JPH0542348A - Continuous casting method - Google Patents

Abstract

PURPOSE:To prevent flowing-out of slag series inclusion into downstream side in a tundish and to obtain a cast slab having no defect by specifying stirring energy density of molten steel in a tundish. CONSTITUTION:Continuous casting is executed so that the stirring energy density of molten steel in the tundish does not exceed 1 watt per 1 ton of the molten steel. When the stirring energy density of molten steel exceeds 1 watt per 1 ton of the molten steel, entrapping of the slag and heat insulating agent is remarkably developed and the quality of cast slab is remarkably deteriorated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting steel, which is a method for preventing surface defects and internal defects of a product due to slag flowing into a tundish from a ladle. The present invention relates to a newly discovered invention regarding agitation energy density.

[0002]

2. Description of the Related Art Non-metallic inclusions contained in molten steel cause nozzle clogging during casting and, when present in a slab, cause various defects in products.

Therefore, great efforts have been made to remove non-metallic inclusions contained in molten steel.

In particular, at the seam portion during continuous casting, the slag in the ladle flows into the tundish, which forms inclusions and causes the above-mentioned problems.

Techniques for preventing the inflow of slag in the ladle into the tundish are (a) a method of adding a lime or the like to the ladle to make it a high-melting compound and solidifying it, and (b) a ladle slag It has been proposed to operate a sliding valve at the bottom of the ladle before it flows into the dish to prevent the ladle slag from flowing into the tundish.

However, the method (a) increases the cost by using an auxiliary material such as lime.

According to this method, even though the upper part of the slag is solidified, the lower part is in contact with the high temperature molten steel,
There is a molten layer of slag, and molten steel slag flows from the bottom of the ladle into the stirring chamber 5 on the tundish entry side at the end of pouring, so this measure alone must completely prevent the inflow of ladle slag 9. Is impossible.

Further, according to the method (b), an excessive amount of molten steel remains in the ladle, which greatly reduces the casting yield of the molten steel 8 in the ladle.

Further, in the final stage of supplying molten steel from the ladle 1 to the tundish 7, a vortex is generated in the molten steel, and the molten steel containing slag itself or a large amount of inclusions in which the slag is suspended is faster in the tundish 7. It flows into the stirring chamber 5.

Since this inflow start time is not constant depending on the amount and physical properties of the ladle slag 9 above the molten steel, it is extremely difficult to determine the inflow time.

In any case, the inflow of the tundish 7 of the ladle slag 9 into the stirring chamber 5 is unavoidable, and the amount of slag accumulated in the stirring chamber 5 increases as the continuous casting proceeds.

Therefore, the slag 9'which has flowed into the stirring chamber 5 and, in addition, a measure for preventing the slag 9'in the tundish including the stirring chamber 5 from being used for the purpose of preventing the temperature drop of the molten steel from flowing out to the downstream side of the tundish. Is important.

By the way, a weir is generally used to prevent the slag flowing into the tundish from flowing out to the mold.

However, in the slab corresponding to the joint portion of continuous casting, defects are generated due to inclusions caused by slag.

Although the cause of this is not clear, it is presumed that the slag has flowed into the mold even at the seam even if a weir is used.

[0016]

SUMMARY OF THE INVENTION The present invention provides a method for preventing the outflow of slag-based inclusions to the downstream side of a tundish, which is the above-mentioned conventional problem.

[0017]

According to the present invention, in a continuous casting process of molten steel, a molten steel is partitioned by a weir 4 between a long nozzle 2 installed at a lower portion of a ladle 1 and a casting nozzle 3 for casting into a mold. When the molten steel 8 is supplied from the ladle 1 to the continuous casting tundish 7 composed of the stirring chamber 5 and the inclusion floating chamber 6, the stirring energy density ε of the molten steel 8 when the molten steel 8 is poured into the stirring chamber 5 is , At least 80 of the total molten steel weight of the front pan
1 watt (W) per 1 ton (t) of molten steel from the time when the injection of the molten steel in the pot is started after the completion of the injection of It is a continuous casting method characterized by casting so as not to exceed the limit.

The method of the present invention will be described in detail below.

When a cause of the slag 9'on the molten metal surface of the stirring chamber 5 flowing out to the downstream side of the tundish 7 was investigated by a water model test, it was found that the stirring energy density ε of the stirring chamber 5 greatly depends on it. It was

The stirring energy-density ε referred to here is a value calculated by the equation (1). ε = (1 / 2ρQV _LN ² ) / (ρ ・ W ・ H ・ L) (1)

Here, ρ is the liquid density, Q is the discharge amount of the long nozzle, V _{LN is the} discharge velocity of the long nozzle, W is the width of the tundish occupied by the molten steel, H is the height of the molten steel, and L is the length of the stirring tank. ..

Generally, as the stirring energy density ε increases, the molten steel flow velocity under the slag 9'becomes faster, and as a result, the slag 9'and, in the worst case, the heat insulating material 11 is caught in the molten steel and flows under the weir 4. Outflow to the downstream side of the street tundish 7.

It has been found that the entrainment of the slag 9'and the heat insulating material 11 becomes remarkable when the stirring energy density ε exceeds 1 W per 1 ton of molten steel, and as a result, the quality of the cast piece is remarkably deteriorated.

