Effect of rare-earth yttrium doping on the microstructure and texture of hot-rolled non-oriented electrical steel

1 Introduction

Non-oriented electrical steel (NOES) is widely recognized as the most extensively used core material in electric motors, generators, and alternators due to its superior magnetic properties, including high permeability, excellent magnetization saturation, and low core losses [1,2,3,4,5]. The presence of the easy magnetization axis <001> in electrical steel makes the {100} texture particularly effective in enhancing magnetic induction. However, the γ-fiber texture (<111>∥ND) and α*-fiber texture ({11 h} <121/h>) can significantly impede domain wall motion, thereby deteriorating magnetic performance [6]. Consequently, optimizing the texture composition by reducing unfavorable texture components while enhancing beneficial ones represents a crucial strategy for improving the magnetic properties of NOES.

To address the increasing performance demands and enhance the proportion of favorable textures in NOES, researchers have conducted extensive in-depth studies focusing on the manufacturing process parameters and chemical composition. The manufacturing process of NOES is relatively complex, involving key steps such as casting, hot rolling, normalizing, cold rolling, and recrystallization annealing [7,8]. Each of these steps plays a crucial role in determining the final recrystallization texture and magnetic properties. After years of research, optimizing process parameters to improve the performance of NOES has reached a bottleneck. Moreover, the chemical composition of electrical steel fundamentally influences the performance of the final product. Currently, many researchers are investigating the composition of NOES, incorporating small amounts of alloying elements to modify its properties. For instance, Zhihao Zhang’s team [9] utilized Nb for microalloying high-silicon steel. The tiny, dispersed Nb precipitates pinned grain boundaries, inhibited grain growth, refined the grain microstructure, and disrupted the ordered rearrangement of adjacent Fe and Si atoms in the matrix. This suppression of the ordered transformation reduced the degree of order in high-silicon electrical steel, thereby improving its hot-rolling plasticity. Previous studies [10] have explored the impact of rare earth elements (REMs) on the magnetic properties of NOES. The addition of REM significantly reduced the density of MnS inclusions, thereby weakening the pinning effect of second-phase particles. This allowed grains to grow sufficiently during annealing, optimizing the recrystallization texture, and ultimately achieving an excellent combination of iron loss and magnetic induction.

In recent years, the modification of NOES by incorporating REMs has become a prominent research topic. Due to the high affinity of REMs for oxygen (O) and sulfur (S), their addition to NOES facilitates the formation of high-melting-point rare earth oxides, sulfides, and oxysulfides. This process purifies the molten iron and suppresses the precipitation of MnS. Additionally, REMs promote the spheroidization of inclusions such as AlN and Al₂O₃, thereby reducing their hindrance to grain boundary migration during recrystallization and grain growth [10]. This ultimately enhances the magnetic properties of the final product. For instance, Ren et al. [11] introduced lanthanum (La) into NOES, where La reacted with impurities to form LaAlO₃ and La₂O₂S. This significantly reduced the quantity of MnS, weakened the {111} <112> texture, and strengthened the {100} <021> texture. Similarly, Li et al. [12] doped Fe–6.5 wt%Si steel with varying amounts of yttrium (Y). They found that the addition of Y resulted in the formation of high-melting-point precipitates, which refined the microstructure during solidification, hot rolling, and annealing processes.

Given that the characteristics of REMs are highly effective even in trace amounts when doped in electrical steel, this study utilizes commercial-grade Fe–3.0 wt%Si steel as the base material and incorporates varying amounts of rare earth yttrium (Y). The research investigates the influence of the addition of Y on the as-cast microstructure and the texture of the hot-rolled microstructure, aiming to provide additional data and theoretical foundations for the production of high-quality NOES.

2 Materials and methods

In this study, 10 mm-thick ingots with varying rare earth Y contents were subjected to hot rolling at 1,050℃, reducing their thickness to 1.5 mm with a rolling reduction rate of 85%. The as-cast microstructure was observed using optical microscopy, while the grain orientation and texture of the hot-rolled microstructure were analyzed using electron backscatter diffraction (EBSD). Additionally, second-phase particles in the ingots were captured using transmission electron microscopy (TEM), and their chemical composition was determined through energy-dispersive spectroscopy (EDS). Table 1 lists the specific chemical compositions of the ingots, with Y0, Y1, Y2, Y3, and Y4 used to denote the compositions of the samples 0#, 1#, 2#, 3#, and 4#, respectively, in the following sections.

