Mandrel bar manufacturing method and mandrel bar manufacturing device

Abstract

To provide a method and an apparatus for manufacturing a mandrel bar capable of performing annealing heat treatment of the mandrel bar without using an atmosphere furnace.SOLUTION: In a manufacturing method for a mandrel bar, a used mandrel bar is subjected to annealing heat treatment and grinding treatment, and then subjected to quenching heat treatment and tempering heat treatment. The manufacturing method determines the hardness of the mandrel bar to be achieved after the annealing heat treatment, determines a heating temperature, a holding time and an upper limit temperature for achieving the determined hardness, determines a grinding range of the mandrel bar, determines the heat treatment conditions for induction heating to achieve the heating temperature and holding time and not to exceed the upper limit temperature in the determined grinding range, performs the grinding treatment after the annealing heat treatment of the mandrel bar under the determined heat treatment conditions, and performs the quenching heat treatment and the tempering heat treatment after the grinding treatment.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 a tube. In this process, a mandrel bar that rolls 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 pulled between grooved rolls to reduce the wall thickness and elongate the tube. The grooved rolls are often installed in multiple stands with a phase difference in the tube circumferential direction so that the outer periphery of the tube can be uniformly reduced. Depending on the length of the tube to be manufactured and the number of stands, the mandrel bar, which is the inner tool, has a long shape of several meters to several tens of meters. In mandrel rolling, the wall thickness is determined by the roll gap of the grooved rolls and the diameter of the mandrel bar that rolls the tube from the inside, so a mandrel bar with high dimensional accuracy over its entire length is required. In addition, the surface of the mandrel bar is transferred to the inner surface of the tube, so good surface quality of the mandrel bar is required. After mandrel rolling, the mandrel bar is gripped at the end by a pulling device, removed from the tube, and recycled for use in mandrel rolling 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, and it is difficult to supply lubricant to the mandrel bar after it is inserted into the inner surface of the tube. Due to these harsh operating conditions, the mandrel bar undergoes diameter reduction and surface seizure depending on the number of times it is used in rolling. When diameter reduction and seizure occur, they have a negative effect on the wall thickness and inner surface quality of the tube being manufactured. For this reason, the amount of diameter reduction and the state of seizure are controlled, and mandrel bars that have a negative effect on product quality are discarded.

[0004] Here, the mandrel bar is long, and various outer diameter sizes of the mandrel bar are required according to the wall thickness of the tube to be manufactured, so the tool cost is high. In order to reduce the tool cost, the mandrel bar used in rolling may be reused for rolling after removing the portion where seizure has occurred and applying heat treatment again to obtain surface hardness.

[0005] As another method for reducing tool costs, for example, Patent Document 1 proposes a method in which the portion of the mandrel bar that comes into contact with the material during rolling can be removed and replaced. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 10-249411 [Patent Document 2] Japanese Patent Publication No. 63-230823 [Patent Document 3] JP 2010-86904 A Summary of the Invention [Problem to be solved by the invention]

[0007] Here, the method of Patent Document 1 requires a change in the structure of the mandrel bar itself to be used. Therefore, in practice, a method of cutting the surface of a mandrel bar after it has been used for rolling and reusing it is widely used. Mandrel bars are manufactured by subjecting them to heat treatments of quenching and tempering so that they are less likely to develop surface defects when used for rolling. Since the mandrel bar has a high surface hardness, it is necessary to perform an annealing heat treatment before cutting to reduce the surface hardness before cutting. However, in order to heat treat a long object such as a mandrel bar in an atmosphere furnace, a very large atmosphere furnace is required.

[0008] In an atmospheric furnace, when a mandrel bar is heated while placed on a hearth or skid, a temperature difference occurs between the part that is in contact with the hearth or skid and the part that is not in contact with the hearth or skid. In this case, a hardness difference occurs in the circumferential and longitudinal directions of the mandrel bar, and the load on the cutting device during cutting is not constant, which may cause unevenness in the surface properties after cutting. Furthermore, if the inside of the mandrel bar is sufficiently heated in a large atmospheric furnace, the energy cost increases.

