JPH0631418A - Continuous casting method - Google Patents

Abstract

PURPOSE:To provide a continuous casting method preventing the development of longitudinal crack near the corner part of a continuously cast slab. CONSTITUTION:Temps. in a mold are measured at plural positions in the casting direction. Tapered amt. of short sides in the mold is adjusted by controlling cooling in the casting direction or according to the casting velocity so as to almost equalize the reducing rate in the casting direction of the heat flux at the long sides of the mold with the reducing rate in the casting direction of the heat flux at the short sides of the mold obtd. with the measured values. By this method, the cast slab having no longitudinal crack near the corner part can be produced at high casting velocity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for preventing imbalance of a solidified shell in a mold and for preventing vertical cracks in the vicinity of a corner of a slab.

[0002]

2. Description of the Related Art In a continuous casting method for steel, as a method for preventing vertical cracking of a slab, for example, a temperature fluctuation amount of a mold copper plate is measured, and an oscillation condition of a mold is minimized so that the temperature fluctuation amount is minimized. To optimize the powder inflow amount (see Japanese Patent Laid-Open No. 56-47245), the heat flux distribution in the width direction of the mold is measured by a plurality of heat flux sensors, and the heat removal amount is determined according to the measured values. The method of changing the type of powder to be charged in each part of the slab width direction so that the temperature becomes uniform in the slab width direction (see Japanese Patent Publication No. 62-43783), the heat removal amount distribution in the slab width direction is measured, and the removal is performed. A method has been proposed in which the heat quantity distribution is compared with the distribution in the standard state to find a deviation, and the vibration condition of high-frequency vibration in the mold width direction is changed so as to eliminate the deviation (see Japanese Patent Application Laid-Open No. 1-241355). .

[0003]

However, the above-mentioned conventional method is not effective as a means for preventing vertical cracks in the vicinity of the corner portion of the cast slab. That is, for vertical cracks that occur in the vicinity of the corners of a rectangular cross-section slab, it is necessary to consider the mold cooling conditions in the casting direction of the two surfaces forming the corners, rather than controlling the mold cooling conditions of the single surface. . However, in the method described in JP-A-56-47245, the temperature fluctuation amount at the position is measured by a thermocouple embedded in the long side of the mold, and the temperature fluctuation amount of the long side of the mold is measured. Since it is a method of controlling the conditions of the oscillation of the mold based on, it is effective in preventing vertical cracking in the widthwise central portion of the slab, the effect of preventing vertical cracking that occurs near the slab corners I hardly get it.

Further, the heat removal amount control method described in the above Japanese Patent Publication No. 62-43783 discloses a heat removal amount based on a heat flux distribution converted by the mold temperature measured by a plurality of sensors embedded in the mold width direction. As described above, not only can vertical cracks that occur in the vicinity of the slab corners be prevented, but this method also causes the mixing of powders with different physical properties, It is almost impossible to obtain the effect of preventing surface defects such as vertical cracks.

Further, the heat removal amount control method described in the above-mentioned Japanese Patent Laid-Open No. 1-241355 is a method of changing the vibration condition of high frequency vibration in the mold width direction based on the heat removal amount distribution in the mold width direction. Although it is possible to optimize the powder inflow amount in the mold width direction and optimally control the heat removal amount distribution, it is not possible to prevent vertical cracking that occurs in the vicinity of the slab corner portion.

As described above, all the conventional techniques adopt the method of controlling the mold temperature or the mold heat flux distribution in the mold width direction on a single surface, and the mold cooling conditions and the casting direction of the two surfaces forming the corner are completely controlled. Since this is not taken into consideration, there is a drawback that vertical cracks that occur in the vicinity of the corners of the cast slab cannot be prevented.

In view of such drawbacks of the prior art, the present invention considers the cooling conditions of the molds in the casting direction of the two faces forming the corners of the slab in the production of the slab with a rectangular cross section. It is intended to propose a continuous casting method capable of preventing the occurrence of vertical cracks at one corner.

[0008]

Means for Solving the Problems The gist of the present invention is to measure the temperature in a mold at a plurality of points in the casting direction as a method for preventing the occurrence of vertical cracks at the corners of a slab having a rectangular cross section. It is characterized by adjusting the casting direction reduction rate of the mold heat flux obtained by the measured value, and also the casting direction reduction rate of the heat flux on the long side of the mold and the casting direction reduction of the heat flux on the short side of the mold. It is characterized by controlling the cooling in the pouring direction so that the rate becomes almost equal,
Adjusting the taper amount of the mold short side according to the casting speed so that the casting direction reduction rate of the heat flux on the long side of the mold and the casting direction reduction rate of the heat flux on the short side of the mold are almost equal. It is characterized by.

[0009]

In the present invention, the method of controlling the decreasing rate (gradient) of the mold heat flux in the casting direction, which is obtained from the temperature of the mold in the casting direction on the two surfaces forming the corners of the cast slab, is as follows. For the reason shown in. That is,
As the casting progresses, the existence of large and small gradients of the heat removal amount drop in the casting direction causes non-uniformity of the initial solidified shell and causes thermal strain. Therefore, even if the heat removal amounts of the two surfaces forming the corner are equal at the solidification starting point in the mold, if the descending gradient of the heat removal amount of the two surfaces is different as the solidification progresses in the casting direction, the corner portion is removed. Thermal strain occurs in the corner, which causes tensile stress in the corners. By controlling the descending gradient of the mold heat flux along the casting direction between these two surfaces in the same pattern on both surfaces,
Excessive tensile stress on the corner can be prevented, and vertical cracks near the corner can be prevented.

Here, as a method for obtaining the heat flux, there are the following methods, for example. A method in which thermocouples are installed at a plurality of positions in the thickness direction of the mold, and the heat flux is obtained from the temperature gradient measured by the thermocouples. A method that calculates the temperature distribution in the mold and the heat flux at each position using a mathematical method, and calculates the heat flux from the correspondence with the measured temperature value.

[0011]

FIG. 1 shows a continuous cast slab according to an embodiment of the present invention together with the state of occurrence of vertical cracks in the vicinity of the corners of the slab, where 1 is the long side surface, 2 is the short side surface,
3 shows a vertical crack in the corner. The vertical cracks 3 that occur near the corner usually occur on the long-side surface 1 side or the short-side surface 2 side within a range of about 50 mm from the corner, but the corner itself may break.

In this embodiment, the mold size is 90.100 m.
With a test continuous casting machine of m thickness x 1000 mm width, 0.0
5 wt% carbon steel was cast under the casting conditions shown in Table 1.

[0013]

[Table 1]

The length of vertical cracking in the vicinity of the corner of the cast piece in this example was evaluated by the length of vertical cracking per unit slab length, and the relationship between the strength of corner vertical cracking and the casting speed is shown in FIG. As is clear from FIG. 2, when the casting speed was increased, the occurrence of vertical corner cracks at the casting speed tended to increase rapidly at the boundary of 3 m / min.

Therefore, the heat flux is determined from the temperature measurement values by thermocouples installed in the casting direction on the long-side copper plate and the short-side copper plate of the mold, and the presence or absence of vertical cracks and the short-side heat removal gradient and the long-side heat removal gradient I investigated the relationship. FIG. 3 is a schematic view of the mold heat flux measurement at that time, and FIG. 4 is the same as the downward h ₁ (45 m) from the meniscus.
shows the results of examining the initial heat flux (meniscus under h ₁₎ to dimensionless in heat flux reduction rate and the corner vertical cracks occurrence or non-occurrence relationship heat flux differential position location and h _{2 (250} mm) of m) Is. The heat flux was determined by the above method.

From the results shown in FIG. 4, vertical vertical cracks do not occur in the region where the heat flux reduction rate is approximately 1: 1 on the long side and the short side (hatched portion in the figure). There was a tendency for cracks to occur.

Further, solidification calculation was performed based on the measured heat flux at each casting speed, and the average value of the solidification shrinkage amount in the width direction of the long side was obtained. As is clear from the relationship, the short side taper is 1.1% of the current
At / m, there was a tendency for excessive indentation and strong cooling. Therefore, when the taper amount on the short side of the mold was slightly loosened, the results shown in Table 2 were obtained. That is, when the taper amount on the short side of the mold is slightly loosened, the region where vertical corner cracks do not occur tends to shift toward the high speed casting side, and the taper amount on the short side is 0.90.
% / M, vertical corner cracks could be prevented even at a casting speed of 5 m / min. On the contrary, on the low casting speed side, since the taper becomes loose and the air gap is formed between the slab and the mold, the heat flux reduction amount on the short side is large, and vertical cracks occur in the corners. Therefore, it is understood that there is an appropriate short side taper amount according to the casting speed.

[0018]

[Table 2]

[0019]

As described above, according to the method of the present invention, by controlling the reduction rate of the heat flux of the two surfaces forming the corner portion of the cast slab, there is no vertical crack in the vicinity of the corner portion and high quality is obtained. The slab can be stably manufactured, and since a slab without vertical cracks in the vicinity of the corner portion can be manufactured even in high speed casting, a high quality slab can be manufactured with high efficiency and high productivity. That is, it has an excellent effect.

[Brief description of drawings]

FIG. 1 is a continuously cast slab according to an embodiment of the present invention,
It is a perspective view which also showed the generation condition of the vertical crack in the corner vicinity of this slab.

FIG. 2 is a diagram showing a relationship between a vertical corner cracking occurrence strength and a casting speed in the same example.

FIG. 3 is a schematic view of mold heat flux measurement in the above-mentioned embodiment.

FIG. 4 is a downward direction h from the meniscus in the above embodiment.
The heat flux difference between the position of ₁ (45 mm) and the position of h ₂ (250 mm) is made dimensionless by the initial heat flux (h ₁ under the meniscus), and the result of the investigation of the relationship between the decrease rate of heat flux and the presence or absence of vertical corner cracks FIG.

FIG. 5 is a diagram showing a relationship between a casting speed and a heat shrinkage amount in the same example.

[Explanation of symbols]

 1 Long side face 2 Short side face 3 Vertical crack

Claims (3)

[Claims] 1. A continuous casting method, wherein the temperature in the mold is measured at a plurality of points in the casting direction, and the reduction rate of the mold heat flux in the casting direction is adjusted by the measured values. 2. The temperature in the mold is measured at a plurality of points in the casting direction, and the decrease rate of the heat flux in the casting direction on the long side of the mold and the heat flux on the short side of the mold, which are obtained from the measured values. A continuous casting method characterized in that cooling in the casting direction is controlled so that the rate of decrease in the casting direction is almost equal to prevent the occurrence of vertical cracks near the corners of the slab. 3. The temperature in the mold is measured at a plurality of points in the casting direction, and the decrease rate of the heat flux in the casting direction on the long side of the mold and the heat flux on the short side of the mold, which are obtained from the measured values. A continuous casting method, characterized in that the taper amount of the short side of the mold is adjusted according to the casting speed so that the reduction rate in the casting direction is almost equal to prevent the occurrence of vertical cracks in the vicinity of the corners of the slab.