Effect of Slab Reheating Temperature on Cold Rolling Texture Evolution of Nb-Containing Grain-Oriented Silicon Steel

Abstract

Texture control of grain-oriented silicon steel is the key factor to ensure the magnetic properties of the finished product. Nb-containing grain-oriented silicon steel with different slab reheating temperatures was hot rolled followed by single-stage or two-stage cold rolling, and the textures were also analyzed. In the single-stage cold rolling process, as the slab reheating temperature is reduced, the intensity of the rotating cube texture {100}<011> and Goss texture {011}<100> drops, and that of the {111}<112> texture increases. In the two-stage cold rolling process, with the decrease in the slab reheating temperature, the intensity of the {111}<112> texture increases from 4.958 to 6.809. At the same slab reheating temperature, the intensity of the rotating cube texture declines more significantly in the two-stage cold rolling process. Finally, two-stage cold rolling with the slab reheating temperature of 1220 °C is found to be more beneficial for the formation of a sharp Goss texture during the second recrystallization. The magnetic induction intensity B800 of the final product is 1.87 T, and the iron loss P1.7/50 is 1.36 W/kg.

1. Introduction

Grain-oriented silicon steel, which is mainly used as the core material in electrical transformers and high-rating generators, is an indispensable and important functional material in the fields of electronics, the electric power industry, and the military industry. It is characterized by a sharp {110}<001> preferred crystallographic orientation referred to as the Goss texture. Grain-oriented silicon steel goes through a series of special processes that are different from other steel products, which finally achieves the sharp Goss ({110}<001>) texture and excellent magnetic properties via secondary recrystallization [1,2,3,4,5].

There are two important problems in the production process of grain-oriented silicon steel. One is the orientation control of the Goss texture, and the other is a series of quality and cost problems caused by the high heating temperature. Strict rolling processes and inhibitors that have an excellent inhibition effect work together to control the orientation of the Goss texture [6,7,8,9]. The inhibition ability of precipitates is found to have a positive correlation with the volume fraction and a negative correlation with the size. To improve the dispersity of precipitates, the inhibitor needs to be completely dissolved in the matrix during slab heating, and the growth rate is small at the precipitation temperature [10,11,12,13]. In order to ensure the inhibiters fully dissolve into the matrix, a higher heating temperature is chosen in the traditional process, which results in up to 5% burning loss, a large amount of slag, short service life of the furnace, coarse grain size, segregation of Si at the grain boundary, edge cracks, and a lower yield [14,15]. So, in order to realize the production of grain-oriented silicon steel with the purpose of high efficiency and energy saving, strip casting technology, which belongs to the short flow process, has been proposed recently [16,17,18,19], as well as low slab reheating temperature technology, which produces the grain-oriented silicon steel by reheating the casting billet at relatively lower temperatures to obtain the plates with high magnetic induction (Hi-B) and low iron loss [20]. So far, they have become research hotspots in the research of grain-oriented silicon steel all over the world [21].

Low slab reheating temperature technology is a process that achieves reheating the casting billet at a lower temperature (1100 °C–1250 °C) to produce the grain-oriented silicon steel by choosing the precipitate with a lower solution temperature as the inhibitor. However, the solution and precipitation behaviors of inhibitors are determined by different chemical compositions and crystal structures. So in our recent studies, the element Nb was added to the grain-oriented silicon steel, and Nb(C,N) particles were found in the Nb-containing silicon steel with a lower solution temperature and better feathers than MnS and AlN, which accordingly can achieve reheating of the slab at a lower temperature [22]. However, there have been limited reported works on Nb-containing grain-oriented silicon steel so far, especially on the effect of a lower slab reheating temperature before hot rolling. Therefore, in this paper, the texture evolution with different slab reheating temperatures in the cold rolling process of Nb-containing grain-oriented silicon steel was studied and the influence of process parameters on the formation, distribution, and evolution of the Goss texture in the cold rolling and annealing process was also explored, which provides a theoretical basis and technical support for the preparation of lo-temperature Hi-B silicon steel.

