JPH0741840A - Ultra low carbon steel melting method - Google Patents

Abstract

(57) [Abstract] [Object] The present invention provides a method for efficiently and economically producing extremely low carbon steel in a ladle type degassing apparatus. [Structure] When performing decarburization treatment of molten steel under reduced pressure using a ladle-type degasser, [C] concentration is 0.0
In the range of 03 mass% or more, molten steel with CaO-containing substance,
By adding one or a mixture of two or more kinds of MgO-containing substances and solidifying the liquid slag, the generation sites of CO bubbles are increased and the decarburization reaction is promoted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a ladle type degassing apparatus in which the content of carbon (hereinafter referred to as [C]) in molten steel is removed to an extremely small amount, for example, 0.0015 mass% or less, The present invention relates to an efficient and economical method for producing ultra low carbon steel.

[0002]

2. Description of the Related Art A ladle type degassing apparatus is disclosed in, for example, R.K. J.
Fruehan ed "VACUUM DEGASSING"
OF STEEL "(AIME, 1990), p. 2
3 to p. As shown in 27, the entire ladle charged with molten steel is charged into a vacuum tank, and under a reduced pressure, an inert gas is blown into the molten steel to decarburize, dehydrogenate and denitrify the molten steel. It is a device for performing degassing treatment such as. When melting ultra low carbon steel,
After roughly decarburizing to about 0.03 mass% in a converter which is a primary refining furnace, it is decarburized to a predetermined [C] concentration in a ladle-type degasser.

Generally, the decarburization reaction of molten steel under reduced pressure is classified into the following three types. That is, (A) a reaction between [C] and oxygen in the molten steel (hereinafter, referred to as [O]) at the interface between the solid and the molten steel having poor wettability with the molten steel, in which case CO bubbles are generated. (B) Reaction between [C] and [O] on the molten steel free surface exposed to a reduced pressure atmosphere. (C) A reaction between [C] and [O] occurring at the interface between the argon bubbles blown into the molten steel and the molten steel. Classified as and.

Of these reactions, the [C] concentration is 0.00
It has been clarified that the reaction of (A) is predominant in the range of 3 mass% or more. The decarburization rate in this region is expressed by equation (1). −dC _c / dt = k (C _c −P _o + P _s ) / K · C _o ) (1) where C _c is [C] concentration, C _o is [O] concentration, t is time, k is Mass transfer capacity coefficient of decarburization reaction, K is reaction [C]
+ [O] = CO (g) equilibrium constant, P _o is vacuum chamber internal pressure, and P _s is slag static pressure.

[0005]

[Problems to be Solved by the Invention] After coarse decarburization in a converter,
When tapping steel in a ladle, slag flows out from the converter into the ladle.
Usually, the slag or low melting point slag CaO-SiO ₂ -Al ₂ O ₃ system, a slug of liquid. In the ladle type degasser, since the liquid slag coats the molten steel surface, the value of P _{s in} the equation (1) becomes large and the decarburization rate becomes small. Furthermore, P _o when the reaction starts of (A) along with shifts to the low pressure side, the higher the [C] concentration at which the reaction is completed in (A). Therefore, it is difficult to rapidly produce ultra low carbon steel.

[0006]

[Means for Solving the Problems] Therefore, in order to solve the above-mentioned problems and easily produce ultra-low carbon steel, a ladle containing the molten steel is placed in a vacuum tank and the molten steel is removed under reduced pressure. When carrying out the decarburization treatment, when the carbon concentration in the molten steel is 0.003 mass% or more, CaO is added to the molten steel.
Containing substances, MgO-containing substances, or a mixture of two or more of them, and solidifying the slag in the ladle, a method for melting ultra-low carbon steel, and using CaO-containing substances quicklime and dolomite Invented a method for melting ultra-low carbon steel, characterized by using dolomite, magnesia brick scrap, magnesia chromite brick scrap as the MgO-containing substance .

[0007]

The present invention will be described in detail below. The present invention is intended to accelerate the decarburization by the reaction (A) among the above three kinds of decarburization reactions to shorten the decarburization treatment time and reduce the reached [C] concentration. What is important in the reaction (A) is that CO bubbles are generated at the interface between the molten steel and a solid substance having poor wettability with the molten steel, such as a ladle refractory. This means that the interface between the solid slag and the molten steel can also serve as a CO bubble generation site. In order to increase the decarburization rate, it is important to increase the CO bubble generation site. Therefore, solidifying the liquid slag and using it as the CO bubble generation site is effective for promoting decarburization. Since the slag that has been inhibiting the decarburization reaction becomes a reaction site that promotes the decarburization reaction, the effect of the reaction of (A) becomes longer in the manifestation period, and the [C] concentration is reduced more quickly, and the reaction is reduced to lower concentrations. You will be able to charcoal.

