Mandrel bar manufacturing method and mandrel bar manufacturing device

Abstract

To provide a manufacturing method for a mandrel bar and a manufacturing apparatus for a mandrel bar that can perform tempering heat treatment of a mandrel bar without using an atmosphere furnace.SOLUTION: A manufacturing method for a mandrel bar is for manufacturing a mandrel bar by performing a hardening heat treatment and then a tempering heat treatment on the mandrel bar, determines the hardness to be achieved by the tempering heat treatment of the mandrel bar, determines a heating temperature, a holding time, and the upper limit temperature required to achieve the determined hardness, determines a range required to achieve the determined hardness of the mandrel bar, determines the induction heating heat treatment conditions that will achieve the heating temperature and holding time within the required range, and also ensure that the maximum temperature is not exceeded, and then performs a tempering heat treatment of the mandrel bar with the determined heat treatment conditions.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a method and an apparatus for manufacturing a mandrel bar. [Background technology]

[0002] Mandrel rolling is one of the processes for reducing the wall thickness of tubes. In this process, a mandrel bar, which reduces the wall thickness from the inside of the tube, is inserted into a blank tube, and both the blank tube and the mandrel bar are drawn between grooved rolls to reduce and elongate the tube. The grooved rolls are often installed in multiple stands, with a circumferential phase difference between them to ensure uniform reduction of the tube's outer circumference. Depending on the length of the tube to be produced and the number of stands, the mandrel bar, which is the internal tool, can be as long as several meters to several tens of meters. In mandrel rolling, the wall thickness is determined by the roll gap between the grooved rolls and the diameter of the mandrel bar, which reduces the tube from the inside, requiring high dimensional accuracy throughout its entire length. Furthermore, since the surface of the mandrel bar is transferred to the inner surface of the tube, good surface quality is essential. After mandrel rolling, the mandrel bar is gripped at its end by a drawing device and removed from the tube, where it is recycled for use in the mandrel rolling process again.

[0003] The mandrel bar generates high contact pressure with the inner surface of the tube being rolled. In the case of hot rolling, the mandrel bar is exposed to high temperatures, making it difficult to supply lubricant after it is inserted into the inner surface of the tube. Under these harsh operating conditions, the surface of the mandrel bar wears due to friction. At the same time, cracks may form on the surface of the mandrel bar due to sudden thermal contraction caused by temperature changes when the mandrel bar is removed from the inner surface of the tube. If the wear or cracks are significant, they will have a negative impact on the wall thickness and inner surface quality of the tube being produced, making the mandrel bar unusable for rolling. For this reason, the mandrel bar must have high surface hardness and toughness.

[0004] As a method for imparting high surface hardness and toughness, for example, Patent Document 1 proposes a method in which a mandrel bar is heated by high-frequency induction heating means to harden the surface, and then the entire bar is heated to temper it. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 8-197112 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-274433 Summary of the Invention [Problem to be solved by the invention]

[0006] Here, the method of Patent Document 1 requires that the entire bar be uniformly heated and tempered in the tempering step, and a very large atmospheric furnace is required to heat treat the entire long object such as a mandrel bar.

[0007] Furthermore, when a mandrel bar is heated in an atmosphere furnace while placed on a hearth or skid, a temperature difference occurs between the portion in contact with the hearth or skid and the portion not in contact with the hearth or skid. In this case, a difference in hardness occurs in the circumferential and longitudinal directions of the mandrel bar, which can cause unevenness in the surface properties of the mandrel bar. Furthermore, if the inside of the mandrel bar is sufficiently heated in a large atmosphere furnace, energy costs increase.

[0008] To address this issue, the present inventors focused on a method of tempering heat treatment using induction heating (IH). The area of ​​a mandrel bar that requires strength and toughness is within a range of several millimeters to several tens of millimeters from the surface, and it is believed that even with induction heating, which preferentially heats the surface, it is possible to temper the required area.

[0009] For example, Patent Document 2 proposes a method of tempering a long object, such as a bar, using induction heating (IH). However, because induction heating generally heats the surface preferentially, for solid, large-diameter objects such as mandrel bars, the radial temperature deviation increases, making it difficult to heat the interior. Furthermore, if the heating temperature is set high enough to sufficiently heat the interior, the maximum surface temperature will be high, potentially preventing the desired heat treatment. Patent Document 2 does not propose a method of tempering the interior using induction heating. Therefore, simply applying conventional technology such as that in Patent Document 2 makes it difficult to temper the required range.

