Hot-rolled steel plate having excellent impact toughness of welded zone and method for manufacturing same
AssigneePosco
InventorsWan-Keun Kim, Duk-Gyu Kim
Life of this patent
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Abstract
The present invention relates to a steel sheet suitable as a material for transporting oil and gas, more particularly, to a hot-rolled steel sheet having excellent impact toughness in a welded zone after electric resistance welding (ERW), and to a hot-rolled steel sheet having excellent impact toughness at a cryogenic temperature of −20° C. or lower, that makes the sheet applied in a cold area, and a method for manufacturing same.
Claims9
A hot-rolled steel sheet having excellent impact toughness in a welded zone, comprising, by weight:
- 0.02 to 0.05% of carbon (C), 0.05 to 0.3% of silicon (Si), 0.4 to 1.2% of manganese (Mn), 0.01% or less of phosphorus (P) (excluding 0%), 0.001% or less of sulfur (S) (excluding 0%), 0.02 to 0.05% of aluminum (Al), 0.02 to 0.06% of niobium (Nb), 0.05 to 0.3% of nickel (Ni), 0.005 to 0.03% of titanium (Ti), 0.002 to 0.008% of nitrogen (N), and 0.0015 to 0.004% of calcium (Ca), with a balance of Fe and other unavoidable impurities,
- wherein the steel sheet satisfies the following Relation 1 and includes polygonal ferrite in an area fraction of 35% or more (less than 100%) as a microstructure, and the crystal grain average size of high angle grains having 15° or more as measured by EBSD is 15 μm or less:
- 3≤Mn/Si≤5 (Relation 1)
- wherein Mn and Si refer to a weight content, respectively.
A method for manufacturing a hot-rolled steel sheet having excellent impact toughness in a welded zone, the method comprising:
- preparing a steel slab which includes, by weight:
- 0.02 to 0.05% of carbon (C), 0.05 to 0.3% of silicon (Si), 0.4 to 1.2% of manganese (Mn), 0.01% or less of phosphorus (P) (excluding 0%), 0.001% or less of sulfur (S) (excluding 0%), 0.02 to 0.05% of aluminum (Al), 0.02 to 0.06% of niobium (Nb), 0.05 to 0.3% of nickel (Ni), 0.005 to 0.03% of titanium (Ti), 0.002 to 0.008% of nitrogen (N), and 0.0015 to 0.004% of calcium (Ca), with a balance of Fe and other unavoidable impurities, and satisfies the following Relation 1;
- heating the steel slab in a temperature range of 1150 to 1300° C.;
- finish hot rolling the heated steel slab in a temperature range of Ar3 to 950° C.;
- after the finish hot rolling, performing cooling to a temperature range of 400 to 650° C. at a cooling rate of 10° C./s or more;
- and
- after the cooling, performing coiling:
- 3≤Mn/Si≤5 (Relation 1)
- wherein Mn and Si refer to a weight content, respectively.
Dependencies are inferred from the claim text.