JP2011011258A - Method for estimating surface layer state of slab and slab surface layer state estimating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correctly estimate whether inclusions and bubbles which can be made into surface defects are present or not in a slab.SOLUTION: This estimating device includes: a temperature acquiring part 101 where the temperature of a measurement position is obtained through thermocouples 10 arranged at a mold 1; a molten steel flow velocity operating part 102 where the flow velocity of a molten steel in the boundary of a solidified shell is calculated from the temperature obataned by the temperature acquiring part 101; a sticking degree deriving part 103 where, in the case the flow velocity of the molten steel is calculated by the molten steel flow velocity operating part 102, based on a beforehand set sticking degree curve expressing the relation between the flow velocity of the molten steel and the sticking degree of inclusions and bubbles, the sticking degree of the inclusions and bubbles corresponding to the flow velocity of the molten steel is derived; and a sticking determination part 104 where the sticking degree of the inclusions and bubbles derived by the sticking degree deriving part 103 and beforehand set threshold are compared, and, in the case the sticking degree of the inclusions and bubbles exceeds the threshold, it is determined that the inclusions and bubbles which can be made into surface defects in the solidified shell corresponding to the measurement position are present.

Description

  The present invention is applied to a slab continuous casting machine that continuously generates a slab from the lower end of a mold by discharging molten steel into the mold, and inclusions and bubbles that may become surface defects when forming a thin steel plate product are slabs. The present invention relates to a slab surface state prediction method and a slab surface state prediction apparatus for predicting whether or not a slab exists.

  In a slab continuous casting machine that continuously generates slab by discharging molten steel into the mold, inclusions such as deoxidation products and bubbles such as argon gas blown into the molten steel flow from the immersion nozzle may adhere to the solidified shell . When this is rolled with inclusions and bubbles adhering to the solidified shell, surface defects such as heges, slivers and blisters may occur in the formed sheet steel product. As in recent years, it is desirable to accurately estimate and treat the occurrence of surface defects at the slab stage in situations where demands on the surface quality of sheet steel products, particularly sheet materials, become severe. .

  The point that the distribution of bubbles and inclusions affects the quality of the slab has already been disclosed in Patent Document 1 and Patent Document 2. For this reason, in these conventional techniques, the flow velocity of molten steel is calculated based on the temperature acquired by the temperature measuring element arranged in the mold, or the flow velocity vector distribution in the entire mold interior is calculated, and the diffusion distribution of inclusions / bubbles is calculated. In other words, estimating the quality of the slab.

Japanese Patent No. 3598078 Japanese Patent No. 3607882

  However, it is not possible to judge whether bubbles or inclusions actually adhere to the solidified shell from the flow rate or flow velocity vector distribution of the molten steel, and inclusions that can cause surface defects when forming thin steel plate products. It is also difficult to predict whether bubbles will be present in the slab.

  In view of the above circumstances, an object of the present invention is to provide a method and an apparatus capable of accurately predicting whether or not inclusions / bubbles that may become surface defects exist in a slab.

  In order to achieve the above object, the present invention is directed to a slab continuous casting machine that continuously generates a slab from the lower end of a mold by discharging molten steel into the mold, and inclusions that can cause surface defects in the generated slab. A method for predicting the state of the surface layer of a slab that predicts whether or not bubbles are present, the temperature at the measurement position being acquired through a temperature measuring element disposed in the mold, and the flow rate of the molten steel at the solidified shell interface from the acquired temperature The flow velocity calculation step for calculating the flow rate is compared with the pre-set critical flow velocity for washing of inclusions / bubbles and the flow velocity of the molten steel calculated in the flow velocity calculation step, and the condition that the calculated molten steel flow velocity is less than the critical flow velocity is washed away. And an adhesion determination step for determining that inclusions / bubbles that may become surface defects exist in the solidified shell corresponding to the measurement position.

  Further, the present invention provides a method for predicting a surface layer state of a slab as described above, wherein a plurality of temperature measuring elements are disposed along the casting direction until the solidified shell reaches a preset thickness starting from the molten steel surface, The temperature when the same portion of the solidified shell passes through the plurality of temperature measuring elements is acquired, and the flow rate of the molten steel is calculated from the acquired temperature, and surface defects can be caused based on the calculated flow rates of the molten steel. It is characterized by determining whether inclusions or bubbles are present.

