JP2779194B2 - Continuous casting of thin cast slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for directly producing thin cast slab from molten metal by a twin drum method, a single drum method, or the like.

[Conventional technology]

2. Description of the Related Art In recent years, in the field of continuous casting of metal, there is a strong demand for the development of a technique for producing a thin cast slab having a shape close to the final shape with the aim of reducing production costs and creating new materials. Various methods have been proposed for this requirement, some of which have reached the level of industrial production. However, the conventional methods are not yet sufficient in terms of productivity, slab quality and the like.

Among these continuous casting methods for thin cast slabs, a twin-drum type using a pair of cooling drums with an internal water cooling mechanism as the main components of the mold, as those using relatively simple equipment, 1 There are a single-drum type using a cooling drum and a drum-belt type in which a pool is formed between the cooling drum and the belt.

FIG. 3 schematically shows a twin drum type continuous casting facility as an example. Molten metal is supplied from an intermediate container such as a tundish 1 to a pool 4 formed between a pair of cooling drums 3a and 3b via an immersion nozzle 2. The heat of the molten metal in the basin 4 is removed through the cooling drums 3a and 3b to form a solidified shell on the peripheral surfaces of the cooling drums 3a and 3b.
This solidified shell grows while being fed to the kissing point 5. Then, the solidified shells formed on the peripheral surfaces of both the cooling drums 3a and 3b are sent out as a thin cast piece 6 integrated at the kissing point 5. Thin cast slab 6
Is sent to the subsequent process via the pinch roll 7 and the like.

During the casting process of the thin cast slab 6, it is necessary to keep the peripheral surfaces of the cooling drums 3a and 3b clean in order to stabilize the formation and growth of the solidified shell. Therefore, a method is known in which casting is continued while the peripheral surfaces of the cooling drums 3a and 3b are polished by the cleaning brush 8 (Japanese Patent Laid-Open No. 60-1844).
No. 49). A method is also known in which ceramics are sprayed onto the peripheral surfaces of the cooling drums 3a and 3b by a drum coater 9 to generate and grow a solidified shell under warm cooling conditions (see JP-A-60-136649).

In these casting methods, it is important to stably maintain the surface properties of the slab at a high level. That is, unlike the case of slabs manufactured by conventional continuous casting equipment, since it was developed with the aim of obtaining a thin cast slab that can reduce the degree of rolling in subsequent steps, The surface properties of the cast slab greatly affect the surface condition of the product.

However, the scum floats on the surface of the molten metal poured into the pool 4. This scum is applied to the cooling drums 3a, 3
It accompanies the molten metal flowing with the rotation of b, and is easily caught in the solidified shell generated and grown on the peripheral surfaces of the cooling drums 3a and 3b. As a result, the thin cast slab 6 is roughened,
Defects such as cracks occur.

In order to solve this drawback, it is conceivable to suppress the generation of scum by setting the pool section 4 to a non-oxidizing atmosphere. However, providing the pool 4 with a protective atmosphere complicates the equipment configuration. In addition, depending on the type and cleanliness of the molten metal, even in a non-oxidizing atmosphere, the pool 4
Oxide and the like may float from molten metal poured into the slag to form scum.

Therefore, the present invention improves the properties of scum generated in the pool, prevents the occurrence of slab defects caused by scum without requiring a special protective atmosphere, and provides excellent quality with less surface cracks. To produce a thin cast slab.

[Means for solving the problem]

In order to achieve the object, the continuous casting method for thin cast slabs of the present invention, when producing a thin cast slab by rapidly cooling and solidifying molten metal on a cooling drum surface, has a viscosity η ( poise) is adjusted to η ≧ 5 or η ≦ 1, or when the viscosity η of the scum is in the range of 1 <η <5, the thermal conductivity λ (Kcal / m · h · deg) of the scum and the scum The scum property is adjusted so as to satisfy the relationship of λ ≧ (η ² + 8η−1) / 16 with the viscosity η.

As a means for adjusting the scum properties, an element having a deoxidizing ability in the molten metal in advance, for example, a method of adding a trace amount of Al, Ca, Ti, etc., and utilizing a chemical reaction accompanying a temperature decrease during pouring, or A method of directly adding a predetermined amount of a compound such as an oxide or a fluoride to the well portion can be used.

[Action]

In the present invention, the viscosity η of the scum floating in the pool is adjusted to a specific range, or when the viscosity η of the scum is not within this specific range, the thermal conductivity λ of the scum is The purpose is to adjust the properties of the scum so as to have a constant relationship with η to prevent the occurrence of surface cracks in the slab.

The present inventors have repeated studies on the relationship between the properties of scum generated in the pool and the occurrence of surface cracks in the slab, and have obtained the results shown in FIGS. 1 and 2. FIG. 1 is a graph showing the relationship between the viscosity η of the scum and the occurrence of surface cracks in the slab, and FIG. 2 is a graph showing the relationship between the thermal conductivity λ of the scum and the occurrence of surface cracks in the slab.

As shown in FIG. 1, in the region I where the viscosity η of the scum is 1 or less, no surface crack of the slab occurs. In this area,
Since the viscosity η of the scum is small, the fluidity is good, and the scum flows uniformly between the cooling drum and the solidified shell in the drum width direction. No splitting occurs.

