KR100554143B1 - Method for AOD working of controlling of crom oxidation - Google Patents

Abstract

In the present invention, when refining stainless steel in the AOD, the ignition pattern is optimized by optimizing the flow rate of the mixed gas, the oxygen / inert gas ratio, the blowing time of the refining gas, etc. It is an object of the present invention to provide an AOD operation method for inhibiting chromium oxidation that can reduce the amount of reduced silicon by proceeding decarburization.

Accordingly, the present invention provides a method for inhibiting chromium oxidation, which includes obtaining a smooth value of decarburization rate and chromium oxidation rate, setting a standard decarburization curve and a standard chromium oxidation curve, and determining a blowing pattern after decarburization. Provide AOD operation methods.

Chromium Oxidation, AOD, Lance, Blowing

Description

Method for AOD working of controlling of crom oxidation

1 is a block diagram of an AOD facility for carrying out the AOD operating method of suppressing chromium oxidation of the present invention.

2 is a view showing the change in the amount of carbon, chromium oxide and the amount of oxygen supplied through the lance and the fuel during one cycle.

Figure 3 shows a simplified form of standard decarburization curve and standard chromium oxide curve for 18% Cr-8% Ni stainless steel.

Figure 4 is a view showing the oxygen flow pattern and the improved blowing pattern required to increase the existing oxygen blowing pattern and decarboxylation efficiency to 80% level.

<Description of the symbols for the main parts of the drawings>

70: calculation unit operation analysis means 71: decarburization speed calculation unit

72: oxygen balance calculator 73: chrome oxidation calculator

The present invention relates to an AOD operating method for manufacturing stainless steel, and more particularly to an AOD operating method for inhibiting chromium oxidation to reduce the production cost by inhibiting the oxidation of chromium when refining stainless steel.

In general, stainless steel manufacturing processes include melting furnaces and ferroalloys to produce stainless molten metals, refining furnaces to control carbon removal, temperature rise and composition of stainless steels, and stainless steel as slabs or blooms. It consists of a continuous casting process to solidify the furnace.

The representative facility used in the refinery process which blows oxygen in this process and performs decarburization, temperature raising, and component control is AOD (Argon Oxygen Decarburization).

The AOD is a lateral take-up device that injects oxygen and inert gas into a stainless molten metal, a top-load lance that supplies oxygen in the upper part of the reaction vessel, and a sub-lance for measuring the temperature of the molten metal and collecting a sample for component analysis during refining. It consists of.

In this AOD process, when oxygen is blown to remove carbon, oxidization of chromium (Cr), which is a valuable metal, is accompanied by oxidation, and after the decarburization, Fe-Si or Al, which has strong oxidizing power, is introduced into the reaction vessel and chromium in the slag. The reduction step is to recover the oxide back into the molten steel.

As such, the AOD operation requires oxidation and reduction reactions in the same reaction vessel and minimizes oxidation of chromium in the decarburizer, so that the flow rate of the mixed gas and the oxygen / inert gas ratio, the upper oxygen flow rate through the blowing pattern, for example, a low odor feeder. The blowing pattern of the oxidation step such as and the lance height, the reducing agent input amount and the input method of the reduction step are very important.

In AOD operations, decarburizers are usually refined in three to five stages.

In the first stage of decarburization, the molten steel flows 1.0 to 1.2 Nm ³ / min · ts, the stolen oxygen flows 0.3 to 0.4 Nm ³ / min · ts, and the low odor nitrogen flows 0.15 tonnes of molten steel in an electric furnace with an initial carbon concentration of 1.5 to 2.5%. Blowing gas of ˜0.20 Nm ³ / min · ts is blown to lower the carbon concentration to 0.5%, which is the critical carbon concentration.

In the decarburization stage 2, the lance oxygen injection is stopped, and the inert gas is also converted from nitrogen to argon (Ar), and the oxygen is 0.6 to 0.7 Nm ³ / min · ts, 0.15 to 0.20 Nm ³ / Ar of min * ts is blown in. In the decarburization step 3, the oxygen / inert gas ratio is lowered to 1: 1, and 0.4 to 0.5 Nm ³ / min · ts of oxygen and 0.4 to 0.5 Nm ³ / min · ts of Ar are respectively injected to obtain a carbon concentration of 0.15%. In the step, 0.2 to 0.25 Nm ³ / min · ts of oxygen and 0.6 to 0.75 Nm ³ / min · ts of Ar are injected at a carbon concentration of 0.07% so that the oxygen / inert gas ratio is 1: 3.

