JP7348138B2 - Induction heating method and induction heating control device - Google Patents

Description

The present invention relates to a technique for monitoring the temperature of a workpiece that is induction heated.

One of the applications of induction heating is metal die forging. The workpiece (metal) is heated by induction before die forging. During induction heating, the temperature of the workpiece is controlled within a predetermined range (for example, 1000°C to 1300°C). If the temperature of the workpiece falls below a predetermined range, the press load on the workpiece during die forging increases, and as a result, the die may be damaged. When the temperature of the workpiece exceeds a predetermined range, the crystal grains of the workpiece become enlarged, which may deteriorate the quality of the workpiece (deterioration of the quality of the workpiece causes fatigue failure of the workpiece).

As a technique for monitoring the temperature of a workpiece to be heated by induction, there is an induction heating method disclosed in Patent Document 1, for example. In this method, the object to be heated is monitored by one or several thermal recording cameras during heating, and the heating is carried out by adjusting the heating parameters or heating conditions until the desired temperature conditions are obtained for said object. It is characterized by being controlled.

Special Publication No. 2002-532836

As mentioned above, if the workpiece is heated excessively during induction heating, the quality of the material of the workpiece will deteriorate. Due to the principle of induction heating, the peak temperature of the workpiece occurs inside the workpiece, not on its surface. Therefore, if the temperature is controlled based on the surface temperature of the work, the work may be heated excessively.

A non-contact temperature sensor is usually used to measure the surface temperature of a workpiece. If oxide scale or the like occurs on the surface of the workpiece, a temperature lower than the actual temperature will be measured, which may cause the workpiece to be excessively heated.

An object of the present invention is to provide an induction heating method and an induction heating control device that can prevent a workpiece from being excessively heated during induction heating.

The induction heating method according to the first aspect of the present invention includes a heating step of inductively heating a workpiece, and an induction heating method based on an actual value of a coil current in the induction heating or an actual value of the surface temperature of the workpiece in the induction heating. an analysis step of determining an analytical value of the internal temperature of the heated workpiece; and a determination of determining whether the monitored temperature exceeds the upper limit temperature of the workpiece, considering the analytical value as the monitored temperature of the workpiece. and a control step of controlling to lower the temperature of the workpiece in the heating step when it is determined that the monitored temperature exceeds the upper limit temperature.

The present inventors focused on the fact that in induction heating, the peak value of the temperature of the workpiece occurs inside the workpiece rather than on its surface, and that the internal temperature of the workpiece cannot be measured by a non-contact temperature sensor. . In the induction heating method according to the first aspect of the present invention, the analytical value of the internal temperature of the workpiece is regarded as the monitored temperature of the workpiece, it is determined whether the monitored temperature exceeds the upper limit temperature of the workpiece, and the monitored temperature is determined to be the upper limit temperature. When the temperature exceeds the temperature, the temperature of the workpiece is controlled to be lowered. Therefore, according to the induction heating method according to the first aspect of the present invention, it is possible to prevent the workpiece from being excessively heated during induction heating.

In the above configuration, the determination step makes the determination by regarding the peak value of the internal temperature among the analyzed values as the monitored temperature.

According to this configuration, the peak value of the internal temperature of the workpiece is regarded as the monitored temperature, and it is determined whether the monitored temperature exceeds the upper limit temperature, thereby more reliably preventing the workpiece from being excessively heated. I can do it.

In the above configuration, the determining step determines whether the monitored temperature exceeds the upper limit temperature by regarding the larger one of the analyzed value and the actual measured value of the surface temperature as the monitored temperature.

The possibility that the actual value of the surface temperature of the workpiece will be larger than the analytical value (for example, the peak value of the internal temperature) cannot be excluded. According to this configuration, the larger of the analytical value and the measured value of the surface temperature of the workpiece is regarded as the monitored temperature, and it is determined whether the monitored temperature exceeds the upper limit temperature, thereby preventing excessive heating of the workpiece. It can be prevented.

In the above configuration, the actual value of the coil current in the induction heating or the actual value of the surface temperature of the workpiece in the induction heating is used as a parameter, and a heat generation term indicating the amount of heat generated inside the workpiece in the induction heating; , further comprising a preparation step of preparing in advance data showing a relationship with the parameter, and a measurement step of measuring the parameter during the heating step, and the analysis step includes determining the heat generation value corresponding to the measured parameter. The term is obtained from the data, the obtained heat generation term is applied to an equation indicating the internal temperature distribution of the workpiece during the induction heating, and the analytical value is obtained using the equation.

The analytical value of the internal temperature of the workpiece can be determined by coupled analysis of the magnetic field and heat transfer. In this case, it is necessary to calculate the magnetic field and calculate the exothermic term based on the calculated magnetic field, which increases the amount of calculation required to calculate the analytical value. According to this configuration, since the heat generation term is prepared in advance, the calculation of the magnetic field and the heat generation term can be omitted when obtaining the analytical value of the internal temperature of the workpiece. Thereby, the amount of calculation necessary for calculating the analytical value can be reduced, and the calculation time for the analytical value can be shortened.

