JP2024018913A - Method for manufacturing mandrel bar and apparatus for manufacturing mandrel bar - Google Patents

Abstract

PROBLEM TO BE SOLVED: 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

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present disclosure relates to a mandrel bar manufacturing method and a mandrel bar manufacturing apparatus.

One of the processes for reducing the thickness of pipes is mandrel rolling. In this process, a mandrel bar that rolls the wall thickness from the inner surface of the tube is inserted into the raw tube, and both the raw tube and the mandrel bar are drawn between grooved rolls to reduce the thickness and stretch the tube. The grooved rolls often have multiple stands, and are installed with a phase difference in the circumferential direction of the pipe so that the outer circumferential length of the pipe can be rolled down uniformly. Depending on the length of the pipe to be manufactured and the number of stands, the mandrel bar, which is an internal tool, has an elongated shape ranging from 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 down the tube from the inside, so a mandrel bar with high dimensional accuracy is required over the entire length. Also, since the surface of the mandrel bar is transferred to the inner surface of the tube, good surface quality of the mandrel bar is required. After mandrel rolling, the mandrel bar is grabbed by the gripping portion at the end by a pulling device, extracted from the tube, and recycled for mandrel rolling again.

The mandrel bar creates high contact pressure with the inner surface of the tube being rolled. In the hot case, the mandrel bar is exposed to high temperatures and it is also difficult to supply lubricant after it is inserted into the inner surface of the tube. Due to such severe usage conditions, the diameter of the mandrel bar decreases depending on the number of times it is used for rolling, and the surface of the mandrel bar gets burned. When diameter shrinkage and seizure occur, the wall thickness and inner surface quality of the manufactured tube are adversely affected. Therefore, the amount of diameter reduction and seizure status are controlled, and mandrel bars that adversely affect product quality are discarded.

Here, the mandrel bar is long and needs to have various outer diameter sizes depending on the wall thickness of the pipe to be manufactured, so the tool cost is high. In order to reduce tool costs, mandrel bars that have been used for rolling are removed from areas where seizures have occurred, and heat treated to obtain surface hardness before being used again for rolling. There is.

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

Japanese Patent Application Publication No. 10-249411 Japanese Unexamined Patent Publication No. 63-230823 Japanese Patent Application Publication No. 2010-86904

Here, in the method of Patent Document 1, it is necessary to change the structure of the mandrel bar used. Therefore, in practice, a method of cutting and reusing the surface of the mandrel bar after it has been used for rolling is widely used. Mandrel bars are manufactured by being subjected to heat treatment of quenching and tempering so that surface flaws are less likely to occur 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.

In an atmospheric furnace, when a mandrel bar is heated while placed on the hearth or skid, a temperature difference occurs between the part that touches the hearth or skid and the part that does not. In this case, a hardness difference occurs in the circumferential direction and the longitudinal direction of the mandrel bar, and the load applied to the cutting device during cutting is not constant, which may result in uneven surface texture after cutting. Furthermore, if the inside of the mandrel bar is sufficiently heated in a large atmospheric furnace, energy costs will increase.

For example, Patent Document 2 and Patent Document 3 propose a method of annealing a long object such as a tube material or a bar material by induction heating (IH). However, in induction heating, the surface is heated preferentially, so for a solid, large-diameter object such as a mandrel bar, the internal temperature deviation in the radial direction becomes large. Furthermore, if the entire furnace is heated to a high temperature in order to sufficiently heat the inside, the maximum temperature reached at the surface may exceed the upper limit temperature. Therefore, if the conventional techniques such as Patent Document 2 and Patent Document 3 are simply applied as they are, it is difficult to sufficiently heat the area to be cut and reduce the hardness.

The present disclosure has been made in view of the above circumstances, and aims to provide a mandrel bar manufacturing method and a mandrel bar manufacturing apparatus that can perform annealing heat treatment of the mandrel bar without using an atmosphere furnace.

As a result of extensive research, the inventors found that it is possible to set a heating pattern that sufficiently heats the innermost part to be cut while suppressing the maximum temperature of the surface using induction heating. The present disclosure has been made based on such knowledge. The summary is as follows.

