KR100948921B1 - Furnace Refining of Ferritic Stainless Steels - Google Patents

Abstract

An external furnace refining method of ferritic stainless steel, the object of which is to provide an external furnace refining method of high-purity deep-processed far-rate stainless steel at low cost. To this end, in the present invention, the step of tapping the ferrite stainless molten steel from the AOD refining furnace with a slag having a basicity (CaO / SiO ₂ ) of 1.6-2.2 ladle of alumina; Injecting fluorite (CaF ₂ ) with the upper slag remaining in the ladle; And performing bottom bubbling in the ladle.

Refinement, low stirring, slag

Description

Refining method of ferrite stainless steel}

1 is a graph showing the relationship between the slag basic and the molten steel sulfur concentration in the refinery,

2 is a graph showing the relationship between the refractory loss (ladle concentration of Al ₂ O ₃ ) according to the ladle material,

3 is a graph showing ladle residual slag amount and temperature drop.

The present invention relates to a method for producing ferritic stainless steel, and more particularly, to an external furnace refining method for ferritic stainless steel used for deep processing.

In general, the ferritic stainless steel for deep processing contains 16-18% by weight of chromium (Cr), 0.12% by weight or less of impurity carbon, and 0.010% by weight or less of sulfur, and thus argon oxygen decarburization (AOD). (Hereinafter referred to as AOD) and vacuum oxygen decarburization (VOD) are refined through decarburization, denitrification and desulfurization in a refining furnace.

The main use of the ferritic stainless steel for deep processing is not only a bright annealing (BA) product but also a flatware, and this requires a spin process such as spinning, so that the spinel is inferior to the surface quality. Minimization of Al ₂ O ₃ -MgO-based spinel inclusions and reduction of the number of cast inclusions are required.

In other words, when the main processing method is deep processing such as bending processing and spinning processing, the requirements for steel should minimize the number and size of inclusions in the material as well as the surface quality.

In addition, Al ₂ O ₃ -MgO-based (hereinafter referred to as spinel) and CaO-TiO ₂ -based inclusions that deteriorate the surface quality are known as defects that cause surface defects as non-extensible inclusions, so these inclusions should also be minimized.

As a refining method for preventing product defects due to spinel, which is a hard inclusion produced in refining stainless steel, Japanese Patent Application Laid-Open Nos. 4-99215, 3-267312, 10-158720, 6-306438 and 8 -104915, and the like are known.

All of these methods apply to a mirror finishing material which is susceptible to surface defects due to inclusions, and is a method of preventing inclusions from extending linearly during rolling of the material.

However, although Japanese Patent Application Laid-Open No. 4-99215 regulates aluminum in the raw material, it is very difficult to regulate Al in the raw material in the stainless steel manufacturing process using scrap.

In addition, Japanese Patent Application Laid-Open Nos. 3-267312 and 10-158720 disclose that the regulation of slag basicity and magnesia concentration can prevent the formation of hard inclusions. Is very difficult.

In Japanese Patent Laid-Open No. 6-306438, in order to prevent the formation of MgO-Al ₂ O ₃ as hard inclusions, the magnesia concentration in the slag is 7 wt% or less, Al ₂ O ₃ is 5 wt% or less, and the basicity is 1.3-1.9. Although suggested, it is quite difficult and stable to control the magnesia concentration in the slag to 5% by weight or less while using magnesia chromite as a refining furnace.

Most of the conventional techniques are characterized in that the basicity of the slag in the refining furnace is adjusted to 2 or less, as in JP-A-8-104915. Thus, when the basicity of the slag is low, the oxygen in the molten steel is increased to Cleanliness deteriorates and it is difficult to remove sulfur in molten steel, which is another function of the refining furnace.

In addition, SiO ₂ inclusions are deoxidation products generated during the decarburization, denitrification, and desulfurization of AOD refining. If these inclusions remain in the material, they may cause bending cracks due to the elongated properties during processing.

Therefore, conventionally, a method of injecting aluminum or titanium (Ti) to minimize SiO ₂ deoxidation products has been applied, but this increases rather hard Al ₂ O ₃ -MgO-based and CaO-TiO ₂ -based hard inclusions causing surface quality problems. There is a disadvantage that the use as a surface stiffener for BA applications is limited. Therefore, there is a need for an economical manufacturing method that does not limit the use of deep processing by minimizing SiO ₂ inclusions while stabilizing surface quality.

