JPS629759A - Method for controlling casting of ingot - Google Patents

Abstract

PURPOSE:To form an ingot having excellent quality by elucidating the association between the distribution condition of defects and steel making conditions, comparing the distribution condition of the defects discriminated by an ultrasonic flaw detection and the standard defect pattern and changing the steel making conditions according to the result of the comparison. CONSTITUTION:Part of the region of the continuously cast ingot or the rolling steel product obtd. from such ingot is subjected to the ultrasonic flaw detection over the entire surface in the transverse direction thereof. The distribution condition of the detects by the result of the flaw detection and the standard defect pattern are compared and the defect condition of the ingot or slab is judged. One or >=2 of the steel conditions such as the casting speed, depth of a immersion nozzle, additives for steel making and others are corrected to eliminate the defects in accordance with the result of the judgement.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention determines the defect state by comparing the ultrasonic flaw detection results of continuously cast slabs or rolled steel products obtained from the slabs with standard defect data. The present invention relates to a slab casting control method that feeds back this information to continuous casting steelmaking conditions.

[Conventional technology]

Generally, testing the performance of a product and feeding it back to the manufacturing process is important for product quality control. In continuous casting, attempts are being made to determine defects in slabs using a simple visual inspection method or ultrasonic flaw detection on the production line, and to reflect the results in steelmaking conditions.

In addition, as a method for determining defects by ultrasonic flaw detection,
There is a technique described in Japanese Patent No. 8-135957.

The technology described in this publication relates to a technology for detecting defects in welded parts. During flaw detection, the height of the reflected echo of the defect and the position of the defect exceeding the threshold value in the X-Y plane and the X-2 plane (when the width direction of the slab is taken as the X-axis) are recorded, and The overall type of defect is discriminated by comparing it with a pattern obtained in advance.

[Problem that the invention seeks to solve]

However, when determining defects in slabs by visual inspection, the defects that can be detected are quite large, and minute defects are overlooked. It was virtually impossible to feed back and utilize the steelmaking conditions.

In addition, in the method of scanning the entire surface of a slab using ultrasonic flaw detection on a continuous casting line to determine defects, the flaw detection speed must follow the casting speed, and the casting speed is usually 600w/5ein or higher. This speed far exceeds the speed of approximately 50 m/sin required for high-precision flaw detection, which has the disadvantage that only large defects can be detected. Therefore, in this case as well, it could not be utilized for feedback to steel manufacturing conditions. In addition, in order to perform ultrasonic flaw detection on slabs on the production line, not only is special equipment required because the slab surface needs to be polished, but it also requires a large number of man-hours and a long time. There was a drawback. That is, there was a drawback that affected productivity.

The welding defect detection technique described in JP-A-58-135957 is similar to the technique of the present invention described later in that the detected defect pattern is compared with a previously obtained defect pattern. There is. However, the flaw detection technology described in the above publication only determines the type of defect, and there is no other description. Even if it is applied to a casting control method that detects defects in a piece and reflects the results of the inspection to steel manufacturing conditions, it will have the same drawbacks as the ultrasonic inspection on the manufacturing line. Even if it were possible to perform flaw detection on continuously cast slabs off-line and reflect the flaw detection results in steelmaking conditions, it would be meaningless unless the technology for the relationship between defects and steelmaking conditions is clarified.

The present invention was developed in view of the above-mentioned conventional circumstances, and it compares the defect distribution state obtained by ultrasonic flaw detection with a standard defect pattern obtained in advance to determine the relationship between the defect distribution state and steel manufacturing conditions. The aim is to improve the quality of slabs and reduce inspection man-hours by elucidating this information and feeding it back to steelmaking conditions.

[Means for solving problems]

The means of the present invention for solving the conventional problems is to carry out ultrasonic flaw detection over the entire width of a continuously cast slab or a part of a rolled steel material obtained from the slab, and detect defects based on the results of the flaw detection. The defect status of the slab or slab is determined by comparing the distribution status of the strands with the standard defect pattern, and based on the results of this determination, the casting speed and immersion nozzle depth are adjusted to correct the defects.

One or more of the casting temperature, steelmaking additives, and other steelmaking conditions are modified.

[For production]

In the present invention, the most important point is that we have clarified the relationship between the distribution state of defects and the steel manufacturing conditions.

