JPH0324220A - Decarbonization of molten steel containing chlorum - Google Patents

Abstract

PURPOSE: To reduce gas cost by feeding CO₂ of a prescribed quantity as treated gas into the molten steel to replace N₂, argon, etc., with CO₂ in blowing oxygen and the treated gas in the molten steel containing ≥10% Cr for decarburization.

CONSTITUTION: In a bottom-blown converter, O₂ and the treated gas as the process gas is blown for decarburization into the molten steel containing ≥10% Cr. CO₂ is used as the treated gas, and the molten steel is decarburized to 0.06% C. CO₂ of 0.2-1.5m³/m³.min (relative to the weight of the molten steel) is additionally fed into the molten steel through a top lance.

COPYRIGHT: (C)1991,JPO

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention resides in the following aspects of the invention:
The present invention relates to a method for decarburizing chromium-containing molten steel having a Cr content of 10% or more in a bottom blast converter.

[Prior Art] Such secondary steel refining of molten steel is performed using oxygen as a process gas and at least one of inert gases argon and nitrogen as a process gas. These refining methods are abbreviated as MRP (Metal Raffinati).
ons process [metal refining method])
, AOD (Argon Oxygen Deca)
r-burization r-argon/oxygen decarburization]
), ASM( Argon Sekundar
M etallurgie [Argon secondary metallurgy 1
) and U B D (Under BottomBl
owing Decarburization,
It is known as [furnace blast decarburization]). West German Patent No. 2
It is known from No. 430,975 that in this case nitrogen and argon are partially replaced by mixing with CO2.

The role of the inert gases argon and nitrogen is to reduce the partial pressure of CO in the melt and in the atmosphere of the processing vessel to the desired working conditions. In this case reactions (1) to (3) are important: 2C + 02 = 2
CO (1)302 +
4C:r-2CrO3
(2) 2Cr + 3CO
= 3 C + Cr203 (3) By diluting oxygen with an inert gas according to the balance of carbon, oxygen, and chromium, decarburization of high chromium-containing molten metal is possible with slight Cr burnout.

t; For example: gL degree 1700℃, the chromium content of the molten metal is C
For r18%, chromium slagging begins at the following CO partial pressure: pcO=i bar C 0.22% or less Pco
= 0.5 bar G O. 13% or less Pco=0
．． 1 bar G O. For this reason, the decarburization of high-chromium-containing melts in bottom-blown converters is carried out in several decarburization phases, depending on different equilibrium conditions. For example, the AOD process utilizes three decarburization phases: In the first phase, the molten metal is heated with a gas mixture consisting mainly of oxygen to the desired working temperature of 1700° C. or higher.

In the second phase, a high inert gas to oxygen ratio is used to prevent chromium slagging.

In other words, Cr/C/P. In the third phase, which maintains an equilibrium of
icency (the rate at which the introduced oxygen reacts with carbon) is accepted, and at the same time the molten metal is reduced to 0.0% in some steps.
It is decarburized to 0.01%.

The amount of inert gas required is very high. Reaction: 02 +
Ar/N2 +2G-2GO+Ar/N2 (4
) indicates that the amount of inert gas (Ar/N2) injected must be twice as large as the amount of oxygen in order to reduce the CO partial pressure to 0-5 bar. Therefore, the lowest CO partial pressure of 0.1 bar can theoretically only be reached using a gas mixture of 1:20 oxygen to inert gas. The CO partial pressure during decarburization using an oxygen/inert gas mixture is higher than the Co partial pressure required for reaction equilibrium. This causes chromium slag formation according to reaction formula (2). However, this reaction does not produce CO. Since the amount of inert gas is kept constant, a decrease in CO partial pressure occurs in the system.

Therefore, the oxidation of carbon according to reaction formula (1) becomes dominant again.

Table l represents the interaction between the oxygen/inert gas ratio and the CO partial pressure resulting from the 'oxygen efficiency', for example: P
co - 0.1 bar is already achieved at an oxygen to inert gas ratio of 1:2 and an efficiency of 10%. As is well known, the "oxygen efficiency" in the AOD method is 75
% and below 10%.

