316 steel plate for fast reactor, and heat treatment method for grain size uniformization control of 316 steel plate for fast reactor

Abstract

The present invention belongs to the field of the heat treatment of steel plates. Disclosed are a 316 steel plate for a fast reactor, and a heat treatment method for grain size uniformization control of a 316 steel plate for a fast reactor. Aimed at the problem of non-uniform distribution of the metallographic structure of existing rolled steel plates in a heat treatment process and the problem of the poor performance of existing rolled steel plates, provided in the present invention is a heat treatment method for grain size uniformization control of a 316 steel plate for a fast reactor. For rolled steel plates with a thickness of 10-85 mm, different solid-solution process steps are used for rolled steel plates with different thicknesses. Before a rolled steel plate is heated to a target temperature, gradient temperatures are set, and heat preservation is performed; and after the rolled steel plate is heated to the target temperature, the heat preservation time is associated with the thickness of the steel plate. In the present invention, a gradient heating process is set to buffer a steel plate, such that an energy gradient can be eliminated before the steel plate is heated to a target temperature, thereby preventing the steel plate from undergoing the phenomena of abnormal grain growth and non-uniform grain distribution in the process of the steel plate being heated to the target temperature; thus, on the basis of ensuring that the mechanical properties of a product meet requirements, uniform grains of grades 4-6 in a full-thickness direction are obtained.

Description

A 316 steel plate for fast reactor and a heat treatment method for controlling its grain size uniformity Technical Field

[0001] The present invention belongs to the technical field of steel plate heat treatment, and more particularly relates to a 316 steel plate for fast reactor and a heat treatment method for controlling grain size homogenization thereof. Background Art

[0002] The 600MW Demonstration Fast Reactor (DFR) is a fourth-generation nuclear power reactor. Due to the unique operating environment, it is constructed extensively of austenitic stainless steel, placing stringent requirements on technical specifications such as chemical composition, purity, corrosion resistance, metallographic structure, and mechanical properties. The 316H stainless steel plates required for the 600MW Demonstration Fast Reactor range from 10mm to 85mm thick and 1400mm to 3400mm wide. The grain size uniformity throughout the thickness must be controlled within the range of 4 to 6 grades, with a grade difference of less than 2 grades. These extremely stringent grain size requirements are particularly stringent. Due to the large amount of deformation during rolling, the metallographic structure exhibits significant anisotropy. Especially for medium and thick plates, the differential deformation distribution can lead to differential dynamic recrystallization or dynamic recovery through the thickness, resulting in a non-uniform as-rolled structure. Conventional solution treatment, involving direct heating to the solution holding temperature, can easily lead to abnormal grain growth and intermixing, failing to meet the grain uniformity assessment requirements for the 600MW Demonstration Fast Reactor project. How to ensure sufficient solid solution of carbides to ensure mechanical properties and corrosion resistance while preventing abnormal grain growth to obtain a uniform structure is a difficulty in manufacturing 316H steel plates for fast reactors.

[0003] For example, Chinese patent application number CN202210977952.3, published on November 11, 2022, discloses a normalized Q345E extra-thick steel plate with core performance and a manufacturing method thereof, with the following components: C: 0.12-0.18%, Si: 0.28-0.5%, Mn: 1.30-1.7%, P≤0.010%, S≤0.008%, Alt: 0.02-0.05%, Nb: 0 0.02-0.03%, V: 0.05-0.07%, Ti: 0.007%-0.02%, Mo: 0.15-0.25%, with the remainder being Fe and residual elements. The present invention utilizes differential temperature rolling during the roughing stage, a post-rolling interval cooling process, and a stepped heating and holding system during the normalizing process, resulting in a more uniform structure through the thickness and improving the core performance of the steel plate, thus meeting the market demand for producing extra-thick Q345E steel plates with guaranteed core performance. However, the patent's disadvantage is that the uniform treatment of extra-thick steel plates fails to ensure a more uniform structure through the thickness of each thickness range.

[0004] Summary of the Invention

[0005] 1. Problems to be solved

[0006] To address the uneven microstructure distribution and poor performance of existing rolled steel plates during heat treatment, the present invention provides a 316 steel plate for fast reactors and a heat treatment method for uniformly controlling its grain size. This method incorporates a gradient heating process to buffer the steel plate, eliminating energy gradients before heating to the target temperature. This prevents abnormal grain growth and uneven distribution during heating. While ensuring the product's mechanical properties meet requirements, the method achieves uniform grain sizes of 4 to 6 throughout the thickness.

