Non-magnetic austenitic stainless steel and manufacturing method thereof

Abstract

The invention discloses non-magnetic austenitic stainless steel and a manufacturing method thereof. The non-magnetic austenitic stainless steel comprises the following chemical components in percentage by mass: 0.036 to 0.059 percent of C, 0.42 to 0.60 percent of Si, 4.08 to 5.62 percent of Mn, 20.60 to 21.05 percent of Cr, 1.52 to 1.69 percent of Mo, 0.11 to 0.20 percent of V, 12.53 to 13.50 percent of Ni, 0.11 to 0.26 percent of Nb, 0.25 to 0.28 percent of N, 0.002 to 0.006 percent of B, 0.010 to 0.050 percent of Al, less than or equal to 0.005 percent of S, less than or equal to 0.02 percent of P, 0.01 to 0.09 percent of RE and the balance of Fe and inevitable impurities. Through reasonable alloy component design and optimization of process parameters, the non-magnetic austenitic stainless steel without high-temperature ferrite is obtained, and the non-magnetic performance of the steel grade is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of stainless steel used in nuclear fusion, and particularly relates to a non-magnetic austenitic stainless steel and a manufacturing method thereof. Background Art

[0002] In the International Thermonuclear Experimental Reactor (ITER) project, the divertor component is a very important part of the magnetic confinement fusion reactor test device. Its main function is to effectively shield impurities from the vessel wall, reduce the pollution of the central plasma, and discharge the particle flow, heat flow from the central plasma, and helium ash generated during the nuclear fusion reaction. The divertor component mainly includes plasma-facing materials, heat sink materials, and structural materials. Among them, the structural materials include the first wall and the blanket structural materials. Currently, there are mainly low-activation structural materials, copper-based alloys (CuNiBe), tantalum-based alloys, niobium-based alloys, molybdenum-based and tungsten-based alloys (Yu Xingzhe, Song Yueqing, etc. Research status and progress of structural materials for fusion reactors. Materials Review. 2008(2): p68-72); due to the extremely harsh working environment of the structural materials, there are combined effects such as high temperature and chemical interaction, changing thermal and mechanical load interaction, radiation, strong neutron flux, protons, and He particles, etc., so it must have good physical properties of radiation-induced swelling resistance, good mechanical properties at room temperature and high temperature, good safety and environmental characteristics, and maintain chemical stability and dimensional stability during the service life.

[0003] Low-activation structural materials mainly focus on oxide precipitation-strengthened alloys, martensitic and ferritic steels, austenitic stainless steels, etc. Chinese Patent ZL200610085908.2 introduced a structural steel material suitable for fusion reactors, mainly a low-activation martensitic steel containing Cr, W, V, Ta, Mn, C, etc.; martensitic or ferritic structural materials have small thermal stress, excellent anti-irradiation performance, and good compatibility with liquid metals. The disadvantage is poor weldability and post-weld heat treatment is required; for nuclear fusion, Cr-Ni type austenitic stainless steels represented by 304L, 316L, and 316LN are often selected as the main structural materials, pursuing high low-temperature strength, low-temperature toughness, low-temperature non-magnetism, and good anti-neutron irradiation performance. An austenitic stainless steel invented in Chinese Patent ZL200910197419.X has a composition range (wt%) of: [Cr] 16.00 - 22.00, [Ni] 8.00 - 12.00, [Mo] 1.00 - 3.00, [N] 0.06 - 0.25, [C] 0.01 - 0.040, [Mn] 1.00 - 4.00, [Si] ≤1.00, [Nb] 0.01 - 0.10, [Ta] 0.01 - 0.10, [Co] 0.030 - 0.100, [P] ≤0.03, [S] ≤0.005, [B] 0.0005 - 0.0018, and the balance is Fe and unavoidable impurities. Its main process is electric furnace + AOD, and then rolled into plates; however, from its composition, ferrite inevitably exists, which will affect the magnetic properties of the steel grade; in addition, although this steel grade has relatively excellent low-temperature mechanical properties, due to the limitation of the composition system, the high-temperature mechanical properties are generally low and cannot meet the high-temperature and high-strength requirements in thermonuclear fusion reactors. Chinese Patent ZL201711008164.9 discloses a Cr-Mn-Ni type austenitic stainless steel for thermonuclear fusion reactors, with a composition range (wt%): [C] 0.02 - 0.04; [Si] 0.7 - 1.0; [Mn] 5.0 - 7.0; [Cr] 20.5 - 22.5; [Ni] 12.5 - 14.5; [Mo] 1.50 - 2.50; [N] 0.25 - 0.30; [Nb] 0.20 - 0.40; [P] ≤0.03; [S] ≤0.01, and the balance is Fe and unavoidable impurities; its room-temperature yield strength is 400 - 480 MPa, tensile strength is 750 - 820 MPa, elongation is 40 - 46%, 250°C high-temperature yield strength is 280 - 350 MPa, tensile strength is 600 - 650 MPa, and relative magnetic permeability is less than 1.01; however, this patent still does not consider the high-temperature ferrite content in the steel, so the magnetism cannot be completely controlled within the range of ≤1.01, and the use of this steel grade still has great limitations.

[0004] Therefore, in order to improve the magnetic properties of the divertor structural materials in a fusion reactor, it is necessary to design a non-magnetic austenitic stainless steel to enhance the stability of the austenite phase in the steel and reduce the content of high-temperature ferrite. Summary of the Invention

[0005] Aiming at the defects existing in the prior art, the object of the present invention is to provide a non-magnetic austenitic stainless steel and its manufacturing method. Through reasonable alloy composition design and optimized process parameters, a non-magnetic austenitic stainless steel without high-temperature ferrite is obtained, ensuring the non-magnetic properties of the steel grade. In addition, the mechanical properties, grain size, and non-metallic inclusion level of this non-magnetic austenitic stainless steel also meet the usage requirements of a thermonuclear fusion reactor.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The first aspect of the present invention provides a non-magnetic austenitic stainless steel, which includes the following chemical components by mass percentage: C: 0.036 - 0.059%, Si: 0.42 - 0.60%, Mn: 4.08 - 5.62%, Cr: 20.60 - 21.05%, Mo: 1.52 - 1.69%, V: 0.11 - 0.20%, Ni: 12.53 - 13.50%, Nb: 0.11 - 0.26%, N: 0.25 - 0.28%, B: 0.002 - 0.006%, Al: 0.010 - 0.050%, S ≤ 0.005%, P ≤ 0.02%, RE: 0.01 - 0.09%, and the balance is Fe and unavoidable impurities.

[0008] Preferably, in the non-magnetic austenitic stainless steel, the contents of C, Si, Mn, Cr, Mo, V, Ni, and Nb satisfy:

[0009] Ferrite content criterion [N](ferrite) = (-0.0354 - 1.0*[C] + 0.0327*[Si] - 0.00101*[Mn] + 0.0521*[Cr] + 0.0418*[Mo] + 0.126*[V] - 0.0464*[Ni] + 0.102*[Nb]) (1 / 0.45) ≤ 0.24;

[0010] Meanwhile, when Cr equivalent Creq = [Cr] + [Mo] + 3*[Si], Ni equivalent Nieq = [Ni] + 21*[C] + 11.5*[N] + 0.5*[Mn], the criterion for Cr equivalent and Ni equivalent satisfies: Nieq > Nieq cal = -10.31708 + 1.04362*Creq;

[0011] Wherein, [C], [Si], [Mn], [Cr], [Mo], [V], [Ni] and [Nb] are respectively the mass percentage contents of the corresponding elements.

[0012] Preferably, the grain size of the non-magnetic austenitic stainless steel is 5.0 - 3.0 grades, and the relative magnetic permeability < 1.01.

[0013] The second aspect of the present invention provides a manufacturing method of a non-magnetic austenitic stainless steel as described in the first aspect of the present invention, comprising the following steps:

[0014] S1, electric furnace smelting, proportioning according to the composition of the non-magnetic austenitic stainless steel, and obtaining electrode bars by means of arc furnace melting, AOD refining, LF refining and casting;

[0015] S2, electroslag remelting, performing electroslag remelting on the electrode bars with a quaternary slag system of CaF2 - Al2O3 - CaO - MgO to obtain electroslag ingots;

[0016] S3, forging and blooming, heating the electroslag ingots to 1200 - 1250 °C, holding for 10 - 20 h, and then obtaining forgings through multiple upsetting and drawing - out forging operations. The reduction ratio of each upsetting is at least 1 / 3 of the height of the electroslag ingot.

[0017] S4, solution heat treatment, heating the forgings to 1040 - 1080 °C, holding for 1.5 - 4.0 h, and then water - cooling.

[0018] Preferably, the step S1 includes the following processes:

[0019] S11, arc furnace melting, using one or more of silicon carbide, silicon balls, and ferrosilicon as reducing agents;

[0020] S12, AOD refining, during the AOD refining process, using Al and Si as deoxidizers, adding electrolytic manganese in 2 - 3 batches according to the upper limit of the composition requirement range, blowing nitrogen to the required nitrogen content, and adding boron iron blocks along with the steel flow when tapping;

[0021] S13, LF refining, finely adjusting the composition of the molten steel during the LF refining process, adding ferroniobium at the initial stage of refining; after adding nitrided ferrochrome, keeping the molten steel static for 30 - 60 min; adding rare earth to the required content before lifting the ladle of the molten steel, and weakly stirring with argon for 20 - 50 min;

[0022] S14, casting, before casting the molten steel, controlling the baking temperature of the ingot mold at 50 - 80 °C, using argon to protect the molten steel during casting, demolding after the mold is cooled for ≥20 h after casting to obtain electrode bars of 10 - 20 tons, and annealing the electrode bars.

[0023] Preferably, in the step S1:

[0024] In the step S12, in the deoxidizer, Al:Si > 1:1, and the reduction intensity is controlled to be 1.2 - 1.4;

[0025] In the step S14, in the annealing treatment, the annealing holding temperature is 780 ± 10 °C, the holding time is 24 - 48 h, and after the holding is completed, it is cooled at a cooling rate of 30 - 40 °C / h to 200 - 250 °C and then air-cooled.

[0026] Preferably, in the step S2, in the quaternary slag system CaF2 - Al2O3 - CaO - MgO, CaF2:Al2O3:CaO:MgO is 65 - 75:15 - 22:3 - 7:3 - 7, and the quaternary slag system CaF2 - Al2O3 - CaO - MgO is used after being baked at 850 ± 10 °C for 4 - 6 h.

[0027] Preferably, in the step S2, during the electroslag remelting process, the steady-state melting rate is controlled at 10 - 18 kg / min; when the electroslag starts to be capped, the weight of the remaining electrode rod ≥ 500 kg, and after the capping and feeding are completed, it is cooled for 60 - 300 minutes and then demolded, and the electroslag ingot is sent to an annealing furnace after demolding and slowly cooled to room temperature.

[0028] Preferably, in the step S3, the heating rate of the electroslag ingot ≤ 100 °C / h;

[0029] During the forging process, the forging start temperature is 1050 °C - 1150 °C, and the final forging temperature ≥ 900 °C.

[0030] Preferably, in the step S3, during the forging process, the number of upsetting and drawing operations is 2 - 4 times;

[0031] During the forging process, when there is a crack tendency or the surface temperature of the forging billet is close to 950 °C, it is immediately reheated in the furnace, and the intermediate reheating temperature is 1180 - 1220 °C, and the holding time is 1.0 - 1.5 h;

[0032] During the forging process, the heating temperature of the last heat treatment is 1150 - 1200 °C, and the deformation amount ≥ 40%.

[0033] The principle of the composition design of the non-magnetic austenitic stainless steel of the present invention is as follows:

[0034] C: mainly enters the matrix in a solid solution state, plays a role in stabilizing austenite, and reduces the formation of high-temperature ferrite in the steel; a part of it combines with Nb and V with nitrogen to form carbonitrides, improving the room-temperature and high-temperature strength of the steel; however, if the carbon content is too high, M 23 C6 carbides are easily formed, resulting in chromium depletion and reducing the corrosion resistance of the steel. Therefore, in the present invention, C is controlled within the range of 0.036 - 0.059%.

[0035] Si: It is mainly used as a deoxidizer during smelting. Together with aluminum, it can improve the non-metallic inclusion index of steel grades with appropriate deoxidation intensity. The addition of Si can strengthen the matrix and improve the high-temperature oxidation resistance of steel. However, since silicon is a ferrite-forming element, it is easy to form high-temperature ferrite in steel, which affects the relative magnetic permeability of steel. Therefore, in this invention, Si is controlled within the range of 0.42 - 0.60%.

[0036] Mn: It replaces part of Ni to stabilize austenite, increases the solubility of nitrogen, promotes work hardening, and improves the strength of steel grades. However, if too much Mn is added, it will reduce the corrosion resistance of steel, especially the ability to resist pitting corrosion and intergranular corrosion. In addition, the increase of Mn promotes work hardening, thus increasing the difficulty of hot and cold processing and affecting the cold processing magnetic properties. Therefore, in this invention, Mn is controlled within the range of 4.08 - 5.62%.

[0037] Cr: It is mainly used to improve the corrosion resistance and oxidation resistance of stainless steel, while increasing the nitrogen solubility in steel and increasing strength. Since Cr is a ferrite-forming element, too high content will cause the formation of high-temperature ferrite in steel. In addition, Cr is also the main element for forming the harmful phase Sigma phase, which will increase the precipitation temperature of Sigma phase and reduce the hot working performance. Therefore, in this invention, the Cr content is controlled within the range of 20.60 - 21.05% to meet the requirements of corrosion resistance and oxidation resistance under harsh working conditions.

[0038] Ni: It is an austenite-forming element, which can prevent the formation of high-temperature ferrite phase during solidification, and can improve the strength, corrosion resistance, anti-magnetism and oxidation resistance of steel. However, it reduces the nitrogen solubility in steel, inhibits the precipitation of strengthening phases such as carbides and nitrides, affects the strength, and increases the cost. In this invention, Ni is controlled within the range of 12.53 - 13.50%.

[0039] S: It will promote the formation of Sigma, resulting in reduced hot plasticity and also affecting the corrosion resistance of steel. Therefore, the sulfur content is controlled below 0.005%, and the lower the content, the better.

[0040] V: It is a strong carbide-forming element, which produces a secondary hardening reaction and improves strength. Appropriate V can also achieve the effect of refining grains. In addition, adding V to steel can improve the morphology of Cr2N-type nitrides and prevent the formation of "false pearlite" structure. However, too high V content will promote the formation of high-temperature ferrite. Therefore, in this invention, the content of vanadium is controlled at 0.11 - 0.20%.

[0041] N: It is an austenite stabilizing element, which forms an interstitial solid solution to improve strength. At the same time, it will improve the corrosion resistance of steel when acting together with Mo. However, if too much N is added, it will affect the high-temperature hot plasticity of steel and cause serious work hardening, reducing the hot workability of steel. Therefore, in this invention, N is controlled within the range of 0.25 - 0.28%.

[0042] Nb: Together with C and N, it forms MX-type carbonitrides, preventing grain growth during high-temperature solution and improving high-temperature strength. However, if the Nb content is too high, it will promote the formation of ferrite phase, produce coarse MX-type carbonitrides and form coarse LAVES phase, reducing the hot workability and fatigue performance of the steel. Therefore, in the present invention, Nb is controlled within the range of 0.11 - 0.26%.

[0043] P: It is easy to form microsegregation during the solidification of molten steel, segregate at grain boundaries, and significantly increase the brittleness of the steel. Therefore, the phosphorus content is controlled below 0.02%, and the lower the content, the better.

[0044] Mo: Together with nitrogen in the steel, it improves the corrosion resistance and pitting resistance of the steel. However, if the molybdenum content is too high, it is easy to promote the formation of high-temperature ferrite and Sigma phase in the steel. Mo is also an expensive metal element, and excessive addition will increase the cost. On the premise of meeting the corrosion resistance, the content of Mo should be reduced as much as possible. Therefore, in the present invention, the content of molybdenum is controlled to be 1.52 - 1.69%.

[0045] B: The main purpose of adding B is to strengthen grain boundaries and improve high-temperature plasticity. However, excessive addition of B will form low-melting borides, which will instead reduce high-temperature plasticity. Therefore, in the present invention, B is controlled within the range of 0.002 - 0.006%.

[0046] Al: It is mainly used as a deoxidizer during melting. Its addition can strengthen the matrix and improve the high-temperature oxidation resistance of the steel. However, since Al is also an element that promotes ferrite formation in austenitic steel, it is easy to form high-temperature ferrite in the steel, affecting the relative magnetic permeability of the steel. Therefore, in the present invention, Al is controlled within the range of 0.010 - 0.050%.

[0047] RE: In the present invention, adding rare earth mainly refines grains, improves hot plasticity, thereby preventing cracking during hot deformation and improving the hot workability of the steel. However, excessive addition of RE will cause the aggregation and growth of rare earth inclusions, resulting in a decline in material properties. Considering that rare earth is prone to oxidation, in the present invention, RE is incorporated at 0.01 - 0.09%.

[0048] Compared with the prior art, the beneficial effects of the present invention are:

[0049] Through reasonable alloy composition design, the present invention obtains a non-magnetic austenitic stainless steel without high-temperature ferrite, ensuring the non-magnetic performance of the steel grade; and the mechanical properties, grain size and non-metallic inclusion level of the steel grade have all met the usage requirements of the thermonuclear fusion reactor; this non-magnetic austenitic stainless steel can also provide assistance for the development of stainless steel with non-magnetic requirements in other fields. Specific embodiments

[0050] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form.

[0051] The non-magnetic austenitic stainless steel provided by the present invention includes the following chemical components by mass percentage: C: 0.036 - 0.059%, Si: 0.42 - 0.60%, Mn: 4.08 - 5.62%, Cr: 20.60 - 21.05%, Mo: 1.52 - 1.69%, V: 0.11 - 0.20%, Ni: 12.53 - 13.50%, Nb: 0.11 - 0.26%, N: 0.25 - 0.28%, B: 0.002 - 0.006%, Al: 0.010 - 0.050%, S ≤ 0.005%, P ≤ 0.02%, RE: 0.01 - 0.09%, and the balance is Fe and inevitable impurities.

[0052] In the above non-magnetic austenitic stainless steel, the contents of C, Si, Mn, Cr, Mo, V, Ni and Nb satisfy: ferrite content criterion [N](ferrite) = (-0.0354 - 1.0 * [C] + 0.0327 * [Si] - 0.00101 * [Mn] + 0.0521 * [Cr] + 0.0418 * [Mo] + 0.126 * [V] - 0.0464 * [Ni] + 0.102 * [Nb]) (1 / 0.45) ≤ 0.24;

[0053] At the same time, when Cr equivalent Creq = [Cr] + [Mo] + 3 * [Si], Ni equivalent Nieq = [Ni] + 21 * [C] + 11.5 * [N] + 0.5 * [Mn], the criterion of Cr equivalent and Ni equivalent satisfies: Nieq > Nieq cal = -10.31708 + 1.04362 * Creq to maintain in the fully austenitic region; where [C], [Si], [Mn], [Cr], [Mo], [V], [Ni] and [Nb] are the mass percentage contents of the corresponding elements respectively.

[0054] The combination of the formed ferrite content criterion and the Cr, Ni equivalent criteria can avoid the occurrence of high-temperature ferrite in the steel of the present invention. The grain size of this non-magnetic austenitic stainless steel is 5.0 - 3.0 grades, and the relative magnetic permeability < 1.01.

[0055] The manufacturing method of the above non-magnetic austenitic stainless steel adopts the processes of electric furnace melting, electroslag remelting, forging and blooming, and solution heat treatment, and specifically includes the following steps:

[0056] S1, electric furnace smelting, according to the composition of non-magnetic austenitic stainless steel, the material is prepared, and the electrode rod is obtained by electric arc furnace melting, AOD refining, LF refining and casting, which specifically includes the following processes:

[0057] S11, electric arc furnace smelting, adding the prepared raw materials into the electric arc furnace, and using one or more of silicon carbide, silicon balls, and ferrosilicon as a reducing agent;

[0058] S12, AOD refining. During the AOD refining process, Al and Si are used as deoxidizers. Electrolytic manganese is added in 2 to 3 batches according to the upper limit of the composition requirement range. Nitrogen is blown to the nitrogen content required by the composition. Boron iron blocks are added along with the steel flow during steel tapping. The boron iron blocks are added according to the B content required by the composition, and the mass percentage of boron in the boron iron blocks is 18 to 20%. In the deoxidizer, Al:Si>1:1, and the reduction strength is controlled to be 1.2 to 1.4.

[0059] S13, LF refining, fine-tune the composition of molten steel during the LF refining process, add ferroniobium at the beginning of refining; calm the molten steel for 30 to 60 minutes after the addition of ferrochromium nitride; add rare earth to the required content before the molten steel is hung, and stir weakly with argon for 20 to 50 minutes. The above-mentioned ferroniobium is added according to the required range of niobium composition of the present invention. In a preferred embodiment, the addition amount of ferroniobium is 2.5 to 2.6 kg / ton of molten steel; wherein the mass percentage content of niobium in ferroniobium is ≥57.0%. Ferrochromium nitride is added according to the required range of nitrogen and chromium composition of the present invention. In a preferred embodiment, the addition amount of ferrochromium nitride is 10 to 12 kg / ton of molten steel; wherein the mass percentage content of N in ferrochromium nitride is ≥5.0%, and the mass percentage content of Cr is ≥60.0%.

[0060] S14, pouring, before pouring molten steel, the baking temperature of the ingot mold is controlled at 50-80°C, and argon is used to protect the molten steel pouring. After pouring, the mold is cooled for ≥20h and then demolded to obtain 10-20 tons of electrode rods (with specifications of F840-970mm). The electrode rods are annealed, wherein the annealing insulation temperature is 780±10°C, the insulation time is 24-48h, and after the insulation is completed, the cooling rate is 30-40°C / h to 200-250°C and then air-cooled.

[0061] S2, electroslag remelting, using the quaternary slag system CaF2-Al2O3-CaO-MgO to electroslag remelt the electrode rod to obtain electroslag ingots;

[0062] Specifically, for large-sized forging products, a crystallizer with a diameter of Φ1000mm-Φ1200mm is selected; the electrode rod is ground before use, the electrode rod is preheated and baked, and the shrinkage end is welded, and the arc starting end is cleaned to reveal the metal color of the main body, and no metal inclusions such as refractory materials should be seen.

[0063] In the selected quaternary slag system CaF2 - Al2O3 - CaO - MgO, the ratio of CaF2:Al2O3:CaO:MgO is 65 - 75:15 - 22:3 - 7:3 - 7, and the weight is 650 - 1000 kg. The quaternary slag system CaF2 - Al2O3 - CaO - MgO is used after being baked at 850 ± 10 °C for 4 - 6 h.

[0064] During the electroslag remelting process, the steady - state melting rate is controlled at 10 - 18 kg / min; when the electroslag starts to be capped, the weight of the remaining electrode rod ≥ 500 kg. After the capping and feeding - in for compensation are completed, it is cooled for 60 - 300 minutes and then demoulded. After the electroslag ingot is demoulded, it is sent into an annealing furnace and slowly cooled to room temperature.

[0065] S3, forging and cogging. Heat the electroslag ingot to 1200 - 1250 °C, hold for 10 - 20 h, and then obtain forgings through multiple upsetting and drawing - out forging operations. The reduction ratio for each upsetting is at least 1 / 3 of the height of the electroslag ingot.

[0066] Specifically, for large - sized electroslag ingots, the heating regime for forging is as follows: the heating rate of the electroslag ingot ≤ 100 °C / h, heat it up to 1200 - 1250 °C, and hold for 10 - 20 h; the forging start temperature is 1050 °C - 1150 °C, and the final forging temperature ≥ 900 °C.

[0067] During the forging process, the number of upsetting and drawing - out operations is 2 - 4 times, and the reduction ratio for each upsetting is at least 1 / 3 of the height of the electroslag ingot. Pay attention to uniform deformation during forging to prevent surface cracks. Once crack tendency appears or the surface temperature of the forging blank is close to 950 °C (for example, the surface temperature detected by an infrared thermometer is 950 + 20 °C), immediately return it to the furnace for heating. The intermediate reheating temperature is 1180 - 1220 °C, and the holding time is 1.0 - 1.5 h. The heating temperature for the last heat is 1150 - 1200 °C, and the deformation amount ≥ 40%.

[0068] S4, solution heat treatment. Perform solution heat treatment on the large - sized forgings obtained after forging: 1040 - 1080 °C × 1.5 - 4.0 h, that is, heat the large - sized forgings obtained after forging to 1040 - 1080 °C, hold for 1.5 - 4.0 h, and then water - cool.

[0069] The non - magnetic austenitic stainless steel and its manufacturing method of the present invention will be further described below with specific examples;

[0070] Example 1

[0071] As shown in Table 1, for the non-magnetic austenitic stainless steel in this embodiment, the mass percentage content (mass%) of its components is as follows: [C] 0.059, [Si] 0.60, [Mn] 5.62, [Cr] 21.05, [Mo] 1.69, [V] 0.20, [Ni] 13.50, [Nb] 0.26, [N] 0.28, [B] 0.006, [Al] 0.050, [S] 0.005, [P] 0.02, [RE] 0.09, and the balance is Fe and inevitable impurities;

[0072] Among them, the contents of [C], [Si], [Mn], [Cr], [Mo], [V], [Ni] and [Nb] satisfy:

[0073] The ferrite content criterion [N](ferrite) = (-0.0354 - 1.0 * [C] + 0.0327 * [Si] - 0.00101 * [Mn] + 0.0521 * [Cr] + 0.0418 * [Mo] + 0.126 * [V] - 0.0464 * [Ni] + 0.102 * [Nb]) (1 / 0.45) = 0.226 ≤ 0.24;

[0074] At the same time, when the Cr equivalent Creq = [Cr] + [Mo] + 3 * [Si] = 24.540 and the Ni equivalent Nieq = [Ni] + 21 * [C] + 11.5 * [N] + 0.5 * [Mn] = 20.769, the Cr and Ni equivalent criteria are satisfied: Nieq > Nieq calculated = -10.31708 + 1.04362 * Creq = 15.293, remaining in the fully austenitic region.

[0075] The preparation process of the non-magnetic austenitic stainless steel in this embodiment adopts electric furnace melting, electroslag remelting, forging and blooming, and solution treatment. The specific preparation process is as follows:

[0076] 1) Electric furnace smelting: Electric furnace smelting includes arc furnace melting, AOD refining, LF refining and casting.

[0077] ① During the arc furnace melting process, reducing agents such as silicon carbide, silicon balls and ferrosilicon are used;

[0078] ② During the AOD refining process, the deoxidizer uses Al:Si ≥ 7:3, and the reduction intensity is controlled at 1.4; electrolytic manganese is added in 3 batches to the upper limit of the composition range, and nitrogen is blown to the required nitrogen content; boron iron blocks are added with the molten steel flow during tapping.

[0079] ③ During the LF refining process, the composition is fine-tuned, and ferroniobium is added at the initial stage of refining. The molten steel is kept calm for 50 min after adding nitrided ferrochromium; rare earth is added to the required content before lifting the ladle, and the weak stirring time is 30 min.

[0080] ④ Casting 20 tons of Φ970mm electrode rods. The baking temperature of the steel ingot mold before casting is controlled at 50-80℃, and the casting is protected by Ar gas. After 20 hours of casting, the electrode rods are demoulded to obtain the electrode rods, and then annealed. The annealing temperature is 780℃±10℃, and the insulation time is 30h. After the insulation is completed, the cooling rate is 30℃ / h to 250℃ and then air-cooled.

[0081] 2) Electroslag remelting

[0082] For large-sized forging products, a crystallizer with a diameter of Φ1200mm is selected; the electrode rod is ground before use, the electrode rod is preheated and baked, and the shrinkage end is welded. The arc starting end is cleaned to reveal the metal color of the main body, and no metal inclusions such as refractory materials should be seen.

[0083] A quaternary slag system is selected: CaF2:Al2O3:CaO:MgO=70:20:5:5, with a weight of 650-700kg, and it is used after being baked at 850°C for 6 hours.

[0084] During the electroslag remelting process, the steady-state melting rate is controlled at 14kg / min; when the electroslag begins to cap, the weight of the remaining electrode rod is ≥500kg; after the capping and shrinkage compensation are completed, the mold is demoulded after cooling for 120 minutes, and the electroslag ingot is sent to the annealing furnace to slowly cool to room temperature after demoulding.

[0085] 4) Forging and forming

[0086] For large-sized electroslag ingots, the forging heating system is to heat up to 1220℃ at a rate of 100℃ / h, and then forge after keeping warm for 15 hours; the start forging temperature is 1100℃; the final forging temperature is ≥950℃. During the forging process, the upsetting and drawing are performed 3 times, and the amount of each upsetting and pressing is at least 1 / 3 of the ingot height. Pay attention to uniform deformation during the forging process to prevent surface cracks. Once cracking tendencies appear or the surface temperature of the forging blank approaches 950℃, it is immediately returned to the furnace for heating. The intermediate heating temperature is 1200℃ and kept warm for 1.5h. The heating temperature of the last fire is 1180℃, and the deformation is controlled to be ≥40%.

[0087] 5) Solution heat treatment

[0088] The large-size forgings after forging are subjected to solution heat treatment: 1065℃×4.0h, water cooling.

[0089] Example 2

[0090] As shown in Table 1, for the non-magnetic austenitic stainless steel in this embodiment, the component mass percentage content (mass%) is as follows: [C] 0.036, [Si] 0.42, [Mn] 4.08, [Cr] 20.60, [Mo] 1.52, [V] 0.11, [Ni] 12.53, [Nb] 0.11, [N] 0.25, [B] 0.004, [Al] 0.030, [S] 0.001, [P] 0.018, [RE] 0.07, and the balance is Fe and unavoidable impurities;

[0091] Among them, the contents of [C], [Si], [Mn], [Cr], [Mo], [V], [Ni] and [Nb] satisfy:

[0092] Ferrite content criterion [N](ferrite) = (-0.0354 - 1.0 * [C] + 0.0327 * [Si] - 0.00101 * [Mn] + 0.0521 * [Cr] + 0.0418 * [Mo] + 0.126 * [V] - 0.0464 * [Ni] + 0.102 * [Nb]) (1 / 0.45) = 0.233 ≤ 0.24;

[0093] Meanwhile, when Cr equivalent Creq = [Cr] + [Mo] + 3 * [Si] = 23.380 and Ni equivalent Nieq = [Ni] + 21 * [C] + 11.5 * [N] + 0.5 * [Mn] = 18.201, the Cr, Ni equivalent criterion is satisfied: Nieq > Nieq calculation = -10.31708 + 1.04362 * Creq = 14.083, remaining in the fully austenitic region.

[0094] The preparation process of the non-magnetic austenitic stainless steel in this embodiment adopts electric furnace melting, electroslag remelting, forging and blooming, and solution treatment. The specific preparation process is as follows:

[0095] 1) Electric furnace smelting: Electric furnace smelting includes arc furnace melting, AOD refining, LF refining and casting.

[0096] ① During the arc furnace melting process, reducing agents such as silicon carbide, silicon balls, and ferrosilicon are used;

[0097] ② During the AOD refining process, the deoxidizer uses Al:Si ≥ 3:2, and the reduction intensity is controlled at 1.3; electrolytic manganese is added in 3 batches to reach the upper limit of the composition range, and nitrogen is blown to the required nitrogen content; boron iron blocks are added with the molten steel flow during tapping.

[0098] ③ During the LF refining process, the composition is finely adjusted, and ferroniobium is added at the initial stage of refining. After adding nitrided ferrochrome, the molten steel is kept still for 30 min; rare earth is added to the required content before ladle lifting, and the weak stirring time is 25 min.

[0099] ④ Pour 20 tons of Φ970mm electrode bars. The baking temperature before pouring the ingot mold is controlled at 70°C, and the pouring is carried out under Ar gas protection. After demolding 18 hours after pouring, the electrode bars are obtained, and then annealing is carried out. The annealing holding temperature is 780°C ± 10°C, the holding time is 32h, and after the holding is completed, it is cooled to 250°C at a cooling rate of 30°C / h and then air-cooled.

[0100] 2) Electro-slag remelting

[0101] For large-sized forging products, a mold with a diameter of Φ1200mm is selected; the electrode bars are used after grinding. The electrode bars are preheated and baked, and the shrinkage cavity end is welded. The starting arc end is cleaned to see the base metal color, and no metal inclusions such as refractory materials are allowed to be seen.

[0102] Select a quaternary slag system: CaF2:Al2O3:CaO:MgO = 71:19:6:4, with a weight of 650 - 700kg, and it is used after being baked at 850°C for 6 hours.

[0103] During the electro-slag remelting process, the steady-state melting rate is controlled at 15kg / min; when the electro-slag starts to cap, the remaining weight of the electrode bar ≥ 500kg; after the capping and feeding are completed, it is cooled for 120 minutes and then demolded. After the electro-slag ingot is demolded, it is sent to an annealing furnace for slow cooling to room temperature.

[0104] 4) Forging and blooming

[0105] For large-sized electro-slag ingots, the forging heating system is that the heating rate is 100°C / h, it is heated to 1220°C, held for 15 hours and then forged; the forging start temperature is 1080°C; the final forging temperature ≥ 950°C. During the forging process, the upsetting and drawing operations are carried out 2 times, and the upsetting reduction amount each time is at least 1 / 3 of the height of the ingot. Pay attention to uniform deformation during forging to prevent surface cracks. Once crack tendency appears or the surface temperature of the forging blank approaches 950°C, immediately send it back to the furnace for heating. The intermediate reheating temperature is 1210°C, and the holding time is 1.5h. The heating temperature of the last heat is 1200°C, and the deformation amount is controlled ≥ 40%.

[0106] 5) Solution heat treatment

[0107] Carry out solution heat treatment on the forged large-sized forgings: 1065°C × 4.0h, water-cooled.

[0108] Example 3

[0109] As shown in Table 1, for the non-magnetic austenitic stainless steel in this embodiment, the mass percentage content (mass%) of its components is: [C] 0.045, [Si] 0.50, [Mn] 4.85, [Cr] 20.85, [Mo] 1.60, [V] 0.15, [Ni] 13.00, [Nb] 0.15, [N] 0.26, [B] 0.005, [Al] 0.030, [S] 0.002, [P] 0.020, [RE] 0.08, and the balance is Fe and unavoidable impurities;

[0110] Among them, the contents of [C], [Si], [Mn], [Cr], [Mo], [V], [Ni] and [Nb] satisfy:

[0111] Ferrite content criterion [N] (ferrite) = (-0.0354 - 1.0 * [C] + 0.0327 * [Si] - 0.00101 * [Mn] + 0.0521 * [Cr] + 0.0418 * [Mo] + 0.126 * [V] - 0.0464 * [Ni] + 0.102 * [Nb]) (1 / 0.45) = 0.229 ≤ 0.24;

[0112] At the same time, when Cr equivalent Creq = [Cr] + [Mo] + 3 * [Si] = 23.950 and Ni equivalent Nieq = [Ni] + 21 * [C] + 11.5 * [N] + 0.5 * [Mn] = 19.360, the Cr and Ni equivalent criteria are satisfied: Nieq > Nieq calculated = -10.31708 + 1.04362 * Creq = 14.678, remaining in the fully austenitic region.

[0113] The preparation process of the non-magnetic austenitic stainless steel in this embodiment adopts electric furnace melting, electroslag remelting, forging and blooming, and solution treatment. The specific preparation process is as follows:

[0114] 1) Electric furnace smelting: Electric furnace smelting includes arc furnace melting, AOD refining, LF refining and casting.

[0115] ① During the arc furnace melting process, reducing agents such as silicon carbide, silicon balls and ferrosilicon are used;

[0116] ② During the AOD refining process, the deoxidizer uses Al:Si ≥ 2:1, and the reduction intensity is controlled at 1.2; electrolytic manganese is added in 3 batches to the upper limit of the composition range, and nitrogen is blown to the required nitrogen content; boron iron blocks are added with the molten steel flow during tapping.

[0117] ③ During the LF refining process, the composition is finely adjusted, and ferroniobium is added at the initial stage of refining. The molten steel is kept still for 35 minutes after adding chromium nitride ferroalloy; rare earth is added to the required content before lifting the ladle, and the weak stirring time is 24 minutes.

[0118] ④ Casting 20 tons of Φ970mm electrode rods. The baking temperature of the steel ingot mold before casting is controlled at 78℃, and the casting is protected by Ar gas. After 22 hours of casting, the electrode rods are demoulded to obtain the electrode rods, and then annealed. The annealing temperature is 780℃±10℃, and the insulation time is 30h. After the insulation is completed, the cooling rate is 30℃ / h to 250℃ and then air-cooled.

[0119] 2) Electroslag remelting

[0120] For large-sized forging products, a crystallizer with a diameter of Φ1200mm is selected; the electrode rod is ground before use, the electrode rod is preheated and baked, and the shrinkage end is welded. The arc starting end is cleaned to reveal the metal color of the main body, and no metal inclusions such as refractory materials should be seen.

[0121] A quaternary slag system is selected: CaF2:Al2O3:CaO:MgO=73:18:5:4, with a weight of 650-700kg, and it is used after being baked at 850°C for 6 hours.

[0122] During the electroslag remelting process, the steady-state melting rate is controlled at 16kg / min; when the electroslag begins to cap, the weight of the remaining electrode rod is ≥500kg; after the capping and shrinkage compensation are completed, the mold is demoulded after cooling for 90 minutes, and the electroslag ingot is demoulded and sent to the annealing furnace to slowly cool to room temperature.

[0123] 4) Forging and forming

[0124] For large-sized electroslag ingots, the forging heating system is to heat up at a rate of 90℃ / h to 1215℃, and then forge after keeping warm for 15 hours; the start forging temperature is 1070℃; the final forging temperature is ≥950℃. During the forging process, the upsetting and drawing are performed twice, and the amount of each upsetting and pressing is at least 1 / 3 of the ingot height. Pay attention to uniform deformation during the forging process to prevent surface cracks. Once cracking tendencies appear or the surface temperature of the forging blank approaches 950℃, return to the furnace for heating immediately. The intermediate heating temperature is 1220℃ and kept warm for 1.5h. The heating temperature of the last fire is 1200℃, and the deformation is controlled to be ≥40%.

[0125] 5) Solution heat treatment

[0126] The large-size forgings after forging are subjected to solution heat treatment: 1065℃×4.0h, water cooling.

[0127] Comparison Example

[0128] Comparative Examples 1 and 2 use the composition shown in Table 1, without adding V, B, Al, RE elements, and are prepared by electric furnace smelting, protective atmosphere electroslag remelting, forging and rolling to obtain stainless steel.

[0129] The austenitic stainless steel without magnetism obtained in Examples 1-3 and the stainless steel obtained in Comparative Examples 1-2 were respectively tested for ferrite, relative magnetic permeability and other indexes such as non-metallic inclusion level, grain size, intergranular corrosion, mechanical properties, etc. The results are shown in Table 2 and Table 3.

[0130] Table 1 Chemical composition of steel (mass%)

[0131]

[0132] Table 2 Test results of ferrite, relative magnetic permeability, non-metallic inclusion level, grain size and intergranular corrosion in the steels of Examples and Comparative Examples

[0133]

[0134] Table 3 Room temperature mechanical properties and high temperature mechanical properties of the steels of Examples and Comparative Examples

[0135]

[0136] Combined with Table 1 and Table 2, compared with Comparative Examples 1 and 2, Examples 1-3 were prepared with the austenitic stainless steel composition of the present invention. No high temperature ferrite appeared in the structure, and the relative magnetic permeability was less than 1.01. At the same time, the non-metallic inclusion level, grain size and intergranular corrosion in its structure all met the use requirements of the thermonuclear fusion reactor; as shown in Table 3, the mechanical properties of the austenitic stainless steel prepared in Examples 1-3 at room temperature and at 250 °C also met the use requirements of the thermonuclear fusion reactor.

[0137] In summary, through reasonable alloy composition design, the present invention obtains austenitic stainless steel without high temperature ferrite, ensuring the non-magnetic property of the steel grade, and the mechanical properties, grain size and non-metallic inclusion level of the steel grade all meet the use requirements of the thermonuclear fusion reactor, and can also provide help for the development of stainless steel with non-magnetic requirements in other fields.

