JP4258341B2 - Manufacturing method of high-strength steel sheet with excellent material uniformity in the longitudinal direction of the steel sheet - Google Patents

Description

  The present invention relates to a method for manufacturing a high-strength steel sheet that is tempered on a line using an induction heating device installed on the rolling line after hot rolling, accelerated cooling or direct quenching.

  Recently, the use of high-strength steel is increasing in steel structures such as pipelines, buildings and bridges. On the other hand, due to the high-precision design in these applications, there is an increasing demand for steel plates with small variations in material properties. In particular, a high-strength steel sheet having a narrow strength range in which the variation in strength in the longitudinal direction of the steel sheet is within a certain range has been required.

  In general, high-strength steel sheets are manufactured by controlled rolling or accelerated cooling after rolling. In controlled rolling, the austenite is not strongly recrystallized in the non-recrystallization temperature region or in the two-phase region, and therefore, uneven strength is likely to occur due to the difference in recrystallization rate and phase fraction due to temperature variation in the longitudinal direction of the steel sheet. Further, even with accelerated cooling after rolling, non-uniformity in strength occurs due to variations in the cooling start temperature and cooling stop temperature in the longitudinal direction of the steel sheet.

  As a countermeasure for reducing these variations, a tempering process in which a steel plate is inserted into a gas combustion furnace and heated to a uniform temperature has been performed. This method uses the fact that the difference in strength after tempering is reduced because the amount of softening due to tempering is generally greater in the portion having higher strength before tempering. However, in the case of the conventional tempering process, since the softening is performed up to a portion that does not need to be softened, there is a limit in reducing the variation in strength, and in some cases, the strength may be lower than the standard strength. Furthermore, there is a problem that the production efficiency is poor due to the offline heat treatment.

Patent Document 1 (Japanese Patent Laid-Open No. 2003-27136) describes a method of reducing the variation in the material properties of a steel sheet by performing a tempering process with an induction heating device. This makes the strength in the longitudinal direction of the steel sheet uniform by changing the heating conditions in the longitudinal direction of the steel sheet and performing induction heating.
JP 2003-27136 A

  The tempering by induction heating described in Patent Document 1 (Japanese Patent Application Laid-Open No. 2003-27136) is capable of tempering only the front end portion and tail end portion of the steel plate. It is an effective means. A dramatic improvement in production efficiency can also be expected by installing an induction heating device on the rolling line and performing tempering online.

  However, when heating while conveying steel on the line where the induction heating device is installed, the waiting time until induction heating by the first induction heating device is different at the tip and tail of the steel, A temperature gradient is produced in the longitudinal direction of the steel sheet by heat dissipation. FIG. 1 schematically shows a temperature distribution in the longitudinal direction of a steel material immediately before being conveyed to the induction heating device. When such a temperature gradient is present, complicated temperature control is required in order to change the heating conditions in the longitudinal direction in accordance with the temperature gradient so as to obtain a uniform temperature after heating and to perform tempering treatment. For example, it is conceivable that the temperature of each part divided in the traveling direction of the steel material is measured, and the optimum heating power of the induction heating device is repeated each time using the temperature value and calculated by an analysis method. However, in this calculation method, the calculation amount is very large and the load on the computer is large.

The present invention was made in order to solve the above-mentioned problems. After hot rolling, accelerated cooling or direct quenching, and then a thick steel plate that is tempered using an induction heating device installed on a rolling line. It is an object of the present invention to provide a manufacturing method that reduces variations in strength in the longitudinal direction of a steel sheet and does not require an analysis method that requires a large amount of calculation such as complicated temperature control and repeated calculation.

The above problems are solved by the following invention. The invention relates to a method for producing a high-strength steel sheet that is subjected to tempering using an induction heating device installed on a rolling line after hot rolling, accelerated cooling or direct quenching, and the first stage induction heating device is entered. Measure the temperature of the top part of the steel material with a temperature detector provided on the side, calculate the target heating temperature for each induction heating device and determine the power to be supplied to the top part of the steel material,
In the part after the head of the steel material, a new target heating temperature is calculated by correcting the target heating temperature for each induction heating device according to the temperature difference between the temperature detected by the temperature detector and the head part. And determining the new power to be supplied to the part behind the head,
In accordance with the movement of the steel material, there is a step of controlling and supplying the determined power to each induction heating device, and a method for producing a thick steel plate having excellent material uniformity in the longitudinal direction of the steel plate is there.

