EP0935008B1 - Iron-cobalt alloy - Google Patents

Description

The present invention relates to an iron-cobalt alloy with characteristics improved mechanics.

Iron-cobalt alloys are well known, and are characterized by both very interesting magnetic properties and by a very great brittleness to the ordinary temperature, which makes their use delicate. In particular, the alloy Fe50Co50, containing 50% by weight of iron and 50% of cobalt, has an induction at very high saturation and good magnetic permeability, but it has the disadvantage of not being able to be cold rolled, which makes it unusable practically. This very great fragility results from the formation, below 730 ° C, approximately, of an ordered α 'phase, resulting from a disorder-order transformation. This disorder-order transformation can be slowed down by adding vanadium, which makes it possible to manufacture an alloy of the iron-cobalt type, containing little about 50% cobalt and about 50% iron, suitable for being cold rolled after very energetic overheating. We have thus proposed an alloy containing approximately 49% cobalt and 2% vanadium, the remainder being iron and impurities. This alloy, which has very good magnetic properties after cold rolling and annealing between 720 ° C and 870 ° C approximately, however has the drawback of requiring special precautions during reheating prior to overheating, in order to limit the magnification of the grain which deteriorates the ductility.

To facilitate reheating before overheating, it has been proposed, in particular in US Patent 3,634,072, to add 0.02% to 0.5% of niobium and possibly from 0.07% to 0.3% of zirconium in order to limit the risk of grain enlargement during reheating. The alloy thus obtained has comparable but no better magnetic properties and ductility than the alloy containing only 2% vanadium. Reheating before overheating is just easier to do.

In addition, it was found that vanadium could be replaced by niobium or tantalum. This is how it was proposed in US patent 4,933,026, an alloy containing at least one element selected from niobium and tantalum in contents such that their sum is between 0.15% and 0.5% (by weight). This alloy which has a ductility comparable to the previous one, with the advantage of being able be annealed at a higher temperature which gives properties magnetic better. But it has the disadvantage of having a resistivity relatively weak electric, which increases losses by induced currents and limits his employment possibilities.

Finally, all of these alloys have mechanical characteristics of insufficient tensile strength for certain applications such as circuits magnetic rotating machines with very high speed of rotation. It is, in In fact, it is hardly possible to obtain a yield strength greater than 480 MPa.

In order to improve these mechanical characteristics, it has been proposed, in particular in international patent application WO 96/36059, an alloy containing essentially (by weight) 48% to 50% cobalt, 1.8% to 2.2% vanadium, 0.15 % to 0.5% niobium and 0.003% to 0.02% carbon, the remainder being iron and impurities. In this patent application it is specified that niobium can be totally or partially replaced by tantalum at the rate of 1 atom of tantalum for 1 atom of niobium, which, taking into account the respective atomic weights of tantalum and niobium, corresponds to more than 2% by weight of tantalum for 1% in niobium weight. In this alloy, niobium (or tantalum), form along the grain seals of the lava phases which prevent the grain from growing, which significantly increases the elastic limit without improving substantially the ductility. For example, after annealing at 720 ° C, the limit elasticity can exceed 600 MPa. However, these mechanical characteristics do not can be obtained with relatively large additions of niobium or tantalum.

The relatively large additions of niobium or tantalum are necessary to obtain a high yield strength while annealing at the top of the recrystallization temperature range, which has the advantage of lead to a low sensitivity of the result obtained at the effective temperature of annealing. However, this solution has the disadvantage of reducing the ability to the alloy with hot rolling.

The object of the present invention is to provide an iron-cobalt alloy having at the times satisfactory ductility, good magnetic properties and improved mechanical characteristics, while having good suitability for hot rolling.

• de 35 % à 55 %, et de préférence de 40 % à 50 % de cobalt,
• de 0,5 % à 2,5 %, et de préférence de 1,5 % à 2,2 % de vanadium,
• au moins un élément pris parmi le tantale et le niobium, en des teneurs telles que 0,02 % ≤ Ta + 2 x Nb ≤ 0,2 %, et de préférence telles que 0,03 %≤ Ta + Nb ≤ 0,15 %, et mieux encore, Nb ≤ 0,03 %,
• de 0,0007 % à 0,007 %, et de préférence de 0,001 % à 0,003 %, de bore,
• moins de 0,05 %, et de préférence, moins de 0,007 % de carbone,

le reste étant du fer et des impuretés résultant de l'élaboration.
De préférence, les impuretés que sont le manganèse, le silicium, le chrome, le molybdène, le cuivre, le nickel et le soufre ont des teneurs telles que : Mn + Si ≤ 0,2 %, Cr + Mo + Cu ≤ 0,2 %, Ni ≤ 0,2 % et S ≤ 0,005 %.To this end, the subject of the invention is an iron-cobalt alloy, the chemical composition of which comprises, by weight:

• from 35% to 55%, and preferably from 40% to 50% of cobalt,
• from 0.5% to 2.5%, and preferably from 1.5% to 2.2% of vanadium,
• at least one element taken from tantalum and niobium, in contents such as 0.02% ≤ Ta + 2 x Nb ≤ 0.2%, and preferably such as 0.03% ≤ Ta + Nb ≤ 0.15 %, and better still, Nb ≤ 0.03%,
• from 0.0007% to 0.007%, and preferably from 0.001% to 0.003%, of boron,
• less than 0.05%, and preferably less than 0.007% carbon,

the remainder being iron and impurities resulting from processing.
Preferably, the impurities that are manganese, silicon, chromium, molybdenum, copper, nickel and sulfur have contents such as: Mn + Si ≤ 0.2%, Cr + Mo + Cu ≤ 0, 2%, Ni ≤ 0.2% and S ≤ 0.005%.

