JP2661375B2 - Method for predicting longitudinal cracks in continuous cast slabs - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for predicting longitudinal cracks in a continuously cast slab in continuous casting of steel.

[0002]

2. Description of the Related Art In continuous casting of steel, particularly in the continuous casting of medium carbon steel slabs having a carbon content of 0.09 to 0.15% by weight, longitudinal cracks often occur on the slab surface. When the carbon content is 0.09 to 0.15% by weight, peritectic solidification occurs and the amount of shrinkage during solidification is large. Therefore, a local gap (hereinafter, referred to as a gap) is generated between the inner surface of the mold and the surface of the slab, and a non-uniform solidified shell is easily generated. As a result, it is considered that cracks occur due to thermal stress.

In order to prevent such vertical cracks, there are (1) a method of optimizing the viscosity of the powder, and (2)
A method has been proposed in which a low heat conductivity metal is joined to the inner surface of a mold copper plate or a groove is formed to reduce the amount of heat removed from molten steel.

On the other hand, vertical cracks tend to occur at the first or second charge in the early stage of casting. There is a need for a countermeasure against this change over time. In addition, the occurrence of vertical cracks during casting is detected only by optical detection at the pinch roll exit side, or by visual inspection at the time of hot maintenance of the slab, and measures are required during casting. It is.

[0005] Therefore, it cannot be said that the above-mentioned measure (1) can be completely prevented due to variations in powder physical properties. In the above-described measure (2), there is an increased risk that breakout occurs due to insufficient solidified shell thickness during high-speed casting. Neither countermeasure is effective for a temporal change in which a vertical crack occurs in the early stage of casting, and it must be said that this is an excessive measure.

[0006] Other gap measuring methods include a method of measuring energy from a powder film (iron and steel,
83-S161) has been proposed. Since this method is also a measurement directly under a mold, it cannot be dealt with in real time and has a serious problem in durability. Therefore,
It is necessary to detect the occurrence of vertical slab cracks in the mold and take preventive measures in real time. As information in the mold at present, there is temperature information from the mold copper plate. Although this information can predict a slab breakout, it is not sensitive enough to predict a vertical crack. There is a need for measures that can be foreseen in the mold and linked to preventive measures.

[0007]

The problem to be solved by the present invention is to predict the occurrence of vertical slab cracks in the mold during continuous casting, and to take appropriate preventive measures immediately to prevent the occurrence of vertical slab cracks. It is an object of the present invention to provide a method for predicting a vertical crack in a slab, which can minimize cracks.

[0008]

According to the present invention, there is provided a method for predicting vertical cracks in a continuous cast slab, wherein the method comprises the steps of: Embedding the inter-metal gap measuring element in the row, measuring the gap between the inner surface of the mold and the slab surface, detecting the unevenness in the width direction of the gap, when the deviation of the gap exceeds 0.2 mm The above problem has been solved by means for predicting the occurrence of vertical slab cracks and controlling the casting conditions.

[0009]

FIG. 2 shows the relationship between the occurrence of vertical cracks and the widthwise gap distribution. If the deviation of the gap in the width direction between adjacent measuring elements is 0.2 mm or more, the probability of the occurrence of vertical cracks will increase sharply. If the deviation of the gap exceeds 0.2 mm, a vertical crack prediction warning is issued and oscillation is performed. What is necessary is just to take measures, such as a change in conditions and a decrease in casting speed.

However, since the limit of the gap amount varies depending on the type of steel, casting speed, and the like, it is necessary to obtain various limit amounts in advance according to casting conditions.

FIG. 3 shows the relationship between the breakout occurrence rate and the gap amount. When the value of the gap amount becomes 0.05 mm or less, the breakout occurrence ratio sharply increases. Therefore,
When the gap amount becomes 0.05 mm or less, a breakout prediction warning is issued, and a countermeasure for reducing the casting speed may be taken.

Since both longitudinal crack prediction and breakout prediction are determined by the gap amount up to 200 mm below the meniscus, the measuring coils are arranged in two rows in the casting direction at a predetermined pitch from the meniscus over the width direction of the inner surface of the mold. An array is sufficient.

When a breakout prediction alarm is issued, casting conditions such as an oscillation change, a powder change, and a decrease in casting speed are controlled.

