Maraging steel and method for improving maraging steel inclusion rating

Abstract

The invention relates to the technical field of maraging steel production, and particularly discloses maraging steel and a method for improving maraging steel inclusion rating, and the method comprises the following steps: step 1, charging Ni, Co, Mo and Fe in selected raw materials into a vacuum induction furnace along with the furnace for smelting, adding a preset amount of granular carbon along with the furnace, heating, stirring and refining after melting down, after refining, adding a predetermined amount of rare earth component, stirring, tapping, and pouring to obtain a maraging steel electrode bar; and secondly, the maraging steel electrode bar is remelted into maraging steel containing 0.2-2.0 wt% of Ti through vacuum arc. According to the method disclosed by the invention, the morphology, the size and the distribution of TiN are effectively controlled by designing certain non-metallic inclusion components and a precipitation mechanism and optimizing a corresponding smelting process, so that the TiN inclusion rating of the maraging steel is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of maraging steel production, and in particular to a maraging steel and a method for improving the inclusion rating of maraging steel. Background Art

[0002] During aging, maraging steels achieve strengthening by the precipitation of intermetallic compounds of strengthening alloying elements such as Co, Mo, Ti, etc. in the Fe-Ni-based supersaturated martensite, and have excellent mechanical properties, such as high strength, good toughness and ductility, and high fatigue strength obtained by aging treatment.

[0003] For typical maraging steels, as the mass content of Ti increases from 0.2% to 1.4%, the yield strength can vary between 1375 MPa and 2800 MPa, divided into five levels of 200 ksi, 250 ksi, 300 ksi, 350 ksi, and 400 ksi. The specific main components are shown in Table 1 below.

[0004] Table 1: Range of main elements of several typical maraging steels and corresponding strength levels wt%

[0005] Currently, the main maraging steel melting method is to select raw materials → melt in a vacuum induction furnace (VIM) → finish the electrode rod → remelt consumably in a vacuum arc furnace (VAR). The maraging steel manufactured by this VIM+VAR double-vacuum process has the advantages of low carbon, low nitrogen, and homogeneity.

[0006] Since maraging steel uses 0.2 - 1.4% Ti as the main alloying element, and Ti is extremely easy to form non-metallic inclusions such as TiN and Ti(C, N) with C and N during the solidification of molten steel. Once the proportion of inclusions with a size ≥ 2 µm in the steel is too large, it will fatally affect the fatigue strength of maraging steel. Therefore, the existing technology has limitations in controlling the morphology of TiN, and the phenomenon of excessive inclusion rating of TiN-type inclusions will occur in production.

[0007] Based on this, the existing technology still needs to be improved. Summary of the Invention

[0008] To solve the above technical problems, embodiments of the present invention propose a maraging steel and a method for improving the inclusion rating of maraging steel to solve the technical problem that the inclusions of the existing maraging steel cannot meet the requirements.

[0009] To solve the above technical problems, on the one hand, some embodiments of the present invention disclose a method for improving the inclusion rating of maraging steel, including: Step 1: Load Ni, Co, Mo, and Fe in the selected raw materials into a vacuum induction furnace for melting along with the furnace, and at the same time add a predetermined amount of granular carbon along with the furnace. After melting is complete, raise the temperature and stir for refining. After refining is completed, add a predetermined amount of rare earth components, stir, and then tap the steel and pour to obtain a maraging steel electrode bar; Step 2: Remelt the maraging steel electrode bar by vacuum arc remelting to obtain a maraging steel containing 0.2 - 2.0 wt% Ti.

[0010] In some embodiments, in Step 1, the addition amount of the granular carbon is 0.0028 - 0.0032% of the total mass of the selected raw materials.

[0011] In some embodiments, in Step 1, the selected raw materials include pure iron, Ni plates, J-Co, J-Mo, titanium rods, and J-Al; Moreover, by mass percentage, in the pure iron, C ≤ 0.01%, S ≤ 0.003%, P ≤ 0.003%, Mn ≤ 0.05%, Si ≤ 0.05%, and N ≤ 0.0025%.

