Electric heating method for steel material

Abstract

To provide an electric heating method for a steel material capable of uniformly heating the steel material over the whole length in the longitudinal direction by a simple method without lowering the heating efficiency of the steel material.SOLUTION: An electric heating method for a steel material includes: a pressing step (step S1) of pressing electrodes against both end portions of the steel material in the length direction with a pressing stress of 3 to 10 MPa; a first heating step (step S2) of electrically energizing the steel material for a predetermined time to heat the steel material; a cooling step (step S3) of thereafter cooling the electrodes with a coolant until the temperature of the electrodes reaches a first target temperature; and a second heating step (step S4) of thereafter restarting the energization of the steel material to heat the steel material until the temperature of the steel material reaches a predetermined second target temperature, while changing the pressing stress set in the pressing step to a pressing stress preset for each temperature of the steel material.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a method for electrically heating steel materials.

Background Art

[0002] As methods for electrically heating steel materials, for example, a method of clamping a plurality of electrodes to the outer peripheral surfaces on both end portions sides in the length direction of the steel material and electrically heating, and a method of pressing electrodes against both end surfaces in the length direction of the steel material and electrically heating are conventionally known. However, in the former method, since current is applied to the steel material through a plurality of clamps, the current is dispersed and applied to the steel material. As a result, the amount of current applied to the end portion of the steel material decreases, and there is a possibility that the end portion is insufficiently heated. In the latter method, the portions where the electrodes contact the steel material are limited to the end surfaces of the steel material. Therefore, compared with the former method, the contact area between the electrode and the steel material decreases, and the resistance value between the electrode and the end surface increases due to the oxide film formed on the end surface. As a result, there is a possibility that the end surface of the steel material is overheated.

[0003] Therefore, methods for uniformly heating steel materials have been conventionally studied. For example, in the method described in Patent Document 1, main electrodes are pressed against each of both end surfaces in the length direction of the steel material, and the outer peripheral surfaces on both end portions sides of the steel material adjacent to each main electrode among the outer peripheral surfaces of the steel material are clamped by auxiliary electrodes. Thereby, since a part of the current applied to the steel material is applied to the steel material through the auxiliary electrodes, it is said that abnormal overheating of both end portions of the steel material can be suppressed.

[0004] Further, Patent Document 2 describes an electric heating furnace configured to suppress local overheating at the end portions of a steel material. In the electric heating furnace, hollow electrodes are pressed against each of both end surfaces in the length direction of the steel material. Inside the hollow electrode, a cooling device is provided so as to be able to advance and retreat with respect to the steel material. When local overheating occurs at the end portion of the steel material, the cooling device is brought into contact with the end portion of the steel material to cool the end portion by the amount of overheating, that is, the amount of heat generation exceeding a preset temperature, by the cooling device.

Prior Art Documents

Patent Document

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the method described in Patent Document 1, in order to uniformly heat the steel material over the entire length in the longitudinal direction, it is necessary to appropriately set the heating time of the steel material through the auxiliary electrode. Specifically, for example, during the energization heating of the steel material, it is necessary to clamp the steel material with the auxiliary electrode according to the heating condition of the steel material and also release the clamp. Therefore, in the method described in Patent Document 1, complicated operations are inevitable, or the equipment may become complicated, and there is room for improvement in these aspects.

[0007] In the electric heating furnace described in Patent Document 2, the end portion of the steel material is cooled by a cooling device by the amount of overheating at the end portion of the steel material, that is, the amount exceeding a predetermined temperature. Therefore, the heating efficiency of the steel material may decrease. Specifically, when the electrode is pressed against the end portion of the steel material for energization heating, the end portion is deformed and flattened by heat and pressure, and the contact between the end portion and the electrode becomes good. On the other hand, as the contact resistance at the electrode gradually increases due to heat, the heat generation amount at the electrode gradually increases. And as the heat generation amount at the electrode increases, the temperature of the end portion of the steel material rises, and in order to make the temperature of the steel material a predetermined temperature, the cooling amount by the cooling device for cooling the end portion of the steel material increases. Due to such a principle, in the electric heating furnace described in Patent Document 2, the heating efficiency of the steel material may decrease, and there is still room for improvement in these aspects.

