Large-size homogenized 2.3 GPa-grade ultrahigh-strength steel and preparation method thereof

Abstract

The invention provides large-size homogenized 2.3 GPa-grade ultrahigh-strength steel and a preparation method thereof, and belongs to the technical field of ultrahigh-strength steel preparation. By optimizing the chemical component proportion and adopting the processes of low-temperature gradient fine grain solidification, forging, quenching, copious cooling, primary medium-temperature tempering and secondary low-temperature tempering, the ultrahigh-strength steel of 200 Kg grade and 2.3 GPa grade with uniform structure and components is prepared, the axial / radial tensile strength Rm of the large-size ultrahigh-strength steel is 2300 + / -25 MPa, and the percentage elongation after fracture A is 12 + / -2%. A low-temperature gradient (less than or equal to 50K / m) fine grain solidification technology is adopted to prepare the 200Kg grade or more steel ingot, the columnar crystal growth is inhibited during solidification, the equiaxed crystal rate is more than or equal to 75%, the macrosegregation is less than or equal to 1.05%, and the structure and component uniformity is improved, so that the consistency of the axial / radial strength and the plasticity / toughness of the ultrahigh-strength steel is improved, and the service requirement of an extreme environment is met.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-high strength steel preparation, and in particular to a large-scale homogenized 2.3 GPa-grade ultra-high strength steel and a preparation method thereof. Background Art

[0002] Improving the defensive and strike capabilities of weapons and equipment and developing highly secure and reliable weaponry are key development strategies for all nations. Penetrating warheads, as a typical kinetic energy attack warhead, are becoming a key weapon for penetrating hard rock and concrete barriers and conducting precision-guided strikes against critical military targets buried deep underground. However, the strength and toughness of existing shell materials no longer meet the requirements for the development of next-generation deep-penetrating warheads. Technologically advanced nations are continuously researching new ultra-high-strength steels to improve deep-penetration performance. A key development direction for future kinetic energy penetrating warheads is high-speed penetration, with penetration speeds reaching 5-6 Ma. The high temperatures and overloads experienced during penetration will be even more severe, placing increasingly stringent demands on shell materials. Furthermore, the hardening and undergrounding of strategically important military targets primarily relies on concrete, requiring shells with superior destructive power and ultra-high strength to shatter concrete at high speeds and penetrate deep into the depths of underground targets. Ultra-high-strength steel is widely used in shells for foreign weaponry, so the high tensile strength (strength) and high elongation (ductility / toughness) required for shells are currently a key research priority. However, the objective reality of the inverse relationship between strength and plasticity, and between strength and toughness, presents technological challenges for strengthening, ductility, and toughening metal materials. To overcome this conflicting relationship, researchers have conducted extensive research to simultaneously improve the strength, plasticity, and toughness of metal materials.

[0003] The strengthening and toughening mechanism of ultra-high-strength steels is primarily secondary hardening. The high strength of secondary hardened ultra-high-strength alloy steels is primarily due to the combined effects of precipitation of alloy carbides during high-temperature tempering and cryogenic treatment of retained austenite. Currently, secondary hardening ultra-high-strength steels, due to their high-cobalt and high-nickel alloying design and ultra-high strength, have become core materials for aircraft landing gear and critical load-bearing components in defense. However, their microstructure and compositional inhomogeneity, as well as poor performance consistency, limit their widespread application. Secondary hardening ultra-high-strength steels exhibit a synergistic effect during high-temperature tempering, resulting from the coherent precipitation of M2C-type alloy carbides and the secondary quenching transformation of retained austenite. However, when secondary hardening reaches its maximum, the steel exhibits poor toughness. While overaging can significantly improve toughness, it also leads to a significant decrease in strength. Current research focuses on achieving greater microstructure uniformity and toughening limits through coordinated manipulation of appropriate chemical composition (e.g., Co addition to increase carbide nucleation density (Co content up to 15%), and partial Cr substitution for Mo to reduce the M2C lattice constant and enhance interface coherence) and optimized fabrication processes. However, for high-alloy secondary hardening steel, since a large amount of alloying elements are added to the steel, the fluidity of the molten steel is reduced, and alloying elements are easily segregated during the pouring and solidification process, resulting in uneven steel composition, poor performance consistency, and a greater risk of failure such as cracking in engineering applications. Therefore, the homogenization smelting process is a research hotspot and difficulty in high-alloy ultra-high strength steel. It can be seen that in order to obtain homogenized ultra-high strength steel, the first thing to solve is to ensure that the metal solution flows fully during the solidification process so that its composition and structure are uniform. However, due to the unreasonable stirring method of the molten steel during the traditional solidification process of the steel ingot, the temperature gradient during the solidification process is large, making the cast structure of the steel ingot a three-crystal zone structure, and the columnar crystal zone is anisotropic, which will seriously reduce its composition uniformity, as well as the consistency of tensile strength, yield strength, plasticity and toughness.

