Method for refining RAFM steel structure through additional particles

Abstract

The invention relates to a method for refining an RAFM steel structure through additional particles. The method comprises the following steps: mixing yttrium nitrate hexahydrate and zirconium nitrate pentahydrate in proportion, adding into deionized water, stirring, drying and roasting to obtain Y-Zr-O powder; mixing the Y-Zr-O powder, ferrotungsten powder and pure iron powder, and performing ball milling, cold pressing and hot sintering to obtain an intermediate alloy block; the preparation method comprises the following steps: preparing materials according to the composition of RAFM, putting the components into a vacuum induction furnace in advance for smelting, putting an intermediate alloy block into molten steel after all the components are smelted, preserving heat, and pouring to obtain a steel ingot containing micro-nano oxide particles. The method is easy and convenient to operate, low in cost and suitable for large-scale production and refinement of the structure of the RAFM steel, and the high-temperature performance of the RAFM steel is improved.

Description

Technical Field

[0001] The present invention relates to a method for refining the structure of low activated ferrite martensite (RAFM) steel by adding external particles, belonging to the technical field of special nuclear power material preparation. Specifically, powder containing micro-nano-level oxides is added during the pouring period of RAFM steel, and then the powder is evenly distributed through appropriate rolling and heat treatment processes, so as to achieve the purpose of refining the RAFM steel structure. Background Art

[0002] Low activation ferrite / martensite (RAFM) steel has excellent thermophysical and mechanical properties such as low radiation swelling and thermal expansion coefficient, high thermal conductivity, etc., and is generally considered to be the preferred candidate cladding structure material for future fusion demonstration reactors and fusion power reactors. However, the high-temperature mechanical properties of existing RAFM steels are insufficient to meet the requirements of the new generation of fission reactors and future fusion reactors, and organizational optimization is urgently needed to improve the high-temperature mechanical properties.

[0003] The most effective strengthening method for RAFM steel is second-phase dispersion strengthening, and the main strengthening phases are fine carbides and oxide phases. However, the working environment of RAFM steel requires it to serve for a long time in an environment above 550°C, which will cause the carbides to coarsen significantly, thereby reducing the high-temperature strength of RAFM steel. In contrast, most oxides are extremely stable particles and are not easy to coarsen at high temperatures. As a precipitated phase, it can make the structure of RAFM steel more stable. At the same time, the fine oxides in the steel can promote the refinement of the RAFM steel structure, thereby improving its high-temperature performance. Therefore, achieving dispersed distribution of fine oxide particles in steel has become an important way to improve the high-temperature performance of RAFM steel.

[0004] Patent CN115029612A (China) adopts a casting method, in which a weak oxidizing gas is introduced during the casting process to react with yttrium in the molten pool to generate oxide particles and disperse them. However, the oxidation time of this method is short, and the yttrium in the molten steel cannot be completely oxidized, and the oxidized products cannot be determined to be all the desired oxides.

[0005] Patents CN201410407312.4 (China), CN201410334753.6 (China), CN102828097A (China), US9039960B2 (USA), and US2015252458A1 (USA) use a process combining mechanical alloying and hot forming to introduce oxide particles. The oxide is dissolved into the alloy powder matrix during the mechanical alloying process and re-precipitated during hot forming, thereby achieving a fine dispersed distribution of the oxide. However, the thermomechanical alloying method has a long process cycle, low production efficiency, and high preparation cost, and large-scale industrial production is difficult.

[0006] Patent CN101967534A (China) mixes nano oxides with high-purity iron powder by ball milling and pressing to prepare nano additive particles, which are then added with the steel flow during the steelmaking process to finally obtain an oxide-containing steel ingot. However, the oxide distribution in the steel obtained by this method is not dispersed enough, and the problem of oxides being easy to float and agglomerate in the molten steel still exists. Summary of the invention

[0007] The purpose of the present invention is to provide a method for refining the structure of RAFM steel by adding particles in view of the problems that the mechanical alloying method and powder hot isostatic pressing / hot extrusion process in the current technology are cumbersome, costly, and cannot be used for large-scale industrial production. The method adds micro-nano oxide powder at the end of smelting to achieve the effect of oxide dispersion distribution in steel; at the same time, tungsten iron powder is added to balance the density of the mixed powder; and after high-temperature rolling, quenching and high-temperature tempering are performed to refine the structure. The present invention is simple to operate, low in cost and suitable for large-scale production of refined RAFM steel structure, improving the high-temperature performance of RAFM steel.

