Patent Application
20250115982
The present disclosure relates to a non-oriented electrical steel sheet and a method of producing the same.
In recent years, energy saving is required due to consideration for the environment such as global warming. In the automotive field, hybrid electric vehicles (HEVs) that use both an engine and a motor, electric vehicles (EVs) driven only by an electric motor, fuel cell electric vehicles (FCEVs), etc. are being developed. The motors used in these vehicles are typically driven in a high frequency range which is advantageous for high speed rotation, in order to enhance efficiency. Non-oriented electrical steel sheets are often used as an iron core material for such drive motors of HEVs and EVs. In order to achieve high motor efficiency, non-oriented electrical steel sheets are strongly required to have lower iron loss in the high frequency range.
Conventionally, the iron loss of non-oriented electrical steel sheets is reduced mainly by adding alloying elements such as Si and Al to enhance specific resistance or by reducing the sheet thickness to reduce eddy current loss. However, when a large amount of alloying elements are added for iron loss reduction, saturation magnetic flux density decreases. A decrease in magnetic flux density causes an increase in copper loss in the motor, leading to lower motor efficiency. Moreover, in order to reduce the thickness of the non-oriented electrical steel sheet, it is necessary to reduce the thickness of the hot-rolled steel sheet or increase the cold rolling reduction in the production process. This lowers productivity.
Accordingly, if a non-oriented electrical steel sheet that has both high magnetic flux density and low iron loss in the high frequency range and also has excellent productivity can be developed, it will greatly contribute to higher efficiency of electric devices.
As a method of achieving low iron loss in the high frequency range, for example, JP H11-343544 A (PTL 1) discloses a method in which 1.5 mass % to 20 mass % of Cr is added to enhance the specific resistance of steel and achieve low iron loss in the high frequency range.
Non-oriented electrical steel sheets are sometimes subjected to stress relief annealing. For example, in the case where a non-oriented electrical steel sheet is used for a stator core of a motor, the non-oriented electrical steel sheet is processed into a core shape by blanking or the like and then subjected to stress relief annealing in order to release the strain introduced during processing.
For example, JP 2006-265720 A (PTL 2) proposes a technique in which final annealing, then skin pass rolling, and then stress relief annealing are performed to improve the magnetic properties after stress relief annealing.
However, since Cr is an element that decreases saturation magnetic flux density, the technique disclosed in PTL 1, which requires the addition of a large amount of Cr, cannot achieve both high magnetic flux density and low high-frequency iron loss.
Moreover, the technique proposed in PTL 2 requires additionally performing skin pass rolling. This causes a decrease in productivity and an increase in production costs.
It could therefore be helpful to provide a non-oriented electrical steel sheet with reduced iron loss in a high frequency range without adding a large amount of alloying elements such as Cr, which causes a decrease in magnetic flux density, and without performing skin pass rolling, which causes a decrease in productivity.
Upon careful examination focusing on the influence of the surface state of a non-oriented electrical steel sheet on its magnetic properties, we discovered that, by limiting the Co content in the steel to a certain range and subjecting the steel sheet after final annealing and before stress relief annealing to acidizing to limit the nitrogen amount in the surface layer to a certain range, iron loss can be reduced without causing a decrease in magnetic flux density.
The present disclosure is based on these discoveries. We thus provide the following.
1. A non-oriented electrical steel sheet comprising a chemical composition containing (consisting of), in mass %, C: 0.0050% or less, Si: 2.0% to 6.5%, Mn: 0.05% to 2.0%, P: 0.10% or less, S: 0.0050% or less, Al: 0.3% to 3.0%, N: 0.0050% or less, Co: 0.0005% to 0.0050%, Ti: 0.0030% or less, Nb: 0.0030% or less, and O: 0.0050% or less, with a balance consisting of Fe and inevitable impurities, wherein at at least one surface of the non-oriented electrical steel sheet, an amount of N existing as AlN in a range from the surface to a depth of 1/20 of a sheet thickness is 0.003 mass % or less, the surface has an oxide layer containing Fe and one or both of Al and Si, and an interface between the oxide layer and a steel substrate of the non-oriented electrical steel sheet has an oxide film consisting of one or both of Al oxide and Si oxide.
