Nanometer dual-precipitation phase reinforced low-alloy ultra-high strength steel and preparation process thereof

Abstract

The application discloses a nano dual-precipitation phase reinforced low-alloy ultra-high-strength steel and a preparation process thereof. The strength of the steel exceeds 2000 MPa through the composite reinforcement of NiAl phases and epsilon-carbide precipitated phases with a nano size of 4-6 nm. The steel is characterized by the following chemical element components and wt% contents: C=0.22-0.38, Si=1.10-2.20, Mn=0.90-2.30, Cr=0.70-2.20, Ni=3.85-5.50, Mo=0.40-1.35, V=0.09-0.35, Al=0.50-2.90, S<=0.0020, P<=0.0050, the total content of carbon, silicon, manganese and molybdenum is <5, the total content of chromium and nickel is <7, and Fe=balance.

Description

Technical Field

[0001] This invention relates to the field of low-alloy steel technology, and in particular to a nano-double precipitate phase reinforced low-alloy ultra-high strength steel and its preparation process. Background Technology

[0002] Low-alloy ultra-high strength steel possesses excellent strength and toughness, and is widely used in key load-bearing structural components across various industrial sectors, such as aircraft landing gear, rocket casings, and engine shafts. Its low alloy content (generally less than 5% total alloy content) and relatively low price offer significant development potential. Currently, low-alloy ultra-high strength steels developed domestically and internationally can be chemically categorized into Si-Mn-Mo, Si-Mn-Cr, Si-Mn-Cr-Mo, Si-Mn-Cr-Ni, and Si-Mn-Cr-Ni-Mo systems. Their strength derives from martensitic phase transformation and dispersion strengthening of ε-carbides, achieving a strength limit approaching 2000 MPa. With the increasing demand for lightweight load-bearing structural components, even higher strength requirements (greater than 2000 MPa) are being placed on low-alloy ultra-high strength steels while still meeting damage limits.

[0003] In recent years, advancements in computer-aided alloy design and nanostructure characterization methods have led to the development of various ultra-high strength and high toughness alloy steels with composite precipitated coherent nanoparticles. For example, Patent Document 1 (CN114150232 B / 202111417024.3): A method for manufacturing an ultra-high strength martensitic aging steel reinforced by a composite of coherent and incoherent nanophases, using coherent NiAl nanophase, coherent Cu-rich nanophase, and incoherent Ni3Ti nanophase, yielded an ultra-high strength steel with a tensile strength of 1200–2000 MPa and an elongation of 5–20%. However, its strength did not meet the requirements, and the combined content of Cr and Ni alloying elements in the steel exceeded 10%. Patent Document 2 (CN114717485 B / Patent No. 202210220378.7: A nano-precipitation strengthened ultra-high strength steel and its preparation method. Using NiAl and NbC phases, an ultra-high strength steel with a tensile strength of 2208 MPa and an elongation of 23% was obtained. However, the process is complex and the Cr and Ni content in the steel is also very high. Patent Document 3 (CN115478212A / 202110604129.3): A method for preparing ultra-high strength steel and bars reinforced by carbides and intermetallic compounds. Using carbides and intermetallic compounds (Ni3Ti, Fe2Mo, etc.), an ultra-high strength steel with a tensile strength exceeding 2200 MPa and an elongation greater than 8% was obtained. However, its total Co and Ni content exceeds 10%, resulting in high cost. Through the research of the above patents, it is not difficult to find that composite precipitation strengthening is mostly used in high-alloy steels, resulting in high alloy costs. Therefore, the purpose of this invention is to provide a nano-dual-precipitate-strength low-alloy ultra-high-strength steel, in which the composite strengthening of NiAl phase and ε-carbide dual-precipitate phase enables the steel to achieve a strength exceeding 2000 MPa, while minimizing costs. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a nano-dual-precipitate-strength low-alloy ultra-high-strength steel and its preparation process. The steel's strength exceeds 2000 MPa through composite strengthening with NiAl phase and ε-carbide precipitates, both with nanoscale dimensions of 4-6 nm. The total wt% content of alloying elements carbon, silicon, manganese, and molybdenum is <5%, and the total wt% content of chromium and nickel is <7%.