As another invention, in the method of manufacturing a high strength steel sheet, after hot rolling, accelerated cooling or direct quenching, and then tempering using an induction heating apparatus installed on the rolling line, the first stage induction heating The step of measuring the temperature of the leading portion of the steel material with a temperature detector provided on the entry side of the device, calculating the target heating temperature for each induction heating device, and determining the power supplied to the leading portion of the steel material When,
From the measured temperature of the top portion of the steel material, the temperature of the tail end portion of the steel material is estimated, a new target heating temperature is calculated by correcting the target heating temperature for each induction heating device, and the tail end of the steel material Determining the power supplied to the part;
In the intermediate portion sandwiched between the head portion and the tail end portion of the steel material, each induction is based on the measured temperature of the tip portion, the estimated temperature of the tail end portion, and the measured temperature of each of the longitudinal intermediate portions. Correcting a target heating temperature for each heating device to calculate a new target temperature, and determining a new power to be supplied to the intermediate portion;
In accordance with the movement of the steel material, there is a step of controlling and supplying the determined power to each induction heating device, and a method for producing a thick steel plate having excellent material uniformity in the longitudinal direction of the steel plate is there.

In these inventions, the step of measuring the longitudinal temperature of the steel sheet after induction heating by a temperature detector installed between the induction heating devices and correcting the target heating temperature of the next induction heating device from the measured temperature When,
And a method of determining a new electric power to be supplied to the next induction heating device based on the corrected target heating temperature. it can.

The above invention is a method for producing a thick steel plate, which is subjected to tempering using an induction heating device installed on a rolling line after hot rolling, accelerated cooling or direct quenching, and a temperature gradient in the longitudinal direction of the steel plate. By controlling the electric power supplied to the induction heating device according to the above, variation in strength in the longitudinal direction of the steel sheet is reduced. In any of the inventions, the temperature of the leading portion of the steel material is measured, the target heating temperature for each induction heating device is calculated based on the measured value, and the power supplied to each induction heating device is controlled. Yes.

According to the present invention, high-strength excellent in material uniformity in the longitudinal direction of the steel sheet is obtained by performing tempering using an induction heating device installed on the rolling line after accelerated cooling or direct quenching of the steel sheet. A steel plate can be manufactured. In particular, high-precision temperature control can be performed without applying an excessive load to the control device that calculates the set power value of the induction heating device.

  Below, the control method for implementing this invention is demonstrated in detail.

  FIG. 2 is a side view showing an example of a schematic configuration of the induction heating apparatus according to the embodiment of the present invention. A plurality of induction heating devices 1 are installed on the line, and a steel material 2 as a material to be heated is heated by each induction heating device 1 while being conveyed from left to right in the figure.

  A temperature detector 3 is provided on the entrance side of the first induction heating device 1 to detect the temperature of the steel material 2 before heating. The detected temperature is input to the control device 4, and the control device 4 calculates the amount of power to be supplied to each induction heating device 1 from the temperature of the steel material 2, and sets the power to the power supply device 5 as a set value. Output. And the electric power supply apparatus 5 controls the electric power of the induction heating apparatus 1 so that it may become a setting value from the control apparatus 4. FIG.

  The control device 4 receives a transport pulse from the transport roller 7, and the control device 4 calculates the transport speed and transport amount of the steel material based on the pulse signal. Moreover, the temperature detector 8 is provided in the exit side of each induction heating apparatus, and it is comprised so that the temperature of the heat-treated steel material 2 can be monitored.

  Next, a method for controlling the temperature of the steel material using the induction heating apparatus of this configuration will be described.

  In the present invention, in order to accurately control the temperature in the moving direction of the steel material, the steel material is virtually divided into a plurality of parts (hereinafter referred to as “virtual parts”) to manage the temperature. The dotted lines marked on the steel material in FIG. 2 indicate the boundaries of the virtual part. The numbers i−1, i, i + 1 described in the virtual part represent the order from the top of the steel material 2.

When a steel material is heated using a plurality of induction heating devices, the heating target temperature in each induction heating device is a temperature restriction condition (for example, the surface temperature is Ac ₁ transformation) that is subject to the final target temperature, power consumption, and heat treatment of the steel material It is determined by factors such as temperature and below. Usually, these conditions are instructed in advance by a host computer or the like for each steel material as a standard for heat treatment, and are input to the control device.

[Supply power control method 1]
First, a procedure for determining the electric power supplied to each induction heating device according to the first embodiment will be described.
(S1) The final target temperature of the steel material to be heated is taken out from the control device and determined.
(S2) When the steel material is transported and passes a predetermined position, a passage detector (not shown) detects “material present” and outputs a signal to the control device. At this timing, the control device reads the temperature of the top portion of the steel material and the conveyance speed of the steel material.
(S3) The heating target temperature in each induction heating device is determined by a method such as heat transfer calculation.
(S4) Of the plurality of induction heating devices installed in the line, the heating target temperature of the jth induction heating device from the top is Tt (j), and the electric energy Pt (j) for obtaining Tt (j) is calculate.
(S5) The calculated set power Pt (j) is output to the power supply device in synchronization with the movement of the leading portion of the steel material.
(S6) The temperature of the i-th imaginary part of the steel material is read as the steel material is conveyed and the longitudinal position of the steel material changes.
(S7) The control device corrects the heating target temperature Tt (j) from the actually measured temperature, and calculates a new heating target temperature Ti (j) for the i-th virtual part from the head.
(S8) New electric power Pi (j) supplied to the induction heating device j is calculated from the new heating target temperature Ti (j).

