L290-grade high-toughness normalized H2S-resistant steel plate and preparation method thereof

Abstract

The invention discloses an L290-grade high-toughness normalized H2S-resistant steel plate and a preparation method thereof. The L290-grade high-toughness normalized H2S-resistant steel plate comprises the following chemical elements in percentage by weight: 0.030-0.049% of C, less than or equal to 0.20% of Si, 0.91-1.20% of Mn, less than or equal to 0.013% of P, less than or equal to 0.0010% of S, 0.01-0.043% of Nb, 0.019-0.043% of Al, 0.01-0.02% of Ti and the balance of Fe. On the basis of a low-carbon and low-manganese component system, MnS inclusions and banded structures in steel are reduced by reducing the content of segregation-prone elements Mn, S and C, strength performance reduction caused by pearlite cluster distribution in the steel and reduction of alloy strengthening elements is reduced, good obdurability is achieved through structure refining control and effective supplement in the Nb alloy precipitation strengthening and rolling process, and the rolling quality is improved. According to the process, normalizing heat treatment realizes uniform and fine distribution of tissues in steel, and the H2S resistance of a steel matrix is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of normalized steel plates for structures, and specifically relates to an L290 grade high-toughness normalized H2S-resistant steel plate and a preparation method thereof. Background Art

[0002] With the rapid and continuous development of the world economy, the demand for normalized H2S-resistant service steel plates for structures and pipelines in cold regions under low-temperature service conditions continues to increase. In the process of service operations in severe cold regions or ultra-low temperature, due to the harsh low-temperature environment, it is inevitable to withstand tension, compression, impact and collision during service. Therefore, the structural steel under this condition is required to have good low-temperature impact toughness, certain strength, and high environmental resistance service requirements to ensure normal use in severe cold regions.

[0003] Many oil and gas resources are often rich in acidic media such as H2S. Pipeline steel will come into contact with H2S and react during the transportation of oil and gas, eventually causing failure of pipeline steel (hydrogen-induced cracking HIC and hydrogen sulfide stress corrosion cracking SSCC), resulting in large economic losses and environmental pollution. The solution to the failure mode of acidic medium transmission pipelines in acidic environments is to use acid-resistant pipeline steel. At present, acid-resistant pipeline steel is mainly used for domestic land pipelines rich in H2S oil and gas pipelines, submarine acid-resistant pipelines, and overseas land acid-resistant pipelines in the Middle East. Compared with pipeline steel, acid-resistant pipeline steel has increased the service requirements in wet H2S acidic environments, and the probability of hydrogen-induced cracking (HIC) and hydrogen sulfide stress corrosion cracking (SSCC) increases sharply. The higher the strength level of pipeline steel and the thicker the thickness specification, the more serious the segregation of the central component structure caused, resulting in greater sensitivity to HIC and SSCC.

[0004] There are three major technical difficulties in the production process of H2S-resistant pipeline steel. First, the purity of molten steel is extremely high. During hydrogen sulfide corrosion, hydrogen atoms easily gather and form cracks at the interface between the two phases, so the higher the purity of molten steel, the better. Second, for certain toughness and acid-resistant pipeline mechanical properties, multiple alloying elements such as alloy strengthening elements C and Mn must be added. The higher the alloying elements, the more serious the component segregation. The uneven organization and banded segregation organization caused by component segregation are very likely to cause hydrogen-induced cracks. Third, for thick-gauge pipelines, there are large differences in the deformation in the thickness direction during rolling, and it is difficult for the deformation to penetrate into the center. The temperature gradient and cooling rate during cooling are also very different, and the uniformity of the organization is difficult to control. It is particularly difficult to obtain a uniform organization in the thickness direction under the conditions of rapid rolling and rapid cooling of hot-rolled TMCP.

[0005] Existing normalized high-toughness steel plates are generally designed with medium-high carbon microalloys, which have good strength and toughness. However, the uneven distribution of pearlite in the normalized structure, especially the banded structure in the structure, seriously affects the H2S performance.