[0010] The present disclosure has been made in view of the above-described circumstances, and has an object to provide a mandrel bar manufacturing method and a mandrel bar manufacturing apparatus that are capable of performing a tempering heat treatment on a mandrel bar without using an atmospheric furnace. [Means for solving the problem]

[0011] As a result of extensive research, the inventors have found that it is possible to use induction heating to set a heating pattern that heats areas requiring strength and toughness to an appropriate level while suppressing the maximum temperature reached on the surface. The present disclosure is based on this finding. The gist of the disclosure is as follows.

[0012] (1) A method for manufacturing a mandrel bar according to one embodiment of the present disclosure includes: A method for manufacturing a mandrel bar, which comprises subjecting the mandrel bar to a quenching heat treatment and then a tempering heat treatment, determining the hardness of the mandrel bar to be achieved after the tempering heat treatment; determining a heating temperature, a holding time, and an upper limit temperature for achieving the determined hardness; determining a range required to achieve the determined hardness of the mandrel bar; determining heat treatment conditions for induction heating that achieve the heating temperature and holding time within the range required to achieve the determined hardness, and that do not exceed an upper limit temperature; The mandrel bar is subjected to a tempering heat treatment under the determined heat treatment conditions.

[0013] (2) As one embodiment of the present disclosure, in (1), The induction heater uses a single coil to heat the mandrel bar.

[0014] (3) As one embodiment of the present disclosure, in (1), The induction heater uses multiple coils to heat the mandrel bar.

[0015] (4) As an embodiment of the present disclosure, in any one of (1) to (3), The heating temperature, the holding time, and the upper limit temperature are determined based on data from an experiment using a steel billet having the same chemical composition as the mandrel bar.

[0016] (5) As an embodiment of the present disclosure, in any one of (1) to (4), The heat treatment conditions are determined based on temperature calculations using FEM coupled with electromagnetic field-thermal conduction analysis.

[0017] (6) An apparatus for manufacturing a mandrel bar according to an embodiment of the present disclosure includes: A mandrel bar manufacturing apparatus that performs a quenching heat treatment on a mandrel bar and then a tempering heat treatment on the mandrel bar, a first determination unit that determines a hardness of the mandrel bar to be achieved after the tempering heat treatment; A second determination unit that determines a heating temperature, a holding time, and an upper limit temperature for achieving the determined hardness; a third determination unit that determines a range in which the determined hardness of the mandrel bar needs to be achieved; a fourth determination unit that determines heat treatment conditions for induction heating such that the heating temperature and holding time are achieved within a range required to achieve the determined hardness, and the upper limit temperature is not exceeded; and a tempering heat treatment execution unit that performs tempering heat treatment on the mandrel bar under the determined heat treatment conditions. [Effects of the Invention]

[0018] According to the present disclosure, there are provided a method and an apparatus for manufacturing a mandrel bar that are capable of performing a tempering heat treatment on a mandrel bar without using an atmospheric furnace. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a diagram for explaining a tempering heat treatment by induction heating in an embodiment of the present disclosure in the case where one coil is used. [Figure 2] FIG. 2 is a diagram for explaining a method for determining heat treatment conditions using the present disclosure. [Figure 3] FIG. 3 shows an example of the results of the experiment. [Figure 4] FIG. 4 is a diagram for explaining a form in which a plurality of coils are used in tempering heat treatment by induction heating according to an embodiment of the present disclosure. [Figure 5] FIG. 5 is a diagram showing the temperature calculation results in the first embodiment. [Figure 6] FIG. 6 is a diagram showing the temperature calculation results in the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, a method for manufacturing a mandrel bar and an apparatus for manufacturing a mandrel bar according to an embodiment of the present disclosure will be described with reference to the drawings.

