JPH06116623A - Method for smelting high purity dead soft steel - Google Patents

Abstract

PURPOSE:To provide a method for smelting a high purity dead soft steel by an RH circulating type degassing apparatus. CONSTITUTION:In the smelting method for the dead soft steel executing decarburizing treatment after decarburizing unoxidized molten steel by the RH circulating type degassing apparatus, before the treatment by the RH apparatus, metallic Al-containing material is added into slag in a ladle to reduce lower oxide in the ladle slag. Thereafter, the flux mainly containing MgO is immediately added into the RH degassing vessel after starting the circulation in the RH apparatus and the decarburizing treatment is successively executed till coming to the aimed carbon concn. or lower. Further, by adding a deoxidizing alloy to execute the deoxidizing treatment, the high purity dead soft steel can be smelted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high cleanliness ultra-low carbon steel.

[0002]

2. Description of the Related Art Conventionally, for producing extremely low carbon steel, undeoxidized molten steel having a low carbon concentration produced in a converter is decarburized by using an RH reflux type degassing device, and then A deoxidizing alloy such as Al is added to perform deoxidizing treatment.

[0003]

[Problems to be Solved by the Invention] However, the RH
The slag in the ladle in the reflux degasser is mixed with the slag flowing out from the converter, so the concentration of lower oxides such as iron oxide is high, so a deoxidizing alloy such as Al is added. When performing deoxidation treatment by means of oxygen, the deoxidizing element dissolved in the molten steel is oxidized by oxygen from the slag to form non-metallic inclusions, and there is a problem that the cleanliness of the molten steel cannot be improved.

As a method for improving the cleanliness of molten steel,
For example, as disclosed in Japanese Patent Application Laid-Open No. 2-30711, Al or Al slag is added to ladle slag to reduce iron oxide in the ladle slag, and then decarburization treatment is carried out by an RH reflux type degasser. Then, a method of performing Al deoxidation is performed.
In order to produce ultra-low carbon steel with a total oxygen concentration [TO] of 20 ppm or less in molten steel by such a method, [T.Fe] in the slag after deoxidation treatment should be at least 2% or less, preferably It is necessary to reduce it to 1% or less, but considering variations in the amount of slag discharged from the converter and an increase in iron oxide concentration in slag during decarburization, [T.Fe] in slag after treatment by RH is considered. In order to reduce the slag in a stable manner, molten steel 1 should be used when deoxidizing slag.
It was necessary to add 0.5 to 1.0 kg of metallic Al per ton to the ladle so that the amount of Al was excessive.

However, when such excess Al is present during decarburization, Al reacts with dissolved oxygen in the molten steel, so that the oxygen concentration in the molten steel is reduced and the decarburization rate is significantly reduced. there were. A method of performing the slag reduction treatment after the decarburization treatment is also conceivable, but this method requires that carbon be picked up from the reducing flux and
Since the reactivity of slag has decreased after the treatment with
Since a stirrer for accelerating the reduction of slag is required and it is necessary to perform deoxidation treatment using RH again after the reduction treatment of slag, the processing time by RH per batch becomes long, and There were problems such as a significant drop in productivity.

It is an object of the present invention to provide a method for melting a high cleanliness ultra-low carbon steel which solves the above problems.

[0007]

The present invention relates to an RH reflux degassing method for melting ultra-low carbon steel, in which undeoxidized molten steel is decarburized by an RH reflux degasser and then deoxidized. Before treatment with the equipment, a metal Al-containing substance is added to the ladle slag to reduce lower oxides such as FeO and MnO in the ladle slag, and then MgO is mainly contained in the RH reflux degasser immediately after the start of reflux. After adding the flux, which is a component, into the RH degassing tank, decarburizing treatment is continued until the target carbon concentration falls below the target carbon concentration, and further deoxidizing alloy is added to perform deoxidizing treatment. It is a method of melting ultra low carbon steel.

[0008]

According to the present invention, when the ladle slag is reduced by the metal Al-containing substance and then decarburized by RH, the flux containing MgO as a main component is immediately removed from the RH degassing tank immediately after the start of RH reflux. By adding it to the inside, a MgO enriched layer is formed between the undeoxidized molten steel and the unreacted metal Al-enriched layer, and the entrainment of the unreacted metal Al into the molten steel is suppressed, whereby the metallic Al Unreacted metal Al
Even if a large amount of carbon is present, it is possible to prevent the decarburization rate from decreasing at the initial stage of decarburization.

In the case where there is no unreacted metallic Al, the conventional method shows an increase in the iron oxide concentration in the slag due to the movement of oxygen from the molten steel during the decarburization treatment. However, in the present invention, the increase in iron oxide concentration in the slag is significantly suppressed by the effect of MgO addition, and the iron oxide concentration in the slag during the deoxidation treatment that is performed subsequent to the decarburization treatment can be reduced, so that it is cleaner. It is possible to melt extremely low carbon steel.

As described above, according to the present invention, even when the degree of reduction of the ladle slag and the degree of residual unreacted metal Al differ depending on, for example, variations in the iron oxide concentration in the converter tap steel slag and the converter outflow slag amount. Since the iron oxide concentration in the deoxidizing slag can be reduced without inhibiting the decarburization reaction, ultra-low carbon steel with higher cleanliness can be efficiently and stably melted.

