JPH09104922A - Production of grain-oriented silicon steel sheet extremely high in magnetic flux density - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a grain-oriented silicon steel sheet extremely high in magnetic flux density by regulating the average grain size of primarily crystallized grains before final finish annealing in a steel slab having a specified componental compsn. and specifying nitriding treatment till the start of secondary recrystallization in final finish annealing after hot rolling. SOLUTION: At the time of producing a grain-oriented silicon steel sheet from a slab contg., by weight, 0.025 to 0.075% C, 2.5 to 4.5% Si, 0.010 to 0.060% acid soluble Al, 0.0010 to 0.0130% N, and the balance Fe, the coiling temp. after the completion of hot rolling is regulated to 650 deg.C. The coil after coiling is subjected to forced cooling at a cooling rate more than that of air cooling to regulate the recrystallizing rate in the center of the sheet thickness of the hot rolled sheet to <=95%, and the average grain size of primarily recrystallized grains till the start of final finish annealing after the completion of decarburizing annealing is regulated to 18 to 35μm. Next, till the start of secondary recrystallization in final finish annealing after the hot rolling, the steel sheet is subjected to nitriding treatment with >=0.0020% by increasing the amt. of nitrogen, and in the case of temp. rising of 900 to 1200 deg.C in the final finish annealing, annealing is executed in an annealing atmosphere under >=15% partial pressure.

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 a grain-oriented electrical steel sheet having excellent magnetic properties, which is used as an iron core of a transformer or the like.

[0002]

2. Description of the Prior Art Unidirectional electrical steel sheets are mainly used for transformers.
It is used as a core material for other electrical equipment and
Magnetic properties such as magnetic properties and iron loss properties are required.
You. As a numerical value showing the excitation characteristic, the strength of a normal magnetic field is 8
Magnetic flux density B at 00 A / m ₈Is used. Also,
As a numerical value showing the iron loss characteristic, the frequency is 50H._TwoAt 1.7
Iron loss W per 1kg when magnetized to Tessler (T)
_17/50You are using

[0003] The magnetic flux density is the largest controlling factor of iron loss characteristics. Generally, the higher the magnetic flux density, the better the iron loss characteristics. In general, when the magnetic flux density is increased, the secondary recrystallized grains become large, and the iron loss characteristics may become poor.
On the other hand, by controlling the magnetic domains, the iron loss characteristics can be improved regardless of the grain size of the secondary recrystallized grains. This unidirectional electrical steel sheet is manufactured by causing secondary recrystallization in the final finishing annealing step and developing a so-called Goss structure having {110} on the steel sheet surface and <001> axis in the rolling direction. . In order to obtain good magnetic properties, it is necessary to highly align <001>, which is the easy magnetic axis, in the rolling direction.

A typical technique for producing such a high magnetic flux density unidirectional electrical steel sheet is Japanese Patent Publication No. 40-15644.
And JP-B-51-13469. In the former, MnS and AlN are used as main inhibitors, and in the latter, MnS, MnSe, Sb, etc. are used. Therefore, in the current technology, it is essential to appropriately control the size, morphology and dispersion state of the precipitates that function as these inhibitors. Speaking of MnS, in the current process, MnS is generated when the slab is heated before hot rolling.
A method is used in which the solid solution is once completely dissolved and then precipitated during hot rolling.

A temperature of about 1400 ° C. is required to completely dissolve the required amount of MnS 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 dedicated to directional magnetic steel is required. 2) The unit energy consumption of the heating furnace is high. 3)
The amount of molten scale increases, and the adverse effects on operation are large, as can be seen from the so-called shavings accumulated in the so-called heating furnace beds.

In order to avoid such a problem, the slab heating temperature should be lowered to the level of ordinary steel.
This means that at the same time, the amount of MnS effective as an inhibitor is reduced or not used at all, and inevitably results in destabilization of secondary recrystallization. Therefore, in order to realize low-temperature slab heating, it is necessary to strengthen the inhibitor in some form with precipitates other than MnS to sufficiently suppress normal grain growth during finish annealing.

As such inhibitors, sulfides, nitrides, oxides, grain boundary precipitation elements, and the like are considered, and known techniques include, for example, the following. In Japanese Patent Publication No. 54-24685, As, Bi, Sn, S
A method of controlling the slab heating temperature in the range of 1050 to 1350 ° C. by including grain boundary segregating elements such as n in the steel is disclosed in Japanese Patent Laid-Open No. 52-24116, in addition to Al, Zr, Ti, It discloses a method for controlling the slab heating temperature in the range of 1100 to 1260 ° C. by containing a nitride forming element such as B, Nb, Ta, V, Cr and Mo.

