EP2675927B1 - Method for producing a grain-oriented flat steel product - Google Patents

Description

The invention relates to a method for producing a grain-oriented steel flat product with minimized magnetic loss values and optimized magnetostrictive properties.

The grain-oriented flat steel products in question here, also referred to in the jargon as "HGO material", are steel bands, in technical language also simply "electrical tapes", or steel sheets, in technical language simply called "electric sheets". From such flat steel products parts are manufactured for electrical engineering applications.

Grain-oriented electrical steel or sheet is particularly suitable for applications in which a particularly low loss of magnetization is in the foreground and high demands are placed on the permeability or polarization. Such requirements exist in particular for parts for power transformers, distribution transformers and higher quality small transformers.

As in detail for example in the EP 1 025 268 B1 is generally explained in the course of the production of Steel flat products, first of all a steel containing (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 each optionally 0.010 to 0.3% Cu, to 0.060% S, to 0.100% P, to 0.2% each of As, Sn, Sb, Te, and Bi, balance iron and unavoidable impurities, to a starting material, such as a Slab, thin slab or a cast strip, potted. The starting material is then subjected, if necessary, to an annealing treatment to be subsequently hot rolled into a hot strip.

After the reeling and optionally additionally performed annealing as well as a likewise optionally completed descaling or pickling treatment, a cold strip is then rolled out of the hot strip in one or more steps, wherein an intermediate annealing can be carried out between the cold rolling steps, if necessary. In the subsequent decarburization annealing, the carbon content of the cold strip is usually significantly reduced to avoid magnetic aging.

After decarburization annealing, an annealing separator, which is typically MgO, is applied to the strip surfaces. The annealing separator prevents the turns of a coil wound from the cold strip from sticking together during the high-temperature annealing that is then carried out. During the high-temperature annealing, which is typically carried out in a hood furnace under protective gas, the texture is created in the cold strip by selective grain growth. Furthermore, forms on the strip surfaces a forsterite layer, the so-called "Glass Film", off. In addition, the steel material is cleaned by diffusion processes occurring during the high-temperature annealing.

Following the high-temperature annealing, the resulting flat steel product is coated with an insulating layer, thermally directed and subjected to low-stress annealing in a final "final annealing". This final annealing can be carried out before or after the assembly of the steel flat product produced in the above-described manner to the blanks required for further processing, wherein the additional stresses arising during the compartmenting process can be reduced by a final annealing after the blanks have been divided. Steel flat products produced in this way generally have a thickness of 0.15 mm to 0.5 mm.

The metallurgical properties of the material, the degrees of cold rolling set in the production of flat steel products and the parameters of the heat treatment steps are coordinated so that targeted recrystallization processes occur. These recrystallization processes lead to the typical for the material "Goss texture", in which the direction of the easiest magnetization in the rolling direction of the finished strips. Grain-oriented flat steel products accordingly have a strongly anisotropic magnetic behavior.

There are various ways of improving the core losses of a grain-oriented flat steel product Methods. For example, the orientational sharpness of the Goss texture of the flat steel product can be improved. Further loss reductions can be achieved by reducing the spacings of the 180 ° domain walls. High tensile stresses in the rolling direction, which are transferred to the steel surface via insulating coatings, also contribute to the reduction of the domain distances and, as a result, to the reduction of the core losses. However, the required tensile stress values can only be realized to a limited extent for technical reasons.

For example, in the DE 18 04 208 B1 or the EP 0 409 389 A2 Another proposed possibility of improving the loss is that partial plastic deformations are produced on the surface of the flat steel product. This can be done for example by a mechanical scoring or piercing the surfaces of the respective flat steel product. The significant improvements in the magnetic properties achieved in this way are offset by the disadvantage that the mechanical treatment of the surface damages the insulating layer of the flat steel product applied thereto. For example, in the case of the formation of transformer plates from such a flat steel product, this can lead to short-circuits in the stacked core of the transformer and to local corrosion.

Attempts to take advantage of mechanical scribing or piercing without destroying the insulation have focused on the use of laser sources ( EP 0 008 385 B1 . EP 0 100 638 B1 . EP 1 607 487 A1 ). Common to the use of lasers based method is that a laser beam focused on the surface of the treated steel flat product and there is a thermal stress in the base material is generated. This leads to the formation of dislocations, at which components of the magnetic flux emerge from the surface of the flat steel product. As a result, the magnetic stray field energy is locally raised, to compensate for the formation of so-called "termination domains" takes place, which are referred to in the jargon as "secondary structures". At the same time, there is a reduction in the main domain distance.

