Low Carbon Low Alloy Steel and Its Heat Treatment Process

Abstract

The present application discloses low-carbon low-alloy steel and its heat treatment process, wherein the heat treatment process of the low-carbon low-alloy steel comprises the following steps in sequence: smelting components to prepare castings; performing a predetermined form of complete annealing treatment on the castings; performing a predetermined form of quenching treatment on the castings; performing a predetermined form of sub-temperature quenching treatment on the castings; and finally performing a predetermined form of tempering treatment on the castings. The low-carbon low-alloy steel prepared in this way can significantly improve the toughness in a low-temperature environment, and at the same time, the elongation after fracture and the end face shrinkage rate can also be improved to a certain extent, and has better mechanical properties. The low-carbon low-alloy steel has more stable performance and a longer service life, and can effectively save costs and reduce resource waste.

Description

Technical Field

[0001] The invention relates to the technical field of metal heat treatment, in particular to low-carbon low-alloy steel and a heat treatment process thereof. Background Art

[0002] After quenching and tempering treatment, low-carbon, low-alloy steel has excellent comprehensive mechanical properties, such as high strength, good toughness, and good wear resistance. It is widely used in transportation, energy, mining and other industries.

[0003] The general heat treatment process of low-carbon, low-alloy cast steel includes three steps: normalizing, quenching and tempering. The parts after heat treatment can adapt to most working conditions. However, the finished products often show insufficient toughness in low temperature environments and are prone to brittle fracture. They are not suitable for use in high-altitude and cold regions such as Northeast my country, Northwest China and Tibet.

[0004] Normally, most parts working in low temperature environments use low-carbon high-alloy steel, which contains precious metals such as nickel and molybdenum. It also has good toughness in low temperature environments and can overcome the above problems. However, in high temperature environments, its strength is low, so its application range is narrow and its manufacturing cost is high. Summary of the invention

[0005] One advantage of the present invention is that it provides a low-carbon, low-alloy steel and a heat treatment process thereof, wherein the low-carbon, low-alloy steel obtained based on the special chemical composition of the low-carbon, low-alloy steel and the unique heat treatment process can significantly improve the toughness in a low-temperature environment, while the elongation after fracture and the end surface shrinkage rate can also be improved to a certain extent, and has better mechanical properties. The low-carbon, low-alloy steel has more stable performance and a longer service life, can effectively save costs and reduce resource waste.

[0006] To achieve at least one of the above advantages of the present invention, the present invention provides a low carbon low alloy steel, consisting of the following components in weight percentage: C: 0.17% to 0.27%, Si: 0.30% to 0.60%, Mn: 0.70% to 1.10%, Cr: 0.60% to 1.00%, Ni: 0.80% to 1.20%, Mo: 0.30% to 0.60%, S≤0.02%, P≤0.02%, and the balance is Fe and unavoidable impurities.

[0007] According to one embodiment of the present invention, the specific chemical composition mass percentages of the low-carbon low-alloy steel are as follows: C: 0.24%, Si: 0.48%, Mn: 0.89%, Cr: 0.72%, Ni: 0.88%, Mo: 0.35%, S≤0.009%, P≤0.013%.

[0008] According to one embodiment of the present invention, the specific chemical composition mass percentages of the low-carbon low-alloy steel are as follows: C: 0.20%, Si: 0.45%, Mn: 0.92%, Cr: 0.70%, Ni: 0.90%, Mo: 0.37%, S≤0.010%, P≤0.012%.

[0009] According to one embodiment of the present invention, the specific chemical composition mass percentages of the low-carbon low-alloy steel are as follows: C: 0.23%, Si: 0.52%, Mn: 0.96%, Cr: 0.65%, Ni: 0.90%, Mo: 0.40%, S≤0.008%, P≤0.010%.