Further, in Japanese Patent Application Laid-Open No. 57-158322, low temperature slab heating is performed by lowering the Mn content and setting the Mn / S ratio to 2.5 or less, and secondary recrystallization is performed by adding Cu. The technology for stabilizing the is disclosed.
Techniques have also been disclosed in which improvements are made from the metallographic side in combination with the reinforcement of these inhibitors. That is,
In JP-A-89433, in addition to Mn, S, Se, Sb,
By adding elements such as Bi, Pb, Sn, and B, and combining them with the columnar crystal ratio of the slab and the secondary cold rolling reduction ratio, 1
A low temperature slab heating of 100 to 1250 ° C is realized. Further, in JP-A-59-190324, an inhibitor is composed mainly of Al and B and nitrogen in addition to S or Se, and secondary annealing is performed by performing pulse annealing at the time of primary recrystallization annealing after cold rolling. The technology for stabilizing the is disclosed.

As described above, in order to realize the low temperature slab heating in the production of grain-oriented electrical steel sheet, great efforts have been made so far. Furthermore, JP-A-59-5652
Japanese Patent Publication No. 2 discloses a technique that enables low temperature slab heating by setting Mn to 0.08 to 0.45% and S to 0.007% or less. With this method, the problem of defective linear secondary recrystallization of the product due to coarsening of the slab crystal grains during heating of the high temperature slab was solved.

By the way, in the production of unidirectional electrical steel sheets, hot-rolled sheet annealing is usually performed for the purpose of making the structure non-uniform after hot rolling, precipitation treatment and the like. For example, in the production method using AlN as the main inhibitor, Japanese Patent Publication No. 40-238
As disclosed in Japanese Patent Publication No. 20, a method of controlling the inhibitor by performing AlN precipitation treatment in hot-rolled sheet annealing has been adopted.

Usually, the grain-oriented electrical steel sheet is manufactured through main processes such as casting-hot rolling-annealing-cold rolling-decarburizing annealing-finish annealing, and requires a large amount of energy. The manufacturing cost is higher than that of the process. In recent years, the review of such a manufacturing process which consumes a large amount of energy has been promoted, and a demand for simplification of the process and energy has been increased. In order to meet such a request, Japanese Patent Publication No. 59-45730 discloses a manufacturing method using AlN as a main inhibitor, in which A in hot rolled sheet annealing is used.
A method has been proposed in which the precipitation treatment of 1N is replaced by high temperature winding after hot rolling.

Certainly, even if the hot-rolled sheet annealing is omitted by this method, the magnetic characteristics can be secured to some extent,
In the usual method of winding in a coil shape of 5 to 20 tons, a local difference in thermal history occurs in the coil during the cooling process, and inevitably the precipitation of AlN becomes non-uniform and the final magnetic characteristics are It varies depending on the location in the coil, resulting in reduced yield.

Further, in Japanese Patent Laid-Open No. 59-50118, a method of manufacturing a grain-oriented electrical steel sheet using MnS, MnSe, and Sb as main inhibitors is a hot rolled steel strip from the finishing final stand to the winding. It has been proposed to suppress the occurrence of strip-shaped secondary recrystallization defects in products by winding a steel strip at a temperature below the temperature determined by the cooling rate.

This method is a technique for suppressing the occurrence of band-shaped secondary recrystallization defects in products caused by high-temperature slab heating, and the production by the single cold-rolling method in which hot-rolled sheet annealing is omitted has not been studied at all. Not not. The present inventors have extensively studied a method for obtaining good magnetic properties in a process of producing a unidirectional electrical steel sheet by low-temperature slab heating and omitting hot-rolled sheet annealing. For example, Japanese Unexamined Patent Publication
Japanese Patent No. 274814 discloses a method of determining the cold rolling rate according to the recrystallization rate of the steel sheet before cold rolling. Further, JP-A-2-274815 discloses a method of lowering the coiling temperature after hot rolling and performing inter-pass aging during cold rolling, and JP-A-3-294427 discloses the coiling temperature after hot rolling. A method for making it particularly low has been disclosed.

As described above, the technical development for improving the magnetic properties has been carried out in relation to the winding condition of hot rolling, the recrystallization rate of the hot rolled sheet, and the cold rolling condition. However, when these techniques are carried out at the manufacturing site, variations occur in the heat history and the winding temperature during hot rolling, and as a result, the magnetic properties of the product after secondary recrystallization annealing fluctuate.

[0016]

Although the method using low temperature slab heating is originally intended to reduce the manufacturing cost, it cannot be industrialized unless it is a technique that can stably obtain good magnetic characteristics. The present inventors, for industrialization of low-temperature slab heating, control the average grain size of primary recrystallization before final finishing annealing, and after hot rolling, nitriding the steel sheet between the start of secondary recrystallization of final finishing annealing. We have built a technology that is based on

While the technology for omitting hot-rolled sheet annealing has been developed based on this technical system, the magnetic properties of the product after secondary recrystallization annealing fluctuate due to variations in the heat history during hot rolling and the winding temperature after hot rolling. There was a problem of doing. The purpose of the present invention is
It is an object of the present invention to provide a method for solving the problem that when the low temperature slab heating and the annealing of the hot rolled sheet are omitted at the same time, the magnetic characteristics fluctuate, which is not preferable.