[0012] In some embodiments, in Step 1, during the vacuum induction furnace melting, the vacuum degree is not greater than 1 Pa, and a power of 200 - 350 KW is used for melting until melting is complete.

[0013] In some embodiments, after the start of heating and stirring for refining, the nitrogen content is controlled by mass percentage. When N ≤ 0.0020%, the early stage of refining is completed and the late stage of refining is entered. Al and Ti are added under stirring, and at the end of refining, N ≤ 0.0015% and C ≤ 0.008% are controlled.

[0014] In some embodiments, in Step 1, the addition amount of the rare earth component is 0.1 kg per ton of the selected raw materials.

[0015] In some embodiments, in Step 1, during the vacuum deep degassing in the refining process, the vacuum degree of the induction furnace is not higher than 1 Pa, and the holding time is not less than 50 min.

[0016] In some embodiments, at the start of the late stage of refining in Step 1, the N content is sampled and analyzed. By mass percentage, if N ≤ 0.0015%, the refining is completed. If N > 0.0015%, graphite granular carbon with an addition amount of 0.002 - 0.003% of the total mass of the added raw materials is added for deoxidation and denitrification to ensure that N ≤ 0.0015% at the end of refining. The late stage of refining starts when N ≤ 0.0020%; Alternatively, in Step 1, at the start of the late stage of refining, the C content is sampled and analyzed. By mass percentage, if C ≤ 0.008%, 0.002 - 0.003% of ultra-pure graphite granular carbon is added while adding Al and Ti.

[0017] In some embodiments, in step two, during vacuum arc remelting, the helium filling pressure is 200 - 300 Pa, and ensure no breakage of the vacuum.

[0018] On the other hand, the embodiments of the present invention also disclose a maraging steel, which is prepared by the aforementioned method; Alternatively, by mass percentage, N ≤ 0.0010%, O ≤ 0.0010%, C ≤ 0.003%; And / or, the RE2O2S - TiN composite inclusions with a size not greater than 2 µm in TiN - type inclusions account for more than 80% of the total number of TiN - type inclusions; Alternatively, according to the rating of ASTM E45 - 18a standard - method D, there are no type A, B, and C inclusions, only type D and type D titanium nitride inclusions.

[0019] Adopting the above - mentioned technical solutions, the present invention has at least the following beneficial effects: A maraging steel and a method for improving the inclusion rating of maraging steel provided by the present invention, through the design of certain non - metallic inclusion components and precipitation mechanisms, and at the same time optimizing the corresponding melting process, effectively control the morphology, size, and distribution of TiN, thereby improving the TiN - type inclusion rating of maraging steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0021] Figure 1 It is a schematic diagram of the typical RE2O2S - TiN type composite inclusion with a size ≤ 2 µm in the maraging steel disclosed by the prior art of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The following will further describe the embodiments of the present disclosure in detail with reference to the drawings and examples. The detailed description and drawings of the following examples are used to illustrate the principles of the present disclosure exemplarily, but cannot be used to limit the scope of the present disclosure. The present disclosure can be implemented in many different forms, not limited to the specific embodiments disclosed herein, but including all technical solutions falling within the scope of the claims.

[0023] These embodiments are provided by the present disclosure to make the present disclosure thorough and complete, and to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that: unless otherwise specifically stated, the relative arrangements of components and steps set forth in these embodiments, the compositions of materials, numerical expressions and values should be construed as merely exemplary, rather than as limitations.

[0024] It should be noted that in the description of the present disclosure, unless otherwise specified, the meaning of "a plurality" is greater than or equal to two; the orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "inner", "outer", etc. is only for the convenience of describing the present disclosure and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation of the present disclosure. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0025] In addition, the "first", "second" and similar terms used in the present disclosure do not denote any order, quantity or importance, but are only used to distinguish different parts. "Vertical" is not strictly vertical, but within the allowable error range. "Parallel" is not strictly parallel, but within the allowable error range. Terms such as "including" or "comprising" mean that the elements before this term cover the elements listed after this term, and do not exclude the possibility of also covering other elements.

