KR101898165B1 - Method for manufacturing ferritic stainless steel having fine cast structure - Google Patents

Abstract

The present invention discloses a method for producing a ferritic stainless steel excellent in liquefaction property by making a cast structure of a ferritic stainless steel casting finer by securing a high equiaxed rate by adding an oxide alloy for nucleation. Manufacturing method cast structure of fine ferrite-based stainless steel in accordance with one embodiment of the present invention, the turn in a cast steel by supplying the molten steel into the mold in a dish, the method for manufacturing the ferritic stainless steel for continuous casting, nucleation containing CaTiO ₃ The ash is added to the molten steel in the tundish and continuously cast.

Description

TECHNICAL FIELD [0001] The present invention relates to a method for manufacturing a ferritic stainless steel,

The present invention relates to a method for producing ferritic stainless steels, and more particularly to a method for producing a ferritic stainless steel casting with fine casting structure by adding a nucleus forming alloy to molten steel in a tundish during continuous casting.

Generally, in the manufacture of ferritic stainless steels, the initially formed cast structure remains unchanged at room temperature because it does not undergo a transformation process during the cooling process, and even if subjected to the rolling and annealing process, The control of the initial casting structure is very important because it is insufficient. Particularly, in the case of a ferritic stainless steel material in which surface quality is important, a cast steel having an equiaxed structure ratio of not more than 40% is subjected to deep drawing of a cold rolled steel sheet During the forming process, the surface of the coil is likely to be parallel to the cold rolling direction, called ridging or roping, and to have thin irregularities. In addition, even if the equilibrium constant is 40% or more, when the size of the equiaxed crystal is large, the driving force required for recrystallization is low, so that a coarse band structure remains and it is difficult to produce a product having excellent ridging properties.

In order to prevent such ridging defects, cold rolled steel and hot rolled steel are conventionally used. However, this causes problems such as a rise in manufacturing cost and a decrease in productivity when manufacturing ferritic stainless steel.

In recent years, as a technique for improving the equiaxed crystal ratio of the ferritic stainless steel casting, a low temperature casting method, a method of adding iron or steel, a method of adding a rare earth element, and an electro-magnetic stirrer have been used.

For example, Patent Document 1 discloses that an Al-Ti-based composite inclusion having a size of 0.3 to 0.5 탆 acts as an equiaxed nucleation seed. In Patent Document 2, the basic composition is set to 0.5 to 3.0, And the degree of equiaxed crystal can be improved by setting the temperature at 20 to 70 ° C. In addition, in Patent Document 3, 30 pieces / m ^{3 of} MgO and MgO-Al ₂ O _{3 each} having a diameter of 0.3 to 5 탆 including 0.002 to 0.02 wt% of Al and less than 0.0005 wt% of Mg were produced So that equiaxed crystals can be secured.

However, these methods utilize the oxides formed in the steelmaking process, which makes it difficult to control the uniform oxides due to the steelmaking operations having a large fluctuation in operation. As a result, There is a possibility that a deviation occurs.

In addition, since Ti is added to ferritic stainless steel, it is known that when the TiN precipitated in molten steel is used as the ferrite solidification nuclei, the solidification structure is likely to be equilibrated and purified. In order to improve the equilibrium constant, sufficient Ti addition is required .

However, since excessive Ti addition causes problems such as nozzle clogging and surface flaws, it is not preferable to simply increase the equilibrium constant by Ti addition. In addition, when the Ti addition amount is less than 0.2 wt%, the TiN crystallization temperature is low and it is difficult to purify the equiaxed crystal.

Japanese Laid-Open Patent Publication No. 1999-323502 (published on November 26, 1999) Japanese Laid-Open Patent Publication No. 2001-049322 (published on February 20, 2001) Japanese Laid-Open Patent Publication No. 2002-030395 (published on Jan. 31, 2002)

An object of the present invention is to provide a method for producing a ferritic stainless steel having a high ridging property by making a cast structure of a ferritic stainless steel casting finer by securing a high equiaxed rate by adding an oxide alloy for nucleation.

A method for producing a ferritic stainless steel having fine casting structure according to an embodiment of the present invention is a method for producing a ferritic stainless steel in which a molten steel in a tundish is supplied to a mold to continuously cast a cast steel, 2) is added and continuously cast.

(1)% (CaO) + % (TiO 2) ≥ 40%

(2) 0.3 ≤% (CaO ) / [% (CaO) +% (TiO 2)] ≤ 0.8

Also, according to an embodiment of the present invention, the nucleation material may include CaTiO ₃ oxide and may be introduced into the molten steel in the tundish.

According to an embodiment of the present invention, the nucleation material may have a size of 0.5 to 15 占 퐉.

