KR100844799B1 - Method for refining the austenite stainless steel having low carbon - Google Patents

Abstract

 In the refining method of the low carbon austenitic stainless steel according to the present invention, in the refining method of a low carbon austenitic stainless steel comprising a decarburization step of decarburizing the low carbon austenitic stainless steel in three stages in an AOD refining furnace, Each decarburization step comprises: setting a decarburization rate of each decarburization step as a dependent variable in a carbon region of 0.1 wt% or more and performing a multiple regression analysis using an operation variable that affects the decarburization speed as an independent variable; Predicting the speed of each decarburization step based on the operating criteria derived through the multiple regression analysis, and deriving the target carbon concentration of each decarburization step through the result of the multiple regression analysis and Monte Carlo method (Monte Carlo Method) ; And setting the carbon concentration of each decarburization step to 0.45%, 0.25%, and 0.10% by adjusting the refining time of each decarburization step, wherein the independent variable is AOD in the first step of decarburization. Initial carbon concentration (wt%), the input amount of ferroalloy per ton of molten steel and quicklime input (kg / ts), and AOD initial molten steel temperature (℃) In the second step of decarburization, it represents the amount of iron alloy (kg / ts) per ton of molten steel introduced in the second step of decarburization and the molten steel temperature (° C.) after the end of the first step of decarburization. Characterized in that the ferroalloy amount (kg / ts) per ton of molten steel to be injected in the third step of decarburization.

AOD refining, austenitic stainless steel, decarburization, regression analysis, simulation

Description

Refining method of low carbon austenitic stainless steel {Method for refining the austenite stainless steel having low carbon}

1 is a graph comparing the AOD refining time when the conventional refining method is applied and the AOD refining time when the refining method of the present invention is applied.

The present invention relates to a method for refining low carbon austenitic stainless steel, and more particularly, to increase decarburization efficiency by adequacy of target carbon concentrations and accurate estimation of decarburization rate during AOD refining of austenitic stainless steel. The present invention relates to a method for refining low-carbon austenitic stainless steel, which can promote the reduction and shorten the refining time.

STS304L stainless steel, a representative low-carbon austenitic stainless steel, is used for tanks in LNG vessels, and STS316L stainless steel is used for pipes and chemical facilities. Conventional refining of low-carbon austenitic stainless steels divides the decarburizer into six stages, sets the target carbon concentration in each stage, and blows gaseous oxygen and inert gas at a predetermined flow rate into the molten metal for decarburization. As it progresses, the oxygen / inert gas ratio is lowered sequentially.

However, the conventional refining method is unstable due to the lack of the operating standards, such as applying a constant decarburization rate step by step without considering the operation factors, and the target carbon concentration of each decarburization step is not optimized. In the case of STS304L stainless steel, the refining time ranges from 60 to 90 minutes with an average of 75 minutes. On the other hand, in the production of low carbon austenitic stainless steel, the refining time of Argon Oxygen Decarburization (AOD) is longer than 10 minutes longer than that of EAF (Electronic Arc Furnace), which is an obstacle to improving the performance ratio.

Therefore, it is necessary to shorten the refining time of low-carbon austenitic stainless steel and establish blow patterns for stabilization of operation. In addition, a large amount of chromium oxide is generated due to excessive oxygen blowing, and the FeSi raw unit for reducing the same increases.

The present invention has been made to solve the above problems, the object of the present invention is to accurately estimate the decarburization rate and target carbon concentration in each carbonization zone of 0.1wt% or more during AOD refining of low carbon austenitic stainless steel. It is to provide a method for refining low carbon austenitic stainless steel that can increase decarburization efficiency and shorten refining time by optimizing.

In the refining method of the low carbon austenitic stainless steel comprising a decarburization step of decarburizing the low carbon austenitic stainless steel according to the present invention in three steps in an AOD refining furnace, each decarburization step In the carbon region of 0.1wt% or more, performing the multiple regression analysis by setting the decarburization rate of each decarburization step as a dependent variable and the operation variable affecting the decarburization speed as an independent variable; Predicting the speed of each decarburization step based on the operating criteria derived through the multiple regression analysis, and deriving the target carbon concentration of each decarburization step through the result of the multiple regression analysis and Monte Carlo method (Monte Carlo Method) ; And setting the carbon concentration of each decarburization step to 0.45%, 0.25%, and 0.10% by adjusting the refining time of each decarburization step, wherein the independent variable is AOD in the first step of decarburization. Initial carbon concentration (wt%), the input amount of ferroalloy per ton of molten steel and quicklime input (kg / ts), and AOD initial molten steel temperature (℃) In the second step of decarburization, it represents the amount of iron alloy (kg / ts) per ton of molten steel introduced in the second step of decarburization and the molten steel temperature (° C.) after the end of the first step of decarburization. Characterized in that the ferroalloy amount (kg / ts) per ton of molten steel to be introduced in the third step of decarburization.

delete

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

In AOD refining, the decarburization reaction is represented by equation (1).