Table 1: Decarburization of high chromium-containing steel by gas mixtures in AOD-, MRP-, UBD-, ASM methods, etc. Oxygen vs. non-oxygen 02 Ratio of active gas partial pressures 100% 5:1 3=l 2; l 1: l 1:2 l Two 3 9 0 9 860 8 0 0 6 6 7 500 4 0 0 00 50% in waste gas in efficiency (Mi833 750 667 500 333 250 30% Rivar 750 667 545 375 230 167 lO% 500 3 7 5 2 8 6 1 6 7 91 63 1:4 333 200 130
48t's 286 167
107 381:6 250 1
43 91 321:7 222
125 79 280:l
O 0 0
In contrast to the 0AOD method, decarburization takes place in only two phases in the MRP method. The process of this method is schematically illustrated in FIG. The decarburization phase is described by regions A and B. C represents the reduction to tapping of the ladle. Below the curve of temperature and carbon concentration, the use of process gases oxygen and process gases argon and nitrogen is described.

In the first decarburization phase A, the slagging of chromium is 1780
It is only inhibited by an increase in temperature to ℃. In the second decarburization phase B, pulsating air blowing is applied. During pulsating blowing, after each blowing time (minute) using oxygen, inert gas is blown for approximately the same amount of time, thereby removing the carbon in the melt.
O partial pressure PCO decreases. The decrease in partial pressure causes a spontaneous reaction between the oxygen and carbon dissolved in the molten metal. This allows decarburization to proceed naturally without oxygen supply.

This is because the high inert gas flow adds sufficient mixing energy into the melt, so that the reaction proceeds until equilibrium is reached.

Modern decarburization processes additionally supply oxygen to the bottom nozzle hole in the first phase with pure oxygen, e.g. To top blow the gas mixture with inert gas,
A top lance is also used. Figure 2 shows AOD-L as an example.
The method is illustrated in a diagram corresponding to FIG. The three decarburization phases are represented by D, E and F, with G representing the use of the top lance and H representing the use of the bottom nozzle hole. moreover,
The ratio of 02 to inert gas in the individual phases is also listed.

OBJECT OF THE INVENTION The object of the invention is to reduce the special gas costs in these methods.

[Manual punch for solving the problem] Starting from the state of the art taken into account in the preamble of claim l of the patent claims, this problem is solved according to the invention by the features stated in the characterizing part of claim l. resolved.

Advantageous embodiments of the invention are described in the claims 2 and below.

In the method of the present invention, the inert gases N2 and A' are approximately 0.0
Starting from the surprising observation that during decarburization up to a carbon content of 0.6%, it is possible to replace not only partially but also completely by CO2. The amount of coz according to the present invention relative to the weight of the molten metal is 0. 2-1. 5 m3/
t-minte JT)? ) In particular, the mixing energy produced in the melt is of a sufficient magnitude to cause the decarburization reaction to proceed under equilibrium conditions. This method includes N2 up to 1
% by volume of industrially pure CO2 is sufficient.

Therefore, C02 can be obtained directly from the C02 tank by evaporation of the liquid phase.

For blowing the CO2, customary devices can be used, such as annular slot nozzles and optionally additionally top lances.

An important effect of CO2 as a process gas is that it causes a decrease in the partial pressure of CO in the melt and in the atmosphere of the converter. Furthermore, a reduction in the N2 and H2 partial pressures is achieved, which leads to degassing of the molten metal. At the same time, loading of the molten metal with N2 and H2 is prevented, which results in a steel that is sufficiently low in nitrogen and hydrogen.

By top-blowing COz into the molten metal through the top France, the partial pressure of CO2 in the atmosphere is diluted in accordance with the amount of CO2. As a result, a decrease in the partial pressure of CO in the molten metal is achieved under the equilibrium conditions between the molten metal and the converter atmosphere, and at the same time, the equilibrium condition for Cr oxidation (reaction formula 2) is shifted toward the lower C content in the molten metal. Moving.

The calculation of the resulting t:CO partial pressure in this case is given in Table 2. The value of this Co partial pressure is the PC when using inert gas nitrogen and/or argon instead of CO2.
Matches the O value.

Table 2: Change in CO partial pressure due to top blowing of CO2 by top lance Ratio of 02 to co2 2:1 l:l 1:2 Partial pressure of CO at 8 below 02 efficiency 50% 30% lO% mbar 667 545 285 500 375 167 333 230 91 667 800 100% 500 Figure 3 shows the inert gas
AOL-L using CO2 instead of Ar8 and N2
In this case, which shows the process of molten metal in a converter, the same decarburization results are obtained. The use of argon is only required below 0.06% carbon.

