TW202221149A - Non-oriented electromagnetic steel sheet and manufacturing method thereof, and hot-rolled steel sheet having good grain growth capability during stress relief annealing and low core loss and high magnetic flux density after the stress relief annealing - Google Patents

Abstract

A non-oriented electromagnetic steel sheet is provided, the chemical composition of which contains, on the basis of mass %, C: 0.0050% or less, Si: 0.10-1.50%, Mn: 0.10-1.50%, sol. Al: 0.0050% or less, N: 0.0030% or less, S: 0.0040% or less, O: 0.0050-0.0200%, and contains one or more elements selected from the group consisting of La, Ce, Zr, Mg, and Ca, 0.0005-0.0200% in total, the remainder is Fe and impurities; the optimal number density N of oxides containing 20-60% of O and 20-60% of Si, on the basis of mass % and with a diameter of 1.0-5.0 [mu]m is 3.0×10 <SP>3</SP> to 10×10 <SP>3</SP> /cm2, and the number density n of oxides containing one or more elements selected from the group consisting of La, Ce, Zr, Mg, and Ca, with a total mass of 1.0 mass % or more, satisfies n/N ≥ 0.01.

Description

Non-oriented electrical steel sheet, method for producing the same, and hot-rolled steel sheet

technical field The present invention relates to a non-oriented electrical steel sheet, a method for producing the same, and a hot-rolled steel sheet that can be used as a material for the non-oriented electrical steel sheet.

Background technique In recent years, with the increasing demand for energy saving in electrical appliances around the world, non-oriented electrical steel sheets used as core materials for rotating machines are also required to have higher performance characteristics. Specifically, for motors of electrical products, which are called high-efficiency models, high-grade materials that increase the content of Si and Al to increase specific resistance and increase crystal grain size have been used. On the other hand, the motors of general-purpose models are also required to be improved in performance, but the cost constraints are severe, so in fact, it is difficult to replace their materials with advanced materials such as high-efficiency models.

The steel sheet required for general-purpose models is a material with a Si content of 1.5% or less, and the iron loss is greatly improved by allowing crystal grains to grow during relaxation annealing performed after punching of the motor core. In order to promote the growth of crystal grains during relaxation annealing, it is effective to reduce the amount of precipitates that are inevitably mixed into the steel, or to make such precipitates harmless.

For example, Patent Document 1 discloses a method for producing an electric iron sheet having excellent magnetic properties, characterized in that a hot-rolled sheet is obtained by hot-rolling a steel billet having C:≦0.065%, Si:≦2.0% , Al: ≦ 0.10%, O: ≦ 0.020%, B/N: 0.50~2.50, and the rest is composed of Fe and unavoidable impurities; one cold rolling or two or more times including intermediate annealing The above-mentioned hot-rolled sheet is made into final size by cold rolling, and further annealing is performed.

Patent Document 2 discloses a non-oriented electrical steel sheet characterized by containing: C: 0.015% or less, Si: 0.1 to 1.0%, sol.Al: 0.001 to 0.005%, Mn: 1.5% or less, S: 0.008% or less, N: 0.0050% or less, TO: 0.02% or less; in the steel, the weight ratio of MnO to the total weight of the three inclusions of SiO ₂ , MnO and Al ₂ O ₃ is 15% or less; The average crystal grain size can be 50 μm or more and the iron loss is low.

Patent Document 3 discloses a non-oriented electrical steel sheet excellent in magnetic properties, characterized 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 adaptable steel Al: 0.1% or less or Zr: 0.05% or less, the remainder is composed of iron and unavoidable impurity elements; in steel, oxides with a diameter of 0.5 μm or more and a size of 5 μm or less are 1000 per 1 cm ² More than 50,000 pieces.

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; and satisfy: (S in Cu sulfide)/(S in steel)≦0.2, or, (S in Cu sulfide)/(Mn sulfide S)≦0.2; in addition, the number density of sulfides containing Cu with a diameter of 0.03 to 0.20 μm in the steel sheet is 0.5 pieces/μm ³ or less.

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 as B/N: 0.5 or more and 1.5 or less, the remainder is composed of Fe and unavoidable impurities; In terms of the number ratio, more than 10% of the composite precipitates are B precipitates; the total distribution density of MnS, Cu ₂ S and their composite sulfides is 3.0×10 ⁵ /mm ² or less; Ti precipitates with a diameter of less than 0.1 μm The distribution density is 1.0×10 ³ pieces/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 However, in a situation where further improvement of the magnetic properties is required, it becomes difficult to manufacture a non-oriented electrical steel sheet with sufficient and stably improved magnetic properties by the above-mentioned conventional method.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a non-oriented electrical steel sheet and a method for producing the same, which have good grain growth during relaxation annealing, low iron loss after relaxation annealing, and relaxation annealing. The back magnetic flux density is high; and there is also provided a hot-rolled steel sheet that can be used as a material for the non-oriented electrical steel sheet.

means of solving problems The present invention has been made in order to solve the above-mentioned problems, and the gist of the present invention is the following non-oriented electrical steel sheet, a method for producing the same, and a hot-rolled steel sheet.

(1) A non-oriented electrical steel sheet, the chemical composition of which contains, in mass %, C: 0.0050% or less, Si: 0.10 to 1.50%, Mn: 0.10 to 1.50%, sol.Al: 0.0050% or less, N: 0.0030 % or less, S: 0.0040% or less, and O: 0.0050 to 0.0200%, and one or more elements selected from the group consisting of La, Ce, Zr, Mg, and Ca: 0.0005 to 0.0200% in total, the remainder: Fe and impurities; Oxides containing 20-60% O and 20-60% Si in mass % and having a diameter of 1.0-5.0 μm, the number density N of which is 3.0 × 10 ³ ~10 × 10 ³ /cm ² ; and in the oxides, oxides containing 1 or more of the aforementioned elements selected from the group consisting of La, Ce, Zr, Mg, and Ca in a total of 1.0% or more by mass %, the number density n of which satisfies the following: Formula (i): n/N≧0.01・・・(i).

