JP7010264B2 - Manufacturing method of grain-oriented electrical steel sheet and grain-oriented electrical steel sheet - Google Patents

Description

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet and a grain-oriented electrical steel sheet.

Generally, grain-oriented electrical steel sheets are manufactured as follows.
First, a steel slab containing Si of about 4.5% by mass or less and an inhibitor component such as MnS, MnSe, and AlN is heated to temporarily dissolve the inhibitor component. The heated steel slab is hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is annealed by hot-rolled sheet if necessary. Next, the hot-rolled steel sheet is subjected to cold rolling once or two or more times with intermediate annealing sandwiched between them to obtain a cold-rolled steel sheet having a final plate thickness. The obtained cold-rolled steel sheet is subjected to primary recrystallization annealing in a wet hydrogen atmosphere to perform primary recrystallization and decarburization. Then, an annealed separator containing magnesium oxide (MgO) as a main component is applied to the cold-rolled steel sheet after primary recrystallization annealing, and then the secondary recrystallization and purification of the inhibitor component are carried out at 1200 ° C. for 5 hours. Perform some final finish annealing.
Through such a process, the grain-oriented electrical steel sheet is manufactured (see Patent Document 1).

MgO applied as an annealing separator reacts with SiO ₂ on the surface of a steel sheet (cold-rolled steel sheet) to form a forsterite film, which is an oxide layer, on the surface of the steel sheet. The forsterite film forms irregularities called anchors at the interface with the steel sheet, and exhibits good adhesion to the steel sheet.

Special Publication No. 52-24499

Patent Document 1 discloses that the surface of a steel sheet after secondary recrystallization is smoothed by removing an oxide layer such as a forsterite film. It is said that good magnetic properties can be obtained by further applying tension to a steel sheet with a smoothed surface.

Examples of the method for smoothing the surface of a steel sheet include mechanical polishing (mechanical polishing) and electrochemical polishing (electrolytic polishing).
However, mechanical polishing may not provide good magnetic properties. It is considered that this is because the crystal structure of the steel sheet is plastically deformed by the stress applied to the steel sheet from the polishing tool used for mechanical polishing.
On the other hand, in electrolytic polishing using an electrolytic solution, stress is not applied to the steel sheet, but a sufficient polishing rate cannot be obtained as compared with mechanical polishing. It is thought that this is because iron ions dissolved in the electrolytic solution exist near the surface of the steel sheet, and the iron ion concentration on the right side of Fe + ^2e- → Fe ²⁺ locally increases near the surface of the steel sheet, making it difficult for electrolysis to proceed. Be done. In order to increase the polishing speed, it may be necessary to apply a high voltage or the like.

Therefore, the present inventors simultaneously perform electrolytic polishing and mechanical polishing using a steel plate after secondary recrystallization as a material to be polished and an electrolytic solution containing abrasive grains on a trial basis (hereinafter, "composite"). It is also called "electrolytic polishing" and may be simply called "polishing").
As a result, when composite electrolytic polishing is performed using an electrolytic solution containing abrasive grains (pH: 6), the material to be polished is taken out from the electrolytic solution and dried in a very short time after polishing. A brown oxide layer was formed on the surface of the surface, and a good surface condition could not be obtained (see FIG. 1). Specifically, the material to be polished after polishing had a surface roughness Ra of 0.43 μm and an oxygen basis weight of 0.08 g / m ² .
However, when the present inventor further investigated the material to be polished after polishing, almost all plastic deformation regions were found inside the material to be polished, even though the material was mechanically polished using a polishing tool. It turned out not.
Therefore, if the conditions for obtaining a good surface condition are found, it is judged that the composite electrolytic polishing can be a method for obtaining good magnetic properties.

The present invention has been made in view of the above points, and an object of the present invention is to provide a method for efficiently manufacturing grain-oriented electrical steel sheets having good surface conditions and magnetic properties.
Another object of the present invention is to provide a grain-oriented electrical steel sheet obtained by the above method.