Thereafter, even if the oxygen supply is stopped and only Ar is injected into 0.4 to 0.5 Nm ³ / min · ts, the carbon remaining by the oxygen remaining in the furnace is lowered to 0.045%. This step is called a rinsing step.

As described above, in the AOD operation, chromium oxidation occurs at about 8 kg / ts per tonne of molten stainless steel in the first stage of decarburization and about 7.5 kg / ts per tonne of molten steel in the decarburization stages 2 to 4.

Therefore, in the decarburization stage 1 where the carbon concentration of molten metal is high, decarburization efficiency is higher than 80%, so chromium oxidation is low and oxygen supply dominates decarburization, but decarburization efficiency in decarburization stages 2-4 is 60%. Regardless of the rate of carbon transfer to the reaction interface, the rate of oxidation of chromium occurs in AOD refining.

18% Cr-8% Ni stainless steel, the world's largest producer, consumes 5.65 Nm ³ / ts of oxygen for the oxidation of valuable metals such as chromium.

This means that about 15.7 kg of chromium oxidizes per tonne of stainless steel. Once oxidized chromium is recovered to molten steel by reducing Si during the final stage of AOD refining, the more chromium oxide, the more Si is required and the higher the cost of manufacturing stainless molten steel.

Therefore, the raw unit of Si input for recovering valuable metals and adjusting components reaches 13.5 kg / ts.

On the other hand, at the carbon concentration of 0.5% or less, the conventional AOD operation in which refining is carried out while maintaining the constant flow rate of the refinery gas and the ratio of oxygen / inert gas at the end of decarburization even though the decarburization rate decreases as the carbon concentration decreases. In the system, excessive oxidation of chromium causes excessive oxidization of chromium.

Therefore, the present invention has been made to solve the problems of the prior art, the purpose of the refining of the flow rate of the mixed gas, oxygen / inert gas ratio, refinery gas through the low odor end of the decarburization when refining stainless steel in AOD The present invention provides an AOD operation method that suppresses chromium oxidation that can minimize the amount of reduced silicon by minimizing chromium oxidation by devising a blow pattern optimized for blowing time.

In order to achieve the above object, the present invention continuously estimates decarburization rate and chromium oxidation rate of 1 hit by using repetitive information periodically receiving information of refining gas and exhaust gas during refining of stainless steel and event information occurring aperiodically. Subdividing the decarburization rate and chromium oxidation rate in each section by subdivision into 1 minute or less and reflecting the decarburization rate and chromium oxidation rate obtained in the step of obtaining the decarburization rate and chromium oxidation rate Determining the blowing pattern after the decarburization step 2 using the standard decarburization curve and the standard chromium oxide curve set in the step of setting the standard decarburization curve and the standard chromium oxide curve, and the setting of the standard decarburization curve and the standard chromium oxide curve. It provides an AOD operating method that inhibits chromium oxidation comprising the step.

Here, in the step of determining the blowing pattern after the decarburization step 2, it is preferable to determine the blowing pattern based on the calculated oxygen flow rate necessary to increase the decarboxylation efficiency to 80% level.

In addition, the blowing pattern after the decarburization step 2 is a decarburization step 2 that is refined for 5 minutes using an oxygen flow rate of 0.5 Nm ³ / min · ts and an argon flow rate of 0.22 Nm ³ / min · ts for AOD; The argon flow is blown to the AOD 0.5 Nm ³ / min · the oxygen flow rate for 1.5 minutes while maintaining the ts the oxygen flow rate for, and the one-minute blow Due to ^{0.5 Nm 3 / min · ts 0.45} Nm 3 / min · ts 3 steps of decarburization followed by blowing at an oxygen flow rate of 0.4 Nm ³ / min · ts for 1.5 minutes, and then blowing at an oxygen flow rate of 0.33 Nm ³ / min · ts for 1 minute; The oxygen flow rate was blown at 0.28 Nm ³ / min · ts for 1.5 minutes while maintaining the flow rate of argon blown to AOD to 0.7 Nm ³ / min · ts, and the oxygen flow rate was 0.22 Nm ³ / for 1 minute. It is preferable to make four steps of decarburization, blowing at min.ts, blowing at an oxygen flow rate of 0.17 Nm ³ /min.ts for the next 1.5 minutes, and blowing at an oxygen flow rate of 0.11 Nm ³ /min.ts for the next minute.

