Methods for controlling the grain corrosion performance and high-temperature strength of thick plates of nitrogen-controlled austenitic stainless steel for nuclear power applications

Abstract

This invention discloses a method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power applications, comprising: controlling the carbon content of raw materials between 0.02% and 0.03%; using a large deformation rolling process to roll the billet into a thick plate during the rolling process; using a chamber furnace solution treatment process in the heat treatment process, controlling the solution temperature at 1020-1080℃, controlling the steel plate temperature entering the furnace to be below 400℃, controlling the heating rate at ≤150℃ / hour when the steel plate temperature in the furnace is below 700℃, controlling the heating rate at ≤200℃ / hour after the steel plate temperature in the furnace reaches 700℃, starting the heat preservation when the steel plate temperature in the furnace reaches the furnace temperature, controlling the heat preservation time to be ≤1min / mm, and rapidly water cooling after exiting the furnace, controlling the interval between the steel plate exiting the furnace and water cooling to be within 2 minutes. This invention achieves control over the grain corrosion performance and high-temperature strength of thick plates of nitrogen-controlled austenitic stainless steel for nuclear power. The pass rates of high-temperature strength and intergranular corrosion performance fully meet the performance requirements of nitrogen-controlled austenitic stainless steel for nuclear power.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel material performance control technology, and more specifically relates to a method for controlling the crystal corrosion performance and high-temperature strength of thick plates of nitrogen-controlled austenitic stainless steel for nuclear power. Background Technology

[0002] Nuclear power is one of the important development directions of the country's emerging energy strategy. With the large-scale construction of nuclear power plants, high requirements have been placed on the localization rate of nuclear power equipment. As raw materials for equipment, steel materials, especially materials for key components, urgently need to be localized.

[0003] Nitrogen-controlled austenitic stainless steel thick plates are mainly used to manufacture reactor internals for nuclear power projects. Reactor internals are the heart of the nuclear island in pressurized water reactor nuclear power plants, supporting and fixing the core assemblies within the reactor pressure vessel. Their working environment is extremely harsh, operating under high temperature, high pressure, and strong radiation, and enduring long-term coolant erosion and vibrations. Therefore, the design and manufacturing quality of the reactor internals' core support plates directly affect the operational safety of the nuclear power plant. To ensure that reactor internals maintain good performance throughout the design life of the nuclear power plant and guarantee its safe operation, the primary technical key is material quality. Therefore, nitrogen-controlled austenitic stainless steel thick plates for nuclear power, as an important nuclear power material, should not only possess sufficient high-temperature strength, corrosion resistance, and radiation resistance, but also good machinability.

[0004] Due to the lack of effective performance control technology, the intergranular corrosion resistance and high-temperature strength of existing nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants are difficult to meet the technical specifications for reactor internals in nuclear power projects. This has greatly affected the localization process of raw materials for nuclear power equipment and restricted the development of the nuclear power industry. Summary of the Invention

[0005] To address the aforementioned technical problems in the existing technology, this invention provides a method for controlling the grain corrosion performance and high-temperature strength of thick nitrogen-controlled austenitic stainless steel plates for nuclear power plants, comprising:

[0006] (1) The carbon content in the raw materials of nitrogen-controlled austenitic stainless steel thick plates for nuclear power is controlled between 0.02% and 0.03% by mass percentage;

[0007] (2) The rolling process adopts a large deformation rolling process to roll the billet into a thick plate of nitrogen-controlled austenitic stainless steel for nuclear power. In the large deformation rolling process, the single-pass reduction rate of the longitudinal rolling stage is controlled to be >13%.

[0008] (3) The rolled thick plate of nitrogen-controlled austenitic stainless steel for nuclear power is subjected to a chamber furnace solution treatment process in the heat treatment process. The solution temperature is controlled at 1020-1080℃, the temperature of the steel plate entering the furnace is controlled at less than 400℃, the heating rate is controlled at ≤150℃ / hour when the temperature of the steel plate in the furnace is below 700℃, and the heating rate is controlled at ≤200℃ / hour after the temperature of the steel plate in the furnace reaches 700℃. When the temperature of the steel plate in the furnace reaches the furnace temperature, the heat preservation begins. The heat preservation time is controlled as follows: heat preservation time / steel plate thickness = 1min / mm. After exiting the furnace, the plate is quickly water-cooled. The water flow of the quenching machine is turned up to the maximum. The interval between the steel plate exiting the furnace and water cooling is controlled within 2 minutes.