(1) A method for manufacturing a mandrel bar according to an embodiment of the present disclosure includes:
A method for manufacturing a mandrel bar, which manufactures a used mandrel bar by subjecting it to annealing heat treatment and cutting treatment, and then subjecting it to quenching heat treatment and tempering heat treatment, the method comprising:
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 to achieve the determined hardness,
determining the cutting range of the mandrel bar;
In the determined cutting range, determine heat treatment conditions for induction heating that achieve the heating temperature and holding time and do not exceed the upper limit temperature,
Performing the cutting treatment after annealing the mandrel bar under the determined heat treatment conditions,
After performing the cutting treatment, quenching heat treatment and tempering heat treatment are performed.

(2) As an embodiment of the present disclosure, in (1),
The induction heating uses one coil to heat the mandrel bar.

(3) As an embodiment of the present disclosure, in (1),
The induction heating uses a plurality of coils to heat the mandrel bar.

(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 experimental data using a steel piece having the same chemical composition as the mandrel bar.

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

(6) A mandrel bar manufacturing apparatus according to an embodiment of the present disclosure includes:
A mandrel bar manufacturing device that performs manufacturing by subjecting a mandrel bar to annealing heat treatment and cutting treatment,
a first determining unit that determines the hardness of the mandrel bar to be achieved after the annealing heat treatment;
a second determining unit that determines the heating temperature, holding time, and upper limit temperature to achieve the determined hardness;
a third determining unit that determines a cutting range of the mandrel bar;
a fourth determining unit that determines heat treatment conditions for induction heating that achieve the heating temperature and holding time and do not exceed an upper limit temperature in the determined cutting range;
An annealing heat treatment execution unit that performs the cutting treatment after annealing the mandrel bar under the determined heat treatment conditions, and performs the quenching heat treatment and the tempering heat treatment after the cutting treatment.

According to the present disclosure, there are provided a mandrel bar manufacturing method and a mandrel bar manufacturing apparatus that can perform annealing heat treatment on a mandrel bar without using an atmospheric furnace.

FIG. 1 is a diagram for explaining an example of annealing heat treatment by induction heating in an embodiment of the present disclosure in which there is one coil. FIG. 2 is a diagram for explaining a method of determining heat treatment conditions using the present disclosure. FIG. 3 is a diagram showing an example of the results of the experiment. FIG. 4 is a diagram for explaining an embodiment of the annealing heat treatment by induction heating in the embodiment of the present disclosure in which there are a plurality of coils. FIG. 5 is a diagram showing the temperature calculation results in Example 1. FIG. 6 is a diagram showing the temperature calculation results in Example 2.

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.

The mandrel bar to be manufactured by the mandrel bar manufacturing method and mandrel bar manufacturing apparatus according to the present embodiment is an internal tool used in the mandrel rolling method. As the steel type of material for manufacturing the mandrel bar, hot work tool steel such as SKD6 or SKD61 specified in JIS is generally used. After the mandrel bar is processed to a predetermined size, it is manufactured through heat treatment of quenching and tempering (initial manufacturing). Mandrel bars are used in the mandrel rolling process in the manufacture of steel pipes, etc. After use, they are subjected to annealing heat treatment, cutting, quenching heat treatment and tempering heat treatment, and then manufactured again (remanufacturing). . In other words, remanufacturing involves processing used mandrel bars. The remanufactured mandrel bar is again used in the mandrel rolling process. The mandrel bar manufacturing method and mandrel bar manufacturing apparatus according to the present embodiment may include the above-mentioned initial manufacturing, but mainly perform the above-mentioned remanufacturing. Hereinafter, the mandrel bar used in the mandrel rolling method may be referred to as a "rolled" mandrel bar. Further, the mandrel bar before and during use in the mandrel rolling method may be referred to as a "before rolling" mandrel bar and a "during rolling" mandrel bar, respectively.

