GB1563853A - Annealing separators - Google Patents

Description

(54) IMPROVEMENTS IN ANNEALING SEPARATORS (71) We, CENTRO SPERIMENTALE METALLURGICO S.. A., of Via di Castel Romano, 00129 Rome Italy and TERNI SOCIETA PER L'INDUSTRIA E L'ELETTRICITA S. p. A, of 122, Viale Castro Pretoria, Rome, Italy, both Italian Bodies Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement :- This invention relates to annealing separators and is concerned with the provision of a new composition of separating agent to be used during the annealing of grain-oriented silicon steel strip which :- (a) has a positive effect on the annealing treatment, reducing overall manufacturing process costs; and (b) improves the magnetic and electrical properties of the annealed strip.

It is a well-known fact that a very critical step in the production of grainoriented silicon steel strip for use as a magnetic material is the final annealing treatment. 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 at the same time eliminates certain impurities, such as sulphides, from the strip, which impurities, although desirable for obtaining the desired orientation of the grains, would impair the magnetic properties of the finished strip.

The final annealing treatment is usually carried out in a batch-type furnace and normally requires considerable time for completion ; holding times of not less than twenty hours at soaking temperature are quite common, even though some proposas do provide for very much shorter annealing cycles. In the annealing treatment, the strip is loaded into the furnace in coils or, in certain cases, in stacks of sheets disposed one on top of the other. This arrangement is partly responsible for the length of the annealing treatment in that it hinders the free flow of the reducing atmosphere (which eliminates the sulphides) between the individual layers of the piled coils or stacks. Furthermore, the individual layers of each coil or stack tend to stick together during annealing.

Annealing separators were introduced originally for the express purpose of eliminating these problems and consisted of refractory powders spread on to the steel strip surface before the strip was wound into coils or arranged in stacks. With the passage of time, it was realised that these refractory powders could carry out additional functions other than keeping the coil layers physically apart and preventing them from sticking together (thus facilitating the free flow of the reducing atmosphere), i. e. reaction with sulphur, facilitating the elimination of this element ; and formation of an adhesive vitreous film capable of insulating the strip both electrically and chemically. Increasingly complex separating agents were, therefore, developed from the original simple-component powders, such as the calcium or magnesium oxide separators described in U. S. Specification No.

2, 492,682 resulting in powders which, in addition to a magnesium oxide base, contained other compounds such as Tir,, VO,, MnO and B203.

At the present moment, annealing separators are available which perform the following functions, either in whole or in part :- a) prevention of adhesion between individual coil layers during annealing in a batch-type ; b) improvement of reducing atmosphere flow between coil layers ; c) reaction with sulphur contained in steel strip d) formation of a protective coating on the ; e) formation of an electrically insulating coating on the strip : f) formation of a coating capable of imparting a slight tension to the strip; and g) provision of a suitable base for the application of a subsequent electrically insulating coating.

A number of efforts have been made in various countries to improve the complex, costly and extremely lengthy manufacturing procedure for the production of grain-oriented silicon steel strip as well as the ferromagnetic properties of the finished product, as can be seen from the numerous patent applications that have been filed and the many technical papers published during the past few years. With the intention of adding a positive contribution to the advancement of technical knowledge in this particular field, the present applicants jointly launched a research project aimed at reducing the length and cost of the manufacturing process and improving some of the electrical and magnetic properties of the finished strip, which project led eventually to the present invention.

It was noted during the course of these research studies that substantial improvementss were possible with regard to product quality, process length and economy when a rare earth metal oxide was added to the magnesium oxide base of the separating agent. Further investigations revealed that astonishing results could be obtained in this way, with far-reaching consequences-some of which were totally unexpected and unforseeable in the light of available knowledge on the subject.

The improvements in question can be summarised as follows :- a) a noticeable positive effect on the surface roughness of the strip. Indeed, in the first instance, small discontinuities were observed under the surface of the strip which were assumed to be inclusions. But more complete analysis of said discontinuities led to the discovery that they were sections of small pit cavities filled with glass film. Thus the formation of the glass film controls the roughness of the strip surface; b) an increase in the denitriding and desulphurising rates, i. e. acceleration of the previously slow steps of the treatment for eliminating impurities in the strip, which steps are responsible for the extremely long annealing times normally ; c) a substantially increased surface resistivity of the coating; and d) an increased adhesion of the coating to the strip.

The above improvements give the following practical advantages :- a) the certainty that in every case the steel strip will benefit from the stretching effect which the coating has on the strip as the result of the different values of the coefficient of thermal expansion in the strip and the coating; it has in fact been ascertained that, for a given strip, a reduction in the surface roughness leads to increased sensitivity to stretching. The resulting tension leads in turn to extremely favourable magnetostrictive values; b) a substantial reduction of the annealing ; c) the possibility, in some cases, of abolishing the additional coating treatment with tensioning and insulating glass, which additional treatment has up to now been considered to be essential ; and d) fewer problems during mechanical treatment of the strip.

A detailed description of the invention is given below, with specific reference to its practical application and to the accompanying drawings, in which Figure la is a micrograph (x 1000 magnification) of the surface layer of a strip treated with an annealing separator containing MgO on ! y, Figure lb is a micrograph (x 1000 magnification) of the surface layer of a strip treated with a known annealing separator, Figure I c is a micrograph (x 1000 magnification) of the surface layer of a treated with an annealing separator according to the present invention, Figure 2 shows magnetostrictive curves obtained for steel strips similar to those shown in Figures lb and lc, Figures 3a and 3b show hysteresis loops obtained for steel strips similar to those shown in Figures lc and lb (at 1. 5 Tesla), Figure 4 illustrates the influences of tension on magnetic losses (at 1. 5 Tesla, and thickness 0.34 mm), Figure 5 is a desulphurisation diagram, and Figure 6 is a denitrification diagram.

According to the invention, an annealing separator for grain-oriented silicon steel strip has a magnesium oxide base and contains at least one compound selected from the group comprising rare earth metal oxides and rare earth metal compounds which yield oxides when subjected to thermal decomposition. The separator may addition contain one or more silicates. The invention also embraces a method of annealing grain-oriented steel strip wherein the strip is coated with an annealing separator as above defined prior to being introduced into an annealing furnace.

In a first embodiment of the present invention the amounts added are preferably such as to give an aggregate content of rare earths (as oxides) in the annealing separator of from 5% to 30% by weight of the total weight and a silicate content of from S ; o to 45% by weight of the total weight. The quantity of the annealing separator deposited on the strip is preferably between 6 and 10 g/m'and the thickness of the final coating should be approximately I to 3 um.

In order to obtain a quantitative evaluation of the improvements obtained using annealing separators according to this invention, five longitudinal parallel strips were prepared (immediately following industrial cold rolling) from a coil of gram-oriented silicon steel strip containing 2.93% by weight Si. This particular method was adopted since it offered the best guarantee of test strip homogeneity.

The five narrow 400 m. long steel strips obtained in this way were each coated with one of the following separating agents :- O-MgO reference standard A-MgO + MnO, (Mo0/MnO, = 95/5) comparison sample B-MgO + MnO2 + B, O, (0.1% as B) comparison sample C-MgO+ rare earth oxides (10% by weight) D-MgO + rare earth oxides (10% by weight) + Na silicate (10% by weight) Figure la shows a steel strip sample coated with an annealing separator containing MgO only and serving as a conventional reference standard. Figures 16 and lc show steel strip samples coated, respectively, with separating agents A and C. The differences between the various test samples are immediately obvious from the micrographs. After examining a great many samples taken at random from the five differently coated strips, an interesting and important fact was noted: the surface roughness of the strip coated with separating agent O was fairly uniform along the entire length of the strip and more or less equal to the level shown in Figure la, but the roughness of the strips coated with separating agents A and B varied instead to some extent, both quantitatively and by location, i. e. samples were obtained at random along the strip length which were either smoother or rougher than the sample shown in Figure I b. Finally, the strips coated with separating agents C and D gave in all cases samples which were almost totally free of surface cavities.

Analysis of the magnetostriction curves (Figure 2) and of the hysteresis loops (Figures 3a and 3b) relative to the samples illustrated in Figures lb and Ic shows the influence of skin roughness on the final magnetic properties of the steel strip. In Figure 2, the curves a-a'and b-b'refer respectively to the strips coated with separating agents A and C. As can be seen from the curves b-b', relative low magnetostriction values are obtained using a separating agent according to this invention; in addition, the magnetostriction value is subject to only a limited variation when the magnetisation increases from zero to a maximum value during the cycle. It can also be observed that the magnetostriction curves remain at very low levels even when the magnetisation reaches values close to the theoretical peak value admissible for the type of steel strip in question; obviously, these conditions give a very low noise level in transformer and other magnetic cores.

Conversely, the magnetostriction curves obtained with strips coated with separating agents currently in use are very similar to those indicated in Figure 2 by the letters a-a'. In this case, the changes produced by magnetostriction in the dimensions of the strip are not only of greater magnitude but are also far more abrupt. Comparision between the two sets of curves shows that, for a magnetisation increase from 1. 2 to 1. 9 Tesla, the magnetostriction increases from-0. 4 x 10-6 to much more than I x 10-6 (curve a) and from approximately -1 x 10-6 to approximately +3 x 10-6 (curve a') using currently available separating agents, while only small magnetostriction changes of around-0. 2 x 10-1 (curve b) or from approximately 4. 5 x 10-6 to approximately-0.8 x 10-6 (curve b') occur using annealing separators according to the present invention.

An even more convincing demonstration of the influence of skin roughness on the final magnetic properties of the steel strip can be obtained by consideration of the diagrams shown in Figures 3a, 3b and 4. In Figures 3a and 3h, the hysteresis loops of strips coated with a separating agent according to this invention (Figure 3a) and with separating agent A (Figure 3b) are compared with the hysteresis loop of the corresponding uncoated steel strip (curves R. and N. R. in Figures 3a and 3b). Figure 4 shows the influence of tension on magnetisation loss for a strip of the type shown in Figure lb, both in the as-rolled condition (curve a) and after removal (by pickling) of the surface layer (curve b). As is immediately evident, the steel strip is unaffected by tension when its surface is rough, but the influence of tension is highly beneficial once roughness have been eliminated.

Figures 5 and 6 show, respectively, the desulphurisation and denitrification diagram for the uncoated steel strip (curve a), for the strip coated with separating agent A (curve c) and for the steel strip coated with separating agent C (curve b).

These diagrams refer to strips which are narrower than those used industrially and cannot therefore be taken as representative of a real life situation; the diagrams have, however, been reproduced for the perfectly legitimate and valid purpose of a comparative evaluation of the curves (and without, therefore, any scales). It should be pointed out, however, that industrial tests have confirmed the validity of the curves from a qualitative standpoint.

As a result, the holding time for steel coils at peak annealing temperature in bell-type furnaces can be reduced by at least 15 o (during some industrial tests, reductions of as much as 50 have been obtained).

With regard to the other advantages obtainable by the use of annealing separators according to the present invention, experimental tests have established that the coating deposited on the steel strip using these new materials during annealing has a far higher surface resistivity and a far greater adhesion than the coatings obtained using the separating agents currently in use. Some significant data are tabulated below for purposes of comparison relative to the five steel strips coated with separating agents O, A, B, C and D and relative to two commercial strips (E, F), together with values extracted from patent literature.

TABLE 1

Surface Resistivity il/cm' Separating Agents Minimum Maximum Average Adhesion** O 0.3 2.6 1.4 > 20 mm.