[0010] In a second aspect, the present application also provides a heat treatment process for low-carbon low-alloy steel, comprising the following steps in sequence:

[0011] S10, smelting the components of the aforementioned low-carbon low-alloy steel to prepare a casting;

[0012] S20, placing the casting in a resistance furnace with a furnace temperature of less than or equal to 200°C, heating the casting at a heating rate of less than or equal to 200°C / h. When the furnace temperature rises to 600°C, the heating rate is less than or equal to 100°C / h. When the furnace temperature rises to 700°C, the heating rate is less than or equal to 150°C. When the furnace temperature rises to 920°C to 950°C, the casting is kept at this temperature for 3h to 4h, then cooled with the furnace, and taken out of the furnace for air cooling when the furnace temperature is less than or equal to 500°C.

[0013] S30, the castings cooled to room temperature by air are placed in a resistance furnace, and the furnace temperature is raised to 400°C at full power, and kept at this temperature for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, and the furnace temperature is raised to 805°C at full power, and kept at this temperature for 60 minutes, and then the furnace temperature is raised to 920°C to 950°C at full power again, and kept at this temperature for 180 to 240 minutes, and then taken out of the furnace and oil-cooled;

[0014] S40, put the castings cooled to room temperature by oil into the resistance furnace, raise the furnace temperature to 400℃ at full power, keep it warm for 10 minutes, then transfer it to the multi-purpose furnace with protective atmosphere, raise the furnace temperature to 805℃ at full power, keep it warm for 60 minutes, then raise the furnace temperature to 830℃~850℃ at full power again, keep it warm for 180~240 minutes, and then take it out of the furnace and cool it by oil;

[0015] S50, tempered, then air cooled out of the furnace.

[0016] According to one embodiment of the present invention, in step S30, the initial furnace temperature of the resistance furnace is less than or equal to 600°C and greater than or equal to 320°C, and in step S40, the initial furnace temperature of the resistance furnace is less than or equal to 600°C and greater than 510°C.

[0017] According to an embodiment of the present invention, in step S30 and step S40, the initial furnace temperature of the protective atmosphere multi-purpose furnace is 800°C to 930°C.

[0018] According to an embodiment of the present invention, in step S50, the tempering is specifically as follows: placing the casting cooled to room temperature by oil into a resistance furnace, raising the furnace temperature to 570° C. to 600° C. at full power, and keeping the temperature for 3 h to 4 h.

[0019] According to an embodiment of the present invention, in step S50, the initial furnace temperature of the resistance furnace is less than or equal to 600°C and greater than 230°C.

[0020] These and other objects, features and advantages of the present invention will be fully reflected in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic flow chart of the heat treatment process for low carbon low alloy steel in the present application is shown. DETAILED DESCRIPTION

[0022] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only examples, and those skilled in the art can think of other obvious variations. The basic principles of the present invention defined in the following description can be applied to other embodiments, variations, improvements, equivalents, and other technical solutions that do not deviate from the spirit and scope of the present invention.

[0023] Those skilled in the art should understand that, in the disclosure of the specification, the orientation or position relationship indicated by the terms "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc. are based on the orientation or position relationship shown in the drawings, which are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the above terms should not be understood as limiting the present invention.

[0024] It is to be understood that the term "one" should be understood as "at least one" or "one or more", that is, in one embodiment, the number of an element may be one, while in another embodiment, the number of the element may be multiple, and the term "one" should not be understood as a limitation on the quantity.

[0025] A low carbon low alloy steel according to a preferred embodiment of the present invention will be described in detail below, wherein the low carbon low alloy steel is composed of the following components in weight percentage: C: 0.17% ~ 0.27%, Si: 0.30% ~ 0.60%, Mn: 0.70% ~ 1.10%, Cr: 0.60% ~ 1.00%, Ni: 0.80% ~ 1.20%, Mo: 0.30% ~ 0.60%, S≤0.02%, P≤0.02%, and the balance is Fe and unavoidable impurities.

