Processing technology of ultra-fine high-strength strip steel

Abstract

The invention relates to the technical field of strip steel processing, in particular to a processing technology of ultra-fine high-strength strip steel, which comprises the following steps: selecting a hot rolled steel coil, and slitting the hot rolled steel coil to obtain a pretreated strip steel coil with a required size; sequentially carrying out pre-pickling and mixed-acid pickling on the pretreated strip steel coil, and then rinsing and drying to obtain an acid-treated strip steel coil; and the strip steel coil subjected to acid treatment is hoisted into a cover type annealing furnace, annealing treatment is carried out, finish rolling and longitudinal shearing splitting are carried out, and the ultra-fine high-strength strip steel is obtained. In the heating stage of annealing treatment, through multi-section stepped heating and heat preservation treatment, the temperature difference between the outer ring position and the middle position of the steel coil is greatly reduced, bonding between the steel coils is effectively prevented, and the problem that the outer ring of the steel coil is subjected to thermal deformation due to too high temperature can be avoided; and meanwhile, in the cooling stage of annealing treatment, low-speed cooling is utilized when the temperature is high, the temperature difference of different positions of the steel ring can be reduced, and the overall quality of the strip steel can be improved through the annealing treatment.

Description

Technical Field

[0001] The invention relates to the technical field of strip steel processing, in particular to a processing technology for ultra-fine high-strength strip steel. Background Art

[0002] Strip steel is a narrow and long steel plate produced by various steel rolling enterprises to meet the needs of industrialized production of various metal or mechanical products in different industrial sectors. Strip steel is supplied in coils and has the advantages of high dimensional accuracy, good surface quality, easy processing, and material saving. It is widely used in the production of cold-formed steel blanks, such as frames, wheel rims, spring leaves, and automotive fine blanking parts.

[0003] When the coil is annealed in a traditional bell-type annealing furnace, the temperature is usually raised to about 400°C at full speed, then the rate is reduced to a specified temperature for insulation treatment, and then the temperature of the coil is quickly reduced to room temperature by air cooling or water cooling. Due to the different heating effects of the outer ring and the middle of the coil, the use of the above annealing process will lead to a large temperature difference between different ring layers, and bonding defects are prone to occur between the ring layers of the steel coil, thereby reducing the quality of the strip. Based on this, a processing technology for ultra-fine high-strength strip steel is proposed. Summary of the invention

[0004] In order to address the deficiencies of the prior art, the present invention provides a processing technology for ultra-fine high-strength strip steel, which greatly reduces the temperature difference at different positions of the steel ring during the annealing process, effectively prevents adhesion between steel coils, and improves the overall quality of the strip steel.

[0005] To achieve the above object, the present invention provides the following technical solution: a processing technology for ultra-fine high-strength strip steel, comprising the following steps:

[0006] (1) Selecting a hot-rolled steel coil and slitting it into strips to obtain a pre-treated strip steel coil of a desired size;

[0007] (2) pre-pickling and mixed acid pickling are performed on the pre-treated strip steel coil in sequence, and then rinsed and dried to obtain the acid-treated strip steel coil;

[0008] (3) The acid-treated strip steel coil is hoisted into a bell-type annealing furnace, annealed, and then finely rolled and longitudinally sheared to obtain ultra-fine high-strength strip steel.

[0009] Preferably, in step (2), the unit tension of the acid-treated steel strip is 29-31 N / mm 2 .

[0010] Preferably, in step (2), the pre-pickling method is as follows: using a sulfuric acid solution with a concentration of 200-240 g / L, pre-pickling at 70-80° C. for 50-60 s; during the pre-pickling, applying variable frequency ultrasonic treatment.

[0011] Preferably, the variable frequency ultrasonic treatment is performed after pre-acid washing for 40-50 seconds.

[0012] Preferably, the frequency of the ultrasonic treatment is 50-100 kHz, and the time of the ultrasonic treatment is 10-15 s.

[0013] Preferably, in step (2), the mixed acid pickling method is as follows: using a concentrated sulfuric acid solution with a concentration of 100-150 g / L and a hydrofluoric acid solution with a concentration of 10-30 g / L, pickling at 35-45°C for 60-70 seconds.

