US2303343A - Silicon steel electrical strip - Google Patents

Description

ing the prior art.

' economics,

Patented Dec. 1, 1942 2,303,343 SILIQON swam. smcrmcu. s'mrr Edwin H. Pakkala, Cleveland, Steel Corporation,

No Drawing. Application January Serial No. 374,402

Engel, Pittsburgh, and Matti H. klahoma, Pa., and Harry. F.

Ohio, assignors to Carnegie-Illinois a corporation of New Jersey Shannon,

4 Claims. (01. 148-12) This invention relates to silicon steel electrical strip, the object being to provide the strip as a mill-processed product having better magnetic properties than can be obtained by follow- In carrying out is to produce a silicon steel with a composition suitable for electrical strip. Generally speaking, this means that the carbon content should be as low as can be obtained in view of current commercial steel refining practices and sound this placing the carbon at a minimum of about .04% and more often around .06%

at the present time. The silicon content should 2% and 5%, it being undesirable to use less than 2% when processing electrical strip to improve its magnetic properties to any great extent, and more than 5% failing to produce suflicient improvement in the magnetic properties to warrant the expense involved. Since the better grades of electrical strip such as used for transformer cores require about, a 3% silicon content, and its manufacture involves cold rollthis invention, the first step ing, introducing .difliculties at the present time in the case of strip containing more than about 3.50% silicon, the silicon content of the steel usually should range between 3% and 3.50%, when the best type of product is desired. The phosphorus and sulphur contents of the steel, like the carbon content, should be kept as low as is practical by means of commercial refining practices, maximums of .010% phosphorus and .020% sulphur being preferred in practicing the present invention. The residual manganese content ordinarily should be no higher than is necessary to overcome the evil effects of sulphur, and no lower: than is consistent with good steel making and rolling practice.

With the above in mind, the steel produced in carrying out the present invention may be advanced as having the following average composition:

unnecessary to bring the hot rolled strip to any particular gauge since the present invention does not necessitate the use of rigidly specified cold rolling reductions. In other words, this hot rolling may be carried out in whatever manner than in a manner dictated by the necessity of bringing the strip to a fixedgauge range. 1

After conditioning the hot rolled strip in the usual manner, it is cold rolled to a thickness permitting a light reduction, subsequently described, to bring it to gauge. As previously indicated, the reduction percentages obtained during this cold rolling are dictated'only by the requirement that the material be brought to the proper thickness. However, the strip should receive suflicient cold reduction to permit its classification as a cold rolled product,'this indicating a of around a 50% reduction.

At this time the strip is highly strained, and it is necessary to anneal it. This is doneby continuously annealing the cold rolled strip in a decarburizing atmosphere with the annealing temperature and time factors adjusted not only to remove all the cold working strains from the strip but also to cause the decarburizing atmosphere to remove substantial amounts of carbon from throughout the thickness of thestrip, thus obtaining considerable columnar grain growth caused by the compositional gradient. In addition to being decarburizing, the atmosphere should also be oxidizing to a degree regulated to cause the formation of a thin, tightly adherent oxide film on the strip without the production of a heavy, loose scale. The production of this film, of course, is not necessary when decarburizing only is desired.

As a specific example of the above step, cold rolled silicon steel-'strip has been passed through a continuous furnace of the type adapted to confine a gaseous atmosphere about the strip. This atmosphere consisted of approximately 12% CO: with the balance inert gas, and it was saturated with moisture at atmospheric pressure Per cent and temperature. The traveling speed of the strip was correlated to the length of the furnace so that the strip remained "in the furnace for approximately six minutes, the temperature in the furnace being about 2000 li'. Temperatures ranging from 1750 Fjto 2400 F. may be used, but all factors must then be regulated to attain the same results. By the use of this practice, all the coldroiling strain was removed and thecarbon content was dropped from .06%

proves most convenient and practical, rather as inch thick and having the average chemicalto less than .02% with consequent columnar grain growth in the case .of strip about .014

composition previously indicated. Also, a thin, tightly adherent oxide film was formed on the strip, excessive oxidation being absent] The talline directions of filmformed was of" a type suitable for use as I interlamellar insulation.

The next step in the present method is to cold work the strip to impart in the same and to then reanneal the strip by the batch method, the predetermined strain and the temperature and time factors of the'reannealing being correlated to effect an appreciable improvement in the magnetic properties of the strip. It is necessary to use the batch annealing method because the time factor of this reannealing is such a long period that continuous annealing is impractical, and during annealing adjacent layers of the strip are prevented from sticking together by the film imparted by the described atmosphere during the first annealing. During the reannealing, this film is preserved by maintaining the strip in an atmosphere that is sufllciently neutral to prevent material reduction of the film or oxidation of the film or strip.

More specifically referring to this cold workin and reannealin step, the working may be done by cold rolling the strip to eflect reductions of from .5% to 5%, a 2% reduction having been found'to produce the maximum results in the case of a steel having the average composition previously advanced, but with its carbon reduced by the first heat treatment as reannealing temperature used is about 1975 F. but rangesirom 1800 F. to 2200 F., the time factor representing a period of about 4 hours at maximum temperature, some '12 hours being required in bringing it to the desired temperature and about threetimes this time being required to effect the coolin from this temperature. The equipment used may be in the form of a batch type furnace, the strip either being placed on the bases in coils or the strip may be sheared into sheet and stacked in the boxes for the treatment. The atmosphere used during this reannealing is not important, providing it is neither strongly oxidizing nor reducing in character. Regardless of the equipment used, the strip is in the form of layers that are pressed together during this reannealing, so as to involve the problem of these layers becoming permanently stuck together, this problem being solved by the oxide film imparted to the strip during the first annealing and which separates the layers so that they cannot stick together.

As a result of this properly critical straining and reannealing step which follows the annealing and decarburizing step, reducing the carbon to less than .02% a product is obtained having new properties. The grain size is unusually large. each grain extending through the thickness of the strip and being from one-half to six inches in diameter and averaging about the majority of the grains being not less than one inch in diameter.

A further, result obtained by the decarburizing step followed by the and annealing is that the grain growth obtained takes place in a msmner1esultin8 in about 75% or more of the grains oriented so that the cryseasiest magnetization are substantially aligned with the rolling direction of the strip. It is thought that the various steps condition the steel so that during the reannealins grains that are improperly oriented are absorbed by *grains that are properly oriented during the growth of the latter.

Furthermore, this product has a core loss in watts per pound, at a magnetic flux density of a predetermined strain described. The

r about .014 inch. It is 10,000 gausses and at 60 cycles, of not greatei than .50, a maximum permeability of at least 13,000 and a permeability of at least 1,500 at a flux density of 16,000 gausses. It is to be understood that this product is made from a steel with the chemical composition like that previously advanced as an average composition, and was processed as described, its thickness being to be noted that this product capable of quantity production rather than one obtained by elaborate treatment of small pieces. Furthermore. the properties here outlined are possessed by the strip after it has been cut and prior to the removal of the resulting cutting strain. In other words, these properties are possessed by the strip while it contains strain of the nature encounis a mill-processed tered when the strip is fabricated in a manner involving cutting, bending, etc.

In common with all electrical strip, more severe straining of the above product lowers its magnetic properties. The straining contemplated here may be that resulting from shearing, punching, stamping, etc., or from bending of the strip as in the forming of a toroidal coil. In such instances, the conventional practice for eleasing such strain is to heat treat the strip by raising its temperature to from 1400" F. to possibly as high as 1650 F., this treatment reimparting the same properties as the strip possessed prior to its straining in the case of electrical strip processed in accordance with the prior art. However,

- in the case of strip resultin from the practice two inches with described critical strainins materially improves of the present invention, this same treatment the magnetic properties of the strip over those it possessed prior to the straining. It is to be appreciated that this heat treatment must be carried out in an atmosphere which does not unduly oxidize the strip nor destroy the quality of the film imparted by the first annealin providing it is desired to maintain this film so that it can function as interlamellar insulation. A reducing atmosphere may be used if the film is not to be preserved.

In the case of strip made from steel having the average composition previously noted and which has been processed in accordance with the present invention, followed by being lightly strained in the manner such as dental to its fabrication as part of a magnetic structure, a heat treatment of from 1400 F. to 1650 F. for from one to three hours lowers its previously stated core loss value to one not greater than .42 watt per pound when determined under the same conditions although values as low as .37 watt per pound have been obtained, its maximum permeability being raised to not less than 15,000 with values up to 30.000 having been obtained and its permeability at 16,000 gausses being raised to not less than 2,500 with values up to 6,000 having been obtained. Here again it is to be noted that these properties are imparted to the steel by a treatment that is entirely practical and which is not unduly expensive.

We claim:

l. A method of making silicon steel electrical strip. said method comprising the steps of proucing a silicon steel with a composition suitable for said product and containing from 2% to 5% silicon and substantially more than about .02% carbon, working said steel to cold rolled strip within 5% of final gauge, continuously-annealing said strip in a decarburizing atmosphere with the annealing temperature and time factors admlght be inci- -at least one halt inch,

iusted to remove all the cold rolling strain from said strip and to also cause said atmosphere to reduce the carbon to less than about .02% throughout the thickness of said strip, said atmosphere being oxidizing to a degree regulated to cause the formation of a thin, adherent oxide film on said strip without the production or heavy,

said strip to-fin'al gauge to impart a predetermined strain therein and annealing said strip by the batch method at temperatures of from 1800' suflicient to increase the average grain size to adjacent layers oi! said strip being prevented from sticking together during said reannealing by said film imparted by said atmosphere during the first-named annealing. and said film being reannealing by I maintaining said strip in an atmosphere that is sufllciently neutral to prevent a material reduction or over-oxidation of said film I". to 2200'1'. for a'time' sumcient to relieve said the carbon to less than about .02% throughout the thickness of said strip,

mation ota thin, adherent oxide film on said strip without the production of heavy, loose scale,

of from 1800 F. to 2200 F.1or a time suflicient to increase the average grain size to at least onehalf inch, adjacent layers of said strip being meor over-oxidation of said film, said strip being subsequently subjected to strain of the nature produced by fabrication operations, .nd said method including reheating said strip to a temperature strain, said reheating efiecting an appreciable improvement in the electrical properties of said strip over those imparted said strip by said reannealing.

3. Electrical silicon steel produced by the method of claim 1.

4. Electrical silicon steel produced by the method of claim 2.

EDWIN H. ENGEL.

MA'I'TI H; P HARRY F. SHANNON.

said atmosphere being oxidizing to a 'degree regulated to cause the for-