TWI751812B - Non-oriented electromagnetic steel sheet, manufacturing method thereof, and hot-rolled steel sheet - Google Patents

Abstract

A non-oriented electrical steel sheet, the chemical composition of which is in mass %: C: 0.0010-0.0050%, Si: 1.50% or less, Mn: 0.10-1.50%, sol.Al: 0.010-0.040%, Ti: 0.0030% or less, Nb: 0.0030% or less, V: 0.0030% or less, Zr: 0.0030% or less, N: 0.0030% or less, S: 0.0040% or less, B: 0.0045% or less, the remainder: Fe and impurities; and satisfy [0.0020≦Ti+ Nb+V+Zr≦0.0120], [0.5≦B/N≦1.5], [sol.B≦0.0005] and [N _AlN ≦0.0005].

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 cost constraints are strict, so in fact, it is difficult to replace their materials with high-quality materials such as high-efficiency models.

The steel sheet required for general-purpose models is a material that has a Si content of 1.5% or less, and that promotes crystal grain growth during relaxation annealing performed after punching of the motor core, thereby significantly improving iron loss.

In addition, recently, consumers have gradually made effective use of scraps generated when iron cores are punched as raw materials for castings. From the viewpoint of securing scrap castability, it becomes necessary to make the Al content of the steel sheet less than 0.05%.

As means for improving the growth of crystal grains during relaxation annealing, for example, Patent Document 1 discloses a method for producing an electric iron sheet having excellent magnetic properties, wherein a hot-rolled sheet is obtained by hot-rolling a steel billet. C: ≦ 0.065%, Si: ≦ 2.0%, Al: ≦ 0.10%, O: ≦ 0.020%, B/N: 0.50~2.50, and the remainder is composed of Fe and inevitable impurities; Cold rolling or cold rolling including intermediate annealing is performed twice or more to make the above-mentioned hot-rolled sheet into final dimensions, 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 It contains Al: 0.1% or less or Zr: 0.05% or less, and the rest is composed of iron and unavoidable impurity elements. In steel, oxides with a diameter of 0.5 μm or more and 5 μm or less in size 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, and 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 precipitates are combined with B precipitates; the _{total distribution density of MnS, Cu 2} S and their composite sulfides is 3.0×10 ⁵ /mm ² or less; the Ti precipitates with a diameter of less than 0.1 μm are 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 reduction in iron loss is required, the above-mentioned conventional method becomes difficult to manufacture sufficiently and stably. In particular, as described in Patent Document 1 and Patent Document 5, B: about 0.002% is added to Al-deoxidized steel to generate BN as a nitride to suppress the precipitation of AlN, which is harmful to crystal grain growth, and even low iron can be obtained in this technology. Compared with the Si deoxidation described in Patent Document 2 and Patent Document 4, there is still a problem that the magnetic flux density becomes lower. Patent Document 5 discloses the effect of increasing the magnetic flux density by adding Sn, but this technique has a problem of increasing cost. In addition, it is considered to promote the coarsening of crystal grains by adding special steps such as skin pass rolling, but in this case, there is still a problem that the manufacturing cost is greatly increased.

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 suppress the generation of AlN and reduce the amount of solid solution B (sol.B). The crystal grains grow well, and the iron loss and magnetic flux density are good. Furthermore, there is 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 is in mass %: C: 0.0010-0.0050%, Si: 1.50% or less, Mn: 0.10-1.50%, sol.Al: 0.010-0.040%, Ti: 0.0030 % or less, Nb: 0.0030% or less, V: 0.0030% or less, Zr: 0.0030% or less, N: 0.0030% or less, S: 0.0040% or less, B: 0.0045% or less, the remainder: Fe and impurities; The electromagnetic steel sheet meets the following formulas (i)~(iv): 0.0020≦Ti+Nb+V+Zr≦0.0120・・・(i) 0.5≦B/N≦1.5・・・(ii) sol.B≦0.0005・・・(iii) N _AlN ≦0.0005・・・(iv) Wherein, the element symbols in the above formulas (i) and (ii) represent the content (mass %) of each element, and the sol. B is the amount (mass %) of solid solution B, and N _AlN in the above formula (iv) is the amount (mass %) of N existing in the form of AlN.

