JPH06104867B2 - Method of heating grain-oriented silicon steel slabs - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for heating a grain-oriented silicon steel slab, and in particular, as a raw material for a grain-oriented silicon steel sheet having excellent magnetic properties in the rolling direction. This paper proposes the results of the development and research on the improvement of the method of uniformly heating the raw steel slab from the surface to the central part for hot rolling.

Such unidirectional silicon steel sheet is composed of secondary recrystallized grains having a plate surface (110) surface and <100> axis aligned in the rolling direction, and has excellent magnetic properties in the direction along the rolling direction. Widely used as a transformer core material.

(Prior Art) In order to develop secondary recrystallized grains having a crystallographic orientation to be used as described above, fine particles conventionally called inhibitors have been used.
The technology adopted is to disperse MnS, MnSe, AlN, and BN in the steel to suppress the growth of crystal grains in other orientations during high-temperature finish annealing.

To control the inhibitor dispersion morphology for that purpose, as is well known, during the slab heating prior to hot rolling, these precipitates are dissolved once and then hot rolled under an appropriate cooling pattern. It is necessary to apply.

By the way, heating of this kind of slab is usually performed at a high temperature of 1300 ° C or higher, and in order to realize sufficient heating up to the center of the slab, in the case of a normal gas combustion type heating furnace, the slab surface temperature is 1350 ° C. However, in this case, a large amount of molten scale is generated, which not only impairs the operability of the heating furnace but also causes surface defects and edge cracks due to grain boundary oxidation.

By the way, in response to the situation in the field mentioned above, Japanese Patent Publication No.
No. 7, in the slab heating below 1300 ℃, the required magnetic properties are not obtained, in addition to such slab heating 1350 ℃ ~ 1400 ℃, especially 1380 ℃ induction heating by an electrical method or A technique for performing resistance heating is disclosed.
However, in the case of induction heating in this known technique, local overheating is likely to occur on the surface of the slab due to the so-called skin effect, and no specific method for avoiding it is mentioned.

Generally, as the slab heating technology by the induction heating furnace, many related to the equipment and the transportation method have been proposed so far, and for example, a billet lifting device for the rigid induction heating furnace and a charging and extracting method for the rigid furnace are shown. Tajiko Ko 51-41053, 51-41053
In addition to these publications, Japanese Patent Publication No. 52-47179, which shows a heating method for preventing the temperature nonuniformity of the slab in the induction heating, and the like. However, there are few cases in which induction heating is applied to a slab that requires high temperature heating such as grain-oriented silicon steel sheet, and there is only one example of Japanese Patent Publication No. 47-14627 listed above. However, this method does not necessarily stabilize the magnetic properties of the product due to the tendency of coarsening of the slab crystal grains because heating is performed at a high temperature of 1350 to 1400 ° C in an induction heating furnace as already mentioned.

On the other hand, the present inventors have previously heated the temperature of the central portion of the slab in a normal gas combustion heating furnace until it reaches 1000 to 1230 ° C. as a method for heating the directional silicon steel slab by the induction heating furnace. Later, a technique was proposed in which the central temperature of the slab reached 1250 to 1350 ° C. in an induction heating furnace capable of controlling an inert gas atmosphere (JP-A-60-145318).

However, the technology of this prior proposal tends to raise only the temperature of the slab surface too much, which causes coarsening of the crystal grains of the surface layer, which results in coarse stretched grains in the hot rolling, resulting in strip fine grains in the final product. However, there is a problem in that it causes a magnetic defect due to.

(Problems to be Solved by the Invention) The purpose of the present invention is to perform various examinations on heating of a slab by an induction heating device, in particular, a temperature rising pattern of a surface layer and a central portion, and various problems described above if suitable frequency control is performed. Based on this finding, the slab heating was carried out at a soaking temperature suitable for hot rolling by heating and raising the surface layer and the central part to a uniform temperature based on this finding and maintaining that temperature. To achieve the establishment of advantageous technology for

(Means for Solving Problems) It is known that the current penetration depth d (cm) in high frequency heating is generally given by the following equation.

