Manganese-copper alloy and preparation method thereof

Abstract

The invention discloses a manganese-copper alloy and a preparation method thereof, the manganese-copper alloy comprises the following components in percentage by mass: 10.0-12.0 wt% of Mn, 4.5-5.5 wt% of Ni, 0.005-0.8 wt% of X, less than or equal to 0.05 wt% of Si, less than or equal to 0.01 wt% of S and the balance of Cu and inevitable impurities, and X is at least one element selected from Fe, Sn, Al and Zn. According to the method, the average resistance temperature coefficient and the resistivity uniformity of the manganese-copper alloy within the specified temperature range can be improved, and the manganese-copper alloy with the low resistance temperature coefficient and the high resistivity uniformity within the wide temperature range is obtained; the average resistance temperature coefficient of the manganese-copper alloy in the temperature zone of 10-80 DEG C is smaller than or equal to 10 ppm / DEG C, the average resistance temperature coefficient of the manganese-copper alloy in the temperature zone of-40-140 DEG C is smaller than or equal to 20 ppm / DEG C, the resistivity is 0.40-0.48 mu omega.m, the resistivity uniformity Z is smaller than or equal to 2%, and application of manganese-copper products to shunt products in the high-end fields of automobiles, energy storage and the like can be met.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of copper alloy, and particularly relates to a manganese-copper alloy and a preparation method thereof. BACKGROUND

[0002] The manganese-copper alloy has a high resistivity and a small resistance temperature coefficient, and is often used for manufacturing shunt products, and then used in the fields of instruments and meters, vehicles, energy storage, charging piles, power supplies, welding equipment and the like. With the rapid development of the field of new energy vehicles, the comprehensive performance of the manganese-copper alloy, including the resistance temperature coefficient and the resistivity uniformity, needs to be further improved.

[0003] In general use scenarios, the temperature of the shunt product is in the range of-40-140 DEG C or even a wider range, and a large temperature rise is generated in the use process, and the resistivity will also change accordingly. With the improvement of the measurement accuracy, higher requirements are put forward for the resistance temperature coefficient of the manganese-copper alloy. At present, the average resistance temperature coefficient of most manganese-copper products is about 20-30 ppm / DEG C (10-80 DEG C), the temperature drift coefficient is relatively high, and the temperature range is relatively narrow, which limits the application of the manganese-copper product in high-end fields such as vehicles and energy storage. In addition, the current national standard requires that the uniformity of the resistance value be less than 5%, but with the development of the shunt product towards high stability, the manganese-copper alloy is required to have higher resistivity uniformity, but the current manganese-copper alloy still has the phenomenon that the resistances of products in the same batch fluctuate greatly. SUMMARY

[0004] The first technical problem to be solved by the application is to provide a manganese-copper alloy with a low resistance temperature coefficient in a wide temperature range and high resistivity uniformity and a preparation method thereof, aiming at the deficiencies of the prior art, which can be applied to shunt products in high-end fields including vehicles and energy storage.

[0005] The technical scheme adopted by the application to solve the above technical problem is that the manganese-copper alloy comprises the following components in mass percentage: Mn: 10.0-12.0 wt%, Ni: 4.5-5.5 wt%, X: 0.005-0.8 wt%, Si<0.05 wt%, S≤0.01 wt%, and the balance is Cu and inevitable impurities, wherein X is at least one element selected from Fe, Sn, Al and Zn.

[0006] Mn: The content of Mn in the Mn-Cu alloy of the present application is 10.0-12.0 wt%. With the increase of temperature, the scattering degree of the lattice vibration of general metal to the free electron will increase, resulting in the increase of the resistance value of the alloy. In comparison, the Mn-Cu alloy has the following other factors affecting the relationship between resistance and temperature: on the one hand, Mn, as one of the main additive elements of the Mn-Cu alloy, will occur atomic segregation (short-range order) in the alloy, and the atomic segregation zone will produce strong scattering to the free electron, resulting in the increase of the resistivity of the alloy; on the other hand, the short-range ordered state is helpful for the "s→d" band transition of the free electron, and the d band of the transition metal has stronger conductive capacity. With the transition of the free electron from the s band to the d band, the conductive capacity of the alloy is enhanced, and the resistivity is reduced. With the increase of temperature, the lattice constant is increased, which promotes the transition of the free electron, so that this part of the resistivity decreases with the increase of temperature. Combining the two influencing mechanisms of "lattice vibration" and "free electron transition", the alloy finally presents the S-shaped "resistance-temperature" relationship of first increasing, then decreasing and then increasing. Therefore, in order to ensure that the temperature range of the alloy is shifted to the target direction, low resistance temperature coefficient is obtained in a wide temperature range, and the resistivity of the alloy is regulated, the content of Mn in the alloy of the present application is controlled to be 10.0-12.0 wt%.

