Non-oriented electrical steel and method for producing non-oriented electrical steel

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates namely to a sheet of non-textured electrical steel with a thickness of 0.29 to 0.4 mm, used as a material for the manufacture of iron cores of electrical machines. The sheet has a composition including the following elements, wt.%: 0.0001 ≤ carbon ≤ 0.007, 0.15 ≤ manganese ≤ 0.2, 3 ≤ silicon ≤ 3.6, 0.7≤ aluminum ≤ 1,3, phosphorus ≤ 0.15, sulfur ≤ 0.006, nitrogen ≤ 0.09, 0.01 ≤ copper ≤ 1, 0.01 ≤ nickel ≤ 1, and 3.85 ≤ Si+Al+Mn ≤ 5.1, the rest consists of iron and inevitable impurities formed during the manufacturing process. The microstructure of the steel sheet consists of ferrite and includes in area fractions 80-100% recrystallized microstructure and 0-20% non-recrystallized microstructure, while the average grain size of the recrystallized microstructure is 20-110 mcm. The sheet has an eddy current loss share of the total iron losses measured at 1 T and 400 Hz of 35-55% when calculated using the Bertotti method, and at the same time has a magnetic polarization at 5000 A / m (J50) of 1.635-1.670 T.EFFECT: improved performance.14 cl, 2 tbl

Description

[0001] The present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof. In particular, the present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof that has low iron loss, in particular low eddy current loss, while having good mechanical properties.

[0002] With the growing global drive for energy conservation in electrical devices, higher performance non-oriented electrical steel sheets are required for use as electrical machine iron core materials, leading to intensive research and development. In particular, the need for small, high-power motors for electric vehicles and the like has recently arisen. These electric vehicle motors have been designed to achieve high rotation speeds and thus high torque with minimal losses. This requires lightweight and high-performance non-oriented electrical steels, which have low losses as a key property.Finding a balance between losses, permeability, polarization, thermal conductivity, tensile strength and yield strength is vital for non-oriented electrical steels.

[0003] The lower the iron losses in an electric machine, the higher its efficiency. Therefore, to reduce iron losses in an electric machine, machine manufacturers have several options. The primary options are either reducing hysteresis losses or reducing eddy current losses to improve the efficiency of their electric machine. Progress is often achieved by combining the two approaches. This invention relates to the second option, namely, reducing eddy current losses in an electric machine. There are two ways to reduce eddy current losses.

[0004] The first approach is to reduce the thickness of the steel sheets used in electric vehicles, for example, to less than 0.35 mm or even lower. Unfortunately, this solution has its limitations due to a reduction in the package fill factor, which reduces the torque achievable for a given vehicle size, and, furthermore, an excessive reduction in the rigidity of certain vehicle components and the emergence of acoustic problems that create an uncomfortable environment for passengers.

[0005] The second approach involves optimizing the elemental composition of the steel sheet, for example, by increasing the amount of alloying elements to limit eddy current losses. Among alloying elements, aluminum and manganese have attractive mechanical and magnetic properties, which can significantly reduce eddy current losses. However, the addition of alloying elements is limited, as at a certain percentage, alloying elements will affect hysteresis and magnetic polarization losses.

[0006] Earlier research and development in the field of high-strength non-oriented electrical steel have led to several methods for producing high-strength non-oriented electrical steel, some of which are listed in the description for a reasonable evaluation of the present invention.

[0007] US2021 / 371948 offers non-oriented electrical steel sheet with average magnetostriction λp - p at 400 Hz and 1.0 T not exceeding 4.5x10 -6 , and a cross-sectional area ratio of recrystallized grains in the rolling direction of the steel sheet of 40-95% and an average grain size of 10 - 40 microns, obtained from a steel slab containing, in mass percent, C: no more than 0.005%, Si: 2.8 - 6.5%, Mn: 0.05 - 2.0%, Al: no more than 3.0%, P: no more than 0.20%, S: no more than 0.005%, N: no more than 0.005%, Ti: no more than 0.003%, V: no more than 0.005% and Nb: no more than 0.005% and satisfying the inequality Si - 2A1 - Mn≥0, by hot rolling, annealing in the hot zone, cold rolling and final annealing under the corresponding cold rolling and final annealing conditions, and The motor core is made of such steel sheet. US2021 / 371948 does not specify the total elongation or eddy current losses.

