Ultrahigh-strength and ultrahigh-toughness steel highly adaptable to extreme stress working conditions and preparation method thereof

Abstract

An ultra-high strength and toughness steel with high adaptability to extreme stress conditions and its preparation method are disclosed, relating to the field of ultra-high strength and toughness steel technology; wherein the chemical composition of the ultra-high strength and toughness steel, by mass percentage, includes: C: 0.24%-0.27%; Si≤0.10%; Mn: 0.85%-1.15%; Cr: 1.50%-2.00%; Ni: 2.40%-2.80%; Mo: 0.25%-0.50%; Nb: 0.05%-0.10%; S≤0.0015%; P≤0.0025%; H≤1.5ppm; O≤20ppm, N≤50ppm; the remainder being iron (Fe) and unavoidable impurities. This ultra-high strength and toughness steel is suitable for parts with effective cross-sectional dimensions of 100-350mm, offering strong versatility. After heat treatment, it has a matrix of tempered martensite and a small amount of retained austenite, combining ultra-high strength with good plasticity and toughness. Its yield strength is ≥1200MPa, tensile strength is ≥1500MPa, elongation is ≥12%, reduction of area is ≥45%, and room temperature impact energy is ≥60J, which is more than 30% higher than that of traditional high-strength steel. This ultra-high strength and toughness steel has excellent comprehensive mechanical properties and is suitable for extreme stress conditions and environments.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength and toughness steel technology, specifically to an ultra-high strength and toughness steel highly adaptable to extreme stress conditions and its preparation method. Background Technology

[0002] With the development of industries such as aerospace, marine engineering, and heavy machinery, higher demands are being placed on high-performance materials. To meet requirements such as structural weight reduction, increased load-bearing capacity, weather resistance, and reliability, high-strength, tough, and large-section steels are needed. Therefore, the development of high-strength steels that also possess good toughness, weather resistance, and the ability to withstand extreme conditions such as low temperatures or high-speed impacts has attracted attention. On the other hand, directly selecting existing high-strength steel grades or simply increasing the content of various chemical elements in steel to redesign the chemical composition to meet application requirements would significantly increase production costs, lead to excessive material waste, and hinder the sustainable development of high-performance steels.

[0003] Conventional medium-carbon high-strength steels possess high strength but relatively low ductility and toughness, making them vulnerable to severe impacts. Therefore, resolving the contradiction between material strength and toughness, and achieving a good balance between the two, is a significant technical challenge. Existing Cr-Ni-Mo series high-strength steels are costly, hindering their large-scale application in modern engineering. In light of these factors, there is an urgent need to develop an ultra-high strength and toughness steel highly adaptable to extreme stress conditions to meet the material requirements of critical equipment. Summary of the Invention

[0004] The purpose of this invention is to propose an ultra-high strength and toughness steel that is highly adaptable to extreme stress conditions and its preparation method, which solves the problem of poor strength and toughness matching in traditional high-strength steel and is suitable for extreme working conditions such as high load and high-speed impact.

[0005] The technical solution adopted in this invention is: an ultra-high strength and toughness steel highly adaptable to extreme stress conditions, wherein the chemical composition of the ultra-high strength and toughness steel, by mass percentage, includes: C: 0.24%-0.27%; Si≤0.10%; Mn: 0.85%-1.15%; Cr: 1.50%-2.00%; Ni: 2.40%-2.80%; Mo: 0.25%-0.50%; Nb: 0.05%-0.10%; S≤0.0015%; P≤0.0025%; H≤1.5ppm; O≤20ppm, N≤50ppm; the remainder being Fe and unavoidable impurities.

