High-strength planet carrier cast steel material and preparation method of high-strength planet carrier

Abstract

The invention discloses a high-strength planet carrier cast steel material which is composed of, by mass, 0.28%-0.35% of C, 0.20%-0.50% of Si, 0.80%-1.20% of Mn, 0.70%-1.00% of Cr, 0.80%-1.20% of Ni, 0.25%-0.50% of Mo, 0.02%-0.05% of Al, smaller than or equal to 0.010% of S, smaller than or equal to 0.015% of P and the balance Fe. In addition, the invention further provides a preparation method of a high-strength planet carrier, low-carbon steel and alloy materials are matched, smelted and cast, and the high-strength planet carrier is obtained. By controlling the content of each element in the high-strength planet carrier cast steel material, the strength, hardness and wear resistance of the material are improved, the hardenability of the material is improved, grains of the material are refined, the impurity content is reduced, and the cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of high-strength planetary carrier cast steel material and preparation process, and specifically relates to a high-strength planetary carrier cast steel material and a preparation method of the high-strength planetary carrier. Background Art

[0002] The planet carrier is the key component of the coal mining machine, and its life directly determines the performance and reliability of the coal mining machine. At present, the planet carrier is processed by forging welding and integral forging.

[0003] The forged and welded structure has high requirements on welding level and quality. The processing technology of the integral forged planetary carrier window is complex, with large processing allowance, low efficiency and long cycle. In addition, the weight of the planetary carrier forging blanks of these two structures is particularly large, which not only requires high raw material procurement costs but also high processing costs.

[0004] Moreover, during the actual casting of the planetary carrier, the stop portion of the planetary carrier is a completely non-machined portion, and the dimensional accuracy requirements are high, which brings great difficulties to the casting production, namely: 1. During assembly, due to the small stop portion space and small assembly gap, interference often occurs or the planetary gear cannot be installed properly, resulting in out-of-tolerance casting size problems; 2. During the pouring and cooling of the casting, since the stop portion is a sand core, the shrinkage compensation path of the molten steel here is narrow, and it is impossible to add a riser subsidy in the stop portion, resulting in serious shrinkage compensation problems in the casting. Shrinkage cavities and shrinkage often occur in the column portion of the stop portion, and cracks occur at the connection between the column and the casting. The quality of the casting is difficult to meet the design requirements and cannot meet the use requirements.

[0005] In order to improve the processing performance of the planetary carrier of high-end coal mining machines and improve the intrinsic quality and stability of the planetary carrier itself, a new type of planetary carrier cast steel material and a planetary carrier made of this material are urgently needed, which can not only meet the requirements of high strength and high reliability, but also reduce the processing difficulty, improve processing efficiency and solve the production bottleneck. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a high-strength planetary carrier cast steel material in view of the above-mentioned deficiencies of the prior art. The high-strength planetary carrier cast steel material improves the strength, hardness and wear resistance of the material, improves the hardenability of the material, refines the grain size of the material, reduces the impurity content and reduces the cost by controlling the content of each element.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: a high-strength planetary carrier cast steel material, characterized in that the cast steel material is composed of the following elements in mass fraction: C 0.28%~0.35%, Si 0.20%~0.50%, Mn 0.80%~1.20%, Cr 0.70%~1.00%, Ni 0.80%~1.20%, Mo0.25%~0.50%, Al 0.02%~0.05%, S≤0.010%, P≤0.015%, and the balance is Fe.

[0008] The invention controls the content of each element, wherein Cr has a great strengthening effect, improves the strength, hardness and wear resistance, and improves the hardenability of the steel; Ni improves the strength of the steel but does not reduce its plasticity, and improves the hardenability of the steel; Mo strengthens ferrite in the steel, improves the strength and hardness of the steel, and improves the hardenability of the steel; Al plays a good deoxidation role in the steel, refines the grains of the steel, improves the strength, and reduces the S and P contents, because when the P content in the steel is greater than 0.1%, iron phosphide is formed and precipitated around the grains, which reduces the plasticity and toughness of the steel; S exists in the form of FeS or FeS-Fe eutectic around the grains of the steel in the molten steel, which reduces the mechanical properties of the steel, and thus obtains a high-strength planetary carrier cast steel material, thereby solving the problems of excessively high Cr and Ni contents and high costs in traditional materials.

