High-toughness light-weight cast aluminum alloy and preparation method thereof

Abstract

The invention discloses a high-toughness lightweight cast aluminum alloy and a preparation method thereof, and relates to the technical field of aluminum alloys. The Al, Er, Sr, Mg, Si and other elements are used for melting preparation, a melting furnace with double stirring blades is adopted, the uniformity of alloy liquid components is effectively improved, grains are refined, the strength and fracture toughness of the alloy are effectively stabilized through three times of step-by-step heating, meanwhile, in the melting process, a certain amount of titanium steam is introduced, an interstitial solid solution is formed between the titanium steam and a matrix, and therefore the alloy strength and fracture toughness are effectively improved. Subsequent induced twin crystal generation is facilitated; after die-casting forming, nano twin crystals are induced to be generated in the aluminum alloy through ultrasonic field hot rolling, and the strength and toughness of the alloy are effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of aluminum alloys, and in particular to a high-strength and lightweight cast aluminum alloy and a preparation method thereof. Background Art

[0002] Among nonferrous metals, aluminum and aluminum alloys are the most commonly used metal structural materials. However, the strength of industrial pure aluminum is very low, so it is not suitable for use as a load-bearing structural component. Usually, a certain proportion of alloying elements is added to pure aluminum to obtain an aluminum alloy with high strength and good casting performance. Aluminum alloys can be divided into two types according to the molding process: cast aluminum alloys and deformed aluminum alloys. Cast aluminum alloys refer to aluminum alloys that are made of parts or blanks by casting methods such as car casting or die casting. Cast aluminum alloys contain a relatively large number of alloying elements, have high strength and excellent plasticity, high temperature performance and cutting performance, and are widely used in aerospace, automobile manufacturing, mechanical electronics, building materials, ships and other fields.

[0003] The rapid development of aerospace, automobile and other industrial fields has led to an increasingly urgent demand for lightweight, high-performance metal materials. Due to its high specific strength and low density, it has received extensive attention and research. The cast structure of aluminum alloys often faces problems such as coarse grains and severe segregation, which will cause the performance of aluminum alloys to fail to meet the production needs of subsequent processing. Summary of the invention

[0004] The object of the present invention is to provide a high-strength and lightweight cast aluminum alloy and a preparation method thereof, so as to solve the problems existing in the prior art.

[0005] In order to solve the above technical problems, the present invention provides the following technical solution: a method for preparing a high-strength and lightweight cast aluminum alloy, comprising the following preparation steps:

[0006] (1) Melting: Add pure aluminum ingots into a furnace with double stirring blades. When the temperature reaches 700°C, turn on the stirring blades, add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, and Al-Mg alloy, and heat to 740°C. Blow a mixed gas consisting of argon and titanium vapor at a flow ratio of 100:1. The blowing time is 100 to 160 seconds, and the blowing depth is 2 cm at the bottom of the melt. Heat to 850°C, add Al-Er alloy, Al-Sr alloy, and Al-Zr alloy, and keep warm for 1 to 2 hours.

[0007] (2) Refining: After the temperature drops to 770°C, add the refining agent, stir for 5 minutes, then cool to 725°C, stop the stirring blade, let stand for 15 minutes, remove the slag, and use 60% argon and 40% nitrogen to blow the alloy liquid from the bottom of the furnace for 15 to 20 minutes;

[0008] (3) Die casting: When the temperature drops to 700°C, die casting is performed, and after cooling to room temperature, the casting is taken out;

[0009] (4) Hot rolling: rolling under the conditions of metal temperature of 450-480°C, total rolling deformation of 50%, 10 hot rolling passes and 5% reduction per pass, and ultrasonic field;

[0010] (5) Homogenization;

[0011] (6) solid solution;

[0012] (7) Time limit.

[0013] Furthermore, the high-strength, toughness and lightweight cast aluminum alloy comprises, by mass percentage, the following elements: Zn 0.58-1.28%, Zr 0.012-0.085%, Mg 2.05-3.8%, Cu 0.11-0.36%, Ti 0.35-0.62%, Si 2.4-3.2%, Er 0.5-1.0%, Sr 0.2-0.6%, and the rest are aluminum and unavoidable impurities, and the content of impurity elements is less than 0.005%.

