Method for controlling carbide non-uniformity of high-carbon chromium bearing steel

Abstract

The invention particularly discloses a control method for carbide nonuniformity of high-carbon-chromium bearing steel, which comprises the following steps: controlling the pulling speed to be 0.85 m / min and the specific water flow to be 0.35 L / Kg in a continuous casting process, and showing that a casting blank model developed by the pulling speed and the specific water flow is 0.35 L / Kg; bearing steel is slightly pressed forwards, the core part quality and dendritic segregation of the casting blank can be effectively improved under the condition that the solid fraction is 40%-100%, and when the solid fraction fs of the center of the casting blank is smaller than 40%, pressing is not carried out; when fs is larger than or equal to 40% and smaller than 50%, the required rolling reduction is larger than or equal to 0.5 mm and smaller than or equal to 2.0 mm; when fs is greater than or equal to 50% and less than 60%, the required rolling reduction is greater than or equal to 1.5 mm and less than or equal to 3.0 mm; when fs is greater than or equal to 60% and less than 90%, the required rolling reduction is greater than or equal to 3.0 mm and less than or equal to 4.0 mm; when 90% < = fs < = 100%, 1.5 mm < = required rolling reduction < = 3.0 mm. By adopting the method, a high-end bearing steel GCr15 product which has the specification of phi 20-60mm and meets the requirements of national standards on strip-shaped and net-shaped carbides can be produced.

Description

Technical Field

[0001] The invention relates to the technical field of special steel smelting, and in particular to a method for controlling carbide heterogeneity of high carbon chromium bearing steel GCr15 for rolling elements with a specification of Φ20-60mm. Background Art

[0002] Bearings are essential core components in mechanical equipment, and the bearing industry is a fundamental and strategic national industry. Bearing steel, one of the most demanding grades of alloy steel, requires high purity, uniform roll weave, and fine grains, earning it the nickname "King of Steel." Furthermore, bearing steel has the most stringent quality requirements of all alloy steels, requiring the most extensive inspection procedures. The quality of bearing steel is recognized worldwide as a key indicator of a country's metallurgical prowess.

[0003] High-carbon chromium bearing steel GCr15 (designated 100Cr6 in Germany, 52100 in the United States, and SUJ2 in Japan, among others) is an excellent grade of steel. Since its invention, it has spanned over a century, and its chemical composition remains unchanged to this day, making it the primary steel used in rolling bearings. Regarding the impact of carbide inhomogeneity on the fatigue performance of GCr15 bearing steel, carbide liquid precipitation, equivalent to inclusions, has the greatest impact on the fatigue life of the bearing steel. Carbide banding affects the brittleness of the steel, while carbide networks affect its toughness. In the absence of bearing steel liquid precipitation, carbide banding has a greater impact on the fatigue life of the bearing steel than carbide networks. However, if the carbide levels meet the requirements of GB / T18254-2016, "High Carbon Chromium Bearing Steel," the impact on the fatigue life of the bearing steel is not significant.

[0004] According to market demand, we developed and produced GCr15 bearing steel for Φ20~60mm pyrotechnical rolling elements. When sampling and testing according to the requirements of the rolling elements, it was found that there was no carbide liquid precipitation in the bearing steel, but the carbide bands and networks often exceeded the 2.5 level required by the national standard "GB / T18254-2016 High Carbon Chromium Bearing Steel", which could not meet the use requirements of the rolling elements.

[0005] Therefore, in order to ensure that the carbide inhomogeneity of rolling element bearing steel meets the standard requirements, it is urgent to develop a new process and method for controlling the carbide inhomogeneity of GCr15 bearing steel. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for controlling the unevenness of carbides in high-carbon chromium bearing steel. By controlling the C and Cr components within the lower limit of the standard requirements during smelting, reducing the continuous casting speed, increasing the continuous casting superheat, pressing forward with light reduction and large reduction amount and light reduction at a solid phase ratio of 40% to 100%, and subjecting the bearing steel to a longer high-temperature diffusion annealing during rolling and water cooling after rolling, the carbide unevenness meets the standard requirements.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] A method for controlling carbide heterogeneity of high-carbon chromium bearing steel is disclosed. The method comprises controlling a casting speed of 0.80 to 0.85 m / min and a water content of 0.30 to 0.35 L / kg during a continuous casting process. A casting model developed at the casting speed and water content indicates that forward pressing under light pressure and light pressure at a solid fraction of 40% to 100% can effectively improve the core quality and dendritic segregation of the casting. When the solid fraction fs at the center of the casting is less than 40%, no pressure reduction is performed.

