Method for detecting defects in slabs during continuous casting

Abstract

To provide a method capable of detecting a slab defect which cannot be detected by conventional technique (temperature measurement of mold temperature by a temperature sensor) in continuous casing.SOLUTION: There is provided a slab defect detection method for continuous casting that comprises: previously finding the relation between temperature measured by a temperature sensor installed at a mold and an increment in temperature of cooling water found from entrance temperature and exit temperature of the cooling water flowing to the mold in the case that a slab has no defect in continuous casting; and detecting the slab having a defect in the continuous casting based upon a deviation of a measured value of the measured temperature or the increment in temperature from an estimated value of the measured temperature or the increment in temperature estimated from the relation.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] This application discloses a method for detecting defects occurring in a slab during continuous casting.

[0002] Defects occurring in a slab during continuous casting become surface defects of the product during rolling in the next process, so they need to be removed by manual work. To avoid this, it may be possible to eliminate the causes of slab defects, but since slab defects may occur due to unexpected causes, a technique for determining slabs that require manual work is also necessary. However, since it is difficult to measure with a general sensor due to high temperature and a large amount of water vapor directly below the mold, it is common to observe the cold slab after casting to determine whether manual work is necessary. However, this method takes time to discover a defective slab, so when a defect is discovered, many slabs have already been cast under the same conditions. As a result, the optimization of operating conditions has been delayed.

[0003] As a technique for detecting slab defects at an early stage, a technique for detecting slab defects in a mold during continuous casting is known. As a general in-mold defect detection method, there is a method that utilizes a temperature sensor (such as a thermocouple or an FBG sensor) installed on a mold copper plate and detects slab defects such as cracks and bleed-breakouts in the mold from the temperature change (for example, Patent Document 1). However, for defects at a location far from the temperature sensor, it is difficult to detect because the temperature change measured by the temperature sensor is small. Conventionally, the detection accuracy has been improved by increasing the density of temperature sensors, but installing a large number of temperature sensors is laborious and costly. In recent years, even for FBG sensors that have come to be used for measuring the temperature of a mold, although it is possible to easily measure at a high density compared to thermocouples, there are cases where the cost becomes high or the number of installations on the mold is limited depending on the shape of the mold copper plate used.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] Defects at positions away from the temperature sensor are difficult to detect by conventional methods (measuring the mold temperature with a temperature sensor). During continuous casting, there is a need for a new method capable of detecting slab defects that could not be detected by conventional methods.

MEANS FOR SOLVING THE PROBLEMS

[0006] As one means for solving the above problems, the present application obtains in advance the relationship between the measured temperature by a temperature sensor installed in the mold and the temperature rise amount of the cooling water obtained from the inlet temperature and the outlet temperature of the cooling water flowing through the mold when no defect occurs in the slab during continuous casting, and during continuous casting of the slab, detects the occurrence of defects in the slab based on the deviation between the measured value of the measured temperature or the temperature rise amount and the estimated value of the measured temperature or the temperature rise amount estimated from the relationship. Disclosed is a method for detecting slab defects in continuous casting.

EFFECTS OF THE INVENTION

[0007] According to the method of the present disclosure, it is possible to detect slab defects that could not be detected by conventional methods (measuring the mold temperature with a temperature sensor) during continuous casting. Further, in the method of the present disclosure, it is not necessary to increase the number of temperature sensors to detect slab defects.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0009] 1. Method for Detecting Slab Defects in Continuous Casting In the method for detecting slab defects of the present disclosure, first, the relationship between the measured temperature by the temperature sensor installed in the mold and the increase in the temperature of the cooling water obtained from the inlet temperature and the outlet temperature of the cooling water flowing through the mold when no defect occurs in the slab during continuous casting is obtained in advance. Then, during the continuous casting of the slab, based on the deviation between the measured value of the measured temperature or the temperature increase amount and the estimated value of the measured temperature or the temperature increase amount estimated from the relationship, the occurrence of defects in the slab is detected.

