JPH10197502A - Central solid phase rate sensing method for cast piece by continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure accurately the central solid phase rate of a cast piece in the solidification process to be conducted for improvement of the quality of cast piece by controlling in good timing the operating conditions of continuous casting properly. SOLUTION: Using an electromagnetic ultrasonic probe of transverse wave type, ultrasonic waves in the transverse mode are incident on the surface of each cast piece fabricated by a continuous casting process, and the transmitted transverse wave is received by a receiver furnished at the contrary surface, and the time for the transverse wave to have penetrated the cast piece is measured by the use of a received signal waveform of the part where the intensity of the received signal exceeds that of the noise signal. From the obtained measurement, the solid phase rate of the central part of the cast piece is determined on the basis of the relationship between the solid phase rate of cast piece determined previously and the time from the incidence of the ultrasonic wave in transverse mode across the thickness to its penetration of the cast piece. Therein the central solid phase rate are signified by the ratio of the solidified phase to the molten phase in that region of the cast piece central part where molten phase and solidified phase are in co-existence.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of detecting a central solid fraction by ultrasonic waves in a cooling process of a continuously cast slab of metal.

[0002]

2. Description of the Related Art In a continuous casting method of metal, a molten metal in a tundish is continuously poured into a mold provided at a lower portion of the tundish through an immersion nozzle, a solidified shell is formed in the mold, and a molten shell is formed from a lower portion of the mold. Continuously drawn as a slab,
This is a method of gradually solidifying molten steel inside a slab from the outside to obtain a slab.

In this continuous casting, the solidification of the molten steel inside the slab drawn from the mold varies depending on the temperature of the molten steel, the speed of drawing the slab, cooling conditions, and the like. Depending on the solidification state, center segregation becomes remarkable, bulging in which the slab abnormally expands, breakout in which the solidified shell is broken and molten steel inside the slab flows out, and the like occur.

[0004] Therefore, it is extremely important to grasp the solidification state in the process of cooling the slab in order to improve the quality of the slab and the productivity.

[0005] Conventional methods for detecting whether the inside of a cast slab drawn from a mold is completely solidified or an unsolidified portion remains include: 1) a tacking method, and 2) a load overload of a rolling roll. And 3) an ultrasonic method.

FIG. 4 is a diagram for explaining a conventional method of detecting an unsolidified portion. These methods will be described below.

1) Riveting method FIG. 4A is a diagram for explaining the riveting method, and is a side sectional view of a cast piece.

In this method, a rivet is driven into a position where the solidification state of the slab is to be checked, and after the slab is completely solidified, the position where the rivet is driven is cut to check whether a molten portion exists.
If there is a molten portion inside the slab at the time when the rivet is driven, the rivet is melted, and the melting state inside the slab at the time of driving the rivet can be determined.

2) Method of Estimating from Load Load of Rolls FIG. 4B is a side sectional view for explaining a method of estimating from load loads of the rolls.

In this method, a rolling roll 4 is provided at a measurement position of a slab, the applied load is measured, and a change in the applied load is used to estimate a state inside the slab.

This method has not been put to practical use because it is not a direct measurement but lacks accuracy.

3) Ultrasonic method FIG. 4C is a side sectional view for explaining the ultrasonic method. This method is a method of providing a probe capable of receiving and receiving transverse ultrasonic waves in the thickness direction of a slab, and estimating the solidification state inside the slab from the intensity of transmitted transverse ultrasonic waves.
However, since the transverse wave does not propagate in the liquid, it can only detect whether or not the inside of the slab is completely solidified.

In recent years, in the process of solidifying a slab in continuous casting, unsolidified rolling has been attempted to reduce the center segregation of the slab by applying rolling reduction to the slab in a state where the inside is unsolidified. In this rolling, it is necessary to perform rolling at an appropriate rolling reduction depending on the solidification state. For example, the thickness and width of the molten portion inside the slab at a predetermined rolling position, if they are larger than appropriate values, increase the rolling reduction to increase the thickness and width of the molten portion. This will reduce the amount of segregation.

As described above, in order to improve the quality of the slab in continuous casting and prevent troubles such as bulging and breakout, it is important to accurately grasp the solidification state in the cooling process. The development of a law is desired.

[0015]

SUMMARY OF THE INVENTION According to the present invention, an appropriate operating condition of a continuous cast slab is controlled in a timely manner, and the solidification state of the slab in the solidification process required for improving the quality of the slab is accurately measured. It is an object to provide a method.

[0016]

Means for Solving the Problems The present inventors have conducted various studies on a method for detecting the solidification state of a slab using ultrasonic waves, and have obtained the following findings.

A) To control the reduction of the slab, it is effective to determine the center solid fraction of the slab as a method of grasping the solidification state inside the slab.

