US2986485A - Annealing process for magnetic steel strip - Google Patents

Description

ANNEALING PROCESS FOR MAGNETIC STEEL STRIP Elmore J. Fitz, Lanesboro, Mass, and Robert E. Waite, East 'Chatham, N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed July 28, 1958, Ser. No. 751,128

8 Claims. (Cl. 148112) The present invention relates to magnetic silicon steel for electrical uses such as in transformers, motors and other electromagnetic apparatus, and more particularly to a process of annealing silicon steel which is adapted to be carried out on a continuous basis.

In the processing of silicon steel strip to produce material of good magnetic and electrical properties, the strip is normally subjected after a series of rolling stages to an annealing treatment in which two purposes are sought to be accomplished. First, the anneal should develop in the steel a crystal structure (texture) so oriented that good magnetic properties are obtained in the strip. Such crystal structure is developed by secondary recrystallization, as explained below. Second, the anneal should remove impurities such as sulfur and carbon which may cause excessive watt loss in the oriented strip. In general, low watt loss cannot be obtained without good crystal orientation and without suitable purification of the material.

By secondary grain growth or secondary recrystallization as referred to herein is meant the process whereby in the final texture producing annealing treatment, strainfree crystal grains grow in size by absorbing each other. Such secondary grain growth follows primary recrystallization, which is a process whereby the distorted grain structure of a cold-worked metal is replaced by a new strain-free grain structure by annealing above a specific minimum temperature. It is the secondary recrystallization that produces the highly preferred orientation sought in high quality magnetic strip, and the orientation thus obtained is completely different from that obtained merely after primary recrystallization. The magnetic strip is generally in a primary recrystallized state when it is ready for the final high temperature anneal. The grains in a primary recrystallized material are on the average considerably smaller than those of a secondary recrystallized material and, also, while primary recrystallized strip has only a small percentage of orientation, e.g. to the large grained secondary recrystallized strip has a much higher degree of orientation with values of 70 to 95% being the rule.

It has previously been found that in order to develop the maximum degree of secondary recrystallization in the annealing stage, the heat treatment is best carried out at a temperature which is lower than the optimum purification temperature. For example, a temperature of 850 C. for grain growth and of 1100 C. for purification has been taught by the prior art. In such prior processes the steel strip was held at the lower temperature for a substantial period of time, at least several hours, to allow adequate grain growth (secondary recrystallization), after which the strip was subjected for several more hours to the higher purifying anneal temperature.

These and other relatively long annealing treatments heretofore considered necessary have been a major obstacle to the development of a continuous annealing treatment in commercial processing of silicon steel strip. Due to the necessity for prolonged treatment in the annealing furnace, the steel strip has conventionally been proc- Patented May 30, 1961 essed in batches in the furnaces, either in the form of coils or stacked laminations. This procedure has many drawbacks. For one thing, it resulted in lack of uniformity in electrical and magnetic properties between different batches or coils, or even between different portions of the same strip. Also it necessitated the application of separator coatings between layers of the strip to prevent them from sticking together durin the annealing process. In the case of the coiled strips, these had to be subsequently flattened by an additional heat treatment to overcome the coil set. A continuous anneal, on the other hand, wherein a single strand of steel is continuously passed through the annealing furnace would not be subject to such difliculties and would markedly facilitate commercial processing of electricalsteel.

It is an object of the present invention to provide an improved method of processing silicon steel which overcomes the above disadvantages of prior processes.

It is another object of the invention to provide a method of treating silicon steel which may be carried out in a continuous process.

It is a particular object of the invention to shorten the time required for the final texture-producing and purifying anneal treatment in the processing of electrical steel so as to adapt such treatment to a continuous process, while affording steel of a high degree of magnetic anisotropy and very low watt loss and other characteristics of high quality electrical steel.

Other objects and advantages of the invention will become apparent from the following description and the appended claims.

To attain the above objects, the present invention provides a method of treating silicon steel strip containing about 1 to 4% silicon which comprises the steps of heating the silicon steel material for about 5 to 60 minutes, and preferably about 15 minutes, at a temperature of about 950 to 1050 for the purpose of initiating the nucleation and growth of crystals of preferred magnetic orientation, and thereafter heating the material at a temperature of about 1175 to 1250 C. to provide for continued growth of crystals of the preferred orientation. For best results, the cooling rate of the thus annealed material is carefully controlled as described hereinafter.

It has been unexpectedly found in accordance with the invention that the low temperature period for initiating secondary recrystallization may be thus considerably reduced below that heretofore considered necessary and that completion of secondary recrystallization can be achieved during the subsequent higher temperature purifying stage to produce high quality steel strip.

A series of tests were made on samples of silicon steel strip containing about 3%% by weight of silicon to determine the effect of dilferent temperatures and periods of heating on the development of magnetic anisotropy in the samples.