(2) The non-oriented electrical steel sheet of the above (1), wherein the chemical composition contains the following in place of a part of the Fe: Sn: 0.50% or less in mass %.

(3) The non-oriented electrical steel sheet according to (1) or (2) above, wherein the average interval between the oxides is 30 to 300 μm.

(4) The non-oriented electrical steel sheet according to any one of the above (1) to (3), which has an average grain size of 30 μm or less, and the average grain size after relaxation annealing is performed at 750° C. for 2 hours. The crystal grain size is 50 μm or more.

(5) A method for producing a non-oriented electrical steel sheet, which is a method for producing the non-oriented electrical steel sheet according to any one of the above (1) to (4), comprising the following steps: Refining step: making molten steel; Continuous casting step: continuous casting of the aforementioned molten steel to produce a steel billet having the chemical composition of claim 1 or claim 2; Hot rolling step: heating the obtained steel billet and then performing hot rolling to make a hot rolled steel sheet; Pickling step: pickling the aforementioned hot-rolled steel sheet; Cold-rolling step: cold-rolling the pickled hot-rolled steel sheet to form a cold-rolled steel sheet; and Finish annealing step: perform finish annealing on the aforementioned cold-rolled steel sheet; In the aforementioned refining step, The oxygen content of the molten steel before adding the alloy is set to 0.010 to 0.050% in mass %, Next, the Si addition amount M1 added to the molten steel and the Si content M2 in the steel billet are adjusted to satisfy the following formula (ii); In the above-mentioned continuous casting step, the nozzle is used, and a part or all of the inner wall of the nozzle that comes into contact with the molten steel is composed of the following material, which is contained in mass %: a total of 3 to 60% One or more oxides selected from the group consisting of La, Ce, Zr, Mg and Ca; 0.90≦M2/M1≦1.10・・・(ii).

(6) The method for producing a non-oriented electrical steel sheet according to (5) above, wherein, in the refining step, the time from the addition of the alloy to the start of the continuous casting step is set within a range of 30 to 180 minutes; In the above-mentioned hot rolling step, after maintaining the temperature of the steel billet in the range of 1050° C. or higher and less than 1150° C. for 15 to 240 minutes, the steel billet is then subjected to hot rolling.

(7) The method for producing a non-oriented electrical steel sheet according to (5) or (6), wherein in the finish annealing step, the temperature of the cold-rolled steel sheet is 800°C or more and less than 850°C.

(8) A hot-rolled steel sheet that can be used as a material for the non-oriented electrical steel sheet according to any one of (1) to (4) above, wherein the chemical composition of the hot-rolled steel sheet contains in mass %: C: 0.0050% or less, Si: 0.10-1.50%, Mn: 0.10-1.50%, sol.Al: 0.0050% or less, N: 0.0030% or less, S: 0.0040% or less, and O: 0.0050-0.0200%, and contain La, Ce, One or more elements of the group consisting of Zr, Mg and Ca: 0.0005~0.0200% in total, the remainder: Fe and impurities; 20~60% O and 20~60% Si in mass %, and the diameter is 1.0 The oxides of ~5.0 μm have a number density N of 3.0×10 ³ to 10×10 ³ pieces/cm ² ; and the aforementioned oxides include those selected from the group consisting of La, Ce, Zr, Mg and Ca. Oxides of one or more kinds of the above-mentioned elements totaling 1.0% or more in mass %, the number density n of which satisfies the following formula (i): n/N≧0.01・・・(i).

(9) The hot-rolled steel sheet according to (8) above, wherein the chemical composition contains the following in place of a part of the Fe: Sn: 0.50% or less in mass %.

(10) The hot-rolled steel sheet according to (8) or (9) above, wherein the average interval between the oxides is 30 to 300 μm.

Invention effect According to the present invention, it is possible to stably provide a non-oriented electrical steel sheet having good grain growth during relaxation annealing and excellent magnetic properties at low cost.

Embodiments of the present invention Form for carrying out the invention When a non-oriented electrical steel sheet is used as a material for mechanical parts such as motor cores, first machining such as punching is performed, and then relaxation annealing is performed at, for example, 750°C for 2 hours. During relaxation annealing, it is necessary to promote the growth of crystal grains in the steel sheet to reduce the iron loss of the steel sheet. Therefore, the non-oriented electrical steel sheet must have the characteristic of promoting crystal grain growth during relaxation annealing.

As one of the factors that suppress the growth of crystal grains during relaxation annealing, for example, inclusions such as MnS having a pinning effect can be mentioned. It has been considered that reducing the amount of element S that causes inclusions can effectively promote the growth of crystal grains during relaxation annealing. However, S is an element that is inevitably mixed into the steel, and the step of removing it increases the manufacturing cost. In addition, it is known that an attempt can be made to control the precipitation state of MnS by the hot rolling conditions, but it cannot be said that the characteristics can be sufficiently improved by this.

Therefore, the inventors of the present application found that, in the production stage of the non-oriented electrical steel sheet material, the iron loss of the steel sheet after relaxation annealing can be improved by allowing fine precipitation of oxides. This is considered to be because MnS, which has a pinning effect, is precipitated on the surface of the refined oxide and becomes harmless. The inventors of the present application have also found that among the oxides, oxides containing 20 to 60% of O and 20 to 60% of Si and having a diameter of 1.0 to 5.0 μm (hereinafter, also referred to as “suitable oxides”) are responsible for the inclusion of In particular, it has a remarkable effect on making the material harmless, and by optimizing the number density, the iron loss of the steel sheet can be improved.

However, the inventors of the present application have further studied repeatedly, and as a result, they have found that only by controlling the number density of suitable oxides as described above, the effect of making MnS harmless cannot be stably obtained, and the crystal grains during relaxation annealing cannot be stably obtained. Growth is inhibited by a certain percentage.