  Here, Pi (j) is expressed by the following equation.

Pi (j) = Pt (j) + ΔPi
ΔPi is power that gives a temperature increase amount of ΔT = Ti−Tt Pt: leading partial power, Pi: power at virtual portion i Tt: leading portion temperature, Ti: temperature of virtual portion i (S9) i-th of steel material The calculated power Pi (j) is output to the power supply apparatus in synchronization with the transport of the virtual part.

  In this way, the effect of the initial temperature difference ΔT can be eliminated by changing the heating target temperature and using all induction heating devices, and the steel material can be uniformly heated, and the load on the induction heating device can be reduced. Can be planned.

[Supply power control method 2]
Next, a procedure for determining the power supplied to each induction heating device according to the second embodiment will be described.
(S1) The final target temperature of the steel material to be heated is taken out from the control device and determined.
(S2) When the steel material is transported and passes a predetermined position, a passage detector (not shown) detects “material present” and outputs a signal to the control device. At this timing, the control device reads the temperature of the top portion of the steel material and the conveyance speed of the steel material.
(S3) The temperature of the tail end portion is estimated from the temperature of the head portion. The temperature estimation method of the tail end part assumes that the temperature at the head and tail end after cooling is the same at the same time, the same position, that is, the head and tail end until entering the induction heating device. Estimate how much the temperature drop due to cooling from the time difference. In addition, in order to increase the accuracy, it is also possible to store the temperature difference between the head portion and the tail end portion of the steel material under the same conditions in the past and estimate the temperature of the tail end portion based on the value. .
(S4) The heating target temperature in each induction heating device for obtaining the final target temperature for the head portion and the tail portion is determined by a method such as heat transfer calculation.
(S5) Of the plurality of induction heating devices installed in the line, the heating target temperature of the leading portion of the jth induction heating device from the head is Tt (j), and the heating target temperature of the tail end portion is Tb (j). , Tt (j) and Tb (j) are calculated as electric power amounts Pt (j) and Pb (j).
(S6) The calculated set power Pt (j) is output to the power supply device in synchronization with the movement of the leading portion of the steel material.
(S7) The temperature of the i-th imaginary part of the steel material is read as the steel material is conveyed and the longitudinal position of the steel material changes.
(S8) The control device corrects the heating target temperature Tt (j) from the measured temperature, and calculates a new heating target temperature Ti (j) for the i-th virtual part from the head.
(S9) New electric power Pi (j) supplied to the induction heating device j is calculated from the new heating target temperature Ti (j).

  Here, Pi (j) is expressed by the following equation.

Pi (j) = (Pt (j) −Pb (j)) / (Tt−Tb) * (Ti−Tt) + Pt (j)
Pt: head partial power, Pb: tail end partial power, Pi: power at virtual part i Tt: tip part temperature, Tb: tail end part temperature, Ti: temperature of virtual part i (S10) i-th virtual of steel The calculated power Pi (j) is output to the power supply device in synchronization with the conveyance of the portion.

  Either of the two methods described above may be used, and can be appropriately selected according to the layout of the device configuration, the capability of the arithmetic device, and the type of steel material (the degree of temperature distribution in the longitudinal direction according to the manufacturing method). In particular, when the temperature change in the longitudinal direction of the steel material is large, if the first embodiment [Supply power control method 1] is used, the calculation error increases as the tail end position is reached. The control method 2] allows more accurate control.

[Supply power control method 3]
A method for performing temperature control with higher accuracy by incorporating FF control into the above embodiment [Supply power control methods 1 and 2] will be described as a third embodiment.

  When the power setting is performed using the mathematical model as in the first and second embodiments, an error may occur in the temperature due to the mathematical model error. For this reason, the temperature of the steel material after heating is measured with a temperature detector installed on the outlet side of the induction heating device, and the electric power is corrected based on the actually measured temperature.

  In FIG. 3, the FF control power calculation device 18 corrects the power based on the measurement signal of the temperature detector 3 installed on the entry side of each induction heating device (including the power supply device) 6. With this correction output, the induction heating device (including the power supply device) 6 adjusts the amount of power supply with this correction output, and controls the temperature of the steel material.

