Method for manufacturing a thin TWIP-steel sheet, thin TWIP-steel sheet, and stator or rotor for an electric machine

Abstract

The invention is related to a method to produce an ultra-thin austenitic steel sheet that may be used in electric motor applications, in particular an ultra-thin TWIP steel sheet for an electric motor rotor. The invention also relates to an ultra-thin austenitic steel sheet, in particular an ultra-thin TWIP steel sheet, produced according to the method.

Description

[0001] METHOD FOR MANUFACTURING A THIN TWIP-STEEL SHEET,

[0002] THIN TWIP-STEEL SHEET, AND STATOR OR ROTOR FOR AN ELECTRIC MACHINE

[0003] FIELD OF THE INVENTION

[0004] The invention is related to a method to produce an ultra-thin austenitic steel sheet that may be used in electric motor (e-motor) applications, in particular an ultra-thin TWIP steel sheet for an electric motor rotor. The invention also relates to an ultra-thin austenitic steel sheet, in particular an ultra-thin TWIP steel sheet, produced according to the method.

[0005] BACKGROUND OF THE INVENTION

[0006] The urgency for reducing carbon dioxide emission has led to a sharp increase in the use of electric propulsion systems in the transport industry, and to a shift from internal combustion engines to electric motors. There is thus a demand for producing electric motors with increased efficiency.

[0007] Some e-motor concepts, for instance a switched reluctance motor (SRM), have a stator with a number of stator poles with each pole having an electric coil. The rotor is provided with a number of rotor poles and has no permanent magnets or coils attached. It is typically made from a soft magnetic material. By energizing the coils of the stator poles a torque is generated by means of which the rotor poles align with the energized stator poles. By energizing the stator poles in a specific sequence, the rotation of the rotor is maintained.

[0008] In a specific type of switched reluctance motors, known as segmental rotor switched reluctance motors (SSRM), the magnetic flux only flows through each individual rotor tooth or pole. In this concept, the rotor core is made of a non-magnetic material that serves to keep the rotor poles of magnetic material electro-magnetically isolated from each other, and provide structural strength to the rotor. To minimise eddy current losses, both the rotor and stator body preferably consists of a stack of multiple thin sheet laminates, each laminate having an electrically insulating coating on both sides.

[0009] To improve efficiency, the SRM and SSRM motors preferably operate at high rotational speeds, which can be as high as 10,000 to 30,000 rpm. At such high speeds, the structural integrity of the thin laminates can become a problem if the material strength is too low.

[0010] Due to it’s non-magnetic properties, aluminium has often been proposed as a suitable material for the rotor core. However, thin gauge aluminium would not be strong enough to maintain the required structural stability at high rotational speeds. Thin high strength stainless steels with austenitic microstructure are considered as an alternative for the fabrication of the core of the magnet carrier, because of its relatively low magnetic permeability, and relatively high resistivity provided mainly by the Cr additions, plus a high corrosion resistance. In common austenitic stainless steel such as 18-8 austenitic grade, the Cr content is over 10 wt.%, so it conducts electricity less than other metals such as copper for a given voltage. The high Cr combined with high Ni lead to a high CO2 footprint, and is also a matter of economy, as the Cr and Ni both have relatively high costs making it more cost-efficient for certain applications to avoid the usage of stainless steels.

[0011] Thus, there is a demand for alternative materials for the rotor core material, which are non-magnetic, thin, high strength, sustainable, and cost effective.

[0012] OBJECT OF THE INVENTION

[0013] It is an object of the invention to provide a high-strength steel sheet and a method of manufacturing such steel sheet, which can be produced in very thin thicknesses, in combination with low magnetic permeability, and may therefore be used in the manufacture of stators or rotors of electric machines. It is a further object of the invention to provide a stator or rotor of an electric machine incorporating such thin steel sheet material, which has high strength and is cost efficient to produce.

[0014] These and further objects are met or exceeded by the method according to claim 1 , the ultra-thin high strength TWIP steel sheet of claim 8, the stator or rotor according to claim 13 and a method of manufacturing a stator or rotor according to claim 14. Advantageous embodiments are set out in the dependent claims and this description.

[0015] Any reference signs in the claims should not be construed as limiting the scope of the appended claims.

[0016] DESCRIPTION OF THE INVENTION

[0017] According to a first aspect of the invention, a method for manufacturing a thin steel sheet is provided. The method has the following steps: a) providing a hot-rolled steel strip having a thickness in a range of 0.8 mm to 4 mm, the strip having the following composition, in weight percent (wt.%): C 0.05-1.25% ;

[0018] Mn 10 - 40%, preferably 10-20%;

[0019] Al 0.1 - 5%, preferably 0.5 to 5%;

[0020] Ni 0.0-2.5%, preferably 0.02 to 0.3%;

[0021] Cr 0.0-5.0%, preferably 0.0 to 0.3%;

[0022] Si 0.0-5.0%, preferably 0.02-1 .0%; V 0.0-0.10%, preferably 0.03-0.10%;

[0023] S 0.0-0.02%;

[0024] P 0.0-0.06%;

[0025] N 0.0-0.02%; incidental elements up to 0.10% each and up to 0.5% in total; the remainder being impurities (viz. residual and tramp impurities from the ironmaking and steelmaking process) and Fe; b) cleaning the surface of the hot-rolled strip in order to remove oxides; c) cold rolling the cleaned hot-rolled with a thickness reduction in a range of 20% to 60%, to obtain a cold rolled strip; d) interannealing the cold rolled strip at a temperature above the Ac3 temperature of the steel for 30 sec. to 60 min., followed by cooling to room temperature at an average cooling rate of 0.01-100 °C / s, in order to obtain an interannealed strip; e) cold rolling the interannealed strip with a thickness reduction in a range between 5% and 40%; f) repeating steps d) and e) up to 6 times; and wherein the final cold rolling step has a thickness reduction in a range of 5% to 30%, preferably of 10% to 20%, to obtain a thin or ultra-thin steel sheet having a thickness in a range between 0.1 mm and 0.5 mm, preferably between 0.1 mm and 0.4 mm, more preferably between 0.15 mm and 0.35 mm.