In the above configuration, the actual measurement value of the coil current in the induction heating is used as a parameter of a coupled analysis, and the analysis step further includes a measurement step of measuring the parameter during the heating step, and the analysis step includes a step of determining whether the magnetic field and the temperature are A location of the workpiece that satisfies the condition of changing in the radial direction of the workpiece and being constant in the axial direction of the workpiece is targeted for the coupled analysis, and the coupled analysis of the one-dimensional magnetic field and heat transfer in the radial direction of the workpiece is performed. The analysis value is determined using analysis.

Coupled analysis of a one-dimensional magnetic field and heat transfer can reduce the amount of calculation compared to a three-dimensional or two-dimensional coupled analysis of a magnetic field and heat transfer. Therefore, according to this configuration, the amount of calculation required to calculate the analytical value of the internal temperature of the workpiece can be reduced, and the calculation time of the analytical value can be shortened.

The induction heating control device according to the second aspect of the present invention converts the analytical value of the internal temperature of the workpiece being induction heated by the induction heating device into the actual value of the coil current in the induction heating or the surface of the workpiece in the induction heating. an analysis unit that determines the temperature based on an actual measured value; a determination unit that considers the analysis value as the monitored temperature of the workpiece and determines whether or not the monitored temperature exceeds the upper limit temperature of the workpiece; and a control unit that controls the induction heating device to lower the temperature of the workpiece when it is determined that the temperature exceeds the upper limit temperature.

The induction heating control device according to the second aspect of the present invention defines the induction heating control method according to the first aspect of the present invention from the viewpoint of the device, and is similar to the induction heating control method according to the first aspect of the present invention. It has the following effects.

According to the present invention, it is possible to prevent a workpiece from being excessively heated during induction heating.

FIG. 1 is a block diagram showing the configuration of an induction heating control device and an induction heating device according to an embodiment. FIG. 2 is an explanatory diagram illustrating an example of data stored in a database. FIG. 7 is an explanatory diagram illustrating another example of data stored in a database. It is a flow chart explaining an induction heating control method concerning an embodiment. It is a graph comparing an analytical value and an actual value regarding the internal temperature of a workpiece during induction heating. It is a graph comparing an analytical value and an actual measurement value regarding the internal temperature of a workpiece at a surface temperature of 1100°C. It is a graph showing the analytical values of the heat generation term of the workpiece obtained using the embodiment at 0 seconds, 1 minute, 5 minutes, and 30 minutes after the start of induction heating. 3 is a graph showing the penetration depth of an induced current generated in a workpiece. It is a graph comparing the analytical value of the internal temperature of the workpiece obtained using the embodiment with the analytical value of the internal temperature of the workpiece obtained using general-purpose software for coupled analysis of magnetic field and heat transfer. It is a block diagram showing composition of an induction heating control device and an induction heating device concerning a 2nd modification of an embodiment. FIG. 3 is a schematic diagram showing the magnetic field at the center in the longitudinal direction of the coil in a state where the coil is tightly wound and the length of the coil is sufficiently large. Temperature distribution from the surface of the workpiece to the inside determined using the second modification of the embodiment and general-purpose software for coupled analysis of magnetic field and heat transfer when the initial value of the surface temperature of the workpiece is 20°C. It is a graph showing the analytical value of. Temperature distribution from the surface of the workpiece to the inside determined using the second modification of the embodiment and general-purpose software for coupled analysis of magnetic field and heat transfer when the initial value of the surface temperature of the workpiece is 500°C. It is a graph showing the analytical value of.

Hereinafter, embodiments of the present invention will be described in detail based on the drawings. In each figure, components given the same reference numerals indicate the same components, and descriptions of the components that have already been described will be omitted.

FIG. 1 is a block diagram showing the configurations of an induction heating control device 1 and an induction heating device 2 according to an embodiment. The induction heating control device 1 controls the induction heating device 2. The induction heating device 2 is a device that inductively heats the workpiece W, and includes a conveying section 21, a coil 22, a power source 23, a thermometer 24, and an ammeter 25. The workpiece W is, for example, a cylindrical elongated metal member with a diameter of 200 to 500 mm. The conveyance unit 21 sends out the work W into the coil 22 by moving the work W in the direction indicated by arrow A from one end of the coil 22 to the other end. The length of the coil 22 is, for example, about 2000 mm. Since the length of the coil 22 is shorter than the length of the workpiece W, a portion P (portion shown in gray) of the workpiece W located inside the coil 22 becomes a heating target.

Power supply 23 supplies alternating current to coil 22 . The alternating current flowing through the coil 22 generates magnetic flux around the coil 22. As a result, an eddy current flows through a portion P of the workpiece W located within the coil 22, generating Joule heat, and the portion P is heated for a predetermined period of time. After a predetermined period of time has elapsed, the transport unit 21 moves the workpiece W in the direction of arrow A. As a result, that portion P is sent to the die forging process, and a new portion P of the workpiece W located within the coil 22 is heated for a predetermined time. The above operations are repeated.

