Method for manufacturing hot-rolled steel sheet for non-oriented electromagnetic steel sheet and method for manufacturing non-oriented electromagnetic steel sheet
Patent Information
- Application Number
- JP2025536890
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2025-06-20
- Publication Date
- 2026-03-05
AI Technical Summary
Conventional methods for producing non-oriented electrical steel sheets using thin slabs face issues with fine precipitates forming during hot rolling, leading to increased iron loss, due to insufficient precipitation of MnS and other compounds, which complicates shape control and increases susceptibility to fracture.
The method involves adding Ca to molten steel and controlling the thin slab continuous casting and hot rolling processes to suppress fine precipitate formation, by setting specific conditions such as Ca content, time from Ca addition to casting, and hot rolling entry temperature, maintaining high slab temperatures during transport, and performing hot rolling in multiple passes with controlled strain rates.
This approach effectively reduces iron loss and maintains good magnetic properties in the non-oriented electrical steel sheets by preventing the formation of fine precipitates, thereby improving the manufacturing process efficiency and product quality.
Abstract
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 from the viewpoint of reducing power consumption, and further reduction in iron loss is being demanded of the non-oriented electrical steel sheets that are used as iron core materials.
[0003] To reduce the iron loss of 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 load during cold rolling, which has led to the problem of increased susceptibility to fracture during cold rolling. While the load during cold rolling can be reduced by reducing the thickness of the hot-rolled steel sheet to be subjected to cold rolling, this increases the 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 (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 slab thickness, the loads in both hot rolling and cold rolling can be reduced. Furthermore, in the method using thin slabs, the continuous casting machine (thin slab caster) is usually directly connected to the hot rolling mill, eliminating the need for slab reheating, thereby significantly reducing energy costs.
[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 predetermined chemical composition and a thickness of 50 mm to 200 mm is transported to a furnace while being maintained at a surface temperature of 800°C or higher, and the thin slab is kept in the furnace at a temperature of 1100°C to 1300°C for a holding time of 60 seconds or longer, and then subjected to predetermined hot rolling to produce a hot-rolled steel sheet for use as a non-oriented electrical steel sheet having a thickness of 0.4 mm to 2.0 mm.
[0006] International Publication No. 2023-095637
[0007] However, in the conventional technology described in Patent Document 1, a thin slab is subjected to hot rolling while being maintained at a high temperature. This means that precipitates in the steel, particularly MnS, do not precipitate or coarsen sufficiently before hot rolling, and instead precipitate finely during hot rolling. This causes a problem in that the fine precipitates refine the steel structure and increase iron loss.
[0008] The present invention aims to solve the above-mentioned problems and to suppress the formation of fine precipitates and the increase in iron loss during the hot rolling process while maintaining a thin slab at a high temperature. In the present invention, a thin slab means a slab having a thickness of 30 mm or more and 180 mm or less.
[0009] As a result of intensive research by the present inventors, it has been found that by adding Ca to molten steel and setting the conditions of the thin slab continuous casting process and the hot rolling process within appropriate ranges, it is possible to suppress the formation of fine precipitates and to suppress the increase in iron loss caused by the casting of thin slabs.
[0010] First, the experiment that led to the development of the present invention will be described.
[0011] [Experiment 1] The inventors considered whether it would be possible to fix S present in steel as CaS, which has a high precipitation temperature, and conducted a study by measuring iron loss. For the measurement, a steel composition containing, by mass%, 0.002% C, 3.00% Si, 0.50% Mn, 0.01% P, 0.0020% S, 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 a base, and Ca was added in amounts ranging from 0.0001 to 0.0030% to obtain molten steel. During continuous casting, 300 seconds after adding Ca to molten steel having a base steel composition, the molten steel was tapped and cast to form a 60 mm thick slab, which was then held at 1100 °C for 8 min in a tunnel furnace and then hot rolled to form a hot-rolled steel sheet with a thickness of 1.6 mm. The surface temperature of the slab at the entry side of the hot rolling mill (hereinafter also referred to as the "entrance temperature of hot rolling") was 980 °C. Hot rolling was performed in five passes, with the strain rate of the first pass of hot rolling being 3.0 / sec. The steel sheet was heated to 980 °C using a bar heater only between the first and second passes. Next, hot-rolled sheet annealing was performed at 980 °C for 30 seconds. The cold-rolled steel sheet was then cold-rolled to a thickness of 0.20 mm, and finally, H was added in a vol% ratio. ２ :N ２ The steel sheets were subjected to finish annealing at 960°C for 10 seconds in a dry atmosphere with a ratio of 0.01 to 0.75. 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.
