DE3782288T2 - METHOD FOR HEAT-TREATING QUICKLY QUICKED FE-6.5% SI-TAPES. - Google Patents

Description

This invention relates to a method for heat treating rapidly quenched Fe-6 to 7, especially 6.5 wt% Si which by controlling the order-disorder reaction gives improved magnetic properties at high induction values.

Fe-6.5 wt.% Si alloy has extremely desirable ferromagnetic properties but poor mechanical properties. It usually has poor ductility and cannot be easily formed into thin ribbons or sheets which can be stamped or wound into predetermined shapes. U.S. Patent No. A-4,649,983 teaches a method of processing Fe-Si alloys containing 6 to 7 wt.% Si to produce thin, ductile ribbons with improved magnetic properties. To produce the ribbon, a flow of molten alloy is expelled through a nozzle and rapidly quenched on the peripheral surface of a rapidly rotating disk, forming a continuous alloy sheet. The so-cast ribbon is then annealed under vacuum at temperatures between 1000°C to 1200°C to obtain a columnar grain structure of a certain size with a <100> fiber texture. This process produces a material having a Power loss of 0.46 W/Kg and an excitation energy of 0.62 VA/Kg at B1.0 T and f=60 Hz, with these properties being isotropic in the band plane.

The said patent does not teach anything regarding induction values above 1.0 T.

The order-disorder phenomenon and the resulting phase diagram of high-grade silicon-iron alloys has been described in the literature. It is known that the order-disorder reaction affects the magnetic properties of materials ranging from those that are structurally sensitive to those that are strengthened. It has been reported that in Fe-6.5 wt% Si, the magnetostriction decreases as the DO3 domain grows and that the magnetic anisotropy decreases as the B2 domain grows. By applying appropriate heat treatments to control the order-disorder reaction, material was produced with a maximum permeability (m) of 52,000 and a coercive force (Hc) of 0.088 Oe at a maximum induction of 1.0 T. However, these properties are not useful for electromagnetic applications such as transformers, generators and motors. In these devices, properties such as low core AC loss at high induction levels (greater than 1.0 T) are essential. No attempt was made to improve the magnetic properties of rapidly quenched Fe-6.5 wt% Si at high induction levels by controlling the order-disorder reaction.

The invention provides a method of heat treating rapidly quenched Fe-Si alloys containing 6 to 7 wt% Si which promotes an order-disorder reaction, thereby improving the magnetic properties at high induction levels, comprising the following steps:

Vacuum annealing at a temperature between 1000ºC and 1200ºC for 1 to 4 hours to achieve < 100> texture with at least twice the random intensity and a grain size of 1-2 mm, and

Annealing at a temperature of 500ºC to 900ºC for 1 to 4 hours and cooling at an appropriate cooling rate to preserve the annealed domain structure, between 100 to 850 nm B2 domains and 5 to 25 nm DO3 domains.

In addition, the invention provides an improved crystal ribbon of a Fe-Si metal alloy containing 6 to 7 weight percent Si, said ribbon having much smaller transverse dimensions than its length, of a Fe-Si alloy containing 6 to 7 weight percent Si, said ribbon:

essentially a <100> texture with at least twice the random intensity;

a grain size of 1-2 mm;

a B2 domain size of 100 to 850 nm;

a DO3 domain size of 5 to 25 nm;

a core AC loss of 1.2 to 1.6 W/Kg at an induction value of B=1.4 T and a frequency of f=60 Hz; and

an excitation energy of 15 to 46 VA/Kg at an induction value of B=1.4 T and a frequency of f=60 Hz .

The strip is sufficiently ductile to be easily punched, wound or otherwise processed into desired shapes. It essentially has isotropic, ferromagnetic properties. These improved magnetic properties mean that the strip is particularly suitable for use in rotors and stators of electromagnetic equipment such as motors and generators, which operate at induction values higher than than 1.0 T.

With reference to the drawings:

Fig. 1 shows dark field transmission electron micrographs of B2 (1a) and DO3 (1b) ordered domain structures using superordinate lattice reflections corresponding to the B2 and DO3 structures diffracted in selected regions in a ribbon annealed for 1 hour under vacuum at 1100ºC and at 825ºC for 1 hour in hydrogen atmosphere.

