JPH06304716A - Method for lubricating gap between mold and cast slab in continuous casting for steel - Google Patents

Abstract

PURPOSE:To stabilize flow of a lubricant into a gap between a mold and a cast slab, to secure the thickness of the lubricant into the gap between the mold and the cast slab and to improve the lubrication. CONSTITUTION:In the continuous casting for steel executing mold oscillation, the mold lubricant is used and the upper part of the mold 1 is covered with a closed chamber 2 and pressurizing gas is supplied into the closed chamber. Pressure having >=20gf/cm<2> is impressed to the mold lubricant floated up on the free surface of the molten steel in the mold, and while forcedly pushing into the gap between the mold 1 and the cast slab 15, the cast slab 15 is drawn out. By this method, the developing ratio of various kinds of surface defects and breakout caused by unevenness of powder film thickness in the longitudinal direction of casting can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of lubricating between a mold and a slab in continuous casting of steel.

[0002]

2. Description of the Related Art Generally, when a steel slab is produced by continuous casting, lubrication with a mold powder has been carried out. At this time, mold vibration is usually used together. However, when the mold is vibrated, an oscillation mark is generally formed in the initial stage of solidification, and when the oscillation mark is deep, it often becomes a starting point of surface defects such as cracks.
Further, due to the mold vibration, the fluctuation of the powder film thickness and the nonuniform cooling of the slab in the longitudinal direction of the casting are caused, and as a result, surface defects such as depletion occur.

In recent years, there has been an increasing demand for strict slab surface quality from the standpoint of improving the orthogonality rate, and a higher viscosity of powder has been aimed at from the viewpoint of preventing inclusion defects caused by powder entrainment. However, the increase in viscosity generally reduces the amount of powder consumed, resulting in deterioration of lubricity between the mold and the slab and an increase in the risk of restraint breakout. Further, due to the increase in viscosity, generally, there is a tendency that an increase in the oscillation mark depth and an increase in surface defects due to non-uniform solidification promotion also occur as a side effect.

As a research on this matter, "Steelmaking Research" No. issued by Nippon Steel Corporation. 324 (1987) p. Three
0 can be exemplified, and the following is made clear. In other words, when using mold powder and performing mold vibration in the casting direction to perform continuous casting, the pressure of the powder film between the mold and the slab at the initial solidification part immediately below the molten steel free surface (meniscus) increases as the mold rises. It is lowest at maximum velocity and maximum at maximum mold descending velocity. When a highly viscous powder is used, this fluctuation is promoted, and if it is remarkable, the pressure of the powder film when the mold is raised may be a negative value. It has been reported that, as a result of the pressure drop of the powder film just below the free surface of the molten steel, the pushing force of the powder film in the casting direction in the mold is decreased, and in a remarkable case, powder backflow occurs.

The backflow of the powder causes a local decrease in the inflow amount of the powder, resulting in an extremely thin portion of the powder film. Mold lubrication becomes poor in the part where the powder film thickness is insufficient, and break-out of restraint property easily occurs. Further, the unevenness of the powder film thickness in the casting longitudinal direction is also promoted, and as a result, the unevenness of lubrication and cooling of the cast piece is promoted, and the surface defects of the cast piece are increased.

[0006] Conventionally, "Materials and Processes" Vol. 4 (1991), in order to stabilize the inflow of mold powder,
P. As seen in 258, measures have been focused on controlling the physical properties of the mold powder. However, since the physical properties of powder generally change significantly depending on conditions such as temperature, it is difficult to ensure proper conditions in actual operation.

As a technique similar to the present invention, which will be described in detail later, there is, for example, a pressurizing type continuous casting facility as disclosed in Japanese Patent Laid-Open No. 3-268848, which improves the responsiveness of molten metal level control. For that reason, the regulation for the volume change amount of the closed chamber is added for that purpose, but the present invention can achieve the following objects without particularly setting such regulation.

[0008]

In view of the above circumstances, the present invention provides a mold lubrication between a mold such as a powder film in a region directly below the molten steel free surface and a slab when performing mold vibration in continuous casting of steel. The purpose of the agent is to prevent backflow through the vibration cycle of the mold, to secure the thickness of the mold lubricant between the mold and the slab, and to improve the lubricity between the mold and the slab. To do.

[0009]

According to the present invention, in continuous casting of steel using a mold lubricant and performing mold vibration including vibration in the casting direction, the upper part of the mold is covered with a closed chamber as shown in FIG. A pressurizing gas is supplied into the closed chamber, and the mold lubricant floated on the molten steel free surface in the mold is pressurized at a pressure of 20 gf / cm ² or more to forcibly press the mold lubricant between the mold and the slab. However, the feature is that the slab is pulled out.

[0010]

[Function] The pressure applied to the mold lubricant on the molten steel free surface in the mold is such that it prevents the mold lubricant in the region immediately below the molten steel free surface invading between the mold and the slab from flowing backward during the cycle of mold vibration. Must be set to.

