A method for preparing ODS steel with bimodal grain size distribution and high dislocation density

Abstract

The invention relates to the technical field of oxide dispersion strengthened steel preparation, in particular to a method for preparing ODS steel with a bimodal grain size distribution and a high dislocation density. The method comprises the following steps: S1: mixing pre-alloyed powder and Y2O3 powder in a mass ratio of 99.65:0.35, wherein the mass composition of the pre-alloyed powder is Fe-9Cr-1.5W-0.2V-0.07Ta-0.1C (wt.%); performing mechanical ball milling in a ball mill under argon protection; placing the ball-milled powder into a stainless steel sheath, and degassing the powder in the stainless steel sheath at a temperature of 450°C until the vacuum degree of the stainless steel sheath reaches 0.002Pa. The method solves the problems of the existing heat treatment process for preparing ODS steel, namely, a relatively complex heat treatment process resulting in high preparation cost and low preparation efficiency, and unsatisfactory microstructural characteristics of the prepared ODS steel.

Description

Technical Field

[0001] The present invention relates to the technical field of oxide dispersion strengthened steel preparation, in particular to a method for preparing ODS steel with bimodal grain size distribution and high dislocation density. Background Art

[0002] Nuclear power plant construction requires a large amount of high-quality steel, including high-end nuclear-grade steel. Low-activation ferritic / martensitic steels have become a leading candidate structural material for fast neutron reactors and fusion reactors due to their low radiation swelling and thermal expansion coefficients, high thermal conductivity, and excellent resistance to liquid metal corrosion. While progress has been made in the development of high-thermal-strength ferritic / martensitic steels, the significant decrease in tensile and creep strength above 600°C remains a major limitation in their ability to withstand the harsh service environments of these steels. Compared to traditional carbide particles, oxides are virtually insoluble in the matrix until the melting point of the matrix material, resulting in higher thermal stability at high temperatures. Therefore, oxide dispersion-strengthened (ODS) steels, which utilize densely distributed nano-sized oxide particles as a reinforcement phase, hold the promise of increasing the service temperature of ferritic / martensitic steels to 650°C and above.

[0003] The preparation of ODS steel primarily involves mechanical alloying, hot consolidation, hot deformation, and subsequent heat treatment. Mechanical alloying of pre-alloyed powder or metal mixed powder with Y2O3 powder, a material with high thermal and chemical stability, is one of the most critical steps. Depending on the Cr content, the matrix of ODS steels can be primarily composed of 9% Cr martensitic steels and 12-22% Cr ferritic steels, differing primarily in whether or not they undergo (complete) austenitic transformation during heating. ODS ferritic steels generally exhibit superior high-temperature mechanical properties to martensitic steels, but their microstructure and mechanical property isotropy are inferior. Highly thermally stable oxide nanoparticles are crucial for achieving excellent high-temperature mechanical properties and radiation resistance in ODS ferritic / martensitic steels. However, methods for controlling oxide nanoparticles are relatively limited. Since grain structure and dislocation structure are also key characteristic elements of ODS steels, controlling the grain size distribution and obtaining a thermally stable dislocation structure are promising approaches to further optimize the high-temperature and radiation resistance of ODS steels. For ODS steel, the existing heat treatment process is a two-step heat treatment process of normalizing + tempering.

[0004] However, practice has shown that there are the following problems in preparing ODS steel through the existing heat treatment process: First, the microstructural characteristics are not ideal. The microstructural characteristics of the ODS steel prepared by this process are carbide + oxide + ferrite, and the grain size does not show a bimodal distribution characteristic, which may further lead to the material's performance under high temperature and irradiation environment being less than ideal; second, the existing heat treatment process is relatively complicated, which requires more time and energy, and is also prone to introduce errors in the process, making the preparation process costly and inefficient.

[0005] Therefore, it is necessary to invent a method for preparing ODS steel with bimodal grain size distribution and high dislocation density to solve the above problems. Summary of the Invention

[0006] In order to solve the problems that the heat treatment process for preparing ODS steel by the existing heat treatment process is relatively complicated, resulting in high preparation cost and low preparation efficiency, and the microstructural characteristics of the prepared ODS steel are not ideal, the present invention provides a method for preparing ODS steel with a bimodal grain size distribution and a high dislocation density.

