JPH0726156B2 - Method for producing grain-oriented electrical steel sheet with excellent magnetic properties and surface properties - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an advantageous method for producing a grain-oriented electrical steel sheet having excellent magnetic properties and few surface defects.

(Prior Art) A unidirectional 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, reflecting the strong demand for energy saving, there is a strong demand for inexpensive supply of grain-oriented electrical steel sheets having excellent characteristics, and it is an important issue how to stabilize the characteristics and reduce the manufacturing cost. Has become.

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, the so-called Goss orientation. Goth bearing 2
In order to develop secondary recrystallized grains, it is necessary to have a so-called inhibitor, which is a dispersed precipitation phase that appropriately suppresses grain boundary migration, and MnSe, MnS, AlN, etc. are generally used as such inhibitors. In this case, it is very important to disperse and precipitate MnSe, MnS, etc. sufficiently during slab heating prior to hot rolling, followed by hot rolling and cooling under appropriate conditions for fine and uniform dispersion precipitation. Therefore, a high slab heating temperature is required for such solid dissolution separation of MnSe, MnS and the like. Therefore, many techniques have been proposed so far for the high-temperature slab heating method for improving the characteristics.

For example, Japanese Patent Publication No. 47-14627, Japanese Patent Publication No. 57-41526, Japanese Patent Publication No.
54-27820, JP 61-0530, JP 59-37330,
JP-A 61-69724, JP-A 61-246317 and JP-A 6-
1-288020 each gazette.

All of the above techniques aim to improve the characteristics by heating the slab at a high temperature. It is well known that heating the slab at a high temperature dissolves the inhibitor sufficiently to improve the properties to some extent, and great efforts have been made in this respect. However, even though the existing equipment and operating technology raised the heating temperature, there was a limit to it. Further, in the past, it was said that not only high temperature but also long time heating was required to sufficiently dissolve and homogenize the inhibitor, and soaking was generally required for 1 to 2 hours. There was also a problem.

(Problems to be Solved by the Invention) As described above, it is difficult to heat the slab to a high temperature in the conventional technology in terms of equipment, and the Si-containing steel is easily oxidized and melted at a high temperature, but the heating efficiency is good. There wasn't. Therefore, despite the great efforts made to raise the heating temperature, the achievable temperature is at most 1400 ° C.
It was said that.

On the other hand, if the slab is heated for a long time in a high temperature range to secure the temperature, abnormal growth of slab crystal grains is induced, and the coarsened crystal grains do not sufficiently recrystallize even in hot rolling, and often secondary recrystallization occurs. In addition to causing crystal defects, fatal surface defects are often generated due to surface grain boundary oxidation or the like.

As described above, with the conventional slab heating technique, it was extremely difficult to satisfy the magnetic properties and the surface properties at the same time.

Incidentally, as a means for solving the above-mentioned problems, recently, Japanese Patent Laid-Open No.
In Japanese Patent Laid-Open No. 109115, there has been proposed a method of heating a slab surface temperature to a temperature range of 1420 to 1495 ° C. for a short time in an induction heating furnace having a low oxidizing atmosphere. However, even in this method, since it is premised that the surface temperature is higher than the central temperature, the secondary recrystallization failure due to the coarsening of the crystal grains when the surface layer of the slab is heated to such a high temperature cannot be avoided. . Therefore, in order to avoid the occurrence of defects due to such high temperature heating, the slab repressing process, that is, 10 to 30% before slab heating
A special process such as a process of applying a certain degree of strain to recrystallize and homogenize the structure is separately required, which causes a large increase in cost.

The present invention advantageously solves the above problems, and proposes a manufacturing method capable of inexpensively obtaining a grain-oriented electrical steel sheet excellent in magnetic characteristics and surface characteristics by performing slab heating under appropriate conditions. The purpose is to do.

(Means for Solving the Problem) As a result of various studies, the inventors have found that the causes of the above-mentioned problems are that the required proper heating temperature distribution in the slab and the heating temperature distribution actually given are It was discovered that this is due to the fact that they did not match well, and the present invention has been completed based on this finding.