Then, after reviewing a method for stabilizing and rendering MnS harmless, it is thought that S is fixed by effectively utilizing one or more kinds selected from La, Ce, Zr, Mg, and Ca. However, if these elements are simply added, coarse inclusions containing these elements are generated, and the effect of making S harmless cannot be sufficiently obtained.

The inventors of the present application tried various methods of adding elements such as La, and as a result found that oxides containing these elements are used in the inner wall of a nozzle used in continuous casting, and these elements are added to molten steel by using the melting loss of the nozzle In this case, La and the like do not form inclusions alone, but are contained in the appropriate oxides described above and are finely dispersed. Thereby, the effect of stably rendering MnS harmless can be obtained. In addition, it is not necessary that all the appropriate oxides contain La or the like, and the effect can be sufficiently exhibited if only a part of them is contained.

The present invention has been completed based on the above findings. Each of the requirements of the present invention will be described below.

1. Chemical composition The chemical composition of the non-oriented electrical steel sheet and the hot-rolled steel sheet according to one embodiment of the present invention will be described. The reason for the limitation of each element is as follows. In addition, "%" concerning content in the following description means "mass %".

C: 0.0050% or less C deteriorates iron loss due to magnetic aging. Therefore, the C content is made 0.0050% or less. The C content should preferably be 0.0030% or less or 0.0020% or less. In addition, since the non-oriented electrical steel sheet of the present embodiment does not require C, the lower limit of the C content is 0%. However, if the removal cost of C mixed with impurities is considered, for example, the lower limit of the C content may be set to 0.0001%, 0.0002%, or 0.0005%.

Si: 0.10~1.50% Si is an element that can effectively increase resistance. In addition, it is an element necessary for the formation of the above-mentioned suitable oxides. However, if the content of Si is greater than 1.50%, the hardness of the non-oriented electrical steel sheet will increase, the magnetic flux density will decrease, and the manufacturing cost will increase. Therefore, the Si content is set to 0.10 to 1.50%. The Si content is preferably 0.20% or more, 0.40% or more, or 0.80% or more. Further, the Si content is preferably 1.40% or less, 1.20% or less, or 1.00% or less.

Mn: 0.10~1.50% Mn not only forms sulfides, but is also an element that can effectively increase the resistance of non-oriented electrical steel sheets. It also has the effect of preventing thermal cracking. However, when the Mn content is excessive, the transformation temperature will drop too much, and the crystal grain size cannot be increased in the relaxation annealing. Therefore, the Mn content is made 0.10 to 1.50% or less. The Mn content is preferably 0.20% or more, 0.40% or more, or 0.80% or more. Further, the Mn content is preferably 1.40% or less, 1.20% or less, or 1.00% or less.

sol.Al: 0.0050% or less Al is generally an element used for steel deoxidation. However, in the present invention, Si is used for deoxidation, so Al is not necessary in the non-oriented electrical steel sheet of the present embodiment. Moreover, when Al is contained excessively, it becomes impossible to form suitable oxide containing Si. Accordingly, the sol.Al content is made 0.0050% or less. The content of sol.Al should preferably be below 0.0045% or below 0.0040%. However, in consideration of the removal cost of Al mixed as an impurity, the lower limit of the sol.Al content may be, for example, 0.0001%, 0.0002%, or 0.0005%.

N: 0.0030% or less N produces nitrides and is an element that may inhibit the growth of crystal grains. Accordingly, the N content should be reduced as much as possible. However, it is industrially difficult to achieve zero content of N mixed into steel as an impurity. In the present invention, the N content is set to be 0.0030% or less as a harmless tolerable amount. In addition, the lower limit value of the N content may be set to 0.0001%, 0.0002%, or 0.0005%.

S: 0.0040% or less S produces a sulfide and is an element that may inhibit the growth of crystal grains. Accordingly, the S content should be reduced as much as possible. However, it is industrially difficult to achieve zero S content mixed into steel as an impurity. In the present invention, it is attempted to make S harmless by depositing S on the oxide surface. However, when the S content is more than 0.0040%, the amount of sulfide precipitation itself increases, it becomes difficult to render S harmless, and the growth of crystal grains is inhibited. Therefore, the S content is made 0.0040% or less. Moreover, the lower limit value of S content may be 0.0001%, 0.0002%, or 0.0005%.

O: 0.0050~0.0200% O is an element necessary for the formation of oxides. When the O content is too small, the necessary oxide amount cannot be secured. On the other hand, when the O content is more than 0.0200%, not only the effect is saturated, but the number density of the appropriate oxides is also excessive, and the appropriate oxides are aggregated. Therefore, the O content is set to 0.0050 to 0.0200%. The O content is preferably 0.0055% or more, 0.0060% or more, or 0.0080% or more. In addition, the O content is preferably 0.0180% or less, 0.0150% or less, or 0.0100% or less.

One or more selected from the group consisting of La, Ce, Zr, Mg and Ca: 0.0005~0.0200% in total By including La, Ce, Zr, Mg, and Ca in the oxide mainly composed of O and Si, the effect of making the sulfide harmless can be obtained more effectively and stably. On the other hand, if the content of these elements is excessively increased, the amount of oxygen in the steel will decrease, and in addition, coarse monomer oxides will be generated, so that the above-mentioned effects cannot be obtained. Therefore, the total content of one or more elements selected from the group consisting of La, Ce, Zr, Mg, and Ca is set to 0.0005 to 0.0200%. The total content of these elements is preferably 0.0008% or more, 0.0010% or more, or 0.0020% or more, and preferably 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0070% or less, or 0.0060% or less. In addition, the functions and effects of La, Ce, Zr, Mg, and Ca are substantially the same in the non-oriented electrical steel sheet of the present embodiment, so the contents of these are specified as total contents.