If the relationship between the two is not clarified, even if the defect distribution state is clarified, it will not be possible to reflect it in the steel manufacturing conditions, and the objectives of improving quality and reducing inspection man-hours will not be achieved. . Generally, defects in steel materials (mainly non-metallic inclusions) originate from the smelting and ingot-forming processes, and are not completely eliminated until they are passed down to the final product.

Therefore, in the present invention, the distribution state of defects obtained by ultrasonic flaw detection is as shown in FIG.
Concentrated in the center, concentrated on one side.

It is classified into three types: intermittent distribution type. In addition, as shown in Figure 1), the distribution state of defects in the thickness direction is classified into standard type, integrated type, mass type, integrated/concentrated on back surface, 9 concentrated on surface, and As shown in (C1) in the same figure,
The distribution state of large and small defects is determined from standard type, large/mass type, small/
It is classified as a centralized type.

Then, we elucidated the causes of defects in the above classification,
As shown in Table 1 (casting speed, immersion nozzle depth, 5 casting temperature, depending on each generation cause).

By modifying one or more of the steelmaking additives and other steelmaking conditions, the generation of defects during the continuous casting process is suppressed and slabs of excellent quality are obtained.

The method of the present invention will be described in more detail below based on embodiments shown in the drawings. The present invention is directed to a method for controlling the casting of slabs in continuous casting, and can also be applied to hot-rolled materials, thick plates, and other steel materials obtained by hot-rolling continuously cast slabs. We will collectively refer to them as part of the concept of .

〔Example〕

As shown in FIG. 2, defects in the slab in this embodiment are detected by cutting a cropped portion 2 of a steel material 1 obtained by rolling a continuously cast slab into a predetermined length using gas, shears, etc. In this embodiment, the strip-shaped region in the width direction formed except for the fish tail portion is subjected to full-surface ultrasonic flaw detection.
The reason for testing the cropped portion 2 of the steel material 1 is that if a continuously cast slab before rolling is cut to a length necessary for testing and used as a test piece, the amount of steel provided will increase; This is because the yield decreases. In addition, the reason why flaws are detected in a part of the steel material 1 in the width direction is that the conditions during continuous casting and the state of the molten metal layer (steel manufacturing conditions) are closely related to the occurrence of defects. This is because the cause of the defect is passed on to subsequent slabs. The cutting length of the cropped portion 2 is the sum of the length A of the uneven portion of the cropped portion 2, the allowance (margin) B, and the length l necessary for ultrasonic flaw detection.

Ultrasonic flaw detection may be performed by any method such as water immersion method or direct contact method, and the feeding of the probe is as shown in Fig. 3.
The entire surface is inspected by reciprocating in a straight line and moving by a predetermined pitch P at both ends. The pitch P is suitably 1 to 10 days. As for the flaw detection results, the detected position of the defect on the x-y plane and the X-Z plane in FIG. 2 and the height of the reflected echo are recorded with a pen type recording stylus or CRT, and are displayed if necessary.

By performing the above-mentioned full-surface flaw detection, the distribution state of defects is analyzed. Analysis of the defect distribution state is performed from three viewpoints. The first is the planar distribution of defects, the second is the distribution of defects in the thickness direction, and the third is the distribution according to the size of the defects themselves. In the first distribution state, the flaw detection area is I, n, IIl, IV in the width direction of the steel material 1.

(2) Divide the defects equally into 5 sections and observe whether the defects are concentrated in any section.The second distribution state is observed to see how many defects exist in each part in the thickness direction, and the third distribution state is the size of the defects. This is to observe the degree of various defects. The distribution of defects obtained through such observation is shown in Figure 1 (
Compare with standard defect patterns obtained in advance shown in a) to (C). The planar distribution state of defects is shown in figure (a) of the same figure.
As shown in (), each section is classified into three types: standard (uniform distribution) type, concentrated distribution type on both sides, concentrated distribution type on one side, concentrated distribution type on one side, and intermittent distribution type. In addition, the distribution state of defects in the thickness direction is shown in figure (b) of the same figure.
As shown in (c) of the same figure, it is classified into standard type, integrated type, large quantity type, integrated/back surface concentrated type, and 9 surface concentrated type. It is classified into small/large-scale type and small/centralized type.

