Semihard Magnetic Alloy and a Process for the Production Thereof

The present invention relates to a semihard magnetic alloy comprising iron, chromium, cobalt and titanium as main components, which has a good workability and is easy to heat treat, and to a process for the production thereof.

At present, semihard magnets are practically used mainly as hysteresis motors, hysteresis brakes and clutches, but magnetic properties required may vary broadly depending upon the nature of final uses. Therefore, there are too many kinds of semihard magnetic alloys having such magnetic properties. Practically, however, typical alloys are limited depending upon the degree of coercive force required. For example, where a coercive force (Hc) of 50 to 160 Oersted is required, Fe-Mn-Ti alloys, Fe-Co-V alloys or Alnico are generally used. Further, where a coercive force (Hc) of 150 to 500 Oersted is required, Fe-Co-V alloys, Alnico or Fe-Ni-Cu alloys are commercially used.

Of these alloys, ones having a particularly high coercive force (Hc) are used in great quantities as hysteresis brakes, clutches and various meters, and the demand of such alloys is rapidly increasing year by year. Thus, the quantities of semihard magnets used are also rapidly increasing. It is well known that Alnico can cover broad ranges of magnetic characteristics by changing the compositions thereof, but they are hard and brittle and, therefore, poor in workability.

Fe-Co-V alloys, i.e., so-called "Vicalloy" are famous as magnets having a good workability, but they have only one and great defect of being expensive because of containing more than 50 % cobalt and more than 10 % vanadium.

Fe-Ni-Cu alloys, i.e., so-called "Cunife" have an excellent workability at normal temperatures and they can readily have a high coercive force, but can only have a relatively lower residual magnetic flux density. Therefore, their uses are limited.

The object of the present invention is to provide semihard magnetic alloys which are free of such defects as mentioned above and cheap, and have a good workability and excellent magnetic properties and further to provide a process for the production thereof. These alloys consist essentially of, by weight, 17 % to 45 % chromium, 3 % to 14 % cobalt, 0.2 % to 10 % titanium and balance substantially iron, and particularly the alloys consisting essentially of 23 % to 35 % chromium, 5 % to 10 % cobalt, 0.3 % to 3 % titanium and balance substantially iron can have excellent magnetic properties. In view of these alloying components, these alloys exhibit a very excellent workability when, for example, forged, rolled, cut, punched or bent under both hot and cold conditions.

In order to give said alloys the best magnetic properties, the solution treatment and subsequent aging are necessary. The solution treatment is necessary for obtaining uniform magnetic properties. The temperatures necessary for the solution treatment of the alloys having said compositions may range normally from 650.degree. C to 1,200.degree. C, preferably from 850.degree. C to 1,085.degree. C.

Further, the aging is necessary for subjecting the alloys rendered uniform by said solution treatment to the uniform precipitation or two-phase separation to produce high coercive force. The following two aging processes can be applied.

One process comprises keeping the solution-treated alloys at a temperature of lower than 650.degree. C for a given period of time and then either cooling said alloys to room temperature in a usual manner (air cooling, water cooling, and etc.) or further maintaining said alloys at a lower temperature for another given period of time and repeating twice or more such maintaining stage (so-called "multi-stage aging process"), except that the maintaining temperature at each of the second and subsequent stages must be lower than that at the last stage. By this multi-stage aging process can be obtained better magnetic properties than by the other process as mentioned below.

The other process comprises continuously cooling the solution-treated alloys from a temperature of not higher than 700.degree. C. In this process, the cooling rate is much lower than usual cooling rates during the cooling of said alloys to at least 550.degree. C, i.e., a cooling rate of not higher than 50.degree. C per hour is selected. Further, the cooling may be set about at a temperature of from 700.degree. C to 580.degree. C. Of course, said alloys are permitted to be maintained at the temperature at which the cooling is set about, for a given period of time. Said multi-stage aging process may be followed by the continuous cooling process.

The present invention is constituted by two main parts: one is an alloy composition capable of providing semihard magnetic properties and the other a process for affording said alloy composition excellent in magnetic properties. The present invention will be explained detailedly below.

