High-plasticity 1500-mpa-grade ultrahigh-strength steel and preparation method therefor

Abstract

High-plasticity 1500-MPa-grade ultrahigh-strength steel and a manufacturing method therefor. The steel comprises the following components in percentages by weight: 0.35-0.40% of C, 1.0-1.8% of Si, 1.5-2.0% of Mn, 0.3-0.6% of Cr, 0.02-0.05% of Al, 0.02-0.05% of Ti, and 0.002-0.02% of B, with the balance being Fe and other inevitable impurities. Moreover, the steel also satisfies: the carbon equivalent Ceq1 of a peritectic reaction zone is greater than 0.17%, and Ceq1=C−0.03Mn−0.06Si−0.222S−0.04P; and the carbon equivalent Ceq2 for welding is smaller than or equal to 0.56%, and Ceq2=C+Mn/20+Si/30+2P+4S. The ultrahigh-strength steel of the present invention has a yield strength of 1000-1300 MPa, a tensile strength of greater than or equal to 1500 MPa, and an elongation at break of greater than or equal to 18%, has a good surface quality, and is particularly suitable for manufacturing structural members and safety members of vehicles with complex shapes and high requirements for a forming performance, such as A/B pillars, side impact beams, longitudinal beams, and bumpers.

1 . A high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa, comprising the following components in weight percentage:
 C: 0.35-0.40%; 
 Si: 1.0-1.8%; 
 Mn: 1.5-2.0%; 
 Cr: 0.3-0.6%; 
 Al: 0.02-0.05%; 
 Ti: 0.02-0.05%; 
 B: 0.002-0.02%; 
 a balance comprising Fe and other unavoidable impurities, wherein the following is satisfied: 
 carbon equivalent in peritectic reaction zone C eq1 >0.17%, C eq1 =C−0.03Mn-0.06Si—0.222S−0.04P; and 
 welding carbon equivalent C eq2 ≤0.56%, C eq2 =C+Mn/20+Si/30+2P+4S. 
 
     
     
         2 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , wherein the balance is Fe and other unavoidable impurities. 
     
     
         3 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , wherein the content of C is 0.36-0.38 wt %. 
     
     
         4 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , wherein the content of Si is 1.4-1.7 wt %. 
     
     
         5 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , wherein the content of Mn is 1.7-2.0 wt %. 
     
     
         6 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , wherein the content of Cr is 0.4-0.6 wt %. 
     
     
         7 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , wherein among the other unavoidable impurities: P is ≤0.015 wt %, S is ≤0.002 wt %, O is ≤0.002 wt %, N is ≤0.004 wt %. 
     
     
         8 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , wherein the ultra-high strength steel has a microstructure of 10%-15% by volume of ferrite+70%-80% by volume of martensite+retained austenite. 
     
     
         9 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 8 , wherein in the ferrite in the microstructure of the ultra-high strength steel, the number of grains with a grain size of ≤5 μm accounts for 90% or more, and the number of grains with a grain size of ≤3 μm accounts for 60% or more. 
     
     
         10 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 8 , wherein an average grain size of the retained austenite in the microstructure of the ultra-high strength steel is ≤2 μm; and/or an average C content C(ra) in the retained austenite satisfies: 1.2 wt %≤C(ra)≤2.0 wt %. 
     
     
         11 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , wherein the ultra-high strength steel has a yield strength of 1000-1300 MPa, a tensile strength of ≥1500 MPa, and an elongation at break of ≥18%. 
     
     
         12 . A method for manufacturing the high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 1 , comprising the following steps:
 1) Smelting and casting 
 Smelting and casting the components according to  claim 1  into a slab, wherein a slab thickness at a continuous casting outlet is controlled to be 55-60 mm, and a continuous casting withdrawal speed is controlled to be 2-5 m/min; 
 2) Slab heating 
 Heating temperature: 1200-1300° C., furnace time: 25-40 min; 
 3) Hot rolling and cooling 
 Performing high-pressure descaling first, controlling a rolling-end temperature at 860-930° C., and then performing laminar cooling to 500-600° C. with a cooling rate controlled at 20-40° C./s, followed by coiling; 
 4) Slow cooling treatment 
 After coiling of a hot-rolled coil, laying down the coil and sealing it in-situ with an insulation cover, or transferring it into a sealed insulation cover for slow cooling; and after treatment in the insulation cover for ≥4 h, opening the cover and taking out the hot-rolled coil; 
 5) Pickling 
 Controlling a pickling speed at 60-150 m/min; 
 6) Annealing 
 Performing continuous annealing at an annealing temperature of 820-900° C.; slow cooling to 690-760° C. at a cooling rate of 3-10° C./s; then rapid cooling to 150-250° C. at a cooling rate of 50-100° C./s; then reheating to 360-460° C., holding for 100-400 s, and finally cooling to room temperature. 
 
     
     
         13 . The method for manufacturing the high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 12 , wherein in step 1), a steel leakage rate during the thin slab continuous casting is controlled to be ≤1%, and a substandard rate due to cracks is controlled to be ≤1.2%. 
     
     
         14 . The method for manufacturing the high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 12 , wherein in step 3), during the high-pressure descaling, water pressure of a first descaling pass is controlled to be not less than 260 bar, and water pressure of a second descaling pass is controlled to be not less than 340 bar; and/or a U-shape coiling mode is used for the coiling in step 3), wherein a coiling temperature is controlled at 550-650° C. within a distance of ≤30 m from a head to a tail of a steel strip. 
     
     
         15 . The method for manufacturing the high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 12 , wherein in the annealing process in step 6), the annealing temperature is 840-870° C., followed by slow cooling to 700-730° C. at a cooling rate of 3-10° C./s; rapid cooling to 170-230° C.; and then reheating to 400-430° C. after the rapid cooling, and holding for 150-300 s; and/or a volume content of hydrogen in a reducing atmosphere in a continuous annealing furnace is controlled to be 10-15%. 
     
     
         16 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 2 , wherein:
 the content of C is 0.36-0.38 wt %; 
 the content of Si is 1.4-1.7 wt %; 
 the content of Mn is 1.7-2.0 wt %; and/or 
 the content of a Cr is 0.4-0.6 wt %. 
 
     
     
         17 . The high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 2 , wherein the ultra-high strength steel has a microstructure of 10%-15% by volume of ferrite+70%-80% by volume of martensite+retained austenite; and/or the ultra-high strength steel has a yield strength of 1000-1300 MPa, a tensile strength of ≥1500 MPa, and an elongation at break of ≥18%. 
     
     
         18 . The method for manufacturing the high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 12 , wherein the components are casted into a slab by thin slab continuous casting. 
     
     
         19 . The method for manufacturing the high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 12 , wherein after the coiling of the hot-rolled coil, allowing it to stay on a reel for ≥3 min, and then sealing it in-situ with an insulation cover, or transferring it into a sealed insulation cover for slow cooling. 
     
     
         20 . The method for manufacturing the high-plasticity ultra-high strength steel having a tensile strength of ≥1500 MPa according to  claim 12 , wherein in the high-plasticity ultra-high strength steel:
 the content of C is 0.36-0.38 wt %; 
 the content of Si is 1.4-1.7 wt %; 
 the content of Mn is 1.7-2.0 wt %; and/or 
 the content of a Cr is 0.4-0.6 wt %.