The present inventors have diligently studied. Specifically, for example, when composite electrolytic polishing was performed using an electrolytic solution (pH: 9) containing abrasive grains, the material to be polished after polishing had a surface roughness Ra of 0.093 μm and an oxygen grain amount. It was 0.01 g / m ² (see FIG. 2). As a result, it was found that the above object can be achieved by setting the pH of the electrolytic solution containing the abrasive grains to a specific range, and the present invention has been completed.

That is, the present invention provides the following [1] to [4].
[1] A method for producing a directional electromagnetic steel sheet having a surface roughness Ra of 0.30 μm or less and an oxygen grain amount of 0.05 g / m ² or less, wherein the steel sheet after secondary recrystallization is produced. A method for manufacturing a directional electromagnetic steel plate, which is used as a material to be polished, and mechanical polishing is performed while performing electrolytic polishing by supplying an electrolytic solution containing abrasive grains and having a pH of more than 7 to the material to be polished. ..
[2] The method for manufacturing a grain-oriented electrical steel sheet according to the above [1], wherein the voltage is applied so that a current of 3 mA / cm ² or more and 160 mA / cm ² or less flows on the surface of the material to be polished.
[3] Before performing the electrolytic polishing and the mechanical polishing on the material to be polished, the oxygen grain amount of the material to be polished is set to 0.5 g / m ² or less in advance in the above [1] or [2]. The method for manufacturing a directional electromagnetic steel plate according to the description.
[4] The surface roughness Ra is 0.30 μm or less, the oxygen grain amount is 0.05 g / m ² or less, and the hysteresis loss at an exciting magnetic flux density of 1.7 T is 0.26 W / kg or less. Directional electrical steel sheet.

According to the present invention, a grain-oriented electrical steel sheet having a good surface condition and magnetic properties can be efficiently manufactured.

It is a photograph which shows the appearance of the material to be polished after composite electrolytic polishing with the electrolytic solution (pH: 6) containing an abrasive grain. It is a photograph which shows the appearance of the material to be polished after composite electrolytic polishing with the electrolytic solution (pH: 9) containing an abrasive grain. It is a photograph which shows the appearance of the material to be polished after the polishing of Example A. It is a photograph which shows the appearance of the material to be polished after the polishing of Example B. It is a photograph which shows the appearance of the material to be polished after the polishing of Example C. It is a photograph which shows the appearance of the material to be polished after the polishing of Example D. It is a photograph which shows the appearance of the material to be polished after the polishing of Example E. It is a photograph which shows the appearance of the material to be polished after the polishing of Example F. It is a photograph which shows the appearance of the material to be polished after the polishing of Example G.

In the method for manufacturing grain-oriented electrical steel sheets of the present invention (hereinafter, also referred to as "manufacturing method of the present invention"), composite electrolytic polishing is performed.
More specifically, the manufacturing method of the present invention is a method for manufacturing a directional electromagnetic steel plate having a surface roughness Ra of 0.30 μm or less and an oxygen grain amount of 0.05 g / m ² or less. The steel plate after secondary recrystallization is used as the material to be polished, and an electrolytic solution containing abrasive grains having a pH of more than 7 is supplied to the material to be polished, and the polishing tool is used to perform electrolytic polishing by applying a voltage. This is a method for manufacturing a directional electromagnetic steel plate, which is mechanically polished using.

<Preparation of abrasive material (steel plate after secondary recrystallization)>
The process up to the secondary recrystallization is not particularly limited, and may be performed according to a conventionally known process. Specifically, for example, there are many steps as follows.
First, inhibitor-forming elements such as C: 0.08% by mass or less, Si: 2.0 to 5.0% by mass, Mn: 0.005 to 0.5% by mass, Al, N, Cu, S, Se, etc. A steel slab containing components such as Ni, Sn, Sb, Cr, P, Bi, B, Nb, and Te that function as an auxiliary inhibitor is hot-rolled to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet is subjected to hot-rolled sheet annealing, if necessary, and then cold-rolled once or two or more times with intermediate annealing sandwiched between them to obtain a cold-rolled steel sheet. Next, the obtained cold-rolled steel sheet (steel sheet) is subjected to primary recrystallization accompanied by decarburization, and then an annealing separator containing MgO is applied, and then final finish annealing is performed for secondary recrystallization. ..
In this way, a steel sheet (abrasive material) after secondary recrystallization is obtained.