Table 3 shows that Ar and N are
The results of decarburization when CO2 is blown in place of CO2 are shown. A method sequence belonging to this, namely the MRP method, is shown in FIG. 4 and corresponds to the method according to FIG.

CO2 gas behaves almost inertly in the gas atmosphere of the converter. This is also possible due to thermal dissociation co2 → CO + ci(C2 (
5) can be ignored. For example co
2 to 1700°C, CO298.08%, COl.
A gas mixture of 28% and 0.64% C is produced.

48275 1.10 0.16 550
462 44 17 11248275
0.16 0.06 462 411
5 4 2545244 0.8
9 0.06 389 148 63
25 12548286 0.85 0.0
5 225 115 60 19
10448289 0.14 0.05 129
113 5 5 15C2
” is the equivalent amount of oxygen produced by the Poud-Har reaction (reaction 26) using CO2.

Table 3 shows the CO injected into the melt through the bottom nozzle hole.
It is recognized that a part of 2 reacts with carbon by Boudouard reaction.

co2 + c → 2CO (6) In this case C2'' is the equivalent amount of oxygen utilized for the decarburization of the molten steel from this reaction. It is recognized that the carbon content of

It is clear that below about 0.06% C, CO2 is reduced by chromium in the melt according to the following equation. This prevents rough decarburization of the molten metal during simultaneous chromium oxidation.

Figure 5 shows the Cr/
A C/PocsF-equilibrium diagram is shown. Similar to Figure 4,
A represents decarburization with CO2 according to the invention up to 0.06% C, B represents pulsatile decarburization, and C represents the reduction caused by the addition of AI and Si.

The chromium and carbon analysis values achieved indicate that a reduction in CO partial pressure has taken place. co. The equilibrium value for achieving the analytical value of oa% is Pco3 0 at 1760°C.
In the range of 0 mbar.

By reducing the slag with silicon and/or aluminum, the metals lost through oxidation, especially chromium, are recovered. For this reason, the slag must be reduced after the decarburization is completed.

Because of the required mixing energy, argon is primarily used as inert gas. Replacement with CO2 can be achieved by replacing C with At and/or Si according to equations (8) and (9).
The reduction of O2 results in carburization of the molten metal. Therefore, it is not necessary to use C02 in this phase.

4Al + 3CO2 → 2A
l203 + 3C (8)Si +
CO2 → Si02 + C
(9)4

[Brief explanation of the drawing]

Figure 1 (a) shows the C content, temperature, and time (win
) diagram, (b) is a block diagram of the gas blowing behavior of Ar, N202, and (a) of FIG. This is a C content/temperature/time (llin) diagram similar to Figure (a), and Figure 3 (a) is the same as Figure 2 (a) when Co2 is blown instead of inert gas N2. C content/temperature/time (w
(b) is a block diagram of the same gas blowing behavior as in FIG. 2(b) in the G phase using the top lance and the H phase using the bottom nozzle hole, and FIG. (a) is C
C content, temperature/time (n+i
(b) is a block diagram of gas blowing behavior similar to FIG. 1(b), and FIG. 5 is a Cr/C-Pc0 equilibrium diagram at various temperatures.

Claims (1)

[Claims] 1. Chromium-containing molten steel having a Cr content of 10% or more,
In a method of decarburizing by blowing oxygen and processing gas as process gases in a bottom blast converter, C0.
A method for decarburizing chromium-containing molten metal, characterized in that CO_2 is exclusively used as a processing gas to decarburize to 0.6%. 2. Process according to claim 1, characterized in that CO_2 is additionally introduced into the melt by means of a top lance. 3. The amount of CO_2 relative to the weight of the molten metal is 0.2 to 1.5
The method according to claim 1 or 2, wherein m^3/t.min. 4. The mixing energy generated in the melt by the amount of CO_2 introduced is sufficient to allow the decarburization reaction to proceed under equilibrium conditions. The method according to claim 3. 5. Any one of claims 1 to 4, wherein the CO_2 is obtained by evaporation from a liquid phase and contains at most 1% by volume of nitrogen.
The method described in section.