[0012] Figure 1 shows the relationship between the amount of Ca added to molten steel and iron loss W １０／４００ It is clear from Figure 1 that iron loss is reduced by controlling the amount of Ca added to molten steel to a range of 0.0010% or more. Furthermore, in the range where iron loss was reduced, the grain size of the steel sheet observed with an optical microscope was coarsened, and when the steel sheet was observed with an SEM, the number of MnS particles was reduced.
[0013] From the above experimental results, the inventors have estimated that the influence of precipitates on the reduction of grain growth was suppressed when the Ca content was in the range of 0.0010% or more. Specifically, when the Ca content was 0.0010% or more, CaS precipitated in the molten steel, reducing the amount of S in the steel, and therefore fine MnS did not precipitate in the subsequent process.
[0014] [Experiment 2] The inventors of the present invention suspected that if CaS precipitates in molten steel, omitting the heat retention step in the tunnel furnace would not have an effect on suppressing the precipitation of fine MnS. Therefore, they investigated the conditions necessary for reducing iron loss by combining the omission of the heat retention step in the tunnel furnace with hot rolling conditions. In the investigation, a molten steel was prepared by adding 0.0020% Ca to a steel composition containing, by mass, 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, and 0.0020% S, with the balance being Fe and unavoidable impurities. During continuous casting, 300 seconds after adding Ca to molten steel having a base steel composition, the molten steel was poured and cast to form a 60 mm thick slab. The slab was transported to a hot rolling mill without heating and hot rolled to form a hot-rolled steel sheet with a thickness of 1.6 mm. The hot-rolling entry temperature was varied in the range of 875°C to 1025°C. Hot rolling was performed in five passes, with the strain rate of the first pass being 3.0 / sec. The steel sheet was heated to 980°C using a bar heater only between the first and second passes. The obtained hot-rolled steel sheet was subjected to the same procedure as in Experiment 1, and the iron loss was measured.
[0015] Figure 2 shows the relationship between the entry temperature and iron loss W １０／４００ The relationship between the temperature and the magnetic properties is shown in Figure 2. As can be seen from Figure 2, when the entry temperature of hot rolling is less than 950°C, iron loss increases. The inventors presumed that this is because the recovery of dislocations introduced during rolling is delayed when the temperature is low, causing fine precipitates such as TiN and TiC, which have a significant effect of suppressing grain growth, to precipitate on the dislocations. Furthermore, this experiment showed that even if the tunnel furnace heat retention is omitted, non-oriented electrical steel sheets with excellent magnetic properties can be obtained by appropriately controlling the hot rolling conditions.
[0016] [Experiment 3] Next, the inventors investigated the conditions for adding Ca to molten steel. For the investigation, a steel composition containing, by mass%, 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 a base. To this, 0.0020% Ca was added to produce molten steel, which was then cast into 60 mm thick slabs. During continuous casting, the time from adding Ca to the molten steel until the molten steel was tapped was varied between 10 and 500 seconds. The cast thin slab was transported to the hot rolling mill without heating and hot rolled to obtain a hot-rolled steel sheet with a thickness of 1.6 mm. The hot rolling entry temperature was 980°C. The hot rolling was performed in five passes, with the strain rate of the first pass being 3.0 / sec. The steel sheet was heated to 980°C using a bar heater only between the first and second passes. The obtained hot-rolled steel sheet was subjected to the same procedure as in Experiment 1, and the iron loss was measured.
[0017] Figure 3 shows the relationship between the time from adding Ca to casting and the iron loss W １０／４００ 3 shows that when the time from the addition of Ca to casting is less than 150 seconds, iron loss increases. The inventors presumed that this is because when the time from the addition of Ca to casting is short, CaS does not coarsen sufficiently in the molten steel, increasing iron loss.
[0018] From the results of Experiments 1 to 3 above, it was found that in both the process in which a slab is maintained at a high temperature using a thin slab caster and transported to a hot rolling mill without passing through a heating furnace and then hot rolled, and the process in which the slab is kept at a high temperature in a tunnel furnace and then hot rolled, an increase in iron loss can be suppressed by adding 0.0010% or more of Ca, setting the time from adding Ca until the molten steel is cast to 150 seconds or more, and setting the hot rolling entry temperature to 950°C or more.
