A cermet shielding material for shielding neutrons and gamma rays and a method for manufacturing the same

Abstract

The application belongs to the technical field of neutron shielding materials, and particularly relates to a metal ceramic shielding material for shielding neutrons and gamma rays. x Mo 1‑x )AlB-MAB solid solution phase compound, which has a monoclinic cubic crystal structure, and a unit cell is formed by alternately stacking MB units and A atom planes; the compound is composed of elements W, Mo, Al or 6061 Al alloy and B, Mo is a solid solution atom of W at the M position, the atomic concentration x of Mo is 5-40 at%, and alloy elements Si and Mg atoms in the 6061 Al alloy are mainly at the A position. The compound metal ceramic shielding material has extremely high concentrations of W and B contents, and has excellent shielding properties for neutrons and g-rays of various energy ranges; moreover, the material has metal and ceramic double characteristics, that is, good mechanical properties, excellent electrical conductivity and thermal conductivity, high thermal stability and high-temperature oxidation resistance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of neutron shielding materials, and particularly relates to a cermet shielding material and a preparation method thereof. BACKGROUND

[0002] In view of different ray generation forms and application characteristics of nuclear technology, the traditional neutron shielding material has been difficult to meet the requirements. In many special small nuclear reactors (such as compact small fusion reactors and mobile small fission reactors) and nuclear facility application sites, it is usually a mixed field of multiple energy rays, and the radiation shielding material not only needs to have high comprehensive shielding efficiency of multiple rays, but also needs to have certain structural mechanical properties, withstand extreme environmental tests, and have high-performance functional / structural integrated neutron / γ ray shielding materials to greatly simplify the shielding structure and realize the miniaturization of the shielding structure.

[0003] Among the typical metal-based functional / structural integrated ray shielding materials, only boron steel is relatively mature. It has good thermal neutron absorption performance, γ ray shielding performance and excellent mechanical properties. Since the addition amount of boron in boron steel is limited, if the ideal neutron absorption effect is to be achieved, the thickness of the boron steel must be increased to improve the neutron absorption capacity; aluminum-based boron carbide composite material, which can add a higher content of boron and also inherits the good mechanical properties and thermal conductivity of aluminum, is the most widely researched and applied neutron absorption material at present. However, it has almost no shielding ability for γ rays, and the use temperature is not high, which also limits its application, such as fusion reactor shielding cladding material. In recent research, W-B / Al composite material (201310176166.4, 201811591037.0) is prepared by adding tungsten and boron elements in the aluminum matrix. Due to the limited addition amount of boron and W, the comprehensive shielding performance of the composite material for fast neutrons and γ rays, high-temperature mechanical properties and corrosion resistance are not ideal. The MH-B-Al (MH: metal hydride) shielding material (201711336252.1, 202210992472.4) prepared by compounding aluminum with titanium hydride, zirconium hydride and boron has good fast neutron moderation and absorption performance, but the general hydride begins to decompose at 300-500℃, and the decomposition will be accelerated under irradiation environment.

[0004] In view of the application defects of the above-mentioned materials, the present application provides a high-density high-temperature functional / structural integrated neutron and γ ray comprehensive shielding material with high shielding performance for neutrons and γ rays in each energy segment, which is expected to be widely applied in various reactors and facilities with mixed energy rays to greatly simplify the shielding structure and realize the miniaturization of the shielding structure. SUMMARY

[0005] The present application aims to provide a cermet shielding material with simple structure and high shielding performance for neutrons and gamma rays in various energy ranges and a preparation method thereof.

[0006] The cermet shielding material for shielding neutrons and gamma rays provided by the present application is (W x Mo 1-x )AlB-MAB phase cermet shielding material, which is a 222 type MAB solid solution phase compound (bulk) with ternary layered nanometer. It is a monoclinic cubic crystal structure with a space group of Cmcm, and the unit cell is formed by alternately stacking MB units and A atoms. The compound is composed of elements W, Mo, Al (or 6061 Al alloy) and B. Mo is a solid solution atom of W at the M site, and the atomic concentration x of Mo is 5-40 at%. The alloy elements Si, Mg and other atoms in the 6061 Al alloy are mainly at the A site. (6061 Al alloy is an Al alloy product model, which is a standard known product.)

