JPH05156361A - Manufacture of grain-oriented electric steel sheet excellent in magnetic property - Google Patents

Abstract

PURPOSE:To manufacture a grain-oriented electric steel sheet having excellent magnetic properties by subjecting the hot rolled slab of high silicon steel having a specified compsn. to hot rolled sheet annealing according to necessary and thereafter executing cold rolling, decarburizing annealing and final finish annealing under specified conditions. CONSTITUTION:The slab of high silicon steel contg., by weight, 0.025 to 0.075% C, 2.5 to 4.5% Si, 0.010 to 0.060% acid soluble A$, 0.0010 to 0.0130% N, <0.014% (S+0.405Se) and 0.05 to 0.8% Mn is heated to <1280 deg.C and is subjected to hot rolling. This hot rolled sheet is subjected to cold rolling for >= two times including final cold rolling at a 60 to 79% draft and, according to necessary, including process annealing into a cold rolled sheet, and after that, decarburizing annealing is executed in a reducing atmosphere. The average grain size of primary recrystallized grains till the start of final finish annealing after the completion of the decarburizing annealing is regulated to 10 to 35mum, and the steel sheet is thereafter subjected to nitriding treatment in an atmosphere contg. an NH3 gas to regulate the content of N to >=0.100%. Next, it is coated with an MgO series separation agent for annealing and is subjected to final finish annealing in an atmosphere of an N2-contg. H2 gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing 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 Related Art Unidirectional electrical steel sheets are mainly used as iron core materials for transformers and other electrical equipment, and are required to have excellent magnetic characteristics such as excitation characteristics and iron loss characteristics. As a numerical value showing the excitation characteristic, the strength of a normal magnetic field is 80
A magnetic flux density B _{8 at} 0 A / m is normally used. In addition, as a numerical value showing the iron loss characteristic, at a frequency of 50 Hz, 1.
Iron loss W per 1kg when magnetized to 7 Tesler (T)
I am using _17/50 . The magnetic flux density is the most dominant factor of the iron loss characteristics, and generally speaking, the higher the magnetic flux density, the better the iron loss characteristics. Generally, when the magnetic flux density is increased, the secondary recrystallized grains become large, which may result in poor iron loss characteristics. On the other hand, by controlling the magnetic domain, 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 to develop a so-called Goss structure having {110} axis on the steel sheet surface and <001> axis in the rolling direction. Has been done. In order to obtain good magnetic properties, it is necessary to highly align <001>, which is the easy magnetization axis, in the rolling direction.

A representative technique for producing such a high magnetic flux density grain-oriented electrical steel sheet is Satoshi Taguchi et al.
0-15644 and Japanese Patent Publication Sho 51
There is a method described in JP-A-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 present 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 present process, after MnS is completely solid-soluted at the time of heating the slab before hot rolling,
A method of precipitating during hot rolling is adopted. A temperature of about 1400 ° C. is necessary to completely form a solid solution of the required amount of MnS for secondary recrystallization. This is higher than the slab heating temperature of ordinary steel by 200 ° C. or more, and this high temperature slab heating treatment has the following disadvantages. 1) A high temperature slab heating furnace exclusively for grain oriented electrical steel is required. 2) The energy intensity of the heating furnace is high. 3) The amount of molten scale increases, and the adverse effect on operation is large, as seen in so-called shaving.