[0018]

The gist of the present invention is as follows. (1) C: 0.025 to 0.075% by weight, S
i: 2.5-4.5%, acid-soluble Al: 0.010-
0.060%, N: 0.0010 to 0.0130%, the balance 1280 slab consisting of Fe and unavoidable impurities
It is heated at a temperature of less than ℃, hot-rolled, and then rolled down to 80
% Of cold rolling, followed by decarburizing annealing and final finishing annealing to produce a grain-oriented electrical steel sheet, wherein the coiling temperature after the hot rolling is set to 650 ° C. or less, and the coil after coiling is used. Is forcedly cooled at a cooling rate of air cooling or more, the recrystallization rate of the thickness center of the hot rolled sheet is set to 95% or less, and the average grain size of primary recrystallized grains after completion of decarburizing annealing is 18 to 18
After the hot rolling, the steel sheet is subjected to a nitriding treatment of 0.0010% or more with a nitrogen increase amount before the secondary recrystallization of the final finish annealing. A method for producing a grain-oriented electrical steel sheet having an extremely high magnetic flux density, characterized by annealing in an annealing atmosphere having a nitrogen partial pressure of 15% or more, and (2) further as a steel component, S + 0.40
5Se: 0.005-0.02%, Mn: 0.05-
(1) A method for producing a grain-oriented electrical steel sheet having an extremely high magnetic flux density, characterized in that a slab containing 0.8% is used, and (3) further heating at a temperature of 800 to 1200 ° C. A method for producing a unidirectional electrical steel sheet having an extremely high magnetic flux density according to (1) or (2), characterized in that the sheet is annealed and the cooling rate between 200 and 650 ° C is set to 5 ° C / sec or more. Is provided.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The grain-oriented electrical steel sheet to which the present invention is applied is obtained by casting a molten steel obtained by a conventional steelmaking method by a continuous casting method or an ingot making method, and if necessary, slabbing Slabs are sandwiched between the steps, and subsequently hot-rolled into hot-rolled sheets. Then, hot-rolled sheets are annealed as necessary, cold rolled once with a rolling reduction of 80% or more, and then de-rolled. It is manufactured by sequentially performing carbon annealing and final finish annealing.

The inventors of the present invention have studied various measures to obtain good magnetic properties while achieving both low-temperature slab heating and omitting hot-rolled sheet annealing, and as a result, control of hot-rolling temperature and hot-rolling We have obtained a new finding that it is extremely effective to combine the following forced cooling, control of the recrystallization rate of the hot-rolled sheet, and control of the nitrogen partial pressure during the final finishing annealing temperature rise. Hereinafter, it demonstrates based on an experimental result.

FIG. 1 shows the influence of the winding temperature after hot rolling and the subsequent cooling conditions on the magnetic properties. In this case, by weight, C = 0.041%, Si = 3.08
%, Acid-soluble Al = 0.031%, N = 0.0062
%, And the balance Fe and unavoidable impurities 2
A 20 ton slab 50 mm thick was produced. And 115
After heating to 0 ° C for about 60 minutes, rough hot rolling in 5 passes, 40mm
After the thickness is changed, the finish hot rolling start temperature and the reduction distribution are variously changed, and the finish hot rolling is performed in 6 passes to obtain a 2.8 mm hot rolled sheet.
It was water-cooled to 0 to 800 ° C and immediately wound into a coil. Then, it was cooled under three conditions of standing cooling, pouring water, immersing in a water tank and cooling.

The hot-rolled coil was subjected to strong reduction rolling of about 88% without annealing the hot-rolled sheet to give a final sheet thickness of 0.335.
mm cold rolled sheet. 840 cold-rolled sheets of these final thicknesses
Decarburization annealing was performed at 150 ° C. for 150 seconds. Then 77
During annealing at 0 ° C. for 30 seconds, NH ₃ gas was mixed into the annealing atmosphere to cause the steel sheet to absorb nitrogen. The amount of N after this nitriding treatment was 0.0198 to 0.0230% by weight, and the average grain size of the primary recrystallized grains was 21 to 26 μm. An annealing separator containing MgO as a main component was applied to the steel sheet after the nitriding, and a final finish annealing was performed at 1200 ° C. for 20 hours. At this time, the nitrogen partial pressure of the annealing atmosphere during the temperature rise is (a) N ₂ : 25%, H ₂ : 75%, (b) N ₂ : 1.
There are two types, 0% and H ₂ : 90%.