The center solid phase ratio is a ratio of the following molten phase and solidified phase in a thin layer of a few mm in which a molten phase and a solidified phase are mixed at the center of a slab.

B) Transverse wave mode ultrasonic waves cannot pass through the molten metal if present in the slab, but before the molten metal is completely solidified. In a part where the solidified phase and the molten phase each having a thickness of several mm are mixed, a transverse wave can be transmitted.

C) The shear wave reception intensity is such that the center solid phase ratio is about 0.5.
25, about 0.7 and 1, it changes abruptly, and becomes constant in the range of about 0.25 to about 0.7, so that the center solid fraction cannot be determined from the reception intensity. .

D) However, the center solid fraction can be determined by measuring the transmission time or resonance frequency of the transverse wave.

The present invention has been made based on such findings, and the gist is as follows.

(1) A transverse wave mode ultrasonic wave is incident from the surface of a continuously cast slab in the thickness direction using a transverse wave type electromagnetic ultrasonic probe, and a transmitted transverse wave is received by a receiver provided on the opposite surface. Then, using the received signal waveform of the portion where the intensity of the received signal exceeds the intensity of the noise signal, the time required for the transverse wave to pass through the continuous cast slab is measured, and the measured value is obtained in advance. Based on the relationship between the center solid fraction of the continuous cast slab and the time required for the transverse wave mode ultrasonic wave to enter the slab after being incident from the surface of the continuous cast slab in the thickness direction. A method for detecting a center solid phase ratio of a continuous cast slab by calculating a center solid phase ratio of a continuous cast slab.

Here, the central solid fraction refers to the ratio of the solidified phase and the molten phase in the region where the molten phase and the solidified phase coexist at the center of the slab, that is, the solidified phase / molten phase.

(2) A transverse wave mode ultrasonic wave is incident from the surface of the continuously cast slab in the thickness direction using a transverse wave type electromagnetic ultrasonic probe, and a transmitted transverse wave is received by a receiver provided on the opposite surface. Then, using the received signal waveform of the portion where the intensity of the received signal exceeds the intensity of the noise signal, determine the resonance frequency, from the resonance frequency, and the previously obtained center solid phase ratio of the continuous cast slab, The center of the continuous cast slab, wherein the center solid phase ratio of the continuous cast slab is determined based on the relationship with the resonance frequency when the shear wave mode ultrasonic wave enters from the surface of the continuous cast slab in the thickness direction. Solid phase ratio detection method.

Here, the central solid fraction refers to the ratio of the solidified phase to the molten phase in the region where the molten phase and the solidified phase are mixed at the center of the slab, that is, the solidified phase / molten phase.

Hereinafter, embodiments of the present invention will be described in detail.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The use of the ultrasonic wave in the shear wave mode is based on the fact that the transit time or resonance frequency of the shear wave in the slab changes with the change in the solid fraction at the center of the slab. This is because it is suitable for seeking.

The intensity of the ultrasonic wave transmitted through the slab is small when the central solid phase ratio is small and cannot be distinguished from a noise signal. Slab transmission time or resonance frequency must be determined. It should be noted that the same applies regardless of whether the slab transmission time or the resonance frequency is used to determine the center solid fraction.
In the method of the present invention, the relationship between the central solid fraction and the slab transmission time or the relationship between the central solid fraction and the resonance frequency must be determined by experiments. The method of obtaining it is to vary the time from when the slab comes out of the mold to the ultrasonic transmission position by changing the drawing speed of the slab, measure the ultrasonic transmission time in the slab, and at each measurement Is determined by heat transfer calculation.

FIG. 1 is a diagram showing the relationship between the center solid fraction and the received signal strength.

FIG. 2 is a diagram showing the relationship between the center solid fraction and the transverse wave transmission time.

FIG. 3 is a diagram showing the relationship between the center solid fraction and the resonance frequency.

These figures are curves obtained by casting a 300 mm thick slab of low carbon steel by a continuous casting experiment apparatus. The received signal exceeds the noise signal at 1 V indicated by an arrow in FIG. 1, and the transmission time or the resonance frequency is obtained using the received waveform at 1 V or more. The reception intensity does not change when the center solid fraction is between 0.25 and 0.7. Therefore, the central solid phase ratio cannot be determined from the reception intensity. On the other hand, the transverse wave transmission time decreases as the central solid fraction increases, and the resonance frequency increases as the central solid fraction increases.

The use of an electromagnetic ultrasonic probe in the method of the present invention requires that ultrasonic waves be incident on a cast piece having a high temperature of about 900 ° C., and that a couplant such as water or oil is used. Ultrasonic probe can not be used. Therefore, an electromagnetic ultrasonic probe that does not require a couplant is used.

[0035]

EXAMPLE A slab having a width of 500 mm and a thickness of 300 mm was cast using low carbon steel molten steel in an experimental continuous casting facility. A transverse wave type electromagnetic ultrasonic probe was used as an ultrasonic wave transmitting and receiving device, and a transverse wave mode ultrasonic wave was made incident in the thickness direction of the slab to measure the receiving intensity.

