US3159511A - Process of producing single-oriented silicon steel - Google Patents

Description

Dec. 1, 1964 Filed May 16, 1962 (are Lass W Core L038 W Core [055 W PRO SATORU TAGUCHI ETAL 1 CESS 0F PRODUCING SINGLE-ORIENTED SILICON STEEL 3 Sheets-Sheet 1 W Fig. I

, 'lm emfure 0f Confinuous anneal mso'c L5- 1.3 Temperature of Contmuaus anneal :75

l l J a0 Cold Rolling Reduction M) l l l l 4| 700 1900 900 moo 000 I200 Temperature [bnfinuous Anneal Fig.6 (A) 20 Temperature 0/ Caminuaus All/1801050; I?

llfi 1F L7 \d/ A6 D IN VENTORSI SATORU TAGUCHI AKIRA SAKAKURA a 04 a6 0.8 /.0 1-2 1-4 -6 8 Z0 Z2 34 26 Secondary old. Roll/n9 Rer/udz'on (Z) Dec. 1, 1964 PROCESS OF PRODUCING SINGLE-ORIENTED SILICON STEEL Filed May 16, 1962 Core Loss w C are L055 W "X SATORU TAGUCHI ETAL 3,159,511

Sheets-She et 2 F i g. 6 (B) Temperafure of Canzr'nuous Annea l/00'C I l l I I I I Secondary Cold Roll/n9 Reduczron Majneir' c Fig'. 4

IN VEN T0R 5A T RI TAGUCH/ I I I l I 0.002 0.004 0.006 0000 0.000 0.0/2 0.0/4 A KIRA SA 0 C Confeni Prior i0 final Anneal KA KRA Dec. 1, 1964 SATORU TAGUCHI ETAL 3,159,511

PROCESS OF PRODUCING SINGLE-ORIENTED SILICON STEEL Filed May 16, 1962 3 Sheets-Sheet 3 INVENTOR3= SATaRu Mal/CHI A IRA SAKA KURA United States Patent 3,159,511 PROCESS 6F PRQDUCING SINGLE-URIENTEB SILICON STEEL Saturn Taguchi andAidra 'Sakairura, Yawata, Japan, as-

signors to Yawata Iron and Steel (30., Ltd., Tokyo,

Japan Filed May 16, 1962, Ser. N 198,395 Claims priority, appiicatioir Japan Nov. 8, 1956 2 Claims. (Cl. 148'111) This invention relates to an improved process for producing single-oriented silicon steel sheets or strips of outstanding magnetic qualities.

This application is a continuation-in-part of our copending patent application Serial No. 669,576, filed July 2, 1957, and now abandoned.

Silicon steel sheets, which have so far been used for iron cores for transformers, generators, and the like, are of a soft magnetic material. Many attempts have been made to control the crystal orientation of the silicon steel sheet in accordance with its use, because the silicon steel has a body centered cubic crystal structure in which three mutually vertical directions of easy magnetization or cube axes 100 are present and, as a consequence thereof, it a magnetic field is applied parallel with one of these directions, the amount of energy necessary to magnetize the silicon steel core will be at a minimum.

The products of the present invention are silicon steel sheets which satisfy these requirements. A silicon steel sheet according to the invention is of the (110) [001] type. In a rolled steel sheet, the direction of easy magnetization 100 is parallel with only one direction which is the rolling direction on the rolling surface, and the two other directions of easy magnetization l00 are so oriented relative to the first-mentioned direction as to form 45 degrees with the rolling surface. This type of silicon steel sheet is called a single-oriented silicon steel sheet and has superior magnetic characteristics specifically in the rolling direction. i

It is an object of our invention to provide a process for the production of single-oriented silicon steel sheets or strips formed of crystals of the (110) [001] orientation and therefore having a high magnetic permeability and a low core loss in the rolling direction, which can be carried out on an industrial scale in a more economic manner than the known methods. 7

It is another object of our invention to provide a process for the production of single-oriented silicon steel sheet or strip having the aforesaid outstanding magnetic properties and characterized by relatively large crystal grains of uniform size and smooth grain boundaries.

More particularly, it is an object of our invention to provide a process for producing single-oriented silicon steel sheets and strips which are characterized by magnetic induction values, in the direction of orientation, which, in a magnetic field of 10 oersteds, exceed 17,000 and preferably even 18,000 gausses coupled with low core losses, which process is more economical than the known method of producing silicon steel with such outstanding magnetic properties.

Single-oriented silicon steel having outstanding magnetic properties must, of course, be carefully distinguished from othernon-oriented silicon steels such as electric Patented Dec. 1, 1964 ice grade, armature grade and motor grade steels of magnetic induction values, at 10 oersteds in the order of 13,000 to 14,000 gausses, with which this invention is not concerned.