[0021] The mandrel bar, which is the target of manufacture by the mandrel bar manufacturing method and mandrel bar manufacturing apparatus according to this embodiment, is an internal tool used in a mandrel rolling process. Hot work tool steels such as SKD6 or SKD61 specified in JIS are generally used as the steel type for manufacturing the mandrel bar. After being machined to a predetermined dimension, the mandrel bar is manufactured by undergoing quenching and tempering heat treatments (initial manufacturing). Mandrel bars are used in the mandrel rolling process in the manufacture of steel pipes and the like, and after use, are subjected to annealing heat treatment, cut, and remanufactured (remanufacturing). The remanufactured mandrel bar is used again in the mandrel rolling process. The mandrel bar manufacturing method and mandrel bar manufacturing apparatus according to this embodiment perform the above-mentioned initial manufacturing or remanufacturing.

[0022] Here, the surface hardness of the mandrel bar after the quenching heat treatment is approximately HV550 to 750 in Vickers hardness. The surface hardness is measured in a range of approximately 0 to 25 mm in the radial direction from the surface (outer surface) of the mandrel bar. Tool steel is generally quenched by induction heating, and quenching of mandrel bars in initial manufacturing and remanufacturing is also performed by induction heating. Induction heating mainly heats the surface. Therefore, the hardness increases near the surface due to the quenching heat treatment.

[0023] <Tempering heat treatment> The mandrel bar manufacturing apparatus according to this embodiment performs tempering heat treatment by induction heating on the mandrel bar after quenching heat treatment.

[0024] To prevent harmful cracks from occurring in the mandrel bar as described above, it is effective to impart high toughness to the mandrel bar by tempering. The range to be toughened is preferably 8 mm or more from the surface, and more preferably 10 mm or more. Furthermore, in a Charpy impact test using a half-size test piece specified in JIS Z 2242:2018 taken from the above range, it is desirable for the mandrel bar to have a toughness value of 4 J or more, and more preferably a toughness value of 16 J or more.

[0025] FIG. 1 is a diagram illustrating a tempering heat treatment by induction heating in the case where one coil 2 is used. The coil 2 is a heating coil for induction heating that heats the mandrel bar 1. The dimensions of the mandrel bar 1 are not particularly limited, but may be, for example, a diameter of approximately 50 mm to 400 mm and a length of approximately 0.4 m to 100.0 m. The dimensions of the coil 2 are also not particularly limited, but since the mandrel bar 1 passes through the coil 2, the inner diameter of the coil 2 is set to be larger than the outer diameter of the mandrel bar 1. The heating frequency of the induction heating is not particularly limited, but in order to efficiently heat the surface of the mandrel bar 1, it is desirable that it be 200 Hz or higher, and more preferably 400 Hz or higher.

[0026] <Transport> In order to perform tempering heat treatment over the entire length of the mandrel bar 1, the conveying device 3 passes the mandrel bar 1 through the coil 2. The conveying method is not limited. For example, the conveying device 3 may convey the mandrel bar 1 by pinching it between pinch rolls and rotating the pinch rolls. Alternatively, the conveying device 3 may move the coil 2 along the longitudinal direction of the mandrel bar 1 without conveying the mandrel bar 1.

[0027] FIG. 2 is a diagram for explaining a method for determining heat treatment conditions (conditions for induction heating in tempering heat treatment). First, the hardness to be achieved after tempering heat treatment is determined, and the necessary temperature and time conditions are determined experimentally. Next, using these temperature and time conditions, heat treatment conditions for achieving the hardness of the mandrel bar 1 within the required range are calculated. Next, the mandrel bar 1 is subjected to tempering annealing under the determined heat treatment conditions. Each item will be described in detail below.

[0028] <Determining hardness> First, as described above, the hardness to be achieved after tempering is determined. The hardness to be achieved after tempering is set as a target hardness that provides sufficient wear resistance to the bar during rolling, for example.

[0029] <Determining temperature, time and upper limit> The temperature and time conditions necessary to achieve the determined hardness after tempering heat treatment are determined by experiment. The experiment may be to temper the same material as the mandrel bar 1 to be cut and investigate the hardness.

