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

Abstract

A non-oriented electrical steel sheet having a composition comprising the following elements: 0.0001% < = carbon < = 0.008%, 0.1% < = manganese < = 1.2%, 2.8% < = silicon < = 3.5%, 0.4% < = aluminum < = 1.2%, 0% < = phosphorus < = 0.15%, 0% < = sulfur < = 0.006%, 0% < = nitrogen < = 0.09%, with 3.5% < = Si + Al + Mn < = 5.5%, 0% < = niobium < = 0.1%, 0% < = titanium < = 0.1%, 0% < = vanadium < = 0.1%, 0% < = chromium < = 1%, 0% < = molybdenum < = 0.5%, 0% < = tungsten < = 0.1%, 0% < = cobalt < = 1%, 0% < = arsenic < = 0.05%, 0.001% < = calcium < = 0.01%, 0% < = copper < = 1%, 0% < = nickel < = 1%, 0% < = boron < = 0.05%, 0% < = lead < = 0.2%, 0% < = tin < = 0.2%, 0% < = antimony < = 0.2%, the remainder consisting of iron and unavoidable impurities resulting from processing, the microstructure of the steel sheet is composed of ferrite, and includes, in area fraction, 80% to 100% of a recrystallized microstructure and 0% to 20% of a non-recrystallized microstructure, in which the average grain size of the recrystallized microstructure is 20 [mu] m to 50 [mu] m.

Description

Technical Field

[0001] The present invention relates to non-oriented electrical steel sheets and a method for manufacturing the same. Specifically, the present invention relates to such non-oriented electrical steel sheets and a method for manufacturing the same, the non-oriented electrical steel sheets having improved magnetic properties such as polarization while having similar mechanical properties in both the transverse direction and the rolling direction at the same time. Background Art

[0002] Due to the improvement of energy-saving achievements of electrical equipment globally, a large amount of research and development work has been invested in the higher performance characteristics required for non-oriented electrical steel sheets used as core materials for rotating machines. Especially recently, as motors for electric vehicles and the like, there is a high demand for small high-power motors. Such electric vehicle motors are designed to enable high-speed rotation, thereby obtaining high torque.

[0003] Early research and development in the field of high-strength non-oriented electrical steel have produced several methods for producing high-strength non-oriented electrical steel. Some of these methods are listed herein to clearly understand the present invention:

[0004] EP2883975 is a high-strength electrical steel sheet suitable as a rotor material for high-speed motors, stably having high strength and also having excellent magnetic properties. This high-strength electrical steel sheet can be obtained by setting its chemical composition to contain by mass%: C: 0.005% or less, Si: greater than 3.5% and 4.5% or less, Mn: 0.01% or greater and 0.10% or less, Al: 0.005% or less, Ca: 0.0010% or greater and 0.0050% or less, S: 0.0030% or less, and N: 0.0030% or less, Ca / S being 0.80 or greater, with the balance being Fe and incidental impurities, and by setting the plate thickness to 0.40 mm or less, setting the non-recrystallized deformed microstructure to 10% or greater and 70% or less, setting the tensile strength (TS) to 600 MPa or greater, and setting the iron loss W 10 / 400 to 30 W / kg or less. However, the steel of EP2883975 cannot exhibit tensile strength characteristics and yield strength characteristics in both the transverse direction and the rolling direction.

[0005] EP3875612 is a non-oriented electrical steel sheet having a predetermined chemical composition satisfying the expression [Si + 0.5×Mn ≥ 4.3], and the average grain size of the base metal is greater than 40 μm and 120 μm or less. EP3875612 cannot exhibit tensile strength characteristics and yield strength characteristics in both the transverse direction and the rolling direction. Summary of the Invention

[0006] An object of the present invention is to solve these problems by manufacturing a non-oriented electrical steel sheet having the following simultaneously:

[0007] - The ultimate tensile strength is 580 MPa or greater in both the transverse direction and the rolling direction, and preferably greater than 600 MPa in both the transverse direction and the rolling direction.

[0008] - The yield strength is 450 MPa or greater in both the transverse direction and the rolling direction, and preferably 470 MPa or greater in both the transverse direction and the rolling direction.

[0009] - The total elongation is 20% or greater in both the transverse direction and the rolling direction.

[0010] - The magnetic polarization at 5000 A / m (J50) is greater than 1.60 T.

[0011] Preferably, such a steel can also have good applicability to rolling and have good weldability and coatability.

[0012] Another object of the present invention is also to enable a method for manufacturing these sheets to be obtained that is compatible with conventional industrial applications and robust to changes in manufacturing parameters.