  Further, the present invention relates to the above-described method for predicting the surface layer state of a slab. Predetermined as a curve, the adhesion determination step is a step of deriving an adhesion degree corresponding to the flow rate of the molten steel calculated in the flow rate calculation step based on the adhesion degree curve, and the derived adhesion degree exceeds a preset threshold value. And a step of determining that inclusions / bubbles that may become surface defects are present.

  In the method for predicting the surface state of a slab as described above, the adhesion curve derives a critical flow velocity for washing corresponding to a critical particle size of inclusions / bubbles which may become surface defects of a product after rolling. When the molten steel flow rate exceeds the derived washing critical flow velocity, the total number of inclusions / bubbles adhering to the solidified shell is set to zero, whereas when the molten steel flow velocity is less than the washing critical flow velocity, the molten steel Determine the critical particle size of inclusions and bubbles adhering to the solidified shell corresponding to the flow velocity and the washing critical flow velocity, and use the distribution function of the inclusion and bubble particle size contained in the molten steel to include the inclusions adhering to the solidified shell. -Created by calculating the total number of bubbles.

  Further, the present invention provides a method for predicting a surface layer state of a slab as described above, wherein a plurality of temperature measuring elements are disposed along the casting direction until the solidified shell reaches a preset thickness starting from the molten steel surface, And the adhesion curve is set according to the arrangement position of the temperature measuring element, the temperature when the same portion of the solidified shell passes through these temperature measuring elements is acquired, and the flow rate of the molten steel is respectively calculated from the acquired temperature. When calculating the degree of adhesion of inclusions / bubbles from the calculated flow velocity of each molten steel based on the adhesion curve corresponding to the measurement position, and any of the derived degrees of adhesion exceeds a preset threshold In addition, it is determined that inclusions / bubbles that may become surface defects exist in a portion corresponding to the measurement position.

  In the slab surface layer state prediction method according to the present invention, a plurality of the temperature measuring elements are arranged from the bottom 50 mm to the thickness of the solidified shell 10 mm starting from the molten steel surface. It is characterized by that.

  The present invention is also applicable to a slab continuous casting machine that continuously generates a slab from the lower end of the mold by discharging molten steel into the mold, and there are inclusions / bubbles that can become surface defects in the generated slab. A surface layer state prediction device for a slab that predicts whether or not a temperature acquisition unit that acquires a temperature at a measurement position through a temperature measuring element disposed in a mold, and at a solidified shell interface from the temperature acquired by the temperature acquisition unit Based on an adhesion curve representing the relationship between a preset molten steel flow rate and inclusion / bubble adhesion, when the molten steel flow rate calculation unit calculates the flow rate of molten steel and the molten steel flow rate calculation unit calculates the molten steel flow rate The adhesion degree deriving part for deriving the adhesion degree of inclusions / bubbles corresponding to the flow rate of the molten steel, and comparing the inclusion / bubble adhesion degree derived by the adhesion degree deriving part with a preset threshold value, With bubbles If the degree exceeds the threshold value, characterized in that the inclusions, bubbles can become surface defects in the solidified shell corresponding to the measurement position and a deposition determining section for determining that there.

  According to the present invention, the washing critical flow velocity required to wash away inclusions and bubbles adhering to the solidified shell is applied as an index, and the flush critical flow velocity is compared with the calculated molten steel flow velocity at the solidified shell interface. Since it is judged whether there are inclusions / bubbles that can become surface defects, it is possible to accurately predict whether inclusions / bubbles that can become surface defects exist in the slab. . This makes it possible to efficiently change the necessity of the surface care for the slab after casting and the change of the assigned grade. It is also possible to prevent the occurrence of surface defects in the thin steel plate product by changing the casting conditions so that inclusions and bubbles are less likely to adhere to the solidified shell.

