Method for producing a boron stainless steel sheet for thermal neutron absorption

Abstract

The present application relates to a kind of boron stainless steel sheet for thermal neutron absorption preparation method, comprising the following process steps: (1) induction melting; (2) atomization and deposition; (3) forging; (4) hot rolling; (5) profile processing.The present application is through innovative process route, reasonable melting, plate processing deformation and heat treatment process, significantly improve the microstructure of material, refine hard and brittle boride particles, and then improve the hot workability of material and the mechanical properties of finished plate, meet the higher requirements of boron stainless steel sheet for thermal neutron absorption.According to the process manufactured by the present application, the performance of the plate is similar to powder metallurgy boron stainless steel, also has excellent comprehensive performance, and the process flow is shorter, and the manufacturing cost is also lower.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials technology, and mainly relates to a method for preparing a boron stainless steel sheet for thermal neutron absorption. Background Technology

[0002] With the rapid development of my country's nuclear power industry, the amount of spent fuel generated by nuclear power plants is increasing year by year, and the safe storage and transportation of spent fuel has attracted widespread attention. Neutron absorbing materials, as key functional materials, play a crucial role in ensuring that spent fuel remains in a subcritical state. Based on boron's excellent thermal neutron absorption properties, boron-containing stainless steel is a commonly used neutron absorbing material, possessing good mechanical properties, structural stability, corrosion resistance, and thermal neutron absorption performance, and has broad application prospects in the field of spent fuel storage and transportation.

[0003] To ensure spent fuel remains in a subcritical state, nuclear power plants are increasingly adopting high-density or extremely high-density storage methods for spent fuel. This has led to increasingly thinner boron-containing stainless steel with increasingly higher boron content. However, boron has very low solid solubility in steel, resulting in the formation of a large amount of low-melting-point boride eutectic in high-boron stainless steel. If traditional smelting / forging processes are used, the slow cooling and solidification of molten steel during the process limits the formation of coarse, hard, and brittle boride structures during solidification. These structures segregate at grain boundaries in a network distribution, severely disrupting the matrix and significantly weakening grain boundary strength. This leads to a sharp decrease in the material's ductility and toughness, significantly deteriorating the steel's hot workability, and causing severe edge cracking in the steel plate. Consequently, the manufacturing of thin boron-containing stainless steel plates is difficult, has a low yield, and will significantly increase manufacturing costs.

[0004] To improve the ductility and toughness of boron-containing stainless steel and enhance its hot workability, Carpenter Corporation in the United States employs pre-alloyed, gas atomization powder metallurgy technology to produce boron-containing stainless steel. This type of boron-containing stainless steel exhibits greater ductility than traditionally cast boron-containing stainless steel, with reduced manufacturing difficulty and increased yield. This is because powder metallurgy boron stainless steel avoids the formation of coarse boride eutectic structures and their continuous network distribution along grain boundaries. The borides in the steel are small and uniformly distributed, and the austenite matrix remains continuous without being interrupted by the boride structure. However, the powder metallurgy process is complex, lengthy, and involves high production costs and energy consumption, resulting in expensive products and limiting the widespread application of powder metallurgy boron stainless steel.

[0005] In summary, improving the hot workability of materials with lower processing costs is a key technical challenge in the preparation of boron-containing stainless steel. Spray forming offers a solution to this problem. Developed from powder metallurgy, spray forming is an advanced rapid solidification technology. Compared to powder metallurgy, it combines liquid metal atomization (rapid solidification) with the deposition of atomized droplets, completing the metallurgical forming process in one step. This results in a shorter process, higher efficiency, and the potential to solve the problems of boron phase segregation and aggregation in traditional casting and forging processes, as well as the increased costs associated with the long processes in powder metallurgy. Summary of the Invention

[0006] To address the challenges of poor hot workability of existing cast and forged boron stainless steel and high costs resulting from the long process flow of powder metallurgy boron stainless steel thin plates, this invention provides a method for preparing boron stainless steel thin plates for thermal neutron absorption. Through an induction melting + spray forming + annealing homogenization process, boron stainless steel spray ingots with fine and uniformly distributed borides are prepared. Based on a processing technology of forging into billets + hot rolling into plates + solution treatment, while ensuring that the billet does not crack, the deformation of the boron stainless steel slab is increased, and the number of hot working processes is reduced. This allows for the refinement and dispersion of the boride microstructure in the finished thin plate, resulting in a boron stainless steel thin plate with excellent comprehensive performance for thermal neutron absorption.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0008] A method for preparing a boron stainless steel sheet for thermal neutron absorption includes the following steps:

[0009] (1) Induction melting (electric arc melting): 304 stainless steel, ferroborone, ferrochrome, ferronickel and other raw materials are proportioned according to requirements and induction melting is carried out to obtain molten steel that meets the composition requirements.

