JPH0259157A - Method for detecting surface defect on continuous casting round cast billet - Google Patents

Abstract

PURPOSE:To detect surface defect on a round cast billet by dividing wall face of a mold into plural steps to height direction and also arranging thermocouples at the same level to circular direction in each division. CONSTITUTION:The wall face of the mold 1 is divided into three to the height direction and the thermocouples 2a-, 3a-, 4a-, are embedded at almost equal interval to the circular direction in each division. Measured temp. values to the height direction t2i, t3i, t4i... show temp. hysteresis at the same position of the cast billet, respectively, and these difference rates t2i-t3i, t3i-t4i... show changing degree of cooling and at the time of being large difference, the crack can be detected. Further, by comparing the measured results to the circular direction and at each step, the crack caused by second factor can be detected.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for detecting surface defects in continuous round slabs, and in particular, the present invention relates to a method for detecting surface defects in continuous round slabs, and in particular, detects multiple surface defects in round slabs that are a major problem in terms of operation and quality during the production stage where round slabs are continuously cast into V-formers. The present invention relates to a detection method based on fluctuations in temperature values detected by thermocouples.

[Conventional technology]

Continuous casting technology has made remarkable progress in recent years, and in particular, technology for manufacturing round slabs by continuous casting has been adopted recently. In addition, when continuously casting round slabs, molds such as tubular molds are used, but during the continuous casting process of round slabs, the solidified shell ruptures and the internal molten steel elutes, causing the round slabs to deteriorate. Surface defects may occur. When such surface defects occur, operations are interrupted and quality problems such as surface flaws and internal cracks occur in the product. On the other hand, when detecting surface defects in continuous round slabs,
The method used is mainly to predict the occurrence of vertical breakout. As a conventional example of a method for detecting the surface defects of a round slab at the manufacturing stage as described above, for example, Japanese Patent Application Laid-Open No. 62-220257
The technique disclosed in the publication No. 1 is known. This technology uses multiple pieces buried in the circumferential direction of the mold wall. ! The presence or absence of a breakout is determined from the fluctuations in individual temperature values detected by the electrocouple, and when a breakout is predicted to occur, the casting speed and amount of cooling water are controlled to prevent a breakout. However, in the conventional example described above, the position where vertical cracking breakout occurs in the round slab does not necessarily match the fluctuation in the temperature value detected by the thermal lightning pair, making it impossible to accurately detect vertical cracking breakout. There was a big flaw. That is, FIG. 6 shows the temperature measurement fii detected by a plurality of thermocouples embedded in the circumferential direction of the mold wall surface in the above conventional example.
! This is a diagram showing the relationship (i.e., temperature history) between fT (unit: °C) and casting time t (unit: seconds). Even during time periods t1 and t2 when temperature measurement giT is stable, the longitudinal direction of the round slab is A cracking breakout has occurred, and even if temperature changes are continuously detected, it is difficult to detect surface defects such as cracks. It is clear that surface defects cannot be identified by (hunting) alone. In this way, in conventional methods, surface defects in round slabs cannot be detected accurately, making it difficult to remove surface defects from round slabs, making it difficult to roll without inspection or unmanned. It was becoming difficult. In addition, if rolling is forcibly carried out without inspection or unmanned under such circumstances, there is a drawback that the number of defects in the product increases, resulting in a decrease in yield and, in turn, an increase in manufacturing costs. Therefore, there is a strong desire to develop a method that can accurately detect surface defects in round slabs at the manufacturing stage, in order to ultimately reduce manufacturing costs by performing rolling without inspection or by hand. .

[Problem to be achieved by the invention]

The present invention has been made in view of this situation, and its object is to provide a method for detecting surface defects in continuously cast round slabs that can accurately detect surface defects in round slabs during the manufacturing stage. .

[Means to achieve the task]

The present invention is a method for detecting surface defects in continuously cast round slabs, in which the wall surface of the mold is divided into multiple stages in the height direction, and thermocouples are arranged in substantially the same position in the circumferential direction for each division. The difference between the maximum and minimum temperature values in the circumferential direction detected within a predetermined time by multiple thermocouples in the same direction section exceeds a predetermined value, or thermocouples at approximately the same position in the circumferential direction The above-mentioned problem has been achieved by determining that a surface defect has occurred in a round slab when the difference in temperature values detected in a pair in the height direction exceeds a predetermined value. .