[0009] For example, Patent Documents 2 and 3 propose a method of annealing a long object such as a tube or bar by induction heating (IH). However, in induction heating, the surface is heated preferentially, so for a solid object with a large diameter such as a mandrel bar, the radial temperature deviation inside becomes large. In addition, if the entire furnace is heated to a high temperature in order to heat the inside sufficiently, the maximum temperature reached on the surface may exceed the upper limit temperature. Therefore, it is difficult to sufficiently heat the area to be cut and reduce the hardness by simply applying conventional techniques such as Patent Documents 2 and 3.

[0010] The present disclosure has been made in consideration 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 an annealing heat treatment on a mandrel bar without using an atmospheric furnace. [Means for solving the problem]

[0011] As a result of intensive research, the inventors have found that it is possible to set a heating pattern using induction heating such that the innermost part to be cut is sufficiently heated 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, comprising the steps of: subjecting a used mandrel bar to an annealing heat treatment and a cutting treatment; and then subjecting the bar to a quenching heat treatment and a tempering heat treatment, determining the hardness of the mandrel bar to be achieved after the annealing heat treatment; Determine the heating temperature, holding time and upper limit temperature for achieving the determined hardness; determining a cutting range of the mandrel bar; Determine heat treatment conditions for induction heating that achieve the heating temperature and holding time in the determined cutting range and do not exceed an upper limit temperature; The mandrel bar is subjected to an annealing heat treatment under the determined heat treatment conditions, and then the cutting treatment is performed; After the cutting process, a quenching heat treatment and a tempering heat treatment are performed.

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

[0014] (3) As an 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 piece 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, A mandrel bar manufacturing apparatus for manufacturing a mandrel bar by subjecting the mandrel bar to an annealing heat treatment and a cutting treatment, A first determination unit that determines a hardness of the mandrel bar to be achieved after the annealing 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 cutting range of the mandrel bar; A fourth determination unit determines heat treatment conditions for induction heating so that the heating temperature and holding time are achieved in the determined cutting range and the upper limit temperature is not exceeded; The method includes an annealing heat treatment execution unit that performs an annealing heat treatment on the mandrel bar under the determined heat treatment conditions, and then performs the cutting treatment, and then performs a quenching heat treatment and a tempering heat treatment after the cutting treatment. Effect of the Invention

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

[0019] [Figure 1] FIG. 1 is a diagram for explaining an embodiment of an annealing heat treatment by induction heating in the present disclosure, in which one coil is used. [Diagram 2] FIG. 2 is a diagram for explaining a method for determining heat treatment conditions using the present disclosure. [Diagram 3] FIG. 3 is a diagram showing an example of the results of the experiment. [Figure 4] FIG. 4 is a diagram for explaining an embodiment of the annealing heat treatment by induction heating in the present disclosure when a plurality of coils are used. [Diagram 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 PREFERRED EMBODIMENTS

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

[0021] The mandrel bar to be manufactured by the mandrel bar manufacturing method and the mandrel bar manufacturing device according to the present embodiment is an inner tool used in the mandrel rolling method. As the steel type of the material for manufacturing the mandrel bar, hot tool steel such as SKD6 or SKD61 specified in JIS is generally used. The mandrel bar is manufactured by processing to a predetermined dimension and then undergoing heat treatments of quenching and tempering (initial manufacturing). The mandrel bar is used in the mandrel rolling process in the manufacture of steel pipes and the like, and after use, is subjected to annealing heat treatment, cut, and subjected to quenching heat treatment and tempering heat treatment, and is manufactured again (remanufacturing). In other words, in remanufacturing, processing is performed on the used mandrel bar. The remanufactured mandrel bar is used again in the mandrel rolling process. The mandrel bar manufacturing method and the mandrel bar manufacturing device according to the present embodiment may include the above initial manufacturing, but mainly performs the above remanufacturing. Hereinafter, the mandrel bar used in the mandrel rolling method may be referred to as the "rolled" mandrel bar. Furthermore, a mandrel bar before and after being used in a mandrel rolling process may be referred to as a "before rolling" mandrel bar and an "in-rolling" mandrel bar, respectively.