The thermometer 24 is a non-contact temperature sensor using a thermocouple, and measures the surface temperature of the workpiece W through a hole penetrated through the center of the coil 22 in the longitudinal direction. Since the heat within the coil 22 is released to the outside from both ends of the coil 22, the temperature at the center of the coil 22 becomes higher than the temperature at both ends of the coil 22. Therefore, the surface temperature of the workpiece W is measured at the center of the coil 22. Temperature data td indicating the surface temperature (actual measurement value of the surface temperature) of the workpiece W measured by the thermometer 24 is sent to the induction heating control device 1 .

The ammeter 25 measures the current flowing through the coil 22 (coil current) during induction heating. Current data id indicating the coil current (actually measured value of the coil current) is sent to the induction heating control device 1.

The induction heating control device 1 includes an arithmetic processing section 11, an IF section 12, an input section 13, and an output section 14. The arithmetic processing unit 11 controls the entire induction heating control device 1 and performs processing necessary for the operation of the induction heating control device 1. The arithmetic processing unit 11 executes the functions of the arithmetic processing unit 11 and hardware such as a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), and an HDD (Hard Disk Drive). Because It is realized by programs and data, etc.

The IF unit 12 is connected to the arithmetic processing unit 11 and inputs and outputs signals and the like to and from external devices under the control of the arithmetic processing unit 11. For example, the IF section 12 receives temperature data td sent from the thermometer 24 and sends it to the arithmetic processing section 11 . The IF section 12 receives the current data ID sent from the ammeter 25 and sends it to the arithmetic processing section 11 . The IF section 12 sends the control signal s1 generated by the temperature control section 114 of the arithmetic processing section 11 to the power supply 23. The power supply 23 controls the alternating current supplied to the coil 22 based on the control signal s1. The IF unit 12 sends the control signal s2 generated by the transport control unit 115 of the arithmetic processing unit 11 to the transport unit 21. The transport unit 21 controls movement of the workpiece W in the direction of arrow A based on the control signal s2. The IF section 12 is realized by an input/output interface circuit.

The input unit 13 is connected to the arithmetic processing unit 11 and is a device for a user to input various information, data, commands, etc. The input unit 13 is realized by a mouse, a keyboard, a touch panel, etc. The output unit 14 is a device that is connected to the arithmetic processing unit 11 and outputs commands, data, etc. input from the input unit 13 under the control of the arithmetic processing unit 11. The output unit 14 is realized by a liquid crystal display, an OLED display (Organic Light Emitting Diode display), or the like.

The arithmetic processing unit 11 includes a database 111, an analysis unit 112, a determination unit 113, a temperature control unit 114, and a transport control unit 115 as functional blocks.

The database 111 stores data indicating the relationship between the coil current and the heat generation term in a table format. FIG. 2A is an explanatory diagram illustrating an example of data stored in the database 111. The heat generation term indicates the amount of heat generated corresponding to the internal temperature distribution of the workpiece W during induction heating. The heating term is determined in advance by performing magnetic field calculations on the magnetic field generated in the coil 22 under a certain coil current. For example, when the coil current i1 is the heat generation term q1, when the coil current i2 is the heat generation term q2, and when the coil current i3 is the heat generation term q3.

Referring to FIG. 1, the analysis unit 112 generates an analysis value of the internal temperature of the workpiece W being induction heated based on the current data id (actual measurement value of the coil current in induction heating) sent from the ammeter 25. seek. Specifically, the analysis unit 112 uses the current data id (actual measured value of the coil current) sent from the ammeter 25 as a parameter, and acquires the heat generation term corresponding to this parameter (actual measured value of the coil current) from the database 111. , the obtained heat generation term is applied to the heat generation term ΔQ in the following equation 1, and using equation 1, the temperature distribution T in the radial direction r of the workpiece W during induction heating (analytical value of the internal temperature of the workpiece W being induction heated) ).

Equation 1 shows an analytical value of the temperature due to induction heating at a point r away from the central axis of the work W in the radial direction (an equation indicating the internal temperature distribution of the work W during induction heating). T indicates the analytical value (temperature), t indicates time, ρ indicates the density of the workpiece W, c indicates the specific heat of the workpiece W, λ indicates the thermal conductivity of the workpiece W, and ΔQ indicates the exothermic term. show. The heat generation term corresponds to the temperature distribution (heat amount) from the surface of the workpiece W to the inside.

Another example of data stored in the database 111 will be explained. Another example is data showing the relationship between the surface temperature of the workpiece W and the exothermic term. FIG. 2B is an explanatory diagram illustrating another example. The heat generation term indicates the amount of heat generated by the workpiece W during induction heating. The exothermic term is determined in advance by performing magnetic field calculations on the magnetic field generated in the coil 22 under the surface temperature of the workpiece W. For example, when the surface temperature is t1, the exothermic term is q1, when the surface temperature is t2, the exothermic term is q2, and when the surface temperature is t3, the exothermic term is q3.

