JPS5868456A - Production of clean steel - Google Patents

Abstract

PURPOSE:To obtain clean steel by efficient floatation of slag by blowing an inert gas into a tundish from its bottom surface in the direction intersecting orthogonally with the flow of the molten steel therein and regulating the agitating flow of the molten steel with a weir provided under the surface of the molten steel. CONSTITUTION:An inert gas is blown from a piping 6 through the refractories 5 of good permeability such as porous brick provided on the bottom surface 1a of a tundish 1 into the molten steel 2 flowing in the tundish from the upper stream side to the down stream side in the direction intersecting orthogonally with the flow of the steel 2 at 0.10-0.38 Nl/cm<2>.min flow rate to agitate the steel 2 and to accelerate the floatation of inclusions. In parallel with said blowing, a weir 7 which is dipped in the steel 2 down to the position of 10-50% the depth of the steel 2 from the surface thereof is provided on the lower stream side near the position where said gas is blown in the direction intersecting orthogonaly with the flow of the steel 2 to regulate the agitating flow of the steel 2 generated by blowing of said inert gas and to accelerate the floatation of the inclusions additionally.

Description

[Detailed Description of the Invention] The present invention aims at cleaning the molten steel by promoting the floating separation of inclusions mixed into the molten steel housed in the tundish for continuous casting at temperatures of °C in the continuous casting of steel. This invention relates to a method for producing clean steel.

In the continuous steel casting process, if inclusions remain in the molten steel being cast, it causes internal defects in the slab. Therefore, conventionally, the following measures have been taken to prevent inclusions from remaining in the molten steel that is cast.

(1) Prevent ladle slag from entering the tundish.

(2) Preventing air oxidation of molten steel.

(3) Promote flotation and separation of inclusions in molten steel in the tundish.

(4) Promote floating separation of inclusions in molten steel in the mold.

Among the above-mentioned measures, the measure in item (3) is also important, and even if molten steel containing a large amount of ladle inclusions is injected into the tundish, it will prevent the inclusions from flotation and separation in the tundish. If this can be promoted, clean slabs can be cast.

Conventionally, as a means for floating and separating inclusions in molten steel in a tundish, weirs made of refractories are provided at various locations in the direction orthogonal to the flow of molten steel flowing from one end of the tundish to the other end. The weir is used to change the flow of molten steel.

FIG. 1 is an explanatory diagram showing an example of the conventional method described above in a partial cross section of a tundish.
A weir 3 made of refractory material is provided in a direction perpendicular to the flow direction of the molten steel 2.

4 is a tundish powder for protecting the hot water surface, and the molten steel 2 poured into one end of the tundish 1 from a ladle (not shown) flows inside the tundish 1 toward the other end as shown by the arrow. When it hits cough 3, the flow changes upward. Therefore, the floating distance of the inclusions in the molten steel is shortened, and the floating separation of the inclusions is promoted.

Figure 2 is a diagram showing the effect of separating inclusions by such a conventional method. The horizontal axis is the particle size of inclusions, and the vertical axis is the amount of inclusions in molten steel. The ratio is based on the amount of inclusions in the molten steel in the mold when no inclusions were present, which is 100L%. As can be seen from the drawings, according to the conventional method described above, the effect of separating large inclusions with a particle size of 300 μm or more is large, but the effect of separating small inclusions with a particle size of less than 300 μm is low.

Moreover, since the number of inclusions present in molten steel is large, the conventional method described above cannot meet the recent high requirements for slab quality, and the small inclusions are also separated and removed. There is a strong desire to develop a method that can do this.

Therefore, as shown in Fig. 3, a refractory material 5 with good air permeability, such as a porous brick, is provided on the bottom surface 1a of the tandye 1 in a direction perpendicular to the flow direction of the molten steel 2, and a pipe 6 is also fed. The inert gas is passed through the refractory 5 to the molten steel 2.
A method has been developed in which the molten steel 2 is caused to flow upward by the fine inert gas bubbles that are blown into the molten steel, and inclusions in the molten steel are adsorbed by the bubbles.

However, in this method, the molten steel in the tundish Ll is stirred by the inert gas blown from the bottom surface 1a of the tundish 1, and as a result, the tundish powder 4 floating on the surface of the tundish is drawn into the molten steel 2.