[0006] A method for preparing ultra-high strength and toughness steel with high adaptability to extreme stress conditions, the method comprising the following steps: S1. Ingredients: Prepare ingredients according to the chemical composition and its mass percentage; S2. Smelting: The proportioned raw materials are smelted in an electric arc furnace and then vacuum refined, and finally cast into alloy ingots. S3. Forging: Forging the alloy ingot formed in step S2, with a heating temperature range of 1150-1200℃, a forging temperature range of 850-1170℃, and a forging ratio of ≥5. S4. Quenching and Tempering Heat Treatment: The forging billet formed after forging in step S3 is subjected to quenching and tempering heat treatment to obtain ultra-high strength and toughness steel. In the quenching and tempering heat treatment, the austenitizing temperature is 870-950℃, and the cooling method is water cooling; the tempering temperature is 180-300℃, and the tempering cooling method is air cooling.

[0007] As a preferred embodiment, after the quenching and tempering heat treatment, the matrix structure of the ultra-high strength and toughness steel is tempered martensite and a small amount of retained austenite.

[0008] As a preferred embodiment, the mechanical properties of the ultra-high strength and toughness steel are: yield strength ≥1200MPa, tensile strength ≥1500MPa, elongation ≥12%, reduction of area ≥45%, and room temperature impact energy ≥60J.

[0009] As a preferred embodiment, in step S3, the effective cross-sectional size of the forging blank is 100-350mm.

[0010] Compared with the prior art, the beneficial effects of the present invention are as follows: The advantages of the present invention lie in leveraging the synergistic effects among chemical elements such as Cr, Ni, Mn, Mo, and Nb: controlling the C content at a low level reduces the level of lattice distortion compared to conventional high-strength steel, improves toughness, and ensures strength by obtaining a fully martensitic structure; since Si is not conducive to improving toughness, Si is controlled at a low level; controlling the Cr content at 1.50%-2.00% improves the hardenability of the material, ensuring that 100-350mm cross-section workpieces are fully hardened and obtain a martensitic structure throughout the cross-section; controlling the Ni content at 2.40%-2.80% is about 20% lower than that of 34CrNi3MoV steel, reducing material costs without significantly impairing the toughness of the material; controlling the unavoidable impurity elements such as S, P, H, O, and N at low levels reduces the anisotropy, cold brittleness tendency, and inclusion content of the steel, reduces crack sources during deformation, and helps to improve toughness. Detailed Implementation

[0011] The present invention will now be described in detail through exemplary embodiments. However, it should be understood that, without further description, elements, structures, and features in one embodiment may be advantageously incorporated into other embodiments.

[0012] It should be noted that, unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "a," "an," or "the," etc., used in the specification and claims of this patent application do not express a limitation on quantity, but rather indicate the presence of at least one; the terms "first," "second," and "third," as used herein, should not be considered as a limitation on the order of components, but are merely for distinguishing different components; the terms "comprising," "including," etc., indicate that the elements or objects preceding "comprising" or "including" encompass the elements or objects listed following "comprising" or "including" and their equivalents, but do not exclude other elements or objects having the same function.

[0013] An ultra-high strength and toughness steel highly adaptable to extreme stress conditions, the chemical composition of the ultra-high strength and toughness steel by mass percentage includes: C: 0.24%-0.27%; Si≤0.10%; Mn: 0.85%-1.15%; Cr: 1.50%-2.00%; Ni: 2.40%-2.80%; Mo: 0.25%-0.50%; Nb: 0.05%-0.10%; S≤0.0015%; P≤0.0025%; H≤1.5ppm; O≤20ppm, N≤50ppm; the remainder is Fe (iron) and unavoidable impurities.

