JPS63212054A - Method for deciding surface defect in cast slab - Google Patents

Abstract

PURPOSE:To quantitatively grasp pattern of variation of molten metal surface and to accurately decide the necessity of cleaning of a cast slab by measuring the variation quantity of the molten metal surface by molten metal surface meters arranged at least near both short sides of a continuous casting mold for the slab. CONSTITUTION:The molten metal surface meters 20 for measuring the variation of the molten metal surface are arranged on the molten metal surface in the mold 1 at two places near the positions of short sides of the mold. The molten metal surface meters 20 having good responsiveness is used there. The pattern of variation obtd. in this way, is compared with the conventional actual result, to decide the necessity of cleaning of the cast slab. In this way, the work can be made to efficient.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for on-line determining the degree of surface defects in slabs during continuous slab casting.

[Conventional technology and its problems]

Figure 2 shows the main parts of the continuous slab casting machine near the slab.
(a) is a horizontal sectional view, and (b) is a vertical sectional view.

The prior art will be explained with reference to this figure.

As shown in FIG. 2, a molten steel flow 4 is discharged from two discharge ports 3 of a submerged nozzle 2 provided at the center of the mold 1, as indicated by arrows. This discharge flow collides with the short side and is divided into two streams, upper and lower, and the upward flow raises the hot water level on the short side. In addition to this, there are small changes in the melt level due to mold vibrations and gas blown from the immersion nozzle, but these are collectively referred to as melt level fluctuations below. A layer 5 of mold powder is formed on the surface of the mold in the mold, which has the functions of preventing oxidation of the molten steel, keeping it warm, lubricating between the slab and the mold, and trapping non-metallic inclusions.

The surface side of this layer is melted and becomes molten powder 6, and the upper side is unmelted and forms powdery powder 7, covering the surface of the hot water. This molten powder 6 mainly plays the role of the above-mentioned powder.

In recent years, high-speed casting of 2 m/win or more has been carried out, and the amount of fluctuation in the molten metal level has also increased, and this problem has come to be greatly focused on. The mechanism of this surface defect generation is thought to be as follows.

Due to the dust rising 8 on the hot water surface shown in Figure 2, the thickness of the molten powder layer on the hot water surface in that area becomes thinner, and the flow of the molten powder between the solidified shell and the mold and the formation of the solidified shell are smooth. Instead, mold powder containing powder is mixed into the solidified shell to form powdery nonmetallic inclusions. In addition, the layer of molten powder between the solidified shell and the mold, which should have the role of lubrication, is uneven and does not provide sufficient lubrication.Furthermore, the heat transfer from the slab to the mold is uneven, causing heat on the surface of the slab. Partly because of the uneven shrinkage, there is a lot of hot water dust rising, especially on the short sides, so many cracks occur.

If the surface properties of the slab are good, it is directly passed through the rolling line and subjected to so-called hot direct rolling (HDR, HotDir).
However, if the surface quality is poor, the slab is cooled to near room temperature, the surface of the slab is treated, and then the slab is heated again and hot rolled.

It goes without saying that if maintenance of the slab is not required, the process will be simplified and this will be a great advantage, but in this case, the problem is how to determine whether or not maintenance of the slab is necessary.

To solve this problem, conventionally, a hot water surface side for controlling the hot water level has been provided in order to keep the hot water level constant by controlling the molten steel injection speed from the immersion nozzle for a constant drawing speed of the slab. By detecting the above-mentioned rising dust on the hot water surface and its wave motion, it is possible to determine whether or not maintenance of the slab is necessary based on the relationship between the level of surface defects, which has been determined in advance through experiments, and the above-mentioned rising dust on the hot water surface and the wave motion. I was judging.

However, since the original purpose of the molten metal surface side lO for controlling the molten metal surface is as described above, it is fixed above the molten metal surface near the immersion nozzle where there is little fluctuation in the molten metal level, so it has a large effect on surface defects of the slab. The amount of variation in the short-side hot water level is not fully understood. Therefore, even though the above-mentioned slab has surface defects, it may be sent to HDR and subsequent processes without being taken care of, resulting in a product that does not meet the specifications.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a method for determining surface defects in a slab, which can determine with high accuracy whether or not maintenance of the slab is necessary.

