An ultra-high strength steel and a method of manufacturing the same

Abstract

This invention provides an ultra-high strength steel in which the rare earth element (RE) content satisfies the following formula: -80 < {[RE] - (a·T[O]} s +b·T[S] m )} / (c·T[N] m <200; where [RE] represents the content of rare earth elements (RE) in the ultra-high strength steel; T[O] s The dissolved oxygen content in ultra-high strength steel; T[S] m T[N] represents the total sulfur content in ultra-high strength steel. m The total nitrogen content in ultra-high strength steel is denoted as ; a, b, and c are correction coefficients one, two, and three, respectively, with values ​​of 2–5, 1.5–2.5, and 0.5–1.5. This invention limits the content of rare earth elements (RE) in ultra-high strength steel by controlling the content of O, S, and N, promoting the nucleation of TiN inclusions in the steel and inhibiting their aggregation and growth. Simultaneously, some TiN forms complexes with RE-O-S, jointly improving the morphology and distribution of inclusions in the steel. Specifically, it transforms the quadrilateral TiN inclusions in the steel into finer spherical, near-spherical, or granular rare earth sulfides, rare earth oxides, and titanium nitride or rare earth oxygen-sulfur-titanium nitride complexes, thereby mitigating stress concentration during deformation and improving the matrix toughness of ultra-high strength steel.

Description

Technical Field

[0001] This invention belongs to the field of ultra-high strength steel technology, specifically relating to an ultra-high strength steel and its preparation method. Background Technology

[0002] Ultra-high strength steel is a core material for critical load-bearing components in high-end equipment, and its performance level is crucial to the safety of equipment service. Taking advanced aircraft landing gear and wind tunnel force measurement devices as examples, the increasingly stringent fatigue or impact load conditions inside these components place higher demands on the strength and toughness of ultra-high strength steel. Although currently commercially available 18Ni series maraging steels have reached ultra-high strength levels of 2.0–2.4 GPa, their toughness is insufficient (A KV The use of steel with a strength of <10J in the aforementioned operating conditions is limited, leaving key components with "no available materials." Therefore, it is urgent to develop steel with ultra-high strength and high toughness to break the material selection dilemma that restricts the selection of key components for high-end equipment.

[0003] Currently, grain refinement and austenite introduction are the main ways to improve the toughness of ultra-high strength steel. These methods often rely on sufficient deformation and phase transformation control. During the preparation process, as the size of ultra-high strength steel components (used for key load-bearing parts of high-end equipment) increases, insufficient deformation or incomplete phase transformation is likely to occur, resulting in low toughness of ultra-high strength steel.

[0004] In view of this, it is necessary to provide an ultra-high strength steel and its preparation method to solve the problem of low toughness of existing ultra-high strength steel. Summary of the Invention

[0005] Therefore, the present invention provides an ultra-high strength steel and its preparation method, which can solve the technical problem of low toughness of existing ultra-high strength steel.

[0006] To address the above problems, this invention provides an ultra-high strength steel, wherein the rare earth element (RE) content in the ultra-high strength steel satisfies the following formula:

[0007] -80<{[RE]-(a·T[O] s +b·T[S] m )} / (c·T[N] m <200;

[0008] Wherein, [RE] represents the content of rare earth element RE in ultra-high strength steel;

[0009] T[O] s The dissolved oxygen content in ultra-high strength steel;

[0010] T[S] m The total sulfur content in ultra-high strength steel;

[0011] T[N]m The total nitrogen content in ultra-high strength steel;

[0012] [RE]、T[O] s 、T[S] m T[N] m All are expressed as a percentage by mass;

[0013] a, b, and c are correction coefficient one, correction coefficient two, and correction coefficient three, respectively, with values ​​of 2–5, 1.5–2.5, and 0.5–1.5.

[0014] Furthermore, the chemical composition of the ultra-high strength steel, by mass percentage, includes: C ≤ 0.01 wt%, Ni: 17.0–18.5 wt%, Co: 11.0–12.5 wt%, Mo: 4.5–5.5 wt%, Ti: 0.9–1.1 wt%, S ≤ 0.005 wt%, P ≤ 0.01 wt%, O ≤ 0.002 wt%, N ≤ 0.0015 wt%, rare earth element RE: 0.001–0.03 wt%, with the balance being Fe;

[0015] Preferably, the rare earth element RE is one of lanthanum (La), cerium (Ce), and yttrium (Y).

[0016] Furthermore, in the ultra-high strength steel: the mass percentage of O is ≤0.001wt%, the mass percentage of N is <0.001wt%, and the mass percentage of rare earth RE is 0.004~0.018wt%.

[0017] Preferably, the mass percentage content of rare earth element RE is 0.0040 to 0.014 wt%.

[0018] Furthermore, in the ultra-high strength steel, more than 50% of the total number of inclusions constitute an average equivalent diameter D. m The inclusions are spherical, near-spherical, or granular in size, ranging from 1 to 5 μm, and are diffusely distributed. The diffusely distributed inclusions include rare earth sulfides RE2S3, rare earth oxides RE2O3, titanium nitride TiN, and rare earth oxysulfide-titanium nitride.

[0019] Furthermore, the ultra-high strength steel has a multiphase structure and original austenitic grains with a grain size greater than or equal to 8. The multiphase structure includes martensite, nano-precipitates, and reversed austenite. The martensite includes martensite lath bundles.

[0020] The nanoprecipitated phase includes a Ni3Ti phase and a molybdenum-rich phase, and the equivalent diameter of the nanoprecipitated phase is 5–50 nm.

[0021] In the multiphase microstructure, the volume percentage of reversed austenite is 5–20 vol.%.

[0022] Preferably, the volume percentage of the reversed austenite is 10-15 vol.%.

[0023] Preferably, the average size of the martensitic lath bundles is less than or equal to 5 μm.

[0024] Furthermore, the properties of the ultra-high strength steel are as follows: yield strength ≥ 2.1 GPa, tensile strength ≥ 2.2 GPa, elongation after fracture ≥ 9%, reduction of area ≥ 50%, and V-notch impact energy ≥ 20-25 J.