[0026] The claimed process steps or method steps are preferably performed in the given sequence. Additional process steps in-between the claimed steps are not excluded, in particular for example additional strip surface cleaning steps to remove the surface oxides to achieve good magnetic properties. In some embodiments, no additional steps other than surface cleaning steps are performed.

[0027] It has been found that a high-manganese twinning induced plasticity (“TWIP”) steel having a composition according to claim 1 may be manufactured at very thin or ultra-thin thicknesses of less than 0.5 mm, preferably less than 0.4 mm, and most preferably less than 0.35 mm, for example at thicknesses between 0.15 mm and 0.3 mm. This has so far been held impossible, because a high work hardening steel like TWIP steel sheet shows cracking when cold rolled to thickness below 0.5 mm. The inventors have demonstrated that such very thin thicknesses may yet be reached by introduction of one or more interannealing steps at carefully controlled temperatures during the cold rolling process. At least one interannealing step at a temperature above the Ac3 temperature is carried out in between two cold rolling steps. The number of interannealing steps may be adapted to the specific material used, the interannealing temperature and time, and the final thickness to be reached. Experiments have been carried out to show that less than 7 interannealing steps, preferably 2 to 5 interannealing steps, suffice to reach the desired ultra-thin thickness. Moreover, a final cold rolling reduction step of up to about 10% to 20% may be performed to obtain the required high strength levels.

[0028] The hot-rolled steel strip has the following composition, in weight percent (wt.%): C 0.05- 1.25, Mn 10 - 40, Al 0.15, Ni 0.0-2.5, Cr 0.0-5.0, Si 0.0-5.0, V 0.0-0.10, S 0.0-0.02, P 0.0-0.06, N 0.0-0.02, incidental elements up to 0.10% each and up to 0.5% in total, the remainder being impurities (viz. residual and tramp impurities from the ironmaking and steelmaking process) and Fe.

[0029] This steel composition is a TWIP steel composition. As known to the skilled person TWIP steels derive their exceptional properties from a specific strengthening mechanism referred to as “twinning” in the austenite phase. In addition to the basic mechanism of dislocation gliding, deformation also occurs by mechanical twinning on the {1 1 1}Y<1 1 2>Ysystem. The formation of mechanical twins during deformation generates high strain hardening, prevents necking and thus exhibits a very high elongation. This mechanism is called Twinning Induced Plasticity.

[0030] The carbon content of the TWIP steel according to the invention is in the range of 0.05- 1.25 wt.%, preferably in a range of 0.05-1.0 wt.%, more preferably in a range of 0.30-0.9 wt.%, most preferably in a range of 0.5-0.8 wt.%. Carbon inhibits the formation of s-martensite by increasing the SFE. Stacking faults are precursors to s-martensite, so increasing the SFE decreases the tendency to form s-martensite. In addition, carbon improves the stability of the austenite, in particular when added within the claimed range.

[0031] The TWIP steel of the invention has a high Mn content to retain the austenitic structure at room temperature. The manganese content is in a range of 10-40 wt.%, more preferred in a range of 10-20 wt.%, most preferably in a range of 12-18 wt.%. Manganese improves the strength of the steel by substitutional solid solution hardening and is an austenite stabilizing element. Moreover, the Stacking Fault Energy (SFE) may be controlled. The manganese content should not be lower than 10 wt.% as this would reduce the SFE of the steel alloy. The manganese range according to the invention provides a stable austenite phase at room temperature due to the synergetic effect with carbon.

[0032] The aluminum content of the steel alloy is 0.1 wt.% to 5 wt.%, more preferably of 0.5 to 5.0 wt%, and most preferably of 0.5 wt.% to 3 wt.%. Aluminium increases the SFE in TWIP steel and prevents carbide formation. Therefore, it is added to the TWIP steel of the present invention. The preferred range is to facilitate the continuous casting and to reduce alloy costs. The nickel content of the invention is in a range of 0.0-2.5 wt.%, preferred in a range of 0.02-0.3 wt.%, most preferably in a range of 0.03-0.12 wt.%. Nickel is an austenite stabilizer similar to Mn, but it is a rather expensive alloying element. It is therefore advantageous that nickel is not required at high levels in the composition of the steel of the present invention. This is opposed to austenitic stainless steel, which often has nickel additions well above 2.5 wt.%.

[0033] The chromium content is between 0.0-5.0 wt.%, preferably in the range of 0.00-0.3 wt.%, more preferred 0.02-0.15 wt.%, most preferred 0.05-0.1 wt.%. A small amount of Cr may be present to improve the passivity of the TWIP steel, thereby improving the corrosion resistance. Too high a Cr amount makes the austenite less stable. Chromium is also a rather expensive alloying element and its presence is to be avoided at substantial levels.

[0034] The silicon content is between 0.0-5.0 wt.%, preferably in a range of 0.02-1.0 wt.%, more preferred in a range of 0.05-0.5 wt.%, most preferably in a range of 0.10 to 0.35 wt.%. A low Si content is purposively used to decrease the Stacking Fault Energy of this TWIP steels and also to obtain a good strip surface quality with respect to oxidation. When it is present, Si also contributes to the strength of the TWIP steel by solid solution strengthening.

