Method for producing hot-rolled steel sheet for non-oriented electromagnetic steel sheet and method for producing non-oriented electromagnetic steel sheet

Abstract

Proposed is a method for producing a hot-rolled steel sheet for a non-oriented electromagnetic steel sheet, the hot-rolled steel sheet being excellent in magnetic characteristics. In the method, a thin slab is hot-rolled. This production method comprises: casting molten steel containing, in mass%, C: ≤ 0.010%, Si: 2.50-5.00%, Mn: 0.10-3.00%, P: ≤ 0.100%, S: 0.0010-0.0050%, Al: ≤ 2.00%, N: ≤ 0.0080%, Cu: ≤ 1.00%, Mo: ≤ 0.050%, Zn: ≤ 0.010%, and Ti: ≤ 0.010%, with the balance being Fe and unavoidable impurities, into a steel slab having a prescribed thickness; subsequently heating the steel slab in a tunnel furnace, with the surface temperature of the steel slab at the exit of the tunnel furnace, the heating time in the tunnel furnace, and the cooling rate of the steel slab from the exit of the tunnel furnace to the entrance of the hot-rolling mill being within predetermined ranges; and subjecting the steel slab to hot-rolling, with the entrance temperature, exit temperature, strain rate during the first pass of the hot-rolling, and coil winding temperature for the hot-rolling mill being within predetermined ranges.

Description

Manufacturing method of hot-rolled steel sheet for non-oriented electrical steel sheet and manufacturing method of non-oriented electrical steel sheet

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

[0002] Non-oriented electrical steel sheets are materials used for the iron cores of motors and generators. ２ There is a strong demand for higher efficiency in electrical equipment to reduce emissions, and as a result, there is a demand for even lower iron loss in non-oriented electrical steel sheets, which are used as iron core materials.

[0003] To reduce iron loss in non-oriented electrical steel sheets, it is effective to increase the resistivity by adding alloying elements such as Si, Al, and Mn, or to reduce the sheet thickness. However, increasing the alloying level and reducing the sheet thickness increases the rolling load during cold rolling. This has led to the problem of the steel sheet being more susceptible to fracture during cold rolling. The rolling load during cold rolling can be reduced by reducing the thickness of the hot-rolled steel sheet to be subjected to cold rolling. However, this increases the rolling load during hot rolling, which has led to the problem of making shape control difficult.

[0004] Therefore, a method for producing non-oriented electrical steel sheets using thinner steel slabs (hereinafter also referred to as thin slabs) than conventional methods has been proposed. In this method, a thin slab is produced using a continuous casting machine called a thin slab caster, and then the thin slab is subjected to rolling. By reducing the thickness of the steel slab, the rolling load in both hot rolling and cold rolling can be reduced. Furthermore, in methods using thin slabs, the continuous casting machine (thin slab caster) is usually directly connected to the hot rolling mill, thereby eliminating the need for slab reheating. In this case, energy costs can be significantly reduced.

[0005] An example of a method for manufacturing a non-oriented electrical steel sheet using a thin slab is the method disclosed in Patent Document 1. Patent Document 1 discloses a technique in which a steel slab having a thickness of 50 mm or more and 200 mm or less is kept at a certain temperature and then hot-rolled to produce a hot-rolled steel sheet having a thickness of 0.4 mm or more and 2.0 mm or less.

[0006] WO 2023 / 095637

[0007] However, the above-mentioned conventional techniques have the following problems to be solved. Specifically, in the method disclosed in Patent Document 1, the steel slab is subjected to hot rolling while maintained at a high temperature. As a result, precipitates in the steel do not sufficiently precipitate and coarsen before hot rolling, and may precipitate finely during hot rolling. These fine precipitates then refine the steel structure, resulting in a problem of increasing the iron loss of the manufactured non-oriented electrical steel sheet.

[0008] Therefore, in order to solve the above-mentioned problems of the prior art, the present invention specifically aims to obtain a hot-rolled steel sheet in which fine precipitation of precipitates is suppressed when a thin slab is hot-rolled while being maintained at a high temperature by heat retention treatment, and also aims to manufacture a non-oriented electrical steel sheet using the hot-rolled steel sheet in which an increase in iron loss is suppressed.

[0009] As a result of extensive research, the inventors have found that by controlling the S content and setting the conditions of the hot rolling process within appropriate ranges, it is possible to suppress the formation of fine precipitates and to suppress an increase in iron loss in non-oriented electrical steel sheets manufactured using thin slabs.

[0010] That is, the method for producing a hot-rolled steel sheet for non-oriented electrical steel sheet according to the present invention, which advantageously solves the above-mentioned problems, comprises, in mass%, C: 0.010% or less, Si: 2.50% or more and 5.00% or less, Mn: 0.10% or more and 3.00% or less, P: 0.100% or less, S: 0.0010% or more and 0.0050% or less, Al: 2.00% or less, N: 0.0080% or less, Cu: 1.00% or less, Mo: 0.050% or less, Zn: 0.010% or less, and Ti: 0.010% or less and optionally further containing: Group A: at least one selected from Sn: 0.20% or less and Sb: 0.20% or less; Group B: at least one selected from Mg: 0.0001% or more and 0.10% or less and REM: 0.0001% or more and 0.10% or less; Group C: B: 0.002% or more and 0.01% or less; Group D: Ni: 0.01% or more and 1.0% or less; Group E: Cr: 0.1% or more and 5.0% or less; Group F: V: 0.001% or more and 0. at least one selected from Nb: 0.001% or more and 0.005% or less, Ta: 0.0001% or more and 0.0020% or less, W: 0.001% or more and 0.050% or less, and Pb: 0.0001% or more and 0.0020% or less; G group: Co: 0.001% or more and 0.100% or less; H group: at least one selected from Ga: 0.0005% or more and 0.0300% or less, and Ge: 0.0005% or more and 0.0300% or less;and I group: As: 0.001% or more and 0.020% or less, with the balance being Fe and inevitable impurities. A method for producing a hot-rolled steel sheet for use in a non-oriented electrical steel sheet, the method comprising: casting a molten steel having a component composition into a steel slab having a thickness of 30 mm or more and 180 mm or less; subsequently, a heat-retention step of retaining the heat of the steel slab in a tunnel furnace; and a hot-rolling step of hot-rolling the steel slab. In the heat-retention step, the surface temperature of the steel slab at the inlet side of the tunnel furnace is set to 850°C or more, and the surface temperature of the steel slab is set to 850°C or more. the surface temperature of the steel slab at the exit side of the tunnel furnace is in the range of 1050°C or more and 1200°C or less, the heat retention time in the tunnel furnace is 8 minutes or more, the hot rolling step is performed under conditions where the cooling rate of the steel slab from the exit side of the tunnel furnace to the entry side of the hot rolling mill is 4°C / s or less, the entry temperature of the hot rolling mill is 950°C or more, the exit temperature of the hot rolling mill is 800°C or more, the strain rate in the first pass of hot rolling is 1.5 / s or more, and the coil winding temperature is 500°C or more;