[0019] The present invention has been completed based on the above findings, and its gist is as follows: [1] A steel sheet containing, by mass%, C: 0.010% or less, Si: 2.50% to 5.00%, Mn: 0.10% to 3.00%, P: 0.100% or less, S: 0.0050% or less, Al: 2.00% or less, N: 0.0080% or less, Cu: 1.00% or less, Ni: 0.50% or less, Cr: 3.00% or less, Mo: 0.050% or less, Zn: 0.010% or less, Ti: 0.010% or less, Sn: 0.20% or less, Sb: 0.20% or less, and Ca: 0.0010% to 0.1000%; Group A: at least one selected from Mg: 0.0001% to 0.10% and REM: 0.0001% to 0.10%; Group B: B: 0.002% to 0.01%; Group C: 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%; Group D: Co: 0.001% to 0.100%; Group E: one or two selected from Ga: 0.0005% to 0.0300% and Ge: 0.0005% to 0.0300%; and 1. A method for producing a hot-rolled steel sheet for use in a non-oriented electrical steel sheet, the method comprising the steps of: continuously casting a slab having a thickness of 30 mm or more and 180 mm or less from molten steel having a chemical composition containing at least one element selected from group F: As: 0.001% or more and 0.020% or less, with the balance being Fe and unavoidable impurities; and hot-rolling the slab, wherein the time from adding Ca to the molten steel until casting is 150 seconds or more; maintaining a surface temperature of the slab at 850°C or more from the time the slab is obtained until it is transported to an inlet side of a hot rolling mill, and setting a surface temperature of the slab at the inlet side of the hot rolling mill to 950°C or more; and hot-rolling includes two or more passes, and is carried out under conditions where a strain rate in the first pass is 5.0 / sec or less, and the steel sheet is heated to 950°C or more by a bar heater between the first and second passes.[2] The method for producing a hot-rolled steel sheet for use as a non-oriented electrical steel sheet according to [1], wherein, after obtaining the slab, the slab is transported to the entry side of the hot rolling mill without heating. [3] The method for producing a hot-rolled steel sheet for use as a non-oriented electrical steel sheet according to [1], comprising a step of subsequently keeping the slab hot in a tunnel furnace after obtaining the slab, in which the surface temperature of the slab at the entry side of the tunnel furnace is 850°C or higher and the surface temperature of the slab at the exit side of the tunnel furnace is 950°C or higher and 1250°C or lower. [4] The method for producing a hot-rolled steel sheet for use as a non-oriented electrical steel sheet according to any of [1] to [3], wherein the slab is a slab tapped from an electric furnace or a converter. [5] 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 any one of [1] to [4]; a hot-rolled sheet annealing step of subjecting the hot-rolled steel sheet to hot-rolled sheet annealing; a cold-rolling step of cold-rolling the hot-rolled steel sheet that has been subject to the hot-rolled sheet annealing to form a cold-rolled steel sheet; and a finish annealing step of subjecting the cold-rolled steel sheet to finish annealing.
[0020] By using the method for manufacturing a hot-rolled steel sheet for non-oriented electrical steel sheet of the present invention, it is possible to suppress the formation of fine precipitates and the increase in iron loss during the hot-rolling process while maintaining a thin slab at a high temperature, and to manufacture a non-oriented electrical steel sheet with good magnetic properties.
[0021] Amount of Ca added to molten steel and iron loss W １０／４００ 1 is a diagram showing the relationship between the entry temperature and iron loss W in hot rolling. １０／４００ 1 is a graph showing the relationship between the time from the addition of Ca to the time of casting and the iron loss W １０／４００ FIG.
[0022] The present invention will be described in detail below.
[0023] [Composition] The reasons for limiting the composition of the molten steel in the present invention will be explained. In the following, "%" is based on mass unless otherwise specified. The composition of the molten steel is that immediately after adding Ca.
[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 the present invention, in order to suppress the magnetic aging, the C content is set to 0.005% or less. The lower limit is not particularly limited and may be 0%, but from the viewpoint of suppressing decarburization costs, it is preferable to set the lower limit to about 0.0001%.
[0025] Si: 2.50% or more and 5.00% or less Si increases the electrical resistance of the steel sheet, and in order to sufficiently reduce iron loss, the addition of 2.50% or more is necessary. Therefore, in the present invention, the Si content is set to 2.50% or more. On the other hand, if it exceeds 5.00%, rolling becomes difficult. Therefore, the Si content is set to 5.00% or less. From the viewpoint of manufacturability, a Si content of 4.00% or less is preferable.
[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 the present invention, 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 to adjust the strength. On the other hand, if the content exceeds 0.10%, the steel becomes embrittled, resulting in a decrease in manufacturability. Therefore, the P content is set to 0.100% or less. The lower limit is not particularly limited and may be 0%, but from the viewpoint of suppressing the cost of dephosphorization, it is preferable to set the lower limit to about 0.01%.
