EP0184637B1 - Process for the manufacture of a magnetic switch element which will demagnetize fast also in a slowly changing magnetic field - Google Patents

Abstract

A process for the manufacture of a magnetic switch element is specified. A fine-crystalline structure is produced by short-term annealing of a cobalt-iron-vanadium alloy at 630 to 900 DEG C, which leads to pulse voltages of over 10 volts when subjected to a slowly changing field under tensional stress in a coil with 1000 windings. This property is reproducible so that inductively operating sensors can be manufactured, for example, for speed transmitters operating down to zero speed.

Description

The invention relates to a method for producing a magnetic switching element made of an alloy containing cobalt-iron and vanadium with approximately equal proportions of cobalt and iron, which, under tension, quickly changes magnetism even when the field changes slowly, the switching element being subjected to a cold deformation of over 80% and then subjected to a heat treatment under protective gas.

In the "Physikalische Zeitschrift" XXXIII, (1932), pages 913 to 923, a magnetic switching element has been described which experiences rapid magnetic reversal even when the field changes slowly, so that a voltage pulse is achieved in transmitters with primary and secondary windings even at very low frequencies becomes. For this purpose, cold-formed and heat-treated wires with approx. 78% nickel, 22% iron are described, which are made by a heat treatment of e.g. Residual stresses are removed for 5 minutes at 800 ° C.

When a tensile stress is applied, the magnetostrictive material experiences a preferred magnetic direction in the direction of the wire axis, so that when the coercive force is reached and exceeded, the magnetization does not yet change. Only when the field strength continues to increase If a critical value increases, a small area is remagnetized at one point on the wire. A magnetic reversal wave propagates from this area, which leads to sudden magnetic reversal of the entire wire. As a prerequisite for achieving this effect, it is stated that a material with small internal stresses, small crystal orientation energy and a high yield strength should be selected.

With such a material it can be achieved that the value of the critical field strength at which the magnetic reversal is initiated is considerably higher than the coercive field strength.

Furthermore, it is known from DE-OS 28 19 305 to produce a magnetic switching wire from a cobalt-iron-vanadium alloy with 4 to 14% vanadium. Periodic twisting gives the wire a magnetically hard sheath and a magnetically soft core, so that the sheath and core can be magnetized in the same direction and also opposite to one another. If a certain reverse magnetization value is selected, it can be achieved that a magnetic reversal occurs in each case during magnetization.

Furthermore, it is known from DE-PS 31 52 008 to produce a jumper wire from two different materials. For switching, a cobalt-iron-vanadium alloy with 4 to 14% vanadium is again proposed, which is cold-worked together with a casing. Shell and core have a different thermal expansion, so that a tension between the shell and core after last cooling of the arrangement. This bracing also means that certain areas of the magnetization loop are only jumped through suddenly, so that even with slow reversal of magnetization jumps occur and voltage pulses are generated in a coil surrounding the material.

With another method, as described in DE-OS 29 33 337 A1, an iron-cobalt-vanadium alloy is also surrounded by a shell made of soft steel and braced against this shell. For this purpose, after cold forming of the wire with the sheath, a heat treatment between 800 and 1100 ° C. is carried out under a protective gas and the tensile stress of such iron-cobalt-vanadium alloys is generated by stretching the wire. The stretching tension is chosen so that the outer shell deforms plastically, while the iron-cobalt-vanadium alloy arranged on the inside is only elastically deformed. As a result, the casing is under pressure and the core is under tension after the stretching process. Again, with slow magnetic reversal, measurable impulses are obtained in a coil surrounding the wire, which are an order of magnitude larger than the impulses that can be achieved with a nickel-iron alloy. With such arrangements, voltage pulses of a few volts peak voltage can be achieved at high tensile stresses and corresponding number of turns.

From US-PS 34 22 407 a method for the heat treatment of a high cobalt alloy with 78 to 95 wt .-% cobalt, 4.5 to 11 wt .-% vanadium, the rest iron, is known, in which a heat treatment in the temperature interval between 150 and 800 ° C for at least 1/2 minute. The alloys have a stepped hysteresis curve.

