A medium frequency induction heat treatment process for core hardening of hot-rolled bars

Abstract

The embodiment of the present application discloses a medium frequency induction heat treatment process for quenching the core of a hot-rolled bar, comprising the following steps: S1, quenching heating, sending a workpiece into a quenching heating furnace, heating the workpiece to a temperature of not less than 800 degrees, and after the temperature of a part of the workpiece is not less than 880 degrees, the workpiece is moved out of the quenching heating furnace along with the conveying system; S2, quenching cooling, spray cooling the workpiece after quenching heating, wherein the cooling water pressure of at least part of the spray cooling is greater than 0.3MPa; S3, tempering heating, sending the workpiece after quenching cooling into a tempering heating furnace to heat the workpiece; S4, tempering cooling, spray cooling the workpiece after tempering heating. The present invention increases the quenching temperature to at least 880 degrees, and then increases the quenching cooling rate by increasing the cooling water pressure, increasing the workpiece temperature and then increasing the cooling rate, so that the workpiece is rapidly cooled from a high temperature to increase the hardenability of the core of the workpiece.

Description

Technical Field

[0001] The embodiments of the present application relate to a metal heat treatment process, and in particular to a medium-frequency induction heat treatment process for core hardening of hot-rolled bars. Background Art

[0002] In the existing technology, high strength M48 studs or wind power anchor bolts are generally used The 42CrMoA hot-rolled black leather material is subjected to medium frequency induction heat treatment, peeling, straightening, rolling, machining, thread rolling, flaw detection and surface treatment; when using When the 42CrMoA hot-rolled black leather material is subjected to medium frequency induction heat treatment, the core quenching hardness after quenching is 43-45HRC and the core metallographic structure tempered sorbite grade after high temperature tempering is at level 4, and the core is not fully quenched.

[0003] The indicators for through hardening are: the metallographic structure of the tempered product is level 2 or above (tempered bainite content ≥ 90%), and the quenched martensite content ≥ 90% (core hardness greater than 51HRC).

[0004] When the metallographic structure of the tempered product is tempered troostite (less than 80%) + ferrite + bainite, the toughness and fatigue resistance of the material are seriously affected. When medium-frequency induction heat-treated bars are processed into high-strength M48 studs or wind power anchor bolts, they are prone to brittle fracture as their use time increases, which directly affects their service life and their risk resistance. Summary of the Invention

[0005] The embodiments of the present application provide a medium frequency induction heat treatment process for core quenching of hot-rolled bars to at least improve or solve the above-mentioned technical problems.

[0006] The embodiments of the present application are implemented through the following technical solutions:

[0007] A medium-frequency induction heat treatment process for quenching the core of a hot-rolled bar comprises the following steps: S1, quenching heating, feeding a workpiece into a quenching heating furnace, heating the workpiece to a temperature not lower than 795°C, and after the temperature of a portion of the workpiece is heated to 875°C-905°C, the workpiece is moved out of the quenching heating furnace along with a conveying system; S2, quenching cooling, spray cooling the workpiece after quenching heating, wherein the cooling water pressure of at least part of the spray cooling is greater than 0.3 MPa; S3, tempering heating, feeding the quenched and cooled workpiece into a tempering heating furnace, heating the workpiece; and S4, tempering cooling, spray cooling the workpiece after tempering heating.

[0008] Furthermore, before step S1, the method further includes: S0, loading, placing the workpiece into the conveying system;

[0009] The conveying system is used to convey the workpiece, so that the workpiece passes through the quenching heating furnace, the quenching cooling mechanism, the tempering heating furnace, and the tempering cooling mechanism in sequence, wherein the quenching cooling mechanism and the tempering cooling mechanism are used to quench and temper the workpiece respectively.

[0010] Furthermore, the quenching cooling mechanism includes two spraying mechanisms arranged in sequence, which spray cool the workpiece twice.

[0011] Furthermore, the quenching heating furnace is sequentially provided with a first quenching temperature detection position and a second quenching temperature detection position along the conveying direction of the workpiece, and positions of the workpiece corresponding to the first quenching temperature detection position and the second quenching temperature detection position are respectively set as the first quenching position and the second quenching position;

[0012] The step of sending the workpiece into the quenching heating furnace to heat the workpiece specifically includes:

[0013] Heat the workpiece in a quenching furnace for at least 242 seconds, and make the temperature of the workpiece at the first quenching position reach 795°C to 805°C, and the temperature of the second quenching position reach 875°C to 905°C;

[0014] In step S2, the cooling water pressure of the first spray cooling is 0.3-0.5 MPa, and the cooling water pressure of the second quenching spray cooling is 0.1 MPa.

