EP1041168A1 - Soft magnetic alloy for a timepiece - Google Patents

Abstract

A soft magnetic iron-nickel alloy, having a relatively low nickel content and containing chromium and cobalt, is new. A novel soft magnetic iron-nickel alloy has the composition (by wt.) 34-40% Ni, 7-10% Cr, 0.5-3% Co, 0.1-1% Mn, ≤ 0.007% ≤ 0.002% S, ≤ 0.004% N, balance Fe and impurities, the sum of N + S + O being ≤ 0.01%.

Description

The invention relates to an economical soft magnetic alloy having good temperature stability of the magnetic permeability and good resistance to oxidation in a humid atmosphere. This alloy can be used, in particular, for the manufacture of the stator of a micro electric stepper motor for watchmaking.

Electric micro motors for watchmaking include a stator generally made of soft magnetic alloy containing about 80% of nickel, a few% molybdenum or copper, the rest being iron. Such alloy has a maximum magnetic permeability of 200,000 to 300,000 on the entire operating temperature range (- 20 ° C, + 60 ° C), therefore, the micro motors thus produced have a very low energy consumption. However, 80% nickel alloys are expensive and oxidize easily in humid atmospheres, which has several drawbacks: use is delicate in certain hot and humid regions; they are bad suitable for the manufacture of watches whose mechanism is visible; they are too much expensive for making cheap watches.

In order to remedy these drawbacks, it has been proposed to replace the alloys with 80% nickel by alloys of the iron-nickel-chromium type containing less than 50% nickel and a few% chromium for the manufacture of watch engines. However, the alloys proposed have, in general, a magnetic permeability that is both insufficient and too sensitive to temperature. This excessive sensitivity of the magnetic permeability to temperature is a drawback. Indeed, a watch motor must function in such a way satisfactory between - 20 ° C and + 60 ° C, which supposes that the permeability magnetic does not vary too much over this temperature range.

Taking into account all the constraints which are imposed on a micro stepping motor for watchmaking, to manufacture the stator of such a motor which is economical, it is desirable to have a soft magnetic alloy which has a saturation induction Bs greater than or equal to 5000 Gauss (0.5 tesla), a maximum relative permeability in direct current µ _{cc, max} greater than 70000, a resistivity ρ sufficient for µ _{cc, max} × ρ> 0.05 Ω.m, a sufficient stability of the magnetic permeability µ _{cc, max} between - 20 ° C and + 60 ° C, improved resistance to oxidation, and a relatively low nickel content. For the magnetic permeability to have sufficient stability, it is desirable that its variation in relative value, with respect to its value at 20 ° C., remain less than 30% over the temperature range considered.

The object of the present invention is to provide an alloy which meets these requirements.

To this end, the subject of the invention is a soft magnetic alloy whose chemical composition includes, in% by weight:
34% ≤ Ni ≤ 40%
7% ≤ Cr ≤ 10%
0.5% ≤ Co ≤ 3%
0.1% ≤ Mn ≤ 1%
the remainder being iron and impurities resulting from processing.

Preferably, the impurities that are O, S, N are such that:
O ≤ 0.007%
S ≤ 0.002%
N ≤ 0.004%
and:
N + S + O <0.01%

It is also preferable for the impurities Si, Al, Ca, Mg to be such that:
If ≤ 0.3%
Al ≤ 0.05%
Ca ≤0.03%
Mg ≤ 0.03%
and :
Si + Al + Ca + Mg + Mn ≤ 1%

This alloy can be used for the manufacture of a magnetic yoke, and in particular, to manufacture the stator of a micro electric stepper motor for watchmaking.

The invention will now be described in more detail and illustrated by examples.