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

1 is a block diagram of an AOD facility for carrying out the AOD operating method of suppressing chromium oxidation of the present invention.

First, according to the above-described drawings, the AOD facility includes a top lance (2) for injecting gas oxygen into the molten steel (1), a horizontal blower (3) for blowing a mixed gas at the bottom of the molten steel (1), and the temperature of the molten metal during refining. And a sub lance 4 for collecting a sample for component analysis, an exhaust gas analyzer 5 for continuously measuring exhaust gas composition every predetermined cycle, and an exhaust gas flow meter 6 for measuring the flow rate of exhaust gas in the same cycle as the exhaust gas analyzer. .

Moreover, the operation analysis means 70 which analyzes operation is comprised by the decarburization rate calculation part 71, the oxygen balance calculation part 72, and the chromium oxidation rate calculation part 73. As shown in FIG.

Here, the information of 70 to 72 of the various hits obtained from the operation analysis means 70 is collected to derive the standard decarburization curve and the standard chromium oxide curve from the standard refining curve section 8, and use the derived result. In this way, the horizontal gas control unit 9 controls the gas mixture of the horizontal gas 3 on-line.

The AOD operation method for suppressing chromium oxidation according to the present invention is a decarburization rate and chromium oxidation rate of 1 hit by using repetitive information that periodically receives information of refinery gas and exhaust gas during refining of stainless steel and event information that occurs aperiodically. The decarburization rate and chromium oxidation rate obtained in the first step of obtaining the smooth value of the decarburization rate and the chromium oxidation rate in each section by subsequently estimating the subdivided into units of 1 minute or less, and obtaining the smooth value of the decarburization rate and the chromium oxidation rate. Degassing using the standard decarburization curve and the standard chromium oxide curve set in the second step of setting the standard decarburization curve and the standard chromium oxide curve reflecting more than 50 hits, and the step of setting the standard decarburization curve and the standard chromium oxide curve The third step is to determine the blowing pattern after the step.

First, in the first step, the cycle information refers to information transmitted to the process computer at regular cycles, and in the AOD process, information related to refinery gas and exhaust gas corresponds to this.

Such periodic information is oxygen and gas injected from the upper lance 2, oxygen and gas injected from the horizontal take 3, and nitrogen and argon taken from the horizontal take 3 to lower partial pressures of CO gas. Is sent to the process computer at regular cycles.

The exhaust system of the AOD is equipped with a gas analyzer such as a mass spectrometer for analyzing the exhaust gas composition, and various analysis values of the exhaust gas are transmitted to the process computer at regular cycle intervals along with the flow rate of the exhaust gas.

Event information refers to information on the instantaneous changes that occur during AOD refining.

Such event information is a representative example of the alloy and subsidiary material input at any time during refining. During the refining process, various additives added to adjust the ingredients, temperature and slag properties are transferred to the process computer along with the event number as well as the type, amount, and time of completion.

In AOD, dilution oxygen gas is blown to lower the CO partial pressure to 1 atm or less, so that decarburization can proceed while inhibiting the oxidation of chromium.

At this time, the decarburized product is the total amount, CO or CO ₂ gas, and there is no oxygen gas remaining in the AOD. Therefore, the decarburization rate can be known from Equation (1) by knowing the concentration of CO and CO ₂ in the exhaust gas and the flow rate of the exhaust gas at any point in refining. .

-dC / dt = K1 (XCO + XCO2) Voff (1)

Where -dC / dt is the decarburization rate expressed in decarburization per unit time in kg / min.

In addition, XCO and XCO2 is the exhaust gas of CO, CO ₂ composition represented by volume percentage at an arbitrary time point of each refinement, Voff is an exhaust gas flow rate in the same time unit Nm ³ / hr, K1 is a unit adjustment factor. In case of refining stainless steel, the oxygen blowing rate supplied to the AOD furnace is expressed by Equation (2).

Vo2 = Vo2, lance + Vo2, tuyere (2)

Where Vo2 is the oxygen oxygen blowing rate supplied to the AOD furnace and the unit is Nm ³ / min.

In addition, Vo2, lance, Vo2, tuyere are oxygen flows blown through lance and tuyere, respectively, and the unit is Nm ³ / min.

If the compositions of CO and CO2 measured continuously from the flue gas analyzer are XCO and XCO2, respectively, the oxygen reacted with carbon at any point of refining among the oxygen supplied to the AOD furnace is Vo2, C as shown in Eq. (3).