2. Materials and Methods

The material used in this study is Nb-containing grain-oriented silicon steel. The original size of the billet is 150 mm × 150 mm × 250 mm. The main chemical composition (wt.%) was 0.42 C, 3.2 Si, 0.08 Mn, 0.016 S, 0.018 Al, 0.045 Nb, and 0.006 N.

The as-cast billets were heated to 1170 °C, 1220 °C, and 1270 °C, respectively, held for 30 min, and then hot rolled to a thickness of 2.5 mm through 7-pass rolling. The hot-rolled plate with an 88% reduction was cold rolled with single-stage cold rolling and two-stage cold rolling. During the two-stage cold rolling process, the first cold rolling reduction was 70% and the second one waas 60%. Between the first and second cold rolling, intermediate annealing was conducted at 900 °C and held for 3 min in a wet atmosphere of 75% H₂ and 25% N₂ with the dew point temperature of 45~55 °C. Synchronous rolling was adopted in all rolling processes. The pre-heating method and rolling lubrication were combined to reduce the edge crack during the rolling process to ensure good surface quality of the finished product. The cold-rolled and annealed plates were analyzed by the Quanta 650 FEI scanning electron microscope (SEM) equipped with a back scattering diffraction (EBSD) system. Finally, the final products P1.7/50 (W/kg) and B800(T) were measured by the MATS-2010SA soft magnetic measuring instrument.

3. Results and Discussion

3.1. Texture Evolution of Single-Stage Cold Rolling

Figure 1 shows the orientation maps of a single-stage cold-rolled plate with different slab reheating temperatures. From Figure 1a, at 1170 °C, the main texture of the cold-rolled plate was distributed dispersedly along the rolling direction as a continuous plate shape. At 1220 °C, the concentration distribution of the rotating cube texture {001}<110> (purple area in Figure 1), which is mostly adjacent to the {114}<8-121> texture (gray), gradually increases. Most {111}<112>(blue) and {111}<132> orientations (green) are found in the same grain. From Figure 1c, at 1270 °C, the {223}<110> texture (green) and the {001}<110> texture are distributed centrally, and the texture types experience rapid declines. By comparing Figure 1a–c, it is known that as the slab reheating temperature increases, the diffused textures gradually concentrate, and the {223}<110> texture and the {001}<110> texture are distributed centrally.

With the increase in rolling temperature, the number of dynamically recrystallized grains increases, the elongated ferrite grains decrease slightly, and the microstructure gradient decreases. There are a large number of {110}<001> grains in the subsurface of the slab after hot rolling at 1270 °C, and the grains gradually rotate from {110}<001> to the {001}<110> phase due to the shear stress in the subsequent cold-rolling process. Finally, this leads to the concentrated distribution of {001}<110> grains in the subsurface of the cold-rolled plate. The main phases of the central layer of the hot-rolled plate are {001}<110> and {111}<112>. Research shows that [23] {001}<110> and {111}<112> orientations produced during cold rolling are metastable orientations, and the crystal may rotate along the following two paths: One is {001}<110>→{112}<110>→{223}<110> and the other is {111}<112>→{111}<110>→{223}<110>. Finally, it leads to the centralized distribution of {223}<110> grains in the central layer of the cold-rolled sheet.