[0010] The amount of increase in the cooling water temperature is a representative value of the heat extraction amount of the entire mold through which the cooling water flows, while the measured temperature value by the temperature sensor installed in the mold is a representative value of the local heat extraction amount around the sensor. In the slab defect detection method of the present disclosure, by combining the mold temperature measurement by the temperature sensor with the amount of increase in the cooling water temperature, slab defects that could not be detected by the conventional method (for example, slab defects passing between sensors during continuous casting) are detected.

[0011] Specifically, the relationship between the measured temperature value by the temperature sensor in advance (when multiple temperature sensors are installed, it may be the total value of the measured temperatures by the multiple temperature sensors) and the amount of increase in the cooling water temperature is sorted out, and it is determined that a slab defect has occurred when the relationship breaks down.

[0012] For example, when a depression-type defect (in addition to depression, defects that lower the mold temperature such as longitudinal cracks and powder entrapment) occurs in the slab in the mold, the contact state between the slab and the mold deteriorates in the part of the depression-type defect, and the temperature of the mold near the depression-type defect decreases. When this temperature decrease is detected by the temperature sensor, it can be determined that a slab defect has occurred according to the conventional logic. On the other hand, when this temperature decrease cannot be detected by the temperature sensor (for example, when there is a defect in an area where no sensor is installed), since the temperature change due to the depression-type defect appears only in the amount of increase in the cooling water temperature, the relationship between the measured value of the temperature sensor and the amount of increase in the cooling water temperature breaks down. When such a breakdown of the relationship is recognized, it can be determined that a slab defect has occurred.

[0013] In addition, even when breakout-related defects occur in the slab within the mold (defects that raise the mold temperature above normal, such as not only breakouts but also a decrease in the solidification shell thickness due to remelting and local defects that do not reach breakouts), they can be detected in the same manner as described above. That is, when a breakout-related defect occurs in the slab, the temperature of the mold near the breakout-related defect rises. If this temperature rise is detected by a temperature sensor, it can be determined that a slab defect has occurred using the same logic as before. On the other hand, when this temperature rise cannot be detected by the temperature sensor, the temperature change due to the breakout-related defect appears only in the amount of increase in the cooling water temperature, so the relationship between the measured value of the temperature sensor and the amount of increase in the cooling water temperature is disrupted. When such a disruption in the relationship is observed, it can be determined that a slab defect has occurred.

[0014] 1.1 Measurement of Mold Temperature by Temperature Sensor As described above, in the method of the present disclosure, the temperature of a local portion of the mold is measured by a temperature sensor installed in the mold. During continuous casting, in a normal state without slab defects, there is no substantial change in the temperature measured by the temperature sensor. However, when a slab defect passes near the portion where the temperature sensor is installed, the temperature measured by the temperature sensor can fluctuate up and down.

[0015] The installation position of the temperature sensor in the mold is not particularly limited. When the opening shape of the mold is a substantially rectangular shape having a long side and a short side, the temperature sensor may be installed on the wall of the long side of the mold, or on the wall of the short side of the mold, or on the walls of both the long side and the short side of the mold. The method of installing the temperature sensor on the mold may be the same as in the prior art.

[0016] The number of temperature sensors in the mold is not particularly limited either. As described above, the method of the present disclosure attempts to detect slab defects in areas where no temperature sensors are installed, and a predetermined effect can be expected regardless of the number of temperature sensors. That is, the number of temperature sensors may be at least one. However, the more temperature sensors there are, the less noise there is. Therefore, from the perspective of further enhancing the detection accuracy of slab defects, a plurality of temperature sensors may be installed in the mold, and four or more temperature sensors may be installed. When a plurality of temperature sensors are installed in the mold, the plurality of temperature sensors may be arranged regularly or randomly. For example, a plurality of temperature sensors may be installed along the casting direction during continuous casting, or a plurality of temperature sensors may be installed along a direction orthogonal to the casting direction during continuous casting, or a plurality of temperature sensors may be installed two-dimensionally along both of these directions.

[0017] The type of temperature sensor is not particularly limited, and a general temperature sensor that can be installed in a mold for continuous casting may be adopted. For example, it may be a thermocouple or an FBG sensor.