The object of the present invention is to provide a method for producing a magnetic switching element with which particularly high magnetic reversal speeds can be obtained reproducibly, so that peak voltages of more than 10 volts in a coil with 1000 turns can be achieved even with tensile stresses from 130 N / mm² are. It has been recognized that a cobalt-iron-vanadium alloy with a vanadium content of more than 1% by weight to ensure the mechanical workability and with less than 6% by weight to avoid precipitations in the heat treatment and by an extremely short final heat treatment have significantly higher and industrially usable remagnetization speeds. A material produced in this way experiences, after reaching a critical field strength (jump field strength), almost complete, abrupt magnetic reversal with measured magnetic reversal speeds of 4 km / sec, so that defined voltage pulses of sufficient height can be achieved.

The object is achieved by a method according to claim 1.

Exemplary embodiments and achievable results are explained in more detail below with reference to FIGS. 1 to 7.

Figure 1a shows schematically a known measuring device for determining the magnetic reversal speed. A wire 1 made of 49% by weight cobalt and iron and 2% by weight of vanadium is clamped in a wall 2 and pulled with a force Z in the direction of arrow 3. The wire is surrounded by a field coil 4. Within the field coil 4 there is a sensor coil 5 and 6, each with 200 turns, at a distance of 200 mm from one another. A start coil 7 is arranged to the left of the sensor coil 5. The field coil 4 is excited for measurement to a value which lies above the coercive field strength of the wire 1 which is not clamped, but is still lower than the jump field strength which would induce a reversal of magnetism in the tensioned wire.

In the same direction as the field coil 4, the starting coil 7 then receives a current pulse which, to the left of the sensor coil 5, generates a field strength in the wire 1 which is above the jump field strength. This creates a magnetic reversal front, as shown in the wire in FIG. 1b. This magnetic reversal front now spreads out within the wire 1, as shown in FIG. 1c, and successively generates a voltage pulse in the sensor coils 5 and 6. The voltage applied to the starting coil 7 is designated V7 in FIG. 1d. FIG. 1d shows a diagram of the voltage U over time T. It can be seen that a short time after the voltage pulse has been applied to the starting coil 7, a voltage pulse V5 in the sensor coil 5 and subsequently a voltage pulse V6 in the sensor coil 6 can be measured. The time interval between these voltage pulses, together with the given distance of the sensor coils 5 and 6 from one another, is a measure of the magnetic reversal rate.

FIG. 2 shows the field strength H over the tensile stress Z, namely the curves of the coercive field strength H _o and the spring field strength H _Sp , at which a reversal of magnetization is initiated, are entered for the cobalt-iron-vanadium alloys mentioned. The difference is referred to as curve H _Sp -H _o . With a tensile stress of 300 N / mm² it already reaches a value of 8 A / cm.

The jumping field strength H _Sp was determined without a pulse in the starting coil by initiating the remagnetization process with the field coil 4 alone.

The coercive field strength H _o , on the other hand, describes the smallest field strength in the field coil 4, at which the magnetic reversal process still takes place using a sufficiently large pulse in the starting coil 7.

In the event that the field generated by the field coil 4 lies above the coercive field strength H _o and below the jump field strength H _Sp , the difference between the field H and H _{o is} decisive for the speed of the magnetic reversal pulse initiated by the starting coil 7. This relationship is shown in FIG. 3. For various tensile loads Z = 15, Z = 60, Z = 160 and Z = 310 N / mm², FIG. 3 shows the dependence of the speed v in km / sec on the driving field H - _Ho .

The voltage pulse in a coil surrounding the wire with slow magnetic reversal is now dependent on this speed v. Figure 4 shows the peak voltage in such a coil with W = 1000 turns and one Tensile stress Z = 310 N / mm² with a cobalt-iron-vanadium alloy and a wire diameter of 0.2 mm.

It has been found that particularly high impulses can only be achieved with relatively low tensile stresses if very specific annealing times in the temperature range from approximately 630 to 900 ° C. are observed.

FIG. 5 shows the measurement results of an alloy with 49% by weight cobalt and iron and 2% by weight vanadium depending on different heat treatments.

FIG. 5 shows the peak voltage Û, measured on a coil with 1000 turns above the tensile stress Z. The curves are labeled K1 to K5 from left to right. The annealing times and the annealing temperatures are given in brackets after the curve names. Annealing is preferably carried out in a nitrogen atmosphere, which allows the temperature to be maintained particularly precisely, since nitrogen is not flared and there are therefore no undesired changes in temperature.