[0008] The object of the present invention is to solve these problems by producing non-oriented electrical steel sheets which simultaneously have a magnetic polarization at 5000 A / m (J50) of 1.635 - 1.670 T, preferably 1.637 T - 1.670 T, more preferably 1.637 - 1.655 T, and a proportion of eddy current losses in the total iron losses of 35 - 55%, preferably 35 - 50% when calculated using the Bertotti method.

[0009] In preferred embodiments, the following additional properties may also be achieved, individually or in combination:

[0010] --tensile strength of 530 MPa or more in both the transverse direction and the rolling direction, and preferably more than 550 MPa in both the transverse direction and the rolling direction;

[0011] - yield strength of 410 MPa or more in both the transverse direction and the rolling direction, preferably 425 MPa or more in both the transverse direction and the rolling direction;

[0012] - total elongation of 10% or more in both the transverse direction and the rolling direction, preferably greater than or equal to 12% in both the transverse direction and the rolling direction;

[0013] -Total loss 14-20W / kg when measured at 1T and 400Hz, preferably 14.5-20W / kg when measured at 1T and 400Hz.

[0014] Preferably, this steel is also suitable for rolling, having adequate stamping ability and coating acceptability.

[0015] Preferably, the hardness is greater than or equal to 185 HV, preferably greater than or equal to 195 HV.

[0016] Another object of the present invention is also to provide a method for producing such sheets that is compatible with traditional industrial applications and is resistant to changes in manufacturing parameters.

[0017] The above object and other advantages of the present invention will become more apparent from a detailed description of a preferred embodiment of the present invention.

[0018] The chemical composition of non-oriented electrical steel includes the following elements in mass percentage.

[0019] Carbon is present in the steel of the present invention in an amount of 0.0001-0.007%. Carbon is a precipitate-forming element and therefore has a negative effect on the magnetic properties of the steel of the present invention. Therefore, the carbon content in the steel of the present invention is 0.0001-0.007%. Since carbon promotes magnetic aging, the carbon content according to the present invention is preferably 0.002-0.007%, more preferably 0.003-0.006%.

[0020] The manganese content in the steel of the present invention is 0.15-0.2%. Manganese provides solid solution strengthening and reduces iron loss by increasing the specific resistance. If manganese is added in an amount exceeding 0.2%, the magnetic flux density may significantly decrease, and the recrystallization of the steel during annealing will be difficult. The preferred manganese content limit is 0.16-0.2%, more preferably 0.17-0.2%.

[0021] The silicon content in the steel of the present invention is 3-3.6%. Silicon is an element that contributes to the increase in strength through solid solution strengthening and is a key element in reducing eddy current losses in iron losses by increasing the resistivity of steel. To achieve these effects, a minimum silicon content of at least 3% is required. However, when the silicon content exceeds 3.6%, rolling becomes difficult, and the magnetic induction of the steel is significantly reduced. The preferred silicon content limit is 3.1-3.55%, more preferably 3.15-3.5%.

[0022] The aluminum content is 0.7-1.3%. Aluminum increases the electrical resistance of the material and can effectively reduce iron loss in steel. When the aluminum content exceeds 1.3%, the magnetic induction of the steel decreases significantly, which also negatively affects the cold rolling performance of the steel. The preferred aluminum content range is 0.8-1.1%, with 0.85-1.09% being more preferred.

[0023] Sulfur is not an essential element, but may be present in steel as an impurity. From the perspective of the present invention, the sulfur content should preferably be as low as possible, but its content should be 0.006% or less to reduce production costs. Furthermore, at higher sulfur content in steel, it forms sulfides, which adversely affect the magnetic properties of the steel of the present invention.

[0024] The phosphorus content in the steel of the present invention is 0-0.15%. Phosphorus reduces ductility in both hot and cold states, particularly due to its tendency to segregate along grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.15%, and preferably below 0.09%.

