Non-oriented electrical steel and method for manufacturing the same

Abstract

Non-oriented electrical steel sheet containing 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%, 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%, 0.0 A non-oriented electrical steel sheet having a composition comprising 0.1%≦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%, with the remainder consisting of iron and unavoidable impurities caused by processing, wherein the microstructure of the steel sheet is made of ferrite and comprises, in area fractions, 80% to 100% of a recrystallized microstructure and 0% to 20% of a non-recrystallized microstructure, and the average grain size of the recrystallized microstructure is 20 microns to 50 microns.

Description

[Technical Field]

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

[0002] Therefore, vigorous research and development efforts have been made, and due to the global increase in the achievement of energy conservation in electrical appliances, higher performance characteristics are required for using non-oriented electrical steel sheets as iron core materials for rotating equipment. In recent years, there has been an increasing demand for small, high-power motors, particularly for use in electric vehicles. Such electric vehicle motors are designed to enable high-speed rotation and thereby obtain high torque.

[0003] Previous research and development in the field of high strength non-oriented electrical steels has resulted in several methods for producing high strength non-oriented electrical steels, some of which are listed herein for a thorough understanding of the present invention.

[0004] European Patent No. 2883975 discloses a high-strength electrical steel sheet suitable as a rotor material for high-speed motors, which has consistently high strength and excellent magnetic properties, and which contains, by mass%, C: 0.005% or less, Si: more than 3.5% and 4.5% or less, Mn: 0.01% or more and 0.10% or less, Al: 0.005% or less, Ca: 0.0010% or more and 0.0050% or less, S: 0.0030% or less, and N: 0.0030% or less, by setting the chemical composition so that Ca / S is 0.80 or more, with the remainder being Fe and incidental impurities, and by setting the thickness to 0.40 mm or less, non-recrystallized deformation microstructure to 10% or more and 70% or less, tensile strength (TS) to 600 MPa or more, iron loss W 10 / 400However, the steel of EP 2 883 975 fails to demonstrate tensile and yield strength properties both transverse and in the rolling direction.

[0005] EP 3875612 discloses a non-oriented electrical steel sheet having a predetermined chemical composition that satisfies the formula [Si + 0.5 × Mn ≧ 4.3] and an average grain size of the main component metals greater than 40 μm and less than 120 μm. EP 3875612 fails to demonstrate tensile and yield strength properties in both the transverse and rolling directions. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] European Patent No. 2883975 [Patent Document 2] European Patent No. 3875612 Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to an ultimate tensile strength of 580 MPa or more in both the transverse and the rolling direction, preferably more than 600 MPa in both the transverse and the rolling direction; Yield strength of 450 MPa or more in both the transverse direction and the rolling direction, preferably 470 MPa or more in both the transverse direction and the rolling direction -Total elongation of 20% or more in both the transverse and the rolling direction Magnetic polarization at 5000 A / m (J50) exceeding -1.60 T The present invention aims to solve these problems by producing a non-oriented electrical steel sheet that simultaneously has the above properties.

[0008] Preferably, such steels have good weldability and coatability and can have good suitability for rolling.

[0009] Another object of the invention is also to make available a method for manufacturing these plates that is compatible with conventional industrial applications and at the same time is robust to changes in manufacturing parameters. [Means for solving the problem]

[0010] The above objects and other advantages of the present invention will become more apparent from the detailed description of preferred embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The chemical composition of non-oriented electrical steel includes the following elements:

[0012] The carbon content in the steel of the present invention is 0.0001% to 0.008%. Carbon is a precipitate-forming element and is therefore detrimental to the magnetic properties of the steel. Therefore, the carbon content in the steel is 0.0001% to 0.008%. Since carbon promotes magnetic aging, the preferred carbon content according to the present invention is 0.0001% to 0.006%, more preferably 0.0001% to 0.005%.

[0013] The manganese content of the steel of the present invention is 0.1% to 1.2%. Manganese provides solid solution strengthening and reduced core loss by increasing the resistivity. If the manganese content exceeds 1.2%, the magnetic flux density can be significantly reduced and recrystallization of the steel is inhibited during annealing. The preferred limits for the presence of manganese are 0.1% to 1.1%, more preferably 0.1% to 1.05%.

[0014] The silicon content of the steel of the present invention is 2.8% to 3.5%. Silicon is an element that contributes to increasing strength through solid solution strengthening and is a key element for reducing eddy current core losses by increasing the resistivity of the steel. The above-mentioned effects require a minimum silicon content of at least 2.8%. However, silicon contents in excess of 3.5% make rolling difficult and significantly reduce the magnetic induction of the steel. The preferred limits for the presence of silicon are 2.9% to 3.4%, more preferably 3% to 3.4%.

