Patent Application
20180237876
The invention relates to a method of producing a grain-oriented electrical steel strip and to a grain-oriented electrical steel strip.
When the present application refers to “electrical steel strips”, this means electrical steel sheets and electrical steel strips produced by rolling steels of suitable composition, and circuit boards or blanks that have been divided therefrom, which are intended for the production of parts for electrical engineering applications.
Grain-oriented electrical steel strips of the type in question here are especially suitable for uses in which the emphasis is on a particularly low cyclic magnetization loss and high demands are made on permeability or polarization. Such demands exist especially in the case of parts for power transformers, distribution transformers and higher-quality small transformers.
As elucidated specifically, for example, in EP 1 025 268 B1, in the course of the production of electrical steel strips, generally a steel comprising (in % by weight) typically 2.5% to 4.0% Si, 0.010% to 0.100% C, up to 0.150% Mn, up to 0.065% Al and up to 0.0150% N, and in each case optionally 0.010% to 0.3% Cu, to 0.060% S, to 0.100% P, and to in each case 0.2% As, Sn, Sb, Te and Bi, the balance being iron and unavoidable impurities, is first cast to give a preliminary material, such as a slab, thin slab or a cast strip. The preliminary material is then, if required, subjected to an annealing treatment and then hot-rolled to give a hot strip.
The resultant hot strip is coiled to give a coil and can then, if required, be subjected to annealing and to a likewise optionally executed descaling or pickling treatment. Then a cold strip is rolled from the hot strip in one or more stages, with performance of intermediate annealing if required between the cold rolling steps in a multistage cold rolling operation effected in multiple steps.
The resultant cold strip then typically undergoes a decarburization anneal, in order to minimize the carbon content of the cold strip for avoidance of magnetic aging.
After the decarburization anneal, an annealing separator is applied to the strip surfaces, which typically comprises MgO. The annealing separator prevents the windings of a coil wound from the cold strip from being welded to one another in a subsequently conducted high-temperature anneal.
During the high-temperature anneal, which is typically conducted in a bell furnace under protective gas, a microstructure texture that makes a significant contribution to the magnetic properties forms in the cold strip as a result of selective grain growth.
At the same time, a forsterite layer forms on the strip surfaces during the high-temperature anneal, often also referred to in the technical literature as “glass film”.
In addition, the steel material is cleaned by diffusion processes that proceed during the high-temperature anneal. Subsequent to the high-temperature anneal, the flat steel product having the forsterite layer which is obtained in this way is coated with an insulation layer, thermally aligned and subjected to stress-relief annealing in a concluding “final anneal”. This final anneal can be effected before or after the finishing of the flat steel product produced in the manner described above to give the blanks required for further processing. By means of a final anneal which is conducted after the blanks have been divided off, the additional stresses that have arisen in the course of the dividing operation can be dissipated. Electrical steel strips produced in such a way generally have a thickness of 0.15 mm to 0.5 mm.
As further elucidated in WO 03/000951 A1, it is likewise prior art that the domain structure can additionally be improved by the application of an insulation layer which exerts a permanent tensile stress on the sheet substrate, and additionally also that by a treatment in which lines of local stresses are generated transverse or oblique to the rolling direction in the flat steel product, the magnetic properties of grain-oriented electrical steel strips can be further improved. Surface structures of this kind can be generated, for example, by local mechanical deformations (EP 0 409 389 A2), laser or electron beam treatments (EP 0 008 385 B1; EP 0 100 638 B1; EP 0 571 705 A2) or etching of trenches (EP 0 539 236 B1).
For example, it is additionally known from EP 0 225 619 B1 that the forsterite film also has an important influence on essential use properties of electrical steel strips. For example, the losses, the noise characteristics in the transformer or else the bond strength of the insulation are affected by the forsterite film between magnetically active base material and insulation layer.
Therefore, the following demands are made in practice on the forsterite film:
Means of optimizing the effect of the forsterite film by chemical additions to the annealing separator applied to the cold-rolled flat steel product prior to the high-temperature anneal are elucidated, for example, in WO 95/25820 A1.