Accordingly, the present invention has been made to solve the above problems, the object of the present invention is to provide a furnace refining method of molten steel to secure high cleanness at low cost for the production of deep ferritic stainless steel.

In order to realize the above object, the present invention is characterized by eliminating the VOD treatment process by optimizing the molten steel stirring conditions in the ladle.

That is, the exothermic refining method of the ferritic stainless steel according to the present invention, the step of tapping the ferrite stainless molten steel with slag having a basicity (CaO / SiO ₂ ) of 1.6-2.2 with a slag of alumina from the AOD refining furnace; Injecting fluorite (CaF ₂ ) with the upper slag remaining in the ladle; And performing bottom bubbling in the ladle.

At this time, the slag before tapping is preferably prepared to contain 2.0 wt% or less of Al ₂ O ₃ and 1.0 wt% or less of TiO ₂ .

In addition, the amount of the upper slag remaining in the ladle is preferably 1.0 to 3.0 tons for the 90 ton ladle, the fluorspar loading is preferably 20-40 kg per ton slag.

In the low stirring step, it is preferable to carry out low stirring for 15 minutes or more at a stirring flow rate of 3.0 to 6.0 liters per ton of molten steel.

Hereinafter, the present invention will be described in detail.

Recently, various refining processes have been developed according to high clean demands, and the ferrite-based stainless steel manufacturing process currently used is shown in Table 1.

Manufacturing process Dissolution C, S removal Leave Exclusion C, S removal Ladle treatment casting Ladle Material One Electric furnace LD converter - skip VOD Low stirring play Magcro 2 blast furnace LD converter - skip VOD Low stirring play Magcro 3 Electric furnace AOD Slag practice - Low stirring play Dolomite The present invention Electric furnace AOD Slag skip - Low stirring play Alumina

Here, the ladle treatment (LT) is the main function of the bottom bubbling for the purpose of adjusting the fine composition and temperature of the molten steel from the refining furnace and the separation of inclusions.

As shown in Table 1, the process 1 to 3 currently used, the process 1 and 2 using the VOD equipment is cast without the tapping operation compared to the process 3, which must be performed after refining in the AOD or LD converter It becomes an advantageous condition for ensuring high cleanliness.

However, for VOD treatment, expensive refractory (magnesia chromite ladle) must be used and productivity is very low due to the additional process.

On the other hand, in the process 3, which omits the VOD process, the economic effect is great, but when the condition of stirring the ladle which is low agitation in the ladle to adjust the composition in the manufacturing of ferritic stainless steel for deep processing is inappropriate, There is a problem of losing value.

In particular, the molten steel in the ladle, which is pulled together with the slag in the AOD refining furnace, is suspended in the slag droplets, so if it is not separated from the flotation, it is difficult to secure the beautiful surface quality and cause processing cracks when used for deep processing. There is a limit to usability. Therefore, when the economical manufacturing process is omitted by omitting the VOD process, the inclusions should be separated and separated as much as possible by optimizing the molten steel stirring conditions.

In general, in case of manufacturing by AOD-LT process, FeCr alloy iron and scraps are dissolved in the electric furnace according to the target chromium concentration, and the impurities such as carbon are removed by injecting oxygen and argon inert gas from the refining furnace. By adding Si alloy iron such as FeSi to lower the chromium oxide and dissolved oxygen levels formed during decarburization, CaO is added to prepare slag to remove sulfur, and the slag is cast into cast ladles.

At this time, since the inclusion of inclusions is inevitable, the removal of the inclusions is very important. In the flotation separation method, the physical properties and amount of the ladle residual slag are very important, and there is a big difference in cleanliness according to the low-odor stirring method.

Each company adopts unique process, and in the case of our company, there is a method of adding CaO powder to prevent reoxidation after slag condition. In this case, the stirring time should be sufficient because fine stirring must be performed. This is possible if the treatment time is not limited, but it is usually impossible to settle for more than 30 minutes due to the limitation of the processing time and the sudden drop in temperature.

