2022 年 63 巻 2 号 p. 112-117
In order to study the microstructure control of continuous casting slab of grain oriented silicon steel, the method of field trial was adopted to analyze the influence of different factors on the microstructure of slabs by adjusting the process parameters that affect the formation and transformation of equiaxed crystals and columnar crystals. Studies have shown that as the superheat increases, the proportion of equiaxed crystals in the grain oriented silicon steel slab decreases. The use of electromagnetic stirring can achieve the casting of grain oriented silicon steel under the condition of the superheat exceeds 40°C and the proportion of equiaxed crystals >50%. The box-type of electromagnetic stirring in secondary cooling zone can be used to effectively control the proportion of equiaxed crystals of the slab from 10% to 75%, and increase the electromagnetic stirring current, which contribute to the linear increasing of the proportion of equiaxed crystals. The increasing of the casting speed and width range is conducive to the proportion of equiaxed crystals in slabs. Meanwhile, moderate electromagnetic stirring intensity (400 A, 3 HZ) is beneficial to the uniformity of the slab composition with the optimal effect of improving the segregation along the thickness direction of the slab. The deviation of the measured and predicted value of the equiaxed crystal proportion of the slab is less, the effective control of the equiaxed crystal ratio can be achieved by adjusting the process parameters.
Fig. 2 The microstructure morphology of the experimental grain oriented silicon steel slab.
Grainoriented silicon steel has the characteristics of low core loss and high magnetic induction, which is an important soft magnetic material mainly used for transformer core manufacturing.1) It is one of the important signs of the production and technological development of high-end steel products in a country.2,3) The Goss texture {110}⟨001⟩ in the finished product of grain oriented silicon steel is the fundamental reason for its excellent magnetic properties.4,5) Under the condition that the silicon content is basically unchanged, the magnetic induction intensity of grain oriented silicon steel is only comparable to that of Goss grains (110)[001], the smaller the average deviation angle of each crystal grain orientation ⟨001⟩ from the rolling direction, the higher the magnetic induction intensity.6,7)
The slab microstructure is the source of the texture evolution of the finished product. The slab microstructure will have a significant impact on the microstructure, texture and property of the subsequent hot rolling, cold rolling and annealing processes, and will affect the property of the finished product.8,9) Studies have shown that in the as-cast microstructure of grain oriented silicon steel, the larger the volume fraction of equiaxed crystals and the smaller the grain size, the better the strength and toughness; while the columnar crystals of the slab will cause uneven microstructure in the finished product, reduce the Goss texture strength, and generate corrugated defects on the surface of the steel plate, which ultimately affect the magnetic properties of the product.10,11) Therefore, the method of increasing the proportion of equiaxed crystal components of grain oriented silicon steel slab and refining the grains are important to improve the comprehensive mechanical properties of the slab.
By optimizing the chemistry composition Sb, Cu,12,13) Cr,14) mechanical vibration15) and continuous casting process,16–20) the equiaxed crystal ratio can be increased, and it is critical to obtain the optimal process parameters for specific steel composition. The patent17) can applied to increase the equiaxed crystal ratio of grain oriented silicon steel slab to more than 80% by controlling the molten steel superheat of the caster and the dynamic soft reduction under the premise of bypassing the electromagnetic stirring in segments. The superheat control window is only 3–10°C, therefore, the existing industrial applications are limited. Chang18) used roller electromagnetic stirring in the secondary cooling zone to study the influence of different continuous casting process parameters on the center equiaxed crystal ratio of grain oriented silicon steel. The equiaxed crystal ratio of the slab can be increased to 39%, but it is still difficult to satisfy the trend of further low iron core loss and high magnetic induction.
Based on industrial experiments, this paper studies the influence of tundish molten steel superheat, electromagnetic stirring current, casting speed and width range on the equiaxed crystal ratio and element segregation in the center of the slab through micro-etching inspection. It provides a reference for realizing the effective control of the slab microstructure.
