Non-oriented electrical steel sheet and method for manufacturing same

Abstract

Provided is a non-oriented electromagnetic steel sheet in which deterioration of grain growth properties due to fine AlN is further suppressed and which is excellent in magnetic properties. This non-oriented electromagnetic steel sheet has a prescribed component composition, and the number density of AlN in the region of a cross-section in the sheet thickness direction excluding the portion from the surface to a depth of 20 µm satisfies conditions (1) to (3). (1) The number density D of AlN having an equivalent circle diameter of 0.2 µm or more is 0.010 / µm2 or less. (2) The ratio Rf of the number density Df of AlN having an equivalent circle diameter of 0.2-1.0 µm to the number density D is 50% or less. (3) In the number density D, the ratio Rc of the number density Dc of AlN having an equivalent circle diameter not less than 5.0 µm is 5% or greater.

Description

Non-oriented electrical steel sheet and its manufacturing method

[0001] The present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same.

[0002] In recent years, due to concerns about the environment such as global warming, ２ There is a demand for reducing emissions and saving energy. In the automotive field in particular, development is underway for hybrid electric vehicles (HEVs) that use both engines and motors, electric vehicles (EVs) that are driven solely by electric motors, and fuel cell electric vehicles (FCEVs).

[0003] In order to improve motor efficiency, motors used in the above-mentioned HEVs, EVs, FCEVs, etc. are generally driven in a high frequency range that is advantageous for high-speed rotation. Therefore, there is a strong demand for low iron loss in the high frequency range for the non-oriented electrical steel sheets used as the iron core material of these motors.

[0004] Conventionally, attempts have been made to reduce iron loss in non-oriented electrical steel sheets by increasing the resistivity through the addition of alloying elements, mainly Si and Al. Among these, Al has a significant effect of increasing the resistivity of steel, and compared to Si, even when added in large amounts, it reduces ductility and toughness to a lesser extent. Therefore, Al is a useful element for achieving both magnetic properties and manufacturability.

[0005] On the other hand, the added Al forms fine AlN in the steel, which inhibits grain growth during annealing. If grain growth is inhibited, the magnetic properties of the final non-oriented electrical steel sheet will deteriorate, so methods for preventing the deterioration of grain growth due to AlN are being investigated.

[0006] For example, Patent Document 1 discloses a technique for suppressing the precipitation of fine AlN by lowering the hot rolling temperature.

[0007] Japanese Patent Application Publication No. 6-279859

[0008] However, the method proposed in Patent Document 1 alone is still insufficient to address the deterioration of grain growth. In order to further improve the magnetic properties of non-oriented electrical steel sheets, a technology is needed that can further suppress the deterioration of grain growth caused by fine AlN.

[0009] The present invention has been developed in view of the above circumstances, and has an object to provide a non-oriented electrical steel sheet that further suppresses the deterioration of grain growth caused by fine AlN and has excellent magnetic properties.

[0010] An example of an experiment conducted by the present inventors to solve the above problems will be described below. In the following description, "%" as a unit of content represents "% by mass" unless otherwise specified.

[0011] (Experiment) To evaluate the effect of AlN on iron loss, the inventors prepared non-oriented electrical steel sheets with different Al contents and evaluated their iron loss. Specifically, molten steel was first subjected to secondary refining at 1560°C and then continuously cast to produce steel slabs with a thickness of 230 mm. In the secondary refining, the Al content of the steel slabs was varied within a range of 0.2 to 2.0% by changing the amount of Al added. The contents of elements other than Al were as follows: C: 0.002%, Si: 3.0%, Mn: 0.5%, P: 0.01%, S: 0.002%, N: 0.004%, Ti: 0.001%, B: 0.0001%, and O: 0.001%. The balance consisted of Fe and unavoidable impurities. The elapsed time from the addition of Al to the start of continuous casting was 350 seconds.

[0012] Next, the steel slab was held at 1000°C for 200 seconds and then heated to 1150°C. The heated steel slab was hot-rolled to obtain a hot-rolled steel sheet. Thereafter, the hot-rolled steel sheet was annealed at 980°C for 30 seconds to obtain a hot-rolled annealed sheet.

[0013] The hot-rolled and annealed steel sheet was then cold-rolled to a thickness of 0.25 mm. The cold-rolled steel sheet was then subjected to final annealing to obtain a non-oriented electrical steel sheet. ２ :N ２ The annealing was carried out in a dry atmosphere with a ratio of 0.01 to 0.75, at an annealing temperature of 960° C. for an annealing time of 10 seconds.

[0014] An Epstein sample having a width of 30 mm and a length of 280 mm was cut out from the rolling direction and the width direction of the obtained non-oriented electrical steel sheet, and the iron loss W １５／５０ was measured using an Epstein tester.

[0015] Figure 1 shows the relationship between Al content and iron loss W １５／５０ 1 is a graph showing the relationship between the Al content and the iron loss. As can be seen from Fig. 1, by setting the Al content to 1.0% or more, it is possible to effectively reduce the iron loss.