[0009] The above-mentioned high-strength planetary carrier cast steel material is characterized in that the cast steel material is composed of the following elements in mass fraction: C 0.30%~0.33%, Si 0.39%~0.48%, Mn 1.05%~1.12%, Cr0.75%~0.83%, Ni 0.83%~0.85%, Mo 0.31%~0.33%, Al 0.02%~0.04%, S≤0.004%, P≤0.012%, and the balance is Fe.

[0010] The above-mentioned high-strength planetary carrier cast steel material is characterized in that the cast steel material is composed of the following elements: C 0.30%, Si 0.39%, Mn 1.05%, Cr 0.82%, Ni 0.85%, Mo 0.33%, Al0.02%, S≤0.004%, P≤0.012%, and the balance is Fe.

[0011] In addition, the present invention provides a process for preparing a high-strength planetary carrier using cast steel material, characterized in that the method comprises the following steps:

[0012] Step 1: preparing a casting mold for a planet carrier;

[0013] Step 2: low carbon steel and alloy materials are prepared according to high strength planetary carrier cast steel materials, and then smelted in an electric arc furnace, and then put into a ladle for refining outside the furnace to obtain molten steel;

[0014] Step 3, casting the molten steel obtained in step 2 into the casting mold prepared in step 1;

[0015] Step 4: After the molten steel cast in step 3 is cooled, demoulding is performed, and then heat treatment is performed to obtain a high-strength planetary carrier.

[0016] The casting mold for preparing a planetary carrier of the present invention includes the processes of shaping, preparing a sand core, placing a cold iron, filling sand and buckling a box, thereby ensuring that the casting mold structure of the planetary carrier is complete and a high-strength planetary carrier can be prepared.

[0017] The above method is characterized in that the amount of raw sand in the casting mold in step 1 is higher than 98%. The present invention ensures the casting effect through the amount of raw sand, and the local thick part of the casting in the casting mold is made of raw sand with higher refractoriness to prevent the raw sand from sintering to form ladle sand or sticky sand.

[0018] The above method is characterized in that the process correction amount of the opening part in the casting mold in step 1 satisfies: 1.5mm in the middle and gradually reduced to 0.5mm on both sides, all fillets are made by the model, and a lining is added on the outside of the column. In the present invention, the steel liquid channel at this part is enlarged by designing the process correction amount and adding the lining during casting, thereby ensuring the dense structure of the column, and after the casting is heat treated, it is cut off and polished to meet the casting size requirements.

[0019] The above method is characterized in that the low carbon steel and alloy material in step 2 are baked to remove water before use. The present invention removes water from the low carbon steel and alloy material by baking to ensure the quality of the high strength planetary carrier.

[0020] The above method is characterized in that the temperature of the electric arc furnace smelting in step 2 is 1550°C to 1650°C, and the refining outside the furnace is subjected to argon blowing treatment when the furnace is discharged, the pressure of the argon blowing is 0.1MPa to 0.3MPa, the time is more than 5min, and the argon blowing is left to stand for more than 5min. The present invention ensures that low-carbon steel and alloy materials are fully melted into molten steel by controlling the temperature of the electric arc furnace smelting, removes impurities and harmful gases by argon blowing, and achieves the purpose of purifying the molten steel. By standing still, it is ensured that the internal gas and inclusions have sufficient time to float to the liquid surface, thereby improving the purification effect.

[0021] The above method is characterized in that the casting temperature in step 3 is 1550°C to 1600°C, and when the molten steel rises to 2 / 3 of the riser, a heat preservation agent is promptly sprinkled, and the heat preservation agent is plant ash, and the amount used is to cover the riser by more than 20 mm, and the heat preservation is performed after the casting. The present invention ensures the casting effect by controlling the casting temperature, and by promptly sprinkling the heat preservation agent when the molten steel rises to 2 / 3 of the riser, it is ensured that the molten steel at the riser can effectively compensate for the shrinkage of the casting, and the heat preservation is performed after the casting to prevent cracks from occurring.