Since the abnormal core loss depends on the distance of the main domains, the losses are minimized by a suitable laser treatment. The laser treatment can improve the corona loss of a grain-oriented flat steel product with a nominal thickness of 0.23 mm typical for these products by more than 10% compared to the untreated state. The loss improvements depend both on the properties of the base material, such as the grain size and texture sharpness, and on the laser parameters, to which the distance L of the lines along which the laser beams are guided onto the respective flat steel product, the contact time t _dwell and the specific Energy density U _s belong. The tuning of these parameters has a decisive influence on the respectively achieved reduction of the re-magnetization losses.

In addition to the re-magnetization losses, noise generation also plays a role in transformers. This is based on a known as magnetostriction physical effect.

Magnetostriction is the change in length of a ferromagnetic material in the direction of its magnetization. By operating a ferromagnetic component, such as a transformer, in an alternating magnetic field, the main 180 ° domains are shifted, which alone, however, does not contribute to the magnetostriction. However, at transitions between the 180 ° major domains to the 90 ° terminal domains, magnetostrictive stresses exist in the material. These form a sound source during operation in the alternating magnetic field and are the cause of the transformer noise.

The introduction of additional 90 ° termination domains, ie of secondary structures, by laser treatment generally leads to an increase in the magnetostriction and thus the noise emissions, in particular during operation of a transformer.

The demands made on minimizing noise in the operation of transformers are constantly increasing. On the one hand, this is due to continuously tightened legal requirements and standards. On the other hand, consumers today generally no longer accept electrical appliances that cause an audible "transformer drone". So today depends the acceptance of large transformers in the vicinity of residential development is crucially dependent on the noise emissions resulting from the operation of such transformers.

A series of laser treatment processes have been proposed that allow both the improvement of losses and better magnetostrictive properties by choosing suitable process parameters ( DE 601 12 357 T2 / EP 1 154 025 B1 . DE 698 35 923 T2 / EP 0 897 016 B1 . EP 2 006 397 A1 . EP 1 607 487 A1 ). However, the optimization of the parameters of the laser treatment has in each case been undertaken only with a view to improving the re-magnetization losses.

In addition to the above-described prior art is from the JP-A-2005 226122 a system with which it should be possible to accurately predict the magnetic properties of an electromagnetic steel sheet after it has been subjected to laser treatment for adjusting its magnetic domain.

Furthermore, it is also from the WO 2009/082155 A1 It is known that by laser treatment, the magnetic properties of a steel sheet for electromagnetic applications can be optimized by refining the magnetic domains so that minimized iron losses are achieved.

Finally, out of the DE 600 04 244 T2 discloses a method for improving the magnetic properties of a steel grain oriented grain steel which has undergone secondary recrystallization finish annealing and is provided with an insulating coating. In this process For example, the steel sheet moved in one direction of movement is continuously scanned with a 10.46 μm wavelength CO2 emission laser in a direction transverse to the direction of movement. The selected process parameters "specific radiant energy", "residence time" and "distance between two successive tracks of the laser beam on the steel sheet" are within 0.1 and 25 mJ / mm ² , 1x10 ^-6s - 1x10 ^-2 s and 2 - 12 mm synchronously and continuously adjusted to optimize the improvement of at least one of the magnetic characteristics of the sheet, with the choice between magnetostriction, induction and core losses continuously measured before and after the laser beam treatment.

Against the background of the prior art discussed above, the object of the invention was to provide a method for producing flat steel products, which are optimally suitable for the production of parts for transformers.

This object has been achieved according to the invention, that in the production of a flat steel product, the operations specified in claim 1 are performed.

Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

1. a) Bereitstellen eines Stahlflachprodukts,
   und
2. b) Laserbehandeln des Stahlflachprodukts, wobei im Zuge der Laserbehandlung in die Oberfläche des Stahlflachprodukts mittels eines mit einer Leistung P von einer Laserstrahlquelle emittierten Laserstrahls linienförmige Verformungen eingeformt werden, die in einem Abstand L angeordnet sind.

In accordance with the prior art described above, a method according to the invention for producing a grain-oriented flat steel product comprises minimized magnetic loss values and optimized magnetostrictive properties the steps

1. a) providing a flat steel product,
   and
2. b) laser treatment of the flat steel product, wherein in the course of the laser treatment in the surface of the flat steel product by means of a laser emitted with a power P from a laser beam laser beam line deformations are formed, which are arranged at a distance L.

Special requirements for the manner of production of the flat steel product provided in step a) do not exist. Thus, the flat steel product provided for the method according to the invention can be produced using the measures generally known to the person skilled in the art and initially based on suitable steel alloys, which are also already well known from the prior art. Of course, this also includes such manufacturing processes and alloys that are not yet known.

According to the invention, the parameters of the laser treatment (step b)) are set so that a flat steel product produced according to the invention not only has minimized magnetization losses, but also optimizes its apparent power S _{1.7 / 50 NACH} after laser treatment.

For this purpose, according to the invention, the apparent power S _{1.7 / 50} of the flat steel product to be treated with the laser beam, determined at a frequency of 50 Hertz and a polarization of 1.7 Tesla, is detected before and after the laser treatment (step b)).

Depending on the difference between the apparent power S _{1.7 / 50 VOR} detected before the laser treatment and the apparent power S _{1.7 / 50 AFR} detected after the laser treatment, the distance a of the linear deformations, the contact time t _dwell , then becomes the parameters of the laser treatment of the laser beam, the specific energy density U _s , the laser power P, the focus size Δs or the _scan speed v _scan varied so that the difference between the respectively detected before and after the laser treatment apparent power S _{1.7 / 50 is} less than 40%.

The parameters of the laser treatment are thus adjusted according to the invention so that an increase in the apparent power S _{1.7 / 50 of} a steel flat product processed according to the invention during laser treatment is limited by setting the parameters of the laser treatment so that the apparent power S ₁ detected after the laser treatment _{, 7/50 FOLLOWS BY THE FOLLOWING CONDITION} : $${\mathrm{S}}_{1.7/50\mathrm{TO}}<1.4\times {\mathrm{S}}_{1.7/50\mathrm{IN\; FRONT}}$$

The increase in the apparent power caused by the laser treatment is accordingly limited according to the invention so that the apparent power after laser cutting is not more than 40% is increased compared to its value on the same workpiece before lasing.

The invention thus takes into account that in the design transformers are usually not the loss of magnetization of each processed flat steel products in the foreground, but the apparent power. Therefore, according to the invention, the parameters of the laser treatment are optimized not only with regard to the re-magnetization losses but also to the apparent powers with the same polarization.

The subject matter of the method according to the invention is thus an optimization of the laser parameters with regard to the minimization of the magnetic reversal loss P _{1.7 / 50} and the apparent power S _{1.7 / 50} . It turned out that minimizing the apparent power minimizes the noise increase. This means that the laser treatment, although superficially causes the refinement of the main domains, which leads to the desired loss reduction, but that this is accompanied by the optimization of the laser treatment according to the invention with respect to the lowest possible apparent power, a comparatively small increase in volume areas with secondary magnetic structures.

In principle, it is conceivable to carry out the laser treatment on electrical sheets or sheet metal blanks. However, it proves to be particularly practical if a flat steel product present as strip material is processed, which passes through the laser treatment in a continuous pass.

In the case that the respective apparent power S _{1.7 / 50} is recorded online before and after the laser treatment during operation and the parameters of the laser treatment are varied online as a function of the difference of the detected apparent powers S _{1.7 / 50} , it is particularly possible promptly respond to changes in the outcome of the laser treatment.

However, it is also possible to perform the detection of the apparent power before and after the laser treatment and the calibration of the laser parameters decoupled in time. For this purpose, samples of the flat steel product can be taken at certain time intervals, the respective apparent power S _{1.7 / 50} can be determined on these samples before and after the laser treatment and the parameters of the laser treatment can be varied depending on the result of this detection. This embodiment makes it possible to carry out the method according to the invention with comparable method and measuring technology.

Practical tests have shown that to achieve optimum apparent power S _{1.7 / 50 it} may be expedient to vary the distance L of the linear deformations in the range from 2 to 10 mm, in particular 4 to 7 mm.

Likewise, a minimization of the change in apparent power S _{1.7 / 50} occurring via the laser treatment can be achieved by varying the exposure time t _{dwell of} the laser beam in the range from 1 × 10 ^-5 s to 2 × 10 ^-4 s.

