JPS6353904B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing cracking of slabs in continuous casting.

In current continuous casting, supplying high-temperature slabs to the rolling heating furnace is a major issue in terms of energy conservation.Therefore, continuous casting operations require high-speed pouring and rapid detection of surface defects. There is a demand for supply of slabs for rolling in a short time. However, when performing high-speed casting, the thickness of the slab solidification shell formed within the slab is thin because the casting speed is high, and the continuous casting mold (hereinafter referred to as
There was a risk of so-called breakout occurring, in which the solidified shell ruptured when a thin part of the solidified shell reached the lower end of the mold within the mold. However, conventionally, it has not been possible to accurately predict the occurrence of this breakout, and therefore,
In order to prevent a breakout, the casting speed was lowered more than necessary, or if a breakout occurred, the operation could be shut down for many hours. In addition, surface defects such as vertical surface cracks are mainly caused by unevenness in the inflow of mold powder between the mold and molten steel (slab), especially due to a local decrease or increase in the amount of heat removed, which causes the solidification shell to change. This occurs due to uneven formation, but conventionally, (1) inspection for defects after rolling;
Surface defects were detected by methods such as (2) visual inspection after cooling the slab, and (3) inspection after extracting and cooling the slab. (2) The heating furnace consumption rate increases due to the need to cool the slab; or (3) complete It had drawbacks such as inability to detect defects.

Breakouts and surface defects as mentioned above
It is a well-known fact that both of these are closely related to the state of contact between the mold and the slab, that is, the state of heat removal.For example, if the contact state between the mold and the slab becomes uneven, the distribution of the amount of heat removed will also be uneven. Therefore, it is considered that surface cracks in slabs can be predicted by measuring the distribution of the amount of heat removed. Therefore, for example,
As shown in FIG. 1, a cooling water passage 1 is formed on the outer surface of a mold side plate 11 forming a mold 10.
A hole 11b is made at the bottom of the hole 1a, and a thermocouple 12 is embedded in the hole 11b.The heat flux is determined by calculation from the temperature gradient detected from the output of the thermocouple that is buried at two points apart in the depth direction. Hey, mold 10
The state of heat removal is being detected. However, in such a method, not only thermal disturbance occurs due to embedding the thermocouple 12, but also the embedding position of the thermocouple 12 is, for example, 1 mm.
Since there is a difference of 5 to 10 degrees Celsius when the temperature goes out of order, it is necessary to embed in an accurate position, which makes the embedding work difficult. In addition, when calculating the amount of heat removed Q using the following formula from the detected temperatures T ₁ and T ₂ of the two thermocouples, the burial interval d, and the thermal conductivity λ of the mold 10, the detected temperature T ₁ ,
Not only is there an error in T ₂ due to thermal disturbance, but there is also an error in the embedding distance d due to the embedding position, which tends to cause errors.

Q=λT ₁ −T ₂ /d (1) Furthermore, the heat flux cannot be directly recorded. In addition, the amount of change in thermocouple output when a breakout or surface flaw occurs is relatively small as shown in Figure 2 (in the case of a breakout), so for example, when detecting a breakout, it is necessary to It is necessary to look at changes in the temperature rise over a short period of time,
It is difficult to judge. Furthermore, with thermocouples, due to factors such as changes in mold thickness due to abrasion of the slab and embedding errors in the thermocouple itself, it is difficult to obtain clear numerical values such as the amount of temperature change at the time of breakout or the amount of temperature change at the time of surface defects. If it cannot be grasped or a vertical crack occurs,
If the change in the value is small, the occurrence of a defect cannot be detected. Furthermore, since a hole is made in the side plate of the mold and a thermocouple is embedded therein, the life of the mold is shortened, and it is difficult to relocate the mold.

The present invention has been made to eliminate the above-mentioned conventional drawbacks, and is capable of predicting cracking in slabs with high sensitivity, simply and reliably under all operating conditions.
It is an object of the present invention to provide a method for preventing cracking of slabs in continuous casting, which can easily and reliably prevent cracking of slabs.

The present invention provides a method for preventing cracking of slabs during continuous casting by measuring heat flux according to the amount of heat removed from the mold using a thin plate type surface heat flux meter disposed on the outer surface of the mold. The heat flux is measured by a flux meter, and when the amplitude of the heat flux waveform representing the temporal change in the heat flux exceeds a predetermined range, the casting speed is reduced, and low-speed casting is performed until the amplitude returns to the original value. This achieves the above object by preventing the occurrence of cracks in the slab.

The present invention utilizes a thin plate type surface heat flux meter developed in recent years. As shown in FIG. 3, this surface heat flux meter 14 is constructed by attaching a heat resistance plate 16 with a thermal conductivity of λ and a sufficiently thin thickness d to the surface of a solid body undergoing heat conduction. After reaching a steady state, the heat flux Q flowing through the thermal resistance plate 16 operates based on the following equation.

Q=λ/dΔT (2) Here, ΔT is the temperature difference between the front and back surfaces of the thermal resistance plate 16. Therefore, if the thermal conductivity λ and the thickness d are known, the heat flux can be determined by electrically measuring the temperature difference ΔT between the detection plates 18 disposed on the front and back surfaces of the thermal resistance plate 16. Q can be found.

This thin-plate type surface heat flux meter (1) does not need to be embedded in a mold and can be measured from the outside surface of cooling water passages, etc., (2) is small and can be installed anywhere, and (3) locally (4) Unlike thermocouples, there is no change in output due to embedding errors, and accurate heat flux values can be obtained just by installing them, even when thermal disturbances occur. , can be confirmed by verification, and (5) like a thermocouple,
There is no need to detect changes from a certain level, and the occurrence of surface cracks can be directly predicted based on the measured heat flux value.

FIG. 4 shows an example of a heat flux waveform representing a temporal change in the heat flux Q obtained by such a heat flux meter 14. The amplitude W of this heat flux waveform varies from the molten steel 22 to the solidified shell 24 as shown in FIG.
a and the mold 10 via the mold powder 25
This indicates the uniformity of the amount of heat removed by the mold powder 25, and when a minute surface crack occurs due to the slag phenomenon caused by abnormal inflow of the mold powder 25, the amplitude W of the crack occurrence location increases. By monitoring the amplitude W of the heat flux waveform, the amplitude W
is greater than a predetermined value, for example, 60×10 ⁴ Kcal/m ² ·hr, so it is possible to predict the occurrence of large surface cracks. The present invention has been made based on such knowledge. In Figure 5, 2
0 is an injection pipe, 24 is a slab, and 15 is a case of the heat flux meter 14.

In addition, if the occurrence of surface cracks is predicted, in order to prevent the progress of surface cracks, for example, the casting speed may be reduced and returned to the original casting speed, or the casting speed may be changed back to the original speed. If the amplitude W of the heat flux waveform does not return to its original value even if the speed is returned to , it is possible to prevent cracking of the slab by changing operating conditions such as changing the mold powder.

Further, the wave height H of the heat flux waveform is proportional to the amount of heat removed between the molten steel 22 and the mold 10, and is
At measurement points of 100 to 300 mm, it is usually 150 to 250 mm.
×10 ⁴ Kcal/m ²・hr (casting speed, mold powder,
(varies depending on taper, etc.). On the other hand, if the solidified shell 24a breaks or becomes thinner, increasing the possibility of breakout, the thermal resistance will decrease and the amount of heat from the molten steel 22 will be rapidly supplied to the mold 10. Therefore, the wave height H suddenly rises to 300×10 ⁴ Kcal/m ² ·hr or more. Therefore, by monitoring the wave height H of the heat flux waveform, the wave height H can be set to a predetermined value, for example, 300×
Since it was 10 ⁴ Kcal/m ² ·hr or more, it is possible to predict the occurrence of cracks in the solidified shell.

In addition, the period of the heat flux waveform indicates a slight change in the gap between the solidified slab shell and the mold, but if this period becomes abnormal, for example, if it becomes extremely long, it may indicate that steady state solidification is not progressing. show. Therefore, cracks can be predicted based on the period.

Furthermore, the occurrence of cracks can be more reliably predicted not only from the individual information on the wave height, amplitude, and period of the heat flux, but also from information on two or three of them.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of continuous casting equipment in which the method for preventing surface cracking of slabs according to the present invention is adopted will be described in detail with reference to the drawings.

In this embodiment, as shown in FIG. 6, molten steel 22 injected from above through an injection pipe 20 is cooled.
A mold 10 for forming the slab 24, a guide roller 26 for guiding the slab 24, a pinch roll 28 for pulling out the slab 24, and a motor 30 for rotationally driving the pinch roll 28.
and a pinch roll drive device 32 for controlling the motor 30, in a continuous casting equipment similar to the conventional one, in a cooling water passage 11a formed in a mold side plate 11 forming the mold 10,
A thin plate type surface heat flux meter 14 is housed in a case 15 (FIG. 5) that has a detection plate made of a material that has approximately the same thermal conductivity as the mold side plate 11 and prevents heat conduction in the direction in which heat flow is not detected. are arranged in close contact with each other by soldering, and the output of the heat flux meter 14 is taken into the signal processing device 36 via the heat extraction converter 34, and the amplitude W of the heat flux waveform is determined by the signal processing device 36. is 60×10 ⁴ Kcal/m ²・hr or more, or when the wave height H is 300×10 ⁴ Kcal/m ²・hr or more, the pinch roll is By controlling the driving device 32, the casting speed is reduced to prevent surface cracking and breakout of the slab, and the alarm 40 is
The system is designed to provide a predictive warning to the operator by activating the system.

The heat flux meter 14 has a mold short piece 11c and a long side 11d, as shown in FIGS. 7 and 8, for example.
The cooling water passages 11a are provided below the normal hot water level, and in the horizontal direction, each cooling water passage 11a or every other cooling water passage 11a is provided, and in the vertical direction, two cooling water passages are provided at every 100 to 200 mm in height.
Approximately 3 to 3 pieces are provided.

The action will be explained below.

As shown in FIG. 7 above, each of the cooling water passages 11a of the mold side plate 11 is provided with a distance of 100 mm from the hot water level in the vertical direction.
Set the heat flux meter 14 at a position of ~300mm, 1.2
While the operation was being carried out at a casting speed of m/min, locally, as shown in Figure 9A, the amplitude W of the heat flux waveform suddenly increased from time _t11 . As shown in Figure B, after a slight delay, the casting speed was once lowered to 0.7 m/min from time _t12 .
As shown in FIG. 9A, the amplitude returned to its original state at time _t13 , making it clear that surface cracking was prevented. Therefore, from time _t13 , the casting speed can be returned to the original 1.2 m/min, and high-speed casting can be resumed. If the amplitude becomes large again when the casting speed is returned to 1.2 m/min, it is possible to prevent the occurrence of surface cracks by other methods such as changing the mold powder.

As explained above, according to the present invention, it is possible to easily and reliably predict the occurrence of cracks in slabs under any operating conditions, and to easily and reliably prevent cracks in slabs. Has excellent effects.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing a state in which a thermocouple for detecting heat removal state is embedded in a continuous casting mold;
FIG. 2 is a diagram showing an example of the output waveform obtained by the thermocouple, and FIG. 3 is a diagram showing the principle of the heat flux meter used in the method for preventing surface cracking of slabs according to the present invention. FIG. 4 is a diagram showing an example of the heat flux waveform obtained by the heat flux meter, and FIG. 5 is a diagram showing the relationship between molten steel and the heat flux meter when the solidification shell is broken. Cross-sectional view showing the relationship, No. 6
The figure is a sectional view, including a partial block diagram, showing the overall configuration of an embodiment of continuous casting equipment in which the method for preventing surface cracking of slabs in continuous casting according to the present invention is adopted. FIG. 8 is a perspective view showing the mounting position of the heat flux meter in the example, FIG. 8 is an enlarged perspective view showing the mounting state of the heat flux meter, and FIG. 9 is the relationship between the heat flux meter output and the casting speed in the example. FIG. 10...Mold for continuous casting, 11...
...Mold side plate, 14...Heat flux meter, Q...Heat flux,
22... Molten steel, 24... Slab, 24a... Solidification shell, 26... Guide roller, 28... Pinch roll, 30... Motor, 32... Pinch roll drive device, 34... Heat extraction converter, 36... Signal processing device, 38... Casting speed control device, 40... Alarm, W... Amplitude of heat flux waveform.

Claims (1)

[Scope of Claims] 1. A method for preventing cracking of a slab during continuous casting by measuring a heat flux corresponding to the amount of heat removed from the mold using a surface heat flux of a thin plate provided on the outer surface of the mold. , the heat flux is measured by the heat flux meter, and when the amplitude of the heat flux waveform representing the temporal change in the heat flux exceeds a predetermined range, the casting speed is reduced, and the casting speed is kept at low speed until the amplitude returns to the original value. A method for preventing cracks in slabs in continuous casting, characterized by preventing cracks in slabs by performing casting.