[0028] S: 0.0050% or less S is an element that forms sulfides and increases iron loss. Therefore, when S is contained, the S content is set to 0.0050% or less, preferably 0.0020% or less. On the other hand, from the viewpoint of iron loss, the lower the S content, the better, so the lower limit of the S content is not limited and may be 0%. However, S is an element that is inevitably mixed into steel as an impurity, and excessive reduction leads to an increase in manufacturing costs. Therefore, from the viewpoint of cost, it is preferable to set the lower limit to about 0.0005%.
[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. The lower limit is not particularly limited and may be 0%, but from the viewpoint of reducing iron loss, it is preferable to set the lower limit to about 0.1%.
[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 desirable 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. More preferably, it is set to 0.0030% or less. Note that the lower limit is not particularly limited and may be 0%, but from the viewpoint of cost, it is preferable to set the lower limit to about 0.0005%.
[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 0.10% or less. On the other hand, the lower limit of the Cu content is not particularly limited and may be 0%, but from the viewpoint of enhancing the effect of adding Cu, the lower limit is preferably set to about 0.01%.
[0032] Ni: 0.50% or less Ni is an element that improves the toughness of steel. However, if the Ni content exceeds 0.50%, the effect saturates, and in addition, the magnetic flux density decreases as the saturation magnetization decreases, which actually increases iron loss. Therefore, when Ni is added, the Ni content is set to 0.50% or less. On the other hand, the lower limit of the Ni content is not particularly limited and may be 0%, but from the viewpoint of enhancing the effect of adding Ni, it is preferable to set the lower limit to about 0.05%.
[0033] Cr: 3.00% or less Cr is an element that increases the resistivity of steel and has the effect of reducing iron loss. However, if the Cr content exceeds 3.00%, not only does the saturation magnetization decrease, but magnetostriction, which is a change in the shape of the steel sheet due to magnetization, increases, resulting in a significant increase in iron loss. Therefore, when Cr is added, the Cr content is set to 3.00% or less. On the other hand, the lower limit of the Cr content is not particularly limited and may be 0%, but from the viewpoint of enhancing the effect of adding Cr, it is preferable to set the lower limit to about 0.05%.
[0034] 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, the lower limit of the Mo content is not particularly limited and may be 0%, but from the viewpoint of strength, it is preferable to set the lower limit to about 0.010%.
[0035] 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 Zn content exceeds 0.010%, the amount of the sulfides increases, which in turn inhibits grain growth and increases iron loss. Therefore, when Zn is added, the Zn content is set to 0.010% or less. On the other hand, the lower limit of the Zn content is not particularly limited and may be 0%, but from the viewpoint of reducing iron loss, it is preferable to set the lower limit to about 0.001%.
[0036] 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 content exceeds 0.010%, the amount of the above-mentioned sulfides and nitrides increases, which in turn inhibits grain growth and increases iron loss. Therefore, when Ti is added, the Ti content is set to 0.010% or less. On the other hand, the lower limit of the Ti content is not particularly limited and may be 0%, but from the viewpoint of strength, it is preferable to set the lower limit to about 0.002%.
[0037] 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 is saturated. Therefore, when Sn is added, the Sn content is set to 0.20% or less, preferably 0.10% or less. On the other hand, the lower limit of the Sn content is not particularly limited and may be 0%, but from the viewpoint of strength, it is preferable that the lower limit be set to about 0.005%.
[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 set to 0.20% or less, preferably 0.10% or less. On the other hand, the lower limit of the Sb content is not particularly limited and may be 0%, but from the viewpoint of strength, it is preferable to set the lower limit to about 0.005%.
[0039] Ca: 0.0010% or More and 0.1000% or Less In the present invention, the Ca content of the molten steel is set to 0.0010% or more and 0.1000% or less. Ca is an element that forms stable sulfides and reduces fine sulfides, thereby improving grain growth and reducing iron loss. By controlling the amount of Ca added to the molten steel to a Ca content of 0.0010% or more and 0.1000% or less, and combining this with the thin slab casting process and hot rolling process described below, it is possible to suppress the formation of fine precipitates and the increase in iron loss caused by thin slab casting. If the Ca content is less than 0.0010%, the above effect cannot be obtained. On the other hand, if the Ca content exceeds 0.1000%, Ca precipitates as Ca-based oxides, which deteriorates iron loss. From the viewpoint of reducing the addition cost, the amount of Ca added is preferably 0.050% or less.
[0040] In the present invention, the molten steel may further optionally contain: Group A: 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 B: B: 0.002% or more and 0.01% or less; Group C: 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 D: Co: 0.001% or more and 0.100% or less; Group E: one or two 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 F: As: 0.001% or more and 0.020% or less; and at least one group selected from the group.
[0041] Group A: 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%.
[0042] 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 obtain 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.