A steel plant uses a vertical-curve double-strand slab continuous caster, with a box-type electromagnetic stirring device in secondary cooling zone. As shown in Fig. 1 that the electromagnetic stirring is installed at the zero segment of the inner curve side in secondary cooling zone, and 0.4 m away from the lower end of the mould. In the present box-type stirrer, to get a compact design that complies with the contractual space requirements, there are no poles. They are placed by 4 spacers made of insulating material. The magnetic behavior of the stirrer remains satisfactory, provided that the stirrer is long enough and the coils are thin enough. A U-shaped magnetic screen made of copper has been implemented to prevent magnetic field form leaking towards areas where it is not needed, actually the back, the top and the bottom of the stirrer. The parameters of the continuous casting machine and the electromagnetic mixing device are shown in Table 1 and Table 2, respectively.
Schematic diagram of installation position of electromagnetic stirring.
According to the growth mechanism of columnar and equiaxed crystals and the transformation mechanism of columnar crystals to equiaxed crystals, the influence of molten steel superheat, electromagnetic stirring intensity, casting speed, and width range on the control of slab microstructure is mainly studied. A micro-etching sample is taken from the third slab of the trial heat. The sample size is 240 mm in thickness and the width is the same as the slab. The calculation method of equiaxed crystal ratio is the thickness of equiaxed crystal at 1/4 of the sample width direction/240 * 100%.
2.2 Element segregation testIn order to study the segregation of the slab composition, chip samples were taken at 1/4 of the width of the slab and every 10 mm in the thickness direction, and the contents of C and S elements were analyzed by LECO high-frequency carbon-sulfur analyzer with a detection accuracy of 0.3 ppm. The test range of element C of the equipment is 0.00006%–6.0%, and that of element S is 0.00006%–0.35%.
Taking 0.5 g to 1.0 g of the above chip samples into the high frequency magnetic field of the furnace with oxygen flow, the sample and flux were heated by induction, and burned in the atmosphere of oxygen. The C and S elements in the sample reacted with oxygen to generate CO, CO2 and SO2 into the gas path system with the carrier gas, and first reached the SO2 detection tank. The infrared absorption spectrum of SO2 (characteristic absorption peak 7400 nm) was used to measure S. Then, CO was converted into CO2 and SO2 into SO3 through hot copper oxide. After that, SO3 was absorbed by the absorbent, and then the gas passed through the CO2 detection cell. The infrared absorption spectrum of CO2 (characteristic absorption peak 4260 nm) was used to determine C.
Among them, the element segregation rate is (the element content at this point-the average value)/the average value of all points. When the segregation rate is less than 0, it is negative segregation, and when it is greater than 0, it is positive segregation.
By adjusting the process parameters that affect the formation of equiaxed crystals and columnar crystals in the slab, the effective control of the equiaxed crystals ratio in the slab from 10% to 75% is realized. Among them, the influence of electromagnetic stirring intensity on the microstructure morphology and central equiaxed crystal of the slab is shown in Fig. 2.
The microstructure morphology of the experimental grain oriented silicon steel slab.
The solidification process of continuous casting slab usually advances from the surface to the center core, and has the characteristics of directional solidification, but the formation of columnar or equiaxed crystals depends on the nucleation in the liquid phase before the solidification interface.21,22) If the molten metal is cast at a very low degree of superheat, the liquid phase is in a supercooled state during the solidification process, and there is a sufficient source of crystal nuclei, columnar crystals cannot be formed to obtain all equiaxed crystal microstructures. On the contrary, under the conditions of directional solidification controlled by strong heat flow, the liquid phase is in an overheated state and cannot be nucleated, and the columnar crystal solidification can be maintained.23–25)
When the casting speed is 1.0 m/min, with 900 A of electromagnetic stirring current and the slab width of 1070 mm, the effect of different superheat on the equiaxed crystal ratio of the slab is compared. It can be seen from Fig. 3 that as the superheat increases, the ratio of equiaxed crystals in the slab shows a decreasing trend, and the use of electromagnetic stirring can achieve the ratio of equiaxed crystals >50% for grain oriented silicon steel with superheat more than 40°C. During the solidification process of molten steel with higher superheat, the steel solidifies in a dendritic shape in the direction of the temperature gradient to form columnar crystals.
