Additively manufactured maraging steel and method of making the same

Abstract

The application discloses a kind of martensitic age steel of additive manufacturing and preparation method thereof, the martensitic age steel, according to mass percentage, its composition is as follows: Ni 16~22%, Co0-5%, Mo 2~6%, Ti 0.2~2.5%, Al 0~1.5%, Re 0~0.3%, wherein Re is selected from at least one of Y or La, the balance is Fe and inevitable impurities.In the application, by reducing Co element, adjusting the ratio of Ni, Ti and Mo in the alloy, while adding a small amount of rare earth (Y and La) element, the alloy has higher M s Under the premise, the raw material cost of alloy powder and the preparation cost of alloy are greatly reduced, and the fine equiaxed grains and high-density nano precipitates formed after aging are formed by combining with the process parameter control of additive manufacturing, so that the mechanical properties of the martensitic age steel are greatly better than those of the traditional casting process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of additive manufacturing die steel, and particularly relates to a kind of martensitic precipitation hardened steel of additive manufacturing and a preparation method thereof. BACKGROUND

[0002] Additive manufacturing is an efficient, flexible and intelligent manufacturing process. Additive manufacturing plays an important role in the formation of complex structural parts. At present, additive manufacturing processes mainly include laser powder bed fusion (L-PBF), laser direct energy deposition (DED) and electron beam melting forming technology. Among the above additive manufacturing processes, L-PBF additive manufacturing process is the most promising. The parts formed by L-PBF have the advantages of high precision and good performance. However, the L-PBF manufacturing process has strict requirements on the powder. At present, there are few martensitic precipitation hardened steel powders suitable for additive manufacturing process, and cracks are easily generated during the forming process. Therefore, developing new powders suitable for additive manufacturing process is the technical difficulty currently faced.

[0003] Martensitic precipitation hardened steel is the preferred material for die steel, because it has high strength, high hardness and good toughness. The mechanical properties of martensitic precipitation hardened steel formed by additive manufacturing are higher than those of traditional castings and forgings, because the additive manufacturing process forms fine equiaxed grains. However, defects such as pores, lock holes and microcracks are easily formed during the additive manufacturing process. At present, 18Ni300 is mainly used for additive manufacturing of martensitic precipitation hardened steel. The patent specification with publication number CN 114351048 A discloses a kind of martensitic precipitation hardened steel powder and its application in additive manufacturing. However, the addition of high content of Co in the alloy increases the preparation cost of martensitic precipitation hardened steel, which limits its wide application. In addition, due to the solid state phase transition, high hardness and non-equilibrium solidification behavior of additive manufacturing martensitic precipitation hardened steel, microcracks are easily generated during the forming process of the printed part. Traditional H13 die steel is prone to crack during additive manufacturing, because the presence of carbon increases the crack sensitivity, thereby reducing the welding performance of H13 steel. The generation of defects during additive manufacturing poses a serious challenge to the mechanical properties and service life of die steel. Developing new alloy compositions and additive manufacturing process control is the most direct and effective method to solve the above problems.

[0004] Traditional carbon-containing steels, such as Fe-0.19C-1.01Mn-1.46Si steel and Fe-0.66C-1.42Cr-0.4Si-0.42Mn-0.07V steel, have excellent mechanical properties (ultimate tensile strength greater than 2 GPa). Patent specification CN 112322991 A discloses a Fe-C-Si-Mn-V low-alloy, medium-manganese, high-strength steel with a yield strength of 2 GPa. However, its chemical composition contains 0.2% to 0.4% by mass of carbon, which can affect the material's weldability. Additive manufacturing processes differ from traditional casting and forging processes. Adding high levels of carbon can reduce the material's weldability, resulting in lower mold quality for AM parts and even causing warping, deformation, and cracking. Therefore, carbon should not be added to maraging steels. To reduce raw material costs, the precious metal content in maraging steels should be reduced. Therefore, the content of elements that form precipitates needs to be increased to enhance strength and hardness. However, the addition of elements (Ti and Al) that form the precipitate phase tends to reduce the manufacturability, which is mainly due to the increase in the crack sensitivity of the additively manufactured maraging steel. According to the Schaeffler phase diagram, the ratio of Ni and Mo content has an important influence on the M s The impact is crucial. The design of maraging steel alloy compositions should consider the effects of the added element content on martensite, austenite, and ferrite. Furthermore, the brittle Laves phase is easily formed during the non-equilibrium solidification process of additive manufacturing, which should be considered in the composition design of additively manufactured maraging steel. Rare earth elements such as Y and La can be appropriately added to additively manufactured maraging steel to further purify the molten steel and grains, ensuring the production of high-quality maraging steel.