[0026] It should also be noted that in the description of the present disclosure, unless otherwise clearly defined and limited, the terms "mounted", "connected" and "coupled" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected, or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present disclosure can be understood according to specific circumstances. When it is described that a specific device is located between a first device and a second device, there may or may not be an intermediate device between the specific device and the first device or the second device.

[0027] All terms used in the present disclosure have the same meaning as understood by those of ordinary skill in the art to which the present disclosure pertains, unless otherwise specifically defined. It should also be understood that terms defined in a general dictionary, such as those, should be construed to have a meaning consistent with their meaning in the context of the relevant art, and should not be interpreted in an idealized or overly formal sense, unless specifically defined as such here.

[0028] Techniques, methods and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, the techniques, methods and devices should be regarded as part of the specification.

[0029] An embodiment of the present invention discloses a method for improving the inclusion rating of maraging steel, which solves the problem of excessive inclusion rating of TiN-type inclusions in existing maraging steel products by controlling the size, distribution or morphology of TiN in maraging steel. The main objective of the present invention is to make TiN-type inclusions less, smaller and more evenly distributed in maraging steel. Specifically, this method involves the following aspects: 1) Reducing the number of TiN-type inclusions: Since Ti is an alloying element, taking advantage of the vacuum in VIM+VAR double vacuum melting, during the VIM melting process, ultra-pure graphite particle carbon is added after charging with the furnace and adding Ti rod stock, and vacuum carbon-oxygen reaction is carried out for deoxidation, decarburization and denitrification to minimize the content of N and C in the steel, especially the content of N. In this way, the generation of TiN and Ti(C,N)-type inclusions during solidification can be reduced, and the number of TiN-type inclusions can be significantly reduced.

[0030] 2) Changing the type of TiN inclusions: After rare earth microalloying at the end of VIM, tapping is carried out to form RE2O2S-TiN composite inclusions with a size of ≤2 µm in the consumable electrode rod, as Figure 1 shown. Even if the peripheral TiN dissolves into the liquid melt during VAR melting, during the solidification process of the metal bath, TiN inclusions will re-precipitate as ≤2 µm RE2O2S-TiN composite inclusions with RE2O2S-type inclusions as the nucleation core.

[0031] 3) Reducing the proportion of the number of large-size TiN-type inclusions: During the VAR melting process, helium is filled to increase the cooling rate. The heterogeneous nucleation of TiN with RE2O2S-type inclusions as the nucleation core will increase the initial temperature of TiN precipitation, that is, the timing of TiN precipitation during solidification (i.e., at a smaller solid fraction) will be relatively advanced. This will result in a larger number and finer size of TiN inclusions precipitated. At the same time, because a large number of heterogeneous nucleations of TiN on the core of RE2O2S inclusions will relatively accelerate the consumption of Ti and N, especially N will be consumed synchronously with the solidification of the molten steel, reducing the amount of N segregated and aggregated at the end of solidification. In this way, the tendency of the size of TiN-type inclusions to grow in the later stage of solidification will be reduced.

[0032] 4) The distribution of TiN-type inclusions is relatively more uniform: By controlling a lower N content to reduce the total amount of TiN precipitation, the heterogeneous nucleation of TiN with RE2O2S inclusions as the core advances the precipitation timing of TiN, the relatively accelerated consumption of N by a large number of heterogeneous nucleations of TiN in the early stage of solidification will lead to a reduced tendency of TiN size growth in the later stage of solidification, and a stronger cooling rate reduces the growth time of TiN. This series of measures can make the radial distribution of TiN inclusions in the ingot relatively more uniform.