[0009] When 40%≤fs<50%, 0.5mm≤required reduction≤2.0mm;

[0010] When 50%≤fs<60%, 1.5mm≤required reduction≤3.0mm;

[0011] When 60%≤fs<90%, 3.0mm≤required reduction≤4.0mm;

[0012] When 90%≤fs≤100%, the required reduction is 1.5mm≤3.0mm.

[0013] Too high a pulling speed cannot effectively reduce the dendritic segregation of the ingot, and too low a pulling speed will lead to other problems, such as overheating and overoxidation. Therefore, the present invention controls the pulling speed to 0.85m / min, which helps to reduce the light dendritic segregation of the ingot, and further helps to reduce the generation of banded carbides in bearing steel.

[0014] Preferably, the C content in the high carbon chromium bearing steel is adjusted to 0.95% to 0.96%, and the Cr content is adjusted to 1.40% to 1.45%, so as to reduce the probability of C and Cr in the steel combining to form carbide (Fe, Cr) 3C.

[0015] Preferably, the superheat of the continuous casting process is controlled at 30-40°C, and through strong cooling (the water content is controlled at 0.30-0.35L / Kg, i.e. strong cooling) and high superheat pouring, more columnar crystals are generated in the billet, especially in the core of the billet, because columnar crystals can more effectively reduce dendritic segregation than equiaxed crystals; when the superheat of bearing steel production is high, a larger reduction amount is adopted in light reduction, which helps to improve the center segregation of the billet, thereby reducing the level of banded structure in the center.

[0016] Preferably, the control method further comprises extending the high-temperature diffusion annealing time, and the high-temperature diffusion annealing time is ≥230 min.

[0017] Preferably, the control method further comprises the step of adding water cooling after rolling, and the cooling bed on the round steel returns to a homogenizing temperature of 650-680°C.

[0018] The bearing steel is subjected to longer high-temperature diffusion annealing and water cooling. The high-temperature diffusion annealing time of the bearing steel is increased to greater than or equal to 230 minutes to make the composition of the ingot more uniform and reduce the dendritic segregation caused by the continuous casting process. The homogenization temperature of the round steel upper cooling bed is controlled to below 700°C to avoid the high-temperature precipitation range of carbides (700-850°C), inhibit the coarse precipitation of carbide networks and carbide bands, and thus reduce the carbide network and band levels.

[0019] Preferably, during the rolling, the temperature of the heating section II is controlled at 1200-1230°C, the temperature of the soaking section is controlled at 1220-1250°C, and the heating time of the heating section II and the soaking section high temperature section is controlled at ≥230 minutes.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] By controlling the C and Cr components at the lower limit of the standard requirements, reducing the continuous casting speed, increasing the continuous casting superheat, applying light reduction and forward pressing, large reduction and light reduction at a solid phase ratio of 40% to 100%, and subjecting the bearing steel to a longer high-temperature diffusion annealing and post-rolling water cooling during rolling, it is possible to produce high-end bearing steel GCr15 products with specifications of Φ20 to 60 mm that meet the requirements for rolling elements used in one-fired materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The spheroidized network carbide 1.0 grade metallographic structure diagram of the round steel of Example 1 is shown;

[0023] Figure 2 The 1.0 grade metallographic structure diagram of the spheroidized banded carbide of the round steel of Example 1 is shown;

[0024] Figure 3 The spheroidized carbide level 1.0 metallographic structure diagram of the round steel of Example 2 is shown;

[0025] Figure 4 The 1.5 grade metallographic structure diagram of the spheroidized banded carbide of the round steel of Example 2 is shown;

[0026] Figure 5 The spheroidized network carbide 2.0 grade metallographic structure diagram of the round steel of Example 3 is shown;

[0027] Figure 6 The figure shows the spheroidized banded carbide 2.0 grade metallographic structure of the round steel of Example 3;

[0028] Figure 7 The 3.0 grade metallographic structure diagram of the spheroidized network carbide of the comparative round steel is shown;

[0029] Figure 8 The metallographic structure diagram of the spheroidized banded carbides of the comparative round steel is shown. DETAILED DESCRIPTION

[0030] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below through examples.