[0043] Group B: 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.
[0044] Group C: 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 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. Therefore, 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 D: 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, since a large amount of Co increases the alloy cost, the upper limit of the Co content is set to 0.100%.
[0050] Group E: One or two elements 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 improves the texture of the steel sheet and increases 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 F: 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] In the composition of the molten steel, the balance other than the above-mentioned components is Fe and inevitable impurities.
[0054] [Manufacturing Conditions for Hot-Rolled Steel Sheet for Non-Oriented Electrical Steel Sheet] The manufacturing method for hot-rolled steel sheet for non-oriented electrical steel sheet of the present invention includes a step of continuously casting a thin slab from molten steel having the above-described chemical composition and a step of hot-rolling the thin slab, and the following manufacturing conditions are adopted during these steps. (1) The time from adding Ca to the molten steel to casting the thin slab is set to 150 seconds or more. Here, the time from adding Ca to the molten steel to casting the thin slab (hereinafter also referred to as "elapsed time") corresponds to the time from the time Ca is added to the molten steel to incorporate Ca into the molten steel until the molten steel is tapped. Here, the Ca added to the molten steel may be pure Ca or a Ca alloy such as CaSi. (2) From the time the thin slab is obtained until it is transported to the inlet side of a hot rolling mill, the surface temperature of the thin slab is maintained at 850°C or higher, and the surface temperature of the thin slab at the inlet side of the hot rolling mill is set to 950°C or higher. (3) Hot rolling including two or more passes is carried out under the condition that the strain rate in the first pass is 5.0 / sec or less, and the steel sheet is heated to 950°C or more by a bar heater between the first and second passes.
[0055] The method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet of the present invention includes the following two methods: (A) A thin slab is produced by a continuous casting method, and the resulting thin slab is transported to the inlet side of a hot rolling mill while maintaining a high temperature without heating (for example, without passing through a heating furnace), with the surface temperature of the thin slab at the inlet side of the hot rolling mill being 950°C or higher; (B) A thin slab is produced by a continuous casting method, and the resulting thin slab is transported to a tunnel furnace while maintaining its surface temperature at 850°C or higher, and is kept at a temperature such that the surface temperature of the slab at the outlet side of the tunnel furnace is 950°C or higher and 1250°C or lower, and then transported to the inlet side of a hot rolling mill, with the surface temperature of the thin slab at the inlet side of the hot rolling mill being 950°C or higher.
[0056] Each step will be described below.
[0057] [Continuous Casting Step] A thin slab is cast by continuous casting from molten steel having the above-described composition. The continuous casting method is not particularly limited, and any known method can be used. The method for adjusting the composition of the molten steel used in continuous casting is also not particularly limited, and any known method can be used. For example, a converter, an electric furnace, a vacuum degasser, and other devices and methods can be used to adjust the composition of the molten steel.
[0058] The chemical composition of the thin slab obtained in the present invention is substantially the same as the chemical composition of the molten steel.
[0059] (Time (elapsed time) after addition of Ca until casting: 150 seconds or more) In the present invention, when casting molten steel whose composition has been adjusted, the time (elapsed time) after addition of Ca until casting is set to 150 seconds or more. If the elapsed time is less than 150 seconds, Ca inclusions such as CaS cannot be sufficiently coarsened in the molten steel, which may cause an increase in iron loss. On the other hand, although there is no particular upper limit on the elapsed time, if it exceeds 1200 seconds, there is an increased risk that Ca will volatilize and escape from the molten steel. Therefore, the elapsed time is preferably 1200 seconds or less, and more preferably 600 seconds or less from the viewpoint of good slab production efficiency.
[0060] (Thickness of thin slab: 30 mm or more and 180 mm or less) In the present invention, thin slabs having a thickness of 30 mm or more and 180 mm or less are manufactured. If the thickness of the slab is less than 30 mm, the surface area relative to the slab volume increases, and the cooling rate of the slab becomes faster, making it impossible to ensure the slab temperature in the hot rolling process. Therefore, the thickness of the slab is set to 30 mm or more. On the other hand, if the thickness of the slab exceeds 180 mm, the rolling load in the hot rolling process increases, increasing the risk of fracture. Therefore, the thickness of the slab is set to 180 mm or less.
[0061] [Transportation Step] The thin slab obtained in the casting step is transported to the inlet side of a hot rolling mill.
[0062] (Surface temperature of thin slab: 850°C or higher) The surface temperature of the thin slab obtained in the casting process is maintained at 850°C or higher until it reaches the inlet side of the hot rolling mill. The thin slab after the casting process is at a high temperature, typically 1000 to 1400°C. In the transporting process, the surface temperature of the thin slab is preferably maintained at 1250°C or lower. It is desirable not to actively cool the thin slab in the transporting process.