ρ: Specific resistance of slab (μΩ · cm) μ: Relative permeability of slab f: Frequency (Hz) where μ and ρ are constants determined by the material of the slab, so the current penetration depth can be changed by changing the frequency. Can be changed. That is, it can be seen that the lower the frequency is, the deeper the current penetration depth becomes, which is advantageous for uniforming the temperature distribution in the slab thickness direction. On the other hand, there is also a problem that it becomes difficult to maintain the surface temperature due to heat radiation because the current density on the surface becomes small. It is necessary to properly select the input power and the frequency so that the slab is heated uniformly in the thickness direction. Important above.

By the way, in the case of grain-oriented silicon steel slabs, as mentioned above, there is a trade-off between the need for high-temperature heating that cannot be seen in other steel types due to the shinhibitor solid solution, and the desire to suppress coarsening of slab crystal grains due to high-temperature heating as much as possible. There are needs.
Therefore, there is a strong demand for ensuring the minimum required temperature, suppressing the maximum temperature in the slab as low as possible, and performing uniform heating in the slab thickness direction. In order to find out the slab heating conditions by the induction heating furnace that satisfies such requirements, the present inventors have conducted various investigations, as a result, found a frequency range suitable for heating the grain-oriented silicon steel slab, and completed the present invention. It was

That is, according to the present invention, firstly, Si: 2.0 to 4.5.wt% Mn: 0.01
~ 0.15 wt% and 0.005 to 0.10 wt% of one or more inhibitors selected from S, Se and Al.
% Directional silicon steel slab containing, when induction heating to a soaking temperature prior to hot rolling, the frequency of this induction heating is controlled to 50 ~ 200Hz, the temperature of the slab center is 130.
By raising the temperature to 0 to 1400 ° C., and then suppressing the input power to 1/2 or less and maintaining the temperature of the center of the slab at the soaking temperature for a predetermined time, a directional silicon steel that solves the above problems. Slab heating method, secondly, when a directional silicon steel slab having the same composition as the above is induction-heated to a soaking temperature prior to hot rolling, the temperature of the central part of the slab is 1300 to 1400 ° C. The frequency of induction heating is controlled to 50 to 100 Hz to increase the temperature until the soaking temperature is reached, and then the input power is suppressed to 1/2 or less and the frequency is controlled to 100 to 200 Hz to control the temperature of the center of the slab. A method for heating a grain-oriented silicon steel slab that solves the above problems is proposed by maintaining the soaking temperature at the soaking temperature for a predetermined time.

Further, in the present invention, as a third method, when the same type of grain-oriented silicon steel slab as that described above is induction-heated to a soaking temperature prior to hot rolling, first in a gas combustion type heating furnace. The central temperature of the slab is heated to 1000 to 1230 ° C., and then the frequency in induction heating is controlled to 50 to 200 Hz to raise the temperature of the central part of the slab to 1300 to 1400 ° C., and then the input power is 1 / A method of holding the temperature of the central portion of the slab to 2 or less and maintaining the temperature of the slab at the soaking temperature for a predetermined time, and as a fourth means for solving the problems, the directional silicon steel slab is soaked prior to hot rolling. When inductively heating to a temperature, first set the center temperature of the slab to 1000 to 12 in a gas combustion type heating furnace.
After heating to 30 ℃, the temperature of the center of the slab is 1300 ~ 14
Until the soaking temperature of 00 ℃ is reached, the induction heating frequency is controlled to 50 to 100Hz to further raise the temperature, and then the input power is suppressed to 1/2 or less and the frequency is controlled to 100 to 200Hz, and the slab center A method for heating a grain-oriented silicon steel slab, which is characterized in that the temperature of the part is kept at the soaking temperature for a predetermined time.

In the present invention, the induction heating is performed in a non-oxidizing atmosphere.

(Function) The material to which the present invention is applied is selected from Si: 2.0 to 4.5 wt% (hereinafter simply expressed as “%”), Mn: 0.01 to 0.15%, and S, Se and Al as an inhibitor component. 1
It is a slab for grain-oriented silicon steel containing 0.005 to 0.10% in total of two or more kinds, and can be obtained by a continuous casting method or an ingot-slab rolling method. The slab thickness is not particularly limited, but is generally 15
Suitable for 0 to 350mm thickness.

Regarding the amount of Si, it is effective in increasing the specific resistance of the steel sheet and reducing iron loss, but if it exceeds 4.5%, the cold ductility is impaired, and if it is less than 2.0%, the iron loss improving effect is weakened and α-γ transformation Due to randomization of the crystal orientation due to, sufficient characteristics cannot be obtained.