[0007] The compound cermet shielding material has extremely high W and B content, and has very excellent shielding performance for neutrons and gamma rays in various energy ranges. On the other hand, the unique structure endows the material with metal and ceramic double characteristics, i.e. good mechanical properties, excellent electrical conductivity and thermal conductivity, high thermal stability and high temperature oxidation resistance. The addition of solid solution atom Mo mainly improves the decomposition temperature of the compound phase. The replacement of Al with 6061 Al alloy forms a multi-element solid solution at the A site, which can improve the mechanical properties of the MAB phase.

[0008] There are few reports on the preparation of WAlB-MAB phase bulk material. Peter has tried to prepare WAlB by hot pressing method, but due to process technology problems, only low W content (Mo x W 1-x )AlB (x>0.75, purity not higher than 74.3±0.8 mol.%) solid solution containing many impurities is synthesized, and WAlB and high W content (W x Mo 1-x )AlB (x>0.25) solid solution cannot be successfully synthesized. The present application uses tungsten powder, molybdenum powder, boron powder and aluminum powder (or 6061 Al alloy powder) in a ratio of (0.6-0.95):(0.05-0.4):(0.8-1.2):(1.0-2.0), and successfully prepares (W x Mo 1-x )-MAB phase material with a phase purity of 92-96 wt.% or more by vacuum hot pressing method.

[0009] The (W x Mo 1-xThe application relates to a preparation method of an AlB-MAB phase cermet shielding material.

[0010] Step 1: raw materials of boron powder, tungsten powder, molybdenum powder and aluminum powder (or 6061 Al alloy powder) are prepared to obtain a raw mixture, and zinc stearate can be added as an active agent to reduce the powder wall sticking and ball sticking in the ball milling process, improve the powder yield, and prevent cold welding;

[0011] Step 2: ball milling media are added to the raw mixture, and the mixture is dry-mixed on a ball mill at a predetermined rotating speed for a predetermined time to obtain a mixed material;

[0012] Step 3: the mixed material is separated from the ball milling media by using a screen to obtain the mixed material;

[0013] Step 4: the dry mixed material is pre-pressed, and then heated and pressurized at a predetermined heating rate and a predetermined pressurizing rate in a vacuum environment or a protective atmosphere, and after the predetermined sintering temperature and the predetermined pressure are reached, sintering is carried out at a predetermined holding time to obtain a sintered block;

[0014] Step 5: the sintered block is cooled to room temperature at a predetermined cooling rate to obtain a high-purity and dense (W x Mo 1-x )AlB-MAB phase ceramic bulk material.

[0015] In step 1, the tungsten powder, the molybdenum powder, the boron powder and the aluminum powder (or the 6061 aluminum alloy powder) are in a molar ratio of W:Mo:B:Al=(0.6-0.95):(0.05-0.4):(0.8-1.2):(1.0-2.0), and the zinc stearate active agent accounts for 0.3-1 wt.% of the raw mixture.

[0016] In step 2, the ball milling media are any one of agate balls, silicon nitride balls, stainless steel balls and zirconia balls, the predetermined rotating speed is 100-300 r / min, and the predetermined dry mixing time is 6-26 h.

[0017] In step 4, the pre-pressing pressure is 7-15 MPa, the vacuum environment pressure is 3x10 -3 ~ 5x10 -1 Pa.

[0018] In step 4, the predetermined heating rate is 5-20 DEG C / min, the predetermined sintering temperature is 800-980 DEG C, the predetermined pressure is 30-60 MPa, and the predetermined holding time is 1-15 h.

[0019] In step 4, after the sintering temperature reaches 200-400 DEG C, the pressure is increased at a predetermined pressurizing rate of 0.1-2 MPa / min to the predetermined pressure.

[0020] In step 5, the predetermined cooling rate is 100-200℃ / h.

[0021] The preparation method of the present application adopts initial materials which are economical and can save cost, and the raw materials used are simple and easy to obtain on the market; the obtained (W x Mo 1-x )AlB ceramic bulk material has high phase purity, and good mechanical properties, thermal conductivity and thermal stability.

[0022] The present application provides a (W x Mo 1-x )AlB-MAB phase cermet shielding material, which can be used as a functional structure integrated neutron shielding material at an environmental temperature of 500-800℃. The shielding material has moderate density, but has strong shielding function for neutrons and gamma rays of various energy segments, high mechanical strength, good thermal stability and corrosion resistance. The present application is particularly suitable for important occasions such as small and mobile reactors and shielding materials for radiotherapy. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a flow chart of the preparation method of the (W x Mo 1-x )AlB-MAB phase cermet shielding material of the present application.