[0014] Preferably, in step (2), desalted water at 60-80° C. is used during rinsing.

[0015] Preferably, in step (3), the hoisting process of the annealing treatment is as follows: hoist the acid-treated strip steel coil and the inner cover onto the furnace in sequence, first introduce nitrogen into the inner cover to purge and expel the air therein; then hoist the heating cover and ignite it, introduce hydrogen into the inner cover to replace the nitrogen, and perform heat treatment; after the heat treatment is completed, remove the heating cover and hoist the cooling cover for cooling treatment, and at the same time introduce nitrogen into the inner cover to expel the hydrogen therein; then remove the cooling cover and the inner cover, and unload the material.

[0016] Preferably, in step (3), the specific method of the annealing treatment is: first, heat to 420-450°C at a rate of 40-50°C / h, and keep warm for 2.5-3.5h; then, heat by 15-20°C each time, and keep warm for 1.0-1.5h, until the temperature reaches 730-750°C, then stop heating, and keep warm for 8-10h; then cool to 400-420°C at a rate of 10-15°C / h, and keep warm for 2-3h; finally, cool to 80-120°C at a rate of 40-50°C / h, and air-cool to room temperature after taking out of the furnace.

[0017] More preferably, the temperature is first raised to 440°C at a rate of 45°C / h and kept at this temperature for 3 hours; then the temperature is raised by 18°C ​​each time and kept at this temperature for 1.2 hours until the temperature reaches 740°C, then the temperature is stopped and kept at this temperature for 8.5 hours; then the temperature is lowered to 410°C at a rate of 12°C / h and kept at this temperature for 2.5 hours; finally, the temperature is lowered to 90°C at a rate of 45°C / h, and the product is taken out of the furnace and air-cooled to room temperature.

[0018] The present invention provides a processing technology for ultra-fine high-strength strip steel, which has the following beneficial effects compared with the prior art:

[0019] In the heating stage of the annealing treatment, the present invention greatly reduces the temperature difference between the outer ring position and the middle position of the steel coil through multi-stage stepped heating and insulation treatment, effectively prevents the adhesion between the steel coils, and avoids the problem of thermal deformation of the outer ring of the steel coil due to excessive temperature; at the same time, in the cooling stage of the annealing treatment, low-rate cooling is used when the temperature is high, which can also reduce the temperature difference at different positions of the steel ring. Through the above annealing treatment, the overall quality of the strip can be improved.

[0020] The present invention applies variable frequency ultrasonic treatment in the pre-pickling stage, and performs ultrasonic treatment a few seconds after the strip enters the pre-pickling stage, so that sufficient hydrogen bubbles generated by entering the oxide layer can burst, thereby stripping the oxide layer; at the same time, variable frequency ultrasonic treatment can generate bubbles of various particle sizes, and the efficiency and effect of pre-pickling can be improved through mutual extrusion and stripping of bubbles of different particle sizes, thereby avoiding the quality reduction of the subsequent steel coil due to inclusions during annealing; in addition, the ultrasonic treatment is performed in the middle stage of the pre-pickling stage, which can ensure the generation of sufficient hydrogen bubbles between the oxide layer and the substrate on the one hand, and prepare for the subsequent bursting and stripping; on the other hand, the ultrasonic treatment time can be greatly shortened, the formation of acid mist can be reduced, and the consumption of sulfuric acid can be reduced.

[0021] The present invention controls the unit tension of the acid-treated strip steel coil to 29-31N / mm 2 During high temperature annealing, the contact force between steel coils can be reduced to reduce the occurrence of bonding. In the actual annealing process, using steel coils with smaller unit tension can only slightly improve the quality of the strip. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0023] Figure 1 This is a schematic diagram of the annealing treatment of the acid-treated steel strip coil in Example 3 of the present invention. DETAILED DESCRIPTION

[0024] The following examples are used to explain the implementation methods of the present application in detail, so that the implementation process of how the present application uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly.

[0025] Example 1

[0026] A processing technology for ultra-fine high-strength strip steel comprises the following steps:

[0027] (1) Select a hot-rolled steel coil (grade SAPH440), cut it longitudinally into strips, and obtain a pre-treated strip steel coil of a desired size;

[0028] (2) The pre-treated steel strip coil is pre-pickled and mixed acid pickled in sequence, then rinsed with desalted water at 60°C and dried to obtain an acid-treated steel strip coil. The unit tension of the acid-treated steel strip coil is controlled to be 31N / mm 2 ;

[0029] The pre-pickling method is as follows: a sulfuric acid solution with a concentration of 200 g / L is used for pre-pickling at 70°C for 60 seconds; during the pre-pickling, variable frequency ultrasonic treatment is applied. The variable frequency ultrasonic treatment is performed after 40 seconds of pre-pickling. The frequencies of the ultrasonic treatment are 50 kHz and 80 kHz, first 50 kHz treatment for 5 seconds, then 80 kHz treatment for 5 seconds, and the total ultrasonic treatment time is 10 seconds.

[0030] The mixed acid pickling method is as follows: use a concentrated sulfuric acid solution with a concentration of 100 g / L and a hydrofluoric acid solution with a concentration of 10 g / L, and pickle at 35°C for 60 seconds.

[0031] (3) The acid-treated strip steel coil is hoisted into a bell-type annealing furnace, annealed, and then finely rolled and longitudinally sheared to obtain ultra-fine high-strength strip steel.

[0032] The specific method of annealing treatment is: first increase the temperature to 420℃ at a rate of 40℃ / h, and keep it warm for 2.5h; then increase the temperature by 15℃ each time, and keep it warm for 1.0h, until the temperature reaches 730℃, stop heating, and keep it warm for 8h; then reduce the temperature to 400℃ at a rate of 10℃ / h, and keep it warm for 2h; finally reduce the temperature to 120℃ at a rate of 40℃ / h, and take it out of the furnace and air cool it to room temperature.

[0033] The mechanical properties of the ultrafine high-strength steel strip obtained in this embodiment were tested, and the specific test results are shown in the following table.

[0034] Table 1 Mechanical properties

[0035] category Tensile strength, MPa Yield strength, MPa Elongation, % Ultra-fine high-strength steel strip 487 415 32 SAPH440 Standard Requirements ≥440 ≥305 ≥30

[0036] Example 2

[0037] A processing technology for ultra-fine high-strength strip steel comprises the following steps:

[0038] (1) Select a hot-rolled steel coil (brand: QSTE500TM), cut it longitudinally into strips, and obtain a pre-treated strip steel coil of a desired size;

[0039] (2) The pre-treated steel strip coil is subjected to pre-pickling and mixed acid pickling in sequence, and then rinsed and dried with desalted water at 80°C to obtain an acid-treated steel strip coil. The unit tension of the acid-treated steel strip coil is controlled to be 29N / mm 2 ;

[0040] The pre-pickling method is as follows: using a sulfuric acid solution with a concentration of 240g / L, pre-pickling at 80°C for 50s; during the pre-pickling, applying variable frequency ultrasonic treatment. The variable frequency ultrasonic treatment is performed after 50s of pre-pickling. The frequency of the ultrasonic treatment is 80-100kHz, first 80kHz treatment for 5s, then 100kHz treatment for 10s, and the total ultrasonic treatment time is 15s.

[0041] The mixed acid pickling method is as follows: use a concentrated sulfuric acid solution with a concentration of 150 g / L and a hydrofluoric acid solution with a concentration of 30 g / L, and pickle at 45°C for 70 seconds.

[0042] (3) The acid-treated strip steel coil is hoisted into a bell-type annealing furnace, annealed, and then finely rolled and longitudinally sheared to obtain ultra-fine high-strength strip steel.

[0043] The specific method of annealing treatment is: first increase the temperature to 450°C at a rate of 50°C / h, and keep it warm for 3.5 hours; then increase the temperature by 20°C each time, and keep it warm for 1.5 hours until the temperature reaches 750°C, then stop heating and keep it warm for 10 hours; then reduce the temperature to 420°C at a rate of 15°C / h, and keep it warm for 3 hours; finally, reduce the temperature to 80°C at a rate of 50°C / h, and take it out of the furnace and air cool it to room temperature.

[0044] The mechanical properties of the ultrafine high-strength steel strip obtained in this embodiment were tested, and the specific test results are shown in the following table.