In addition, there are limitations in the manufacture of clean steel as the conditions for absorbing the injured inclusions into the slag are not established. Failure to completely remove the flocculated inclusions results in the Al ₂ O ₃ -MgO high melting point crystallization in the inclusions, which makes it difficult to separate flotations, so the Al content in the molten steel must be kept low.

Therefore, the choice of ladle material is also very important. That is, when using alumina-based ladles, the presence of Al ₂ O ₃ -MgO-based spinel inclusions deteriorating the surface quality is inevitable.

If the material of the cast ladle refractory material is Dolomic-based (CaO-MgO-based), it will cause operation obstacles due to the attachment now, and there is a management difficulty such as hygroscopicity of the refractory, so it is not economical method.

Since the method of leaving the residual slag requires the steel agitation when the ferroalloy and the coolant are added, the refractory damage and the workability are not good, and there is no method of accurately predicting the agitation strength, which causes the cleanliness deviation. Therefore, in order to use the method of leaving residual slag, slag fluidity must be secured and low odor stirring conditions must be optimized. If these conditions are not satisfied, the slag may be mixed.                     

Therefore, the present invention proposes a suitable low stirring conditions that can maximize the separation of inclusions for the purpose of high cleanliness of molten steel while leaving the residual slag and using alumina ladle for the economic production of ferritic stainless steel for deep processing Realize out-of-furnace refining methods.

That is, the present invention optimizes the ladle molten steel stirring conditions in the ladle treatment process in order to omit the VOD treatment process, which is a factor of increasing the manufacturing cost, which is the biggest disadvantage of the VOD process in the VOD process, which is a favorable manufacturing process for high clean steel, Cr: 16.0 A low cost manufacturing process for deep-processed ferritic stainless steel (also referred to as STS430 steel) with a component condition of ˜18.0 wt%, C: 0.12 wt% or less and S: 0.01 wt% or less is realized.

To this end, first, the molten metal containing 1.8-2.0% by weight of carbon, 0.02-0.05% by weight of sulfur, and 16-18% by weight of chromium dissolved in an electric furnace was deoxidized in an AOD refining furnace. Phosphorus argon and nitrogen are blown to remove carbon and nitrogen at the same time.

Next, slag basicity (CaO / SiO ₂ ) was prepared at 1.6-2.2 to remove sulfur, and the slag prepared at the Al ₂ O ₃ concentration of the slag was 2.0% or less and the TiO ₂ concentration was 1.0% or less. Leave to

At this time, the reason for limiting the slag basicity to 1.6-2.2 is as follows. That is, when the slag basicity is lower than 1.6, as shown in FIG. 1, there is a limit in removing sulfur, which is an impurity element in the steel. If the slag basicity is lower than 1.6, raw materials having a low sulfur content in the molten metal of the electric furnace are used. There is a problem that additional costs are generated because it has to be used, or preliminary dehydration in the molten metal. Normally, sulfur content should be managed at 80ppm or less in consideration of pick up during tapping, and the product requirement level is 100ppm or less.

If the slag basicity is higher than 2.2, the slag melting point is increased and the fluidity is lowered, and when the low stirring using the ladle residual slag is performed, the ferroalloy for the fine adjustment of the temperature and composition in the ladle becomes impossible. In addition, as can be seen from the embodiment, the incidence of spinel, which is the cause of surface quality inferiority, is increased.

Therefore, the basicity of the slag is preferably 1.6-2.2.

In addition, the reason for limiting the material of the ladle to the alumina material is as follows. Among the applicable ladles, Mag-Cro-based (MgO-Cr ₂ O ₃ ) ladles are used for VOD treatment due to their high fire resistance and have a disadvantage in that they are not economical due to their high cost, and dolomide materials (CaO-MgO-based) They have a disadvantage of low productivity and uneconomic because they are so attached that they cause trouble in operation and have to perform additional work to remove them.

On the other hand, alumina (Al ₂ O ₃ ) ladles were adopted by the company because of their low cost and good operability, but surface defects caused by alumina remained a chronic problem. Therefore, it is very important to solve the problems caused by alumina (mainly spinel generation) while using an alumina ladle, and in the present invention, it was solved by optimizing the ladle stirring conditions.

