JP2008031495A - Manufacturing method of grain-oriented electrical steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a grain-oriented electrical steel sheet having high permeability and workability. <P>SOLUTION: When manufacturing a grain-oriented electrical steel sheet by performing the final finish annealing so as not to generate a ceramic coating film on a surface with a steel slab containing ≤0.02% C and 1.0-5.0% Si and having the inhibitor-less composition as a base material, the cumulative draft when the surface temperature of the steel sheet during the hot rolling is ≥950°C is ≥75% and the cumulative draft when the surface temperature of the steel sheet is ≥1,050°C is ≥20%. The required time in the temperature rising process of the initial annealing after the hot rolling between 500 and 900°C is within 100 seconds. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a unidirectional electrical steel sheet that is suitable for use mainly in iron core materials such as small transformers such as EI cores, rotary machines, generator cores, and ignition coils.

Conventional technology

A unidirectional electrical steel sheet (hereinafter referred to as a unidirectional electrical steel sheet) oriented in the {110} <001> orientation, the so-called Goss orientation, contains a component called an inhibitor in the steel, and the steel slab is heated at a high temperature. After solidifying the inhibitor by heating, the inhibitor is finely precipitated in the hot rolling process, and after cold rolling, recrystallization annealing is performed which also serves as decarburization, and then an annealing separator mainly composed of MgO is added. In general, the above crystal orientation is obtained by applying a phenomenon called secondary recrystallization during final finishing annealing at a high temperature for a long time. Here, MgO, which is the main component of the annealing separator, reacts with the silica formed on the surface of the steel sheet by recrystallization annealing performed in a humid atmosphere during the final finish annealing, so that forsterite (Mg ₂ SiO ₄ ) Form a layer. This forsterite layer not only imparts tension to the steel sheet itself, but also acts as an intermediate layer between the vitreous insulating tension coating and the ground iron baked on the surface after final finish annealing. Thus, although the forsterite layer is a by-product due to the annealing separator, it is considered essential for the unidirectional electrical steel sheet that requires a reduction in iron loss due to the insulating tension coating.

On the other hand, the forsterite layer has the above-mentioned usefulness, but because it is ceramic, it has a high hardness, and there is a problem that the wear of the mold is remarkable when an iron core is manufactured by stamping. is doing.
In addition, when having a vitreous insulating tension coating on the forsterite layer, the problem is that the adhesion during bending is inferior to that of a coating containing an organic material used as an insulating coating for non-oriented electrical steel sheets. There is a point.
Because of these problems, it has been difficult to industrially use unidirectional electrical steel sheets as iron cores that require punching or bending.

In order to solve the above-mentioned problems, it is conceivable to first produce a unidirectional electrical steel sheet that does not form a ceramic film such as forsterite, and to form the same insulating film as the non-oriented electrical steel sheet. .
As such a technique, a method of removing the forsterite film by a method such as pickling or grinding after the final finish annealing can be considered, but this method not only increases the cost but also deteriorates the surface properties and reduces the magnetic properties. There is a problem that the characteristics are deteriorated.

Further, for example, Patent Document 1 discloses a technique for suppressing the formation of forsterite by blending a chemical in an annealing separator mainly composed of MgO applied to final finish annealing, and Patent Document 2 further describes Mn. A technique for suppressing the formation of forsterite by applying an annealing separator mainly composed of silica or alumina to the material to be contained is disclosed.
However, these techniques require purification annealing at a high temperature to remove the inhibitor component, which increases the manufacturing cost and increases the crystal grain size of the product and impairs workability. There was a problem.

In contrast to these techniques, Patent Document 3 discloses a method in which goss-oriented crystal grains are developed at a low temperature using a material that does not contain an inhibitor component, and a purification annealing or a special annealing separator is not required. According to this method, a unidirectional electrical steel sheet having no forsterite film can be produced at low cost without the need for purification annealing.
However, the unidirectional electrical steel sheet obtained by this method has a problem in terms of workability because the secondary recrystallization grain size is not sufficiently controlled.

Since the crystal grain size of ordinary cold-rolled steel sheets and non-oriented electrical steel sheets is about several μm to several hundred μm, the influence of crystal orientation during processing is averaged and does not appear remarkably. On the other hand, in the unidirectional electrical steel sheet, since the diameter of the secondary recrystallized grains reaches several mm to several tens mm, there is a problem that an adverse effect due to crystal orientation occurs during processing.
Specifically, it is a phenomenon in which the dimensional accuracy at the time of punching deteriorates due to the deviation of the crystal orientation of the secondary recrystallized grains forming the steel sheet, and the secondary recrystallized grain size increases accordingly. become. For such a phenomenon, it is advantageous to make the secondary recrystallization grain size fine.

  As a method for obtaining a fine goth orientation, there are methods disclosed in Patent Documents 4 and 5. Magnetic steel sheets by these methods are excellent in workability, but because the crystal grain size is fine, it is difficult to improve the orientation integration, and the magnetic permeability equivalent to that of ordinary unidirectional electrical steel sheets is obtained. I can't. Furthermore, since the grain boundary density is high, the total amount of magnetic poles generated at the crystal grain boundary is large, and in particular, there is a problem that the magnetic permeability is low in a low magnetic flux density region near 1.0 T.