[0022] The above method is characterized in that the heat treatment process in step 4 is: heating to 920°C and then normalizing for 3 hours, followed by air cooling, then heating to 620°C for primary tempering for 3 hours, followed by air cooling, then heating to 880°C and then keeping warm for 3 hours, followed by oil cooling quenching, and finally heating to 570°C for secondary tempering for 3.5 hours, and oil cooling. The present invention improves the performance of the high-strength planetary carrier by performing heat treatment in steps, ensuring that the various performance indicators of the high-strength planetary carrier meet or exceed the requirements for use.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The present invention controls the content of each element in the high-strength planet carrier cast steel material, improves the strength, hardness and wear resistance of the material, improves the hardenability of the material, refines the grain of the material, and controls the impurity content, thereby solving the problem of excessively high Cr and Ni content and high cost in traditional materials.

[0025] 2. The present invention uses low-carbon steel and alloy materials as raw materials to prepare high-strength planetary carriers by casting, and improves its intrinsic performance through heat treatment, thereby achieving the purpose of reducing raw material manufacturing costs, simplifying processing procedures, and shortening processing cycles. In addition, its overall stability is further improved, ensuring the quality and service life of the high-strength planetary carrier, and meeting the requirements of coal mining machines for high-strength planetary carriers.

[0026] 3. In the present invention, the process correction amount is designed during casting, and the lining is added to enlarge the steel liquid channel in this part, so as to ensure the compactness of the column structure. After the casting is heat treated, it is cut off and polished to meet the casting size requirements, thereby improving the quality of the high-strength planetary carrier.

[0027] 4. The yield strength of the high-strength planetary carrier cast steel material of the present invention is greater than 900MPa, the tensile strength is greater than 1008MPa, and the hardness is greater than 293HB. The yield strength of the prepared high-strength planetary carrier is greater than 945MPa, the tensile strength is greater than 1045MPa, and the hardness is greater than 296HB.

[0028] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a schematic diagram of the process correction amount of the crotch opening part in the present invention.

[0030] Figure 2 This is a physical picture of the high-strength planetary carrier prepared in Example 1 of the present invention.

[0031] Figure 3 This is a SEM image of the high-strength planetary carrier prepared in Example 2 of the present invention.

[0032] Figure 4 3 is a SEM image of the high-strength planetary carrier prepared in Example 3 of the present invention. DETAILED DESCRIPTION

[0033] Figure 1 It is a schematic diagram of the process correction amount of the crotch opening part in the present invention, Figure 1 The planet carrier in the figure is a cast planet carrier. Figure 1 It can be seen that the opening part of the planetary carrier is in the shape of a window. The middle of the window shape is the opening part. The middle correction amount is 1.5mm, which gradually decreases to 0.5mm on both sides of the opening part, and a filler is added on the outside of the column.

[0034] Example 1

[0035] The cast steel material of this embodiment is composed of the following elements: C 0.33%, Si 0.48%, Mn 1.05%, Cr 0.83%, Ni 0.83%, Mo 0.32%, Al 0.03%, S 0.005%, P 0.011%, and the balance is Fe.

[0036] This embodiment includes the following steps:

[0037] Step 1, prepare a casting mold for the planetary carrier; the amount of raw sand in the casting mold is higher than 98%; the process correction amount of the opening part in the casting mold meets the following requirements: 1.5mm in the middle and gradually reduced to 0.5mm on both sides, all fillets are made through the model, and a lining is added on the outside of the column;

[0038] Step 2: After baking and removing water from low-carbon steel and alloy materials according to high-strength planetary carrier cast steel materials, they are mixed, and then smelted in an electric arc furnace at a temperature of 1550°C to 1650°C, and then put into a ladle for refining outside the furnace. When taken out of the furnace, argon gas is blown at a pressure of 0.1MPa to 0.3MPa for more than 5 minutes. After the argon gas blowing is completed, it is allowed to stand for more than 5 minutes to obtain molten steel;

[0039] Step 3, casting the molten steel obtained in step 2 in the casting mold prepared in step 1 at a temperature of 1550°C to 1600°C, and when the molten steel rises to 2 / 3 of the riser, timely sprinkle plant ash insulation agent in an amount to cover the riser by more than 20 mm, and perform insulation after casting;

[0040] Step 4: After the molten steel cast in step 3 is cooled, demold it, heat it to 920°C and then normalize it for 3 hours, then air cool it, then heat it to 620°C and temper it for 3 hours, then air cool it, then heat it to 880°C and keep it for 3 hours, followed by oil quenching, and finally heat it to 570°C and temper it for 3.5 hours, and oil cool it to obtain a high-strength planetary carrier.

