JP2015160239A - Method and apparatus for surface defect determination for continuously cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a method and an apparatus for surface defect determination for a continuously cast slab, enabling a high quality slab to be produced.SOLUTION: The method for surface defect determination for a continuously cast slab related to the present invention comprises: a cast mold copper plate temperature acquisition step of acquiring a cast mold copper plate temperature measured by thermocouples (temperature measuring elements) 5; a main component analysis step of calculating a main component score by carrying out the main component analysis on the basis of the acquired cast mold copper plate temperature; and a main component score determination step of determining presence or absence of defects in the surface of a cast metal on the basis of the calculated main component score. The thermocouples 5 are embedded in a cast mold long side and are arranged in such a manner that: in the casting direction, a position of the uppermost stage temperature measuring element is within 250 mm from a molten metal surface control level, a position of the lowermost stage temperature measuring element is away from the molten metal surface control level by 500 mm or more, and a gap between adjacent temperature measuring elements is 250 mm or less; and in the cast mold width direction, positions of the temperature measuring elements installed at points closest to both short sides are within 250 mm along a direction from the position of intersection of a short side face with a long side face of a sab width being a measurement object toward a center in the cast mold width, and a gap between adjacent temperature measuring elements is 250 mm or less.

Description

  The present invention relates to a method for determining a surface defect of a continuous casting slab in which a temperature measuring element is embedded in a mold used for continuous casting of a slab, and a slab surface defect is determined based on a temperature measurement value of a mold copper plate temperature obtained by the temperature measuring element. And an apparatus.

In recent years, quality requirements for high-grade steel products have become stricter, and there is a demand for higher quality from the slab stage, that is, from the continuous casting stage.
One of the surface defects at the product stage is that caused by mold powder spread on the molten steel surface in the mold. For example, when the surface flow velocity of the molten steel surface in the mold is too fast, it is known that mold powder is caught in the molten steel, and this mold powder causes surface defects. Further, even when the amount of fluctuation of the molten steel surface in the mold is large, the mold powder is trapped in the molten steel and causes surface defects as described above.
Alternatively, deoxidation products such as alumina (Al ₂ O ₃ ), Ar gas bubbles, or other foreign substances present in the molten steel may be trapped in the solidified shell as a cause of surface defects. When rolling without removing the mold powder or the like captured by these solidified shells, a surface flaw defect called “hege sliver” or the like occurs in the steel sheet product after rolling, and the yield of the steel sheet product is reduced.

  Therefore, in the continuous casting process, in order to prevent the occurrence of such defects, it is common to apply a magnetic field to the molten steel in the mold to control the molten steel flow in the mold and prevent entrapment and trapping of mold powder and the like. (For example, refer to Patent Document 1).

However, even if the molten steel flow is controlled by applying a magnetic field to the molten steel, it is difficult to completely suppress the above slab surface defects because entrainment and trapping of mold powder, etc. occur due to operational fluctuations. It is.
Therefore, by detecting the change in the molten steel flow in the mold with the temperature change of the thermocouple embedded in the mold copper plate, the presence or absence of defects on the surface of the slab is determined, and based on the determination result, some measures are taken in the continuous casting process. Techniques for taking or slab care have been proposed.

  For example, in Patent Document 2, a plurality of temperature measuring elements are arranged in the width direction on the back side of the long-side copper plate of a continuous casting mold to measure the temperature distribution in the width direction of the long-side copper plate, and the measured temperature distribution Any of the magnetic field strength of the magnetic field generator attached to the mold, the casting speed, the immersion depth of the immersion nozzle, and the amount of Ar blown into the immersion nozzle so that the difference between the maximum value and the minimum value is 12 ° C. or less. Adjusting one or more is disclosed.

  Further, in Patent Document 3, the mold copper plate temperature or the heat flux at each point in the width direction of the continuous casting mold is measured and the distribution in the mold width direction is monitored, and the distribution state in the mold width direction is temporal. A surface defect detection method for determining that vertical cracks have occurred on the surface of a slab when there has been a significant change to the above is disclosed.

  Furthermore, in Patent Document 4, while applying a moving magnetic field that causes the molten steel in the mold to turn in the horizontal direction, the copper plate temperature is measured using a temperature measuring element embedded in the back side of the long copper plate, and the axis of the mold space is When the measurement results of the respective temperature measuring elements arranged at the symmetrical positions as the symmetry axis are compared, and the ratio of the lower measured temperature to the higher measured temperature of both becomes smaller than 0.85 Discloses a method for determining that a defect has occurred on the surface of a slab.

  In Patent Document 5, a large number (many types) of operation data is converted so that it can be expressed by a small number (small types) of variables (referred to as feature amounts), and the correspondence between the feature amounts that are representative values and the quality data is disclosed. A technology for predicting the quality of a product using a performance database is disclosed.

JP-A-10-305353 International Publication No. 2000/51763 Japanese Patent Laid-Open No. 2-151356 JP 2009-214150 A Japanese Patent No. 5169096

However, the above prior art has the following problems.
That is, in Patent Document 2 and Patent Document 3, the occurrence of surface defects is determined based on the temperature distribution in the width direction of the mold, but when the molten steel in the mold is horizontally swung using a moving magnetic field, The mold copper plate temperature distribution is basically asymmetrical in the width direction with respect to the mold axis center, and the copper plate temperature distribution pattern in the mold width direction changes depending on the casting speed, slab width, magnetic field strength, etc. The occurrence of defects on the surface of the slab cannot be determined from the level of simple copper plate temperature or the difference between the maximum and minimum values. In other words, Patent Document 2 and Patent Document 3 are based on casting conditions in which the temperature distribution in the mold width direction is substantially uniform, and cannot be applied to the case where the molten steel in the mold is swirled horizontally.