[0016] Furthermore, the number density of AlN in the cross section of the non-oriented electrical steel sheet was measured in a region excluding the portion from the surface to a depth of 20 μm, and the following D and Dc were obtained. The measurements were performed using the method described in the Examples below. - Number density D of AlN having an equivalent circle diameter of 0.2 μm or more - Number density Dc of AlN having an equivalent circle diameter of 5.0 μm or more

[0017] Next, from the obtained D and Dc, the proportion Rc (%) of the number density Dc of AlN particles having a circle equivalent diameter of 5.0 μm or more was calculated.

[0018] Fig. 2 is a graph showing the relationship between Al content and Rc. As can be seen from Fig. 2, steel sheets with an Al content of 1.0% or more have a significantly higher Rc than steel sheets with an Al content of less than 1.0%.

[0019] From the above results, it is clear that iron loss can be reduced by increasing the proportion Rc of relatively coarse AlN, and that in order to achieve this, the Al content must be 1.0% or more.

[0020] Based on the above results, the inventors further investigated the influence of AlN and found that by controlling the conditions of secondary refining, the amount of fine AlN, which is a cause of deterioration in grain growth, is reduced, and the amount of coarse AlN, which is harmless to grain growth, is increased.

[0021] The present invention has been completed based on the above findings, and the gist and configuration of the present invention are as follows.

[0022] 1. A non-oriented electrical steel sheet having a chemical composition, by mass%, of C: 0.010% or less, Si: 1.0 to 5.0%, Mn: 0.05 to 5.0%, P: 0.10% or less, S: 0.010% or less, Al: 1.0 to 3.0%, N: 0.010% or less, Ti: 0.010% or less, B: 0.0010% or less, and O: 0.005% or less, with the balance being Fe and unavoidable impurities, wherein the number density of AlN in a region of a cross section in the sheet thickness direction excluding a portion from the surface to a depth of 20 μm satisfies the following conditions (1) to (3): (1) The number density D of AlN having an equivalent circle diameter of 0.2 μm or more is 0.010 particles / μm. ２ (2) In the number density D, the proportion Rf of the number density Df of AlN particles having an equivalent circle diameter of 0.2 to 1.0 μm is 50% or less. (3) In the number density D, the proportion Rc of the number density Dc of AlN particles having an equivalent circle diameter of 5.0 μm or more is 5% or more.

[0023] 2. The non-oriented electrical steel sheet according to 1 above, wherein the chemical composition further includes, in mass%, at least one selected from the group consisting of Sn: 0.001 to 0.20%, Sb: 0.001 to 0.20%, Ca: 0.0001 to 0.10%, Mg: 0.0001 to 0.10%, REM: 0.0001 to 0.10%, Mo: 0.002 to 0.20%, Zn: 0.0005 to 0.0050%, Ni: 0.01 to 1.0%, Cr: 0.05 to 5.0%, and Cu: 0.005 to 1.0%.

[0024] 3. The non-oriented electrical steel sheet according to 1 or 2 above, wherein the chemical composition further includes, in mass%, at least one selected from the group consisting of V: ​​0.001 to 0.050%, Nb: 0.001 to 0.005%, Ta: 0.0001 to 0.0020%, W: 0.001 to 0.050%, Pb: 0.0001 to 0.0020%, Co: 0.001 to 0.100%, Ga: 0.0005 to 0.0300%, Ge: 0.0005 to 0.0300%, and As: 0.001 to 0.020%.

[0025] 4. A method for producing the non-oriented electrical steel sheet according to any one of 1 to 3 above, comprising: a slab production step of producing a slab having the chemical composition by secondary refining molten steel at a molten steel temperature of 1520°C or higher and then continuously casting it, a slab heating step of holding the slab at a holding temperature of 950 to 1050°C for a holding time of 180 seconds or more and then heating it to a maximum temperature of 1100°C or higher, a hot rolling step of hot-rolling the heated slab to form a hot-rolled steel sheet, an annealing step of annealing the hot-rolled steel sheet to form a hot-rolled annealed sheet, a cold rolling step of cold-rolling the hot-rolled annealed sheet to form a cold-rolled steel sheet, and a final annealing step of final annealing the cold-rolled steel sheet, wherein Al is added to the molten steel in the secondary refining, and the elapsed time from the addition of Al to the start of the continuous casting is 300 seconds or more.

[0026] According to the present invention, it is possible to provide a non-oriented electrical steel sheet that is excellent in magnetic properties and that further suppresses the deterioration of grain growth caused by fine AlN. The non-oriented electrical steel sheet of the present invention has reduced iron loss, particularly in the high frequency range, and is extremely suitable for use as an iron core material for motors such as those for electric vehicles.

[0027] Furthermore, when manufacturing an iron core using a non-oriented electrical steel sheet, it is common to punch the non-oriented electrical steel sheet into a desired shape and then perform stress relief annealing to release the strain caused by punching. In the chemical composition of the non-oriented electrical steel sheet of the present invention, the re-solution temperature of the precipitated AlN is very high, exceeding 1000°C, so even if stress relief annealing is performed at a general temperature (700 to 800°C), the above-mentioned effects of the present invention are not impaired.

