JP4325451B2 - Method for detecting surface defect of continuous cast slab and removing method thereof - Google Patents

Description

  The present invention relates to a method for detecting occurrence of surface defects caused by inclusions generated during casting by a continuous casting method, and further removing a surface defect portion in a slab based on the detection result.

  Generally, in the operation of a continuous casting machine, in order to obtain a high quality slab, various methods are adopted so that the flow state in the continuous casting mold is within a certain range. For example, as an indicator of the molten steel flow state, the molten steel flow velocity measurement in the mold shown in Non-Patent Document 1 is performed.

In Patent Document 1, a temperature measuring element is embedded in a horizontal direction at a position directly below the molten metal surface in the mold cooling plate, and the solidification latent heat is calculated from the temperature information obtained from the temperature, the molten steel temperature, and the casting speed. A technique for obtaining a suitable molten steel flow velocity by extracting a heat transfer amount by molten steel flow and converting it by a heat transfer engineering method is disclosed.
The goal of these techniques is to find the optimum conditions for continuous casting.

  Further, Patent Document 2 discloses a control technique for detecting molten metal level fluctuations and controlling molten steel in a mold using electromagnetic force to realize casting under optimum conditions.

  The above three technologies are all aimed at controlling the casting conditions within a certain range, but until the abnormal flow and fluctuations in the molten metal level, which are mainly caused by clogging of the immersion nozzle during casting, are completely suppressed. Therefore, a slab containing surface defects caused by inclusions is sent to a hot rolling process and further to a cold rolling process, resulting in the occurrence of product defects. When such a product defect occurs, the defective part must be cut and removed, so that the product yield is significantly reduced. In some cases, it may be necessary to re-roll the alternative material and re-manufacture everything, which may affect the delivery date of the product.

On the other hand, the detection method by a slab surface flaw disclosed in Patent Documents 3 and 4 is known as a slab surface defect detection technique. However, these disclosed techniques only detect irregularities on the surface of the slab due to surface cracks, and are not techniques for detecting defects in which inclusions are captured on the surface of the slab.
Further, Patent Document 5 discloses a technique for identifying a location where a mold flux has penetrated to the outside of a slab from a change in temperature of the mold, but today, most of surface defects are inside the solidified shell. It is thought that this technology has already lost its practicality.
CAMP-ISIJ, 11 (1998), 181 Japanese Unexamined Patent Publication No. 2000-246413 JP-A-11-90600 JP-A-11-90599 JP 2000-317595 A JP-A-106653

  In order to detect the surface defect occurrence position of the slab in continuous casting at the time of slab casting, the product defect occurs after obtaining information such as the molten steel flow velocity and molten metal surface level as an index of molten steel flow in the mold. Where it is essential to closely examine the correspondence with the position, each of the above-described conventional techniques is a method for controlling the casting conditions to suppress the occurrence of defects caused by inclusions. And it has not been studied until now regarding its removal. In addition, other defect detection techniques described above are also for cracks on the slab surface that are relatively easy to detect, or defects due to the biting of the mold flux, and for detecting defects caused by inclusions generated in the steelmaking process. The problem was left where the effect could not be demonstrated.

  Therefore, an object of the present invention is to propose a method for detecting an inclusion-induced defect on a slab surface and a method for removing an inclusion-induced defect, which are accurate and have high immediacy.

  In order to advantageously solve the above-described problems of the prior art, the inventors have conducted research on the correspondence between the molten steel flow velocity in the mold and the occurrence of product defects, and have developed the present invention. That is, the gist of the present invention is as follows.

In continuous casting in which molten metal is injected from the upper part of the mold and the cast slab is continuously pulled out from the lower part of the mold, the flow rate of the molten metal immediately below the molten metal surface in the mold is spaced apart at intervals not exceeding 300 mm in the mold width direction. The continuous casting casting is characterized in that it is determined that surface defects caused by inclusions occur when the measurement value is measured at a time interval not exceeding 10 seconds and the measured value falls below 0.13 m / s. A method for detecting surface defects on a piece.

