KR960000325B1 - Mold flux of continuous casting - Google Patents

Abstract

The mold flux comprises CaO of 28-42 wt.%, SiO2 of 29.0-43.0 wt.%, MgO of below 5.0 wt.%, Al2O3 of below 6.0 wt.%, F of 6.0-10.0 wt.%, Na2O of below 14.0 wt.%, B2O3 of below 1.0 wt.%, total carbon of 1.0-6.0 wt.%, and the balance inevitable impurities with basicity ratio of 1.2-1.3% and crystalline structure ratio of over 60.0%.

Description

Continuous casting mold flux with high crystallinity

1 is a graph showing the relationship between crystallinity and basicity, F, Al ₂ O ₃ , B ₂ O ₃ and MgO content of the mold flux.

2 is a cross-sectional photograph of the slag film taken while using the mold flux having different crystallinity in actual cycle.

3 is a graph showing the relationship between the crystallinity and the duty-free crack generation index.

The present invention relates to a mold flux for continuous casting, and more particularly, has a high crystallinity that can reduce cracks in duty free, which is one of the major defects occurring on the surface of cast steel, an intermediate product produced by the continuous casting method. It relates to a mold flux for continuous casting.

In general, molten steel containing a carbon component in the range of 0.08-0.l4wt% tends to generate longitudinal cracks (ie, vertical cracks) on the surface during solidification through a continuous casting process.

Thus, these grades are often called crack-sensitive grades.

These duty-free cracks are due to mechanical and thermal stresses acting on the surface of the cast steel. When the duty-free cracks occur, the final products are present as defects. In order to reduce the occurrence of cracks, especially in the case of continuous casting of crack-sensitive steels, various improvements have been proposed and implemented.

Mold is a device that determines the outer shape of cast steel, which is an intermediate product, when solidification of molten steel is started in the continuous casting process, and the thin cast steel (hereinafter referred to as “Shell”) that only solidifies the surface in contact with the water cooled mold. The mold flux is used as a lubricant while continuously vibrating with a vibrator to prevent breakage of the solidification shell due to friction between molds (hereinafter referred to as breakout breakout). As slag, it is injected onto molten steel in the mold during continuous casting to form an unmelted layer, an anti-melting layer, and a molten slag layer. Slag film is formed and this slag film is sprinkled to cool the slab as it is followed along with the slab drawn down the continuous casting machine. As is to be consumed by the wash water.

In this process, the mold flux plays various roles in addition to the lubrication action between the mold and the solidification shell.

That is, it reduces friction between cast and mold (lubrication), absorbs and dissolves non-metallic inclusions from molten steel as a separate float, protects molten steel from the atmosphere, prevents reoxidation, and suppresses heat release to the atmosphere. Heats and acts as a heat transfer medium from the cast to the mold.

Among the roles mentioned above, the most relevant to duty-free cracks is the heat transfer medium function. In order to reduce the duty-free cracks, the passion stress applied to the solidification shell should be minimized by inducing uniform and gentle cooling in the slab width direction.

For uniform and gentle cooling, a uniform slag film should be formed in the slab width direction, and uniform molten slag should be introduced beforehand. Conventionally, uniform molten slag keeps the viscosity of the mold flux within an appropriate range. It has been known to be the most effective.

That is, as the main method for selecting the mold flux for crack sensitive steel, conventionally, the basicity (CaO (wt%) / SiO ₂ (wt%)) of the mold flux is 1.0 to 1.1, and the viscosity (1300 ° C, poise) is drawn out. It controlled so that it might become 1.0 to 3.5 times (m / min).

The composition of the conventional mold flux is shown in Table 1 below.

TABLE 1

The present invention is to provide a mold flux for continuous casting having a high crystallinity that can reduce the generation of cracks in duty-free during continuous casting of crack-sensitive steel, by selecting the mold flux based on the crystalline rate, unlike in the conventional method, There is a purpose.

Hereinafter, the present invention will be described in detail.

The present invention, in the mold flux for continuous casting, wt%, CaO: 28 to 42.0%, SiO ₂ ; 29.0 to 43.0%, MgO: 5.0% or less, Al ₂ O ₃ : 6.0% or less, F: 6.0-10.0%, Na ₂ 0: 14% or less, B ₂ O ₃ : 10% or less, total carbon , 1.0 to 6.0%, and other unavoidable impurities, and the basicity is 1.2 to 1.3; And it relates to a mold flux for continuous casting having a high side crystallinity of 60.0% or more crystallinity.

Hereinafter, the present invention will be described in more detail.

The CaO and SiO ₂ is a component for maintaining a low melting point region, the content of CaO is preferably limited to 28.0 to 42.0%, SiO ₂ to 29.0 to 43.0%.

However, when the CaO (wt%) / SiO ₂ (wt%) ratio is 1.2 or less, the crystallinity of the mold flux and the basicity [CaO (wt%) / SiO ₂ (wt%)] are shown in FIG. As also shown in a), the crystallinity decreases rapidly, and in the case of 1.3 or more, the melting point of the flux increases, so the CaO / SiO ₂ ratio is preferably limited to 1.2 to 1.3.

The MgO is added to lower the melting point of the flux, but when the amount is 5.0% or more, as shown in FIG. 1 (b) showing the relationship between the crystalline rate of the mold flux and the MgO content, the crystalline rate rapidly increases. Since it is lowered, it is preferable to limit the amount of MgO added to 5.0% or less.

