JP3000305B2 - Method for estimating surface defects of slab slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for estimating a surface defect of a slab slab, and more particularly to a method of estimating a surface defect of a medium-carbon or ultra-low-carbon slab slab by using a level detector.

[0002]

2. Description of the Related Art A normal casting situation in continuous casting of a slab will be described with reference to FIGS. The molten steel 2 housed in a tundish or the like is injected into the mold 6 via the immersion nozzle 4. However, as the casting time elapses, as shown in FIG. 2, if the discharge hole 4A of the immersion nozzle 4 loses symmetry due to adhesion of Al ₂ O ₃ or the like or erosion, the discharge flow 2A of molten steel in the mold 6 becomes uneven. Water level 2 at the approaching meniscus 8
B is disturbed. A molten mold powder 10 coats the surface of the molten steel 2 on the molten metal surface, and a powdery powder 10 immediately after being poured is placed on the molten mold powder 10.
A is also floating, so if the surface of the molten metal is disturbed, in addition to the molten mold powder 10, the powdered powder 10A is rolled in,
This may enter between the mold 6 and the solidified shell 12 and become one of the causes of surface defects as non-metallic inclusions. Originally, the role of the mold powder is to cover the surface of the molten steel to prevent oxidation and keep the heat, to lubricate between the mold 6 and the solidified shell 12 and to trap nonmetallic inclusions. The layer of the molten mold powder 10 becomes non-uniform due to turbulence and swelling, lubrication is not sufficiently performed, and heat conduction of the slab to the mold 6 becomes non-uniform. There were also problems such as doing. If the surface properties of the slab are good, hot direct rolling is also possible, but if the surface properties are poor, the slab is cooled to near normal temperature, the slab is cleaned, and then heated again to heat it. Since it is necessary to perform cold rolling, it is an important technique to estimate the presence or absence of a surface defect of a slab and the necessity of maintenance.

Conventionally, in continuous casting of slab cast slabs, a method is disclosed that uses a level gauge to prevent the turbulence and swelling of molten steel as described above and to estimate the presence or absence of surface defects on the cast slabs. Have been. For example, there are JP-A-64-2772, JP-A-63-212054 and JP-A-63-168262. The outline of these conventional technologies will be described. JP-A-64-2772: The gist of the present invention is as follows. That is, "a plurality of thermocouples are arranged in a vertical direction near the surface of the mold on the short side of the mold for continuous casting of slabs and embedded, and a thermometer is disposed on the surface of the surface near the short side of the mold to form a thermocouple. And a change in the temperature and the level of the molten metal in a steady state, and the rate of change in the steady state, and the output from the level gauge are input to a computer. That is, the present invention uses a thermocouple and a level gauge embedded in the center of the short side and the long side of the mold to predict the degree of surface defects of the slab by knowing the fluctuation state of the powder layer. is there. However, in this method, since the detector is provided in the center of the short side and the long side, the detection sensitivity is low when the slab is large, and the symmetry of the discharge hole of the immersion nozzle in the slab thickness direction is impaired. The detection accuracy in the case is also insufficient.

JP-A-63-212054: The gist of the present invention is as follows. That is, "a method for determining a surface defect of a slab, wherein a level gauge is provided on a level of the molten metal near at least both short sides of the continuous casting mold of the slab and the level variation of the molten metal is measured." is there. In the present invention, a level gauge is provided near both short sides. However, since the level is a fixed position, the detection accuracy remains questionable depending on the slab size and the type of immersion nozzle. Similar to the method of -2772, the detection accuracy when the symmetry in the slab thickness direction of the discharge hole of the immersion nozzle is impaired is insufficient.

