JPH04362133A - Production of thick grain-oriented silicon steel plate excellent in magnetic property - Google Patents

Abstract

PURPOSE:To improve the magnetic properties of a thick grain-oriented silicon steel plate used for iron core for electrical equipment. CONSTITUTION:A slab having a composition consisting of C, Si, acid soluble Al, N, <=0.014% (S+0.405Se), 0.05-0.8% Mn, and the balance Fe with inevitable impurities is heated up to <1280 deg.C and hot-rolled. Subsequently, the resulting hot rolled plate is subjected, without hot rolled plate annealing, to cold rolling at 60-79% reduction of area into a plate of 0.4-1.0mm thickness. Then, the thick grain-oriented silicon steel plate is produced by the known method. At this time, hot rolling finishing temp. is regulated to 600-850 deg.C and cumulative reduction of area in the final three passes in hot rolling is regulated to >=30% and further the average grain size of the primary recrystallized grains after the completion of decarburizing annealing is regulated to 18-30mum, and, in the course between the completion of hot rolling and the initiation of secondary recrystallization in final finish annealing nitriding treatment is applied to the steel plate.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing unidirectional electrical steel sheets having excellent magnetic properties and used as cores of transformers and the like.

0002

BACKGROUND OF THE INVENTION Unidirectional electrical steel sheets are mainly used as core materials for transformers and other electrical equipment, and are required to have excellent magnetic properties such as excitation properties and iron loss properties. As a numerical value representing the excitation characteristic, the strength of the magnetic field is 800A.
A magnetic flux density B8 at /m is usually used. Also,
The numerical value representing iron loss characteristics is 1.7 at a frequency of 50Hz.
Iron loss W per 1 kg when magnetized to Tesler (T)
I am using 17/50. Magnetic flux density is the most dominant factor in iron loss characteristics, and generally speaking, the higher the magnetic flux density, the better the iron loss characteristics. In general, when the magnetic flux density is increased, secondary recrystallized grains become larger, which may result in poor iron loss characteristics. On the other hand, by magnetic domain control, the iron loss characteristics can be improved regardless of the grain size of the secondary recrystallized grains.

[0003] This unidirectional electrical steel sheet is produced by causing secondary recrystallization in the final finish annealing process and developing a so-called Goss structure with a {110} axis on the steel sheet surface and a <001> axis in the rolling direction. Manufactured. In order to obtain good magnetic properties, it is necessary to highly align <001>, which is the axis of easy magnetization, in the rolling direction. Typical manufacturing techniques for such high magnetic flux density unidirectional electrical steel sheets include the methods described in Japanese Patent Publication No. 40-15644 by Satoru Taguchi et al. and Japanese Patent Publication No. 13469 No. 1987 by Takuichi Imanaka et al. . MnS and AlN in the former, MnS in the latter
, MnSe, Sb, etc. are used as main inhibitors. Therefore, in current technology, it is essential to properly control the size, morphology, and dispersion state of these precipitates that function as inhibitors. As for MnS,
In the current process, MnS is once completely dissolved in solid solution during slab heating before hot rolling, and then precipitated during hot rolling. A temperature of about 1400° C. is required to completely dissolve the amount of MnS required for secondary recrystallization. This is more than 200° C. higher than the slab heating temperature of ordinary steel, and this high temperature slab heating treatment has the following disadvantages.

1) A high-temperature slab heating furnace exclusively for grain-oriented electrical steel is required. 2) The energy consumption rate of the heating furnace is high. 3) The amount of molten scale increases, which has a large negative impact on operations as seen in so-called slag scraping. In order to avoid such problems, the slab heating temperature can be lowered to the same level as for ordinary steel, but this also means that the amount of MnS, which is effective as an inhibitor, must be reduced or not used at all. This inevitably leads to destabilization of secondary recrystallization. Therefore, in order to realize low-temperature slab heating, it is necessary to strengthen the inhibitor in some way with precipitates other than MnS to sufficiently suppress normal grain growth during final annealing. As such inhibitors, in addition to sulfides, nitrides, oxides, grain boundary precipitated elements, etc. can be considered, and examples of known techniques include the following.