In order to avoid such problems, the slab heating temperature should be lowered to the level of ordinary steel, but this is also effective as an inhibitor of MnS.
Is meant to be used in a small amount or not used at all, which necessarily causes destabilization of secondary recrystallization. Therefore, in order to realize low-temperature slab heating, M
It is necessary to strengthen the inhibitor with precipitates other than nS to sufficiently suppress normal grain growth during finish annealing.
Such inhibitors include 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,
A method of controlling the slab heating temperature in the range of 1050 to 1350 ° C. by containing grain boundary segregation elements such as Bi, Sn, and Sb in the steel is disclosed, and in JP-A-52-24116, in addition to Al, Zr, Ti, B, Nb, Ta, V, C
There is disclosed a method in which the slab heating temperature is set to a range of 1100 to 1260 ° C. by containing a nitride-forming element such as r or Mo. Further, in JP-A-57-158322, low temperature slab heating is performed by reducing the Mn content and setting the Mn / S ratio to 2.5 or less.
Discloses a technique for stabilizing the secondary recrystallization by adding. On the other hand, a technique was also disclosed in which improvement was made from the side of the metal structure in combination with reinforcement of these inhibitors. That is, in JP-A-57-89433, S, S
Elements such as e, Sb, Bi, Pb, Sn, and B are added, and the columnar crystal ratio of the slab and the secondary cold rolling reduction are combined to achieve low temperature slab heating at 1100 to 1250 ° C. Further, in JP-A-59-190324, an inhibitor is mainly composed of Al and B and nitrogen in addition to S or Se, and secondary annealing is performed by performing pulse annealing during primary recrystallization annealing after cold rolling. The technology that stabilizes is presented. Thus, in order to realize low temperature slab heating in the production of grain-oriented electrical steel sheets, great efforts have been made so far.

Now, in JP-A-59-56522, Mn is 0.08 to 0.45% and S is 0.007.
A technique has been disclosed that enables low temperature slab heating by controlling the content to be not more than%. By 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.

[0008]

The fact that low-temperature slab heating is possible has a great industrial advantage as described above, but it is not necessary to put extra effort into production control.
It is not preferable in terms of cost reduction. Therefore, on the premise of low-temperature slab heating, it is necessary to reduce the labor such as production control and design the process to stabilize the magnetic characteristics at a high level. For this purpose, it may be totally acceptable to increase the number of steps. The present invention provides a method for manufacturing a grain-oriented electrical steel sheet which satisfies such requirements and has excellent magnetic properties.

[0009]

The gist of the present invention is as follows. That is, (1) C: 0.025 to 0.075% by weight%, Si: 2.5 to 4.5
%, Acid-soluble Al: 0.010 to 0.060%, N:
0.0010 to 0.0130%, S + 0.405Se:
A slab containing 0.014% or less, Mn: 0.05 to 0.8%, and the balance being Fe and inevitable impurities is 1
After heating at a temperature of less than 280 ° C., hot rolling is performed, and the obtained hot rolled sheet includes final cold rolling with a rolling reduction of 60 to 79%, and if necessary, is subjected to two or more cold rollings with intermediate annealing. , Then, decarburization annealing, in the method of producing a final finishing annealing to produce a grain-oriented electrical steel sheet, after decarburization annealing is completed, the average particle size of the primary recrystallized grains until the start of final finishing annealing is 10 to 35 μm,
A method for producing a grain-oriented electrical steel sheet having excellent magnetic properties, which comprises subjecting a steel sheet to a nitriding treatment after hot rolling and before the start of secondary recrystallization of final finish annealing.

(2) The steel sheet immediately before the start of secondary recrystallization in the final finish annealing described in the preceding paragraph contains N: 0.0100% or more by weight%, which is an excellent unidirectional property of magnetic properties. It is a method of manufacturing an electromagnetic steel sheet.

[0011]

The function of the grain-oriented electrical steel sheet of the present invention is
The molten steel obtained by the conventional steelmaking method is cast by a continuous casting method or an ingot making method, and if necessary, the slab is separated by a slab, and then hot-rolled into a hot-rolled sheet. If necessary, hot-rolled sheet annealing is performed, and this hot-rolled sheet includes final cold rolling with a rolling reduction of 60 to 79%, and intermediate annealing is performed 2.
It is manufactured by performing cold rolling more than once and then sequentially performing decarburization annealing and final finishing annealing.