[0007] The present invention is achieved by adopting the following technical solutions:

[0008] A method for preparing ODS steel with a bimodal grain size distribution and high dislocation density comprises the following steps:

[0009] S1: Pre-alloyed powder and Y2O3 powder were mixed in a mass ratio of 99.65:0.35, and the mass composition of the pre-alloyed powder was Fe-9Cr-1.5W-0.2V-0.07Ta-0.1C (wt.%); mechanical ball milling was performed in a ball mill with argon protection. The purpose of mechanical ball milling was to ensure that the pre-alloyed powder and Y2O3 powder were fully mixed and evenly dispersed, which helped to achieve uniform distribution in the material, thereby improving its performance; the milled powder was placed in a stainless steel sleeve, and the powder in the stainless steel sleeve was heated at a temperature of 450 ° C. Degassing is performed until the vacuum degree of the stainless steel sheath reaches 0.002 Pa. The degassing treatment is used to reduce the gas content in the mixture formed by the pre-alloyed powder and the Y2O3 powder to prevent the generation of bubbles in subsequent operations. The stainless steel sheath is sealed and welded, and then placed in a hot isostatic pressing device for curing and molding. The molding temperature is 1150°C, the holding time is 3 hours, and the molding pressure is 150MPa. The vacuum degree is maintained by sealing and welding the stainless steel sheath to ensure that it will not be contaminated by external gases during the hot isostatic pressing process. The steel is then cooled to room temperature with the furnace to obtain hot isostatically pressed 9Cr-ODS martensitic steel.

[0010] S2: The hot isostatically pressed 9Cr-ODS martensitic steel was austenitized at 1100 °C for 0.5 h and water quenched to room temperature;

[0011] S3: The water-quenched 9Cr-ODS martensitic steel was heated to 1100°C at 20°C / s and held for 10 min. It was then cooled to 700°C-610°C for different holding times until the ferrite phase transformation was complete. The steel was then cooled to room temperature at 40°C / s to obtain an ODS steel with a bimodal grain size distribution and high dislocation density.

[0012] Furthermore, the ball-to-material ratio in the ball mill is 15:1.

[0013] Furthermore, the ball milling speed in the ball mill is 400 r / min.

[0014] Furthermore, the ball milling time in the ball mill is 45 h.

[0015] The key to the present invention lies in the composition design of the pre-alloyed powder, the process parameters of hot isostatic pressing, and the isothermal temperature and holding time of the subsequent isothermal heat treatment process. The mass composition of the pre-alloyed powder is Fe-9Cr-1.5W-0.2V-0.07Ta-0.1C (wt.%), which determines that the 9Cr-ODS martensitic steel can undergo an austenitic phase transformation during heating and a ferrite phase transformation and a martensitic phase transformation during cooling. The hot isostatic pressing process parameters will affect the distribution state of the oxide nanoparticles. The 9Cr-ODS martensitic steel undergoes a ferrite phase transformation during the isothermal heat treatment process. Due to the different nucleation rates of ferrite at the original austenite grain boundaries and within the grains, as well as the kinetic factors of the ferrite phase transformation, the ferrite grain sizes formed at the original austenite grain boundaries and within the grains are different, resulting in a bimodal distribution. During the ferrite transformation, the movement of the ferrite / austenite interface coupled with the oxide nanoparticles creates a high density of dislocations within the ferrite grains. This results in an ODS steel with a bimodal grain size distribution and a high density of dislocations.

[0016] During the isothermal heat treatment process, the selection of isothermal temperature and isothermal time are crucial. If the isothermal temperature is too high, ferrite phase transformation will not occur; if the isothermal temperature is too low, martensite phase transformation will occur; if the isothermal time is insufficient, the ferrite transformation cannot be complete and a fully ferrite structure cannot be obtained.

[0017] The present invention performs certain composition design and preparation process adjustment on ODS steel in the early stage. In the subsequent heat treatment process, only one heat treatment step is required, that is, simply controlling the isothermal temperature and time to effectively control the grain size distribution in the ODS steel. At the same time, high-density dislocations can be obtained inside the ferrite grains. The process is simple and highly targeted, which is of great significance to improving the high-temperature strength and radiation resistance of ODS ferrite steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1This is an electron backscatter diffraction (EBSD) photograph of the 9Cr-ODS martensitic steel of the present invention after isothermal heat treatment at 700°C.

[0019] Figure 2 This is a transmission electron microscope (TEM) photograph of the 9Cr-ODS martensitic steel of the present invention after isothermal heat treatment at 700°C.

[0020] Figure 3 This is an electron backscatter diffraction (EBSD) photograph of the 9Cr-ODS martensitic steel of the present invention after isothermal heat treatment at 640°C.

[0021] Figure 4 This is a transmission electron microscope (TEM) photograph of the 9Cr-ODS martensitic steel of the present invention after isothermal heat treatment at 640°C.

[0022] Figure 5 This is an electron backscatter diffraction (EBSD) photograph of the 9Cr-ODS martensitic steel of the present invention after isothermal heat treatment at 610°C.

[0023] Figure 6 This is a transmission electron microscope (TEM) photograph of the 9Cr-ODS martensitic steel of the present invention after isothermal heat treatment at 610°C. DETAILED DESCRIPTION

[0024] The pre-alloyed powder of the present invention has the following composition by mass percentage: C=0.1%, Cr=9%, W=1.5%, V=0.2%, Ta=0.07%, and the remainder is Fe. The Y2O3 powder of the present invention has a size distribution of 30nm to 50nm.

