CS207458B1 - annealing separator - Google Patents

Description

The present invention relates to an annealing separator for grain oriented silicon steel strips based on magnesium oxide.

Annealing separators have a positive effect on the thermal, in particular annealing treatment and reduce the overall cost of the manufacturing process, and on the other hand, improve the magnetic and electrical properties of the annealed strip.

It is a well known fact that the final annealing treatment is a critical step in the production of grain oriented silicon steel strips for use as a magnetic material. The annealing treatment not only determines the selective growth of grains having a particular orientation with respect to the plane and direction of rolling, but also removes certain impurities (e.g. sulfides) from the strip, which, although necessary in the actual production process to obtain the desired grain orientation, influenced the magnetic properties of the finished strip.

This annealing treatment is typically carried out in intermittent furnaces and normally requires significant completion times; for example, dwell times of at least twenty hours at equalization temperature are quite common, although, according to some patents, much shorter annealing cycles are possible. In this annealing, the strip is introduced into the furnace in coils or, in certain cases, in bundles of sheets stacked on top of each other. This arrangement in part affects the annealing treatment time because it prevents the free flow of the reducing atmosphere (which removes sulfides) between the individual layers of stacked coils or bundles. Furthermore, the individual coil or bundle layers tend to adhere to each other during annealing.

The aforementioned separators, hereinafter referred to as "annealing separators", were originally introduced for the express purpose of eliminating the above problems and consisted of refractory powders spread on the surface of the steel strip before the strip was wound into coils or bundled. Over time, it has been found that these refractory powders can perform other functions than simply keeping the coil layers physically apart and preventing them from sticking together (thereby facilitating the free flow of the reducing atmosphere), i.e., they can react with sulfur to facilitate removal of the element and further may form an adhesive glassy film capable of insulating the web both electrically and chemically. Therefore, increasingly complex release agents have been developed from the original powders of a single component, such as the calcium or magnesium oxide separators described in U.S. Pat. No. 2,492,682, and finally there were powders which, in addition to the magnesium oxide base, contained other compounds such as Tg, V2O5, MnO?

A clear and understandable front view of the progressive development of the annealing separator design can be found in various recent patents filed by ARMCO (British Pat. Nos. 1,108,949 and 1095,903) or by ΝΓΡΡΟΝ STEEL, Co. (British Pat. No. 1,183,092) and U.S. Pat. Nos. 3,868,280 and 3,676,227 and many other applicants, which additionally include silicate-based release agents.

Annealing separators are available at present, which either fully or partially perform the following functions:

- avoidance of adhesion between the individual coil layers in the annealing process in intermittent furnaces;

Improving the stream of reducing atmosphere between the coil layers;

- reaction with sulfur expelled from the steel strip;

Forming a protective coating on the belt;

Forming an electrically insulating coating on the strip;

Forming a coating capable of imparting a slight belt tension;

Forming a suitable base for the subsequent electrical insulating coating.

Numerous attempts have been made in various countries to improve the complex, costly, and extremely lengthy manufacturing process of a silicon-oriented silicon steel strip as well as to improve the ferromagnetic properties of the finished product, as can be seen from numerous patents filed at that time and many technical work published in the last few years. In order to contribute positively to the advancement of technical knowledge in this special field, the applicants jointly developed a research project aimed at reducing the duration and cost of the manufacturing process and improving certain electrical and magnetic properties of the finished strip, ultimately leading to the invention described and claimed in this application .

The above mentioned drawbacks are eliminated by an annealing separator for grain oriented silicon steel strips based on magnesium oxide according to the invention, characterized in that the separator comprises at least one compound selected from the group comprising rare earth metal oxides, for example carbon dioxide, lanthanum, neodymium and praseodymium and rare earth metal compounds, such as cerium, lanthanum, neodymium and praseodymium hydroxides, which form oxides upon heat treatment.

According to a preferred embodiment of the invention, the rare earth content of the coating ranges from 5% to 30% of the total weight of the coating, calculated as oxides.

According to a further embodiment of the invention, the rare earth metal oxides are generally of a particle size which pass through a sieve having a mesh size of less than 50.4 per cm ^{2 of} sieve, with 35 to 55% by weight of these oxides having a particle size that does not pass through the sieve; With a mesh number of 77.5 per cm ^{2 of} sieve and a coating content thereof in the range of 0.8 to 7% of the total weight of the coating.

