JPH08225826A - Vacuum decarburization of molten steel - Google Patents

Abstract

PURPOSE: To economically and efficiently remove carbon by blowing a gas essentially containing CO into molten steel in the region having specific carbon concentration at its early decarburizing phase when the molten steel accommodated in ladle is decarburized with evacuation/vacuum. CONSTITUTION: When decarburizing the molten steel accommodated in ladle with evacuation/vacuum, in the early phase of its decarburizing and in the region in which (C) is >=0.0020(mass%), OG gas (recovered gas at converter) essentially containing CO or COG gas (recovered gas at coke furnace) is blown into molten steel for the purpose of agitating circulating the molten steel. By this method, the decarburizing treatment of molten steel is executed at cost lower than that of conventional way.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a decarburizing method for economically and efficiently removing carbon [C] contained in molten steel under reduced pressure and vacuum.

[0002]

2. Description of the Related Art Carbon contained in steels and alloys is used in the case of steel sheets for automobiles and beverage cans.
A very small amount is required to improve workability and prevent aging.

Generally, in the steelmaking industry, decarburization treatment of molten steel or molten steel alloy (hereinafter simply referred to as molten steel) is performed, for example, as shown in pages 671 to 685 of Ironmaking, Ironmaking, 3rd Edition Handbook of Steelmaking II. Using various vacuum refining equipment,
It is carried out by utilizing the reaction represented by the formula (1).

[C] + [O] = CO (1) RH (Reinstahl Hutten) which is a typical equipment for vacuum decarburization refining
Werks Heraus) In vacuum degassing equipment, Ar is generally used as a gas for molten steel circulation. As a special case, as a denitrification reaction acceleration or decarburization reaction acceleration measure A
A technique of blowing a gas in which r is mixed with CO or H ₂ is disclosed (JP-A-57-117817 and JP-A-3-61).
No. 315), a technique of using CO ₂ as a circulating gas has been applied (WO94-29488). Generally, Ar and H ₂ are expensive, and in order to produce an extremely low carbon steel at low cost, it is used for molten steel circulation. It is urgent to reduce the cost of blowing gas or stirring gas. On the other hand, CO ₂ is low in cost and is an effective means for adding an oxygen source, but there is a concern that the molten steel temperature may decrease due to the decomposition reaction.

[0005]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention
An object of the present invention is to provide a method for carrying out vacuum treatment economically and efficiently for decarburizing and refining.

[0006]

The present invention has solved the above-mentioned problems, and its gist is as follows. In a method of decarburizing molten steel contained in a ladle using decompression / vacuum, at least [C] at the initial stage of decarburization is 0.0020.
A method for vacuum degassing and decarburizing molten steel, wherein the gas species blown into the molten steel for stirring the molten steel or circulating the molten steel in the region of (mass%) or more is a gas containing CO as a main component.

[0007]

[Function] Decarburization of molten steel in RH is roughly divided into the following reactions.

CO bubbles are generated inside the molten steel, and
It is released to the gas phase in the vacuum chamber.

The CO generated on the free surface of the molten steel is released into the gas phase inside the vacuum / vacuum chamber.

Of these, [C] is approximately 0.0020 (m
(ass%) or more, most of the decarburization reaction proceeds by the CO bubble bioreaction of the above item. Capacity factor kov decarburization speed by this reaction, as shown in FIG. 1, when the decompression rate conditions in the vacuum chamber is constant, the ratio Q _m / W with molten steel circulating rate Q _m and the molten steel weight W _{m It} increases with increasing _m . However, the value of k _{ov is} a value defined by the equation (2), and the molten steel recirculation velocity Q _m can be determined by the equation (3) (Trans.I).
SIJ, 28 (1988), p. 305).

[0011]

[Equation 1]

Q _m = α · F _gas ^1/3 · D ^4/3 · {In (P _in / P _t } ^1/3 (ton / min) (3) [C] _e : Apparent equilibrium [C ] Concentration (mass%) α: Constant (11.8) F _gas : Injection gas amount (Nl / min) D: Immersion pipe diameter (m) P _in , P _t : Pressure at gas injection position, inside vacuum chamber Pressure (mmHg) That is, the present inventors have found that the value of Q _m / W _m has almost the same effect regardless of whether Ar injection or OG gas injection in which CO is the main component is injected. ] Of about 0.0050 (mass%) or more is sufficient for decarburization treatment, OG gas blowing with CO as a main gas is sufficient as reflux gas.The cost ratio of Ar and OG gas is 20/1. Yes, by changing Ar to OG gas as the reflux gas,
It is possible to significantly reduce the cost of decarburization.