[Measures for Solving the Problems] The method for determining surface defects in slabs according to the present invention involves placing the molten metal surface side above the molten metal surface at least near both short sides of a slab continuous casting mold. It is characterized by measuring the amount of surface variation.

A plurality of molten metal surface sides arranged above the molten metal surface in the mold measure the amount of molten metal level fluctuation caused by the flow of molten steel injected into the mold, and at each position where the molten metal surface side is arranged, Determine the appropriate range of hot water level fluctuation according to operating conditions from the relationship between the hot water level fluctuation and surface defects obtained in advance from actual values, and compare this with the hot water level fluctuation from the hot water surface side. The degree of surface defects in the slab is determined by using the following methods, and it is determined whether or not surface treatment of the slab is necessary.

〔Example〕

An embodiment based on the present invention will be described with reference to FIG.

FIG. 1(a) is a horizontal sectional view, and FIG. 1(b) is a vertical sectional view, and parts common to those in FIG. 2 related to the prior art are given the same numbers and will be explained unless particularly necessary for the sake of clarity. omitted.

In addition to the hot water surface side 10 for controlling the hot water level, which is already fixed and installed above the hot water surface in the mold, two hot water surface sides 20 for measuring hot water level fluctuations are arranged at two locations near the short sides of the mold. ing. The hot water surface side 20 can be easily moved two-dimensionally to a desired position on the hot water surface, and has faster response than the hot water surface side 10 for controlling the hot water level.

The process for determining whether or not the slab requires maintenance and therefore whether or not it is suitable for HDR (hot direct rolling) when the molten metal surface side 20 is installed in the mold as described above will be explained.

Table 1 shows three typical cases 1, 2, and 3 of variations in hot water level. This is done by dividing the two mold short sides A and B and the immersion nozzle into a part close to the short sides (indicated as "edge") and a part close to the immersion nozzle (indicated as "middle") to increase the amount of fluctuation in the melt level at four locations. , medium, and small.

Table 1 Pattern of hot water level fluctuation The amount of hot water level fluctuation is often large at the positions near the short sides, in Table 1, at the A end and B end. In some cases, there will be more. In order to cope with such a case, the molten metal surface side 20 is periodically moved mainly parallel to the long side, so that all fluctuations in the molten metal level in the mold can be grasped.

The pattern of fluctuation thus grasped can be used to determine whether or not maintenance of the slab is necessary by referring to the relationship between the fluctuation pattern, amount of fluctuation, and surface defects, which have been known in advance by examining actual results.

Regarding the position where the hot water surface side lO for hot water level control is located, the output from the hot water surface side 10 can be used to measure the amount of hot water level fluctuation; In order to cover the entire area, since the hot water surface side 10 is installed so as to be movable up and down, it is only necessary to temporarily move it upward.

〔Effect of the invention〕

The surface of the molten metal according to the present invention can measure the molten metal level fluctuation over the entire area in the mold, and the pattern of the molten metal surface fluctuation can be quantitatively grasped. It is possible to determine whether maintenance is necessary,
This increases the amount of slabs sent to the HDR (hot direct rolling) process, which has a significant effect on reducing the cost of hot-rolled products.

[Brief explanation of the drawing]

FIG. 1 is a view showing the main parts of the hot water surface side and the vicinity of the mold according to the present invention, and FIG. 2 is a view showing the main parts of the mold and the vicinity according to the prior art. 1...Mold, 2...Immersion nozzle, 3...Discharge port,
4... Molten steel flow, 5... Mold powder, 6...
Molten powder, 7... Powder powder, 8... Rising of the hot water level, 9... Solidified shell, 10... Hot water surface side for controlling the hot water level, 20... For measuring hot water level fluctuations. Hot water side.

Claims (1)

[Claims] A method for determining surface defects in a slab, characterized by measuring the amount of fluctuation in the melt level by placing the melt surface above the melt surface at least near both short sides of a continuous casting mold for the slab.