  For example, in the first and second embodiments 1 and 2 [supplied power control methods 1 and 2], the heating target temperature Tt (j) at the head of the jth induction heating device and before the (j + 1) th induction heating device When a difference occurs in the measured temperature of the head portion measured by the installed temperature detector, the power correction is necessary in the (j + 1) th induction heating device in order to compensate for this temperature difference. That is, a new heating target temperature Tt (j + 1) is calculated based on the actually measured temperature, and a new power Pt (j + 1) supplied to the induction heating device (j + 1) is calculated from the new heating target temperature Ti (j + 1). .

By performing such FF control, temperature control with higher accuracy becomes possible, and a high-strength steel sheet excellent in material uniformity in the longitudinal direction can be manufactured.
In addition, although the said embodiment demonstrated the Example of the steel plate with the temperature distribution which temperature fell gradually in the longitudinal direction of a steel plate, this invention is not limited to this embodiment.

For example, depending on the accelerated cooling conditions, the temperature may be lower at the leading end and the tail end of the steel plate than at the intermediate portion. In such a case, the variation in temperature distribution in the longitudinal direction can be eliminated by setting the heating target temperatures at the head and tail ends higher than those at the intermediate portion. The length of the head and tail ends to be heated to a high temperature is set in advance based on conventional knowledge depending on the type of steel sheet and manufacturing conditions, and the length is heated to a constant temperature. The intermediate portion may be heated by the method of Embodiment 1 or 2 [Supply power control method 1, 2].

  Examples of the present invention will be described. After hot-rolling steels having chemical components shown in Table 1, accelerated cooling was performed to produce various steel plates. The manufacturing conditions are shown in Table 2. The steel sheet after rolling was tempered using an induction heating apparatus arranged in series on the line, and the strength distribution in the longitudinal direction of the steel sheet was measured by a tensile test.

Table 3 shows the tensile test results. By performing tempering with an induction heating apparatus using the temperature control method of the present invention, the variation in strength in the longitudinal direction of the steel sheet was greatly reduced.
In particular, when FF control was combined, the variation was the smallest.

The figure which shows typically the temperature distribution of the longitudinal direction of the steel materials just before conveyed by an induction heating apparatus. The side view which shows an example of schematic structure of the induction heating apparatus which concerns on embodiment of invention. The figure which shows the structure of FF control of an induction heating apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Induction heating apparatus 2 Steel material 3 Temperature detector 4 Control apparatus 5 Electric power supply apparatus 6 Induction heating apparatus (Including electric power supply apparatus)
7 Transport roller 8 Temperature detector (exit side)
18 FF control power calculation device

Claims (3)

After hot rolling, after accelerated cooling or direct quenching, in a method for producing a high-strength steel sheet that performs tempering using an induction heating device installed on a rolling line,
Measure the temperature of the top part of the steel material with a temperature detector provided on the entrance side of the induction heating device in the first stage, calculate the target heating temperature for each induction heating device, and supply the power to the top part of the steel material A step of determining
In the part after the head of the steel material, a new target heating temperature is calculated by correcting the target heating temperature for each induction heating device according to the temperature difference between the temperature detected by the temperature detector and the head part. And determining a new power to be supplied to the portion after the head,
A method for producing a high-strength steel sheet excellent in material uniformity in the longitudinal direction of the steel sheet, comprising the step of controlling and supplying the determined power to each induction heating device in accordance with the movement of the steel material .
After hot rolling, after accelerated cooling or direct quenching, in a method for producing a high-strength steel sheet that performs tempering using an induction heating device installed on a rolling line,
Measure the temperature of the top part of the steel material with a temperature detector provided on the entrance side of the induction heating device in the first stage, calculate the target heating temperature for each induction heating device, and supply the power to the top part of the steel material A step of determining
From the measured temperature of the top portion of the steel material, the temperature of the tail end portion of the steel material is estimated, a new target heating temperature is calculated by correcting the target heating temperature for each induction heating device, and the tail end of the steel material Determining the power supplied to the part;
In the intermediate portion sandwiched between the head portion and the tail end portion of the steel material, induction heating is performed based on the measured temperature of the tip portion, the estimated temperature of the tail end portion, and the measured temperature of each of the intermediate portions. Correcting the target heating temperature for each device, calculating a new target heating temperature, and determining new power to be supplied to the intermediate part; and
A method for producing a high-strength steel sheet excellent in material uniformity in the longitudinal direction of the steel sheet, comprising the step of controlling and supplying the determined power to each induction heating device in accordance with the movement of the steel material .
Measuring the temperature in the longitudinal direction of the steel sheet after induction heating by a temperature detector installed between the induction heating devices, and correcting the target heating temperature of the next induction heating device from the measured temperature;
3. A step of determining a new electric power to be supplied to the next induction heating device based on the corrected target heating temperature, and having a high material uniformity in the longitudinal direction of the steel sheet according to claim 1 or 2. A method for producing a strength steel plate.

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