Patent Application
20240410029
The present invention relates to a finish annealing facility that is preferably used for the production of electrical steel sheets, a finish annealing method and an electrical steel sheet production method that use this facility, and a non-oriented electrical steel sheet with excellent magnetic properties.
An electrical steel sheet, for example, a non-oriented electrical steel sheet is typically produced as follows: Steel adjusted to a predetermined ingredient composition is melted to be formed into a steel material (slab) by a method such as a continuous casting method, and then the slab is subjected to hot-rolling to form a hot-rolled sheet. The hot-rolled sheet is subjected to hot-band annealing as necessary, and then one cold rolling or two or more cold rollings with intermediate annealing between each cold rolling is performed thereon to obtain a cold-rolled sheet having a final sheet thickness (product sheet thickness). The cold-rolled sheet is then subjected to finish annealing for imparting desired magnetic properties thereto, and an insulation coating is formed thereon as necessary.
As a technique for performing annealing on a steel sheet, for example, Patent Literature 1 discloses a technique that performs annealing on a steel sheet in a continuous annealing furnace having a heating zone, a heat retention zone (soaking zone), and a cooling zone, with the atmosphere of each zone set to a gas composition containing 1 to 10 vol % hydrogen and, as the rest, 90 to 99 vol % nitrogen and unavoidable impurities.
However, the present inventors found that, when finish annealing is performed on a non-oriented electrical steel sheet in the atmosphere disclosed in Patent Literature 1, desired iron loss properties are not often stably achieved.
Therefore, the present invention aims to solve the above-described problem with the prior art, to provide a finish annealing facility that can achieve excellent iron loss properties, to propose a finish annealing method of an electrical steel sheet and a production method of the steel sheet that use the finish annealing facility, and to stably provide a non-oriented electrical steel sheet having excellent iron loss properties.
Toward the solution to the above-described problem, the present inventors vigorously conducted studies with a focus on the influence of the atmosphere inside an annealing furnace of a finish annealing facility on the magnetic properties of an electrical steel sheet. As a result, we learned that the reason why the magnetic properties were unstable in the prior art was due to the nitriding of a steel sheet that occurs during finish annealing. We found that the nitriding of a steel sheet could be inhibited by performing finish annealing in an atmosphere satisfying certain conditions, and that thereby a lower iron loss in a finish-annealed steel sheet could be stably achieved, which led to the development of the present invention.
The present invention based on these insights is as follows.
The present invention can effectively prevent nitriding of a steel sheet that occurs during finish annealing, thereby making it possible to stably produce a non-oriented electrical steel sheet with low iron loss. Thus, the present invention can stably provide a non-oriented electrical steel sheet that is suitable as a core material for motors of hybrid electric vehicles, electric vehicles, vacuum cleaners, high-speed generators, compressors of air conditioners, machine tools, and others.
First, experiments that inspired the development of the present invention will be described.
A steel material (slab) having an ingredient composition including C: 0.0021 mass %, Si: 3.7 mass %, Mn: 0.4 mass %, P: 0.01 mass %, S: 0.0016 mass %, Al: 0.6 mass %, N: 0.0022 mass %, Ti: 0.0014 mass %, Nb: 0.0010 mass %, and O: 0.0025 mass %, with the rest being Fe and unavoidable impurities was hot-rolled into a hot-rolled sheet having a sheet thickness of 1.8 mm. The hot-rolled sheet was subjected to hot-band annealing at 900° C. for 30 seconds, pickled, and then cold rolled into a cold-rolled sheet having a final sheet thickness of 0.25 mm. The cold-rolled sheet was then subjected to finish annealing at 1000° C. for 10 seconds using a horizontal continuous annealing furnace having a heating zone, a soaking zone, and a cooling zone. In the process, the furnace atmosphere in the soaking zone was set to a mixed gas atmosphere of H2:N2=20:80 as a vol % ratio (dew point: −55° C.) or an atmosphere of 100 vol % Ar gas (dew point: −55° C.).