Furthermore, according to recent research by the inventors, in the flow velocity distribution diagram in the depth direction of molten steel in the tundish in Figure 4, the flow velocity in the I-I line section of Figure 3 is indicated by the black circle, and it is found that inert gas The flow velocity of the molten steel at the position where it is blown into the molten steel is the same as the part just below the molten metal surface, and the molten steel near the bottom of the tundish flows in the opposite direction to the upstream side, so inclusions in the floating process are drawn into the vicinity of the bottom of the tundish by this molten steel flow. I found out that it can happen.

From the above-mentioned viewpoint, the present invention has been developed to produce a clean slab by efficiently flotation-separating all inclusions mixed in molten steel in a tundish, regardless of their particle size. The present invention provides a method for producing steel, in which molten steel flowing in a tundish from one end to the other end has a 0. 10-0. 38 Nil
/crL TiA is injected with an inert gas at a flow rate of TiA to stir the molten steel, and is provided downstream in the vicinity of the inert gas injection position in a direction perpendicular to the flow of the molten steel.
A weir that is immersed at the surface of the molten steel to a position of 10 to 50% of the depth of the molten steel rectifies the agitation flow of the molten steel caused by the injection of the inert gas, and floats and separates inclusions mixed into the molten steel. It is characterized in that it accelerates the cleaning of the molten steel.

Next, the present invention will be explained with reference to examples and drawings.

FIG. 5 is a partial sectional view of a tundish showing an example of the method of this invention, FIG. 6 is a partial plan view of the same, and FIG. 7 is a sectional view taken along the line A--A in FIG. 6. As shown in the drawings, also in the method of this invention, a refractory material 5 with good air permeability, such as a porous brick, is provided on the bottom surface 1a of the tundish 1 in a direction perpendicular to the flow direction of the molten steel 2. Blowing inert gas into the molten steel 2 through the refractory 5 through the pipe 6 provided along the lower surface thereof is the same as the conventional method described above.

In the method of this invention, the above-mentioned five well-ventilated refractories are placed on the downstream side of the molten steel near the inert gas injection position.
A refractory weir (hereinafter referred to as upper weir) 7 is provided from the upper surface of the molten steel in a direction perpendicular to the flow direction of the molten steel to a position 10 to 50 inches below the molten steel surface, and the upper weir 7 This rectifies the stirring flow of molten steel generated by blowing inert gas.

As a result, the molten steel 2 flowing inside the tundish 1 is
As shown in the figure, only on the upstream side of the upper weir 7, the refractory 5 with good air permeability is also stirred by the inert gas blown in, and on the downstream side of the upper weir 7, a strong flow occurs in the molten steel just below the molten metal surface. do not have.

In FIG. 4, the flow velocity distribution of molten steel in the section marked by the line ■-■ in FIG. 5 is shown by white circles. As is clear from the comparison with the flow velocity distribution of the conventional method shown by the black circles mentioned above in the figure, in the case of the method of this invention, the flow velocity is almost the same from near the surface of the tundish to near the bottom, and near the bottom of the tanditsu, the flow velocity is almost the same. No flow flows backward to the upstream side.

Figure 8 shows that when the method of this invention is carried out, the ratio of inclusions that pass through the tundish and are brought into the mold is 100% when no weir is installed, as determined by slime analysis.
It is shown as % together with the conventional example. In the drawing,
A is when no weir is installed, B is when a weir is installed on the bottom of the tundish shown in Figure 1, C is when the upper weir is installed by immersing it to the molten metal level or 30% of the molten steel depth, ■ ) is 0 inert gas from the bottom of the tanditshu,
When injecting at a flow rate of 22 Nl/crL mm, E injects inert gas from the bottom of the tundish at a flow rate of 0. This is the case where it is immersed and installed up to the 30th position. As can be seen from the drawings, the method of this invention was able to significantly reduce the amount of inclusions in molten steel compared to the conventional method.

FIG. 9 shows the proportion of inclusions for each particle size in the molten steel in the mold when the method of the present invention is implemented, with the case where no weir is installed as 100%, and the conventional examples B, C, and D described above. is shown with. In the drawings, open circles indicate B, open triangles indicate C, open squares indicate D, and closed circles indicate the method of this invention.

As can be seen from the drawings, according to the method of the present invention, it was possible to accurately separate small inclusions of about 80 to 300 microns, which were difficult to separate using conventional methods.