[0031] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0032] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive of other embodiments.

[0033] Banded carbides are secondary carbides formed by dendritic segregation during the solidification of molten steel. If banded carbides are located on rolling elements, they can severely impact the contact fatigue life of the bearing. Reticular carbides refer to excess secondary carbides that precipitate in a network along the austenite grain boundaries in hypereutectoid steel. They typically form during post-rolling cooling. Slow post-rolling cooling results in more carbide precipitation, leading to severe network carbides. Fast post-rolling cooling results in less carbide precipitation, resulting in lighter network carbides. Because the formation mechanisms of banded and network carbides in bearing steel differ, they must be controlled separately.

[0034] The following describes in detail the method for controlling the unevenness of carbides in high-carbon chromium bearing steel according to the present invention through three embodiments. The composition and proportion of the bearing steel GCr15 described in the three embodiments are shown in Table 1.

[0035] Table 1 GCr15 composition requirements

[0036]

[0037] Example 1:

[0038] The production method of 5 batches of GCr15 steel, including 1 batch of bearing steel GCr15, is as follows:

[0039] (1) KR desulfurization, S content is 0.001%.

[0040] (2) Argon is blown throughout the bottom blowing process of the LD converter. In order to reduce the O content in the steel, the final C content is 0.17%. Ferrosilicon and high carbon ferromanganese are added starting from 1 / 3 of the steel being tapped, followed by ductile ferrochrome, then ordinary recarburizer, and finally top slag lime.

[0041] (3) Add refining slag to the LF refining furnace to produce white slag and maintain it, while ensuring the fluidity of the top slag. Use aluminum particles for strong deoxidation. Add the deoxidizer evenly and do not add it in a concentrated manner. Fine-tune the C and Cr components in the LF furnace to control the C content to 0.94% and the Cr content to 1.41%.

[0042] (4) It is strictly forbidden to add alloy to RH, return to LF furnace to increase temperature, perform vacuum treatment and soft blowing stirring.

[0043] (5) The continuous casting adopts an integral tundish, which is cleaned and emptied by argon before pouring. The argon blowing time is 3 minutes. The casting is protected throughout the whole process and the pouring is started outside the tank. The continuous casting speed is 0.85m / min and the water content is 0.35L / Kg. Under this speed and water content, the developed casting model shows that the solid phase ratio of roller 1# is 42%, the solid phase ratio of roller 2# is 53%, the solid phase ratio of roller 3# is 64%, the solid phase ratio of roller 4# is 73%, the solid phase ratio of roller 5# is 89%, and the solid phase ratio of roller 6# is 100%. According to production practice, it is found that the forward pressing with light pressure and the large pressing amount and the light pressing at the solid phase ratio of 40% to 100% can improve the casting process. Based on best production practices, a reduction of 1.0mm for the 1# roll, 2.5mm for the 2# and 6# rolls, and 3.5mm for the 3#, 4#, and 5# rolls, for a total reduction of 16.5mm, achieved optimal improvements in core quality and dendritic segregation. The continuous casting superheat was controlled at 36°C. To ensure uniform composition and strand quality, advanced electromagnetic stirring was employed in the mold and at the end of solidification. At the end of the pour, a 5.5t holdover steel was retained in the tundish. Composition measurements during continuous casting revealed a C content of 0.95% and a Cr content of 1.41%. 240mm×300mm strands were then drawn.