A test specimen measuring 180 mm in length and 30 mm in width was cut out from the finish-annealed steel sheet, with the longitudinal direction in an L direction (rolling direction) and with the longitudinal direction in a C direction (perpendicular to rolling direction), and each iron loss W10/400 was measured by the Epstein test.
The measurement results of the iron loss are shown in
Further, the finish-annealed steel sheet was analyzed for a content Ns (mass %) of N present as AlN (N as AlN) in the 1/20-sheet-thickness layer and a content Nt (mass %) of N present as AlN (N as AlN) across the entire sheet thickness by an electroextraction method. A relationship between analytical values of these contents and the iron loss 10/400 is organized and shown in
The present inventors have conducted experiments to explore a method for preventing nitriding in finish annealing and mitigating the precipitation of fine nitrides AlN.
One possible method for inhibiting nitriding is to reduce the nitrogen content in the atmosphere inside the annealing furnace. However, reducing the nitrogen content in the atmosphere in the heating zone hinders the formation of a stable alumina layer on the surface of the steel sheet. As a result, nitriding of the surface layer of the steel sheet in the soaking zone is conversely promoted, which may lead to deterioration of the magnetic properties or to appearance failures, such as surface defects, due to pick-up on the rolls in the furnace being promoted. Thus, it is considered necessary to contain a certain amount of nitrogen in at least the atmosphere in the heating zone of the annealing furnace. Accordingly, in the experiments to be described below, the nitrogen content in the furnace atmosphere in the heating zone was controlled to 80 vol %.
First, the following experiment was conducted to examine the influence of a nitrogen partial pressure in the furnace atmosphere during the finish annealing on the magnetic properties.
A steel material (slab) having an ingredient composition including C: 0.0021 mass %, Si: 3.7 mass %, Mn: 0.4 mass %, P: 0.01 mass %, S: 0.0016 mass %, Al: 0.6 mass %, N: 0.0022 mass %, Ti: 0.0014 mass %, Nb: 0.0010 mass %, and O: 0.0025 mass %, with the rest being Fe and unavoidable impurities was hot-rolled into a hot-rolled sheet having a sheet thickness of 1.8 mm. The hot-rolled sheet was subjected to hot-band annealing at 900° C. for 30 seconds, pickled, and then cold-rolled into a cold-rolled sheet having a final sheet thickness of 0.25 mm. The cold-rolled sheet was then subjected to finish annealing with the furnace atmosphere in the soaking zone set to an atmosphere of a mixed gas of H2 and N2, while the content of N2 was varied within a range of 0 to 100 vol %. The dew point of the furnace atmosphere was controlled to −55° C. (constant).
A test specimen measuring 180 mm in length and 30 mm in width was cut out from the finish-annealed steel sheet, with the longitudinal direction in the L direction (rolling direction) and with the longitudinal direction in the C direction (perpendicular to rolling direction), and iron loss W10/400 was measured by the Epstein test. The results are shown in
The following experiment was conducted to examine a temperature region where nitriding occurs in finish annealing.
A steel material (slab) having an ingredient composition including C: 0.0021 mass %, Si: 3.7 mass %, Mn: 0.4 mass %, P: 0.01 mass %, S: 0.0016 mass %, Al: 0.6 mass %, N: 0.0022 mass %, Ti: 0.0014 mass %, Nb: 0.0010 mass %, and O: 0.0025 mass %, with the rest being Fe and unavoidable impurities was hot-rolled into a hot-rolled sheet having a sheet thickness of 1.8 mm. The hot-rolled sheet was subjected to hot-band annealing at 900° C. for 30 seconds, pickled, and then cold-rolled into a cold-rolled sheet having a final sheet thickness of 0.25 mm. The cold-rolled sheet was then subjected to finish annealing by holding the steel sheet at a soaking temperature for 10 seconds while varying the soaking temperature within a range of 800 to 1050° C., with the furnace atmosphere in the soaking zone set to a mixed gas atmosphere of H2:N2=20:80 as a vol % ratio (dew point: −55° C.) in which nitriding occurs.