Next, the relationship between the defects classified as described above and steel manufacturing conditions will be explained with reference to Table 1. In addition, in Table 1, the symbols of each English letter shown in each category of the distribution state of defects in the thickness direction and the distribution state of large and small defects are as shown in Figure 1 (b) and Figure (e).
).

First, the planar distribution state of defects and its response! ! ! The treatment method under II condition will be explained. The distribution of defects in this case was categorized by equally dividing the specimen into five regions I to V in the X-axis direction (width direction), as shown in FIG. 1 (al). Naturally, the equal division of the area is not limited to five.

(l) Standard (uniform distribution) type refers to a state in which defects are uniformly distributed throughout.

In this case, the current steel manufacturing conditions are judged to be good because a product with uniform steel composition and cleanliness can be obtained.
Continue continuous casting while maintaining the same steelmaking conditions.

(2) 0 Both sides concentrated type refers to the case where the defects are concentrated in the area of distribution month and the area of V.

This is because the upward spout of the immersion nozzle points directly to both sides of the scene, so fluctuations in the molten steel supplied from the tundish to the mold occur in the direction of the short side of the mold (
This is because a turbulent flow occurs in the Y-axis direction of the steel material, and inclusions mainly composed of deoxidation products in the molten steel are likely to be captured by the solidified shell in this region.

Another cause is that powder, slag, etc. are more likely to get caught. This phenomenon is likely to occur if the immersion nozzle is too shallow.

It is also related to the fact that the casting speed is too fast, so the casting speed should be made slower.

(3) The centrally concentrated distribution type is the opposite of the above-mentioned both sides concentrated distribution type, and refers to the case where defects are concentrated in the region (■). The downflow collides with the short side and splits into an upflow and a downflow, but the downflow further bends toward the center of the width and collides with the same downflow coming from the opposite side at the center, resulting in the above-mentioned flow at the collision part. This is because inclusions are more likely to be captured by the solidified shell. This type differs from the above-mentioned both-side concentrated type because the immersion nozzle is too deep. Furthermore, if the casting speed is too high, the flow velocity of the above-mentioned downward flow becomes even higher, and the amount of inclusions trapped in the region (2) increases.

However, fluctuations in the molten steel inside the mold are small because the depth of the immersion nozzle is deep, making it difficult for powder, slag, etc. to be caught in the mold.

Therefore, in this case, the steel manufacturing conditions may be treated by making the immersion nozzle shallower and slowing down the casting speed.

(4) 8-side concentrated distribution type refers to a case where defects are concentrated in either the l region or the V region. This is caused by one of the nozzles being blocked, or by a difference in the distance from the nozzle to the left and right short sides, that is, the nozzle jetting speed becomes unbalanced on the left and right sides, and the casting speed is too fast. It is. Therefore, to suppress the unilateral flow of molten steel in the mold and to slow down the pouring speed, specifically, measures such as installing a new nozzle and a steel pipe or correcting the nozzle position are taken.

(5) The zero-intermittent distribution type is a case where defects are concentrated alternately in the areas (1) to (2). This occurs because the direction of the molten steel jet from the nozzle is not perpendicular to the short side of the mold, but tangential to the long side. In other words, the jet flow rotates from the long side surface to the short side surface, and the position where the jet flow attenuates and finally forms a stagnation is ■
Since this is the area marked with and ■, a large number of inclusions are captured there. As a countermeasure, the direction of the immersion nozzle may be corrected.

By the way, casting speed is mentioned as a countermeasure for steelmaking conditions in all cases except for standard type and intermittent distribution type, but this means that if the casting speed is high when the distribution of defects is segregated, segregation will occur. This is because the degree of This is well known to those skilled in the art and will not be described here.

Next, the distribution of defects in the thickness direction and countermeasures for steel manufacturing conditions will be explained. Note that the distribution state in this case is determined for each of the regions I to V shown in FIG. 1 (f8).

(1). The standard type is as shown in figure (b) of the 1st wJ,
A state in which defects are distributed along a gentle curve with the apex at 1/4 of the thickness of the steel material (slab), and the number N of defects distributed at 1/4 of the plate thickness is large because:
This is because the continuous casting machine is of a curved type, and the reason for this is also well known to those skilled in the art, and the explanation will be omitted. In this case as well, a product with a nearly uniform distribution of ingredients can be obtained, so the i! Sewing is performed under IWi conditions, and there is no need to take any other measures.