(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 of the above (1) or (2), which has an average grain size of 30 μm or less, and, The average crystal grain size after relaxation annealing was carried out under the condition of holding at 750° C. for 2 hours was 50 μm or more.

(4) 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 (3), comprising the following steps: Steel-making step: manufacturing a steel billet having the chemical composition of (1) or (2) above; 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 hot rolling step, Before performing hot rolling: keep the above-mentioned steel billet temperature in the range of 1000~1050℃ for more than 30 minutes, and set the cumulative reduction ratio in the temperature range of 900~1000℃ to more than 70%; After hot rolling: the temperature of the hot-rolled steel sheet is kept within a range of 700° C. or higher and less than 780° C. for 30 minutes or longer.

(5) The method for producing a non-oriented electrical steel sheet according to (4) above, wherein, in the finish annealing step, heating is performed at an average heating rate of 20°C/s or more to a maximum attainable temperature, and the maximum attainable temperature is 800°C or higher and less than 850°C, and the time until the temperature of the cold-rolled steel sheet reaches 800°C or higher is set to 15 seconds or less.

(6) A hot-rolled steel sheet that can be used as a material for the non-oriented electrical steel sheet of any one of the above (1) to (3), wherein the chemical composition of the hot-rolled steel sheet is in mass %: C: 0.0010 to 0.0050%, Si: 1.50% or less, Mn: 0.10-1.50%, sol.Al: 0.010-0.040%, Ti: 0.0030% or less, Nb: 0.0030% or less, V: 0.0030% or less, Zr: 0.0030% or less, N: 0.0030% Below, S: 0.0040% or less, B: 0.0045% or less, the remainder: Fe and impurities; and the hot-rolled steel sheet satisfies the following equations (i) to (iv): 0.0020≦Ti+Nb+V+Zr≦0.0120・・・(i) 0.5≦B/N≦1.5・・・(ii) sol.B≦0.0005・・・(iii) N _AlN ≦0.0005・・・(iv) Among them, the above equations (i) and (ii) The element symbol in the above indicates the content (mass %) of each element, sol.B in the above formula (iii) is the amount of solid solution B (mass %), and N _AlN in the above formula (iv) is in the form of AlN. The amount of N present (mass %).

(7) The hot-rolled steel sheet according to (6) above, wherein the chemical composition contains the following in place of a part of the Fe: Sn: 0.50% or less in mass %.

Invention effect According to the present invention, a non-oriented electrical steel sheet with good crystal grain growth after relaxation annealing and good iron loss and magnetic flux density after relaxation annealing can be stably provided at low cost.

Embodiments of the present invention Form for carrying out the invention The inventors of the present application investigated the reason why the magnetic flux density of Al-deoxidized steel containing about 0.002% of B is lower than that of Si-deoxidized steel, focusing on the existence form of B. B has the effect of suppressing the precipitation of AlN which is extremely harmful to the growth of crystal grains. This is because B is more likely to generate nitrides than Al. However, the inventors of the present application discovered that when the non-oriented electrical steel sheet contains an excessive amount of B, B exists in a solid solution state and reduces the magnetic flux density (see FIG. 1 ).

If the B content is reduced, the amount of solid solution B will not increase due to excess B, but AlN will be generated due to excess N, and the growth of crystal grains will be deteriorated. Accordingly, it is desirable to increase the B content as much as possible within the range where solid solution B does not occur, but it is industrially difficult to control the B content with such high precision. This is because in addition to Al and B, the elements that generate nitrides include Ti, Nb, V, Zr, etc. The existence of these elements must also be taken into account when controlling the B content, but because these are usually impurities It is difficult to accurately grasp the amount of non-oriented electrical steel sheets. In addition, the N content of the non-oriented electrical steel sheet may fluctuate during the steelmaking process, so it is difficult to accurately grasp the amount of B to be added. In this way, it is extremely difficult industrially to contain B in a non-oriented electrical steel sheet in order to suppress the formation of AlN.

