Effect of Hot Deformation Temperature on Texture Formation Regularity of CGO Silicon Steel under High Deformation Rate

The influence of hot deformation temperature on microstructure and texture of CGO silicon steel under the condition of a high deformation rate (100 s) was studied by SEM and EBSD techniques. The results indicate that the typical microstructures at room temperature consist of ferrite and pearlite under different hot deformation temperatures. The higher deformation temperature is beneficial to obtain a more uniform recrystallization microstructure and lower pearlite content. Cubic texture {100}⟨001⟩ and rotated cubic texture {100}⟨011⟩ are dominant texture components in the tested steels, and {111}⟨112⟩ texture in γ fiber is also strong, the intensity of which is higher than that of {111}⟨110⟩ texture. Goss texture {110}⟨001⟩ is weak. With the rising of the hot deformation temperature, {100}⟨011⟩ texture decreases firstly and then increases at 1100C.When the hot deformation temperature raises from 800C to 900C, {111}⟨112⟩ texture shows an increasing trend, while {111}⟨110⟩ texture content is stable. When the temperature further increases to 1100C and 1150C, {111}⟨112⟩ and {111}⟨110⟩ textures are slightly weakened.


Introduction
Grain-oriented silicon steel is an important soft magnetic material which is widely applied in electronic and electric power industry and mainly used for the manufacture of transformers and so forth [1,2].The metallurgical factors which affect the magnetic properties include the steel composition, microstructure, crystallographic texture, grain size, and internal stress, of which microstructure and texture are important influencing factors [3].Due to the obvious inheritance of the textures [4] during manufacturing, the hot rolling textures have significant effect [5] on the texture evolution during the following process of grain-oriented silicon steel.The research on the origin and evolution of textures is beneficial for the optimization of texture control in the production processes.The deformation rate applied to grain-oriented silicon steel in actual hot rolling process is relatively high.Despite numerous researches on the effect of the hot rolling temperature on the microstructure and texture of grain-oriented silicon steel at present, the research under the condition of a high strain rate is not much.Liu et al. [6] studied hot rolling texture distribution at different temperatures using a thermal simulator under low deformation rate condition.In this study, the texture evolution during hot rolling process was simulated by uniaxial compression.The results show that gamma textures which have strong inheritance play an important role in the subsequent process and improve the concentration of textures after secondary recrystallization.Zhang et al. [7] studied hot rolling textures of 3.2% Si grain-oriented silicon steel and the inheritance of textures between hot rolling and subsequent processes, this research focused on microstructure, and texture evolution of the static recrystallization occurred in the process of annealing after the deformation of CGO silicon steel.The results show that there is no obvious heterogeneity along the thickness direction after primary recrystallization, and {111}⟨110⟩ texture is the strongest texture component, and there is a certain intensity of the Goss texture.The texture formation mechanism is different between dynamic recrystallization and static recrystallization.The texture is definitely different between hot compression and hot rolling which is used industrially, and Zhichao et al. [8,9] have done some 2 Advances in Materials Science and Engineering researches on the formation and evolution of textures in hot rolling process, but the thermal compression process is not involved in the same deformation conditions, and this process has significant impact on the texture evolution during hot deformation.The rough rolling of CGO steel usually has a high rolling speed which is closed to 100 s −1 , so that the texture evolution during this process is difficult to analyze, and limited work is available in the literature on this impact.It is therefore important to summarize the texture evolution law under the large deformation rate with different temperatures.This paper investigate the effect of hot deformation temperature on the microstructure and texture of grain-oriented silicon steel under the condition of a high deformation rate, which provides reference to the texture control during the manufacturing process of grain-oriented silicon steel.

Experimental Materials and Methods
Experimental materials are cut from an ingot which has a chemical composition of 0.065% C, 2.95% Si, 0.15% Mn, and 0.027% S and processed into cylinder specimens.The experimental process is illustrated in Figure 1.The specimens were heated to 1300 ∘ C at 10 ∘ C/s, held for 3 min, and then cooled, respectively, to 800, 850, 900, 1100, and 1150 ∘ C at 4 ∘ C/s and conducted uniaxial compression with the same deformation rate of 100 s −1 .Test samples were cut along the axis of compression specimen and their microstructures in center layer were analyzed by field emission scanning electron microscopy (SEM) after mechanical polishing and etching.The orientation distributions were measured by electron back-scattered diffraction (EBSD) attached to Zeiss ultra 55 SEM.The experimental data were analyzed by Channel 5 software and the contents of typical textures were obtained.