Japanese Patent Publication No. 6-49948 JP-A-8-134542 JP 2000-129356 JP JP 2000-160305 A Japanese Patent Laid-Open No. 2001-303214

  The present invention has been developed in view of the above situation, and an object thereof is to propose an advantageous method for producing a unidirectional electrical steel sheet having both high magnetic permeability and workability.

Now, the inventors have developed the technique disclosed in Patent Document 3 for developing a Goth orientation structure from a material that does not contain an inhibitor component, and has developed a method for achieving both workability and magnetic properties.
As a result, in order to ensure the workability and the magnetic characteristics at the same time, it was determined that it is important to appropriately control the particle size distribution of the secondary recrystallized grains, which was the basis of the present invention.

That is, from the viewpoint of workability, secondary recrystallized grains having an excessively coarse grain size should not be generated, and from the viewpoint of securing magnetic properties, the ratio of secondary recrystallized grains having a fine grain size is constant. It is important to suppress the following.
In order to obtain such a secondary recrystallized structure, it is advantageous to make the hot-rolled sheet structure uniform by using a material containing C in an amount sufficient to cause γ transformation during hot rolling. If the content of C in the slab is large, the load of decarburization annealing in the production process is large, which is disadvantageous in terms of production cost.

  Therefore, as a result of intensive studies on a method for developing a good secondary recrystallized structure even with a material having a low C content, the reduction ratio at a specific temperature or higher in hot rolling is increased, and this identification is performed. It is possible to control the reduction rate in the higher temperature range in the temperature range higher than the temperature, and to appropriately control the temperature increase condition in the first annealing after hot rolling, especially the time required in the temperature range of 500 to 800 ° C. As a result, the present invention has been found to be effective.

That is, the gist configuration of the present invention is as follows.
(1) After heating a steel slab containing C: 0.02% or less and Si: 1.0-5.0%, Al: 100 ppm or less, N: 50 ppm or less to a temperature of 1100-1300 ° C. Hot roll and then hot-rolled sheet annealing and then cold-rolled to the final sheet thickness, or hot-rolled sheet annealing is performed twice with intermediate annealing between the final sheet thickness and After that, a method for producing a unidirectional electrical steel sheet comprising a series of steps in which recrystallization annealing is performed and then final finishing annealing is performed so that a ceramic film is not formed on the surface, followed by flattening annealing and baking an insulating coating. In
Cumulative rolling reduction at a steel sheet surface temperature of 950 ° C or higher during hot rolling is 75% or higher, and the cumulative rolling reduction at a rolling steel roll surface on the rolling roll entry side of hot rolling is 1050 ° C or higher is 20%. A method for producing a unidirectional electrical steel sheet as described above, characterized in that the temperature raising process of the first annealing after hot rolling: the required time between 500-900 ° C. is within 100 seconds.

(2) Steel slab is further mass%, Mn: 0.02-2.0%, Ni: 0.005-2.0%, Sn: 0.01-2.0%, Sb: 0.005-0.5%, Cu: 0.01-2.0%, Mo: 0.01 The method for producing a unidirectional electrical steel sheet according to the above (1), comprising one or more selected from ˜0.50% and Cr: 0.01 to 2.0%.

  According to the present invention, by reducing the amount of C added in the slab, decarburization annealing in the process after hot rolling is performed at a low temperature or in a short time, or using a material that can be omitted, A unidirectional electrical steel sheet can be obtained without impairing the magnetic permeability at a relatively low magnetic flux density of about 1.0 T, and with little die wear during punching and good dimensional accuracy.

  In order to obtain a unidirectional electrical steel sheet having both an excellent processing method and magnetic properties, the present invention does not allow a hard coating on the surface, and the secondary recrystallized grain size distribution is not excessively fine. It has been completed by newly finding out that it is effective to make uniform, and an index of the particle size distribution necessary for that purpose is shown, and a manufacturing method thereof is presented.

The present invention will be specifically described below.
First, the experimental results that led to the present invention will be described in detail. Unless otherwise specified, “%” in relation to ingredients means mass%.
A steel slab containing C: 0.04%, Si: 3.3% and Mn: 0.04% and reduced to Al: 40 ppm, S: 20 ppm, N: 20 ppm, O: 15 ppm was produced by continuous casting. A specimen was taken from this steel slab, heated to 1250 ° C, hot-rolled under various conditions, then subjected to hot-rolled sheet annealing at a holding temperature of 1000 ° C, and then cold-rolled to 0.35 mm. After making the final thickness of the sheet thick, hydrogen: nitrogen = 25: 75 by volume ratio, dew point: annealing at 850 ° C for both decarburization and recrystallization, and then annealing the alumina powder After applying as a separating agent and performing a final finish annealing at a holding temperature of 900 ° C., a semi-organic coating solution composed of dichromate and resin was applied and baked to produce a prototype steel plate.

