JP2735896B2 - High temperature heating method for silicon steel slab. - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for heating silicon steel slabs at a high temperature, and particularly to an ultra-high temperature method without causing deterioration of surface properties and magnetic properties of a product plate. The slab heating in the furnace is made possible.

(Prior Art) A grain-oriented electrical steel sheet is mainly used as an iron core material of a transformer or a generator, and is required to have a high magnetic flux density and a low iron loss.

By the way, in recent years, in view of the strong demand for energy saving, inexpensive supply of grain-oriented electrical steel sheets with excellent magnetic properties has been strongly desired, but in addition to stabilizing the magnetic properties described above, Ensuring reliable reliability has become an important issue.

In order to obtain a grain-oriented electrical steel sheet having excellent magnetic properties, it is basically necessary to obtain a secondary recrystallized structure highly integrated in the {110} <001> orientation, so-called Goss orientation. In order to develop secondary recrystallized grains with Goss orientation, it is necessary to have a so-called inhibitor, which is a dispersed precipitate phase that moderately suppresses grain boundary migration, and MnSe, MnS, AlN, etc. are generally used as such inhibitors. Have been. In this case, it is very important to disperse and precipitate finely and uniformly by dissolving MnSe, MnS, etc. sufficiently at the time of slab heating prior to hot rolling, then hot rolling under appropriate conditions and then cooling. And
It is said that a high slab heating temperature is required for solid dissolution of MnSe, MnS, and the like.

For this reason, many efforts have been made to improve the slab heating temperature sufficiently.

Recently, a heating method using an induction heating method has been developed as a method capable of performing the above-described high-temperature slab heating. In a heating furnace using such an induction heating method, it has been confirmed that heating to a sufficiently high temperature with high accuracy is extremely effective for improving characteristics, but on the other hand, there are some disadvantages associated with high-temperature heating. As expected. That is the generation of slag during high-temperature heating, and several techniques have been proposed for the purpose of preventing the generation of slag.

Techniques proposed as a solution to the above-mentioned problem include JP-A-60-145318, JP-A-61-69927, and JP-A-61-699.
No. 24 and Japanese Patent Application Laid-Open No. 62-130219 disclose techniques. All of these techniques aim at reducing the oxidation loss by lowering the oxygen concentration in the furnace at the time of high-temperature heating, or preventing the generation of flaws due to high-temperature oxidation.

For example, Japanese Patent Application Laid-Open No. 60-145318 discloses that a large amount of slag is generated on the slab surface during high-temperature heating, which not only impairs the operability of the heating furnace but also causes surface flaws. It has been proposed that when the slab surface temperature is 1250 ° C. or more, the O ₂ content in the heating atmosphere should be 1% or less, and that the upper limit of the heating temperature in the gas combustion furnace should be 1230 ° C.

Further, Japanese Patent Application Laid-Open No. 61-69927 discloses that a large amount of slag causes furnace wall erosion and yield reduction, hot coil ear roughness due to grain boundary oxidation of the slab surface during high-temperature heating, and final decarburization caused by slab surface decarburization. In order to prevent magnetic degradation of the product and to prevent the slab columnar crystals from becoming coarse, heating in an electric heating furnace should be performed at a temperature of 1310 to 1350 ° C, the atmosphere should be limited to non-oxidizing, and the fuel combustion furnace Upper limit of soaking temperature at 1250
It has been suggested that the temperature should be in ° C.

Further, in Japanese Patent Application Laid-Open No. 61-69924, when a slab is heated to a high temperature by an induction heating method, the slab surface temperature is 1325 ° C.
If the temperature exceeds 1325 ° C, the O ₂ concentration
It proposes that it should be controlled below 10%. The examples show an example in which the heating temperature is 1350 ° C., the O ₂ concentration is 10% or less, and the heating temperature is 1370 ° C. and the O ₂ concentration is 1% or less.

Further, in Japanese Patent Application Laid-Open No. 62-130219, in order to prevent the yield from decreasing and the generation of molten slag which seriously hinders the operation of the heating furnace, the O ₂ concentration in the atmosphere is calculated by the following formula: O ₂ (%) = 36.4-5.0 ln T (° C) It is proposed that the temperature be kept below. The specific values are 0.55% or less at 1300 ° C, 0.36% or less at 1350 ° C,
Although the range of 0.18% or less at 00 ° C is shown, this is simply a mathematical expression based on thermodynamic common sense that if the O ₂ concentration is reduced, it becomes difficult for molten slag to be generated with this component. It does not provide any insight.