[0033] Specifically, this method may include: 1) Select raw materials of maraging steel, using pure iron, Ni plates, J-Co, J-Mo, titanium rods, J-Al. The surfaces of the raw materials must be derusted and clean, dry, and free of oil. Pure iron (C ≤ 0.01%, S ≤ 0.003%, P ≤ 0.003%, Mn ≤ 0.05%, Si ≤ 0.05%, N ≤ 0.0025%).

[0034] Among them, J is the abbreviation of metal. J-Co, J-Mo, and J-Al represent metal cobalt, metal molybdenum, and metal aluminum respectively. Such expressions in the raw materials generally refer to relatively high-purity metal cobalt, metal molybdenum, and metal aluminum. Maraging steel refers to maraging steel containing 0.2 - 2.0 wt% Ti, in which intermetallic compounds precipitate from supersaturated martensite based on Fe-Ni to achieve the strengthening purpose. As a preferred embodiment, raw materials with lower S, P, and N contents can be selected, especially pure iron and titanium rods.

[0035] 2) Melt the raw materials in step 1) using a vacuum induction furnace. Ni, Co, Mo, and Fe are charged with the furnace, and about 0.003 wt% of granular carbon is added with the furnace at the same time. When starting to melt, the required vacuum degree is ≤ 1 Pa, and medium or high power is used for melting until it is melted clean. Heat up and stir for refining, controlling N ≤ 0.0020% in the early stage of refining. After stirring at the end of the early stage of refining, add Al and Ti, controlling N ≤ 0.0015% and C ≤ 0.008% at the end of refining. Add 0.01% rare earth and stir thoroughly before tapping the steel, and pour maraging steel electrode rods containing 0.2 - 2.0 wt% Ti. The electrode rods are subjected to finishing such as grinding the surface, flattening the head, and removing shrinkage cavities.

[0036] Medium power generally refers to 200 - 250 KW, and high power generally refers to 300 - 350 KW.

[0037] As a preferred embodiment, medium power melting can be selected. The melting time is longer, and the vacuum degassing effect is better when the solid metal material melts and drips on the surface. After the furnace charge is melted clean, during the vacuum deep degassing in the refining period, the vacuum degree of the induction furnace is not higher than 1 Pa, and the holding time is not less than 50 min. In the later stage of refining, the N content can be sampled and analyzed. If N ≤ 0.0015%, after stirring for 5 minutes, go to the next step. If N ≥ 0.0015%, a small amount of ultra-pure graphite granular carbon can be added for deoxidation and denitrification. The key points for controlling the refining time and strengthening stirring are to ensure that N ≤ 0.0015% at the end of the early stage of refining. In the later stage of refining, the C content can also be sampled and analyzed. If C ≤ 0.008%, about 0.002% of ultra-pure graphite granular carbon can be added while adding Al and Ti. In this way, carbon-oxygen reaction for deoxidation, decarburization, and denitrification is carried out within the effective vacuum pressure depth on the surface of the molten steel, reducing the influence of the addition of Ti rod material on increasing the N content of the molten steel. At the end of the vacuum deep denitrification in refining, 0.1 kg / t rare earth (RE) or mixed rare earth (RE) can be added through the vacuum bin for deep desulfurization, deep deoxidation, and modification of inclusions in maraging steel.

[0038] 3) Remelt the electrode bar in step 2) by vacuum arc remelting to obtain maraging steel containing 0.2 - 2.0 wt% Ti. Low melting speed smelting ensures that the molten pool is sufficient to reach the edge. Helium filling enhances the cooling capacity of the consumable ingot. The helium filling pressure is 200 - 300 Pa to ensure no breakage of the vacuum. As a preferred embodiment, the prepared maraging vacuum arc remelting consumable steel ingot containing 0.2 - 2.0 wt% Ti has N ≤ 0.0010%, O ≤ 0.0010%, and C ≤ 0.003%.