[0007] Ni: The content of Ni in the Mn-Cu alloy of the present application is 4.5-5.5 wt%. Ni is dissolved in the Mn-Cu alloy, which can effectively improve the solidification structure of the alloy, increase the resistivity of the alloy, reduce the average resistance temperature coefficient of the alloy, and improve the corrosion resistance of the alloy. In addition, the presence of Ni makes the matrix phase of the alloy stable, which can inhibit the segregation of Mn in the alloy and improve the organizational stability of the alloy. In order to play the above-mentioned roles, the content of Ni is preferably more than 4.5 wt%. On the other hand, if the content of Ni is higher than 5.5 wt%, the resistivity of the alloy will exceed the ideal value. Therefore, the content of Ni in the alloy of the present application is controlled to be 4.5-5.5 wt%.

[0008] X: The content of X in the Mn-Cu alloy of the present application is 0.005-0.8 wt%, and X is at least one element selected from Fe, Sn, Al and Zn. X element is dissolved in the Mn-Cu alloy, which can affect the "s→d" transition process of the free electron, so that the peak point of "resistance-temperature" moves to the high temperature direction, resulting in the increase of the temperature drift accuracy at -40℃ and the decrease of the temperature drift accuracy at 140℃ in the range of -40-140℃. If the content of the above-mentioned X element is too low, it cannot play the role of regulating the required resistance temperature coefficient in a certain temperature range. On the other hand, if the addition content of X element is too high, it may result in the decrease of the resistivity stability, corrosion resistance and processability of the alloy. Therefore, the content of X element in the alloy of the present application is controlled to be 0.005-0.8 wt%.

[0009] Si: The content of Si in the Mn-Cu alloy of the present application is ≤0.05wt%. Si is an impurity element in the Mn-Cu alloy of the present application. If the content of Si is too high, Si element is easy to combine with elements including Mn to form second phase after annealing in the later processing, thus leading to the decrease of the content of Mn element in solid solution in the melt, resulting in the decrease of the resistivity of the alloy. Further, the resistivity of the alloy is difficult to be stably controlled due to the unstable content of Si, thus leading to the decrease of the uniformity of the resistivity of the alloy. Therefore, the content of Si in the alloy of the present application is controlled to be ≤0.05wt%.

[0010] S: The content of S in the Mn-Cu alloy of the present application is ≤0.01wt%. S is an impurity element in the Mn-Cu alloy of the present application. If the content of S is too high, S is easy to combine with elements including Mn to form S-containing compound in the process of melting and casting, thus leading to the gradual decrease of the content of Mn element in solid solution in the melt, resulting in the gradual decrease of the resistivity of the alloy, i.e. the uniformity of the resistivity of the alloy becomes poor. In order to reduce the adverse effect of the existence of S element on the uniformity of the resistivity, the content of S should be reduced as much as possible in the alloy of the present application. Therefore, the content of S in the alloy of the present application is controlled to be ≤0.01wt%.

[0011] The cross section of the Mn-Cu alloy of the present application is observed by using a scanning electron microscope. Preferably, the number density of S-containing compound with a length of 0.3μm or more in the microstructure of the cross section of the Mn-Cu alloy of the present application is 7.5×10 3 2 The following. In the Mn-Cu alloy of the present application, S, which is an unavoidable impurity, combines with Mn, Ni and X, which are essential elements, to form S-containing compound, which is distributed in the matrix phase in the form of submicron to about 4μm. If the S-containing compound is too much, the uniformity of the resistivity of the alloy will be decreased. In particular, when the size of the S-containing compound is 0.3μm or more, the amount of elements other than S in the S-containing compound, which are helpful to the performance, cannot be ignored. The S-containing compound with a size of 0.3μm or more is easy to lead to the high resistivity of the alloy, and the adverse effect on the uniformity of the resistivity of the alloy becomes more obvious. In addition, the coarse S-containing compound with a size of more than 0.3μm is easy to gather along the grain boundary or the processing direction, which also has adverse effect on the processability of the alloy. Therefore, the present application inhibits the precipitation of excessive harmful coarse S-containing compound in the alloy, so as to improve the uniformity of the resistivity of the alloy and avoid the deterioration of the processability of the alloy. Specifically, the number density of S-containing compound with a length of 0.3μm or more in the alloy of the present application is controlled to be 7.5×10 3 2 The following.