[0014] A method for preparing ultra-high strength and toughness steel with high adaptability to extreme stress conditions includes the following steps: S1. Batching: Prepare materials according to the above chemical composition and its mass percentage; in the specific embodiments below, in order to achieve the target chemical composition, the sintering rate of each element is estimated according to the parameters of the smelting equipment, and the amount of raw materials added is calculated. For example, the raw materials include the following raw materials by mass percentage: 92.0-93.5% scrap steel and pig iron, 2.64-3.29% micro-carbon chromium alloy, 2.38-2.59% nickel plate, 0.45-0.76% molybdenum plate, 0.90-1.17% ferromanganese, 0.07-0.15% ferroniobium, and 0.21-0.24% carbon raiser; S2. Smelting: The proportioned raw materials are smelted in an electric arc furnace and then refined in a vacuum furnace, and finally cast into alloy ingots. Specifically, the proportioned raw materials are smelted in an electric arc furnace and then refined in a vacuum furnace. During the smelting process, P ≤ 0.002% and C ≤ 0.06% are controlled. During the refining process, [O] ≤ 30ppm is controlled. During the vacuum process, the molten steel is kept in a high vacuum environment of ≤ 10Pa for more than 20 minutes to further reduce the gas content. Finally, the molten steel is poured into alloy ingots at 1570-1580℃. S3. Forging: The alloy ingot formed in step S2 is forged. The heating temperature range is 1150-1200℃, the forging temperature range is 850-1170℃, and the forging ratio is ≥5 to obtain a forging billet. The effective cross-sectional size of the forging billet is 100-350mm. S4. Quenching and Tempering Heat Treatment: The forging billet formed after forging in step S3 is subjected to quenching and tempering heat treatment to obtain ultra-high strength and toughness steel. In the quenching and tempering heat treatment, the austenitizing temperature is 870-950℃, and the cooling method is water cooling; the tempering temperature is 180-300℃, and the tempering cooling method is air cooling.

[0015] After quenching and tempering heat treatment, the matrix structure of ultra-high strength and toughness steel is tempered martensite and a small amount of retained austenite. The mechanical properties of ultra-high strength and toughness steel are: yield strength ≥1200MPa, tensile strength ≥1500MPa, elongation ≥12%, reduction of area ≥45%, and room temperature impact energy ≥60J.

[0016] The chemical composition design basis for this invention in manufacturing ultra-high strength and toughness steel is as follows: Carbon (C), as the main strengthening element in steel, dissolves in ferrite to form interstitial solid solutions. Through its interaction with iron atoms, it hinders dislocation movement, thereby significantly improving the strength and hardness of steel. However, with increasing carbon content, the toughness of steel usually decreases, leading to embrittlement. This is because increased carbon content increases lattice distortion and resistance to dislocation movement, making it prone to stress concentration under external forces, resulting in reduced toughness. The ultra-high strength and toughness steel mentioned in this invention specifies a carbon mass fraction of 0.24%-0.27%, ensuring high strength by achieving a fully martensitic structure while maintaining ductility and toughness based on a lath martensitic structure with a lower carbon content.

[0017] Manganese (Mn) acts as a solid solution element in steel, dissolving in ferrite (α-Fe) and austenite (γ-Fe) to form substitutional solid solutions. Through this solid solution strengthening mechanism, the size difference and electron cloud structure between manganese and iron atoms cause lattice distortion, thereby increasing the resistance to dislocation movement and improving the strength and hardness of the steel. An appropriate amount of manganese can improve the toughness of steel, mainly due to its deoxidizing and desulfurizing effects and its improvement of inclusion morphology. However, excessive manganese content may lead to a decrease in toughness, as too much manganese increases the steel's brittleness. In the ultra-high strength and toughness steel mentioned in this invention, the mass fraction of manganese is limited to 0.85%-1.15%, a range that helps to obtain high strength while retaining good plasticity and toughness.

[0018] The main role of chromium (Cr) is to improve hardenability, improve the microstructure of steel, enhance mechanical properties, and make the material more suitable for large-section billets. Chromium dissolves into ferrite, causing lattice distortion and producing solid solution strengthening, increasing the strength and hardness of ferrite; however, excessive chromium can reduce the material's toughness. Chromium forms various highly stable alloy carbides with carbon, such as Cr23C6 and Cr7C3, which can effectively hinder dislocation movement, further improving the strength and hardness of steel; however, excessive chromium content can affect the material's plasticity and toughness. The ultra-high strength and toughness steel of this invention limits the chromium mass fraction to 1.50%-2.00%, a content that ensures the steel's hardenability and strength while maintaining a certain level of toughness.

