KR101092191B1 - Method for manufacturing stainless steel - Google Patents

Abstract

The present invention is a method of manufacturing a stainless steel that can effectively replace the fluorspar in a range that does not impair desulfurization efficiency by mixing calcium aluminate and cryolite with flux at a higher basicity than the fluorspar when desulfurization of the stainless steel via the refining operation. It is about. Method for manufacturing stainless steel according to the present invention is a method for manufacturing stainless steel produced through the process of electric furnace-AOD- ladle- tundish-continuous casting, the refining in the AOD, decarburization and denitrification step by oxygen blowing; Slag and strong deoxidation by deoxidation after the decarburization and denitrification; And desulfurization step by desulfurization (CaO) and flux (Flux) after the deoxidation; including, but using a mixed flux of calcium aluminate and cryolite as the flux in the desulfurization step. By such a configuration, excellent results can be obtained in terms of desulfurization rate and desulfurization efficiency, and the concentration of fluorine components harmful to the environment and refractory materials can be reduced by half compared to the use of fluorspar.

AOD, stainless steel, desulfurization, cryolite, fluorspar, fluorine, calcium aluminate

Description

Method for manufacturing stainless steel

The present invention relates to a method for manufacturing stainless steel, and more particularly, can be used to replace the fluorite which is not only gradually depleted in the desulfurization stage of stainless steel via the refining operation, but also causes an increase in the fluorine content harmful to the environment and the human body. It relates to a method for producing stainless steel using flux.

Generally, steelmaking, including the refining process of stainless steel, consists of a furnace-AOD-ladle-continuous casting process.

Among these, in the case of AOD (Argon Oxygen Decarburization) process, the molten metal (high carbon and sulfur concentration) manufactured by dissolving scrap and raw materials in an electric furnace is transferred to a refining furnace. Thereafter, decarburization is carried out by oxygen blowing, deoxidation step of removing oxygen from slag and steel by adding a deoxidizer, and desulfurization agent for desulfurization and flux for securing slag fluidity to remove sulfur in the steel. Is done.

In general, the desulfurization reaction may be represented by the following [Formula 1].

^{[S] + (O 2 -} ) = (S 2 -) + [O]

Here, [S] and (O) means that S is present in the metal phase and O is present in the slag phase. When desulfurizing stainless steel using CaO in [Formula 1] it may be represented by the following [Formula 2].

(CaO) + [S] = (CaS) + [O]

In this case, in order to improve the desulfurization efficiency, it is generally easy to increase the CaO content, but as the CaO content increases, the melting point of the slag or rises. As a result, not only the liquid fraction which contributes to desulfurization is reduced, but also the slag fluidity is deteriorated, so that desulfurization efficiency is lowered.

In order to solve the problem of deterioration of slag liquidity, a large amount of fluorite is being used in addition to CaO desulfurization in the operation of stainless steel refining furnaces, but the price is rising due to the gradual depletion of fluorspar, which may increase the cost burden in the steel industry in the future. There is this. Crucially, fluorspar has half the amount of fluorine, which inevitably contains a large amount of fluorine that is harmful to the environment and human body. The treatment of slag generated during this process is mainly dependent on recycling or landfilling into roadbed. However, slag generated in the stainless steel manufacturing process using a large amount of fluorspar due to the tightening water quality and soil environment regulations for fluorine may not be recycled and landfilled in the future, and thus, fluorine-containing slag treatment may become a serious problem. . To address this, many techniques have been reported to explore new processes and alternatives that can minimize or in-place replacement of in-process fluorspar inputs.

Looking at the overall technology trends, patents for fluorspar reduction technology in steelmaking processes began to be filed since the mid-1970s, and have been filed in the late 1970s and early 1980s when fluorspar supply and demand were imbalanced. In the late 1990s, when environmental regulations began to strengthen in earnest, the number of applications has remained steady within 20. By country, Japan, Korea, the United States, and Europe, Nippon Steel Co., Ltd. (NSC) occupies the largest share, and JFE and SUMITOMO occupy the following, leading the development by Japan. You can see that we are here. These technologies have been mainly developed for carbon steel, and the number of publications is low for stainless steel containing large amounts of chromium (Cr). It is not easy to find a fluorspar substitute in stainless steel that naturally contains Cr ₂ O ₃ of high melting point compared to carbon steel containing FeO of low melting point of slag. Patents for fluorspar reduction reported so far are known as follows.

