JP3507232B2 - Manufacturing method of unidirectional electrical steel sheet with large product thickness - Google Patents

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 a large product thickness, 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 excellent in magnetic characteristics such as excitation characteristics and iron loss characteristics. Required for reduced energy loss. As a characteristic value representing the excitation characteristic, a magnetic flux density B ₈ at a magnetic field strength of 800 A / m is standardized by JIS and is normally used. As a characteristic value indicating energy loss, the energy loss (iron loss) W _17/50 per 1 kg of steel sheet when magnetized to 1.7 Tesler (T) at a frequency of 50 Hz is also JI.
It is standardized by S.

The magnetic flux density is the most dominant factor of iron loss, and generally, the higher (larger) the magnetic flux density is, the better the iron loss characteristics are. Further, generally, when the magnetic flux density becomes high, the secondary recrystallized grains become large, and the iron loss may deteriorate. In this case, as is widely known, by controlling the magnetic domain, iron loss can be improved regardless of the grain size of the secondary recrystallization.

This unidirectional electrical steel sheet has a so-called Goss structure having {110} on the surface of the steel sheet and <001> axis in the rolling direction by causing secondary recrystallization in the final finishing annealing step. 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 manufacturing technique for such a high magnetic flux density unidirectional electrical steel sheet has been developed for a long time, and in Japan, a method using MnS and AlN as so-called inhibitors (Japanese Patent Laid-Open No. 40-1).
5644), a method using MnS, MnSe, Sb and the like (JP-A-51-13469). In these cases, complete solid solution of the inhibitor at the hot rolled sheet stage is required, and it is necessary to set the heating temperature of the steel ingot (slab) to 1350 ° C. or higher during actual hot rolling.

There are a number of disadvantages and inconveniences in heating at this high temperature. Therefore, attempts have been made to lower the heating temperature of the steel ingot (slab) during the hot rolling. JP-A-59-56522 discloses one of them. Many inventions have been made as the development of this technology, and a method of performing nitriding in the temperature rising process from decarburization annealing to final finishing annealing to form an inhibitor (Japanese Patent Laid-Open Nos. 62-45285 and 60-179855). ), And a method of performing a nitriding treatment using a mixed gas of hydrogen, nitrogen, and ammonia under a condition in which the strip is run (JP-A-2-77525, JP-A-1-82400, and JP-A-3-180).
460 and JP-A-1-317592) have been proposed.

Further, a method of controlling the primary recrystallized grain size at an intermediate stage after completion of primary recrystallization during decarburization annealing and before completion of secondary recrystallization during final annealing (Japanese Patent Laid-Open No. 3-294425).
Japanese Patent Application Laid-Open No. 2-96275, Japanese Patent Application Laid-Open No. 2-59
No. 020 and Japanese Patent Laid-Open No. 1-82393) have also been proposed. By the way, the unidirectional electrical steel sheets are mainly used by being laminated as an iron core of a transformer. Particularly for a large-sized power transformer, the work of stacking the iron cores is a manual work, and thus the workability (productivity) is improved as the thickness of the unidirectional electrical steel sheet is increased. Therefore, there has been a strong demand for manufacturing thick (for example, more than 0.40 mm) unidirectional electrical steel sheets with excellent magnetic properties. However, in the production of the grain-oriented electrical steel sheet, it has been essential in the prior art to incorporate carbon to some extent. The purpose of containing carbon is high temperature (1350 ° C or higher)
In the case of slab heating, there are measures such as preventing abnormal grain growth during slab heating, securing a transformation phase for adjusting the texture of primary recrystallization, and securing a solid solution amount of the inhibitor. In addition, as in the case of the present invention, the nitriding treatment in the state where the strip is run by heating the slab below 1280 ° C. is performed as described above.
It was required to contain some carbon for the purpose of. On the other hand, carbon is 3 in the final product of the grain-oriented electrical steel.
If it is present at 0 ppm or more, magnetic aging will occur and the commercial value will be lost. Therefore, in the decarburization annealing process, the carbon content is forced to 30%.
It should be below ppm. In this case, if the plate thickness is thick, it takes time to decarburize, the productivity is remarkably reduced, and the cost is increased. Therefore, the product thickness of unidirectional electrical steel sheet is currently 0.3.
It is standardized in JIS etc. with a maximum of 5 mm. As a method of reducing the carbon content, as a substitute for the above, a method of once pre-rolling (breakdown) to make fine crystal grains before hot rolling and heating is adopted. However, also in this case, the cost is inevitably increased. Further, there is a limit to the reduction of C.