Hereinafter, the clarification process of the present invention will be described.

The inventors first began by investigating the internal properties of a grain-oriented electrical steel sheet containing silicon.

The distribution of coarse precipitates of continuously cast slabs containing Si: 3.4wt% (simply expressed as%) and Se: 0.02%, that is, precipitates such as (Mn, Fe) Se with a diameter of 5μm or more in the slab thickness direction, Examined. The results are shown in FIG.

As is clear from the figure, coarse precipitates (inhibitors) that are extremely difficult to dissociate and dissolve are approximately 1 / thick in thickness than the slab surface.
It was confirmed that the number was low up to around 5, and increased sharply toward the center.

Such coarse precipitates are formed during slab solidification.However, in the usual slab continuous casting method with a slab thickness of about 100 to 300 mm, it is necessary to avoid the formation of such coarse precipitates despite many efforts. Not yet successful.
In fact, under normal casting conditions, it was confirmed that there is no significant change in the size and dispersion state of these precipitates even if the cooling water ratio is increased or the solidified portion is subjected to electromagnetic stirring treatment. Of course, it was observed that larger precipitates were unevenly distributed in the central segregation portion and the internal crack generation portion.

Even if these precipitates are present, it is not possible to detect the component change in the slab thickness direction due to the presence or absence of them with the usual macrochemical analysis method, which is different from the conventionally known macrocenter segregation. It is a thing with.

Generally, above the melting temperature, the dissociation and dissolution process of the precipitate is limited by the diffusion rate of the solute elements constituting the precipitate. Therefore, as the size of the precipitate increases, the dissolution time increases in proportion to the square of the size of the precipitate. Therefore, the coarse inhibitor existing in the central layer is much less likely to dissolve than the surface layer.

If it is desired to completely dissolve the inhibitor in a short time, the central layer requires a higher temperature than the surface layer.

On the other hand, however, it is known that in the silicon steel slab, the crystal grains are likely to coarsen when heated at a high temperature, which causes secondary recrystallization failure or surface defect. In particular, since the slab surface layer has columnar crystals, abnormal growth of crystal grains is extremely likely to occur. Therefore, in contrast to the dissolution of the inhibitor for preventing the change of the slab structure, the steel plate surface is preferably at a low temperature.

FIG. 2 shows the temperature distribution in the slab thickness direction when heat treatment is performed by a general slab heating method.

Typical cross-sectional temperature distribution of the slab heated by the latest high-temperature heating type walking beam type gas heating furnace is as shown by the curve (2), and the surface temperature is at least several tens of degrees Celsius compared to the central part. Is high. On the other hand, the optimum heating temperature distribution considering the slab internal structure expected from FIG. 1 is as shown by the curve (1). Therefore, especially when trying to shorten the heating time as much as possible, it is absolutely necessary to secure the heating temperature.

Conventional walking beam gas heating furnace ignoring equipment capacity (this is mainly heated by radiation from the surface)
The temperature distribution in the slab when the temperature of the central part of the slab is raised to a temperature at which the inhibitor sufficiently dissolves is as shown by the curve (3). In this case, the slab surface becomes the melting temperature of silicon steel, which is impossible in principle. Alternatively, even before melting, the grain boundary portion and the like become vulnerable, and significant surface defects are generated.

Thus, it was extremely difficult for the existing heating method to achieve both complete dissolution of the inhibitor and prevention of coarsening of the slab structure.

Therefore, the inventors set out to develop a heating method which is different from the conventional heating method and which is preferable in principle and economically.

As a result, when electromagnetic induction heating was applied as the slab heating means and the treatment was performed under specific conditions, unexpected results were achieved in achieving the intended purpose.

The present invention is based on the above findings.