[0008] The present invention has been made to solve the above-described problems, and an object thereof is to provide an electric heating method for a steel material that can uniformly heat the entire length of the steel material in the longitudinal direction by a simple method without reducing the heating efficiency of the steel material.

Means for Solving the Problems

[0009] In order to achieve the above object, the present invention [1] An electric heating method for a steel material that heats the steel material by energizing the steel material, comprising: a pressing step of pressing electrodes against both ends of the steel material in the length direction thereof with a pressing force of 3 to 10 Mpa; a first heating step of energizing the steel material through the electrodes for a predetermined time to heat the steel material; a cooling step of supplying a cooling medium to the electrodes after stopping the energization of the steel material in the first heating step and cooling the electrodes with the cooling medium so that the temperature of the electrodes becomes a first target temperature; and then, restarting the energization of the steel material and heating the steel material so as to reach a predetermined second target temperature, and a second heating step of changing the pressing force set in the pressing step to a pressing force preset for each temperature of the steel material. [2] The electric heating method for a steel material according to [1], wherein in the cooling step, the cooling amount of the electrodes is controlled by controlling the flow rate of the cooling medium. [3] The electric heating method for a steel material according to [1] or [2], wherein in the second heating step, the pressing force set in the pressing step is decreased stepwise or continuously as the temperature of the steel material rises.

Effects of the Invention

[0010] According to the present invention, it is possible to uniformly heat the entire length of the steel material in the longitudinal direction by a simple method without reducing the heating efficiency of the steel material.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0012] Hereinafter, an example of an embodiment of the present invention (hereinafter referred to as the present embodiment) will be described. FIG. 1 is a perspective view showing an example of an electric heating device for a steel material to which the method for electrically heating a steel material according to the present embodiment can be applied. The electric heating device 1 shown in FIG. 1 is configured to press electrodes 2 against both ends of the steel material 3 in the length direction of the steel material 3 and energize the steel material 3 in that state to heat the steel material 3. That is, the steel material 3 is sandwiched between a pair of electrodes 2. A thermometer (hereinafter referred to as a steel material thermometer) 4 for measuring the temperature of the steel material 3 heated by energization is installed at substantially the center of the steel material 3 in the length direction of the steel material 3. The steel material thermometer 4 may be a conventionally known one, and examples of the steel material thermometer 4 include a contact thermometer using a thermocouple and a non-contact thermometer using infrared rays. Further, the steel material thermometer 4 is electrically connected to a control device described later so that the measured temperature of the steel material 3 can be input to the control device.

[0013] (Electrode) In the example shown in FIG. 1, electrodes 2 are arranged on each of both sides of the steel material 3 in the length direction of the steel material 3. Each electrode 2 is configured to be pressed against each of both end faces of the steel material 3 in the length direction of the steel material 3. Note that, instead of pressing the electrodes 2 against both end faces of the steel material 3, the electrodes 2 may be configured to be pressed against each of both ends in the length direction of the steel material 3. Alternatively, the electrodes 2 may be configured to clamp both end portions of the steel material 3.

[0014] In order to suppress heat generation of the electrode 2 itself due to energization, the electrode 2 is preferably made of a material having an electrical resistivity of 10 -6 Ωm or less. Further, since the electrode 2 is in direct contact with the steel material 3 heated by energization, it is preferably made of a material having excellent heat resistance. Furthermore, as will be described later, since the electrode 2 is cooled by a cooling medium, the electrode 2 is preferably made of a material having good thermal conductivity. Therefore, examples of the material constituting the electrode 2 include, but are not limited to, copper tungsten, molybdenum copper, and carbon electrodes.

[0015] A thermometer for measuring the temperature of the electrode 2 (hereinafter referred to as the electrode thermometer) 5 is provided on the electrode 2. The electrode thermometer 5 may be a conventionally known one, and examples of the electrode thermometer 5 include a contact thermometer using a thermocouple and a non-contact thermometer using infrared rays. Further, the electrode thermometer 5 is electrically connected to a control device described later, and the temperature of the electrode 2 measured by the electrode thermometer 5 can be input to the control device.