[0005] Additively manufactured maraging steel parts typically require post-processing, such as heat treatment and hot isostatic pressing, to further eliminate defects such as residual stress, porosity, and texture, and to reduce anisotropy in mechanical properties. Optimizing the additive manufacturing process can improve the quality and mechanical properties of printed parts. For maraging steel, parts constructed using selective laser melting additive manufacturing typically exhibit good build quality, meeting the application requirements of conventional mold steels. Heat treatment increases mold steel strength but reduces ductility. Therefore, additive manufacturing of maraging steels must be performed within appropriate build and heat treatment windows. Considering part integrity and practicality, high-temperature solution aging and direct aging are commonly used heat treatment processes for additively manufactured maraging steels. Direct aging, by contrast, has a lesser impact on mold surface quality, primarily due to its lower aging temperature.

[0006] The existing problems of the additive manufacturing maraging steel technology are: 1) the addition of cobalt element in the maraging steel leads to the increase of raw material cost; the maraging steel is the preferred material of the die steel, however, the die steel usually contains the addition of carbon element to precipitate carbide, so as to improve the strength and hardness thereof; the addition of the carbon element leads to the reduction of the welding performance, which is not conducive to the additive manufacturing process; 2) in the additive manufacturing process, due to the high internal stress and the high hardness of the martensite, the crack sensitivity of the traditional maraging steel is increased, and the quality of the printed part is further reduced. SUMMARY

[0007] In view of the defects of the prior art, a first object of the present application is to provide a high-strength additive manufacturing maraging steel.

[0008] A second object of the present application is to provide a preparation method of the additive manufacturing maraging steel.

[0009] The present application balances the M s , alloy cost, phase composition and crack sensitivity, adopts more inexpensive raw materials, controls the cost of the maraging steel through reasonable component design, and combines the additive manufacturing process parameter control, to form fine equiaxed grains (1-2 μm) and high-density nano precipitates (Ni3(Ti, Al, Mo) formed after aging, so as to obtain the maraging steel with excellent mechanical properties superior to the traditional casting process.

[0010] In order to achieve the above object, the present application adopts the following technical scheme:

[0011] The additive manufacturing maraging steel provided by the present application has the following composition in terms of mass percentage: Ni 16-22%, Co 0-5%, Mo 2-6%, Ti 0.2-2.5%, Al 0-1.5%, Re 0-0.3%, wherein Re is selected from at least one of Y or La, and the balance is Fe and inevitable impurities.

[0012] The maraging steel provided by the present application has a lower cobalt content, which is beneficial to the precipitation of Mo-rich phase during the aging process, and further improves the strength and hardness of the material; in addition, a small amount of Co can increase the M s of the material, increase the volume fraction of the martensite, and the addition of Al can improve the M s, promote the formation of martensitic structure, while making the aging process of Ni3Al precipitation increase strength, in addition, can also be used as deoxidizer, prevent the increase of oxygen content in the liquid steel, adding Ti and Mo in the aging process of Ni3(Ti, Mo), improve the strength of the material, while adding Y and La elements can improve the solid solubility of martensitic steel. That is, in the present application, by reducing the Co element, adjusting the ratio of Ni, Ti and Mo in the alloy, while adding a small amount of rare earth (Y and La) elements, in order to ensure that the alloy has higher M s Under the premise, greatly reduces the raw material cost of alloy powder and the preparation cost of alloy, and can improve the mechanical properties of the material. In addition, the martensitic steel of the present application contains almost no C element affecting the welding performance, and the impurities C, P and S are all <0.03%, so the welding performance / printability is improved.

[0013] However, it should be pointed out that the addition amount of Mo and Ti in the dual-phase structure martensitic steel of the present application is not higher than 6% and 2.5% respectively, because adding higher content of Mo and Ti in the martensitic steel will lead to the precipitation of phase supersaturation and consume a large amount of Ni and Fe, reducing the contribution of martensitic structure to strength and toughness. In addition, too high Ti content is easy to increase the oxygen content in the steel, which is not conducive to obtaining better strength and plasticity combination.

[0014] Preferably, in the martensitic steel, the mass ratio of Ni:Mo:Ti is 17-21:3-5:0.5-2.5, preferably 18.5-20.5:3.5-5:0.6-1.8.

[0015] The inventors found that by controlling the ratio of Ni, Mo and Ti in the martensitic steel within the above range, the M s higher, and can obtain better mechanical properties through additive manufacturing process.

[0016] Further preferably, the martensitic steel, by mass percent, its composition is as follows: Ni 18-20%, Co 1-5%, Mo 3-5%, Ti 0.5-2%, Al 0-1.2%, Re 0-0.3%, wherein Re is selected from at least one of Y or La, and the balance is Fe and inevitable impurities.