(Problems to be Solved by the Invention) As described above, various studies have conventionally been made on the improvement of the high-temperature heating technique using the induction heating or the like. The problem common to the prior arts was how to prevent the generation of a large amount of slag due to high-temperature heating. Certainly, the generation of a large amount of slag due to high-temperature heating is undesirable because it lowers the yield and the operating efficiency, but this is not a new problem caused by changing the heating method to induction heating. Simply increasing the temperature increased the oxidation reaction rate, and merely increased the amount of slag.

Therefore, it can be easily estimated that to prevent this, it is effective to lower the O ₂ concentration or set the upper limit of the heating temperature. Therefore, as apparent from the prior art cited above, the upper limit of the heating temperature is 1400 ° C. or less.

However, slab heating at higher temperatures is advantageous for improving the magnetic properties based on the complete solid solution of the inhibitor and the high integration of Goss orientation.

Therefore, the inventors conducted various manufacturing experiments using an induction heating furnace that can actually heat to an extremely high temperature of 1400 ° C. or more. As a result, it was found that various fatal defects which were not experienced at all in the conventional gas heating furnace having a heating temperature of about 1400 ° C. or less occur in the subsequent decarburization annealing process.

In other words, do not generate a large amount of molten slag
When a slab heated to an extremely high temperature of 00 ° C. or more was used as a material, it was confirmed that extremely good magnetic property values were obtained as in the conventional knowledge. However, when such a process was applied, intermediate annealing was inserted. In the decarburization annealing step after the two cold rollings, extremely undesirable surface defects appeared. This defect was completely different from a defect with a surface crack that became apparent after hot rolling or cold rolling as conventionally known. This decarburization annealing step is an important step before the secondary recrystallization, and is performed on the steel sheet surface simultaneously with decarburization.
This is to form a thin, uniform and dense oxide film of Si. To form such a coating soundly prevents the inhibitor from decomposing from the steel sheet surface and promotes the secondary recrystallization at a high temperature in a stable manner. It is extremely important to guarantee. Therefore, in the ordinary decarburization step, not only the control of the heating rate but also the strict control of the atmosphere corresponding to each temperature. However, in an induction furnace with a pure N ₂ atmosphere, 14
When a slab heated to an extremely high temperature of 00 ° C. or more was used as a material, a healthy film was not formed in the decarburization process despite normal processing conditions, and it was found that this caused a film defect. is there.

As a result of close investigation on this point, it was found that almost no dense oxide film in which Si, which is usually observed, was concentrated was formed in such a defective portion. Therefore, it has been found that this defect is not only a simple defect in appearance but also a serious defect that locally significantly degrades the properties (mainly, coating properties) or reliability of the magnetic steel sheet.

Heretofore, there has been no knowledge about this type of defect, and, of course, no technology has been proposed for preventing this defect or suggesting a method for preventing it.

The present invention proposes a technique for effectively preventing the occurrence of serious defects due to the high-temperature heating of such a slab, and enables the practical use of a slab high-temperature heating capable of obtaining stable and good characteristics.

(Means for Solving the Problems) The present invention has been developed based on a new finding that poor formation of an oxide film during decarburization annealing is caused by formation of a de-Si layer during high-temperature heating. Things.

That is, the present invention provides a method for heating a silicon-containing steel slab, then performing hot rolling, and then performing one or two cold rolling operations including intermediate annealing, followed by decarburizing annealing and then final finishing annealing. In the method for producing a grain-oriented silicon steel sheet comprising a series of steps of applying, when the slab is heated, first, the O ₂ concentration in the atmosphere is 2%.
% Under the following conditions, and heated to the slab surface temperature reaches the temperature range of from 1,000 to 1,170 ° C., subsequently O ₂ concentration in the following atmosphere 3000 ppm, the slab center temperature: heating to a temperature range of 1,380 to 1,470 ° C. This is a method for heating a silicon steel slab at a high temperature, which comprises performing a soaking treatment for 5 to 25 minutes in this temperature range.

In the slab heating of the present invention, it is preferable to use a gas combustion furnace when heating the low-temperature slab in the first stage and use an electric heating furnace such as an induction heating furnace or an electric resistance furnace when heating the high-temperature slab in the second stage.

Hereinafter, the investigation results and the experimental results that led to the invention will be described.