[0025] On the other hand, the present invention provides a method for preparing the ultra-high strength steel according to any one of the above claims, comprising the following steps:

[0026] Smelting: According to the chemical composition of the ultra-high strength steel, the raw materials are smelted and cast to obtain steel ingots;

[0027] Annealing: The steel ingot is subjected to annealing treatment; the holding time t during the annealing treatment. h >(1.5D I ) / 100h, where D I These are the cross-sectional dimensions of the steel ingot obtained after casting, in mm;

[0028] Forging: Forging is performed on annealed steel ingots to obtain forging materials;

[0029] Solution heat treatment: The forging material is subjected to solution heat treatment; the solution treatment time t s >(1.5~2.5)D b / 100h, where D b The cross-sectional dimensions of the forged material obtained after forging are in mm;

[0030] Aging treatment: The forging material after solution heat treatment is subjected to aging treatment to obtain the ultra-high strength steel.

[0031] Furthermore, the drawing ratio Y in the forging process is ≥10, where Y=(D I / D b ) 2 .

[0032] Furthermore, smelting is carried out using either vacuum induction refining (VIM) or vacuum induction refining (VIM) combined with vacuum arc remelting (VAR).

[0033] Preferably, rare earth element RE is added during VIM refining; the refining time before adding rare earth element RE is t1≥60min, and the refining time after adding rare earth element RE is t2≥30min;

[0034] Preferably, the vacuum degree P during VIM refining is... v≤1~5Pa;

[0035] Preferably, the tapping time t3 after the smelting process is completed is ≤30min.

[0036] Furthermore, the annealing temperature is 1000–1100°C; and / or

[0037] The forging process is performed at a temperature of 800–1200°C; and / or

[0038] The solution heat treatment temperature is 800–900°C; and / or

[0039] The aging treatment temperature is 460–540℃; the aging treatment time is 1–16 hours.

[0040] Based on the above method, the combination of "rare earth modified inclusions" and "construction of multi-level multiphase structure" yields an ultra-high purity multi-level multiphase structure. Without compromising precipitation strengthening, the V-notch impact toughness of 2.2 GPa grade ultra-high strength steel is increased to 20-25 J and above through multiple toughening mechanisms such as reducing stress concentration, inhibiting crack nucleation, and hindering crack propagation. This is 2-3 times the impact toughness of existing steels of the same strength grade. Furthermore, the heat treatment process is easy to operate and less affected by the "size effect" of the steel.

[0041] The ultra-high strength steel and its preparation method provided by this invention have the following beneficial effects:

[0042] 1. This invention limits the content of rare earth elements (RE) in ultra-high strength steel by controlling the content of O, S, and N, thereby promoting the nucleation of TiN inclusions in the steel and inhibiting their aggregation and growth. At the same time, some TiN forms a complex with RE-OS, which together improves the morphology and distribution of inclusions in the steel. Specifically, the quadrilateral TiN inclusions in the steel are transformed into finer spherical, near-spherical, or granular rare earth sulfides, rare earth oxides, and titanium nitride or rare earth oxygen-sulfur-titanium nitride complexes that are diffusely distributed. This can reduce stress concentration during deformation and thus improve the matrix toughness of ultra-high strength steel.

[0043] 2. Further, the chemical composition of the ultra-high strength steel of the present invention includes: C ≤ 0.01 wt%, Ni: 17.0–18.5 wt%, Co: 11.0–12.5 wt%, Mo: 4.5–5.5 wt%, Ti: 0.9–1.1 wt%, S ≤ 0.005 wt%, P ≤ 0.01 wt%, O ≤ 0.002 wt%, N ≤ 0.0015 wt%, rare earth element RE: 0.001–0.03 wt%, and the balance being Fe; according to the above chemical composition, a phase transformation is performed to obtain the steel with… The multi-level complex structure of "martensite + nano-precipitates + austenite" can effectively inhibit crack nucleation and hinder crack propagation, further improving the toughness and plasticity of ultra-high strength steel. Specifically, after aging treatment, nano-scale precipitates, such as Ni3Ti or Mo-rich phases, will precipitate in the matrix. At the same time, due to the addition of rare earth elements, the size of the martensite lath bundles can be refined, and the Mo, Ni and Ti elements in the steel matrix can be segregated (i.e., chemical heterogeneity is formed), thereby promoting the formation of reverse austenite. More reverse austenite is beneficial to the plasticity and toughness of ultra-high strength steel.

[0044] 3. Furthermore, by controlling the target cross-sectional dimensions of the material before and after forging, a larger drawing-to-forging ratio can be obtained, achieving a larger deformation amount. This allows for the thorough breaking down of coarse grains, and the use of deformation energy storage promotes recrystallization, thereby improving the grain size and obtaining a microstructure with a pre-austenite grain size greater than or equal to level 8. Refined pre-austenite grains are a sufficient condition for obtaining even finer martensitic subgrains. Martensitic lath bundles, as one of the key subgrain components, are the smallest toughening unit controlling ultra-high strength steel. Fine martensitic lath bundles achieve toughening by hindering crack propagation and increasing its path. Therefore, limiting the pre-austenite grain size and obtaining smaller martensitic lath bundles can further improve the toughness of ultra-high strength steel. Attached Figure Description

[0045] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0046] Figure 1 The morphology of the original austenite grains in the ultra-high strength steel in the embodiments and comparative examples of this invention is as follows:

[0047] Figure 2 This is the inverse pole diagram of the aged martensite of the ultra-high strength steel in the embodiments and comparative examples of the present invention;

[0048] Figure 3 This is a typical aging state multi-phase microstructure diagram of ultra-high strength steel in the embodiments of the present invention;

[0049] Figure 4 Here are the scanning electron microscope images and compositional distribution of RE-OS-TiN in Example 3;

[0050] Figure 5 The images show the scanning electron microscope (SEM) morphology and compositional distribution of inclusions in the ultra-high strength steel in Comparative Example 1.