The lower limit of the amount of Mn is set to 0.01% so as to prevent cracking due to hot brittleness, and the upper limit is set to 0.15% so as not to raise the dissociation solid solution temperature of MnS and MnSe.

S, Se and Al are finely dispersed in the steel in the form of MnS, MnSe and AlN and act as an inhibitor.The lower limit of the total amount of these is 0.
005% is the minimum amount to function as an inhibitor, the upper limit of 0.10% is mainly determined by economic reasons.

In addition to these, B, Sb, Sn,
Grain boundary segregation elements such as As, Pb, Bi, Cu and Mo are known, and their addition does not impair the effects of the present invention. Needless to say, when Al is used as the inhibitor component, the amount of N that balances with this is required.

The slab having the above components is heated prior to hot rolling, but is heated directly in an induction heating furnace, or after being heated in advance to 1000 to 1230 ° C. in a normal gas combustion type heating furnace if necessary, Especially in the induction heating furnace, the core temperature is set to 13 depending on the amount and type of the inhibitor in order to form a solid solution.
Heat between 00 ℃ and 1400 ℃ for 10 minutes or more. The former method is more advantageous in combination with the latter gas combustion type heating because it takes time to make the slab temperature uniform and the electric power cost is high. At that time, the slab center temperature during charging of the induction heating furnace is preferably set to 1000 to 1230 ° C. If the upper limit temperature is higher than that, "stickiness" will occur and cause surface defects, so it is regulated.

Further, the surface properties of the slab can be improved by setting a non-oxidizing atmosphere, particularly preferably an inert gas atmosphere, during induction heating.

Next, as for the soaking temperature, if the temperature of the central part of the slab is less than 1300 ° C, the required magnetic properties cannot be obtained, while if it exceeds 1400 ° C, the slab crystal grains become coarse and the magnetic properties deteriorate.
Limited to 00-1400 ℃.

Keep the soaking temperature at least 10 during induction heating.
Although it is set to a minute, if this is less than that time, the soaking is insufficient.

During induction heating, the temperature of the center of the slab is reached as quickly as possible to a predetermined soaking temperature of 1300 ° C or more and 1400 ° C or less (this temperature is determined by the amount of MnS, Se, Al), and during this time It is an object of the present invention to carry out uniform heating so as not to cause an abnormally high temperature, and therefore it is necessary to carry out the heating frequency at 50 Hz or more and 200 Hz or less (more preferably 70 Hz or more and 180 Hz or less).

The frequency is set to the above range because the frequency is 50Hz to 1kHz.
This is based on a heating experiment using the test induction heating device 1 shown in FIG. 1, which can be changed stepwise, and evaluated by the temperature distribution in the slab 2 when heating and soaking for a certain period of time. The slab 2 has a thickness of 200 mm, a width of 500 mm and a length of 600 mm,
Slab 2 was inserted vertically as shown in the figure. In addition, this device 1 is capable of independently controlling the three-zone input power in the height direction. Further, in the experiment, the apparatus 1 has a seal structure capable of heating the entire slab in an inert gas atmosphere in order to reduce heat dissipation of the slab as much as possible and suppress generation of slag at high temperature.

Fig. 2 shows that a 3.1% silicon steel slab with the above dimensions was preheated in a gas combustion type heating furnace at 1200 ° C for 3 hours and the center temperature was about 1150.
After the temperature reaches ℃, it is charged into the induction heating device shown in Fig. 1 and the input power is kept constant until the surface temperature reaches 1400 ℃.
FIG. 6 is a diagram showing a temperature distribution in the slab thickness direction at the time when the input power is reduced to ½ or less and the temperature is balanced for 15 minutes after being heated at n. Each temperature was measured by making a hole in the slab and inserting a thermocouple. The current frequency in the induction heating device
Temperature difference in the slab thickness direction is 50 ° C in the range of 50Hz to 200Hz
It can be seen that it is possible to perform uniform heating with a temperature difference of 30 ° C. or less at a frequency of 70 Hz to 150 Hz, in particular.