FIG. 1 is a sectional view conceptually showing a mold of a slab continuous casting machine to which the present invention is applied. FIG. 2 is a graph showing the relationship between the casting distance and the minimum distance from the surface of the slab to the inclusions / bubbles obtained by measuring the surface defects generated in the thin steel plate product. FIG. 3 is a conceptual diagram showing the arrangement positions of the temperature measuring elements with respect to the mold shown in FIG. FIG. 4 is a graph showing an adhesion curve applied in the present invention. FIG. 5 is a graph showing the relationship between the particle size of inclusions / bubbles required to generate the adhesion curve shown in FIG. 4 and the molten steel flow velocity at the solidified shell interface. FIG. 6 is a graph showing the relationship between the particle size and number ratio of inclusions / bubbles necessary for generating the adhesion curve shown in FIG. FIG. 7 is a diagram illustrating a correspondence relationship between the position of the temperature measuring element disposed on the mold and the adhesion curve to be applied. FIG. 8 is a chart showing the relationship between the determination of the presence or absence of surface defects according to the present invention and the actual surface defects of the coil. FIG. 9 is a block diagram showing an apparatus for predicting the surface layer state of the slab in the slab continuous casting machine shown in FIG. FIG. 10 is a chart showing the specifications of the slab continuous casting machine in which the test of the example was performed. FIG. 11 is a chart showing casting conditions in which tests of the examples were conducted. FIG. 12 is a chart showing the component ranges of steel subjected to the tests of the examples. FIG. 13 is a chart showing the relationship between the determination of the presence or absence of surface defects and the actual surface defects of the coil in the example.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of a slab surface state prediction method and a slab surface state prediction apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 shows a mold 1 of a slab continuous casting machine to which the present invention is applied. A mold 1 of a slab continuous casting machine is configured to include a pair of opposed mold long sides 1a and a pair of opposed mold short sides 1b installed between the mold long sides 1a. The molten steel Y is injected into the inside through 2. The molten steel Y injected into the mold 1 is cooled inside the mold 1 to form a solidified shell SH, and is drawn out below the mold 1 to form a slab. Both the mold long side 1a and the mold short side 1b are made of a copper plate.

  The mold 1 is provided with a temperature measuring element, for example, a thermocouple 10 (see FIG. 3) for detecting the temperature on the mold long side 1a. The thermocouple 10 is disposed in the mold 1 because the molten steel flow velocity at the solidified shell interface is calculated from the temperature of the copper plate constituting the mold 1 and surface defects are formed when the thin steel plate product is formed based on the calculated molten steel flow velocity. This is to predict whether inclusions / bubbles that can be present are present in the generated slab. Hereinafter, in order to predict the adhesion of inclusions / bubbles that cause surface defects occurring in the thin steel plate product, it will be described where the thermocouple 10 is optimally disposed in the mold 1.

  First, the present inventors collected a large number of surface defects such as baldness and sliver that were actually generated in a sheet steel product. The thickness cross section of the steel sheet was observed with a microscope at a position including the defect, and the depth from the steel sheet surface to inclusions / bubbles was measured. Considering the ratio of the steel plate thickness to the slab thickness, the depth from the steel plate surface to inclusions / bubbles was converted to the distance from the slab surface to inclusions / bubbles. The conversion result is shown in FIG.

  The horizontal axis in FIG. 2 is the casting speed, and the vertical axis is the minimum distance from the surface of the slab to the inclusions / bubbles. As is clear from FIG. 2, it was found that inclusions / bubbles that become surface defects in the thin steel plate products are distributed within a range of 1 to 10 mm from the surface of the slab. Therefore, what is necessary is just to acquire the molten steel flow velocity about the range until the thickness of the solidification shell SH becomes 10 mm.

  However, the actual mold surface varies depending on various effects such as the vibration of the mold 1 itself, the relationship between the molten steel Y discharged from the immersion nozzle 2 and the drawing speed. Moreover, according to actual measurement, it was found that the molten steel Y was flowing at the same flow rate 100 mm below the mold surface. For this reason, when the thermocouple 10 is disposed, the mold hot water surface is not provided as the upper limit position, but the lower limit of 50 mm from the mold hot water surface where the influence of the molten metal surface fluctuation is small is set as the optimum upper limit position.

Considering these conditions, the positions where the thermocouple 10 is disposed on the long mold side 1a are a plurality of points between the position determined by the following formula (1) from the lower 50 mm starting from the mold surface.
y = V × (10 / k) ² (1)

Here, y is a distance (m) from the mold surface to the position where the thermocouple 10 is disposed, V is a casting speed (m / min), k is a solidification coefficient in the mold 1 (mm / min ^{1 / 2} ). The solidification coefficient k varies depending on the continuous casting machine, but is usually 15 to 23.

The above formula (1) is derived as d = 10 in the following formula (2) representing the thickness d of the solidified shell SH.
d = k√t (2) where t = y / V (min)

  The lowermost thermocouple 10 is preferably in the y position, but if it is difficult to dispose the thermocouple 10 due to equipment, it may be disposed above the thermocouple 10.

