JP2626795B2 - Continuous casting of molten steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a continuous casting method of molten steel for producing defect-free clean steel slabs having few inclusions and good surface properties.

(Conventional technology) In the continuous casting method of steel generally implemented, molten steel produced in a converter or the like is received in a tundish and injected into a mold through an immersion nozzle. A solidified shell is formed, and the solidified shell is configured to be drawn out from immediately below the mold while being cooled by spraying. In this case, it is well known that the mold vibrates up and down and lubrication is performed by pouring powder into the meniscus for the purpose of preventing seizure between the slab and the mold and reducing frictional resistance.

However, in such a conventional mold vibration method, the flow of the molten powder between the inner wall of the mold and the solidified shell of the slab is not uniform because the frequency is low, and the original purpose cannot be completely achieved. With the vibration of the mold, a stripe pattern called an oscillation mark is formed on the slab surface, and this oscillation mark contributes to the surface flaw. Also, the powder injected as a lubricant melts at the meniscus, and when the casting speed is high, the molten powder is caught in the molten steel due to the molten steel flow near the meniscus in the mold, causing deterioration of the internal quality of the slab. .

Japanese Patent Application Laid-Open No. Sho 62-144856 discloses that, in order to solve these problems, a heating zone and a cooling zone are sequentially provided below the upper vessel so that the steel is always in a molten state on the inner wall of the heating zone. A method is disclosed in which a mold is vibrationless, powderless, and nozzleless in which the amount of heat is supplied to the inner wall of the cooling zone and the amount of cooling is controlled so that molten steel always starts to solidify on the inner wall of the cooling zone.

(Problems to be Solved by the Invention) In this method, as shown in FIG. 3, a heating zone 5 and a cooling zone 6 are sequentially provided below the upper vessel 4, and steel is always formed on the inner wall of the heating zone 5. Heat is supplied so as to be in a molten state, and further, by controlling the cooling amount so that molten steel always starts to solidify on the inner wall of the cooling zone 6, mold vibration can be reduced.
It is designed to be cast without powder or nozzle. However, in this method, the heating zone 5 and the cooling zone 6
Therefore, a large temperature gradient could not be secured in the vicinity of these joints due to heat transfer between the heating zone 5 and the cooling zone 6, and the solidification starting point of the molten steel 3 could not be controlled uniformly. Further, there is a risk that thermal distortion is caused due to a difference in thermal expansion between the heating zone 5 and the cooling zone 6, which significantly damages constituent members and sometimes causes accidents such as hot water leakage.

The present invention solves the above-mentioned problems, secures a large temperature gradient at the junction between the heating zone and the cooling zone, controls the solidification starting point to be constant, and has a thermal strain due to a difference in thermal expansion near the junction. It is an object of the present invention to stably produce a defect-free clean steel slab having good surface properties of a slab by solving the problem and preventing leakage of hot water.

(Means for Solving the Problems) The present invention has been made to advantageously solve such a problem, and the gist of the present invention is that an upper container in which a heating zone and a cooling zone are sequentially provided at a bottom portion. In the continuous casting method of molten steel using a method, a gap is provided at a boundary between a heating zone and a cooling zone, and the inside of the gap is sealed with an inert gas or a liquid mixture of a lubricant to solidify a solidification starting point. It is a continuous casting method.

(Operation) Hereinafter, the operation of the present invention will be described in detail with reference to the drawings.

FIG. 1 is an explanatory diagram showing the entire embodiment of the present invention, and FIG. 2 is an explanatory diagram showing details of how to provide an interval.

In the present invention, as shown in FIG. 1, an interval 8 is provided between the heating zone 5 and the cooling zone 6, and an inert gas such as argon is injected into the interval 8 via a pressure header 9 and a fluid pressure adjusting valve 10. Or by encapsulating, and by reducing the amount of heat transfer between the heating zone 5 and the cooling zone 6, a large temperature gradient can be ensured at the joint, and the solidification starting point can be kept constant. it can. Specifically, for example, 2mm
By providing the Ar interval 8 of the heating zone 5 and the cooling zone 6
The heat flow rate at the junction with is approximately halved.

The injection pressure in this case requires a molten steel head pressure at a portion where the gap 8 and the molten steel 3 are in contact with each other, and a pressure drop due to viscous resistance generated when the fluid flows through the gap 8, that is, a supply pressure corresponding to the pressure loss. Specifically, it is necessary to satisfy the condition represented by the following equation.

When sealed: P ≧ H × ρ _Fe where h: distance from meniscus to the interval [m] ρ _Fe : density of molten steel (= 7000 [Kg / m ³ ]) ν: kinematic viscosity coefficient of lubricant to be injected [m ² / sec] b: gap thickness [m] v: injection lubricant injection speed [m / sec] g: gravitational acceleration (= 9.8 [m / sec ² ]) ρ _L : injection lubricant density [kg / m ³ ] ρ: pressure at the time of injection or sealing [kg / m ² ] Further, the difference between the amount of thermal expansion of the heating zone 5 and the amount of thermal expansion of the cooling zone 6 can be absorbed at intervals of 8 and the heating zone 5 And cooling zone 6
The casting can be stably performed without giving excessive thermal strain to the constituent material of the present invention, so that the constituent material does not suddenly break during operation.