[0035] Vanadium may be present in the steel in a range of 0.0-0.10 wt.%, more preferred in a range of 0.003-0.10 wt.%. Vanadium increases the strength properties of the steel by forming fine carbides, nitrides and carbo-nitrides. These precipitates will also refine the austenite grain size, contributing to the strength and high ductility of the TWIP steel of this invention. V is most effective up to 0.10 wt.%, and at high levels may only add to the costs of the steel.

[0036] Sulfur may be present in the steel of the invention in a range of 0.0 to 0.02 wt.%, more preferred 0.0 to 0.010 wt.%, most preferably up to 0.0 to 0.0080 wt.%. The presence of sulfur is to be limited to avoid the formation of excessive amounts of sulfide inclusions which may promote cracking during cold rolling.

[0037] The content of phosphorous may be up to 0.06 wt.%, more preferred up to 0.05 wt.%, most preferred up to 0.03 wt.%. The presence of phosphorous is to be limited to avoid grain boundary embrittlement.

[0038] Nitrogen may be present in an amount of up to 0.02 wt.%, preferably in a range of 0.0005-0.02 wt.%, more preferred in a range of 0.002-0.012 wt.%. Nitrogen has a stabilizing effect on the austenite, synergistic to that of C and Mn. In addition, N causes solid solution strengthening of austenite and forms nitride or carbo-nitride precipitates in combination with V and C.

[0039] Impurities (viz. residual and tramp impurities from the ironmaking and steelmaking process), and incidental elements (e.g., Cu, Mo, Nb, Sn, Ti) up to 0.10 wt.% each and up to 0.5 wt.% in total. In an embodiment the incidental elements are present up to 0.10 wt.% each. In an embodiment the total of incidental elements does not exceed 0.30 wt.%. The remainder being iron. The level of incidental elements should be kept as low as possible. Notably Cu should not exceed 0.10 wt.%, and preferably not exceed 0.07 wt.%, as it may have an adverse effect on the surface quality of the very thin steel sheet. As the method of the invention requires multiple rolling and interannealing steps the surface quality might otherwise be affected by too high a Cu content, where the surface quality is important in the preferred envisaged application by stacking multiple thin steel sheets having no metallic surface coating as part of a stator or rotor of an electric machine.

[0040] In the first process step a), a hot-rolled steel strip of defined composition and having a thickness in a range of 0.8 mm to 4 mm, more preferred 1.2 mm to 3 mm, most preferred 1.5 mm to 2.5 mm is provided. The invention is not limited by the casting method or the hot rolling method. The hot-rolled steel strip may be produced by any possible method. For example, it may be cast and direct-rolled in a continuous process, in particular in a continuous steel-casting facility. Alternatively, the steel may be cast and hot-rolled in separate, discontinuous process steps. The cast steel is hot-rolled or direct-rolled down to the required thickness of 0.8 mm to 4 mm.

[0041] In an embodiment, the hot rolled strip has been manufactured by casting, homogenizing and hot rolling, as explained above. The homogenization treatment may be carried out between casting and hot rolling at a temperature in a range of 1100°C and 1300°C, preferably in a range of 1150°C to 1250°C, for a soaking time between 1 and 24 hours, preferably between 2 and 10 hours, and more preferably between 2 and 5 hours.

[0042] Optionally, the hot-rolled steel strip may be annealed after hot-rolling, for example at a temperature of 700°C to 900°C. In an embodiment, the hot-rolled steel strip is provided in a continuous annealed condition. In an embodiment, the method also includes the steps of casting, hot-rolling and optionally annealing the steel slab or strip to be provided in step a).

[0043] The surface, in particular the rolling surface, of the hot-rolled strip is cleaned in step b) after the steel strip has cooled to room temperature, to remove oxides (scale) from the surface. The cleaning may be performed chemically and / or mechanically. The scale on the hot-rolled steel strips can be removed either by pickling in an acid solution (e.g., sulfuric or hydrochloric acid) at warm temperatures (about 70-120°C) or by a combination of pickling and mechanical brushing of the strip surface. Next the strip is cold rolled in step c) with a thickness reduction in the range of 20-60%, preferably 30-55%, more preferred 35-50%, to obtain a cold rolled strip. This first cold rolling reduction is preferably to an intermediate thickness of 0.5-2.5 mm, more preferred 0.6-2.0 mm. The first cold rolling reduction may lead to a work hardening effect and a respective increase in yield strength and ultimate tensile strength. This effect is desirable, but normally would prevent further cold reduction due to excessive high roll forces or strip failure, and which is overcome by the method according to this invention.

[0044] Therefore, the inventive process introduces an interannealing step d) at a temperature above the Ac3 temperature for 30 sec. to 60 min., more preferred 1 min. to 30 min., most preferred 1-10 min followed by cooling to room temperature at an average cooling rate of 0.01- 100°C / s, more preferably of 1.0-50°C / s, most preferably of 10-40°C / s, whereby an interannealed strip is obtained. This interannealing will cause an austenite content of >95 wt.% to be obtained in the strip microstructure, which is sufficient to have good cold rollability. If austenite content is less than 95%, then the second phases may promote cracking during cold rolling due to the presence of considerable amount of interfaces of softer phase and harder phase. It has been found that the interannealing has to be performed at a temperature above the Ac3 temperature. Otherwise, the material is likely to crack at the transverse edges in the subsequent cold rolling steps, due to the presence of significant amounts of second phase, for example >5 wt.%.

