TWI757156B - Hot-rolled steel sheet for non-oriented electrical steel sheet and method for producing the same - Google Patents

Abstract

The hot-rolled steel sheet for non-oriented electrical steel sheet of this case has a solid solution Ti content of 0.0005% or less, and Ti carbides with an equivalent circle diameter of 10 nm or more and 50 nm or less exist in the grains of the fertilized iron grains and the grains. boundary; Ti carbides existing in the above grain boundaries have more than 10% and less than 100% are composite precipitation with Mn sulfide, and the number density of Ti carbides existing in the above grain boundaries is 0.1 /μm or less.

Description

Hot-rolled steel sheet for non-oriented electrical steel sheet and method for producing the same

Field of Invention The present invention relates to a hot-rolled steel sheet for non-oriented electrical steel sheets and a manufacturing method thereof.

Background of the Invention In recent years, due to the increasing demand for energy saving in electrical appliances around the world, non-oriented electrical steel sheets used as iron core materials for rotating machines are also required to have higher performance characteristics.

High-efficiency models of electric motors use high-grade non-oriented electrical steel sheets. In general, for high-grade non-oriented electrical steel sheets, the content of Si and Al is increased to increase specific resistance and control the grain size to be coarse.

On the other hand, for general-purpose models, general-purpose grade non-oriented electrical steel sheets are used in electric motors. In recent years, there has been a demand for improved performance for the motors of connected models, but for general-purpose models, the cost constraints are severe, and it is difficult to replace the non-oriented electrical steel sheets used with high-grade electromagnetic steel sheets like high-efficiency models. .

In general, general-purpose grades of non-oriented electrical steel sheets have a chemical composition with a low Si content. In such general-purpose grade non-oriented electrical steel sheets, for example, the iron loss characteristics are improved by promoting crystal grain growth during relaxation annealing applied after the punching of the motor core.

In terms of a method for improving the growth of crystal grains during relaxation annealing, the following technical proposals have been proposed so far.

For example, Patent Document 1 discloses a method for producing an electric iron sheet having excellent magnetic properties, which is characterized in that a hot rolled sheet is obtained by hot rolling a steel billet, and then cold rolling is performed once or twice including intermediate annealing. The above-mentioned cold rolling is used to make the obtained hot-rolled sheet into the final size, and further annealing is performed; the steel billets are C:≦0.065%, Si:≦2.0%, Al:≦0.10%, O:≦0.020%, B /N: ≦0.50~2.50, the remainder is composed of Fe and inevitable impurities.

Patent Document 2 discloses a non-oriented electrical steel sheet, which has an average grain size after magnetic annealing of 50 μm or more and less iron loss, and is characterized by containing: C: 0.015% or less, Si: 0.1 to 1.0%, sol. Al: 0.001~0.005%, Mn: 1.5% or less, S: 0.008% or less, N: 0.0050% or less, TO: 0.02% or less; among them, the steel contains three kinds of inclusions: SiO ₂ , MnO, and Al ₂ O3 The weight ratio of MnO is 15% or less based on the total weight.

Patent Document 3 discloses a non-oriented electrical steel sheet excellent in magnetic properties, characterized in that it contains, in % by weight: C: 0.01% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.1% or more and 1.5% or less , and according to the deoxidation method of the steel, it contains: Al: 0.1% or less or Zr: 0.05% or less, and the rest is composed of iron and unavoidable impurity elements; among them, for the oxides in the steel, the diameter is 0.5 μm or more and Those with a size of 5 μm or less are 1,000 or more and 50,000 or less per 1 cm ² .

Patent Document 4 discloses a non-oriented electrical steel sheet containing, in mass %, C: 0.0050% or less, Si: 0.05 to 3.5%, Mn: 3.0% or less, Al: 3.0% or less, S: 0.008% or less, P : 0.15% or less, N: 0.0050% or less, Cu: 0.2% or less; it satisfies: (S of Cu sulfide)/(S in steel)≦0.2, or, (S of Cu sulfide)/(Mn sulfide S)≦0.2; and in the steel sheet, the number density of sulfides containing Cu with a diameter of 0.03 to 0.20 μm is 0.5/μm ³ or less.

Moreover, Patent Document 5 discloses a non-oriented electrical steel sheet characterized by containing, in mass %, Si: 1.5% or less, Mn: 0.4% or more and 1.5% or less, Sol.Al: 0.01% or more and 0.04% or less , Ti: 0.0015% or less, N: 0.0030% or less, S: 0.0010% or more and 0.0040% or less, B is 0.5 or more and 1.5 or less in terms of B/N, and the rest is composed of Fe and unavoidable impurities; containing Mn More than 10% of the sulfides are composite precipitates with B precipitates; the total distribution density of MnS, Cu ₂ S and their composite sulfides is below 3.0×10 ⁵ /mm ² ; the diameter is less than 0.1 μm The distribution density of the Ti precipitates is 1.0×10 ³ /mm ² or less.

prior art literature Patent Literature Patent Document 1: Japanese Patent Laid-Open No. 54-163720 Patent Document 2: Japanese Patent Laid-Open No. 63-195217 Patent Document 3: Japanese Patent Application Laid-Open No. 3-104844 Patent Document 4: Japanese Patent Laid-Open No. 2004-2954 Patent Document 5: International Publication No. 2005/100627

Summary of Invention The problem to be solved by the invention In the techniques of Patent Documents 1 to 5, it is an attempt to reduce iron loss by promoting the growth of crystal grains during relaxation annealing. However, in these technologies, there is a new problem that the magnetic flux density decreases as the iron loss decreases. As a result, in general-purpose grade non-oriented electrical steel sheets with limited chemical components, there is a problem that both low iron loss and high magnetic flux density cannot be achieved at a high level.