[0044] (6) The steel from this furnace was rolled into Φ20mm GCr15 round steel. During rolling, the temperature of the heating section II was controlled at 1200-1230°C, the temperature of the soaking section was controlled at 1220-1250°C, and the heating time of the heating section II and the soaking section high temperature section was controlled at 235 minutes. After rolling, the round steel was water-cooled, and the return homogenization temperature on the cooling bed was controlled at 650°C. After spheroidizing annealing, the Φ20mm GCr15 round steel from this furnace was tested for carbide heterogeneity, network carbides, band carbides, and carbide liquid precipitation. The results all met the requirements for rolling element bearing steel. The specific test results are shown in Table 2.

[0045] Example 2

[0046] The production method of 6 heats of GCr15 steel was as follows:

[0047] (1) KR desulfurization, S content is 0.002%.

[0048] (2) Argon is blown throughout the bottom blowing process of the LD converter to reduce the O content in the steel. The final C content is 0.19%. Ferrosilicon and high carbon ferromanganese are added starting from 1 / 3 of the steel being tapped, followed by ductile ferrochrome, then ordinary recarburizer, and finally top slag lime.

[0049] (3) Add refining slag to the LF refining furnace to produce white slag and maintain it, while ensuring the fluidity of the top slag. Use aluminum particles for strong deoxidation. Add the deoxidizer evenly and do not add it in a concentrated manner. Fine-tune the C and Cr components in the LF furnace to control the C content to 0.94% and the Cr content to 1.43%.

[0050] (4) It is strictly forbidden to add alloy to RH, return to LF furnace to increase temperature, perform vacuum treatment and soft blowing stirring.

[0051] (5) The continuous casting adopts an integral tundish, which is cleaned and emptied by argon before pouring. The argon blowing time is 4 minutes. The casting is protected throughout the whole process and the pouring is started outside the tank. The continuous casting speed is 0.80m / min and the water content is 0.30L / Kg. Under this speed and water content, the developed casting model shows that the solid phase ratio of roller 1# is 46%, the solid phase ratio of roller 2# is 57%, the solid phase ratio of roller 3# is 66%, the solid phase ratio of roller 4# is 75%, the solid phase ratio of roller 5# is 92%, and the solid phase ratio of roller 6# is 100%. According to production practice, it is found that the forward pressing with light pressure and the large pressing amount and the light pressing at the solid phase ratio of 40% to 100% can improve the casting process. Based on best production practices, the core quality and dendritic segregation of the ingot were optimized with a reduction of 1.2mm for the 1# roll, 2.7mm for the 2# and 6# rolls, and 3.3mm for the 3#, 4#, and 5# rolls, for a total reduction of 16.5mm. The continuous casting superheat was controlled at 33°C. To ensure uniform composition and ingot quality, advanced electromagnetic stirring was employed in the mold and at the end of solidification. At the end of the pour, 6 tons of excess steel was retained in the tundish. Composition measurements during continuous casting revealed a C content of 0.95% and a Cr content of 1.43%. The ingots were then drawn into 240mm x 300mm ingots.

[0052] (6) The steel from this furnace was rolled into Φ40mm GCr15 round steel. During rolling, the temperature of the heating section II was controlled at 1200-1230°C, the temperature of the soaking section was controlled at 1220-1250°C, and the heating time of the heating section II and the soaking section high temperature section was controlled at 243 minutes. After rolling, the steel was water-cooled, and the return homogenization temperature on the cooling bed was controlled at 660°C. After spheroidizing annealing, the GCr15 round steel of Φ20mm in this furnace was tested for carbide heterogeneity, network carbides, band carbides, and carbide liquid precipitation. The results all met the requirements for rolling element bearing steel. The specific test results are shown in Table 2.

[0053] Example 3

[0054] Continuous casting produced 8 heats of GCr15 steel, including 1 heat of bearing steel GCr15. The production method is as follows:

[0055] (1) KR desulfurization, S content is 0.001%.

[0056] (2) Argon is blown throughout the bottom blowing process of the LD converter. In order to reduce the O content in the steel, the final C content is 0.17%. Ferrosilicon and high carbon ferromanganese are added starting from 1 / 3 of the steel being tapped, followed by ductile ferrochrome, then ordinary recarburizer, and finally top slag lime.