[0041] Figure 2 This is a physical picture of the high-strength planetary carrier prepared in this embodiment. Figure 1 It can be seen that the surface of the high-strength planet carrier prepared in this embodiment is smooth.

[0042] Example 2

[0043] The cast steel material of this embodiment is composed of the following elements: C 0.30%, Si 0.39%, Mn 1.05%, Cr 0.82%, Ni 0.85%, Mo 0.33%, Al 0.02%, S 0.004%, P 0.012%, and the balance is Fe.

[0044] This embodiment includes the following steps:

[0045] Step 1, prepare a casting mold for the planetary carrier; the amount of raw sand in the casting mold is higher than 98%; the process correction amount of the opening part in the casting mold meets the following requirements: 1.5mm in the middle and gradually reduced to 0.5mm on both sides, all fillets are made through the model, and a lining is added on the outside of the column;

[0046] Step 2: After baking and removing water from low-carbon steel and alloy materials according to high-strength planetary carrier cast steel materials, they are mixed, and then smelted in an electric arc furnace at a temperature of 1550°C to 1650°C, and then put into a ladle for refining outside the furnace. When taken out of the furnace, argon gas is blown at a pressure of 0.1MPa to 0.3MPa for more than 5 minutes. After the argon gas blowing is completed, it is allowed to stand for more than 5 minutes to obtain molten steel;

[0047] Step 3, casting the molten steel obtained in step 2 in the casting mold prepared in step 1 at a temperature of 1550°C to 1600°C, and when the molten steel rises to 2 / 3 of the riser, timely sprinkle plant ash insulation agent in an amount to cover the riser by more than 20 mm, and perform insulation after casting;

[0048] Step 4: After the molten steel cast in step 3 is cooled, demold it, heat it to 920°C and then normalize it for 3 hours, then air cool it, then heat it to 620°C and temper it for 3 hours, then air cool it, then heat it to 880°C and keep it for 3 hours, followed by oil quenching, and finally heat it to 570°C and temper it for 3.5 hours, and oil cool it to obtain a high-strength planetary carrier.

[0049] Figure 3 is a SEM image of the high-strength planetary carrier prepared in this embodiment. Figure 3 It can be seen that the organization is fine tempered troostite.

[0050] Example 3

[0051] The cast steel material of this embodiment is composed of the following elements: C 0.30%, Si 0.43%, Mn 1.12%, Cr 0.75%, Ni 0.83%, Mo 0.31%, Al 0.04%, S 0.006%, P 0.007%, and the balance is Fe.

[0052] This embodiment includes the following steps:

[0053] Step 1, prepare a casting mold for the planetary carrier; the amount of raw sand in the casting mold is higher than 98%; the process correction amount of the opening part in the casting mold meets the following requirements: 1.5mm in the middle and gradually reduced to 0.5mm on both sides, all fillets are made through the model, and a lining is added on the outside of the column;

[0054] Step 2: After baking and removing water from low-carbon steel and alloy materials according to high-strength planetary carrier cast steel materials, they are mixed, and then smelted in an electric arc furnace at a temperature of 1550°C to 1650°C, and then put into a ladle for refining outside the furnace. When taken out of the furnace, argon gas is blown at a pressure of 0.1MPa to 0.3MPa for more than 5 minutes. After the argon gas blowing is completed, it is allowed to stand for more than 5 minutes to obtain molten steel;

[0055] Step 3, casting the molten steel obtained in step 2 in the casting mold prepared in step 1 at a temperature of 1550°C to 1600°C, and when the molten steel rises to 2 / 3 of the riser, timely sprinkle plant ash insulation agent in an amount to cover the riser by more than 20 mm, and perform insulation after casting;

[0056] Step 4: After the molten steel cast in step 3 is cooled, demold it, heat it to 920°C and then normalize it for 3 hours, then air cool it, then heat it to 620°C and temper it for 3 hours, then air cool it, then heat it to 880°C and keep it for 3 hours, followed by oil quenching, and finally heat it to 570°C and temper it for 3.5 hours, and oil cool it to obtain a high-strength planetary carrier.