[0005] In Japanese Patent Publication No. 54-24685, As
, Bi, Sn, Sb, and other grain boundary segregation elements are included in the steel to increase the slab heating temperature to a range of 1050 to 1350°C. Japanese Patent Publication No. 52-2411
In Publication No. 6, in addition to Al, Zr, Ti, B, Nb, Ta
, V, Cr, Mo, and other nitride-forming elements have been disclosed to increase the slab heating temperature to a range of 1100 to 1260°C. Also, JP-A-57-1583
In Publication No. 22, low-temperature slab heating is performed by lowering the Mn content and making the Mn/S ratio 2.5 or less,
Furthermore, a technique for stabilizing secondary recrystallization by adding Cu is disclosed. On the other hand, techniques have also been disclosed in which improvements are made from the metal structure side in combination with reinforcement of these inhibitors. That is, in JP-A-57-89433, in addition to Mn, S, Se, Sb, Bi, Pb, Sn,
By adding elements such as B and combining this with the columnar crystallinity of the slab and the secondary cold rolling reduction,
We have achieved low-temperature slab heating of ℃. Furthermore, JP-A-5
In Publication No. 9-190324, in addition to S or Se,
The inhibitor is composed mainly of Al, B and nitrogen,
A technique has been disclosed that stabilizes secondary recrystallization by performing pulse annealing during primary recrystallization annealing after cold rolling. As described above, great efforts have been made to realize low-temperature slab heating in the production of grain-oriented electrical steel sheets.

[0006] Previously, in Japanese Patent Application Laid-Open No. 59-56522, Mn was 0.08 to 0.45% and S was 0.00%.
A technology has been disclosed that enables low-temperature slab heating by heating the slab to 7% or less, and this method solves the problem of linear secondary recrystallization defects in products caused by coarsening of slab crystal grains during high-temperature slab heating. It was done.

[0007]

[Problems to be Solved by the Invention] In recent years, there has been an increasing need to use grain-oriented electrical steel sheets in place of the currently used high-grade non-oriented electrical steel sheets for applications such as core materials for turbine generators. Regarding the above-mentioned uses, compared to other uses of non-oriented electromagnetic steel sheets, magnetic properties in one direction are considered to be more important, so the need for using grain-oriented electromagnetic steel sheets has increased. On the other hand, the main manufacturing process of grain-oriented electrical steel sheets after hot rolling is hot-rolled sheet annealing - cold rolling - decarburization annealing - finishing annealing, which is the main process after hot rolling of non-oriented electrical steel sheets. It is more complicated than cold rolling-annealing. Therefore, in terms of manufacturing cost, grain-oriented electrical steel sheets are considerably more expensive than non-oriented electrical steel sheets.

Furthermore, in a normal pickling line or a tandem cold rolling line, there is a limit to the thickness that can be passed through the sheet, and if a thick cold rolled material is passed through the sheet, breakage may occur. Therefore, if a thick material with a thickness of 0.5 mm or the like is to be manufactured by cold rolling once, there is an upper limit to the thickness of the cold-rolled material, so it is necessary to reduce the cold-rolling rate. Furthermore, in the production of grain-oriented electrical steel sheets, hot-rolled sheets are usually annealed for the purpose of making the structure non-uniform, precipitation treatment, etc. after hot rolling. For example, in the production method using AlN as the main inhibitor,
As shown in Publication No. 3820, AlN
A method has been adopted to control the inhibitor by performing a precipitation treatment.

In recent years, the manufacturing process of grain-oriented electrical steel sheets, which consumes a large amount of energy, has been reviewed, and there has been a growing demand for simplification of the process and energy. In order to meet these demands, in a manufacturing method using AlN as the main inhibitor, a method was developed in which the AlN precipitation treatment during hot-rolled sheet annealing was replaced by high-temperature coiling after hot rolling (Japanese Patent Publication No. 59
-45730) was proposed. It is true that magnetic properties can be maintained to some extent even if hot-rolled sheet annealing is omitted using this method, but in the normal method of winding into a 5-20 ton coil, A difference in thermal history occurs depending on the location, inevitably resulting in non-uniform precipitation of AlN, and the final magnetic properties vary depending on the location within the coil, resulting in a decrease in yield.

