JPH03453A - Continuous casting mold for restraining corner crack in casting billet - Google Patents

Abstract

PURPOSE:To reduce surface temp. difference between a major side mold and a minor side mold and to prevent vertically cracking fault in a casting billet by reducing the depths of slitting grooves on the major side copper plate in the mold for casting carbon steel bloom toward the corner part sequentially. CONSTITUTION:By burying stainless steel square material 3 into the slitting grooves 2 for cooling on the major side mold 1 in the copper or copper alloy- made mold for casting the carbon steel bloom, the depths of the grooves 2 are reduced toward the corner part sequentially to make the mold having the little difference of the surface temps. between the major side mold 1 and the major side mold. By this method, the development of solidified shell is uniformly executed at the major side mold and the minor side mold, and the casting billet fault is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a continuous casting mold that suppresses corner cracks in slabs.

<Conventional technology> Due to the geometrical dimensions of the cross section of the bloom casting mold, the long side is strongly cooled and the short side is warmly cooled, so that the temperature on the long side and short side is evenly distributed on the mold surface closest to the corner. As a result, a temperature difference also occurs in the meniscus temperature of the molten steel, so vertical cracking defects are likely to occur near the corners of the bloom slab, which is the product.

The larger the bloom, the more likely these defects will occur, and the more likely these defects will occur when medium-skinned steel is cast in the peritectic reaction region.

To explain in more detail, carbon in steel is 0.15 to 0.18%.
The steel type has a large shrinkage within the mold, is difficult to remove heat from the mold wall, and is extremely prone to cracking due to the peritectic reaction. It is also generally known that repeated shrinkage and recuperation during solidification causes austenite grains to grow within the shell after solidification, making it extremely susceptible to cracking.

As a means for alleviating strong cooling on the long side, there is
The method of forming a large number of slit grooves only on the inner surface of the long side of the mold as shown in Japanese Patent Publication No. 180649, and the method disclosed in Japanese Patent Application Laid-Open No. 61-9275
The method of slow cooling by making grooves on the inner surface of both the long and short sides as shown in Publication No. 6, and also the method of
There is a method shown in Japanese Patent No. 11735 in which a large number of recesses are formed on the entire surface or a part of a mold, and several shapes of recesses are proposed.

However, as shown in these publications, the depth of the grooves and recesses is If it is less than several micrometers to several hundred micrometers, the molten steel will be pulled in, making it extremely shallow, and it will not be able to withstand long-term wear from the solidified shell of the molten steel. Typically, a mold with a flat inner surface is used.

<Problems to be Solved by the Invention> As a result of two-dimensional thermal analysis of a conventional continuous casting mold, the inventor found that at the corners closest to the long side and short side,
It was discovered that there was a significant temperature difference between the long side and the short side.

Figure 1 shows the analysis results. In the figure, a is the long side surface temperature of the conventional specification mold, b is the short side surface temperature of the conventional specification mold, and the temperature at the corner is 151°C on the long side. The short side is 270'C. That is, the long side is cooled extremely strongly, the short side is cooled extremely slowly, and the temperature difference between the two is 119°C, which is a large loss of balance.

In addition, when edge prints were taken from a cut sample of a slab in which the carbon component was in the peritectic reaction zone, grain growth was confirmed at the short side corner position 15III1, where solidification is slowest due to the above temperature difference. It was found that vertical cracks at one corner were concentrated in this area.

Based on these analyzes and observations, the occurrence of vertical cracks in slab corners will be explained in detail. On the short sides of the corners, solidification is significantly delayed due to slow cooling. On the other hand, since the long side is strongly cooled, solidification occurs first, so the solidification slow shell near the corner of the short side receives tensile stress and separates from the mold wall, creating a gap between it and the mold. Molten powder flows into this gap, and the local solidification delay further progresses. When the slab comes into contact with a large amount of cooling water at the position where it passes through the lower end of the mold, it rapidly shrinks, and the shrinkage reaction causes vertical cracks in areas where solidification is delayed. As mentioned above, this phenomenon occurs noticeably in medium-coal steel grades with large solidification shrinkage, but is not noticeable in high-order steel grades and low-odor steel grades. Furthermore, it occurs frequently when the cooling water temperature drops in winter, and is extremely likely to occur when the water supply temperature falls below 26°C.

It is an object of the present invention to provide a continuous casting mold that eliminates the drawbacks of the conventional method and suppresses corner cracks.

<Means for Solving the Problems> In order to solve the problems of the above-mentioned conventional method, the present invention employs the following means. In other words, by gradually decreasing the depth of the cooling slit grooves on the long side of the mold toward the corners, the mold has a small difference in surface temperature between the long side mold surface and the short side surface temperature, which prevents corner cracks in the slab. can be suppressed. Furthermore, a mold having the same effect can be obtained by applying a material such as 5US304, which has lower thermal conductivity than copper or copper alloy, to the bottom of the slit groove of the long-side copper plate and gradually decreasing the depth of the groove toward the corner. A mold having the same effect can also be obtained by making the width of the slit groove of the long side copper plate gradually smaller toward the corner. Furthermore, a mold in which the distance between the slit grooves of the long-side copper plate is gradually increased toward the corner portion can also be used to obtain a mold having the same effect.

<Examples> The present invention will be described in detail below with reference to Examples and Comparative Examples. Fig. 2 is a long side cross-sectional view of an example of the present invention, in which a square piece of 5US304 with a width of 511IffI and a height of 16 mm is embedded in the slit groove closest to the corner of the long side copper plate, and the effective depth of the groove is 9 mm. An SUS square piece with a width of 5M and a height of 5 meters was placed in the slit groove adjacent to the groove, making the effective depth of the groove 20 mm. FIG. 3 is an embodiment of the present invention, that is, a short side view paired with FIG. 2. Figure 4 shows the long side of the conventional model. Fifth
The figure is a view of the short side that is the opposite of FIG. 4.

As described above, the mold surface temperature distribution of a continuous casting mold that combines the long side in FIG. 4 and the short side in FIG. 5 is as shown in a and b in FIG. 1. The results of two-dimensional thermal analysis of the surface temperature of the mold according to the embodiment of the present invention, that is, the long side of FIG. 2 and the short side of FIG.
Also shown in the figure, C is the long side surface temperature and d is the short side surface temperature.Compared to the conventional method, the temperature at the corner part is 191°C on the long side and 245°C on the short side. C, and the temperature difference between the long side and the short side is 54°C, which has been greatly improved. This mold is suitable for suppressing corner cracks in slabs by reducing the temperature difference between the long side and the short side.

The conditions for the two-dimensional thermal analysis shown in Fig. 1 are: for both the conventional type and the example of the present invention, the mold side pull-out speed is 0.8 m/min, the amount of cooling water is 800 f/min on the long side; 500
f 7 sides/min, cooling water temperature 40"C.

The conventional slit groove has a width of 5 mm on the long side and a depth of 25 m.
There are 16 rows of grooves, and the short side is 5 mm wide and 25 mm deep.
In one example of the present invention, the long side has a width of 5 mm, the groove closest to the corner is 9 m deep, and a square piece of US304 with a width of 511 Il and a height of 16 mm is placed on the mold surface side of the groove. The groove adjacent to this groove has a width of 51 Wl and a depth of 20!
+l[11, further add width 5mm and height 5ff on the mold surface side
The other grooves are 5 mm wide and 25 mm deep, with a total of 16 rows, and the short side has 12 rows of 25 mm deep grooves.

FIG. 6 shows the results of comparative casting of two types of steels shown in the table below using both the conventional type and one example of the present invention.

As can be seen from Fig. 6, the corner crack occurrence rate in the conventional type is 77.5% on average for steel types A and B, and in one example of the present invention, it is 5%, which is a sufficient effect. It was confirmed.

<Effects of the Invention> As described above, by gradually relaxing the cooling capacity of the cooling slit grooves on the long sides toward the corners, the difference in cooling capacity between the cooling slits on the long sides and the short sides is minimized. To provide a continuous casting mold having the feature that solidification shell formation is performed as evenly as possible on the long side and short side by reducing the temperature difference between the long side and the short side of molten steel in contact with the molten steel. It is suitable for applications that do not cause slab defects.

[Brief explanation of the drawing]

Figure 1 is a temperature distribution diagram of the mold surface obtained by two-dimensional thermal analysis of a conventional method and an example of the present invention, Figure 2 is an explanatory diagram of a long side cross section showing an example of the present invention, and Figure 3 is an illustration of the present invention. Figure 4 is an explanatory diagram of the cross section of the long side of the conventional type, and Figure 5 is an explanatory diagram of the cross section of the short side that is paired with the long side of Figure 4. The explanatory diagram, FIG. 6, is a graph comparing the incidence of corner cracks in the cast sides of the conventional type and one example of the present invention. 7th
8 and 9 are detailed explanatory diagrams of the present invention. In the figure, a: b: Surface temperature of the long side of the conventional mold Surface temperature of the short side of the conventional mold Surface temperature of the long side of an example of the present invention Surface temperature of the short side of an example of the present invention Long side mold length Cooling slit groove of side mold Square bar 4 inside cooling slit groove: Short side mold 5: Fat layer from corner of cooling slit groove of short side mold (mm) Figure 2 Figure 3 Figure 40, 80 p<q(r Figure 9

Claims (1)

[Claims] 1. In a mold for casting a carbon steel bloom made of copper or copper alloy, and in which the cooling of the copper plate is of the slit groove type, the depth of the slit groove of the long side copper plate is directed toward the corner part. A continuous casting mold characterized by successively shallower molds. 2. The continuous casting mold according to claim 1, wherein a metal member having a lower thermal conductivity than copper or a copper alloy is applied to the bottom of the slit groove of the long side copper plate. 3. A carbon steel bloom casting mold made of copper or copper alloy, and a slit groove type mold for cooling the copper plate,
A mold for continuous casting, characterized in that the width of the slit groove in the long side copper plate gradually decreases toward the corner. 4. A mold for casting a carbon steel bloom made of copper or copper alloy, and a slit groove type mold for cooling the steel plate, characterized in that the distance between the slit grooves on the long sides of the copper plate is gradually increased toward the corners. Continuous casting mold.