As is apparent from FIG. 1, the coiling temperature after hot rolling was 650 ° C. or less, continuous forced cooling was performed, and the nitrogen partial pressure in the annealing atmosphere during the final finish annealing temperature rise was 25.
%, Which is a high condition, is a necessary condition for realizing B ₈ ≧ 1.95T. The present inventors examined the findings shown in FIG. 1 in more detail. FIG. 2 shows the winding temperature 650 after hot rolling in FIG.
After forced cooling was continued at a temperature of ℃ or below, the relationship between the recrystallization rate at the plate thickness center and the magnetic properties was shown when the nitrogen partial pressure in the annealing atmosphere during the final finish annealing was 25%. It is a thing.

As is apparent from FIG. 2, assuming that the nitrogen partial pressure of the annealing atmosphere during the final finish annealing temperature increase is 25%, the hot rolling temperature is 650 ° C. or less and the cooling is conducted forcibly. The condition that the recrystallization rate at the plate thickness center of the hot rolled plate is 95% or less is a sufficient condition for achieving B ₈ ≧ 1.95T. Although the mechanism of the phenomenon shown in FIGS. 1 and 2 is not always clear, the present inventors consider as follows.

The metallurgical changes caused by lowering the coiling temperature after hot rolling include changes in the transformation phase from pearlite to bainite, changes from grain boundary precipitation of carbonitrides to intragranular precipitation, and Fe. A change from ₃ C to ε-carbide and a change from Si ₃ N ₄ to Fe ₁₆ N ₂ are considered.
Furthermore, it is considered that the effect of the forced cooling subsequent to the winding after hot rolling is to suppress the coarsening of the intragranular precipitates and sufficiently generate the tongue ring of dislocation during cold rolling. The region of high dislocation concentration around the intragranular fine carbonitride serves as a site for generating random orientation during cold rolling recrystallization, so the cold rolling recrystallization texture (primary recrystallization texture) is randomized.
This randomization is not always a good tendency from the viewpoint of the conventional metallurgy for producing a grain-oriented electrical steel sheet. However, the present inventors have noticed that {100} <025> oriented grains decrease in the primary recrystallization texture.

The {100} <025> oriented grains tend to be larger in size than the other oriented grains, and two equivalent {100} <025> oriented grains have a Σ5-corresponding orientation relationship. Therefore, during the final finish annealing, they tend to corrode each other to form coarse grains. When coarse grains increase in the primary recrystallized plate, the driving force for grain growth of {110} <001> oriented secondary recrystallized grains locally decreases, and the secondary recrystallization is delayed.

As a result, in the case of secondary recrystallization in the normal temperature rising process, the secondary recrystallization completion temperature rises, the inhibitor strength rapidly decreases, and the grain boundary character dependence of grain boundary migration decreases. Therefore, the degree of integration of {110} <001> orientation of the secondary recrystallization texture decreases, and in an extreme case, secondary recrystallization failure occurs. Therefore, by combining low temperature winding with subsequent forced cooling, {100} <0 in the primary recrystallization texture.
It is estimated that the reduction of the 25> azimuth is linked to obtaining good magnetic characteristics.

Next, the synergistic effect of the above description of the coiling temperature after hot rolling and the subsequent forced cooling and control of the recrystallization rate of the hot rolled sheet is estimated as follows. Regarding the precipitation of carbides in the grains during low-temperature winding, if carbon diffuses to the grain boundaries during the cooling process up to room temperature after winding, it precipitates as Fe ₃ C, but if it does not diffuse to the grain boundaries, Precipitates in the grains.
On the other hand, setting the recrystallization rate of the hot-rolled sheet to a predetermined amount or less is equivalent to securing a high dislocation density in the grains, and securing the amount of dislocations as precipitation sites for carbonitrides in the grains. It makes sense.

In addition, the orientation that is difficult to recrystallize in the hot-rolled sheet is the {100} system orientation, and many of these orientation grains are present in the sheet thickness center layer. The reason why there is an upper limit of the recrystallization rate at the plate thickness center of the hot rolled plate for improving the magnetic properties is that it is necessary to secure the {100} system orientation of the plate thickness center in an unrecrystallized state. . It can be understood that the {100} orientation after hot rolling is in a non-recrystallized state because the dislocation density as the precipitation nucleus of the carbonitride is high and the carbonitride is likely to be intragranularly precipitated. That is, the upper limit value of the recrystallization rate of the hot-rolled sheet for producing the synergistic effect of the coiling temperature after hot rolling and the forced cooling that follows is the carbonitride in the grains of the {100} -oriented grains of the hot-rolled sheet. It is considered that this is a necessary condition for precipitating.