[0016] In addition, in this embodiment, in order to suppress overheating of the electrode 2, each electrode 2 is cooled by a cooling medium. Specifically, a cooling flow path for the cooling medium is formed inside the electrode 2, and heat exchange is performed between the cooling medium flowing through the cooling flow path and the electrode 2 to cool the electrode 2. FIG. 2 is a diagram showing an example of the cooling flow path of the cooling medium in the electrode 2. In the example shown in FIG. 2, the electrode 2 is formed in a flat plate shape, and a cooling flow path 6 for the cooling medium is formed in an annular shape along the four sides of the electrode 2 inside the electrode 2. Further, as shown in FIG. 1, a supply port 7 for supplying the cooling medium to the cooling flow path 6 is formed above the electrode 2 in the vertical direction of the energization heating device 1 and the electrode 2, and a discharge port 8 for discharging the cooling medium from the cooling flow path 6 is formed below the electrode 2 in the vertical direction of the electrode 2. Here, the above-mentioned overheating of the electrode 2 means that the temperature of the electrode 2 exceeds a predetermined temperature, and the predetermined temperature corresponds to the first target temperature in this embodiment. In this embodiment, the flow rate of the cooling medium flowing through the cooling flow path 6 is controlled so that the temperature of the electrode 2 becomes equal to or lower than the first target temperature.

[0017] The cooling medium is a medium that transfers heat in a sensible heat state, and examples of the cooling medium include water and oil. In the example shown in FIG. 1, the pump 9 supplies the cooling medium to each electrode 2, and the cooling medium whose temperature has risen due to heat exchange with the electrode 2 is supplied to the cooler 10 for cooling. Therefore, the electrode 2, the pump 9, and the cooler 10 are connected in series by piping 11 such as a hose or a pipe so that the cooling medium can circulate between them. In this way, a circulation path for the cooling medium is formed. Note that a tank (not shown) for storing the cooling medium and an on-off valve (not shown) for blocking the flow of the cooling medium may be provided in the above-mentioned circulation path.

[0018] (Power supply device) A power supply device 12 for applying current to each electrode 2 is connected to the electrode 2 via a wire 13. The power supply device 12 may be a conventionally known one, and may be either a DC power supply device for applying DC electricity to the electrode 2 or an AC power supply device for applying AC electricity to the electrode 2. When a current of 1 kA or more is applied from the power supply device 12 to the electrode 2, the wire 13 is preferably a bus bar. When a current lower than 1 kA is applied from the power supply device 12 to the electrode 2, the wire 13 is preferably a cable that is easier to handle than a bus bar.

[0019] (Pressing device) A configuration in which a steel material 3 is sandwiched between a pair of electrodes 2 will be described. A fixing base 14 is fixed to a mounting surface such as a floor (not shown). One of the pair of electrodes 2, i.e., an electrode 2a, is fixed to the fixing base 14. In the following description, the one electrode 2a will be referred to as the fixed electrode 2a. The other electrode 2b is configured to approach and separate linearly with respect to the fixed electrode 2a. In the following description, the other electrode 2b will be referred to as the movable electrode 2b.

[0020] An actuator for approaching and separating the movable electrode 2b with respect to the fixed electrode 2a is provided on the side opposite to the fixed electrode 2a across the movable electrode 2b in the moving direction of the movable electrode 2b with respect to the fixed electrode 2a. Examples of the actuator include a single-acting or double-acting pressing device 15 using oil, air, or the like as a pressure medium.

[0021] First, the case where the above-described actuator is a double-acting pressing device 15 will be described. The pressing device 15 has a cylinder (not shown) and a piston 16 that moves back and forth in the cylinder in its axial direction. A movable electrode 2b is connected to the piston 16. Also, a pressure medium can be supplied to each of the two spaces partitioned by the piston 16 and the cylinder by a pressure pump (not shown), and each space is configured to be dischargeable. Then, the pressure medium is supplied to one of the two spaces by the pressure pump, and the pressure medium is discharged from the other space. By doing so, the piston 16 is moved from one space side to the other space side.