The inventors have surprisingly found that when added 0.0007% to 0.007% by weight, or better still 0.001% to 0.003%, of boron, to an alloy iron-cobalt containing, moreover, from 0.5% to 2.5%, or better still from 1.5% to 2.2%, of vanadium as well as a small amount of elements such as tantalum and niobium, we significantly increased the yield strength of the alloy, while retaining satisfactory magnetic characteristics and having very good suitability hot rolling.

As an example and comparison, we have developed alloys A and B according to the invention and alloy C according to the prior art. With these alloys, we made 2 mm strips by hot rolling at around 1200 ° C thick which have been hyper-soaked by cooling in less than 1 second between 800 ° C and 100 ° C. The strips thus obtained were cold rolled to obtain strips 0.35 mm thick. These cold rolled strips then have been annealed, in accordance with the state of the art, at temperatures between 700 ° C and 900 ° C so as to give them the properties of use. We then measured the mechanical and magnetic characteristics obtained. Alloys A and B have been hot-rolled without difficulty, ie without the appearance of corner cracks.

The chemical compositions were as follows (the complement being iron): Co V Your Nb B VS mn Yes Cr Or Cu S P AT 48.5 1.98 - 0.044 0.0022 0,011 0,102 0.06 0.04 0.11 0.01 0,004 0.005 B 48.1 1.9 0.17 - 0.0012 0.005 0.05 0.05 0.02 0.2 0.01 0,002 0.005 VS 48.7 1.97 - 0.064 - 0,010 0.09 0.05 0.04 0.12 0.01 0,003 0.005 The mechanical characteristics obtained after annealing at 725 ° C, 760 ° C and 850 ° C were (Re0,2 = elastic limit; HV = Vickers hardness):

• les valeurs de l'induction magnétique B (en tesla), pour des excitations magnétiques H en courant continu de 20 Oe = 1600 A/m, 50 Oe = 4000 A/m et 100 Oe = 8000 A/m ;
• le champ coercitif Hc en A/m,
• les pertes ferromagnétiques (en W/kg) à 400 Hz pour une induction sinusoïdale de 2 tesla de valeur crête.

The magnetic characteristics measured were:

• the values of magnetic induction B (in tesla), for magnetic excitations H in direct current of 20 Oe = 1600 A / m, 50 Oe = 4000 A / m and 100 Oe = 8000 A / m;
• the coercive field Hc in A / m,
• ferromagnetic losses (in W / kg) at 400 Hz for a sinusoidal induction of 2 tesla peak value.

• après un recuit à 725 °C
  
• après un recuit à 760 °C :
  
• après un recuit à 850 °C :
  

These values were:

• after annealing at 725 ° C
  
• after annealing at 760 ° C:
  
• after annealing at 850 ° C:
  

These results show that, while having very magnetic properties close to those of alloy C according to the prior art, alloys A and B in accordance with the invention have significantly higher mechanical characteristics, since the yield strength can exceed 500 MPa, these characteristics are comparable to those obtained with alloys according to the prior art having 0.3% niobium.

Claims (8)

1. Iron-cobalt alloy characterized in that its chemical composition comprises, by weight: 35% ≤ Co ≤ 55% 0.5% ≤ V ≤ 2.5% 0.02% ≤ Ta + 2 x Nb ≤ 0.2% 0.0007% ≤ B ≤ 0.007% C ≤ 0.05% the balance being iron and impurities resulting from the smelting operation.
2. Iron-cobalt alloy according to Claim 1, characterized in that: 1.5% ≤ V < 2.2%.
3. Iron-cobalt alloy according to Claim 1 or Claim 2, characterized in that: 0.03% ≤ Ta + Nb ≤ 0.15%.
4. Iron-cobalt alloy according to Claim 1, 2 or 3, characterized in that: Nb ≤ 0.03%.
5. Iron-cobalt alloy according to Claim 1, 2, 3 or 4, characterized in that: 0.001% ≤ B ≤ 0.003%.
6. Iron-cobalt alloy according to any one of Claims 1 to 5, characterized in that: C ≤ 0.007%.
7. Iron-cobalt alloy according to any one of Claims 1 to 6, characterized in that the impurities resulting from the smelting operation have contents such that: Mn + Si ≤ 0.2% Cr + Mo + Cu ≤ 0.2% Ni ≤ 0.2% S ≤ 0.005%.
8. Iron-cobalt alloy according to any one of Claims 1 to 7, characterized in that: 40% ≤ Co ≤ 50%.