[0015] Furthermore, the workpiece after quenching and cooling is sent to a tempering heating furnace to heat the workpiece, specifically including: heating the workpiece to 635° C. to 645° C., and the heating time in the tempering heating furnace is at least 531 seconds.

[0016] Furthermore, in step S4, the cooling water pressure for spray cooling is 0.1 MPa.

[0017] Furthermore, the quenching heating furnace is a medium frequency induction furnace and adopts a two-stage heating method, the power of the first stage heating is 600kw; the power of the second stage heating is 150kw.

[0018] Furthermore, the workpiece has a diameter of 46±0.5 mm and a length of 6 m.

[0019] Furthermore, the workpiece is made of 42CrMoA steel.

[0020] Furthermore, the 42CrMoA steel is composed of the following components in weight percentage:

[0021] C: 0.41%, Si: 0.26%, Mn: 0.78%, P: 0.011%, Cu: 0.02%, Cr: 1.1%, Ni: 0.06%, Mo: 0.24%, S: 0.004%, H: 0.006%, and the balance is Fe and unavoidable impurities.

[0022] The beneficial effects are:

[0023] Compared with the prior art, a medium-frequency induction heat treatment process for quenching the core of a hot-rolled bar in an embodiment of the present application increases the quenching temperature to partially reach 875°C to 905°C, and then increases the cooling water pressure to increase the quenching cooling rate. By increasing the workpiece temperature and the cooling rate, the workpiece is quickly cooled from a high temperature to increase the hardenability of the core of the workpiece. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The following is a further detailed description of the specific implementation of the embodiment of the present application with reference to the accompanying drawings, wherein:

[0025] Figure 1 It is a level 1 tempered martensite metallographic structure;

[0026] Figure 2 It is a 2nd level tempered martensite metallographic structure;

[0027] Figure 3 It is a level 3 tempered martensite metallographic structure;

[0028] Figure 4 It is a level 4 tempered martensite metallographic structure. DETAILED DESCRIPTION

[0029] In order to enable those skilled in the art to better understand the technical solutions of the embodiments of the present application, the embodiments of the present application are described in detail below with reference to the accompanying drawings. The description in this section is merely exemplary and explanatory and should not have any limiting effect on the scope of protection of the embodiments of the present application.

[0030] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0031] It should be noted that the terms "center", "up", "down", "left", "right", "vertical", "horizontal", "inside", "outside", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings, or are the orientation or positional relationship in which the products of the embodiments of the present application are usually placed when in use. They are only for the convenience of describing the embodiments of the present application 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 operated in a specific orientation, and therefore cannot be understood as limiting the embodiments of the present application. In addition, the terms "first", "second", "third", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0032] Furthermore, terms such as "horizontal," "vertical," and "overhanging" do not necessarily imply that a component must be absolutely horizontal or overhanging, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal than "vertical," and does not mean that the structure must be completely horizontal, but rather that it can be slightly tilted.

[0033] In the description of the embodiments of the present application, it should also be noted that, unless otherwise expressly specified or limited, the terms "disposed," "installed," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in the embodiments of the present application based on the specific circumstances.

[0034] like Figure 1 As shown, a medium frequency induction heat treatment process for core hardening of hot-rolled bars includes steps S1, S2, S3, and S4.

[0035] S1, quenching heating, the workpiece is sent into the quenching heating furnace, the workpiece is heated to a temperature not lower than 795 degrees Celsius, and the temperature of some parts of the workpiece is heated to 875 degrees Celsius to 905 degrees Celsius, and the workpiece is moved out of the quenching heating furnace along with the conveying system;

[0036] S2, quenching cooling, spray cooling the workpiece after quenching heating, wherein the cooling water pressure of at least part of the spray cooling is greater than 0.3MPa; usually after quenching cooling, sampling is taken to test the core hardness, and multiple sampling can be performed.

[0037] S3, tempering heating, sending the workpiece after quenching and cooling into the tempering heating furnace to heat the workpiece;

[0038] S4, tempering cooling, spray cooling the workpiece after tempering heating. Usually sampling is carried out after tempering cooling to obtain the core metallographic structure grade.