[0022] Next, the case where the above-described actuator is a single-acting pressing device 15 will be described. Even in the case of the single-acting pressing device 15, the pressing device 15 has a cylinder (not shown) and a piston 16 that moves back and forth in the cylinder in its axial direction. A movable electrode 2b is connected to the piston 16. Also, among the two spaces partitioned by the piston 16 and the cylinder, a pressure medium is supplied to the space on the opposite side of the movable electrode 2b with the piston 16 interposed therebetween, and the piston 16 and the movable electrode 2b are moved toward the fixed electrode 2a side by the pressure in that space. Further, a return spring (not shown) is provided, and the piston 16 is returned to its original position by the elastic force of the return spring.

[0023] Note that the above-described actuator only needs to be configured to be able to approach and separate the movable electrode 2b from the fixed electrode 2a. Therefore, instead of the single-acting or double-acting pressing device 15 described above, the actuator may be a device having a cam mechanism or a feed screw mechanism that converts rotational motion into linear motion, and a motor connected to these mechanisms so as to be able to transmit power. That is, the rotational motion generated by the motor is converted into linear motion by the cam mechanism or the feed screw mechanism. By doing so, the movable electrode 2b may be configured to move back and forth with respect to the fixed electrode 2a.

[0024] Further, the pressing device 15 is provided with a pressure sensor 17 for measuring the pressing force when the movable electrode 2b is pressed against the steel material 3. The pressure sensor 17 may be a conventionally known one. Further, the pressure sensor 17 is electrically connected to a control device described later, and the pressure measured by the pressure sensor 17 can be input to the control device. Alternatively, instead of the pressure sensor 17, it may be configured to estimate the pressing force when the movable electrode 2b is pressed against the steel material 3 based on the stroke amount of the piston 16 of the pressing device 15.

[0025] (Control device) The control device 18 is mainly composed of a microcomputer, and is configured to perform calculations based on the input signal and arithmetic expressions and data stored in advance, and output the calculation result as a control command signal. Examples of the input signal include the current temperature of the steel material 3 measured by the steel material thermometer 4, the current temperature of the electrode 2 measured by the electrode thermometer 5, and the current pressing force when the movable electrode 2b is pressed against the steel material 3 measured by the pressure sensor 17. Examples of the control command signal include a signal for the pressing device 15 to change the stroke amount of the piston 16, a signal for the power supply device 12 to change the current applied to the electrode 2, and a signal for the cooler 10 that cools the cooling medium and the pump 9 that changes the flow rate and flow volume of the cooling medium. Examples of the data stored in the control device 18 include, for example, the target heating temperature of the steel material 3 by the energization heating device 1, the target temperature of the electrode 2 or the upper and lower limit values, and the target pressing force or the upper and lower limit values of the pressing force when the movable electrode 2b is pressed against the steel material 3.

[0026] Next, the energization heating method for steel materials according to the present embodiment will be described.

[0027] (Pressing step) FIG. 3 is a diagram for explaining an example of each step of the method for electric heating of a steel material according to the present embodiment. When the steel material 3 is disposed between a pair of electrodes 2 of the electric heating device 1, the pressing device 15 is operated by the control device 18, and the movable electrode 2b moves toward the fixed electrode 2a. As a result, the electrodes 2 are pressed against both ends of the steel material 3 in the length direction thereof with a predetermined pressing force (step S1). This step S1 corresponds to the pressing step of the present embodiment. In step S1, the pressing force for pressing the movable electrode 2b against the steel material 3 can be measured by the pressure sensor 27. As an example, the pressing force is preferably within a predetermined range of pressing force. The range of the pressing force is, for example, 3 to 10 MPa. This is because, in the first heating step (step S2) described below, the unevenness and oxide film on the end face of the steel material 3 are deformed or broken by the pressing force set in the pressing step and the heat generated in the first heating step, so as to improve the contact state between each electrode 2 and both end faces of the steel material 3. That is, at the time of the pressing step (step S1), the contact area between the electrode 2 and the steel material 3 is smaller than that at the time of the first heating step (step S2).