It is likewise known that the properties and effect of the forsterite film are also affected by the process steps that the steel substrate undergoes in the production of grain-oriented electrical steel strips prior to the application of the annealing separator. An indicator here is the composition of the oxide layer present on the steel substrate prior to the bell anneal, which leads to reproducible glass films in the case of a high-temperature anneal executed according to the prior art with subsequent relaxation annealing and optional additionally conducted laser treatment.
In the consideration of the relationship between the condition of the steel substrate in the production of grain-oriented electrical steel strips prior to the application of the annealing separator and the high-temperature anneal and the resultant properties of the forsterite film obtained, the main emphasis in the literature is typically on the interdigitation of the forsterite with the steel substrate, since the adhesion of the composite composed of forsterite film and insulation coating which is formed in the subsequent steps is significantly dependent thereon.
For example, JP 2004/191217 A1 has proposed improving the bond strength of the insulation layer by optimizing the uppermost oxide layer by means of examinations on the basis of Fourier transform infrared spectrometry “FTIR”. For this purpose, an infrared beam is guided onto the surface at a defined angle and the directed reflection is measured. Since multiple reflections occur within the material, depending on the angle of incidence, it is possible to measure only the uppermost portion of the oxide layer. Therefore, this method can permit only conclusions about the bond strength; it is not possible to use it to determine other properties, for example the later tensile strength.
Therefore, the already published JP 2004/191217 A1 or other technical publications, for example the article “Rapid quantitative analysis of fayalite and silica formed during decarburization of electrical steel” by Jung et al., published in Surface Interface Analysis 2012, 44, 270-275, or the article “Characterization of chemical information and morphology for in-depth oxide layers in decarburized electrical steel with glow discharge sputtering”, likewise by Jung et al., published in Surface Interface Analysis 2013, 45, 1119-1128, discuss a combination with invasive techniques, for example rf-GDOES or else wet chemistry. However, these methods do not permit any conclusions as to the molecular composition of the oxide layer or the forsterite layer produced thereon, since the removal of the surface disrupts the molecular composition. Even a combination with microscopic methods as proposed in the second article by Jung et al. cited above does not result in any conclusions that would allow the oxide layer to be described in such a way as to permit direct conclusions to be drawn for practical use.
Against the background of the prior art, the problem addressed was that of specifying a method of producing grain-oriented electrical steel strips with which the surface constitution of the respective flat steel product can be adjusted in a controlled manner prior to the application of the annealing separator such that a forsterite film with optimal effect in terms of the magnetic properties of the electrical steel strip to be produced is obtained.
The invention has solved this problem by following the procedure of the method specified in claim 1 in the production of grain-oriented electrical steel strips.
Advantageous configurations of the invention are specified in the dependent claims and are elucidated specifically hereinafter, as is the general concept of the invention.
According to the invention, in the production of grain-oriented electrical steel strips, the steps that are typically envisaged for this purpose in the prior art are implemented.
These include
It will be apparent that the method of the invention may comprise further steps which are conducted in the conventional production of electrical steel strips in order to achieve optimized magnetic properties or properties that are important for practical use. These include, for example, reheating of the precursor obtained after the casting of the steel, descaling of the hot strip prior to the cold rolling or, in the case of the multistage performance of cold rolling, intermediate annealing conducted in a conventional manner between the cold rolling stages in each case.
Crucial factors here in deciding whether, in the production of electrical steel strips, a surface constitution of the respective flat steel product prior to the application of the annealing separator which enables the reliable formation of a forsterite film with optimal effect in terms of the magnetic properties of the electrical strip to be produced is obtained are, in accordance with the invention,
0.5×area(Fe2SiO4)≤area(αSiO2)≤2×area(Fe2SiO4).
