JPH03104814A - Ladle refining method - Google Patents

Abstract

PURPOSE:To surely prevent the [N] pickup of a molten steel by supplying gaseous CO2 into a ladle cap to maintain a gaseous CO2 atmosphere in the gaseous phase in contact with the molten steel at the time of blowing a stirring gas to the molten steel while heating the molten steel with a submerged arc in the ladle. CONSTITUTION:The cap is put on the ladle contg. the molten steel in an undeoxidized state in LF equipment and the gaseous CO2 is supplied and discharged to and from the ladle by the gas feed and discharge pipes mounted to the cap to maintain the gaseous CO2 atmosphere in the ladle. On the other hand, a slag forming material and metallic Al are added from an adding hole to the molten steel and the electrode immersed in the molten steel is energized to heat the molten steel with the submerged arc, by which the molten steel is deoxidized. Gaseous Ar is supplied to an injection lance to stir the molten steel upon completion of the substitution with the gaseous CO2. The outdoor air is infiltered from the spacing of the cap into the inside by the fluctuation in the pressure in the ladle at this time and slag is discharged to expose the molten steel on the part of the melt surface by the floating of the stirring gas. Since the gaseous CO2 exists in the gaseous phase, the [O] level at the gas-liquid boundary rises and the [N] pickup to the molten steel is effectively prohibited.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a ladle refining method in which an electrode is immersed in slag to submerged arc heat the molten steel and a stirring gas is blown into the molten steel.

[Prior Art] In recent years, low-nitrogen steel has come to be used for surface-treated steel sheets such as tin plates and outer panels of automobile bodies, and there is a demand for technology for rapidly and stably manufacturing this type of steel sheet. In order to continuously cast thin slabs for low-nitrogen steel sheets, from the converter process to the casting process, contamination of the molten steel by nitrogen gas in the atmosphere, that is, preventing [N] pick-up, is necessary to reduce the nitrogen content of the molten steel. It is necessary to control the amount.

Usually, molten steel tapped from a converter has a high amount of dissolved oxygen [0] and a high amount of impurities such as sulfur [S] and phosphorus [P], so it is removed by ladle refining method (LF method) before casting. Treat with acid, desulfurization, and dephosphorization.

In the conventional ladle refining method, the ladle is covered with a lid, the inside of the ladle is replaced with argon gas, and stirring gas and auxiliary raw materials are injected into the molten steel while submerged arc heating the molten steel in an argon gas atmosphere.

[Problems to be Solved by the Invention] However, in the conventional ladle refining method, the pressure inside the ladle fluctuates due to the injection of stirring gas, and outside air enters the ladle through the gap in the lid. The exposed surface of the molten steel is contaminated by the intruding outside air due to the floating gas, and [N] pickup occurs.

For this reason, when producing low-nitrogen steel by the ladle refining method, various [N] surprise prevention measures have been conventionally taken, as shown in (1) to (3) below.

(1) Inert gas seal method (2) Vacuum treatment method (V-K I P) (3) Electromagnetic stirring method However, all of the above (1) to (3) have high running costs and equipment costs. , not suitable for length. Therefore, after ladle refining, it is necessary to further denitrify the molten steel using an RH degassing facility.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a ladle refining method that can reliably prevent [N] surprise-up of molten steel at low cost.

[Means for Solving the Problems] A ladle refining method according to the present invention is a ladle refining method in which electrodes are immersed in slag to submerged arc heat the molten steel and stirring gas is blown into the molten steel. It is characterized by supplying it into the ladle lid and creating a carbon dioxide gas atmosphere in the gas phase that comes into contact with the molten steel.

The inventors replaced the inside of the ladle with various gases and discovered that [N
] I researched the pickup. As a result, it was found that a gas containing oxygen as a component, such as carbon dioxide gas, is suitable as a replacement gas.

In this case, it is also possible to replace the atmosphere inside the ladle with oxygen gas. However, although oxygen gas has a high effect of preventing [N] 2 pickup, it is not suitable as a replacement gas because the amount of deoxidizing agent added increases, a large amount of alumina is generated, and the cleanliness of molten steel decreases.

[Function] In the ladle refining method according to the present invention, when stirring gas is blown into the molten steel, slag is removed from the molten steel in the floating region of the stirring gas, the molten steel surface is partially exposed, and the inside of the ladle is removed. The molten steel tends to be contaminated by the air that has entered the atmosphere, but since carbon dioxide gas, which is an oxidizing gas, exists at the gas-liquid interface, the [0] concentration at the interface increases, and the [N ] Bick-up is effectively prevented.