Cr ₂ O ₃ +3 [C] = 2 [Cr] + 3CO (g) ------------ (1)

As for the oxidation rate of chromium and carbon in AOD refining, most of the oxygen injected in the tuyere zone is consumed for chromium oxidation in the initial stage of refining, and Cr ₂ O ₃ is bulk with Ar or N ₂ . (1) The opinion of oxidizing carbon by reaction is dominant.

In the vicinity of the tuyere zone, most of the blown oxygen forms FeO together with chromium oxide, but is immediately reduced by chromium. It is assumed that the mass transfer of carbon from the bulk molten steel bed to the molten steel / bubble interface dominates the reaction rate.

In the high carbon region where the carbon concentration in molten steel is high enough, the chromium oxide rises from the molten steel surface and reacts with the carbon to be reduced.

In the region where the carbon concentration in the molten metal is 0.1 wt% or more, the driving force for the mass transfer of carbon to the bubble surface is sufficiently high, so the oxygen blowing rate adopted in the actual operation is insufficient to oxidize the carbon in the high carbon region.

That is, the decarburization rate through the chromium oxide in the high carbon region is mainly determined by the oxygen blowing rate, FO ₂ , and the decarburization rate is represented by the formula (2).

d [% C] / dt = f (FO ₂ ) = α ---------------- (2)

Where α is the rate constant in units of% / min and is affected by operating variables. If decarburization continues and the carbon concentration in molten steel is lowered, the produced chromium oxide will not react with carbon and remain in slag. When carbon moves through the bulk to the Ar-CO bubble, the average decarburization rate in the low carbon region where the carbon concentration is 0.1wt% or less by introducing β, a rate constant proportional to the mean mass transfer coefficient in the molten phase ( 3)

d [% C] / dt = -β [% C] ----------------- (3)

Where β is empirically determined value and unit is min ^-1 and [% C] is carbon concentration at any refining time. Since the decarburization mechanism in the area where the carbon concentration is 0.1 wt% or more, that is, decarburization in the first to third stages is the oxygen supply rate, the decarburization rate during this period is constant if the operating conditions such as the blowing oxygen flow rate and the oxygen / inert gas ratio are constant. The decarburization rate in the decarburization nth step can be calculated from the equation (4). Where n = 1, 2, 3.

(d [% C] / dt) _n = ([% C] _n-1 -[% C] _n ) / Δt _n = α _n ------------- (4)

(4) where (d [% C] / dt ) n is the decarburization rate (wt% / min), [ % C] n is a carbon concentration (wt%) after the decarburization n-th stage exit at the n-th stage decarburization, △ t _n is the refining time (min) of the n-th step of decarburization, and α _n is the rate constant (wt% / min) in the n-th step of decarburization. In the first stage of decarburization, [% C] _n-1 = [% C] ₀ , ie the initial carbon concentration of AOD. In the AOD refining of low carbon austenitic stainless steel, after the nth step of decarburization, the component analysis and temperature measurement are performed. It can be calculated easily by

In the first to third stages of decarburization with carbon concentration of more than 0.1wt%, the decarburization rate of each decarburization stage was set as a dependent variable, and multiple regression analysis was carried out using the operation variable affecting the decarburization rate as an independent variable. Based on the Monte Carlo simulation, the target carbon concentrations for each stage were derived.

The Monte Carlo method is a useful simulation method in the engineering community that provides an approximation of the answer to a problem from a probability distribution when the exact mathematical model is not known. The basic idea is to look at an input variable as a probability function, generate a random number, take only the appropriate value, and discard the rest, to get the closest result.

By setting the lower limit of decarburization speed and the standard deviation and the standard deviation of the operation variables adopted in the multiple regression analysis, the Monte Carlo method was used to simulate the operation and the optimum target carbon concentration was obtained. The target carbon concentrations of the first and third steps of decarburization are 0.45%, 0.25% and 0.10%, respectively.

Unlike conventional AOD refining methods for low carbon austenitic stainless steels, the present invention relies on the average value of the decarburization rate for each decarburization stage calculated from the operation results for each heat. In the present invention, decarburization is performed using multiple regression analysis results. The speed was corrected, and the average value of the decarburization speed was obtained to set the standard decarburization speed.