[0051] Figure 6 The images show the metallographic and scanning electron microscope (SEM) images of the undissolved elemental RE particles in Comparative Example 3. Detailed Implementation

[0052] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0053] To meet the performance requirements of ultra-high strength steel for large-size components in high-end equipment under conditions of severe fatigue or impact loading, the present invention aims to provide a method for preparing ultra-high strength steel, the specific scheme of which is as follows:

[0054] A type of ultra-high strength steel, wherein the content of rare earth element RE in the ultra-high strength steel satisfies the following formula:

[0055] -80<{[RE]-(a·T[O] s +b·T[S] m )} / (c·T[N] m <200;

[0056] Wherein, [RE] represents the content of rare earth element RE in ultra-high strength steel;

[0057] T[O] s The dissolved oxygen content in ultra-high strength steel;

[0058] T[S] m The total sulfur content in ultra-high strength steel;

[0059] T[N] m The total nitrogen content in ultra-high strength steel;

[0060] [RE]、T[O] s 、T[S] m T[N] m All are expressed as a percentage by mass;

[0061] a, b, and c are correction coefficient one, correction coefficient two, and correction coefficient three, respectively, with values ​​of 2–5, 1.5–2.5, and 0.5–1.5.

[0062] Rare earth elements RE and O mainly form RE2O3 and RE-OS-TiN, rare earth elements RE and S mainly form RE2S3 and RE-OS-TiN, and rare earth elements RE and N mainly form RE-OS-TiN. Therefore, taking into full account the consumption of rare earth elements RE by the chemical reaction with O and S in steel, as well as the necessary content of RE to participate in the TiN modification effect, the content of the four elements is limited.

[0063] In some embodiments, the chemical composition of the ultra-high strength steel, by weight percentage, includes: C ≤ 0.01 wt%, Ni: 17.0–18.5 wt%, Co: 11.0–12.5 wt%, Mo: 4.5–5.5 wt%, Ti: 0.9–1.1 wt%, S ≤ 0.005 wt%, P ≤ 0.01 wt%, O ≤ 0.002 wt%, N ≤ 0.0015 wt%, rare earth element RE: 0.001–0.03 wt%, with the balance being Fe;

[0064] Preferably, the rare earth element RE is one of lanthanum (La), cerium (Ce), and yttrium (Y).

[0065] In some embodiments, in the ultra-high strength steel: the mass percentage of O is ≤0.001wt%, the mass percentage of N is <0.001wt%, and the mass percentage of rare earth RE is 0.004~0.018wt%.

[0066] Preferably, the mass percentage content of rare earth element RE is 0.004 to 0.014 wt%.

[0067] In some embodiments, more than 50% of the total number of inclusions in the ultra-high strength steel is of average equivalent diameter D. m The inclusions are spherical, near-spherical, or granular in size, ranging from 1 to 5 μm, and are diffusely distributed. The diffusely distributed inclusions include rare earth sulfides RE2S3, rare earth oxides RE2O3, titanium nitride TiN, and rare earth oxysulfide-titanium nitride. Preferably, more than 75% of the total number of inclusions in the steel are the above-mentioned diffusely distributed inclusions. More preferably, more than 95% of the total number of inclusions in the steel are the above-mentioned diffusely distributed inclusions.

[0068] In some embodiments, the ultra-high strength steel has a multiphase structure and original austenitic grains with a grain size greater than or equal to 8. The multiphase structure includes martensite, nano-precipitates, and reversed austenite; the martensite includes martensite lath bundles.

[0069] The nano-precipitated phases include Ni3Ti phase and molybdenum-rich phase, with an equivalent diameter of 5–50 nm, and are dispersed within the ultra-high strength steel matrix.

[0070] In the multiphase microstructure, the volume percentage of reversed austenite is 5–20 vol%.

[0071] Preferably, the volume percentage of reversed austenite is 5–15 vol%; more preferably, it is 10–15 vol%.

[0072] The original austenite grain size is greater than or equal to grade 8; preferably, it is greater than or equal to grade 9; more preferably, it is greater than or equal to grade 10.

[0073] The average size of the martensitic lath bundles is less than or equal to 5 μm; preferably, less than or equal to 2.5 μm; more preferably, less than or equal to 1.5 μm. The martensitic lath bundles are a type of martensitic subgrain between martensitic lath blocks and martensitic laths, consisting of regions enclosed by high-angle grain boundaries of 55–65°.

[0074] Generally, for ultra-high strength steels, sufficient refinement of the martensitic multi-level structure (proto-austenite grains, martensitic laths, martensitic lath bundles, and martensitic laths, etc.) helps improve toughness and plasticity. Specifically, refined proto-austenite grains are a sufficient condition for obtaining even finer martensitic subgrains, while martensitic lath bundles are considered the smallest toughening unit controlling this type of ultra-high strength steel, achieving toughening by hindering crack propagation and increasing its path. Therefore, considering the toughening effect that mainly controls the multi-level structure of the matrix, the sizes of proto-austenite grains and martensitic lath bundles are limited.

[0075] In some implementations, the properties of ultra-high strength steel are as follows: yield strength ≥ 2.1 GPa, tensile strength ≥ 2.2 GPa, elongation after fracture ≥ 9%, reduction of area ≥ 50%, and V-notch impact energy ≥ 20 J.

[0076] On the other hand, the present invention provides a method for preparing the ultra-high strength steel according to any one of the above claims, comprising the following steps:

[0077] Smelting: The raw materials are prepared according to the chemical composition of ultra-high strength steel, and then smelted and cast to obtain steel ingots; high-purity metal materials that have been thoroughly degreased and dried are used in the smelting process.

[0078] Annealing: Annealing (homogenization) of steel ingots; holding time t during annealing. h >(1.5D I ) / 100h, where D I These are the cross-sectional dimensions of the steel ingot obtained after casting, in mm;

[0079] Forging: Forging is performed on annealed steel ingots to obtain forging materials;

[0080] Solution heat treatment: Solution heat treatment of forgings; solution treatment time t s >(1.5~2.5)D b / 100h, where D b The cross-sectional dimensions of the forged material obtained after forging are in mm;

[0081] Aging treatment: The forging material after solution heat treatment is aged to obtain ultra-high strength steel.