[0063] When the above (A) and (B) are adopted, they can be carried out as follows.
[0064] (Case (A) Above) In the above (A), the thin slab obtained in the continuous casting process is transported to the inlet side of the hot rolling mill without heating (for example, without passing through a heating furnace) while maintained at a high temperature. In this transport process, the temperature of the thin slab after the continuous casting process decreases. In the present invention, however, the surface temperature of the thin slab at the inlet side of the hot rolling mill is set to 950°C or higher. Therefore, under the conditions of (A), the thin slab is transported to the inlet side of the hot rolling mill while it is still at 950°C or higher. In this transport process, it is desirable to transport the thin slab as quickly as possible to avoid a decrease in its temperature. Furthermore, in the transport process, the thin slab may be cut before being transported to the inlet side of the hot rolling mill. However, from the viewpoint of suppressing a decrease in the temperature of the thin slab, it is desirable to transport the thin slab directly to the inlet side of the hot rolling mill without cutting it.
[0065] (Case (B) above) -Temperature of thin slab at the entrance side of the tunnel furnace: 850°C or higher- In case (B) above, the thin slab obtained in the casting process is transported to a tunnel furnace used for heat retention treatment, and then transported to the entrance side of the hot rolling mill. In this case, it is important to transport the thin slab to the entrance side of the tunnel furnace while maintaining the surface temperature of the thin slab at 850°C or higher. If the surface temperature of the thin slab falls below 850°C, the energy required to reheat the slab increases, and the energy-saving effect cannot be obtained. In the transport process, the thin slab may be cut and then transported to the furnace. However, from the perspective of suppressing a decrease in the temperature of the thin slab, it is desirable to transport the thin slab directly to the tunnel furnace without cutting it.
[0066] - Heat retention process: temperature of thin slab at tunnel furnace outlet: 950°C to 1250°C - The thin slab transported to the tunnel furnace is kept at a temperature of 950°C to 1250°C at the tunnel furnace outlet so that the surface temperature of the thin slab is maintained at a temperature of 950°C to 1250°C. By performing the heat retention process, a drop in the temperature of the thin slab is prevented, and the load of hot rolling is reduced.
[0067] If the temperature of the thin slab at the exit of the tunnel furnace is less than 950°C, the temperature during hot rolling will be too low, causing fine precipitates to precipitate and increasing iron loss. Therefore, the temperature at the exit of the tunnel furnace is set to 950°C or higher, preferably 1100°C or higher. On the other hand, the temperature at the exit of the tunnel furnace is set to 1250°C or lower, preferably 1200°C or lower, in order to effectively prevent precipitates in the steel from dissolving and precipitating finely in subsequent processes, which would increase iron loss.
[0068] - Heat retention process: Heat retention time - The heat retention time (the time required for the thin slab to pass through the tunnel furnace) is not particularly limited, but if it is less than 5 minutes, the temperature of the thin slab will not reach the furnace temperature, preventing a drop in the temperature of the thin slab and not fully demonstrating the effect of reducing the hot rolling load. Therefore, the heat retention time is preferably 5 minutes or more. On the other hand, there is no particular limit to the upper limit of the heat retention time, but if it exceeds 30 minutes, the effect will saturate and the furnace length of the tunnel furnace will increase, resulting in increased equipment costs. Therefore, the heat retention time is preferably 30 minutes or less, and more preferably 15 minutes or less.
[0069] The heating method in the heat retention treatment is not particularly limited, and any method such as induction heating, gas furnace, or electric furnace can be used. ２ From the viewpoint of reduction, it is preferable to use induction heating or an electric furnace, which are heating methods that use electrical energy.
[0070] [Hot Rolling Step] The thin slab is transported to a hot rolling mill and hot rolled to form a hot rolled steel sheet.
[0071] (Surface temperature of thin slab at the entry side of hot rolling mill: 950°C or higher) If the surface temperature of the thin slab at the entry side of the hot rolling mill (entry side temperature of hot rolling) is less than 950°C, the recovery of dislocations is delayed, and fine precipitates such as TiC and TiN precipitate on the dislocations. Therefore, the entry side temperature of hot rolling is set to 950°C or higher. On the other hand, the upper limit of the temperature can be set to 1250°C or lower, preferably 1200°C or lower, and a higher temperature within this range is more desirable.
[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 2, for example, 10 or less, or even 6 or less.
