Decarburization Annealing Research Articles

Microstructure and Texture Evolution of Fe-Si Steels after Hot Dipping and Diffusion Annealing

A homogenous intensity distribution along the cube texture fibre is important to achieve an easy magnetization in non-oriented electrical steels. Several alternatives have been discussed in literature to achieve this goal namely, tertiary recrystallization (surface energy controlled), decarburization annealing, two step cold rolling (strain induced boundary migration), twin-roll thin strip casting (directional solidification), phase transformation (surface energy anisotropy) and columnar grains formation (selective grain growth). In the present study, a hypoeutectic Al-Si alloy was deposited on the surface of cold rolled Fe-Si steels with a hot dipping simulator and subsequently annealed at 1000°C for different times. This procedure was developed previously in order to enrich the substrate with Al and/or Si and consequently improve their resistivity. Of specific interest was the formation of columnar grains in the low Fe-Si steel after annealing. These columnar grains were found to grow from the surface towards the centre of the substrate. The microstructure and texture in the columnar grains were significantly different than those in the middle of the material. Therefore, the evolution of these features during processing was studied in detail in this work.

Rapid quantitative analysis of fayalite and silica formed during decarburization of electrical steel

Subscales on surfaces are affected by the temperature and oxidation potential during decarburization annealing of electrical steel containing 3 wt% silicon. Knowledge of the structural and chemical properties of the surface oxide layer subscales permits the control of high‐temperature oxidation processes in the electrical steel. In the present work, the oxide layers were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectrometry, and glow discharge optical emission spectrometry (GD‐OES). The main oxide compounds formed within the subscales during decarburization annealing of the electrical steel were fayalite (Fe2SiO4) and silica (SiO2). The fayalite and silica contents were quantitatively determined by wet analysis via the galvanostatic electrolysis method, and these oxide content measurements were compared with the fayalite content determined by FTIR spectrometry and the silica determined by GD‐OES. The results determined by rapid methods and wet analysis showed good agreement. The present findings show that FTIR spectrometry and GD‐OES measurements may be used for the rapid quantitative analysis of fayalite and silica in surface oxide layers during the manufacture of electrical steel. Copyright © 2011 John Wiley & Sons, Ltd.

Internal oxidation of Fe–3%Si alloys annealed under H2O–H2 atmosphere

AbstractInternal oxidation phenomena during decarburization annealing are very important in the manufacturing process of grain oriented electrical steel. In the present study, the internal oxidation of nearly pure Fe–3%Si alloy was investigated, as a basic study for decarburization annealing. Annealing was performed at various $P_{{\rm H}_{{\rm 2}} {\rm O}} /P_{{\rm H}_{{\rm 2}} } $ ratios and at 1123 K for 300 s. The transition of external to internal oxidation took place at $P_{{\rm H}_{{\rm 2}} {\rm O}} /P_{{\rm H}_{{\rm 2}} } $ around 0.01. The morphologies of oxide in the internal oxidation zone (IOZ) were filamentary or needle‐like shapes and different from the reported oxide shapes in industrial electrical steel. These peculiar morphologies affected the kinetic aspects such as the depth and the enrichment of oxide in the IOZ.

A TEM Study of Oxide Layers Formed during Decarburization Annealing of Electrical Steel

It is very important to know the morphology and chemical properties of subscales of oxide layers on surface for controlling and understanding high temperature oxidation in electrical steel. In present work, the oxide layers were investigated by various methods of transmission electron microscopy (TEM) such as scanning transmission electron microscopy (STEM), nanobeam electron diffraction (NBD), energy dispersive X-ray spectrometry (EDS), and electron energy loss spectrometry (EELS). The high-angle annular dark field (HAADF) of STEM could be a useful analysis technique to study the morphology of the oxide layers. The main oxides formed in the subscales during the decarburization annealing were fayalite, iron oxides, and silica, which were identified by EDS, NBD and EELS. The crystalline fayalites were found both in the surface region within several tens nanometers and in the region within a micrometer surrounding silica, and the atomic configuration in the unit cell of the fayalite was presented. Amorphous silica was formed both in the upper region of the subscales with a spherical shape and in the interface between the spherical silica and the iron matrix with a lamellar shape. TEM could be useful technique to characterize morphologies, microstructures and elemental compositions of oxides, and to understand the oxidation mechanism for the manufacture of the high quality electrical steel.