  On the other hand, Patent Document 4 captures a change in the temperature of a copper plate accompanying a change in flow in a mold and performs defect determination. In this method, it is determined that the flow velocity reduction region is a region where defects occur, but if there are no deoxidation products, Ar bubbles, mold powder, or other foreign substances that cause defects in the region, the solidified shell Is not a product defect because nothing is captured. That is, this method may detect and determine the occurrence of defects excessively.

  Patent Document 5 is a product quality prediction technique using a database, but a facility / control system for storing / managing / reading the database is required, and there is a concern that the capital investment cost will rise. Moreover, in the Example of patent document 5, although the product quality prediction example using a mold copper plate temperature is shown, it is mentioned regarding the number of copper plate temperature data required for quality prediction (the number of thermocouples embedded in a copper plate). Absent.

  The present invention has been made to solve the above-described problems, and obtains a surface defect determination method and apparatus for a continuous cast slab capable of producing a high-quality slab without increasing capital investment costs. For the purpose.

(1) A method for determining surface defects of a continuous cast slab according to the present invention is a continuous casting in which a mold copper plate temperature is measured by a temperature measuring element embedded in the long side of the mold, and a slab surface defect is determined based on the measured temperature value. A method for determining surface defects of a slab,
The arrangement of the temperature measuring element embedded in the long side of the mold,
Regarding the casting direction, the position of the top temperature sensor is within 250 mm from the molten metal surface control level, the position of the bottom temperature sensor is 500 mm or more away from the molten metal surface control level, and the distance between adjacent temperature sensors. Is 250 mm or less,
Regarding the mold width direction, the position of the temperature measuring element installed at the location closest to both short sides is changed from the position of the intersection of the short side surface and long side surface of the slab width to be measured toward the mold width center. Within 250 mm, and the interval between adjacent temperature measuring elements is 250 mm or less,
A mold copper plate temperature acquisition step of acquiring a mold copper plate temperature measured by the temperature measuring element arranged as described above,
A principal component analysis step of calculating a principal component score by performing a principal component analysis based on the acquired mold copper plate temperature;
And a principal component score determining step for determining whether or not a defect has occurred on the surface of the slab based on the calculated principal component score.

(2) Further, in the device described in (1) above, the position of the lowest temperature measuring element is within 750 mm in the casting direction from the molten metal surface control level.

(3) In the above (1) or (2), the principal component score determining step determines that a defect has occurred when the principal component score exceeds a predetermined threshold value. It is.

(4) The continuous casting slab surface defect determination device according to the present invention measures the mold copper plate temperature with a temperature measuring element embedded in the long side of the mold, and continuously determines the continuous casting slab surface defect based on the measured temperature value. A device for determining surface defects of a cast slab,
A temperature measuring element group embedded in the mold long side, a mold copper plate temperature acquisition means for acquiring a mold copper plate temperature measured by each temperature measuring element of the temperature measuring element group, and based on the acquired mold copper plate temperature A principal component analysis means for calculating a principal component score by performing a principal component analysis, and a principal component score determination means for determining the presence or absence of defects on the slab surface based on the calculated principal component score,
The arrangement of the temperature measuring elements constituting the temperature measuring element group,
Regarding the casting direction, the position of the top temperature sensor is within 250 mm from the molten metal surface control level, the position of the bottom temperature sensor is 500 mm or more away from the molten metal surface control level, and the distance between adjacent temperature sensors. Is 250 mm or less,
Regarding the mold width direction, the position of the temperature measuring element installed at the location closest to both short sides is changed from the position of the intersection of the short side surface and long side surface of the slab width to be measured toward the mold width center. The distance between adjacent temperature measuring elements is 250 mm or less.

(5) Further, in the device described in (4) above, the position of the lowest temperature measuring element is within 750 mm in the casting direction from the molten metal surface control level.

(6) In the above (4) or (5), the principal component score determining means determines that a defect has occurred when the principal component score exceeds a predetermined threshold. It is.

  In the present invention, with regard to the arrangement of the temperature measuring elements, in the casting direction, the position of the uppermost temperature measuring element is within 250 mm from the molten metal surface control level, and the position of the lowermost temperature measuring element is 500 mm or more from the molten metal surface control level. The distance between adjacent temperature measuring elements is 250 mm or less, and in the mold width direction, the position of the temperature measuring element installed at a location closest to both short sides is the short side surface of the slab width to be measured. The copper plate of the mold measured by the temperature measuring elements arranged as described above, within 250 mm along the direction from the position of the intersecting line of the long side surface to the center of the mold width, and the interval between the adjacent temperature measuring elements is 250 mm or less. A mold copper plate temperature acquisition step for acquiring temperature, a principal component analysis step for calculating a principal component score by performing principal component analysis based on the acquired mold copper plate temperature, and a slab table based on the calculated principal component score By having a main component score determination step of determining whether the occurrence of defects, without rising the capital expenditure can determine presence of defects based on the measured temperature with high accuracy. Moreover, a high quality slab can be manufactured by appropriately treating defects on the surface of the slab based on the determination result.