[0018] 1.2 Cooling water temperature rise The cooling water temperature rise is obtained from the inlet temperature and the outlet temperature of the cooling water flowing through the mold. The inlet temperature and the outlet temperature of the cooling water may be directly measured by a temperature sensor or the like, or may be indirectly measured based on heat transfer calculations or the like.

[0019] As described above, when a defect occurs in the slab in the mold, the cooling water temperature rise changes. That is, when a depression-type defect occurs in the slab in the mold, the heat transfer amount from the slab to the mold decreases, so the cooling water temperature rise also decreases. Also, when a breakout-type defect occurs in the slab in the mold, the heat transfer amount from the slab to the mold increases, so the cooling water temperature rise also increases.

[0020] The cooling water may flow through the entire mold or only through a part of the mold. For example, when the opening shape of the mold is a substantially rectangular shape having a long side and a short side, the cooling water may flow through the wall on the long side of the mold, or may flow through the wall on the short side of the mold, or may flow through the walls on both the long side and the short side. The cooling water may flow independently on the long side and the short side of the mold. When the cooling water flows independently on the long side and the short side of the mold, it becomes easier to narrow down whether the slab defect occurred on the long side or the short side. In this regard, in the method of the present disclosure, the amount of increase in the cooling water temperature may be measured at a plurality of locations. The form of the cooling water flow path in the mold may be the same as that of the conventional one.

[0021] The mold wall through which the cooling water flows and the mold wall on which the temperature sensor is installed may be the same wall or different walls. Regardless of the position of the temperature sensor and the position of the cooling water flow path, the method of the present disclosure can detect slab defects occurring in regions where the temperature sensor is not installed.

[0022] 1.3 Relationship between the measured temperature by the temperature sensor and the amount of increase in the cooling water temperature According to the findings of the present inventors, there is a certain correlation between the measured temperature by the temperature sensor installed in the mold and the amount of increase in the cooling water temperature obtained from the inlet temperature and the outlet temperature of the cooling water flowing through the mold when no defects occur in the slab during continuous casting. For example, in a normal state where no defects occur in the slab, when the measured temperature by the temperature sensor increases, the amount of increase in the cooling water temperature also increases accordingly.

[0023] The relationship between the measured temperature by the temperature sensor and the amount of increase in the cooling water temperature may be formulated. For example, as shown in the examples described later, it is also possible to organize the relationship between the measured temperature by the temperature sensor and the amount of increase in the cooling water temperature as a linear function. The relationship between the measured temperature by the temperature sensor and the amount of increase in the cooling water temperature when no defects occur in the slab during continuous casting may be determined based on past or current operating results, or may be determined by simulation or the like.

[0024] The relationship between the measured temperature by the temperature sensor and the increase in the cooling water temperature can vary depending on the type of molten metal being continuously cast and other casting conditions. In the method of the present disclosure, the relationship between the measured temperature by the temperature sensor and the increase in the cooling water temperature may be pre-arranged for each continuous casting condition. In actual continuous casting, according to the conditions of the continuous casting, the optimal relationship that matches (or is closest to) the conditions of the continuous casting may be selected from among a plurality of pre-arranged relationships.

[0025] 1.4 Detection of occurrence of defects in the slab In the method of the present disclosure, during continuous casting of the slab, based on the deviation between the measured value of the above-mentioned measured temperature or temperature increase and the estimated value of the measured temperature or temperature increase estimated from the above-mentioned relationship, the occurrence of defects in the slab is detected.

[0026] For example, during continuous casting of the slab, the mold temperature is measured by a temperature sensor installed in the mold. Then, from the measured value of the temperature measured by the temperature sensor and the above-mentioned relationship obtained in advance, an estimated value of the increase in the cooling water temperature that could occur when no defects occur in the slab is obtained. When the above-mentioned relationship is formulated as a mathematical formula, by substituting the measured value of the temperature measured by the temperature sensor into the mathematical formula, the estimated value of the increase in the cooling water temperature that could occur when no defects occur in the slab is determined. On the other hand, during continuous casting of the slab, in parallel with the measurement of the mold temperature by the above-mentioned temperature sensor, the increase in the cooling water temperature is measured from the inlet temperature and the outlet temperature of the cooling water. When a deviation exceeding the threshold occurs between the measured value of the increase in the cooling water temperature and the estimated value of the increase in the cooling water temperature estimated as described above, it can be determined that a slab defect has occurred in a portion that cannot be detected by the temperature sensor.