Sn: 0.50% or less In the present invention, Sn is not essential. However, Sn has the effect of inhibiting the nitridation and oxidation of the surface of the steel sheet during relaxation annealing, and it is also an element that effectively increases the magnetic flux density. In view of the above, Sn may be contained in an appropriate amount. However, even if more than 0.50% of Sn is contained, the manufacturing cost will increase in addition to the saturation of the effect. Therefore, in order to contain it, at this time, the content of Sn is made 0.50% or less. The Sn content is preferably 0.45% or less, 0.40% or less, or 0.30% or less. In addition, in order to obtain the above-mentioned effects, the Sn content is preferably 0.01% or more, 0.02% or more, 0.03% or 0.05%.

In the chemical composition of the non-oriented electrical steel sheet and the hot-rolled steel sheet of the present embodiment, the remainder is Fe and impurities. Impurities refer to components that are mixed with raw materials such as ore, scraps, or various factors in the production process during the industrial production of steel, and are acceptable within the range that does not adversely affect the non-oriented electrical steel sheet of the present embodiment.

2. Oxides The oxides contained in the non-oriented electrical steel sheet and the hot-rolled steel sheet of the present embodiment will be described. The non-oriented electrical steel sheet and the hot-rolled steel sheet of the present embodiment contain 20 to 60% by mass of O and 20 to 60% of Si, and an appropriate oxide having a diameter of 1.0 to 5.0 μm. The number density N of the suitable oxides is 3.0×10 ³ to 10×10 ³ pieces/cm ² . Since the number density of suitable oxides is measured on the cross-section of non-oriented electrical steel sheet and hot-rolled steel sheet, it is specified as the number per unit area.

As described above, sulfides such as MnS, which inhibit the growth of crystal grains in the non-oriented electrical steel sheet, are rendered harmless by oxides. Its mechanism is presumed as follows. When the billet of non-oriented electrical steel sheet material is cast, oxides are formed first, and then MnS is precipitated. At this time, MnS will precipitate on the oxide surface. When a large amount of oxides having a predetermined particle size is generated in molten steel, the number of MnS precipitation increases, whereby MnS becomes harmless.

The oxide that can effectively make MnS finely dispersed, its chemical composition in mass %: contains 20~60% of O and 20~60% of Si. It is considered that oxides with chemical compositions outside this range tend to be difficult to precipitate MnS on the surface. Accordingly, in the non-oriented electrical steel sheet and the hot-rolled steel sheet of the present embodiment, among the oxides, the number density of oxides having the above-mentioned chemical composition is specified.

In addition, from the viewpoint of promoting the growth of crystal grains, among the oxides having the above-mentioned chemical composition, those having a diameter of 1.0 to 5.0 μm are effective. Oxides with a diameter of less than 1.0 μm are not suitable because they inhibit the growth of crystal grains. In addition, when the amount of oxides having a diameter larger than 5.0 μm increases, the number density of oxides decreases. Accordingly, in the non-oriented electrical steel sheet and the hot-rolled steel sheet of the present embodiment, the number density of oxides having a diameter of 1.0 to 5.0 μm is specified.

A suitable oxide satisfying the above requirements has a number density N of 3.0×10 ³ to 10×10 ³ pieces/cm ² . When the number density N of suitable oxides is less than 3.0×10 ³ /cm ² , the number of MnS precipitation points is insufficient, and it is impossible to make MnS harmless. On the other hand, when the number density N of suitable oxides exceeds 10×10 ³ pieces/cm ² , it becomes difficult to uniformly disperse the oxides. In other words, if the number density of the appropriate oxides is excessive, the appropriate oxides will agglomerate, and the effect of finely dispersing MnS cannot be obtained. The number density N of suitable oxides is preferably 3.5×10 ³ pieces/cm ² or more, 4.0×10 ³ pieces/cm ² or more, or 5.0×10 ³ pieces/cm ² or more.

In addition, for oxides whose chemical composition and particle size do not satisfy the above requirements (eg, oxides with a diameter of less than 1.0 μm and oxides with a diameter of more than 5.0 μm), the number density should be reduced as much as possible. However, in non-oriented electrical steel sheets and hot-rolled steel sheets with the above chemical compositions, when the number density N of suitable oxides is controlled at 3.0×10 ³ to 10×10 ³ pieces/cm ² , the elements from which the oxides are supplied will be reduced. The appropriate oxide is formed and consumed. In this case, the generation of oxides that do not satisfy the above-mentioned requirements, such as oxides with a diameter of less than 1.0 μm and oxides with a diameter of more than 5.0 μm, is inevitably suppressed. Accordingly, the number density of oxides that do not satisfy the above requirements does not need to be specified.

Furthermore, in the non-oriented electrical steel sheet of the present embodiment, the appropriate oxides contain at least one element selected from the group consisting of La, Ce, Zr, Mg, and Ca in a total of 1.0% in mass % For the above oxides, the number density n satisfies the following formula (i): n/N≧0.01・・・(i).

La, Ce, Zr, Mg, and Ca (hereinafter also referred to as "La, etc.") generate not only oxides but also sulfides, but the inclusions generated by these elements will be coarse to a diameter of 5 μm or more. On the other hand, the oxide containing O and Si as the main components can exist in a relatively fine and dispersed state such as 1.0 to 5.0 μm in diameter as described above. In this case, La and the like are contained in an oxide mainly composed of O and Si, whereby these elements can be dispersed at a high density that cannot be achieved by oxides or sulfides of La and the like. Then, it is considered that La or the like captures S of the impurity element and forms sulfides on the oxides, thereby effectively and stably rendering S harmless.

In order to obtain the above-mentioned effects, the total concentration of La and the like in the appropriate oxide is 1.0 mass % or more. Moreover, the number density n of the suitable oxide containing La etc. (henceforth "Oxide containing La etc.") is 1% or more of the number density N of the suitable oxide. In other words, the value of n/N is 0.01 or more. The total concentration of La and the like in the appropriate oxide may be 5.0 mass % or more, 10.0 mass % or more, or 20.0 mass % or more.