In the case of another example of the database 111, the analysis unit 112 calculates the temperature of the workpiece W being induction heated based on the temperature data td (actual measurement value of the surface temperature of the workpiece W during induction heating) sent from the thermometer 24. Find the analytical value of the internal temperature. Specifically, the analysis unit 112 uses the temperature data td (actual measured value of surface temperature) sent from the thermometer 24 as a parameter, and acquires the exothermic term corresponding to this parameter (actual measured value of surface temperature) from the database 111. , by applying the obtained heat generation term to the heat generation term ΔQ in Equation 1, and using Equation 1, the temperature distribution T in the radial direction r of the workpiece W during induction heating (analytical value of the internal temperature of the workpiece W being induction heated) seek.

The determining unit 113 regards the analytical value of the internal temperature of the workpiece W obtained by the analyzing unit 112 as the monitored temperature of the workpiece W, and determines whether the monitored temperature exceeds the upper limit temperature of the workpiece W. The lower limit temperature of the workpiece W is compared with the surface temperature of the workpiece W measured by the thermometer 24. That is, the determining unit 113 determines whether the surface temperature indicated by the temperature data td is lower than the lower limit temperature.

When the determination unit 113 determines that the monitored temperature exceeds the upper limit temperature, the temperature control unit 114 generates a control signal s1 that controls to lower the temperature of the workpiece W that is being heated by induction, and the IF unit 12 A control signal s1 is sent to the power supply 23. The control signal s1 here is a signal that turns off the power supply 23.

When the power source 23 is turned off, no alternating current is supplied to the coil 22, so induction heating stops and the temperature of the workpiece W gradually decreases. When the determination unit 113 determines that the surface temperature is lower than the lower limit temperature, the temperature control unit 114 generates a control signal s1 that controls increasing the temperature of the workpiece W, and the IF unit 12 transmits the control signal s1 to the power supply 23. send. The control signal s1 here is a signal that turns on the power supply 23.

The transport control unit 115 generates a control signal s2. The IF unit 12 sends a control signal s2 to the transport unit 21. The transport unit 21 transports the work W into the coil by moving the work W in the direction of arrow A based on the control signal s2.

An induction heating control method according to an embodiment will be described. FIG. 3 is a flowchart explaining this. The data stored in the database 111 is shown in the example shown in FIG. 2A. A portion P (portion shown in gray) of the workpiece W located within the coil 22 is heated by induction. In order to control the temperature of the portion P within a predetermined range, induction heating includes a heating process in which the power source 23 is turned on and the portion P is heated, and a cooling process in which the power source 23 is turned off and heat is radiated from the portion P. include. Induction heating is performed for a predetermined period of time (for example, 60 minutes).

Referring to FIGS. 1 and 3, a user operates input unit 13 and inputs a command to start induction heating. The thermometer 24 successively measures the surface temperature of the workpiece W, and sends temperature data td to the arithmetic processing section 11 via the IF section 12. The ammeter 25 sequentially measures the coil current flowing through the coil 22 and sends current data id to the arithmetic processing section 11 via the IF section 12.

The temperature control unit 114 stores the predetermined time in advance, and starts measuring time by operating a timer included in the temperature control unit 114. The temperature control unit 114 determines whether a predetermined time has elapsed (S1). That is, the temperature control unit 114 determines whether a predetermined time has elapsed since the start of induction heating.

Since induction heating has started, the temperature control unit 114 determines that the predetermined time has not elapsed (No in S1), and generates a control signal s1 to turn on the power supply 23. The IF section 12 sends this control signal s1 to the power supply 23. As a result, the power supply 23 is turned on, alternating current is supplied to the coil 22, and the portion P of the workpiece W located inside the coil 22 is heated (S2). This is the heating process.

The analysis unit 112 refers to the data shown in FIG. 2A stored in the database 111 and obtains the heat generation term corresponding to the coil current (actual measured value of the coil current) indicated by the current data id sent from the ammeter 25. Then, the obtained heat generation term is applied to the heat generation term ΔQ in the above equation (1), and an analytical value (internal temperature distribution of the workpiece W) is obtained using equation (1). The analysis unit 112 obtains analysis values at regular time intervals.

The determination unit 113 stores the upper limit temperature of the workpiece W in advance, and regards the peak value of the analysis value (the maximum value of the internal temperature of the workpiece W) as the monitoring temperature, and determines whether the peak value of the analysis value exceeds the upper limit temperature. It is determined whether or not there is one (S3). The determination unit 113 makes this determination on the analysis values obtained by the analysis unit 112 at regular time intervals. When the determination unit 113 determines that the peak value of the analysis value does not exceed the upper limit temperature (No in S3), the process returns to process S1.

When the determination unit 113 determines that the peak value of the analysis value exceeds the upper limit temperature (Yes in S3), the temperature control unit 114 determines whether the predetermined time has elapsed (S4). When the temperature control unit 114 determines that the predetermined time has not elapsed (No in S4), it generates a control signal s1 to turn off the power supply 23. The IF section 12 sends this control signal s1 to the power supply 23. As a result, the power supply 23 is turned off, alternating current is no longer supplied to the coil 22, and heat is radiated from the portion P (S5). This is the cooling process.