Here, the surface hardness of the mandrel bar after the quenching heat treatment is approximately HV550 to 850 in terms of Vickers hardness. The surface hardness of the mandrel bar after the tempering heat treatment is approximately HV350 to 550 in terms of Vickers hardness. The surface hardness during and after rolling is the same as that after 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, hardening of tool steel is performed by induction heating, and hardening of mandrel bars during initial manufacturing is also performed by induction heating. In induction heating, the surface is mainly heated. Therefore, hardening increases the hardness near the surface. The above-mentioned surface hardness of about HV400 to 550 is the hardness increased by the heat treatment of quenching, and is the hardness in the range of about 0 to 25 mm in the radial direction from the outer surface.

<Annealing heat treatment>
The mandrel bar manufacturing apparatus according to the present embodiment performs an annealing heat treatment on the rolled mandrel bar by induction heating. FIG. 1 is a diagram for explaining an example of annealing heat treatment by induction heating in which there is only one coil 2. 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 as an example, the diameter may be about 50 mm to 400 mm, and the length may be 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 induction heating is not particularly limited, but in order to efficiently heat the surface of the mandrel bar 1, it is preferably 200 Hz or more, and more preferably 400 Hz or more.

<Transportation>
In order to perform the annealing heat treatment over the entire length of the mandrel bar 1, the conveying device 3 causes the mandrel bar 1 to pass through the inside of the coil 2. The form of transportation is not limited. The conveying device 3 may convey the mandrel bar 1 by, for example, sandwiching the mandrel bar 1 between pinch rolls and rotating the pinch rolls. Further, the conveying device 3 may move the coil 2 along the longitudinal direction of the mandrel bar 1 without conveying the mandrel bar 1.

FIG. 2 is a diagram for explaining a method for determining heat treatment conditions (induction heating conditions 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, heat treatment conditions for achieving these temperature and time conditions within the cutting range of the mandrel bar 1 are determined by calculation. Next, the mandrel bar 1 is subjected to an annealing heat treatment under the determined heat treatment conditions. The details of each item will be explained below.

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

<Determination of temperature, time and upper limit value>
The temperature and time conditions required to achieve the determined hardness (or lower hardness) after the annealing heat treatment are determined through experiments. In the experiment, the same material as the mandrel bar 1 to be cut may be annealed and heat treated to investigate the hardness.

As a result of intensive study by the inventors, it was found that heating with a tempering parameter (tempering parameter) above a certain level is necessary in order to lower the hardness. It was also found that heating the material above a certain temperature causes the material to become hardened, increasing its hardness. Through experiments, the heating temperature and holding time to achieve the determined hardness are determined, and the upper limit temperature at which the material does not become hardened (the upper limit of the temperature of annealing heat treatment) is determined.

FIG. 3 is a diagram showing an example of the results of the experiment. The vertical axis indicates the heating temperature (°C). The horizontal axis shows the retention time (seconds). “〇” indicates a Vickers hardness of HV350 or less. Further, "x" indicates a Vickers hardness greater than HV350. The example in FIG. 3 shows that heating with a tempering parameter of 17,200 or more is effective for lowering the hardness. Further, it has been shown that it is effective to set the upper limit temperature to 900°C so that the material does not become hardened. The tempering parameter (TP) is determined by the following formula.

TP=(Temp+273)×(20+log(Time/3600))

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

<Determining the cutting range>
The range to be cut in order to reuse the mandrel bar 1, that is, the number of millimeters to be cut in the radial direction from the surface is determined. In this range, it is necessary to lower the hardness by annealing heat treatment.

<Determination of heat treatment conditions>
Heat treatment conditions for induction heating are determined so that the heating temperature and holding time are achieved over the entire determined cutting range, and the upper limit temperature is not exceeded.

Due to the characteristics of induction heating, the surface is heated preferentially. Therefore, the heat treatment conditions for induction heating are determined so that the innermost position of the cutting range achieves the determined heating temperature and holding time. Here, if the heating temperature determined at the innermost position is achieved, the temperature will be higher at the radially outer positions. Therefore, the heat treatment conditions for induction heating are determined so that the maximum temperature at the surface position where the temperature is highest does not exceed the upper limit temperature.