[Effect]

The present invention was made based on the fact that the vertical cracking breakout of round slabs is also affected by the temperature history from when the slab starts solidifying in the mold until it exits the mold. be. That is, the inventor considered the phenomenon shown in FIG. 6 as follows. The reason why vertical cracking breakout occurs in round slabs is, firstly, one point in the circumferential direction of the round slab is locally cooled too much and cracks, and secondly, cooling is slow and the solidified shell is cracked. This is a lick where the thin part cracks due to the tensile force associated with solidification and shrinkage. Therefore, although it is possible to detect the occurrence of vertical cracking breakout to some extent from the fluctuations in the temperature values detected by multiple thermocouples arranged circumferentially on the mold wall surface, the fluctuations in the temperature values Even if the amount of cracking is small, if the mold and slab are not in contact for a certain period of time, fir cracking should occur. It is also necessary to investigate whether there was any further contact. Furthermore, by examining the temperature fluctuations in the circumferential direction and the temperature fluctuations in the height direction of the mold wall surface, it is thought that vertical cracking breakout due to the second factor described above can also be detected. Therefore, in the present invention, the wall surface of the mold is divided into a plurality of stages in the height direction, and thermocouples are arranged in substantially the same position in the circumferential direction for each section. are arranged in a matrix. In other words, it is possible to detect vertical cracking breakout in a round slab to some extent by just using multiple thermocouples buried in the same vertical section in the circumferential direction of the mold wall surface, but if By detecting the temperature history of the round slab with thermocouples buried in approximately the same position, vertical breakout of the round slab can be accurately detected. Regarding the circumferential direction, a plurality of thermocouples are used for each section divided in the height direction of the mold wall surface to determine, for example, the predetermined time required for the slab at the start of solidification to pass through the mold (at the casting speed Vc,
Time required for the slab to pass through length 1 of the mold J/Vc
) or more, and the difference ΔT between the maximum and minimum temperature values is
For example, it is monitored whether the following formula (1) is satisfied. When it is determined that the following formula (1) is satisfied through such monitoring, it is determined that a surface defect has occurred in the round slab, and the round slab is placed on an inspection line. ΔT>a/d-bu (1) However, a satisfies 2xlO'<a<6xlO',
This is called the crack sensitivity constant, and as shown in Figure 2, the constant d, which is determined by the steel type of the round slab, is a value taken to check the detection sensitivity of the thermocouple when measuring temperature. The installation depth of the pair (depth from the surface) (ll16),
It is used to evaluate the degree of development of the solidified shell, and is the distance of the thermocouple from the meniscus (nua).If the thermocouple installation depth d is constant, it is also possible to evaluate using only °C. be. By the way, the above equation (1) was obtained through experiments as follows. Figure 2 shows 5 vertical breakouts.
This is a diagram obtained by experiment with the temperature difference on the mold wall surface when the temperature difference occurs at 0% or more. In the diagram, the vertical axis is the temperature difference at the same height (
In other words, it is the difference (ΔT) between the maximum and minimum temperature values detected within the above predetermined time by multiple thermocouples buried in the circumferential direction of the same category, and the horizontal axis is the thermocouple is the product (dXJ2) of the thickness of the mold at the location where it is installed (d This is a region where no breakout occurs, and region B has a vertical cracking breakout of 50
% or more, and region C is a region where vertical cracking breakouts occur frequently (for example, 80% or more). Furthermore, similar experiments were carried out by changing the g4 types of round slabs, and a distribution diagram (not shown) in which region B in Figure 2 was shifted toward the lower left or upper right of Figure 2 was obtained. . From these experimental results, when a is a constant determined by the steel type of the round slab (so-called vertical crack sensitivity constant), under the conditions of 2xlO'< a <6xlO', in region B of Fig. 2, etc. It turns out that (ΔT)x(dxA)>a holds true. From this formula, the vertical cracking breakout of a round slab is 50%.
When the above occurs, the temperature difference ΔT in the circumferential direction of the mold wall surface becomes ΔT>a/(d−β), and the above equation (1) is derived. In the experiment for creating Figure 2, as shown in Figure 1, the wall surface of the mold 1 was divided into three sections in the height direction, and six thermocouples 2a to 2 were placed in each section in the circumferential direction. 2f, 3a-3f
, 4a~4f (2d~2f, 3d~3f, 4d~4
f (not shown) was buried at approximately the same time. Also, these thermocouples each have 1801111.340 from the meniscus.
At a distance of IllIM and 540 nn, six thermocouples were arranged in the circumferential direction of the mold wall, resulting in a total of 18 thermocouples arranged in a matrix on the mold wall. Furthermore,
180111.340 n11 from meniscus respectively
, and 540), the thickness of the mold copper plate was 911.7 mm and 5 mm, respectively. Note that when the above formula (1) holds true, measures such as controlling the casting speed and adjusting the cooling of the mold are taken to avoid further occurrence of breakout. or,
The accuracy improves as more thermocouples are embedded in the circumferential direction of the mold wall surface, but three or more thermocouples are sufficient. On the other hand, in the height direction, in order to check the thermal history, thermocouples are provided in at least two stages, and the thermocouples are arranged at the same position in the drawing direction of the round slab. Then, a crack can be detected when a difference in temperature values in the height direction (for example, adjacent ones) detected by thermocouples at substantially the same position in the circumferential direction exceeds a predetermined threshold. That is, the temperature measurements in the height direction tzi, t3i, tri...
(Fig. 1) shows the temperature history of the same part of the slab, and this difference (tzi t31), (t3i
A large or small value of j4i>... represents a sudden change in cooling, so a crack can be detected when this difference is large. Furthermore, by comparing the measurement results in the circumferential direction and at each stage, it is also possible to detect cracks caused by the second factor.