Japanese Laid-Open Patent Publication No. 2002-285217 uses a desulfurizing agent containing Na ₂ O, which is known to reduce slag melting point such as silicate (Na ₂ O.SiO ₂ ) in quicklime, and the content of Na ₂ O in slag is 6wt%. As a result, a report on a technique for reducing fluorspar is reported. However, when Na ₂ O is used in a large amount, it is considered that not only deterioration of the working environment due to volatilization but also rapid increase in erosion of the refractory material may adversely affect the refractory life.

In Japanese Patent Laid-Open Nos. 1996-325620 and 1999-131127, fluorite (Colemanite, Ca ₂ B ₆ O ₁₁ · 5H ₂ O), Yulec, which is a B ₂ O ₃ -containing substance as an alternative to fluorspar in the production of stainless steel One of the sites (Ulexite, NaCaB ₅ O ₉ · 8H ₂ O), Kernite (Kernite, Na ₂ B ₄ O ₇ · 4H ₂ O) and Borax (Borax, Na ₂ B ₄ O ₇ · 10H ₂ O) The technique of adding and increasing the fluidity of slag and desulfurization is demonstrated. However, the addition of B ₂ O ₃ mentioned in the above patent into slag not only causes new environmental problems due to boron elution from slag during slag recycling, but also due to pick-up of boron components into molten steel during operation. There exists a problem that it is difficult to apply about steel other than boron addition steel.

In Japanese Patent Laid-Open No. 1984-177313, in the reduction deoiling method of chromium-containing molten steel, SiO ₂ which is detrimental to desulfurization in slag composition after decarburization is reduced to 10 wt% or less, and CaO / Al ₂ O ₃ is added as a new basicity concept. Al is added as a reducing agent so as to be 0.8 ~ 1.4, and CaO is added as a coagent to provide a description of the possibility of operation of intangible stone by CaO-Al ₂ O ₃ -based slag. However, in order to ensure slag fluidity by Al ₂ O ₃ alone, Al ₂ O ₃ must be increased in a large amount of slag, which causes accelerated erosion of the refractory during refining operation using CaO-MgO-based dolomite as a refractory, resulting in reduced lifetime. Will cause problems. In addition, during the operation of a refining furnace based on silicon (Si) deoxidation, high melting point Al ₂ O ₃ inclusions may be generated by Al pick-up in molten steel, thereby causing problems in post-processing quality.

Japanese Laid-Open Patent Publication No. 1998-088216 discloses a technique that does not use fluorite by dissolving Al in molten iron and then physically injecting or impeller stirring to supply MgO to the molten iron and desulfurize it. This process is preferably carried out before refining in the concept of pretreatment, and it is thought to be excellent in terms of efficiency, but also requires the introduction of equipment to secure mechanical stirring force, which requires economical investment. There is this.

As such, there is a need for a flux material that can replace the use of fluorspar, which causes an increase in fluorine that is harmful to the environment and the human body in the desulfurization operation in a refinery.

Accordingly, an object of the present invention is to mix and add calcium aluminate and cryolite with flux at a higher basicity than fluorite when desulfurization of stainless steel via smelting operation, thereby introducing slag fluidity without introducing new equipment or contamination by flux components into molten steel. The present invention provides a method for refining stainless steel that can effectively replace fluorspar in a range that does not deteriorate and does not impair desulfurization efficiency.

In addition, another object of the present invention is to find a theoretical basis for using cryolite as a flux to reduce the content of fluorine in the slag, and finally to produce a stainless steel that can be produced in an environmentally friendly recycling slag by applying it to the actual process in a stable manner To provide a way.

In the method of manufacturing a stainless steel produced through a process of electric furnace-AOD-ladle-tundish-continuous casting, the refining in the AOD, decarburization and denitrification by oxygen blowing; Slag and strong deoxidation by deoxidation after the decarburization and denitrification; And desulfurization by desulfurization (CaO) and flux (Flux) input after the deoxidation. However, in the desulfurization step, a mixed flux of calcium aluminate and cryolite is used.