The present invention has been made in view of such a problem. An object of the present invention is to provide a hot-rolled steel sheet for non-oriented electrical steel sheet and a method for producing the same, which can have both low iron loss and high magnetic flux density even if the chemical composition is limited.

means of solving problems The gist of the present invention is as follows.

(1) The hot-rolled steel sheet for non-oriented electrical steel sheets according to one aspect of the present invention contains the following chemical components in mass %: C: 0.0010% or more and 0.0050% or less, Si: 0.1% or more and less than 0.5%, Mn: 0.1% or more and 0.5% or less, Al: 0.1% or more and 0.5% or less, total-Ti: 0.0010% or more and 0.0030% or less, N: 0.0010% or more and 0.0030% or less, S: more than 0.0015% and less than 0.0040%, Nb: 0% or more and 0.0030% or less, V: 0% or more and 0.0030% or less, Zr: 0% or more and 0.0030% or less, Sn: 0% or more and 0.100% or less, The rest is composed of Fe and impurities, The amount of solid solution Ti is below 0.0005%; When observed on the observation plane parallel to the rolling direction and the plate width direction, Ti carbides with an equivalent circle diameter of 10 nm or more and 50 nm or less exist in the grains and grain boundaries of the fertilized iron grains; The above-mentioned Ti carbides existing in the above-mentioned crystal grains have more than 10% and less than 100% of the above-mentioned Ti carbides are composite precipitation with Mn sulfides, and, The number density of the above-mentioned Ti carbides existing in the above-mentioned grain boundary is 0.1 pieces/μm or less. (2) The hot-rolled steel sheet for non-oriented electrical steel sheet described in (1) above may also contain the following chemical components in mass %: Sn: 0.010% or more and 0.100% or less. (3) A method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to an aspect of the present invention is a method for producing the hot-rolled steel sheet for a non-oriented electrical steel sheet according to the above (1) or (2), comprising: Follow the steps below: Casting step: cast molten steel to obtain steel billets, The steel billet contains the following as chemical components in % by mass: C: 0.0010% or more and 0.0050% or less, Si: 0.1% or more and less than 0.5%, Mn: 0.1% or more and 0.5% or less, Al: 0.1% or more and 0.5% or less, total-Ti: 0.0010% or more and 0.0030% or less, N: 0.0010% or more and 0.0030% or less, S: more than 0.0015% and less than 0.0040%, Nb: 0% or more and 0.0030% or less, V: 0% or more and 0.0030% or less, Zr: 0% or more and 0.0030% or less, Sn: 0% or more and 0.100% or less, The remainder consists of Fe and impurities; and Hot rolling step: hot rolling the aforementioned steel billet to obtain a hot rolled steel sheet; and In the aforementioned hot rolling step, Before hot rolling, the steel billet is heated and kept in a temperature range of 1150°C or more and 1200°C or less for 10 minutes or more and 60 minutes or less, and is given 20% or more and 30% in the final pass of finishing hot rolling. the following shrinkage; After hot rolling, the steel sheet is kept in a temperature range of 800° C. or more and less than 900° C. for 15 minutes or more and 30 minutes or less.

Invention effect According to the above aspects of the present invention, it is possible to provide a hot-rolled steel sheet for non-oriented electrical steel sheets and a method for manufacturing the same, which can have both low iron loss and high magnetic flux density even if the chemical composition is limited.

Embodiments of the present invention Form for carrying out the invention Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the scope of the present invention. In addition, in the following numerical limitation range, the lower limit value and the upper limit value are included in the range. For values marked "greater than" or "less than", the value is not included in the range of values. In addition, unless otherwise stated, "%" about the content of each element means "mass %".

Regarding the Al-added steel containing about 0.002% of Ti, the present inventors investigated the reason why the magnetic flux density after relaxation annealing is lower than that before relaxation annealing, focusing on the existence form of Ti.

In general, in Al-added steel, coarse AlN tends to be generated in the steel-making step or the hot-rolling step. The generated coarse AlN itself does not harm the growth of crystal grains. However, once coarse AlN is formed, the N in the steel decreases and the precipitation of TiN is suppressed. Once the precipitation of TiN is suppressed in this way, the amount of solute Ti in the steel increases. The inventors of the present invention have found that when the amount of solid solution Ti is high in the hot rolled steel sheet, the magnetic flux density decreases when a non-oriented electrical steel sheet is produced and subjected to relaxation annealing (see FIG. 1 ).

As for the method of reducing the solid-solution Ti content of the hot-rolled steel sheet, it is generally considered that the Ti content of the steel billet can be reduced. If the Ti content of the billet is reduced, the amount of solid solution Ti in the hot-rolled steel sheet is also reduced; as a result, the growth of crystal grains becomes stable during relaxation annealing as a non-oriented electrical steel sheet, and the magnetic flux after relaxation annealing can be suppressed. Density drops. However, Ti contained in the steel billet is an impurity element. If the Ti content of the steel billet is reduced to such an extent that the decrease in the magnetic flux density can be avoided, the manufacturing cost of the non-oriented electrical steel sheet will increase, which is undesirable.

Therefore, the inventors of the present application have devoted themselves to researches to achieve a method for obtaining good magnetic properties when producing a general-purpose grade non-oriented electrical steel sheet even if the steel billet contains Ti as an impurity. As a result, the present inventors have found that it is appropriate to precipitate Ti contained as an impurity as Ti nitrides and Ti carbides as much as possible at the stage of hot rolling the steel sheet, thereby reducing the amount of Ti dissolved in the steel.

