EP4663796A1

EP4663796A1 - Non-oriented electromagnetic steel sheet, method for producing same, and rotary electric machine containing same

Info

Publication number: EP4663796A1
Application number: EP24753385.4A
Authority: EP
Inventors: Tesshu Murakawa
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-02-09
Filing date: 2024-02-07
Publication date: 2025-12-17
Also published as: MX2025009263A; TWI893648B; JPWO2024166942A1; TW202432856A; JP7644404B2; CN120659900A; KR20250133704A; WO2024166942A9; WO2024166942A1

Abstract

Novel non-oriented electrical steel sheet excellent in magnetic aging resistance and resistant to stretcher strain and a method of production of the same and rotating electrical machine containing the same, that is, non-oriented electrical steel sheet having a chemical composition comprised of, by mass%, C: 0.0100% or less, Si: 2.6% or more and 4.5% or less, Mn: 0.10% or more and 3.00% or less, P: 0.15% or less, S: 0.0040% or less, N: 0.0040% or less, Al: 0.10% or more and 2.00% or less, one or more elements selected from Sn and Sb: 0 to 0.200%, Cr: 0.001 to 5.000%, and predetermined amounts of optional added elements (Ni, Cu, Ca, Mg, REM, Ti, B, O) and balance: Fe and impurities, wherein a difference: AI. in lower yield point before and after aging when imparting 8% strain then aging at 100°C for 1 hour is less than 40 MPa, a method of production of the same, and a rotating electrical machine containing the same.

Description

FIELD

The present invention relates to non-oriented electrical steel sheet, a method of production of the same, and a rotating electrical machine containing the same.

BACKGROUND

Recently, issues with the global environment have become the focus of attention. Demands for tackling energy savings have been becoming increasingly loud. In the midst of all of this, higher efficiency of electrical equipment has been strongly called for in recent years. Due to this, there have been increasingly strong calls for improved magnetic properties in the non-oriented electrical steel sheet widely used as core materials of motors etc. In recent years, this trend has become particularly widespread in motors for electric vehicles and hybrid vehicles in which advances have been made in achieving higher efficiency of motors.
In motors for driving automobiles etc., the temperature sometimes rises during use. If magnetic aging occurs in this temperature region, only naturally the magnetic properties will deteriorate along with time starting from the time of delivery of the material.
Further, in motors for driving automobiles etc., sometimes stretcher strain occurs when crimping and bending the core in a helical shape. Due to this, the dimensional precision of the product falls and as a result sometimes magnetic properties as desired cannot be obtained.
PTL 1 tries to provide core material for rotary machines excellent in bendability and iron loss properties. Specifically, it describes adding Ti in 0.01 to 0.1% to ultralow C steel whereby a material in which the C is completely immobilized, the aging resistance is improved, and bendability is excellent is obtained.
PTL 2 tries to provide, at a low cost, non-oriented electrical steel sheet excellent in strength and magnetic properties suitable as a core material of motors for electrical vehicles and specifically proposes stamping out a rotor and stator from non-oriented electrical steel sheet with a specific chemical composition and annealing only the stator to remove strain and thereby achieve both high strength for rotor use and low iron loss for stator use.

[CITATION LIST]

[PATENT LITERATURE]

[PTL 1] Japanese Patent No. 4325235
[PTL 2] Japanese Patent No. 5228379

SUMMARY

[TECHNICAL PROBLEM]

Up to now as well, non-oriented electrical steel sheet excellent in magnetic properties, workability, and strength has been studied from various angles. However, PTL 1 requires addition of Ti, but Ti acts disadvantageously on magnetic properties. Further, PTL 2 stamps out a rotor and stator from the same steel sheet and anneals only the stator to remove strain, but if it were possible to obtain the rotor and stator from different steel sheets and combine them or to suitably select whether to anneal the rotor and stator to remove strain, there would be greater freedom of design in the products.
An object of the present invention is, in consideration of this situation, to provide a novel non-oriented electrical steel sheet excellent in resistance to magnetic aging and resistant to formation of stretcher strain and a method of production of the same and a rotating electrical machine containing the same.