The determination unit 113 stores the lower limit temperature of the workpiece W in advance, and determines whether the surface temperature (actual measurement value of the surface temperature) indicated by the temperature data td sent from the thermometer 24 is smaller than the lower limit temperature. (S6). When the determination unit 113 determines that the surface temperature is equal to or higher than the lower limit temperature (No in S6), the process returns to step S4.

When the determination unit 113 determines that the surface temperature is lower than the lower limit temperature (Yes in S6), the temperature control unit 114 determines whether the predetermined time has elapsed (S7). When the temperature control unit 114 determines that the predetermined time has not elapsed (No in S7), the process returns to process S2. When the temperature control unit 114 determines that the predetermined time has elapsed (Yes in S7, Yes in S1, Yes in S4), the induction heating of the portion P to be heated ends. The transport unit 21 moves the workpiece W in the direction indicated by the arrow A and sends out the part P outside the coil 22, and a new part P of the workpiece W located inside the coil 22 becomes a heating target.

It will be explained with reference to FIGS. 4 and 5 that the analytical value of the internal temperature of the workpiece W obtained using the embodiment is almost the same as the actually measured value of the internal temperature of the workpiece W. The actual value of the internal temperature of the workpiece W is a value measured with the temperature detection part (thermocouple) of the thermometer 24 inserted into a hole formed inside the workpiece W from the surface thereof. FIG. 4 is a graph comparing analytical values and actual measured values regarding the internal temperature of the workpiece W during induction heating. The horizontal axis of the graph indicates the induction heating time. The vertical axis of the graph indicates the internal temperature. The actual measurement value is the actual measurement value of the internal temperature at a point 68 mm deep from the surface of the workpiece W. The analytical value is an analytical value of the internal temperature at a point 68 mm deep from the surface of the workpiece W, which was determined using the embodiment. It can be seen that the analytical value is almost the same as the actual measured value.

FIG. 5 is a graph comparing analytical values and actual measured values regarding the internal temperature of the workpiece W at a surface temperature of 1100°C. The horizontal axis of the graph indicates the depth from the surface of the workpiece W. The vertical axis of the graph indicates the internal temperature. The round plot shows the actual values of the internal temperature at depths of 0 mm, 40 mm, 70 mm, and 190 mm of the workpiece W at a distance d2 (d2>d1) along the longitudinal direction of the coil 22 from one end surface of the coil 22. show. The analytical value is an analytical value of the internal temperature of the workpiece W obtained using the embodiment. It can be seen that the analytical value is almost the same as the actual measured value.

FIG. 6 is a graph showing the analytical values of the heat generation term of the workpiece W obtained using the embodiment at 0 seconds, 1 minute, 5 minutes, and 30 minutes after the start of induction heating. . The surface temperatures of the workpiece W 0 seconds, 1 minute, 5 minutes, and 30 minutes after the start of induction heating were 20°C, 610.97°C, 842.07°C, and 1006.5°C. The horizontal axis of the graph indicates the depth from the surface of the workpiece W. The vertical axis of the graph indicates the amount of heat generated per unit volume of the workpiece W, and corresponds to the heat generation term.

At the beginning of induction heating (0 seconds, 1 minute), the amount of heat generated on the surface of the workpiece W is larger than the amount of heat generated inside the workpiece W, but as the induction heating time elapses (after 5 minutes, 30 minutes). It can be seen that there are points inside the workpiece W where the amount of heat generated is larger than that on the surface of the workpiece W. This can be explained theoretically as follows.

FIG. 7 is a graph showing the penetration depth of the induced current generated in the workpiece W. The horizontal axis of the graph indicates the distance x from the surface of the workpiece W (depth from the surface). δ is the penetration depth. The vertical axis of the graph indicates the current density of the induced current.

Induction heating is performed by electrical resistance to a current flowing through the workpiece W by electromagnetic induction. Therefore, the heat generation distribution due to induction heating depends on the current density distribution. The current density is highest on the surface of the workpiece W, and has a distribution that decreases exponentially as it goes inside the workpiece W. Generally, the total current (|i|) that is continuously distributed from current density i0 to 0 is assumed to be a conductor with a width value of δ (penetration depth), and a current with uniform current density i0 is I think it flows. The penetration depth (δ) is expressed by equation (2) in FIG.

In equation (2), ρ is the resistivity (μΩ・cm) of the workpiece W, and μ is the relative magnetic permeability of the workpiece W (μ>1 for magnetic materials, μ = 1), f is the frequency (Hz) of the coil current.

As can be seen from equation (2), the penetration depth (δ) is given by the physical properties of the work W and the frequency of the coil current, regardless of the dimensions of the work W.

