Maraging steel and preparation method thereof

Abstract

The preparation method comprises the steps that maraging steel raw materials are selected, and high-oxygen pure iron with the C / O mass ratio being 0.75-0.85 is adopted as the iron raw materials; smelting the maraging steel raw material by using a vacuum induction furnace, and pouring to obtain an electrode bar; and remelting the electrode bar by using vacuum arc to obtain the maraging steel cast ingot. The high-strength and high-toughness alloy material comprises the following components in percentage by mass: 17.0%-19.0% of Ni, 14.0%-16.5% of Co, 5.5%-7.5% of Mo, 1.0%-1.5% of Ti, 0.03%-0.08% of Al, less than 0.003% of C, less than 0.001% of N, less than 0.001% of O, less than 0.002% of P, less than 0.001% of S, less than 0.01% of Si, less than 0.01% of Mn, 0.0001%-0.01% of RE, 0.0001%-0.003% of B and the balance of Fe. The requirements of ultra-low carbon, ultra-low oxygen, ultra-low nitrogen and high purity of the ultra-pure maraging steel can be met, the smelting time of a vacuum induction furnace can be shortened, the efficiency is improved, and the ultra-pure maraging steel has extremely high application and popularization value.

Description

Technical Field

[0001] The present invention relates to the technical field of maraging steel production, and particularly to a maraging steel and a preparation method thereof. Background Art

[0002] During aging, maraging steel achieves strengthening by the precipitation of intermetallic compounds of strengthening alloying elements such as Co, Mo, Ti, and Al in the Fe-Ni-based supersaturated martensite, and has 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 Ti content 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.

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

[0005] Currently, the highest-strength-level maraging steel for mature commercial applications is 18Ni(350) of the 350 grade, whose tensile strength can reach 2500 MPa and yield strength can reach 2400 MPa, and it also has certain plasticity and toughness. There are also 400-grade maraging steels 13Ni400 and 18Ni400 with a tensile strength of 2800 MPa and a yield strength of 2750 MPa, but their plasticity and toughness are relatively reduced compared to the 350 grade, and the plasticity and toughness test data are unstable and show a certain brittleness. Currently, there is no large-scale commercialization and engineering application.

[0006] In order to ensure the stable ductility and toughness of the 400-grade maraging steel with a tensile strength of 2800 MPa and a yield strength of 2750 MPa. Since Ti is the main alloying element of the strengthening phase Ni3Ti in maraging steel, C, N and Ti will form non-metallic inclusions TiC, TiN, Ti(CN); Ti, Al, Mn, Si and O will form non-metallic inclusions such as oxides; Ti, Mn and S will form non-metallic inclusions such as MnS, TiS; P, S are prone to segregation at grain boundaries; these will all affect the ductility and toughness of maraging steel, especially TiS will increase the brittle fracture sensitivity of ultra-high-strength maraging steel. Therefore, an ultra-high-purity 400-grade maraging steel is needed, and an appropriate preparation process is adopted to improve the purity and prepare such an ultra-high-purity maraging steel ingot.

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

[0008] To solve the above technical problems, an embodiment of the present invention provides a maraging steel and a preparation method thereof to solve the technical problem that the mechanical properties 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 maraging steel, which, by mass percentage, includes: Ni: 17.0% - 19.0%, Co: 14.0% - 16.5%, Mo: 5.5% - 7.5%, Ti: 1.0% - 1.5%, Al: 0.03% - 0.08%, C < 0.003%, N < 0.001%, O < 0.001%, P < 0.002%, S < 0.001%, Si < 0.01%, Mn < 0.01%, RE: 0.0001% - 0.01%, B: 0.0001% - 0.003%, and the balance is Fe.

[0010] In some embodiments, the tensile strength ≥ 2800 MPa and the yield strength ≥ 2750 Mpa.

[0011] On the other hand, some embodiments of the present invention also disclose a preparation method of the foregoing maraging steel, including: Step 1: Select raw materials for the maraging steel. Among them, the iron raw material uses high-oxygen pure iron with a C / O mass ratio of 0.75 - 0.85; Step 2: Melt the raw materials of the maraging steel using a vacuum induction furnace and cast to obtain electrode rods; Step 3: Remelt the electrode rods using vacuum arc remelting to obtain maraging steel ingots.