[0045] Table 2 Mechanical properties

[0046] category Tensile strength, MPa Yield strength, MPa Elongation, % Ultra-fine high-strength steel strip 692 545 18 QSTE500TM Standard Requirements ≥550 ≥500 ≥17

[0047] Example 3

[0048] A processing technology for ultra-fine high-strength strip steel comprises the following steps:

[0049] (1) Select a hot-rolled steel coil (QSTE650TM), cut it longitudinally into strips, and obtain a pre-treated strip steel coil of a desired size;

[0050] (2) The pre-treated steel strip coil is sequentially pre-pickled and mixed acid pickled, then rinsed with desalted water at 70°C and dried to obtain an acid-treated steel strip coil. The unit tension of the acid-treated steel strip coil is controlled to be 30N / mm 2 ;

[0051] The pre-pickling method is as follows: a sulfuric acid solution with a concentration of 220 g / L is used for pre-pickling at 78°C for 56 seconds; during the pre-pickling, variable frequency ultrasonic treatment is applied. The variable frequency ultrasonic treatment is performed after 45 seconds of pre-pickling. The frequencies of the ultrasonic treatment are 50 kHz and 100 kHz, first 50 kHz treatment for 5 seconds, then 100 kHz treatment for 7 seconds, and the total ultrasonic treatment time is 12 seconds.

[0052] The mixed acid pickling method is as follows: using a concentrated sulfuric acid solution with a concentration of 125 g / L and a hydrofluoric acid solution with a concentration of 18 g / L, pickling at 40°C for 66 seconds.

[0053] (3) The acid-treated strip steel coil is hoisted into a bell-type annealing furnace, annealed, and then finely rolled and longitudinally sheared to obtain ultra-fine high-strength strip steel.

[0054] The specific method of annealing treatment is: first increase the temperature to 440°C at a rate of 45°C / h and keep it warm for 3 hours; then increase the temperature by 18°C ​​each time and keep it warm for 1.2 hours until the temperature reaches 740°C, then stop heating and keep it warm for 8.5 hours; then reduce the temperature to 410°C at a rate of 12°C / h and keep it warm for 2.5 hours; finally reduce the temperature to 90°C at a rate of 45°C / h, take it out of the furnace and air cool it to room temperature.

[0055] The mechanical properties of the ultrafine high-strength steel strip obtained in this embodiment were tested, and the specific test results are shown in the following table.

[0056] Table 3 Mechanical properties

[0057] category Tensile strength, MPa Yield strength, MPa Elongation, % Ultra-fine high-strength steel strip 875 696 16 QSTE650TM Standard Requirements ≥700 ≥650 ≥14

[0058] Comparative Example 1

[0059] A processing technology for ultra-fine high-strength strip steel comprises the following steps:

[0060] Steps (1)-(2) are the same as those in Example 3. The difference is that:

[0061] (3) The acid-treated strip steel coil is hoisted into a bell-type annealing furnace, annealed, and then finely rolled and longitudinally sheared to obtain ultra-fine high-strength strip steel.

[0062] The specific method of annealing treatment is: first increase the temperature to 440°C at a rate of 45°C / h and keep it warm for 3 hours; then increase the temperature at a rate of 18°C / h until the temperature reaches 740°C, stop heating, and keep it warm for 8.5 hours; then decrease the temperature to 410°C at a rate of 12°C / h and keep it warm for 2.5 hours; finally decrease the temperature to 90°C at a rate of 45°C / h, take it out of the furnace and air cool it to room temperature.

[0063] The mechanical properties of the ultrafine high-strength steel strip obtained in this comparative example were tested, and the specific test results are shown in the following table.

[0064] Table 4 Mechanical properties

[0065] category Tensile strength, MPa Yield strength, MPa Elongation, % Ultra-fine high-strength steel strip 780 662 15 QSTE650TM Standard Requirements ≥700 ≥650 ≥14

[0066] Comparative Example 2

[0067] A processing technology for ultra-fine high-strength strip steel comprises the following steps:

[0068] Steps (1)-(2) are the same as those in Example 3. The difference is that:

[0069] (3) The acid-treated strip steel coil is hoisted into a bell-type annealing furnace, annealed, and then finely rolled and longitudinally sheared to obtain ultra-fine high-strength strip steel.