[0039] The above embodiments of the present invention have improved the current smelting process. In the current process, no additional carbon is added during the vacuum induction furnace smelting of maraging steel. After improvement, about 0.003% of granular carbon is added with the furnace. In the initial stage of melting clearance, due to the granular carbon added with the furnace, a shallow molten pool will form at the bottom, and the carbon content in the molten pool will be relatively higher, which is more conducive to vacuum carbon-oxygen reaction for deoxidation, decarburization, and denitrification, and is more conducive to controlling the nitrogen content in the steel at a lower level. And about 0.002% of ultra-pure graphite granular carbon is added when conditions permit in the later stage of refining, so that carbon-oxygen reaction for deoxidation, decarburization, and denitrification can be carried out in one step within the effective vacuum pressure depth on the surface of the molten steel, reducing the impact of adding Ti rod on increasing the N content in the molten steel. The improved process can control the nitrogen content in the steel at a lower level. In addition, 0.01% rare earth is added and fully stirred at the end of smelting before tapping, and composite inclusions with RE2O2S as the core and TiN coating will form in the poured electrode bar.

[0040] Example 1 A method for improving the TiN type inclusion rating of maraging steel specifically includes the following operation steps: (1) Obtain a consumable ingot by smelting using the method of vacuum induction + vacuum consumable. The specific chemical composition (mass%): C: 0.0018; Ni: 18.3; Co: 7.96; Mo: 5.02; Ti: 0.39; Al: 0.13; RE: 0.0019, and the rest is Fe.

[0041] (2) Further: Select high-quality raw materials for vacuum induction melting, using pure iron, Ni plates, J-Co, J-Mo, titanium rods, and J-Al. The surface of the raw materials must be rust-removed and clean, dry, and free of oil. Pure iron (C: 0.005%, S: 0.0015%, P: 0.003%, Mn: 0.005%, Si: 0.005%, N: 0.0015%).

[0042] (3) Further: Add 0.003% of granular carbon with the furnace during the smelting process. When starting to melt, the required vacuum degree is ≤ 1 Pa, medium or high power melting, and melting clearance. Heat up and stir for refining. At the end of the early stage of refining, N: 0.0019%.

[0043] (4) Further, Al and Ti are added at the beginning of refining and 0.002% of granular carbon is added at the same time. At the end of refining, N: 0.0014%, C: 0.004%. 0.01wt% rare earth is added and fully stirred before steel is tapped, and a 606mm Ø martensitic aging steel electrode rod containing 0.88wt% Ti is cast. The electrode rod is polished, flattened, and shrinkage holes are removed.

[0044] (5) Further, the electrode rods are remelted by vacuum arc into a 660 mm Ø maraging steel consumable ingot containing 0.39 wt% Ti. The melting rate is 5.5 kg-4.5 / min while ensuring that the molten pool is sufficient to the edge. Helium is filled to 250 Pa to enhance the cooling capacity of the consumable ingot.

[0045] (6) Furthermore, the consumable ingot is homogenized for 1250*48h, and then the blank is upset and forged into a 250mm Ø bar.

[0046] Example 2 A method for improving the rating of TiN inclusions in maraging steel, comprising the following steps: (1) A consumable ingot was obtained by smelting using the vacuum induction + vacuum consumable method. The specific chemical composition (mass %) is: C: 0.0015; Ni: 18.01; Co: 9.89; Mo: 4.58; Ti: 0.88; Al: 0.14; RE: 0.0018, and the rest is Fe.

[0047] (2) Further, the raw materials for vacuum induction melting are selected, such as pure iron, Ni plate, J-Co, J-Mo, titanium rod, and J-Al. The surface of the raw materials must be rust-free and the surface must be clean, dry, and free of oil stains, pure iron (C: 0.005%, S: 0.0020%, P: 0.003%, Mn: 0.005%, Si: 0.005%, N: 0.0015%).

[0048] (3) Further, 0.003% of granular carbon is added to the furnace during the smelting process. The vacuum degree is required to be ≤1Pa when the melting begins. Medium or high power melting is used to melt the molten metal. The temperature is raised and stirred for refining. At the end of the early stage of refining, N is 0.0018%.