[0025] The nitrogen content is limited to 0.09% to minimize the precipitation of aluminum nitrides during solidification, which adversely affect the magnetic properties of the steel.

[0026] Titanium is an optional element, and when added to the steel of the present invention, its content is 0-0.1%. It forms titanium nitrides, which appear during solidification of the casting. The titanium content is limited to 0.1% to avoid the formation of titanium nitrides, which adversely affect the magnetic properties of the steel of the present invention. Titanium content below 0.001% has no effect on the steel of the present invention.

[0027] Niobium is present in the steel of the present invention in amounts of 0-0.1% and is suitable for the formation of carbonitrides, which increase the strength of the steel of the present invention through precipitation strengthening. Niobium will also influence the size of microstructural components by precipitating as carbonitrides. However, niobium content above 0.1% is not economically viable due to the saturation effect.

[0028] Vanadium is present in the steel of the present invention in an amount of 0-0.1%, and effectively increases the strength of the steel by forming carbides or carbonitrides, and the upper limit of the content is 0.1% from an economic point of view.

[0029] Chromium is an optional element for the steel of the present invention, and its content is 0-1%. Chromium provides strength to the steel through solid solution strengthening, but when the content exceeds 1%, it degrades the surface quality of the steel.

[0030] Molybdenum is an optional element, with a content of 0-0.5% in the steel of this invention. Molybdenum promotes coarsening of carbides and thus reduces iron loss. At a content of more than 0.5%, the effect of improving iron loss is saturated.

[0031] Tungsten is an optional element, with a content of 0-0.1% in the steel of the present invention. Tungsten has the same effect of coarsening carbides and reducing iron loss as Mo. However, when the addition amount is less than 0.001% by weight, the above-mentioned effect cannot be sufficiently achieved, whereas when it exceeds 0.1% by weight, the effect of improving iron loss is saturated.

[0032] Cobalt is an optional element, accounting for 0-1% of the steel in this invention. Cobalt increases the magnetic moment of the Fe alloy and has the effect of increasing the magnetic flux density and reducing iron loss. However, adding less than 0.01% by weight does not sufficiently achieve the above effects, and adding more than 1% by weight significantly increases raw material costs.

[0033] Arsenic is an optional element, the content of which in the steel of the present invention is 0-0.05%. As is an element that promotes grain boundary segregation and improves texture, thereby reducing iron loss. The above effect is achieved with an addition of at least 0.001% by weight. However, As is an element that causes grain boundary embrittlement, and this negative effect becomes particularly noticeable when added in amounts exceeding 0.05% by weight. Therefore, it is preferable to add As in an amount of 0.001-0.05% by weight.

[0034] Nickel can be added as an optional element in an amount of 0-1% to increase the strength of the steel of the present invention, as well as to improve its strength and relative elongation. However, when the nickel content is more than 1%, it causes a deterioration in ductility. In a preferred embodiment, the nickel content remains less than 0.04%. In another preferred embodiment, the nickel content is at least 0.01%, preferably 0.01-0.04%.

[0035] Copper can be added as an optional element in an amount of 0-1% to increase the strength and elongation of the steel of the present invention. However, if the content exceeds 1%, the surface quality may deteriorate. In a preferred embodiment, the copper content is at least 0.01%.

[0036] Boron is an optional element for the steel of the present invention and may be present in an amount of 0 - 0.05%. Boron forms boronitrides and imparts additional strength to the steel of the present invention when added in an amount of at least 0.0001%.

[0037] Calcium may optionally be present in the steel of the present invention in an amount of 0.001 - 0.01%. Calcium promotes the refining of steel by binding harmful sulfur in globular form, thereby slowing down its negative effects.

[0038] Other elements such as Sn, Pb, or Sb may be added individually or together in the following proportions: Sn ≤ 0.2%, Pb ≤ 0.2%, and Sb ≤ 0.2%. Up to the specified maximum content, these elements enable grain refinement during solidification. In a preferred embodiment, the Sn content is less than 0.04%.

[0039] The rest of the steel is made up of iron and inevitable impurities formed during the manufacturing process.