That is, the present invention is C: 0.01 to 0.08%, Si: 2.5 to 4.0%, Mn: 0.03 to 0.10%, any one or two kinds of S and Se: 0.01 to 0.0
6% and sometimes additionally at least one selected from Sb, Mo, Cu and Sn:
A silicon steel slab containing 0.005 to 0.2% or / and Al: 0.01 to 0.1%, the balance of which is substantially Fe, is cold-rolled once after hot rolling or twice with intermediate annealing. Prior to hot rolling, in order to produce a grain-oriented electrical steel sheet by a series of steps in which rolling is performed to a final thickness, decarburization / primary recrystallization annealing is performed, and then final finishing annealing is performed, the frequency: 30Hz or more, by electromagnetic induction heating of less than 300Hz, while raising the slab center to 1400 ~ 1470 ℃, for the slab surface, strengthen the heat removal from the surface, 10 ~ 100 ℃ lower than the center temperature The present invention is a method for producing a grain-oriented electrical steel sheet having excellent magnetic properties and surface properties, which is characterized by performing a slab heat treatment.

The present invention was developed based on the finding that the temperature distribution in the slab as shown in FIG. 3 can be achieved by applying the electromagnetic induction heating method and appropriately controlling the frequency and the input power. .

That is, in the conventional method, the heating method was mainly due to radiation, or even when the induction heating method was adopted, only the heating efficiency was so much focused that only the surface layer portion was heated intensively. .

On the other hand, the inventors have found that it is particularly important to heat the central part of the slab to a high temperature, and as a result of examining the method of heating the central part, the frequency during induction heating is appropriately controlled, Moreover, the temperature distribution conventionally required has been successfully obtained by appropriately controlling the heat radiation from the slab surface.

The frequency of induction heating should be 30Hz or more and 300Hz (less than) so that heat can penetrate into the slab as much as possible, and the temperature of the inner wall of the furnace should be heated to an extremely high temperature of 1400 ℃ or more and heat dissipation from the surface should be increased to some extent It is important to control the ambient temperature, and the combination of these two factors has made it possible to control the temperature distribution in the slab thickness direction.

Specific means for removing heat from the surface of the steel sheet include (1) increasing the gas flow rate in the furnace atmosphere to enhance the cooling effect on the surface, (2) increasing slightly the radiation heat dissipation to the refractories around the furnace, Specifically, arrange a cooling pipe inside or outside the adiabatic refractory so that the magnitude of heat removal can be controlled.
Alternatively, it is effective to cool the peripheral heat insulating material with a gas to control heat removal, and mechanically change the thickness.

By combining the above means, it became possible to control the surface temperature within a range of 0 to 50 ° C lower than the center.

In order to sufficiently enhance the above effects in the present invention,
The slab surface temperature during heating is at least 10 ° C above the core temperature
Needs to be low. However, if it is too low, the cost will rise, so the lower limit is lower than the center temperature.
The value was set to be 100 ° C lower. A more preferable surface temperature is a temperature range 20 to 50 ° C. lower than the core temperature.

The application of electromagnetic induction heating to the heating of slabs for grain-oriented silicon steel sheets has already been proposed in JP-A-62-103322. The technology disclosed in the publication relates to an improvement in a method of heating the temperature of the central portion of the slab to 1300 to 1400 ° C. by controlling the frequency during induction heating, and heating uniformly from the surface to the central portion. Is. As described above, the above-mentioned technique has a maximum and sole purpose to make the heating temperature distribution in the slab uniform, and further sets the upper limit of the optimum slab center heating temperature to 1400 ° C based on the premise. Therefore, this technology only covers the center of the slab.
The technical content is completely different from this invention, which is intended to obtain excellent characteristics by heating to over 1400 ° C.

Generally, it is well known that uniform heating at high temperature for a long time is indispensable for uniform dissolution of the inhibitor, and long-time heating was usually applied. However, on the other hand, when the slab is heated at a high temperature for a long time, the crystal grains are coarsened, causing secondary recrystallization defects and surface defects.

Therefore, under normal slab heating conditions, it was unavoidable to achieve uniform dissolution of the inhibitor at the expense of crystal grain coarsening.

As a result of various studies on the conditions for preventing crystal grain coarsening and for the complete dissolution of the inhibitor, the inventors have found that the soaking time is within the range shown in FIG. Have also determined that they can achieve this. Therefore, in this invention, the slab center heating temperature is limited to the range of 1400 to 1470 ° C. Also, the slab surface temperature is 10 to 10% higher than the core temperature as described above.
It is desirable that the temperature is 0 ° C. lower and the soaking time is in the range of 5 to 20 minutes.

