Selective laser melting forming super-high strength and toughness low alloy steel, and preparation method and application thereof

Abstract

The application belongs to the technical field of additive manufacturing alloy steel and its heat treatment, and discloses a super-high strength and toughness low-alloy steel formed by selective laser melting, and a preparation method and application thereof. The chemical composition of the super-high strength and toughness low-alloy steel is C: 0.20-0.40%, Si: 0.80-1.80%, Mn: 0.4-1.5%, Cr: 0.40-1.40%, Ni: 0.5-1.50%, Cu: 0.40-1.5%, Mo: 0.1-0.5%, S≤0.015%, P≤0.025%, Si / C≥4, Ni / Cu≥0.8, and the balance is Fe. The low-alloy steel with a strength and ductility product of 18.5-29.5 GPa% is prepared by optimizing the alloy composition and heat treatment process and utilizing the synergistic deformation between the multi-phase structures. The application has low production cost and overcomes the problem of low strength and toughness product and low strength and ductility product of the additive manufacturing low-alloy steel.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing alloy steel and its heat treatment technology, specifically relating to a selective laser melting forming of ultra-high strength and toughness low alloy steel and its preparation method and application. Background Technology

[0002] Ultra-high strength low-alloy steel is generally a structural steel with a total alloy element content of less than 5%, a room temperature yield strength ≥1380MPa or a tensile strength ≥1470MPa, and good plasticity. It has been widely used in aerospace, nuclear power, and transportation fields, such as aircraft landing gear, rocket engine casings, heavy-duty train bogies, and high-speed rail brake discs. Additive manufacturing, also known as 3D printing, is a technology that uses the gradual accumulation of materials to manufacture solid parts. It not only effectively avoids the problems of long production processes and complex process chains in traditional parts manufacturing, but is also an effective way to achieve lightweight equipment and structural innovation. Selective laser melting (SLM) is an additive manufacturing technology that uses a laser as a heat source. Its high forming accuracy and excellent comprehensive performance of components make it commonly used to manufacture high-end parts. The combination of SLM and ultra-high strength low-alloy steel can not only reduce costs and increase efficiency, but also give rise to a new generation of lightweight, multi-functional integrated high-end equipment.

[0003] The strength-ductility product (tensile strength × elongation) is the amount of plastic deformation that a material can absorb per unit volume before fracture, and it is an important indicator of material safety. Due to the rapid melting and cooling during additive manufacturing, the molten pool exhibits typical non-equilibrium solidification characteristics, resulting in a certain gap between the strength-ductility product of additively manufactured low-alloy steel and that of forgings and rolled products produced by traditional processes.

[0004] Chinese patent CN 117600492 A discloses a method for forming ultra-high strength low-alloy steel using laser selective melting technology. The resulting alloy steel has a tensile strength of approximately 1750 MPa, an elongation of 9%, and a strength-ductility product of only 15.8 GPa%. Significantly increasing the content of precious metal elements such as Ni, Co, Cr, Mo, and V is currently an effective way to improve the strength-ductility product of alloy steel. Chinese patent CN114411067A discloses a medium-carbon hot-work die steel material and an additive manufacturing method based on it. Its total alloy content is approximately 9%, with Cr as the main alloying element. After heat treatment, the resulting alloy steel has a tensile strength of up to 1800 MPa, an elongation of approximately 10%, and a strength-ductility product of approximately 18.0 GPa. Chinese patent CN112831721A discloses a method for preparing SLM high-strength-ductility-product steel, with a total alloy content of approximately 10%, the main alloying element being Ni. After tempering, the additively manufactured ultra-high-strength steel exhibits a yield strength ≥927 MPa, tensile strength ≥1650 MPa, total elongation ≥15.2%, and a strength-ductility-product ≥25.4 GPa. Chinese patent CN117403145A discloses an additively manufactured ultra-high-strength steel and its preparation method, with a total alloy content of approximately 25%, and Ni and Co contents both exceeding 9.5%. After heat treatment, the alloy steel achieves a tensile strength of 2320 MPa, an elongation of approximately 12%, and a strength-ductility-product of approximately 27.8 GPa. However, the increase in alloy element content leads to an increase in production costs.

