JP2003048045A - Method for uniformly heating molten steel in tundish - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a means for uniformly heating molten steel in a tundish without deterioration of the quality in a cast slab, increase of the cost and erosion of the refractory. SOLUTION: In a method for heating the molten steel in the tundish for continuous casting with the plasma arc, such as to show in figure 2, the tundish in which the molten steel depth b at the heating position with plasma torches 4 is <=1 m and a horizontal distance a from a molten steel pouring hole to a molten steel spouting hole becomes at >=4 times of the molten steel depth b at the heating position and also, the plasma torches 4 composed of pair of a cathode and an anode parallel arranged with the molten steel flow from a molten steel pouring hole 2 to a molten steel spouting hole 6 in the tundish, are disposed and heated. It is desirable to separate the disposed positions of the plasma torches to >=1000 mm from the position of the molten steel pouring hole, and to heat at 700-1300 mm of the distance between the torches.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for uniformly heating molten steel in a tundish by plasma arc in continuous casting.

[0002]

2. Description of the Related Art Generally, in continuous casting of steel, after the molten steel from a ladle is once received in a tundish, it is supplied to one or a plurality of molds for casting. At that time, the degree of superheat of the molten steel in the mold (“molten steel temperature” −
The "solidification temperature of molten steel") is usually determined by the degree of superheat of molten steel in the ladle. In this case, if the degree of superheat of the molten steel in the ladle is low, nozzle clogging or skinning on the surface of the molten metal in the mold may prevent stable operation, and non-metallic inclusions contained in the molten steel. It is known that the floating separation of the cast iron becomes insufficient and the quality of the slab deteriorates.

For this reason, conventionally, means for compensating the temperature of molten steel by heating the molten steel in the tundish with a plasma arc has been adopted. This plasma arc heating means can supply a large amount of heat to the molten steel in a short time. Normally, plasma is applied from above to the molten steel in the tundish to prevent the molten steel temperature from decreasing. It is attempted to raise the temperature of molten steel.

However, the heating by the plasma arc has a problem that the temperature distribution of the molten steel in the tundish varies because the heat is locally supplied in the vicinity of the ignition point. In particular, when heating from the upper part of the molten steel, high temperature molten steel accumulates in the upper part of the tundish and is not sufficiently mixed with the low heat molten steel flowing in the lower part, so that the heat deposition efficiency decreases and the melting loss of refractory increases. It was a remarkable result.

Therefore, the following proposals have been made in order to solve such problems. a) A method of agitating the molten steel in the tundish at the time of heating by a plasma arc so as to make the molten steel temperature uniform (see JP-A-59-107755). b) A method in which a weir is provided at the bottom of the tundish near the pouring position of the molten steel to change the flow of the molten steel so as to make the molten steel temperature uniform (see Japanese Patent No. 2834657). c) An electrostatic magnetic field is applied in the direct current path formed by the plasma arc of the molten steel in the tundish to control the direction of the molten steel flow so that it does not become a simple flow, thereby making the molten steel temperature uniform. Method for achieving this (Japanese Patent Laid-Open No. 6-11451
(See Japanese Patent Publication No. 1).

However, the following problems have been pointed out in each of these proposed methods. That is, in the method of gas-stirring the molten steel in the tundish during heating, some of the fine bubbles in the tundish are brought into the mold and adhere to the solidification shell, causing defects in the slab or on the surface. In addition, there is a problem that the torch life is shortened due to splash or the like generated during stirring.

In the method of providing the weir on the bottom of the tundish, while the shape of the weir is maintained, a corresponding effect is recognized, but when the weir is melted and lost due to long-term use, the effect of equalizing the molten steel temperature is obtained. Disappears. Further, there is a problem that the refractory cost is increased by providing the weir, and the existence of the weir hinders the discharge of the residual steel in the tundish to deteriorate the yield.

Further, in the method of controlling the molten steel flow by applying an electrostatic magnetic field, it is necessary to arrange complicated equipment in the tundish, so that the cost is increased and the equipment is restricted (the layout of the equipment and the restrictions on the attached equipment). Etc.) is caused.

[0009]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The object of the present invention is to solve the above problems pointed out when heating molten steel in a tundish by using a heating means by a plasma arc, and to provide a large-scale incidental equipment which causes an increase in cost. It is to establish means for uniformly heating the molten steel in the tundish by plasma arc without causing melting loss of refractory and deterioration of slab quality.