Further, JP-A-4-323 and JP-A-4
In Japanese Patent Laid-Open No. 324, the conditions for hot rolling of a thick grain-oriented electrical steel sheet without hot-rolled sheet annealing are specified.
Of course, a thick grain-oriented electrical steel sheet can also be manufactured by this method. In this case, only the primary recrystallization texture is modified in the production of grain-oriented electrical steel, but the inhibitor is adjusted only in the hot rolling process, and the stability of magnetic properties and iron loss There was a limit to improvement.

[0008]

As described above, the decarburization step is inevitably present in the production of the grain-oriented electrical steel sheet. Therefore, in the case of thick material (0.30 mm or more), the carbon before decarburization annealing is used. It is important to reduce the content as much as possible to improve the productivity. The present invention provides a method of reducing the carbon content in order to secure the magnetic properties and reduce the load in such decarburization annealing.

As is well known, the decarburization annealing process of the grain-oriented electrical steel sheet is continuously performed and has the following three functions. That is, (1) primary recrystallization, (2) decarburization,
(3) It has three functions of forming a surface oxide layer. These 3
Careful operation is required to make the two functions work well. When the product is thick as in the present invention, it has been difficult to economically combine the conventional methods (2) and (3).

That is, when the plate thickness is large, the time required for decarburization becomes long (approximately the decarburization time becomes long in proportion to the square of the thickness). On the other hand, when the steel sheet is allowed to stay in the decarburizing atmosphere (oxidizing atmosphere) for a long time, the oxide layer on the surface becomes thick and the decarburizing property is reduced, and it becomes very difficult to reduce the residual carbon to 30 ppm or less. Further, if the oxide layer on the surface becomes extremely thick, defects will occur in the primary film (film mainly composed of forsterite) after secondary recrystallization annealing, and the commercial value will be significantly reduced. Therefore, it is necessary to reduce the carbon content.

Further, what is required is to produce a thick grain oriented electrical steel sheet having good magnetic loss as well as magnetic flux density among the magnetic characteristics, by manufacturing with a low carbon content material.

[0012]

Means for Solving the Problems The inventors of the present invention have made earnest studies and, as a main technique, carry out a nitriding treatment in a state where a slab heating temperature is lower than 1280 ° C. and a strip is allowed to run after decarburization annealing. In the production method of unidirectional electrical steel sheet, if the product sheet thickness and the final cold rolling rate are within certain ranges, it is possible to produce unidirectional electrical steel sheet with good magnetism even if the carbon content is lower than before. Found out.

In the first place, in order to obtain good magnetic properties (secondary recrystallization texture having a good Goss orientation with good directionality) in the production of grain-oriented electrical steel sheet, the primary recrystallization aggregation after decarburization annealing is required. It is necessary to optimize and secure inhibitor strength. Conventionally, in order to optimize the primary recrystallization texture in the production of grain-oriented electrical steel sheets, a certain degree of transformation phase was required during the final cold rolling after the hot rolling. For this reason, carbon is included. However, 12
It has been found that in the slab heating method of less than 80 ° C., the degree of optimization of the primary recrystallization texture may be small.
Quantitatively, this optimization degree corresponds to the grain boundary theory of Harase et al. (Eg, Japanese Examined Patent Publication 7-26155, The Japan Institute of Metals, No. 59).
Vol. 9, No. 9 (1995) 917-924). That is, Goss oriented grains ({110} <011>)
And Σ9 corresponding orientation grains exist even in a small amount, and the value is all Σ9
Largest (strong) and good Go in the azimuth distribution
Secondary recrystallization occurs in the ss orientation. However, the stability and sharpness of this Goss orientation expression depend on the strength of the inhibitor.

In the method according to the present invention, it has been found that the inhibitor strength can be arbitrarily controlled as strip nitriding so that only good Goss-oriented grains can be selectively expressed. As a method for maximizing the Σ9 value for this Goss-oriented grain in the entire distribution, there are mainly texture before cold rolling, grain size, and cold rolling rate, of which the cold rolling rate is the most influential factor. Therefore, by defining this, it becomes possible to manufacture a grain-oriented electrical steel sheet having a thick product sheet.

