JPS59104421A - Decarburization of chromium-containing molten steel - Google Patents

Abstract

PURPOSE:To eliminate problems such as decrease of decarburizing speed, the oxidation of a metal or the like, in a decarburizing process by blowing O2-gas and dilution gas into molten steel, by specifying the blow-in condition of the aforementioned both gases in three regions due to the C-content in the molten steel. CONSTITUTION:In a decarburizing process by blowing protective gas and a gaseous mixture of O2-gas and dilution gas into molten steel from the outer and the inner pipes of a double pipe tuyere, the C-content in the molten steel divided into three regions, that is, a region of up to 0.7%, a region to 0.05-0.25% from 0.7% and a region to up to a final C-value from 0.25-0.05%. In this state only the O2-gas, a gaseous mixture continuously changed in the ratio of the O2-gas and the dilution gas corresponding to the C-content and only the dilution gas are successively and respectively blown into the molten steel in these regions from the inner pipe. By this method, the conservation of a required reducing agent, shortening of a reduction time, prevention of the molten loss of a refractory material and enhancing effect of refining efficiency are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for decarburizing molten steel by simultaneously blowing diluent gas and oxygen gas.

In the decarburization refining of molten steel, the oxidation of the metals in the steel should be controlled as much as possible, and the decarburization should be carried out preferentially.The oxidation of chromium contained in the steel should be suppressed as much as possible, and the reducing agent necessary to reduce the chromium oxide should be used. It is extremely important to reduce the amount of slag forming agent and to carry out highly efficient decarburization refining to improve operational efficiency and prevent refractory erosion.

In general, the lower the steel bath temperature, the lower the [C] concentration in molten steel.
The higher the O gas partial pressure, the higher its equilibrium value, so efficient decarburization can be achieved by increasing the steel bath temperature and lowering the 00 partial pressure. However, increasing the steel bath temperature increases the wear and tear on the refractories that hold the molten steel, so there is a limit to the improvement in decarburization efficiency by increasing the steel bath temperature.

On the other hand, as a method to reduce the 00 partial pressure, diluent gas and oxygen gas are simultaneously injected from the inner pipe of a double-tube tuyere, and as the decarburization reaction progresses, the diluent gas becomes enriched according to the concentration of [C] in the molten steel. For example, there is an AOD method in which the ratio of diluent gas and oxygen gas is changed stepwise.

In normal refining, as decarburization progresses, the contribution of oxygen gas injection to the decarburization reaction decreases, the contribution to metal oxidation increases, and the amount of input reducing agent increases. Therefore, by enriching the diluent gas using the AOD method, it is possible to lower the 960 partial pressure and increase the contribution of oxygen gas to the decarburization reaction, thereby suppressing metal oxidation and preventing an increase in the amount of reducing agent. However, in the AOD method, the cost of diluent gas increases due to enrichment of the diluent gas, the decarburization rate decreases due to a decrease in the amount of oxygen gas blown, and the refining time increases, resulting in an increase in the cost of side personnel. One way to increase the decarburization rate is to increase the oxygen blowing speed, but this method is not recommended due to equipment limitations such as blowing pressure, blowing nozzle diameter, dust collection hood, cooling capacity, etc., and operational problems such as splashing. Blowing gas velocity is limited. For this reason, the decarburization rate could not be sufficiently increased because the oxygen blowing rate reached the maximum rate depending on the ratio with the diluent gas selected at that time.

The present invention is a method for decarburizing molten steel by simultaneous injection of diluent gas and oxygen gas, which solves various problems such as a decrease in decarburization rate and metal oxidation. This is Notooshi.

In a method of decarburizing refining by injecting a protective gas from the outer tube of a double-tube tuyere and a mixed gas of diluent gas and oxygen gas from the inner tube into the molten steel, (1) C] Injecting only oxygen gas from the inner tube of the double tube tuyere in the range up to 0.7% content, (2) In molten steel [C) Content from 0.7% to 0.05~02
(3) Injecting diluent gas and oxygen gas from the inner pipe in the five-stroke region while continuously changing the ratio of dilution gas and oxygen gas according to the [C] content in the molten steel; (3) [C] content in the molten steel is 0; A method for decarburizing chromium-containing molten steel, characterized in that only diluent gas is blown into the inner tube in the range from .25 to .005 to the final [C] value.The present invention will be described in detail below.