[0049] (4) Further, in the early stage of refining, Al and Ti are added and 0.002% of granular carbon is added. At the end of refining, N: 0.0015%, C: 0.003%. 0.01wt% rare earth is added and fully stirred before steel is tapped. A 606mm Ø martensitic aging steel electrode rod containing 0.88wt% Ti is cast. The electrode rod is polished, flattened, and shrinkage holes are removed.

[0050] (5) Further: The electrode bar is remelted by vacuum arc remelting into a maraging steel consumable ingot with a diameter of Ф660mm containing 0.88wt% Ti. The melting rate is 5.5kg - 4.5 / min while ensuring that the molten pool reaches the edge sufficiently. Helium is filled at 250Pa to enhance the cooling capacity of the consumable ingot.

[0051] (6) Further: After homogenization treatment of the consumable ingot at 1250 * 48h, it is upset and pierced to form a billet, and then forged into a Ф150mm bar.

[0052] Example 3 A method for improving the TiN - type inclusion rating of maraging steel, specifically including the following operating steps: (1) A consumable ingot is smelted by the method of vacuum induction + vacuum consumable. The specific chemical composition (mass%): C: 0.002; Ni: 18.04; Co: 12.01; Mo: 4.44; Ti: 1.41; Al: 0.13; RE: 0.0017, and the rest is Fe.

[0053] (2) Further: The raw materials for vacuum induction melting are carefully selected, using pure iron, Ni plates, J - Co, J - Mo, titanium rods, and J - Al. The surfaces of the raw materials must be rust - removed and clean, dry, and free of oil. Pure iron (C: 0.005%, S: 0.0015%, P: 0.003%, Mn: 0.005%, Si: 0.005%, N: 0.0015%).

[0054] (3) Further: 0.003% of granular carbon is added to the furnace during the melting process. The required vacuum degree at the start of melting is ≤1Pa, and medium or high - power melting is carried out until melting is complete. Then, the temperature is raised, stirred, and refined. At the end of the early stage of refining, N: 0.0017%.

[0055] (4) Further: At the end of the early stage of refining, Al and Ti are added with stirring, and at the same time, 0.002% of granular carbon is added. At the end of the refining stage, N: 0.0012%, C: 0.003%. 0.01wt% rare earth is added and stirred thoroughly, and then the steel is tapped and poured into a Ф606mm electrode bar of maraging steel containing 0.88wt% Ti. The electrode bar is subjected to finishing such as grinding the surface, flattening the head, and removing the shrinkage cavity.

[0056] (5) Further: The electrode bar is remelted by vacuum arc remelting into a maraging steel consumable ingot with a diameter of Ф660mm containing 0.39wt% Ti. The melting rate is 5.5kg - 4.5 / min while ensuring that the molten pool reaches the edge sufficiently. Helium is filled at 250Pa to enhance the cooling capacity of the consumable ingot.

[0057] (6) Further: After homogenization treatment of the consumable ingot at 1250 * 48h, it is upset and pierced to form an intermediate billet of Ф250mm, and then hot - rolled into a Ф50mm bar.

[0058] Table 2 shows the partial compositions of 250, 300, and 350 grades of maraging steel in Examples 1 - 3. It can be seen that the N content can be reduced to less than 5 ppm, the S content is controlled below 20 ppm, the O content can be controlled to below 10 ppm, and there is also residual RE below 20 ppm. The present invention uses the standard test method for determining the inclusion content in steel ASTM E45 - 18a (2023) to measure the inclusion content, and the results are shown in Tables 3 to 6.