[0011] In a more preferred embodiment of the method for producing a hot-rolled steel sheet for use as a non-oriented electrical steel sheet according to the present invention, the molten steel is tapped from an electric furnace or a converter.

[0012] Furthermore, the present invention provides a method for manufacturing a non-oriented electrical steel sheet that advantageously solves the above-mentioned problems, comprising: a hot-rolled steel sheet manufacturing step of manufacturing a hot-rolled steel sheet by any one of the above-mentioned methods for manufacturing a hot-rolled steel sheet for a non-oriented electrical steel sheet; a hot-rolled sheet 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 finish annealing step of finish annealing the cold-rolled steel sheet.

[0013] According to the present invention, a hot-rolled steel sheet can be obtained in which the formation of fine precipitates is suppressed when a thin slab is subjected to a heat-holding treatment and hot-rolled while being maintained at a high temperature. This hot-rolled steel sheet can be used to produce a non-oriented electrical steel sheet with good magnetic properties.

[0014] S content and iron loss W of non-oriented electrical steel sheets in Experiment 1 １０／４００1 is a graph showing the relationship between the temperature at the entry side of the steel slab into the hot rolling mill and the iron loss W in the hot rolling of non-oriented electrical steel sheets in Experiment 2. １０／４００ 1 is a graph showing the relationship between the strain rate and iron loss W in the first pass of hot rolling for non-oriented electrical steel sheets in Experiment 3. １０／４００ 10 is a graph showing the relationship between

[0015] Before describing embodiments of the present invention, we first describe the experiments that inspired the development of the present invention. In the following description, "%" representing chemical composition is based on mass unless otherwise specified. <Experiment 1> The inventors focused on controlling MnS, which precipitates finely in steel and increases iron loss, and conducted iron loss measurements. A steel composition containing 0.002% C, 3.00% Si, 0.50% Mn, 0.01% P, 0.50% Al, 0.0020% N, 0.01% Cu, 0.010% Mo, 0.001% Zn, 0.002% Ti, with the balance being Fe and unavoidable impurities, was used as the base. S was added to this composition in a range of 0.0001 to 0.0070%, and the resulting molten steel was cast into a 60 mm thick steel slab. The steel slab was then maintained at 1100°C in a tunnel furnace. The hot rolling entry temperature was 960°C, the strain rate in the first pass of the hot rolling was 5.0 / s, and the coiling temperature was 520°C, resulting in a hot-rolled steel sheet with a thickness of 1.6 mm. Next, the hot-rolled steel sheet was annealed at an ambient temperature of 980°C for 30 seconds. Thereafter, the cold-rolled steel sheet was obtained with a thickness of 0.30 mm. Finally, the H ２ :N ２ The steel sheets were subjected to finish annealing at a temperature of 960°C for 10 seconds in a dry atmosphere with a carbon content of 25:75. Epstein samples measuring 30 mm in width and 280 mm in length were cut out from the steel sheets in the rolling direction and the width direction, and the iron loss W １０／４００ was measured using an Epstein tester.

[0016] Figure 1 shows the relationship between the S content in steel sheets and iron loss W １０／４００ It is clear from Figure 1 that iron loss is reduced by controlling the S content within the range of 0.0010 to 0.0050%. Furthermore, the grain size of the steel sheet observed with an optical microscope was coarsened in the range where iron loss was reduced.

[0017] From the above experimental results, the inventors estimated that the influence of precipitates on the reduction of grain growth was suppressed when the S content was within the above range. Specifically, when the S content was less than 0.0010%, the number of precipitated MnS was small, so that precipitates such as TiN and TiC, which have lower precipitation temperatures than MnS, could not precipitate using MnS as nuclei. Therefore, these precipitates were finely dispersed, reducing grain growth. When the S content exceeded 0.0050%, the number of MnS increased, reducing grain growth.

[0018] <Experiment 2> In light of the above results, we investigated the conditions necessary for controlling the hot rolling process to precipitate coarse precipitates and reduce fine precipitates. A steel composition containing 0.002% C, 3.00% Si, 0.50% Mn, 0.01% P, 0.50% Al, 0.0020% N, 0.01% Cu, 0.010% Mo, 0.001% Zn, 0.002% Ti, 0.0020% S, with the balance being Fe and unavoidable impurities, was used as the base steel. The resulting molten steel was then cast into a 60 mm thick steel slab. The steel slab was then maintained at 1100°C in a tunnel furnace. The hot rolling entry temperature was changed in the range of 875 to 1025°C, the strain rate in the first pass of hot rolling was set to 5.0 / s, and the coil winding temperature was set to 520°C, to produce hot rolled steel sheets with a thickness of 1.6 mm. The obtained hot rolled steel sheets were subjected to the same procedure as in Experiment 1, and the iron loss was measured.