[0082] In some embodiments, the drawing ratio Y in the forging process is ≥10, where Y = (D I / D b ) 2 The target cross-sectional dimensions (D) of the material before and after forging. I and D b On the one hand, it determines the time for annealing and solution heat treatment, and on the other hand, it can obtain a larger drawing-to-forging ratio and achieve a larger deformation amount, thereby ensuring that coarse grains can be fully broken and that deformation energy storage can be used to promote recrystallization, thereby improving the grain grade and obtaining a microstructure with an original austenite grain size greater than or equal to 8.

[0083] Steel ingots obtained by VIM or VIM+VAR smelting are generally cylindrical ingots, therefore the above cross-sectional dimensions are diameters.

[0084] In some implementations, smelting is carried out using vacuum induction refining (VIM) or vacuum induction refining combined with vacuum arc remelting (VAR).

[0085] Preferably, rare earth element RE is added during VIM refining; the refining time before adding rare earth element RE is t1≥60min, and the refining time after adding rare earth element RE is t2≥30min;

[0086] Preferably, the vacuum degree P during VIM refining is... v ≤1~5Pa;

[0087] Preferably, the tapping time t3 after the smelting process is completed is ≤30min.

[0088] Generally, for high Co-Ni martensitic aging ultra-high strength steel, after the VIM+VAR two-stage special metallurgical process, the main type of inclusion in the steel is quadrilateral TiN, which has an adverse effect on the toughness and plasticity of the steel matrix. Adding RE during the VIM process can promote TiN nucleation and inhibit its aggregation and growth. At the same time, some TiN can form a complex with RE-OS, which together improves the morphology and distribution of inclusions, thereby reducing stress concentration during deformation and ensuring the toughness and plasticity of the matrix.

[0089] On the other hand, the addition of rare earth element RE can produce martensitic lath bundles with a small average grain size (less than or equal to 5 μm) after solution heat treatment, and cause the Mo, Ni and Ti elements in the steel matrix to segregate (i.e. form chemical heterogeneity), thereby promoting the formation of reverse austenite; thus forming a multi-level complex structure of "martensite + nano-precipitates + austenite", which can effectively inhibit crack nucleation and hinder crack propagation, and further improve the toughness and plasticity of ultra-high strength steel.

[0090] Prolonged refining of molten steel before the addition of rare earth element (RE) can thoroughly purify the steel and reduce its oxygen content. Adding RE and increasing the refining time promotes the full integration of RE with the molten steel, enhancing the modification effect on inclusions. Vacuum level directly affects the RE oxidation rate and the removal rate of gaseous elements from the molten steel, while tapping time also affects the amount of RE oxidized in the molten steel. Therefore, all these factors together determine the RE loss during the smelting and casting process and the final RE recovery rate.

[0091] In some embodiments, the annealing temperature is 1000–1100°C; and / or

[0092] The forging temperature is 800–1200℃; and / or

[0093] The solution heat treatment temperature is 800–900℃; and / or

[0094] The aging treatment temperature is 460–540℃; the aging treatment time is 1–16 hours.

[0095] The present invention will be further described below with reference to specific embodiments and comparative examples. In this embodiment, the rare earth element RE is high-purity metallic lanthanum La (purity ≥ 99.5%).

[0096] Example 1

[0097] This embodiment provides a method for preparing ultra-high strength steel, including the following steps:

[0098] Smelting: VIM smelting was carried out according to the chemical composition in Table 1, and steel ingots with a diameter of 300 mm were obtained after casting. High-purity metal materials that have been thoroughly degreased and dried were used in the smelting process. The refining time before adding rare earth lanthanum (La) was 70 min, the refining time after adding it was 40 min, the vacuum degree during the refining process was 3 Pa, and the tapping time after refining was 20 min.

[0099] Annealing: The steel ingot with a diameter of 300 mm was annealed at 1100℃ for 6 hours;

[0100] Forging: The annealed steel ingot is forged to obtain a forging material with a diameter of 70mm. The initial forging temperature is 1150℃ and the final forging temperature is 830℃.

[0101] Solution heat treatment: The forging was solution heat treated at 820℃ for 2 hours;

[0102] Aging treatment: The forging material after solution heat treatment is aged at 480℃ for 4 hours to obtain ultra-high strength steel.

[0103] Table 1. Chemical composition of the ultra-high strength steel in Example 1 (unit: wt%)

[0104] 0.003 17.9 11.1 4.87 0.98 0.0017 0.0007 0.0012 0.004 0.0059 margin

[0105] As shown in Table 1, in this embodiment, [La] is 0.0059 wt%, and T[O] is... s It is 0.0017wt%, T[S] m The content is 0.0012 wt%, T[N] m The value is 0.0007wt%, and the correction factors a, b, and c take values ​​of 3, 2, and 1 respectively. Substituting these values ​​into the above data, we obtain {[RE]-(a·T[O]}. s +b·T[S] m )} / (c·T[N] m It is approximately -2.3.

[0106] Table 2 shows the distribution of the main inclusions in the ultra-high strength steel prepared in this embodiment. It can be seen that the main components of the inclusions in the steel in this embodiment are TiN, RE2S3 and RE2O3, accounting for 59%, 34% and 7% respectively. The proportion of fine inclusions with a size in the range of 1 to 5 μm is 98.6%, only 1.4% of the inclusions have a size of 5 to 10 μm, and there are no larger inclusions with a size greater than 10 μm.

[0107] Figure 1 a shows the morphology of the original austenite in the solid solution state in this embodiment. The original austenite grain grade was evaluated as grade 10 using the intercept method, and the elongation ratio Y = 18.4 ≥ 10 was calculated.