In addition, the number density N of suitable oxides is measured by the following procedure. The oxides contained in the non-oriented electrical steel sheet or the hot-rolled steel sheet are observed with a scanning electron microscope (SEM). The observation magnification was set to 1000 times. The observation field area was 25 mm ² , and the number of observation points was 4 (in other words, the total observation field area was 100 mm ² ). Here, the chemical composition of each oxide is measured by an energy dispersive X-ray analyzer (EDS) attached to the SEM, and it is determined whether each oxide contains 20 to 60% of O and 20 to 60% in mass %. The Si person.

Then, the equivalent circle diameter of the cross-sectional area of the oxide is regarded as the diameter of the oxide, and an electron microscope photograph is taken with a transmission electron microscope (TEM). 1.0~5.0μm. From these results, oxides containing 20 to 60% by mass of O and 20 to 60% of Si and having a diameter of 1.0 to 5.0 μm were regarded as appropriate oxides, and appropriate oxides were identified in each electron microscope photograph. the location of the oxide. Then, the number density of suitable oxides is calculated by dividing the number of suitable oxides contained in all electron micrographs by the sum of the field areas of all electron micrographs. In addition, although agglomeration of a plurality of oxides may be observed, in this case, the equivalent circle diameters are measured individually, and those with diameters of 1.0 to 5.0 μm are judged as suitable oxides, and the total number of the oxides is counted.

In addition, the ratio (n/N) of the number density n of oxides containing La etc. with respect to the number density N of suitable oxides is obtained by the following procedure. Use the energy dispersive X-ray analyzer (EDS) attached to the TEM to measure the chemical composition of each appropriate oxide, and determine whether each appropriate oxide contains a compound selected from the group consisting of La, Ce, Zr, Mg and Ca. One or more elements in total of 1.0 mass % or more. If it is an appropriate oxide and contains at least 1.0 mass % in total of one or more elements selected from the group consisting of La, Ce, Zr, Mg, and Ca, it is regarded as an oxide containing La and the like, and each The position of oxides containing La etc. in the electron micrograph. Then, the ratio of oxides containing La or the like to the appropriate number of oxides (n/ N).

In addition, in order to uniformly disperse the appropriate oxides without agglomerating them, the average interval between the appropriate oxides is preferably 30 to 300 μm. By setting the average interval between suitable oxides to be 30 μm or more, the aggregation and distribution of suitable oxides can be suppressed, and the effect of making MnS harmless can be more reliably exhibited. In addition, in the appropriate oxide aggregated portion, the deterioration of the mechanical properties and the electromagnetic properties of the non-oriented electrical steel sheet can be suppressed. Accordingly, the average interval between suitable oxides is preferably 30 μm or more.

In the same way, if the average interval between suitable oxides is 300 μm or less, the suitable oxides are appropriately dispersed, and MnS precipitation points can be sufficiently ensured. Therefore, the average interval between suitable oxides is preferably 300 μm or less. The average interval between suitable oxides is more preferably 35 μm or more, 40 μm or more, or 50 μm or more. Moreover, the average interval between suitable oxides is more preferably 280 μm or less, 250 μm or less, or 220 μm or less.

The average interval of the appropriate oxides is obtained by the following procedure: through the above procedure, the particle size and position information of the appropriate oxides in each electron microscope photograph are identified, and based on this, the distance between the appropriate oxides is measured and the average value is calculated, thus obtained. In addition, when agglomeration of a plurality of oxides is observed, the appropriate oxides are close to each other, so the distance may be 0. However, in the present invention, in this case, the distance 0 is not used in the calculation of the average value. In other words, even if the number density of oxides is the same, when partial aggregation occurs, the average interval becomes larger.

3. Crystal particle size The crystal grain size of the non-oriented electrical steel sheet of the present embodiment is not particularly limited. As described above, since the non-oriented electrical steel sheet is used after machining and relaxation annealing, the crystal grain size varies depending on the stress relaxation annealing conditions. Considering the above-mentioned actual use conditions, as long as the grain growth during relaxation annealing is good, it is not necessary to specify the crystal grain size at the stage of the non-oriented electrical steel sheet. However, when the average crystal grain size is 30 μm or less, the punching workability is improved. Therefore, the average crystal grain size can also be specified to be 30 μm or less. Known techniques can be suitably used for the means for making the average crystal grain size 30 μm or less.

In general, non-oriented electrical steel sheets are supplied for machining and relaxation annealing after shipment. When the average crystal grain size after the relaxation annealing is 50 μm or more, the iron loss characteristics are greatly improved. Since the non-oriented electrical steel sheet of the present embodiment has appropriately controlled chemical composition and oxide state, the average grain size after relaxation annealing at 750° C. for 2 hours is 50 μm or more. In addition, in the case of an actual product, the relaxation annealing conditions are not limited to the above-mentioned conditions, and the annealing temperature and time may be appropriately changed in consideration of both equipment limitations and the promotion of crystal grain growth.

The average grain size of the non-oriented electrical steel sheet can be obtained by the following method. The L section (section parallel to the rolling direction) of the non-oriented electrical steel sheet was ground and etched, and observed with an optical microscope. The observation magnification was 100 times, the observation field area was 0.5 mm ² , and the number of observation points was 3. The average grain size of the non-oriented electrical steel sheet was determined by applying JIS G 0551:2013 "Steel - Microscopic test method for grain size" to these optical micrographs.

4. Manufacturing method The manufacturing method of the non-oriented electrical steel sheet of the present embodiment includes a refining step, a continuous casting step, a hot rolling step, a pickling step, a cold rolling step, and a finish annealing step. Of these, the refining step and the continuous casting step are particularly important for oxide control.

(a) Refining step In the refining step, molten steel is produced. In this step, the composition of the steel billet is prepared by adding alloying elements to the molten steel. After the predetermined alloy is added to the molten steel, the amount of oxides generated will gradually increase in the process until the molten steel solidifies through the continuous casting step described later. In addition, oxides that float and are caught in the slag are also generated. Therefore, in order to set the number density of suitable oxides to 3.0×10 ³ to 10×10 ³ pieces/cm ² , the oxygen content of the molten steel before alloying is adjusted to 0.010 to 0.050% by mass. When the amount of oxygen is insufficient, the number density of the generated oxides will be insufficient. On the other hand, when the amount of oxygen is excessive, the number density of oxides increases excessively and the oxides aggregate.

