A 2500mpa grade high-strength high-plasticity low-alloy steel and a preparation method thereof

Abstract

The application relates to the field of super-high-strength steel preparation and provides a 2500MPa-grade high-strength high-plasticity low-alloy steel and a preparation method, the method comprises the following steps: alloy smelting, forging, hot rolling, austenitizing and tempering treatment. The alloy steel is composed of the following components in percentage by mass: C: 0.5-0.8%, Si: 0.5-2.0%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: <=3%, Mo: <=2%, V: <=0.5%, Nb: <=0.5%, the rest is Fe and inevitable impurity elements, the residual austenite accounts for 5-13% of the volume of the alloy steel, the yield strength is 1800-2100 MPa, the tensile strength is 2500-2800 MPa, and the total elongation is 8-11%. The preparation process is simple, the production flow is short, the cold rolling process is omitted, the cost is saved, the overall process has low requirements on equipment, and the application prospect is good.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength steel preparation technology, and in particular to a 2500MPa grade high-strength, high-ductility, low-alloy steel and its preparation method. Background Technology

[0002] Currently, against the backdrop of global energy shortages, the development of a new generation of ultra-high-strength steel is an important way to achieve lightweight structural materials, thereby realizing energy conservation, emission reduction, and greenhouse gas emission reduction, and promoting the sustainable development of the steel industry. Lightweight design strategies are essential for the next generation of high-performance structural materials. Strength and plasticity are the most important performance indicators for structural materials, but ultra-high-strength steels generally exhibit an "inverted relationship" between strength and plasticity, a problem particularly pronounced at ultra-high strength levels. Ultra-high-strength alloy steels above 2500 MPa face a series of problems, including poor elongation due to high strength, high alloy costs, and complex manufacturing processes. For example, 18Ni(350) martensitic aging steel with a tensile strength of 2450 MPa requires the solid solution of large amounts of precious metal elements such as Ni, Co, and Mo to obtain high strength. The high alloy cost limits its application range, and the plasticity of martensitic aging steel is relatively low, with a particularly significant decrease in elongation when the strength exceeds 2500 MPa. Secondly, existing high-strength steels often employ demanding production processes, such as high-strain warm rolling and cold rolling, which place high demands on rolling mills and other equipment, making them unsuitable for widespread application.

[0003] Against this backdrop, developing ultra-high strength steels with excellent strength and ductility through appropriate composition design and simple preparation processes presents significant challenges. Low-alloy steels, such as 300M and 4340 steels, are typical representatives of ultra-high strength steels. Their low alloy cost and superior comprehensive performance make them the preferred materials for major load-bearing components in aerospace and defense industries, and they represent a cutting-edge topic and research hotspot in materials science. Ultra-high strength low-alloy steels are typically high-carbon steels. Their composition incorporates significant amounts of Mn, Si, Cr, and Ni to improve hardenability, ensuring that quenching after complete austenitization yields a fully martensitic structure or a multiphase structure of martensite and a small amount of retained austenite. The addition of appropriate amounts of Mo, V, and Nb allows for the precipitation of high-density microalloyed carbides within the martensitic matrix, strengthening the matrix.

[0004] Currently, ultra-high strength steels with tensile strengths above 2500 MPa suffer from high alloying costs, complex processes, and low plasticity. Therefore, overcoming the strength-plasticity inversion problem in high-strength steel urgently needs to be addressed. To solve these problems, this invention provides a low-alloy steel with a tensile strength >2500 MPa, characterized by possessing ultra-high strength while retaining high plasticity, resulting in excellent comprehensive mechanical properties. Among the martensitic steels reported to be 2500 MPa and above, few achieve a total elongation >8%.