[0073] (Strain rate in the first pass of hot rolling: 5.0 / sec or less) If the strain rate in the first pass of hot rolling exceeds 5.0 / sec, the rate at which dislocations, which are the driving force for precipitation, are introduced exceeds the rate at which the dislocations recover, resulting in the precipitation of fine precipitates such as nitrides during hot rolling. Therefore, the strain rate in the first pass of hot rolling is set to 5.0 / sec or less. On the other hand, although there is no particular restriction on the lower limit of the strain rate, a strain rate of 1.0 / sec or more is preferable because productivity decreases when the strain rate is less than 1.0 / sec.
[0074] Here, the strain rate ε′ is calculated using the following equations (1) and (2). Here, ε' is the strain rate in the first pass of hot rolling, and h １ is the slab thickness before hot rolling, and h ０ is the slab thickness after one pass of hot rolling, and t ｃis the time required for the first pass of hot rolling, R is the roll diameter for the first pass of hot rolling, and v ０ is the threading speed of the steel slab.
[0075] (Heating of steel sheet by bar heater between first and second passes of hot rolling: 950°C or higher) During hot rolling, the steel sheet is heated to 950°C or higher by a bar heater between the first and second passes. If the steel sheet is not heated by a bar heater, the second rolling pass is performed with dislocations introduced, resulting in the precipitation of fine precipitates that adversely affect magnetic properties during the second rolling pass. Furthermore, if the steel sheet is heated to less than 950°C, dislocation recovery is insufficient, so the steel sheet is heated to 950°C or higher. Although the upper limit is not particularly limited, from the viewpoint of productivity, 1050°C or lower is preferable. Heating between passes other than between the first and second passes is not particularly limited; heating may be performed only between the first and second passes, and heating may not be performed between other passes. When heating is performed between passes other than between the first and second passes, the heating temperature can be 950°C or higher and 1050°C or lower. Heating can also be performed using a bar heater.
[0076] Any heater can be used as the bar heater as long as it can perform the above heating. The bar heater may be one that heats the entire width of the steel material (or rough bar), or one that heats only the widthwise ends (edge heater). However, from the viewpoint of uniforming the temperature distribution in the sheet width direction, it is preferable to use a heater that can heat the entire width of the steel material. As a heater that can heat the entire width of the steel material, for example, a solenoid-type induction heating device is preferably used, in which a heating coil is wound in a cylindrical shape and the steel material is induction-heated by passing through the coil. When a solenoid-type induction heating device is used, both the surface layer and the widthwise ends of the steel sheet are heated.
[0077] (Conditions at the Outlet Side of the Hot Rolling Mill) The temperature of the steel sheet at the outlet side of the hot rolling mill is not particularly limited, but is preferably not less than 800° C. At the outlet side of the hot rolling mill, the steel sheet can be wound into a coil.
[0078] The thickness of the steel sheet at the delivery side in hot rolling 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 becomes excessively long, resulting in reduced productivity. Therefore, from the viewpoint of productivity, it is preferable that the delivery thickness in hot rolling be 0.4 mm or more. On the other hand, if the delivery thickness exceeds 2.0 mm, the load in cold rolling increases. Therefore, from the viewpoint of reducing the load in cold rolling, it is preferable that the delivery thickness in hot rolling be 2.0 mm or less.
[0079] [Manufacturing conditions for non-oriented electrical steel sheet] A manufacturing method of a non-oriented electrical steel sheet according to an embodiment of the present invention includes a hot-rolled steel sheet manufacturing step of manufacturing a hot-rolled steel sheet by the above-described manufacturing method, a hot-rolled sheet annealing step of subjecting the hot-rolled steel sheet to hot-rolled sheet annealing, a cold-rolling step of cold-rolling the hot-rolled steel sheet that has been subject to the hot-rolled sheet annealing to form a cold-rolled steel sheet, and a finish annealing step of subjecting the cold-rolled steel sheet to finish annealing.
[0080] The steps of hot-rolled sheet annealing, cold rolling, and finish annealing are not particularly limited, and can be carried out by known methods.
[0081] After the hot-rolled sheet annealing, it is also preferable to carry out 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. The methods for pickling and insulating coating are not particularly limited, and can be carried out by known methods.
[0082] The present invention will be specifically explained with reference to examples, but the present invention is not limited to the following description.
[0083] Example 1 A 60 mm thick slab having the chemical composition shown in Table 1 was produced by continuous casting. In continuous casting, the time from the addition of Ca to the casting of molten steel was 300 seconds. Next, the slab was transported to the inlet side of a hot rolling mill without heating and subjected to five passes of hot rolling to obtain a hot-rolled steel sheet. In the hot rolling process, the surface temperature of the slab at the inlet side of the hot rolling mill was 1000°C, and the strain rate in the first pass of hot rolling was 3.0 / sec. Only between the first and second passes, the steel sheet was heated to the bar heater heating temperature shown in Table 2 using a bar heater. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 1000°C for 30 seconds to obtain a hot-rolled annealed sheet. Next, the hot-rolled annealed sheet was cold-rolled to obtain a cold-rolled steel sheet with a thickness of 0.20 mm. Thereafter, the cold-rolled steel sheets were subjected to finish annealing at 1000°C for 10 seconds to obtain non-oriented electrical steel sheets.