2. The non-oriented electrical steel sheet according to 1., wherein the chemical composition further contains, in mass %, one or more groups out of the following groups A to F:
3. A method of producing a non-oriented electrical steel sheet, the method comprising: subjecting a steel material to cold rolling once or twice or more with intermediate annealing therebetween to obtain a cold-rolled steel sheet, the steel material having a chemical composition containing, in mass %, C: 0.0050% or less, Si: 2.0% to 6.5%, Mn: 0.05% to 2.0%, P: 0.10% or less, S: 0.0050% or less, Al: 0.3% to 3.0%, N: 0.0050% or less, Co: 0.0005% to 0.0050%, Ti: 0.0030% or less, Nb: 0.0030% or less, and O: 0.0050% or less, with a balance consisting of Fe and inevitable impurities; subjecting the cold-rolled steel sheet to final annealing and stress relief annealing to produce a non-oriented electrical steel sheet, the stress relief annealing being performed at a soaking temperature of 750° C. to 950° C. in an atmosphere having a nitrogen content of 50 vol % or more; and subjecting, after the final annealing and before the stress relief annealing, a surface of the steel sheet to acidizing for 1 second to 60 seconds using an acid containing at least one selected from the group consisting of hydrochloric acid, phosphoric acid, sulfuric acid, and nitric acid at a total concentration of 3 wt % to 30 wt %.
4. The method of producing a non-oriented electrical steel sheet according to 3., comprising: subjecting, before the cold rolling, the steel material to hot rolling to obtain a hot-rolled steel sheet; and subjecting the hot-rolled steel sheet to hot-rolled sheet annealing to obtain a hot-rolled and annealed sheet, wherein the hot-rolled and annealed sheet is subjected to the cold rolling.
5. The method of producing a non-oriented electrical steel sheet according to 3, or 4., wherein the chemical composition further contains, in mass %, one or more groups out of the following groups A to F:
It is thus possible to provide a non-oriented electrical steel sheet with reduced iron loss in a high frequency range without causing a decrease in magnetic flux density or productivity. In detail, since low high-frequency iron loss can be achieved by adding a small amount of Co and controlling the nitrogen amount in the surface layer by acidizing, addition of a large amount of alloying elements such as Cr, which causes a decrease in magnetic flux density, is unnecessary. Moreover, excellent magnetic properties can be achieved without skin pass rolling, which causes a decrease in productivity. In the case of performing skin pass rolling, skin pass rolling equipment needs to be newly installed. Acidizing, on the other hand, can be performed using cleaning equipment included in typical steel sheet production lines, without loss of productivity.
In the accompanying drawings:
First, experiments that led to development of the presently disclosed techniques will be described. In the specification, “%” as a unit of content refers to “mass %” unless otherwise specified. The unit of acid concentration is “wt %”.
A plurality of non-oriented electrical steel sheets with different Co contents were each produced according to the following procedure, and the iron loss in the high frequency range of each of the obtained non-oriented electrical steel sheets was evaluated.
Steel containing C: 0.0019%, Si: 3.1%, Mn: 0.8%, P: 0.01%, S: 0.0015%, Al: 0.9%, N: 0.0018%, Ti: 0.0010%, Nb: 0.0008%, O: 0.0021%, and Co: 0.0001% to 0.01% with the balance consisting of Fe and inevitable impurities was obtained by steelmaking in a vacuum furnace and cast into a steel ingot. The steel ingot was hot rolled to obtain a hot-rolled steel sheet with a sheet thickness of 2.0 mm, and the hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 960° C. for 30 seconds to obtain a hot-rolled and annealed sheet. The hot-rolled and annealed sheet was pickled and then cold rolled to obtain a cold-rolled steel sheet with a final sheet thickness of 0.25 mm.
The obtained cold-rolled steel sheet was subjected to final annealing at 850° C. for 10 seconds in an atmosphere of H2:N2=20:80 in vol %, and then subjected to acidizing. In the acidizing, the steel sheet was immersed in 10% hydrochloric acid for 10 seconds. After this, stress relief annealing was performed at 850° C. for 2 hours in an atmosphere having a nitrogen content of 100 vol % to obtain a non-oriented electrical steel sheet.
Test pieces of 30 mm in width and 180 mm in length were cut out from the rolling direction (L direction) and transverse direction (C direction) of the non-oriented electrical steel sheet obtained in the foregoing manner, and the high-frequency iron loss of the test pieces was evaluated. Specifically, the iron loss W10/400 of the test pieces in the (L+C) direction at a maximum magnetic flux density of 1.0 T and a frequency of 400 Hz was measured by Epstein testing.
As can be seen from the results illustrated in
In order to investigate the cause of the decrease in iron loss by the addition of a small amount of Co, a section in the rolling direction of each non-oriented electrical steel sheet produced in the above experiment was observed using a scanning electron microscope (SEM). As a result, in each non-oriented electrical steel sheet having a Co content outside the range of 0.0005% to 0.0050%, precipitation of fine AlN was observed in the range from the surface of the steel sheet to a depth of 1/20 of the sheet thickness (hereafter also referred to as “surface layer”). In each non-oriented electrical steel sheet having a Co content in the range of 0.0005% to 0.0050%, on the other hand, AlN hardly precipitated in the surface layer.