Figure 1 shows the weight concentration of [C] in molten steel (hereinafter simply [C] in molten steel) during decarburization refining using a double-tube tuyere to inject oxygen gas and diluent gas into molten steel from the inner tube and protective gas from the outer tube. The relationship between the oxygen argon ratio (02/Ar) and the ratio of the blown oxygen gas consumed for decarburization (hereinafter referred to as decarburization oxygen efficiency or η0) is shown. [C] in molten steel is 0.7%
In the above range, the 02/Ar ratio is infinite, that is, even if only oxygen is used for blowing, η0 is almost equal to the 02/Ar ratio=4/1. In other words, if the content of molten steel is less than 0.7%, the decarburization oxygen efficiency when only oxygen gas is injected, and the decarburization oxygen efficiency when oxygen gas and diluent gas are injected at a predetermined ratio. There is no difference between Therefore, [C] in molten steel is 0.7%
Until then, if the injection of diluent gas from the inner tube of the double-pipe tuyere is stopped and refining is performed with only oxygen gas injection, the amount of oxygen injection gas corresponding to the amount of injection gas can be increased since there is no dilution gas injection. The decarburization rate can be increased and the refining time can be shortened.

In this process, [CX 0.7%) in molten steel is predicted by the following method.

Since the decarburization-oxygen efficiency when only oxygen gas is injected is approximately 80% constant, the analysis value of [C] in the molten steel at the start of injection, the amount of molten steel, and the newly mixed in from the coolant and alloys added afterwards [C] C) Calculate the weight of [C] contained in the molten steel from the amount, and find out 0.7
Determine the amount of oxygen required to decarburize up to %, and inject the calculated amount of oxygen gas so that the [C] in the molten steel becomes 07%.

The amount of molten steel is determined using the following formula.

(Amount of molten steel charged + Amount of quantum alloy removed - Amount of metal oxidation - Amount of oxidation) Figure 2 shows the oxygen gas /
The relationship between the dilution gas ratio, [C] in molten steel, and decarburization oxygen efficiency is shown.

When oxygen gas/diluent gas -4/'1 is selected, the decarburization oxygen efficiency up to 0.7% [C] in molten steel is the saturation value (approximately 80
%), but below the coefficient [C] in molten steel of 0.7, it decreases as the [C] in molten steel decreases.

That is, if it is less than 0.7%, as the [C] content in the molten steel decreases, the oxidation of the metal components in the molten steel increases, and the amount of reducing agent consumed after blow-off increases.

Therefore, in the conventional method, the gas ratio for enriching the diluent gas, for example, oxygen gas/diluent gas=2/1, is selected at a stage before metal oxidation increases to a certain extent. Blowing is continued at this gas ratio, and if metal oxidation increases, a gas ratio further enriching the diluent gas is selected and blowing is continued. In the conventional method, decarburization was carried out by switching the oxygen gas/diluent gas ratio in 3 to 6 steps as in the previous method, so it was inevitable that a region [c] would occur in which the decarburization oxygen efficiency decreased. In the figure, C)+/Ar=
The decarburization oxygen efficiency when 4/1 to 2/1 to 1/2 molten steel [c] is blown to 0.5% to 0.25% is shown by the solid arrow.

In order to solve this decrease in decarburization oxygen efficiency, the oxygen gas/diluent gas ratio must be changed from time to time to achieve a saturated decarburization oxygen efficiency (approximately 80%) depending on the [C] in the molten steel. It is possible. In this case, the decarburization oxygen efficiency is, for example, in molten steel [C]
When decarburization occurs from 0.7% to 0.25%, the transition is as shown by the dotted line arrow in the figure.

That is, the shaded area in the figure represents the effect of improving decarburization oxygen efficiency by the continuous switching method, which reduces the amount of oxidized metal and accordingly reduces the amount of reducing agent, which is a great advantage in terms of cost.

At this time, the gas ratio of 021 Ar is ηo = (a + bγ) [Chi Chi a: constant depending on the steel type b: constant depending on the number of furnace ports [chi (:) 1: in molten steel [C] Using the formula of predicted value, η (1';80% and γ=f(C%s i
) can be used to find the gas ratio that should be set.

During this time, the amount of molten steel is ascertained, the required amount of decarburization is determined, and the amount of oxygen to be blown is determined.

When the oxygen gas/diluent gas ratio is continuously changed according to [C] in molten steel, [C+] is expressed as [C] content/amount of molten steel x 1400, so it changes from time to time [C]
] It is necessary to understand the amount of molten steel, which changes from time to time, as well as the content.

