Low-frequency magnetic screening made from a soft magnetic alloy

Abstract

The invention relates to a magnetic screening for frequency fields between 50 Hz and 3000 Hz, made from a soft magnetic alloy of the following composition in wt. %: 30%≦Ni≦40%, 0%≦Cu+Co≦4%, 5%≦Cr+Mo≦17%, 5%≦Cr 0%≦Nb≦2%, Mn≦0.35%, Si≦0.2%, C≦0.050%, O≦0.0160%, S≦0.0020%, B≦0.0010%, optionally at least one element selected from magnesium and calcium in amounts such that the sum thereof remains below 0.1 %, the rest being iron and production impurities. The chemical composition furthermore satisfies the following relationships: Cr+Mo≦0.8×Ni+0.9×(Co+Cu) 18.4; Cr+Mo≦4×Ni+3×(Co+Cu)−124; 4×(Cr+Mo)≧125−3×Ni. The invention further relates to use of said alloy for the production of low-frequency magnetic screening.

Description

The present invention relates to low-frequency magnetic shielding made from a soft magnetic alloy and to the use of this alloy for the production of low-frequency shielding. In the context of the present invention, the term “low-frequency” denotes frequencies between 50 Hz and 3000 Hz.

Magnetic shielding is produced from a high-permeability magnetic alloy and, in particular, from an alloy of the Fe—Ni80 type containing approximately 80% of nickel. They have permeability in a direct field μ_cc of more than 100000 and permeability in an alternating field at 300 Hz, μ_{300 Hz} of more than 10000. Furthermore, these alloys have a coercive field Hc of less than 20 mOe and saturation induction Bs of more than 6000 Gauss. However, these alloys are very expensive as they have a high nickel content.

Alloys of the Fe—Ni36 type containing approximately 36% of nickel are used to produce less expensive shielding. However, these alloys have permeability in a direct field μ_cc between only 20000 and 30000 and permeability in an alternating field at 300 Hz, μ_{300 Hz} between 8000 and 9000, a coercive field Hc between 50 and 100 mOe and saturation induction Bs of approximately 13000 Gauss. With these magnetic properties the shielding obtained is less effective than shielding produced from Fe—Ni80 alloy.

It has also been proposed, for example in U.S. Pat. No. 5,158,624, to use an alloy containing 35 to 40% of nickel and 5 to 14% of chromium, the remainder being iron and impurities and the composition satisfying the relationships 3×Ni−5×Cr≦80 and Ni−Cr≧25. Furthermore, the contents of oxygen, sulphur and boron have to be strictly controlled; in particular the oxygen content has to be kept at less than 0.005%. In addition, the alloy contains 0.5% manganese, approximately 0.2% of silicon, approximately 0.01% of aluminum. This alloy has permeability in an alternating field at 300 Hz, μ_{300 Hz} of between 9400 and 14900, a coercive field Hc between 10 and 80 mOe and saturation induction Bs between 5000 G and 8200 G.

This alloy has the advantage of having higher permeability μ_{300 Hz} than the alloy Fe—Ni36 and of containing chromium, and this gives it some resistance to corrosion, but its better permeability may only be obtained with very low oxygen contents, and this restricts the production thereof. In addition, it would be desirable to have an inexpensive alloy having even better magnetic permeability.

The object of the present invention is to propose an inexpensive soft magnetic alloy, which is suitable for the production of low-frequency magnetic shielding, is more effective and less restrictive to produce than known alloys.

The invention accordingly relates firstly to magnetic shielding for frequency fields between 50 Hz and 3000 Hz, made from a soft magnetic alloy of the following composition in % by weight: 30%≦Ni≦40% 0%≦Cu+Co≦4% 5%≦Cr+Mo≦17% 5%≦Cr 0% ≦Nb≦2% Mn≦0.35% Si≦0.2% C≦0.050% O≦0.0160% S≦0.0020% B≦0.0010%, optionally at least one element selected from magnesium and calcium in amounts such that the sum thereof remains below 0.1%, the remainder being iron and production impurities, the chemical composition furthermore satisfying the following relationship: Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4 Cr+Mo≦4×Ni+3×(Co+Cu)−124 4×(Cr+Mo)≧125−3×Ni.