In view of such a situation, the inventors of the present application have intensively studied a method for obtaining favorable magnetic properties which is acceptable when the amount of B is excessive or insufficient relative to the amount of N, and as a result, obtained the following findings.

First, when the amount of B is excessive with respect to the amount of N, a method of generating B precipitates other than BN during hot rolling is examined. As a result, it was found that the amount of B precipitates other than BN can be increased by setting the temperature and cumulative reduction ratio during hot rolling within appropriate ranges. Specifically, it is desirable to perform hot rolling under the condition that the cumulative reduction ratio is 70% or more in the temperature range of 900 to 1000°C. This effect is presumed to be due to the promotion of B precipitation by hot rolling.

Thereafter, the hot-rolled steel sheet is heat-treated at a temperature range of 700°C or higher and lower than 780°C for 30 minutes or more, whereby B precipitates other than the above-mentioned BN generated by processing can be grown. As a result, in finish annealing of the steel sheet after cold rolling or relaxation annealing of the steel sheet (non-oriented electrical steel sheet) after finish annealing, the crystal grain growth becomes favorable, and low iron loss and high magnetic flux density can be realized.

Next, a method for preventing the formation of AlN, which is harmful to the growth of crystal grains, is examined when the amount of B is insufficient relative to the amount of N. As a result, it was found that heat treatment is performed if the steel billet before hot rolling is kept in the temperature range of 1000 to 1050°C for 30 minutes or more, and the steel sheet after hot rolling is kept in the temperature range of 700°C or more and less than 780°C for 30 minutes. By performing the above heat treatment, the formation of AlN can be avoided, and in the finish annealing of the steel sheet after cold rolling, or the relaxation annealing of the non-oriented electrical steel sheet after finish annealing, the growth of crystal grains becomes good, and low iron can be obtained. loss and high magnetic flux density.

Based on the above findings, it was found that in Al-deoxidized steel to which about 0.002% of B was added, even if no special steps were added to promote crystal grain coarsening, the conditions of low iron loss and high magnetic flux density could be achieved, and the present invention was completed. .

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: 0.0010~0.0050% C has the effect of fixing the solid solution B in the form of carbide. However, when C is contained in an amount greater than 0.0050%, the iron loss will be deteriorated due to magnetic aging. Therefore, the C content is set to 0.0010 to 0.0050%. The C content is preferably 0.0015% or more, 0.0020% or more, or 0.0025% or more. In addition, the C content is preferably 0.0045% or less, 0.0040% or less, or 0.0035% or less.

Si: 1.50% or less Si is an element that can effectively increase resistance. However, when the Si content is greater than 1.50%, the hardness will increase, the magnetic flux density will decrease, and the manufacturing cost will increase. Therefore, the Si content is made 1.50% or less. The Si content is preferably 1.30% or less, 1.00% or less, or 0.80% or less. Although the lower limit of the Si content is 0%, in order to obtain the above-mentioned effects, the Si content is preferably 0.10% or more, 0.20% or more, or 0.50% or more.

Mn: 0.10~1.50% Mn is a sulfide-generating element, and is preferably contained in an appropriate amount from the viewpoint of promoting the growth of crystal grains. However, when the Mn content is more than 1.50%, the transformation temperature decreases, and it becomes difficult to control the structure of the hot-rolled steel sheet, and it becomes impossible to promote the growth of crystal grains, and the iron loss deteriorates. In addition, the saturation magnetic flux density drops significantly. Therefore, the Mn content is set to 0.10 to 1.50%. The Mn content is preferably 0.30% or more, 0.50% or more, or 0.70% or more. Further, the Mn content is preferably 1.20% or less, 1.00% or less, or 0.80% or less.

sol.Al: 0.010~0.040% Al is an element necessary for steel deoxidation. When the content of sol.Al (Al existing in a solid solution state) is less than 0.010%, a stable deoxidation effect cannot be obtained, and problems such as nozzle clogging occur. On the other hand, from the viewpoint of effective utilization of waste materials by consumers, the less Al content, the better. Therefore, the sol.Al content is set to 0.010 to 0.040%. The sol.Al content is preferably 0.015% or more, 0.020% or more, or 0.025% or more. In addition, the sol.Al content is preferably 0.035% or less, 0.030% or less, or 0.028% or less.

