JPH0422554A - Continuous casting method - Google Patents

Abstract

PURPOSE:To obtain a cast slab without any surface defect by using dummy bar setting suitable number of thermocouples at the upper end so that the temp. sensing contacts come to the specific depth from outer peripheral surface of the cast slab and the specific position at the upper part thereof, detecting temp. signals and controlling cooling water flow rates. CONSTITUTION:A mold wall is formed by dividing into two or more steps and downstream side mold wall 2 except the most upstream side mold wall 1 is constituted by dividing to width direction of the cast slab with plural cooling water guide plates 3, and the cooling water guide plate constituting pair of the downstream side mold wall is constituted mutually as attachable/detachable. A slit 8 for gas blowing is arranged at between the upstream side mold 1 and the downstream side mold 2. The dummy bar 9 setting the suitable number of thermocouples 10 at the upper end so that the temp. sensing contacts come to at the position of 0.5-20mm depth to inside from the outer peripheral surface of cast slab and 10mm apart from the upper end thereof upward, is used. The thermocouples are intermally chilled in solidified shell at the initial stage of casting and temp. signals from the thermocouples are detected and cooling water flow rates in the downstream side mold are controlled. By this method, the cast slab without any surface defect can be stably produced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a continuous casting method, and particularly to a method at the start of casting.

(Prior Art) A continuous casting mold usually has a length of 600 mm = 1200 mm, and the inner wall of the mold is made of a material having high thermal conductivity, such as copper or a copper alloy.

When casting is performed using such a mold, the molten steel is indirectly cooled by a cooling medium (e.g. water) supplied inside the mold wall, and solidification progresses gradually from the part in contact with the mold wall, forming a solidified shell. As the thickness of the solidified shell grows to the extent that it can withstand the hydrostatic pressure of the internal molten steel, the solidified shell contracts, creating a void between the mold wall and the solidified shell.

Particularly in a mold with a rectangular cross section, the solidified slab shell that contacts the center of the broad wall of the mold tends to bulge outward due to the internal molten steel pressure and comes into relatively good contact with the mold wall, but There is a tendency for voids to appear conspicuously in the lower part of the surface side.

This generation of voids significantly reduces the efficiency of heat transfer from the slab to the mold wall, greatly inhibits the growth of the solidified shell of the slab, and causes quality defects such as surface vertical cracks due to uneven thickness of the solidified shell. This is often a major cause of breakout due to damage to the solidified shell. This is a major fundamental problem with the current continuous casting equipment, and is the biggest obstacle to achieving high-speed casting.

Therefore, in the specification and drawings of Patent Application No. 36643 of 1999, the present applicant proposes a method to prevent the formation of a solidified shell by preventing the mold cooling ability from decreasing due to the gap between the slab formed at the bottom of the mold and the mold. In order to prevent severe wear on the mold walls by increasing uniformity and improving lubrication, "In continuous casting assembly molds with a rectangular cross section, one or both of the two opposing mold walls are covered with slabs. The mold is divided into two or more stages in the casting direction, and the downstream mold wall except the most upstream mold wall is divided in the width direction of the slab by a plurality of cooling water guide plates, forming a pair of downstream mold walls. We have proposed a continuous casting mold in which the cooling water guide plates are movable toward and away from each other.

Furthermore, the applicant has stated that: ■ In the specification and drawings of Patent Application No. 229895 of 1999, ``any one or both of the two pairs of opposing mold walls are arranged in two or more stages in the slab casting direction. Continuous casting characterized in that a slit for blowing out gas is provided between the upstream mold and the downstream mold or at the bottom of the upstream mold in an assembled mold that is divided into two parts, and is located 30' or more and less than 90' downward from the horizontal plane. ``Continuous casting mold'' characterized in that a suction slit for drainage and exhaust gas is provided at least at the top of the casting mold and the downstream mold, which is connected to a vacuum system reduced from atmospheric pressure.

(Problems to be Solved by the Invention) If the invention of item (2) proposed by the present applicant is used, it is possible to operate the system without any problems in general, but the following points to be improved have been found. Ta. That is, it has been found that the surface quality of the slab strongly depends on the cooling conditions in the lower mold, and that the following problems arise in setting these conditions.

It has been described in the previous invention that non-uniform cooling of the slab surface causes quality defects such as surface vertical cracks, and is also a major factor in breakout due to solidified shell damage. Methods to prevent this include setting the distance between the cooling water guide plate and the slab online or controlling the flow rate of the cooling water, but it is not possible to directly detect the cooling status of the slab and take action. Because there is no such thing, it is sometimes insufficient. That is, the cooling condition of the slab is greatly influenced by the conditions (flow rate, thickness) of the water film formed between the slab surface and the cooling water guide plate.

However, when the slab is cooled, the thickness of the water film changes due to thermal contraction, and the flow rate of the water film changes accordingly, so the cooling conditions also change. Furthermore, the thickness of the water film changes again.

Therefore, it is very difficult to indirectly determine cooling conditions by only controlling these two factors, and there is no guarantee.

The present invention has been made in view of such problems,
The purpose is to directly detect and control the cooling status of slabs in the downstream mold, thereby enabling stable operations and providing a method for casting slabs without quality defects. .

