JPH0824998A - Belt type continuous casting method - Google Patents

Abstract

(57) [Summary] [Object] The present invention prevents the occurrence of vertical cracking of medium carbon steel and casts a thin slab without defects when casting a thin slab with a belt type continuous casting machine. The present invention provides a continuous casting method that can be performed. [Structure] In belt-type continuous casting, a coating agent having a particle diameter of 50 μm or less containing 75% by weight or more of graphite powder is applied to the outermost layer of the one or more coating agents applied to the surface of the belt, which is in contact with molten steel, and cast. By carburizing the surface layer of the slab and avoiding the δ-γ transformation, a thin slab without vertical cracks can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a belt-type continuous casting technique for obtaining thin ingots having a thickness of 100 mm or less from molten steel by continuous casting, and particularly to the practice of this technique. The present invention relates to a method for producing a slab having good surface quality without vertical cracks.

[0002]

2. Description of the Related Art Conventionally, as one of continuous casting methods, for example, a partial region of a traveling route is provided with a predetermined interval, which is disclosed in, for example, JP-A-58-107255 and JP-A-1-293956. A pair of endless metal belts facing each other, a metal belt sandwiched between the metal belts, and a pair of blocks that move in synchronization with the thin slab form a cross-sectional shape corresponding to the desired slab, and those metal belts. And the block group are guided and supported by guide rolls and guide rails so as to rotate along a predetermined movement path, and a fluid film formed by ejecting a cooling fluid on the back surface of the metal belt between the guide rolls While cooling the metal belt, molten steel is injected from above the casting space through an injection nozzle and solidified along the mold wall of the metal belt or block group. To generate, and configured to derive from the casting space via the guide roll from the bottom of the resulting slab by the growth of solidification nuclei, the belt type continuous casting machine referred to as the so-called "belt caster" is proposed.

The metal belt used in such a belt-type casting machine is usually made of steel, but the belt surface is in direct contact with the molten metal, so that the heat load is very large, and the belt is easily deformed or damaged. Further, the deformation of the belt causes fluctuations in the molten metal surface and uneven cooling of the slab, which causes quality deterioration such as surface cracking of the slab. Therefore, many belt protection measures for belt casters have been proposed. For example, in JP-A-63-220949, an inorganic coating agent such as refractory ceramics whose thickness and thermal conductivity are specified is applied to the surface of a belt to prevent the belt from being deformed and to prevent surface defects caused by the deformation of the belt. A method of preventing the occurrence has been proposed. However, the coating on the surface of the belt focuses on improving the durability of the belt, and is unsatisfactory in terms of the surface quality of the slab, particularly in terms of preventing cracking.

On the other hand, there are many proposals focusing on gentle cooling and lubrication to prevent cracking. JP-A 1-210155
Japanese Patent Publication proposes a method of using a coating agent containing 5 to 30% by weight of graphite powder as a lubricant in order to prevent the crack caused by the solidification shrinkage by allowing the solidification shrinkage of the slab to proceed smoothly. Has been done. However, when the content of the graphite powder is 5 to 30% by weight, the friction coefficient is 0.3 or more and the lubrication is insufficient, and particularly the amount of C in the molten steel is 0.09 to 0.1.
The occurrence of vertical cracks cannot be prevented with 6% by weight of medium carbon steel.

Further, in Japanese Unexamined Patent Publication No. 1-197049, it is important to form a solidified shell of uniform thickness in order to prevent the vertical cracking of the medium carbon steel. In order to achieve this, a method has been proposed in which the particle size of the coating agent and the coating area are defined.
However, there is a limit to the slow cooling that can be achieved in terms of equipment and operation, and vertical cracking of medium carbon steel cannot be prevented in that range.

Further, in Japanese Patent Laid-Open No. 5-138307, a coating thickness is secured for obtaining stable heat removal required to obtain a solidified shell having a uniform thickness, and a solidified shell of a slab accompanying shrinkage of the slab is provided. In order to improve the lubricity between the belts, a coating with a high melting point aiming at slow cooling and a coating with a low melting point aiming at improving the lubricity as a second layer are applied on the surface side of the belts. The method of using is proposed. However, when a low melting point coating is used for low friction, the molten low melting point coating agent reacts with the coating on the belt side, and stable uniform cooling is impossible for a long time.
There is a problem that the coating melted by the meniscus flows unevenly between the belt slabs, and the inflowing portion is strongly non-uniformly solidified, rather causing vertical cracking.

[0007]

As described above, as for the coating agent for the metal belt of the belt type continuous casting method, measures have been found to sufficiently secure the quality of the slab, especially to prevent the vertical cracking of medium carbon steel. Absent. In view of these circumstances, it is an object of the present invention to prevent vertical cracking of a slab in belt type continuous casting.

[0008]

The present invention provides the following means in order to solve the above-mentioned problems. In continuous belt type casting, of the one or more coating agents applied to the belt surface, the outermost layer in contact with the molten steel is coated with a coating agent having a particle size of 50 μm or less containing 75% by weight or more of graphite powder and cast. Belt type continuous casting method.

