Preparation method of thermal neutron shielding steel

Abstract

The invention belongs to the technical field of metal materials and radiation protection, and particularly relates to a preparation method of thermal neutron shielding steel. According to the method, efficient spray forming of the B4C / 304 stainless steel composite material is achieved through a gas-solid two-phase flow driving mechanism under the negative pressure environment, B4C ceramic particles are evenly dispersed and embedded into a 304 stainless steel matrix, and due to the strong driving force of gas-solid two-phase flow, firm interface bonding is formed between the B4C particles and the matrix. According to the process, gas atomization and solid-phase medium particles are combined, a gas-solid two-phase mixed flow is formed, the atomization efficiency is greatly improved, the powder particle size is refined, reinforced phase distribution is optimized, and a new way is opened up for preparation of the high-performance thermal neutron shielding steel composite material.

Description

Technical Field

[0001] The invention belongs to the technical field of metal materials and radiation protection, and particularly relates to a method for preparing thermal neutron shielding steel. Background Art

[0002] As a form of energy with high efficiency, clean and environmentally friendly characteristics, nuclear energy is widely used in military, energy, medical and other fields, and has become a key choice for countries in responding to energy demand and environmental challenges. However, with the rapid development of my country's nuclear industry, higher requirements are placed on the comprehensive performance of neutron shielding materials required for high-performance spent fuel storage and transportation. Current thermal neutron shielding materials are usually made by mixing polymers such as polyethylene and polyurethane with neutron shielding materials. The production process of this shielding material is complicated and cumbersome, and the melting point is relatively low, about below 0°C, and it will become hard and brittle, lacking the ability to resist impact and vibration. In addition, B4C ceramic particles are introduced into the composite material by methods such as powder metallurgy to obtain a higher thermal neutron absorption cross section. However, this process has a high production cost, the shielding effect is not ideal, and the size and distribution state of the B4C particles have a certain influence on the mechanical properties and interface bonding of the composite material, which cannot meet the actual application needs in the field of radiation protection technology. Therefore, it is necessary to develop novel preparation methods to meet the current market demand. Summary of the invention

[0003] In view of the problems in the prior art, the present invention provides a method for preparing thermal neutron shielding steel.

[0004] The technical solution of the present invention is: A method for preparing thermal neutron shielding steel comprises the following steps: Step (1) adopts a double vacuum chamber closed structure device to realize negative pressure environment spraying: the double vacuum chamber structure device is divided into a melting chamber, a tundish chamber and a spray deposition chamber, the tundish chamber is connected to the melting chamber and the spray deposition chamber, and the connection and isolation are realized by a casting plug valve; the melting chamber is provided with a melting ladle, and the liftable tundish is preheated to a temperature outside the chamber and then placed in the tundish chamber; the melting chamber is connected to a high vacuum pump group, and the vacuum degree of the melting chamber is ≤5×10 -2 Pa; the jet deposition chamber is equipped with a high-speed vacuum pump group to maintain the ambient pressure ≤5000Pa under the jet gas input condition; Step (2) performing vacuum medium frequency induction melting of a 304 matrix alloy in a melting chamber, wherein the composition of the 304 matrix alloy is: Cr 17.0-20%, Ni 10.0-16.0%, Mn 1.0-2.5%, B 0.1-0.5%, Ti 0.1-0.5%, C < 0.04%, N ≤ 10 ppm, and the remainder is Fe; the melting amount is 1-10 t; Step (3) After the smelting of step (2), the superheat of the 304 stainless steel alloy solution is controlled to be +40-80°C, the sealing door of the tundish cabin is closed, the smelting ladle is subjected to bottom argon blowing for deslagging and degassing, the control valve a is opened, the pouring plug plate is closed, and then the pouring plug plate valves of the smelting cabin and the jet deposition cabin are opened, the high-speed vacuum pump group is started, the tundish is lowered to a distance of 400-800 mm from the deposition billet, the smelting ladle controls the flow rate through the bottom slide gate and injects the 304 stainless steel alloy solution into the tundish to stabilize the liquid level of 100-180 mm, and the tundish preheating temperature is ≥1200°C; Step (4) Surface modification of B4C particles: B4C particles are mixed with 304 stainless steel, Fe or Al powder by ball milling in liquid argon, the mass fraction of B4C particles is more than 95%, and the grinding ball material is super S280 stainless steel; wherein the B4C particle size is less than 2 μm, the metal powder particle size is 20-30 μm, the ball milling time is more than 24 hours, the ball milling speed is 280-320 r / min, the metal powder is promoted to adhere to the surface of the B4C particles, and the B4C particles and the 304 stainless steel alloy solution are fully wetted, wherein the particles provide a solidification heat transfer ratio of more than 35%; Step (5) The B4C particles after surface modification in step (4) are uniformly mixed with the inert gas Ar in a gas-solid two-phase flow mixing and preheating device and preheated to form a gas-solid two-phase flow, and then enter the injection mechanism (11) in two ways through the two-phase flow pipeline. The 304 stainless steel alloy solution is ejected through the injection mechanism. The inert gas Ar blows the B4C particles into the alloy solution being ejected through the two-phase flow pipeline. The two converge and are deposited on the deposition blank. Step (6) the gas-solid two-phase flow fully disperses the 304 stainless steel alloy solution flowing out of the two pouring ingots of the injection mechanism into uniform droplets in the injection deposition chamber, and completes the deposition blank injection on the deposition plate; After the injection in step (7) is completed, the temperature of the top of the ingot is controlled to be ≤1350°C, and the temperature of the bottom of the ingot is controlled to be ≤1250°C. After the ingot is taken out, it is sent to a 1180°C forging and holding furnace for 2 hours to equalize the temperature. It is then forged into a square billet for hot rolling. The final rolling thickness is 3.0~12mm. The billet has good plasticity and no edge cracks are generated during forging and hot rolling.