In the case where a fiber laser is used as the laser source, the laser power P in the fiber lasers available today can be varied in the range from 200 to 3000 W in order to minimize the change in the apparent power S _{1.7 / 50} occurring via the laser treatment become. Fiber lasers have the particular advantage here of allowing close focusing of the laser beam. For example, track widths of less than 20 μm can be achieved with a fiber laser.

However, it is also possible to use a CO ₂ laser in the implementation of the method according to the invention as a laser source. Due to the fact that in such a laser, the laser beam can not be so strongly focused, here in the currently available CO ₂ lasers, a variation of the laser power P in the range 1000 to 5000 W for the purpose of minimizing the entering via the laser treatment Change in Apparent power S _{1,7 / 50} displayed.

Of course, the inventive method can preferably be carried out on such flat steel products, which is occupied at least with an insulating layer. In addition, between the insulating layer and the steel substrate of the flat steel product, for example, a glass or forsterite layer may still be present.

Fig. 1

ein Diagramm, in dem die Verlustverbesserung ΔP_1,7/50 und Scheinleistungsänderung ΔS_1,7/50 über den Abstand L der Laserspuren aufgetragen sind;

Fig. 2

ein Diagramm, in dem aus der gemessenen Längenänderung berechnete Geräusche N als Funktion der Polarisation J dargestellt sind.

To demonstrate the effect of the invention, the following examples of a procedure according to the invention have been investigated. Show it:

Fig. 1

a diagram in which the loss improvement ΔP _{1.7 / 50} and apparent power change ΔS _{1.7 / 50} over the distance L of the laser tracks are plotted;

Fig. 2

a diagram in which from the measured change in length calculated noise N as a function of the polarization J are shown.

As part of systematic investigations, various parameters of the operational laser device were varied with a 1 kW multimode fiber laser. The parameters to be optimized were the distance L of the laser lines, the laser power P, the focus size Δs and the _scan speed v _scan .

The empirical evaluation of a test matrix showed that variations of the above mentioned parameters can lead to drastic changes of the apparent power with significant improvements of the core losses.

As an example shows Fig. 1 Loss improvement ΔP _{1,7 / 50} (symbolized by filled squares) and apparent power change ΔS _{1,7 / 50} (symbolized by empty circles) as a function of the distance L of the laser tracks. The reference quantities are in each case the changes ΔP _{1.7 / 50 of} the power loss P _{1.7 / 50} and the change ΔS _{1.7 / 50 of} the apparent power S _{1.7 / 50 in} relation to the un-lasered state, ie the state before the laser treatment ( Step b)) specified.

By varying the focus size Δs and the _scan speed v _scan , ie the speed with which the laser is moved, different exposure times t _{dwell of} the laser beam were generated on the surface of the flat steel product present as strip material. The relationship between t _dwell , Δs and v _scan can be described as follows: $${\mathrm{t}}_{\mathrm{dwell}}=\mathrm{\Delta }\mathrm{s}/{\mathrm{v}}_{\mathrm{scan}}$$

The range of exposure times ranging from 1.times.10.sup.- ⁵ seconds to ^{2.times.10.sup.-4} seconds results in a certain range with equal improvements in the magnetization losses P _{1.7 / 50} in different apparent power changes ΔS _{1.7 / 50} . It was found that with minimized apparent power changes ΔS _{1.7 / 50} an optimal noise behavior of the treated flat steel product is achieved.

The following examples show the influence of exposure time t _dwell on core loss P _{1.7 / 50} and apparent power S _{1.7 / 50} :
0.23 mm thick steel strips were laser-treated. The contact time t _dwell was varied on the basis of the above-explained relationships.

After measuring the magnetic characteristics, the changes summarized in Table 1 below ΔP _{1.7 / 50} , ΔS _{1.7 / 50 of} the magnetization losses P _{1.7 / 50} and the apparent power S _{1.7 / 50 were} : Table 1 sample P [W] Δs [mm] t _dwell [s] ΔP _{1.7 / 50} [%] ΔS _{1.7 / 50} [%] 1 900 5 9.9 ^* 10-5 -12 + 70 2 900 5 6.6 ^* 10-5 -13 + 46 3 900 5 3,3 ^* 10-5 -13 + 18

The samples were examined below for their magnetostrictive properties and used to calculate the expected noise during operation. A method was used to calculate the noise from the magnetostriction measurements, as described in the IEC Technical Report IEC 62581 TR and in the publication of E. Reiplinger, "Assessment of grain-oriented transformer sheets with respect to transformer noise", Journal of Magnetism and Magnetic Materials 21 (1980), 257-261 , have been published.