The effect of superheat on the equiaxed crystal ratio of the slab.
When the superheat is dissipated and the temperature of the molten steel is between the liquidus line and the solidus line, tiny equiaxed crystal nucleus grows with further cooling to form an equiaxed crystal. In the case of high superheat, a large amount of heat released during the solidification of the slab cannot be transferred in time, and the growth of columnar crystals is developed, resulting in an increase in the ratio of columnar crystals.
3.2 The effect of electromagnetic stirring intensity on equiaxed crystal ratioThe strong flow of the melt caused by electromagnetic stirring can break and erode the dendrite arms. Some of the dendritic fragments will serve as additional nuclei during the solidification of the molten metal, and the other part of the solute-rich dendritic fragments will be carried away from the dendrites by the molten steel flow for remelting. Therefore, the appearance of more nucleation bases and the temperature homogenization brought by the remelting of dendritic fragments will promote the formation of more equiaxed crystals.26) At the same time, the strong flow can greatly accelerate the heat and mass transfer of the liquid core, so that the superheat disappears rapidly, the two-phase zone rapidly expands, the diffusion boundary layer of the solidification front becomes thinner, the concentration gradient increases, and undercooling composition of the two-phase zone increases. It is conducive to the development of equiaxed crystals, so as to improve the equiaxed crystal ratio of the slab.
Steel is magnetic at normal temperature. When the temperature of the steel exceeds the Curie point, which is 760°C, it becomes non-magnetic, and the temperature of molten steel far exceeds the Curie point. The reason why it can be stirred is also because of its conductivity with the characteristics of non-magnetic. When the electromagnetic stirrer is given a multi-phase (two-phase or three-phase) alternating current, a rotating magnetic field that rotates around the axis is generated. When it cuts molten steel, an induced current is generated in it, that is
| \begin{equation*} I=\sigma\times E=\sigma\times (v \times B) \end{equation*} |
Among them, I: induced current density; σ: conductivity of molten steel; E: induced electric potential; v: relative velocity of magnetic field and molten steel; B: magnetic induction intensity.
The current interacts with the magnetic field to produce electromagnetic force, namely:
| \begin{equation*} F=I\times B=\sigma\times(v\times B)\times B \end{equation*} |
The electromagnetic force drives the movement of the molten metal. The change of the alternating electromagnetic field is related to the working frequency of electromagnetic stirring. The alternating electromagnetic field (B) produced by the electromagnetic stirrer is related to the intensity of the working current. Therefore, the stirring force produced by the electromagnetic stirring is affected by the working frequency and current of the electromagnetic stirring.
In the experiment, we changed the electromagnetic stirring force by adjusting the working current of electromagnetic stirring, to study its influence on the growth of the slab microstructure. The directional solidification characteristics of the slab determine the effect of electromagnetic stirring position on the slab microstructure. After the slab exits the mould, a large temperature gradient is generated due to forced cooling in the secondary cooling zone, which is beneficial to the growth of columnar crystals. As the distance from the mould outlet is farther, the liquid core area of the slab becomes smaller. The earlier electromagnetic stirring is applied after the slab exits the mould, the more conducive to the interruption and melting of the columnar crystals when the columnar crystals are not completely solidified in the early stage of formation. In the case of high superheat, the temperature gradient can be reduced in time to hinder the growth of columnar crystals and form opportunities that are conducive to the formation of equiaxed crystals.
It can be seen from Fig. 4 that the electromagnetic stirring current has a significant effect on the equiaxed crystal ratio. When the electromagnetic stirring current is increased, the equiaxed crystal ratio of the slab increases linearly from 10% to 75%. At the same time, according to Fig. 1, after electromagnetic stirring is used, the equiaxed grain size is significantly reduced, and the equiaxed grain ratio is increased with smaller grain size of the as-cast microstructure for silicon steel, which can increase the strength and toughness of the slab.
The influence of electromagnetic stirring current on the equiaxed crystal ratio of the slab.