The effect of controlling the primary recrystallized grain size between the completion of decarburization annealing and the start of final finish annealing, which is a feature of the present invention, will be described based on experimental results. FIG. 1 is a graph showing the effect of the primary recrystallized grain size on the magnetic flux density of the product. In this experiment, the weight% is C = 0.053%,
Si = 3.25%, acid-soluble Al = 0.015 to 0.0
43%, N = 0.333 to 0.0095%, S = 0.
A slab containing 007% and Mn = 0.14% and having a balance of Fe and unavoidable impurities and having a thickness of 40 mm was heated to 1150 ° C., and a hot rolled sheet having a thickness of 2.3 mm was formed by 6 passes. The hot-rolled sheet was pickled, then cold-rolled at a rolling reduction of about 42% to obtain 1.34.
mm thickness. Thereafter, this steel sheet was soaked at 1100 ° C. for 30 seconds, subjected to intermediate annealing by gradually cooling to 900 ° C. and then rapidly cooling, and then cold rolled at a rolling reduction of 75% to obtain a 0.335 mm thick cold rolled sheet. Then, this cold rolled sheet is heated to 780 to 1000 ° C for 1 hour.
Decarburization annealing was performed for 50 seconds. At this time, the annealing atmosphere was a mixed gas of 25% N ₂ and 75% H ₂ , and the dew point was 62 ° C. After that, during the heat treatment of holding at 750 ° C. for 30 seconds, NH ₃ gas was mixed in the atmosphere gas to subject the steel sheet to nitrogen treatment. The N content of the steel sheet after nitriding was 0.0179 to 0.
It was 0208% by weight. The average grain size (equivalent circle diameter) of the primary recrystallized grains of this steel sheet was measured using an optical microscope and an image analyzer. Then, an annealing separator containing MgO as a main component is applied to the steel sheet after nitriding, and N ₂ : 25%, H ₂ :
A final finishing annealing was carried out by raising the temperature to 1200 ° C. at a rate of 15 ° C./hr in a 75% mixed gas and holding the temperature in H ₂ : 100% at 1200 ° C. for 20 hours.

As is apparent from FIG. 1, good magnetic properties of B ₈ ≧ 1.85 T are obtained when the average grain size of the primary recrystallized grains is in the range of 10 to 35 μm.

Although the reason why the relationship shown in FIG. 1 is established is not always clear, the present inventors presume as follows. That is, it is considered that the mechanism of influence of the temporary recrystallization texture on the secondary recrystallization orientation can be explained by using the concept of corresponding orientation. In the case of 60-79% low pressure final cold rolling as in the present invention, the primary recrystallization texture does not develop so sharply. Therefore, the orientation corresponding to the secondary recrystallization orientation does not become so strong.
In such a case, it is considered that the nuclei having a large product of the abundance of the secondary recrystallized nuclei and the corresponding orientation amount are likely to be secondary recrystallized. At this time, the critical value required for secondary recrystallization of the above product becomes important, and if this critical value is too high, secondary recrystallized grains having various orientations will be generated. Since the reciprocal of the primary recrystallized grain size is considered to be proportional to the driving force for grain growth of the secondary recrystallized grain, the smaller the average grain size of the primary recrystallized grains, the lower the upper critical value, the larger the It seems that the critical value is high. Therefore, as shown in FIG.
It can be said that there was an appropriate value of the average particle size of the primary recrystallization in m. In the case where it exceeds 10 μm, the secondary recrystallization is poor in all cases, and it is understood that the appropriate range of the critical value and the appropriate range of the primary recrystallized grain size correspond to each other.

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. When C is less than 0.025% by weight (hereinafter simply referred to as%), the secondary recrystallization becomes unstable, and even when the secondary recrystallization is performed, B ₈ > 1.80 (T)
Since it is difficult 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 significant, so Si is set to 4.5% or less. 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.
It is preferably 3.2% or more.

Al is AlN necessary for stabilizing secondary recrystallization.
Alternatively, in order to secure (Al, Si) nitride, 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%, "blister of steel plate" called blister occurs, so the content is made 0.0130% or less.

