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

Abstract

We propose a method for manufacturing hot-rolled steel sheets for non-oriented electrical steel sheets with excellent magnetic properties by hot-rolling thin slabs. In mass%, C: ≦0.010%, Si: 2.50-5.00%, The manufacturing method includes casting molten steel having a chemical composition containing Mn: 0.10 to 3.00%, P: ≦0.100%, S: 0.0010 to 0.0050%, Al: ≦2.00%, N: ≦0.0080%, Cu: ≦1.00%, Mo: ≦0.050%, Zn: ≦0.010%, Ti: ≦0.010%, with the balance being Fe and unavoidable impurities, into a steel slab of a predetermined thickness, subsequently holding the steel slab at a temperature in a tunnel furnace, and hot rolling in which the surface temperature of the steel slab at the exit of the tunnel furnace, the holding time in the tunnel furnace, and the cooling rate of the steel slab from the exit of the tunnel furnace to the entry of a hot rolling mill are all within predetermined ranges, and the entry temperature and exit temperature of the hot rolling mill, the strain rate in the first pass of hot rolling, and the coiling temperature are all within predetermined ranges.

Description

[Technical Field]

[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. [Background technology]

[0002] Non-oriented electrical steel sheets are materials used for the iron cores of motors and generators. In recent years, there has been a strong demand for higher efficiency in electrical equipment in order to reduce CO2 emissions. Therefore, 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 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 issue of steel sheets being more susceptible to fracture during cold rolling. The rolling load during cold rolling can be reduced by thinning the hot-rolled steel sheet used for cold rolling. However, this increases the rolling load during hot rolling, 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, so that slab reheating is often omitted. 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 constant 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. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2023 / 095637 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned conventional techniques have the following problems to be solved. That is, in the method disclosed in Patent Document 1, the steel slab is subjected to hot rolling while being 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. [Means for solving the problem]

[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 comprising: 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 0.050% or less, Nb: 0.00 at least one selected from the group consisting of 1% 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 the group consisting of 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 A method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet, the method comprising the steps of: casting a molten steel having a composition containing at least one element selected from the group consisting of elements mentioned above, with the balance being Fe and inevitable impurities, into a steel slab having a thickness of 30 mm to 180 mm; subsequently, a heat-retention step of retaining the temperature 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 entrance side of the tunnel furnace is set to 850°C or higher, and the surface temperature of the steel slab at the exit side of the tunnel furnace is set to 850°C or higher. The hot rolling process is characterized in that the surface temperature is in the range of 1050°C or higher and 1200°C or lower, the heat retention time in the tunnel furnace is 8 minutes or longer, and the hot rolling process is carried out 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 lower, the entry temperature of the hot rolling mill is 950°C or higher, the exit 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.

[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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a graph showing the relationship between the S content and iron loss W10 / 400 of non-oriented electrical steel sheets in Experiment 1. [Figure 2] 10 is a graph showing the relationship between the temperature of the steel slab on the inlet side to the hot rolling mill and the iron loss W10 / 400 in hot rolling of non-oriented electrical steel sheets in Experiment 2. [Figure 3] 10 is a graph showing the relationship between strain rate and iron loss W10 / 400 in the first pass of hot rolling for non-oriented electrical steel sheets in Experiment 3. DETAILED DESCRIPTION OF THE INVENTION

[0015] Before describing the embodiments of the present invention, the experiments that led to the development of the present invention will be described. 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. The steel composition used was 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. S was added to the base steel in a range of 0.0001 to 0.0070%. The resulting molten steel was then cast into a 60 mm thick steel slab. The steel slab was then heated to 1100°C in a tunnel furnace. The hot rolling entry temperature was 960°C, the strain rate in the first pass of hot rolling was 5.0 / s, and the coil winding 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 for 30 seconds at an ambient temperature of 980°C. After that, the cold-rolled steel sheet was obtained with a thickness of 0.30 mm, and finally, it was subjected to finish annealing for 10 seconds at a temperature of 960°C in a dry atmosphere with a vol% ratio of H2:N2 = 25:75. Epstein samples with a width of 30 mm and a length of 280 mm were cut out from the steel sheet 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 Figure 1 shows the relationship between iron loss and S content. It is clear that iron loss is reduced by controlling the S content within the range of 0.0010 to 0.0050%. Furthermore, when observed with an optical microscope, the grain size of the steel sheet 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 to control the hot rolling process to precipitate coarse precipitates and reduce the amount of fine precipitates. The steel composition used was 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. The resulting molten steel was then cast into a 60 mm thick steel slab. The slab was then heated to 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 Figure 2 shows that iron loss increased when the entry temperature of hot rolling fell below 950°C. The inventors speculated that this was because strain is introduced during rolling, which increases the driving force for precipitates, and that when the steel sheet temperature is low, precipitates such as TiN and TiC precipitate simultaneously with MnS and become finely dispersed. They also speculated that when the steel sheet temperature is low, the grain growth of precipitates is suppressed, resulting in finer 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 steel. 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 set to 960°C, the strain rate during the first pass was varied between 0.2 and 4.2 / s, and the coiling temperature was set to 520°C. A 1.6 mm thick hot-rolled steel sheet was obtained. 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 Figure 3 shows that iron loss increased when the strain rate in the first pass of hot rolling was set to less than 1.5 / s. The inventors presumed that because the driving force for precipitation was small when the strain rate was low, MnS did not precipitate sufficiently in the first pass of hot rolling, and instead precipitated in subsequent passes where the rolling temperature was low, preventing fine precipitates such as TiN and TiC from precipitating around the 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, embodiments for carrying out the present invention will be specifically described. However, the present invention is not limited to these embodiments.