If a large number of carbonitrides can be precipitated in the grains of {100} -oriented grains in the state of hot-rolled sheet, dislocations in carbonitrides will occur in the grains of {100} -oriented grains during the subsequent cold rolling. It is considered that tangling occurs and many high strain regions are formed in the grains. Since these high strain regions serve as nucleation sites of random orientation and primary recrystallization orientation, {100} <025 which is likely to occur from the position of {100} orientation of the hot rolled sheet in the cold rolled recrystallization texture. > It is considered that oriented grains are relatively reduced. As described above, the reduction of {100} <025> oriented grains is caused by {110} <00 at the time of secondary recrystallization.
1> It is considered to be useful for increasing the degree of orientation integration.

On the other hand, as shown in FIG. 2, the reason why the control of the annealing atmosphere of the final finish annealing is an essential condition of the present invention is considered as follows. Fine nitrides (Fe ₁₆ N ₂ ) due to high cooling rate following winding after hot rolling
However, this precipitate decomposes when the temperature of decarburization annealing rises, and fine AlN precipitates. Since this fine AlN has a strong inhibitory effect on grain growth and controls the crystal grain size within an appropriate range,
It is necessary to raise the decarburization annealing temperature and extend the time. In the case of grain growth in a state where the inhibitor strength is high, since the grain boundary character dependence of grain boundary movement is emphasized, some oriented grains are likely to grow abnormally to form a mixed grain structure. This phenomenon causes the destabilization of the secondary recrystallization, so the nitrogen partial pressure of the annealing atmosphere in the secondary recrystallization temperature range during the final finish annealing is increased to suppress the decomposition of the inhibitor during the secondary recrystallization. It seems necessary to do.

Next, the reasons for limiting the constituent features of the present invention will be described. First, the reasons for limiting the slab components and the slab heating temperature will be described in detail. C is 0.025% by weight
(Hereinafter simply abbreviated as%), the secondary recrystallization becomes unstable, and even when secondary recrystallization occurs, B ₈ > 1.80.
Since (T) is hard to obtain, it was set to 0.025% or more. on the other hand,
If C is too much, the decarburization annealing time becomes long and it is not economical, so the content was made 0.075% or less.

If Si exceeds 4.5%, cracking during cold rolling becomes remarkable, so Si is set to 4.5% or less. On the other hand, if it is less than 2.5%, the specific resistance of the material is too low, and the low iron loss required for the transformer core material cannot be obtained.
Desirably, it is at least 3.2%. Since Al secures AlN or (Al, Si) N necessary for stabilizing secondary recrystallization, 0.010% or more is required as acid-soluble Al. If the acid-soluble Al exceeds 0.060%, the AlN of the hot-rolled sheet becomes unsuitable and the secondary recrystallization becomes unstable, so the content was made 0.060% or less.

N is 0.00 in the ordinary steelmaking work.
Since it is difficult to make it less than 10% and it is not economically preferable, it is set to 0.0010% or more, while 0.01
When it exceeds 30%, "steel plate surface blistering" called blister occurs, so the content was made 0.0130% or less. Mn
Even if S and MnSe are present in the steel, it is possible to improve the magnetic characteristics by properly selecting the conditions of the manufacturing process. However, if S and Se are high, secondary recrystallization defective portions called linear fine grains tend to occur, and in order to prevent the generation of this secondary recrystallization defective portion, (S + 0.40
It is desirable that 5Se) ≦ 0.014%. Also,
If S or Se exceeds the above value, the probability of occurrence of defective secondary recrystallization is increased no matter how the manufacturing conditions are changed, which is not preferable. In addition, the time required for purification in the final finish annealing is too long, which is not preferable. From such a viewpoint, it is meaningless to increase S or Se unnecessarily.

If the Mn content is less than 0.05%, the shape (flatness) of the hot-rolled sheet obtained by hot rolling, especially the side edges of the strip, becomes corrugated, which lowers the product yield. On the other hand, if the Mn content exceeds 0.8%, the magnetic flux density of the product is lowered, which is not preferable. Therefore, the upper limit of the Mn content is set to 0.8%. In addition, Sn, Sb, Cr, Cu, Ni, B, T known as inhibitor constituent elements
There is no problem in containing a small amount of i or the like.

The heating temperature of the slab is limited to less than 1280 ° C. for the purpose of cost reduction in the same manner as ordinary steel. Preferably it is 1200 ° C or lower. In the subsequent hot rolling process, after heating a slab with a thickness of 100 to 400 mm,
Both of them consist of rough hot rolling and finish hot rolling performed in multiple passes. The methods of rough hot rolling and finish hot rolling are not particularly limited, but it should be noted that the recrystallization rate of the hot rolled sheet is controlled as described later. When the hot-rolled sheet annealing is omitted as in the present invention, it is preferable in terms of controlling the primary recrystallized grain size that precipitation of AlN is performed during hot rolling. Therefore, the precipitation temperature range of AlN is 800 to 10
It is preferable to take a temperature history of hot rolling so as to stay in the temperature range of 00 ° C for a long time.