Table 1 shows the penetration depth of carbon steel. The penetration depth increases as the frequency of the coil current becomes lower, and increases rapidly in the non-magnetic region (region of temperature above the Curie point). Therefore, as shown in FIG. 6, the heat generation distribution is such that only the surface of the workpiece W is heated because the temperature of the workpiece W is low at the beginning of induction heating. After that, when the surface temperature exceeds the Curie point and the surface becomes a non-magnetic region, the amount of heat generated at the surface decreases and the penetration depth increases, so the heat generation distribution shifts to the inside of the work W. .

The above is a theoretical explanation that as the induction heating time elapses, a point where the amount of heat generated is larger than the surface of the workpiece W is generated inside the workpiece W.

In the embodiment, the analysis value of the internal temperature of the workpiece W is regarded as the monitored temperature of the workpiece W, and it is determined whether the monitored temperature exceeds the upper limit temperature of the workpiece W (S3). When the temperature of the workpiece W is lowered (Yes in S3), control is performed to lower the temperature of the workpiece W (S5). Therefore, according to the embodiment, it is possible to prevent the workpiece W from being excessively heated during induction heating.

FIG. 8 compares the analytical value of the internal temperature of the workpiece W obtained using the embodiment with the analytical value of the internal temperature of the workpiece W obtained using general-purpose software for coupled analysis of magnetic field and heat transfer. This is a graph. The horizontal axis of the graph indicates the depth from the surface of the workpiece W. The vertical axis of the graph indicates the internal temperature of the workpiece W.

Lines (A) to (G) show analytical values of the internal temperature of the workpiece W obtained using the embodiment. The line (A) shows the analysis value 20 seconds after the start of induction heating. The line (B) shows the analysis value 1 minute after the start of induction heating. The line (C) shows the analysis value 5 minutes after the start of induction heating. Line (D) shows the analysis value 10 minutes after the start of induction heating. Line (E) shows the analysis value 20 minutes after the start of induction heating. The line (F) shows the analysis value 40 minutes after the start of induction heating. The line (G) shows the analysis value 60 minutes after the start of induction heating.

The plots (a) to (g) show analytical values of the internal temperature of the workpiece W obtained using general-purpose software for coupled analysis of magnetic field and heat transfer. The plot in (a) shows the analytical values 20 seconds after the start of induction heating. The plot in (b) shows the analytical values 1 minute after the start of induction heating. The plot in (c) shows the analytical values 5 minutes after the start of induction heating. The plot in (d) shows the analytical values 10 minutes after the start of induction heating. The plot in (e) shows the analytical values 20 minutes after the start of induction heating. The plot in (f) shows the analytical values 40 minutes after the start of induction heating. The plot in (g) shows the analysis values 60 minutes after the start of induction heating.

The analytical value of the internal temperature of the workpiece W obtained using the embodiment almost matches the analytical value of the internal temperature of the workpiece W obtained using general-purpose software for coupled analysis of magnetic field and heat transfer. I understand.

When calculating the analytical value of the internal temperature of the workpiece W using general-purpose software for coupled analysis of magnetic field and heat transfer, it is necessary to calculate the magnetic field and calculate the exothermic term based on the calculated magnetic field, so it is necessary to calculate the analytical value. The amount of calculation required increases. According to the embodiment, by using the database 111 storing the data shown in FIG. 2A or 2B, it is possible to omit magnetic field calculation and heat generation term calculation when obtaining the analytical value of the internal temperature of the workpiece W. . Therefore, the amount of calculation required to calculate the analytical value of the internal temperature of the workpiece W can be reduced, and the time required to calculate the analytical value can be shortened. Specifically, the calculation time for the analytical value of the internal temperature of the workpiece W obtained using the embodiment is about 1 minute, and there is no problem in practical use.

A first modification of the embodiment will be described. The first modification differs from the embodiment in processing S3 in FIG. In the first modification, the determination unit 113 regards the larger of the analytical value of the internal temperature of the work W and the actual measured value of the surface temperature of the work W as the monitored temperature, and determines whether the monitored temperature exceeds the upper limit temperature. (processing S3). The actual value of the surface temperature of the workpiece was measured at the same timing as the actual value of the coil current used to calculate the analytical value.

The possibility that the actual measured value of the surface temperature of the workpiece W is larger than the analyzed value of the internal temperature of the workpiece W (for example, the peak value of the internal temperature) cannot be excluded. According to the first modification, the larger of the analytical value of the internal temperature of the workpiece W and the actual measured value of the surface temperature of the workpiece W is regarded as the monitored temperature, and it is determined whether the monitored temperature exceeds the upper limit temperature. Therefore, excessive heating of the workpiece W can be prevented.

A second modification of the embodiment will be described. The second modification differs from the embodiment in how the analytical value of the internal temperature of the workpiece W is determined. FIG. 9 is a block diagram showing the configurations of an induction heating control device 1a and an induction heating device 2 according to a second modification of the embodiment. The calculation processing section 11 further includes an analysis section 116 in addition to a determination section 113, a temperature control section 114, and a conveyance control section 115.

The analysis unit 116 uses a coupled analysis of the magnetic field and heat transfer in the work W that is being induction heated to obtain an analytical value of the surface temperature of the work W that is being induction heated.