[0015] The aluminum content is 0.4% to 1.2%. Aluminum increases the electrical resistivity of the material and can effectively reduce the iron loss of the steel. If the aluminum content exceeds 1.2%, the magnetic induction of the steel is significantly reduced and is also detrimental to the rollability of the steel during cold rolling of the steel of the present invention. The preferred limit for the presence of aluminum is 0.7% to 1.2%, more preferably 0.8% to 1.1%.

[0016] Although sulfur is not an essential element, it may be contained in the steel as an impurity, and from the viewpoint of the present invention, the sulfur content is preferably as low as possible, but not more than 0.006%, from the viewpoint of production costs. Furthermore, if higher sulfur is present in the steel, the sulfur combines to form sulfides that are detrimental to the magnetic properties of the present invention.

[0017] The phosphorus content of the steel of the present invention is between 0% and 0.15%, and phosphorus reduces hot and cold ductility due to its tendency to segregate, especially at grain boundaries, or to co-segregate with manganese, and for these reasons its content is limited to 0.15%, preferably less than 0.09%.

[0018] Nitrogen is limited to 0.09% to minimize the precipitation of aluminum nitrides during solidification, which is detrimental to the magnetic properties of the steel.

[0019] Titanium is an optional element, and when added to the steel of the present invention, the content is 0% to 0.1%. Titanium forms titanium nitrides that appear during solidification of the casting. For this reason, the amount of titanium is limited to 0.1% to avoid the formation of titanium nitrides, which are detrimental to the magnetic properties of the steel of the present invention. Titanium contents below 0.001% have no effect on the steel of the present invention.

[0020] Niobium is present in the steel of the present invention in an amount of 0% to 0.1% and is suitable for forming carbonitrides to increase the strength of the steel of the present invention by precipitation hardening. Niobium also influences the size of the microstructure by precipitating as carbonitrides. However, niobium contents above 0.1% are not economically interesting due to saturation effects.

[0021] Vanadium is present in the steel of the present invention in an amount of 0 to 0.1%, and is effective in increasing the strength of the steel by forming carbides or carbonitrides, with the upper limit being 0.1% from an economical viewpoint.

[0022] Chromium is an optional element for the steel of the present invention, between 0% and 1%. Chromium provides strength to the steel through solid solution strengthening, but if used in excess of 1%, it will detract from the surface finish of the steel.

[0023] Molybdenum is an optional element that constitutes between 0% and 0.5% of the steel of the present invention. Mo has the effect of coarsening carbides, thereby reducing iron loss. If Mo exceeds 0.5%, the effect of improving iron loss saturates.

[0024] W: 0 to 0.1% Like Mo, W has the effect of coarsening carbides and reducing iron loss. However, if the amount added is less than 0.001% by mass, this effect cannot be fully achieved, while if the amount added exceeds 0.1% by mass, the effect of improving iron loss saturates.

[0025] Cobalt 0-1% Cobalt is an element that increases the magnetic moment of an Fe alloy, thereby increasing magnetic flux density and reducing iron loss. However, if the amount added is less than 0.01 wt%, these effects cannot be fully achieved, while if the amount added exceeds 1 wt%, the raw material cost increases significantly.

[0026] As: 0 to 0.05% As is a grain boundary segregation element, and has the effect of improving texture and thereby reducing iron loss. The above effect is obtained by adding 0.001 wt% or more. However, As is an element that causes grain boundary embrittlement, and this adverse effect becomes particularly significant when added in excess of 0.05 wt%. Therefore, it is preferable that As be added in the range of 0.001 to 0.05 wt%.

[0027] Nickel may 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 to improve its strength and elongation, however, if its content exceeds 1%, nickel causes a decrease in ductility.

[0028] Copper may 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, if its content exceeds 1%, copper may deteriorate the surface morphology.

[0029] Boron is an optional element for the steel of the present invention and may be present at 0% to 0.05%. Boron forms boron nitrides and imparts additional strength to the steel of the present invention when added in an amount of at least 0.0001%.

[0030] Calcium may optionally be present in the steel of the present invention in an amount of 0.001% to 0.01%. Calcium contributes to the refining of the steel by binding harmful sulfur inclusions in a spherical form, thereby retarding the harmful effects of sulfur.

[0031] Other elements such as Sn, Pb or Sb can 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 indicated, these elements allow for grain refinement during solidification. The remainder of the steel composition consists of iron and unavoidable impurities resulting from processing.