[0012] ​​Preferably, the average grain size of the manganese-copper alloy of the present invention is 5–40 μm. The inflection point of the S-shaped temperature-resistance curve of the manganese-copper alloy is affected by the average grain size; as the average grain size increases, the inflection point shifts towards higher temperatures. If the average grain size of the manganese-copper alloy of the present invention exceeds 40 μm, it will cause an increase in the temperature coefficient of resistance of the alloy. On the other hand, when the average grain size of the alloy is less than 5 μm, the inflection point shifts towards lower temperatures, and the temperature coefficient of resistance of the alloy will also exceed the target range. Therefore, based on the composition of the alloy of the present invention, in order to obtain the required low temperature coefficient of resistance over a wider temperature range, the average grain size of the manganese-copper alloy of the present invention is controlled within 5–40 μm.

[0013] Preferably, the manganese-copper alloy of the present invention has an average temperature coefficient of resistance ≤10ppm / ℃ in the temperature range of 10~80℃, an average temperature coefficient of resistance ≤20ppm / ℃ in the temperature range of -40~140℃, a resistivity of 0.40~0.48μΩ·m, and a resistivity uniformity Z≤2%, which can meet the application requirements of manganese-copper products in shunt products in high-end fields such as automobiles and energy storage.

[0014] The manganese-copper alloy of this invention can be made into plates, strips, bars, wires, etc., as needed.

[0015] The second technical problem to be solved by the present invention is to provide a method for preparing a manganese-copper alloy, comprising the following steps: melting and casting → hot deformation → planing or milling → cold deformation → heat treatment → finished product, wherein the cold deformation and subsequent heat treatment constitute a cycle step, and the cycle step is repeated several times. In the melting and casting process, a covering agent with a thickness of ≥5cm is applied to the surface of the copper alloy melt. The covering agent is pre-baked at a baking temperature of ≥500℃ and a baking time of ≥6h.

[0016] Melting and Casting: The required alloying elements are proportioned according to the design. Copper and nickel raw materials are first added and smelted. After the materials are completely melted, a copper-manganese master alloy is added. After heating, other raw materials are added in descending order of melting point and smelted to obtain a copper alloy melt. During the smelting process, refining agents and degassing sheets are added. After the materials are completely melted, the copper alloy melt is poured into a holding furnace, and a covering agent is added to the holding furnace to form a covering agent with a thickness of ≥5cm on the surface of the copper alloy melt. The covering agent is pre-baked at a baking temperature of ≥500℃ for a baking time of ≥6h. Afterward, the composition ratio is adjusted and slag is removed. After the copper alloy melt is held at a temperature until it is uniformly mixed, horizontal continuous casting or semi-continuous casting is carried out. The smelting and casting temperatures are 1150~1400℃. Because manganese (Mn) is highly reactive, it is prone to oxidation and burn-off during alloy casting. Furthermore, the high melt temperature and high Mn content in the casting environment cause elements, including Mn, to easily combine with sulfur (S) to form sulfur-containing compounds, leading to changes in the final alloy resistivity. Therefore, to mitigate Mn burn-off and reduce the formation and coarsening of sulfur-containing compounds, strict control is needed during the smelting and casting processes through material selection and the use of covering agents, ensuring the subsequent production of alloy products with the desired resistivity range. On one hand, this invention adds manganese in the form of a copper-manganese master alloy during alloy smelting. On the other hand, a covering agent is applied to the surface of the copper alloy melt during casting. However, if the covering agent thickness is too small, the baking temperature is too low, or the baking time is too short, excessive sulfur-containing compounds will be introduced into the melt. Therefore, to improve the coverage and reduce the impact of sulfur-containing compounds on resistivity, this invention controls the thickness of the covering agent applied to the surface of the copper alloy melt to be ≥5 cm, and the covering agent is pre-baked at a temperature ≥500℃ for a time ≥6 hours. The coating agents that can be selected in the preparation method of this invention include graphite flakes, charcoal, anhydrous borax, graphite carburizing agents, etc.