Ti: 0.0030% or less Ti generates nitrides and significantly deteriorates grain growth. However, since Ti is an element mixed into steel in the form of impurity, it is industrially difficult to achieve zero Ti content. In addition, a very small amount of Ti has the effect of suppressing the formation of AlN. On the other hand, when its content is excessive, crystal grain growth is deteriorated. Therefore, the Ti content is made 0.0030% or less. The Ti content is preferably 0.0025% or less, 0.0020% or less, or 0.0015% or less. On the other hand, in order to obtain the above effects, the Ti content is preferably 0.0005% or more, 0.0008% or more, 0.0010% or more, or 0.0012% or more.

Nb: 0.0030% or less Nb forms nitrides and significantly deteriorates grain growth. However, since Nb is an element mixed into steel in the form of an impurity, it is industrially difficult to achieve zero Nb content. In addition, a very small amount of Nb has the effect of inhibiting the formation of AlN. On the other hand, when its content is excessive, crystal grain growth is deteriorated. Therefore, the Nb content is made 0.0030% or less. The Nb content is preferably below 0.0025%, below 0.0020% or below 0.0015%. On the other hand, in order to obtain the above-mentioned effects, the Nb content is preferably 0.0005% or more, 0.0008% or more, 0.0010% or more, or 0.0012% or more.

V: 0.0030% or less V generates nitrides and significantly deteriorates grain growth. However, since V is an element mixed into steel in the form of impurities, it is industrially difficult to achieve zero V content. In addition, a very small amount of V has the effect of inhibiting the formation of AlN. On the other hand, when its content is excessive, crystal grain growth is deteriorated. Therefore, the V content is made 0.0030% or less. The V content is preferably 0.0025% or less, 0.0020% or less, or 0.0015% or less. On the other hand, in order to obtain the above-mentioned effects, the V content is preferably 0.0005% or more, 0.0008% or more, 0.0010% or more, or 0.0012% or more.

Zr: 0.0030% or less Zr generates nitrides and significantly deteriorates grain growth. However, since Zr is an element mixed into steel in the form of impurities, it is industrially difficult to achieve zero Zr content. In addition, a very small amount of Zr has the effect of inhibiting the formation of AlN. On the other hand, when its content is excessive, crystal grain growth is deteriorated. Therefore, the Zr content is made 0.0030% or less. The Zr content is preferably 0.0025% or less, 0.0020% or less, or 0.0015% or less. On the other hand, in order to obtain the above-mentioned effects, the Zr content is preferably 0.0005% or more, 0.0008% or more, 0.0010% or more, or 0.0012% or more.

Any one of Ti, Nb, V and Zr may be contained alone, or two or more of them may be contained in combination. However, when the total content of these elements is too small, the effect of inhibiting the formation of AlN cannot be obtained, and on the other hand, when the content is too large, the growth of crystal grains is deteriorated. Therefore, the total content of these elements must satisfy the following formula (i). 0.0020≦Ti+Nb+V+Zr≦0.0120・・・(i) Here, the element symbol in the above formula (i) represents the content (mass %) of each element.