Preferably, it is preferable for the silicon content to be less than 0.15%, for the manganese content to be more than 0.05%, and for the sum of cobalt and copper contents to be more than 0.015%. The oxygen content may be more than 0.0050%.

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

The alloy according to the invention contains the following, in % by weight:

• • more than 30% of nickel for obtaining good magnetic properties and, in particular, adequate magnetic permeability and saturation induction, but less than 40%, because nickel is an expensive element and, above 40%, does not improve the desired magnetic properties;
  • one or more elements selected from copper and cobalt, the sum of their contents being between 0% and 4% and preferably more than 0.015%, and more preferably more than 0.5%, and yet more preferably more than 1% to increase the saturation induction Bs and obtain high magnetic permeability when the nickel content is relatively low;
  • one or more elements selected from chromium and molybdenum, the sum of their contents being between 5% and 17%, and the chromium content being more than 5%. These elements increase the magnetic permeability and reduce the coercive field, providing the contents thereof are not too high. Furthermore, to obtain the desired magnetic properties, namely Bs>4000 G at 40° C. and good magnetic permeability, the Cr, Mo, Ni, Cu and Co contents have to be such that: Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4 and Cr+Mo≦4×Ni+3×(Co+Cu)−124 and, to have good magnetic permeability, the Cr, Mo and Ni contents have to be such that: 4×(Cr+Mo)≧125−3×Ni;
  • optionally up to 2% of niobium to increase the mechanical strength;
  • less than 0.35% and preferably more than 0.05% of manganese and less than 0.20% and preferably less than 0.15% of silicon. These elements are required for production, but the inventors have found, in a novel manner, that, by limiting the contents of these elements, the magnetic permeability at 300 Hz, μ_{300 Hz}, is substantially increased, even with oxygen contents which may be as high as 0.0160%;
  • less than 0.0500% of carbon, less than 0.0020% of sulphur, less than 0.0010% of boron, less than 0.0200% of nitrogen and less than 0.0160% of oxygen. These limits to the contents of impurities enable high magnetic permeability to be obtained. It should be noted, however, that the oxygen content may be more than 0.0050%, without adversely affecting the magnetic properties, and this enables the alloy to be produced more easily and more economically, which is desirable;
  • optionally magnesium or calcium in amounts of which the sum may be as high as 0.1000% and must preferably be less than 0.0500%, but more than 0.0010%, in order to form magnesium or calcium oxides which facilitate the mechanical cutting of parts in strips.

The remainder of the composition is iron and optionally impurities.

Using this alloy, strips are produced, for example, by hot rolling then cold rolling. At the final thickness, the strips are subjected to annealing at least at 1050° C. and preferably at more than 1100° C., also preferably in a hydrogen reducing atmosphere or in a mixture of steam and hydrogen. After annealing, cooling to ambient temperature preferably has to be carried out at slow speed, in other words necessitates more than 1 hour to be able to reach 200° C. in order to optimize the magnetic permeability at 300 Hz.

The following magnetic properties are obtained in the strips obtained in this way and also having a thickness of 0.4 mm: μ_{300 Hz}>15000 μ_cc>40000 Bs>4000 G Hc<100 mOe

These properties allow production of magnetic shielding which is very effective in low-frequency fields, but also in direct fields (for example, terrestrial field).

As an example, the alloys designated 1 to 21 according to the invention were produced and the alloys designated 22 to 32 were provided as a comparison. The compositions and the properties of these alloys are shown in Tables 1 and 2, and the magnetic properties of the alloys are shown in Tables 3 and 4.

The magnetic properties were measured on 0.6 mm thick strips in the case the coercive fields Hc, expressed in mOe, and in the case of permeability in a direct field gcc which was measured at 0° C. and at 40° C. The saturation induction Bs, expressed in Gauss, was measured at 40° C. The magnetic permeability in an alternating field at 30 Hz, μ_{300 Hz}, was measured at 40° C. on 0.4 mm thick strips. The alloys were produced under vacuum in an induction furnace then cast in the form of hot-rolled then cold-rolled ingots to provide strips from which samples were cut and were then annealed for four hours at 1170° C. under pure dry hydrogen, with rapid cooling if they were intended to measure permeability in a direct field and slow cooling if they were intended to measure permeability in an alternating field.