[0005] Furthermore, in the above-mentioned method for preparing thermal neutron shielding steel, the number of pores counted on the solidification end surface in the spray deposition chamber is ≤5 / 10mm 2 , and the pore size is ≤1μm, and all the pores are pressed together after forging in step (7).

[0006] Furthermore, in the above-mentioned method for preparing a thermal neutron shielding steel, a gas-solid two-phase flow is used to drive the liquid phase, and the solid phase exists simultaneously as a coolant, a neutron adsorption functional phase and a strengthening phase. After hot rolling in step (7), the mass fraction of B4C in the hot-rolled plate matrix is ​​greater than 3%, the B4C particle size is controlled to be 800-1200nm, and is evenly distributed on the matrix without grain boundary segregation phenomenon. The matrix yield strength is greater than 350MPa, the tensile strength is greater than 700MPa, and the room temperature elongation is greater than 25%.

[0007] Furthermore, in the above-mentioned method for preparing thermal neutron shielding steel, the preheating temperature of the surface-modified B4C particles in step (5) is ≥600°C, and the powder feeding rate of the B4C ceramic particles with a particle size of 500~1200nm is 300~1250g / min.

[0008] Furthermore, in the above-mentioned method for preparing thermal neutron shielding steel, in step (1), two nozzles are horizontally arranged at the lower edge of the tundish, the nozzle diameter is φ4~10mm, the single nozzle flow rate is 20~60kg / min, the nozzle outlet flow rate is 1.0~1.8m / min, the nozzle position is opposite to the 1 / 3 and 2 / 3 positions of the deposited billet radius, and the outer nozzle aperture area is 20% larger than the inner aperture.

[0009] Furthermore, in the above-mentioned method for preparing a thermal neutron shielding steel, the number of openings of the jet deposition ring of the jet mechanism in step (5) is 12 to 20, the aperture is 4 to 8 mm, and the focal length is 60 to 80 mm below the normal plane; the jet deposition ring is made of heat-resistant stainless steel, and the inner surface is plated with hard chrome, HRC52 or above; the inclination angle of the jet deposition ring is -30 to +30°, and the scanning frequency is 0 to 2 Hz; the gas injection pressure is 0.4 to 0.7 MPa, and the gas injection volume of a single jet deposition ring is 15 to 45 m 3 / min, particle loading 500~2500g / min.

[0010] Furthermore, in the above-mentioned method for preparing thermal neutron shielding steel, during the spray deposition process of step (6), the distance between the deposition disk and the lower end of the spray deposition ring is controlled to be 200~450mm, the diameter of the deposition billet is 400~600mm, the rotation speed of the deposition disk is 40~80r / min, the descending speed is 50~120mm / min, and the surface molten pool depth is controlled to be 1~2mm.