[0138] It should be noted that the above examples are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A non-magnetic austenitic stainless steel, characterized in that, Including the following chemical components in mass percentage: C: 0.036-0.059%, Si: 0.42-0.60%, Mn: 4.08-5.62%, Cr: 20.60-21.05%, Mo: 1.52-1.69%, V: 0.11-0.20%, Ni: 12.53-13.50%, Nb: 0.11-0.26%, N: 0.25-0.28%, B: 0.002-0.006%, Al: 0.010-0.050%, S≤0.005%, P≤0.02%, RE: 0.01-0.09%, and the balance is Fe and unavoidable impurities.

2. The non-magnetic austenitic stainless steel according to claim 1, characterized in that, In the non-magnetic austenitic stainless steel, the contents of C, Si, Mn, Cr, Mo, V, Ni and Nb satisfy the following requirements: Ferrite content criterion [N](ferrite) = (-0.0354 - 1.0*[C] + 0.0327*[Si] - 0.00101*[Mn] + 0.0521*[Cr] + 0.0418*[Mo] + 0.126*[V] - 0.0464*[Ni] + 0.102*[Nb]) (1 / 0.45) ≤ 0.24; At the same time, when the Cr equivalent Creq = [Cr] + [Mo] + 3* [Si], and the Ni equivalent Nieq = [Ni] + 21* [C] + 11.5* [N] + 0.5* [Mn], the Cr equivalent and Ni equivalent criteria meet: Nieq> Nieq = -10.31708 + 1.04362*Creq; Wherein, [C], [Si], [Mn], [Cr], [Mo], [V], [Ni] and [Nb] are the mass percentage contents of the corresponding elements respectively.

3. The non-magnetic austenitic stainless steel according to claim 2, characterized in that, The grain size of the non-magnetic austenitic stainless steel is 5.0-3.0, and the relative magnetic permeability is less than 1.

01.

4. A manufacturing method of the non-magnetic austenitic stainless steel according to any one of claims 1 to 3, characterized in that, The following steps are involved: S1, electric furnace smelting, according to the composition of non-magnetic austenitic stainless steel, the material is prepared, and the electrode rod is obtained by electric arc furnace melting, AOD refining, LF refining and casting; S2, electroslag remelting, using the quaternary slag system CaF2-Al2O3-CaO-MgO to electroslag remelt the electrode rod to obtain electroslag ingots; S3, forging and blanking, heating the electroslag ingot to 1200-1250°C, keeping it warm for 10-20 hours, and then performing multiple upsetting and drawing forging to obtain forgings, with the downstroke of each upsetting being at least 1 / 3 of the height of the electroslag ingot. S4, solution heat treatment, heat the forging to 1040-1080℃, keep it warm for 1.5-4.0h, and then cool it with water.

5. The manufacturing method of the non-magnetic austenitic stainless steel according to claim 4, characterized in that, The step S1 includes the following process: S11, electric arc furnace smelting, using one or more of silicon carbide, silicon balls, and ferrosilicon as reducing agents; S12, AOD refining, in the AOD refining process, Al and Si are used as deoxidizers, electrolytic manganese is added in 2 to 3 batches according to the upper limit of the composition requirement range, nitrogen is blown to the nitrogen content required by the composition, and ferroboron blocks are added with the steel flow when tapping; S13, LF refining, fine-tune the composition of molten steel during the LF refining process, add ferroniobium at the beginning of refining; calm the molten steel for 30 to 60 minutes after adding ferrochromium nitride; add rare earth to the required content before hanging the molten steel, and stir it weakly with argon for 20 to 50 minutes; S14, pouring, before pouring molten steel, the baking temperature of the ingot mold is controlled at 50-80°C, and argon gas is used to protect the molten steel pouring. After the pouring is completed, the mold is cooled for ≥20 hours and then demolded to obtain 10-20 tons of electrode rods, which are annealed.

6. The manufacturing method of the non-magnetic austenitic stainless steel according to claim 5, characterized in that, In step S1: In the step S12, in the deoxidizer, Al:Si > 1:1, and the reduction intensity is controlled to be 1.2 - 1.4; the boron iron block is added according to the B content required by the composition. In the step S14, in the annealing treatment, the annealing holding temperature is 780 ± 10 °C, the holding time is 24 - 48 h, and after the holding is completed, it is cooled at a cooling rate of 30 - 40 °C / h to 200 - 250 °C and then air-cooled.

7. The manufacturing method of the non-magnetic austenitic stainless steel according to claim 4, characterized in that, In the step S2, in the quaternary slag system CaF2 - Al2O3 - CaO - MgO, CaF2:Al2O3:CaO:MgO is 65 - 75:15 - 22:3 - 7:3 - 7, and the quaternary slag system CaF2 - Al2O3 - CaO - MgO is used after being baked at 850 ± 10 °C for 4 - 6 h.

8. The manufacturing method of the non-magnetic austenitic stainless steel according to claim 4, characterized in that, In the step S2, in the electroslag remelting process, the steady-state melting rate is controlled at 10 - 18 kg / min; when the electroslag starts to be capped, the weight of the remaining electrode rod ≥ 500 kg, and after the capping and feeding are completed, it is cooled for 60 - 300 minutes and then demolded. After the electroslag ingot is demolded, it is sent into an annealing furnace and slowly cooled to room temperature.

9. The manufacturing method of the non-magnetic austenitic stainless steel according to claim 4, characterized in that, In the step S3, the heating rate of the electroslag ingot ≤ 100 °C / h. During the forging process, the forging start temperature is 1050 °C - 1150 °C, and the final forging temperature ≥ 900 °C.

10. The manufacturing method of the non-magnetic austenitic stainless steel according to claim 4, characterized in that, In the step S3, during the forging process, the number of upsetting and drawing operations is 2 - 4 times. During the forging process, when a crack tendency appears or the surface temperature of the forging billet is close to 950 °C, it is immediately reheated in the furnace. The intermediate reheating temperature is 1180 - 1220 °C, and the holding time is 1.0 - 1.5 h. During the forging process, the heating temperature of the last heat treatment is 1150 - 1200 °C, and the deformation amount ≥ 40%.

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