The secondary recrystallized structure of the obtained prototype steel sheet was investigated by macro etching with acid, and 20 ring-shaped samples with an outer diameter of 100 mm and an inner diameter of 80 mm were punched with a press punching device. The outer diameter was measured every 45 °. Here, the variation in outer diameter was quantified by the following index Δφ (%).
Δφ (%) = (maximum outer diameter−minimum outer diameter) / average outer diameter × 100 (1)

The secondary recrystallized structure was determined by visually observing the secondary recrystallized grain boundary after macro etching with acid, and the particle size distribution was examined with an image analyzer. The particle size of the secondary recrystallized grains was determined by measuring the area S of each secondary recrystallized grain by image analysis and setting the equivalent circle diameter determined by the following equation.
Equivalent circle diameter r = 2 (S / π) ^0.5

FIG. 1 shows the results of investigating the relationship between the area ratio of crystal grains having an equivalent circle diameter of 20 mm or more and Δφ.
As shown in the figure, when the area ratio of crystal grains with an equivalent circle diameter of 20 mm or more increases, Δφ increases and dimensional accuracy (punchability) deteriorates. However, if this area ratio is 15% or less, Variation in punching dimensional accuracy is small.
The reason for this is that when coarse crystal grains exist, the influence of the specific crystal grain orientation becomes stronger without averaging the influence of the crystal grains whose crystal orientation is slightly shifted during the deformation process of the material during the punching process. This is probably because the dimensional accuracy deteriorates, but if the number of coarse crystal grains is reduced, such deterioration is suppressed.

  JP 2002-212687 A discloses a technique for making fine crystal grains present in secondary recrystallized grains as “unidirectional electromagnetic copper plate with good iron loss and punching workability and manufacturing method thereof”. However, the fine crystal grains here have only the effect of contributing to the improvement of the iron loss, and do not improve the dimensional accuracy during the punching process.

Next, FIG. 2 shows the result of examining the relationship between the area ratio of crystal grains having an equivalent circle diameter of 3 mm or less and the relative permeability μr _10/50 at 50 Hz and 1.0 T.
As shown in the figure, by limiting the area ratio of crystal grains having an equivalent circle diameter of 3 mm or less to 20% or less, a relative magnetic permeability of 20000 or more is obtained.
The decrease in the permeability when the ratio of the secondary recrystallized grains having such a fine grain size is increased is considered to be mainly caused by the decrease in the permeability due to the magnetic pole generated at the grain boundary.

  From the above results, by setting the area ratio of secondary recrystallized grains having an equivalent circle diameter of 20 mm or more to 15% or less and the area ratio of secondary recrystallized grains having an equivalent circle diameter of 3 mm or less to 20% or less, It has been clarified that it is possible to maintain high dimensional accuracy during punching while securing a high relative permeability of 20000 or more.

Then, next, inventors examined the manufacturing method for obtaining the product of the above secondary recrystallized structures stably. Here, when the slab contains a C amount sufficient to cause the γ transformation during hot rolling, a secondary recrystallized structure having a uniform crystal grain size distribution is obtained due to the homogenizing effect of the structure due to the γ transformation. Although it has been clarified by research prior to the present invention that it is easy to be done, if the C content in the slab is high, the decarburization annealing takes a long time, so the amount of C addition in the slab is reduced. Is advantageous in terms of manufacturing cost.
Therefore, the inventors have been required from the viewpoint of dimensional accuracy at the time of punching even in the case of a steel component in which the amount of C in the slab is 0.02% or less and does not cause γ transformation during hot rolling. The method of obtaining the next recrystallized structure was examined.
The experimental results are shown below.

  C: 0.0010 to 0.04%, Si: 3.3% and Mn: 0.04%, Al: 40ppm, S: 20ppm, N: 20pm, O: Reduced to 15ppm, thickness: 200mm steel slab for continuous casting After heating at 1150 ° C. for 2 hours, hot rolling was performed to obtain a hot rolled sheet having a thickness of 2.2 mm. At this time, the hot rolling rough rolling start temperature is 1120 ° C (no waiting before rolling) and 1020 ° C (waiting before rolling), the end temperature is 950 ° C, the steel strip thickness after rough rolling and the rough rolling The required time at each stage of the rolling pass was changed.

Next, a test material was collected from the obtained hot rolled sheet and subjected to hot rolled sheet annealing at 1000 ° C. for 60 seconds. In this hot-rolled sheet annealing, the time required for the temperature raising process: 500-900 ° C. was 60 seconds. Next, after pickling, it is cold-rolled to a thickness of 0.35 mm and decarburized and recrystallized annealed in a volume ratio of hydrogen: nitrogen = 25: 75, dew point: 50 ° C. and held at 850 ° C. for 100 seconds. After the application, a final finish annealing was performed in an atmosphere of N ₂ : 25% and Ar: 75%, which was held at 900 ° C. for 30 hours. Thereafter, the steel sheet was macro-etched to reveal the secondary recrystallization structure, and the particle size distribution was investigated by image analysis.

FIGS. 3 and 4 show the cumulative reduction ratio at 950 ° C. or more, the area ratio of crystal grains having an equivalent circle diameter of 20 mm or more, and the area ratio of crystal grains having an equivalent circle diameter of 3 mm or less during hot rolling, respectively. The result of investigating the relationship is shown.
In these figures, the comparative method refers to a cumulative rolling reduction rate of 12% when the temperature of the material to be rolled on the entry side of each rolling roll is 1050 ° C. or higher, and the temperature rising process of hot-rolled sheet annealing: between 500 to 900 ° C. When the required time is 60 seconds, the one-side invention method is that the temperature of the material to be rolled on each roll entry side is 1050 ° C. or higher, the cumulative rolling reduction is 20%, and the temperature raising process of hot-rolled sheet annealing: 500 This is a case where the required time between ˜900 ° C. is 60 seconds.