Even if MnS and MnSe are present in the steel, it is possible to improve the magnetic properties by appropriately selecting the conditions of the manufacturing process. However, if S and Se are high, secondary recrystallization defects called linear fine grains tend to occur. To prevent the generation of secondary recrystallization defects, (S + 0.405Se) ≦ 0.014 Should be%. 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 undesirably long, and it is meaningless to increase S or Se unnecessarily from this viewpoint.

The lower limit of Mn is 0.05%. 0.0
If it is less than 5%, the shape (flatness) of the hot-rolled sheet obtained by hot rolling and the side edge portion of the strip become wavy, which causes a problem of lowering the product yield. On the other hand, the Mn content is 0.
If it exceeds 8%, the magnetic flux density of the product is lowered, which is not preferable, so the upper limit of the amount of Mn was set to 0.8%.

In addition to this, a small amount of Sn, Sb, Cr, Cu, Ni, B, Ti, etc., which are inhibitor constituent elements, may be contained.

The slab heating temperature is limited to less than 1280 ° C. for the purpose of cost reduction in the same manner as ordinary steel. It is preferably 1200 ° C or lower. Subsequent to slab heating, hot rolling is performed by a usual method to obtain a hot rolled sheet. Then, the final cold rolling with a rolling reduction of 60 to 79% is included, and if necessary, intermediate annealing is performed, and cold rolling is performed twice or more. If the final cold rolling reduction is less than 60%, {111} <112> etc. {1
10} <001> is too small to obtain a high magnetic flux density, and if it exceeds 79%, a high magnetic flux density can be obtained, but an appropriate range of primary recrystallized grain size becomes narrow, which is preferable. It is disadvantageous in that the magnetic characteristics are stably obtained. Therefore, the final cold rolling reduction rate is defined as 60 to 79%.

The steel sheet after cold rolling is subjected to decarburization annealing, application of an annealing separating agent, and final finishing annealing by a usual method to obtain a final product. The average grain size of the primary recrystallized grains after the completion of decarburization annealing and before the start of final finishing annealing is set to 10 to 35 μm as shown in FIG. 1 within the range of B ₈ (T).
This is because a good magnetic flux density of 1.85 can be obtained. Then, it is defined that the steel sheet is subjected to the nitriding treatment before the start of the secondary recrystallization of the final finish annealing after the hot rolling, in the process premised on the low temperature slab heating as in the present invention, it is necessary for the secondary recrystallization. This is because the inhibitor strength tends to be insufficient.
The method of nitriding is not particularly limited, and it is a method of mixing NH ₃ gas into the annealing atmosphere after decarburization annealing to perform nitriding, a method of using plasma, a nitride is added to an annealing separating agent, and a final finishing annealing is performed. Any method may be used, such as a method of absorbing nitrogen generated by decomposition of nitrides in the temperature into the steel sheet, or a method of nitriding the steel sheet by increasing the N ₂ partial pressure in the atmosphere of final annealing. The nitriding amount is not particularly limited, but 1 ppm or more is necessary.

In the final finish annealing, for example, in the case of annealing in H ₂ or the like, denitrification is likely to occur from the steel sheet, and even if a nitriding treatment is performed before the final finish annealing, secondary recrystallization occurs. It is also considered that there is a shortage of precipitates for tightening and obtaining good magnetic properties. In the steel sheet immediately before the start of secondary recrystallization, it is more preferable that the N content is 0.0100% or more by weight in order to obtain good magnetic properties. The final finish annealing conditions are not particularly limited, but annealing in an atmosphere with a N ₂ partial pressure of 1% or more is preferable from the viewpoint of stabilizing secondary recrystallization.