B as used hereinafter, designates the magnetic flux density in a'magnetic field of 10 oersteds and therefore, the better the parallel relation of the axis of easy magnetization with the rolling direction, the higher the value shown by B and the less the electric power'loss in case the sheet is used as an'iron core. The value of B in the rolling direction as shown by single-oriented silicon steel sheet marketed today is 17,000 to 18,000 gausses. It is considered that, in case B is lower than 17,000 gausses, the sheet does not perform as a singleoriented silicon steel sheet.

In order to fully appreciate the progress achieved by the process of the present invention over the presently used techniques, it is necessary to review briefly the more recent development of single-oriented silicon steels.

Heretofore silicon steel having low core loss and orientation magnetizable in a singledirection of rolling has been required for use in transformers, motors, and gen erators, etc. Several methods of producing such silicon steel by the standard cold rolling procedure are known, for example, US. Patent Numbers 1,965,559 to Goss, and 2,535,420 to Jackson. Goss proposed the combination of cold rolling the steel with a reduction in thickness of more than 50% two times and a suitable heat treatment,

and Jacksons method comprises a drastic cold rolling with a reduction in thickness of substantially 73% to 84%, one slight cold rolling with a'reduction of 2% to 0.4%, and an appropriate heat treatment. Both patents referred to require two steps of cold rolling procedure.

U.S. Patents 2,287,466 to Carpenter and 2,287,467 to Carpenter and Jackson, on the other hand, obtain straightgrain permeability in silicon steels, coupled with low core and hysteresis losses from silicon steels by hot rolling a silicon steel containing from about 1 to 4% of silicon to a hot-rolled sheet, then subjecting the hot-rolled sheet to a cold rolling reduction in a single stage by about 67 to 83%, and finally heat treating the steel at temperatures between 700 and 1200 C. Preferred are pickling preparatory to the cold rolling stage, and dividing the latter stage into two phases with intermediate open anneal at about 730 to 900 C. Moreover, the unpiclded material is advantageously subjected to a decarburizing box anneal at 650 to 925 C. for two 'to twenty-four hours prior to cold rolling.

Decarburization has been recommended because it was recognized that carbon present in electrical steels in amounts substantially greater than 0.02%, as iron caribide, adversely affects the electrical properties.

The decarburizati-on can also be carried out after cold rolling, for'instance cold rolling in two stages, 'by first decarburizing the cold rolled silicon steel strip containing from about 1.5 to 4% of silicon at a temperature of about 800 C. and then holding the strip at a tempera ture of about 850 to 900 C. in a non-oxidizing atmosphere for about four to twenty-four hours and thereafter subjecting the'strip to a heat treatment at higher temperature between 900 and 1200 C. (British Patent 667,279 to Thompson-Houston Co. and US. Patent 2,534,141 to Morill et a1.), thereby achieving perfection of the texture of the cold rolled strip and relief of mechanical strain.

The magnetic induction thus achieved by the various known treatments involving a single cold rolling step in single-oriented silicon steel at constant magnetizing force of 10 oersteds was in the order of 16,000 gausses and with two cold-rolling steps up to 17,300 gausses, until Littm-ann and Heck (US. Patent 2,599,340), Rickett (US. Patent 2,826,520), Crede and co-workers (US. Patent 2,867,557) and others taught that even higher magnetic induction values could be attained, if the silicon steel ingots having the conventional silicon content of electrical steels were subjected to an extensive heat treatment prior to and/or during the hot rolling treatment and, in any event, prior to any cold rolling treatment, whether single or multi-stage.

The conventional practice is to reheat and hot-roll slabs. For example, in US. Patent 2,307,391 to Cole or 2,158,065 is mentioned the following slab reheating hot rolling:

Ingotheating at 100 to 1260 C. blooming slabs reheating at 1000 to 1260- hot-rolling-ehot-rolled sheet or strip This hot rolled steel is the starting material for the subsequent cold rolling and heat treatment steps which have been discussed hereinbefore. It had also been recognized that a heat treatment subsequent to the hot rolling and prior to the cold rolling treatment, either in the nature of a box anneal or as a coninuous treatment at about 750 to 1100 C. would be valuable.

The magnetic induction value which has so far been 16,600 to 17,620 has now been improved by Littmann et al. to be 17,000 to 18,350 and by Crede et al. to be on the average 18,090 at 10 oersteds by subjecting the abovementioned ingots and/ or slabs to a special heat treatment at higher temperatures. Thus, Littmann and Heck heat the ingots to about 1290 C., then roll into slabs, heat the slabs in a slab furnace to about or even above 1375 C., without burning, hot roll the slabs as rapidly as possible to about 5.5 to 9 times the final thickness, then open anneal at 760 to 1100 C. and then pickle and subject the hot rolled material to the earlier known cold rolling and annealing treatment.