[0009] As a specific implementation method, in the above-mentioned method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power, the solution treatment temperature is controlled at 1060±10℃.

[0010] As one specific implementation method, in the above-mentioned method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power, the thickness specification of the nitrogen-controlled austenitic stainless steel thick plates for nuclear power is ≥50mm.

[0011] The present invention provides a method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants. This method, through control of raw material carbon content, large deformation rolling process, and solution treatment process, solves the current technical difficulties in controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants. It achieves control over these performance characteristics, ensuring that the high-temperature strength fully meets the specified mechanical property requirements for nitrogen-controlled austenitic stainless steel for nuclear power plants, and that the intergranular corrosion performance pass rate reaches 100%. This method lays the foundation for the localization of reactor internals materials in nuclear power projects and fills the technical gap in controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1a The image shows a metallographic diagram of a thick plate of nitrogen-controlled austenitic stainless steel for nuclear power plants, which is rolled using conventional small deformation methods.

[0014] Figure 1bThe image shows a metallographic diagram of a nitrogen-controlled austenitic stainless steel thick plate for nuclear power plant, rolled with large deformation using the method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plate for nuclear power plant according to the present invention.

[0015] Figure 2 This is a schematic diagram showing the effect of heat treatment temperature on the yield strength and tensile strength of steel plates for the same solution treatment time;

[0016] Figure 3 The curves showing the variation of the yield strength of steel plates under different solution temperatures and solutions times are presented.

[0017] Figure 4 The curves showing the variation of tensile strength of steel plates under different solution temperatures and solutions times are presented. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0019] In the method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants of the present invention, the term "thick plate" refers to a steel plate with a thickness specification of ≥50mm.

[0020] The carbon content (C) of nitrogen-controlled austenitic stainless steel for nuclear power plants is required to be ≤0.035% by mass percentage. The specified mechanical properties are: room temperature yield strength Rp0.2 ≥ 210 MPa, room temperature tensile strength Rm ≥ 520 MPa, 350℃ high-temperature yield strength Rp0.2 ≥ 135 MPa, and 350℃ high-temperature tensile strength Rm ≥ 294 MPa. The intergranular corrosion tolerance of nitrogen-controlled austenitic stainless steel for nuclear power plants must reach 100%. As is known in the art, the main challenges in controlling the intergranular corrosion performance and high-temperature strength of thick plates of nitrogen-controlled austenitic stainless steel for nuclear power plants include:

[0021] I. In order to ensure that the thick plates of nitrogen-controlled austenitic stainless steel for nuclear power plants have both good resistance to intergranular corrosion and high-temperature strength, it is necessary to reasonably control the carbon content in the raw materials. If the carbon content control range is narrow, the smelting difficulty is high; if the carbon content control range is wide, the resistance to intergranular corrosion and high-temperature strength cannot be guaranteed.

[0022] II. Due to the limited design of billet thickness and small compression ratio, thick plates of nitrogen-controlled austenitic stainless steel for nuclear power with a thickness of ≥50mm have a low qualification rate in meeting the specified mechanical property requirements for nitrogen-controlled austenitic stainless steel for nuclear power.

[0023] 3. For thick austenitic stainless steel plates with a thickness of ≥50mm used in nuclear power plants, improper heat treatment processes can cause uneven microstructure (uneven grains), which in turn affects the intergranular corrosion resistance of the thick austenitic stainless steel plates used in nuclear power plants.

[0024] Therefore, the method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants of the present invention generally achieves the control of crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants through the following technical means: (1) controlling the carbon content in the raw materials of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants, (2) using a large deformation rolling process in the rolling process, and (3) using a chamber furnace solution treatment process in the heat treatment process. Specifically, the method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants of the present invention includes:

[0025] (1) Control of carbon content in nitrogen-controlled austenitic stainless steel thick plates for nuclear power applications

[0026] The carbon content in the raw materials of nitrogen-controlled austenitic stainless steel thick plates for nuclear power is controlled between 0.02% and 0.03% by mass percentage. The principle and optimization design research results are as follows:

[0027] The specified values ​​for the chemical composition of nitrogen-controlled austenitic stainless steel for nuclear power are shown in Table 1, and the specified values ​​for its mechanical properties are shown in Table 2.