[0011] Advantages and beneficial effects of the present invention: (1) The present invention combines the gas-solid two-phase flow dynamics principle and vacuum jet deposition technology to propose a method for preparing thermal neutron shielding steel materials. The method utilizes the gas-solid two-phase flow driving mechanism under negative pressure to achieve efficient jet forming of B4C / 304 stainless steel composite materials. In this process, the B4C ceramic particles are evenly dispersed and embedded in the 304 stainless steel matrix. Thanks to the strong driving force of the gas-solid two-phase flow, a strong interface bond is formed between the B4C particles and the matrix. This unique process not only promotes the rapid solidification of the alloy, but also significantly improves the mechanical properties, corrosion resistance and high temperature resistance of the material, ensuring that it can still maintain excellent shielding effectiveness and stability when facing a high radiation environment.

[0012] (2) Since the deposition process in a vacuum environment reduces material oxidation and contamination, the subsequent processing costs are reduced. This process combines gas atomization with solid medium particles to form a gas-solid two-phase mixed flow, which greatly improves the atomization efficiency, refines the powder particle size, optimizes the distribution of the reinforcing phase, and opens up a new way for the preparation of high-performance thermal neutron shielding steel composite materials. This is of great significance to the demand for high-performance materials in my country's nuclear industry, military industry, aerospace and other fields.

[0013] (3) The present invention adopts a self-designed vacuum chamber structure and successfully performs spray deposition in a negative pressure environment of ≤5000Pa (≤0.05 bar). B4C particles act as a cooling medium to improve the heat absorption capacity of the mixed medium, reduce the airflow input and flow rate, and quickly discharge the bubbles in the micro-molten pool under negative pressure, eliminating most of the solidification shrinkage cavities and looseness. The remaining part is also a negative pressure vacuum bubble, which is convenient for forging or extrusion pressing.

[0014] (4) During the spray forming process of the present invention, the molten droplets are rapidly cooled under high-speed airflow, and the cooling rate can reach 102~103K / s, which is a rapid solidification, much higher than powder metallurgy, casting and other technologies. This rapid solidification characteristic, on the one hand, makes the grain refinement play a role in fine grain strengthening; on the other hand, it makes B4C as a reinforcing phase uniformly distributed in the matrix, and the B4C particles act as second phase particles to hinder dislocation movement, thereby improving the mechanical properties of the material.

[0015] (5) The temperature of the formed billet of the present invention is relatively high. The billet can be directly forged or perforated and rolled after high-temperature sampling and uniformization, which can eliminate the slow cooling and heating steps and significantly reduce the heat emission of forming. After the forging and hot rolling process, the B4C / 304 stainless steel composite material makes the B4C mass fraction in the matrix more than 3%, evenly distributed on the matrix, and there is no grain boundary segregation phenomenon. The matrix yield strength is more than 350MPa, the tensile strength is more than 700MPa, and the room temperature elongation is more than 25%. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1This is a schematic diagram of the process for preparing thermal neutron shielding steel according to the present invention; In the figure: 1-high vacuum pump group; 2-control valve a; 3-smelting chamber; 4-smelting ladle; 5-tundish; 6-pouring gate valve; 7-tundish chamber sealing door; 8-tundish chamber; 9-gas-solid two-phase flow mixing preheating device; 10-two-phase flow pipeline; 11-injection mechanism; 12-injection deposition chamber; 13-deposition blank; 14-deposition plate; 15-control valve b; 16-solid phase particle cyclone separator; 17-control valve c; 18-high suction speed vacuum pump group. DETAILED DESCRIPTION

[0017] The schematic diagram of the process for preparing thermal neutron shielding steel according to the present invention is as follows Figure 1 As shown, the specific implementation methods of the present invention are further described in detail below in conjunction with the accompanying drawings and examples of the specification. The following examples are used to illustrate the present invention but cannot be used to limit the scope of the present invention.