The solidification coefficient in the mold 1 of the slab continuous casting machine for which the data was measured was 19.3 mm / min ^1/2 . Using this solidification coefficient and the distance from the slab surface to the inclusions / bubbles, the distance from the mold surface to the lower side was calculated backwards and indicated on the vertical axis. As a result, in the example shown in FIG. 2, it was found that inclusions / bubbles that cause surface defects of the thin steel plate products were adhered to the lower side of the mold surface within a range of 500 mm. Therefore, it is understood that the molten steel flow velocity at the solidified shell interface should be known within a range from 50 mm below the mold hot metal surface to 500 mm below.

  On the other hand, in the sheet width direction of the thin steel plate product, surface defects existed randomly, and the distribution of surface defects was not particularly biased. Therefore, in the width direction of the mold 1, that is, in the mold long side 1 a, it is necessary to know the molten steel flow velocity in the entire region.

  The interval in the mold width direction of the thermocouple 10 is determined by how much spatial wavelength the spatial fluctuation of the molten steel flow velocity in the mold 1 has. The present inventors have already disclosed in Japanese Patent No. 3386051 regarding the spatial fluctuation of the molten steel flow velocity. That is, the force received from the molten steel flow by the refractory rod with one end immersed in the mold surface is measured by the load cell, and the molten steel flow velocity profile along the mold width direction in the vicinity of the mold surface is measured. The spatial wavelength of the molten steel flow velocity distribution was determined to be about 800 to 1800 mm. Therefore, in order to detect this spatial variation, the thermocouple 10 may be disposed in the mold width direction at intervals of 200 to 450 mm or less. However, at a position close to the mold surface, the thickness of the initial solidified shell SH has fluctuations in the mold width direction. For this reason, about the position close to the mold surface, the thermocouple 10 is arranged at an interval smaller than the above-mentioned interval, and the influence of fluctuation in the mold width direction of the thickness of the initial solidified shell SH is obtained by taking the spatial variation average. It was decided to eliminate.

  Next, after converting the copper plate temperature acquired through the thermocouple 10 to the molten steel flow velocity, what kind of treatment was performed to examine whether the adhesion state of inclusions / bubbles could be accurately detected.

  As a result of investigation by the present inventors, it was found that the size (particle size) of inclusions / bubbles due to surface defects of the thin steel plate product was about 300 μm or more. On the other hand, the washing critical flow velocity required to prevent the inclusions / bubbles from adhering to the solidified shell interface is shown in the literature (Yamada et al .: Materials and Processes, 12 (1999), 682) by model calculation. ing. From both findings, it was assumed that inclusions / bubbles that could become surface defects of the thin steel plate product were likely to adhere to the solidified shell SH when the molten steel flow velocity at the solidified shell interface was less than the critical flow velocity (= 0.1 m / s).

  As described above, the molten steel flow velocity at the solidified shell interface is calculated based on the measured temperature of the thermocouple 10 arranged two-dimensionally in the mold 1 from the examination results of the two points, and the molten steel flow rate to the solidified shell SH is calculated based on the molten steel flow velocity. The state of inclusion / bubble adhesion was predicted as follows.

As shown in FIG. 3, the thermocouple 10 was arrange | positioned to the copper plate which comprises the casting_mold | template long side 1a. In the second and third stages, the thermocouples 10 are arranged at intervals of 200 to 450 mm, and as described above, the first stage is the fluctuation of the thickness of the initial solidified shell SH in the mold width direction. In order to eliminate the influence, the interval in the mold width direction of the thermocouple 10 is made fine. From the copper plate temperature measured by these thermocouples 10, the molten steel flow velocity at the solidified shell interface is calculated by the following equations (3) and (4).

  When the calculated molten steel flow velocity is lower than the above-described critical flow velocity for washing, it is possible for the molten steel flow rate to be less than the critical flow velocity in the same manner in the range having a radius of 1/2 of the spatial fluctuation wavelength of the molten steel flow velocity with the thermocouple 10 as the center. It was supposed to have a sex. Specifically, as shown in FIG. 3, the thermocouples 10 of the second and third stages are divided into measurement columns in a plurality of mold width direction sections with the second and third stage thermocouples 10 as the center in the width direction. If there is a thermocouple 10 that is washed away with a flow velocity lower than the critical flow velocity, it is determined that there is a possibility of surface defects due to the inclusion of inclusions / bubbles in the measurement row. It is possible to judge whether or not there is a surface defect in the width direction of the coil in the coil of the thin steel plate product, but how much inclusions / bubbles causing the defect are from the surface of the coil plate. This is because the depth is not known unless the thickness cross section of the coil is examined for each defect.

