Non-oriented electrical steel sheet and method for manufacturing the same

Abstract

The objective of the present invention is to inexpensively provide an excellent non-oriented magnetic steel sheet achieving both punching workability and low iron loss. Provided is a non-oriented magnetic steel sheet having a component composition of C, Si, Mn, P, S, Al, N, Pb, and Ca, and including no more than 100 pieces / mm2 of Pb precipitates having a particle diameter of 10-100 nm, and between 0.1 pieces / mm2 and 1,000 pieces / mm2 (both inclusive) of Pb-Ca composite precipitates having a particle diameter of 0.5-5.0 μm. Also provided is a method for manufacturing a non-oriented magnetic steel sheet, the method comprising a hot rolling step, a hot-rolled sheet annealing step, a cold rolling step, and a finish annealing step. In the hot rolling step, a steel slab is heated at between 460°C and 540°C (both inclusive) for 5 min or more in a first phase, and at between 1,060°C and 1,140°C (both inclusive) for 20 min or more in a second phase. In the finish rolling, the steel sheet temperature FET on the inlet side of a first pass is set to between 940°C and 1100°C (both inclusive), and the reduction rate of the first pass is set to 40% or more. In the hot-rolled sheet annealing step, the annealing temperature is set to between 900°C and 1100°C (both inclusive).

Description

[Technical Field]

[0001] This invention relates to a non-oriented electrical steel sheet with excellent punching processability and magnetic properties, and a method for manufacturing the same. [Background technology]

[0002] Non-oriented electrical steel sheets used in the drive motors of electric and hybrid vehicles require low iron loss to reduce energy loss. While adding elements such as Si and Al has been proposed as an effective way to achieve low iron loss, this method significantly degrades the punching processability when using the electrical steel sheets as motor cores. Poor punching processability also leads to premature wear of the punching dies.

[0003] Therefore, to solve this problem, various technologies for improving the punching processability of steel sheets have been proposed to date.

[0004] Patent Document 1 discloses a method for manufacturing non-oriented electrical steel sheets, characterized by controlling the mass ratio of Si and Al. Specifically, the objective is to provide a non-oriented electrical steel sheet that has excellent magnetic properties as an electrical equipment core material, as well as excellent punching processability that facilitates forming into electrical equipment cores. The non-oriented electrical steel sheet has excellent magnetic properties and punching processability, characterized by containing, by mass%, C: 0.003% or less, Si: 1.0% to 3.0%, Al: 0.1% to 3.0%, and Mn: 0.1% to 1.0%, with the Al and Si content satisfying the relationship 0.2 ≤ Al / (Si + Al) ≤ 0.6, the remainder being Fe and unavoidable impurity elements, and having a yield ratio expressed as (yield strength / tensile strength) of 0.6 or more and a Vickers hardness of 200 or less.

[0005] Furthermore, Patent Document 2 discloses a method characterized by the addition of Cr to the component composition. Specifically, the objective is to provide an electrical steel sheet with low iron loss after finish annealing and excellent punching processability. The sheet contains, by mass%, C: 0.005% or less, P: 0.05% or less, Si: 1-3.5%, Mn: 0.05-1.5%, Al: 0.1-3%, S: 0.02% or less, N: 0.005% or less, Cr: 0.2-3%, with the remainder being substantially Fe, a Vickers hardness of 190 or less on the steel sheet surface, and 30 oxide inclusions / mm² with a diameter of 1 μm or more on the steel sheet cross-section. 2 The following are characteristics of a non-oriented electrical steel sheet with excellent punching processability. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-214758 [Patent Document 2] Japanese Patent Publication No. 2003-27195 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, the prior art disclosed in the above-mentioned patent document has the following problems.

[0008] The technology described in Patent Document 1 requires the addition of 2.29% or more Al compared to the examples, which presents problems with the manufacturability of the steel sheet. Furthermore, the technology described in Patent Document 2 has the problem of high manufacturing costs due to the addition of expensive Cr.

[0009] This invention was developed in view of the above-mentioned problems of the prior art, and aims to provide an excellent non-oriented electrical steel sheet that achieves both punching processability and low iron loss at a low cost. [Means for solving the problem]

[0010] The non-oriented electrical steel sheet according to the present invention, which advantageously solves the above problems, is configured as follows. [1] In mass%, C: 0.005% or less, Si: 2.5% to 5.0%, Mn: 0.1% to 3.0%, P: 0.100% or less, S: 0.005% or less, Al: 2.0% or less, N: 0.005% or less, Pb: 0.00010% to 0.00200%, and Contains Ca: 0.0010% to 0.020%, and optionally further contains the following A group Contains the ingredients, Group A: One or more elements selected from Mo: 0.001-0.050%, Zn: 0.001-0.010%, Ti: 0.001-0.010%, Sn: 0.001-0.200%, and Sb: 0.001-0.200%. 、 The composition consists of Fe and unavoidable impurities, and the number density of Pb precipitates with a particle size of 10-100 nm is 100 particles / mm³. 2 The following conditions apply, and the number density of Pb-Ca composite precipitates with a particle size of 0.5-5.0 μm is 0.1 particles / mm³. 2 More than 1000 pieces / mm 2 This is a non-oriented electrical steel sheet characterized by satisfying the following conditions.

