JPS6219745A - Detecting method for surface defect of continuous cast slab - Google Patents

Abstract

PURPOSE:To detect the defect of the corner part of a slab by providing a temp. sensitive element at each corner part of the mold side constituting the continu ous casting mold for slab, by detecting the respectively opposing two pairs of differential temp. of each corner part and by finding the difference thereof. CONSTITUTION:A mold 1 is composed of the mold side 20 of a long side 2 and short side 3. Temp. sensitive elements 5a-5d, 50a-50d are buried as well at the part of the mold side 20 corresponding to the corner part 4a of a slab. The differential temp. of the opposing two pairs of corner parts with the direc tion of the short axis X as the reference, namely the differential temp. DELTAT1(5a-5b), DELTAT2(5c-5d) are detected for instance and the difference delta in (DELTAT1-DELTAT2) is found. The surface crack is detected as well by taking in advance the difference delta of the case of the breakout (BO) due to the surface crack being caused as the threshold value and by comparing the difference delta in mea surement with the threshold value thereof. Since, therefore, the difference deltain the opposing two pairs of differential temp. DELTAT1 and DELTAT2 of the corner part is compared with the threshold value delta, the surface crack can be detected and the BO can be forcast with high accuracy.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is directed to a continuous casting method for manufacturing square slabs, which is capable of eliminating defects occurring on the surface of the slab, particularly surface cracks occurring at the corners of the slab. This paper relates to a surface defect detection method for early detection. This type of defect detection is necessary to prevent the occurrence of casting troubles such as breakouts (hereinafter referred to as BO) caused by such defects.

[Conventional technology]

As is well known, in continuous casting for producing square slabs, molten steel is poured into a mold having the same cross-sectional shape as the slab, the molten steel is solidified from the surface in contact with the mold, and after being formed into a predetermined cross-section, it is poured into the lower part of the mold. The slab is produced by drawing it out more continuously. The initial solidification state of the molten steel in the mold has an important influence on the continuous casting operation.

For example, if powder used as a lubricant flows unevenly into the mold during the initial solidification process, various defects will occur on the surface of the solidified shell.

It is known that these surface defects tend to concentrate at the corners of slabs, and when these surface defects occur, the strength of the solidified shell decreases, causing molten steel to flow out directly under the mold. Manufactured slabs require care.

In particular, when the BO occurs, it takes a long time to recover, which significantly reduces productivity. In addition, in recent years, efforts have been made to increase the speed of continuous casting and to directly connect continuous casting and rolling processes (hereinafter referred to as direct rolling), but the occurrence of surface defects is a major hindrance to their implementation. It had become. Therefore, it is desirable to detect defects while they are in the mold and to take measures to prevent BO when defects are detected.

By the way, as a conventional method for detecting casting abnormalities such as the surface defects mentioned above, for example, as shown in Japanese Patent Publication No. 59-46703, temperature sensing elements are arranged at close intervals in the width direction of the mold.
An excursion of the mold temperature from a steady level to a lower temperature side detected by the temperature sensing element may cause cracks on the slab surface, and an excursion from a steady level to a higher temperature side may cause rupture of the solidified shell or B.
A common method was to detect O.

[Problems to be Solved by the Invention] In the conventional method, cracks occur on the surface of the slab when the detected temperature of only a portion of the mold where temperature sensing elements are densely arranged in the width direction deviates from a steady level to a low temperature side. It was assumed that this would occur and was detected. For this reason, the detection areas for surface cracks are limited, and there are many cases of misjudgment or oversight, and in particular, surface defects that occur at the corners of slabs can hardly be detected.

The present invention aims to improve the problems of the conventional method, that is, to detect defects at corners.

[Means for solving problems]

The means of the present invention for solving the above-mentioned problems is to embed temperature-sensing elements at substantially the same level in each corner of the side of the mold constituting a continuous casting mold for square slabs, and to embed temperature-sensing elements in advance at the short axis of the mold. Alternatively, a surface crack occurrence threshold is set based on the correlation between the temperature difference ΔTl, ΔT2 difference δ between both corner parts facing each other with respect to the long axis, etc., and the surface cracks in the corner part of the slab that have occurred in the past in actual operation, and then The temperature differences ΔTi, ΔT2 are determined from the temperature detected by the temperature sensing element during continuous casting.
, and the difference δ thereof are sequentially calculated, and when the difference δ falls within the surface crack occurrence threshold, it is detected as a surface crack of the slab.

[Effect]

Now, defects such as surface cracks that occur at the corner portions of slabs and BO caused by such defects are considered to be due to stress concentration occurring at delayed solidification portions of the corner portions of slabs. The solidification delay and stress concentration caused by the solidification delay are influenced by various factors such as the temperature of the molten steel injected into the mold, the amount of taper on the short side of the mold, the inflow of powder and the lubrication state, and the casting speed.

