JP2021164927A - Method for detecting cast slab defect in continuous casting - Google Patents

Abstract

To provide a method for detecting a cast slab defect in continuous casting, which can accurately detect depletion of a cast slab and displays the occurrence prediction position of depletion as a map of a cast slab surface.SOLUTION: A plurality of temperature sensors are installed laterally in a row in a cast slab width direction inside a mold wall. A temperature changing speed is calculated by time-differentiating the measured temperature time change of each temperature sensor. A time region where the temperature changing speed has become equal to or lower than a preset critical temperature changing speed is set as a depletion detection time region. The time when each longitudinal position of a cast slab passes the temperature sensor is associated with the depletion detection time region to set a depletion detection region 20 of the cast slab. The depletion detection region 20 on the surface of the cast slab is identified in a two-dimensional region in the cast slab width direction and a casting direction to generate a depletion position prediction map 21.SELECTED DRAWING: Figure 4

Description

The present invention relates to a method for detecting slab defects in continuous casting, and more particularly to a method for detecting dent defects on the surface of slabs called compression as slab defects.

Various surface defects occur on the surface of slabs in which molten metal is continuously cast. As a kind of surface defect, a dent defect on the surface of a slab called depletion is known. Depression is likely to occur in so-called periteum carbon steel having a C content of about 0.07 to 0.18% by mass, or medium carbon steel having a C content of about 0.5%. In addition, the tendency of depletion to occur may change due to continuous casting conditions, for example, changes in the quality of the mold powder used in the mold.

It is difficult to measure the dent of a slab with a continuous casting machine under high temperature conditions and due to the large amount of water vapor. Therefore, it is common to cool the slab after continuous casting and measure the cold slab. However, with this method, it takes time to find the depletion, so when the depletion is found, many slabs have already been cast under the same conditions. Therefore, when the depletion occurs frequently due to the change of the casting conditions, the optimization of the operating conditions is delayed, and the result is that the depletion occurrence abnormality occurs in a large amount of slabs.

In addition, the depletion portion often carries the risk of the presence of cracks in the recessed portion of the coagulation shell. If the cracked slab is rolled as it is in the subsequent process, it may cause surface defects of the product. Therefore, in order to confirm the depletion portion and eliminate the possibility that harmful cracks have occurred, it is necessary to clean it with a hand scarf or the like at the slab stage slightly deeper than the depletion depth. Conventionally, the decompression of cold pieces has been visually observed and identified, and the cracked portion has been removed with a hand scarf or the like. In this method, it is necessary to visually observe the entire surface of the slab in detail, which is troublesome and has a problem that detection omission is likely to occur.

When the depletion is generated, the contact state between the mold and the slab deteriorates, and the temperature of the mold copper plate drops. A method of detecting the occurrence of compression by detecting this temperature drop is known.

According to Patent Document 1, when continuously casting molten steel containing C = 0.08 to 0.15%, the mold wall temperature at a predetermined position below the molten metal surface of the continuous casting mold is 20 ° or more and 30 ° from the predetermined value in the normal state. It descends by less than, and then changes to an increase toward the above-mentioned predetermined value, and when the rate of increase change exceeds 2 ° C./sec, the surface layer of the solidified shell in the mold is cracked during width rolling after casting. A method for detecting an abnormal dent due to solidification shrinkage of a slab in a mold for detecting the occurrence of a continuous vertical or horizontal dent is disclosed.

In Patent Document 2, the temperature of the inner surface of the mold is continuously measured over the entire circumference, and the measured temperature curve at the measurement site rapidly decreases with the passage of time and then rapidly rises again. A method of detecting the occurrence of vertical depletion by showing a waveform is disclosed.

Patent Document 3 includes a first temperature sensor arranged on the side surface of the mold and a second temperature sensor arranged downstream thereof, and the first temperature difference between the first temperatures detected before and after is continuous. Then, in the first period showing a negative value, the first index that indexes the degree of depression during the period is calculated, and the temperature of the second temperature sensor is detected at a timing later than that of the first temperature sensor to detect the second temperature difference. Is calculated and the degree of depression is used as the second index, and the first index and the second index are multiplied to generate the third index. A method for determining the temperature is disclosed.

Special Fair 3-77944 Gazette Japanese Unexamined Patent Publication No. 10-193065 Japanese Unexamined Patent Publication No. 2010-279957

By the methods described in Patent Documents 1 and 2, the occurrence of depletion can be predicted to some extent, but when harmful depletion is generated but cannot be detected (referred to as "not detected"), the occurrence of depletion is predicted but actually. Occurred at a non-negligible frequency when no harmful depletion occurred (referred to as "overdetection"). Further, the method described in Patent Document 3 requires that a first temperature sensor and a second temperature sensor are provided above and below the mold, which complicates the configuration of the detection device. A first object of the present invention is to provide a method for detecting slab defects in continuous casting, which can more accurately detect the occurrence of slab compression without making the detection device excessively complicated. ..

Depression sites often carry the risk of cracks being present in the recessed portion of the initial solidification shell. Therefore, when detecting slab defects, it is preferable if the predicted position of occurrence of depletion can be displayed as a map of the slab surface, because the maintenance operator can quickly find cracks on the slab surface. A second object of the present invention is to provide a method for detecting slab defects in continuous casting, which can display the predicted occurrence position of compression as a map of the slab surface.

That is, the gist of the present invention is as follows.
[1] In continuous steel casting, a plurality of temperature sensors are installed in a horizontal row in the slab width direction inside the mold wall.
Set the critical temperature change rate as a negative value in advance.
The temperature change rate obtained by time-differentiating the measured temperature change with time of each temperature sensor is obtained, and the time range in which the temperature change rate is equal to or lower than the critical temperature change rate is defined as the compression detection time range, and each of the cast slabs in the length direction. The time when the position passes the temperature sensor and the compression detection time region are made to correspond to each other as a slab compression detection region, and the compression detection region on the slab surface is specified by a two-dimensional region extending in the slab width direction and the casting direction. A method for detecting slab defects in continuous casting.
[2] Of the temperature sensors provided in the width direction of the slab, regarding the space between the temperature sensors adjacent to each other, the temperature at each position in the width direction is assumed to change linearly in the width direction in the space. The method for detecting slab defects in continuous casting according to [1], wherein the depletion detection time region is estimated at each position in the width direction and used for the identification of the depletion detection region.

In the present invention, a plurality of temperature sensors are installed in a horizontal row in the width direction of the slab inside the mold wall, the temperature change rate obtained by time-differentiating the measured temperature change with time of each temperature sensor is obtained, and the temperature change rate is predetermined. The time region where the critical temperature change rate is equal to or less than the critical temperature change rate is defined as the depletion detection time region, and the time when each position in the length direction of the cast slab passes through the temperature sensor and the depletion detection time region are associated with each other to correspond the slab depletion. By specifying the compression detection region on the slab surface as the detection region as a two-dimensional region extending in the slab width direction and the casting direction, it is possible to detect the occurrence of slab compression more accurately and predict the occurrence of depletion. The position can be displayed as a map of the slab surface.

It is a figure which shows the temperature sensor installation position in a mold. It is the figure which corresponded the decompression occurrence position of a slab and the temperature measurement result, (A) is the decompression occurrence position of a slab, (B) is the time-dependent change of temperature at a temperature measurement position, (C) is a temperature measurement position. It is a figure which shows the time-dependent change of the temperature change rate in. (A) is a figure which shows the decompression occurrence position of a slab, and (B) is a figure which shows the two-dimensional distribution of the temperature change rate of a slab as a map. (A) is a diagram showing the compression generation position of the slab, and (B-1) to (B-4) are two-dimensional distributions of parts where the cooling rate (negative temperature change rate) is faster than the critical temperature change rate. Is displayed as a map.

Continuous casting was performed using a continuous casting apparatus having a mold shape in which the width of the slab to be cast was 600 mm and the thickness of the slab was 100 mm. The height of the mold (distance from the upper end 2 of the mold to the lower end 3 of the mold) is 700 mm (see FIG. 1). The position of the meniscus 4 in the mold is 80 mm from the upper end 2 of the mold. On one of the long sides 1 of the mold, temperature sensors (5A to 5G) are installed in a horizontal row in the slab width direction at positions as shown in FIG. The position of the temperature sensor in the height direction is 615 mm from the upper end 2 of the mold and 535 mm from the meniscus 4. An FBG (Fiber Bragg Grating) is used as the temperature sensor, and the temperature sensor is provided at a depth of 5.6 mm from the mold surface.

The molten steel components to be cast are as shown in Table 1, and a component composition in which compression is likely to occur on the surface of the slab is selected. The casting speed was 1 m / min.

During continuous casting, the temperature measured by the temperature sensor (5A to 5G) was recorded, and the depletion of the cast slab was visually inspected. The compression generation position 9 on the long side surface of the slab 8 is indicated as a white circle in FIG. 2 (A). The longitudinal direction of FIG. 2A is the casting direction, and the orthogonal direction thereof is the width direction of the slab 8. The temperature measurement result of the temperature sensor 5G is shown in FIG. 2 (B). In FIG. 2A, the straight line portion described as “temperature measurement position (5G)” is the slab portion where the temperature was measured by the temperature sensor 5G. Since the casting speed is constant at 1 m / min, the position in the casting direction in FIG. 2 (A) and the position on the horizontal axis (time axis) in FIG. 2 (B) can be made to correspond one-to-one. Further, FIG. 2C is drawn with the time derivative of the temperature measured by the temperature sensor 5G as the vertical axis.