Vo2, C = 1/2 (XCO + XCO2) Voff (3)

Oxygen that is not used for decarburization of oxygen taken in AOD refining oxidizes more than 90% of chromium, so the rate of chromium oxidation is expressed by (4).

-dCr / dt = K2K3 (Vo2-Vo2, C) (4)

Where -dCr / dt is the rate of chromium oxidation in units of kg / min expressed as chromium oxidation per unit time.

On the other hand, K2 is a ratio of 0.8 to 0.95 as the ratio of oxygen to chromium oxidation in oxygen not used for decarburization, and K3 is a unit conversion factor taking units of kg / Nm ³ .

Next, the second step is to set the standard decarburization curve and the standard chromium conversion curve. First, the metal oxidation tendency of each decarburization step is as follows.

FIG. 2 is a view showing a change in the amount of carbon, chromium oxide, and oxygen supplied through a lance and a fuel during one cycle.

In the first stage of decarburization, about 0.12 Nm ³ / ts of oxygen is supplied for 5 seconds, and it can be seen that much oxidation of chromium occurs at the beginning and the end of stage 1.

At the beginning of the first stage, the metal oxides can reach 0.07 Nm ³ / ts for 5 seconds, which is very high, accounting for more than 50% of the injected oxygen.

As the rate of decarburization increases gradually, the amount of metal oxides decreases gradually, so that almost all of the oxygen supplied to the decarburization stage is used only for decarburization, so that metal oxidation rarely occurs.

In the early stage of decarburization, the decarburization rate is relatively high, so the metal oxidation rate is relatively low, but the metal oxidation rate tends to increase as the decarburization rate decreases.

In the third stage of decarburization, only about half of the blowing oxygen is used for decarburization, and the rest is consumed for oxidation of valuable metals such as chromium.

In the fourth stage of decarburization, the amount of oxygen supplied is greatly reduced, so that metal oxidation does not occur much, but as the decarburization rate decreases, the metal oxidation rate increases slightly.

In order to derive the standard decarburization curve and the standard chromium oxidation curve, the total decarburizer was divided into several sections in units of 10 cycles for each hit, and the average decarburization rate and the average chromium oxidation rate were calculated.

This data processing was performed for more than 50 hits and the average value of the average oxidation rate was calculated for the corresponding section.

As a result, a simple standard decarburization curve and a standard chromium oxide curve for 18% Cr-8% Ni stainless steel in AOD refining can be derived as shown in FIG. 3.

This standard curve can be used to establish an oxygen injection continuous control pattern for inhibiting chromium oxidation.

Next, the third step is to determine the blowing pattern after the second step of decarburization. First, in the first step of decarburization, the supply of oxygen dominates the decarburization reaction so that the oxygen injection flow rate is lowered to suppress the oxidation of chromium generated in the first step. This may cause a prolongation of refining time.

In the present invention, the focus is on the inhibition of chromium oxidation in the decarburization stages 2 to 4 in which the mass transfer of carbon to the reaction interface is not rate, but rate.

In the decarburization stages 2 to 4, the decarburization rate is independent of the oxygen supply rate, so reducing the oxygen injection flow rate does not lower the decarburization rate or extend the refining time.

The decarburization efficiency of the second stage of decarburization varies from 45 to 53% in a narrow range, with an average decarburization efficiency of 50%.

In the decarburization stage 3, the deoxygenation efficiency varies greatly from 53% to 81%, with an average decarbonation efficiency of 66%.

In addition, the decarboxylation efficiency in step 4 varies even more, from 40% to almost 100%, with an average decarbonation efficiency of 72%.

In order to increase the decarboxylation efficiency to 80%, which is the first level of decarburization, it is most desirable to lower oxygen that is excessively blown.

Figure 4 is a view showing the oxygen flow pattern and the improved blowing pattern required to increase the existing oxygen blowing pattern and decarboxylation efficiency to 80% level.

Summarizing the optimum blow pattern that can suppress the chromium oxidation as follows.

In the second stage of decarburization, the oxygen flow rate is lowered from 0.67 Nm ³ / min · ts to 0.5 Nm ³ / min · ts for 5 minutes.

In the first stage of decarburization, the initial 1.5 minutes were blown with an oxygen flow rate of 0.5 Nm ³ / min · ts as in the previous method, and after 1 minute, the oxygen flow rate was 0.45 Nm ³ / min · ts, and after 1.5 minutes 0.4 Nm ³ / After 1 minute, the oxygen flow rate is lowered to 0.33 Nm ³ / min · ts.