When the metallic Al is charged into the ladle, the amount of the metallic Al added is preferably in the range of 0.3 to 1 kg / t per ton of molten steel. 0.3 kg
If it is less than / t, the reduction of iron oxide in the slag may be insufficient, and if it is more than 1 kg / t, it is not efficient because the residual amount of unreacted metal Al increases. Mg added at the start of RH treatment
The amount of O 2 -based flux added is MgO per ton of molten steel.
The range of 1 to 4 kg / t is good. If the MgO content is less than 1 kg / t, the barrier effect between unreacted metallic Al and molten steel is not sufficient, and if the amount of metallic Al added to the ladle slag exceeds 1 kg / t, decarburization inhibition at the initial stage of decarburization is observed. If the amount of MgO added to the ladle slag exceeds 4 kg / t, the deoxidation rate during deoxidation treatment tends to decrease, and the absorption rate of alumina inclusions into the slag decreases. Conceivable.

[0012]

EXAMPLES The results of carrying out the method of the present invention in an RH having a molten steel amount of 260 tons are described below. 0.04% carbon concentration in converter
Immediately after tapping the undeoxidized molten steel that has been decarburized to the ladle,
After adding an Al slag with a pure Al content of 50% to the ladle slag, 300 Nl / min of Ar gas was blown from the porous tuyere provided at the bottom of the pot and stirred for 3 minutes to reduce the slag. After that, vacuum decarburization treatment was performed using RH as shown in Fig. 1. About 30 seconds after the evacuation was started, the reflux was started.
% Or more of MgO clinker was added. At the beginning of decarburization,
Top blowing of oxygen gas was performed in the vacuum chamber at a blowing rate of 30 Nm ³ / min for about 3 minutes. After carrying out decarburization until the target carbon concentration became 25 ppm or less, metallic Al was added to adjust the Al concentration in the molten steel within the range of 0.04 to 0.05%, and deoxidation was performed for 10 minutes after adding Al.

The amount of MgO added at the start of RH treatment is 2 kg /
Figure 2 shows the relationship between the basic unit of metal Al for reduction treatment of ladle slag and the total oxygen concentration [TO] in molten steel after deoxidation in the case of t. When the basic unit of metal Al is 0.4 kg / t or more, the total oxygen concentration [TO] in molten steel can be stably reduced to 30 ppm or less. Moreover, even if the basic unit of metal Al is 1 kg / t or more,
It can be seen that the effect on the cleaning of the molten steel is small only by increasing the unreacted content.

FIG. 3 shows the relationship between the basic unit of metal Al for the reduction treatment of ladle slag and the decarburization treatment time and the effect of addition of MgO on the relation. By adding 2 kg / t of MgO,
Even when a large amount of Al is added, the adverse effect on the decarburization rate is smaller than that in the conventional method in which MgO is not added. The basic unit of metal Al for reducing ladle slag is 1.0 kg / t
Fig. 4 shows the relationship between the amount of MgO added and the decarburizing treatment time at the start of the treatment with RH in the case of. In the method of the present invention in which MgO is added, the decarburization time is significantly shortened as compared with the conventional method in which MgO is not added.If the MgO basic unit is 1.0 kg / t or more, decarburization inhibition by addition of Al slag is observed. Can be prevented.

Metal Al unit consumption for ladle slag reduction is 1.0 kg /
the amount of MgO added at the start of treatment with RH in the case of t
The relationship of [TO] is shown in Fig. 5. Compared with the conventional method in which MgO is not added, in the method of the present invention in which MgO is added, [TO] tends to be slightly reduced, and it is considered that reoxidation of slag during decarburization is suppressed. However, when the amount of MgO added is further increased to 4 kg / t or more, [TO] tends to increase, which is considered to be because the absorption rate of alumina inclusions in the slag decreases.

[0016]

As described above, according to the present invention, it is possible to produce a high-cleanliness ultra-low carbon steel with improved cleanliness of the molten steel without inhibiting decarburization during the decarburization treatment of the ultra-low carbon steel. Becomes

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an apparatus for carrying out the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a metal Al unit for ladle slag reduction treatment and [TO] after deoxidation treatment.

FIG. 3 is a characteristic diagram showing a relationship between a metal Al unit for ladle slag reduction treatment and decarburization treatment time.

FIG. 4 is a relational characteristic diagram between the amount of MgO added at the start of RH treatment and the decarburization treatment time.

FIG. 5 is a graph showing the relationship between [TO] and the amount of MgO added at the start of RH treatment.

Claims (1)

[Claims] 1. A method for melting ultra-low carbon steel in which undeoxidized molten steel is decarburized by an RH reflux degasser and then deoxidized, in which ladle slag is added before treatment by the RH reflux degasser. Addition of metallic Al-containing substance to FeO in ladle slag,
After reducing low-grade oxides such as MnO, and then adding flux containing MgO as a main component to the RH degassing tank immediately after the start of reflux in the RH reflux degasser, decarburization is continued until the carbon concentration falls below the target level. A process for producing a high cleanliness ultra-low carbon steel, which comprises performing a deoxidizing treatment by adding a deoxidizing alloy.