JPH0787976B2 - Online slab surface defect detection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an on-line slab surface defect detection method in continuous casting in which slab surface defects are detected online during casting.

[Conventional technology]

Among the problems that occur during continuous casting, in a so-called medium carbon steel with a carbon content of 0.08 to 0.16% as a typical example, since a peritectic reaction is involved in the solidification process during casting, the slab surface due to transformation shrinkage This is a major problem because vertical cracks, cracks in the corners, depletion, etc. are likely to occur and high-speed stable casting in continuous casting is difficult.
Therefore, conventionally, the occurrence of surface defects in the slab was detected by visually observing the slab or by other inspection methods, but such a method requires much time and manpower for the inspection, and the surface defects are found. At that time, a large amount of slabs have already been produced, and it is necessary to care for these slabs and the distribution becomes very complicated.

As a method for solving the above drawbacks, for example, there is a "method for detecting vertical cracks in a slab during continuous casting" disclosed in Japanese Patent Laid-Open No. 62-192243. This is to embed a plurality of thermocouples in a mold, measure the temperature, determine the temperature change amount at each measurement point, and detect the presence or absence of vertical cracking by checking whether this change amount exceeds a predetermined value. .

[Problems to be Solved by the Invention]

Therefore, in order to confirm the function of the above proposal by experiments, the present inventors embed a plurality of thermocouples in the mold wall, measure the temperature, and obtain the temperature change amount at each point with time, As a result of investigating the relationship with one-sided surface properties, very large vertical cracks could be detected to some extent, but fine vertical cracks could hardly be detected (see Fig. 11).

FIG. 11 (a) shows the maximum temperature change amount of the slab without surface defects, and FIG. 11 (b) shows the maximum temperature change amount of the surface defect generation slab and the slab sketch corresponding to this change amount. However, as is clear from the figure, it is difficult to detect surface defects by the amount of temperature change with this conventional method.

The present invention has been made in view of such a conventional drawback, and detects the change over time in the mold width direction temperature or the balance of the heat flux distribution, rather than the temperature change amount at each point, to obtain a slab. The purpose is to detect surface defects including surface cracks and depletion online.

[Means for Solving the Problems]

The present invention measures the temperature or heat flux at each point in the width direction of the continuous casting mold, observes the temperature or heat flux distribution, and based on the change over time of the temperature or heat flux distribution, the surface of the slab An on-line slab surface defect detection method characterized by determining the presence or absence of a defect, wherein the change detection means is a temperature or heat flow with each adjacent measurement point at each point in the mold width direction. The difference between the fluxes is given a sign of ±, and it is detected online based on the arrangement of this code, or the change in the order from the lower temperature or heat flux, or the change over time in the temperature or heat flux distribution, There is a method of detecting online based on the change of the minimum point of each measurement point distribution.

[Action]

The present invention is the knowledge obtained by the above-mentioned configuration, that is, when the slab has no surface defects, the temperature distribution or heat flux distribution in the mold width direction shows no temporal change. The above is based on the fact that the distribution largely changes with time when surface defects occur. That is, it is possible to know the surface property of the slab online during casting by detecting the time variation of the temperature or heat flux distribution, and if it changes, it is judged that the surface defect has occurred, and if it has not changed, the surface property is good. Is.

〔Example〕

The present invention will be described below with reference to the drawings, tables and the like. First to
FIG. 10 shows an embodiment of the present invention.

FIG. 1 is a perspective view showing a continuous casting mold according to an embodiment. A thermocouple as a temperature measuring device is embedded in the meniscus portion in the width direction of the copper plate of this mold, and the thermoelectromotive force of the thermocouple is converted into an electric signal corresponding to the temperature by the converter and taken into the calculator.

This signal is taken in at a constant cycle (for example, every second).
Then, the temperature distribution or heat flux distribution in the mold width direction is calculated and output to the CRT screen. However, when the CRT screen is output, the temperature or heat flux in the mold width direction is interpolated by the spline function. Further, in the case of heat flux calculation, another thermocouple is embedded by changing the depth (see FIGS. 2 and 8).
Figure (a)), and calculate based on the temperature difference. Figure 2 shows the temperature measurement points and thermocouple positions during the experiment. CA thermocouple (1mmφ, sheath length 1.5m, heat resistant type compensating lead wire) is used as the thermocouple. Also, of the thermocouple numbers, the odd number is 5 mm from the surface.
, The even ones are embedded at the 15 mm position.