[0057] (3) Add refining slag to the LF refining furnace to produce white slag and maintain it, while ensuring the fluidity of the top slag. Use aluminum particles for strong deoxidation. Add the deoxidizer evenly and do not add it in a concentrated manner. Fine-tune the C and Cr components in the LF furnace to control the C content to 0.95% and the Cr content to 1.40%.

[0058] (4) It is strictly forbidden to add alloy to RH, return to LF furnace to increase temperature, perform vacuum treatment and soft blowing stirring.

[0059] (5) The continuous casting adopts an integral tundish, which is cleaned and emptied by argon before pouring. The argon blowing time is 2 minutes. The casting is protected throughout the whole process and the pouring is started outside the tank. The continuous casting speed is 0.83m / min and the water content is 0.33L / Kg. Under this speed and water content, the developed casting model shows that the solid phase ratio of roller 1# is 43%, the solid phase ratio of roller 2# is 54%, the solid phase ratio of roller 3# is 65%, the solid phase ratio of roller 4# is 74%, the solid phase ratio of roller 5# is 91%, and the solid phase ratio of roller 6# is 100%. According to production practice, it is found that the forward pressing with light pressure and large pressure reduction and light pressure reduction at a solid phase ratio of 40% to 100% can improve the casting process. Based on best production practices, the core quality and dendritic segregation of the ingot were optimized with a reduction of 1.1mm for the 1# roll, 2.6mm for the 2# and 6# rolls, and 3.4mm for the 3#, 4#, and 5# rolls, for a total reduction of 16.5mm. The continuous casting superheat was controlled at 34°C. To ensure uniform composition and ingot quality, advanced electromagnetic stirring was employed in the mold and at the end of solidification. At the end of the pour, a 5t excess steel was retained in the tundish. Composition measurements during continuous casting revealed a C content of 0.96% and a Cr content of 1.40%. The ingots were then drawn into 240mm x 300mm ingots.

[0060] (6) The steel from this furnace was rolled into Φ60mm GCr15 round steel. During rolling, the temperature of the heating section II was controlled at 1200-1230°C, the temperature of the soaking section was controlled at 1220-1250°C, and the heating time of the heating section II and the soaking section high temperature section was controlled at 237 minutes. After rolling, the steel was water-cooled, and the return homogenization temperature on the cooling bed was controlled at 670°C. After spheroidizing annealing, the GCr15 round steel with a diameter of Φ20mm from this furnace was tested for carbide heterogeneity, network carbides, band carbides, and carbide liquid precipitation. The results all met the requirements for rolling element bearing steel. The specific test results are shown in Table 2.

[0061] Comparative Example

[0062] The production method of 5 batches of GCr15 steel, including 1 batch of bearing steel GCr15, is as follows:

[0063] (1) KR desulfurization, S content is 0.001%.

[0064] (2) Argon is blown throughout the bottom blowing process of the LD converter. In order to reduce the O content in the steel, the final C content is 0.21%. Ferrosilicon and high carbon ferromanganese are added starting from 1 / 3 of the steel being tapped, followed by ductile ferrochrome, then ordinary recarburizer, and finally top slag lime.

[0065] (3) Add refining slag to the LF refining furnace to produce white slag and maintain it, while ensuring the fluidity of the top slag. Use aluminum particles for strong deoxidation. Add the deoxidizer evenly and do not add it in a concentrated manner. Fine-tune the C and Cr components in the LF furnace to control the C content to 0.99% and the Cr content to 1.50%.

[0066] (4) It is strictly forbidden to add alloy to RH, return to LF furnace to increase temperature, perform vacuum treatment and soft blowing stirring.