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

[0024] C: 0.010% or less C is a harmful element that causes magnetic aging in the product 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. There is no particular lower limit, but from the viewpoint of suppressing decarburization costs, it is preferable that the lower limit of the C content is set to about 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 will precipitate, which will actually worsen 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 to adjust strength. On the other hand, if the P content exceeds 0.100%, the steel becomes embrittled, resulting in reduced manufacturability. Therefore, the P content is set to 0.100% or less. Although there is no particular lower limit, from the viewpoint of suppressing dephosphorization costs, 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 precipitating in combination with precipitates such as TiC and TiN, which have adverse effects 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 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 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, improving strength. However, if the Mo content exceeds 0.050%, iron loss tends to increase, so when Mo is added, the Mo content is set to 0.050% or less. On the other hand, there is no lower limit for the Mo content, but 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 has the effect of reducing iron loss. However, if the Zn content exceeds 0.010%, the amount of these sulfides increases, inhibiting grain growth and increasing iron loss. Therefore, the Zn content is set to 0.010% or less. On the other hand, there is no lower limit for the Zn content, but 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, there is no lower limit for the Ti content, but 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 an amount exceeding 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% or more and 0.10% or less 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. However, if the Mg content exceeds 0.10%, the effect saturates and costs increase unnecessarily, so the upper limit is set to 0.10%. Therefore, the Mg content is preferably 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. However, 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% to 0.10%. REM is a collective term for 17 elements, including 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 achieve 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% to 1.0%.

[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 achieve 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 drops significantly due to a decrease in the saturation magnetic flux density, so the upper limit is set at 5.0%. Therefore, the Cr content is preferably in the range of 0.1% to 5.0%.

[0044] Group F: 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 V: 0.001% or more and 0.050% or less V 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 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, so 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 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 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 steel sheets 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% or more and 0.0300% or less Ga is an element that improves the texture of steel sheets and increases magnetic flux density, and can be added as needed. To achieve these effects, the Ga content should be 0.0005% or more. However, adding a large amount of Ga saturates the effect and increases alloy costs, 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 improves the texture of steel sheets and increases magnetic flux density, and can be added as needed. To achieve these effects, the Ge content should be 0.0005% or more. However, adding a large amount of Ge saturates the effect and increases alloy costs, 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 as needed. 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, oxygen (O), and the content of about 0.0050% is permitted. Of the above elements, contents below the preferred range for effective action are permitted as unavoidable impurities because they do not affect the magnetic properties of the product.

[0054] [Manufacturing conditions for hot-rolled steel sheets for non-oriented electrical steel sheets] Next, the manufacturing conditions for manufacturing a hot-rolled steel sheet for a non-oriented electrical steel sheet using molten steel having the above-mentioned 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, hot-rolled steel sheets are manufactured under the following conditions: the entry temperature of the hot rolling mill is 950°C or higher, the exit 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 the molten steel is tapped from a converter or an electric furnace. For example, melting reduced iron or scrap in an electric furnace is preferable because it contributes to reducing CO2 emissions.

[0057] [Continuous casting process] The molten steel with the adjusted composition is then 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: 30mm to 180mm The continuous casting process produces steel slabs with a thickness of 30 mm or more and 180 mm or less. If the thickness of the steel slab is less than 30 mm, the surface area relative to the slab volume becomes large, and the cooling rate of the steel slab becomes fast. 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. During the transport 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 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 exit side of the tunnel furnace, the steel slab is subjected to heat-holding treatment so that the surface temperature is between 1050°C and 1200°C for a heat-holding time of at least 8 minutes. This heat-holding 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 and finish annealing processes when manufacturing non-oriented electrical steel sheet.