[0005] Chinese Patent Publication No. CN 104911501A discloses an ultra-high strength high-carbon dislocation-type martensitic steel with a tensile strength of 2150-2400 MPa and an elongation of 6-10%, and its preparation method. The chemical composition is as follows: C: 0.6-0.85%; Si: 0.01-0.8%; Mn: 0.1-0.5%; Cr: 0.8-2.0%; Cu: 0.05-0.4%; Ni: 0.05-0.3%; Ti: 0.02-0.1%; V: 0.02-0.2%; Nb: 0.02-0.15%; P: <0.02%; S: <0.02%, with the balance being Fe. Although the steel invented by this patent has good overall mechanical properties, the rolling process requires rolling at an intermediate temperature of 500-700℃ with a rolling yield of 50-90%, resulting in relatively high deformation resistance, which is not conducive to industrial application.

[0006] Chinese Patent Publication No. CN 113604753B discloses an ultra-high strength martensitic aging steel reinforced by nano-Mo clusters and a high-density B2-NiAl phase, and its preparation method. The chemical composition is: Ni: 10%–18%, Co: 4%–16%, Mo: 3%–9%, ​​Al: 0.5%–6%, with the balance being iron and unavoidable impurities. Although this patented steel possesses an ultra-high strength of up to 2600 MPa, its elongation is very limited (<4%), and the alloy contains a large amount of the noble metal elements Ni and Co, resulting in high costs.

[0007] It can be seen that the invention patents with Chinese patent publication number CN 104911501A and CN 113604753B both prepared ultra-high strength steel through different means. However, the intermediate temperature large deformation process used in the former is not suitable for industrial production, while the alloy proposed by the latter is too expensive and not suitable for large-scale application. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a 2500MPa grade high-strength, high-ductility low-alloy steel and its preparation method. Through optimized alloy composition design, a low-alloy steel with both ultra-high strength and high ductility is invented, whose mechanical properties exceed those of most martensitic steels. Under the preferred composition and process of this invention, the tensile strength is >2500MPa while maintaining an excellent elongation of 11%, and the comprehensive mechanical properties are higher than all currently reported martensitic steels. Furthermore, the process of this invention is simple to operate and suitable for large-scale industrial applications.

[0009] The present invention adopts the following technical solution:

[0010] On one hand, the present invention provides a method for preparing 2500MPa grade high-strength, high-ductility, low-alloy steel, comprising:

[0011] S1. Alloy smelting: Alloy raw materials with a set ratio are smelted in a vacuum induction melting furnace and cast into steel ingots;

[0012] S2. Forging: The steel ingot is forged several times at a first set temperature to form a square billet, which is then air-cooled to room temperature.

[0013] S3. Hot rolling: The billet is hot rolled at a set temperature to obtain a hot-rolled plate, which is then water-cooled or air-cooled to room temperature.

[0014] S4. Austenitization: The hot-rolled plate obtained in step S3 is heated to the temperature for complete austenitization, and then the hot-rolled plate is quenched in water or oil to room temperature to obtain a quenched plate.

[0015] S5. Tempering treatment: The quenched plate obtained in step S4 is tempered at a second set temperature.

[0016] In addition to any of the possible implementations described above, another implementation is provided in which the mass percentage of the alloy raw materials in step S1 is: C: 0.5-0.8%, Si: 0.5-2.0%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: ≤3%, Mo: ≤2%, V: ≤0.5%, Nb: ≤0.5%, with the balance being Fe and unavoidable impurity elements.

[0017] In addition to any of the possible implementations described above, another implementation is provided in which, in step S1, the alloy is smelted and then cast into a casting equipment to solidify and obtain a 25kg billet or steel ingot.

[0018] In addition to any of the possible implementations described above, another implementation is provided in which, during step S2, the first set temperature during forging is above the austenitizing temperature of the steel ingot, the steel ingot is held at 1150-1250℃ for ≥1h before forging, and the final forging temperature is ≥800℃.

[0019] In addition to any of the possible implementations described above, another implementation is provided in which, in step S2, the dimensions of the billet are 30mm thick × 100mm wide.

[0020] In addition to any of the possible implementations described above, another implementation is provided in which the specific steps of hot rolling in step S3 are as follows: the billet is held at 1150-1250℃ for 0.5-3 hours; after exiting the furnace, it is first subjected to no less than two passes of rough rolling, with an initial rolling temperature of 1100-1200℃ and a rough rolling reduction rate of ≥30% per pass, and then subjected to multiple passes of finish rolling, with the final rolling temperature controlled at 800-950℃.