[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 value is 12.5 W / kg or less.
[0085]
[0086] From the results shown in Table 1, it can be seen that non-oriented electrical steel sheets with good core loss properties can be obtained by using molten steel having a chemical composition that satisfies the conditions of the present invention. Note that in Table 1, Nos. 7, 13, 19, and 23 are examples that could not be evaluated because they fractured during cold rolling.
[0087] (Example 2) A steel having a component composition containing, in mass%, 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%, and Ca: 0.002%, with the balance being Fe and inevitable impurities, was melted, and a hot-rolled steel sheet for non-oriented electrical steel sheet was produced under the conditions shown in Table 2. 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. Next, the hot-rolled annealed sheet was subjected to cold rolling to obtain a cold-rolled steel sheet having a thickness of 0.30 mm. Thereafter, the cold-rolled steel sheets were subjected to finish annealing at 1000°C for 10 seconds to obtain non-oriented electrical steel sheets.
[0088] 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 value is 12.5 W / kg or less.
[0089]
[0090] From the results shown in Table 2, it can be seen that by adopting manufacturing conditions that satisfy the requirements of the present invention, a non-oriented electrical steel sheet with good core loss properties can be obtained.
[0091] By using the method for manufacturing a hot-rolled steel sheet for non-oriented electrical steel sheet of the present invention, it is possible to suppress the formation of fine precipitates and the increase in iron loss during the hot-rolling process while maintaining a thin slab at a high temperature, and to manufacture a non-oriented electrical steel sheet with good magnetic properties.
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: 0.0050% or less, Al: 2.00% or less, N: 0.0080% or less, Cu: 1.00% or less, Ni: 0.50% or less, Cr: 3.00% or less, Mo: 0.050% or less, Zn: 0.010% or less, Ti: 0.010% or less, Sn: 0.20% or less, Sb: 0.20% or less; Ca: 0.0010% or more and 0.1000% or less Contains Additionally, optionally Group A: 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 B: B: 0.002% or more and 0.01% or less; Group C: 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 D: Co: 0.001% or more and 0.100% or less; Group E: one or two 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 F: As: 0.001% or more and 0.020% or less; At least one group selected from A method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet, comprising: a step of continuously casting a slab having a thickness of 30 mm or more and 180 mm or less from molten steel having a component composition containing The time from adding Ca to the molten steel until casting is set to 150 seconds or more, maintaining a surface temperature of the slab at 850°C or higher from the time the slab is obtained until the time the slab is transported to the inlet side of a hot rolling mill, and setting the surface temperature of the slab at the inlet side of the hot rolling mill to 950°C or higher; The hot rolling includes two or more passes, is performed under conditions where a strain rate in the first pass is 5.0 / sec or less, and the steel sheet is heated to 950°C or more by a bar heater between the first pass and the second pass. A manufacturing method for hot-rolled steel sheets for non-oriented electrical steel sheets.
2. The method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to claim 1, wherein after the slab is obtained, the slab is transported to an inlet side of the hot rolling mill without being heated.
3. 2. The method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to claim 1, further comprising a step of heat-keeping the slab in a tunnel furnace after obtaining the slab, wherein the surface temperature of the slab at the entry side of the tunnel furnace is 850°C or higher and the surface temperature of the slab at the exit side of the tunnel furnace is 950°C or higher and 1250°C or lower.
4. The method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the slab is a slab tapped from an electric furnace or a converter.
5. 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 method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to any one of claims 1 to 3; a hot-rolled sheet annealing step of subjecting the hot-rolled steel sheet to hot-rolled sheet annealing; a cold-rolling step of cold-rolling the hot-rolled steel sheet that has been subject to the hot-rolled sheet annealing to form a cold-rolled steel sheet; and a finish annealing step of subjecting the cold-rolled steel sheet to finish annealing.
6. 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 described in claim 4; a hot-rolled sheet annealing process for subjecting the hot-rolled steel sheet to hot-rolled sheet annealing; a cold-rolling process for cold-rolling the hot-rolled steel sheet that has been subject to hot-rolled sheet annealing to form a cold-rolled steel sheet; and a finish annealing process for subjecting the cold-rolled steel sheet to finish annealing.