[0045] Thereafter, interannealed strip may be further cold rolled in a step e) with a thickness reduction in the range between 5-40%, more preferred between 10-35%, even more preferably between 5-25%.

[0046] The interannealing step is repeated with the above-identified time and temperature combination procedure, followed by a cold rolling step. This interannealing followed by cold rolling may be repeated until an ultra-thin thickness in the range between 0.1 and 0.5 mm, preferably between 0.1 and 0.4 mm, more preferred 0.15 and 0.35 mm, most preferred between 0.15 and 0.25 mm is reached.

[0047] The steps d) of interannealing and e) of cold rolling are repeated up to 6 times, preferably 1 to 4 times, more preferred 2 to 3 times. It has been found that this is sufficient to reach the claimed thin or ultra-thin thickness. For economic reasons, as few interannealing steps as possible are carried out. Depending on the interannealing temperature and time, the selected alloying composition, fewer or more interannealing steps may be required. Generally, the higher the selected interannealing temperature and the lower the content of alloying elements in the TWIP steel, the fewer interannealing steps will be required to reach the desired ultra-thin thickness. The last interannealing heat treatment in any series of interannealing treatments should be performed above the Ac3 temperature of the steel to ensure a maximum content of austenite in the microstructure.

[0048] Cold rolling and interannealing may be carried out in a continuous process, in particular in a continuous process line similar to a continuous annealing line. Therein, it is possible that the steel strip is continuously processed by alternately annealing and cold rolling the strip. Alternatively, the interannealing treatment may be carried out in a batch annealing process, wherein a cold rolled strip is coiled directly after the cold rolling. The coil is transported into a suitable annealing furnace, which is heated to the annealing temperature. The coil will thus be batch-annealed in the annealing furnace, and will be cooled down to ambient temperature in the batch annealing furnace as a coil as is well known in the art. The coil is then transferred back to the cold rolling mill, where the next cold rolling step will be carried out.

[0049] In a preferred embodiment, the cold rolling step e) is carried out by thickness reduction between 10%-30%, more preferred between 10%-20%. Thereby, the thickness of the steel strip may be reduced significantly, while at the same time avoiding cracking.

[0050] In an embodiment, the interannealing is performed at a temperature range between 650°C and 1100°C, preferably between 700°C and 1000°C, more preferably between 750°C and 950°C, and most preferably between 850°C and 950°C. It has been shown in experiments that this interannealing temperature will allow to reach the required thin or ultra-thin thickness, while at the same time preserving the desired property of the TWIP steel sheet. Below 650°C or below the Ac3 temperature, the cold rolling step will lead to unacceptable cracking of the cold rolled strip and will thus not allow to reach the desired ultra-thin thickness. Above 1100°C, the mechanical properties of the steel sheet will be degraded, in particular the yield strength and ultimate tensile strength will be reduced. Also, in order to save energy, lower temperatures are preferred. However, it is nevertheless desirable to choose a temperature which is higher than Ac3+50°C, more preferred higher than Ac3+100 °C, most preferred higher than Ac3+150°C. Thereby, although the interannealing needs to be carried out at a higher temperature, fewer interannealing steps are required, thereby, overall less heating energy is required. This will result in a most preferred temperature range of between 850°C and 950°C for most alloying compositions.

[0051] In an embodiment, the interannealed strip surface is cleaned in order to remove oxides before a next cold rolling step. It has been shown that oxides may form in the interannealing process and that better results may be obtained by removing them before each cold rolling step. This may be performed by chemical or mechanical processes, as described above for cleaning of the hot rolled strip, for example by dipping in a suitable cleaning solution or by scraping the surface mechanically. This cleaning step may be performed directly before the cold rolling in the same line, or in an off-line process.

[0052] In an embodiment, the interannealing is performed in a protective atmosphere, in particular in an atmosphere comprising nitrogen, argon, hydrogen, or suitable mixtures thereof. A vacuum may also be used, particularly when manufacturing on a small scale. With that the formation of oxides is reduced. The protective atmosphere preferably contains less than 10000 ppm oxygen, more preferably less than 1000 ppm oxygen, in order to reduce the formation of oxides to an acceptable level.

[0053] In an embodiment, the first cold rolling step c) is carried out up to an intermediate thickness of in a range of about 0.5-2.5 mm, and preferably in a range of about 0.5-2.0 mm. Thereby, the thickness of the strip is already reduced by a significant amount, thereby allowing to reach an ultra-thin thickness in the next cold rolling steps.

[0054] In accordance with the invention, the final or the last cold rolling reduction step in a series of cold rolling-interannealing cycles is in a range of 5% to 30%, preferably of 5% to 20%, and more preferably of 8% to 20%, may be performed to obtain the required high strength levels of the ultra-thin TWIP steel sheet.

[0055] After the final or last cold rolling reduction the thin TWIP steel sheet is not subjected to any recovery heat treatment in order to maintain all propertiesy at the required level.

[0056] After the final or last cold rolling reduction the thin TWIP steel sheet is not provided with any metallic coating on one or both of its rolling faces. Thus the thin TWIP steel sheet is not provided with a pure zinc, or zinc alloy, or magnesium alloy, nor with an aluminium alloy coating.

[0057] It has been found that with the method according to the invention a thin or an ultra-thin high-strength TWIP steel sheet can be manufactured, being devoid of a metallic surface coating, and having low magnetic permeability and therefore ideally suitable for use in the manufacture of stators or rotors of electric machines. The TWIP steel sheet has two or more, and preferably all, of the following properties: yield strength (Rp) > 750 MPa, and preferably > 750 MPa; ultimate tensile strength (Rm) > 900 MPa, and preferably > 950 MPa;

[0058] A80 total elongation > 13%, and preferably > 15%; relative magnetic permeability < 1.8, preferably < 1.6, and more preferably < 1.4; electrical resistivity > 0.8 pQ-m; and a microstructure having an austenite content >98 wt.% as measured by X-ray diffraction.