The summary is as follows. (1) By weight ratio , C: 0.010 to 0.040%, Si: 2.5 to 4.0%, acid-soluble Al: 0.020 to 0.040%, N: 0.005 to 0. 010%, 0.005 to 0.015 at least one of S and Se
%, Mn: 0.05 to 0.8%, 128 slabs with the balance Fe and unavoidable impurities
Heating at a temperature of less than 0 ° C., hot rolling, hot-rolled sheet annealing, cold rolling at least once with intermediate annealing sandwiched between hydrogen, nitrogen, and decarburization annealing, and running strips.
Nitriding is performed in a mixed gas of ammonia, and then firing is performed.
In the manufacturing method of the grain-oriented electrical steel sheet, which is subjected to the final finish annealing by applying the blunt separating agent , the product thickness (t: mm) is 0.30 mm.
≦ t ≦ 0.65 mm and final cold rolling ratio: R (%)

[0016]

[Equation 2]

A method for producing a grain-oriented electrical steel sheet having a large product thickness, characterized in that (2) Further, in addition to the steel composition of the above (1), Sn, S
A method for producing a grain-oriented electrical steel sheet having a large product thickness, which contains 0.02 to 0.3% of at least one of b, Cr and P. (3) Further, in addition to the steel composition of the above (1) and (2),
A method for producing a grain-oriented electrical steel sheet having a large product thickness, characterized by containing Cu in an amount of 0.03 to 0.30%. (4) Furthermore, in addition to the steel composition of the above (1), (2), and (3), Ni is contained in an amount of 0.03 to 0.3%. Steel plate manufacturing method.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
First, the reason for limiting the plate thickness in the present invention will be described. When the product plate thickness is less than 0.30 mm, the carbon content is 0.040%
Even if the content is 0.060% or less, decarburization up to 30 ppm or less of carbon in decarburization annealing is not rate-determining but surface oxide layer formation is rate-determining, and there is little merit in reducing carbon. Therefore, the plate thickness is 0.30 mm or more. Further, if the plate thickness is less than 0.30 mm, if the carbon content is small and the primary recrystallization texture is not adequately adequate, the degree of deterioration of the inhibitor (mainly AlN) during secondary recrystallization annealing will be fast and good. No good Goss-oriented grains can be obtained. Of course, it is possible to control the atmosphere during the secondary recrystallization and suppress the decomposition of the inhibitor to obtain good Goss-oriented grains.
In this case, many defects occur in the primary film mainly composed of forsterite, and the yield is remarkably reduced.

On the other hand, the upper limit of 0.65 mm has a carbon content of 0.05 in order to secure productivity with a plate thickness larger than this.
This is because it must be less than 10%. When the carbon content is less than 0.010%, it is difficult to favorably form the primary film containing forsterite as a main component and to stabilize the secondary recrystallization having good Goss orientation.

Next, the reasons for limiting the component composition of the electromagnetic steel slab used as the starting material in the present invention are as follows.
C was set to 0.010 to 0.040%. 0.010%
If less than the above, it was described above. On the other hand, if it exceeds 0.040%, the productivity in the decarburization step is significantly impaired, which is outside the scope of the present invention.

If the Si content is less than 2.5%, good iron loss cannot be obtained. On the other hand, if it exceeds 4.0%, it becomes difficult to perform steel sheet processing such as cold rolling at room temperature due to brittleness. S and Se are 0.015% or less, desirably 0.013% or less. One-way electromagnetic produced by manufacturing a hot-rolled sheet at a slab heating temperature of 1280 ° C. or lower, and then making an inhibitor after the decarburization annealing step such as strip nitriding after hot-rolled sheet annealing and cold rolling. This is because in the steel sheet, a large amount of S and Se hinders the grain growth of the primary recrystallized grains and is harmful. If it is less than 0.005%, due to unavoidable fluctuation factors in operation in hot rolling (temperature history difference on and between skids, fluctuation of hot rolling temperature due to acceleration of rolling speed, etc.),
Since the variation in the grain growth of the primary recrystallized grains is likely to occur, the product cannot be manufactured industrially stably.