The degree of anisotropy produced in the tested samples is shown in Table I below as percent orientation, these values being obtained by the integrated disc test well known in the art. The latter test involves the use of an instrument which compares the voltage developed in a magnetic flux field by a disc of the test sample with that developed-with a standard specimen having a known degree of magnetic anisotropy. Since the degree of magnetic anisotropy is a function of the amount of texture, i.e., crystal orientation in the strip, it is possible to callbrate the disc tester in terms of percent of perfect crystal orientation.

In the annealing tests, the specimens were initially heated in a tube furnace at selected nucleating temperatures of 950 C., 975 C., 1000 C. and 1025 C. and thereafter given a heat treatment of 1175 C. for 8 hours. Table I below shows the results of these tests at different times and temperatures, the values shown being in percent anisotropy based on an average of samples:

Table l Percent Nucleating'lemp. and Time Anisot- 30 min 93 60 min 93 975 0.:

10 min 93 30 min 91 60 min 89 1,000 0.:

8 min 90 m 88 1,026" 0.:

15min 84 As a basis for comparison, 30 samples of similar silicon steel material were given a standard anneal at 1175 C. for 8 hours without any prior heat treatment. Such anneals are batch type and the thermal inertia of the stacked pieces insures that they will pass through the grain growth temperature range slowly enough to allow sufficient time for proper texture development in accordance with the teaching of the prior art. The average value for these 30 samples was 86% anisotropy.

it will be seen from these results that in all cases where the initial heat treatments were in the range of 950 C. to 1000 C. the anisotropy obtained was greater than that obtained in the samples given the standard 1175 C. anneal. It is also noteworthy that the shorter times at each temperature resulted in as high or higher a degree of magnetic anisotropy as the longer periods at the same temperature. The results obtained were further unexpected in demonstrating that substantially higher secondary recrystallization temperatures than heretofore considered desirable could provide entirely satisfactory degrees of orientation.

In connection with the above tests, samples were checked for percentage orientation at the end of various times at the respective nucleating temperatures. From this it was found that the time for complete anisotropic development at these temperatures was between 1 and 16 hours. It seemed evident, therefore, that in the procedure carried out as described only a small degree of secondary recrystallization is developed during the relatively short periods at the lower temperatures and that the necessary grain growth was for the most part accomplished and completed during the higher purifying anneal stage. This is in contrast to the prior art teaching which has assumed that adequatesecondaryrecrystallization could be carried out only byholding the steel strip at temperatures below 900 C. for periods of not less than four hours duration.

It has further been found in connection with the invention that if the second stage anneal is carried out at a temperature in the vicinity of 1200 C. and the rate of cooling thereafter is carefully controlled to a maximum of 100 C. per minute a substantial reduction can be made in the time required for the second stage anneal with little or no sacrifice in the quality of steel produced.

Table 11 below shows the results of tests conducted on silicon steel strip using the two stage treatment of the invention at l000 and then 1200 C., the percent texture shown being the average value of the twelve lots From these results it is again definite that good crystal development can be achieved by a partial growth for a short time at a low temperature followed by completion at a higher temperature. The data also indicates that lengthening the period of the first stage permits a shortening of a second stage to attain similar or better, percent texture.

On the basis of numerous tests, a particular annealing cycle was established which proved suitable for consistently producing high quality magnetic steel. This cycle consisted of heating the silicon steel strip for about 15 minutes at 1000 C., 15 minutes at 1225 C., and cooling at a maximum rate of C. per minute. This cycle calls for a total elapsed time of about 42 minutes, a period which is considerably shorter than the many hours of annealing treatment customary in the prior art processes. The watt losses of ten lots of silicon steel samples 14 mils thick at a flux density of 15,000 subjected to the foregoing cycle were found on the average to be about'.64 watt per pound, and the magnetic anisotropy was 92%, values which are quite satisfactory, especially considering the rapidity of the annealing process.

As a consequence of the markedly shortened annealing periods thus provided, it becomes practical to treat the steel strip in a continuous annealing process. Thus, a continuous anneal may be carried out by moving cold rolled silicon steel strip of indefinite length and typically 14 mils thick through adjoining furnace chambers having the temperature levels set forth in the foregoing cycle, i.e., 1000 C. and 1225 C., and at a speed such that it takes 15 minutes for movement through each furnace chamber. Thereafterthe strand may be moved through a cooling chamber where it is cooled at the stated maximum rate of 100 C. per minute.

While the above described cycle appears particularly satisfactory for the purposes of the invention, the invention is not limited to the specific times and temperatures stated therefor. Tests have shown, for example, that the 15 minute period for the second stage anneal at 1225 C. could be further reduced without substantial sacrifice in watt loss or degree of crystal orientation. Material so treated at 1225 C. in the second stage for 10 minutes showed a watt loss of .65 watt per pound and a percent texture of 92%, while that similarly treated for 5 minutes had the same watt loss and a percent texture of 91%, these values each being based on an average of five samples. In general, the initial stage temperature may be in the range of 900 C. to 1050 C. and for a period of 5 to 60 minutes, and the secondstage temperature in the range of 1175 C. to 1250 C. for a period of l to 30 minutes.

It has further been found that in order to obtain the benefits of the invention it is essential that'the second stage high temperature anneal follow immediately upon the first stage low temperature treatment without any intervening cooling period, since tests have indicated that where an intermediate cooling period is provided, the percent orientation ultimately achieved-was substantially lower than in the absence of such an intervening cooling period.

The'annealing treatments described are preferably carried out in reducing atmospheres in the furnace, and while hydrogen may typically be employed for this purpose, other known types of reducing atmospheres could be used if desired. a

The expressions sheet material and sheet as used in the appended claims are intended to include such forms as sheets, strips, tapes and other laminar shapes.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the artwithoutactual-ly departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations ascome within the'true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. The method of treating cold rolled electrical silicon steel strip containing 1 to 4 percent silicon which comprises the steps of heating said strip for 5 to 60 minutes at a temperature of 950 to 1050 C. to initiate secondary recrystallization therein, and immediately thereafter heating said strip at a temperature of 1175 to 1250 C. without cooling the same between the heating steps for providing continued secondary recrystallization in said silicon steel strip.

2. The method of treating cold rolled electrical silicon steel strip containing 1 to 4 percent silicon which comprises heating said silicon steel strip for to 30 minutes at a temperature of 950 to 1050 C., immediately thereafter heating said strip for l to 30 minutes at a temperature of 1200 to 1250 C. without cooling the same between the heating steps, and thereafter cooling said strip at a rate of not more than 100 C. per minute.

3. The method of treating cold rolled electrical silicon steel strip containing 1 to 4 percent silicon which comprises heating said silicon steel strip for about minutes at about 1000 C., immediately thereafter heating said strip for about 15 minutes at about 1225 C. without cooling the same between the heating steps, and thereafter cooling said strip at a rate of not more than 100 C. per minute.

4. The method of processing of electrical silicon steel sheet material containing 1 to 4% silicon which comprises continuously moving an elongated strip of said material in sequence through a reducing atmosphere at a temperature of 950 to 1050" C., and then immediately through a reducing atmosphere at a temperature of 1175 to 1250 C. without cooling the material between the heating steps.

5. The method of processing electrical silicon steel sheet material containing 1 to 4 percent silicon which comprises continuously moving an elongated strip of said material through a reducing atmosphere at a temperature of 950 to 1050 C. and then immediately through a reducing atmosphere at a temperature of 1200 to 1250 C. without cooling the material between the heating steps, the speed of said material being such as to subject the same to aforementioned temperatures for respective periods of about 10 to 30 minutes and 1 to 30 minutes.

6. The method of processing electrical silicon steel sheet material containing 1 to 4 percent silicon which comprises continuously moving an elongated strip of said material through a reducing atmosphere at a temperature of 950 to 1050 C. and then immediately through a reducing atmosphere at a temperature of 1200 to 1250" C. without cooling the material between the heating steps, the speed of said material being such as to subject the same to the aforementioned temperatures for respective periods of about 10 to 30 minutes and l to 30 minutes, and thereafter cooling said strip at a rate of not more than 100 C. per minute.

7. The method of processing electrical silicon steel sheet material containing 1 to 4 percent silicon which comprises continuously moving an elongated strip of said material through a reducing atmosphere at a temperature of about 1000 C. and then immediately through a reducing atmosphere at a temperature of about 1225 C. without cooling the material between the heating steps, the speed of said material being such as to subject the same to the aforementioned temperatures for respective periods of about 15 minutes each, and thereafter cooling said strip at a rate of not more than 100 C. per minute.

8. In the processing of electrical silicon steel sheet material containing 1 to 4% silicon, the steps of heating such sheet material for 5 to minutes at a temperature of 950 to 1050 C., and immediately thereafter heating said sheet material at a temperature above about 1175 C. without cooling the same between said heating steps.

References Cited in the file of this patent UNITED STATES PATENTS 1,965,559 Gross July 3, 1934 2,209,687 Crafts July 30, 1940 2,303,343 Engel et al Dec. 1, 1942 2,307,391 Cole et a1. Jan. 5, 1943 2,534,141 Morrill et a1 Dec. 12, 1950 2,867,559 May Jan. 6, 1959 FOREIGN PATENTS 778,987 Great Britain July 17, 1957

Claims (1)

1. 2. THE METHOD OF TREATING COLD ROLLED ELECTRICAL SILICON STEEL STRIP CONTAINING 1 TO 4 PERCENT SILICON WHICH COMPRISES HEATING SAID SILICON STEEL STRIP FOR 10 TO 30 MINUTES AT A TEMPERATURE OF 950 TO 1050*C., IMMEDIATELY THEREAFTER HEATING SAID STRIP FOR 1 TO 30 MINUTES AT A TEMPERATURE OF 1200 TO 1250*C. WITHOUT COOLING THE SAME BETWEEN THE HEATING STEPS, AND THEREAFTER COOLING SAID STRIP AT A RATE OF NOT MORE THAN 100*C. PER MINUTE.