[0019] Figure 2 shows the relationship between the entry temperature and iron loss W １０／４００ The relationship between the temperature and the iron loss is shown in Figure 2. As can be seen from Figure 2, when the entry temperature of hot rolling falls below 950°C, iron loss increases. The inventors presumed that this is because strain is introduced during rolling, which increases the driving force for precipitate precipitation, and that when the temperature of the steel sheet is low, precipitates such as TiN and TiC precipitate simultaneously with MnS and become finely dispersed. They also presumed that when the temperature of the steel sheet is low, the grain growth of precipitates is suppressed, resulting in fine precipitates.

[0020] <Experiment 3> Next, the effect of strain rate during the first pass of hot rolling on the morphology of precipitates was investigated. A steel composition containing 0.002% C, 3.00% Si, 0.50% Mn, 0.01% P, 0.50% Al, 0.0020% N, 0.01% Cu, 0.010% Mo, 0.001% Zn, 0.002% Ti, 0.0020% S, with the balance being Fe and unavoidable impurities, was used as the base. The resulting molten steel was then cast into a 60 mm thick steel slab. The steel slab was then maintained at 1100°C in a tunnel furnace. Then, the hot rolling entry temperature was set to 960°C, the strain rate in the first pass of the hot rolling was changed in the range of 0.2 to 4.2 / s, and the coil winding temperature was set to 520°C, to obtain hot-rolled steel sheets with a thickness of 1.6 mm. The obtained hot-rolled steel sheets were subjected to the same procedure as in Experiment 1, and the iron loss was measured.

[0021] Figure 3 shows the relationship between the strain rate and iron loss W during the first pass of hot pressing. １０／４００ 3 shows that when the strain rate in the first pass of hot rolling is set to less than 1.5 / s, iron loss increases. The inventors presumed that when the strain rate is low, the driving force for precipitation is small, so MnS does not precipitate sufficiently in the first pass of hot rolling, and precipitates in subsequent passes where the rolling temperature is low, and therefore fine precipitates such as TiN and TiC cannot precipitate around MnS.

[0022] From the results of Experiments 1 to 3 above, it was found that even when a thin slab caster is used, an increase in iron loss can be suppressed by setting the S content to 0.0010% or more and 0.0050% or less, setting the hot rolling entry temperature to 950°C or more, and setting the strain rate in the first pass of hot rolling to 1.5 / s. The present invention is based on the above findings. Hereinafter, specific embodiments for carrying out the present invention will be described. However, the present invention is not limited to these embodiments.

[0023] <Composition> The reasons for limiting the composition of the molten steel in this embodiment will be explained below. The cast thin slab also has the same composition as the molten steel.

[0024] C: 0.010% or less C is a harmful element that causes magnetic aging in the finished steel sheet, forming carbides and degrading iron loss. Therefore, in this embodiment, in order to suppress the magnetic aging, the C content is set to 0.010% or less. Preferably, the C content is 0.005% or less. Although there is no particular lower limit, from the viewpoint of suppressing decarburization costs, it is preferable that the lower limit of the C content be approximately 0.0001%.

[0025] Si: 2.50% or more and 5.00% or less In order to increase the electrical resistance of the steel sheet and sufficiently reduce iron loss, the addition of 2.50% or more of Si is necessary. Therefore, in this embodiment, the Si content is set to 2.50% or more. On the other hand, if the Si content exceeds 5.00%, rolling becomes difficult. Therefore, the Si content is set to 5.00% or less. From the viewpoint of manufacturability, the Si content is preferably 4.00% or less.

[0026] Mn: 0.10% or more and 3.00% or less Like Si and Al, Mn has the effect of increasing the electrical resistance of the steel sheet and reducing iron loss. Therefore, in this embodiment, the Mn content is set to 0.10% or more. On the other hand, if the Mn content exceeds 3.00%, Mn carbides precipitate, which actually worsens iron loss. Therefore, the Mn content is set to 3.00% or less. A preferable range is 0.20% or more and 1.00% or less.

[0027] 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 embrittled, resulting in a decrease in manufacturability. Therefore, the P content is set to 0.100% or less. Although there is no particular lower limit, from the viewpoint of suppressing the cost of dephosphorization, the P content is preferably set to 0.001% or more. Furthermore, from the viewpoint of enhancing the effect of adding P, the P content is more preferably set to 0.005% or more, and even more preferably set to 0.010% or more.

[0028] S: 0.0010% or more and 0.0050% or less S exists mainly as MnS in steel, and can be rendered harmless by complex precipitation with precipitates such as TiC and TiN, which have a negative effect on magnetic properties when precipitated alone. Therefore, in this embodiment, the S content is set to 0.0010% or more. On the other hand, if the S content exceeds 0.0050%, the amount of MnS precipitated increases, inhibiting grain growth and increasing iron loss. Therefore, the S content is set to 0.0050% or less. Preferably, the S content is set to 0.0010% or more and 0.0040% or less.

[0029] Al: 2.00% or less Like Si, Al has the effect of increasing the electrical resistance of the steel sheet and reducing iron loss. However, if the Al content exceeds 2.00%, rolling becomes difficult. Therefore, the Al content is set to 2.00% or less. From the viewpoint of improving castability, the Al content is preferably set to 1.50% or less. On the other hand, although there is no lower limit for the Al content, from the viewpoint of the balance between iron loss and manufacturability, the Al content is preferably 0.20% or more, more preferably 0.30% or more, and even more preferably 0.50% or more.