Therefore, the present inventors focused on the recrystallization phenomenon after the final pass of finish hot rolling, which had received little attention in the past, and utilized this phenomenon to develop a manufacturing method using one cold rolling with a strong reduction of 80% or more. proposed a method of omitting hot-rolled sheet annealing (Japanese Patent Application No. 1-85540, Japanese Patent Application No. 1-85541). These technologies are characterized by creating a fine recrystallized structure in the hot-rolled sheet through strong reduction during the final three passes of finish hot rolling and holding at a high temperature after the completion of hot rolling. It has become possible to both heat the slab at a high temperature and omit hot-rolled sheet annealing.

Regarding hot rolling of unidirectional electrical steel sheets, secondary recrystallization failure (generation of linear fine grains connected in the rolling direction) due to coarse growth of slab crystal grains during high-temperature slab heating (for example, 1300° C. or higher) ) to prevent 960° during hot rolling.
A method has been proposed in which coarse crystal grains are divided by performing recrystallization high reduction rolling at a temperature of ~1190°C and a reduction rate of 30% or more per pass (Japanese Patent Publication No. 37172/1983).
. Although this method certainly reduces the generation of linear fine particles,
The manufacturing process is based on hot-rolled sheet annealing.

[0012] In addition, in a manufacturing method using MnS, MnSe, Sb as an inhibitor,
A method has been proposed in which MnS and MnSe are uniformly and finely precipitated by continuous hot rolling at a temperature of 10°C at a reduction rate of 10% or more and subsequent cooling at a cooling rate of 3°C/sec or more to improve magnetic properties. (Japanese Unexamined Patent Publication No. 51-2071
Publication No. 6). Another method is to perform hot rolling at a low temperature, suppress the progress of recrystallization, and prevent {110}<001> oriented grains formed by shear deformation from being reduced by subsequent recrystallization, thereby improving magnetic properties. has been proposed (Japanese Patent Publication No. 59-32526, Japanese Patent Publication No. 59-35415). In these methods as well, 1 without annealing the hot rolled sheet.
Production by the re-cold rolling method has not even been considered. In addition, when hot-rolling silicon steel slabs containing ultra-low carbon, we conduct low-temperature, large-reduction hot rolling that accumulates strain in the hot-rolled plate, and then recrystallize in the subsequent hot-rolled plate annealing to create the coarse crystal grains characteristic of ultra-low carbon materials. A method has been proposed (Japanese Patent Publication No. 59-34212). However, even in this method, production by a one-time cold rolling method without hot-rolled sheet annealing has not even been considered.

[0013] Therefore, the present inventors have developed a technique (patent application No.
85540, Japanese Patent Application No. 1-85541) are of great significance.

[0014]

Means for Solving the Problems According to the present invention, C by weight
:0.021~0.075%, Si:2.5~4.5%
, acid-soluble Al: 0.010-0.060%, N: 0.
0030-0.0130%, S+0.405Se:0.
0.014% or less, Mn: 0.05 to 0.8%, and the balance consists of Fe and inevitable impurities.
Heating at a temperature below ℃, hot rolling, followed by cold rolling at a rolling reduction of 60 to 79% without hot-rolled sheet annealing, followed by decarburization annealing and final finish annealing to 0.4 to 1 .0mm
In a method for manufacturing thick unidirectional electrical steel sheets, the hot rolling end temperature is 600 to 850°C, the cumulative reduction rate in the final three passes of hot rolling is 30% or more, and after completion of decarburization annealing, final finish annealing is performed. The average grain size of primary recrystallized grains until the start is 1
By applying nitriding treatment to the steel sheet after hot rolling and before the start of secondary recrystallization during final finish annealing, a thick unidirectional electrical steel sheet with excellent magnetic properties can be stably obtained. .

[0015]

[Operation] The unidirectional electrical steel sheet targeted by the present invention is
Molten steel obtained by conventional steel-making methods is cast by continuous casting or ingot making, followed by a blooming process as necessary to form a slab, followed by hot rolling into a hot-rolled plate, and then hot rolling. Cold rolling with a rolling reduction of 60 to 79% without plate annealing,
Manufactured by sequentially performing decarburization annealing and final finish annealing.