[0028] In addition, if the above-described pressing force is smaller than the predetermined range of pressing force, it may be difficult to deform or break the unevenness and oxide film on the end face of the steel material 3 in the first heating step. As a result, local heating may occur between the electrode 2 and the end face of the steel material 3, and the electrode 2 may be damaged by melting. On the other hand, if the pressing force is larger than the predetermined range of pressing force, the steel material 3 or the electrode 2 may be deformed in the first heating step. The operation state of the pressing device 15 may be controlled by the control device 18, or may be performed by an operator.

[0029] (First Heating Step) Next, as shown in FIG. 3, in the first heating step (step S2), a predetermined target time and current are applied to the steel material 3 from the power supply device 12 via the pair of electrodes 2. As a result, the steel material 3 is heated. This is to increase the temperatures of both end faces of each electrode 2 and the steel material 3 to make them easier to deform. Specifically, immediately before the start of the first heating step, unevenness and oxide films (not shown) exist on both end faces of the steel material 3, and the contact area between the electrode 2 and the steel material 3 is small. For the steel material 3 in such a state, short-time energization heating is performed in the first heating step. As a result, the contact portion between the electrode 2 and the steel material 3 becomes locally high in temperature. Thereby, the deformation resistance of the surface of the electrode 2 that contacts the steel material 3 and the end face of the steel material 3 is reduced. Further, when the electrode 2 is pressed against the end face of the steel material 3 in that state, the unevenness and oxide film on the surface of the end face of the steel material 3 are deformed or broken, and the true contact area between the electrode 2 and the end face of the steel material 3 increases. In this way, the contact state between each electrode 2 and both end faces of the steel material 3 can be improved. Note that the control of the operating state of the power supply device 12 may be performed by the control device 18, or may be performed by an operator.

[0030] The above-mentioned target time is preferably set between 1 and 5 seconds, but it is preferably changed as appropriate according to the material of the steel material 3 and the surface state of each end face of the steel material 3. For example, after starting the energization, when the actual voltage between the electrodes 2 becomes 1.5 times or less of the voltage theoretical value applied to the steel material 3 represented by the product of the electrical resistance value of the steel material 3 at room temperature and the applied current value, the energization is cut off and the energization heating is terminated. Or, when the temperature of the contact portion between the electrode 2 and the steel material 3 reaches 1200 °C or higher, the energization is cut off and the energization heating is terminated. Or, when 5 seconds have elapsed after starting the energization, the energization is cut off and the energization heating is terminated. Alternatively, the relationship between the material of the steel material 3 before the energization heating, the surface state of each end face of the steel material 3, and the energization heating time when the contact state between each electrode 2 and both end faces of the steel material 3 becomes good may be obtained in advance, and the target time may be set based on the previously obtained relationship and the surface state. The reason for terminating the energization when the voltage between the electrodes becomes 1.5 times or less of the voltage theoretical value is that the voltage between the electrodes after cooling the electrodes and the steel material becomes a value close to the voltage theoretical value. That is, it is considered that the true contact area has increased sufficiently. The reason for terminating the energization when the temperature of the contact portion between the electrode and the steel material reaches 1200 °C or higher is to suppress the melting of the steel material at 1400 °C or higher. The reason for terminating the heating when 5 seconds have elapsed after starting the energization is that if the energization continues for a long time with a poor contact state between the electrode and the steel material, the contact portion between the electrode and the steel material may melt. That is, if the energization continues for a long time in a state where the true contact area between the electrode and the steel material is small, the contact portion between the electrode and the steel material may melt.