In the fourth stage of decarburization, the blowing was performed at an oxygen flow rate of 0.28 Nm ³ / min · ts for 1.5 minutes, and the oxygen flow rate was 0.22 Nm ³ / min · ts after 1 minute, and 0.17 Nm ³ / min · ts after 1.5 minutes. After minutes, lower the oxygen flow rate to 0.11 Nm ³ / min · ts.

Example

AOD operation results 124 heats and AOD operation results 158 heats to which the present invention was applied were analyzed. In the conventional AOD operation, the oxygen injection amount was 6.19 Nm ³ / ts from the second stage of decarburization to the end point of decarburization, but in the operation to which the present invention is applied, the oxygen injection amount of 1.28 Nm ³ / ts is reduced to 4.91 Nm ³ / ts.

In the prior art, after the second step of decarburization, 6.44 kg / ts of chromium was oxidized, but the amount of chromium oxidation lost when the present invention was applied was 3.61 kg / ts, which prevented the oxidation of 2.83 kg / ts of chromium.

Therefore, it can be seen that most of the oxygen at this stage contributed to the inhibition of chromium oxidation.

In addition, in the conventional base, the raw unit of Si input for recovering valuable metals and adjusting components reached 13.5 kg / ts, but since the new blowing pattern of the present invention was applied, Si of 1.9 kg / ts was reduced to 11.6 kg / ts.

Therefore, it can be seen that the inhibition of chromium oxidation has a direct effect on the reduction of the amount of reduced silicon.

Although the refining time after the decarburization step 2 differs depending on the carbon concentration after the end of the first step, the refining time level is approximately 15.2 minutes in the prior art and 14.9 minutes in the present invention.

Comparison table of the effect according to the prior art and the present invention is shown in Table 1 below.

As described above, the AOD operation method of inhibiting chromium oxidation according to the present invention reduces chromium oxidation, thereby reducing expensive chromium consumption, thereby reducing silicon consumption, thereby reducing production cost.

Claims (3)

In the AOD operation method for inhibiting chromium oxidation to suppress the oxidation of chromium during the refining of stainless steel, When refining the stainless steel, repetitive information for periodically receiving refined gas and flue gas information and event information generated aperiodically are used to continuously estimate the decarburization rate and chromium oxidation rate of 1 hit, and subdivided into units of 1 minute or less. Obtaining smooth values of decarburization rate and chromium oxidation rate in each section; Setting a standard decarburization curve and a standard chromium oxidation curve by reflecting the decarburization rate and the chromium oxidation rate obtained in the step of obtaining the smooth value of the decarburization rate and the chromium oxidation rate by 50 hits or more; AOD for inhibiting chromium oxidation comprising the step of determining the blowing pattern after the decarburization step 2 using the standard decarburization curve and the standard chromium oxide curve set in the step of setting the standard decarburization curve and standard chromium oxidation curve Method of operation. The method of claim 1, The step of determining the blowing pattern after the second step of decarburization is AOD operation method for inhibiting chromium oxidation, characterized in that to determine the blowing pattern on the basis of the calculated oxygen flow rate to reach the deoxygenation efficiency to 80% level. The method of claim 2, The blowing pattern after the second step of decarburization is Decarburization step 2, refining for 5 minutes with an oxygen flow rate of 0.5 Nm ³ / min · ts and argon flow rate of 0.22 Nm ³ / min · ts The argon flow is blown to the AOD 0.5 Nm ³ / min · the oxygen flow rate for 1.5 minutes while maintaining the ts and blown to a 0.5 Nm ³ / min · ts, then a minute oxygen flow rate 0.45 Nm ³ for a / min · 3 steps of decarburization, which is blown at ts, and then blown at an oxygen flow rate of 0.4 Nm ³ / min · ts for 1.5 minutes, and blown at an oxygen flow rate of 0.33 Nm ³ / min · ts for 1 minute. The oxygen flow rate was blown at 0.28 Nm ³ / min · ts for 1.5 minutes while maintaining the flow rate of argon blown to the AOD to 0.7 Nm ³ / min · ts, and the oxygen flow rate was 0.22 Nm ³ for the next 1 minute. blown at / min · ts, blown at an oxygen flow rate of 0.17 Nm ³ / min · ts for the next 1.5 minutes, and blown at an oxygen flow rate of 0.11 Nm ³ / min · ts for one minute, followed by four steps of decarburization. AOD operation method for inhibiting chromium oxidation.

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