High-strength steel sheet having excellent bendability and formability and method for manufacturing same
Provided are a high-strength steel sheet having excellent bendability and formability, and a method for manufacturing same. The steel sheet includes: 0.05 to 0.12% of carbon (C), 2.0 to 3.0% of manganese (Mn), 0.5% or less (excluding 0%) of silicon (Si), 1.0% or less (excluding 0%) of chromium (Cr), 0.1% or less (excluding 0%) of niobium (Nb), 0.1% or less (excluding 0%) of titanium (Ti), 0.0025% or less (excluding 0%) of boron (B), 0.02 to 0.05% of aluminum (sol.Al), 0.05% or less (excluding 0%) of phosphorus (P), 0.01% or less (excluding 0%) of sulfur (S), 0.01% or less (excluding 0%) of nitrogen (N), with a balance of Fe and inevitable impurities, and 35 to 50% of ferrite and 35 to 45% of bainite, and a balance of martensite, the ferrite comprising, by area fraction: 8 to 15% of non-recrystallized ferrite and 27 to 35% of recrystallized ferrite, as a microstructure.
1 . A high-strength steel sheet having excellent bendability and formability, the steel sheet comprising:
by weight, 0.05 to 0.12% of carbon (C), 2.0 to 3.0% of manganese (Mn), 0.5% or less excluding 0% of silicon (Si), 1.0% or less excluding 0% of chromium (Cr), 0.1% or less excluding 0% of niobium (Nb), 0.1% or less excluding 0% of titanium (Ti), 0.0025% or less excluding 0% of boron (B), 0.02 to 0.05% of aluminum (sol.Al), 0.05% or less excluding 0% of phosphorus (P), 0.01% or less excluding 0% of sulfur(S), 0.01% or less excluding 0% of nitrogen (N), with a balance of Fe and inevitable impurities; and
a microstructure comprising, by area fraction, 35 to 50% of ferrite, 35 to 45% of bainite, and a balance of martensite,
wherein the ferrite includes a non-recrystallized ferrite, by area fraction, in a range of 8 to 15% of a total amount of the microstructure, and a recrystallized ferrite, by area fraction, in a range of 27 to 35% of the total amount of the microstructure.
2 . The high-strength steel sheet having excellent bendability and formability of claim 1 , wherein the microstructure further comprises, by 20% or less excluding 0% of a martensite phase.
3 . The high-strength steel sheet having excellent bendability and formability of claim 1 , wherein the microstructure further comprises, by area fraction, 3% or less excluding 0% of a retained austenite phase.
4 . The high-strength steel sheet having excellent bendability and formability of claim 1 , further comprising: a tensile strength of 980 MPa or more, a yield strength of 550 to 650 MPa, and a total elongation of 12% or more.
5 . The high-strength steel sheet having excellent bendability and formability of claim 1 , further comprising: a 3-point bending angle of 90 degrees or more.
6 . The high-strength steel sheet having excellent bendability and formability of claim 1 , further comprising: a thickness of 0.5 to 2.5 mm.
7 . A metho for manufacturing the high-strength steel sheet having excellent bendability and formability of claim 1 , comprising operations of:
preparing a steel slab including by weight:
0.05 to 0.12% of carbon (C), 2.0 to 3.0% of manganese (Mn), 0.5% or less excluding 0% of silicon (Si), 1.0% or less excluding 0% of chromium (Cr), 0.1% or less excluding 0% of niobium (Nb), 0.1% or less excluding 0% of titanium (Ti), 0.0025% or less excluding 0% of boron (B), 0.02 to 0.05% of aluminum (sol.Al), 0.05% or less excluding 0) of phosphorus (P), 0.01% or less excluding 0% of sulfur(S), 0.01% or less excluding 0% of nitrogen (N), with a balance of Fe and inevitable impurities;
heating the steel slab at a temperature within a range of 1100 to 1300° C.;
subjecting the heated steel slab to hot rolling to manufacture a hot-rolled steel sheet;
coiling the hot-rolled steel sheet at a temperature within a range of 400 to 700° C.;
cooling the hot-rolled steel sheet to room temperature after coiling;
subjecting the cooled hot-rolled steel sheet to cold rolling to manufacture a cold-rolled steel sheet;
continuously annealing the cold-rolled steel sheet;
primary cooling the steel sheet to a temperature within a range of 650 to 700° C. at an average cooling rate of 1 to 10° C./s after the continuous annealing; and
secondary cooling the steel sheet to a temperature within a range of 300 to 580° C. at an average cooling rate of 5 to 50° C./s after the primary cooling,
wherein the cold rolling is performed in 7 passes or less, and a total reduction ratio is 55 to 70%.
8 . The method for manufacturing a high-strength steel sheet having excellent bendability and formability of claim 7 , wherein, in the hot rolling, finish hot rolling is performed at an outlet temperature of Ar3 or higher to 1000° C.
9 . The method for manufacturing a high-strength steel sheet having excellent bendability and formability of claim 7 , wherein the cooling after the coiling is performed at a cooling rate of 0.1° C./s or less excluding 0° C./s.
10 . The method for manufacturing a high-strength steel sheet having excellent bendability and formability of claim 7 , wherein the continuous annealing is performed in a facility having a heating zone, a soaking zone, and a cooling zone, wherein the heating zone and the soaking zone are controlled to be in a temperature within a range of 770 to 810° C.
11 . The method for manufacturing a high-strength steel sheet having excellent bendability and formability of claim 7 , further comprising an operation of:
performing an overaging treatment after the secondary cooling,
wherein the overaging treatment is performed for 200 to 800 seconds.
12 . The method for manufacturing a high-strength steel sheet having excellent bendability and formability of claim 7 , wherein when the thickness of the hot-rolled steel sheet is 4 mm or more, the cold rolling is performed in 15 to 20 passes using a reversing mill.