Magnetic Alloy

Abstract

A magnetic alloy that is ductile and can be cold rolled containing by weight about 0.5-2.5 percent vanadium, 45-52 percent cobalt, at least one element selected from the group consisting of about 0.02-0.5 percent niobium and about 0.07-0.3 percent zirconium, and the balance iron except for incidental impurities.

Description

This invention relates to a magnetic alloy and, more particularly, to a vanadium-iron-cobalt alloy having improved ductility.

As pointed out in U.S. Pat. No. 3,024,141 granted to R. E. Burket et al. on Mar. 6, 1962, a well-known soft magnetic alloy contains about 45 to 52 percent cobalt, about 45 to 52 percent iron, and about 0.5 to 2.5 percent vanadium except for incidental impurities. The alloy has highly desirable magnetic properties particularly when prepared so as to contain about 2 percent vanadium, 49 percent cobalt, and 49 percent iron. Unfortunately, it has proven to be extremely difficult on a commercial scale to reproduce consistently the desired combination of magnetic properties and ductility. In the case of small experimental test specimens, there is no difficulty in heating them to about 1,650.degree. F. to transform a small proportion of the ferrite to austenite which on reversion to secondary ferrite along the grain boundaries of the much coarser primary ferrite ensures the fine-grain size necessary for ductility. However, in the case of the coils usually provided for cold rolling which may weigh from about 500 to 1,000 pounds or more, heating of the entire mass through the subtransformation range extending from about 1,400.degree. to 1,600.degree. F. cannot be carried out fast enough in conventional batch heating equipment so as to avoid grain growth and consequent embrittlement of the alloy.

Said U.S. Pat. No. 3,024,141, in order to overcome the loss of ductility when such alloys were heat treated in the usual way, describes a two-stage heat treatment to replace the annealing treatment theretofore used of heating at about 1,650.degree. F., which consists of preheating the alloy to about 1,020.degree. to 1,120.degree. F. for about 3-10 hours followed, without intermediate cooling, by heating between 1,425.degree.-1,475.degree. F. between 1.5-3 hours, then quenching to 30.degree.-50.degree. F. and after which the temperature is permitted to rise to room temperature. However, that process leaves much to be desired in that it does not provide the optimum or best possible ductility that can be had in such alloys.

Having in mind that vanadium is a well-known grain-refining agent and is often included in alloys to provide that effect, and having in mind also that the addition of a fourth alloying element to the V-Co-Fe alloy would be expected to adversely affect the alloy's desired magnetic properties, it was therefore very much unexpected to discover that small but controlled additions of the elements niobium and zirconium made possible the attainment of the desired degree of ductility without resort to special heat treatments or equipment and without adversely affecting the magnetic properties. Titanium and aluminum, well known for use as grain refiners, were also found to be ineffective in the V-Co-Fe alloy.

The objects and advantages of the present invention are attained by providing an alloy which by weight consists essentially of 0.5-2.5 percent vanadium, 45-52 percent cobalt, 45-52 percent iron, one or both of 0.02-0.5 percent niobium and 0.07-0.3 percent zirconium, plus incidental impurities. By incidental impurities it is intended no more than about 2 percent and preferably less than about 1 percent of other elements that may be present for one reason or another including those elements which may be present in amounts ranging up to a few hundredths of a percent and those which may be present in amounts ranging up to a few tenths of a percent. For example, carbon should not exceed about 0.03 percent and preferably no more than about 0.015 percent, manganese should not exceed about 0.8 percent and preferably no more than about 0.3 percent, silicon should not exceed about 0.4 percent and preferably no more than about 0.2 percent, phosphorus and sulfur should each not exceed about 0.02 percent and preferably no more than about 0.01 percent, chromium should not exceed about 0.1 percent, nickel should not exceed about 0.8 percent and preferably no more than about 0.3 percent and molybdenum should not exceed about 0.2 percent and preferably no more than about 0.1 percent.

For best magnetic properties, cobalt and iron are each included in an amount of about 48.5-49.5 percent and preferably in equal amounts. The larger amounts of vanadium, that is above about 1.5 percent, give the better mechanical properties, and about 1.8-2.2 percent vanadium is preferred.

Niobium in an amount ranging from about 0.02 percent has a significant effect upon the ductility of the alloy, but when present in amounts above about 0.5 percent, niobium adversely affects the magnetic properties of the alloy so that a minimum induction of 18,500 gauss with a magnetizing force of 3 oersteds after a 4-hour anneal at about 1,500.degree.-1,550.degree. F. cannot be attained. Preferably from about 0.05- 0.3 percent niobium is included. When zirconium instead of niobium is used for its effect on controlling the ductility of the alloy, at least about 0.07 percent zirconium is required and preferably at least about 0.1 percent is used. Above about 0.3 percent zirconium adversely affects the magnetic properties of the alloy so that the desired minimum induction of 18,500 gauss under the conditions just stated cannot be attained. Niobium is considered a more desirable additive than zirconium for this purpose of ensuring the required ductility; but, if desired, zirconium can be used alone, or both elements can be used so long as their combined content is not so large as to adversely affect the magnetic properties of the alloy.

The alloy is prepared, worked and formed into products using conventional techniques. It can be melted in air as by means of an electric arc furnace or it can be melted under a controlled atmosphere using well-known vacuum techniques. After being melted and cast as an ingot, it is forged into billets or slabs from a furnace temperature of about 1,950.degree.-2,250.degree. F. After the usual surface preparation, it is then hot rolled to strip, also from a furnace temperature of about 1,950.degree.-2,250.degree. F. and formed into a coil while still hot. The thus-formed strip is an intermediate product substantially thicker than the finished size which is then formed by cold working to the final thickness or gauge. It is to be noted that a somewhat higher forging and rolling temperature than 2,250.degree. F. can be used if desired.

In its hot rolled condition, the strip is too brittle to be cold worked, and, as in the case of the prior alloy described in said U.S. Pat. No. 3,024,141, it is necessary to heat treat the present alloy in order to condition the coiled strip by increasing its ductility so that it can be cold worked successfully. Such heat treatment hitherto has had two objectives. One objective was to heat the entire mass of the material being treated to or just above the ferrite-to-austenite transformation temperature so as to inhibit grain growth and then quench. For the alloy of the present invention and the prior art V-Co-Fe alloy, the ferrite-austenite transformation temperature is substantially the same and is about 1,650.degree. F. Variations in composition will, as is known, shift the transformation point, but it can be readily determined. The other objective is to ensure the disordered condition of the alloy by rapidly quenching through the order-disorder temperature range which for the type of alloys under discussion occurs at about 1,350.degree. F.

The presence of both phenomena provides a greater assurance of the required degree of ductility because when both are substantially uniformly present throughout the microstructure of the material, the possibility of localized areas of brittleness is precluded.

To this end, the hot rolled material of the present invention, prior to cold rolling, is annealed preferably at or just above the ferrite-to-austenite transformation temperature which occurs at about 1,650.degree. F. The duration of the heat treatment is preferably just long enough to bring the entire mass to the desired temperature. Because the material is most conveniently heated while coiled, some portions require longer heating than others, and, therefore, some parts of the mass may be in the grain-growth temperature range, which extends from just above the order-disorder temperature to the neighborhood of or just above the ferrite-austenite transformation temperature, for as long as 2 to 3 hours. It is an important advantage of the alloy of the present invention that it can withstand such long periods in the grain-growth temperature range without the grain-growth characteristic of the prior alloy and the consequent loss in ductility. After being brought to heat, the alloy is then quenched rapidly in water and cold rolled to final size. Prior to cold rolling, the material may be pickled or otherwise prepared as desired.

The finished material is given a final annealing treatment in dry hydrogen, which, as desired, may or may not be in the presence of a magnetizing force. The final anneal can be carried out at a temperature ranging from about 1,400.degree. to 1,550.degree. F. and preferably is carried out at about 1,500.degree. to 1,550.degree. F. for 4 hours.

As was pointed out hereinabove, it was very much unexpected to discover that niobium and zirconium could be successfully used to improve the ductility of the V-Co-Fe alloy while such well-known grain refiners as vanadium, aluminum and titanium failed to work. The best explanation now available, based on X-ray diffraction and electron microscopic studies, appears to be that niobium and zirconium are unique in that when either is present in the alloy within the ranges stated herein, it forms a very fine intermetallic compound along the grain boundaries known as Laves phase which is very small compared to the grain structure of the alloy matrix. The intermetallic compound known as Laves phase is usually in the form AB.sub.2 where A and B are metallic elements. Corresponding examinations of specimens of the V-Co-Fe alloy with and without the other grain-refining elements aluminum and titanium failed to disclose any Laves phase, and did not have the desired ductility.

EXAMPLE NO. 1

As an example of this invention, a 13-inch wide ingot was cast containing by weight about 1.95 percent vanadium, 48.93 percent cobalt, 0.14 percent niobium and the balance iron except for incidental impurities, which amounted to no more than about 0.4 percent. The ingots were forged to 9-inch by 3-inch slabs, cogged to 7.5 -inch by 3-inch, cut in half and then hot rolled to 7.5-inch by 0.1-inch thick strip forming coils weighing about 500 pounds each. Hot working was carried out from a furnace temperature of about 2,150.degree. F. The coils were then heated for 2 hours at about 1,650.degree. F. followed by quenching in water. The as-quenched hardness was found to be Rockwell B 96 with a grain size of ASTM 7-8. The 0.2 percent yield strength was measured at 66,000 and 68,300 p.s.i. Ultimate tensile strength was found to be 108,500 and 108,900 p.s.i. Elongation was measured as 19.3 and 17.6 percent in 2 inches, values which are considered excellent for V-Co-Fe alloys. Cold rolling to thicknesses of from 0.01 to 0.016 inch were carried out without difficulty. Strip, 0.016-inch thick, was annealed for 4 hours at about 1,530.degree. F. in dry hydrogen and then, when subjected to a magnetizing force of 10 oersteds, had an induction of 22,100 gauss, when subjected to a magnetizing force of 50 oersteds had an induction of 23,450 gauss, and when subjected to a magnetizing force of 100 oersteds had an induction of 23,900 gauss.