[0070] The specific method of annealing treatment is: first increase the temperature to 440°C at a rate of 45°C / h, and keep it warm for 3 hours; then increase the temperature at a rate of 18°C / h until the temperature reaches 740°C, stop heating, and keep it warm for 8.5 hours; then reduce the temperature to 90°C at a rate of 45°C / h, take it out of the furnace and air cool it to room temperature.

[0071] The mechanical properties of the ultrafine high-strength steel strip obtained in this comparative example were tested, and the specific test results are shown in the following table.

[0072] Table 5 Mechanical properties

[0073]

[0074]

[0075] Comparative Example 3

[0076] A processing technology for ultra-fine high-strength strip steel comprises the following steps:

[0077] Steps (1) and (3) are the same as those in the corresponding steps in Example 3. The difference is that:

[0078] (2) The pre-treated steel strip is subjected to pre-pickling and mixed acid pickling in sequence. The pre-pickling method is as follows: a sulfuric acid solution with a concentration of 220 g / L is used for pre-pickling at 78°C for 56 seconds; the mixed acid pickling method is as follows: a concentrated sulfuric acid solution with a concentration of 125 g / L and a hydrofluoric acid solution with a concentration of 18 g / L are used for pickling at 40°C for 66 seconds, and then rinsed and dried with desalted water at 70°C to obtain an acid-treated steel strip. The unit tension of the acid-treated steel strip is controlled to be 30 N / mm 2 .

[0079] The mechanical properties of the ultrafine high-strength steel strip obtained in this comparative example were tested, and the specific test results are shown in the following table.

[0080] Table 6 Mechanical properties

[0081] category Tensile strength, MPa Yield strength, MPa Elongation, % Ultra-fine high-strength steel strip 821 680 15 QSTE650TM Standard Requirements ≥700 ≥650 ≥14

[0082] Comparative Example 4

[0083] A processing technology for ultra-fine high-strength strip steel comprises the following steps:

[0084] Steps (1) and (3) are the same as those in the corresponding steps in Example 3. The difference is that:

[0085] (2) The pre-treated steel strip coil is sequentially pre-pickled and mixed acid pickled, then rinsed with desalted water at 70°C and dried to obtain an acid-treated steel strip coil. The unit tension of the acid-treated steel strip coil is controlled to be 30N / mm 2 ;

[0086] The pre-pickling method is as follows: using a sulfuric acid solution with a concentration of 220 g / L, pre-pickling at 78°C for 56 seconds; during the pre-pickling, single-frequency ultrasonic treatment is applied. The single-frequency ultrasonic treatment is performed after 45 seconds of pre-pickling. The frequency of the ultrasonic treatment is 100 kHz, and the total time of the ultrasonic treatment is 12 seconds.

[0087] The mixed acid pickling method is as follows: using a concentrated sulfuric acid solution with a concentration of 125 g / L and a hydrofluoric acid solution with a concentration of 18 g / L, pickling at 40°C for 66 seconds.

[0088] The mechanical properties of the ultrafine high-strength steel strip obtained in this comparative example were tested, and the specific test results are shown in the following table.

[0089] Table 7 Mechanical properties

[0090] category Tensile strength, MPa Yield strength, MPa Elongation, % Ultra-fine high-strength steel strip 846 687 15 QSTE650TM Standard Requirements ≥700 ≥650 ≥14

[0091] Comparative Example 5

[0092] A processing technology for ultra-fine high-strength strip steel comprises the following steps:

[0093] Steps (1) and (3) are the same as those in the corresponding steps in Example 3. The difference is that:

[0094] (2) The pre-treated steel strip coil is sequentially subjected to pre-pickling and mixed acid pickling. The pre-pickling method is as follows: a sulfuric acid solution with a concentration of 220 g / L is used for pre-pickling at 78° C. for 56 seconds; the mixed acid pickling method is as follows: a concentrated sulfuric acid solution with a concentration of 125 g / L and a hydrofluoric acid solution with a concentration of 18 g / L are used for pickling at 40° C. for 66 seconds, and then rinsed and dried with desalted water at 70° C. to obtain an acid-treated steel strip coil. The unit tension of the acid-treated steel strip coil is controlled to be 33 N / mm 2 .