? Embodiments] Hereinafter, embodiments of the present invention will be briefly described with reference to the accompanying drawings.

Undeoxidized tapped molten steel is stored in a ladle and transported to the LF equipment. The ladle is mounted on a traveling truck, and the rails of the traveling truck extend from the LF equipment to the RH degassing equipment and the foundry, respectively. The molten steel in the ladle is covered with slag. The ladle is covered with a lid. A gas supply pipe and a gas exhaust pipe are respectively attached to the lid, so that the gas in the ladle is exhausted while the CO2 gas is supplied into the ladle. A flow rate control valve is attached to the base end side of the CO2 gas supply pipe. Furthermore, an addition hole is provided in the lid, so that alloy materials and auxiliary materials can be added into the pot.

Three electrodes pass through the lid near the center, each tip immersed in the slag layer covering the molten steel. Further, an injection lance passes through the vicinity of the periphery of the lid, and its tip is immersed in the molten steel. The proximal end of the injection lance is connected to an argon gas supply source and a border supply source.

Next, the case of ladle refining of low nitrogen steel will be explained.

The molten steel before treatment is in an undeoxidized state, and the amount of [C] is approximately 40
0 ppll, [0] amount is about 600 ppm, and [N] amount is about 10 to 15 ppI1. When the ladle arrives at the LP equipment, the lid is placed on it, CO2 gas is supplied, and the inside of the ladle is heated to C.
Create an O2 gas atmosphere. Meanwhile, slag material and metal aluminum are added to the molten steel in the pot through the addition hole, and electricity is applied to the electrode.
Submerged arc heating of molten steel. This deoxidizes the molten steel and reduces the [0]ffi to a level of about 1-5 ppm.

When CO2 gas replacement is completed, argon gas is supplied to the injection lance to gas-stir the molten steel. By blowing in stirring gas, the pressure inside the pot fluctuates, and outside air enters the pot through the gap in the lid.

By the way, slag is removed in the region where the stirring gas floats to the surface, and molten steel is exposed in a part of the scene. When the exposed surface of the molten steel comes into contact with the intruding outside air, the nitrogen gas contained in the outside air tries to dissolve into the molten steel, but since CO2 gas exists in the gas phase, the [0] level at the gas-liquid interface increases and the molten steel [N] Bickup to [N] is effectively prevented. As a result, the molten steel after treatment has a [N] content of approximately 15 ppm.

FIG. 1 is a graph showing a comparison between processing under a CO2 gas atmosphere and processing under an argon gas atmosphere, with the horizontal axis representing the processing time and the vertical axis representing the amount of [N] picked up. In the figure, white circles indicate the results of processing under a CO2 gas atmosphere (Example), and black circles indicate the results of processing under an argon gas atmosphere (Comparative Example). As is clear from the figure,
In the comparative example, a [N] pick-up amount of 0.2 ppm per minute was obtained, whereas in the example, a [N] pick-up amount of 0.01 to 0.02 ppm per minute was obtained. It was possible to significantly reduce the amount of [N] picked up compared to the conventional method.

In the above example, CO2 gas was used as the replacement gas in the pot in the form of I2, but the present invention is not limited to this.
Steam gas (H20 gas) may be mixed with CO2 gas.

[Effects of the Invention] According to the present invention, [N] surprise of molten steel can be reliably prevented at low cost by the simple operation of supplying carbon dioxide gas into the ladle. Since carbon dioxide gas is cheaper than argon gas, the manufacturing cost of low nitrogen steel can be significantly reduced compared to conventional methods.

In addition, the post-process RH degassing treatment can be omitted,
It is also possible to simplify logistics within the steel factory.

As a result, it is expected that significant cost reductions will be realized in the integrated production system for low nitrogen steel.

[Brief explanation of drawings]

FIG. 1 is a graph diagram for explaining the effects of the embodiment of the present invention.

Claims (1)

[Claims] In the ladle refining method, in which stirring gas is blown into the molten steel while electrodes are immersed in slag to submerged arc heat the molten steel, carbon dioxide gas is supplied into the ladle lid and the gas phase in contact with the molten steel is made into a carbon dioxide gas atmosphere. A ladle refining method characterized by: