Investigation of microstructure and texture as related to primary recrystallization of grain-oriented silicon steel

Grain-oriented silicon steel requires a very strong Goss {110}<001> texture and as such a comprehensive control of thermo-mechanical processing parameters is required to achieve a strict control of texture. In particular the cold rolling and decarburization annealing process parameters have a determining effect on the grain size, the grain size distribution and the crystal texture distribution. In this study, the effect of cold rolling reduction on texture distribution in cold-rolled sheet has been analyzed. Secondary recrystallization of grain-oriented silicon steel is also highly dependent on the microstructure and texture after primary recrystallization. The influence of annealing temperature and carbides on the primary recrystallization process has also been investigated by measurements of grain growth and texture following primary recrystallization. By optimizing the microstructure and the texture of primary recrystallization of grain-oriented silicon steel, it is shown that a grain-oriented silicon steel exhibiting high magnetic induction low iron loss has been developed at Shougang Steel.


Introduction
Grain-oriented silicon steel is a important soft magnetic material used for efficient transfer of electrical energy to magnetic energy.A high-degree of control of secondary recrystallization is crucial for the improvement of magnetic properties of grain-oriented silicon steel, and as such a comprehensive understanding of the full thermo-mechanical processing schedule is required in order to achieve a strict control of the final texture.Many studies [1][2][3][4] have been carried out on secondary recrystallization where this recognized to be influenced by many factors, including inhibitor strength, cold rolling parameters, the primary recrystallization structure and texture, and the high-temperature annealing temperature and atmosphere.In particular, the microstructure and texture after primary recrystallization have an important influence on subsequent secondary recrystallization during the high temperature annealing process [5].
In the present investigation the grain-oriented silicon steel undergoes decarburization annealing treatment after high reduction and aging rolling on twenty-high roll mill.The effect of reduction rate and aging treatment during cold rolling both have an influence on the primary recrystallization during decarburization annealing [6].Moreover, the heating rate, annealing temperature and time of decarburization annealing also have an influence on the primary recrystallization microstructure and texture [7].Here we focus on research into the effects of different cold rolling reductions and of the   The inherent non-uniformity in plastic deformation results also in a non-uniform cold-rolled microstructure in the form of deformation bands, transition bands, shear bands, etc.The dislocation density in the sheet after high reduction rolling is very high, resulting in a high stored energy of deformation.Local spatial variation, both within and between texture components, in the stored energy of deformation will affect the nucleation of recrystallization and the growth of recrystallization nuclei.The experimental finding of a shift in dominant orientation with increasing cold rolling reduction, from {211} <011> to {100} <011>, is in agreement with previously reported observations [8].
Goss nuclei mainly exist in shear bands caused by intense local shear deformation.Such shear bands represent regions of non-uniform deformation undergoing intense local shearing, i.e., a concentrated local area of shear strain.Shear bands are common in the plastic deformation of BCC metals and the formation of shear bands is influenced by many factors, including the initial grain size, the grain orientation, and the deformation temperature.The volume fraction and average size of Goss, {110} <001>, texture regions after different levels of cold-reduction are shown in figure 3.With increasing amount of cold-rolling reduction, the Goss texture fraction shows a decreasing trend, and remains stable after 87% reduction.The average size of Goss regions tends to decrease with increasing cold-rolling reduction, and also remains stable after 87% reduction.A more detailed investigation shows that the local texture around Goss regions has a significant intensity close to the {111} <112> component.