[0010] (2) Atomization and deposition: The molten steel is poured into the preheated tundish and flows out through the guide pipe into the atomizer. The molten steel is atomized by dual nozzles and nitrogen. The atomized droplets are accelerated by high-pressure nitrogen and deposited on the rotating receiving substrate to form a cylindrical deposition billet.

[0011] (3) Homogenization annealing: The deposited billet is placed in an annealing furnace and heated, heated, held and cooled to obtain a homogenized sprayed billet;

[0012] (4) Forging: The homogenized annealed sprayed ingot is placed in a high-temperature furnace and forged into a thick plate after heating, temperature rise and heat preservation;

[0013] (5) Hot rolling: The thick plate is put back into the high temperature furnace, and after heating, temperature rise and heat preservation, the thin plate of the required thickness is hot rolled out;

[0014] (6) Profile processing: The hot-rolled sheet is solution treated, then straightened and cut to length to obtain the finished sheet.

[0015] The mass fractions of each component in the molten steel in step (1) are as follows: C: 0.01-0.06%, B: 1.50-1.90%, Si≤1.0%, Mn≤2.0%, P≤0.035%, S≤0.030%, Ni: 12.0-15.0%, Cr: 18.0-20.0%, N≤0.10%, Co≤0.20%, with the remainder being Fe and unavoidable impurities.

[0016] In step (2), the molten steel pouring temperature is 1550~1570℃, the atomization pressure is 0.40~0.50MPa, the diameter of the guide tube is 5.0~7.0mm, the substrate rotation speed is 50~80rpm, and the tundish preheating temperature is 900℃~1000℃.

[0017] The homogenization annealing in step (3) has a heating temperature of 800~900℃, a heating rate of 1~5℃ / min, a holding time of 48~144h, and a cooling rate of 1~5℃ / min.

[0018] The forging in step (4) adopts a multi-fire forging process, with the maximum forging reduction in a single fire not exceeding 50%, the heating temperature being 1050~1150℃, the heating rate being 5~10℃, the holding time being 60~90min, and the final forging temperature not lower than 900℃.

[0019] The hot rolling in step (5) adopts a multi-pass hot rolling process, with the maximum hot rolling reduction per pass not exceeding 60%, the heating temperature being 1050~1150℃, the heating rate being 5~10℃, the holding time being 60~90min, and the final rolling temperature not lower than 900℃.

[0020] The solution temperature in step (6) is 1040~1100℃. Let the solution time be t, the plate thickness be h, the unit of t be min, and the unit of h be mm. Then t = (2~4) min / mm×h.

[0021] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0022] 1. This invention adopts a spray forming process of "atomization + deposition" to obtain material properties similar to those of powder metallurgy. However, compared with the powder metallurgy process, the process flow is greatly shortened, reducing the equipment commonly used in powder metallurgy such as screening, mixing, pressing and sintering, thereby effectively reducing the current manufacturing cost of powder metallurgy boron stainless steel.

[0023] 2. This invention effectively overcomes the problems of poor plasticity and forming difficulty of boron-containing stainless steel by combining homogenized annealing and multi-fire forging, making the size and thickness of the finished sheet material controllable and more operable.

[0024] 3. The boron-containing stainless steel sheet prepared by this invention has fine borides that are evenly distributed in the austenitic matrix. Its comprehensive performance is superior to that of high-boron stainless steel prepared by traditional casting method and is close to the level of powder metallurgy boron stainless steel. The effect is very significant.

[0025] 4. The preparation method of this invention requires relatively simple equipment and has relatively low cost. The plate has excellent mechanical properties, and the boride is fine and has good uniform distribution, which meets the design and use requirements of neutron absorbing materials for critical control of spent fuel storage and transportation in nuclear power plants and nuclear radiation protection. Attached Figure Description

[0026] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0027] Figure 1 This is a microstructure diagram of the high-boron stainless steel sheet obtained in Embodiment 1 of the present invention;

[0028] Figure 2 This is a microstructure diagram of the high-boron stainless steel sheet obtained in Example 2 of the present invention;

[0029] Figure 3 This is a microstructure diagram of the high-boron stainless steel sheet obtained in Example 3 of the present invention;

[0030] Figure 4 This is a microstructure diagram of the high-boron stainless steel sheet obtained in Comparative Example 1 of the present invention.

