JP6903996B2 - Non-oriented electrical steel sheet - Google Patents

Description

The present invention relates to non-oriented electrical steel sheets having a small iron loss. The non-oriented electrical steel sheet obtained in the present invention is suitable for applications requiring extremely low iron loss, such as the iron core of large electric machinery and equipment.

The demand for iron loss reduction for non-oriented electrical steel sheets continues. In order to reduce iron loss, various measures are taken such as reduction of impurities in steel, optimization of crystal grain size, improvement of texture, increase of intrinsic resistance, and reduction of plate thickness. The most important thing in realizing them is the composition design of the alloy. Regarding the improvement of iron loss by adjusting the components, many techniques have been proposed, such as not only increasing the intrinsic resistance but also detoxifying the precipitates and improving the texture.

Patent Document 1 proposes a technique for reducing iron loss by adding Sn.

Patent Document 2 proposes a technique of adding Sn or Sb in order to suppress the deterioration of iron loss due to the formation of a nitride layer on the surface of the steel sheet when the S contained in the steel is very low.

Patent Document 3 proposes a technique for detoxifying fine inclusions by adding REM.

Japanese Unexamined Patent Publication No. 55-158252 Japanese Unexamined Patent Publication No. 10-317111 Japanese Unexamined Patent Publication No. 2005-336503

The iron loss of non-oriented electrical steel sheets has been steadily reduced, and an object of the present invention is to further reduce this. Then, an object of the present invention is to provide a material suitable for an application requiring a very low iron loss, such as a large rotary machine.

The present inventors have diligently studied the composition of components for reducing the iron loss of non-oriented electrical steel sheets. As a result, it was found that the iron loss can be reduced depending on the contents of Al, Mn, and Sn. In the present invention, the Al content is limited in order to reduce the hysteresis loss, and therefore a large amount of Mn is contained for the purpose of compensating for the reduced intrinsic resistance. However, the content of Mn tends to deteriorate iron loss by forming a nitride on the surface of the steel sheet at the time of final annealing in which it is cold-rolled and recrystallized. In order to suppress this nitriding, a small amount of Sn is added according to the Mn content.

The present invention has been made based on the above findings, and the gist thereof is as follows.

[1] In mass%, C: 0.0030% or less, Si: 2.7 to 3.3%, Al: 0.2 to 0.5%, Mn: 0.5 to 2.0%, S: A steel sheet containing 0.0040% or less and N: 0.0040% or less, further containing Sn in an amount satisfying the formula (1), and the balance consisting of Fe and impurities, which is obtained by the formula (2). _{A non-oriented electrical steel sheet having a nitrogen content [N] s} of 0.0040% by mass or less in a portion having a depth of up to 20 μm from the outermost surface of the ground iron portion.

0.04-(0.01 / [Mn]) ≤ [Sn] ≤ 0.1 ... Equation (1)
[N] _s = (t × [N] ₁ − (t-40) × [N] ₂ ) / 40 ・ ・ ・ Equation (2)

Here, [Sn] and [Mn] are the contents of Sn and Mn (mass%), respectively, t is the thickness of the steel sheet excluding the insulating coating (μm), and [N] ₁ is the entire steel sheet excluding the insulating coating. Nitrogen content (mass%) and [N] ₂ are the nitrogen content (mass%) contained in the steel sheet after removing 20 μm (excluding the insulating coating) from both surfaces.

[2] The above [1], which contains at least one of B: 0 to 0.0005%, REM: 0 to 0.03%, and Ca: 0 to 0.005% in place of a part of the Fe. ] Non-oriented electrical steel sheet.

According to the present invention, it is possible to reduce the iron loss of non-oriented electrical steel sheets.

Effect of Mn and Al on iron loss in samples prepared by hydrogen box annealing. Effect of Mn and Al on iron loss in samples prepared by continuous annealing. Effect of Mn and Al on the difference in nitrogen content due to the annealing method. Changes in iron loss due to the addition of Sn. Changes in the nitrogen content of the surface layer due to the addition of Sn. Effect of Mn and Sn on surface nitrogen content.