A 2.5 25 12 20mm.

B 2 26 13 20 mm.

C 35 200 110 < 5 mm.

D 30 230 tO0 < 5 mm.

E 10* 200* 150* 20 mm.

F 10* 140* 110* 20mm.

British Patent No. 1,183,092 @4 26 ( > 100*) 20 mm.

U. S. Patent No. 3,868,280 1.5 24 - > 20 mm.

*Values referring to steel subsequently provided with an additional coating of tensioning glass.

**Data indicating the maximum diameter of rod around which steel strip can be bent through 180 without damage to, or detachment of, the coating.

In a second embodiment of the present invention, it is also possible to use lesser amounts of rare earth metal oxides, provided they are micronized, i. e. provided the rare earth metal oxides are of a particle size less than 325 mesh, and include a fraction of between 35 and 55% in weight of more than 500 mesh.

In this case, it is possible to use only the stoichiometric amounts of rare earth metal oxides required to react completely with sulphur or other impurities. In addition, even smaller amounts can be used if some of the advantages arising from the use of rare earth oxides (such as rapid desulphurisation or denitrification) are considered to be of less importance. In this embodiment the amount of rare earth metal oxides is between 0.8 and 7% by weight of the total weight of the separator.

However, when using said lesser amounts of rare earth metal oxides, it is necessary to use an additional coating of low-melting-point, insulating and tensioning glass.

Nevertheless, since the glass film obtained following this second embodiment is exceptionally thin, compact and adherent, a very favorable space factor can be obtained.

A set of steel strips similar to those previously used was prepared and each strip was coated with one of the following separating agents :- a) Rare earth metal oxides 0.8% by weight, the balance MgO b) 1. 40/ c) " " " " 2.4% " " " " " d) " " " " 4.0% " " " " " e) " " " " 5.6% " " " " " f) 7. 00//o g) Rare earth metal oxides 10% by weight, the balance MgO 04''/ h) " " " " 20% " " " " " i) " " " " 0.4% " " " " " The amount of deposited separator was between 6 and 8 g/m. Each of said compositions was deposited using two particle sizes, ie. smaller than 325 mesh (indicated by the relevant letter followed by 1), and between 140 and 270 mesh (indicated by the relevant letter followed by 2). Thus al indicates a separating agent containing 0.8 weight % of rare earth metal oxides and having a particle size of less than 325 mesh, the balance being MgO, while a2 indicates the same composition of annealing separator, in which the particle size of rare earth metal oxides ls between 140 and 270 mesh. Each strip was subsequently coated with a tensioning and insulating composition, and subjected to the traditional final annealings.

The most important characteristics were measured and are tabulated in the following Table The Franklin resistivity was measured following A. S. T. M.

Standard A 344-60 T; the adhesion was measured by bending the coated strip through 180 around cylinders of different diameters and recording the minimum diameter down to which the inside film did not have cracks. The thickness of each strip was 0.30 mm.

TABLE 2

%Distribution of Franklin Resistivity Strip Glass film thickness m Final thickness of insulating layer m 0-39,9 /cm 40-99,9 /cm 100-999 /cm 1000 + /cm Adhesion mm Permeability Core losses W 17/50 # a1 0.4 1.5 2 48 50 - 23 1900 1.11 3.0 - 25 60 15 18 1.08 a2 uneven 2.1 80 20 - - > 35 1890 1.16 3.1 75 25 - - 1.15 b1 0.4 1.6 - 40 50 10 18 1913 1.09 2.9 - 15 40 45 14 1.05 b2 uneven 2.0 70 30 - - > 35 1905 1.16 3.5 70 20 10 - 1.16 c1 0.5 1.6 - 8 60 32 12 1915 1.09 3.0 - 4 40 56 < 10 1.05 c2 uneven 2.8 10 50 40 - 20 1905 1.13 3.8 - 55 45 - 20 1.13 d1 0.4 1.7 - 8 50 42 10 1917 1.08 3.1 - 4 35 61 < 10 1.04 d2 0.4 1.7 - 25 65 10 15 1910 1.09 3.3 - 10 60 30 13 1.06 e1 0.5 1.9 - 10 40 50 < 10 1910 1.10 3.1 - 8 45 47 < 10 1.06 e2 0.5 2.0 - 20 50 30 12 1908 1.10 3.1 - 18 45 37 15 1.08 f1 0.7 2.1 - 5 50 45 15 1906 1.10 3.6 - - 50 50 12 1.08 f2 0.7 2.1 - 10 60 30 14 1910 1.08 3.5 - 10 50 40 14 1.06 TABLE 2(continued)