Typically, in the production process of a non-oriented electrical steel sheet, nitrogen in the atmosphere diffuses into the steel sheet during final annealing, resulting in the formation of nitrides such as AlN in the surface layer (such a surface layer into which nitrides have precipitated is also referred to as “nitrided layer”). However, the above experimental results suggest that, in each non-oriented electrical steel sheet having a Co content in the range of 0.0005% to 0.0050%, the amount of nitrides (AlN) in the surface layer was reduced for some reason and consequently the iron loss decreased.
Then, the concentration of N existing as AlN in the range from the surface of the non-oriented electrical steel sheet to a depth of 1/20 of the sheet thickness (hereafter referred to as “surface layer nitridation amount”) was analyzed by electrolytic extraction.
As can be seen from the results illustrated in
Further, the surface of each non-oriented electrical steel sheet that was found to have decreased in iron loss was observed using a transmission electron microscope (TEM). The observation revealed that an oxide layer containing Fe and one or both of Al and Si was formed at the surface of the steel sheet and an oxide film consisting of one or both of Al oxide and Si oxide was formed at the interface between the oxide layer and the steel substrate. Further investigation found that the oxide layer was formed at the surface of the steel sheet by the acidizing and the oxide film was formed at the interface between the oxide layer and the steel substrate by the subsequent stress relief annealing.
These experimental results suggest that the oxide layer and the oxide film effectively inhibited nitridation during the stress relief annealing and consequently the iron loss decreased.
Next, in order to evaluate the influence of the acidizing conditions on the magnetic properties of non-oriented electrical steel sheets, acidizing was performed under various conditions and the high-frequency iron loss of each of the obtained non-oriented electrical steel sheets was measured.
Specifically, first, steel containing C: 0.0024%, Si: 3.6%, Mn: 0.5%, P: 0.01%, S: 0.0021%, Al: 1.0%, N: 0.0022%, Ti: 0.0011%, Nb: 0.0007%, O: 0.0020%, and Co: 0.0023% with the balance consisting of Fe and inevitable impurities was obtained by steelmaking in a vacuum furnace and cast into a steel ingot. The steel ingot was hot rolled to obtain a hot-rolled steel sheet with a sheet thickness of 1.8 mm, and the hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 980° C. for 30 seconds to obtain a hot-rolled and annealed sheet. The hot-rolled and annealed sheet was pickled and then cold rolled to obtain a cold-rolled steel sheet with a final sheet thickness of 0.20 mm.
The obtained cold-rolled steel sheet was subjected to final annealing at 950° C. for 10 seconds in an atmosphere of H2:N2=15:85 in vol %, and then subjected to acidizing. The acidizing was performed by immersing the steel sheet in hydrochloric acid for 10 seconds. Here, the concentration of the hydrochloric acid used was varied in the range of 1% to 50%. After this, stress relief annealing was performed at 800° C. for 2 hours in an atmosphere having a nitrogen content of 100 vol % to obtain a non-oriented electrical steel sheet.
Test pieces of 30 mm in width and 180 mm in length were cut out from the rolling direction (L direction) and transverse direction (C direction) of the non-oriented electrical steel sheet obtained in the foregoing manner, and the high-frequency iron loss (W10/400) of the test pieces was evaluated. Specifically, the iron loss W10/400 of the test pieces in the (L+C) direction at a maximum magnetic flux density of 1.0 T and a frequency of 400 Hz was measured by Epstein testing. Moreover, the concentration of N existing as AlN in the range from the surface of the non-oriented electrical steel sheet to a depth of 1/20 of the sheet thickness was measured by electrolytic extraction.
Furthermore, non-oriented electrical steel sheets were produced in the same manner as in Experiment 2 except that the concentration of hydrochloric acid in acidizing was 20% and the acidizing time (immersion time) was varied in the range of 0.5 seconds to 100 seconds, and the high-frequency iron loss (W10/400) and the surface layer nitridation amount were measured.
As can be seen from the results illustrated in
The present disclosure is based on these discoveries. Embodiments of the present disclosure will be described in detail below, although the present disclosure is not limited to such embodiments.
The reasons for limiting the chemical composition of the non-oriented electrical steel sheet to the foregoing range in the present disclosure will be described.
C is a harmful element that degrades iron loss by magnetic aging. In detail, if the C content is excessively high, C forms carbides that precipitate over time, causing an increase in iron loss. The C content is therefore limited to 0.0050% or less. The C content is preferably 0.0040% or less. Although no lower limit is placed on the C content, the C content is preferably 0.0001% or more from the viewpoint of reducing the decarburization costs in the refining process.
Si is an element that has the effect of enhancing the specific resistance of steel and reducing iron loss. Si also has the effect of enhancing the strength of steel through solid solution strengthening. In order to achieve these effects, the Si content is 2.0% or more, and preferably 2.5% or more. If the Si content is more than 6.5%, slab cracking is likely to occur during production, resulting in decreased productivity. The Si content is therefore 6.5% or less, and preferably 6.0% or less.