Alloys 1 to 21 all have a coercive field of substantially less than 100 mOe, permeability in a direct field of more than 40000, at both 0° C. and 40° C., permeability in an alternating field at 300 Hz of more than 15000 and saturation induction of more than 4000 G.

	TABLE 1
	In % by weight	In ppm
Item	Ni	Cr	Mo	Co	Cu	Mn	Si	Nb	C	S	P	N	O	B
1	38.94	9.14	<0.005	0.042	0.022	0.323	0.163	traces	65	13	41	28	67	<10
2	36.52	9.02	0.0057	0.065	0.020	0.315	0.168	traces	55	14	43	27	80	<10
3	36.88	9.04	<0.005	0.044	0.022	0.310	0.155	traces	45	14	43	27	100	<10
4	37.64	6.98	<0.005	0.005	0.017	0.333	0.144	traces	45	14	<30	13	93	<10
5	33.66	7.95	—	—	—	0.188	<0.01	—	41	9	34	29	120	<10
6	33.55	8.17	<0.005	0.014	<0.01	0.172	0.016	traces	160	<5	<30	21	35	<10
7	37.63	9.31	0.023	0.503	0.094	0.293	<0.01	0.007	86	8	<30	27	90	<10
8	37.95	9.56	0.0056	1.42	0.020	0.289	0.017	traces	83	9	<30	30	84	<10
9	37.86	10.55	<0.005	0.962	0.018	0.299	0.019	traces	49	10	<30	27	140	<10
10	39.49	9.6	0.0097	1.02	<0.01	0.287	0.021	0.006	96	10	30	29	29	<10
11	37.75	9.54	—	1.02	—	0.300	—	—	91	6	<20	6.6	62	<10
12	37.66	9.19	<0.005	1.02	<0.01	0.178	0.105	traces	150	6	<30	14	25	<10
13	35.8	9.05	—	1.04	—	0.300	—	—	83	<5	<20	<5	99	<10
14	35.7	9.17	<0.005	1.03	<0.01	0.173	0.111	traces	130	<5	<30	12	64	<10
15	35.77	5.6	<0.005	1.01	<0.01	0.306	0.035	traces	94	<5	40	8.3	75	<10
16	37.74	5.76	<0.005	0.969	<0.01	0.308	0.033	traces	92	<5	38	7.5	58	<10
17	35.85	5.89	<0.005	2.85	<0.01	0.308	0.031	traces	83	<5	34	5.5	52	<10
18	35.79	8.92	—	3.03	—	0.290	—	—	90	<5	<20	9.6	69	<10
19	37.77	5.8	0.0086	2.87	<0.01	0.298	0.033	traces	69	<5	37	8.8	83	<10
20	37.45	8.72	—	3.06	—	0.300	—	—	89	<5	<20	12	68	<10
21	31.84	8.23	<0.005	3.07	<0.01	0.174	0.013	traces	150	6	<30	10	19	<10

Alloys 22 to 32, given as a comparison, show the significance of the limits imposed on the chemical composition.

	TABLE 2
	By % weight	In ppm
Item	Ni	Cr	Mo	Co	Cu	Mn	Si	Nb	C	S	P	N	O	B
22	31.84	8.2	<0.005	0.011	<0.01	0.173	0.018	traces	150	5	<30	10	24	<10
23	33.46	4.88	0.014	0.014	0.011	0.133	0.018	traces	66	11	<30	19	94	<10
24	33.78	2.02	2.03	traces	<0.01	0.186	<0.01	traces	150	<5	<30	10	13	<10
25	33.78	0.019	2.21	traces	<0.01	0.183	<0.01	traces	130	<5	<30	10	34	<10
26	37.69	3.14	<0.005	1.06	<0.01	0.296	0.031	traces	90	<5	35	<5	57	<10
27	33.7	8	—	2.07	—	0.187	<0.01	traces	66	15	38	7	180	<10
28	33.96	2.64	<0.005	1.96	<0.01	0.259	0.032	—	89	<5	35	5.1	85	<10
29	33.83	5.1	<0.005	2.02	<0.01	0.152	<0.01	traces	67	7	<30	7.2	110	<10
30	31.68	8.03	0.027	0.01	2.97	0.176	0.018	traces	120	7	31	54	67	<10
31	30.14	2.09	<0.005	traces	2.99	0.193	<0.01	0.005	130	<5	<30	10	29	<10
32	32.29	1.87	0.082	traces	3.92	0.166	0.014	0.007	83	9	35	7.6	86	<10

