JPH0657854B2 - Method of heating grain-oriented silicon steel slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) Regarding improvement of heating method for performing advantageous hot rolling on a silicon steel slab as a material of a unidirectional silicon steel sheet having excellent magnetic properties in the rolling direction In this specification, the results of the development research on the proper application of induction heating are described below.

Needless to say, unidirectional silicon steel plate has {110} on the plate surface.
It is widely used as a core material for transformers because it is composed of secondary recrystallized grains having <100> axes aligned in the plane and rolling direction and has excellent magnetic properties in the direction along the rolling direction.

In order to develop secondary recrystallized grains with such crystallographic orientation, fine MnS, MnSe, AIN, and BN called inhibitors are dispersed and precipitated in the steel, and other orientations occur during high-temperature finish annealing. It is necessary to effectively suppress the growth of the crystal grains.

As a control of the inhibitor dispersion morphology for that purpose, as is well known, after these precipitates are once solid-dissolved during slab heating prior to hot rolling, hot rolling is performed under an appropriate cooling pattern. It is necessary to carry out rolling.

Here, slab heating is usually performed at a high temperature of 1300 ° C or higher, but in the case of an ordinary gas combustion type heating furnace to realize sufficient heating to the center of the slab, the surface temperature of the slab is 1350 ° C.
In general, 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.

(Prior Art) Japanese Patent Publication No. 47-14627 discloses 1300 for the above-mentioned points.
1350 ℃ ~ 1400 ℃, in addition to such slab heating, because the required magnetic properties cannot be obtained by slab heating below ℃,
In particular, it is disclosed to perform induction heating or resistance heating by an electric method at a temperature of 1380 ° C., but in the case of induction heating, local heating on the slab surface due to the so-called skin effect is likely to occur, and regarding its control, Not touched.

In general, as for the slab heating method using an induction heating furnace, several patents and utility models related to equipment and transportation methods have been proposed so far, and a method for inserting and extracting a steel strip lifting device for a rigid induction heating furnace and a rigid furnace was shown. In addition to JP-B-51-41052, JP-B-51-41053, and JP-B-52-47179, which shows a heating method for preventing temperature nonuniformity of the slab in induction heating. However, there are few cases where induction heating is applied to slabs that require high temperature heating such as grain-oriented silicon steel, and there is only one Japanese Patent Publication No. 4714627 listed above, but this method has already been used. In an induction heating furnace as touched
Since the high temperature heating of 1350 ℃ to 1400 ℃ is performed under the grate of local overheating on the surface of the slab, the magnetic properties of the product are not always stable due to the tendency of crystal grains to coarsen.

(Problems to be Solved by the Invention) The inventors of the present invention first tried to slab-heat silicon steel with an induction heating furnace.

That is, the slab center temperature is 1000 ~
After heating until reaching 1230 ° C, preferably in an induction heating furnace with a substantially non-oxidizing atmosphere, to allow the slab to reach 1250 ~ 1380 ° C at its core temperature, where the advantages of induction heating furnace One of them is that it can be heated up to a predetermined temperature in a short time, and especially the surface temperature is easy to increase. Therefore, in order to increase the input power in the early stage of heating, the heat drop between the surface of the slab and the center of the slab is used in the final stage. By doing so, it was found that efficient slab heating is possible in a short time.

However, if the surface temperature of the slab is set too high, the crystal grains of the surface layer become coarse, as described above, and these become coarse elongated grains in hot rolling, which causes magnetic defects due to strip fine grains in the final product. It was

The inventors have proposed a method for heating a slab by an induction heating furnace, in particular,
It is intended to solve the above-mentioned problems and realize advantageous slab heating based on the results of various studies on the temperature rising patterns of the surface layer and the central portion.