Here, in addition to the output of induction heating, the conveyance speed of the mandrel bar 1 may be determined as the heat treatment conditions so that the above heating temperature, holding time, etc. are achieved. In determining the output of induction heating, a heat transfer calculation or the like may be performed based on the temperature history of the mandrel bar 1 or the like. For example, calculations may be performed using FEM (Finite Element Method), which combines electromagnetic field and thermal conduction analysis. Through calculation, the induction heating output and the conveyance speed of the mandrel bar 1 are determined so as to achieve the predetermined heating temperature and holding time and not to exceed the upper limit temperature. Then, the annealing heat treatment of the mandrel bar 1 is performed under the determined heat treatment conditions.

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 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 calculations made when determining the heat treatment conditions. Further, an inlet thermometer 5 for measuring the temperature of the mandrel bar 1 may be installed on the inlet side of the coil 2. The output of the coil 2 may be adjusted such that the surface temperature at the mandrel bar 1 on the input side of the coil 2 follows the calculations when determining the heat treatment conditions. Here, the outlet thermometer 4 and the inlet thermometer 5 may be non-contact measuring devices such as radiation thermometers, or may be contact thermometers.

<Multiple coil configuration>
FIG. 4 is a diagram for explaining an embodiment in which there are a plurality of coils 2 regarding annealing heat treatment by induction heating. To reduce the radial temperature deviation of the mandrel bar 1, which is a solid and large-diameter object, by arranging a plurality of coils 2 at a distance (inter-coil distance) and heating them as shown in Fig. 4. I can do it. After being heated by the first coil 2 (coil 2 on the upstream side in the conveying direction) and before being heated by the second coil 2 (coil 2 on the downstream side in the conveying direction), the diameter of the mandrel bar 1 changes. This is because the temperature distribution becomes smaller due to directional heat transfer. The number of coils 2 may be three or more.

An outlet thermometer 4 for measuring the temperature of the mandrel bar 1 may be installed on the outlet 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 at the exit side of the second coil 2 follows the calculation when determining the heat treatment conditions. Further, an inlet thermometer 5 for measuring the temperature of the mandrel bar 1 may be installed on the inlet side of the first coil 2. The output of the coil 2 may be adjusted so that the surface temperature at the mandrel bar 1 on the input side of the first coil 2 follows the calculations made when determining the heat treatment conditions. Further, an intermediate thermometer 6 for measuring the temperature of the mandrel bar 1 may be installed between the two coils 2. The output of the coils 2 may be adjusted so that the surface temperature of the mandrel bar 1 between the two coils 2 follows the calculations when determining the heat treatment conditions.

The manufacturing apparatus for the mandrel bar 1 according to the present embodiment may be an apparatus that includes or controls the annealing heat treatment apparatus shown in FIGS. 1 and 4, and may 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 a computer that manages, for example, 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. It may be. Here, the method of determining the heat treatment conditions shown in FIG. 2 may be implemented by the CPU of the computer. Data used in the heat treatment conditions (including experimental data) may be entered from a computer input device or via a network and stored in memory or a hard disk drive. Further, the above-described process of “determining hardness” may be executed by the first determining unit. The process of "determining temperature, time, and upper limit value" described above may be executed by the second determining unit. The above process of "determining the range to be cut" may be executed by the third determining unit. The process of "determining heat treatment conditions" described above may be executed by the fourth determining unit. Further, the annealing heat treatment, cutting treatment, hardening heat treatment, and tempering heat treatment of the mandrel bar 1 may be executed by the annealing heat treatment execution section. When one or more programs stored in the memory are read by the CPU of the computer, the CPU is used as a first determining section, a second determining section, a third determining section, a fourth determining section, and an annealing heat treatment execution section. You can make it work. As described above, the mandrel bar 1 manufacturing apparatus according to the present embodiment includes the first determining section, the second determining section, the third determining section, the fourth determining section, and the annealing heat treatment execution section, and Execute manufacturing method 1. Here, the cutting process after the annealing heat treatment of the mandrel bar 1 may be performed by a cutting process execution unit, and the manufacturing apparatus for the mandrel bar 1 may further include a cutting process execution unit. At this time, like 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 processing execution unit.