Alloy 22 has a chromium content which is too high to satisfy the conditions Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4 and Cr+Mo≦4×Ni+3×(Co+Cu)−124, and its saturation induction is very low.

Alloys 23, 24, and 25 have chromium contents which are too high to satisfy the condition 4×(Cr+Mo)≧125−3×Ni, and their permeability in a direct field is substantially less than 40000.

Alloy 26 does not satisfy the relationship Cr≧5%, and its permeability in an alternating field at 300 Hz is substantially less than 15000.

Alloy 27 has an oxygen content of more than 160 ppm and its permeability in a direct field is substantilly less than 40000.

Alloys 28 and 29 do not satisfy the relationship 4×(Cr+Mo)≧125−3×Ni, and alloy 28 does not satisfy the condition Cr≧5%. On the one hand, their permeability in a direct field is substantially less than 40000, but, in particular, their permeability in an alternating field is substantially less than 15000.

Alloy 30 does not satisfy the conditions Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4 and its permeability in a direct field is substantially less than 40000.

Alloy 31 does not satisfy the conditions Cr+Mo≦4×Ni+3×(Co+Cu)−124 and 4×(Cr+Mo)≧125−3×Ni, and its magnetic permeability is inadequate in both an alternating field and a direct field.

	TABLE 3
	Magnetic properties
	Item	Hc	Bs	μcc 0° C.	μcc 40° C.	μ300 Hz
	1	28	6800	61400	62400	17900
	2	23	5500	64000	50300	23200
	3	22	6000	69000	59600	21700
	4	39	8100	69200	67200	21100
	5	18	4500	47500	44700	17800
	6	10	4200	80300	72000	16000
	7	22	6500	67600	67400	21400
	8	26	6700	66800	64700	19100
	9	22	5600	48100	77200	20800
	10	21	7200	72600	72100	18600
	11	22	6800	98300	93000	18500
	12	14	6800	132900	118300	22000
	13	24	5700	71500	66900	22100
	14	22	5700	86900	78900	22400
	15	44	8400	42700	57100	17900
	16	34	9500	74900	93400	19400
	17	45	9100	52500	60500	17000
	18	27	6700	84200	97600	19100
	19	44	10000	48100	68200	17600
	20	25	7500	91900	84100	20800
	21	13	4700	51600	65000	17900

	TABLE 4
	Magnetic properties
	Item	Hc	Bs	μcc 0° C.	μcc 40° C.	μ300 Hz
	22	7	500	45800	—	13600
	23	36	7000	24200	29100	14400
	24	28	7400	33300	31900	14200
	25	33	8200	29100	26600	14100
	26	54	11700	48200	59200	13700
	27	32	5900	26600	37600	20000
	28	85	10200	18000	22900	10800
	29	69	8400	14100	21300	13000
	30	26	4700	31700	32200	15300
	31	33	6500	20500	21400	11100
	32	73	10400	12500	18100	—

Alloy 32 does not satisfy the conditions 4×(Cr+Mo)≧125−3×Ni, and its magnetic permeability is very inadequate.

Claims

1. Magnetic shielding for frequency fields between 50 Hz and 3000 Hz, made from a soft magnetic alloy of the following composition in % by weight:

2. Shielding according to claim 1, further characterised in that:

3. Shielding according to claim 1, further characterised in that:

4. Shielding according to claim 1, further characterised in that:

5. Shielding according to claim 1, further characterised in that:

6. Use of a soft magnetic alloy of which the composition is as defined in claim 1, for the production of magnetic shielding for frequency fields between 50 Hz and 3000 Hz.