Rickett soaked the slabs, at somewhat lesser temperatures at 1220 to 1240, for a minimum of eight hours, rather than 1375 C., in order to avoid a condition known as dripping, due to melting at that higher temperature, of oxides of iron and silicon mixed in proportions commonly found on the surface of silicon steel slabs. Hot rolling was then carried out at temperatures at about or in excess of 956 C.

Still more recently, this slab-reheating process was further modified by Crede and co-workers who heat the ingots to a temperature of over 1260 C. for a minimum of hours, which comprise heating for at least three hours to not less than 1345 C., then slab the steel to obtain a hot slab having a thickness of from to 1 /2 inches and a temperature of not less than 1095 C. and then immediately subject the hot slab to a series of hot reductions to produce a strip of a thickness of 0.060 to 0.10 inch before the temperature of the strip decreases below 870 C., so that the direct hot Working from the heated ingot to the hot rolled strip is carried out without an intermediate reheating of the steel. The steel is then subjected to two cold rolling treatments with intermediate open anneal at 870 to 1010 C., and, after termination of cold reduction, an open anneal in a decarburizing atmosphere at 760 to 850 C., and finally a box anneal in dry hydrogen at 1150 to1200 C.

In this manner, Crede and co-workers obtained singleoriented steel, the magnetic induction of which, at oersteds, amounted to 17,600 to 18,690 gausses, with an average of 18,090 gausses, and which satisfactorily low watt losses. On the other hand, even when an ingot is heated to the conventional temperature of 1000 to 1260 C. and is directly hot-rolled to obtain a hot-rolled sheet and the sheet is treated in two cold-rolling steps, a value of only 16,000 to 17,200 gausses will be obtained. The direct hot rolling with intermediate anneal at 870 C. to 1010 C. only affords a product which shows magnetic induction values of 16,000 to 17,620 gausses; direct hot rolling with subsequent single cold rolling treatment, i.e. without intermediate annealing affords only silicon steel strip with magnetic induction values, at 10 oresteds, below 16,000 gausses.

The microsegregations of various elements in the steel are made more homogeneous by the conventionally practised slab reheating process than by the direct hot-rolling invented by Crede. Such microsegregation, particularly of phosphorus and manganese, was believed to be responsible for the erratic response of silicon steels to treatments designed to produce a grain-oriented structure.

Unless the steel is substantially completely homogenized, by the heat treatment of the slabs, such segregations were expected to persist throughout the processing to develop magnetic properties and interfere with the attainment of complete grain orientation and therefore optimum magnetic properties, particularly permeability. Therefore, the conventionally practised slab reheating process is adopted in the present invention.

We have now discovered that it is possible to obtain silicon steel products with outstanding magnetic properties equal to those attained by Crede and co-workers, namely magnetic induction values, at 10 oersteds, which exceed 17,000, and even 18,000 gausses, and core losses in the order of 1.25 watts per kilogram, at 15 kilogausses and 50 cycles per second (Epstein test), by the improved process according to the present invention, which comprises the slab reheating process to a temperature not higher than 1260 C. and only a single cold rolling stage without intermediate annealing, but which requires, in contrast to the above-mentioned earlier processes, a careful control of the aluminum content of the silicon steel material being treated, and a control of the temperature of the continuous anneal which follow the single-stage cold rolling treatment prior to the final anneal.

The aluminum content of the silicon steel must be held between 0.010 and 0.030% by weight in the silicon steel ingot at the beginning of the hot rolling treatment.

Moreover, the temperature during the open anneal following the cold rolling stage must not exceed 1050" C., or it will be impossible to attain the outstanding magnetic properties, hitherto achieved exclusively by the more complicated and less economical processes described above, by a single cold rolling stage, without intermediate anneal between a plurality of cold rolling phases.

Other features and objects of our invention will become apparent from the following description thereof when taken in conjunction with the accompanying drawings in which:

FIG. 1 represents the curves showing the relation between cold rolling and core loss after the final anneal or" silicon steel sheet or strip treated by the process according to our invention.

FIG. 2 illustrates the curves showing the relation between the temperature of continuous anneal and core loss after the aforesaid final anneal.

PEG. 3 represents the curves showing the relation between aluminum content prior to the final box anneal and core loss after the final box anneal in the process according to our invention.

KG. 4 gives the curves showing the relation between carbon content prior to the final box anneal and core loss after the final anneal.

FIG. 5 gives macrostructure photographs (actual size) showing the relation between aluminum content prior to the final anneal and crystal grain size after the final anneal with reference to six different samples, A, B, C, D, E and F.