[0026] Second, reference Figure 1 The present application also provides a heat treatment process for the above-mentioned low-carbon low-alloy steel, which comprises the following steps in sequence:

[0027] S10, smelting the components of the aforementioned low-carbon low-alloy steel to prepare a casting;

[0028] S20, placing the casting in a resistance furnace with a furnace temperature of less than or equal to 200°C, heating the casting, wherein the heating rate is less than or equal to 200°C / h, when the furnace temperature rises to 600°C, the heating rate is less than or equal to 100°C / h, when the furnace temperature rises to 700°C, the heating rate is less than or equal to 150°C, when the furnace temperature rises to 920°C-950°C, heat preservation for 3h-4h, then cooling with the furnace, and when the furnace temperature is less than or equal to 500°C, taking the casting out of the furnace and air cooling, so that the casting can be completely annealed;

[0029] S30, placing the castings cooled to room temperature by air into a resistance furnace, raising the furnace temperature to 400°C at full power, keeping the temperature for 10 minutes, then transferring to a protective atmosphere multi-purpose furnace, raising the furnace temperature to 805°C at full power, keeping the temperature for 60 minutes, then raising the furnace temperature to 920°C to 950°C at full power again, keeping the temperature for 180 to 240 minutes, taking out of the furnace and oil cooling, thereby quenching;

[0030] S40, the castings cooled to room temperature by oil are placed in a resistance furnace, and the furnace temperature is raised to 400°C at full power, and kept at this temperature for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, and the furnace temperature is raised to 805°C at full power, and kept at this temperature for 60 minutes, and then the furnace temperature is raised to 830°C to 850°C at full power again, and kept at this temperature for 180 to 240 minutes, and then taken out of the furnace and cooled by oil, thereby performing sub-temperature quenching;

[0031] S50, tempering, followed by air cooling, wherein the tempering specifically comprises: placing the castings cooled to room temperature by oil into a resistance furnace, raising the furnace temperature to 570°C to 600°C at full power, and keeping the temperature for 3h to 4h. In addition, the initial furnace temperature of the resistance furnace is less than or equal to 600°C and greater than 230°C.

[0032] It should be particularly emphasized that in step S30, the initial furnace temperature of the resistance furnace is less than or equal to 600°C and greater than or equal to 320°C. At the same time, in step S40, the initial furnace temperature of the resistance furnace is less than or equal to 600°C and greater than 510°C, so that a higher initial furnace temperature can be obtained.

[0033] After the room temperature casting is placed in the resistance furnace, under the influence of the casting, the resistance furnace in step S30 will be cooled to 130℃~280℃, and the resistance furnace in step S30 will be cooled to 190℃~280℃. On this basis, the resistance furnace in step S30 and step S40 will raise the furnace temperature to 400℃ at full power. In this way, on the one hand, the heat treatment efficiency can be greatly improved, that is, the production efficiency can be improved. On the other hand, after verification by a large amount of test data, based on the high heat atmosphere formed by the initial furnace temperature, and then full power heating, the heat treatment effect on the casting, especially in a low temperature environment, including the mechanical properties including elongation after fracture and end face shrinkage can be improved to a certain extent, and its toughness can be significantly improved. However, whether it is the initial furnace temperature of the resistance furnace in step S30 and step S40, or the initial furnace temperature of the resistance furnace in step S50, it is not easy to exceed 600℃, which will affect the performance of the casting.

[0034] Among them, if the above-mentioned initial furnace temperature of the resistance furnace is set only in step S30 and step S40, the mechanical properties and toughness including the elongation after fracture and the end surface shrinkage rate can be greatly improved. However, if the initial furnace temperature with a predetermined range is set in steps S30, S40 and S50 at the same time, the mechanical properties and toughness including the elongation after fracture and the end surface shrinkage rate can be greatly improved. The specific effect data can be seen in the subsequent mechanical properties comparison table.

[0035] In addition, in step S30 and step S40, the initial furnace temperature of the protective atmosphere multi-purpose furnace is 800°C to 930°C, and its effect is similar to the initial furnace temperature of the resistance furnace, which will not be described in detail here.

[0036] Example 1: The specific chemical composition mass percentages of the sample are as follows: C: 0.24%, Si: 0.48%, Mn: 0.89%, Cr: 0.72%, Ni: 0.88%, Mo: 0.35%, S≤0.009%, P≤0.013%.