[0027] Alternatively, during the continuous casting of the slab, measure the inlet temperature and the outlet temperature of the cooling water flowing through the mold, and measure the increase in the cooling water temperature from the difference between them. Then, based on the measured value of the increase in the cooling water temperature and the above-mentioned relationship obtained in advance, obtain the estimated value of the measured temperature by the temperature sensor that could be possible when no defect occurs in the slab. When the above relationship is formulated as a mathematical formula, substituting the measured value of the increase in the cooling water temperature into the mathematical formula determines the estimated value of the measured temperature by the temperature sensor that could be possible when no defect occurs in the slab. On the other hand, during the continuous casting of the slab, parallel to the measurement of the increase in the cooling water temperature, measure the mold temperature by a temperature sensor. When a deviation exceeding the threshold occurs between the measured value of the temperature measured by this temperature sensor and the estimated value of the measured temperature by the temperature sensor estimated as described above, it can be determined that a slab defect has occurred in a portion that cannot be detected by the temperature sensor.

[0028] Alternatively, show the above-mentioned relationship obtained in advance as a function represented by a straight line or a curve on a graph. The straight line or curve shown on the graph coincides with the plot on the graph of the estimated value of the measured temperature by the temperature sensor and the estimated value of the increase in the cooling water temperature that could be possible when no defect occurs in the slab. Then, during the continuous casting of the slab, measure the mold temperature by a temperature sensor and the increase in the cooling water temperature, and plot these measured values on the above-mentioned graph. When the plotted position is separated from the above-mentioned straight line or curve by more than the threshold, it can be determined that a slab defect has occurred in a portion that cannot be detected by the temperature sensor.

[0029] In the method of the present disclosure, the "deviation" between the measured value and the estimated value includes, in addition to the difference between the measured value and the estimated value and the ratio of the measured value to the estimated value, the way of deviation over time from the above-mentioned relationship obtained in advance (how much deviation occurs with respect to the above-mentioned relationship per predetermined time), and other such concepts. The "deviation" between the measured value and the estimated value may be a bias between the measured value and the estimated value. As described above, in the method of the present disclosure, the occurrence of slab defects is detected using the breakdown of the relationship between the measured value of the temperature sensor and the amount of increase in the cooling water temperature as an index, and this "breakdown" of the relationship corresponds to the above-mentioned "deviation". There is no particular limitation on what specific index to adopt as the deviation, and an appropriate one may be selected in consideration of detection accuracy and the like.

[0030] There is no particular limitation on the "threshold value" that serves as a criterion for determining whether or not slab defects have occurred. An appropriate threshold value may be set in consideration of temperature fluctuations (vibrations) and noise that inevitably occur when there are no slab defects. Also, different threshold values may be set for each continuous casting condition.

[0031] As described above, according to the method of the present disclosure, during continuous casting, it is possible to detect slab defects that could not be detected by the conventional method (measuring the mold temperature with a temperature sensor). Further, the method of the present disclosure enables the detection of slab defects occurring in regions that cannot be detected by temperature sensors, and it is not necessary to increase the number of temperature sensors to detect such slab defects.