[0008] The technical solution adopted by the present invention to solve the technical problem is: A method for refining RAFM steel structure by adding particles, the method comprising the following steps: (1) Yttrium nitrate hexahydrate and zirconium nitrate pentahydrate are mixed in proportion and added to deionized water, and then heated and stirred with an electromagnetic stirrer until the solution becomes transparent; Among them, the mass ratio is yttrium nitrate hexahydrate: zirconium nitrate pentahydrate = 6:5; Add 20-30 g of yttrium nitrate hexahydrate per 100 ml of deionized water; (2) placing the stirred solution in a 100°C-160°C forced air drying oven and drying for 12-24 hours to obtain a precursor powder; (3) placing the precursor powder in a muffle furnace and heating it to 500°C~1200°C for calcining for 4~6 hours to obtain Y-Zr-O powder; (4) Y-Zr-O powder, tungsten iron powder and pure iron powder are mixed and added into a planetary ball mill for ball milling to obtain a mixed powder with micro-nano-scale oxide dispersion; The mass ratio is Y-Zr-O powder: tungsten iron powder: pure iron powder = 1:1~2:4~10; the ball milling time is 12~90 hours (5) placing the mixed powder in a hydraulic press to cold press into a block, and then placing it in a tubular furnace for hot sintering to obtain an intermediate alloy block; Among them, the sintering temperature is 1000℃~1200℃, and the sintering time is 1~2h.

[0009] (6) preparing materials according to the composition of the substrate, placing each component in a vacuum induction furnace for smelting in advance, and keeping the temperature for 10 to 20 minutes after all the components are melted, placing the intermediate alloy block into the molten steel and keeping the temperature at 1600° C. to 1800° C. for 5 to 10 minutes, and then pouring to obtain a steel ingot containing micro-nano-scale oxide particles; wherein the mass ratio is substrate: intermediate alloy block = 1: 0.005 to 0.1; (7) The steel ingot is heated to 1100-1250 °C and kept at this temperature for 20-30 min, and then rolled using a slot rolling mill with a rolling deformation of 40-70%. After rolling, the sample is quenched at 980-1050 °C for 20-30 min, and then subjected to high-temperature tempering for 1-2 h at a high-temperature tempering temperature of 650-800 °C, thereby obtaining a low-activation ferrite martensitic steel with improved performance.

[0010] The substrate is specifically RAFM steel.

[0011] By using vacuum induction furnace smelting technology, the oxide powder, ferrotungsten powder and pure iron powder are pre-milled to make the oxide powder dispersed in the intermediate alloy block. Then, it is put into the molten steel in the later stage of smelting. After rolling and quenching and tempering heat treatment process, RAFM steel with uniform distribution of micro-nano level oxide is obtained, so as to achieve the effect of dispersion strengthening and refine the structure of RAFM steel.

[0012] The essential features of the present invention are: In current technology, oxide dispersion strengthened steels are mostly prepared by powder metallurgy, but the powder metallurgy method cannot be used for large-scale industrial production and is relatively costly.

[0013] The main production step of the present invention is casting, which is conducive to large-scale industrial production. Different from other casting methods that generate oxides in steel (such as endogenous methods), the present invention uses tungsten iron powder with higher density to balance oxide powder with lower density, thereby solving the problem that oxide particles are easy to float in molten steel. The addition of tungsten iron powder can improve the wetting angle between oxide particles and molten steel, making it easier to disperse and distribute in steel. In addition, the present invention uses ball milling technology to pre-refine the oxide and disperse it in tungsten iron powder and pure iron powder, thereby solving the problem that oxides are easy to agglomerate in molten steel. After casting, the steel is further hot-rolled, quenched and tempered at high temperature to further improve its performance.

[0014] Beneficial effects of the present invention: 1. Compared with the current mainstream preparation process of oxide dispersion strengthened steel - powder metallurgy, the present invention eliminates the complex and time-consuming hot forming process. In addition, the present invention can achieve single-batch mass production, which can effectively reduce costs and improve production efficiency.

[0015] 2. The addition of micro-nano oxide particles can effectively refine the original austenite grains and martensite lath bundles, and can reduce the size of the precipitated phase while increasing the density.

[0016] 3. The steel ingot is then subjected to appropriate rolling and heat treatment processes to further refine the structure and increase the elongation from 18.5% (non-oxide steel) to 25.2% (oxide steel). BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is an EBSD photograph of the oxide-free CLAM steel after heat treatment in Example 1; Figure 2 This is an EBSD photograph of the oxide-containing CLAM steel after heat treatment in Example 1; Figure 3 This is a SEM photograph of the precipitated phase of the oxide-free CLAM steel after heat treatment in Example 1; Figure 4 This is a SEM photograph of the precipitated phase of the oxide-containing CLAM steel after heat treatment in Example 1; DETAILED DESCRIPTION

[0018] Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. When the following description refers to the drawings, the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Instead, they are merely examples of devices and methods consistent with some aspects of the present invention as detailed in the appended claims.

[0019] The experimental steel used in this case is CLAM steel, and composite oxides of Y and Zr are introduced into the steel by casting.