[0059] Partial compositions of maraging steel in Examples 1 - 3 (wt%) in Table 2

[0060] Tables 3, 5, and 6 show the grading results of various inclusions at R / 2 of Ф250 mm bars of 18Ni250 steel, Ф150 mm bars of 18Ni300 steel, and Ф50 mm bars of 18Ni350 steel in Examples 1 - 3 according to the ASTM E45 - 18a standard - D method. It can be seen from Table 2 that the cleanliness of the prepared maraging steel is very high, so the D method (low inclusion content) is used for grading. It can be seen from Tables 3, 5, and 6 that there are no type A, B, and C inclusions in the steel, only type D and D titanium nitride inclusions. Among them, there is only 1 field of view number of grade 0.5 for the fine D type. The fine D titanium nitride type is only grade 0.5, and the field of view numbers are 9, 17, and 21 respectively.

[0061] Table 3 Inclusion grading at R / 2 of Ф250 mm bar of 18Ni250 steel in Example 1 (non - metallic inclusions - D method)

[0062] Table 4 shows the grading results of D titanium nitride inclusions at different radial positions of the Ф250 mm bar of 18Ni250 steel in Example 1. It can be seen from the table that the distribution of D titanium nitride inclusions is very uniform from the edge to 1 / 4R, and there is only the fine D titanium nitride type of grade 0.5, and the field of view numbers are 9, 8, 9, and 10 respectively. There is a slightly increasing phenomenon at the center position. The field of view number of the fine D titanium nitride type at the core is 15, and the field of view number of the coarse type of grade 0.5 is 1. This indicates that the radial distribution of D titanium nitride inclusions is very uniform. It can be seen that the difference in the field of view numbers of the fine D titanium nitride type of grade 0.5 at any two positions in the radial direction is not more than 4, and the difference in the field of view numbers of the coarse D titanium nitride type of grade 0.5 is not more than 1.

[0063] Table 4 Grading of D titanium nitride inclusions at different radial positions of the Ф250 mm bar of 18Ni250 steel in Example 1 (non - metallic inclusions - D method)

[0064] Table 5 Inclusion Rating at R / 2 of Ф150mm Bar of 18Ni300 Steel in Example 2 (Non-Metallic Inclusions - D Method)

[0065] Table 6 Inclusion Rating at R / 2 of Ф50mm Bar of 18Ni350 Steel in Example 3 (Non-Metallic Inclusions - D Method)

[0066] The method for improving the inclusion rating of maraging steel disclosed in the above embodiments of the present invention adds ultra-pure graphite particle carbon after furnace charging and adding Ti bar stock, and conducts vacuum carbon-oxygen reaction for deoxidation, decarburization and denitrification, effectively controlling the N content of the maraging steel after the VIM+VAR melting process below 15 ppm. At the same time, rare earth microalloying is carried out at the end of VIM and then tapping is carried out, so as to form RE2O2S-TiN composite inclusions with a size of ≤2um in the consumable electrode bar. The peripheral TiN in the RE2O2S-TiN composite inclusions in the consumable electrode will completely dissolve into the liquid melt when the end of the consumable electrode melts, drips and is in the molten pool during the vacuum arc consumable remelting process. When the melt solidifies, the TiN inclusions will precipitate again with RE2O2S-type inclusions as the nucleation core during the solidification process of the metal molten pool. And the RE2O2S-type inclusions as the TiN nucleation core will increase the initial temperature of the heterogeneous nucleation precipitation of TiN inclusions, that is, the precipitation time of TiN inclusions will be relatively advanced. Since TiN is heterogeneously nucleated on the core of RE2O2S inclusions, it will relatively accelerate the consumption of Ti and N, especially N will be consumed synchronously with the solidification of the molten steel, reducing the amount of N segregating and accumulating towards the solidification end. In this way, the tendency of the TiN-type inclusions to grow in size in the later stage of solidification will be reduced. Thus, the TiN-type inclusions in the maraging steel are less, smaller and more evenly distributed.

[0067] So far, the embodiments of the present disclosure have been described in detail. To avoid obscuring the concept of the present disclosure, some details well known in the art have not been described. Those skilled in the art can fully understand how to implement the technical solutions disclosed here based on the above description.

[0068] Although some specific embodiments of the present disclosure have been described in detail by way of examples, those skilled in the art should understand that the above examples are only for illustration and not for limiting the scope of the present disclosure. Those skilled in the art should understand that the above embodiments can be modified or partial technical features can be equivalently replaced without departing from the scope and spirit of the present disclosure. In particular, as long as there is no structural conflict, the various technical features mentioned in each embodiment can be combined in any way.

Claims

1. A method for improving inclusion rating of maraging steel, characterized in that: include: Step 1: Ni, Co, Mo and Fe in the selected raw materials are loaded into a vacuum induction furnace for melting, and a predetermined amount of granular carbon is added into the furnace. After melting, the temperature is raised and stirred for refining. After refining, a predetermined amount of rare earth components is added, stirred, and then steel is tapped and cast to obtain a maraging steel electrode rod; Step 2: Remelting the maraging steel electrode rod into maraging steel containing 0.2-2.0 wt% Ti by vacuum arc.

2. The method for improving inclusion rating of maraging steel according to claim 1, characterized in that: In step 1, the amount of granular carbon added is 0.0028-0.0032% of the total mass of the selected raw material.

3. The method for improving inclusion rating of maraging steel according to claim 1, characterized in that: In step 1, the selected raw materials include pure iron, Ni plate, J-Co, J-Mo, titanium rod, and J-Al; Moreover, in terms of mass percentage, C≤0.01%, S≤0.003%, P≤0.003%, Mn≤0.05%, Si≤0.05%, and N≤0.0025% in pure iron.

4. The method for improving inclusion rating of maraging steel according to claim 1, characterized in that: In step 1, when melting in a vacuum induction furnace, the vacuum degree is not greater than 1Pa, and a power of 200-350KW is used for melting and clear melting.

5. The method for improving inclusion rating of maraging steel according to claim 1, characterized in that: After the start of heating and stirring refining, the nitrogen content is controlled in terms of mass percentage. When N≤0.0020%, the early stage of refining is completed and the late stage of refining begins. Al and Ti are added under stirring, and N≤0.0015% and C≤0.008% are controlled at the end of refining.

6. The method for improving inclusion rating of maraging steel according to claim 1, characterized in that: In step 1, the rare earth component is added in an amount of 0.1 kg per ton of selected raw material.

7. The method for improving inclusion rating of maraging steel according to claim 5, characterized in that: In step 1, the vacuum degree of the induction furnace during vacuum deep degassing in the refining process is not higher than 1 Pa, and the holding time is not less than 50 minutes.

8. The method for improving inclusion rating of maraging steel according to claim 1, characterized in that: In step 1, sampling and analyzing the N content at the beginning of the late refining period is performed, and if N is less than or equal to 0.0015%, the refining is completed; if N is greater than or equal to 0.0015%, 0.002-0.003% of the total weight of the raw material is added with graphite particles for deoxidation and denitrification to ensure that N is less than or equal to 0.0015% at the end of the refining period. The late refining period starts when N is less than or equal to 0.0020%. Alternatively, in step 1, sampling is performed at the beginning of the late refining period to analyze the C content, and if C is ≤ 0.008% by mass, 0.002-0.003% of ultrapure graphite particles are added at the same time as Al and Ti.

9. The method for improving inclusion rating of maraging steel according to claim 1, characterized in that: In step 2, the helium filling pressure during vacuum arc remelting is 200-300Pa, and it is ensured that there is no air break.

10. A maraging steel, characterized in that: Prepared by the method according to any one of claims 1 to 9; Or, in terms of mass percentage, N≤0.0010%, O≤0.0010%, C≤0.003%; and / or, RE2O2S-TiN composite inclusions not larger than 2µm in the TiN inclusions account for more than 80% of the total number of TiN inclusions; Alternatively, according to the rating of ASTM E45-18a standard - Method D, there are no A, B, or C type inclusions, but only D type and D titanium nitride type inclusions.

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