[0014] Furthermore, in step (1), the bottom stirring blade of the double stirring blades rotates counterclockwise, and the upper stirring blade rotates clockwise, the distance between the upper and lower stirring blades is 35 cm, and the stirring blade speed is 60 to 80 rpm.

[0015] Furthermore, the argon gas flow rate in step (1) is 100 mL / min.

[0016] Furthermore, the amount of the refining agent added in step (2) is 1.5‰ of the weight of the alloy liquid.

[0017] Furthermore, the refining agent in step (2) is prepared by mixing potassium chloride, calcium carbonate, graphite powder, hexachloroethane and sodium chloride in a mass ratio of 32-40:2-8:1-6:10-15:20-32.

[0018] Furthermore, the intensity of the ultrasonic field in step (4) is 1 to 50 Kw / m 2 , the sound field frequency is 10~40kHz.

[0019] Furthermore, the homogenization in step (5) is carried out at 425-450° C. for 20 h, followed by air cooling to room temperature.

[0020] Furthermore, the solid solution in step (6) is: kept at 420-440° C. for 2 hours, and then taken out of the furnace and quenched in water.

[0021] Furthermore, the aging in step (7) is as follows: keeping the temperature at 140-160°C for 1-2 hours, then raising the temperature to 200-220°C and keeping the temperature for 30 minutes.

[0022] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0023] The present invention utilizes Al, Er, Sr, Mg, Si and other elements for melting and preparation, adopts a melting furnace with double stirring blades, and when the metal is initially in a liquid state, turns on the stirring blades, and the upper and lower stirring blades rotate in different directions to cause the bottom molten liquid to surge to the top, so that the metal molten liquid is heated evenly, the melting effect is accelerated, and the alloy elements are prevented from being segregated, the uniformity of the alloy liquid composition is effectively improved, and impurities can be separated from the molten liquid, so as to achieve the purpose of improving the strength and toughness of the cast aluminum alloy, and through three step-by-step heating, the harmful phase can be promoted to dissolve, and then the network distribution of the harmful phase can be broken, so as to stabilize the strength and fracture toughness of the alloy; in addition, the addition of Er and Sr can refine the grains, and cooperate with the step-by-step heating to fully grow, strengthen the effect of pinning dislocations, and improve the strength and toughness of the cast aluminum alloy.

[0024] During the melting process, a certain amount of titanium vapor is introduced to form an interstitial solid solution with the matrix, and the stirring of the stirring blades can prevent the formation of too large a solid solution, which can effectively reduce the stacking fault energy, which is beneficial to the subsequent induction of twin formation, and the titanium compound precipitated through solid solution provides additional strength increment for the matrix; after the metal is molten and hydraulically cast, a large number of dislocations are introduced into the alloy through hot rolling, and the ultrasonic field impact effect is used to promote the rapid movement of dislocations in the aluminum alloy material, thereby inducing the formation of nano twins inside the aluminum alloy, providing a large number of nucleation sites for the formation of precipitation phases in the subsequent natural aging process, accelerating the precipitation of the second phase, making the precipitation phase in the alloy evenly distributed and having a smaller size, thereby improving the strength and toughness of the alloy. DETAILED DESCRIPTION

[0025] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0026] Example 1

[0027] (1) Melting: Add pure aluminum ingots into a furnace with double stirring blades. When the temperature is raised to 700°C, turn on the stirring blades. The bottom stirring blade rotates counterclockwise, and the upper stirring blade rotates clockwise. The distance between the upper and lower stirring blades is 35 cm. The stirring blade speed is 60 rpm. Add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, and Al-Mg alloy. Raise the temperature to 740°C. Blow into a mixed gas consisting of argon and titanium vapor at a flow ratio of 100:1. The blowing time is 100 s. The blowing depth is 2 cm at the bottom of the melt. Raise the temperature to 850°C. Add Al-Er alloy, Al-Sr alloy, and Al-Zr alloy. Keep warm for 1 to 2 h. The argon flow rate is 100 mL / min.