[SOLUTION TO PROBLEM]

According to the present invention, the following aspects are provided:

[1] Non-oriented electrical steel sheet having a chemical composition comprised of, by mass%, C: 0.0100% or less, Si: 2.6% or more and 4.5% or less, Mn: 0.10% or more and 3.00% or less, P: 0.15% or less, S: 0.0040% or less, N: 0.0040% or less, Al: 0.10% or more and 2.00% or less, one or more elements selected from Sn and Sb: 0 to 0.200%, Cr: 0.001 to 5.000%, Ni: 0 to 5.000%, Cu: 0 to 5.000%, Ca: 0 to 0.020%, Mg: 0 to 0.0200% and rare earth elements (REM): 0 to 0.020%, Ti: less than 0.0100%, B: 0% to 0.0050%, O: 0% to 0.0200%, and balance: Fe and impurities, wherein a difference: A.I. in lower yield point before and after aging when imparting 8% strain then aging at 100°C for 1 hour is less than 40 MPa.
[2] The non-oriented electrical steel sheet according to [1], wherein the chemical composition further includes Mo: 0% to 0.0200%.
[3] The non-oriented electrical steel sheet according to [1] or [2], wherein in the following formula, Cev≤0.0055% is satisfied. Cev = 12 × ([C(mass%)]/12 + [N(mass%)]/14 - [B(mass%)]/11 - [Ti(mass%)]/48 - [Cr(mass%)]/104 - [Mo(mass%)]/192) where, [element symbol (mass%)] indicates the content (mass%) of that element in the chemical composition, 0 being entered when that element is not included.
[4] A method of production of non-oriented electrical steel sheet according to any one of [1] to [3], comprising producing non-oriented electrical steel sheet having a chemical composition comprised of, by mass%, C: 0.0100% or less, Si: 2.6% or more and 4.5% or less, Mn: 0.10% or more and 3.00% or less, P: 0.15% or less, S: 0.0040% or less, N: 0.0040% or less, Al: 0.10% or more and 2.00% or less, one or more elements selected from Sn and Sb: 0 to 0.200%, Cr: 0.001 to 5.000%, Ni: 0 to 5.000%, Cu: 0 to 5.000%, Ca: 0 to 0.020%, Mg: 0 to 0.0200% and rare earth elements (REM): 0 to 0.020%, Ti: less than 0.0100%, B: 0% to 0.0050%, O: 0% to 0.0200%, and balance: Fe and impurities by steps including steelmaking, hot rolling, annealing the hot rolled sheet, pickling, cold rolling, finish annealing, and overaging, during which making the overaging temperature of the overaging 50°C to 500°C and holding in the 50°C or more temperature region for 10 seconds or more.
[5] The method of production of non-oriented electrical steel sheet according to [4], wherein the chemical composition further includes Mo: 0% to 0.0200%.
[6] The method of production of non-oriented electrical steel sheet according to [4] or [5] further comprising, at the overaging, pressing the steel sheet against a less than diameter 500 mm roll to bend it in at least part of the step of holding at the 50°C to 500°C temperature region.
[7] The method of production of non-oriented electrical steel sheet according to any one of [4] to [6] further comprising, after the overaging, pressing the steel sheet against a less than diameter 500 mm roll to bend it.
[8] A method of production of non-oriented electrical steel sheet according to any one of [1] to [3], comprising producing non-oriented electrical steel sheet having a chemical composition comprised of, by mass%, C: 0.0100% or less, Si: 2.6% or more and 4.5% or less, Mn: 0.10% or more and 3.00% or less, P: 0.15% or less, S: 0.0040% or less, N: 0.0040% or less, Al: 0.10% or more and 2.00% or less, one or more elements selected from Sn and Sb: 0 to 0.200%, Cr: 0.001 to 5.000%, Ni: 0 to 5.000%, Cu: 0 to 5.000%, Ca: 0 to 0.020%, Mg: 0 to 0.0200% and rare earth elements (REM): 0 to 0.020%, Ti: less than 0.0100%, B: 0% to 0.0050%, O: 0% to 0.0200%, and balance: Fe and impurities by steps including steelmaking, hot rolling, annealing the hot rolled sheet, pickling, cold rolling, finish annealing, and bending, during which, in the bending, pressing the steel sheet against a less than diameter 500 mm roll to bend it.
[9] The method of production of non-oriented electrical steel sheet according to [8], wherein the chemical composition further comprises Mo: 0% to 0.0200%.
[10] A rotating electrical machine comprising a stator, rotor, and a housing holding the stator and rotor, in which a core material of the stator or a core material of the rotor is comprised of a non-oriented electrical steel sheet according to any one of [1] to [3].

[ADVANTAGEOUS EFFECTS OF INVENTION]

The non-oriented electrical steel sheet provided according to the present invention is excellent in magnetic aging resistance and resistant to stretcher strain. Further, the non-oriented electrical steel sheet can be obtained by the method of production provided by the present invention. Furthermore, the rotating electrical machine provided by the present invention is excellent in magnetic aging resistance and is resistant to stretcher strain at the time of working.
Here, "excellent in magnetic aging resistance" typically indicates an iron loss deterioration rate of less than 0.9. The "iron loss deterioration rate" is the value of the increase in iron loss before and after aging at 200°C for 24 hours divided by the iron loss before aging.
"Resistant to stretcher strain" will be explained in detail later, but corresponds to an A.I. of less than 40 MPa.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view for explaining the method for finding an A.I.

DESCRIPTION OF EMBODIMENTS

The present invention will be explained for each constituent requirement.
The reasons for limiting the chemical composition of the steel are as follows: Note that in the following explanation, unless otherwise indicated, the contents of the elements will be deemed to be expressed by mass% with the word "mass" omitted.

<C: 0.0100% or Less>

C has the action of raising the iron loss and causing magnetic aging. Therefore, C is made 0.0100 mass% or less. Preferably it is 0.001 to 0.004 mass%.

<Si: 2.6% or More and 4.5% or Less>

Si has the action of increasing the specific resistance of steel or reducing the iron loss. To obtain this action, 2.6% or more is necessary. On the other hand, if Si is more than 4.5%, the steel becomes brittle and the rollability falls. Therefore, Si is 2.6 to 4.5%. Preferably it is 3.0 to 3.5 mass%.

<Mn: 0.10% or More and 3.00% or Less>

Mn has the action of raising the specific resistance of steel or making the sulfides coarser and rendering them harmless. To obtain this action, 0.10% or more is necessary. On the other hand, if Mn is more than 3.00 mass%, a drop in the magnetic flux density and a rise in cost are invited and cracking easily occurs at the time of cold rolling. Therefore, Mn is made 0.10 to 3.00%. Preferably it is 0.1 to 0.5 mass%.

<P: 0.15% or Less>

P is an element necessary for increasing the strength of steel sheet or for improving the stampability, but if more than 0.15% is added, the steel sheet becomes brittle, therefore the content is made 0.15% or less. Preferably, it is 0.01 to 0.10%.

<S: 0.0040% or Less>

S is limited to 0.0040% or less. S precipitates as sulfides in steel and causes the crystal grain growth and iron loss to deteriorate. If more than 0.0040%, the deterioration in crystal grain growth and deterioration in iron loss become remarkable, so the content is limited to 0.004% or less. The lower limit is not particularly prescribed, but with the usual method of production, making it 0.0005% or less is difficult. Preferably it is 0.0010 to 0.0030%.

<N: 0.0040% or Less>

N, if contained in a large amount, forms nitrides and causes the magnetic properties to deteriorate, so the upper limit has to be made 0.0040%. The lower limit is not particularly prescribed, but if considering current steelmaking technology, de facto 0.0001% becomes the lower limit. Preferably, it is 0.0003 to 0.0020%.

<Al: 0.10% or More and 2.00% or Less>

Al is effective as a deoxidizer and further can make the nitrides coarser to render them harmless. Further, like Si, it increases the specific resistance of steel and reduces the iron loss. To obtain this action, 0.10% or more is necessary. However, if more than 2.00%, the steel becomes brittle and the rollability falls. Therefore, Al is 0.10 to 2.00%. Preferably it is 0.20 to 1.50%.

Sn and Sb are effective for improving texture and suppressing nitriding and oxidation. Further, at 0.005% or more, Sn and Sb suppress the action of C whereby the aging resistance is improved. If too great, the effect becomes saturated and further embrittlement of the steel, suppression of crystal grain growth, and other detrimental effects arise, therefore the total is made 0.200% or less. Preferably it is 0.030 to 0.150%.

<Cr: 0.001% or More and 5.000% or Less>

Cr has the ability to form carbides and nitrides. It immobilizes solid solution C and solid solution N and has the effect of suppressing stretcher strain. For this reason, the lower limit is 0.001% or more. On the other hand, Cr lowers the saturated magnetic flux density of steel sheet. If more than 5.000%, the issue of the cost of addition becomes greater, therefore the content is limited to 5.000% or less. Preferably it is 0.100 to 4.000%.