[0030] Here, the inventors have conducted extensive research and found that heating at a certain tempering parameter (tempering parameter) or higher is necessary to reduce the hardness, based on the hardness after quenching heat treatment. They also found that if the heating temperature is too high or the holding time is too long, the hardness will be reduced too much. Among the heating conditions experimentally found to result in an excessive reduction in hardness, the lowest heating temperature value may be used as the upper limit temperature to prevent the material hardness from being reduced too much. In this way, the heating temperature, holding time, and upper limit temperature for achieving the target hardness are determined through experimentation. Here, in the experiment, a long holding time is preferably 60 seconds or more, and more preferably 100 seconds or more. A short holding time is preferably 20 seconds or less, and more preferably 10 seconds or less.

[0031] Figure 3 shows an example of experimental results. The vertical axis represents the heating temperature (°C). The horizontal axis represents the holding time (seconds). "◯" indicates a Vickers hardness of HV400 to 550. "X" indicates a Vickers hardness greater than HV550. "△" indicates a Vickers hardness less than HV400. The example in Figure 3 shows that heating with a tempering parameter of 15,400 or higher is effective for reducing hardness. Furthermore, among the heating conditions that resulted in excessive reduction in hardness obtained through the experiment, the lowest heating temperature was 800°C, indicating that using 800°C as the upper limit temperature to prevent excessive reduction in material hardness is effective. Here, the heating conditions that resulted in excessive reduction in hardness were those marked with "△," indicating a Vickers hardness less than HV400. The tempering parameter (TP) can be calculated using the following formula:

[0032] TP=(Temp+273)×(20+log(Time / 3600))

[0033] Here, Temp is the heating temperature [°C], and Time is the holding time [seconds].

[0034] <Determining the range of hardness that needs to be achieved> The range in which the mandrel bar 1 must achieve hardness is determined. The hardness must be adjusted to fall within this target range by tempering heat treatment. The range in which the mandrel bar 1 must achieve hardness is determined, for example, as a range of 0 to 10 mm in the radial direction from the outer surface, depending on the required specifications.

[0035] <Determining heat treatment conditions> The heat treatment conditions for induction heating are determined so that the heating temperature and holding time are achieved within the entire range of hardness that needs to be achieved, and the upper limit temperature is not exceeded.

[0036] Due to the characteristics of induction heating, the surface is heated preferentially. Therefore, the heat treatment conditions of induction heating are determined so that the innermost position within the range where hardness needs to be achieved achieves the determined heating temperature and holding time. Here, once the determined heating temperature is achieved at the innermost position, the temperature will be higher at positions radially outward. Therefore, the heat treatment conditions of induction heating are determined so that the maximum temperature reached at the surface position where the temperature is highest does not exceed the upper limit temperature.

[0037] Here, the heat treatment conditions may be determined, such that the heating temperature and holding time are achieved, including the conveying speed of the mandrel bar 1 in addition to the induction heating output. The induction heating output may be determined by performing a heat transfer calculation based on the temperature history of the mandrel bar 1. For example, the calculation may be performed using a finite element method (FEM) that couples electromagnetic field and heat conduction analysis. The induction heating output and the conveying speed of the mandrel bar 1 are determined by the calculation so that the predetermined heating temperature and holding time are achieved and the upper limit temperature is not exceeded. Then, the mandrel bar 1 is subjected to a tempering heat treatment under the determined heat treatment conditions.

[0038] Referring again to FIG. 1 , an outlet thermometer 4 for measuring the temperature of the mandrel bar 1 may be installed on the outlet side of the coil 2. In the tempering heat treatment of the mandrel bar 1, the output of the coil 2 may be adjusted so that the surface temperature of the mandrel bar 1 on the outlet side of the coil 2 follows the calculations made when determining the heat treatment conditions. Also, an entry thermometer 5 for measuring the temperature of the mandrel bar 1 may be installed on the entry side of the coil 2. The output of the coil 2 may be adjusted so that the surface temperature of the mandrel bar 1 on the entry side of the coil 2 follows the calculations made when determining the heat treatment conditions. Here, the outlet thermometer 4 and the entry thermometer 5 may be non-contact measuring devices such as radiation thermometers, or may be contact thermometers.

