Economical easily-formed boron-containing stainless steel for thermal neutron absorption and method of making same

Abstract

The application discloses an economical boron-containing stainless steel for hot neutron absorption, which comprises the following steps: (1) component design; (2) smelting molten steel according to the set component in a vacuum environment, applying electromagnetic stirring in a smelting process, and carrying out casting under a nitrogen protection atmosphere to obtain a casting blank; (3) homogenizing heat treatment of the casting blank for 240-720 min between Ac3 and the melting point of boride (T f ); air cooling to room temperature after discharging; (4) after removing casting defects, inclusions and slag foreign matters by grinding the casting blank after heat treatment, carrying out hot deformation and solid solution treatment; the hot deformation heating temperature and the solid solution treatment temperature are respectively T f 10-80 DEG C and 30-100 DEG C. The application is aimed at the characteristics of high cost and poor hot plasticity of the traditional boron-containing stainless steel for hot neutron absorption, reduces the use of precious metal Ni, improves the hardness of the matrix structure, enhances the deformation coordination of the matrix and boride, greatly improves the hot plasticity, and improves the hot deformation capacity of the material.

Description

Technical Field

[0001] This invention relates to the field of steel alloy materials technology, specifically to an economical, easily formable, boron-containing stainless steel for thermal neutron absorption and its preparation method. Background Technology

[0002] Boron-containing stainless steel is an important protective functional material, exhibiting excellent thermal neutron absorption capabilities in spent fuel storage pools. Due to its harsh service conditions, it requires superior corrosion resistance, high tensile strength, hardness, and impact toughness, along with a certain degree of elongation and weldability for easy processing or welding into the required components. Traditional boron-containing stainless steel for thermal neutron absorption is mainly modified 304-based austenitic stainless steel, referencing ASTM A887, resulting in A-grade and B-grade series products with varying boron contents and mechanical properties. These products contain a high amount of the precious metal Ni (12~15 wt.%), and as the boron content increases, a large amount of hard and brittle eutectic borides precipitates at the grain boundaries, leading to a sharp decrease in thermoplasticity and a very low yield. Therefore, for stainless steels with high boron content, both the alloying and processing costs are relatively high, especially for wide slab materials primarily produced by hot rolling.

[0003] Researchers have developed boron-containing stainless steels of various specifications using specialized forming processes such as powder metallurgy, spray forming, 3D printing, strip casting, and stack rolling, resulting in improved thermoplasticity. However, due to limitations in the processes themselves, including equipment, forming technology, and production procedures, as well as the diverse requirements for product specifications, the manufacturing cost remains high, limiting industrialization and forcing the industry to seek alternative materials. Therefore, developing a low-cost, easily formable boron-containing stainless steel is crucial.

[0004] Based on the above research background, this invention provides an economical, easily formable boron-containing stainless steel for thermal neutron absorption. Addressing the high cost and poor thermoplasticity of traditional boron-containing stainless steel for thermal neutron absorption, this invention utilizes a novel alloy composition and forming process design to reduce the use of the precious metal Ni. Simultaneously, it improves the hardness of the matrix structure through solid solution strengthening, deformation strengthening, and precipitation strengthening, enhancing the deformation compatibility between the matrix and the hard, brittle borides, significantly improving thermoplasticity and enhancing the material's hot deformation capability. Summary of the Invention

[0005] The purpose of this invention is to address the above-mentioned problems by providing an economical, easily formable boron-containing stainless steel for thermal neutron absorption and its preparation method.

[0006] The objective of this invention is achieved as follows: an economical, easily formable, boron-containing stainless steel for thermal neutron absorption, comprising the following chemical mass percentages: B: 1.7~3.5%, Cr: 12.5~17.5%, Ni: 2.5~7.5%, Mn: 4.0~10.0%, Si≤0.50%, C: 0.02~0.1%, Nb≤0.30%, N≤0.30%, (P+S)≤0.05%, with the remainder being Fe and unavoidable impurities.

