JP4639497B2 - Method for refining high carbon steel - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for refining high carbon steel such as tool steel, bearing steel, or tire cord steel.
[0002]
[Prior art]
Up to now, the melting of high carbon steel has been mainly carried out by secondary refining using an RH type or DH type vacuum degassing apparatus after rough decarburization in a converter. This is because decarburization, dehydrogenation, deoxidation, and the like can be performed at high levels by stirring undeoxidized or semi-deoxidized molten steel under high vacuum to induce a CO reaction. However, as the demand for high carbon steel and special steel increases, production is limited only by the RH or DH degassing equipment. Therefore, in order to effectively use the existing equipment, a technique for melting high carbon steel has been required even in the VOD equipment which is usually used mainly for refining stainless steel. As a technique for melting high carbon steel using equipment similar to VOD, the atmospheric pressure is set to 30 to 150 Torr (3.95 to 19.74 kPa), as shown in Japanese Patent Publication No. 1-46563. In addition, there is a technique for performing deoxidation / denonmetal inclusion treatment by injecting an inert gas from the bottom to induce a strong CO reaction.
[0003]
[Problems to be solved by the invention]
In the invention disclosed in Japanese Examined Patent Publication No. 1-46563, the upper end of the ladle is covered with an upper hermetic cover connected to the vacuum exhaust device via an exhaust duct, and the pressure in the upper space of the ladle is 30 to 150 torr. While maintaining a low degree of vacuum, an inert gas is blown from the bottom of the ladle to induce a CO gas boiling reaction to stir the molten steel and slag.
[0004]
However, if the above method is applied to the vacuum processing up to a high vacuum level of 1 torr (131.6 Pa) using the VOD equipment, the following problems occur and the operation becomes difficult. A malfunction occurred.
[0005]
Unlike the facility disclosed in Japanese Patent Publication No. 1-46563, the VOD facility is configured to accommodate the entire ladle in a vacuum vessel and perform a decompression process. When an intense CO reaction is induced by blowing an inert gas from the bottom of the ladle under vacuum, as shown in Japanese Patent Publication No. 1-46563 using such VOD equipment, When the killing process was performed, a sudden CO reaction occurred, and the slag and molten steel foamed and expanded, causing a so-called overflow problem in which the slag and molten steel were scattered in the vacuum vessel. When slag or molten steel was scattered in the vacuum vessel, there were problems such as burning the sliding nozzle drive provided at the bottom of the ladle and making it impossible to process the next charge.
[0006]
In the present invention, when a high carbon molten steel after rough decarburization in a decarburization furnace or the like is subjected to secondary refining under a vacuum using a VOD facility, a rapid gas accompanying a decarburization reaction also in a high carbon region. An object of the present invention is to provide a technique for stably processing without generating overflow (slag from a ladle or outflow of molten steel).
[0007]
[Means for Solving the Problems]
The present invention has been made to solve the above problems, and in order to prevent overflow when performing acid feed heat treatment or vacuum kill treatment using a VOD facility, a rapid CO reaction in molten steel is required. In order to do so, it was considered that the molten steel being heated by acid supply or being killed should be sufficiently deoxidized. For example, in the case of deoxidation with Al, if Al is sufficiently dissolved in the molten steel being killed, oxygen in the molten steel should be consumed for the combustion of Al and no overflow with a rapid CO reaction should occur. The present invention has been reached from the idea that there is. That is, in the present invention, when high-carbon steel is vacuum-refined using a VOD facility, the C concentration in the molten steel is measured before the start of the vacuum refining, and is balanced with the C concentration in the molten steel under the pressure in the vacuum vessel. After obtaining the oxygen activity , obtaining the concentration of the deoxidizing material that is in equilibrium with the oxygen activity , adding the deoxidizing material to the molten steel so that the concentration of the deoxidizing material is equal to or higher, and then performing vacuum refining. This is a feature of refining high carbon steel. Here, the deoxidizing material refers to Al, Si, Ti, etc. that are usually used for deoxidizing molten steel.