[0062] Surface temperature of steel slab at the exit of the tunnel furnace: 1050℃ to 1200℃ If the temperature of the steel slab is below 1050°C, the precipitates do not coarsen sufficiently. Therefore, the precipitates remain fine in 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 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 during the hot-rolled sheet annealing process and finish annealing process is inhibited, 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 minutes or more If the dwell time in the tunnel furnace is less than 8 minutes, 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 in the product increases. Therefore, the dwell time is set to 8 minutes or more. On the other hand, there is no particular upper limit to the dwell time, but if it exceeds 30 minutes, the effect saturates and the tunnel furnace becomes longer, resulting in increased equipment costs. Therefore, the dwell time is preferably set to 30 minutes or less, and more preferably 15 minutes 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 steel slab is transported to a hot rolling mill, where it is hot rolled, wound into a coil, and subjected to a hot rolling process to produce a hot-rolled steel sheet. In this embodiment, the hot rolling process is performed under conditions that satisfy the following (1) to (5), and it is important for reducing iron loss that fine precipitates such as TiC and TiN are precipitated compositely using MnS as a nucleus. (1) Cooling rate of steel slab from the exit of the tunnel furnace to the entry of the hot rolling mill: 4°C / s or less (2) Surface temperature of 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 steel plate at the exit of the hot rolling mill: 800°C or higher (5) Coil winding temperature: 500°C or higher

[0066] Cooling rate of steel slab: 4℃ / s or less After the heat-holding process, the steel slab reacts with oxygen in the air, forming scale on its surface. If the slab is subjected to hot rolling with the remaining scale, it will be rolled in and cause 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 cause the temperature to fall far below the temperature at which MnS precipitates, resulting in the precipitation of fine precipitates such as TiC and TiN, and increased iron loss in 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 steel slab at the entrance of hot rolling mill: 950°C or more 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. For this reason, the entry temperature of the hot rolling mill is set to 950°C or higher. On the other hand, there is no particular upper limit for 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 during 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 during the first pass of hot rolling must be 1.5 / s or higher. While there is no particular upper limit for the strain rate during the first pass of hot rolling, an excessively high strain rate can lead to the precipitation of fine precipitates such as TiC and TiN. Therefore, the strain rate during the first pass of hot rolling should preferably be 4.0 / s or lower. Here, the strain rate was calculated using equations (1) and (2) of Equation 1 below.

[0069]

number

[0070] Here, ε (in italics) is the strain rate (1 / s) in the first pass of hot rolling, h0 is the thickness of the steel slab before hot rolling (m), h1 is the thickness of the rolled material after the first 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, and ν0 is the speed of the rolled material (m / min).

[0071] Steel plate temperature at the hot rolling mill exit: 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 with MnS as the nucleus becomes impossible. For this reason, 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 upper limit for the temperature of the hot-rolled steel sheet at the delivery side of the hot rolling mill, and it is preferable to keep the temperature closer to the temperature of the material being rolled at the entry side of the hot rolling mill, as 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 outlet 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 outlet 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 outlet thickness in finish rolling be 0.4 mm or more. On the other hand, if the outlet 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 outlet thickness in finish rolling be 2.0 mm or less.

[0074] Coil winding temperature: 500°C or higher If the coil winding temperature is below 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. Therefore, the coil winding temperature is set to 500°C or higher. On the other hand, there is no particular upper limit for the coil winding temperature, but coiling at high temperatures may cause deformation of the coil. Therefore, it is preferable that the coil winding temperature be 550°C or lower.

[0075] [Manufacturing conditions for non-oriented electrical steel sheets] The method for producing a non-oriented electrical steel sheet in this embodiment includes a hot-rolled steel sheet production step of producing a hot-rolled steel sheet by the above-mentioned production 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 hot-rolled steel sheet annealing, cold rolling, and finish annealing steps of finish annealing are not particularly limited and 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. [Example]

[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 heat retention 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 retention time was 10 minutes. Next, the scale on the surface of the steel slab was removed using high-pressure water, and the steel slab was then subjected to five passes of hot rolling and wound into a coil to produce a hot-rolled steel sheet. In the hot rolling process, the cooling rate from the tunnel furnace exit to the hot rolling mill was 3°C / s, and the surface temperature of the steel slab at the hot rolling mill entry side 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 hot rolling mill exit side was 900°C, and the coil winding 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 cold-rolled 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 with 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. For a plate thickness of 0.30 mm, the iron loss W 10 / 400 is good if it is 13.50W / 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 is contained as an unavoidable impurity.

[0081] [Table 1]

[0082] <Example 2> A steel having a chemical 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 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 cold-rolled 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 with 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. For a plate thickness of 0.30 mm, the iron loss W 10 / 400 is good if it is 13.50W / 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] [Table 2]

Claims

1. 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 Contains Furthermore, if necessary, 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: 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 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 Group I: As: 0.001% or more and 0.020% or less Contains at least one element selected from A method for producing a hot-rolled steel sheet for use in a non-oriented electrical steel sheet, the method comprising: casting molten steel having a component composition with the balance being Fe and unavoidable impurities into a steel slab having a thickness in the range of 30 mm to 180 mm; 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 higher, the surface temperature of the steel slab at the outlet side of the tunnel furnace is set to a range of 1050°C or higher and 1200°C or lower, and the heat retention time in the tunnel furnace is set to 8 minutes or longer, 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 a non-oriented electrical steel sheet according to claim 1, wherein the molten steel is tapped from an electric furnace or a converter.

3. 3. A method for producing a non-oriented electrical steel sheet, comprising: a hot-rolled steel sheet production step of producing a hot-rolled steel sheet by the production method of a hot-rolled steel sheet for a non-oriented electrical steel sheet according to claim 1; 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.

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