Rapidly Solidified High Silicon Steel with Minor Boron Addition That Improves Magnetic Properties

This patent application claims the benefit of U.S. Provisional Patent Application No. 63/647,192, filed May 14, 2024, the entire teachings and disclosure of which are incorporated herein by reference thereto.

This invention was made with government support under DE-AC-02-07CH11358 awarded by the Department of Energy. The government has certain rights in the invention.

This invention generally relates to an electrical steel and, in particular, to an electrical steel having 6.5 wt % Si and minor additions of boron configured for production of melt-spun ribbons.

The soft magnetic market for electric machines and power electronics is dominated by electrical steel containing about 3.2% silicon (Fe-3.2% Si, wt. %). Fe-3.2% Si gained popularity mainly due to its excellent performance/cost ratio, which can be partly attributed to its low materials cost and high processability. With the increasing demand for higher power densities, newer generation electric machines are designed to operate at higher frequencies. Such high-frequency operation can only be effective in improving motor power density if the iron losses of the materials can be maintained at a relatively low level, which depends on the materials' thickness, electrical resistivity, magnetocrystalline anisotropy, and permeability. Among these factors, electrical resistivity is particularly important for reducing eddy current at high frequency. Though Fe-3.2% Si has a favorable saturation magnetic flux density as high as 2.0 T, its electrical resistivity is only 52 μΩ cm.

The electrical resistivity of iron can be significantly increased by alloying with metalloid elements. The most potent element in increasing the electrical resistivity of iron discovered so far is silicon. With Si content increasing from zero to 3.2%, the resistivity increases from 10 to 52 μΩ-cm; when Si further increases to 6.5%, the resistivity will increase to 82 μΩ-cm. Such increase is effective in reducing the core loss. For example, the loss W10/400 (i.e., induction of 1 T at 400 Hz) of Fe-3.2% Si and Fe-6.5% Si is 14 and 6, respectively. However, the increased Si content promotes brittleness due to the formation of embrittling B2 and D0phases, and Fe-6.5% Si cannot be processed using the traditional cost-effective cold roll process.

Research shows that the brittleness issue of the high silicon steel can be mitigated with rapid solidification. Planar flow casting has been found suitable for producing ductile Fe-6.5% Si ribbon. albeit there are limitations on the width and surface quality of the ribbon. These limitations originate from the high melting point of Fe-6.5% Si, which is ˜400° C. higher than a typical amorphous alloy that can be mass-produced by planar flow casting. The ribbon quality is also affected by surface tension and liquid viscosity, which in turn affects the melt pool behavior on the quench wheel.

According to the Fe—Si—B ternary phase diagram, boron forms multiple eutectics with reduced melting points. Boron's effect on the surface tension of Fe—Si melt has not been reported, but it is known to reduce the surface tension of Fe at 1550° C. Similar behavior is expected for Fe—Si melt with boron alloying, which is expected to result in better wetting. Boron is also expected to reduce the viscosity of the melt. Reduced surface tension and viscosity suggest better flow characteristics leading to better processability.

Fe-6.5% Si has previously been alloyed with boron in minor amounts (75 ppm to 530 ppm) to produce rolled plates. The boron additions helped to increase the grain boundary cohesion and grain refinement, which enabled the warm rolling (˜500° C.) of Fe-6.5% Si sheets. Further, boron alloying into directionally solidified Fe-6.5% Si helps to increase the alloy's undercooling, homogenization, strength, and ductility. However, an understanding of the effect of boron in melt-spun Fe-6.5% Si with respect to processability and magnetic and mechanical properties is lacking.

In a first aspect, embodiments of the disclosure relate to a method of preparing a ribbon of electrical steel comprising 6.5 wt % Si. In this method, a stream of the electrical steel in a molten form is ejected onto an outer surface of a rotating wheel. The stream of electrical steel is cooled on the outer surface of the rotating wheel to form the ribbon. The ribbon is removed from the outer surface of the rotating wheel by its moment of inertia. The ribbon has a thickness as measured perpendicular to the ribbon surface of 0.1 mm or less. The electrical steel includes from 100 ppm to 2100 ppm of B.