[0059] Tensile tests are performed at room temperature with A80 specimen geometry with tensile testing parallel to the rolling direction according to EN 10002-1 / 150 6892-1.

[0060] It has been found the above set of properties remain within the specified range also after curing of an organic coating applied on the steel sheet, for example in the manufacturing of a stator or rotor of an electric machine (e.g., 200°C / 1 hour treatment) as well as after an in-service temperature rise of for example up to about 300°C, when the electric machine is in operation.

[0061] According to another aspect, the invention relates to a thin TWIP steel sheet, in particular an ultra-thin TWIP steel sheet having a thickness in a range of 0.1 to 0.5 mm, preferably of 0.1 mm and 0.4 mm, more preferred between 0.15 mm and 0.35 mm, being devoid of a metallic surface coating, and having a relative magnetic permeability < 1.8, an electrical resistivity > 0.8 pQ-m, and a microstructure having an austenite content > 98 wt.%. The magnetic permeability is the proportionality between an induced internal field and the strength of an external applied magnetic field. The relative magnetic permeability of a material is the ratio of the magnetic permeability of the material to that of vacuum and which is assumed to be 1. A low relative magnetic permeability of around 1 means that the material’s internal field does not interact with the externally applied fields, generating no magnetism or attraction by them. In an embodiment of the ultra-thin TWIP steel sheet the relative magnetic permeability is < 1.6, and preferably < 1 .4.

[0062] The TWIP steel sheet has a composition consisting of as specified in claim 1 and with preferred embodiments set out in this description. Preferably, it has been manufactured by a method as claimed and described herein.

[0063] The ultra-thin TWIP steel sheet has a yield strength of > 750 MPa, preferably > 800 MPa, in combination with an ultimate tensile strength of > 900 MPa, preferably > 950 MPa, and a total elongation A80 > 13%, preferably of > 15%. It has been found that these high values of properties are even preserved if the ultra-thin TWIP steel sheet is manufactured into a stator or rotor of an electric machine. In particular, these properties are maintained also after curing of an organic coating at a temperature in the range of up to 300°C, more preferred at 160°C to 260°C, for 20 sec. to 30 min. In addition, the properties are preferably maintained at elevated temperatures, which may occur during operation of the electric machine. In particular, these properties are also present after extended exposure to a temperature above 80°C, in particular between 80°C and 200°C, preferably between 100°C and 180°C, for more than 30 min., more preferred for more than 1 hour, for example for 10 hours.

[0064] According to a further aspect, the invention is directed to the use of the ultra-thin TWIP steel sheet as described herein in the manufacture of a stator or rotor of an electric machine. For example, the ultra-thin TWIP steel sheet according to an embodiment of the invention may be cut into suitable parts, for example by stamping. The parts may be stacked, wherein for example for about 200 to 3000, preferably for about 800 to 1000 steel parts are stacked together. Preferably, the steel sheets are stacked in the axial direction of the electric machine to build the hub of a stator or rotor.

[0065] The invention is also directed to a method for manufacturing such a stator or rotor of an electric machine. The method may comprise cutting the steel sheet into parts having the suitable form corresponding to the cross-section of the required rotor or stator. The sheet parts are then coated and stacked together. The coating is preferably an organic coating which may also be used to laminate the sheet parts together. The stacking is followed by a step of curing the organic coating at a temperature in the range of about 160°C to 240°C for about 20 sec to 30 min. Thereby, the stack of ultrathin steel sheet parts are baked together and will form nonmagnetic components for a stator or rotor of an electric machine. Joining of the magnetic rotor poles to the non-magnetic rotor core can be done via any joining method, for example as disclosed in patent documents EP2994977 and EP2994978, either before or prior to the coating process.

[0066] In an embodiment, the invention is further directed to a stator or rotor of an electric machine comprising an ultra-thin TWIP steel sheet according to an embodiment of the invention, in particular a stack of multiple ultra-thin steel sheet parts. The electric machine may be an electric motor or an electric generator or an electric machine which is able to act both ways, and which is preferably used in electric vehicles, in order to allow both electric propulsion and reuse of braking energy.

[0067] BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The invention will now be described by means of preferred embodiments and examples with reference to the attached drawings. In the drawings:

[0069] Fig. 1 shows schematically a rotor body laminate of a non-magnetic material;

[0070] Fig. 2 shows a cross-section through a stack of ultra-thin steel parts; Fig. 3 is a process diagram of an embodiment of a method according to the invention;

[0071] Fig. 4 shows an illustration of the cold rolling experiments, showing the interannealing temperature versus the thickness reduction;

[0072] Fig. 5 shows the yield strength (Rp) for various steel sheet products at various final annealing temperature and various in service conditions;

[0073] Fig. 6 shows the tensile strength (Rm) for various steel sheet products at various final annealing temperature and various in service conditions;

[0074] Fig. 7 shows the total elongation (A80) for various steel sheet products at various final annealing temperature and various in service conditions;

[0075] Fig. 8 is a graph of austenite fraction, as measured by X-ray diffraction, for various steels at various final annealing temperatures and various in service conditions;

[0076] Fig. 9 shows the electrical resistivity for various steel sheet products at various final annealing temperature and various in service conditions;

[0077] Fig. 10 shows the relative magnetic permeability for various steel sheet products at various final annealing temperature and various in service conditions.