[0013] The above objects and other advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention. Detailed Description

[0014] The chemical composition of the non-oriented electrical steel contains the following elements:

[0015] Carbon is present in the steel of the present invention in an amount of 0.0001% to 0.008%. Carbon is a precipitate-forming element and is therefore harmful to the magnetic properties of the steel of the present invention. Therefore, the amount of carbon present in the steel of the present invention is 0.0001% to 0.008%. Since carbon promotes magnetic aging, the preferred content of carbon according to the present invention is 0.0001% to 0.006%, and more preferably 0.0001% to 0.005%.

[0016] The manganese content of the steel of the present invention is 0.1% to 1.2%. Manganese provides solid solution strengthening and reduces iron loss by increasing the specific resistance. When the amount of manganese added is higher than 1.2%, the magnetic flux density may be significantly reduced, and the recrystallization of the steel during annealing will be hindered. The preferred limit of the presence of manganese is 0.1% to 1.1%, and more preferably 0.1% to 1.05%.

[0017] The silicon content of the steel of the present invention is 2.8% to 3.5%. Silicon is an element that helps to increase strength through solid solution strengthening and is a key element for reducing the eddy current loss of iron loss by increasing the specific resistance of the steel. The above effects require a minimum silicon content of at least 2.8%. However, when the amount of silicon content exceeds 3.5%, rolling becomes difficult and the magnetic induction of the steel will be significantly reduced. The preferred limit of the presence of silicon is 2.9% to 3.4%, and more preferably 3% to 3.4%.

[0018] The aluminum content is 0.4% to 1.2%. Aluminum increases the resistivity of the material and can effectively reduce the iron loss of the steel. When the aluminum content is present at more than 1.2%, the magnetic induction of the steel will be significantly reduced, and this is also harmful to the cold rolling rollability of the steel of the present invention. The preferred limit of the presence of aluminum is 0.7% to 1.2%, and more preferably 0.8% to 1.1%.

[0019] Sulfur is not an essential element but may be included as an impurity in the steel. From the perspective of the present invention, the sulfur content is preferably as low as possible, but from the perspective of manufacturing cost, the sulfur content is 0.006% or less. In addition, if there is more sulfur in the steel, it will combine to form sulfides, which are harmful to the magnetic properties of the present invention.

[0020] The phosphorus component of the steel of the present invention is 0% to 0.15%. Phosphorus reduces hot ductility and cold ductility, especially due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.15%, and preferably less than 0.09%.

[0021] Nitrogen is limited to 0.09% to minimize the precipitation of aluminum nitride during solidification (which is harmful to the magnetic properties of the steel).

[0022] Titanium is an optional element, and when added to the steel of the present invention at 0% to 0.1%, it forms titanium nitride that appears during the solidification of the casting product. Therefore, the amount of titanium is limited to 0.1% to avoid forming titanium nitride that is harmful to the magnetic properties of the steel of the present invention. When the titanium content is less than 0.001%, no effect is imparted to the steel of the present invention.

[0023] Niobium is present in the steel of the present invention at 0% to 0.1% and is suitable for forming carbonitrides to increase the strength of the steel of the present invention through precipitation hardening. Niobium will also affect the size of the microstructure components by precipitating as carbonitrides. However, due to the saturation effect, a niobium content higher than 0.1% is not economically interesting.

[0024] Vanadium is present in the steel of the present invention at 0% to 0.1% and effectively enhances the strength of the steel by forming carbides or carbonitrides. From an economic perspective, the upper limit is 0.1%.

[0025] Chromium is an optional element of the steel of the present invention, ranging from 0% to 1%. Chromium provides strength to the steel through solid solution strengthening, but when used at more than 1%, it impairs the surface finish of the steel.

[0026] Molybdenum is an optional element of the steel of the present invention, ranging from 0% to 0.5%.

[0027] Mo has the effect of coarsening carbides and thus reducing iron loss. When it exceeds 0.5%, the effect of improving iron loss saturates.

[0028] W: 0% to 0.1%

[0029] Like Mo, W has the effect of coarsening carbides and reducing iron loss. However, when the addition amount is less than 0.001% by mass, the above effects cannot be fully obtained, and when it exceeds 0.1% by weight, the effect of improving iron loss saturates.

[0030] Cobalt 0% to 1%

[0031] Cobalt is an element that increases the magnetic moment of the Fe alloy and has the effects of increasing the magnetic flux density and reducing iron loss. However, when the addition amount is less than 0.01% by weight, the above effects cannot be fully obtained, and when it exceeds 1% by weight, the raw material cost increases significantly.