Mn is an element that has the effect of enhancing the specific resistance of steel and reducing iron loss, as with Si. Mn also has the effect of forming sulfides and improving hot brittleness. In order to achieve these effects, the Mn content is 0.05% or more, and preferably 0.1% or more. If the Mn content is more than 2.0%, slab cracking is likely to occur during production, resulting in decreased productivity. The Mn content is therefore 2.0% or less, and preferably 1.5% or less.
If the P content is excessively high, cold rollability degrades. The P content is therefore 0.10% or less, and preferably 0.05% or less. No lower limit is placed on the P content, and the lower limit may be 0%. However, since P is an element that is highly effective in enhancing specific resistance and reducing eddy current loss, P may be optionally added within the foregoing range. From the viewpoint of reducing eddy current loss, the P content is preferably 0.001% or more and more preferably 0.003% or more.
S forms sulfides as precipitates and inclusions and decreases productivity (hot rollability) and the magnetic properties of the non-oriented electrical steel sheet. The S content is therefore 0.0050% or less, and preferably 0.0030% or less. Since a lower S content is better, no lower limit is placed on the S content, and the lower limit may be 0%. Excessively reducing the S content, however, causes an increase in production costs, and accordingly the S content is preferably 0.0001% or more and more preferably 0.0005% or more.
Al is an element that has the effect of enhancing the specific resistance of steel and reducing iron loss, as with Si. If the Al content is more than 3.0%, however, the steel becomes brittle and slab cracking is likely to occur during production. The Al content is therefore 3.0% or less, preferably 2.0% or less, and more preferably 1.5% or less. If the Al content is less than 0.3%, Al forms fine nitrides that precipitate, causing an increase in iron loss. The Al content is therefore 0.3% or more, and preferably 0.4% or more.
N is a harmful element that forms nitrides and degrades magnetic properties. The N content is therefore 0.0050% or less, and preferably 0.0040% or less. Since a lower N content is better, no lower limit is placed on the N content, and the lower limit may be 0%. Excessively reducing the N content, however, causes an increase in production costs, and accordingly the N content is preferably 0.0005% or more and more preferably 0.0010% or more.
Co is an element that has the effect of inhibiting nitridation of the surface layer of the steel sheet during final annealing, as mentioned above. The Co content is therefore 0.0005% or more, and preferably 0.001% or more. If the Co content is more than 0.0050%, an outer oxide layer formed during final annealing increases in thickness, hindering the formation of an oxide layer by acidizing. The Co content is therefore 0.0050% or less, and preferably 0.0040% or less.
Ti is a harmful element that forms fine carbonitrides that precipitate and increases iron loss. The adverse effect of Ti is particularly noticeable when the Ti content is more than 0.0030%. The Ti content is therefore 0.0030% or less, and preferably 0.0020% or less. Since a lower Ti content is better, no lower limit is placed on the Ti content, and the lower limit may be 0%. Excessively reducing the Ti content, however, causes an increase in production costs, and accordingly the Ti content is preferably 0.0001% or more and more preferably 0.0005% or more.
Nb is a harmful element that forms fine carbonitrides that precipitate and increases iron loss. The adverse effect of Nb is particularly noticeable when the Nb content is more than 0.0030%. The Nb content is therefore 0.0030% or less, and preferably 0.0020% or less. Since a lower Nb content is better, no lower limit is placed on the Nb content, and the lower limit may be 0%. Excessively reducing the Nb content, however, causes an increase in production costs, and accordingly the Nb content is preferably 0.0001% or more and more preferably 0.0005% or more.
O is a harmful element that forms oxides and degrades magnetic properties. The O content is therefore limited to 0.0050% or less. The O content is preferably 0.0040% or less. Since a lower O content is better, no lower limit is placed on the O content, and the lower limit may be 0%. Excessively reducing the O content, however, causes an increase in production costs, and accordingly the O content is preferably 0.0005% or more and more preferably 0.0010% or more.
A non-oriented electrical steel sheet in one embodiment of the present disclosure has a chemical composition containing the above-described components with the balance consisting of Fe and inevitable impurities.
The chemical composition may optionally further contain one or both of Sn and Sb in the following amounts.
Sn is an element that has the effect of improving recrystallized texture and improving magnetic flux density and iron loss. In the case of adding Sn, the Sn content is 0.005% or more in order to achieve the effect. If the Sn content is more than 0.20%, the effect is saturated. The Sn content is therefore 0.20% or less.
Sb is an element that has the effect of improving recrystallized texture and improving magnetic flux density and iron loss, as with Sn. In the case of adding Sb, the Sb content is 0.005% or more in order to achieve the effect. If the Sb content is more than 0.20%, the effect is saturated. The Sb content is therefore 0.20% or less.
The chemical composition may optionally further contain at least one selected from the group consisting of Ca, Mg, and REM (rare earth metal) in the following amount.
Ca, Mg, and REM are components that have the effect of forming stable sulfides and improving grain growth. In the case of adding at least one selected from the group consisting of Ca, Mg, and REM, the total content of Ca, Mg, and REM is 0.0005% or more in order to achieve the effect. If the total content is more than 0.020%, the effect is saturated. The total content is therefore 0.020% or less.
The chemical composition may optionally further contain at least one selected from the group consisting of Cu, Cr, and Ni in the following amount.
Cu, Cr, and Ni are elements that have the effect of enhancing the specific resistance of steel and further reducing iron loss. In the case of adding at least one of Cu, Cr, and Ni, the total content of Cu, Cr, and Ni is 0.03% or more in order to achieve the effect. Since excessive addition decreases magnetic flux density, the total content is 1.0% or less.
The chemical composition may optionally further contain one or both of Ge and Ga in the following amount.
Ge and Ga are elements that have the effect of improving texture. In the case of adding one or both of Ge and Ga, the total content of Ge and Ga is 0.0005% or more and preferably 0.0020% or more in order to achieve the effect. If the total content is more than 0.01%, the effect is saturated and the alloy cost increases. The total content is therefore 0.01% or less, and preferably 0.0050% or less.
The chemical composition may optionally further contain Zn in the following amount.
Zn is an element that has the effect of inhibiting nitridation during final annealing. In the case of adding Zn, the Zn content is 0.001% or more, and preferably 0.002% or more. If the Zn content is more than 0.05%, Zn forms sulfides, causing an increase in iron loss. The Zn content is therefore 0.05% or less, and preferably 0.01% or less.
The chemical composition may optionally further contain one or both of Mo and W in the following amount.
Mo and W are elements that have the effect of improving high-temperature strength. Adding one or both of Mo and W can suppress surface defects (scab) in non-oriented electrical steel sheets. The steel sheet according to the present disclosure is high-alloy steel and its surface is easily-oxidizable, and thus the likelihood of scab caused by surface cracking is high. By adding one or both of Mo and W, such cracking can be suppressed. In the case of adding one or both of Mo and W, the total content of Mo and W is 0.001% or more and preferably 0.005% or more in order to sufficiently achieve the effect. If the total content is more than 0.05%, the effect is saturated and the alloy cost increases. The total content is therefore 0.05% or less, and preferably 0.020% or less.
As demonstrated in the foregoing experiments, there is a strong correlation between the surface layer nitridation amount and the high-frequency iron loss, and the iron loss decreases significantly when the surface layer nitridation amount is 0.003% or less. Hence, in the present disclosure, the surface layer nitridation amount at at least one surface (i.e. at least one side) of the non-oriented electrical steel sheet is 0.003% or less. Since a lower surface layer nitridation amount is better from the viewpoint of reducing the high-frequency iron loss, no lower limit is placed on the surface layer nitridation amount, and the lower limit may be 0%. From the viewpoint of production, however, the surface layer nitridation amount may be 0.0001% or more, and may be 0.0005% or more. The “surface layer nitridation amount” herein refers to the amount of N existing as AlN in the range from the surface of the non-oriented electrical steel sheet to a depth of 1/20 of the sheet thickness. The surface layer nitridation amount can be measured by electrolytic extraction analysis. More specifically, the surface layer nitridation amount can be measured by the method described in the EXAMPLES section. It is preferable that the surface layer nitridation amount at both surfaces (sides) of the non-oriented electrical steel sheet satisfies the foregoing condition.
The non-oriented electrical steel sheet according to the present disclosure has, at at least one surface, an oxide layer containing Fe and one or both of Al and Si. The oxide layer is formed by acidizing after final annealing and before stress relief annealing, as described later.
The oxide layer may be a Fe—Al-based oxide layer containing Fe and Al, a Fe—Si-based oxide layer containing Fe and Si, or a Fe—Si—Al-based oxide layer containing Fe, Si, and Al. The Fe concentration in the oxide layer is not limited, but is preferably less than 100 mass % relative to the total concentration of Al and Si. Fe may be dissolved in the oxide, or may exist as a precipitate or an oxide. The oxide layer may optionally further contain other elements, for example, one or both of Co and Mn.
The thickness of the oxide layer is not limited, but is preferably 50 nm or more from the viewpoint of further inhibiting nitridation during stress relief annealing. If the oxide layer is excessively thick, not only nitridation is accelerated but also the stacking factor of the steel sheet decreases. Therefore, from the viewpoint of further inhibiting nitridation and improving the stacking factor, the thickness of the oxide layer is preferably 200 nm or less. The thickness of the oxide layer can be measured by scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS). More specifically, the thickness of the oxide layer can be measured by the method described in the EXAMPLES section.
The oxide layer may be amorphous, crystalline, or a composite oxide of amorphous and crystalline. The ratio of the crystalline phase to the total of the amorphous phase and the crystalline phase contained in the oxide layer is preferably in the range of 0% or more and 30% or less and more preferably in the range of 0% or more and 10% or less. The ratio of the crystalline phase can be measured by observing the oxide layer using a scanning transmission electron microscope (STEM). Specifically, first, a STEM image is obtained with about 2,000,000 to 10,000,000 magnification using a beam diameter that provides a resolution of 1 nm or less. In the obtained STEM image, the region where the lattice fringes can be seen is the crystalline phase and the region where the maze pattern can be seen is the amorphous phase, and the areas of both regions are determined. From this result, the ratio of the area of the crystalline phase to the total area of the amorphous phase and the crystalline phase is calculated, and the ratio expressed as a percentage is taken to be the ratio of the crystalline phase.
The non-oriented electrical steel sheet according to the present disclosure has an oxide film consisting of one or both of Al oxide and Si oxide at the interface between the oxide layer and the steel substrate. In other words, the non-oriented electrical steel sheet according to the present disclosure includes the oxide film on the surface of the steel substrate and includes the oxide layer on the surface of the oxide film.
The thickness of the oxide film is not limited. However, given that high-frequency iron loss decreased significantly as a result of acidizing with an acid concentration of 3% to 30% and an acidizing time of 1 second to 60 seconds and the thickness of the oxide film in this case was 10 nm to 40 nm as mentioned above, the thickness of the oxide film is preferably in the range of 10 nm or more and 40 nm or less.
The oxide film may be composed of a single film or a plurality of films. For example, the oxide film may be an Al-rich oxide film, a Si-rich oxide film, or a multilayer film in which an Al-rich oxide film and a Si-rich oxide film are stacked. In particular, an oxide film in which an Al-rich oxide film and a Si-rich oxide film are alternately deposited is preferable.
Here, the “Al-rich oxide film” is defined as an oxide film in which the Al concentration is higher than the Si concentration. The Al-rich oxide film may be a film consisting essentially of Al oxide. Likewise, the “Si-rich oxide film” is defined as an oxide film in which the Si concentration is higher than the Al concentration. The Si-rich oxide film may be a film consisting essentially of Si oxide.
The coverage of the oxide film is not limited, but is preferably 80% or more and more preferably 90% or more. Although no upper limit is placed on the coverage, the upper limit may be 100%. The coverage herein is the coverage of the oxide film on the surface of the steel sheet.
The structure of the oxide film can be measured by observing the interface between the oxide layer and the steel substrate using STEM-EDS.
In detail, the thickness and layer structure of the oxide film can be measured by observing the interface between the oxide layer and the steel substrate using STEM-EDS. Specifically, EDS-line profiles at the interface between the oxide layer and the steel substrate are obtained at intervals of 0.5 nm. From the obtained elemental distributions of Al, Si, and O, the thickness and layer structure of the oxide film can be determined.
The coverage of the oxide film can be measured using STEM-EDS as well. Specifically, first, an elemental analysis map of the interface between the oxide layer and the steel substrate of the steel sheet is obtained using STEM-EDS with at least 100,000 magnification. The length of the oxide film covering the steel substrate of the steel sheet is determined from the elemental distributions of Al, Si, and O in the obtained elemental analysis map. The determined length of the oxide film is divided by the length of the steel substrate of the steel sheet to calculate the coverage of the oxide film. In other words, the coverage of the oxide film herein is a percentage expressing the ratio of the length of the oxide film to the length of the steel substrate of the steel sheet, as measured by STEM-EDS.
Next, a method of producing a non-oriented electrical steel sheet in one embodiment of the present disclosure will be described.
In one embodiment of the present disclosure, a non-oriented electrical steel sheet can be produced by subjecting a steel material (slab) having the above-described chemical composition to the following treatments (1) to (4) sequentially:
In another embodiment of the present disclosure, hot rolling and hot-rolled sheet annealing may be performed before the cold rolling. In other words, in the other embodiment, a non-oriented electrical steel sheet can be produced by subjecting a steel material (slab) having the above-described chemical composition to the following treatments (1) to (6) sequentially:
As the steel material, any steel material having the above-described chemical composition may be used. A typical example of the steel material is a steel slab.
The method of producing the steel material is not limited, and the steel material may be produced by any method. For example, steel is prepared by steelmaking in a refining process using a converter, an electric furnace, a vacuum degasser, etc., and then made into a steel material by a continuous casting method or an ingot casting and blooming method. A thin slab or thinner cast steel of 100 mm or less in thickness produced by a direct casting method may be used as the steel material.
