GB2451219A

GB2451219A - Soft magnetic iron-cobalt based alloy

Info

Publication number: GB2451219A
Application number: GB0813522A
Authority: GB
Inventors: Witold Pieper; Joachim Gerster
Original assignee: Vacuumschmelze GmbH and Co KG
Current assignee: Vacuumschmelze GmbH and Co KG
Priority date: 2007-07-27
Filing date: 2008-07-24
Publication date: 2009-01-28
Anticipated expiration: 2028-07-24
Also published as: GB2451219B; GB0813522D0; KR20090012145A; DE102007035774B4; KR101060094B1; DE102007035774A1; HK1127158A1; DE102007035774B9

Abstract

A soft magnetic alloy comprises (by weight): 10-22% Co, 0-4 % V, 1.5-5 % Cr, 0 <Mn < 1 %, 0-1 % Mo, 0.5-1.5 % Si, 0.1-1.0 % Al, and the remainder iron. The total amount of chromium, manganese, molybdenum, aluminium, silicon and vanadium lies in the range 4.0-9.0 %. The alloy may be used to make the core of an actuator, the core of an internal combustion engine solenoid valve, the core of a fuel injection valve, a stator or rotor of an electric motor or a yoke for an electromagnetic actuator or a solenoid valve. The alloy can be processed by melting, hot forming, cold forming and a final anneal at a temperature in the range 700-1100 {C in an inert gas, hydrogen or vacuum.

Description

1 2451219 Soft magnetic iron-cobalt-based alloy and process for manufacturing it The invention relates to a soft magnetic iron-cobalt-based alloy which has a cobalt content of 10 to 22 percent by weight, and to a process for manufacturing the alloy and a process for manufacturing semi-finished products from the alloy, particularly but not exclusively, magnetic components for actuator systems.

Soft magnetic iron-cobalt-based alloys have a high saturation magnetisation and can therefore be used to develop electromagnetic actuator systems with high forces and/or small dimensions. These alloys are typically used in solenoid valves such as solenoid valves for fuel injection systems in internal combustion engines, for example.

Soft magnetic iron-cobalt-based alloys with a cobalt content of 10 to 22 percent by weight are known, for example, from US 7,128,790. When these alloys are used in fast-switching actuators it the switching frequency is limited due to the eddy currents which occur. Moreover, improvements in the strength of the magnet cores used in continuous operation in high frequency actuator systems are also desired.

The present invention seeks, therefore, to provide an alloy which is better suited to use as a magnet core in fast-switching actuators.

According to the present invention there is provided a soft magnetic alloy which consists essentially of 10 percent by weight S Co 5 22 percent by weight, 0 percent by weight s V s 4 percent by weight, 1.5 percent by weight S Cr 5 5 percent by weight, 0 percent by weight < Mn < 1 percent by weight, 0 percent by weight S Mo 5 1 percent by weight, 0.5 percent by weight s Si s 1.5 percent by weight, 0.1 percent by weight s Al �= 1.0 percent by weight and the remainder iron, the content of the elements chromium and manganese and molybdenum and aluminium and silicon and vanadium being 4.0 percent by weight S (Cr+Mn+Mo+Al+Si�V) 5 9.0 percent by weight.

In the invention a soft magnetic alloy consists essentially of 10 percent by weight �= Co 22 percent by weight, 0 percent by weight S V S 4 percent by weight, 1.5 percent by weight �= Cr 5 5 percent by weight, 0 percent by weight < Mn < 1 percent by weight, 0 percent by weight S Mo 5 1 percent by weight, 0.5 percent by weight S Si 5 1.5 percent by weight, 0.1 percent by weight �= Al 1.0 percent by weight and the remainder iron, the content of the elements chromium and manganese and molybdenum and aluminium and silicon and vanadium being 4.0 percent by weight �= (Cr+Mn+Mo+Al�Si+V) 9.0 percent by weight.

The term "essentially" indicates the inclusion of incidental impurities. The alloy preferably has a maximum of 200 ppm nitrogen, a maximum of 400 ppm carbon and a maximum of 100 ppm oxygen.