[0006] For example, Chinese patent CN117385150A discloses a method for manufacturing a low-cost L360PSL1 normalizing-free pipeline steel pipe, whose chemical composition by mass percentage is C: 0.18-0.22%, Si: 0.2-0.35%, Mn: 1.50-1.60%, P: ≤0.020%, S: ≤0.015%, Al: 0.005-0.040%; its manufacturing process is also disclosed. Chinese patent CN111254362B discloses a method for producing Gr.B grade normalized pipeline steel, the chemical composition of which is C=0.9-0.12%, Si=0.20-0.30%, Mn=1.20%-1.35%, P≤0.015%, S≤0.0025%, Alt=0.020-0.042%, Nb=0.015-0.030%, Ti=0.015-0.020%, B≤0.0005%, Pcm=0.17-0.20%, the remainder is Fe and unavoidable impurities. All of them are medium-high carbon microalloys, and although they have good toughness, the uneven pearlite distribution in the normalized structure, especially the banded structure in the structure, seriously affects the H2S performance. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a L290 grade high-toughness normalized H2S resistant steel plate and a preparation method to achieve good strength, toughness and H2S resistance in view of the insufficient H2S resistance of the current medium and high carbon normalized pipelines.

[0008] To achieve the above object, the present invention provides a L290 grade high toughness normalized H2S resistant steel plate, wherein the chemical elements include by weight percentage: C 0.030-0.049%, Si≤0.20%, Mn 0.91-1.20%, P≤0.013%, S≤0.0010%, Nb 0.01-0.043%, Al 0.019-0.043%, Ti 0.01%-0.02%, and the balance is Fe.

[0009] By using low carbon and medium-low manganese components combined with Nb microalloying, the Mn content in the steel is adjusted to 0.91-1.20%, and the S content in the steel is reduced to below 0.002%, thereby reducing the contents of C, Mn, and S, which are easy to segregate, and improving the HIC and SSCC resistance of the steel. Due to the insufficient strength of the steel matrix caused by low carbon and medium-low manganese content, the precipitation strength of the steel is improved by adding Nb alloy elements, which makes up for the insufficient strength caused by low carbon and medium-low manganese content.

[0010] The specific functions and contents of the chemical elements added in the present invention are:

[0011] C: Carbon is a cheap and effective strengthening element. Carbon is the main element that affects the strength, toughness and weldability of pipeline steel. As the carbon content increases, the weldability deteriorates, the toughness decreases, the segregation intensifies, and the HIC resistance decreases. As the strength level of steel increases, the C content in pipeline steel tends to decrease. Therefore, the present invention controls C to below 0.030-0.049%.

[0012] Si: Si is dissolved in steel and plays a role in solid solution strengthening. Si in steel can reduce the solubility of carbon in austenite. The Si element strongly inhibits the precipitation of carbides along grain boundaries during the bainite transformation process, increases the grain boundary bonding force, and improves toughness. When the silicon content is too high, the plasticity and toughness of the material will be significantly reduced, and the weldability of the steel will be reduced. Therefore, in order to avoid the significant deterioration of the plasticity and toughness of the steel due to the addition of excessive silicon, the present invention controls the Si content to below 0.20%.

[0013] Mn: Mn is a basic alloying element for HSLA steel for pipelines. Manganese has four main functions in steel: (1) reducing the γ→α phase transformation temperature, delaying the austenite phase transformation, reducing the amount of pearlite and refining the ferrite grain size. High Mn can promote the nucleation of acicular ferrite; (2) increasing the solubility product of Nb (C, N) in austenite, reducing its tendency to precipitate prematurely in austenite, reducing the size of precipitated carbides, and promoting precipitation strengthening effects; (3) inherent beneficial effects on toughness. (4) When the Mn content in the steel increases excessively, it is easy to cause component segregation, forming a pearlite band in the center of the plate thickness. When the banded structure is ≥2%, it generally cannot pass the acid-resistant HIC and SSCC tests, and Mn and S in the steel easily form MnS inclusions. Segregation and MnS inclusions are very likely to cause hydrogen induced cracking (HIC) and stress corrosion cracking (SSCC) in the service environment of submarine acid-resistant pipelines. Acid-resistant submarine pipelines generally reduce the manganese content in steel on the basis of pipeline steels of the same steel grade. Therefore, the present invention generally controls the amount of Mn added to 0.91-1.20%.

