JPH0315189A - Induction heating method for slab - Google Patents

Abstract

PURPOSE:To prevent corner sections from being insufficiently heated, and thereby obtain an uniform distribution in temperature in a range including interiors by selecting current energizing a heating coil within a specified range determined by the thickness and the specific resistance of a slab. CONSTITUTION:A slab 1 is concentrically arranged in the inside of a rectangular coil 2 which is winded around in such a way that it is in a form roughly similar to the axial cross section of the slab 1 to be heated, so that the specified magnitude of current is applied to the heating coil 2 for the specified period of time. The both ends of the coil 2 are connected with a heating power supply 3 through a frequency converting section 4 so that current with frequencies converted by the converting section 4 is thereby applied to the coil. Namely, it is designed that current with frequencies f(Hz) within the range limited by a formula which includes the thickness tw(mm) and the specific resistance rho(muOMEGA-cm) of the slab 1, is applied to the coil 2. This permits the depth in generating induction current within the slabe 1 to be optimized so that the uniform distribution in temperature can thereby be obtained in the range from the surface to the center section in the direction of thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an induction heating method for uniformly heating a slab as a rolled material, particularly a titanium slab, to a predetermined temperature.

[Conventional technology]

Conventional induction heating methods for induction heating slabs to the required temperature in the rolling process include, for example, rIron a
nd Steel Engineer ;Septe
umber 1979J, pages 50 to 55.

FIG. 6 is a schematic perspective view showing the implementation state of this induction heating method. In the figure, reference numeral 1 denotes a slab to be heated, and the slab 1 is placed in a rectangular heating coil 2 whose outer shape is approximately similar to that of the slab 1, so that each side of both sides is parallel to each other with a predetermined gap between them. arranged concentrically. Both ends of the heating coil 2 are connected to a heating power source 3, and after placing the slab 1 as described above, by passing current generated from the heating power source 3 through the heating coil 2, the slab 1 is heated by electromagnetic induction.
An induced current is generated near the surface of the slab 1, and the slab 1 is heated by the heat generated as the current flows. As the heating power source 3, a commercial AC power source is generally used, and in this case, the heating coil 2 has a commercial frequency (5011z or 60Hz).
z) is applied.

[Problem to be solved by the invention]

Now, when inductively heating slab 1 in this way,
In order to improve the plate thickness accuracy after rolling, which is a subsequent process, it is strongly desired that the slab 1 be heated uniformly over the entire cross section.
In this field, uniformity of heating conditions is an important issue. On the other hand, an important factor in determining the quality of the heating state is how deep the induced current is generated from the surface of the slurp 1 when the heating coil 2 is energized. Although the vicinity is sufficiently heated, there is insufficient heat transfer in the central part in the thickness direction, resulting in an insufficiently heated part near the center, resulting in a large temperature gradient in the thickness direction of the slurp l. On the other hand, if the induced current is generated at a deep depth, a relatively uniform temperature distribution can be obtained in the thickness direction of the slab 1, but as a result of inhibiting the flow of the induced current at the corners, each There is a drawback that local heating defects occur near the corners.

The generation depth δ (mm) of the induced current is given by the following well-known formula.

δ− 50.3 Jρ/ (μ・f)・
...(11 However, ρ is the specific resistance (μΩ-cm), which is a physical property value specific to the material of the slab 1 to be heated, μ is also the magnetic impermeability, and r is the frequency (Hz) of the heating power source 3. For example, when a titanium slab 1 is placed in a heating coil 2 connected to a heating power source 3 with a commercial frequency of 60 Hz and heated by induction, the specific resistance ρ of the titanium material at room temperature to 950°C is 140 μΩ. -cm, and since the non-permeability μ is 1, the induced current generation depth δ is 76.8 mm from equation (1). Thickness of slab 1 of 150+n+
In the conventional induction heating method that uses a commercial power source as the heating power source 3, heating defects occur at the corners of the slab l, making it impossible to obtain a uniform heating state. There was a problem in that it was difficult to obtain the desired thickness accuracy when rolling the slab 1 in a subsequent process.

The present invention has been made in view of the above circumstances, and provides an induction heating method for slabs that prevents the occurrence of heating defects at corners and makes it possible to obtain as uniform a temperature distribution as possible throughout the interior. The purpose is to provide

[Means to solve the problem]

In the induction heating method of a slab according to the present invention, a slab to be heated is placed in a rectangular heating coil that is arranged concentrically inside the slab, and the slab thickness tw (mm) and the specific resistance ρ (μΩ1c
Induction heating is performed by passing a current having a frequency f (Hz) within the range defined by the formula including m).