[0039] <Multiple coil configuration> FIG. 4 is a diagram illustrating a tempering heat treatment by induction heating when there are multiple coils 2. By arranging multiple coils 2 at a distance (coil-to-coil distance) and heating them as shown in FIG. 4, it is possible to reduce the radial temperature deviation of the mandrel bar 1, which is a solid object with a large diameter. This is because the temperature distribution is reduced due to heat transfer in the radial direction of the mandrel bar 1 after it is heated by the first coil 2 (the coil 2 on the upstream side in the conveying direction) and before it is heated by the second coil 2 (the coil 2 on the downstream side in the conveying direction). There may be three or more coils 2.

[0040] An outlet thermometer 4 may be installed on the outlet side of the second coil 2 to measure the temperature of the mandrel bar 1. In the tempering heat treatment of the mandrel bar 1, the output of the coil 2 may be adjusted so that the surface temperature of the mandrel bar 1 on the outlet side of the second coil 2 follows the calculations used to determine the heat treatment conditions. Also, an inlet thermometer 5 may be installed on the inlet side of the first coil 2 to measure the temperature of the mandrel bar 1. The output of the coil 2 may be adjusted so that the surface temperature of the mandrel bar 1 on the inlet side of the first coil 2 follows the calculations used to determine the heat treatment conditions. Also, an intermediate thermometer 6 may be installed between the two coils 2 to measure the temperature of the mandrel bar 1. The output of the coil 2 may be adjusted so that the surface temperature of the mandrel bar 1 between the two coils 2 follows the calculations used to determine the heat treatment conditions.

[0041] The mandrel bar 1 manufacturing apparatus according to this embodiment may include the tempering heat treatment apparatus shown in FIGS. 1 and 4 or may control the tempering heat treatment apparatus, and may be configured to include, for example, a computer. The mandrel bar 1 manufacturing apparatus may execute the above-described mandrel bar 1 manufacturing method using, for example, a computer. The computer may be, for example, a computer that manages the manufacture of the mandrel bar 1 and the mandrel rolling process using the mandrel bar 1. The configuration of the computer is not particularly limited and may include, for example, a memory (storage device), a CPU (processing device), a hard disk drive (HDD), a communication control unit for connecting to a network, a display device, and an input device. Here, the method for determining the heat treatment conditions shown in FIG. 2 may be implemented by the computer's CPU. Data (including experimental data) used in the heat treatment conditions may be input from the computer's input device or via a network and stored in the memory or hard disk drive. The "hardness determination" process may be executed by a first determination unit. The "temperature, time, and upper limit value determination" process may be executed by a second determination unit. The "hardness range determination" process may be executed by a third determination unit. The above-mentioned "determination of heat treatment conditions" process may be executed by a fourth determination unit. Furthermore, the execution of the tempering heat treatment of the mandrel bar 1 may be executed by a tempering heat treatment execution unit. When one or more programs stored in memory are loaded by the CPU of a computer, the CPU may function as a first determination unit, a second determination unit, a third determination unit, a fourth determination unit, and a tempering heat treatment execution unit. In this way, the mandrel bar 1 manufacturing device according to this embodiment includes a first determination unit, a second determination unit, a third determination unit, a fourth determination unit, and a tempering heat treatment execution unit, and executes the mandrel bar 1 manufacturing method.

[0042] As described above, the manufacturing method and manufacturing apparatus for the mandrel bar 1 according to this embodiment can perform a tempering heat treatment on the mandrel bar 1 without using an atmosphere furnace, thanks to the above-described configuration and steps. In other words, according to the present disclosure, the tempering heat treatment on the mandrel bar 1 can be realized without the need for a large atmosphere furnace. Therefore, the problem of a temperature difference occurring between the portion in contact with the hearth or skid and the portion not in contact with the hearth or skid does not occur. According to the present disclosure, the portion to be cut is heated in a concentrated manner by induction heating, which reduces energy costs compared to an atmosphere furnace.

[0043] (Example) The effects of the present disclosure will be specifically described below based on examples, but the present disclosure is not limited to the contents of the examples.

[0044] In this example, a mandrel bar was manufactured.