[0011] The method for manufacturing non-oriented electrical steel sheets according to the present invention, which advantageously solves the above problems, is configured as follows. [2] Contains, by mass%, C: 0.005% or less, Si: 2.5% to 5.0%, Mn: 0.1% to 3.0%, P: 0.100% or less, S: 0.005% or less, Al: 2.0% or less, N: 0.005% or less, Pb: 0.00010% to 0.00200%, Ca: 0.0010% to 0.020%, and optionally further containing the following A group Contains the ingredients, Group A: One or more elements selected from Mo: 0.001-0.050%, Zn: 0.001-0.010%, Ti: 0.001-0.010%, Sn: 0.001-0.200%, and Sb: 0.001-0.200%. 、 A hot rolling process in which a steel slab having a component composition consisting of the residue and Fe and inevitable impurities is heated and hot-rolled to obtain a hot-rolled steel sheet; a hot-rolled sheet annealing process in which the hot-rolled steel sheet is annealed to obtain a hot-rolled annealed sheet; a cold rolling process in which the hot-rolled annealed sheet is subjected to two or more cold rollings sandwiching one cold rolling or intermediate annealing to obtain a cold-rolled steel sheet; and a finish annealing process in which the cold-rolled steel sheet is subjected to finish annealing. In the hot rolling process, the heating of the steel slab includes two steps, the first step being at 460°C or higher and 540°C or lower for 5 minutes or more, and the second step being at 1060°C or higher and 1140°C or lower for 20 minutes or more. The finish rolling of the hot rolling is such that the temperature of the steel sheet on the entrance side of the first pass is 940°C or higher and 1100°C or lower, and the reduction ratio of the first pass is 40% or higher. In the hot-rolled sheet annealing process, the annealing temperature is 900°C or higher and 1100°C or lower. This is a method for manufacturing a non-oriented electromagnetic steel sheet. [3] The method for manufacturing a non-oriented electromagnetic steel sheet according to [2] above, wherein the steel slab is a thin slab continuously cast with a thickness of 70 mm or more and 200 mm or less.

Effects of the Invention

[0012] According to the present invention, it is possible to manufacture an excellent non-oriented electromagnetic steel sheet that achieves both low iron loss and punching workability even in the case of a high Si-containing steel.

Brief Description of the Drawings

[0013] [Figure 1] It is a graph showing the relationship between the Pb content in the steel sheet and the curl height of the punching test piece. [Figure 2] It is a graph showing the relationship between the Pb content in the steel sheet and the number density of Pb precipitates having a particle size of 10 to 100 nm. [Figure 3] It is a graph showing the influence of the presence or absence of Pb addition on the relationship between the Ca content in the steel sheet and the iron loss W 10 / 400. [Figure 4] It is a graph showing the relationship between the Ca content in the steel sheet and the number density of Pb-Ca composite precipitates having a particle size of 0.5 to 5.0 μm.

Modes for Carrying Out the Invention

[0014] The inventors investigated various steel compositions and discovered that adding Pb improved the punching processability of non-oriented electrical steel sheets. Therefore, they focused on the combination of reducing iron loss through high alloying and improving punching processability through Pb addition, and after further investigation, they found that adding Ca and controlling Pb precipitates is effective in obtaining low iron loss and high punching processability, leading to the development of the present invention.

[0015] First, I will explain the experiment that led to the development of this invention. <Experiment 1> The inventors focused on the effect of Pb addition in order to develop a non-oriented electrical steel sheet with excellent punching processability, and measured punching processability and iron loss to clarify the influence of Pb on the processability and magnetic properties of the steel sheet. The test material was a steel slab with a base composition of C:0.002%, Si:3.4%, Mn:0.3%, P:0.01%, S:0.002%, N:0.002%, Al:1.2%, with the remainder being Fe and unavoidable impurities, to which Pb was added in a range of 0 to 0.005%. These test materials were heated at 1100°C for 20 mins, and then hot-rolled to produce hot-rolled steel sheets with a thickness of 2.0 mm. For the first pass of hot rolling, the temperature at the entry side was 1050°C, resulting in a reduction ratio of 50% of the steel sheet, and the temperature at the exit side of the final pass was 950°C. Next, hot-rolled sheet annealing was performed at 980°C for 30 seconds. After that, cold rolling was performed to produce a cold-rolled steel sheet with a thickness of 0.25 mm, and finally, finish annealing was performed at 960°C for 10 seconds in a dry atmosphere with a vol% ratio of H2:N2 = 25:75.

[0016] Thus, an Epstein test sample measuring 30 mm in width and 280 mm in length was cut from the rolling direction and width direction of the steel plate to determine the iron loss W. 10 / 400 The efficiency was measured using an Epstein tester. For punching workability, 17mm squares were punched out of the obtained steel sheet using a SKD11 die in 2 x 10 5 The evaluation was based on the burr height at the end face of the test specimen after punching it a number of times.