Fig. 2 shows typical micrographs of the grain size and grain morphology of a Fe-6.5 wt% Si ribbon annealed at 1100°C for 1 hour.

Fig. 3 shows a (200) pole pattern of a Fe-6.5 wt% Si ribbon which was annealed at 1100ºC for 1 hour.

For the purposes of this invention and as used in the present description, a strip is a slender body whose transverse dimensions are much less than its length. Such a strip may be in the form of a ribbon, strip or sheet which is narrow or wide and has a regular or irregular cross-section. Also for the purposes of this invention, a strip is considered ductile if it can be bent to a radius of ten times its thickness without breaking.

It is well known that single iron crystals have a cubic crystal structure and can be magnetized very easily in the <100> direction, less easily in the <110> direction, and least easily in the <111> direction. These directions are expressed in normal crystallographic rotation. This magnetic anisotropy exerts a strong influence on static hysteresis losses during alternating magnetization. In the core of rotary machines, the magnetic field lies in the plane of the sheet, but the The angle between the field and the longitudinal direction of the sheet changes as the core is rotated. It is therefore desirable to have a material with a texture such that the "hard" (i.e., most difficult to magnetize) <111> direction is not in the plane of the sheet. A <100>"fiber" texture (i.e., a texture in which all the grains have a <100> direction normal to the sheet surface and in all possible rotational positions about this normal) is the most desirable ferromagnetic material in rotary equipment because the sheet then has isotropic ferromagnetic properties in its own plane. A material is considered to have substantially ferromagnetic properties if its ferromagnetic properties, as defined by its BH curve, do not vary by more than 20% when measured in any direction in the plane of the sheet.

The term "texture" as used herein expresses the predominant orientation of the crystal grains in the metal, compared with a control sample in which the crystals are randomly oriented. The texture can be determined by conventional means such as X-ray diffraction and electron beam diffraction analysis.

The present invention provides a method for processing cast ribbons of Fe-Si alloys containing 6 to 7 wt% Si to obtain optimal B2 and DO3 domain structures. A ribbon processed by the method according to the present invention is ductile and has improved magnetic properties such as energy loss and excitation energy at high induction levels. Generally speaking, the ribbon is rapidly solidified and then treated by a two-step annealing process. In step (ii) the required cooling rates are achieved simply by furnace cooling in a hydrogen atmosphere.

In rapidly solidified Fe-Si alloys containing 6 to 7 wt% Si, which are subjected to only the first annealing step, the B2 domain is about 160 nm and there is no evidence of DO3 domains. AC core losses and excitation energy in these materials, although favorable at induction levels below about 1.0 T, increase rapidly at higher induction levels. After the second annealing step of this invention, both B2 and DO3 domains are present and AC core losses and excitation energy are significantly improved at induction levels above about 1.2 T.

A typical example of the B2 and DO3 domain structure in a Fe-6.5 wt% Si alloy subjected to the heat treatment of this invention is shown in Fig. 1. The domain size is strongly dependent on the annealing temperature and only weakly dependent on the annealing time. Annealing in the lower range of this invention (500°C to 700°C) results in a large domain size in the ribbon. A smaller domain size can be achieved by annealing at the higher temperatures of this invention (700°C to 900°C). In general, higher second step annealing temperatures and longer annealing times result in smaller B2 and DO3 domain sizes and lower AC core losses and excitation energy at high induction levels. Preferably, the second step annealing is carried out at temperatures between 790°C and 860°C. Fe-Si ribbon annealed by this preferred process has a B2 domain size of 100-250 nm, a DO3 domain size of 5 to 10 nm, an AC core loss of 1.2 to 1.5 W/Kg, and an excitation energy of 15 to 26 VA/Kg, with the AC core loss and excitation energy being measured at an induction value of 1.4 T and a frequency of 60 Hz.

The retained ductility and improved magnetic Properties of rapidly solidified Fe-Si alloys containing 6 to 7 wt.% Si arise from the refinement of the size of the ordered domains. Advantageously, such alloys, when subjected to the two-step annealing process of this invention, prove to be very suitable for use in rotating electromagnetic apparatus operating at induction levels above about 1.2 T.