In the desulfurization step, basicity (CaO / SiO ₂ ) is set in the range of 1.9 to 2.7.

In addition, the calcium aluminate is added to 15 ~ 35wt% of the total slag weight.

In addition, the cryolite is charged with 1.8 ~ 3wt% of the total slag weight.

In addition, the calcium aluminate includes CaO and Al ₂ O ₃ .

In addition, the cryolite includes CaF ₂ and Na ₃ AlF ₆ .

In addition, the slag produced by the manufacturing method as described above has a fluorine concentration of 0.5wt% or less.

As described above, according to the present invention, the desulfurization operation is performed by mixing calcium aluminate and cryolite with flux at a higher basicity than the fluorspar when desulfurization of the stainless steel via the refining operation, thereby achieving an equivalent or higher level of the existing desulfurization efficiency. I can keep it.

In addition, by significantly reducing the concentration of fluorine that is harmful to humans and the environment to a level that does not cause environmental problems, a method for desulfurization of stainless steel that can form stainless slag that can be recycled stably without violating environmental regulations in the future is provided. It can be secured.

Hereinafter, a method of manufacturing stainless steel according to the present invention will be described in detail with reference to the drawings showing an embodiment of the present invention.

1 is a comparison of a conventional process and a process according to the invention.

Referring to Figure 1, in the production of stainless steel via the refining furnace according to the present invention was derived a refining method that can effectively reduce the fluorine content in the slag without using a large amount of fluorite.

Looking at the operation of the refining furnace, after the molten metal produced in the electric furnace to reach the refining furnace through the ladle, the charged molten metal is subjected to decarburization and denitrification by oxygen blowing (1, 5). Then, after the deoxidation step (2, 6) to remove the slag and oxygen in the steel by the deoxidizer input, after the desulfurization step (3, 7) in the steel by the flux input for desulfurization and flux securing, the final molten steel After checking the temperature and ingredients, the process starts with the refinery.

Among them, calcium aluminate (CaO · Al ₂ O ₃ ) and cryolite (Na ₃ ) are used at the basicity upward conditions without using fluorite (3), which is a means for securing the flowability of slag in the desulfurization step. A composite substitute 7 of AlF ₆ ) was added. This significantly reduces the concentration of fluorine, a human and environmental hazardous substance, to 0.5wt% while maintaining the equivalent level of desulfurization efficiency when using existing fluorspar, which can produce slag that can be safely recycled without causing environmental problems. Desulfurization refining technology of steel.

The calcium aluminate and cryolite phase used as a fluorspar substitute in the present invention are X-ray diffraction (XRD) analysis results of CaO Al ₂ O _{3 for} calcium aluminate, and some CaF ₂ (Fluorspar) for cryolite. It was confirmed that most of Na ₃ AlF ₆ (Sodium Cryolite), and X-ray fluorescence spectroscopy (XRF; X-Ray Flourescence Spectrometry) analysis performed for quantitative analysis are shown in Table 1 below.

Fluorite Substances XRD analysis results Calcium aluminate SiO ₂ , wt% CaO, wt% Al ₂ O ₃ , wt% MgO, wt% 5 40 52 3 cryolite Ca, wt% Al, wt% Na, wt% F, wt% 6 14 27 53

Here, calcium aluminate is an oxide containing CaO and Al ₂ O ₃ as main components, and some SiO ₂ and MgO components.In the case of cryolite, Al, Na and F components are the main components, and some Ca is CaF ₂ form. It can be seen that it exists.

Figure 2 is a state diagram showing the change in melting point according to the Al ₂ O ₃ content in CaO-SiO ₂ -Al ₂ O ₃ ternary system.

Referring to Figure 2, in the present invention will be described a series of theoretical background to find a fluorspar substitute using the raw material. The core of the present invention can be represented by a series of processes using calcium aluminate, elevating basicity, and using cryolite.

First, the background of the use of calcium aluminate, as shown in Fig. 2, when the CaO / SiO ₂ ratio is 1.3 or more in the CaO-SiO ₂ -10wt% MgO slag system, 2CaO · SiO ₂ solid phase starts to form in the slag, and the melting point of the slag is 1600 It rises more than degreeC. Such a solid compound reduces the fraction of the liquid phase in the slag, thereby causing a decrease in the desulfurization efficiency. At this time, as shown in FIG. 2, when Al ₂ O ₃ is added in slag, as shown in CaO-SiO ₂ -Al ₂ O ₃ -10wt% MgO state diagram, melting point of slag until Al ₂ O ₃ is increased to a certain concentration Will decrease proportionally. There will be many ways to add such Al ₂ O ₃ .