[0004] Based on this, it is very important to provide a method for preparing large-scale homogenized 2.3GPa-level ultra-high strength steel. Summary of the Invention

[0005] The purpose of the present invention is to provide a large-scale homogenized 2.3GPa-grade ultra-high-strength steel and a preparation method thereof, which is used to solve the technical problem that it is impossible to prepare 200Kg-grade and above 2.3GPa-grade ultra-high-strength steel with uniform structure and composition in the existing technology, and at the same time improve the consistency of its axial / radial strength and plasticity / toughness.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a large-size homogenized 2.3 GPa-grade ultra-high strength steel, comprising the following chemical components in mass percentage: C 0.20-0.30%, Si≤0.015%, Mn≤0.015%, Ni 12.0-15.0%, Cr 2.0-4.0%, Mo 1.0-2.0%, Al 0.08-0.12%, Co 12.0-15.0%, Nb0.015-0.025%, V 0.05-0.10%, W 0.8-1.2%, P≤0.01%, S≤0.01%, and the balance Fe.

[0008] The present invention also provides a method for preparing large-scale homogenized 2.3 GPa-grade ultra-high strength steel, comprising the following steps:

[0009] 1) Mixing raw materials except Al and then performing vacuum melting to obtain molten steel;

[0010] 2) Feeding Al filaments into the molten steel, followed by refining and casting to obtain a steel ingot;

[0011] 3) The steel ingot is subjected to three-stage and three-draw forging, quenching, deep cooling, one medium-temperature tempering, and two low-temperature tempering in sequence to obtain large-sized homogenized 2.3GPa-grade ultra-high strength steel.

[0012] Furthermore, the temperature of the vacuum melting is 1600-1700° C., the vacuum degree of the vacuum melting is 1-5 Pa, and the power of the vacuum melting is ≥200 KW.

[0013] Furthermore, the diameter of the Al filament is 0.5-1.2 mm, the speed of feeding the Al filament is 0.05-1.5 m / s, and the mass of the Al filament is 0.08-0.12% of the total mass of the molten steel.

[0014] Furthermore, the refining power is ≥260KW; the pouring vacuum is 1-5Pa, and the pouring power is 80-120KW.

[0015] Furthermore, after the pouring, low temperature gradient fine grain solidification is performed, and the low temperature gradient fine grain solidification is performed by radial / tangential bidirectional mechanical stirring, or radial / tangential bidirectional magnetic field coupled stirring;

[0016] The ratio of the tangential mechanical stirring speed to the radial mechanical stirring speed is 3-6:1-3.

[0017] Furthermore, the three-drilling and three-drawing forging is to heat the steel ingot to 1100-1150° C., keep it warm for 5-8 hours, have a forging ratio of ≥6, and a final forging temperature of ≥950° C.

[0018] Furthermore, the quenching temperature is 850-950° C., and the quenching time is 1-2 hours;

[0019] The deep cooling temperature is -80 to -70°C, and the deep cooling time is 0.5 to 2 hours.