[0039] The present invention provides a medium-frequency induction heat treatment process for quenching the core of a hot-rolled bar. The process increases the quenching temperature to at least 880 degrees, and then increases the cooling water pressure to increase the quenching cooling rate. By increasing the workpiece temperature and the cooling rate, the workpiece is quickly cooled from a high temperature to increase the hardenability of the workpiece core.

[0040] Usually the core hardening index is: the metallographic structure of the tempered product is level 2 or above (tempered bainite content ≥ 90%), and the quenched martensite content is ≥ 90% (core hardness is greater than 51HRC).

[0041] The tempered bainite content can be determined by metallographic method (refer to GB / T 13320 Metallographic structure rating diagram and evaluation method for steel die forgings), such as Figures 1 to 4 Shown are four levels of tempered troostite metallographic structures.

[0042] Take a sample at the center of the tempered workpiece and measure the tempered martensite content in the metallographic structure. The quenched martensite content can be determined by the hardness method, by taking a sample at the center of the quenched workpiece and measuring the Rockwell hardness to determine the martensite content. The corresponding relationship between martensite content and Rockwell hardness is shown in Table 1 below:

[0043]

[0044] For example, when the C content of round steel is 0.42, if its hardness is ≥51, it means that its quenched martensite content is ≥90%.

[0045] In a possible embodiment, before step S1, the process further includes: step S0, loading the workpiece into the conveying system;

[0046] The conveying system is used to transport workpieces, allowing them to pass through the quenching heating furnace, quenching cooling mechanism, tempering heating furnace, and tempering cooling mechanism in sequence. The quenching cooling mechanism and tempering cooling mechanism are used to quench and cool the workpieces, respectively. The conveying system passes through the quenching heating furnace and tempering heating furnace, feeding the workpieces during heating. After processing, the workpieces are conveyed out and enter the next process, achieving continuous process processing and automated station switching, improving the consistency and automation of the processing process and increasing production efficiency.

[0047] In a possible embodiment, the quenching and cooling mechanism includes two spraying mechanisms arranged in sequence, which spray cool the workpiece twice to ensure the quenching and cooling effect.

[0048] In a possible embodiment, the quenching heating furnace is sequentially provided with a first quenching temperature detection position and a second quenching temperature detection position along the conveying direction of the workpiece. The first quenching temperature detection position and the second quenching temperature detection position are provided with a temperature detection mechanism (such as a temperature sensor) to detect the temperature of the workpiece position below them. The positions of the workpiece corresponding to the first quenching temperature detection position and the second quenching temperature detection position are respectively set as the first quenching position and the second quenching position. After the workpiece enters the quenching heating furnace, the temperature of its first quenching position is detected by the temperature detection mechanism of the first quenching temperature detection position, and the temperature of its second quenching position is detected by the temperature detection mechanism of the second quenching temperature detection position, so as to achieve more comprehensive temperature monitoring of the workpiece. And usually, the distance between the first quenching temperature detection position and the second quenching temperature detection position is adapted to the length of the workpiece to detect the temperature at both ends of the workpiece. In step S2, the workpiece is fed into a quenching furnace and heated. Specifically, the workpiece is heated in the quenching furnace for at least 242 seconds, and the temperature of the workpiece at the first quenching position reaches 795°C to 805°C, and the temperature of the workpiece at the second quenching position reaches 875°C to 905°C. When the workpiece is gradually transported out of the quenching furnace along the conveying system, when the workpiece moves from the first quenching position to the second quenching position, its temperature has been heated to 875°C to 905°C, thereby ensuring the consistency of the workpiece temperature when it leaves the quenching furnace. At the same time, the workpiece movement time is not wasted, and the workpiece is heated simultaneously during the conveying process, thereby improving the heat treatment efficiency. The heating time of at least 242 seconds can ensure the heating time and allow the interior of the workpiece to be heated thoroughly. The heating time, the temperature of the first quenching position, and the temperature of the second quenching position can be selected according to the situation and are all within the allowable range of this process; for example, when the heating time is 250 seconds, the temperature of the first quenching position reaches 795°C, and the temperature of the second quenching position reaches 875°C; or when the heating time is 260 seconds, the temperature of the first quenching position reaches 805°C, and the temperature of the second quenching position reaches 905°C.