In continuous casting, from the time when the injection of 80% of the total molten steel weight of the front ladle is completed to the time of starting the injection of molten steel of the rear ladle and the time of injecting 20% of the total molten steel amount, the following is performed. Problems arise.

That is, casting of the slab is carried out while the ladle is replaced, and therefore the amount of hot water in the tundish is reduced.

Further, the head of the molten steel in the ladle is also lowered, and the inflow amount of the molten steel into the tundish is also reduced.

In order to recover this at an early stage, it is general to operate by opening the opening of the sliding valve to a large value in the initial stage of casting of the next ladle. Therefore, the stirring energy density ε of the molten steel in the stirring chamber 5 is ε. Increases instantly.

Therefore, it was found that the stirring energy density ε of the molten steel in the stirring chamber 5 in this time zone exceeds 1 W per ton of molten steel.

The molten steel equivalent to 20% of the total molten steel in the ladle before the ladle is exchanged during continuous casting is the molten steel with a large amount of inclusions in which the slag is suspended. If it flows into the stirring chamber 5 at the final stage of casting and is cast under a strong molten steel stirring energy density ε, it tends to flow out to the downstream side of the tundish.

Therefore, the present invention limits the weight of molten steel before and after the ladle is replaced.

By the way, in order to reduce ε, the falling energy of molten steel is reduced. It is possible to reduce the so-called pouring speed, but it is difficult to keep the pouring speed below a certain speed in terms of quality and productivity, and especially when the ladle is replaced, the initial pouring speed of the rear pan should be increased as much as possible.

Under this constraint, it is conceivable to increase the volume of the stirring chamber 5.

In order to increase the volume of the stirring chamber 5, any one or all of the width and depth of the tundish and the length of the stirring chamber may be increased. As a result of the water model test, It has been found that increasing the depth of molten steel is most effective.

It is preferable to increase the width in order to improve the shape of the tundish, but there is a limit to the size of the tundish.

Further, if the length of the stirring chamber is increased, the upper weir is changed to the downstream side when the total length of the tundish is restricted, but in this case, the length of the inclusion floating area on the downstream side is changed. It becomes shorter and naturally there is a limit.

Therefore, it is most effective to make the stirring chamber deep in order to reduce ε without significantly changing the tundish shape.

[0037]

EXAMPLES Next, examples of the present invention will be described.

The present invention is currently applied to all molten steel having an iron-based component that is cast by a continuous casting machine.

C 0.04%, Si 0.01%, Mn
0.23%, P 0.022%, S0.010%, So
l. Al 0.050%.

As for the continuous casting conditions, one ladle was 100 tons and 500 tons of molten steel was continuously cast. The thickness of the mold is 2
The width is 50 mm and the width is 1150 mm.

The tundish has a stirring chamber length of 2700
mm, the bottom width was fixed to 800 mm, and the depth of molten steel was changed to change ε.

The distance between the weir 4 and the tundish 7 was fixed at an optimum length.

FIG. 2 shows the relationship between the stirring energy density ε of the stirring chamber 5 of the tundish 7 shown in FIG. 1 and the inclusion index on the downstream side of the tundish obtained by casting in the various tundish shapes described above. Is.

It can be seen that the inclusion index increases as the stirring energy density ε increases.

In FIG. 2, when the stirring energy density ε exceeds 1 watt per ton of molten steel, the inclusion index exceeds 5.

When slabs having various inclusion indices were rolled into a base plate for deep drawing cans, it became clear that surface defects and internal defects were generated when the inclusion index exceeded 5.

Therefore, it was confirmed that when the stirring energy density ε was cast under the condition of 1 watt or less per 1 ton of molten steel, a slab without surface defects or internal defects of the original plate for deep drawing cans was obtained. ..

[0048]

As described above, according to the present invention, it is possible to prevent the slag flowing into the tundish from flowing out to the downstream side in the tundish, and to obtain a high cleanliness cast piece having no surface defects or internal defects. It greatly contributes to casting.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a tundish.

FIG. 2 is an explanatory diagram showing a relationship between an inclusion index and a stirring energy density.

[Explanation of symbols]

 1 Ladle 2 Long Nozzle 3 Injection Nozzle 4 Weir 5 Stirring Chamber 6 Inclusion Floating Chamber 7 Tundish 8 Molten Steel 9, 9'Slag 10 Sliding Valve 11 Heat Insulation Material

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Issei Umezawa 1 Fuji-machi, Hirohata-ku, Himeji City Nippon Steel Corporation Hirohata Works

Claims (1)

[Claims] 1. A molten steel stirring chamber 5 partitioned by a weir 4 between a long nozzle 2 installed at the bottom of a ladle 1 and a casting nozzle 3 for casting a mold in a continuous casting process of molten steel, and floating of inclusions. Tundish 7 for continuous casting consisting of chamber 6
When the molten steel 8 is fed from the ladle 1 to the stirring ladle 1, the stirring energy density ε of the molten steel 8 at the time of pouring into the stirring chamber 5 is continued from the time when at least 80% of the total molten steel weight of the preceding ladle has been poured. After injecting the molten steel in the pan, the time from the beginning to inject 20% of the total molten steel weight is 1
A continuous casting method, characterized in that casting is performed so as not to exceed 1 watt (W) per ton (t).