[0005] The technical solution of the present invention is as follows:

[0006] A nano-double precipitate phase reinforced low-alloy ultra-high strength steel, characterized in that it comprises the following chemical elements and their wt% content: C=0.22~0.38, Si=1.10~2.20, Mn=0.90~2.30, Cr=0.70~2.20, Ni=3.85~5.50, Mo=0.40~1.35, V=0.09~0.35, Al=0.50~2.90, S≤0.0020, P≤0.0050, total content of carbon, silicon, manganese and molybdenum <5%, total content of chromium and nickel <7%, Fe= balance.

[0007] This includes strengthening the steel with a tensile strength exceeding 2000 MPa through composite reinforcement using NiAl phase and ε-carbide precipitates, both with nanometer sizes of 4–6 nm.

[0008] It includes tensile strength up to 2100 MPa, yield strength ≥ 1600 MPa, elongation ≥ 8.0%, area shrinkage ≥ 30%, and impact energy ≥ 45 J.

[0009] A process for preparing nano-dual-precipitation phase reinforced low-alloy ultra-high-strength steel is characterized by comprising the following chemical element composition and wt% content of metallurgical raw materials: C=0.22~0.38, Si=1.10~2.20, Mn=0.90~2.30, Cr=0.70~2.20, Ni=3.85~5.50, Mo=0.40~1.35, V=0.09~0.35, Al=0.50~2.90, S≤0.0020, P≤0.0050, total content of carbon, silicon, manganese and molybdenum <5%, total content of chromium and nickel <7%, Fe= balance.

[0010] Includes the following steps:

[0011] Step 1, smelting process;

[0012] Step 2, forging process;

[0013] Step 3, post-forging annealing process;

[0014] Step 4, heat treatment process.

[0015] Step 1 includes smelting using a vacuum induction furnace and vacuum consumable remelting.

[0016] Step 2 includes performing the following forging process on the steel ingots obtained from smelting: three forging and three drawing processes.

[0017] Step 1: The initial forging temperature is 1070±10℃, the forging is upsetting to 1 / 2 of the original height, and the drawing is elongating to 4 / 5 of the original height;

[0018] Step 2: Heat to 1070±10℃, build up to 1 / 2 of the original height, and draw up to 4 / 5 of the original height;

[0019] Step 3: Heat to 1070±10℃, upset to 1 / 2 of the original height, and then forge or roll into bars or billets of the corresponding size according to product requirements. The final forging temperature of the above process is ≥850℃.

[0020] Step 3 includes heating the forging to 650-670℃ and holding it for 10 hours, then furnace cooling it to 260-290℃ and holding it for 10 hours, then heating it to 890℃ and holding it for ≥12 hours, air cooling it to 260-290℃ and holding it for 10 hours, finally heating it to 670℃ and holding it for ≥20 hours, then furnace cooling it to 150℃ and air cooling it out of the furnace.

[0021] Step 4 includes heating the annealed forging to 950℃~1100℃, holding it at that temperature for 1~2 hours, then oil quenching it to room temperature, followed by heating it to 200℃~300℃, holding it at that temperature for 2~3 hours, and then air cooling it to room temperature.

[0022] The technical effects of this invention are as follows: This invention provides a nano-double precipitate phase strengthened low alloy ultra-high strength steel and its preparation process. Its strength comes from the martensitic phase transformation and the composite strengthening of nano-sized (4-6nm) NiAl phase and ε-carbide double precipitate phase. The tensile strength can reach 2100MPa, the yield strength ≥1600MPa, the elongation ≥8.0%, the reduction of area ≥30%, and the impact energy ≥45J.