As shown in Fig. 5, the ratio of equiaxed crystals at different casting speeds after using electromagnetic stirring shows that when the casting speed is increased by 0.1 m/min, the ratio of equiaxed crystals increases by about 0.7%–3.2%. As the casting speed increases, the solidified slab shell becomes thinner, the temperature gradient in the solid-liquid two-phase zone decreases, and the growth of columnar crystals becomes slow. Under the crushing and melting erosion of electromagnetic stirring, it is not conducive to the growth of columnar crystals toward the center of the slab, the proportion of equiaxed crystals increases. At the same time, under the same condition of other process, the proportion of equiaxed crystals at the 1320 mm of width is 3.7%–7.0% higher than that of the 1070 mm of width. With the same thickness of the slab, the slab width increases and the liquid phase area of the slab increases, the heat dissipation slows down. Under the action of electromagnetic stirring, the temperature gradient decreases, which is beneficial to the generation of equiaxed crystals.
Proportion of equiaxed crystals under different casting speeds and width.
The formation of the Goss texture of grain-oriented silicon steel is not only affected by the microstructure of the slab, but also by the uniformity of the chemistry composition distribution. The uneven composition will affect the abnormal growth of secondary recrystallized grains, which is not conducive to the form the uniform magnetic properties of finished products. Therefore, in the study of improving the microstructure and composition of the slab, the uniformity of the composition distribution in the slab must be considered.
During the crystallization process, the alloy composition of the solidified steel is different from that of the original molten steel. This phenomenon is caused by the selective crystallization. The solubility of alloying elements in the solid and liquid phases is different. Generally speaking, the solubility in the solid phase of steel is lower than that in the liquid phase, that is, the distribution concentration of alloying elements in the solid phase of steel is less than the distribution concentration in the liquid phase. Therefore, during the crystallization process of steel, a large amount of solute will precipitate and accumulate at the crystallization front, and the concentration of solute in the solid phase is lower than the original concentration. This phenomenon is called selective crystallization. With the continuous enrichment of the mother liquor solute, the concentration continues to rise, and as the temperature continues to drop, the molten steel will eventually solidify. Therefore, the solute content in the solid phase of the final solidified part will be higher than the original concentration. Therefore, during the entire solidification process, the solute concentration distribution in the slab microstructure is uneven. The first solidified part has a low solute content, while the last solidified part has a high solute content. This uneven composition is called composition segregation.
It can be seen from Fig. 6 and Fig. 7 that under the conditions of different electromagnetic stirring currents, there is obvious positive segregation of C and S at the 1/2 (120 mm) of the slab thickness, and the most obvious is at 200 A. When the electromagnetic stirring current is 900 A, C and S have obvious negative segregation at 40 mm and 200 mm in the thickness direction of the slab. When the electromagnetic stirring current value is reduced, the negative segregation is obviously weakened. In summary, weak stirring is not conducive to the homogenization of the solidified components in the center of the slab, while excessive stirring leads to accelerated mass transfer, resulting in the accumulation of easily segregated elements, and the segregation rate increases. Therefore, moderate electromagnetic stirring intensity (400 A) is conducive to the uniformity of the slab composition and weaken the segregation.
Segregation rate of C element in thickness direction under different electromagnetic stirring.
Segregation rate of S element in thickness direction under different electromagnetic stirring.
Through the statistics of the production data of different continuous casting process parameters, with the regression software Origin for polynomial fitting, the relationship between the ratio of equiaxed crystals of grain-oriented silicon steel slab and superheat, electromagnetic stirring current, casting speed, and slab width is shown as below:
| \begin{equation*} m = 4.91 + 0.28\,a + 0.0602\,b - 0.107\,c + 0.00739\,d \end{equation*} |
Among them, m: equiaxed crystal ratio, %; a: casting speed, m/min; b: electromagnetic stirring current, A; c: superheat, °C; d: slab width, mm.
Table 3 above is the comparison between the measured data and the predicted results. The deviation between the measured and predicted values of the equiaxed crystal ratio of the slab is relatively small, and the effective control of the equiaxed crystal ratio of the slab can be achieved by adjusting the process parameters.