[0024] Figure 2 is an X-ray diffraction refinement spectrum of the (W x Mo 1-x )AlB-MAB phase cermet shielding material prepared in Example 1 of the present application.

[0025] Figure 3 is a bending stress-strain curve of the (W x Mo 1-x )AlB-MAB phase cermet shielding material prepared in Example 4 of the present application.

[0026] Figure 4 is a scanning electron microscope (SEM) image of the (W x Mo 1-x )AlB-MAB phase cermet shielding material prepared in Example 1 of the present application.

[0027] Figure 5 is the change of Cf252 source neutron transmittance of different materials with shielding thickness.

[0028] Figure 6 is the change of Co60-1.25MeV gamma ray transmittance of different materials with shielding thickness. DETAILED DESCRIPTION

[0029] The application will be further described in the following with specific examples. Figure 1 is (W x Mo 1-x )AlB-MAB phase cermet shielding material prepared in Embodiment 1 to Embodiment 6 of the application.

[0030] Embodiment 1, a high-purity and dense high-temperature neutron and gamma ray comprehensive shielding material (W 0.95 Mo 0.05 )AlB ceramic bulk material, comprising the following steps:

[0031] Step S1, raw materials tungsten powder, molybdenum powder, boron powder and aluminum powder are weighed according to the molar ratio W:Mo:B:Al=0.95:0.05:1:1.3, and 0.7wt.% of zinc stearate active agent is added according to the total mass of the raw materials, to obtain a raw mixture, and the total mass of the raw mixture is about 100g.

[0032] Step S2, the raw mixture is placed in a stainless steel ball mill jar, and stainless steel balls are added to the stainless steel mill jar, the ball-to-material ratio is 2:1, and the mass ratio of medium balls (d=10mm) to small balls (d=6mm) is 1:1. The stainless steel ball mill jar containing the raw mixture is evacuated before ball milling, and the mixed material is obtained by dry mixing on a planetary ball mill at 300r / min for 6 hours after good vacuum.

[0033] Step S3, the mixed material and the ball milling medium are separated by a sieve with a pore size of 50 mesh (0.28mm) to obtain the mixed material.

[0034] Step S4, the mixed material is placed in a graphite mold, the graphite mold is placed in a hot pressing furnace for pre-pressing at 10MPa, and then heated in a vacuum environment (pressure ~ 3E-1Pa) at a heating rate of 10℃ / min. After the sintering temperature reaches 300℃, the pressure is increased at a predetermined pressure increasing rate of 0.5MPa / min, and finally sintered at a sintering temperature of 830℃, a pressure of 35MPa, and a holding time of 5 hours. The X-ray diffraction refinement pattern is shown in Figure 2 , and the scanning electron microscope (SEM) image is shown in Figure 3 .

[0035] Step S5, after the holding and pressure increasing are completed, the temperature is lowered to room temperature at a cooling rate of 150℃ / h to obtain a dense (W 0.95 Mo 0.05 )AlB ceramic material.

[0036] The (W 0.95 Mo 0.05 )AlB ceramic material prepared in this embodiment has a purity of 96.3wt.%, a density of 87.2%, and a bending strength of 440MPa.

[0037] Example 2, a high-purity dense (W 0.80 Mo 0.20 )AlB ceramic bulk material is prepared, comprising the following steps:

[0038] Step S1, raw materials tungsten powder, molybdenum powder, boron powder, aluminum powder are dosed according to the molar ratio W:Mo:B:Al=0.8:0.2:1:1.3, tungsten powder 62.5g, molybdenum powder 8.2g, boron powder 4.6g, aluminum powder 14.9g, add 0.7wt.% zinc stearate activator, dosing, the total mass of the original dosing is about 90g.

[0039] Step S2, place the original dosing in a stainless steel ball mill jar, add stainless steel balls to the stainless steel ball mill jar, ball-to-material ratio 2:1, medium ball (d=10mm):small ball (d=6mm) mass ratio 1:1, vacuumize the stainless steel ball mill jar before ball milling, after good vacuumization, dry mix for 15 hours with a ball mill at 200r / min to obtain a mixture.