[0020] Furthermore, the temperature of the first medium-temperature tempering is 460-520° C., the time of the first medium-temperature tempering is 4-6 hours, and the steel is water-cooled after tempering;

[0021] The temperature of the secondary low-temperature tempering is 220-250° C., the time of the secondary low-temperature tempering is 1-3 hours, and air cooling is performed after tempering.

[0022] Beneficial effects of the present invention:

[0023] During smelting, the present invention adds Al filaments and adopts radial / tangential bidirectional mechanical stirring or radial / tangential bidirectional magnetic field coupling stirring process to make the molten steel fully flow during solidification, realize the homogenization of the temperature field during the solidification process of the ultra-high strength steel ingot, and achieve low temperature gradient (≤50K / m) fine crystal solidification; through a reasonable control process, Al filaments with a diameter of 0.5 to 1.2 mm are fed into the steel melt, and through oxidation reaction, radial / tangential bidirectional mechanical stirring or radial / tangential bidirectional magnetic field coupling stirring is adopted to strengthen the stirring of the steel melt and control the interface concentration gradient of aluminum oxide particles in the steel melt, thereby forming nano-scale oxides. This type of nano-oxide can serve as heterogeneous nucleation to refine the matrix structure; at the same time, radial / tangential bidirectional mechanical stirring, or radial / tangential bidirectional magnetic field coupling stirring process, is used to make the molten steel fully flow during solidification, and can also achieve uniformity of the temperature field during the solidification process of ultra-high strength steel ingots, achieving low temperature gradient (≤50K / m) fine grain solidification, inhibiting columnar crystal growth, and its equiaxed crystal ratio ≥75%, macro segregation ≤1.05%, making its composition and structure uniform, thereby improving the consistency of axial / radial strength and plasticity / toughness of 200Kg grade and above 2.3GPa grade ultra-high strength steel, and meeting the service requirements of extreme environments;

[0024] The preparation method of the present invention is highly operable, and its composition and structure are uniform and controllable. Compared with traditional melting and solidification processes, the present invention adopts radial / tangential bidirectional mechanical stirring, or radial / tangential bidirectional magnetic field coupling stirring process, to make the molten steel fully flow during solidification, and can prepare 200Kg grade and above 2.3GPa grade ultra-high strength steel with uniform structure and composition, which is beneficial to simultaneously improve the consistency of its axial / radial strength and plasticity / toughness, and can be promoted and applied industrially. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a graph showing the axial tensile stress-strain curve of the ultra-high strength steel prepared in Example 1 of the present invention;

[0026] Figure 2 The microstructure and temperature gradient diagram of the ultra-high strength steel ingot prepared in Example 1 of the present invention;

[0027] Figure 3 This is the microstructure and temperature gradient diagram of the ultra-high strength steel ingot prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The present invention provides a large-size homogenized 2.3 GPa-grade ultra-high strength steel, comprising the following chemical components in mass percentage: C 0.20-0.30%, Si≤0.015%, Mn≤0.015%, Ni 12.0-15.0%, Cr 2.0-4.0%, Mo 1.0-2.0%, Al 0.08-0.12%, Co 12.0-15.0%, Nb0.015-0.025%, V 0.05-0.10%, W 0.8-1.2%, P≤0.01%, S≤0.01%, and the balance Fe.

[0029] The large-size ultra-high strength steel mentioned in the present invention mainly refers to 200Kg grade and above 2.3GPa grade ultra-high strength steel.

[0030] In the present invention, by adopting a reasonable chemical component ratio and a radial / tangential bidirectional mechanical stirring or a radial / tangential bidirectional magnetic field coupled stirring process, the molten steel is allowed to fully flow during solidification, and the temperature field is homogenized during the solidification process of the ultra-high strength steel ingot, thereby achieving low temperature gradient (≤50K / m) fine grain solidification, inhibiting the growth of columnar crystals, and achieving an equiaxed crystal ratio of ≥75% and a macrosegregation of ≤1.05%, thereby making the composition and structure uniform, thereby improving the consistency of the axial / radial strength and the plasticity / toughness of 200Kg grade and above 2.3GPa grade ultra-high strength steel, and its promotion and application are of great significance.