[0022] Here, the surface hardness of the mandrel bar after the quenching heat treatment is about HV550 to 850 in Vickers hardness. The surface hardness of the mandrel bar after the tempering heat treatment is about HV350 to 550 in Vickers hardness. The surface hardness during and after rolling is the same as that after the tempering heat treatment. The surface hardness is the hardness in the range of about 0 to 25 mm in the radial direction from the surface (outer surface) of the mandrel bar. Generally, the quenching of tool steel is performed by induction heating, and the quenching of the mandrel bar in the initial manufacturing is also performed by induction heating. In the induction heating, the surface is mainly heated. Therefore, it is the vicinity of the surface that the hardness increases by the quenching heat treatment. The above surface hardness of about HV400 to 550 is the hardness in the range of about 0 to 25 mm in the radial direction from the outer surface where the hardness has increased by the quenching heat treatment.

[0023] <Annealing heat treatment> The mandrel bar manufacturing apparatus according to the present embodiment performs an annealing heat treatment by induction heating on the mandrel bar after rolling. FIG. 1 is a diagram for explaining the annealing heat treatment by induction heating in a form in which 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 about 50 mm to 400 mm and a length of about 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 preferable that the frequency is 200 Hz or more, and more preferably 400 Hz or more.

[0024] <Transport> In order to perform the annealing 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 form of conveyance 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.

[0025] FIG. 2 is a diagram for explaining a method for determining the heat treatment conditions (conditions for induction heating in annealing heat treatment). First, the hardness to be achieved after annealing heat treatment is determined, and the necessary temperature and time conditions are determined based on experiments. Next, the heat treatment conditions for achieving these temperature and time conditions in the cutting range of the mandrel bar 1 are calculated. Next, the mandrel bar 1 is subjected to annealing heat treatment under the determined heat treatment conditions. Each item will be described in detail below.

[0026] <Determination of hardness> First, as described above, the hardness to be achieved after the annealing heat treatment is determined. The hardness to be achieved after the annealing heat treatment is set as a target value that is cuttable depending on the capabilities of a cutting device used in the cutting process, for example.

[0027] <Determining temperature, time and upper limit> The temperature and time conditions necessary to achieve the determined hardness after annealing heat treatment (or a hardness less than the determined hardness) are obtained by an experiment. The experiment may be to anneal the same material as the mandrel bar 1 to be cut and investigate the hardness.

[0028] Here, the inventors have conducted extensive research and found that heating at a certain tempering parameter or higher is necessary to reduce the hardness. They also found that heating to a certain temperature or higher results in the material being quenched and the hardness increasing. Through experiments, the heating temperature and holding time for achieving the determined hardness are determined, and the upper limit temperature (upper limit of the temperature for annealing heat treatment) at which the material is not quenched is also determined.

[0029] FIG. 3 shows an example of the experimental results. The vertical axis indicates the heating temperature (°C). The horizontal axis indicates the holding time (seconds). "◯" indicates that the Vickers hardness is HV350 or less. Also, "×" indicates that the Vickers hardness is greater than HV350. The example in FIG. 3 shows that heating with a tempering parameter of 17200 or more is effective for reducing hardness. It also shows that setting the upper limit temperature to 900°C is effective to prevent the material from becoming quenched. The tempering parameter (TP) can be calculated using the following formula.

[0030] TP=(Temp+273)×(20+log(Time / 3600))

[0031] Here, Temp is the heating temperature [°C], and Time is the holding time [seconds].

[0032] <Determining the cutting area> The area to be cut in order to reuse the mandrel bar 1, i.e., how many mm to cut in the radial direction from the surface, is determined. It is necessary to reduce the hardness of this area by annealing heat treatment.

[0033] <Determining heat treatment conditions> The heat treatment conditions for induction heating are determined so that the heating temperature and holding time are achieved throughout the determined cutting range, and the upper limit temperature is not exceeded.

[0034] 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 of the cutting range 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 with the highest temperature does not exceed the upper limit temperature.