[0010]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies while repeating many tests in order to achieve the above-mentioned object. As a result, "a plasma arc is applied to heat molten steel in a tundish. In this case, uniform heating of the molten steel in the tundish is significantly improved simply by modifying the arrangement of the plasma torch and the shape of the tundish, and nozzle clogging due to low heat, skinning of the molten steel surface in the mold, and uneven heating It will be possible to stably prevent melting of refractory materials. ''
I was able to obtain new knowledge.

The present invention has been made on the basis of the above findings and the like, and provides the following method for uniformly heating molten steel in a tundish. Plasma molten steel in tundish for continuous casting
In the method of heating with a plasma torch, the molten steel depth at the heating position by the plasma torch is 1 m or less, and the horizontal distance from the molten steel injection hole to the molten steel discharge hole is at least 4 times the molten steel depth at the heating position. The tundish is used, and a plasma torch composed of a pair of cathode and anode is arranged in parallel with the molten steel flow from the molten steel injection hole to the molten steel discharge hole of the tundish to heat. , Uniform heating method for molten steel in tundish. Position the plasma torch from the molten steel injection hole position 1
Separated by 000 mm or more, and the distance between torches is 700 to 1
The method for uniformly heating molten steel in a tundish according to the above item, wherein heating is performed at 300 mm.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The operation of the present invention will be described below while introducing the embodiments. First, in the method for uniformly heating molten steel in a tundish according to the present invention, as shown in FIG. 1, the cathode and anode are parallel to the molten steel flow from the molten steel injection hole to the molten steel discharge hole of the tundish. Arranging and heating a pair of plasma torches is one of the most important requirements. This point can also be confirmed by the following test example.

FIG. 2 is a schematic explanatory view of the test apparatus of the present invention. Reference numeral 1 is a ladle, 2 is a molten steel injection hole from the ladle to the tundish (composed of a gas seal pipe), and 3 is. Tundish, 4 is a plasma torch, 5 is molten steel, 6 is a molten steel discharge hole from the tundish to the mold (comprising an immersion nozzle), and 7 is a mold for solidifying the molten steel. Further, a in the figure is a horizontal distance between the molten steel injection hole 2 from the ladle to the tundish and the molten steel discharge hole 6 from the tundish to the mold, and b
Indicates the molten steel depth at the heating position by the plasma torch. Here, the molten steel depth b is the distance from the molten steel surface directly under the plasma torch to the refractory at the tundish bottom,
If the plasma torch is a dual-type torch, the deeper value is used. Further, c in the figure is the horizontal distance from the molten steel injection hole 2 from the ladle to the tundish to one side of the plasma torch 4, and d is the distance between the torches.

In the apparatus shown in FIG. 2, the molten steel 5 stored in the tundish 3 from the ladle 1 through the molten steel injection hole 2 is heated by the plasma arc generated from the plasma torch 4, and the molten steel discharge hole is formed. It is injected into the mold 7 through 6 (immersion nozzle) and solidifies to form a slab.

In this test apparatus, the molten steel injection hole to the tundish and the molten steel discharge hole from the tundish to the mold are each in one strand form, but the molten steel injection hole to the tundish is one. Similar results are obtained in the case of a multi-strand machine in which a plurality of molten steel discharge holes are provided from the tundish to the mold only at one place.

Now, casting is carried out using the apparatus shown in FIG. 2 and under the casting conditions shown in Table 1 below, with the plasma torch arranged parallel to the molten steel flow and with the plasma torch arranged vertically. Then, the temperature distribution of the molten steel in the tundish, the heat deposition efficiency, and the local melting loss of the refractory due to the difference in the torch arrangement were investigated.