[0019] Nickel (Ni) can reduce the resistance to dislocation movement in steel, allowing dislocations to move more evenly during deformation and thus improving the steel's toughness. However, excessive nickel content can promote the segregation of certain impurity elements (such as phosphorus and tin) at grain boundaries during tempering, reducing grain boundary bonding strength and increasing temper brittleness, leading to a significant decrease in impact toughness. Furthermore, nickel is a relatively expensive metal with high prices on the global market; adding large amounts of nickel to steel significantly increases raw material costs. The ultra-high strength and toughness steel mentioned in this invention limits the nickel mass fraction to 2.40%-2.80%, a range that provides good toughness while controlling production costs.

[0020] Molybdenum (Mo) is primarily used to improve the strength, hardness, and wear resistance of steel. It also improves the toughness and impact resistance of steel by refining grains and inhibiting grain boundary embrittlement. Molybdenum enhances the hardenability and tempering resistance of steel, suppresses temper brittleness, and allows the steel to achieve superior performance during heat treatment, making it an important alloying element in the manufacture of high-strength low-alloy steel. In the ultra-high strength and toughness steel mentioned in this invention, the molybdenum mass fraction is limited to 0.25%-0.50%. This content range allows the material to achieve good hardenability and toughness while controlling material costs.

[0021] Niobium (Nb) combines with carbon and nitrogen in steel to form numerous fine, dispersed Nb(C,N) carbonitrides. These particles effectively hinder dislocation movement, thereby significantly increasing the yield strength and tensile strength of the steel. Simultaneously, they strongly inhibit austenite grain growth, refining the final microstructure (such as ferrite and pearlite), achieving "fine-grain strengthening." This not only improves strength but also typically enhances toughness and lowers the brittle-to-ductile transition temperature. Using niobium carbonitride (Nb(C,N)) as the core carrier, through mechanisms such as grain refinement, precipitation strengthening, delayed recrystallization, and improved high-temperature performance, a synergistic improvement in the strength, toughness, and processability of steel is achieved, making it a key element in microalloyed steel.

[0022] Through the microalloying of Nb, a large number of fine, dispersed Nb(C,N) particles are formed in the steel. These particles not only play a role in precipitation strengthening, but more importantly, they pin grain boundaries during high-temperature forging and heat treatment, effectively inhibiting austenite grain growth and achieving fine-grain strengthening. This fine-grain strengthening mechanism is the key to the significant improvement in plasticity and toughness, especially impact toughness, of the material of this invention while maintaining high strength.

[0023] To facilitate understanding and implementation of the present invention, more specific and detailed embodiments are provided below for reference. The advantages of the present invention will become apparent from the description and performance results of the following specific embodiments. Unless otherwise specified, the raw materials used in the following embodiments are all commercially available.

[0024] Example 1

[0025] The chemical composition of the ultra-high strength and toughness steel is formulated by mass percentage as follows: C: 0.24%; Si: 0.06%; Mn: 0.95%; Cr: 1.60%; Ni: 2.48%; Mo: 0.30%; Nb: 0.09%; S: 0.0010%; P: 0.0022%; H: 0.9ppm; O: 15ppm; N: 42ppm; the remainder is iron (Fe) and unavoidable impurities.

[0026] The materials are prepared according to the formula. During the alloy composition ratio process, the element burn-off rate is estimated according to the equipment parameters, and the amount added is calculated according to the mass percentage of the alloy elements. The feeding ratio (weight percentage) of this embodiment is: 93.29% scrap steel and pig iron, 2.64% micro-carbon chromium alloy, 2.30% nickel plate, 0.45% molybdenum plate, 0.97% ferromanganese, 0.13% ferroniobium, and 0.21% carbon raiser. The alloy ingot is prepared by a composite smelting method (electric arc furnace smelting and vacuum refining smelting).

[0027] The alloy ingot is forged at a heating temperature of 1190℃; the forging temperature range is 850-1170℃, and the forging ratio is ≥5 to obtain a forging billet with an effective cross-sectional size of 100-350mm.

[0028] The forging billet is subjected to quenching and tempering heat treatment, wherein the austenitizing temperature is 930℃ and the cooling method is water cooling; the tempering temperature is 220℃ and the tempering cooling method is air cooling.