[0030] N: 0.0080% or less N is a harmful element that forms fine nitrides, inhibits grain growth, and increases iron loss, so it is preferable to reduce it as much as possible. In particular, if the N content exceeds 0.0080%, the above-mentioned adverse effects become significant, so the N content is set to 0.0080% or less. Preferably, it is set to 0.0030% or less. On the other hand, from the viewpoint of iron loss, the lower the N content, the better, so the lower limit of the N content is not limited and may be 0%. However, since N is an element that is inevitably mixed into steel as an impurity, excessive reduction in N content increases manufacturing costs. Therefore, from the viewpoint of cost, the N content is preferably set to 0.0001% or more, and more preferably 0.0005% or more.

[0031] Cu: 1.00% or less Cu is an element that has the effect of improving the magnetic flux density of steel sheets. However, if the Cu content exceeds 1.00%, it causes hot embrittlement and surface defects. Therefore, when Cu is added, the Cu content is set to 1.00% or less. From the viewpoint of balancing magnetic properties and cost, the Cu content is preferably set to 0.1% or less. On the other hand, although there is no lower limit for the Cu content, from the viewpoint of enhancing the effect of adding Cu, the Cu content is preferably set to 0.01% or more.

[0032] Mo: 0.050% or less Mo reacts with C to form carbides at grain boundaries, thereby improving strength. However, if the Mo content exceeds 0.050%, iron loss tends to increase. Therefore, when Mo is added, the Mo content is set to 0.050% or less. On the other hand, although there is no lower limit for the Mo content, from the viewpoint of strength, it is preferable that the Mo content be 0.010% or more.

[0033] Zn: 0.010% or less Zn reacts with S to form coarse sulfides, suppressing the precipitation of fine sulfides such as MnS and reducing iron loss. However, if the content exceeds 0.010%, the amount of the sulfides increases, inhibiting grain growth and increasing iron loss. Therefore, the Zn content is set to 0.010% or less. On the other hand, although there is no lower limit for the Zn content, from the viewpoint of reducing iron loss, it is preferable that the Zn content be 0.001% or more.

[0034] Ti: 0.010% or less Ti reacts with C and N to form carbides and nitrides at grain boundaries, and thus has the effect of improving strength, similar to Mo. However, if the Ti content exceeds 0.010%, the amount of the above-mentioned carbides and nitrides increases, which in turn inhibits grain growth and increases iron loss. Therefore, the Ti content is set to 0.010% or less. On the other hand, although there is no lower limit for the Ti content, from the viewpoint of strength, it is preferable that the Ti content be 0.002% or more.

[0035] In this embodiment, the molten steel has a component composition containing C: 0.010% or less, Si: 2.50% or more and 5.00% or less, Mn: 0.10% or more and 3.00% or less, P: 0.100% or less, S: 0.0010% or more and 0.0050% or less, Al: 2.00% or less, N: 0.0080% or less, Cu: 1.00% or less, Mo: 0.050% or less, Zn: 0.010% or less, and Ti: 0.010% or less, with the balance being Fe and unavoidable impurities.

[0036] In this embodiment, the composition of the molten steel may further optionally contain at least one group of elements selected from the following groups A to I:

[0037] Group A: At least one selected from Sn: 0.20% or less and Sb: 0.20% or less Sn: 0.20% or less Sn is an element that has the effect of suppressing nitriding and oxidation of the surface layer and reducing iron loss. However, even if added in excess of 0.20%, the effect saturates. Therefore, when Sn is added, the Sn content is preferably 0.20% or less, and more preferably 0.10% or less. On the other hand, from the viewpoint of enhancing the above effect, the Sn content is preferably 0.005% or more.

[0038] Sb: 0.20% or less Sb is an element that has the effect of suppressing nitriding and oxidation of the surface layer and reducing iron loss. However, even if added in an amount exceeding 0.20%, the effect saturates. Therefore, when Sb is added, the Sb content is preferably 0.20% or less, and more preferably 0.10% or less. On the other hand, from the viewpoint of enhancing the above effect, the Sb content is preferably 0.005% or more.

[0039] Group B: At least one selected from Mg: 0.0001% to 0.10% and REM: 0.0001% to 0.10% Mg: 0.0001% to 0.10% Mg is an element that fixes S as sulfides and contributes to reducing iron loss. To achieve this effect, the Mg content should be 0.0001% or more. On the other hand, if the Mg content exceeds 0.10%, the effect saturates and costs increase unnecessarily, so the upper limit is set to 0.10%. Therefore, it is preferable that Mg be contained in the range of 0.0001% to 0.10%.

[0040] REM: 0.0001% or more and 0.10% or less REM (rare earth metal elements) are a group of elements that fix S as sulfides and contribute to reducing iron loss. To achieve this effect, the REM content should be 0.0001% or more. On the other hand, if the REM content exceeds 0.10%, the effect saturates and costs increase unnecessarily, so the upper limit is set to 0.10%. Therefore, the REM content is preferably in the range of 0.0001% or more and 0.10% or less. REM is a collective term for 17 elements: Sc, Y, and lanthanides.

[0041] Group C: B: 0.002% or more and 0.01% or less B has the effect of forming fine carbides in steel and increasing the strength of the steel sheet. To obtain this effect, the B content should be 0.002% or more. On the other hand, if the B content exceeds 0.01%, excessive carbides are formed and iron loss deteriorates, so the upper limit is set to 0.01%. Therefore, the B content is preferably in the range of 0.002% or more and 0.01% or less.

[0042] Group D: Ni: 0.01% or more and 1.0% or less Ni is an element that improves the toughness of steel and can be added as needed. To obtain this effect, the Ni content should be 0.01% or more. However, if the Ni content exceeds 1.0%, the effect saturates, so the upper limit of the Ni content is set to 1.0%. Therefore, the Ni content is preferably in the range of 0.01% or more and 1.0% or less.