The content of [C] in the molten steel was determined as predicted above because approximately 80 g of the injected oxygen gas is consumed for decarburization.
If the content weight and the amount of oxygen gas blown are known, the amount of decarburization can be determined, and therefore the [c] content can be determined. The amount of molten steel can be determined by determining the amount of oxidized metal from the amount of oxygen gas blown and the decarburization efficiency of 80%, since oxygen other than that consumed for decarburization of the injected oxygen gas oxidizes the metal. In addition to the amount of molten steel calculated in , the amount of molten steel can be grasped.

The amount of oxidation of the metal component is a [Si] + b [cr: ] + c [
Mn) −+−d[Fe]. At this time, a g
b g c, d are each 81 + Cr + M
It is a coefficient of the amount of oxidation loss of n g Fe.

Of the oxygen consumed in metal oxidation, it is preferentially consumed in St oxidation.

The [C] weight concentration at that time is calculated from the [C] content and the amount of molten steel determined as described above, and the ratio of oxygen gas/octane gas is selected accordingly and gas blowing is performed. After a minute time has elapsed, the amount of decarburization and the amount of metal oxidation are calculated using the above method from the minute amount of oxygen injected at that time, the amount of [C] in the molten steel is calculated, and the ratio of oxygen gas/dilution gas is selected accordingly, and the gas blow do.

In this way, [c] in the molten steel changes from 0.7% to [C1] in the molten steel.
It becomes possible to perform extremely efficient decarburization by continuously switching the gas ratio in the range of 0.25 to 0.05%-i.

During this process, carbon in molten steel is analyzed and predicted [I
Modifying the CE value is extremely effective in soils to prevent Ar gas loss or reducing agent loss.

Figure 3 shows the percentage of (c) in molten steel in this process.
An example in which the /Ar ratio is continuously changed is shown.

Figure 4 shows the relationship between [C] in molten steel and the decarburization reaction rate in the conventional method where oxygen gas and argon gas were injected at a ratio of 1:2 and when only argon gas was injected once. show. In the figure, the decarburization reaction rate when only argon gas is injected is lower than the conventional method when [C] in molten steel exceeds 0.25, but when [C] in molten steel is 0.25~ At 0.15%, it is equivalent, and in the range where [C] in molten steel is lower than 0.15'%, it is higher than the conventional method.

In other words, after the [C] in the molten steel is decarburized to 0.25% or less, even if the oxygen gas injection is stopped, the [C] in the molten steel will be decarburized by the oxygen in the molten steel only by dilution with inert gas. It is. During decarburization, oxygen in the slag moves into the molten steel as the oxygen in the molten steel decreases, and the chromium carbide in the slag is reduced, which is very convenient.

In order to actively promote the transfer of oxygen in the chromium oxide to molten steel, it is necessary to limit the amount of metal oxide required for decarburization remaining in the slag at the same time as inert gas injection. It is desirable to add a reducing agent or, if necessary, a reducing agent and a slag forming agent.

By blowing inert gas, the chances of contact between the syslag and the molten steel increase, and the oxygen in the chromium oxide migrates into the molten steel, so that the oxygen in the molten steel reacts efficiently with [C] in the molten steel, resulting in reduction. Decarburization progresses significantly. Naturally, it needs to be the final [C] value.

In this process, the metal oxides in the slag are supplied with the oxygen necessary for decarburization, causing a reaction with [C] in the molten steel, and during this time, no oxygen is blown into the molten steel, so the metal oxides are not reduced. The amount of reducing agent required is reduced, and the reduction reaction can be completed with a smaller amount of reducing agent than that used in conventional methods.

Furthermore, at the end of decarburization, the reduction reaction is already in progress, and even if the reduction stage begins, it is only necessary to add the reducing agent and slag-forming agent for the amount that is insufficient for reduction, and the reduction can be significantly reduced. It is possible to shorten the reduction time, prevent corrosion of refractories, and improve refining efficiency.

In the present invention, the captured gas injected from the outer pipe of the double-pipe tuyere includes inert gas such as argon, carbon dioxide gas, nitrogen gas,
Water vapor or the like can be used, and other well-known gases such as inert gas such as argon, nitrogen gas, etc. can be used as the diluent gas blown from the inner tube. In addition to stainless steel, the present invention can also be applied to other chromium-containing steel types that undergo dilution decarburization.

Next, an example in which the method of the present invention is applied to an AOD method and a conventional AOD refining method will be compared and explained with reference to FIG.