Curve K5 is an example of insufficient annealing treatment (15 seconds at 650 ° C), residual stresses remain in the wire; the wire is still curved, with the result that pulses of significant size only occur at tensile stresses above 400 N / mm².

Even with half the glow time (8 seconds) at a higher temperature (750 ° C), higher tension pulses result at lower tensile stresses, which reach 12 V at 480 N / mm² (curve K3).

If the glow time is extended again to 15 seconds at this temperature, a higher voltage pulse is obtained in accordance with curve K2 with an even lower tensile stress, but only reaches 11 volt peak voltage at 400 N / mm². The micrograph of this sample shows the beginning of recrystallization with a small grain diameter of 1.5 to 2 µm. A further extension of the glow time to 30 seconds (curve K1) further improves the values for small tensile stresses. Peak voltages of 14 volts were measured at 310 N / mm². The micrograph of this sample shows a clear recrystallization with fine grain in the range of 2 to 3 µm.

A further extension of the annealing time according to curve K4 with 60 seconds at 750 ° C in turn worsens the values, since the recrystallization continues and an average grain size of 10 µm is measured in the micrograph. At an annealing temperature of 750 ° C and the material used, there is an optimal range of about 20 to 40 seconds of annealing time, which results in particularly high tensile stress values at low tensile stresses. It is essential, depending on the temperature and the alloy used, to determine the annealing time which clearly reveals a recrystallization of the structure, but is dimensioned so short that there is still no significant grain growth.

This optimum glow time will decrease at higher temperatures compared to the example mentioned and will increase at lower temperatures. However, temperatures above 900 ° C are not suitable, since the wire will then heat up unevenly and furthermore Phase conversion can result. The resulting martensite would disrupt the crystal lattice and further deteriorate the values that could be achieved.

Temperatures too low, e.g. below 630 ° C, embrittlement of the material, excretions can occur and internal tensions can be built up again. For this reason, glow times of over 120 seconds are necessary to achieve good results, i.e. high pulse voltages, no longer suitable.

Since the material must have low crystal anisotropy in addition to high magnetostriction, the cobalt content should fluctuate between 30 and 60%. The vanadium content should be well below 10%. Experiments with the known material Co₅₂V₁₀Fe₃₈ have led to excretions in the extremely short glow time, which clearly worsened the pulse behavior. In addition to vanadium, the alloy can also contain other components, for example nickel, niobium or molybdenum, but chromium may not be present or only in very small amounts below 1%, since this significantly deteriorates the magnetic properties (permeability).

After the heat treatment, the material must no longer be heated to very high temperatures (above 400 ° C) in order to maintain the optimal structure. For this reason, the wire should be elastically deformed either by clamping it in a holding device or, in the case of a sheath or a core with different yield strength, by stretching and thus held under tension.

The short annealing times can preferably be carried out in a continuous furnace in which the speed of the wire and the length of the wire section in the furnace determine the heat treatment time.

Since essentially a certain material heat-treated according to a certain method under tensile stress is decisive for the effect that can be achieved, the invention is not restricted to a circular wire cross-section. It can be any elongated material shape, e.g. rolled strip with rectangular or square cross-section.

Claims (5)

1. A process for the production of a magnetic switch element in wire- or strip form comprising an alloy, substantially containing 30 to 60% by weight cobalt, 1 to 6% by weight vanadium and the remainder iron, by cold working of the switch element and subsequent thermal treatment in the presence of a shield gas for a period of between 3s and 120s at a temperature of between 900 and 630°C wherein, in order to obtain a fine-crystalline structure, the high temperatures are used for shorter periods and vice versa.
2. A process as claimed in Claim 1, characterised in that for the thermal treatment the magnetic switch element is drawn through a continuous furnace in wire- or strip form.
3. A process as claimed in Claim 1, characterised in that nitrogen or another, non-combustible gas is used as shield gas.
4. A process as claimed in Claim 1, characterised in that the thermal treatment is carried out for a period of 8 to 60s at a temperature of 800 to 650°C.
5. A process as claimed in Claim 1, characterised in that the thermal treatment is carried out for a period of between 20 and 40s at 730 to 770°C.

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