JPH105952A - Flux for continuous casting of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cast the slab free from surface defects by specifying the surface tension of the flux for continuously casting steel at 1500 deg.C and the viscosity at 1300 deg.C in the range of the respective prescribed values. SOLUTION: In the flux for continuous casting, the surface tension at 1500 deg.C is in the range of 0.4∼0.6N/m, and the viscosity at 1300 deg.C is in the range of 0.05-0.5Pa's respectively. When the surface tension at 1500 deg.C exceeds 0.6N/m, inclusions and bubbles are left on the interface to generate surface defects. On the other hand, when the surface tension is smaller, the molten flux can be included in the molten steel, and the lower limit to prevent this phenomenon is 0.4N/m. When the viscosity of the molten flux at 1300 deg.C is lower than 0.05Pa's, not only inclusion of the molten flux into the molten steel is generated but also the excessive flow-in is generated to cause the surface cracking. When the viscosity exceeds 0.5Pa's, the heat flux in a mold is increased due to insufficient flow-in of the flux to cause defective lubrication, depression, and surface cracking.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a flux for continuous casting of steel.

[0002]

_2. Description of the Related Art In a general continuous casting flux, the main components are CaO and SiO ₂ , and in order to adjust the viscosity and the freezing point, these main components are alkali or alkaline earth metal oxides, carbonates and fluorine. Compound has been added.

[0003] Regarding the physical properties of such a flux, the viscosity and the freezing point are adjusted to adjust the inflow amount, and the basicity and the freezing point are adjusted to adjust the heat removal in the mold.
In some cases, the chemical composition of the flux is adjusted according to the composition of the molten steel.

Japanese Patent Application Laid-Open No. Hei 3-77753 discloses that, in addition to the concentration of Al ₂ O ₃ and SiO _{2 in} a flux, the concentration of other CaO, Na ₂ O, F, and MgO is defined. Has been proposed to enable the casting of steel containing 2% or more. this is,
In casting Al-containing steel, A
This prevents the inflow failure of the flux due to the transition of l.

The present inventors have disclosed in Japanese Patent Application Laid-Open No. 5-15955 a flux applicable to high-speed casting by specifying the carbonate concentration, viscosity, melting point and basicity in the flux. This is for the purpose of securing the inflow of flux in high-speed casting.

The main purpose of any of the above fluxes is to stably lubricate.

[0007]

However, with the progress of continuous casting technology, strict requirements have been made on the quality of cast slabs in addition to smooth casting. In particular, in the case of a slab for a thin plate, there are problems such as a bubble defect and an inclusion defect on the surface thereof, or a defect due to entrainment of molten flux into molten steel.
These also pose a problem in slabs for thick plates, blooms or billets for steel pipes, though to varying degrees. As for the stainless steel sheet, the scale loss at the time of rolling is small, and the surface property is regarded as important for its application. Therefore, even a small bubble defect becomes a quality problem.

In order to stabilize the lubrication, the flux must have a low viscosity and a low freezing point, but if these properties are provided, it becomes a disadvantageous condition for the above-mentioned entrainment.

[0009] Inclusions and bubbles in the molten steel in the continuous casting mold float through the interface between the molten steel and the molten flux. Heretofore, knowledge of these detailed behaviors has not been sufficiently obtained, and there has been no report on a flux for continuous casting that takes into account the interface detachment phenomenon of inclusions and bubbles in relation to the physical property values.

[0010] It is an object of the present invention to prevent entrapment of a molten flux into molten steel and to promote the transfer of bubbles at the interface between the molten steel and the flux and nonmetallic inclusions in the molten steel into the flux, It is an object of the present invention to provide a continuous casting flux capable of preventing surface defects.

[0011]

The gist of the present invention resides in the following steel continuous casting flux.

The range of surface tension at 1500 ° C. is 0.
A flux for continuous casting of steel, wherein the range of viscosity at 4 to 0.6 N / m and 1300 ° C. is 0.05 to 0.5 Pa · s.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Inclusions and bubbles in molten steel in a continuous casting mold float when passing through the interface between the molten steel and the molten flux if the surface tension of the molten flux is too high. It remains at the interface between the molten steel and the molten flux and causes surface defects of the slab. On the other hand, if the surface tension is too low, the molten flux is entrained in the molten steel, resulting in a surface defect of the slab.

The present inventor has elucidated these phenomena, and in order to find a flux for continuous casting that allows air bubbles and inclusions to easily pass through the interface and prevent entrainment of the molten flux, the steel type, the molten steel, The relationship between the surface tension and viscosity of the flux was investigated in detail, and the phenomena occurring near the interface between the molten steel and the molten flux were investigated equilibrium.

The forces acting on the inclusions and bubbles reaching the above-mentioned interface are the surface tension of the molten steel and the molten flux and the interfacial tension between the molten steel and the molten flux. The present inventor has considered these forces thermodynamically, and has found a balance condition between the bubble and the interface between inclusions. That is, if the sum of the above interfacial tension and the surface tension of the molten flux is equal to the surface tension of the molten steel, inclusions and bubbles remain at the interface.

Therefore, if an attempt is made to move inclusions and air bubbles into the molten flux, the surface tension of the molten flux is reduced, the surface tension of the molten steel is increased, and the interfacial tension between the molten steel and the molten flux is reduced. It may be lowered.