[0031] (Cooling step) On the one hand, in the first heating step, in order to cause local heating at the contact surface between the electrode 2 and the steel material 3, the electrode 2 becomes hot. Generally, when the steel material 3 becomes hot, its electrical resistance value increases. Therefore, in the first energization heating step, if the energization heating continues, as the temperature of the steel material 3 rises, the electrical resistance value of the steel material 3 increases, and accordingly, the heat generation amount of the steel material 3, that is, the temperature of the steel material 3 rises, and the above-mentioned electrical resistance value further increases. As a result, the contact surface between the electrode 2 and the steel material 3, the electrode 2, and both ends of the steel material 3 may be overheated. Therefore, in the subsequent cooling step (step S3), a cooling medium is supplied to the internal cooling flow path of the electrode 2 to cool the electrode 2.

[0032] Specifically, after the elapse of the target time, the application of current to the electrode 2 is immediately stopped or cut off. In addition, a cooling medium is supplied to the cooling flow path 6 in the electrode 2. The cooling of the electrode 2 by the cooling medium is performed until the temperature of the electrode 2 measured by the electrode thermometer 5, that is, the measured value, drops to a predetermined first target temperature. When increasing the cooling rate of the electrode 2 by the cooling medium, in order to increase the flow rate of the cooling medium, the motor rotation speed of the pump 9 is increased. That is, the flow rate of the cooling medium is increased to increase the cooling amount of the electrode 2. When the temperature of the electrode 2 reaches the predetermined first target temperature, the supply of the cooling medium to the cooling flow path 6 in the electrode 2 is stopped. This may be done by stopping the pump 9, or the supply of the cooling medium to the above-mentioned cooling flow path 6 may be stopped by closing an on-off valve (not shown) provided in the pipe 11. Note that the control of the operating state of the pump 9 may be performed by the control device 18, or may be performed by an operator. In addition, the functional means for executing this step S3 corresponds to the cooling means of the present embodiment.

[0033] In addition, in the above-mentioned first heating step (step S2) and cooling step (step S3), the pressing force for pressing the electrode 2 against both end faces of the steel material 3 is controlled to be substantially constant at the pressing force set in the pressing step (step S1). This is to maintain the situation where the true contact area increases in the first heating step.

[0034] (Second heating step) Next, energization of the electrode 2 is resumed, and the steel material 3 is energized and heated until the temperature at the central portion in the length direction of the steel material 3 reaches a preset second target temperature (step S4). This step S4 corresponds to the second heating step of the present embodiment. The application of current to the electrode 2 may be continuously performed until the temperature at the central portion of the steel material 3 reaches the preset second target temperature, or may be performed in a pulsed manner at regular time intervals. Also, the amount of current is controlled by pulse width modulation (PWM) of the power supply device 12, or more simply, by tapping switching of the transformer or the like.

[0035] Also, in the second heating step, the temperature of the steel material 3 gradually rises. Therefore, as the temperature of the steel material 3 rises, the pressing device 15 is controlled to gradually reduce the pressing force for pressing the electrode 2 against both end faces of the steel material 3. For example, the pressing force set in the pressing step (step S1) is gradually or continuously reduced as the temperature of the steel material 3 rises. This is to avoid welding of the steel material 3 and the electrode 2 to each other due to contact in a high-temperature and high-pressure state. For example, an optimum pressing force is obtained in advance for each temperature of the steel material 3, and based on the relationship between the temperature and the pressing force, and the temperature of the steel material 3 measured by the steel material thermometer 4, the pressing device 15 is controlled to adjust the pressing force for pressing the electrode 2 against both end faces of the steel material 3. Note that the control of the operating states of the power supply device 12 and the pressing device 15 may be performed by the control device 18, or may be performed by an operator.

[0036] According to the present embodiment described above, in the first heating step, the contact area between the electrode 2 and both end faces of the steel material 3 can be increased, and their contact state can be improved. Also, by cooling the electrode 2 in the subsequent cooling step, local heating between the electrode 2 and both end faces of the steel material 3 can be suppressed. That is, a state can be achieved in which current easily flows almost uniformly in the length direction of the steel material 3. As a result, in the second heating step, the steel material 3 can be heated almost uniformly over the entire length in the length direction of the steel material 3, which is unprecedented. Also, since the steel material 3 is heated in a state where the above-described local heating is suppressed, the heating efficiency of the steel material 3 does not decrease, or a decrease in heating efficiency can be suppressed.

