JPH07276006A - Twin-belt continuous casting method - Google Patents

Abstract

(57) [Summary] [Objective] In the twin-belt continuous casting method, a twin-belt continuous casting method capable of efficiently casting alumina-based inclusion accumulation zones to cast a slab without surface defects. provide. [Structure] Unsolidified part-curved twin-belt continuous casting When continuously casting thin slab slabs, the vertical length H (cm) below the mold and the radius of curvature R ( c
m), thin slab thickness D (cm), casting speed V (m / min), and the defect prevention index I is 0.185 or more. Here, the defect prevention index I = (1.6942 + 0.0867 * D + 0.
001283 * H-0.07765 * V + 0.0006
It is represented by 019 * R) / D.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a twin belt type continuous casting method used as a continuous casting method for steel (carbon steel, stainless steel, etc.) in the field of iron and steel manufacturing, and particularly causes a surface defect of a steel strip. The present invention relates to a casting method for suppressing the formation of an alumina-based inclusion accumulation zone.

[0002]

2. Description of the Related Art In recent years, in the field of continuous casting of iron and steel, there has been an increasing demand for reducing the thickness of cast slabs and increasing the speed of casting. To meet these demands, for example, a pair of endlessly moving molds is used. Development of a twin-belt continuous casting method using a belt and a pair of moving short-side blocks that move endlessly is in progress.

In this twin-belt type continuous casting machine, for example, as shown in FIG. 4, the mold m has three pulleys p.
A pair of metal belts ba, bb provided with a cooling structure w that are wound around a, pb, pc and move endlessly, and a pair of endlessly move between the pair of metal belts near their side ends. The movable short side blocks ca and cb are formed, and molten steel s is injected into the mold from the tundish 1d through the injection nozzle e, and cooled and solidified in the mold.
In the unsolidified state, the support roll r vertically-curves-horizontally supports and pulls out to cast a thin slab sc (see, for example, JP-A-1-254354).

Conventionally, in continuous casting by such an unsolidified portion-curved twin belt type continuous casting machine, due to passage of the curved portion in the unsolidified state, the upper surface layer of the cast piece obtained by casting It is known that the alumina-based inclusion accumulating zone is easily trapped in the part, and when the steel strip is manufactured by rolling this slab, a flaw is generated on the surface of the steel strip. However, no clear measures have been disclosed as to what conditions can be used to suppress the formation of inclusion accumulation zones at positions where surface defects occur in steel strip products.

Conventionally, as a method for preventing the occurrence of product defects due to inclusions, for example, Japanese Patent Laid-Open No. 5-769.
No. 93 “Continuous casting method for cold rolled carbon steel slab”
In order to prevent inclusions from being trapped in the solidified shell, it has been proposed to use a continuous casting machine having a vertical portion of 20 m or more, and to perform electromagnetic stirring at a position within 1.5 m from the meniscus. However, this method requires excessive equipment and is costly.

[0006] Further, Japanese Patent Laid-Open No. 1-180753 proposes to supply a molten flux to a molten steel surface in a mold to absorb and dissolve inclusions floating in the mold and discharge the inclusions out of the system. . However, in this method, when casting at a high speed, the molten flux itself is caught in the molten steel and cast, which causes defects in the slab. As described above, in the conventional method, it is difficult to efficiently and sufficiently suppress the occurrence of steel strip surface flaws caused by the formation of inclusion accumulation zones in the surface layer of the cast slab.

[0007]

SUMMARY OF THE INVENTION The present invention is a continuous casting method using a twin belt type continuous casting machine, in which the position of the alumina-based inclusion accumulation zone is controlled to cast a slab that does not cause a product surface defect, A twin-belt continuous casting method capable of improving the quality of a steel strip obtained by rolling this.