[0031] Figure 5 This is a microstructure diagram of the high-boron stainless steel sheet obtained in Comparative Example 2 of the present invention. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention. Example 1

[0033] The method for preparing the boron stainless steel sheet for thermal neutron absorption of the present invention is carried out in sequence according to the following steps:

[0034] (1) Induction melting (electric arc melting): 304 stainless steel, ferroboron, ferrochrome, ferronickel and other raw materials are mixed according to the required proportions and induction melting is carried out to finally obtain a molten steel with the following chemical composition: C: 0.028%, B: 1.58%, Si: 0.75%, Mn: 1.23%, P: 0.015%, S: 0.010%, Ni: 13.54%, Cr: 18.95%, N: 0.070%, Co: 0.19%, with the remainder being Fe and unavoidable impurities;

[0035] (2) Atomization and deposition: 1570℃ molten steel is poured into a 1000℃ preheated tundish. The molten steel flows out through a Ø6mm guide pipe and enters the atomizer. The molten steel is atomized by dual nozzles and nitrogen. The atomized droplets are accelerated by 0.50MPa nitrogen and deposited on a rotating receiving substrate with a rotation speed of 50rpm to form a cylindrical deposition billet with a diameter of Ø380mm.

[0036] (3) Homogenization annealing: The cylindrical deposited billet with a diameter of Ø380mm is placed in the annealing furnace and heated to 800℃ at a heating rate of 1.5℃ / min, held for 48h, and then cooled to 500℃ at a rate of 1℃ / min, and then cooled with the furnace.

[0037] (4) Forging: The homogenized annealed deposited billet is placed in a high-temperature furnace, heated to 1150℃, heated at a rate of 10℃ / min, and held for 90min before being freely forged into a thick plate with h=80mm. Multi-fire forging is used, with a final forging temperature of 900℃ and a forging reduction of no more than 50% per fire.

[0038] (5) Hot rolling: The forged thick plate is put back into the high temperature furnace and heated to 1150℃ at a heating rate of 10℃ / min. After holding for 60min, it is hot rolled into the required plate with h=6mm. Multi-fire hot rolling is adopted, and the maximum hot rolling reduction per fire does not exceed 60%.

[0039] (6) Profile processing: The hot-rolled plate is subjected to solution treatment at 1040℃ for 30 minutes, and then straightened and cut to length to obtain finished plates with borides uniformly distributed in austenite and a density exceeding 99% TD.

[0040] The microstructure of the plate obtained according to this embodiment is as follows: Figure 1 As shown. From Figure 1 The microstructure diagram shows that the material is nearly fully dense, with fine, dark gray granular borate particles no larger than 10 μm evenly distributed in the austenitic matrix, exhibiting microstructure characteristics similar to high-performance powder metallurgy boron stainless steel. Example 2

[0041] The method for preparing the boron stainless steel sheet for thermal neutron absorption of the present invention is carried out in sequence according to the following steps:

[0042] (1) Induction melting (electric arc melting): 304 stainless steel, ferroboron, ferrochrome, ferronickel and other raw materials are proportioned according to requirements and induction melting is carried out to finally obtain molten steel with the following chemical composition: C: 0.026%, B: 1.71%, Si: 0.65%, Mn: 1.33%, P: 0.018%, S: 0.010%, Ni: 14.05%, Cr: 18.57%, N: 0.060%, Co: 0.18%, with the remainder being Fe and unavoidable impurities;

[0043] (2) Atomization and deposition: 1560℃ molten steel is poured into a tundish preheated to 950℃. The molten steel flows out through a Ø5.8mm guide pipe and enters the atomizer. The molten steel is atomized by dual nozzles and nitrogen. The atomized droplets are accelerated by 0.40MPa nitrogen and deposited on a rotating receiving substrate with a rotation speed of 70rpm to form a cylindrical deposition billet with a Ø380mm diameter.

[0044] (3) Homogenization annealing: The cylindrical deposited billet with a diameter of Ø380mm is placed in an annealing furnace and heated to 850℃ at a heating rate of 2.0℃ / min, held for 72h, and then cooled to 500℃ at a rate of 1.5℃ / min, and then cooled with the furnace.