The invention proceeds here from the finding that, firstly, the bond strength of the forsterite film on the steel substrate is controlled solely by the uppermost atomic layers of the oxide layer, whereas the stress transmitted to the base material can be modified only within certain limits. In order, however, to further increase the tensile stress, it is necessary, for example, in the case of an annealing separator consisting essentially of MgO, to change the morphological arrangement of the magnesium atoms in the matrix of the SiO4 tetrahedra within the forsterite film formed from the annealing separator.
For this purpose, according to the findings of the invention, it is necessary not just to control the molecular composition of the near-surface layer and the atomic composition of the oxide layer, but also to molecularly characterize and influence the entire oxide layer in a controlled manner.
In order to assure this, in accordance with the invention, the oxide layer is characterized by means of “diffuse reflectance Fourier transformation infrared spectroscopy”, also referred to as “DRIFT method” for short. In the DRIFT method, an IR light beam is directed onto the sample surface by means of concave mirrors and the reflected light is also detected by means of concave mirrors (see Beasley et al., “Comparison of transmission FTIR, ATR and DRIFT spectra”, Journal of Archeological Science, Vol. 46, June 2014, pages 16-22). This enables the evaluation of deeper-lying oxide layers and hence a deeper analysis of the molecular components in the oxide layer. On the basis of the result of the DRIFT analysis, the process parameters in the subsequent processing of the flat steel products are then adjusted such that an oxide layer favorable for the formation of an optimally adhering forsterite film that simultaneously exerts optimally high tensile stresses is formed on the steel substrate.
The analysis of the DRIFT spectrum of the oxide layer present on the surface of the flat steel product after the cold rolling should be checked continuously in order firstly to detect the quality of the oxide film across the entire surface of the flat steel product in question for each batch of electrical steel strips. Secondly, the information derived in accordance with the invention from the DRIFT spectrum allows optimization of the results in the production of subsequent batches of electrical steel strips. If the DRIFT spectrum shows that the ratio of the proportions of α-SiO2 and fayalite (Fe2SiO4) molecules does not meet the specifications of the invention, for this purpose, the process steps in the method of the invention that have been implemented up to the application of the annealing separator are adjusted. In other words, the steel analysis, the parameters for the hot strip anneal, the parameters for the cold rolling and the parameters for the oxidation/primary recrystallization anneal are adjusted such that the condition set in accordance with the invention for the molecular proportions that show in the DRIFT spectrum
0.5×area(Fe2SiO4)≤area(αSiO2)≤2×area(Fe2SiO4).
is satisfied.
The oxidation/primary recrystallization anneal can be combined in a manner known in practice with a decarburization anneal, in which the carbon content of the steel substrate is minimized, and a nitriding treatment which is likewise optionally conducted in a manner known per se, which has the aim of increasing the nitrogen content of the steel substrate.
The area(αSiO2) and area(Fe2SiO4) can be determined here for the peaks representing the proportion of the αSiO2 and Fe2SiO4 molecules in a manner known per se (see Foley, “Equations for chromatographic peak modeling and calculation of peak area”, Analytical Chemistry, Vol. 59, Aug. 1, 1987, pages 1984-1985) as the area enclosed by the respective peak and its baseline, the start and end of the baseline being determined by the two foot points F1, F1′; F2, F2′ of the respective peak, i.e. the points where the line of the spectrum gives way to the respective peak (see
Typically, in the method of the invention, the cold rolling (step f)) is conducted in at least three cold rolling steps, typically with an intermediate anneal between the cold rolling steps in a manner known per se, in order to eliminate the cold solidifications that arise in each preceding cold rolling step and to assure rollability for the subsequent rolling step. In the practical implementation of the method of the invention, the hot strip is likewise optionally subjected to a hot strip anneal in a manner which is likewise known, in order to assure optimal cold rollability.