(Means for solving the problem) The central temperature of the slab is necessary for dissociation of the inhibitor to form a solid solution.
Considering the usefulness of heating the slab in the range of 1250 to 1380 ℃ for at least 10 minutes, especially during this slab, when the center temperature of the slab is 1300 ℃ or more, the surface temperature (Ts) and the center temperature ( Tc) temperature difference is -40 ℃ ≤ Ts-Tc ≤
It has been found that abnormal growth of crystal grains on the surface of the slab can be advantageously suppressed by setting the temperature difference to -30 ° C ≤ Ts-Tc ≤ -5 ° C at a temperature of 20 ° C, especially at a slab center temperature of 1320 ° C or higher. , Came to complete this invention.

The present invention is selected from C: 0.025 to 0.080% by weight (hereinafter simply indicated by%), Si: 2.0 to 4.5%, Mn: 0.02 to 0.10%, and S, Se, Al and B as an inhibitor component. Using a silicon steel slab having a composition containing at least one of 0.005 to 0.10% in total, a unidirectional silicon steel sheet having a thickness of 0.15 to 0.5 mm is produced by a process including hot rolling and subsequent cold rolling. In the production, the slab center temperature Tc is 1300 to 1380 ° C by the temperature rising in the induction heating furnace during the slab heating prior to the hot rolling and the temperature keeping for at least 10 minutes in the furnace through this temperature rising.
Under the heat conduction with the slab surface in the temperature range within the range of Ts, the surface temperature Ts does not fall below 40 ° C compared to the central temperature Tc and exceeds 20 ° C, which is a temperature range that does not rise.
Is adjusted, and then hot rolling is started, which is a method for heating the directional silicon steel slab.

Here, the surface temperature Ts of the slab is adjusted by adjusting the temperature range of the center temperature Tc of the slab at 1320 ° C or higher to fall within the range of 5 to 30 ° C. It is even more desirable to have an oxidizing atmosphere.

This invention advantageously prevents overheating on the slab surface,
It is possible to stabilize the electromagnetic characteristics.

In particular, the present invention regulates the amount of C and the amount of inhibitor in addition to the amount of Si, and regulates the temperature difference between the surface and the center in the heating of such a component-oriented directional silicon steel slab. It was possible to avoid magnetic defects due to coarsening. Especially, setting the lower limit of the C content to 0.025% has a good result in improving the structure during hot rolling, and the upper limit of the slab heating temperature at the center is set to the above value. It can be raised to 1380 ° C without overheating.

When the temperature is raised by the induction heating furnace near the high temperature,
By setting a substantially non-oxidizing atmosphere, and particularly preferably an inert atmosphere, the surface properties of the slab can be improved.

(Function) The material to which this invention is applied is C: 0.025 to 0.080%, Si:
A slab for grain-oriented silicon steel containing 2.0 to 4.5%, Mn: 0.02 to 0.10%, and a total of 0.005 to 0.10% of one or more selected from S, Se, Al and B as an inhibitor component. It is obtained by continuous casting or slab rolling of steel ingot. Although the slab thickness is not particularly limited, a thickness of 150 to 350 mm is generally suitable.

C is controlled to 0.025 to 0.080%,
It passes through the + γ region and effectively improves the hot rolling texture, which results in an upper limit of the slab center temperature of 1380 ° C.
It has already been mentioned that it can be increased to.

The amount of Si increases the specific resistance of the steel sheet and is effective in reducing iron loss, but 4.
If it exceeds 5%, the cold ductility is impaired, and if it is less than 2.0%, the iron loss improving effect is weakened and sufficient properties cannot be obtained due to randomization of the crystal orientation due to α-γ transformation.

The lower limit of the amount of Mn is to prevent cracking due to hot brittleness, and the upper limit is 0.10% in order not to raise the dissociation solid solution temperature of MnS and MnSe.
Regulated by.

S, Se, Al and B are finely dispersed in the steel in the form of MnS, MnSe, AIN and BN, respectively, and act as inhibitors. The lower limit of 0.005% of these total amounts is the minimum amount that functions as an inhibitor. , The upper limit of 0.10% is mainly determined by economic reasons. Needless to say, when Al and B are used as the inhibitor components, the amount of N is required to be balanced with them.