In a second aspect, embodiments of the disclosure relate to the fabrication method in which the ribbon of electrical steel comprises a saturation magnetization of at least 17.6 kG (1400 kA/m).

In a third aspect, embodiments of the disclosure relate to the method according to the first aspect or the second aspect in which the ribbon of electrical steel has a Vickers hardness (HV) in a range from 300 to 500.

In a fourth aspect, embodiments of the disclosure relate to the method according to any of the first aspect to the third aspect in which, under a maximum magnetic flux density of 10 kG (1 T) with excitation source in direct current mode, a maximum permeability (μ) of the ribbon of electrical steel is greater than 1000.

In a fifth aspect, embodiments of the disclosure relate to the method according to any of the first aspect to the fourth aspect in which, under a maximum magnetic flux density of 10 kG (1 T) with excitation source in direct current mode, coercivity (H) of the ribbon of electrical steel is less than 0.7 Oe (55 A/m).

In a sixth aspect, embodiments of the disclosure relate to the method according to any of the first aspect to the fifth aspect in which, under a maximum magnetic flux density of 10 kG (1 T) with the excitation source in an alternating current mode at a frequency of 400 Hz, an iron loss of the ribbon of electrical steel is 12 W/kg or less.

In a seventh aspect, embodiments of the disclosure relate to the method according to any of the first aspect to the sixth aspect in which the ribbon of electric steel includes about 14 wt % or less of Fe—Si—B eutectic phase.

In an eighth aspect, embodiments of the disclosure relate to an electrical steel structure. The electrical steel structure includes a ribbon having a length, a width, and a thickness. The length is perpendicular to the width, and the thickness is perpendicular to both the length and the width. The ribbon is made of Fe-6.5% Si and 100 ppm to 2100 ppm of B. The thickness of the ribbon is 0.1 mm or less.

In a ninth aspect, embodiments of the disclosure relate to the electrical steel structure of the eighth aspect in which the ribbon has a saturation magnetization of at least 17.6 kG (1400 kA/m).

In a tenth aspect, embodiments of the disclosure relate to the electrical steel structure of the eighth aspect or the ninth aspect in which the ribbon exhibits a Vickers hardness (HV) in a range from 300 to 500.

In an eleventh aspect, embodiments of the disclosure relate to the electrical steel structure of any of the eighth aspect to the tenth aspect in which, under a maximum magnetic flux density of 10 kG (1 T) with excitation source in direct current mode, a maximum permeability (μ) of the ribbon is at least 1000.

In a twelfth aspect, embodiments of the disclosure relate to the electrical steel structure of any of the eighth aspect to the eleventh aspect in which, under a maximum magnetic flux density of 10 kG (1 T) with excitation source in direct current mode, coercivity (H) of the ribbon is less than 0.7 Oe (55 A/m).

In a thirteenth aspect, embodiments of the disclosure relate to the electrical steel structure of any of the eighth aspect to the twelfth aspect in which, under a maximum magnetic flux density of 10 kG (1 T) with excitation source in alternating current mode at a frequency of 400 Hz, an iron loss of the ribbon is 12 W/kg or less.

In a fourteenth aspect, embodiments of the disclosure relate to the electrical steel structure of any of the eighth aspect to the thirteenth aspect in which the ribbon comprises about 14 wt % or less of Fe—Si—B eutectic phase.

In a fifteenth aspect, embodiments of the disclosure relate to an electrical component including a plurality of laminated layers of the electrical steel structure according to any of the eighth aspect to the fourteenth aspect.

In a sixteenth aspect, embodiments of the disclosure relate to an electrical component according to the fifteenth aspect in which the electrical component is at least part of a stator or rotor of an electric motor.

In a seventeenth aspect, embodiments of the disclosure relate to an electrical component according to the fifteenth aspect in which the electrical component is at least part of a core of a transformer.

In an eighteenth aspect, embodiments of the present disclosure relate to an electrical component comprising a plurality of laminated layers of the electrical steel structure according to any of the eleventh aspect to the seventeenth aspect.