As shown in Figures 8 and 9, such effects of the method of the present invention cannot be obtained simply by blowing inert gas into the molten steel, but in addition, A tracer was added to the position corresponding to the receiving steel, and the time from receiving the steel to discharging the steel into the mold was investigated using a water model experiment. As a result, it was found that when an upper weir is not used, the time required for the molten steel supplied to the tundish to be discharged into the mold is extremely short, and from this it can be inferred that the flow velocity of molten steel just below the molten metal surface is quite high. Ta.

Figure 10 shows the inert gas added to the molten steel at a rate of 0.22Nl/
The relationship between the immersion depth of the upper weir in molten steel when injected at a flow rate of cwt, min and the amount of inclusions brought into the mold after passing through the tundish is Amounts are shown as 100%. As can be seen from the drawing, when the immersion depth of the upper weir in the molten steel is 10 to 50 inches of the total molten steel depth, the inclusions are effectively separated and floated, but the immersion depth is 10%. When the ratio is less than 50 or more than 50, the effect of separating and flotating inclusions becomes low.

Further, if the immersion depth of the upper weir exceeds 50%, the cross-sectional area through which molten steel passes through the tundish becomes narrow, and a flow of molten steel against the bottom surface of the tundish occurs, which deteriorates the effect of flotation and separation of inclusions.

Therefore, the immersion depth of the tundish into the molten steel is the minimum depth to prevent the molten steel directly below the molten steel surface at the inert gas injection position from flowing directly downstream, that is, 10 times the molten steel depth. It should be ~50chi.

Figure 11 shows the relationship between the amount of inert gas blown into molten steel and the amount of inclusions brought into the mold after passing through the tundish.
Shown as 100%. As you can see from the drawing,
An inert gas is passed through a refractory with good air permeability into the molten steel, and the gas per unit area of the refractory is 0. 10-0. 38
Nl/a1. m1yr (When blowing at an amount of D', an excellent effect is obtained in separating and flotating inclusions, but when the blowing amount is less than 0.10 Nl/lyj0miy+, stirring of the molten steel is weak, and 0.38 Nl/crl .

If it goes beyond deception, on the other hand, the stirring will be too strong, and the effect will be diminished.

As described above, according to the method of the present invention, inclusions mixed in molten steel can be efficiently floated and separated in the tundish, regardless of their particle size. This provides the industrial advantage of being able to cast clean steel.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of a conventional inclusion flotation separation means, and FIG. 2 is a diagram showing the effect of separating inclusions by the conventional method.
FIG. 3 is an explanatory view showing another example of the conventional inclusion flotation separation means, FIG. 4 is an explanatory view showing the flow velocity distribution of molten steel in the tundish, and FIGS. 5 to 7 are explanatory views showing the present invention. Fig. 5 is a partial sectional view of the tundish, Fig. 6 is a plan view of Fig. 5, Fig. 7 is a sectional view taken along line A-A in Fig. 6, and Fig. 8 is the amount of inclusions in molten steel according to the method of this invention. Figure 9 is a diagram showing the proportion of inclusions for each particle size in molten steel obtained by the method of the present invention in comparison with the conventional example. FIG. 11 is a diagram showing the relationship between the depth and the amount of inclusions in molten steel, and FIG. 11 is a diagram showing the relationship between the amount of inert gas blown and the amount of inclusions in molten steel. In the drawings, 1... Tandish, 2... Molten steel, 3... Conventional weir, 4... Powder, 5... Refractory material with good air permeability, 6... Piping, 7... Upper cough. Fig. 1 Grain size of intercalated metal (OJ) of molten steel;

Claims (1)

[Claims] For the molten steel flowing inside the tundish from one end side to the other end, from the bottom of the tundish in a direction perpendicular to the flow of the molten steel, 0° 10 to 0.38 N l /cd, -
A flow rate of inert gas is injected to stir the molten steel, and at the same time, the molten steel depth is set from the molten steel surface to the molten steel surface, which is provided on the downstream side near the inert gas injection position in a direction perpendicular to the flow of the molten steel. The weir, which is immersed to a position of 10 to 50 inches, rectifies the stirring flow of the molten steel caused by the injection of the inert gas, promotes floating separation of inclusions mixed in the molten steel, and cleans the molten steel. A method for producing clean steel, characterized by: