High strength steel sheet having excellent workability and method for manufacturing same
AssigneePosco
InventorsJae-Hoon Lee, Min-Seo Koo, Tae-Oh Lee
Life of this patent
Estimating the term…
Abstract
Provided is a steel sheet and a method for manufacturing 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. The steel sheet includes: 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.
Claims9
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, 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], [Relational Expression 2], and [Relational Expression 3].
- 0.4≤[H]F/[H]TM+B+γ≤0.9 [Relational Expression 1]
- where [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, and [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,
- V(1.2 μm, γ)/V(γ)≥0.1 [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 the 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 the fraction (vol %) of the retained austenite in a lath form, and V(γ) is the fraction (vol %) of the retained austenite of the steel sheet.
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:
- 0.25 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 heated steel sheet for 50 seconds or more;
- cooling (primary cooling) the heated 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 cooled 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 cooled steel sheet in the temperature within a range for 5 seconds or more;
- cooling (tertiary cooling) the cooled 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 tertiarily cooled steel sheet to a temperature within a range of 350 to 550° C., and maintaining (tertiary maintaining) the heated steel sheet in the temperature within a range for 10 seconds or more;
- cooling (quaternary cooling) the heated steel sheet to a temperature within a range of 250 to 450° C., and maintaining (quaternary maintaining) the quaternarily cooled steel sheet in the temperature within a range for 10 seconds or more;
- and
- cooling (fifth cooling) the quaternarily cooled steel sheet to room temperature.
Dependencies are inferred from the claim text.