In a nineteenth aspect, embodiments of the present disclosure relate to the electrical component of the eighteenth aspect in which the electrical component comprises at least part of a stator or rotor of an electric motor.

In a twentieth aspect, embodiments of the present disclosure relate to the electrical component of the eighteenth aspect in which the electrical component comprises at least part of a core of a transformer.

Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.

Embodiments of the present disclosure relate to a method of melt-spinning Fe-6.5% Si electrical steel alloyed with boron and to a ribbon produced by the method. As will be discussed more fully below, the addition of boron to Fe-6.5% Si electrical steel improves the cooling of the melt when contacting a spinning wheel during the melt-spinning process. Specifically, the boron lowers the melting temperature of the electrical steel and improves the wettability of the electrical steel on the surface of the spinning wheel. The improved wettability increases the contact time between the surface of the spinning wheel and the electrical steel, allowing for increased heat dissipation from the resulting ribbon.

Additionally, the boron does not substantially affect the ductility of the electrical steel such that long continuous ribbons of uniform width and thickness can be produced. Notwithstanding, the ribbon retains sufficient hardness to provide good deformation when processed into laminated structures for electric motor or transformer parts through conventional methods, such as stamping.

Further, the addition of a small amount of boron surprisingly enhances magnetic properties of the electrical steel ribbon, including doubling permeability and decreasing coercivity. The effect of boron on the properties of the Fe-6.5% Si electrical steel is far greater than would be expected from the small amounts of boron added (e.g., in a range of 100 ppm to 2100 ppm). These and other aspects and advantages of the disclosed method of melt-spinning an Fe-6.5% Si electrical steel having alloying additions of boron and ribbon produced by same will be described more fully below and in relation to the figures provided herewith. These embodiments are provided by way of illustration and not limitation.

Fe-6.5% Si is too brittle for typical cold rolling and stamping to make electric motor and transformer components. Melt-spinning offers a way to rapidly solidify the electrical steel to retain some ductility, but the melting temperature of Fe-6.5% Si is too high (by at least 300° C.) for melt-spinning using existing apparatuses. By adding boron to the Fe-6.5% Si electrical steel composition, the enhanced wettability allows the melt to stay on the wheel for an extra of time, which will allow more heat to be dissipated through the wheel. Moreover, the addition of boron lowers the melting temperature, which reduces the overall amount of the heat to be dissipated. Together, these two effects allow for the melt-spinning of Fe-6.5% Si, and the ribbons formed therefrom have advantageous mechanical and magnetic properties.

depicts an apparatusfor melt-spinning of a ribbon of Fe-6.5% Si alloyed with boron. As shown in, the apparatusincludes a cruciblecontaining metal feedstock. The metal feedstockis Fe-6.5% Si alloyed with boron. In one or more embodiments, the Fe-6.5% Si comprises 100 ppm to 2100 ppm of boron, in particular 500 ppm to 1500 ppm of boron. This level of boron addition is more than mere impurity levels, which would be less than 100 ppm (e.g., 10 to 50 ppm), and reflects an intentional addition of boron to the electrical steel composition to achieve the desired properties as discussed below. Advantageously, the addition of boron helps to decrease the melting temperature, lowering the temperature needed to perform the melt-spinning process. In one or more embodiments, the ribbon of electric steel comprises up to 14 wt % of Fe—Si—B eutectic composition.

Disposed around the crucibleis a heating element. In one or more embodiments, the heating elementis, for example, an inductive heating element, among other possibilities. The heating elementis configured to melt the metal feedstock, and the molten metal feedstockis forced through a nozzleof the crucibleas a streamof molten metal. In one or more embodiments, the metal feedstockis forced through the nozzleusing pressurized gas, such as an inert gas (e.g., argon).