In the surface defect detection method described above , after determining that a surface defect caused by inclusions has occurred, the surface portion of the slab containing the surface defect occurrence corresponding portion is removed by grinding or welding. A method for removing surface defects from continuously cast slabs.

  According to the present invention, the flow velocity value in the vicinity of the solidification interface in the mold is evaluated to accurately detect the occurrence of a defect due to inclusions, and the corresponding position of the defect is accurately removed based on the detection result. be able to.

The details of the configuration of the present invention, particularly the contents found by the inventors will be described below.
In continuous casting experiments conducted by the inventors, the flow rate of molten steel near the solidification interface in the mold was measured, and slab defects after casting were observed. It was found that a defect occurred at the position of the cast slab (cold rolled product).
FIG. 1 discloses the relationship between the position of the slab in the longitudinal direction, the solidification interface flow velocity, and the position corresponding to the occurrence of product defects on the slab. As shown in FIG. While measuring the solidification interface flow velocity in B, the defects on the slab after casting were observed. As a result, it was found that a slab defect occurred when the flow velocity at position B decreased.

  Here, the position A is a portion in the mold near the short side, where the flow velocity is generally large, and the position B is a portion where the molten steel flow is likely to stagnate in the vicinity of the immersion nozzle, and the flow velocity at the position B is particularly 0.13. Many defects occurred when below m / s. This means that when the flow velocity at the solidified shell interface exceeds this value, inclusions or bubbles easily adhere to the solidified shell and cannot stay, but when it falls below 0.12 m / s. It means that it is surely captured by the shell.

  The flow velocity measurement position is the solidification interface just below the mold surface in the mold, but it is about -50 mm (near) just below the meniscus. The reason for limiting to this position is that the solidified shell thickness is about 1 mm, and the inclusions / bubbles trapped in this thickness portion are most likely to be surface defects in consideration of the scale generated in the steel plate manufacturing process.

  Next, the experimental results to obtain this knowledge were statistically processed, and the flow velocity value corresponding to the defect was determined in units of 0.01 m / s for the total number of defects investigated. FIG. 2 shows the composition of the flow velocity value obtained here in terms of the cumulative frequency rate, with the composition ratio as the vertical axis and the flow velocity of the molten steel at the solidification interface as the horizontal axis. As shown in this figure, it can be seen that when the flow velocity serving as the threshold is less than 0.13 m / s, the defect detection rate is 95% or more.

  Further, in FIG. 2, in each case where the threshold value of the flow velocity is a numerical value indicated on the horizontal axis, the ratio at which the corresponding defect does not exist, that is, erroneous detection is indicated by a broken line. As shown here, when the flow velocity serving as the threshold exceeds 0.13 m / s, so-called overdetection that causes the determination of the presence of a defect despite the fact that there is no defect is abruptly increased. Therefore, the threshold value of the flow velocity at the time of detecting the defect is set to 0.13 m / s, more preferably 0.12 m / s.

Next, the inventors conducted research on the measurement position of the flow velocity. As a result, the measurement was performed at a plurality of points spaced in the width direction of the mold, specifically, the position of the thermocouple T as shown in FIG. It is preferable to perform the measurement at a measurement point embedded in. However, this interval is up to 300 mm. The reason is that it is possible to detect almost all defects in the slab at this interval. In particular, according to the study by the inventors, it has been confirmed that when it exceeds 300 mm, the rate of missing the defect position exceeds 30%. Therefore, it is preferable that the measurement position of the flow velocity is a plurality of points spaced in the width direction of the mold, and the distance is 300 mm or less. On the other hand, an increase in the number of measurement points leads to an increase in cost, so this interval should be 40 mm or more.
The measurement point is embedded in the thickness direction of the metal mold (M), for example, at a position of 13 mm from the inner surface of the molded steel plate, and in the mold drawing direction, for example, at a position about 50 mm below the meniscus. Is preferred.
The position of the measurement point is preferably determined appropriately in consideration of the structure of the mold, for example, the interval between the cooling slits, the bolt for fixing the temperature measuring element, and the like.