[0018] In the following embodiments, a double vacuum chamber closed structure device is used to realize negative pressure environment spraying: the double vacuum chamber structure device is divided into a melting chamber 3, a tundish chamber 8 and a spray deposition chamber 12, the tundish chamber 8 connects the melting chamber 3 and the spray deposition chamber 12, and the connection and isolation are realized by the casting gate valve 6; the melting chamber 3 is provided with a melting ladle 4, and the liftable tundish 5 is preheated to temperature outside the chamber and then placed in the tundish chamber 8; the melting chamber 3 is connected to the high vacuum pump group 1, and the spray deposition chamber 12 is separately provided with a high-speed vacuum pump group 18. Example 1

[0019] In this embodiment, a method for preparing thermal neutron shielding steel includes the following steps: Step (1) Open the control valve a2, close the pouring gate valve 6, and adjust the vacuum degree of the smelting chamber 3 to ≤5×10 -2 Pa, the ambient pressure under the injection gas input condition is ≤5000Pa; two nozzles are arranged horizontally at the lower edge of the tundish 5, the nozzle diameter is φ4mm, the single nozzle flow rate is 20kg / min, the nozzle outlet flow rate is 1.0m / min, the nozzle position is directly opposite to the 1 / 3 and 2 / 3 positions of the deposited billet radius, and the outer nozzle aperture area is 20% larger than the inner aperture; Step (2) performing vacuum medium frequency induction melting of a 304 base alloy in a melting chamber 3, wherein the composition of the 304 base alloy is: Cr17.0%, Ni10%, Mn1%, B0.1%, Ti 0.1%, C 0.04%, N≤10ppm, and the remainder is Fe; the melting amount is 1 t; Step (3) After the smelting of step (2), the superheat of the 304 stainless steel alloy solution is controlled to be +40°C, and the smelting ladle 4 is used for bottom blowing argon deslagging and degassing treatment. The control valve a2 and the tundish cabin sealing door 7 are closed, and then the control valve b15 and the control valve c17 are opened. Then, the pouring gate valve 6 of the smelting cabin 3 and the jet deposition cabin 12 is opened, and the high-speed vacuum pump group 18 is started. The tundish 5 is lowered to a distance of 400 mm from the deposition billet. The smelting ladle 4 controls the flow rate through the bottom slide gate and injects the 304 stainless steel alloy solution into the tundish to stabilize the liquid level at 100 mm. The tundish 5 is preheated to a temperature of 1200°C; Step (4) Surface modification of B4C particles: B4C particles are mixed with 304 stainless steel, Fe or Al powder by ball milling in liquid argon, the mass fraction of B4C particles is 95%, and the grinding ball material is super S280 stainless steel; wherein the B4C particle size is 2 μm, the metal powder particle size is 20 μm, the ball milling time is 24 hours, the ball milling speed is 280 r / min, the metal powder is promoted to adhere to the surface of the B4C particles, and the B4C particles and the 304 stainless steel alloy solution are fully wetted, wherein the particles provide 35% of the solidification heat transfer ratio; Step (5) The surface-modified B4C particles in step (4) are mixed uniformly with the inert gas Ar in the gas-solid two-phase flow preheating device 9 and preheated to form a gas-solid two-phase flow, and are divided into two routes to enter the injection mechanism 11 through the two-phase flow pipeline 10, and the 304 stainless steel alloy solution is sprayed out through the injection mechanism 11. The inert gas Ar blows the B4C particles into the alloy solution being sprayed through the two-phase flow pipeline 10, and the two converge and are deposited on the deposition blank 13; the preheating temperature of the surface-modified B4C particles is 600°C, and the powder feeding rate of 500nm B4C ceramic particles is 300g / min; the number of openings of the injection deposition ring of the injection mechanism 11 is 12, the aperture is 4mm, and the focal length is 60mm below the normal plane; the injection deposition ring is made of heat-resistant stainless steel, and the inner surface is plated with hard chrome, HRC52 or above; the inclination angle of the injection deposition ring is -30°, and the scanning frequency is 0Hz; the gas injection pressure is 0.4MPa, and the injection volume of a single injection deposition ring is 15m 3 / min, particle loading 500g / min; Step (6) The 304 stainless steel alloy solution flowing out of the two pouring ingots of the injection mechanism 11 is fully dispersed into uniform droplets in the injection deposition chamber 12 by gas-solid two-phase flow, and the deposition blank 13 is sprayed on the deposition plate 14; during the injection deposition process, the deposition plate 14 is controlled to be 200 mm away from the lower end of the injection deposition ring, the diameter of the deposition blank 13 is 400 mm, the rotation speed of the deposition plate is 40 r / min, the descending speed is 50 mm / min, and the surface molten pool depth is controlled to be 1 mm; After the injection in step (7), the top temperature of the ingot is controlled at 1350°C and the bottom temperature of the ingot is controlled at 1250°C. After the ingot is taken out, it is sent into a forging and holding furnace at 1180°C for 2 hours, and then forged into a square billet for hot rolling. The final rolling thickness is 3.0 mm. The mass fraction of B4C in the hot-rolled plate matrix is ​​3%. The B4C particle size is controlled at 800 nm and is evenly distributed on the matrix without grain boundary segregation. The matrix yield strength is 350 MPa, the tensile strength is 700 MPa, and the room temperature elongation is 25%. The number of pores on the solidification end surface in the spray deposition chamber is 5 / 10mm 2 , and the pore size is 1 μm, and all the pores are pressed together after forging in step (7). Example 2