[0032] 2. Continuous casting method The technology of the present disclosure also has an aspect as a continuous casting method for slabs. That is, the continuous casting method for slabs of the present disclosure obtains in advance the relationship between the measured temperature by a temperature sensor installed in the mold and the amount of increase in the temperature of the cooling water obtained from the inlet temperature and the outlet temperature of the cooling water flowing through the mold when no defects occur in the slab during continuous casting, during the continuous casting of the slab, based on the deviation between the measured value of the measured temperature or the amount of temperature increase and the estimated value of the measured temperature or the amount of temperature increase estimated from the relationship, the occurrence of defects in the slab is detected, When no defect occurs in the ingot slab, continuous casting is continued without changing the continuous casting conditions. When a defect occurs in the ingot slab, the continuous casting conditions may be changed or the continuous casting may be stopped. More specifically, when a defect occurs in the ingot slab, for example, the casting speed may be changed or the set value of the electromagnetic device may be changed. Further, when a defect occurs in the ingot slab, a maintenance process may be performed to remove the defect. When a defect occurs in the ingot slab, in the maintenance process, for example, the scarfing (grinding) amount may be increased compared to the normal time when no defect occurs in the ingot slab, or maintenance may be performed by hand scarfing while visually checking for defects. Since the details are obvious from the above description, further explanation is omitted here.

[0033] 3. Supplementary In addition, during continuous casting, when a depression defect and a breakout defect occur simultaneously, it can be said that it is difficult to detect the defect of the ingot slab by the method of the present disclosure. However, it is usually impossible for a depression defect and a breakout defect to occur simultaneously. In the unlikely event that a situation occurs where a depression defect and a breakout defect occur simultaneously, it is considered that an obvious abnormality has occurred in the continuous casting, and the abnormality can be easily detected by a method other than the method of the present disclosure.

[0034] The method of the present disclosure may be additionally implemented when it is determined that there is no ingot slab defect in the conventional method using a temperature sensor. That is, during continuous casting of the ingot slab, the method of the present disclosure may be implemented for the purpose of preventing omission of ingot slab defect detection by the conventional method using a temperature sensor.

[0035] The method of the present disclosure is applicable regardless of the shape and type of the ingot slab. The ingot slab may be a square ingot slab or a round ingot slab. Further, the type of metal constituting the ingot slab is not particularly limited, and for example, it may be steel or a metal other than steel.

Example

[0036] Examples according to the present invention are shown below. The present invention is not limited to this single conditional example. In the present invention, various conditions can be adopted without departing from the gist and as long as the object is achieved.

[0037] 1. Test conditions Using a test continuous casting machine, a continuous casting test of the slab was carried out. The continuous casting conditions are shown in Table 1, and the steel composition (mass%) of the slab is shown in Table 2. The balance other than the components shown in Table 2 is Fe and impurities. Also, Fig. 1 shows the number and positions of the temperature sensors (FBG sensors) installed in the mold of the test continuous casting machine. As shown in Fig. 1, a plurality of temperature sensors were installed on the long side wall of the mold.

[0038]

Table 1

[0039]

Table 2

[0040] 2. Test results As a result of the casting test, a slab having a plurality of concave defects called depressions as shown in Fig. 2 was obtained. Regarding the depression located at the center in the slab width direction (for example, the region X indicated by the white-line ellipse in Fig. 2), since it is far from the temperature sensor position, it did not appear as a temperature change in the temperature sensor.

[0041] Fig. 3 shows the relationship between the average increase in cooling water temperature ΔT every 5 seconds in the defect-free section in the above continuous casting test w and the total measured temperature ΣT by the temperature sensor s It can be seen that there is a linear relationship between the two. Also, it can be said that defects occur when these relationships break down. For example, ΣT at the time when a defect corresponding to the region X in Fig. 2 occurs s and ΔT wWhen plotted on FIG. 3, it can be seen that they are plotted at positions deviating from the linear relational expression.

[0042] Here, for quantitative arrangement, the estimated value of the total measured temperature by the temperature sensor obtained by substituting the actually measured value ΔT of the average cooling water temperature rise into the above relational expression (the estimated value that should be possible when there is no slab defect) is T w w total total total w total ), and the ratio of the two is defined as I (= T / T w total ). The location where the ratio I is below 100% indicates that "the heat extraction detected by the sensor is small compared to the actual heat extraction", that is, "there is a depression defect that cannot be detected by the sensor". The correspondence between the ratio I and the defect position of the slab is shown in FIG. 4. In the lower graph of FIG. 4, the portion corresponding to the location where a depression that cannot be detected only by the temperature sensor occurs among the depressions in the slab is indicated by double arrows. It can be confirmed that there is a depression between the sensors at the location where the ratio I is significantly below 100%. In view of this, for example, a threshold value can be set for the ratio I, and for the portion below the threshold value, it can be determined that a depression has occurred.