N: 0.0030% or less N forms nitrides and inhibits the growth of crystal grains. In terms of conditions that do not worsen the growth of crystal grains, the upper limit of the N content is set to 0.0030%. The N content is preferably 0.0025% or less, 0.0020% or less, or 0.0015% or less. In addition, the N content should be reduced as much as possible, but since N is an element mixed into steel in the form of impurities, it is industrially difficult to achieve zero N content. In the present invention, in addition to the premise that N is contained to some extent, the lower limit of the N content is defined by the relational expression of the B content described later. On the other hand, the lower limit value of the N content may be separately defined. The N content may be, for example, 0.0008% or more, 0.0010% or more, or 0.0012% or more. _{In addition, the N content means the N content of all forms including N AlN} , which will be described later, and N constituting BN, and the like.

S: 0.0040% or less S forms sulfides that significantly deteriorate grain growth. 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 is preferably 0.0035% or less, 0.0030% or less, or 0.0025% or less. The lower limit of the S content is 0%, but the S content may be set to 0.0008% or more, 0.0010% or more, or 0.0012% or more in consideration of refining costs.

B: 0.0045% or less B is an element necessary for suppressing the formation of AlN which is harmful to the growth of crystal grains. Therefore, the B content is in the range of 0.0045% or less and is defined according to the above-mentioned N content. Specifically, the B content is controlled so as to satisfy the following formula (ii). In addition, this B content means the B content of all forms including solid solution B (sol.B) and B which form a precipitate, such as BN. Setting the value of B/N to 0.5 to 1.5 is one of the important means to reduce the amount of solid solution B and suppress the generation of AlN. In addition, the value of B/N is preferably 0.6 or more, 0.7 or more, or 0.8 or more. Moreover, B/N is preferably 1.4 or less, 1.3 or less, or 1.0 or less. 0.5≦B/N≦1.5・・・(ii) Here, the element symbol in the above formula (ii) represents the content (mass %) of each element.

Moreover, in this invention, in addition to B content, sol.B content is also prescribed|regulated. The content of sol.B is set below 0.0005%, which is the upper limit that does not affect the magnetic flux density. In other words, the sol.B content must satisfy the following formula (iii). sol.B≦0.0005・・・(iii)

The content of sol.B should preferably be below 0.0004% or below 0.0003%. In addition, the content of sol.B should be reduced as much as possible, so the lower limit is 0%. On the other hand, the content of sol.B may be defined as 0.00005% or more, 0.00010% or more, or 0.00015% or more.

In the present invention, the sol.B content is determined by the following procedure. First, a test piece was cut out from a non-oriented electrical steel sheet or a hot-rolled steel sheet, and about 0.4 g was electrolyzed ^{with 10% acetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm 2 .} The solution used for the electrolysis was filtered through a filter with a pore size of 0.2 μm, and the B content in the extraction residue was measured by ICP emission spectrometry with respect to the extraction residue collected on the filter paper. Then, the value obtained by subtracting the B content in the extraction residue from the B content in the steel is determined as the sol.B content.

Sn: 0.50% or less In the present invention, Sn is not essential, so the lower limit of the content is 0%. From the viewpoint of reducing the alloy cost, the Sn content should be reduced as much as possible. However, Sn has the effect of increasing the magnetic flux density. In addition, Sn also has the effect of suppressing nitridation and oxidation of the steel sheet surface during annealing. In addition, when sol.Al: 0.010 to 0.040% is contained, Sn is particularly easy to nitride. Therefore, Sn may be contained as needed. Specifically, the Sn content is preferably 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, even if the Sn content is too high, the effect is saturated, so the Sn content may be set to 0.40% or less, 0.30% or less, 0.20% or less, 0.10% or less, 0.09% or less, or 0.08% or less.

2. The content of N (hereinafter referred to as “N _AlN ”) constituting AlN in the precipitate is set to 0.0005% or less as the upper limit which does not affect the growth of crystal grains. In other words, the N _AlN content must satisfy the following formula (iv). N _AlN ≦0.0005・・・(iv)

The N _AlN content is preferably 0.0004% or less or 0.0003% or less. In addition, the _{content of N AlN} should be reduced as much as possible, so the lower limit of the content is 0%. On the other hand, the _{content of N AlN} may be defined as 0.00005% or more, 0.00010% or more, or 0.00015% or more.

