2013 Volume 53 Issue 7 Pages 1211-1214
A fine dispersion of precipitates is a key requirement in the manufacturing process of Fe-3%Si grain oriented electrical steel. In the production of high permeability grain oriented steel precipitate particles of copper and manganese sulphides and aluminium nitride delay normal grain growth during primary recrystallization, causing preferential growth of grains with Goss orientation during secondary recrystallization. The sulphides precipitate during the hot rolling process. The aluminium nitride particles are formed during hot rolling and the hot band annealing process. In this work AlN precipitation during hot deformation of a high permeability grain oriented 3%Si steel is examined. In the study, transfer bar samples were submitted to controlled heating, compression and cooling treatments in order to simulate a reversible hot rolling finishing. The samples were analyzed using the transmission electron microscope (TEM) in order to identify the precipitates and characterize size distribution. Precipitate extraction by dissolution method and analyses by inductively coupled plasma optical emission spectrometry (ICP-OES) were used to quantify the precipitation. The results allowed to describe the precipitation kinetics by a precipitation-time-temperature (PTT) diagram for AlN formation during hot rolling.
Electrical steels are used in electric power applications, typically as magnetic core materials for transformers, electric motors and generators. The sharp texture in grain-oriented electrical steels is developed through secondary recrystallization. A basic metallurgical principle of secondary recrystallization is the inhibition of normal grain growth by the second phase particles present during primary recrystallization.1)
It is well known that manganese sulfide (MnS), copper sulphide (CuS) and aluminum nitride (AlN) have been extensively used grain growth “inhibitors” in electrical steels. The morphology, volume fraction and particle size distribution of precipitates have considerable importance in improving the final texture and, therefore, the magnetic properties of grain oriented steels. As large particles only exhibit a very small pinning effect on grain boundaries, it is important to know how to produce a fine dispersion of precipitates during hot rolling and first annealing. Thus, detailed and clear description of precipitation at high temperatures is of great interest.2)
The most crucial point in manufacturing grain oriented silicon steel with high permeability, using AlN as the inherent inhibitor is to ensure that the AIN is finely precipitated in the processes from steel making through hot-rolled sheet annealing.
The solubility product of AlN in silicon-steel, both in ferrite3) and austenite,5) have been described in the literature and are expressed by Eqs. (1) and (2), respectively:
| (1) |
| (2) |
From a microstructural point of view, the diversity of AlN precipitate morphologies and the existence of two crystallographic phases: stable hexagonal (wurtzite) and metastable cubic structures.
Generally, it was observed that in steel, aluminum nitrides nucleate into transition coherent cubic structure in order to decrease the nucleation energy barrier via the interfacial term and during their growth they could transform into hexagonal (wurtzite) structure.4)
Precipitation of AlN in silicon steel have been studied by Iwayama and Haratami6) and by Oh.7) In this work the precipitates formed during hot deformation were characterized and the AlN precipitation kinetics was investigated. The results are discussed and compared to those obtained by Iwayama and Haratami6) and Oh et al.7)
Transfer bar samples of Fe-3%Si Steel, which chemical composition is shown in Table 1, were used to prepare compression specimens with 90 mm in height and 10 mm in diameter. The specimens were initially heated to 1370°C for 30 min and rapidly cooled in cold water. To prevent oxidation during heating and soaking the samples were sealed in quartz tubes under vacuum. Compression tests were performed in a Gleeble 3500 machine as schematically shown in Fig. 1. The tests were designed to study precipitation under conditions that simulate a reversible hot rolling finishing: The specimens were heated in the Gleeble machine to 1350°C for 5 minutes, cooled at 25°C/s to the test temperature (1200°C, 1100°C, 1000°C and 900°C), held for 3 s for equalization before the first deformation pass; deformation of 40% was applied at a rate of 0.13 s–1, followed by soaking at the test temperature for different soaking times (1, 10, 100, 1000 s) before the second deformation of 40% was applied, at the same deformation rate, followed by fast cooling in water.
| Element | Si | C | N | Al | S | Mn | Cu |
|---|---|---|---|---|---|---|---|
| %At | 3.028 | 0.069 | 0.0094 | 0.0216 | 0.0272 | 0.0560 | 0.0941 |
Annealing curve and thermomechanical treatment schedule for compression testing.
In order to characterize the precipitates formed during the thermo-mechanical treatments, high resolution transmission electron microscopy observations were performed on carbon extraction replicas. Samples were prepared using standard techniques. The observations were conducted in a field-emission gun JEOL 2200FS and Tecnai G2 F20 microscopes operated at 200 kV, equipped with a slow-scan CCD camera and energy dispersive X-ray spectroscopy (EDS). To quantify the precipitation of AlN, precipitate particles were extracted by chemical etching, dissolved in acid solution and analysed by inductively coupled plasma optical emission spectrometry (ICP-OES).
For all test conditions the precipitate particles were identified by EDS analysis and selected area diffraction. For each kind of particle, sulphide, nitride or co-precipitate of sulphide and nitride, the particle size distribution was obtained from around 200 particles analysed per sample.
