Maraging steel and method for improving inclusion rating of maraging steel

Abstract

The present invention relates to the technical field of maraging steel production, specifically disclosing a maraging steel and a method for improving the inclusion rating of maraging steel. The method comprises: first, charging selected raw materials containing medium nickel, cobalt, molybdenum, and iron into a vacuum induction furnace for melting, simultaneously adding a predetermined amount of granular carbon, melting the raw materials, heating them, stirring, and refining them; then, adding a predetermined amount of rare earth elements, stirring, and then tapping and casting the steel to obtain a maraging steel electrode rod; and second, remelting the maraging steel electrode rod using a vacuum arc to produce maraging steel containing 0.2 to 2.0 wt% Ti. The method improves the TiN inclusion rating of the maraging steel by designing specific non-metallic inclusion compositions and precipitation mechanisms, and optimizing the corresponding melting process to effectively control the morphology, size, and distribution of TiN.

Description

Technical Field

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

[0002] During the aging process, maraging steel is strengthened by the precipitation of intermetallic compounds of strengthening alloying elements such as Co, Mo, and Ti in the Fe-Ni based supersaturated martensite. It has excellent mechanical properties such as high strength, good toughness and ductility, as well as high fatigue strength obtained through aging treatment.

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

[0004] Table 1: Main element ranges and corresponding strength levels of several typical maraging steels (wt%)

[0005]

[0006] Currently, the main maraging melting method involves selecting raw materials, melting in a vacuum induction furnace (VIM), finishing electrodes, and then remelting in a vacuum arc furnace (VAR). This dual vacuum process, VIM+VAR, produces maraging steel with low carbon, low nitrogen, and a homogeneous quality.

[0007] Maraging steel contains 0.2-1.4% Ti as the primary alloying element. During solidification, Ti readily reacts with carbon and nitrogen to form non-metallic inclusions such as TiN and Ti(C, N). If the proportion of these inclusions ≥2µm in the steel increases significantly, they can severely impact the fatigue strength of the maraging steel. Consequently, existing methods have limitations in controlling TiN morphology, leading to excessive TiN inclusion ratings during production.

[0008] Based on this, the existing technology still needs to be improved. Summary of the Invention

[0009] To solve the above technical problems, embodiments of the present invention provide a maraging steel and a method for improving inclusion rating of maraging steel, so as to solve the technical problem that the inclusion rating of maraging steel in the prior art cannot meet the requirements.

[0010] To solve the above technical problems, in one aspect, some embodiments of the present invention disclose a method for improving inclusion rating of maraging steel, comprising:

[0011] Step 1: Ni, Co, Mo, and Fe in the selected raw materials are charged into a vacuum induction furnace for melting, and a predetermined amount of granular carbon is added into the furnace. After melting, the mixture is heated and stirred for refining. After refining, a predetermined amount of rare earth elements is added, stirred, and then tapped and cast to obtain a maraging steel electrode rod.

[0012] Step 2: remelting the maraging steel electrode rod into maraging steel containing 0.2-2.0 wt% Ti by using vacuum arc.

[0013] In some embodiments, in step 1, the amount of granular carbon added is 0.0028-0.0032% of the total mass of the selected raw material.

[0014] In some embodiments, in step 1, the selected raw materials include pure iron, Ni plate, J-Co, J-Mo, titanium rod, and J-Al;

[0015] 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.

[0016] In some embodiments, in step 1, during melting in a vacuum induction furnace, the vacuum degree is no greater than 1 Pa, and a power of 200-350 kW is used for melting and clear melting.

[0017] In some embodiments, after the start of heating and stirring refining, the nitrogen content is controlled in percentage by mass. 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 at the end of refining, N ≤ 0.0015% and C ≤ 0.008%.

[0018] In some embodiments, in step 1, the amount of rare earth component added is 0.1 kg per ton of selected raw material.

[0019] In some embodiments, 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.

[0020] In some embodiments, in step 1, at the beginning of the late refining stage, a sample is taken to analyze the N content, 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 refining. The late refining stage begins when N is less than or equal to 0.0020%.

[0021] Alternatively, in step 1, at the beginning of the late refining stage, a sample is taken to analyze the C content, and if C is ≤ 0.008% by mass, 0.002-0.003% of ultrapure graphite particles are added simultaneously with the addition of Al and Ti.