[0040] Step S3, separate the mixture and the ball milling medium with a 50 mesh sieve to obtain the mixture.

[0041] Step S4, place the mixture in a graphite mold, place the graphite mold in a hot press furnace for pre-pressing at 10MPa, then heat at a heating rate of 10℃ / min in a vacuum environment (pressure <1E-2Pa), after the sintering temperature reaches 300℃, press at a predetermined pressurization rate of 30MPa / h, finally sinter at a sintering temperature of 870℃, a pressure of 40MPa, and a holding time of 5 hours.

[0042] Step S5, after the holding time ends, cool to room temperature at a cooling rate of 120℃ / h to obtain a dense (W 0.80 Mo 0.20 )AlB ceramic material.

[0043] The (W 0.80 Mo 0.20 )AlB ceramic material prepared in this example has a purity of 94.6wt.%, a density of 92%, and a bending strength of 376MPa.

[0044] Example 3, a high-purity high-temperature-resistant neutron and gamma ray comprehensive shielding material (W 0.95 Mo 0.05 )6061AlB ceramic bulk is prepared, comprising the following steps:

[0045] Step S1, raw materials tungsten powder, molybdenum powder, boron powder, 6061 aluminum alloy powder are weighed according to the molar ratio W:Mo:B:6061Al=0.95:0.05:1:1.3, wherein the tungsten powder is 74.42g, the molybdenum powder is 2.04g, the boron powder is 4.61g, and the 6061 aluminum alloy powder is 15.56g, and 0.5wt.% of zinc stearate active agent is added according to the total mass of the raw materials to obtain a raw mixture, and the total mass of the raw mixture is about 100g.

[0046] Step S2, the raw mixture is placed in a stainless steel ball mill jar, stainless steel balls are added to the stainless steel ball mill jar, the ball-to-material ratio is 2:1, and the mass ratio of medium balls (d=10mm) to small balls (d=6mm) is 1:1. The stainless steel ball mill jar is vacuumized before ball milling, and the mixed material is obtained by dry mixing in a planetary ball mill at 300r / min for 15 hours after vacuumization.

[0047] Step S3, the mixed material and the ball milling medium are separated by a sieve with a pore size of 50 meshes to obtain the mixed material.

[0048] Step S4, the mixed material is placed in a graphite mold, and then heated in a vacuum environment (pressure <6E-3Pa) at a heating rate of 10℃ / min. After the sintering temperature reaches 300℃, the pressure is increased at a predetermined pressure increasing rate of 0.7MPa / min. Finally, sintering is carried out under the parameter conditions of a sintering temperature of 800℃, a pressure of 35MPa, and a holding time of 5 hours.

[0049] Step S5, after the holding time ends, the temperature is lowered to room temperature at a cooling rate of 150℃ / h to obtain a dense (W 0.95 Mo 0.05 )6061AlB ceramic material.

[0050] The (W 0.95 Mo 0.05 )6061AlB ceramic material prepared in this embodiment has a purity of 92wt.%, a density of 85.6%, and a bending strength of 484MPa.

[0051] Example 4, preparation of a high-purity dense (W 0.80 Mo 0.20 )6061AlB ceramic bulk material, comprising the following steps:

[0052] Step S1, raw materials tungsten powder, molybdenum powder, boron powder, 6061 aluminum alloy powder are weighed according to the molar ratio W:Mo:B:Al=0.8:0.2:1:1.3, wherein the tungsten powder is 74.42g, the molybdenum powder is 2.04g, the boron powder is 4.61g, and the 6061 aluminum alloy powder is 15.56g, and 0.5wt.% of zinc stearate active agent is added according to the total mass of the raw materials to obtain a raw mixture, and the total mass of the raw mixture is about 100g.

[0053] Step S2, the raw ingredients are placed in a stainless steel ball mill jar, stainless steel balls are added to the stainless steel ball mill jar, the ball-to-material ratio is 2:1, the mass ratio of medium balls (d=10 mm) to small balls (d=6 mm) is 1:1, the stainless steel ball mill jar is vacuumed before ball milling, and the dry mixing is performed on the ball mill at 300 r / min for 15 hours to obtain a mixture.

[0054] Step S3, the mixture and the ball milling medium are separated by a sieve with a mesh size of 50 meshes to obtain the mixture.