[0017] Figure 1 shows the relationship between the amount of Pb in the steel sheet and the burr height. From Figure 1, it is clear that the burr height decreases with the addition of Pb, meaning that the punching processability improves. On the other hand, adding Pb to steel significantly increased iron loss. Furthermore, the average grain size of the steel sheet, as observed with an optical microscope, decreased with the addition of Pb. To investigate the cause of this microstructure refinement, we observed precipitates in steel sheets using a transmission electron microscope (TEM) with a thin-film method. In steel sheets with added Pb, numerous Pb precipitates with a particle size of 10-100 nm were observed. We then identified the number of these fine Pb precipitates that inhibit grain growth and adversely affect iron loss.

[0018] Figure 2 shows the relationship between the amount of Pb and the number density of Pb precipitates in the 10-100 nm range. From Figure 2, it can be seen that the number density of Pb precipitates increases when Pb is added. Therefore, the reason for the increase in iron loss is thought to be that the decrease in grain growth due to the increase in Pb precipitates led to an increase in hysteresis loss.

[0019] <Experiment 2> The effect of Ca addition on iron loss in non-oriented electrical steel sheets containing Pb was investigated. Specifically, a steel composition with the following mass% composition as a base: C:0.002%, Si:3.4%, Mn:0.3%, P:0.01%, S:0.002%, N:0.002%, Al:1.2%, Pb:0.0010%, with the remainder being Fe and unavoidable impurities, was used as the test material. Ca was added to this composition in a range of 0 to 0.05%. This test material was subjected to hot rolling and finish annealing using the same method as in Experiment 1. From the resulting steel sheet, an Epstein test sample with a width of 30 mm and a length of 280 mm was cut out in the rolling direction and the width direction, and the iron loss W was measured. 10 / 400 It was measured using an Epstein apparatus.

[0020] Iron loss W of the obtained steel plate 10 / 400When measured, even when the amount of Ca was changed, the effect of suppressing the increase in iron loss due to Pb was not obtained. However, when precipitate observation was performed by a transmission electron microscope (TEM) using a thin film method, Pb-Ca composite precipitates with a particle size of 0.5 to 5.0 μm were observed. In addition, the Pb-Ca composite precipitate in the present invention was defined as a precipitate in which the ratio of Pb was 10% or more and the ratio of Ca was 20% or more in terms of the atomic ratio obtained by energy dispersive X-ray spectrometry (EDS). Therefore, the inventors investigated the control of Pb-Ca composite precipitates and the suppression of the increase in iron loss due to Pb.

[0021] <Experiment 3> In a non-oriented electrical steel sheet containing Pb and Ca, the influence of the slab reheating conditions before hot rolling on iron loss and punching workability was investigated. That is, based on a steel composition having C: 0.002%, Si: 3.4%, Mn: 0.3%, P: 0.01%, S: 0.002%, N: 0.002%, Al: 1.2%, Pb: 0.0010% by mass%, and the balance being Fe and unavoidable impurities, steel slabs were used as test materials with Ca added in the range of 0 to 0.05% varied therein. Also, a steel slab without Pb was prepared as a comparative material.

[0022] These test materials were heated in two stages at 500°C for 5 minutes and then at 1100°C for 20 minutes, and then hot rolled into a hot rolled sheet with a thickness of 2.0 mm. The temperature on the entrance side of the first pass of hot rolling was 1050°C with a reduction ratio of the steel sheet of 50%, and the temperature on the exit side of the final pass was 950°C. Next, hot rolled sheet annealing was carried out at 980°C for 30 seconds. Then, cold rolling was performed to obtain a cold rolled sheet with a thickness of 0.25 mm, and finally, finish annealing was carried out at 960°C for 10 seconds in a dry atmosphere with a volume ratio of H2:N2 = 25:75.

[0023] Thus, from the rolling direction and the sheet width direction of the obtained steel sheet, Epstein test samples with a width of 30 mm × a length of 280 mm were cut out for iron loss W 10 / 400The efficiency was measured using an Epstein tester. For punching workability, 17mm squares were punched out of the obtained steel sheet using a SKD11 die in 2 x 10 5 The evaluation was based on the burr height at the end face of the test specimen after punching it a number of times.

[0024] Figure 3 shows the amount of Ca in steel plates and iron loss W in Pb-added and Pb-free materials. 10 / 400 The relationship is shown. From Figure 3, it is clear that adding Ca to Pb-added material reduces iron loss. Compared with Pb-less material, the reduction in iron loss due to Ca addition is thought to be because Ca suppressed the increase in iron loss caused by Pb.

[0025] Using a thin-film method, the precipitates were observed using a transmission electron microscope (TEM), revealing Pb-Ca composite precipitates with particle sizes ranging from 0.5 to 5.0 μm. The number of these Pb-Ca composite precipitates was identified. Figure 4 shows the relationship between the number density of Pb-Ca composite precipitates and the amount of Ca. Figure 4 shows that as the amount of Ca increased with Ca addition, the number density of Pb-Ca composite precipitates increased. This indicates a decrease in Pb precipitates, which are a factor in microstructure refinement.