[0017]

[Table 1]

Here, the "temperature distribution of molten steel in the tundish" means the depth 100 mm, 300 mm, 5 from the surface of the molten steel directly above the immersion nozzle (molten steel discharge hole) of the tundish.
Spot temperature measurement was performed at four points of 00 mm and 700 mm, and evaluation was made by the temperature difference between the maximum and minimum values. The "heat deposition efficiency" is calculated from the "temperature at the bath depth 500 mm near the molten steel injection hole 2 from the ladle to the tundish" and the "average temperature measured at the four points just above the immersion nozzle". The molten steel temperature rise ΔT in the dish was calculated, and this ΔT was used to calculate by the following formula. Heat transfer efficiency η = (sput-put x specific heat of molten steel x ΔT) / (plasma output)

On the other hand, "local melting loss of refractory material" means that plasma heating is not performed uniformly on the molten steel and the refractory material of the equipment is melted at a specific position and its thickness changes. However, here, the value obtained by dividing the amount of change (mm) at the portion with the largest amount of melting loss by the plasma heating time (min) is defined as the "local amount of melting of refractory material".

First, FIG. 3 shows a comparison of the investigation results of "temperature distribution of molten steel in tundish" and "heat deposition efficiency" depending on the arrangement of plasma torches. From FIG. 3 as well, it is understood that the temperature distribution in the case where the torch is arranged in parallel to the molten steel flow is smaller than that in the case where the torch is arranged vertically, and the heat deposition efficiency is increased.

Next, FIG. 4 shows the results of the investigation of the "local erosion amount of the refractory material" depending on the arrangement state of the plasma torch. From FIG. 4 as well, it can be seen that the amount of local melting loss of the refractory is smaller when the torch is arranged parallel to the molten steel flow than when it is arranged vertically.

The above test results show that the "line connecting the torch tips of the cathode and the anode" is "the line connecting the molten steel injection hole from the ladle and the molten steel discharge port to the mold". It is clearly shown that the parallel arrangement of the plasma torches is a very important requirement for the uniform heating of the molten steel in the tundish.

Further, in the method for uniformly heating molten steel in a tundish according to the present invention, the shape of the tundish used is also important, and the molten steel depth b at the heating position by the plasma torch shown in FIG. It is necessary to secure a tundish shape in which the horizontal distance a from the molten steel injection hole 2 to the molten steel discharge hole 6 is 4 m or more and 4 times or more the molten steel depth b at the heating position.

That is, the present inventors have arranged a plasma torch 4 consisting of a pair of cathode and anode in parallel with the molten steel flow from the molten steel injection hole 2 to the molten steel discharge hole 6 and then " When the horizontal distance a "from the molten steel injection hole 2 to the molten steel discharge hole 6 and the" molten steel depth b at the heating position by the plasma torch "are changed, the" temperature distribution of molten steel in the tundish (temperature difference ) ”And“ heat deposition efficiency ”. The casting conditions at this time are as shown in Table 2, and various horizontal distances a and molten steel depth b shown in Table 2 were combined for casting.

[0025]

[Table 2]

FIG. 5 shows "molten steel depth b at heating position".
3 is a graph showing the results of an investigation of “temperature difference of molten steel” according to “horizontal distance a from molten steel injection hole to molten steel discharge hole”. From FIG. 5, it can be confirmed that if the molten steel depth at the heating position is 1 m or less and a / b ≧ 4, the temperature difference of the molten steel is within 5 ° C. and the molten steel is uniformly mixed. Further, FIG. 5 shows that the molten steel depth at the heating position is 1
Even if m / m is less than a / b <4, the horizontal distance required for mixing cannot be secured, the temperature difference of molten steel is 5 ° C. or more, and even if a / b ≧ 4, the molten steel depth at the position is 1m
It also shows that the temperature variation becomes large when the value exceeds.

FIG. 6 shows the "temperature difference of molten steel" investigated.
Fig. 7 is a graph showing the relationship between "heat deposition efficiency" and "local melting loss of refractory material". From Fig. 6, the temperature difference of molten steel is 5
It can be seen that the heat treatment efficiency and the amount of local erosion of the refractory material remarkably increase when the temperature exceeds ℃.

These results show that “a / b ≧ 4” and “b ≦ 1” for “horizontal distance a from molten steel injection hole to molten steel discharge hole” and “molten steel depth b at heating position” shown in FIG.
By satisfying the condition of "m", it is possible to secure uniform heating of the molten steel in the tundish, improve the heat deposition efficiency and reduce the melting loss of the refractory.

