BR112013020657B1 - ORIENTED ELECTRIC STEEL SHEET AND METHOD FOR PRODUCTION - Google Patents

Abstract

Patent specification: "Non-oriented electric steel sheet and method for producing it". The present invention relates to a low cost non-oriented electric steel sheet which has excellent magnetic properties and mechanical properties as well as excellent steel sheet quality. The non-oriented electric steel plate has a chemical composition containing by weight% Si: 5.0? or less, mn: 2.0? or less, al: 2.0? or less, and p: 0.05? or less, in a range that satisfies formula (1) and, furthermore, c: 0.008? or more and 0.040? or less; n: 0.003? or less, and ti: 0.04? or less, in a range that satisfies formula (2), with the balance composed of fe and the incidental impurities. 300? 85 [si?]? 16 [mn?]? 40 [al?]? 490 [p?]? 430 ????? (1) 0.008? you *? 1.2 [c?] ????? (2) where you *? you? 3.4 [no].

Description

Descriptive Report of the Invention Patent for ELECTRIC STEEL SHEET NOT ORIENTED AND METHOD FOR PRODUCTION OF THE SAME.

TECHNICAL FIELD [001] The present invention relates to a non-oriented electric steel sheet, and in particular to an unoriented electric steel sheet having high strength and excellent fatigue properties, as well as excellent magnetic properties which is suitably used for components that are subjected to high stress, typically electric motors for turbine generators, electric vehicles and hybrid vehicles, or rotors for machinery with high speed rotation, such as servo motors for robots, machine tools or the like, and a method for production. In addition, the present invention provides the non-oriented electric steel sheet described above, at a low cost compared to the conventional technique.

BACKGROUND TECHNIQUE [002] As recent advances in electric motor systems have enabled frequency control of electrical power sources, more and more motors are offering variable speed operation and allowing high speed rotation at frequencies higher than commercial frequency . In such motors that allow high speed rotation, the centrifugal force acting on a rotating body is proportional to the rotation radius and increases in proportion to the square of the rotation speed. Consequently, in particular, rotor materials for medium or large high speed motors require high strength.

[003] In addition, in DC reverse control motors of the IPM type (permanent internal magnet), which was used for engines in hybrid vehicles, such as electric motors such as electric motors

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2/34 or compressor engines, stress is concentrated in portions between grooves to embed magnets in a rotor and the external circumference of the rotor, or in narrow bridge portions several millimeters wide between the grooves to embed magnets. Since engines can be reduced in size, with increasing speed of rotation, there is an increasing demand to increase the speed of rotation of engines, such as electric motors for hybrid vehicles with space and weight restrictions. As such, high strength materials are advantageously used as core materials for use in high speed rotors or motors.

[004] On the other hand, since rotating equipment such as motors or generators make use of the electromagnetic phenomenon, the core materials of the iron cores of rotating equipment need to have excellent magnetic properties. In particular, it is necessary for high speed motor rotors to assume low iron loss at high frequency; the loss of iron at high frequency would, on the other hand, lead to an increase in core temperature due to eddy current induced by a high frequency magnetic flux, causing thermal demagnetization of the embedded permanent magnets, reducing the efficiency of the motor, etc. . Therefore, there is a demand for such an electric steel plate as a material for rotors that has high strength and excellent magnetic properties.

[005] Steel reinforcement mechanisms include reinforcement of the solid solution, reinforcement of precipitation, reinforcement of the crystal grain, hardening, etc. To date, a number of highly resistant, non-oriented electrical steel sheets have been considered and proposed to meet requirements, such as those of high speed motor rotors. [006] As an example that uses the reinforcement of the solid solution, for example, JP 60-238421 A (PTL 1) proposes a method to increase the strength of the steel by adding elements such as Ti, W,

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Mo, Mn, Ni, Co or Al to steel with the purpose of increasing mainly the Si content from 3.5% to 7.0%, and, in addition, achieving the reinforcement of the solid solution. In addition, in addition to the solid solution reinforcement methods described above, JP 62-112723 A (PTL 2) proposes a method to improve the magnetic properties by controlling the crystal grain size in the range of 0.01 mm to 5 , 0 mm by manipulating the conditions of the final annealing.

[007] However, when these methods are applied to production in the factory, production in the factory may be more prone to problems such as fracture of the sheet in a lamination line after hot rolling, which would cause the reduction in yield and the stopping the production line by need. Sheet fracture can be reduced if cold rolling is carried out in warm conditions at sheet temperatures of hundreds of degrees Fahrenheit, in which case, however, process control problems will be a considerable concern, such as adapting the equipment to the warm lamination, tighter production restrictions, etc.

[008] In addition, as a technique that uses the precipitation of carbonitrides, JP 06-330255 A (PTL 3) proposes a technique that makes use of the reinforcement due to the precipitation and the grain refining effects provided by carbonitrides in steel, steel containing Si in the range of 2.0% or more and less than 4.0%, C in the range of 0.05% or less, and one or two elements between Nb, Zr, Ti and V in the range of 0.1 < (Nb + Zr) / 8 (C + N) <1.0, and 0.4 <(Ti + V) / 4 (C + N) <4.0. Similarly, JP 02-008346 A (PTL 4) proposes a technique, in addition to the characteristics described in PTL 3, of adding Ni and Mn in a total amount of 0.3% or more and 10% or less to steel for reinforcement of the solid solution, and also add Nb, Zr, Ti and V in the same ratios as those described in PTL 3 to the steel, to balance the high resistance with the magnetic properties.

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4/34 [009] However, if these methods are applied to obtain high resistance, problems arise that not only inevitably cause the deterioration of magnetic properties, but also make the resulting products susceptible to surface defects such as precipitated crusts, internal defects, etc., resulting in lower product quality and, moreover, subject to a reduction in yield due to the removal of defects and fracture problems during the production of steel sheets, resulting in an increased cost. In addition, the technique described in PTL 4 will lead to an even greater increase in cost because it involves adding an expensive solid solution reinforcing element, such as Ni.

