JP2018165383A - Nonoriented electromagnetic steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a non-oriented electrical steel sheet suitable for an application in which a very low iron loss is required. SOLUTION: In non-oriented electrical steel sheets, the Al content is limited in order to reduce hysteresis loss, and therefore Mn is contained in a large amount for the purpose of compensating for the reduced intrinsic resistance, and surface nitriding manifested by Mn is suppressed. Therefore, a small amount of Sn is added according to the Mn content, and the nitrogen content [N] s contained in the portion of the steel sheet having a depth of up to 20 μm from the outermost surface is 0.0040% by mass or less. And. [Selection diagram] Fig. 4

Description

  The present invention relates to a non-oriented electrical steel sheet having a small iron loss. The non-oriented electrical steel sheet obtained by the present invention is suitable for applications that require extremely low iron loss, such as iron cores for large electric machine 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 specific resistance, and reduction in sheet thickness. The most important thing in realizing them is the alloy composition design. For improving iron loss by adjusting the components, many techniques have been proposed, including not only increasing the specific resistance but also detoxifying precipitates and improving the texture.

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

  In Patent Document 2, when S contained in steel is very low, a nitride layer is formed on the surface of the steel sheet and the iron loss is deteriorated. In order to suppress this, a technique of adding Sn or Sb is proposed.

  Patent Document 3 proposes a technique that renders fine inclusions harmless by adding REM.

JP-A-55-158252 JP-A-10-317111 JP 2005-336503 A

  Although the iron loss of the non-oriented electrical steel sheet has been steadily reduced, the present invention aims to further reduce this. And it aims at providing the material suitable for the use as which a very low iron loss is calculated | required, such as a large rotator, for example.

  The present inventors diligently studied about the component composition for reducing the iron loss of the non-oriented electrical steel sheet. As a result, it was found that the iron loss can be reduced by the contents of Al, Mn, and Sn. In the present invention, in order to reduce hysteresis loss, the Al content is limited, so that a large amount of Mn is contained for the purpose of compensating for the lowered specific resistance. However, the inclusion of Mn makes it easy to generate a nitride on the steel sheet surface and deteriorate iron loss during the final annealing for cold rolling recrystallization. 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] By 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 plate that contains 0.0040% or less and N: 0.0040% or less, further contains Sn in an amount that satisfies the formula (1), and the balance is Fe and impurities, and is obtained by the formula (2). A non-oriented electrical steel sheet, wherein the nitrogen content [N] _s contained in a portion having a depth of 20 μm from the outermost surface of the base iron portion is 0.0040 mass% or less.

0.04- (0.01 / [Mn]) ≦ [Sn] ≦ 0.1 Formula (1)
[N] _s = (t × [N] ₁ − (t−40) × [N] ₂ ) / 40 Expression (2)

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

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

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

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

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

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

  For magnetic measurement, the sample was sheared to 55 mm × 55 mm, which was the sample size for magnetic measurement, and the sample was subjected to surface pressure to bind and anneal, and a small sample for cross-sectional structure observation 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, an ideal sample can be obtained in evaluating the magnetic characteristics without the influence of nitriding or oxidation and without residual strain.

  Based on JIS C 2556 “Magnetic Steel Sheet Single Sheet Magnetic Characteristic Testing Method”, AC magnetic measurement was performed, and DC magnetic measurement was performed using a Cioffi type magnetometer. In that case, the value in the table was used for the density. Further, based on JIS G 0551 “steel—microscopic test method for crystal grain size”, the average crystal grain size of each sample was determined. Based on these data, the relationship between the average crystal grain size and the iron loss in each alloy can be obtained.

  According to the relationship diagram, the total iron loss W15 / 50 was the smallest when the average grain size was 140 μm in almost any alloy. FIG. 1 shows the total iron loss, hysteresis loss, and eddy current loss when the average crystal grain size is 140 μm, and shows the influence of Mn and Al on them.

  From this figure, it can be seen that the effect of Mn concentration on the hysteresis loss is small and increases as the Al concentration increases. On the other hand, eddy current loss decreases as Mn and Al increase. The reason why the hysteresis loss increases as Al increases is not clear. Moreover, it is considered that the eddy current loss reflects a change in specific resistance. The total iron loss obtained by adding the two has a tendency to decrease as the amount of Mn increases, and is the smallest when the Al content is about 0.2 to 0.5%.