Figure 2 shows the orientation distribution functions (ODFs) at the φ2 = 45° section of the surface texture of the hot-rolled plates and cold-rolled plates with different slab heating temperatures. From Figure 2a–c, different slab heating temperatures can be seen to have an obvious influence on the hot-rolled texture. At 1170 °C, the hot-rolled texture is dominated by a copper-type texture {112}<111>, a brass texture {011}<211>, and a Goss texture {011}<100>, as well as a small amount of a cube texture {001}<100>. With the increase in the slab heating temperature, the intensity of the Goss texture increases, while the cube texture {001}<100> disappears, and a small amount of the rotating cube texture {001}<110> appears. When the slab heating temperature rises to 1270 °C, there is no obvious change in texture type, but the intensity of the rotating cube texture {100}<011> increases significantly. After cold rolling, the texture of the plates changed significantly. Comparing Figure 2d–f, with the increase in the slab heating temperature, the texture intensity clearly increases. The intensity of the {111}<110> texture in the cold-rolled plate is the highest, which reaches 18.786, while at 1270 °C, the intensity of the rotating cube texture {100}<011> is the highest, at about 27.015, and the intensity of the {111}<112> texture is reduced to 0.160.

3.2. Texture Evolution of Two-Stage Cold Rolling

Figure 3 shows the orientation maps of two-stage cold-rolled plates with different slab reheating temperatures. Figure 3a–c shows the orientation distributions of the first cold-rolled plates. It can be seen that the texture consists of a strong α-fiber texture and a weak γ-fiber texture. The α-fiber texture mainly contains the {001}<110> texture (purple) and the {223}<110> texture (green), and the γ-fiber texture includes the {111}<112> texture (blue) and the {111}<132> texture (dark green). Figure 3d–f shows the orientation distributions of the annealed plates. After intermediate annealing, the proportion of the {001}<010> texture (orange) increases, especially the proportion of γ-fiber at 1220 °C, which clearly increases including the {111}<112> texture (blue), the {111}<132> texture (dark green), and a small amount of the {111}<110> texture (yellow). After the second cold rolling with a 60% reduction, there was no obvious change in the texture type.

Figure 4 shows the orientation distribution functions (ODFs) at the φ2 = 45°section of the surface layer texture of the hot-rolled, annealed, and two-stage cold-rolled plates with different slab reheating temperatures. From Figure 4a–c, the textures of hot-rolled plates are all dominated by the Goss texture, copper-type texture, and brass texture, and the texture intensities differ slightly from each other. Figure 4d–f shows ODFs of the first cold-rolled plates with a 70% reduction ratio. It can be seen that the rotating cube texture, {111}<112> texture, and {111}<110> texture are all observed in the first cold-rolled plates with different slab reheating temperatures. At 1270 °C and 1220 °C, the {100}<001> cube texture can be found, while at 1170 °C, it disappears completely. In addition, the {111}<110> texture at 1270 °C is obviously weaker than that at 1220 °C and 1170 °C. Figure 4g–i shows ODFs of the annealed plate with different slab reheating temperatures. After intermediate annealing, the γ-fiber texture was clearly weakened, while the cube texture was enhanced. Moreover, it is worth noting that the {111}<112> texture is retained at 1220 °C, which is more beneficial for the formation of the sharp {111}<112> texture during the second cold rolling. Figure 4j–l shows ODFs of the second cold-rolled plates with a 60% reduction. It can be found that the {111}<112> texture became the strongest texture in the cold-rolled plates. Moreover, the γ-fiber texture was enhanced significantly, and there was no significant difference in the texture type.

After secondary cold rolling, there is a typical microstructure heterogeneity along the thickness direction, and there are significant differences under different slab heating temperatures. When the slab heating temperature is 1170 °C, the lower slab heating temperature has a great influence on the solid solution precipitation behavior of the inhibitor. The unfavorable slab heating temperature leads to a decrease in the inhibition ability of the inhibitor, which cannot effectively inhibit the growth of the primary re-crystallized grains and leads to a larger grain size after the second cold rolling. When the slab heating temperature is 1220 °C, the surface grains are fine and uniform, the precipitates are fine and dispersed, the amount of precipitates in the central layer is small, the grain size increases, and the distribution is relatively uniform. When the slab heating temperature is 1270 °C, due to the high temperature, the development of dynamic recrystallization is relatively perfect, which increases the grain size of the original structure and has a significant impact on the grain size of cold rolling, forming uneven large-sized strip ferrite grains. Combined with the above analysis, it can be seen that the grain size is the largest at a slab heating temperature of 1270 °C, and the microstructure heterogeneity along the thickness direction is poor. When the slab heating temperature is 1170 °C, the average grain size of the one-quarter layer is similar to that of the central layer, and there are several mixed crystal areas. The microstructure of the secondary cold-rolled sheet at 1220 °C is better than that of the other two processes.