[0037] The heat treatment process of low carbon low alloy steel includes the following steps in sequence:

[0038] S10, melting components of low-carbon low-alloy steel to prepare castings;

[0039] S20, loading the casting into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 25° C., rising to 600° C. in 3 hours, then rising to 700° C. in 1 hour, then rising to 930° C. in 2 hours, keeping the temperature for 3 hours, then cooling with the furnace for 16 hours, the temperature drops to 490° C., and then air cooling after being taken out of the furnace;

[0040] S30: loading the casting into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 560° C., heating the furnace to 400° C. at full power in 30 minutes, keeping the temperature for 10 minutes, transferring the casting into a protective atmosphere multi-purpose furnace, wherein the initial furnace temperature of the protective atmosphere multi-purpose furnace is 920° C., heating the furnace to 805° C. at full power in 70 minutes, keeping the temperature for 60 minutes, heating the furnace to 930° C. at full power in 50 minutes, keeping the temperature for 180 minutes, and oil cooling the casting after it is taken out of the furnace;

[0041] S40: The casting is loaded into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 560°C, and the temperature is raised to 400°C at full power in 30 minutes, and kept warm for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, wherein the initial furnace temperature of the protective atmosphere multi-purpose furnace is 920°C, and the temperature is raised to 805°C at full power in 70 minutes, and kept warm for 60 minutes, and the temperature is raised to 850°C at full power in 15 minutes, and kept warm for 180 minutes, and then taken out of the furnace and oil-cooled.

[0042] S50: loading the casting into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 390° C., heating to 600° C. at full power for 1.5 hours, keeping the temperature for 3 hours, and air cooling after taking out of the furnace.

[0043] Example 2: The specific chemical composition mass percentages of the sample are as follows: C: 0.20%, Si: 0.45%, Mn: 0.92%, Cr: 0.70%, Ni: 0.90%, Mo: 0.37%, S≤0.010%, P≤0.012%.

[0044] The heat treatment process of low carbon low alloy steel includes the following steps in sequence:

[0045] S10, melting components of low-carbon low-alloy steel to prepare castings;

[0046] S20: loading the casting into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 28° C., rising to 600° C. in 3 hours, rising to 700° C. in 1 hour, rising to 940° C. in 2 hours, keeping the temperature for 3.5 hours, and then cooling with the furnace for 18 hours until the temperature drops to 486° C., and then air cooling out of the furnace;

[0047] S30: The casting is loaded into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 350°C, and the temperature is raised to 400°C at full power in 45 minutes, and kept at this temperature for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, wherein the initial furnace temperature of the protective atmosphere multi-purpose furnace is 920°C, and the temperature is raised to 805°C at full power in 70 minutes, and kept at this temperature for 60 minutes, and the temperature is raised to 940°C at full power in 50 minutes, and kept at this temperature for 210 minutes, and then taken out of the furnace and oil-cooled;

[0048] S40: The castings are loaded into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 550°C, which is heated to 400°C at full power in 30 minutes, kept warm for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, wherein the initial furnace temperature of the protective atmosphere multi-purpose furnace is 920°C, which is heated to 805°C at full power in 70 minutes, kept warm for 60 minutes, heated to 850°C at full power in 15 minutes, kept warm for 210 minutes, and then taken out of the furnace and oil-cooled.

[0049] S50: The castings are loaded into a box-type resistance furnace, where the initial furnace temperature is 360°C, heated to 600°C at full power for 1.8 hours, kept at that temperature for 3.5 hours, and then air-cooled after being taken out of the furnace.

[0050] Example 3: The specific chemical composition mass percentages of the sample are as follows: C: 0.23%, Si: 0.52%, Mn: 0.96%, Cr: 0.65%, Ni: 0.90%, Mo: 0.40%, S≤0.008%, P≤0.010%.

[0051] The heat treatment process of low carbon low alloy steel includes the following steps in sequence:

[0052] S10, melting components of low-carbon low-alloy steel to prepare castings;

[0053] S20: loading the casting into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 23° C., rising to 600° C. in 3 hours, rising to 700° C. in 1 hour, rising to 920° C. in 2 hours, keeping the temperature for 3 hours, and then cooling with the furnace for 15 hours until the temperature drops to 498° C., and then air cooling out of the furnace;