First, the experiments that led to the invention of this case will be described.

<Experiment 1>
The target alloys with different component compositions of Al and Mn shown in Table 1 are melted by vacuum melting, and then hot-rolled (heating temperature 1150 ° C., finishing temperature 850 ° C., finished plate thickness 2.3 mm) and hot-rolled. Plate annealing (equalizing temperature 870 ° C., soaking time 60 seconds) and cold rolling were performed to obtain a cold rolled plate of 0.5 mm. Next, in order to obtain a sample for evaluation, the cold rolled plate was box-annealed with 100% hydrogen and a DRY atmosphere.

For magnetic measurement, after shearing to the sample size of 55 mm × 55 mm for magnetic measurement, the sample was bound and subjected to annealing by applying surface pressure, and a small sample for observing the cross-sectional structure was also subjected to annealing at the same time. The soaking temperature was 800 to 1000 ° C., and the soaking time was 2 hours. By performing annealing by such a method, it is possible to obtain an ideal sample for evaluating magnetic properties without the influence of nitriding or oxidation and without residual strain.

Based on JIS C 2556 "Electromagnetic Steel Sheet Single Plate Magnetic Characteristic Test Method", AC magnetic measurement was performed, and DC magnetic measurement was performed using a Cioffi type magnetic measuring device. At that time, the values in the table were used for the density. Further, the average crystal grain size of each sample was determined based on JIS G 0551 “Steel-Crystal Particle Size Microscopic Test Method”. Based on these data, the relationship between the average crystal grain size and iron loss in each alloy can be obtained.

According to the relationship diagram, the total iron loss W15 / 50 was the smallest in almost all alloys when the average crystal grain size was 140 μm. In FIG. 1, the total iron loss, the hysteresis loss, and the eddy current loss were obtained when the average crystal grain size was 140 μm, and the effects of Mn and Al on them were shown.

Looking at this figure, it can be seen that the influence of the Mn concentration on the hysteresis loss is small and increases as the Al concentration increases. On the other hand, the eddy current loss decreases as both Mn and Al increase. The cause of the increase in hysteresis loss as Al increases is not clear. The eddy current loss is considered to reflect the change in the intrinsic resistance. The total iron loss, which is the sum of these two, tends to decrease as the amount of Mn increases, and becomes the smallest when the amount of Al is about 0.2 to 0.5%.

<Experiment 2>
Experiment 1 was the evaluation result of the magnetic characteristics when ideal annealing (box annealing) was performed. In Experiment 2, a 0.5 mm cold-rolled plate produced by the same method as in Experiment 1 was annealed in a continuous annealing furnace capable of simulating actual annealing. The tension applied to the steel sheet was 0.3 kgf / mm ² , the atmosphere was 70% nitrogen, 30% hydrogen, DRY, the soaking temperature was 850 to 1050 ° C., and the soaking time was 30 seconds. From the material after annealing, a 55 mm × 55 mm magnetic measurement sample and a small piece for cross-sectional observation were cut out, and the magnetic properties and crystal structure were evaluated. The evaluation method is the same as in Experiment 1.

FIG. 2 shows the effects of the Mn content and the Al content on the total iron loss when the average crystal grain size is 140 μm. Compared with the total iron loss in FIG. 1, the iron loss value is larger as a whole, and the deterioration of iron loss due to increasing Mn to 0.2%, 0.5%, 0.7%. The amount is getting bigger. For each alloy, the amount of nitrogen was measured between the case of ideal annealing (box annealing) in Experiment 1 and the case of simulated annealing (continuous annealing) in Experiment 2, and the effects of Mn content and Al content on the difference. Is shown in FIG.

The measurement of nitrogen content was based on JIS G 1228 "Iron and Steel-Nitrogen Quantification Method". From this figure, it can be seen that when the actual machine simulated annealing (continuous annealing) is performed, nitriding becomes easier as the Mn content increases. Therefore, it can be said that the increase in the amount of deterioration of iron loss with Mn is due to the nitriding of the steel sheet surface during annealing.