3.3. Analysis on the Texture of Single-Stage Cold Rolling with Different Slab Reheating Temperatures

Figure 5 show the orientation distribution intensities along the α-fiber, γ-fiber, and η-fiber textures. From Figure 5, after cold rolling, the textures in the α-fiber and γ-fiber are significantly enhanced, while those in the η-fiber weakens. A sharp {001}<110> texture in the α-fiber and a strong {111}<112> texture in the γ-fiber also appear. Meanwhile, the Goss texture clearly weakens, and the diffused α-fiber and γ-fiber textures start to be strengthened while the η-fiber texture begins to diffuse. From Figure 5d, the strongest α-fiber texture is the rotating cube texture with an intensity of 27.015 at 1270 °C, followed by that at 1220 °C and 1170 °C, whose intensities are 17.1147 and 13.836, respectively. The above phenomenon illustrates that with the increase in the slab reheating temperature, the intensity of the rotating cube texture in the single-stage cold-rolled plates increases dramatically, which is bad for the formation of the Goss grain. From Figure 5e, the {111}<112> texture at 1170 °C is the strongest in the γ-fiber with the intensity of 3.045. With the increase in the slab reheating temperature, the intensity of the {111}<112> texture sharply weakens. When the slab reheating temperature is 1220 °C and 1270 °C, the intensity of the {111}<112> texture is 2.350 and 1.283, respectively.

By comparing Figure 5d,f, it can be concluded that the rotating cube texture, the {111}<112> texture, and the Goss texture of the cold-rolled plate have clearly changed as the slab heating temperature increases from 1170 °C to 1270 °C. (1) The intensity of the rotating cube texture increases from 13.836 to 27.015, while those at 1170 °C and 1220 °C slightly differ. (2) The intensity of the {111}<112> texture decreases from 3.045 to 1.283, but that of the {111}<112> texture at 1170 °C and 1220 °C slightly differs. (3) The intensity of the Goss texture increases from 0.170 to 0.972, but that of the Goss texture at 1220 °C and 1270 °C differs a little. From the analysis above, it is determined that the cold-rolled plate at 1220 °C is more beneficial for the formation of the high-component Goss texture during secondary recrystallization.

There are two dynamic processes in the hot rolling of grain-oriented silicon steel. One is plastic deformation. The plastic deformation is mainly based on the movement of dislocations. The movement of dislocations will cause the distribution of various defects in the collective to become dense and improve the defect density. Second, with the change in temperature, there will be a dynamic recrystallization process, including recovery, nucleation, and grain growth. Recovery will repair the defects inside the crystal to a certain degree and reduce the defects inside the crystal. In these two processes, one process generates the deformation texture, and the other process generates the recrystallization texture. The two processes either occur simultaneously or alternately and interact with each other. There are two kinds of textures produced during cold rolling, whereby one is the α-fiber texture parallel to the rolling direction, while the other is the γ-fiber texture parallel to the normal direction. After comparison, the {001}<110> rotating cube texture in the α-fiber of hot-rolled plate samples at different heating temperatures has no obvious change. This is because the rotating cube texture is located on the {001} low-energy surface and is not easily swallowed, resulting in a slight change in the orientation density of grains with the rotating cube orientation. During cold rolling, the strength of the {001} < 110 > texture will increase with the increase in deformation. At the same time, the {111} < 112 > texture and strength will also change.

3.4. Analysis on the Texture of Two-Stage Cold Rolling with Different Slab Reheating Temperatures

Figure 6 shows the orientation distribution intensities along the α-fiber, γ-fiber, and η-fiber after the first cold rolling with different slab reheating temperatures. From Figure 6a, the strongest texture in α-fiber is the {223}<110> texture at 1220 °C, and the intensity is 16.184. For the rotating cube texture, the intensities at different slab reheating temperatures differ slightly and lie between 5.368 and 9.541. From Figure 6b, the strongest texture in the γ-fiber is the {111}<112> texture at 1170 °C, and its intensity is 5.679, while at 1220 °C, the {111}<112> texture is the weakest with an intensity of only 1.984. From Figure 6c, the strongest texture in the η-fiber is the {001}<100> texture at 1220 °C, and its intensity is 1.435. The Goss texture is weaker with three different slab reheating temperatures, and the difference is less than 0.1.

Figure 7 shows the orientation distribution intensities along the α-fiber, γ-fiber, and η-fiber after annealing with different slab reheating temperatures. From Figure 7a, the strongest texture in the α-fiber is the {113}<110> texture at 1270 °C, and its intensity is 6.098. The second strongest texture in the α-fiber is the rotating cube texture at 1270 °C, and its intensity is 5.255. The intensities of the rotating cube texture at 1170 °C and 1220 °C are 1.688 and 2.350, respectively, which are similar to each other. From Figure 7 and Figure 8, the rotating cube texture after intermediate annealing is weakened slightly, and the decline in intensities is minimal at 1270 °C. From Figure 7b, the strongest texture in the γ-fiber is the {111}<112> texture at 1220 °C, and its intensity is 3.687. The intensities of the {111}<112> texture at 1170 °C and 1270 °C are 1.523 and 1.364, respectively, which are similar to each other. From Figure 7c, as the slab reheating temperature increases from 1170 °C to 1270 °C, the intensity of the Goss texture increases slightly from 1.389 to 2.087 and 2.849.

Figure 8 shows the orientation distribution intensities along the α-fiber, γ-fiber, and η-fiber after the second cold rolling with different slab reheating temperatures. From Figure 8a, the strongest texture in the α-fiber is the {223}<110> texture at 1170 °C with an intensity of 4.905. The strongest rotating cube texture was shown at 1270 °C with and intensity of 1.394. From Figure 8b, the strongest texture intensity of the γ-fiber is the {111}<112> texture at 1170 °C, and its intensity is 6.809. As the slab reheating temperature increases, the intensity of the {111}<112> texture decreases gradually at 1220 °C and 1270 °C, and the intensities are 6.444 and 4.958, respectively. From Figure 8c, the strongest texture in the η-fiber is the {001}<100> texture at 1220 °C, and its intensity is 3.800. It can also be concluded that the Goss texture is relatively weak at three slab reheating temperatures, and the intensity lies between 0.01 and 0.07.

According to XRD and EBSD results from published articles, it is known that the substructure is rarely observed in the rotating cube texture, which inevitably results in lower storage energy in grains [24]. It has been demonstrated by experiments that a substructure (a shear band) with high energy is the key to forming Goss grains [25]. Moreover, the rotating cube texture tends to be adjacent to the brass texture, and even transforms to brass texture, which are both bad for the subsequent nucleation and growth of Goss grains during the second recrystallization. From experimental results, it is known that the intensity of the rotating cube texture in two-stage cold-rolled plates with a slab reheating temperature of 1220 °C is the lowest, and the intensity of {111}<112> differs slightly from that at 1170 °C. Besides, there is no obvious difference of the Goss texture with different slab reheating temperatures. So, a slab reheating temperature of 1220 °C is more beneficial for the formation of high-component Goss grains. Meanwhile, by comparing the texture type and intensity of two cold-rolling technologies, it is found that with the same slab reheating temperature, the fraction of the rotating cube texture obtained from two-stage cold rolling is much lower than that from single-stage cold rolling, and the {111}<112> texture is clearly enhanced. Therefore, two-stage cold rolling with a slab reheating temperature of 1220 °C is more beneficial for the formation of a sharp Goss texture.