[0009]

In order to solve the problems of the prior art, the present inventors first examined the mechanism of vertical cracking in medium carbon steel, which is particularly susceptible to cracking. As a result, vertical cracking is likely to occur in medium carbon steel because the thin solidification delay part of the solidification shell is lifted by the strain associated with the δ-γ transformation, and further the solidification delay becomes severe and the heat shrinkage strain is likely to concentrate.
It was clarified that due to the strain associated with the δ-γ transformation, a bending moment acts on the solid-liquid interface side of the solidification-delayed portion to cause internal cracking, which is the starting point of vertical cracking.

As described above, in order to prevent the vertical cracking of the medium carbon steel, it has been proved to be most effective to suppress the influence of strain associated with the δ-γ transformation. As a method of suppressing the influence of strain, a method of improving the lubrication between the belt and the slab has been conventionally used, but as described above, the vertical cracking of the medium carbon steel cannot be prevented. Therefore, the inventors have conducted various studies on methods for suppressing the δ-γ transformation itself, and have obtained the following findings. That is, if the surface layer is carburized to increase the C concentration in the surface layer to a region where δ-γ transformation does not occur, that is, a region where δ does not occur, cracking due to strain associated with δ-γ transformation can be prevented.

As a method of carburizing the surface layer of the cast slab,
It is effective to carburize the surface of the belt in contact with the surface of the slab with a coating agent containing C. Therefore, next, the amount of C in the belt coating agent for preventing the occurrence of vertical cracks was examined. As a result, as shown in FIG. 1, it was found that vertical cracking did not occur if the coating agent contained 75% by weight or more of C.

Further, as a result of investigating the influence of the particle size of the coating agent, it was found that vertical cracking occurs when the particle size exceeds 50 μm as shown in FIG. It is considered that this is because when the particle size becomes large, the contact area between the coating agent and the slab becomes small and the amount of carburization becomes insufficient.

The strain generated by thermal contraction regardless of the δ-γ transformation can be relaxed by the lubricity of graphite.

The coating thickness of the coating agent is required to maintain the function as a lubricant between the belt and the slab and protect the belt, and is generally 30 to 200 μm.
When the coating agent of the present invention is applied to a belt which has been previously subjected to ceramics thermal spraying for imparting heat insulation property, it is carburized on the surface of the slab and functions as a lubricant. Since it is sufficient, a thickness of 10 to 30 μm is sufficient.

The method of applying the coating agent includes a method of applying the coating agent to the belt in advance and a method of applying the coating agent to the belt in advance and a method of continuously applying the coating agent to the belt every one rotation of the belt during casting by using a spray gun or the like. The latter method is suitable because the coating agent of (1) will be worn away by contact with the slab.

The present invention has been made by applying a new action based on the above findings, and thereby the object of the present invention has been achieved.

[0017]

EXAMPLE Using a belt type continuous casting method as shown in FIG. 3, a medium carbon steel molten steel having the composition shown in Table 1 was used, casting speed was 6 m / min, slab size was 1200 mm wide, 75 mm thick thin casting. Pieces were continuously cast. At this time, the ratio of graphite in the coating agent and the particle size were variously changed and applied to the belt. In each case, the total length of vertical cracks generated per 1 m of slab length (1200 mm width × 1000 mm length) (generation of vertical cracks) Rate m
/ M) is shown in Table 2. It can be seen that vertical cracking does not occur when a coating agent containing 75% by weight or more of graphite and having a particle size of 50 μm or less is used.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

As described above, the present invention is a coating agent containing 75% by weight or more of graphite powder having a particle diameter of 5 to 50 μm in the outermost layer in contact with molten steel among one or more coating agents applied to the belt surface. It is possible to increase the C concentration in the surface layer portion of the medium carbon steel and to avoid the δ-γ transformation by applying, and it is possible to stably produce a slab with good surface quality without vertical cracking.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a relationship between a graphite ratio in a coating agent and a vertical crack occurrence rate.

FIG. 2 is an explanatory diagram showing a relationship between a particle size of a coating agent and a vertical crack occurrence rate.

FIG. 3 is a schematic cross-sectional view showing a twin-belt type continuous casting apparatus as an example of a continuous casting apparatus for casting thin cast pieces.

[Explanation of symbols]

 1 Tundish 2 Sliding nozzle 3 Injection nozzle 4 Top pulley 5 Drying device 6 Coating spray device 7 Tension pulley for tension control of metal belt 8 Metal belt 9 Foot roll

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akio Kasama No. 1 Nishinosu, Oita City, Oita Prefecture Shin-Nippon Steel Co., Ltd. Oita Works

Claims (1)

[Claims] 1. In a belt-type continuous casting, a particle diameter of at least 75% by weight of graphite powder is contained in the outermost layer of one or more coating agents applied to the surface of the belt, which is in contact with molten steel.
A belt-type continuous casting method, characterized in that a coating agent having a thickness of not more than μm is applied and casting is performed.