(2) The integrated type refers to a case where the number N of defects distributed is extremely concentrated at 1/4 of the board thickness. This indicates that a large amount of inclusions were present in the tundish and, by extension, in the molten steel in the ladle, and was caused by insufficient separation and removal of these inclusions. Therefore, one measure to improve steelmaking conditions is to strengthen ladle treatments such as bubbling or vacuum degassing.Another measure is to heat the molten steel in the tundish or to supply the molten steel to the tundish. By setting the water temperature high, etc., the ΔT (
The difference between the casting temperature and the liquidus temperature) can be increased, or Ca can be added in the tundish to reduce the size of defects, and furthermore, contact between the inside of the mold and the outside air can be prevented. The Ar seal may be strengthened to prevent the temperature of the molten steel from dropping, and the pouring speed may be slowed down.

(3) The mass type is similar to the above-mentioned integrated type,
This is a case where the number N of defects increases over the entire width direction compared to the integrated type. This is because there are more inclusions in the molten steel in the ladle and tundish than in the case of the integrated type, and countermeasures need to be strengthened even more than in the case of the integrated type.

(4) The accumulation/back surface concentration type refers to a type in which defects are concentrated on the front surface and the number N of defects is large across the entire width direction. Countermeasures for the steel manufacturing conditions are the same as in the case of the high-volume type described above. However, in order to prevent inclusions from concentrating on the surface area, it is necessary to use a powder with low viscosity and a high rate of sludge formation, and to absorb the inclusions on the surface of the molten steel in the mold. Although only the shape concentrated on the front surface is shown in FIG. 1(b), the shape may also be concentrated on the back surface.

(5) 8 Surface concentration type refers to a case where defects are concentrated on the front side or the back side. The countermeasure for 0M steel conditions, where the molten steel scene in the mold changes and impurities such as powder and slag containing inclusions are dragged into the contact surface with the mold, is to change the nozzle shape to a wider diameter. , or change the ejection direction downward. In addition, by changing the powder to one with higher viscosity, it is possible to suppress fluctuations in the liquid level in the mold.If necessary, the oscillation frequency can be increased to change the shape of the solidified shell at the meniscus to prevent the powder from being entrained. Take measures to create a shape that is less likely to occur. In addition, except for the standard type,
A common measure for all of these is to reduce the casting speed, which is a well-known measure that promotes the flotation and separation of inclusions in the mold and their absorption into the slag.

Finally, the distribution state depending on the size of the defects themselves (particularly the size in the width direction) and countermeasures for the steel manufacturing conditions will be explained with reference to FIG. 1(e).

(1) In the diagram (C) of FIG. 1, a is a standard type.

Although this standard type has many defects of about 100μ,
The number of defects in the range of 00 to 500μ is very small, and the product quality is good. Therefore, maintaining the set steelmaking conditions,
No separate countermeasures are required.

(2) Large-sized/large-volume type refers to a case where the number N of defects in the standard type is large overall, and this is due to insufficient processing in the ladle and insufficient processing in the tundish. Measures to improve steelmaking conditions include strengthening ladle treatments such as bubbling or vacuum gas treatment, increasing ΔT in the tundish, and adding Ca.

What is necessary is to strengthen the Ar seal. The casting speed shall be slow.

(3) Small/intensive defects refer to those in which the number N of small defects of 300μ or less is large, and the number N of smaller defects of about 100μ is extremely large.This is due to inclusions in molten steel. The separation flotation speed depends on the inclusion particle size, and the larger the particle size, the faster it floats, so this small and concentrated type was caused by inadequate flotation removal treatment of inclusions in the ladle and tundish. It is. Therefore, countermeasures under steelmaking conditions should include increasing ΔT and strengthening the Ar seal in the ladle treatment by bubbling and the tundish.
Addition of Ca is not necessary. Also, in this case, measures are taken to slow down the casting speed.

As in the above-mentioned embodiment, the present invention elucidates the relationship between the defect distribution state and steel manufacturing conditions, compares the defect distribution state detected by ultrasonic flaw detection with standard defect patterns, categorizes it, and applies the results to steel manufacturing conditions. By reflecting this in the conditions, high quality slabs can be manufactured. Furthermore, as a result, the number of inspection steps can be reduced.