[0035] Here, in order to achieve the above heating temperature and holding time, the conveying speed of the mandrel bar 1 may be determined as the heat treatment conditions in addition to the induction heating output. In determining the induction heating output, a heat transfer calculation may be performed 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 an electromagnetic field-thermal conduction analysis. The induction heating output and the conveying speed of the mandrel bar 1 are determined by the calculation so that the previously determined heating temperature and holding time are achieved and the upper limit temperature is not exceeded. Then, the annealing heat treatment of the mandrel bar 1 is performed under the determined heat treatment conditions.

[0036] Referring again to FIG. 1 , an exit thermometer 4 for measuring the temperature of the mandrel bar 1 may be provided on the exit side of the coil 2. In the annealing 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 exit side of the coil 2 follows the calculation when determining the heat treatment conditions. Also, an entry thermometer 5 for measuring the temperature of the mandrel bar 1 may be provided 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 calculation when determining the heat treatment conditions. Here, the exit thermometer 4 and the entry thermometer 5 may be non-contact measuring devices such as radiation thermometers, or may be contact thermometers.

[0037] <Multiple coil configuration> Fig. 4 is a diagram for explaining an embodiment of the annealing heat treatment by induction heating when there are multiple coils 2. By arranging multiple coils 2 at a distance (coil-to-coil distance) as shown in Fig. 4 and heating them, 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 becomes smaller due to the radial heat transfer 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.

[0038] An exit thermometer 4 for measuring the temperature of the mandrel bar 1 may be provided on the exit side of the second coil 2. In the annealing 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 exit side of the second coil 2 follows the calculation when determining the heat treatment conditions. Also, an entry thermometer 5 for measuring the temperature of the mandrel bar 1 may be provided on the entry side of the first 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 first coil 2 follows the calculation when determining the heat treatment conditions. Also, an intermediate thermometer 6 for measuring the temperature of the mandrel bar 1 may be provided between the two coils 2. 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 calculation when determining the heat treatment conditions.

[0039] The manufacturing apparatus for the mandrel bar 1 according to the present embodiment may include the annealing heat treatment apparatus of FIG. 1 and FIG. 4 or may be an apparatus for controlling the annealing heat treatment apparatus, and may be configured to include, for example, a computer. The manufacturing apparatus for the mandrel bar 1 may execute the above-mentioned manufacturing method for the mandrel bar 1 by, for example, a computer. The computer may be, for example, a computer that manages the manufacturing 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 in FIG. 2 may be implemented by the CPU of the computer. Data (including experimental data) used in the heat treatment conditions may be input from the input device of the computer or via a network and stored in the memory or the hard disk drive. In addition, the above-mentioned process of "determining hardness" may be executed by the first determination unit. The above-mentioned process of "determining temperature, time, and upper limit value" may be executed by the second determination unit. The above-mentioned process of "determining the cutting range" may be executed by the third determination unit. The above-mentioned process of "determining heat treatment conditions" may be executed by the fourth determination unit. Moreover, the execution of the annealing heat treatment, cutting treatment, quenching heat treatment, and tempering heat treatment of the mandrel bar 1 may be executed by the annealing heat treatment execution unit. When one or more programs stored in the memory are read by the CPU of the computer, the CPU may function as a first determination unit, a second determination unit, a third determination unit, a fourth determination unit, and an annealing heat treatment execution unit. In this way, the manufacturing device of the mandrel bar 1 according to the present embodiment includes a first determination unit, a second determination unit, a third determination unit, a fourth determination unit, and an annealing heat treatment execution unit, and executes the manufacturing method of the mandrel bar 1. Here, the execution of the cutting treatment after the annealing heat treatment of the mandrel bar 1 may be executed by the cutting treatment execution unit, and the manufacturing device of the mandrel bar 1 may further include a cutting treatment execution unit. At this time, similar to the other functional units, when the program stored in the memory is read by the CPU of the computer, the CPU may function as a cutting treatment execution unit.