Therefore, as shown in FIG. Temperature-sensing elements 5a to 5d, 50a to 50d such as thermocouples are buried in the areas where the mold temperature changes, and the temperature changes are detected. investigated.

FIG. 3 shows the temperature values detected by the temperature sensing elements 5a to 5d buried in the long side 2 of the temperature sensing elements, and at the point indicated by the broken line a, the BO There has occurred. As can be seen in Fig. 3, even when the BO occurs, there is little change in the individual temperature measurements, and unlike the conventional method, the phenomenon in which the individual temperature measurements are biased from the steady level to the low temperature side is captured and the surface With the method of detecting defects, it was not possible to detect the surface cracks and the BO caused by them.

However, the measured temperature values on the sides are shown in the dashed line X in FIG.
Between the two pairs of corner portions facing each other with respect to the short axis direction shown in, that is, in this example, 5a and 5b, and 5c and 5d,
It was found that the temperatures at the corners of the same long side 2 were significantly eccentric.

Focusing on this knowledge, the inventors first determined the temperature difference between two pairs of corner portions facing each other with respect to the short axis direction We further investigated and researched the correlation between the temperature difference (hereinafter referred to as the temperature difference between corners) and BO caused by surface defects that actually occurred in the past.6 Figure 1 shows an example of the results. The temperature difference ΔTl (5a-5b), ΔT2 (5cm5) between both corners represented by 5a and 5b and 5c and 5d is
d) is expressed as an index (a value obtained by multiplying the temperature difference by a display coefficient) on the vertical and horizontal axes, respectively, to show the relationship with the casting results. In FIG. 1, O indicates a normal slab with no surface defects, and * indicates a slab in which BO has occurred due to surface cracks. Also, the dashed dotted line indicates ΔT1=ΔT2. In other words, the difference δ between the temperature differences ΔTl and ΔT2 between the two pairs of corner portions is zero, and if the difference δ is zero, no surface cracks occur at all. However, when the difference δ determined by, for example, (8T1-ΔT2) became less than 12 or more than +2, BO caused by surface cracks frequently occurred. In FIG. 1, the solid line b1 indicates the difference δ=+2, and the solid line b2 indicates the difference δ=-2. In this example, the difference δ is indicated by the diagonal line G in FIG.
Within the range of -2 to +2 shown by , it was possible to produce normal slabs with no surface defects. On the contrary, it was found that when the difference δ is outside the shaded area 6, the rate of occurrence of BO due to surface defects is extremely high. In the present invention, the area outside the six hatched areas is defined and used as the slab surface defect occurrence threshold (hereinafter simply referred to as defect occurrence threshold).

The defect occurrence threshold is determined in advance according to various operating conditions such as the size and steel type of the slab, the temperature of the molten steel poured into the mold, the amount of taper on the short side of the mold, the type and amount of powder used, and the casting speed. All you have to do is find it and set it appropriately.

Therefore, the temperature differences ΔTi and ΔT2 are determined based on the temperature detection value detected during continuous casting by a temperature sensing element embedded in the corner part of the mold side 20, and the difference δ is calculated from the ΔTi and ΔT2.
is calculated sequentially, and the error δ is compared with the defect occurrence threshold,
A surface defect may be detected when the difference δ falls within the defect occurrence threshold.

As the temperature sensing element, in addition to the above-mentioned thermocouple that directly measures the temperature of the side of the mold, for example, a well-known heat flux meter that measures heat flux from the amount of heat passing per unit area of the side of the mold may be used. It is also possible to change the thickness of the mold side,
It is effective because the initial solidification state within the mold can be accurately detected without being affected by changes in the absolute temperature of the mold side due to changes in the temperature of the cooling water.

By the way, the buried position of the temperature sensing element is long side 2 or short side 3 if it corresponds to the corner part 4a of the slab 4 to be manufactured.
Either is fine. In the experience of the present inventors, the corner 4 of the slab 4
Approximately 150 mm from b to long side 2.

For short side 3, it was possible to effectively exhibit the above function as long as it was within about 501 Illn. In a well-known variable width mold in which the slab width can be changed by moving the short side 3, it is preferable to embed a temperature sensing element in the short side 3. It is also possible to adopt a method in which a plurality of temperature sensing elements are buried at predetermined intervals, and the temperature sensing element buried in the optimum position is selected and used depending on the slab width at the time of operation. be. In addition, in the casting direction, it is preferable to lower the surface level in the mold 1 by about 100+ nm or more, and within this range, one step or multiple steps at appropriate intervals in the casting direction.
The above judgment may be made at each stage.