<Experiment 3>
Next, as a method for preventing iron loss deterioration due to nitriding, the effect of adding Sn, which is an element that easily segregates on the surface and is considered to have little adverse effect on magnetism, was investigated. The alloys having the composition shown in Table 2 were melted, and a 0.5 mm cold-rolled plate was obtained by the same method as in Experiment 1 and subjected to annealing in a continuous annealing furnace. The soaking temperature is 1050 ° C., the soaking time is 30 seconds, and the atmosphere is 70% nitrogen, 30% hydrogen, and DRY. FIG. 4 shows the change in iron loss W15 / 50 due to Sn. The addition of Sn improved the iron loss.

Next, the nitrogen content in the surface layer of the annealed sample was examined. The region of the surface layer shall be in the range of 20 μm on one side, and the nitrogen content of that portion shall be [N] _s . To determine the nitrogen content in the surface layer, first _{measure the nitrogen content [N] 1} of the sample after annealing, then remove both surfaces by 20 μm by chemical polishing, and then remove the nitrogen content of the sample after removal. Amount [N] ₂ is measured. Using the measured [N] ₁ and [N] ₂ and the plate thickness t = 500 μm, [N] _s is calculated by the following equation.

[N] _s = (t × [N] ₁ − (t-40) × [N] ₂ ) / 40 ... Equation (2)

_{The change of [N] s} obtained in this way due to Sn is shown in FIG. The addition of Sn reduces the nitrogen concentration on the surface. That is, by adding Sn, nitriding during annealing after cold rolling was prevented, and deterioration of iron loss was suppressed.

<Experiment 4>
As shown in Table 3, the alloys in which the amounts of Mn and Sn were changed were melted in vacuum, a cold-rolled plate was prepared by the same method as in Experiment 1, and continuous annealing was performed in the same manner as in Experiments 2 and 3. For each of the obtained samples, the nitriding [N] _s on the surface of the steel sheet was determined by the same method as in Experiment 3, and the case of 40 ppm or less was marked with ◯, the case of above was marked with x, and the relationship between Mn and Sn is shown in FIG. It was. It can be seen that in order to reduce the amount to 40 ppm or less, it is necessary to increase the amount of Sn added as the amount of Mn increases. When the boundary between ◯ and × is fitted by a mathematical formula, it can be generally expressed as [Sn] = 0.04-0.01 / [Mn].

Based on these experiments, further studies have been carried out to reach the present invention. Hereinafter, the provisions of various requirements will be described in detail.

First, the reasons for limiting the component composition contained in the non-oriented electrical steel sheet of the present invention will be described. Hereinafter, "%" for a component means "mass%".

C: 0.0030% or less C forms carbides in the steel and deteriorates iron loss, so the content is set to 0.0030% or less. On the other hand, since it has an effect of suppressing nitriding of the surface during finish annealing and improving iron loss, 0.0005% or more may be added. The preferred range for addition is 0.0005 to 0.0025%, more preferably 0.0005 to 0.0020%, and even more preferably 0.0005 to 0.0015%.

Si: 2.7-3.3%
Si is an element effective for increasing the natural resistance of steel and reducing iron loss. In the present invention, it is 2.7% or more. On the other hand, if the amount is too large, the toughness of the steel deteriorates and manufacturing becomes difficult, so the content should be 3.3% or less. The preferred range is 2.8 to 3.2%, more preferably 2.9 to 3.1%.

Al: 0.2-0.5%
Al raises the solution temperature of AlN, suppresses fine precipitation of AlN in the steps after slab heating, and contributes to reduction of iron loss. To enjoy the effect, the lower limit is set to 0.2%. On the other hand, as revealed in Experiment 1, Al has an effect of increasing the hysteresis loss, so in the present invention, the upper limit is set to 0.5%. The preferred range is 0.2 to 0.4%, more preferably 0.2 to 0.3%.