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments. Example 1 (1) First, prepare Y 4 Zr 3 O 12 Powder, the powder is composed of Y(NO 3 ) 3 6H 2 O and Zr(NO 3 ) 4 ·5H 2O made of (Y(NO 3 ) 3 6H 2 O and Zr(NO 3 ) 4 ·5H 2 O ratio 6:5), first 186g Y (NO 3 ) 3 6H 2 O and 155gZr(NO 3 ) 4 ·5H 2 O was dissolved in 800 ml of ionized water until transparent, and then placed in a DL-101 electric constant temperature blast drying oven and heated to 140 ° C for 24 hours to obtain a precursor, and then the precursor was calcined (calcination temperature 500 ° C, time 240 minutes) to obtain Y 4 Zr 3 O 12 powder; (2) Y 4 Zr 3 O 12 Powder, tungsten iron powder (60%W) and pure iron powder (purity 99.99%) were put into a ball mill for ball milling (Y 4 Zr 3 O 12 The ratio of powder, tungsten iron powder and pure iron powder is 1:1:5, and the ball-to-material ratio is 10:1). After ball milling for 60 hours, Y 4 Zr 3 O 12 A mixed powder in which tungsten iron powder is completely dispersed in pure iron powder; (3) The mixed powder was placed in a cold press (total load 157 kN, stabilization time 5 min) and pressed into cylindrical blocks with a thickness of 10 mm and a diameter of 20 mm in batches. The blocks were then placed in a tubular furnace and heated to 1100 °C and sintered for 60 min to obtain intermediate alloy blocks.

[0021] (3) Prepare the materials according to the composition of CLAM steel. Place 77.1% pure iron, 9% chromium, 1.5% tungsten, 0.15% tantalum, and 0.2% vanadium (all in mass percentage, the purity of each material is 99.99%) in a crucible of a vacuum induction furnace. When the vacuum degree is less than 10 Pa, start heating until all of them are melted. (4) Continue heating and keep the temperature for 15 minutes when the temperature reaches 100°C (1600°C) above the liquidus. (5) Fill with argon gas to 1 kPa, and then add the remaining components of steel (0.9% carbon, 0.15% silicon, 0.45% manganese) and the master alloy block in sequence. The mass ratio of steel to master alloy block is 1:0.03; (6) High power heating to 1700 °C and stirring for 5 min, adjusting the temperature and pouring into the mold, cooling in the furnace, and finally obtaining a square steel ingot of 150 mm × 150 mm × 200 mm; (7) Then, a 40 mm × 40 mm × 150 mm rolling sample was taken from the center along the casting direction. The sample was first treated by keeping it at 1150 °C for 0.5 h, and then rolled using a Φ350 mm × 850 mm two-roll groove rolling mill to a 21.9 mm × 21.9 mm sample with a deformation of about 70% (rolled to the 7th pass). (8) After rolling, the sample was quenched at 980 °C for 0.5 h and then subjected to high-temperature tempering for 1.5 h at a high-temperature tempering temperature of 750 °C to obtain CLAM steel with refined structure.

[0022] Comparative Example 1 (no oxide powder added) (1) Prepare the materials according to the composition of CLAM steel, and put 77.1% pure iron, 9% chromium, 1.5% tungsten, 0.15% tantalum, and 0.2% vanadium (all by mass percentage) into the crucible of a vacuum induction furnace. When the vacuum degree is less than 10 Pa, start heating until all of them are melted; (2) Continue heating and keep the temperature for 15 minutes when the temperature reaches 100 °C (1600 °C) above the liquidus; (3) Fill with argon gas to 1 kPa, and then add the remaining components of steel, such as 0.9% carbon, 0.15% silicon and 0.45% manganese; (4) High power heating to 1700 °C and stirring for 5 min, adjusting the temperature and pouring into the mold, cooling in the furnace, and finally obtaining a square steel ingot of 150 mm × 150 mm × 200 mm; (5) Then, a 40 mm × 40 mm × 150 mm rolling sample was taken from the center along the casting direction. The sample was first treated with a 1150 °C insulation treatment for 0.5 h, and then rolled using a Φ350 mm × 850 mm two-roller groove rolling mill to a 21.9 mm × 21.9 mm sample with a deformation of about 70% (rolled to the 7th pass). (6) After rolling, the sample was quenched at 980 °C for 0.5 h and then subjected to high-temperature tempering for 1.5 h at a high-temperature tempering temperature of 750 °C to obtain CLAM steel without oxide addition.

[0023] Figure 1 and Figure 2EBSD microstructure photos of the samples were obtained for Example 1 and Example 1, respectively, and they showed the same microstructure. From the results, after hot rolling, quenching and tempering heat treatment, the grains of the CLAM steel without oxides were relatively coarse, while the grain refinement effect of the CLAM steel containing oxides was more obvious. After adding oxides, the average grain size was refined from 1.67μm to 1.49μm.