[0040] 3.85% ≤ Si + Al + Mn ≤ 5.5%

[0041] The non-oriented electrical steel sheet according to the invention necessarily contains silicon, manganese, and aluminum, so that their total content is 3.85-5.5% by weight. When the total content of Si, Mn, and Al is less than 3%, it is impossible to achieve the required mechanical properties along with magnetic properties. However, when the total content of Si, Mn, and Al exceeds 5.5%, the steel becomes hardened, making rolling difficult. Preferably, the content limit of Si, Mn, and Al is 3.9-5.2%, more preferably 4-5%.

[0042] The microstructure of non-oriented electrical steel will be described in detail below, and the percentage composition is given in area fractions.

[0043] The microstructure consists of ferrite. The steel of the present invention has a recrystallized microstructural region occupying 80-100% of the area, with an average grain size of 20-110 μm. The recrystallized structure with a high degree of recrystallization is due to homogeneous enrichment with silicon, which improves the magnetic properties of the steel of the present invention. The controlled grain size ensures the required mechanical properties in both the transverse direction and the rolling direction. Preferably, the degree of recrystallization is 90-100%. Preferably, the average grain size of the present invention is 20-100 μm, more preferably 20-90 μm.

[0044] The steel of the present invention may have a non-recrystallized microstructural region whose proportion is 0 to 20%, the preferred degree of non-recrystallization is 0 to 10%, more preferably 0 to 5%.

[0045] In addition to the above-mentioned microstructure, the microstructure of non-oriented electrical steel does not contain microstructural components such as martensite, bainite, pearlite and cementite.

[0046] The steel according to the invention can be produced by any suitable method. However, it is preferable to use the method according to the invention, which will be described in detail as a non-limiting example.

[0047] This preferred method involves producing a semi-finished product from a steel casting with the chemical composition of the steel according to the invention. Casting can be carried out either as ingots or continuously as thin slabs or thin strips, i.e., with a thickness ranging from approximately 240 mm for slabs to several tens of millimeters for thin strips.

[0048] For example, a slab casting is produced using steel with the chemical composition according to the invention, and then the slab is reheated at a reheating temperature of 1050-1250°C to achieve a uniform temperature throughout the slab. At temperatures below 1050°C, rolling is difficult, and the load on the rolling mill is too great. At temperatures above 1250°C, alloys with high silicon content become very soft and may sag, making them difficult to process.

[0049] The reheating temperature of the slab is preferably 1100 - 1220°C, more preferably 1130 - 1190°C.

[0050] The reheated slab is subjected to hot rolling. The hot rolling end temperature affects the final microstructure of the steel after hot rolling and is 750-950°C. At a rolling end temperature below 750°C, recrystallization is limited, and the microstructure is severely deformed. A temperature above 950°C indicates a higher impurity content in the solid solution and the possible subsequent formation of precipitates, as well as deterioration of the magnetic properties. The preferred rolling end temperature is 840-900°C, more preferably 850-875°C.

[0051] The hot-rolled steel sheet obtained in this way is immediately cooled at a cooling rate of at least 10°C / s to the hot-rolled steel sheet coiling temperature, which also affects the quality of the hot-rolled steel sheet; it is 500 to 750°C. Coiling at a temperature below 500°C will not ensure sufficient recovery, while this metallurgical stage is necessary to maintain magnetic properties. At a temperature above 750°C, a thick oxide layer is formed, which complicates subsequent processing steps such as cold rolling and / or pickling. Preferably, the cooling rate is less than or equal to 200°C / s. Preferably, the coiling temperature is 510 to 560°C, more preferably 530 to 560°C.

[0052] Hot rolled steel sheet is coiled and cooled to room temperature before optional hot annealing.

[0053] The hot-rolled steel sheet may be subjected to an optional step of removing scale generated during hot rolling before optional hot-zone annealing. The hot-rolled sheet is then optionally subjected to hot-zone annealing. Such annealing is carried out at a temperature of 650-1100°C, preferably for at least 10 seconds and no more than 96 hours, with the temperature preferably maintained in the range of 700-1070°C, more preferably 780-1000°C. Thereafter, an optional step of removing scale from the hot-rolled steel sheet may be performed, for example, by pickling.