In addition, the inventors of the present application have also obtained the knowledge that the above-mentioned Ti nitrides have a sufficiently large precipitation size so as not to inhibit the growth of crystal grains. The following knowledge was obtained. For example, in the stage of hot rolling a steel sheet, if Ti carbides and Mn sulfides are compounded and precipitated, and the number of Ti carbides that are individually precipitated in the crystal grain boundaries is controlled, a non-oriented electrical steel sheet can be produced. During relaxation annealing, crystal grains will grow steadily. The idea is revealed below.

First, in order to reduce the amount of solid-solution Ti in the hot-rolled steel sheet, it is necessary to allow Ti in the steel to precipitate as Ti nitrides. As a result of the review, it was found that if the heating of the billet before hot rolling is properly controlled, TiN will be preferentially precipitated over AlN, and the amount of solid solution Ti in the hot rolled steel sheet can be reduced to a certain extent. In this way, in order to preferentially precipitate TiN over AlN, the steel billet is heated to a temperature range of 1150°C or higher and 1200°C or lower before hot rolling, and the steel billet is kept within this temperature range for 10 minutes or more and 60 minutes or less. Can.

However, as described above, it is not enough to simply precipitate Ti in the steel as Ti nitrides. In order to sufficiently reduce the amount of solid-solution Ti in the hot-rolled steel sheet, it is necessary to further precipitate Ti in the steel as Ti carbides. At this time, it is necessary to make Ti carbide and Mn sulfide composite precipitation in the ferrite grains and restrain it from precipitating alone toward the ferrite grain boundaries, so as not to hinder the growth of crystal grains. The review results show that if the reduction ratio in the final pass of finishing hot rolling is controlled, and the temperature of the steel sheet after hot rolling is properly controlled, Ti carbides can be precipitated in a suitable form. In order to appropriately precipitate Ti carbides in this way, a reduction ratio of 20% or more and 30% or less can be applied in the final pass of finishing hot rolling, and after hot rolling, the steel sheet is heated to 800°C or more and It is kept for more than 15 minutes and less than 30 minutes within the temperature range of less than 900°C.

As described above, by controlling the temperature before hot rolling, Ti in the steel is precipitated in the form of TiN, and by controlling the rolling during the final pass of finishing hot rolling and controlling the temperature after hot rolling, the Ti in the steel is Ti is further precipitated in the form of TiC. As a result, the amount of solid-solution Ti in the hot-rolled steel sheet decreases, and crystal grains grow stably when a non-oriented electrical steel sheet is produced and subjected to relaxation annealing, so that low iron loss and high magnetic flux density can be achieved.

In addition, the precipitation of TiN by controlling the temperature before hot rolling is large enough to not hinder the growth of crystal grains. On the other hand, although TiC precipitated at the crystal grain boundary significantly deteriorates the growth of crystal grains, TiC compounded on Mn sulfide in the crystal grains does not deteriorate the growth of crystal grains. Accordingly, it is necessary to make TiC complex and precipitate with sulfide in the ferrite grains, and to suppress its separate precipitation toward the ferrite grain boundaries.

That is, in the hot-rolled steel sheet for non-oriented electrical steel sheet of the present embodiment, Ti in the steel is precipitated in the form of Ti nitride and Ti carbide to reduce the amount of solid-solution Ti. The precipitation form of Ti carbides avoids hindering the growth of crystal grains. As a result, even as a general-purpose grade with limited chemical composition, it is possible to combine low iron loss with high magnetic flux density at a high level.

In addition, in the hot-rolled steel sheet for non-oriented electrical steel sheet of the present embodiment, since it is premised on reducing the amount of solid-solution Ti, it is preferable to reduce the content of nitride-forming elements other than Ti, namely Nb, V, and Zr, so as to avoid wasteful consumption. N in steel. For example, the content of each of Nb, V, and Zr may be 0.0030% or less.

<Chemical composition of hot-rolled steel sheet> Regarding the hot-rolled steel sheet used for the non-oriented electrical steel sheet of the present embodiment, the reason for limiting the chemical composition of the steel will first be described.

In the present embodiment, the hot-rolled steel sheet contains basic elements and optionally optional elements in terms of chemical components, and the remainder is composed of Fe and impurities.

C (carbon) is an essential element. When the C content is too large, the iron loss of the non-oriented electrical steel sheet is deteriorated due to magnetic aging. Therefore, the C content is made 0.0050% or less. On the other hand, from the viewpoint of avoiding the formation of solid solution B, the C content is made 0.0010% or more. The C content may be 0.0045% or less, 0.0040% or less, or 0.0035% or less. Moreover, the C content may be 0.0015% or more, 0.0020% or more, or 0.0025% or more.

Si (silicon) is an essential element. Si is an element effective for increasing the electrical resistance of the non-oriented electrical steel sheet. However, when the Si content is too large, the hardness of the non-oriented electrical steel sheet increases, the magnetic flux density decreases, and the cost increases. For the chemical composition of the general grade, the Si content is set to be less than 0.5%. The Si content may be set to 0.4% or less. On the other hand, in order to obtain the above-mentioned effects, the Si content is made 0.1% or more. The Si content may be set to 0.20% or more.

Mn (manganese) is an essential element. Mn is a sulfide-forming element, and is preferably contained in an appropriate amount from the viewpoint of promoting the growth of crystal grains. Therefore, the Mn content is made 0.1% or more. The Mn content may be 0.20% or more. On the other hand, considering the structure control of the hot-rolled sheet and the decrease in the saturation magnetic flux density of the non-oriented electrical steel sheet, the Mn content was made 0.5% or less. The Mn content may be set to 0.4% or less.