[0025] A method for preparing ODS steel with a bimodal grain size distribution and high dislocation density comprises the following steps:

[0026] S1: The pre-alloyed powder and Y2O3 powder were mixed in a mass ratio of 99.65:0.35, and mechanically ball-milled in a QM-3SP4 planetary ball mill with a ball-to-material ratio of 15:1, a ball-to-material ratio of 400 r / min, and a ball-milling time of 45 h under argon protection. The milled powder was placed in a stainless steel sheath, and the powder in the stainless steel sheath was degassed at a temperature of 450°C until the vacuum degree of the stainless steel sheath reached 0.002 Pa. The stainless steel sheath was sealed and welded, and then placed in a hot isostatic pressing equipment for curing and molding at a molding temperature of 1150°C, a holding time of 3 h, and a molding pressure of 150 MPa. The steel was then cooled to room temperature with the furnace to obtain hot isostatically pressed 9Cr-ODS martensitic steel.

[0027] S2: The hot isostatically pressed 9Cr-ODS martensitic steel was austenitized at 1100 °C for 0.5 h and water quenched to room temperature;

[0028] S3: The water-quenched 9Cr-ODS martensitic steel was heated to 1100°C at 20°C / s and held for 10 min. It was then cooled to 700°C-610°C for different holding times until the ferrite phase transformation was complete. The steel was then cooled to room temperature at 40°C / s to obtain an ODS ferritic steel with a bimodal grain size distribution and high dislocation density.

[0029] The following are specific embodiments of the present invention, but the present invention is not limited to the following embodiments.

[0030] Example 1:

[0031] The pre-alloyed powder has a mass composition of Fe-9Cr-1.5W-0.2V-0.07Ta-0.1C (wt.%). The pre-alloyed powder was mixed with Y2O3 powder in a mass ratio of 99.65:0.35 and mechanically milled in a QM-3SP4 planetary ball mill with a ball-to-material ratio of 15:1, a milling speed of 400 r / min, and a milling time of 45 hours under argon protection. The milled powder was placed in a stainless steel sheath and degassed at 450°C until the vacuum level in the sheath reached 0.002 Pa. The sheath was sealed and welded, and then placed in a hot isostatic pressing (HIP) machine for curing and molding at a molding temperature of 1150°C, a holding time of 3 hours, and a molding pressure of 150 MPa. The steel was then cooled to room temperature to obtain HIP-molded 9Cr-ODS martensitic steel.

[0032] The hot isostatically pressed 9Cr-ODS martensitic steel was then austenitized at 1100°C for 0.5h and water quenched to room temperature. An isothermal heat treatment process was then performed, heating to 1100°C at 20°C / s and holding for 10min, followed by cooling to 700°C and holding for 20min. After the ferrite phase transformation was completed, the steel was cooled to room temperature at 40°C / s to obtain an ODS steel with a bimodal grain size distribution and a high dislocation density, and the ODS steel was an ODS ferritic steel.

[0033] Figure 1 This is an electron backscatter diffraction (EBSD) photograph of the 9Cr-ODS martensitic steel after isothermal heat treatment at 700°C in Example 1. As can be seen from the figure, after isothermal heat treatment, the matrix of the 9Cr-ODS martensitic steel is transformed into irregular ferrite, and the ferrite grain size is 3.3±2.8μm; Figure 2 This is a transmission electron microscope (TEM) photograph of the 9Cr-ODS martensitic steel after isothermal heat treatment at 700°C in Example 1. Unlike traditional ODS steel, high-density dislocations are observed in the ODS steel after ferrite phase transformation.

[0034] Example 2:

[0035] The pre-alloyed powder has a mass composition of Fe-9Cr-1.5W-0.2V-0.07Ta-0.1C (wt.%). The pre-alloyed powder was mixed with Y2O3 powder in a mass ratio of 99.65:0.35 and mechanically milled in a QM-3SP4 planetary ball mill with a ball-to-material ratio of 15:1, a milling speed of 400 r / min, and a milling time of 45 hours under argon protection. The milled powder was placed in a stainless steel sheath and degassed at 450°C until the vacuum level in the sheath reached 0.002 Pa. The sheath was sealed and welded, and then placed in a hot isostatic pressing (HIP) machine for curing and molding at a molding temperature of 1150°C, a holding time of 3 hours, and a molding pressure of 150 MPa. The steel was then cooled to room temperature to obtain HIP-molded 9Cr-ODS martensitic steel.

