Ultra-high-strength steel having excellent plasticity and method for manufacturing same
Disclosed is an ultra-high-strength steel having excellent plasticity, comprising in mass percent the chemical elements: C: 0.26-0.30 wt %; Si: 0.8-1.00 wt %; Mn: 2.80-3.30 wt %; Al: 0.04-0.08 wt %; with the balance being Fe and other inevitable impurities. Also disclosed is a manufacturing method for manufacturing the ultra-high-strength steel having excellent plasticity, comprising the following steps: (1) smelting and thin slab continuous casting; (2) heating; (3) hot rolling, wherein an oxide scale on the surface of a hot-rolled steel strip has a thickness of ≤6 μm, and (FeO+Fe 3 O 4 )≤40 wt % in the oxide scale on the surface of the hot-rolled strip steel; (4) acid pickling or acid pickling and cold rolling; and (5) continuous annealing: annealing at 800-920° C. and performing slow cooling at 3-10° C./s to 690-760° C.; performing fast cooling to 250-350° C. at 50-100° C.; and then heating to 360-460° C., maintaining the temperature for 100-400 s and cooling to room temperature.
1 . An ultra-high-strength steel having excellent plasticity, comprising chemical elements in mass percentages of:
C: 0.26-0.30 wt %;
Si: 0.8-1.00 wt %;
Mn: 2.80-3.30 wt %;
Al: 0.04-0.08 wt %;
a balance of Fe and unavoidable impurities;
wherein the ultra-high strength steel having excellent plasticity has a microstructure comprising 20 vol. %-40 vol. % ferrite, 50 vol. %-70 vol. % martensite, and retained austenite, and
wherein the ultra-high strength steel having excellent plasticity has a yield strength of 850-1000 MPa, a tensile strength of 1180-1300 MPa, a uniform elongation of ≥11%; and an elongation at break of 15%-20%.
2 . The ultra-high strength steel having excellent plasticity according to claim 1 , wherein the mass percentages of the chemical elements satisfy at least one of:
C: 0.26-0.28 wt %;
Si: 0.9-1.00 wt %;
Mn: 2.9-3.1 wt %.
3 . The ultra-high strength steel having excellent plasticity according to claim 1 , further comprising at least one of the following chemical elements:
0<Cr≤0.05 wt %;
0<Mo≤0.05 wt %;
0<Nb≤0.03 wt %;
0<Ti≤0.05 wt %;
0<V≤0.03 wt %;
0<B≤0.001 wt %.
4 . The ultra-high strength steel having excellent plasticity according to claim 3 , wherein the mass percentages of the chemical elements satisfy at least one of:
0<Cr≤0.03 wt %;
0<Mo≤0.03 wt %;
0<Nb≤0.01 wt %;
0<Ti≤0.03 wt %;
0<V≤0.01 wt %.
5 . The ultra-high strength steel having excellent plasticity according to claim 1 , wherein among unavoidable impurities: P≤0.01 wt %, S≤0.01 wt %, N≤0.006 wt %.
6 . The ultra-high strength steel having excellent plasticity according to claim 1 , wherein ferrite comprises 90% or more of grains of 10 μm or less, and 60% or more of grains of 5 μm or less.
7 . The ultra-high strength steel having excellent plasticity according to claim 1 , wherein an average grain size of retained austenite is ≤2 μm; and/or an average C content in retained austenite is ≥1.1 wt %.
8 . A manufacturing method for the ultra-high-strength steel having excellent plasticity according to claim 1 , comprising steps:
(1) smelting and thin slab continuous casting: controlling a slab thickness at an exit end in the continuous casting at 55-60 mm;
(2) heating;
(3) hot rolling: thickness of an oxide scale on a surface of a hot-rolled steel strip: ≤6 μm; (FeO+Fe 3 O 4 ) in the oxide scale on the surface of the hot-rolled steel strip: ≤40 wt %;
(4) pickling or pickling+cold rolling;
(5) continuous annealing: annealing at 800-920° C.; slowly cooling to 690-760 20 C. at a cooling rate of 3-10° C./s to obtain a certain proportion of ferrite; rapidly cooling to 250-350° C. at a cooling rate of 50-100° C./s to transform austenite partially to martensite; reheating to 360-460° C., holding for 100-400 s, and finally cooling to room temperature.
9 . The manufacturing method according to claim 8 , wherein in the step (1), a drawing speed in the continuous casting is controlled at 2-5 m/min.
10 . The manufacturing method according to claim 8 , wherein in step (2), the slab is heated to 1200-1300° C.
11 . The manufacturing method according to claim 8 , wherein in the step (3), a finishing rolling temperature is controlled at 860-930° C., and a coiling temperature is 450-600° C.
12 . The manufacturing method according to claim 8 , wherein in step (4), when a step of pickling+cold rolling is employed, an amount of deformation is controlled at 40%-70%.
13 . The manufacturing method according to claim 8 , wherein in step (5), the continuous annealing process is controlled to satisfy at least one of:
annealing temperature of 820-870° C.;
slowly cooling to 700-730° C. at a cooling rate of 3-10° C./s;
rapidly cooling to 270-330° C.;
after rapid cooling, reheating to 400-430° C. and holding for 150-300 s;
controlling a volume content of hydrogen in a reducing atmosphere in a continuous annealing furnace at 10-15%.
14 . The ultra-high strength steel having excellent plasticity according to claim 2 , further comprising at least one of the following chemical elements:
0<Cr≤0.05 wt %;
0<Mo≤0.05 wt %;
0<Nb≤0.03 wt %;
0<Ti≤0.05 wt %;
0<V≤0.03 wt %;
0<B≤0.001 wt %.
15 . The manufacturing method according to claim 8 , wherein the ultra-high-strength steel having excellent plasticity further comprises at least one of the following chemical elements:
0<Cr≤0.05 wt %;
0<Mo≤0.05 wt %;
0<Nb≤0.03 wt %;
0<Ti≤0.05 wt %;
0<V≤0.03 wt %;
0<B≤0.001 wt %.
16 . The manufacturing method according to claim 8 , wherein ferrite comprises 90% or more of grains of 10 μm or less, and 60% or more of grains of 5 μm or less; and/or, wherein an average grain size of retained austenite is ≤2 μm; and/or an average C content in retained austenite is ≥1.1 wt %.