JP2003275852A - Method and apparatus for continuously casting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for continuously casting a steel with which mixed layer of mold flux and oxide scale on the surface of a cast slab causing uneven cooling in a secondary cooling zone can efficiently by removed. <P>SOLUTION: The mold flux and the oxide film (the mixed layer 6 thereof) stuck on the surface of the cast slab 5 are removed at the interval from a mold 1 to first support rolls 4. It is desirable that this removal is performed so as to collide water jet stream with the collide pressure of ≥10 N/cm<SP>2</SP>on the cast slab surface by using a high pressure water spray nozzle 2. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel continuous casting method and apparatus, and more particularly to a steel continuous casting method and a steel continuous casting method capable of effectively preventing the occurrence of slab surface layer defects due to uneven cooling in the upper part of the secondary cooling zone. Regarding the device.

[0002]

2. Description of the Related Art When a steel slab is manufactured by continuous casting using a water-cooled mold, molten steel is pulled out while solidifying in the mold. However, this solidified shell has an outer surface with a certain degree of uneven cooling. As a result, unevenness is likely to occur, resulting in further uneven growth of the solidified shell. In the worst case, vertical cracks occur on the surface of the slab in the mold or in the secondary cooling zone.

Such vertical cracks on the surface of the slab are frequently generated in medium carbon steel mainly having a carbon content of 0.10 mass%. When vertical cracks occur, it is necessary to remove flaws and defects (hereinafter referred to as "maintenance") before sending the slab to the rolling process. Since the tendency of defects to occur significantly increases as the casting speed increases, the casting speed in today's general slab continuous casting machines has increased, for example, by about 1.5 to 3 times compared to 10 years ago. Also contributes to an increase in maintenance work.

Therefore, the unevenness of solidified shell growth due to uneven cooling is an obstacle to the application of direct feed heating (hot charge) or direct feed rolling (direct charge), which is being widely applied due to its excellent economic efficiency. At the same time, it is an obstacle to high-speed casting, which is a requirement for productivity improvement. In order to prevent the occurrence of surface vertical cracks in such a continuous cast slab, the initial stage of solidification (hereinafter,
In the initial solidification), it is considered important to carry out uniform slow cooling to homogenize the solidified shell growth and suppress the formation of "claws" (collapse of the solidified shell). The technology is disclosed.

On the other hand, uniform cooling in the secondary cooling zone is also
Although it is a very important factor for obtaining a sound slab, there is only a cooling method such as mist spray, which sprays spray water uniformly on the slab.

[0006]

By the way, according to the research and study by the present inventors, even if uniform cooling is achieved in the mold and in the secondary cooling zone, high-speed casting such that the casting speed exceeds 2 m / min. In the above, the case where uniform cooling of the slab is hindered frequently occurs for the following reason. That is, due to the mold flux discharged from the mold and non-uniformly attached to the slab, 1) an increase in the scale generation rate on the slab surface, 2) adhesion due to a decrease in the melting point of the scale on the slab surface and the mold flux mixed layer This is a non-uniform cooling of the cast slab due to the unevenness of the thin and thick oxide scale layers on the surface of the cast due to the mechanism of strengthening the properties.

Oxide scale has a thermal conductivity that is about an order of magnitude smaller than that of the solidified shell, and causes non-uniform cooling as a heat transfer resistance layer in the secondary cooling zone after the mold. This non-uniform cooling is one of the causes of surface vertical cracking of the slab and fluctuation of the molten metal level in the mold due to unsteady bulging, and it acts as a factor that hinders high quality maintenance and stable operation in the continuous casting process of steel. . FIG. 3 is a graph showing the correlation between the amount of mold flux attached to the slab and the surface quality of the slab in such a case. Here, as the data on the horizontal axis of FIG. 3, a value calculated from the concentration of CaO 3 contained in the short side surface scale of the cast slab before the hot rolling furnace was used.

The present invention provides a continuous casting method and apparatus for steel capable of effectively removing the mixed layer of mold flux and oxide scale on the surface of the cast slab, which causes non-uniform cooling in the secondary cooling zone. With the goal.

[0009]

DISCLOSURE OF THE INVENTION The present invention is characterized in that, in a continuous casting method for steel, the mold flux and oxide film adhering to the surface of the slab between the mold and the first slab support roll are removed. It is a continuous casting method for steel. It is preferable that the mold flux and the oxide film are removed by causing a water jet to impinge on the surface of the slab with a collision pressure of 10 N / cm ² or more. Further, the present invention, in the continuous casting apparatus for steel, has high-pressure water spraying means for causing a water jet to impinge on the surface of the slab between the mold and the first slab support roll at a collision pressure of 10 N / cm ² or more. It is a continuous casting device for steel characterized by the above.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the non-uniform cooling in the secondary cooling zone, which is non-uniformly adhered to the surface of the slab from the mold to the first slab support roll which becomes the inlet of the secondary cooling zone Since the flux (mold flux) and oxide film (oxide scale), which are the facilitating factors, are removed, the spray cooling in the subsequent secondary cooling zone is made uniform and strengthened, and the solidification of the slab is uneven or local. The solidification delay can be eliminated, and defects such as vertical cracks on the surface of the slab and instability factors can be reduced.

As a means for removing the flux and the oxide film, any physical method can be applied. For example, a method of spraying high-pressure water on the surface of the slab (a method of spraying high-pressure water), a method of pressing the surface of the slab with a roll having irregularities, a method of rubbing the surface of the slab with a brush of high-temperature strength alloy wire, etc. Can be mentioned. However, running cost, certainty of effect, ease of maintenance, ease of equipment construction,
Furthermore, when considering the effect of improving the productivity of the continuous casting machine by strengthening the cooling by high collision force spray as an additional effect,
A method using high-pressure water spray is preferable.