[0054] S30: The casting is loaded into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 320°C, and the temperature is raised to 400°C at full power in 39 minutes, and kept at this temperature for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, wherein the initial furnace temperature of the protective atmosphere multi-purpose furnace is 9150°C, and the temperature is raised to 805°C at full power in 70 minutes, and kept at this temperature for 60 minutes, and the temperature is raised to 920°C at full power in 50 minutes, and kept at this temperature for 180 minutes, and then taken out of the furnace and oil-cooled;

[0055] S40: The casting is loaded into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 545°C, and the temperature is raised to 400°C at full power in 30 minutes, and kept at this temperature for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, wherein the initial furnace temperature of the protective atmosphere multi-purpose furnace is 925°C, and the temperature is raised to 805°C at full power in 70 minutes, and kept at this temperature for 60 minutes, and the temperature is raised to 840°C at full power in 15 minutes, and kept at this temperature for 180 minutes, and then taken out of the furnace and oil-cooled;

[0056] S50: The castings are loaded into a box-type resistance furnace, wherein the initial furnace temperature of the resistance furnace is 230°C, the temperature is raised to 600°C at full power for 2.0h, the temperature is kept for 3h, and the castings are air-cooled after being taken out of the furnace.

[0057] The mechanical properties of the castings obtained in Examples 1 to 3 are compared with those of the castings obtained by conventional quenching and tempering treatment as shown in the following table:

[0058]

[0059]

[0060] Mechanical properties comparison table

[0061] Therefore, in the present application, compared with the normalizing + quenching + tempering process in the prior art, this special heat treatment process is used to replace normalizing with complete annealing, which can refine the grains, reduce the grain size difference, and thus make the structure uniform. In addition, after a special sub-temperature quenching treatment process, the heat preservation is carried out in the range of 20°C below Ac3, which can refine the grains, increase a small amount of tough phase "strand-shaped ferrite" structure and evenly distribute it. On the basis of combining the above various treatment methods, the low-temperature toughness of the product obtained by heat treatment can be significantly improved. At the same time, the elongation after fracture and the end shrinkage rate can also be improved to a certain extent, so that the low-carbon low-alloy steel provided in the present application not only has high mechanical properties, but also has good low-temperature toughness. At the same time, the overall performance is stable and the product has a longer service life, which can save costs and reduce resource waste.

[0062] It should be understood by those skilled in the art that the embodiments of the present invention described above and shown in the accompanying drawings are only examples and do not limit the present invention. The advantages of the present invention have been fully and effectively achieved. The functional and structural principles of the present invention have been demonstrated and explained in the embodiments, and the embodiments of the present invention may be deformed or modified in any way without departing from the principles.

Claims

1. Low carbon low alloy steel, characterized in that: It is composed of the following components in weight percentage: C: 0.17% to 0.27%, Si: 0.30% to 0.60%, Mn: 0.70% to 1.10%, Cr: 0.60% to 1.00%, Ni: 0.80% to 0.90%, Mo: 0.30% to 0.60%, S≤0.02%, P≤0.02%, and the balance is Fe and unavoidable impurities; The heat treatment process of the low carbon low alloy steel comprises the following steps in sequence: S10, smelting the components of the low-carbon low-alloy steel to prepare a casting; S20, placing the casting in a resistance furnace with a furnace temperature of less than or equal to 200°C, heating the casting at a heating rate of less than or equal to 200°C / h. When the furnace temperature rises to 600°C, the heating rate is less than or equal to 100°C / h. When the furnace temperature rises to 700°C, the heating rate is less than or equal to 150°C. When the furnace temperature rises to 920°C to 950°C, the casting is kept at this temperature for 3h to 4h, then cooled with the furnace, and taken out of the furnace for air cooling when the furnace temperature is less than or equal to 500°C. S30, the castings cooled to room temperature by air are placed in a resistance furnace, and the furnace temperature is raised to 400°C at full power, and kept at this temperature for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, and the furnace temperature is raised to 805°C at full power, and kept at this temperature for 60 minutes, and then the furnace temperature is raised to 920°C to 950°C at full power again, and kept at this temperature for 180 to 240 minutes, and then taken out of the furnace and oil-cooled; S40, put the castings cooled to room temperature by oil into the resistance furnace, raise the furnace temperature to 400℃ at full power, keep it warm for 10 minutes, then transfer it to the multi-purpose furnace with protective atmosphere, raise the furnace temperature to 805℃ at full power, keep it warm for 60 minutes, then raise the furnace temperature to 830℃~850℃ at full power again, keep it warm for 180~240 minutes, and then take it out of the furnace and cool it by oil; S50, place the castings that have been oil-cooled to room temperature into a resistance furnace, raise the furnace temperature to 570℃~600℃ at full power, keep it warm for 3h~4h, and then take it out of the furnace and air-cool it.