[0007] A method for preparing an economical, easily formable boron-containing stainless steel for thermal neutron absorption includes the following steps: Step 1: Steel melting: Prepare smelting raw materials according to the set composition requirements of the economical, easily formable boron-containing stainless steel for thermal neutron absorption, melt the smelting raw materials into molten steel under vacuum, and apply electromagnetic stirring for 4-10 minutes after adding alloying materials and during steel refining. Casting is carried out under a nitrogen protective atmosphere to obtain a billet; Step 2: Homogenization heat treatment: Heat the billet at T=(T Ac3 ~T f The billet undergoes homogenization heat treatment, with a holding time of 240~720 min. After being removed from the furnace, the billet is air-cooled to room temperature. In the formula, T is the heat treatment temperature in °C. Ac3 The Ac3 temperature of the cast billet, in °C; T f The value is the melting point of the boride in the billet, in °C; Step 3: Grinding: Remove casting defects at the head and tail of the heat-treated billet, and grind the surface to remove surface inclusions, pits, and slag foreign matter; Step 4: Hot deformation and solution treatment: Heat the ground billet under T... f After holding at 10~80℃ below the required temperature for 120~240 minutes, heat deformation is performed to obtain the desired shape and size; then, at T... f Solution treatment at 30-100℃ below the specified temperature for 60-120 minutes, followed by water cooling to room temperature; where T f The value represents the melting point of the boride in the billet, expressed in °C.

[0008] In step one above, elements such as B, N, C, and Nb in the smelting raw materials should be added in an alloying manner. When calculating the added alloy raw materials, the content of other elements and impurities introduced by the alloy should be accurately measured to ensure that the actual element content conforms to the chemical mass percentage composition range of the economical easy-to-form thermal neutron absorbing boron-containing stainless steel.

[0009] In step two above, the material's T Ac3 T f The holding time can be obtained through testing with a thermal analyzer; it is the actual duration after the steel plate reaches the set temperature.

[0010] In step three above, grinding can be done by cold machining. After grinding, the surface of the billet should be free of casting defects and the roughness Ra should not be higher than 8μm.

[0011] In step four above, hot deformation includes, but is not limited to, hot rolling, hot forging, hot extrusion, and hot drawing. The holding time is the actual duration after the steel plate reaches the set temperature.

[0012] The beneficial effects of the present invention are: (1) Optimize the alloy composition design and propose to replace the precious metal Ni with Mn+N and Nb microalloying in the boron-containing stainless steel for thermal neutron absorption, thereby reducing material costs while enhancing the deformation coordination of the matrix and borides and improving the thermoplasticity of the material.

[0013] (2) This invention organically combines the physical metallurgical principles such as solid solution strengthening, deformation strengthening, and precipitation strengthening with the processing technology such as steel melting, heat treatment, and hot deformation, and proposes a new alloy composition and processing technology, which greatly reduces the material manufacturing cost and processing difficulty, and can be applied industrially. Detailed Implementation

[0014] This invention optimizes alloy composition design, proposing the use of Mn+N and Nb microalloying to replace the precious metal Ni in boron-containing stainless steel for thermal neutron absorption. This reduces material costs while enhancing the deformation compatibility of the matrix and borides, thus improving the material's thermoplasticity. This invention organically combines the principles of physical metallurgy, such as solid solution strengthening, deformation strengthening, and precipitation strengthening, with processing techniques such as steel smelting, heat treatment, and hot deformation. It proposes a new alloy composition and processing technology, significantly reducing material manufacturing costs and processing difficulty, enabling industrial application.

[0015] The method of the present invention includes the following steps: 1. Composition design: The chemical composition by mass percentage is as follows: B: 1.7~3.5%, Cr: 12.5~17.5%, Ni: 2.5~7.5%, Mn: 3.0~10.0%, Si≤0.50%, C: 0.02~0.1%, Nb≤0.30%, N≤0.30%, (P+S)≤0.05%, and the remainder is Fe and unavoidable impurities.