[0054] This hot rolled steel sheet is then subjected to cold rolling to obtain a cold rolled steel sheet with a reduction ratio of 50-95%. Preferably, with a reduction of 60-95%, more preferably 75-95%.

[0055] After this, the cold-rolled steel sheet is subjected to heat treatment, which will give the steel of the present invention the required mechanical properties and microstructure.

[0056] The cold rolled steel sheet is heated starting from room temperature at a heating rate HR1 of at least 1°C / s to an annealing temperature Tsoak, which is 800-1175°C, preferably 810-1165°C, and more preferably 1000-1150°C. In a preferred embodiment, the heating rate HR1 is at least 2°C / s, more preferably at least 5°C / s.

[0057] The cold rolled steel sheet is kept at Tsoak temperature for 10~5000 seconds to ensure recrystallization of 80~100%.

[0058] Then, the cold-rolled steel sheet is cooled, starting from the temperature Tsoak, the cold-rolled steel sheet is cooled at a cooling rate CR1 in the range of 1-150°C / s to a temperature T1 in the range of 20-300°C. In a preferred embodiment, the cooling rate CR1 is 3-120°C / s. Preferably, the temperature T1 is 20-200°C.

[0059] The cold rolled steel sheet thus obtained has a thickness of 0.29-0.4 mm, more preferably 0.29-0.38 mm, even more preferably 0.29-0.37 mm.

[0060] The cold-rolled steel sheet is then cooled to room temperature to produce a non-oriented electrical steel sheet. The non-oriented electrical steel sheet of the present invention can be further coated with insulation, an organic or inorganic coating, or a combination of both to improve insulation.

[0061] EXAMPLES

[0062] The following tests, examples, explanatory material, and tables presented in the description are not limiting and are to be considered for illustrative purposes only. They illustrate the advantageous features of the present invention.

[0063] Steel sheets of different steel compositions are presented in Table 1, where the steel sheets were manufactured according to the process parameters specified in Table 2. Table 3 presents the evaluation results of the obtained properties.

[0064] All steels in Table 1 have a nitrogen content of less than 0.09%.

[0065] Table 2 shows the hot rolling and annealing parameters applied to the cold-rolled steel sheets to impart the required mechanical and magnetic properties of non-oriented electrical steel to the steels in Table 1. All inventive steels I1 to I3 are cooled after hot rolling at a cooling rate of 15°C / s, and after winding, all inventive steels I1 to I3 are subjected to hot zone annealing at a temperature of 800°C for 12 hours. In addition, for the inventive samples, the cold rolling reduction is 80%, and the heating rate HR1 to the annealing holding temperature is 5°C / s. The T1 temperature for all inventive samples is 25°C, and the cooling rate CR1 is 5°C / s.

[0066] All steels produced according to the parameters specified in Table 2 have a recrystallized microstructure % greater than 95% recrystallization and a grain size of 20 - 110 μm.

[0067] Table 1

[0068]

[0069] Table 2

[0070] Trials Reheating (°C) FRT (Final Rolling Temperature) (°C) Winding (°C) Compression (%) Tsoak (°C) Annealing time (s) Thickness after cold rolling (mm) I1 1150 860 545 80 1020 40 0,295 I2 1150 860 545 80 1020 40 0,303 I3 1150 860 545 80 1020 40 0,309

[0071] Table 3 presents the results of various mechanical tests carried out in accordance with the standards. The tensile strength, total elongation, and yield strength are measured in accordance with NF EN ISO 6892-1, and the magnetic properties J50 and total iron loss at 1 T and 400 Hz are measured in accordance with IEC 60404-2. Eddy current losses are calculated using the Bertotti method, published under the title “General Properties of Power Losses in Soft Ferromagnetic Materials” by Giorgio Berttoti published in IEEE TRANSACTIONS ON MAGNETICS, Vol. 24, No. 1 of January”. Equation 2 defines the classical losses, which are denoted as (P class ), which for the purposes of the present invention are referred to as eddy current losses.

[0072] The average grain size of the recrystallized microstructure was measured according to ASTM E112 96(02) by the line intersection method.