How to obtain the analytical value of the surface temperature of the workpiece W will be explained. The analysis unit 116 subjects the part of the workpiece W, where the magnetic field and temperature change in the radial direction of the workpiece W and satisfy the condition of being constant (almost constant) in the axial direction of the workpiece W, to the coupled analysis. The analytical values are obtained using a coupled analysis of the one-dimensional magnetic field and heat transfer in the radial direction. It is a coupled analysis of the magnetic field and heat transfer using only the magnetic field.) The above conditions are, for example, when the length of the coil 22 is sufficiently large relative to the diameter of the workpiece W, the coil 22 is tightly wound, and the length of the coil 22 is sufficiently large.

explain in detail. When the length of the coil 22 is sufficiently large relative to the diameter of the workpiece W, the temperature of the workpiece W in the axial direction of the workpiece W (the longitudinal direction of the coil 22) is constant (approximately constant). In this case, the temperature of the work W changes in the radial direction of the work W. Therefore, it can be considered that a temperature distribution (one-dimensional temperature distribution) occurs only in the radial direction of the workpiece W.

FIG. 10 is a schematic diagram showing the magnetic field at the longitudinal center of the coil 22 in a state where the coil 22 is tightly wound and the length of the coil 22 is sufficiently large. The dotted line indicates the magnetic field. When the coil 22 is tightly wound and the length of the coil 22 is sufficiently large, a magnetic field is generated in the longitudinal direction (z) of the coil 22 in the longitudinal center of the coil 22, and a magnetic field is generated in the radial direction of the coil 22. No magnetic field is generated at (r). In this case, the magnetic field in the longitudinal direction (z) of the coil 22 changes in the radial direction (r) of the coil 22. Therefore, it can be considered that a magnetic field distribution (one-dimensional magnetic field distribution) occurs only in the radial direction of the coil 22 (radial direction of the workpiece W).

From the above, when the length of the coil 22 is sufficiently large relative to the diameter of the workpiece W, the coil 22 is tightly wound, and the length of the coil 22 is sufficiently large, the magnetic field and temperature of the workpiece W are This satisfies the condition that it changes in the radial direction and remains constant (almost constant) in the axial direction of the workpiece W.

The coupled analysis of one-dimensional magnetic field and heat transfer will be explained in detail. Induction heating occurs due to electrical resistance to a current flowing through the workpiece W by electromagnetic induction. The electromagnetic induction phenomenon is explained by the Maxwell equation, and by calculating the magnetic field based on the value of the coil current flowing through the coil 22, the winding density of the coil 22, and the temperature distribution of the workpiece W, the current density in the workpiece W can be calculated. distribution can be obtained. Specifically, the strength _HZ of the magnetic field at the longitudinal center of the coil 22 at a point r away from the central axis of the workpiece W in the radial direction is obtained by solving equation (3). σ represents the electrical conductivity of the workpiece W, μ represents the relative magnetic permeability of the workpiece W, f represents the frequency of the coil current, and j represents the imaginary unit.

Equation (3) is called a zero-order Bessel differential equation, and the general solution is expressed by Equation (4).

Here, I ₀ is a Bessel function of the first kind and zero order, K ₀ is a Bessel function of the second kind and zero order, and A and B are constants. An externally applied magnetic field H ₀ is given as a boundary condition for finding equation (4) (that is, finding the values of constants A and B). The externally applied magnetic field H ₀ is expressed by equation (5). I indicates the value of the coil current (actually measured value of the coil current), and N indicates the winding density of the coil 22.

Since the value I of the coil current (actual measured value of the coil current) is a value that fluctuates from moment to moment, calculation accuracy can be improved by reflecting the current data ID (Fig. 9) sent from the ammeter 25 in equation (5). It is possible to improve Based on the above equations (3) to (5), the strength H _Z of the magnetic field at the longitudinal center of the coil 22 can be determined.

When it is assumed that the magnetic field component is only in the longitudinal direction (z direction) of the coil 22, only the circumferential direction component of the electric field is effective, and by modifying the Maxwell equation, an equation showing the circumferential electric field component E _φ can be obtained. (6) is obtained.

At this time, since the eddy current density i _φ is given by equation (7), the current density distribution inside the workpiece W can be determined from equation (7). The amount of heat generated inside the workpiece W is obtained using the current density distribution determined by the equation (7) and the equation (8).

Equation (9) is calculated by using the obtained calorific value q as the exothermic term ΔQ in the heat transfer analysis of Equation (9), and the analytical value of the temperature distribution from the surface to the inside of the workpiece W in the induction heating process is Desired. ρ indicates the density of the workpiece W, c indicates the specific heat of the workpiece W, λ′ indicates the thermal conductivity of the workpiece W, T indicates the analytical value (temperature), and t indicates the time.