Al (aluminum) is an essential element. Al is a deoxidizing element of steel. From the viewpoint of securing the stable deoxidation effect and suppressing the formation of fine AlN, the content of Al is set to 0.1% or more. On the other hand, when the Al content is too large, AlN will be preferentially precipitated over TiN, which hinders the reduction of the amount of solid-solution Ti through the precipitation of TiN. Therefore, the Al content is made 0.5% or less. Preferably, the Al content can also be set to 0.3% or less or 0.2% or less.

Ti (titanium) is an element mixed into the steel billet. If the Ti content is set to zero, the manufacturing cost will increase. Therefore, for the chemical composition of general-purpose grades, the total-Ti content is set to 0.0010% or more. The total-Ti content can also be set to be greater than 0.0020%. On the other hand, when the total-Ti content is too large, it becomes difficult to reduce the amount of solid solution Ti. Therefore, the total-Ti content is made 0.0030% or less. In addition, the so-called total-Ti means: Ti contained in a solid solution in the steel, Ti contained in precipitates such as TiN and TiC, and Ti which is the sum of the two.

Nb (niobium) is an element of choice. Nb generates nitrides and consumes N in the steel, so it may prevent the reduction of the amount of solid-solution Ti by generating TiN. However, Nb is an element mixed into the steel billet. Excessively reducing the Nb content will lead to an increase in manufacturing cost. Therefore, the Nb content is set to 0.0030% or less in consideration of TiN production and manufacturing costs. Preferably, the Nb content can also be set to 0.0025% or less, 0.0020% or less, or 0.0015% or less. The lower the Nb content, the better, and the lower limit may also be 0%. However, in consideration of industrial productivity, the Nb content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

V (vanadium) is an element of choice. V generates nitrides and consumes N in the steel, so that the reduction of the amount of solid-solution Ti by the generation of TiN may be prevented in some cases. However, V is an element mixed into the steel billet. Excessive reduction of the V content leads to an increase in the manufacturing cost. Therefore, the V content is set to 0.0030% or less in consideration of TiN production and manufacturing costs. Preferably, the V content can also be set to 0.0025% or less, 0.0020% or less, or 0.0015% or less. The lower the V content, the better, and the lower limit may also be 0%. However, in consideration of industrial productivity, the V content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

Zr (zirconium) is an optional element. Zr forms nitrides and consumes N in the steel, so it may prevent the reduction of the amount of solid-solution Ti through the formation of TiN. However, Zr is an element mixed into the steel billet. Excessive reduction of the Zr content leads to an increase in the manufacturing cost. Therefore, the Zr content is set to 0.0030% or less in consideration of TiN production and manufacturing costs. Preferably, the Zr content can also be set to 0.0025% or less, 0.0020% or less, or 0.0015% or less. The lower the Zr content, the better, and the lower limit may also be 0%. However, in consideration of industrial productivity, the Zr content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

N (nitrogen) is an essential element for forming nitrides. It is generally considered that in non-oriented electrical steel sheets, nitrides are detrimental to the growth of crystal grains. However, the inventors of the present invention have found that if Ti is fixed in the form of Ti nitrides such as TiN using N to reduce the amount of solid-solution Ti in the hot-rolled steel sheet, the decrease in magnetic flux density after relaxation annealing can be suppressed. Therefore, the N content is made 0.0010% or more. The N content may be 0.0012% or more, 0.0015% or more, or 0.0020% or more. On the other hand, when too much N is contained, crystal grain growth is inhibited, which is not preferable. Therefore, the N content is made 0.0030% or less. The N content may be set to 0.0025% or less.

S (sulfur) is an essential element for forming Mn sulfides. It is generally considered that in a non-oriented electrical steel sheet, sulfides deteriorate grain growth, so the S content should be reduced as much as possible. However, the inventors of the present application have obtained the knowledge that an appropriate amount of sulfide functions as a TiC precipitation nucleus, thereby rendering TiC harmless. TiC usually precipitates at the crystal grain boundaries of the fertile iron grains before the crystal grain growth, thereby significantly deteriorating the crystal grain growth. On the other hand, TiC which is compound-precipitated on the sulfide is precipitated in the crystal grains of the ferric iron grains, and thus does not deteriorate the growth of the crystal grains. In order to compound precipitation of TiC on the sulfide, the S content is set to be more than 0.0015%. The S content may be set to 0.0020% or more, or more than 0.0020%. On the other hand, when too much S is contained, crystal grain growth is inhibited, which is not preferable. In particular, when the S content is more than 0.0040%, the amount of sulfide precipitation increases and the growth of crystal grains is inhibited. Therefore, the S content is made 0.0040% or less. The S content may be set to 0.0035% or less, 0.0030% or less, or 0.0025% or less.

Sn (tin) is an element of choice. The lower limit of the Sn content may also be 0%. However, Sn has the effect of increasing the magnetic flux density. In addition, Sn also has the effect of suppressing nitriding and oxidation of the steel sheet surface during annealing. Therefore, Sn may be contained as needed. For example, the Sn content may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, even if the Sn content is too large, the effect is saturated, so the Sn content may be 0.100% or less, 0.090% or less, or 0.080% or less.

The remainder of the chemical composition consists of Fe and impurities. In addition, the impurity refers to an element that does not impair the effect of the present embodiment even if it is contained, and an element that is mixed from ores and scraps as raw materials, or from the manufacturing environment when industrially producing steel sheets. The upper limit of the total content of impurities may be, for example, 5%.