JP-A-63-168262: The gist of the present invention is as follows. That is, "a light guide connected to an image processing apparatus is attached to a through hole formed in a portion corresponding to a meniscus on a long side or a short side constituting a continuous casting mold, and hot water during casting is passed through the light guide. A level change is detected from the observation image obtained by directly observing the level, and the detected level change is set based on the correlation between the level change obtained in advance and the occurrence of surface defects in the mold. And estimating the occurrence of a surface defect when the detected molten metal level variation exceeds the allowable variation, and estimating a slab surface defect in the continuous casting. The method. " That is, the present invention is a method of providing a detector at a meniscus portion on a long side or a short side of a mold, detecting a fluctuation of a molten metal level, and estimating a surface defect of a slab.
However, this method has a fixed installation position similarly to the method of JP-A-63-212054, and therefore has a problem in detection accuracy depending on the size of the slab and the type of immersion nozzle.

[0006]

In each of the above prior arts, a detector is provided at a meniscus portion on a long side or a short side of a mold to detect a fluctuation amount of a molten metal level to estimate a surface defect of a slab. However, in Japanese Patent Application Laid-Open No. Sho 63-212054, since the installation position of the level gauge is fixed, there is a problem in the detection accuracy depending on the size of the slab and the type of nozzle, and Japanese Patent Application Laid-Open Nos. 64-2772 and 63-168262. Although the amount of fluctuation of the molten metal surface and its pattern due to the position in the width direction of the mold are considered, but the difference due to the position in the thickness direction at a fixed position in the slab width direction is not considered, In addition to poor detection sensitivity, the occurrence of a defect at a locally limited position is not often detected. SUMMARY OF THE INVENTION An object of the present invention is to overcome the above-mentioned problems of the prior art and significantly improve detection sensitivity.

[0007]

The gist of the present invention is as follows. That is, both the long side and the short side of the continuous casting mold are provided with the level detectors, respectively, and the level detectors provided on the long side are at two or more locations on both sides of the immersion nozzle discharge hole, respectively. A limit value for the level change of the molten steel level suitable for the type of slab steel, size, and type of immersion nozzle is provided. If even one of the plurality of detected levels of the molten metal level exceeds the limit value, surface defects occur in the slab. And a method for estimating a surface defect of a slab slab.

An embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a plan view showing a casting state of a narrow slab, and FIG. 2 shows a state in which a deposit 14 is generated in a discharge hole 4A of an immersion nozzle 4, clogging occurs and a flow path of the discharge hole 2A is disturbed. It is a schematic plan view shown. FIG. 3 is a schematic plan view showing molten steel flow 2 during casting of a wide slab. FIG. 4 is a schematic plan view showing a state during casting when the angle of the discharge hole 4A of the immersion nozzle 4 is changed. In the present invention, as shown in FIGS. 1 to 4, each of the long side 6A and the short side 6B of the mold 6 has a level detector 16A, 16B, 16 using a thermocouple or the like.
C and 16D are provided, and at least two detectors 16A and 16C on the long sides 6A and 6C of the mold are provided on both sides of the discharge hole 4A of the immersion nozzle.

[0009] A limit value of the level change corresponding to the type of the slab steel to be cast, the size of the mold, the type of the immersion nozzle, and the inclination angle of the discharge hole is determined. If any one location exceeds the set level limit of the molten metal level, it is presumed that a surface defect has occurred in the slab.

There are various appearances of surface defects of steel. For example, as shown in FIG. 5, a medium carbon steel slab has many corner crack defects, and in the case of a slab for an ultra-low carbon steel sheet, FIG. As shown in FIG.
This is a “bulging defect” which occurs at a position several hundred mm from the corner on the C side. Although the cause of the generation of these surface defects is as described above, details of the slab surface defects generated near the meniscus 8 in the mold 6 will be described with reference to FIG. The molten steel 2 injected into the mold 6 from a tundish or the like is rapidly cooled from a portion in contact with the mold 6, forms a solidified shell 12 and contracts in volume, and forms a gap 18 between the molten steel 2 and the mold 6. As shown in FIG.
Since the molten layer 10 of the mold powder is covered, the gap 18 is filled, and normally functions as a lubricant between the mold 6 and the solidified shell 12. However, since the powder layer 10A is covered on the mold powder molten layer 10, when the molten metal surface is disturbed or swells, the mold powder powder layer 10A is involved to form nonmetallic inclusions. In addition, when the cooling of the formed solidified shell 12 becomes uneven, a surface defect as shown in FIGS. 5 and 6 results. "Scratch" 20 generated in these slabs 12A or
As shown in FIG. 10, there is a strong correlation between a surface defect such as a “bulge defect” 22 generated in the extremely low carbon steel slab slab 12A shown in FIG. .

