High-purity austenitic stainless steel for nuclear power and preparation method and application of high-purity austenitic stainless steel

Abstract

The invention relates to high-purity austenitic stainless steel for nuclear power and a preparation method and application of the high-purity austenitic stainless steel. In the preparation method, the Co content is reduced from the source by screening a low-cobalt raw material; by selecting an AOD-LF-continuous casting-ESR purification process, the effects of efficiently removing [H] and T [O] and reducing the size of inclusions are achieved; by using the continuous casting billet as a consumable electrode of ESR, the ferrite content in stainless steel can be reduced, a segregation band is reduced, and the process time is shortened; the plate is subjected to high-pressure rolling and gradient solution treatment, so that the grain uniformity can be obviously improved, and the grade difference between the center and the surface layer of the stainless steel is smaller than or equal to 1 grade. According to the preparation method, the purity, the service performance and the production efficiency of the stainless steel material are remarkably improved, and the stainless steel material is suitable for nuclear reactor pressure vessel internal components.

Description

Technical Field

[0001] The present invention relates to the technical field of austenitic stainless steel, and in particular to high-purity austenitic stainless steel for nuclear power, a preparation method thereof, and applications thereof. Background Art

[0002] The energy steel sector continues to develop, and research into materials related to hydrogen storage and transportation is gaining momentum. Austenitic stainless steel, with its excellent corrosion resistance and robust toughness at both room and low temperatures, is widely recognized as a suitable material for pressure vessel plates used in hydrogen storage and transportation.

[0003] The pressure vessel steel plates used in hydrogen storage containers have a complex service environment, involving low temperatures, high pressures, and the presence of hydrogen. As the industry continues to evolve, equipment is moving towards larger sizes, higher performance, reduced weight, and longer service lives. This trend places more stringent demands on the raw materials used to manufacture hydrogen storage equipment. Traditional low-carbon alloy container steel plates are no longer strong enough to meet the manufacturing requirements of high-end hydrogen storage pressure vessel equipment in terms of strength and toughness. In addition, the high residual ferrite content and large grain size in the material, as well as the corrosive environment present in the application scenario, will have a significant negative impact on the overall performance of the material.

[0004] At present, the existing stainless steel preparation technology for nuclear power faces the following technical bottlenecks:

[0005] 1. Poor control of impurity elements: The traditional scrap steel recycling process results in a high level of cobalt (Co) residue, which has an adverse effect on the mechanical properties of stainless steel.

[0006] 2. Excessive gas content: When using the RH / VOD process, the dehydrogenation rate is only 80%, and the residual hydrogen ([H]) and oxygen ([O]) in the finished product are relatively high.

[0007] 3. Obvious structural defects: The ferrite content in the center area of ​​the continuous casting billet is high, and the grain size difference is large.

[0008] 4. Low process efficiency: The traditional RH / VOD refining process has a low yield rate and there is a risk of excessive alumina (Al2O3) inclusions; the die-casting electroslag process is not only inefficient but also costly. Summary of the Invention

[0009] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a high-purity austenitic stainless steel for nuclear power and a preparation method and application thereof.

[0010] To achieve the above object, the technical solution adopted by the present invention is:

[0011] In a first aspect, the present invention provides a method for preparing high-purity austenitic stainless steel for nuclear power, comprising the following steps:

[0012] S1. Preparation of low-cobalt nickel-iron alloy liquid: mixing low-cobalt laterite nickel ore and low-cobalt iron ore powder, adding a carbon source, and smelting at 1400-1600° C., adding rare earth pre-melted slag during the smelting process to obtain a low-cobalt nickel-iron alloy liquid with a Co content of ≤0.04wt%; wherein the mass ratio of the low-cobalt laterite nickel ore, low-cobalt iron ore powder, carbon source and rare earth pre-melted slag is (8-15):(8-15):(1-3):1,

[0013] S2. Primary smelting in AOD furnace: Add nickel-iron alloy liquid into AOD furnace, measure the temperature at 1600-1700℃, start bottom blowing of oxygen, add low-cobalt scrap steel and electrolytic manganese during oxygen blowing to perform alloying operation, stop oxygen blowing when the carbon content in the molten steel is ≤0.005wt%, add ferrosilicon, fluorite and lime, and then blow argon and yttrium oxide from the bottom until T[O] in the molten steel is ≤15ppm, stop blowing argon, and tap after slag removal. Obtaining AOD primary steel liquid; wherein the mass ratio of the nickel-iron alloy liquid, low-cobalt scrap steel, electrolytic manganese, ferrosilicon, fluorite, lime and yttrium oxide is (20-30): (5-10): 1: (0.7-1): (1.5-2.5): (0.3-0.5): (0.006-0.016); the tapping temperature is 1600-1700° C., and the tapping target is: Co ≤ 0.04wt%;

[0014] S3, LF furnace refining: adding low-fluorine slag to the AOD primary steel liquid, blowing argon gas from the bottom and stirring, and refining until T[O] in the steel liquid is ≤10ppm to obtain LF refined steel liquid; wherein, the amount of the low-fluorine slag added is 1.5-3.0% by weight of the AOD primary steel liquid, within which the slag layer thickness is ensured to be ≥30mm, covering the steel liquid surface to prevent secondary oxidation, while also avoiding excessive slag resulting in increased heat loss and saturated precipitation of Al2O3 in the slag (deteriorating fluidity);

[0015] S4, continuous casting billet preparation: the LF refined molten steel is continuously cast to obtain a continuous casting billet;

[0016] S5, ESR treatment: using the continuous casting billet as a consumable electrode to perform electroslag remelting and continuous directional solidification to obtain a steel ingot;

[0017] S6, hot rolling: sending the steel ingot to a rolling mill for rolling, water cooling, and obtaining a plate;

[0018] S7. Gradient solution treatment: The plate is first kept at 1020-1070°C for 30-60 minutes, then kept at 1090-1120°C for 1.5-2.5 hours, and cooled to room temperature to obtain high-purity austenitic stainless steel for nuclear power.