[0024] Figure 3 and Figure 4 The SEM microstructure photos of the samples were obtained from Example 1 and Example 1, and the precipitated phases were analyzed to find that after adding the oxide, the precipitated phase (M 23 C 6 ) increased significantly, their size became finer and their density increased, which indicated that the addition of oxides effectively promoted the 23 C 6 Phase precipitation. Through room temperature tensile test - according to the national standard GB / T228-2002 bar tensile specimen preparation standard, room temperature tensile specimens were prepared with a gauge length of 25mm and a total length of 80mm. The universal testing machine AGS-X-50KN was used for room temperature tensile testing at a tensile speed of 1mm / min. Three tensile tests were conducted on the CLAM steel tensile specimens containing oxides and those without oxides. The lengths of the two groups of specimens before and after the three experiments were measured and the average value was taken to calculate the elongation. The results showed that the elongation ranged from 18.5% to 25.2%, indicating that the addition of oxides can effectively refine the grains of CLAM steel and promote M 23 C 6 The precipitation of , makes its plastic toughness improve, thus increasing the elongation.

[0025] Example 2 The other steps are the same as those in Example 1, except that the mass ratio of the steel material to the master alloy block is changed from 1:0.03 to 1:0.06; The obtained material has a slightly finer structure and a slightly improved elongation than that of Example 1; Example 3 The other steps are the same as those in Example 1, except that the ratio of Y4Zr3O12 powder, ferrotungsten powder and pure iron powder is changed from 1:1:5 to 1:1:10; The obtained material structure refinement effect is slightly worse than that of Example 1, but is still refined compared to Comparative Example 1, and the elongation is improved.

[0026] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the specification and practicing the present invention. This application is intended to cover any modification, use or adaptation of the present invention, which follows the general principles of the present invention and includes common knowledge or customary techniques in the art that are not invented by the present invention. The specification and examples are intended to be exemplary only, and the true scope and spirit of the present invention are indicated by the following claims. It should be understood that the present invention is not limited to the implementation method described above, and various microalloying element types, contents and action positions can be changed without departing from its scope.

[0027] Matters not covered by the present invention are known technologies.

Claims

1. A method for refining RAFM steel structure by adding particles, characterized in that the method comprises the following steps: (1) Yttrium nitrate hexahydrate and zirconium nitrate pentahydrate are mixed in proportion and added to deionized water, and then heated and stirred with an electromagnetic stirrer until the solution becomes transparent; in, The mass ratio is yttrium nitrate hexahydrate: zirconium nitrate pentahydrate = 6:5; (2) drying the stirred solution at 100° C. to 160° C. for 12 to 24 hours to obtain a precursor powder; (3) placing the precursor powder in a muffle furnace and heating it to 500°C~1200°C for calcining for 4~6 hours to obtain Y-Zr-O powder; (4) Y-Zr-O powder, tungsten iron powder and pure iron powder are mixed and added into a planetary ball mill for ball milling to obtain a mixed powder with micro-nano-scale oxide dispersion; The mass ratio is Y-Zr-O powder: tungsten iron powder: pure iron powder = 1:1~2:4~10; the ball milling time is 12~90 hours; (5) placing the mixed powder into a hydraulic press to cold press into a block, and then placing it into a tubular furnace for hot sintering to obtain an intermediate alloy block; Among them, the sintering temperature is 1000℃~1200℃, and the sintering time is 1~2h; (6) preparing materials according to the composition of the substrate, placing each component in a vacuum induction furnace for smelting in advance, and keeping the temperature for 10 to 20 minutes after all the components are melted, placing the intermediate alloy block into the molten steel, keeping the temperature at 1700° C. to 1800° C. for 5 to 10 minutes, and then pouring to obtain a steel ingot containing micro-nano-scale oxide particles; wherein the mass ratio is substrate: intermediate alloy block = 1: 0.005 to 0.1; (7) The steel ingot is heated to 1100-1250 °C and kept at this temperature for 20-30 min, and then rolled using a slot rolling mill with a rolling deformation of 40-70%. After rolling, the sample is quenched at 980-1050 °C for 20-30 min, and then subjected to high-temperature tempering for 1-2 h at a high-temperature tempering temperature of 650-800 °C, thereby obtaining a low-activation ferrite martensitic steel with improved performance.

2. The method for refining RAFM steel structure by adding external particles as claimed in claim 1, characterized in that 20-30 g of yttrium nitrate hexahydrate is added per 100 ml of deionized water.

3. The method for refining RAFM steel structure by adding particles as claimed in claim 1, characterized in that The substrate is specifically RAFM steel.

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