[0005] The low strength-ductility product and high alloy element content both hinder the application of SLM ultra-high strength steel. Improving the strength-ductility product of SLM ultra-high strength low alloy steel is an urgent problem to be solved in the current field of additive manufacturing alloy steel. Summary of the Invention

[0006] In order to overcome the shortcomings and deficiencies of the prior art, the primary objective of this invention is to provide an ultra-high strength and toughness selective laser melting forming low alloy steel.

[0007] Another objective of this invention is to provide a method for preparing ultra-high strength and toughness selective laser melting (SLM) formed low-alloy steel. This ultra-high strength and toughness SLM formed low-alloy steel is based on traditional ultra-high strength low-alloy steel, with the addition of an appropriate amount of Cu. After solution treatment, sub-temperature quenching, and tempering heat treatment, a multiphase microstructure is obtained, primarily composed of martensite, supplemented with small amounts of ferrite, retained austenite, and nano-sized copper-rich phases. This multiphase microstructure maintains high strength while exhibiting good plasticity, displaying a high strength-ductility product. Furthermore, the preparation process of this SLM formed ultra-high strength and toughness low-alloy steel is simple and low-cost.

[0008] Another object of the present invention is to provide the application of the above-mentioned ultra-high strength and toughness selective laser melting forming low alloy steel.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] A selective laser melting forming ultra-high strength and toughness low alloy steel, wherein the chemical composition of the ultra-high strength and toughness low alloy steel is as follows (by mass percentage): C: 0.20-0.40%, Si: 0.80-1.80%, Mn: 0.4-1.5%, Cr: 0.40-1.40%, Ni: 0.5-1.50%, Cu: 0.40-1.5%, Mo: 0.1-0.5%, S≤0.015%, P≤0.025%, with the balance being Fe.

[0011] Preferably, the chemical composition of the ultra-high strength and toughness low alloy steel is: C: 0.22-0.38%, Si: 1.0-1.60%, Mn: 0.6-1.20%, Cr: 0.60-1.20%, Ni: 0.7-1.30%, Cu: 0.50-1.20%, Mo: 0.2-0.4%, S≤0.010%, P≤0.015%, with the balance being Fe;

[0012] The ultra-high strength and toughness low alloy steel has a yield strength ≥1200MPa, tensile strength ≥1520MPa, elongation ≥11.5%, and strength-ductility product ≥18.5GPa.

[0013] Preferably, the mass ratios of Si and C, Ni and Cu in the chemical composition of the ultra-high strength and toughness selective laser melting forming low alloy steel simultaneously satisfy the following conditions: Si / C ≥ 4.0, Ni / Cu ≥ 0.8.

[0014] Preferably, the mass ratios of Si and C, Ni and Cu in the chemical composition of the ultra-high strength and toughness selective laser melting forming low alloy steel simultaneously satisfy the following conditions: Si / C ≥ 4.5, Ni / Cu ≥ 1.0.

[0015] A method for preparing ultra-high strength and toughness low alloy steel by selective laser melting includes the following steps:

[0016] S1) Powder preparation: Obtain alloy steel powder with the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel;

[0017] S2) Sample printing: Alloy steel powder is printed into shape using a selective laser melting device to obtain a printed part;

[0018] S3) Sub-temperature quenching: The printed part obtained in S2 is first austenitized at a certain temperature, then air-cooled to the two-phase region temperature, then held at the two-phase region temperature, and finally quenched and cooled to room temperature to obtain a sub-temperature quenched sample.

[0019] S4) Tempering: Temper the sub-temperature quenched sample obtained in step S3, and then air-cool it to room temperature to obtain the final product.

[0020] Preferably, the particle size of the alloy steel powder in step S1 is 15-53 μm.