Recrystallization and texture evolution after decarburizing annealing
Cold-rolled sheet samples with a final thickness of 0.27 mm were cut into 30 × 100 mm (RD) samples for laboratory-based decarburization annealing.Decarburization temperatures of 750 ℃, 780 ℃, 790 ℃, 800 ℃, 810 ℃, 820 ℃, 830 ℃, 840 ℃, 850 ℃, 860 ℃, 870 ℃, 880 ℃, 900 ℃, and 920 ℃ were investigated, in case each with a soaking time of 120 s.Carbon analysis was carried out using a LECO CS844 instrument.Texture and microstructure investigations were carried out based on EBSD measurements (using an Oxford Instruments system coupled to a Symmetry S2 detector), with metallographic examination carried out in a cross section of each sample defined by the rolling and normal directions.
The main factors affecting the decarburization rate are the oxidation energy, the decarburization temperature and time, and the silicon content.Decarburization annealing is carried out in a wet hydrogen atmosphere where a higher dew point of the furnace atmosphere results in a higher decarburization rate.Oxidation of Al, Si, and Mn takes place in a humid atmosphere, forming a dense internal oxide layer of Al 2 O 3 and SiO 2 , thereby reducing the decarburization rate.As such the decarburization rate is fast in the initial stage of the annealing process, both because the carbon content is high (driving a fast diffusion rate of carbon) and as there is no internal oxide layer formation.The variation in carbon content for samples annealed at different decarburization temperatures under laboratory conditions is shown in figure 4. The carbon content is below 30 ppm in the sample annealed at 920 °C, which meets the product requirement.Because recrystallization and grain growth involve the migration of high angle grain boundaries migration, significant changes in texture take place after primary recrystallization.The primary recrystallization texture depends on various metallurgical parameters including the chemical compositions, the cold-rolling temperature and reduction level, the heating rate during annealing, and the heating temperature and annealing time.
Figure 5 shows the typical microstructure after decarburization annealing at 820 °C, 840 °C, 860 °C, 880 °C, and 900 °C.It can be seen that the primary recrystallization texture contains a strong {114} <418> texture component, and that the {111} <112> texture strength is significantly enhanced compared to the as-deformed state.A statistical analysis of the proportion of important textures after primary recrystallization shows that the volume fraction of the {114}<418> components always largest, the {111} <112> and {100}<021> components always either 2 nd or 3 rd largest, and the volume fraction of Goss grains is generally less than 1%.In comparison, after cold-rolling the volume fraction of the {112} <110> is always largest, with the {114}<418>, {111}<110>, and {001} <110> The nuclei during recrystallization are expected to originate from highly distorted regions of the deformed (cold-rolled) matrix.Some new {111}<112> grains nucleate at grain boundaries with {111}<110> deformed grains.The large orientation difference between {111}<112> and {111}<110> ensures that {111}<112> nuclei have a high angle grain boundary with the {111}<110> deformed matrix, which is favourable for grain boundary migration.Primary recrystallized grains of {114} <418> orientation will nucleate at the grain boundaries of deformed grains of α-fibre texture, which similarly form high angle grain boundaries with α-fibre texture volumes.As such, grains with {114}<418> texture have a growth advantage compared to any α-fibre texture nuclei during recrystallization, which leads to a texture transfer from the -fibre to the {114}<418> orientation during primary recrystallization.Several studies [9][10] have indeed shown the primary recrystallization of grain-oriented silicon steel after high reduction has strong a {114}<418> texture.
A quantitative examination of the grain sizes of grains with of different orientation shows that grains with {100}<021> and {114}<418> orientations are larger, while {111}<112> orientation are smaller than the average grain size.The proportion of {111}<112> grains decreases with increasing annealing temperature, while the proportion of {114}<418> grains increases with increasing temperature.The {114} <418> grains nucleated from α-fibre texture boundaries result in high angle grain boundaries with other α-fibre texture regions, such that the {114}<418> grains can have a growth advantage during primary recrystallization.It is notable that both {114}<418> and {111}<112> recrystallized grains can exhibit a Σ9 relationship with Goss orientation grains.As such, the grain boundary relationship of {114}<418> and {111}<112> grains with Goss grains satisfies both the coincidence site lattice (CSL) model and the high-energy (HE) boundary model, both of which have been proposed as an explanation for the abnormal growth of Goss grains with sharp orientation.

Final product texture and magnetic properties
The final high temperature annealing ensures a strong Goss {110} <001> texture following secondary recrystallization.During the early stage of the high-temperature annealing process, inhibitor particles will prevent normal growth of grains by Zener pinning.After a certain critical temperature, the inhibitors will undergo Ostwald ripening, resulting in the abnormal growth of Goss grains.The Goss deviation angle of the final product is shown in figure 6 in the form of a {200} pole figure obtained by X-ray measurements.The red boxes indicate the positions of {200} poles for the exact Goss orientation.For this product the deviation angle for Goss grains is calculated as 2.5° (where the deviation angle is obtained by comparing the strongest peak position of the sample with the ideal peak positions position in red.), and the magnetic induction, B 800, is 1.93 T.

Figure 6. {200} pole figure
showing the Goss texture for the final product.Red boxes mark the positions of the {200} poles for the ideal Goss orientation, used for calculation of the Goss deviation angle.

Conclusions 1)
With increasing cold rolling reduction the proportion of α-fibre texture gradually increases.The proportion of Goss texture, {110}<001>, decreases with increasing reduction and is stable after 87% reduction.The size of Goss crystal regions shows a decreasing trend with increasing reduction and is stable after 87% reduction.2) Primary recrystallization shows that {114}<418> grains are always largest, {111} <112>,{100}<021> grains are always 2 nd or 3 rd largest, and the volume fraction of Goss grains is generally less than 1%. 3) In the primary recrystallized state the proportion of {111}<112> grains decreases and the proportion of {114}<418> grains increases with increasing decarburization annealing temperature.The grain size of {114}<418> grains is larger than the average grain size, while the grain size of {111}<112> grains is smaller than the average grain size.4) A strong Goss texture is obtained after secondary recrystallization in the final product, deviating from the ideal Goss orientation by 2.5°.The resulting grain-oriented silicon steel has good magnetic properties of B 800 = 1.93 T.

Figure 1
Figure 1 presents texture component maps, obtained from electron back-scatter diffraction (EBSD) investigations, corresponding to the different investigated rolling reductions, showing also the variation in local microstructure.An increasing amount of cold rolling reduction, results in a greater stored energy of deformation, and also in more inhomogeneous slip deformation.Both of these factors can have an influence on the primary recrystallization during subsequent decarburization annealing.

Figure 1 .
Figure 1.Texture fraction maps for samples cold-rolled to different reductions.

Figure 2 .
Figure 2. Effect of rolling reduction on texture fraction intensity.

Figure 3 .
Figure 3.Effect of rolling reduction on the size and volume fraction (%) of Goss regions.

Figure 4 .
Figure 4. Variation in carbon content with decarburization temperature.

Figure 5 .
Figure 5. EBSD maps showing the orientations, classified into different texture components, after primary recrystallization annealing at different decarburization temperatures.

Table 1 .
Experiment parameters of samples used for investigation of cold-rolling reduction.