JPH04143054A - Method for preventing cracking breakout in continuous casting - Google Patents

Abstract

PURPOSE:To easily and surely prevent cracking breakout by measuring molten metal surface level near the short side in a mold at the time of continuously casting and lowering casting velocity in the case variation of the molten metal surface exceeds the preset upper limit value. CONSTITUTION:Vortex sensors 7 are set near surfaces of both short sides 4B in the mold to measure the molten metal surface level near these. Successively, the detected signal of vortex sensor 7 is outputted to an abnormal detecting instrument 8 and the variation Ai of molten metal surface level per unit time is compared with the preset value Ao of upper limit within which the cracked breakout can be prevented, and in the case of Ai>Ao, the casting velocity V is lowered with a casting velocity control unit 9 and this detected result and treated result are displayed on a monitor 10.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention is aimed at preventing leakage cable breakout caused by cracks directly under the mold due to uneven solidification within the mold, especially when producing medium-carbon materials in continuous casting. The present invention relates to a method for preventing severe breakout.

<Prior art> In continuous casting, molten steel poured into a mold is primarily cooled by the mold to form a solidified shell on the surface of the molten steel, and the solidified shell is grown while being guided and supported in a subsequent secondary cooling zone to continue casting. It is a method of continuously manufacturing pieces, and the biggest operational problem with such continuous casting is breakout, where molten steel leaks due to rupture of the solidified shell.

Such breakouts cause great damage when they occur, so it is necessary to predict and prevent them from occurring, and as casting speeds increase, predicting and preventing breakouts has become extremely important. ing.

Various breakout prediction/prevention methods have been proposed in the past, including the following methods.

(i) Japanese Patent Publication No. 59-46703 The mold temperature is detected by disposing a number of temperature detecting elements in the width direction of the mold, and the mold temperature of the plurality of adjacent temperature detecting elements suddenly deviates from a steady level to a high temperature side. A method of predicting the occurrence of rupture or breakout of the solidified shell by

6D JP-A No. 2-52158 A temperature detection element is embedded in the opposing short sides of the mold, and a drifting breakout caused by a biased discharge flow from a submerged nozzle is detected from the deviation of the temperature values of both opposing short sides. Method.

GiD JP-A No. 60-191648 A surface temperature detector is placed directly under the mold to measure the temperature distribution in the width direction of the slab directly under the mold, and the temperature distribution is compared with the normal temperature distribution to identify any abnormality or temperature distribution. A method for detecting double skin and restrictive breakout based on abnormalities in temporal changes.

GV) JP-A-57-14449 A method of detecting breakout by measuring the shape of the short side of the slab between the mold and the support roll group and comparing the measured value with a reference value.

<Problems to be Solved by this Invention> Breakouts include restrictive breakouts that occur when the mold and solidified shell seize together, cracking breakouts that occur when the surface of the solidified shell cracks directly under the mold, and powder that breaks out from the mold. There are two main types of powder entrainment breakout that occur due to excessive flow between the mold and the solidified shell.Method (1) is mainly used to prevent restraint breakout that is accompanied by seizure breakage within the mold. With this method, it is not possible to accurately detect cracking breakout during casting of medium-carbon materials, where transverse cracking and temperature regulation occur directly under the mold without shell breakage within the mold.

Furthermore, method 00 cannot similarly detect if there is no drifting flow from the submerged nozzle.

Meanwhile, Gi confirming the solidified shell formation directly under the mold.
In methods D and GV), there is a problem with the durability of the detector under high temperature and high humidity conditions, and the detection is performed after the breakout-prone area reaches the bottom edge of the mold, so it is difficult to prevent breakout after detection. The problem is that it is difficult.

This invention was made to solve the above-mentioned problems, and its purpose is to easily detect cracking breakouts that occur due to cracks directly under the mold during casting of medium-carbon materials by detecting them at the upper part of the mold. Another object of the present invention is to provide a method for preventing cracking breakout in continuous casting, which can reliably prevent breakout.

<Means for Solving the Problems> As shown in FIGS. 1 and 2, the present invention solves the cracking breakout that occurs due to cracks directly under the mold due to uneven solidification within the mold during continuous casting. In this method, the hot water level near the short side of the mold is detected by a hot water level detection sensor 7 (
The abnormality detection device 8 compares the amount of fluctuation Ai of the hot water level per unit time with the preset upper limit value that can prevent the occurrence of cracking breakout (for example, with an eddy current sensor), and then In this case, the casting speed (2) is reduced.

The casting speed (■) is reduced to V, where Ai≦〇, or a predetermined lower limit speed ν. Try to lower it to .

Further, the casting speed (1) may be reduced by either automatic deceleration by the casting speed control device 9 or manual deceleration by an operator monitored by the monitor 10.

Effect> Cracking breakout is a phenomenon that occurs in certain types of steel, and as shown in Figure 3, when the [C] concentration is 0.09 to 0.
It occurs in 15% of medium carbon materials. Steel with this [C] concentration is known to have a high incidence of vertical cracking in slabs during boat fishing, and is a steel type that is likely to cause uneven formation of solidified shells.

During casting of such steel types, near the short side of the mold (10
When the occurrence of hot water level fluctuation (within 0 mm) increases,
The degree of semi-uniform formation of the solidified shell in the casting direction increases, and solidification delayed areas occur.