In this method using high-pressure water spray, as shown in FIG. 1, for example, as shown in FIG. 1, a jet of water impinges on the surface of the slab 5 between the mold 1 and the first slab support roll 4 (1st). This can be carried out by disposing the high-pressure water spray nozzle (high-pressure water spraying means) 2 to be provided. However, the collision pressure of the water jet on the surface of the slab is
If it is less than 10 N / cm ² , the mixed layer 6 of the flux and oxide film adhering to the cast piece 5 discharged from the mold 1 is destroyed,
Since it cannot be removed, the collision pressure of the water jet on the surface of the slab is 10N.
It is necessary to determine the spraying conditions such as the nozzle type of the high-pressure water spray nozzle 2, the spray distance, the nozzle back pressure, and the sprayed water amount so that the spray pressure is not less than / cm ^2, and spraying.

As a nozzle type for obtaining a high collision pressure, for example, a descaling type which forms a substantially linear spray pattern as shown in FIG. 2A is preferable. On the other hand, the normal secondary cooling spray nozzle 3 as shown in FIG. 1 is a flat type that forms an elliptical spray pattern as shown in FIG. 2 (B). With a flat type nozzle
It is difficult to obtain a collision pressure of 10 N / cm ² or more.

In the present invention, the upper limit of the collision pressure is not particularly limited, but it is possible to apply an excessive collision pressure to a high temperature solidified shell (solidified shell) having a thickness of about 10 to 20 mm. Since there is concern about damage to operations such as steel leak accidents due to shell breakage, it should be avoided as much as possible. Further, the present invention manifests its effect remarkably in a high-speed casting operation in which the casting speed (slab drawing speed) is 2 m / min or more, so that it is preferable to carry out the present invention during such a high-speed casting operation.

[0015]

Example A medium carbon steel containing 0.11% by mass of carbon and having a high cracking susceptibility was prepared by the continuous casting apparatus having the configuration shown in FIG.
When casting under the conditions of a molten steel superheat degree of 25K and a casting speed of 2.4 m / min, a high pressure water spray nozzle 2 was used as shown in FIG.
Using two types of descaling type A and flat type B that form the spray patterns of (A) and (B) respectively,
A water jet was sprayed on the slab under various conditions shown in Table 1. The amount of water sprayed was 70 L / min per 1 m of slab width under each condition.

Table 1 shows the quality primary failure index (based on the condition C1) due to the occurrence of vertical cracks in the slab investigated on the outlet side of the secondary cooling zone under each condition. Impact pressure of water jet slab at 10 N / c
In the comparative examples (conditions C1 to C3) in which the mixed layer 6 of the flux and the oxide film could not be destroyed and removed because it was less than m ² , a slight improvement was observed in the condition C3, but sufficient quality could be obtained. Not done. On the other hand, the embodiment of the present invention (conditions E1 to E) in which the cast layer collision pressure of the water jet was set to 10 N / cm ² or more to destroy and remove the mixed layer of the flux and the oxide film
In 4), a remarkable quality improvement effect could be obtained.

[0017]

[Table 1]

Further, when a general low carbon steel containing 0.04% by mass of carbon was cast under the same trial conditions as above, in the comparative example, fluctuations in the molten metal surface in the mold due to unsteady bulging in the secondary cooling zone were scattered. On the other hand, in the Examples, uniform strong cooling in the secondary cooling zone was promoted, the shell thickness was increased, and unsteady bulging was suppressed, so there was no such fluctuation in the molten metal level in the mold.

[0019]

According to the present invention, in the continuous casting of steel, the secondary cooling which is unevenly adhered to the surface of the slab from the mold to the first slab support roll which becomes the inlet of the secondary cooling zone. Since the flux and oxide film, which are factors that promote cooling non-uniformity in the strip, are removed, the spray cooling in the subsequent secondary cooling zone is made uniform and enhanced, resulting in uneven solidification of the cast slab or local It has an excellent effect that the solidification delay can be eliminated, and defects such as vertical cracks on the surface of the slab and unsteady factors such as unsteady bulging can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the present invention.

FIG. 2 is a high-pressure water spray (A) according to the present invention and a normal secondary cooling spray (water spray or mist spray).
It is a spray pattern comparison figure with (B).

FIG. 3 is a graph showing the correlation between the amount of residual flux on the surface of a slab and the quality reject index due to the occurrence of vertical crack defects, which is the basis of the present invention.

[Explanation of symbols]

1 Mold (water-cooled copper plate mold) 2 High-pressure water spray nozzle (high-pressure water spraying means) 3 Ordinary secondary cooling spray (water spray or mist spray) nozzle 4 Cast strip support roll 5 Cast strip (continuous cast strip) 6 Mixed layer of flux (mold flux) and oxide film

Claims (3)

[Claims] 1. A continuous casting method for steel, comprising removing a mold flux and an oxide film adhering to the surface of the slab between the mold and the first support roll. 2. The mold flux and oxide film are removed by causing a water jet to impinge on the surface of the slab at a collision pressure of 10 N / cm ² or more.
A method for continuously casting steel as described. 3. A continuous casting device for steel, wherein the slab surface between the mold and the first support roll immediately below the mold is 10
A continuous casting apparatus for steel, comprising high-pressure water spraying means for colliding a water jet with a collision pressure of N / cm ² or more.