After finishing hot rolling, after air cooling for several seconds,
It is water-cooled at 20 to 200 ° C./sec, and the steel sheet is wound in a coil shape of 5 to 20 tons. The cooling process of the steel sheet is not particularly limited, but it should be noted that the recrystallization rate of the hot rolled sheet is controlled, as will be described later. Further, as in the present invention, when omitting the hot rolled sheet annealing,
It is preferable to prolong the air-cooling time after finish hot rolling to positively precipitate AlN from the viewpoint of controlling the primary recrystallized grain size.

It is necessary to set the coiling temperature of the steel sheet after hot rolling to 650 ° C. or lower, and to forcibly cool the coil after coiling at a cooling speed higher than that of air cooling. This is because, as shown in FIG. 1, setting this range is a necessary condition for obtaining good magnetic characteristics of B ₈ ≧ 1.95T. The lower limit of the coiling temperature is not particularly limited, but it is industrially not easy to coil the coiling at a temperature of less than 200 ° C., and therefore 200 to 65 is possible.
The range of 0 ° C is industrially preferable.

The method of forced cooling after winding is not particularly limited. In any method such as immersing the coil in a water tank, pouring water or water mist into the coil, immersing the coil in an oil tank, etc., precipitation of fine carbonitrides is prevented and coarsening is prevented by increasing the cooling rate. effective. The recrystallization rate at the plate thickness center of this hot rolled sheet was defined as 95% or less. This is because, as shown in FIG. 2, good magnetic properties of B ₈ ≧ 1.95 T can be obtained in this range. The lower limit of the recrystallization rate is not particularly limited, but if the recrystallization rate is reduced, the cold rolling rate for improving the magnetic properties tends to be reduced. The method for controlling the recrystallization rate is not particularly limited, but the finishing hot rolling temperature,
In particular, a method of controlling the finish hot rolling finish temperature, a method of controlling the reduction distribution of the finish hot rolling, a method of controlling the time until the start of water cooling after the finish hot rolling can be used.

Subsequently, the hot-rolled sheet is 800 to 1200 ° C.
The hot rolled sheet is annealed at the temperature of 200 to 65 during the cooling process.
It is cooled at a cooling rate of 5 ° C / sec or more between 0 ° C. These treatments are more preferable for stabilizing the magnetic properties at a high level. These conditions are necessary to once dissolve the fine carbonitrides in the hot-rolled sheet and then to uniformly precipitate the fine carbonitrides. In the case of the present invention, the state of dispersion of carbonitrides in the hot-rolled sheet is finer and more uniform than in the ordinary hot-rolled sheet, so that the heat cycle control effect of the hot-rolled sheet annealing can be easily obtained.

The annealed steel sheet is then cold-rolled, but the reduction ratio of cold-rolling is set to 80% or more. In this range, an appropriate amount of {110 is included in the primary recrystallization texture. } <
This is because an appropriate amount of corresponding oriented grains ({111} <112>, etc.) that are likely to be silkworm eroded when the 001> oriented grains and their {110} <001> oriented grains are subjected to secondary recrystallization. The steel sheet after such cold rolling is subjected to decarburization annealing, applying an annealing separating agent, and finally finishing annealing by a usual method to obtain a final product. Here, the average grain size of the primary recrystallized grains from the completion of decarburization annealing to the start of final finish annealing is set to 18 to 35 μm, because a good magnetic flux density is easily obtained within this value range, and the grain size is This is because there is little change in the magnetic flux density due to fluctuations.

After the hot rolling and before the start of secondary recrystallization in the final finish annealing, it is specified that the steel sheet is subjected to a nitriding treatment of 0.0020% or more with a nitrogen increase amount, that is, low temperature slab heating as in the present invention. This is because the process premised on (2) tends to lack the inhibitor strength required for secondary recrystallization. The method of nitriding is not particularly limited, and after decarburization annealing, a method of nitriding by subsequently mixing NH ₃ gas in an annealing atmosphere, a method of nitriding using nitrogen atmosphere center plasma,
A method in which nitride is added to an annealing separator, and nitrogen generated by decomposition of nitride during absorption of temperature in final finish annealing is absorbed in a steel sheet,
Any method such as a method of increasing the N ₂ partial pressure in the atmosphere of final finish annealing and nitriding the steel sheet may be used. For the amount of nitriding,
In order to exert the inhibitor effect sufficiently, the amount of nitrogen increase needs to be 0.0020% or more.

When the temperature of the final finish annealing is increased, 900 to 1200 ° C.
The annealing atmosphere was defined as a nitrogen partial pressure of 15% or more.
This is because a fine nitride (Fe ₁₆ N ₂ ) is formed because the cooling rate following the winding after annealing is high, and this precipitate is decomposed at the time of decarburization annealing temperature rise to generate fine AlN. Is deposited. This fine AlN has a strong grain growth inhibitory force and controls the crystal grain size within an appropriate range. Therefore, it is necessary to raise the decarburization annealing temperature or extend the time. In the case of grain growth in a state where the inhibitor strength is high, since the grain boundary character dependence of grain boundary movement is emphasized, some oriented grains grow abnormally and a mixed grain structure tends to occur. This phenomenon causes destabilization of the secondary recrystallization. Therefore, the nitrogen partial pressure of the annealing atmosphere in the secondary recrystallization temperature range of 900 to 1200 ° C. during the final finish annealing is set to 15% or more. It is necessary to suppress the decomposition of the inhibitor during the next recrystallization.