Effect of Hot Band Annealing on the Microstructure and Mechanical Properties of Low Carbon Electrical Steels

AbstractMagnetic properties of grain non-oriented low-C electrical steels are improved when the hot rolled strip is annealed (HBA) prior to cold rolling and final annealing treatments. This improvement results from development of a {100}<uvw> texture in the large grained ferrite microstructure produced during final annealing. HBA at 800–850 °C results in rapid decarburization and elimination of carbide particles which have caused concerns about the suitability of the mechanical properties in the final product. In this work, samples taken from a hot rolled electrical steel coil are subjected to HBA during 150 minutes at 850 °C, cold rolled and finally annealed three minutes at temperatures between 700 and 1000 °C. The resulting tensile properties are compared with those of samples subjected to a similar processing route but without the HBA treatment and samples of industrially semi-processed grain non-oriented electrical steel decarburized 16 hours at 750 °C. It is shown that the yield strength of samples with and without HBA depends on the final grain size according to the Hall-Petch relationship; the final grain size depends strongly on annealing temperature. However, the HBA treatment causes the strength to decrease by a factor of about 2.5 and the ductility to increase by a factor of about 1.5. It is observed that the microstructure and tensile properties of the semi-processed electrical steel subjected to a final decarburization annealing are identical to those observed in material subjected to HBA in the present work. These results indicate that the HBA treatment not only improves the magnetic properties but also leads to a significant reduction of production time for grain non-oriented electrical steels.

Effect of the temperature and dew point of the decarburization process on the oxide subscale of a 3% silicon steel

The oxide subscale formed on the decarburization annealing of 3% Si–Fe was investigated using microscopy and spectroscopy techniques. It was found that the morphology as well as the molecular structure of the subscale are affected by temperature and dew point. The results suggest that there is an optimum level of internal oxidation and an optimum fayalite/silica ratio in the subscale to achieve a oriented grain silicon steel having a continuous and smooth ceramic film and low core loss.

Formación de microporosidad interna en polvos de acero inoxidable: cinética y mecanismo

The internal microporosity of stainless steel powders is obtained by a technology developed in the Metallurgical Research Center (CIME) in collaboration with ISPETP, which consists of carbon enrichment of alloy during the fusion process, and after powder atomization a subsequent decarburization annealing. The internal microporosity, which can reach up to 10 volume percent of the steel particle, reduces powder density and improves powder compressibility, while costs for technology installation are also reduced. In this paper the technology for obtaining the microporosity, the mathematical models of the process, and the structural transformations undergone by stainless steel powder are shown. It is concluded that for carbon contents lower than 0.05% internal microporosity tends to disappear.

Influence of heat treatment before secondary recrystallization on magnetic properties of 3% Si grain-oriented steel containing Sb

The secondary recrystallization of 3% Si grain-oriented steel containing Sb has been investigated. The mechanism of the improvement of the magnetic properties under the conditions of Sb addition and a constant temperature treatment during secondary recrystallization has been clarified by three experiments. It has been found that (1) Sb segregation in grain boundaries during decarburization annealing retards decarburization and grain growth, so that the {110} intensity increases and the grain size decreases with increasing Sb; (2) the constant temperature treatment for final annealing enhances the Sb segregation in grain boundaries, and therefore maintains the {110} intensity in the specimen with the addition of Sb; and (3) the addition of Sb increases the inhibiting force. These three effects enable secondary recrystallization such that many (110)[001] nuclei grow rapidly, surrounded by small primary grains, leading to high B 8 values.

Rapid tribological data generation for PM materials using mapping technique and atlas : T.S. Eyre et al (Brunel University, Uxbridge, UK)

The influence of decarburization annealing parameters on two heat samples with A1N as inhibitor with different nitrogen contents has been studied. The obtained results support the hypothesis that the increased surface oxidation at decarburization with higher dewpoint enables higher nitridation in the initial phase of final annealing in the nitrogen containing atmosphere, and therefore it intensifies the inhibition of the primary grain growth. The nitridation has a positive influence on heats with relatively low N contents.