[0095] The mechanical properties of the ultrafine high-strength steel strip obtained in this comparative example were tested, and the specific test results are shown in the following table.

[0096] Table 8 Mechanical properties

[0097] category Tensile strength, MPa Yield strength, MPa Elongation, % Ultra-fine high-strength steel strip 870 688 16 QSTE650TM Standard Requirements ≥700 ≥650 ≥14

[0098] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A processing technology for ultra-fine high-strength strip steel, characterized in that: The following steps are involved: (1) Selecting a hot-rolled steel coil and slitting it longitudinally to obtain a pre-treated strip steel coil of a desired size; (2) pre-pickling and mixed acid pickling are performed on the pre-treated strip steel coil in sequence, and then rinsed and dried to obtain the acid-treated strip steel coil; (3) The acid-treated strip steel coil is hoisted into a bell-type annealing furnace, annealed, and then finely rolled and longitudinally sheared to obtain ultra-fine high-strength strip steel.

2. The processing technology of ultra-fine high-strength steel strip according to claim 1 is characterized in that: In step (2), the unit tension of the acid-treated steel coil is 29-31 N / mm 2 .

3. The processing technology of ultrafine high-strength steel strip according to claim 1, characterized in that: In step (2), the pre-pickling method is as follows: using a sulfuric acid solution with a concentration of 200-240 g / L, pre-pickling at 70-80° C. for 50-60 seconds; during the pre-pickling, applying variable frequency ultrasonic treatment.

4. The processing technology of ultrafine high-strength steel strip according to claim 3 is characterized in that: The variable frequency ultrasonic treatment is carried out after pre-acid washing for 40-50 seconds.

5. The processing technology of ultrafine high-strength steel strip according to claim 3 is characterized in that: The frequency of the ultrasonic treatment is 50-100 kHz, and the time of the ultrasonic treatment is 10-15 s.

6. The processing technology of ultrafine high-strength steel strip according to claim 1, characterized in that: In step (2), the mixed acid pickling method is as follows: using a concentrated sulfuric acid solution with a concentration of 100-150 g / L and a hydrofluoric acid solution with a concentration of 10-30 g / L, pickling at 35-45°C for 60-70 seconds.

7. The processing technology of ultrafine high-strength steel strip according to claim 1, characterized in that: In step (2), desalted water at 60-80° C. is used during rinsing.

8. The processing technology of ultrafine high-strength steel strip according to claim 1, characterized in that: In step (3), the hoisting process of the annealing treatment is as follows: hoist the acid-treated strip steel coil and the inner cover onto the furnace in sequence, first introduce nitrogen into the inner cover to purge and expel the air therein; then hoist the heating cover and ignite it, introduce hydrogen into the inner cover to replace the nitrogen, and perform heat treatment; after the heat treatment is completed, remove the heating cover and hoist the cooling cover for cooling treatment, and at the same time introduce nitrogen into the inner cover to expel the hydrogen therein; then remove the cooling cover and the inner cover, and unload the material.

9. The processing technology of ultrafine high-strength steel strip according to claim 1, characterized in that: In step (3), the specific method of the annealing treatment is: first, heat the temperature to 420-450°C at a rate of 40-50°C / h, and keep it warm for 2.5-3.5h; then, heat it up by 15-20°C each time, and keep it warm for 1.0-1.5h until the temperature reaches 730-750°C, then stop heating, and keep it warm for 8-10h; then cool it down to 400-420°C at a rate of 10-15°C / h, and keep it warm for 2-3h; finally, cool it down to 80-120°C at a rate of 40-50°C / h, and then take it out of the furnace and air cool it to room temperature.

10. The processing technology of ultrafine high-strength steel strip according to claim 9, characterized in that: First, increase the temperature to 440℃ at a rate of 45℃ / h and keep it warm for 3h; then increase the temperature by 18℃ each time and keep it warm for 1.2h until the temperature reaches 740℃, then stop heating and keep it warm for 8.5h; then decrease the temperature to 410℃ at a rate of 12℃ / h and keep it warm for 2.5h; finally, decrease the temperature to 90℃ at a rate of 45℃ / h, take it out of the furnace and air cool it to room temperature.

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