[0043] Group E: Cr: 0.1% or more and 5.0% or less Cr has the effect of increasing the resistivity of steel and reducing iron loss. To obtain this effect, the Cr content should be 0.1% or more. 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, so the upper limit is set to 5.0%. Therefore, the Cr content is preferably in the range of 0.1% or more and 5.0% or less.

[0044] Group F: at least one selected from V: 0.001% to 0.050%, Nb: 0.001% to 0.005%, Ta: 0.0001% to 0.0020%, W: 0.001% to 0.050%, and Pb: 0.0001% to 0.0020%; V: 0.001% to 0.050%. V is an element that has the effect of increasing the strength of the steel sheet and can be added as needed. To achieve this effect, the V content should be 0.001% or more. However, if the V content exceeds 0.050%, fine precipitates will form in the steel sheet, increasing iron loss. Therefore, the upper limit of the V content is set to 0.050%.

[0045] Nb: 0.001% or more and 0.005% or less Nb is an element that has the effect of increasing the strength of steel sheet and can be added as needed. To obtain this effect, the Nb content should be 0.001% or more. However, if the Nb content exceeds 0.005%, fine precipitates will form in the steel sheet, increasing iron loss, so the upper limit of the Nb content is set to 0.005%.

[0046] Ta: 0.0001% or more and 0.0020% or less Ta is an element that has the effect of increasing the strength of the steel sheet and can be added as needed. To obtain this effect, the Ta content should be 0.0001% or more. However, if the Ta content exceeds 0.0020%, fine precipitates will form in the steel sheet, increasing iron loss, so the upper limit of the Ta content is set to 0.0020%.

[0047] W: 0.001% or more and 0.050% or less W is an element that has the effect of increasing the strength of the steel sheet and can be added as needed. To obtain this effect, the W content should be 0.001% or more. However, if the W content exceeds 0.050%, fine precipitates will form in the steel sheet, increasing iron loss, so the upper limit of the W content is set to 0.050%.

[0048] Pb: 0.0001% or more and 0.0020% or less Pb is an element that has the effect of increasing the strength of steel sheet and can be added as needed. To obtain this effect, the Pb content should be 0.0001% or more. However, if the Pb content exceeds 0.0020%, fine precipitates will form in the steel sheet, increasing iron loss, so the upper limit of the Pb content is set to 0.0020%.

[0049] Group G: Co: 0.001% or more and 0.100% or less Co is an element that has the effect of increasing the magnetic flux density of the steel sheet and can be added as needed. To achieve this effect, the Co content should be 0.001% or more. However, adding a large amount of Co increases the alloy cost, so the upper limit of the Co content is set to 0.100%.

[0050] H group: at least one selected from Ga: 0.0005% to 0.0300% and Ge: 0.0005% to 0.0300% 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 needed. To achieve this effect, the Ga content should be 0.0005% or more. However, adding a large amount of Ga saturates the effect and increases the alloy cost, so the upper limit of the Ga content is set to 0.0300%.

[0051] Ge: 0.0005% or more and 0.0300% or less 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 needed. To obtain such effects, the Ge content should be 0.0005% or more. However, adding a large amount of Ge saturates the effect and increases the alloy cost, so the upper limit of the Ge content is set to 0.0300%.

[0052] Group I: As: 0.001% or more and 0.020% or less As is an element that has the effect of increasing the strength of steel sheets and can be added appropriately. To obtain this effect, the As content should be 0.001% or more. However, if the As content exceeds 0.020%, the risk of fracture during cold rolling increases. Therefore, the upper limit of the As content is set to 0.020%.

[0053] Incidentally, unavoidable impurities include, for example, O (oxygen), and the content of about 0.0050% is permissible. Of the above elements, contents below the preferred range for effective action are permissible as unavoidable impurities because they do not affect the magnetic properties of the product.

[0054] [Manufacturing Conditions for Hot-Rolled Steel Sheet for Non-Oriented Electrical Steel Sheet] Next, manufacturing conditions for manufacturing a hot-rolled steel sheet for non-oriented electrical steel sheet using molten steel having the above-mentioned chemical composition will be described.

[0055] The method for producing a hot-rolled steel sheet for non-oriented electrical steel sheet according to this embodiment includes a step of casting molten steel having the above-described chemical composition into a steel slab having a thickness of 30 mm to 180 mm, followed by a heat-retention step of retaining the slab in a tunnel furnace, and a hot-rolling step of hot-rolling the slab. In the heat-retention step, the surface temperature of the steel slab at the entrance of the tunnel furnace is set to 850°C or higher, and the surface temperature of the steel slab at the exit of the tunnel furnace is set to 1050°C or higher and 1200°C or lower. The heat-retention time in the tunnel furnace is set to 8 minutes or longer, and the cooling rate of the steel slab from the exit of the tunnel furnace to the entry of the hot rolling mill is set to 4°C / s or lower. In the hot rolling process, the hot-rolled steel sheet is produced under the following conditions: the inlet temperature of the hot rolling mill is 950°C or higher, the outlet temperature of the hot rolling mill is 800°C or higher, the strain rate in the first pass of hot rolling is 1.5 / s or higher, and the coil winding temperature is 500°C or higher.

[0056] [Molten Steel Melting Process] The method for adjusting the composition of molten steel is not particularly limited and can be any method. For example, a converter, an electric furnace, a vacuum degasser, or other devices and methods can be used to adjust the composition of molten steel. It is preferable that molten steel is tapped from a converter or an electric furnace. For example, if reduced iron or scrap is melted in an electric furnace, CO２ This is preferable as it contributes to reducing emissions.

[0057] [Continuous Casting Step] Next, the molten steel with the adjusted composition is continuously cast to produce a steel slab. The method for continuous casting is not particularly limited, and can be carried out according to a conventional method.