3 Results

The as-cast macrostructures of ingots with different rare earth Y contents are shown in Figure 1(a–e). Overall, the as-cast microstructure of NOES without the addition of rare earth exhibits typical coarse-grained characteristics, with grain sizes reaching centimeter scale, as shown in Figure 1(a). With the addition of rare earth Y, the as-cast microstructure is significantly refined. For the Y1 composition, most grains are refined, but some larger grains remain, indicating incomplete refinement, as shown in Figure 1(b). Under the Y2 composition, the grains are fully refined, with an average grain size of approximately 700 μm, as shown in Figure 1(c). The refinement effects of the Y3 and Y4 compositions are similar, with most grains measuring 1,100 μm, while localized regions exhibit even finer grains, as shown in Figure 1(d) and (e). By statistically analyzing the average grain sizes of the experimental steel with different compositions, as shown in Figure 1(f), it is evident that the addition of Y significantly refines the grains, reducing the average grain size from 2,000 μm to a range of 1,100–700 μm.

Figure 1

As-cast microstructure and average grain size of NOES with different Y contents: (a) Y0, (b) Y1, (c) Y2; (d) Y3, (e) Y4, and (f) average grain size.

The second-phase particles in the as-cast microstructure were characterized and analyzed using TEM, as shown in Figure 2(a)–(d). According to relevant studies, REMs exhibit high affinity for elements such as O, S, and P, and their addition leads to the formation of corresponding oxides, oxysulfides, sulfides, or phosphides [12]. After introducing Y into the ingot, new second-phase particles were formed, as shown in Figure 2(a)–(d). EDS analysis revealed that Y primarily combines with S and O to form two types of second-phase particles, Y₂O₂S and YS, which exhibit regular cubic and octahedral shapes with an average size of approximately 200 nm. Research indicates that high-melting-point second-phase particles generated by REMs can serve as nucleation substrates, increasing the nucleation rate during solidification and thereby refining the microstructure [13]. The significant grain refinement effect observed in this study is attributed to the heterogeneous nucleation effect of the newly formed Y₂O₂S and YS particles after the addition of rare earth Y. The nucleation effectiveness was quantitatively evaluated using the edge-to-edge matching (E2EM) model [14], which calculates the crystallographic compatibility of Y₂O₂S and YS particles. The results indicate that the lattice mismatch of both rare earth compounds is below the effective nucleation threshold of 10%, meeting the requirements for nucleation agents during solidification. Thus, these particles can act as nucleation agents in the melt, achieving grain refinement. However, due to the addition of low content of rare earth Y, the size of the Y-containing second-phase particles does not exhibit a clear functional relationship with the amount of Y added.

Figure 2

TEM bright field image of Y inclusions in the as-cast microstructure: (a) Y1, (b) Y2, (c) Y3, and (d) Y4.

Figure 3(a1–e3) displays the microstructure texture maps of the full thickness of hot-rolled sheets with different Y contents, with a deviation angle of 15°. The hot-rolled microstructure of NOES without the addition of Y exhibits typical characteristics of traditional NOES hot-rolled microstructures: the surface layer consists of fine equiaxed grains, with the center layer composed of elongated grains, and the subsurface layer being a mixture of equiaxed and elongated grains [8], as shown in Figure 3(a)–(1). In terms of texture, as shown in Figure (a-2), the hot-rolled sheet without the addition of Y is predominantly characterized by shear textures such as Goss and brass, with the center layer featuring deformed elongated microstructures and dominated by γ-fiber texture. In contrast, the experimental steels with the addition of rare earth Y exhibit significant differences, as shown in Figure (b1–e1). First, their as-cast grain sizes are smaller, but after hot rolling, the grains become noticeably coarser compared to those without the addition of Y, showing a transformation contrary to the conventional inheritance law of microstructure. Specifically, the fine dynamically recrystallized grains in the surface layer of the hot-rolled sheet are significantly reduced, while the subsurface and center layers consist of large elongated grains with a more uniform microstructure. Moreover, as the Y content increases, the grain size gradually increases. The addition of rare earth Y significantly alters the traditional hot-rolled texture distribution: the {100} plane texture and its variant {113} <361>, as well as the Goss texture, are significantly enhanced, while the brass texture and γ-fiber texture are relatively weakened. The macrotexture analysis results of the hot-rolled sheets, as shown in Figure 4(a) and (b), further confirm the texture regulation effect of rare earth Y: the texture without the addition of Y exhibits sharp γ-fiber characteristics, while the texture modified by rare earth Y shows a stronger brass texture. This is primarily due to the significant increase in the grain size, the substantial reduction in the number of statistically analyzed grains, and the abnormal growth trend of some oriented grains.