[0029] Example 2

[0030] The chemical composition of the ultra-high strength and toughness steel is formulated by mass percentage as follows: C: 0.27%; Si: 0.06%; Mn: 1.05%; Cr: 1.92%; Ni: 2.75%; Mo: 0.40%; Nb: 0.07%; S: 0.0011%; P: 0.0015%; H: 1.0ppm; O: 17ppm; N: 44ppm; the remainder is iron (Fe) and unavoidable impurities.

[0031] The materials are prepared according to the formula. During the alloy composition ratio process, the element burn-off rate is estimated according to the equipment parameters, and the amount added is calculated according to the mass percentage of the alloy elements. The feeding ratio (weight percentage) in this embodiment is: 92.24% scrap steel and pig iron, 3.18% micro-carbon chromium alloy, 2.56% nickel plate, 0.61% molybdenum plate, 1.07% ferromanganese, 0.10% ferroniobium, and 0.24% carbon raiser. The alloy ingot is prepared by a composite smelting method (electric arc furnace smelting and vacuum refining smelting).

[0032] The alloy ingot is forged at a heating temperature of 1190℃; the forging temperature range is 850-1170℃, and the forging ratio is ≥5 to obtain a forging billet with an effective cross-sectional size of 100-350mm.

[0033] The forging billet is subjected to quenching and tempering heat treatment, wherein the austenitizing temperature is 930℃ and the cooling method is water cooling; the tempering temperature is 220℃ and the tempering cooling method is air cooling.

[0034] Example 3

[0035] The chemical composition of the ultra-high strength and toughness steel is formulated by mass percentage as follows: C: 0.25%; Si: 0.08%; Mn: 0.89%; Cr: 1.73%; Ni: 2.60%; Mo: 0.46%; Nb: 0.09%; S: 0.0011%; P: 0.0020%; H: 1.0ppm; O: 17ppm; N: 45ppm; the remainder is iron (Fe) and unavoidable impurities.

[0036] The materials are prepared according to the formula. During the alloy composition ratio process, the element burn-off rate is estimated according to the equipment parameters, and the amount added is calculated according to the mass percentage of the alloy elements. The feeding ratio (weight percentage) of this embodiment is: 92.77% scrap steel and pig iron, 2.86% micro-carbon chromium alloy, 2.42% nickel plate, 0.70% molybdenum plate, 0.90% ferromanganese, 0.13% ferroniobium, and 0.22% carbon raiser. The alloy ingot is prepared by a composite smelting method (electric arc furnace smelting and vacuum refining smelting).

[0037] The alloy ingot is forged at a heating temperature of 1190℃; the forging temperature range is 850-1170℃, and the forging ratio is ≥5 to obtain a forging billet with an effective cross-sectional size of 100-350mm.

[0038] The forging billet is subjected to quenching and tempering heat treatment, wherein the austenitizing temperature is 930℃ and the cooling method is water cooling; the tempering temperature is 220℃ and the tempering cooling method is air cooling.

[0039] To further demonstrate the superiority of the ultra-high strength and toughness steel of this invention, a comparison with commonly used steel grades 34CrNi3MoV and 30Cr2Ni2Mo in the standard is shown in Table 1: Table 1. Comparison of chemical composition (mass fraction, %) of the material of this invention with 34CrNi3MoV and 30Cr2Ni2Mo materials.

[0040] Comparative Example 1 This comparative example is 34CrNi3MoV steel conforming to GB / T 33084-2016 standard. Its chemical composition, by mass percentage, includes: C: 0.32%; Si: 0.27%; Mn: 0.65%; Cr: 1.30%; Ni: 3.10%; Mo: 0.28%; V: 0.12%; the remainder being iron (Fe) and unavoidable impurities.

[0041] Comparative Example 2 This comparative example is a 30Cr2Ni2Mo steel conforming to GB / T 3077-2015 standard. Its chemical composition, by mass percentage, includes: C: 0.30%; Si: 0.25%; Mn: 0.70%; Cr: 1.95%; Ni: 2.10%; Mo: 0.40%; the remainder being iron (Fe) and unavoidable impurities.