[0026] Furthermore, the height of the burr hardly changed even when calcium was added. These results demonstrate that by performing a two-stage slab heating process with a component composition that includes Ca in addition to Pb, it is possible to produce non-oriented electrical steel sheets with excellent punching workability while suppressing the increase in iron loss.

[0027] The non-oriented electrical steel sheet according to this embodiment will be described below. <Component composition of non-oriented electrical steel sheet> The chemical composition of non-oriented electrical steel sheets is as follows (in mass%): C: 0.005% or less, Si: 2.5% to 5.0%, Mn: 0.1% to 3.0%, P: 0.100% or less, S: 0.005% or less, Al: 2.0% or less, N: 0.005% or less, Pb: 0.00010% to 0.00200%, Ca: 0.0010% to 0.020%. Each component is explained below. In the following explanation, "%" representing the content of a component means "mass%" unless otherwise specified.

[0028] C: 0.005% or less Carbon (C) is a harmful element that causes magnetic aging in the product plate, forming carbides and degrading iron loss. Therefore, to suppress magnetic aging, the carbon content should be 0.005% or less. Although there is no specific lower limit for the carbon content, it is preferable to set it at around 0.0001% from the viewpoint of suppressing decarburization costs.

[0029] Si: 2.5% or more and 5.0% or less Si has the effect of increasing the electrical resistance of steel sheets and reducing iron loss. To sufficiently reduce iron loss, an addition of 2.5% or more is necessary. Therefore, the Si content should be 2.5% or more. On the other hand, if the Si content exceeds 5.0%, not only does rolling become difficult, but the punching processability also decreases. Therefore, the Si content should be 5.0% or less. Furthermore, from the viewpoint of manufacturability, a Si content of 4.0% or less is preferable.

[0030] Mn: 0.1% or more and 3.0% or less Like Si and Al, Mn increases the electrical resistance of steel sheets and reduces iron loss. Therefore, the Mn content should be 0.1% or more. On the other hand, if the Mn content exceeds 3.0%, Mn carbides will precipitate, which will actually worsen iron loss. Therefore, the Mn content should be 3.0% or less. Preferably, the Mn content is in the range of 0.2% to 1.0%.

[0031] P:0.100% or less P has the effect of increasing the strength of steel and can be used for strength adjustment. On the other hand, if the P content exceeds 0.100%, the steel becomes brittle, leading to a decrease in manufacturability. Therefore, the P content should be 0.100% or less. Although there is no specific lower limit for the P content, from the viewpoint of suppressing the cost of eliminating P, it is preferable to keep it at around 0.001 mass%.

[0032] S: 0.005% or less S is a harmful element that inhibits grain growth and increases iron loss by forming fine sulfides, so it is desirable to reduce its content as much as possible. In particular, the above adverse effects become significant when the S content exceeds 0.005%, so it should be kept below 0.005%. More preferably, the S content should be below 0.003%.

[0033] Al: 2.0% or less Al, like Si, increases the electrical resistance of steel sheets and reduces iron loss. However, if the Al content exceeds 2.0%, not only does rolling become difficult, but the punching processability also decreases. Therefore, the Al content should be kept below 2.0%. However, if the Al content is in the range of 0.01% or more and less than 0.1%, fine AlN precipitates and iron loss increases; therefore, the Al content is preferably in the range of 0.1% or more and 2.0% or less.

[0034] N: 0.005% or less Since nitrogen (N) is a harmful element that inhibits grain growth by forming fine nitrides and increases iron loss, it is desirable to reduce its content as much as possible. In particular, the above adverse effects become significant when the N content exceeds 0.005%, so it should be kept below 0.005%. More preferably, the N content should be 0.003% or less.

[0035] Pb: 0.00010% or more and 0.00200% or less Adding lead (Pb) can improve punching processability. Therefore, the Pb content should be 0.00010% or higher. On the other hand, if the Pb content exceeds 0.00200%, the Pb itself will form numerous fine precipitates, inhibiting grain growth and increasing iron loss. Therefore, the Pb content should be between 0.00010% and 0.00200%.

[0036] Ca: 0.0010% or more and 0.020% or less Ca reacts with S to form CaS, creating coarse precipitates that suppress the precipitation of fine sulfides such as MnS, thereby reducing iron loss. Furthermore, CaS and CaO precipitate together with Pb, suppressing the precipitation of fine Pb precipitates. Therefore, the Ca content should be 0.0010% or higher. On the other hand, exceeding 0.020% increases the amount of CaS and CaO, which in turn inhibits grain growth and increases iron loss. Therefore, the Ca content should be 0.020% or less.

[0037] The above describes the basic composition of the non-oriented electrical steel sheet according to the embodiment, but optionally, the following A group It can contain ingredients. Group A; one or more selected from Mo: 0.001-0.050%, Zn: 0.001-0.010%, Ti: 0.001-0.010%, Sn: 0.001-0.200%, and Sb: 0.001-0.200%. above

[0038] Mo: 0.001~0.050% Mo reacts with C to form carbides at grain boundaries, thereby improving strength. Therefore, the Mo content should be 0.001% or higher. However, since iron loss tends to increase when the Mo content exceeds 0.050%, the Mo content should be 0.050% or lower.