[0016] The inventors of the present invention found it necessary to lower the rolling reduction due to limitations on the thickness of the cold-rolled material, and extensively studied ways to improve the magnetic properties with a rolling reduction of less than 80%. As a result, new findings were obtained that lowering the hot rolling end temperature to 600 to 850°C and increasing the cumulative reduction rate of 30% or more in the final three passes of hot rolling are effective in obtaining good magnetic properties. .

A detailed explanation will be given below based on experimental results. FIG. 1 is a graph showing the influence of the cumulative reduction rate of the final three passes of hot rolling and the end temperature of hot rolling on the magnetic flux density of the product. Here, C: 0.042% by weight, Si: 3.11% by weight, acid-soluble Al: 0.031% by weight, N: 0.0069% by weight, S: 0.007% by weight, Mn: 0.14 A 40 mm thick slab containing Fe and unavoidable impurities was heated to 1150° C. for 6 passes to form a 2.0 mm thick hot rolled plate. At this time, pass schedules were conducted under various conditions. In addition, the hot rolling start temperature is 700 to 1100°C.
And so. At this time, the hot rolling end temperature was 575 to 1041°C. After hot rolling, air cooling for 1 second, water cooling to 550°C,
A winding simulation was performed in which the sample was held at ℃ for 1 hour and then cooled in a furnace. Thereafter, this hot-rolled sheet was pickled and then cold-rolled at a rolling reduction of 75% to obtain a cold-rolled sheet with a thickness of 0.50 mm.

Next, the temperature was maintained at 840°C for 400 seconds, and the temperature was heated to 860°C.
Decarburization annealing was performed by holding at ℃ for 20 seconds. After that, 75
During the heat treatment held at 0°C for 30 seconds, NH3 was added to the atmospheric gas.
Gas was mixed in to cause nitrogen absorption in the steel plate. At this time, the amount of N in the steel plate was 0.0192 to 0.0211% by weight. The average grain size of primary recrystallized grains throughout the thickness of this steel plate was measured using an optical microscope and an image analyzer.22
It was ~25 μm. Next, M was applied to this nitrided plate.
An annealing separator containing gO as a main component was applied, and final annealing was performed.

As is clear from FIG. 1, a good magnetic flux density of B8≧1.88 (T) can be obtained when the cumulative reduction rate of the final three passes of hot rolling is 30% or more and the hot rolling end temperature is 850° C. or less. There is. The reason why the relationship shown in FIG. 1 is established is not necessarily clear, but the inventors of the present invention speculate as follows. It has been known that the cold rolling ratio is a controlling factor for the cold rolling recrystallization texture, and in particular, the controlling factor for the secondary recrystallization orientation is {110}<001>, {111}.
}<112> oriented grain abundance is important. This {110}<001> oriented grain in the recrystallized texture is 60-7
It reaches a maximum at a rolling reduction of 0%, and decreases as the rolling reduction increases in a range of over 70%. On the other hand, {111}<112> oriented grains in the recrystallized texture account for approximately 90%
It tends to increase as the rolling reduction increases within the range of rolling reduction. On the other hand, in hot rolling, crystal rotation during rolling, dynamic recovery during rolling, static recovery between passes, and static or quasi-dynamic recrystallization occur. In recrystallization in hot rolling, if nucleation-growth type recrystallization occurs, a texture change occurs. Temperature is important from the viewpoint of recrystallization in hot rolling, and lowering the temperature makes recovery and recrystallization less likely to occur.
The metallographic structure and texture are close to those of cold-rolled steel. As in the present invention, by setting the hot rolling end temperature to 850°C or lower and setting the cumulative reduction rate of the final three passes of hot rolling to 30% or more, the metallographic structure and texture of the hot rolled sheet are changed as if the reduction rate was 1 to 1. By successfully creating a product that looks as if it had been cold-rolled by about 30%, the optimum cold-rolling rate for maximizing magnetic flux density has been increased to 60-79%.
It is considered that they succeeded in reducing the amount to a certain degree.