[0049] In step S2, the cooling water pressure of the first spray cooling is 0.3-0.5MPa, and the cooling water pressure of the second quenching spray cooling is 0.1MPa. Increasing the first cooling water pressure increases the speed of the first cooling of the high-temperature workpiece, and rapidly cools the workpiece from the quenching furnace. The actual grain size of austenite has a greater impact on the hardenability of steel. Coarse austenite grains can shift the C curve to the right, reducing the critical cooling rate of steel. Increasing the quenching temperature and extending the quenching time can increase its hardenability, allowing the core of the workpiece to be hardened. The workpiece is heated in the quenching furnace for 242 seconds, so that its core also obtains sufficient heating and temperature, ensuring that the quenching heat can reach its core position, providing a basis for subsequent core hardening.

[0050] In one possible embodiment, the workpiece, after being tempered and cooled, is fed into a tempering furnace for heating, specifically comprising heating the workpiece to a temperature of 635°C to 645°C, with the tempering furnace heating time being at least 531 seconds. Specifically, the tempering furnace is provided with a first tempering temperature detection position and a second tempering temperature detection position, sequentially along the workpiece conveying direction. The first and second tempering temperature detection positions are provided with temperature detection mechanisms (e.g., temperature sensors) for detecting the temperature of the workpiece below them. The positions of the workpiece corresponding to the first and second tempering temperature detection positions are designated as the first and second tempering positions, respectively. After the workpiece enters the tempering furnace, the temperature of its first tempering position is detected by the temperature detection mechanism at the first tempering temperature detection position, and the temperature of its second tempering position is detected by the temperature detection mechanism at the second tempering temperature detection position, thereby achieving more comprehensive temperature monitoring of the workpiece. Typically, the spacing between the first and second tempering temperature detection positions is adapted to the length of the workpiece to detect the temperatures at both ends of the workpiece. In step S3, the workpiece is heated in the tempering furnace for at least 531 seconds to ensure that the core of the workpiece is sufficiently heated and reaches the required temperature. The first tempering position and the second tempering position of the workpiece are both heated to 635°C to 645°C, and then sent to the subsequent process position; specifically, the heating time can be 531 seconds or longer than 531 seconds, and the temperature of the workpiece after heating can be 635°C, 640°C, 645°C, etc., all within the allowable range of this process.

[0051] In a further embodiment, in step S4, the cooling water pressure of the spray cooling is 0.1 MPa to cool the tempered heated workpiece.

[0052] In a possible embodiment, the quenching heating furnace is a medium frequency induction furnace and adopts a two-stage heating method. The power of the first stage heating is 600kw, and the power of the second stage heating is 150kw, to ensure sufficient power to heat the workpiece so that the workpiece reaches the required temperature.

[0053] In a possible embodiment, the workpiece has a diameter of 46±0.5 mm and can be used as a base material for high-strength M48 studs or wind power anchor bolts. The workpiece length is 6 m, so that longer parts can be processed at one time, thereby improving processing efficiency.

[0054] In a possible embodiment, the workpiece is made of 42CrMoA steel. 42CrMoA steel has few internal defects and relatively good internal quality.

[0055] In a possible embodiment, the 42CrMoA steel is composed of the following components in weight percentage:

[0056] C: 0.41%, Si: 0.26%, Mn: 0.78%, P: 0.011%, Cu: 0.02%, Cr: 1.1%, Ni: 0.06%, Mo: 0.24%, S: 0.004%, H: 0.006%, and the balance is Fe and unavoidable impurities.

[0057] Table 2 below shows the experimental data of the embodiments of the present invention and the comparative embodiments.

[0058]

[0059]

[0060] Example 1:

[0061] S0, loading, placing the workpiece into the conveying system;

[0062] S1: Quenching heating: the workpiece is heated in the quenching heating furnace for 242 seconds, and the temperature of the workpiece at the first quenching position reaches 800 degrees, and the temperature at the second quenching position reaches 880 degrees, and then the workpiece is moved out of the quenching heating furnace along with the conveying system;

[0063] S2, quenching cooling, spray cooling the workpiece after quenching heating, the cooling water pressure of the first spray cooling is 0.3MPa, and the cooling water pressure of the second quenching spray cooling is 0.1MPa;

[0064] S3, tempering heating, sending the quenched and cooled workpiece into the tempering heating furnace and heating for 531 seconds, and heating the first tempering position and the second tempering position of the workpiece to 640 degrees;

[0065] S4, tempering cooling, spray cooling the workpiece after tempering heating, and the cooling water pressure of the spray cooling is 0.1MPa.