Among them, an aluminum (Al) metal is added as an effective method to increase Al ₂ O ₃ by oxidation, or an alumina brick or alumina composite oxide that enables direct increase of Al ₂ O ₃ . There will be an input.

However, in the case of the former, when directly injecting aluminum (Al) which is expensive and has high deoxidizing power, it may directly react with other elements or refractory in the slag, causing negative problems. If alumina brick is added, the slag reaction rate will be delayed due to the slow melting rate.

Therefore, in the present invention, calcium aluminate having a low melting point and a relatively low melting point of about 1400 ° C. and fast melting rate was selected as an increase source of Al ₂ O ₃ . At this time, the desulfurization effect by the increase of the CaO component which plays a role of desulfurization in calcium aluminate may be expected.

3 is a graph showing a change in sulfur concentration in the steel according to the addition amount of calcium aluminate at a basic level of 1.5, Figure 4 is a graph showing a change in sulfur concentration in the steel according to the amount of fluorspar addition at a basicity of 1.5.

As shown in Figures 3 and 4, it can be seen that the desulfurization efficiency is increased proportionally when the calcium aluminate in the slag is increased at a basicity of 1.5 and a temperature of 1600 ° C. However, this also can be seen to be insufficient compared to the degree of desulfurization when using the fluorspar of FIG.

In other words, as the concentration of calcium aluminate was increased to 0 wt%, 8 wt%, 15 wt% and 23 wt%, the concentration of sulfur in the steel gradually decreased to about 0.016 wt%. However, as the concentration of fluorspar is increased to 0wt%, 2wt%, 5wt%, 8wt%, 12wt% and 23wt% in FIG. 4, the sulfur concentration in the steel is gradually decreased up to about 0.007wt%. That is, since the effect of the case where calcium aluminate is added to the flux instead of the fluorite is remarkably reduced, calcium aluminate alone cannot replace the existing fluorite.

Accordingly, it is necessary to change the other conditions to improve the desulfurization efficiency, and it is necessary to consider the effect by basicity. In general, the desulfurization capacity (C _S ) and the sulfur distribution ratio (L _S ) are defined by the following equations (3) and (4).

C _{S = K 1 * a O2 - /} f S2 - = (wt% S 2 -) * a 0 (or P O2 1/2) / f s * [wt% S]

L _S = (wt% S) / [wt% S] = CS * fS / a0 (or PO21 / 2)

logL _S = logC _S + logf _S -loga ₀ (or -1 / 2logP _O2 )

Where K ₁ is the equilibrium constant value of Equation 1, a _M is the activity of M material, f _N is the activity coefficient of N material, (wt% L) and [wt% L] is the weight ratio of L material in slag and metal , P _O2 is O ₂ Mean partial pressure of gas, respectively. Here, the desulfurization ability is a function that depends on the properties of the slag itself and the sulfur distribution ratio is a function that depends simultaneously on the slag and metal properties. In order to accelerate the desulfurizing reaction as can be seen the two expression activity of the desulfurization in the performance point of view sulfur ions in the slag coefficient (f _{S2 -)} reduction and (a _O2 activity of the oxygen ions in the slag by the basicity increases, such as the use of CaO flux _- ) An increase would be advantageous. In terms of the sulfur distribution ratio, the use of slag having excellent desulfurization ability (C _S ) that can absorb a lot of sulfur, the increase of the activity coefficient of sulfur in metal (f _S ) through metal composition adjustment, and the activity of oxygen in metal through deoxidation A decrease in degree (a ₀ ) would be advantageous. Among the above methods for promoting desulfurization, increasing the basicity is known as the most effective method.

Figure 5 is a graph showing the change in sulfur concentration in the steel when using calcium aluminate in basicity 2.1, Figure 6 is a graph showing the change in sulfur concentration in the steel according to the amount of fluorspar in basicity 2.1.