Also, according to one embodiment of the present invention, the nucleation material may be uniformly distributed in the base metal and added to the molten steel in the form of a wire.

According to an embodiment of the present invention, the base metal may have a melting point lower than the molten steel temperature in the tundish by at least 10 ° C.

According to an embodiment of the present invention, the molten steel in the tundish can be agitated.

The ferritic stainless steel manufacturing method according to the embodiment of the present invention may exhibit fine equiaxed crystals having an average diameter of 1.5 mm or less and an equiaxed retention ratio of 70% or more, thereby exhibiting superior ridging properties.

1 is a flowchart schematically showing a method of manufacturing a ferritic stainless steel according to the present invention.
2 is an optical microscope photograph showing the cross-sectional structure of the nucleus forming alloy according to the present invention.
Fig. 3 is a graph showing the average diameters of equiaxed crystals of the examples and comparative examples of the present invention.
FIG. 4 is a graph illustrating the size and number of oxides containing CaTiO ₃ phase in a cast steel according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

1 is a flowchart showing a method of manufacturing a ferritic stainless steel according to an embodiment of the present invention.

The ferritic stainless steel having a fine cast structure according to the present invention is characterized in that the ferritic molten steel which has undergone the refining step of deoxidizing molten steel by injecting a deoxidizing agent into the ferritic molten steel, the component adjusting step of the refined molten steel, And adding a nucleation alloy to the ferritic molten steel, and continuously casting the ferritic molten steel to which the nucleating alloy is added by moving the ferritic molten steel through a dipping nozzle in a tundish to a mold have.

Various types of oxides, nitrides and boron compounds have been prepared and tested in order to attain the object of the present invention to obtain a fine equiaxed crystal structure. As a result, it is concluded that the following composition range and size range can act as an equiaxed nucleation site and obtain a fine equiaxed crystal.

The present inventors have found in Korean Patent Laid-Open Publication No. 10-2015-0075320 that the oxide composition of molten steel can be controlled during the refining step of the ferritic stainless steel to function as an equiaxed nucleation site. When the oxide composition of the molten steel is controlled under the conditions disclosed in the above-mentioned patent publications, CaTiO ₃ and Al ₂ O ₃ -MgOTiO ₂ oxides are crystallized in the cooling process, and CaTiO ₃ oxide is more effective for the isotropic nucleation of these crystallization phases Respectively. Based on the results of the present invention, the present invention provides a method for producing CaTiO ₃ A method of inducing equiaxed crystal formation by directly dispersing the phases uniformly in the molten steel without forming them by the refining process has been developed.

Although many studies have been made on the direct injection of oxide powders among the equiaxed crystal formation methods using oxides, it is very difficult to see the effect actually. Typical reasons are that the specific gravity difference between the oxide and the molten steel is so large that it is difficult to uniformly disperse in the molten steel, and the dispersed oxides easily float and separate, making it difficult to act as a nucleation site. Accordingly, in the present invention, a method of finding effective oxides in ferritic molten steel and effectively dispersing them in molten steel has been extensively studied, and the following method has been developed.

Manufacturing method cast structure of fine ferrite-based stainless steel in accordance with one embodiment of the present invention, molten steel in, tundish above the nucleation material containing CaTiO ₃ according as the turn supply the molten steel in the dish to the mold continuously cast steel cast by Lt; / RTI > The nucleating agent acts as a primary ferrite nucleation site at the time of solidification of the cast steel to form a fine equiaxed crystal structure.

In order to uniformly inject the nucleation material fine particles into the tundish during the continuous casting process, when the fine particles are injected in the component adjustment step, most of the continuous casting process requires a waiting time, If the fine particles are injected in the casting process, the injecting apparatus is complicated and can not be uniformly dispersed as a whole. Accordingly, the tundish, which is a step of supplying molten steel from the ladle and supplying the molten steel having the inclusions reduced to the mold, is an optimal position for uniformly distributing the fine particles.

The nucleating agent comprises CaTiO ₃ and may be uniformly distributed in the form of powder in the matrix. In order to uniformly distribute the nucleating material in the molten steel, the role of the base metal, which is a base material, is important. In the present invention, a nucleating alloy having uniform distribution of the nucleating material in the base metal is used.

The nucleation material may include oxides in the molten steel, including CaTiO ₃ . CaTiO ₃ is effective in its pure form, but it can also be lowered in melting point to improve wettability with molten steel. The composition of each oxide contained in the nucleating material is prepared so as to satisfy the following formulas (1) and (2) Can be used.