In the present invention, the N _AlN content is determined by the following procedure. First, a test piece was cut out from a non-oriented electrical steel sheet or a hot-rolled steel sheet, and about 0.4 g was electrolyzed ^{with 10% acetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm 2 .} The solution used for this electrolysis was filtered through filter paper with a pore size of 0.2 μm, and the extraction residue collected on the filter paper was subjected to ICP emission spectrometry to measure the Al content in the extraction residue. Then, since all the Al in the extraction residue was considered to be in the form of AlN, the N content in the extraction residue was calculated by multiplying the Al content in the extraction residue by 14/27, and it was determined as the N _AlN content.

In addition, as described above, in the present invention, the state of the precipitate is very important, but the state of the precipitate does not need to be particularly defined. This is because it is technically difficult to define the state of the precipitate because the precipitate is very fine. It was also confirmed that the amount of N _AlN constituting the precipitates and the like are within the above range, whereby the precipitates can be well controlled and the magnetic properties of the non-oriented electrical steel sheet can be improved.

3. Crystal particle size The average 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 average grain size varies depending on the 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 average grain size in the stage of the non-oriented electrical steel sheet. On the other hand, the average crystal grain size is an important factor from the viewpoint of improving punching workability. When the average grain size of a non-oriented electrical steel sheet to be used for punching is 30 μm or less, the punching workability is improved. Therefore, the average grain size of the non-oriented electrical steel sheet after finish annealing may be 30 μm or less. As means for making the average crystal grain size 30 μm or less, known techniques can be suitably employed.

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 obtained by applying JIS G 0551:2013 "Steel - Microscopic test method for grain size" to these optical micrographs.

4. Manufacturing method The method for producing a non-oriented electrical steel sheet according to the present embodiment includes a steel making step, a hot rolling step, a pickling step, a cold rolling step, and a finish annealing step.

(a) Steel making step In the steel-making step, a steel billet having the above-mentioned chemical composition is produced by performing suitable refining and casting. The manufacturing conditions in the steel-making step are not particularly limited, and known conditions can be appropriately employed.

(b) Hot rolling step In the hot rolling step, the billet obtained in the continuous casting step is heated and then hot rolled 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 precipitate state of the hot-rolled steel sheet are common to those of the non-oriented electrical steel sheet of the present embodiment.

The hot rolling step is an important step for controlling precipitates and securing magnetic properties. In the hot rolling step, the temperature of the steel billet is kept in the range of 1000 to 1050° C. for more than 30 minutes before the hot rolling is performed. Next, hot rolling is performed so that the cumulative reduction ratio in the temperature range of 900 to 1000° C. becomes 70% or more. Then, after hot-rolling, the temperature of the hot-rolled steel sheet is kept in the range of 700° C. or higher and less than 780° C. for 30 minutes or longer.

As described above, it is technically extremely difficult to judge whether the B content is excessive or insufficient relative to the N content in the steel-making step. However, by satisfying the above-mentioned manufacturing conditions in the hot rolling step, it is possible to cope with the excess or deficiency of the B content relative to the N content. The details of each case are as follows.

First, when the B content is excessive relative to the N content, the amount of AlN can be suppressed by immobilizing N with B, so that the N _AlN content can be made 0.0005% or less. On the other hand, the sol.B content may be greater than 0.0005% due to the presence of remaining B. Therefore, it is necessary to generate B precipitates other than BN to suppress the solid solution B content. Although B will generate carbides, the precipitation temperature of B carbides is relatively low. Therefore, the cumulative reduction ratio in the temperature range of 900 to 1000° C. is set to 70% or more, whereby the precipitation of B carbides can be promoted.

In addition, although hot rolling under the above conditions promotes the precipitation of B carbides, a part of B may remain in a solid solution state and remain in the steel sheet after hot rolling. However, after the hot rolling, the temperature of the hot-rolled steel sheet is kept in the range of 700° C. or higher and less than 780° C. for 30 minutes or more, whereby the solid solution B can be precipitated. This is because the B carbides do not precipitate even when the holding temperature is lower than 700°C, and melt again at 780°C or higher.