(Operation) The reason why the component composition of the raw material in the present invention is limited to the above range will be described.

C: This element is useful for making the structure uniform and fine during hot rolling and cold rolling and for developing the Goss orientation, and it is necessary to add at least 0.01% or more. However, if the content exceeds 0.08%, the Goss orientation will be disturbed, so the upper limit is 0.08%.
Stipulated in.

Si: Increases the specific resistance of the steel sheet and effectively contributes to the reduction of iron loss, but if it exceeds 4.0%, the cold rolling property is impaired, while if it is less than 2.5%, not only the specific resistance decreases, but also the secondary purification. Since the crystal orientation is randomized by α-γ transformation during the final high temperature annealing performed for recrystallization, and a sufficient iron loss improving effect cannot be obtained, the Si content was limited to the range of 2.5 to 4.0%.

Mn: At least 0.03% is required to prevent hot embrittlement, but if it is too much, the magnetic properties deteriorate, so the upper limit was made 0.10%.

Se and S: Both are essential elements as inhibitors that control the secondary recrystallization of grain-oriented silicon steel sheets. From the viewpoint of securing suppression power, it is necessary to contain at least 0.01%.
If it exceeds 06%, the effect is impaired, so the lower and upper limits were made 0.01% and 0.06%, respectively.

As inhibitors, in addition to the above S and Se, Sb, Mo, Cu and Sn
Etc. are also effectively matched.

In particular, the coexistence of Sb and Mo is practically important, and its effect is well known. Further, when Al is added, it is known that good characteristics can be obtained by the single rolling method. In principle, the coexistence of these elements does not impair the effects of the present invention.

Since Cu, Sn, Sb, and Mo have their effects utilized in the total amount of 0.005 to 0.2% and Al in the range of 0.01 to 0.1%, they are also utilized in the present invention in the above range of components. I decided.

(Example) Example 1 C: 0.045%, Si: 3.20%, Mn: 0.08%, Se: 0.025%, Sb: 0.02%
And a continuous casting slab of 240 mm thickness containing Mo: 0.01% and the balance being substantially Fe, while the slab temperature does not fall below 800 ° C, the following heating methods (1) Walking beam type conventional Type gas heating method, (2) conventional electromagnetic induction heating method (frequency: 300Hz), (3) improved electromagnetic induction heating method (frequency: 100Hz) according to the present invention, the slab center temperature is more than 1350 ℃ Heated to

The slab after heating is finished by hot rolling according to the usual method to a thickness of 2.0 mm, and further cold-rolled to 0.5 mm, and then at 1000 ° C for 5
After the intermediate annealing for 2 minutes, the product is finished in a secondary cold rolling to a product thickness of 0.23 mm, and then in a 50% H ₂ + 50% N ₂ atmosphere (80 at a dew point of 60 ° C).
(0 ° C.), decarburization annealing was performed for 3 minutes, the annealing separating agent MgO was applied, and then box annealing was performed in hydrogen at 1200 ° C. for 10 hours to obtain a product.

Table 1 shows the results of an examination of the estimated temperature distribution in the slab, the magnetic property distribution of the product coil, and the surface properties.

(1) In the conventional heating method of the walking beam system, since the slab heating requires a long time and is heated only by the radiation from the surface, the surface temperature is always considerably higher than the center. Further, since the atmosphere cannot be controlled during heating, surface oxidation was unavoidable, and significant surface defects occurred due to high temperature heating.

In fact, we tried to heat the slab at a high temperature in order to secure the slab center temperature, but at most the slab center temperature was raised to 1370 ° C, at which time the surface temperature was 1410 ° C and the surface was exposed to the atmosphere for more than 1 hour. The values of magnetic flux density and iron loss were also insufficient. It is presumed that the cause of this is that the solid solution of the inhibitor, especially the coarse precipitate in the central part of the slab and in the vicinity of internal cracks, which is unavoidable in ordinary casting, was insufficient.
Further, heating at high temperature for a long time causes the slab structure to become coarse, which causes secondary recrystallization failure.