[0039] Zn: 0.001~0.010% Zn reacts with S to form coarse sulfides, which suppress the precipitation of fine sulfides such as MnS, thereby reducing iron loss. Therefore, the Zn content should be 0.001% or higher. On the other hand, if the Zn content exceeds 0.010%, the amount of the above-mentioned sulfides increases, which in turn inhibits grain growth and increases iron loss. Therefore, the Zn content should be 0.010% or less.

[0040] Ti: 0.001~0.010% Similar to Mo, Ti reacts with C to form carbides, thereby suppressing grain growth and improving toughness. Therefore, the Ti content should be 0.001% or higher. However, since iron loss tends to increase when the Ti content exceeds 0.010%, the Ti content should be 0.010% or lower.

[0041] Sn: 0.001~0.200% Similar to P, adding Sn significantly improves the texture, increases magnetic flux density, and reduces iron loss. Therefore, the Sn content should be 0.001% or higher. On the other hand, if the Sn content exceeds 0.200%, the effect saturates and manufacturability decreases. Therefore, the Sn content should be 0.200% or less.

[0042] Sb: 0.001~0.200% Similar to P and Sn, adding Sb significantly improves the texture, increases magnetic flux density, and reduces iron loss. Therefore, the Sb content should be 0.001% or higher. On the other hand, if the Sb content exceeds 0.200%, the effect saturates and manufacturability decreases. Therefore, the Sb content should be 0.200% or less.

[0043] As: 0.020% or less As has the effect of refining the crystal grains and improving strength. On the other hand, if the content exceeds 0.020%, it may cause cold rolling fracture. Therefore, the As content should be 0.020% or less.

[0044] Nb: 0.005% or less Nb has the effect of refining crystal grains and improving strength. On the other hand, if the content exceeds 0.005%, it generates fine precipitates and increases iron loss. Therefore, the Nb content should be 0.005% or less.

[0045] W: 0.050% or less W has the effect of refining the crystal grains and improving strength. On the other hand, if the content exceeds 0.050%, it generates fine precipitates and increases iron loss. Therefore, the W content should be 0.050% or less.

[0046] V:0.050% or less V has the effect of refining the crystal grains and improving strength. On the other hand, if the content exceeds 0.050%, it generates fine precipitates and increases iron loss. Therefore, the V content should be 0.050% or less.

[0047] Ta: 0.0020% or less Ta has the effect of refining crystal grains and improving strength. On the other hand, if the content exceeds 0.0020%, it generates fine precipitates and increases iron loss. Therefore, the Ta content should be 0.0020% or less.

[0048] Cu: 0.01~1.0% Cu has the effect of improving the strength of steel plates through aging treatment. Therefore, the Cu content should be 0.01% or more. On the other hand, if the content exceeds 1.0%, precipitates will form, which will actually increase iron loss. Therefore, the Cu content should be between 0.01% and 1.0%.

[0049] Ni: 0.01~1.0% Ni improves the toughness of steel sheets and increases productivity. Therefore, the Ni content should be 0.01% or more. On the other hand, if the Ni content exceeds 1.0%, the effect saturates and costs increase. Therefore, the Ni content should be between 0.01% and 1.0%.

[0050] B: 0.0020% or less B improves the toughness of steel plates and enhances productivity. However, if the B content exceeds 0.0020%, iron loss increases. Therefore, the B content should be 0.0020% or less.

[0051] Cr: 0.01~3.0% Cr increases the electrical resistance of steel plates and reduces iron loss. Therefore, the Cr content should be 0.01% or higher. On the other hand, if the Cr content exceeds 3.0%, the magnetic flux density decreases. Therefore, the Cr content should be 3.0% or less.

[0052] Mg: 0.0001~0.005% Mg forms coarse sulfides with sulfur in steel, which reduces iron loss. Therefore, the Mg content should be 0.0001% or more. On the other hand, adding more than 0.005% of Mg will actually increase iron loss. Therefore, the Mg content should be between 0.0001% and 0.005%.

[0053] REM: 0.001~0.05% REM has the effect of coarsening sulfides in steel and reducing iron loss. Therefore, the REM content should be 0.001% or more. On the other hand, if the content exceeds 0.05%, it will actually lead to an increase in iron loss. Therefore, the REM content should be 0.001 to 0.05%.

[0054] Co:0.10% or less Co improves the texture of steel sheets and increases magnetic flux density. However, if the Co content exceeds 0.10%, the effect saturates, and costs increase. Therefore, the Co content should be 0.10% or less.

[0055] Ge: 0.030% or less Ge improves the texture of steel sheets and increases magnetic flux density. However, exceeding 0.030% Ge content leads to saturation of the effect and increased costs. Therefore, the Ge content should be kept below 0.030%.