[0025]

EXAMPLES Examples will be described below. [Example 1] C: 0.055 wt%, Si: 3.24 wt%, Mn: 0.15 wt%, S: 0.007 wt%,
Acid-soluble Al: 0.029% by weight, N: 0.0076% by weight, balance Fe and unavoidable impurities 4
After heating the 0 mm thick slab at a temperature of 1150 ° C. for 60 minutes,
Hot rolling was started at 1080 ° C. to obtain a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was pickled and then cooled at a rolling reduction of about 34% to a thickness of 1.52 mm. After that, this steel plate is 1050
After the treatment at 30 ° C. for 30 seconds (soaking), the intermediate annealing was performed by gradually cooling to 900 ° C. and then rapidly cooling. Then, it was cooled at a rolling reduction of about 78% to obtain a cold-rolled sheet having a thickness of 0.335 mm.

830 ° C. × 150 seconds (soaking) on this cold rolled sheet
Decarburization annealing was applied. The average grain size (circle equivalent diameter) of the steel sheet after decarburization annealing was 22 μm. Next, during the heat treatment of holding the steel plate at 750 ° C. for 30 seconds, NH ₃ gas was mixed into the atmosphere gas and subjected to the two conditions of nitriding treatment and non-treatment. The N content of the steel sheet after nitriding under the conditions was 0.0188% by weight. After that, an annealing separator having MgO as a main component was applied to the steel sheet, and final finish annealing was performed by a known method. Table 1 shows the experimental conditions and the results of the magnetic properties of the products.

[0027]

[Table 1]

[Example 2] C: 0.048% by weight, S
i: 3.23% by weight, Mn: 0.14% by weight, S: 0.
007% by weight, acid-soluble Al: 0.018% by weight, N:
A 40 mm thick slab containing 0.0089% by weight and the balance Fe and unavoidable impurities was added at a temperature of 1250 ° C. to 60
Immediately after heating for a minute, hot rolling was performed to obtain a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was pickled and then cooled at a rolling reduction of about 42% to have a thickness of 1.34 mm. Then 98 on this steel plate
After 0 ° C. × 2 minutes (soaking), it was air-cooled to 600 ° C. and then subjected to an intermediate annealing of rapid cooling, followed by cooling at a reduction rate of 75%,
A cold-rolled sheet having a thickness of 335 mm was used.

Thereafter, a: 800 ° C. × 150 was applied to this cold rolled sheet.
Seconds (soaking), b: 840 ° C. × 150 seconds (soaking), two levels of decarburization annealing. In this case, the average grain size (equivalent circle diameter) of the primary recrystallized grains was a: 9 μm, b: 13 μm. This steel sheet was subjected to the nitriding treatment described in Example 1. The N content of the steel sheet after nitriding under the conditions is 0.0169 to
It was 0.0181% by weight. After that, an annealing separator containing MgO as a main component was applied, and final finish annealing was performed by a known method. Table 2 shows the experimental conditions and the magnetic properties of the products.

[0030]

[Table 2]

[Example 3] C: 0.040% by weight, S
i: 3.30% by weight, Mn: 0.14% by weight, S: 0.
006% by weight, acid-soluble Al: 0.048% by weight, N:
A 40 mm-thick slab containing 0.0051% by weight and the balance Fe and unavoidable impurities at a temperature of 1100 ° C.
Immediately after heating for a minute, hot rolling was performed to obtain a hot rolled sheet having a thickness of 2.3 mm. After soaking this hot rolled sheet for 2 minutes at 1000 ° C, 60
A hot-rolled sheet was annealed by air-cooling to 0 ° C. and then rapidly cooled, then pickled and cold-rolled at a reduction rate of 50% to obtain a 1.15 mm-thick cold-rolled sheet. Thereafter, this cold-rolled sheet was subjected to an intermediate annealing of 1000 ° C. × 2 minutes (soaking), then gradually cooled to 900 ° C. and then rapidly cooled. Then, it is cooled at a reduction rate of about 75%, and
A cold rolled sheet having a thickness of 285 mm was used.