  Furthermore, it is preferable that the time interval at the time of measuring the flow velocity value not exceed 10 seconds. That is, when 10 seconds are exceeded, 25% of defects are still missed. Here, as the flow velocity measurement time interval is shorter than 10 seconds, it is more advantageous in preventing defects from being overlooked. However, even if it is shorter than 0.5 seconds, improvement in detection accuracy cannot be expected. Because, as will be described later, the mold copper plate temperature is used for the desired flow velocity measurement method, the temperature detection time constant (rise time, fall time) is greater than 0.5 s, and fluctuations with a shorter period cannot be detected. Because.

  Inclusion-related defects contained in the slabs detected by the method described above were obtained by tracking the inside of the machine by calculation from the speed at which the mold-corresponding part moves at the time of detection. The position of the slab corresponding to the defect is marked, and when the number of corresponding parts is small, only that part is ground, and when the number of the corresponding parts is large, the entire defect occurrence surface is uniformly ground or melted and removed. Accordingly, it is possible to substantially suppress the occurrence of defects due to inclusions in subsequent processes.

  By the way, the defect caused by inclusions increases the occurrence rate due to the presence of many inclusions themselves, but it is always necessary that the inclusions remain in the steel and be captured by the solidified shell. Is a condition. It was clarified in the present invention that the probability of inclusion supplementation increases when the flow rate is small qualitatively, and inclusion supplementation can be suppressed by giving a sufficient flow rate to the solidification interface. More specifically, the so-called cleaning effect has a threshold value in the range of 0.13 to 0.12 m / s.

  The method for measuring the flow velocity is not particularly limited, but, for example, the flow velocity is continuously measured at a certain time interval in the mold width direction during casting more than the method described in Non-Patent Document 1. It is more preferable to obtain the estimated flow velocity based on the temperature of the mold copper plate described in Patent Document 1.

  Low carbon to ultra low carbon steel (0.0002 to 0.04 mass% C), the final product of which is a cold-rolled steel sheet or its surface-treated product, using a mold flux with a viscosity of 0.4 to 4.O Poise, casting speed of 1.5 to 3.0 In continuous casting at m / min, slab surface defects were detected and removed. Here, the flow velocity measurement of the molten steel was performed at 200 points at 8 points in the width direction of the mold according to the method described in Patent Document 1, and the measurement result was automatically determined and determined based on the determination result. Marking was performed at the defect determination position on the slab, and the defect corresponding position was ground and removed by 1.5 mm in the slab thickness direction.

Here, the thickness to be removed by grinding was determined in the range of 1 to 2 mm based on the flow velocity measurement position and the casting speed, and the average value was 1.5 mm. In addition, shortening the flow velocity measurement position interval is advantageous in preventing missed defects, but increasing the number of measurement points leads to an increase in cost, so it was set to 200 mm for economic reasons. For the flow rate measurement time interval, flow rate measurement with a period of 0.5 seconds was performed.

For comparison, defects were detected by the method disclosed in Patent Document 5 and defects were removed from the corresponding position cast (conventional example). In any case, the occurrence of defects after cold rolling was investigated, and when defects were detected and removed according to the present invention, the occurrence of defects in the cold rolled steel sheet was drastically reduced to 1/10 compared to the conventional case.

It is a figure which shows the relationship between the solidification interface flow velocity and generation | occurrence | production of a defect. It is the diagram which represented the structure of the flow-velocity value corresponding to a defect with the cumulative frequency rate. It is a figure which shows the measurement location in a casting_mold | template.

Claims (2)

In continuous casting in which molten metal is injected from the upper part of the mold and the cast slab is continuously pulled out from the lower part of the mold, the flow rate of the molten metal immediately below the molten metal surface in the mold is spaced apart at intervals not exceeding 300 mm in the mold width direction. The continuous casting casting is characterized in that it is determined that surface defects caused by inclusions occur when the measurement value is measured at a time interval not exceeding 10 seconds and the measured value falls below 0.13 m / s. A method for detecting surface defects on a piece. In the surface defect detection method according to claim 1 , after determining that a surface defect due to inclusions has occurred, the surface portion of the slab containing the surface defect occurrence corresponding portion is ground or removed by grinding. A method for removing surface defects from a continuous cast slab characterized by

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