[0028] Table 1 Chemical composition (mass percentage, %)

[0029] Specified value ≤0.035 ≤1.0 ≤2.0 ≤0.03 ≤0.015 control value 0.02～0.028 0.4～0.8 1.5～2.0 ≤0.03 ≤0.01 Cr Ni N Co Specified value 18.5～20.0 9.0～10.0 ≤0.08 ≤0.2 control value 19.0～20.0 9.5～10.0 0.06～0.08 ≤0.05

[0030] Table 2 Mechanical Properties

[0031] room temperature ≥210 ≥520 350℃ ≥135 ≥294

[0032] To improve the strength of steel, the upper limit of carbon content needs to be controlled. However, to ensure excellent resistance to intergranular corrosion and stress corrosion cracking, the carbon content needs to be minimized. Therefore, controlling the carbon content presents a contradiction between ensuring resistance to intergranular corrosion and ensuring mechanical strength. Table 3 shows the actual test results of intergranular corrosion and mechanical properties of nitrogen-controlled austenitic stainless steel for nuclear power applications under different carbon contents.

[0033] Table 3. Intergranular corrosion and mechanical properties at different carbon contents

[0034]

[0035] It can be seen that, with other elements having a certain content, when the carbon content is ≤0.02%, the mechanical properties of the steel plate at 350℃ are unqualified (the yield strength Rp0.2 at 350℃ is 120~133MPa, which does not meet the specified value requirement of ≥135MPa). When the carbon content is above 0.03%, the intergranular corrosion test of the steel plate is unqualified (the intergranular corrosion pass rate is 65%, which does not meet the requirement of 100% intergranular corrosion pass rate).

[0036] Therefore, in the method for controlling the grain corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants according to the present invention, the carbon content in the raw materials of the nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants is controlled by mass percentage between 0.02% and 0.03%, which can ensure the requirements of grain corrosion performance and high-temperature strength of the nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants, and achieve a reasonable match between the two. That is, when the carbon content is between 0.02% and 0.03%, the intergranular corrosion qualification rate of the nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants reaches 100%, the yield strength Rp0.2 at 350℃ is 140-155MPa, which meets the specified value requirement of ≥135MPa, and the tensile strength Rm at 350℃ is 420-435MPa, which meets the specified value requirement of ≥294MPa.

[0037] (2) Large deformation rolling process is adopted in the rolling process.

[0038] A large deformation rolling process is used to roll the billet into thick plates of nitrogen-controlled austenitic stainless steel for nuclear power. In this large deformation rolling process, the single-pass reduction rate in the longitudinal rolling stage is controlled to be >13%.

[0039] The principles and optimization design results of the above-mentioned large deformation rolling process in the rolling process are as follows:

[0040] To investigate the effect of deformation on grain size, the same billet was rolled into steel plates of the same thickness using different passes with varying deformation amounts. The first group was rolled using conventional small deformation, completed in 13 passes (rolling schedule shown in Table 4). The second group was rolled using large deformation, completed in 9 passes (rolling schedule shown in Table 5). Subsequently, all plates underwent solution treatment at the same furnace temperature of 1070℃ and a holding time of 1 min / mm. The microstructure and mechanical properties of the steel plates were compared experimentally. The results of the mechanical property tests are shown in Table 6, and the results of the microstructure tests are shown in Table 7. Metallographic images obtained from the microstructure tests are shown in [Table 6]. Figure 1a (Small deformation rolling) and Figure 1b (Large deformation rolling)

[0041] Table 4 Rolling Procedure for Small Deformation

[0042]

[0043] Table 5 Rolling Reduction Procedure for Large Deformation Amount

[0044]

[0045]

[0046] Table 6 Comparison of Mechanical Performance Tests

[0047]

[0048] Table 7 Comparison of Microstructure Experiments

[0049] Small deformation 65 3 Large deformation 65 5.5

[0050] Comparisons of mechanical property tests and microstructure tests show that steel plates rolled with large deformation exhibit significantly superior mechanical properties compared to those rolled with small deformation. Steel plates rolled with small deformation have coarser grains, with a grain size of only grade 3, while steel plates rolled with large deformation have finer grains, reaching a grain size of grade 5.5. Therefore, in the method for controlling the grain corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants in this invention, a large deformation rolling process is used to roll the billet into thick nitrogen-controlled austenitic stainless steel plates for nuclear power plants. In the longitudinal rolling stage of this large deformation rolling process, the single-pass reduction rate is controlled to be >13%. By employing a large deformation rolling process in the rolling process, the cast grains can be fully broken down, thereby eliminating mixed grains and coarse grains, achieving the goal of refining the grains and improving high-temperature strength.