The alloy disclosed in the invention has a higher specific resistivity than the binary Cc-Fe alloy leading to the suppression of eddy currents, the saturation polarisation being reduced as little as possible while at the same time the coercive field strength H is increased as little as possible. This is achieved by the addition by alloying of the non-magnetic elements, in particular of the elements chromium and manganese and molybdenum and aluminium and silicon and vanadium in the content disclosed in the invention which lies between 4.0 and 9.0 percent by weight.

Cr and Mn show a strong increase in resistivity at a low reduction in saturation. At the same time the annealing temperature corresponding to the upper boundary of the ferritic phase is lowered. This is not, however, desired since it leads to poorer soft magnetic properties.

Al, V and Si also increase the electrical resistivity whilst at the same time raising the annealing temperature. Thus it is possible to specify an alloy with high resistivity, high saturation and a high annealing temperature and thus with good soft magnetic properties.

Moreover, due to its Al and Si contents the alloy has greater strength. The alloy is cold formable and ductile in its fully annealed state. An elongation value of AL > 2% or AL> 20% is measured in tensile tests. This alloy is suitable for use as a magnet core in a fast-acting actuator system such as a fuel injection valve of an internal combustion engine.

The requirements demanded of a soft magnetic cobalt-iron-based alloy for an actuator system are contradictory. In the binary alloy a higher cobalt content leads to a high saturation magnetisation J of approximately 9 mT per 1 percent by weight Co (starting from 17 percent by weight Co) and thus permits smaller dimensions and greater system integration or higher actuator forces with the same dimensions. At the same time, however, the costs of the alloy increase. As the Co percentage increases, soft magnetic properties such as permeability, for example, deteriorate. Above a cobalt content of 22 percent by weight, the increase in saturation due to the addition by alloying of further Co is less. The alloy should also have a high specific electrical resistivity and good soft magnetic properties.

This alloy therefore has a cobalt content of 10 percent by weight Co �= 22 percent by weight. A lower cobalt content reduces the raw materials cost of the alloy, thereby making it suitable for applications subject to high cost pressure such as those in the automotive sector, for example. Maximum permeability is high within this range, leading to more favourable lower driver currents when used as an actuator.

In further embodiments the alloy has a cobalt content of 14 percent by weight Co 22 percent by weight and 14 percent by weight �= Co 20 percent by weight.

The soft magnetic alloy of the magnet core has chromium and manganese contents which lead to a higher specific electrical resistivity p in the annealed state with lower saturation reduction. This higher specific resistivity permits shorter switching times in an actuator since eddy currents are reduced. At the same time, the alloy has high saturation and high permeability Px and therefore retains good soft magnetic properties.

The elements Si and Al in the alloy provide improved alloy strength without substantially reducing its soft magnetic properties. Due to the addition by alloying of Si and Al it is possible to significantly increase the strength of the alloy by solid solution hardening without a significant reduction in magnetic properties.

The aluminium and vanadium contents disclosed in the invention permit a higher annealing temperature which leads to good soft magnetic properties of the coercive field strength H and maximum permeability High permeability is desired since it leads to low drive currents when the alloy is used as a magnet core or a flow conductor of an actuator.

In one embodiment the alloy has silicon content of 0.5 percent by weight Si 1.0 percent by weight.

The Mo content has been kept low in order to prevent the formation of carbides which could lead to a reduction in magnetic properties.

In addition to Cr and Mn, a small molybdenum content is also favourable since it is characterised by a good increase in resistivity to reduction in saturation ratio.

In one embodiment the aluminium and silicon content is 0.6 percent by weight �= All-Si 1.5 percent by weight in order to avoid the brittleness and processing problems which can occur with high combined aluminium and silicon contents.

In one embodiment the content of chromium and manganese and molybdenum and aluminium and silicon and vanadium is 6.0 percent by weight �= Cr-'-Mn+Mo+Al+Si�V 9.0 percent by weight.

Alloys with the aforementioned compositions can have a specific electrical resistivity of p > 0.50 p)m or p > 0.55 pOrn or p > 0.60 pC)m or p > 0.65 pOrn. This value provides an alloy which when used as a magnet core of an actuator system produces lower eddy currents. This permits the use of the alloy in actuator systems with fast switching times.