[0014] P, S: Phosphorus is easy to cause segregation in pipeline steel, which will also deteriorate welding performance, significantly reduce the low-temperature impact toughness of steel, and increase the brittle transition temperature. Sulfur is the main element that affects the HIC and SSC resistance of pipeline steel. Sulfur is easy to combine with manganese to form MnS inclusions, and sulfur also affects the low-temperature impact toughness of pipeline steel. Therefore, pipeline steel should minimize the adverse effects of P and S elements on the performance of steel. The P content should be controlled below 0.013%, and the S content should be controlled below 0.002%. Inclusion modification treatment and other technologies should be used to make the inclusions in the steel spheroidized and evenly distributed to reduce their adverse effects.

[0015] Nb: Since niobium can significantly increase the recrystallization temperature of steel, a high Nb content can ensure a high austenite recrystallization temperature, thereby obtaining a fine structure containing a large number of deformation bands. At the same time, when Nb reaches a certain content in microalloyed controlled rolled steel, fine Nb (C, N) particles can be precipitated and strengthened during the rolling cooling process, thereby improving the strength of the steel. Therefore, the present invention controls the Nb content to 0.01-0.043%.

[0016] Ti: In controlled rolling low-carbon pipeline steel, adding 0.02% titanium can refine the grains and improve the yield strength and toughness of the steel. This performance improvement is mainly related to the fact that titanium can increase the recrystallization temperature of steel and the austenite grain coarsening temperature, thereby controlling the grain size during continuous casting and heating. At the same time, adding Ti to Nb steel can extend the precipitation incubation period of NbC, so that the carbide precipitation start time of Nb-Ti composite steel is later than that of Nb steel, so that the precipitate is finer and more dispersed. Since Ti can combine with N at high temperature to form TiN particles, the addition of Ti is beneficial to the grain control of the heat-affected zone during welding, which is also very beneficial to improving the toughness of the heat-affected zone of welding. Therefore, the present invention controls the Ti content to 0.01-0.02%.

[0017] Al: Al is the main deoxidizing element in steel, which can significantly reduce the oxygen content in steel. At the same time, the combination of aluminum and nitrogen forms AlN, which can effectively refine the grains. However, when the aluminum content in steel exceeds a certain amount, it is easy to cause a significant increase in aluminum oxide inclusions, reduce the cleanliness of the steel, and is not conducive to toughness. Therefore, the present invention generally controls the Als content to 0.01-0.043%.

[0018] Furthermore, the C content is 0.044-0.049%.

[0019] Furthermore, the Mn content is 0.91-1.20%.

[0020] Furthermore, the Nb content is 0.010-0.015% or 0.030-0.043%.

[0021] Furthermore, the Ti content is 0.015% to 0.02%.

[0022] Also provided is a method for preparing the L290 grade high-toughness normalized H2S resistant steel plate as described above, wherein the process flow is blast furnace molten iron → molten iron desulfurization → converter top and bottom composite blowing → argon blowing → LF furnace → RH treatment → continuous casting → sending the ingot for heating → hot rolling in a rolling mill → cooling → coiling → finishing → heat treatment;

[0023] The temperature of the steel billet in the heating furnace is 1140-1180°C;

[0024] The rolled steel plate is subjected to normalizing heat treatment at a temperature of 850-920°C and a holding time of plate thickness + 5-10 min. The unit of plate thickness is mm.

[0025] When the billet is heated in the heating furnace, the heating system is strictly controlled to ensure that the micro-alloying elements are fully dissolved and the austenite grains are prevented from being significantly coarsened. The original austenite grain size is controlled by low temperature (1140-1180℃) heating. The finishing process is low-temperature rolling, which inhibits the tendency of grain growth and refines the final rolling structure. Rapid cooling after rolling promotes the transformation of ferrite to small-sized ferrite and pearlite. The low carbon content in the steel also reduces the large pearlite and banded structure in the steel. The rolled steel plate is subjected to normalizing heat treatment at a heat treatment temperature of 850-920℃ and a holding time of plate thickness (mm) + 5-10min. After normalizing heat treatment, the structure in the steel is more uniform and refined.