Figure 5a shows the conventional AOD method. In order to efficiently decarburize the carbon in molten steel, the ratio of oxygen and argon to be blown into the molten steel is changed stepwise according to the change in the amount of carbon in the molten steel. decarburize to avoid oxidation as much as possible. However, in general, about 2% of chromium in molten steel is oxidized, so after decarburization, slag forming agents (CaO, etc.) and reducing agents (Fe-8t, etc.) are used.
is added, the molten steel is stirred by inert gas injection for an appropriate period of time, oxidized chromium is reduced, and the molten steel is desulfurized using a slag-forming agent added at the same time as the reducing agent, and then the steel is tapped.

In contrast, in the present invention, as shown in FIG.
To reduce [C] from 0.7% to 0.7%, blowing with only oxygen gas from the inner tube of the tuyere is performed to reduce [C] in the molten steel from 0.7% to 0.25~0.
．． Up to 0.05%, a mixed gas of oxygen and diluent gas is blown while continuously changing the oxygen and diluent gas ratio so that η0 is about 80%, and [C] in the molten steel is from 0.25 to 0.05%. Until the final step [C], decarburization and reduction are performed by blowing only Ar.

Since η0 of the blown oxygen gas can be maintained extremely high, it is possible to keep the amount of metal oxidation low.

According to the method of the present invention, it is possible to suppress the amount of chromium oxidation, which conventionally was around 2% at the end of decarburization, to around 1.4%. It was possible to reduce the reducing agent amount index to about 70, with the modulus being 100. In addition, the slag-forming agent amount index was able to be reduced from 100 in the conventional method to about 70.

Examples of the present invention in decarburization refining of stainless steel will be shown below.

Example 1 Decarburization using only oxygen was performed from the start of blowing until the EC) in the molten steel reached 0.7%, and then decarburization was performed until the [C] in the steel reached 0.25% while continuously changing the oxygen gas and diluent gas ratios. After carbonization, decarburization using only Ar gas was carried out until the [C] content in the steel became 0.06%. After that, a reducing agent and a slag-forming agent were added to reduce and desulfurize the steel, and the steel was tapped.

Example 2 From the start of blowing [C], decarburization was performed using only oxygen until [C] in the molten steel reached 0.7%, and then, while continuously changing the oxygen gas and diluent gas ratios, [C] in the molten steel reached 0.7%. Decarburize at 15%-t, switch to include Ar ff gas, and at the same time add metal elements necessary for decarburization (FeOycr203 * MnO5
Fe-8t and CaO were added to limit the amount of NiO remaining in the slag, and decarburization and reduction with Si proceeded simultaneously, and decarburization was performed in a trench where [C] in the steel became 0.06%.

After that, the required amount of reducing agent and slag-forming agent were added, and the steel was reduced and desulfurized and tapped.

Table 1 shows the Cr yield, reducing agent amount, and slag forming agent amount for each example and conventional example.

Table 1

[Brief explanation of drawings]

Figures 1 and 2 show the decarburization oxygen efficiency (η0) and the molten steel [C
] and the ratio of oxygen and argon to be blown into the molten steel (02/A
Figure 3 is a diagram showing the relationship between [C] in molten steel and the 02/Ar ratio injected into the molten steel, and Figure 4 is a diagram showing the relationship between the decarburization reaction rate ([%C) and 1 'In the molten steel at the time of bare blow-stop [CE
It is a figure showing the relationship between 9 degrees (weight ratio) and the ratio of oxygen to argon (02/Ar) blown into molten steel. FIG. 5 is a diagram showing an outline of the embodiment, and FIG. 5(a) is a conventional example, and FIG.
b) is an example of the present invention. Fig. 2 During molten steel [C] (OA) 7 Fu% 4 CCJ C%) Fig. 4 Arm 4'#' CC transfer at the end of eruption (Weight % 5)
zo (a9 5th ko (b)

Claims (1)

[Claims] In a method for decarburizing refining by injecting a protective gas from the outer tube of a double tube panel and a mixed gas of diluent gas and oxygen gas from the inner tube into molten steel, ) Injecting only oxygen gas from the inner tube of the double tube tuyere in the range where the [C] content in the molten steel is up to 0.7%, (2) The [C] content in the molten steel is from 0.7% to 0. .05~02
(3) Injecting the diluent gas into the oxygen gas from the inner pipe in the 5th region while continuously changing the ratio of the diluent gas to the oxygen gas in accordance with the [C] content in the molten steel. (3) The [C] content in the molten steel is 0. A method for decarburizing chromium-containing molten steel, characterized in that only diluent gas is blown from the inner tube in the range from .25 to 0.05% to the final [C] value.