Each thermocouple is 150 mm above the top of the mold. Further, the heat flux q is calculated at each point at a constant cycle based on the temperature difference between the two thermocouples embedded in the depth direction. That is, Here, q: heat flux (Kcal / m ² h) k: heat conductivity coefficient of copper plate mold (Kcal / mhk) D: difference in depth between two thermocouples (m) Fig. 3 shows heat flux q Calculate at each point in the mold width direction,
The graph which interpolated the value by the spline function is shown.

Then, the temperature or heat flux is measured and the number of changes of the distribution is investigated, and if the value exceeds a threshold value, it is judged that the surface defect of the slab has occurred. The distribution change frequency is plotted by plotting the lowest point of the heat flux in relation to the position, and when the position change occurs, the number is counted as a change (change in distribution).

FIG. 4 shows that the surface defect of the slab was judged to have occurred when the above measurement results changed from the distribution state of (a) to the distribution state of (b) after 4 seconds in FIG. However, when there is no change as shown in (a) and (b) of FIG.

As the detection means for knowing the change state of the distribution at this time,
When the difference in temperature or heat flux from the adjacent measurement point becomes low as shown in FIG. 7A, a sign of + is given when the difference becomes high, and the order of arrangement of the signs changes with time (FIG. (B) (a) -b) or a number having a low temperature difference is assigned (Fig. 6), and the order of arrangement of the numbers changes with time (a- (b) in the same figure). In such cases, it is judged that a defect has occurred on the surface of the slab. The presence or absence of this distribution change can be immediately dealt with by the defect occurrence by constantly monitoring the CRT. FIG. 7 shows the results of examining the number of distribution changes for each slab using this method. From this, it can be seen that the correspondence between the surface defect index and the number of distribution changes is well taken.

Next, at the point where the heat flux is the lowest, that is, at the point where heat removal is considered to be the most delayed, it is considered that the shell is the thinnest and vertical cracks and the like are likely to occur. If the heat flux distribution does not change, it is considered that the point where the heat flux is the lowest (minimum point) does not change with time. Therefore, we examined how the point with the lowest heat flux changes with time by the method shown in FIG. The results are shown in Fig. 10. As is clear from this, the minimum point of heat flux (that is, the point where the shell is considered to be the thinnest) frequently changes with time in the slab with surface defects, while the slab with no surface defects is present. Then you can see that there is almost no change. Therefore, by monitoring the temporal change of the minimum point of the heat flux distribution by CRT, it is possible to establish an online detection system for slab surface defects.

Next, in order to understand the heat removal behavior in the meniscus part, which is considered to have a great influence on the surface defects of the medium carbonaceous material, a thermocouple (two in the depth direction) was placed in the width direction of the mold copper plate meniscus part. We investigated the relationship between the surface texture and the change in heat removal amount due to changes in embedding and operating conditions. as a result,
Even with medium carbonaceous materials, there is no unevenness in temperature (heat flux) distribution in low-speed casting, but unevenness occurs in high-speed casting. That is, the distribution was monitored while performing high-speed casting at 1.7 m / min in the method based on the temporal change of the minimum point.
Since the minimum point of the distribution was detected by another thermocouple and the change in the distribution was detected, the casting speed was immediately reduced to 1.1 m / min to make the distribution uniform. And after homogenizing the distribution, speed up,
The casting speed was returned to 1.7 m / min and the casting was continued. As a result, when observing the surface of the cast slab after casting, a defect was found in the portion corresponding to the time when the movement of the minimum point occurred, and no defect occurred, and sufficient accuracy for detection was obtained. I was able to confirm that I had it.

Next, in the method of changing the order of the code sequence with time, three of the seven detection points were changed, and it was determined that the distribution had changed, so the casting speed was changed from 1.7 m / min to 1.
It decreased to 1 m / min, and after stabilizing the distribution, the speed was increased again as described above.

Fig. 9 shows that the temperature distribution and heat flux distribution were measured in an experiment and the correspondence with the surface properties of the slab was investigated. It was found that the distribution shape of the slab changed little with time.

