JPH0577014A - Method for preventing longitudinal crack on short side of continuously cast slab and breakout - Google Patents

Abstract

PURPOSE:To prevent the longitudinal crack in the short side of a cast slab and the breakout by embedding measuring elements for gap between metals in the casting direction of the inner surface in the short side of a mold, measuring the gap between the inner surface in the short side of the mold and the surface of the cast slab and controlling taper in the short side of the mold. CONSTITUTION:The coils measuring the gap are embedded in two lines at the prescribed size downward from meniscus in the casting direction of the short side 11 of the mold 1. The taper is changed at any time while measuring the gap during casting to keep the gap constant. By this method, the shell thickness in the short side of the cast slab is uniformized and the longitudinal crack is prevented and the breakout caused by this can be prevented.

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 cracking and breakout of continuously cast slabs.

[0002]

2. Description of the Related Art In continuous casting of steel, particularly in continuous casting of a medium carbon steel slab having a carbon content of 0.09 to 0.15% by weight, longitudinal cracks often occur on the surface of the slab. 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 nonuniformly solidified shell is easily generated. As a result, it is believed that thermal stress causes cracking.

Therefore, as a method of preventing such vertical cracking, (1) a method of optimizing the viscosity of powder, (2)
There has been proposed a method of lowering the amount of heat removed from the molten steel by joining a metal having a low thermal conductivity to the inner surface of the mold copper plate or forming a groove.

On the other hand, vertical cracks tend to occur between the first and second charges in the initial casting stage. There is a demand for measures to cope with this change over time. In addition, vertical cracks generated during casting can only be detected by optical detection on the pinch roll exit side or visual inspection during hot slab maintenance, and countermeasures must be taken during casting. Is.

Therefore, it cannot be said that the above countermeasure (1) can be completely prevented due to the dispersion of the powder physical properties. In the above measure (2), there is an increased risk of breakout due to insufficient solidified shell thickness during high speed casting. With respect to the change over time in which vertical cracking occurs in the initial stage of casting, none of the measures have an effect, and it must be said that it is an excessive treatment.

Further, this tendency is strong on the short side shell side,
There is a problem that short side vertical cracks and breakouts due to the cracks frequently occur.

As a method of measuring the gap between the mold and the slab, a method of measuring the energy from the powder film (iron and steel, 83-S161) has been proposed. Since this method also measures directly under the 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 slab vertical cracks in the mold and take preventive measures in real time. The current information in the mold is temperature information from the mold steel plate. This information can predict breakout of the slab, but is not sensitive enough to predict vertical cracking. A measure that can be predicted in the mold and linked to preventive measures is desired.

[0008]

The problem to be solved by the present invention is to change the short side taper in accordance with the amount of the gap between the mold and the slab during continuous casting, thereby causing a break caused by short side vertical cracking. It is to prevent out.

[0009]

A method for preventing vertical cracking and breakout of a short side of a continuously cast slab according to the present invention uses a multi-taper mold for forcibly deforming the short side of the mold in the continuous casting method for steel. , Embedding intermetallic gap measuring elements in a plurality of rows at a predetermined pitch in the casting direction of the inner surface of the short side of the mold, measuring the gap between the inner surface of the mold short side and the surface of the slab by the element, The above problem is solved by means of controlling the taper of the short side of the mold based on the measured value.

[0010]

The intermetallic gap measuring element used in the method of the present invention is, for example, the patent application of the present applicant (Japanese Patent Application No. 2-29853).
No. 6) may be used. The method for measuring the gap between the surfaces of the metal bodies of the invention according to this patent application is such that one of the metal bodies arranged facing each other is separated by an appropriate length in a plane parallel to the surface facing the other, and a pair of coils Is embedded, and a low-frequency exciting current is applied to one of these coils,
When it is made to function as a transmission coil that generates a magnetic field, and when the energy of this magnetic field propagates through both metal bodies and the gap between them and captures the induced current induced in the other coil, this induced current contains the magnetic field energy Utilizing the fact that a phase delay and a decrease in strength occur depending on the difference in propagation paths, particularly the magnitude of the gap, the phase change and / or the strength change between the exciting current and the induced current of the receiving coil are detected to Identify the size of the gap.

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

On the other hand, the applicant of the present invention has proposed a multi-taper mold in which a pressurizing device is attached to the copper plate on the short side of the mold and the short side is forcibly deformed so as to match the profile of the slab (Japanese Patent Laid-Open Publication No. HEI-6) ..

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

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

Therefore, while measuring the gap during casting,
Keep the gap constant by changing the taper from time to time. As a result, it is possible to make the shell thickness of the short side of the cast slab uniform, prevent vertical cracks, and prevent breakout resulting therefrom.

FIG. 4 shows the relationship between the vertical crack occurrence rate and the short-side casting direction gap distribution. When the deviation of the gap amount in the short side casting direction between the adjacent measuring elements becomes 0.2 mm or more, the probability of vertical cracking rapidly increases, so the gap amount is 0.2 mm.
If it exceeds, a vertical crack prediction warning will be issued and the taper rate will be changed.

However, since the limit amount of the gap varies depending on the steel type, the casting speed, etc., it is necessary to obtain the limit amount in advance in accordance with the casting conditions.

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

Therefore, by controlling the mold short side taper ratio so that the gap is 0.05 mm or more and the deviation is less than 0.2 mm, vertical cracking and breakout can be prevented.

Since both vertical crack prediction and breakout prediction are determined by the gap amount up to 200 mm below the meniscus, it is sufficient to arrange the measurement coils from the meniscus in two rows in the casting direction.

[0021]

Embodiments of the method of the present invention will be described with reference to FIGS.

Medium carbon steel (C = 0.10%), 200 mm thickness × in a 1-point straightening continuous casting machine with a bending radius of 10 m
A 1800 mm wide slab was continuously cast at a casting speed of 3.0 m / min. No. On the short side 11 of the one-strand mold 1, gap measuring coils 2 were embedded in two rows of 50 mm and 150 mm below the meniscus in the casting direction. No. Two
As a strand, a conventional mold was used for comparison.

In the first strand, as described above, the gap is 0.05 mm or more and the gap amount deviation is 0.2 m.
The short side taper of the mold was controlled at any time so that it was less than m.
The result is shown in FIG. Vertical cracking is about 1/5 of the conventional method
It becomes the following.

FIG. 1 shows a row of a mechanism for changing the taper rate by providing an extrusion device 3 on the back surface of the short side 11 of the mold 1.

[0025]

According to the present invention, it is possible to prevent vertical cracks on the short sides of a slab and breakouts resulting therefrom, and it is possible to omit the maintenance process and to significantly improve the yield.

[Brief description of drawings]

FIG. 1 is a partial side view of a multi-taper 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 changes in the solidified shell thickness of a cast slab.

FIG. 4 is a graph showing the relationship between the gap amount deviation and the 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: Extrusion device, 11: Short side.

Claims (1)

[Claims] 1. In a continuous casting method for steel, a multi-taper mold for forcibly deforming a short side of a mold is used, and a gap between metals is measured in a plurality of rows at a predetermined pitch in a casting direction on an inner surface of the short side. A continuous cast slab consisting of embedding an element, measuring the gap between the inner surface of the mold short side and the surface of the slab with the element, and controlling the taper of the short side of the mold based on the measured value. Method for preventing short side vertical cracking and breakout.