[0021] In addition to any of the possible implementations described above, another implementation is provided in which, in step S3, a hot rolling process is used to directly hot roll the billet to a target thickness of 1.0 to 5.0 mm, without the need for subsequent cold rolling.

[0022] In addition to any of the possible implementations described above, another implementation is provided in which, in step S4, the complete austenitizing temperature is 850–1000°C and the holding time is 2–30 min.

[0023] In addition to any of the possible implementations described above, another implementation is provided in which, in step S5, the second set temperature during tempering is 100-300°C and the holding time is 0.1-48h, so as to reduce the internal stress caused by water quenching or oil quenching.

[0024] In addition to any of the possible implementations described above, another implementation is provided in which, in step S5, the microstructure after tempering is a multiphase structure of martensite and austenite, wherein the retained austenite accounts for 5-13% of the volume of the ultra-high strength and high plasticity low alloy steel, and at this time the yield strength of the steel is 1800-2100 MPa, the tensile strength is 2500-2800 MPa, and the total elongation is 8-11%.

[0025] On the other hand, the present invention provides a 2500MPa grade high-strength, high-ductility low-alloy steel, wherein the ultra-high-strength, high-ductility low-alloy steel is obtained by the above-described preparation method, and the retained austenite accounts for 5-13% of the volume of the ultra-high-strength, high-ductility low-alloy steel; the mass percentage of the composition of the ultra-high-strength, high-ductility low-alloy steel is: C: 0.5-0.8%, Si: 0.5-2.0%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: ≤3%, Mo: ≤2%, V: ≤0.5%, Nb: ≤0.5%, with the balance being Fe and unavoidable impurity elements.

[0026] In addition to any of the possible implementations described above, another implementation is provided in which the ultra-high strength and high plasticity low alloy steel has a yield strength of 1800-2100 MPa, a tensile strength of 2500-2800 MPa, and a total elongation of 8-11%.

[0027] The design concept and function of the main elements in this invention are explained below:

[0028] Carbon: C is the most effective element for improving interstitial solid solution strengthening and strength in martensitic steel. Higher C content results in finer martensite and more significant dislocation strengthening. It should be noted that excessive C content increases the stability of retained austenite, which is detrimental to achieving high strength. Based on these considerations, the C content is controlled within the range of 0.5% to 0.8%.

[0029] Mn: The addition of Mn can increase the hardenability of steel and delay the transformation of pearlite and bainite; however, Mn is a strong austenite stabilizing element, which lowers the Ms point of steel. Excessive Mn content will lead to Mn segregation and an increase in the amount of residual austenite after quenching, which is not conducive to obtaining high strength. In summary, this invention limits the Mn content to the range of 0.2% to 2%.

[0030] Si: Si has a significant solid solution strengthening effect in martensite and can suppress the recovery of dislocations in quenched martensite during tempering to maintain high strength. However, excessive Si content will significantly reduce the toughness and machinability of steel. Therefore, this invention controls the Si content in the range of 0.5% to 2.0%.

[0031] Cr: Cr can significantly improve the hardenability and corrosion resistance of steel, refine the microstructure, and strongly delay the transformation of pearlite and bainite, thereby stabilizing the martensitic structure. However, excessive Cr can easily form coarse carbides. Taking all factors into consideration, its content is limited to 0.2% to 2%.

[0032] Nickel: Ni can improve the plasticity of the martensitic matrix through solid solution and also improve the hardenability of steel. However, excessive Ni content will significantly increase the cost of steel and stabilize the residual austenite. Therefore, the Ni content should be controlled between 0 and 3%.

[0033] Molybdenum (Mo): The addition of Mo improves the hardenability of steel. Mo can prevent the precipitation of phases along grain boundaries, thereby avoiding intergranular fracture and improving fracture toughness. Mo readily forms carbides in high-carbon steel, contributing significantly to the strength of the steel of this invention; however, excessive Mo addition will substantially increase the cost of the steel. Based on the effect of Mo on steel, the Mo content is controlled at 0–2%.

