Economical easy-to-form boron-containing stainless steel for thermal neutron absorption and preparation method thereof

Abstract

The invention discloses economical easy-to-form boron-containing stainless steel for thermal neutron absorption. A preparation method comprises the following steps: (1) component design; (2) molten steel is smelted in a vacuum environment according to the set components, electromagnetic stirring is applied in the smelting process, casting is conducted in a nitrogen protective atmosphere, and a casting blank is obtained; (3) the casting blank is subjected to homogenization heat treatment for 240-720 min between the Ac3 temperature and the boride melting point temperature (Tf), and the casting blank is air-cooled to the room temperature after being discharged out of the furnace; (4) grinding the casting blank subjected to heat treatment to remove casting defects, inclusions and slag foreign matters, and then carrying out thermal deformation and solution treatment; the thermal deformation heating temperature and the solution treatment temperature are respectively 10-80 DEG C and 30-100 DEG C below Tf. Aiming at the characteristics that traditional boron-containing stainless steel for thermal neutron absorption is high in cost and poor in thermoplasticity, through new alloy component design and forming process design, the use of precious metal Ni is reduced, meanwhile, the matrix structure hardness is improved, the deformation coordination of a matrix and boride is enhanced, the thermoplasticity is greatly improved, and the thermal deformation capacity of the material is improved.

Description

Technical Field

[0001] The invention relates to the technical field of steel alloy materials, and in particular to an economical and easily formed boron-containing stainless steel for thermal neutron absorption and a preparation method thereof. Background Art

[0002] Boron-containing stainless steel is an important protective functional material with good thermal neutron absorption capacity in spent fuel storage pools. Due to its relatively harsh service conditions, it needs to have excellent corrosion resistance, high tensile strength, hardness, impact toughness and other comprehensive properties, as well as a certain elongation and weldability, so as to facilitate processing or welding into required parts. Traditional boron-containing stainless steel for thermal neutron absorption is mainly based on modified 304-based austenitic stainless steel, with the reference standard being ASTM A887, forming a series of A and B products with different boron contents and mechanical properties. This type of product contains a high content of precious metal Ni (12~15wt.%). At the same time, as the boron content in the steel increases, a large amount of hard and brittle eutectic borides will precipitate at the grain boundaries, resulting in a sharp drop in thermoplasticity and a very low yield rate. Therefore, for stainless steel with a higher boron content, the material alloy cost and processing cost are relatively high, especially for wide slab materials that are mainly formed by hot rolling.

[0003] Researchers have prepared boron-containing stainless steel of different specifications through some special molding processes, such as powder metallurgy, injection molding, 3D printing, thin strip continuous casting, and stacked rolling, and the thermoplasticity of the material has been improved. However, due to the limitations of the process itself, including equipment, molding technology, production process, etc., as well as the diversified requirements of product specifications, the material manufacturing cost is relatively high, and industrialization is limited, resulting in the industry having to find other alternative materials. Therefore, it is essential to develop a low-cost, easy-to-form boron-containing stainless steel.

[0004] Based on the above research background, the present invention provides an economical and easy-to-form boron-containing stainless steel for thermal neutron absorption. In view of the high cost and poor thermoplasticity of traditional boron-containing stainless steel for thermal neutron absorption, through new alloy composition design and molding process design, the use of precious metal Ni is reduced, while the matrix structure hardness is improved by solid solution strengthening, deformation strengthening, precipitation strengthening and other methods, the deformation coordination between the matrix and hard and brittle boride is enhanced, the thermoplasticity is greatly improved, and the thermal deformation ability of the material is improved. Summary of the invention

[0005] The purpose of the present invention is to provide an economical and easy-to-form boron-containing stainless steel for thermal neutron absorption and a preparation method thereof in view of the above problems.

[0006] The object of the present invention is achieved by providing an economical, easily formed boron-containing stainless steel for thermal neutron absorption, wherein the 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%, and the remainder is Fe and unavoidable impurities.

[0007] A method for preparing an economical and easy-to-form boron-containing stainless steel for thermal neutron absorption comprises the following steps: Step 1: molten steel smelting: preparing smelting raw materials according to the set component requirements of the economical and easy-to-form boron-containing stainless steel for thermal neutron absorption, smelting the smelting raw materials into molten steel under a vacuum environment, applying electromagnetic stirring for 4 to 10 minutes after adding alloy raw materials and during molten steel refining, and casting under a nitrogen protective atmosphere to obtain a cast billet; Step 2: homogenization heat treatment: subjecting the cast billet to a temperature of T = (T Ac3 ~T f ) for homogenization heat treatment, the holding time is 240~720min, and the billet is air-cooled to room temperature after being taken out of the furnace; where T is the heat treatment temperature, unit ℃; T Ac3 is the Ac3 temperature of the ingot, in °C; T f is the melting point of the boride in the ingot, in °C; Step 3: Grinding: remove the casting defects at the head and tail of the ingot after heat treatment, and grind the surface to remove surface inclusions, pits and slag foreign matter; Step 4: Thermal deformation and solution treatment: The ground ingot is placed on T f After keeping the temperature at 10-80℃ for 120-240min, heat deformation is performed to obtain the required shape and size; then, f Solution treatment at 30~100℃ for 60~120min, then water-cooling to room temperature; where T f It is the melting point of boride in the ingot, in °C.