[0028] (2) Refining: After the temperature drops to 770°C, add a refining agent in an amount of 1.5‰ of the alloy liquid weight, stir for 5 minutes, cool to 725°C, stop the stirring blade, let stand for 15 minutes, skim the slag, and use 60% argon and 40% nitrogen to blow the alloy liquid from the bottom of the furnace for 15 minutes; the refining agent is a mixture of potassium chloride, calcium carbonate, graphite powder, hexachloroethane, and sodium chloride in a mass ratio of 32:2:1:10:20;

[0029] (3) Die casting: When the temperature drops to 700°C, die casting is performed, and after cooling to room temperature, the casting is taken out;

[0030] (4) Hot rolling: rolling under the conditions of metal temperature of 450°C, total rolling deformation of 50%, 10 hot rolling passes and 5% pressure reduction per pass, and ultrasonic field; the intensity of the ultrasonic field is 10Kw / m 2 , the sound field frequency is 10kHz;

[0031] (5) Homogenization: homogenize at 425°C for 20 h, then air cool to room temperature;

[0032] (6) Solution treatment: 420℃ for 2h, then quench in water;

[0033] (7) Aging: Keep at 140℃ for 1 hour, then raise the temperature to 200℃ and keep at this temperature for 30 minutes to obtain a high-strength, tough and lightweight cast aluminum alloy.

[0034] Furthermore, the high-strength, toughness and lightweight cast aluminum alloy comprises, by mass percentage, the following elements: Zn 0.58%, Zr 0.012%, Mg 2.05%, Cu 0.11%, Ti 0.35%, Si 2.4%, Er 0.5%, Sr 0.2%, and the rest being aluminum and unavoidable impurities, with the impurity element content being less than 0.005%.

[0035] Example 2

[0036] (1) Melting: Add pure aluminum ingots into a furnace with double stirring blades. When the temperature is raised to 700°C, turn on the stirring blades. The bottom stirring blade rotates counterclockwise, and the upper stirring blade rotates clockwise. The distance between the upper and lower stirring blades is 35 cm. The stirring blade speed is 70 pm. Add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, and Al-Mg alloy. Raise the temperature to 740°C and blow a mixed gas composed of argon and titanium vapor at a flow ratio of 100:1. The blowing time is 130 s and the blowing depth is 2 cm at the bottom of the melt. Raise the temperature to 850°C, add Al-Er alloy, Al-Sr alloy, and Al-Zr alloy, and keep warm for 1.5 h. The argon flow rate is 100 mL / min.

[0037] (2) Refining: After the temperature drops to 770°C, add a refining agent in an amount of 1.5‰ of the alloy liquid weight, stir for 5 minutes, cool to 725°C, stop the stirring blade, let stand for 15 minutes, skim the slag, and use 60% argon and 40% nitrogen to blow the alloy liquid from the bottom of the furnace for 18 minutes; the refining agent is a mixture of potassium chloride, calcium carbonate, graphite powder, hexachloroethane, and sodium chloride in a mass ratio of 36:5:3.5:12:26;

[0038] (3) Die casting: When the temperature drops to 700°C, die casting is performed, and after cooling to room temperature, the casting is taken out;

[0039] (4) Hot rolling: rolling under the conditions of metal temperature of 465°C, total rolling deformation of 50%, 10 hot rolling passes and 5% pressure reduction per pass, and ultrasonic field; the intensity of the ultrasonic field is 25Kw / m 2 , the sound field frequency is 20kHz;

[0040] (5) Homogenization: homogenization treatment is performed at 430°C for 20 h, and then air-cooled to room temperature;

[0041] (6) Solution treatment: 435℃ for 2h, then quench in water;

[0042] (7) Aging: Keep at 150°C for 1.5 hours, then raise the temperature to 210°C and keep at this temperature for 30 minutes to obtain a high-strength, tough and lightweight cast aluminum alloy.