[0021] Preferably, the sample printing parameters in step S2 are as follows: laser power 220W-300W, scanning speed 800-1100mm / s, scanning spacing 0.11±0.05mm, powder layer thickness 40±20μm, laser spot diameter 100±20μm, substrate preheating temperature 100±20℃, scanning strategy: strip scanning and layer-by-layer rotation, initial rotation angle 30±10°, rotation increment 67±20°; the density of the printed part in step S2 is ≥99.4%.

[0022] Preferably, the austenitizing treatment in step S3 is performed at a temperature of 880-960℃ and the holding time is 2-4 hours.

[0023] The temperature of the two-phase region in step S3 is 740-800℃, and the holding time is 1-3h;

[0024] The quenching medium used for cooling in step S3 is PAG quenching fluid.

[0025] Preferably, the tempering temperature in step S4 is 200-300℃, and the holding time is 3-6h.

[0026] The above-mentioned laser melting and forming of ultra-high strength and toughness low alloy steel is used in aircraft landing gear, engine mounts or high-speed rail brake disc components.

[0027] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0028] (1) The ultra-high strength and toughness selective laser melting forming low-alloy steel obtained by this invention is based on traditional ultra-high strength low-alloy steel. By adding an appropriate amount of Cu element and optimizing the alloy content of C, Si, Ni, etc., combined with appropriate laser melting and sub-temperature quenching heat treatment processes, a multiphase structure with martensite as the main component and a small amount of ferrite, retained austenite, and nano-sized copper-rich phase is obtained. The solid solution strengthening of martensite and the dispersion strengthening of copper-rich phase give the multiphase structure ultra-high strength, while the good plasticity of ferrite and the phase transformation plasticizing effect (TRIP effect) of retained austenite give the multiphase structure excellent plasticity. The ultra-high strength and toughness selective laser melting forming low-alloy steel obtained by this invention has a tensile strength of 1520-1910 MPa, while its total elongation is 11.5-17.6%, and the strength-ductility product is 18.5-29.5 GPa*%, showing a good strength-toughness match.

[0029] (2) The method for preparing ultra-high strength and toughness selective laser melting forming low alloy steel provided by this invention is simple and easy to control. The performance of the product reaches the level of laser melting forming high alloy steel, and it has excellent cost-effectiveness. The ultra-high strength and toughness selective laser melting forming low alloy steel prepared by this invention has a very wide range of application prospects. Attached Figure Description

[0030] Figure 1 (a) and (b) in the figure are Kikuchi belt contrast images of the alloy steel produced in Example 3 and Comparative Example 4 of the present invention, respectively.

[0031] Figure 2 (a) and (b) are phase composition diagrams of the alloy steels produced in Example 3 and Comparative Example 4 of the present invention, respectively. The white part is martensite and ferrite with body-centered cubic structure, and the blue part is residual austenite phase with face-centered cubic structure.

[0032] Figure 3 The tensile stress-strain curves are for alloy steels in Example 3 and Comparative Examples 1-4. Detailed Implementation

[0033] The following detailed embodiments further illustrate the content of the present invention. However, the scope of the present invention is not limited to the following embodiments, and conventional techniques can be referred to for process parameters not specifically specified.

[0034] Example 1: A low-alloy steel with ultra-high strength and toughness formed by selective laser melting

[0035] The chemical composition of the ultra-high strength and toughness selective laser melting forming low alloy steel is as follows: C: 0.22, Si: 1.0%, Mn: 0.6%, Cr: 0.8%, Ni: 1.2%, Cu: 1.1%, Mo: 0.3%, S: 0.005%, P: 0.010%, with the balance being Fe, Si / C = 4.5, Ni / Cu = 1.1.

[0036] The preparation method of the ultra-high strength and toughness selective laser melting forming low alloy steel includes the following steps:

[0037] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel.

[0038] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were: laser power 240W, scanning speed 900mm / s, scanning spacing 0.11mm, powder layer thickness 40μm, laser spot diameter 100μm, substrate preheating temperature 100℃, and scanning strategy: strip scanning and layer-by-layer rotation with an initial rotation angle of 30° and a rotation increment of 67°, resulting in a printed part with a density of 99.5%.

