US3413165A - Hot rolling process for making grain oriented silicon iron sheet - Google Patents

Description

United States Patent 3,413,165 HOT ROLLING PROCESS FOR MAKING GRAIN ORIENTED SILICON IRON SHEET Gordon John Philip Buchi, Stafford, and Desmond Gritfiths, Altrincharn, England, assignors to The English Electric Company Limited, London, England, a British company No Drawing. Continuation-impart of application Ser. N 0. 409,972, Nov. 9, 1964. This application June 21, 1967, Ser. No. 647,604 Claims priority, application Great Britain, Nov. 13, 1963, 44,881/ 63 Claims. (Cl. 148-111) ABSTRACT OF THE DISCLOSURE Grain oriented silicon-iron sheet is rolled to final thickness entirely by hot rolling in effectively two stages. Each hot rolling stage may comprise more than one pass. The temperature in the second stage during which the sheet is rolled to its final thickness determines the type of texture which will be imparted to the sheet in the course of a recrystallization anneal. Rolling at temperatures above 1000 C. in the second stage favors the formation of a Goss-type texture, whereas when the temperature is below 900 C. the formation of a cube-type texture is favored.

This invention relates to grain-oriented silicon-iron alloy sheet and processes for its production, and is a continuation-impart of application Ser. No. 409,972, filed Nov. 9, 1964, now abandoned.

Such sheet is used especially in the electrical industry, for example for transformer cores. By a grain-oriented silicon-iron alloy sheet is meant a sheet (which expression includes a strip) comprising an alloy of silicon in iron, the remainder being impurities, wherein either (a) the majority of the silicon iron grains in the sheet lie within of the so-called Goss or roof-top type texture in which a (110) plane of the cubic silicon-iron crystal lattice lies parallel to the sheet surface and an easy magnetic direction [001] lies parallel to the rolling direction of the sheet, or (b) the majority of the grains in the strip have a cube-type texture in which a (100) plane of the cubic silicon-iron crystal lattice lies within 10 of the sheet surface, and hence each grain has two mutuallyperpendicular easy magnetic directions lying within 10 of the sheet surfaces.

Before describing the invention it is necessary to describe brieily some known processes for making grainoriented silicon-iron alloy sheet.

In 1935, N.P. Goss, in the Transactions of the American Society of Metals, 1935, vol. 23, p. 511, described a process for producing 3% silicon-iron strip having the roof-top texture expressed in Miller indices as (110) [001]. A current commercial version of this process comprises the following steps:

(1) Hot rolling a cast ingot to strip 0.1-0.2 in. thick.

(2) Cold rolling the strip in stages with an intermediate annealing stage after each cold rolling stage, down to a thickness of 0.3-0.4 in.

(3) Heat treatment at SOD-900 C. in wet hydrogen to reduce the carbon content of the strip.

(4) Final cold reduction to a thickness of 0.013 in.

(5) Annealing at 1100-1200 C. to effect secondary recrystallisation and to reduce the sulphur content of the strip.

It is during the final step that the oriented silicon-iron grains are formed by secondary recrystallisation. The detailed mechanism of the Goss process is still not clear,

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but is known to be dependent on the presence of a dispersed precipitate in the strip at the outset of the secondary recrystallisation process. This precipitate is in general manganese sulphide, and it is accordingly essential for adequate quantities of manganese and sulphur to be included in the ingot. It is believed that grains having the [001] orientation break away more readily from the dispersed precipitate particles than do grains having other orientations. Thus the precipitate inhibits growth of the said grains having other orientations, and the mechanism operating in processes of this kind is therefore conveniently called a precipitate inhibition mecha- IllSII].

During the final annealing treatment the manganese sulphide precipitate is decomposed, the manganese going into solution in the alloy and the sulphur being removed by a surface-vapour phase reaction. The annealing stage must therefore be continued long enough for all the sulphur to disappear, and this is a disadvantage of all processes involving precipitate inhibition, as it considerably increases the time required for the process.

A further disadvantage of precipitate inhibition processes is that only roof-top texture can be produced thereby.