[C] is 0.0020 (mass)
%), The CO in the blown OG gas is decomposed by the following equation, which may hinder the progress of the decarburization reaction of the molten steel. Therefore, in the region where [C] is 0.0050 (mass%) or less, the molten steel reflux gas or the molten steel stirring gas is required to be a gas species containing no carbon content. Ar is optimal in this case.

CO → [C] + [O] (4) In carrying out the present invention, the blowing gas containing CO as a main component is OG gas which is a recovery gas in the converter or a coke oven. COG, which is a recovered gas of the above, is used. COG
The cost of gas is 1/10 of that of Ar, and it is possible to significantly reduce the cost of decarburization.

However, when a hydrocarbon-based gas is contained in the blown gas containing CO as a main component, the hydrocarbon is decomposed by the reaction of the formula (5) and the decarburization reaction is hindered. In order to prevent this, it is sufficient to contain water in the same mole as carbon in the contained hydrocarbon. That is, it is possible to prevent the hydrocarbon gas in the gas from changing into CO and H ₂ by the reaction of the equation (6), and carbon being absorbed by the molten steel.

C _x H _y = x [C] + y [H] (5) C _x H _y + xH ₂ O = xCO + (x + y) H ₂ (6) In the gas used for carrying out the present invention Fluctuations in CO concentration have little effect on the decarburizing effect obtained by the present invention.

[0017]

【Example】

Example 1 300 tons of molten steel was decarburized using an RH degassing facility. However, the inner diameter of the immersion pipe is 65 to 75 cm, and the blowing gas for molten steel circulation is Ar, CO, OG gas, and CO.
It is G gas, and its blowing amount is 1500 to 4000 (N
1 / min) (the typical composition of the blown gas is shown in Table 1). However, a gas containing COG gas containing the same mole of water as CH ₄ was also used.

The pumping capacity of the pump is constant, and the pressure change in the vacuum chamber is almost the same even if the reflux ratio changes.
After in, the vacuum chamber pressure reached 5 mmHg or less.
The [O] concentration is in the range of 0.045 to 0.060 mass%.

FIG. 1 shows the relationship between the capacity coefficient k _{ov of the} decarburization rate and the reflux ratio Q _m / W _m . However, k _ov is a value when [C] is 0.0030 (mass%) or more.

The value of k _ov is Q _m /
It increases with an increase in W _m , the results of the method of the present invention and the conventional method are almost the same, and economical decarburization treatment is possible.

[0021]

[Table 1]

Example 2 300 tons of molten steel was decarburized using an RH degassing facility. However, the inner diameter of the dipping pipe was 65 cm, the blowing gas for molten steel circulation was CO gas, OG gas, and COG gas, and the blowing amount was 2500 (Nl / min). The [O] concentration is in the range of 0.045 to 0.060 mass%. 11 minutes after the start of decarburization, [C] is 0.0
When reaching about 030 (mass%), the blowing gas is switched to Ar, and the blowing rate of Ar is 2500 (N
1 / min). The pumping capacity is constant,
The pressure changes in the vacuum chambers were almost the same in both cases. However, Table 1 shows the typical composition of the blown gas.

FIG. 2 shows the change with time of [C]. For comparison, the change with time of [C] during the decarburization treatment by the usual Ar blowing is shown. The results of the method of the present invention and the conventional method are almost the same, and economical decarburization treatment is possible.

[0024]

EFFECTS OF THE INVENTION The decarburization treatment of molten steel can be carried out at a lower cost than the conventional method, and a more economical decarburization treatment is possible.

[Brief description of drawings]

FIG. 1 is a drawing showing the relationship between k _ov and Q _m / W _m .

FIG. 2 is a drawing showing a change with time of [C] during decarburization treatment.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akito Kiyose 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Steel Works

Claims (3)

[Claims] 1. A method for decarburizing molten steel stored in a ladle by using decompression / vacuum, in which molten steel is agitated or agitated at least in a region where [C] is 0.0020 (mass%) or more in the initial stage of decarburization. A vacuum decarburization method for molten steel, characterized in that a gas having CO as a main component is used as a gas species to be blown into the molten steel for circulating the molten steel. 2. The vacuum decarburization method for molten steel according to claim 1, wherein OG gas or COG gas is used as a gas containing CO as a main component. 3. The vacuum decarburizing method for molten steel according to claim 2, wherein when COG is used, water is added so that the number of moles is the same as the number of moles of carbon in the hydrocarbon gas. Vacuum decarburization method.