Mn: 0.5-2.0%
In the present invention, as shown above, since the upper limit of the Al content is limited, the intrinsic resistance tends to be insufficient. Since Mn also has an action of increasing the intrinsic resistance, in the present invention, 0.5% or more of Mn is added in order to increase the insufficient intrinsic resistance. However, excessive addition makes the steel embrittlement, so the upper limit is 2.0%. The preferred range is 0.5 to 1.5%, more preferably 0.5 to 1.3%, and even more preferably 0.5 to 1.0%.

S: 0.0040% or less Since S forms a precipitate and deteriorates iron loss, it should be 0.0040% or less. It is preferably 0.002% or less, more preferably 0.001% or less.

N: 0.0040% or less N forms a precipitate and deteriorates iron loss, so it should be 0.0040% or less. It is preferably 0.002% or less, more preferably 0.001% or less.

Sn: 0.04-(0.01 / [Mn]) ≤ [Sn] ≤ 0.10.
As shown in the previous experiments 2 to 4, when the Mn content is increased, the surface of the steel sheet is easily nitrided during finish annealing and the iron loss is deteriorated. However, when Sn is added, the surface nitriding is suppressed and the iron loss is suppressed. Will also be improved. The required Sn addition amount at that time increases depending on the Mn addition amount. As shown in FIG. 6, the required amount of Sn added is 0.04- (0.01 / [Mn] when the contents (% by mass) of Sn and Mn are [Sn] and [Mn], respectively. ]) ≤ [Sn]. On the other hand, the excessive content of Sn deteriorates the toughness of the steel and promotes the peeling of the insulating coating. Therefore, the upper limit is set to 0.10%. It is preferably 0.08% or less, and more preferably 0.06% or less.

B: 0 to 0.0005%
Like C, B also has the effect of suppressing surface nitriding during finish annealing and improving iron loss, and can be added. B is not an essential element in the present invention, but in order to enjoy this effect, addition of 0.0001% or more is preferable. On the other hand, excessive addition deteriorates iron loss, so the upper limit is set to 0.0005%. The cause of the deterioration of iron loss due to excessive addition is not clear, but it is presumed that a fine compound containing B is formed. A more preferable range when added is 0.0001 to 0.0003%, and even more preferably 0.0001 to 0.0002%.

[N] _{s : 0.0040% by mass or less As found in Experiment 3, the larger the average amount of nitrogen [N] s} contained in the portion of the steel sheet at a depth of up to 20 μm from the outermost surface, the more iron. Loss W15 / 50 deteriorates. From FIGS. 4 and 5, if [N] _s is 0.0040% or less, there is no deterioration of iron loss. Therefore, in the present invention, [N] _s is set to 0.0040% or less. Preferably, it is 0.0030% or less.

Here, [N] _s is the following when the nitrogen content of the entire steel sheet is [N] ₁ , the nitrogen content of the sample after removing 20 μm on both surfaces is [N] ₂ , and the thickness of the steel sheet is t. Obtained by the formula. At that time, the nitrogen content is measured in accordance with JIS G 1228 “Iron and Steel-Nitrogen Quantification Method”. Also, the thickness of the insulating coating and the amount of nitrogen contained therein are not included.

[N] _s = (t × [N] ₁ − (t-40) × [N] ₂ ) / 40 ... Equation (2)

<Other elements>
In order to fix S by forming coarse sulfate or sulfide and suppress the formation of fine sulfide, REM may be added in the range of 0.03% or less. REM is a general term for a total of 17 elements, which is obtained by adding Sc with an atomic number of 21 and Y with an atomic number of 39 to 15 elements from La with an atomic number of 57 to Lu with an atomic number of 71. Since Ca has the same effect, it may be contained in the range of 0.005% or less.

Other harmful impurity elements are preferably reduced as much as possible, and Ti, Nb, and V are particularly preferably 0.005% or less.

The rest are unavoidable impurities and Fe.

Next, a manufacturing method for realizing the present invention will be described.

The non-directional electromagnetic steel plate of the present invention is obtained by hot-rolling a slab having a component composition in a specified range to obtain a hot-rolled plate, and then subjecting the hot-rolled plate to hot-rolling and annealing to obtain a hot-rolled annealed plate. It can be manufactured by cold-rolling once or twice or more with intermediate annealing in between to obtain a cold-rolled plate, and then performing finish annealing on the cold-rolled plate. The hot-rolled plate annealing may be replaced by self-annealing with the heat of the coil wound after the hot-rolling.