[0056] Ga: 0.030% or less Ga improves the texture of steel sheets and increases magnetic flux density. However, exceeding 0.030% Ga content leads to saturation of the effect and increased costs. Therefore, the Ga content should be 0.030% or less.

[0057] The chemical composition of the non-oriented electrical steel sheet according to this embodiment contains the above-mentioned elements, with the remainder being Fe and unavoidable impurities.

[0058] <Form of precipitates> Next, we will explain the precipitates in non-oriented electrical steel sheets. Because it significantly affects grain boundary pinning, the number density of Pb precipitates with a particle size of 10-100 nm should be 100 particles / mm³. 2 The following range will be controlled. In addition, to control the number of Pb precipitates, the number density of Pb and Ca composite precipitates with a particle size of 0.5 to 5.0 μm will be 0.1 particles / mm³. 2 More than 1000 pieces / mm 2 Control within the following range. Here, the particle size of the precipitate is evaluated by its equivalent circular diameter from the projected area measured with a microscope.

[0059] <Microstructure of non-oriented electrical steel sheet> Next, we will explain the microstructure of non-oriented electrical steel sheets. As described above, it is preferable to control the number density of Pb precipitates and composite precipitates of Pb and Ca to set the average grain size of the steel sheet between 60 μm and 300 μm. If the average grain size of the steel sheet is less than 60 μm, iron loss increases. On the other hand, if the structure of the steel sheet becomes too coarse, i.e., if the average grain size exceeds 300 μm, the toughness deteriorates, making it prone to fracture during processing.

[0060] <Manufacturing method for non-oriented electrical steel sheets> Next, a method for manufacturing non-oriented electrical steel sheets according to this embodiment will be described. The manufacturing of non-oriented electrical steel sheets is carried out through a process that includes hot rolling, hot-rolled sheet annealing, cold rolling, and finish annealing of steel slabs adjusted to the above-mentioned component composition.

[0061] Slab Slabs can be manufactured by melting steel with a composition suitable for the present invention in a conventional refining process consisting of a converter and vacuum degassing treatment, and then using conventional continuous casting or ingot-molding rolling methods. From the viewpoint of reducing environmental impact (reducing CO2 emissions), an electric furnace method using scrap instead of blast furnace pig iron as the steel raw material (iron source) is preferred. When manufacturing slabs by melting scrap in an electric furnace, the Pb content is high due to the Pb contained in the scrap, which has the advantage of reducing the cost of adding Pb. Furthermore, since the scrap also contains elements such as Mo, Ti, and Zn that can be optionally added in the present invention, it is possible to improve the properties of the steel sheet while reducing the environmental impact.

[0062] Hot rolling process The slabs are reheated before hot rolling, using at least two heating devices with different power outputs installed in succession to perform heating in two stages. The first heating stage aims to dissolve the elemental Pb to increase fluidity and facilitate composite precipitation with Ca. The heating temperature should be between 460°C and 540°C, and the heating time should be 5 minutes or longer. If the slab heating temperature falls below 460°C, the Pb cannot be dissolved, making composite precipitation of Ca and Pb difficult. On the other hand, if the temperature exceeds 540°C, Pb will preferentially exist as an elemental material rather than precipitation with Ca, making it difficult to precipitate composite precipitates of Pb and Ca. There is no specific upper limit for the heating time, but from the viewpoint of reducing operating costs, it is preferable to set it to around 10 minutes.

[0063] The second heating stage aims to increase the slab temperature and facilitate rolling. The heating temperature should be between 1060°C and 1140°C, and the heating time should be 20 minutes or longer. If the slab heating temperature exceeds 1140°C, precipitates such as AlN and MnS will dissolve in the steel, re-precipitation in subsequent processes will inhibit grain growth, and iron loss will increase. On the other hand, if the temperature falls below 1060°C, the hot rolling load will increase, and it will become impossible to secure the finish rolling entry temperature (hereinafter, the rolling entry temperature will also be referred to as FET). There is no specific upper limit for the heating time, but from the viewpoint of reducing operating costs, it is preferable to set it to around 30 minutes.

[0064] Hot rolling consists of rough rolling, which uses multiple single-stand rolling mills to roll the slab into a sheet bar of a predetermined thickness, and finish rolling, which follows the rough rolling and continues rolling to the target thickness using a finish rolling mill consisting of multiple stands. The rough rolling can be done using conventional methods.

[0065] In finish rolling following rough rolling, the entry temperature of the steel sheet in the first pass, i.e., the finish rolling entry temperature FET (°C), should be 940°C or higher to promote recrystallization and grain growth. On the other hand, if the temperature exceeds 1100°C, some of the precipitates containing MnS and Pb will dissolve and precipitate in subsequent processes, increasing iron loss, so it should be kept below 1100°C. In addition, to neutralize precipitates such as MnS and AlN that inhibit grain growth and increase iron loss, the reduction ratio in the first pass should be 40% or higher. There is no upper limit specified for the reduction ratio, but considering manufacturability, a range of 40-50% is preferable.

[0066] In hot rolling, the exit temperature in the final pass of finish rolling is preferably 930°C or higher, from the viewpoint of promoting recrystallization and grain growth of the hot-rolled steel sheet and improving its magnetic properties.