[0026] The present invention adopts low carbon, low P, S-medium and low manganese components in combination with Nb microalloying system, reduces the content of C, Mn and S, which are easy to segregate components of the material, reduces the content of large-sized MnS in steel and segregation band structure, and improves the HIC and SSCC resistance of steel. At the same time, the heating and rolling process control means are adopted to ensure the solid solution of Nb and control the grain size during the rolling process, effectively inhibit the growth of the structure and the final rolling structure during the hot rolling process, thereby ensuring the grain size and structure type of the structure under the rolling state, and the core segregation band and the rolled structure are uniformed by normalizing heat treatment, ensuring that the final structure is ferrite and dispersed fine pearlite structure. In terms of performance, the low carbon, medium and low manganese components reduce the proportion of pearlite strengthening phase in the steel, and are effectively supplemented by (Nb, Ti) (C, N) precipitation strengthening and structure refinement control during the Nb and Ti rolling process, ensuring the rolled performance, and by controlling the normalizing process, utilizing the pinning effect of (Nb, Ti) (C, N) on the austenite grain boundary and the fine structure obtained by normalizing cooling to ensure the normalizing performance.

[0027] Furthermore, the superheat degree during continuous casting is 10-25°C.

[0028] Furthermore, when the temperature of the steel billet in the heating furnace is 1140-1150° C., the holding time is 160-260 min; when the temperature of the steel billet in the heating furnace is greater than 1150 and ≤1180° C., the holding time is 60-100 min.

[0029] Furthermore, the hot rolling of the rolling mill is divided into two stages. The first stage is rough rolling, in which 5 to 7 large deformation rolling passes are carried out in the austenite zone at a temperature of 950 to 1100°C, with a pass reduction rate of 10 to 15%, and the thickness of the intermediate billet after rough rolling is 40 to 60 mm; the second stage is finishing rolling, in which the finishing rolling start temperature is ≤950°C, rolling is carried out in the austenite non-recrystallization zone, the reduction rate of each pass is greater than 10%, the cumulative reduction rate of finishing rolling is 60%, and the final rolling temperature is 810 to 860°C.

[0030] Furthermore, the cooling rate is 5-20°C / s, and the final cooling temperature is 550-650°C.

[0031] The present invention adopts low carbon-medium and low manganese components with Nb microalloying, combines BOF-LF-RH-CC smelting process, controlled rolling and controlled cooling TMCP rolling and normalizing heat treatment process, realizes low-level control of harmful elements P, S, O, H, and adopts low-temperature heating to control the original austenite grain size during the hot rolling process, and adopts low-temperature rolling during the finishing rolling process to inhibit the tendency of grain growth and refine the final rolling structure, and rapidly cools after rolling to promote the rapid transformation of ferrite, increase the proportion of small and medium-sized structures, and achieve the goal of organizational strengthening. Through the normalizing heat treatment process, the uneven structure in the rolled state is further uniformed and refined to meet the various index requirements of the HIC A solution acid resistance test. The physical and chemical performance requirements of the L290 grade high-toughness H2S resistant steel plate are: yield Rt0.5≥290MPa, tensile strength Rm415~655MPa, elongation≥28%, -30℃ impact ℃ KV2≥55J, -20℃ DWTT SA≥85%, and the acid resistance test A solution has good H2S corrosion resistance. .

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention is based on a low-carbon and low-manganese component system, reduces the MnS inclusions and banded structure in the steel by reducing the contents of the easily segregated elements Mn, S, and C, reduces the clustered distribution of pearlite in the steel, reduces the decline in strength performance caused by the reduction of alloy strengthening elements, and effectively supplements it through Nb alloy precipitation strengthening and microstructure refinement control during rolling to achieve good strength and toughness. In terms of process, normalizing heat treatment realizes uniform and fine distribution of the structure in the steel, and improves the H2S resistance of the steel matrix. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is the metallographic structure diagram of the rolled plate at 1 / 4 of the thickness at 200 times;

[0034] Figure 2 This is the metallographic structure diagram at 200 times the center of the rolled state;

[0035] Figure 3 This is the 200 times metallographic structure diagram of the 1 / 4 of the centripetal part of the normalized rolling;

[0036] Figure 4 This is a 200 times metallographic structure diagram of the 1 / 2 of the center part of the normalized rolling. DETAILED DESCRIPTION

[0037] The present invention is further described in detail with reference to the following specific examples.

[0038] The specific implementation steps are carried out using acid-resistant pipeline steel with a thickness of 6 to 25.4 mm.