[0007] 2. Technical solution

[0008] To solve the above problems, the present invention adopts the following technical solutions.

[0009] A heat treatment method for controlling the grain size uniformity of 316 steel plates for fast reactors is disclosed. Specifically, different solution treatment steps are used for rolled steel plates with thicknesses ranging from 25 to 85 mm. The specific steps are as follows:

[0010] For rolled steel plates with a thickness of 25 to 60 mm, the steel plates are heated to a first gradient temperature after rolling and held at that temperature for 0.5 to 1.5 hours; then heated to a target temperature, held at that temperature, and cooled to a cooling temperature; the holding time after heating to the target temperature corresponds to the thickness of the steel plate, and the number of minutes of the holding time is equal to the number of millimeters of the thickness of the steel plate; the difference between the first temperature gradient and the target temperature is 40°C to 60°C; the first temperature gradient is determined by the thickness of the rolled plate and the grain state of the rolled state; the first temperature gradient is 980 to 1020°C;

[0011] For rolled steel plates with a thickness of 60 to 85 mm, the steel plates are heated to a second gradient temperature after rolling and held at that temperature for 0.5 to 1 hour; then heated to the target temperature, held at that temperature, and cooled to the cooling temperature; the holding time after heating to the target temperature corresponds to the thickness of the steel plate, and the number of minutes of the holding time is equal to the number of millimeters of the thickness of the steel plate; the difference between the second temperature gradient and the target temperature is 80°C to 100°C; the second temperature gradient is determined by the thickness of the rolled plate and the grain state of the rolled state, and the second temperature gradient is 940 to 980°C;

[0012] The target temperature of the steel plate is greater than 1040°C.

[0013] Furthermore, when the steel plate is heated to the target temperature, it is kept warm and then cooled by water cooling until the surface temperature of the steel plate is less than 80°C.

[0014] Furthermore, water cooling is performed until the surface temperature of the steel plate is less than 80°C, and the interval time from leaving the furnace to starting water cooling is controlled to be ≤5 minutes.

[0015] Furthermore, before the solution treatment process is carried out on the steel plate, the final rolling temperature of the steel plate is controlled to be ≥950°, and the rolled steel plate is water-cooled before the solution treatment process is carried out.

[0016] Furthermore, when the steel plate is directly heated to the target temperature or the steel plate is heated from the first gradient temperature to the target temperature or the steel plate is heated from the second gradient temperature to the target temperature, the maximum heating rate of the heating equipment is used to heat the steel plate to the target temperature.

[0017] Furthermore, the target temperature of the rolled steel plate with a thickness of 25 to 85 mm in the solutionizing process is 1040° C. to 1060° C.

[0018] A 316 steel plate for fast reactors is provided using the heat treatment method for controlling the grain size uniformity of the 316 steel plate for fast reactors as described above, wherein the grain size uniformity of the 316 steel plate in the full thickness direction is within the range of 4 to 6, and the full thickness grain size range is less than 2. 3. Beneficial effects

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The present invention provides a gradient heating process for rolled steel plates of different thicknesses during solution heating, thereby buffering the steel plates and eliminating the energy gradient before heating to the target temperature, thereby avoiding abnormal grain growth and uneven distribution during heating to the target temperature. Different gradient temperatures are set for different thicknesses, and different grain size distributions are caused by different thicknesses during rolling. Therefore, different gradient temperatures are selected to eliminate the energy gradient, so that the final steel plates have uniform grain distribution in the thickness direction and meet the requirements. At the same time, a connection is established between the holding time after heating to the target temperature and the thickness of the steel plates, thereby avoiding resource consumption caused by too long a holding time, and failing to obtain uniform grain distribution in the full thickness direction of the steel plates due to too short a holding time. The entire heat treatment method ensures that the steel plates are fully solutionized while avoiding the occurrence of mixed crystals during high-temperature solution. On the basis of ensuring that the mechanical properties of the products meet the requirements, uniform 4-6 grains are obtained in the full thickness direction.