[0017] More preferably, the martensitic steel, by mass percent, its composition is as follows: Ni 18.5-19.5%, Co 2-4%, Mo 3.5-5%, Ti 0.6-1.8%, Al 0.1-1%, Re 0.02-0.2%, wherein Re is selected from at least one of Y or La, and the balance is Fe and inevitable impurities.

[0018] The alloying elements of the above preferred maraging steel are used to obtain the maraging steel with the best performance.

[0019] Preferably, the maraging steel has a martensite start temperature (M s ) of 80-210℃. By the component design of the present application, the martensite start temperature (M s ) is 80-210℃, and the martensite finish temperature (M s In the above range, it is beneficial to form a high volume fraction of martensite in the additive manufacturing process and improve the strength / hardness of the material.

[0020] Preferably, the maraging steel has a dual-phase structure of martensite and austenite, wherein the volume fraction of the martensite is 97-99%, and the volume fraction of the austenite is 1-3%.

[0021] In the present application, the maraging steel has a dual-phase structure of martensite and austenite, the martensite functions to improve the strength and hardness, and a small amount of austenite functions to induce stress-induced martensite phase transition and maintain the plasticity of the material. However, since a high volume fraction of martensite is a prerequisite for improving the mechanical properties, the present application effectively controls the content of the austenite stabilizing element Ni within the design range of the present application, so that the volume fraction of the austenite is only 1-3% under the synergistic effect of the other components of the present application, and the best comprehensive mechanical properties are finally obtained.

[0022] Preferably, the grains of the base body in the maraging steel are equiaxed grains with a grain size of 1-2μm, and the maraging steel contains a cellular structure with a size of about 200-900nm.

[0023] The grains in the maraging steel provided by the present application are fine equiaxed grains, wherein the grain size of the maraging steel after powder printing is 1-1.5μm, and the grain size of the aged state after aging treatment is 1-2μm. The grain refinement effectively improves the strength and toughness of the material, the equiaxed grains are beneficial to the plastic deformation of the material and the suppression of the generation of micro-cracks, and in addition, the maraging steel provided by the present application contains a large amount of cellular structure (dislocation cell),

[0024] The functions of the fine cellular structure include the following two aspects: 1) the cellular structure is beneficial to the slip of the grains during the deformation of the material, and the mechanical property anisotropy is reduced. 2) the fine cellular structure increases the formation of dislocations at the cell walls, and the dislocation strengthening is one of the reasons for the increase of the yield strength of the material.

[0025] In a preferred embodiment, the maraging steel further contains Ni3(Ti, Mo, Al) precipitates, the size of which is 4.9 to 6.5 nm. The maraging steel of the present invention also contains a large number of cellular structures with fine grains, resulting in excellent mechanical properties.

[0026] In a preferred embodiment, the maraging steel has a tensile strength of 1600-2100 MPa, a yield strength of 1500-1900 MPa, an elongation of 2-8%, and a hardness of 52-56 HRC.

[0027] The maraging steel provided by the present invention, after powder printing, has a tensile strength of 1000-1200 MPa, an elongation of 5-20%, and a hardness of 25-35 HRC in the molded (printed) state. After aging and precipitation of strengthening phases, the tensile strength reaches 1600-2100 MPa, the yield strength reaches 1500-1900 MPa, the elongation reaches 2-8%, and the hardness reaches 52-56 HRC. The maraging steel provided by the present invention combines excellent hardness, strength, and ductility. Furthermore, the molded maraging steel is free of cracks and microcracks, resulting in a high yield rate.

[0028] The present invention also provides a method for preparing maraging steel by additive manufacturing, comprising preparing various metal raw materials according to a designed ratio, performing vacuum induction melting and casting to obtain a steel ingot, atomizing the steel ingot to obtain spherical powder particles, additively manufacturing the spherical powder particles to obtain a maraging steel blank, and subjecting the maraging steel blank to an aging treatment to obtain the maraging steel.

[0029] All metal raw materials in the present invention are pure metals with a purity higher than 99.5%.

[0030] The alloy composition designed according to the present invention is cheaper than that of traditional maraging steel. The spherical powder particles obtained by gas atomization are suitable for additive manufacturing. The molded parts obtained by additive manufacturing are free of cracks and microcracks and have high hardness and strength.

[0031] In a preferred embodiment, the vacuum induction melting process is as follows: first, Al, Ti, and Re are wrapped with aluminum paper, and then 1 / 2Ni, 1 / 3Fe, Co, Mo, 1 / 2Ni, and 2 / 3Fe are placed in a melting furnace from bottom to top. After heating until melted, the paper-wrapped Al, Ti, and Re are added to the melting furnace, melted, and the melt obtained by melting is cast into a mold to obtain a steel ingot.