The percentage of the elements aluminium and silicon in the alloy disclosed in the invention produces an alloy with a yield strength of R2> 340 MPa. This higher alloy strength is able to lengthen the service life of the alloy when used as a magnet core of an actuator system. This is attractive when the alloy is used in high frequency actuator systems such as fuel injection valves in internal combustion engines.

The alloy disclosed in the invention has good soft magnetic properties and good strength and a high specific electrical resistivity. In further embodiments the alloy has a saturation of J(400A/cm)> 1.00 T or> 2.0 T and/or a coercive field strength H of < 3.5 Ncm or H < 2.0 A/cm or H < 1.0 A/cm and/or a maximum permeability Pm> 1000 or p,>2000.

The content of chromium and manganese and molybdenum and aluminium and silicon and vanadium disclosed in the invention lies between 4.0 percent by weight und 9.0 percent by weight. Due to this high content it is possible to provide an alloy which has a higher electrical resistivity of p> 0.6 pCm and a low coercive field strength H of < 2.0 Ncm. This combination of properties is particularly suitable for use in fast-switching actuators.

The invention also provides for a soft magnetic core or flow conductor for an electromagnetic actuator made of an alloy in accordance with one of the preceding embodiments. In various different embodiments this soft magnetic core is a soft magnetic core for a solenoid valve of an internal combustion engine, a soft magnetic core for a fuel injection valve of an internal combustion engine, a soft magnetic core for a direct fuel injection valve of a spark ignition engine or a diesel engine and a soft magnetic component for electromagnetic valve adjustment such as an inlet/ outlet valve.

The various actuator systems such as solenoid valves and fuel injection valves have different requirements in terms of strength and magnetic properties. The requirements can be met by selecting an alloy with a composition which lies within the aforementioned ranges.

The invention also provides for a fuel injection valve of an internal combustion engine with a component made of a soft magnetic alloy in accordance with one of the preceding embodiments. In further embodiments the fuel injection valve is a direct fuel injection valve of a spark ignition engine and a direct fuel injection valve of a diesel engine.

In further embodiments the invention provides for a yoke, that is a component, formed of a magnetically soft material for the purpose of guiding, containing or concentrating the magnetic flux, in an electromagnetic actuator, and/or a soft magnetic rotor and stator for an electric motor made of an alloy in accordance with one of the preceding embodiments.

The invention also provides for a process for manufacturing semi-finished products from a cobalt-iron alloy in which workpieces are manufactured initially by melting and hot forming a soft magnetic alloy which consists essentially of 10 percent by weight Co 22 percent by weight, 0 percent by weight �= V 4 percent by weight, 1.5 percent by weight �= Cr �= 5 percent by weight, 0 percent by weight < Mn < 1 percent by weight, 0 percent by weight �= Mo 1 percent by weight, 0.5 percent by weight �= Si �= 1.5 percent by weight, 0.1 percent by weight �= Al �= 1.0 percent by weight and the remainder iron, the content of the elements chromium and manganese and molybdenum and aluminium and silicon and vanadium being 4.0 percent by weight �= (Cr+Mn+Mo+Ak-Si+V) 9.0 percent by weight. A final annealing process is then carried out.

This alloy can be melted by means of various different processes. All current techniques including air melting and Vacuum Induction Melting (VIM), for example, are possible in theory. In addition, an arc furnace or inductive techniques may also be used. Treatment by Vacuum Oxygen Decarburization (VOD) or Argon Oxygen Decarbutization (AOD) or Electro Slag Remelting (ESR) improves the quality of the product.

The VIM process is the preferred process for manufacturing the alloy since using this process it is on one hand possible to set the contents of the alloy elements more precisely and on the other easier to avoid non-metallic inclusions in the solidified alloy.

Depending on the semi-finished products to be manufactured, the melting process is followed by a range of different process steps.