[0020] In this embodiment, a method for preparing thermal neutron shielding steel includes the following steps: Step (1) Open the control valve a2, close the pouring gate valve 6, and adjust the vacuum degree of the smelting chamber 3 to ≤5×10 -2 Pa, the ambient pressure under the injection gas input condition is ≤5000Pa; two nozzles are arranged horizontally at the lower edge of the tundish 5, the nozzle diameter is φ6mm, the single nozzle flow rate is 40kg / min, the nozzle outlet flow rate is 1.4m / min, the nozzle position is directly opposite to the 1 / 3 and 2 / 3 positions of the deposited billet radius, and the outer nozzle aperture area is 20% larger than the inner aperture Step (2) performing vacuum medium frequency induction melting of a 304 base alloy in a melting chamber 3, wherein the composition of the 304 base alloy is: Cr15%, Ni12%, Mn2%, B1%, Ti 0.3%, C 0.04%, N≤10ppm, and the remainder is Fe; the melting amount is 2t; Step (3) After the smelting of step (2), the superheat of the 304 stainless steel alloy solution is controlled to be +60°C, and the bottom argon is blown to remove slag and degassing by the melting ladle 4. The control valve a2 and the sealing door 7 of the tundish cabin are closed, and then the control valve b15 and the control valve c17 are opened. Then, the pouring gate valve 6 of the melting cabin 3 and the spray deposition cabin 12 is opened, and the high-speed vacuum pump group 18 is started. The tundish 5 is lowered to a distance of 600 mm from the deposition billet. The melting ladle 4 controls the flow rate through the bottom slide gate and injects the 304 stainless steel alloy solution into the tundish to stabilize the liquid level at 140 mm. The preheating temperature of the tundish 5 is 1300°C. Step (4) Surface modification of B4C particles: B4C particles are mixed with 304 stainless steel, Fe or Al powder by ball milling in liquid argon, the mass fraction of B4C particles is 95%, and the grinding ball material is super S280 stainless steel; wherein, the B4C particle size is 1.5 μm, the metal powder particle size is 25 μm, the ball milling time is 30 hours, the ball milling speed is 300 r / min, the metal powder is promoted to adhere to the surface of the B4C particles, and the B4C particles and the 304 stainless steel alloy solution are fully wetted, wherein the particles provide 35% of the solidification heat transfer ratio; Step (5) The surface-modified B4C particles in step (4) are mixed uniformly with the inert gas Ar in the gas-solid two-phase flow preheating device 9 and preheated to form a gas-solid two-phase flow, and are divided into two routes to enter the injection mechanism 11 through the two-phase flow pipeline 10, and the 304 stainless steel alloy solution is sprayed out through the injection mechanism 11, and the inert gas Ar blows the B4C particles into the alloy solution being sprayed through the two-phase flow pipeline 10, and the two converge and are deposited on the deposition blank 13; the preheating temperature of the surface-modified B4C particles is 800°C, and the powder feeding rate of 800nm ​​B4C ceramic particles is 800g / min; the number of openings of the injection deposition ring of the injection mechanism 11 is 14, the aperture is 6mm, and the focal length is 70mm below the normal plane; the injection deposition ring is made of heat-resistant stainless steel, and the inner surface is plated with hard chrome, HRC52 or above; the inclination angle of the injection deposition ring is 0°, and the scanning frequency is 1Hz; the gas injection pressure is 0.5MPa, and the injection volume of a single injection deposition ring is 30m 3 / min, particle loading 1000g / min; Step (6) The 304 stainless steel alloy solution flowing out of the two pouring ingots of the injection mechanism 11 is fully dispersed into uniform droplets in the injection deposition chamber 12 by gas-solid two-phase flow, and the deposition blank 13 is sprayed on the deposition plate 14; during the injection deposition process, the deposition plate 14 is controlled to be 350 mm away from the lower end of the injection deposition ring, the diameter of the deposition blank 13 is 500 mm, the rotation speed of the deposition plate is 60 r / min, the descending speed is 80 mm / min, and the surface molten pool depth is controlled to be 1.5 mm; After the injection in step (7), the top temperature of the ingot is controlled at 1350°C and the bottom temperature of the ingot is controlled at 1250°C. After the ingot is taken out, it is sent into a forging and holding furnace at 1180°C for 2 hours, and then forged into a square billet for hot rolling. The final rolling thickness is 6.0 mm. The mass fraction of B4C in the hot-rolled plate matrix is ​​3%. The B4C particle size is controlled at 1000 nm and is evenly distributed on the matrix without grain boundary segregation. The matrix yield strength is 350 MPa, the tensile strength is 700 MPa, and the room temperature elongation is 30%; The number of pores on the solidification end surface in the spray deposition chamber is 8 / 10mm 2 , and the pore size is 1 μm, and all the pores are pressed together after forging in step (7). Example 3