[0008]

The present invention uses a mold formed of a pair of belts that move endlessly in synchronism with the speed of a slab and a pair of moving short side blocks composed of a plurality of blocks. , When the thin slab slab is continuously cast by the unsolidified-curved continuous casting machine, the vertical portion length H below the mold
(Cm), radius of curvature R (cm) of bent portion of cast slab, thickness D (cm) of thin slab cast, equipment specification that the defect prevention index I determined by casting speed V (m / min) is 0.185 or more, The twin-belt continuous casting method is characterized in that continuous casting is performed under casting conditions. Here, the defect prevention index I is I = (1.6942 + 0.0867 * D + 0.0012)
83 * H-0.07765 * V + 0.0006019 *
R) / D.

[0009]

According to the present invention, in the twin-belt type continuous casting method, rolling is performed by controlling the position of the alumina-based inclusion accumulation zone on the surface of the slab to prevent defects in the steel strip product due to the inclusion of alumina diameter. Slabs that do not generate can be stably cast.

The present invention will be described below. A characteristic feature of the slab cast by the curved continuous casting machine is that inclusion accumulation zones are formed near the surface layer of the top surface of the slab. When this generation position is shallower than a specific depth from the surface layer (close to the surface layer), surface defects occur when the steel strip product is manufactured by rolling. The specific depth depends on the slab thickness.

The present inventors cast a slab by the twin belt type continuous casting method, and analyzed the mechanism of generation of alumina diameter inclusions and the relationship between the accumulation position of inclusions and the steel strip surface flaw generation. Fig. 1 (a) shows the thickness 5 cast at a casting speed of 7 m / min.
It shows the alumina inclusion index distribution in the thickness direction of a 0 mm thin slab cast, where alumina-based inclusions are accumulated in the center of the cast. Further, FIG. 1 (b) shows that a conventional curved type general continuous casting machine having a surface flaw has a thickness of 250.
Fig. 2 shows the index distribution of alumina inclusions in the thickness direction when a slab of mm is cast at a casting speed of 1.5 m / min. The alumina-based inclusions are thin slabs with a thickness of 50 mm as shown in Fig. 2. It is accumulated on the surface side larger than in the case of.

The present inventors rolled the above-mentioned slab and rolled it to 3 mm.
It has been found that when a thick steel strip product is manufactured, the cast product of FIG. 1 (b) has no product defects and the cast product of FIG. 1 (a) has product defects. The present invention specifies the specifications and casting conditions of the continuous casting machine for obtaining the position of the alumina inclusion accumulation zone, which does not cause steel strip surface defects when casting a thin slab cast piece with a twin-belt type continuous casting machine. Inspiration,
The optimum specific range was found, and the present invention was completed. The continuous casting machine specification here means the length H of the vertical portion that vertically supports the thin slab slab in the unsolidified state immediately below the mold.
(Cm), refers to the bending radius (casting radius) R (cm) of the bending portion that supports the unsolidified slab, and the casting conditions are thin slab casting thickness D (cm), casting speed V (m) / min).

In the present invention, a defect prevention index is found based on the specifications and casting conditions of a continuous casting machine that does not cause defects in a thin slab cast piece (steel strip), and a thin slab cast in an unsolidified state directly below the mold is found. The length H (cm) of the vertical part that vertically supports the piece,
By controlling (setting, changing) the bending radius (casting radius) R (cm) of the bending portion that supports the unsolidified slab in a curved manner,
The feature is that the defect prevention index is set within a specific range. The defect prevention index I in the present invention is I = (1.6942 + 0.0867 * D + 0.0012)
83 * H-0.07765 * V + 0.0006019 *
R) / D.

Each of these numerical values was obtained from a large number of experiments by arranging the relationship between the surface defects of the steel strip product caused by alumina-based inclusions and the doors where the inclusions are accumulated, from the specifications of the continuous casting machine and the casting conditions. Is. FIG. 2 shows the relationship between the defect prevention index and the surface defects due to the alumina inclusions in the steel strip obtained by hot rolling the slab. When the defect prevention index is 0.185 or more. In addition, it is shown that surface defects due to alumina inclusions do not occur in the steel strip.