[0022] In some embodiments, in step 2, the helium filling pressure during vacuum arc remelting is 200-300 Pa, and it is ensured that there is no air breakage.

[0023] On the other hand, an embodiment of the present invention further discloses a maraging steel, which is prepared by the aforementioned method;

[0024] Alternatively, calculated by mass percentage, N≤0.0010%, O≤0.0010%, C≤0.003%;

[0025] and / or, among the TiN inclusions, RE2O2S-TiN composite inclusions not larger than 2µm account for more than 80% of the total number of TiN inclusions;

[0026] 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.

[0027] By adopting the above technical solution, the present invention has at least the following beneficial effects:

[0028] The present invention provides a maraging steel and a method for improving the inclusion rating of the maraging steel. By designing a specific non-metallic inclusion composition and precipitation mechanism and optimizing the corresponding smelting process, the morphology, size, and distribution of TiN are effectively controlled, thereby improving the TiN inclusion rating of the maraging steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0030] Figure 1 Schematic diagram of typical RE2O2S-TiN composite inclusions with a size of ≤2 μm in maraging steel disclosed in the prior art of the present invention. DETAILED DESCRIPTION

[0031] The following embodiments of the present disclosure are further described in detail with reference to the accompanying drawings and examples. The detailed description of the following examples and the accompanying drawings are intended to illustrate the principles of the present disclosure, but are not intended to limit the scope of the present disclosure. The present disclosure can be implemented in many different forms and is not limited to the specific embodiments disclosed herein, but rather includes all technical solutions within the scope of the claims.

[0032] The present disclosure provides these embodiments in order to make this 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 arrangement of parts and steps, the composition of materials, numerical expressions and numerical values ​​set forth in these embodiments should be interpreted as merely exemplary, and not as limiting.

[0033] It should be noted that, in the description of this disclosure, unless otherwise specified, "plurality" means greater than or equal to two; terms such as "upper," "lower," "left," "right," "inner," and "outer" indicating directions or positional relationships are intended solely to facilitate and simplify the description of this disclosure, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on this disclosure. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0034] In addition, the terms "first," "second," and similar terms used in this disclosure do not denote any order, quantity, or importance, but are merely used to distinguish different parts. "Perpendicular" does not mean perpendicular in the strict sense, but rather means within the tolerance range. "Parallel" does not mean parallel in the strict sense, but rather means within the tolerance range. "Include" or "comprising" and similar terms mean that the elements preceding the term include the elements listed after the term, and do not exclude the possibility of also including other elements.

[0035] It should also be noted that, in the description of this disclosure, unless otherwise expressly specified or limited, the terms "installed," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to direct connections or indirect connections through an intermediary. Those skilled in the art will understand the specific meanings of the above terms in this disclosure depending on the specific circumstances. When a specific device is described as being located between a first device and a second device, there may or may not be an intervening device between the specific device and the first or second device.

[0036] All terms used in this disclosure have the same meaning as understood by one of ordinary skill in the art to which this disclosure belongs, unless otherwise specifically defined. It should also be understood that terms defined in, for example, common dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant art, and should not be interpreted in an idealized or highly formal sense, unless explicitly defined as such herein.

[0037] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, they should be considered part of the specification.

[0038] The present invention discloses a method for improving inclusion rating in maraging steel. This method addresses the problem of excessive TiN inclusion ratings in existing maraging steel products by controlling the size, distribution, or morphology of TiN in the steel. The primary goal of the present invention is to reduce the number of TiN inclusions in maraging steel, making them smaller and more evenly distributed. Specifically, this method involves the following aspects:

[0039] 1) Reduce the number of TiN inclusions: Because Ti is an alloying element, the vacuum advantage of VIM+VAR double vacuum melting is utilized. Ultra-pure graphite carbon particles are added to the furnace and after adding Ti rods during the VIM melting process to carry out vacuum carbon-oxygen reaction for deoxidation, decarburization and denitrification. This can minimize the N and C contents in the steel, especially the N content. This can reduce the formation of TiN and Ti(C, N) inclusions during solidification and significantly reduce the number of TiN inclusions.