[0017] Hot deformation: The temperature of hot deformation is above 720℃, and the total deformation is above 50%. Hot deformation helps to eliminate internal defects generated in the alloy during the melting and casting process, and at the same time plays a role in ensuring the microstructure of the alloy and meeting the performance requirements of the alloy. Hot deformation can be selected from hot forging, hot extrusion, hot rolling, etc.

[0018] Planing or milling: Removing impurities from the surface of a material by planing or milling.

[0019] Cold deformation: The processing rate of each cold deformation pass is 5-30%, and the total processing rate is 20-95%. Cold deformation improves the dimensional accuracy and tolerance consistency of the alloy. Cold deformation processes include cold drawing, rolling, etc., and straightening processes can be implemented as needed. Preferably, the cold deformation processing rate before the finished product in the final pass is ≤20%.

[0020] Heat treatment: The preparation method of this invention includes at least one heat treatment step. Heat treatment serves to eliminate processing stress and ensure that the alloy can be processed into the required small size. Preferably, in the preparation method of this invention, the final heat treatment step before the finished product is a staged heat treatment, i.e., a first-stage heat treatment is performed at 500–670℃ for 0.5–10 hours, followed by a second-stage heat treatment at 670–750℃ for 0.5–8 hours. Recrystallization occurs in the alloy during the first-stage heat treatment, which reduces internal defects such as vacancies and dislocations, lowering the resistivity of the alloy. Furthermore, performing the first-stage heat treatment within the temperature range of 500–670℃ stabilizes the average grain size in the alloy. Within the second-stage heat treatment temperature range of 670–750°C, vacancies in the alloy further diffuse, accelerating the formation of the low-temperature stable inhomogeneous solid solution "K state". Through the above-mentioned staged heat treatment, the microstructure of the alloy is effectively optimized, ensuring the uniformity of the internal structure of the alloy. Thus, the desired temperature coefficient of resistance and resistivity uniformity are controlled and obtained. In addition, if the cooling rate is slow during the cooling process after heat treatment, the average grain size of the alloy is prone to coarsening. Therefore, preferably, in the preparation method of the present invention, the average cooling rate after heat treatment is controlled at 2°C / min or higher.

[0021] Optionally, before finishing the product, the alloy is subjected to pre-finishing online cleaning at a speed of 15-45 m / min and a temperature of 20-100℃. During online cleaning, a single billet passes through an acid pickling chamber, where the oxide scale is removed under the action of acidic medium and wiping.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. This invention improves the average temperature coefficient of resistance and resistivity uniformity within a specified temperature range by adding Mn, Ni and X elements, while controlling the content of impurity elements Si and S, and further controlling the number density of coarse S-containing compounds in the microstructure of the cross-section of the manganese copper alloy, thereby obtaining a manganese copper alloy with a low temperature coefficient of resistance and high resistivity uniformity over a wider temperature range.

[0024] 2. The manganese-copper alloy of this invention has an average temperature coefficient of resistance ≤10ppm / ℃ in the temperature range of 10~80℃, an average temperature coefficient of resistance ≤20ppm / ℃ in the temperature range of -40~140℃, a resistivity of 0.40~0.48μΩ·m, and a resistivity uniformity Z≤2%, which can meet the application requirements of manganese-copper products in shunt products in high-end fields such as automobiles and energy storage. Attached Figure Description

[0025] Figure 1 The image shows a scanning electron microscope (SEM) image (10000×) of the cross-section of the manganese-copper alloy in Example 1.

[0026] Figure 2 The image shows a scanning electron microscope (SEM) image (10000×) of the cross-section of the manganese-copper alloy in Comparative Example 3. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the embodiments.