[0044]

【Example】

[Example 1] C: 0.051% (wt%, the same applies hereinafter),
Si: 3.08%, acid-soluble Al: 0.032%, N:
Twelve 10-ton slabs containing 0.0061% and having a balance of Fe and unavoidable impurities and having a thickness of 250 mm were produced.

After soaking the slab at 1110 ° C. for about 60 minutes, hot rolling is immediately started, and the hot rolling of 5 passes is carried out at 40 ° C.
After the thickness was set to mm, a 6-pass hot rolling was performed to obtain a hot rolled coil having a thickness of 2.8 mm. The final hot rolling finish temperature was 851 to 898 ° C. Then, after hot rolling, it is air-cooled for about 2 seconds, water-cooled for about 5 seconds, and (a) 705 to 718 ° C., (b) 611 to 625.
℃, (c) 515 ~ 523 ℃ rolled up at three levels, (1)
It was cooled in two ways: air cooling and (2) immersion cooling in a water tank. The hot-rolled coil was cold-rolled at a rolling reduction of about 88% to obtain a 0.335-mm cold-rolled sheet without annealing the hot-rolled sheet at 840 ° C.
Then, decarburization annealing was performed for 150 seconds. After that, 75
Annealing is carried out by holding at 0 ° C for 30 seconds.
Nitrogen was absorbed in the steel sheet by mixing ₃ gases. The N content of this steel sheet after nitriding was 0.0189 to 0.0230%. The average grain size of the primary recrystallized grains after the nitriding treatment was 22 to 24 μm. Next, an annealing separator having MgO as a main component was applied to this steel sheet, and the final finishing annealing was performed at 15 ° C./hour to 1200 ° C. and holding at 1200 ° C. for 20 hours. As an atmosphere condition for this final finish annealing, (I) during temperature increase up to 1200 ° C. N ₂ : 10
%, H ₂ : 90%, 100% H ₂ during retention at 1200 ° C.,
(II) During heating up to 800 ° C., N ₂ : 10%, H ₂ : 90
%, From 800 ° C. to 1200 ° C. N ₂ : 50%, H ₂ : 5
Two treatments were performed: 0%, 100% H ₂ during 1200 ° C. retention. Table 1 shows the experimental conditions and the results of magnetic properties.

[0046]

[Table 1]

Example 2 (1) C: 0.048%, Si: 3.17%, Mn:
0.03%, S: 0.002%, acid-soluble Al: 0.0
27%, N: 0.0060%, (2) C, Si, acid-soluble Al, N content is the same as (1), and Mn: 0.10.
%, S: 0.007%, and four 10 ton slabs having a thickness of 250 mm and consisting of the balance Fe and unavoidable impurities were manufactured. After soaking the slab at 1120 ° C. for about 60 minutes, hot rolling was immediately started, and after 5 passes of rough hot rolling to 40 mm thickness, 6 passes of finish hot rolling to form a hot rolled coil of 2.6 mm thickness. did. At this time, the finish hot rolling finish temperature is 901 to 956 ° C.
Met.

Then, after hot rolling, it was air-cooled for about 3 seconds, water-cooled for about 5 seconds, wound at 457 to 470 ° C., and immersion-cooled in a water tank. Such a hot rolled coil is (a) without hot rolled sheet annealing,
(B) Hold at 1000 ° C. for 3 minutes and then increase to 650 ° C.
After cooling at ℃ / sec, cooling was performed from 650 ° C to 200 ° C at a cooling rate of 20 ° C / sec, followed by air cooling. This coil was cold rolled at a rolling reduction of about 89% to obtain a 0.285 mm thick cold rolled coil.

Next, this cold rolled coil was subjected to 150 ° C. at 835 ° C.
Decarburization annealing was performed for 2 seconds. After that, 3 at 770 ℃
Annealing was carried out for 0 second, and NH ₃ gas was mixed in the annealing atmosphere to cause nitrogen absorption in the steel sheet. The N content of the steel sheet after nitriding was 0.0201 to 0.0213%. The average grain size of the primary recrystallized grains after the nitriding treatment was 22 to 25 μm.

Then, an annealing separator containing MgO as a main component is applied to the steel sheet, the temperature is raised to 1200 ° C. at 10 ° C./hour, and the temperature is held at 1200 ° C. for 20 hours. Final finish annealing was performed. Table 2 shows the experimental conditions and the results of magnetic properties.