The drawing speed of the slab is controlled in order to change the time required for the slab to pass through the ultrasonic transmission position, and the time from when the slab exits the mold to the ultrasonic transmission position (hereinafter, the time from the completion of pouring). ) From 1000 seconds to 3500 seconds, and the reception intensity during that time was measured.

Based on the measured intensity, the transverse wave slab transmission time and resonance frequency were determined.

FIG. 5 is a diagram showing the relationship between the intensity of the received signal and the time from the completion of the casting.

The received signal has an intensity of 1v [received signal magnitude (S) / about 1800 seconds after completion of casting (point A).
Noise signal magnitude (N) = 2] was detected for about 5 minutes.

Next, in about 2 minutes, the intensity increases to about 80% of the maximum received signal strength of the completely coagulated portion (point B → C).
point). Thereafter, the maximum value is reached in about 5 to 10 minutes (point D).

FIG. 6 is a diagram showing the relationship between the slab transmission time of transverse waves and the time from the completion of casting. The transmission time is
It is obtained from the waveform after point B in FIG.

Also, the time between the time when the mold is removed and the time when the measurement is performed is 10
The central solid fraction at each time from 00 to 3500 seconds was calculated by heat transfer calculation.

FIG. 7 is a diagram showing the relationship between the center solid fraction obtained by the above heat transfer calculation and the time from the completion of casting.

FIG. 8 is a diagram showing the relationship (a) between the obtained center solid fraction and the received signal strength and the transverse wave transmission time (a), and the relationship (b) to the resonance frequency. When the reception intensity was 1.25 V or less, it was not possible to measure a transverse wave due to a noise signal.

By obtaining this figure in advance, it becomes possible to determine the center solid fraction by measuring the transverse wave transmission time or the resonance frequency in actual operation.

[0046]

According to the method of the present invention, it is possible to accurately determine the center solid phase ratio at an arbitrary position in the slab cooling process of continuous casting, and to improve the quality of the slab and improve bulging in operation. Trouble can be prevented and stable operation is possible, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a center solid phase ratio and a received signal strength.

FIG. 2 is a diagram illustrating a relationship between a center solid fraction and a transverse wave transmission time.

FIG. 3 is a diagram showing a relationship between a center solid fraction and a resonance frequency.

FIG. 4 is a diagram for explaining a conventional method of detecting an unsolidified portion.

FIG. 5 is a diagram arranged in relation to the intensity of a received signal and the time from the completion of casting.

FIG. 6 is a diagram illustrating the relationship between the slab transmission time of transverse waves and the time from the completion of casting.

FIG. 7 is a diagram showing a relationship between a calculated solid phase ratio and a time from completion of casting.

FIG. 8 is a diagram illustrating a relationship between a solid phase ratio, a received signal strength, a transverse wave transmission time, and a resonance frequency.

Claims (2)

[Claims] 1. A method for producing a continuously cast slab from a surface thereof in a thickness direction.
A transverse wave mode ultrasonic wave is incident using a transverse wave type electromagnetic ultrasonic probe, and a transmitted transverse wave is received by a receiver provided on the opposite surface,
Using the received signal waveform of the part where the strength of the received signal exceeded the strength of the noise signal, the time required for the transverse wave to pass through the continuous cast slab was measured, and the measured value was determined in advance. Continuous casting is performed based on the relationship between the center solid fraction of the continuous casting slab and the time required for transverse wave mode ultrasonic waves to penetrate through the slab after entering in the thickness direction from the surface of the continuous casting slab. A method for detecting a center solid fraction of a continuously cast slab, wherein a center solid fraction of a piece is obtained. Here, the central solid fraction refers to the ratio of the solidified phase and the molten phase in the region where the molten phase and the solidified phase are mixed at the center of the slab, that is, the solidified phase / molten phase. 2. A continuous casting slab from the surface thereof in the thickness direction.
A transverse wave mode ultrasonic wave is incident using a transverse wave type electromagnetic ultrasonic probe, and a transmitted transverse wave is received by a receiver provided on the opposite surface,
Using the received signal waveform of the portion where the intensity of the received signal exceeds the intensity of the noise signal, the resonance frequency is determined, and from the resonance frequency, the center solid fraction of the continuous cast slab previously determined and the transverse wave mode The central solid phase of the continuous cast slab is obtained by determining the center solid phase ratio of the continuous cast slab based on the relationship with the resonance frequency when the ultrasonic wave is incident from the surface of the continuous cast slab in the thickness direction Rate detection method. Here, the central solid fraction refers to the ratio of the solidified phase and the molten phase in the region where the molten phase and the solidified phase are mixed at the center of the slab, that is, the solidified phase / molten phase.