[0032] In the present application, 1 / 2 Ni means that the Ni raw material taken according to the design proportion is divided into two parts by mass ratio, and one part is taken, 1 / 3 Fe means that the Fe raw material taken according to the design proportion is divided into three parts by mass ratio, and one part is taken, and 2 / 3 Fe means that the Fe raw material taken according to the design proportion is divided into three parts by mass ratio, and two parts are taken.

[0033] The inventor found that the non-uniformity of the molten liquid can be avoided by first putting Fe, Ni and Mo into the crucible according to the above proportions and order, melting in vacuum to avoid oxidation, and finally adding Al, Ti and rare earth elements to avoid element volatilization. After the melting is completed, the molten melt obtained by melting is poured into a mold to obtain a steel ingot of a certain size and shape. According to the above feeding mode, a uniform steel ingot with alloy design composition can be obtained. It should be pointed out that if the feeding order is unreasonable, on the one hand, the composition may not be uniform; on the other hand, due to the large difference in melting point, it will be difficult to melt part of the metal during the melting process, resulting in melting failure.

[0034] In a preferred embodiment, the process of gas atomization is as follows: first, the steel ingot is subjected to secondary melting in a vacuum environment to obtain molten steel, and the molten steel is kept at 1550-1650℃ for 10-15min, then the molten steel is poured into a gas atomization cavity and gas atomized under argon to obtain spherical powder particles. The molten steel is blown and solidified by high-speed argon to form powder particles. The powder particles are spherical powder with good flowability, which is suitable for additive manufacturing.

[0035] In a preferred embodiment, the particle size of the spherical powder particles is 15-53μm.

[0036] In a preferred embodiment, the additive manufacturing method is powder bed melting.

[0037] In a preferred embodiment, the process parameters of additive manufacturing are as follows: laser power is 195-300W, preferably 190-290W, scanning speed is 900-1250mm / s, preferably 950-1250mm / min, printing spacing is 75-120μm, preferably 80-120μm, printing layer thickness is 25-45μm, and energy density is 75-95J / mm 3 .

[0038] By controlling the process parameters of additive manufacturing within the above range, the performance of the final obtained maraging steel is optimal. If the parameters are unreasonable, the energy density will be higher or lower than the optimal forming process window of the material, which will further destroy the formation of equiaxed crystal or cellular structure.

[0039] Under the above optimized laser process parameters, the maraging steel formed has a density greater than 99%.

[0040] Preferably, the scanning strategy in the process of additive manufacturing is strip scanning, the rotation angle between layers is 67°, and the dwell time of the scraper between the current layer and the next layer is 10-15s.

[0041] The inventors found that the performance of the final martensitic age steel is optimal when using strip scanning, because on the one hand, strip scanning is scanning in regional blocks, avoiding the accumulation of a large amount of heat when the temperature is higher than M s and the volume fraction of martensite cannot be obtained. On the other hand, strip scanning effectively reduces the temperature gradient of the formed piece, preventing the generation of large internal stress.

[0042] Preferably, the temperature of the aging treatment is 450-550℃, and the time of the aging treatment is 0.5-6h.

[0043] Further preferably, the temperature of the aging treatment is 480-510℃, and the time of the aging treatment is 1-4h.

[0044] Through the aging treatment, nanoscale Ni3(Ti, Mo, Al) is formed in the martensite matrix.

[0045] Principle and advantage

[0046] The advantages of the existing additive manufacturing martensitic age steel technology include flexibility, intelligence, and personalized customization, etc. However, the disadvantages of additive manufacturing mainly focus on relatively low forming quality and high manufacturing cost. The cost of additive manufacturing mainly includes two parts: raw material cost and processing cost. Among them, the cost of raw materials can be controlled by component modification. For example, reducing the noble metal elements in traditional martensitic age steel. In addition, by increasing the formation elements titanium and aluminum of precipitates, in-situ precipitation of nanometer precipitates can be induced and the number of precipitates in the processing process can be controlled, thereby reducing the processing time. The present application specifically reduces the content of cobalt while increasing the content of titanium and rare earth elements, and combines the laser cycle preheating effect in the process of additive manufacturing, which effectively improves the mechanical properties of the martensitic age steel. Compared with the as-cast, the strength of the low-cobalt martensitic age steel created by the present application is at least increased by ~ 200MPa. The increase in strength can be attributed to the precipitates induced by the laser repeated preheating in the process of additive manufacturing and the high volume fraction of martensite. Therefore, the low-cobalt martensitic age steel of the present application is more suitable for additive manufacturing process. This is because the alloy composition of the present application reduces the amount of Mo-rich brittle phase in the aging process. In addition, the titanium, aluminum and rare earth content of the present application is moderate, which is more conducive to improving the forming quality and avoiding the generation of micro-cracks in the formed piece.

[0047] Wherein, the application provides a martensite aging steel alloy component suitable for additive manufacturing, which has the following advantages: (1) Ti in the alloy component is more beneficial to the precipitation of nanoparticles in the aging process, Ni3Ti has a semi-coherent orientation relationship with the martensite matrix, which is beneficial to the strengthening of the matrix while retaining good plasticity; the addition of trace rare earth elements (Y and La) in the steel can purify the molten steel and grains, reduce the impurity content in the martensite aging steel, and improve the printing quality and performance; it can also improve the solid solubility of the martensite steel, (2) Unlike traditional martensite aging steels, the martensite aging steel of the application does not contain C element which affects the welding performance, thus improving the welding performance / printability; (3) Compared with traditional single-phase cobalt-containing martensite aging steels, the dual-phase structure martensite aging steel of the application has a printing state martensite volume fraction of 97-99% and an austenite volume fraction of 1-1% in the preferred scheme. Martensite can improve the strength of the material, and as a precipitated phase, austenite can improve the plasticity of the material by undergoing martensite transformation during deformation.

[0048] Specifically, in the application, by reducing the Co element, adjusting the ratio of Ni, Ti and Mo in the alloy, and adding rare earth elements (Y and La), the alloy has a relatively high M s Under the premise, the preparation cost of the alloy is reduced, and the mechanical properties of the material are improved.

[0049] Mo mainly plays a role in the martensite aging steel by dissolving into the matrix during austenitizing to form a supersaturated solid solution. During the subsequent aging process, intermetallic compounds such as Ni3Mo, FeMo, Fe2Mo and Fe7Mo6 are precipitated to strengthen the matrix.

[0050] Ti mainly plays a role in the martensite aging steel by dissolving into the matrix during austenitizing to form a supersaturated solid solution. During the subsequent aging process, intermetallic compounds such as Ni3Ti are precipitated to strengthen the matrix.

[0051] Al mainly plays a role in the martensite aging steel by dissolving into the matrix during austenitizing to form a supersaturated solid solution. During the subsequent aging process, intermetallic compounds such as Ni3Al are precipitated to strengthen the matrix. In addition, Al has the following advantages: on the one hand, Al can improve the M s , which is beneficial to the formation of martensite structure; on the other hand, Al can also be used as a deoxidizer during alloy smelting to remove residual oxygen content in the atmosphere.

[0052] Y and La elements can also improve the solid solubility of the martensite steel, and the main role is to purify the molten steel and austenite grains, and reduce harmful elements such as O, P and S inclusions in the steel.

[0053] Under the optimized composition and process, the strength and hardness of the martensite aging steel of the application can be comparable to the traditional cobalt-containing martensite aging steel. It can be used as a candidate material for tool and die steel.

[0054] The martensite aging steel with the obtained dual-phase structure has reasonable component design, does not contain elements affecting welding performance and noble metals, has excellent mechanical properties, low alloy preparation cost, and has good application prospect, and can be widely applied to technical fields such as industrial mold manufacturing, aviation and automobile industry.

[0055] Compared with the prior art, the main advantages of the application include: 1) the powder raw material has simple composition, the raw material is cheap, and the preparation cost is lower; it is suitable for industrialized mass production, 2) the martensite aging steel of the application has good forming quality (no cracks, high weldability, good mechanical properties), and does not need to be treated by hot isostatic pressing in the later period, and only needs to be aged for several hours at a lower temperature to achieve the strength and hardness required for industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 SEM (scanning electron microscope) photograph of the powder morphology of Example 1;

[0057] Figure 2 Tensile mechanical properties of Example 1, Example 3 and Comparative Example 1 in the printed state;

[0058] Figure 3 Rockwell hardness (HRC) of Example 3 in the formed state and aged state.

[0059] Figure 4 Low magnification transmission electron microscope (TEM) photograph of Example 1, showing dislocation cell structure;

[0060] Figure 5 High magnification TEM photograph of Example 1, showing nano-precipitate structure;

[0061] Figure 6 SEM photograph of the formed martensite aging steel of Example 2, showing cell structure;

[0062] Figure 7 Optical micrograph of the comparative example. DETAILED DESCRIPTION

[0063] The application will be further described below in conjunction with the drawings and specific examples. It should be understood that these examples are only used to illustrate the application and not to limit the scope of the application. The experimental methods not specified in the following examples are usually carried out under conventional conditions, or under the conditions recommended by the manufacturer.

[0064] Example 1

[0065] The martensitic age hardening steel suitable for additive manufacturing described in the embodiment is prepared by using Fe, Ni, Mo, Ti and Y with a purity higher than 99.9%, according to the alloy composition ratio designed in Table 1, and then by carrying out the processes of batching, smelting, gas atomization powdering, additive manufacturing process control and aging. Finally, the high-strength martensitic age hardening steel is obtained. The specific process is as follows.