[0045] (Example 1: When there is one coil) A mandrel bar made of hot work tool steel (SKD6) having the chemical composition shown in Table 1 was subjected to induction quenching heat treatment, followed by the induction tempering heat treatment described in the above embodiment. The remainder not shown in Table 1 is Fe and unavoidable impurities. The mandrel bar had a diameter of 120 mm. The mandrel bar had a length of 20,000 mm. The target hardness after tempering heat treatment was set to HV400 to 550, which would ensure sufficient wear resistance of the bar. Furthermore, it was also targeted that this hardness be achieved within a range of 0 to 10 mm radially from the outer surface. After the mandrel bar was subjected to quenching heat treatment, the end of the mandrel bar was cut and the Vickers hardness was measured within a range of 0 to 10 mm radially from the outer surface. The hardness was found to be in the range of HV595 to 608.

[0046] [Table 1]

[0047] Experiments were conducted in advance using SKD6 steel billets with the same chemical composition as the mandrel bar. After heat treatment using the same temperature pattern as the quenching performed in the mandrel bar manufacturing, the hardness reached HV546 when held at 600°C for 60 seconds. Based on these results, it was decided that the heating temperature should be 600°C and the holding time should be 60 seconds (tempering parameter 15908). In addition, experiments were conducted under multiple conditions, changing the heating temperature and holding time. As a result, the hardness was less than HV400 under multiple conditions. The lowest heating temperature among these conditions was 790°C. Based on these results, it was decided that the upper limit temperature should be 790°C.

[0048] Tempering heat treatment was performed using a tempering heat treatment device as shown in Figure 1. The inner diameter of the coil is 140 mm. The outer diameter of the coil is 180 mm. The length (coil length) is 200 mm. The heating frequency of the induction heating is 3000 Hz. An outlet thermometer, which is a radiation thermometer, is installed at the outlet side of the coil to measure the temperature of the mandrel bar. The position of the coil is fixed. The mandrel bar is transported at a constant speed by pinch rolls installed on the inlet and outlet sides of the coil.

[0049] Prior to the tempering heat treatment, temperature calculations were performed using FEM coupled with electromagnetic field and heat conduction analysis. Figure 5 shows the results of calculations of the temperature changes over time at the surface and at a point 10 mm from the surface (two evaluation points). If the mandrel bar transfer speed was 1.5 mm / s and the temperature of the mandrel bar at the coil exit was 744°C, the time the point 10 mm from the surface remained above 600°C would be 61 seconds, and the maximum surface temperature would be 746°C, below the upper limit temperature. Here, the point 10 mm from the surface corresponds to the innermost point within the range where hardness must be achieved. Therefore, the mandrel bar transfer speed was set to 1.5 mm / s, and the induction heating output was adjusted so that the temperature reading on the exit thermometer would be 744°C.

[0050] Using the above settings, the entire length of the mandrel bar was subjected to a tempering heat treatment. After the tempering treatment, the ends of the mandrel bar were cut and the Vickers hardness was measured in the range of 0 to 10 mm in the radial direction from the outer surface. The hardness was found to be in the range of HV462 to 538, which was within the target range.

[0051] (Example 2: Multiple coils) A mandrel bar made of hot work tool steel (SKD6) having the chemical composition shown in Table 1 was subjected to induction quenching heat treatment, followed by the induction tempering heat treatment described in the above embodiment. The remainder not shown in Table 1 is Fe and unavoidable impurities. The mandrel bar had a diameter of 170 mm. The mandrel bar had a length of 20,000 mm. The target hardness after tempering heat treatment was HV400 to 550, which ensures sufficient wear resistance of the bar. Furthermore, it was targeted that this hardness be achieved within a range of 0 to 10 mm radially from the outer surface. The end of the mandrel bar after quenching heat treatment was cut and the Vickers hardness was measured within a range of 0 to 10 mm radially from the outer surface. The hardness was found to be in the range of HV590 to 602.

[0052] Experiments were conducted in advance using SKD6 steel billets with the same chemical composition as the mandrel bar. After heat treatment using the same temperature pattern as the quenching performed in the mandrel bar manufacturing, the hardness reached HV548 when held at 600°C for 60 seconds. Based on these results, it was decided that the heating temperature should be 600°C and the holding time should be 60 seconds (tempering parameter 15908). In addition, experiments were conducted under multiple conditions, changing the heating temperature and holding time. As a result, the hardness was less than HV400 under multiple conditions. The lowest heating temperature among these conditions was 790°C. Based on these results, it was decided that the upper limit temperature should be 790°C.