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a graph showing the concentration of C and Al in molten steel on the horizontal axis and the equilibrium oxygen activity on the vertical axis. When starting vacuum refining of high-carbon steel using VOD equipment, sampling is performed to measure C% in molten steel, and the necessary deoxidation element concentration (%) from the oxygen activity in equilibrium with the C% (FIG. 1 shows an example in which Al is used as a desulfurization element.). The decarburization reaction is prevented by adding additional Al and constantly maintaining the Al concentration in the killed above the limit Al concentration (%). And the generation | occurrence | production of overflow (outflow of slag and a bullion into the vacuum tank) is prevented by suppressing generation | occurrence | production of CO gas accompanying a decarburization reaction. For example, as indicated by broken lines 1, 2 and 3 in FIG. 1, when the degree of vacuum is 100 Torr (13.16 kPa) and [C] = 0.45%, the limit Al concentration = 0.015%. Therefore, it is only necessary to keep the Al concentration in the killed at 0.015% or more.
[0009]
Here, vacuum refining refers to the entire process under reduced pressure using a VOD facility. In other words, in addition to the dehydrogenation reaction and the floating separation treatment of the deoxidized product, the alloy material addition / dissolution treatment, the addition of Al and the heat treatment of the molten steel by oxygen blowing from the top blowing oxygen lance, the desulfurization or desulfurization of the molten steel by slag Includes acid treatment.
[0010]
【Example】
A carburizing agent is added to the molten steel decarburized and refined in the converter at the time of steel removal to the ladle, the [C] concentration is set to 0.45%, and this molten steel is degassed and deoxidized by a VOD facility. Vacuum refining was carried out mainly for floating separation. The amount of molten steel was 180 tons. Before starting the vacuum refining, a molten steel sample was taken and analyzed for [C]. At this time, the [C] concentration was 0.45% by mass. Since the ultimate vacuum at the time of vacuum refining at the operation standard was 100 torr (131.6 Pa), the oxygen activity a _{0 in the} steel was adjusted to equilibrate with 0.45% by mass of [C] under this pressure as shown in FIG. Obtained from the graph, the [Al] concentration in the molten steel in equilibrium with the oxygen activity a ₀ in the steel was estimated to be 0.015% by mass. Therefore, before starting the vacuum refining, Al is added to the molten steel, and the [Al] concentration in the molten steel is adjusted to 0.030% by mass to start the vacuum refining, and the final vacuum is 100 torr (131.6 Pa). Vacuum refining for a minute was performed. As a result, it was possible to operate without causing overflow of molten steel or slag.
[0011]
As a comparative example, vacuum refining was started without adding Al to the molten steel with [C] = 0.45 mass%, and Al equivalent to 0.030 mass% was reached after the ultimate vacuum reached 100 torr (131.6 Pa). The process to add was performed. After the vacuum refining was completed, the top of the vacuum vessel was opened and the ladle was removed from the vacuum vessel. A large amount of molten steel was blown into the vessel, and part of the piping for supplying gas to the porous plug at the bottom of the ladle Was found to be burned out.
[0012]
【The invention's effect】
By carrying out the present invention, VOD vacuum treatment of high carbon steel can be performed stably without overflow trouble, and recovery work of slag / molten steel scattered in the vacuum vessel, which has occurred due to overflow trouble so far Stable treatment became possible without causing various problems such as operation hindrance and burning of piping at the bottom of the ladle.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between [C], [Al]% and equilibrium oxygen activity.
[Explanation of symbols]
1, 2, 3 dashed line

Claims (1)

When vacuum refining high carbon steel using VOD equipment, C concentration in the molten steel is measured before the start of vacuum refining, and the oxygen activity that balances with the C concentration in the molten steel under the pressure in the vacuum vessel is obtained. High-carbon, characterized in that after obtaining a deoxidizer concentration that balances with its oxygen activity, vacuum refining is performed after the deoxidizer is added to the molten steel so that the deoxidizer concentration is equal to or higher than the deoxidizer concentration Steel refining method.

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