[0019] The raw materials of the present invention are low-cobalt laterite nickel ore (Co≤0.05%) and low-cobalt iron ore powder (Co≤0.02%), which reduces the input of cobalt from the source and reduces the Co content of the final alloy (from 0.2-0.25% of traditional scrap steel to ≤0.05%). At the same time, in the preparation process of the low-cobalt nickel-iron alloy liquid in step S1, adding rare earth pre-melted slag containing yttrium oxide can reduce the T[O] content. Specifically, yttrium oxide can dissolve in molten steel during the smelting process and undergo a reduction reaction. The reaction formula is: Y2O3→2[Y]+3[O]. The oxygen affinity of yttrium (Y) (ΔG°f, Y2O3=-1820kJ / mol) is much higher than that of cobalt (Co) (ΔG°f, CoO=-214kJ / mol). The dissolved yttrium will preferentially capture oxygen in the molten steel, and a dynamic cyclic reaction of 2[Y]+3[O]→Y2O3 will occur. This process reduces the local oxygen potential of the molten steel, thereby freezing the oxidation reaction of cobalt ([Co]+[O]→CoO), forcing cobalt to remain in the metallic state in the molten steel. During the primary refining process in an AOD furnace, powdered yttrium oxide is directly sprayed into the molten steel, causing it to react with the metallic Co remaining in the molten steel. The specific reaction steps are: Y2O3 (powder spray) + 2[Co] (molten steel) → 2[Y] + 2CoO↑ (gasification), 3Y2O3 + 4[Co] → 6[Y] + 2Co3O4↑ (gasification). Yttrium oxide reacts with elemental cobalt during this process to form the readily volatile cobalt oxides (CoO / Co3O4). Bottom-blown argon quickly removes the gaseous cobalt oxides from the molten pool, while the [Y] generated in this step can continue to participate in the deoxidation cycle. During the yttrium oxide injection process, part of the yttrium oxide will remain in the slag, which can react with the cobalt oxide that has not yet been vaporized, thereby achieving deep capture of the slag phase (removal of residual cobalt). The specific reaction is: 3Y2O3 (in the slag) + 2CoO (unvaporized) → 2Y3CoO6 (yttrium cobaltate, solid solution in the slag). Therefore, the present invention can significantly reduce the Co content in the AOD furnace primary refining. In the LF furnace refining process, low-fluorine slag (CaF2≤30%) and bottom blowing of argon are used to further deoxidize to T[O]≤10ppm. At the same time, the gradient solution treatment with staged temperature increase in step S7 of the present invention can eliminate residual stress in the alloy, homogenize the structure, and inhibit ferrite precipitation.

[0020] This invention addresses the technical issues of low process efficiency caused by low yield and high costs by replacing die casting with continuous casting + ESR. This reduces the traditional die casting electroslag process from 192 hours to 105 hours, reducing the production cycle by 45.3%, while also improving the yield rate (avoiding shrinkage cavities and looseness in die casting). Furthermore, the Y2O3 and low-fluorine slag can be recycled, reducing auxiliary material consumption and waste residue emissions, lowering environmental costs.

[0021] Preferably, in step S1, the smelting step specifically comprises: first, pre-reduction treatment at 1400-1490°C for 1-2 hours, adding rare earth pre-slag, and then final reduction treatment at 1550-1600°C for 2-3 hours. The present invention further promotes sulfur removal and further reduces the Co content of the nickel-iron alloy liquid through two-stage high-temperature smelting.

[0022] Preferably, the carbon source is coke and anthracite in a mass ratio of (2-4): 1. More preferably, the mass ratio of coke to anthracite is 3:1.

[0023] Preferably, the low-cobalt laterite nickel ore comprises the following components in weight percentage: Fe 0.8-1.5%, Co 0.005-0.05%, MgO 1-5%, SiO2 15-30%, and the balance being Ni and unavoidable impurities.

[0024] Preferably, the low-cobalt iron ore powder comprises the following components in percentage by mass: SiO2 3-10%, Co 0.001-0.02wt%, S 0.001-0.5wt%, P 0.001-0.1wt%, and the balance is Fe and unavoidable impurities.

[0025] Preferably, the rare earth pre-melted slag comprises the following raw materials in percentage by mass: Y2O3 0.01-0.05%, Al2O3 30-35%, SiO2 4-19%, and the balance is CaO.

[0026] Preferably, the low-cobalt scrap steel comprises the following components in weight percentage: Co 0.005-0.01wt%, C 0.03-0.1wt%, and the balance is Fe and unavoidable impurities.

[0027] Preferably, the particle size of the rare earth pre-melted slag is 0.5-1.0 mm; the rare earth pre-melted slag is prepared by mixing various raw materials and baking the mixture at 250±10°C.

[0028] Preferably, the low-fluorine slag contains the following components in weight percentage: CaF2 25-35% and Al2O3 15-25%, with the balance being CaO. Preferably, the low-fluorine slag in step S3 is pre-melted into spheres with a particle size of 10-15 mm.

[0029] Preferably, in step S2, the flow rate of bottom-blown oxygen is 2800-3200 L / min, and the flow rate of bottom-blown argon is 1150-1250 L / min; in step S3, the flow rate of bottom-blown argon is 750-850 L / min.

[0030] Preferably, in step S4, the cross-sectional dimensions of the continuous casting machine are 220×1580 mm, the casting speed is controlled to be 0.65-0.75 m / min, and the water volume distribution in the secondary cooling zone of the continuous casting machine is: 1.5 L / kg steel in the upper part and 0.8 L / kg steel in the lower part;

[0031] Preferably, in step S4, the surface of the continuous casting billet is ground to a grinding depth of ≥3 mm to ensure that all cracks and oxide layers are removed, and then used as an ESR consumable electrode to avoid the coarse grains and center looseness of traditional die casting.

[0032] Preferably, the specific operation of step S5 is as follows: inserting the continuous casting billet into the slag pool for melting to obtain molten metal droplets, the molten metal droplets enter the crystallizer through the slag pool, and are cooled and solidified under the condition of vacuum degree ≤10Pa to obtain steel ingots; wherein the slag in the slag pool comprises the following components in weight percentage: CaF2 35-45% and Al2O3 15-25%, and the balance is CaO; the initial slag thickness in the slag pool is 120-150mm; the melting rate is controlled to be 2.0-2.5kg / min; the droplet diameter is ≤3mm; the crystallizer size is 400×1580mm; the flow rate of the crystallizer cooling water is 50-70m 3 / h, axial temperature gradient ≥100℃ / cm; solidification rate is 15-20mm / min, ensuring columnar crystal growth along the axial direction (segregation bandwidth ≤10mm). Dehydrogenation can be achieved in the ESR (electroslag remelting) step. The specific principles are: 1) Vacuum droplet degassing: under vacuum ([H]) → H2↑; 2) The slag in the crystallizer is high-fluorine slag, which can achieve hydrogen fixation. The specific reaction method is F-+H+→HF↑ (chemical adsorption dehydrogenation). After ESR treatment, the [H] in the steel ingot can be reduced to ≤1ppm, which is a significant improvement compared to the traditional RH / VOD process (dehydrogenation rate is only 80%).