Example

[0037] Next, an example in which the method for electric current heating of a steel material according to this embodiment is applied to the electric current heating of a steel material will be described.

[0038] In the example, ordinary steel (SS400) of 160 mm × 10 m was used as the steel material. An electrode made of copper tungsten (CuW70) was used as the electrode. A DC power supply device was used as the power supply device, and a direct current was applied from the DC power supply device to the steel material in the first heating step and the second heating step. Also, in the second heating step, in an environment at room temperature, the steel material was heated for 40 seconds to the target temperature. The target temperature was set to 1200°C. Other conditions and results are summarized in Table 1.

[0039]

Table 1

[0040] Comparative Example 1 is an example in which the first heating step and the cooling step of this embodiment were not carried out, and heating was performed toward a target temperature of 1200°C. In this Comparative Example 1, as shown in Table 1, the temperature difference between both ends and the central portion in the length direction of the steel material was 300°C.

[0041] On the other hand, in Example 1 and Example 2 to which the method for electric current heating of a steel material according to this embodiment was applied, as shown in Table 1, the above-described temperature difference became 50°C. In Example 1 and Example 2, compared with Comparative Example 1, it was possible to perform electric current heating almost uniformly over the entire length in the length direction of the steel material.

[0042] Example 3 is an example in which the time of the first heating step is shorter than that of Example 1. In this Example 3, as shown in Table 1, the above-mentioned temperature difference was enlarged compared with Example 1. This is presumably because the shortening of the heating time in the first heating step caused insufficient destruction of the oxide film and deformation of the unevenness. However, even if the destruction of the oxide film and the deformation of the unevenness are insufficient compared with Example 1, the first heating step is still passed through. Therefore, the contact area between the electrode and both end faces of the steel material increased, and the above-mentioned temperature difference could be reduced compared with Comparative Example 1.

[0043] Example 4 is an example in which energization heating was performed in the same manner as in Example 1 on the steel material without any treatment. In this Example 4, as shown in Table 1, the above-mentioned temperature difference was enlarged compared with Example 1. This is presumably because, due to the lack of treatment, the destruction of the oxide film and the deformation of the unevenness in the first heating step were insufficient compared with Example 1. However, even if the destruction of the oxide film and the deformation of the unevenness are insufficient compared with Example 1, the first heating step is still passed through. Therefore, the contact area between the electrode and both end faces of the steel material increased, and the above-mentioned temperature difference could be reduced compared with Comparative Example 1.

[0044] Example 5 is an example in which untreated steel material was used, the heating time in the first heating step was set longer than that of Example 1, and energization heating was performed in the same manner as in Example 1 except for that. Although the steel material used in Example 5 was untreated, the destruction of the oxide film and the deformation of the unevenness in the first heating step were sufficiently performed. Therefore, as shown in Table 1, the above-mentioned temperature difference became comparable to that of Example 1.

[0045] Example 6 is an example in which compressed air was used for cooling a pair of electrodes, and the cooling time and the flow rate in the cooling step were increased compared with Example 1, and energization heating was performed in the same manner as in Example 1 except for that. The thermal conductivity of air is lower than that of water. However, in Example 6, by increasing the cooling time and increasing the flow rate of the compressed air, as shown in Table 1, the temperature difference could be reduced to the same level as that of Example 1.

[0046] Comparative Example 2 is an example where the pressure in the pressing step is higher than the preferred range of the present embodiment, that is, higher than 3 to 10 Mpa. In Comparative Example 2, the fact that the pressure in the pressing step was higher than that in Example 1 caused the steel material to buckle in the subsequent first heating step, and it was not possible to perform electric heating.

[0047] Comparative Example 3 is an example where the pressure in the pressing step is lower than the preferred range of the present embodiment. In Comparative Example 3, the electrodes were melted and damaged in the first heating step. This is because the low pressure in the first heating step caused the unevenness and oxide film on both end faces of the steel material to not be deformed or broken. As a result, the contact area between the electrodes and both end faces of the steel material became smaller than that in Example 1, and it is considered that local heating occurred at the contact portions between them.