[0108] After aging treatment, the matrix exhibits a refined multi-level martensitic structure. Figure 2 a indicates that the average size of the martensite lath bundles is 1.5 μm, and X-ray diffraction experiments determined that the steel contains 13.7% reversed austenite. Figure 3 (a,b) shows the microstructure of martensitic lath bundles. Figure 3 (c, d) show the distribution of the Ni3Ti nano-precipitate and the Mo-rich phase (indicated by the arrows) after aging, as well as the elongated or spherical reversed austenite. The properties of the ultra-high strength steel in this embodiment are shown in Table 15.

[0109] Table 2. Types and number / percentage of inclusions of different sizes in Example 1

[0110]

[0111] Example 2

[0112] This embodiment provides a method for preparing ultra-high strength steel, including the following steps:

[0113] Smelting: VIM smelting was carried out according to the chemical composition in Table 3, and steel ingots with a diameter of 300 mm were obtained after casting. High-purity metal materials that have been thoroughly degreased and dried were used in the smelting process. The refining time before adding rare earth lanthanum (La) was 65 min, the refining time after adding La was 35 min, the vacuum degree during the refining process was 2.5 Pa, and the tapping time after refining was 15 min.

[0114] Annealing: The steel ingot with a diameter of 300 mm was annealed at 1150℃ for 6 hours;

[0115] Forging: The annealed steel ingot is forged to obtain a forging material with a diameter of 70mm. The initial forging temperature is 1150℃ and the final forging temperature is 830℃.

[0116] Solution heat treatment: The forging was solution heat treated at 820℃ for 2 hours;

[0117] Aging treatment: The forging material after solution heat treatment is aged at 480℃ for 4 hours to obtain ultra-high strength steel.

[0118] Table 3 Chemical composition of the ultra-high strength steel in Example 2 (unit: wt%)

[0119] 0.0054 18.0 11.1 4.96 0.97 0.0011 0.0006 0.0012 0.004 0.0041 margin

[0120] As shown in Table 3, in this embodiment, [La] is 0.0041 wt%, and T[O] is... s It is 0.0011 wt%, T[S] m The content is 0.0012 wt%, T[N] m The value is 0.0006wt%, and the correction factors a, b, and c take values ​​of 2, 1.5, and 0.5 respectively. Substituting these values ​​into the above data, we obtain {[RE]-(a·T[O]}. s +b·T[S] m )} / (c·T[N] m The value is approximately 0.33.

[0121] Table 4 shows the distribution of the main inclusions in the ultra-high strength steel prepared in this embodiment. It can be seen that the main components of the inclusions in the steel in this embodiment are TiN, RE2S3 and RE2O3, accounting for 81%, 12% and 5% respectively. The proportion of fine inclusions with a size in the range of 1 to 5 μm is 99.6%, only 0.4% of the inclusions have a size of 5 to 10 μm, and there are no larger inclusions with a size greater than 10 μm.

[0122] Figure 1 b shows the morphology of the original austenite in the solid solution state in this embodiment. The original austenite grain grade was evaluated as 9.5 using the intercept method, and the elongation ratio Y = 18.4 ≥ 10 was calculated.

[0123] After aging treatment, the matrix exhibits a refined multi-level martensitic structure. Figure 2 b indicates that the average size of the martensite lath bundles is 4.5 μm, and X-ray diffraction experiments determined that the steel contains 5.3% reversed austenite. The properties of the ultra-high strength steel in this embodiment are shown in Table 15.

[0124] Table 4. Types and proportions of inclusions of different sizes in Example 2

[0125]

[0126] Example 3

[0127] This embodiment provides a method for preparing ultra-high strength steel, including the following steps:

[0128] Smelting: VIM smelting was carried out according to the chemical composition in Table 5, and steel ingots with a diameter of 300 mm were obtained after casting. High-purity metal materials that have been thoroughly degreased and dried were used in the smelting process. The refining time before adding rare earth lanthanum (La) was 80 min, the refining time after adding it was 50 min, the vacuum degree during the refining process was 3.5 Pa, and the tapping time after refining was 10 min.

[0129] Annealing: The steel ingot with a diameter of 300 mm was annealed at 1150℃ for 8 hours;

[0130] Forging: The annealed steel ingot is forged to obtain a forging material with a diameter of 80mm. The initial forging temperature is 1180℃ and the final forging temperature is 850℃.

[0131] Solution heat treatment: The forging was solution heat treated at 850℃ for 3 hours;

[0132] Aging treatment: The forging material after solution heat treatment is aged at 500℃ for 2 hours to obtain ultra-high strength steel.

[0133] Table 5. Chemical composition of the ultra-high strength steel in Example 3 (wt%)

[0134] 0.0060 17.9 11.1 4.90 1.04 0.0014 0.0008 0.0021 0.004 0.013 margin

[0135] As shown in Table 5, in this embodiment, [La] is 0.013 wt%, and T[O] is... s It is 0.0014 wt%, T[S] m The content is 0.0021 wt%, T[N] m The value is 0.0008wt%, and the correction factors a, b, and c take values ​​of 5, 2.5, and 1.5 respectively. Substituting these values ​​into the above data, we obtain {[RE]-(a·T[O]}. s +b·T[S] m )} / (c·T[N] m It is approximately 0.625.

[0136] Table 6 shows the distribution of the main inclusions in the ultra-high strength steel prepared in this embodiment. It can be seen that the main inclusions in the steel are TiN, RE2S3, and RE2O3, accounting for 12%, 40%, and 47% respectively. Fine inclusions with a size range of 1–5 μm account for 97.4%, with only 2.6% of inclusions having a size of 5–10 μm. There are no larger inclusions larger than 10 μm. Furthermore, scanning electron microscopy and energy dispersive spectroscopy (EDS) were used to analyze the inclusions. Figure 4 The RE-OS-TiN composite inclusions can be observed, which are nearly spherical or ellipsoidal in shape and smaller than 5 μm in size. The above characteristics also exist in other embodiments, which will not be presented individually.

[0137] Figure 1 c shows the morphology of the original austenite in the solid solution state in this embodiment. The original austenite grain grade was evaluated as 10.5 using the intercept method, and the elongation ratio Y = 14.1 ≥ 10 was calculated.