Here, consider a case where an oxide layer is excessively formed on the surface of the material to be polished before performing the composite electrolytic polishing. In this case, the resistance differs between the portion covered with the oxide layer and the steel plate (base iron) portion, so when a voltage is applied for electrolytic polishing, the current density that flows becomes non-uniform, and electrolysis proceeds locally. Cheap. As a result, the formation of surface irregularities on the material to be polished may be promoted, or the material to be polished may be easily oxidized after polishing.

Therefore, the oxygen grain amount of the material to be polished before performing the composite electrolytic polishing is preferably 0.5 g / m ² or less, more preferably 0.4 g / m ² or less, and further preferably 0.3 g / m ² or less. It is preferable, and 0.2 g / m ² or less is particularly preferable. It is needless to say that the lower the oxygen basis weight is, the more preferable it is, and 0.0 g / m ² may be used.

The oxygen basis weight is a value obtained by obtaining the oxygen amount of the entire material to be polished (total thickness) by chemical analysis and converting it into the basis weight per unit area (hereinafter, the same applies).

The method for reducing the oxygen content of the material to be polished before performing the composite electrolytic polishing is not particularly limited, but for example, oxygen of the steel plate to be polished is applied from the application of the annealing separator to the secondary recrystallization. Examples thereof include a method of reducing the amount of graining, and specific examples thereof include a method of preventing the formation of a forsterite film by using an annealing separator described in JP-A No. 64-62476.

Such a quenching separator contains, for example, magnesium oxide (MgO) and at least one chloride selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides.
The chloride content is, for example, 0.5 to 50 parts by mass, preferably 2 to 40 parts by mass, and more preferably 3 to 20 parts by mass with respect to 100 parts by mass of magnesium oxide.
The annealing separator may further contain an additive such as titanium oxide (TiO ₂ ) in an amount of 0.5 to 5 parts by mass with respect to 100 parts by mass of magnesium oxide.

Further, the oxygen basis weight of the material to be polished may be reduced in advance by removing the oxide layer by mechanical polishing. In this case, it is preferable to perform mechanical polishing using Scotch paper in order to prevent strain from being introduced into the deep part of the material to be polished.
When this mechanical polishing is performed, it is preferable that the oxygen basis weight of the material to be polished after the mechanical polishing is reduced to 0.5 g / m ² or less.

<Composite electrolytic polishing>
Next, composite electrolytic polishing is performed on the prepared material to be polished (steel plate after secondary recrystallization). Specifically, an electrolytic solution having a pH of more than 7 containing abrasive grains is supplied to the material to be polished. Then, a voltage is applied between the polishing tool and the material to be polished so that a predetermined amount of current flows on the surface of the material to be polished. As a result, electrochemical polishing (electrolytic polishing) is performed. At the same time as this electrolytic polishing, mechanical polishing (mechanical polishing) is also performed on the material to be polished using a polishing tool. Since the composite electrolytic polishing also performs mechanical polishing, it is more efficient than the case where only electrolytic polishing is performed.

At this time, as the electrolytic solution, an alkaline electrolytic solution having a pH of more than 7 is used instead of the generally used neutral or acidic electrolytic solution.
As a result, an oxide layer is not formed on the surface of the material to be polished until the material to be polished is taken out from the electrolytic solution and dried after polishing, and a good surface condition can be obtained. The reason for this is not clear, but it is thought that the surface of the material to be polished has hydroxy groups to protect the surface.
The material to be polished after being taken out from the electrolytic solution is thoroughly washed with water to remove the electrolytic solution and abrasive grains remaining on the surface. After that, it is sufficiently dried.