<Ni: 0% or More and 5.000% or Less>

Ni is an effective element able to raise the strength of steel sheet without causing embrittlement too much. However, it is expensive, so is added in accordance with the required strength. If added, the upper limit, considering cost, is made 5.000%. Preferably it is 1.000 to 4.000%.

<Cu: 0% or More and 5.000% or Less>

Cu increases the hardness of steel sheet. If more than 5.000%, the issue of the cost of addition becomes greater, therefore the content is limited to 5.0% or less. Preferably it is 0.100 to 4.000%.

<Ca: 0% or More and 0.020% or Less>

Ca is intentionally added from the viewpoint of improving grain growth. However, it is expensive, so is added in accordance with need. If added, the upper limit, considering cost, is made 0.020% or less. Preferably it is 0.008% or less.

<Mg: 0% or More and 0.0200% or Less>

Mg is intentionally added from the viewpoint of improving grain growth. However, it is expensive, so is added in accordance with need. If added, the upper limit, considering cost, is made 0.0200% or less. Preferably it is 0.0080% or less.

Rare earth elements are added for the purpose of improving grain growth by suppressing precipitation of impurities. However, they are expensive, so are added in accordance with need. If added, the upper limit, considering cost, is made 0.020%. Preferably it is 0.001 to 0.008%.

<Ti: Less Than 0.0100%>

Ti has the ability to form carbides and nitrides. It immobilizes solid solution C and solid solution N and has the effect of suppressing stretcher strain. However, Ti forms fine nitrides or carbides. It causes remarkable deterioration of the crystal grain growth in stress relief annealing and causes deterioration of the magnetic properties, therefore is limited to less than 0.0100%. Preferably, it is 0.0030% or less. Ti may also be 0%, but unavoidably enters. The content may be 0.0020% or less.

<B: 0% or More and 0.0050% or Less>

If adding Al, this precipitates as fine AlN and causes deterioration of the magnetic properties, therefore simultaneously, B is added. If more than 0.0050%, the magnetic properties are liable to deteriorate due to the excess B, therefore the upper limit is made 0.0050%. Preferably the content is 0.0030 to 0.0040%.

<O: 0% or More and 0.0200% or Less>

O forms precipitates to obstruct grain growth during annealing and cause deterioration of the magnetic properties, so the content is made 0.0200% or less. Preferably it is 0.0030% or less. O may also be 0%, but sometimes is unavoidably contained and may also be 0.0020% or less.

<Mo: 0% or More and 0.0200% or Less>

Mo has the ability to form carbides and nitrides. It immobilizes solid solution C and solid solution N and has the effect of suppressing stretcher strain. Therefore, Mo may be 0% or more and may be 0.0030% or more. However, Mo forms precipitates to obstruct grain growth during annealing and cause deterioration of the magnetic properties, so the content is made 0.0200% or less.
The chemical composition of the non-oriented electrical steel sheet according to the present embodiment may contain, in addition to the above elements, Nb, V, Zr, Ce, Bi, and W in 0.10% or less each to an extent not affecting the properties of the non-oriented electrical steel sheet.

<Balance: Fe and Impurities>

The balance is comprised of Fe and impurities. The "impurities" mean elements entering from the raw materials such as ore and scrap and from the production environment etc. when industrially producing a slab or steel. Further, this does not exclude addition of known elements replacing Fe anticipating known effects to an extent not causing loss of the effects of the present invention.

<Cev>

Stretcher strain is due to solid solution C and solid solution N. The Cev defined by the following formula is an indicator relating to the solid solution C and solid solution N. If too large, it is believed stretcher strain would easily occur. For this reason, Cev is preferably ≤0.0055%. Cev may also be 0.0054% or less and preferably is 0.0025 or less, more preferably is 0.0015 or less, still more preferably is 0.0000 or less. The lower limit value of Cev is not prescribed, but may be -0.5861 or more. Cev = 12 × ([C(mass%)]/12 + [N(mass%)]/14 - [B(mass%)]/11 - [Ti(mass%)]/48 - [Cr(mass%)]/104 - [Mo(mass%)]/192)
In the above formula, [element symbol (mass%)] indicates the content (mass%) of that element in the chemical composition of the non-oriented electrical steel sheet. For example, [C(mass%)] indicates the content (mass%) of C in the chemical composition of non-oriented electrical steel sheet. Further, 0 is entered if the chemical composition of the non-oriented electrical steel sheet does not contain that element.