[0045] (4) Forging: The homogenized annealed deposited billet is placed in a high-temperature furnace, heated to 1100℃, heated at a rate of 10℃ / min, and held for 90min before being freely forged into a thick plate with h=100mm. Multi-fire forging is used, with a final forging temperature of 900℃ and a forging reduction of no more than 50% per fire.

[0046] (5) Hot rolling: The forged thick plate is put back into the high temperature furnace and heated to 1100℃. The heating rate is 10℃ / min. After holding for 60min, it is hot rolled into the required plate with h=3mm. Multi-fire hot rolling is adopted, and the maximum hot rolling reduction in a single fire does not exceed 50%.

[0047] (6) Profile processing: The hot-rolled plate is subjected to solution treatment at 1100℃ for t=25min, and then straightened and cut to length to obtain finished plate with borides uniformly distributed in austenite and a density of more than 99%TD.

[0048] The microstructure of the plate obtained according to this embodiment is as follows: Figure 2 As shown. From Figure 2 The microstructure diagram shows that the material is also nearly fully dense, with fine, dark gray granular borate particles no larger than 10 μm evenly dispersed in the austenitic matrix, exhibiting similar high-performance microstructure characteristics to powder metallurgy boron stainless steel. Example 3

[0049] The method for preparing the boron stainless steel sheet for thermal neutron absorption of the present invention is carried out in sequence according to the following steps:

[0050] (1) Induction melting (electric arc melting): 304 stainless steel, ferroboron, ferrochrome, ferronickel and other raw materials are mixed according to the required proportions and induction melting is carried out to finally obtain a molten steel with the following chemical composition: C: 0.035%, B: 1.86%, Si: 0.85%, Mn: 1.13%, P: 0.014%, S: 0.010%, Ni: 13.82%, Cr: 19.14%, N: 0.070%, Co: 0.20%, with the remainder being Fe and unavoidable impurities;

[0051] (2) Atomization and deposition: 1550℃ molten steel is poured into a tundish preheated to 900℃. The molten steel flows out through a Ø7.0mm guide pipe and enters the atomizer. The molten steel is atomized by dual nozzles and nitrogen. The atomized droplets are accelerated by 0.45MPa nitrogen and deposited on a rotating receiving substrate with a rotation speed of 80rpm to form a cylindrical deposition billet with a Ø380mm diameter.

[0052] (3) Homogenization annealing: The cylindrical deposited billet with a diameter of Ø380mm is placed in an annealing furnace and heated to 850℃ at a heating rate of 3.0℃ / min, held for 144h, and then cooled to 500℃ at a rate of 4.5℃ / min, and then cooled with the furnace.

[0053] (4) Forging: The homogenized annealed deposited billet is placed in a high-temperature furnace, heated to 1050℃, heated at a rate of 5℃ / min, and held for 90min before being freely forged into a thick plate with h=85mm. Multi-fire forging is used, with a final forging temperature of 900℃ and a forging reduction of no more than 50% per fire.

[0054] (5) Hot rolling: The forged thick plate is put back into the high temperature furnace and heated to 1050℃. The heating rate is 5℃ / min. After holding for 60min, it is hot rolled into the required plate with h=2.5mm. Multi-fire hot rolling is adopted, and the maximum hot rolling reduction in a single fire does not exceed 50%.

[0055] (6) Profile processing: The hot-rolled plate is subjected to solution treatment at 1100℃ for t=20min, and then straightened and cut to length to obtain finished plate with borides uniformly distributed in austenite and a density exceeding 99%TD.

[0056] The microstructure of the plate obtained according to this embodiment is as follows: Figure 3 As shown. From Figure 3 The microstructure diagram shows that the material is also nearly fully dense, with fine, dark gray granular borate particles no larger than 10 μm evenly dispersed in the austenitic matrix, exhibiting the same microstructure characteristics as high-performance Grade A boron stainless steel. Comparative Example 1

[0057] Comparative Example 1 uses a melting and casting process. Steps (1), (4) to (6) of Comparative Example 1 are the same as those of Example 1. The difference is in steps (2) and (3).

[0058] The molten steel obtained in step (1) is molded into induction ingots with a specification of Ø300mm by die casting, and then electroslag remelted to obtain boron stainless steel electroslag remelted ingots with a specification of Ø360mm. Then, boron stainless steel plates are obtained by steps (4) to (6) in Example 1.