[0055] Step S4, the mixture is placed in a graphite mold, the graphite mold is placed in a hot-pressing furnace for pre-pressing at 10 MPa, then the graphite mold is heated in a vacuum environment (pressure <5E-3 Pa) at a heating rate of 10 ℃ / min, the graphite mold is pressurized at a predetermined pressurizing rate of 0.4 MPa / min after the sintering temperature reaches 300 ℃, and finally the graphite mold is sintered at a sintering temperature of 920 ℃, a pressure of 35 MPa, and a holding time of 5 hours.

[0056] Step S5, after the holding time ends, the temperature is lowered to room temperature at a cooling rate of 150 ℃ / h to obtain a dense (W 0.80 Mo 0.20 )AlB ceramic material. The bending stress-strain curve of the (W Figure 4 Mo

[0057] The (W 0.80 Mo 0.20 )6061AlB ceramic material prepared in this embodiment has a purity of 94.0 wt.%, a density of 96%, and a bending strength of 600 MPa.

[0058] Example 5, preparation of a high-purity dense high-temperature-resistant neutron and gamma ray comprehensive shielding material (W 0.95 Mo 0.05 )Al 1.8 B ceramic bulk material, including the following steps:

[0059] Step S1, the raw materials tungsten powder, molybdenum powder, boron powder, and aluminum powder are weighed according to the molar ratio W:Mo:B:Al=0.95:0.05:1:1.8, and 0.7 wt.% of zinc stearate activator is added according to the total mass of the raw materials to obtain the raw ingredients.

[0060] Step S2, the raw ingredients are placed in a stainless steel ball mill jar, stainless steel balls are added to the stainless steel ball mill jar, the ball-to-material ratio is 2:1, the mass ratio of medium balls (d=10 mm) to small balls (d=6 mm) is 1:1, the stainless steel ball mill jar is vacuumed before ball milling, and the dry mixing is performed on the ball mill at 300 r / min for 15 hours.

[0061] Step S3: Separate the mixture and the ball milling media using a 50-mesh sieve to obtain the mixture.

[0062] Step S4: Place the mixture in a graphite mold, place the graphite mold in a hot press furnace and pre-press it at 10 MPa. Then, heat it in a vacuum environment (pressure <3E-3Pa) at a heating rate of 10℃ / min. After the sintering temperature reaches 300℃, pressurize it at a predetermined pressurization rate of 0.5 MPa / min. Finally, sinter it under the parameter conditions of 800℃, 35 MPa, and holding at temperature and pressure for 5 hours.

[0063] Step S5, after the heat preservation and pressure holding are completed, the temperature is lowered to room temperature at a rate of 150℃ / h to obtain a dense (W) 0.95 Mo 0.05 )6061Al 1.8 B. Ceramic materials.

[0064] The (W) prepared in this embodiment 0.95 Mo 0.05 Al 1.8 The B ceramic material has a purity of 92.7 wt.%, a density of 86.6%, and a flexural strength of 386 MPa.

[0065] According to Examples 1 to 5, the high-purity dense (W) x Mo 1-x The method for preparing AlB ceramic bulk material involves using tungsten powder, molybdenum powder, boron powder, and aluminum powder (or 6061 aluminum alloy powder) as raw materials, with zinc stearate added as an activator to obtain the raw materials. Then, ball milling media are added to the raw materials, and the mixture is dry-mixed in a ball mill for a predetermined time to obtain a blend. The blend is then sieved. The blend is pre-pressed, and subsequently heated and pressurized under a vacuum or protective atmosphere at predetermined heating and pressurization rates until a predetermined sintering temperature and pressure are reached. Sintering is then performed for a predetermined holding time to obtain a sintered block. Finally, the sintered block is cooled to room temperature at a predetermined cooling rate to obtain high-purity, dense (W) ceramic bulk material. x Mo 1-x AlB ceramic bulk material. The initial materials used in the above process are relatively economical and cost-effective. The raw materials used are simple, readily available on the market, inexpensive, and easy to implement in the process.

