JPS6316218B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for controlling the level of molten metal in a mold of continuous casting equipment. Generally, in continuous casting equipment, as shown in Fig. 1, molten steel is poured from a tundish 3 through a nozzle 2 immersed in a water-cooled mold 1.
The solidified slab 4 is pulled out by pinch rolls (not shown) installed below. The level of the molten metal in the mold 1 must be controlled so that molten steel does not overflow from the mold, and for this purpose a level meter 5 is provided above the mold. This level meter 5 is generally of the eddy current type and can measure with an accuracy of about 1 mm. The detected hot water level signal is led to the controller 6, where calculations mainly based on proportionality and integration are performed, and then the nozzle opening degree is output to the sliding nozzle control device 7. The sliding nozzle control device 7 gives a signal to a motor 9 that drives the sliding nozzle 8, and the sliding nozzle 8 opens and closes to change the flow rate of molten steel from the tundish 3. The opening degree of the sliding nozzle 8 is detected by an interlocking opening degree meter 10 and inputted to the controller 6. In the figure, V indicates the drawing speed, and W indicates the tundish weight. The above is the mold level control method that has been generally implemented in the past, but in recent years, the level meter 5 has become more accurate, and changes in the pinch roll drawing speed and tundish weight that cause noise in this control system. With the development of technology to compensate for changes, etc.
The accuracy of controlling the hot water level has improved dramatically. An example is shown in FIG. The hot water level remains stable regardless of changes in drawing speed, and is mostly controlled within ±5 mm. In the figure, α indicates ±5 mm. For reference, in conventional equipment, as shown in Table 1,
±5mm accounts for 93% of the total casting length, and ±6mm accounts for 100%.

[Table] On the other hand, it is known that mold level control not only prevents molten steel from overflowing from the mold, but also has a significant relationship with surface defects on slabs. Figure 3 shows the relationship between the fluctuation of the melt level and the wrinkling of the slab. The image shows slight hot water wrinkles,
is an extremely large hot water wrinkle. According to this, the larger the amount of hot water level fluctuation, the worse the water wrinkle gauge, and the larger the hot water level fluctuation rate (when positive), which is obtained by differentiating the amount of hot water level fluctuation, the worse the hot water wrinkle rating. There is. Further, FIG. 4 shows the relationship between the rising rate of the hot water level and the hot water wrinkle rating, and FIG. 5 similarly shows the relationship between the rising rate of the hot water level and the size of inclusions. It is thought that this is because the higher the rising speed of the melt level, the larger the melt wrinkles, and the inclusions that should float at this time are captured by the solidified shell. As mentioned above, when the level control becomes more precise, the change in the level becomes smaller and the rate of rise in the level becomes smaller, resulting in slabs with a better surface, and as a result, the size of inclusions becomes smaller. As a result, it has become possible to omit processes such as inspection for inclusions. However, increasing the precision of hot water level control has posed serious problems. This is localized erosion caused by powder from the immersion nozzle. Normally, alumina graphite is used for the immersion nozzle 2, but when the level of the molten metal in the mold is maintained constant, it is thrown onto the molten steel, and the lubricating powder between the mold and the molten steel accumulates on the surface. Locally eroded,
A hole will open and it will become unusable. FIG. 6 is a partially enlarged view of the submerged nozzle, with the eroded portion indicated by 11. Normally, in continuous casting operations, molten steel is poured from a ladle containing molten steel (not shown) into the tundish 3 shown in Fig. 1, but in order to increase productivity, the molten steel is cast continuously from multiple ladles (not shown). Continuous-continuous casting is carried out by casting into a tundish 3 and operating it. The number of ladles usually corresponds to the number of charges in a converter, etc., so the important issue is how to cast as many charges as possible in one continuous casting operation. If the immersion nozzle is damaged during casting, subsequent casting will be impossible, resulting in major operational troubles. In the experience of the present invention, 5 is an example in which a submerged nozzle designed to withstand charges becomes unusable in the middle of 3 charges, or continuous casting is impossible for more than 2 charges. Ta. This situation has arisen as a result of the recent increase in the precision of hot water level control, and when the hot water level control accuracy is low as in the past, the hot water level is changing appropriately. Therefore, the position of the erosive powder also changes, and as a result, the positions in contact with the immersion nozzle 2 are equalized, thereby achieving a longer service life of the immersion nozzle. The present invention was made by focusing on the respective advantages of the recent high-precision hot water level control method and the conventional low-precision hot water level control method. This realizes a lifespan comparable to that of conventional immersion nozzles. In other words, the quality of slabs has been improved due to highly accurate level control, but in order to maintain a constant level, the level position of the immersion nozzle is intensively eroded by powder. On the contrary, this is prevented by changing the hot water level. In the present invention, the target hot water level set by the controller 6 (conventionally, it was set to a constant value by the operator) is raised and lowered by a time function as shown in FIG. In this case, as shown in Figure 8,
This is done by adding the output of the triangular wave generating circuit 12 to the circuit that calculates the difference between the feedback signal L of the hot water level and the target level, but if the controller 6 is composed of a microcomputer, this can be easily done. A triangular wave generation circuit 12 can be incorporated.
The period T and amplitude A of the triangular wave to be added to this target value are given by a program, and usually may be constant. However, it is important to ensure the quality of the slab by setting the rate of change of this triangular wave V (the amount of change in the melt level per unit time) to a value of V=A/T/4=4A/T1mm/sec (1) It is important on In other words, from Figures 3, 4, and 5, the rate of rise in the hot water level is 2.
If it exceeds mm/sec, the hot water wrinkles on the surface of the slab will become large, and as a result, inclusions will be trapped, so it is necessary to suppress the hot water level rising speed to 2 mm/sec or less. On the other hand, in the latest hot water level control method, as shown in Table 2, the hot water level rising speed is controlled to 1 mm/sec or less.