As shown in the results of Figure 5, after increasing the desulfurization slag basicity from 1.5 to 2.1, it can be seen that not only increases the desulfurization rate but also decreases the concentration of sulfur in the final steel when 23 wt% of calcium aluminate is added. In fact, this can be achieved by increasing the desulfurization by basicity even when using fluorspar as shown in FIG. In other words, when the calcium aluminate was added 23wt% sulfur concentration in the steel was reduced to about 0.01wt%, the sulfur concentration in the steel was reduced to about 0.004wt% by adding 23wt% of fluorspar. Therefore, even in the case of using fluorspar in the operation of conventional refining by simply increasing the basicity, it is believed that the effect of increasing the desulfurization.

However, it is still difficult to solve the problem of increasing the concentration of fluorine in slag caused by using a large amount of fluorite. Therefore, it would be necessary to find excellent desulfurization efficiencies as in the case of using fluorspar while significantly lowering the concentration of fluorine. In the present invention, the result of raising the basicity to the calcium aluminate can solve the fluorine problem, but to solve the problem of desulfurization results that do not reach the use of fluorite using the characteristics of the fluorine ion on the slag structure. The following findings have been reported on the role of fluorine ions on the slag structure.

Scheme 1 shows the effect of fluorine ions on the CaO-SiO ₂ -CaF ₂ slag structure, and Scheme 2 shows the effect of fluorine ions on the CaO-Al ₂ O ₃ -CaF ₂ slag structure.

^{_{[Si 2 O 7] 6- +}} 2F - = 2 [SiO 3 F] 3- + O 2 -

^{_{[Al 2 O 7] 8- +}} 2F - = 2 [AlO 3 F] 4- + O 2 -

As in Scheme 1 and Scheme 2, as the fluorine ion in the slag enters the slag structure is simplified, so that the slag viscosity is drastically reduced. This decrease in viscosity may increase slag flowability and increase desulfurization.

Therefore, in the present invention, a method of forming slag that can be stably recycled while maintaining a desulfurization effect equivalent to or higher than that of existing fluorite by effectively utilizing fluorine that should be removed within a small amount that does not cause environmental problems. Was created.

The uniqueness of the present invention is not only to use a small amount of existing fluorite but also to explore a new material that can help more effectively desulfurization. That is, in the present invention, a material called cryolite containing Na and Al, which may not only increase the fluorine component but also bring about further desulfurization effect, was used.

FIG. 7 is a graph showing changes in sulfur concentration in steel when calcium aluminate is mixed with fluorspar and cryolite at a basicity of 2.1.

Referring to FIG. 7, it can be seen that desulfurization is increased only by adding a small amount of fluorine as well as excellent in terms of desulfurization rate and efficiency when injecting cryolite into calcium aluminate as compared to when fluorite is added to calcium aluminate. That is, when 2wt% fluorspar is added to 23wt% calcium aluminate, the sulfur concentration in steel is 0.01wt%, while when 2wt% cryolite is added to 23wt% calcium aluminate, the sulfur concentration in steel is 0.007wt% As low as.

Here, the reason for the better desulfurization results in the case of cryolite than fluorite is that the oxygen concentration in the steel is reduced by the aluminum (Al) contained in the crystallite, which can be seen in Equation 4 by increasing the distribution ratio by reducing the oxygen activity Is believed to be the cause. In fact, after the experiment on the STS304 steel (Fe-18wt% Cr-8wt% Ni), the final oxygen concentration in the steel was about 40ppm for fluorspar, but about half as low as 20ppm for cryolite.

8 is a graph showing the desulfurization efficiency according to the amount of desulfurization flux added and mixed use.

Desulfurization efficiency is 65-75% level when using fluorite at 8-15 wt% in basicity 1.5 as shown in FIG. 8, while slag basicity is raised to 2.1 and a mixture of 23 wt% calcium aluminate and 2 wt% cryolite in slag is used. In this case, it can be seen that the desulfurization efficiency is 80% or more than the previous level. Here, the desulfurization efficiency showing how much sulfur is removed from the steel is expressed by the following equation.

% Desulfurization efficiency = 100 * ([initial sulfur concentration]-[sulfur concentration after desulfurization]) / [initial sulfur concentration]

Therefore, from the above theoretical approach, fluorine concentration in the final slag can be effectively controlled within the range that does not impair the existing desulfurization efficiency by using a higher basicity and a mixed flux of calcium aluminate and cryolite in the desulfurization operation. It can be reduced.