Regarding the portion where the straight line of the “temperature measurement position (5G)” in FIG. 2 (A) and the white circle at the compression generation position 9 overlap (indicated by the down arrow in FIGS. 2 (B) and 2 (C)), FIG. 2 (B) ) And the behavior of the temperature change in FIG. 2 (C), the temperature in FIG. 2 (B) decreases with time at the compression generation position 9 in FIG. 2 (A), and the same time axis. It was found that the temperature change in FIG. 2C corresponds to the negative maximum position at the position. In FIG. 2C, a alternate long and short dash line is drawn at a position where the temperature change is -4 ° C / sec, and the portion where the temperature change rate is larger than the alternate long and short dash line is displayed as “Temperature high-speed decrease region 10”. There is. At the compression generation position 9 in FIG. 2 (A), the temperature change rate in FIG. 2 (C) exceeds -4 ° C / sec in the negative direction (the absolute value of the temperature change rate exceeds 4 ° C / sec). It is clear.

Next, using the temperature measurement results of all the temperature sensors (5A to 5G) arranged in a row in the width direction of the mold, an attempt was made to create a map of the temperature change on the surface of the slab. FIG. 3A is the same as FIG. 2A, and the compression generation position 9 on the surface of the slab 8 is indicated by a white circle.

In the slab width direction, the temperature is measured discretely by seven temperature sensors (5A to 5G). When creating a map of temperature change, regarding the space between adjacent temperature sensors (for example, the space between temperature sensor 5A and temperature sensor 5B), the width direction is assumed that the temperature change in the width direction between both temperature sensors is a straight line. The temperature at each position was estimated.
Further, based on the actual casting speed of the slab, the time when each position of the cast slab in the length direction passes through the temperature sensor is associated with the temperature measurement result by the temperature sensor.
Then, a two-dimensional map of the slab surface was created with the brightness distribution shown at the bottom of FIG. 3 (B) for the temperature change rate at each position on the slab surface (casting direction and slab width direction). , The map shown in FIG. 3 (B) was obtained. In FIGS. 3 (A) and 3 (B), the positional relationships of the slabs in the width direction and the casting direction correspond to each other. The whitish portion of the map of FIG. 3 (B) (the portion where the temperature change is large in the negative direction) has a positional relationship with the white circle portion recognized as the compression generation position 9 in FIG. 3 (A). I understand. The alternate long and short dash line in each of FIGS. 3 (A) and 3 (B) indicates the measurement point of the temperature sensor 5G. Since the temperature sensor 5G is the outermost temperature measurement point in the slab width direction and the temperature is not estimated for the outside of the temperature sensor 5G, the width in which the white part is drawn on the map in FIG. The directional end is the position of the temperature sensor 5G.

Next, the critical temperature change rate is set as a negative value at a pitch of 1 ° C./sec from -3 ° C / sec to -6 ° C / sec, and the temperature change rate is negative on the same map as in FIG. 3 (B). The part where the value is below the critical temperature change rate (temperature high-speed decrease region 10) is outlined, and the other part is black, and is drawn in FIGS. 4 (B-1) to (B-4) (hereinafter, “temperature high-speed decrease”). It is called "Map 11". FIG. 4 (B-1) shows a critical temperature change rate of -3 ° C / sec, FIG. 4 (B-4) shows a critical temperature change rate of -6 ° C / sec, and each figure has a critical temperature change rate of 1 ° C. It differs at the / sec pitch. When the critical temperature change rate is a small negative value (for example, FIG. 4 (B-1) (critical temperature change rate is -3 ° C / sec)), there are many white areas, and in addition to the actual compression generation position 9. , The part where the depletion was not observed is also included, and this part is the over-detection of the depletion. On the other hand, when the critical temperature change rate is a large negative value (for example, FIG. 4 (B-4) (critical temperature change rate is -6 ° C / sec)), there are few white areas and compression is observed. Nevertheless, there is a part that is not specified as a white part, and this part is not detected as a depletion. Then, there is an appropriate amount in the critical temperature change rate. For example, in the example shown in FIG. 4, FIG. 4 (B-2) (critical temperature change rate is -4 ° C./sec) is calculated based on the temperature measurement result. The correspondence between the white part of the map and the compression occurrence position 9 observed in the actual slab was the best.