[0039] Process flow: blast furnace molten iron → molten iron desulfurization → converter top and bottom composite blowing → argon blowing → LF furnace → RH treatment → continuous casting → sending ingot for heating → hot rolling in rolling mill → controlled cooling → coiling → finishing (opening) → heat treatment.

[0040] Hot metal pretreatment, desulfurization through KR desulfurization station, end point [S] ≤ 0.0050%, slag removal is required to reach the "0" point.

[0041] The converter is used for oxygen blowing decarburization, and the strong oxidizing atmosphere of the converter is used to remove P from the molten steel, with the end point P≤0.015%.

[0042] LF ladle refining performs white slag making operation to reduce the oxygen content in slag and molten steel, uses high basicity slag to remove S content in steel to ≤0.0020%, and carries out Nb alloying operation.

[0043] The RH treatment time is ≥12min; the circulation time after adding the alloy is ≥5min, Ti-Fe is added according to the target composition, and the chemical composition of each element is fine-tuned according to the target composition requirements. After the vacuum is ended, the composition should be within the internal control range.

[0044] During casting, the superheat is controlled at 10-25°C, casting is carried out at a constant speed, the fan-shaped segment support guide rollers are kept centered, the roller spacing is reduced, the roll gap status of the casting machine is checked and adjusted regularly, the slab size accuracy is controlled to prevent bulging, dynamic light pressure is used to improve the segregation of the continuous casting slab, and the slab low-multiple rating is controlled within C1.0. When conditions permit, electromagnetic stirring is used to increase the axial crystal ratio and improve the slab segregation grade.

[0045] Heating and insulation of the heating furnace. The heating temperature is 1140-1180℃; according to different charging temperatures, when the temperature of the billet in the heating furnace is 1140-1150℃, the insulation time is 160-260min; when the temperature of the billet in the heating furnace is greater than 1150 and ≤1180℃, the insulation time is 60-100min.

[0046] Hot rolling is a two-stage rolling process. In the first stage, rough rolling is performed in a high-temperature austenite zone above 950°C for 5 to 7 large deformation rolling passes, with a pass reduction rate of 10 to 15%, and the thickness of the intermediate billet after rough rolling is 40 to 60 mm; in the second stage, finishing rolling is performed, with the finishing rolling start temperature set to ≤ 950°C, and rolling is performed in the austenite non-recrystallization zone, with a reduction rate of more than 10% for each pass, a cumulative reduction rate of 60% for finishing rolling, and a final rolling temperature of 810 to 860°C.

[0047] After finishing rolling, accelerated cooling is performed to obtain a uniform pearlite + ferrite structure to prevent a substantial increase in the proportion of large-size ferrite in the structure. The accelerated cooling rate is 5 to 20 °C / s, and the final cooling temperature is designed to be 550 to 650 °C.

[0048] Table 1 is a list of chemical components and weight percentages of various embodiments of the present invention

[0049] Elemental composition C Si Mn P S Nb Al Ti Example 1 0.047 0.11 1.03 0.010 0.0010 0.040 0.040 0.020 Example 2 0.044 0.15 1.03 0.012 0.0008 0.033 0.035 0.017 Example 3 0.030 0.15 1.20 0.012 0.0009 0.010 0.019 0.018 Example 4 0.046 0.10 0.90 0.013 0.0010 0.030 0.034 0.011 Example 5 0.045 0.20 0.98 0.012 0.0009 0.033 0.019 0.018 Example 6 0.030 0.15 1.05 0.011 0.0010 0.032 0.045 0.018 Example 7 0.044 0.14 1.05 0.009 0.0009 0.043 0.036 0.016 Example 8 0.049 0.18 0.91 0.009 0.0009 0.005 0.030 0.015 Example 9 0.030 0.15 1.14 0.012 0.0009 0.011 0.019 0.020 Example 10 0.044 0.14 1.10 0.009 0.0009 0.014 0.036 0.016

[0050] Table 2 is a list of the rolling and cooling temperatures of the preparation process of each embodiment of the present invention

[0051] Serial number Heating temperature(℃) Rough rolling end temperature (℃) Finish rolling start temperature (℃) Finishing rolling temperature(℃) Final cooling temperature(℃) Example 1 1180 962 900 825 630 Example 2 1165 965 900 800 580 Example 3 1150 950 910 830 560 Example 4 1170 970 905 810 550 Example 5 1185 965 900 850 640 Example 6 1190 980 950 860 650 Example 7 1180 961 925 825 600 Example 8 1170 960 900 830 610 Example 9 1180 965 915 835 590 Example 10 1182 960 902 825 600