Next, the reasons for limiting the constituent elements of the present invention will be described. First, the reasons for limitations regarding the components of the slab and the slab heating temperature will be explained in detail. C is 0.021% by weight (
If B8 is less than 1.80 (hereinafter simply abbreviated as %), secondary recrystallization becomes unstable, and even if secondary recrystallization is performed, B8 > 1.80 (
Since T) is difficult to obtain, the content was set at 0.021% or more. On the other hand, C
If it increases too much, the decarburization annealing time becomes long and is not economical, so it is set to 0.075% or less.

[0021] If Si exceeds 4.5%, cracking during cold rolling becomes significant, so it is set to 4.5% or less. Moreover, if it is less than 2.5%, the specific resistance of the material will be too low and the low core loss required for a transformer core material cannot be obtained, so it is set at 2.5% or more. It is preferably 3.2% or more. Al is AlN or (Al,Si)nitrid, which is necessary for stabilizing secondary recrystallization.
To ensure es, 0.010% as acid-soluble Al
The above is necessary. If acid-soluble Al exceeds 0.060%, the AlN of the hot rolled sheet becomes inappropriate and secondary recrystallization becomes unstable, so it was set to 0.060% or less.

[0022]N is 0.00 in normal steelmaking work.
Since it is difficult and economically unfavorable to reduce the content to less than 30%, it is set to 0.0030% or more, while 0.01%
If it exceeds 30%, "blistering" on the surface of the steel plate will occur, so it was set to 0.0130% or less. Mn
Even if S and MnSe are present in steel, it is possible to improve the magnetic properties by appropriately selecting the manufacturing process conditions. However, when S and Se are high, secondary recrystallization defects called linear fine grains tend to occur, and in order to prevent the occurrence of secondary recrystallization defects, (S + 0.4
5Se)≦0.014%. If S or Se exceeds the above value, the probability that secondary recrystallization defects will occur increases, which is undesirable, no matter how the manufacturing conditions are changed. Also, the time required for purification in final finish annealing is undesirable, and from this point of view, S
Alternatively, there is no point in increasing Se unnecessarily.

The lower limit of Mn is 0.05%. 0.0
If it is less than 5%, the shape (flatness) of the hot-rolled sheet obtained by hot rolling, especially the side edges of the strip, becomes wavy, resulting in a problem of lowering the product yield. On the other hand, if the Mn content exceeds 0.8%, the magnetic flux density of the product decreases, which is undesirable, so the upper limit of the Mn content was set to 0.8%. In addition, Sn and Sb, which are known as inhibitor constituent elements,
, Cr, Cu, Ni, B, Ti, etc. may be contained in trace amounts.

[0024] The slab heating temperature was limited to less than 1280°C for the purpose of reducing costs by making it comparable to ordinary steel. Preferably it is 1200°C or less. In the subsequent hot rolling process, after heating the slab, which is usually 100 to 400 mm thick,
Both consist of rough hot rolling and finishing hot rolling performed in multiple passes. The rough hot rolling method is not particularly limited and may be carried out by a conventional method. The feature of the present invention is the finish hot rolling that follows the rough hot rolling. Finish hot rolling is usually performed by high-speed continuous rolling of 4 to 10 passes. Normally, the rolling reduction in finishing hot rolling is such that the rolling reduction is high in the first stage and the rolling reduction is lower towards the latter stage to obtain a good shape. The rolling speed is usually 100-3000m/
The time between passes is 0.01 to 100.
seconds. What is limited in the present invention is only the hot rolling end temperature and the cumulative reduction rate of the final three passes of hot rolling, and other conditions are not particularly limited. It is also preferable to perform the process under strong pressure because it causes some recrystallization and improves the structure.

Next, the reasons for limiting the above hot rolling conditions will be described. The hot rolling end temperature was 600 to 850°C. 850
If it exceeds .degree. C., recrystallization tends to occur, and as shown in FIG. 1, in the case of a low cold rolling rate as in the present invention, the magnetic flux density decreases, which is not preferable. On the other hand, if it is less than 600°C, rolling becomes difficult, which is not preferable.