The above-mentioned chemical components may be measured by a general method for analyzing steel. For example, the chemical components may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, a 35 mm square test piece is taken from the steel sheet, and the test piece is measured based on the conditions of a calibration curve (calibration curve) prepared in advance using ICPS-8100 manufactured by Shimadzu Corporation (measuring device). chemical composition. In addition, C may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas melting-thermal conductivity method.

<Ti carbide> Regarding the hot-rolled steel sheet for the non-oriented electrical steel sheet of the present embodiment, the characteristics of Ti carbide (TiC) will be described next.

As described above, in this embodiment, the chemical components and the production conditions are controlled in a complex and inseparable manner, and the Ti precipitates contained in the hot-rolled steel sheet are controlled. In particular, in this embodiment, the precipitation of Ti carbides alone at the crystal grain boundaries of the fertilized iron grains is suppressed.

For the hot-rolled steel sheet used for the non-oriented electrical steel sheet according to the present embodiment, Ti carbides having an equivalent circle diameter of 10 nm or more and 50 nm or less when observed on an observation plane parallel to the rolling direction and the sheet width direction Exist in grains and grain boundaries of fertilizer grain iron grains; wherein The above-mentioned Ti carbides existing in the above-mentioned crystal grains have 10% or more and 100% or less of the composite precipitation with Mn sulfide, and, The number density of the above-mentioned Ti carbides existing in the above-mentioned grain boundary is 0.1 pieces/μm or less.

In the present embodiment, as the size of the Ti carbide that most affects the growth of crystal grains, the Ti carbide with an equivalent circle diameter of 10 nm or more and 50 nm or less is controlled. In the hot-rolled steel sheet for non-oriented electrical steel sheets of the present embodiment, Ti carbides of the above-mentioned size are contained in the grains and grain boundaries of the ferrite grains.

If more than 10% and less than 100% of Ti carbides are compounded with Mn sulfide in the Ti carbides existing in the grains of the iron grains of the fertilizer grains, they exist alone in the iron grains of the fertilizer grains. The number density of the above-mentioned Ti carbides at the grain boundaries of the grains can be suppressed to 0.1 pieces/μm or less. Among the Ti carbides existing in the grains of the iron grains of the fertilizer grains, there should preferably be more than 20%, more than 30%, more than 40%, or more than 50% of the Ti carbides that are related to Mn. Sulfide compound precipitation.

If the number density of Ti carbides existing in the grain boundaries of the fertilized iron grains is 0.1 pieces/μm or less, the growth of the crystal grains will not be inhibited. The number density of Ti carbides existing at the grain boundaries is preferably 0.05 pieces/μm or less, 0.01 pieces/μm or less, or 0.005 pieces/μm or less. For Ti carbides existing in grain boundaries, the smaller the number density, the better, so the lower limit can also be zero.

The method for analyzing Ti carbides existing in grains and grain boundaries of ferrite grains is as follows. (1) The hot-rolled steel sheet is cut along the rolling direction and the width direction, and small test pieces are taken. The surface of the small test piece (the rolling surface parallel to the rolling direction and the plate width direction) is ground to any depth, and the back side of the small test piece (relative to the rolling surface) is also ground, thereby the small test piece is ground. Made into a thin film. (2) Observe the polished surface with a transmission electron microscope, and find out whether the presence of inclusions with an equivalent circle diameter of 10 nm or more and 50 nm or less is within a grain or a grain boundary. (3) The composition of each inclusion was measured by EDS (energy dispersive X-ray analyzer) attached to a transmission electron microscope. (4) Inclusions with an atomic ratio of Mn and S about 1:1 are judged as Mn sulfides, and inclusions with an atomic ratio of Ti and C about 1:1 are judged as Ti carbides, and the number of these is calculated. . The number of composite precipitations of the above-mentioned Mn sulfide and the above-mentioned Ti carbide was also calculated. (5) Based on the above calculation results, the ratio of the number of Ti carbides compounded and precipitated with Mn sulfide in the grains of the ferrite iron grains was calculated. (6) Similarly, the number density of Ti carbides in contact with and existing in the grain boundaries of the fertilized iron grains was calculated. The number density is determined as a value obtained by dividing the number of Ti carbides in contact with and existing in the grain boundaries of the fertile iron grains by the total length of the grain boundaries. In addition, the area of the measurement region is at least 100 μm ² . If the sum of the measurement areas is at least 100 μm ² , the number of measurement locations and the size of the measurement field of view do not need to be particularly limited.

<Amount of solid solution Ti> Regarding the hot-rolled steel sheet used for the non-oriented electrical steel sheet of the present embodiment, the amount of solid-solution Ti will be described next.

In the hot-rolled steel sheet for the non-oriented electrical steel sheet of the present embodiment, Ti in the steel is precipitated in the form of Ti nitrides and Ti carbides, thereby reducing the amount of solid-solution Ti in the steel. Specifically, in the hot-rolled steel sheet for the non-oriented electrical steel sheet of the present embodiment, the amount of solid-solution Ti is 0.0005% or less in mass %. When the amount of solute Ti in the hot-rolled steel sheet is 0.0005% or less, even if a non-oriented electrical steel sheet is prepared and a relaxation annealing is performed, the decrease in the magnetic flux density can be suppressed. That is, even if the chemical composition is limited as a general-purpose grade, it is possible to have both low iron loss and high magnetic flux density at a high level. The amount of solid solution Ti is preferably 0.0003% or less, or 0.0001% or less. The smaller the value of the amount of solid solution Ti, the better, so the lower limit can also be zero.