[0021] (2) The steel plate in the present invention is heated to the target temperature and kept at this temperature for a certain period of time before being cooled by water cooling. The water cooling method has a fast cooling speed and can cool a large area of ​​the steel plate at the same time, effectively improving the cooling efficiency and achieving a significant cooling effect at a low cost. At the same time, the temperature of the steel plate after water cooling is controlled at a certain temperature, and the time from being taken out of the furnace to the start of water cooling is controlled to ensure the solid solution strengthening effect.

[0022] (3) The present invention strictly controls the final rolling temperature of the steel plate to avoid the final rolling temperature being too high, which may cause the grains to become coarse and difficult to control and treat in the subsequent solution treatment; and to avoid the final rolling temperature being too low, which may cause incomplete recrystallization process and result in large residual stress affecting its mechanical properties; and when the steel plate is heated to the target temperature, it is heated quickly by increasing the temperature as quickly as possible to improve the overall heating efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a schematic diagram of the rolled grain size of a 10mm to 20mm rolled steel plate;

[0024] Figure 2 is a schematic diagram of the grain size of a 25 mm to 55 mm rolled steel plate;

[0025] FIG3 is a schematic diagram of the grain size of a 60 mm to 85 mm rolled steel plate;

[0026] FIG4 is a schematic diagram of the grain size of a 10 mm to 20 mm rolled steel plate after heat treatment;

[0027] FIG5 is a schematic diagram of the grain size of 25 mm to 55 mm rolled steel plates after heat treatment;

[0028] FIG6 is a schematic diagram of the grain size of 60 mm to 85 mm rolled steel plates after heat treatment. DETAILED DESCRIPTION

[0029] The present invention is further described below with reference to specific embodiments and accompanying drawings.

[0030] In the present application, the 316H stainless steel thick plate required in the 600MW demonstration fast reactor has a thickness of 10mm to 85mm and a width of 1400 to 3400mm. Due to its relatively thin thickness, the metallographic structure is significantly anisotropic due to the large deformation of the slab during the rolling process; especially for medium and thick plates, the particularity of the deformation distribution difference will promote the difference in dynamic recrystallization or dynamic recovery in the thickness direction, forming an uneven structure in the rolled state. Therefore, the present application provides a heat treatment method for controlling the grain size uniformity of 316 steel plates for fast reactors, so that while ensuring sufficient solid solution of carbides to ensure mechanical properties and corrosion resistance, it prevents abnormal growth of grains to obtain a uniform structure.

[0031] Please refer to Figures 1 to 3. Figure 1 is a schematic diagram of the rolled grain size of 10mm to 20mm rolled steel plates; Figure 2 is a schematic diagram of the rolled grain size of 25mm to 55mm rolled steel plates; and Figure 3 is a schematic diagram of the rolled grain size of 60mm to 85mm rolled steel plates.

[0032] Example 1

[0033] A heat treatment method for controlling the grain size homogenization of 316 steel plates for fast reactors is disclosed. For rolled steel plates with a thickness of 25 to 85 mm, different solution treatment steps are employed for rolled steel plates of different thicknesses. It is noted that for steel plates with a thickness of 10 to 25 mm, after rolling, the rolled grains are mostly uniformly deformed grains, as shown in FIG1 . Therefore, the rolled steel plates can be directly heated to the target temperature as quickly as possible, then held at that temperature and then cooled. This method prevents abnormal grain growth within the steel plates during the rapid heating process, effectively ensuring their mechanical properties and corrosion resistance while obtaining a uniform metallographic structure.

[0034] For rolled steel plates with a thickness of 25 to 85 mm, the grain distribution in the rolled state after rolling is complex, so a different solution treatment process is used. The target temperature of the steel plate is greater than 1040°C, specifically 1040°C to 1060°C. The process is as follows:

[0035] For rolled steel plates with a thickness of 25 to 60 mm, the grain size after rolling is relatively fine (grade 6-7), and the surface of the steel plate even reaches grade 7-8, as shown in FIG2 ; therefore, the specific operation in the solution process is as follows: after rolling, the steel plate is heated to a first gradient temperature (980° C. to 1020° C.) and kept warm for 0.5 h to 1.5 h; then heated to a target temperature (1040° C. to 1060° C.), kept warm, and then cooled to a cooling temperature; the holding time after heating to the target temperature corresponds to the thickness of the steel plate, and the number of minutes of the holding time is equal to the number of millimeters of the thickness of the steel plate (i.e., the holding time is 25 to 60 min); the difference between the first temperature gradient and the target temperature is 40° C. to 60° C.; of course, the first temperature gradient is determined by the thickness of the rolled plate and the grain state of the rolled state;

[0036] For rolled steel plates with a thickness of 25 to 60 mm, the grain size after rolling is relatively small and uniform. Therefore, the difference between the first gradient temperature and the target temperature is only 40 to 60 degrees Celsius, which is a small temperature difference. The setting of the first gradient temperature and the holding time forms a low-temperature holding step, thereby eliminating the energy gradient and avoiding the situation where direct heating to the target temperature will cause abnormal grain growth and uneven distribution.