[0040] 2) Change the type of TiN inclusions: Perform rare earth microalloying at the end of VIM and then tap the steel to form RE2O2S-TiN composite inclusions ≤ 2µm in the consumable electrode rod, e.g. Figure 1 Even though the peripheral TiN will dissolve into the liquid phase during VAR smelting, during the solidification of the metal pool, TiN inclusions will re-precipitate into RE2O2S-TiN composite inclusions ≤ 2 μm with RE2O2S inclusions as the nucleus.

[0041] 3) Reducing the proportion of large TiN inclusions: Helium filling during VAR melting increases the cooling rate. The heterogeneous nucleation of TiN using RE2O2S inclusions as nuclei raises the initial precipitation temperature, significantly advancing the timing of TiN precipitation during solidification (i.e., at a lower solid fraction). This results in a larger number of smaller TiN inclusions. Furthermore, the large-scale heterogeneous nucleation of TiN on RE2O2S inclusion cores accelerates the consumption of Ti and N, particularly N, which is consumed synchronously with steel solidification, reducing the amount of N segregated toward the end of solidification. This reduces the tendency of TiN inclusions to grow in size in the later stages of solidification.

[0042] 4) More uniform distribution of TiN inclusions: By controlling the nitrogen content to a lower level, the total amount of TiN precipitation is reduced; TiN precipitation is advanced through heterogeneous nucleation with RE2O2S inclusions as the core; the relatively accelerated consumption of nitrogen by large quantities of heterogeneous nucleation of TiN in the early solidification stage reduces the tendency of TiN size growth in the later solidification stage; and a faster cooling rate reduces the growth time of TiN. This series of measures can make the radial distribution of TiN inclusions in the steel ingot more uniform.

[0043] The method may specifically include:

[0044] 1) Select maraging steel raw materials, including pure iron, nickel plate, J-Co, J-Mo, titanium rod, and J-Al. The raw material surface must be cleaned of rust and clean, dry, and free of oil and dirt. Pure iron (C ≤ 0.01%, S ≤ 0.003%, P ≤ 0.003%, Mn ≤ 0.05%, Si ≤ 0.05%, N ≤ 0.0025%).

[0045] J is the abbreviation for metal. J-Co, J-Mo, and J-Al represent cobalt, molybdenum, and aluminum, respectively. These terms in raw materials generally refer to high-purity cobalt, molybdenum, and aluminum. Maraging steel refers to steel that contains 0.2-2.0 wt% Ti and is strengthened by precipitation of intermetallic compounds within the Fe-Ni-based supersaturated martensite. As a preferred embodiment, raw materials with lower S, P, and N contents can be selected, particularly pure iron and titanium rods.

[0046] 2) The raw materials from step 1) are melted in a vacuum induction furnace. Ni, Co, Mo, and Fe are charged to the furnace, along with approximately 0.003 wt% of granulated carbon. The vacuum level should be ≤1 Pa at the start of melting. Melt at medium or high power until the melt is clear. The temperature is raised and stirred for refining, controlling the nitrogen content to ≤0.0020% in the early stages of refining. Al and Ti are added after stirring in the early stages of refining, controlling the nitrogen content to ≤0.0015% and the carbon content to ≤0.008% at the end of refining. 0.01% rare earth is added and stirred thoroughly before tapping. Maraging steel electrode rods containing 0.2-2.0 wt% Ti are then cast. The electrode rods are then finished by surface polishing, flattening, and removal of shrinkage cavities.

[0047] Medium power generally refers to 200-250KW, and high power generally refers to 300-350KW.

[0048] 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. After the charge is melted, the vacuum degree of the induction furnace is not higher than 1Pa during the vacuum deep degassing during the refining period, and the holding time is not less than 50min. In the later stage of refining, sampling and analysis of N content can be carried out. If N≤0.0015%, stir for 5 minutes and then proceed 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 of controlling the refining time and strengthening the stirring are to ensure that N≤0.0015% at the end of the early stage of refining. In the later stage of refining, sampling and analysis of C content can also be carried out. 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 deoxidation, decarburization and denitrification are carried out within the effective vacuum pressure depth on the surface of the molten steel, reducing the impact of the addition of Ti rods on the increase in the N content of the molten steel. At the end of refining vacuum deep denitrification, 0.1 kg / t rare earth (RE) or mixed rare earth (RE) can be added through the vacuum silo to carry out deep desulfurization, deep deoxidation and modification of maraging steel inclusions.