[0073]

Claims

1. A sheet of non-oriented electrical steel having a thickness of 0.29 to 0.4 mm and a composition comprising the following elements, wt.%: 0.0001 ≤ carbon ≤ 0.007, 0.15 ≤ manganese ≤ 0.2, 3 ≤ silicon ≤ 3.6, 0.7 ≤ aluminum ≤ 1.3, phosphorus ≤ 0.15, sulfur ≤ 0.006, nitrogen ≤ 0.09, where 3.85 ≤ Si+Al+Mn ≤ 5.1, 0.01 ≤ copper ≤ 1, 0.01 ≤ nickel ≤ 1, the remainder consists of iron and inevitable impurities formed during the manufacturing process, the microstructure of the said steel sheet consists of ferrite and includes in area fractions 80-100% recrystallized microstructure and 0-20% non-recrystallized microstructure, while the average grain size of the recrystallized microstructure is 20-110 μm, and the proportion of eddy current losses in the total losses in iron, measured at 1 T and 400 Hz, is 35-55% when calculated by the Bertotti method, and at the same time has a magnetic polarization at 5000 A / m (J50) of 1.635-1.670 T.

2. A sheet of non-oriented electrical steel according to item 1, the composition of which includes 3.1-3.55% silicon.

3. A sheet of non-oriented electrical steel according to claim 1 or 2, the composition of which includes 0.002-0.007% carbon.

4. A sheet of non-oriented electrical steel according to any one of paragraphs 1-3, the composition of which includes 0.8-1.1% aluminum.

5. A sheet of non-oriented electrical steel according to any one of paragraphs 1-4, the composition of which includes 0.16-0.2% manganese.

6. A sheet of non-oriented electrical steel according to any one of paragraphs 1-5, in which the content of non-recrystallized microstructure is 0-10%.

7. A sheet of non-oriented electrical steel according to any one of paragraphs 1-6, in which the content of recrystallized microstructure is 90-100%.

8. A non-oriented electrical steel sheet according to any one of claims 1 to 7, wherein the tensile strength of said steel sheet is at least 530 MPa in both the transverse direction and the rolling direction.

9. A sheet of non-oriented electrical steel according to any one of paragraphs 1 to 8, having a yield strength of 410 MPa or more in both the transverse direction and the rolling direction.

10. A sheet of non-oriented electrical steel according to any one of claims 1 to 9, having a total elongation of at least 10% in both the transverse direction and the rolling direction.

11. A method for producing a sheet of non-oriented electrical steel according to any one of paragraphs 1-10, comprising the following sequential stages: - preparation of a semi-finished product from steel having a composition according to any of paragraphs 1-5; - reheating the specified semi-finished product to a temperature of 1050-1250°C; - rolling of the specified semi-finished product at a hot rolling end temperature of 750-950°C to obtain a hot-rolled steel sheet; - then the hot rolled steel sheet is cooled immediately after completion of hot rolling from the hot rolling end temperature to the winding temperature of 500-750°C with a cooling rate of at least 10°C / s; - then winding of hot-rolled steel sheet in the winding temperature range of 500-750°C; - annealing in the hot zone of hot-rolled steel sheet at a temperature of 650-1100°C for 10 seconds to 96 hours; - cold rolling of the said hot-rolled steel sheet with a degree of compression of 50-95% to obtain a cold-rolled steel sheet; - then annealing the said cold-rolled steel sheet, wherein the heating for annealing is started from room temperature to an annealing temperature Tsoak of 800-1175°C, with a heating rate HR1 of at least 1°C / s; - then annealing is carried out at annealing temperature for 10-5000 seconds; - then the cold-rolled steel sheet is cooled, starting from the annealing temperature to the temperature T1, which is in the range of 300-20°C, with a cooling rate CR1 of 1-150°C / s; - then cooling to room temperature to obtain a sheet of non-oriented electrical steel.

12. The method according to claim 11, wherein the annealing temperature Tsoak is 810-1165°C.

13. The method according to item 11 or 12, wherein the temperature T1 is 200-20°C.

14. The method according to any one of paragraphs 11-13, wherein the cooling rate of CR1 is 3-120°C / s.

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