Al combines with N to form AlN,
In the present invention, since it is essential to form (Al, Si) N by nitriding steel after the post-process, that is, after completion of primary recrystallization, a certain amount or more of free Al is required. Therefore, sol. 0.020 as Al
Add 0.040%. If the content of Mn is too small, the secondary recrystallization becomes unstable, while if it is too large, it becomes difficult to obtain a product having a high magnetic flux density. The proper content is 0.
It is 05 to 0.8%. Preferably, 0.070-0.
3%.

If N is less than 0.005%, the development of secondary recrystallized grains becomes poor. On the other hand, when it exceeds 0.010%, swelling of steel plate called blister occurs. It has been reported that P has the effect of improving the primary recrystallization texture. If the P content is low, this effect is small, and the cost of steelmaking increases, so the lower limit is made 0.02%. Regarding the upper limit, if it exceeds 0.30%, P tends to segregate at the grain boundaries to cause brittle fracture, and industrial production becomes difficult. Therefore, the upper limit is set to 0.30%.

Sn and Sb are, as has been conventionally said,
In the primary recrystallization texture, it has an effect of increasing {110} <001> oriented grains and an effect of uniformly precipitating sulfide. Therefore, in the present invention, Cu-S, M
The effect of controlling the precipitation of sulfides such as n-S is enhanced. Furthermore, when Sn and Sb are added in a large amount, oxidation during decarburization annealing is less likely to occur, and primary recrystallized grain growth tends to be less likely to occur. For this reason, the decarburization annealing temperature is set to the conventional 820-84.
There is no choice but to raise it by 20 ° C from 0 ° C. This is the direction that facilitates the formation of the primary coating of the grain-oriented electrical steel sheet. In addition, since the secondary recrystallized grain size is reduced by adding Sb and Sn, the iron loss (particularly low magnetic field iron loss) becomes better than that without addition. On the other hand, when Sb or Sn is less than 0.02%, the secondary recrystallized grains do not become so small. Further, if Sb or Sn exceeds 0.30%, nitriding treatment after decarburization annealing becomes difficult, which is not suitable for industrial production.

Cr is added to secure the amount of oxygen after decarburization annealing necessary for forming a forsterite film.
If it is less than 0.02%, the amount of oxygen becomes extremely small. On the other hand, if it exceeds 0.30%, the amount of oxygen increases extremely, and good forsterite is not formed. In addition, the magnetic flux density also decreases. If the Cu content is less than 0.03%, the effect is small. On the other hand, if it exceeds 0.5%, Cu-S precipitates are coarsened and the effect is reduced. Further, during hot rolling, the frequency of occurrence of so-called "Cu hegging" defects sharply increases. Preferably, it is 0.05 to 0.10%.

If Ni is less than 0.03%, the effect is small, and if it exceeds 0.3%, it is effective as shown in JP-A-5-306410, but it becomes expensive. Therefore, the upper limit is 0.30%. Addition of Cr and Ni further improves the effect of the present invention, and it is sufficient to add only an amount commensurate with cost. Next, the necessity of annealing the hot rolled sheet will be described. As described above, JP-A-4-323 and JP-A-4-324 define the hot rolling conditions when hot-rolled sheet annealing is not performed. In this case, hot rolling substitutes for hot rolled sheet annealing. But in this case
The primary recrystallization texture has a small Goss orientation, and according to the corresponding grain boundary theory, the grain size of the final product tends to be large and the iron loss tends to be poor. An effective means for securing the amount of Goss-oriented grains in the primary recrystallization texture, reducing the grain size of the final product, and improving iron loss is to perform hot-rolled sheet annealing. In fact, the above-mentioned patent publications both describe that the magnetic flux density is improved, but there is no description relating to the improvement of iron loss.