[0034] Vanadium and niobium (V and Nb), as microalloying elements, can form nanoscale carbides with carbon (C). These precipitates pin grain boundaries, resulting in uniform and fine austenite grains, and ultimately refine the martensite structure, thereby improving the plasticity of the matrix. They also produce a high precipitation strengthening effect, contributing to the strength of the sample steel. However, excessive V and Nb will consume C in the martensite and increase the cost of the steel. Therefore, V and Nb should be controlled within the range of 0–0.5%.

[0035] The possible reasons why the alloy steel of this invention possesses both high strength and high ductility are analyzed as follows:

[0036] 1. Reasonable element design: By controlling the content of alloying elements, a residual austenite structure with a volume fraction of 5-13% is retained in the martensitic matrix structure, thus maintaining high plasticity on the basis of high strength. The inventors found in practice that if the volume fraction of the residual austenite structure is not within this range, the high strength and high plasticity of the alloy steel cannot be maintained at the same time.

[0037] 2. Directly following the large reduction hot rolling in step S3 with water or air cooling to room temperature, combined with the appropriate austenitizing treatment in step S4, yields a fine microstructure and high interfacial density to coordinate plastic deformation, thus promoting high strength and high plasticity. Experiments have shown that the large reduction in step S3 (reduction rate ≥30% per rough rolling pass) effectively refines the microstructure. In step S4, the complete austenitizing temperature is 850–1000℃, and the holding time is 2–30 min. If the holding time exceeds 30 min, the microstructure becomes significantly larger, and the plasticity decreases significantly. Appropriate holding time also effectively refines the microstructure.

[0038] 3. The high strength and high plasticity of this invention cannot be achieved by only having 1 or only having 2 as described above. Only by simultaneously possessing 1 and 2, combined with other steps in the process, can the high-strength, high-plasticity, low-alloy steel of this application be obtained.

[0039] The beneficial effects of this invention are as follows: the preparation process of this invention is simple, the production process is short, the cold rolling process is eliminated, the cost is saved to a certain extent, and the overall process does not have high requirements for equipment, thus it has good prospects for industrial application. Attached Figure Description

[0040] Figure 1 The diagram shown is a flowchart illustrating a method for preparing 2500MPa-grade high-strength, high-ductility, low-alloy steel according to an embodiment of the present invention.

[0041] Figure 2 The image shown is a scanning electron microscope (SEM) image of Embodiment 3 of the present invention.

[0042] Figure 3 The tensile stress-strain curves of Examples 2 and 3 are shown.

[0043] Figure 4 The image shown is a transmission electron microscope (TEM) image after tempering in Example 3. Detailed Implementation

[0044] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be noted that the technical features or combinations of technical features described in the following embodiments should not be considered in isolation, but can be combined with each other to achieve better technical effects.

[0045] like Figure 1 As shown in the figure, an embodiment of the present invention provides a method for preparing 2500MPa grade ultra-high strength and high plasticity low alloy steel, comprising:

[0046] S1. Alloy smelting: Alloy raw materials with a set ratio are smelted in a vacuum induction melting furnace and cast into steel ingots;

[0047] S2. Forging: The steel ingot is forged several times at a first set temperature to form a square billet, which is then air-cooled to room temperature.

[0048] S3. Hot rolling: The billet is hot rolled at a set temperature to obtain a hot-rolled plate, which is then water-cooled or air-cooled to room temperature.

[0049] S4. Austenitization: The hot-rolled plate obtained in step S3 is heated to the temperature for complete austenitization, and then the hot-rolled plate is quenched in water or oil to room temperature to obtain a quenched plate.

[0050] S5. Tempering treatment: The quenched plate obtained in step S4 is tempered at a second set temperature.

[0051] In one specific embodiment, in step S1, the mass percentage of the alloy raw materials is: C: 0.5-0.8%, Si: 0.5-2.0%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: ≤3%, Mo: ≤2%, V: ≤0.5%, Nb: ≤0.5%, with the balance being Fe and unavoidable impurity elements.