The surface defect is presumed to be due to the surface being exposed to the oxidizing atmosphere at high temperature for a long time.

On the other hand, an electromagnetic induction type slab heating method has been studied recently, and the results obtained by heating with a typical heating heat pattern are shown in Table 1 (2).

In this case, although the temperature could be made sufficiently high, as a characteristic of electromagnetic induction heating, only the vicinity of the surface became particularly high. As a result, secondary recrystallization failure due to coarsening of the slab crystal and accompanying deterioration of magnetic properties were observed, and the expected magnetic properties were not obtained when the slab was heated to such a high temperature. In addition, non-uniformity of the insulating coating due to surface defects caused by coarse particles in the surface layer was also observed.

On the other hand, when the method of the present invention, namely (3) the improved electromagnetic induction heating method is used for heating, the surface temperature is 1410 ° C, but the central temperature reaches 1430 ° C, and the central inhibitor is sufficiently dissolved and the surface layer is An ideal temperature distribution was achieved in which the vicinity was not heated to an unnecessarily high temperature.

In this case, in order to lower the temperature of the surface from the center, rapid heating by the induction heating method is adopted, and at the same time, the temperature of the heat insulating material is lowered by forced cooling in the latter half of the heating (the heat removal is increased), and The temperature of the chisel was effectively controlled and lowered. If you add it for reference, the heat radiation by radiation becomes extremely large at an ultrahigh temperature of about 1400 ° C.
(The size is 4 of the absolute temperature of the heating object temperature and the ambient temperature.
The temperature of the heating slab surface could be easily changed by only changing the temperature of the outer insulation (as is known to be proportional to the power difference).

As a result, magnetic flux density and iron loss were sufficiently good,
Moreover, it is possible to extremely reduce the magnetic characteristic defective portion which usually occurs to some extent. Further, since the surface was not heated to an unnecessarily high temperature, the surface defect occurrence rate was also very small.

Example 2 Molten steels having various compositional compositions shown in Table 2 were each treated with 150 m
After casting in a m-thick mold, slab rolling to a thickness of 100 mm, and then the obtained slab was subjected to electromagnetic induction heating (frequency control) in the thickness direction temperature control by the same ambient temperature and control as in the heating method (3) of Example 1. : 100 Hz) and ordinary electric resistance heating to obtain a temperature distribution as shown in Table 3, and then hot rolling with 5 passes to finish a hot rolled sheet with a thickness of 2.4 mm.

Then, for steels A to D and H to J, a 0.23 mm thick cold was applied by a normal double cold rolling and double annealing method, and for steels E to G, a normal single cold rolling and single annealing method. It was a rolled sheet.

Dew point: 800 in a 60 ° C (50% H ₂ + 50% N ₂ ) atmosphere
After decarburization and primary recrystallization annealing at ℃ for 3 minutes, MgO
After applying an annealing separator containing
Final finishing annealing was performed at 1200 ° C for 10 hours.

Table 4 shows the results of examining the magnetic properties, the secondary recrystallization anomaly occurrence rate, and the surface defect occurrence rate of each obtained product plate.

As is clear from the results shown in Table 4, when the slab heat treatment according to the present invention is performed, both excellent magnetic properties and good surface properties are obtained at the same time.

(Effects of the Invention) Thus, according to the present invention, a grain-oriented electrical steel sheet having not only excellent magnetic properties but also few surface defects can be obtained at low cost without the need for special treatment.