(1)% (CaO) + % (TiO 2) ≥ 40%

(2) 0.3 ≤% (CaO ) / [% (TiO 2) +% (CaO)] ≤ 0.8

The size distribution of the nucleating material acting as nucleation in the superfine ferrite phase in the molten steel should satisfy 0.5 to 15 탆. In the case where the diameter of the oxide is less than 0.5 탆, the barrier energy of the interface is large due to the large activation energy barrier required for nucleation, so that nucleation of the ferrite phase is difficult. On the other hand, oxides having a particle size exceeding 15 mu m are not easily retained in the molten steel due to their ease of flotation and separation, and thus do not contribute to nucleation.

2 is an optical photograph showing a cross-sectional structure of a nucleus forming alloy according to an embodiment of the present invention. As shown in FIG. 2, it can be seen that the nucleating material is uniformly distributed in the base metal. In the actual operation, when the alloy for nucleation in the molten steel is injected, the base metal is preferentially melted and the nucleus material distributed in the inside is dispersed into the molten steel. In particular, the molten alloy is gradually melted from the surface of the nucleus forming alloy It is possible to prevent coagulation of the nucleators due to dispersion of the nucleators.

For smooth dissolution of the nucleation alloy, the properties of the base metal should be such that the melting point is lower than the molten steel temperature in the tundish. Preferably, the material should be selected to have a melting point of at least 10 ° C lower than the molten steel temperature, and may be made of carbon steel or stainless steel depending on the application.

The alloy for nucleation may be added to the molten steel by mechanical mixing. For example, the alloy for nucleation may be prepared in a wire form and may be added by a wire-feeding method using a steel tube.

Further, the molten steel in the tundish can be agitated to uniformly disperse the nucleating material in the molten steel. For example, the molten steel may be stirred by rotating the stirring device such as an impeller provided on the outer surface of the tube tube to uniformly disperse the nucleating material in the molten steel.

Hereinafter, the present invention will be described in detail with reference to the following examples. However, the following examples are intended to illustrate the present invention in more detail, but the scope of the present invention is not limited to these examples.

C: not more than 0.02%, N: not more than 0.015%, Si: not more than 0.5%, S: not more than 0.5%, and the like. Ferritic stainless steel containing not more than 0.01% of Cr, 9 to 30% of Cr, not more than 2.0% of Mn, not more than 2.0% of Si, not more than 0.1% of Ti, not more than 0.6% of Nb and the balance Fe and unavoidable impurities Using an induction melting furnace. Nucleating alloys made by wire coating were mechanically mixed with 0.2% carbon steel powder by varying the composition range of the nucleating material, and the nucleation alloy was fed into the molten steel in the tundish at a constant speed in synchronism with the casting speed by wire feeding. At this time, the temperature of the molten steel in the tundish was adjusted to 1,545 ° C on an average, and the injection of the nucleus forming alloy was maintained at about 0.05% in terms of a mass ratio for about 5 minutes and cast at a rate of 1.0mpm into a rectangular mold.

The cast specimens were cut at 1/2 point in the height direction, polished, and subjected to hexagonal etching. The images were acquired through optical microscope photography and the average diameter of equiaxed specimens was measured by image analysis technique. The equiaxed crystal is defined as a crystal having a ratio of the long side to the short side of 2 or less, and is expressed as an average equiaxed diameter in terms of a circular diameter. Table 1 below shows the composition of the nucleating agent and thus the diameters of equiaxed crystals.

division Nucleation Re-composition (%) Equilibrium Diameter (mm) CaO TiO ₂ Al ₂ O ₃ CaO + TiO ₂ CaO / (CaO + TiO ₂₎ Average Deviation Example 1 50 50 0 100 0.50 0.98 0.55 Example 2 30 40 30 70 0.43 1.12 0.57 Example 3 50 30 20 80 0.63 0.99 0.59 Example 4 30 30 40 60 0.50 1.17 0.58 Example 5 30 10 60 40 0.75 1.18 0.61 Comparative Example 1 0 0 0 0 - 3.55 2.01 Comparative Example 2 100 0 0 100 0.00 2.99 1.89 Comparative Example 3 0 0 100 0 - 3.05 2.05 Comparative Example 4 0 100 0 100 1.00 3.45 2.57 Comparative Example 5 10 40 50 50 0.20 2.89 1.87 Comparative Example 6 0 40 60 40 0.00 3.31 2.11 Comparative Example 7 10 30 60 40 0.25 3.17 2.45 Comparative Example 8 50 10 40 60 0.83 3.44 2.25 Comparative Example 9 20 70 10 90 0.22 2.74 2.25 Comparative Example 10 10 10 80 20 0.50 3.25 2.11

In Comparative Examples 1 to 4, nucleation re-penetration or nucleation alloy was produced by using CaO, TiO ₂ , and Al ₂ O ₃ alone. In Examples 2 to 5 and Comparative Examples 5 to 10, the effect of finer definition of equiaxed crystals was examined while changing the composition of the nucleating agent.

Referring to Table 1, it can be seen that Example 1 formed a fine equiaxed crystal by using a pure CaTiO ₃ phase as a nucleation material.

In the case of Examples 2 to 5, it was confirmed that the equations (1) and (2) were satisfied, and the sizes of the equiaxed crystals became finer in the range of 1 mm. On the other hand, in Comparative Examples 5 to 9, the equations (1) were satisfied but the equiaxed definition size was not miniaturized because the formula (1) was satisfied, And it was confirmed that the size of the equiaxed definition was not miniaturized.

In Comparative Examples 1 to 4, the evaluation was made using oxides other than CaTiO ₃ as proposed in the present invention, and all of the isometric definition refinement results could not be obtained.

From the results of Table 1, it was possible to set the composition range of the effective nucleating reoxides. Next, the average diameters of equiaxed crystals and average equiaxed diameters of the casts according to the present invention using the nucleus-forming reoxidation composition of Example 1 and the quenched materials according to Comparative Example 1 were examined.

Si: not more than 0.01%, Cr: 9 to 30%, Mn: not more than 2.0%, Si: not more than 2.0%, Ti: not more than 0.1%, C: not more than 0.02% % Or less, Nb: 0.6% or less, the balance Fe and unavoidable impurities were dissolved by using a 50 kg vacuum induction melting furnace. Nucleating alloys made by wire coating were mechanically mixed with 0.2% carbon steel powder by varying the composition range of the nucleating material, and the nucleation alloy was fed into the molten steel in the tundish at a constant speed in synchronism with the casting speed by wire feeding. At this time, the temperature of the molten steel in the tundish was adjusted to 1,545 ° C on an average, and the injection of the nucleus forming alloy was maintained at about 0.05% in terms of a mass ratio for about 5 minutes and cast at a rate of 1.0mpm into a rectangular mold.

The produced cast steel was cut in the width direction, polished, and then subjected to hexagonal macro-etching to measure the diameters of equiaxed crystals. The measurement range of the equiaxed crystals was limited to the 1/4 thickness range from the center in the thickness direction. In addition, two out of the slabs produced by each ladle were selected to evaluate the average equilibrium constant and the average equilibrium diameter.

division Equilibrium constant (%) Equilibrium Diameter (mm) Analysis Week Number Average Deviation Average Deviation Comparative Example 51 4 3.70 1.03 32 Example 75 5 0.99 0.38 35

Based on the results shown in Table 2, the average equiaxed crystal size of the ferritic stainless steels of Examples and Comparative Examples is plotted in FIG. As a result of the evaluation, the average equilibrium rate is 51% and the equiaxed diameter is 3.70mm. On the other hand, in the case of the embodiment, the average equilibrium constant is increased to 75% and the equiaxed diameter is reduced to 1 mm or less.

FIG. 4 is a graph illustrating the size and number of oxides containing CaTiO ₃ phase in a cast steel according to the embodiment. The size of the oxide was evaluated by the maximum diameter, and the size was distributed in the range of 0.5 to 10 mu m. About 30% of the nucleation materials contained in the nucleation alloy remained in the final cast, and it could be assumed that the ferrite was formed by heterogeneous nucleation due to the residual nucleation material. That is, the fine equiaxed crystal formation of the examples can be judged by the effect of the nucleating material among the nucleation alloys proposed in the present invention.

It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims and their equivalents. It is to be understood that the technical idea is included in the scope of the present invention.

Claims (6)

A method of manufacturing a ferritic stainless steel in which molten steel in a tundish is supplied to a mold to continuously cast a cast steel,
A nucleating material containing CaTiO ₃ oxide is injected into the molten steel in the tundish to continuously cast the molten steel,
Wherein the nucleation material satisfies the following formulas (1) and (2): " (1) "
(1)% (CaO) + % (TiO 2) ≥ 40%
(2) 0.3 ≤% (CaO ) / [% (CaO) +% (TiO 2)] ≤ 0.8
(And where,% (CaO),% ( TiO 2) refers to the weight% of CaO, TiO ₂₎ delete The method according to claim 1,
Wherein the nucleus forming material has a fine casting structure having a size of 0.5 to 15 占 퐉. The method according to claim 1,
Wherein the nucleus forming material is uniformly distributed in a matrix and is added to the molten steel in the form of a wire. 5. The method of claim 4,
Wherein the base metal has a melting point lower than that of the molten steel in the tundish by at least 10 캜. The method according to claim 1,
Wherein the molten steel in the tundish is molten.

Images