[0078] DESCRIPTION OF EMBODIMENTS

[0079] Similar elements are indicated in the drawings using the same reference signs.

[0080] Any reference signs in the claims should not be construed as limiting the scope of the appended claims.

[0081] Fig. 1 is taken from patent document EP2994978 and incorporated herein by reference. It shows a finished rotor laminate 4 with the joined laminate 1 and 2 and wherein the annular laminate 3 has been machined such that separate pole elements 5 are obtained. A rotor is assembled by stacking a number of these rotor laminates in a manner known in the art. In particular, multiple TWIP steel sheet parts, made from TWIP steel sheet and / or by the method according to an embodiment of the invention, are stacked on top of each other, wherein such a stack is show in cross-section in Fig. 2.

[0082] Fig. 2 shows a stack of TWIP steel sheet parts 14, each having a thickness d. The thickness d is preferably 0.15 mm to 0.25 mm, for example 0.2 mm. The steel sheet parts 16 are laminated together by coating 18, which is spaced between and around the steel sheet part 16 and which may be cured using a heat treatment at a moderate temperature of for example 140°C to 250°C. The stack 14 of steel sheet parts 16 may be used for example in the manufacturing of a stator or rotor for an electric machine.

[0083] Fig. 3 illustrates a process flow of the method according to the invention. In step 20, a hot rolled steel slab or strip in the claimed thickness is provided. Any oxide on the rolling surface is removed in step 22, for example by pickling in an acid solution. In step 24, a first cold rolling step is carried out with a thickness reduction in a range of 20% to 60%, to obtain a cold- rolled strip. In step 26, the cold-rolled strip is interannealed at a temperature above the Ac3 temperature e.g. for 30 sec. to 60 min., followed by cooling to room temperature or ambient temperature at an average cooling rate of 0.01 to 100°C / s in order to obtain an interannealed strip. The interannealing may be carried out either continuously or discontinuously, in particular in a batch annealing process of the coiled strip. After the interannealing, the steel is sufficiently softened to carry out another cold rolling step 30, wherein the second - and any further - cold rolling steps are carried out with a thickness reduction in a range between 5% and 40%. Before the cold-rolling step 30, any oxides which have formed on the surface of the interannealed strip are preferably removed, for example by pickling in step 28.

[0084] As indicated by the arrow from step 30 to step 26, steps 26-30 may be repeated, depending on the amount of cold reduction desired, as well as on the steel composition and the specific annealing temperature chosen. For example, 1-6, preferably 1-3 repetitions may be applied. The ultra-thin TWIP steel sheet receives its last or final cold rolling reduction step, here step g) (step 32), of up to about 5% to 30% to obtain the required high strength levels and thus without being followed by an interannealing step above the Ac3 temperature. Next the steel sheet is coiled in step 34 and ready for storage or shipment and for further processing, in particular for use in the manufacturing of stators or rotors for electric machines.

[0085] The invention will now be illustrated with reference to non-limiting comparative and examples according to the invention.

[0086] EXAMPLE 1

[0087] Two TWIP steel sheets having compositions within the scope of the invention, named A and B, were studied. The chemical composition of each steel is listed in Table 1. The TWIP steel materials A and B were provided as cold-rolled and continuously annealed (810°C) sheets, having a thickness of 1.4 mm and 1.7 mm, respectively. Table 1 : Chemical composition of the TWIP steels and comparative steel (REF) in wt.%. Balance is iron and process impurities. Rolling trials were performed on the two TWIP steels A and B. The alloys were cold rolled in multiple steps, with interannealing steps in between, using different interannealing temperatures as presented in Table 2. The experiments were carried out on laboratory scale of testing. The annealing time was always 1 hour. The first cold rolling step was generally done with a 35-38% reduction.

[0088] Table 2: Summary of cold rolling processes with various annealing temperatures.

[0089] For the A alloy and an interannealing temperature of 500°C, a total of 6 interannealings were required to reach 0.7 mm from 1.7 mm. For the A alloy and an interannealing temperature of 600°C, which is still below the Ac3 temperature of the material, a total of 5 times interannealing were required to reach 0.6 mm from 1.7 mm. However, in these trials, the cold rolling process had to be stopped before the desired ultra-thin thickness of 0.2 mm could be reached, because the material started to crack.

[0090] For the A alloy and an interannealing temperature of 700°C, which is above the Ac3 temperature for this material, in total 7 times interannealing was required to reach 0.21 mm from 1.7 mm. For the A alloy and an interannealing temperature of 900°C, in total 5 times interannealing was required to reach 0.2 mm from 1.7 mm. For the B alloy and an interannealing temperature of 700°C, which is above the Ac3 temperature for this material, in total 5 times interannealing was required to reach 0.2 mm from 1.4 mm. For the B alloy and an interannealing temperature of 900°C, in total 5 times interannealing was required to reach 0.2 mm from 1.4 mm.

[0091] It was observed that interannealing at 700°C or above is needed for these alloys to reach the required 0.2 mm thickness. An interannealing temperature of 900°C reduces the number of interannealing steps, at least for steel B, and may therefore be preferably, at least for some alloy compositions, over a lower interannealing temperature.

[0092] These experiments are summarized in Fig. 4, which presents each rolling trial as a horizontal line, with the annealing steps marked as circles. The graph places the interannealing temperature versus thickness. From Fig. 4, it becomes evident that interannealing temperatures of 500°C or 600°C are too low to reach the desired ultra-thin thickness of 0.2 mm, even when multiple interannealing steps are carried out.