[0058] Steel slab thickness: 30 mm or more and 180 mm or less In the above continuous casting process, steel slabs with a thickness of 30 mm or more and 180 mm or less are produced. If the thickness of the steel slab is less than 30 mm, the surface area relative to the slab volume increases, and the cooling rate of the steel slab increases. As a result, the temperature of the steel slab cannot be maintained in the hot rolling process. Therefore, the thickness of the steel slab is set to 30 mm or more. On the other hand, if the thickness of the steel slab exceeds 180 mm, the rolling load in the hot rolling process increases. This increases the risk of steel strip breakage. Therefore, the thickness of the steel slab is set to 180 mm or less.

[0059] [Transportation Process] Next, the steel slab produced in the continuous casting process is transported to a tunnel furnace where it is used for heat retention treatment. In the transportation process, it is important to transport the steel slab to the entrance of the tunnel furnace while maintaining the surface temperature of the steel slab at 850°C or higher. In other words, in this embodiment, the steel slab is transported so that its surface temperature does not fall below 850°C between the time it is produced in the continuous casting process and the time it reaches the furnace. If the surface temperature of the steel slab falls below 850°C, the energy required to reheat the steel slab increases, and the energy-saving effect cannot be obtained. Therefore, the surface temperature of the steel slab at the entrance of the tunnel furnace is set to 850°C or higher. Preferably, it is 900°C or higher. Although there is no upper limit, considering that the steel slab is cooled to completely solidify during continuous casting, the upper limit of the surface temperature of the steel slab is preferably about 1200°C.

[0060] In the transporting step, the steel slab may be transported to the tunnel furnace after being cut, but from the viewpoint of suppressing a decrease in the temperature of the slab, it is preferable to transport the steel slab directly to the tunnel furnace without cutting it.

[0061] [Heat Retention Process] Next, at the outlet side of the tunnel furnace, heat retention treatment is performed so that the surface temperature of the steel slab is 1050°C or higher and 1200°C or lower for a heat retention time of 8 minutes or longer. The heat retention treatment promotes the precipitation and coarsening of precipitates such as MnS and AlN. This neutralizes fine precipitates that inhibit grain growth in the hot-rolled sheet annealing process and finish annealing process when manufacturing non-oriented electrical steel sheet.

[0062] Surface temperature of the steel slab at the exit side of the tunnel furnace: 1050°C or higher and 1200°C or lower. If the temperature of the steel slab is lower than 1050°C, the precipitates do not coarsen sufficiently. As a result, the precipitates remain fine on the slab, inhibiting grain growth during hot-rolled sheet annealing and finish annealing. This increases the iron loss of the product. Therefore, the surface temperature of the steel slab at the exit side of the tunnel furnace, i.e., the dwelling temperature, is set to 1050°C or higher, preferably 1100°C or higher. On the other hand, if the dwelling temperature is higher than 1200°C, the precipitation of precipitates does not progress. Therefore, fine precipitates precipitate during hot rolling after the dwelling treatment. As a result, grain growth is inhibited during the hot-rolled sheet annealing process and finish annealing process, increasing the iron loss of the product. Therefore, the dwelling temperature is set to 1200°C or lower, preferably 1150°C or lower.

[0063] Heat retention time: 8 min or more If the heat retention time in the tunnel furnace is less than 8 min, the precipitation and coarsening of precipitates do not proceed sufficiently. As a result, grain growth during hot-rolled sheet annealing and finish annealing is hindered, and iron loss of the product increases. Therefore, the heat retention time is set to 8 min or more. On the other hand, there is no particular upper limit to the heat retention time, but if it exceeds 30 min, the effect saturates and the length of the tunnel furnace increases, resulting in increased equipment costs. Therefore, the heat retention time is preferably set to 30 min or less, and more preferably 15 min or less.

[0064] The heating method in the heat retention treatment is not particularly limited, and any method such as induction heating, a gas furnace, or an electric furnace can be used. However, when a gas furnace is used, a large amount of scale is generated on the surface of the steel sheet due to combustion gas. Therefore, when removing the scale in the hot rolling process described below, water is sprayed vigorously, which increases the cooling rate of the steel slab. Therefore, from the viewpoint of maintaining the temperature of the steel slab at a high temperature, it is preferable to use induction heating or an electric furnace, which are heating methods that use electrical energy.

[0065] [Hot Rolling Process] Next, the hot rolling process is carried out, in which the steel slab is transported to a hot rolling mill, hot rolled, and wound into a coil to produce a hot-rolled steel sheet. In this embodiment, the hot rolling process is carried out under conditions that satisfy the following (1) to (5), and it is important for reducing iron loss that MnS is used as a nucleus to cause the complex precipitation of fine precipitates such as TiC and TiN. (1) Cooling rate of the steel slab from the exit side of the tunnel furnace to the entry side of the hot rolling mill: 4°C / s or less (2) Surface temperature of the steel slab at the entry side of the hot rolling mill: 950°C or more (3) Strain rate in the first pass of hot rolling: 1.5 / s or more (4) Temperature of the steel sheet at the exit side of the hot rolling mill: 800°C or more (5) Coil winding temperature: 500°C or more

[0066] Cooling rate of steel slab: 4°C / s or less. After the heat-holding process, the steel slab reacts with oxygen in the air to form scale on its surface. If the slab is subjected to hot rolling with the remaining scale, the scale will be incorporated, resulting in surface defects. Therefore, after leaving the tunnel furnace, the steel slab is descaled with high-pressure water. This removes the surface scale and cools the steel slab. If the cooling rate is greater than 4°C / s, the areas of the steel slab exposed to the high-pressure water will be rapidly cooled locally. This will significantly lower the temperature at which MnS precipitates, causing the precipitation of fine precipitates such as TiC and TiN, which will increase the iron loss of the product. Therefore, the cooling rate is set to 4°C / s or less. On the other hand, a faster cooling rate is preferable from the perspective of scale removal. Therefore, the cooling rate of the steel slab is preferably 2°C / s or more.