[0031] In the present invention, the C content is preferably 0.22-0.28%, more preferably 0.24-0.26%, calculated by mass percentage.

[0032] In the present invention, the Si content is preferably ≤0.01%, more preferably ≤0.005%, calculated as a percentage by mass.

[0033] In the present invention, the Mn content is preferably ≤0.01%, more preferably ≤0.005%, calculated as a percentage by mass.

[0034] In the present invention, the Ni content is preferably 12.5-14.5% by mass, and more preferably 13-14% by mass.

[0035] In the present invention, the Cr content is preferably 2.5 to 3.5%, more preferably 2.8 to 3.2%, by mass percentage.

[0036] In the present invention, the content of Mo is preferably 1.2-1.8% by mass, and more preferably 1.4-1.6% by mass.

[0037] In the present invention, the Al content is preferably 0.09-0.11%, more preferably 0.10%, calculated by mass percentage.

[0038] In the present invention, the Co content is preferably 12.5-14.5% by mass, and more preferably 13-14% by mass.

[0039] In the present invention, the Nb content is preferably 0.018-0.022% by mass, and more preferably 0.02% by mass.

[0040] In the present invention, the V content is preferably 0.06 to 0.09%, more preferably 0.07 to 0.08%, calculated by mass percentage.

[0041] In the present invention, the W content is preferably 0.85-1.1%, more preferably 0.9-1.0%, calculated by mass percentage.

[0042] In the present invention, the P content is preferably ≤0.005%, and the S content is preferably ≤0.005%, calculated by mass percentage.

[0043] The present invention also provides a method for preparing large-scale homogenized 2.3 GPa-grade ultra-high strength steel, comprising the following steps:

[0044] 1) Mixing raw materials except Al and then performing vacuum melting to obtain molten steel;

[0045] 2) Feeding Al filaments into the molten steel, followed by refining and casting to obtain a steel ingot;

[0046] 3) The steel ingot is subjected to three-stage and three-draw forging, quenching, deep cooling, one medium-temperature tempering, and two low-temperature tempering in sequence to obtain large-sized homogenized 2.3GPa-grade ultra-high strength steel.

[0047] In the present invention, the temperature of the vacuum melting is 1600-1700°C, preferably 1650-1630°C; the vacuum degree of the vacuum melting is 1-5Pa, preferably 2-4Pa, more preferably 3Pa; the power of the vacuum melting is ≥200KW, preferably ≥230KW.

[0048] In the present invention, the diameter of the Al filament is 0.5-1.2 mm, the speed of feeding the Al filament is 0.05-1.5 m / s, preferably 0.08-1.2 m / s, and more preferably 1.0 m / s; the mass of the Al filament is 0.08-0.12% of the total mass of the molten steel, preferably 0.09-0.11%, and more preferably 0.10%.

[0049] In the present invention, the refining power is ≥260KW, preferably ≥280KW; the pouring vacuum is 1-5Pa, preferably 2-4Pa, more preferably 3Pa; the pouring power is 80-120KW, preferably 90-110KW, more preferably 100KW.

[0050] In the present invention, after the pouring, low temperature gradient fine grain solidification is performed, and the low temperature gradient fine grain solidification is performed by radial / tangential bidirectional mechanical stirring, or radial / tangential bidirectional magnetic field coupled stirring, and the ratio of the tangential mechanical stirring speed to the radial mechanical stirring speed is 3 to 6:1 to 3, preferably 4:3. The present invention adopts low temperature gradient fine grain solidification to make the molten steel flow fully. Since the melt at the edge of the mold (i.e., away from the center of the mold) is lower in temperature than the melt in the center of the mold, under the forced action of the radial shear flow, the melt with a lower temperature at one edge continuously moves toward the center of the mold, thereby reducing the temperature of the central melt. At the same time, the melt in the center of the mold continuously moves toward the other edge, thereby increasing the temperature of the melt at the other edge; and under the forced action of the tangential shear flow, the melt at the other edge is brought back to this edge, and the above movement is repeated, thereby achieving uniformity of the temperature field during the solidification process of the ingot, achieving low temperature gradient (≤50K / m) fine grain solidification, inhibiting the growth of columnar crystals, and making its composition and structure uniform.