[0016] 2. Steel melting: Prepare smelting raw materials according to the set composition, melt steel in a vacuum environment, apply electromagnetic stirring during the smelting process, and cast in a nitrogen protective atmosphere to obtain a billet.

[0017] 3. Homogenization heat treatment: The cast billet is subjected to heat treatment at T=(T Ac3 ~T f The billet undergoes homogenization heat treatment, holding for 240-720 minutes, and is then air-cooled to room temperature after being removed from the furnace. Where T is the heat treatment temperature in °C; T0 Ac3 The Ac3 temperature of the material, in °C; T f The melting point of the borides in the steel plate is expressed in °C.

[0018] 4. Grinding: Remove casting defects at the head and tail of the heat-treated billet and grind the surface to remove macroscopic inclusions and foreign matter such as slag.

[0019] 5. Hot deformation and solution treatment: The ground steel billet is then subjected to T... f After holding at 10~80℃ below the required temperature for 120~240 minutes, heat deformation is performed to obtain the desired shape and size; then, at T... f Solution treatment at 30-100℃ below the specified temperature for 60-120 minutes, followed by water cooling to room temperature; where T f is the melting point of the borides in the steel plate, in °C.

[0020] In step 2 above, elements such as B, N, C, and Nb in the smelting raw materials should be added in an alloying manner. When calculating the added alloy raw materials, the content of other elements and impurities introduced by the alloy should be accurately measured to ensure that the actual element content conforms to the composition range described in step 1.

[0021] In step 3 above, the material's T Ac3 T f The holding time can be obtained through testing with a thermal analyzer; it is the actual duration after the steel plate reaches the set temperature.

[0022] In step 4 above, grinding can be done by cold machining. The casting defects on the surface of the steel billet should be removed so that there are no obvious shrinkage cavities, shrinkage holes and large inclusions.

[0023] In step 5 above, hot deformation includes, but is not limited to, hot rolling, hot forging, hot extrusion, and hot drawing. The holding time is the actual duration after the steel plate reaches the set temperature.

[0024] The smelting raw materials used in the embodiments of the present invention have clean surfaces and are free of oxidizing impurities. The permissible deviation of the chemical composition of the finished steel product adopts the standard GB / T222.

[0025] In this embodiment of the invention, heat treatment is performed using a box-type resistance heating furnace, and timing begins after the furnace temperature reaches the set temperature. Example 1

[0026] The raw materials were smelted according to the set chemical composition ratio (mass fraction): B: 1.92%, Cr: 16.5%, Ni: 5.5%, Mn: 5.0%, Si: 0.30%, C: 0.02%, Nb: 0.1%, N: 0.1%, (P+S) 0.045%, with the remainder being Fe and unavoidable impurities. In the raw materials, B was obtained as a ferroborone alloy, C was T10, Nb was ferroniobium, and N was chromium nitride. After the raw materials were charged into the furnace, they were smelted under vacuum. Electromagnetic stirring was applied for 5 minutes after the addition of alloying materials and during steel refining. Casting was then carried out under a nitrogen protective atmosphere to obtain a billet.

[0027] The T of the test steel was measured by a thermal analyzer. Ac3 and T f The temperatures are 840℃ and 1975℃ respectively, so the homogenization heat treatment temperature for the billet is selected as 1170℃, and the holding time after reaching the temperature is 600min.

[0028] After heat treatment, the casting billet is processed by cold machining methods such as sawing, grinding, and milling to remove casting defects at the head and tail, and the surface is ground to remove surface inclusions, pits, and slag foreign objects.

[0029] The ground steel billet was held at 1160℃ for 180 minutes and then hot-rolled to obtain a hot-rolled slab with a thickness of 60 mm. It was then solution treated at 1100℃ for 90 minutes and then water-cooled to room temperature after being taken out of the furnace. Example 2

[0030] The method is the same as in Example 1, except that: (1) Chemical composition ratio of smelting raw materials (mass fraction): B: 2.25%, Cr: 17.0%, Ni: 7.0%, Mn: 6.0%, Si: 0.50%, C: 0.05%, Nb: 0.05%, N: 0.08%, (P+S) 0.04%, the remainder being Fe and unavoidable impurities.