[0067] Surface temperature of the steel slab at the entry side of the hot rolling mill: 950°C or higher If the surface temperature of the steel slab at the entry side of the hot rolling mill, i.e., the entry temperature, is less than 950°C, the driving force for precipitation becomes large. As a result, fine precipitates such as TiC and TiN precipitate simultaneously with MnS, and composite precipitation cannot occur with MnS as a nucleus. Therefore, the entry temperature of the hot rolling mill is set to 950°C or higher. On the other hand, there is no particular restriction on the upper limit of the entry temperature of the hot rolling mill, and it is preferable to set the temperature closer to the surface temperature of the steel slab at the exit side of the tunnel furnace, as this can suppress the precipitation of TiC and TiN.

[0068] Strain rate in the first pass of hot rolling: 1.5 / s or more. If the strain rate in the first pass of hot rolling is less than 1.5 / s, the rate at which dislocations, which are the driving force for precipitation, are introduced will exceed the rate at which dislocations recover. Therefore, MnS precipitation will not proceed sufficiently during hot rolling. Therefore, the strain rate in the first pass of hot rolling is set to 1.5 / s or more. On the other hand, there is no particular upper limit for the strain rate in the first pass of hot rolling, but if the strain rate is too high, even fine precipitates such as TiC and TiN will precipitate. Therefore, the strain rate in the first pass of hot rolling is preferably 4.0 / s or less. Here, the strain rate was calculated using the following equations (1) and (2) of Equation 1.

[0069]

[0070] Here, ε (with dots) is the strain rate (1 / s) in the first pass of hot rolling, h ０ is the thickness of the steel slab before hot rolling (m), h １ is the thickness of the rolled material after one pass of hot rolling (m), t ｃ is the time (s) required for the first pass of hot rolling, R is the roll diameter (m) of the first pass of hot rolling, ν ０ indicates the speed (m / min) of the material being rolled.

[0071] Temperature of the steel sheet at the delivery side of the hot rolling mill: 800°C or higher If the temperature of the hot-rolled steel sheet at the delivery side of the hot rolling mill is less than 800°C, the driving force for precipitation becomes large. As a result, fine precipitates such as TiC and TiN precipitate simultaneously with MnS, and composite precipitation cannot occur with MnS as a nucleus. Therefore, the temperature of the hot-rolled steel sheet at the delivery side of the hot rolling mill is set to 800°C or higher. On the other hand, there is no particular restriction on the upper limit of the temperature of the hot-rolled steel sheet at the delivery side of the hot rolling mill, and it is preferable that the temperature is closer to the temperature of the material to be rolled at the entry side of the hot rolling mill, since this can suppress the precipitation of TiC and TiN.

[0072] The number of passes in the hot rolling is not particularly limited, and can be any number of passes equal to or greater than one.

[0073] The delivery thickness in finish rolling of hot rolling, i.e., the thickness of the finally obtained hot-rolled steel sheet, is not particularly limited and may be any thickness. However, if the delivery thickness is less than 0.4 mm, the overall length of the steel sheet may become excessively long, which may reduce productivity. Therefore, from the viewpoint of productivity, it is preferable that the delivery thickness in finish rolling be 0.4 mm or more. On the other hand, if the delivery thickness exceeds 2.0 mm, the load in cold rolling may become excessive. Therefore, from the viewpoint of reducing the load in cold rolling, it is preferable that the delivery thickness in finish rolling be 2.0 mm or less.

[0074] Coil winding temperature: 500°C or higher If the coil winding temperature is lower than 500°C, the driving force for precipitation of TiC and TiN will be weak. As a result, aging precipitation will not occur in the coiled state. Therefore, fine precipitation will occur in subsequent processes, increasing iron loss. For this reason, the coil winding temperature is set to 500°C or higher. On the other hand, there is no particular upper limit to the coil winding temperature, but there is a risk of the coil deforming if it is wound at a high temperature. Therefore, it is preferable that the coil winding temperature be set to a temperature of 550°C or lower.

[0075] [Manufacturing conditions for non-oriented electrical steel sheet] The manufacturing method of the non-oriented electrical steel sheet in this embodiment includes a hot-rolled steel sheet manufacturing step of manufacturing a hot-rolled steel sheet by the above manufacturing method, a hot-rolled steel sheet annealing step of annealing the hot-rolled steel sheet to form a hot-rolled annealed steel sheet, a cold-rolling step of cold-rolling the hot-rolled annealed steel sheet to form a cold-rolled steel sheet, and a finish-annealing step of finish-annealing the cold-rolled steel sheet. The above-mentioned steps of hot-rolled steel sheet annealing, cold rolling, and finish-annealing can be performed according to a conventional method.

[0076] After the hot-rolled sheet annealing, it is also preferable to perform pickling before cold rolling. Furthermore, after the finish annealing, it is preferable to form an insulating coating on the surface of the obtained non-oriented electrical steel sheet. There are no particular limitations on the pickling and the formation of the insulating coating, and they can be performed according to a conventional method.