[0033] At the same time, the ESR stage uses a high temperature gradient (≥100℃ / cm) and a low melting rate (2.0-2.5kg / min) to form a columnar crystal structure growing along the axial direction, reducing the central ferrite content from 15-20% in the traditional process to ≤0.5%, and the segregation bandwidth ≤10mm;

[0034] Preferably, in step S6, the steel ingot is heated at 1200-1300°C, first rolled, then rolled in three passes, and finally rolled and water-cooled to obtain the plate; wherein the reduction ratio of the first rolling pass is 35±2%; in the three rolling passes, the first rolling pass is forward rolling at 1150-1250°C with a reduction ratio of 30±2%, the second rolling pass is reverse rolling at 1120-1180°C with a reduction ratio of 32±2%, and the third rolling pass is forward rolling at 1020-1060°C with a reduction ratio of 28±2%. The cooling rate of water cooling is ≥30°C / s, and the termination temperature of water cooling is 20-40°C. The reduction ratio of each hot rolling pass is 26-37%, and the coarse grains are broken by severe deformation to promote austenite recrystallization, so that the full-thickness grain size difference is ≤1 level. Moreover, through multi-pass high-reduction rolling, the number of rolling passes is reduced, efficiency is improved, and grain unevenness caused by the traditional single-stand low reduction rate (≤20%) is avoided.

[0035] In a second aspect, the present invention provides a high-purity austenitic stainless steel for nuclear power, which is prepared by the preparation method of the high-purity austenitic stainless steel for nuclear power in the first aspect.

[0036] In a third aspect, the present invention provides the use of the high-purity austenitic stainless steel for nuclear power in the second aspect in the preparation of nuclear power pressure vessel plates.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] In the preparation method of stainless steel of the present invention, laterite nickel ore is used to smelt low-cobalt nickel-iron alloy (Co≤0.05%) to replace part of scrap steel, and the yttrium-based rare earth adsorption technology in AOD refining is combined to make the Co content in the stainless steel ≤0.04wt%. Compared with the traditional use of scrap steel (Co content of 0.2-0.25%), the present invention reduces the Co content from the source by more than 85% through component control; AOD-LF-continuous casting-ESR purification is used to make [H]≤1ppm, T[O]≤10ppm, and the size of hard inclusions ≤10μm, achieving the effect of efficient gas purification and impurity removal of [H] and T[O]; the purified molten steel is continuously cast to obtain a continuous casting billet, and the continuous casting billet is used as a self-refining agent of ESR. After ESR treatment, the consumable electrode has a central ferrite content of ≤0.5% and a segregation band width of ≤10mm. In contrast, the central ferrite content of the conventional crystallizer treatment is 15-20%, which reduces the ferrite content and shrinks the segregation band. Furthermore, the present invention uses a combination of continuous casting and ESR, which shortens the working time compared to the conventional film casting and ESR combination, thereby improving process efficiency. The present invention uses a rolling process with a reduction rate of more than 26% per pass, resulting in uniform grains on the surface and in the center of the stainless steel, with a grade difference of ≤1 level. The present invention uses gradient solid solution to heat treat the hot-rolled plate, which, compared to a single-temperature solid solution, improves the grain uniformity of the plate and achieves a full-thickness grain size difference of ≤1 level. Therefore, the present invention systematically addresses the four major technical bottlenecks of the conventional process through raw material optimization, multi-stage refining, directional solidification, and thermal processing innovation, achieving the effects of impurity control, gas purification, structural uniformity, and efficiency improvement. This process provides a high-purity, high-performance stainless steel material solution for nuclear power, aerospace, and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a microstructure diagram of the steel plate at 1 / 4 of the thickness prepared by the process of Example 4;

[0040] Figure 2 The microstructure of the steel plate prepared by the process of Example 4 at 1 / 2 of the thickness;

[0041] Figure 3 This is a microstructure diagram of the steel plate at 1 / 4 of the thickness prepared by the process of Comparative Example 3;

[0042] Figure 4 This is a microstructure diagram of the steel plate at 1 / 2 of the thickness prepared by the process of Comparative Example 3;

[0043] Figure 5 This is a microstructure diagram of the steel plate at 1 / 4 of the thickness prepared by the process of Example 1;

[0044] Figure 6 This is a microstructure diagram of the steel plate prepared by the process of Example 1 at 1 / 2 of the thickness. DETAILED DESCRIPTION

[0045] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0046] The sources of raw materials used in the embodiments and comparative examples are as follows:

[0047] Electrolytic manganese: The manufacturer is Ningxia Tianyuan Manganese Industry Group Co., Ltd.

[0048] Ferrosilicon: The manufacturer is Anyang Huatuo Metallurgical Co., Ltd., and the model is Ferrosilicon 72;

[0049] Fluorite: The manufacturer is Lingshou County Zhongheng Mineral Products Processing Plant;

[0050] Lime: The manufacturer is Lingshou County Zhongheng Mineral Products Processing Plant.

[0051] Unless otherwise specified, other materials, reagents, etc. used in the Examples and Comparative Examples can be obtained from commercial sources.

[0052] Example 1

[0053] A method for preparing high-purity austenitic stainless steel for nuclear power, comprising the following steps:

[0054] S1. Preparation of low-cobalt nickel-iron alloy liquid: mixing low-cobalt laterite nickel ore and low-cobalt iron ore powder, adding a carbon source, pre-reducing the mixture at 1460° C. for 1.5 hours, adding rare earth pre-melted slag, and finally reducing the mixture at 1580° C. for 2.5 hours to obtain a low-cobalt nickel-iron alloy liquid; wherein the Co content in the low-cobalt nickel-iron alloy liquid is ≤0.04wt%; and the mass ratio of the low-cobalt laterite nickel ore, low-cobalt iron ore powder, carbon source, and rare earth pre-melted slag is 10:11:2:1;

[0055] The carbon source is coke and anthracite in a mass ratio of 3:1;

[0056] The low-cobalt laterite nickel ore comprises the following components in weight percentage: Fe 1%, Co 0.02%, MgO 4%, SiO2 18%, and the balance being Ni and unavoidable impurities;

[0057] The low-cobalt iron ore powder comprises the following components in weight percentage: SiO2 6%, Co 0.01%, S 0.2%, P 0.03%, and the balance is Fe and unavoidable impurities;

[0058] The rare earth pre-melted slag comprises the following raw materials in percentage by mass: 0.03% Y2O3, 23% Al2O3, 18% SiO2, and the balance CaO; the particle size of the rare earth pre-melted slag is 0.5 to 1.0 mm; the rare earth pre-melted slag is prepared by mixing the raw materials and baking at 250° C. for 1 hour;

[0059] S2, AOD furnace primary refining: adding nickel-iron alloy liquid into the AOD furnace, measuring the temperature at 1680°C, starting bottom blowing of oxygen, adding low-cobalt scrap steel and electrolytic manganese during the oxygen blowing process for alloying operation, stopping oxygen blowing when the carbon content in the steel liquid is ≤0.005wt%, adding ferrosilicon, fluorite and lime, and then bottom blowing argon and spraying yttrium oxide until T[O] in the steel liquid is ≤15ppm, stopping argon blowing, and tapping after slag removal to obtain AOD primary steel liquid; wherein the mass ratio of the nickel-iron alloy liquid, low-cobalt scrap steel, electrolytic manganese, ferrosilicon, fluorite, lime and yttrium oxide is 25:8:1:0.75:2:0.4:0.01; the tapping temperature is 1680°C, and the tapping target has been determined to be Co0.005wt%; wherein the low-cobalt scrap steel includes the following components in weight percentage: Co 0.008wt%, C0.04wt%, the balance is Fe and inevitable impurities; the flow rate of bottom blowing oxygen is 3000L / min, and the flow rate of bottom blowing argon is 1200L / min;

[0060] S3. LF furnace refining: adding low-fluorine slag to the AOD primary steel liquid, blowing argon at the bottom and stirring, and refining until T[O] in the steel liquid is ≤10ppm to obtain LF refined steel liquid; wherein the amount of the low-fluorine slag added is 2% by weight of the AOD primary steel liquid; the low-fluorine slag contains the following components in weight percentage: CaF2 30% and Al2O3 20%, with the balance being CaO; the low-fluorine slag needs to be pre-melted into spherical shape before addition, with a particle size of 10-15mm; the bottom blowing argon flow rate is 800L / min, and the stirring speed is 600rpm;

[0061] S4. Continuous casting slab preparation: LF refined steel liquid was continuously cast, and the slab surface was ground to a depth of 5 mm to obtain a continuous casting slab. The cross-sectional dimensions of the continuous casting slab were 220×1580 mm, the casting speed was 0.7 m / min, and the water distribution in the secondary cooling zone during continuous casting was: 1.5 L / kg steel in the upper portion and 0.8 L / kg steel in the lower portion.

[0062] S5. ESR treatment: The continuous casting billet is used as a consumable electrode for electroslag remelting and continuous directional solidification to obtain a steel ingot; the specific steps are: inserting the continuous casting billet into a slag pool for melting to obtain metal droplets, the metal droplets enter the crystallizer through the slag pool, and are cooled and solidified under a vacuum of 5 Pa to obtain a steel ingot; the slag in the slag pool contains the following components in weight percentage: CaF2 30% and Al2O3 20%, with the balance being CaO; the initial slag thickness in the slag pool is 140 mm; the melting rate is 2.5 kg / min, and the diameter of the metal droplets is ≤ 3 mm; the size of the crystallizer is 400×1580 mm; the cooling water flow rate in the crystallizer is 60 m 3 / h, axial temperature gradient is 120℃ / cm; solidification rate is 18mm / min;

[0063] S6. Hot rolling: a steel ingot with a thickness of 400 mm is heated at 1250°C for 8 hours, firstly subjected to the first rolling, then to the third rolling, and finally subjected to the final rolling, and water-cooled to obtain a plate with a thickness of 90 mm; the reduction rate of the first rolling is 35%, and the ingot thickness is rolled from 400 mm to 260 mm; in the three rolling passes: the first pass: forward rolling is carried out at a temperature of 1200°C, with a reduction rate of 30%, and the ingot thickness is rolled from 260 mm to 182 mm; the second pass: reverse rolling is carried out at a temperature of 1150°C, with a reduction rate of 32%, and the ingot thickness is rolled from 182 mm to 124 mm; the third pass: forward rolling is carried out at a temperature of 1050°C, with a reduction rate of 28%, and the ingot thickness is rolled from 124 mm to 89 mm; the final rolling temperature is 980°C, the cooling rate of water cooling is 50°C / s, and then cooled to room temperature;

[0064] S7. Gradient solution treatment: The plate is first kept at 1020°C for 60 minutes, then kept at 1090°C for 2.5 hours, and water-cooled to room temperature to obtain high-purity austenitic stainless steel for nuclear power.

[0065] Example 2

[0066] A method for preparing high-purity austenitic stainless steel for nuclear power, comprising the following steps:

[0067] S1. Preparation of low-cobalt nickel-iron alloy liquid: mixing low-cobalt laterite nickel ore and low-cobalt iron ore powder, adding a carbon source, pre-reducing the mixture at 1450° C. for 2 hours, adding rare earth pre-melted slag, and finally reducing the mixture at 1550° C. for 3 hours to obtain a low-cobalt nickel-iron alloy liquid; wherein the Co content of the low-cobalt nickel-iron alloy liquid is ≤0.04wt%; and the mass ratio of the low-cobalt laterite nickel ore, low-cobalt iron ore powder, carbon source, and rare earth pre-melted slag is 8:15:1:1;

[0068] The carbon source is coke and anthracite in a mass ratio of 2:1;

[0069] The low-cobalt laterite nickel ore comprises the following components in weight percentage: Fe 0.8%, Co 0.05%, MgO 1%, SiO2 15%, and the balance being Ni and unavoidable impurities;

[0070] The low-cobalt iron ore powder comprises the following components in weight percentage: SiO2 3%, Co 0.02%, S 0.5%, P 0.1%, and the balance is Fe and unavoidable impurities;

[0071] The rare earth pre-melted slag comprises the following raw materials in percentage by mass: 0.01% Y2O3, 20% Al2O3, 15% SiO2, and the balance CaO; the particle size of the rare earth pre-melted slag is 0.5 to 1.0 mm; the rare earth pre-melted slag is prepared by mixing the raw materials and baking at 240° C. for 1 hour;

[0072] S2, AOD furnace primary refining: adding nickel-iron alloy liquid into the AOD furnace, measuring the temperature at 1600°C, starting bottom blowing of oxygen, adding low-cobalt scrap steel and electrolytic manganese during the oxygen blowing process for alloying operation, stopping oxygen blowing when the carbon content in the steel liquid is ≤0.005wt%, adding ferrosilicon, fluorite, and lime, and then bottom blowing argon and spraying yttrium oxide until T[O] in the steel liquid is ≤15ppm, stopping argon blowing, and tapping after slag removal to obtain AOD primary steel liquid; wherein the mass ratio of the nickel-iron alloy liquid, low-cobalt scrap steel, electrolytic manganese, ferrosilicon, fluorite, lime and yttrium oxide is 20:5:1:0.7:1.5:0.3:0.006; the tapping temperature is 1600°C, and the tapping target is Co0.01wt%; the low-cobalt scrap steel includes the following components in weight percentage: Co 0.01wt%, C 0.1wt%, the balance is Fe and unavoidable impurities; the flow rate of bottom blowing oxygen is 2800L / min, and the flow rate of bottom blowing argon is 1150L / min;

[0073] S3. LF furnace refining: adding low-fluorine slag to the AOD primary steel liquid, blowing argon at the bottom and stirring, and refining until T[O] in the steel liquid is ≤10ppm to obtain LF refined steel liquid; wherein the amount of the low-fluorine slag added is 1.5% by weight of the AOD primary steel liquid; the low-fluorine slag contains the following components in weight percentage: CaF2 25% and Al2O3 15%, and the balance is CaO; the low-fluorine slag needs to be pre-melted into spherical particles with a particle size of 10mm before adding; the bottom blowing argon flow rate is 750L / min, and the stirring speed is 400rpm;

[0074] S4. Continuous casting slab preparation: LF refined steel liquid was continuously cast, and the slab surface was ground to a depth of 3 mm to obtain a continuous casting slab. The cross-sectional dimensions of the continuous casting slab were 220×1580 mm, the casting speed was 0.7 m / min, and the water distribution in the secondary cooling zone during continuous casting was: 1.5 L / kg steel in the upper portion and 0.8 L / kg steel in the lower portion.

[0075] S5. ESR treatment: The continuous casting billet is used as a consumable electrode for electroslag remelting and continuous directional solidification to obtain a steel ingot; the specific steps are: inserting the continuous casting billet into a slag pool for melting to obtain metal droplets, the metal droplets enter the crystallizer through the slag pool, and are cooled and solidified under a vacuum degree of 10 Pa to obtain a steel ingot; wherein the slag in the slag pool contains the following components in weight percentage: CaF2 35% and Al2O3 15%, and the balance is CaO; the initial slag thickness in the slag pool is 120mm; the melting rate is 2.0kg / min, and the diameter of the metal droplets is ≤3mm; the size of the crystallizer is 400×1580mm; the cooling water flow rate in the crystallizer is 50m 3 / h, axial temperature gradient ≥100℃ / cm; solidification rate 15mm / min;

[0076] S6. Hot rolling: a steel ingot with a thickness of 443 mm was heated at 1200°C for 8 hours, firstly subjected to the first rolling, then to the third rolling, and finally subjected to the final rolling, and water-cooled to obtain an 80 mm plate; the first rolling reduction was 33%, and the ingot thickness was rolled from 443 mm to 297 mm; in the three rolling passes: the first pass was forward rolling at a temperature of 1150°C, with a reduction of 28%, and the ingot thickness was rolled from 297 mm to 214 mm; the second pass was reverse rolling at 1120°C, with a reduction of 30%, and the ingot thickness was rolled from 214 mm to 108 mm; the third pass was forward rolling at a temperature of 1020°C, with a reduction of 26%, and the ingot thickness was rolled from 108 mm to 80 mm; the final rolling temperature was 950°C, the water cooling rate was 60°C / s, and the ingot was cooled to room temperature;

[0077] S7. Gradient solution treatment: The plate is first kept at 1020°C for 60 minutes, then kept at 1090°C for 2.5 hours, and water-cooled to room temperature to obtain high-purity austenitic stainless steel for nuclear power.

[0078] Example 3

[0079] A method for preparing high-purity austenitic stainless steel for nuclear power, comprising the following steps:

[0080] S1. Preparation of low-cobalt nickel-iron alloy liquid: mixing low-cobalt laterite nickel ore and low-cobalt iron ore powder, adding a carbon source, pre-reducing the mixture at 1490° C. for 1 hour, adding rare earth pre-melted slag, and finally reducing the mixture at 1600° C. for 2 hours to obtain a low-cobalt nickel-iron alloy liquid; wherein the Co content in the low-cobalt nickel-iron alloy liquid is ≤0.04 wt %; and the mass ratio of the low-cobalt laterite nickel ore, low-cobalt iron ore powder, carbon source, and rare earth pre-melted slag is 15:8:3:1;

[0081] The carbon source is coke and anthracite in a mass ratio of 4:1;

[0082] The low-cobalt laterite nickel ore comprises the following components in weight percentage: Fe 1.5%, Co 0.005%, MgO 5%, SiO2 30%, and the balance being Ni and unavoidable impurities;

[0083] The low-cobalt iron ore powder comprises the following components in weight percentage: SiO2 10%, Co 0.008%, S 0.1%, P 0.03%, and the balance being Fe and unavoidable impurities;

[0084] The rare earth pre-melted slag comprises the following raw materials in percentage by mass: 0.05% Y2O3, 30% Al2O3, 20% SiO2, and the balance CaO; the particle size of the rare earth pre-melted slag is 0.5 to 1.0 mm; the rare earth pre-melted slag is prepared by mixing the raw materials and baking at 260°C;

[0085] S2, AOD furnace primary refining: adding nickel-iron alloy liquid into the AOD furnace, measuring the temperature at 1700°C, starting bottom blowing of oxygen, adding low-cobalt scrap steel and electrolytic manganese during the oxygen blowing process for alloying operation, stopping oxygen blowing when the carbon content in the molten steel is ≤0.005wt%, adding ferrosilicon, fluorite, and lime, then bottom blowing argon and spraying yttrium oxide, stopping argon blowing when T[O] in the molten steel is ≤15ppm, skimming and tapping to obtain AOD primary steel liquid; wherein the mass ratio of the nickel-iron alloy liquid, low-cobalt scrap steel, electrolytic manganese, ferrosilicon, fluorite, lime and yttrium oxide is 30:10:1:1:2.5:0.5:0.016; the tapping temperature is 1700°C, and Co0.003wt% has been measured at the time of tapping; the flow rate of bottom blowing oxygen is 3200L / min, and the flow rate of bottom blowing argon is 1250L / min;

[0086] S3. LF furnace refining: low-fluorine slag is added to the AOD primary steel liquid, argon is blown from the bottom and stirred, and the steel liquid is refined until T[O] is ≤10ppm to obtain LF refined steel liquid; wherein the low-fluorine slag is added in an amount of 3.0% by weight of the AOD primary steel liquid; the low-fluorine slag contains the following components in weight percentage: CaF2 35% and Al2O3 25%, with the balance being CaO; the low-fluorine slag is pre-melted into spherical particles with a particle size of 10-12mm before addition; the bottom-blown argon flow rate is 850L / min, and the stirring speed is 1000rpm;

[0087] S4. Continuous casting slab preparation: LF refined steel liquid was continuously cast, and the slab surface was ground to a depth of 5 mm to obtain a continuous casting slab. The cross-sectional dimensions of the continuous casting slab were 220×1580 mm, the casting speed was 0.7 m / min, and the water distribution in the secondary cooling zone during continuous casting was: 1.5 L / kg steel in the upper portion and 0.8 L / kg steel in the lower portion.

[0088] S5. ESR treatment: The continuous casting billet is used as a consumable electrode for electroslag remelting and continuous directional solidification to obtain a steel ingot; the specific steps are: inserting the continuous casting billet into a slag pool for melting to obtain metal droplets, the metal droplets enter the crystallizer through the slag pool, and are cooled and solidified under a vacuum degree of 10 Pa to obtain a steel ingot; the slag in the slag pool contains the following components in weight percentage: CaF2 45% and Al2O3 25%, and the balance is CaO; the initial slag thickness in the slag pool is 150mm; the melting rate is 2.5kg / min, and the diameter of the metal droplets is ≤3mm; the size of the crystallizer is 400×1580mm; the cooling water flow rate in the crystallizer is 70m 3 / h, axial temperature gradient is 150℃ / cm; solidification rate is 20mm / min;

[0089] S6. Hot rolling: a steel ingot with a thickness of 443 mm was heated at 1200°C for 8 hours, firstly subjected to the first rolling, then to the third rolling, and finally subjected to the final rolling, and water-cooled to obtain an 80 mm plate; the first rolling reduction was 33%, and the ingot thickness was rolled from 443 mm to 297 mm; in the three rolling passes: the first pass was forward rolling at a temperature of 1150°C, with a reduction of 28%, and the ingot thickness was rolled from 297 mm to 214 mm; the second pass was reverse rolling at 1120°C, with a reduction of 30%, and the ingot thickness was rolled from 214 mm to 108 mm; the third pass was forward rolling at a temperature of 1020°C, with a reduction of 26%, and the ingot thickness was rolled from 108 mm to 80 mm; the final rolling temperature was 950°C, the water cooling rate was 60°C / s, and the ingot was cooled to room temperature;

[0090] S7. Gradient solution treatment: The plate is first kept at 1020°C for 60 minutes, then kept at 1090°C for 2.5 hours, and water-cooled to room temperature to obtain high-purity austenitic stainless steel for nuclear power.

[0091] Example 4

[0092] The difference between Example 4 and Example 1 is that Example 1 is rolled five times, and the thickness of the final plate is set to 90 mm, as follows:

[0093] The 400mm thick steel ingot is heated at 1250℃ for 8h, then rolled in four passes, and then final rolled and water-cooled to obtain a 90mm thick plate. The rolling parameters are: first pass reduction, reduction rate of 25%, the ingot thickness is rolled from 400mm to 300mm, and then three passes are rolled: the first pass: forward rolling at a temperature of 1200℃, reduction rate of 25%, the ingot thickness is rolled from 300mm to 225mm, the second pass: reverse rolling at 1150℃, reduction rate of 25%, The ingot thickness is rolled from 225mm to 169mm. The third pass: forward rolling is carried out at a temperature of 1050℃, with a reduction rate of 25%, and the ingot thickness is rolled from 169mm to 127mm. The fourth pass: forward rolling is carried out at a temperature of 1050℃, with a reduction rate of 25%, and the ingot thickness is rolled from 127mm to 95mm. The fifth pass: forward rolling is carried out at a temperature of 1050℃, with a reduction rate of 5.3%, and the ingot thickness is rolled from 95mm to 90mm. The final rolling temperature is 980℃, and the water cooling rate is 50℃ / s.

[0094] Comparative Example 1

[0095] The difference between Comparative Example 1 and Example 1 is that yttrium oxide is not added to the rare earth pre-melted slag in Comparative Example 1, and an equal amount of CaO is used to make up for the missing amount.

[0096] Comparative Example 2

[0097] The difference between Comparative Example 2 and Example 1 is that yttrium oxide is not sprayed in step S2 of Comparative Example 2.

[0098] Comparative Example 3

[0099] The difference between Comparative Example 3 and Example 1 is that: Comparative Example 3 does not perform gradient solution treatment, but only performs single solution treatment at 1050° C. for 3 hours.

[0100] Comparative Example 4

[0101] The difference between Comparative Example 4 and Example 1 is that in Comparative Example 4, the continuous casting process in step S4 is replaced by die casting, and the die casting method includes the following steps:

[0102] The LF refined steel liquid obtained in step S3 is poured into a cast steel ingot mold by a bottom casting method. After pouring, it is solidified unidirectionally from bottom to top for 36 hours. After solidification, the steel ingot is hoisted out, and the ingot mold is removed to obtain a rough casting blank. 15% to 20% of the top of the rough casting (shrinkage cavities, segregation areas) and 5% to 10% of the bottom (inclusion-rich areas) are cut off, and then rolled at 1200°C to obtain a mold casting blank.

[0103] Performance data

[0104] The following tests were performed on the stainless steel plates of Examples 1-4 and Comparative Examples 1-4:

[0105] 1. Co content detection

[0106] The Co content of the stainless steel plates in each group was tested, and the specific data are shown in Table 1.

[0107] Table 1 Co content of each group of stainless steel plates

[0108] sample Co content / % Example 1 0.003 Example 2 0.012 Example 3 0.002 Example 4 0.004 Comparative Example 1 0.011 Comparative Example 2 0.009 Comparative Example 3 0.003 Comparative Example 4 0.004

[0109] 2. Room temperature tensile test

[0110] According to GB / T 228.1-2021 "Tension test of metallic materials Part 1: Room temperature test method", use a universal material testing machine, start the testing machine, apply axial tensile force to the sample, and the testing machine automatically records the deformation process and force of the sample until the sample breaks. Calculate the tensile strength R m and yield strength Rp 0.2 , Rp 0.2 It refers to the stress corresponding to the plastic deformation of the sample of 0.2%. The data are shown in Table 2.