[0052] In one specific embodiment, in step S1, after the alloy is smelted, it is cast into a casting equipment and solidified to obtain a 25kg billet or steel ingot.

[0053] In one specific embodiment, in step S2, the first set temperature during forging is above the austenitizing temperature, the steel ingot is held at 1150-1250℃ for ≥1h before forging, and the final forging temperature is ≥800℃.

[0054] In one specific embodiment, in step S2, the dimensions of the billet are 30mm thick × 100mm wide.

[0055] In one specific embodiment, step S3, the specific steps of hot rolling are as follows: the billet is held at 1150-1250℃ for 0.5-3 hours; after exiting the furnace, it undergoes at least two rough rolling passes, with an initial rolling temperature of 1100-1200℃, and a rough rolling reduction rate for each pass.

[0056] ≥30%, then multiple passes of finishing rolling, with the final rolling temperature controlled at 800~950℃.

[0057] In one specific embodiment, in step S3, only hot rolling is used to directly hot roll the billet to the target thickness of 1.0 to 5.0 mm, and no further cold rolling is required.

[0058] In one specific embodiment, in step S4, the complete austenitizing temperature is 850–1000°C, and the holding time is 2–30 min.

[0059] In one specific embodiment, in step S5, the second set temperature during tempering is 100-300°C, and the holding time is 0.1-48h, in order to reduce the internal stress caused by water quenching or oil quenching.

[0060] In one specific embodiment, in step S5, the microstructure after tempering is a multiphase structure of martensite and austenite, wherein the retained austenite accounts for 5-13% of the volume of the ultra-high strength and high plasticity low alloy steel. At this time, the yield strength of the steel is 1800-2100 MPa, the tensile strength is 2500-2800 MPa, and the total elongation is 8-11%.

[0061] This invention discloses an ultra-high strength, high plasticity, low alloy steel with a tensile strength >2500MPa. The mass percentage composition of the ultra-high strength, high plasticity, low alloy steel is as follows: C: 0.5-0.8%, Si: 0.5-2.0%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: ≤3%, Mo: ≤2%, V: ≤0.5%, Nb: ≤0.5%, with the balance being Fe and unavoidable impurity elements.

[0062] The ultra-high strength and high plasticity low alloy steel is obtained by the above preparation method, and the retained austenite accounts for 5-13% of the volume of the ultra-high strength and high plasticity low alloy steel.

[0063] The ultra-high strength and high plasticity low alloy steel has a yield strength of 1800-2100 MPa, a tensile strength of 2500-2800 MPa, and a total elongation of 8-11%.

[0064] The present invention will be further described below with reference to specific embodiments.

[0065] The chemical composition of the alloys in each embodiment is shown in Table 1, the main austenitizing and tempering process parameters are shown in Table 2, and the mechanical properties are shown in Table 3.

[0066] Table 1 shows the chemical composition (wt.%) of each embodiment.

[0067] serial number C Mn Si Cr Ni Mo V Nb Example 1 0.53 1.5 1 2 1.5 2 0.2 0.1 Example 2 0.61 0.6 1.4 0.8 0.8 0.5 0.1 / Example 3 0.65 1.0 1.6 1.2 2.1 0.5 0.3 0.05 Example 4 0.65 0.2 1.6 0.8 1.0 0.5 0.3 / Example 5 0.72 1.8 0.5 0.3 0.2 0.06 0.1 0.2

[0068] Table 2 Main austenitizing and tempering process parameters of the embodiments of the present invention.

[0069]

[0070] After austenitization and tempering processes were carried out according to the process parameters in Table 2 in Examples 1-5, the yield strength, tensile strength and total elongation of the steel were analyzed by tensile tests, and the results are shown in Table 3.