<A.I. Less Than 40 MPa>

The non-oriented electrical steel sheet according to the present embodiment has an A.I. of less than 40 MPa.
Here, "A.I." is an abbreviation for "Aging Index" and is also called the "Ageing Index". The A.I. is found by obtaining a tension sample of JIS No. 13B from the steel sheet, imparting 8% strain, then aging at 100°C for 1 hour and finding the difference of the stress at 8% strain before aging and the lower yield point after aging. The strain is based on the original gauge length and expresses the elongation from there as a percentage. The test speed at that time is, by stroke control, 7.2 mm/min (strain speed at original gauge length of 60 mm corresponds to 0.002/s). FIG. 1 is a view for generally explaining the method of finding the A.I. First, a sample is subjected to a tensile test. 8% strain is imparted at room temperature (23°C). Further, the stress when imparting 8% strain (that is, the stress at 8% strain before aging) is recorded. After that, the sample after the tensile test is aged at 100°C for 1 hour, cooled down to room temperature (23°C), then again subjected to a tensile test to make the sample break. The lower yield point after the aging is found from the stress-strain curve in the tensile test and the difference from the stress at 8% strain before aging is found. At that time, if a clear yield phenomenon and work hardening are observed by the stress-strain curve, the smallest value of the stress after the yield phenomenon and before the work hardening is made the lower yield point. If work hardening was not clear such as in FIG. 1, the location with the lowest stress between location of the yield phenomenon to the location of 2% lower elongation from the elongation at the time of fracture is made the lower yield point. The rest of the matters relating to measurement are based on JIS Z 2241(2011). If the A.I. is 40 MPa or more, at the time of forming the core, stretcher strain occurs and the dimensional precision deteriorates. Further, magnetic aging occurs and the magnetic properties deteriorate over time. In general the smaller the A.I., the better the effect of suppression of stretcher strain and the aging resistance. For this reason, the A.I. is preferably 35 MPa or less, more preferably 30 MPa or less.

Next, the reasons for limitation of the production conditions in this invention and the suitable production conditions will be explained. In general, the method of production of non-oriented electrical steel sheet includes the steps of steelmaking, slab casting, slab reheating, hot rolling, annealing the hot rolled sheet, pickling, cold rolling, final annealing (finish annealing), aging and/or bending, and application of an insulation coating. The steps may be performed in the order described and the order of the steps may be suitably adjusted.

(Steelmaking)

Converter steelmaking, degasification, and other ordinary steelmaking methods are used to adjust the steel to the above chemical composition.

(Slab Casting)

The slab casting for obtaining steel having the above chemical composition is made the conventional continuous casting. To facilitate slab heating, there is nothing stopping using the blooming method for the continuously cast slab, but the cost rises, so this should be avoided as much as possible.
In the slab casting, in known continuous casting, the initial thickness is not particularly limited but a slab of a range of about 150 mm to 300 mm, preferably a range of 200 mm to 250 mm, is produced.

(Slab Reheating)

The condition of the slab reheating temperature before hot rolling is extremely important in production of non-oriented electrical steel sheet. This is because it is related to the dissolution and precipitation of impurity elements. To prevent fine precipitation of compounds containing impurity elements, the slab reheating temperature is preferably 1250°C or less. Of course, if it were possible to reduce the absolute values of the contents of the main harmful elements of S, N, etc., the slab heating temperature could be raised but this is not practical in industrial production.

(Hot Rolling)

The hot rolling conditions of the slab for the non-oriented electrical steel sheet are in principle low temperature extraction and high temperature rolling, but by performing process annealing, the coiling temperature does not have to be made extremely high. Rather, high temperature rolling is not preferable from the viewpoint of descaling. That is, in the finishing hot rolling, it is sufficient to set that conditions so that the finishing entry temperature is made 900°C to 1000°C, the exit temperature is made 830 to 900°C, and the coiling temperature is made 600°C to 700°C.

(Annealing of Hot Rolled Sheet)

The hot rolled sheet can also be annealed, but even if applying such annealing, while the magnetic properties in the rolling direction can be improved, they are not greatly improved and the cost remarkably rises, therefore whether or not using this is required should be judged based on the desired properties.