The Al ₂ O ₃ is added to adjust the viscosity of the flux, but when the addition amount is 6.0% or more, as shown in (c) of FIG. 1 showing the relationship between the crystalline rate of the mold flux and the Al ₂ O ₃ content Since the crystallinity decreases rapidly, the amount of Al ₂ O ₃ added is preferably limited to 6.0% or less.

F is a component added to lower the melting point and the viscosity of the flux. When the addition amount is 6.0% or less, as shown in (d) of FIG. 1 showing the relationship between the crystalline rate of the mold flux and the F content, If the crystallinity drops sharply and is more than 10%, the erosion resistance to refractory materials such as immersion nozzles is increased, so the content of F is preferably limited to 6.0 to 10%.

In order to reduce the melting point and viscosity of the Na ₂ O flux, the addition and withdrawal is a component, and the increase rate of the addition effect is lowered at 14% or more, and it is uneconomical, so the content of Na ₂ 0 is limited to 14% or less. It is desirable to.

The B ₂ O _{3 is} also added to lower the melting point and viscosity of the flux. When the addition amount is 1.0% or more, also in (e) of FIG. 1 showing the relationship between the crystalline rate of the mold flux and the B ₂ O ₃ content. As shown, since the crystallinity decreases abruptly, the amount of B ₂ O ₃ added is preferably limited to 1.0% or less.

In addition, the C is a component that acts as the aggregate of the flux, when the content is less than 1%, the role as aggregate is not sufficiently exhibited, and when the content is 6% or more, the amount of aggregate becomes too large to facilitate melting of the flux. Since it is not made, the content of C is preferably limited to 1 to 6%.

The mold flux of the present invention, which is formed as described above, has a melting point of 960 to 1250 ° C, a viscosity of 0.6 to 3.7 (1300 ° C, Poise), and a crystalline rate of 60% or more.

An example of the preferable crystalline rate measuring method in this invention is as follows.

First, 20 g of the mold flux is decarburized in an electric furnace maintained at 600 to 700 ° C., and then the decarburized mold flux is placed in a platinum furnace to be melted using an electric furnace. The molten mold flux is poured into a platinum container which is water cooled from below to solidify and cool.

The solidified and cooled mold flux shows a flat plate shape (hereinafter referred to as “Mold Flux Film”), and the cross section of the mold flux film is divided into a transparent glassy layer and an opaque crystalline layer.

After measuring the average value of the total thickness of the mold flux film and the opaque nodular layer thickness on the cross section of the thus prepared mold flux film, the crystallinity rate is determined by the following formula.

Crystalline rate (%) = (nodular layer thickness in mold flux film / total thickness of mold flux film) × 100

In the present invention, the decrease in the duty-free crack generation as the crystallinity increases is that when the molten slag solidifies into crystalline, microbubbles are present in the crystalline filling due to the solidification shrinkage, and these bubbles are molten steel through the slag film. This is because the thermal transfer between the shell and the mold is delayed, resulting in a gentle and uniform cooling effect on the solidified shell, thereby reducing thermal stress on the solidified shell.

Therefore, in order to obtain the above-mentioned effect, it is preferable to limit the crystalline rate to 60% or more.

Here, the crystallinity rate is an index indicating the tendency for the molten slag to solidify into crystalline.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

Mold flux scales having the composition, basicity, and crystallinity as shown in Table 2 below, for cracking sensitive steel grades under the same casting conditions, for two months in a continuous continuous casting machine, with regard to binding breakout, duty free crack generation index, and other application problems It investigated and the results are shown in Table 3 below. Moreover, after taking a slag film, it observed with the optical microscope, and the result is shown in FIG.

TABLE 2

TABLE 3

The duty-free crack incidence index in Table 3 is the duty-free crack incidence rate which is the value obtained by dividing the number of the duty-free crack incidence cast parts by the number of cast parts cast using the mold flux in the case of crack sensitive steel grade casting using the mold flux. Is the value obtained by dividing the vertical cracking rate of the mold flux having a crystalline rate of 0.7%.

As shown in Table 3, it can be seen that the case of applying the mold flux of the present invention is superior in the crack generation index in terms of duty free compared to the case of applying the conventional mold flux.

As shown in FIG. 2, it can be seen that the crystallinity of the actual slag film is increased when the crystallinity is high.

Example 2

Except for changing the crystalline rate as shown in FIG. The index was measured and the measurement result is shown in FIG. 3 in relation to the crystallinity rate.

As shown in FIG. 3, as the crystalline rate increases, there is a clear tendency that the incidence of duty-free cracks decreases. This decrease tends to be the largest at around 60% and the crystalline rate is higher than that. It can be seen that the effect is maintained when the crystallinity rate is around 60%.

Therefore, it can be seen that the occurrence of duty-free cracks can be reduced when casting a crack-sensitive steel grade by applying a mold flux having a crystalline rate of 60% or more.

Claims (1)

In the mold flux for continuous casting, in wt%, CaO: 28 to 42%, SiO ₂ : 29.0 to 43.0%, MgO: 5.0% or less, Al ₂ O ₃ : 60% or less, F: 6.0 to 10.0%, Na ₂ O: 14.0% or less, B ₂ 0 ₃ : 1.0% or less, Total Carbon: 1.0-6.0%, and other unavoidable impurities, and have a basicity of 1.2 to 1.3; And a crystalline rate of 60.0% or more, the mold casting for continuous casting having a high crystal.