It is explanatory drawing of the surface defect determination apparatus of the continuous casting slab which concerns on one embodiment of this invention. It is explanatory drawing of arrangement | positioning of the temperature measuring element in the experiment for demonstrating the process which led to this invention. It is a graph which shows the relationship between the mold copper plate temperature which concerns on the experiment for demonstrating the process which led to this invention, and casting time (the 1). It is a graph which shows the relationship between the mold copper plate temperature which concerns on the experiment for demonstrating the process which led to this invention, and casting time (the 2). It is a graph which shows the relationship between the mold copper plate temperature which concerns on the experiment for demonstrating the process which led to this invention, and casting time (the 3). It is a graph showing the principal component analysis result which concerns on the experiment for demonstrating the process which led to this invention. It is a flowchart of the surface defect determination method of the continuous casting slab which concerns on one embodiment of this invention. It is a graph of the experimental result which concerns on Example 2 of this invention.

  Before explaining in detail the surface defect determination method and apparatus for a continuous cast slab according to an embodiment of the present invention, the background to the present invention will be described first.

<Background to the Present Invention>
The inventors of the present invention investigated the profile of the mold long side copper plate temperature profile in the mold casting direction and the mold width direction under various casting conditions in a slab continuous casting machine. In that case, the thermocouple 5 was embedded as a temperature measuring element in the almost identical location facing each other in the opposite mold long side copper plates (see FIG. 1), and the temperature of each of the mold long side copper plates was measured.

The arrangement of the thermocouple 5 on the long side copper plate will be described with reference to FIG.
As for the casting direction, as shown in FIG. 2, the thermocouple 5 was provided so that there were a total of 9 stages from the A stage to the I stage at a pitch of 100 mm starting from the point where the distance from the molten metal level control level was 50 mm. . The molten metal surface control level is a molten metal surface level targeted for automatic injection amount control when molten steel is poured from a tundish into a mold.
In the mold width direction, 16 rows of thermocouples 5 from the first row to the 16th row were provided at a pitch of 130 mm. The distance from the upper surface of the mold copper plate to the hot water level control level is 50 mm. Thus, by embedding the thermocouple 5 over almost the entire long side surface of the mold, the copper plate temperature profile of the entire mold can be measured.
Using the mold 3 having such a thermocouple group, a slab was manufactured while measuring the copper plate temperature distribution under various casting conditions. Furthermore, the manufactured slab was rolled, surface defects were continuously measured with an on-line surface defect meter, and the measurement results of the surface defect occurrence position in the product and the mold copper plate temperature were compared and investigated.

  In the surface defect measurement, it was found that scabs were generated in the product at a position corresponding to the position of the second row of the thermocouple 5 in FIG. And, when the time zone in which the slab position having the cause of the defect was cast was examined from the found hege occurrence position, it was found that the hege was generated in the time zone from 1100 to 1250 seconds from the start of casting. .

  The measurement results of the A-stage to I-stage copper plate temperatures at the second row position including this time zone are shown in FIGS. 3 to 5, the vertical axis represents the mold copper plate temperature (° C.), the horizontal axis represents the casting time (sec .: second) from the start of casting, and FIG. 3 (b) is the second row B stage, FIG. 3 (c) is the second row C stage, FIG. 4 (d) is the second row D stage, FIG. 4 (e) is the second row E stage, FIG. 4 (f) is the second row F stage, FIG. 5 (g) is the second row G stage, FIG. 5 (h) is the second row H stage, and FIG. 5 (i) is the second row I stage copper plate temperature. Each measurement result is shown.

The inventors first examined whether a defect could be determined by simply capturing the temperature change behavior of the mold copper plate.
However, as shown in FIG. 3 to FIG. 5, although some changes are observed in the temperature behavior of some thermocouples 5 in the time period when the shave is generated, the same temperature change behavior is observed in the time period when there is no shave. Therefore, it is considered that the defect determination is difficult only from the temperature change behavior of the mold copper plate.
In addition, it is not easy from the viewpoint of data processing to determine a slight temperature change behavior among a large number of embedded thermocouples 5.

Therefore, the present inventors tried various analyzes in order to find the peculiarities of the mold copper plate temperature in the time of occurrence of the beard, and found a determination method using principal component analysis. Principal component analysis is a statistical analysis method that synthesizes new variables representing features from many observed variables, and is a method suitable for finding singular points.
Hereinafter, a specific method of principal component analysis will be described by taking as an example the case where the number of thermocouples 5 embedded in the mold copper plate is 72.
When it is assumed that temperature data can be acquired from 72 thermocouples 5 per second, time series data X of temperature for t seconds is expressed by the following equation (1). However, in formula (1), T represents temperature.

The time series data X of temperature can be approximated by, for example, weighting three main components (first to third main components) and combining them. Here, the first principal component to the third principal component are representative patterns (changes in value with respect to time) for expressing time series data of temperature.
Although the number of principal components can be set arbitrarily, there may be no significant difference in the trend only because the variables increase unnecessarily. .
The first principal component to the third principal component are represented by 72-dimensional vectors (y ₁ , y ₂ , y ₃ ) represented by the following formula (2) (basic vectors).