Decarburization annealing of grain-oriented silicon steel with AlN as inhibitor

The influence of decarburization annealing parameters on two heat samples with A1N as inhibitor with different nitrogen contents has been studied. The obtained results support the hypothesis that the increased surface oxidation at decarburization with higher dewpoint enables higher nitridation in the initial phase of final annealing in the nitrogen containing atmosphere, and therefore it intensifies the inhibition of the primary grain growth. The nitridation has a positive influence on heats with relatively low N contents.

The effects of decarburization annealing on the microstructure and magnetic properties of semiprocessed motor lamination steels

Epstein packs of five compositions of steel were decarburized in exothermic gas for various times at either of two temperatures and dewpoints. The rate controlling process for decarburization was identified. Moreover, the variables affecting and the values of the optimum magnetic properties were determined, as were the times for optimum annealing. In addition, the effects of Sb on optimum decarburization time and properties were noted and explained.

The microstructure change of low-carbon electrical steels by residual aluminum

The fully processed low-carbon electrical steels are generally fabricated through vacuum degassing to reduce the carbon level and to avoid the need for any further decarburization annealing treatment. This investigation was conducted on eighteen heats of such steels with aluminum content ranging from 0.001% to 0.011% which was believed to come from the addition of ferroalloys.The sizes of all the observed grains are less than 24 μm, and gradually decrease as the content of aluminum is increased from 0.001% to 0.007%. For steels with residual aluminum greater than 0. 007%, the average grain size becomes constant and is about 8.8 μm as shown in Fig. 1. When the aluminum is increased, the observed grains are changed from the uniformly coarse and equiaxial shape to the fine size in the region near surfaces and the elongated shape in the central region. SEM and EDAX analysis of large spherical inclusions in the matrix indicate that silicate is the majority compound when the aluminum propotion is less than 0.003%, then the content of aluminum in compound inclusion increases with that in steel.

Reactions during decarburization annealing of electrical steel

AbstractThe decarburization and surface oxidation reactions were observed during the decarburization annealing cycle of a 3 wt–%Si regular grain oriented electrical steel. Annealing was carried out at four H2O/H2 ratios in the range 0·14–0·7 and three temperatures (787, 843, and 899°C) for time periods of up to 30 min. Gravimetric, chemical, and microstructural analyses indicate that the oxidation and decarburization reactions are kinetically related; e.g. at lower H2O/H2 ratios, less total oxidation and decarburization are observed at the higher soak temperatures. This behaviour is reversed as the ambient H2O/H2 ratio is increased. The microstructural analysis indicates the presence of an amorphous phase and finely dispersed particles of silica (SiO2) and fayalite (Fe2 SiO4) in the scale. These data were analysed on the basis of the transition between internal and external oxidation and the data compared favourably with values predicted by existing theories.MST/139

Reactions during decarburization annealing of electrical steel

Reactions during decarburization annealing of electrical steel

Reduction and decarburization annealing of atomized powders in converted gas

1. Laboratory experiments have demonstrated the feasibility of reducing a stationary layer of cast iron powder at a specific load of 14.2 g/cm2 (bed height 40 mm). Reduction under these conditions is possible thanks to the high porosity of the resultant sponge, amounting, according to measurements, to 60–65%. 2. Use of converted natural gas widens the permissible range of fluctuation of the O/C ratio in starting cast iron powders to ±0.1, compared with ±0.02 for the self-annealing process, and enables iron powders to be produced by a single-stage process meeting the oxygen content requirements of GOST 9849-74 standard for grade PZh3. 3. To lower the carbon potential of converted gas (without decreasing its reducing power) and obtain iron powders meeting the carbon content requirements of GOST 9849-74 standard for grade PZh3, it is necessary to raise the temperature in the last zone of the furnace., This may be accomplished by suitably changing the positions of the burners or by reversing the direction of flow of the gas (concurrent flow).