According to FIGS. 3 and 4, when the amount of material C is 0.040%, the temperature rising process of hot-rolled sheet annealing: when the required time between 500 to 900 ° C. is 60 seconds, during hot rolling, 950 By simply setting the cumulative rolling reduction above 75 ° C to 75% or more, the area ratio of secondary grains with a circle equivalent diameter of 20 mm or more is 15% or less, and at the same time the area ratio of secondary grains with a circle equivalent diameter of 3 mm or less Can be reduced to 20% or less.
On the other hand, in a material having a material C amount of 0.0020%, a desired secondary recrystallization structure is not obtained only by applying these methods, and the temperature of the material to be rolled is in a temperature range of 1050 ° C. or higher. It can be seen that a secondary recrystallized structure suitable for improving the punching dimensional accuracy is obtained by increasing the cumulative rolling reduction to 20%.

Next, in FIG. 5 and FIG. 6, in a material having a material C amount of 0.0020%, during hot rolling, the cumulative rolling reduction at 950 ° C. or higher is set to 75%, and the temperature rising process of hot-rolled sheet annealing: 500 to 900 Cumulative rolling reduction and equivalent circle diameter when the temperature of the material to be rolled on each roll entry side is 1050 ° C or higher when the required time between ° C is 60 seconds and the equivalent circle diameter: equivalent area ratio of crystal grains of 20mm or more The diameter: The result of having investigated about the relationship with the area ratio of the crystal grain of 3 mm or less is shown.
As shown in FIG. 5 and FIG. 6, the area ratio of crystal grains having an equivalent circle diameter of 20 mm or more is 15% or less by setting the cumulative reduction ratio at a temperature of the rolled material: 1050 ° C. or more to 20% or more. In addition, it can be seen that a product having an area ratio of crystal grains having an equivalent circle diameter of 3 mm or less and 20% or less is obtained.

As described above, for a material having a low C content, it is necessary to adopt the hot rolling method and the hot-rolled sheet annealing method of the present invention.
The reason why the distribution of secondary recrystallized grain size is improved by ensuring a cumulative reduction ratio of 75% or higher at 950 ° C or higher during hot rolling is that the surface layer of the steel sheet is subjected to strong strain at high temperatures. It is considered that crystal grain growth during hot rolling progresses moderately and the structure after hot rolling is made uniform, and as a result, the structure after secondary recrystallization is made uniform. In particular, it is considered that the generation of fine grains and coarse grains having a secondary recrystallization grain size is suppressed.

  Thus, by ensuring a high reduction ratio in the high temperature region of hot rolling, a uniform hot-rolled sheet structure is formed, which also affects the recrystallization annealing structure. As a result, the particle size distribution of secondary recrystallized grains It can be said that the area ratio of coarse particles having a particle diameter (equivalent circle diameter) of 20 mm or more and fine particles having a particle diameter (equivalent circle diameter) of 3 mm or less is reduced.

However, in the case of a steel having a low C content in the slab and a composition that does not cause γ transformation during hot rolling, only a cumulative reduction of 75% or higher at 950 ° C. or higher is ensured during hot rolling. Then, since it is difficult to achieve a uniform hot-rolled sheet structure, it is presumed that a secondary recrystallized structure required from the viewpoint of ensuring punching accuracy cannot be obtained.
The reason for this is considered to be that when the γ transformation does not occur during rolling, recrystallization due to the phase transformation does not easily occur during rolling, so that the influence of the slab structure tends to remain.

In the case of such a material having a low C composition, it has been found that it is advantageous to set the cumulative rolling reduction at a high temperature range of 1050 ° C. or higher at the initial stage of hot rolling as high as 20% or higher.
The reason for this is considered to be that the generation and growth of recrystallized grains from the deformed structure is promoted by applying a large strain in the high temperature region of hot rolling. When γ transformation is involved, the hot rolled structure can be made uniform by ensuring a sufficient amount of rolling down to a temperature of about 950 ° C, along with a decrease in the γ phase rate as the temperature decreases during rolling. On the other hand, when there is no γ transformation, it is better to give a large strain in the high temperature range, generate recrystallized grains from the deformed structure, and then grow the grains appropriately in the subsequent rolling process. It is considered advantageous for homogenization. On the other hand, when a strong strain is applied after the temperature of the steel sheet during hot rolling is reduced, the subsequent grain growth does not proceed and the crystal grains become fine, and the grains are partially formed by abnormal grain growth during hot-rolled sheet annealing. It is estimated that it becomes coarse and eventually leads to non-uniform secondary recrystallized structure.
For this reason, it is considered important to increase the rolling reduction ratio of the initial rolling pass at the highest temperature in hot rolling to a predetermined value in the case of a material having a low C content. It can be said that it is advantageous to keep the entrance temperature in the first pass of the hot rolling high.