[0043] Furthermore, the high-strength, toughness and lightweight cast aluminum alloy comprises, by mass percentage, the following elements: Zn 1.09%, Zr 0.055%, Mg 3.12%, Cu 0.21%, Ti 0.47%, Si 2.87%, Er 0.78%, Sr 0.4%, and the rest being aluminum and unavoidable impurities, and the content of impurity elements is less than 0.005%.

[0044] Example 3

[0045] (1) Melting: Add pure aluminum ingots into a furnace with double stirring blades. When the temperature is raised to 700°C, turn on the stirring blades. The bottom stirring blade rotates counterclockwise, and the upper stirring blade rotates clockwise. The distance between the upper and lower stirring blades is 35 cm. The stirring blade speed is 80 rpm. Add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, and Al-Mg alloy. Raise the temperature to 740°C and blow a mixed gas composed of argon and titanium vapor at a flow ratio of 100:1. The blowing time is 160 s and the blowing depth is 2 cm at the bottom of the melt. Raise the temperature to 850°C, add Al-Er alloy, Al-Sr alloy, and Al-Zr alloy, and keep warm for 2 h. The argon flow rate is 100 mL / min.

[0046] (2) Refining: After the temperature drops to 770°C, add a refining agent in an amount of 1.5‰ of the alloy liquid weight, stir for 5 minutes, cool to 725°C, stop the stirring blade, let stand for 15 minutes, skim the slag, and use 60% argon and 40% nitrogen to blow the alloy liquid from the bottom of the furnace for 20 minutes; the refining agent is a mixture of potassium chloride, calcium carbonate, graphite powder, hexachloroethane, and sodium chloride in a mass ratio of 40:8:6:15:32;

[0047] (3) Die casting: When the temperature drops to 700°C, die casting is performed, and after cooling to room temperature, the casting is taken out;

[0048] (4) Hot rolling: rolling under the conditions of metal temperature of 480°C, total rolling deformation of 50%, 10 hot rolling passes and 5% pressure reduction per pass, and ultrasonic field; the intensity of the ultrasonic field is 35Kw / m 2 , the sound field frequency is 30kHz;

[0049] (5) Homogenization: homogenization treatment is performed at 450°C for 20 h, and then air-cooled to room temperature;

[0050] (6) Solution treatment: 440℃ for 2h, then quench in water;

[0051] (7) Aging: Keep at 160°C for 2 hours, then raise the temperature to 220°C and keep at this temperature for 30 minutes to obtain a high-strength, tough and lightweight cast aluminum alloy.

[0052] Furthermore, the high-strength, toughness and lightweight cast aluminum alloy comprises, by mass percentage, the following elements: Zn 1.28%, Zr 0.085%, Mg 3.8%, Cu 0.36%, Ti 0.62%, Si 3.2%, Er 1.0%, Sr 0.6%, and the rest being aluminum and unavoidable impurities, with the impurity element content being less than 0.005%.

[0053] Comparative Example 1

[0054] The difference between Comparative Example 1 and Example 2 lies in step (1). Step (1) is changed to: add pure aluminum ingot into a furnace with double stirring blades, and when the temperature is raised to 700°C, turn on the stirring blades, the bottom stirring blade rotates counterclockwise, and the upper stirring blade rotates clockwise. The distance between the upper and lower stirring blades is 35 cm, and the stirring blade speed is 70 pm. Add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, and Al-Mg alloy, and heat to 740°C. Blow into a mixed gas consisting of argon and titanium vapor at a flow ratio of 100:1. The blowing time is 130 s, the blowing depth is 2 cm from the bottom of the melt, and the temperature is raised to 850°C. Add Al-Er alloy and Al-Zr alloy and keep warm for 1.5 h. The argon flow rate is 100 mL / min. The remaining steps are the same as Example 2.

[0055] Comparative Example 2

[0056] The difference between Comparative Example 2 and Example 2 lies in step (1). Step (1) is changed to: add pure aluminum ingot into a furnace with double stirring blades, and when the temperature is raised to 700°C, turn on the stirring blades, the bottom stirring blade rotates counterclockwise, the upper stirring blade rotates clockwise, the distance between the upper and lower stirring blades is 35 cm, the stirring blade speed is 70 pm, add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, Al-Mg alloy, and heat to 740°C. Blow into a mixed gas consisting of argon and titanium vapor at a flow ratio of 100:1. The blowing time is 130 s, the blowing depth is 2 cm from the bottom of the melt, and the temperature is raised to 850°C. Add Al-Sr alloy and Al-Zr alloy and keep warm for 1.5 h. The argon flow rate is 100 mL / min. The remaining steps are the same as Example 2.