[0052] Table 3 is a list of performance test results of various embodiments of the present invention

[0053]

[0054] Through the physical and chemical comprehensive performance data of the 10 examples 1 to 10, it can be seen that the comprehensive performance of the 10 groups of examples can fully meet the technical index requirements of L245 and L290, and have good anti-H2S corrosion performance. The representative microstructure is shown in the attached figure, where Figure 1 and Figure 2 The microstructures of 1 / 4 and 1 / 2 plate thickness in the rolled state are: Figure 3 and Figure 4 The 1 / 4 and 1 / 2 thickness structures of the rolled steel plate after normalizing heat treatment. As can be seen from the figure, in the rolled state, even if a low-carbon composition design is adopted, the rolled structure is uneven, especially in the center of the rolled plate thickness, there is an obvious segregation band. Such segregation bands have a great influence on H2S performance and are very likely to cause HIC cracks. After normalizing heat treatment, the uniformity of the structure in the steel is greatly improved. Even the banded segregation in the 1 / 2 thickness of the rolled plate is significantly improved after normalizing heat treatment. After normalizing, a relatively uniform ferrite + a small amount of dispersed pearlite is obtained. Therefore, the structure after normalizing performs better in terms of resistance to H2S corrosion.

Claims

1. A L290 grade high toughness normalized H2S resistant steel plate, characterized by: The chemical elements include by weight percentage: C 0.030-0.049%, Si≤0.20%, Mn 0.91-1.20%, P≤0.013%, S≤0.0010%, Nb 0.01-0.043%, Al 0.019-0.043%, Ti 0.01%-0.02%, and the balance is Fe.

2. According to claim 1, the L290 grade high toughness normalized H2S resistant steel plate is characterized by: The C content is 0.044-0.049%.

3. The L290 grade high toughness normalized H2S resistant steel plate according to claim 1, characterized in that: The Mn content is 0.91-1.20%.

4. The L290 grade high toughness normalized H2S resistant steel plate according to claim 1, characterized in that: The Nb content is 0.010-0.015% or 0.030-0.043%.

5. The L290 grade high toughness normalized H2S resistant steel plate according to claim 1, characterized in that: The Ti content is 0.015% to 0.02%.

6. A method for preparing L290 grade high toughness normalized H2S resistant steel plate as claimed in claim 1, wherein the process flow is blast furnace molten iron → molten iron desulfurization → converter top and bottom composite blowing → argon blowing → LF furnace → RH treatment → continuous casting → heating of cast billet → hot rolling in rolling mill → cooling → coiling → finishing → heat treatment; characterized in that: The temperature of the steel billet in the heating furnace is 1140-1180°C; The rolled steel plate is subjected to normalizing heat treatment at a temperature of 850-920°C and a holding time of plate thickness + 5-10 min. The unit of plate thickness is mm.

7. The preparation method according to claim 6, characterized in that: The superheat degree during continuous casting is 10-25°C.

8. The preparation method according to claim 6, characterized in that: When the temperature of the steel billet in the heating furnace is 1140-1150° C., the insulation time is 160-260 minutes; when the temperature of the steel billet in the heating furnace is greater than 1150 and ≤1180° C., the insulation time is 60-100 minutes.

9. The preparation method according to claim 6, characterized in that: The hot rolling of the rolling mill is divided into two stages. The first stage is rough rolling, in which 5 to 7 passes of large deformation rolling are carried out in the austenite region at a temperature of 950 to 1100° C., with a pass reduction rate of 10 to 15%, and the thickness of the intermediate billet after rough rolling is 40 to 60 mm. In the second stage, the finishing rolling, the starting rolling temperature of the finishing rolling is ≤950°C, and the austenite non-recrystallization zone rolling is carried out. The reduction rate of each pass is greater than 10%, the cumulative reduction rate of the finishing rolling is 60%, and the final rolling temperature is 810-860°C.

10. The preparation method according to claim 6, characterized in that: The cooling rate is 5-20°C / s, and the final cooling temperature is 550-650°C.

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