[Example]

Hereinafter, the effects of the present invention on fluorspar through the embodiments of the present invention will be described.

First, prior to the introduction of a full-fledged example, the process of finding the optimum range of each factor for basicity, calcium aluminate and cryolite, which are important factors introduced in the present invention, will be described.

9 is a graph showing desulfurization efficiency according to basicity.

9 shows the desulfurization efficiency of slag according to basicity. At this time, the remaining amount of calcium aluminate and the amount of cryolite were maintained at an appropriate value that does not inhibit desulfurization and the experiment was performed. The same applies to the two series of cases. As shown in the results, the desulfurization efficiency tends to increase according to the basicity, but if it is excessively increased, the desulfurization efficiency decreases due to the lack of slag fluidity. On the contrary, the desulfurization efficiency decreases because the slag desulfurization ability decreases. Therefore, in the case of using fluorspar, the basicity range is 1.9-2.7, considering that the desulfurization efficiency is 70% to 80%.

10 is a graph showing the desulfurization efficiency according to the amount of cryolite added.

As can be seen in FIG. 10 in the same manner as in FIG. 9, even when the amount of cryolite is added too little, the slag desulfurization ability is lowered and the final desulfurization efficiency is decreased, and the desulfurization efficiency is proportionally increased when added in excess. However, this eventually increases the concentration of fluorine in the slag, which causes problems in recycling. Therefore, it is necessary to use cryolite properly in a range that does not violate environmental regulations, and an appropriate level of cryolite consumption is 3wt, based on the criteria that satisfy the fluorine concentration of 0.8 mg / liter or less in drinking water, which is the most severe environmental regulation in Japan. It is considered to be less than%. Of course, the amount of fluorine elution corresponding to 3 wt% of cryolite consumption was actually analyzed through the waste process test method, and the corresponding amount of elution was 0.5 mg / liter. Here, 3wt% is the initial dose, so when used at a high temperature of 1600 ℃, due to the nature of the fluorine-containing material, it is actually left in the slag in a smaller amount due to volatilization, so the initial titer of crystallite corresponding to 0.8mg / liter elution of the final slag after operation May increase further. In addition, the minimum amount of cryolite that can exhibit a desulfurization effect (desulfurization efficiency 75% or more) similar to fluorspar injection by the cryolite input is 1.8wt%. Based on the above results, the appropriate level of cryolite input is estimated to be 1.8 ~ 3wt%.

11 is a graph showing desulfurization efficiency according to calcium aluminate addition amount.

Referring to FIG. 11, when the calcium aluminate is added in an excessive amount, the slag desulfurization ability by the CaO fraction decrease will be a problem, and when the lack thereof, the slag fluidity decreases, which will eventually lead to a reduction in the desulfurization efficiency. Therefore, the appropriate amount of calcium aluminate addition amount is considered to be 15-35wt%.

In conclusion, from the above results, in the desulfurization operation of stainless steel refining furnace, the basic conditions of 1.9 ~ 2.7, cryolite 1.8 ~ 3 wt% and calcium aluminate 15 to 35 wt% may be selected. Where wt% of cryolite and calcium aluminate is the weight ratio to the total amount of desulfurized slag.

From now on, the conditions derived from the present invention in earnest will be introduced using the specific results for the stainless steel desulfurization results according to a preferred embodiment. STS304 (Fe) with the basicity of 2.1, cryolite 2 ~ 2.3wt% and calcium aluminate 23wt% as the intermediate value of each condition considered to be able to confirm the effect stably within the appropriate condition range derived above. Desulfurization experiments of -18 wt% Cr-8 wt% Ni) and STS430 (Fe-16 wt% Cr) steels were conducted.

As the base material used in the desulfurization experiment, the initial [S] concentration was 300 ppm through vacuum dissolution, and other components were generally used to produce stainless steel corresponding to the STS304 and STS430 compositions. And slag include CaO, SiO ₂ and MgO were mixed powder was prepared a uniform Master Slag through melting, results 49wt% CaO - it was confirmed that the _{7wt% MgO - 44wt% SiO 2} . After the desulfurization experiment was performed by adding fluorite or calcium aluminate and cryolite mixed flux as desulfurization material (CaO) and flux in the steel and master slag.