[0051] In the present invention, the three-drilling and three-drawing forging is to heat the steel ingot to 1100-1150°C, preferably 1100-1130°C, more preferably 1100-1120°C; keep it warm for 5-8 hours, preferably 6 hours; the forging ratio is ≥6, preferably 6-9, more preferably 7-8; the final forging temperature is ≥950°C, preferably 980-1050°C, more preferably 990-1020°C.

[0052] In the present invention, the quenching temperature is 850-950° C., preferably 870-910° C., more preferably 880-900° C.; the quenching time is 1-2 h, preferably 1.2-1.8 h, more preferably 1.4-1.6 h.

[0053] The deep cooling temperature is -80 to -70°C, preferably -78 to -72°C, and more preferably -76 to -73°C; the deep cooling time is 0.5 to 2 hours, preferably 0.7 to 1.8 hours, and more preferably 1 to 1.5 hours.

[0054] In the present invention, the temperature of the first medium-temperature tempering is 460-520°C, preferably 470-510°C, more preferably 480-500°C; the time of the first medium-temperature tempering is 4-6 hours, preferably 4.5-5.5 hours, more preferably 5 hours; water cooling after tempering;

[0055] The temperature of the secondary low-temperature tempering is 220-250° C., preferably 225-245° C., more preferably 230-240° C.; the time of the secondary low-temperature tempering is 1-3 hours, preferably 1.5-2.5 hours, more preferably 2 hours; and air cooling is performed after tempering.

[0056] In the present invention, the low temperature gradient fine grain solidification + forging + quenching + deep cooling + one medium temperature tempering + two low temperature tempering processes are adopted to regulate the microstructure of 2.3GPa grade ultra-high strength steel, and the matrix structure is mainly a tempered martensite structure.

[0057] The technical solutions provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0058] Example 1

[0059] Accurately weighed, 200kg per furnace. The chemical composition of ultra-high strength steel is: C 0.26%, Si 0.01%, Mn 0.01%, Ni 12.5%, Cr 2.6%, Mo 1.3%, Al 0.085%, Co 13.6%, Nb 0.022%, V 0.08%, W 0.85%, P 0.002%, S 0.002%, balance Fe;

[0060] The large-size ultra-high strength steel mainly refers to 200Kg grade and above 2.3GPa grade ultra-high strength steel, ingot size: diameter Height h 350mm.

[0061] The raw materials corresponding to the chemical composition of the ultra-high strength steel are added in a manner such that large pieces are close to the inner wall of the crucible and small pieces are close to the center of the crucible. The furnace cover is closed, the vacuum pump is turned on, and the vacuum degree of the melting furnace is evacuated to 3.0 Pa for vacuum melting. The vacuum melting temperature is 1650° C. The vacuum melting process is as follows: the melting furnace power is set to 200 kW, and the temperature is increased by electricity. When the metal in the crucible begins to melt, the power is increased to 220 kW for melting. When the melting occurs and splashing occurs, the power is reduced to 200 kW, the vacuum pump is turned off, and argon is filled into the furnace to increase the vacuum pressure of the furnace body to 200 Pa. After the end, molten steel is obtained.