[0023] The steel of this invention is alloyed by adding Cr, Mo, Ni, V, and Al. Cr and Mo can play a role in solid solution strengthening and alloy carbide strengthening; Ni can improve the plasticity and toughness of the steel and form a 4-6 nm NiAl phase with Al; trace amounts of V can form MC-type structures that hinder the growth of the original austenite grains, thus refining the grain size. The content of impurity elements is controlled by a dual vacuum smelting process of vacuum induction and vacuum arc remelting. After appropriate heat treatment, the microstructure of the steel is ensured to be lath martensite + thin film austenite + ε-carbide + nano-scale NiAl phase, giving the material both ultra-high strength and good toughness. The mechanical properties of the steel of this invention are tested according to national standards (GB / T 228.1-2018, GB / T229-2007) for quasi-static tensile and impact toughness, meeting the following requirements: tensile strength of ≥2100 MPa, yield strength of ≥1600 MPa, elongation of ≥8.0%, reduction of area of ​​≥30%, and impact energy of ≥45 J. Detailed Implementation

[0024] The present invention will now be described with reference to the embodiments.

[0025] A nano-dual-precipitate-strength low-alloy ultra-high-strength steel comprises the following chemical elements and their wt% contents: C=0.22~0.38, Si=1.10~2.20, Mn=0.90~2.30, Cr=0.70~2.20, Ni=3.85~5.50, Mo=0.40~1.35, V=0.09~0.35, Al=0.50~2.90, S≤0.0020, P≤0.0050, total content of carbon, silicon, manganese, and molybdenum <5%, total content of chromium and nickel <7%, Fe=balance. The steel's tensile strength exceeds 2000 MPa due to the composite strengthening by NiAl phase and ε-carbide precipitates, both with nanoscale dimensions of 4~6 nm. It also exhibits tensile strength up to 2100 MPa, yield strength ≥1600 MPa, elongation ≥8.0%, reduction of area ≥30%, and impact energy ≥45 J.

[0026] A preparation process for a nano-dual-precipitation phase strengthened low-alloy ultra-high-strength steel includes preparing metallurgical raw materials according to the following chemical element composition and their wt% content: C=0.22~0.38, Si=1.10~2.20, Mn=0.90~2.30, Cr=0.70~2.20, Ni=3.85~5.50, Mo=0.40~1.35, V=0.09~0.35, Al=0.50~2.90, S≤0.0020, P≤0.0050, total content of carbon, silicon, manganese and molybdenum <5%, total content of chromium and nickel <7%, Fe=balance. The process includes the following steps: Step 1, smelting process; Step 2, forging process; Step 3, post-forging annealing process; Step 4, heat treatment process.

[0027] Step 1 includes smelting using a vacuum induction furnace and vacuum consumable remelting. Step 2 includes performing the following three-stage forging process on the smelted steel ingot: Step 1, initial forging temperature is 1070±10℃, upsetting to 1 / 2 of the original height, and drawing to 4 / 5 of the original height; Step 2, heating to 1070±10℃, upsetting to 1 / 2 of the original height, and drawing to 4 / 5 of the original height; Step 3, heating to 1070±10℃, upset to 1 / 2 of the original height, and then forging or rolling into bars or billets of the corresponding dimensions according to product requirements. The final forging temperature of the above process is ≥850℃. Step 3 includes heating the forging to 650-670℃ and holding for 10 hours, then furnace cooling to 260-290℃ and holding for 10 hours, followed by heating to 890℃ and holding for ≥12 hours, air cooling to 260-290℃ and holding for 10 hours, and finally heating to 670℃ and holding for ≥20 hours, followed by furnace cooling to 150℃ and air cooling. Step 4 includes heating the annealed forging to 950℃-1100℃, holding for 1-2 hours, oil quenching to room temperature, then heating to 200℃-300℃, holding for 2-3 hours, and finally air cooling to room temperature.

[0028] C: As the main solid solution strengthening element in the steel of this invention, it is strengthened through interstitial atom solid solution after martensite transformation and ε-carbides precipitated during low-temperature tempering. Increased C content promotes carbide formation and improves strength; however, when the C content exceeds 0.5%, brittle fracture easily occurs at the original austenite grain boundaries. Taking all factors into consideration, the C content in this invention is between 0.22% and 0.38%.