The streamis directed onto a spinning wheel. As shown in, the wheelis depicted as rotating, which allows the streamto contact an uncovered surface of the wheel. In this way, the surface of the wheelimmediately cools the streamof molten metal to produce a solidified ribbon. To facilitate cooling, in one or more embodiments, the wheelis cooled with a fluid, such as water. In one or more embodiments, the wheelis rotating at a speed of at least 3 m/s (tangential speed), in particular a speed of at least 7 m/s (tangential speed). The degree of cooling is dependent, at least in part, on the length of time that the stream/ribbon/is in contact with the outer surface of the wheel. According to embodiments of the present disclosure, contact between the stream/ribbon/is enhanced based on the boron additions to the Fe-6.5% Si, which improves the wettability of the molten electrical steel on the wheel. Further, in one or more embodiments, the ribbon of electric steel is in contact with the outer surface of the rotating wheelover an arcuate distance (D) of at least 25 mm. In one or more embodiments, the rotating wheel has a diameter of 250 mm or more. In one or more embodiments, the ribbon of electric steel is in contact with the rotating wheelfor at least 5° of rotation (as denoted by rotation angle θ in).

The rapid solidification of the molten metal to form the ribbonallows the Fe-6.5% Si metal to bypass or partially bypass the ordered phases (specifically, B2 and D0) that result in brittleness. As will be discussed more fully below, the ribbonexhibits sufficient ductility for melt-spin processing while retaining sufficient hardness for stamping procedures. Electrical steel ribbonsaccording to the present disclosure may be used in electrical componentsas shown in, such as statorsor rotorsof electric motors or in transformer cores. The structures may be formed using a stack of ribbonsthat may be stamped to near net-shape and laminated together. In one or more embodiments, at least some of the ribbonswithin the stack are coated with an insulating material (e.g., varnish) or separated by a sheet of insulating material (e.g., paper). For example, a layer of insulation (e.g., varnish or paper) is provided between every layer or up to every ten layers of ribbons. In one or more embodiments, the laminated ribbonsare joined together using one or more pins extending through the stack, using welds, or using adhesives, amongst other possibilities.

While the ductility allows for continuous ribbons of uniform width and thickness to be produced and handled, the ribbonsshould be sufficiently hard to allow for stamping into stator, rotor, or transformer coreshapes to be used in laminations. The ductility of the electrical steel ribbons is described below in relation percentage of samples that withstand parallel plate bending tests. In one or more embodiments, the ribbon of electrical steel comprises a Vickers hardness (HV) in a range from 300 to 500, in particular in a range from 370 to 430.

In one or more embodiments, the ribbonsproduced via melt-spinning have a thickness (dimension of ribbonperpendicular to the outer surface of the wheel) of 0.1 mm or less, in particular in a range from 0.05 mm to 0.1 mm and most particularly about 0.03 mm. The thickness of the ribboncan be controlled, e.g., based on the speed of the spinning wheeland the rate of flow of the streamof molten metal. In one or more embodiments, the ribbonshave a width in a range from 1 mm to 300 mm. Commercially, melt-spun ribbonsare typically produced having widths of about 50 mm or about 250 mm. Further, in one or more embodiments, the ribbonsmay be melt-spun to lengths up to 1000 m. While not particularly limited, the ribbonstypically have a length of at least 10 m when produced via melt-spinning.

Advantageously, the ribbonof Fe-6.5% Si electrical steel comprising boron additions according to the present disclosure exhibits desirable magnetic properties for use as components in electric motors or transformers. In one or more embodiments, the ribbon of electrical steel comprises a saturation magnetization of at least 17.6 kG (1400 kA/m). In one or more embodiments, under a maximum magnetic flux density of 10 kG (1 T) with excitation source in direct current mode, a maximum permeability (μ) of the ribbon of electrical steel is at least 4000. In one or more embodiments, under a maximum magnetic flux density of 10 kG (1 T) with excitation source in direct current mode, coercivity (H) of the ribbon of electrical steel is less than 0.7 Oe (55 A/m). In one or more embodiments, under a maximum magnetic flux density of 10 kG (1 T) with excitation source in an alternating current mode at a frequency of 400 Hz, an iron loss of the ribbon of electrical steel is 12 W/kg or less.

Fe-6.5% Si electrical steel was first pre-alloyed by arc melting of high-purity iron and silicon chunks (>99.9%) in an arc furnace under an argon atmosphere. High-purity boron chunks were then added to the iron-silicon melt. In particular, the boron chunks were submerged in the iron-silicon melt to minimize the mass loss (<0.1% mass loss after melting). In the samples prepared, boron was added in an amount in a range from 0.01 wt % to 2.24 wt %. The samples were flipped at least three times to ensure homogeneity during each arc melting. The arc melted buttons were then drop cast into 10 mm diameter rods.