By the way, in carrying out the method for uniformly heating molten steel in a tundish according to the present invention, the distance between the torches of the plasma torch is set to 700 to 1300 mm, and the position of the plasma torch is set. 1000 from the molten steel injection hole position
It is better to heat them apart from each other by mm or more. This is for the following reason. That is, when the torch distance d of the plasma torch shown in FIG.
However, if it exceeds 1300 mm, the heat transfer efficiency will be significantly reduced due to the dispersion of the heated portion. Also,
If the horizontal distance c from the molten steel injection hole 2 to the plasma torch 4 shown in FIG. 2 is smaller than 1000 mm, local melting damage of the refractory gas seal pipe in the molten steel injection hole 2 tends to occur.

In order to investigate the influence of the torch distance d of the plasma torch, the inventors of the present invention parallel to the molten steel flow from the molten steel injection hole 2 to the molten steel discharge hole 6 in order to investigate the influence of the torch distance d. A casting test was conducted under the casting conditions shown in Table 3 (conditions in which the distance d between the torches was variously changed) using the apparatus shown in FIG.

[0031]

[Table 3]

The results of this casting test are shown in Table 4.

[0033]

[Table 4]

As is clear from the results shown in Table 4, when the distance d between the torches of the plasma torch is widened, the heat deposition efficiency is lowered, and when they are too close to each other, a discharge is generated between the torches, which is stable. The heating becomes impossible. Therefore, it is desirable that the torch distance d is set to 700 to 1300 mm to ensure a heat-transfer efficiency of 80% or more under stable heating operation.

Next, from the molten steel injection hole 2 to the plasma torch 4
In order to investigate the influence of the horizontal distance c up to, the casting test was conducted under the casting conditions shown in Table 5 (conditions in which the horizontal distance c was variously changed).

[0036]

[Table 5]

The results of this casting test are shown in FIG. As can be seen from the results shown in FIG. 7, when the horizontal distance c from the molten steel injection hole position to the plasma torch is less than 700 mm, the amount of local erosion of the molten steel injection pipe increases rapidly. Therefore, it is desired that the horizontal distance c be 1000 mm or more.

[0038]

As described above, according to the present invention, the molten steel in the tundish can be uniformly heated by the plasma arc with high heat deposition efficiency without using a special device. Therefore, local melting damage of the tundish refractory is prevented, and stable continuous casting operation with a low refractory cost can be realized, which brings industrially useful effects.

[Brief description of drawings]

FIG. 1 is an explanatory diagram related to the arrangement of a molten steel flow and a plasma torch in a tundish.

FIG. 2 is a schematic explanatory diagram relating to the test apparatus of the present invention.

FIG. 3 is a graph showing, in comparison, investigation results relating to the relationship between the arrangement of plasma torches, “temperature distribution (temperature difference) of molten steel in the tundish” and “heat deposition efficiency of molten steel”.

FIG. 4 is a graph showing a result of an examination related to a relationship between an arrangement state of plasma torches and “amount of local erosion of refractory”.

FIG. 5 is a graph showing the results of an investigation of “temperature difference of molten steel” according to “molten steel depth b at heating position” and “horizontal distance a from molten steel injection hole to molten steel discharge hole”.

FIG. 6 is a graph showing a relationship between “temperature difference of molten steel” and “heat deposition efficiency” and “local melting loss of refractory”.

FIG. 7 is a graph showing the results of investigation of the horizontal distance from the molten steel injection hole to the plasma torch and the amount of local erosion damage in the molten steel injection pipe.

[Explanation of symbols]

1 ladle 2 Molten steel injection hole from ladle to tundish 3 tundish 4 Plasma torch 5 Molten steel 6 Molten steel discharge holes from tundish to mold 7 Mold

Claims (2)

[Claims] 1. A method of heating molten steel in a tundish for continuous casting by a plasma arc, wherein the molten steel depth at a heating position by a plasma torch is 1 m or less, and the molten steel injection hole to the molten steel discharge hole. A tundish whose horizontal distance is at least four times the molten steel depth at the heating position is used, and the pair of cathode and anode is parallel to the molten steel flow from the molten steel injection hole to the molten steel discharge hole of the tundish. A method for uniformly heating molten steel in a tundish, which comprises arranging and heating a plasma torch comprising 2. A plasma torch is arranged at a distance of 1000 mm or more from a molten steel injection hole position, and heating is performed with a torch distance of 700 to 1300 mm.
The method for uniformly heating molten steel in a tundish according to claim 1.