JPH0734131A - Induction heating method for slab - Google Patents

Abstract

PURPOSE:To heat steel slabs for hot rolling uniformly over the entire part by inductively heating the parts overheated in a gas heating furnace while cooling these part with a cooling gas at the time of heating the steel slabs in the gas heating furnace and an induction heating furnace. CONSTITUTION:After the plural steel slabs 1 are heated in the gas heating furnace 6, the slabs are transferred toward the induction heating furnace 2 by the table for slab transfer. The surface temp. distributions of the slabs 1 are measured by a slab surface thermometer group during the course of the transfer and the overheated parts of the slabs 1 are detected. The slabs 1 are transferred from the table 5 for transfer into the induction heating furnace 2 by a slab transfer machine 8 and are energized and reheated by an induction coil 4. The overheated parts of the slabs 1 detected by the thermometer group 7 are inductively heated while these parts are cooled by blowing the cooling gas, such as gaseous nitrogen of <=5% oxygen content thereto from a piping 3. The temp. distributions of the surfaces of the slabs 1 are measured by the thermometer placed in the furnace even during the induction heating and the cooling gas is blown to the high-temp. parts, by which the steel slabs heated to the uniform temp. distribution are obtd. The slabs are then hot rolled by a roughing mill 9.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for heating steel products such as slabs for grain-oriented electrical steel sheets immediately before hot rolling, and particularly, an induction heating furnace (commonly called an I furnace. It is a name for standing and heating in a furnace in a shape, and the slab width direction corresponds to the height in the furnace).

[0002]

2. Description of the Related Art Grained electrical steel sheets (hereinafter referred to as "Grain O")
The slab, which is an abbreviation of lented (GO material), is heated to 1350 ° C. or higher before hot rolling. The reason for induction heating the GO slab in the I furnace is that the slab structure is not coarsened,
This is for re-dissolving the inhibitor precipitate. If I
If a non-uniform heating part occurs inside the GO slab in the furnace, defective magnetic properties (hereinafter referred to as “magnetic defect”) due to insufficient re-solidification of the inhibitor, rolling trouble due to partial melting of the GO material, and troubles with frequent surface defects. And so on.
As a countermeasure, conventionally, the induction heating coil of the I furnace is divided into a plurality of stages in the vertical direction (Fig. 6), and the input power to each coil is adjusted to eliminate the non-uniformity of the temperature in the GO slab width direction. I am trying.

The heating of the GO slab is performed by first heating the GO slab in the gas combustion type preheating furnace at about 1200 ° C., reinserting the GO slab extracted from the GO slab into the induction heating furnace, and further heating it to a higher temperature. In the induction heating, as disclosed in Japanese Patent Laid-Open No. 51-22136, the induction heating increases the magnetic flux density in the central portion of the slab and causes overheating, so that the overheating portion is cooled. It was proposed to carry out induction heating while blowing gas. However, this problem could be solved without blowing gas by controlling the magnetic flux density by dividing the induction heating coil described above in the height direction of the I furnace. Further, the induction heating of the induction coil division method has a drawback that the temperature distribution in the longitudinal direction of the slab becomes uneven. To solve this problem, Japanese Patent Laid-Open No.
As described in No. 11717, partition walls are provided at several locations in the longitudinal direction of the furnace to partition the interior of the furnace, and a slab is heated and homogenized while flowing a gas whose flow rate is controlled in each room. A method has been proposed.

However, all of these existing slab temperature control methods uniformly eliminate the temperature unevenness that occurs during induction heating, and cannot eliminate local heating unevenness in the longitudinal and width directions of the slab. Was the way. The reason for this is that in induction heating, there are two types: temperature unevenness that occurs during heating of the slab and temperature unevenness that results from temperature unevenness that already exists when the induction heating furnace is charged. It has been the former problem, that is, the problem during slab induction heating, that has been proposed as a measure for eliminating temperature unevenness. Furthermore, the temperature difference between the surface and the inside of the slab that occurs during induction heating can be erased by setting a holding time after heating.