Next, Si is added to the molten steel. Here, the Si addition amount M1 (mass %) added to the molten steel and the Si content M2 (mass %) in the steel billet finally obtained in the subsequent continuous casting step are determined to satisfy the following formula (ii). Here, the Si addition amount M1 added to the molten steel is a value (%) obtained by dividing the total mass of Si added to the molten steel by the total mass of the molten steel. The Si content M2 of the steel billet is the Si content in the chemical composition of the steel billet, and is substantially equivalent to the Si content of the hot-rolled steel sheet and the non-oriented electrical steel sheet obtained from the steel billet. 0.90≦M2/M1≦1.10・・・(ii)

When the Si content M2 of the billet is too small relative to the Si addition amount M1 added to the molten steel, and is less than 0.90 times that of M1, Si is mostly captured in the form of SiO ₂ to the slag and discharged out of the molten steel, and excessive Si deoxidation is carried out. . Therefore, the oxide number density of the steel billet cannot be set within an appropriate range. On the other hand, when the Si content M2 of the steel billet is too large relative to the Si addition amount M1 added to the molten steel, and is more than 1.10 times that of M1, Si deoxidation will not be carried out, but the number of oxides will increase excessively and oxides will occur. Agglomeration, etc.

In addition, as described above, the Si content M2 in the molten steel is substantially equal to the value of the Si content of the finally obtained hot-rolled steel sheet and non-oriented electrical steel sheet. Accordingly, the Si content M2 in the molten steel is set to 0.10 to 1.50%.

In addition, in consideration of the time required for the coarse oxides to float from the molten steel, the time from the addition of the alloy to the start of the continuous casting step is preferably 30 minutes or more. Also, if the time from the addition of the alloy to the start of the continuous casting step is too long, the fine oxides will not remain in the molten steel. Therefore, from the viewpoint of ensuring this, the time from the addition of the alloy to the casting is The time should be set to 180 minutes or less. By adjusting the amount of O and the amount of Si added before the alloy is added within the time range, the appropriate average interval specified above can be obtained.

(b) Continuous casting step The molten steel produced in the refining step is continuously cast in the continuous casting step to produce a steel billet having the above-described chemical composition. This step is an important step for allowing a part of the appropriate oxide to contain La or the like. If La et al. are added by means of rare earth metal alloys (mischmetall), a violent reaction will occur between these elements and the molten steel, resulting in a significant reduction in the amount of oxygen in the steel and the mixing of impurity elements in the slag into the molten steel. In addition, even the generated oxide grows very coarse with a diameter larger than 5 μm.

Therefore, in the production method of the present embodiment, in the continuous casting step, a nozzle is used, and a part or all of the inner wall of the nozzle that comes into contact with the molten steel is made of the following material, which is % by mass In total, 3 to 60% of oxides containing La and the like are contained. La and the like are supplied to the steel through the melt loss that would come into contact with the inner wall of the nozzle of the molten steel. By this method, when the molten steel contains La, etc., it will not affect the oxygen and slag in the steel, and can achieve the compounding of the subsequent Si main oxide and these elements, and can effectively make the sulfide become. harmless.

(c) Hot rolling step In the hot rolling step, after heating the billet obtained in the continuous casting step, hot rolling is performed to prepare a hot rolled steel sheet. Through this step, a hot-rolled steel sheet according to an embodiment of the present invention can be produced. In addition, the steps following the hot rolling step do not substantially affect the chemical composition and oxide state. Therefore, as described above, the chemical composition and oxide state of the hot-rolled steel sheet are common to those of the non-oriented electrical steel sheet of the present embodiment.

By setting the heating temperature of the steel billet before hot rolling to less than 1150° C., the appropriate oxides can be uniformly dispersed, and the average interval between the appropriate oxides can be adjusted to an appropriate range. Therefore, the heating temperature of the steel billet should preferably be set to less than 1150°C. In addition, from the viewpoint of securing the rollability, the lower limit of the heating temperature of the billet before hot rolling is preferably 1050°C. More preferably, after maintaining the temperature range above 1050℃ and less than 1150℃ for 15~240 minutes, hot rolling is performed on the steel billet immediately.

In addition, the reduction ratio in the hot rolling step is not particularly limited, but is preferably 90% or more. In addition, the thickness of the obtained hot-rolled steel sheet is not particularly limited, but is preferably 1.0 to 4.0 mm, more preferably 2.0 to 3.0 mm.

(d) Pickling step In the pickling step, pickling is performed on the hot-rolled steel sheet obtained in the hot-rolling step. The pickling conditions are not particularly limited, and may be within the usual range in the production conditions of non-oriented electrical steel sheets.

(e) Cold rolling step In the cold rolling step, cold-rolling is performed on the pickled hot-rolled steel sheet to prepare a cold-rolled steel sheet. The cold rolling conditions are not particularly limited, and may be within the usual range in the production conditions of non-oriented electrical steel sheets. For example, the reduction ratio in the cold rolling step is preferably 50 to 95%, and more preferably 75 to 85%.

(f) Finish annealing step In the finish annealing step, finish annealing is performed on the cold-rolled steel sheet obtained in the cold rolling step. In the finish annealing step, when the maximum attained temperature (cold-rolled steel sheet temperature) is 850°C or higher, the crystal grain size becomes too large, which may cause defects when punching is performed before relaxation annealing. In order to avoid this, the maximum reaching temperature should preferably be set to less than 850°C. In addition, when the maximum attained temperature is less than 800° C., the recrystallization may be insufficient, and a defect may occur during punching. In order to avoid this, the maximum reaching temperature should preferably be set to 800°C or higher. In addition, in order to prevent the crystal grain size from becoming too large and the occurrence of defects when punching is performed before relaxation annealing, the time for the temperature of the cold-rolled steel sheet to reach 800°C or higher is preferably 15 seconds or less.