Further, even in the region III where the viscosity η of the scum is 5 or more, the surface crack of the slab does not occur. In this region, since the viscosity η of the scum is extremely high and there is almost no fluidity, the scum only floats in the pool and the phenomenon that the scum is caught between the cooling drum and the solidified shell is less likely to occur,
Therefore, no slab cracks occur.

In the region II where the viscosity η of the scum is more than 1 and less than 5, the surface cracks of the slab tend to occur. In this region, since there is moderate fluidity, a phenomenon occurs in which a part of the scum is caught between the cooling drum and the solidification shell, and a part of the scum floats in the pool. For this reason, the scum entrainment between the cooling drum and the solidified shell is not uniform in the drum width direction, and a scum entrained portion and a non-entrained portion exist in the slab. Solidification becomes uneven, and cracks tend to occur on the surface of the slab.

From these facts, it was found that in order to prevent the occurrence of surface cracks in the cast slab, the viscosity η of the scum in the pool portion should be maintained at 1 or less or 5 or more.

However, depending on the type of steel to be cast, it may be difficult to maintain the viscosity η of the scum in the pool in a range of 1 or less or 5 or more. For example, in the case of steels whose chemical composition range is very strict, such as nuclear power materials, some of the elements added for the purpose of adjusting the viscosity of scum remain in molten steel. become unable.

On the other hand, in the region II where the viscosity η of the scum is more than 1 and less than 5, there are times when slab cracks occur and times when they do not occur. It was also found that, even in the region II where the viscosity η of the scum is more than 1 and less than 5, if the thermal conductivity λ has a certain relationship with the viscosity η of the scum, no slab cracks occur. That is, as shown in the figure, when the thermal conductivity λ is lower than the curve in the figure with respect to the viscosity η of the scum, slab cracks occur (indicated by ● in the figure). If it is higher than above, no slab cracks occur (marked with ○ in the figure). The critical curve that does not cause this crack is λ = (η ²
+ 8η-1) / 16. The reason that the slab crack does not occur when the thermal conductivity λ is on or above this curve is that when the thermal conductivity λ is somewhat large, for example, even if the scum is unevenly wound, This is presumed to be because no solidification delay until cracking occurs.

From the above, when it is difficult to maintain the viscosity η of the scum in the pool portion at 1 or less or 5 or more, the thermal conductivity λ satisfies the relationship of λ ≧ (η ² + 8η−1) / 16. By adjusting the scum properties as described above, the occurrence of cracks in the cast slab can be prevented.

〔Example〕

Molten stainless steel with SUS 304 composition and temperature of 1500 ℃
The molten metal was poured into the pool 4 between the cooling drums 3a and 3b shown in FIG. 3 to produce a thin cast piece 6 having a thickness of 2 mm and a width of 800 mm at a casting speed of 80 m / min.

At this time, a scum having a thickness of about 0.5 mm was floating on the surface of the molten stainless steel poured into the pool section 4. Therefore, the processing method A: tundish method addition treatment with Ca in the molten steel of B: tundish method addition treatment of Al in the molten steel of C: addition of CaO in the water reservoir portion processing method D: a CaF ₂ in the basin section The viscosity η and the thermal conductivity λ of the scum were adjusted by the addition. As a result, as shown in Table 1, the surface defects of the cast thin cast slab were reduced.

In addition, in the above example, the case where the twin drum type continuous casting machine was used was described. However, the present invention is not limited to this, and other continuous casting methods such as a single-drum method and a drum-belt method can be used as long as the molten metal is rapidly cooled and solidified on the peripheral surface of the cooling drum to form a solidified shell. It goes without saying that the same applies to the method.

〔The invention's effect〕

As described above, in the present invention, the scum is accompanied by the flow of the molten metal flowing with the rotation of the cooling drum by adjusting the physical properties of the scum floating on the surface of the basin. Is prevented or uniform, or even if the scum entrainment is non-uniform, solidification non-uniformity is reduced. Therefore, the solidified shell formed and grown on the peripheral surface of the cooling drum becomes uniform, and it becomes possible to manufacture a thin cast piece free from defects such as surface roughness and cracks caused by scum.

[Brief description of the drawings]

1 and 2 are graphs showing the relationship between the scum viscosity and thermal conductivity and the occurrence of slab cracks, and FIG. 3 is a schematic diagram showing a twin drum type continuous casting machine. 1: Tundish, 2: Immersion nozzle 3a, 3b: Cooling nozzle, 4: Pool basin 5: Kissing point, 6: Thin cast slab 7: Pinch roll, 8: Cleaning brush 9: Drum coater

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) B22D 11/06 330 B22D 11/06 340

Claims (1)

(57) [Claims] When the molten metal is rapidly cooled and solidified on a cooling drum surface to produce a thin cast slab, whether the viscosity η (poise) of the scum floating in the pool is adjusted to η ≧ 5 or η ≦ 1. ,
Alternatively, when the viscosity η of the scum is in the range of 1 <η <5, λ ≧ (η ² + 8η−1) is set between the thermal conductivity λ of the scum and the viscosity η of the scum. ) A continuous casting method for thin cast slabs, wherein the relationship of / 16 is maintained.