A test specimen was cut out from each finish-annealed steel sheet, and the content Ns (mass %) of N present as AlN (N as AlN) in the 1/20-sheet-thickness layer was analyzed by an electroextraction method. The results are shown in
The following experiment was conducted to examine the influence of the dew point of the furnace atmosphere during finish annealing on the magnetic properties.
A steel material (slab) having an ingredient composition including C: 0.0021 mass %, Si: 3.7 mass %, Mn: 0.4 mass %, P: 0.01 mass %, S: 0.0016 mass %, Al: 0.6 mass %, N: 0.0022 mass %, Ti: 0.0014 mass %, Nb: 0.0010 mass %, and O: 0.0025 mass %, with the rest being Fe and unavoidable impurities was hot-rolled into a hot-rolled sheet having a sheet thickness of 1.8 mm. The hot-rolled sheet was subjected to hot-band annealing at 900° C. for 30 seconds, pickled, and then cold-rolled into a cold-rolled sheet having a final sheet thickness of 0.25 mm. The cold-rolled sheet was then subjected to finish annealing at 1000° C. for 10 seconds. In the process, the furnace atmosphere was set to 100% Ar in a region where the temperature of the furnace atmosphere was 900° C. or higher, while the dew point was varied within a range of −60 to −10° C.
A test specimen measuring 180 mm in length and 30 mm in width was cut out from each finish-annealed steel sheet, with the longitudinal direction in the L direction (rolling direction) and with the longitudinal direction in the C direction (perpendicular to rolling direction), and iron loss W10/400 was measured by the Epstein test. The results are shown in
The present inventors conducted the following experiment to examine the influence of cooling strain introduced during cooling as one of the factors other than the atmosphere that may adversely affect the iron loss properties in finish annealing.
A steel material (slab) having an ingredient composition including C: 0.0021 mass %, Si: 3.7 mass %, Mn: 0.4 mass %, P: 0.01 mass %, S: 0.0016 mass %, Al: 0.6 mass %, Ti: 0.0014 mass %, Nb: 0.0010 mass %, O: 0.0025 mass %, and N: 0.0022 mass %, with the rest being Fe and unavoidable impurities was hot-rolled into a hot-rolled sheet having a sheet thickness of 1.8 mm. The hot-rolled sheet was subjected to hot-band annealing at 900° C. for 30 seconds, pickled, and then cold-rolled into a cold-rolled sheet having a final sheet thickness of 0.25 mm. The cold-rolled sheet was then subjected to finish annealing including a soaking process at 1000° C. for 30 seconds and cooling, with the furnace atmosphere in the temperature region of 900° C. or higher set to H2:N2=20:80 (dew point: −55° C.) in which nitriding occurs. In the process, the average cooling rate from the soaking temperature of 1000° C. to 900° C. was varied within a range of 10 to 15° C./s. The reason for setting the temperature region in which the cooling rate was to be varied to the temperature region of 900° C. or higher is that the cooling rate in this temperature region is considered to have a significant influence on cooling strain.
A test specimen measuring 180 mm in length and 30 mm in width was cut out from each finish-annealed steel sheet, with the longitudinal direction in the L direction (rolling direction) and with the longitudinal direction in the C direction (perpendicular to rolling direction), and each iron loss W10/400 was measured by the Epstein test. The results are shown in
Next, the ingredient composition of the steel material (slab) used to produce the non-oriented electrical steel sheet of the present invention will be described.
It is preferable that the slab used to produce the non-oriented electrical steel sheet of the present invention contain, as basic ingredients, Si: 2.8 to 6.5 mass %, Mn: 0.1 to 2.0 mass %, and Al: 0.3 to 2.0 mass %.