Next, when the B content is insufficient relative to the N content, the sol.B can be made 0.0005% or less, but it is necessary to suppress the generation of AlN. In the present invention, by generating nitrides of Ti, Nb, V, and Zr, the generation of AlN, which is detrimental to crystal grain growth, is suppressed. Since these nitrides are relatively fine, they must be sufficiently grown in this step. Therefore, before the hot rolling is performed, the temperature of the steel billet is kept in the range of 1000~1050°C for more than 30 minutes, and after the hot rolling is performed, the temperature of the hot-rolled steel sheet is kept in the range of 700°C or more and less than 780°C. 30 points or more. Thereby, N is fixed by Ti, Nb, V, and Zr, and the amount of AlN is suppressed, and the _{content of N AlN} can be made 0.0005% or less.

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.

(c) 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.

(d) 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 ranges 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%.

(e) 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, conditions are not particularly limited, and known conditions can be appropriately used. However, it is suitable because the magnetic flux density can be increased by increasing the heating rate of the cold-rolled steel sheet. Accordingly, the heating rate in the finish annealing step is preferably set to 20° C./s or more. The so-called heating rate here is the value obtained by dividing the difference between the heating start temperature and the soaking temperature of the cold-rolled steel sheet by the time from the heating start temperature to the soaking temperature, that is, from the heating start temperature to the soaking temperature. The average heating rate up to the hot temperature.

In addition, 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. On the other hand, when the maximum attained temperature is less than 800° C., recrystallization may be insufficient, and failure 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 The steel making step, the hot rolling step, the pickling step, the cold rolling step and the finish annealing step are sequentially performed, 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.

[Table 1]

[Table 2]

With respect to the obtained non-oriented electrical steel sheet, the contents _{of sol.B and N AlN were measured by the following methods.}

First, a test piece was cut out from a non-oriented electrical steel sheet, and about 0.4 g was electrolyzed ^{with 10% acetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm 2 .} The solution used for this electrolysis was filtered through filter paper with a pore size of 0.2 μm, and the B content in the extraction residue was measured by ICP emission spectrometry with respect to the extraction residue collected on the filter paper. Then, the value obtained by subtracting the B content in the extraction residue from the B content in the steel is determined as the sol.B content.

Similarly, a test piece was cut out from a non-oriented electrical steel sheet, and about 0.4 g was electrolyzed ^{with 10% acetone-1% tetramethylammonium chloride/methanol at a current density of 20 mA/cm 2 .} The solution used for this electrolysis was filtered through filter paper with a pore size of 0.2 μm, and the extraction residue collected on the filter paper was subjected to ICP emission spectrometry to measure the Al content in the extraction residue. Then, the N content in the extraction residue was obtained by multiplying the Al content in the extraction residue by 14/27, and it was determined as the N _AlN content.

For the contents of sol.B and N _AlN , the measured values were obtained from five steel sheets, and averaged, and used as the respective measurement results.

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).

The above evaluation results are shown in Table 3. In addition, the characteristic evaluation was implemented using 5 sheets of steel plates. In addition, in Table 3, about the iron loss, the average value and the maximum value are shown, and about the magnetic flux density, the average value and the minimum value are shown.

[table 3]

As shown in Table 3, it was found that Test Nos. 1 to 8, 29, and 30, which satisfy 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 18 specified in the present invention, and as a result, at least one of iron loss and magnetic properties was deteriorated.

In addition, in Test Nos. 19 to 24, steels with insufficient B content relative to N content were used, and the manufacturing conditions were not suitable, so the N _AlN content exceeded the upper limit, resulting in deterioration of iron loss. In addition, in Test Nos. 25 to 28, since the steel with an excessive B content relative to the N content was used, the manufacturing conditions were unsuitable, so the solid solution B content was larger than the upper limit, resulting in the deterioration of the magnetic flux density.