[0010] In addition, as a technique that uses hardening, JP 2005-113185 A (PTL 5) proposes a technique to increase the resistance of steel containing Si in the range of 0.2% to 3.5% by allowing the worked microstructure remains in the steel material. Specifically, PTL 5 describes means that do not perform heat treatment after cold rolling or, if performed, retain the steel material at 750 ° C for a maximum of 30 seconds, preferably at 700 ° C or less, more preferably at 650 ° C or less, 600 ° C or less, 550 ° C or less, and 500 ° C or less. PTL 5 reports the actual results indicating that the worked microstructure ratio is 5% with annealing at 750 ° C for 30 seconds, 20% with annealing at 700 ° C for 30 seconds, and 50% with annealing at 600 ° C for 30 seconds . In this case, there is a problem that such low annealing temperatures lead to an insufficient correction of the laminating strips. Improperly shaped steel sheets have the problem that it would lead to a lower stacking factor after being worked on a motor core in a stacked shape, a non-uniform stress distribution when rotating at a high speed like a rotor, etc. There is another problem that the reason for the grains worked

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5/34 for recrystallized grains varies greatly with steel compositions and annealing temperatures, making it difficult to obtain stable properties. In addition, a non-oriented electric steel sheet is usually subjected to final annealing using a continuous annealing furnace, which is generally maintained in an atmosphere containing at least several percent hydrogen gas to reduce the oxidation of the surfaces of the sheet metal. steel. To perform low temperature annealing at temperatures below 700 ° C in such continuous annealing equipment, there will be tremendous operational restrictions, such as the need to consume time to change oven temperature settings, replacing the atmosphere in the oven to avoid the hydrogen explosion, etc.

[0011] In view of the aforementioned technical fundamentals, the inventors of the present invention proposed in JP 2007186790 A (PTL 6) a high strength electrical steel plate balancing the ability to correct the shape of the steel plate with the ability to reinforce by non-recrystallized microstructures during annealing, a steel plate that is obtained by adding sufficiently and excessively in relation to C and N to a silicon steel plate with reduced levels of C and N and thus increasing the recrystallization temperature of the silicon plate. This method still has a difficulty due to the fact that it can increase the cost of the alloy due to the relatively high content of Ti, cause variations in the mechanical properties due to the remaining recrystallized microstructures, etc.

LIST OF QUOTES

Patent Literature

PTL 1: JP 60-238421 A

PTL 2: JP 62-112723 A

PTL 3: JP 6-330255 A

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PTL 4: JP 2-008346 A

PTL 5: JP 2005-113185 A

PTL 6: JP 2007-186790 A

SUMMARY OF THE INVENTION

Technical Problem [0012] As described above, some proposals were made in non-oriented, high-strength electrical steel sheets. In these proposals made until today, however, it has not been possible until now to use, with the use of common equipment for the production of electric steel sheets, such non-oriented high strength electric steel sheet in an industrially stable manner with good yield and low cost. which has good magnetic properties in addition to high tensile strength and high fatigue strength and, in addition, meets the quality requirements of the steel sheet, such as those relating to surface defects, internal defects, sheet shape, etc. In particular, the high-strength electrical steel plates that have been supplied so far for high speed motor rotors are in a situation where the resulting rotors will be subjected to the inevitable generation of heat due to their magnetic properties, that is, high loss of high frequency iron, which necessarily represents limitations in specifying the design of the engines.

[0013] Therefore, an objective of the present invention is to provide a high strength non-oriented electric steel sheet at low cost, having excellent magnetic and quality properties of the steel sheet, and a method for producing it. Specifically, an objective of the present invention is to provide means to produce such an unoriented electric steel sheet in an industrially stable and low cost manner that has both a tensile strength of 650 MPa or more, desirably 700 MPa or more, and good properties of low iron loss at high frequency so that, for example,

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For example, a steel material having a sheet thickness of 0.35 mm has a W10 / 400 value of 40 W / kg or less, desirably 35 W / kg or less.

Solution to the Problem [0014] The inventors of the present invention have carried out intense studies on electrical steel sheets with oriented grain that can achieve the objectives described above at a high level and methods for producing them. As a result, the inventors revealed that the amounts and ratio of Ti and C to be added to steel are deeply responsible for the balance between the strength properties and the magnetic properties of an electric steel sheet, and that an electric steel sheet with high strength having excellent properties can be produced in a stable and low cost by optimizing the amount of Ti carbide precipitation.

[0015] That is, the present invention is based on the following discoveries:

(A) The growth of the crystal grains of an electric steel sheet during the final annealing can be inhibited by the presence of a relatively small amount of Ti carbides, with which reinforcement by refining the crystal grains can be achieved.

(B) The presence of excess Ti carbides does not contribute to the effective inhibition of the growth of crystal grains, but instead has adverse effects such as causing more surface defects, and internal defects, degrading the quality of the steel plate, contributing to fractures etc. At this point, surface defects such as crusts and internal defects are significantly reduced by controlling the quality of the Ti to be added to the steel within a suitable range.

[0016] On the other hand, Ti nitrides are formed at higher temperatures than Ti carbides. Thus, they are less effectivePetition 870180033011, of 24/04/2018, p. 11/42

8/34 times to inhibit the growth of the crystal grains and are not useful for controlling the refining of the crystal grains intended by the present invention. Therefore, in an approach to inhibit the growth of crystal grains by controlling the amount of Ti carbides, it is desirable to reduce the N content in a stable manner. This is entirely different from conventional approaches that use precipitation reinforcement, where the effects of C and N are treated in the same way.

(C) In a steel plate with refined crystal grains, the solute C has the effect of not only increasing the tensile strength, but also improving the fatigue property essentially necessary for a rotor material that rotates at high speed.

(D) The main alloy components that are normally added for the purpose of reducing iron losses by increasing the electrical resistance of an electric steel plate are Si, Al and Mn. The three substitute connecting elements also have the effect of improving the reinforcement of the solid steel solution. Consequently, the balance between high resistance and low iron loss is effectively guaranteed based on the reinforcement of the solid solution by these elements. However, there is a limit to the addition of these elements since an excessive addition leads to the weakening of the steel and presents difficulty in the production of steel. The addition of Si-based compounds is desirable to satisfy the requirements of reinforcing the solid solution, low iron loss and productivity in the most efficient way.