On the other hand, regarding the latter problem, that is, the temperature unevenness caused by the heating of the slab in the gas combustion type preheating furnace, the contact portion between the hearth skid and the slab is heated higher than the other portions on the low temperature side. It occurs due to the difficulty, and the high temperature side occurs because the temperature near the combustion flame of the burner is preferentially increased. Then, this high-temperature part is carried over when induction heating is performed in the next step and becomes even hotter (overheated) than other parts, leading to problems such as excessive oxidation of the slab surface and increase of scale loss, and production of GO material. It also has a negative impact on both quality.

[0006]

In view of the latter problem described above, the present invention eliminates the overheated portion generated before the slab is inserted into the induction heating furnace in the induction heating furnace before hot rolling. The purpose is to propose a method for making the temperature of the entire slab uniform.

[0007]

Means for Solving the Problems The inventor has made extensive studies in order to achieve the above object. However, in the normal heating of the slab in the I furnace, when a trouble such as a broken pipe of the seal gas on the hearth or the furnace occurs. We focused on the experience that the seal gas was directly blown onto the slab and that part was cooled. The present invention is
This was done based on that experience, and local cooling is performed while blowing a cooling gas to the slab overheating portion during induction heating. That is, the present invention, after heating the slab in the preheating furnace, the furnace wall is divided into a plurality of longitudinal and lateral directions,
When inserting into an induction heating furnace in which a gas outlet is provided in each section and performing induction heating, the temperature distribution of the slab surface is measured before insertion into the induction heating furnace, and the slab is overheated by the preheating. It is an induction heating method for a slab, which is characterized by detecting a portion and performing induction heating while blowing a cooling gas from a gas outlet near the overheated portion. Furthermore, in order to ensure the effect, the present invention detects the high temperature portion by measuring the temperature distribution of the slab surface during the induction heating, and the induction heating while blowing the cooling gas from the gas outlet near the high temperature portion. The method for induction heating of a slab according to claim 1, wherein the method is continued.

In this case, oxygen is used as the cooling gas.
Gases containing less than 100% are suitable.

[0009]

In the present invention, when heating the slab before hot rolling, temperature unevenness that has already existed prior to insertion into the induction heating furnace is detected in advance, and that portion is locally gas cooled in the induction heating furnace. However, since the entire slab is heated, the temperature of the entire slab becomes uniform. Further, even in the induction heating furnace, the temperature distribution of the slab is measured to confirm the homogenization and the high temperature part is locally cooled, so that the homogenizing effect is further promoted. The contents of the present invention will be described below with reference to FIGS.

First, FIG. 2 shows an equipment layout for slab heating. As shown in FIG. 2, when the GO slab 1 preheated in the gas heating furnace 6 is conveyed to the I furnace 2, the surface temperature distribution of the GO slab is measured by the surface thermometer group 7 installed on the way. The measurement result is shown in FIG. 3, which detects the overheated portion of the GO slab generated in the gas preheating furnace. This overheated portion corresponds to the portions a, b and c in FIG. Referring back to FIG. 2, the GO slab 1 is loaded into the I furnace 2 by the slab transfer machine 8 and the induction heating is started. Next, a gas (with a content of [O ₂ ] ≦ 5%) having a predetermined flow rate is ejected from the cooling gas block 15 shown in FIG. 1 and is blown to the above-mentioned overheated portion of the GO slab for a predetermined time in the furnace. After that, as shown in FIG. 4, thermometers (thermometers themselves are not shown) installed at a plurality of positions in the I-furnace 2,
The GO slab surface temperature distribution during the induction heating is measured, and it is rechecked whether or not the above-mentioned heating unevenness is eliminated. If not solved, the above gas blowing action is further added to the high temperature portion.

Here, the cooling gas is preferably [O ₂ ] ≦ 5%. As shown in FIG. 5, the oxygen content of the cooling gas is 5%.
In the case of the above, excessive oxidation of the slab surface occurs in the I furnace, which lowers the cooling capacity and causes a defective undercoating in the product. Furthermore, during heating in the I furnace, the scale loss from the GO slab increases, the yield decreases, and the operational troubles due to the generated slag also increase.