A brief description will be given to procedures performed desulfurization experiment, the first charging the base material in an alumina crucible, and the target master slag and desulfurization material (CaO) in the upper After raising gives blowing an inert gas to 1600 ℃ basicity (CaO / SiO ₂₎ After calculation, it was added. Fluorite, or cryolite and calcium aluminate mixed fluxes, as flux, was introduced into the crucible from the top in the same manner. After the desulfurization experiment, the sulfur concentration in molten steel specimens was analyzed using the LECO C / S simultaneous analyzer, and the fluorine concentration in slag was selectively used according to the concentration range. In other words, by using ion chromatography (Ion Chromatography) at low concentrations (<1wt%) and X-ray fluorescence beads (XRF) at high concentrations (> 1wt%) To improve the accuracy.

Table 2 below shows the result of desulfurization while changing the content of fluorspar at a basicity of 1.5 for STS304 steel as a conventional example. As shown in the results, the desulfurization efficiency is increased by increasing the content of fluorspar, but the fluorine concentration in the final slag also increases. In addition, when the fluorite use is reduced to lower the fluorine concentration, the desulfurization efficiency is significantly lowered, resulting in quality problems. Therefore, it is difficult to solve both the quality and fluorine problems by simply using fluorspar.

division Initial input Experiment result Master slag (g) CaO (g) Fluorite (g) CaO / SiO ₂ [S] (wt%) after dehydration Desulfurization Efficiency
(%) (F) _slag
(wt%) Conventional Example 1 49.0 14.5 1.5 1.5 0.0183 36.9 1.05 Conventional Example 2 48.0 14.0 3.0 1.5 0.0141 51.4 2.05 Conventional Example 3 46.5 13.5 5.0 1.5 0.0095 67.2 3.10 Conventional Example 4 42.5 12.5 10.0 1.5 0.0075 74.1 7.20 Conventional Example 5 38.5 11.5 15.0 1.5 0.0066 77.2 10.85

[Table 3] and [Table 4] below show the results of the desulfurization of STS304 and STS430 steels using the cryolite and calcium aluminate mixed fluxes instead of fluorite in basicity 2.1 as an example of the invention.

division Initial input Experiment result Master
slag (g) CaO (g) cryolite
(g) Calcium aluminium
Nate (g) CaO / SiO2 [S] (wt%) Desulfurization Efficiency
(%) (F) _slag
(wt%) Inventory 1 32.0 16.5 1.5 15 2.1 0.0063 79 0.50 Inventory 2 30.2 18.5 1.3 15 2.1 0.0054 82 0.29 Inventory 3 30.0 18.5 1.5 15 2.1 0.0056 81 0.40 Honorable 4 30.0 18.5 1.5 15 2.1 0.0043 86 0.39 Inventory 5 30.0 18.5 1.5 15 2.1 0.0042 86 0.34 Inventive Example 6 30.0 18.5 1.5 15 2.1 0.0051 83 0.42

division Initial input Experiment result Master
slag (g) CaO (g) cryolite
(g) Calcium aluminium
Nate (g) CaO / SiO2 [S] (wt%) Desulfurization Efficiency
(%) (F) slag
(wt%) Inventory 1 30.0 18.5 1.5 15 2.1 0.0031 89 0.38 Inventory 2 30.0 18.5 1.5 15 2.1 0.0037 88 0.35 Inventory 3 30.0 18.5 1.5 15 2.1 0.0047 84 0.40 Honorable 4 30.0 18.5 1.5 15 2.1 0.0030 90 0.34 Inventory 5 30.0 18.5 1.5 15 2.1 0.0042 86 0.44

12 is a desulfurization graph of STS304 steel using calcium aluminate and cryolite mixed flux at basicity 2.1, and FIG. 13 is a desulfurization graph of STS430 steel using calcium aluminate and cryolite mixed flux at basicity 2.1.