[0039] S3) Sub-temperature quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 960°C, held for 2 hours for austenitization treatment, then air-cooled to 800°C, and then sent to a heat treatment furnace to be held at 800°C for 1.5 hours. Finally, it is sent to a tank containing PAG quenching liquid to be cooled to room temperature to obtain a sub-temperature quenched sample.

[0040] S4) Tempering: The sub-temperature quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours. It is then air-cooled to room temperature to obtain the final product.

[0041] Example 2: A low-alloy steel with ultra-high strength and toughness formed by selective laser melting

[0042] The chemical composition of the ultra-high strength and toughness selective laser melting forming low alloy steel is as follows: C: 0.26, Si: 1.2%, Mn: 0.7%, Cr: 0.7%, Ni: 1.0%, Cu: 1.0%, Mo: 0.2%, S: 0.007%, P: 0.012%, with the balance being Fe, Si / C = 4.6, Ni / Cu = 1.0.

[0043] The preparation method of the ultra-high strength and toughness selective laser melting forming low alloy steel includes the following steps:

[0044] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel.

[0045] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were: laser power 220W, scanning speed 1000 mm / s, scanning spacing 0.11mm, powder layer thickness 40 μm, laser spot diameter 100 μm, substrate preheating temperature 100 ℃, and scanning strategy: strip scanning and layer-by-layer rotation with an initial rotation angle of 30° and a rotation increment of 67°, resulting in a printed part with a density of 99.4%.

[0046] S3) Sub-temperature quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 920°C, held for 2 hours for austenitization treatment, then air-cooled to 785°C, and then sent to a heat treatment furnace to be held at 785°C for 2 hours. Finally, it is sent to a tank containing PAG quenching liquid to be cooled to room temperature to obtain a sub-temperature quenched sample.

[0047] S4) Tempering: The sub-temperature quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours. It is then air-cooled to room temperature to obtain the final product.

[0048] Example 3: A low-alloy steel with ultra-high strength and toughness formed by selective laser melting

[0049] The chemical composition of the ultra-high strength and toughness selective laser melting forming low alloy steel is as follows: C: 0.30, Si: 1.4%, Mn: 0.7%, Cr: 0.8%, Ni: 0.9%, Cu: 0.8%, Mo: 0.2%, S: 0.006%, P: 0.012%, with the balance being Fe, Si / C = 4.7, Ni / Cu = 1.1.

[0050] The preparation method of the ultra-high strength and toughness selective laser melting forming low alloy steel includes the following steps:

[0051] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel.

[0052] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were: laser power 220W, scanning speed 1000 mm / s, scanning spacing 0.11mm, powder layer thickness 40 μm, laser spot diameter 100 μm, substrate preheating temperature 100 ℃, and scanning strategy: strip scanning and layer-by-layer rotation with an initial rotation angle of 30° and a rotation increment of 67°, resulting in a printed part with a density of 99.6%.

[0053] S3) Sub-temperature quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 900°C, held for 2 hours for austenitization treatment, then air-cooled to 770°C, and then sent to a heat treatment furnace to be held at 770°C for 2 hours. Finally, it is sent to a tank containing PAG quenching liquid to be cooled to room temperature to obtain a sub-temperature quenched sample.

[0054] S4) Tempering: The sub-temperature quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours. It is then air-cooled to room temperature to obtain the final product.

[0055] Example 4: A low-alloy steel with ultra-high strength and toughness formed by selective laser melting

[0056] The chemical composition of the ultra-high strength and toughness selective laser melting forming low alloy steel is as follows: C: 0.34, Si: 1.6%, Mn: 0.7%, Cr: 0.6%, Ni: 0.9%, Cu: 0.8%, Mo: 0.2%, S: 0.006%, P: 0.012%, with the balance being Fe, Si / C = 4.7, Ni / Cu = 1.1.

[0057] The preparation method of the ultra-high strength and toughness selective laser melting forming low alloy steel includes the following steps:

[0058] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel.