[0017] Based on these findings, the inventors of the present invention found that the use of balanced reinforcement properties of the solid solution with the use of substitute linkers composed mainly of Si, refining of the crystal grains with Ti carbides, and reinforcement of the solid solution with an interstitial element of C can provide a non-oriented electric steel plate that has high strength, excellent fatigue properties under the conditions

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9/34 conditions of use and, in addition, excellent magnetic and quality properties of the steel sheet, without adding substantially extra restrictions to the production of steel sheets or additional steps to the normal production of non-oriented electric steel sheets, and they also discovered a necessary method for their production. As a result, the inventors completed the present invention.

[0018] That is, the main features of the present invention are as follows:

[0019] (i) A non-oriented electric steel plate comprising, in mass%:

Si: 5.0% or less;

Mn: 2.0% or less;

Al: 2.0% or less; and

P: 0.05% or less, in a range that satisfies formula (1), and the steel sheet also comprises, in% by mass:

C: 0.008% or more and 0.040% or less;

N: 0.003% or less; and

Ti: 0.04% or less, in a range that satisfies formula (2), the balance being composed of Fe and incidental impurities:

300 <85 [Si%] + 16 [Mn%] + 40 [Al%] + 490 [P%] <430 ···· (1)

0,008 <Ti * <1,2 [C%] ···· (2) where Ti * = Ti - 3,4 [N%], and [Si%], [Mn%], [Al%], [ P%], [C%] and [N%] respectively represent the contents (% by mass) of the indicated elements.

[0020] (ii) The electric steel plate not oriented according to item (i) above, in which the contents of Si, Mn, Al and P are, in% by mass,

Si: more than 3.5% but not more than 5.0%,

Mn: 0.3% or less,

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Al: 0.1% or less, and

P: 0.05% or less.

[0021] (iii) The electric steel plate not oriented according to item (i) or (ii) above, also comprising, in mass%, at least one element between:

Sb: 0.0005% or more and 0.1% or less;

Sn: 0.0005% or more and 0.1% or less;

B: 0.0005% or more and 0.01% or less;

Ca: 0.001% or more and 0.01% or less;

Rare Earth Metals (REM): 0.001% or more and 0.01% or less;

Co: 0.05% or more and 5% or less;

Ni: 0.05% or more and 5% or less; and

Cu: 0.2% or more and 4% or less.

[0022] (iv) A method for producing a non-oriented electric steel plate, comprising:

subject the steel plate to soaking where the steel plate is retained at a soaking temperature of 1000 ° C to 1200 ° C, the steel plate containing, in mass%,

Si: 5.0% or less,

Mn: 2.0% or less,

Al: 2.0% or less, and

P: 0.05% or less, in a range that satisfies formula (1), and the steel plate also containing, in mass%,

C: 0.008% or more and 0.040% or less,

N: 0.003% or less, and

Ti: 0.04% or less, in a range satisfying formula (2);

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11/34 subjecting the steel plate to subsequent hot rolling to obtain a hot rolled steel material;

then subject the steel material to cold rolling or warm lamination once, or twice or more with intermediate annealing performed between them, to be finished at the final thickness of the sheet, and subject the steel material to final annealing where, before of the final annealing, the steel material is subjected to heat treatment at least once where the steel material is retained at temperatures of 800 ° C or more and 950 ° C or less for 30 seconds or more, and subsequently to the final annealing at 700 ° C or more and 850 ° C or less:

300 <85 [Si%] + 16 [Mn%] + 40 [Al%] + 490 [P%] <430 (1)

0.008 <Ti * <1.2 [C%] (2) where Ti * = Ti - 3.4 [N%].

[0023] (v) The method for producing an electric steel plate not oriented according to item (iv) above, where the contents of Si, Mn, Al and P are, and,% by mass,

Si: more than 3.5% but not more than 5.0%,

Mn: 0.3% or less,

Al: 0.1% or less, and

P: 0.05% or less.

[0024] (vi) The method for producing an electric steel plate not oriented according to item (iv) or (v) above, in which the steel plate also contains, in mass%, at least one element between:

Sb: 0.0005% or more and 0.1% or less;

Sn: 0.0005% or more and 0.1% or less;

B: 0.0005% or more and 0.01% or less;

Ca: 0.001% or more and 0.01% or less;

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Rare Earth Metals: 0.001% or more and 0.01% or less;

Co: 0.05% or more and 5% or less;

Ni: 0.05% or more and 5% or less; and

Cu: 0.2% or more and 4% or less.

Advantageous Effects of the Invention [0025] In accordance with the present invention, a non-oriented electrical steel sheet can be provided which is excellent in both mechanical and magnetic properties necessary for a rotor material for high speed rotary motors, and which have excellent quality of the steel plate in terms of crusts, shape of the plate, etc. The present invention also allows for the stable production of such non-oriented electric steel sheets with high yield, without incurring a significant increase in cost or imposing severe restrictions on production or requiring extra steps, compared to the normal production of electric steel sheets with grain oriented. Therefore, the present invention is applicable in the field of engines, such as electric motors for electric vehicles and hybrid vehicles or servomotors for robots and machine tools, where the demand for a higher speed of rotation tends to grow in the future. Thus, the present invention has a high industrial value and makes a significant contribution to the industry.

BRIEF DESCRIPTION OF THE DRAWINGS [0026] The present invention will also be described in relation to the accompanying drawings, in which:

FIG. 1 is a graph illustrating the relationship between Ti content and tensile strength;

FIG. 2 is a graph illustrating the relationship between Ti content and iron losses; and FIG. 3 is a graph illustrating the relationship between the Ti content

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13/34 and the defect rate of crusts on the surface.

DESCRIPTION OF EMBODIMENTS [0027] The experimental results inherent to the present invention will be described in detail below.

[0028] That is, the inventors of the present invention investigated in detail how Ti, which is the main forming element of carbonitrides, affects the quality of the steel sheet in terms of reinforcement by precipitation, recrystallization, grain growth behavior, crusts , etc. As a result, it was found that Ti has significantly different effects, in particular when added so that the resulting Ti content is equal to or less than the total C and N content in the atomic fraction, and has an optimal addition range for meet the requirements at a high level in relation to high strength as well as the magnetic properties and quality of the steel sheet. The main experimental results will be described below. The percentage% of each steel component represents% by mass, unless otherwise specified.

