JPS63281753A - Method for judging validity of mold powder for continuous casting - Google Patents

Abstract

PURPOSE:To decide validity of mold powder so as to obtain a continuously cast slab without longitudinal crack by observing variation of conductive heat from mold wall in the specific range at both ends of long sides in the mold and comparing these variations. CONSTITUTION:Below molten steel surface in the mold at the time of continuously casting, the variations of conductive heats from mold walls at the long side ranges in the mold, at which are within 100mm below the meniscus and within one fifth of length of the long sides 12 from the short sides 10 in the mold respectively, are observed and compared. Then, measurement of the conductive heat is executed with plural thermocouples buried in the mold walls at the long side ranges in the mold and the variations of the conductive heats are compared at the total four ranges in both ends of the long sides 12 in the mold. Difference between the max. temp. and the min. temp. for each powder measured by using these thermocouples is found, and these differences are compared, to decide that the powder having the min. difference thereof is excellent.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a method for determining the superiority and inferiority of mold powder for continuous casting,
In particular, the present invention relates to a method for determining the superiority or inferiority of mold powder for continuous casting by monitoring and comparing the amount of heat removed from the mold wall in a specific region.

(Prior art) There are various surface defects on slabs produced by continuous casting (hereinafter referred to as "continuously cast slabs"), but in steel types where (C) falls within the peritectic region,
Vertical cracks and corner cracks are likely to occur, which is a major impediment to improving productivity.

Conventionally, countermeasures have been taken against such surface flaws, such as optimizing operating conditions or optimizing mold powder.

However, there was no technology that could detect vertical cracks and corner cracks in continuously cast slabs in advance or at the same time at the beginning of their occurrence.

In other words, they were simply making a visual judgment based on the actual casting results. Therefore, measures against vertical cracking, such as mold powder? 8 [It was merely a matter of actually measuring the thickness and liter consumption, checking the state of slag hair formation, etc. Furthermore, when considering individual mold powders, there was no clear standard for confirming the superiority or inferiority of those mold powders.

Although much research has already been conducted into the causes and mechanisms of vertical cracking, it has not yet reached the stage where it can be said to be fully elucidated.

“Tetsu to Hagane” No. 67 (1981 No. 8 121O-12
(page 19) discloses an example in which the temperature of the mold wall is measured in order to elucidate the mechanism of occurrence of vertical cracks. This is based on the prior knowledge that if a powder that causes many vertical cracks is used, mold temperature fluctuations near the meniscus will increase. It is already known that when a vertical crack occurs, the amount of heat removed through the mold wall increases in that area, but if a vertical crack has already occurred, it cannot be called a prediction, and conventional visual inspection The judgment and its practical technical significance are largely the same.

Furthermore, the variation in the amount of heat removed when there is a slight vertical crack is unknown. In any case, it can be said that nothing concrete has been disclosed regarding the correlation between the two.

Note that monitoring the mold wall temperature as a method for improving casting defects in continuous casting is described in, for example, Japanese Patent Laid-Open No. 6
No. 0-99467 (method for detecting shell breakage by monitoring mold wall temperature) and JP-A-61-226154 (method for predicting breakout by monitoring mold wall temperature). However, these methods do not disclose where on the mold wall the temperature is to be measured and monitored. The prevention of vertical cracking and the correlation between vertical cracking and the amount of heat removed have not been clarified at all.

(Problems to be Solved by the Invention) Thus, an object of the present invention is to provide a practical means of judgment when comparing the superiority of continuously cast slabs without vertical cracks and mold powder.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present inventor has continued various studies and found that one of the causes of vertical cracking in continuously cast slabs is the mold of molten mold powder. We focused on the fact that this was a locally uneven inflow into the gap between the inner wall and the slab. At that time, if such a break occurs in the molten powder film, the heat insulation effect will be lost, so it is expected that there will be a significant change in the amount of heat removed. When we measured the amount of heat removed in the central region of the long sides of the mold, we found that, unexpectedly, there was a significant increase in the amount of heat removed at the corners of the mold.

Moreover, it was found that even before the occurrence of vertical cracks, there is a large variation range depending on the thickness of the molten powder film.

At that time, we learned that the range of variation in the amount of heat removed by each mold powder is small for an excellent mold powder, and large for a bad mold powder, so we performed continuous casting using several mold powders, and The present invention was achieved based on the realization that it is easy to determine the superiority or inferiority of mold powder by comparing the amount of variation in the amount of heat removed for each type of mold powder.

Figure 1 shows two types of mold powders A and B having the compositions shown in Table 1, and a casting speed of 1.20 to 1.50 m/min.
win, slab size 1240 width x 270 thickness (am)
, Steel type C-0, below the molten steel surface in the mold when continuous casting is performed under the conditions of 10 to 0.16%, within 100 ia + m from the meniscus, and 11 of the total length of the long side of the mold.
This is a graph representing the amount of variation in the amount of heat removed from the mold wall in a total of four long side regions of the mold located closer to the short side of the mold and located within a distance of 5. The molten powder was adjusted so that it always existed on the molten steel surface at a thickness of 5 to 10 mm.

From the graph in Figure 1, it can be seen that from the center area of the long side of the mold,
It can be seen that the amount of heat removed in the corner area is large and its variation is also large, and that the variation range for each mold powder is very small (half of that) for excellent mold powders, such as mold powder A. . Temperature measurements in the center region cannot capture such trends or differences.

[・Here, the gist of the present invention is that, during continuous casting, the molten steel surface in the mold is below, within 100 cm below the meniscus, and from the short side of the mold to 175 cm of the total length of the long side of the mold. The i! This is a method for determining the superiority or inferiority of mold powder for continuous casting.