In contrast, a principal component score a _i ^j is calculated for each time-series data of temperature. The principal component score a _i ^j is a weighting value for the first to third principal components described above. By paying attention to these three principal component scores a _i ^j , It is possible to extract singular points from
The time series data X of the temperature expressed by the equation (1) is expressed by the following equation (3) using the first to third principal components (y ₁ , y ₂ , y ₃ ) and the principal component score a _i ^j. Can be approximated by

The principal component analysis can be performed instantaneously by using general-purpose statistical analysis software, and the principal component score can be calculated. Results of principal component analysis on all measured temperature data (ie, “1-A” to “16-I”) of the long side surface during the casting period of the same charge as shown in FIGS. Is shown in FIG. 6 for each main component.
In FIG. 6, the vertical axis represents the absolute value of the principal component score, the horizontal axis represents the casting time (sec .: second), FIG. 6 (a) is the first principal component score, and FIG. The second principal component score, FIG. 6C, is a graph of the third principal component score.
As described above, the principal component score is a new variable that characterizes a large number of temperature data, corresponding to the respective weights of the first principal component to the third principal component. It shows that there is specificity.
In addition, although a negative value may appear for the principal component score, this time the purpose is to investigate the variation, so the evaluation was performed by taking the absolute value of the principal component score.

FIG. 6 clearly shows that all of the first principal component to the third principal component score greatly fluctuate in the vicinity of 1100 seconds to 1250 seconds, which is the time of occurrence of the beard. Thus, the present inventors have found that surface defect determination can be performed by performing principal component analysis on the measured temperature data and monitoring fluctuations in the principal component score.
It is preferable that the copper plate temperature data to be subjected to principal component analysis is limited to data within the same charge. This is because, for example, when the molten steel component and the molten steel temperature are different between the charge and the next charge, a case where the copper plate temperature behavior is different occurs.

  Next, the present inventors have examined the optimal arrangement of the thermocouple 5 embedded in the mold long side surface for each of the casting direction and the mold width direction in making the determination by the principal component analysis described above. This will be described below.

<Regarding the arrangement of the thermocouple in the casting direction>
Regarding the arrangement of the thermocouple 5 in the casting direction, the installation range and the thermocouple interval were examined, and the following findings (knowledge i and knowledge ii) were obtained.
In the following description, “position” represents a position in the casting direction from the molten metal level control level.

≪Knowledge about installation range of thermocouple in casting direction (position of uppermost and lowermost stages of thermocouple 5) ≫
When the uppermost position of the thermocouple 5 was below 250 mm from the molten metal surface control level, there was a case in which the occurrence of casting defects in the very surface layer of the slab was overlooked. Therefore, it is desirable that the uppermost stage of the thermocouple 5 be within a range of 250 mm from the level control level (Knowledge i-1).

  If the lowermost position of the thermocouple 5 is 500 mm or less, the molten steel flow due to the discharge flow from the immersion nozzle can be sufficiently captured, and casting defects (inclusions, bubbles) that cause surface defects of the product Of these, the occurrence of casting defects in a relatively deep position of about 8 to 12 mm from the slab surface is not overlooked. Therefore, it is desirable that the upper limit value of the lowest position of the thermocouple 5 is 500 mm or less (knowledge i-2).

However, it is desirable that the lowermost position of the thermocouple 5 has a lower limit value of 750 mm (Knowledge i-3). The reason is as follows.
At a position below 750 mm from the molten metal level control level, a solidified shell has already been sufficiently formed, and even if mold powder and deoxidation products resulting from surface defects are captured at this position, it is difficult to reflect on the copper plate temperature. In addition, since the mold powder and deoxidation product captured at a position below 750 mm from the molten metal level control level are relatively inside the slab, it is considered that surface defects are less likely to occur even when rolled.
Therefore, it is desirable that the lower limit value of the lowest position of the thermocouple 5 is 750 mm. In other words, it is sufficient to embed the thermocouple 5 in the range of the molten metal surface control level to the casting direction 750 mm, and it is not always necessary to embed the thermocouple 5 below that, and even if embedded, the thermocouple cost increases. It will be.

  From the above findings i-1 to i-3, it is desirable that the thermocouple 5 be arranged in the casting direction so that the uppermost level is within the range of 250 mm from the molten metal surface control level and the lowermost level is within the range of 500 mm to 750 mm. (Findings i).

≪Knowledge about interval of thermocouple casting direction≫
It has been clarified that surface defects can be sufficiently determined if the distance between adjacent thermocouples in the range of the above-described molten metal surface control level to 750 mm in the casting direction is 250 mm or less (Knowledge ii). When the distance between the thermocouples in the casting direction was larger than 250 mm, there was a case in which the behavior of generation of scabs was overlooked.

<Regarding the arrangement of thermocouples in the mold width direction>
Similar to the arrangement of the thermocouple 5 in the casting direction, the installation range and the interval of the thermocouple 5 in the mold width direction were examined, and the following findings (knowledge iii and knowledge iv) were obtained. .

≪Knowledge about installation range of thermocouple in mold width direction≫
It is desirable that the thermocouple 5 installed at a location near both short sides in the mold width direction is within a range of 250 mm or less from the intersection of the short side surface and the long side surface in the mold width direction (knowledge iii). This is because when the thermocouple 5 is not in the above range, a case of overlooking the behavior of casting defect generation in the vicinity of the mold short side was observed.