<Experiment 2>
Experiment 1 was an evaluation result of magnetic characteristics when ideal annealing (box annealing) was performed. In Experiment 2, a 0.5 mm cold-rolled sheet produced by the same method as in Experiment 1 was annealed in a continuous annealing furnace that can simulate actual machine 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 850 to 1050 ° C., and the soaking time was 30 seconds. A 55 mm × 55 mm sample for magnetic measurement and a small piece for cross-sectional observation were cut out from the annealed material, and the magnetic properties and crystal structure were evaluated. The evaluation method is the same as in Experiment 1.

  FIG. 2 shows the influence of the Mn content and the Al content on the total iron loss when the average crystal grain size is 140 μm. Compared to the total iron loss in FIG. 1, the iron loss value is larger overall, and the iron loss is deteriorated by increasing Mn to 0.2%, 0.5%, and 0.7%. The amount is getting bigger. For each alloy, the amount of nitrogen in the case of ideal annealing (box annealing) in Experiment 1 and in the case of actual machine simulated annealing (continuous annealing) in Experiment 2 is measured, and the influence of Mn content and Al content on the difference Is shown in FIG.

  The measurement of the nitrogen content was based on JIS G 1228 “Iron and steel-nitrogen determination method”. From this figure, it can be seen that when actual machine simulated annealing (continuous annealing) is performed, nitriding becomes easier as the Mn content increases. Therefore, it can be said that the deterioration amount of the iron loss with Mn is caused by nitriding of the steel sheet surface during annealing.

<Experiment 3>
Next, the effect of addition of Sn, which is an element that easily segregates on the surface and is considered to have little adverse effect on magnetism, was investigated as a method for preventing iron loss deterioration accompanying nitriding. Alloys having the component compositions shown in Table 2 were melted, 0.5 mm cold-rolled plates were 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, the atmosphere is 70% nitrogen, 30% hydrogen, and DRY. FIG. 4 shows changes in iron loss W15 / 50 due to Sn. Iron loss was improved by adding Sn.

Next, the nitrogen content in the surface layer of the annealed sample was examined. The region of the surface layer is in a range of 20 μm on one side, and the nitrogen content in that part is [N] _s . In order to determine the nitrogen content in the surface layer, first the nitrogen content [N] ₁ of the sample after annealing is measured, then both surfaces are removed by 20 μm by chemical polishing, and the nitrogen content of the sample after removal is measured. The quantity [N] ₂ is measured. Using the measured [N] ₁ and [N] ₂ and the plate thickness t = 500 μm, [N] _s is obtained by the following equation.

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

FIG. 5 shows the change of [N] _s obtained by Sn in this way. The surface nitrogen concentration is reduced by the addition of Sn. 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, alloys with varying amounts of Mn and Sn were vacuum-melted, cold-rolled plates were produced by the same method as in Experiment 1, and subjected to continuous annealing as in Experiments 2 and 3. For each of the obtained samples, nitriding [N] _s on the steel sheet surface was obtained in the same manner as in Experiment 3, and the relationship between Mn and Sn is shown in FIG. It was. It can be seen that in order to achieve 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 X is fitted by a mathematical expression, it can be generally expressed as [Sn] = 0.04−0.01 / [Mn].

  Based on these experiments, further studies have been made and the present invention has been achieved. Hereinafter, the definition 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 “% by mass”.

C: 0.0030% or less C forms carbides in steel and deteriorates iron loss, so the content is made 0.0030% or less. On the other hand, 0.0005% or more may be added because it has the effect of suppressing nitriding of the surface during finish annealing and improving iron loss. The preferable range in the case of adding is 0.0005 to 0.0025%, more preferably 0.0005 to 0.0020%, and still more preferably 0.0005 to 0.0015%.

Si: 2.7 to 3.3%
Si is an effective element for increasing the specific 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 it becomes difficult to manufacture. A preferable range is 2.8 to 3.2%, and 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 iron loss reduction. In order to enjoy the effect, the lower limit is made 0.2%. On the other hand, as revealed in Experiment 1, Al has the effect of increasing hysteresis loss, so the upper limit is set to 0.5% in the present invention. A preferable range is 0.2 to 0.4%, more preferably 0.2 to 0.3%.