[0059] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were: laser power 240W, scanning speed 1000 mm / s, scanning spacing 0.11mm, powder layer thickness 40 μm, laser spot diameter 100 μm, substrate preheating temperature 100 ℃, and scanning strategy: strip scanning and layer-by-layer rotation with an initial rotation angle of 30° and a rotation increment of 67°, resulting in a printed part with a density of 99.6%.

[0060] S3) Sub-temperature quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 900°C, held for 2 hours for austenitization treatment, then air-cooled to 755°C, and then sent to a heat treatment furnace to be held at 755°C for 3 hours. Finally, it is sent to a tank containing PAG quenching liquid to be cooled to room temperature to obtain a sub-temperature quenched sample.

[0061] S4) Tempering: The sub-temperature quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours. It is then air-cooled to room temperature to obtain the final product.

[0062] Example 5: A low-alloy steel with ultra-high strength and toughness formed by selective laser melting

[0063] The chemical composition of the ultra-high strength and toughness selective laser melting forming low alloy steel is as follows: C: 0.38, Si: 1.8%, Mn: 0.8%, Cr: 0.6%, Ni: 0.8%, Cu: 0.7%, Mo: 0.2%, S: 0.008%, P: 0.010%, with the balance being Fe, Si / C = 4.5, Ni / Cu = 1.1.

[0064] The preparation method of the ultra-high strength and toughness selective laser melting forming low alloy steel includes the following steps:

[0065] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel.

[0066] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were: laser power 240W, scanning speed 900 mm / s, scanning spacing 0.11mm, powder layer thickness 40 μm, laser spot diameter 100μm, substrate preheating temperature 100 ℃, and scanning strategy: strip scanning and layer-by-layer rotation with an initial rotation angle of 30° and a rotation increment of 67°, resulting in a printed part with a density of 99.6%.

[0067] S3) Sub-temperature quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 880°C, held for 2 hours for austenitization treatment, then air-cooled to 740°C, and then sent to a heat treatment furnace again to be held at 740°C for 2 hours. Finally, it is sent to a tank containing PAG quenching liquid to be cooled to room temperature to obtain a sub-temperature quenched sample.

[0068] S4) Tempering: The sub-temperature quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours. It is then air-cooled to room temperature to obtain the final product.

[0069] Comparative Example 1: Cu-free sub-temperature quenching-tempering selective zone laser melting forming of low alloy steel

[0070] The chemical composition of the laser-melted low-alloy steel is: C: 0.30, Si: 1.4%, Mn: 0.7%, Cr: 0.8%, Ni: 0.9%, Mo: 0.2%, S: 0.006%, P: 0.012%, with the balance being Fe, and Si / C = 4.7.

[0071] The method for preparing low-alloy steel by selective laser melting includes the following steps:

[0072] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above low alloy steel element composition ratio;

[0073] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were: laser power 220W, scanning speed 1000 mm / s, scanning spacing 0.11mm, powder layer thickness 40 μm, laser spot diameter 100 μm, substrate preheating temperature 100 ℃, and scanning strategy: strip scanning and layer-by-layer rotation with an initial rotation angle of 30° and a rotation increment of 67°, resulting in a printed part with a density of 99.6%.

[0074] S3) Sub-temperature quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 900°C, held for 2 hours for austenitization treatment, then air-cooled to 770°C, and then sent to a heat treatment furnace to be held at 770°C for 2 hours. Finally, it is sent to a tank containing PAG quenching liquid to be cooled to room temperature to obtain a sub-temperature quenched sample.

[0075] S4) Tempering: The sub-temperature quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours. It is then air-cooled to room temperature to obtain the final product.

[0076] Comparative Example 2: Low-alloy steel with Ni / Cu ratio of 0.5, formed by sub-temperature quenching-tempering and selective laser melting.