[0066] Step 1: The charging sequence in the crucible is 1 / 2 Ni, 1 / 3 Fe, Mo, 1 / 2 Ni, 2 / 3 Fe. Start vacuumizing, and start heating and melting when the vacuum degree reaches the required value. After Fe, Ni and Mo are melted, Ti and Y wrapped with aluminum foil in advance are added to the crucible in the charging chamber.

[0067] Step 2: After the alloy is melted, stop the power and cool down, and then cast at a stable speed. After casting, stay for 5-10 minutes, and then take out the ingot mold by breaking the vacuum.

[0068] Step 3: Put the ingot into the crucible of the gas atomization equipment to carry out secondary melting at 1600-1700℃ for 15 min. When the steel liquid boils, start introducing into the gas atomization cavity and blow the liquid with high-pressure inert gas. Particles with different particle size ranges are obtained.

[0069] Step 4: Screen the above powder particles to obtain spherical powder with a particle size range of 15-53 μm.

[0070] Step 5: Use the laser powder bed fusion (L-PBF) additive manufacturing process to form a block with a size of 30 mm x 14 mm x 10 mm. The substrate used is a common stainless steel material. The laser power used is 285 W, the scanning speed is 1100 mm / s, the scanning interval is 110 μm, the powder layer thickness is 30 μm, the scanning strategy is a strip scanning mode, and the rotation angle between layers is 67°. The downtime of the scraper between the current layer and the next layer is 10 seconds.

[0071] Step 6: After the formed block is cut from the substrate using wire electrical discharge machining, the surface is polished using 2000-mesh SiC sandpaper, and then the density and porosity are tested.

[0072] Step 7: The above alloy block is aged using a box muffle furnace at a temperature of 500℃ for 4 h, and then cooled to room temperature.

[0073] Step 8: The mechanical properties and microstructure of the formed and heat-treated samples are characterized.

[0074] Example 2

[0075] The martensitic age hardening steel suitable for additive manufacturing described in this embodiment, the alloying elements are selected with purity higher than 99.9% of Fe, Ni, Mo, Ti, Al and Y, according to the alloying component ratio designed in Table 1 to carry out batching, smelting, gas atomization powdering, additive manufacturing process control and aging, finally get the high strength martensitic age hardening steel. The specific implementation steps and methods are as follows:

[0076] Step 1: Fe, Ni, M, Al, Ti and Y are directly added to the crucible of the gas atomization equipment.

[0077] Step 2: Start vacuumizing, and start heating and melting when the vacuum degree reaches the required value.

[0078] Step 3: After the liquid steel boils, start introducing into the gas atomization cavity and blowing the liquid with high-pressure inert gas. Get particles of different particle size ranges.

[0079] Step 4: Screen the above powder particles to get spherical powder with a particle size range of 15-53 μm.

[0080] Step 5: Use laser powder bed fusion (L-PBF) additive manufacturing process to form a 30mm×14mm×10mm block, and the substrate used is ordinary stainless steel material. The laser power used is 285W, the scanning speed is 1100mm / s, the scanning interval is 110μm, and the powder layer thickness is 30μm. The scanning strategy is strip scanning mode, and the rotation angle between layers is 67°. The stop time of the scraper between the current layer and the next layer is 10 seconds.

[0081] Step 6: After the formed block is cut from the substrate using wire cut electrical discharge machining, the surface is polished using 2000 mesh SiC sandpaper, and then density and porosity tests are performed.

[0082] Step 7: Use a box muffle furnace to age the above alloy block, and cool to room temperature.

[0083] Step 8: Perform mechanical property and microstructure characterization on the as-formed and heat-treated samples.

[0084] Example 3

[0085] Step 1: The martensitic age hardening steel suitable for additive manufacturing described in this embodiment, the alloying elements are selected with purity higher than 99.9% of Fe, Ni, Mo, Ti, Al and La, according to the alloying component ratio designed in Table 1 to carry out batching, smelting, gas atomization powdering, additive manufacturing process control and aging, finally get the high strength martensitic age hardening steel. The specific implementation steps and methods are as follows:

[0086] Step 1: The sequence of the charge added into the crucible is 1 / 2 Ni, 1 / 3 Fe, Co, Mo, 1 / 2 Ni, 2 / 3 Fe, vacuum is started, when the vacuum degree reaches the requirement, start heating and melting. After Fe, Ni, Mo are melted, Al, Ti and La wrapped with aluminum foil in advance are added into the crucible in the charging chamber.

[0087] Step 2: After the alloy is melted, power is cut off to cool down, and the temperature reaches the casting temperature, and then the casting is carried out with power on at a smooth speed. After casting, stay for 5-10 minutes, break the vacuum and take out the ingot mold.

[0088] Step 3: Put the steel ingot into the crucible of the gas atomization equipment for secondary melting, when the steel liquid boils, start to lead into the gas atomization cavity and blow the liquid with high pressure inert gas. Get particles of different particle size ranges.