(Pickling)

The hot rolled sheet or the hot rolled annealed sheet can be subjected to any treatment for descaling before the cold rolling or finish annealing so as to remove the oxides formed on the steel sheet, that is, the layer of scale. Pickling is the most generally used descaling method. An aqueous solution of one or more inorganic acids is used to chemically wash the steel sheet surface. While there is nothing stopping performing descaling, if the annealing atmosphere is good and the scale layer of the surface layer is sparse, pickling may also be unnecessary.

(Cold Rolling)

The cold rolling is performed in one operation or performed split into two or more operations interposed with annealing. One cold rolling operation means performing one or more cold rolling passes without annealing interposed. In either type of cold rolling, the final rolling reduction is preferably made 70 to 95%.
If performing the cold rolling two times interposed by annealing, the annealing is preferably performed at 750 to 1200°C for 30 seconds to 10 minutes.
If performing the cold rolling split into two or more operations interposed with annealing, the magnetic properties tend to become uniform. The number of the cold rolling operations is suitably selected considering the desired magnetic properties and production costs.
The thickness of the final product is obtained at this stage. Overall, the finished product thickness may be made less than 0.35 mm. The smallest sheet thickness may practically be made 0.20 mm if considering the capacities of the production facilities and the handling ability. Note that if the final product thickness is determined, the thickness of the hot rolled steel strip is automatically determined by the cold rolling rate.

(Final Annealing: Finish Annealing)

The finish annealing is greatly affected by the time and temperature. For shortening the annealing time, a higher temperature is better, but at the present, this is prescribed by the specifications of the continuous casting furnace facilities. 1075°C is the highest temperature. At the low temperature side, if less than 950°C, a somewhat longer time of soaking becomes necessary.
Furthermore, to improve the texture, the patent literature ( Japanese Examined Patent Publication No. 06-051889 ) proposes heating to a 750 to 1150°C temperature by 133°C/s or more. These is nothing stopping applying this art.
If the holding time in the finish annealing is less than 20 seconds, the grain growth will not be sufficient. Even if more than 90 seconds, no further grain growth will occur, therefore the time is made 20 to 90 seconds in range.

(Overaging)

By performing overaging after the finish annealing or during the cooling in the finish annealing, the solid solution C in the steel sheet bonds with the Fe atoms to form cementite, the solid solution C becomes increasingly immobilized, and the effect of suppressing magnetic aging and stretcher strain is improved. The overaging temperature in the overaging is made 50°C or more and 500°C or less. If less than 50°C, the effect sometimes is not sufficiently obtained. If more than 500°C, sometimes the solid solution C cannot be kept immobilized. The upper limit of the overaging is preferably made 300°C or less. The lower limit may be made 100°C or more, preferably is made 200°C or more. Further, the time in the overaging is made 10 seconds or more. If less than 10 seconds, the effect of overaging sometimes becomes unable to be sufficiently obtained. The upper limit of the time in the overaging is not particularly prescribed, but the effect becomes saturated, so it is desirably made 60 minutes or less.

(Bending)

By performing the bending after the finish annealing, the solid solution C and solid solution N in the steel sheet penetrate the dislocations in the Fe atom arrangement, the solid solution C and solid solution N become increasingly immobilized, and the effect of suppressing magnetic aging and stretcher strain is improved. The bending is performed by wrapping the steel sheet around a less than diameter 500 mm roll. That is, the bending is performed by pressing the steel sheet against a less than diameter 500 mm roll. With a diameter 500 mm or more roll, the steel sheet cannot be sufficiently bent and the effect sometimes cannot be sufficiently obtained. The lower limit of the roll diameter is not particularly prescribed, but the smaller the diameter, the larger the bending and greater the liability of the steel sheet breaking etc., therefore the diameter may be made 300 mm or more. Further, based on the tangential direction at the contact point of the roll cross-sectional circle and the steel sheet (sheet surface horizontal direction of steel sheet in flat state) (0 degree), the bending angle of the steel sheet may be made 5 degrees or more and 20 degrees or less. If less than 5 degrees, the steel sheet cannot be sufficiently bent and sometimes the effect cannot be sufficiently obtained. If the bending angle is too large, the steel sheet is liable to break etc., therefore the angle may be made 20 degrees or less. The upper limit of the bending angle may be made 15 degrees or less. The lower limit may be made 8 degrees or more or may be made 10 degrees or more.
Note that skin pass rolling differs from bending in the disturbance of the Fe atomic arrangement in the steel. The introduction of dislocations also differs. Therefore, it is difficult to obtain the same effects as bending. In terms of costs as well, bending is advantageous. Note that bending enables the above effect to be obtained even if performed once.
Further, the leveling is performed by wrapping the steel sheet around a roll. This is done for the purpose of straightening the sheet wound up in a coil shape, eliminating warping and bending of the coil, and rendering the sheet flat. Further, the roll diameter is small. Typically, it is 100 mm or less. Therefore, the leveling differs from the bending in the present embodiment.
Overaging and bending may be jointly used. The overaging and bending may be performed simultaneously or the bending may be performed after the overaging. By jointly using these, the effect of suppressing magnetic aging and stretcher strain can be improved synergistically.