[0008] In the above step 1, the elements such as B, N, C, Nb in the smelting raw materials should be added in the form of alloys. When calculating the added alloy raw materials, the content of other elements and impurities brought into the alloy should be accurately measured to ensure that the actual element content meets the chemical mass percentage composition range of the economical, easy-to-form boron-containing stainless steel for thermal neutron absorption as described in claim 1.

[0009] In the above step 2, the material T Ac3 , T f It can be obtained through thermal analyzer testing. The holding time is the actual duration after the steel plate reaches the set temperature.

[0010] In the above step 3, grinding can be performed by cold machining. After grinding, the surface of the ingot should be free of casting defects and the roughness Ra should not be higher than 8 μm.

[0011] In the above step 4, thermal deformation includes but is not limited to hot rolling, hot forging, hot extrusion, and hot drawing, and 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) optimizing the alloy composition design, proposing to use Mn+N and Nb microalloying to replace the precious metal Ni in boron-containing stainless steel for thermal neutron absorption, thereby reducing material costs and enhancing the deformation coordination of the matrix and the boride, thereby improving the thermoplasticity of the material.

[0013] (2) The present invention organically combines physical metallurgical principles such as solid solution strengthening, deformation strengthening, and precipitation strengthening with processing techniques such as molten steel smelting, heat treatment, and thermal deformation, and proposes a new alloy composition and processing technology, which greatly reduces material manufacturing costs and processing difficulty and can be applied industrially. DETAILED DESCRIPTION

[0014] The present invention optimizes the alloy composition design and proposes to replace the precious metal Ni with Mn+N and Nb microalloying in boron-containing stainless steel for thermal neutron absorption, thereby reducing the material cost while enhancing the deformation coordination of the matrix and the boride and improving the material thermoplasticity. The present invention organically combines the physical metallurgical principles such as solid solution strengthening, deformation strengthening, and precipitation strengthening with processing techniques such as molten steel smelting, heat treatment, and thermal 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.

[0015] The method of the present invention comprises the following steps: 1. Composition design: the chemical composition by mass percentage is: 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 rest is Fe and unavoidable impurities.

[0016] 2. Molten steel smelting: The smelting raw materials are configured according to the set composition, and the molten steel is smelted in a vacuum environment. Electromagnetic stirring is applied during the smelting process, and casting is performed under a nitrogen protective atmosphere to obtain a cast billet.

[0017] 3. Homogenization heat treatment: Heat the ingot at T = (T Ac3 ~T f ) for homogenization heat treatment, the holding time is 240~720min, and the billet is air-cooled to room temperature after being taken out of the furnace. Where, T is the heat treatment temperature, unit ℃; T Ac3 is the Ac3 temperature of the material, in °C; T f is the melting point of the boride in the steel plate, in °C; 4. Grinding: Remove the casting defects at the head and tail of the heat-treated ingot, and grind the surface to remove macro inclusions, slag and other foreign matter.

[0018] 5. Thermal deformation and solution treatment: The ground steel billet is f After keeping the temperature at 10-80℃ for 120-240min, heat deformation is performed to obtain the required shape and size; then, f Solution treatment at 30~100℃ for 60~120min, then water-cooling to room temperature; where T f It is the melting point of boride in steel plate, in ℃.

[0019] In the above step 2, elements such as B, N, C, Nb in the smelting raw materials should be added in the form of alloys. When calculating the added alloy raw materials, the content of other elements and impurities brought into the alloy should be accurately measured to ensure that the actual element content meets the composition range described in step 1.

[0020] In step 3 above, the material’s T Ac3 , T f It can be obtained through thermal analyzer testing. The holding time is the actual duration after the steel plate reaches the set temperature.

[0021] In the above step 4, the grinding can be carried out by cold machining, and the casting defects on the surface of the clean steel billet should be removed so that there is no obvious shrinkage, shrinkage cavity and large inclusions.

[0022] In the above step 5, thermal deformation includes but is not limited to hot rolling, hot forging, hot extrusion, hot drawing, etc., and the holding time is the actual duration after the steel plate reaches the set temperature.

[0023] The smelting raw materials used in the embodiments of the present invention have a clean surface and are free of oxidized impurities, and the allowable deviation of the chemical composition of the finished steel product adopts the standard GB / T222.