In case of increasing the ladle agitation flow rate under the ladle stirring condition, the amount of Al ₂ O _{3 in} the slag before the low agitation (shown as B / B in FIG. ₂ ) is 1.5 weight as shown in FIG. 2. What was% is raised to 8.0% by weight, which indicates that the ladle refractory loss is seriously raised.

This increases the Al ₂ O ₃ concentration in the inclusions as well as the refractory loss, and promotes the Al ₂ O ₃ -MgO high melting point inclusion called spinel to deteriorate the surface quality.

This is also the reason for limiting the Al ₂ O ₃ concentration in the refining slag before tapping to 2.0% by weight or less. In addition, the reason for limiting the content of TiO _{2 in} the slag before refining to 1.0% by weight or less is that the slag droplets not suspended and removed during tapping remain as CaO-TiO ₂ high melting point inclusions in the cast steel. This is because it causes inferiority.

Next, 1.0 to 3.0 tons per 90 tons of ladle is not completely removed without removing the upper slag in the ladle, and the slag low stirrer fluorite (CaF ₂ ) is added 20-40 kg per ton of slag to secure the fluidity of the slag. Increasing the inclusion absorption capacity by the method, then low-odor stirring is carried out for at least 15 minutes at a stirring flow rate of 3.0-6.0 L per tonne of molten steel.

The reason for limiting ladle residual slag to 1.0-3.0 ton per 90 ton ladle is as follows. As shown in FIG. 3, when the slag is completely excluded and there is no residual slag amount, fine stirring must be performed to prevent the formation of molten steel oxide such as SiO ₂ -based inclusions.

On the other hand, when the amount of residual slag is more than 3.0 tons, it is difficult to check the stirring strength and to input ferroalloy and coolant. Therefore, the amount of upper slag remaining inside the ladle is preferably 1.0-3.0 ton per 90 ton ladle.

In addition, the reason why the fluorspar is added is that the temperature is high at the time of tapping, so the fluidity and the inclusion absorbing ability are good, but the fluidity of the slag worsens with the temperature drop, and the inclusion absorbing ability is worsened. Therefore, the injured inclusions can be mixed back into the molten steel, so as to prevent this.

If the fluorspar is less than 20 kg per ton slag, the above-mentioned function cannot be performed. On the other hand, if more than 40 kg per ton of slag is added, ladle refractory loss may occur, resulting in excessive Al ₂ O ₃ in the alumina ladle. It dissolves and promotes spinel crystallization which affects surface quality, resulting in poor surface quality.

Therefore, the fluorspar input amount is preferably 20-40 kg per ton of slag.

And, when low stirring in the ladle, the reason for limiting the stirring flow rate and time to more than 3.0-6.0L and 15 minutes per ton of molten steel, respectively, to maximize the inclusion flotation separation effect while preventing the ladle residual slag from remixing To do this.

As shown in the examples, when the agitation flow rate is less than 3.0L per ton of molten steel, the inclusion flotation effect is small, whereas when the agitation flow rate is more than 6.0L per ton of molten steel, re-incorporation of slag occurs. Therefore, the stirring flow rate is preferably 3.0-6.0 liters per tonne of molten steel.

In addition, minimum time is required to ensure inclusion time, but if less than 15 minutes, the number of inclusions in the cast cannot be lowered to the required level. This is expected to add a temperature compared to the standard that maintained the conventional 8 minutes or more, but may be possible because the temperature drop effect during low stirring using ladle residual slag is smaller than the conventional as shown in FIG.

Figure 3 shows the degree of temperature drop during 15 minutes low stirring operation according to the amount of slag residual slag. As shown in Figure 3 shows that the temperature drop is generally reduced to less than 15 ℃ when leaving the residual slag 2.0 tons at 30 ℃ in the case of complete exclusion.

If the low stirring operation is longer than 15 minutes, the tapping temperature from the smelting furnace is increased and the service life of the ladle low odor porous plug is not economical.

Hereinafter, the present invention will be described in more detail with reference to Examples.

As shown in Table 1, ferrite stainless steel for deep working was manufactured under the conditions of Samples 1 to 28, and the spinel generation rate, the number of cast inclusions, and the processing results are shown in Table 1 together.