Figure 3

Hot-rolled EBSD images with different Y contents (IPF, several typical textures MAPPING, ODF): (a1–a3) Y0, (b1–b3) Y1, (c1–c3) Y2, (d1–d3) Y3, and (e1–e3) Y4.

Figure 4

Macro texture components of hot-rolled organization: (a) φ ₁ = 0°, φ ₂ = 45°, (b) Φ = 55°, and φ ₂ = 45°.

The quantitative statistics of the texture type proportions in the hot-rolled sheets are shown in Figure 5. Overall, in the absence of rare earth Y, the brass texture accounts for the highest proportion in the subsurface layer. After the addition of rare earth Y, the texture composition undergoes significant changes. Specifically, the proportions of shear textures such as brass and γ decrease to varying degrees, while the proportions of λ texture and {113} <361> texture increase to varying degrees.

Figure 5

Proportion of texture in hot-rolled plates.

Based on the kernel average misorientation (KAM) theory, the geometric dislocation density can be quantitatively calculated to characterize the uniformity of plastic deformation in materials. Regions with higher KAM values typically correspond to greater degrees of plastic deformation or higher defect densities. The KAM maps of hot-rolled microstructures with different Y contents are shown in Figure 6(a)–(e). The sample without the addition of rare earth Y exhibits a distinct gradient distribution of dislocation density: the surface and subsurface layers have higher dislocation density intensities, while the center layer has a relatively lower dislocation density. This distribution characteristic is closely related to the dynamic recrystallization in the surface layer and the grain recovery and growth mechanism in the center layer during hot rolling. In contrast, after the addition of rare earth Y, as shown in Figure 6(b)–(e), the dislocation density is significantly reduced, mainly concentrated in the 1–2 intensity range, but the trend of higher dislocation density in the surface and subsurface layers compared to the center layer remains. To further validate these results, the grain boundary microstructure was characterized using TEM, as shown in Figure 7(a) and (b). The sample without the addition of Y shows high-density dislocation tangles at the grain boundaries, while the addition of rare earth Y significantly reduces the dislocation density at the grain boundaries. This observation is consistent with KAM analysis, jointly confirming the positive role of rare earth Y in reducing the dislocation density and improving the material performance. This purifying effect of REMs is primarily attributed to their strong affinity for impurity elements, which reduces lattice defects by forming stable rare earth compounds, thereby lowering the dislocation density.

Figure 6

KAM diagram of hot-rolled organization: (a) Y0, (b) Y1, (c) Y2, (d) Y3, and (e) Y4.

Figure 7

TEM bright-field at the grain boundary: (a) without the addition of rare earth Y and (b) with the addition of rare earth Y.

4 Discussion

The influence of dislocation density on NOES is mainly reflected in two aspects. First, the level of dislocation density directly affects the magnetic permeability and magnetic losses of NOES. Excessively high dislocation density can hinder the movement of magnetic domain walls, thereby increasing hysteresis losses and reducing the material’s magnetic permeability. Second, dislocation density impacts the strength and toughness of NOES. High dislocation density typically indicates the presence of numerous defects within the material, which can act as crack initiation sites under stress, leading to increased brittleness and reduced resistance to impact and fracture. Reducing dislocation density can enhance the toughness and strength of silicon steel, improving its mechanical properties. After the addition of rare earth Y, the purification effect of rare earth significantly reduces the dislocation density in the hot-rolled microstructure, facilitating the production of high-quality NOES.