As described above, the pressure fluctuation of the powder film between the mold and the slab just below the free surface of the molten steel during the mold vibration cycle is larger as the viscosity of the powder is higher, and the powder reverses when the pressure of the powder film rises and the mold rises. Is likely to occur. The pressure fluctuation of this powder film is
Since the total amplitude is at most about several tens of gf / cm ² , applying a pressure equal to or higher than this can prevent powder backflow and increase the pressure of the powder film just below the molten steel free surface. Came to the knowledge of.

The viscosity of the powder used practically is 0.
Is about 5～15Poise, which corresponds to, but applied pressure required to prevent reverse current powder is on the order of ^{2~20gf / cm 2, 20gf / cm} 2 as the increase powder thickness shown in FIG. 2 Since it was recognized that the above pressure was necessary, the applied pressure was set to 20 gf / cm ² or more in the present invention.

According to the present invention, the pressurizing gas in the closed chamber installed above the mold pressurizes the mold powder floated on the free surface in the mold and the powder film between the mold and the slab continuous with the mold powder. . As a result, by sufficiently securing the pressure in the closed chamber, it becomes possible to prevent the backflow of the powder in the region immediately below the molten steel free surface through the cycle of the mold vibration, and to secure the powder film thickness to secure the mold and the slab. An improvement in lubricity can be achieved.

[0014]

EXAMPLES Examples of the present invention will be shown below. FIG. 1 is a casting apparatus according to the present invention. When injecting the molten steel in the molten steel holding container 8 heated by the induction furnace 9 into the water-cooled copper mold 1 through the immersion nozzle 7, the powder 16 was supplied from the powder container 14 into the mold immediately after starting the injection of the molten steel.
A closed chamber 2 was provided above the mold 1, and Ar gas pressurized by a compressor 13 was introduced into the closed chamber. The pressure of the Ar gas was adjusted by the pressure adjusting valve 11. As the mold lubricant, a mold powder mainly composed of an oxide is generally used, but as long as it has a similar lubricating function and becomes a liquid or a solid-liquid mixed state on the molten steel free surface, the existing Not limited to mold powder. The pressurizing gas is preferably an inert gas such as Ar from the viewpoint of preventing molten steel contamination, but N ₂ gas, air or the like may be used. The slab 15 is pulled out by the pulling-out drive mechanism 4 via the dummy bar 3 while performing vibration of the mold by the vibrator 6.

Table 1 shows the implementation conditions. The powder used has two kinds of viscosity. Further, the case where no pressure was applied to the Ar gas in the closed chamber was also investigated and compared with the case where pressure was applied.

After the casting reached a steady state, the supply of molten steel to the mold was stopped, the slab drawing was stopped, the slab was cooled, the slab was taken out from the mold, and the condition of the slab was observed. Figure 2
Shows the measurement results of the powder film thickness. The powder film thickness indicates the average value and range of 48 points measured in the steady part. The powder film thickness decreased due to the increase in casting speed and the increase in powder viscosity, but the increase in powder film thickness was confirmed in all cases by pressurizing the closed room air, and the minimum value of the powder film thickness also increased. . Here, the minimum value of the film thickness when a pressure of 20 gf / cm ² is applied approximately corresponds to the average value of the film thickness when no pressure is applied. Furthermore, when the surface quality of the slab was investigated under the conditions shown in Table 1, powder B was used, the applied pressure was 50 gf / cm ^2, and the casting speed was 0.5.
At m / min, the oscillation mark depth was 0.45 mm in the conventional method without applying pressure, but was 0.15 mm.
It became the following. At the same time, the surface cracks were drastically reduced, and the crack occurrence rate was 15% to 20% in the conventional method, but stably achieved 5% or less, and the breakout occurrence rate was 16% in the conventional method as shown in FIG. What was 0.7% became 0%.

[0017]

[Table 1]

[0018]

As described above, according to the present invention, the flowability of powder between the mold and the slab can be improved, and in particular, the shortage of the powder film thickness can be avoided. As a result, the surface defects of the slab and the breakout occurrence rate can be greatly reduced, which is an industrially advantageous invention.

[Brief description of drawings]

FIG. 1 shows a casting apparatus according to the present invention.

FIG. 2 shows how the powder film thickness changes when the pressure applied to the gas in the mold is changed in the apparatus of FIG.

FIG. 3 shows breakout occurrence rates of the conventional method and the method according to the present invention.

[Explanation of symbols]

 1 Water-Cooled Copper Mold 2 Sealed Chamber 3 Dummy Bar 4 Dummy Bar Extraction Mechanism 5 Suspension 6 Mold Vibrator 7 Immersion Nozzle 8 Molten Steel Holding Container 9 Induction Furnace 10 Storage Chamber 11 Pressure Regulator 12 Pressure Gauge 13 Compressor 14 Powder Container 15 Cast Piece 16 Mold Powder

Claims (1)

[Claims] 1. In continuous casting of steel with vibration of a mold, a mold lubricant is used, and the mold upper part is covered with a closed chamber, and a pressurizing gas is supplied into the closed chamber so that the molten steel free surface in the mold is covered. A mold for continuous casting of steel, characterized in that a pressure of 20 gf / cm ² or more is applied to the floated mold lubricant to forcibly push the mold lubricant between the mold and the slab to pull out the slab. And lubrication method between slabs.