[0024] In the embodiment of the present invention, the heat treatment adopts a box-type resistance heating furnace, and the timing starts after the furnace temperature rises to the set temperature. Example 1

[0025] The raw materials are 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%, and the rest are Fe and unavoidable impurities. Among the raw materials, B is obtained as ferroboron alloy, C is T10, Nb is ferroniobium, and N is chromium nitride. After the smelting raw materials are charged into the furnace, they are melted under a vacuum environment. After adding the alloy raw materials and during the refining of the molten steel, electromagnetic stirring is applied for 5 minutes respectively, and casting is carried out under a nitrogen protective atmosphere to obtain a cast billet.

[0026] The thermal analyzer test showed that the T Ac3 and T f They are 840℃ and 1975℃ respectively, so the homogenization heat treatment temperature of the ingot is selected to be 1170℃, and the holding time after reaching the temperature is 600min.

[0027] The heat-treated ingot is processed by cold machining such as sawing, grinding, and milling machines to remove casting defects at the head and tail, and the surface is ground to remove surface inclusions, pits, and slag foreign matter.

[0028] The ground steel billet was kept at 1160°C for 180 min and then hot rolled to obtain a hot-rolled slab with a thickness of 60 mm. It was then solution treated at 1100°C for 90 min and water-cooled to room temperature after being taken out of the furnace. Example 2

[0029] The method is the same as 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%, and the rest is Fe and unavoidable impurities.

[0030] (2) Electromagnetic stirring was applied during the smelting process for 7 min after adding alloy raw materials and during the refining of molten steel. (3) The TAc3 and T f They are 850℃ and 1985℃ respectively. The homogenization heat treatment temperature, thermal deformation heating temperature and solution treatment temperature of the ingot are selected as 1160℃, 1150℃ and 1120℃ respectively.

[0031] The above embodiments have been used to describe the features and effects of the present invention in detail, but are not limited to these embodiments. Without departing from the concept of the present invention, there may be more other equivalent embodiments. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An economical and easily formed boron-containing stainless steel for thermal neutron absorption, characterized in that: Its chemical mass percentage is: 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%, and the rest are Fe and unavoidable impurities.

2. A method for preparing an economical and easily moldable boron-containing stainless steel for thermal neutron absorption, characterized in that: The following steps are involved: Step 1: molten steel smelting: prepare smelting raw materials according to the set composition requirements of economical easy-to-form boron-containing stainless steel for thermal neutron absorption, melt the smelting raw materials into molten steel under vacuum environment, apply electromagnetic stirring for 4 to 10 minutes after adding alloy raw materials and during molten steel refining, and cast under nitrogen protective atmosphere to obtain ingot; Step 2: Homogenization heat treatment: Heat the ingot at T = (T Ac3 ~T f ) for homogenization heat treatment, the holding time is 240~720min, and the billet is air-cooled to room temperature after being taken out of the furnace; where T is the heat treatment temperature, unit ℃; T Ac3 is the Ac3 temperature of the ingot, in °C; T f is the melting point of the boride in the ingot, in °C; Step 3: Grinding: Remove the casting defects at the head and tail of the heat-treated ingot, and grind the surface to remove surface inclusions, pits and slag foreign matter; Step 4: Thermal deformation and solution treatment: The ground ingot is placed on a T f After keeping the temperature at 10-80℃ for 120-240min, heat deformation is performed to obtain the required shape and size; then, f Solution treatment at 30~100℃ for 60~120min, then water-cooling to room temperature; where T f It is the melting point of boride in the ingot, in °C.

3. The method for preparing an economical and easily moldable boron-containing stainless steel for thermal neutron absorption according to claim 2, characterized in that: In the above step 1, the elements such as B, N, C, Nb in the smelting raw materials should be added in the form of alloys. When calculating the added alloy raw materials, the content of other elements and impurities brought into the alloy should be accurately measured to ensure that the actual element content meets the chemical mass percentage composition range of the economical, easy-to-form boron-containing stainless steel for thermal neutron absorption as described in claim 1.

4. The method for preparing an economical and easily moldable boron-containing stainless steel for thermal neutron absorption according to claim 2, characterized in that: In the above step 2, the material T Ac3 , T f It can be obtained through thermal analyzer testing. The holding time is the actual duration after the steel plate reaches the set temperature.

5. The method for preparing an economical and easily moldable boron-containing stainless steel for thermal neutron absorption according to claim 2, characterized in that: In the above step 3, grinding can be performed by cold machining. After grinding, the surface of the ingot should be free of casting defects and the roughness Ra should not be higher than 8 μm.

6. The method for preparing an economical and easily moldable boron-containing stainless steel for thermal neutron absorption according to claim 2, characterized in that: In the above step 4, thermal deformation includes but is not limited to hot rolling, hot forging, hot extrusion, and hot drawing, and the holding time is the actual duration after the steel plate reaches the set temperature.