Experiment 1 [0029] Steel samples, which have compositions composed mainly of silicon (Si): 4.0% to 4.1%, manganese (Mn): 0.03% to 0.05%, aluminum (Al): 0.001% or less, phosphorus (P): 0.007% to 0.009%, and sulfur (S): 0.001% to 0.002%, containing substantially constant amounts of carbon (C): 0.024% to 0.026% and nitrogen (N): 0.001 % to 0.002%, and different amounts of titanium (Ti) in the range of 0.001% to 0.36%, were contained by the production of steel in a vacuum melting furnace. These steel samples were heated to 1100 ° C and then subjected to hot rolling to be finished to a thickness of 2.1 mm, respectively. Then, the steel samples were subjected to hot strip annealing at 900 ° C for 90 seconds and also to cold rolling to be finished to a thickness of

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14/34 sura of 0.35 mm, after which the occurrence of crust defects on the surfaces of the steel plates (crust size per unit area) was determined. Subsequently, the steel samples were subjected to final annealing at 800 ° C for 30 seconds and evaluated for their mechanical properties (using JIS No. 5 tensile specimens cut parallel to the rolling direction) and magnetic properties ( using Epstein specimens cut in the rolling direction and in the transverse direction, measuring the loss of iron W10 / 400 with a magnetization flux density of 1.0 T and a frequency of 400 Hz). The results of the research on tensile strength, magnetic property and the occurrence of a crust defect on the surface are described in FIGS. 1, 2 and 3 as a function of Ti content, respectively.

[0030] Initially, as illustrated in FIG. 1, the tensile strength increases with the addition of Ti. However, this effect has been found to be less pronounced within a range of Ti content indicated by A (range A) in FIG. 1 where the Ti content is lower, while stable improvements in resistance are observed within a range of Ti content indicated by B (range B) in the figure. In addition, even other improvements in strength are achieved within a range indicated by C (range C) in the figure where the Ti content is highest. In the observation of the steel structure in these regions, it was found that in strip B, the steel structure contains homogeneous microstructures with a crystal grain size of 10 pm or less, while in strip A it involves more grown crystal grains than in strip B, particularly mixed grain size microstructures with partial grain growth. On the other hand, in range C, the steel structure assumes a multi-phase of non-recrystallized grains and recrystallized grains.

[0031] FIG. 2 illustrates the relationship between IT Content and W10 / 400 iron loss. While good iron-loss properties are obtained

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15/34 of those in range A with the least iron loss, as illustrated in FIG. 1, track A shows lower resistance levels. On the other hand, although high strength materials are obtained in bands C and D in FIG. 2, iron loss is also high in these ranges. In contrast, band B offers materials that have iron loss properties almost as good as those in band A, while the resistance and yield strength is comparable to those obtained in band C.

[0032] On the other hand, as illustrated in FIG. 3, the crust defect rate begins to increase when the Ti content exceeds 0.04%, and continues to increase until around the point where the equivalent ratio of Ti element to C and N is equal to 1, where a substantially constant rate of crust generation is achieved. Assuming constant levels of C and N, the amount of Ti carbonitride precipitates continues to increase until around a point at which this equivalent element ratio is equal to 1, and then remains constant. Thus, it is considered that the amount of Ti carbonite precipitates is relative to the amount of crust generation. Thus, it is considered that the amount of Ti carbonite precipitates is related to the amount of crust generation.

[0033] These results revealed that by controlling the Ti content within the B range, it becomes possible to balance high resistance and low iron loss, while reducing crust defects that would cause a reduction in yield and fracture problems in the plate and be directly linked to an increase in production cost. That is, it is advantageous to contain Ti in an amount of 0.04% or less in terms of reducing crust defects, as long as it is sufficient for the formation of a certain amount of Ti carbonitrides.

[0034] In addition, as a result of other studies conducted with the same components except for the steel described above and N content and with varying N levels, it was also found that the limit

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16/34 lower than the Ti content for which high resistance can be obtained increases with increasing the N content. Still other studies have revealed that it is necessary to satisfy a ratio of 0.008 <Ti * (where Ti * = Ti 3.4 [N%]). Thereafter, it is believed that since Ti carbides make a major contribution to increasing strength while Ti nitrides contribute less, it is more important to control Ti carbides.

[0035] These results revealed that by controlling the Ti content to a level in the B range, it becomes possible to balance high resistance and low iron loss, while reducing crust defects that would otherwise cause a reduction in yield and a sheet fracture problem and being directly linked to an increase in production cost.

Experiment 2 [0036] Then, to investigate details of the influence of Ti carbonitrides, steel samples having the compositions shown in Table 1 were prepared by producing steel in a vacuum melting furnace to obtain steel sheets, each having a thickness of 0.35 mm, following the same procedure as in Experiment 1. The C and N levels of the steel samples were varied using the steel sample a, which has low C and N levels, as a reference. Steel samples c and d contain C and N so that their total content is within a predetermined range. The rate of crust defects on the surface, the loss of iron and the tensile strength of the resulting samples are shown in Table 2. While steel b, c and d show an increase in strength compared to sample a, comparing samples c and d which have substantially the same total amount of C and N to assess the effect of adding C and N, it can be seen that the steel sample c, which has a lower N content, has greater strength. In the observation of the microstructures,

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17/34 found that steel samples are listed as a> d> b> c in decreasing order of crystal grain size, as is the case with decreasing order of tensile strength.

Table 1 (mass%)

Steel Si Mn Al P Ç N You The 4.33 0.07 0.0005 0.010 0.0019 0.0021 0.0302 B 4.32 0.05 0.0010 0.010 0.0240 0.0009 0.0295 ç 4.29 0.03 0.0007 0.010 0.0293 0.0009 0.0298 d 4.25 0.08 0.0018 0.020 0.0249 0.0052 0.0301

Table 2

Steel W10 / 400 (W / kg) Tensile strength TS (MPa) Fatigue strength limit FS (MPa) Resistance ratio FS / TS Surface crust defect rate (m / m2) The 26.9 641 535 0.83 0.000 B 33.0 722 630 0.87 0.003 ç 32.5 730 665 0.91 0.003 d 31.0 676 540 0.80 0.004