[Examples] Hereinafter, the present invention as described above will be described in more detail with reference to the drawings. FIG. 3 is a diagram for explaining this embodiment. In the figure, 1 is made of a copper plate material and is made of a tubular mold with an outer diameter of 175 nm and a thickness of 5I, and 2a to 2C (and 2f to 2f, not shown) are molds in the circumferential direction from the meniscus to the position of 18 (Il) on the mold wall surface. Thermocouples 3a to 3C (and 3d to 3f not shown) are buried at approximately equal intervals in
) are thermal pairs buried at approximately equal intervals in the circumferential direction at positions 34 Oram from the meniscus in the mold wall surface. In Figure 3, from the time when molten steel is poured into mold 1 and the slab begins to solidify! The temperature was measured using IL, couplers 2a to 2f, and 3a to 3f. For the temperature measured in this way, every 2 minutes,
Find the difference ΔT' between the temperature values of the thermocouple corresponding to the height direction,
A graph of the relationship between the temperature difference ΔT' in the height direction and the casting time t is shown in FIG. 4. That is, FIG. 4 is a diagram showing the relationship between the temperature difference ΔT' in the height direction and the speed time t. When the temperature difference ΔT' becomes 40°C or more, the vertical cracking breakout occurs at 80 It was confirmed that this occurs with a probability of more than %. On the other hand, FIG. 5 is a diagram showing the results of tracking the product care index from March 1985 to May 1986 by applying the present invention to a round slab continuous casting machine. In this figure, the smaller the value of the product care index on the vertical axis, the better the product, ie, the less occurrence of vertical breakout. As is clear from FIG. 5, the present invention was implemented to perform non-inspection rolling by changing the casting speed based on longitudinal crack prediction.
Since November 1998, the product care index (external surface defect occurrence rate) has decreased significantly, and by implementing the present invention, vertical cracking breakout of round slabs (i.e., surface defects of round slabs) has been reduced significantly. It can be seen that the occurrence is significantly reduced. It should be noted that the present invention is not limited to the above-described embodiments, and can be modified in various ways. A threshold value may be provided, and whether or not vertical breakout has occurred may be determined based on whether or not the threshold value is exceeded. Effects of the Invention As explained in detail above, according to the present invention, it becomes possible to detect the temperature history inside the mold during the manufacturing stage of the round slab, and the temperature history in the mold can be detected more accurately than in the conventional example. surface defects can be detected. Therefore, it is possible to significantly reduce the occurrence of surface defects in round slabs during the manufacturing stage while performing rolling without inspection or manually, and even if surface defects have already occurred, further occurrence can be effectively prevented. . In addition, the inflow status of mold powder can be estimated by predicting the occurrence of breakout, which is useful for determining the quality of mold powder and determining the appropriate rM of mold powder.
It has the advantage that it can also be used for

[Brief explanation of the drawing]

FIG. 1 is a perspective view for explaining the present invention, and FIG. 2 is a perspective view for explaining the present invention.
A diagram showing experimentally determined temperature differences on the mold wall surface when 50% or more of cracking breakout occurs; Figure 3 is a perspective view for explaining the present invention in detail; Figure 4 is a height Figure 5 is a diagram showing the relationship between the difference in temperature values in the direction and casting time. Figure 5 is a diagram showing changes in the product care index. Figure 6 is
FIG. 2 is a diagram showing the relationship between temperature measurements continuously detected by thermocouples embedded in the circumferential direction of the mold wall and casting time. 1...Mold, 2a-2c, 3a-3c, 4a-4c--
·thermocouple. Figure 2 Mae also input fsu O Tsuyarage 01 Nona 1st generation C6/Matai Figure 3 Figure tog. t (Suzo 84 intervals (什)) Toden X, Se, 43 low! =)

Claims (1)

[Claims] (1) In a method of detecting surface defects of the round slab based on temperature values detected by a plurality of thermocouples arranged on the wall surface of the mold during continuous casting of the round slab, the method comprises: The wall surface is divided into multiple stages in the height direction, and thermocouples are placed at approximately the same position in the circumferential direction for each division, and the thermocouples in the same height direction are detected within a predetermined time. The difference between the maximum and minimum temperature values in the circumferential direction exceeds a predetermined value, or the difference in temperature values in the height direction detected by thermocouples at approximately the same position in the circumferential direction exceeds a predetermined value. A method for detecting surface defects in continuously cast round slabs, characterized in that when the value exceeds a value, it is determined that a surface defect has occurred in the round slab.