[0028] Al content and iron loss W １５／５０ 1 is a graph showing the relationship between Al content and Rc.

[0029] The present invention will be described in detail below. Note that the following description is of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below.

[0030] [Non-oriented electrical steel sheet] A non-oriented electrical steel sheet according to one embodiment of the present invention has a predetermined component composition, and in addition, the number density of AlN is controlled to satisfy specific conditions.

[0031] (Composition) First, the reasons for limiting the composition of the non-oriented electrical steel sheet of the present invention will be explained. As will be described later, when producing the non-oriented electrical steel sheet of the present invention, a steel slab having this composition may be used.

[0032] C: 0.010% or less C is an element that forms carbides and adversely affects iron loss characteristics. In particular, if the C content exceeds 0.010%, the adverse effects become significant. Therefore, the C content is set to 0.010% or less, preferably 0.004% or less. On the other hand, the lower limit of the C content is not particularly limited and may be 0%. However, excessive reduction will increase costs. Therefore, the C content is preferably set to 0.0001% or more.

[0033] Si: 1.0 to 5.0% Si is an element that has the effect of increasing the resistivity of steel and reducing iron loss. Si also has the effect of increasing the strength of steel through solid solution strengthening. To achieve these effects, the Si content is set to 1.0% or more, preferably 1.5% or more, and more preferably 2.0% or more. On the other hand, if the Si content exceeds 5.0%, the magnetic flux density decreases significantly due to a decrease in saturation magnetic flux density. In addition, the toughness decreases, increasing the possibility of fracture during the cold rolling process. Therefore, the Si content is set to 5.0% or less, preferably 4.5% or less, and more preferably 4.0% or less.

[0034] Mn: 0.05 to 5.0% Like Si, Mn is an element that is effective in increasing the resistivity and strength of steel. To achieve this effect, the Mn content is set to 0.05% or more, preferably 0.1% or more. On the other hand, if the Mn content exceeds 5.0%, the precipitation of MnC is promoted, resulting in deterioration of magnetic properties. Therefore, the Mn content is set to 5.0% or less, preferably 3.0% or less.

[0035] P: 0.10% or less P is an element used to adjust the strength (hardness) of steel. However, if the P content exceeds 0.10%, toughness decreases and cracks tend to occur during processing. Therefore, the P content is set to 0.10% or less, preferably 0.08% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0%. However, since excessive reduction increases costs, the P content is preferably set to 0.001% or more, and more preferably 0.003% or more.

[0036] S: 0.010% or less S is an element that forms fine precipitates and adversely affects iron loss characteristics. In particular, if the S content exceeds 0.01%, the adverse effects become significant. Therefore, the S content is set to 0.010% or less, preferably 0.008% or less, and more preferably 0.005% or less. On the other hand, the lower limit of the S content is not particularly limited and may be 0%. However, since excessive reduction increases costs, the S content is preferably set to 0.0001% or more, and more preferably 0.0003% or more.

[0037] Al: 1.0 to 3.0% Like Si, Al is an element that increases the resistivity of steel and reduces iron loss. However, when fine AlN precipitates in steel, grain growth deteriorates and magnetic properties deteriorate. To suppress the adverse effects of this fine AlN, the Al content must be 1.0% or more. This is because, as shown in Experiment 1 above, an Al content of 1.0% or more increases the proportion of coarse AlN. Therefore, the Al content is set to 1.0% or more, preferably 1.5% or more. On the other hand, an Al content exceeding 3.0% promotes nitriding of the steel sheet surface, resulting in deterioration of magnetic properties. Therefore, the Al content is set to 3.0% or less.

[0038] N: 0.010% or less N is an element that forms fine precipitates and adversely affects iron loss characteristics. In particular, if the N content exceeds 0.010%, the adverse effects become significant. Therefore, the N content is set to 0.010% or less, preferably 0.008% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, from the viewpoint of facilitating the precipitation of coarse AlN, the N content is preferably set to 0.001% or more, more preferably 0.003% or more.

[0039] Ti: 0.010% or less Ti is an element that reacts with N in steel to form nitrides. Adding Ti can further suppress the precipitation of fine AlN. However, if the Ti content exceeds 0.010%, the increase in Ti precipitates actually worsens grain growth. Therefore, the Ti content is set to 0.010% or less. On the other hand, the lower limit of the Ti content is not particularly limited and may be 0%. However, in order to enhance the effect of adding Ti, it is preferable that the Ti content be 0.001% or more.

[0040] B: 0.0010% or less B is an element that reacts with N in steel to form nitrides. Adding B can further suppress the precipitation of fine AlN. However, if the B content exceeds 0.0010%, the increase in B precipitates actually worsens grain growth. Therefore, the B content is set to 0.0010% or less. On the other hand, the lower limit of the B content is not particularly limited and may be 0%. However, in order to enhance the effect of adding B, it is preferable that the B content be 0.0001% or more.