Further, for the purpose of improving the magnetic properties and the mechanical properties, the slab may contain at least one element selected from: P: 0.10 mass % or less, Sn: 0.005 to 0.20 mass %, Sb: 0.005 to 0.20 mass %, Ca: 0.0005 to 0.020 mass %, Mg: 0.0005 to 0.020 mass %, REM: 0.0005 to 0.020 mass %, Cr: 0.01 to 1.0 mass %, Co: 0.01 to 1.0 mass %, Ni: 0.01 to 1.0 mass %, Cu: 0.01 to 1.0 mass %, Mo: 0.001 to 0.1 mass %, and W: 0.001 to 0.1 mass %.
C, S, N, Ti, Nb, and O are harmful elements that adversely affect the magnetic properties by forming carbonitrides to be precipitated, forming oxides, or forming sulfides. Therefore, it is preferable to limit these elements as follows: C: 0.0050 mass % or less, S: 0.0050 mass % or less, N: 0.0050 mass % or less, Ti: 0.0030 mass % or less, Nb: 0.0030 mass % or less, and O: 0.0050 mass % or less.
Next, the production method of the non-oriented electrical steel sheet of the present invention will be described.
The non-oriented electrical steel sheet of the present invention can be produced by a conventional known production process as follows: A steel material (slab) having the above-described ingredient composition is hot-rolled into a hot-rolled-sheet, and the hot-rolled sheet is subjected to hot-band annealing, if necessary, and then to one cold rolling or two or more cold rollings with intermediate annealing between each cold rolling to obtain a cold-rolled sheet having a final sheet thickness (product sheet thickness). Next, the cold-rolled sheet is subjected to finish annealing, and an insulation coating is formed thereon as necessary. In the following, the production method will be specifically described.
The slab serving as the material of the non-oriented electrical steel sheet can be produced by melting steel having the above-described ingredient composition complying with the present invention by a commonly known refining process that uses a converter, an electric furnace, a vacuum degassing device, etc., and then performing a usual continuous casting method or an ingot-making blooming method, or can be formed into a thin cast slab with a thickness of 100 mm or less by a direct casting method.
The slab is then hot-rolled into a hot-rolled sheet using a commonly known method and conditions. The slab is usually hot-rolled after being heated to a predetermined temperature in a heating furnace, but may be hot-rolled immediately after casting, without being reheated. In the case of a thin cast slab, it may be hot-rolled, or may be subjected to the subsequent steps without being hot rolling.
The hot-rolled sheet having been hot-rolled is subjected to hot-band annealing, if necessary. It is preferable to perform the hot-band annealing under soaking conditions of 800 to 1100° C. for 180 seconds or less. When the soaking temperature is lower than 800° C., the effect of hot-band annealing is too small to sufficiently achieve a magnetic property improving effect. On the other hand, when the soaking temperature exceeds 1100° C. and the soaking time exceeds 180 seconds, the crystal grains are coarsened to facilitate brittle failure (sheet fracture) during cold rolling or impair productivity, which is disadvantageous in terms of the production cost. More preferable conditions are 850 to 1000° C. for 60 seconds or less.
It is preferable to set the dew point of the atmosphere in the heating zone and the soaking zone in the hot-band annealing to 0° C. or higher but not higher than 70° C. When the dew point is lower than 0° C., an oxidized layer formed on the surface of the steel sheet during annealing is likely to be removed during pickling, so that nitriding may be promoted during finish annealing at a later stage and the magnetic properties may deteriorate. On the other hand, when the dew point exceeds 70° C., conversely, an oxidized film that is difficult to remove by pickling is formed during annealing, which hinders the pickling efficiency or increases the load of cold rolling, thereby impairing productivity.
The steel sheet after the hot rolling or hot-band annealing is then formed into a cold-rolled sheet with a final sheet thickness by one cold rolling or two or more cold rollings with intermediate annealing between each cold rolling. It is preferable that the final sheet thickness, i.e., the product sheet thickness, be 0.30 mm or less, from the viewpoint of reducing the iron loss.