In order to solidify the liquid slag, it is effective to add one or a mixture of two or more of CaO-containing substance and MgO-containing substance having a high melting point to the molten steel.
The reasons for limiting the [C] concentration when one or more of CaO-containing substances and MgO-containing substances are added to molten steel will be described. As described above, the [C] concentration is 0.003 ma.
In the range of less than ss%, CO bubbles are hardly generated, and even if the slag is solidified to increase the generation sites of CO bubbles, the decarburization reaction cannot be promoted. On the other hand, in the range where the [C] concentration is 0.003 mass% or more, the decarburization rate increases as the CO bubble generation site increases. Therefore, CaO-containing substance, MgO
The concentration of [C] when one or more of the contained substances is added to the molten steel is 0.003 mass% or more. CaO
As the contained substance, quicklime or dolomite is advantageous because it is inexpensive and has a good effect of increasing the melting point of slag.

Further, since the MgO-containing substance is inexpensive, it has a good effect of increasing the melting point of slag,
Dolomite, magnesia brick waste, and magnesia chromite brick waste are advantageous. The method of adding the CaO-containing substance or the mixture of one or more of the MgO-containing substances to the molten steel is to drop it from above the ladle onto the surface of the molten steel, or to use a lance immersed in the molten steel as the carrier gas. Any method of blowing into molten steel may be used, and the effect is the same. The CaO-containing substance and the MgO-containing substance added to the molten steel are preferably powdery or granular in terms of efficiently solidifying the slag.

[0010]

【Example】

Example 1 Initial component is [C]; 0.03 mass%, [Si];
0.1 mass% or less, [Mn]; 0.01 to 0.5 m
%, [P]; 0.005-0.02 mass%,
[S]; 0.003 to 0.02 mass%, [Al];
0.002 mass% or less, [O]; 0.03 to 0.0
A molten steel having a mass of 8 tons and a weight of 300 tons was decarburized by using a ladle type degassing device. The amount of slag before decarburization was about 2 tons. [C] concentration is 0.003mas
In the range of s% or more, 500 kg of quicklime was added to the molten steel, and decarburization treatment was performed while solidifying the slag. The change with time of the [C] concentration at this time is shown in FIG. Comparative Example 1 also shown in FIG. 1 is a case where decarburization treatment was performed without adding quick lime. In Comparative Example 2, the [C] concentration is 0.00
In the range of 3 mass% or more, quicklime is not added,
This is the case where the slag is solidified by adding quick lime to the molten steel in the range where the [C] concentration is less than 0.003 mass%.
In Comparative Example 1 and Comparative Example 2, the reached [C] concentration in the decarburization treatment for 20 minutes is 0.002 mass%, whereas the method of the present invention can decarburize up to 0.0015 mass%.

Example 2 Initial components are [C]; 0.03 mass%, [Si];
0.1 mass% or less, [Mn]; 0.01 to 0.5 m
%, [P]; 0.005-0.02 mass%,
[S]; 0.003 to 0.02 mass%, [Al];
0.002 mass% or less, [O]; 0.03 to 0.0
A molten steel having a mass of 8 tons and a weight of 300 tons was decarburized by using a ladle type degassing device. The amount of slag before decarburization was about 2 tons. [C] concentration is 0.003mas
In the range of s% or more, 500 kg of the slag solidifying agent shown in Table 1 was added to the molten steel, and decarburization treatment was performed while solidifying the slag. Table 1 also shows the reached [C] concentration after 20 minutes of decarburization treatment.

[0012]

[Table 1]

In the comparative example, [C] after decarburization treatment for 20 minutes
While the concentration is 0.002 mass%, according to the method of the present invention, up to 0.0015 mass% or less [C]
The concentration can be reduced.

[0014]

EFFECTS OF THE INVENTION According to the method of the present invention, an ultra low carbon steel having a [C] concentration of 0.0015 mass% or less can be easily produced in a ladle type degassing apparatus without extending the decarburizing time. Became.

[Brief description of drawings]

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

Claims (3)

[Claims] 1. When a ladle charged with molten steel is charged into a vacuum chamber and decarburization treatment of molten steel is carried out under reduced pressure,
A pole characterized by adding a CaO-containing substance, a MgO-containing substance, or a mixture of two or more thereof to the molten steel so that the carbon concentration in the molten steel is 0.003 mass% or more, and solidifying the slag in the ladle. Method for melting low carbon steel. 2. The method for melting ultra-low carbon steel according to claim 1, wherein quicklime and dolomite are used as the CaO-containing substance. 3. The method for melting ultra-low carbon steel according to claim 1, wherein dolomite, magnesia brick waste, and magnesia chromite brick waste are used as the MgO-containing substance.