[0048] Comparative Example 4 is an example where the pressure was kept constant in the second heating step. As the temperature of the steel material rises in the second heating step, the deformation resistance of the steel material and the electrodes decreases, but in Comparative Example 4, the pressure is kept constant. Therefore, the electrodes and both end portions of the steel material were welded to each other, and it was not possible to perform electric heating.

[0049] Note that the surface state described in Table 1 refers to the surface states of both end faces of the steel material. "No treatment" means that the steel material was left as it was cut to a predetermined size by a circular saw. Therefore, there are unevenness of the circular saw blade and oxide scale on both end faces of the steel material. "Grinder treatment" means a state where both end faces of the steel material in the no-treatment state were polished using a grinder. Therefore, the unevenness of the circular saw blade and the oxide scale are removed from both end faces of the steel material, or the unevenness of the circular saw blade and the oxide scale are less than those of both end faces of the steel material in the no-treatment state.

[0050] Figures 4 to 7 listed in the column of the pressure pattern in Table 1 show the pressure pattern of the pressing force for pressing the electrodes against the steel material by the pressing device in the second heating step. That is, the pressure pattern shown in Figure 4 is a pressure pattern in which the above-described pressing force is gradually decreased according to the temperature of the steel material. Specifically, when starting the heating in the second heating step, the above-described pressing force is set to 10 MPa. Thereafter, when the temperature at the central portion in the length direction of the steel material reaches 800 °C, the pressing force is set to 1 MPa. Further thereafter, when the temperature at the central portion of the steel material reaches 1000 °C, the pressing force is set to 0 MPa. The pressure pattern shown in Figure 5 is a pressure pattern in which the pressing force at the start of heating in the second heating step is set to 1 MPa and maintained. The pressure pattern shown in Figure 6 is a pressure pattern in which the pressing force at the start of heating in the second heating step is set higher than the pressure pattern shown in Figure 4, and the pressing force is gradually decreased according to the temperature of the steel material. Specifically, when starting the heating in the second heating step, the above-described pressing force is set to 50 MPa. Thereafter, when the temperature at the central portion in the length direction of the steel material reaches 800 °C, the pressing force is set to 1 MPa. Further thereafter, when the temperature at the central portion of the steel material reaches 1000 °C, the pressing force is set to 0 MPa. The pressure pattern shown in Figure 7 is a pressure pattern in which the pressing force at the start of heating in the second heating step is set to 10 MPa, which is higher than the example shown in Figure 5, and maintained.

Explanation of Signs

[0051] 1 Electric heating device 2 Electrode 3 Steel material 4 Thermometer for steel material 5 Thermometer for electrode 6 Cooling channel 7 Supply port 8 Discharge port 9 Pump 10 Cooler 11 Pipe 12 Power supply device 13 Electric wire 14 Fixed base 15 Pressing device 16 cylinders 17 pressure sensor 18 control device

Claims

1. A method for electric current heating of steel materials, which heats the steel materials by applying an electric current to the steel materials, comprising: a pressing step of pressing electrodes against both ends of the steel material in the length direction of the steel material with a pressing force of 3 to 10 MPa; a first heating step of heating the steel material by applying an electric current to the steel material through the electrodes for a predetermined time; a cooling step of supplying a cooling medium to the electrodes after stopping the application of the electric current to the steel material in the first heating step, and cooling the electrodes with the cooling medium so that the temperature of the electrodes reaches a first target temperature; thereafter, restarting the application of the electric current to the steel material and heating the steel material so that it reaches a predetermined second target temperature, and a second heating step of changing the pressing force set in the pressing step to a pressing force preset for each temperature of the steel material; A method for electric current heating of steel materials.

2. In the cooling step, the cooling amount of the electrodes is controlled by controlling the flow rate of the cooling medium. The method for electric current heating of steel materials according to Claim 1.

3. In the second heating step, the pressing force set in the pressing step is decreased stepwise or continuously as the temperature of the steel material rises. The method for electric current heating of steel materials according to Claim 1 or 2.

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