The material to be polished (that is, the grain-oriented electrical steel sheet obtained by the production method of the present invention) after the composite electrolytic polishing thus obtained has a good surface condition. Specifically, the surface roughness Ra is 0.30 μm or less, and the oxygen basis weight is 0.05 g / m ² or less.

The surface roughness Ra is an arithmetic mean roughness (Ra) measured in accordance with JIS B 0601: 2013 (hereinafter, the same applies).

Further, the grain-oriented electrical steel sheet obtained by the manufacturing method of the present invention has almost no strain on its surface. Therefore, the magnetic characteristics are good. Specifically, 0.26 W / kg or less can be achieved as a hysteresis loss (history loss) at an exciting magnetic flux density of 1.7 T.

The pH of the electrolytic solution is preferably 8 or more because the above-mentioned effects are more excellent. The upper limit is not particularly limited, but the pH of the electrolytic solution is preferably 10 or less.
The type of the electrolytic solution is not particularly limited as long as the pH is within the above range, but an electrolytic solution containing sodium nitrate as a main liquid composition is preferable.
Examples of the components of the abrasive grains contained in the electrolytic solution include Al ₂ O ₃ and SiO ₂ . Such abrasive grains are dispersed in the electrolytic solution and used.

If the amount of current flowing on the surface of the material to be polished for applying a voltage is too small, the speed of electrolytic polishing may not be sufficient, and as a result, the desired surface condition may not be obtained.
Therefore, the amount of current flowing through the surface of the material to be polished is preferably 3 mA / cm ² or more, more preferably 4 mA / cm ² or more, and further preferably 5 mA / cm ² or more because the desired surface condition can be easily obtained. preferable.
On the other hand, if this amount of current is too large, the iron ion concentration on the surface of the material to be polished may increase and the efficiency may decrease. Further, due to the formation of the oxide layer, the surface condition may be insufficient before the material to be polished is taken out from the electrolytic solution and dried after polishing.
Therefore, the amount of current flowing through the surface of the material to be polished is preferably 160 mA / cm ² or less, more preferably 130 mA / cm ² or less, and further preferably 100 mA / cm ² or less because the desired surface condition can be easily obtained. preferable.

As for other conditions, devices, etc. in composite electrolytic polishing, conventionally known conditions, devices, etc. in electrolytic polishing and mechanical polishing can be appropriately adopted.

Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to these.

<Preparation of abrasive material (steel plate after secondary recrystallization)>
First, Si: 3.2% by mass, Mn: 0.03% by mass, Al: 0.02% by mass, N: 0.006% by mass, and Se: 0.005% by mass are contained, and the balance is Fe. And prepared a steel slab consisting of unavoidable impurities. The prepared steel slab was heated to 1350 ° C. and then hot-rolled to obtain a hot-rolled steel sheet having a plate thickness of 2.0 mm.
The obtained hot-rolled steel sheet is annealed at 1000 ° C. for 60 seconds, then pickled, and then cold-rolled once to obtain a cold-rolled steel sheet having a thickness of 0.23 mm. Obtained.
The obtained cold-rolled steel sheet was subjected to primary recrystallization annealing with a soaking temperature of 830 ° C. and a soaking time of 2 minutes. The primary recrystallization annealing was carried out in a moist atmosphere with 50% by volume of hydrogen + 50% by volume of nitrogen and a dew point of 55 ° C. in all steps. Decarburization was also performed at the same time.
An annealing separator was applied to the cold-rolled steel sheet (steel sheet) after primary recrystallization annealing. As the annealing separator, an annealing separator in which 2 parts by mass of titanium oxide (TiO ₂ ) and a predetermined amount of magnesium chloride (MgCl ₂ ) were mixed with 100 parts by mass of magnesium oxide (MgO) was used. The amount (unit: parts by mass) of magnesium chloride (MgCl ₂ ) with respect to 100 parts by mass of magnesium oxide (MgO) is shown in Table 1 below.
Then, for secondary recrystallization, final finish annealing was performed at 1150 ° C. for 5 hours of soaking. In this way, a steel sheet after secondary recrystallization, which is the material to be polished, was obtained.
The oxygen basis weight of the obtained material to be polished (oxygen basis weight before polishing, unit: g / m ² ) was measured by the above-mentioned method. The measurement results are shown in Table 1 below.