As an operation variable affecting the decarburization rate, in the first stage of decarburization, the initial carbon concentration of AOD (wt%), the amount of ferroalloy and the amount of quicklime input per ton of molten steel in the first stage (kg / ts), and AOD (Audi) initial molten steel temperature (° C), and in the case of the second stage of decarburization, the amount of ferroalloy (kg / ts) per ton of molten steel introduced in the second stage and the end of the first stage of decarburization Is the molten steel temperature (℃), and in the case of decarburization step 3, the input amount of ferroalloy (kg / ts) per ton of molten steel input in step 3 was selected, and the decarburization rate due to the operation variable was performed by performing multiple regression analysis. The correction amount is calculated and shown in Table 1.

Table 1. Decarburization Speed Correction by Operation Variables

  Decarburization nth stage   Standard decarburization rate (% / min) Operation variables AOD initial carbon concentration (%) n-stage ferroalloy input (kg / ts) 1st step quicklime | input (kg / ts) Molten steel temperature at the end of n-1 stage (℃)  Reference value 0.1% increase  Reference value 5kg / ts increase correction value  Reference value Correction value at 5kg / ts increase  Reference value Correction value at 10 ℃ One 0.090 1.6 0.005 35 -0.0020 30 -0.0010 1520 0.0015 2 0.045 x - 15 -0.0010 x - 1680 -0.0005 3 0.030 x - 15 -0.0005 x - x -

In the first stage of decarburization, the initial carbon concentration of AOD was 1.7wt%, and the input amount of iron alloy and quicklime was 40kg / ts and 35kg / ts, respectively, and the initial molten steel temperature of AOD was At 1530 ° C., the decarburization rate of the first step of decarburization is corrected to 0.0935% / min.

1 is a graph comparing the AOD refining time when the conventional refining method is applied and the AOD refining time when the refining method of the present invention is applied.

When the low-carbon austenitic stainless steel refining method of the present invention is applied to a practical industry, the effect of shortening refining time is shown in comparison with the conventional method. In the conventional method, the AOD refining time is about 75 minutes, whereas in the present invention, the refining method provided by the present invention as 70 minutes has an excellent refining time shortening effect.

According to the present invention, the reason why the refining time of the AOD refining method of the conventional low carbon austenitic stainless steel is delayed is to always apply a constant decarburization rate without considering operation factors and optimize the target carbon concentration in each decarburization step. It is possible to reduce the AOD refining time by identifying the operating variables affecting the decarburization speed.The target carbon concentration and decarburization for each decarburization stage is analyzed through statistical analysis and simulation from the performance. The object of the present invention was achieved by correcting the decarburization rate in stages.

The foregoing merely illustrates preferred embodiments of the present invention and those skilled in the art to which the present invention pertains may make modifications and variations to the present invention without departing from the spirit and gist of the invention as set forth in the appended claims. It must be recognized.

As described above, according to the refining method of the low carbon austenitic stainless steel according to the present invention, it is possible to increase the decarburization efficiency and shorten the refining time by accurately estimating the decarburization rate for each step of decarburization in the carbon region and by optimizing the target carbon concentration. It works.

Claims (2)

In the refining method of a low carbon austenitic stainless steel comprising a decarburization step of decarburizing a low carbon austenitic stainless steel in three stages in an AOD refining furnace, Each decarburization step, In the carbon area of 0.1wt% or more, Performing a multiple regression analysis by setting the decarburization speed of each decarburization step as a dependent variable and an operation variable affecting the decarburization speed as an independent variable; Predicting the speed of each decarburization step based on the operating criteria derived through the multiple regression analysis, and deriving the target carbon concentration of each decarburization step through the result of the multiple regression analysis and Monte Carlo method (Monte Carlo Method) ; And Adjusting the refining time of each decarburization step to set the carbon concentration of each decarburization step to 0.45%, 0.25%, and 0.10%, The independent variable Initial carbon concentration (wt%) of AOD in the first stage of decarburization, input amount of ferroalloy and quicklime (kg / ts) per ton of molten steel input in the first stage of decarburization, and AOD ) Indicates the initial molten steel temperature (℃), In the second step of decarburization, the amount of iron alloy (kg / ts) per ton of molten steel introduced in the second step of decarburization and the molten steel temperature (° C.) after the completion of the first step of decarburization, A method for refining low carbon austenitic stainless steel, characterized in that the third step of decarburization indicates the amount of iron alloy (kg / ts) per ton of molten steel introduced in the third step of decarburization. delete

Images