This defect prevention index is set according to the range of the steel type to be cast, the length H of the vertical portion below the mold, the bending radius (casting radius) R of the curved portion of the slab, and the slab size (thickness D) range. However, in the present invention, these setting conditions are considered in advance so that the defect prevention index I becomes 0.185 or more. The defect prevention index is an index of the distance between the accumulation position of alumina-based inclusions in the cast and the surface of the cast, and the small index means that the position where the inclusions are accumulated is relatively close to the cast surface. Is present in. When this defect prevention index is smaller than the upper limit value of 0.185, alumina-based inclusions appear on the surface of the steel strip product when rolled, resulting in surface defects of the steel strip product.

Although the present invention is applied in the design stage of a continuous casting machine, the vertical portion length H (c
m), the radius of curvature R (cm) of the slab bending part, etc. are often fixed conditions, and if the slab size (thin slab slab thickness D) is determined, in actual operation When the present invention is applied, the casting speed is changed so that the defect prevention index I is 0.185 or more. Recently, a continuous casting machine that can change the slab width, change the mold, change the casting radius, etc. has also been proposed.When using such a continuous casting machine to perform continuous casting, not only the casting speed, The length H (cm) of the vertical portion below the mold and the radius of curvature R (cm) of the bent portion of the slab may be changed. In both cases, the defect prevention index I is 0.1.
Change it to be 85 or more.

[0017]

Example 1 An example of the present invention will be described below. Figure 3
Shows an outline of a twin-belt continuous casting in-line hot finish rolling process embodying the present invention. First, in this twin-belt continuous casting machine, molten steel 2 was supplied to the tundish 1, and when the molten steel 2 was accumulated in the tundish 1 to some extent, the sliding nozzle 3 installed in the tundish 1 was operated to melt the molten steel. 2, a pair of belts 7a and 7b, which are wound between two pulleys 5a and 5b and a tension adjusting pulley 5c whose position is controlled by a tension control device 6 through an injection nozzle 4, and endlessly rotationally move, and this pair of belts. It is poured into a mold 9 formed by a pair of moving short side blocks 8a and 8b composed of a large number of blocks which move endlessly between both side ends.

The belts 7a and 7b form the long sides of the mold 9, and the back surface at the position in contact with the molten steel is constructed to be cooled by the cooling water flowing in the cooling box 10, and A large number of movable short-side blocks 8a and 8b form the short sides of the mold 9, and are connected to a chain 11 that moves endlessly via a connecting link (not shown) and move along the guide rails 12 to move. In the process, it is configured to come into contact with the molten steel after being cooled by the cooling device 13 arranged along the moving path.

The molten steel 2 is made up of the cooled belts 7a, 7a.
In contact with the operating surface of b, it is cooled and removed of heat to solidify,
The support roll 14 disposed below the vertical axis (length H) clamps and pulls it out at the curved portion (radius R of the curved portion), carries it out as a thin-walled cast piece 15, and passes through the heat-retaining furnace 16 for in-line hot rolling. The steel strip 18 is introduced into a rolling mill 17 and hot-rolled.

In the twin-belt type continuous casting in-line hot rolling process configured as described above, thin cast pieces of low carbon steel are continuously cast, and thin cast slabs are in-lined from the obtained thin slab pieces. With respect to the surface defect occurrence situation of the steel strip obtained by hot rolling with, in the case of within the scope of the present invention,
Confirmation experiments were conducted when the results were outside the scope of the present invention. The occurrence of defects on the surface of the steel strip was visually confirmed. In the conventional example, a slab slab having a thickness of 250 mm was used, and after continuous casting, hot rolling was carried out through a hot rough rolling mill to produce a steel strip.