[0062] At a speed of 0.9m / s, Al filaments with a diameter of 1mm and a length of 100.3m are bundled and fed into the molten steel for deoxidation until the oxygen content is ≤10ppm. After the steel is melted, samples are taken to detect the content of each element in the molten steel. The vacuum pump is turned on and the refining power is set to 260KW. After observing the metal raw materials in the crucible through the observation hole, the refining is continued until the molten steel begins to splash. The power is turned off to cool down and the stirring of the melt is strengthened to promote the redox reaction of the molten steel. The ingot mold and launder are hoisted, the vacuum pump is turned on, and the vacuum is evacuated to within 3Pa. Then the power is turned on at 100KW. The static pressure before pouring is 0.13V. After 30 minutes, the crucible is tilted by the operator to pour; after the pouring is completed, radial / tangential bidirectional mechanical stirring is started (the ratio of the tangential mechanical stirring speed to the radial mechanical stirring speed is 4:3), and the temperature is cooled while stirring, and then the vent valve is opened to break the vacuum, the furnace door is opened, and the ingot mold is taken out and disassembled to obtain an ultra-high strength steel ingot, followed by subsequent corresponding three-pier and three-draw forging, the forging temperature is 1100°C, the heat preservation is 6h, the final forging temperature is 950°C, and the bar is forged (forging ratio 7) to prepare the required specifications and dimensions;

[0063] The steel rod is quenched at 900°C for 1 hour, then deep-cooled at -73°C for 1 hour, tempered once at 490°C for 5 hours, water-cooled after tempering, and finally tempered twice at 220°C for 2 hours. After that, it is air-cooled to room temperature to obtain ultra-high strength steel, the matrix structure of which is mainly tempered martensite structure.

[0064] Example 2

[0065] Accurately weighed, 200kg per furnace. The chemical composition of ultra-high strength steel is: C 0.25%, Si 0.01%, Mn 0.01%, Ni 13.5%, Cr 2.5%, Mo 1.4%, Al 0.09%, Co 14.0%, Nb 0.02%, V 0.09%, W 0.92%, P 0.0017%, S 0.002%, balance Fe;

[0066] The large-size ultra-high strength steel mainly refers to 200Kg grade and above 2.3GPa grade ultra-high strength steel, ingot size: diameter Height h 350mm.

[0067] The raw materials corresponding to the chemical composition of the ultra-high strength steel are added in a manner such that large pieces are close to the inner wall of the crucible and small pieces are close to the center of the crucible. The furnace cover is closed, the vacuum pump is turned on, and the vacuum degree of the melting furnace is evacuated to 3.0 Pa for vacuum melting. The vacuum melting temperature is 1630° C. The vacuum melting process is as follows: the melting furnace power is set to 210 kW, and the temperature is increased by electricity. When the metal in the crucible begins to melt, the power is increased to 230 kW for melting. After melting until splashing occurs, the power is reduced to 200 kW, the vacuum pump is turned off, and argon is filled into the furnace to increase the vacuum pressure of the furnace body to 230 Pa. After completion, molten steel is obtained.

[0068] At a speed of 1m / s, Al filaments with a diameter of 1mm and a length of 106.2m are bundled and fed into the molten steel for deoxidation until the oxygen content is ≤10ppm. After the steel is melted, samples are taken to detect the content of each element in the molten steel. The vacuum pump is turned on and the refining power is set to 270KW. After observing the metal raw materials in the crucible through the observation hole, the refining is continued until the molten steel begins to splash. The power is turned off to cool down and the stirring of the melt is strengthened to promote the redox reaction of the molten steel. The ingot mold and launder are hoisted, the vacuum pump is turned on, and the vacuum is evacuated to within 3Pa. Then the power is turned on at 90KW and the steel is left to stand for 2 hours before pouring. 5 minutes to promote the floating of oxide inclusions and improve the purity of ultra-high strength steel, and then the operator tilts the crucible for pouring; after pouring is completed, radial / tangential bidirectional mechanical stirring is turned on (the ratio of the tangential mechanical stirring speed to the radial mechanical stirring speed is 4:3), and cooling is performed while stirring, and then the vent valve is opened to break the vacuum, the furnace door is opened, and the ingot mold is taken out and disassembled to obtain an ultra-high strength steel ingot, followed by subsequent corresponding three-pier and three-draw forging, the forging temperature is 1150°C, the heat preservation is 6h, the final forging temperature is 1000°C, and the bar is forged (forging ratio 8) to prepare the required specifications and dimensions;

[0069] The steel rod was quenched at 890°C for 1 hour, then deep-cooled at -73°C for 1 hour, and then tempered at 500°C for 5 hours. After tempering, it was water-cooled, and finally tempered at 230°C for 2 hours. After that, it was air-cooled to room temperature to obtain ultra-high strength steel, the matrix structure of which was mainly martensite tempered structure.