An intermetallic gap measuring element used in the method of the present invention is disclosed in, for example, a patent application of the present applicant (Japanese Patent Application No. Hei 2-298).
No. 536). The method for measuring an inter-surface gap of a metal body according to the invention according to this patent application includes a method in which one of metal bodies arranged opposite to each other is separated by a suitable length in a plane parallel to a surface opposed to the other pair of coils. Buried,
A low-frequency excitation current is applied to one of these coils to function as a transmission coil that generates a magnetic field, and the energy of this magnetic field propagates through the gap between the two metal bodies and between them, and to the other coil. When the induced current to be induced is captured, this induced current utilizes the difference in the propagation path of the magnetic field energy, in particular, the fact that the phase lag and the strength decrease according to the size of the gap occur. A phase change and / or a change in intensity with the induced current is detected to determine the size of the gap.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for predicting a vertical crack in a continuous cast slab of the method will be described with reference to FIGS.

As shown in FIG. 1, a predetermined pitch P (for example, 100 mm) is set in the width direction of the inner surface 11 of the continuous casting mold 1.
The intermetallic gap measuring elements 2 are buried in a plurality of rows (two rows in the illustrated example) in the pouring direction, and a gap (gap) between the mold inner surface 11 and the slab surface is measured. The unevenness in the width direction of the gap is detected, the occurrence of vertical cracks in the slab is predicted, and casting conditions such as oscillation change are controlled.

The elements 2 are provided in two rows separated from the position of the meniscus 3 by a distance d ₁ (for example, 50 mm) and further below the meniscus 3 by a distance d ₂ (for example, 100 mm).

The installation interval P of the intermetallic gap measuring element 2 (for example, the exciting coil) is preferably about 100 mm in consideration of mutual interference and the measuring range. Since vertical cracks in the slab occur in the early stage of casting, (meniscus) 3 to 200 m
m. Therefore, it is preferable that the element 2 be provided with a plurality of (preferably two) rows in the casting direction at a predetermined pitch over the width direction of the inner surface of the mold.

In a one-point straightening continuous casting machine having a curvature radius of 10 m, a medium carbon steel (C = 0.10%), a thickness of 200 mm ×
A slab having a width of 1800 mm was continuously cast at a casting speed of 3.0 m / min. No. Nineteen coils for gap measurement according to the method of the present invention were buried in a one-strand mold at a pitch of 100 mm in the width direction, and a total of 38 coils / one side in two rows of 50 mm and 150 mm below the meniscus. No. The two strands used the conventional mold as a comparison.

FIG. 4 shows the state of occurrence of vertical cracks during continuous casting under the above conditions. No. with conventional mold. With two strands, vertical cracks occur frequently in the early stage of casting, and the casting length is about 60
m. No. 1 was obtained from the mold based on the method of the present invention. In the case of one strand, the deviation of the gap amount became 0.2 mm or more in the initial stage of casting, and a vertical cracking warning was issued. Therefore, the oscillation stroke was reduced from 6 mm to 5 mm.
Did not lead to the occurrence of vertical cracks. Then, after the casting speed reaches a steady state,
mm, vertical cracks no longer occurred, and casting was continued under normal conditions.

[0021]

According to the present invention, vertical cracks, especially in the initial stage of continuous casting, are greatly reduced, and a maintenance step can be omitted and a hot charge ratio can be improved.

[Brief description of the drawings]

FIG. 1 is a partially broken perspective view of a mold to which the method of the present invention is applied.

FIG. 2 is a graph showing a relationship between a gap amount deviation between adjacent coils and a slab vertical crack strength.

FIG. 3 is a graph showing a relationship between a gap amount and a breakout occurrence rate.

FIG. 4 is a graph showing the result of applying the method of the present invention and the conventional method to an actual machine.

[Explanation of symbols]

 Reference Signs List 1 mold 2 measuring element 3 meniscus 11 mold inner surface

Claims (1)

(57) [Claims] In a continuous casting method for steel, intermetallic gap measuring elements are buried in a plurality of rows at a predetermined pitch and in a casting direction over a width direction of an inner surface of a casting mold for continuous casting. Measuring the gap between, detecting the unevenness in the width direction of the gap, predicting the occurrence of vertical slab cracks when the deviation of the gap exceeds 0.2 mm, and controlling the casting conditions. Of predicting vertical cracks in continuous cast slabs.