FIG. 6 is a diagram showing the relation between the rate of reduction of the thickness in the secondary whereby it is removed as carbon oxide.

, cold-rolling and the core loss after the final annealing in case the secondary cold-rolling is made.

Silicon steel adapted for carrying out the improved process of producing single-oriented steel strip or sheets,

according to our invention must contain from 2.0 to 4.0% .by weight of silicon, silicon contents of 2.9 to 3.2% being preferred.

' The starting material must further contain the above defined amounts of aluminum. The carbon content should be between 0.02 and 0.06%, about 0.04% being preferred. v;

The carbon content is not to limit the composition of the material of the present invention.- However, it is difficult to reduce the carbon content to less than 0.02% in the industrially obtained ordinary ingot. If the carbon content is greater than 0.06%, troublesome decarburization treatment will be required in order to eliminate the detrimental eifect'of the residual carbon on the magnetic characteristics of the product. Therefore, a content of 0.02 to 0.06% C is recommended in the starting material for the process ofthe present in vention.

Analyses of typical steels useful as starting materials in the process according to the invention are, for instance:

However, we do not intend to limit the range of the constituents in the above except as to the ranges of silicon and aluminum. The aluminum content of the conventional single-oriented silicon steel is usually below 0.010%. Higher aluminum contents have sometimes been used, but optimal results were obtained, for instance, by

Crede and co-workers only with silicon steels in which the Al content was from 0 to below 0.010%.

We have discovered not only that the B value improves as the aluminum content is increased above thehitherto preferred upper limit of 0.010%, but also the phenomenon that a peak of magnetic characteristics is an tained when the aluminum content is 0.020%, while the magnetic values decrease again at higher aluminum contents, especially above 0.030% A1.

Many attempts have been made to add aluminum to various types of non-oriented steels with a view to improving the magnetic properties of the latter. For example, in US. Patent 2,445,632 to Williams, the temperature at which the 'y phase (austenite phase) appears is elevated by adding 0.010 to 1.00% aluminum, and carbon which would dissolve into the steel if aluminum were not present, remains undissolved and combines with oxygen, Further, it is mentioned in US. Patent 2,378,321 to Pakkala that, when about 0.20% aluminum is added, a proper crystal state will be obtained even without the danger of a sample of a high carbon content cutting a 'y-loop. However, the products produced by these known methods are nonoriented silicon steel sheets and their industrial uses are different from those of the products of the present inven tion. Theobject of adding aluminum in the known methods is to prevent the 7 phase from appearing, where as, in the present process, the 7 phase would not be expected to and does not appear, irrespective of the presence or absence of aluminum. In the past, the art had, therefore, considered the presence of aluminumin silicon steel used as starting material in the production of singletempt to use a determined aluminum content. In fact, it is quite unexpected that, when aluminum is added within the above stated limits, the magnetic permeability ime proves, because altuninum was believed to form oxide such as A1 0 in the steel which would be detrimental to the magnetic characteristics. Therefore aluminum has been avoided where feasible and its content has usually been required to be less than 0.010%, except where it is unavoidably present in the steel derived from Al-rich raw material. In contrast to the above-described beliefs, it is the most important feature of the present invention to limit the aluminum content to 0.010 to 0.030%.

The ingot in the present invention is heated to a maximum of 1260 C. and is bloomed in a blooming mill (slab mill) so as to obtain a slab of conventional dimensions, and the slab is then reheated to 1000 to 1260" C.

and is converted to a hot-rolled strip of l to 3 mm. thickness in a hot-rolling mill. Such strip is then pickled and subjected to a single cold-rolling treatment during which it is reduced directly, without any intermediate annealing to a final gauge, for instance, between 0.1 and 0.5 mm.

Of course, the above dimensions are given by Way of example only. v The cold rolled strip or sheet is then subjected to a continuous anneal within the range of about 750 to 950 C. for a short period of time, of about one to four.

minutes, and preferably of about three minutes.

The resulting product is finally box annealed at a temperature within the range of about 1000 to 1200 C. for several hours. 4

Optimal results are obtained with silicon steel having 2 to 4% by weight of silicon and 0.020% of aluminum, if the steel, first hot rolled by the usual slab reheating process and then cold rolled in a single stage with a reduction of is then subjected to continuous anneal at about 875 C. for three minutes and then to a box anneal at 1200 C. for twenty hours.

We have found that the attainment of optimal results depends primarily on the presence of critical amounts of aluminum in acid-soluble form in the treated silicon steel.