[0041] O: 0.005% or less O is an element that forms inclusions in molten steel and adversely affects iron loss characteristics. In particular, if the O content exceeds 0.005%, the adverse effects become significant. Therefore, the O content is set to 0.005% or less, preferably 0.0030% or less. On the other hand, the lower limit of the O content is not particularly limited and may be 0%. However, since excessive reduction increases costs, the O content is preferably set to 0.0005% or more, and more preferably 0.0008% or more.

[0042] The chemical composition of a non-oriented electrical steel sheet according to one embodiment of the present invention contains the above elements, with the balance being Fe and unavoidable impurities.

[0043] The above-mentioned composition may further optionally contain at least one selected from the group consisting of Sn, Sb, Ca, Mg, REM, Mo, Zn, Ni, Cr, and Cu in the following amounts:

[0044] Sn: 0.001 to 0.20% Sn is an element that is effective in further improving magnetic flux density and reducing iron loss by improving texture. When Sn is added, the Sn content is set to 0.001% or more to obtain this effect. On the other hand, if the Sn content exceeds 0.20%, the effect saturates and costs increase unnecessarily. Therefore, the Sn content is set to 0.20% or less.

[0045] Sb: 0.001 to 0.20% Sb is an element that is effective in further improving magnetic flux density and reducing iron loss by improving texture. When Sb is added, the Sb content is set to 0.001% or more to obtain this effect. On the other hand, if the Sb content exceeds 0.20%, the effect saturates and costs increase unnecessarily. Therefore, the Sb content is set to 0.20% or less.

[0046] Ca: 0.0001 to 0.10% Ca is an element that fixes S as sulfides and contributes to further reducing iron loss. When Ca is added, the Ca content is set to 0.0001% or more to obtain the above effect. On the other hand, if the Ca content exceeds 0.10%, the effect saturates and costs increase unnecessarily. Therefore, the Ca content is set to 0.10% or less.

[0047] Mg: 0.0001 to 0.10% Mg is an element that fixes S as sulfides and contributes to further reducing iron loss. When Mg is added, the Mg content is set to 0.0001% or more to obtain the above-mentioned effect. On the other hand, if the Mg content exceeds 0.10%, the effect saturates and costs increase unnecessarily. Therefore, the Mg content is set to 0.10% or less.

[0048] REM: 0.0001 to 0.10% REM (rare earth metals) are a group of elements that fix S as sulfides and contribute to further iron loss reduction. When REM is added, the REM content is set to 0.0001% or more to obtain the above-mentioned effect. On the other hand, if the REM content exceeds 0.10%, the effect saturates and costs increase unnecessarily. Therefore, the REM content is set to 0.10% or less.

[0049] Mo: 0.002 to 0.20% Mo has the effect of forming fine carbides in steel and increasing the strength of the steel sheet. When Mo is added, the Mo content is set to 0.01% or more to obtain this effect. On the other hand, if the Mo content exceeds 0.20%, excessive carbides are formed, which actually deteriorates the iron loss. Therefore, the Mo content is set to 0.20%.

[0050] Zn: 0.0005 to 0.0050% Zn is an element that is effective in further improving magnetic flux density and reducing iron loss by improving texture. When Zn is added, the Zn content is set to 0.0005% or more to obtain this effect. On the other hand, if the Zn content exceeds 0.0050%, the effect saturates and costs increase unnecessarily. Therefore, the Zn content is set to 0.0050% or less.

[0051] Ni: 0.01 to 1.0% Ni is an element that improves the toughness of steel and can be added as desired. When Ni is added, the Ni content is set to 0.01% or more in order to obtain the above-mentioned effect. On the other hand, if the Ni content exceeds 1.0%, the effect saturates. Therefore, the Ni content is set to 1.0% or less.

[0052] Cr: 0.05 to 5.0% Cr is an element that has the effect of increasing the resistivity of steel and further reducing iron loss. When Cr is added, the Cr content is set to 0.05% or more to obtain this effect. On the other hand, if the Cr content exceeds 5.0%, the magnetic flux density decreases significantly due to a decrease in the saturation magnetic flux density. Therefore, the Cr content is set to 5.0% or less.

[0053] Cu: 0.005 to 1.0% Cu is an element that improves the toughness of steel and can be added as desired. When Cu is added, the Cu content is set to 0.005% or more in order to obtain the above-mentioned effect. On the other hand, if the Cu content exceeds 1.0%, the effect saturates. Therefore, the Cu content is set to 1.0% or less.

[0054] The above-mentioned composition may further optionally contain at least one element selected from the group consisting of V, Nb, Ta, W, Pb, Co, Ga, Ge, and As in the following amounts:

[0055] V: 0.001 to 0.050% V is an element that has the effect of increasing the strength of steel sheet and can be added as desired. When V is added, the V content is set to 0.001% or more to obtain this effect. However, if the V content exceeds 0.050%, fine precipitates will form in the steel sheet, increasing iron loss. Therefore, the V content is set to 0.050% or less.

[0056] Nb: 0.001 to 0.005% Nb is an element that has the effect of increasing the strength of steel sheet and can be added as desired. When Nb is added, the Nb content is set to 0.001% or more to obtain this effect. However, if the Nb content exceeds 0.005%, fine precipitates will form in the steel sheet, increasing iron loss. Therefore, the Nb content is set to 0.005% or less.