[Effect]

In the present invention, the heating coil has a frequency f within the above range.
By passing a current with a current of
This also prevents the occurrence of heating defects at corners.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on drawings showing embodiments thereof. FIG. 1 is a schematic perspective view showing the implementation state of the method for induction heating a slab according to the present invention (hereinafter referred to as the method of the present invention).

In carrying out the method of the present invention, as in the conventional method, a rectangular heating coil 2 is wound so as to be substantially similar to the axial cross section of the slurp 1 to be heated, and a heating power source 3 is used as a power source for energizing the coil. However, in the method of the present invention, unlike the conventional method, both ends of the heating coil 2 are not directly connected to the heating power source 3, but a frequency converter 4 using a thyrisk inverter or the like is used. The heating coil 2 is connected through the heating coil 2, and a current having a frequency converted by the frequency converter 4 is applied to the heating coil 2. In the method of the present invention, as shown in the figure, a slab l is placed inside the heating coil 2.
are arranged concentrically, and each side of the slab 1 in the width direction and thickness direction and each side of the heating coil 2 in the longitudinal direction and a direction perpendicular thereto are substantially parallel with a predetermined gap between them. This is carried out by energizing the heating coil 2 with a predetermined magnitude of current for a predetermined period of time.

Due to this energization, an induced current is generated inside the slab 1 by electromagnetic induction, and the slab 1 is heated by the heat generated by the energization of this induced current, but as described above, the current applied to the heating coil 2 is In order to optimize the generation depth δ of the induced current in the slab 1 instead of the commercial frequency (50Hz or 6011z) in the heating power source 3, the frequency converter 4
This provides the slab 1 with a uniform temperature distribution in the thickness direction up to the inside and prevents the occurrence of poorly heated portions near the corners.

As shown in (1) 2 above, the induced current generation depth δ in the slab 1 is determined by the specific resistance ρ, the non-permeability μ, and the heating coil 2.
It is determined by the frequency r of the current flowing through the heating coil 2, and among these values, ρ and μ are physical property values specific to the slab 1, so the generation depth δ of the induced current and the frequency of the current flowing through the heating coil 2 There is a unique correspondence between them. On the other hand, the induced current generation depth δ is closely related to the temperature distribution within the slab 1, as described above. Focusing on the above, the present inventor has developed a general thickness dimension (150
A heating experiment was conducted by applying various frequencies f to the current flowing to the heating coil 2 in the slab 1 having a thickness of 2 mm to 250 ml, and from the results, the thickness tw (mm to 250 ml) of the slab 1 was determined.
m), that is, as shown in Figure 1, the following equation includes the length of the shorter side in the axial cross section of the slab 1 and the specific resistance ρ (μΩ-cm), which is a physical property value specific to the material of the slab 1. It has been found that at a frequency f (Hz) within the range shown, the induced current generation depth becomes appropriate and a desirable heating state is realized.

Some of the experimental results are shown in FIGS. 2 to 4.

All of these are 200mm thick, 1000mm wide,
A titanium slab 1 with a length of 8,000 mm is heated for about 60 minutes to raise the temperature from room temperature to approximately 800'c, and then heated for about 60 minutes from one corner E of the slab 1 in the width direction.
It shows the results of measuring the temperature distribution from the surface to the center in the thickness direction in the +nn+ range.
Figure 2 shows that the frequency loss f of the current flowing to the heating coil 2 is 60I.
1z, FIG. 3 shows the case where f is 13011z, and FIG. 4 shows the case where f is 20011z.

First, in Fig. 2, it is clear that a heating defect where the temperature measurement result is lower than 800°C occurs near the corner E, but on the other hand, there is a heating defect in the area from the surface to the center in the thickness direction. A relatively uniform temperature distribution was obtained throughout. This is because the frequency f is too small. On the other hand, in Fig. 4, a temperature measurement result of about 820°C to 840°C is obtained near the corner E, and no heating defects occur, but on the other hand, the temperature near the surface is high and the thickness is high. There is a large temperature gradient that decreases toward the center in the horizontal direction. This is because the frequency f is excessive.

On the other hand, in FIG. 3, a temperature distribution close to that shown in FIG. 4 is obtained near the corner E, and no heating defects are observed, and other parts are similar to those shown in FIG. 2. near,
There is a relatively uniform temperature distribution, and from this, 1
A frequency of 30 Hz can be said to be an appropriate frequency.