[0093] Both interannealing at 700°C and 900°C allowed to reach the desired ultrathin thickness of 0.2 mm, wherein the number of interannealing steps appeared to be lower for the alloy A.

[0094] EXAMPLE 2

[0095] Based on the experience from the rolling trials in example 1 , further tests were performed on TWIP steel A.

[0096] Steel A was provided in a cold rolled and continuously annealed (at 810°C) and pickled condition at a thickness of 1.7 mm. In Table 3 and Figures 5 to 10, this material is referred to as “1.7mm TWIP-AR”. This material was further reduced to 0.2 mm thickness, using a rolling schedule that included 10 cold rolling steps with interannealing steps in between. The interannealing steps were performed at 900°C for 3 minutes. The last interannealing step should be performed at a temperature above Ac3 to ensure maximum austenite amount in the microstructure. After the second and each further interannealing step, the surface was cleaned in order to remove oxides by pickling. However, no pickling was performed after the optional last annealing step performed after the last cold rolling step. The initial thickness was 1.7 mm, and the further thicknesses after each cold rolling steps were 1.1 mm, 1.0 mm, 0.9 mm, 0.75 mm, 0.6 mm, 0.5 mm, 0.4 mm, 0.3 mm, and 0.25 mm, resulting in the final thickness of 0.20 mm and hence a final cold rolling reduction of 20% in accordance with the invention. In the Table 3 and Figures 5 to 10, this material is referred to as “0.2mm TWIP-CR”.

[0097] From these 0.2 mm cold rolled TWIP steel sheets, samples were cut for various tests along the rolling direction. In particular, A80 tensile tests were performed according to ISO6892- 1. The austenite content was measured using X-Ray diffraction (XRD). The electrical resistivity was measured using a AOIP 0M21 micro-ohm meter, where the resistivity (p) was calculated by p = Rxwxt / L from a 10x300 mm sample, where R is the measured electrical resistance, w the sample width, t the sample thickness and L the sample length. The magnetic permeability was measured on the same sample by means of inductance spectra in the frequency range of 375 Hz-48 kHz measured by an air-core differential sensor with a magnetic field strength of 100 A / m.

[0098] To evaluate the performance of the sheet product under different circumstances, the 0.2 mm TWIP-CR sheet material was further subjected to a series of post-cold rolling annealing treatments (1), as well as two in-service conditions relevant for the e-motor applications (2 and 3), namely:

[0099] A. A post-cold rolling final annealing treatment in the temperature range of 500°C to 900°C for 3 minutes, the results of which are shown in Fig.5A to Fig. A respectively. This process step of final annealing is an optional process step performed after the last cold rolling step when manufacturing the TWIP sheet material in accordance with the invention, the temperature of which should be controlled to a certain maximum temperature to avoid adverse effects on the properties.

[0100] B. A final annealing treatment in the temperature range of 500°C to 900°C for 3 minutes as in process A. followed by a curing treatment at 200°C for 1 hour as is relevant for an organic coating, the results of which are shown in Fig.5B to Fig.10B respectively;

[0101] C. A final annealing treatment in the temperature range of 500°C to 900°C for 3 minutes as in process A. followed by a simulation for the in-service temperature rise of 300°C for 1 hour, the results of which are shown in Fig. 5C to Fig.lOC respectively.

[0102] After these heat treatments, the mechanical properties, austenite content, electric resistivity and relative magnetic permeability of the samples were determined similar as described above for the original 0.2 mm TWIP sheet material without further heat treatment. To investigate the effect of the oxide layer that was formed during annealing on the magnetic properties, the 0.2 mm TWIP-CR material, having the final 20% cold reduction and no further heat treatments, was also tested in a pickled condition. In the Table 3 and Figure 10, this material is referred to as “0.2mm TWIP-PL” and shows that pickling to remove the oxide surface layer significantly reduces the relative magnetic permeability.

[0103] An overview of the materials and conditions is provided in Table 3.

[0104] Table 3: Material conditions tested in example 2. The measured tensile properties for the 1.7 mm and 0.2 mm cold rolled TWIP steels after various heat treatments are presented in Figures 5, 6 and 7, that show the yield strength, tensile strength and total elongation, respectively. It can be seen that the 0.2 mm TWIP material, that was produced with a 20% final cold reduction, has strength and elongation properties in line with the minimum target values required for the preferred application or use of the TWIP steel sheet in accordance with the invention. When the material is subjected to additional annealing cycles well above 700°C, viz. at 800°C or 900°C, the material softens leading to too low a strength. It is also shown in these figures that the cold rolled TWIP material still has sufficient mechanical properties after the in-service conditions. Figure 8 shows the measured austenite content of the 1.7 and 0.2 mm TWIP steels after various heat treatments. The high austenite fractions that were measured show that the strengthening observed for the cold rolled samples is achieved via twinning of the austenitic phase, and not via formation of secondary phases.

[0105] Figure 9 shows the measured electric resistivity of the 1.7 and 0.2 mm TWIP steels after various heat treatments. Although there is some variation depending in the annealing, or postheating condition, all materials have an electric resistivity well above the minimum requirement for the preferred application or use of the TWIP steel sheet in accordance with the invention.

[0106] Figure 10 shows the measured electric resistivity of the 1.7 and 0.2 mm TWIP steels after various the heat treatments. Most materials have a magnetic permeability below the maximum requirement for the preferred envisaged application or use of the TWIP steel sheet in accordance with the invention. At the same time, the magnetic permeability of the 0.2 mm TWIP material is consistently higher than that of the 1.7 mm TWIP material, even though these steels are based on the same composition, and both have a similar >98% austenite content. Reason for this was found to be in the surface oxides. Additional samples that were pickled after the last annealing and cold rolling step, showed a much lower magnetic permeability, comparable to the 1.7 mm TWIP-AR.