[0012] In some embodiments, in Step 1, the selection of raw materials for the maraging steel includes: high-oxygen pure iron with a C / O ratio of 0.75 - 0.85, Ni plates, J-Co, J-Mo, titanium rods, J-Al; among them, in the high-oxygen pure iron with a C / O mass ratio of 0.75 - 0.85, by mass percentage, C: 0.012% - 0.068%, O: 0.016% - 0.080%, N ≤ 0.001%, P < 0.002%, S < 0.001%, Si ≤ 0.005%, Mn ≤ 0.01%.

[0013] In some embodiments, in Step 2, during the vacuum induction furnace melting, 81% - 85% of the high-oxygen pure iron of the Ni, Co, and Mo dosing amounts is charged into the furnace. When starting to melt, the required vacuum degree ≤ 1 Pa, and melt until the melt is clear; The temperature is raised and stirred for refining, and the refining enters the early stage. When sampling and analysis show that N is ≤ 0.0020%, the early stage of refining ends. Stirring and adding Al, Ti, and the remaining high-oxygen pure iron enters the late stage of refining. When sampling and analysis show that N is ≤ 0.0015% and C is ≤ 0.003%, the refining ends. Add 0.005-0.01wt% rare earth and 0.002-0.003wt% boron, stir thoroughly, then tap steel and cast electrode rods; The proportion of high oxygen pure iron added in batches is calculated by mass ratio.

[0014] In some embodiments, the electrode rod obtained by casting is annealed and finished before proceeding to step three.

[0015] In some embodiments, in step three, during vacuum arc remelting, the molten pool is ensured to be sufficiently close to the edge during smelting, the vacuum degree is ≤0.1 Pa, and helium is filled to enhance the cooling capacity of the consumable ingot.

[0016] In some embodiments, in step 2, during the melting to clear melting process, the melting power is 200-350KW.

[0017] In some embodiments, in step 2, after the charge is melted, the vacuum degree of the induction furnace is not higher than 1 Pa during vacuum deep degassing in the refining period, and the holding time is not less than 40 minutes.

[0018] In some embodiments, sampling is performed at the end of refining to analyze the N content. If N≤0.0015%, stirring is performed for 5 minutes before proceeding to the next step. If N>0.0015%, refining is performed for another 20 minutes before sampling to ensure that N≤0.0015% at the end of refining.

[0019] By adopting the above technical solution, the present invention has at least the following beneficial effects: The present invention provides a maraging steel and a preparation method thereof, wherein a high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75-0.85 is used to replace conventional pure iron, and is added in batches with the furnace together with Al and Ti. The high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75-0.85 enhances the efficiency of carbon-oxygen reaction denitrification under vacuum conditions during vacuum induction melting, thereby expanding the advantages. At the same time, carbon and oxygen in a specific ratio in pure iron are used for carbon-oxygen reaction and simultaneous deoxidation, decarbonization and denitrification, and the powerful denitrification kinetics of carbon-oxygen reaction are used for denitrification. The C / O ratio of 0.75-0.85 can obtain ultra-low carbon, ultra-low nitrogen and ultra-low oxygen at the end of the carbon-oxygen reaction under extreme vacuum conditions. The requirements of ultra-low carbon, ultra-low oxygen, ultra-low nitrogen and high purity of the ultra-high purity maraging steel can be met, the smelting time of the vacuum induction furnace can be shortened, the efficiency can be improved, and the value of popularization and application is extremely high. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0021] Figure 1 It is a flowchart of a preparation method of a maraging steel disclosed in some embodiments of the present invention. Detailed implementation manners

[0022] The following will further describe the implementation manners of the present disclosure in detail with reference to the accompanying drawings and embodiments. The detailed description of the following embodiments and the accompanying drawings are used to exemplarily illustrate the principles of the present disclosure, 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 in the text, 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 the components and steps, the components of the materials, the numerical expressions and values described in these embodiments 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 relationships indicated by the terms "upper", "lower", "left", "right", "inner", "outer", etc. are 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 cannot 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. Words such as "including" or "comprising" mean that the elements before the word cover the elements listed after the word, 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 specified and defined, the terms "installed", "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, an intermediate device may or may not exist between the specific device and the first device or the second device.

[0027] All terms used in the present disclosure have the same meanings as those 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 interpreted as having meanings consistent with their meanings 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] Some embodiments of the present invention disclose an ultra-high purity maraging steel and a method for preparing an ingot thereof. It can endow the 18Ni400 grade maraging steel with a tensile strength of 2800 MPa and a yield strength of 2750 MPa with certain and stable toughness and plasticity.

[0030] To achieve the above object, the present invention is implemented by the following technical solutions: An ultra-high purity maraging steel and a method for preparing an ingot thereof, comprising the following steps: The chemical composition (by mass percentage) of the maraging steel is: Ni: 17.0% - 19.0%, Co: 14.0% - 16.5%, Mo: 5.5% - 7.5%, Ti: 1.0% - 1.5%, Al: 0.03% - 0.08%, C < 0.003%, N < 0.001%, O < 0.001%, P < 0.002%, S < 0.001%, Si < 0.01%, Mn < 0.01%, RE: 0.0001% - 0.01%, B: 0.0001% - 0.003%, and the balance is Fe.

[0031] The method for preparing the ultra-high purity maraging steel ingot is obtained by smelting using the method of (selecting raw materials + vacuum induction melting and casting consumable electrodes + finishing consumable electrodes + vacuum arc consumable remelting) to obtain the ultra-high purity maraging steel ingot. As Figure 1 shown, it includes: 1) Select raw materials of maraging steel, and use high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, 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. The high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85 (C: 0.012% - 0.068%, [O]: 0.016% - 0.080%, [N] ≤ 0.001%, P < 0.002%, S < 0.001%, Si ≤ 0.005%, Mn ≤ 0.01%). Preferably, raw materials with lower S, P, Si, and Mn contents can be selected, especially pure iron and titanium rods.

[0032] 2) Melt the raw materials in step 1) using a vacuum induction furnace. Ni, Co, Mo, and high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85 are charged with the furnace. Add 5 / 6 of the total amount of high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85. When starting to melt, the required vacuum degree is ≤ 1 Pa, and medium or high power is used for melting until it is completely melted. Heat up and stir for refining, and control the nitrogen content N ≤ 0.0020% in the early stage of refining. After stirring in the early stage of refining, add Al, Ti, and 1 / 6 of the total amount of high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85. Control the nitrogen content N ≤ 0.0015% and carbon content C ≤ 0.003% in the final stage of refining. Add 0.005 - 0.01 wt% rare earth and 0.002 - 0.003 wt% boron, stir well, and then tap the steel and pour the electrode rods. The electrode rods are subjected to finishing operations such as annealing, surface polishing, end flattening, and shrinkage cavity cutting.

[0033] Preferably, medium power can be selected for melting, with a longer melting time until completely melted, and a better vacuum degassing effect when the solid metal materials melt and drip.

[0034] After the furnace charge is completely melted, during the deep vacuum 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 40 min.

[0035] In the later stage of refining, sample and analyze [N]. If [N] ≤ 0.0015%, proceed to the next step after stirring for 5 minutes. If [N] ≥ 0.0015%, sample can be taken again after another 20 min of refining. The key points for controlling the refining time and enhanced stirring are to ensure that [N] ≤ 0.0015% at the end of the early stage of refining.

[0036] At the end of the deep vacuum denitrification in the refining period, add 0.005 - 0.01 wt% rare earth (RE) through the vacuum bin for deep desulfurization, deep deoxidation, and modification of inclusions in maraging steel.

[0037] At the end of the deep vacuum denitrification in the refining period, add 0.002 - 0.003 wt% boron (B) through the vacuum bin. B is mainly used to strengthen the grain boundaries of maraging steel.

[0038] 3) Remelt the electrode bar obtained in step 2) by vacuum arc remelting. During low melting rate smelting, ensure that the molten pool is sufficient to reach the edge. The vacuum degree is ≤0.1 Pa. Charge helium to enhance the cooling capacity of the consumable ingot, and the helium charging pressure is 200 - 300 Pa to ensure no air leakage, so as to prepare an ultra-high purity maraging steel ingot.

[0039] The ultra-high purity maraging steel prepared by the above method has a tensile strength ≥2800 MPa and a yield strength ≥2750 MPa, and at the same time has certain and stable toughness and plasticity. The ingot preparation process is the same as the conventional vacuum induction furnace (VIM) + vacuum arc consumable remelting (VAR) process route. However, a high oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85 is used to replace the conventional pure iron. The high oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85 is added batch by batch with the furnace charge and together with Al and Ti, which enhances the efficiency of denitrification by the carbon-oxygen reaction under vacuum conditions during vacuum induction melting and expands the advantages. At the same time, the specific proportion of carbon and oxygen in the pure iron can be used for carbon-oxygen reaction to deoxidize, decarburize and denitrify simultaneously. The powerful denitrification kinetics of the carbon-oxygen reaction can be used for denitrification. The C / O ratio of 0.75 - 0.85 can obtain ultra-low carbon, ultra-low nitrogen and ultra-low oxygen at the end point of the carbon-oxygen reaction under extreme vacuum conditions. It can meet the requirements of ultra-low carbon, ultra-low oxygen, ultra-low nitrogen and high purity of this ultra-high purity maraging steel, shorten the melting time of the vacuum induction furnace, improve the efficiency, and has extremely high popularization and application value.

[0040] Generally, in the conventional method of smelting maraging steel in a vacuum induction furnace in the prior art, the time of the extended vacuum high-temperature carbon-oxygen reaction is used to ensure a very low nitrogen content, and even two or more vacuum high-temperature carbon-oxygen reaction refinements are used for denitrification. However, carbon and oxygen are not deliberately added to the charge. As the vacuum high-temperature carbon-oxygen reaction proceeds, the carbon and oxygen contents in the steel become lower and lower, the carbon-oxygen reaction becomes weaker and weaker, and the denitrification ability becomes weaker and weaker. Furthermore, when the oxygen content in the molten steel decreases to a certain extent, MgO in the crucible will decompose and transfer oxygen to the molten steel. At this time, long-term vacuum high-temperature carbon-oxygen reaction refining will reduce the life of the crucible refractory, and the molten steel will also be contaminated by Mg, forming Mg-containing non-metallic inclusions, which will affect the purity.

[0041] Example 1 An ultra-high purity maraging steel and its ingot preparation method specifically include the following operation steps: (1) Select maraging steel raw materials, including high oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, 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. The composition of the high oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85 is wt: C: 0.018%, [O]: 0.022%, [N] ≤ 0.001%, P < 0.002%, S < 0.001%, Si ≤ 0.005%, Mn ≤ 0.01%.

[0042] (2) The Ni plate, J-Co, J-Mo, and high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, which is 5 / 6 of the batching amount, are charged into the vacuum induction furnace for melting along with the furnace. When starting to melt, the required vacuum degree is ≤1 Pa, and melting is carried out with a power of 250 KW until the melt is clear. Then, heating and stirring refining are carried out. During the deep degassing in the refining period, the vacuum degree of the induction furnace is not higher than 1 Pa. In the early stage of refining, N ≤ 0.0020%. Al and Ti are added during stirring, and the high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, which is 1 / 6 of the batching amount, is added. Control the N ≤ 0.0015% and C ≤ 0.003% at the end of refining. The total refining holding time is 50 min. 0.008% rare earth and 0.003% boron are added, and after sufficient stirring, the steel is tapped and poured into a φ360 mm electrode bar. The electrode bar is subjected to finishing operations such as surface grinding, end flattening, and shrinkage cavity cutting.

[0043] Melting with a small power (less than 200 KW) will result in a longer melting period and low melting efficiency. However, in principle, small-power melting should not lead to a decline in product quality.

[0044] (3) The electrode bar is remelted by vacuum arc using a φ406 mm diameter ingot mold. Low melting speed melting of 2.9 - 3.2 kg / min is adopted while ensuring that the molten pool is sufficient to reach the edge. The vacuum degree is ≤0.1 Pa. The helium charging pressure is 200 - 300 Pa to enhance the cooling capacity of the consumable ingot. An ultra-high purity maraging steel ingot is prepared.

[0045] (4) By mass percentage, after detection, in the main alloy components of the ingot, Ni: 18.1%, Co: 15.28%, Mo: 6.52%, Ti: 1.19%, Al: 0.04%, and the rest is Fe.

[0046] (5) The residual element composition of the ingot is shown in Table 2.

[0047] Example 2 An ultra-high purity maraging steel and its ingot preparation method specifically include the following operating steps: (1) Select maraging steel raw materials, using high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, Ni plate, J-Co, J-Mo, titanium rod, and J-Al. The surfaces of the raw materials must be rust-removed and clean, dry, and free of oil. The composition of the high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85 is: C: 0.024%, [O]: 0.045%, [N] ≤ 0.001%, P < 0.002, S < 0.001, Si ≤ 0.005%, Mn ≤ 0.01%.

[0048] (2) The Ni plate, J-Co, J-Mo, and high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, which is 5 / 6 of the batching amount, are charged into a vacuum induction furnace for melting along with the furnace. When starting to melt, the required vacuum degree is ≤1 Pa, and it is melted with a power of 300 KW until completely melted. Then, it is heated up and stirred for refining. During the vacuum deep degassing in the refining period, the vacuum degree of the induction furnace is not higher than 1 Pa. In the early stage of refining, N ≤ 0.0020%. Al and Ti are added with stirring, and high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, which is 1 / 6 of the batching amount, is added. The N content at the end of refining is controlled to be ≤0.0015%, and the C content is ≤0.003%. The total refining holding time is 42 min. 0.005% rare earth and 0.0025% boron are added, stirred well, and then tapped for pouring the φ360 mm electrode bar. The electrode bar is subjected to finishing operations such as surface grinding, end flattening, and shrinkage cavity cutting.

[0049] (3) The electrode bar is remelted by vacuum arc remelting using a φ406 mm diameter ingot mold, with a low melting rate of 2.9 - 3.2 kg / min, while ensuring that the molten pool reaches the edge sufficiently. The vacuum degree is ≤0.1 Pa. The helium filling pressure is 200 - 300 Pa to enhance the cooling capacity of the consumable ingot. An ultra-high purity maraging steel ingot is prepared.

[0050] (4) By mass percentage, after testing, the main alloying components of the ingot are Ni: 18.05%, Co: 15.58%, Mo: 6.62%, Ti: 1.27%, Al: 0.04%, and the rest is Fe.

[0051] (5) The residual element components of the ingot are shown in Table 2.

[0052] Example 3 An ultra-high purity maraging steel and its ingot preparation method, specifically including the following operating steps: (1) Select raw materials for maraging steel, using high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, Ni plate, J-Co, J-Mo, titanium rod, and J-Al. The surfaces of the raw materials must be derusted and clean, dry, and free of oil. The composition of the high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85 is: C: 0.025%, [O]: 0.046%, [N] ≤ 0.001%, P < 0.002, S < 0.001, Si ≤ 0.005%, Mn ≤ 0.01%.

[0053] (2)The Ni plate, J-Co, J-Mo, and high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 5 / 6 of the batching amount and 0.75 - 0.85 are charged into a vacuum induction furnace for melting along with the furnace. When starting to melt, the required vacuum degree is ≤1 Pa, and it is melted with a power of 330 KW until the melt is clear. Then, it is heated up and stirred for refining. During the vacuum deep degassing in the refining period, the vacuum degree of the induction furnace is not higher than 1 Pa. In the early stage of refining, N ≤ 0.0020%. Stir and add Al, Ti, and high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 1 / 6 of the batching amount and 0.75 - 0.85, and control N ≤ 0.0015% and C ≤ 0.003% at the end of refining. The total refining holding time is 42 min. Add 0.01% rare earth and 0.0025% boron, stir well and then tap the steel, and pour the φ606 mm electrode bar. The electrode bar is subjected to finishing operations such as polishing the surface, flattening the head, and cutting off the shrinkage cavity.

[0054] (3)The electrode bar is remelted by vacuum arc remelting using a φ660 mm diameter ingot mold, with a low melting rate of 4.7 - 5.2 kg / min, while ensuring that the molten pool reaches the edge sufficiently. The vacuum degree is ≤0.1 Pa. The helium filling pressure is 200 - 300 Pa to enhance the cooling capacity of the consumable ingot. An ultra-high purity maraging steel ingot is prepared.

[0055] (4)By mass percentage, after detection, the main alloying components of the ingot are Ni: 17.95%, Co: 15.36%, Mo: 6.73%, Ti: 1.31%, Al: 0.04%, and the rest is Fe.

[0056] (5)The residual element composition of the ingot is shown in Table 2.

[0057] Comparative Example 1 The preparation of the ultra-high purity maraging steel ingot for the comparative example includes the following operating steps: (1)Select high-quality maraging steel raw materials, including low-carbon, nitrogen, and oxygen pure iron, Ni plate, J-Co, J-Mo, titanium rod, and J-Al. The surfaces of the raw materials must be rust-removed and clean, dry, and free of oil. The composition of the pure iron is: C: 0.004%, [O]: 0.0033%, [N]: 0.002%, P: 0.0018, S: 0.003, Si: 0.0032%, Mn: 0.015%.

[0058] (2)The Ni plates, J-Co, J-Mo, and pure iron are charged into a vacuum induction furnace for melting. When starting to melt, the required vacuum degree is ≤1 Pa, and melting is carried out with a power of 300 KW until melting is complete. Then, the temperature is raised and stirred for refining. During the refining period, when deep degassing is carried out under vacuum in the induction furnace, the vacuum degree is not higher than 1 Pa. In the early stage of refining, N≤0.0020%, and Al and Ti are added while stirring to control N≤0.0020% and C≤0.005% at the end of refining. The total refining holding time is 90 min. 0.005% rare earth and 0.0025% boron are added, stirred well, and then tapped for casting a φ360 mm electrode bar. The electrode bar is subjected to finishing operations such as surface polishing, end flattening, and shrinkage cavity removal.

[0059] Since the comparative example did not use high-oxygen pure iron with an ultra-low nitrogen C / O ratio of 0.75 - 0.85, but only used conventional pure iron with low nitrogen and carbon and oxygen, during the refining period, it was only possible to require N≤0.0020% and C≤0.005% to end the refining because extending the refining time further was basically meaningless as carbon-oxygen reaction could not be used for decarburization, deoxidation, and denitrification, and the molten steel had reached equilibrium. In this comparative example, at 90 min, N≤0.0020% and C≤0.005% were already satisfied, reaching the end of the refining period.

[0060] (3)The electrode bar is remelted by vacuum arc remelting using an ingot mold with a diameter of φ406 mm, and low melting speed melting is carried out at 3.3 - 3.6 kg / min with a vacuum degree ≤0.1 Pa. The helium filling pressure is 200 - 300 Pa to enhance the cooling capacity of the consumable ingot. An ultra-high purity maraging steel ingot is prepared.

[0061] (4)In terms of mass percentage, after testing, the main alloy components of the ingot are Ni: 17.65%, Co: 15.46%, Mo: 6.75%, Ti: 1.35%, Al: 0.039%, and the rest is Fe.

[0062] (5)The residual element components of the ingot are shown in Table 2.

[0063] Table 2 Residual Element Components (ppm) of the Ultra-High Purity Maraging Steel Ingots in Examples 1 - 3

[0064] During the vacuum induction furnace melting in Embodiments 1-3 of the present invention, the melting rate is controlled to strengthen the degassing and removal of residual elements during the vacuum consumable process, and at the same time, the composition of the consumable ingot obtained is more uniform. The total refining time in the refining stage is 50 min, 42 min, and 42 min respectively; during the vacuum induction furnace melting in Comparative Example 1, the total refining time is 90 min respectively. The optimal value of the total refining time is 40-50 min, and the refining can only be ended when the indicators in the later stage of refining are reached (for example, N≤0.0015%, C≤0.003% in the embodiments, N≤0.0020%, C≤0.005% in the comparative example), otherwise, the time in the later stage of refining is continued to be extended. It is extremely difficult for the comparative example to reach the index of "N≤0.0015%, C≤0.003%" according to engineering experience, or it needs a very long refining time, such as more than 4 hours to reach this index. At this time, the target composition of the refining end point will be compromised (in this comparative example, N is compromised from ≤0.0015% to ≤0.0020%, and C is compromised from ≤0.003% to ≤0.005%). Without efforts on the raw materials, sometimes the refining time is too long to reach this index, and even the dissolution of refractories will make the steel unclean.