  Here, in order to determine whether or not there is a possibility of a surface defect due to inclusion / bubble adhesion, in the present embodiment, an adhesion curve representing the relationship between the molten steel flow velocity and the inclusion / bubble adhesion is shown. It is set in advance as a judgment criterion.

  FIG. 4 is a graph showing the adhesion curve, and shows the relationship between the molten steel flow velocity on the horizontal axis and the degree of adhesion of inclusions / bubbles to the solidified shell SH (adhesion degree) on the vertical axis. This adhesion curve shows the limit particle size of inclusions / bubbles adhering to the solidified shell SH for each molten steel flow rate shown in FIG. 5, and the rosin Ramler distribution of inclusions / bubbles contained in the molten steel Y shown in FIG. It is set from. Here, the numerical value on the vertical axis in FIG. 6 is a value obtained based on the calculation formula of the number distribution in the Rosin-Rammler distribution.

  Hereinafter, a method of creating the adhesion curve shown in FIG. 4 will be described. Since the smallest particle size of inclusions / bubbles that can be a surface defect of the product after rolling, that is, the critical particle size for surface defect is 140 μm, the flushing corresponding to the critical particle size for surface defect is 140 μm from FIG. When the critical flow velocity (hereinafter referred to as “washing critical flow velocity for surface defects”) is obtained, it is 0.2 m / s. It is necessary to consider separately when the molten steel flow velocity exceeds the critical flow velocity for washing away the surface defects.

  First, when the molten steel flow rate exceeds the critical flow rate for washing away surface defects, inclusions / bubbles do not adhere to the solidified shell SH. Therefore, in this case, the total number of inclusions / bubbles adhering to the solidified shell SH is plotted as zero and plotted on the adhesion curve.

  Next, let us consider the case where the molten steel flow velocity is less than the critical flow velocity for washing out surface defects. For example, when the molten steel flow rate is 0.15 m / s, the particle size corresponding to the molten steel flow rate of 0.15 m / s is 180 μm from FIG. 5, and inclusions / bubbles with a particle size of 180 μm or less adhere to the solidified shell SH. I understand.

  Next, the number of inclusions / bubbles distributed in the range of 140 μm or more and 180 μm or less (shaded portion in FIG. 6) is obtained from FIG. Further, the integrated value of inclusions and bubbles is calculated for each molten steel flow rate by changing the molten steel flow rate, and plotted on the adhesion curve. By the above method, the adhesion curve shown in FIG. 4 can be obtained.

  The critical particle size of inclusions and bubbles adhering to the solidified shell SH and the distribution of inclusions and bubbles contained in the molten steel Y differ depending on the position from the mold surface. Accordingly, as shown in FIG. 7, the adhesion curve is set individually according to the measurement position of the mold copper plate by the thermocouple 10. In any adhesion curve, the adhesion has a positive value when the molten steel flow velocity (solidification interface flow velocity) at the solidified shell interface is washed away and falls below the critical flow velocity.

  Here, the second stage and the third stage have a lower washing critical flow velocity of inclusions / bubble adhesion than the first stage because they are disposed in the second stage and the third stage rather than the first stage. Since the measurement position of the thermocouple 10 has a large distance from the mold surface to the bottom, that is, the distance from the slab surface to the solidified shell interface is large, the particle size of inclusions / bubbles that cause surface defects in thin steel plate products However, the critical value is larger at the measurement positions of the second stage and the third stage than the first stage. Therefore, according to the literature (Yamada et al .: Materials and Processes, 12 (1999), 682), when inclusions / bubbles have a large particle size and the solidification rate at the solidified shell interface decreases, the inclusions / bubbles are washed away. The critical flow velocity required for this is also reduced.

  The temperature when the same portion of the solidified shell SH passes is measured by the thermocouple 10 disposed in each of the first stage, the second stage, and the third stage in the mold copper plate, and the molten steel flow velocity calculated from each measured temperature is obtained. Find the degree of adhesion of the corresponding inclusions / bubbles. The maximum value of these three adhesion degrees is defined as the adhesion degree of the measurement row. When the degree of adhesion exceeds a preset threshold value, it is determined that there is a possibility of a surface defect due to inclusion / bubble adhesion in the mold width direction section corresponding to the measurement sequence.