[0036] The hot isostatically pressed 9Cr-ODS martensitic steel was then austenitized at 1100°C for 0.5h and water quenched to room temperature. An isothermal heat treatment process was then performed, heating to 1100°C at 20°C / s and holding for 10min, followed by cooling to 640°C and holding for 70min. After the ferrite phase transformation was completed, the steel was cooled to room temperature at 40°C / s to obtain an ODS steel with a bimodal grain size distribution and a high dislocation density, and the ODS steel was an ODS ferritic steel.

[0037] Figure 3 This is an electron backscatter diffraction (EBSD) photograph of the 9Cr-ODS martensitic steel after isothermal heat treatment at 640°C in Example 2. As can be seen from the figure, compared with the grain size after the ferrite phase transformation at 700°C in Example 1, the ferrite grains after the isothermal heat treatment at 640°C are refined to 1.7±1.6 μm; Figure 4 This is a transmission electron microscope (TEM) photograph of the 9Cr-ODS martensitic steel after isothermal heat treatment at 640° C. in Example 2. The ferrite matrix contains a high density of dislocations.

[0038] Example 3:

[0039] The pre-alloyed powder has a mass composition of Fe-9Cr-1.5W-0.2V-0.07Ta-0.1C (wt.%). The pre-alloyed powder was mixed with Y2O3 powder in a mass ratio of 99.65:0.35 and mechanically milled in a QM-3SP4 planetary ball mill with a ball-to-material ratio of 15:1, a milling speed of 400 r / min, and a milling time of 45 hours under argon protection. The milled powder was placed in a stainless steel sheath and degassed at 450°C until the vacuum level in the sheath reached 0.002 Pa. The sheath was sealed and welded, and then placed in a hot isostatic pressing (HIP) machine for curing and molding at a molding temperature of 1150°C, a holding time of 3 hours, and a molding pressure of 150 MPa. The steel was then cooled to room temperature to obtain HIP-molded 9Cr-ODS martensitic steel.

[0040] The hot isostatically pressed 9Cr-ODS martensitic steel was then austenitized at 1100°C for 0.5h and water quenched to room temperature. An isothermal heat treatment process was then performed, heating to 1100°C at 20°C / s and holding for 10min, followed by cooling to 610°C and holding for 3h. After the ferrite phase transformation was completed, the steel was cooled to room temperature at 40°C / s to obtain an ODS steel with a bimodal grain size distribution and a high dislocation density, and the ODS steel was an ODS ferritic steel.

[0041] Figure 5 This is an electron backscatter diffraction (EBSD) photograph of the 9Cr-ODS martensitic steel after isothermal heat treatment at 610°C in Example 3. As can be seen from the figure, after isothermal heat treatment, the matrix of the 9Cr-ODS martensitic steel is transformed into irregular ferrite, and the dual distribution feature of the ferrite grain size is more obvious; Figure 6 This is a transmission electron microscope (TEM) photograph of the 9Cr-ODS martensitic steel after isothermal heat treatment at 610°C in Example 3. Unlike traditional ODS steel, high-density dislocations are observed in the ODS steel after ferrite phase transformation.

[0042] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing ODS steel with bimodal grain size distribution and high dislocation density, characterized by: The following steps are involved: S1: Pre-alloyed powder and Y2O3 powder were mixed in a mass ratio of 99.65:0.35, and the mass composition of the pre-alloyed powder was Fe-9Cr-1.5W-0.2V-0.07Ta-0.1C (wt.%); mechanical ball milling was performed in a ball mill with argon protection; the ball-milled powder was placed in a stainless steel sheath, and the powder in the stainless steel sheath was degassed at a temperature of 450°C until the vacuum degree of the stainless steel sheath reached 0.002Pa; the stainless steel sheath was sealed and welded, and then placed in a hot isostatic pressing equipment for curing and molding. The molding temperature was 1150°C, the holding time was 3h, and the molding pressure was 150MPa. The steel was then cooled to room temperature with the furnace to obtain hot isostatically pressed 9Cr-ODS martensitic steel. S2: The hot isostatically pressed 9Cr-ODS martensitic steel was austenitized at 1100 °C for 0.5 h and water quenched to room temperature; S3: The water-quenched 9Cr-ODS martensitic steel was heated to 1100°C at 20°C / s, held for 10 min, then cooled to 610°C and held for 3 h until the ferrite phase transformation was completed. The steel was then cooled to room temperature at 40°C / s to obtain a bimodal grain size distribution and a high dislocation density ODS steel.

2. The method for preparing an ODS steel with a bimodal grain size distribution and a high dislocation density according to claim 1, characterized in that: In step S1: the ball-to-material ratio in the ball mill is 15:

1.

3. The method for preparing an ODS steel with a bimodal grain size distribution and a high dislocation density according to claim 1, wherein: In step S1: the ball milling speed is 400 r / min.

4. The method for preparing an ODS steel with a bimodal grain size distribution and a high dislocation density according to claim 1, wherein: In step S1: the ball milling time is 45 hours.

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