[0057] Ta: 0.0001 to 0.0020% Ta is an element that has the effect of increasing the strength of steel sheet and can be added as desired. When Ta is added, the Ta content is set to 0.0001% or more to obtain this effect. However, if the Ta content exceeds 0.0020%, fine precipitates will form in the steel sheet, increasing iron loss. Therefore, the Ta content is set to 0.0020% or less.

[0058] W: 0.001 to 0.050% W is an element that has the effect of increasing the strength of steel sheet and can be added as desired. When W is added, the W content is set to 0.001% or more to obtain this effect. However, if the W content exceeds 0.050%, fine precipitates will form in the steel sheet, increasing iron loss. Therefore, the W content is set to 0.050% or less.

[0059] Pb: 0.0001 to 0.0020% Pb is an element that has the effect of increasing the strength of steel sheet and can be added as desired. When Pb is added, the Pb content is set to 0.0001% or more to obtain this effect. However, if the Pb content exceeds 0.0020%, fine precipitates will form in the steel sheet, increasing iron loss. Therefore, the Pb content is set to 0.0020% or less.

[0060] Co: 0.001 to 0.100% Co is an element that has the effect of increasing the magnetic flux density of the steel sheet and can be added as desired. When Co is added, the Co content is set to 0.001% or more to obtain this effect. However, increasing the Co content too much increases the alloy cost. Therefore, the Co content is set to 0.100% or less.

[0061] Ga: 0.0005 to 0.0300% Ga is an element that has the effect of improving the texture of the steel sheet and increasing the magnetic flux density, and can be added as desired. When Ga is added, the Ga content is set to 0.0005% or more to obtain this effect. However, adding a large amount of Ga saturates the effect and increases the alloy cost. Therefore, the Ga content is set to 0.0300% or less.

[0062] Ge: 0.0005 to 0.0300% Ge is an element that has the effect of improving the texture of the steel sheet and increasing the magnetic flux density, and can be added as desired. When Ge is added, the Ge content is set to 0.0005% or more to obtain this effect. However, adding a large amount of Ge saturates the effect and increases the alloy cost. Therefore, the Ge content is set to 0.0300% or less.

[0063] As: 0.001 to 0.020% As is an element that has the effect of increasing the strength of steel sheet and can be added as desired. When As is added, the As content is set to 0.001% or more to obtain this effect. However, if the As content exceeds 0.020%, the risk of fracture during cold rolling increases. Therefore, the As content is set to 0.020% or less.

[0064] (Number Density of AlN) Next, the number density of AlN in the non-oriented electrical steel sheet of the present invention will be described. In the non-oriented electrical steel sheet of the present invention, the number density of AlN in the region of the cross section in the sheet thickness direction, excluding the portion from the surface to a depth of 20 μm, must satisfy the following three conditions: (1) The number density D of AlN having a circle equivalent diameter of 0.2 μm or more is 0.010 particles / μm ２ (2) In the number density D, the proportion Rf of the number density Df of AlN particles having an equivalent circle diameter of 0.2 to 1.0 μm is 50% or less. (3) In the number density D, the proportion Rc of the number density Dc of AlN particles having an equivalent circle diameter of 5.0 μm or more is 5% or more.

[0065] The reasons for limiting the above conditions are explained below. Note that AlN precipitated by nitriding during the annealing process is present in the surface layer of the non-oriented electrical steel sheet, particularly in the portion from the surface of the steel sheet to a depth of 20 μm. However, the AlN present in such a surface layer has almost no effect on grain growth. Therefore, in the present invention, the number density of AlN in the region excluding the portion from the surface to a depth of 20 μm in the cross section of the non-oriented electrical steel sheet in the sheet thickness direction is used as the "number density of AlN."

[0066] (1) D: 0.010 pieces / μm ２ Hereinafter, AlN in steel sheets is a harmful substance that pins grain boundaries and deteriorates grain growth. ２ If the number density D exceeds 0.010 pieces / μm, the magnetic properties of the non-oriented electrical steel sheet will deteriorate due to the above effect. ２ Preferably 0.006 particles / μm or less ２ The lower limit of the number density D is not particularly limited, and is 0 particles / μm ２ It may be.

[0067] The reason why the number density of AlN particles having an equivalent circle diameter of 0.2 μm or more is specified is because it is difficult to measure the number density of AlN particles having an equivalent circle diameter of less than 0.2 μm. On the other hand, in non-oriented electrical steel sheets manufactured by the method disclosed in this specification, there is almost no AlN particles having an equivalent circle diameter of more than 10.0 μm. Therefore, there is no problem in using the number density D of AlN particles having an equivalent circle diameter of 0.2 to 10.0 μm.