  Moreover, the reason why the particle size distribution of secondary recrystallized grains cannot be made uniform simply by optimizing the heating rate of hot-rolled sheet annealing is that the pinning effect by solute nitrogen is due to the non-uniformity of the hot-rolled structure. It is presumed to be non-uniform in response. That is, the desired secondary recrystallized structure is achieved for the first time by optimizing the time required for the temperature raising process of the subsequent hot-rolled sheet annealing after optimizing the reduction ratio in the high temperature region of hot rolling and making the structure uniform. Is considered to be achieved.

Furthermore, it is considered that the effect of the temperature increase rate as described above appears in the first annealing after hot rolling. That is, when hot-rolled sheet annealing is omitted and intermediate annealing is performed, the temperature raising process of intermediate annealing corresponds to this.
In the prior art disclosed in Japanese Patent Application Laid-Open No. 9-316537, etc., the heating rate of hot-rolled sheet annealing and intermediate annealing has been appropriately controlled. In these techniques, AlN or the like is used. The purpose was to properly precipitate the inhibitors.
On the other hand, the method for optimizing the required time in the temperature raising process of the first annealing after hot rolling or intermediate annealing, that is, the first annealing after hot rolling, in the manufacturing method of the electrical steel sheet not using the inhibitor as in the present invention, It is intended to suppress the precipitation of impurities, and its purpose is different from that of the conventional technique.

Next, the reason for limitation regarding the manufacturing method of the present invention will be described.
First, the reason why the component composition of the steel slab, which is a material, is limited to the above range will be described.
C: 0.02% or less C is a useful element that homogenizes the structure after hot rolling by improving the γ transformation and improves the magnetic properties. However, if it remains in the iron base part of the product, it will increase the iron loss due to the aging effect. Therefore, the final product must be reduced to 0.0050% or less. For this reason, even when an appropriate amount of C is added for the purpose of improving the structure during hot rolling, it is necessary to remove C from the steel at any point in the subsequent manufacturing process. For this reason, a method of applying decarburization annealing that also serves as recrystallization annealing after the final cold rolling is generally used, but since sufficient decarburization requires an appropriate annealing time, production efficiency decreases. Resulting in. In order to reduce such a decrease in productivity due to the addition of C in steel, it is preferable to reduce the amount of C added to the material. However, in this case, as described above, the γ transformation during hot rolling does not occur, or the γ phase ratio decreases, and improvement of the hot rolled structure cannot be expected.
In view of such a situation, the present invention has found a method for obtaining a good hot-rolled structure even if the amount of C in the material is small by devising hot rolling conditions.
That is, by using the method of the present invention, it is possible to reduce the C addition amount to 0.02% or less without causing deterioration of the structure of the hot-rolled sheet, so the C amount in the slab is limited to the above range. did. Note that the C content is preferably 0.01% or less, more preferably 0.0050% or less.

Si: 1.0-5.0%
Si not only increases electric resistance and reduces iron loss, but also has the effect of stabilizing the iron BCC structure and enabling annealing at high temperatures. However, if the Si content is less than 1.0%, a sufficient iron loss reduction effect cannot be obtained. On the other hand, if the Si content exceeds 5.0%, not only the magnetic flux density is lowered but also the secondary workability of the product is remarkably deteriorated. The amount was limited to the range of 1.0-5.0%.

Al: 100 ppm or less, N: 50 ppm or less The present invention is based on the manufacturing technology of a unidirectional electrical steel sheet that does not use an inhibitor as described in Patent Document 3 described above. Therefore, by reducing the impurity elements, the degree of orientation accumulation of secondary recrystallized grains is improved by utilizing the difference in the ease of movement of grain boundaries depending on the grain boundary structure. For this purpose, it is particularly necessary to limit the above range for Al and N. In addition to Al and N, B, Nb, V, S, Se and P are preferably reduced to 50 ppm or less.
In addition, about N, in order to acquire the pinning effect by solid solution N, it is advantageous to make it contain 10 ppm or more in a slab.

In addition, when it contains other components, it is preferable to set it as the following range.
Mn: 0.02 to 2.0%
Mn is an element useful for improving hot workability. However, if the content is less than 0.02%, the effect of addition is poor, and the magnetic properties are somewhat disadvantageous. On the other hand, if it exceeds 2.0%, the magnetic flux density is reduced. Therefore, when Mn is added, the above range is preferable. In addition, almost all of Mn contained in the slab stage remains in the iron base portion of the product.

Ni: 0.005-2.0%
Ni is a useful element that improves the magnetic properties by improving the structure, and can be added as necessary. Here, if the content is less than 0.005%, the improvement of the magnetic properties is not sufficient. On the other hand, if it exceeds 2.0%, secondary recrystallization becomes unstable and the magnetic properties deteriorate, so when adding Ni The above range is preferable. In addition, almost all of the Ni contained in the slab stage remains in the iron base portion of the product.

Sn: 0.01-2.0%, Sb: 0.005-0.5%, Cu: 0.01-2.0%, Mo: 0.01-0.50%, Cr: 0.01-2.0%
Any of the above elements is a useful component for improving iron loss, and can be added alone or in combination as required. Here, when the content is less than the lower limit, the effect of improving the iron loss is poor. On the other hand, when the upper limit is exceeded, secondary recrystallization becomes unstable and the magnetic properties deteriorate, so each element is contained in the above range. It is preferable. For any of the above elements, almost the entire content in the slab stage remains in the iron base portion of the product.