[0066] The final metallographic structure of the core of the tempered product is level 2, and the quenching hardness of the core of the quenched product measured by three samples is 51.5HRC, 52HRC, and 52.5HRC respectively.

[0067] Example 2:

[0068] S0, loading, placing the workpiece into the conveying system;

[0069] S1: Quenching heating: the workpiece is heated in the quenching heating furnace for 242 seconds, and the temperature of the workpiece at the first quenching position reaches 800 degrees, and the temperature at the second quenching position reaches 880 degrees, and then the workpiece is moved out of the quenching heating furnace along with the conveying system;

[0070] S2, quenching cooling, spray cooling the workpiece after quenching heating, the cooling water pressure of the first spray cooling is 0.5MPa, and the cooling water pressure of the second quenching spray cooling is 0.1MPa;

[0071] S3, tempering heating, sending the quenched and cooled workpiece into the tempering heating furnace and heating for 531 seconds, and heating the first tempering position and the second tempering position of the workpiece to 640 degrees;

[0072] S4, tempering cooling, spray cooling the workpiece after tempering heating, and the cooling water pressure of the spray cooling is 0.1MPa.

[0073] The metallographic structure of the core of the final tempered product is level 1, and the quenching hardness of the core of the quenched product measured by three samples is 54HRC, 55HRC, and 56.5HRC respectively.

[0074] Example 3:

[0075] S0, loading, placing the workpiece into the conveying system;

[0076] S1: Quenching heating: the workpiece is heated in the quenching heating furnace for 242 seconds, and the temperature of the workpiece at the first quenching position reaches 800 degrees, and the temperature of the workpiece at the second quenching position reaches 900 degrees, and then the workpiece is moved out of the quenching heating furnace along with the conveying system;

[0077] S2, quenching cooling, spray cooling the workpiece after quenching heating, the cooling water pressure of the first spray cooling is 0.3MPa, and the cooling water pressure of the second quenching spray cooling is 0.1MPa;

[0078] S3, tempering heating, sending the quenched and cooled workpiece into the tempering heating furnace and heating for 531 seconds, and heating the first tempering position and the second tempering position of the workpiece to 640 degrees;

[0079] S4, tempering cooling, spray cooling the workpiece after tempering heating, and the cooling water pressure of the spray cooling is 0.1MPa.

[0080] The final metallographic structure of the core of the tempered product is level 2, and the quenching hardness of the core of the quenched product measured by three samples is 51.5HRC, 52HRC, and 52.5HRC respectively.

[0081] Example 4:

[0082] S0, loading, placing the workpiece into the conveying system;

[0083] S1: Quenching heating: the workpiece is heated in the quenching heating furnace for 242 seconds, and the temperature of the workpiece at the first quenching position reaches 800 degrees, and the temperature of the workpiece at the second quenching position reaches 900 degrees, and then the workpiece is moved out of the quenching heating furnace along with the conveying system;

[0084] S2, quenching cooling, spray cooling the workpiece after quenching heating, the cooling water pressure of the first spray cooling is 0.5MPa, and the cooling water pressure of the second quenching spray cooling is 0.1MPa;

[0085] S3, tempering heating, sending the quenched and cooled workpiece into the tempering heating furnace and heating for 531 seconds, and heating the first tempering position and the second tempering position of the workpiece to 640 degrees;

[0086] S4, tempering cooling, spray cooling the workpiece after tempering heating, and the cooling water pressure of the spray cooling is 0.1MPa.

[0087] The final metallographic structure of the core of the tempered product is level 1, and the quenching hardness of the core of the quenched product measured by three samples is 57HRC, 56.5HRC, and 56HRC respectively.

[0088] It can be seen from Table 2 that, compared with the embodiments of the present application, the temperature of the second quenching position of the workpiece in comparative examples 1 to 9 is lower, resulting in the core not being fully quenched, the core metallographic structure being lower, and the core quenching hardness being no higher than 49, which is significantly different from the technical effect of the present application.