[0067] Furthermore, it is more preferable to hot-roll thin slabs with a thickness of 70 mm to 200 mm using a thin slab caster that integrates a continuous casting machine and rolling equipment. This is because, since the slab is not cooled to room temperature, composite precipitates with Ca can be formed before Pb precipitates as a single molecule.

[0068] Hot-rolled sheet annealing process Next, the hot-rolled steel sheet is subjected to hot-rolled sheet annealing, which involves holding it at a predetermined temperature for 1 second or more. Hot-rolled sheet annealing recrystallizes the structure of the hot-rolled sheet, and the annealing temperature is set to 900°C or higher to obtain good magnetic properties. On the other hand, if the annealing temperature exceeds 1100°C, some of the precipitates containing MnS and Pb will dissolve and precipitate in subsequent processes, increasing iron loss, so the annealing temperature should be set to 1100°C or lower. Pickling, cold rolling, and finish annealing after hot-rolled sheet annealing can be done by conventional methods.

[0069] Cold rolling process Cold rolling is a process in which a hot-rolled, annealed sheet that has undergone pickling is cold-rolled to obtain the final thickness of the product sheet. There are no particular restrictions on this cold rolling, as long as the final thickness is achieved. However, if the reduction ratio in cold rolling is too low, the strength of the steel sheet after finish annealing may decrease significantly, so the reduction ratio in cold rolling is preferably 50% or more, more preferably 70% or more. Furthermore, cold rolling is not limited to one step, and two or more cold rolling steps may be performed with intermediate annealing in between as needed. In this case, the conditions for intermediate annealing can also be any commonly used conditions, and there are no particular restrictions.

[0070] Finish annealing process Finish annealing of cold-rolled sheets is a process of annealing cold-rolled sheets, which have reached their final thickness through cold rolling, to impart desired magnetic and strength properties. To sufficiently resolve the strain introduced during cold rolling through recrystallization and obtain good magnetic properties, the finish annealing temperature of the cold-rolled sheet is preferably 850°C or higher. Furthermore, to obtain even better magnetic properties, the finish annealing temperature of the cold-rolled sheet is more preferably 880°C or higher. On the other hand, if the finish annealing temperature of the cold-rolled sheet is too high, the recrystallized structure becomes too coarse and the magnetic properties deteriorate, so the finish annealing temperature of the cold-rolled sheet is preferably 1050°C or lower. More preferably, the finish annealing temperature of the cold-rolled sheet is 1030°C or lower. [Examples]

[0071] Embodiments of the present invention will be further described by reference to examples. It should be noted that the present invention is not limited to the manufacturing conditions and product performance shown in the following examples. Within the scope of the present invention, the desired performance can be achieved by the embodiments.

[0072] In the converter-vacuum degassing refining process, steels No. 1 to 104 having the component compositions shown in Tables 1-1 to 1-4 were melted and formed into slabs by continuous casting. The slabs were then heated in two steps: 500°C for 5 minutes and then 1100°C for 20 minutes, followed by hot rolling to produce hot-rolled steel sheets with a thickness of 2.0 mm. The temperature at the entry side of the first pass of hot rolling was 1050°C, resulting in a reduction ratio of 50% of the steel sheet, and the temperature at the exit side of the final pass was 950°C. Next, the rolled sheet was annealed at 980°C for 30 seconds. Subsequently, the material was cold-rolled to a thickness of 0.25 mm, and finally, it was finished annealed at 960°C for 10 seconds in a dry atmosphere with a vol% ratio of H2:N2 = 25:75.

[0073] From the rolling direction and width direction of the steel plate thus obtained, an Epstein test sample measuring 30 mm in width and 280 mm in length was cut out, and an Epstein test was performed in accordance with JIS C 2550-3 (2019) to determine the iron loss W. 10 / 400 The following was measured: Iron loss W at a plate thickness of 0.25 mm. 10 / 400 A value of 14 W / kg or less is considered good.

[0074] Furthermore, 17mm square test pieces were cut from the obtained steel plate and 2×10mm squares were cut using an SKD11 mold. 5 The burr height on the end face of the test piece was measured after punching. A burr height of 30 μm or less indicates good punching processability.

[0075] Furthermore, the number density of Pb precipitates and Pb-Ca composite precipitates in the finished annealed plate was measured by TEM using the thin-film method. The equivalent circular diameter was calculated from the projected area measured by microscopy for both the crystal grain size and the precipitate size. These measurement results are shown in Tables 2-1 to 2-4.

[0076] Tables 2-1 to 2-4 show that by controlling the component composition of the steel material within the range of the present invention, non-oriented electrical steel sheets with excellent iron loss and punching workability can be obtained.

[0077] In the converter-vacuum degassing refining process, the composition by mass percent was C:0.0025%, Si:2.85%, Mn:0.30%, P:0.006%, S:0.0018%, N:0.0025%, Al:1.20%, Pb:0.0006%, Ca:0.0 0 Steel containing 4% was melted and formed into slabs using a continuous casting method. Then, the slabs were reheated, followed by hot rolling and hot-rolled sheet annealing under the conditions shown in Table 3. Subsequently, the material was cold-rolled to a thickness of 0.25 mm, and finally, it was finished annealed at 960°C for 10 seconds in a dry atmosphere with a vol% ratio of H2:N2 = 25:75.