[0037] These samples were also investigated for their fatigue properties. Tests were conducted in a traction-to-traction mode with a stress ratio of 0.1 at a frequency of 20 Hz, where the fatigue resistance limit is defined as the stress that allows a sample to survive 10 million cycles of stress amplitude. Their results are also shown in Table 2. Although the trend is observed that materials having a higher tensile strength TS have a higher limit of resistance to FS fatigue, the FS / TS strength ratio differs for different materials. In this case, the steel sample c gave the best result. On the other hand, the steel sample d does not improve the fatigue strength limit for its high tensile strength. Given these

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18/34 circumstances, and as a result of our detailed investigations of the microstructures of the steel sample d, many precipitates, presumably TiN precipitates having a grain size of more than 5 mm were sporadic by the microstructures, and these precipitates were estimated to contribute to the source of fatigue fracture. It should be noted here that nitrogen reacts with titanium at relatively high temperatures of 1100 ° C or more and tends to precipitate as TiN crude. It was believed, therefore, that TiN tends to provide origins of fatigue fracture and is less effective compared to Ti carbides to inhibit the growth of crystal grains, which is one of the objectives of the present invention.

[0038] On the other hand, when comparing steel samples b and c, it was also found that the steel sample c gives better results in terms of tensile strength and fatigue strength limit, and is particularly characterized by its strength limit relatively high fatigue and high FS / TS resistance ratio. Since steel samples b and c have substantially the same levels of Ti and N, they exhibit similar precipitation behavior of Ti nitrides and Ti carbides. It is believed that the difference between them is attributed to the difference in the amount of solute carbon. Consequently, it is estimated that the presence of solute carbon reduced the occurrence and spread of fractures and increased the limit of resistance to fatigue due to lock dislocations introduced during repeated stress cycles as discovered in the fatigue test. Therefore, it is also important to ensure the formation of solute carbon.

[0039] Based on the results of the experiments described above, the inventors of the present invention have done further studies on how these factors, including Ti carbides, Ti nitrides and solute carbon, with the addition of relatively small amounts of Ti,

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19/34 affect the steel structure, the surface quality of the steel sheet, as well as the mechanical properties and the magnetic properties of the steel sheets. As a result, the inventors discovered the rules in detail applicable to these factors and developed the present invention.

[0040] The present invention will now be described in detail below for each requirement.

[0041] Initially, the fundamentals for the limitations in relation to the main steel components are described.

[0042] The steel of the present invention contains Si: 5.0% or less, Mn: 2.0% or less, Al: 2.0% or less, and P: 0.05% or less, in a range that meets formula (1):

300 <85 [Si%] + 16 [Mn%] + 40 [Al%] + 490 [P%] <430 ..... (1) [0043] An object of the present invention is to provide an electric steel sheet having high strength and excellent magnetic properties at low cost. To that end, it is necessary to achieve reinforcement of the solid solution above a certain level by means of the four major alloy components described above. Thus, it is important to specify the contents of the four major alloy components as described later. And add these components to the steel so that the total amount of these alloy components is within a range that satisfies formula (1) above, considering individual contributions to the reinforcement of the solid solution. That is, if formula (1) gives a result less than 300, the resistance of the resulting material is insufficient, while if formula (1) gives a result greater than 430, there are more problems with the fractures of the plates at the time of production steel plates, leading to a deterioration in productivity and a significant increase in production costs.

[0044] The following describes the fundamentals for limiting the individual levels of the four major alloy components.

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Si <5.0% [0045] Silicon (Si) is generally used as a deoxidizer and one of the main elements that are contained in a non-oriented electric steel plate and has the effect of increasing the electrical resistance of the steel to reduce its loss of iron. In addition, Si is an element that is positively added to the non-oriented electric steel plate because it is able to achieve greater tensile strength, greater fatigue resistance, and less iron loss at the same time and in a more balanced way if compared to other reinforcing elements of the solid solution, such as Mn, Al or Ni, which are added to the grain-oriented electric steel plate. For this purpose, it is advantageous to contain Si in the steel in an amount of 3.0% or more, more preferably exceeding 3.5%. However, above 5.0% the degradation of toughness will be pronounced, which would require highly sophisticated control during the passage of the sheet and the rolling processes, resulting in lower productivity. Therefore, the upper limit of the Si content should be 5.0% or less.

Mn <2.0% [0046] Manganese (Mn) is effective in improving the hot brittleness properties, and also has the effect of increasing the electrical resistance of the steel to reduce its iron loss and increase the resistance of the steel by strengthening of the solid solution. Thus, Mn is preferably contained in the steel in an amount of 0.01% or more. However, the addition of Mn is less effective in improving the strength of the steel compared to Si and its excessive addition leads to the weakening of the resulting steel. Therefore, the Mn content must be 2.0% or less.

Al <2.0% [0047] Aluminum (Al) is an element that is generally used in refining steel as a strong deoxidizing element. In addition, as in the case of Si and Mn, Al also has effects of increasing resistance

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21/34 electrical steel to reduce iron loss and increase steel strength by reinforcing the solid solution. Therefore, Al is preferably contained in steel in an amount of 0.0001% or more. However, the addition of Al is less effective in improving the strength of steel compared to Si and its excessive addition leads to the weakening of the resulting steel. Therefore, the Al content must be 2.0% or less.

P <0.05% [0048] Phosphorus (P) is extremely effective in increasing the strength of steel because it offers a significantly high capacity to reinforce the solid solution even when added in relatively small amounts. Thus, P is preferably contained in the steel in an amount of 0.005% or more. However, an excessive addition of P leads to weakening of the steel due to segregation, causing intergranular fracture or a reduction in the rolling capacity. Therefore, the P content is limited to 0.05% or less.

[0049] Additionally, among these main connecting elements Si, Mn, Al and P, a Si based alloy design is advantageous to balance the reinforcement of the solid solution / low iron loss and productivity in a more efficient way. That is, it is advantageous to contain Si in the steel in an amount of more than 3.5% to optimize the balance of properties of the non-oriented electric steel sheet, where the contents of the remaining three elements are preferably controlled as follows: Mn: 0, 3% or less, Al: 0.1% or less, and P: 0.05% or less. The rationale for the upper limit limitations is as described above.