Although the thickness of the non-oriented electrical steel sheet produced through the above steps is not particularly limited, it is preferably 0.1 to 1.0 mm, more preferably 0.2 to 0.7 mm.

Hereinafter, the present invention will be described in more detail with examples, but the present invention is not limited to these examples.

Example A refining step and a continuous casting step are performed under various conditions to manufacture a steel billet, and a hot rolling step, a pickling step, a cold rolling step, and a finish annealing step are sequentially performed on the obtained steel billet, thereby producing a non-oriented electrical steel sheet. The chemical compositions of the non-oriented electrical steel sheets are shown in Table 1, and the manufacturing conditions thereof are shown in Table 2. In addition, about each steel plate, it manufactured 5 times under the same conditions. In addition, in all the steel sheets, the time for the temperature of the steel sheet to reach 800° C. or higher in the finish annealing step was 15 seconds or less.

Except for Test No. 24, the contents of La, Ce, Zr, Mg, and Ca in the steel were adjusted only by melting the nozzle material in the continuous step. On the other hand, as for Test No. 24, the composition was adjusted by adding alloying elements to the molten steel in the refining step.

[Table 1]

[Table 2]

The obtained non-oriented electrical steel sheet was measured by the following method: the number density N of suitable oxides, the ratio of the number density n of oxides containing La etc. to the number density N of suitable oxides (n /N), the average interval of suitable oxides, and the average crystal grain size. Then, the measured values were obtained from five steel sheets and averaged, and used as the respective measurement results.

The oxides contained in the non-oriented electrical steel sheet were observed by SEM at an observation magnification of 1000 times. The observation field area was 25 mm ² , and the number of observation points was 4 (in other words, the total observation field area was 100 mm ² ). Here, the chemical composition of each oxide is measured by EDS attached to the SEM, and it is determined whether each oxide contains 20 to 60% of O and 20 to 60% of Si in mass %.

Then, the equivalent circle diameter of the oxide cross-sectional area was regarded as the diameter of the oxide, and an electron microscope photograph was taken using a TEM, and it was determined whether the equivalent circle diameter of each oxide was 1.0 to 5.0 μm through image analysis of the photograph. From these results, oxides containing 20 to 60% by mass of O and 20 to 60% of Si and having a diameter of 1.0 to 5.0 μm were regarded as suitable oxides, and were identified in each electron microscope photograph. The location of the appropriate oxide. Then, the number density of suitable oxides is calculated by dividing the number of suitable oxides contained in all electron micrographs by the sum of the field areas of all electron micrographs.

In addition, the chemical composition of each appropriate oxide was measured using EDS attached to TEM, and it was determined whether each appropriate oxide contained one or more elements selected from the group consisting of La, Ce, Zr, Mg, and Ca in total 1.0 Mass % or more. If it is an appropriate oxide and contains 1.0 mass % or more of one or more elements selected from the group consisting of La, Ce, Zr, Mg, and Ca in total, it is regarded as an oxide containing La, etc., and each electron microscope photograph is identified. The position where oxides such as La are contained. Then, the ratio (n/N) of oxides containing La, etc. to the appropriate number of oxides is calculated by dividing the number of oxides containing La, etc. contained in all electron micrographs by the number of appropriate oxides .

The average interval of the appropriate oxides is obtained by the following procedure: through the above procedure, the particle size and position information of the appropriate oxides in each electron microscope photograph are identified, and based on this, the distance between the appropriate oxides is measured and the average value is calculated, thus obtained.

In addition, the cross section of the non-oriented electrical steel sheet L was ground and etched, and observed with an optical microscope. The observation magnification was 100 times, the observation field area was 0.5 mm ² , and the number of observation points was 3. The average grain size of the non-oriented electrical steel sheet was determined by applying JIS G 0551:2013 "Steel - Microscopic test method for grain size" to these optical micrographs.

Next, the obtained non-oriented electrical steel sheet was held at 750° C. for 2 hours to perform relaxation annealing. The following characteristic evaluations were performed on the non-oriented electrical steel sheet after relaxation annealing.

(A) Iron loss after relaxation annealing The iron loss (W15/50) of the steel sheet after relaxation annealing was measured in accordance with JIS C 2552:2014 "Non-oriented electrical steel strip". The W15/50 of the steel sheet after relaxation annealing was a non-oriented electrical steel sheet of 5.0 W/kg or less, which was judged to be excellent in iron loss characteristics after relaxation annealing.

(B) Magnetic flux density after relaxation annealing The magnetic flux density (B ₅₀ ) of the steel sheet after relaxation annealing was measured in accordance with JIS C 2552:2014 "Non-oriented electrical steel strip". The B ₅₀ of the steel sheet after relaxation annealing was a non-oriented electrical steel sheet of 1.70 T or more, and it was judged that the magnetic flux density after relaxation annealing was excellent.

(C) Grain growth in relaxation annealing The average grain size of the steel sheet after relaxation annealing was measured using the same measuring method as the average grain size of the above-mentioned non-oriented electrical steel sheet. The non-oriented electrical steel sheet having an average crystal grain size of 50 μm or more after relaxation annealing was judged to have good grain growth during relaxation annealing.

(D) Punching workability The punching workability was evaluated using a non-oriented electrical steel sheet before relaxation annealing and after finish annealing. Specifically, the steel plate is punched with a clearance of 7% or more and 12% or less of the thickness of the steel plate. The burr height of the punched part was measured. About the sample whose burr height was 30 micrometers or less, the punching workability was judged as "good" (symbol A). About the sample whose burr height was more than 30 micrometers and 100 micrometers or less, the punching workability was judged as "ok" (symbol B). About the sample whose burr height was more than 100 micrometers, the punching workability was judged as "defect" (symbol C).

The above evaluation results are shown in Table 3.