[0021] In this embodiment, a method for preparing thermal neutron shielding steel includes the following steps: Step (1) Open the control valve a2, close the pouring gate valve 6, and adjust the vacuum degree of the smelting chamber 3 to ≤5×10 -2 Pa, the ambient pressure under the injection gas input condition is ≤5000Pa; two nozzles are arranged horizontally at the lower edge of the tundish 5, the nozzle diameter is φ10mm, the single nozzle flow rate is 60kg / min, the nozzle outlet flow rate is 1.8m / min, the nozzle position is directly opposite to the 1 / 3 and 2 / 3 positions of the deposited billet radius, and the outer nozzle aperture area is 20% larger than the inner aperture; Step (2) performing vacuum medium frequency induction melting of a 304 matrix alloy in a melting chamber 3, wherein the composition of the 304 matrix alloy is: Cr 20%, Ni 16.0%, Mn 2.5%, B 0.5%, Ti 0.5%, C 0.04%, N ≤ 10ppm, and the remainder is Fe; the melting amount is 10t; Step (3) After the smelting of step (2), the superheat of the 304 stainless steel alloy solution is controlled to be +80°C, and the smelting ladle 4 is used for bottom blowing argon deslagging and degassing treatment. The control valve a2 and the tundish cabin sealing door 7 are closed, and then the control valve b15 and the control valve c17 are opened. Then, the pouring gate valve 6 of the smelting cabin 3 and the spray deposition cabin 12 are opened, and the high-speed vacuum pump group 18 is started. The tundish 5 is lowered to a distance of 800 mm from the deposition billet. The smelting ladle 4 controls the flow rate through the bottom slide gate and injects the 304 stainless steel alloy solution into the tundish to stabilize the liquid level at 180 mm. The tundish 5 is preheated to a temperature of 1400°C. Step (4) Surface modification of B4C particles: B4C particles are ball-milled and mixed with 304 stainless steel, Fe or Al powder in liquid argon, with a mass fraction of B4C particles of 95% and a grinding ball made of super S280 stainless steel; wherein the particle size of B4C is 2 μm and the particle size of metal powder is 30 μm, and the ball milling is performed for 1 hour, followed by a pause of 15 minutes; the ball milling is performed for 1 hour, followed by a pause of 10 minutes; and then the ball milling is performed for 1 hour, followed by a pause of 5 minutes, and the ball milling speed is 320 r / min, to promote the adhesion of metal powder to the surface of B4C particles and to promote the full wetting of B4C particles and 304 stainless steel alloy solution, wherein the particles provide 35% of the solidification heat transfer ratio; Step (5) The surface-modified B4C particles in step (4) are mixed uniformly with the inert gas Ar in the gas-solid two-phase flow preheating device 9 and preheated to form a gas-solid two-phase flow, and are divided into two routes to enter the injection mechanism 11 through the two-phase flow pipeline 10, and the 304 stainless steel alloy solution is sprayed out through the injection mechanism 11, and the inert gas Ar blows the B4C particles into the alloy solution being sprayed through the two-phase flow pipeline 10, and the two converge and are deposited on the deposition blank 13; the preheating temperature of the surface-modified B4C particles is 800°C, and the powder feeding rate of 1200nm B4C ceramic particles is 1250g / min; the number of openings of the injection deposition ring of the injection mechanism 11 is 16, the aperture is 8mm, and the focal length is 80mm below the normal plane; the injection deposition ring is made of heat-resistant stainless steel, and the inner surface is plated with hard chrome, HRC52 or above; the injection deposition ring inclination angle is +30°, and the scanning frequency is 2Hz; the gas injection pressure is 0.7MPa, and the injection volume of a single injection deposition ring is 45m 3 / min, particle loading 2500g / min; Step (6) The 304 stainless steel alloy solution flowing out of the two pouring ingots of the injection mechanism 11 is fully dispersed into uniform droplets in the injection deposition chamber 12 by gas-solid two-phase flow, and the deposition blank 13 is sprayed on the deposition plate 14; during the injection deposition process, the deposition plate 14 is controlled to be 450 mm away from the lower end of the injection deposition ring, the diameter of the deposition blank 13 is 600 mm, the rotation speed of the deposition plate is 80 r / min, the descending speed is 120 mm / min, and the surface molten pool depth is controlled to be 2 mm; After step (7) the spraying is completed, the top temperature of the ingot is controlled at 1350°C and the bottom temperature of the ingot is controlled at 1250°C. After the ingot is taken out, it is sent into a forging and holding furnace at 1180°C for 2 hours and then forged into a square billet for hot rolling. The final rolling thickness is 12.0 mm. The mass fraction of B4C in the hot-rolled plate matrix is ​​3%. The B4C particle size is controlled at 1200 nm and is evenly distributed on the matrix without grain boundary segregation. The matrix yield strength is 350 MPa, the tensile strength is 700 MPa, and the room temperature elongation is 25%. The number of pores on the solidification end surface in the spray deposition chamber is 6 / 10mm 2 , and the pore size is 1 μm, and all the pores are pressed together after forging in step (7).