(Formation of Insulation Coating)

Electrical steel sheet is used stacked up, so to secure interlayer resistance, an insulation coating may be applied to the surface. One containing Cr has been used in the past, but Cr free coatings are also being developed recently. Either is all right.

EXAMPLES

Molten steel was continuously cast to prepare a 250 mm thick slab so as to obtain steel having each of the chemical compositions shown in the following Table 1. Next, the above slab was hot rolled to prepare a hot rolled sheet. The slab reheating temperature at that time was 1200°C, the finishing temperature at the finish rolling was 850°C, the coiling temperature at the time of coiling was 650°C, and the finished sheet thickness was 2.0 mm. Next, in the above hot rolled sheet, as the annealing of the hot rolled sheet, appealing was performed at 900 degrees for 1 minute. The sheet was pickled to remove the scale and cold rolled to a 0.25 mm thickness. Further, finish annealing was performed at 800°C for 30 seconds. After that, aging described in Table 1 was performed. Further, in some samples, as bending, the sheet was run wrapped around a φ400 mm roll to bend it by a bending angle of 10 degrees (tangential direction of roll cross-sectional circle was made 0 degree). That is, bending was performed by pressing the steel sheet against the roll. The bending was performed during the overaging or after the overaging when performing overaging and was performed after the finish annealing when not performing overaging. Note that steel sheet produced under the same conditions as No. 101 except for performing the overaging during the cooling of the finish annealing (No. 127) was confirmed to have magnetic properties and an A.I. equal to those of No. 101
Next, the magnetic properties of iron loss W10/400 (iron loss at maximum magnetic flux density of 1.0T and frequency of 400 Hz) and the magnetic flux density B50 (magnetic flux density at magnetizing force of 5000A/m) were measured. For the measurement sample, a 55 mm square test piece was taken. The average values of the properties in the rolling direction and width direction were found. For magnetic measurement, measurement was conducted using a device able to measure a 55 mm square test piece and further smaller test piece based on the magnetic circuit described in JIS C 2556 (2015). The measurement results are shown in Table 1. After magnetic measurement, the sample was aged for magnetic measurement at 200°C for 24 hours. The iron loss W10/400 was measured. The value of the increase in iron loss divided by the iron loss W10/400 before aging is defined as the iron loss deterioration rate after aging. It is shown in Table 1. Further, A.I. is also measured. The results are shown in Table 1.
Nos. 101 to 122 and 127 are excellent in composition and A.I. No. 123 is not subjected to either overaging and bending, therefore is high in A.I. Nos. 124 and 126 do not contain Cr and are high in A.I. No. 125 contains Ti over the upper limit value, therefore is high in iron loss. Further, the comparative examples high in A.I. also tend to be high in iron loss deterioration rate after aging.
For evaluation of the stretcher strain, the yield elongation (described in JIS Z2241) at the tension before aging (8% strain) in an A.I. test was investigated. If yield elongation occurs, the visible surface defect stretcher strain occurs. In examples with an A.I. of less than 40 MPa, in each case yield elongation did not occur, but in examples with an A.I. of 40 MPa or more, yield elongation occurred. From this, lowering the A.I. (to less than 40 MPa) enabled stretcher strain to be suppressed. Note that the "---" in the column "yield elongation" of the table indicates yield elongation did not occur.