[0050] In addition, C, N and Ti are also important elements in the present invention. This is because it is important to inhibit the growth of the crystal grains during the annealing of the steel sheet without the use of an adequate amount of fine Ti carbides and to develop the capacity of reinforcing the refining of the crystal grain. For that purpose

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22/34 therefore, it is necessary to contain C: 0.008% or more and 0.040% or less, N: 0.003% or less, and Ti: 0.04% or less in steel in a range that satisfies formula (2):

0.008 <Ti * <1.2 [C%] (2) where Ti * = Ti - 3.4 [N%].

0.008% <C <0.040% [0051] Carbon (C) must be contained in steel in an amount of 0.008% or more. That is, a carbon content of less than 0.008% makes it difficult to provide stable precipitation of fine Ti carbides and results in an insufficient amount of C solute, in which case another improvement in fatigue strength is no longer possible. On the other hand, an excessive addition of C leads to the deterioration of magnetic properties, while becoming a factor responsible for an increase in cost, such as making hardening more pronounced during cold rolling and causing the plate to fracture, forcing more cycles rolling mill due to an increased rolling load, etc. Therefore, the upper limit of C is limited to 0.04%.

N <0.003% [0052] Nitrogen (N) forms nitrides with Ti, which are, however, formed at higher temperatures than Ti carbides. Thus, N is less effective in inhibiting the growth of crystal grains and not it is very effective in refining the crystal grains. In contrast, N sometimes causes adverse effects such as providing a source for fatigue fractures. Therefore, the N content is limited to 0.003% or less. In addition, without limitation, the lower limit is preferably about 0.0005% in terms of degassing capacity in steel production and to avoid deterioration in productivity due to the long duration of refining.

Ti <0.04%

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23/34 [0053] The control of titanium (Ti) carbides is important in the present invention. Ti tends to form nitrides instead of carbides at high temperatures, so it is necessary to control the amount of Ti that forms carbides. If the amount of Ti that is capable of forming is denoted as Ti *, Ti * is represented as the Ti content minus the atomic equivalent with N, that is:

Ti * = Ti - 3,4 [N%] [0054] To allow the added amount of Ti to precipitate as Ti carbides to increase the strength of the steel, while inhibiting the growth of the crystal grains to prevent an increase in the loss of iron from steel, it is necessary to use an adequate amount of C and satisfy Ti *> 0.008. On the other hand, if the Ti content is increased in relation to the C content, there is a reduction in the amount of C solute, in which case another improvement in fatigue resistance is no longer possible. Therefore, it is also necessary to satisfy, at the same time, Ti * <1,2 [C%].

[0055] In addition, if the Ti content exceeds 0.04%, as previously described in relation to FIG. 3, more crust defects will occur and steel plate quality and yield will be reduced, resulting in an increase in cost. Therefore, the upper limit of the Ti content should be 0.04%.

[0056] The present invention may also contain elements different from the elements mentioned above without impairing the effects of the invention. For example, the present invention may contain antimony (Sb) and tin (Sn), each of which has an effect of improving the magnetic properties of steel in the range of 0.0005% to 0.1%; boron (B), which has an effect of increasing the strength of the steel grain edge, in the range of 0.0005% to 0.01%; Ca and Rare Earth Metals, each of which has an effect of controlling the shape of the oxide and sulfide and improving the magnetic properties of steel, in the range

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24/34 from 0.001% to 0.01%; Co and Ni, each of which has the effect of improving the magnetic flux density of steel, in the range of 0.05% to 5%; and Cu, which is expected to provide precipitation resistance through aging precipitation, in the range of 0.2% to 4%, respectively.

[0057] The rationale for the limitations in relation to a production method according to the present invention will be described below. In the present invention, the production process from steel production to cold rolling can be carried out according to methods commonly used for general production of electrical steel sheets with oriented grain. For example, a steel, which was prepared by steel production and refined with predetermined components in a converter or electric oven, can be subjected to continuous casting or block production after conventional casting to obtain steel plates, which in turn can subjected to processing steps, including hot rolling, optional hot strip annealing, cold rolling, final annealing, insulation and cooking coating application, etc., to produce steel sheets. In these steps, the conditions for adequately controlling the state of precipitation will be described below. It should be noted that hot strip annealing can optionally be performed after hot rolling, and that cold rolling can be performed once or twice or more with intermediate annealing performed between them.

[0058] Steel plates made up of the chemical compositions mentioned above must be subjected to hot rolling at a heating temperature of the plate of 1000 ° C or more up to 1200 ° C or less. That is, if the plate heating temperature is less than 1000 ° C, it is not possible to achieve an effect of inhibiting the growth of the crystal grains during the final annealing of

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25/34 in a sufficient manner due to the precipitation and growth of the Ti carbides during the heating of the plates. Alternatively, if the heating temperature of the plate is greater than 1200 ° C, this is not only disadvantageous in terms of cost, but also causes deformation in the plate due to the reduction of resistance to high temperature, which interferes, for example, with extraction steel plates of the heating furnace, resulting in less operating capacity. Therefore, the heating temperature of the plate must be within the range of 1000 ° C to 1200 ° C. In addition, hot rolling itself is not rolled to a particular type and can be carried out under the conditions, for example, of finishing temperature of the hot rolling in the range of 700 ° C to 950 ° C and winding temperature of 750 ° C or less.

[0059] The resulting hot-rolled steel materials are then subjected to optional hot strip annealing and cold rolling or warm rolling once, or twice or more with intermediate annealing performed between them is finished to a final thickness of sheet before final annealing, it is important to subject the steel materials to heat treatment at least once where the steel materials are retained at temperatures of 800 ° C or more and 950 ° C or less for 30 seconds or more. This heat treatment can allow the precipitation of Ti carbides in microstructures before the final annealing and thus inhibit the growth of the crystal grains during the final annealing.

[0060] That is, if the heat treatment described above is carried out at temperatures below 800 ° C, the resulting precipitation may be insufficient, while above 950 ° C, the effect of inhibiting the growth of the crystal grains during annealing end would be insufficient due to the growth of precipitates.

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26/34 [0061] Additionally, the heat treatment mentioned above is preferably carried out in combination with either hot strip annealing or intermediate annealing before final annealing.

[0062] The subsequent final annealing can be carried out at 700 ° C or more and 850 ° C or less to control the microstructure of the recrystallized grains in a fine and homogeneous state, providing an electric steel sheet having high strength and excellent magnetic properties. . If the final annealing is carried out at temperatures below 700 ° C the resulting recrystallization is insufficient, while above 850 ° C the crystal grains are liable to grow even when applying the present invention, resulting in a reduction in the resistance of the products . After this final annealing, the steel materials are subjected to processes for the application and firing of an insulating coating to obtain the final products.