Regarding the hot-rolled microstructure, the microstructure of the sample without the addition of Y can be explained by the influence of microstructural inheritance. The fine equiaxed grains in the hot-rolled sheet are generated through dynamic recrystallization, while the elongated grains in the center layer are formed through dynamic recovery. The partially recrystallized state in the subsurface layer of the hot-rolled sheet results from the combined effects of dynamic recovery and recrystallization during hot rolling [15]. The surface layer, subjected to intense shear deformation by the rollers, accumulates high internal deformation energy and experiences the fastest temperature increase, meeting the conditions for dynamic recrystallization first, thus forming fine equiaxed grains after hot rolling. In contrast, the lower strain rate in the center layer results in lower stored energy, delaying dynamic recrystallization. Driven by the higher deformation temperature, the center layer primarily undergoes dynamic recovery, leading to the formation of elongated grains. The subsurface layer, as a transition region between the surface and center layers, experiences intermediate temperatures and strain rates, resulting in a mixed microstructure of equiaxed and elongated grains. In summary, the gradient microstructure observed in the thickness direction of hot-rolled sheets is a common characteristic of electrical steel [8].

In contrast, the hot-rolled microstructure with the addition of rare earth Y does not exhibit the typical three-layered microstructure. Instead, it shows significant abnormalities. The mechanism behind the abnormal microstructure formation in Y-added hot-rolled sheets is illustrated in Figure 8. After the addition of Y, which readily combines with O, S, and P, the melt is purified, reducing the dislocation density and lowering the dislocation energy within the organization. During hot rolling, the reduced stored energy in the microstructure increases the energy required for critical dynamic recrystallization in the surface layer. Additionally, studies have shown that the addition of REMs causes second-phase particles to segregate at grain boundaries [16,17], exerting a strong pinning effect that hinders grain boundary movement and significantly inhibits the recrystallization process in NOES. Under the combined effects of these factors, the dynamic recrystallization process is delayed, weakening dynamic recrystallization in the subsurface layer while having a lesser impact on dynamic recovery. This ultimately results in the formation of larger, elongated grains. Furthermore, the reduction in dislocation density decreases the number of grains formed by strain-induced boundary migration [18,19], providing space for grain recovery and growth in the ingot. This leads to the formation of equiaxed grains exceeding 100 μm in size in the subsurface to center layers.

Figure 8

Schematic diagram of the influence of rare earth Y on hot-rolled microstructure and texture.

In terms of texture, after the addition of rare earth Y, the fine grains in the subsurface layer largely disappear, replaced by larger grains grown through recovery. This region still experiences significant shear forces during rolling, forming shear textures such as the Brass texture. Due to the tendency of yttrium-containing secondary-phase particles to segregate near grain boundaries, they exert a strong pinning effect on the adjacent grain boundaries, as illustrated in Figure 9(a)–(d). This phenomenon significantly restricts the recovery and growth of grains with shear textures, such as the brass texture. The Goss texture in the center layer primarily arises from the rotation of cube-oriented grains around the <100>∥RD axis. The reduction in fine recrystallized grains in the subsurface layer provides space for the growth of Goss-oriented grains, resulting in the formation of a sharp Goss texture after hot rolling.

Figure 9

Grain orientation near inclusions: (a) micro-morphology of inclusions, (b) energy spectrum of inclusions, (c) phase diagram around inclusions, and (d) IPF around inclusions.

5 Conclusions

In this experiment, commercial-grade Fe–3.0% wt Si NOES was used as the base material, with the addition of varying amounts of rare earth Y to investigate the effects of rare earth Y on the as-cast and hot-rolled microstructures of NOES. A comprehensive analysis was conducted to understand the reasons behind the abnormal microstructure inheritance phenomenon observed in the Y-containing hot-rolled microstructure. The main conclusions are summarized as follows:

1. REM Y has strong deoxygenation and desulfurization abilities and can react with O and S to form high-melting-point second-phase particles. These particles can serve as effective nucleating agents during the solidification process, promoting the formation of fine-grained microstructures. Meanwhile, during the subsequent cooling process, they generate strong pinning effects at grain boundaries, inhibiting grain growth and significantly refining the microstructure.

2. The purification effect of Y reduces impurity-induced lattice defects, lowering dislocation density and stored energy within the material. Concurrently, the Y-containing second-phase particles segregate at grain boundaries, hindering boundary migration and dynamically recrystallized grain formation. This dual mechanism eliminates fine recrystallized grains in surface/subsurface layers at hot rolling, shifts the microstructure toward coarse elongated grains, and homogenizes dislocation distribution across thickness layers (surface > subsurface > center).

3. An appropriate amount of rare earth Y can significantly pin the brass-oriented grain boundaries, inhibit the development of brass components, and enhance the {100} and its variant hot-rolled textures. Considering the grain size and texture of the hot-rolled plate, the addition of 0.012% Y is recommended to prepare NOES.