The following examples are presented for a more complete understanding of the invention.

example 1

A strip of Fe-6.5 wt% Si alloy was cast using the planar flow casting process disclosed in U.S. Patent No. 4,331,739. The as-cast strip had a 100% columnar grain structure with an average grain size of 2.3 x 10-5 m and essentially no second phase particles at the grain boundaries. The strip had an almost random texture. The material was vacuum annealed at 1100°C for 1 hour to achieve the desired <100> texture and optimum grain size. Figure 2 shows representative micrographs of grain size and grain morphology in a strip annealed at 1100°C for 1 hour. This annealed band exhibits a strong <100> texture with up to 44 times random intensity, as shown in Fig. 3 .

The domain structure was observed in a transmission electron microscope (UEM) dark field of the superior lattice reflections corresponding to the B2 and DO3 structures. The extent of the B2 domain in the ribbon annealed at 1100ºC for 1 hour is about 160 nm. No evidence of DO3 domain was found in this ribbon.

The magnetic properties (AC core loss and excitation energy) of this annealed tape are listed in Table 1. These measurements were made by winding the samples after heat treatment, with 100 primary and secondary turns. Measurements for core loss were made using a Dranetz 3100 sampling network comparator. Primary current was determined from the voltage across a 0.1 ohm non-inductive resistor in the primary circuit. Resistive losses in the primary circuit were eliminated by measuring the induced secondary voltage. The network comparator sampled these voltage waveforms and calculated the total loss. Excitation energy was calculated using rms voltmeter measurements of the same voltage waveforms. A Hewlett-Packard 9836 computer was used to control both the network comparison solver and frequency generator, and via an IEEE 488 rail to log data from these two, as well as from rms and integrating voltmeters. A computer program allowed the induction, as calculated by the integrating voltmeter, to be automatically set to preselected values and then all readings were logged. The computer calculated values for core loss and excitation energy per kilogram. Voltage feedback from the secondary windings was necessary to maintain a sinusoidal excitation flux due to the high excitation currents at high induction values. Air-core flux compensators were also used because of these high charging currents.

Table 1

AC core loss and excitation energy of rapidly solidified Fe-6.5 wt% Si ribbon annealed at 1100ºC for 1 hour, values measured at f=60 Hz. Induction values Core loss Excitation energy

Examples 2-10

Samples of materials cast and annealed as in Example 1 were subjected to a second annealing treatment at temperatures ranging from 500ºC to 900ºC for periods ranging from 1 hour to 4 hours in a hydrogen atmosphere. After annealing, the furnace power was turned off and the sample was allowed to cool to room temperature. Samples were prepared for microstructural analysis using the UEM and for measurement of magnetic properties as described in Example 1. The following examples illustrate the effect of heat treating on the domain extent and magnetic properties of Fe-6.5 Weight% Si tape.

The size of the B2 and DO3 domains as determined by UEM analysis is listed in Table 2 for the different annealing temperatures and times.

Table 2

Effect of heat treatment on the B2 and DO3 domain size of Fe-6.5 wt% Si after annealing at 1100ºC for 1 h. Example Annealing temperature (ºC) Annealing time (hours) B2 Domain size (mm) DO3 Domain size (mm)

Examples 2-10 illustrate that the order-disorder reaction in Fe-6.5 wt% Si, as indicated by the change in the size of the B2 and DO3 domains, is strongly influenced by the secondary annealing temperature, and this relatively independent of the annealing time.

The AC core loss and excitation energy at various induction values measured at 60 Hz are given for these examples in Tables 3-8. No data are given for Examples 7 and 8 due to unavailability of material to construct test specimens.

