JPH05239532A - Method for melting extreme-low carbon steel - Google Patents

Abstract

PURPOSE:To easily obtain an extreme-low carbon steel by adding a solid material containing gas component to molten steel having a prescribed or lower carbon concn. in the reduced pressure, increasing gas-liquid reaction interface area by the generated gas and promoting decarburizing reaction. CONSTITUTION:A molten steel having <=0.005mass% carbon concn. is incorporated in a vacuum vessel, and to this molten steel, a solid material [e.g. Ca(OH)2, TiH2, etc.], containing a gas component is added. Then, by the gas instantaneously decomposed and generated, the gas-liquid reaction interface area is increased to promote the decarburizing reaction. In such way, the extreme-low carbon steel having <=0.001mass% carbon content in the molten steel is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vacuum degassing apparatus for molten steel in which the content of carbon (hereinafter referred to as [C]) in the molten steel is removed to an extremely small amount, for example, 0.001 mass% or less. The present invention relates to an efficient and economical method for producing low carbon steel.

[0002]

2. Description of the Related Art Generally, in the steelmaking industry, decarburization treatment of molten steel is carried out, for example, in the 3rd edition of the Iron and Steel Handbook II, Ironmaking and Steelmaking 671-6.
It is carried out using a vacuum decarburizer as shown on page 85. However, the [C] concentration is 0.005mas
When it is s% or less, the decarburization rate sharply decreases, and it is not easy to rapidly reduce the [C] content to an extremely small amount. It is said that this is because the decarburization reaction accompanied by the generation of CO bubbles from the inside of the molten steel is reduced, and the decarburization reaction mainly occurs on the molten steel free surface or the interface between the blown Ar bubbles and the molten steel. .. Therefore, the [C] concentration is 0.005
In the region of mass% or less, measures are taken to increase the gas-liquid reaction interface area to accelerate the decarburization reaction.

For example, in an RH vacuum degasser,
[C] H ₂ gas, H ₂ + for the purpose of increasing the gas-liquid reaction interface area and strong stirring of molten steel in the region of 0.01 mass% or less
Japanese Patent Publication No. 60-21207 discloses a method of accelerating the decarburization reaction by injecting hydrogen-containing substances such as Ar gas and ammonia gas into molten steel and performing hydrogen gas boiling.

[0004]

In the case of this method, when the hydrogen-containing gas is blown into the molten steel, it is possible to stably blow the gas into the molten steel due to abnormal melting damage of the porous brick or the immersion lance for blowing the gas. It is difficult. Furthermore, since hydrogen is once dissolved in molten steel, a step of removing hydrogen after the decarburization treatment is necessary. This leads to an increase in processing time and is economically disadvantageous.

[0005]

The gist of the present invention is that the decarburization treatment of molten steel under reduced pressure is performed in a vacuum chamber in a region where the carbon concentration in the molten steel is 0.005 mass% or less. In the method for smelting ultra-low carbon steel, a solid substance containing a gas component is added to the molten steel.

[0006]

The present invention will be described in detail below. The essence of the present invention is to increase the gas-liquid reaction interface area by the gas generated by momentary decomposition when a gas component-containing substance is added to molten steel. Generally, the decarburization reaction of molten steel under reduced pressure is roughly classified into the following three types. (1) Reaction between [C] and [O] inside the molten steel and on the refractory surface. In this case, CO bubbles are generated. (2) Reaction between [C] and [O] on the molten steel free surface exposed to the reduced pressure atmosphere. (3) Reaction between [C] and [O] that occurs at the interface between the argon bubbles blown into the molten steel and the molten steel.

Of these reactions, the [C] concentration is 0.00
It has been clarified that the reaction (1) is predominant in the region of more than 5 mass%. In this region, CO bubbles are actively generated from inside the molten steel, and even if a gas component-containing substance is added to the molten steel to increase the gas-liquid reaction interface area, the effect of promoting the decarburization reaction is small. On the other hand, when the [C] concentration is 0.
In the range of 005 mass% or less, the decarburization reaction is (2)
The reaction mainly occurs at the molten steel free surface in (3) and the interface between the argon bubbles in (3) and the molten steel. In this region, increasing the gas-liquid reaction interface area is important for promoting the decarburization reaction. Therefore, the [C] concentration when the gas component-containing substance is added is 0.005 mass%.