[0031] (2) Electromagnetic stirring is applied for 7 minutes during the smelting process after the addition of alloy raw materials and during the refining of molten steel.

[0032] (3) The TAc3 and T of the test steel were tested by a thermal analyzer. f The temperatures are 850℃ and 1985℃ respectively. The homogenization heat treatment temperature, hot deformation heating temperature, and solution treatment temperature of the billet are selected as 1160℃, 1150℃, and 1120℃ respectively.

[0033] The features and effects of the present invention have been described in detail above through embodiments, but the invention is not limited to these embodiments. Many other equivalent embodiments can be implemented without departing from the concept 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. An economical, easily formable boron-containing stainless steel for thermal neutron absorption, characterized in that: Its chemical mass percentages are: B: 1.7~3.5%, Cr: 12.5~17.5%, Ni: 2.5~7.5%, Mn: 4.0~10.0%, Si≤0.50%, C: 0.02~0.1%, Nb≤0.30%, N≤0.30%, (P+S)≤0.05%, with the remainder being Fe and unavoidable impurities; A method for preparing an economical, easily formable boron-containing stainless steel for thermal neutron absorption includes the following steps: Step 1: Steel melting: Prepare the smelting raw materials according to the set composition requirements of the economical easy-to-form thermal neutron absorbing boron-containing stainless steel, melt the smelting raw materials into molten steel in a vacuum environment, apply electromagnetic stirring for 4~10 minutes after adding alloy raw materials and during steel refining, and cast the billet under a nitrogen protective atmosphere. Step 2: Homogenization heat treatment: The cast billet is subjected to a heat treatment at T=(T Ac3 ~T f The billet undergoes homogenization heat treatment, with a holding time of 240~720 min. After being removed from the furnace, the billet is air-cooled to room temperature. In the formula, T is the heat treatment temperature in °C. Ac3 The Ac3 temperature of the cast billet, in °C; T f The melting point of the boride in the billet is given in °C. Step 3: Grinding: Remove casting defects at the head and tail of the heat-treated billet and grind the surface to remove surface inclusions, pits and slag foreign objects; Step 4: Hot deformation and solution treatment: The ground billet is then subjected to T... f After holding at 10~80℃ below the required temperature for 120~240 minutes, heat deformation is performed to obtain the desired shape and size; then, at T... f Solution treatment at 30-100℃ below the specified temperature for 60-120 minutes, followed by water cooling to room temperature; where T f The value represents the melting point of the boride in the cast billet, expressed in °C.

2. The economical, easily formable, boron-containing stainless steel for thermal neutron absorption according to claim 1, characterized in that: In step one above, the B, N, C, and Nb elements in the smelting raw materials should be obtained by adding them in an alloying manner. When calculating the added alloy raw materials, the content of other elements and impurities introduced by the alloy should be accurately measured to ensure that the actual element content conforms to the chemical mass percentage composition range of the economical easy-to-form thermal neutron absorbing boron-containing stainless steel.

3. The economical, easily formable, boron-containing stainless steel for thermal neutron absorption according to claim 1, characterized in that: In step two above, the material's T Ac3 T f The holding time is obtained through testing with a thermal analyzer; it is the actual duration after the steel plate reaches the set temperature.

4. The economical, easily formable, boron-containing stainless steel for thermal neutron absorption according to claim 1, characterized in that: In step three above, the grinding is done by cold machining. After grinding, the surface of the billet should be free of casting defects and the roughness Ra should not be higher than 8μm.

5. The economical, easily formable, boron-containing stainless steel for thermal neutron absorption according to claim 1, characterized in that: In step four above, hot deformation includes, but is not limited to, hot rolling, hot forging, hot extrusion, and hot drawing. The holding time is the actual duration after the steel plate reaches the set temperature.