[0078] From the rolling direction and width direction of the steel plate thus obtained, an Epstein test sample measuring 30 mm in width and 280 mm in length was cut out, and an Epstein test was performed in accordance with JIS C 2550-3 (2019) to determine the iron loss W. 10 / 400 The following was measured: Iron loss W at a plate thickness of 0.25 mm. 10 / 400 A value of 14 W / kg or less is considered good.

[0079] Furthermore, 17mm square test pieces were cut from the obtained steel plate and 2×10mm squares were cut using an SKD11 mold. 5 The burr height on the end face of the test piece was measured after punching. A burr height of 30 μm or less indicates good punching processability.

[0080] Furthermore, the number density of Pb precipitates and Pb-Ca composite precipitates in the finished annealed plate was measured by TEM using the thin-film method. The equivalent circular diameter of the crystal grain and precipitate grain size was calculated from the projected area measured by microscopy. These measurement results are shown in Table 4.

[0081] Table 4 shows that by controlling the steel sheet manufacturing conditions within the scope of the present invention, a non-oriented electrical steel sheet that achieves both low iron loss and excellent punching workability can be obtained.

[0082] Table 1-1

[0083] Table 1-2

[0084] Table 1-3

[0085] Table 1-4

[0086] Table 2-1

[0087] Table 2-2

[0088] Table 2-3

[0089] Table 2-4

[0090] Table 3

[0091] Table 4

Claims

1. In mass percent, C: 0.005% or less, Si: 2.5% or more and 5.0% or less, Mn: 0.1% or more and 3.0% or less, P: 0.100% or less, S: 0.005% or less, Al: 2.0% or less, N: 0.005% or less, Pb: 0.00010% or more and 0.00200% or less, Ca: 0.0010% or more and 0.020% or less It contains, The following components of Group A are optionally included: Group A: One or more elements selected from Mo: 0.001-0.050%, Zn: 0.001-0.010%, Ti: 0.001-0.010%, Sn: 0.001-0.200%, and Sb: 0.001-0.200%. The composition consists of Fe and unavoidable impurities as the remainder. The metallographic structure has an average grain size of 60 μm or more and 300 μm or less. The number density of Pb precipitates with a particle size of 10-100 nm is 100 particles / mm². ２ The following: The number density of Pb-Ca composite precipitates with a particle size of 0.5 to 5.0 μm is 0.1 particles / mm³. ２ More than 1000 pieces / mm ２ A non-oriented electrical steel sheet characterized by satisfying the following conditions.

2. In mass percent, C: 0.005% or less, Si: 2.5% or more and 5.0% or less, Mn: 0.1% or more and 3.0% or less, P: 0.100% or less, S: 0.005% or less, Al: 2.0% or less, N: 0.005% or less, Pb: 0.00010% or more and 0.00200% or less, Ca: 0.0010% or more and 0.020% or less It contains, The following components of Group A are optionally included: Group A: One or more elements selected from Mo: 0.001-0.050%, Zn: 0.001-0.010%, Ti: 0.001-0.010%, Sn: 0.001-0.200%, and Sb: 0.001-0.200%. A hot rolling process involves heating and hot rolling a steel slab having a composition consisting of Fe and unavoidable impurities to produce a hot-rolled steel sheet, The process involves annealing the aforementioned hot-rolled steel sheet to obtain a hot-rolled annealed sheet, A cold rolling process to obtain a cold-rolled steel sheet by subjecting the hot-rolled annealed sheet to one cold rolling or two or more cold rolling processes with an intermediate annealing in between, A finish annealing step is performed on the cold-rolled steel sheet, Includes, In the hot rolling process, the heating of the steel slab includes a two-stage step in which the first stage is heated to 460°C or higher and 540°C or lower for 5 minutes or more, and the second stage is heated to 1060°C or higher and 1140°C or lower for 20 minutes or more. In the finish rolling of the hot rolling, the temperature of the steel sheet on the entry side of the first pass is set to 940°C or higher and 1100°C or lower, and the reduction ratio of the first pass is set to 40% or higher. In the aforementioned hot-rolled sheet annealing process, the annealing temperature is set to 900°C or higher and 1100°C or lower. The average grain size of the metal microstructure is set to be between 60 μm and 300 μm. The number density of Pb precipitates with a particle size of 10 to 100 nm is set to 100 particles / mm² or less. A method for manufacturing non-oriented electrical steel sheets, characterized in that the number density of Pb-Ca composite precipitates having a particle size of 0.5 to 5.0 μm is 0.1 particles / mm² or more and 1000 particles / mm² or less.

3. The method for manufacturing a non-oriented electrical steel sheet according to claim 2, characterized in that the steel slab is a thin slab continuously cast to a plate thickness of 70 mm or more and 200 mm or less.

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