[0059] Microstructure of a steel plate as shown in Comparative Example 1 Figure 4 As shown, the material is also dense, but the dark gray boride structure is significantly different from that of Example 1. Its borides are mostly elongated strips with large size differences, ranging from about 3 to 20 μm, and there is a relatively obvious segregation phenomenon. It does not have the microstructure characteristics of high-performance Grade A boron stainless steel. Comparative Example 2

[0060] Comparative Example 2 uses powder metallurgy process. Steps (1), (4) to (6) of Comparative Example 2 are the same as those of Example 1. The difference is in steps (2) and (3).

[0061] The molten steel obtained in step (1) is atomized into powder by nitrogen or other inert gas, and boron stainless steel powder passing through 300 mesh is collected by sieving. The powder is then mixed, pressed and sintered to obtain boron stainless steel sintered ingots. The boron stainless steel plates are then obtained by steps (4) to (6) in Example 1.

[0062] The microstructure of the steel plate in Comparative Example 2 is as follows: Figure 5 As shown, the material is dense. Compared with Example 1, its structure has slightly more pores, but the dark gray boride is in the form of finer particles, with a size not exceeding 5μm. It is uniformly dispersed in the austenitic matrix and has obvious microstructure characteristics of Grade A boron stainless steel.

[0063] As can be seen from the above embodiments and comparative examples, the boron-containing stainless steel sheet of the present invention has fine borides that are uniformly distributed in the austenitic matrix, which is significantly better than high-boron stainless steel prepared by traditional casting methods and close to the level of powder metallurgy boron stainless steel. It has good hot workability and mechanical properties, and meets the higher requirements of boron stainless steel thin sheets for thermal neutron absorption.

[0064] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a boron stainless steel sheet for thermal neutron absorption, the method comprising the following steps: (1) Induction melting: 304 stainless steel, ferroborone, ferrochrome and ferronickel raw materials are proportioned according to requirements and induction melting is carried out to obtain molten steel that meets the composition requirements; (2) Atomization and deposition: The molten steel is poured into the preheated tundish and flows out through the guide pipe into the atomizer. The molten steel is atomized by dual nozzles and nitrogen. The atomized droplets are accelerated by high-pressure nitrogen and deposited on the rotating receiving substrate to form a cylindrical deposition billet. (3) Homogenization annealing: The deposited billet is placed in an annealing furnace and heated, heated, held and cooled to obtain a homogenized sprayed billet; (4) Forging: The homogenized annealed sprayed ingot is placed in a high-temperature furnace and forged into a thick plate after heating, temperature rise and holding. (5) Hot rolling: The thick plate is put back into the high temperature furnace, and after heating, temperature rise and heat preservation, the thin plate of the required thickness is hot rolled out; (6) Profile processing: The hot-rolled sheet is solution treated, then straightened and cut to length to obtain the finished sheet; In step (2) atomization and deposition, the steel molten casting temperature is 1550~1570℃, the atomization pressure is 0.40~0.50MPa, the diameter of the guide tube is 5.0~7.0mm, the substrate rotation speed is 50~80rpm, and the tundish preheating temperature is 900℃~1000℃. In step (3) homogenization annealing, the heating temperature is 800~900℃, the heating rate is 1~5℃ / min, the holding time is 48~144h, and the cooling rate is 1~5℃ / min; The forging in step (4) adopts a multi-fire forging process. The maximum forging reduction in a single fire is no more than 50%. The heating temperature is 1050~1150℃, the heating rate is 5~10℃, the holding time is 60~90min, and the final forging temperature is not lower than 900℃. The hot rolling in step (5) adopts a multi-pass hot rolling process, with the maximum hot rolling reduction per pass not exceeding 60%, the heating temperature being 1050~1150℃, the heating rate being 5~10℃, the holding time being 60~90min, and the final rolling temperature not lower than 900℃.

2. The method for preparing a boron stainless steel sheet for thermal neutron absorption according to claim 1, characterized in that... In step (1) induction melting, the mass fraction of each component in the molten steel is as follows: C: 0.01-0.06%, B: 1.50-1.90%, Si≤1.0%, Mn≤2.0%, P≤0.035%, S≤0.030%, Ni: 12.0-15.0%, Cr: 18.0-20.0%, N≤0.10%, Co≤0.20%, with the remainder being Fe and unavoidable impurities.

3. The method for preparing a boron stainless steel sheet for thermal neutron absorption according to claim 1, characterized in that... The solution temperature in step (6) is 1040~1100℃. Let the solution time be t, the plate thickness be h, the unit of t be min, and the unit of h be mm. Then t = (2~4) min / mm×h.

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