3.1. Effect of Rare Earth Y on the Type and Morphology of Inclusions in Hot-Rolled Plates and Normalized Plates
The statistical results of the type and quantity of inclusions in Y-bearing and Y-free hot-rolled and normalized plates are shown in
Table 2 and
Table 3, where N represents the total number of inclusions per unit area, and the d
mean is the average size of inclusions in the corresponding silicon steel plate. The analysis shows that the inclusions in the test steel without rare earth Y are mainly MnS, Al
2O
3 single-phase inclusions, and Al
2O
3-MnS composite inclusions. With the progress of rough rolling, finishing rolling, and normalizing process, the total number of inclusions in the test steel increases gradually, but the type of inclusions remains the same. With the addition of rare earth Y, the amount of rare earth sulfur oxide inclusions in the test steel increases greatly, the Al
2O
3-MnS type inclusions are transformed into Y
xS
y-Y
2O
2S rare earth inclusions, and the single-phase MnS inclusion disappears. In addition, there are a small amount of large-sized Al
2O
3 inclusions.
Figure 2 shows the composition and morphology of inclusions in rough-rolled plates without Y. Elliptical MnS inclusions and irregular Al
2O
3-MnS type inclusions are mainly found in the rough-rolled plates. Due to the high solid solution temperature of MnS, the size of MnS inclusions in the rough-rolling stage is relatively large, with most of the MnS sizes being around 2 μm. The solid solution temperature of Al
2O
3 is very high. Before the steel liquid solidifies, Al
2O
3 already exists. During the cooling process of the steel liquid, the particles that precipitate later are easy to precipitate and grow with the existing particles as the core. Therefore, a large part of the inclusions in the rough-rolled plate are composite inclusions with Al
2O
3 as the core; as the size of Al
2O
3 as the core itself is about 2 μm, the generated composite inclusions are larger in size, around 5 μm. Due to the small reduction in rough rolling, the deformation of inclusions is also less affected.
Figure 3 shows the composition and morphology of inclusions in the Y-free precision rolled steel plate. Compared with the rough-rolled plate, the main types of inclusions have not changed, mainly consisting of MnS inclusions and Al
2O
3-MnS composite inclusions. Unlike rough-rolled plates, MnS inclusions change from elliptical to elongated shapes because MnS has high plasticity and extends with deformation under large deformation variables. The Al
2O
3-MnS composite inclusion is formed by Al
2O
3 as the core, which has the characteristic of high hardness. Under large reduction, the Al
2O
3-MnS composite inclusion does not deform and its size does not change.
Figure 4 shows the composition and morphology of inclusions in a normalized plate without Y. The inclusions in the normalized plate after normalization treatment are mainly elliptical MnS and irregular Al
2O
3-MnS. Unlike rough and finish-rolled plates, MnS inclusions are finer, with a size of around 600 nm. This is because during normalization treatment, as the temperature increases, MnS solidly dissolves into the austenite phase, and as the temperature decreases, fine and dispersed MnS precipitates. However, there are still some large-sized Al
2O
3-MnS composite inclusions remaining in the normalization plate, which is because the lower normalization temperature did not completely dissolve MnS.
With the addition of rare earth Y, the composition and morphology of inclusions in the steel are shown in
Figure 5. The inclusions in rough-rolled plate, finish-rolled plate and normalizing plate are all transformed into spherical or ellipsoidal Y
xS
y-Y
2O
2S type inclusions. There are no MnS or MnS-Al
2O
3 type inclusions in Y-bearing silicon steel. Rare earth Y is easy to combine with oxygen and sulfur in molten steel to form spherical yttrium sulfide, yttrium sulfur oxide or other type inclusions containing yttrium, which can effectively inhibit the formation of long strip MnS inclusions in the steel. Due to the strong deoxidization ability of rare earth, most of the Al
2O
3 inclusions in steel are also modified into rare earth sulfur oxides. Spherical or ellipsoidal rare earth sulfides are dispersed in the steel matrix and maintain their original morphology during rolling [
20,
21,
22,
23,
24]. The residual Al
2O
3 inclusions have a larger size and a strip-shaped morphology, with a relatively small quantity, which will not affect the mechanical properties of the steel.