Claims

Non-oriented electrical steel sheet having a chemical composition comprised of, by mass%, C: 0.0100% or less, Si: 2.6% or more and 4.5% or less, Mn: 0.10% or more and 3.00% or less, P: 0.15% or less, S: 0.0040% or less, N: 0.0040% or less, Al: 0.10% or more and 2.00% or less, one or more elements selected from Sn and Sb: 0 to 0.200%, Cr: 0.001 to 5.000%, Ni: 0 to 5.000%, Cu: 0 to 5.000%, Ca: 0 to 0.020%, Mg: 0 to 0.0200% and rare earth elements (REM): 0 to 0.020%, Ti: less than 0.0100%, B: 0% to 0.0050%, O: 0% to 0.0200%, and balance: Fe and impurities, wherein a difference: A.I. in lower yield point before and after aging when imparting 8% strain then aging at 100°C for 1 hour is less than 40 MPa.
The non-oriented electrical steel sheet according to claim 1, wherein the chemical composition further includes Mo: 0% to 0.0200%.
The non-oriented electrical steel sheet according to claim 1 or 2, wherein in the following formula, Cev≤0.0055% is satisfied. Cev = 12 × ([C(mass%)]/12 + [N(mass%)]/14 - [B(mass%)]/11 - [Ti(mass%)]/48 - [Cr(mass%)]/104 - [Mo(mass%)]/192) where, [element symbol (mass%)] indicates the content (mass%) of that element in the chemical composition, 0 being entered when that element is not included.
A method of production of non-oriented electrical steel sheet according to any one of claims 1 to 3, comprising producing non-oriented electrical steel sheet having a chemical composition comprised of, by mass%, C: 0.0100% or less, Si: 2.6% or more and 4.5% or less, Mn: 0.10% or more and 3.00% or less, P: 0.15% or less, S: 0.0040% or less, N: 0.0040% or less, Al: 0.10% or more and 2.00% or less, one or more elements selected from Sn and Sb: 0 to 0.200%, Cr: 0.001 to 5.000%, Ni: 0 to 5.000%, Cu: 0 to 5.000%, Ca: 0 to 0.020%, Mg: 0 to 0.0200% and rare earth elements (REM): 0 to 0.020%, Ti: less than 0.0100%, B: 0% to 0.0050%, O: 0% to 0.0200%, and balance: Fe and impurities by steps including steelmaking, hot rolling, annealing the hot rolled sheet, pickling, cold rolling, finish annealing, and overaging, during which making the overaging temperature of the overaging 50°C to 500°C and holding in the 50°C or more temperature region for 10 seconds or more.
The method of production of non-oriented electrical steel sheet according to claim 4, wherein the chemical composition further includes Mo: 0% to 0.0200%.
The method of production of non-oriented electrical steel sheet according to claim 4 or 5, further comprising, at the overaging, pressing the steel sheet against a less than diameter 500 mm roll to bend it in at least part of the step of holding at the 50°C to 500°C temperature region.
The method of production of non-oriented electrical steel sheet according to any one of claims 4 to 6, further comprising, after the overaging, pressing the steel sheet against a less than diameter 500 mm roll to bend it.
A method of production of non-oriented electrical steel sheet according to any one of claims 1 to 3, comprising producing non-oriented electrical steel sheet having a chemical composition comprised of, by mass%, C: 0.0100% or less, Si: 2.6% or more and 4.5% or less, Mn: 0.10% or more and 3.00% or less, P: 0.15% or less, S: 0.0040% or less, N: 0.0040% or less, Al: 0.10% or more and 2.00% or less, one or more elements selected from Sn and Sb: 0 to 0.200%, Cr: 0.001 to 5.000%, Ni: 0 to 5.000%, Cu: 0 to 5.000%, Ca: 0 to 0.020%, Mg: 0 to 0.0200% and rare earth elements (REM): 0 to 0.020%, Ti: less than 0.0100%, B: 0% to 0.0050%, O: 0% to 0.0200%, and balance: Fe and impurities by steps including steelmaking, hot rolling, annealing the hot rolled sheet, pickling, cold rolling, finish annealing, and bending, during which, in the bending, pressing the steel sheet against a less than diameter 500 mm roll to bend it.
The method of production of non-oriented electrical steel sheet according to claim 8, wherein the chemical composition further comprises Mo: 0% to 0.0200%.
A rotating electrical machine comprising a stator, rotor, and a housing holding the stator and rotor, in which a core material of the stator or a core material of the rotor is comprised of a non-oriented electrical steel sheet according to any one of claims 1 to 3.