Prior to the final anneal, aluminum is present in the steel as an acid-insoluble compound consisting principally of A1 0 and as an acid-soluble compound consisting principally of AlN. As shown in Table I below, the amount of the acid-insoluble aluminum remains substantially constant at about 0.005% A1, no matter how large or small the total amount of aluminum may be. The amount of the acid-soluble aluminum depends upon the total amount of aluminum. Therefore, crystal grain growth after the final anneal depends upon the amount of the acid-soluble aluminum consisting principally of AlN present in the steels at the beginning of final anneal. FIG. 3 indicates the relationship between the aluminum (AlN) content prior to the final anneal, core loss after the final anneal, and magnetic induction in which the steel possessing the chemical analysis is shown in Table I or similar analysis with different contents of aluminum is bloomed and hot-rolled, then cold rolled with a reduct'ionin thickness of 80 percent, annealed continuously at the temperature of 850 C. for three minutes, and finally annealed at a temperature of 1200 C. for twenty hours. Asstated hereinbefore, magnetic properties after the final anneal are of the desired high order when the total amount of aluminum is within the range of 0.010% to 0.030%, and they decrease rapidly when aluminum is above 0.040%. g V

Specifically, when the aluminum content is 0.020%, B will be of such high value as 17,800 gausses. This is a value which has hitherto never been obtained in the production of single-oriented steel involving the'conventional slab reheating process.

In accordance with the foregoing, the aluminum content is held above 0.010 and up to 0.030% in the present invention. It will now be shown that a superior singleoriented silicon steel sheet can be obtainedin a single cold-rolling treatment by adding the above-defined small amount of aluminum.

FIGURE 6A shows the variation of the core loss value as against varying aluminum contents in the case that a stock having such chemical composition as is shown also in Table II was heated to 1200 C. and was bloomed to be converted to a slab, the slab was again heated to 1250 C. and was hot-rolled to a hot-rolled sheet of 1.6mm. thickness and the sheet Was pickled, then coldrolled at a rate of depression of about 80%, continuously annealed at 850 C. for three minutes, then secondarily cold-rolled to reduce the thickness variously and finally is unexpectedly obtained, due to the effect of the abovedefined aluminum content, without such secondary coldrolling step, than with the same.

It is made clear from the above that, in case a silicon steel stock with the above-defined small amount of aluminum of 0.010 to 0.030% according to the present invention is used, and is subjected to a treatment including only a single cold-rolling step, there will be obtained a superior single-oriented silicon steel sheet having a magnetic induction B exceeding 17,000 gausses and low core loss value, even though no special high temperature heating is applied to the stock in hot-Working it.

TABLE I Chemical analysis (percent) Sample Step Al O Si Mn P S Cu Ni Cr Al in all Acidsoluble AI Chemical analysis (percent) Magnetic test (Epstein test) Sample Step N T Core loss (W/lrg.) Magnetic induction (gauss) N in all N as AlN W10/50 715/50 13 B5 1310 B Norns:

1* Material analysis. 2* Analysis aiter continuous anneal. 3* Analysis after final anneal. W/kg. =Watts per kilogram. W10/50=10 kilogausses at a frequency of 50 cps. W15/50=Watts 15 kilogausses at a frequency of 50 cps. B =B at 3 oersteds.

annealed. The signs A, B, C, D and F of the curves correspond respectively to the sample numbers A, B, C, D and F in Table I. As will be readily seen from these data, in case aluminum is less than 0.010%, i.e. the conventional steel is used, and if secondary cold-rolling at a rate of depression of about 1% is applied, the core loss value of the product will be lower, Whereas, in case aluminum is in the range between 0.010 to 0.030%, and if the secondary cold-rolling is applied, the core loss value .will be even higher and the magnetic properties will deteriorate. Thus, it is another most important feature of the present invention that a secondary cold-rolling is avoided, for, as is made clear from the above, a singleoriented silicon steel sheet of better magnetic properties Cold rolling with a drastic reduction influences the metal structure after a subsequent continuous anneal. The relation between cold rolling and magnetic properties fter the final anneal is given in PEG. 1. The curves shown in FIG. 1 are plotted based on the test in which a silicon steel of the composition listed in Table II below is bloomed and is then hot-rolled to be a hotrolled sheet 1.6 mm. thick, then cold rolled with various reductions in thickness, then annealed continuously at the temperature of 850 C. and at the temperature of 1050 C. for three minutes, respectively, and finally annealed at the temperature of 1200 C. for twenty hours; the relation between cold rolling and core loss after the final anneal as plotted clearly indicates that a silicon 9 steel of low core loss is produced when the amount of reduction in thickness is chosen within the range of 65% to 85%.

TABLE II Analysis of Ingot and of Treated lllazerial After Each Step (Percent by Weight) After In the drawings and the specification, we use the technical term WIS/50 which shows watts 15 kilogausses at a frequency of 50 cycles per second, and Watts loss per kilogram.