(Means for Solving the Problem) As a result of conducting various experiments and research to achieve the above object, the inventor has found that (1) an appropriate number of thermocouples are installed at the upper end of the dummy bar; The temperature of the solidified shell is continuously measured during the initial stage of casting, that is, from the start of drawing of the slab to the time when it exits the downstream mold.

(2) The thermocouple's temperature-sensing contact should be located at least 10 m1 above the upper end of the dummy par, and the thermocouple should be placed at a depth of 0.5 to 20III11 inward from the outer peripheral surface of the cast slab. Place the pair at the top of the dummy bar.

(3) During the initial period of casting shown in (1) above, the cooling conditions of each part are determined by controlling the cooling water flow rate of the cooling water guide plate portion corresponding to the thermocouple based on the output signal (temperature) from each thermocouple. Control.

(4) The surface cooling conditions of the slab reach a nearly steady state at the initial stage of casting, and unless there is a major change in the conditions during casting, the cooling conditions will not change significantly depending on the initial setting.

It turned out that.

That is, the continuous casting method according to the present invention divides one or both of two pairs of opposing mold walls of a continuous casting assembly mold having a rectangular cross section into two or more stages in the slab casting direction. At the same time, the downstream mold wall excluding the most upstream mold wall is divided in the slab width direction by a plurality of cooling water guide plates, and the respective cooling guide plates constituting the pair of downstream mold walls are separated from each other. In addition to being movable toward and away from the upstream mold, a slit for gas blowing is provided between the upstream mold and the downstream mold, or at the bottom of the upstream mold at a distance of 30° or more from the horizontal plane.
Continuous casting molds with a downward angle of less than 90 degrees, or continuous casting molds with suction slits for drainage and exhaust gas connected to a vacuum system that is lower than atmospheric pressure, at least at the top of the downstream mold. It is a method of continuous casting using
20 kats deep and at least 10mm from its upper edge
Using a dummy par with an appropriate number of thermocouples installed on its upper end so that the temperature-sensing contacts are placed at separate positions, the thermocouples are wrapped in the solidified shell at the early stage of casting, and the temperature signals from the thermocouples are detected. The gist of this is to control the flow rate of cooling water in the downstream mold.

In the present invention, it is less than 0.5 degrees to install the thermocouple at the upper end of the dummy so that the temperature-sensing contact point of the thermocouple is located at a depth of 0.5 to 20 degrees inside the outer peripheral surface of the cast slab. In this case, there is a high possibility that the thermocouple will be exposed from the skin of the slab and will measure the temperature of the cooling water in addition to the temperature of the solidified shell. On the other hand, if it exceeds 20 m1, the thermal resistance of the solidified shell will slow down the response to changes in the cooling status of the slab surface, resulting in poor cooling control accuracy.

In addition, in the present invention, the temperature-sensitive junction of the thermocouple is installed at a distance of at least 10 m from the upper end of the dummy bar, because if the distance is less than 10 m, the influence of sensible heat changes of the dummy bar will appear. This is because the temperature of the solidified shell cannot be detected.

Note that the temperature-sensitive contact may be located as far away from the top of the dummy as possible, but if it is too far away, the thermocouple may melt due to the heat of the molten metal, and there is a high possibility that it will break or bend. This distance should be determined so that the casting speed is constant when the tip of the thermocouple enters the downstream mold due to drawing of the slab, and it is preferable to set this distance to a maximum and not to separate it any further.

(Example) The method of the present invention will be explained below based on the accompanying drawings.

Figure 1 shows the method of the present invention, which was filed in Japanese Patent Application No. 1-2298 by the applicant.
It shows the case where it is applied to the mold proposed in the specification and drawings of No. 95, where (a) is a front view in cross section, (b) is a side view, and (c) shows how the thermocouple is attached to the dummy bar. FIG.

First, the mold to which the method of the present invention is applied will be briefly explained.

In FIG. 1, 1 is an upstream mold, and 2 is a downstream mold. Of these, the upstream mold 1 usually has a tapered mold wall or two opposing pairs of parallel mold walls.

On the other hand, the downstream mold 2 is composed of, for example, a plurality of strip-shaped cooling water guide plates 3, each of which has a central cylinder 4, for example.
It is connected to a moving device such as the like through a link 5 so that the pair of mold wall surfaces can move toward and away from each other. In addition, 6 in FIG. 1 represents a spring, and 7 represents an immersion nozzle.

Reference numeral 8 denotes a gas blowing slit interposed, for example, between the upstream mold 1 and the downstream mold 2, and gas is blown from this slit 8 toward the slab during operation.

Although not shown, a suction slit is provided at the top of the cooling water guide plate 3 constituting the wall of the downstream mold 2, and a water supply port row and a drain port row are provided alternately below the suction slit. The pressure is detected by pressure detectors installed in these water supply parts and drainage parts, and the cylinder 4
This allows each cooling water guide plate 3 to be moved.

At the start of continuous casting using a mold having such a configuration, a dummy par 9 having an appropriate number of thermocouples arranged thereon is used as described below.

