JPS6247621B2 - - Google Patents

Description

[Detailed description of the invention] The present invention deals with non-metallic inclusions in molten steel, especially fine particles.
This invention relates to a filter used to remove Al ₂ O ₃ based inclusions. Nonmetallic inclusions that are present in molten steel and cannot be separated and removed before solidification is completed have an adverse effect on the workability and mechanical properties of the steel, as well as the surface quality of the finished product. In recent years, the quality required for steel materials has become increasingly strict, and so-called screen steel with fewer impurities is now in demand. A conventional method for removing impurities is to add a refining agent (flux) such as CaO, CaF ₂ or Na ₂ CO ₃ to hot metal or molten steel to perform desulfurization and dephosphorization treatment. Furthermore, degassing of H ₂ , N _{2 ,} etc. is carried out using a vacuum degassing device such as an RH type or a DH type. Furthermore, removal of deoxidation products, that is, nonmetallic inclusions, is also an important point in cleaning molten steel. Particularly in the case of Al-deoxidized steel, the deoxidized product Al ₂ O ₃ is difficult to float and remains in the slab, which causes deterioration of product quality. Normally, in the case of continuous casting, these non-metallic inclusions are floated and separated in a tundish and the molten steel is poured into a mold as clean molten steel. But 100
Currently, it is difficult to float and separate microscopic nonmetallic inclusions smaller than µ or inclusions in the form of Al ₂ O ₃ clusters from steel, and no appropriate removal technology has been established. . The present inventors have found through various experiments that calcareous refractories that come into contact with molten steel tend to react with Al ₂ O ₃ in the steel.
We obtained the knowledge that Al ₂ O ₃ is easily absorbed. An example of the experimental connection is shown in FIG. This figure shows the surface of a calcareous refractory (93% CaO, balance MgO) used in a continuous casting tandem.
This is the Al ₂ O ₃ content distribution, and it can be seen that Al ₂ O ₃ has penetrated to about 20 mm from the refractory surface. Also,
This test result was obtained after casting 4ch (400t), but the calcareous refractory was hardly eroded and the reaction products (a(CaO), b
This shows that there is no contamination of (Al ₂ O ₃ )), indicating that the reaction product itself is not the source of inclusions. This means that Al ₂ O ₃ in the molten steel can be separated and removed by bringing the calcareous refractory into contact with the molten steel, and the first point of the present invention is to actively utilize this phenomenon. Next, the second point is that inside the tundish,
As shown in FIGS. 2 and 3A and 3B, cylinders 1 made of calcareous refractories are stacked one on top of the other. In other words, the stacking positions are as shown in Figure 2.
One or more arbitrary positions between the injection long noise 3 into the tundish 2 and the casting nozzle 4 into the mold. The direction in which the cylinders are stacked is such that the axis of the cylinders matches the direction of the flow of molten steel in the tundish.This increases the chance of contact between the cylinders and molten steel, and also straightens the flow of molten steel after passing through this filter. This prevents the slag and heat insulating agent on the surface of the molten steel in the tundish from becoming entangled. In addition, numeral 5 in the figure indicates a holding plate for preventing floating. Next, the differences between the filter of the present invention and related prior art will be described. First of all, let's talk about the difference between this technique and the conventional technique, which has a large number of through holes.With this technology, the molten steel comes into contact only with the inner surface of the through holes, and the contact area of the molten steel is small. On the other hand, since the filter of the present invention is a cylindrical refractory material and is made of multi-layered refractories, the molten steel contacts both the inner and outer surfaces of the cylinder, significantly increasing the contact area, and as a result, the effect of adsorbing inclusions is reduced. Extremely large. Next, JP-A-56-1252 proposes a filter made of a refractory shelf. The difference between this shelf filter and the filter of the present invention is that, first, in the case of a shelf filter, it is necessary to fix each filter to a tundish tray, making it extremely difficult to construct a tundish tray. The filter of the invention is extremely easy to install by stacking the cylinders in a tundish and finally pressing them down with a single holding plate. Next, in the case of the second shelf filter, the contact with the molten steel is a plane contact, whereas in the filter of the present invention, the contact is a curved surface with a large number of cylinders, and the contact area with the molten steel is larger in the filter of the present invention. The larger the size, the greater the ability to adsorb inclusions. Another reason for stacking them in a cylindrical shape is to allow sufficient preheating gas to pass through when preheating the tundish.
This is to make it easier to heat the steel to a temperature that will prevent the molten steel from solidifying in the filter section when pouring molten steel. In other words, JP-A-56-
When a filter is made of granular refractories like 47509, subtle differences in the way the granular refractories are piled up create unevenness in the passage of the gas flow, resulting in part of the filter becoming clogged and causing the molten steel to become clogged. As a result, it became difficult to obtain the desired casting speed. However, by forming the calcareous refractory into a cylindrical shape as in the present invention, the entire filter is preheated uniformly, and troubles such as clogging of molten steel do not occur. However, if the area ratio of the space in the cross section of the filter decreases or the length of the cylinder increases, the contact rate with the molten steel will increase, but the pressure loss of the molten steel at the filter will increase, and the shape of the tundish will change. Since the process becomes complicated, it is necessary to consider the shape of the tundish, the amount of casting per unit time, etc. to make it appropriate. The specific effects of the present invention will be explained below using Examples and Comparative Examples. Example After tapping the steel from the converter, the molten steel as shown in Table 1, which was finely adjusted in the RH vacuum degassing equipment, was heated to a wall thickness of 40 mm and an outer diameter of 80 mm.
25 pieces of lime-based cylindrical refractories with a length of 100 mm are stacked in 5 tiers of 3, 4, 5, 6, and 7 from the bottom of the TD, and stacked from above to prevent these refractories from floating. The sample was placed in a tundish equipped with a filter held down by a high alumina refractory holding plate, and cast into a mold using an immersion nozzle under the conditions shown in Table 1. As a result, TO=25ppm, Al ₂ O ₃ inclusions were
Extremely clean steel with 53 pieces/ ^25mm2 was obtained by microscopic observation, and the magnetic detection defect occurrence rate in finished tinplate was 2 pieces/1000.
^m2 , which was a very good result. Comparative Example As a comparative test with the example, molten steel with almost the same composition (shown in Table 1) was melted in the same process as the example, passed through a normal tundish without a filter, and placed into the mold with an immersion nozzle to form a molten steel with the same composition as the example. Casting was performed under the same casting conditions. As a result, TO=38ppm, _{Al2O3 -} based inclusions, 124
pieces/25mm ² , magnetic detection defect occurrence rate in finished tinplate is 6
KE/1000m ² The cleanliness and product quality of the cast slab were inferior compared to the case where an inclusion filter was installed. As is clear from the Examples and Comparative Examples above, it can be said that the filter for removing non-metallic inclusions according to the present invention is extremely useful. 【table】

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the change in Al ₂ O ₃ content from the surface of the lime-based refractory of the tundish after casting, and Fig. 2 and Fig. 3 A and B show the nonmetallic inclusion removal filter according to the present invention. It is a diagram when it is installed in a tandem tray. 1... Lime-based cylindrical refractory material, 5... Anti-surfacing holding plate.

Claims (1)

[Claims] 1. Between the injection position from the ladle of the continuous casting tundish and the injection position from the tundish into the continuous casting mold, a multi-layered lime-based cylindrical refractory is placed in the direction of flow of molten steel in the tundish.
Install it in one or more locations with the axial direction of the cylinder parallel,
A filter for removing non-metallic inclusions from molten steel, characterized in that it adsorbs minute Al ₂ O ₃ inclusions.