Microstructures of Experimental Steels.
The phase diagrams of the experimental steels are calculated using Thermo-Calc software.Figure 2(a) presents the effect of carbon content on the microstructure of the steel.Figure 2(b) presents the dependence of the relative qualities of austenite, ferrite, cementite, and so forth on the temperature.It can be seen from Figure 2 that when the C content is in a range of 0.04-0.78%, the high temperature microstructure at 1200 ∘ C is single-phase austenite.During the cooling process, the proeutectoid ferrite occurs firstly, and two-phase range of austenite and ferrite exists.With the formation of ferrite, the concentration of carbon in ferrite is very low and reduces with the decreasing of temperature, and C atoms diffuse to the surrounding austenite.This process is accompanied with the decreasing of temperature and the formation of ferrite continuously.At the five experimental temperatures between 800 and 1100 ∘ C, deformation specimens are at the "ferrite + austenite" two-phase range, the austenite has the highest relative content at 1180 ∘ C, and the content will reduce with the decreasing of temperature.under a high deformation rate.Figure 3(f) illustrates the average grain size of different specimens.Typical microstructures consist of ferrite and pearlite, and the microstructure morphologies of the samples show significant differences under various deformation temperatures.The recrystallization grains of the specimens deformed at 850 and 900 ∘ C are coarse, and their grain size distributions are inhomogeneous.At the deformation temperatures of 1100 ∘ C and 1150 ∘ C, the recrystallization degrees of part grains are low and the content of pearlite is higher.With the increases of deformation temperature, the recrystallization grain size decreases gradually and the grain size distribution is more uniform.It can be obtained from Figure 2(b) that, with the increasing of deformation temperature, the austenite proportion becomes larger and the supersaturation of C in austenite decreases.
The deformed austenite recrystallizes sufficiently, leading to fine and uniform size of deformed austenite.At the same time, crystal distortion is small and is not conducive for the diffusion of the C atoms from inside of grains to grain boundaries where pearlite nucleates, which lead to the difficulty of alternative growth of pearlite lamellae.The pearlite layer is thinner after high temperature deformation and pearlite area is smaller.In addition, due to the increasing of deformed austenite, pearlite distribution becomes more uniform after phase transformation.During 800∼900 ∘ C, recrystallization degree is weak, distortion of deformed austenite is large, and it is conductive to the diffusion of C atoms.Coarse pearlite is formed after phase transformation and distributed intensively because of the decreasing of deformed austenite.During rolling in two-phase range of ferrite and austenite, the degree of recrystallization is lower at low temperature.At a higher deformation temperature, ferrite and austenite are compressed and form dislocation-substructure inside of the grains [9].The higher temperature provides driving force for the grain growth, which leads to the recrystallization and the growth of ferrite grains in a wide range.When the deformation temperature is low, austenite grains are elongated without recrystallization and dislocation-substructures accumulated within grains, which become nucleation sites of phase transformation.The elongated austenite grain boundaries restrict the growth of newly formed microstructures.So it can be observed from Figure 3(b) that the grain bands did not grow up sufficiently between the gaps of larger size grains.During the cooling, ferrite and cementite will generate inside of austenite and the newly formed ferrite has small size because of the limitation of austenite grain boundaries.At lower deformation temperature, there are long strip recovery microstructures which remain in the matrix, such as Figures 3(b) and 3(c).

Textures of Experimental Steels.
The initial textures before high temperature deformation are distributed randomly because the experimental materials are cut from forged CGO silicon steel.The orientation image maps of the experimental steels with high deformation rate at different deformation temperatures were shown in Figure 4 and the area fractions of different textures were listed in Table 1 and Figure 5. Various colors represent grains with different orientation, Goss {110}⟨001⟩ oriented grains is red, {111}⟨121⟩ oriented grains is yellow, {111}⟨100⟩ oriented grains is blue, cubic oriented grains is brown, rotated cube {100}⟨011⟩ oriented grains is green, brass-type {110}⟨112⟩ oriented grains is purple, and copper-type {112}⟨111⟩ oriented grains is blue.
As can be seen in Table 1, in general, cube texture {100}⟨001⟩ and rotated cube texture {100}⟨011⟩ are dominant texture components at different hot deformation temperatures.In addition, the contents of {111}⟨112⟩ in  fiber and copper-type {112}⟨111⟩ texture are also higher.Goss texture {110}⟨001⟩ is weak and its content is low.The contents of texture components will change at various deformation temperatures.For instance, with the rising of hot deformation temperature, the content of rotated cube texture {100}⟨011⟩ decreases first and then increases further.The content of {111}⟨112⟩ texture and copper-type texture {112}⟨111⟩ increase first and then reduce.
From Table 1 and Figure 6 it can be seen that the content of Goss texture {110}⟨001⟩ is low at most of the hot deformation temperatures.Goss texture of hot rolled plate originates from the broken columnar crystal with ⟨100⟩ direction in casting billet [14][15][16].During the hot deformation process, Goss texture formed by shear strain will transit to other orientations under high deformation rate because of the migration of the high angle grain boundaries.Goss oriented grains will transform to brass-type texture {110}⟨112⟩ and copper-type {112}⟨111⟩ texture because of the orientation conversion relationship.The intensity of copper-type texture {112}⟨111⟩ is higher than the Goss texture {110}⟨001⟩ which can be seen in Table 1.During the cold rolling, Goss oriented grains descend from the broken columnar grains with ⟨100⟩ direction of casting billet and are prone to transit to {223}⟨110⟩ texture along certain path.However, it is difficult to obtain the final {223}⟨110⟩ texture because of the restriction of certain conditions.In brief, the content of Goss texture {110}⟨001⟩ is low in the steels which experienced thermal deformation.
Rotated cube texture {100}⟨011⟩ is the common component in hot rolled plate.From Table 1 and Figure 5 it can be seen that, with the rising of deformation temperature, the content of {100}⟨011⟩ texture decreases first and then increases.The texture characteristics of single crystal under different rolling reductions indicate that when the rolling reduction was greater than 60%, {211}⟨011⟩ and {100}⟨011⟩ texture components are enhanced because of {211}⟨111⟩ slip system during thermal deformation [17,18], which explains the fact that the content of rotated cube texture {100}⟨110⟩ is high at all the deformation temperature.The rotated cube texture {100}⟨110⟩ is difficult to recrystallize [19] and should be eliminated in the subsequent process.The typical texture during austenite recrystallization is cubic texture.All steels with different deformation temperature therefore have a high proportion of {100}⟨001⟩ cubic texture.However, the cubic texture is an unfavorable component which is formed by the initial columnar crystal in grain-oriented silicon steel.Researches [10] indicated that, in the subsequent process, the repeated rolling and annealing can weaken the heredity of {100} texture and decrease the {100}⟨001⟩ cubic texture.
During the hot compression with high strain rate of CGO silicon steel, scattered  fiber texture merged into {100}⟨011⟩ texture and cube {100}⟨011⟩ texture was formed because of strong rotation of grains.Under high stain rate, cubic texture {100}⟨001⟩ keeps stable and takes a great proportion.Copper texture {112}⟨111⟩ and brass texture {110}⟨112⟩ are typical shear textures.Brass texture {110}⟨112⟩ is derived from shear layer which forms at the subsurface of the plate during hot rolling and is closely related to Goss texture {110}⟨001⟩.The formation of brass texture {110}⟨112⟩ is due to the inhibition of grain boundaries.The rotation of Goss oriented grains along TD direction is impeded so that the grains rotate along ND direction instead.

Conclusions
(1) The typical microstructures at room temperature of CGO silicon steel after hot deformation are ferrite and pearlite, and there are some differences in grain size distributions and morphologies.A high hot deformation temperature is benefit to obtain a more uniform recrystallized microstructure and lower pearlite  content.A low deformation temperature leads to higher pearlite content and partial recrystallization.
(3) With the rising of deformation temperature, the content of the rotated cube {100}⟨011⟩ texture decreases first and then increases when the deformation temperature further achieved to 1100 ∘ C; when the deformation temperature raises from 800 ∘ C to 900 ∘ C, {111}⟨112⟩ texture shows an increasing trend, and {111}⟨110⟩ texture is more stable.When the hot deformation temperature continues to increase to 1100 ∘ C and 1150 ∘ C, {111}⟨112⟩ texture and {111}⟨110⟩ texture are slightly weakened.

Figure 2 :
Figure 2: Thermodynamic calculated phase diagram.(a) Fe-C phase diagram; (b) the relative amount of different phases change with temperature.

Figure 6 :
Figure 6: Texture contents of samples under different hot deformation temperatures in tested steel.

Table 1 :
Texture contents of samples under different hot deformation temperatures in tested steels (%).