This cold-rolled sheet was subjected to two levels of decarburization annealing: a: 850 ° C. × 150 seconds (soaking), b: 950 ° C. × 150 seconds (soaking). In this case, the average grain size (equivalent circle diameter) of the primary recrystallized grains was a: 26 μm, b: 37 μm. This steel sheet was subjected to the nitriding treatment described in Example 1. The N content of the steel sheet after nitriding under the conditions is 0.0193 to
It was 0.0201% by weight. After that, an annealing separator containing MgO as a main component was applied, and final finish annealing was performed by a known method. Table 3 shows the experimental conditions and the magnetic properties of the products.

[0033]

[Table 3]

[Example 4] C: 0.045% by weight, S
i: 3.25% by weight, Mn: 0.15% by weight, S: 0.
007% by weight, acid-soluble Al: 0.030% by weight, N:
A 40 mm thick slab containing 0.0087 wt% and Sn: 0.050 wt% and the balance Fe and unavoidable impurities was heated at a temperature of 1150 ° C. for 60 minutes and immediately hot-rolled to a thickness of 2.3 mm. It was used as a hot rolled sheet. The hot-rolled sheet was pickled and then cold-rolled at a rolling reduction of about 34% to a thickness of 1.52 mm. After that, the steel plate was heated at 1100 ° C for 30 seconds (soaking).
After the above treatment, an intermediate annealing is performed in which the material is gradually cooled to 900 ° C and then rapidly cooled, and then cooled at a reduction rate of about 78% to obtain a thickness of 0.335 mm.
It was a thick cold-rolled sheet.

This cold-rolled sheet was heated at 820 ° C. for 150 seconds (soaking).
Decarburization annealing was applied. The average grain size (equivalent circle diameter) of the steel sheet after decarburization annealing was 21 μm. Then, during the heat treatment of holding this steel plate at 750 ° C. for 30 seconds, NH ₃ gas was mixed in the atmosphere gas to perform nitriding treatment and no treatment.
The condition was processed. The N content of the steel sheet after nitriding under the conditions is
It was 0.0124% by weight. After that, an annealing separator containing MgO as a main component was applied, the temperature was raised to 1200 ° C. at 15 / hr, and final finishing annealing was performed at 1200 ° C. for 20 hours. At this time, in a 20-hour retention at 1200 ° C,
It is performed in an annealing atmosphere of H ₂ 100%, and in the temperature rising process, a: N ₂ 25%, H ₂ 75%, b: H ₂ 100% 2.
The standard atmosphere conditions were used. In addition, 90 during final finish annealing
Samples were taken out of the furnace at intervals of 25 ° C. from 0 ° C. to 1200 ° C., and the N content of the samples immediately before the start of secondary recrystallization was investigated. Secondary recrystallization did not occur in the sample that was not subjected to the nitriding treatment. For samples that have undergone nitriding,
In the atmosphere a, secondary recrystallized grains were observed at 1050 ° C., and the N content of the steel sheet at 1025 ° C. (immediately before the start of secondary recrystallization) was 0.0146 wt%. On the other hand, in the case of the atmosphere b, secondary recrystallized grains were observed at 1075 ° C and 1050 ° C.
The N content of the steel sheet (immediately before the start of secondary recrystallization) was 0.009.
It was 5% by weight. Table 4 shows the experimental conditions and the magnetic properties of the products.

[0036]

[Table 4]

[Example 5] C: 0.048% by weight, S
i: 3.31% by weight, Mn: 0.15% by weight, S: 0.
007% by weight, acid-soluble Al: 0.031% by weight, N:
A 40 mm thick slab containing 0.0069% by weight and the balance Fe and unavoidable impurities at a temperature of 1150 ° C.
Immediately after heating for a minute, hot rolling was performed to obtain a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was pickled and then cold rolled at a rolling reduction of about 49% to obtain a cold rolled sheet having a thickness of 1.18 mm. Thereafter, the cold-rolled sheet was treated at 1050 ° C. for 30 seconds (soaking), then 90
An intermediate annealing was performed in which the steel was gradually cooled to 0 ° C. and then rapidly cooled, and then cooled at a rolling reduction of about 78% to obtain a 0.260 mm-thick cold-rolled sheet.