The effect of hot-rolled sheet annealing will be described below.
Coupled with the specific effect of cold rolling reduction,
Improves magnetic properties (especially iron loss value) in grain-oriented electrical steel
It has a special effect. Next, the cold rolling reduction
The range will be described based on FIGS. 1 and 2. Figure 1
Product thickness (mm) and final cold rolling rate in the case of claim 1
(%) Is shown. In Fig. 1, the ◎ mark indicates magnetism.
Bundle density (B₈) Is 1.88T or more, ○ indicates B ₈≧ 1.8
0T, △ mark is B₈A region of <1.80T is shown. Mark x
Is an area where a secondary recrystallization defect occurs. In the present invention
Then B₈≧ 1.80T is regarded as a region with good magnetic characteristics
It In this way, the thicker the product, the better the final cold
The range of rolling reduction is widening because of the primary recrystallization texture.
Even if the quality is not very good, the
This is because the deterioration of the target is slow. Show the upper and lower limits
The expression is

[0028]

[Equation 3]

[0029]

[0030]

EXAMPLES Examples of the present invention will be described below. <Example 1> Steel ingots having the components shown in Table 1 were manufactured by an ordinary method, and after heating the slab at 1100 to 1150 ° C, the hot rolled sheet was finished by ordinary hot rolling to have a hot rolled sheet thickness of 1.4 to 6.0 mm. Then 112
Annealed hot-rolled sheet for 2 minutes at 0 ° C, 180-22 after pickling
0.22-0.2 after warm rolling at 0 ° C. for at least 2 passes.
Cold rolled to 60 mm. After that, at 820 ° C to 840 ° C,
N ₂ : 25%, H ₂ : 75% in atmosphere gas, dew point 6
Decarburization primary recrystallization annealing was performed at 2 ° C. for 70 seconds to 150 seconds.

Then, the total nitrogen content is adjusted to 195 to 210.
Strip nitriding treatment was performed at ppm to apply an annealing separator , and finish annealing was performed. This finish annealing is 10-20
The temperature is raised at a temperature of ℃ / hour, and the atmosphere is N ₂ : 25%, H ₂ : 7
It was 5%. Then at 1200 ° C. for 20 hours, H ₂ : 1
A purification annealing of 00% was performed. Then, a tension coating and a smoothing treatment which are usually used were applied. The results are shown in Table 2.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

In the conventional GO production, it is necessary to contain some C in order to secure the transformation phase. For this reason, a product having a large thickness requires a long time for decarburization and is costly. In the present invention, it is possible to reduce the cost by reducing the C content.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a product thickness and a final cold reduction when a hot rolled sheet is annealed in the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hisakazu Kitagawa 1-1 Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Shin Nippon Steel Co., Ltd. Yawata Works (72) Inventor Katsuro Kuroki Tobata-ku, Kitakyushu-shi, Fukuoka 59 Nippon Steel Plant Design Co., Ltd., Nakahara No. 46 (56) Reference JP-A-59-56522 (JP, A) JP-A-62-45285 (JP, A) JP-A-2-77525 (JP, A) JP-A-3-294425 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/12 C22C 38/00 303 C22C 38/06 C22C 38/60 H01F 1/16

Claims (4)

(57) [Claims] 1. By weight ratio, C: 0.010 to 0.040%, Si: 2.5 to 4.0%, acid-soluble Al: 0.020 to 0.040%, N: 0.005 to 0.010%, 0.005 to 0.015 at least one of S and Se
%, Mn: 0.05 to 0.8%, 128 slabs with the balance Fe and unavoidable impurities
Heating at a temperature of less than 0 ° C., hot rolling, hot-rolled sheet annealing, cold rolling at least once with intermediate annealing sandwiched between hydrogen, nitrogen, and decarburization annealing, and running strips.
In the method for producing a grain-oriented electrical steel sheet in which a nitriding treatment is performed in a mixed gas of ammonia, and then an annealing separator is applied and final finishing annealing is performed, the product thickness (t: mm) is 0.30 mm.
≦ t ≦ 0.65 mm, and the final cold rolling rate: R (%) is given by A method for producing a grain-oriented electrical steel sheet having a large product thickness, characterized by: 2. The production of a grain-oriented electrical steel sheet having a large product thickness according to claim 1, further containing 0.02 to 0.30% of at least one of Sn, Sb, Cr and P. Method. 3. The method for producing a grain-oriented electrical steel sheet having a large product sheet thickness according to claim 1, further comprising 0.03 to 0.30% Cu. 4. The method for producing a grain-oriented electrical steel sheet according to claim 1, 2 or 3, further comprising 0.03 to 0.3% of Ni.