[0077] Example 1 A 60 mm thick steel slab having the chemical composition shown in Table 1 was produced by continuous casting. Without cutting, the steel slab was transported to a tunnel-type electric furnace while maintaining its surface temperature at 850°C or higher, and subjected to a heat-holding treatment in the electric furnace. The surface temperature of the steel slab at the exit of the tunnel furnace was approximately 1100°C, and the heat-holding time was 10 minutes. Next, scale on the surface of the steel slab was removed using high-pressure water, and the steel slab was subjected to five passes of hot rolling and wound into a coil to obtain a hot-rolled steel sheet. In the hot rolling process, the cooling rate from the exit of the tunnel furnace to the hot rolling mill was 3°C / s, and the surface temperature of the steel slab at the entry of the hot rolling mill was 1000°C. The strain rate in the first pass of hot rolling was 3.0 / s, the temperature of the steel sheet at the exit of the hot rolling mill was 900°C, and the coiling temperature was 520°C.

[0078] The obtained hot-rolled steel sheet was subjected to hot-rolled annealing at 1000°C for 30 seconds to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet was then subjected to cold rolling to obtain a cold-rolled steel sheet with a thickness of 0.30 mm. The cold-rolled steel sheet was then subjected to finish annealing at 1000°C for 10 seconds to obtain a non-oriented electrical steel sheet.

[0079] 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 １０／４００was measured using an Epstein tester. １０／４００ It is good if the power consumption is 13.50 W / kg or less.

[0080] As can be seen from the results shown in Table 1, non-oriented electrical steel sheets with good iron loss properties can be obtained with a chemical composition that satisfies the conditions of the present invention. Note that Test Nos. 7, 13, 19, and 23 could not be evaluated because they broke during production. In the chemical composition column, "-" indicates that the component was not added or that the component was contained as an unavoidable impurity.

[0081]

[0082] Example 2 A steel having a component composition containing C: 0.002%, Si: 3.00%, Mn: 0.50%, P: 0.01%, S: 0.0030%, Al: 0.50%, N: 0.0020%, Cu: 0.01%, Mo: 0.010%, Zn: 0.001%, Ti: 0.002%, with the balance being Fe and unavoidable impurities was melted, and a steel slab was continuously cast to produce a hot-rolled steel sheet for use as a non-oriented electrical steel sheet under the conditions shown in Table 2.

[0083] The obtained hot-rolled steel sheet was subjected to hot-rolled annealing at 1000°C for 30 seconds to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet was then subjected to cold rolling to obtain a cold-rolled steel sheet with a thickness of 0.30 mm. The cold-rolled steel sheet was then subjected to finish annealing at 1000°C for 10 seconds to obtain a non-oriented electrical steel sheet.

[0084] 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 １０／４００ was measured using an Epstein tester. １０／４００ It is good if the power consumption is 13.50 W / kg or less.

[0085] As can be seen from the results shown in Table 2, under the manufacturing conditions that satisfy the requirements of the present invention, a non-oriented electrical steel sheet with good core loss properties can be obtained.

[0086]

Claims

1. Containing, by mass%, C: 0.010% or less, Si: 2.50% or more and 5.00% or less, Mn: 0.10% or more and 3.00% or less, P: 0.100% or less, S: 0.0010% or more and 0.0050% or less, Al: 2.00% or less, N: 0.0080% or less, Cu: 1.00% or less, Mo: 0.050% or less, Zn: 0.010% or less, and Ti: 0.010% or less, and optionally further containing: Group A: at least one selected from Sn: 0.20% or less and Sb: 0.20% or less; Group B: at least one selected from Mg: 0.0001% or more and 0.10% or less and REM: 0.0001% or more and 0.10% or less; Group C: B: 0.002% or more and 0.01% or less; D Group: Ni: 0.01% or more and 1.0% or less; E Group: Cr: 0.1% or more and 5.0% or less; F Group: at least one selected from V: 0.001% or more and 0.050% or less, Nb: 0.001% or more and 0.005% or less, Ta: 0.0001% or more and 0.0020% or less, W: 0.001% or more and 0.050% or less, and Pb: 0.0001% or more and 0.0020% or less; Group G: Co: 0.001% or more and 0.100% or less; H 1. A method for producing a hot-rolled steel sheet for use in a non-oriented electrical steel sheet, the method comprising: casting a molten steel having a component composition consisting of at least one element selected from group I: Ga: 0.0005% or more and 0.0300% or less and Ge: 0.0005% or more and 0.0300% or less; and group I: As: 0.001% or more and 0.020% or less, with the balance being Fe and inevitable impurities, into a steel slab having a thickness in the range of 30 mm or more and 180 mm or less, subsequently carrying out a heat retention step of retaining the steel slab in a tunnel furnace, and a hot rolling step of hot-rolling the steel slab, wherein in the heat retention step, the surface temperature of the steel slab at the entrance side of the tunnel furnace is set to 850°C or more, and the surface temperature of the steel slab at the exit side of the tunnel furnace is set to 1050°C or more and 1200°C or less, and the heat retention time in the tunnel furnace is 8 minutes or more, the hot rolling step is performed under conditions satisfying the following: a cooling rate of the steel slab from the exit side of the tunnel furnace to the entry side of the hot rolling mill is 4°C / s or less; an entry temperature of the hot rolling mill is 950°C or more; an exit temperature of the hot rolling mill is 800°C or more; a strain rate in the first pass of hot rolling is 1.5 / s or more; and a coil winding temperature is 500°C or more.

2. The method for producing a hot-rolled steel sheet for use as a non-oriented electrical steel sheet according to claim 1, wherein the molten steel is tapped from an electric furnace or a converter.

3. A method for manufacturing non-oriented electrical steel sheet, comprising: a hot-rolled steel sheet manufacturing process for manufacturing hot-rolled steel sheet by the method for manufacturing hot-rolled steel sheet for non-oriented electrical steel sheet according to claim 1 or 2; a hot-rolled sheet annealing process for annealing the hot-rolled steel sheet to form a hot-rolled annealed sheet; a cold-rolling process for cold-rolling the hot-rolled annealed sheet to form a cold-rolled steel sheet; and a finish annealing process for finish annealing the cold-rolled steel sheet.

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