[0057] Comparative Example 3

[0058] The difference between Comparative Example 3 and Example 2 lies in step (1). Step (1) is changed to: add pure aluminum ingot into a furnace, heat it to 700°C, add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, and Al-Mg alloy, heat it to 740°C, blow in a mixed gas consisting of argon and titanium vapor at a flow ratio of 100:1, the blowing time is 130s, the blowing depth is 2cm at the bottom of the melt, heat it to 850°C, add Al-Er alloy, Al-Sr alloy, and Al-Zr alloy, and keep warm for 1.5h; the argon flow rate is 100mL / min; the remaining steps are the same as Example 2.

[0059] Comparative Example 4

[0060] The difference between Comparative Example 4 and Example 2 lies in step (1). Step (1) is changed to: add pure aluminum ingot into a furnace with double stirring blades, and when the temperature is raised to 700°C, turn on the stirring blades, the bottom stirring blade rotates counterclockwise, the upper stirring blade rotates clockwise, the distance between the upper and lower stirring blades is 35 cm, the stirring blade speed is 70 pm, add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, Al-Mg alloy, raise the temperature to 850°C, add Al-Er alloy, Al-Sr alloy, Al-Zr alloy, and keep warm for 1.5 hours; the remaining steps are the same as Example 2.

[0061] Comparative Example 5

[0062] The difference between Comparative Example 5 and Example 2 is that step (4) is different. Step (4) is changed to: rolling under the conditions of metal temperature of 465°C, total rolling deformation of 50%, 10 hot rolling passes and 5% pressure reduction per pass; the remaining steps are the same as Example 2.

[0063] Effect example

[0064] Table 1 below shows the performance analysis results of the high-strength, toughness and lightweight cast aluminum alloys of Examples 1 to 3 of the present invention and Comparative Examples 1 to 5.

[0065] Table 1

[0066] Tensile strength(MPa) Yield strength (MPa) Elongation after break (%) Example 1 544 503 10.8 Example 2 552 510 11.2 Example 3 548 505 10.1 Comparative Example 1 501 452 8.1 Comparative Example 2 502 455 8.5 Comparative Example 3 489 438 9.5 Comparative Example 4 511 463 9.0 Comparative Example 5 521 488 9.7

[0067] The present invention utilizes Al, Er, Sr, Mg, Si and other elements for melting and preparation, and adopts a melting furnace with double stirring blades to effectively improve the uniformity of alloy liquid components, and can also promote the separation of impurities from the melt, thereby achieving the purpose of improving the strength and toughness of the cast aluminum alloy, and through three-step temperature rise, it can promote the dissolution of harmful phases, thereby breaking the network distribution of harmful phases, and then stabilizing the strength and fracture toughness of the alloy; in addition, the addition of Er and Sr can refine the grains, and with the step-by-step temperature rise, fully grow, strengthen the effect of pinning dislocations, and improve the strength and toughness of the cast aluminum alloy. During the melting process, a certain amount of titanium vapor is introduced to form an intermetallic space with the matrix. The stirring of the stirring blade can prevent the formation of too large a solid solution, which can effectively reduce the stacking fault energy, which is beneficial to the subsequent induction of twin formation, and the titanium compound precipitated by the solid solution provides additional strength increment for the matrix; after the metal is molten and hydraulically cast, a large number of dislocations are introduced into the alloy through hot rolling, and the ultrasonic field impact effect is used to promote the rapid movement of dislocations in the aluminum alloy material, thereby inducing the formation of nano twins inside the aluminum alloy, providing a large number of nucleation sites for the formation of precipitation phases in the subsequent natural aging process, accelerating the precipitation of the second phase, making the precipitation phase in the alloy evenly distributed and having a smaller size, thereby improving the strength and toughness of the alloy.