Therefore, in the present invention, a limit value of the change in the level of the molten metal level, which is suitable for the type of the slab steel, the size of the mold, the type of the immersion nozzle, etc. , 6C, at least 2 places, short sides 6B, 6D
A plurality of level detectors 1 at least at one location
6 is provided to detect the level change of the molten metal level. If any one of the values exceeds the limit value, it is estimated that surface defects 20 and 22 have occurred in the slab 12A.

[0012]

According to the present invention, a level detector is provided on each of the long side and the short side of the mold, and the level detector on the long side is provided on both sides of the discharge hole of the immersion nozzle. Provided at more than one place, further set the limit value of the level of metal level change to suit the type of slab steel, the type of immersion nozzle, if even one point of the plurality of detected level of the level exceeds the limit,
Since the method of estimating the occurrence of surface defects in the slab was taken,
The following effects were obtained. (A) The detection sensitivity of the surface defect of the slab according to the present invention is sensitive. For example, the crack detection rate of the medium carbon material is 65% according to the conventional method, but is 95% according to the method according to the present invention. Could be improved. In addition, the detection rate of blistering defects of extremely low carbon materials,
35% by the conventional method can be increased to 90% by the method of the present invention. (B) The method for estimating a surface defect by the configuration of a molten metal level detector of a cast slab according to the present invention provides a process computer with a large number of determination patterns such as a type of a target surface defect, a detection position, a mold size, a casting speed and casting conditions. Inputting can be performed quickly and accurately, and the operation is simple.

[Brief description of the drawings]

FIG. 1 is a plan view showing a state of detecting a molten metal level during casting of a narrow slab according to the present invention.

FIG. 2 is a plan view showing a situation in which deposits are deposited on a discharge hole of a submerged nozzle and a non-uniform discharge steel flow occurs.

FIG. 3 is a plan view showing a state of detecting a molten metal level similar to that in FIG. 1 during wide slab casting.

FIG. 4 is a plan view showing an installation position of a molten metal level detector according to the present invention in a mold when a discharge angle of an immersion nozzle is changed.

FIG. 5 is a perspective view showing a state of occurrence of a crack defect in a medium carbon steel slab.

FIG. 6 is a perspective view showing a state of occurrence of a blister defect in an extremely low carbon steel slab.

FIG. 7 is a front sectional view showing a conventional continuous casting into a mold.

FIG. 8 is a sectional view taken along the line AA of FIG. 7;

FIG. 9 is an enlarged partial cross-sectional view showing the vicinity of a mold meniscus for explaining a surface defect generation mechanism of a slab.

FIG. 10 is a diagram showing a correlation between a molten metal level fluctuation index and a surface defect occurrence index of a slab.

[Explanation of symbols]

2 molten steel 2A discharge flow 2B molten surface level 4 immersion nozzle 4A discharge hole 6 mold 6A long side 6B short side 8 meniscus 10 mold powder 10A powdery powder 12 solidified shell 14 attached matter 16 (16A, 16B, 16C, 16D) Level detector 18 gap 20 crack defect 22 blister defect

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-168262 (JP, A) JP-A-1-284471 (JP, A) JP-A-64-2772 (JP, A) JP-A-4- 143056 (JP, A) JP-A-62-151256 (JP, A) JP-A-63-212054 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/16 104 B22D 11/18

Claims (1)

(57) [Claims] The present invention relates to a casting mold for continuous casting, in which a long level detector is provided on each of both long sides and a short side, and at least two locations are provided on both sides of the immersion nozzle discharge hole. In addition, a slab steel type, size, and a limit value of the level change corresponding to the type of the immersion nozzle are provided. If even one of the plurality of detected level levels exceeds the limit, the surface of the slab is surfaced. A method for estimating a surface defect of a slab slab, comprising estimating occurrence of a defect.