[0138] After aging treatment, the matrix exhibits a refined multi-level martensitic structure. Figure 2 c indicates that the average size of the martensite lath bundles is 1.3 μm, and X-ray diffraction experiments determined that the steel contains 5.8% reversed austenite. The properties of the ultra-high strength steel in this embodiment are shown in Table 15.

[0139] Table 6. Types and proportions of inclusions of different sizes in Example 3

[0140]

[0141] Example 4

[0142] This embodiment provides a method for preparing ultra-high strength steel, including the following steps:

[0143] Smelting: VIM smelting was carried out according to the chemical composition in Table 7, and steel ingots with a diameter of 300 mm were obtained after casting. High-purity metal materials that have been thoroughly degreased and dried were used in the smelting process. The refining time before adding rare earth lanthanum (La) was 85 min, the refining time after adding it was 55 min, the vacuum degree during the refining process was 3 Pa, and the tapping time after refining was 12 min.

[0144] Annealing: The steel ingot with a diameter of 300 mm was annealed at 1150℃ for 8 hours;

[0145] Forging: The annealed steel ingot is forged to obtain a forging material with a diameter of 80mm. The initial forging temperature is 1180℃ and the final forging temperature is 850℃.

[0146] Solution heat treatment: The forging was solution heat treated at 850℃ for 3 hours;

[0147] Aging treatment: The forging material after solution heat treatment is aged at 500℃ for 2 hours to obtain ultra-high strength steel.

[0148] Table 7. Chemical composition (wt%) of the ultra-high strength steel in Example 4.

[0149] 0.0064 17.8 11.1 4.86 1.03 0.0014 0.0005 0.0017 0.004 0.018 margin

[0150] As shown in Table 7, in this embodiment, [La] is 0.018 wt%, and T[O] is... s It is 0.0014 wt%, T[S] m The content is 0.0017 wt%, T[N] m The value is 0.0005wt%, and the correction factors a, b, and c take values ​​of 4, 2, and 1 respectively. Substituting these values ​​into the above data, we obtain {[RE]-(a·T[O]}. s +b·T[S] m )} / (c·T[N] m It is approximately 18.

[0151] Table 8 shows the distribution of the main inclusions in the ultra-high strength steel prepared in this embodiment. It can be seen that the main components of the inclusions in the steel in this embodiment are TiN, RE2S3 and RE2O3, accounting for 8%, 29% and 62% respectively. The proportion of fine inclusions with a size in the range of 1 to 5 μm is 98.1%, only 1.9% of the inclusions have a size of 5 to 10 μm, and there are no larger inclusions with a size greater than 10 μm.

[0152] Figure 1 Figure d shows the morphology of the original austenite in the solid solution state in this embodiment. The original austenite grain grade was evaluated as 9.5 using the intercept method, and the elongation ratio Y = 14.1 ≥ 10 was calculated.

[0153] After aging treatment, the matrix exhibits a refined multi-level martensitic structure. Figure 2 d indicates that the average size of the martensite lath bundles is 1.1 μm, and X-ray diffraction experiments determined that the steel contains 5.1% reversed austenite. The properties of the ultra-high strength steel in this embodiment are shown in Table 15.

[0154] Table 8. Types and proportions of inclusions of different sizes in Example 4

[0155]

[0156]

[0157] Comparative Example 1

[0158] This comparative example provides a method for preparing ultra-high strength steel, including the following steps:

[0159] Smelting: According to the chemical composition in Table 9, VIM smelting was carried out, and after casting, a steel ingot with a diameter of 300mm was obtained. High-purity metal materials that have been thoroughly degreased and dried were used in the smelting process. The total refining time was 90min, the vacuum degree during the refining process was 3.5Pa, and the tapping time after refining was 15min.

[0160] Annealing: The steel ingot with a diameter of 300 mm was annealed at 1150℃ for 8 hours;

[0161] Forging: The annealed steel ingot is forged to obtain a forging material with a diameter of 80mm. The initial forging temperature is 1180℃ and the final forging temperature is 850℃.

[0162] Solution heat treatment: The forging was solution heat treated at 850℃ for 3 hours;

[0163] Aging treatment: The forging material after solution heat treatment is aged at 500℃ for 2 hours to obtain ultra-high strength steel.

[0164] Table 9. Chemical composition of the ultra-high strength steel in Comparative Example 1 (unit: wt%)

[0165] 0.0060 17.8 11.2 4.89 0.95 0.0011 0.0007 0.0016 0.004 margin

[0166] Table 10 shows the distribution of the main inclusions in the ultra-high strength steel prepared in this comparative example. Figure 5 The scanning electron microscope (SEM) images and compositional distribution of inclusions in the ultra-high strength steel prepared in this comparative example are shown in Table 10 and... Figure 5 As can be seen from the data, the inclusions in the comparative steel are mainly composed of TiN ( Figure 5Of the total, 99 wt% were small inclusions with a size range of 1–5 μm, 93.8% were small inclusions with a size range of 5–10 μm, and there were no large inclusions larger than 10 μm.

[0167] Figure 1 e shows the morphology of the original austenite in the solid solution state in this comparative example. The original austenite grain grade was evaluated as 8.5 using the intercept method, and the elongation ratio Y = 14.1 ≥ 10 was calculated.

[0168] After aging treatment, the matrix exhibits a refined multi-level martensitic structure. Figure 2 The value of e indicates that the average size of the martensite lath bundles is 5.5 μm, and X-ray diffraction experiments determined that the steel contains 3.2% reversed austenite. The properties of the ultra-high strength steel in this comparative example are shown in Table 15.

[0169] Table 10 shows the types and proportions of inclusions of different sizes in Comparative Example 1.

[0170]

[0171] Comparative Example 2

[0172] This comparative example provides a method for preparing ultra-high strength steel, including the following steps:

[0173] Smelting: VIM smelting was carried out according to the chemical composition in Table 11, and steel ingots with a diameter of 300 mm were obtained after casting. High-purity metal materials that have been thoroughly degreased and dried were used in the smelting process. The refining time before adding rare earth lanthanum (La) was 70 min, the refining time after adding it was 40 min, the vacuum degree during the refining process was 3 Pa, and the tapping time after refining was 20 min.