<Polishing>
In Example A, the obtained material to be polished was simply mechanically polished using Scotch paper.
On the other hand, in Examples B to G, the obtained material to be polished was subjected to composite electrolytic polishing using an electrolytic solution containing abrasive grains. More specifically, an electrolytic solution containing sodium nitrate in which Al ₂ O ₃ abrasive grains were dispersed was used. Further, Table 1 below shows the pH and current density of the electrolytic solution (the amount of current applied to the surface of the material to be polished, unit: mA / cm ² ).

<Evaluation>
The appearance of the material to be polished after polishing was observed. The external photographs of Examples A to G are shown in FIGS. 3 to 9, respectively. The appearance state (“mirror surface”, etc.) is shown in Table 1 below.
Further, the surface roughness Ra (unit: μm) and the oxygen basis weight (unit: g / m ² ) of the material to be polished after polishing were measured by the above-mentioned method. The measurement results are shown in Table 1 below.
Further, the hysteresis loss (unit: W / kg) at an exciting magnetic flux density of 1.7 T was obtained from the hysteresis loop. This result is also shown in Table 1 below.

<Summary of evaluation results>
First, the examples B to G in which the composite electrolytic polishing is performed are compared. In Examples B to G, since composite electrolytic polishing accompanied by mechanical polishing is performed, the efficiency is better than that of simple electrolytic polishing.
However, in Example G in which the pH of the electrolytic solution is 7, the material to be polished after polishing has an appearance of "reoxidation", a surface roughness Ra of 0.5 μm, and an oxygen basis weight of 0. It was 08 g / m ² . The hysteresis loss was 0.30 W / kg. That is, the surface condition and magnetic properties were insufficient.

On the other hand, in Examples B to F in which the pH of the electrolytic solution is higher than 7, the surface roughness Ra of the material to be polished after polishing is 0.30 μm or less, and the oxygen basis weight is 0.05 g. It was less than / m ² . The hysteresis loss was 0.26 W / kg or less. That is, both the surface condition and the magnetic characteristics were good.

Comparing Examples B to F, Examples C, E and Examples have an oxygen roughness before polishing of 0.5 g / m ² or less and a current density of 3 mA / cm ² or more and 160 mA / cm ² or less. F had a smaller surface roughness Ra value and a smaller oxygen content than Example B and Example D, which did not satisfy these conditions. The value of hysteresis loss was also smaller.

In Example A in which mechanical polishing was performed, the hysteresis loss was 0.35 W / kg. It is considered that this is because the crystal structure was plastically deformed by the stress of mechanical polishing.

Claims (3)

A method for manufacturing grain-oriented electrical steel sheets having a surface roughness Ra of 0.30 μm or less and an oxygen basis weight of 0.05 g / m ² or less.
The steel plate after secondary recrystallization is used as the material to be polished.
A method for manufacturing a directional electromagnetic steel plate, in which an electrolytic solution containing abrasive grains having a pH of more than 7 is supplied to the material to be polished, and mechanical polishing is performed while performing electrolytic polishing by applying a voltage. The method for manufacturing a directional electromagnetic steel plate according to claim 1, wherein the voltage is applied so that a current of 3 mA / cm ² or more and 160 mA / cm ² or less flows on the surface of the material to be polished. The directional electromagnetic steel plate according to claim 1 or 2, wherein the oxygen grain amount of the material to be polished is 0.5 g / m ² or less in advance before the electrolytic polishing and the mechanical polishing of the material to be polished are performed. Manufacturing method.

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