Normally, when operating in stainless steel refining furnace, the basicity is in the range of 1.6 to 1.8, and the fluorite concentration in the slag is in the range of 8 to 10 wt%. In Table 2, the corresponding desulfurization efficiency can be regarded as approximately 70 to 80%. Compared with the desulfurization efficiency, the results of the present invention show that the desulfurization efficiency is equal to or higher than that when using fluorite at the basicity of 2.1 and using calcite and calcium aluminate mixed fluxes. It can be seen that it can be applied without.

12 and 13 show the desulfurization behavior over time for STS304 and STS430 steels, respectively. can confirm. In addition to the desulfurization efficiency, it can be seen that the concentration of fluorine in the final slag is 0.5 wt% or less in the STS304 and STS430, which is about 10 times lower than the fluorine concentration of 5 wt%. In this case, simply calculating the fluorine concentration in the slag relative to the input amount can be expected to be about 1wt% level, but only half of the value is estimated due to the reduction of volatilization due to the nature of the fluorine-containing material. As a result of the volatilization experiment using the actual cryolite, it was experimentally confirmed that more than half of the volatilization was possible. In addition, the fluorine dissolution test using slag corresponding to 0.5wt% resulted in 0.5mg / l, which is lower than 0.8mg / l, which is the regulation value of fluorine in Japanese drinking water. This means that by allowing only a small amount of fluorine in the present invention, the desulfurization efficiency can be improved and the fluorine environmental regulation problem can be solved.

In conclusion, by applying the conditions of using a mixed flux of calcium aluminate and cryolite in the basicity upstream conditions derived from the present invention, it is possible to prevent the quality problems of the recycling and landfilling problems that occur when the fluorine regulations are strengthened in the future. At the same time, it was possible to derive a method for desulfurization of stainless steels that could solve the environmental regulations.

Although the technical spirit of the present invention has been described in detail according to the above-described preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

1 is a comparison of a conventional process and a process according to the invention.

Figure 2 is a state diagram showing the melting point change according to the content of Al ₂ O ₃ in CaO-SiO ₂ -Al ₂ O ₃ ternary system.

Figure 3 is a graph showing the change in sulfur concentration in the steel according to the calcium aluminate addition amount at a basicity of 1.5.

Figure 4 is a graph showing the change in sulfur concentration in the steel according to the amount of fluorspar at the basicity 1.5.

5 is a graph showing changes in sulfur concentration in steel when using calcium aluminate at a basicity of 2.1.

Figure 6 is a graph showing the change in sulfur concentration in the steel according to the amount of fluorspar in basicity 2.1.

FIG. 7 is a graph showing changes in sulfur concentration in steel when calcium aluminate is mixed with fluorspar and cryolite at basicity 2.1.

8 is a graph showing desulfurization efficiency according to desulfurization flux addition amount and mixed use.

9 is a graph showing desulfurization efficiency according to basicity.

10 is a graph showing the desulfurization efficiency according to the amount of cryolite added.

11 is a graph showing desulfurization efficiency according to calcium aluminate addition amount.

12 is a desulfurization graph of STS304 steel using calcium aluminate and cryolite mixed flux at basicity 2.1.

FIG. 13 is a graph of desulfurization of STS430 steel using calcium aluminate and cryolite mixed flux at basicity 2.1.

Claims (7)

In the manufacturing method of stainless steel manufactured by the process of electric furnace-AOD- ladle- tundish-continuous casting, Refining in the AOD, Decarburization and denitrification by oxygen blowing; Slag and strong deoxidation by deoxidation after the decarburization and denitrification; And Desulfurization by desulfurization (CaO) and flux (Flux) after the deoxidation; including; In the desulfurization step using a mixed flux of calcium aluminate and cryolite, the basic method (CaO / SiO ₂ ) to operate in a range of 1.9 to 2.7. delete The method of claim 1, The calcium aluminate is a method of manufacturing stainless steel to inject 15 ~ 35wt% of the total slag weight. The method of claim 1, The cryolite is a method of manufacturing stainless steel injecting 1.8 ~ 3wt% of the total slag weight. The method of claim 1, The calcium aluminate is a method for producing stainless steel containing CaO and Al ₂ O ₃ . The method of claim 1, The cryolite is a method for producing stainless steel containing CaF ₂ and Na ₃ AlF ₆ . The method of claim 1, The slag produced by any one of claims 1 to 6 has a fluorine concentration of 0.5wt% or less.

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