[0012]

EXAMPLES C: 0.05%, Si: 3.5%, Mn:
0.08%, Se: 0.025%, Sb: 0.030
%, N: 25 ppm of the GO slab was inserted into the gas preheating furnace 6 and heated in a heating cycle with an extraction temperature target of 1180 ° C. After extracting the slab 1 from the gas heating furnace 6 after a predetermined time, the slab surface temperature was measured by the surface thermometer 7 shown in the equipment layout of FIG. The result was as shown in FIG. Next, this GO slab is charged into the I furnace 2,
(B), (E) block 1 of the gas cooling device 2 shown in FIG.
5, N ₂ gas with a flow rate of 10 Nm ³ / hr (O ₂ ≦ 1.
1) from the (K) block with a flow rate of 15 Nm ³ / hr N
_Two gases were sprayed onto the surface of the overheated part of the GO slab.
After 15 minutes, when the surface temperature of the slab was measured in the I-furnace 2, the portion A in FIG. 3 was still 10 ° C. higher than the peripheral portion.
8 Nm ³ / h from the (B) block of the gas cooling device of FIG.
R ₂ gas was blown for 15 minutes. The hot coil after rolling the GO slab subjected to the above-mentioned treatment and the hot coil after the rolling of the GO slab extracted from the gas heating furnace in the same manner and not subjected to the above-mentioned treatment at all were measured for the iron loss value level in the longitudinal direction. A comparison is shown in FIG. The hot coil treated according to the present invention has a more uniform iron loss level in the longitudinal direction of the coil.

[0013]

As described above, according to the present invention, the temperature of the entire slab is made uniform in the induction heating furnace before hot rolling.
The iron loss value of the rolled product became uniform in the longitudinal direction, and the product acceptance rate was increased by 10% compared with the past.

[Brief description of drawings]

FIG. 1 is a gas piping diagram for local cooling of an in-furnace slab according to the present invention (a side view seen from the slab 1 surface side in FIG. 6).

FIG. 2 is an equipment layout between a gas heating furnace of a GO slab, heating of an I furnace, and rough rolling.

FIG. 3 is an example of a slab surface temperature distribution before charging into an I furnace.

FIG. 4 is a view showing the installation state of a radiation thermometer for controlling heating of the I furnace according to the present invention.

FIG. 5 shows the relationship between the oxygen concentration in the cooling gas, the yield of quality defects, and the occurrence rate of operational troubles.

FIG. 6 shows a conventional slab charging situation in an I-furnace and an induction heating coil installation situation.

FIG. 7 is an iron loss value in the coil longitudinal direction.

[Explanation of symbols]

1 slab (GO slab) 2 induction heating furnace (I furnace) 3 cooling gas pipe 4 induction coil 5 slab transfer table 6 gas combustion type preheating furnace 7 slab surface thermometer group 8 slab transfer machine 9 rough rolling mill 10 gas flow rate Total 11 Gas flow control valve 12 Gas shutoff valve 13 One side of slab 14 Three side of slab 15 Cooling gas nozzle block (from A to L 12
There are pieces)

Claims (2)

[Claims] 1. After heating a slab in a preheating furnace, the furnace wall is divided into a plurality of parts in the vertical and horizontal directions, and the induction heating is performed by inserting the slab into an induction heating furnace in which a gas outlet is provided in each division part. Before insertion into the heating furnace, the temperature distribution of the slab surface is measured, and the overheated portion of the slab due to the preheating is detected,
An induction heating method for a slab, characterized by performing induction heating while blowing a cooling gas from a gas outlet close to the overheating portion. 2. The induction heating is continued while the temperature distribution of the slab surface is measured during the induction heating to detect a high temperature portion, and the cooling gas is blown from the gas outlet near the high temperature portion. The slab induction heating method according to claim 1.