[0077] The chemical composition of the low alloy steel formed by selective laser melting is as follows: C: 0.30, Si: 1.4%, Mn: 0.7%, Cr: 0.8%, Ni: 0.4%, Cu: 0.8%, Mo: 0.2%, S: 0.006%, P: 0.012%, with the balance being Fe, Si / C = 4.7, Ni / Cu = 0.5.

[0078] The method for preparing low-alloy steel by selective laser melting includes the following steps:

[0079] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel.

[0080] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were as follows: laser power: 220W, scanning speed: 1000 mm / s, scanning spacing: 0.11 mm, powder layer thickness: 40 μm, laser spot diameter: 100 μm, substrate preheating temperature: 100 ℃, scanning strategy: strip scanning and layer-by-layer rotation, initial rotation angle: 30°, rotation increment: 67°, to obtain a printed part with a density of 99.6%.

[0081] S3) Sub-temperature quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 900°C, held for 2 hours for austenitization treatment, then air-cooled to 770°C, and then sent to a heat treatment furnace to be held at 770°C for 2 hours. Finally, it is sent to a tank containing PAG quenching liquid to be cooled to room temperature to obtain a sub-temperature quenched sample.

[0082] S4) Tempering: The sub-temperature quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours. It is then air-cooled to room temperature to obtain the final product.

[0083] Comparative Example 3: Low-alloy steel with Si / C ratio of 2.0, formed by sub-temperature quenching-tempering and selective laser melting.

[0084] The chemical composition of the low alloy steel formed by selective laser melting is as follows: C: 0.30, Si: 0.6%, Mn: 0.7%, Cr: 0.8%, Ni: 0.9%, Cu: 0.8%, Mo: 0.2%, S: 0.006%, P: 0.012%, with the balance being Fe, Si / C = 2, Ni / Cu = 1.1.

[0085] The method for preparing low-alloy steel by selective laser melting includes the following steps:

[0086] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel.

[0087] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were as follows: laser power: 220W, scanning speed: 1000 mm / s, scanning spacing: 0.11 mm, powder layer thickness: 40 μm, laser spot diameter: 100 μm, substrate preheating temperature: 100 ℃, scanning strategy: strip scanning and layer-by-layer rotation, initial rotation angle: 30°, rotation increment: 67°, to obtain a printed part with a density of 99.6%.

[0088] S3) Sub-temperature quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 900°C, held for 2 hours for austenitization treatment, then air-cooled to 770°C, and then sent to a heat treatment furnace to be held at 770°C for 2 hours. Finally, it is sent to a tank containing PAG quenching liquid to be cooled to room temperature to obtain a sub-temperature quenched sample.

[0089] S4) Tempering: The sub-temperature quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours, and then air-cooled to room temperature to obtain the final product.

[0090] Comparative Example 4: Low-alloy steel formed by conventional quenching-tempering selective laser melting

[0091] The chemical composition of the low alloy steel formed by selective laser melting is as follows: C: 0.30, Si: 1.4%, Mn: 0.7%, Cr: 0.8%, Ni: 0.9%, Cu: 0.8%, Mo: 0.2%, S: 0.006%, P: 0.012%, with the balance being Fe, Si / C = 4.7, Ni / Cu = 1.1.

[0092] The method for preparing the quenched-tempered selective laser melting and forming low alloy steel includes the following steps:

[0093] S1) Powder preparation: Alloy steel powder with a particle size of 15-53μm was obtained according to the above-mentioned elemental composition ratio of ultra-high strength and toughness low alloy steel.

[0094] S2) Sample Printing: Selective laser melting (SLM) equipment was used for printing. The forming chamber of the SLM equipment was evacuated and filled with high-purity nitrogen (purity ≥99.99%) as a protective atmosphere. The forming process parameters were as follows: laser power: 200W, scanning speed: 1000 mm / s, scanning spacing: 0.11 mm, powder layer thickness: 40 μm, laser spot diameter: 100 μm, substrate preheating temperature: 100 ℃, scanning strategy: strip scanning and layer-by-layer rotation, initial rotation angle: 30°, rotation increment: 67°, to obtain a printed part with a density of 99.6%.

[0095] S3) Quenching: After separating the printed part obtained in S2 from the substrate, the printed part is first sent to a heat treatment furnace, heated to 900°C, and held for 2 hours for austenitization treatment. Then it is sent to a tank containing PAG quenching liquid to cool to room temperature to obtain a quenched sample.

[0096] S4) Tempering: The quenched sample obtained in step S3 is sent back into the heat treatment furnace and heated to 250°C and held for 4 hours, and then air-cooled to room temperature to obtain the final product.

[0097] Example and Comparative Material Performance Tests

[0098] The relevant mechanical properties were tested according to the methods specified in GB / T228.1-2010, the standard for room temperature tensile testing of metallic materials. The test results are listed in Table 1.

[0099] Table 1

[0100]

[0101] The mechanical property values ​​in Table 1 are the average of three measurements.

[0102] As shown in Table 1, the laser-melted low-alloy steel prepared by this invention has a tensile strength of 1520-1910 MPa and an elongation of 11.5-17.6%, exhibiting a good strength-plasticity match. Therefore, the strength-plasticity product is as high as 18.5-29.5 GPa*. (1) The Cu-rich nanophase not only enhances the strength of the matrix but also improves the TRIP effect of RA. Therefore, under similar composition and the same laser melting and heat treatment process, the tensile strength, elongation, and strength-ductility product of Example 3 (containing Cu) are increased by 10.5% (160 MPa), 47.9%, and 63.9% (11.5 GPa*%), respectively, compared with Comparative Example 1 (without Cu). (2) Ni can inhibit the segregation of Cu at grain boundaries and improve the strength and toughness of the material. Therefore, under similar composition and the same laser melting and heat treatment process, the tensile strength, elongation, and strength-ductility product of Example 3 (Ni / Cu=1.1) are increased by 8.2% compared with Comparative Example 2 (Ni / Cu=0.5). (127MPa), 60% and 76.6% (12.8GPa*%); (3) Si can suppress the precipitation of carbides in the two-phase region and during tempering. Therefore, under similar composition and the same laser melting forming and heat treatment process, the tensile strength, elongation and strength-ductility product of Example 3 (Si / C=4.7) are 6.3% (99MPa), 135.7% and 150% (17.7GPa*%) higher than those of Comparative Example 3 (Si / C=2), respectively; (4) Although sub-temperature quenching reduces the content of martensite in the multiphase structure, it increases the content of RA and reduces the thermal stress of quenching. Therefore, under the same composition and laser melting forming process but different heat treatment process, the tensile strength of Example 3 (sub-temperature quenching-tempering) is 18.3% (382 MPa) lower than that of Comparative Example 2 (conventional quenching-tempering), but the elongation and strength-ductility product are 214.3% and 156.5% (18GPa*%) higher than those of Comparative Example 2 (conventional quenching-tempering).

[0103] Figure 1 A contrast image of Kikuchi strip made of alloy steel, such as Figure 1 As shown, the matrix structure of the materials under both processes in Example 3 and Comparative Example 4 is lath-shaped, but the lath size in Comparative Example 4 is slightly smaller than that in Example 3.

[0104] Figure 2 The phase composition diagram of alloy steel is shown below. Figure 2 As shown, the volume fraction of retained austenite in Example 3 was approximately 7.1%, while the volume fraction of retained austenite in Comparative Example 4 was approximately 4.2%, indicating a significant increase in the volume fraction of retained austenite compared to Comparative Example 4.

[0105] The significant difference between this invention and existing laser-melted low-alloy steels lies in the optimized mass ratio of Si to C and Ni to Cu to suppress carbide precipitation and Cu segregation. Simultaneously, sub-critical quenching is used to achieve the formation of nanoscale copper-rich phases and a suitable amount of retained austenite. Ultimately, through the synergistic control of alloy composition and sub-critical quenching heat treatment, a multiphase microstructure exhibiting both high strength and excellent toughness is obtained. This overcomes the strength-toughness mismatch problem currently found in laser-melted low-alloy steels, achieving performance levels comparable to laser-melted high-alloy steels, and demonstrating superior cost-effectiveness.