Continuous annealing for a short time after the cold rolling process is carried out, so that the cold rolled crystals may be of a recrystallized structure, and to eliminate detrimental carbon in this step in order to meet the required limits therefor, discussed supra. This annealing is usually carried out in a neutral or reducing atmosphere, but need not be limited to these types of atmosphere. However, an atmosphere containing hydrogen and water vapor is advantageous in view of decarburization. In an atmosphere containing hydrogen in amounts as small as 10% by volume and a small amount of Water vapor, the effect of decarburization is fully achieved Within a relatively short time. In general, decarburization is effected in an atmosphere containing in the range of 0.1% to 0.5% of E by volume, based on the amount of hydrogen irrespective of the content of the latter in the atmosphere.

The influence exercised by the temperature applied at continuous anneal upon the magnetic properties of the final product is given in FIG. 2, which shows the test results of core loss in which a silicon steel containing the composition given in. Table II is bloomed and is then hot-rolled to a hot-rolled sheet of 1.6 mm. thickness, then cold rolled with a reduction of 80%, continuously annealed at different temperatures between 700 and 1200 C. for three minutes, respectively, and finally annealed at the temperature of 1200 C. for twenty hours. The temperature range of 750 to 950 C. is preferred with a view to obtaining a final product having optional magnetic properties, and decarburization is advantageously effected within that temperature range.

However, if the temperature of the continuous annealing is higher than the temperature of 1050 C.

which is .the critical upper limit of the temperature range in which the process according to our invention is applicable, the magnetic properties of the metal will be only improved when it is subjected to a secondary cold rolling with a reduction in thickness of 1.0% and the most advantageous feature of the invention will thus not be realized. I

In the above, in case the temperaturefor the continuous annealing is hi her than the temperature specified in the present invention, if the second cold-rolling or such weak cold-rolling as of a rate of depression of about 1.0% is further applied after the continuous annealing, the magnetic characteristics of the final product will be better and therefore the above-mentioned feature of the present invention will be lost. That is to say, FIGURE 63 shows the variation of the core loss value as against varying aluminum contents in the case that a silicon steel sheet having such chemical composition as is shown in Table II was first heated to 1200 C., was bloomed,

10 was heated to 1250 C., was hot-rolled to a hot-rolled sheet of 1.6 mm. thickness, was then pickled, was coldrolled at a rate of depression of about was con.- tinuously annealed at 1100 C. for three minutes, was secondarily cold-rolled to be variously reduced in thick. ness and was finally annealed at 1200 C. for 20 hours. As will be understood by comparing FIGURES 6A and 6B, in case the continuously annealing temperature is 1100" C. which is higher than the range of 75.0 to 950 C. specified in the present invention, reduction of the sheet thickness by a secondary cold-rolling will be required irrespective of the aluminum content. Temperature control within the above limits during the continuous anneal is therefore an important factor in the process of thepresent invention. I

A highly oriented crystal grain favored with smooth boundary is developed in the silicon steel by the final anneal, thereby alfording a final product of excellent magnetic properties. The short time continuous anneal serves the purpose of improving the recrystallized struc ture by the development of crystal grain growth of large and uniform size. As a commercially feasible condition, We prefer the temperature range of 750 to 950 C. mentioned hereinabove for the continuous anneal. The recrystallized structure of the metal after an anneal within this temperature range has a relatively large grain of uniform size, while at an anneal of a higher temperature than 1050 C. a crystal grain of irregular and coarse size is grown, so that a highly oriented grain growth cannot be expected after the final anneal.

Continuous anneal of about one minute is sufiicicnt to' develop recrystallized structure on the cold rolled steel, and an anneal of less than four minutes is sufiicient if annealing is combined with decarburizing. If the carbon content of the hot rolled steel is exceedingly small, annealing of about one minute is sufiicient. This applies particularly when the steel with hot mill scales thereon has been box annealed to decarburize it directly following hot rolling. If the carbon content of the silicon steel has decreased so low due to decarburizing effect by boxannealing the hot-rolled strip with rnill scales deposited thereon that the carbon does not interfere with grain growth at the final annealing core loss after, the improved results will be obtained even though the steel is subjected directly to the final anneal without recourse to a short time continuous anneal after the cold rolling step. How ever, a short time continuous anneal is preferable in order to improve a consistent quality of the final product.

Moreover, far-reaching decarburization-Will be hardly effected in case the steel is hot-rolled and is then continuously annealed with mill scales deposited thereon, therefore a short timecontinuous anneal after cold rolling is usually required.

The final box anneal is preferably carried out in a neutral or reducing atmosphere at a higher temperature than 1000 C. for at least five hours in order to obtain a grain-oriented silicon steel having low core loss. It is desirable .to anneal the steel for a number of hours at an elevated temperature, but an anneal at a higher temperature than 1250" C. for more than forty hours is not ef fective in spite of an elevated temperature and long hours. We prefer an annealing temperature adapted for commercial production Within the range of 1000 to 1200" C. in our invention, but the advantageous effect of our process is not lost even though the anneal is carried out at a itemperature above the range referred to.

FIG. ,4 gives thecurves showing the relation between carbon content prior to the final anneal and core loss after the final anneal in which the steel having analysis of sample D or similar analysis is hot rolled, then cold rolled in a single stage with a reduction in thickness of 80 percent, annealed continuously at the temperature of 850 C. for three minutes, and finally annealed at the temperature of 1200 C. for twenty hours, which indicate clearly that carbon has a considerably adverse effect on 1 1 the steel, and suggest that 0.008% C. is requisite prior to the final anneal.

With an appropriate control of the aluminum content of the silicon steel stock in accordance with the invention, core losses after the final anneal decreases considerably, and crystal grain size of average 1 to 2 cm. in diameter is obtained.

As fully described hereinabove, our invention cornprises the usual slab reheating hot rolling process of silicon steel stock containing 2.04.0% Si, and 0.0l0.030% Al, then cold rolling in a single step, continuously annealing at 750-950 C. for a short time, and finally annealing at 1000-1200 0; therefore, our process is very simple compared with the most recent processes, yet it provides for a commercial production of single oriented silicon steel with the best hitherto achieved magnetic properties including low core loss in an inexpensive manner.

In Table III, there are compared the optimal results obtained with the process according to the invention, and

these samples after the final anneal changes as shown in FIG. which clearly demonstrates the effect of the amount of aluminum present therein.

EXAMPLE 2 Hot rolled silicon steel strip of the composition of sample D in Table I with hot mill scale thereon is boxannealed at 750 C. for about 5 hours immediately after rot rolling (analysis after box-anneal: 0.010% C, 2.95% Si, 0.017% Al in all, 0.015% acid-soluble Al, 0.008% Ti, 0.0102% N in all, 0.0072% N as AlN). The strip is then pickled, cold rolled in a single strip with a reduction in thickness of 80% to the final gauge, 0.33 mm., and then annealed continuously at 1200 C. for twenty hours. The test values after the final anneal are a core loss of 1.08 watts per kilogram at kilogausses at 50 cycles, and a magnetic induction of 18,050 gausses at 10 oersteds.

EXAMPLE 3 Silicon steel strip box-annealed at 750 C. with hot mill those obtained with several of known processes. scale thereon after hot rolling, as in Example 2 (analysis TABLE III [Temperatures in 0.]

Hot Ingot A1, Ingot Slab Reheat- Rolling Open Cold Process and/or Designation of Steel Percent Soaking Temp. ing of Temp. Anneal Rolling Temp. Slab at first Stand Armeo Transeor Mir U.S. Steel USS Transformer 52 M-l4 Yawata Hilite Cor H12.

Below 0.0L 1, 230 1, 060 760-1, 005 Single. Littmann et al U.S. Pat. 2,599,340 Below 0.01.. 1, 280 1, 000 7604, 005 Double. 0.01 1, 200 1, 150 1, 400 700-1, 095 Double. Crede et al U.S. Iat. 2,867,557 Below 0.0L. 1, 320 1, 230 None 1,120 760-1, 095 Double. Present Appln.:

Ex. 1 0.020- 1, 230 1, 250 1,060 Single. Ex. 2 0.020 1, 230 1, 250 1, 060 750 Single.

Intermediate Magnetic Core loss, Anneal Final Continuous Final Induction W/kg. (15 Process and/or Designation of Steel between cold Gauge Anneal Anneal B10 kilcgausses Rolling (mm.) (gausses) cycles) Steps Armco Transcor M14 0.035 13, 200 U.S. Steel USS Transformer 52 M-14 0.035 13, 300 Yawata Hilite Cor Hi1 0 Littemann et a1 U.S. Pat. 2,599,340.- 1 0 1 5 Credo et a1 U.S. Pat. 2,867,557 870-1, 010 760-850 1, 1504: 200 18, 090 Below 1.4. Present Appln:

Ex. 1 None 0. 33 850 1, 200 17,800 1.11. Ex. 2 None 0.33 1, 200 18,050 1.08.

It will be understood that the details described hereinbefore and in the following non-limitative example may readily be modified by those skilled in the art withlout departing from the spirit and scope of the invention. It is therefore intended that these details be interpreted as being illustrative and not in a limiting sense.