  A series of processes such as the above-described copper plate temperature measurement, molten steel flow velocity calculation, adhesion degree derivation, and surface defect possibility determination are performed at a constant cycle along the casting direction, for example, every second, and the surface for each measurement position. Judgment of the possibility of defects and the surface defect inspection results of the actually formed thin steel sheet coil were put together and counted. The tabulation results are shown in FIG.

  In FIG. 8, N1 is a case where it is determined that there is no possibility of a defect, and the actual thin steel plate coil has no surface defect. N4 is a case where it is determined that there is a possibility of a defect, and the actual thin steel sheet coil also has a surface defect.

  On the other hand, N2 in FIG. 8 is a case of “missing” in which it is determined that there is no possibility of a defect, but the actual thin steel plate coil has a surface defect. However, since the surface inspection result of the thin steel plate coil is used for the presence / absence of defects, there is a possibility that rollable surface defects are mixed in a process downstream of casting. Therefore, if a large number of sheet steel coils are matched, and N2 / (N2 + N4) is equal to the ratio of the rollability defect to the surface defects of the sheet steel coil, the castability, that is, the inclusion / bubble solidified shell SH It can be said that there are few missed surface defects due to adhesion to the surface. Further, if N2 / (N2 + N4) is significantly different from the ratio of rolling defects in the surface defects of the thin steel sheet coil, N2 / (N2 + N4) is equivalent to the ratio of rolling defects in the surface defects of the thin steel sheet coil. The above threshold value may be adjusted so that In addition, if a means for directly measuring inclusions / bubbles in the slab surface layer instead of surface defect inspection of a thin steel plate coil, for example, an ultrasonic flaw detector is used, the threshold value is adjusted so that N2 becomes almost zero. can do.

  FIG. 9 shows an apparatus for predicting the surface layer state of a slab in a slab continuous casting machine by the method described above. In FIG. 9, the control means 100 includes inclusions / bubbles that may cause surface defects in a slab generated by a slab continuous casting machine based on a preset program and data when a detection signal is given from the thermocouple 10. It has a temperature acquisition unit 101, a molten steel flow rate calculation unit 102, an adhesion degree derivation unit 103, and an adhesion determination unit 104.

  The temperature acquisition unit 101 acquires the copper plate temperature at the measurement position in the mold 1 according to the detection result of the thermocouple 10 disposed in the mold 1. When the temperature acquisition unit 101 acquires the copper plate temperature, the molten steel flow rate calculation unit 102 calculates the flow rate of the molten steel Y at the solidified shell interface at the measurement position according to the above equations (3) and (4). The calculated molten steel flow velocity is given to the adhesion degree deriving unit 103 together with information on the measurement position at which the copper plate temperature is measured. The degree-of-adhesion deriving unit 103 is based on the molten steel flow rate given from the molten steel flow rate calculating unit 102 and the information on the measurement position at which the copper plate temperature is measured, and the relationship between the molten steel flow rate stored in the memory 105 in advance and the inclusion / bubble adhesion degree. The adhesion degree of inclusions / bubbles corresponding to the molten steel flow velocity is derived based on the adhesion curve representing In the memory 105, an individual adhesion degree curve is set according to the measurement position of the mold copper plate by the thermocouple 10. The adhesion degree deriving unit 103 selects a corresponding adhesion degree curve in accordance with the measurement position information, and derives the adhesion degree of inclusions / bubbles corresponding to the molten steel flow rate based on the selected adhesion degree curve and the molten steel flow rate. The adhesion determination unit 104 compares the adhesion level given from the adhesion level deriving unit 103 with a preset threshold value, and based on the comparison result, inclusions / bubbles that may cause surface defects in the solidified shell SH corresponding to the measurement position. Whether or not exists.