[0040] As described above, the manufacturing method of the mandrel bar 1 and the manufacturing device of the mandrel bar 1 according to the present embodiment can perform the annealing heat treatment of the mandrel bar 1 without using an atmosphere furnace by the above-mentioned configuration or process. That is, according to the present disclosure, the annealing heat treatment of 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 part in contact with the hearth or skid and the part not in contact with the hearth or skid does not occur. According to the present disclosure, the load on the cutting device during cutting is constant, and no unevenness occurs in the surface properties after cutting. In addition, according to the present disclosure, the part to be cut is heated intensively by induction heating, so that the energy cost can be reduced compared to an atmosphere furnace. In addition, according to the present disclosure, the heat treatment conditions are optimized so that the maximum reaching temperature of the surface does not exceed the upper limit temperature even in the case of a solid product such as the mandrel bar 1. As a result, problems such as damage to the blade of the cutting device are eliminated.

[0041] (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.

[0042] In this example, a hot mandrel rolling process was carried out in the production of seamless steel pipe.

[0043] (Example 1: One coil) A mandrel bar made of hot work tool steel (SKD6) having the chemical composition shown in Table 1 was subjected to the induction heating annealing heat treatment described in the above embodiment. The balance not shown in Table 1 is Fe and unavoidable impurities. The subject mandrel bar was used for hot mandrel rolling. The mandrel bar had a diameter of 120 mm. The mandrel bar had a length of 20,000 mm. In order to reuse this mandrel bar, a 10 mm cut was made in the radial direction. 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, and the hardness was in the range of HV495 to 529.

[0044] [Table 1]

[0045] The cutting device used in the cutting process carried out after the annealing heat treatment was capable of cutting up to HV350, so the target hardness was set to HV350 or less.

[0046] A preliminary experiment was conducted using a SKD6 steel billet with the same chemical composition as the mandrel bar. After annealing heat treatment with the same temperature pattern as the quenching and tempering performed in the initial production of the mandrel bar, the hardness became HV341 when held at 700°C for 80 seconds. From this result, it was decided that the heating temperature should be 700°C and the holding time should be 80 seconds (tempering parameter 17851). It was also found that the hardness increased sharply when the heating temperature was increased above 900°C. From this result, it was decided that the upper limit temperature should be 900°C.

[0047] Annealing heat treatment was performed using the annealing heat treatment equipment 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 exit thermometer, which is a radiation thermometer, is installed at the exit 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 entrance and exit sides of the coil.

[0048] Prior to the annealing heat treatment, temperature calculations were performed using FEM coupled with electromagnetic field-thermal conduction analysis. Figure 5 shows the results of calculating the temperature change over time at the surface and at a point 10 mm from the surface (two evaluation points). If the mandrel bar transport speed is 1.0 mm / s and the temperature of the mandrel bar at the coil exit is 814°C, the time for the point 10 mm from the surface to be held at 700°C or higher is 82 seconds, and the maximum surface temperature is 816°C, which is below the upper limit temperature. Here, the point 10 mm from the surface corresponds to the innermost position of the range where hardness needs to be reduced. Therefore, the mandrel bar transport speed was set to 1.0 mm / s, and the induction heating output was adjusted so that the measurement value of the thermometer at the exit side was 814°C.

[0049] With the above settings, annealing heat treatment was performed on the entire length of the mandrel bar. After the annealing heat treatment, the mandrel bar was cut 10 mm in the radial direction, and cutting was successful over the entire length, with no problems such as damage to the cutting device blades. In addition, when the surface of the mandrel bar after cutting was visually inspected, no unevenness in the surface properties was observed. After the cutting treatment, the mandrel bar was subjected to quenching heat treatment and tempering heat treatment. The mandrel bar that had been quenched and tempered was used in the hot mandrel rolling process, and no unevenness in the surface properties was observed in the rolled pipe.

[0050] (Example 2: Multiple coils) The mandrel bar made of hot work tool steel (SKD6) having the chemical composition shown in Table 1 above was subjected to the induction heating annealing heat treatment described in the above embodiment. The mandrel bar in question was one that had been used in hot mandrel rolling. The mandrel bar had a diameter of 170 mm. The mandrel bar had a length of 20,000 mm. In order to reuse this mandrel bar, a 10 mm cut was made in the radial direction. 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, and the hardness was in the range of HV496 to 531.

[0051] The cutting device used in the cutting process carried out after the annealing heat treatment was capable of cutting up to HV350, so the target hardness was set to HV350 or less.