The Goss process described above suffers from another serious disadvantage, which is that cold rolling is necessary. The main disadvantage of this is that, since the addition of silicon to iron embrittles the iron, it is not possible to cold roll an alloy containing more than about 4 /z% of silicon. Thus if cold rolling is required, the alloy must be a low-silicon alloy having no more than this amount. In general, however, the terms low-silicon and high silicon refer to silicon contents respectively of up to about 6% and approximately from 6 to 10%.

The cold rolling stages are in fact eliminated in a process described in US. Patent 3,144,363 to R. G. Aspden and G. N. Facaros, in which the ingot is reduced to its final thickness by hot rolling in two stages, with an intermediate anneal (heat treatment) between the hot rolling stages. The latter are at 1000 C. and 750 C. (reducing to 350 C. as the metal cools) respectively. Secondary recrystallisation is effected by annealing at 10001250 C.

The Aspden process, however, is another involving precipitate inhibition, to which end manganese and sulphur have to be added to the original melt, to give 0.1-0.5% manganese and 0.01 to 1% sulphur. Copper, titanium and chromium, or any of them, may be used in place of some or all of the manganese.

Thus the Aspden process has the following disadvantages:

(a) the inclusion of additives is necessary to effect crystallisation according to the desired grain orientation;

(b) this orientation can only be of Goss (roof-top) texture; and

(c) the final annealing stage must be of a long enough duration for all the sulphur to be removed.

Furthermore, Aspdens process is limited to steels having not more than 7% silicon.

Applicants are also aware of US. Patents 3,105,782 to Walter, and 3,266,955 to Taguchi et a1. Taguchi et al. describes processes for producing strip having double orientation, that is to say partly of Goss texture and partly of (100) [011] texture. This is achieved by a timeconsuming and clumsy process involving cross-rolling. The Taguchi process also suffers from some of the disadvantages previously mentioned, in that it involves cold rolling and is therefore confined to low silicon steels (24% and it requires the addition of aluminum to the melt. This produces aluminum nitride precipitate, which appears to act as an inhibitor in the same way as other inhibitors mentioned above, though in this case it apparently limits the grains to those having a particular plane in the rolling plane, and therefore, in common with other precipitate inhibition processes, suffers from the disadvantage that it is not possible to choose cube or Goss texture at will.

Walter describes a process for low-silicon steel (up to 6% silicon), involving cold rolling with its attendant disadvantages. In this case cube or Goss texture can be chosen according to the time taken for the final anneal, the initial growth being of cube textures which gradually change to Goss if annealing is prolonged. Walter propounds the theory that there is a critical oxygen content above which the texture is of the cube type, and below which Goss texture can be produced under certain conditions, the mechanism involved being a surface energy control mechanism brought about by the oxygen and other impurities. It can however only be used for sheets of relatively thin cross-section and low silicon content.

The objects of the present invention include the following:

(1) To produce grain-oriented silicon-iron alloy sheet having a high or low silicon content as desired, the process being the same in principle whatever the silicon content;

(2) To produce grain-oriented silicon-iron alloy sheet in which the silicon content may lie anywhere, not only in the range 27% but also the range 710%.

(3) To produce grain-oriented silicon-iron alloy sheet without requiring the essential addition of sulphur, manganese, aluminum or other additives to the melt, while allowing the optional use of such additives if desired and allowing silicon-iron of commercial grades, including the usual impurities, to be used successfully;

(4) To produce grain-oriented silicon-iron alloy sheet having a cube or Goss texture, or a mixture of both, as desired without recourse to such artifices as cross-rolling and without having to increase the final annealing time if it is required to produce Goss texture as against cube texture; and

(5) To produce grain-oriented silicon-iron alloy sheet by a process involving no cold rolling and no annealing before the sheet has been reduced to substantially its final thickness.

We have found that all these five objects are achieved by a novel process in which the ingot (which expression includes a slab) is reduced to its final thickness in what are effectively two hot-rolling stages, followed by a final anneal to effect secondary recrystallisation in which the grain texture is formed by a surface energy controlled mechanism. The type of texture in the final product is determined entirely by control of the temperature in the second of two hot rolling stages.