The morphology and composition of micro inclusions in the hot-rolled plate of the test steel were observed by TEM and EDS. The results are shown in
Figure 6 and
Figure 7. Spherical MnS-SiO
2 composite precipitates with a size less than 200 nm are mainly found in Y-free steel. From the element distribution diagram, it can be seen that the distribution of S element is relatively high, and MnS precipitates with SiO
2 as the core composite. In the Y-bearing steel, there are spherical particles with a size of about 20 nm, which are distributed in an aggregated state. The EDS analysis results show that the particles are mainly composed of O, Si, S, Mn, and Y. No Al containing particles and single fine MnS precipitation were found in the experimental steel [
25]. From the distribution maps of various elements, it can be seen that small inclusions adhere to the SiO
2 matrix, with a relatively low content of Mn element. The aggregated small particles are mostly rare earth oxide sulfides.
3.2. Effect of Rare Earth Y on Inclusions Composition in the Hot-Rolled Plate and Normalized Plate
Inclusion data in 30 consecutive fields of view were collected using a field emission scanning electron microscope (FE-SEM) with a magnification of 500 times. The density of inclusions is expressed by
N, as shown in Formula (1), that is, the number of inclusions within a certain size range detected in the unit field of view area of the sample, unit: pieces/mm
2. Among them, the
Nt represents the total number of inclusions in all fields of view, unit: pieces, and
At represents the total area of all fields of view, unit: mm
2. The inclusion area fraction is expressed by the percentage of the inclusion area to the total field of view area in a certain size range detected in the steel, % [
11,
26].
The quantity density and area fraction of inclusions in different size ranges in the rough-rolled plates are shown in
Figure 8. With the addition of rare earth Y, the number of inclusions in the rough-rolled plate decreased as a whole, in which the inclusions in the size range of 0.5~2 μm decreased by 29%. However, the number of larger size inclusions increased significantly, and the inclusions in the size range of 2~4 μm increased by about 1.5 times. Compared with the samples without rare earth Y, the area fraction of inclusions in Y-bearing steel increases generally. The area fraction of inclusions smaller than 4 μm is 0.171%, and the area fraction of inclusions larger than 4 μm is 0.086%.
The quantity density and area fraction of inclusions in different size ranges in the finish-rolled plate are shown in
Figure 9. With the addition of rare earth Y, the number of inclusions in the finish-rolled plate is greatly reduced, in which the number of inclusions within the size range of 0.5~2 μm is reduced by 48%, and the number of inclusions within the size range of 2~4 μm is increased by 14%. The area fraction of inclusions smaller than 4 μm in finish-rolled steel plates containing yttrium decreased to some extent. However, there are many inclusions with a size larger than 4 μm in the Y-bearing finish-rolled plate. The area fraction of inclusions in the Y-bearing finish-rolled plate increased obviously. Compared with the samples without rare earth, the area fraction of inclusions in Y-bearing steel is obviously higher.
The quantity density and area fraction of inclusions in different size ranges in the normalized plate are shown in
Figure 10. With the addition of rare earth Y, the number of inclusions in the normalized plate is still reduced overall, among which the number of small size inclusions in the range of 0.5~2 μm is reduced by 38%, and the inclusions in the range of 2~4 μm are reduced by 11%. Similar to the hot-rolled plate, there are still many inclusions larger than 4 μm in the Y-containing normalizing plate. These large-sized inclusions lead to a significant increase in the area fraction of inclusions in normalized plates containing yttrium. Compared with the samples without rare earth, the area fraction of inclusions with the size of 0.5~2 μm reduces by 0.069%, with the size of 2~4 μm increases by 0.017%, with the size of 4~6 μm increases by 0.064%, and that of the inclusions larger than 6 μm increases by 0.048%. The addition of rare earth yttrium reduces the area fraction of small-sized inclusions and increases the area fraction of large-sized inclusions.
The addition of rare earth yttrium inhibits the nucleation of small-sized inclusions such as MnS and Al2O3, and reduces the number and area fraction of small-sized inclusions in the steel. At the same time, large-sized rare earth containing inclusions are formed in the steel. With rough rolling, finishing rolling, and normalization progress, the number of small-sized inclusions in Y-free steel increases significantly. This is due to the uniform and fine precipitation of MnS during the rolling process, and the number of large-sized inclusions has no obvious change. According to the statistical results of inclusions area fraction, the area fraction of small-sized inclusions in Y-free steel increases obviously with the progress of the rolling and annealing processes. The number of inclusions in Y-bearing steel increases slightly in the three processes, and the area fraction of small-sized inclusions decreases, while that of large-sized inclusions increases. The author believes that this is due to the heating treatment of different processes, which increases the size of rare earth inclusions within a certain size range. In general, due to the relatively large amount of rare earth in the experimental steel, the number of inclusions larger than 4 μm increases, but the average size of the inclusions only increases about 0.60 μm.