[0052] A preliminary experiment was conducted using a SKD6 steel billet with the same chemical composition as the mandrel bar. After annealing heat treatment with the same temperature pattern as the quenching and tempering performed in the initial production of the mandrel bar, the hardness became HV338 when held at 700°C for 80 seconds. From this result, it was decided that the heating temperature should be 700°C and the holding time should be 80 seconds (tempering parameter 17851). It was also found that the hardness increased sharply when the heating temperature was increased above 900°C. From this result, it was decided that the upper limit temperature should be 900°C.

[0053] The annealing heat treatment was performed using the annealing 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 exit thermometer, which is a radiation thermometer, is installed at the exit side of the coil to measure the temperature of the mandrel bar. The coil position is fixed. The mandrel bar is transported at a constant speed by pinch rolls installed on the entrance and exit sides of the coil.

[0054] Prior to the annealing heat treatment, temperature calculations were performed using FEM coupled with electromagnetic field-thermal conduction analysis. Figure 6 shows the results of calculating 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 transport speed is set to 1.0 mm / s and the temperature of the mandrel bar at the coil exit is 813°C, the time for the position 10 mm from the surface to be held at 700°C or higher is 83 seconds, and the maximum temperature reached at the surface is 816°C, which is below the upper limit temperature. Here, the position 10 mm from the surface corresponds to the innermost position of the range where hardness needs to be reduced. Therefore, the distance between the coils was set to 80 mm, the mandrel bar transport speed was set to 1.0 mm / s, and the induction heating output was adjusted so that the measurement value of the exit thermometer was 813°C.

[0055] With the above settings, annealing heat treatment was performed on the entire length of the mandrel bar. After the annealing heat treatment, the mandrel bar was cut 10 mm in the radial direction, and cutting was successful over the entire length, with no problems such as damage to the cutting device blades. In addition, when the surface of the mandrel bar after cutting was visually inspected, no unevenness in the surface properties was observed. After the cutting treatment, the mandrel bar was subjected to quenching heat treatment and tempering heat treatment. The mandrel bar that had been quenched and tempered was used in the hot mandrel rolling process, and no unevenness in the surface properties was observed in the rolled pipe.

[0056] (Comparative Example 1: Without annealing heat treatment) In Comparative Example 1, cutting was performed without performing annealing heat treatment on the mandrel bar after rolling. Except for not performing annealing heat treatment, the conditions were the same as in Example 1. As a result, problems such as damage to the blade of the cutting device occurred, and cutting of 10 mm in the radial direction could not be performed at any position on the mandrel bar.

[0057] (Comparative Example 2: When the surface is heated too high) Annealing heat treatment was performed on a mandrel bar made of hot work tool steel (SKD6) having the chemical composition shown in Table 1 above. The mandrel bar in question was used in hot mandrel rolling. The mandrel bar had a diameter of 120 mm. The mandrel bar had a length of 20,000 mm. In order to reuse this mandrel bar, a 10 mm cut was made in the radial direction. 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, and the hardness was in the range of HV494 to 529.

[0058] Annealing heat treatment was performed using the annealing heat treatment equipment 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 exit thermometer, which is a radiation thermometer, is installed at the exit 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 entrance and exit sides of the coil.

[0059] The mandrel bar was moved at a speed of 1.0 mm / s, and the output of the induction heater was adjusted so that the measurement value of the exit thermometer became 950°C.

[0060] With the above settings, the entire length of the mandrel bar was annealed. When the mandrel bar after the annealing was cut 10 mm in the radial direction, problems such as damage to the cutting blade of the cutting device occurred, and it was not possible to cut 10 mm in the radial direction at any position on the mandrel bar. This is thought to be because the temperature of the mandrel bar surface became too high, causing it to become hardened and becoming highly hard.

[0061] (Comparative Example 3: When the surface is cooled too much) The mandrel bar after rolling similar to that in Example 2 was subjected to annealing heat treatment in the same coil as that in Example 2.

[0062] The mandrel bar was moved at a speed of 1.0 mm / s, and the output of the induction heater was adjusted so that the measurement value of the exit thermometer was 750°C.