Mn: 0.5 to 2.0%
In the present invention, since the upper limit of the Al content is limited as described above, the specific resistance tends to be insufficient. Since Mn also has the effect of increasing the specific resistance, in the present invention, 0.5% or more of Mn is added in order to increase the insufficient specific resistance. However, excessive addition causes embrittlement of the steel, so the upper limit is made 2.0%. A preferable range is 0.5 to 1.5%, more preferably 0.5 to 1.3%, and still more preferably 0.5 to 1.0%.

S: 0.0040% or less Since S forms precipitates and deteriorates iron loss, it is made 0.0040% or less. Preferably it is 0.002% or less, More preferably, it is 0.001% or less.

N: 0.0040% or less N forms precipitates and deteriorates iron loss, so the content is made 0.0040% or less. Preferably it is 0.002% or less, More preferably, it is 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 reduced. Will be improved. The necessary Sn addition amount at that time increases depending on the Mn addition amount. As shown in FIG. 6, the necessary addition amount of Sn is 0.04- (0.01 / [Mn) when the Sn and Mn contents (mass%) are [Sn] and [Mn], respectively. ]) ≦ [Sn]. On the other hand, an excessive content of Sn deteriorates the toughness of the steel or promotes the peeling of the insulating coating. Therefore, the upper limit is made 0.10%. Preferably it is 0.08% or less, More preferably, it is 0.06% or less.

B: 0 to 0.0005%
B, like C, can be added because it has the effect of suppressing surface nitriding during finish annealing and improving iron loss. 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 degrades iron loss, so the upper limit is made 0.0005%. The reason why the iron loss is deteriorated by excessive addition is not clear, but it is presumed that a fine compound containing B is formed. The more preferable range in the case of adding is 0.0001 to 0.0003%, and more preferably 0.0001 to 0.0002%.

[N] _s : 0.0040% by mass or less As understood from Experiment 3, as the average amount of nitrogen [N] _s contained in a portion having a depth of 20 μm from the outermost surface of the steel plate portion of the steel plate increases as iron increases The loss W15 / 50 deteriorates. From FIG. 4 and FIG. 5, when [N] _s is 0.0040% or less, there is no deterioration of iron loss. Therefore, in the present invention, [N] _s is made 0.0040% or less. Preferably, it is 0.0030% or less.

Here, [N] _s is the following when the nitrogen content of the whole steel plate is [N] ₁ , the nitrogen content [N] ₂ of the sample after removing both surfaces by 20 μm, and the plate thickness of the steel plate is t. Obtained by the formula. At that time, the nitrogen content is measured in accordance with JIS G 1228 “Iron and steel-nitrogen determination method”. Also, the thickness of the insulating coating and the amount of nitrogen it contains 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 sulfates and sulfides and suppress the formation of fine sulfides, REM may be added in a range of 0.03% or less. REM is a generic name for a total of 17 elements including 15 elements from La with atomic number 57 to Lu with 71 and Sc with atomic number 21 and Y with atomic number 39. Since Ca has the same effect, it may be contained in a 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 balance is inevitable impurities and Fe.

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

  The non-oriented electrical steel sheet of the present invention is a hot-rolled sheet obtained by hot rolling a slab having a component composition within a specified range, and hot-rolled annealed sheet by subjecting the hot-rolled sheet to hot-rolled annealed sheet. It is possible to manufacture by cold-rolling a sheet by subjecting it to cold rolling two or more times with intermediate annealing or intermediate annealing, and subjecting the cold-rolled sheet to finish annealing. You may replace hot-rolled sheet annealing with self-annealing with the heat of the coil wound up after hot rolling.

  When hot-rolled sheet annealing is performed, the hot rolling has a slab heating temperature of 1100 to 1200 ° C, a finishing temperature of 800 to 950 ° C, and a winding temperature of 550 to 650 ° C, so that the magnetic properties 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 thickness after hot rolling is set so that the cold rolling reduction ratio is 75% or more and 90% or less according to the thickness of the final product. By setting 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 texture after cold rolling recrystallization is improved and the magnetic flux density is increased. However, if it is too large, the toughness of the steel decreases, so 200 μm or less is preferable. 80-120 micrometers is more preferable.