The cold-rolled sheet is subjected to finish annealing, which is the most important step in the present invention, and, if necessary, an insulation coating is applied thereto to obtain a product sheet. It is preferable that the soaking temperature in this finish annealing be within a range of 900 to 1200° C. from the viewpoint of allowing the crystal grains to grow large and reducing the iron loss. When the soaking temperature is lower than 900° C., the crystal grains do not grow sufficiently, while when the soaking temperature exceeds 1200° C., the crystal grains coarsen excessively or a disadvantage in terms of the heat energy cost is caused. A more preferable range is 1000 to 1100° C. The influence of the soaking temperature on the grain growth during finish annealing is substantially predominant, while the influence of the soaking time is minor. Therefore, the soaking time can be adjusted according to the length of the annealing furnace or the productivity taken into account.
The present invention is characterized in that the continuous annealing furnace shown in
Here, it is important in the present invention that the furnace atmosphere of the continuous annealing furnace needs to be appropriately controlled from the viewpoint of preventing nitriding of the surface of the steel sheet that occurs during finish annealing. In particular, it is necessary to set the furnace atmosphere in the second region, i.e., the temperature region where the atmospheric temperature is 900° C. or higher, to an atmosphere of one type of gas among N2, H2, and a rare gas or an atmosphere of a mixed gas of two or more types of these gases, and to set the nitrogen content in the atmosphere to 30 vol % or less. For example, the atmosphere of a mixed gas of H2:N2=80:20 as a vol % ratio is preferable. Further, from the viewpoint of preventing nitriding and oxidation, the dew point of the atmosphere needs to be set to −40° C. or lower. The nitrogen content is preferably 20 vol % or less, and the dew point is preferably −50° C. or lower. Since the dew point of a gas that can be supplied as an industrial gas is about −60° C., the lower limit of the dew point is necessarily about −60° C.
In the present invention, it is preferable that, in the cooling zone at a later stage which is included as a part of the second region, the cooling rate from the soaking temperature to 900° C. be controlled to 15° C./s or less. As described above, when the cooling rate exceeds 15° C./s, the iron loss increases due to cooling strain. A more preferable cooling rate is 10° C./s or less.
Further, in the present invention, it is preferable that the first region located upstream of the second region, i.e., the region where the furnace atmosphere temperature in the heating zone is lower than 900° C., be set to have an atmosphere having a nitrogen content of 80 vol % or more and a dew point of −40° C. or lower. This is from the viewpoint of forming an alumina layer on the steel sheet surface which functions to prevent nitriding in the subsequent temperature region of 900° C. or higher (second region) or prevent pick-up on the rolls in the furnace. A more preferable nitrogen content is 90 vol % or more.
It is desirable that the heating rate of the steel sheet in the first region of the finish annealing facility of the present invention be set to 100° C./s or more. This is because, when the heating rate is 100° C./s or more, the priority of recrystallization in the {111} orientation decreases, causing the orientation of recrystallized grains to become random, which has an enhancing effect on the magnetic flux density. On the other hand, a high heating rate that is too high raises concerns such as that, as the shape of the steel sheet cannot be corrected, the magnetic properties may deteriorate due to strain, or that heating unevenness may occur in the sheet width direction, resulting in larger variation in the iron loss value in the sheet width direction. Therefore, it is preferable that the upper limit of the heating rate be about 300° C./s.
In the case where the heating rate of the steel sheet in the first region is set to 100° C./s or more, it is desirable that, as shown in
In the third region following the second region, i.e., the cooling zone in which the furnace atmosphere temperature is lower than 900° C., since there is little risk of introducing cooling strain or nitriding, the atmosphere and the cooling rate are not particularly limited, and rapid cooling may be performed in a 100% nitrogen gas atmosphere, for example.