EXAMPLE 1 Ingots of silicon steel stock having the six different compositions A to F shown in Table I are treated, as follows:

An ingot of each silicon steel stock was heated to 1230 C., was bloomed, was further heated to 1250 C. and was hot-rolled until a hot-rolled steel strip of 1.65 mm. thickness was obtained.

The hot-rolled strip is then pickled, and then cold rolled with a reduction in thickness of 80 percent in a single step to the final gauge, 0.33 mm. Thereafter, it is continuously annealed at 850 C. for three minutes, and finally annealed at 1200 C. for twenty hours. Core loss and magnetic induction values obtained after the anneal have been reported in Table I. Particularly, the sample D containing 0.017% A1 prior to the final anneal affords a core loss of 1.11 watts per kilogram at 15 kilogausses at 50 cycles, a magnetic induction of 17,800 gausses at 10 oersteds, and high orientation. The macrostructure of after box-anneal: 0.007% C, 2.96% Si, 0.015% A1 in all, 0.011% acid-soluble Al, 0.006% Ti, 0.0079% N in all, 0.0049% N as AlN) is pickled, then cold rolled in a single step with a reduction in thickness of to the final gauge, 0.33 111111., and finally annealed at 1200 C. for twenty hours. Core loss and magnetic induction after the final anneal are 1.31 watts per kilogram at 15 kilogausses at 50 cycles and 17,300 gausses at 10 oersteds.

We claim:

1. In a process for producing single-oriented silicon steel sheet, which comprises blooming a silicon steel ingot containing 24% by weight Si to produce a silicon steel stock, hot-rolling said stock, pickling said rolled material, subjecting said material to a single cold-rolling, reducing the stock thickness by 65 to thereby producing the sheet of the final thickness, continuously annealing the material in a decarburizing atmosphere at a temperature of between 750 to 950 C., and finally annealing at a temperature of 1,000 to 1,200 C., thereby producing a silicon steel sheet of secondary recrystallization and of good grain-orientation in the direction of cold-rolling, and of a magnetic induction, at 10 oersteds, of greater than 17,000 gausses, the improvement, in combination therewith, wherein (a) said ingot is heated to a temperature of maximally 1260 C. and then bloomed to produce silicon steel 13 14 stock which is reheated to a temperature of maxisentially of 0.02% by weight of Al, 2 to 4% by weight m-ally 1260 C., and of Si, with the balance being essentially iron. (b) said stock consisting essentially of 0.01 to 0.03%

by Weight of 2 4% by Weight of Si, with the References Cited in the file of this patent balance being essentially iron. 5 2. In a process for producing silicon steel sheet as UNITED STATES PATENTS claimed in claim 1, wherein the ingot is heated to a temperature of about 1230 C. and then bloomed to produce 312 233 2 :1

a silicon steel stock, said stock being reheated to a temperatu re of about 1250 C. and said stock consisting eslo 2867557 Crede 1959

Claims (1)

1. IN A PROCESS FOR PRODUCING SINGLE-ORIENTED SILICON STEEL SHEET, WHICH COMPRISES BLOOMING A SILICON STEEL INGOT CONTAINING 2-4% BY WEIGHT SI TO PRODUCE A SILICON STEEL STOCK, HOT-ROLLING SAID STOCK, PICKLING SAID ROLLED MATERIAL, SUBJECTING SAID MATERIAL TO A SINGLE COLD-ROLLING, REDUCING THE STOCK THICKNESS BY 65 TO 85%, THEREBY PRODUCING THE SHEET OF THE FINAL THICKNESS, CONTINUOUSLY ANNEALING THE MATERIAL IN A DECARBURIZING ATMOSPHERE AT A TEMPERATURE OF BETWEEN 750 TO 950*C., AND FINALLY ANNEALING AT A TEMPERATURE OF 1,000 TO 1,200*C., THEREBY PRODUCING A SILICON STEEL SHEET OF SECONDARY RECRYSTALLIZATION AND OF GOOD GRAIN-ORIENTATION IN THE DIRECTION OF COLD-ROLLING, AND OF A MAGNETIC INDUCTION, AT 10 OERSTEDS, OF GREATER THAN 17,000 GAUSSES, THE IMPROVEMENT, IN COMBINATION THEREWITH, WHEREIN (A) SAID INGOT IS HEATED TO A TEMPERATURE OF MAXIMALLY 1260*C. AND THEN BLOOMED TO PRODUCE SILICON STEEL STOCK WHICH IS REHEATED TO A TEMPERATURE OF MAXIMALLY 1260*C., AND (B) SAID STOCK CONSISTING ESSENTIALLY OF 0.01 TO 0.03% BY WEIGHT OF AL, 2 TO 4% BY WEIGHT OF SI, WITH THE BALANCE BEING ESSENTIALLY IRON.

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