[0029] Si (Si): During the smelting process, Si is added to steel as a deoxidizer and increases the fluidity of the molten steel. Additionally, Si inhibits the formation of cementite, improving the tempering resistance of the steel, thus allowing the tempering temperature of the steel in this invention to be below the brittle temperature range. However, when the Si content is in the range of 0.8% to 1.0%, the plasticity and toughness of the steel decrease significantly. Excessive Si content reduces the solubility of Mo in the steel matrix, leading to residual alloy carbides during quenching and heating, which impairs the toughness of the steel. Considering all factors, the Si content in this invention is controlled between 1.10% and 2.20%.

[0030] Mn: As a good deoxidizer and desulfurizer added to steel, Mn can replace some Ni to stabilize austenite and reduce production costs. However, excessive Mn content will lower the Ms point, reduce the strength of the steel, and high Mn content is detrimental to corrosion resistance. Taking all factors into consideration, the Mn content in this invention is controlled between 0.90% and 2.30%.

[0031] Cr: As an additive to improve the hardenability of steel, Cr has a combined effect of solid solution strengthening and alloy carbide formation, which can further improve the strength of steel. However, high Cr content leads to the formation of coarse eutectic carbides, which reduces the ductility and toughness of steel. Furthermore, excessively high Cr content can also lower the Ms point, leading to the formation of twinned martensite and impairing the toughness of the steel. Considering all factors, the Cr content in this invention is controlled between 0.70% and 2.20%.

[0032] Ni, as an alloying element that expands the austenite phase region, makes it less prone to decomposition of screw dislocations, thus facilitating cross-slip and significantly improving the toughness of steel. Furthermore, Ni combines with Al to form nano-NiAl phases, increasing the strength of the steel. However, Ni is a scarce resource in my country; excessively high content not only increases costs but also lowers the Ms point, increasing the content of retained austenite in the steel and reducing its strength. Considering all factors, the Ni content in this invention is controlled between 3.85% and 5.50%.

[0033] Mo, as a strong carbide-forming element, can improve the tempering stability of the steel of this invention. Furthermore, when present simultaneously with Mn and Cr, Mo can suppress or reduce temper brittleness caused by other elements. When the Mo content is around 0.5%, temper brittleness in the steel can be essentially eliminated, and second-type temper brittleness can also be effectively prevented. However, Mo is an alloying element that expands the ferrite phase region; excessively high levels will reduce the ductility and toughness of the steel and increase its cost. Considering all factors, the Mo content in this invention is controlled between 0.40% and 1.35%.

[0034] V: As a microalloying element, adding a small amount of V can form MC-type carbides, which hinder the migration of the original austenite grain boundaries during quenching, thereby refining the grains and improving both the strength and toughness of the steel. However, excessive addition will reduce the toughness. Taking all factors into consideration, the V content in this invention is controlled between 0.09% and 0.35%.

[0035] Al: As the main element forming the nano-NiAl phase, a high concentration of nano-NiAl phase can effectively improve the strength of steel. In addition, Al can also be added to steel as a deoxidizer to purify the molten steel. However, excessive Al will promote ferrite formation and reduce the ductility and toughness of steel. Taking all factors into consideration, the Al content should be controlled between 0.50% and 2.90%.

[0036] The requirements for controlling the content of harmful impurities in the steel of this invention are: S≤0.0020%, P≤0.0050%.

[0037] Example: Test steels numbered 1-3 were smelted using vacuum induction and vacuum arc remelting, and their chemical compositions are shown in Table 1. The smelted steel ingots underwent a three-stage forging process: Stage 1: Initial forging temperature was 1070℃, upsetting to 1 / 2 of the original height, and drawing to 4 / 5 of the original height; Stage 2: Temperature was increased to 1070℃, upsetting to 1 / 2 of the original height, and drawing to 4 / 5 of the original height; Stage 3: Temperature was increased to 1070℃, upsetting to 1 / 2 of the original height, and finally precision forging into a round ingot with a diameter of 370mm and a length of 3200mm. The forgings were heated to 650–670℃ and held for 10 hours, then furnace cooled to 260–290℃ and held for 10 hours. They were then heated to 890℃ and held for ≥12 hours, air-cooled to 260–290℃ and held for 10 hours, and finally heated to 670℃ and held for ≥20 hours. Afterward, they were furnace cooled to 150℃ and then air-cooled. The heat treatment regime in Table 2 was used. Table 3 shows the quasi-static mechanical properties of steels in Examples 1-3, with tensile strengths all reaching 2100 MPa and impact energy exceeding 45 J.