[0032] As: 0% to 0.05%

[0033] As is a grain boundary segregation element and has the effect of improving the structure and thus reducing iron loss. The above effects are obtained by adding not less than 0.001% by weight. However, As is an element that causes grain boundary embrittlement, and when it is added at more than 0.05% by weight, this adverse effect becomes particularly significant. Therefore, As is preferably added in the range of 0.001% to 0.05% by weight.

[0034] Nickel can be added as an optional element in an amount of 0% to 1% to increase the strength of the steel of the present invention and improve its strength and elongation. However, when its content is higher than 1%, nickel causes ductility deterioration.

[0035] Copper can be added as an optional element in an amount of 0% to 1% to increase the strength and elongation of the steel of the present invention. However, when its content is higher than 1%, it may deteriorate the surface appearance.

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

[0037] Calcium may optionally be present in the steel of the present invention and may be from 0.001% to 0.01%. Calcium contributes to the refining of the steel by binding harmful sulfur contents into a globular form, thereby impeding the harmful action of sulfur.

[0038] Other elements such as Sn, Pb or Sb may be added individually or in combination in the following proportions: Sn ≤ 0.2%, Pb ≤ 0.2% and Sb ≤ 0.2%. Up to the maximum content levels shown, these elements enable grain refinement during solidification. The remainder of the composition of the steel consists of iron and inevitable impurities resulting from processing.

[0039] 3.5% ≤ Si + Al + Mn ≤ 5.5%

[0040] The non-oriented electrical steel sheet according to the present invention must contain silicon, manganese and aluminum such that the total content is from 3.5 wt% to 5.5 wt%. When the total content of Si, Mn and Al is less than 3.5%, it is not possible to achieve the mechanical properties as well as the magnetic properties in both the transverse direction and the rolling direction. However, when the total content of Si, Mn and Al exceeds 5.5%, the steel hardens and rolling becomes difficult.

[0041] The microstructure of the non-oriented electrical steel will now be described in detail, all percentages being area fractions.

[0042] The microstructure consists of ferrite. The steel of the present invention has a recrystallized microstructure region of 80% to 100% by area fraction, wherein the average grain size of the grains is from 20 μm to 50 μm. The recrystallized structure with a high degree of recrystallization is due to uniform silicon enrichment, which improves the magnetic properties of the steel of the present invention. The controlled grain size ensures the mechanical properties in both the transverse direction and the rolling direction. The preferred degree of recrystallization is from 90% to 100%. The preferred average grain size of the present invention is from 20 μm to 45 μm, and more preferably from 20 μm to 40 μm.

[0043] The steel of the present invention may have a non-recrystallized microstructure region of 0% to 20% by area fraction, the preferred degree of recrystallization being from 0% to 10%, and more preferably from 0% to 5%.

[0044] In addition to the above microstructure, the microstructure of the non-oriented electrical steel does not contain microstructure components such as martensite, bainite, pearlite and cementite.

[0045] The steel according to the present invention can be manufactured by any suitable method. However, as a non-limiting example, the method according to the present invention which will be detailed is preferably used.

[0046] Such a preferred method involves providing a semi-finished casting of steel having the chemical composition of the steel according to the present invention. The casting can be made into an ingot or continuously into the form of a slab or strip, i.e., with a thickness ranging from about 220 mm for a slab to several tens of millimeters for a strip.

[0047] For example, a casting in the form of a slab is cast with the chemical composition according to the present invention and then reheated. The reheating temperature of the slab is 1050 °C to 1250 °C until the temperature is uniform throughout the slab. Below 1050 °C, rolling becomes difficult and the forces on the rolling mill will be too high. Above 1250 °C, high-silicon steel grades become very soft and may show a slight sag, thus becoming difficult to handle.

[0048] The reheated slab is subjected to hot rolling, where the finishing temperature of the hot rolling affects the final hot-rolled microstructure and occurs between 750 °C and 950 °C. When the finishing temperature is below 750 °C, recrystallization is restricted and the microstructure is highly deformed. Above 950 °C will mean more impurities in the solid solution and possible precipitation and deterioration of magnetic properties therefrom.

[0049] Then, the hot-rolled steel sheet thus obtained is immediately cooled to the coiling temperature of the hot-rolled steel sheet at a cooling rate of at least 10 °C / second, which also affects the hot-rolled steel sheet; it occurs between 500 °C and 750 °C. Coiling at a temperature below 500 °C will not allow sufficient recovery, and this metallurgical step is necessary for magnetic properties. Above 750 °C, a thick oxide layer will appear, which will cause difficulties in subsequent processing steps such as cold rolling and / or pickling. Preferably, the cooling rate will be less than or equal to 200 °C / second.