Slab heating temperature: 1100-1300 ℃
If the heating temperature of the slab is below 1100 ° C, a sufficient temperature cannot be maintained during hot rolling, so that a sufficient reduction ratio cannot be obtained in a high temperature range. As a result, a uniform hot rolled sheet structure is obtained. Since it cannot be obtained, the particle size distribution of secondary recrystallization becomes non-uniform. On the other hand, when the heating temperature of the slab exceeds 1300 ° C., the growth of crystal grains during heating proceeds, the structure after hot rolling becomes non-uniform, and a desired secondary recrystallized structure cannot be obtained. Therefore, the slab heating temperature was limited to the range of 1100-1300 ° C. More preferably, it is the range of 1150-1250 degreeC.
In particular, in the present invention, since the amount of C contained in the slab is limited to 0.02% or less, the γ phase ratio during hot rolling is low, and it is not possible to make the hot rolled structure uniform by γ transformation. For this reason, from the viewpoint of preventing coarsening of crystal grains during slab heating, the heating temperature needs to be 1300 ° C. or lower.

Cumulative rolling reduction at a steel plate surface temperature of 950 ° C or higher during hot rolling: 75% or higher As described above, the cumulative rolling reduction at a steel plate surface temperature of 950 ° C or higher is set to 75% or higher. Since the area ratio of the next recrystallized grains can be reduced, the above range is adopted. The surface temperature of the material to be rolled is defined as the temperature immediately after rolling. Therefore, if there is a difference between the temperature measurement time and the rolling end time, the surface temperature of the material to be rolled decreases with time unless a special heat treatment is applied during rolling. It is possible to calculate the cumulative reduction in the temperature range of 950 ° C or higher. A more preferable cumulative rolling reduction is 80% or more.

Cumulative rolling reduction when the surface temperature of the material to be rolled on the rolling roll entry side of hot rolling is 1050 ° C or higher: 20% or more As previously, by limiting the C content in the slab low, In the case where the homogenization of the hot rolling structure due to the γ transformation cannot be expected, such disadvantages can be compensated by appropriately controlling the hot rolling conditions at the initial stage of hot rolling. That is, by performing rolling so that the cumulative rolling reduction in a high temperature range of 1050 ° C. or higher is 20% or higher, the recrystallization structure of the hot rolled sheet is appropriately coarsened due to strain introduced in the high temperature range, The structure after annealing after hot rolling is made uniform. In hot rolling conditions outside the above range, such effects do not occur, and the frequency of formation of coarse or fine secondary recrystallized grains in the final product increases, and the dimensional accuracy after processing is impaired. , Limited to the above range. A more preferable cumulative rolling reduction is 30% or more.
The surface temperature of the material to be rolled is defined as the temperature immediately before rolling in each roll. Therefore, if there is a discrepancy between the temperature measurement time and the rolling start time, unless the special heat treatment is applied during rolling, the surface temperature of each rolled roll is assumed to decrease depending on the time. It is possible to calculate the cumulative reduction amount at a temperature of 1050 ° C. or higher.

Of hot-rolled sheet annealing or intermediate annealing, the first annealing temperature rising process after hot rolling: 500 to 900 ° C required time: within 100 seconds As mentioned above, the first annealing temperature rising process after hot rolling : If the required time between 500 and 900 ° C exceeds 100 seconds, the precipitation amount of impurities in the steel increases, the amount of dissolved N decreases, and sufficient pinning effect cannot be obtained during secondary recrystallization annealing, In the secondary recrystallized grains, coarse grains or fine grains are generated. Therefore, in order to suppress the generation of coarse and fine crystal grains in the secondary recrystallized grains, the temperature raising process of the first annealing after hot rolling: the required time between 500 and 900 ° C is within 100 seconds. It is necessary to. Moreover, in order to acquire this effect, it is necessary to make the hot-rolled board structure | tissue uniform previously by setting it as said hot-rolling conditions.
Temperature raising step of the first annealing after hot rolling: A more preferable required time between 500-900 ° C. is 60 seconds or less.

Final finish annealing without ceramic coating In the present invention, in order to prevent die wear during punching and ensure adhesion during bending, no ceramic coating is formed on the surface during final finishing annealing. Thus, after the planarization annealing, an inorganic, organic or semi-organic insulating coating is applied. For this reason, in the final finish annealing, it is necessary to prevent the formation of a ceramic film such as forsterite on the steel sheet surface. Therefore, when using an annealing separator, it is preferable to select a substance having low reactivity with the steel plate such as alumina powder or silica powder. It is also possible to use a conventional annealing separator such as MgO. In this case, in order to prevent the formation of forsterite, the ultimate temperature of final finish annealing may be limited to a low temperature range of about 1000 ° C. or less. In addition, when an appropriate amount of surface segregation type elements such as Sb, Sn, and Bi are contained in the steel, the final temperature of final finish annealing is limited to a low temperature range of about 950 ° C or less, so that no annealing separator is applied. However, adhesion between steel strips can be prevented.