[0032] 3.5%≦Si+Al+Mn≦5.5% The non-oriented electrical steel sheet according to the present invention must contain silicon, manganese, and aluminum in a total content of 3.5% to 5.5% by weight. If the total content of Si, Mn, and Al is less than 3.5%, it will be impossible to achieve the desired mechanical properties both transverse to and in the rolling direction, along with the desired magnetic properties. However, if the total content of Si, Mn, and Al exceeds 5.5%, the steel will harden, making rolling difficult.

[0033] The microstructure of non-oriented electrical steels will now be described in detail, with all percentages being area fractions.

[0034] The microstructure is made up of ferrite. The steel of the present invention has an area fraction of 80% to 100% recrystallized microstructure regions with grains having an average grain size of 20 microns to 50 microns. The recrystallized structure with a high degree of recrystallization is due to homogeneous silicon enrichment, which improves the magnetic properties of the steel of the present invention. The controlled grain size ensures mechanical properties both transverse to and in the rolling direction. The preferred degree of recrystallization is 90% to 100%. The preferred average grain size for the present invention is 20 microns to 45 microns, more preferably 20 microns to 40 microns.

[0035] The steel of the present invention may have an area fraction of non-recrystallized microstructure regions between 0% and 20%, with the preferred degree of recrystallization being between 0% and 10%, more preferably between 0% and 5%.

[0036] In addition to the above microstructure, the microstructure of non-oriented electrical steels does not include microstructural constituents such as martensite, bainite, pearlite and cementite.

[0037] The steel according to the invention can be produced by any suitable method, however, it is preferred to use the method according to the invention detailed below, by way of non-limiting example.

[0038] Such a preferred method consists in providing a semi-finished casting of steel having the chemical composition of the steel according to the invention, which casting can be ingot or can be continuously in the form of thin slabs or thin strip, i.e. thicknesses ranging from about 220 mm for slabs to up to several tens of millimeters for thin strip.

[0039] For example, castings in the form of slabs are cast with the chemical composition according to the present invention and then reheated, with the slab reheating temperature being between 1050°C and 1250°C until the temperature becomes uniform throughout the slab. Below 1050°C, rolling becomes difficult and the force on the mill becomes too high. Above 1250°C, high silicon grades become very soft and may exhibit some warping, making them difficult to handle.

[0040] The reheated slab is then subjected to hot rolling, which is performed between 750°C and 950°C, with the hot rolling finish temperature playing a role in the final hot rolled microstructure. Finish rolling temperatures below 750°C limit recrystallization and result in a highly distorted microstructure. Temperatures above 950°C mean more impurities in solid solution, which can result in precipitation and deterioration of magnetic properties.

[0041] The hot-rolled steel sheet thus obtained is then immediately cooled to the coiling temperature of the hot-rolled steel sheet, which also serves the same purpose for the hot-rolled steel sheet, at a cooling rate of at least 10°C / s, and the cooling is carried out at a temperature of 500°C to 750°C. Coiling at temperatures below 500°C does not allow sufficient recovery to occur, but this metallurgical step is necessary for magnetic properties. Above 750°C, a thick oxide layer appears, which causes difficulties for subsequent processing steps such as cold rolling and / or pickling. Preferably, the cooling rate is not more than 200°C / s.

[0042] The coiled hot rolled steel sheet is then cooled to room temperature and then subjected to an optional hot roll annealing.

[0043] Prior to the optional hot-rolled sheet annealing, the hot-rolled steel sheet may be subjected to an optional descaling step to remove scale formed during hot rolling. The hot-rolled sheet is then subjected to the optional hot-rolled sheet annealing, which is preferably carried out at a temperature of 650°C to 1100°C for at least 10 seconds and up to 96 hours, with the temperature preferably being maintained at 700°C to 1070°C, more preferably 720°C to 1050°C. The optional descaling step of the hot-rolled steel sheet may then be carried out, for example, by pickling the sheet.

[0044] This hot-rolled steel sheet is then subjected to cold rolling to obtain a cold-rolled steel sheet having a thickness reduction of 35 to 90%.

[0045] The cold rolled steel sheet is then heat treated, which gives the steel of the present invention the required mechanical properties and microstructure.

[0046] The cold-rolled steel sheet is then heated, starting from room temperature, at a heating rate HR1 of at least 1°C / s to an annealing temperature Tsoak of 800°C to 875°C, preferably 810°C to 865°C. In a preferred embodiment, the heating rate HR1 for heating is at least 2°C / s, more preferably at least 5°C / s.

[0047] The cold rolled steel sheet is maintained at Tsoak for 10 seconds to 5000 seconds to ensure 80% to 100% recrystallization.