[Table] Therefore, when changing the hot water level target value, select the period and amplitude of the triangular wave to be added, as shown in equation (1), and the hot water level rise rate of 1 mm/ Even if seconds are added, the total rising speed of the hot water level is 2 mm/second. In the example, the amplitude was 10 mm, the period was 1 hour, and the rate of change V was about 0.01 mm/sec. Figure 9 shows the fluctuations in the hot water level in this case, and it shows that the short-cycle fluctuation width of the hot water level is within ±5 mm, and at the same time, the control that follows the long-term changes in the target value is performed. You can see that As a result, since the contact position of the powder on the immersion nozzle changes every moment, local erosion by the powder does not occur, making it possible to perform 5-charge continuous casting, and the nozzle erosion status after completion can be changed. 1st
As shown in Figure 0, it is uniform. Next, as a comparative example, the target value of the hot water level was controlled to be as constant as possible without periodically raising and lowering it. As a result, as shown in Figure 2, the hot water level is controlled within a range of fluctuation of 5 mm with respect to the target value (0 level).
There is no periodic vertical movement in relation to the target value of the hot water level. For this reason, as shown in FIG. 6, the immersion nozzle for injecting molten steel was locally eroded as shown in the eroded portion 11, and holes were opened, making it unusable. This occurs because the lubricating powder on the molten steel surface is always in contact with approximately the same lengthwise portion of the nozzle. As another comparative example, the target change rate V of the hot water level is set to 1.
It was periodically moved up and down at more than 7 mm/sec. That is, the hot water level was varied with a period of 5 minutes and a swing width of 525 mm. As a result, the rising speed of the hot water level is 8 mm/sec, which comes from the accuracy of the hot water level control system itself.
mm/sec, and as a result, the hot water wrinkle rating deteriorated to B to C, as shown in FIGS. 3 and 4. In the present invention, since the target value is automatically changed, there is no need for the operator to be concerned with the level of the hot water. Furthermore, although the method of the present invention is effective for high-precision hot water level control, if the rate of change V is kept sufficiently small, it can also be applied to conventional control that has a wide range of hot water level fluctuations, and has a long nozzle life. Although the effect of oxidation is small, it can be expected. Furthermore, in the present invention, the change in the target hot water level is made into a triangular wave, but it may be a sine wave as long as it satisfies equation (1), or a sawtooth wave that increases the velocity in the direction of the drop in the hot water level. But it's okay. This is because the rising speed of the molten metal surface has a negative effect on the slab surface as shown in Figures 3, 4, and 5, but the falling speed has no effect. Regarding how to change the target value of the hot water level, it is fine if the triangular wave function shown in Fig. 7 is generated by an analog circuit, but if it is performed by a digital circuit or calculation, microscopically it will look like a step. The rate of change will be microscopically excessive, but considering the response time of the control system, find the average rate of change for 1 to 2 seconds, and even if this is 1 mm/sec or less, However, this is consistent with the intent of the present invention. As described above, according to the method of the present invention, not only can high-quality slabs be obtained, but also the number of charges in continuous-to-continuous casting can be increased, which has great effects in terms of productivity. be.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of molten metal level control in continuous casting.
Figure 2 is an explanatory diagram of hot water level control accuracy, Figure 3 is a graph of the relationship between hot water level rising rate and hot water wrinkles, Figure 4 is a graph of the relationship between hot water level rising rate and hot water wrinkle rating, Figure 5 is a diagram showing the relationship between the rising speed of the hot water level and the size of inclusions, Figure 6 is an explanatory diagram of the erosion status of the immersion nozzle, Figure 7 is a diagram showing the relationship between the hot water level and time, and Figure 8 is a diagram showing the relationship between the hot water level and time. FIG. 9 is a block diagram of the generating circuit, FIG. 9 is an explanatory diagram of the fluctuation of the hot water level according to the present invention, and FIG. 10 is an explanatory diagram of the state of erosion of the immersion nozzle when the present invention is implemented. 1; mold, 2; immersion nozzle, 3; tundish, 4; slab, 5; level meter, 6; controller,
7; Sliding nozzle control device, 8; Sliding nozzle, 9; Motor, 10; Sliding nozzle opening gauge, 11; Immersion nozzle erosion part, 1
2; Triangular wave generation circuit.

Claims (1)

[Claims] 1 The level signal of the continuous casting of steel by a continuous casting machine equipped with a constant level control device is guided to the controller that sets the target level, and the difference between the actual level and the target level is calculated. In a device that adjusts the nozzle opening degree based on this, the target value of the molten steel level in the mold, which is set in this controller, is periodically raised and lowered, and the rate of change at this time is V (molten steel per unit time). A mold surface level control method, characterized in that the amount of surface level change) is 1 mm/sec or less.