[0068] (2) Rf: 50% or less. Given the same amount of AlN precipitated, the smaller the size of AlN, the greater the grain boundary pinning effect. Therefore, by lowering the proportion of the number density D of AlN with a relatively small equivalent circle diameter, the adverse effects on grain growth can be reduced and the magnetic properties of the non-oriented electrical steel sheet can be improved. Specifically, the proportion Rf of the number density Df of AlN with an equivalent circle diameter of 0.2 to 1.0 μm is set to 50% or less, preferably 40% or less. The lower limit of Rf is not limited and may be 0%. Rf can be calculated using the following formula: Rf (%) = (Df / D) × 100

[0069] (3) Rc: 5% or more. For the same amount of precipitates, the larger the size of the precipitates, the smaller the grain boundary pinning effect. Therefore, by reducing the proportion of the number density D of AlN particles with a relatively large equivalent circle diameter, the adverse effects on grain growth can be reduced and the magnetic properties of the non-oriented electrical steel sheet can be improved. Specifically, the proportion Rc of the number density D of AlN particles with an equivalent circle diameter of 5.0 μm or more is set to 5% or more, preferably 10% or more. The upper limit of Rc is not particularly limited, but can be set to, for example, 25% or less. Rc can be calculated using the following formula: Rc (%) = (Dc / D) × 100

[0070] As described above, in the non-oriented electrical steel sheet manufactured by the method disclosed in this specification, there is almost no AlN with an equivalent circle diameter exceeding 10.0 μm, and therefore there is no problem in using the number density Dc of AlN with an equivalent circle diameter of 5.0 to 10.0 μm.

[0071] [Manufacturing Method] Next, a method for manufacturing a non-oriented electrical steel sheet according to one embodiment of the present invention will be described. The non-oriented electrical steel sheet of the present invention can be manufactured by sequentially carrying out the following steps (1) to (6). Each step will be specifically described below. (1) Slab manufacturing step (2) Slab heating step (3) Hot rolling step (4) Annealing step (5) Cold rolling step (6) Final annealing step

[0072] (1) Slab Manufacturing Process First, a slab (steel slab) having the above-described chemical composition is manufactured. Specifically, molten steel is subjected to secondary refining and then continuously cast into a slab. Secondary refining is a process for removing impurity elements from molten steel and adding alloying elements. In the present invention, Al is added to molten steel in the secondary refining so that the Al content in the final non-oriented electrical steel sheet falls within the above-described range.

[0073] The molten steel to be subjected to the secondary refining is not particularly limited and can be produced by any method. Typically, molten steel produced in a converter is used, but it can also be produced in an electric furnace using scrap as the iron source.

[0074] In the present invention, in order to control the number density of AlN in the finally obtained non-oriented electrical steel sheet, it is important to control the conditions of the slab manufacturing process as follows.

[0075] - Time elapsed from addition of Al to start of continuous casting: 300 seconds or more According to the studies of the present inventors, by setting the time from addition of Al in secondary refining to start of continuous casting to 300 seconds or more, the number of coarse AlN particles having a size of 5.0 μm or more precipitated in the molten steel can be increased. Therefore, the elapsed time is set to 300 seconds or more. On the other hand, there is no particular upper limit to the elapsed time. However, since an excessively long time leads to a decrease in productivity, it is preferable to set it to 400 seconds or less, and more preferably to set it to 350 seconds or less.

[0076] Molten steel temperature: 1520°C or higher According to the studies of the present inventors, in order to precipitate coarse AlN during secondary refining, it is necessary to raise the molten steel temperature to a sufficiently high temperature during secondary refining. Therefore, the molten steel temperature during secondary refining is set to 1520°C or higher, preferably 1540°C or higher. On the other hand, although there are no particular restrictions on the upper limit of the molten steel temperature, if the molten steel temperature is excessively high, AlN will partially dissolve and precipitate as fine AlN in a subsequent process, which will deteriorate grain growth, so the upper limit is set to 1600°C. Preferably, the upper limit is 1540°C or higher and 1580°C or lower.

[0077] The method for continuously casting molten steel into a slab is not particularly limited, and can be carried out in accordance with a conventional method.

[0078] (2) Slab Heating Step Next, prior to hot rolling, the slab is heated. Specifically, the slab is held at a holding temperature of 950 to 1050°C for a holding time of 180 seconds or more, and then heated to a maximum temperature of 1100°C or more. The reasons for limiting these conditions are explained below.

[0079] Holding temperature: 950-1050°C Holding time: 180 seconds or more According to the inventors' research, holding the slab at a specific temperature during the slab heating process can suppress the precipitation of fine AlN. This is because the nitrogen dissolved in the steel precipitates as TiN or BN during the holding process, suppressing the precipitation of the dissolved nitrogen as fine AlN in subsequent processes. To achieve this effect, the holding temperature is set to 950°C or higher, preferably 975°C or higher. Furthermore, the holding temperature is set to 1050°C or lower, preferably 1025°C or lower. Similarly, to achieve this effect, the holding time is set to 180 seconds or longer. While there is no particular upper limit to the holding time, an excessively long holding time reduces productivity. Therefore, the holding time is preferably set to 250 seconds or less.