The Fe-6.5% Si rods were melted and heated to 1650° C. inside a quartz crucible using induction heating. The meltstock was injected onto a rotating copper wheel spinning at 20 m/s tangential speed with an overhead pressure of 120 Torr. The nozzle orifice was 0.81 mm in diameter, and the melt spinning chamber was filled with ⅓ of He after 3 vacuum flushes. To probe the melt spinning process, a 12-bit complementary metal-oxide semiconductor (CMOS) high-speed camera was set up and focused on the side profile of the ribbon as the ribbon was being produced.

The melting behavior of the alloys was measured by differential scanning calorimetry (NETZSCH, DSC 404). The heating rate was 10° C./min. The cross-sectional micrographs of the samples were taken using a scanning electron microscope (SEM) (Teneo, FEI Inc) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. The X-ray diffraction (XRD) patterns were collected via Bruker DaVinci D8 system equipped with a Cu target. Microhardness tests were conducted on polished cross-sections using a microhardness tester (LECO LM 247AT). The magnetic properties of the alloys were measured using Vibrating Sample Magnetometer (VSM) (Versalab, Quantum Design, Inc.) up to 30 kOe (237 kA/m) magnetic field. The closed-loop magnetic measurement was done using a computer-automated soft magnetic test station (model SMT-700, KJS Associates/Magnetic Instrumentation) with a single-strip test fixture. The melt-spun ribbons were annealed in a sealed quartz tube filled with Ar at 1100° C. for 2 hours and then were assembled into a plate 18 mm wide by 65 mm long for the magnetic measurement. A magnesium oxide (MgO) coating was applied to the ribbons to prevent inter-ribbon adhesion during annealing and was removed after the annealing. The densities of the alloys needed for the flux density calculation were measured on drop-casted samples using Archimedes' method.

The DSC curves inshow the melting behavior of the alloys. The melting onset of the liquidus is depressed, accompanied by the formation of a eutectic phase with boron alloying. The eutectic temperature (1145-1160° C.) in(first dip in each of the B-alloyed curves) correlates well with the eutectic temperature in Fe—B or Fe—Si—B phase diagram. The onsets of the liquidus and the eutectic are plotted inas a function of boron content. The liquidus onset for the Fe-6.5% Si was 1415° C., and it was lowered by 15° C. with 0.21 wt % of boron, which was further lowered to 1350° C. with 1.10 wt % of boron. When boron is added to 2.24 wt % (close to the eutectic composition), the melting point of Fe-6.5% Si is fully depressed to the eutectic temperature of 1145° C., which is 270° C. lower than its original melting point. The 2.24 wt % (10 at %) of boron addition will bring the alloy into the amorphous region. The formation of an amorphous structure is confirmed on melt-spun ribbon with 2.24 wt % boron alloying through XRD studies (not shown). By estimating the heat of fusion or change in enthalpy in melting (area under the curve),shows the relative fractions of the solid solution and eutectic phases. The amount of eutectic phase increases rapidly with increasing boron additions. The amount of eutectic is ˜14.2% with 0.21 wt % of boron addition, where it increases to 54.1% and 100% with 1.10 wt % and 2.24 wt % of boron additions, respectively.

The Fe-rich side of the Fe—B phase diagram consists of FeB and Fe solid solution with a maximum solubility of 0.02 at % with a eutectic temperature of 1175° C. According to the 10 at. % Si vertical section of Fe—Si—B pseudo-binary phase diagram, a ternary eutectic is present between Fe, FeB, and FeSiB. The eutectic temperature is 1112° C. The eutectic temperature is in good agreement with the lower temperature peak by the DSC (˜1150° C.), considering a higher Si % (12.14 at %) in Fe-6.5% Si alloy.