In addition, it is difficult to ensure uniformity in processing accuracy when the size of the gap is 1 mm or less, and as a result, it is impossible to uniformly inject the lubricant in the width direction. A two-phase flow occurs between the upper and lower portions, and gas or lubricating oil is injected into the molten steel side at the upper portion, and molten steel is inserted at the lower portion of the interval 8, so that only the pressure control of the header 9 provided behind the interval 8 requires Since the interface with the molten steel cannot be fully controlled, the size of the interval 8 is appropriately 1 mm to 5 mm.

The present inventors conducted various investigation tests on the detailed shape of the gap 8, and as a result, as shown in FIG. As shown in the figure, it has been found that by providing the gap 8 with an inclination so that the side in contact with the molten steel faces downward, the molten steel can be uniformly held in the width direction without being inserted into the gap 8. Note that the inclination of the interval 8 need not be the entire interval 8, but need only be inclined at least in the vicinity of the molten steel 3 as shown in FIG. 2 (c).
Further, the inclination angle may be basically any number as long as the inclination angle is downward, and may be arbitrarily determined as necessary for manufacturing the heating zone 5 and the cooling zone 6.

Further, since the lubricating powder used in the conventional continuous casting method cannot be used in this submerged solidification method, the cooling zone 6
In this case, 5% to 30% of a liquid lubricant such as a mineral oil or a vegetable oil, or a powdery solid lubricant was contained from 8 spaces in order to prevent the occurrence of seizure between the steel and the slab. By injecting and sealing the liquid, the occurrence of this seizure can be reliably prevented, and the amount of heat transfer between the heating zone 5 and the cooling zone 6 can be reduced.

Example 1 Example 1 On a cooling zone 6 of a continuous casting machine for casting a low carbon aluminum killed steel slab for a thin plate having a slab width of 1600 mm and a slab thickness of 245 mm at a casting speed of 1.85 m / min, FIG. A heating zone 5 having a length of 200 mm as shown, and an upper container 4 having a capacity of 2 tons were installed above the heating zone. A heating coil capable of supplying 0.5 MW of electric energy is buried in the heating zone 5, and a gap 8 of about 2 mm is provided between the heating zone 5 and the cooling zone 6. A header 9 was provided. The inside 100 mm of the interval 8 had a shape of FIG. 2 (c) inclined downward by 10 degrees from horizontal.

The length from the molten steel 3 bath surface in the upper container 4 to the interval 8 is approximately
400mm, pressure 0.28kg / cm corresponding to this molten steel head
^When the casting was carried out by filling argon gas from the pressure header 9 in step ² , a clean steel slab with good surface properties and no defect was able to be stably manufactured for 20 hours or more without significant damage to the heating member.

Example 2 In the same manner as in Example 1, in the upper container 4 and the continuous casting machine shown in FIG. 1, instead of argon gas, a solid lubricant of several μm was used instead of the argon gas between the heating zone 5 and the cooling zone 6. Vegetable oil (rapeseed oil) mixed with 10% of boron nitride (BN) as an agent was supplied at a header 9 pressure of 0.5 kg / cm ² , the slab pull-out resistance was reduced to about 1/2, and the slab surface properties However, it was able to produce about 300 ch of stable and clean defect-free steel stably without any operational troubles such as breakout caused by seizure.

Comparative Example As shown in FIG. 3, in a continuous casting machine in which a heating zone 5 and a cooling zone 6 were directly connected, a slab width of 1600 mm and a slab thickness of 245 mm
Casting speed of 1.85m / min for low carbon alkylated slab
After casting for about 5 hours, a slab with good internal quality and surface properties could be produced. After the casting was completed, the mold was examined in detail. Since the crack was observed, the next casting could not be continued, and the mold had to be taken off-line and repaired.

(Effect of the Invention) As described in detail above, by applying the present invention, there is no operational trouble such as hot water leakage or seizure breakout, and a slab excellent in surface texture and internal quality can be stably obtained. And can be cast.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an embodiment of the present invention, and FIGS.
FIG. 3 (c) is an explanatory view showing details of how to provide an interval, and FIG. 3 is an explanatory view showing a conventional submerged solidification method. 1 ... ladle for molten steel, 2 ... long nozzle, 3 ... molten steel, 4
... upper container, 5 ... heating zone, 6 ... cooling zone, 7 ... solidified shell, 8 ... interval, 9 ... pressure header, 10 ... fluid pressure regulating valve.

Claims (1)

(57) [Claims] In a continuous casting method of molten steel using an upper vessel having a heating zone and a cooling zone sequentially connected to a bottom portion, an interval is provided at a boundary between the heating zone and the cooling zone, and an inert gas or lubrication is provided in the interval. A continuous casting method for molten steel, characterized in that the solidification starting point is fixed by sealing with a liquid mixture of an agent.