[0080] - Maximum temperature: 1100°C or higher If the maximum temperature in the slab heating step is low, the load on the rolling rolls in the subsequent hot rolling step will increase, reducing manufacturability. Therefore, the maximum temperature is set to 1100°C or higher. On the other hand, there is no particular upper limit to the maximum temperature, but if it is too high, costs will increase. Therefore, it is preferable that the maximum temperature be 1200°C or lower.

[0081] (3) Hot Rolling Step Next, the heated slab is hot rolled to obtain a hot rolled steel sheet. The hot rolling conditions are not particularly limited, and the hot rolling may be carried out in accordance with a conventional method.

[0082] (4) Annealing Step Next, the hot-rolled steel sheet is annealed to obtain a hot-rolled annealed sheet. Such annealing of a hot-rolled steel sheet is generally called hot-rolled sheet annealing. By carrying out the annealing, the hot-rolled sheet is normalized. The conditions for the annealing are not particularly limited, but it is preferable to anneal under conditions of 900°C or higher and 1050°C or lower.

[0083] After the annealing, the hot-rolled and annealed sheet may be pickled prior to the subsequent cold rolling. The pickling can be carried out in accordance with a conventional method.

[0084] (5) Cold Rolling Step Next, the hot-rolled annealed sheet is cold-rolled to obtain a cold-rolled steel sheet. The conditions for the cold rolling are not particularly limited, and may be performed according to a conventional method. Typically, the cold rolling can be performed using a tandem mill. The number of stands in the tandem mill is not particularly limited, but is preferably 3 or more, and more preferably 4 or more. The number of stands is preferably 7 or less, and more preferably 6 or less. Furthermore, the reduction in the cold rolling is not particularly limited, but it is preferable that the total reduction is 80% or more and less than 95%.

[0085] (6) Final annealing step: The cold-rolled steel sheet is then final-annealed to produce a non-oriented electrical steel sheet. The conditions for the final annealing are not particularly limited, and the final annealing may be performed according to a conventional method. For example, the cold-rolled steel sheet may be heated to an annealing temperature in a non-oxidizing atmosphere and held at the heating temperature for a certain period of time. The annealing temperature is preferably, for example, 800 to 1200°C. The time for holding at the heating temperature (holding time) is preferably 5 to 60 seconds. After the holding time has elapsed, the steel sheet may be cooled by any method.

[0086] After the final annealing, an insulating coating may be applied to the surface of the obtained non-oriented electrical steel sheet. The insulating coating is not particularly limited and can be formed according to a conventional method.

[0087] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0088] [Production of Non-oriented Electrical Steel Sheets] Non-oriented electrical steel sheets having the chemical compositions shown in Tables 1 and 2 were produced by the following procedure.

[0089] First, the molten steel was subjected to secondary refining and then continuously cast to produce a slab having the above-mentioned composition and having a thickness of 230 mm. The molten steel temperature in the secondary refining and the elapsed time from the addition of Al to the start of the continuous casting were as shown in Tables 3 and 4.

[0090] Next, the slab was heated under the conditions shown in Tables 3 and 4, and then hot-rolled to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was annealed by a known method in which it was held at 950°C for 30 seconds, to obtain a hot-rolled annealed sheet.

[0091] The hot-rolled and annealed steel sheets were pickled and then cold-rolled to obtain cold-rolled steel sheets having a thickness of 0.25 mm. The cold-rolling was carried out at room temperature using a tandem mill.

[0092] The cold-rolled steel sheet was subjected to final annealing and then an insulating coating was applied to obtain a final non-oriented electrical steel sheet. The final annealing was performed by a known method of holding the steel sheet at 1000°C for 10 seconds in a non-oxidizing atmosphere. The insulating coating was also performed by a common method.

[0093] [Number Density of AlN] Next, the number density of AlN was measured for each of the obtained non-oriented electrical steel sheets, and D, Rf, and Rc were determined. Specifically, first, a test piece for observing precipitates was taken from the center of the sheet width of the non-oriented electrical steel sheet. Next, the taken test piece was embedded in resin so that the cross section in the sheet thickness direction became the observation surface. Thereafter, the cross section was observed in a 1.0 mm wide section by scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX). ２ The above range was observed, and the number density of AlN was measured in the region excluding the portion from the surface to a depth of 20 μm. The following three values ​​were obtained as the number density: D, the number density of AlN particles having an equivalent circle diameter of 0.2 μm or more; Df, the number density of AlN particles having an equivalent circle diameter of 0.2 to 1.0 μm; Dc, the number density of AlN particles having an equivalent circle diameter of 5.0 μm or more.

[0094] From the obtained D and Df, the proportion Rf (%) of the number density Df of AlN particles having an equivalent circle diameter of 0.2 to 1.0 μm was calculated. Similarly, from the obtained D and Dc, the proportion Rc (%) of the number density Dc of AlN particles having an equivalent circle diameter of 5.0 μm or more was calculated. The obtained D, Rf, and Rc are shown in Tables 5 and 6.