Specifically, if the surface tension of the molten flux exceeds 0.6 N / m at 1500 ° C., inclusions and air bubbles remain at the interface due to the above-mentioned relationship, and these become surface defects of the slab. Was clarified from the results of physical property measurement. On the other hand, if the surface tension of the molten flux is low, as described above, the molten flux may be entrained in the molten steel, and the lower limit of the surface tension that can prevent this is due to the experimental results using a water model and a low melting point metal. According to this, it was 0.4 N / m. The reason why the temperature is limited to 1500 ° C. as described above is that the normal casting temperature is near this temperature.

Here, it is originally necessary to define the interfacial tension between the molten steel and the molten flux.

However, since this interfacial tension is difficult to measure and is an unsuitable property value for flux composition design and quality control, the present inventor has determined that interfacial tension and surface tension are not independent property values. Focusing on this, these physical property values were examined in detail. As a result, it was confirmed that in the range of the surface tension of the molten flux, inclusions and bubbles can be transferred into the molten flux under the conditions including the interfacial tension.

The surface tension of the molten steel does not need to be particularly limited since it does not substantially change in the case of a normal iron alloy.

The condition of the viscosity of the molten flux is not directly related to promoting inclusions and bubbles from the interface. However, if the viscosity of the molten flux at 1300 ° C. is lower than 0.05 Pa · s, not only the molten flux is entrained in the molten steel, but also a surface crack of the slab occurs due to excessive inflow. At this time, even if the inclusions and bubbles are separated from the interface and reduced, a cast slab having good surface properties cannot be obtained. On the other hand, 0.5Pa
If it exceeds s, the heat flux in the mold increases due to insufficient flux inflow, and poor lubrication, depression (surface defects such as lateral cracks) and surface cracks occur. The reason why the above viscosity range is set to 1300 ° C. is based on the usual practice that the viscosity of the flux has a value at this temperature as a representative value.

As described above, a continuous casting flux having a surface tension range of 0.4 to 0.6 N / m at 1500 ° C. and a viscosity range of 0.05 to 0.5 Pa · s at 1300 ° C. is used. This makes it possible to obtain a steel slab with few surface defects.

The flux of the present invention comprises Ca, Si, A
1, Na, Mg, Li, K, Mn, Fe, Ba and B
And the rest or all or part of the oxide or fluoride powder and the carbon powder. The specific composition is determined according to the casting conditions within a range that satisfies the above conditions of surface tension and viscosity, and components other than those described above may be added.

The flux of the present invention can be applied irrespective of the type of steel, carbon content and type of continuous casting machine. In particular, a great effect is exhibited when stainless steel, ultra-low carbon steel, or a casting speed range of 0.5 to 3 m / min. If the continuous casting machine is a vertical type, it can be applied to any type, but in order to sufficiently obtain the effect of the flux of the present invention, a type in which the inside of the mold is vertical is desirable. The shape of the slab may be any of billets, blooms, slabs, etc., but it is effective to use the flux of the present invention particularly in the case of a slab having severe surface properties.

[0025]

EXAMPLE A continuous casting test using a flux having a composition shown in Table 2 was conducted on a molten steel having a composition shown in Table 1 to investigate a bubble defect, a flux entrapment defect, and a depression on the surface of a slab.

[0026]

[Table 1]

[0027]

[Table 2]

The continuous casting machine was a one-point correction type having a curvature radius of 10 m, a slab having a cross section of 1000 mm in width and 150 mm in thickness, and a casting speed of 1.0 m / min.

Casting was possible with any of the fluxes. The evaluation of the slab surface was performed using an index that allowed no defect at grade 5 and rolling at grade 3 or higher.
The investigation results are shown in FIGS.

FIG. 1 is a diagram showing a comparison of cellular defects on the slab surface with indices. As shown in FIG. 1, the cellular defect became worse in Test No. 3 (Comparative Example) in which the surface tension exceeded the upper limit.

FIG. 2 is a diagram showing the incorporation defects of the flux on the slab surface by comparing the indices. As shown in FIG. 2, the entrapment defect of the flux deteriorated in Test No. 3 (Comparative Example) in which the surface tension was lower than the lower limit and in Test No. 6 (Comparative Example) in which the viscosity was lower than the lower limit.

FIG. 3 is a diagram showing the depletion on the slab surface compared by an index. As shown in FIG. 3, the depletion deteriorated in Test No. 6 (Comparative Example) in which the viscosity was lower than the lower limit or in Test No. 7 (Comparative Example) in which the viscosity was higher than the upper limit.

On the other hand, in any of the examples of the present invention, no surface defects and no depletion were observed in the properties of bubbles and flux entrainment.

[0034]

By using the flux for continuous casting of steel of the present invention, it is possible to cast a slab without surface defects.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing a comparison of bubble defects on a slab surface by an index.

FIG. 2 is a diagram showing a comparison between indices of flux entrainment defects of a flux on a slab surface.

FIG. 3 is a diagram showing the depletion on the slab surface compared by an index.

Claims (1)

[Claims] 1. A surface tension at 1500 ° C. of 0.4 N / m.
A flux for continuous casting of steel, having a viscosity of not less than 0.6 N / m and not more than 0.05 Pa · s at 1300 ° C. and not more than 0.5 Pa · s.