The results are shown in Table 2 together with the conventional case or the case outside the scope of the present invention. The conditions shown below were adopted for casting and rolling. (1) Continuous casting conditions Continuous casting machine specifications Vertical part length H: 0 to 300 mm Curved part radius (casting radius) R: 300 to 1050 cm Steel type: carbon steel (component composition is shown in Table 1) Casting temperature: 1530 to 1570 ° C Casting speed V: 1 to 15 m / min Slab size: Thickness D 50 to 100 mm x width 1300 mm Belt material: Carbon steel Size: Thickness 1.2 mm x width 1500 mm Cooling: Cooling water sprayed on the back side Moving short side block Material: Copper Size : 50-100 mm x 100 mm x 50 mm Cooling condition: Spraying cooling water from outside (2) Rolling condition Steel strip size: 1.2-4 mm thickness x 1300 mm width Rolling temperature: 900-1000 ° C

[0022]

[Table 1]

[0023]

[Table 2]

As shown in Table 2, the defect prevention index is 0.18.
In the present invention of 5 or more, the steel band surface defect generation index is all 0, which means that no surface defect is generated.

On the other hand, in the conventional example, the defect prevention index is 0.184 or less and the steel strip surface defect generation index is 0.5 to 5.5, which means that the product cannot be manufactured without care. There were many things.

From the above, in the present invention, the defect prevention index is set, and the specifications and the casting conditions of the continuous casting machine are set so that the value becomes 0.185 or more, and the thin film with no surface defects is generated. A twin-belt continuous casting method capable of casting a slab slab and rolling the thin slab slab to obtain a steel strip having no surface defects.

Although this embodiment is applied to the continuous casting in-line hot rolling process, the present invention does not generate surface defects when the continuous casting process is independent and hot rolling is performed. It is also positioned as a twin-belt continuous casting method capable of obtaining a steel strip free from surface defects, and is not applied only to the continuous casting in-line hot rolling process.

[0028]

According to the present invention, in the twin-belt type continuous casting method, by suppressing the formation of the alumina-based inclusion accumulation zone on the surface of the cast piece, it is possible to stably cast the cast piece having no surface defect, The slab can be rolled to stably manufacture a steel strip having no defects.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an alumina index for each position of a slab in a slab by a twin-belt type continuous casting method, and FIG.
The case of a thin slab slab having a thickness of mm, and the diagram (b) show the case of a slab having a thickness of 250 mm by a conventional method (continuous casting method of a fixed water-cooled copper mold system).

FIG. 2 is an explanatory view showing a relationship between surface defects and a defect prevention index of a steel strip obtained by hot rolling a slab by a twin belt type continuous casting method.

FIG. 3 is a schematic explanatory view of a twin-belt type continuous casting machine according to an embodiment of the present invention, in which (a) is a side sectional view and (b) is a schematic sectional view taken along the line Aa-Ab in (a). .

FIG. 4 is a side cross-sectional schematic explanatory view showing an example of a known twin belt type continuous casting machine.

[Explanation of symbols]

 1: Tundish 2: Molten steel 3: Sliding nozzle 4: Injection nozzle 5a, 5b: Pulley 5c: Tension adjusting pulley 6: Tension control device 7a, 7b: Belt 8a, 8b: Moving short side block 9: Mold 10: Cooling box 11: Chain 12: Guide rail 13: Cooling device 14: Support roll 15: Thin slab cast piece 16: Heat-retaining furnace 17: In-line hot rolling machine 18: Steel strip

Claims (1)

[Claims] 1. An unsolidified portion-curved continuous using a mold formed of a pair of belts that move endlessly in synchronization with the speed of a slab and a pair of moving short-side blocks composed of a plurality of blocks. When continuously casting thin slab cast pieces by a casting machine, the vertical length H (cm) below the mold, the radius of curvature R (cm) of the bent portion of the cast pieces, the thin slab cast piece thickness D (cm), the casting speed V
A twin-belt continuous casting method, characterized in that continuous casting is performed under equipment specifications and casting conditions in which a defect prevention index I determined by (m / min) is 0.185 or more. Here, the defect prevention index I is I = (1.6942 + 0.0867 * D + 0.0012)
83 * H-0.07765 * V + 0.0006019 *
R) / D.