In the case of performing hot rolling, the steel material is hot rolled to obtain a hot-rolled steel sheet. The hot rolling conditions are not limited, and hot rolling may be performed under commonly known conditions.
Typically, the steel material is reheated in a heating furnace or the like and then rolled. Alternatively, the cast steel material may be immediately hot rolled without reheating.
In the case of not performing hot rolling, the steel material can be directly subjected to cold rolling. In the case where hot rolling is omitted, it is preferable to use a thin slab or thinner cast steel as the steel material.
In the case where hot rolling is performed, the obtained hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled and annealed sheet. The hot-rolled sheet annealing conditions are not limited. If the soaking temperature is less than 800° C., however, the effect of hot-rolled sheet annealing is small and the magnetic property improving effect may be insufficient. Accordingly, the soaking temperature in the hot-rolled sheet annealing is preferably 800° C. or more and more preferably 850° C. or more. If the soaking temperature is more than 1100° C., the production costs increase. In addition, due to coarsening of crystal grains, embrittlement fracture (breakage) tends to occur during subsequent cold rolling. The soaking temperature is therefore preferably 1100° C. or less and more preferably 1000° C. or less.
The soaking time in the hot-rolled sheet annealing is preferably 3 minutes or less and more preferably 1 minute or less, from the viewpoint of ensuring productivity. Although no lower limit is placed on the soaking time, the soaking time is preferably 1 second or more and more preferably 5 seconds or more.
Next, cold rolling is performed. In the case where hot rolling and hot-rolled sheet annealing are performed, the hot-rolled and annealed sheet is subjected to cold rolling to obtain a cold-rolled steel sheet having a final sheet thickness. In the case where hot rolling and hot-rolled sheet annealing are not performed, the steel material is directly subjected to cold rolling to obtain a cold-rolled steel sheet having a final sheet thickness. Cold rolling may be performed only once, or twice or more. In the case of performing cold rolling twice or more, intermediate annealing is performed between cold rolling. The cold rolling conditions are not limited, and cold rolling may be performed under commonly known conditions.
The final sheet thickness in the cold rolling, that is, the sheet thickness of the non-oriented electrical steel sheet finally obtained, is not limited and may be any thickness. If the non-oriented electrical steel sheet is excessively thick, however, iron loss increases. The final sheet thickness is therefore preferably 0.35 mm or less. If the non-oriented electrical steel sheet is excessively thin, it is difficult to handle the steel sheet during the production processes such as cold rolling and annealing. The final sheet thickness is therefore preferably 0.10 mm or more.
The cold-rolled steel sheet obtained as a result of the cold rolling is subjected to final annealing. The final annealing conditions are not limited, and final annealing may be performed under commonly known conditions. From the viewpoint of coarsening the crystal grain size and further reducing iron loss, the soaking temperature in the final annealing is preferably 700° C. to 1200° C. and more preferably 800° C. to 1100° C. For the same reason, the soaking time in the final annealing is preferably 1 second to 120 seconds and more preferably 5 seconds to 60 seconds.
After the final annealing, acidizing is performed on the surface of the steel sheet before subsequent stress relief annealing. For the acidizing, it is necessary to use an acid containing at least one selected from the group consisting of hydrochloric acid, phosphoric acid, sulfuric acid, and nitric acid at a total concentration of 3% to 30%, in order to form a desired oxide film during stress relief annealing and inhibit nitridation. The total concentration of the acid is preferably 5% to 25%. For the same reason, the acidizing time is 1 second to 60 seconds, and preferably 2 seconds to 20 seconds.
After the acidizing, it is preferable to form an insulating coating on the surface of the non-oriented electrical steel sheet. The insulating coating may be formed under commonly known conditions. As the insulating coating, an organic coating containing resin is preferable from the viewpoint of ensuring good blanking workability. If weldability is given importance, on the other hand, a semi-organic coating or an inorganic coating is preferable.
For production of a stator core of a motor, for example, steel sheets after final annealing may be processed into a core shape by blanking or the like, stacked and fixed together, and then subjected to stress relief annealing. Hence, the non-oriented electrical steel sheet according to the present disclosure also includes a core-shaped steel sheet obtained as a result of steel sheets after final annealing being processed into a core shape by blanking or the like, stacked and fixed together, and then subjected to stress relief annealing.
Stress relief annealing is performed at a soaking temperature of 750° C. to 950° C. in an atmosphere having a nitrogen content of 50 vol % or more. If the soaking temperature is less than 750° C., the iron loss improving effect by stress relief annealing is insufficient. If the soaking temperature is more than 950° C., it is difficult to ensure insulation between the stacked steel sheets. The soaking time in the stress relief annealing is not limited, but is preferably 0.1 hours to 10 hours. In the present disclosure, excellent iron loss property can be achieved even when stress relief annealing is performed in an atmosphere mainly composed of nitrogen gas which is relatively inexpensive. Accordingly, the upper limit of the nitrogen content in the atmosphere is not limited and may be 100%.