• plus de 34 % de nickel pour obtenir une induction à saturation et perméabilité magnétique suffisantes. Mais, pour obtenir un alliage économique, et compte tenu notamment d'une addition de chrome, la teneur en nickel doit rester inférieure à 40 %.
• de 7 % à 10 % de chrome pour améliorer la résistance à l'oxydation et augmenter la perméabilité magnétique à basse température ; lorsque la teneur en nickel est comprise entre 34 % et 40 %, une telle teneur en chrome améliore sensiblement la perméabilité magnétique entre - 40 °C et 0 °C.
• de 0,5 % à 3 % de cobalt pour obtenir une stabilité en température suffisante de la perméabilité magnétique. En effet, les inventeurs ont constaté de façon inattendue que, pour des teneurs en nickel comprises entre 34 % et 40 % et des teneurs en chrome comprises entre 7 % et 10 %, une addition modérée de cobalt améliorait sensiblement la stabilité en température de la perméabilité magnétique, entre - 20 °C et + 60 °C.
• de 0,1 % à 1 % de manganèse, et de préférence plus de 0,2 %, pour désoxyder l'alliage et fixer le soufre.
• le reste de la composition est constitué de fer et d'impuretés résultant de l'élaboration.

The chemical composition of the soft magnetic alloy comprises, in% by weight:

• more than 34% nickel to obtain an induction with sufficient saturation and magnetic permeability. However, to obtain an economical alloy, and especially taking into account the addition of chromium, the nickel content must remain below 40%.
• from 7% to 10% chromium to improve the resistance to oxidation and increase the magnetic permeability at low temperatures; when the nickel content is between 34% and 40%, such a chromium content appreciably improves the magnetic permeability between - 40 ° C and 0 ° C.
• from 0.5% to 3% of cobalt to obtain sufficient temperature stability of the magnetic permeability. Indeed, the inventors have unexpectedly found that, for nickel contents between 34% and 40% and chromium contents between 7% and 10%, a moderate addition of cobalt significantly improves the temperature stability of the magnetic permeability, between - 20 ° C and + 60 ° C.
• from 0.1% to 1% of manganese, and preferably more than 0.2%, to deoxidize the alloy and fix the sulfur.
• the rest of the composition consists of iron and impurities resulting from the production.

The impurities are, in particular, oxygen, sulfur, nitrogen, silicon, aluminum, calcium and magnesium.

• la teneur en oxygène reste inférieure ou égale à 0,007 %, la teneur en azote reste inférieure ou égale à 0,004 %, la teneur en soufre reste inférieure ou égale à 0,002 %, et la somme O + N + S des teneurs en oxygène, azote et soufre, reste inférieure ou égale à 0,01 % ;
• les teneurs résiduelles en éléments désoxydants tels que Si, Al, Ca, Mg restent inférieures ou égales à 0,3 % pour le silicium, 0,05 % pour l'aluminium, et 0,03 % pour le calcium ou pour le magnésium ; le calcium et le magnésium ont l'avantage de permettre la formation de petits oxydes qui rendent l'alliage plus facilement découpable.

All these impurities have an unfavorable effect on the magnetic properties, therefore, in order to obtain satisfactory magnetic properties, it is preferable that:

• the oxygen content remains less than or equal to 0.007%, the nitrogen content remains less than or equal to 0.004%, the sulfur content remains less than or equal to 0.002%, and the sum O + N + S of the oxygen, nitrogen contents and sulfur, remains less than or equal to 0.01%;
• the residual contents of deoxidizing elements such as Si, Al, Ca, Mg remain less than or equal to 0.3% for silicon, 0.05% for aluminum, and 0.03% for calcium or for magnesium; calcium and magnesium have the advantage of allowing the formation of small oxides which make the alloy more easily cutable.

In addition, it is preferable that the sum Mn + Si + Al + Ca + Mg of manganese, silicon, aluminum, calcium and magnesium contents, residue less than or equal to 1%.

The contents of other impurities such as phosphorus and boron, must also remain as low as possible.

The alloy thus defined, which is of the Fe-Ni-Cr-Co type, can be rolled to hot, then cold, and possibly annealed under hydrogen at a temperature greater than or equal to 900 ° C for more than an hour, and preferably between 1100 ° C and 1200 ° C for 1 to 4 hours. High annealing temperature under hydrogen has the advantage of eliminating, at least partially, certain precipitates of sulphides or nitrides which have an unfavorable effect on the magnetic properties.