Other inhibitors include Sb, Sn, As, Pb, Bi, C
Although grain boundary segregation type elements such as u and Mo are known, addition of these elements does not impair the effect of the present invention.

The above-mentioned slab is heated prior to hot rolling, but is heated directly in an induction heating furnace, or after being heated up to about 1250 ° C. in a normal gas combustion type heating furnace, and is preferably substantially non-heated. In an induction heating furnace in an oxidizing atmosphere, the central temperature is especially dependent on the amount and type of dissociating inhibitor solid solution.
Heat at 1250 ℃ ~ 1380 ℃ for 10 minutes or more. At this time, it is necessary to prevent abnormally high temperature heating on the slab surface. Especially when the center temperature exceeds 1300 ° C, the temperature difference between the surface temperature Ts and the center temperature Tc is -40 ° C.
Setting ≤Ts-Tc≤20 ° C is important for stabilizing the magnetic properties of the final product.

Generally, in the slab heating by the induction heating furnace, although internal heat is generated, the slab surface layer is more likely to be heated faster, so it is important to select the frequency or control the input power pattern. That is, before or after the central portion temperature reaches 1300 ° C., the input power is gradually reduced to balance the temperature drop of the surface due to heat dissipation and the internal heat generation amount, and thereby the temperature difference between the surface layer of the slab and the center of the present invention. You can make it enter the condition.

Here, the core temperature of the slab has a lower limit temperature of at least 1250 ° C. due to the required amount of solid solution of the inhibitor, while 13
At temperatures higher than 80 ° C, magnetism is adversely affected by the coarsening of slab crystal grains. At the beginning, it was stated that silicon steel tends to generate a large amount of slag at high temperatures. For that reason, the fluidity of firelite increases, and at temperatures above 1250 ° C, the O ₂ content in the atmosphere is less than 1% of non-oxidizing gas. It goes without saying that it is more desirable to have an atmosphere.

Figure 1 summarizes the results of an investigation of the effects of the slab center temperature Tc and the surface-center temperature difference during soaking during heating of the slab in an induction heating furnace, that is, while maintaining temperature control, on the final magnetic properties. Is shown.

In this experiment, Si 3.05%, Mn 0.078%, Se 0.023%, Sb
A 200 mm thick silicon steel slab containing 0.030% was preheated in a gas heating furnace until the center temperature reached 1150 ° C.
It was transferred to an induction heating furnace and heated at various temperatures with the central temperature ranging from 1220 ℃ to 1400 ℃.

At this time, after the center temperature Tc of the slab reaches the specified temperature
Temperature difference Ts between the surface temperature Ts when heating is continued for 15 minutes (note that Ts changed with time, but the average temperature during that time)
-Tc is the vertical axis and the center temperature is the horizontal axis.
Magnetic flux density B of the final product finished to 0.30 mm thickness by the spinning method
I arranged it with ₁₀ values.

In the experimental example, the central temperature regulated as a condition of the present invention
1.91T at -40 ℃ ≦ Ts−Tc ≦ 20 ℃ at 1300 ℃ to 1380 ℃
Higher magnetic flux density B ₁₀ is obtained, especially the core temperature is 1320 ℃
At −1380 ° C., −10 ° C. ≦ Ts−Tc ≦ 5 ° C., B _{10 of} 1.93 T or more is obtained.

Here, the slab center temperature Tc is the temperature of the slab center part where the temperature during heating hardly rises, and the slab center temperature Tc was measured by directly inserting a thermocouple.

Fig. 2 shows an example of measuring the temperature change of the slab during heating by the induction heating furnace. The power supply power is reduced immediately before the center temperature Tc of the slab rises to 1300 ° C to suppress the surface temperature from exceeding 1380 ° C. While turning off the power after the temperature difference between the surface layer and the center is reduced due to heat conduction between the surface layer and the center, the surface temperature Ts becomes lower and the center temperature Tc is approximately 1340 ° C. The figure shows how the slab surface and center temperature change when held for 15 minutes at a temperature Ts lower by about 20 ° C. It can be seen that the conditions of the present invention are satisfied at a slab center temperature of 1300 ° C or higher.

