Through the low-magnification inspection of the oriented silicon steel casting slab, the influence of the superheat of the tundish molten steel, the electromagnetic stirring current, the strength of the secondary cooling water, the drawing speed and the cross-sectional size on the center equiaxed crystal rate of the casting slab was studied. The results show that under the test conditions, when the superheat of the molten steel in the tundish is controlled within the range of 14～18 ℃, the change of the central equiaxed crystal rate caused by the difference of superheat is small, and the variation range is only 0%～3%; The electromagnetic stirring current and the slab cross-sectional size have obvious effects on the central equiaxed crystal ratio. With the increase of the stirring current, the central equiaxed crystal ratio increases, and when the slab cross-sectional size increases, it shows a decreasing trend. ; The second cold water strength and pulling speed have relatively little influence on the central equiaxed crystal ratio. With the increase of the second cold water strength, the central equiaxed crystal ratio shows a decreasing trend, and when the pulling speed increases, it shows an increasing trend .
Oriented silicon steel is an indispensable and important soft magnetic functional material for the electric power and electronic industries. It has the characteristics of high magnetic inductance and low iron loss. It is mainly used in large and medium-sized transformer cores. Oriented silicon steel is praised as a "tower spire product" and "steel artwork" because of its long and complex production lines, high production process control requirements, and many performance-influencing factors. The manufacturing technology and product quality of silicon steel sheets are considered to be an important indicator of the production and technological development of high-end steel products in a country. High-end silicon steel products are also one of the important directions for the future development of the steel industry. The columnar crystal structure developed in the silicon steel continuous casting billet will cause serious internal cracks and center segregation, and cause the hot-rolled sheet to appear a coarse band structure. Therefore, the columnar crystal structure developed in the silicon steel slab should be suppressed, and the equiaxed crystal ratio in the slab should be increased. This paper compares the slab structure under different control methods, observes the macrostructure of the oriented silicon steel slab, and analyzes the influence of different continuous casting process parameters on the center equiaxed crystal ratio of the oriented silicon steel slab.
1 Continuous casting process parameters and research methods
1.1 Continuous casting process parameters
The steel zone of a steel mill adopts the process flow of "hot metal pretreatment □ converter smelting □ RH refining □ continuous casting" to produce oriented silicon steel, in which the continuous casting process uses a two-stream straight arc slab continuous caster and adopts two cold zone roller electromagnetic stirring , The two pairs of electromagnetic rollers are respectively located at 4.0 and 6.8m away from the liquid level of the crystallizer. Since the superheat of the molten steel in the tundish has an important influence on the equiaxed crystal ratio of the slab center, low superheat pouring is beneficial to increase the equiaxed crystal ratio. Therefore, the electromagnetic stirring current, the strength of the secondary cooling water, the pulling speed and the cross-sectional size have an important influence on the orientation. Under the influence of the equiaxed crystal rate of the silicon steel casting slab center, the superheat of the molten steel in the tundish of the test heat should be controlled as low as possible in a narrower range. In this study, the change range of the molten steel superheat in the tundish was 14-18 ℃. In the test furnace, the third block of the double-stream casting slab was taken from the transverse low-magnification sample. The continuous casting process parameters and corresponding low-magnification sample numbers of the test furnaces are shown in Table 1. The cooling intensity of the cooling curve No. 1 is greater than that of the cooling curve No. 2. The amount of water increased by about 10%.
1.2 Research methods
The low-magnification structure of the oriented silicon steel slab is obtained by the hot acid method, and the central equiaxed crystal ratio is calculated to analyze the influence of different continuous casting process parameters on the center equiaxed crystal ratio of the oriented silicon steel slab. The hot acid method is a low-magnification inspection method for steel widely used in steel mills. After taking a sample of a continuous casting slab with a thickness of 100 mm and a width larger than 1/2 of the slab width, it is processed by a milling machine to a surface roughness less than 1.6. μm, the sample is acid-etched with a hydrochloric acid solution at a temperature of 60-85 ℃ for 15-25 minutes, and then the surface of the casting slab sample is cleaned with alkaline water, and the macrostructure of the casting slab can be observed. The specific process is shown in Figure 1. The typical low-magnification structure of the continuous casting billet is composed of three crystal bands, from the outside to the inside, there are small equiaxed crystal belts, columnar crystal belts and central equiaxed crystal belts. The schematic diagram is shown in Figure 2.