The gas component-containing substance to be added to the molten steel for promoting decarburization is preferably a substance which decomposes immediately upon contact with the molten steel to generate gas and does not contain a carbon source.
Therefore, as a substance containing a gas component, Ca (O
H) ₂ , Mg (OH) ₂ , Fe (OH) ₂ , TiH ₂ ,
MgH ₂ , VH ₂ , ZrH ₂ , and TiFeH ₂ are used. Furthermore, even if these substances are added alone,
Two or more kinds may be mixed and added, and the decarburization promoting effect is the same.

When Ca (OH) ₂ or Mg (OH) ₂ is added to the molten steel, bubbles are generated by the following reaction. Ca (OH) ₂ → CaO + H ₂ O → CaO + [O] + H
₂ Mg (OH) ₂ → MgO + H ₂ O → MgO + [O] + H
_{2 When a} hydrogen-containing gas is blown into molten steel, it is difficult to stably blow the gas into the molten steel due to abnormal melting damage of the porous brick and the immersion lance for blowing the gas. Furthermore, since hydrogen is once dissolved in molten steel, a step of removing hydrogen after the decarburization treatment is necessary. This leads to an increase in processing time and is economically disadvantageous.

When a solid substance is added to molten steel as in the present invention, there is no abnormal melting loss of porous brick or lance,
Since the amount of hydrogen dissolved in the molten steel is small, the dehydrogenation step is unnecessary. The gas component-containing substance may be added either by adding it from above the molten steel or by immersing the lance in the molten steel and blowing an inert gas into the molten steel as a carrier gas.

The present invention can be applied to various vacuum degassing devices such as RH, DH and VOD. further,
The present invention is also effective in promoting the denitrification reaction that occurs at the gas-liquid interface as in the decarburization reaction.

[0012]

【Example】

Example 1 Initial component is [C]; 0.02 mass%, [Si];
0.1 mass% or less, [Mn]; 0.01 to 0.5 m
%, [P]; 0.005-0.02 mass%,
[S]; 0.003 to 0.015 mass%, [A
l]; 0.002 mass% or less and a weight of 300 tons of molten steel were decarburized using an RH vacuum degassing apparatus. When the [C] concentration reached 0.005 mass%, the molten steel in the vacuum chamber was filled with Ca (O
H) ₂ 300 kg was added continuously. FIG. 1 shows the change with time of the [C] concentration at this time. Comparative Example 1
[C] concentration is 0.01 mass% or less 0.005mass
In the area of more than s%, the molten steel in the vacuum tank has a grain size of 1 to 2 m from above.
3 is a change with time in [C] concentration when 300 kg of Ca (OH) _{2 of} m was continuously added. [C] concentration is 0.00
In the region of more than 5 mass%, the decarburization promoting effect is comparatively small even if the gas component-containing substance is added, and the [C] concentration after the decarburization treatment for 20 minutes is about 0.0014 mass%. When the gas component-containing substance is added in the region where the [C] concentration is 0.005 mass% or less, the decarburization promoting effect is large, and the [C] concentration after the decarburization treatment for 20 minutes is 0.0008 m.
It became ass%.

Example 2 Initial component is [C]; 0.02 mass%, [Si];
0.1 mass% or less, [Mn]; 0.01 to 0.5 m
%, [P]; 0.005-0.02 mass%,
[S]; 0.003 to 0.015 mass%, [A
l]; 0.002 mass% or less and a weight of 300 tons of molten steel were decarburized using an RH vacuum degassing apparatus. When the [C] concentration reached 0.005 mass%, 300 kg of the gas component-containing substance shown in Table 1 having a particle size of 1 to 2 mm was continuously added to the molten steel in the vacuum chamber from above. The [C] concentration 20 minutes after the start of the decarburization treatment is also shown in Table 1. Comparative Example 2 is a case where CaCO ₃ containing a carbon source was added. When CaCO ₃ is added to the molten steel, the generated CO ₂ is reduced and carbon is dissolved in the molten steel, so the decarburization promoting effect is smaller than that of the gas component-containing substance used in the present invention. [C] concentration of 0.
It is about 0015 mass%. On the other hand, when the gas component-containing substance used in the present invention was added, after the decarburization treatment for 20 minutes, the [C] concentration was 0.001 mass.
It can be reduced to s% or less.

[0014]

[Table 1]

[0015]

According to the present invention, the [C] concentration is 0.001.
It has become possible to easily produce an extremely low carbon steel having a mass% or less.

[Brief description of drawings]

FIG. 1 is a diagram showing a change with time of a [C] concentration.

Claims (1)

[Claims] 1. When carrying out decarburization treatment of molten steel under reduced pressure, the carbon concentration in the molten steel is 0.005 mass.
% Or less, a method for melting ultra-low carbon steel, characterized in that a solid substance containing a gas component is added to the molten steel in the vacuum chamber.