%Distribution of Franklin Resistivity Strip Glass film thickness m Final thickness of insulating layer m 0-39,9 /cm 40-99,9 /cm 100-999 /cm 1000 + /cm Adhesion mm Permeability Core losses W 17/50 # g1 1.1 2.5 - 40 60 - 16 1900 1.12 3.8 - 40 50 10 13 1.10 g2 0.6 1.9 - 5 50 45 12 1913 1.07 3.6 - - 65 35 12 1.05 h1 1.8 3.1 15 35 50 - 20 1890 1.16 4.2 - 35 60 5 15 1.10 h2 0.7 2.1 - - 60 40 12 1910 1.09 3.6 - - 55 45 12 1.07 i1 Irregular 1.3 30 45 25 - > 35 1880 1.17 3.2 20 20 60 - 30 1.17 In conclusion, the use of annealing separators according to this invention gives the following substantial advantages:-a) almost complete elimination of cavities in the surface layer, guaranteeing a greatly uniformity of magnetostriction values and more efficient hysteresis loops, b) possibility of reducing to a considerable extent the holding time at peak annealing temperature, and c) the possibility of abolishing the necessity for an additional insulating and tensioning glass coating on strips used for certain light-duty electromagnetic applications, thereby avoiding the difficult operation of applying the extra coating to the vitreous film formed on the strip during batch annealing. The technical and economic benefits resulting from this improvement can be appreciated from the data given in Table 1. Examination of these data shows that the average surface resistivity values obtained using separating agents according to the present invention are comparable (in absolute values) to those obtained currently for highpermeability silicon steel strips available on the market today with a double coating, i.e. the glass film originating from the separating agent and the superimposed insulating and tensioning coating (for example, strips treated with the separating agent marketed under the Registered Trade Mark "CARLITE").

This last-mentioned advantage leads to another, equally important advantage, i. e. the reduction of the total thickness of the finished strip by the elimination of the need for a double coating or by the thinness of the glass film and (since ferromagnetic performance is unimpaired) the consequent increase in space factor by approximately 1% a particularly important advantage in the case of large cores.

Claims (7)

1. WHAT WE CLAIM IS : 1. An annealing separator for grain-oriented silicon steel strip, said separator having a magnesium oxide base and containing at least one compound selected from the group comprising rare earth metal oxides and rare earth metal compounds which yield oxides when subjected to thermal decomposition.
2. 2. An annealing separator according to claim l, the rare earth content of which is between 5 . and 30/by weight (as oxides) of the total weight.
3. 3. An annealing separator according to claim 1, wherein the rare earth metal oxides have a particle size smaller than 325 mesh, with a fraction of between 35 and 55% in weight of more than 500 mesh, the content of said oxides being between 0.8 and 7% by weight of the total weight.
4. 4. An annealing separator according to claim 1, 2 or 3, additionally containing one or more silicates.
5. 5. An annealing separator according to claim 4, the silicate content of which is between 5% and 45% by weight of the total weight.
6. 6. An annealing separator according to claim l, and substantially as hereinbefore described.
7. 7. A method of annealing grain-oriented silicon steel strip wherein the strip is coated with an annealing separator according to any one of the preceding claims prior to being introduced into an annealing furnace.