This cold-rolled sheet was subjected to two levels of decarburization annealing: a: 820 ° C. × 150 seconds (soaking), b: 850 ° C. × 150 seconds (soaking). In this case, the average grain size (equivalent circle diameter) of the primary recrystallized grains was a: 21 μm, b: 24 μm. This steel sheet was subjected to the nitriding treatment described in Example 1.
The N content of the steel sheet after nitriding was 0.0195 to 0.0210% by weight. After that, an annealing separating agent containing MgO as a main component is applied, and the temperature is raised to 1200 ° C. at 10 / hr, and 120
A final finish annealing was carried out at 0 ° C for 20 hours. At this time, the temperature was kept at 1200 ° C. for 20 hours in an annealing atmosphere of H ₂ 100%, and in the temperature rising process, a: N ₂ 25%,
H ₂ 75%, b: N ₂ 75%, and H ₂ 25% were set as two levels of atmospheric conditions. During the final finish annealing, the sample was taken out from the furnace under the conditions described in Example 4, and the N content of the sample immediately before the start of secondary recrystallization was investigated. In the case of decarburizing annealing condition a and final finishing annealing atmosphere condition A, secondary recrystallized grains were observed at 1050 ° C., and a steel plate at 1025 ° C. (immediately before the start of secondary recrystallization)
The amount of N was 0.0198% by weight. In the case of decarburization annealing condition a and final finishing annealing atmosphere condition B, secondary recrystallized grains were observed at 1075 ° C., and the N content of the steel plate at 1050 ° C. (immediately before the start of secondary recrystallization) was 0.0254 wt. %Met. In the case of decarburization annealing condition b and final finishing annealing atmosphere condition A, secondary recrystallized grains are observed at 1075 ° C. and 1050 ° C.
The N content of the steel sheet (immediately before the start of secondary recrystallization) was 0.018
It was 4% by weight. In the case of decarburization annealing condition b and final finishing annealing atmosphere condition B, secondary recrystallized grains are observed at 1100 ° C., and the N content of the steel plate at 1075 ° C. (immediately before the start of secondary recrystallization) is 0.0245 wt. %Met. Table 5 shows the experimental conditions and the magnetic properties of the products.

[0039]

[Table 5]

[0040]

As described above, in the present invention,
The final cold rolling reduction rate of the process of performing cold rolling two or more times is 6
0 to 79%, the primary recrystallization grain size is 10 to 35 μm after the completion of decarburization annealing and before the start of final finish annealing, and after hot rolling, the steel sheet is formed before the start of secondary recrystallization of final finish annealing. Nitriding treatment was performed, and N of the steel sheet immediately before the start of secondary recrystallization
By controlling the amount, good magnetic properties can be stably obtained under the low temperature slab heating condition, so that its industrial effect is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a primary recrystallized grain size (equivalent circle diameter) and a magnetic flux density of a product.

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[Procedure amendment]

[Submission date] November 17, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction content]

Although the reason why the relationship shown in FIG. 1 is established is not always clear, the present inventors presume as follows. That is, for the secondary recrystallization orientation
The mechanism of influence of primary recrystallization texture can be explained by using the concept of corresponding orientation. In the case of 60-79% low pressure final cold rolling as in the present invention, the primary recrystallization texture does not develop so sharply. Therefore, the orientation corresponding to the secondary recrystallization orientation does not become so strong.
In such a case, it is considered that the nuclei having a large product of the abundance of the secondary recrystallized nuclei and the corresponding orientation amount are likely to be secondary recrystallized. At this time, the critical value required for secondary recrystallization of the above product becomes important, and if this critical value is too high, secondary recrystallization
Does not occur and is too low, secondary recrystallized grains having various orientations will be generated. Since the reciprocal of the primary recrystallized grain size is considered to be proportional to the driving force of the grain growth of the secondary recrystallized grains, the smaller the average grain size of the primary recrystallized grains, the lower the upper critical value, the larger the It seems that the critical value is high. Therefore, as shown in FIG. 1, it can be said that there is an appropriate value for the average particle size of primary recrystallization in the range of 10 to 35 μm. In the case where it exceeds 10 μm, the secondary recrystallization is poor in any case, and it is understood that the appropriate range of the critical value and the appropriate range of the primary recrystallized grain size correspond to each other.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