As mentioned above, if the target of the coupled analysis is a location on the workpiece W that satisfies the condition that the magnetic field and temperature change in the radial direction of the workpiece W and are constant (almost constant) in the axial direction of the workpiece W, then the primary It will be explained that even in the original coupled analysis of the magnetic field and heat transfer, the analytical value of the surface temperature of the workpiece W is sufficiently reliable. FIGS. 11 and 12 show the analytical values of the temperature distribution from the surface to the inside of the workpiece W obtained using the second modification of the embodiment, and the analysis values obtained using general-purpose software for coupled analysis of magnetic field and heat transfer. 2 is a graph comparing the calculated temperature distribution from the surface of the workpiece W to the inside thereof with an analytical value. The value of the current flowing through the coil 22 (actually measured value of coil current) was 4500 A, and the winding density of the coil 22 was 31.5 (1/m). 11 and 12 show cases where the initial temperature of the actual measured value of the surface temperature of the workpiece W is 20° C. and 500° C., respectively. The horizontal axis of the graph indicates the distance (depth) from the surface of the workpiece W. The vertical axis of the graph indicates the analytical value of the temperature of the workpiece W. When the distance from the surface of the workpiece W is 0, an analytical value of the surface temperature of the workpiece is shown.

In FIGS. 11 and 12, the data shown by lines show the analytical values of the temperature distribution from the surface to the inside of the workpiece W obtained using the second modification of the embodiment, and the data shown by plots show the analysis value of the temperature distribution from the surface to the inside of the workpiece W obtained using the second modification of the embodiment. The analytical values of the temperature distribution from the surface to the inside of the workpiece W obtained using general-purpose heat transfer coupled analysis software are shown. Analysis value after 20 seconds after initial temperature measurement, analysis value after 1 minute, analysis value after 5 minutes, analysis value after 10 minutes, analysis value after 20 minutes, analysis value after 40 minutes. It was found that for all of the analytical values, the data shown by the line gave similar results to the data shown by the plot. Therefore, according to the second modification of the embodiment, the same analytical values as the general-purpose software for the coupled analysis of magnetic field and heat transfer can be obtained, so the reliability is similar to that of the general-purpose software for the coupled analysis of the magnetic field and heat transfer. has.

In the second modification of the embodiment, the analytical value of the surface temperature of the workpiece W is determined by coupled analysis of the one-dimensional magnetic field and heat transfer, so that the amount of calculation can be reduced. In fact, the calculation time for the analytical value of the surface temperature of the work W obtained using the second modification of the embodiment is about several minutes, and there is no problem in practical use.

1, 1a Induction heating control device 2 Induction heating device 22 Coil 24 Thermometer W Workpiece P Part of the workpiece located inside the coil (part to be heated)
td Temperature data id Current data s1 Control signal s2 Control signal

Claims (5)

A heating process of induction heating the workpiece,
an analysis step of determining an analytical value of the internal temperature of the workpiece being induction heated, based on the actual measurement value of the coil current in the induction heating or the actual measurement value of the surface temperature of the workpiece in the induction heating;
a determination step of determining whether or not the monitored temperature exceeds an upper limit temperature of the workpiece by considering the larger one of the analysis value and the actual measurement value of the surface temperature as the monitored temperature of the workpiece;
An induction heating method, comprising: a control step of controlling to lower the temperature of the workpiece in the heating step when it is determined that the monitored temperature exceeds the upper limit temperature. The induction heating method according to claim 1, wherein the determination step makes the determination using a peak value as the analysis value among the analysis values of the internal temperature obtained in the analysis step. a heat generation term that uses the actual value of the coil current in the induction heating or the actual value of the surface temperature of the workpiece in the induction heating as a parameter, and indicates the amount of heat generated inside the workpiece in the induction heating; a preparation step of preparing in advance data showing the relationship between the
further comprising a measuring step of measuring the parameter during the heating step,
The analysis step includes:
obtaining the exothermic term corresponding to the measured parameter from the data;
3. The induction heating method according to claim 1 , wherein the acquired exothermic term is applied to an equation indicating the internal temperature distribution of the workpiece in the induction heating, and the analytical value is obtained using the equation. Further comprising a measuring step of using the actual measured value of the coil current in the induction heating as a parameter of a coupled analysis and measuring the parameter during the heating step,
In the analysis step, a location of the workpiece that satisfies the condition that the magnetic field and temperature change in the radial direction of the workpiece and remain constant in the axial direction of the workpiece is targeted for the coupled analysis, and the magnetic field and temperature in the radial direction of the workpiece are The induction heating method according to claim 1 or 2 , wherein the analytical value is obtained using the coupled analysis of a one-dimensional magnetic field and heat transfer. an analysis unit that calculates an analytical value of the internal temperature of the workpiece being induction heated by the induction heating device based on an actual value of the coil current in the induction heating or an actual value of the surface temperature of the workpiece in the induction heating;
a determination unit that determines whether or not the monitored temperature exceeds an upper limit temperature of the workpiece by considering the larger one of the analysis value and the actual measurement value of the surface temperature as the monitored temperature of the workpiece;
An induction heating control device comprising: a control section that controls the induction heating device to lower the temperature of the workpiece when it is determined that the monitored temperature exceeds the upper limit temperature.