The alloy of the present invention is fundamentally a Fe-Cr-Co base alloy having titanium incorporated therein. The Fe-Cr-Co base alloy is well known as Spinodal alloy. Recently, it has been found that a Fe-Cr-Co base alloy having molybdenum or tungsten incorporated therein is usable as a permanent magnet. However, this alloy has excellent magnetic properties while, in view of the aspect of microstructure, the solution treatment must be made at a higher temperature since at high temperature range a non-magnetic austenite phase is stable and tends to remain at room temperature. Normally, a solution treating temperature of above 1,300.degree. C is required, but there is industrially considerable difficulty in solution treating at such temperature. Further, the melting should be conducted in vacuum or a non-oxidizing atmosphere and the fluidity of a melt is poor, because of the high chromium content. Thus, the improvements of the above defects are strongly desired.

The object of the present invention is to provide a semihard magnetic alloy being different from such hard magnetic alloy as mentioned above and having a relatively low coercive force. According to the present invention, it has been found that such defects as mentioned above can not only be completely removed, but also better hot workability can be obtained by adding an amount of titanium to the alloy.

In the alloy of the present invention chromium is one of the most essential components. If the chromium content is below 17 %, the two-phase separation (or spinodal decomposition), which is necessary to obtain semihard magnetic properties, does not take place. If the chromium content is above 45 %, the .sigma. phase which has a conspicuously adverse influence on workability cannot be prevented from occurring. Thus, the alloy containing the chromium content in the range of 17 % to 45 % is satisfactory to a semihard magnet, but in view of the aspect of magnetic properties, the chromium content in the range of 23 % to 35 % gives the best results.

Cobalt plays an important role in the twophase separation in association with chromium while, in view of the aspect of microstructure, it has a close relation to the .gamma. phase. With respect to the two-phase separation, the lower limit of the cobalt content is 3 %. If the cobalt content is below 3 %, a coercive force of only 40 Oersted or less is only obtained. The coercive force is predominantly increased with the cobalt content increased, but if the cobalt content exceeds 14 %, the solution treatment becomes very difficult and, as a result, at industrially available solution treating temperatures of below 1,300.degree. C excellent magnetic properties are difficult to obtain. With respect to both the facts that the heat treatment can be easily carried out industrially and excellent magnetic properties can be obtained, the cobalt content in the range of 5 % to 10 % is preferred.

Titanium is essential to the alloy of the present invention and it is necessary to add in an amount of not less than 0.2 % to the alloy of the present invention. However, the titanium content exceeding 10 % deteriorates the workability under hot and cold conditions. With respect to magnetic properties and workability, the optimum titanium content is in the range of 0.3 % to 3 %.

Carbon, magnesium and calcium from a furnace body and raw materials, and manganese from deoxidizing agents are permitted to be incorporated as impurities into the alloy in amounts of up to 0.1 % carbon and up to 1 % calcium, magnesium and manganese. The incorporation of such impurities in the indicated amounts has no adverse influence on both the magnetic properties and workability.

The solution treatment is important for homogenizing the microstructure of the alloy to obtain uniform magnetic properties. As mentioned above, the addition of titanium serves to remarkably relieve the conditions of the solution treatment. That is, Fe-Cr-Co alloys free of titanium usually require rapid cooling such as water cooling from a temperature of higher than 1,300.degree. C, but when a small amount of titanium (not less than 0.2 %) is added the high temperature area in which austenite is stable becomes very narrower. It has been experimentally confirmed that Fe-Cr-Co alloys having 0.5 % to 1 % titanium added thereto are possible to solution treat over an entire high temperature area (above about 700.degree. C). This is industrially very significant. Thus, the addition of titanium is greatly advantageous.

In order for the alloy of the present invention to exhibit the optimum magnetic properties, the aging subsequent to the solution treatment is necessary. The combination of the specified alloy compositions and the heat treating conditions makes it possible to obtain the optimum magnetic properties.