[0037] For rolled steel plates with a thickness of 60 to 85 mm, the steel plates are heated to a second gradient temperature (940°C to 980°C) after rolling and kept warm for 0.5 to 1.5 hours; then heated to a target temperature (1040°C to 1060°C), kept warm, and then cooled to a cooling temperature; the holding time after heating to the target temperature corresponds to the thickness of the steel plate, and the number of minutes of the holding time is equal to the number of millimeters of the thickness of the steel plate; the difference between the first temperature gradient and the target temperature is 80 to 100°C; of course, the second temperature gradient is determined by the thickness of the rolled plate and the grain state of the rolled state;

[0038] For rolled steel plates with a thickness of 60-85 mm, the grains are stratified across the thickness after rolling, with the middle grains being approximately 4 levels coarser and the surface grains being approximately 6 levels finer, as shown in Figure 3. The left side of Figure 3 shows the surface grain size, the middle shows the grain size at point T / 4, and the right side shows the grain size at point T / 2. After rolling, the grain size is large but unevenly distributed. Therefore, a second gradient temperature is set. This temperature is significantly different from the target temperature and is lower (940°C to 980°C). This prevents the middle grains from growing while better eliminating the energy gradient, resulting in a uniform grain distribution across the thickness.

[0039] The present invention provides a gradient heating process for rolled steel plates of different thicknesses during solution heating, thereby buffering the steel plates and eliminating energy gradients before heating to the target temperature, thereby avoiding abnormal grain growth and uneven distribution during heating to the target temperature. Different gradient temperatures are set for different thicknesses, and different grain size distributions are caused by different thicknesses during rolling. Therefore, different gradient temperatures are selected to eliminate energy gradients, so that the final steel plates have uniform grain distribution in the thickness direction and meet the requirements. At the same time, a connection is established between the holding time after heating to the target temperature and the thickness of the steel plates, thereby avoiding resource consumption caused by too long a holding time, and failing to obtain uniform grain distribution in the full thickness direction of the steel plates if the holding time is too short. The entire heat treatment method ensures sufficient solid solution of the steel plates while avoiding the occurrence of mixed crystals during high-temperature solid solution, and obtains uniform 4-6 level grains in the full thickness direction on the basis of ensuring that the mechanical properties of the products meet the requirements.

[0040] Furthermore, regardless of the thickness range, after the steel plate is heated to the target temperature and held at that temperature, it is cooled using water cooling until the surface temperature is <80°C. This water cooling method offers high cooling efficiency, enabling simultaneous cooling of large areas of the steel plate while also being cost-effective. Furthermore, the interval between cooling to a surface temperature <80°C and the start of water cooling, from exiting the furnace to the start of water cooling, is controlled to ≤5 minutes to ensure the solution strengthening effect.

[0041] Furthermore, before the solution treatment, the steel plate's final rolling temperature is controlled to be ≥950°C. This prevents excessively high final rolling temperatures, which can lead to coarse grains and make them difficult to control and treat during subsequent solution treatment. It also prevents excessively low final rolling temperatures, which can lead to incomplete recrystallization and result in high residual stresses that affect mechanical properties. The steel plate is then water-cooled after rolling before the solution treatment.

[0042] At the same time, when the steel plate is directly heated to the target temperature or the steel plate is heated from the first gradient temperature to the target temperature or the steel plate is heated from the second gradient temperature to the target temperature, the maximum heating rate of the heating equipment is used to heat the steel plate to the target temperature, thereby improving the overall heating efficiency, thereby shortening the entire heat treatment time and increasing the efficiency.