If the shell thickness directly under the mold in this delayed solidification area is 2 or less of the thick part of the shell where solidification has progressed normally, the third
As shown in the figure, transverse cracks A occur from the delayed solidification area, and there is a risk of breakout.

Fluctuations in the level of the hot water and the degree of solidification shell formation have a relationship as shown in FIG. 5, and the amount of fluctuation in the level of the hot water approaches the upper limit. (5mm/5ec in this figure), there is a risk that transverse breakout will occur. Therefore, a hot water level detection sensor 7 is installed near the short side of the mold (within 100 cm) to measure the amount of hot water level fluctuation, and when this reaches the upper limit value,
By outputting a warning from the abnormality detection device 8 and lowering the casting speed automatically or manually, cracking breakout can be prevented.

When lowering the casting speed, fluctuations in the melt level will inevitably occur due to changes in the casting speed, so even if the speed is set at a safe speed at which transverse cracking breakout does not occur, that is, even if uneven solidification of the solidified shell occurs, the shell thickness at the delayed solidification part will be reduced. For example, the lower limit speed VO is 10 mm or more (for example, 1.2 m/min,
After the hot water level has stabilized, the hot water level fluctuation amount Ai reaches the upper limit value A again. It is preferable to increase the speed and continue casting within the range below.

<Embodiment> The present invention will be described below based on an illustrative embodiment. As shown in FIG. 2, the continuous casting equipment supplies molten steel 5 in a ladle 1 to a mold 4 through a tundish 2 and an immersion nozzle 3.
The molded material is cast into the mold, guided and supported by a group of guide and support rolls 6, and pulled out by a drive roll 6A at a speed of {circle around (2)}.

The mold 4 is an assembled mold combining a pair of long sides 4A and a pair of short sides 4B.

In such a configuration, an eddy current sensor 7 is installed within 100 W from the surface of both short sides 4B of the mold to measure the level of the molten metal in the vicinity of both short sides.

The detection signal of the eddy current sensor 7 is output to the abnormality detection device 8, and the abnormality detection device 8 determines the amount of fluctuation in the hot water level per unit time to a preset upper limit that can prevent the occurrence of cracking breakout. For example, compared to 5 (mm/sec), 8,〉8. In this case, the casting speed controller 9 lowers the casting speed (2), and the monitor 10 displays the detection results, processing results, etc.

The casting speed control device 9 sets a lower limit speed v0 at which the shell thickness of the solidification delayed portion becomes 10 mm or more, for example, 1.2 Cm/mi.
n ), and after the hot water level stabilized, A8 became A.

The drive roll 6A is controlled to continue casting by increasing the speed within the range below.

When the present invention was carried out using the apparatus configured as described above under the following conditions, the results shown in FIG. 7 were obtained.

0 Slab dimensions = 700-1600mm (width) x 25
0 to 300 (thickness) 0 upper limit value Ao: 5 (mm/sec) (hot water level fluctuation amount) 0 lower limit speed V. : 1.2 (m/min) (casting speed) 0 Molten metal level detection sensor: Eddy current sensor As shown in Figure 7, before the implementation of this invention, there were about 5 cases of cracking breakouts in medium carbon materials per year. However, by implementing the present invention, it was possible to completely prevent the cracking breakout of the medium carbon material. The present invention is based on (
C) By applying the method only to steel types in which cracking breakout occurs at a concentration of 0.09 to 0.15%, cracking breakout can be reliably predicted and prevented.

<Effects of the Invention> As mentioned above, the present invention measures the molten metal level near the short sides of the mold and measures the molten metal level in order to prevent cracking breakout that occurs due to cracks directly under the mold during casting of medium-carbon materials. Since the casting speed is reduced when the amount of level fluctuation exceeds a preset upper limit, cracking breakout can be easily and reliably prevented.

In addition, since uneven solidification of the shell in the mold can be predicted based on fluctuations in the molten steel level in the mold, the problem of durability of the detector can be alleviated by detecting it at the upper part of the mold.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an apparatus for implementing the method according to the present invention, FIG. 2 is an overall schematic diagram showing continuous casting equipment,
Figure 3 is a schematic cross-sectional view showing the non-uniform formation of solidified shell due to fluctuations in the melt level, Figure 4 is a graph showing the relationship between the carbon concentration in molten steel and the frequency of short side cracking breakout, and Figure 5 is the level of the melt level. Figure 6 is a graph showing the relationship between the amount of variation and solidification shell unresponsiveness - Figure 6 is a graph showing the relationship between casting speed and the frequency of short-side cracking breakout occurrence, and Figure 7 is a graph showing the frequency of cracking breakout occurrence. be. 1...Ladle, 2...Tandishu 3
... Immersion nozzle, 4 ... Mold 4A ... Mold long side, 4B ... Mold short side 5 ... Molten steel,
6... Guide support roll group 6A... Drive roll 7... Molten metal level detection (eddy current) sensor 8... Abnormality detection device 9... Casting speed control device 1
0... Monitor 11... Solidified shell diagram number 1 diagram

Claims (1)

[Claims] (1) A method for preventing cracking breakout caused by cracking directly under the mold due to uneven solidification within the mold during continuous casting, which measures the level of the molten metal near the short sides of the mold, and Compare the amount of fluctuation A_i of the hot water surface level with a preset upper limit value A_o that can prevent the occurrence of cracking breakout,
A method for preventing cracking breakout in continuous casting, characterized by reducing the casting speed when _i>A_o.