[0089] Step 4: Screen the above powder particles to get spherical powder with particle size range of 15-53 μm.

[0090] Step 5: Use laser powder bed fusion (L-PBF) additive manufacturing process to form a block of 30 mm x 14 mm x 10 mm, and the substrate used is ordinary stainless steel material. The laser power used is 285 W, the scanning speed is 1100 mm / s, the scanning interval is 110 μm, and the powder layer thickness is 30 μm. The scanning strategy is strip scanning mode, and the rotation angle between layers is 67°. The dwell time of the scraper between the current layer and the next layer is 10 seconds.

[0091] Step 6: After the formed block is cut from the substrate using wire electrical discharge machining, the surface is polished using 2000 mesh SiC sandpaper, and then density and porosity tests are carried out.

[0092] Step 7: Use a box muffle furnace to age the above alloy block, and cool to room temperature.

[0093] Step 8: Perform mechanical property and microstructure characterization on the formed and heat treated samples.

[0094] Comparative Example 1

[0095] The martensitic age hardening steel of the comparative example is selected from Ni, Mo, Cr, Ti, Al, V, Si, Cu with a purity higher than 99.9%, the balance is Fe and inevitable impurities, and is prepared according to the alloy composition ratio designed in Table 1, and is subjected to alloying, melting, gas atomization powdering, additive manufacturing process control and aging to obtain the high-strength martensitic age hardening steel. The specific implementation steps and methods are as follows:

[0096] Step 1: Put the prepared alloy raw materials in step 1 into the crucible of the gas atomization equipment, then start vacuumizing, then start melting, and when the steel liquid is completely melted, pour the alloy liquid into the gas atomization cavity to obtain metal powder by blowing the liquid droplets with high-pressure inert argon gas.

[0097] Step 2: Screen the above powder particles to obtain spherical powder with a particle size range of 15-53 pm.

[0098] Step 3: Use the laser powder bed fusion (L-PBF) additive manufacturing process to form a 30 mm x 14 mm x 10 mm block, and the substrate used is a common stainless steel material. The laser power used is 285 W, the scanning speed is 1100 mm / s, the scanning interval is 110 pm, and the powder layer thickness is 30 pm.

[0099] Step 4: After the formed block is cut from the substrate using wire electrical discharge machining, the surface is polished using 2000 mesh SiC sandpaper, and then density and porosity tests are performed.

[0100] Step 5: Age the above alloy block using a box muffle furnace, and cool to room temperature.

[0101] Step 6: Perform mechanical property and microstructure characterization on the as-formed and heat-treated samples.

[0102] Table 1 Alloy compositions of examples 1-3 and comparative examples

[0103]

[0104] Test Example 1 Powder particle size and formed piece density test

[0105] The particle size distribution of the martensitic age hardening steel powder is tested using a laser particle size analyzer. The powder particle size range is 15-53 pm. In addition, the sphericity of the powder is observed using a scanning electron microscope (SEM), as shown in Figure 1 SEM of Example 1. The results show that the designed and prepared alloy powder has high sphericity and good flowability, which meets the requirements of the additive manufacturing process. The density of the additive manufactured sample is measured using the Archimedes drainage method. The specific method is: after the alloy block is polished flat using sandpaper, it is placed in a balance to measure its mass in air, denoted as M; then it is placed in the water of the measuring instrument to measure its mass as m, the actual density of the alloy is p = p0m / M, p0 is the density of water. Each sample is measured 3 times. The theoretical density is the density of the melted alloy, denoted as p1, and the density is

[0106] Test Example 2 Mechanical property test

[0107] After the samples were cut from the substrate using wire electrical discharge machining, part of the samples were directly aged. The samples were polished using 400-2000 mesh SiC sandpaper to remove the indicated oxide layer thickness. The tensile mechanical properties were tested using an electronic universal testing machine. The tensile samples were dog bone shaped, with a gauge length of 9.5 mm, a width of 2 mm, and a thickness of 1.5 mm. The strain rate was 10 -3 / s -1 The strain variable was obtained using a digital video extensometer, Figure 2 The tensile mechanical properties of Examples 1, 3, and the comparative example are shown in Table 2. The average and standard deviation of the tensile mechanical properties of Examples 1-3 and the comparative example in the as-formed and aged states are shown in Table 2. The Rockwell hardness of the maraging steel was tested using a 150 kgf load, Figure 3 The hardness of Example 3 in the as-formed and aged states is shown in Table 2.

[0108] Table 2 Tensile mechanical properties of Examples 1-3 and the comparative example

[0109]

[0110] Microstructure characterization of Test Example 3

[0111] The melt pool and channel morphology of the as-formed samples in the build direction and horizontal direction were observed using optical microscopy (OM). The porosity of the samples was analyzed using OM, as well as defects under different forming process conditions, such as Figure 7 The OM images of the comparative example are shown in Table 2. The grain size and texture of the maraging steel were measured using an electron backscatter diffraction (EBSD) instrument. The test sample was prepared using an electrolytic double jet instrument, and the etching solution used was a 10% perchloric acid and 90% alcohol solution, with a temperature of -30°C. The same method was used to prepare a transmission electron microscope (TEM) test sample, and the cellular structure and nanometer precipitates in the maraging steel were observed, Figure 3 and Figure 4 The TEM images of Example 2 in the as-formed and aged states are shown in Table 2, Figure 6 The SEM image of the as-formed maraging steel of Example 2 is also shown in Table 2, and the cellular structure can be clearly seen;

[0112] In summary, the dual-phase maraging steel of the present application has excellent mechanical properties, and the alloy composition is reasonably designed. A certain content of austenite and martensite dual-phase structure can be obtained through simple solid solution and aging treatment. The high-density dislocations and nanometer precipitates formed in the martensite matrix have a great effect on the mechanical properties. The present application has important significance for the development of high-strength steel alloys.

[0113] It is to be understood that even though numerous characteristics and embodiments of the application have been set forth in the foregoing disclosure, the details can be varied without departing from the application, which is defined by the claims.

Claims

1. A method for preparing maraging steel by additive manufacturing, characterized in that: Various metal raw materials are prepared according to the designed proportions, vacuum induction melting and casting are performed to obtain steel ingots, the steel ingots are atomized to obtain spherical powder particles, the spherical powder particles are additively manufactured to obtain maraging steel billets, and the maraging steel billets are subjected to aging treatment to obtain maraging steel; The additive manufacturing method is powder bed fusion; The additive manufacturing process parameters are as follows: laser power of 195-300 W, scanning speed of 900-1250 mm / s, printing pitch of 75-120 μm, printing layer thickness of 25-45 μm, and energy density of 75-95 J / mm 3 ; During the additive manufacturing process, the scanning strategy is a strip scanning method, and the rotation angle between layers is 67 o , the pause time between the scraper from the current layer to the next layer is 10-15s; The aging treatment temperature is 480°C to 510°C, and the aging treatment time is 0.5 to 6 hours; The maraging steel has the following composition, calculated by mass percentage: Ni 16-22%, Co 0-5%, Mo 2-6%, Ti 0.2-2.5%, Al 0-1.5%, Re 0.02-0.3%, wherein Re is selected from Y and / or La, and the balance is Fe and unavoidable impurities; The maraging steel M s 80-210 ℃; The maraging steel has a dual-phase structure of martensite and austenite, wherein the volume fraction of martensite is 97-99% and the volume fraction of austenite is 1-3%; The grains of the matrix in the maraging steel are equiaxed grains with a grain size of 1-2 μm. The maraging steel contains a cellular structure with a size of 200-900 nm.

2. The method for preparing maraging steel by additive manufacturing according to claim 1, characterized in that: The vacuum induction melting process is as follows: first, aluminum paper is wrapped around Al, Ti, and Re, and then 1 / 2 Ni, 1 / 3 Fe, Co, Mo, 1 / 2 Ni, and 2 / 3 Fe are placed in a melting furnace from bottom to top. After heating until melted, the paper-wrapped Al, Ti, and Re are added to the melting furnace, melted, and the melt obtained by melting is cast into a mold to obtain a steel ingot.

3. The method for preparing maraging steel by additive manufacturing according to claim 1, characterized in that: The gas atomization process is as follows: first, the steel ingot is subjected to secondary melting in a vacuum environment to obtain molten steel, and the molten steel is kept at 1550-1650°C for 10-15 minutes, and then the molten steel is poured into a gas atomization chamber and gas atomized under argon gas to obtain spherical powder particles; The particle size of the spherical powder particles is 15-53 μm.

4. The method for preparing maraging steel by additive manufacturing according to claim 1, characterized in that: In the maraging steel, the mass ratio of Ni:Mo:Ti is 17-21:3-5:0.5-2.

5.

5. The method for preparing maraging steel by additive manufacturing according to claim 1, characterized in that: The maraging steel has the following composition, calculated by mass percentage: Ni 18-20%, Co 1-5%, Mo 3-5%, Ti 0.5-2%, Al 0-1.2%, Re 0.02-0.3%, wherein Re is selected from Y and / or La, and the balance is Fe and unavoidable impurities.

6. The method for preparing maraging steel by additive manufacturing according to claim 1, characterized in that: The maraging steel has a tensile strength of 1600-2100 MPa, a yield strength of 1500-1900 MPa, an elongation of 2-8%, and a hardness of 52-56 HRC.

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