[0174] Annealing: The steel ingot with a diameter of 300 mm was annealed at 1100℃ for 6 hours;

[0175] Forging: The annealed steel ingot is forged to obtain a forging material with a diameter of 100mm. The initial forging temperature is 1180℃ and the final forging temperature is 900℃.

[0176] Solution heat treatment: The forging was solution heat treated at 820℃ for 3 hours;

[0177] Aging treatment: The forging material after solution heat treatment is aged at 500℃ for 2 hours to obtain ultra-high strength steel.

[0178] Table 11 Chemical composition of ultra-high strength steel in Comparative Example 2 (unit: wt%)

[0179] 0.0079 17.7 11.1 5.03 1.01 0.0007 0.0009 0.0013 0.004 0.0065 margin

[0180] Table 12 shows the distribution of the main inclusions in the ultra-high strength steel prepared in this comparative example. It can be seen that the main components of the inclusions in the steel in this comparative example are TiN, RE2S3 and RE2O3, accounting for 56%, 36% and 8% respectively. The proportion of fine inclusions with a size in the range of 1 to 5 μm is 98%, only 2.0% of the inclusions have a size of 5 to 10 μm, and there are no larger inclusions with a size greater than 10 μm.

[0181] Figure 1 f shows the morphology of the original austenite in the solid solution state in this comparative example. The original austenite grain grade was evaluated as level 6 using the intercept method, and the elongation ratio Y = 9 < 10 was calculated.

[0182] After aging treatment, the matrix exhibits a refined multi-level martensitic structure. Figure 2 f indicates that the average size of the martensite lath bundles is 8 μm, and X-ray diffraction experiments determined that the steel contains 5.5% reversed austenite. The properties of the ultra-high strength steel in this comparative example are shown in Table 15.

[0183] Table 12 shows the types and proportions of inclusions of different sizes in Comparative Example 2.

[0184]

[0185] Comparative Example 3

[0186] This comparative example provides a method for preparing ultra-high strength steel, including the following steps:

[0187] Smelting: VIM smelting was carried out according to the chemical composition in Table 13, and steel ingots with a diameter of 300 mm were obtained after casting. High-purity metal materials that have been thoroughly degreased and dried were used in the smelting process. The refining time before adding rare earth lanthanum (La) was 85 min, the refining time after adding La was 55 min, the vacuum degree during the refining process was 3.5 Pa, and the tapping time after refining was 10 min.

[0188] Annealing: The steel ingot with a diameter of 300 mm was annealed at 1160℃ for 8 hours;

[0189] Forging: The annealed steel ingot is forged to obtain a forging material with a diameter of 80mm. The initial forging temperature is 1180℃ and the final forging temperature is 900℃.

[0190] Solution heat treatment: The forging was solution heat treated at 850℃ for 3 hours;

[0191] Aging treatment: The forging material after solution heat treatment is aged at 500℃ for 2 hours to obtain ultra-high strength steel.

[0192] Table 13 Chemical composition of ultra-high strength steel in Comparative Example 3 (unit: wt%)

[0193] 0.0030 17.8 11.2 5.06 0.094 0.0010 0.0006 0.0010 0.0014 0.066 margin

[0194] Table 14 shows the distribution of the main inclusions in the ultra-high strength steel prepared in this comparative example. It can be seen that the main components of the inclusions in the steel in this comparative example are TiN, RE2S3 and RE2O3, accounting for 3%, 15% and 84% respectively. The proportion of fine inclusions with a size in the range of 1 to 5 μm is 98.2%, only 1.8% of the inclusions have a size of 5 to 10 μm, and there are no large inclusions larger than 10 μm.

[0195] Figure 1 g shows the morphology of the original austenite in the solid solution state in this comparative example. The original austenite grain grade was evaluated as grade 9 using the intercept method, and the elongation ratio Y = 14.1 ≥ 10 was calculated.

[0196] After aging treatment, the matrix exhibits a refined multi-level martensitic structure. Figure 2 g indicates that the average size of the martensite lath bundles is 2.5 μm, and X-ray diffraction experiments determined that the steel contains 3.5% reversed austenite. The properties of the ultra-high strength steel in this comparative example are shown in Table 15.

[0197] Table 14 shows the types and proportions of inclusions of different sizes in Comparative Example 2.

[0198]

[0199] Table 15 presents the mechanical properties of the examples and comparative examples after solution treatment and aging. Firstly, compared to the impact toughness (A) of Examples 1-4... KV =20~31J), because no rare earth La was added in Comparative Example 1, the inclusions were mainly quadrilateral TiN, and the proportion of inclusions with a size greater than 5μm was relatively high, which resulted in insufficient impact toughness (A KV =12J); In contrast, Comparative Example 3 added 0.066% rare earth La, which is outside the required range for rare earth addition. Although the inclusions were modified and refined, from Figure 6 It can be seen that, due to excessive addition, large undissolved elemental rare earth La particles are formed in the matrix, which in turn destroys the homogeneity and continuity of the matrix, causing embrittlement, a significant decrease in tensile plasticity, and difficulty in testing A. KV The specific values; unlike the two comparative examples mentioned above, although Comparative Example 2 added 0.006% rare earth La, meeting the requirements for rare earth addition range and achieving the effect of modifying and refining inclusions, the small forging deformation (which can be seen from the forging elongation ratio) resulted in a lower original austenite grain grade after forging, leading to a larger martensite lath bundle size and making it difficult to further improve toughness. Therefore, Comparative Example 2 had poor impact toughness (A).KV =16J).