Surprising results with far-reaching consequences, some of which were totally unexpected and unpredictable, have proved to be possible in this way, based on the available knowledge of the problem.

The improvements achieved by the invention can be summarized as follows:

A noticeable positive effect on the roughness of the strip surface. Initially, however, small discontinuities were observed below the surface of the strip, which were considered inclusions. However, a more detailed analysis of these discontinuities led to the conclusion that these were sections of small depressions filled with glass film. In this way, the glass film controls the roughness of the strip surface;

- an increase in de-nitrided and desulfurization rates, i.e., acceleration of slow processing steps to remove strip contaminants, which caused the extremely long annealing times that were normally required;

A significantly increased surface specific resistance of the coating;

- increased adhesion of the coating to the belt.

The above improvements lead to the following practical advantages:

- to the absolute certainty that, in any case, the steel strip will benefit from the tensioning phenomenon by which the coating is applied to the strip. acts as a result of different values of the coefficient of thermal expansion in the strip and in the coating;

- it has been found that for a given belt type, a reduction in surface roughness results in greater susceptibility to tension. The resulting voltage leads in turn to extremely favorable magnetostriction values;

- to substantially reduce the end annealing time;

- in some cases, the possibility of removing the additional coating by tensioning and insulating glass, which has so far been considered essential;

- fewer problems associated with mechanical belt processing.

A detailed description of the invention will now be given with reference to its practical use in conjunction with the accompanying drawings.

Giant. Ia is a photomicrograph (100x magnification) of the surface layer of the annealed treated strip. a separator containing only MgO, Fig. 1b is a photomicrograph (at 1000X magnification) of the strip surface layer

Fig. 1c is a photomicrograph (at 100X magnification) of the surface layer of the annealed separator strip according to the invention; Fig. 2 shows the magnetostriction curves obtained for the steel strips in the manner of the strips shown in Figs. 1b and 1c. Figures 3a and 3b show the hysteresis loops obtained for steel strips similar to those shown in Figures 1c and 1b (at 1.5 Tesla); Figure 4 shows the effect of tensioning on magnetic losses (at 1.5 Tesla, thickness 0, Fig. 5 is a desulfurization diagram and Fig. 6 is a denitrification diagram.

According to a first embodiment of the invention, the added amounts are preferably such that the rare earth aggregates (such as oxides) contained in the annealing separators range from 5% to 30% of the total weight. The amount of annealing separator deposited on the strip was preferably 6 to 10 g per m ² . The thickness of the final coating should be about 1 to 3 microns.

In order to obtain a quantitative evaluation of the improvements obtained using the annealing separators of the invention, longitudinal parallel strips (just after industrial cold rolling) were prepared from a grain oriented silicon steel strip containing 2.93 weight percent Si. This method was chosen because it gave the best guarantee of homogeneity of the test strip.

The five narrow strips of 400 m long obtained in this way were each coated with one of the following release agents:

O - MgO reference standard

A - MgO + MnO2 (MgO / MnO2 = 95/5) comparative sample

B - MgO + MnO2 + B2O3 (0.1% w / w, as B) comparative sample

C - MgO + rare earth oxides (10 wt.%)

D - MgO + rare earth oxides (10% w / w) + sodium silicate (10% w / w).

Giant. 1a shows a sample of a steel strip coated with an annealing separator containing only MgO and selected as a common reference standard.

Giant. 1b and 1c show samples of steel strips coated with release agents A and C, respectively. · The differences between the different test samples are immediately apparent from the micrographs. After examining a large number of samples taken randomly from five differently coated strips, an interesting and important fact was found: the surface roughness of the strip coated with release agent 0 was considerably uniform along the length of the strip and more or less equal to the level shown in Fig. instead, the release agents A and B varied to some extent, both in quantity and in position, i.e. samples were obtained which along the length of the strip were either smoother or rougher than the sample shown in FIG. 1b. Finally, the strips coated with the separating agents C and D gave in all cases samples which were almost completely free of surface cavities.

The view of the magnetostriction curves (Fig. 2) and the hysteresis loops (Figs. 3a and 3b) referring to the samples of Figs. 1b and 1c show the effect of surface roughness on the final magnetic properties of the steel strip.

In FIG. 2, the curves a, a ‘and b, b‘ refer to the strips coated with the separating agents A and B respectively. C. As can be seen from curves b, b poměrně, relatively low values of magnetostriction are obtained using a release agent constructed in accordance with the invention; furthermore, magnetostriction values exhibit only limited variation as the magnetization increases from zero to a maximum value during one cycle. It may also be observed that the magnetostriction curves remain at very low levels, although magnetization reaches values close to the theoretical peak values permissible for the type of steel strip under consideration; it is clear that these conditions give very low noise levels in the transformer and other magnetic cores.

In contrast, the magnetostriction curves obtained with the strips coated with commonly used release agents are very similar to those shown in FIG. 2 letters a, and ‘. In this case, the magnitude of the magnitude changes in the strip dimensions are not only larger in size, but are also more severe.

Comparison between the two sets of curves shows that for magnetization below 1.2 to 1.9 Tesla the magnetostriction increases from -0.4.10 ^-6 to more than 1.10 ~ ⁶ (curve a) and from approximately -110 · 6 · to approximately 3.. 10 ^-6 (curve a ') using the conventional type separation agents, while the magnetostrictional variations around -0.6 are relatively small. 10 ⁶ (curve b), or from a value of about ^-6 to about -0,5.10 -0,8.10 ^-6 (curve b ') using ^annealed: Olfaction separators, arranged according to the invention.

Even more convincing evidence of the effect of surface roughness on the final magnetic properties of the steel strip is given by the diagrams reproduced in Figures 3a, 3b and 4. In Figures 3a and 3b the hysteresis loops of the strips are coated with a Separating Agent constructed according to the invention (Fig. 3a) and separation Reagent A (FIG. 3b) is compared with the hysteresis loop of the corresponding uncoated steel strip (curve R. and curve NR in FIGS. 3a and 3b). Giant. 4 shows the effect of voltage on the loss of magnetization for the strip of the type shown in FIG. 1b, both in the state

Ί after rolling (curve a) and after removal of the surface layer by pickling (curve b) As is immediately apparent, the steel strip is not affected by stress when its surface is rough, but the stress effect is beneficial once the roughness has been removed .

Giant. 5 and 6 show desulfurization and denitrification diagrams for uncoated steel strip (curve a), for strip coated with release agent A (curve c) and for steel strip coated with release agent C (curve b). These diagrams relate to lanes that are narrower than the lanes used industrially and thus cannot represent the real situation; however, these diagrams are shown for the fully justified and valid purpose of the comparative evaluation. curves (and therefore without any scales). It should be stressed that industrial tests have already confirmed the validity of the curves in qualitative terms.

As a guide, the residence time for coils at the peak annealing temperature in the bell furnace may be reduced by at least · 15% (in some industrial tests, · even 50% was achieved).

With respect to other advantages achievable by the use of annealing separators constructed according to the invention, experimental tests have shown that the coating deposited by these new materials on the steel strip during annealing has a much higher surface specific resistance and a much greater adhesion than coatings obtained conventionally. release agents used. Some significant data are shown in the following table for the relative comparison of five steel strips coated with release agents O, A, B, C and D and two industrial strips E, F together with the values found in the patent literature.

TABLE 1 separační specific surface resistance reagents Ω / cm ² Adhesion average diameter

O 0.3 2.6 1.4 > 20 mm AND 2.5 25 12 20 mm В 2 26 13 20 mm C 35 200 100 ALIGN! <5 mm D 30 230 100 ALIGN! <5 mm E 10 ’ 200 ’ 150 * 20 mm F 10 ’ 140 ’ 110 ’ 20 mm British patent No. 1 183 092 -4 26 (> 100 ’) - 20 mm U.S. Patent No. 3,868,280 1.5 24 - > 20 mm

* Values related to a steel stripe dated with an additional coating of stretch glass.

** Data · The maximum diameter of the bar around which the steel strip can be bent 180 ° without damaging the coating or detaching it.

According to a second embodiment of the invention, it is also possible to use smaller amounts of rare earth metal oxides, provided that they are micronized, i.e. provided that the rare earth metal oxides are all of a particle size that passes through a sieve having less than 50.4 mesh. The 1 cm ² sieve and the fraction 35 to 55% by weight have a particle size that does not pass through the sieve ¹ with a mesh number of 77.5 per 1 cm ² sieve.

In this case, only stoichiometric amounts of rare earth metal oxides can be used to fully react, for example, with sulfur, and even higher amounts if the benefits associated with a higher content of rare earth oxides (such as rapid desulfurization or denitrification) can or must be considered less important.

In this embodiment, the amount of rare earth metal oxides is between 0.8 and 7 wt%.

When using the smaller amounts of rare earth metal oxides, an additional coating of low-melting, insulating and stretching glass is required.

However, since the glass film obtained according to the second embodiment is exceptionally thin, compact and adhesive, a very favorable spatial factor can be achieved.

A set of steel strips similar to those used above was prepared, and each strip is coated with one of the following release agents:

a) rare earth oxides 0.8% by weight, the rest of MgO,

b) rare earth oxides 1.4% by weight, the rest of MgO,

c) rare earth oxides 2.4% by weight, the rest of MgO,

d) rare earth oxides 4.0% by weight, the rest of MgO,

e) rare earth oxides 5.6% by weight, the rest of MgO,

f) rare earth oxides · 7.0 wt%, rest of MgO,

g) rare earth oxides 10% by weight, the rest of MgO,

(h) rare earth oxides of 20% by weight, the remainder of MgO;

i) rare earth oxides 0.4% by weight, the rest MgO.

The amount of separator deposited was between 6 and 8 g per m ² .

Each of these substances was deposited using two particle sizes, i.e. particles that pass through a sieve with a smaller number of meshes than

50.4 per 1 cm ^{2 of} sieve (indicated by the appropriate letter followed by "1") and particles passing through sieves with a mesh count of 21.7 to 41.8 per 1 cm ^{2 of the} sieve (indicated by the relevant letter followed by "2").

Thus, ai denotes a release agent containing 0.8% by weight of a rare-earth metal oxide having a particle size that passes through a sieve having a mesh size of less than 50.4 per cm ^{2 of} sieve, the rest of MgO, while ag denotes the same annealing separator composition, in which the particle size of the rare earth metal oxides is such that they pass through sieves having a mesh count of from 21.7 to 41.8 per cm ^{2 of} sieve.

Each strip was then coated with a tensioning and insulating material and subjected to conventional end annealing or cooling.

The most important properties were measured and compiled in Table 2. The specific Franklin resistance was measured according to ASTM standard A 344-6C T; adhesion was measured by bending the coated strip 180 ° around rollers of different diameters, and the minimum diameter at which the inner film did not show a crack was recorded. The strip thickness was 0.30 mm.

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In conclusion, the use of annealing separators constructed according to the invention gives the following substantial advantages:

- almost complete removal of the wells in the upper layer, which ensures greater uniformity of magnetostrictive values and a more efficient hysteresis loop.

- Possibility to reduce largely the dwell time at peak annealing temperature.

Ability to eliminate the need for an additional insulating and tensioning glass coating on strips for certain less strenuous electrical applications, thereby eliminating the difficulty of applying a special coating to the glass film formed on the strip during intermittent annealing. The technical and economic benefits resulting from this improvement can be identified from the data compiled in Table 1.

Examination of these data shows that the average surface specific resistance values obtained using the release agents of the invention are in absolute terms comparable to those commonly reported for high permeability silicon steel strips available on the market today - and are double coated, i.e. a glass coating originating from and on the release agent. deposited insulation and tension coatings [for example, for strips treated with a release agent sold under the CARLITEJ trademark.

This latter advantage leads to a second equally important advantage, i.e. reducing the total thickness of the finished strip by eliminating the need for a double coating or achieving a thin glass web, and since ferromagnetic performance is the same, an increase in the space factor of approximately 1 This is a particularly important advantage in the case of large-sized cores.

Claims (3)

I will invent the object An annealing separator for grain-oriented silicon steel strips based on magnesium oxide, characterized in that the separator comprises at least one compound selected from the group consisting of rare earth metal oxides such as cerium oxide, lanthanum, neodymium, praseodymium and rare earth metal compounds, such as cerium, lanthanum, neodymium-praseodymium hydroxides, which form oxides in the heat treatment. Second annealing separator according to claim 1, characterized -se that -Contents rare earth in the coating -rozsahu from about 5% to about _30/0 of the total weight of the coating, as počítáno-, oxides. • 3. 6. The annealing separator of claim 1 wherein the rare earth metal oxides have a particle size which passes through a sieve having less than about 50.4 mesh per cm &lt; 2 &gt; They do not pass through a sieve having a mesh number of 77.5 per cm ^{2 of} sieve and have a coating content in the range of 0.8 to 7% of the total weight of the coating.