When hot-rolled sheet is annealed, hot rolling is performed by setting the slab heating temperature in the range of 1100 to 1200 ° C., the finishing temperature in the range of 800 to 950 ° C., and the winding temperature in the range of 550 to 650 ° C., so that the magnetic characteristics of the final product are good. Become. When self-annealing is performed, the finishing temperature is preferably 900 to 1000 ° C., and the winding temperature is preferably 750 to 850 ° C. The plate thickness after hot rolling is such that the cold rolling reduction ratio is 75% or more and 90% or less according to the plate thickness of the final product. Within this range, a high magnetic flux density can be obtained.

The average ferrite crystal grain size of the hot-rolled annealed plate is preferably 60 μm or more because the larger the average ferrite crystal grain size, the better the texture after cold-rolled recrystallization and the higher the magnetic flux density. However, if it is too large, the toughness of the steel will decrease, so 200 μm or less is preferable. More preferably, it is 80 to 120 μm.

In the final annealing after cold rolling (finish annealing), there are optimum conditions for obtaining the optimum average ferrite crystal grain size in the final product plate and for suppressing nitriding and oxidation of the surface of the steel sheet.

The average ferrite grain size of the product plate is preferably 80 to 250 μm in order to obtain a low iron loss. More preferably, it is 100 to 180 μm. Therefore, it is desirable that the temperature at the time of soaking in the final annealing is 950 ° C. or higher.

On the other hand, nitriding of the surface of the steel sheet can be prevented by regulating the components contained in the steel, and the iron loss of the product can be made good. A low iron loss can be stably obtained. For that purpose, the temperature of the steel sheet at the time of soaking is preferably 1050 ° C. or lower, more preferably 1025 ° C. or lower.

Further, the atmosphere gas is a mixed gas of nitrogen and hydrogen, and the smaller the volume ratio of nitrogen, the more the surface nitriding is suppressed. The nitrogen ratio in the mixed gas is preferably 90% or less, more preferably 80% or less, further 70% or less, and further preferably 60% or less. However, if the ratio of nitrogen is lowered, the ratio of hydrogen must be increased inevitably, and the equipment specifications and operating conditions for ensuring safety become strict. In order to ensure safety within a realistic cost range, the nitrogen ratio is preferably 50% or more.

Further, if the atmospheric dew point at the time of soaking is too low, the surface of the steel sheet tends to be nitrided, so that the temperature is preferably −30 ° C. or higher, further −20 ° C. or higher, further −15 ° C. or higher, further −10 ° C. or higher. On the other hand, if the atmospheric dew point is too high, the surface of the steel sheet is likely to be oxidized, and the magnetic flux density is lowered due to the oxidation of the surface of the steel sheet. Therefore, the atmospheric dew point at the time of soaking must be 0 ° C. or lower.

Here, it is considered that the reason why nitriding is easily performed when the plate temperature at the time of soaking is high is that nitrogen molecules in the atmosphere are decomposed into atomic nitrogen and easily diffused in the steel sheet. Further, it is considered that the reason why nitriding is easily performed when the ratio of nitrogen in the mixed gas is high is that there is a condition that the nitrogen concentration is the rate-determining factor for steel sheet nitriding. On the other hand, the reason why nitriding is more likely to occur when the atmospheric dew point is low is presumed to be related to the oxide formation on the surface, but it is unknown.

After finish annealing, if necessary, an insulating film can be formed on the surface to obtain the non-oriented electrical steel sheet of the present invention.