[0111] 3. High temperature tensile test

[0112] According to GB / T228.2-2015 "High-temperature tensile test of metal materials", the test temperature is 350℃, and the tensile strength R is calculated. m and yield strength Rp 0.2 , Rp 0.2 , data see Table 2.

[0113] 4. Low temperature impact test

[0114] In accordance with GB / T 229-2020, "Metallic Materials Charpy Pendulum Impact Test Method," the impact specimen was placed on the anvil of the testing machine, with the specimen notch aligned with the direction of the pendulum's impact to ensure correct specimen positioning. The test temperature was set to -195°C. The pendulum was raised and released, impacting the specimen, breaking it. The testing machine automatically recorded the impact absorbed energy (KV2) measured by the 2mm pendulum blade on the V-notch specimen in joules. The data is shown in Table 2.

[0115] 5. Yield rate: Tested in accordance with GB / T 246-2017 "Hydraulic Test Methods for Metallic Tubes". Qualified products are determined as follows: surface crack length ≤ 0.5D and depth ≤ 0.1t, no delamination or inclusion exposure is allowed, and ovality deviation ≤ 15% of the original outer diameter.

[0116] 6. Grain and ferrite test of stainless steel plate at 1 / 4 thickness and 1 / 2 thickness

[0117] The stainless steel plates of Example 1, Example 4 and Comparative Examples 3-4 were tested for grain size at 1 / 4 and 1 / 2 of the plate thickness according to the standard test method for average grain size determination of ASTM E112-2013. The standard is divided into grades 00 to 10 (the larger the number, the finer the grain). The ferrite content at 1 / 4 and 1 / 2 of the plate thickness in the stainless steel was determined by metallographic method. The data are shown in FIG. Figure 1-6 ;

[0118] Table 1 Performance data of each group of samples

[0119]

[0120] Depend on Figure 1-2 It can be seen that the rolling reduction rate of Example 4 is lower than 26%, the grain order at 1 / 4 of the steel plate thickness is 7.5, and the ferrite is 0.2%. The grain order at 1 / 2 of the steel plate thickness is 2.5, and the ferrite is 0.3%. This shows that the grain size difference between the surface and the center of the stainless steel plate is large, the grain size difference is 5, the central ferrite is not much different from that of the surface, and there is no obvious central segregation.

[0121] Depend on Figure 3-4 It can be seen that Comparative Example 3 only underwent a single solid solution treatment. The grain size at 1 / 4 of the steel plate thickness was level 7, with ferrite 1.2%. The grain size at 1 / 2 of the steel plate thickness was level 4, with ferrite 8.9%. This indicates that the grain size difference between the surface and center of the stainless steel plate is level 3, the central ferrite is higher than that of the surface, and the central segregation is prominent.

[0122] Depend on Figure 5-6 It can be seen that the stainless steel of Example 1 has a grain size of 6.5 and ferrite of 0.3% at 1 / 4 of the steel plate thickness, and a grain size of 6 and ferrite of 0.4% at 1 / 2 of the steel plate thickness, which indicates that Example 1 undergoes three passes of rolling in hot rolling and is simultaneously subjected to gradient solution treatment, so that the difference in grain size between the surface and the center is small, the grain size difference is only 0.5, the organization is uniform, and at the same time, there is no obvious difference between the central ferrite and the surface, and the improvement in the center is prominent. This may be because the rolling with a high reduction rate breaks up the coarse grains through severe deformation, promotes the recrystallization of austenite, and makes the full thickness grain size difference ≤1 level. The gradient solution treatment adopts staged heating to eliminate residual stress and homogenize the organization, thereby inhibiting the precipitation of ferrite.

[0123] As can be seen from Table 2, combined with the data of Example 1 and Example 4, its tensile properties at room temperature and high temperature and low-temperature impact resistance are significantly lower than those of Example 1. This may be because the low rolling reduction rate will lead to incomplete recrystallization of austenite, increase the grain size difference between the surface and the center, and coarsen the grains, resulting in a decrease in mechanical properties and a corresponding increase in the risk of brittle fracture, thereby reducing the yield rate.

[0124] As can be seen from Table 1-2, combined with the data of Example 1 and Comparative Example 1-2, the reduction of the Co content in the stainless steel is mainly due to the selection of raw materials, and mainly relies on the rare earth pre-melted slag and the combination of yttrium oxide sprayed in AOD to remove Co. Therefore, the Co content in the stainless steel of Comparative Example 1-2 is significantly higher than that of Example 1, and its mechanical properties and temperature resistance are also significantly reduced. This may be because the solid solubility of cobalt in austenitic stainless steel is low. When the cobalt content is too high, it will segregate at the grain boundaries to form brittle phases, such as Laves phases, resulting in a decrease in the toughness of the material. It is easy to crack under stress, which reduces the ductility and impact toughness of the material and increases the risk of brittle fracture.

[0125] Combining the data of Example 1 and Comparative Example 3, it can be seen that its tensile properties at room temperature and high temperature and low-temperature impact resistance are significantly lower than those of Example 1. This may be because the gradient heating eliminates the hot rolling residual stress (from ≥200 MPa to ≤50 MPa), the single solid solution increases the residual stress, and the stress is superimposed during high-temperature stretching, thereby reducing the tensile strength.

[0126] Combining the data from Example 1 and Comparative Example 4 reveals that the room temperature and high temperature tensile properties and low-temperature impact resistance are significantly lower than those of Example 1. This is likely due to the increased V segregation and Al₂O₃ inclusion density at the center of the die-cast ingot, which leads to greater fluctuations in room temperature yield strength. Die casting can also lead to uncontrolled ferrite content, reducing high-temperature tensile properties and increasing low-temperature brittleness, significantly impacting yield.