Defects occurred at ultra-high temperature heating of 1400 ° C or higher C: 0.05wt%
(Hereinafter simply shown as%), Si: 3.4%, Mn: 0.07% and Se: 0.
FIG. 5 shows a photograph of a cross-sectional metal structure in the vicinity of the surface of the steel slab containing 025% and the balance substantially consisting of Fe.

As is clear from the figure, the surface layer of such a slab is composed of a porous metal layer (No. 1 in the figure) and an oxidized substance (2). As a result of the investigation, it was confirmed that the porous metal layer 1 had decreased from about 3%, which should normally contain Si, to a level at which it could hardly be detected. Conversely, it was confirmed that Si was concentrated in the oxide layer 2. Si is an element that is very easily oxidized, and a steel containing about 3% of Si forms a dense oxide film in a short time during decarburization annealing. However, if a de-Si layer is formed on the surface layer, no oxide film is formed at all in an ordinary atmosphere.

The formation of such a de-Si layer on the surface during slab heating was found to be the direct cause of the uneven oxide film.
The de-Si layer generated during slab heating is very difficult to peel off,
Normal descaling methods such as high-pressure water cannot be sufficiently removed. Therefore, in order to prevent the occurrence of defects during decarburization annealing, it is necessary to use a heat treatment method that does not form such a de-Si layer during slab heating.

Therefore, a heating method that can prevent the formation of the de-Si layer was examined. First, an experiment was performed to confirm the effect of lowering the oxygen concentration during high-temperature heating. In the laboratory, C: 0.05%, Si: 3.4%,
A steel slab containing Mn: 0.07% and Se: 0.025 and substantially the balance of Fe was heated at 1430 ° C for 30 minutes or 60 minutes in a nitrogen atmosphere having various oxygen concentrations, and then subjected to a normal treatment. After hot rolling, primary cold rolling, intermediate annealing and secondary cold rolling under the conditions, decarburization treatment was performed at 950 ° C. for 3 minutes in a hydrogen atmosphere with a dew point of 30 ° C.

FIG. 1 shows the results of an examination of the effect of the O ₂ concentration on the oxide film thickness after slab heating and the rate of non-uniform oxidation of the annealed plate in the above experiment. In this case, the rate of non-uniform oxidation is a comparison of the rate of occurrence per unit area of 10 cm × 10 cm.

As is conventionally known, the oxide film thickness was certainly reduced by reducing the oxygen concentration. Especially when the oxygen concentration
When the content was set to 0.5% or less, the oxide film became very thin, and no generation of so-called molten slag was observed.

However, even when the oxygen concentration was reduced, the incidence of non-uniform oxidation defects hardly decreased. However, there was some dependence on the soaking time.

From the above results, it was confirmed that formation of the de-Si layer at such a high temperature could not be solved at all simply by lowering the oxygen concentration in the atmosphere.

Next, the influence of ordinary gas furnace heating conditions prior to heating at an ultra-high temperature of 1430 ° C. was investigated in the same manner as in the aforementioned experiment.
The high-temperature heating condition was fixed at 1430 ° C. for 60 minutes, and the heating temperature of the gas furnace was varied in advance. At this time, the holding time at each heating temperature was 60 minutes.

FIG. 2 shows the effect of the gas furnace heating temperature on the rate of non-uniform oxidation, using the O ₂ concentration in the atmosphere as a parameter.

As is clear from the figure, the effect of the heating temperature of the gas furnace was hardly recognized. The effect of the atmosphere in the furnace also tended to be slightly lower when the O ₂ concentration was lower, but it did not reach the point where it could be improved.

Therefore, the viewpoint was changed further, and the O ₂ concentration and heating temperature during heating in the gas furnace were changed over a wide range, and the influence on the scale thickness and the structure of the ground iron interface at that time was investigated. FIG. 3 shows the results.

As is clear from the figure, as a general tendency, as conventionally known, the lower the heating temperature was, the smaller the scale generation amount was. The effect of O ₂ concentration on scale thickness was also investigated.
Although there was a slight difference between the O ₂ concentration of 5% and 1%, it was not always clear.

However, when the heating temperature was 1150 ° C. or lower, the scale thickness changed greatly only when the O ₂ concentration was low. In addition, when these samples were heated at 1430 ° C for 20 minutes, their cross-sections were observed. Only when the O ₂ concentration of the gas heating furnace was 1% and the heating temperature was 1150 ° C or lower, it was found that the formation of a de-Si layer was prevented. , An improvement effect was observed.