For example, divide the long side at the upper end of dummy par 9 into four equal parts,
In addition, thermocouples 10 are arranged at three locations (six locations in total) at positions 5 m inside from the mold wall surface so that their temperature-sensing contacts are 30 m above the upper end of the dummy par 9.

As described above, by using the dummy par 9 in which the thermocouple 10 is arranged, it becomes possible to detect the temperature at a position approximately 5 degrees inside from the surface of the slab at the start of casting, and the mold is adjusted based on this detected temperature. The cooling conditions are optimally controlled.

FIG. 2 shows the results of measuring the temperature at a position about 5 degrees inward from the surface of the slab using the thermocouple IO.

In this case, the steady casting speed is 1.4 m/sin,
A steady casting speed was reached in 25 seconds from the start of hot water supply, and the tip of the thermocouple (thermal contact) reached the downstream mold in about 27 seconds. If the cooling conditions shown in FIG. 2(a) are not controlled, the cooling at both ends is too strong compared to the center, resulting in a large temperature difference.

On the other hand, when the cooling conditions were controlled based on the temperature detected by the thermocouple, it was possible to make the cooling conditions of each part of the slab similar, as shown in FIG.

It has been found that by detecting and optimally controlling the cooling conditions of the downstream mold at the initial casting stage in this way, it is possible to produce slabs with almost no surface defects over the entire length of the slab thereafter.

Next, the results of experiments conducted to confirm the effects of the present invention will be explained. A continuous casting machine equipped with the mold shown in Fig. 1 casts low carbon aluminum killed steel of 50 tons per heat at a casting speed of 2 to 6 m/1 lin to a thickness of 105 m.
, a slab with a width of 1050 mm was manufactured.

At this time, in the upstream mold, Ca0S i
o 2- AI 20. I used a type of powder. Then, the flow rate of cooling water in the downstream mold was set at 6 to 40 m/sec.
Casting was performed by changing the temperature within the range of c.

When starting operation using a dummy paddle equipped with thermocouples arranged as in the present embodiment shown in FIG. 1, the amount of cooling water from the cooling water guide plate constituting the downstream mold is determined based on the temperature detected by the thermocouples. Fig. 3 shows a comparison of the defect occurrence rate on the slab surface when the temperature was controlled and when the thermocouple was not placed and the amount of cooling water was not controlled.

As is clear from Fig. 3, as the casting speed increases, the incidence of surface defects on the slab increases in the conventional method, but in the method of the present invention, the defect incidence is generally small and does not change much. Not yet.

This is because in the conventional method, as the casting speed increases, the amount of cooling water in the downstream mold increases, so the deformation due to rapid cooling of the high-temperature solidified shell becomes large, and the amount of cooling water is not directly controlled. This is because the defect occurrence rate increases.

(Effects of the Invention) As explained above, according to the present invention, it is possible to stably produce a slab without surface defects.

Note that the arrangement positions and the number of thermocouples are not limited to those shown in this embodiment, and can be changed as appropriate within the scope of the present invention. Furthermore, the dummy par on which the thermocouple is placed is not limited to the conventional one shown in this example, but it goes without saying that it may be placed on the dummy par proposed by the applicant in the patent application dated April 9, 1990. be.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the method of the present invention, in which (a) is a front view in cross section, (b) is a side view, (c) is a drawing showing how the thermocouple is attached to the dummy par, and Fig. 2 Figure 3 shows an example of measurement of slab temperature using a thermocouple; (a) shows a case without cooling control; (b) shows a case with cooling control; Fig. 3 shows the method of the present invention and the conventional method. It is a comparison diagram of the effects of the methods. 1 is an upstream mold, 2 is a downstream mold, 3 is a cooling water guide plate, 4 is a cylinder, 9 is a dummy bar, and IO is a thermocouple. @Figure 2 (a) #4 #3@A.3g!藺<sec>Warming P8 Unborn 3
Between one company (9c) (a) Figure 1 Figure 3

Claims (1)

[Claims] (1) Two opposing continuous casting assembly molds with a rectangular cross section
One or both of the pair of mold walls is divided into two or more stages in the slab casting direction, and the downstream mold wall except the most upstream mold wall is formed with a plurality of cooling water guide plates. The respective cooling guide plates, which are divided in the width direction of the slab and constitute the paired downstream mold walls, are configured to be movable towards and away from each other, and between the upstream mold and the downstream mold, or between the upstream mold. A continuous casting mold with a slit for blowing out gas at the bottom thereof at an angle of 30° or more and less than 90° downward from the horizontal plane, or connected to at least the top of the downstream mold to a vacuum system whose pressure is reduced from atmospheric pressure. This is a method of continuous casting using a continuous casting mold equipped with suction slits for drainage and exhaust gas, and the cast slab is cast at a depth of 0.5 to 20 mm inside the outer circumferential surface of the cast slab, and at its upper end. Using a dummy bar with an appropriate number of thermocouples installed on its upper end so that the temperature-sensing junction is located at least 10 mm upward from A continuous casting method characterized by controlling the flow rate of cooling water in a downstream mold by detecting a temperature signal.