[0051]

[Table 2]

[Example 3] C: 0.050%, Si:
3.21%, acid-soluble Al: 0.032%, N: 0.0
Two 10-ton slabs containing 065% and having a balance of Fe and unavoidable impurities and having a thickness of 250 mm were produced. This slab was soaked at a temperature of 1200 ° C. for about 60 minutes, immediately followed by hot rolling, and after 5 passes of rough hot rolling to 40 mm thickness, finish hot rolling with 6 passes to heat 2.3 mm thick. It was a rolled coil.

At this time, the finish hot rolling start temperature was set to 1100.
C., 1050.degree. C., 1000.degree. C. under three conditions, the plate thickness and reduction distribution of each pass of finish hot rolling are (a) 40 → 20 →
10 → 5 → 4 → 3 → 2.3mm (50 → 50 → 50 → 20
(→ 25 → 23%), (b) 40 → 30 → 18 → 11 → 7
→ 4 → 2.3mm (25 → 40 → 39 → 36 → 43 → 43
%). Then, after the hot rolling was finished, the mixture was air-cooled for about 3 seconds, wound at 501 to 532 ° C., and immersed and cooled in a water tank. After that, the reduction ratio of about 8 without hot-rolled sheet annealing.
Cold rolled at 5% to make a 0.335 mm thick cold rolled sheet at 835 ° C.
Decarburization annealing for 150 seconds, followed by 3 at 770 ° C
During the annealing for 0 second, NH ₃ gas was mixed into the steel sheet to absorb nitrogen. The N content of this steel sheet after nitriding is 0.01
It was 90 to 0.0217%. Then, on this steel sheet,
Applying an annealing separator containing O as the main component, and applying 1200 ° C for 2
A final finish annealing was carried out for 0 hour. The annealing atmosphere during the temperature rise of this final finish annealing was N ₂ : 40%, H ₂ : 60%.
And

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

[0055]

[Table 3]

[0056]

According to the present invention, the winding temperature after hot rolling is controlled, forced cooling is performed after winding, the recrystallization rate of the hot rolled sheet is controlled, and after the completion of decarburizing annealing until the start of final finishing annealing. The average grain size of the primary recrystallized grains is controlled, and a predetermined amount of nitriding treatment is applied to the steel sheet after hot rolling until the start of secondary recrystallization in the final finish annealing, and the annealing atmosphere at the time of final finish annealing temperature rise is controlled. By doing so, even if the hot rolled sheet annealing is omitted by the low temperature slab heating, good magnetic characteristics can be stably obtained, so that its industrial effect is extremely large.

[Brief description of the drawings]

FIG. 1 is a graph showing the influence of the winding temperature after hot rolling, the subsequent cooling conditions, and the atmospheric conditions during the final finish annealing temperature increase on the magnetic properties.

FIG. 2 is a graph showing the relationship between the recrystallization rate at the plate thickness center of the hot rolled sheet and the magnetic characteristics.

Claims (3)

[Claims] 1. By weight ratio, C: 0.025 to 0.075%, Si: 2.5 to 4.5%, acid-soluble Al: 0.010 to 0.060%, N: 0.0010. A slab containing 0.0130% and the balance consisting of Fe and unavoidable impurities is heated at a temperature of less than 1280 ° C., hot rolled, and subsequently cold rolled at a reduction rate of 80% or more, and then decarburized and annealed. In the method of producing a grain-oriented electrical steel sheet by performing final finish annealing, the coiling temperature after the hot rolling is set to 650 ° C. or less, and the coil after coiling is forcibly cooled at a cooling rate of air cooling or higher to heat the coil. The recrystallization rate at the plate thickness center of the rolled sheet is set to 95% or less, and the average grain size of the primary recrystallized grains after completion of decarburization annealing until the start of final finishing annealing is 18 to 35.
μm, the steel sheet is subjected to a nitriding treatment of 0.0020% or more with a nitrogen increase amount before the secondary recrystallization of the final finishing annealing after hot rolling, and the temperature of 900 to 1200 ° C. during the final finishing annealing is increased. A method for producing a grain-oriented electrical steel sheet having an extremely high magnetic flux density, characterized by annealing in an annealing atmosphere having a nitrogen partial pressure of 15% or more. 2. A steel component further comprising S + 0.40 in a weight ratio.
5Se: 0.005-0.020%, Mn: 0.05-
The slab containing 0.8% is used, The manufacturing method of the grain-oriented electrical steel sheet with extremely high magnetic flux density of Claim 1 characterized by the above-mentioned. 3. The magnetic flux density according to claim 1, wherein the hot-rolled sheet is annealed at a temperature of 800 to 1200 ° C., and the cooling rate between 200 and 650 ° C. is 5 ° C./sec or more. Of highly oriented grain-oriented electrical steel sheet.