Example 1 [0063] Steel samples having the compositions shown in Table 3 were obtained by producing steel in a vacuum melting furnace, heated to 1100 ° C, and then subjected to hot rolling to be a thickness of 2.1 mm. Then, the samples were subjected to hot strip annealing at 900 ° C for 90 seconds and also to cold rolling to be finished to a thickness of 0.35 mm. At that time, an assessment was made of the occurrence of crust defects on the surfaces of steel sheets, using the crust size per unit area as a reference. Subsequently, the samples were subjected to final annealing for 30 seconds under two different conditions at 750 ° C and 800 ° C respectively. Then, the specimens were cut parallel to the rolling direction of the samples of the steel sheets thus obtained for tensile and fatigue tests. In addition, the magnetic properties

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27/34 were evaluated based on the loss of iron with a magnetic flux density of 1.0 T and a frequency of 400 Hz of the Epstein test specimens that were cut from the samples in the lamination direction and in the transversal direction, respectively. The results of the evaluation are shown in Table 4.

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Table 3 (% by mass)

Steel Si Mn Al P Ç N You Formula (1) You* Grades 1 4.08 0.08 0.0010 0.012 0.0250 0.0015 0.0010 354 -0.0041 Comparative Example 2 4.10 0.05 0.0010 0.010 0.0247 0.0013 0.0189 354 0.0145 Example of the Invention 3 4.05 0.04 0.0004 0.018 0.0251 0.0016 0.0349 354 0.0295 Example of the Invention 4 4.08 0.05 0.0015 0.011 0.0245 0.0012 0.0641 353 0.0600 Comparative Example 5 4.02 0.04 0.0020 0.017 0.0258 0.0017 0.1164 351 0.1106 Comparative Example 6 4.07 0.08 0.0019 0.014 0.0260 0.0019 0.1630 354 0.1565 Comparative Example

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Table 4

Steel Crust defect rate on the surface (m / m ² ) Annealing at 800 ° C Annealing at 750 ° C Grades W10 / 400 (W / kg) Resist. traction TS (MPa) Resistance limit fatigue FS (MPa) Resistance ratio FS / TS W10 / 400 (W / kg) Resist. traction TS (MPa) Resistance limit The fatigue FS (MPa) Resistance ratio FS / TS 1 0.000 27.4 634 540 0.85 33.4 707 570 0.81 Comparative example 2 0.000 31.5 710 635 0.89 33.9 727 650 0.89 Example of the invention 3 0.005 33.7 715 650 0.91 34.6 731 665 0.91 Example of the invention 4 0.159 42.7 722 600 0.83 44.4 737 620 0.84 Comparative example 5 0.189 46.5 726 560 0.77 48.3 744 575 0.77 Comparative example 6 0.211 48.0 734 565 0.77 51.0 750 580 0.77 Comparative example

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[0064] It can be seen from Table 4 that steel sample No. 1, which has a Ti * value outside the scope of the present invention, has significantly different properties depending on the temperatures of the fine annealing, which is considered problematic in terms of quality control. On the other hand, steel samples containing an adequate amount of Ti show minor differences in their properties depending on the temperatures of the final annealing, producing high tensile strength in a stable manner. However, if compared to steel samples 2 and 3 having steel compositions within the range specified by the present invention, steel samples 4, 5 and 6, each having a Ti content outside the scope of the present invention have a limit from fatigue strength not so high to its high tensile strength and have lower crust rate and magnetic properties.

Example 2 [0065] Steel samples having the compositions shown in Table 5 were obtained by producing steel in a vacuum melting furnace, heated to 1050 ° C, and then subjected to hot lamination to have a thickness of 2 , 1 mm. Then the samples were subjected to hot strip annealing at 850 ° C for 120 seconds and then cold rolling to be finished to a thickness of 0.35 mm. At that time, an assessment of the occurrence of crust defects was made, using the crust size per unit area as a reference. Subsequently, the steel samples were subjected to final annealing at 800 ° C for 30 seconds. Then, specimens were cut parallel to the rolling direction of the steel plate samples thus obtained by tensile test and fatigue test. In addition, the magnetic properties were evaluated based on the loss of iron with a magnetization flux density of 1.0 T and a frequency of 400 Hz of the Epstein test specimens

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31/34 that were cut from the samples in the rolling direction and in the transversal direction, respectively. Their results are also shown in Table 6.

[0066] Additionally, steel sample No. 18, which does not satisfy the ratio of formula (1) specified by the present invention, experienced plate fracture during cold rolling, and then was not subjected to the subsequent evaluation process.

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Table 5 (% by weight)

Steel Si Mn Al P Ç N You others Formula (1) You* Grades 7 3.05 0.15 0.3500 0.018 0.0165 0.0014 0.0174 - 284 0.0126 Comparative example 8 3.75 0.08 0.0010 0.019 0.0043 0.0015 0.0172 - 329 0.0121 Comparative example 9 3.78 0.05 0.0008 0.014 0.0159 0.0017 0.0166 - 329 0.0108 Example of the invention 10 4.01 0.04 0.0001 0.015 0.0135 0.0013 0.0154 - 349 0.0109 Example of the invention 11 4.01 0.04 0.0004 0.015 0.0320 0.0016 0.0148 - 349 0.0093 Example of the invention 12 4.05 0.05 0.0004 0.013 0.0572 0.0016 0.0166 - 351 0.0111 Comparative example 13 4.03 0.01 0.0004 0.001 0.0175 0.0041 0.0168 - 343 0.0027 Comparative example 14 4.82 0.04 1.0300 0.018 0.0158 0.0016 0.0188 - 419 0.0133 Example of the invention 15 3.02 0.88 0.7000 0.010 0.0289 0.0016 0.0333 - 317 0.0278 Example of the invention 16 3.55 0.59 1,2100 0.010 0.0294 0.0021 0.0328 - 344 0.0256 Example of the invention 17 4.30 0.11 0.1800 0.012 0.0285 0.0025 0.0322 - 380 0.0236 Example of the invention

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Continuation...