≪Knowledge about the interval in the mold width direction of thermocouple≫
In continuous casting, the casting width changes according to the required dimensions of the product. The casting width under the conditions investigated this time was 1000 mm to 2100 mm. In this case, it was found that the surface defects can be sufficiently determined if the distance between adjacent thermocouples in the mold width direction is 250 mm or less (knowledge iv). . When the distance between thermocouples in the mold width direction was larger than 250 mm, there was a case where the behavior of hege generation was overlooked.
In addition, said knowledge i-knowledge iv are demonstrated in the below-mentioned Example.

  Next, the present inventors paid attention to the threshold value of the principal component score. In general, there are serious and minor surface defects, and depending on the customer who handles steel sheet products, there are cases where minor surface defects can be used. And the yield of steel sheet products can be increased by determining the occurrence of defects when the threshold value is exceeded. In this investigation, when any of the first principal component score to the third principal component score is 2.5 or more, it tends to be a relatively serious surface defect.

  Based on the above, a surface defect determination device 1 (hereinafter simply referred to as “surface defect determination device 1”) for a continuous cast slab according to an embodiment of the present invention will be described with reference to FIG.

<Surface defect determination device>
As shown in FIG. 1, the surface defect determination device 1 includes a thermocouple 5 as a temperature measuring element embedded in the mold 3 and an arithmetic device 7 for determining a surface defect based on a temperature measurement value by the thermocouple 5. I have.
Hereinafter, the arrangement of the thermocouple 5 and the configuration of the arithmetic unit 7 will be described in detail.

≪Thermocouple arrangement≫
Regarding the arrangement of the thermocouple 5 in the casting direction, as described above, the uppermost stage is within a range of 250 mm from the molten metal level control level, and the lowermost stage is located at a distance of 500 mm or more from the molten metal surface control level (from Knowledge i). The interval between adjacent thermocouples 5 was set to 250 mm or less (knowledge ii).
In addition, regarding the arrangement of the thermocouple 5 in the mold width direction, as described above, the position of the temperature measuring element installed at the location closest to both short sides is the intersection of the short side surface and the long side surface of the slab width to be measured. From the position of the line, the distance between adjacent thermocouples 5 was 250 mm or less (knowledge iv) within 250 mm along the direction toward the center of the mold width (knowledge iii).
That is, with respect to the casting direction, at least one thermocouple 5 is arranged in the range of 250 mm from the molten metal level control level, and similarly, the lowest stage of the thermocouple 5 is arranged at a position 500 mm or more apart, and the mold width direction As for, at least one row of thermocouples 5 is arranged in a range of 250 mm from the short side of the mold.
By measuring the temperature with the thermocouple group arranged in this manner, the copper plate temperature profile of the entire mold 3 can be measured. Moreover, it is desirable to arrange the thermocouple 5 as a temperature measuring element on the long side surface of the mold 3 within a range of 750 mm from the molten metal surface control level toward the casting direction (from knowledge i).

As an example of the arrangement of the thermocouples 5, for example, in the casting direction, the first stage thermocouple 5 is arranged at a position of 50 mm from the molten metal surface control level, and the interval between adjacent thermocouples 5 in the casting direction is 100 mm. Eight stages are arranged in such a way (up to the H stage in FIG. 2). In the mold width direction, 15 rows are arranged so that the interval between adjacent thermocouples 5 in the mold width direction is 130 mm.
The intervals between the thermocouples 5 in the casting direction and the mold width direction need only be within the above ranges, and may be arranged at equal intervals, not necessarily at equal intervals, the structure and size of the mold 3, etc. It may be appropriately adjusted according to the above.
In the above description, an example in which the thermocouple 5 is used as the temperature measuring element is shown. However, any temperature measuring element can be used as long as it can accurately measure the copper plate temperature, such as an optical fiber sensor. It doesn't matter.

≪Calculation device≫
The arithmetic unit 7 is constituted by a computer, and obtains a mold copper plate temperature acquisition means 9 for acquiring the mold copper plate temperature measured by the thermocouple 5, and performs a principal component analysis based on the acquired template copper plate temperature to perform a principal component score. And a principal component score determining means 13 for determining the presence or absence of defects on the surface of the slab based on the calculated principal component score.

The mold copper plate temperature acquisition means 9 can acquire the mold copper plate temperature at predetermined measurement time intervals.
The predetermined time interval is desirably an interval of 1 second to 10 seconds. The reason is as follows. In order to detect the temperature fluctuation, 1 second or more and 10 seconds or less are sufficient, and when the temperature is acquired at an interval shorter than 1 second, it becomes easy to pick up the influence of disturbance such as mold vibration. In addition, measurement at intervals exceeding 10 seconds increases the risk of overlooking fluctuations due to the occurrence of abnormalities.
Moreover, when the period of the periodic change of the molten steel flow in the mold (for example, the molten steel discharge flow rate from the left and right discharge ports alternately fluctuates alternately) is measured, it is about 10 to 30 seconds. By measuring at a measurement time interval shorter than the minimum value of 10 seconds, which is the minimum value of this cycle, it is possible to capture temperature changes caused by periodic changes in molten steel flow. The time interval is preferably 10 seconds or less.