3.5. Analysis of Texture and Magnetic Properties of the Final Product

The secondary cold-rolled plate with a slab heating temperature of 1220 °C has a low rotating cubic texture, a sharp {111}<112> texture, and a weak Goss texture. Therefore, the sample is annealed at a high temperature to determine its microstructure and magnetic properties. Figure 9a shows the orientation image maps along the thickness direction of the secondary cold-rolled sheet annealed at 1220 °C. From Figure 9a, after high-temperature annealing, the main texture of the sample is the Goss texture, and the texture content of other orientations is very small. Figure 9b shows the {001} polar diagram of a secondary cold-rolled plate annealed at 1220 °C. From Figure 9b, after high-temperature annealing, the deviation angle of the Goss texture is small, the orientation is accurate, and the sharpness is high. In the second cold-rolling process, AlN, Nb(C,N), and other precipitates began to precipitate, which hindered the normal growth of primary recrystallized grains and pinned them at the grain boundary. In the second cold-rolling process, they pinned the movement of dislocations and the migration of the grain boundary. They effectively inhibit the grain growth of the cold-rolled sheet and control the {111}<112> texture grain size, so as to facilitate the final Goss grain phagocytosis. According to the theory of coincidence site lattice (CSL) mode [26,27], the Σ9 grain boundary density around the Goss grains is high, and the Σ9 grain boundary is considered to have low interface energy, which means that inhibitors are effective for Σ9. The pinning effect of the grain boundary is very weak. On the contrary, Σ3 grain boundaries are not easy to migrate, and the migration rate of Σ5 grain boundary is between the two. Research shows that [24,28] {111}<112>-oriented grain boundaries are similar to the Goss grain Σ9 grain boundary, and Σ9 grain boundaries are high-energy grain boundaries with high mobility and a high diffusion coefficient. The existence of these grain boundaries greatly reduces the pinning force of inhibitors on Gaussian grains, causing them to first eliminate the “bondage” of inhibitors in the process of secondary recrystallization, and swallow other oriented grains to ensure that high-component Goss grains are finally obtained.

Table 1. Magnetic properties of final products.

Magnetic Properties	Secondary Cold Rolled Plate
B800/T	1.87
P1.7/50/W/kg	1.36

shows the magnetic induction intensity and iron loss of the final product. From Table 1, after high-temperature annealing, the magnetic induction intensity B₈₀₀ of the final product is 1.87 T and the iron loss P_1.7/50 is 1.36 W/kg. It can be seen that the sample reached the magnetic property parameters of high-magnetic-polarization-grade steel sheet with brand 27QG110. Through comparison, it can be found that the low slab reheating temperature technology is of great significance to improve the production process, save production costs, and obtain the oriented silicon steel with excellent performance.

4. Conclusions

(1)

During the single-stage cold-rolling process, as the slab reheating temperature reduces from 1270 °C to 1170 °C, the intensity of the rotating cube texture {100}<011> is reduced from 27.015 to 13.836, and that of the Goss texture reduces from 0.927 to 0.170. Conversely, the intensity of {111}<112> increases from 1.283 to 3.045.

(2)

During the two-stage cold rolling process, when the slab reheating temperature is 1220 °C, the rotating cube texture {100}<011> is the lowest. As the slab reheating temperature is reduced from 1270 °C to 1170 °C, the intensity of the {111}<112> texture increases, and the intensity values are 4.958, 6.444, and 6.809, respectively. Meanwhile, there is no obvious difference between the intensities of the Goss texture.

(3)

Two-stage cold rolling with a slab reheating temperature of 1220 °C is more beneficial for the formation of a sharp Goss texture during the second recrystallization. The magnetic induction intensity B800 of the final product is 1.87 T, and the iron loss P1.7/50 is 1.36 W/kg.