2. The low carbon low alloy steel according to claim 1, characterized in that: The specific chemical composition mass percentages are as follows: C: 0.24%, Si: 0.48%, Mn: 0.89%, Cr: 0.72%, Ni: 0.88%, Mo: 0.35%, S≤0.009%, P≤0.013%.

3. The low carbon low alloy steel according to claim 1, characterized in that: The specific chemical composition mass percentages are as follows: C: 0.20%, Si: 0.45%, Mn: 0.92%, Cr: 0.70%, Ni: 0.90%, Mo: 0.37%, S≤0.010%, P≤0.012%.

4. The low carbon low alloy steel according to claim 1, characterized in that: The specific chemical composition mass percentages are as follows: C: 0.23%, Si: 0.52%, Mn: 0.96%, Cr: 0.65%, Ni: 0.90%, Mo: 0.40%, S≤0.008%, P≤0.010%.

5. Heat treatment process of low carbon low alloy steel, characterized in that: The following steps are included in sequence: S10, smelting the components of the low-carbon low-alloy steel according to any one of claims 1 to 4 to prepare a casting; S20, placing the casting in a resistance furnace with a furnace temperature of less than or equal to 200°C, heating the casting at a heating rate of less than or equal to 200°C / h. When the furnace temperature rises to 600°C, the heating rate is less than or equal to 100°C / h. When the furnace temperature rises to 700°C, the heating rate is less than or equal to 150°C. When the furnace temperature rises to 920°C to 950°C, the casting is kept at this temperature for 3h to 4h, then cooled with the furnace, and taken out of the furnace for air cooling when the furnace temperature is less than or equal to 500°C. S30, the castings cooled to room temperature by air are placed in a resistance furnace, and the furnace temperature is raised to 400°C at full power, and kept at this temperature for 10 minutes, and then transferred to a protective atmosphere multi-purpose furnace, and the furnace temperature is raised to 805°C at full power, and kept at this temperature for 60 minutes, and then the furnace temperature is raised to 920°C to 950°C at full power again, and kept at this temperature for 180 to 240 minutes, and then taken out of the furnace and oil-cooled; S40, put the castings cooled to room temperature by oil into the resistance furnace, raise the furnace temperature to 400℃ at full power, keep it warm for 10 minutes, then transfer it to the multi-purpose furnace with protective atmosphere, raise the furnace temperature to 805℃ at full power, keep it warm for 60 minutes, then raise the furnace temperature to 830℃~850℃ at full power again, keep it warm for 180~240 minutes, and then take it out of the furnace and cool it by oil; S50, tempered, then air cooled out of the furnace.

6. The heat treatment process for low carbon low alloy steel according to claim 5, characterized in that: In step S30, the initial temperature of the resistance furnace is less than or equal to 600°C and greater than or equal to 320°C. In step S40, the initial temperature of the resistance furnace is less than or equal to 600°C and greater than 510°C.

7. The heat treatment process for low carbon low alloy steel according to claim 6, characterized in that: In step S30 and step S40, the initial furnace temperature of the protective atmosphere multi-purpose furnace is 800°C to 930°C.

8. The heat treatment process for low carbon low alloy steel according to any one of claims 5 to 7, characterized in that: In step S50, the tempering is specifically as follows: placing the casting cooled to room temperature by oil into a resistance furnace, raising the furnace temperature to 570°C to 600°C at full power, and keeping the temperature for 3h to 4h.

9. The heat treatment process for low carbon low alloy steel according to claim 8, characterized in that: In step S50, the initial furnace temperature of the resistance furnace is less than or equal to 600°C and greater than 230°C.

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