Moreover, by setting the heating rate in the stress relief annealing to 50° C./hr to 800° C./hr, it is possible to form an oxide film having a structure in which a Si-rich oxide film and an Al-rich oxide film are alternately deposited.
In order to determine the effects according to the present disclosure, non-oriented electrical steel sheets were produced according to the following procedure and their magnetic properties were evaluated.
Steel slabs having the chemical compositions shown in Tables 1 to 3 were heated at 1120° C. for 30 minutes, and then hot rolled to obtain hot-rolled steel sheets of 1.9 mm in sheet thickness. The hot-rolled steel sheets were then subjected to hot-rolled sheet annealing at 970° C. for 30 seconds to obtain hot-rolled and annealed sheets. The hot-rolled and annealed sheets were pickled, and then cold rolled to obtain cold-rolled steel sheets having the final sheet thicknesses shown in Tables 4 to 6.
Subsequently, the cold-rolled steel sheets were subjected to final annealing at 820° C. for 10 seconds in an atmosphere of H2:N2=20:80 in vol %, and then subjected to acidizing under the conditions shown in Tables 4 to 6. After the acidizing, stress relief annealing was performed under the conditions shown in Tables 4 to 6 to obtain non-oriented electrical steel sheets.
In Comparative Example No. 14 with an excessively high Mn content, slab cracking occurred during hot rolling and consequently production was discontinued. Likewise, in Comparative Example No. 15 with an excessively high Al content and Comparative Example No. 18 with an excessively high Si content, slab cracking occurred during hot rolling and consequently production was discontinued. In Comparative Example No. 21 with an excessively high P content, cracking occurred during cold rolling and consequently production was discontinued.
Next, test pieces were collected from each of the obtained non-oriented electrical steel sheets, and electrolytic extraction analysis was conducted to measure the amount of N existing as AlN in the range from one surface of the non-oriented electrical steel sheet to a depth of 1/20 of the sheet thickness (surface layer nitridation amount). The measurement results are shown in Tables 7 to 9.
An oxide layer containing Fe and one or both of Al and Si was formed at the surface of the obtained non-oriented electrical steel sheet. The thickness of the oxide layer was measured by scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDS). Specifically, EDS-line profiles were obtained at intervals of 0.5 nm, and the thickness of the oxide layer was determined from the obtained elemental distributions of Al, Si, and O. The measurement results are shown in Tables 7 to 9.
Further, the thickness and layer structure of the oxide film formed at the interface between the oxide layer and the steel substrate in the obtained non-oriented electrical steel sheet were measured using STEM-EDS. Specifically, EDS-line profiles at the interface were obtained at intervals of 0.5 nm, and the thickness and layer structure of the oxide film were determined from the obtained elemental distributions of Al, Si, and O.
In Tables 7 to 9, each example marked with “Si, Al” had a membranous oxide film containing Si and Al and having a structure in which a Si-rich oxide film and an Al-rich oxide film are alternately deposited. Each example marked with “Al” had a membranous oxide film containing Si and Al and composed only of an Al-rich oxide film.
The coverage of the oxide film was also measured using STEM-EDS. Specifically, first, an elemental analysis map of the interface between the oxide layer and the steel substrate of the steel sheet was obtained using STEM-EDS with at least 100,000 magnification. The length of the oxide film covering the steel substrate of the steel sheet was determined from the elemental distributions of Al, Si, and O in the obtained elemental analysis map. The determined length of the oxide film was divided by the length of the steel substrate of the steel sheet to calculate the coverage of the oxide film. The measurement results are shown in Tables 7 to 9.
Moreover, a sample was collected from each of the obtained non-oriented electrical steel sheets, test pieces of 30 mm in width and 180 mm in length were cut out from the rolling direction (L direction) and the transverse direction (C direction), and the iron loss W10/400 in the (L+C) direction was measured by Epstein testing. In this example, the high-frequency iron loss was determined as good in the case where the measured iron loss W10/400 was less than the iron loss reference value (W/kg) given by the following formula (1):
The measurement results are shown in Tables 7 to 9. As can be seen from these results, each non-oriented electrical steel sheet satisfying the conditions according to the present disclosure exhibited reduction in iron loss in the high frequency range. According to the present disclosure, it is possible to reduce iron loss in the high frequency range without addition of a large amount of alloying elements, which causes a decrease in magnetic flux density. The non-oriented electrical steel sheet according to the present disclosure is thus suitable for use as a material for motor cores of hybrid electric vehicles, electric vehicles, high-speed generators, air conditioner compressors, vacuum cleaners, machine tools, etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-015256 | Feb 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/001969 | 1/23/2023 | WO |