[0053] Tempering heat treatment was carried out using the tempering heat treatment equipment shown in Figure 4. Both coils have the same dimensions. The inner diameter of the coil is 190 mm. The outer diameter of the coil is 230 mm. The length (coil length) is 200 mm. The heating frequency of the induction heating is 3000 Hz. An outlet thermometer, which is a radiation thermometer, is installed on the outlet side of the coil to measure the temperature of the mandrel bar. The position of the coil is fixed. The mandrel bar is transported at a constant speed by pinch rolls installed on the inlet and outlet sides of the coil.

[0054] Prior to the tempering heat treatment, temperature calculations were performed using FEM coupled with electromagnetic field and heat conduction analysis. Figure 6 shows the results of calculations of the temperature change over time at the surface and at a point 10 mm from the surface (two evaluation points). The distance between the coils was set to 80 mm. If the mandrel bar transfer speed was set to 1.5 mm / s and the temperature of the mandrel bar at the coil exit was 735°C, the time for the point 10 mm from the surface to be maintained at 600°C or higher would be 63 seconds, and the maximum surface temperature would be 736°C, below the upper limit temperature. Here, the point 10 mm from the surface corresponds to the innermost point within the range where hardness must be achieved. Therefore, the distance between the coils was set to 80 mm, the mandrel bar transfer speed was set to 1.5 mm / s, and the induction heating output was adjusted so that the temperature reading on the exit thermometer would be 735°C.

[0055] Using the above settings, the entire length of the mandrel bar was subjected to a tempering heat treatment. After the tempering heat treatment, the ends of the mandrel bar were cut and the Vickers hardness was measured in the range of 0 to 10 mm in the radial direction from the outer surface. The hardness was found to be in the range of HV459 to 537, which was within the target range.

[0056] (Comparative Example 1: When the surface is heated too high) A mandrel bar made of hot work tool steel (SKD6) having the chemical composition shown in Table 1 was subjected to induction heating quenching heat treatment and then induction heating tempering heat treatment. The balance not shown in Table 1 is Fe and unavoidable impurities. The mandrel bar had a diameter of 120 mm and a length of 20,000 mm. After quenching heat treatment, the end of the mandrel bar was cut and the Vickers hardness was measured in the range of 0 to 10 mm radially from the outer surface. The hardness was found to be in the range of HV596 to 605.

[0057] Tempering heat treatment was performed using a tempering heat treatment device as shown in Figure 1. The inner diameter of the coil is 140 mm. The outer diameter of the coil is 180 mm. The length (coil length) is 200 mm. The heating frequency of the induction heating is 3000 Hz. An outlet thermometer, which is a radiation thermometer, is installed at the outlet side of the coil to measure the temperature of the mandrel bar. The position of the coil is fixed. The mandrel bar is transported at a constant speed by pinch rolls installed on the inlet and outlet sides of the coil.

[0058] The mandrel bar was conveyed at a speed of 1.5 mm / s, and the output of the induction heater was adjusted so that the temperature reading on the outlet thermometer reached 810°C.

[0059] Using the above settings, a tempering heat treatment was performed over the entire length of the mandrel bar. After the tempering treatment, the end of the mandrel bar was cut and the Vickers hardness was measured in the range of 0 to 10 mm in the radial direction from the outer surface. The hardness was in the range of HV324 to 480, which was outside the target range. This is thought to be because the surface temperature of the mandrel bar became too high, resulting in a decrease in surface hardness.

[0060] (Comparative Example 2: When the surface is cooled too much) The mandrel bar after the quenching heat treatment similar to that of Example 2 was subjected to a tempering heat treatment in the same coil as that of Example 2.

[0061] The mandrel bar was conveyed at a speed of 1.5 mm / s, and the output of the induction heater was adjusted so that the temperature reading on the outlet thermometer reached 650°C.

[0062] Using the above settings, a tempering heat treatment was performed over the entire length of the mandrel bar. After the tempering heat treatment, the ends of the mandrel bar were cut and the Vickers hardness was measured in the range of 0 to 10 mm radially from the outer surface. The hardness was in the range of HV509 to 580, which was outside the target range. This is thought to be because the heating temperature was too low, and the heating temperature and holding time were insufficient, resulting in an insufficient reduction in hardness. In addition, Charpy tests were performed using half-size test pieces as specified in JIS Z 2242:2018. The test pieces were taken from the range of 0 to 10 mm radially from the outer surface. The toughness value was a low value of 2J, indicating a high risk of harmful cracks occurring in the mandrel bar.

[0063] (Comparative Example 3: When an atmospheric furnace was used) A tempering heat treatment was carried out in an atmosphere furnace on the mandrel bar after rolling in the same manner as in Example 2. In the tempering heat treatment, the mandrel bar was inserted into an atmosphere furnace at a furnace temperature of 630°C, and after 80 minutes, was removed and air-cooled.

[0064] After the tempering heat treatment, the end of the mandrel bar was cut and the Vickers hardness was measured in the range of 0 to 10 mm in the radial direction from the outer surface. The hardness was in the range of HV459 to 542, which was within the target range. However, the energy cost was 1.5 times higher than in Example 2. This is because, in the case of an atmospheric furnace, it is necessary to heat the entire large furnace to a high temperature, which results in a large energy cost for heating.

[0065] Furthermore, visual inspection of the surface of the mandrel bar after the tempering heat treatment revealed unevenness in the circumferential and longitudinal directions of the surface. This is thought to be due to the fact that the heating was performed in an atmospheric furnace, which resulted in a temperature difference between the part of the mandrel bar that was in contact with the skid and the part that was not, resulting in a difference in hardness after the tempering heat treatment. Such unevenness can be transferred to the product during rolling and can cause quality defects.

[0066] Although the embodiments of the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art would easily be able to make various modifications or alterations based on the present disclosure. Therefore, it should be noted that these modifications and alterations are included within the scope of the present disclosure. For example, the functions included in each component or step can be rearranged so as not to cause logical inconsistencies, and multiple components or steps can be combined or divided into one. The embodiments of the present disclosure can also be realized as a storage medium on which a program executed by a processor included in an apparatus is recorded. It should be understood that these are also included within the scope of the present disclosure. [Explanation of symbols]

[0067] 1 mandrel bar 2 coils 3. Conveyor equipment 4 Outlet thermometer 5 Inlet thermometer 6 Intermediate thermometer

Claims

1. A method for manufacturing a mandrel bar, which comprises subjecting the mandrel bar to a quenching heat treatment and then a tempering heat treatment, determining the hardness of the mandrel bar to be achieved after the tempering heat treatment; determining a heating temperature, a holding time, and an upper limit temperature for achieving the determined hardness; determining a range required to achieve the determined hardness of the mandrel bar; determining heat treatment conditions for induction heating that achieve the heating temperature and holding time within the range required to achieve the determined hardness, and that do not exceed an upper limit temperature; and subjecting the mandrel bar to a tempering heat treatment under the determined heat treatment conditions.

2. The method for manufacturing a mandrel bar according to claim 1 , wherein the induction heating uses one coil to heat the mandrel bar.

3. The method for manufacturing a mandrel bar according to claim 1 , wherein the induction heating uses a plurality of coils to heat the mandrel bar.

4. 4. The method for manufacturing a mandrel bar according to claim 1, wherein the heating temperature, the holding time, and the upper limit temperature are determined based on data from an experiment using a steel billet having a chemical composition similar to that of the mandrel bar.

5. The method for manufacturing a mandrel bar according to any one of claims 1 to 3, wherein the heat treatment conditions are determined based on temperature calculations using FEM coupled with electromagnetic field-thermal conduction analysis.

6. A mandrel bar manufacturing apparatus that performs a quenching heat treatment on a mandrel bar and then a tempering heat treatment on the mandrel bar, a first determination unit that determines a hardness of the mandrel bar to be achieved after the tempering heat treatment; a second determination unit that determines a heating temperature, a holding time, and an upper limit temperature for achieving the determined hardness; a third determination unit that determines a range in which the determined hardness of the mandrel bar needs to be achieved; a fourth determination unit that determines heat treatment conditions for induction heating such that the heating temperature and holding time are achieved within a range required to achieve the determined hardness, and the upper limit temperature is not exceeded; a tempering heat treatment execution unit that performs a tempering heat treatment on the mandrel bar under the determined heat treatment conditions.

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