The steps after hot rolling for the slab heated under the conditions of the present invention are not particularly different from usual ones, and one or two cold rolling including intermediate annealing and decarburization annealing depending on the amount and kind of the inhibitor and Subsequent high temperature box annealing can produce a unidirectional silicon steel sheet having a thickness of 0.15 to 0.50 mm.

(Example) C 0.040%, Si 3.30%, Mn melted by continuous casting
When hot rolling a 250 mm thick silicon steel slab containing 0.065%, S 0.025%, Sb 0.030%, B 0.0015%, the temperature of the slab center was 1150 in a gas combustion type heating furnace.
After heating until it reaches ℃, it is immediately charged into the induction heating furnace, and the center temperature Tc of the slab (1320 ℃) and the surface temperature Ts (13
50 ° C), the power supply is turned off when the temperature difference reaches + 30 ° C, and the above temperature difference is reversed due to heat conduction between the surface layer and the center, and the surface temperature is 1340 ° C and the central temperature is 1350 ° C. The temperature is maintained for 20 minutes at the bottom for extraction and 3.0m by hot rolling.
A hot rolled sheet with a thickness of m was obtained. Then cold rolled to 0.70 mm, 950 ℃,
After performing intermediate annealing in a hydrogen-nitrogen mixed gas for 3 minutes, 0.23 mm thickness was finished by secondary cold rolling. After that, decarburization was performed in wet hydrogen at 800 ° C. for 3 minutes, MgO was applied, and then box annealing was performed in hydrogen at 1200 ° C. and 10 hr.

The electromagnetic characteristics of the final product thus obtained were as follows, and a high magnetic flux density grain-oriented silicon steel sheet with good iron loss was obtained.

W _17/50 : 0.85 W / kg, B ₁₀ : 1.92T (Effect of the invention) According to the present invention, the local abnormal rise of the surface temperature is effectively suppressed during the heating for the hot rolling of the silicon steel slab. Moreover, the temperature lowering and heating required for solid solution of the precipitate that serves as an inhibitor is only possible, and the magnetic properties of the product steel strip can be effectively improved.

[Brief description of drawings]

Figure 1 shows the slab center temperature Tc extracted by heating the unidirectional slab for silicon steel in an induction heating furnace and the surface temperature Ts at this time.
The temperature difference Ts-Tc (° C) with the magnetic flux density B ₁₀ (T) of the final product
FIG. 2 is a graph showing changes in slab surface-center temperature when the slab is heated in an induction heating furnace.

Claims (3)

[Claims] 1. C: 0.025 to 0.080% by weight Si: 2.0 to 4.5% by weight Mn: 0.02 to 0.10% by weight, and 0.005% of at least one selected from S, Se, Al and B as an inhibitor component. 0.15 to 0.5 m by a process including a raw material of a silicon steel slab having a composition of about 0.10 wt% and hot rolling and subsequent cold rolling.
In producing an m-thickness unidirectional silicon steel sheet; by heating the slab prior to hot rolling and by raising the temperature in an induction heating furnace and then maintaining the temperature for at least 10 minutes in the furnace after this heating. Center temperature (Tc) of slab is 1300
Under the heat conduction with the slab surface layer in the temperature range of 〜 1380 ℃, the temperature is not lower than 40 ℃ compared to the above core temperature (Tc), and still higher than 20 ℃. A method for heating a grain-oriented silicon steel slab, which comprises adjusting the surface temperature (Ts) of the slab, which is a difference, and then starting hot rolling. 2. The surface temperature (Ts) of the slab is adjusted to 1320 ° C.
This is 5 for the center temperature (Tc) of the slab above.
The method according to claim 1, wherein the temperature difference is within the range of -30 ° C. 3. The method according to claim 1, wherein the induction heating furnace has a substantially non-oxidizing atmosphere.