The formation of intermetallics and its eutectics (especially in) in the as-cast samples (having 0 wt %, 0.2 wt %, 0.5 wt %, and 1 wt % B) can be identified in the XRD patterns ofand the SEM microstructure in, having 0.2 wt %, 0.5 wt %, and 1 wt % B, respectively. The shift in the XRD for the 0.02 wt % B is reversed compared to rest suggests that the composition of the matrix is not taking up all the solutes. The micrographs show that the fraction of intermetallic phase increases with increasing boron addition. The XRD patterns and the phase diagram analysis suggest that the intermetallic could be FeSiB. The intermetallic effectively pins the gain growth of the Fe-6.5% Si solid solution phase, which may lead to lower eddy current losses at high frequencies. The cooling rate (˜10° C./s) for the as-spun sample seems fast enough to prevent intermetallic precipitation where only BCC solid solution phase is present in the XRD patterns ().

To study the physical properties of the alloy with boron alloying, the saturation magnetization Ms and the microhardness were measured. As shown in, overall, the Ms drops almost linearly with boron alloying. Boron, being a paramagnetic element, dilutes the ferromagnetic moment of iron. The drop in Ms is minor, being only 0.5 kG/at % B. Interestingly, saturation magnetization Ms increased with 0.04 wt % boron alloying, which has previously been reported in boron additions to electrical steel. Below a critical concentration, boron segregation in the grain boundary was believed to result in increased grain coarsen, leading to higher flux density (permeability) and lower core losses. As shown in, the microhardness of the samples does not follow a monotonic relationship with boron content. Rather, the hardness drops initially, then rise with increasing boron content. It is well known that hardness is closely related to grain size (Hall-Petch relationship) and intermetallic formation. The formation of the intermetallic phase and refined grains () with boron alloying (>0.10 wt %) are responsible for the increase in hardness observed. The hardness of Fe-6.5% Si is also closely related to its ordering degree. A lower ordering degree resulting from faster cooling can significantly reduce the hardness of Fe-6.5% Si. Boron alloying forms a low melting eutectic that aids the cooling of Fe-6.5% Si. The lower ordering degree due to faster cooling is responsible for the initial (B≤0.10 wt %) hardness reduction. Such an initial drop in hardness is consistent with the reported increased ductility with minor amounts of boron additions.

High-quality ribbon/tape production (i.e., uniform thickness and width) of Fe-6.5% Si depends heavily on the flow characteristics of the alloy during the melt spinning or planar flow casting process. An essential parameter for the flow characteristics is the melt-wheel contact distance. For Fe-6.5% Si, a longer wheel contact distance resulted from a more stable melt pool is desirable as it improves the cooling of the ribbon. Longer wheel contact distance also has significant practical importance. It helps heat management during manufacturing (by removing heat from the ribbon to the wheel and avoiding ribbons leaving the wheel red hot) and aids in controlling detached angles when a ribbon detachment mechanism is used. The ribbon(melt)-wheel contact distance was evaluated via a high-speed camera by imaging the side profile of the ribbon as it is being formed. The white triangle symbols inmark where the ribbon is detached from the wheel for Fe-6.5% Si containing 0 wt % B and for Fe-6.5% Si containing 0.21 wt % B, respectively. It quantitatively shows an improved wheel contact with boron alloying. This is direct evidence of better flow characteristics from added boron owing to improved wetting and reduced viscosity. Boron alloyed ribbons also are longer and more continuous, with improved surface quality.

A parallel plate bending test was also performed to characterize the ductility of the as-spun ribbon. This test quantifies the bending strain of the sample when bending the ribbon between two plates until the ribbon breaks. The distance of the two parallel plates is noted, and together with the thickness of the ribbon were used to calculate the bending strain. Due to the high ductility of the ribbon, the number of ribbons that broke when the two plates completely close, i.e., the spacing of the parallel plates becomes zero, were counted. The likelihood of fracture (out of 10 tests for each composition) are 0%, 10%, 10%, and 50% for the 0%, 0.046%, 0.1%, and 0.207% boron alloyed samples, respectively. It suggests that minor boron alloying (≤0.10 wt %) has a minimum impact on the ductility of the samples. However, there is a noticeable ductility decrease when 0.21 wt % boron is added due to excess intermetallic formation.