  Specifically, each copper plate temperature is acquired by the temperature acquisition unit 101 from the detection results of the thermocouples 10 arranged in the first stage, the second stage, and the third stage. Each molten steel flow velocity is calculated. The copper plate temperature acquired by the temperature acquisition unit 101 through the thermocouples 10 arranged in the first to third stages is that when the same portion of the solidified shell SH passes during casting. When the molten steel flow velocity calculated by the molten steel flow velocity calculation unit 102 is given, the adhesion degree deriving unit 103 derives the adhesion degree corresponding to each molten steel flow velocity. The adhesion determination unit 104 given the adhesion degree sets the maximum adhesion degree from the plurality of adhesion degrees derived for the same location of the solidified shell SH as the maximum adhesion degree of the solidified shell SH corresponding to the measurement position. When the maximum degree of adhesion exceeds a preset threshold, it is determined that there are inclusions / bubbles that can become surface defects. The determination result is stored in the memory 105 and is output via the output means 106 such as a display or a printer. Therefore, it is possible to efficiently change the necessity of surface care for the slab after casting and the change of the assigned grade based on the output result. In addition, it is possible to prevent the occurrence of surface defects in the thin steel plate product by changing the casting conditions such that inclusions and bubbles are less likely to adhere to the solidified shell SH.

  The specifications of the slab caster that was tested are as shown in FIG. 10, and the test was performed according to the casting conditions shown in FIG. The component range of the steel during the test is as shown in FIG.

The thermocouple 10 has a first stage 187 mm (= 97 mm below the mold surface), a second stage 272 mm (= 182 mm below the mold surface), and a third stage 502 mm (= the mold surface) from the upper end of the mold copper plate. 412 mm below). Assuming that the solidification coefficient in the mold 1 is 19.3 mm / min ^1/2 as described above, y = 429.54 mm, and the position where the thermocouple 10 is disposed is downward from the mold surface as described above. The condition between 50 mm and the position determined by the above equation (1) is satisfied. The distance from the tip of the temperature measuring part of each thermocouple 10 to the solidified shell side surface of the mold copper plate is 21.6 mm. The interval in the mold width direction of the thermocouple 10 disposed in the first stage was 50 mm, and the interval in the mold width direction of the thermocouple 10 disposed in the second and third stages was 100 mm. The type of the thermocouple 10 used is JIS-T.

  By the above method, the possibility of surface defects with respect to the measurement position and surface defect inspection of the actually formed thin steel plate coil were determined for 100 slabs for automobile outer plates, and the results were compared and tabulated. The tabulation results are shown in FIG. In this embodiment, since the temperature is detected by the thermocouple 10 at 15 intervals in the mold width direction and at a pitch of 1 second in the casting direction, a total of 562,500 evaluation sections are obtained with 100 slabs.

  In the judgment of the possibility of surface defects according to the present invention, 3119 (= N4) were judged as “with surface defects” by the actual surface defect meter as judged as “inclusions / bubbles”. On the other hand, in the determination of the possibility of surface defects according to the present invention, there are 1832 (= N2) that are determined as “no inclusions / bubbles” and determined as “surface defects” in an actual surface defect meter. However, since the thin steel plate actually measured is a coil, it includes rolling surface defects. In the case of a steel plate production line for automobile outer plates, it is statistically known that the cause ratio of surface defects detected by a coil surface defect meter is about 60% for castability and about 40% for rollability. In this embodiment, N4 / (N4 + N2) = 0.63. That is, most of the surface defects detected as N2 are rollable, and it can be judged that the surface defects of castability can be predicted with almost no oversight. In the determination of the possibility of surface defects, there were 16218 (= N3) that were judged as “inclusions / bubbles” and judged as “no surface defects” by an actual surface defect meter. This is because the molten steel flow velocity at the solidified shell interface is washed away and the critical flow velocity is interrupted, so that inclusions and bubbles are in an environment where the inclusions and bubbles are likely to adhere. This is considered to mean that was not present in the vicinity of the solidified shell interface.

  In the above-described embodiment, three stages of temperature measuring elements are arranged along the casting direction, but may be two stages or less, or four stages or more. If the number of temperature measuring elements is increased, a large number of molten steel flow velocities can be acquired along the thickness direction of the solidified shell SH, and the determination of the presence or absence of surface defects can be made more finely.

DESCRIPTION OF SYMBOLS 1 Mold 1a Mold long side 1b Mold short side 2 Immersion nozzle 10 Thermocouple 100 Control means 101 Temperature acquisition part 102 Molten steel flow velocity calculation part 103 Adhesion degree deriving part 104 Adhesion judgment part 105 Memory 106 Output means SH Solidified shell Y Molten steel

Claims (7)