[table 3]

As shown in Table 3, it can be seen that Test Nos. 1 to 8, 22, 23, and 25 to 27, which meet the requirements of the present invention, stably exhibited excellent magnetic properties. On the other hand, the chemical composition did not satisfy Test Nos. 9 to 16 prescribed in the present invention, and as a result, at least one of iron loss and magnetic properties was deteriorated. In addition, since Test No. 17 did not contain any of La, Ce, Zr, Mg, and Ca, oxides containing La and the like were not formed. Therefore, although the average value of iron loss decreases, since MnS cannot be stabilized and rendered harmless, the maximum value of iron loss increases.

In Test Nos. 18 to 21, due to unsuitable manufacturing conditions, the number density of suitable oxides fell outside the specified range, resulting in deterioration of iron loss. In addition, in Test No. 24, the composition was adjusted by adding Mg and Ce to the molten steel in the refining step without effectively utilizing the melting of the nozzle material, so that the oxides containing La and the like were not sufficiently formed. As a result, the average value of iron loss decreased, but since MnS could not be stabilized and rendered harmless, the maximum value of iron loss increased.

Industrial availability According to the present invention, it is possible to stably provide a non-oriented electrical steel sheet having good grain growth during relaxation annealing and excellent magnetic properties at low cost. Accordingly, the present invention has extremely high industrial applicability.

(none)

Claims (10)

A non-oriented electrical steel sheet, the chemical composition of which contains in mass %: C: 0.0050% or less, Si: 0.10-1.50%, Mn: 0.10-1.50%, sol.Al: 0.0050% or less, N: 0.0030% or less, S: 0.0040% or less, O: 0.0050-0.0200%, and one or more elements selected from the group consisting of La, Ce, Zr, Mg, and Ca: 0.0005-0.0200% in total, the remainder: Fe and impurities; Oxide containing 20-60% O and 20-60% Si in mass % and having a diameter of 1.0-5.0 μm, the number density N of which is 3.0×10 ³ -10×10 ³ pieces/cm ² ; and Among the above-mentioned oxides, oxides containing one or more kinds of the above-mentioned elements selected from the group consisting of La, Ce, Zr, Mg, and Ca in a total of 1.0% or more in mass %, and the number density n of which satisfies the following (i ) formula: n/N≧0.01・・・(i). The non-oriented electrical steel sheet according to claim 1, wherein the chemical composition contains the following in place of a part of the Fe: Sn: 0.50% or less in mass %. The non-oriented electrical steel sheet according to claim 1 or claim 2, wherein the average interval between the oxides is 30 to 300 μm. The non-oriented electrical steel sheet according to any one of claim 1 to claim 3, the average grain size of which is 30 μm or less, and the average grain size after relaxation annealing is performed at 750° C. for 2 hours is 50 μm or more. A method for manufacturing a non-oriented electrical steel sheet, which is a method for manufacturing the non-oriented electrical steel sheet as claimed in any one of claim 1 to claim 4, comprising the following steps: Refining step: making molten steel; Continuous casting step: continuous casting of the aforementioned molten steel to produce a steel billet having the chemical composition of claim 1 or claim 2; Hot rolling step: heating the obtained steel billet and then performing hot rolling to make a hot rolled steel sheet; Pickling step: pickling the aforementioned hot-rolled steel sheet; Cold-rolling step: cold-rolling the pickled hot-rolled steel sheet to form a cold-rolled steel sheet; and Finish annealing step: perform finish annealing on the aforementioned cold-rolled steel sheet; In the aforementioned refining step, The oxygen content of the molten steel before adding the alloy is set to 0.010 to 0.050% in mass %, Next, the Si addition amount M1 added to the molten steel and the Si content M2 in the steel billet are adjusted to satisfy the following formula (ii); In the aforementioned continuous casting step, the following nozzle is used, and a part or all of the inner wall of the nozzle that contacts the aforementioned molten steel is composed of the following material, which is contained in mass %: a total of 3 to 60% including optional One or more oxides selected from the group consisting of La, Ce, Zr, Mg and Ca; 0.90≦M2/M1≦1.10・・・(ii). The method for producing a non-oriented electrical steel sheet according to claim 5, wherein, in the refining step, the time from the addition of the alloy to the start of the continuous casting step is set within a range of 30 to 180 minutes; In the above-mentioned hot rolling step, after maintaining the temperature of the steel billet in the range of 1050° C. or higher and less than 1150° C. for 15 to 240 minutes, the steel billet is then subjected to hot rolling. The method for producing a non-oriented electrical steel sheet according to claim 5 or claim 6, wherein in the finish annealing step, the temperature of the cold-rolled steel sheet is set to 800°C or more and less than 850°C. A hot-rolled steel sheet that can be used as a material for the non-oriented electrical steel sheet according to any one of claim 1 to claim 4, wherein the chemical composition of the hot-rolled steel sheet in mass % contains: C: 0.0050% or less, Si: 0.10 ~1.50%, Mn: 0.10~1.50%, sol.Al: 0.0050% or less, N: 0.0030% or less, S: 0.0040% or less, and O: 0.0050~0.0200%, and contains La, Ce, Zr, Mg One or more elements of the group formed by and Ca: 0.0005~0.0200% in total, the remainder: Fe and impurities; 20~60% O and 20~60% Si in mass %, and the diameter is 1.0~5.0μm The oxides whose number density N is 3.0×10 ³ to 10×10 ³ pieces/cm ² ; and the aforementioned oxides include one or more selected from the group consisting of La, Ce, Zr, Mg and Ca The above-mentioned elements are oxides with a total of 1.0% or more in mass %, and their number density n satisfies the following formula (i): n/N≧0.01・・・(i). The hot-rolled steel sheet according to claim 8, wherein the chemical composition contains the following in place of a part of the Fe: Sn: 0.50% or less in mass %. The hot-rolled steel sheet according to claim 8 or claim 9, wherein the average interval between the oxides is 30 to 300 μm.