(% in large scale)

Steel Si Mn Al P Ç N You others Formula (1) You* Grades 18 4.60 0.59 1,2100 0.010 0.0296 0.0011 0.0311 - 454 0.0293 Comparative example 20 4.11 0.08 0.0009 0.011 0.0167 0.0021 0.0217 Sn: 0.043 356 0.0145 Example of the invention 21 4.30 0.18 0.2530 0.007 0.0145 0.0009 0.0191 B: 0.003 382 0.0160 Example of the invention 22 4.25 0.09 0.2310 0.018 0.0181 0.0011 0.0155 Ca: 0.003 381 0.0117 Example of the invention 23 4.22 0.15 0.0830 0.015 0.0226 0.0016 0.0185 Rare Earth Metals: 0.004 372 0.0130 Example of the invention 24 3.98 0.25 0.2250 0.013 0.0284 0.0018 0.0355 Co: 0.25 358 0.0293 Example of the invention 26 3.87 0.18 0.2760 0.011 0.0336 0.0013 0.0347 Cu: 0.22 348 0.0302 Example of the invention

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Table 6

Steel Surface crust defect rate (m / m2) W10 / 400 (W / kg) Resist. traction TS (MPa) Resistance limit fatigue FS (MPa) Resistance ratio FS / TS Grades 7 0.001 28.6 625 510 0.82 Comparative example 8 0.000 32.7 673 535 0.79 Comparative example 9 0.001 34.5 685 624 0.91 Example of the invention 10 0.005 32.2 708 631 0.89 Example of the invention 11 0.005 31.2 705 650 0.92 Example of the invention 12 0.230 38.7 694 575 0.93 Comparative example 13 0.110 36.8 701 540 0.77 Comparative example 14 0.005 28.8 779 715 0.92 Example of the invention 15 0.035 34.5 668 607 0.91 Example of the invention 16 0.026 33.3 703 645 0.92 Example of the invention 17 0.039 33.5 735 680 0.93 Example of the invention 18 - - - - - Comparative example 20 0.004 31.2 707 635 0.90 Example of the invention 21 0.006 33.4 733 640 0.87 Example of the invention 22 0.003 31.6 729 640 0.88 Example of the invention 23 0.003 32.0 721 633 0.88 Example of the invention 24 0.007 33.3 723 645 0.89 Example of the invention 26 0.008 33.5 706 608 0.86 Example of the invention [0067] Can be vis1 Table 6 shows that each of the

steel according to the present invention has less crusts, good iron loss properties and high tensile strength, as well as high fatigue strength.

Claims (6)

1. Electric steel sheet not oriented, characterized by having the chemical composition consisting of, in% by mass: Si: 5.0% or less; Mn: 2.0% or less; Al: 2.0% or less; and P: 0.05% or less, in a range that satisfies formula (1), and C: 0.008% or more and 0.040% or less; N: 0.003% or less; and Ti: 0.04% or less, in a range that satisfies formula (2), and the balance being Fe and the incidental impurities: 300 <85 [Si%] + 16 [Mn%] + 40 [Al%] + 490 [P%] <430 (1) 0.008 <Ti * <1.2 [C%] (2) where Ti * = [Ti%] - 3.4 [N%]. 2. Electric steel sheet not oriented according to claim 1, characterized by the fact that the contents of Si, Mn, Al and P are,% by mass, Si: more than 3.5% but not more than 5.0%, Mn: 0.3% or less, Al: 0.1% or less, and P: 0.05% or less. 3. Electric steel plate not oriented according to claim 1 or 2, characterized in that the chemical composition additionally contains, in% by mass, at least one of: Sb: 0.0005% or more and 0.1% or less; Sn: 0.0005% or more and 0.1% or less; B: 0.0005% or more and 0.01% or less; Petition 870180028312, of 04/09/2018, p. 37/53 2/3 Ca: 0.001% or more and 0.01% or less; Rare Earth Metals: 0.001% or more and 0.01% or less; Co: 0.05% or more and 5% or less; Ni: 0.05% or more and 5% or less; and Cu: 0.2% or more and 4% or less. 4. Method for producing a non-oriented electric steel sheet, characterized by comprising: subjecting a steel plate to flooding, where the steel plate is retained at a flooding temperature of 1000 ° C to 1200 ° C, the steel plate having a chemical composition consisting of, in mass%, Si: 5.0% or less, Mn: 2.0% or less, Al: 2.0% or less, and P: 0.05% or less, in a range that satisfies formula (1), and C: 0.008% or more and 0.040% or less, N: 0.003% or less, and Ti: 0.04% or less, in a range that satisfies formula (2) and the balance being Fe and incidental impurities; subjecting the steel sheet to subsequent hot rolling to obtain a hot rolled steel material; then subject the steel material to cold rolling or warm rolling once, or twice or more with intermediate annealing performed between them, to be finished in a final plate thickness; and subject the steel material to final annealing, in which, before final annealing, the steel material is subjected to heat treatment at least once, where the steel material is retained in a Petition 870180028312, of 04/09/2018, p. 38/53 3/3 temperature of 800 ° C or more and 950 ° C or less for 30 seconds or more, and subsequent to final annealing at 700Ό or more and 850 ° C or less: 300 <85 [Si%] + 16 [Mn%] + 40 [Al%] + 490 [P%] <430 ..... (1) 0.008 <Ti * <1.2 [C%] (2), where Ti * = [Ti%] - 3.4 [N%]. 5. Method for producing electric steel sheet not oriented according to claim 4, characterized by the fact that the contents of Si, Mn, Al and P are, in mass%, Si: more than 3.5% but not more than 5.0%, Mn: 0.3% or less, Al: 0.1% or less, and P: 0.05% or less. 6. Method for producing an electric steel sheet not oriented according to claim 4 or 5, characterized by the fact that the chemical composition additionally contains, in mass%, at least one of: Sb: 0.0005% or more and 0.1% or less, Sn: 0.0005% or more and 0.1% or less, B: 0.0005% or more and 0.01% or less, Ca: 0.001% or more and 0.01% or less, Rare Earth Metals: 0.001% or more and 0.01% or less, Co: 0.05% or more and 5% or less, Ni: 0.05% or more and 5% or less, and Cu: 0.2% or more and 4% or less.