Fig. 2 shows the noise N calculated from the measured change in length as a function of polarization J.

The solid curve is in Fig. 2 the reference state before the laser treatment ("without laser treatment"), wherein the measured values, which form the basis of this curve, are symbolized by circles filled with black.

The dashed line whose measurements are indicated by unfilled squares is in Fig. 2 the noise development in a laser treatment again, which has led to a change in the apparent power S _{1.7 / 50} by + 70%.

The narrower dashed line, whose measured values are characterized by unfilled triangles, is given in Fig. 2 the noise development in a laser treatment again, which has led to a change in the apparent power S _{1.7 / 50} by + 46%.

The dotted line whose measurements are indicated by unfilled circles indicates Fig. 2 the noise development in a laser treatment again, in which the parameters of the laser treatment have been selected in accordance with the invention so that the change in apparent power S _{1.7 / 50} has remained limited to + 18%.

The change ΔP _{1.7 / 50 of} the power loss P _{1.7 / 50} achieved with the laser treatment was in each case - 13% compared to the initial state before the laser treatment.

The calculated noises with the inventively achieved, optimized apparent power changes of ΔS = +18% are therefore always smaller than in the initial state.

If, on the other hand, no attention is paid to the apparent power, a noise increase of 1.1 to 1.5 dB is observed for comparable loss improvements.

Out Fig. 2 Although it is apparent that at high levels of the transformer to 1.7 Tesla, for example, the differences in noise between a treated steel flat product according to the invention and a conventionally treated flat steel product only small. However, they are still there given systematically. In addition, these differences are immediately apparent with a lower modulation of the transformer, ie with smaller magnetic polarizations.

By optimizing the laser parameters according to the invention so that the difference between the apparent power S _{1.7 / 50} measured before and after the laser treatment is less than 40%, an effective minimization of the power losses P _{1.7 / 50} on the one hand can be achieved on the other hand but also minimize the noise emission during operation. It is irrelevant whether the inventively carried out comparison of measured before and after the laser treatment values of the apparent power S _{1,7 / 50} online on-line or takes place in time decoupled held calibrations.

Claims (10)

1. Method for producing a grain-oriented flat steel product that is intended for the manufacture of parts for electrotechnical applications and has minimised magnetic loss values and optimised magnetostrictive properties, including the operations

   a) providing a flat steel product,

   b) laser-treating the flat steel product, wherein, in the course of the laser treatment, linear deformations, which are arranged with a spacing a, are formed into the surface of the flat steel product by means of a laser beam emitted by a laser beam source with a power P,

   characterised in that the apparent power S_1.7/50 of the flat steel product before and after laser treatment (operation b)) at a frequency of 50 Hertz and a polarisation of 1.7 Tesla is measured, and in that as parameters of the laser treatment the spacing a between the linear deformations, the dwell time t_dwell of the laser beam, the specific energy density U_s, the laser power P, the focus size Δs or the scan speed v_scan are varied such that the difference between the apparent power S_1.7/50 measured before and after treatment is less than 40%.
2. Method according to Claim 1, characterised in that the laser treatment is continuous.
3. Method according to any one of the preceding claims, characterised in that the respective apparent power S_1.7/50 before and after laser treatment in continuous operation is measured online and the parameters of the laser treatment are varied online depending on the difference between the apparent powers S_1.7/50.
4. Method according to either Claim 1 or Claim 2, characterised in that the samples of the flat steel product are taken at certain intervals, the apparent power S_1.7/50 of each of these samples before and after laser treatment is determined and the parameters of the laser treatment are varied depending on the results of these measurements.
5. Method according to any one of the preceding claims, characterised in that the spacing a between the linear deformations is varied in the range from 2-10 mm.
6. Method according to Claim 5, characterised in that the spacing a between the linear deformations is varied in the range from 4-7 mm.
7. Method according to any one of the preceding claims, characterised in that the dwell time t_dwell of the laser beam is varied in the range from 1 x 10^-5 s to 2 x 10^-4 s.
8. Method according to any one of the preceding claims, characterised in that a fibre laser is used as a laser source and the power P is varied in the range from 200 - 3000 W.
9. Method according to any one of the preceding claims, characterised in that a CO₂ laser is used as a laser source and the power P is varied in the range from 1000 - 5000 W.
10. Method according to any one of the preceding claims, characterised in that the flat steel product is coated with an insulation layer.

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