[0042] The mechanical properties of the forged billets of the materials listed in the above embodiments and comparative examples were tested. The billet cross-sectional dimensions were 150×150mm. After quenching and tempering, samples were taken at T / 2 of the billet to test the mechanical properties. The results are shown in Table 2. Table 2 Mechanical Performance Test Results

[0043] Compared to conventional high-strength steel, the material of this invention contains a relatively low amount of carbon (C), which is beneficial for obtaining a low-carbon lath martensite structure. It also contains appropriate amounts of Cr and Ni, which improve hardenability, allowing parts with a cross-section of 100-350mm to achieve a fully developed lath martensite structure. Simultaneously, Ni has a strong grain-refining effect. These factors enable large-section workpieces of 100-350mm to maintain strength while improving toughness.

[0044] Referring to Table 2, the yield strength of the materials in each embodiment of the present invention can reach over 1200 MPa, the tensile strength can reach over 1500 MPa, the elongation can reach over 12%, the reduction of area can reach over 45%, and the room temperature impact energy (U-notch) can reach over 60 J. All indicators can meet the design target values. In contrast, the plasticity and toughness of the comparative material are significantly lower, and there is a certain risk of failure under strong impact conditions.

[0045] In summary, this invention proposes an ultra-high strength and toughness steel that is highly adaptable to extreme stress conditions. It has excellent comprehensive mechanical properties, and its impact energy for large cross-section workpieces is more than 30% higher than that of traditional ultra-high strength steel. It has a high degree of adaptability to extreme stress conditions such as high-speed impact, making it a high-quality steel that meets practical applications.

[0046] The parts not described in detail in the above embodiments are existing technologies.

[0047] It should be noted that although the present invention has been described through the above embodiments, the present invention may have many other embodiments. Without departing from the spirit and scope of the present invention, those skilled in the art can obviously make various corresponding changes and modifications to the present invention, but all such changes and modifications should fall within the scope of protection of the appended claims and their equivalents.

Claims

1. A high-strength and high-toughness steel highly adaptable to extreme stress conditions, characterized in that, The chemical composition of the ultra-high strength and toughness steel, by mass percentage, includes: C: 0.24%-0.27%; Si≤0.10%; Mn: 0.85%-1.15%; Cr: 1.50%-2.00%; Ni: 2.40%-2.80%; Mo: 0.25%-0.50%; Nb: 0.05%-0.10%; S≤0.0015%; P≤0.0025%; H≤1.5ppm; O≤20ppm, N≤50ppm; the remainder being Fe and unavoidable impurities. The preparation method of ultra-high strength and toughness steel includes the following steps: S1. Ingredients: Prepare ingredients according to the chemical composition and its mass percentage; S2. Smelting: The proportioned raw materials are smelted in an electric arc furnace and then vacuum refined, and finally cast into alloy ingots. S3. Forging: The alloy ingot formed in step S2 is forged. The heating temperature range is 1150-1200℃, the forging temperature range is 850-1170℃, and the forging ratio is ≥5 to obtain a forging billet. S4. Quenching and tempering heat treatment: The forging billet obtained in step S3 is subjected to quenching and tempering heat treatment to obtain ultra-high strength and toughness steel forgings; in the quenching and tempering heat treatment, the austenitizing temperature is 870-950℃ and the cooling method is water cooling; the tempering temperature is 180-300℃ and the tempering cooling method is air cooling. The effective cross-sectional size of the forging blank is 100-350mm.

2. The ultra-high strength and toughness steel according to claim 1, characterized in that: In step S4, after the quenching and tempering heat treatment, the matrix structure of the ultra-high strength and toughness steel forging is tempered martensite and a small amount of retained austenite.

3. The ultra-high strength and toughness steel according to claim 1, characterized in that: In step S4, after the quenching and tempering heat treatment, the mechanical properties of the ultra-high strength and toughness steel forging are: yield strength ≥1200MPa, tensile strength ≥1500MPa, elongation ≥12%, reduction of area ≥45%, and room temperature impact energy ≥60J.

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