As described above, the method for manufacturing the mandrel bar 1 and the apparatus for manufacturing the mandrel bar 1 according to the present embodiment can perform annealing heat treatment on the mandrel bar 1 without using an atmospheric furnace by the above-described configuration or steps. That is, according to the present disclosure, annealing heat treatment of the mandrel bar 1 can be realized without requiring a large atmospheric furnace. Therefore, there is no problem in which a temperature difference occurs between a portion that is in contact with the hearth or skid and a portion that is not in contact with the hearth or skid. According to the present disclosure, the load applied to the cutting device during cutting is constant, and no unevenness occurs in the surface texture after cutting. Further, according to the present disclosure, since the part to be cut is intensively heated by induction heating, energy costs can be reduced compared to an atmospheric furnace. Further, according to the present disclosure, by optimizing the heat treatment conditions, even in the case of a solid product such as the mandrel bar 1, the maximum temperature reached on the surface is prevented from exceeding the upper limit temperature. As a result, problems such as damage to the blade of the cutting device are eliminated.

(Example)
Hereinafter, the effects of the present disclosure will be specifically explained based on Examples, but the present disclosure is not limited to the contents of the Examples.

In this example, a hot mandrel rolling process was performed in manufacturing a seamless steel pipe.

(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 annealing heat treatment by induction heating as described in the above embodiment. The remainder not shown in Table 1 is Fe and unavoidable impurities. The mandrel bar in question is one used for hot mandrel rolling. The diameter of the mandrel bar is 120 mm. The length of the mandrel bar is 20000 mm. In order to reuse this mandrel bar, a 10 mm cut is made in the radial direction. When 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 HV495 to 529.

The target hardness was set to HV350 or less because the cutting device used in the cutting process performed after the annealing heat treatment was capable of cutting as long as the hardness was HV350 or less.

Preliminary experiments were conducted using a SKD6 steel billet with a similar chemical composition to the mandrel bar. After annealing heat treatment using the same temperature pattern as the quenching and tempering performed in the initial manufacture of the mandrel bar, the hardness was HV341 when held at 700° C. for 80 seconds. From this result, it was decided to set the heating temperature to 700°C and the holding time to 80 seconds (tempering parameter 17851). Furthermore, it was found that when the heating temperature was increased above 900°C, the hardness increased rapidly. Based on this result, it was decided to set the upper limit temperature to 900°C.

Annealing heat treatment was performed using an annealing heat treatment apparatus as shown in FIG. 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 induction heating is 3000Hz. An exit thermometer, which is a radiation thermometer, is installed on 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 conveyed at a constant speed by pinch rolls installed on the input and output sides of the coil.

Prior to annealing heat treatment, temperature calculations were performed using FEM coupled with electromagnetic field and thermal conduction analysis. FIG. 5 shows the results of calculating the temporal change in temperature at the surface and at a location 10 mm from the surface (two evaluation points). If the conveyance speed of the mandrel bar is 1.0 mm/s and the temperature of the mandrel bar on the coil exit side is 814°C, the time that the position 10mm from the surface is maintained at 700°C or higher is 82 seconds, and the maximum temperature on the surface is The reached temperature is 816°C, which is below the upper limit temperature. Here, the position 10 mm from the surface corresponds to the innermost position within the range where hardness needs to be lowered. Therefore, the conveyance speed of the mandrel bar was set to 1.0 mm/s, and the output of induction heating was adjusted so that the measured value of the exit side thermometer was 814°C.

With the above settings, annealing heat treatment was performed on the entire length of the mandrel bar. When the mandrel bar after the annealing heat treatment was cut by 10 mm in the radial direction, the entire length was successfully cut, and no problems such as damage to the blade of the cutting device occurred. Further, when the surface of the mandrel bar after cutting was visually checked, no unevenness in surface quality was observed. After the cutting process, the mandrel bar was subjected to quenching heat treatment and tempering heat treatment. A mandrel bar that had undergone quenching heat treatment and tempering heat treatment was used in a hot mandrel rolling process, and no unevenness in surface texture was observed in the rolled pipe.

(Example 2: When there are multiple coils)
Annealing heat treatment by induction heating as described in the above embodiment 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 is one used for hot mandrel rolling. The diameter of the mandrel bar is 170 mm. The length of the mandrel bar is 20000 mm. In order to reuse this mandrel bar, a 10 mm cut is made in the radial direction. When 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 HV496 to 531.

The target hardness was set to HV350 or less because the cutting device used in the cutting process performed after the annealing heat treatment was capable of cutting as long as the hardness was HV350 or less.

Preliminary experiments were conducted using a SKD6 steel billet with a similar chemical composition to the mandrel bar. After annealing heat treatment with the same temperature pattern as the quenching and tempering performed in the initial manufacture of the mandrel bar, the hardness was HV338 when held at 700° C. for 80 seconds. From this result, it was decided to set the heating temperature to 700°C and the holding time to 80 seconds (tempering parameter 17851). Furthermore, it was found that when the heating temperature was increased above 900°C, the hardness increased rapidly. Based on this result, it was decided to set the upper limit temperature to 900°C.

Annealing heat treatment was performed using an annealing heat treatment apparatus as shown in FIG. Both coil dimensions are the same. 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 induction heating is 3000Hz. An exit thermometer, which is a radiation thermometer, is installed on 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 conveyed at a constant speed by pinch rolls installed on the input and output sides of the coil.

Prior to annealing heat treatment, temperature calculations were performed using FEM coupled with electromagnetic field and thermal conduction analysis. FIG. 6 shows the results of calculating the temporal change in temperature at the surface and at a location 10 mm from the surface (two evaluation points). The distance between the coils was 80 mm. If the conveyance speed of the mandrel bar is 1.0 mm/s and the temperature of the mandrel bar on the coil exit side is 813°C, the time that a position 10 mm from the surface is maintained at 700°C or higher will be 83 seconds, and the maximum temperature on the surface will be The reached temperature is 816°C, which is below the upper limit temperature. Here, the position 10 mm from the surface corresponds to the innermost position within the range where hardness needs to be lowered. Therefore, the distance between the coils was set to 80 mm, the conveyance speed of the mandrel bar was set to 1.0 mm/s, and the output of induction heating was adjusted so that the measured value of the outlet thermometer was 813°C.

With the above settings, annealing heat treatment was performed on the entire length of the mandrel bar. When the mandrel bar after the annealing heat treatment was cut by 10 mm in the radial direction, the entire length was successfully cut, and no problems such as damage to the blade of the cutting device occurred. Further, when the surface of the mandrel bar after cutting was visually checked, no unevenness in surface quality was observed. After the cutting process, the mandrel bar was subjected to quenching heat treatment and tempering heat treatment. A mandrel bar that had undergone quenching heat treatment and tempering heat treatment was used in a hot mandrel rolling process, and no unevenness in surface texture was observed in the rolled pipe.

(Comparative example 1: without annealing heat treatment)
As Comparative Example 1, cutting was performed without annealing the rolled mandrel bar. The conditions are the same as in Example 1 except that annealing heat treatment is not performed. 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.

(Comparative Example 2: When the surface temperature is 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 is one used for hot mandrel rolling. The diameter of the mandrel bar is 120 mm. The length of the mandrel bar is 20000 mm. In order to reuse this mandrel bar, a 10 mm cut is made in the radial direction. When 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 HV494 to 529.

Annealing heat treatment was performed using an annealing heat treatment apparatus as shown in FIG. 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 induction heating is 3000Hz. An exit thermometer, which is a radiation thermometer, is installed on 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 conveyed at a constant speed by pinch rolls installed on the input and output sides of the coil.

The conveyance speed of the mandrel bar was 1.0 mm/s, and the output of induction heating was adjusted so that the measured value of the exit thermometer was 950°C.

With the above settings, annealing heat treatment was performed on the entire length of the mandrel bar. When cutting 10 mm in the radial direction on the mandrel bar after annealing heat treatment, problems such as damage to the blade of the cutting device occurred, making it impossible to cut 10 mm in the radial direction at any position on the mandrel bar. Ta. This is thought to be because the surface temperature of the mandrel bar became too high, resulting in a hardened state and increased hardness.

(Comparative Example 3: When the surface temperature is too low)
The same rolled mandrel bar as in Example 2 was subjected to annealing heat treatment using the same coil as in Example 2.

The conveyance speed of the mandrel bar was 1.0 mm/s, and the output of induction heating was adjusted so that the measured value of the exit thermometer was 750°C.

With the above settings, annealing heat treatment was performed on the entire length of the mandrel bar. When cutting 10 mm in the radial direction on the mandrel bar after annealing heat treatment, problems such as damage to the blade of the cutting device occurred, making it impossible to cut 10 mm in the radial direction at any position on the mandrel bar. Ta. This is considered to be because the heating temperature was low, so that sufficient heating temperature and holding time were not ensured in the part near the center of the cutting range, and the hardness was not lowered sufficiently.

(Comparative Example 4: When using an atmosphere furnace)
The same rolled mandrel bar as in Example 2 was subjected to annealing heat treatment in an atmospheric furnace and then cut. For the annealing heat treatment, a mandrel bar was inserted into an atmosphere furnace with a furnace temperature of 700°C, and after 120 minutes, extraction was performed and air cooling was performed.

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, compared to Example 1, the energy cost was 1.4 times higher. This is because, in the case of an atmospheric furnace, it is necessary to heat the entire large furnace to a high temperature, which increases the energy cost for heating.

Further, when the surface of the mandrel bar after cutting was visually checked, there was unevenness in the surface texture in the circumferential direction and the longitudinal direction. This is 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. A mandrel bar that had undergone 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, it was found that the surface texture was uneven in the circumferential direction and in the longitudinal direction. This is because the unevenness on the mandrel bar surface was transferred.

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 changes or modifications based on the present disclosure. It should therefore be noted that these variations or modifications are included within the scope of this disclosure. For example, the functions included in each component or each step can be rearranged to avoid logical contradictions, and multiple components or steps can be combined or divided into one. It is. Embodiments according to the present disclosure can also be realized as a storage medium recording a program executed by a processor included in the device. It is to be understood that these are also encompassed within the scope of the present disclosure.

1 Mandrel bar 2 Coil 3 Conveying device 4 Outlet thermometer 5 Inlet thermometer 6 Intermediate thermometer

Claims (6)

A method for manufacturing a mandrel bar, which manufactures a used mandrel bar by subjecting it to annealing heat treatment and cutting treatment, and then subjecting it to quenching heat treatment and tempering heat treatment, the method comprising:
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 to achieve the determined hardness,
determining the cutting range of the mandrel bar;
In the determined cutting range, determine heat treatment conditions for induction heating that achieve the heating temperature and holding time and do not exceed the upper limit temperature,
Performing the cutting treatment after annealing the mandrel bar under the determined heat treatment conditions,
A method for manufacturing a mandrel bar, comprising performing quenching heat treatment and tempering heat treatment after performing the cutting process. The method for manufacturing a mandrel bar according to claim 1, wherein the induction heating heats the mandrel bar using one coil. The method for manufacturing a mandrel bar according to claim 1, wherein the induction heating heats the mandrel bar using a plurality of coils. The heating temperature, the holding time, and the upper limit temperature are determined based on experimental data using a steel piece having the same chemical composition as the mandrel bar, according to any one of claims 1 to 3. A method of manufacturing a mandrel bar. 4. The method for manufacturing a mandrel bar according to claim 1, wherein the heat treatment conditions are determined based on temperature calculation by FEM coupled with electromagnetic field-thermal conduction analysis. A mandrel bar manufacturing device that performs manufacturing by subjecting a mandrel bar to annealing heat treatment and cutting treatment,
a first determining unit that determines the hardness of the mandrel bar to be achieved after the annealing heat treatment;
a second determining unit that determines the heating temperature, holding time, and upper limit temperature to achieve the determined hardness;
a third determining unit that determines a cutting range of the mandrel bar;
a fourth determining unit that determines heat treatment conditions for induction heating that achieve the heating temperature and holding time and do not exceed an upper limit temperature in the determined cutting range;
A mandrel comprising: an annealing heat treatment execution unit that performs the cutting treatment after annealing the mandrel bar under the determined heat treatment conditions, and performs a quenching heat treatment and a tempering heat treatment after the cutting treatment. Bar manufacturing equipment.

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