[0089] Compared with the embodiments of the present application, the quenching speed (heating time in the quenching furnace) and the tempering speed (heating time in the tempering furnace) of Comparative Examples 10 to 15 and Comparative Examples 17 to 22 are shorter, resulting in the core not being heated through, affecting the tempering effect and the quenching effect, resulting in the core not being quenched through, the core metallographic structure being low, and the core quenching hardness being no higher than 49, which is significantly different from the technical effect of the present application.

[0090] Compared with the present application, the quenching cooling water pressure 1 (cooling water pressure of the first spray cooling) of Comparative Example 16 and Comparative Example 23 is lower, resulting in insufficient quenching cooling speed and incomplete quenching of the core, which is significantly different from the technical effect of the present application.

[0091] The above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the embodiments of the present application should be included within the scope of the technical solutions of the present application.

Claims

1. A medium frequency induction heat treatment process for core hardening of hot rolled bars, characterized in that: The steps include: S1, quenching heating, sending the workpiece into the quenching heating furnace, heating the workpiece to a temperature not lower than 795°C, and after the temperature of some parts of the workpiece is heated to 875°C to 905°C, the workpiece is moved out of the quenching heating furnace along with the conveying system; the workpiece is made of 42CrMoA steel; the diameter of the workpiece is 46±0.5mm; S2, quenching cooling, spray cooling the workpiece after quenching heating, wherein the cooling water pressure of at least part of the spray cooling is greater than 0.3 MPa; S3, tempering heating, sending the workpiece after quenching and cooling into the tempering heating furnace to heat the workpiece; S4, tempering cooling, spray cooling the workpiece after tempering heating; The quenching cooling mechanism includes two spray mechanisms arranged in sequence, which spray cool the workpiece twice; The quenching heating furnace is sequentially provided with a first quenching temperature detection position and a second quenching temperature detection position along the conveying direction of the workpiece, and the positions of the workpiece corresponding to the first quenching temperature detection position and the second quenching temperature detection position are respectively set as the first quenching position and the second quenching position; the distance between the first quenching temperature detection position and the second quenching temperature detection position is adapted to the length of the workpiece so as to detect the temperature at both ends of the workpiece; The step of sending the workpiece into the quenching heating furnace to heat the workpiece specifically includes: Heat the workpiece in a quenching furnace for at least 242 seconds, and make the temperature of the workpiece at the first quenching position reach 795°C to 805°C, and the temperature of the second quenching position reach 875°C to 905°C; In step S2, the cooling water pressure of the first spray cooling is 0.3-0.5 MPa, and the cooling water pressure of the second quenching spray cooling is 0.1 MPa.

2. A medium frequency induction heat treatment process for core hardening of hot-rolled bars according to claim 1, characterized in that: Before step S1, the following steps are also included: Step S0: loading the workpiece into the conveying system; The conveying system is used to convey the workpiece, so that the workpiece passes through the quenching heating furnace, the quenching cooling mechanism, the tempering heating furnace, and the tempering cooling mechanism in sequence, wherein the quenching cooling mechanism and the tempering cooling mechanism are used to quench and temper the workpiece respectively.

3. A medium frequency induction heat treatment process for core hardening of hot rolled bars according to claim 2, characterized in that: The workpiece after quenching and cooling is sent to a tempering heating furnace to heat the workpiece, specifically including: heating the workpiece to 635° C. to 645° C., and the heating time in the tempering heating furnace is at least 531 seconds.

4. A medium frequency induction heat treatment process for core hardening of hot-rolled bars according to claim 3, characterized in that: In step S4, the cooling water pressure for spray cooling is 0.1 MPa.

5. The medium frequency induction heat treatment process for core hardening of hot-rolled bars according to claim 2, characterized in that: The quenching heating furnace is a medium frequency induction furnace and adopts a two-stage heating method. The power of the first stage heating is 600kw; the power of the second stage heating is 150kw.

6. The medium frequency induction heat treatment process for core hardening of hot-rolled bars according to claim 1, characterized in that: The workpiece has a diameter of 46±0.5 mm and a length of 6 m.

7. A medium frequency induction heat treatment process for core hardening of hot rolled bars according to claim 1, characterized in that: The 42CrMoA steel consists of the following components in weight percentage: Composition: C: 0.41%, Si: 0.26%, Mn: 0.78%, P: 0.011%, Cu: 0.02%, Cr: 1.1%, Ni: 0.06%, Mo: 0.24%, S: 0.004%, H: 0.006%, and the balance is Fe and unavoidable impurities.

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