For the samples after acid leaching, the center equiaxed crystal area must be marked first. The boundary between the columnar crystal region and the central equiaxed crystal region of the billet is not a straight line in the strict sense. However, in order to facilitate the calculation of the central equiaxed crystal ratio, an approximate method is used to average the corresponding low-magnification samples under different continuous casting process parameters. A straight line is used to distinguish the boundary between the columnar crystal area and the central equiaxed crystal area, and the central equiaxed crystal area is considered to be rectangular in shape. Then measure and calculate the central equiaxed crystal area and total surface area of the slab, and calculate the equiaxed crystal ratio in the center of the slab according to formula (1).
In the formula: E is the center equiaxed crystal rate, %; Sc is the center equiaxed crystal area area, mm2; Ss is the total surface area of the slab, mm2.
2 Result analysis
The low-magnification microstructure and central equiaxed crystal region of the cast slab under different continuous casting process conditions are shown in Figure 3. The calculated center equiaxed crystal rate is shown in Table 2.
2.1 The influence of the superheat of the molten steel in the tundish
In order to analyze the influence of the superheat of the molten steel in the test heat on the center equiaxed crystal rate, it is necessary to compare and analyze the center equiaxed crystal rate of the slab under the same process conditions except the superheat to determine the center caused by the superheat The size of the change in equiaxed crystal ratio. Under the test conditions, the same continuous casting process parameters except superheat can be divided into 4 categories, as shown in Table 3. The comparison of the superheat and the equiaxed crystal rate of the center of the cast slab under the 4 types of continuous casting process conditions is shown in Table 4. It can be seen from Table 4 that under the conditions of different types of continuous casting process parameters, the range of superheat difference is 1～4 ℃, and the range of center equiaxed crystal rate difference is 0%～3%; When the heat is different, there is a certain difference in the center equiaxed crystal rate. The largest difference between superheat and equiaxed crystal rate is category IV. The difference in superheat is 4°C, the center equiaxed crystal rate is 3%, and when the superheat is low, the corresponding center equiaxed crystal rate is higher. Type I corresponds to a 1°C difference in superheat and a 1% difference in center equiaxed crystal ratio. Also, when the degree of superheat is low, the corresponding center equiaxed crystal ratio is higher. Type II and Type III superheat degrees differ by 1 and 3 ℃ respectively, and the corresponding center equiaxed crystal rates are the same. According to Table 2, under the test conditions, the degree of superheat is controlled within the range of 14～18 ℃, and the change range of the equiaxed crystal rate of the slab center under different continuous casting process conditions is 23%～39%. The variation range of the center equiaxed crystal ratio caused by the difference is only 0% to 3%, which is relatively small. Therefore, when analyzing the influence of other continuous casting process parameters on the central equiaxed crystal rate under the experimental conditions, the influence of superheat on it can be ignored.
2.2 The influence of electromagnetic stirring current
Figure 4 shows the isometric crystal rate of the cast slab center under different electromagnetic stirring current conditions when the cross-section size is 230mm×1290mm, the pulling speed is 0.90m/min, and the No. 1 cooling curve is adopted. It can be seen from Figure 4 that the average values of the equiaxed crystal ratios at the center of the slab under the conditions of 200A-5 Hz and 300A-5 Hz are 24% and 39%, respectively. In the case of the same electromagnetic stirring frequency, the current is increased from 200 to 300 A, and the center equiaxed crystal rate is increased from 24% to 39%, with a difference of 15%. Therefore, appropriately increasing the electromagnetic stirring current can effectively increase the center equiaxed crystal rate of the oriented silicon steel billet. This is because the columnar crystals formed by stirring the molten steel are interrupted, and the broken columnar crystal grains become the core for the formation of equiaxed crystals; at the same time, electromagnetic stirring increases the flow of molten steel and strengthens the convection and the heat transfer between solid and liquid. It reduces the degree of molten steel superheat, inhibits the directional growth of columnar crystals, and promotes the growth of equiaxed crystals, thereby increasing the center equiaxed crystal ratio of the cast slab.
