JP2661380B2 - Method for preventing short side vertical cracking and breakout of continuous cast slab - 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 preventing short side vertical cracks and breakouts in continuous cast slabs.

[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.

Furthermore, this tendency is strong on the short side shell side,
There is a problem that short side vertical cracks and breakouts caused by the cracks occur frequently.

As a method for measuring a gap between a mold and a slab, a method for 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 cracks in the slab in the mold and take preventive measures in real time. As information in a mold at present, there is temperature information from a mold steel 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.

[0008]

The problem to be solved by the present invention is to change the short-side taper in accordance with the gap between the mold and the slab during continuous casting, thereby reducing breakage caused by short-side vertical cracks. To prevent out.

[0009]

SUMMARY OF THE INVENTION The present invention provides a method for preventing short side vertical cracks and breakout of a continuous cast slab. Using a tapered mold,
Embedding the intermetallic gap measuring elements at a predetermined pitch in the thickness direction of the inner surface of the short side of the mold, and in a plurality of rows in the casting direction, the gap between the inner surface of the short side of the mold and the surface of the slab by the element. Is measured, and the taper of the short side of the mold is controlled based on the measured value so that the gap is 0.05 mm or more, and the deviation of the gap in the casting direction is less than 0.2 mm. The above problem has been solved by the following means.

[0010]

The 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. 2-29853).
No. 6). 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. And a low-frequency exciting current is applied to one of these coils,
It functions as a transmission coil that generates a magnetic field, and when the energy of this magnetic field propagates through the gap between the two metal bodies and captures the induced current induced in the other coil, this induced current includes the magnetic field energy Utilizing the difference in the propagation paths, especially the fact that the phase lag and the intensity decrease corresponding to the size of the gap occur, the phase change and / or the intensity change between the exciting current and the induced current of the receiving coil are detected to detect the phase change and / or the intensity change. Identify the dimensions of the gap.

It has been found that the gap between both coils needs to be about 100 mm in consideration of mutual interference and measurement range. Therefore, in the casting direction of the short side of the mold, 100
By embedding coils at a pitch of mm, the state of gap distribution in the casting direction can be measured.

On the other hand, the present applicant has proposed a multi-tapered mold in which a pressing device is attached to a copper plate on a short side of a mold and the short side is forcibly deformed to match the profile of a slab (Japanese Patent Laid-Open No. 2-247059). Gazette).

By attaching the above-described intermetallic gap measuring element to the short side of the multi-taper mold, it is possible to always maintain a constant gap.

The short side profile of a slab differs depending on the type of steel, casting speed, and mold cooling strength. Even under the same conditions, as shown in FIG. 3, unevenness of the solidified shell thickness, that is, a change in the gap between the mold and the slab appears. Therefore, even if the mold taper is determined according to the steel type and the casting speed, it may change during casting.

Therefore, while measuring the gap during casting,
Change the taper from time to time to keep the gap constant. This makes it possible to make the slab short-side shell thickness uniform, prevent vertical cracks, and prevent breakout due to the cracks.

FIG. 4 shows the relationship between the vertical crack generation rate and the gap distribution in the short side casting direction. If the deviation of the gap amount in the short side casting direction between adjacent measuring elements is 0.2 mm or more, the probability of the occurrence of vertical cracks increases rapidly.
If it exceeds 2 mm, a warning for predicting a vertical crack is issued and the taper ratio is changed.

However, since the limit amount of the gap 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. 5 shows the relationship between the breakout occurrence rate and the gap amount. When the value of the gap becomes 0.05 mm or less, the breakout occurrence ratio sharply increases. Therefore, when the gap becomes 0.05 mm or less, a breakout prediction alarm is issued and the taper ratio is changed.

Accordingly, if the short side taper ratio of the mold is controlled so that the gap becomes 0.05 mm or more and the deviation becomes less than 0.2 mm, vertical cracks and breakout can be prevented.

Since both the prediction of longitudinal cracks and the prediction of breakout are determined by the gap amount up to 200 mm below the meniscus on the inner surface of the mold short side, the measurement coil is moved from the meniscus to a plurality of steps in the casting direction on the inner surface of the mold short side (in the embodiment, 2
It is sufficient to arrange them in a plurality of rows (two rows in the embodiment) in the thickness direction of the inner surface of the short side of the mold.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the method of the present invention will be described with reference to FIGS.

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. As shown in FIG. 2, on the short side 11 of the one-strand mold 1, the gap measuring coils 2 are arranged in two stages of 50 mm and 150 mm below the meniscus in the casting direction and in two rows in the thickness direction of the short side of the mold. Buried. No. The two strands used the conventional mold as a comparison.

As described above, the first strand has a gap of 0.05 mm or more and a gap amount deviation of 0.2 m.
m, the short side of the mold was controlled as needed.
FIG. 6 shows the result. Vertical cracking is about 1/5 of the conventional method
It becomes below.

FIG. 1 shows a row of mechanisms for changing the taper ratio by providing an extruder 3 on the back side of the short side 11 of the mold 1.

[0025]

According to the present invention, it is possible to prevent short-side vertical cracks in the cast slab and breakout caused by the short-side cracks, to omit a maintenance step, and to greatly improve the yield.

[Brief description of the drawings]

FIG. 1 is a partial side view of a multi-tapered mold to which the method of the present invention has been applied.

FIG. 2 is a perspective view of an installation example of a gap measuring coil used in the method of the present invention.

FIG. 3 is a graph showing a variation of a solidified shell thickness of a slab.

FIG. 4 is a graph showing a relationship between a deviation of a gap amount and a vertical crack occurrence rate.

FIG. 5 is a graph showing a relationship between a gap amount and a breakout occurrence rate.

FIG. 6 is a graph showing the effect of the method of the present invention.

[Explanation of symbols]

1: mold, 2: gap measuring coil, 3: extruder, 11: short side.

Claims (1)

(57) [Claims] In a continuous casting method for steel, a multi-tapered mold for forcibly deforming a short side of a mold is used, and at a predetermined pitch over a thickness direction of an inner surface of a short side of the mold.
And, burying the intermetallic gap measuring elements in a plurality of rows in the pouring direction, measuring the gap between the inner surface of the short side of the mold and the surface of the slab with the element, the gap is 0.05 mm or more, and, A method of preventing short-side longitudinal cracking and breakout of a continuous cast slab, comprising controlling a taper of a short side of the mold based on the measured value so that a deviation of a gap in a casting direction is less than 0.2 mm. .