As the principal component analysis means 11, for example, general-purpose statistical analysis software may be used.
As a determination method in the principal component score determination means 13, for example, a predetermined threshold value is set, and when the principal component score exceeds the threshold value, it is determined that a defect has occurred.

  Since the surface defect determination apparatus 1 as described above does not use a database as in the technique of Patent Document 5, the capital investment cost does not increase, and a large amount of operation data is not required, which is simple.

  A surface defect determination apparatus 1 using a specific example of a surface defect determination method (hereinafter, simply referred to as “surface defect determination method”) of a continuous casting slab using the surface defect determination apparatus 1 configured as described above. Along with this operation, description will be made with reference to other figures as appropriate based on the flowchart of FIG.

<Surface defect judgment method for continuous casting slab>
As shown in FIG. 7, the method for determining surface defects of a continuous cast slab according to an embodiment of the present invention includes a mold copper plate temperature acquisition step (S1) for acquiring a mold copper plate temperature measured by a thermocouple 5, and the acquisition. A principal component analysis step (S3) for calculating a principal component score by performing a principal component analysis based on the obtained mold copper plate temperature, and determining the presence or absence of defects on the slab surface based on the calculated principal component score And a principal component score determination step (S5).
Each step will be described below.

≪Mold copper plate temperature acquisition process≫
First, in the mold copper plate temperature acquisition step, the mold copper plate temperature measured by the thermocouple 5 is acquired using the mold copper plate temperature acquisition means 9 (S1).
The mold copper plate temperature data is acquired from the thermocouple 5 embedded in the mold 3 at intervals of 1 second to 10 seconds (see FIGS. 3 to 5).

≪Principal component analysis process≫
Next, based on the mold copper plate temperature acquired in the mold copper plate temperature acquisition step, a principal component analysis is performed using the principal component analysis means 11 to calculate a principal component score (S3).
As described above, the number of principal components can be arbitrarily set. However, since there may be no significant difference in the trend only by increasing the variables, in this example, from the first principal component to the third principal component. did. Moreover, although a negative value may appear regarding the principal component score, the absolute value was calculated so that the fluctuation of the principal component score can be grasped (see FIG. 6).
As shown in FIG. 6, the principal component score fluctuates greatly in the period of 1100 seconds to 1250 seconds, which is the time of occurrence of betting, and it is possible to determine the occurrence of defects on the slab surface based on the principal component score.

≪Principal component score determination process≫
Next, based on the principal component score calculated in the principal component analysis step, the presence or absence of defects on the slab surface is determined using the principal component score determination means 13 (S5).
As a determination method, for example, a threshold value is set, and when any of the first to third principal component scores exceeds the threshold value, it is determined that a defect has occurred. When the threshold value is set to 2.5 and the principal component score shown in FIG. 6 is determined, the determination is as follows.
Although the threshold value 2.5 was not exceeded in the second principal component score (see FIG. 6B) in the time range of 1100 seconds to 1250 seconds, which is the time of occurrence of the shave, the first principal component score and the third principal component score ( In FIG. 6 (a) and FIG. 6 (b)), the defect occurrence is determined to be “present”.

The cast slab is transported to the rolling step (S9) after being subjected to a slab treatment (S7) based on the determination result.
Here, as a slab treatment, for example, if it is determined that there is a defect occurrence, the surface of the part is cared with a scarf or a grinder to remove the surface defect, and then the slab is conveyed to the rolling process, When it is determined that there is no defect occurrence, it may be conveyed to the rolling process without surface maintenance.

  As described above, in the present embodiment, the thermocouple 5 as the temperature measuring element is arranged in the casting direction, the position of the uppermost temperature measuring element is within 250 mm from the molten metal surface control level, and the lowest level measuring element is measured. The position of the temperature element is 500 mm or more away from the molten metal surface control level, the distance between adjacent temperature measurement elements is 250 mm or less, and in the mold width direction, the temperature measurement element installed at the location closest to both short sides Obtain the temperature of the mold copper plate by setting the position within 250 mm along the direction from the intersection of the short side and long side of the slab width to be measured to the center of the mold width, and the interval between adjacent temperature measuring elements is 250 mm or less. By performing the process, the principal component analysis process, and the principal component score determination process, it is possible to determine the occurrence of slab surface defects with high accuracy without increasing capital investment costs, and an appropriate slab process based on the determination result. It can be produced efficiently excellent surface quality of the slab by the applied.

A specific experiment for confirming the effect of the arrangement of the thermocouple 5 in the method for determining surface defects of a continuous cast slab according to the present invention was performed, and the results will be described below.
In the experiment, the surface defect determination apparatus 1 shown in FIG. 1 is used to determine the surface defect by changing the arrangement of the thermocouple 5 and evaluate the actual surface defect probability.
The thermocouple 5 was arranged so that all of Invention Example 1 to Invention Example 9 satisfied the findings (i) to (iv) described in the above embodiment. Specific arrangement will be described later.

A JIS-T type thermocouple is used as the thermocouple 5, and its installation is performed by drilling from the surface contacting the mold frame of the long copper plate to the surface contacting the molten steel, and the hot junction contacts the bottom of the drilled tip. So buried. The distance from the bottom of the tip to the surface of the copper plate contacting the molten steel was 13 mm.
Thus, the aluminum killed molten steel was cast using the continuous casting mold in which the thermocouple 5 was installed. The casting thickness was 220 to 300 mm, the casting width was 1000 to 2100 mm, and the molten steel throughput was 3.0 to 7.5 ton / min. The molten steel discharge angle of the molten steel discharge hole of the immersion nozzle was set to 5 ° to 45 ° downward, and the immersion depth (however, the distance from the meniscus to the upper end of the molten steel discharge hole) was 180 mm to less than 300 mm. Ar gas was used as the inert gas blown from the immersion nozzle.