[0067] The continuous casting adopts an integral tundish. The tundish is cleaned and emptied with argon before pouring. The argon blowing time is 3 minutes. The casting is protected throughout the whole process and pouring is started outside the tank. The continuous casting speed was 0.90 m / min, and the water content was 0.15 L / kg. The developed strand model showed that the solid fractions of roll 1# were 37%, roll 2# 45%, roll 3# 54%, roll 4# 62%, roll 5# 71%, roll 6# 80%, roll 7# 100%, and roll 8# 100%. The corresponding reductions were: roll 4# 1.5 mm, roll 5# 2.5 mm, roll 6# 4 mm, roll 7# 3.5 mm, and roll 8# 2 mm, for a total reduction of 13.5 mm. The continuous casting superheat was controlled at 23°C. Advanced electromagnetic stirring in the mold and at the end of solidification was employed to ensure compositional uniformity and strand quality. A 5.5 t steel reserve was maintained in the tundish at the end of the pour. The composition of the continuous casting was measured, and the C content was 1.00%, the Cr content was 1.50%, and the continuous casting was drawn into 240mm×300mm ingots.

[0068] (6) This furnace steel was rolled into Φ20mm GCr15 round steel. During rolling, the temperature of the heating section II was controlled at 1200-1230°C, the temperature of the soaking section was controlled at 1220-1250°C, and the heating time of the heating section II and the soaking section high temperature section was controlled at 155 minutes. No water cooling was performed after rolling. After spheroidizing annealing, this furnace of Φ20mm GCr15 round steel was tested for carbide heterogeneity, network carbides, band carbides, and carbide liquid precipitation. The results did not meet the requirements for rolling element bearing steel. The specific test results are shown in Table 2.

[0069] Table 2 Carbide inhomogeneity test results of bearing steel GCr15 after spheroidizing annealing

[0070]

[0071] It can be seen from the above embodiments that by adopting the present invention, during smelting, the C and Cr components are controlled at the lower limit of the standard requirements, the continuous casting speed is reduced, the continuous casting superheat is increased, light reduction and forward pressing and large reduction amount and light reduction are performed at a solid phase ratio of 40% to 100%, and the bearing steel is subjected to long-term high-temperature diffusion annealing and water cooling after rolling, thereby producing high-end bearing steel GCr15 products with specifications of Φ20 to 60 mm that meet the requirements of single-fired material for rolling elements.

[0072] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for controlling the unevenness of carbides in high carbon chromium bearing steel, characterized in that: The control method is to control the casting speed to 0.80-0.85 m / min and the water content to 0.30-0.35 L / Kg during the continuous casting process. The casting model developed under the casting speed and water content shows that the forward pressing of the bearing steel under light pressure and the light pressure under the solid phase ratio of 40%-100% can effectively improve the quality of the core of the casting and the dendritic segregation. When the solid phase ratio fs of the casting center is less than 40%, no pressure reduction is performed. When 40%≤fs<50%, 0.5mm≤required reduction≤2.0mm; When 50%≤fs<60%, 1.5mm≤required reduction≤3.0mm; When 60%≤fs<90%, 3.0mm≤required reduction≤4.0mm; When 90%≤fs≤100%, the required reduction is 1.5mm≤3.0mm.

2. The method for controlling carbide inhomogeneity of high carbon chromium bearing steel according to claim 1, characterized in that: The C content in the high carbon chromium bearing steel is adjusted to 0.95% to 0.96%, and the Cr content is adjusted to 1.40% to 1.45%.

3. The method for controlling carbide heterogeneity of high carbon chromium bearing steel according to claim 1, characterized in that: The superheat degree of the continuous casting process is controlled at 30-40°C.

4. The method for controlling carbide heterogeneity of high carbon chromium bearing steel according to claim 1, characterized in that: The control method further includes extending the high-temperature diffusion annealing time, wherein the high-temperature diffusion annealing time is ≥230 minutes.

5. The method for controlling carbide heterogeneity of high carbon chromium bearing steel according to claim 1, characterized in that: The control method further comprises the step of adding water cooling after rolling, wherein the upper cooling bed of the round steel returns to a homogenizing temperature of 650-680°C.

6. The method for controlling carbide heterogeneity of high carbon chromium bearing steel according to claim 5, characterized in that: During the rolling, the temperature of the heating section II is controlled at 1200-1230° C., the temperature of the soaking section is controlled at 1220-1250° C., and the heating time of the heating section II and the soaking section high temperature section is controlled at ≥230 minutes.

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