[0070] Comparative Example 1

[0071] Ultra-high strength steel (chemical composition: C 0.28%, Si 0.01%, Mn 0.01%, Ni 12.8%, Cr2.63%, Mo 1.32%, Al 0.095%, Co 13.8%, Nb 0.02%, V 0.085%, W 0.86%, P 0.0018%, S 0.002%, balance Fe) was prepared according to the existing traditional methods (electric furnace / converter melting-refining-vacuum treatment-conventional die casting-electroslag-forging-heat treatment); ingot size: diameter Height h 350mm).

[0072] Comparative Example 2

[0073] The same as Example 1, except that no radial / tangential bidirectional mechanical stirring is performed after the pouring is completed, and the steel ingot is obtained after direct cooling.

[0074] Comparative Example 3

[0075] The same as Example 2, except that the Al filaments are not fed separately, but all raw materials are directly mixed and then vacuum melted.

[0076] The ultra-high strength steels prepared in Examples 1 to 2 and Comparative Examples 1 to 3 were tested for mechanical properties, C element macrosegregation, and ingot equiaxed crystal ratio. The tensile method for the mechanical property test was carried out in accordance with GB / T 228.1-2010; the C element macrosegregation was carried out in accordance with YB / T 4397-2014G; and the ingot equiaxed crystal ratio test was carried out in accordance with GB / T 226-2015. The test results are shown in Table 1.

[0077] Table 1 Test results of ultra-high strength steel prepared in Examples 1-2 and Comparative Examples 1-3

[0078]

[0079]

[0080] It can be seen from the data in Table 1 that the ultra-high strength steels Rm and A prepared in Examples 1 to 2 both meet the tensile strength Rm = 2300 ± 25 MPa, elongation after fracture A = 12 ± 2%, and the mechanical properties, macro-segregation of C elements and the proportion of equiaxed crystals in the ingot are significantly higher than those in Comparative Examples 1 to 3. The ultra-high strength steel samples prepared in the embodiments of the present invention are tested by special equipment for ultra-high strength steel. The proportion of equiaxed crystals in the prepared ultra-high strength steel ingots and the mechanical properties of the ultra-high strength steel materials are significantly higher than those in Comparative Examples 1 to 3. At the same time, the axial / radial strength and plasticity uniformity of the ultra-high strength steels prepared in Examples 1 to 2 are higher than those in Comparative Examples 1 to 3, which shows that this method can significantly improve the uniformity of the structure and composition of large-size 200Kg grade and above 2.3GPa grade ultra-high strength steels, and can simultaneously improve the consistency of their axial / radial strength and plasticity / toughness.

[0081] from Figures 1 to 3 It can be seen that the yield strength R p0.2 The tensile strength Rm is 1938MPa, the tensile strength Rm is 2285MPa, the elongation after fracture A is 10.58%, and the equiaxed crystal ratio of the ingot can reach 92%. However, the equiaxed crystal ratio of ultra-high strength steel ingots prepared by existing traditional methods is only 50%.

[0082] As can be seen from the above embodiments, the present invention provides a large-scale homogenized 2.3GPa grade ultra-high strength steel and its preparation method. The present invention optimizes the chemical composition ratio, adds Al filaments during smelting and adopts radial / tangential bidirectional mechanical stirring, or radial / tangential bidirectional magnetic field coupling stirring process, so that the molten steel solidifies and flows fully, realizes the homogenization of the temperature field during the solidification process of the ultra-high strength steel ingot, achieves low temperature gradient (≤50K / m) fine grain solidification, inhibits columnar crystal growth, and its equiaxed crystal ratio is ≥75%, and macro segregation is ≤1.05%, so that its composition and structure are uniform, thereby improving the axial / radial strength and plastic / toughness consistency of 200Kg grade and above 2.3GPa grade ultra-high strength steel, meeting the service requirements of extreme environments, and its promotion and application are of great significance. The large-scale ultra-high strength steel of the present invention has an axial / radial tensile strength Rm = 2300±25MPa and an elongation after fracture A = 12±2%. The preparation method of the present invention is highly operable, and its composition and structure are uniform and controllable. Compared with traditional melting and solidification processes, the present invention can produce 200Kg grade and above 2.3GPa grade ultra-high strength steel with uniform structure and composition, which is beneficial to simultaneously improve the consistency of its axial / radial strength and plasticity / toughness, and can be promoted and applied industrially.