As described above, the most preferable value of the critical temperature change rate can be determined in advance as a negative value based on the correspondence with the position of the depletion observed in the actual slab. Hereinafter, it is also referred to as “selected critical temperature change rate”. Then, the temperature change rate obtained by time-differentiating the measured temperature with time of each temperature sensor is obtained, and the time range in which the temperature change rate is equal to or less than the critical temperature change rate (selected critical temperature change rate) is defined as the compression detection time range. The time when each position in the length direction of the cast slab passes through the temperature sensor and the compression detection time region correspond to each other to form the slab compression detection region 20, and the compression detection region 20 on the slab surface is defined as the slab width direction. It can be specified in a two-dimensional region over the casting direction. Hereinafter, it is referred to as "depression position prediction map 21". In the example shown in FIG. 4, the selected critical temperature change rate is -4 ° C./sec, and FIG. 4B-2 (critical temperature change rate is -4 ° C./sec) is the compression position prediction map 21.

When the distance between the temperature sensors installed in the mold width direction is narrow, the temperature high-speed decrease map 11 (depression) is based only on the temperature measurement result of the temperature sensor installation position without performing the processing to supplement the data for the space between the temperature sensors. The position prediction map 21) can be drawn. Further, after defining the temperature high-speed drop region 10 in the straight line portion corresponding to the temperature sensor installation position, the boundary of the temperature high-speed drop region 10 at the temperature sensor position is connected by a straight line between the adjacent temperature sensors in a two-dimensional manner. It can also be set to the temperature high-speed drop region 10 (depression detection region 20). If the distance between the temperature sensors in the mold width direction is 50 mm or less, the compression position prediction map can be created by performing the above data processing.

When the distance between the temperature sensors installed in the mold width direction is wide, the temperature at each position in the width direction is estimated assuming that the temperature change in the width direction is a straight line for the space between adjacent temperature sensors, and each in the width direction. The depletion detection time region may be obtained at the position and used for specifying the depletion detection region 20. If the distance between the temperature sensors in the mold width direction is 150 mm or less, the compression position prediction map can be created by performing the above data processing.

Suitable embodiments of the temperature sensor used in the present invention will be described.
The mold wall on which the temperature sensor is installed may be determined according to the state of occurrence of compression in the slabs cast in the continuous casting facility. If it is sufficient to detect only the compression on the long side of the slab, the temperature sensor may be installed only on the long side of the mold. If it is necessary to detect the position where the compression occurs on the short side surface of the slab, install a temperature sensor on the short side surface of the mold.

There is no problem with the installation position of the temperature sensor in the height direction of the mold as long as it is below the meniscus, but it is desirable to install it at a position closer to the lower end of the mold because the accuracy becomes higher as it gets closer to the lower end of the mold. The depth from the surface of the mold on the molten steel side to the temperature sensor may be at least 5 mm because the inner surface may be cut due to wear of the mold. As a type of temperature sensor, an FBG (Fiber Bragg Grating) or a thermocouple can be used.

Hereinafter, the description of the concept of the present invention will be supplemented.
The present invention is based on the finding that depletion, which is a surface defect of slabs, occurs in the mold. When the depletion is generated, the contact state between the mold and the slab deteriorates, and the temperature of the mold copper plate drops. The present invention was made based on the idea of detecting depletion by detecting this temperature drop. This temperature change is measured by a plurality of temperature sensors (thermoelectric pair, etc., FBG sensor) installed on the mold copper plate (in the width direction).

1 Long side of the mold 2 Upper end of the mold 3 Lower end of the mold 4 Meniscus 5A-5G Temperature sensor 8 Shards 9 Depression generation position 10 High-speed decrease area 11 Temperature high-speed decrease map 20 Depression detection area 21 Depression position prediction map

Claims (2)

In continuous steel casting, multiple temperature sensors are installed in a horizontal row in the slab width direction inside the mold wall.
Set the critical temperature change rate as a negative value in advance.
The temperature change rate obtained by time-differentiating the measured temperature change with time of each temperature sensor is obtained, and the time range in which the temperature change rate is equal to or lower than the critical temperature change rate is defined as the compression detection time range, and each of the cast slabs in the length direction. The time when the position passes the temperature sensor and the compression detection time region are made to correspond to each other as a slab compression detection region, and the compression detection region on the slab surface is specified by a two-dimensional region extending in the slab width direction and the casting direction. A method for detecting slab defects in continuous casting. Of the temperature sensors provided in the slab width direction, for the space between the temperature sensors adjacent to each other, the temperature at each position in the width direction is estimated assuming that the temperature changes linearly in the width direction in the space. The method for detecting slab defects in continuous casting according to claim 1, wherein the compression detection time region is obtained at each position in the width direction and used for the identification of the compression detection region.

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