Table 3

Effect of heat treatment on AC core loss and excitation energy of Fe-6.5 wt% Si after annealing at 1100ºC for 1 hour, measured at an induction value of B=0.6 T, frequency f=60 Hz. Example Annealing temperature (ºC) Annealing time (hours) Core loss (W/Kg) Excitation force (VA/Kg)

Table 4

Effect of heat treatment on AC core loss and excitation energy of Fe-6.5 wt% Si after annealing at 1100ºC for 1 hour, measured at an induction value of B=0.8 T, frequency f=60 Hz. Example Annealing temperature (ºC) Annealing time (hours) Core loss (W/Kg) Excitation force (VA/Kg)

Table 5

Effect of heat treatment on AC core loss and excitation energy of Fe-6.5 wt% Si after annealing at 1100ºC for 1 hour, measured at an induction value of B1.0 T, frequency f=60 Hz. Example Annealing temperature (ºC) Annealing time (hours) Core loss (W/Kg) Excitation force (VA/Kg)

Table 6

Effect of heat treatment on AC core loss and excitation energy of Fe-6.5 wt% Si after annealing at 1100ºC for 1 hour, measured at an induction value of B=1.2 T, frequency f=60 Hz. Example Annealing temperature (ºC) Annealing time (hours) Core loss (W/Kg) Excitation energy (VA/Kg)

Table 7

Effect of heat treatment on AC core loss and excitation energy of Fe-6.5 wt% Si after annealing at 1100ºC for 1 hour, measured at an induction value of B=1.3 T, frequency f=60 Hz. Example Annealing temperature (ºC) Annealing time (hours) Core loss (W/Kg) Excitation energy (VA/Kg)

Table 8

Effect of heat treatment on AC core loss and excitation energy of Fe-6.5 wt% Si after annealing at 1100ºC for 1 hour, measured at an induction value of B=1.4 T, frequency f=60 Hz. Example Annealing temperature (ºC) Annealing time (hours) Core loss (W/Kg) Excitation energy (VA/Kg)

The foregoing examples clearly illustrate that rapidly solidified Fe-Si alloys containing 6 to 7 wt.% Si, and preferably 6.5 wt.% Si, exhibit an improvement in AC core loss and excitation energy at high induction levels when processed by the process of this invention, as compared to those subjected to only one annealing step. The improvement in core loss and excitation energy is due to the refinement of the domain structure as indicated in Table 2. The refinement of domain size and, consequently, the magnetic properties are markedly enhanced when the second annealing step of this invention is carried out at temperatures in the preferred range, 800°C to 900°C.

Claims (10)

1. A process for heat treating rapidly quenched Fe-Si alloys containing 6 to 7 wt% Si which promotes an order/disorder reaction and thereby improves the magnetic properties at high induction values, comprising the following steps: Vacuum annealing at a temperature between 1000ºC and 1200ºC for 1 to 4 hours to obtain a < 100> texture with at least twice the random intensity and a grain size of 1-2 mm; and Annealing at a temperature of 500ºC to 900ºC for 1 to 4 hours and cooling at a rate sufficient to preserve the annealed domain structure in the range of 100 to 850 nm B2 domain and 5 to 25 nm DO3 domain. 2. The method of claim 1, wherein said vacuum annealing step is carried out at a temperature between 1075°C and 1125°C. 3. A method according to claim 2, wherein said second annealing step is carried out at a temperature between 790°C and 860°C. 4. A method according to claims 1, 2 or 3, wherein said second annealing step is carried out in a hydrogen atmosphere. 5. A method according to any one of claims 1 to 4, wherein said cooling after said second annealing step is carried out in a hydrogen atmosphere. 6. A method according to any one of claims 1 to 5, wherein said cooling after said second annealing step occurs at a rate of between 15 and 35°C per minute. 7. Ribbon, with a slender body and transverse dimensions much less than its length, made of a Fe-Si alloy containing 6 to 7 Weight% Si, wherein said tape: essentially a < 100> texture with at least twice the random intensity, a grain size of 1-2 mm, a B2 domain size of 100 to 850 nm, a DO3 domain size of 5 to 25 nm, an AC core loss of 1.2 to 1.6 W/Kg at an induction value of B=1.4 T and a frequency of f=60 Hz, and an excitation energy of 15 to 46 VA/Kg at an induction value of B=1.4 T and a frequency of f=60 Hz. 8. A tape according to claim 7, in which said B2 domain size is 100 to 250 nm. 9. A belt according to claim 7 or 8, wherein said AC core loss is 1.2 to 1.50 W/Kg, at an induction value of B=1.4 T and a frequency of f=60 Hz. 10. A belt according to claim 7, 8 or 9, wherein said AC core loss is 1.2 to 1.50 W/Kg at an induction value of B=1.4 T and at a frequency of f=60 Hz.