[0083] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A large-scale homogenized 2.3GPa grade ultra-high strength steel, characterized by: The steel comprises the following chemical components in mass percentage: C 0.20-0.30%, Si≤0.015%, Mn≤0.015%, Ni 12.0-15.0%, Cr 2.0-4.0%, Mo1.0-2.0%, Al 0.08-0.12%, Co 12.0-15.0%, Nb0.015-0.025%, V 0.05-0.10%, W0.8-1.2%, P≤0.01%, S≤0.01%, and the balance is Fe.

2. A method for preparing the large-scale homogenized 2.3 GPa grade ultra-high strength steel according to claim 1, characterized in that: The following steps are involved: 1) Mixing raw materials except Al and then performing vacuum melting to obtain molten steel; 2) Feeding Al filaments into the molten steel, followed by refining and casting to obtain a steel ingot; 3) The steel ingot is subjected to three-stage and three-draw forging, quenching, deep cooling, one medium-temperature tempering, and two low-temperature tempering in sequence to obtain large-sized homogenized 2.3GPa-grade ultra-high strength steel.

3. The method for preparing large-scale homogenized 2.3 GPa grade ultra-high strength steel according to claim 2, characterized in that: The temperature of the vacuum melting is 1600-1700° C., the vacuum degree of the vacuum melting is 1-5 Pa, and the power of the vacuum melting is ≥200 KW.

4. The method for preparing large-scale homogenized 2.3 GPa grade ultra-high strength steel according to claim 2 or 3, characterized in that: The diameter of the Al filaments is 0.5-1.2 mm, the speed of feeding the Al filaments is 0.05-1.5 m / s, and the mass of the Al filaments is 0.08-0.12% of the total mass of the molten steel.

5. The method for preparing large-scale homogenized 2.3 GPa grade ultra-high strength steel according to claim 4, characterized in that: The refining power is ≥260KW; the pouring vacuum degree is 1-5Pa, and the pouring power is 80-120KW.

6. The method for preparing large-scale homogenized 2.3 GPa grade ultra-high strength steel according to claim 2, 3 or 5, characterized in that: After the pouring, low temperature gradient fine grain solidification is performed, and the low temperature gradient fine grain solidification is performed by radial / tangential bidirectional mechanical stirring, or radial / tangential bidirectional magnetic field coupling stirring; The ratio of the tangential mechanical stirring speed to the radial mechanical stirring speed is 3-6:1-3.

7. The method for preparing large-scale homogenized 2.3 GPa grade ultra-high strength steel according to claim 6, characterized in that: The three-pier three-draw forging is to heat the steel ingot to 1100-1150° C., keep it warm for 5-8 hours, set the forging ratio to ≥6, and set the final forging temperature to ≥950° C.

8. The method for preparing large-scale homogenized 2.3 GPa grade ultra-high strength steel according to claim 2 or 7, characterized in that: The quenching temperature is 850-950°C and the quenching time is 1-2h; The deep cooling temperature is -80 to -70°C, and the deep cooling time is 0.5 to 2 hours.

9. The method for preparing large-scale homogenized 2.3 GPa grade ultra-high strength steel according to claim 8, characterized in that: The temperature of the first medium-temperature tempering is 460-520°C, the time of the first medium-temperature tempering is 4-6 hours, and water cooling is performed after tempering; The temperature of the secondary low-temperature tempering is 220-250° C., the time of the secondary low-temperature tempering is 1-3 hours, and air cooling is performed after tempering.

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