[0043] Example 2

[0044] A 316 steel plate for fast reactors that uses the aforementioned heat treatment method for grain size homogenization control. The grain size uniformity of the 316 steel plate throughout its thickness is within the range of 4 to 6, with a range of less than 2. Its mechanical properties and corrosion resistance meet the technical requirements of fast reactors. The measured mechanical properties of 316H rolled steel plate are shown in Table 1; the grain size test results are shown in Table 2. For grain size photographs after using the heat treatment method of this application, please refer to Figures 4 to 6. Figure 4 is a schematic diagram of the grain size after heat treatment of 10mm to 20mm; Figure 5 is a schematic diagram of the grain size after heat treatment of 25mm to 50mm; Figure 6 is a schematic diagram of the grain size after heat treatment of 60mm to 85mm, and a typical photograph of grains after solid solution treatment. The left side of Figures 4 to 6 shows the grain size on the steel plate surface, the middle shows the grain size at T / 4 of the steel plate, and the right side shows the grain size at T / 2 of the steel plate.

[0045] Table 1 Mechanical properties test results of 316H rolled steel plate

[0046] Table 2 Test results of grain size and corrosion resistance of 316H rolled steel plate

[0047] The examples described in the present invention are merely descriptions of the preferred embodiments of the present invention and are not intended to limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made to the technical solutions of the present invention by engineers and technicians in this field should fall within the scope of protection of the present invention.

Claims

1. A heat treatment method for controlling the grain size homogenization of 316 steel plate for fast reactor, characterized in that: For rolled steel plates with a thickness of 25 to 85 mm, different solid solution process steps are used for rolled steel plates of different thicknesses; the details are as follows: For a rolled steel plate with a thickness of 25 to 60 mm, the steel plate is heated to a first gradient temperature after rolling and kept warm for 0.5 to 1.5 hours; then heated to a target temperature, kept warm and cooled to a cooling temperature; the holding time after heating to the target temperature corresponds to the thickness of the steel plate, and the number of minutes of the holding time is equal to the number of millimeters of the thickness of the steel plate; the first temperature gradient is determined by the thickness of the rolled plate and the grain state of the rolled state; the first temperature gradient is 980 to 1020°C; For a rolled steel plate with a thickness of 60 to 85 mm, the steel plate is heated to a second gradient temperature after rolling and kept warm for 0.5 to 1 hour; then heated to a target temperature, kept warm, and cooled to a cooling temperature; the holding time after heating to the target temperature corresponds to the thickness of the steel plate, and the number of minutes of the holding time is equal to the number of millimeters of the thickness of the steel plate; the second temperature gradient is determined by the thickness of the rolled plate and the grain state of the rolled state, and the second temperature gradient is 940 to 980°C; The target temperature of the steel plate is greater than 1040°C.

2. The heat treatment method for controlling the grain size homogenization of 316 steel plate for fast reactor according to claim 1, characterized in that: When the steel plate is heated to the target temperature, it is kept warm and then cooled by water cooling until the surface temperature of the steel plate is less than 80°C.

3. The heat treatment method for controlling the grain size uniformity of 316 steel plate for fast reactor according to claim 2, characterized in that: Water cool to the surface temperature of the steel plate <80℃, and the interval time from taking out of the furnace to starting water cooling is controlled to ≤5min.

4. The heat treatment method for controlling the grain size uniformity of 316 steel plate for fast reactor according to claim 1, characterized in that: Before the steel plate is subjected to the solution treatment process, the final rolling temperature of the steel plate is controlled to be ≥950°, and the steel plate after rolling is water-cooled before the solution treatment process.

5. The heat treatment method for controlling the grain size uniformity of 316 steel plate for fast reactor according to claim 1, characterized in that: When the steel plate is directly heated to the target temperature or the steel plate is heated from the first gradient temperature to the target temperature or the steel plate is heated from the second gradient temperature to the target temperature, the maximum heating rate of the heating device is used to heat the steel plate to the target temperature.

6. The heat treatment method for controlling the grain size uniformity of 316 steel plate for fast reactor according to claim 5, characterized in that: The target temperature of the rolled steel plate with a thickness of 10 to 85 mm in the solution treatment process is 1040°C to 1060°C.

7. A 316 steel plate for fast reactor using the heat treatment method for controlling the grain size uniformity of the 316 steel plate for fast reactor as claimed in claims 1 to 6, characterized in that: The grain size of the 316 steel plate for stacking has a grain uniformity in the range of 4 to 6 in the full thickness direction, and the range is less than 2.

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