Next, when induction heating a 3.1% silicon steel slab of the same size, heat at 70 Hz until the slab center temperature reaches a soaking temperature of 1350 ° C, then increase the input power at the time of heating P
As opposed to The temperature distribution in the thickness direction of the slab was examined when the temperature was changed to 70Hz, 100Hz, 150Hz, 200Hz and soaked for 15 minutes. The results are shown in FIG. After reaching uniform temperature, set the frequency to 10
When the temperature is increased to 0 Hz or more and 200 Hz or less, the temperature difference in the thickness direction is small, and the difference is remarkable especially at the end portion in the slab length direction where the temperature variation is large. Thus, in induction heating of the slab, by controlling the frequency during heating and after reaching the soaking temperature, and the input power within the range of the present invention, it is possible to prevent the magnetic deterioration due to excessive high temperature heating or insufficient temperature. .

The slab heated by the method of the present invention is immediately subjected to hot rolling according to a known method,
Hot rolled steel strip with a thickness of 1.5 to 3.5 mm. After that, depending on the cold rolling process conditions suitable for the inhibitor component composition, the product sheet thickness of 0.10 to 0.50 mm thickness is obtained by performing cold rolling once or twice with intermediate annealing, followed by decarburization annealing and subsequent high temperature finish annealing and further insulation. It will be the final product due to the coating process,
Compared with the conventional slab heating method, there is less variation in electromagnetic characteristics and low iron loss can be stably obtained.

(Example) Example 1 A 200 mm thick silicon steel continuous casting slab containing C: 0.048%, Si: 3.28%, Mn: 0.082%, and S: 0.032% was charged into an induction heating furnace immediately after casting. , In N ₂ gas atmosphere, full power heating is performed at a frequency of 90Hz for about 25 minutes until the slab center temperature reaches 1370 ° C, and then while maintaining 1370 ° C, the input power is increasing 1/2 to 1/1 / The temperature was gradually lowered to 5 and kept for 10 minutes to make the temperature uniform. At this time, two conditions were tested: one in which the frequency after reaching the soaking temperature was 90 Hz, and one in which the frequency was increased to 125 Hz. The slab after heating is immediately hot-rolled to a hot rolled steel strip with a thickness of 2.0 mm, and the primary cold rolling is performed to 75
% Reduction rate, intermediate annealing at 950 ° C. for 2 minutes in hydrogen, and secondary cold rolling to a product thickness of 0.23 mm. Then 800
℃ −3min After decarburization annealing in wet hydrogen and applying MgO, 1200
Final annealing at 10 ° C. for 10 hours was carried out in hydrogen to carry out secondary recrystallization and purification of the steel sheet. After that, the separating agent was removed and an insulating coating was applied to obtain a final product. The electromagnetic characteristics of the final product are as follows. In each case, a grain-oriented silicon steel sheet with good magnetism was obtained. Especially, in the slab induction heating, the one in which the frequency during heating and during soaking was switched has less magnetic variation. It has good characteristics.

Example 2 C: 0.053%, Si: 3.30%, Mn: 0.080%, Se: 0.030%, Sb:
A 200 mm thick continuously cast silicon steel slab containing 0.030% was preheated in a gas combustion type heating furnace at a slab center temperature of 1150 ° C.
Until the slab center temperature reaches 1380 ° C in an N ₂ gas atmosphere in an induction heating furnace for 9 min.
Full-power heating is performed at a frequency of 0 Hz, and then the input power is raised to 1/2 to 1 while the center temperature is maintained at 1380 ° C.
The temperature was gradually lowered to / 4 and kept for 15 minutes to make the temperature uniform.

At this time, two conditions were tested: one performed at a frequency of 90 Hz after reaching soaking, and one raised to 120 Hz. The slab after heating is immediately hot-rolled into a hot-rolled steel strip having a thickness of 3.0 mm, the primary cold rolling is performed at a reduction rate of 75%, the intermediate annealing is performed at 1000 ° C for 2 min in hydrogen, and then the secondary cooling is performed. The product thickness is 0.30mm. Then, decarburization annealing was performed in wet hydrogen at 800 ° C. for 3 minutes, MgO was applied, and then final annealing at 860 ° C., 25 Hr + 1200 ° C., 10 Hr was performed in hydrogen to carry out secondary recrystallization and purification of the steel sheet. After that, the separating agent was removed and an insulating coating was applied to obtain a final product. The electromagnetic characteristics of the final product are as follows. In each case, a grain-oriented silicon steel sheet with good magnetism was obtained. Especially, in the slab induction heating, the one in which the frequency during heating and during soaking was switched has less magnetic variation. It has good characteristics.