[0065] So far, the 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 are not described. Those skilled in the art can fully understand how to implement the technical solutions disclosed herein based on the above description.

[0066] 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 some 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 technical features mentioned in each embodiment can be combined in any way.

Claims

1. A maraging steel, characterized in that: In terms of mass percentage, it includes: Ni: 17.0%-19.0%, Co: 14.0%-16.5%, Mo: 5.5%-7.5%, Ti: 1.0%-1.5%, Al: 0.03%-0.08%, C<0.003%, N<0.001%, O<0.001%, P<0.002%, S<0.001%, Si<0.01%, Mn<0.01%, RE: 0.0001%-0.01%, B: 0.0001%-0.003%, and the balance is Fe.

2. The maraging steel according to claim 1, characterized in that: Tensile strength ≥2800 MPa, yield strength ≥2750 MPa.

3. The method for preparing maraging steel according to claim 1 or 2, characterized in that: include: Step 1: Selecting maraging steel raw materials, wherein the iron raw material is high-oxygen pure iron with a C / O mass ratio of 0.75-0.85; Step 2: melting the maraging steel raw material in a vacuum induction furnace and casting to obtain an electrode rod; Step 3: remelting the electrode rod by vacuum arc to obtain a maraging steel ingot.

4. The method for preparing maraging steel according to claim 3, characterized in that: In step 1, the selected maraging steel raw materials include: high oxygen pure iron with a C / O ratio of 0.75-0.85, Ni plate, J-Co, J-Mo, titanium rod, and J-Al; wherein, in terms of mass percentage, in the high oxygen pure iron with a C / O mass ratio of 0.75-0.85, C: 0.012%-0.068%, O: 0.016%-0.080%, N ≤0.001%, P <0.002, S <0.001, Si ≤0.005%, and Mn ≤0.01%.

5. The method for preparing maraging steel according to claim 3, characterized in that: In step 2, when melting in a vacuum induction furnace, Ni, Co, Mo, and 81%-85% of high-oxygen pure iron are loaded into the furnace. When melting begins, the vacuum degree is required to be ≤1Pa, and the melting is performed until it is clear. The temperature is raised and stirred for refining, and the refining enters the early stage. When sampling and analysis show that N is ≤ 0.0020%, the early stage of refining ends. Stirring and adding Al, Ti, and the remaining high-oxygen pure iron enters the late stage of refining. When sampling and analysis show that N is ≤ 0.0015% and C is ≤ 0.003%, the refining ends. Add 0.005-0.01wt% rare earth and 0.002-0.003wt% boron, stir thoroughly, then tap steel and cast electrode rods; The proportion of high oxygen pure iron added in batches is calculated by mass ratio.

6. The method for preparing maraging steel according to claim 5, characterized in that: After the electrode rod obtained by casting is annealed and finished, the process proceeds to step three.

7. The method for preparing maraging steel according to claim 3, characterized in that: In step three, during vacuum arc remelting, the molten pool is ensured to be sufficient to the edge while melting, the vacuum degree is ≤0.1Pa, and helium is filled to enhance the cooling capacity of the consumable ingot.

8. The method for preparing maraging steel according to claim 5, characterized in that: In step 2, during the melting to clear process, a power of 200-350KW is used for melting.

9. The method for preparing maraging steel according to claim 5, characterized in that: In step 2, after the charge is melted, the vacuum degree of the induction furnace is not higher than 1 Pa during vacuum deep degassing in the refining period, and the holding time is not less than 40 minutes.

10. The method for preparing maraging steel according to claim 5, characterized in that: At the end of refining, take samples to analyze the N content. If N ≤ 0.0015%, stir for 5 minutes and then proceed to the next step. If N > 0.0015%, continue refining for another 20 minutes and take samples to ensure that N ≤ 0.0015% at the end of refining.

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