This alloy has a saturation induction Bs greater than 5000 Gauss, at 70 ° C, a maximum relative magnetic permeability in direct current µ _{cc, max} greater than 70,000 at 20 ° C, an electrical resistivity p greater than 70 µΩ.cm at 20 ° C, and a temperature stability of the maximum relative magnetic permeability defined for a temperature T, by: Δµ cc, max (T) / µ cc, max (20 ° C) ≤ 30%

In this formula, Δµ _{cc, max} (T) represents the variation of µ _{cc, max} between 20 ° C and T, and µ _{cc, max} (20 ° C) represents the permeability in direct current at 20 ° C.

In addition, given its chromium content, the alloy has a good resistance to oxidation in a humid atmosphere.

By way of example, washers with an inside diameter of 20 mm and an outside diameter of 30 mm were produced, cut from cold-rolled strips 0.6 mm thick in alloys according to the invention and in alloys given as for comparison, produced by vacuum fusion of pure raw materials. The washers were annealed under hydrogen at 1170 ° C for 4 hours. We measured the induction at saturation Bs at 70 ° C, the coercive field Hc at 20 ° C, the electrical resistivity ρ at 20 ° C, the maximum relative magnetic permeability in direct current µ _{cc, max} at 20 ° C and the maximum value of its relative variation | Δµ _{cc, max} (T) / µ _{cc, max} (20 ° C) | over the temperature range - 20 ° C, + 60 ° C (for short, this maximum variation value is called Δµ / µ).

The chemical compositions of the alloys 1 to 4, corresponding to the invention, and 5 to 17 given by way of comparison, are indicated in Table 1 and the magnetic characteristics, in Table 2. rep Or Cr Co Mn VS Yes P NOT O S N + O + S 1 35.79 8.92 3.03 0.29 0.009 0.03 0.002 0.001 0.0069 0.0005 0.0084 2 37.45 8.72 3.06 0.3 0.0089 0.03 0.002 0.0012 0.0068 0.0005 0.0085 3 37.75 9.54 1.02 0.3 0.0091 0.03 0.002 0.0007 0.0062 0.0005 0.0074 4 39.49 9.6 1.02 0.287 0.0096 0.021 0.003 0.0029 0.0029 0.001 0.0068 5 35.8 9.05 1.04 0.3 0.0083 0.03 0.002 0.0005 0.009 0.0005 0.0100 6 37.63 9.31 0.5 0.293 0.0086 0.01 0.003 0.0027 0.009 0.0008 0.0125 7 37.95 9.56 1.42 0.289 0.0083 0.017 0.003 0.003 0.0084 0.0009 0.0123 8 36.54 9.03 0.096 0.306 0.006 0.164 0.007 0.0027 0.008 0.0014 0.0121 9 36.97 9.02 0.04 0.293 0.0046 0.15 0.0057 0.0027 0.010 0.002 0.0147 10 37.82 8.95 0.002 0.48 0.005 0.013 0.004 0.0042 0.0066 0.0042 0.0150 11 35.85 5.89 2.85 0.308 0.0083 0.031 0.0034 0.0006 0.0052 0.0005 0.0063 12 37.69 3.14 1.06 0.296 0.009 0.031 0.0035 0.0005 0.0057 0.0005 0.0067 13 37.74 5.76 0.97 0.308 0.0092 0.033 0.0038 0.0008 0.0058 0.0005 0.0071 14 35.77 5.6 1.01 0.306 0.0094 0.035 0.004 0.0008 0.0075 0.0005 0.0088 15 37.77 5.8 2.87 0.287 0.0069 0.033 0.0037 0.0009 0.0083 0.0005 0.0097 16 33.96 2.64 1.96 0.259 0.0089 0.032 0.0035 0.0051 0.0085 0.0005 0.0141 17 37.86 10.55 0.96 0.299 0.0049 0.019 0.003 0.0027 0.014 0.001 0.0177 rep Bs (G) Hc (Oe) ρ (µΩ.cm) µ _{cc, max} Δµ / µ (%) 1 5800 27.4 92 92700 26 2 6800 24.5 94.4 87500 16 3 6000 21.9 93.2 95400 9 4 6500 20.6 98.5 72000 2 5 4800 23.9 91.1 70000 16 6 5500 22 92.9 67000 4 7 5800 25.7 93.5 67000 12 8 4300 23.3 95 78400 55 9 4700 22 96 67000 37 10 5400 15.5 95 76500 48 11 8400 45.4 90.9 53200 53 12 11000 54.3 82.6 54200 33 13 8700 33.9 90.2 83600 54 14 7600 44.5 90.5 49700 65 15 9400 44.2 90.9 57900 61 16 9200 85 83.5 20200 60 17 4700 21.9 96.2 62000 57

The comparison of samples 1 to 7 on the one hand and 8 to 17 on the other hand, shows that an addition of 0.5% to 3% of cobalt combined with a content nickel between 34% and 40% and a chromium content between 7% and 10%, very significantly improves the temperature stability Δµ / µ of the magnetic permeability in direct current. In particular, samples 8 to 10, which have nickel and chromium contents in accordance with the invention, but which contain practically no cobalt, always have a value of Δµ / µ greater than 30, while for samples 1 to 7, Δµ / µ is always less than 30.

Similarly, samples 11 to 17, which contain cobalt, but of which the chromium contents are outside the limits of the invention, have values Δµ / µ greater than 30.

In addition, the comparison of samples 1 to 4 (in accordance with the invention) whose oxygen contents are less than 0.007% and whose sums of nitrogen, oxygen and sulfur contents are less than 0.01%, have an induction at saturation Bs greater than 5000 Gauss, and a maximum relative magnetic permeability in direct current µ _{cc, max} greater than 70000 at 20 ° C, whereas samples 5 to 7 which do not satisfy the conditions of oxygen content or of sum N + O + S, have either a saturation induction of less than 5000 Gauss, or a magnetic permeability in direct current of less than 70,000 at 20 ° C. In all cases, the resistivity is greater than 90 µΩ.cm; the product µ _{cc, max} × ρ is greater than 0.05 Ω.m.

With the alloy according to the invention, micro stators can be manufactured stepper motors for watchmaking, both economical, having good resistance to oxidation by a humid atmosphere, and having good performances.

Due to the saturation induction greater than 5000 Gauss, the torque electromagnetic applied to the rotor is always much higher than the torques resistant.

Due to the magnetic permeability greater than 70,000 (at 20 ° C), the magnetic reluctance of the circuit remains low, which allows the use of a coil not too big.

Due to the high electrical resistivity, the induced currents are limited, which allows to obtain low energy losses.

Due to the presence of more than 7% chromium, the resistance to oxidation is good.

Finally, this alloy is significantly more economical than alloys with 80% nickel.

Claims (4)

Soft magnetic alloy of the iron-nickel type, characterized in that its chemical composition includes, in% by weight:
34% ≤ Ni ≤ 40%
7% ≤ Cr ≤ 10%
0.5% ≤ Co ≤ 3%
0.1% ≤ Mn ≤ 1%
O ≤ 0.007%
S ≤ 0.002%
N ≤ 0.004%
with N + S + O ≤ 0.01%
the remainder being iron and impurities resulting from processing. Alloy according to claim 1 characterized in that the impurities that are Si, Al, Ca, Mg are such that:
If ≤ 0.3%
Al ≤ 0.05%
Ca <0.03%
Mg ≤ 0.03%
and,
Si + Al + Ca + Mg + Mn ≤ 1% Use of an alloy according to any one of claims 1 to 2 for the manufacture of a magnetic yoke. Use according to claim 3 characterized in that the cylinder head magnetic constitutes the stator of a micro electric stepper motor for watchmaking.