Industrial availability According to the present invention, a non-oriented electrical steel sheet with good crystal grain growth after relaxation annealing and good iron loss and magnetic flux density after relaxation annealing can be stably provided at low cost. Therefore, the present invention has extremely high industrial applicability.

FIG. 1 is a graph showing the relationship between the residual B amount and the magnetic flux density.

(none)

Claims (7)

A non-oriented electrical steel sheet, the chemical composition of which is in mass %: C: 0.0010~0.0050%, Si: 1.50% or less, Mn: 0.10~1.50%, sol.Al: 0.010~0.040%, Ti: 0.0030% or less, Nb: 0.0030% or less, V: 0.0030% or less, Zr: 0.0030% or less, N: 0.0030% or less, S: 0.0040% or less, B: 0.0045% or less, the remainder: Fe and impurities; and the non-oriented electrical steel sheet The following equations (i)~(iv) are satisfied: 0.0020≦Ti+Nb+V+Zr≦0.0120・・・(i) 0.5≦B/N≦1.5・・・(ii) sol.B≦0.0005・・・(iii) N _AlN ≦0.0005・・・(iv) Wherein, the element symbols in the above formulae (i) and (ii) represent the content (mass %) of each element, and sol.B in the above formula (iii) is a solid The amount of dissolved B (mass %), N _AlN in the above formula (iv) is the amount of N (mass %) that exists in the form of AlN. 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 of claim 1 or 2, the average grain size of which is 30 μm or less, and, The average crystal grain size after relaxation annealing was carried out under the condition of holding at 750° C. for 2 hours was 50 μm or more. A method for manufacturing a non-oriented electrical steel sheet is a method for manufacturing the non-oriented electrical steel sheet as claimed in any one of claim 1 to claim 3, comprising the following steps: Steel-making step: manufacturing steel billets with 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 hot rolling step, Before performing hot rolling: keep the above-mentioned steel billet temperature in the range of 1000~1050℃ for more than 30 minutes, and set the cumulative reduction ratio in the temperature range of 900~1000℃ to more than 70%; After hot rolling: the temperature of the hot-rolled steel sheet is kept within a range of 700° C. or higher and less than 780° C. for 30 minutes or longer. The method for producing a non-oriented electrical steel sheet according to claim 4, wherein in the aforementioned finishing annealing step, the heating is performed at an average heating rate of 20°C/s or more to a maximum reaching temperature, and the maximum reaching temperature is 800°C or more and less than 850°C °C, and the time until the temperature of the cold-rolled steel sheet reaches 800°C or more is set to 15 seconds or less. A hot-rolled steel sheet, which can be used as a material for the non-oriented electrical steel sheet according to any one of claim 1 to claim 3, the chemical composition of the hot-rolled steel sheet in mass %: C: 0.0010~0.0050%, Si: 1.50 % or less, Mn: 0.10 to 1.50%, sol.Al: 0.010 to 0.040%, Ti: 0.0030% or less, Nb: 0.0030% or less, V: 0.0030% or less, Zr: 0.0030% or less, N: 0.0030% or less, S : 0.0040% or less, B: 0.0045% or less, the remainder: Fe and impurities; and the hot-rolled steel sheet satisfies the following formulas (i) to (iv): 0.0020≦Ti+Nb+V+Zr≦0.0120・・・( i) 0.5≦B/N≦1.5・・・(ii) sol.B≦0.0005・・・(iii) N _AlN ≦0.0005・・・(iv) Among them, the elements in the above equations (i) and (ii) The symbol indicates the content (mass %) of each element, sol.B in the above formula (iii) is the amount of solid solution B (mass %), and N _AlN in the above formula (iv) is N that exists in the form of AlN Amount (mass %). The hot-rolled steel sheet according to claim 6, wherein the chemical composition contains the following in place of a part of the Fe: Sn: 0.50% or less in mass %.

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