[0050] Then the coiled hot-rolled steel sheet is cooled to room temperature and then subjected to optional hot-strip annealing.

[0051] The hot-rolled steel sheet can be subjected to an optional scale-removing step to remove the scale formed during hot rolling before the optional hot-strip annealing. Then the hot-rolled sheet is subjected to optional hot-strip annealing, such hot-strip annealing being carried out at a temperature of 650 °C to 1100 °C, preferably for at least 10 seconds and not exceeding 96 hours, the temperature preferably being maintained at 700 °C to 1070 °C, and more preferably 720 °C to 1050 °C. Thereafter, the optional scale-removing step of such a hot-rolled steel sheet can be carried out by pickling of such a sheet, for example.

[0052] Then the hot-rolled steel sheet is subjected to cold rolling with a thickness reduction rate of 35% to 90% to obtain a cold-rolled steel sheet.

[0053] Thereafter, the cold-rolled steel sheet is heat-treated, which will impart the necessary mechanical properties and microstructure to the steel according to the present invention.

[0054] Then, the cold-rolled steel sheet is heated. The heating starts from room temperature, and the cold-rolled steel sheet is heated to an annealing temperature T of 800°C to 875°C, preferably 810°C to 865°C, at a heating rate HR1 of at least 1°C / second. 均热 In a preferred embodiment, the heating rate HR1 for heating is at least 2°C / second, and more preferably at least 5°C / second.

[0055] The cold-rolled steel sheet is held at T 均热 for a time of 10 seconds to 5000 seconds to ensure 80% to 100% recrystallization.

[0056] Then, the cold-rolled steel sheet is cooled. The cooling starts from T 均热 and the cold-rolled steel sheet is cooled to a temperature T1 in the range of 20°C to 300°C at a cooling rate CR1 of 1°C / second to 150°C / second. In a preferred embodiment, the cooling rate CR1 is 3°C / second to 120°C / second. The preferred temperature of T1 is 20°C to 200°C.

[0057] Then, the cold-rolled steel sheet is cooled to room temperature to obtain a non-oriented electrical steel sheet.

[0058] The non-oriented electrical steel sheet of the present invention can optionally be coated with an insulating organic coating or an inorganic coating or a combination of both to improve the insulation.

[0059] Examples

[0060] The following tests, examples, graphical examples, and tables presented herein are non-limiting in nature and must be considered solely for illustrative purposes and will show the advantageous features of the present invention.

[0061] Steel sheets made of steels with different compositions are summarized in Table 1, where the steel sheets are produced according to the process parameters specified in Table 2, respectively. Thereafter, Table 3 summarizes the microstructures of the steel sheets obtained during the test, and Table 4 summarizes the evaluation results of the properties obtained.

[0062]

[0063]

[0064] Table 3 illustrates the results of tests in terms of area fraction for determining the microstructures of both the inventive steel and the reference steel on different microscopes such as a scanning electron microscope according to the standard, and the grain sizes of the inventive examples and the reference examples are measured by the linear intercept method according to ASTM E112 standard.

[0065] The results are noted herein:

[0066] Table 3:

[0067] Test Recrystallized Non-recrystallized Grain size I1 100 0 26 I2 100 0 30 I3 100 0 32 I4 100 0 29 I5 100 0 26 R1 100 0 <![CDATA 113 > R2 100 0 <![CDATA 100 > R3 100 0 <![CDATA 107 > R4 100 0 <![CDATA 101 >

[0068] I = According to the present invention; R = reference; underlined values: not according to the present invention.

[0069] Table 4

[0070] Summarizes the results of multiple mechanical tests carried out according to the standards. For the tests, the ultimate tensile strength and the yield strength were tested according to the NF EN ISO 6892-1 standard, and the J50 magnetic properties were measured according to the NF EN 60404-2 standard.

[0071]

[0072] I = According to the present invention; R = reference; underlined values: not according to the present invention.