If strips are to be manufactured for subsequent pressing into parts, the ingot produced in the melting process is formed by blooming into a slab ingot. Blooming refers to the forming of the ingot into a slab ingot with a rectangular cross section by a hot rolling process at a temperature of 1250°C, for example. After blooming, any scale formed on the surface of the slab ingot is removed by grinding. Grinding is followed by a further hot rolling process by means of which the slab ingot is formed into a strip at a temperature of 1250°C, for example. Any impurities which have formed on the surface of the strip during hot rolling are then removed by grinding or pickling, and the strip is formed to its final thickness which may be within a range of 0.1 mm to 0.2 mm by cold rolling. Ultimately, the strip is subjected to a final annealing process. During this final annealing any lattice imperfections produced during the various forming processes are removed and crystal grains are formed in the structure.

The manufacturing process for producing turned parts is similar. Here, too, the ingot is bloomed to produce billets of quadratic cross-section. On this occasion, the so-called blooming process takes place at a temperature of 1250°C, for example. The scale produced during blooming is then removed by grinding. This is followed by a further hot rolling process in which the billets are formed into rods or wires with a diameter of up to 13 mm, for example. Faults in the material are then corrected and any impurities formed on the surface during the hot rolling process removed by planishing and pre-turning. In this case, too, the material is then subjected to a final annealing process.

The final annealing process can be carried out within a temperature range of 700°C to 100°C. In one embodiment, final annealing is carried out within a temperature range of 750°C to 850°C. The final annealing process may be carried out in inert gas, in hydrogen or in a vacuum.

Conditions such as the temperature and duration of final annealing can be selected such that after final annealing the alloy has deformation parameters under tensile testing including an elongation at rupture value of> 2% or AL > 20%.

In a further embodiment the alloy is cold formed prior to final annealing.

Claims

1. Soft magnetic alloy which consists essentially of 10 percent by weight �= Co 22 percent by weight, 0 percent by weight �= V �= 4 percent by weight, 1.5 percent by weight �= Cr �= 5 percent by weight, 0 percent by weight < Mn < 1 percent by weight, 0 percent by weight �= Mo �= 1 percent by weight, 0.5 percent by weight �= Si �= 1.5 percent by weight, 0.1 percent by weight Al �= 1.0 percent by weight and the remainder iron, the content of the elements chromium and manganese and molybdenum and aluminium and silicon and vanadium being 4.0 percent by weight �= (Cr+Mn+Mo+Al+Si+V) �= 9.0 percent by weight.

2. Soft magnetic alloy in accordance with claim 1 characterised by a cobalt content of 14 percent by weight Co �= 22 percent by weight.

3. Soft magnetic alloy in accordance with claim 2, characterised by a cobalt content of 14 percent by weight �= Co �= 20 percent by weight.

4. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by a vanadium content of 0 percent by weight �= V 2 percent by weight.

5. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by a molybdenum content of 0 percent by weight < Mo <0.5 percent by weight.

6. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by a manganese content of 0.4 percent by weight < Mn < 1.0 percent by weight.

7. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by a silicon content of 0.5 percent by weight Si 1.0 percent by weight.

8. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by an aluminium and silicon content of 0. 6 percent by weight Ali-Si �= 2 percent by weight.

9. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by a chromium and manganese and molybdenum and aluminium and silicon and vanadium content of 6.0 percent by weight Cr+Mn+Mo+Al+Si+V 9.0 percent by weight.

10. Soft magnetic alloy in accordance with any one of the preceding claims, characterised in that under tensile testing in its final annealed state the alloy has an elongation at rupture value of A1 > 2%.

II. Soft magnetic alloy in accordance with claim 10, wherein under tensile testing in its final annealed state the alloy has an elongation at rupture value of A1 > 20%.

12. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by a specific electrical resistivity of p > 0.50 pOrn.

13. Soft magnetic alloy in accordance with claim 12, characterised by a specific electrical resistivity of p > 0.55 pOrn.

14. Soft magnetic alloy in accordance with claim 13, characterised by a specific electrical resistivity of p > 0.60 pOrn.

15. Soft magnetic alloy in accordance with claim 14, characterised by a specific electrical resistivity of p > 0.65 pQm.

16. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by a yield strength of R1,o2 > 340 MPa.

17. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by saturation at J(400Ncm)> 1.0 T. 16. Soft magnetic alloy in accordance with claim 17, characterised by saturation at J(400Ncm) > 2.00 T. 19. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by

a coercive field strength H of < 3.5 A/cm.

20. Soft magnetic alloy in accordance with claim 19, characterised by

a coercive field strength H of < 2.0 A/cm.

21. Soft magnetic alloy in accordance with any one of the preceding claims, characterised by a maximum permeability of p> 1000.

22. Soft magnetic alloy in accordance with claim 21, characterised by a maximum permeability of p > 2000.

23. Soft magnetic core for an electromagnetic actuator made of an alloy in accordance with any one of claims I to 22.

24. Soft magnetic core for a solenoid valve of an internal combustion engine made of an alloy in accordance with any one of claims 1 to 22.

25. Soft magnetic core for a fuel injection valve of an internal combustion engine made of an alloy in accordance with any one of claims 1 to 22.

26. Soft magnetic core for a direct fuel injection valve of a spark ignition engine made of an alloy in accordance with any one of claims 1 to 22.

27. Soft magnetic core for a direct fuel injection valve of a diesel engine made of an alloy in accordance with any one of claims 1 to 22.

28. Fuel injection valve of an internal combustion engine with a component made of a soft magnetic alloy in accordance with any one of claims 1 to 22.

29. Fuel injection valve in accordance with claim 28, wherein the fuel injection valve is a direct fuel injection valve of a spark ignition engine.

30. Fuel injection valve in accordance with claim 28, wherein the fuel injection valve is a direct fuel injection valve of a diesel engine.

31. Soft magnetic stator for an electric motor made of an alloy in accordance with any one of claims 1 to 22.

32. Soft magnetic rotor for an electric motor made of an alloy in accordance with any one of claims I to 22.

33. Soft magnetic component, for an electromagnetic valve adjustment system for an inlet and/or outlet valve used in an engine, made of an alloy in accordance with any one of claims 1 to 22.

34. A yoke formed of a magnetically soft material for the purpose of guiding, containing or concentrating the magnetic flux in an electromagnetic actuator, made of an alloy in accordance with any one of claims I to 22.

35. A yoke formed of a magnetically soft material for the purpose of guiding, containing or concentrating the magnetic flux in a solenoid valve, made of an alloy in accordance with any one of claims 1 to 22.

36. Process for manufacturing semi-finished products from a cobalt-iron-based alloy in which workpieces are manufactured initially by melting (1) and hot forming (4, 10) a soft magnetic alloy which consists essentially of 10 percent by weight Co 22 percent by weight, 0 percent by weight V 4 percent by weight, 1.5 percent by weight Cr 5 percent by weight, 0 percent by weight < Mn < 1 percent by weight, 0 percent by weight �= Mo 1 percent by weight, 0.5 percent by weight �= Si 1.5 percent by weight, 0.1 percent by weight Al 1.0 percent by weight and the remainder iron, the content of the elements chromium and manganese and molybdenum and aluminium and silicon and vanadium being 4.0 percent by weight �= Cr+Mn+Mo+Al+Si+V �= 9.0 percent by weight, and a final annealing process (7, 12) being carried out.

37. Process in accordance with claim 36, wherein final annealing (7, 12) is carried out within a temperature range of 700°C to 1100°C.

38. Process in accordance with claim 37, wherein final annealing (7, 12) is carried out within a temperature range of 750°C to 850°C.

39. Process in accordance with any one of claims 36 to 38, wherein final annealing is carried out such that after final annealing the alloy has deformation parameters under tensile testing including an elongation at rupture value of A> 2%.

40. Process in accordance with claim 39, wherein final annealing is carried out such that after final annealing the alloy has deformation parameters under tensile testing including an elongation at rupture value of AL > 20%.

41. Process in accordance with any one of claims 36 to 40, wherein the alloy is cold formed prior to final annealing (7, 12).

42. Process in accordance with one of claims 36 to 41, wherein the alloy is subjected to a final annealing process in inert gas, hydrogen or in a vacuum.

43. A soft magnetic alloy substantially as described herein with reference to the embodiments.

44. Process for manufacturing semi-finished products from a cobalt-iron-based alloy, substantially as described herein