[0127] In summary, the stainless steel produced by the preparation method of the present invention has the characteristics of ultra-low impurities, no segregation and highly uniform structure, and also improves production efficiency, so that the stainless steel has good high-temperature performance and good toughness and strength, and is suitable for nuclear reactor pressure vessel internal components.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing high-purity austenitic stainless steel for nuclear power, characterized in that: The following steps are involved: S1. Preparation of low-cobalt nickel-iron alloy liquid: mixing low-cobalt laterite nickel ore and low-cobalt iron ore powder, adding a carbon source, and smelting at 1400-1600° C., adding rare earth pre-melted slag during the smelting process to obtain a low-cobalt nickel-iron alloy liquid with a Co content of ≤0.04wt%; wherein the mass ratio of the low-cobalt laterite nickel ore, low-cobalt iron ore powder, carbon source and rare earth pre-melted slag is (8-15):(8-15):(1-3):1, S2. Primary smelting in AOD furnace: Add nickel-iron alloy liquid into AOD furnace, measure the temperature at 1600-1700℃, start bottom blowing of oxygen, add low-cobalt scrap steel and electrolytic manganese during oxygen blowing to perform alloying operation, stop oxygen blowing when the carbon content in the molten steel is ≤0.005wt%, add ferrosilicon, fluorite and lime, and then blow argon and yttrium oxide from the bottom until T[O] in the molten steel is ≤15ppm, stop blowing argon, and tap after slag removal. Obtaining AOD primary steel liquid; wherein the mass ratio of the nickel-iron alloy liquid, low-cobalt scrap steel, electrolytic manganese, ferrosilicon, fluorite, lime and yttrium oxide is (20-30): (5-10): 1: (0.7-1): (1.5-2.5): (0.3-0.5): (0.006-0.016); the tapping temperature is 1600-1700° C., and the tapping target is: Co ≤ 0.04wt%; S3, LF furnace refining: adding low-fluorine slag to the AOD primary steel liquid, blowing argon gas from the bottom and stirring, and refining until T[O] in the steel liquid is ≤10ppm to obtain LF refined steel liquid; wherein the amount of the low-fluorine slag added is 1.5-3.0% by weight of the AOD primary steel liquid; S4, continuous casting billet preparation: the LF refined molten steel is continuously cast to obtain a continuous casting billet; S5, ESR treatment: using the continuous casting billet as a consumable electrode to perform electroslag remelting and continuous directional solidification to obtain a steel ingot; S6, hot rolling: sending the steel ingot to a rolling mill for rolling, water cooling, and obtaining a plate; S7. Gradient solution treatment: The plate is first kept at 1020-1070°C for 30-60 minutes, then kept at 1090-1120°C for 1.5-2.5 hours, and cooled to room temperature to obtain high-purity austenitic stainless steel for nuclear power.

2. The method for preparing high-purity austenitic stainless steel for nuclear power according to claim 1, characterized in that: In step S1, the smelting step is specifically as follows: first, pre-reduction treatment is performed by keeping the temperature at 1400-1490° C. for 1-2 hours, rare earth pre-slag is added, and then final reduction treatment is performed by keeping the temperature at 1550-1600° C. for 2-3 hours.

3. The method for preparing high-purity austenitic stainless steel for nuclear power according to claim 1, characterized in that: The carbon source is coke and anthracite in a mass ratio of (2-4):

1.

4. The method for preparing high-purity austenitic stainless steel for nuclear power according to claim 1, characterized in that: The raw materials for preparing the austenitic stainless steel include at least one of the following (I) to (V): (I) The low-cobalt laterite nickel ore comprises the following components in weight percentage: Fe 0.8-1.5%, Co 0.005-0.05%, MgO 1-5%, SiO2 15-30%, and the balance being Ni and unavoidable impurities; (II) The low-cobalt iron ore powder comprises the following components in percentage by weight: SiO2 3-10%, Co 0.001-0.02wt%, S 0.001-0.5wt%, P 0.001-0.1wt%, and the balance is Fe and unavoidable impurities; (III) the rare earth pre-melted slag comprises the following raw materials in percentage by mass: Y2O3 0.01-0.05%, Al2O3 30-35%, SiO2 4-19%, and the balance CaO; (IV) the low-cobalt steel scrap comprises the following components in weight percentage: Co 0.005-0.01 wt%, C 0.03-0.1 wt%, and the balance Fe and unavoidable impurities; (V) The low-fluorine slag contains the following components in weight percentage: CaF2 25-35% and Al2O3 15-25%, with the balance being CaO.

5. The method for preparing high-purity austenitic stainless steel for nuclear power according to claim 1, characterized in that: In step S2, the flow rate of bottom-blown oxygen is 2800-3200 L / min, and the flow rate of bottom-blown argon is 1150-1250 L / min; in step S3, the flow rate of bottom-blown argon is 750-850 L / min.

6. The method for preparing high-purity austenitic stainless steel for nuclear power according to claim 1, characterized in that: In step S4, the cross-sectional size of the continuous casting machine is 220×1580 mm, the casting speed is controlled to be 0.65-0.75 m / min, and the water volume distribution in the secondary cooling zone of the continuous casting machine is: 1.5 L / kg steel in the upper part and 0.8 L / kg steel in the lower part.

7. The method for preparing high-purity austenitic stainless steel for nuclear power according to claim 1, characterized in that: In step S5, the slag in the slag pool includes the following components in weight percentage: CaF2 35-45% and Al2O3 15-25%, with the balance being CaO; the initial slag thickness in the slag pool is 120-150 mm; the melting rate is controlled to be 2.0-2.5 kg / min; the droplet diameter is ≤3 mm; the crystallizer size is 400×1580 mm; the flow rate of the crystallizer cooling water is 50-70 m 3 / h, axial temperature gradient ≥100℃ / cm; solidification rate 15~20mm / min; vacuum degree of ESR treatment ≤10Pa.

8. The method for preparing high-purity austenitic stainless steel for nuclear power according to claim 1, characterized in that: In step S6, the steel ingot is heated at 1200-1300°C, first rolled, then rolled in three passes, and finally rolled and water-cooled to obtain the plate; wherein the reduction rate of the first rolling pass is 35±2%; in the three rolling passes: the first rolling pass: forward rolling is performed at 1150-1250°C, with a reduction rate of 30±2%, the second rolling pass: reverse rolling is performed at 1120-1180°C, with a reduction rate of 32±2%, the third rolling pass: forward rolling is performed at 1020-1060°C, with a reduction rate of 28±2%; and / or the cooling rate of water cooling is ≥30°C / s, and the termination temperature of water cooling is 20-40°C.

9. A high-purity austenitic stainless steel for nuclear power, characterized in that: The high-purity austenitic stainless steel for nuclear power is prepared by the preparation method of any one of claims 1 to 8.

10. Use of the high-purity austenitic stainless steel for nuclear power according to claim 9 in the preparation of nuclear power pressure vessel plates.

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