According to a preferred embodiment of the present invention, an appropriate number of thermocouples are embedded in the mold wall in the long side region of the mold, and by monitoring temperature changes in the mold wall, changes in the amount of heat removed from the mold wall are compared. I will.

In the present invention, the temperature measurement point, that is, the area, is firstly
It must be below the surface of the molten steel in the mold, but this is a small area (meaning that it is in contact with the molten steel. Also, it must be within 100s+a+ of the meniscus, so that it must be below the surface of the molten steel. However, in that region, a strong shell has already formed, so large fluctuations in the amount of heat removed cannot be detected.Furthermore, the measurement region is set to a distance within 175 mm of the total length of the long side of the mold. The reason why this is limited to the long side region of the mold near the short side of the mold is that the amount of variation in the heat removal (■) shown in Figure 1 above is greatest in this region.This so-called corner part is also cooled significantly. Moreover, it is a region that receives external forces such as thermal stress associated with cooling in a concentrated manner, and therefore the influence of the uneven molten film of the mold powder becomes noticeable.Thickness of the molten film 1
This is due to changes in the insulation effect due to changes in temperature.

As already mentioned, when vertical cracks occur or are about to occur, it means that breaks in the molten film of the mold powder have occurred or are about to occur. In such a case, the molten metal and the inner wall of the mold come into direct contact, and the temperature of the mold wall surface increases, causing an abnormal amount of heat to be removed. In this way, if an abnormality occurs in the amount of heat removed, that is, if there is a sudden increase in the amount of heat removed, it will appear as a temperature rise on the mold wall surface, and this will be detected as a temperature change using a thermocouple. It detects it.

According to the present invention, as mentioned above, unexpectedly, such temperature changes are noticeable at the mold corner parts, especially before the occurrence of vertical cracks, and moreover, the temperature changes for each mold powder are The range of fluctuation is small for excellent mold powder,
Inferior molding powder will be thick (in other words, a uniform molten powder coating will not be formed).

Thus, according to the present invention, it is possible to determine an excellent powder that can supply molten powder with a uniform thickness to the gap between the mold and the inner wall of the mold during the casting period.

(Operation) Next, the present invention will be described in more detail with reference to the accompanying drawings.

FIG. 2 is a schematic explanatory diagram of temperature measurement points on the mold wall in the present invention.

As shown in Figure 2, the continuous casting mold usually has a short side of 10 mm.
and a long side 12, one of which is the fixed side 1.
4, the other side becomes the free side 16, and is moved according to the required dimensions. According to the present invention, temperature measurement points can be provided on either long side, but in the illustrated example, the free side long side 1
Only the measurement point provided at 6 (indicated by the xj mark in the figure) is visible.Normally, this is done by embedding a thermocouple in the mold wall.

These measurement points are within 10 hm from the meniscus.
It may be located anywhere within the long side region within 175 mm of the total long side from the short side, usually within 200 mm, but it is preferably as close to the corner as possible. The number of measurement points is also not particularly limited, but preferably two or more measurement points are provided at one location.

After setting the measuring points in this way, continuous casting is started according to conventional methods. In that case, use 2
When there are the above types of molding powder, use them one after another, measure the amount of heat removed during the period of use of each molding powder, and measure the difference between the maximum and minimum values, that is, the range of variation in the amount of heat removed. Calculate. During the casting period, the amount of heat removed is constantly changing. After calculating the variation range for each mold powder in this way, they are compared, and the one with the smallest variation range is judged to be excellent. A small variation range means that there is little variation in the thickness of the molten powder coating during the casting period, and the heat insulation effect at that time is uniform. It is preferable to record the data of a plurality of measurement points, respectively, and to determine the fluctuation range for all the measurement points.

According to a specific aspect of the present invention, in order to improve measurement accuracy, the tips of the thermocouples are all placed at a certain depth from the inner wall of the mold (e.g. 10 to 151), and the electromotive force is The temperature is measured continuously, converted to temperature, and then the difference between the highest and lowest temperatures is taken and stored. The same operation is repeated for different types of powder, the variation ranges of each are compared, and the one with the smallest variation is judged to be excellent.

(Effects of the Invention) As detailed above, according to the present invention, mold powder can be determined even during continuous casting operation, and in that case, superiority or inferiority can be determined before vertical cracks occur. be. In particular, the present invention has great significance in the sense that it has established criteria for determining mold powder, for which there were no clear criteria in the past. Particularly in light of the recent situation where the development of mold powder suitable for high-speed casting has become an urgent issue, the effects of the present invention are recognized to be excellent.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the temperature measurement points on the long side of the mold and the amount of heat removed; and FIG. 2 is a schematic explanatory diagram of the temperature measurement points in the present invention.

Claims (3)

[Claims] (1) During continuous casting, the mold wall is located below the molten steel surface in the mold, within 100 mm below the meniscus, and within a distance of 1/5 of the total length of the long side of the mold from the short side of the mold. A method for determining the superiority or inferiority of mold powders for continuous casting, characterized in that a mold powder with a small amount of variation is selected as superior by comparing the amount of variation in the amount of heat removed from the mold powder. (2) The method according to claim 1, wherein fluctuations in the amount of heat removed from the mold wall are compared by monitoring temperature changes using a plurality of thermocouples embedded in the mold wall in the long side region of the mold. (3) The method according to claim 1 or 2, wherein the amount of variation in the amount of heat removed is compared in a total of four mold long side regions at both ends of each mold long side.