[0063] With the above settings, the entire length of the mandrel bar was annealed. After the annealing, the mandrel bar was cut 10 mm in the radial direction, but problems such as damage to the cutting blade occurred, and it was not possible to cut 10 mm in the radial direction at any position on the mandrel bar. This is thought to be because the heating temperature was low, and therefore sufficient heating temperature and holding time were not secured in the area near the center of the cutting range, and the hardness did not decrease sufficiently.

[0064] (Comparative Example 4: When an atmospheric furnace is used) Annealing heat treatment was performed in an atmosphere furnace and cutting was performed on the mandrel bar after rolling similarly to Example 2. In the annealing heat treatment, the mandrel bar was inserted into an atmosphere furnace at a furnace temperature of 700° C., and after 120 minutes, it was removed and air-cooled.

[0065] When the mandrel bar after the annealing heat treatment was cut by 10 mm in the radial direction, the cutting was successful over the entire length. However, 1.4 times the energy cost was required compared to Example 1. This is because, in the case of an atmospheric furnace, the entire large furnace needs to be heated to a high temperature, and the energy cost for heating is large.

[0066] Furthermore, when the surface of the mandrel bar after cutting was visually inspected, there were unevennesses in the circumferential and longitudinal directions of the surface properties. This was because heating was performed in an atmospheric furnace, so there was 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 annealing heat treatment. After the cutting process, the mandrel bar was subjected to quenching heat treatment and tempering heat treatment. The mandrel bar that had been subjected to the quenching heat treatment and tempering heat treatment was used in the hot mandrel rolling process. When the surface of the rolled pipe was visually inspected, there were unevennesses in the surface properties in the circumferential and longitudinal directions. This was because the unevenness of the mandrel bar surface was transferred.

[0067] 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 can easily make various modifications or corrections based on the present disclosure. Therefore, it should be noted that these modifications or corrections are included in the scope of the present disclosure. For example, the functions included in each component or each step can be rearranged so as not to be logically inconsistent, and multiple components or steps can be combined into one or divided. The embodiments of the present disclosure can also be realized as a storage medium on which a program executed by a processor included in the device is recorded. It should be understood that these are also included in the scope of the present disclosure. [Explanation of symbols]

[0068] 1 Mandrel bar 2 Coil 3. Conveyor 4 Outlet thermometer 5 Inlet thermometer 6 Intermediate thermometer

Claims

1. A method for manufacturing a mandrel bar, comprising the steps of: subjecting a used mandrel bar to an annealing heat treatment and a cutting treatment; and then subjecting the bar to a quenching heat treatment and a tempering heat treatment, determining the hardness of the mandrel bar to be achieved after the annealing heat treatment; Determine the heating temperature, holding time and upper limit temperature for achieving the determined hardness; determining a cutting range of the mandrel bar; Determine heat treatment conditions for induction heating that achieve the heating temperature and holding time in the determined cutting range and do not exceed an upper limit temperature; The mandrel bar is subjected to an annealing heat treatment under the determined heat treatment conditions, and then the cutting treatment is performed. The method for manufacturing a mandrel bar includes performing a quenching heat treatment and a tempering heat treatment after the cutting treatment.

2. The method of claim 1 , wherein the induction heating uses one coil to heat the mandrel bar.

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

4. The method for manufacturing a mandrel bar according to any one of claims 1 to 3, wherein the heating temperature, the holding time, and the upper limit temperature are determined based on data from an experiment using a steel slab having a similar chemical composition 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 performed by FEM coupled with electromagnetic field-thermal conduction analysis.

6. A mandrel bar manufacturing apparatus that performs annealing heat treatment and cutting treatment on a mandrel bar, A first determination unit that determines a hardness of the mandrel bar to be achieved after the annealing 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 cutting range of the mandrel bar; a fourth determination unit that determines heat treatment conditions for induction heating so that the heating temperature and holding time are achieved in the determined cutting range and the upper limit temperature is not exceeded; and an annealing heat treatment execution unit that performs the cutting treatment after performing the annealing heat treatment on the mandrel bar under the determined heat treatment conditions, and that performs a quenching heat treatment and a tempering heat treatment after performing the cutting treatment.

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