2.3 The influence of the intensity of the secondary cold water
Figure 5 shows the central equiaxed crystal rate of the cast slab under different cooling curve conditions when the cross-section size is 230mm×1290mm, the pulling speed is 0.90m/min, and the electromagnetic stirring parameter is 200A-5 Hz. It can be seen from Fig. 5 that the equiaxed crystal ratios of the slab when the cooling curves No. 1 and No. 2 are used are 24% and 28%, respectively. Under the two cooling conditions, when the No. 1 cooling curve, which is stronger than the No. 2 cooling curve, is used, the center equiaxed crystal rate is reduced by 4%. This is because when the cooling intensity is high, the unsolidified molten steel in the slab has a larger temperature gradient, and the degree of subcooling at the front of the solid-liquid interface increases, which is conducive to the formation of developed columnar crystals, but suppresses the equiaxed crystals in the center of the slab. Formation and growth.
However, since the cooling curve No. 1 does not increase much (approximately 10%) compared to the cooling curve No. 2, the equiaxed crystal rate of the slab center corresponding to the No. 1 curve is reduced to a certain extent, but it corresponds to the No. 2 cooling curve. The equiaxed crystal rate of the slab center is not much different. 2.4 The impact of pulling speed
Figure 6 shows the center equiaxed crystal rate of the cast slab under different drawing speed conditions when the cross-sectional size is 230mm×1290mm, the No. 2 cooling curve, and the electromagnetic stirring parameter is 200 A-5 Hz. It can be seen from Fig. 6 that the equiaxed crystal ratios of the slab center under the conditions of pulling speeds of 0.90 and 1.0 m/min are 28% and 30%, respectively. When the drawing speed is higher, the center equiaxed crystal ratio increases slightly, with a difference of 2%. It can be seen that an appropriate increase in the drawing speed can increase the center equiaxed crystal ratio of the oriented silicon steel slab to a certain extent. This is because as the drawing speed increases, the temperature gradient and solidification rate in the two-phase zone of the slab solidification decrease, the undercooling at the front of the solid-liquid interface decreases, the primary dendrite spacing of the slab increases, and the columnar crystals transform to equiaxed crystals. In advance, the equiaxed crystal rate increases.
2.5 The influence of the section size of the cast slab
Figure 7 shows the isometric crystal rate of the slab center under different cross-sectional dimensions when the pulling speed is 1,000 m/min, the No. 2 cooling curve, and the electromagnetic stirring parameter is 200A-5 Hz. It can be seen from Figure 7 that the average values of the equiaxed crystal rate of the slab center under the conditions of 230mm×1290mm and 230mm×1060mm in cross-section are respectively 29% and 39%, under the two cross-sectional size conditions, the center equiaxed crystal rate differs by 10%. It can be seen that the center equiaxed crystal rate of the oriented silicon steel billet with small cross-section is higher under the same cooling process conditions.
3 Conclusion
(1) Under the test conditions, when the change range of the superheat degree of the molten steel in the tundish is 14～18℃, the change of the center equiaxed crystal rate of the oriented silicon steel slab caused by the difference of the degree of superheat is small, and the change range is only 0%. ~ 3%. Therefore, when analyzing the influence of other continuous casting process parameters on the center equiaxed crystal ratio of the oriented silicon steel billet, the influence of superheat on it can be ignored.
(2) The effect of electromagnetic stirring current and slab cross-sectional size on the center equiaxed crystal rate is more obvious. The center equiaxed crystal rate is higher when the stirring current is large, and the center equiaxed crystal rate is lower when the slab cross-sectional size is larger. . Under the condition of electromagnetic stirring frequency of 5Hz, when the stirring current is increased from 200 to 300A, the center equiaxed crystal rate increases from 24% to 39%; the cross-sectional size changes from 230 mm×1060 mm to 230 mm×1290 mm, the center is equal to The axial crystal ratio is reduced from 39% to 29%.
(3) The secondary cooling water strength and pulling speed have relatively small influence on the central equiaxed crystal ratio. When the secondary cooling water has a higher intensity, the central equiaxed crystal ratio will decrease slightly, but when the pulling speed is higher, the central equiaxed crystal ratio will increase slightly. . When the secondary cooling water curve changes from No. 2 with a lower cooling intensity to No. 1 with a higher cooling intensity, the center equiaxed crystal ratio is reduced from 28% to 24%; the pulling speed is increased from 0.90 to 1. At 00m/min, the center equiaxed crystal rate increased from 28% to 30%.
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