As has been described above, the finish annealing facility of the present invention requires the annealing furnace to be divided into the first region, the second region, and the third region with the temperature of the furnace atmosphere of 900° C. as the boundaries, where the gas composition and the dew point of the atmosphere need to be individually controlled in each region. Therefore, it is preferable that the annealing furnace be divided into a plurality of segments in the forward direction of the steel sheet as shown in
Since an H2 gas has good heat transfer properties, including an H2 gas in the atmospheric gas in the second region has the effect of reducing cooling unevenness in the sheet thickness direction during cooling and thereby reducing deterioration of the magnetic properties due to cooling strain. However, since an H2 gas is an explosive gas, it is preferable to prevent the gas from flowing into the first region or the third region where the temperature is lower than 900° C. From this viewpoint, it is preferable to provide an inter-zone seal (slit) between the first region and the second region and between the second region and the third region as shown in
An example in which a finish annealing facility of the present invention was applied to the production of a non-oriented electrical steel sheet of the present invention will be described below.
Steel sheets after cold rolling (cold-rolled sheets) for non-oriented electrical steel sheets having ingredient compositions containing the ingredients shown in Table 1, with the rest being Fe and unavoidable impurities, were threaded through an annealing facility having a horizontal continuous annealing furnace composed of the heating zone, the soaking zone, and the cooling zone shown in
A test specimen was taken from each finish-annealed steel sheet thus obtained, and the content Ns (mass %) of N present as AlN (N as AlN) in the 1/20-sheet-thickness layer and the content Nt (mass %) of N present as AlN (N as AlN) across the entire sheet thickness were analyzed by an electroextraction method. Further, a test specimen measuring 180 mm in length and 30 mm in width was cut out from each finish-annealed steel sheet, with the longitudinal direction in the L direction (rolling direction) and with the longitudinal direction in the C direction (perpendicular to rolling direction), and iron loss W10/400 was measured by the Epstein test.
The measurement results are also shown in Table 1. The standard iron loss values shown in Table 1 are the upper limit values of iron loss required for each steel sheet. Since the iron loss depends largely on the sheet thickness, the required iron loss value varies according to the sheet thickness. Therefore, the standard iron loss values were calculated using the following Formula:
W
10/400 (W/kg)=8+20 t, where t is the sheet thickness (mm).
The results show that when the finish annealing facility of the present invention is applied to the production of a non-oriented electrical steel sheet and finish annealing satisfying the conditions of the present invention is performed, nitriding in finish annealing is inhibited, so that a non-oriented electrical steel sheet with low iron loss can be stably produced.
Eight coils of steel sheets after cold rolling (cold-rolled sheets) produced in Example 1 were prepared, each having a sheet thickness of 0.25 mm and an ingredient composition containing Si: 3.7 mass %, Mn: 0.4 mass %, and Al: 0.6 mass %, with the rest being Fe and unavoidable impurities. Of these coils, four coils were threaded through the annealing facility shown in
A test specimen was extracted from each finish-annealed steel sheet thus obtained, and the content Ns (mass %) of N present as AlN (N as AlN) in the 1/20-sheet-thickness layer and the content Nt (mass %) of N present as AlN (N as AlN) across the entire sheet thickness were analyzed by an electroextraction method. Further, a test specimen measuring 180 mm in length and 30 mm in width, with the longitudinal direction in the L direction (rolling direction) and with the longitudinal direction in the C direction (perpendicular to rolling direction) was cut out from each finish-annealed steel sheet, and iron loss W10/400 and magnetic flux density B50 were measured by the Epstein test.
The measurement results are also shown in Table 2. The results show that when rapid heating is performed in the heating process of finish annealing using the annealing facility having the rapid heating zone provided in the first region, the magnetic flux density can be further enhanced than when rapid heating is not performed.
The technology of the present invention can be applied to the production of not only iron core materials of drive motors of HEVs. EVs, and FCEVs but also iron core materials of motors of compressors of air conditioners, machine tools, high-speed generators, vacuum cleaners, and others.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-179145 | Nov 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/038167 | 10/13/2022 | WO |