  In the present invention, since the impurity elements in the slab are reduced, the final finish annealing does not need to reach as high a temperature as the conventional unidirectional electrical steel sheet, and therefore, the adhesion between the steel strips can be advantageously prevented. . Moreover, it can be said that the low temperature reached in the final finish annealing is extremely advantageous in terms of construction and maintenance of the annealing furnace or running cost.

Next, manufacturing conditions recommended in the present invention will be described.
The annealing temperature in the first annealing after hot rolling is preferably about 950 to 1100 ° C. This is because when the first annealing temperature after hot rolling is below 950 ° C., recrystallization of the unrecrystallized structure after hot rolling does not proceed, leading to a low degree of orientation accumulation of secondary recrystallized grains. When the first annealing temperature after rolling exceeds 1100 ° C, the grain size before cold rolling becomes coarse, and the volume fraction of the {111} <112> orientation becomes markedly reduced in the structure after cold rolling. This is because the degree of orientation of crystal grains decreases. A more preferable annealing temperature is 950 to 1050 ° C.

  The recrystallization annealing temperature after cold rolling is preferably about 850 to 1050 ° C. This is because when the recrystallization annealing temperature is lower than 850 ° C., the primary grain structure becomes fine and the driving force of secondary recrystallization grain growth becomes excessive. As a result, the area ratio of grains with an equivalent circle diameter of 20 mm or more is 15 On the other hand, when the recrystallization annealing temperature exceeds 1050 ° C, the area ratio of crystal grains with an equivalent circle diameter of 3 mm or less exceeds 20% due to insufficient driving force for secondary recrystallization grain growth. Because it becomes.

Furthermore, C contained in the slab needs to be removed from the steel by decarburization annealing. This is because if the C residual amount in the steel of the product is high, the iron loss is deteriorated due to the aging effect. Decarburization annealing can be combined with recrystallization annealing by performing recrystallization annealing in a decarburizing atmosphere, which is advantageous in simplifying the manufacturing process. In addition, it is effective to simplify the decarburization after cold rolling by performing auxiliary decarburization by setting the atmosphere of hot-rolled sheet annealing or intermediate annealing to a decarburizing atmosphere. It is also possible to perform decarburization annealing after secondary recrystallization annealing.
In the present invention, even if the C content in the material slab is lowered to 0.02% or less, a secondary recrystallized structure having a uniform particle size distribution can be obtained. Therefore, such decarburization annealing is performed at a low temperature in a short time. It is possible. Moreover, decarburization annealing is also omissible because the amount of C in a slab shall be 0.0050% or less.

  Next, the annealing temperature in the final finish annealing is preferably about 850 to 1000 ° C. This is because when the annealing temperature is lower than 850 ° C., the primary grain size immediately before the generation of secondary recrystallized grains becomes too small, and crystals with an equivalent circle diameter of 20 mm or more due to excessive driving force for secondary recrystallized grain growth. When the area ratio of the grains exceeds 15%, on the other hand, when the temperature exceeds 1000 ° C, the primary grain size immediately before the occurrence of secondary recrystallized grains becomes excessive and the driving force is insufficient. This is because the area ratio exceeds 20%. In final finish annealing, a method is used in which secondary recrystallization is started or completed while maintaining a constant temperature, and then the temperature is raised to a high temperature within a range where adhesion between steel plates does not occur, so that very fine crystal grains disappear. It is also possible to do.

Example 1
A steel slab with a thickness of 200mm, which contains C: 0.0020% and Si: 3.2%, and has a composition reduced to Al: 50ppm and N: 30ppm, is manufactured by continuous casting. After heating for 1 hour at the indicated temperature, it was hot-rolled to a thickness of 2.2 mm using a three-series rough rolling mill and a seven-stand tandem rolling mill. Subsequently, hot-rolled sheet annealing was performed at 1000 ° C. for 60 seconds, and the time required between 500 ° C. and 900 ° C. was variously changed as shown in Table 1 during the temperature raising process in this annealing. Subsequently, a final sheet thickness of 0.35 mm was obtained by cold rolling, and then recrystallization annealing was performed for 20 seconds at 900 ° C. in a nitrogen atmosphere.
Next, after applying silica as an annealing separator to the surface of the steel sheet, a final finish annealing was performed in a nitrogen atmosphere at 900 ° C. for 50 hours. Thereafter, the silica was washed and removed, followed by flattening annealing, and then a semi-organic coating liquid composed of dichromate and resin was applied and baked at 300 ° C. to obtain a product.
In the above manufacturing method, by adjusting the time from the extraction of the slab from the heating furnace to the start of rolling or the time between each roll during rough rolling, the cumulative rolling reduction at 950 ° C. or higher during rough rolling As shown in Table 1, the rolling reduction when the surface temperature of the material to be rolled on the rolling roll entry side was 1050 ° C. or higher was varied. In addition, the start temperature of finish rolling subsequent to rough rolling was set to 930 ° C in all cases.
The area ratio occupied by crystal grains having an equivalent circle diameter of 3 mm or less and the area ratio occupied by crystals having an equivalent circle diameter of 20 mm or more on the steel sheet surface of the product thus obtained were examined. Moreover, the Epstein test piece was extract | collected from the obtained product, and the magnetic characteristic was measured. Further, 20 ring-shaped samples having an outer diameter of 100 mm and an inner diameter of 80 mm were punched out, and Δφ defined by the equation (1) was evaluated.
The obtained results are also shown in Table 1.

As is clear from the table, all of the unidirectional electrical steel sheets obtained according to the present invention had excellent magnetic properties with a relative permeability μr _10/50 of 20000 or more, and the dimensional variation of processed products was small.

Example 2
A silicon steel slab with a thickness of 250 mm, containing the constituent elements shown in Table 2 and having the balance of Fe and inevitable impurities, is manufactured by continuous casting, and heated at 1250 ° C. for 1 hour in a gas heating furnace Afterwards, cumulative rolling reduction of 1050 ° C or higher in the first stage of rough rolling: 25%, end temperature of rough rolling: 950 ° C, plate thickness after rough rolling: 50mm (rough rolling rolling reduction: 80%), finishing rolling start temperature: The thickness was set to 2.2 mm by hot rolling at 930 ° C., and then pickled, and then rolled to 1.0 mm by cold rolling. Next, after intermediate annealing at 1050 ° C. for 30 seconds in a nitrogen atmosphere, a final thickness of 0.27 mm was obtained by cold rolling. During the temperature raising process in the intermediate annealing, the required time between 500 and 900 ° C. was 60 seconds. Next, annealing was performed for both decarburization and recrystallization that was held at 900 ° C. for 10 seconds in a nitrogen atmosphere.
Next, after applying silica as an annealing separator to the surface of the steel sheet, final finishing annealing was performed in an argon atmosphere at 900 ° C. for 50 hours. Thereafter, the silica was washed away with water and subjected to planarization annealing, and then a semi-organic coating liquid composed of dichromate and resin was applied and baked at 300 ° C. to obtain a product.
The area ratio occupied by crystal grains having an equivalent circle diameter of 3 mm or less and the area ratio occupied by crystals having an equivalent circle diameter of 20 mm or more on the steel sheet surface of the product thus obtained were examined. Moreover, the Epstein test piece was extract | collected from the obtained product, and the magnetic characteristic was measured. Further, 20 ring-shaped samples having an outer diameter of 100 mm and an inner diameter of 80 mm were punched out, and Δφ defined by the equation (1) was evaluated.
The obtained results are also shown in Table 2.

  As shown in the table, in the case of the product according to the present invention, the dimensional accuracy at the time of punching was good, and at the same time, the magnetic characteristics were also excellent.

It is a figure which shows the relationship between the area ratio of a crystal grain with a particle size (equivalent circle diameter) of 20 mm or more and an index Δφ (%) of dimensional accuracy deterioration. It is a figure which shows the relationship between the area ratio of the crystal grain whose particle size (equivalent circle diameter) is 3 mm or less, and relative permeability μr _10/50 at 50 Hz and 1.0 T. It is a figure which shows the relationship between the cumulative reduction rate in 950 degreeC or more during hot rolling, and the area ratio of the secondary recrystallized grain 20 mm or more in equivalent circle diameter. It is a figure which shows the relationship between the cumulative reduction rate in 950 degreeC or more and the area ratio of the secondary recrystallized grain of 3 mm or less diameter during hot rolling. It is a figure which shows the relationship between the cumulative rolling reduction in hot rolling roll entrance temperature: 1050 degreeC or more, and the area ratio of the secondary recrystallized grain of circle equivalent diameter: 20 mm or more. It is a figure which shows the relationship between the cumulative rolling reduction in hot rolling roll entrance temperature: 1050 degreeC or more, and the area ratio of the secondary recrystallized crystal grain of circle equivalent diameter: 3 mm or less.

Claims (2)

A steel slab containing, by mass%, C: 0.02% or less and Si: 1.0-5.0%, Al: 100ppm or less, N: 50ppm or less, heated to a temperature of 1100-1300 ° C, and then hot-rolled Then, after hot-rolled sheet annealing, it is cold-rolled to the final sheet thickness, or after two cold-rolling sandwiching intermediate annealing without hot-rolled sheet annealing to the final sheet thickness, In the method for producing a unidirectional electrical steel sheet comprising a series of steps in which recrystallization annealing is performed and then final finishing annealing is performed so that a ceramic film is not formed on the surface, and then flattening annealing is performed, and an insulating coating is baked.
Cumulative rolling reduction at a steel sheet surface temperature of 950 ° C or higher during hot rolling is 75% or higher, and the cumulative rolling reduction at a rolling steel roll surface on the rolling roll entry side of hot rolling is 1050 ° C or higher is 20%. A method for producing a unidirectional electrical steel sheet as described above, characterized in that the temperature raising process of the first annealing after hot rolling: the required time between 500-900 ° C. is within 100 seconds.   Steel slab is further mass%, Mn: 0.02-2.0%, Ni: 0.005-2.0%, Sn: 0.01-2.0%, Sb: 0.005-0.5%, Cu: 0.01-2.0%, Mo: 0.01-0.50% 1 or 2 or more types selected from Cr and 0.01-2.0%, The manufacturing method of the unidirectional electrical steel sheet of Claim 1 characterized by the above-mentioned.

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