[0200] Table 15 Mechanical property results of Examples 1-4 and Comparative Examples 1-3

[0201]

[0202]

[0203] The above embodiments and comparative examples show that, through rare earth alloying and microstructure control, the present invention can obtain an ultra-pure steel matrix, modify or refine inclusions, and a multi-level multiphase microstructure. Under the premise of ensuring the strengthening of nano-precipitates, this multiple toughening effect can enable the V-notch impact energy of 2.2 GPa ultra-high strength steel to reach 20-25 J and above, with a maximum of 31 J. Compared with steel without rare earth alloying or insufficient refinement of multi-level multiphase microstructure, its impact toughness can be improved by 2-3 times.

[0204] It will be readily understood by those skilled in the art that, without conflict, the advantageous technical features of the above-mentioned methods can be freely combined and superimposed.

[0205] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention. The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the protection scope of the present invention.

Claims

1. An ultra-high strength steel, characterized in that, The rare earth element (RE) content in the ultra-high strength steel satisfies the following formula: -80<{[RE]-(a·T[O] s +b·T[S] m )} / (c·T[N] m )<200; Wherein, [RE] represents the content of rare earth element RE in ultra-high strength steel; T[O] s The dissolved oxygen content in ultra-high strength steel; T[S] m The total sulfur content in ultra-high strength steel; T[N] m The total nitrogen content in ultra-high strength steel; [RE]、T[O] s 、T[S] m T[N] m All are expressed as a percentage by mass; a, b, and c are correction coefficient one, correction coefficient two, and correction coefficient three, respectively, with values ​​of 2~5, 1.5~2.5, and 0.5~1.

5. In the ultra-high strength steel, more than 50% of the total number of inclusions constitute the average equivalent diameter. D m The inclusions are spherical, near-spherical, or granular in size, ranging from 1 to 5 μm, and are diffusely distributed. The diffusely distributed inclusions include rare earth sulfides RE2S3, rare earth oxides RE2O3, titanium nitride TiN, and rare earth oxygen-sulfur-titanium nitride complexes.

2. The ultra-high strength steel according to claim 1, characterized in that, The chemical composition of the ultra-high strength steel includes: C ≤ 0.01 wt%, Ni: 17.0~18.5 wt%, Co: 11.0~12.5 wt%, Mo: 4.5~5.5 wt%, Ti: 0.9~1.1 wt%, S ≤ 0.005 wt%, P ≤ 0.01 wt%, O ≤ 0.002 wt%, N ≤ 0.0015 wt%, rare earth element RE: 0.001~0.03 wt%, and the balance being Fe.

3. The ultra-high strength steel according to claim 2, characterized in that, The rare earth element RE is one of lanthanum (La), cerium (Ce), and yttrium (Y).

4. The ultra-high strength steel according to claim 2, characterized in that, In the ultra-high strength steel: the mass percentage of O is ≤0.001wt%, the mass percentage of N is <0.001wt%, and the mass percentage of rare earth RE is 0.004~0.018wt%.

5. The ultra-high strength steel according to claim 2, characterized in that, In the ultra-high strength steel: the mass percentage of O is ≤0.001wt%, the mass percentage of N is <0.001wt%, and the mass percentage of rare earth RE is 0.004~0.014wt%.

6. The ultra-high strength steel according to claim 1, characterized in that, The ultra-high strength steel has a multiphase structure and original austenitic grains with a grain size greater than or equal to 8. The multiphase structure includes martensite, nano-precipitates, and reversed austenite. The martensite includes martensite lath bundles. The average size of the martensitic lath bundles is less than or equal to 5 μm; The nanoprecipitated phase includes a Ni3Ti phase and a molybdenum-rich phase, and the equivalent diameter of the nanoprecipitated phase is 5~50 nm. In the multiphase microstructure, the volume percentage of reversed austenite is 5-20 vol.%.

7. The ultra-high strength steel according to claim 1, characterized in that, The properties of the ultra-high strength steel are as follows: yield strength ≥ 2.1 GPa, tensile strength ≥ 2.2 GPa, elongation after fracture ≥ 9%, reduction of area ≥ 50%, and V-notch impact energy ≥ 20 J.

8. A method for preparing ultra-high strength steel according to any one of claims 1 to 7, characterized in that, Includes the following steps: Smelting: According to the chemical composition of the ultra-high strength steel, the raw materials are smelted and cast to obtain steel ingots; Annealing: The steel ingot is subjected to annealing treatment; the holding time of the annealing treatment is... t h >(1.5 D I ) / 100h, where, D I These are the cross-sectional dimensions of the steel ingot obtained after casting, in mm; Forging: Forging is performed on annealed steel ingots to obtain forging materials; Solution heat treatment: The forging material is subjected to solution heat treatment; the duration of the solution treatment is... t s > (1.5~2.5) D b / 100h, of which, D b The cross-sectional dimensions of the forged material obtained after forging are in mm; Aging treatment: The forging material after solution heat treatment is subjected to aging treatment to obtain the ultra-high strength steel.

9. The method for preparing ultra-high strength steel according to claim 8, characterized in that, The drawing ratio Y in the forging process is ≥ 10, where Y = ( D I / D b ) 2 .

10. The method for preparing ultra-high strength steel according to claim 8, characterized in that, In the smelting step, smelting is carried out using either Vacuum Induction Refining (VIM) or Vacuum Induction Refining (VIM) + Vacuum Arc Remelting (VAR).

11. The method for preparing ultra-high strength steel according to claim 8, characterized in that, Rare earth element RE is added during VIM refining; the refining time before adding rare earth element RE is t1≥60min, and the refining time after adding rare earth element RE is t2≥30min.

12. The method for preparing ultra-high strength steel according to claim 8, characterized in that, Vacuum degree P during VIM refining v The Pa value is 1~5 Pa.

13. The method for preparing ultra-high strength steel according to claim 8, characterized in that, The tapping time t3 after the smelting process is completed is ≤30min.

14. The method for preparing ultra-high strength steel according to claim 8, characterized in that, The annealing temperature is 1000~1100℃; and / or The forging process is performed at a temperature of 800~1200℃; and / or The solution heat treatment temperature is 800~900℃; and / or The aging treatment temperature is 460~540℃; the aging treatment time is 1~16h.

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