3.3. Morphological Characteristics of Inclusions in Test Steel after Solid Solution Heat Treatment
Samples with the size of 10 mm × 8 mm × 8 mm were taken from two kinds of test steels. Put the sample into a 1350 °C tube furnace, keep it for 20 min, and then put it into cold water to cool to room temperature. The morphology of inclusions is shown in
Figure 11 and
Figure 12. The inclusions in Y-free steel are mainly irregular Al
2O
3 and strip or spherical MnS-Al
2O
3 inclusions, and no MnS inclusions are detected. This is because the solution temperature of MnS in the steel is about 1320 °C, and the sample is heated to 1350 °C and kept for 20min in the experiment, which made MnS a solid solution again. The inclusions in the Y-bearing steel are still spherical YxSy-Y
2O
2S inclusions, and the high-temperature heating does not affect the inclusion types of the Y-bearing steel.
The quantity density and area fraction of inclusions in the rough-rolled plate heated at high temperature are shown in
Figure 13. Compared with Y-bearing steel and Y-free steel, the number of small inclusions in the size range of 0.5~2 μm in the Y-free steel is reduced by 41%, and inclusions in the size range of 2~4 μm are reduced by 62%. However, there are still many large-sized inclusions in the Y-bearing steel, which are larger than 4 μm. The inclusions area fraction of Y-free steel is also significantly lower than that of Y-bearing steel. According to
Figure 6 and
Figure 11, since most MnS inclusions in the steel are redissolved at high temperatures, the number and area fraction of inclusions in the Y-free steel are significantly reduced. However, the area fraction of inclusions in Y-bearing steel increases slightly, which is consistent with the results of the above experiments. The high heating temperature before hot rolling makes the MnS and other inhibitors in the grain-oriented silicon steel dissolve again, and fine particles are precipitated during hot rolling. With the addition of rare earth yttrium, the inclusions in steel are transformed into high melting point rare earth inclusions due to the denaturation effect of rare earth on inclusions. With the high heating temperature at 1350 °C, only the area fraction of inclusions in Y-bearing steel increased slightly.
3.4. Influence of Rare Earth Y on the Formability of Hot-Rolled Plate
Figure 14a,b represent the local average orientation difference (LAM) of hot-rolled plates, Y-free and Y-bearing, respectively. By scanning the orientation difference between adjacent data points in the grain using EBSD, and statistical analysis of LAM, the orientation change inside the grain of plastic deformed metal after deformation is analyzed. The small angle orientation difference between grains can form substructure grain boundaries and dislocations, so although dislocation density cannot be directly measured, the relative size of dislocation density can be measured through statistical LAM [
27]. The dislocation density increases with the increase of the deformation variable, so the LAM value is indirectly positively correlated with the dislocation density, that is, the higher the overall dislocation density, the greater the LAM value.
The LAM values are shown in
Figure 15, where the abscissa represents the LAM value and the ordinate represents the relative frequency. The peak values of LAM of hot-rolled plates without and containing Y appeared at 1.05° and 0.75°, respectively, and the average values of local orientation difference were 1.72° and 1.40°, respectively. This shows that the number of data points with higher local misorientation decreases after adding rare earth. There are strip MnS inclusions and irregular Al
2O
3 inclusions in the traditional hot-rolled plate of grain-oriented silicon steel. Due to the presence of these inclusions, when the interface deformation is incompatible with the matrix, dislocation accumulation is easy to occur at the interface, which makes the stress and strain distribution more uneven [
28]. Stress concentration areas are easily formed at the interface. In addition, there is an area between the inclusion and the steel matrix where the stress gradient is not obvious. If the inclusion is close enough, that is, the number density of the inclusion is relatively high, the stress fields around the inclusion may interact and cause stress concentration [
29,
30]. With the addition of rare earth, the inclusions such as MnS and Al
2O
3 in the hot-rolled plate are transformed into spherical rare earth compounds, and the number of small- and medium-sized inclusions in steel is significantly reduced. This improves the mechanical properties of the hot-rolled silicon steel plate and avoids the cracking problem caused by dislocation accumulation during the hot-rolling process.