[Brief description of drawings]

FIG. 1 is a graph showing the state of distribution of coarse precipitates in the slab thickness direction, FIG. 2 is a temperature distribution diagram in the slab thickness direction after slab heat treatment according to the conventional method, and FIG. 3 is according to the method of the present invention. FIG. 4 is a graph showing the temperature distribution in the slab thickness direction after slab heating, and FIG. 4 is a graph showing the conditions for grain coarsening and the complete dissolution of the inhibitor in terms of the relationship between slab heating temperature and soaking time.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yoshiaki Iida 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division (56) Reference JP-A-63-100128 (JP, A) JP-A-62 -10214 (JP, A)

Claims (4)

[Claims] 1. C: 0.01 to 0.08 wt%, Si: 2.5 to 4.0 wt%, Mn: 0.03 to 0.10 wt%, any one or two of S and Se: 0.01 to 0.0
A silicon steel slab containing 6 wt%, and the balance of which is substantially Fe, is subjected to hot rolling, one time or intermediate annealing.
After performing cold rolling once to obtain the final plate thickness, decarburizing / primary recrystallization annealing, and then final finishing annealing, the grain-oriented electrical steel sheet is manufactured by hot rolling. Previously, the frequency of 30Hz or more, less than 300Hz by electromagnetic induction heating, while raising the slab center to 1400 ~ 1470 ℃,
For the slab surface, strengthen the heat removal from the surface,
A method for producing a grain-oriented electrical steel sheet excellent in magnetic properties and surface properties, which comprises performing a slab heat treatment at a temperature 10 to 100 ° C lower than a central temperature. 2. C: 0.01 to 0.08 wt%, Si: 2.5 to 4.0 wt%, Mn: 0.03 to 0.10 wt%, any one or two of S and Se: 0.01 to
0.06 wt%, at least one selected from Sb, Mo, Cu and Sn: 0.005 to 0.2 wt%, the rest is a composition of Fe, the silicon steel slab, after hot rolling, A grain-oriented electrical steel sheet is manufactured by a series of processes in which cold rolling is performed once or two times with intermediate annealing sandwiched to obtain the final plate thickness, decarburization and primary recrystallization annealing are performed, and then final finish annealing is performed. In doing so, prior to hot rolling, the frequency: 30 Hz or more, by electromagnetic induction heating of less than 300 Hz, while raising the slab center to 1400 ~ 1470 ° C., for the slab surface, strengthen the heat removal from the surface. And a slab heat treatment at a temperature 10 to 100 ° C. lower than the central temperature, which is a method for producing a grain-oriented electrical steel sheet having excellent magnetic properties and surface properties. 3. C: 0.01 to 0.08 wt%, Si: 2.5 to 4.0 wt%, Mn: 0.03 to 0.10 wt%, any one or two kinds of S and Se: 0.01 to 0.0
A silicon steel slab containing 6 wt%, Al: 0.01 to 0.1 wt%, and the balance of which is substantially Fe, is hot-rolled and then cold-rolled once or twice with intermediate annealing. Prior to hot rolling, a frequency of 30Hz or more is used to manufacture grain-oriented electrical steel sheets by a series of processes in which decarburization / primary recrystallization annealing is performed after the final sheet thickness is applied, and then final finish annealing is performed. , The temperature of the slab center is raised to 1400 to 1470 ° C by electromagnetic induction heating of less than 300 Hz, while the heat removal from the slab surface is strengthened, resulting in a temperature 10 to 100 ° C lower than the center temperature. A method for manufacturing a grain-oriented electrical steel sheet having excellent magnetic properties and surface properties, which is characterized by performing a slab heat treatment. 4. C: 0.01 to 0.08 wt%, Si: 2.5 to 4.0 wt%, Mn: 0.03 to 0.10 wt%, any one or two of S and Se: 0.01 to 0.0
6% by weight, at least one selected from Sb, Mo, Cu and Sn: 0.005 to 0.2% by weight, Al: 0.01 to 0.1% by weight, with the balance being essentially Fe composition silicon steel slab After hot rolling, the steel sheet is subjected to one or two cold rolling steps with an intermediate anneal between them to obtain the final thickness, followed by decarburization / primary recrystallization anneal, followed by a final finish anneal. Prior to hot rolling in producing a grain-oriented electrical steel sheet by a frequency: 30Hz or more, by electromagnetic induction heating of less than 300Hz, the slab center is heated to 1400 ~ 1470 ℃, while the slab surface, the surface A method for producing a grain-oriented electrical steel sheet having excellent magnetic properties and surface properties, which is characterized by performing a slab heat treatment at a temperature 10 to 100 ° C. lower than a central temperature by enhancing heat removal from the steel.