[0095] [Magnetic Properties] Next, the iron loss of the obtained non-oriented electrical steel sheet was measured to evaluate its magnetic properties. Specifically, a test piece for magnetic measurement, 30 mm wide and 280 mm long, with the length direction being the rolling direction and the direction perpendicular to the rolling direction, was taken from the non-oriented electrical steel sheet. Next, the iron loss W １０／４００ The measurement was performed by the Epstein method in accordance with JIS C2550-1:2011.

[0096] The measurement results are shown in Tables 5 and 6. １０／４００ If the magnetic flux density was 12.5 W / kg or less, the non-oriented electrical steel sheet was judged to have excellent magnetic properties. Note that, for Nos. 28 and 36, the properties could not be evaluated because they were broken during the cold rolling process.

[0097] As can be seen from the results in Tables 5 and 6, all of the non-oriented electrical steel sheets that satisfied the conditions of the present invention had excellent magnetic properties. In contrast, the non-oriented electrical steel sheets that did not satisfy the conditions of the present invention had poor magnetic properties.

[0098]

[0099]

[0100]

[0101]

[0102]

[0103]

Claims

1. In mass percent, C: 0.010% or less, Si: 1.0-5.0%, Mn: 0.05-5.0%, P: 0.10% or less, S: 0.010% or less, Al: 1.0-3.0%, N: 0.010% or less, Ti: 0.010% or less, B: 0.0010% or less, and O: Includes 0.005% or less. It has a component composition consisting of the remainder being Fe and unavoidable impurities. A non-oriented electrical steel sheet in which the number density of AlN in the region of the cross-section in the thickness direction, excluding the portion from the surface to a depth of 20 μm, satisfies the following conditions (1) to (3). (1) The number density D of AlN with an equivalent circle diameter of 0.2 μm or more is 0.010 particles / μm ２ below. (2) Of the number density D, the proportion Rf of the number density Df of AlN having an equivalent circle diameter of 0.2 to 1.0 μm is 50% or less. (3) Of the number density D, the proportion Rc of AlN having an equivalent circle diameter of 5.0 μm or more in number density Dc is 5% or more.

2. The above component composition is, in mass%, Sn: 0.001-0.20%, Sb: 0.001 to 0.20%, Ca: 0.0001-0.10%, Mg: 0.0001-0.10%, REM: 0.0001-0.10%, Mo: 0.002-0.20%, Zn: 0.0005-0.0050%, Ni: 0.01 to 1.0%, Cr: 0.05–5.0%, and Cu: 0.005-1.0%, The non-oriented electrical steel sheet according to claim 1, further comprising at least one selected from the group consisting of the following.

3. The above component composition is, in mass%, V: 0.001-0.050%, Nb: 0.001 to 0.005%, Ta: 0.0001 to 0.0020%, W: 0.001-0.050%, Pb: 0.0001 to 0.0020%, Co: 0.001 to 0.100%, Ga: 0.0005-0.0300%, Ge: 0.0005–0.0300%, and As: 0.001 to 0.020%, The non-oriented electrical steel sheet according to claim 1 or 2, further comprising at least one selected from the group consisting of the following.

4. A method for manufacturing a non-oriented electrical steel sheet according to claim 1 or 2, A slab manufacturing process for producing a slab having the above-mentioned component composition by second-stage refining of molten steel at a molten steel temperature of 1520°C or higher, followed by continuous casting. The slab is heated in a slab heating step, which involves holding the slab at a holding temperature of 950 to 1050°C for a holding time of 180 seconds or more, and then heating it to a maximum temperature of 1100°C or higher. A hot rolling process in which the heated slab is hot-rolled to produce a hot-rolled steel sheet, Annealing process to obtain a hot-rolled annealed sheet by annealing the aforementioned hot-rolled steel sheet, A cold rolling process in which the hot-rolled annealed sheet is cold-rolled to produce a cold-rolled steel sheet, and This includes a final annealing step for performing final annealing on the cold-rolled steel sheet, In the aforementioned secondary refining, Al is added to the molten steel. A method for manufacturing non-oriented electrical steel sheets, wherein the elapsed time from the addition of Al to the start of continuous casting is 300 seconds or more.

5. A method for manufacturing a non-oriented electrical steel sheet according to Claim 3, A slab manufacturing process for producing a slab having the above-mentioned component composition by second-stage refining of molten steel at a molten steel temperature of 1520°C or higher, followed by continuous casting. The slab is heated in a slab heating step, which involves holding the slab at a holding temperature of 950 to 1050°C for a holding time of 180 seconds or more, and then heating it to a maximum temperature of 1100°C or higher. A hot rolling process in which the heated slab is hot-rolled to produce a hot-rolled steel sheet, Annealing process to obtain a hot-rolled annealed sheet by annealing the aforementioned hot-rolled steel sheet, A cold rolling process in which the hot-rolled annealed sheet is cold-rolled to produce a cold-rolled steel sheet, and This includes a final annealing step for performing final annealing on the cold-rolled steel sheet, In the aforementioned secondary refining, Al is added to the molten steel. A method for manufacturing non-oriented electrical steel sheets, wherein the elapsed time from the addition of Al to the start of continuous casting is 300 seconds or more.