  Moreover, the magnetic field generator applied the moving magnetic field of the direction which opposes each along the mold long side copper plate which opposes, and provided the flow which turns in the horizontal direction along the solidification shell interface to the molten steel in a casting_mold | template.

  The cast slab is transported to the rolling process without being treated with a scarf or grinder, and is subjected to hot rolling, cold rolling, etc., and surface defects are continuously detected with an on-line surface defect meter. Measured, the defect occurrence position was made to correspond to the slab position, the measured copper plate temperature was compared with the result of principal component analysis, and the surface defect probability was evaluated.

  The principal component analysis was performed on all copper plate temperature measurement values of the same charge, and the principal component analysis was performed once for each charge (a plurality of charge copper plate temperature measurement values were not collectively analyzed.) Further, here, when any one of the first principal component score to the third principal component score has a variation of 2.5 or more, it is determined as the surface defect “present”, and is determined as the surface defect “present”. A value obtained by dividing the number of points by the number of surface defect occurrences of the actual product was evaluated as a surface defect probability. In addition, both the inventive example and the comparative example are evaluated for a casting amount of 200 charges (about 300 tons per charge).

For comparison, the same evaluation was performed so that the arrangement of the thermocouple 5 was out of the range in any of the findings (i), (ii), and (iv) (Comparative Examples 1 to 2). Comparative Example 6).
In Comparative Example 1, the thermocouples 5 are arranged at the same intervals as in Example 4 of the present invention, but at a position exceeding the range of the molten metal surface control level to the casting direction 750 mm so as to be outside the range of the knowledge (i). One stage is arranged more (9 stages × 16 columns).
In Comparative Example 2 and Comparative Example 3, the interval between thermocouples in the casting direction is made larger than 250 mm so that it is outside the range of the knowledge (ii).
In Comparative Example 4, the interval between thermocouples in the mold width direction is made larger than 250 mm so that it is outside the range of knowledge (iv).
In Comparative Example 5, the uppermost thermocouple is installed at a position of 280 mm, which is outside the range of 250 mm from the molten metal surface control level, so that it is outside the range of the knowledge (i).
In Comparative Example 6, the lowermost thermocouple is installed at a position of 450 mm, which is outside the range of 500 to 750 mm from the molten metal surface control level, so as to be outside the range of knowledge (i).
In any case of Invention Example 1 to Invention Example 9 and Comparative Examples 1 to 6, the measurement time interval by the arranged thermocouple 5 was set to 5 seconds.
Table 1 shows the arrangement of the thermocouples 5 of the present invention example 1 to the present invention example 9 and the comparative example 1 to the comparative example 6 and the surface defect ratio (%).

  As shown in Table 1, in Inventive Example 1 to Inventive Example 9, the surface defect middle ratio was a result exceeding 80%, and good results were obtained.

  Although the comparative example 1 also has a high surface defect ratio of 91%, it is the same ratio as that of the present invention example 4 in which the thermocouple 5 is arranged in substantially the same manner, and the thermocouple cost increases compared to the present invention example 4. It is disadvantageous in terms. Thus, as explained in the knowledge (i) of the above embodiment, since there is no great difference even if it is arranged beyond the range of the molten metal surface control level to the casting direction 750 mm, the thermocouple is arranged within the range. It is preferable. However, this does not exclude the case where the thermocouple is disposed beyond the range of 750 mm in the casting direction.

In Comparative Examples 2 and 3 where the thermocouple spacing in the casting direction was larger than 250 mm and in Comparative Example 4 where the thermocouple spacing in the mold width direction was larger than 250 mm, the hit ratio was a low value of 46 to 53%. From these, knowledge (ii) and knowledge (iv) were verified. In Comparative Examples 5 and 6, the hit ratio was 64 to 67%, which was lower than that of the present invention.
When the surface defect positions of the products in Comparative Examples 2 to 6 were investigated, most of the cases overlooked the surface defects generated between the thermocouples.

  Next, the verification using the temperature data by the thermocouple obtained when casting using the mold of Invention Example 1 in Table 1 was performed. Specifically, the principal component analysis is performed after the temperature data of the thermocouple is omitted so that the distance from the short side of both ends of the slab to be measured to the outermost thermocouple exceeds 250 mm, and surface defects are detected. The impact on hit rate was investigated. As a result, the hit ratio, which was 81% in Invention Example 1 in Table 1, decreased to 56%. The distance from the short sides of the slab ends to the outermost thermocouples became too large, resulting in overlooking surface defects. That is, as explained as knowledge (iii), it was found that the distance from the short side of both ends of the slab to the outermost thermocouple is preferably 250 mm or less.

Next, specific experiments for confirming the effects of the surface defect determination method using the surface defect determination apparatus 1 shown in FIG. 1 were performed, and the results will be described below.
In the experiment, the presence / absence of a surface defect is determined using the surface defect determination device 1 and a care procedure is performed based on the determination (invention example), and the slab is manufactured without introducing the surface defect determination device 1 In (Comparative Example), the product yield is compared.

