JP2009521599A - Method for producing ferritic stainless steel with fine solidification structure and ferritic stainless steel produced thereby - Google Patents

Abstract

  Ferrite with a fine solidification structure that controls the concentration of alumina inclusions in molten steel to maximize the effect of forming effective TiN that acts as a heterogeneous nucleation site of ferrite during solidification to improve the equiaxed crystal ratio , A ferritic stainless steel produced thereby, a stage in which a decarburization reaction is performed, a stage in which Al is added, a composite deoxidation stage, an alloying stage, and a primary discrimination stage And a secondary discrimination step and a continuous casting step.

Description

  The present invention relates to a method for producing a ferritic stainless steel having a fine solidified structure and a ferritic stainless steel produced by the method, and more specifically, the concentration of alumina inclusions in molten steel is controlled to make ferrite non-uniform during solidification. The present invention relates to a method for producing a ferritic stainless steel having a fine solidification structure that maximizes the production effect of effective TiN acting as a nucleation site and improves the equiaxed crystal ratio, and a ferritic stainless steel produced thereby.

  In general, 0.2 to 0.5% Ti may be added to ferritic stainless steel having a Cr concentration of 10 to 30% for the purpose of improving corrosion resistance. In this case, TiN is formed according to the following formula 1 from steelmaking to the continuous casting process, and when the size and distribution are appropriate, it acts as a non-uniform nucleation site for ferrite during solidification of the molten steel, resulting in fine solidification. You can get an organization.

[Formula 1]
Ti + N = TiN
As described above, the following techniques have been reported in order to refine the solidified structure by controlling the TiN generation process from steelmaking to the continuous casting process.

  In US Pat. No. 5,868,875, Cr: 8-25%, Mn: 0.1-1.5%, Si: 1.5% or less, N: 0.05% or less, C: 0.08% or less In the case of Ti deoxidizing molten steel of Al: 0.01% or less, the concentration of Ti is (% Ti / 48) / [(% C / 12) + (% N / 14)]> 1.5. Proposed to meet.

  In addition, in the specification of European Patent No. 924313, when [% Ti] × [% N] ≧ 0.14 × [% Al] is satisfied among Ti, Al and N concentrations, 50% or more slab equiaxed shaft It was proposed that the crystallinity could be secured.

  However, as in the above-mentioned patent, when the molten stainless steel is performed by Ti deoxidation, if the concentration of oxygen in the molten steel is high, most of the charged Ti may be oxidized to the form of Ti oxide. Yes, this can act as a factor that inhibits the production of TiN according to Equation 1 above.

  Moreover, since the production | generation of excess Ti oxide can cause the clogging of a submerged nozzle and the steel-making defect in the surface of a slab in a continuous casting process, there is a limit in the application only by simple Ti deoxidation.

  In European Patent No. 1491646, Cr: 10-20%, C: 0.001-0.01%, Si: 0.01-0.3%, Mn: 0.01-0.3%, N : It was proposed that slabs with a fine solidification structure can be produced by adding 2 to 50 ppm Mg to molten steel of 0.001 to 0.02% and Ti: 0.05 to 0.3%. .

That is, by adding 50 ppm or less of Mg to ferritic stainless steel, 17.4 (Al ₂ O ₃ ) +3.9 (MgO) +0.3 (MgAl ₂ O ₄ ) +18.7 (CaO) ≦ 500 And (Al ₂ O ₃ ) + (MgO) + (MgAl ₂ O ₄ ) + (CaO) ≧ 95 (each component is mol% standard) It is proposed that the inclusions act as heterogeneous nucleation sites of ferrite during solidification of molten steel.

However, in such a case, the simultaneous control of the concentration of each oxide is impossible, that is, the concentration of a specific component such as Al ₂ O ₃ is very large, or the concentration of MgAl ₂ O ₄ is extremely high. Although the above equation can be satisfied even when the temperature is low, a fine solidified structure cannot be easily obtained. This is because when the concentration of Al ₂ O ₃ in the inclusion is excessively high, the lattice mismatch between Al ₂ O ₃ / TiN or Al ₂ O ₃ / ferrite is also large, that is, the interfacial energy is large. This is because it hardly acts as a non-uniform nucleation site of ferrite during solidification.

In Japanese Patent Publication No. 2002-030324, a CaO—SiO ₂ system in equilibrium with molten steel of Cr: 10-30%, Si: 0.2-3.0%, Ti: 0.05-0.3% is used. The basicity of the slag is controlled to 1.2 to 2.4, and [% Al] / [% Ti] = 0.01 to 0.1 in the molten steel, that is, [% Ti] / [% Al] = 10. It was proposed that a slab having an equiaxed crystal ratio of 70% or more can be obtained by controlling in the range of ˜100.

  However, when operating at a composition of high Ti and low Al in the above concentration range, a large amount of Ti oxide may be generated, which causes nozzle clogging during casting and surface defects of slabs. obtain.

In Japanese Patent No. 2000-160229, CaO and Al are introduced so that the basicity of the CaO-Al ₂ O ₃ slag is in the range of 0.7 to 2.5 during the scouring of vacuum decarburization. Then, stirring the molten steel using Ar for 5 minutes or more, and adding Ti so that the TiN single nitride has an area ratio of 0.01% or more, an equiaxed crystal ratio of slab of 60% or more is obtained. Proposed to be able to get.

  However, in practice, it is not easy to appropriately form TiN independent of the oxidation-inclusions in the molten stainless steel in which inclusions in the oxide form are distributed. This is because in the formation of TiN, the oxidation-inclusions act as non-uniform nucleation sites, and TiN is crystallized with the oxidation-inclusions as nuclei when actually observing the inside of a slab with a fine solidified structure. This is consistent with the fact that the form of oxide-TiN composite inclusions is distributed in large quantities.

In Japanese Patent No. 2004-043838, in molten steel refining of Cr: 9 to 30%, it is made to be in the range of [% Ti] * [% N] = 0.0007 to 0.004 in the molten steel, and oxygen The oxygen activity in the molten steel is measured using a sensor, and after adding an appropriate deoxidizer so as to be in the range of log ₀ = -5 to -3, the equiaxed crystal ratio is improved by casting. Proposed to be able to.

  However, it is known that it takes a relatively long time to measure the activity of oxygen in molten steel using an oxygen sensor every time stainless steel is refined, and its accuracy is somewhat reduced. Therefore, it is judged that there are some vague points in applying to actual operations.

  Therefore, the present invention has been made in view of the above circumstances, and its purpose is to control the concentration of alumina inclusions in molten steel by the combined deoxidation of Si / Mn / Al / Ti in the vacuum decarburization refining process. By maximizing the effect of producing effective TiN that acts as a non-uniform nucleation site for ferrite during solidification, a slab of ferritic stainless steel with a high equiaxed crystal ratio is produced, resulting in excellent formability. That is, an object of the present invention is to provide a method for producing a ferritic stainless steel having a fine solidification structure with few ridge defects and a ferritic stainless steel produced thereby.

In order to achieve the above object, the method for producing a ferritic stainless steel having a fine solidification structure according to the present invention includes a step in which a decarburization reaction is performed by blowing oxygen from above the molten steel in a vacuum decarburization ladle, turning the steps of decarburization turns on the Al for the reduction of Cr ₂ O ₃ in the molten steel was performed, a deoxidizing agent to the molten steel Al is turned on for the reduction of the Cr ₂ O ₃ A composite deoxidation stage, an alloying stage in which an alloying metal is introduced into the molten steel, a primary discrimination stage in which the Al concentration in the molten steel is analyzed to determine whether the Al concentration is within a set value range, and the Al When the concentration satisfies the set value, a second determination step of determining whether the concentration of alumina inclusions in the final molten steel by stirring with an inert gas corresponds to the target value, and the concentration of the alumina inclusions When the target value is met, the molten steel is continuously cast. Comprising the that stage.

  Here, the deoxidizer may be Si and Mn in the composite deoxidation step, and the alloying metal may be 0.2 wt% to 0.4 wt% Ti in the alloying step.

In addition, the concentration of alumina inclusions in the molten steel in the continuous casting stage satisfies the following conditions,
[Al] ^alumina <70 ppm
^{(Here, [Al] alumina = [Al} ] total - [Al] dissolved)
It is preferable that the molten steel components satisfy the following conditions in the continuous casting stage.

[Si] + [Mn] = 0.5% to 1.0%
Where% is mass%,
Furthermore, it is preferable that the final composition of the smelting slag in the vacuum decarburization smelting ladle satisfies the following conditions.

1.1 ≦ (% CaO) / (% Al ₂ O ₃ ) ≦ 1.4
4 ≦ (% TiO ₂ ) / (% SiO ₂ ) ≦ 6
Where% is mass%,
Moreover, the molten steel is 80 to 85 tons.

  Preferably, the set value of the Al concentration in the first discrimination step is 0.05 mass% to 0.12 mass%, and when the Al concentration is less than 0.05 mass%, the molten steel is 80 tons. The step of additionally charging 30 kg to 40 kg of Al to ˜85 tons is further included. When the Al concentration is 0.12% by mass or more, 250 kg to 300 kg of 80 to 85 tons of molten steel is used. The method further includes a step of adding quick lime.

  And the concentration target value of the alumina inclusion in the second discrimination step is 70 ppm or less.

  The ferritic stainless steel having a fine solidified structure according to the present invention is manufactured by the method for producing a ferritic stainless steel having a fine solidified structure according to the present invention, and the concentration of alumina inclusions is 70 ppm or less. The crystallinity is 40% or more.

  The method for producing a ferritic stainless steel having a fine solidification structure according to the present invention and the ferritic stainless steel produced thereby depended on technologies such as control of casting temperature and control of electromagnetic stirring force that have been implemented in the past. Compared with the slab manufacturing method with a fine solidification structure, effective TiN is effectively generated by precise control of the concentration of molten steel components such as Si and Mn and alumina inclusions in the vacuum decarburizing and refining process, thereby improving operational safety. Ferritic stainless steel with high solidification structure and high equiaxed crystal ratio and high uniaxial crystal ratio. As a result, ferritic stainless steel with excellent formability, that is, fine solidification structure with few ridge defects Steel can be obtained.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a flowchart of a method for producing a ferritic stainless steel having a fine solidified structure according to a preferred embodiment of the present invention.

The method for producing a ferritic stainless steel having a fine solidification structure according to a preferred embodiment of the present invention includes a step (S10) in which a decarburization reaction is performed by blowing oxygen from an upper part of molten steel in a vacuum decarburization ladle. A step of introducing Al to reduce the Cr ₂ O _{3 in} the molten steel subjected to the decarburization reaction (S20), and a deoxidizer to the molten steel in which Al has been added to reduce the Cr ₂ O ₃ A composite deoxidation step (S30) in which the alloy is introduced, an alloying step (S40) in which the alloyed metal is introduced into the molten steel, and an Al concentration in the molten steel are analyzed to determine whether the Al concentration is within a set value range In the first discrimination step (S50), when the Al concentration satisfies the set value, stirring is performed using an inert gas (S60) to determine whether the concentration of alumina inclusions in the final molten steel corresponds to the target value. Performing a second discrimination step (S70), When the concentration of alumina inclusions satisfies the target value, comprising the steps (S80) to continuously cast the molten steel.

  Here, the set value of the Al concentration in the first discrimination step (S50) is 0.05 to 0.12% by mass (S51), and Al is less than 0.05% by mass (S52). It is preferable to further include a step (S56) of adding quick lime when the amount is 0.12% by mass or more (S56).

  The deoxidizer in the composite deoxidation step (S30) is Si and Mn, and the alloying metal in the alloying step (S40) is Ti.

  FIG. 2 is a graph showing changes in the slab equiaxed crystal ratio by controlling the concentration of alumina inclusions, and FIG. 3 is a graph showing the results of FIG. 2 as distributed data.

  Referring to FIGS. 2 and 3, it can be seen that the equiaxed crystal ratio of the slab increases as the concentration of alumina inclusions in the molten steel decreases. Here, the density | concentration of a suitable alumina inclusion can be set to 70 ppm or less, and when such conditions are satisfy | filled, the slab of 40-100% of equiaxed crystal ratio can be obtained. At this time, when the concentration of the alumina inclusions exceeds 70 ppm, the production of effective TiN is suppressed, so that it is impossible to secure the target equiaxed crystal ratio.

In order to control the molten steel components as described above, Al is introduced after the completion of the decarburization reaction by blowing oxygen from the upper part of 80 to 85 tons of molten steel in a vacuum oxygen decarburization (VOD) ladle and oxidized. Cr ₂ O ₃ in the slag generated from the base is reduced.

In the reduction step of Cr ₂ O ₃ by the addition of Al, Si and Mn are added to perform composite deoxidation, and then Ti is added to control the target composition.

  Based on the input of Ti, the Al concentration in the molten steel is primarily analyzed after about 5 minutes, and when the primary Al concentration is excessively low or high, additional Al or quick lime is added respectively. Then, continuous casting is performed by controlling the concentration of alumina inclusions in the final molten steel so as to meet the target range by stirring with an inert gas at the bottom of the ladle.

In the molten steel after oxygen blowing for decarburization in the vacuum decarburization step, oxygen having a concentration corresponding to the equilibrium of [Cr] / (Cr ₂ O ₃ ) as shown in the following formula 2 exists.

[Formula 2]
2 [Cr] +3 [O] = (Cr ₂ O ₃ )
According to literature already known, the oxygen concentration in about 20% of [Cr] molten steel equilibrated with Cr ₂ O ₃ at 1650 ° C. is known to be at a level of 0.06 to 0.07%. . Therefore, for deoxidation of molten steel and reduction of Cr ₂ O _{3 in} the slag, Al, which is an efficient deoxidation element, is added to induce reactions as shown in the following formulas 3 and 4.

[Formula 3]
2 [Al] +3 [O] = (Al ₂ O ₃ )
[Formula 4]
(Cr ₂ O ₃ ) +2 [Al] = (Al ₂ O ₃ ) +2 [Cr]
However, when performing deoxidation of Al as in the above formulas 3 and 4, a large amount of Al ₂ O ₃ inclusions are present in the molten steel, and the alumina inclusions formed at this time are aggregated and Although it grows and floats or is removed, in the case of a fine size of several μm level, it stays in the molten steel until casting.

Table 1 below shows the equiaxed crystal ratio of the slab and the presence or absence of defects in comparison with [% Al] ^alumina and [% Si] + [% Mn] in the conventional example compared to the case of the present invention.

  As shown in Table 1, in the inventive examples, a high slab equiaxed crystal ratio of 40% or more can be obtained as compared with the conventional examples.

On the other hand, TiN is formed in the molten steel by the reaction of the above formula 1, but the formation time differs depending on the composition and temperature of the molten steel. If TiN is formed in a ladle or tundish before solidification of molten steel, TiN may be produced by homogeneous nucleation and growth due to reaction of Ti atoms and N atoms in the molten steel. In particular, it is advantageous to nucleate at the third interface, for example at the oxide-inclusion / molten steel interface. At this time, as the oxidation-inclusions that provide non-uniform nucleation sites of TiN, Al ₂ O ₃ , MgO, TiO _x , MgO—Al ₂ O ₃ , CaO—TiO _x , MgO—Al ₂ O may be used. _3- TiO _{x and the} like, and as an indirect index indicating the ease of heterogeneous nucleation of TiN on the surface of these inclusions, the degree of lattice mismatch defined by the following formula 5 and δ can be mentioned.

Here, l _oxide and l _TiN mean the lattice constants of oxidation-inclusions and TiN crystals, respectively, and the larger the δ between the two substances, the less likely it is to act as a non-uniform nucleation site.