  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 steel plate surface.

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

  On the other hand, nitriding of the steel sheet surface can be prevented by regulating the components contained in the steel, and the iron loss of the product can be made good, but by producing it under optimum conditions, nitriding is further suppressed, Low iron loss can be obtained stably. For this purpose, the temperature of the steel sheet during soaking is preferably 1050 ° C. or lower, more preferably 1025 ° C. or lower.

  The atmosphere gas is a mixed gas of nitrogen and hydrogen, and surface nitridation is suppressed as the volume ratio of nitrogen is smaller. 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, and the equipment specifications and operating conditions for ensuring safety become severe. In order to ensure safety within a practical cost range, the proportion of nitrogen is preferably 50% or more.

  Further, if the atmospheric dew point at the time of soaking is too low, the steel sheet surface is likely to be nitrided. 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 during soaking needs to be 0 ° C. or less.

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

  After the 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 component composition shown in Table 4 was melted in vacuum, and the resulting ingot was subjected to rough rolling at a heating temperature of 1150 ° C. to obtain a rough bar having a thickness of 40 mm. Thereafter, the heating temperature was 1150 ° C., the finishing temperature was 850 ° C., Finish rolling with a winding temperature of 400 ° C. and a finish thickness of 2.0 mm was performed to obtain a hot-rolled sheet. The obtained hot-rolled sheet was subjected to hot-rolled sheet annealing at 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 sheet. These cold-rolled sheets were subjected to finish annealing using a continuous annealing furnace that imitated an actual machine production line. The tension applied to the steel sheet was 0.3 kgf / mm ² , the furnace atmosphere was 70% nitrogen, 30% hydrogen, dew point −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. From the formula (2), the surface nitrogen amount [N] s was determined. Table 4 shows [N] s of each sample. Further, 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 of 55 mm × 55 mm was cut out and subjected to magnetic measurement with a single plate magnetic measuring device. In that case, the density of each sample used the value of Table 4 calculated from a component composition. Table 4 shows the iron loss W15 / 50 and the magnetic flux density B50. W15 / 50 was 2.4 W / kg or less as good characteristics. According to the present invention, a good iron loss could 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 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 determined in the same manner as in Example 1, and are shown in Table 5. As the soaking temperature was higher, the amount of surface nitrogen increased, and the iron loss tended to deteriorate. Moreover, when the atmospheric dew point is in the range of −15 ° C. or more and 5 ° C. or less, very good characteristics with W15 / 50 of 2.3 W / kg or less can be obtained. However, at a 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 surface nitrogen amount and the better the iron loss.

  According to the present invention, it is possible to reduce the iron loss of a non-oriented electrical steel sheet, and it is particularly suitable for an iron core of a device that requires a low iron loss, such as a large rotating device.

Claims (2)

% By 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: 0.0040% or less, and N: 0.0040% or less,
Containing
Furthermore, it contains Sn in an amount satisfying the formula (1),
The balance consists of Fe and impurities,
Non-directionality characterized in that the nitrogen content [N] _s contained in a portion having a depth from the outermost surface of the steel plate portion of the steel sheet determined by the formula (2) to 20 μm is 0.0040 mass% or less. Electrical steel sheet.
0.04- (0.01 / [Mn]) ≦ [Sn] ≦ 0.1 Formula (1)
[N] _s = (t × [N] ₁ − (t−40) × [N] ₂ ) / 40 Expression (2)
Here, [Sn] and [Mn] are the contents (mass%) of Sn and Mn, t is the plate thickness (μm) of the steel plate excluding the insulating coating, and [N] ₁ is the whole steel plate excluding the insulating coating. The nitrogen content (% by mass) of [N] ₂ is the nitrogen content (% by mass) contained in the steel plate after removing both surfaces by 20 μm (excluding the insulating coating). Instead of a part of the Fe,
B: 0 to 0.0005%,
REM: 0-0.03%, and Ca: 0-0.005%
The non-oriented electrical steel sheet according to claim 1, comprising at least one of the following.

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