Al is AlN necessary for stabilizing secondary recrystallization.
Alternatively , in order to secure (Al, Si) N , acid-soluble A
0.01% or more is required as l. Acid soluble Al
Is more than 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.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0023

[Correction method] Change

[Correction content]

The steel sheet after cold rolling is subjected to decarburization annealing, application of an annealing separating agent, and final finishing annealing by a usual method to obtain a final product. The average grain size of the primary recrystallized grains after completion of decarburization annealing and before the start of final finishing annealing is set to 10 to 35 μm as shown in FIG. 1 within the range of B ₈ (T) ≧
This is because a good magnetic flux density of 1.85 can be obtained. Then, it is defined that the steel sheet is subjected to the nitriding treatment before the start of the secondary recrystallization of the final finish annealing after the hot rolling, in the process premised on the low temperature slab heating as in the present invention, it is necessary for the secondary recrystallization. This is because the inhibitor strength tends to be insufficient.
The nitriding method is not particularly limited, and a method of denitrifying and then subsequently nitriding by mixing NH ₃ gas in an annealing atmosphere, a method of using plasma, a method of adding a nitride to an annealing separator and adding a final finishing annealing step Any method such as a method of absorbing nitrogen formed by decomposition of nitrides in the temperature into the steel sheet or a method of nitriding the steel sheet by increasing the N ₂ partial pressure in the atmosphere of final annealing is applicable. The nitriding amount is not particularly limited, but 1 ppm or more is necessary.

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[0040]

As described above, in the present invention,
The final cold rolling reduction rate of the process of performing cold rolling two or more times is 6
0 to 79%, the average grain size of the primary recrystallized grains is 10 to 35 μm after the completion of decarburization annealing and before the start of final finish annealing, and after the hot rolling until the start of secondary recrystallization of final finish annealing. By nitriding the steel sheet and controlling the N content of the steel sheet immediately before the start of secondary recrystallization, good magnetic properties can be stably obtained under low-temperature slab heating conditions. Is extremely large.

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FIG. 1 is a diagram showing the relationship between the average grain size (equivalent circle diameter) of primary recrystallized grains and the magnetic flux density of a product.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Internal reference number FI technical display location H01F 1/16 B 7371-5E (72) Inventor Akira Kono No. 1 Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka No. 1 inside Nippon Steel Co., Ltd. Yawata Works

Claims (2)

[Claims] 1. By weight%, C: 0.025 to 0.075%, Si: 2.5 to 4.5%, acid-soluble Al: 0.010 to 0.060%, N: 0.0010 to 0. After heating a slab containing 0.0130%, S + 0.405Se: 0.014% or less, Mn: 0.05 to 0.8% and the balance of Fe and inevitable impurities at a temperature of less than 1280 ° C., Hot-rolled, the resulting hot-rolled sheet including final cold rolling with a rolling reduction of 60 to 79%,
If necessary, perform cold rolling twice or more with intermediate annealing.
Then, in the method of producing a unidirectional electrical steel sheet by performing decarburization annealing and final finishing annealing, after decarburization annealing is completed, the average grain size of the primary recrystallized grains until the start of final finishing annealing is 10 to 35 μm.
m, a nitriding treatment is performed on the steel sheet after hot rolling and before the start of secondary recrystallization in the final finish annealing, a method for producing a unidirectional electrical steel sheet having excellent magnetic properties. 2. A steel sheet immediately before the start of secondary recrystallization in the final finish annealing contains N: 0.0100% or more by weight%. Manufacturing method of grain-oriented electrical steel sheet.