[0107] These results allow the conclusion that the proposed thin TWIP steel sheet is a highly suitable material for use in electric machines.

[0108] Any reference signs in the claims should not be construed as limiting the scope of the appended claims.

Claims

CLAIMS1. Method for manufacturing an ultra-thin steel sheet, comprising the steps of: a) providing (20) a hot-rolled steel strip having a thickness in a range of 0.8 mm to4 mm, the slab or strip having the following composition, (in wt.%):C 0.05-1.25%,Mn 10-40%, preferably 10-20%,Al 0.1-5%, preferably 0.5 to 5%,Ni 0.0-2.5%, preferably 0.02-0.3%,Cr 0.0-5.0%, preferably 0.0-0.3%,Si 0.0-5.0%, preferably 0.02-1 .0%,V 0.0-0.10%, preferably 0.03-0.10%,5 0.0-0.02%,P 0.0-0.06%,N 0.0-0.02%, incidental elements up to 0.10% each and up to 0.5% in total, the remainder being impurities and Fe; b) cleaning (22) the surface of the hot-rolled strip in order to remove oxides; c) cold rolling (24) the cleaned strip with a thickness reduction in a range of 20% to 60%, to obtain a cold rolled strip; d) interannealing (26) the cold rolled strip at a temperature above the Ac3 temperature of the steel for 30 sec to 60 min., followed by cooling to room temperature at an average cooling rate of 0.01-100 °C / s, in order to obtain an interannealed strip; e) cold rolling (28) the interannealed strip with a thickness reduction in a range between 5% and 40%; f) repeating steps d) and e) up to 6 times; and the final cold rolling step has a thickness reduction in a range of 5% to 30%, preferably of 5% to 20%, to obtain a steel sheet having a thickness in a range of 0.1 to 0.5 mm, preferably of 0.1 to 0.4 mm, more preferably of 0.15 to 0.35 mm.

2. The method according to claim 1 , wherein steps d) and e) are 1 to 4 times, preferably 2 to 3 times.

3. The method according to claim 1 or 2, wherein the cold rolling step e) is carried out with a thickness reduction between 10% and 30%, more preferably between 10% and 20%.

4. The method according to any one of the preceding claims, wherein the cold rolling step c) is carried out up to an intermediate thickness of 0.5-2.0 mm.

5. The method according to any one of the preceding claims, wherein the interannealing is performed at a temperature in a range between 650°C and 1100°C, preferably between 700°C and 1000°C, more preferably between 850°C and 950°C.

6. The method according to any one of the preceding claims, wherein the surface of the interannealed strip is cleaned to remove oxides prior to a cold rolling step.

7. The method according to any one of the preceding claims, wherein the interannealing step(s) is / are performed in a protective atmosphere, preferably wherein the protective atmosphere contains less than 10000 ppm oxygen, more preferred less than 1000 ppm oxygen.

8. The method according to any one of the preceding claims, wherein the aluminium content is in a range of 0.5 to 5.0 wt.%, and preferably of 0.5 to 3 wt.%.

9. The method according to any one of the preceding claims, wherein the nickel content is in a range of 0.02 to 0.3 wt.%, and preferably of 0.03 to 0.12 wt.%.

10. The method according to any one of the preceding claims, wherein the silicon content is in a range of 0.02 to 1.0 wt.%, preferably of 0.05 to 0.5 wt.%, and more preferably of 0.10 to 0.35 wt.%.

11. An ultra-thin TWIP steel sheet (16) having a composition consisting of, in wt.%:C 0.05-1.25%,Mn 10-40%, preferably 10-20%,Al 0.1-5%, preferably 0.5 to 5%,Ni 0.0-2.5%, preferably 0.02-0.3%,Cr 0.0-5.0%, preferably 0.0-0.3%,Si 0.0-5.0%, preferably 0.02-1 .0%,V 0.0-0.10%, preferably 0.03-0.10%,S 0.0-0.02%,P 0.0-0.06%,N 0.0-0.02%, incidental elements up to 0.10% each and up to 0.5% in total, the remainder being impurities and Fe, having a thickness in a range of 0.1 to 0.5 mm, preferably of 0.1 to 0.4 mm, more preferably of 0.15 to 0.35 mm, and having a relative magnetic permeability <1.8, an electrical resistivity > 0.8 Q-m, and a microstructure having an austenite content > 98 wt.%.

12. The ultra-thin TWIP steel sheet (16) according to claim 11 , further having the following properties:• Yield strength > 750 MPa, preferably > 800 MPa,• Ultimate tensile strength > 900 MPa, preferably > 950 MPa,• A80 total elongation > 13%, preferably > 15%.

13. The ultra-thin TWIP steel sheet (16) according to claim 11 or 12 having been manufactured by the method of any one of claims 1 to 10.

14. Use of an ultra-thin TWIP steel sheet (16) according to any one of claims 11 to 13 in the manufacturing of a stator or rotor (2) of an electric machine.

15. A stator or rotor (2) of an electric machine, comprising an ultrathin TWIP steel sheet (16) according to any one of claims 11 to 13, in particular a stack of multiple ultra-thin steel sheet parts.

16. A method of manufacturing a stator or rotor of an electric machine according to claim 15, comprising a step of applying and curing an organic coating applied on a stack of multiple ultra-thin steel sheet parts, preferably the curing is at a temperature in a range of 160°C to 240°C for 20 sec to 30 min.

Images