(Effect of the Invention) As described above, according to the present invention, the grain-oriented silicon steel slab has a soaking temperature (1300-
Induction heating can be performed effectively up to 1400 ° C, and grain oriented silicon steel sheets with excellent electromagnetic properties can be economically manufactured.

[Brief description of drawings]

FIG. 1 is a cutaway perspective view of an essential part of an induction heating furnace in which the heating conditions (frequency) of the present invention are investigated, and FIG. 2 is a difference in heating frequency when a directional silicon steel slab is heated in the induction heating furnace. Fig. 3 is a diagram showing the temperature distribution in the thickness direction of the central part of the slab by Fig. 3, and Fig. 3 shows the temperature dependence of the directional silicon steel slab in the induction heating furnace and the heating temperature after the soaking temperature is changed. It is a figure which shows the temperature distribution of the slab center and the slab end part in the slab thickness direction. 1 ... Induction heating device, 2 ... Slab

Claims (8)

[Claims] 1. A direction of containing Si: 2.0 to 4.5 wt%, Mn: 0.01 to 0.15 wt%, and 0.005 to 0.10 wt% of one or more inhibitors selected from S, Se and Al. When the induction silicon steel slab is induction heated to the soaking temperature prior to hot rolling, the frequency of this induction heating is 50 to 200H.
The temperature of the central part of the slab is raised to 1300 to 1400 ° C. by controlling to z, and then the input power is suppressed to 1/2 or less and the temperature of the central part of the slab is kept at the soaking temperature for a predetermined time. A method for heating a grain-oriented silicon steel slab. 2. The heating method according to claim 1, wherein the induction heating is performed in a non-oxidizing atmosphere. 3. A direction of containing Si: 2.0 to 4.5 wt%, Mn: 0.01 to 0.15 wt%, and 0.005 to 0.10 wt% of one or more inhibitors selected from S, Se and Al. When the induction silicon steel slab is induction-heated to a soaking temperature prior to hot rolling, the frequency of induction heating is 50 to 50 until the temperature of the slab center reaches a soaking temperature of 1300 to 1400 ° C.
The temperature is controlled to 100 Hz to raise the temperature, and thereafter the input power is suppressed to 1/2 or less and the frequency is controlled to 100 to 200 Hz to maintain the temperature of the slab center portion at the soaking temperature for a predetermined time. Method for heating grain-oriented silicon steel slabs. 4. The heating method according to claim 3, wherein the induction heating is performed in a non-oxidizing atmosphere. 5. A direction in which Si: 2.0 to 4.5 wt%, Mn: 0.01 to 0.15 wt% and 0.005 to 0.10 wt% of one or more inhibitors selected from S, Se and Al are contained. When the induction silicon steel slab is induction-heated to a soaking temperature prior to hot rolling, first the central temperature of the slab is heated to 1000 to 1230 ° C. in a gas combustion type heating furnace, and then the frequency in induction heating is changed. The temperature of the slab center is controlled to 50 to 200 Hz to raise the temperature of the slab to 1300 to 1400 ° C, and then the input power is suppressed to 1/2 or less and the temperature of the slab center is maintained at the soaking temperature for a predetermined time. A method for heating a grain-oriented silicon steel slab, which comprises: 6. The heating method according to claim 5, wherein the induction heating is performed in a non-oxidizing atmosphere. 7. A direction in which Si: 2.0 to 4.5 wt%, Mn: 0.01 to 0.15 wt% and 0.005 to 0.10 wt% of one or more inhibitors selected from S, Se and Al are contained. When the induction silicon steel slab is induction-heated to a soaking temperature prior to hot rolling, first the central temperature of the slab is heated to 1000 to 1230 ° C. in a gas combustion type heating furnace, and then the slab center part is heated. Until the temperature reaches the soaking temperature of 1300 to 1400 ° C, the frequency in induction heating is controlled to 50 to 100 Hz to further raise the temperature, and then the input power is suppressed to 1/2 or less and the frequency is 100% or less.
A method for heating a grain-oriented silicon steel slab, comprising controlling the temperature of the center of the slab to be maintained at the soaking temperature for a predetermined time by controlling the temperature to ˜200 Hz. 8. The heating method according to claim 7, wherein the induction heating is performed in a non-oxidizing atmosphere.