[0049] 3) The electrode rods from step 2) are remelted using a vacuum arc to form a maraging steel containing 0.2-2.0 wt% Ti. Melting is performed at a low melting rate while ensuring the molten pool reaches the edges. Helium is then filled to enhance the cooling capacity of the consumable ingot. The helium filling pressure is 200-300 Pa to ensure that no air is broken. As a preferred embodiment, the prepared maraging vacuum arc remelted consumable steel ingot containing 0.2-2.0 wt% Ti has nitrogen ≤ 0.0010%, oxygen ≤ 0.0010%, and carbon ≤ 0.003%.

[0050] The above-described embodiments of the present invention improve upon existing melting processes. The current process does not add additional carbon during the vacuum induction furnace melting of maraging steel. This improvement allows for the addition of approximately 0.003% granular carbon. During the initial clearing stage, this added granular carbon creates a shallow molten pool at the bottom, resulting in a relatively high carbon content. This facilitates vacuum carbon-oxygen reaction deoxidation, decarburization, and denitrification, further helping to control the nitrogen content in the steel to a lower level. Furthermore, in the later stages of refining, when conditions permit, approximately 0.002% of ultrapure graphite granular carbon is added. This allows for a single-step carbon-oxygen reaction deoxidation, decarburization, and denitrification within the effective vacuum pressure depth of the molten steel surface, mitigating the effect of the addition of Ti rods on the increased nitrogen content in the molten steel. This improved process can further control the nitrogen content in the steel. Furthermore, by adding 0.01% rare earth metals at the end of the melting process and thoroughly stirring the steel before tapping, composite inclusions with a core of RE2O2S and a TiN coating are formed in the cast electrode rods.

[0051] Example 1

[0052] A method for improving the rating of TiN inclusions in maraging steel comprises the following steps:

[0053] (1) A consumable ingot was obtained by smelting using the vacuum induction + vacuum consumable method. The specific chemical composition (mass %) is: 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.

[0054] (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 raw material surface must be cleaned of rust 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%).

[0055] (3) Furthermore, 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 is used for melting and the smelting is clear. The temperature is raised and stirred for refining. At the end of the early refining period, N is 0.0019%.

[0056] (4) Further, during the initial refining phase, Al and Ti are stirred and added, along with 0.002% particulate carbon. At the end of refining, 0.0014% N and 0.004% C are added. 0.01wt% rare earth is added, stirred thoroughly, and then the steel is tapped. A Ø606mm maraging steel electrode rod containing 0.88wt% Ti is then cast. The electrode rod is then polished, flattened, and shrinkage cavities removed.

[0057] (5) Furthermore, the electrode rods were remelted using a vacuum arc to form a Ø660 mm consumable ingot of maraging steel containing 0.39 wt% Ti. The melting rate was 5.5 kg / min, while ensuring that the molten pool was sufficiently deep. Helium was then added to the ingot to enhance its cooling capacity.

[0058] (6) Furthermore, the consumable ingot is homogenized for 1250*48h, and then the blank is forged to form a 250mm Ø bar.

[0059] Example 2

[0060] A method for improving the rating of TiN inclusions in maraging steel comprises the following steps:

[0061] (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.

[0062] (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 raw material surface must be cleaned of rust and clean, dry, and free of oil. Pure iron (C: 0.005%, S: 0.0020%, P: 0.003%, Mn: 0.005%, Si: 0.005%, N: 0.0015%).

[0063] (3) Furthermore, 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 is used for melting and the smelting is clear. The temperature is raised and stirred for refining. At the end of the early refining period, N is 0.0018%.

[0064] (4) Further, during the initial refining phase, Al and Ti are stirred and added, while 0.002% of particulate carbon is added. At the end of refining, 0.0015% of nitrogen and 0.003% of carbon are added. 0.01wt% of rare earth is added and stirred thoroughly before tapping the steel. A Ø606mm maraging steel electrode rod containing 0.88wt% of Ti is then cast. The electrode rod is then polished, flattened, and shrinkage cavities removed.

[0065] (5) Furthermore, the electrode rods were remelted using a vacuum arc to form a Ø660 mm consumable ingot of maraging steel containing 0.88 wt% Ti. The melting rate was 5.5 kg / min, while ensuring that the molten pool was sufficiently deep. Helium was then added to the ingot to enhance its cooling capacity.

[0066] (6) Furthermore, the consumable ingot is homogenized for 1250*48h, and then the blank is upset and forged into a 150mm Ø bar.