According to the present invention, the aging is preferably carried out at a maintaining temperature of about 650.degree. C or less, but it may be carried out at a maintaining temperature of up to about 700.degree. C for a short period of time. However, aging the alloy at a temperature of higher than 700.degree. C and at a temperature between 650.degree. C and 700.degree. C for a long period of time does not only deteriorate magnetic properties, but also promotes the precipitation of the .sigma. phase which has an adverse influence on workability and, as a result, such aging brings about embrittlement of the alloy.

It has been found that the optimum aging conditions for the alloy of the present invention, under which conditions very excellent magnetic properties can be obtained, are:

1. a range between 650.degree. C and 500.degree. C is divided into at least two stages and the aging is carried out in a first stage at a temperature from 650.degree. C down to 600.degree. C for 0.1 to 5 hours and then similar agings carried out at subsequent stages, each of which is at a temperature of 5.degree. to 30.degree. C lower than that of the last stage, for 0.2 to 20 hours for each stage (multi-stage aging process); or

2. the aging is carried out by continuously cooling the alloy from a temperature of 700.degree. C to 580.degree. C down to at least 550.degree. C at a cooling rate of not higher than 50.degree. C per hour (continuous cooling process).

The continuous cooling process (2) above gives particularly good magnetic properties and can be readily carried out.

The following examples illustrate the present invention.

An alloy consisting essentially of 22.3 % chromium, 6.5 % cobalt, 0.91 % titanium and balance iron at 1,200.degree. C was cooled in air and then aged at 630.degree. C for 1 hour, at 590.degree. C for 5 hours and further at 550.degree. C for 20 hours. Thus, a semihard magnetic alloy having a residual magnetic flux density (Br) of 7,000 Gauss and a coercive force (Hc) of 103 Oersted was obtained.

An alloy consisting essentially of 35.0 % chromium, 10.3 % cobalt, 1.88 % titanium and balance iron at 1,200.degree. C was cooled in air and then aged at 630.degree. C for 1 hour, at 600.degree. C for 3 hours, at 570.degree. C for 10 hours and further at 540.degree. C for 20 hours. Thus, excellent magnetic properties, i.e., Br of 8,300 Gauss and HC of 475 Oersted were obtained.

An alloy having the same composition as in Example 2 at 1,000.degree. C was cooled in air, heated to 630.degree. C, cooled down to 500.degree. C at an average rate of 40.degree. per hour and then air cooled. Br of 9,000 Gauss and Hc of 230 Oersted were thus obtained.

An alloy consisting essentially of 28.7 % chromium, 7.9 % cobalt, 1.52 % titanium and balance iron at 900.degree. C was cooled in air and then aged at 630.degree. C for 15 hours. As a result, Br was 6,500 Gauss and Hc 60 Oersted. Another alloy having the same composition was maintained at 600.degree. C for 30 minutes and then continuously cooled down to 500.degree. C for 30 hours. As a result, excellent magnetic properties, i.e., Br of 8,200 Gauss and Hc of 515 Oersted were obtained.

As mentioned above, the alloy of the present invention exhibits excellent magnetic properties particularly when the aging is properly carried out. Various magnetic properties are possible to obtain through convenient combinations of the continuous cooling aging process and the multi-stage aging process.

FIGS. 1 and 2 show the relations between the composition and the magnetic properties, particularly the coercive force (Hc) and the residual magnetic flux density (Br), respectively under constant solution heating and aging conditions. That is, FIGS. 1 and 2 show changes of Hc and Br, respectively, with contour lines for alloys having a titanium content of 1.5 % and various chromium and cobalt contents, which were solution treated at 1,000.degree. C and then aged at 600.degree. C for 1 hour, at 580.degree. C for 3 hours and further at 530.degree. C for 20 hours.

As mentioned above, according to the present invention, the alloys having the same magnetic properties as those of the Fe-Co-V alloys or Alnico alloys can be readily obtained, and they are inexpensive as compared with the Fe-Co-V alloys and have very good mechanical properties. Thus, the alloys of the present invention are practically excellent semihard magnetic alloys.