[0043] In addition, in the above, a slab in which a depression, which is a defect that reduces the mold temperature, has occurred was shown, but in the method of the present disclosure, the type of slab defect to be detected is not limited to depression defects, and may be breakout defects. When a breakout defect occurs between the sensors, it is considered that the above ratio I exceeds 100% significantly, and based on this, it can be determined that a breakout defect has occurred.

[0044] 3. Supplementary We also considered the case where the number of temperature sensors installed in the mold was changed. Specifically, in the above continuous casting test, after reducing the number of temperature sensors adopted as the measured values of the measured temperature to four in the casting direction, the same analysis as above was performed. Fig. 5 shows the installation positions of the temperature sensors in the mold adopted as the measured values of the measured temperature. Fig. 6 shows the correspondence between the above ratio I and the defect positions of the slab. As shown in Fig. 6, the position of the slab defect that cannot be detected only by the temperature sensor coincides with the position where the above ratio I is much lower than 100%. That is, it can be seen that regardless of the number of temperature sensors, the above method can accurately detect slab defects that cannot be detected only by the temperature sensors.

[0045] In addition, although the noise increases, the number of temperature sensors installed in the mold may be one. Also, from the above results, it is clear that the desired effect can be expected regardless of the position of the temperature sensor and the flow location of the cooling water. For example, in the above, the case where the temperature sensor is installed on the long-side wall of the mold was exemplified, but even when the temperature sensor is installed on the short-side wall of the mold, the same method can accurately detect slab defects that cannot be detected only by the temperature sensors.

[0046] Also, in the above embodiment, as a specific example of the "deviation" between the measured value of the measured temperature or the temperature rise amount by the temperature sensor and the estimated value of the measured temperature or the temperature rise amount estimated from a predetermined relationship obtained in advance, the "ratio I" between the measured value and the estimated value was exemplified. However, the "deviation" that can be adopted in the method of the present disclosure is not limited to this. The difference between the measured value and the estimated value may be adopted, and when the difference is large, it may be determined that a slab defect that cannot be detected only by the temperature sensor has occurred. Alternatively, the deviation of the measured value from the estimated value over time (how much the measured value deviates from the above relationship (estimated value) per a predetermined time) may be monitored, and when the deviation is large, it may be determined that a slab defect that cannot be detected only by the temperature sensor has occurred. Alternatively, using other "deviations" as indicators, slab defects that cannot be detected only by the temperature sensor may be detected.

[0047] In addition, in the above embodiment, a form of continuously casting a rectangular slab made of steel is shown. However, the technology of the present disclosure is also applicable to the case of continuously casting a slab made of other metal types or having other slab shapes.

[0048] As described above, in order to detect a slab defect in the mold at a point away from the temperature sensor during continuous casting of the slab, it can be said that it is effective to adopt the following method. (1) Determine in advance the relationship between the measured temperature by the temperature sensor installed in the mold and the temperature rise amount of the cooling water obtained from the inlet temperature and the outlet temperature of the cooling water flowing through the mold when no defect occurs in the slab during continuous casting. (2) During the actual continuous casting of the slab, based on the deviation between the measured value of the measured temperature or the temperature rise amount and the estimated value of the measured temperature or the temperature rise amount estimated from the relationship, detect the occurrence of a defect in the slab.

Claims

【Claim 1】 When there are no defects in the slab during continuous casting, determine in advance the relationship between the measured temperature by the temperature sensor installed in the mold and the temperature rise of the cooling water obtained from the inlet temperature and the outlet temperature of the cooling water flowing through the mold. During continuous casting of the slab, based on the deviation between the measured value of the measured temperature and the estimated value of the measured temperature estimated from the measured value of the temperature rise and the relationship, or the deviation between the measured value of the temperature rise and the estimated value of the temperature rise estimated from the measured value of the measured temperature and the relationship, detect the occurrence of defects in the slab. A method for detecting slab defects in continuous casting.

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