FIG. 5 shows the case where currents having various frequencies f including the above-mentioned three frequencies are applied to the heating coil 2. It is a graph which shows the temperature measurement result at each temperature measurement point shown attached.

Note that ■ to ■ are all 300 mm in the width direction from the corner E.
Temperature measurement points are arranged in the thickness direction at mm positions, ■ are set at the center in the thickness direction, ■ are set near the surface, and ■ ~
Each of the temperature measuring points (2) is set as a point that approximately divides the area between (2) and (2) into four equal parts. In addition, ■ to ■ are all temperature measurement points arranged on a straight line extending from the corner E to the center in the thickness direction with an inclination of 45 degrees, and ■ is set at a position of 50 mm each in the width direction, and ■ is set very close to the corner E, and ■ is set approximately at the center between ■ and ■, and ■ is approximately at the center between ■ and ■. are set respectively.

From FIG. 5, it is clear that as the frequency r decreases, the temperature values at the temperature measurement points from ■ to ■ tend to decrease rapidly, and it is clear that a heating defect occurs near the corner E. As the frequency f increases, the
It is clear that the temperature difference between (1) and (2) tends to increase, and a large temperature gradient occurs in the thickness direction. The limiting equation for the frequency f shown in equation (2) above is set to satisfy the conditions that the temperature drop near the corner E is small and that an excessive temperature gradient does not occur in the thickness direction of the slap 1.

If the specific resistance value of titanium material 140 μΩ-cm in the temperature range from room temperature to 950°C and the thickness t, = 200 mm of the slab 1 are substituted for the specific resistance ρ in equation (2), the appropriate frequency range is , 93.501z)≦f≦171.0(}12) ={
3), which is shown in FIG. When the frequency f of the current flowing through the heating coil 2 is 60 il z, and when the frequency f of the current flowing through the heating coil 2 is 200 Hz,
(3), and in the former case, a low-temperature area occurs near the corner E as shown in Figure 2, and in the latter case, as shown in Figure 4. As described above, a large temperature gradient occurs in the thickness direction, and the temperature near the surface becomes excessively high. On the other hand, the frequency f of the conducting current
When is 130 Hz, this is a frequency within the appropriate range shown by equation (3), so as shown in FIG. 3, a substantially uniform temperature distribution is obtained inside the slab 1.

In the above explanation, the slab 1 made of titanium was described, but the formula (2) for limiting the appropriate frequency can also be applied to the slab 1 made of other materials, and the method of the present invention is applicable to the slab 1 made of other materials. Using the thickness dimension t8 representing the size and the specific grinding resistance ρ determined according to the material of the slab l, (2
) This can be easily implemented by setting the appropriate frequency f determined in step 2 in the frequency conversion section 4.

〔Effect of the invention〕

As detailed above, in the method of the present invention, a current having a frequency within the range defined by (2) including the thickness of the slab to be heated and the specific resistance of the slab is passed through the heating coil. Therefore, the generation depth of the induced current in the slab C is optimized, and a temperature distribution as uniform as possible from the surface of the slab to the center in the thickness direction is obtained, and heating defects at the corners are prevented. The present invention has excellent effects such as no occurrence of such occurrence and high plate thickness accuracy during rolling, which is a subsequent process.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view showing the implementation state of the method of the present invention, and FIGS. 2, 3, and 4 show cases in which the frequencies of the current applied to the heating coil are 60 Hz, 130 Hz, and 200 Hz, respectively.
Figure 5 showing the temperature distribution inside the slab in each case where z
The figure is a graph showing how the temperature values change at various temperature measurement points inside the slab when the frequency of the current applied to the heating coil is changed. Figure 6 shows the implementation of the conventional slab induction heating method. It is a typical perspective view showing a state. ■... Slab 2... Heating coil 3... Heating power source 4... Frequency conversion unit Note that in the drawings, the same reference numerals indicate the same or equivalent parts. 1... Slab 2... Heating coil

Claims (1)

[Claims] (1) A method in which a slab to be heated is arranged concentrically within a rectangular heating coil having a cross-sectional shape substantially similar to the slab, and the slab is induction heated by electromagnetic induction caused by energization of the coil, comprising: The frequency f (Hz) of the current applied to the heating coil is selected within the range determined by the following formula including the thickness t_w (mm) of the slab and the specific resistance ρ (μΩ-cm). Induction heating method for slabs. ▲Contains mathematical formulas, chemical formulas, tables, etc.▼