<Example 1>
The steel having the composition shown in Table 4 was melted in vacuum, and the obtained ingot was roughly rolled at a heating temperature of 1150 ° C. to obtain a coarse bar having a thickness of 40 mm. A hot-rolled plate was obtained by performing finish rolling at a winding temperature of 400 ° C. and a finish thickness of 2.0 mm. The obtained hot-rolled plate was annealed with a hot-rolled plate having a soaking temperature of 870 ° C. and a soaking time of 60 seconds, and then subjected to cold rolling to obtain a 0.5 mm cold-rolled plate. The cold-rolled plates were finish-annealed using a continuous annealing furnace that imitated the actual production line. The tension applied to the steel sheet was 0.3 kgf / mm ² , the atmosphere in the furnace was 70% nitrogen, 30% hydrogen, and the dew point was −30 ° C., the soaking temperature was 1025 ° C., and the soaking time was 30 seconds.

Using the nitrogen content [N] 1 of the obtained steel sheet, the nitrogen content [N] 2 after removing both surfaces of the steel sheet by 20 μm, and the steel sheet thickness t (= 0.5 mm) before removal. , The amount of nitrogen [N] s on the surface was determined from the formula (2). The [N] s of each sample is shown in Table 4. In addition, the cross-sectional structure was observed to determine the ferrite crystal grain size, which is also shown in Table 4. Further, a sample for magnetic measurement having a size of 55 mm × 55 mm was cut out, and magnetic measurement was performed with a single plate magnetic measuring device. At that time, the values in Table 4 calculated from the component composition were used for the density of each sample. Table 4 shows the iron loss W15 / 50 and the magnetic flux density B50. Good characteristics were obtained when W15 / 50 was 2.4 W / kg or less. According to the present invention, good iron loss can be obtained.

<Example 2>
The hot-rolled annealed plates used in the tests of Table 4 and D12 of Example 1 were subjected to cold-rolling to obtain cold-rolled plates having plate thicknesses of 0.3 mm, 0.35 mm, and 0.5 mm. The obtained cold-rolled sheet was subjected to finish annealing under various conditions shown in Table 5 to obtain a non-oriented electrical steel sheet. [N] s and iron loss W15 / 50 of the obtained steel sheet were obtained by the same method as in Example 1, and are shown in Table 5. The higher the soaking temperature, the larger the amount of surface nitrogen, and the more the iron loss tended to deteriorate. Further, in the range where the atmospheric dew point is −15 ° C. or higher and 5 ° C. or lower, very good characteristics of W15 / 50 of 2.3 W / kg or lower can be obtained. However, at the dew point of 5 ° C., the magnetic flux density decreased. Further, the lower the volume ratio of nitrogen in the mixed gas, the smaller the amount of surface nitrogen and the better the iron loss tended to be.

According to the present invention, it is possible to reduce the iron loss of non-oriented electrical steel sheets, and it is particularly suitable for the iron core of equipment such as large rotating equipment, which requires low iron loss.

Claims (2)

By mass%
C: 0.0030% or less,
Si: 2.7-3.3%,
Al: 0.2-0.5%,
Mn: 0.5-2.0%,
S: 0.0040% or less, and N: 0.0040% or less,
Contains,
Further, an amount of Sn that satisfies the formula (1) is contained, and
The rest consists of Fe and impurities
_{Non-directional characteristic that the amount of nitrogen [N] s} contained in the portion having a depth of up to 20 μm from the outermost surface of the base steel portion of the steel sheet obtained by the formula (2) is 0.0040% by mass or less. Electromagnetic steel plate.
0.04-(0.01 / [Mn]) ≤ [Sn] ≤ 0.1 ... Equation (1)
[N] _s = (t × [N] ₁ − (t-40) × [N] ₂ ) / 40 ・ ・ ・ Equation (2)
Here, [Sn] and [Mn] are the contents of Sn and Mn (mass%), respectively, t is the thickness of the steel sheet excluding the insulating coating (μm), and [N] ₁ is the entire steel sheet excluding the insulating coating. Nitrogen content (mass%) and [N] ₂ are the nitrogen content (mass%) contained in the steel sheet after removing 20 μm (excluding the insulating coating) from both surfaces. Instead of a part of the Fe
B: 0 to 0.0005%,
REM: 0 to 0.03%, and Ca: 0 to 0.005%
The non-oriented electrical steel sheet according to claim 1, which contains at least one of.

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