Further, as a result of the above investigation, according to the literatures so far, it is said that the surface defect may be caused by the nonuniform solidification in the width direction immediately below the meniscus, but from the present experiment, even the distribution was nonuniform. However, it was concluded that surface defects are unlikely to occur unless the distribution changes significantly over time.

By comparing the heat flux distributions of the slab with surface defects and the slab with no surface defects, the distribution of the slabs with surface defects showed a marked change. Therefore, it is possible to construct an online detection system for surface defects by constantly monitoring the heat flux distribution. In addition, it is considered possible to quantitatively evaluate operating factors by using the same index.

Table 1 below shows the surface defect detection rates according to the conventional method and the present invention.
Table 2 shows changes in surface defect occurrence rate, breakout occurrence rate, and average pouring speed before and after the operation of the present invention.

[Advantages of the Invention] As shown in Tables 1 and 2 above, the present invention has an extremely high surface defect detection rate as compared with the conventional method. Further, the operation of the present invention has the effect that the surface defect occurrence rate is drastically reduced from 10% to 2% and the breakout due to the surface defects is also reduced.

Furthermore, as explained above, the present invention allows surface defects to be detected online during casting. Therefore, it is possible to suppress the progress of defects by taking appropriate actions when surface defects are detected, and to simplify the surface property inspection process and physical distribution. Further, since it is considered that stable casting can be performed at higher speeds than at present, the significance of the present invention is extremely high.

[Brief description of drawings]

FIG. 1 is a schematic diagram of the constitution of the present invention, FIG. 2 is a diagram showing an embodiment of a position where a thermocouple is embedded in a continuous casting mold, FIG. 3 is a graph in which a heat flux distribution is interpolated by a spline function, and FIG. (A) shows a change in heat flux distribution when there is a surface defect, and (b) shows a graph showing a state in which the heat flux distribution does not change when there is no surface defect, and FIG. 5 shows temperature or heat flux. Signs are given to adjacent differences. Fig. 6 (a) shows changes in the distribution, Fig. 6 (b) shows changes in the arrangement of the signs, and Fig. 6 shows changes in temperature or heat flux. Numbers are given to adjacent differences, and FIG. 7A shows the variation of the distribution.
FIG. 7B is a diagram showing the change in the arrangement of the numbers, FIG. 7 is a diagram showing the relationship between the surface defect index and the number of distribution changes, and FIG.
The figure shows the change over time in the minimum point of the temperature or heat flux distribution,
FIG. 9 is a sketch diagram showing a state of a slab surface defect corresponding to a change in temperature or heat flux distribution with time. FIG. 9A shows a case where there is a defect, and FIG. 9B shows a case where no defect occurs. Fig. 10 shows the relationship between the transition state of the heat flux minimum point (minimum point) and the presence or absence of surface defects. The figure (a) is the first charge, and the figure (b) is the second charge. Fig. 11 (c) shows the case of the third charge, Fig. 11 (a) shows the relationship between the maximum temperature change and the time of the slab without surface defects, and Fig. 11 (b) shows the surface defects. It is the figure which matched the maximum temperature change amount with respect to the time of a generated slab, and this with a slab sketch.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hirogo Yamane, Inventor Hiroshima Yamane, 1-chome, Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture (No address) Inside the Mizushima Works, Kawasaki Steel Co., Ltd. (56) Reference JP-A-56-95461 (JP, A) JP 61-200453 (JP, A) JP 63-30146 (JP, A)

Claims (3)

[Claims] 1. A temperature or heat flux at each point in the width direction of a continuous casting mold is measured, the temperature or heat flux distribution is monitored, and a slab of a slab is measured based on a change with time of the temperature or heat flux distribution. An online slab surface defect detection method characterized by determining the presence or absence of a surface defect. 2. The change in the temperature or heat flux distribution with time,
The difference between the temperature and the heat flux from the adjacent measurement point is given a sign of ±, and the online detection is performed based on the arrangement of the signs or the change in the order from the lower temperature or heat flux. ) A method for detecting surface defects on a slab as described above. 3. The change in the temperature or heat flux distribution with time,
The online slab surface defect detection method according to claim 1, wherein the detection is performed online based on a change in the minimum point of each measurement point distribution.