Claims

1. A method for preparing thermal neutron shielding steel, characterized in that: The following steps are involved: Step (1) adopting a double vacuum chamber closed structure device to realize negative pressure environment spraying: the double vacuum chamber structure device is divided into a melting chamber, a tundish chamber and a spray deposition chamber, the tundish chamber is connected to the melting chamber and the spray deposition chamber, and the connection and isolation are realized by a casting gate valve; the melting chamber is provided with a melting ladle, and the liftable tundish is preheated to temperature outside the chamber and then placed in the tundish chamber; The smelting chamber is connected to a high vacuum pump group, and the vacuum degree of the smelting chamber is ≤5×10 -2 Pa; the jet deposition chamber is equipped with a high-speed vacuum pump group to maintain the ambient pressure ≤5000Pa under the jet gas input condition; Step (2) performing vacuum medium frequency induction melting of a 304 matrix alloy in a melting chamber, wherein the composition of the 304 matrix alloy is: Cr 17.0-20%, Ni 10.0-16.0%, Mn 1.0-2.5%, B 0.1-0.5%, Ti 0.1-0.5%, C < 0.04%, N ≤ 10 ppm, and the remainder is Fe; the melting amount is 1-10 t; Step (3) After the smelting of step (2), the superheat of the 304 stainless steel alloy solution is controlled to be +40-80°C, the sealing door of the tundish cabin is closed, the smelting ladle is subjected to bottom argon blowing for deslagging and degassing, the control valve a is opened, the pouring gate valve is closed, and then the pouring gate valves of the smelting cabin and the jet deposition cabin are opened, the high-speed vacuum pump group is started, the tundish is lowered to a distance of 400-800 mm from the deposition billet, the smelting ladle controls the flow rate through the bottom slide gate and injects the 304 stainless steel alloy solution into the tundish to stabilize the liquid level of 100-180 mm, and the tundish preheating temperature is ≥1200°C; Step (4) Surface modification of B4C particles: B4C particles are mixed with 304 stainless steel, Fe or Al powder by ball milling in liquid argon, the mass fraction of B4C particles is more than 95%, and the grinding ball material is super S280 stainless steel; wherein the B4C particle size is less than 2 μm, the metal powder particle size is 20-30 μm, the ball milling time is more than 24 hours, the ball milling speed is 280-320 r / min, the metal powder is promoted to adhere to the surface of the B4C particles, and the B4C particles and the 304 stainless steel alloy solution are fully wetted, wherein the particles provide a solidification heat transfer ratio of more than 35%; Step (5) The B4C particles surface-modified in step (4) are uniformly mixed with inert gas Ar in a gas-solid two-phase flow mixing and preheating device, and then preheated to form a gas-solid two-phase flow, and are divided into two routes to enter the injection mechanism through the two-phase flow pipeline, and the 304 stainless steel alloy solution is sprayed out through the injection mechanism, and the inert gas Ar blows the B4C particles into the spraying alloy solution through the two-phase flow pipeline, and the two converge and are deposited on the deposition blank; Step (6) the gas-solid two-phase flow fully disperses the 304 stainless steel alloy solution flowing out of the two pouring ingots of the injection mechanism into uniform droplets in the injection deposition chamber, and completes the deposition blank injection on the deposition plate; After the spraying in step (7) is completed, the temperature of the top of the ingot is controlled to be ≤1350°C, and the temperature of the bottom of the ingot is controlled to be ≤1250°C. After the ingot is taken out, it is sent to a 1180°C forging and holding furnace for 2 hours, and then forged into a square billet for hot rolling. The final rolling thickness is 3.0~12mm.

2. The method for preparing thermal neutron shielding steel according to claim 1, characterized in that: The number of pores on the solidification end surface in the spray deposition chamber is ≤5 / 10mm 2 , and the pore size is ≤1μm, and all the pores are pressed together after forging in step (7).

3. A method for preparing thermal neutron shielding steel according to claim 1, characterized in that: After hot rolling in step (7), the mass fraction of B4C in the hot-rolled plate matrix is ​​above 3%, the B4C particle size is controlled to be 800-1200 nm, and the B4C is evenly distributed on the matrix without grain boundary segregation.

4. A method for preparing thermal neutron shielding steel according to claim 1, characterized in that: The preheating temperature of the surface-modified B4C particles in step (5) is ≥600°C, and the powder feeding rate of the B4C ceramic particles with a particle size of 500-1200 nm is 300-1250 g / min.

5. A method for preparing thermal neutron shielding steel according to claim 1, characterized in that: In step (1), two nozzles are arranged horizontally at the lower edge of the tundish, with a nozzle diameter of φ4~10mm, a single nozzle flow rate of 20~60kg / min, a nozzle outlet flow rate of 1.0~1.8m / min, and the nozzle position is opposite to the 1 / 3 and 2 / 3 positions of the deposited billet radius. The outer nozzle aperture area is 20% larger than the inner aperture area.

6. A method for preparing thermal neutron shielding steel according to claim 1, characterized in that: In step (5), the number of openings of the jet deposition ring of the jet mechanism is 12 to 20, the aperture is 4 to 8 mm, and the focal length is 60 to 80 mm below the normal plane; the jet deposition ring is made of heat-resistant stainless steel, and the inner surface is plated with hard chrome, HRC52 or above; the inclination angle of the jet deposition ring is -30 to +30°, and the scanning frequency is 0 to 2 Hz; the gas injection pressure is 0.4 to 0.7 MPa, and the gas injection volume of a single jet deposition ring is 15 to 45 m 3 / min, particle loading 500~2500g / min.

7. A method for preparing thermal neutron shielding steel according to claim 1, characterized in that: During the spray deposition process of step (6), the distance between the deposition disk and the lower end of the spray deposition ring is controlled to be 200-450 mm, the diameter of the deposition billet is 400-600 mm, the rotation speed of the deposition disk is 40-80 r / min, the descending speed is 50-120 mm / min, and the surface molten pool depth is controlled to be 1-2 mm.

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