The arrangement of the thermocouple 5 in the present invention example is an arrangement corresponding to the present invention examples 1 to 9 in Table 1.
In addition, the principal component analysis was performed on all copper plate temperature measurement values of the same charge as in the first embodiment, and the principal component analysis was performed once for each charge (collecting the copper plate temperature measurement values of a plurality of charges together). Do not do principal component analysis.)
Based on the surface defect determination, for slabs whose principal component score exceeds the threshold, the slab surface is treated with a scarf and / or grinder and conveyed to the rolling process, while the slab has a principal component score less than the threshold. On the other hand, the slab was conveyed to the rolling process in an uncleaned state.

  Regarding the casting conditions conducted in the experiment, the aluminum killed molten steel was cast in the same manner as in Example 1. The casting thickness, the casting width, the molten steel throughput, the molten steel discharge angle of the molten steel discharge hole of the immersion nozzle, the immersion depth, and the blowing from the immersion nozzle The kind of the inert gas was the same as in Example 1, and the flow was imparted in the same manner by applying a moving magnetic field.

The slab is subjected to hot rolling, cold rolling, etc., and surface defects are continuously measured with an on-line surface defect meter. When surface defects are detected at the product stage, the defect is cleaned and cut off. (Slab treatment), product yield was evaluated.
The product yield was evaluated by a value obtained by dividing the product weight that could be shipped as a product by the casting weight. FIG. 8 shows a graph for comparing the product yield indexes of the present invention example and the comparative example. In FIG. 8, the product yield index of the comparative example was set to 100. As shown in FIG. 8, the product yield index of the example of the present invention was 106, and an improvement of 6% was realized.

  As described above, by performing the continuous casting by applying the surface defect determination method of the present invention, it is possible to detect the occurrence of surface defects before the rolling process and perform slab treatment, and to achieve a slab with excellent surface quality. It was proved that it can be manufactured efficiently and the product yield can be improved.

DESCRIPTION OF SYMBOLS 1 Surface defect determination apparatus 3 Mold 5 Thermocouple 7 Arithmetic apparatus 9 Mold copper plate temperature acquisition means 11 Principal component analysis means 13 Principal component score determination means

Claims (6)

A method for determining a surface defect of a continuous cast slab, in which a mold copper plate temperature is measured by a temperature measuring element embedded in the mold long side, and a slab surface defect is determined based on the temperature measurement value,
The arrangement of the temperature measuring element embedded in the long side of the mold,
Regarding the casting direction, the position of the top temperature sensor is within 250 mm from the molten metal surface control level, the position of the bottom temperature sensor is 500 mm or more away from the molten metal surface control level, and the distance between adjacent temperature sensors. Is 250 mm or less,
Regarding the mold width direction, the position of the temperature measuring element installed at the location closest to both short sides is changed from the position of the intersection of the short side surface and long side surface of the slab width to be measured toward the mold width center. Within 250 mm, and the interval between adjacent temperature measuring elements is 250 mm or less,
A mold copper plate temperature acquisition step of acquiring a mold copper plate temperature measured by the temperature measuring element arranged as described above,
A principal component analysis step of calculating a principal component score by performing a principal component analysis based on the acquired mold copper plate temperature;
A method for determining surface defects of a continuous cast slab, comprising: a principal component score determining step for determining whether or not a defect has occurred on a slab surface based on the calculated principal component score.   2. The method for determining surface defects of a continuous cast slab according to claim 1, wherein the position of the lowest temperature measuring element is set within 750 mm in the casting direction from the level control level.   3. The surface defect determination method for a continuous casting slab according to claim 1, wherein the principal component score determination step determines that a defect has occurred when the principal component score exceeds a predetermined threshold value. A surface defect determination device for a continuous casting slab that measures a mold copper plate temperature by a temperature measuring element embedded in the long side of the mold and determines a continuous casting slab surface defect based on the temperature measurement value,
A temperature measuring element group embedded in the mold long side, a mold copper plate temperature acquisition means for acquiring a mold copper plate temperature measured by each temperature measuring element of the temperature measuring element group, and based on the acquired mold copper plate temperature A principal component analysis means for calculating a principal component score by performing a principal component analysis, and a principal component score determination means for determining the presence or absence of defects on the slab surface based on the calculated principal component score,
The arrangement of the temperature measuring elements constituting the temperature measuring element group,
Regarding the casting direction, the position of the top temperature sensor is within 250 mm from the molten metal surface control level, the position of the bottom temperature sensor is 500 mm or more away from the molten metal surface control level, and the distance between adjacent temperature sensors. Is 250 mm or less,
Regarding the mold width direction, the position of the temperature measuring element installed at the location closest to both short sides is changed from the position of the intersection of the short side surface and long side surface of the slab width to be measured toward the mold width center. A surface defect determination device for a continuously cast slab, wherein the distance between adjacent temperature measuring elements is 250 mm or less.   The surface defect determination device for a continuous cast slab according to claim 4, wherein the position of the lowest temperature measuring element is set within 750 mm in the casting direction from the level control level.   6. The continuous defect slab surface defect determination apparatus according to claim 4, wherein the principal component score determination means determines that a defect has occurred when the principal component score exceeds a predetermined threshold.