[0066] Furthermore, Examples 1 to 5 of the present invention are the first to creatively synthesize dense (W) x Mo 1-x AlB bulk material, obtained (W x Mo 1-xAlB ceramic bulk materials have a purity of up to 92–96 wt.%, a density of up to 85–96%, and a flexural strength of 370–600 MPa. To evaluate the sintered (W... x Mo 1-x The shielding effect of AlB ceramic bulk materials on neutrons and gamma rays is first theoretically demonstrated by W... 0.95 Mo 0.05 AlB and W 0.8 Mo 0.2 AlB materials were compared with some typical shielding materials, and their neutron and gamma-ray transmittance was calculated using the Monte Carlo program (MCNP5), such as... Figure 5 , 6 As shown. Furthermore, the sintered sample was compared with a 252Cf neutron source and... 60 The shielding performance of Co-1.25MeV gamma rays was tested. For samples 3 and 4, when W 0.95 Mo 0.05 AlB at a thickness of 33mm and W 0.8 Mo 0.2 AlB at a thickness of 29mm 252 The Cf neutron source had transmittances of 45.6% and 49.6%, respectively, and shielding effects of 54.4% and 50.4%, respectively, which were 1.4 and 1.28 times lower than typical neutron / gamma-ray shielding materials such as boron steel. 60 γ-ray transmittance of Co-1.25MeV, W 0.95 Mo 0.05 AlB and W 0.8 Mo 0.2 The linear attenuation coefficients of AlB were 0.606 cm⁻¹. -1 and 0.513cm -1 Boron steel is 0.39cm. -1 It can be demonstrated that the (WxMo1-x)AlB ceramic bulk material prepared by this invention has a good shielding effect against neutrons and gamma rays, and possesses a certain structural strength. It can be widely used as a functional / structural integrated shielding material in small and mobile reactors, radiation medicine and other fields.

Claims

1. A cermet shielding material for shielding neutrons and gamma rays, characterized by, For (W x Mo 1-x )AlB -MAB solid solution compound, with monoclinic cubic crystal structure, space group Cmcm, its unit cell is stacked alternately by MB units and A atom plane; the compound is composed of W, Mo, Al or 6061 Al alloy and B, Mo is a solid solution atom in M position, and the alloy elements Si and Mg atoms in 6061 Al alloy are mainly in A position; The molar ratio of tungsten, molybdenum, boron and aluminum is W:Mo:B:Al = (0.6-0.95):(0.05-0.4):(0.8-1.2):(1.0-2.0).

2. A method of producing a cermet shielding material as claimed in claim 1, characterized in that The specific steps are as follows: Step 1: The powder of boron, tungsten, molybdenum and aluminum or 6061Al alloy is prepared as raw material to obtain the original ingredients; Step 2: The ball milling medium is added to the original ingredients, and the dry ball milling is carried out on the ball mill; Step 3: The mixture is separated from the ball milling medium by a screen to obtain the mixture; Step 4: The dry mixture is pre-pressed, and then heated and pressurized at a predetermined heating rate and a predetermined pressurizing rate in a vacuum environment or a protective atmosphere, and after reaching a predetermined sintering temperature and a predetermined pressure, sintering is carried out for a predetermined holding time to obtain a sintered block; Step 5, cooling the sintered block to room temperature to obtain high-purity dense (W x Mo 1-x ) AlB -MAB phase ceramic bulk material.

3. The preparation method according to claim 2, characterized in that, In step 1, the molar ratio of tungsten, molybdenum, boron and aluminum is W:Mo:B:Al = (0.6-0.95):(0.05-0.4):(0.8-1.2):(1.0-2.0).

4. The production method according to claim 3, characterized by, In step 2, the ball milling medium is any one of agate ball, silicon nitride ball, stainless steel ball and zirconium oxide ball, the ball milling speed is 100-300 r / min, and the dry ball milling time is 6-26 h.

5. The preparation method according to claim 4, characterized in that, In step 4: The pre-pressing pressure is 7-15 MPa, and the pressure in the vacuum environment is 3x10 -3 ~5x10 -1 Pa. The predetermined heating rate is 5-20 ℃ / min, the predetermined sintering temperature is 800-980 ℃, the predetermined pressure is 30-60 MPa, and the predetermined holding time is 1-15 h; Wherein, after the sintering temperature reaches 200-400 ℃, the pressure is increased at a rate of 0.1-2 MPa / min to the predetermined pressure.

6. The production method according to claim 5, wherein In step 5, the cooling rate is 200-250 ℃ / h.

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