[0071] Table 3 Mechanical properties of various embodiments of the present invention

[0072]

[0073] The alloy scanning electron microscope (SEM) image of Embodiment 3 of the present invention is shown below. Figure 2 As shown, the martensitic structure after tempering is very fine. The engineering stress-strain curves of Example 3 after tempering are shown below. Figure 3 As shown in Table 3 and Figure 3 It is evident that the low-alloy steel provided by this invention possesses extremely high strength and excellent plasticity. The preferred composition of the steel has a yield strength of 2016 MPa, a tensile strength of 2808 MPa, and a total elongation of 11.2%.

[0074] The composition and process of Example 2 not only achieve a tensile strength of up to 2800 MPa, but also maintain a total elongation of 11%. The comprehensive mechanical properties of Example 3 are higher than all martensitic steels reported to date.

[0075] Figure 4 This is a transmission electron microscope (TEM) image after tempering in Example 3. High-density nanoscale carbides were extracted from the steel using carbon film replication technology. It can be seen that the size of most carbides is <30 nm. Figure 4 It is known that a large number of nano carbide particles exist in the martensitic matrix after tempering, which can produce excellent strengthening effect.

[0076] While several embodiments of the present invention have been provided herein, those skilled in the art should understand that modifications can be made to these embodiments without departing from the spirit of the invention. The above embodiments are merely exemplary and should not be construed as limiting the scope of the invention.

Claims

1. A method for preparing 2500MPa grade high-strength, high-ductility, low-alloy steel, characterized in that, The method includes: S1. Alloy smelting: Alloy raw materials with a set ratio are smelted in a vacuum induction melting furnace and cast into steel ingots; the mass percentage of the alloy raw materials is: C: 0.5-0.8%, Si: 0.5-2.0%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: 0.2-3%, Mo: 0.06-2%, V: 0-0.5%, Nb: 0-0.5%, with the balance being Fe and unavoidable impurity elements; S2. Forging: After holding the steel ingot at 1150-1250℃ for ≥1h, it is forged into a square billet at a temperature above the austenitizing temperature, with a final forging temperature of ≥800℃, and then air-cooled to room temperature. S3. Hot rolling: After holding the billet at 1150-1250℃ for 0.5-3 hours, it is rolled at 1100-1200℃, and then subjected to rough rolling of no less than two passes with a reduction rate of ≥30% per pass and multiple passes of finish rolling, directly hot rolling to the target thickness of 1.0-5.0 mm, without any subsequent cold rolling or warm rolling. After rolling, it is cooled to room temperature by water cooling or air cooling to obtain a hot-rolled plate. S4. Austenitization: The hot-rolled plate is heated to 850-1000℃ and held for 2-30 minutes to achieve complete austenitization. Then, it is quenched in water or oil to room temperature to obtain a quenched plate. S5. Tempering treatment: The quenched plate is tempered at 100-300℃ for 0.1-48h. After step S5, the microstructure of the steel is a multiphase structure of martensite and retained austenite, with a retained austenite volume fraction of 5-13%, a yield strength of 1800-2100MPa, a tensile strength of 2500-2800MPa, and a total elongation of 8-11%.

2. The method for preparing 2500MPa grade high-strength, high-ductility, low-alloy steel as described in claim 1, characterized in that, In step S3, the final rolling temperature of hot rolling is controlled at 800~950℃.

3. A 2500MPa grade high-strength, high-ductility, low-alloy steel, characterized in that, The high-strength, high-ductility, low-alloy steel is obtained by the preparation method described in any one of claims 1-2, wherein the retained austenite accounts for 5-13% of the volume of the high-strength, high-ductility, low-alloy steel; the mass percentage of the alloy raw materials is: C: 0.5-0.8%, Si: 0.5-2.0%, Mn: 0.2-2.0%, Cr: 0.2-2%, Ni: 0.2-3%, Mo: 0.06-2%, V: 0-0.5%, Nb: 0-0.5%, with the balance being Fe and unavoidable impurity elements.

4. The 2500MPa grade high-strength, high-ductility, low-alloy steel as described in claim 3, characterized in that, The high-strength, high-ductility, low-alloy steel has a yield strength of 1800~2100MPa, a tensile strength of 2500~2800MPa, and a total elongation of 8~11%.

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