[0068] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations within the meaning and scope of the equivalent elements of the claims be included in the invention. Any marking in a claim should not be considered as limiting the claim to which it relates.

Claims

1. A method for preparing a high-strength and lightweight cast aluminum alloy, characterized in that: The method comprises the following preparation steps: (1) Melting: Add pure aluminum ingots into a furnace with double stirring blades. When the temperature reaches 700°C, turn on the stirring blades, add Al-Cu alloy, Al-Si alloy, Al-Zn alloy, and Al-Mg alloy, and heat to 740°C. Blow a mixed gas consisting of argon and titanium vapor at a flow ratio of 100:

1. The blowing time is 100 to 160 seconds, and the blowing depth is 2 cm at the bottom of the melt. Heat to 850°C, add Al-Er alloy, Al-Sr alloy, and Al-Zr alloy, and keep warm for 1 to 2 hours. (2) Refining: After the temperature drops to 770°C, add the refining agent, stir for 5 minutes, then cool to 725°C, stop the stirring blade, let stand for 15 minutes, remove the slag, and use 60% argon and 40% nitrogen to blow the alloy liquid from the bottom of the furnace for 15 to 20 minutes; (3) Die casting: When the temperature drops to 700°C, die casting is performed, and after cooling to room temperature, the casting is taken out; (4) Hot rolling: rolling under the conditions of metal temperature of 450-480°C, total rolling deformation of 50%, 10 hot rolling passes and 5% reduction per pass, and ultrasonic field; (5) Homogenization; (6) solid solution; (7) Time limit.

2. The method for preparing a high-strength and lightweight cast aluminum alloy according to claim 1, characterized in that: The high-strength and light-weight cast aluminum alloy comprises, by mass percentage, Zn 0.58-1.28%, Zr0.012~0.085%, Mg2.05~3.8%, Cu0.11~0.36%, Ti0.35~0.62%, Si2.4~3.2%, Er0.5~1.0%, Sr0.2~0.6%, and the rest are aluminum and unavoidable impurities, and the content of impurity elements is less than 0.005%.

3. The method for preparing a high-strength and lightweight cast aluminum alloy according to claim 1, characterized in that: In step (1), the bottom stirring blade of the double stirring blade rotates counterclockwise, and the upper stirring blade rotates clockwise. The distance between the upper and lower stirring blades is 35 cm, and the stirring blade speed is 60-80 rpm.

4. The method for preparing a high-strength and lightweight cast aluminum alloy according to claim 1, characterized in that: The argon gas flow rate in step (1) is 100 mL / min.

5. The method for preparing a high-strength and lightweight cast aluminum alloy according to claim 1, characterized in that: The amount of refining agent added in step (2) is 1.5‰ of the weight of the alloy liquid.

6. The method for preparing a high-strength and light-weight cast aluminum alloy according to claim 1, characterized in that: The refining agent in step (2) is prepared by mixing potassium chloride, calcium carbonate, graphite powder, hexachloroethane and sodium chloride in a mass ratio of 32-40:2-8:1-6:10-15:20-32.

7. The method for preparing a high-strength and lightweight cast aluminum alloy according to claim 1, characterized in that: The intensity of the ultrasonic field in step (4) is 1 to 50 Kw / m 2 , the sound field frequency is 10~40kHz.

8. The method for preparing a high-strength and light-weight cast aluminum alloy according to claim 1, characterized in that: Step (5) of homogenization: homogenization treatment is carried out at 425-450° C. for 20 h, and then air-cooled to room temperature.

9. The method for preparing a high-strength and lightweight cast aluminum alloy according to claim 1, characterized in that: The solid solution in step (6) is kept at 420-440° C. for 2 h, and then removed from the furnace and quenched in water.

10. The method for preparing a high-strength and lightweight cast aluminum alloy according to claim 1, characterized in that: The aging process in step (7) is as follows: keeping the temperature at 140-160°C for 1-2 hours, then raising the temperature to 200-220°C and keeping the temperature for 30 minutes.