Figure 2 shows the extraction replica micrograph and EDS spectrum of a sample deformed at 900°C with 1000 s holding time between deformations. There are particles with spherical morphology and particles with spherical morphology associated with a cubic morphology. The particles with spherical morphology were identified as copper sulphide (CuxS) precipitates and the particles with spherical morphology associated with a cubic morphology were identified as CuxS+AlN precipitates, where the spherical part corresponds to the CuxS and the cubic morphology to AlN. It is possible to observe also a difference in contrast between CuxS and AlN precipitates: CuxS shows darker contrast with the carbon layer whereas AlN contrast is weak. At 900°C similar precipitates were observed even at 1 s soaking time between deformations, showing that under the test conditions of the present work incubation time for AlN precipitation at 900°C is below 1 s. The frequency of CuxS+AlN particles increased with soaking time and the average particle size was 50 nm.
TEM carbon extraction replica micrograph showing precipitation of CuxS+AlN (a) and EDS spectrum of the CuxS+AlN particles (b).
Selected area diffraction has shown that the CuxS particles are hexagonal close-packed with lattice parameters a=0.3794 nm and c=1.6341 nm; and that the AlN particles are face centred cubic crystals with lattice parameter a=0.3956 nm, see Fig. 3.
Micrograph of CuxS+AlN precipitate (a), EDS spectrum (b) and diffraction pattern ((c) and (d)) for the sample performed at 900°C.
At 1000°C test temperature the spherical particles were identified as CuxS and (Cu,Mn)S. The later tended to increase in frequency with soaking time. The AlN precipitates appear associated with CuxS, as described before, and with (Cu,Mn)S. Nitride precipitates, associated with sulphides, were observed even at 1 s soaking time, but the frequency of such precipitates increased with soaking, particularly above 100 s soaking. Average AlN co-precipitate particle size increased with soaking time, from 39 nm at 1 s soak to 73 nm at 1000 s soak.
By selected area diffraction to kinds of copper sulphides were identified: Hexagonal close-packed as described above, and face centred cubic with lattice parameter a=0.5582 nm. Based on average EDS results the face centred cubic particles could be describe as Cu1.8S. The (Cu,Mn)S precipitates also showed face centred cubic structure with lattice parameter a=0.559 nm.
The amount of Al as precipitate, measured by dissolution and ICP-OES analysis, for the different test temperatures and soaking times, is shown in Fig. 4. To estimate the fraction of precipitation for the different test temperatures and soaking times, in relation to the equilibrium, it was assumed that Al in the precipitates is present only in AlN particles. The amount of Al as AlN in the equilibrium was calculated from Eqs. (1) and (2) and of the equilibrium percentages of ferrite and austenite at the different temperatures. The equilibrium percentage of Al as AlN precipitate, as a function of temperature, is also shown in Fig. 4. For high test temperatures and long soaking times the values of Al as AlN tend to approach the calculated equilibrium values. According with Fig. 4, AlN precipitation starts at 1124°C.
The content of Al as AlN precipitate for the different test temperatures and soaking times.
Figure 5 shows the PTT curve for aluminum nitride obtained in this work, showing start precipitation and 50% precipitation, compared with precipitation start obtained by Haratani6) and by Oh.7) In order to elaborate this curve it was calculated the ratio between the values of Al as AlN precipitate found by ICP-OES analyses and the values of Alp calculated as described earlier.
PPT curves for AlN in 3%Si–Fe obtained from this work.
For a temperature of 900°C, the high dislocation density produced with the initial deformation and the lower recovery rate propitiates a higher nucleation rate, hence the higher fraction precipitated for permanence time of 1 s and 10 s. As the growth rate of particles is lower at 900°C than at 1000°C, because of the lower diffusion coefficient, and the volumetric fraction of precipitate on the equilibrium is higher (900°C), for long permanency time the percentage precipitated is lower than 1000°C.
The shortest precipitation time in Haratani’s work was 14 s at 1150°C; in Oh’s work, 95 s at 1000°C; in the present work, less than 1 s at a temperature lower than 900°C.
The differences observed between the present work and the works of Haratani6) and Oh7) can be explained based on chemical composition of silicon steels used in each case, the use of deformation to induce precipitation and the deformation rates applied in the experiments.
Precipitation temperature is determined by solubility product and the relative amounts of ferrite and austenite in the structure. Although the steel used in the present experiment had higher solubility product than in references6) and,7) it had higher carbon content and so higher percentage of austenite. The equilibrium temperatures for start precipitation, calculated based on Eqs. (1) and (2) and the equilibrium fractions of ferrite and austenite, as explained above, were 1288°C, 18.65% obtained by Haratani and 1160°C, 27.50% obtained by Oh.
The nitride particles observed in the present work were always associated with sulphide particles, particularly CuxS, that tended to precipitate over the AlN particle. A more detailed description of sulphide precipitation kinetics will be the subject of another paper.
- The PTT diagrams determined by the present compression test for AIN precipitation in the dual-phase 3% Si electrical steel are generally C-shaped, with the nose located under at 900°C, for time lower than 1 second, and its precipitation doesn’t finish before 1000 seconds of test.
- The precipitation start curve obtained here is moved to shorter times because the chemical composition of silicon steels used and the use of deformation to induce precipitation and the deformation rates applied in the experiments,
- CuS precipitate has a hexagonal close-packed structure and AlN has a face centered cubic structure.