It will be understood that each hot rolling stage may comprise more than one pass. The first hot rolling stage reduces the ingot to an intermediate thickness, and the resulting sheet is then subjected to the second hot rolling stage in which it is rolled to its final thickness at a temperature which is kept substantially constant throughout the stage and which is in the range 750900 C. if cube texture is desired, or 10004200 C. if Goss texture is desired. If the temperature for this stage is between 900 and 1000 C., there will result a mixture of cube and Goss textures, the ultimate proportion depending on the actual temperature for this stage.

It will be understood that references in this specification and in the claims to cube texture or Goss texture in the final product imply that all or nearly all the siliconiron grains are oriented according (as the case may be) to cube or Goss texture as hereinbefore defined. The presence of impurities may produce a few grains oriented differently from the majority. It will also be understood that the temperatures of 900 C. and 1000 C. specified above as the upper limit for ultimately obtaining cube texture, and the lower limit for Goss texture, respectively,

are not sharply-defined boundaries. It the second hot rolling stage is carried out at 875 C., for example, there will be a small proportion of Goss-oriented grains in the final product. If this stage is carried out at 775 C. this proportion will be smaller. It follows that to obtain equal proportions of cube and Goss textures the second hot rolling stage will be carried out at about 950 C.

No annealing is necessary before the sheet is reduced to its final thickness: thus the only heat to which it need be subjected between the end of the first hot rolling stage and the end of the second hot rolling stage is that necessary to obtain and maintain the temperature required for the second hot rolling stage.

Grain-oriented silicon-iron alloy sheet made by processes according to the invention is included in the scope of the invention.

Various processes according to the invention for the production of grain-oriented silicon-iron alloy sheet according to the invention, containing from 2 to 10 percent by weight of silicon, remainder iron and impurities, will now be described by way of example.

In a first example, a silicon-iron ingot is first hot-rolled to produce a strip having an intermediate thickness of 0.05 in. to 0.02 in., at a temperature of about 1000 C. to 1200 C., care being taken to ensure that the sheet after this first hot-rolling stage consists of equiaxed small grains.

The hot-rolled sheet is then reduced in a second hot rolling stage to its final thickness, or substantially its final thickness, in a number of passes at a temperature between 750 C. and l200 C., the steel being reheated between the hot-rolling operations to maintain the required temperature. The optimum reduction in thickness on each pass during rolling, and the number of reheating stages necessary during rolling, are dependent on the temperature at which the rolling is carried out, on the silicon content of the alloy being rolled, and the type and power of the rolling mill being used. It has been found desirable to carry out all these reheating operations under non-oxidising conditions, i.e. in a reducing atmosphere or in a vacuum so as to prevent excessive scaling of the metal before further rolling takes place. Prior to each reheating operation, scale formed in the previous rolling operation is removed.

The sheet is descaled by pickling, or by sand-blasting or slot-blasting, after each hot-rolling operation. After descaling the sheet will have rather a rough surface, which may be made smoother after the final hot-rolling operation by mechanical or chemical polishing. Alternatively, the descaled sheet may be subjected to a pinch pass on a cold mill which smoothes the surface but which reduces the thickness by less than 1 percent.

It Will thus be appreciated that the silicon-iron alloy sheet is brought to its final thickness, or substantially to its final thickness, entirely by hot rolling operations.

The sheet is then annealed, in either the smoothed or unsmoothed condition, at a temperature between 1050 C. and 1300 C. in a closely-controlled atmosphere capable of chemically reducing silica, or alternatively in a vacuum o f less than 10" mm. Hg, to effect secondary recrystallisation. The duration of the annealing stage will depend on the temperature and on the purity and thickness of the sheet, but preferably will be between 2 and 48 hours.

It has been found that the temperature at which the strip is rolled to its final thickness has a significant effect on the product of the secondary recrystallisation process. Rolling at temperatures above l00O C. favors the formation of a Goss-type texture close to the [001] orientation. At temperatures below 900 C. the formation of a cube-type texture (001) [lzkl] is favored. The nature of the cube-type texture found is also dependent upon the rolling schedule.