For example, a hexagonal system _Al 2 _{O 3} structure, face-centered cubic (fcc) while in between the TiN structure is a level of δ _Al2O3-TiN ≒ 0.1, MgO same face-centered cubic structure or in between the MgAl ₂ O ₄ spinel and TiN is level _{δ MgO (or spinel) -TiN ≒} 0.0002. Therefore, it is expected that MgO-based inclusions act more easily as non-uniform nucleation sites of TiN than Al ₂ O ₃ -based inclusions.

  Accordingly, when a large amount of alumina inclusions are present in the molten steel, TiN nucleation is reduced, and as a result, it can be a cause of lowering of heterogeneous nucleation of ferrite during solidification.

Therefore, in order to produce a slab having a finely solidified structure, it is essential to reduce alumina inclusions in the molten steel. At the same time, in order to reduce TiO _x -based inclusions, composite desorption is performed by introducing Si / Mn. Tried to get acid effect.

FIG. 4 is a graph showing the reduction of inclusions of Al ₂ O ₃ and TiO _x by the method for producing a ferritic stainless steel having a fine solidified structure according to a preferred embodiment of the present invention, as compared with the conventional example.

Referring to FIG. 4, when the existing molten steel has a level of [Si] + [Mn] = 0.1 to 0.4% and is constant at [Al] = 0.03%, Ti ₃ O _{5 The} criticality at which no single inclusion is formed is [Ti] ≈0.45%, whereas [Si] + [Mn] = 0.6 to 0.9%, and [Al] = 0.03% It can be seen that Ti ₃ O ₅ production can be suppressed to a concentration of about 0.5% [Ti]. Further, when the concentration of [Si] + [Mn] is low and [Ti] = 0.4% is constant in the molten steel, a critical [Al] ≈0.027% in which no Ti ₃ O ₅ inclusion is formed. When the concentration of [Si] + [Mn] is high and [Ti] = 0.4%, Ti ₃ O ₅ inclusions are formed up to a level of [Al] = 0.024%. I understand that it is not done.

  As described above, [Si] + [Mn] in molten steel is introduced so as to meet the target concentration and% range, and in order to control the concentration of alumina inclusions to 70 ppm or less, about 5 When the minutes have elapsed, the [Al] concentration in the molten steel is primarily analyzed.

When the concentration of Al to be analyzed is less than 0.05%, sufficient reduction of Cr ₂ O ₃ is not performed, and when it is 0.12% or more, the concentration of the final alumina inclusion exceeds 70 ppm. Therefore, after the analyzed Al concentration is less than 0.05% and 0.12% or more, 30 to 40 kg of Al is added or 250 to 300 kg of quick lime is additionally added. By stirring with an inert gas at the bottom of the ladle, the concentration of alumina inclusions in the final molten steel is controlled to 70 ppm or less, and then continuous casting can promote the generation of effective TiN in the molten steel. .

Here, when less than 30 kg of Al is added when the Al concentration is less than 0.05%, the addition effect is low with respect to 80 to 85 tons of molten steel, so efficient reduction of Cr ₂ O ₃ . If Al exceeding 40 kg is added, the Al concentration may exceed 0.12%. Therefore, the amount of Al added is preferably 30 to 40 kg.

  Moreover, if less than 250 kg of quick lime is added when the Al concentration is 0.12% or more, surface defects in the final product may be caused. If more than 300 kg of quick lime is added, the final alumina inclusions Since the concentration can exceed 70 ppm, the amount of quicklime added is preferably 250 to 300 kg.

The final composition of the slag slag in the vacuum decarburization smelting ladle is 1.1 ≦ (% CaO) / (% Al ₂ O ₃ ) ≦ 1.4, and 4 ≦ (% TiO ₂ ) / (% SiO ₂ ) ≦ 6.

When (% CaO) / (% Al ₂ O ₃ ) is less than 1.1, the concentration of alumina inclusions may exceed 70 ppm, and (% CaO) / (% Al ₂ O ₃ ) is 1. If it exceeds 4, surface defects or the like in the final product may be caused. Therefore, it is preferable that 1.1 ≦ (% CaO) / (% Al ₂ O ₃ ) ≦ 1.4.

Moreover, when (% TiO ₂ ) / (% SiO ₂ ) is less than 4, the effect of suppressing the generation of excessive Ti oxide by Si is reduced, and (% TiO ₂ ) / (% SiO ₂ ) is reduced. If it exceeds 6, there is a risk of reducing the formation of TiN which acts as an uneven nucleation site of ferrite when solidifying as an excessive Ti oxide, so 4 ≦ (% TiO ₂ ) / (% SiO ₂ ) It is preferable that ≦ 6.

  As such a method, a ridge generated at the time of forming a final cold-rolled product as well as manufacturing a slab having a fine solidification structure by securing an equiaxed crystal ratio of 40% or more with a slab having a thickness of 200 to 220 mm. Defects can be reduced.

  Examples of the present invention will be described below.

Iron scrap and alloyed iron were melted in an electric furnace so as to have a composition of Fe-17% Cr, and the steel was put into a ladle at a temperature of about 1780 ° C. through a rough decarburization process in an AOD refining furnace. The ladle has molten steel and slag, and the slag was removed by a mechanical method in order to increase the efficiency of vacuum decarburization. At this time, the temperature of the molten steel was measured to be about 1600 ° C. The ladle is moved to a vacuum decarburizing and scouring stand (Stand) and covered with a vacuum cover. After the decarburization reaction is completed by supplying gaseous oxygen using a lance at the upper part of the molten steel, the temperature of the molten steel reaches about 1670 ° C. The composition of the molten steel was analyzed as shown in Table 2 below.

After completion of the oxygen blowing, about 320 kg of Al was added for reduction of Cr ₂ O ₃ in the slag and deoxidation of the molten steel under a vacuum atmosphere. At this time, Si and Mn were added together with Al to suppress the generation of Ti oxide, and Ti was added in a sponge shape so that the level of [Ti] = 0.3% in the molten steel.

  [Al] = 0.04% in the molten steel at 5 minutes after the introduction of the alloyed iron. As described above, after adding 30 kg of Al, Ar was supplied at the bottom of the ladle. The molten steel was stirred for about 20 minutes. After completion of the refining process under vacuum, the temperature of the molten steel was about 1600 ° C., and a coolant or the like was added in the atmospheric state in order to control the casting temperature of about 1550 ° C.

After the ladle treatment process, the ladle was transferred to the continuous casting process, and the molten steel components in the final tundish were analyzed as shown in Table 3 below.

It can be seen that the concentration of [Al ^alumina ] existing as alumina in the molten steel components in Table 3 is 40 ppm, which satisfies the range proposed in the present invention.

  At this time, the N concentration in the molten steel is about 110 ppm, and reacts with 0.30% [Ti] to contribute to the generation of effective TiN and obtain a slab with a fine solidified structure as shown in FIG. Can do.

  Although the technical idea of the present invention has been specifically described by the preferred embodiment, it should be well understood that the embodiment is for the purpose of explanation and not for the limitation. Moreover, those skilled in the art in the technical field of the present invention can understand that various embodiments are possible within the scope of the technical idea of the present invention.

  As described above, the method for producing a ferritic stainless steel having a fine solidification structure according to the present invention and the control of the casting temperature and the electromagnetic stirring force that have been carried out by the ferritic stainless steel produced thereby, etc. Compared with the slab manufacturing method, which has a solidified structure that depends on the technology, effective TiN is effectively generated by precise control of the concentration of molten steel components such as Si and Mn and alumina inclusions in the vacuum decarburization refining process. The slab of ferritic stainless steel with high operational safety, fine solidification structure and high equiaxed crystal ratio is manufactured. As a result, solidification structure with excellent formability, that is, with few ridge defects. Fine ferritic stainless steel can be obtained.

FIG. 1 is a flowchart of a method for producing a ferritic stainless steel having a fine solidified structure according to a preferred embodiment of the present invention. FIG. 2 is a graph showing changes in the equiaxed crystal ratio of the slab by controlling the concentration of alumina inclusions. FIG. 3 is a graph showing the result of FIG. 2 as distributed data. FIG. 4 is a graph showing the reduction of Al ₂ O ₃ and TiO _x inclusions by the method for producing a ferritic stainless steel having a fine solidification structure according to a preferred embodiment of the present invention, as compared with the conventional example. FIG. 5 shows a form of oxide-TiN composite inclusions in which a solidified structure is distributed inside a fine slab by a method for producing a ferritic stainless steel having a fine solidified structure according to a preferred embodiment of the present invention. It is an electron micrograph which shows this compared with a prior art example.