The character of the oxide layer on the cold strip obtained after the cold rolling can then be influenced via the steel composition smelted in step a) and the adjustment of the parameters for the optional hot strip anneal, for the cold rolling and for the oxidation/primary recrystallization anneal, taking account of the inventive measure that follows in each case, where the measures in question can be utilized in combination with one another or alternatively to one another:
kH=Tmax/(8×DPmax+10×K)
kC=Tob/(2×Ab)
For the oxidation/primary recrystallization anneal (step g)) an index kOx is determined by the formula
kOx=Tox/(5×DPox)
Then the parameters Tmax, DPmax, K, Tob, Ab, Tox and DPox for steps a), f), g) of the process of the invention and the composition of the steel substrate processed in accordance with the invention are adjusted such that the indices kH, kC and kOx satisfy the conditions
% Sn/% Cu≤kC≤3×(% Sn/% Cu+% Cr+kH) (1)
¼×(kH+kC+% Sn/% Cu)≤kOx≤2×(kH+kC+% Sn/% Cu+% Cr) (2)
and, if a hot strip anneal is conducted,
γ1150/100×3≤kH≤γ1150/100×15 (3)
where
γ1150=694×% C−23×% Si+64.8
and with % C the carbon content of the steel melt, with % Sn the Sn content of the steel melt, with % Cu the copper content of the steel melt and with % Cr the chromium content of the steel melt, each reported in % by weight.
The parameter γ1150 is the percentage alpha/gamma conversion, which is elucidated in detail in EP0600181.
It has been found that, surprisingly, an oxide layer produced in accordance with the invention enhances the diffusion of nitrogen into the steel base material when the steel substrate processed in accordance with the invention is subjected to a nitriding process as described, for example, in EP 0 950 120 A1.
Suitable annealing separators for the purposes of the invention are especially those conventional annealing separators which consist predominantly, i.e. typically to an extent of at least 85% by weight, of MgO.
In principle, it is conceivable to execute the high-temperature anneal in a continuous run. However, a particularly advantageous high-temperature annealing method in relation to the desired optimization of the magnetic properties and the practical utility of electrical steel strips produced in accordance with the invention has been found to be a high-temperature anneal (step i)) conducted in the form of a bell anneal. The temperatures for the high-temperature anneal are typically in the temperature range of 1000-1250° C. known per se for this purpose.
In accordance with the above elucidations, a grain-oriented electrical steel strip of the invention comprises a cold-rolled steel substrate consisting of a steel comprising (in % by weight) 2.0-4.0% Si, up to 0.100% C, up to 0.065% Al and up to 0.020% N, and in each case optionally up to 0.5% Cu, up to 0.060% S and likewise optionally in each case up to 0.3% Cr, Mn, Ni, Mo, P, As, Sn, Sb, Se, Te, B or Bi, the balance being iron and unavoidable impurities, wherein a forsterite film present on said steel substrate features a higher peak at the wavenumber of 977 cm−1 than at the wavenumber of 984 cm−1 in a spectrum recorded by means of diffuse reflectance Fourier transformation infrared spectroscopy. An electrical steel strip of this kind can especially be produced by employing the method of the invention.
The carbon content of the electrical steel strip having the characteristics of the invention is typically at least 0.01% by weight, but may also be lower as a result of the process steps implemented in the course of its production, especially in the case of corresponding performance of the optional decarburization anneal.
The invention is elucidated in detail hereinafter by working examples. The figures show:
Melts A-F with the compositions specified in table 1 have been smelted and cast to give a 65 mm strand, from which thin slabs have been divided as intermediate product.
In 28 experiments, cold strips for the production of grain-oriented electrical steel strips have been produced from the thin slabs in the manner described hereinafter.
After reheating to a reheating temperature of typically 1170° C., the thin slabs have been hot-rolled to give a hot strip having a thickness of typically 2.3 mm, which has then been coiled to give a coil. The coiling temperature was typically 540° C.
Subsequently, the respective hot strip has been subjected to a hot strip anneal in which it has been through-heated in each case at a maximum temperature Tmax under an atmosphere having a maximum dew point Dpmax, and after which it has been cooled to room temperature in each case with a cooling rate K.