For slab continuous casting machines that continuously generate slab from the lower end of the mold by discharging molten steel into the mold, predict whether inclusions or bubbles that may cause surface defects exist in the generated slab A method for predicting the surface state of a slab,
A flow rate calculation step of acquiring the temperature at the measurement position through a temperature measuring element arranged in the mold and calculating the flow rate of the molten steel at the solidified shell interface from the acquired temperature,
Compare the preset flow critical flow velocity of inclusions and bubbles with the flow velocity of the molten steel calculated in the flow velocity calculation step, and if the calculated molten steel flow velocity is less than the critical flow velocity, the solidification corresponding to the measurement position A method for predicting a surface state of a slab, comprising: an adhesion determination step for determining that inclusions / bubbles that may become surface defects exist in the shell. A plurality of temperature measuring elements are arranged along the casting direction until the solidified shell reaches a preset thickness starting from the molten steel surface,
The temperature when the same portion of the solidified shell passes through the plurality of temperature measuring elements is acquired, and the flow rate of the molten steel is calculated from the acquired temperature, and surface defects can be caused based on the calculated flow rates of the molten steel. The method for predicting a state of a surface layer of a slab according to claim 1, wherein it is determined whether or not inclusions / bubbles are present. The relationship between the molten steel flow velocity and the inclusion / bubble adhesion degree is set in advance as an adhesion curve so as to have a positive value only when the molten steel flow velocity is washed away and below the critical flow velocity,
The adhesion determination step includes
A step of deriving an adhesion corresponding to the flow rate of the molten steel calculated in the flow rate calculation step based on the adhesion curve;
The method for predicting the surface layer state of a slab according to claim 1, further comprising: determining that there are inclusions / bubbles that may become surface defects when the derived adhesion degree exceeds a preset threshold value. The adhesion curve is
Deriving the critical flow velocity for washing, which corresponds to the critical particle size of inclusions and bubbles, which can be surface defects of the product after rolling,
When the molten steel flow velocity exceeds the derived washing critical flow velocity, the total number of inclusions / bubbles adhering to the solidified shell is set to zero,
On the other hand, when the molten steel flow rate is lower than the flushing critical flow rate, the critical particle size of inclusions / bubbles attached to the solidified shell corresponding to the molten steel flow rate and the flushing critical flow rate is obtained, and the inclusions contained in the molten steel The method for predicting the state of the surface layer of a slab according to claim 3, wherein the surface layer state prediction method is created by calculating the total number of inclusions and bubbles adhering to the solidified shell using a distribution function of bubble particle size. Multiple temperature measuring elements are arranged along the casting direction until the solidified shell reaches the preset thickness starting from the molten steel surface, and the adhesion curve is set according to the position of the temperature measuring element. And
Obtain the temperature when the same location of the solidified shell has passed through the plurality of temperature measuring elements, calculate the flow rate of the molten steel from the acquired temperature, respectively, and calculated based on the adhesion curve corresponding to the measurement position, respectively When the degree of adhesion of inclusions / bubbles is derived from the flow rate of the molten steel, and any of the derived degrees of adhesion exceeds a preset threshold, inclusions / bubbles that may cause surface defects are found at the site corresponding to the measurement position. It is judged that it exists, The surface layer state prediction method of the slab of Claim 3 characterized by the above-mentioned.   6. The slab according to claim 2, wherein a plurality of the temperature measuring elements are arranged from a lower side of 50 mm to a thickness of the solidified shell of 10 mm starting from the molten steel surface. Method for predicting the surface layer of For slab continuous casting machines that continuously generate slab from the lower end of the mold by discharging molten steel into the mold, predict whether inclusions or bubbles that may cause surface defects exist in the generated slab A slab surface state prediction device,
A temperature acquisition unit for acquiring the temperature of the measurement position through a temperature measuring element arranged in the mold;
A molten steel flow rate calculation unit for calculating a flow rate of the molten steel at the solidified shell interface from the temperature acquired by the temperature acquisition unit;
When the molten steel flow velocity is calculated by the molten steel flow velocity calculation unit, based on the adhesion curve representing the relationship between the molten steel flow velocity and the inclusion / bubble adhesion degree set in advance, the inclusion / bubble flow corresponding to the molten steel flow velocity is calculated. An adhesion degree deriving unit for deriving the degree of adhesion;
When the adhesion degree of inclusions / bubbles derived by the adhesion degree deriving unit is compared with a preset threshold value, and the adhesion degree of inclusions / bubbles exceeds the threshold value, surface defects in the solidified shell corresponding to the measurement position An apparatus for predicting a state of a surface layer of a slab, comprising: an adhesion determination unit that determines that inclusions / bubbles that can be formed are present.

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