In this case, Samples 14 to 18 correspond to Examples 1 to 5 according to the present invention, respectively.                     

Sample Basicity (CaO / SiO ₂ ) Residual Slag (Ton) CaF ₂ (kg / t-slag) Stir Flow Rate (ℓ / t-steel) Stirring time (minutes) Spinel incidence (%) Cast inclusions (pcs / 50㎠) Processing result One 2.5 0.5 0 3 12 100 90 Surface defect 2 2.3 0.5 0 4 13 100 90 Surface defect 3 2.7 2.8 20 3 12 100 45 Surface defect 4 2.2 2.5 25 3 20 100 60 Surface defect 5 1.6 0.5 0 2 5 0 200 Processing crack 6 1.7 0.5 0 2 8 0 90 Inadequate 7 1.9 0.5 0 One 9 5 100 Inadequate 8 1.6 0.5 20 2 8 80 120 Surface defect 9 1.8 0.5 40 One 8 100 150 Surface, machining failure 10 1.7 0.8 20 One 12 5 60 Inadequate 11 1.8 1.2 40 2 15 0 50 Inadequate 12 1.6 1.6 60 2 8 100 15 Surface defect 13 1.7 2.0 80 2 18 100 20 Surface defect 14 (Example 1) 1.9 2.2 20 4 15 0 5 Good 15 (Example 2) 1.6 2.5 30 4 15 5 15 Good 16 (Example 3) 1.8 2.5 30 3 17 20 20 Good 17 (Example 4) 1.6 2.4 20 4 20 5 10 Good 18 (Example 5) 1.8 1.7 40 3 25 20 15 Good 19 1.6 1.6 60 4 20 50 5 Good 20 1.7 2.3 80 4 8 100 20 Surface defect 21 1.9 4.0 0 6 8 5 120 Inadequate 22 1.6 3.5 0 6 18 0 65 Inadequate 23 1.8 2.0 0 7 15 0 35 Inadequate 24 1.6 2.0 20 7 15 0 50 Inadequate 25 1.7 2.5 40 8 17 0 65 Inadequate 26 1.9 2.4 60 8 20 60 40 Inadequate 27 1.6 2.8 80 8 20 100 40 Surface defect 28 1.8 2.5 80 8 15 100 50 Surface defect

Here, the spinel incidence rate represents the percentage of the inclusions in which the spinel is determined among the slab inclusions as a percentage.

As shown in Table 2, the deep-processed ferritic stainless steel according to Examples 1 to 5 of the present invention has a low spinel incidence rate of 20% or less and less than 20 cast iron inclusions, which is a cause of poor surface quality. Of course, it can be used as a general purpose material for deep processing, and it can secure high quality while omitting the VOD process.

As described above, the present invention has the effect of eliminating the VOD treatment process by optimizing the molten steel stirring conditions in the ladle and using alumina ladle to provide a low-cost and high-clean ferrite stainless outside furnace refining method.

Claims (6)

An argon oxygen decarburization (AOD) refiner was used to ferrite stainless molten steel having a basicity (CaO / SiO ₂ ) of 1.6-2.2 and containing 2.0 wt% or less of Al ₂ O ₃ and 1.0 wt% or less of TiO ₂ . Stepping on the ladle made of alumina together with the slag; Injecting fluorite (CaF ₂ ) with the upper slag remaining in the ladle; And Performing bottom bubbling in the ladle; Furnace refining method of ferritic stainless steel comprising a. delete The method of claim 1, The out-of-furnace refining method of ferritic stainless steel, wherein the upper slag is left 1.0-3.0 tons per 90 tons ladle in the ladle. The method of claim 1, The fluorspar is an out-refining method of ferritic stainless steel is added 20-40 kg per ton of slag. The method of claim 4, wherein The low stirring step of the furnace refining method of ferritic stainless steel low stirring 15 minutes or more at a stirring flow rate of 3.0-6.0 L per tonne of molten steel. The method according to any one of claims 1, 3, 4, and 5, The ferritic stainless steel includes 16.0 to 18.0 wt% of chromium (Cr), 0.12 wt% or less of carbon (C), and ferrite stainless steel, which is a ferrite stainless steel for deep processing including sulfur (S) to 0.01 wt% or less. Steel off-road refining method.

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