[0048] The cold-rolled steel sheet is then cooled, starting from Tsoak, at a cooling rate CR1 of 1°C / s to 150°C / s to a temperature T1 in the range of 20°C to 300°C. In a preferred embodiment, the cooling rate CR1 is 3°C / s to 120°C / s. The preferred T1 temperature is 20°C to 200°C.

[0049] Thereafter, the cold-rolled steel sheet is cooled to room temperature to obtain a non-oriented electrical steel sheet.

[0050] The non-oriented electrical steel sheets of the present invention may be optionally coated with an insulating, organic or inorganic coating, or a combination of both, to improve dielectric isolation. [Example]

[0051] The following tests, examples, figurative illustrations and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, illustrating advantageous features of the present invention.

[0052] Steel plates made from steels with different compositions are summarized in Table 1, and the steel plates are manufactured according to the process parameters specified in Table 2. Table 3 then summarizes the microstructures of the steel plates obtained during the tests, and Table 4 summarizes the results of the evaluation of the obtained properties.

[0053] [Table 1]

[0054] Table 2 Table 2 summarizes the hot rolling and annealing process parameters that were performed on the cold rolled steel sheets in Table 1 to give the steels in Table 1 the mechanical and magnetic properties required to become non-oriented electrical steels.

[0055] Table 2 is as follows:

[0056] [Table 2]

[0057] Table 3 illustrates the results of tests carried out according to standards with different microscopes, such as scanning electron microscopes, to determine the microstructure of both the inventive and reference steels in terms of area fraction, the grain size of both the inventive and reference examples being measured by the straight intercept method according to the ASTM E112 standard. The results are given here.

[0058] [Table 3]

[0059] Table 4 The results of various mechanical tests carried out in accordance with the standards are summarized here: for the tests, the ultimate tensile strength and yield strength were tested in accordance with the NF EN ISO 6892-1 standard, and the J50 magnetic properties were measured in accordance with the NF EN 60404-2 standard.

[0060] [Table 4]

Claims

1. 1. A non-oriented electrical steel sheet comprising the following elements, expressed in weight percent: 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% 3.5%≦Si+Al+Mn≦5.5% and 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% wherein the remainder of the composition is composed of iron and unavoidable impurities caused by processing, and the microstructure of the steel sheet is made of ferrite and includes, in area fractions, 80% to 100% of a recrystallized microstructure and 0% to 20% of a non-recrystallized microstructure, and the average grain size of the recrystallized microstructure is 20 microns to 50 microns.

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

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

4. The non-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the composition contains 0.7% to 1.2% 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% 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. 8. The non-oriented electrical steel sheet according to claim 1, wherein the steel sheet has a tensile strength of at least 580 MPa in both the transverse direction and the rolling direction.

9. 9. The non-oriented electrical steel sheet according to claim 1, wherein the steel sheet has a total elongation of at least 20% in both the transverse and rolling directions.

10. A method for producing a non-oriented electrical steel sheet, comprising the following successive steps: - providing a steel composition according to any one of claims 1 to 5; - reheating said semi-finished product to a temperature between 1050°C and 1250°C; - rolling the semi-finished product, the hot rolling finishing temperature being 750°C to 950°C, to obtain a hot-rolled steel plate; - cooling the hot rolled sheet immediately after finishing the hot rolling - then cooling said hot rolled steel sheet from the finish of hot rolling to a coiling temperature range of 500°C to 750°C at a cooling rate of at least 10°C / s. thereafter, coiling the hot-rolled steel sheet at a coiling temperature range of 500°C to 750°C. - optionally subjecting the hot rolled steel sheet to a descaling process; - optionally performing a hot rolled sheet anneal at 650°C to 1100°C for a period of 10 seconds to 96 hours; - optionally subjecting the hot rolled steel sheet to a descaling process; - cold rolling the hot-rolled steel sheet at a reduction ratio of 35 to 90% to obtain a cold-rolled steel sheet; - then annealing the cold-rolled steel sheet, the heating for annealing starting from room temperature and at a heating rate HR1 of at least 1°C / s up to an annealing temperature range Tsoak of 800°C to 875°C; - then carrying out an annealing at the annealing temperature for a period of 10 to 5000 seconds; - then cooling the cold-rolled steel sheet starting from the annealing temperature at a cooling rate CR1 of between 1°C / s and 150°C / s to a temperature T1 of between 300°C and 20°C; - Then, cooling to room temperature to obtain a non-oriented electrical steel sheet. A method comprising:

11. 11. The method of claim 10, wherein the Tsoak temperature for annealing is between 810°C and 865°C.

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

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

14. Use of a steel sheet according to any one of claims 1 to 9 or produced according to the method of claims 10 to 13 for the manufacture of parts for electric vehicles or electric machines.

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

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