[0067] Example 3

[0068] A method for improving the rating of TiN inclusions in maraging steel comprises the following steps:

[0069] (1) A consumable ingot was obtained by smelting using the vacuum induction + vacuum consumable method. The specific chemical composition (mass %) is: 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.

[0070] (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 raw material surface must be cleaned of rust 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%).

[0071] (3) Furthermore, 0.003% of granular carbon is added to the furnace during the smelting process. The vacuum degree is required to be ≤1Pa when melting begins. Medium or high power is used for melting and clear melting is performed. The temperature is raised and stirred for refining. At the end of the early refining period, N is 0.0017%.

[0072] (4) Further, during the initial refining phase, Al and Ti are stirred and added, along with 0.002% particulate carbon. At the end of refining, 0.0012% N and 0.003% C are added. 0.01wt% rare earth is added, stirred thoroughly, and then the steel is tapped. A Ø606mm maraging steel electrode rod containing 0.88wt% Ti is then cast. The electrode rod is then polished, flattened, and shrinkage cavities removed.

[0073] (5) Furthermore, the electrode rods were remelted using a vacuum arc to form a Ø660 mm consumable ingot of maraging steel containing 0.39 wt% Ti. The melting rate was 5.5 kg / min, while ensuring that the molten pool was sufficiently deep. Helium was then added to the ingot to enhance its cooling capacity.

[0074] (6) Furthermore, the consumable ingot is homogenized for 1250*48h, then the ingot is upset and drawn, forged into a 250mm diameter intermediate billet, and hot rolled into a 50mm diameter bar.

[0075] Table 2 shows the partial composition of the 250, 300, and 350 grade maraging steels used in Examples 1-3. It can be seen that the nitrogen content can be reduced to below 5 ppm, the sulfur content is controlled below 20 ppm, the oxygen content is controlled below 10 ppm, and residual RE is contained below 20 ppm. The present invention employed the standard test method for inclusion content in steel, ASTM E45-18a(2023), to determine inclusion content. The results are shown in Tables 3 to 6.

[0076] Table 2 Partial composition of the medium maraging steel of Examples 1-3 (wt%)

[0077]

[0078] Tables 3, 5, and 6 show the inclusion rating results for various types of inclusions at the R / 2 position for 18Ni250 steel bars (250 mm diameter), 18Ni300 steel bars (150 mm diameter), and 18Ni350 steel bars (50 mm diameter) in Examples 1-3, according to ASTM E45-18a - Method D. Table 2 shows that the maraging steel produced has a very high cleanliness level, so Method D (low inclusion content) was used for rating. Tables 3, 5, and 6 show that the steel contains no Class A, B, or C inclusions, only Class D and D titanium nitride inclusions. The D inclusions each have a field number of only 0.5. The D titanium nitride inclusions have a field number of only 9, 17, and 21, respectively.

[0079] Table 3 Rating of inclusions at R / 2 of 18Ni250 steel Ø250 mm bar in Example 1 (non-metallic inclusions - D method)

[0080]

[0081] Table 4 shows the rating results of D titanium nitride inclusions at different radial positions of the 18Ni250 steel Ø250mm bar in Example 1. It can be seen from the table that the D titanium nitride inclusions are very evenly distributed from the edge to 1 / 4R, with only D titanium nitride fine class 0.5 inclusions, and the field numbers are 9, 8, 9, and 10, respectively. There is a slight increase in the center position, with the field number of D titanium nitride fine class 0.5 inclusions in the center being 15 and the field number of coarse class 0.5 inclusions being 1. This shows that the radial distribution of D titanium nitride inclusions is very even. It can be seen that the difference in the field number of D titanium nitride fine class 0.5 inclusions at any two locations in the radial direction is no more than 4, and the difference in the field number of D titanium nitride coarse class 0.5 inclusions is no more than 1.

[0082] Table 4 Rating of titanium nitride inclusions at different radial positions (D method) of 18Ni250 steel Ø250 mm bar in Example 1 (non-metallic inclusions-D method)

[0083]

[0084] Table 5 Rating of inclusions at R / 2 of 18Ni300 steel Ø150 mm bar in Example 2 (non-metallic inclusions - D method)

[0085]

[0086] Table 6 Rating of inclusions at R / 2 of 18Ni350 steel Ø50 mm bar of Example 3 (non-metallic inclusions - D method)

[0087]

[0088] The method for improving inclusion rating in maraging steel disclosed in the above-described embodiments of the present invention effectively controls the nitrogen content of maraging steel to below 15 ppm after undergoing the VIM+VAR melting process by adding ultrapure graphite particles and then adding Ti rods. This involves vacuum carbon-oxygen reaction deoxidation, decarburization, and denitrification. Furthermore, rare earth microalloying is performed at the end of the VIM process before tapping, thereby forming RE2O2S-TiN composite inclusions ≤2 μm in size in the consumable electrode rod. The peripheral TiN in these RE2O2S-TiN composite inclusions in the consumable electrode completely dissolves into the liquid phase during melting, dripping, and remelting of the consumable electrode tip during vacuum arc remelting. During melt solidification, TiN inclusions reprecipitate within the metal pool, nucleating on the RE2O2S inclusions. RE2O2S inclusions, acting as TiN nuclei, raise the initial temperature for heterogeneous nucleation and precipitation of TiN inclusions, significantly advancing the precipitation time of TiN inclusions. The heterogeneous nucleation of TiN on RE2O2S inclusion cores accelerates the consumption of Ti and N. In particular, N is consumed simultaneously with steel solidification, reducing the amount of N segregating toward the end of solidification. This reduces the tendency of TiN inclusions to grow in size during the later stages of solidification, resulting in fewer, smaller, and more uniformly distributed TiN inclusions in maraging steel.

[0089] Thus far, various embodiments of the present disclosure have been described in detail. To avoid obscuring the concept of the present disclosure, some details known in the art have not been described. Based on the above description, those skilled in the art can fully understand how to implement the technical solutions disclosed herein.

[0090] Although some specific embodiments of the present disclosure have been described in detail through examples, those skilled in the art will understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. Those skilled in the art will understand that the above embodiments may be modified or some technical features may be replaced with equivalents without departing from the scope and spirit of the present disclosure. In particular, as long as there are no structural conflicts, the various technical features mentioned in the various embodiments may be combined in any manner.

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 charged into a vacuum induction furnace for melting, and a predetermined amount of granular carbon is added into the furnace, wherein the amount of granular carbon added is 0.0028-0.0032% of the total mass of the selected raw materials; after melting, the temperature is raised and stirred for refining. After the temperature is raised and stirred for refining, the nitrogen content is controlled in terms of mass percentage. When N is less than or equal to 0.0020%, the early refining stage is completed, and the late refining stage begins. Al and Ti are added under stirring, and N is controlled to be less than or equal to 0.0015% and C to be less than or equal to 0.008% at the end of refining. After the refining is completed, a predetermined amount of rare earth components is added, stirred, and then the 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; In step 1, sampling and analyzing the nitrogen content at the beginning of the late refining stage, and calculating the nitrogen content by mass percentage, if nitrogen is less than or equal to 0.0015%, refining is completed; if nitrogen is greater than or equal to 0.0015%, graphite particles of carbon at a concentration of 0.002-0.003% of the total weight of the raw material are added for deoxidation and denitrification to ensure that nitrogen is less than or equal to 0.0015% at the end of refining. The late refining stage starts when nitrogen is less than or equal to 0.0020%. The amount of rare earth component added is 0.1 kg per ton of selected raw material; In step 1, at the beginning of the late refining stage, a sample is taken to analyze the C content, calculated as a mass percentage. If C is ≤ 0.008%, 0.002-0.003% of ultrapure graphite particles are added at the same time as Al and Ti.

2. 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, in pure iron, C≤0.01%, S≤0.003%, P≤0.003%, Mn≤0.05%, Si≤0.05%, and N≤0.0025%.

3. 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 1 Pa, and a power of 200-350KW is used for melting and clear melting.

4. The method for improving inclusion rating of maraging steel according to claim 1, 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.

5. 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.

6. A maraging steel, characterized in that: The TiN inclusions contain RE2O2S-TiN composite inclusions no larger than 2µm, accounting for more than 80% of the total number of TiN inclusions, and are prepared by the method according to any one of claims 1 to 5; Calculated by mass percentage, N≤0.0010%, O≤0.0010%, C≤0.003%; According to the grading of ASTM E45-18a standard - D method, there are no A, B, or C type inclusions, only D type and D titanium nitride type inclusions.

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