[0051] (3) The heat treatment process adopts chamber furnace solution treatment process.

[0052] The rolled thick austenitic stainless steel plates for nuclear power use are subjected to a chamber furnace solution treatment process in the heat treatment process. The solution temperature is controlled at 1020-1080℃, the temperature of the steel plate entering the furnace is controlled at below 400℃, the heating rate is controlled at ≤150℃ / hour when the temperature of the steel plate in the furnace is below 700℃, and the heating rate is controlled at ≤200℃ / hour after the temperature of the steel plate in the furnace reaches 700℃. When the temperature of the steel plate in the furnace reaches the furnace temperature, the heat preservation begins, and the heat preservation time is controlled at: heat preservation time / steel plate thickness = 1min / mm. After exiting the furnace, the plates are rapidly water-cooled with the water flow of the quenching machine turned up to the maximum. The interval between the steel plate exiting the furnace and water cooling is controlled within 2 minutes.

[0053] In one specific implementation, the solution temperature in the chamber furnace solution treatment process is controlled at 1060±10℃.

[0054] The principles and optimization design results of the above-mentioned chamber furnace solution treatment process in the heat treatment process are as follows:

[0055] Optimization of heat treatment solution temperature: With the same solution time, as the solution temperature increases, the yield strength Rp0.2 at 350℃ and the tensile strength Rm at 350℃ of the steel plate show a certain decreasing trend. (See [reference needed]) Figure 2 According to the metallographic structure of the steel plate, when the solution solution temperature is in the range of 1020 to 1080℃, the grain recovery is sufficient. However, when the solution solution temperature exceeds 1080℃, grain coarsening is not significant. Furthermore, only when the solution solution temperature is above 1020℃ can the intergranular corrosion pass rate reach 100%. Therefore, in the method for controlling the grain corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants of the present invention, the solution solution temperature is controlled at 1020 to 1080℃, and preferably at 1060 ± 10℃.

[0056] Optimization of heat treatment holding time: Tests were conducted on the yield strength and tensile strength of steel plates at different solution treatment temperatures and holding times. Figure 3 The curves showing the variation of the yield strength Rp0.2 of the steel plate under different solution treatment temperatures and different holding times are presented. Figure 4 The curves showing the variation of the tensile strength Rm of the steel plate under different solution treatment temperatures and different holding times are presented. Figure 3 and Figure 4 It can be seen that with the extension of the holding time, the yield strength Rp0.2 and tensile strength Rm of the steel plate at 350℃ also show a certain decreasing trend. According to the metallographic structure of the steel plate, the grain size difference at different solution treatment temperatures for holding times of 10 min (experimental steel plate thickness 10 mm × 1 min / mm) and 30 min (experimental steel plate thickness 10 mm × 3 min / mm) is grade 0.5. The decrease in yield strength and tensile strength is also attributed to grain coarsening. Therefore, in the method for controlling the grain corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants of the present invention, the holding time is controlled according to the start of timing at the temperature: holding time / steel plate thickness = 1 min / mm.

[0057] Optimization of Heating Rate in Heat Treatment: The main effect of heating rate on the mechanical properties of steel plates at 350℃ lies in its influence on the uniformity of the steel plate's microstructure. Studies have shown that a higher heating rate results in greater inconsistency between the grain recovery and growth on the steel plate's surface and in the center. The grains on the steel plate surface are significantly coarser than those in the normal microstructure, and since intergranular corrosion test samples are taken from the steel plate surface, this increases the steel plate's susceptibility to intergranular corrosion. Excessively fast heating rates also increase the temperature difference between the furnace temperature and the steel plate, causing the furnace temperature to reach the solution treatment temperature while the steel plate temperature remains below it. Therefore, rationally controlling the furnace inlet temperature and heating rate can reduce the grain size difference between the steel plate's surface and the 1 / 4 thickness region. Table 8 shows the experimental results for the grain size at the steel plate's surface and 1 / 4 thickness region under two conditions: no control of furnace inlet temperature and heating rate, and control of furnace inlet temperature and heating rate.

[0058] Table 8. The Influence of Furnace Temperature and Heating Rate Control on Grain Uniformity of Steel Plates

[0059]

[0060] Therefore, in the method for controlling the grain corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants of the present invention, the furnace inlet temperature and heating rate are controlled to make the grains uniform on the surface of the steel plate and at 1 / 4 of the plate thickness. The specific control method is as follows: the furnace inlet temperature of the steel plate is controlled to be below 400°C, the furnace temperature is controlled to be 1060±10°C, the heating rate is controlled to be ≤150°C / hour when the temperature of the steel plate in the furnace is below 700°C, and the heating rate is controlled to be ≤200°C / hour after the temperature of the steel plate in the furnace reaches 700°C.

[0061] In summary, this invention addresses the specific performance requirements of nitrogen-controlled austenitic stainless steel thick plates for nuclear power applications. By studying the influence of hot rolling deformation on grain size, it refines the grains and improves high-temperature performance. Optimizing the hot rolling process, specifically by fully utilizing the capacity of a four-high mill and employing large deformation rolling at the beginning of rolling, effectively breaks down the cast grains, avoiding mixed grains and coarse grains. This lays a solid foundation for subsequent solution heat treatment, intergranular corrosion prevention, and high-temperature strength assurance. Furthermore, by considering the formation mechanism of intergranular corrosion in austenitic stainless steel, it identifies numerous factors influencing intergranular corrosion performance in process design and heat treatment. Through reasonable control of carbon content and the development of a scientific heat treatment regime, a good balance between high-temperature strength and intergranular corrosion resistance is achieved in nitrogen-controlled austenitic stainless steel thick plates.

[0062] Compared with existing technologies, the method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants in this invention solves the technical difficulties in controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants by controlling the carbon content of raw materials, large deformation rolling process, and solution treatment process. It achieves control over the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants. Its high-temperature strength fully meets the mechanical property requirements of nitrogen-controlled austenitic stainless steel for nuclear power plants, and its intergranular corrosion performance qualification rate reaches 100%. The method for controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants in this invention lays the foundation for the localization of reactor internals materials for nuclear power projects and fills the technical gap in controlling the crystal corrosion performance and high-temperature strength of nitrogen-controlled austenitic stainless steel thick plates for nuclear power plants.

[0063] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A method for controlling the grain corrosion performance and high-temperature strength of thick nitrogen-controlled austenitic stainless steel plates for nuclear power plants, characterized in that... include: (1) The carbon content in the raw materials of nitrogen-controlled austenitic stainless steel thick plates for nuclear power is controlled between 0.02% and 0.03% by mass percentage; (2) The rolling process adopts a large deformation rolling process to roll the billet into a thick plate of nitrogen-controlled austenitic stainless steel for nuclear power. In the large deformation rolling process, the single-pass reduction rate in the longitudinal rolling stage is controlled to be >13%. (3) The nitrogen-controlled austenitic stainless steel thick plate for nuclear power obtained by rolling adopts a chamber furnace solution treatment process in the heat treatment process. The solution temperature is controlled at 1060±10℃, the temperature of the steel plate entering the furnace is controlled at less than 400℃, the heating rate is controlled at ≤150℃ / hour when the temperature of the steel plate in the furnace is below 700℃, and the heating rate is controlled at ≤200℃ / hour after the temperature of the steel plate in the furnace reaches 700℃. When the temperature of the steel plate in the furnace reaches the furnace temperature, the heat preservation begins. The heat preservation time is controlled as follows: heat preservation time / steel plate thickness = 1min / mm. After exiting the furnace, it is quickly water-cooled. The water flow of the quenching machine is turned up to the maximum. The interval between the steel plate exiting the furnace and water cooling is controlled within 2 minutes, so that the grains on the surface of the steel plate and at 1 / 4 of the plate thickness are uniform. The thickness specification of the nitrogen-controlled austenitic stainless steel thick plate for nuclear power is ≥50mm.

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