wherein the martensite phase is contained in an area fraction of 10 to 30%.
4 . The steel sheet of claim 1 , wherein the steel sheet includes a zinc-based plating layer formed on at least one surface thereof.
5 . The steel sheet of claim 1 , wherein the steel sheet has a tensile strength of 590 MPa or more, and
a relation between a strain hardening index (N1) measured in a strain section of 4 to 6%, a strain hardening index (N4) measured in a strain section of 10 to uniform elongation (%), a tensile strength (TS), a total elongation (TE), and a uniform elongation (UE) satisfies the following Relational Expression 2:
( TS×TE×UE×N 1× N 4)≥14,000[Relational Expression 2]
where a unit is MPa %.