In addition, although Ti in the steel is precipitated in the form of Ti nitrides and Ti carbides to reduce the amount of solid-solution Ti, the Ti nitrides are large enough to prevent the growth of crystal grains because the size of the precipitation is large. Therefore, in the hot-rolled steel sheet for the non-oriented electrical steel sheet according to the present embodiment, if the amount of solute Ti is 0.0005% or less, the precipitation form of Ti nitrides (for example, particle size and number density, etc.) is not particularly limited. ). On the other hand, Ti carbides inhibit the growth of crystal grains. Therefore, in the hot-rolled steel sheet for the non-oriented electrical steel sheet of the present embodiment, the precipitation form of Ti carbides is controlled as described above, in addition to the amount of solute Ti being 0.0005% or less.

The quantitative method of solid solution Ti is as follows. (1) The total-Ti content of the hot-rolled steel sheet (the total amount of Ti constituting the Ti precipitates and the solid solution Ti) was determined by chemical analysis. (2) By electrolysis, the hot-rolled steel sheet is dissolved and the residue is extracted. (3) The residue components were analyzed by ICP, and the Ti content of the residue was determined. (4) The Ti content of the Ti precipitates of the hot-rolled steel sheet was subtracted from the total-Ti content of the hot-rolled steel sheet, and the obtained value was regarded as the solid-solution Ti amount of the hot-rolled steel sheet. In addition, by electrolysis, both Ti nitride and Ti carbide can be extracted as residues. Accordingly, the difference between the Ti content of the residue obtained by the electrolysis method and the total-Ti content of the hot-rolled steel sheet can be regarded as the solid-solution Ti content of the hot-rolled steel sheet.

<Average grain size> In the hot-rolled steel sheet of the present embodiment, the average crystal grain size is not particularly defined. However, in the case of a non-oriented electrical steel sheet for supplying to punching, the smaller the average grain size, the more suppressed the occurrence of burr during punching and improved workability. Therefore, in the non-oriented electrical steel sheet after finish annealing and before relaxation annealing, the average crystal grain size may be 30 μm or less.

In order to control the average grain size of the non-oriented electrical steel sheet after finishing annealing and before relaxation annealing to 30 μm or less using the hot-rolled steel sheet of the present embodiment, a known technique can be suitably used. The measurement of the average grain size of the non-oriented electrical steel sheet after finishing annealing and before relaxation annealing can be carried out by the following method: observe the cross section of the steel sheet with an optical microscope, draw a straight line in the thickness direction of the sheet, and calculate the difference between the straight lines. The method of the number of crystal grains (the so-called counting method prescribed in Appendix B of JIS G 0551:2013).

<Manufacturing method of hot-rolled steel sheet> Next, a method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to the present embodiment will be described.

The manufacturing method of the hot-rolled steel sheet of the present embodiment includes the following steps: Casting step: cast molten steel to obtain steel billets, The steel billet contains the following as chemical components in % by mass: C: 0.0010% or more and 0.0050% or less, Si: 0.1% or more and less than 0.5%, Mn: 0.1% or more and 0.5% or less, Al: 0.1% or more and 0.5% or less, total-Ti: 0.0010% or more and 0.0030% or less, N: 0.0010% or more and 0.0030% or less, S: more than 0.0015% and less than 0.0040%, Nb: 0% or more and 0.0030% or less, V: 0% or more and 0.0030% or less, Zr: 0% or more and 0.0030% or less, Sn: 0% or more and 0.100% or less, The remainder consists of Fe and impurities in the billet; and Hot-rolling step: hot-rolling the above-mentioned steel billet to obtain a hot-rolled steel sheet; and In the above hot rolling step, Before hot rolling, the above-mentioned steel billet is heated and kept in the temperature range of 1150°C or more and 1200°C or less for 10 minutes or more and 60 minutes or less, and applied to 20% or more and 30% in the final pass of finishing hot rolling. the following shrinkage; After hot rolling, the steel sheet is kept in a temperature range of 800°C or more and less than 900°C for 15 minutes or more and 30 minutes or less.

In the casting step, molten steel is obtained by refining so that the chemical composition of the finally obtained hot-rolled steel sheet falls within the above-mentioned range, and the molten steel is cast to obtain a steel billet. The chemical composition of the steel billet is the same as that of the hot-rolled steel sheet described above. In addition, in the hot-rolled steel sheet of the present embodiment, the amount of solid solution Ti is particularly important, but the control of the amount of solid solution Ti can be performed by a subsequent hot rolling (hot rolling) step or the like. Accordingly, the manufacturing conditions in the steel-making step are not particularly limited, and known conditions can be appropriately employed.

In the hot rolling step, the steel billet after the casting step is hot rolled to obtain a hot rolled steel sheet. This hot rolling step is an important step to control the amount of solid solution Ti by controlling the precipitates.

First, it is necessary to suppress the generation of AlN and promote the precipitation of TiN. For this purpose, the steel billet before hot rolling is heated to a temperature range of 1150° C. or more and 1200° C. or less, and kept in this temperature range for 10 minutes or more and 60 minutes or less, and then hot-rolling the steel billet.

This heating condition promotes the precipitation of TiN, but only under this heating condition, a part of Ti remains in a solid solution state and remains in the hot-rolled steel sheet (hot-rolled steel sheet). Therefore, it is necessary to promote the precipitation of TiC. However, when precipitating TiC, it is necessary to control TiC so as not to separate out of the fertile iron grain boundaries alone.