Claims

1. An non-oriented electrical steel sheet, in weight percentage, its composition contains the following elements: 0.0001% ≤ Carbon ≤ 0.008% 0.1% ≤ Manganese ≤ 1.2% 2.8% ≤ Silicon ≤ 3.5% 0.4% ≤ Aluminum ≤ 1.2% 0% ≤ Phosphorus ≤ 0.15% 0% ≤ Sulfur ≤ 0.006% 0% ≤ Nitrogen ≤ 0.09% where 3.5% ≤ Si + Al + Mn ≤ 5.5% and may contain one or more of the following optional elements: 0% ≤ Niobium ≤ 0.1% 0% ≤ Titanium ≤ 0.1% 0% ≤ Vanadium ≤ 0.1% 0% ≤ Chromium ≤ 1% 0% ≤ Molybdenum ≤ 0.5% 0% ≤ Tungsten ≤ 0.1% 0% ≤ Cobalt ≤ 1% 0% ≤ Arsenic ≤ 0.05% 0.001% ≤ Calcium ≤ 0.01% 0% ≤ Copper ≤ 1% 0% ≤ Nickel ≤ 1% 0% ≤ Boron ≤ 0.05% 0% ≤ Lead ≤ 0.2% 0% ≤ Tin ≤ 0.2% 0% ≤ Antimony ≤ 0.2% The remaining part is composed of iron and inevitable impurities caused by processing. The microstructure of the steel sheet is composed of ferrite, and contains, in area fraction: 80% to 100% of recrystallized microstructure, 0% to 20% of non-recrystallized microstructure, where the average grain size of the recrystallized microstructure is 20 microns to 50 microns.

2. The non-oriented electrical steel sheet according to claim 1, wherein the composition contains 2.9% to 3.4% of silicon.

3. The non-oriented electrical steel sheet according to claim 1 or 2, wherein the composition contains 0.0001% to 0.006% of carbon.

4. The non-oriented electrical steel sheet according to claims 1 to 3, wherein the composition contains 0.7% to 1.2% of aluminum.

5. The non-oriented electrical steel sheet according to any one of claims 1 to 4, wherein the composition contains 0.1% to 1.1% of manganese.

6. The non-oriented electrical steel sheet according to any one of claims 1 to 5, wherein the amount of non-recrystallized microstructure is 0% to 10%.

7. The non-oriented electrical steel sheet according to any one of claims 1 to 6, wherein the amount of recrystallized microstructure is 90% to 100%.

8. The non-oriented electrical steel sheet according to any one of claims 1 to 7, wherein the tensile strength of the steel sheet in both the transverse direction and the rolling direction is at least 580 MPa.

9. The non-oriented electrical steel sheet according to any one of claims 1 to 8, wherein the total elongation of the steel sheet in both the transverse direction and the rolling direction is at least 20%.

10. A method for producing a non-oriented electrical steel sheet, comprising the following sequential steps: - Providing a steel composition according to any one of claims 1 to 5; - Reheating the semi-finished product to a temperature of 1050 °C to 1250 °C; - Rolling the semi-finished product to obtain a hot-rolled steel sheet, wherein the hot-rolling finish rolling temperature should be 750 °C to 950 °C; - Immediately cooling the hot-rolled steel sheet after the hot rolling ends; - Then cooling the hot-rolled steel sheet from the end of hot rolling to a coiling temperature range of 500 °C to 750 °C at a cooling rate of at least 10 °C / second; - Thereafter coiling the hot-rolled steel sheet within the coiling temperature range of 500 °C to 750 °C; - Optionally, perform a scale removal process on the hot-rolled steel sheet; - Optionally, perform hot-strip annealing on the hot-rolled steel sheet at a temperature of 650°C to 1100°C for a time of 10 seconds to 96 hours; - Optionally, perform a scale removal process on the hot-rolled steel sheet; - Cold-roll the hot-rolled steel sheet at a reduction rate of 35% to 90% to obtain a cold-rolled steel sheet; - Subsequently, annealing is performed on the cold-rolled steel sheet, wherein the heating for annealing starts from room temperature to an annealing temperature range T of 800 °C to 875 °C 均热 , wherein the heating rate HR1 is at least 1 °C / second; - Then perform annealing at the annealing temperature for a time of 10 seconds to 5000 seconds; - Then cool the cold-rolled steel sheet, the cooling starting from the annealing temperature to a temperature T1 of 300°C to 20°C, where the cooling rate CR1 is 1°C / second to 150°C / second; - Then cool to room temperature to obtain a non-oriented electrical steel sheet.

11. The method according to claim 10, wherein the T for annealing 均热 temperature is from 810 °C to 865 °C.

12. The method according to claim 10 or 11, wherein the temperature T1 is 200°C to 20°C.

13. The method according to any one of claims 10 to 12, wherein the cooling rate CR1 is 3°C / second to 120°C / second.

14. Use of the steel sheet according to any one of claims 1 to 9 or the steel sheet produced by the method according to claims 10 to 13 for manufacturing components of an electric vehicle or an electric machine.

15. A vehicle or an electric machine, comprising a component obtained according to claim 14.

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