[0106] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing ultra-high strength and toughness low-alloy steel by selective laser melting, characterized in that, The chemical composition of ultra-high strength and toughness low alloy steel formed by selective laser melting, by mass percentage, is as follows: C: 0.20-0.40%, Si: 0.80-1.80%, Mn: 0.4-1.5%, Cr: 0.40-1.40%, Ni: 0.5-1.50%, Cu: 0.40-1.5%, Mo: 0.1-0.5%, S≤0.015%, P≤0.025%, with the balance being Fe; The chemical composition of the ultra-high strength and toughness low alloy steel formed by selective laser melting simultaneously satisfies the following conditions: Si / C ≥ 4.0, Ni / Cu ≥ 0.

8. Includes the following steps: S1) Powder preparation: Obtain alloy steel powder with the elemental composition ratio of ultra-high strength and toughness low alloy steel formed by selective laser melting; S2) Sample printing: Alloy steel powder is printed into shape using a selective laser melting device to obtain a printed part; S3) Sub-temperature quenching: The printed part obtained in S2 is first austenitized at a certain temperature, then air-cooled to the two-phase region temperature, then held at the two-phase region temperature, and finally quenched and cooled to room temperature to obtain a sub-temperature quenched sample. S4) Tempering: Temper the sub-temperature quenched sample obtained in step S3, and then air-cool it to room temperature to obtain the final product. The austenitizing treatment in step S3 is carried out at a temperature of 880-960℃ and a holding time of 2-4 hours. The temperature of the two-phase region in step S3 is 740-800℃, and the holding time is 1-3h; The quenching medium used for cooling in step S3 is PAG quenching fluid; The tempering process in step S4 is performed at a temperature of 200-300℃ for 3-6 hours.

2. The method for preparing ultra-high strength and toughness low-alloy steel by selective laser melting according to claim 1, characterized in that, The chemical composition of the selected laser melting-formed ultra-high strength and toughness low alloy steel, by mass percentage, is as follows: C: 0.22-0.38%, Si: 1.0-1.60%, Mn: 0.6-1.20%, Cr: 0.60-1.20%, Ni: 0.7-1.30%, Cu: 0.50-1.20%, Mo: 0.2-0.4%, S≤0.010%, P≤0.015%, with the balance being Fe; The selected area laser melting formed ultra-high strength and toughness low alloy steel has a yield strength ≥1200MPa, tensile strength ≥1520MPa, elongation ≥11.5%, and strength-ductility product ≥18.5GPa.

3. The method for preparing ultra-high strength and toughness low-alloy steel by selective laser melting according to claim 1, characterized in that, The chemical composition of the ultra-high strength and toughness low alloy steel formed by selective laser melting simultaneously satisfies the following conditions: Si / C ≥ 4.5, Ni / Cu ≥ 1.

0.

4. The method for preparing ultra-high strength and toughness low-alloy steel by selective laser melting according to claim 1, characterized in that, The particle size of the alloy steel powder in step S1 is 15-53 μm.

5. The method for preparing ultra-high strength and toughness low-alloy steel by selective laser melting according to claim 1, characterized in that, The sample printing parameters in step S2 are as follows: laser power 220W-300W, scanning speed 800-1100mm / s, scanning spacing 0.11±0.05mm, powder layer thickness 40±20μm, laser spot diameter 100±20μm, substrate preheating temperature 100±20℃, scanning strategy: strip scanning and layer-by-layer rotation, initial rotation angle 30±10°, rotation increment 67±20°; The density of the printed part in step S2 is ≥99.4%.

6. A selective laser melting forming method for ultra-high strength and toughness low-alloy steel, characterized in that, It is prepared by the method described in any one of claims 1 to 5.

7. The application of the selective laser melting forming ultra-high strength and toughness low alloy steel according to claim 6 in aircraft landing gear, engine brackets or high-speed rail brake disc components.

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