On the other hand, the cumulative reduction rate in the final three passes of finish hot rolling was set to 30% or more. If the value is less than this, the effect of cold rolling on the metallographic structure and texture will not be sufficient, and as shown in FIG.
As shown in the figure, in the case of low reduction cold rolling as in the present invention, the magnetic flux density of the product decreases, which is not preferable. Although the upper limit of the cumulative reduction rate of the final three passes is not particularly limited, it is industrially difficult to apply a cumulative reduction of 99.9% or more.

[0027] After the final pass of hot rolling, it is cooled in air for usually about 0.1 to 100 seconds, then cooled in water, wound up at a temperature of 300 to 700°C, and slowly cooled. Although this cooling process is not particularly limited, it is preferable to perform rapid cooling within 5 seconds after hot rolling in order to suppress recrystallization. This hot-rolled sheet was then subjected to a rolling reduction of 60 without being annealed.
~79% cold rolling is performed to obtain a cold rolled sheet with a thickness of 0.4 to 1.0 mm.

The reason why the thickness of the cold-rolled sheet is defined as 0.4 to 1.0 mm is for the purpose of the present invention to obtain a thick unidirectional electrical steel sheet. Moreover, if it exceeds 1.0 mm, decarburization annealing takes too much time, which is not preferable. This reduction rate is 60 to 79
% because if the cold-rolled material is too thick, it is likely to break during pickling or passing through the cold-rolling line, so it is necessary to reduce the cold-rolling rate. This upper limit value comes from the plate thickness limitation of the cold-rolled material, while the lower limit value was determined from the need to maintain the magnetic flux density at a high level.

[0029] This cold rolling method is not particularly limited. Either tandem method or reverse method is acceptable. There is no particular limitation on the number of passes, but
There is no point in taking over 100 passes unnecessarily. The cold-rolled steel sheet is subjected to decarburization annealing, application of an annealing separator, and final finish annealing in a conventional manner to obtain a final product. The reason why the average grain size of the primary recrystallized grains after the completion of decarburization annealing and before the start of final annealing is set to 18 to 30 μm is because within this value range, a good magnetic flux of B8 (T)≧1.88 is achieved. This is because the density can be stably obtained.

[0030]The reason why the steel sheet is nitrided after hot rolling and before the start of secondary recrystallization during final finish annealing is that in a process based on low-temperature slab heating such as the present invention, secondary recrystallization is required. This is because the inhibitor strength required for recrystallization tends to be insufficient. The nitriding method is not particularly limited, and after decarburization annealing, NH3 is added to the annealing atmosphere.
A method of nitriding by mixing gas, a method of using plasma,
A method of adding nitrides to the annealing separator and allowing the steel sheet to absorb the nitrogen produced by decomposition of the nitrides during temperature rise during final annealing.
Any method may be used, such as increasing the N2 partial pressure in the final annealing atmosphere and nitriding the steel plate. Although the amount of nitriding is not particularly limited, it is required to be 1 ppm or more.

[0031]

[Example] An example will be explained below. Example 1 C: 0.038% by weight, Si: 3.08% by weight, Mn:
0.15% by weight, S: 0.006% by weight, acid-soluble Al
After heating a 40 mm thick slab containing: 0.028% by weight, N: 0.0065% by weight, and the balance consisting of Fe and unavoidable impurities at a temperature of 1150°C, ■ 1000°C,
■Start hot rolling at 900℃, ■800℃, 40→23→
It was hot rolled with a pass schedule of 14 → 9 → 6 → 3.5 → 2 (mm) to obtain a 2.0 mm hot rolled sheet. At this time, the hot rolling end temperature was 912° C. in the case of (1), 825° C. in the case of (2), and 741° C. in the case of (2), and in this case, the cumulative reduction rate of the final three passes was 78%. After hot rolling, a coiling simulation was performed in which the product was air cooled for 1 second, then water cooled to 550°C, held at 550°C for 1 hour, and then cooled in a furnace. Thereafter, this hot-rolled sheet was pickled and then cold-rolled at a rolling reduction of 75% to obtain a cold-rolled sheet with a thickness of 0.50 mm.

Next, the temperature was maintained at 840°C for 300 seconds, and then heated to 860°C.
Decarburization annealing was performed by holding at ℃ for 20 seconds. After that, 75
During the heat treatment held at 0°C for 30 seconds, NH3 was added to the atmospheric gas.
Gas was mixed in to cause nitrogen absorption in the steel plate. At this time, the amount of N in the steel plate was 0.0197 to 0.0214% by weight. Further, the average grain size of primary recrystallized grains in the entire thickness of this steel plate was measured using an optical microscope and an image analyzer, and was found to be 23 to 25 μm. Next, an annealing separator containing MgO as a main component was applied to the nitrided steel sheet, and final finish annealing was performed using a known method.

Table 1 shows the experimental conditions and magnetic properties of the product.

[0034]

[Table 1]

Example 2 C: 0.051% by weight, Si: 3.23% by weight, Mn:
0.14% by weight, S: 0.007% by weight, acid-soluble Al
A 40 mm thick slab containing: 0.031 wt% N, 0.0065 wt% N, and the remainder Fe and unavoidable impurities was heated at a temperature of 1100°C, and then hot rolled in 6 passes to a thickness of 2.3 mm. It was made into a hot rolled sheet. At this time, the reduction distribution is 40 →
20 → 10 → 5 → 4 → 3 → 2.3 (mm). At this time, the hot rolling start temperature was set to (1) 1000°C and (2) 850°C. In this case, the hot rolling end temperature is 894°C in case of ■;
In the case of
%Met. After hot rolling, air cooling for 1 second, then water cooling to 450°C,
A winding simulation was performed in which the sample was held at 450° C. for 1 hour and then cooled in a furnace. Thereafter, this hot-rolled sheet was pickled and then cold-rolled in the same direction at a rolling reduction of 78% to obtain a cold-rolled sheet with a thickness of 0.50 mm.

[0036] Next, hold at 830°C for 300 seconds, and
Decarburization annealing was performed by holding at 0°C for 20 seconds. After that, 7
During the heat treatment held at 50°C for 30 seconds, NH was added to the atmospheric gas.
3 Gas was mixed in to cause nitrogen absorption in the steel plate. At this time, the amount of N in the steel plate is 0.0208 to 0.0219% by weight.
Met. Further, the average grain size of primary recrystallized grains in the entire thickness of this steel plate was measured using an optical microscope and an image analyzer, and was found to be 22 to 24 μm. Next, an annealing separator containing MgO as a main component was applied to the nitrided steel plate, and final finish annealing was performed using a known method.

Table 2 shows the experimental conditions and magnetic properties of the product.

[0038]

[Table 2]

Example 3 C: 0.033% by weight, Si: 3.15% by weight, Mn:
0.15% by weight, S: 0.006% by weight, acid-soluble Al
After heating a 30 mm thick slab containing Fe: 0.029% by weight, N: 0.0071% by weight, and the balance consisting of Fe and unavoidable impurities at a temperature of 1150°C, hot rolling was performed.
．． It was made into a 3 mm hot rolled sheet. In this case, the reduction distribution is 30 →
20 → 13 → 8 → 5 → 3.0 → 2.3 (mm). The hot rolling start temperature was set to (1) 1100°C and (2) 900°C. At this time, the hot rolling end temperature was 935°C in the case of (1) and 837°C in the case of (2). The cumulative rolling reduction rate of the final three passes was 71%. After hot rolling, a winding simulation was performed in which the material was air cooled for 1 second, water cooled to 500° C., held at 500° C. for 1 hour, and then cooled in a furnace. Thereafter, this hot-rolled sheet was pickled and then cold-rolled at a rolling reduction of 78% to obtain a cold-rolled sheet with a thickness of 0.50 mm. At this time, when the thicknesses were 1.8 mm, 1.2 mm, and 0.8 mm, aging treatment was performed at 100° C. for 5 minutes (soaking). Next, decarburization annealing was performed by holding the temperature at 835° C. for 400 seconds. Thereafter, during heat treatment at 750°C for 30 seconds, NH3 gas was mixed into the atmospheric gas to cause nitrogen absorption in the steel plate. At this time, the amount of N in the steel plate is 0.0198 to 0.02
It was 15% by weight. Further, the average grain size of primary recrystallized grains in the entire thickness of this steel plate was measured using an optical microscope and an image analyzer, and was found to be 21 to 23 μm. Then,
An annealing separator containing MgO as a main component was applied to the steel plate after the nitriding treatment, and final annealing was performed using a known method.

Table 3 shows the experimental conditions and magnetic properties of the product.

[0041]

[Table 3]

Example 4 C: 0.035% by weight, Si: 3.13% by weight, Mn:
0.15% by weight, S: 0.007% by weight, acid-soluble Al
: 0.031% by weight, N: 0.0065% by weight, Sn:
A 40 mm thick slab containing 0.07% by weight and the remainder Fe and unavoidable impurities was heated at a temperature of 1150° C. and then hot rolled to obtain a 2.3 mm hot rolled sheet. Reduction distribution 40 → 18 → 8 → 4 → 3 → 2.5 → 2.3 (m
m). Hot rolling start temperature: ■1050℃, ■700℃
And so. At this time, the hot rolling end temperature was 891°C in the case of (1) and 635°C in the case of (2), and in this case, the cumulative reduction rate of the final three passes was 43%. After hot rolling, a coiling simulation was performed in which the product was air cooled for 1 second, then water cooled to 550°C, held at 550°C for 1 hour, and then cooled in a furnace. After that, this hot rolled sheet is pickled,
Next, it was cold-rolled at a rolling reduction of 74% to obtain a cold-rolled sheet with a thickness of 0.60 mm. Next, decarburization annealing was performed by holding at 840°C for 350 seconds and then at 860°C for 20 seconds. Thereafter, during heat treatment held at 750° C. for 30 seconds, NH3 gas was mixed into the atmospheric gas to cause nitrogen absorption in the steel plate. At this time, the amount of N in the steel plate is 0.0184 to 0.0203
% by weight. Further, the average grain size of primary recrystallized grains in the entire thickness of this steel plate was measured using an optical microscope and an image analyzer, and was found to be 23 to 25 μm. Next, an annealing separator containing MgO as a main component was applied to the nitrided steel plate, and final finish annealing was performed using a known method.

Table 4 shows the experimental conditions and magnetic properties of the product.

[0044]

[Table 4]

[0045]

[Effects of the Invention] As explained above, in the present invention,
To perform nitriding treatment on a steel sheet by controlling the end temperature of hot rolling, the cumulative reduction rate of the final three passes of hot rolling, and the average grain size of primary recrystallized grains between the completion of decarburization annealing and the start of final finish annealing. Accordingly, it is possible to omit hot-rolled sheet annealing and obtain a thick unidirectional electrical steel sheet having good magnetic properties at a low cold rolling rate, so the industrial effect is extremely large.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of the cumulative reduction rate of the final three passes of hot rolling and the end temperature of hot rolling on the magnetic flux density of the product.

Claims (1)

[Claims] [Claim 1] C: 0.021-0.075% by weight
, Si: 2.5-4.5%, Acid-soluble Al: 0.010
~0.060%, N:0.0030~0.0130%,
S+0.405Se: 0.014% or less, Mn: 0.0
A slab containing 5 to 0.8% and the remainder consisting of Fe and unavoidable impurities is heated at a temperature below 1280°C, hot rolled, and subsequently reduced to a rolling reduction of 60 to 60% without hot-rolled plate annealing.
In a method of manufacturing a thick unidirectional electrical steel sheet with a thickness of 0.4 to 1.0 mm by performing 79% cold rolling, followed by decarburization annealing and final finish annealing, the hot rolling end temperature is set at 600 to 850 mm.
℃, the cumulative reduction rate of the final three passes of hot rolling is 30% or more, and the average grain size of primary recrystallized grains from the completion of decarburization annealing to the start of final finish annealing is 18 to 30 μm. A method for producing a thick unidirectional electrical steel sheet with excellent magnetic properties, characterized by subjecting the steel sheet to nitriding treatment before the start of secondary recrystallization after final finish annealing.