Claims (15)

A stage where a decarburization reaction is performed by blowing oxygen from the top of the molten steel in the vacuum decarburization ladle;
Injecting Al for reduction of Cr ₂ O _{3 to} the molten steel subjected to the decarburization reaction;
A combined deoxidation step of introducing a deoxidizer into the molten steel into which Al has been introduced for the reduction of the Cr ₂ O ₃ ;
An alloying step of introducing an alloying metal into the molten steel;
A primary discrimination step of analyzing the Al concentration in the molten steel and discriminating whether the Al concentration is within a set value range;
When the Al concentration satisfies a set value, a second determination step of determining whether the concentration of alumina inclusions in the final molten steel corresponds to the target value by stirring with an inert gas;
And a step of continuously casting the molten steel when the concentration of the alumina inclusions satisfies a target value.   The method for producing a ferritic stainless steel having a fine solidification structure according to claim 1, wherein the deoxidizer is Si and Mn in the composite deoxidation step.   The method for producing a ferritic stainless steel having a fine solidification structure according to claim 1 or 2, wherein the alloying metal is Ti in the alloying step.   The said Ti is 0.2 mass%-0.4 mass%, The manufacturing method of the ferritic stainless steel whose solidification structure is fine of Claim 3. The method for producing a ferritic stainless steel having a fine solidification structure according to any one of claims 1 to 4, wherein the concentration of alumina inclusions in the molten steel satisfies the following condition in the continuous casting stage.
[Al] ^alumina <70 ppm
(Where [Al] ^alumina = [Al] ^total- [Al] ^dissolved ) The method for producing a ferritic stainless steel having a fine solidification structure according to any one of claims 1 to 5, wherein the molten steel component satisfies the following condition in the continuous casting stage.
[Si] + [Mn] = 0.5% to 1.0%
(However,% is% by mass.) The method for producing a ferritic stainless steel having a fine solidification structure according to any one of claims 1 to 6, wherein a final composition of the smelting slag in the vacuum decarburization smelting ladle satisfies the following condition.
1.1 ≦ (% CaO) / (% Al ₂ O ₃ ) ≦ 1.4
4 ≦ (% TiO ₂ ) / (% SiO ₂ ) ≦ 6
(However,% is% by mass.)   The method for producing a ferritic stainless steel having a fine solidification structure according to any one of claims 1 to 7, wherein the molten steel is 80 to 85 tons.   9. The ferritic stainless steel having a fine solidification structure according to claim 1, wherein a set value of the Al concentration in the first discrimination step is 0.05 mass% to 0.12 mass%. Production method.   The method for producing a ferritic stainless steel having a fine solidification structure according to claim 9, further comprising the step of additionally adding Al when the Al concentration is less than 0.05 mass%.   The method for producing a ferritic stainless steel having a fine solidified structure according to claim 10, wherein 30 to 40 kg of Al is added to 80 to 85 tons of molten steel.   The method for producing a ferritic stainless steel having a fine solidification structure according to claim 9, further comprising the step of additionally charging quick lime when the Al concentration is 0.12% by mass or more.   The said quicklime is a manufacturing method of ferritic stainless steel with a fine solidification structure of Claim 12 with which 250 kg-300 kg is supplied with respect to said molten steel 80 tons-85 tons.   The method for producing a ferritic stainless steel having a fine solidification structure according to any one of claims 1 to 13, wherein a concentration target value of the alumina inclusion in the second discrimination step is 70 ppm or less.   A ferritic stainless steel produced by the method according to claim 1, wherein the concentration of alumina inclusions is 70 ppm or less, and the solidification structure of the cast slabs having an axial crystal ratio of 40% or more is fine.