[0038] Table 1 Chemical composition (wt%) of embodiments of the present invention

[0039]

[0040] Table 2 Heat Treatment Regulations

[0041]

[0042] Table 3 Quasi-static mechanical properties

[0043]

[0044] Contents not described in detail in this specification are prior art known to those skilled in the art. It is hereby indicated that the above description is intended to help those skilled in the art understand this invention, but does not limit the scope of protection of this invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essential content of this invention fall within the scope of protection of this invention.

Claims

1. A nano-dual-precipitation phase reinforced low-alloy ultra-high-strength steel, characterized in that, The chemical composition and their wt% content include the following: C=0.22~0.38, Si=1.10~2.20, Mn=0.90~2.30, Cr=0.70~2.20, Ni=3.85~5.50, Mo=0.40~1.35, V=0.09~0.35, Al=0.50~2.90, S≤0.0020, P≤0.0050, total content of carbon, silicon, manganese and molybdenum <5%, total content of chromium and nickel <7%, Fe=balance; The microstructure of the steel consists of lath martensite + thin film austenite + ε-carbide + nanoscale NiAl phase; This includes strengthening the steel with a tensile strength exceeding 2000 MPa through composite reinforcement of NiAl phase and ε-carbide precipitates, both with nanometer sizes of 4–6 nm.

2. The nano-dual-precipitation phase reinforced low-alloy ultra-high-strength steel according to claim 1, characterized in that, It includes tensile strength up to 2100 MPa, yield strength ≥ 1600 MPa, elongation ≥ 8.0%, area shrinkage ≥ 30%, and impact energy ≥ 45 J.

3. A preparation process for nano-dual-precipitation phase reinforced low-alloy ultra-high-strength steel, characterized in that, Metallurgical raw materials are prepared according to the following chemical element composition and their wt% content: C=0.22~0.38, Si=1.10~2.20, Mn=0.90~2.30, Cr=0.70~2.20, Ni=3.85~5.50, Mo=0.40~1.35, V=0.09~0.35, Al=0.50~2.90, S≤0.0020, P≤0.0050, total content of carbon, silicon, manganese and molybdenum <5%, total content of chromium and nickel <7%, Fe=balance; Includes the following steps: Step 1, smelting process; Step 2, forging process; Step 3, post-forging annealing process; Step 4, heat treatment process; Step 3 includes heating the forging to 650-670℃ and holding it for 10 hours, then furnace cooling it to 260-290℃ and holding it for 10 hours, then heating it to 890℃ and holding it for ≥12 hours, air cooling it to 260-290℃ and holding it for 10 hours, finally heating it to 670℃ and holding it for ≥20 hours, then furnace cooling it to 150℃ and air cooling it out of the furnace. Step 4 includes heating the annealed forging to 950℃~1100℃, holding it at that temperature for 1~2 hours, then oil quenching it to room temperature, followed by heating it to 200℃~300℃, holding it at that temperature for 2~3 hours, and then air cooling it to room temperature.

4. The preparation process of nano-dual-precipitation phase strengthened low-alloy ultra-high-strength steel according to claim 3, characterized in that, Step 1 includes smelting using a vacuum induction furnace and vacuum consumable remelting.

5. The preparation process of nano-dual-precipitation phase strengthened low-alloy ultra-high strength steel according to claim 3, characterized in that, Step 2 includes performing the following forging process on the steel ingots obtained from smelting: three forging and three drawing processes. Step 1: The initial forging temperature is 1070±10℃, the forging is upsetting to 1 / 2 of the original height, and the drawing is elongating to 4 / 5 of the original height; Step 2: Heat to 1070±10℃, build up to 1 / 2 of the original height, and draw up to 4 / 5 of the original height; Step 3: Heat to 1070±10℃, upset to 1 / 2 of the original height, and then forge or roll into bars or billets of the corresponding size according to product requirements. The final forging temperature of the above process is ≥850℃.

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