[0057] Figure 4is a SEM image of the high-strength planetary carrier prepared in this embodiment. Figure 4 It can be seen that the organization is fine tempered troostite.

[0058] The cast steel materials of Examples 1 to 3 were tested for performance together with 42CrMo steel and ZG34CrNiMo steel. The results are shown in Table 1. The mass fractions of the elements in 42CrMo steel and ZG34CrNiMo steel are shown in Table 2.

[0059] Table 1

[0060]

[0061]

[0062] Table 2

[0063]

[0064] It can be seen from Table 1 that the cast steel materials of Examples 1 to 3 have excellent mechanical properties and fully meet the use requirements. Combining Table 1 and Table 2, it can be seen that the cast steel materials of Examples 1 to 3 have reduced Cr and Ni contents, and at the same time have excellent mechanical properties and reduce the preparation cost.

[0065] The mass fraction of each element in the high-strength planetary carriers prepared in Example 2 and Example 3 was tested and the results are shown in Table 3. The mechanical properties test results are shown in Table 4.

[0066] Table 3

[0067]

[0068]

[0069] Table 4

[0070]

[0071] It can be seen from Table 3 and Table 4 that the high-strength planetary carriers prepared in Example 2 and Example 3 have excellent mechanical properties.

[0072] Example 4

[0073] The cast steel material of this embodiment is composed of the following elements: C 0.28%, Si 0.50%, Mn 0.80%, Cr 1.00%, Ni 0.80%, Mo 0.50%, Al 0.02%, S≤0.010%, P≤0.015%, and the balance is Fe.

[0074] This embodiment includes the following steps:

[0075] Step 1, prepare a casting mold for the planetary carrier; the amount of raw sand in the casting mold is higher than 98%; the process correction amount of the opening part in the casting mold meets the following requirements: 1.5mm in the middle and gradually reduced to 0.5mm on both sides, all fillets are made through the model, and a lining is added on the outside of the column;

[0076] Step 2: After baking and removing water from low-carbon steel and alloy materials according to high-strength planetary carrier cast steel materials, they are mixed, and then smelted in an electric arc furnace at a temperature of 1550°C to 1650°C, and then put into a ladle for refining outside the furnace. When taken out of the furnace, argon gas is blown at a pressure of 0.1MPa to 0.3MPa for more than 5 minutes. After the argon gas blowing is completed, it is allowed to stand for more than 5 minutes to obtain molten steel;

[0077] Step 3, casting the molten steel obtained in step 2 in the casting mold prepared in step 1 at a temperature of 1550°C to 1600°C, and when the molten steel rises to 2 / 3 of the riser, timely sprinkle plant ash insulation agent in an amount to cover the riser by more than 20 mm, and perform insulation after casting;

[0078] Step 4: After the molten steel cast in step 3 is cooled, demold it, heat it to 920°C and then normalize it for 3 hours, then air cool it, then heat it to 620°C and temper it for 3 hours, then air cool it, then heat it to 880°C and keep it for 3 hours, followed by oil quenching, and finally heat it to 570°C and temper it for 3.5 hours, and oil cool it to obtain a high-strength planetary carrier.

[0079] Example 5

[0080] The cast steel material of this embodiment is composed of the following elements: C 0.35%, Si 0.20%, Mn 1.20%, Cr 0.70%, Ni 1.20%, Mo 0.25%, Al 0.05%, S≤0.010%, P≤0.015%, and the balance is Fe.

[0081] This embodiment includes the following steps:

[0082] Step 1, prepare a casting mold for the planetary carrier; the amount of raw sand in the casting mold is higher than 98%; the process correction amount of the opening part in the casting mold meets the following requirements: 1.5mm in the middle and gradually reduced to 0.5mm on both sides, all fillets are made through the model, and a lining is added on the outside of the column;

[0083] Step 2: After baking and removing water from low-carbon steel and alloy materials according to high-strength planetary carrier cast steel materials, they are mixed, and then smelted in an electric arc furnace at a temperature of 1550°C to 1650°C, and then put into a ladle for refining outside the furnace. When taken out of the furnace, argon gas is blown at a pressure of 0.1MPa to 0.3MPa for more than 5 minutes. After the argon gas blowing is completed, it is allowed to stand for more than 5 minutes to obtain molten steel;