The hot strips have subsequently been cold-rolled in five passes to give a cold strip in each case. The mean surface temperature Tob of the cold strip during the last three cold rolling passes and the total decrease in thickness Ab achieved over the last three cold rolling passes have been determined here.
The cold strips obtained after the cold rolling have been subjected to a combined annealing treatment in which a decarburization under an atmosphere with a maximum dew point Dpdec and a maximum annealing temperature Tdec, an oxidation/primary recrystallization at a maximum annealing temperature Tox and a maximum dew point Dpox, and in some selected samples a nitriding treatment at a maximum temperature Tnit under an atmosphere with a maximum dew point Dpnit have been conducted.
An oxide layer was present on each of the cold strips thus obtained, for each of which a DRIFT spectrum has been recorded.
In addition, for the samples, the indices kH, kC and kOx and the areas “area(Fe2SiO4)” and “area(αSiO2)” that are taken up by the peaks attributed to the Fe2SiO4 molecules and the αSiO2 molecules at the wavenumber 980 cm−1 (Fe2SiO4 molecules) and 1250 cm−1 (αSiO2 molecules), and the ratio “area(Fe2SiO4)/area(αSiO2)” have been determined. Samples for which the area(Fe2SiO4)/area(αSiO2) ratio is in the range of 0.5-2 meet the requirements of the invention.
Subsequently, an annealing separator that consisted to an extent of 90% of MgO has been applied to the cold strips.
The cold strips that have thus been coated have been subjected to a high-temperature anneal conducted as a bell anneal, the maximum temperature of which was 1200° C. First of all, in a manner known per se, the cold strips have been kept here under an atmosphere consisting to an extent of 75% by volume of hydrogen and of 25% by volume of nitrogen, and finally in a cleaning phase under an atmosphere consisting to an extent of 100% by volume of hydrogen.
After the cooling, a forsterite film was present on each of the samples, of which a DRIFT spectrum has again been recorded in each case. In the DRIFT spectra of the forsterite film recorded for the 28 samples, the peak heights at the wavenumbers of 977 cm−1 and 984 cm−1 have been determined and compared to one another. Samples for which the peak height at the wavenumber of 977 cm−1 is smaller than at the wavenumber of 984 cm−1 are not in accordance with the invention.
In table 2, for examples 1-28 of the alloys A-F specified in table 1 of which the steel substrate of the respective sample consisted, the temperature Tmax and the maximum dew point Dpmax for the hot strip anneal, the cooling rate K in the subsequent cooling, the mean surface temperature Tob of the cold strip during the last three cold rolling passes and the total decrease in thickness Ab achieved over the last three cold rolling passes, the maximum dew point Dpdec and the maximum annealing temperature Tdec for the decarburizing anneal, the maximum annealing temperature Tox and the maximum dew point Dpox of the oxidation/primary recrystallization anneal, the maximum temperature Tnit and the maximum dew point Dpnit of the optionally conducted nitriding treatment are specified.
In table 3, for examples 1-28, the indices kH, kC and kOx and the values of “area(Fe2SiO4)”, “area(αSiO2)”, the ratio F/αS, i.e. the ratio “area(Fe2SiO4)”/“area(αSiO2)”, the peak heights at the wavenumbers 977 cm−1 and 984 cm−1, the difference 977-984, i.e. the “peak height at wavenumber 977 cm−1”−“peak height at wavenumber 984 cm−1” are reported. In addition, table 3 emphasizes which of examples 1-28 are inventive and which are not.
The tensile stress exerted on the respective steel substrate by the forsterite film obtained on samples 1-28 was typically 6 MPa for the inventive samples.
All measurements on samples 1-28 were conducted with conventional analysis units. A Bruker Tensor 27 from Bruker Corporation was used for the FT-IR measurements, which were conducted either in the state after the decarburizing anneal or after the bell anneal, and a “Praying Mantis” cell, which is manufactured by HARRICK SCIENTIFIC PRODUCTS INC., for the DRIFT measurements.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 114 358.5 | Aug 2015 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/070316 | 8/29/2016 | WO | 00 |