Therefore, the experiment was continued in detail to clarify these improvement conditions, and finally the following conditions were obtained.

That is, in gas furnace heating, the heating temperature is reduced and
It is important to reduce the O ₂ concentration to reduce the scale thickness. The O ₂ concentration depends on the gas flow velocity on the slab surface, but as shown in FIG. 4, it was found that the necessary condition was O ₂ concentration: 2% or less. Further, the heating temperature greatly changes from 1170 ° C. to the scale generation amount, so it is necessary to set the upper limit temperature to 1170 ° C. Above this temperature
It is presumed that a large change occurs in the steel containing Si because the oxide starts to melt. On the other hand, if the heating temperature is too low,
Since a large amount of energy is required during electric heating, which is inconvenient, the lower limit of the heating temperature is set to 1000 ° C.

The final slab heating after heating the gas furnace requires 1380 ° C or higher for complete dissolution of the inhibitor. It has been found that the heating needs to be performed in as short a time as possible because the depth becomes deep and a fatal defect is likely to occur.

Therefore, in the present invention, in order to perform such high-temperature heating in a short time, the final slab heating is performed in an electric heating furnace such as an induction heating furnace or an electric resistance furnace. The atmosphere in this 3000ppm or less O ₂ concentration, particularly preferably 1000ppm
It was confirmed that it was necessary to: That is, the reason for lowering the O ₂ concentration is that, as shown in FIG. 1, the oxide layer is formed when the oxygen concentration is increased, and the oxide layer formed in the gas heating furnace is advanced to the inside of the steel sheet. That's why. Here, the O ₂ concentration should not exceed 3,000 ppm if the generated slag during gas furnace heating is small, but if the generated slag is large, the O ₂ concentration during high-temperature heating should be further reduced to 1000 ppm or less. Is desirable.

As for the soaking time, a certain time was required for dissolving the inhibitor, and the lower limit was 5 minutes. However, if it is too long, a de-Si layer is easily formed, so the upper limit is set to 25 minutes. Furthermore, if the heating temperature is too high, the slab considerably dissolves depending on the component system, so the upper limit was set to 1470 ° C.

(Operation) As the silicon-containing steel which is the material of the present invention, any of the conventionally known component compositions are suitable, and typical compositions are as follows.

C: 0.01 to 0.10% C is an element that is useful not only for uniform micronization of the structure during hot rolling and cold rolling but also for the development of goth surroundings, and is preferably added at least 0.01% or more. However, if the content exceeds 0.10%, the Goss orientation is rather disturbed. Therefore, the upper limit is preferably about 0.10%.

Si: 2.0-4.5% Si increases the specific resistance of the steel sheet and effectively contributes to the reduction of iron loss. However, if it exceeds 4.5%, the cold-rolling property is impaired, whereas if it is less than 2.0%, the specific resistance only decreases. Not secondary recrystallization
Since the crystal orientation is randomized by the α-γ transformation during the final high-temperature annealing performed for the annealing, and a sufficient iron loss improvement effect cannot be obtained, the Si content is preferably set to about 2.0 to 4.5%.

Mn: 0.02 to 0.12% Mn needs to be at least about 0.02% in order to prevent hot embrittlement, but if it is too much, magnetic properties are degraded, so the upper limit is preferably set to about 0.12%.

As inhibitors, there are so-called MnS, MnSe-based and AlN-based. In the case of MnS and MnSe, at least one selected from Se and S: 0.005 to 0.06
% Se, S are all effective elements as inhibitors for controlling secondary recrystallization of grain-oriented silicon steel sheets. From the viewpoint of securing the suppressing force, at least about 0.005% is required, but if it exceeds 0.06%, the effect is impaired. Therefore, the lower and upper limits are preferably set to about 0.01% and 0.06%, respectively.

In the case of the AlN system, Al: 0.005 to 0.10%, N: 0.004 to 0.015% The range of Al and N is also set to the above range for the same reason as in the case of the MnS and MnSe systems described above. Here, the above-mentioned MnS, MnSe-based and AlN-based can be used in combination.

Inhibitor components include S, Se, Al, Cu,
Sn, Cr, Ge, Sb, Mo, Te, Bi, P, and the like are also advantageously used, so that a small amount of each of them can also be contained. Here, the preferred addition ranges of the above components are Cu, Sn, Cr: 0.01, respectively.
0.15%, Ge, Sb, Mo, Te, Bi: 0.005 to 0.1%, P: 0.01 to 0.2
%, And each of these inhibitor components can be used alone or in combination.

The slab is intended to be a continuously cast one or a lump from an ingot, but it goes without saying that a slab which has been continuously cast and then re-pumped is also included.

(Example) Example 1 Continuously cast a steel containing 0.045% of C, 3.2% of Si, 0.08% of Mn, and 0.020% of Se, and the remainder substantially composed of Fe, and the slab of the steel in the atmosphere was In a gas-fired slab heating furnace in which the O ₂ concentration has been changed, after heating under conditions where the soaking temperature has been variously changed, immediately in the induction heating furnace in which the atmosphere can be controlled, various O ₂ concentrations ( The rest is soaked in N ₂ ) atmosphere under various conditions of soaking temperature and soaking time.
Hot rolling was performed in a normal process. Further pickling according to the usual method,
Following the annealing, primary cold rolling, intermediate annealing, and secondary cold rolling, primary recrystallization annealing also serving as decarburizing annealing was performed. The annealing conditions were a hydrogen atmosphere with a dew point of 55 ° C and an annealing temperature of 800 ° C.
In addition to observing the steel sheet surface after the decarburizing annealing treatment, the surface coating properties after the final finish annealing and the insulating coating treatment were further investigated.

 Table 1 shows the obtained results.

As is clear from the table, in the steel sheet heated in an appropriate range according to the present invention, no oxide film nonuniformity occurred at all, and further, there was no poor appearance and no deterioration in insulation resistance. No.1
In No. 2, the product appearance was good, but the magnetic flux density was extremely low due to the low heating temperature and could not withstand actual use.

Example 2 Steel slabs having various compositions shown in Table 2 were subjected to an O ₂ concentration of 1.1.
% Heat treatment at 1140 ° C for 40 minutes in a gas heating furnace, immediately put it into an induction heating furnace, heat it to 1410 ° C in 1 hour, hold it for 20 minutes, and then hot roll in the usual process did.
Thereafter, the same processing as in Example 1 was performed.

Table 2 shows the results obtained by examining the insulating film properties of the product sheets thus obtained. In all cases, good properties were obtained when treated according to the present invention.

(Effects of the Invention) Thus, according to the present invention, by controlling the atmosphere and temperature not only at the time of heating at an ultra-high temperature by electric heating but also at the time of heating the gas furnace prior thereto, decarburization and primary recrystallization annealing in the subsequent process It is possible to heat a slab to an ultra-high temperature of 1380 ° C or more without fear of the occurrence of non-uniform oxidation, which was a concern at times, and to obtain a stable electromagnetic steel sheet with excellent surface properties and good magnetic properties. Can be.

[Brief description of the drawings]

FIG. 1 shows the oxygen concentration and oxide film thickness in the slab induction heating furnace,
FIG. 2 is a graph showing the relationship between the rate of occurrence of oxidation non-uniformity, FIG. 2 is a graph showing the effect of the gas furnace heating temperature on the oxidation non-uniformity using oxygen concentration as a parameter, and FIG. Fig. 4 is a graph showing the effect of the gas furnace heating temperature with the oxygen concentration as a parameter. Fig. 4 is a graph showing the effect of the oxygen concentration in the furnace on the scale thickness. Fig. 5 is silicon containing heated at an extremely high temperature. It is a cross section metallographic photograph near the surface of a steel slab.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Michio Komatsubara, Inventor 1 Kawasaki-cho, Chiba-shi, Chiba Prefecture Kawasaki Steel Corporation In-house Research & Development Division (72) Inventor Takahiro Suga 1-1-1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corporation Company Technology Research Division

Claims (1)

(57) [Claims] 1. A silicon-containing steel slab is heated, hot-rolled, and then cold-rolled once or twice with intermediate annealing, followed by decarburizing annealing and then final finishing annealing. In the method for producing a grain-oriented silicon steel sheet comprising a series of steps, in the above slab heating, first, the O ₂ concentration in the atmosphere is 2%.
Under the following conditions, and heated to the slab surface temperature reaches the temperature range of from 1,000 to 1,170 ° C., subsequently O ₂ concentration in the following atmosphere 3000 ppm, the slab center temperature: heating to a temperature range of 1,380 to 1,470 ° C., A high-temperature heating method for a silicon-containing steel slab, wherein a soaking treatment is performed for 5 to 25 minutes in this temperature range.