[0084] Step 3, casting the molten steel obtained in step 2 in the casting mold prepared in step 1 at a temperature of 1550°C to 1600°C, and when the molten steel rises to 2 / 3 of the riser, timely sprinkle plant ash insulation agent in an amount to cover the riser by more than 20 mm, and perform insulation after casting;

[0085] Step 4: After the molten steel cast in step 3 is cooled, demold it, heat it to 920°C and then normalize it for 3 hours, then air cool it, then heat it to 620°C and temper it for 3 hours, then air cool it, then heat it to 880°C and keep it for 3 hours, followed by oil quenching, and finally heat it to 570°C and temper it for 3.5 hours, and oil cool it to obtain a high-strength planetary carrier.

[0086] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent changes made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A high-strength planetary carrier cast steel material, characterized in that: The cast steel material is composed of the following elements in mass fraction: C 0.28%-0.35%, Si 0.20%-0.50%, Mn 0.80%-1.20%, Cr 0.70%-1.00%, Ni 0.80%-1.20%, Mo 0.25%-0.50%, Al 0.02%-0.05%, S≤0.010%, P≤0.015%, and the balance is Fe.

2. The high-strength planet carrier cast steel material according to claim 1, characterized in that: The cast steel material is composed of the following elements in mass fraction: C 0.30%-0.33%, Si 0.39%-0.48%, Mn 1.05%-1.12%, Cr 0.75%-0.83%, Ni 0.83%-0.85%, Mo 0.31%-0.33%, Al 0.02%-0.04%, S≤0.004%, P≤0.012%, and the balance is Fe.

3. The high-strength planet carrier cast steel material according to claim 1, characterized in that: The cast steel material consists of the following elements in mass fractions: C 0.30%, Si 0.39%, Mn 1.05%, Cr 0.82%, Ni 0.85%, Mo 0.33%, Al 0.02%, S≤0.004%, P≤0.012%, and the balance is Fe.

4. A process for preparing a high-strength planet carrier using the cast steel material according to any one of claims 1 to 3, characterized in that: The method comprises the following steps: Step 1: preparing a casting mold for a planet carrier; Step 2: low carbon steel and alloy materials are prepared according to high strength planetary carrier cast steel materials, and then smelted in an electric arc furnace, and then put into a ladle for refining outside the furnace to obtain molten steel; Step 3, casting the molten steel obtained in step 2 into the casting mold prepared in step 1; Step 4: After the molten steel cast in step 3 is cooled, demoulding is performed, and then heat treatment is performed to obtain a high-strength planetary carrier.

5. The method according to claim 4, characterized in that The amount of raw sand in the casting mold in step 1 is higher than 98%.

6. The method according to claim 4, characterized in that The process correction amount of the opening part in the casting mold described in step 1 meets the requirements: 1.5mm in the middle and gradually decreasing to 0.5mm on both sides. All fillets are made through the model, and lining is added on the outside of the column.

7. The method according to claim 4, characterized in that The low carbon steel and alloy materials described in step 2 are baked to remove water before use.

8. The method according to claim 4, characterized in that The temperature of the electric arc furnace smelting in step 2 is 1550°C to 1650°C, and the refining outside the furnace is subjected to argon blowing when the product is taken out of the furnace. The pressure of the argon blowing is 0.1MPa to 0.3MPa, and the time is more than 5 minutes. After the argon blowing is completed, the product is allowed to stand for more than 5 minutes.

9. The method according to claim 4, characterized in that The casting temperature in step three is 1550°C to 1600°C. When the molten steel rises to 2 / 3 of the riser, a heat-insulating agent is promptly sprinkled. The heat-insulating agent is wood ash, and the amount used is to cover the riser by more than 20 mm. After the casting, heat preservation is performed.

10. The method according to claim 4, characterized in that The heat treatment process in step 4 is: heating to 920°C and then normalizing for 3 hours, followed by air cooling, then heating to 620°C for primary tempering for 3 hours, followed by air cooling, then heating to 880°C and keeping warm for 3 hours, followed by oil quenching, and finally heating to 570°C for secondary tempering for 3.5 hours and oil cooling.

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