Two specific examples will now be given of processes according to the invention.

Example 1 A vacuum melted silicon-iron ingot containing 3% by weight of silicon is hot-rolled at a temperature of 1150 C. to a thickness of 0.120 in. The band so produced is then processed according to the following schedule.

(a) Descaled by sand blasting.

(b) Heated to 800 C., held at temperature for 10 minutes in an atmosphere of forming gas.

(c) Hot-rolled to a thickness of 0.052 in.

(d) Descaled by pickling.

(e) Heated to 800 0, held at temperature for 10 minutes in an atmosphere of forming gas.

(f) Hot-rolled to a thickness of 0.026 in.

(g) Descaled by pickling.

(h) Heated to 800 C., held at temperature for 10 minutes in an atmosphere of forming gas.

(i) Hot-rolled to a final thickness of 0.013 in.

(j) Descaled by pickling.

(k) Surface smoothed by pinch pass on cold mill.

(1) Degreased.

(n1) Annealed at 1150 C. for 24 hours in a hydrogen atmosphere of dew-point less than --50 C., to eifect secondary recrystallisation.

The resultant sheet exhibits a substantially cubeoriented secondary recrystallisation texture.

Example 2 A vacuum-melted silicon-iron ingot containing 6% by weight of silicon is hot-rolled at a temperature of 1200 C. to a thickness of 0.125 in. and is then processed as follows:

(a) Descaled by shot blasting.

(b) Heated at 1150 C. for 5 minutes in an atmosphere of nitrogen to which has been added 10% of hydrogen.

() Hot-rolled to a thickness of 0.050 in.

(d) Descaled by pickling.

(e) Heated at 1150 C. for minutes in the same atmosphere of nitrogen and hydrogen.

(f) Hot-rolled to a thickness of 0.020 in.

(g) Descaled by pickling.

(h) Heated at 1150 C. for 5 minutes in the same atmosphere of nitrogen and hydrogen.

(i) Hot-rolled to a thickness of 0.016 in.

(j) Descaled by pickling.

(k) Heated at 1150 C. for 5 minutes in the same atmosphere of nitrogen and hydrogen.

(1) Hot-rolled to a final thickness of 0.013 in.

6 (m) Descaled by pickling. (n) Smoothed by chemical polish. (o) Degreased. (p) Annealed at 1200 C. for 24 hours in a hydrogen atmosphere of dewpoint less than C.

The resultant sheet exhibits a substantially Goss-oriented secondary recrystallisation texture.

We claim:

1. A process for making grain-oriented silicon-iron alloy sheet in which an ingot comprising silicon and iron remainder impurities, is hot-rolled to a strip having an intermediate thickness in a first hot-rolling stage, which is followed by a second hot rolling stage wherein the strip is reduced to a final thickness in a number of passes, after which the strip is annealed under non-oxidising conditions to effect secondary recrystallisation, in which the improvement is characterized in that the only heat to which the strip is subjected before being reduced to its final thickness is that necessary to obtain the temperature required for said second hot rolling stage, and by maintaining the temperature throughout said second hot rolling stage within a range of 750-900 C. whereby a cube texture is obtained by said secondary recrystallisation anneal.

2. A process according to claim 1, wherein the temperature through said second hot rolling stage is maintained within the range of 10001200 C. whereby a Goss texture is obtained by said secondary recrystallisation anneal.

3. A process according to claim 1 wherein the temperature throughout said second hot rolling stage is maintained within the range of 900 to 1000 C. whereby a mixture of cube and Goss textures is obtained by said secondary recrystallisation anneal.

4. A process according to claim 1, wherein the silicon content of the ingot is in the range 210% by weight.

5. A process according to claim 1, wherein said intermediate thickness is less than 0.025 in.

References Cited UNITED STATES PATENTS 2,113,537 4/1938 Heimenz 148-111 3,147,157 9/1964 Grenoble 148--111 3,164,496 1/ 1965 Hibbard et al 148-l20 HYLAND BIZOT, Primary Examiner.

P. WEINSTEIN, Assistant Examiner.