In this case, a reduction of 20% or more and 30% or less is applied in the final pass of finishing hot rolling, and after hot rolling, it is kept in the temperature range of 800°C or more and less than 900°C for 15 minutes or more and 30 minutes. minutes or less. Thereby, the Mn sulfide precipitated during hot rolling acts as a nucleus, and TiC is compositely precipitated. When the above conditions are met, TiC will be compounded with Mn sulfide in the ferrite grains, which can inhibit the separate precipitation of TiC in the ferrite grain boundaries. Specifically, the number ratio of TiC compounded and precipitated with Mn sulfide in the iron grains of the fertilizer grains is 10% or more and less than 100% by number, and the TiC that is separately precipitated in the grain boundaries of the fertilizer grains, The number density thereof is 0.1 pieces/μm or less. By controlling the precipitates in this way, the amount of solute Ti in the hot-rolled steel sheet can be made 0.0005% or less. Therefore, the crystal grain growth during relaxation annealing becomes stable, and even if the chemical composition is limited as a general-purpose grade, it can still have both low iron loss and high magnetic flux density at a high level.

When the reduction ratio of the final pass of finishing hot rolling is less than 20% or more than 30%, TiC will separate and finely precipitate, and the crystal grain growth after relaxation annealing will become unstable. Even if the temperature is maintained after hot rolling at 800° C. or lower, TiC is singly and finely precipitated, and crystal grain growth after relaxation annealing becomes unstable. On the other hand, when temperature holding is performed at a temperature higher than 900° C., solid solution Ti cannot be precipitated. In addition, even if holding at a temperature of 800°C or higher and 900°C or lower, if the holding time is shorter than 15 minutes or longer than 30 minutes, composite precipitation cannot sufficiently occur, and crystal grain growth during relaxation annealing may become unstable.

<Manufacturing method of non-oriented electrical steel sheet> Next, a method for producing a non-oriented electrical steel sheet using the hot-rolled steel sheet for the non-oriented electrical steel sheet according to the present embodiment will be described.

The manufacturing method of the non-oriented electrical steel sheet of the present embodiment uses the above-mentioned hot-rolled steel sheet, and it can be carried out as follows: The hot-rolled steel sheet is manufactured by satisfying the above-mentioned manufacturing conditions, and the obtained hot-rolled steel sheet is subjected to pickling and cold rolling without annealing the hot-rolled sheet; The cold-rolled material after the above-mentioned cold-rolling is heated at a heating rate of 20°C/sec or higher, and soaked at a temperature of 850°C or lower.

As described above, a hot-rolled steel sheet is produced through a casting step and a hot-rolling step, and the hot-rolled steel sheet is subjected to a pickling step, a cold rolling (cold rolling) step, and a finish annealing step without applying the hot-rolled sheet annealing. . Among these steps, the conditions of the pickling step and the cold rolling step are not particularly limited, and known conditions can be appropriately used.

The finishing annealing step is a step of heating, soaking and cooling the cold-rolled steel sheet after the cold-rolling step. The conditions of the finish annealing step are also not particularly limited, and known conditions can be appropriately used. However, in addition to using the hot-rolled steel sheet for the non-oriented electrical steel sheet according to the present embodiment, the heating rate of the steel sheet in the heating process of the finishing annealing step is set to 20° C./sec or more, whereby the non-oriented electromagnetic steel sheet can be improved. Magnetic flux density of the steel plate. Accordingly, the heating rate in the heating process of the finishing annealing step may be set to 20° C./sec or more. The so-called heating rate here is the value obtained by dividing the difference between the starting heating temperature and the soaking temperature of the hot-rolled steel sheet by the time from the starting heating temperature to the soaking temperature; that is, from the starting heating temperature to the soaking temperature Average heating rate up to soaking temperature.

Further, in the soaking process of the finishing annealing step, the annealing temperature (soaking temperature) can also be specified not to be greater than 850°C. When the annealing temperature is higher than 850°C, in the composite precipitation of TiC and sulfide controlled in the hot rolling step, TiC will become solid solution Ti again, which may make the growth of crystal grains unstable. Accordingly, the heating rate in the heating process of the finishing annealing step is preferably set to 20°C/sec or more, and the annealing temperature during the soaking process of the finishing annealing step is preferably set to 850°C or less.

The non-oriented electrical steel sheet obtained by the above steps can be suitably used as a material for electric appliances such as motors. When manufacturing a motor member, the non-oriented electrical steel sheet is subjected to machining such as punching, and relaxation annealing. As a standard, the relaxation annealing conditions are set to: annealing temperature of 750°C and holding time of 2 hours. However, the annealing temperature and time may be appropriately changed in consideration of both equipment limitations and the promotion of crystal grain growth. [Example 1]

One aspect and effect of the present invention will be described in more detail with examples, but the conditions in the examples are only examples of conditions used to confirm the practicability and effects of the present invention, and the present invention is not limited to this condition example. The present invention can adopt various conditions as long as it does not depart from the gist of the present invention and achieves the object of the present invention.

The chemical composition-adjusted steel billet is hot-rolled, and a hot-rolled steel sheet with a thickness of 2.5 mm is coiled. Tables 1A to 1B show the chemical components of the hot-rolled steel sheets, and Tables 2A to 2E show the hot-rolling conditions. In addition, the chemical composition of the steel billet and the chemical composition of the hot-rolled steel sheet are the same except for the amount of solid solution Ti.

With respect to the manufactured hot-rolled steel sheet, the amount of solid-solution Ti and Ti carbide were analyzed based on the above-described method. The results are shown in Tables 3A to 3E.

In addition, after the hot-rolled steel sheet was pickled, it was cold-rolled to 0.5 mm to prepare a cold-rolled steel sheet, and was further subjected to finish annealing under the conditions shown in Tables 2A to 2E to obtain a non-oriented electrical steel sheet.