Next, with reference to FIGS. 4 and 5, an example of a specific defect distribution state and the results obtained when steel manufacturing conditions are reflected thereon based on Table 1 will be described. The casting conditions and flaw detection conditions for the sample are as follows.

A. Casting conditions Steel type: 5S41. Aβ killed steel slab size =
1800m x 220 va x 8000m tundish: 30 ton capacity - Ar gas atmosphere immersion nozzle: upward 2-hole immersion nozzle depth? 120n+ Mold oscillation frequency: 80cpo+Pouring speed: 800m/1Iin ΔT: 20℃ Continuous casting machine: Curved type (curvature 15m) Ladle processing: Not used B, sample size = 1800mm x 19m x 30
0 mC6 ultrasonic deep wound condition method: Water immersion method, oblique angle method (45°) Probe: Point convergence type (convergence diameter 2J5) Sensitivity: 5TB-^2,1”Xl, 8Q%+6db Frequency: 5MHz scanning : Automatic scanning (pitch P-1 cutting) Figure 4 shows the hot rolling of the slab cast under the above-mentioned casting conditions, and the cropped section 2 (see Figure 2) of the rolled steel material as described above. This figure shows the distribution of defects when the flaws are detected under sonic testing conditions. The defect pattern belongs to the concentrated distribution type on both sides. In addition, the pattern (in the figure of Fig. 4) is the distribution state of defects in the thickness direction, and the first
In Figure 1) and Table 1, it belongs to the integrated type. Furthermore, the diagram in FIG. 4 (C1 is the distribution state of large and small defects, and in FIG. 1 (e) and Table 1, the defects belong to the small and concentrated type.

In the experiment, test pieces manufactured under the above-mentioned steel-making conditions were subjected to measures based on the steel-making conditions shown in Table 1 according to each of the above-mentioned types.

Specifically, when a test piece produced under these steelmaking conditions with immersion nozzle depth: 140 mm, casting speed: 700 m/sin, and ΔT: 30°C was subjected to ultrasonic testing in the same manner, no. The defect distribution shown in FIG. 5 was obtained. Figures (a) to (C) of this Figure 5
) correspond to figures (a) to (C) of FIG. 4 above. As is clear from FIG. 5, the number N of defects and the size of the defects themselves have decreased significantly, and
In the table, it belongs to each standard type. In other words, a slab of excellent quality was obtained.

By the way, the present invention is not limited to the above-mentioned embodiments, and can be modified as appropriate. It is possible to add slug conditions, etc. Although the explanation is omitted for reflecting the information on the steelmaking conditions, feedback may be provided directly online, or feedback may be provided in a batch manner.

〔Effect of the invention〕

As explained above, in the present invention, the distribution state of defects! ! ! By elucidating the relationship with steel conditions, comparing the distribution of defects determined by ultrasonic flaw detection with standard defect patterns, and changing steel-making conditions accordingly, we can produce slabs of superior quality. It is possible to do so. Moreover, as a result, it is possible to reduce the number of inspection steps.

[Brief explanation of drawings]

The drawings are all related to the present invention; Fig. 1 is a drawing explaining the distribution of defects, Fig. 2 is a perspective view of the entire slab showing the flaw detection points and cut points of the steel material subjected to ultrasonic flaw detection, and Fig. Figure 3 is a plan view of the cropped part explaining the scanning method of ultrasonic flaw detection, Figure 4 is a diagram showing the distribution of defects according to the flaw detection results of a concrete product, and Figure 5 is the diagram after changing the ism conditions. It is a drawing showing the distribution state of defects according to the flaw detection results of a specific product. 1... Slab (m material) 2... Crop section patent applicant Sumitomo Metal Industries Co., Ltd. Agent
Patent Attorney Toshihiko Uchida (Defect Medium Cμm) Figure 1

Claims (1)

[Claims] 1. Perform ultrasonic flaw detection on the entire width of a continuously cast slab or a rolled steel material obtained from the slab, and compare the defect distribution state based on the inspection results with the standard defect pattern. Determine the defect status of the piece or slab, and adjust one or more of the following steelmaking conditions: pouring speed, immersion nozzle depth, pouring speed, steelmaking additives, and other steelmaking conditions to correct the defect based on the results of this determination. A method for controlling slab casting, characterized by correcting the following.