It may also be noted that as a test of ductility, material of this composition was hot rolled into 2-inch wide by 0.1-inch thick strip and quenched in water. Test specimens were then cut from the strip and were heat treated for 2 hours at temperatures ranging from 1,400.degree. to 1,650.degree. F. and water quenched. Each specimen was then subjected to 120.degree. bends until at least surface cracks appeared. The material was found to be ductile following each heat treatment.

As further illustrations of the present invention, examples 2-5 were prepared as experimental 17-pound vacuum induction heats having analyses in weight percent as indicated in Table I. --------------------------------------------------------------------------- TABLE I

Ex. No. V Co Nb Zr __________________________________________________________________________ 2 2.04 49.07 0.05 -- 3 2.00 48.89 0.31 -- 4 1.99 49.10 -- 0.12 5 1.97 48.83 -- 0.27 __________________________________________________________________________

In Examples 2-5 the balance was iron except for incidental impurities which amounted to about 0.28 percent for example 2, about 0.25 percent for example 3, about 0.28 percent for example 4, and about 0.28 percent for example 5. The heats were cast into ingots and then forged from a furnace temperature of about 2,150.degree. F. to 2-inch by 1-inch bars. Following surface preparation, the bars were hot rolled from the same furnace temperature of 2,150.degree. F. to 0.1-inch thick strip from which 3/4-inch test specimens were cut for further testing. Test specimens of each of the examples 2-5 were heat treated for 2 hours at temperatures extending from 1,400.degree. to 1,650.degree. F. in 50.degree. intervals. Each specimen following its particular heat treatment was quenched in water. The ductility of the heat-treated test specimens was then evaluated by bending each through 120.degree. and noting the number of 120.degree. bends that could be made before each failed. The results are set out in table II opposite each example and under the temperature at which the particular specimens were heat treated.

The grain size of each specimen was measured and found to be ASTM 6 or smaller.

Material from each of the examples 2-5, that had been hot rolled and quenched as was described, was then heated at 1,650.degree. F. for one hour, quenched in water, and then cold rolled to strip 0.014-inch thick. Each specimen was then heated in dry hydrogen at 1,430.degree. F. for 2.5 hours. Specimens of each example which had been thus treated were then subjected to three different magnetizing fields of 3, 6 and 12 oersteds; the induction in gauss was measured and is set out in table III. --------------------------------------------------------------------------- TABLE III

Ex. No. 3 Oe. 6 Oe. 12 Oe. __________________________________________________________________________ 2 20.3000 21,400 22,200 3 20,300 21,100 22,200 4 18,800 20,300 21,300 5 18,600 20,200 21,300 __________________________________________________________________________

Because of its ductility and magnetic properties, this alloy is well suited for use in making cold rolled magnetic laminations. Depending upon the use intended, such laminations may range in thickness from about 0.004 to 0.016 inch or larger. The alloy can also be readily formed into cold rolled strip for use as tape cores which typically range in thickness from about 0.001 to 0.006 inch.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

Claims

What is claimed is:

1. A magnetic alloy that is ductile and can be cold rolled when quenched after being for up to about 3 hours at a temperature above its order-disorder transformation temperature and as high as just above its ferrite-austenite transformation temperature, which consists by weight essentially of about 0.5-2.5 percent vanadium, about 45-52 percent cobalt, at least one element selected from the group consisting of 0.02-0.5 percent niobium and 0.07-0.3 percent zirconium, and the balance essentially iron except for incidental impurities.

2. The alloy of claim 1 which contains about 1.5-2.5 percent vanadium.

3. The alloy of claim 2 which contains about 48.5- 49.5 percent cobalt.

4. The alloy of claim 3 which contains about 48.5-49.5 percent iron.

5. The alloy of claim 1 which contains about 0.05-0.3 percent niobium.

6. The alloy of claim 1 which contains about 0.1-0.3 percent zirconium.

7. The alloy of claim 5 which contains about 1.8-2.2 percent vanadium, about 48.5-49.5 percent cobalt, and about 48.5-49.5 percent iron.

8. The alloy of claim 6 which contains about 1.8-2.2 percent vanadium, about 48.5-49.5 percent cobalt, and about 48.5-49.5 percent iron.