According to the following steps and the criteria of pass/fail, characteristic evaluation is carried out to determine whether the various non-oriented electrical steel sheets obtained in the above steps are (A) low in iron loss after relaxation annealing and (B) after relaxation annealing Non-oriented electrical steel sheet with high magnetic flux density.

(A) Iron loss after relaxation annealing The iron loss (W15/50, W15/60) of the steel sheet after relaxation annealing (annealing temperature 750°C and holding time 2 hours) was measured in accordance with JIS C 2552:2014 "Non-oriented electrical steel strip". Then, the iron loss was separated by the dual frequency method, and the non-oriented electrical steel sheet whose W15/50 hysteresis loss after relaxation annealing was 2.6 W/kg or less was judged as the iron loss characteristics after relaxation annealing Excellent. dual frequency method Iron loss of W15/50 divided by frequency 50・・・α Iron loss of W15/60 divided by frequency 60・・・β Hysteresis loss of W15/50=α×50-(β-α)×250

(B) Magnetic flux density after relaxation annealing The magnetic flux density (B50) of the steel sheet after relaxation annealing (annealing temperature 750°C and holding time 2 hours) was measured according to JIS C 2552:2014 "Non-oriented electrical steel strip". Then, a value obtained by dividing B50 by the saturation magnetic flux density Bs of the steel sheet, that is, a non-oriented electrical steel sheet having a B50/Bs of 0.820 or more, is judged to be excellent in magnetic flux density after relaxation annealing. In addition, the saturation magnetic flux density Bs of the steel sheet can be obtained by substituting the chemical composition of the steel sheet into the following formula. Saturation magnetic flux density Bs=2.1561-0.0413×Si-0.0198×Mn-0.0604×Al

The above evaluation results are shown in Table 3A to Table 3E. As shown in Tables 1A to 3E, the examples of the present invention can have both low iron loss and high magnetic flux density because the chemical composition, the amount of solid solution Ti, and the Ti carbide are satisfied. On the other hand, since the comparative example does not satisfy any one of the chemical composition, the amount of solid solution Ti, or the Ti carbide, it is impossible to have both low iron loss and high magnetic flux density.

[Table 1A]

[Table 1B]

[Table 2A]

[Table 2B]

[Table 2C]

[Table 2D]

[Table 2E]

[Table 3A]

[Table 3B]

[Table 3C]

[Table 3D]

[Table 3E]

industrial availability According to the above aspects of the present invention, it is possible to provide a hot-rolled steel sheet for non-oriented electrical steel sheets and a method for manufacturing the same, which can have both low iron loss and high magnetic flux density even if the chemical composition is limited. Therefore, the industrial applicability is high.

FIG. 1 is a graph showing the relationship between the amount of solute Ti in the hot-rolled steel sheet and the magnetic flux density B50 after relaxation annealing when making a non-oriented electrical steel sheet.

Claims (3)

A hot-rolled steel sheet for non-oriented electrical steel sheets, characterized in that it contains the following chemical components in mass %: C: 0.0010% or more and 0.0050% or less, Si: 0.1% or more and less than 0.5%, Mn: 0.1% or more and 0.5% or less, Al: 0.1% or more and 0.5% or less, total-Ti: 0.0010% or more and 0.0030% or less, N: 0.0010% or more and 0.0030% or less, S: more than 0.0015% and less than 0.0040%, Nb: 0% or more and 0.0030% or less, V: 0% or more and 0.0030% or less, Zr: 0% or more and 0.0030% or less, Sn: 0% or more and 0.100% or less, The rest is composed of Fe and impurities, The amount of solid solution Ti is below 0.0005%; When observed on the observation plane parallel to the rolling direction and the plate width direction, Ti carbides with an equivalent circle diameter of 10 nm or more and 50 nm or less exist in the grains and grain boundaries of the fertilized iron grains; The above-mentioned Ti carbides existing in the above-mentioned crystal grains have more than 10% and less than 100% of the above-mentioned Ti carbides are composite precipitation with Mn sulfides, and, The number density of the above-mentioned Ti carbides existing in the above-mentioned grain boundary is 0.1 pieces/μm or less. As claimed in claim 1, the hot-rolled steel sheet for non-oriented electrical steel sheet contains the following chemical components in mass %: Sn: 0.010% or more and 0.100% or less. A method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet, which is a method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet as claimed in claim 1 or 2, characterized by comprising the following steps: Casting step: cast molten steel to obtain steel billets, The steel billet contains the following as chemical components in % by mass: C: 0.0010% or more and 0.0050% or less, Si: 0.1% or more and less than 0.5%, Mn: 0.1% or more and 0.5% or less, Al: 0.1% or more and 0.5% or less, total-Ti: 0.0010% or more and 0.0030% or less, N: 0.0010% or more and 0.0030% or less, S: more than 0.0015% and less than 0.0040%, Nb: 0% or more and 0.0030% or less, V: 0% or more and 0.0030% or less, Zr: 0% or more and 0.0030% or less, Sn: 0% or more and 0.100% or less, The remainder consists of Fe and impurities; and Hot rolling step: hot rolling the aforementioned steel billet to obtain a hot rolled steel sheet; and In the aforementioned hot rolling step, Before hot rolling, the steel billet is heated and kept at a temperature range of 1150°C or more and 1200°C or less for 10 minutes or more and 60 minutes or less, and 20% or more and 30% are applied in the final pass of finishing hot rolling. the following shrinkage; After hot rolling, the steel sheet is kept in a temperature range of 800°C or more and less than 900°C for 15 minutes or more and 30 minutes or less.

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