[0028] According to the alloy compositions of Examples 1-10 and Comparative Examples 1-2 in Table 1, copper and nickel raw materials were first added and smelted. After the materials were completely melted, a copper-manganese master alloy was added. After heating, other raw materials were added in descending order of melting point and smelted to obtain a copper alloy melt. During the smelting process, refining agents and degassing sheets were added. After the materials were completely melted, the copper alloy melt was poured into a holding furnace, and a covering agent was added to the holding furnace to form graphite flakes with a thickness of ≥5 cm on the surface of the copper alloy melt as a covering agent. The covering agent was pre-baked at a baking temperature of ≥500℃ for a baking time of ≥6 hours. After that, the composition ratio was adjusted and slag was removed. After the copper alloy melt was held at a temperature until it was uniformly mixed, horizontal continuous casting or semi-continuous casting was carried out. The smelting and casting temperatures were 1150-1400℃. Next, hot deformation is carried out at a temperature above 720℃ with a total deformation of more than 50%. The hot-deformed wire blank is then subjected to multiple cycles of planing, cold deformation, heat treatment, and stretching to prepare the desired product. Samples of manganese-copper alloy wire were produced. The cold deformation rate for each pass was 5-30%, with a total processing rate of 20-95%. The final heat treatment process before finishing the product involved a graded heat treatment: a first-stage heat treatment at 500-670℃ for 0.5-10 hours, followed by a second-stage heat treatment at 670-750℃ for 0.5-8 hours. The average cooling rate after heat treatment was controlled to be above 2℃ / min.

[0029] Comparative Example 3: The difference between Comparative Example 3 and the preparation methods of Examples 1-10 and Comparative Examples 1-2 is that unbaked graphite flakes were used as a covering agent during the casting process.

[0030] The characteristics of the wire samples of Examples 1-10 and Comparative Examples 1-3 were evaluated under the following conditions.

[0031] Alloy element testing: ICP analysis was used to test the content of elements such as Mn, Ni, Fe, Sn, Al, Zn, and Si, and a carbon-sulfur analyzer was used to test the S content.

[0032] Resistivity and resistivity uniformity: Tests were conducted according to GB / T 6146-2010 "Test Method for Resistivity of Precision Resistance Alloys," using the Bridge method. Samples were 1 m in length, and the ambient temperature was 20℃. The unit of resistivity was μΩ·m. The actual resistivity was measured at any two sections (a and b) from the same batch of wire samples, according to the formula... Calculate the resistivity uniformity Z, where ρ a The resistivity of segment a is represented by ρ, with units of μΩ·m; b This represents the resistivity of segment b, expressed in μΩ·m.

[0033] Average Temperature Coefficient of Resistance (TCR): The resistivity at each selected temperature point is measured, and the temperature T-accuracy (R) is calculated and fitted from the test data. T -R 20 ) / R 20 Accuracy curve, according to formula TCR={(R max -R min ) / R 20} / (T max -T min Calculate the average temperature coefficient of resistance, where R max R represents the maximum resistance value (Ω) on the accuracy curve. min R represents the minimum resistance value (Ω) on the accuracy curve. 20 T represents the resistance value (Ω) measured at 20℃. max T represents the highest temperature (°C) within a temperature range. min The lowest temperature (°C) represents the temperature range. Among them, the average temperature coefficient of resistance in the temperature range of 10 to 80°C is selected as the actual test temperature points at 10°C, 20°C, and 80°C. The average temperature coefficient of resistance in the temperature range of -40 to 140°C is selected as the actual test temperature points at -40°C, 0°C, 20°C, 80°C, and 140°C. The sample length is ≥55cm.

[0034] Microstructure observation: The grain size of the wire sample was observed using an optical microscope, and the average grain size was calculated based on the results. The cross-section of the sample was observed using a scanning electron microscope. A measurement field of 28 μm × 21 μm was randomly selected. Secondary electron images were obtained under the conditions of a working distance WD 7 mm, accelerating voltage 20 keV, and current 999 pA. Point analysis of particles in the secondary electron phase was performed using EDS. Particles with an analytical concentration of S greater than 1.0 wt% were considered as S-containing particles (i.e., S-containing compounds). The number of S-containing particles with a major diameter greater than 0.3 μm in the measurement field was counted. For 20 non-repeating measurement fields, the number density of S-containing particles was calculated. The total number of particles in each measurement area was divided by the total area of ​​the measurement area to calculate the number density of S-containing particles with a major diameter greater than 0.3 μm in the wire sample. The sum of the major diameters and the number of particles containing the S phase can be determined using the image processing software built into the scanning electron microscope. For particles whose parts are cut off by the boundary, the major diameter is determined by the part of the particle located within the measurement area. Figure 1 and Figure 2 The images are scanning electron microscope (SEM) images of the cross-sections of manganese-copper alloys from Example 1 and Comparative Example 3, respectively.

[0035] Table 1 shows the chemical composition and some preparation processes of the alloys in each embodiment and comparative example. Table 2 shows the microstructure and properties of the alloys in each embodiment and comparative example. As can be seen from Table 2, the number density of S-containing compounds in the microstructure of the wire samples in Examples 1-10 is ≤7.5 × 10⁻⁶. 3 pcs / mm 2 The average grain size is 5–40 μm; the wire samples of Examples 1–10 have an average temperature coefficient of resistance ≤10 ppm / ℃ in the temperature range of 10–80℃ and an average temperature coefficient of resistance ≤20 ppm / ℃ in the temperature range of -40–140℃, a resistivity of 0.40–0.48 μΩ·m, and a resistivity uniformity Z≤2%, indicating that the manganese copper alloy of the present invention has a low temperature coefficient of resistance in a wide temperature range while exhibiting high resistivity uniformity, and is suitable for shunt products including high-end fields such as automobiles and energy storage.

[0036] The elemental content of the wire sample in Comparative Example 1 exceeds the design scope of this invention, and the number density of S compounds in the wire sample in Comparative Example 1 is too high. Therefore, it cannot fully achieve the values ​​of the manganese-copper alloy of this invention in terms of resistance temperature coefficient, resistivity uniformity, and other properties.

[0037] The elemental content of the wire sample in Comparative Example 2 exceeds the design scope of this invention, and it does not meet the requirements in terms of resistivity and average temperature coefficient of resistance.

[0038] The elemental content of the wire sample in Comparative Example 3 exceeds the design scope of this invention. Unbaked graphite flakes were used as a covering agent during the casting process. The number density of coarse S-containing compounds and the average grain size in the microstructure of the wire sample obtained are significantly different from those in the embodiments of this invention. It does not meet the requirements in terms of resistivity, average temperature coefficient of resistance and resistivity uniformity.

[0039]

[0040]

Claims

1. A manganese-copper alloy, characterized in that, The manganese-copper alloy has the following mass percentage composition: Mn: 10.0-12.0 wt%, Ni: 4.5-5.5 wt%, X: 0.005-0.8 wt%, Si≤0.05 wt%, S≤0.01 wt%, with the balance being Cu and unavoidable impurities, wherein X is at least one element selected from Fe, Sn, Al, and Zn.

2. The manganese-copper alloy according to claim 1, characterized in that, In the microstructure of this manganese-copper alloy cross-section, the number density of sulfur-containing compounds with a major diameter greater than 0.3 μm is 7.5 × 10⁻⁶. 3 pcs / mm 2 the following.

3. The manganese-copper alloy according to claim 1, characterized in that, The average grain size of this manganese-copper alloy is 5–40 μm.

4. The manganese-copper alloy according to claim 1, characterized in that, The manganese-copper alloy has an average temperature coefficient of resistance ≤10ppm / ℃ in the temperature range of 10~80℃, an average temperature coefficient of resistance ≤20ppm / ℃ in the temperature range of -40~140℃, a resistivity of 0.40~0.48μΩ·m, and a resistivity uniformity Z≤2%.

5. The method for preparing the manganese-copper alloy according to any one of claims 1 to 4, characterized in that, The process includes the following steps: casting → hot deformation → planing or milling → cold deformation → heat treatment → finished product. The cold deformation and subsequent heat treatment constitute a cycle step, which is repeated several times. In the casting process, a covering agent with a thickness of ≥5cm is applied to the surface of the copper alloy melt. The covering agent is pre-baked at a temperature of ≥500℃ for a time of ≥6h.

6. The preparation method according to claim 5, characterized in that, The final heat treatment process before the finished product is a staged heat treatment, which involves first performing a first-stage heat treatment at 500–670℃ for 0.5–10 hours, and then performing a second-stage heat treatment at 670–750℃ for 0.5–8 hours.

7. The preparation method according to claim 5, characterized in that, The average cooling rate after heat treatment is controlled to be above 2℃ / min.

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