Furthermore, as shown in FIG. 2, the distance between the opposing corners for determining the temperature T1 and ΔT2 is in the short axis direction
, or the long axis direction Y, or depending on the situation, it is possible to set the center point Z of the cross section of the mold as a reference. Table 1 below shows typical combinations.

Table 1 In Table 1, the same marks (○ or x) represent opposing corner parts.

〔Example〕

The present invention was carried out in a continuous casting facility that manufactures slabs with a width of 1000 mm and a thickness of 250 mm.

In this example, as shown in FIG. 4, a thermocouple was embedded as a temperature sensing element 5 on the long side 2 at a distance of about 50 mm from the corner 4b of the slab 4. The buried position of the thermocouple in the casting direction was 200 mm below the scene level, and in one step.

Temperature difference ΔT1 (5a 5b) between the corners when two-surface cracking actually occurred during normal operation using the above mold at a casting speed of 1.6 m/min.

ΔT2 (5cm5d) and its difference The defect occurrence threshold was investigated from δ (ΔT1−ΔT2).

FIG. 5 shows the defect occurrence threshold set based on the above investigation results, and the area other than the diagonal and IG is the defect occurrence threshold.

Next, the temperature differences ΔTi and ΔT2 between the corners and the difference δ between the corners were successively calculated from the temperature values measured by the thermocouple during operation. FIG. 6 shows the variation of the difference δ, and the difference δ became the defect generation threshold at time t1. Therefore, slab 4
It was determined that a surface defect had occurred, and an alarm was immediately issued and the casting speed was reduced to 0.5 m/min. As a result, BO could be prevented. By continuously monitoring the occurrence of surface defects in this way and continuing operations.

There was no BO caused by surface cracks.

〔Effect of the invention〕

By implementing the present invention, it is now possible to accurately detect surface cracks that occur at the corners of slabs, and the BO
It has become possible to predict with high accuracy.

As a result, it was possible to completely prevent adverse effects such as deterioration in the quality of the slab, drop in the temperature of the slab, and poor matching with subsequent processes, which were caused by operational actions due to misjudgment that frequently occurred in the past.

[Brief explanation of drawings]

FIG. 1 is a graph showing an example of the defect generation threshold obtained based on the present invention, FIG. 2 is a cross-sectional view of a mold showing a situation in which a temperature-sensitive element is embedded in the well-known side of the mold, and FIG. 3 is the same as that shown in FIG. It is a graph which shows the transition situation of the temperature value of each temperature sensing element in . Figures 4, 5, and 6 show specific examples based on the present invention. Figure 4 is a cross-sectional view of the mold showing how the temperature sensing element is buried, and Figure 5 shows the defect occurrence threshold. The graph in FIG. 6 is a graph showing the change in the difference δ. 1: Mold 2: Side 3: Short side 2o: Mold side 5a-5d, 50a-50b: Temperature sensing element East 1 Figure ΔT1 (Horikei ○ Procedural correction band (self-proposed) P/Mon 21, 1985: Sun 3, Relationship with the case of the person making the amendment Patent applicant address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (665) Nippon Steel Corporation Representative Takeshi 1
) Yutaka 4, Agent 103 Phone: 03-864-60
52 Address 2-27-6-5 Higashi Nihonbashi, Chuo-ku, Tokyo Subject of amendment Claims column 6 of the specification
, Contents of the amendment (1) The entire text of the Claims column on the first page of the specification is corrected as follows. ``2. Claimed scope In each corner of the mold side constituting the continuous casting mold for rectangular slabs, a temperature sensing element is embedded at approximately the same level, and two pairs of opposing temperatures of each corner are preliminarily determined. Difference ΔTi
, ΔT2, and the surface cracks at the slab corners that occurred during actual operation, a surface crack occurrence threshold is set, and then the temperature difference ΔTi, ΔT2 is determined from the temperature detected by the temperature sensing element during continuous casting. , and the difference δ thereof are sequentially calculated, and when the difference δ falls within the surface crack occurrence threshold, it is detected as a surface crack of the slab. "that's all

Claims (1)

[Claims] Temperature-sensing elements are embedded at approximately the same level in each corner of the mold side constituting the continuous casting mold for square slabs, and the temperature difference ΔTi between two opposing pairs of each corner is set in advance.
, ΔT2, and the surface cracks at the slab corners that occurred during actual operation, a surface crack occurrence threshold is set, and then the temperature difference ΔTi, ΔT2 is determined from the temperature detected by the temperature sensing element during continuous casting. , and the difference δ thereof are sequentially calculated, and a slab surface crack is detected when the difference δ falls within the surface crack occurrence threshold.