KR100941841B1 - A method of manufacturing austenite stainless steel - Google Patents

Abstract

The present invention relates to a method for producing austenitic stainless steel, the method for producing austenitic stainless steel according to the present invention AOD refining process and casting ladle refining method for producing austenitic stainless steel, AOD Adjusting the basicity of the slag to 1.5 ~ 1.9 in the refining furnace, adjusting the alumina concentration of the slag to 3% or less, adjusting the deoxidizer concentration to 0.4 to 0.5% and lowering the inclusions in the molten steel in the casting ladle. Characterized in that it comprises a.

Austenitic, Stainless Steel, Cleanliness, Surface Quality

Description

Manufacturing method of austenitic stainless steel {A METHOD OF MANUFACTURING AUSTENITE STAINLESS STEEL}

The present invention relates to a method for producing austenitic stainless steel, and more specifically, to control slag basicity (ratio of calcium oxide (CaO) and silicon oxide (SiO ₂ )) and molten steel temperature of AOD refining furnace, Refractories are limited to reduce the alumina concentration of molten steel in AOD refineries. In addition, in order to reduce the inclusions in the molten steel in the casting ladle treatment by adjusting the casting conditions to minimize the inclusions to prevent surface defects caused by the inclusions in the processing of stainless steel products.

In general, austenitic stainless steel has a very important surface quality, but austenitic stainless steel has excellent ductility, so that surface defects occur easily, and microscopic defects are not managed under normal conditions. Is a high gloss product needed. There are many factors affecting the surface defects of such austenitic stainless steels, but among them, the composition (particularly, Al ₂ O ₃ spinel inclusions) and the number of high melting point nonmetallic inclusions are a major problem. In other words, if an inclusion remains on the surface of the product, it may damage the surface or cause cracking. However, since non-metallic inclusions are inevitably generated by deoxidation of molten steel, incorporation of slag, reoxidation, etc., there is a demand for a method of lowering the composition of the inclusions and minimizing the number of inclusions. To this end, it is manufactured using a secondary refining facility such as Vacuum Oxygen Decarburization (VOD) or Ladle Furnace (LF), which is disadvantageous in terms of manufacturing cost.

FIG. 1A is a view showing the shape of a hard inclusion of stainless steel, and FIG. 1B is a view showing a defect of a cold rolled coil surface by the hard inclusion.

In manufacturing stainless steel, oxygen gas is blown into molten steel by argon-oxygen decarburization (AOD) to remove carbon. At this time, organic metals such as chromium and iron are oxidized to form chromium oxides, causing loss of molten steel component. Therefore, to reduce this, a silicon alloy such as FeSi is added as a deoxidizer together with a basic flux mainly composed of calcium oxide (CaO), and the molten steel is stirred with an inert gas to promote deoxidation and removal of inclusions. However, in the case of deoxidation by the addition of silicon, silica (SiO ₂ ) is generated by the reaction as in Scheme 1 below.

[Si] + 2 [O] = (SiO ₂ )

In addition, since the aluminum alloy-containing material in the silicon alloy, when the concentration of aluminum in the molten steel is a certain value or more, magnesium aluminate-based spinel (MgAl ₂ O _{4 of the} shape as shown in Figure 1a) SPINEL) or alumina (Al ₂ O ₃ ) inclusions are generated by the reaction as in Scheme 2 below, and high melting point inclusions are inevitably present in the molten steel.

2 [Al] + 3 [O] = (Al ₂ O ₃ )

The high melting point inclusions generated as above result in surface defects of the product as shown in FIG. 1B or cracks during processing.

In addition, slag is suspended in molten steel during the reduction refining process and the transfer from the refining furnace to the casting ladle, and the suspended slag particles act as nuclei for inclusion growth. Silica and magnesium aluminate-based spinel, alumina and slag particles react with each other to form inclusions. The fine non-metallic inclusions produced as described above are suspended as fine solid or liquid particles in the molten steel at 1600-1700 ° C., so that the coagulation and growth between the particles are difficult and do not float to the upper part of the molten steel by buoyancy and remain in the molten steel continuously. Done.

When these inclusions are cast into cast ladles in an AOD refining furnace, they react with the slag incorporated into the molten steel and change into low melting point inclusions. However, as the temperature of the molten steel continues to drop until it solidifies in the casting process, the oxidation of aluminum causes a change in the composition of the inclusions, thereby increasing the concentration of alumina in the inclusions. In addition, aluminum in molten steel has a low solubility due to a decrease in molten steel temperature, so that the inclusions are changed into high melting point inclusions by alumina in combination with oxygen in the molten steel, and finally, slag inclusions and magnesium aluminate which adversely affect the product surface quality. The precipitated phase of the system spinel inclusions is combined.

 In order to control the components, number, size, etc. of the inclusions, various techniques are applied to Korean Patent Nos. 2002-0022275, Korean Patent Nos. 2001-0063536, and Korean Patent Nos. 2004-0056706, but it is not applicable to the actual process. Difficulties or limitations in general application in ways that are only possible for special steel grades and require additional processing.

In addition, Japanese Patent No. 2001-164312 for improving cleanliness is applicable only when the titanium content is high, it is difficult to control stably, and it is difficult to see it as a fundamental cleanness improvement method. Japanese Patent Laid-Open No. 11-199917 for harmless non-metallic inclusions in a solidified structure is very difficult to control the composition, which is not practical and there is a fear of clogging the nozzle. Japanese Patent Application Laid-Open No. 07-188861, which discloses a method for separating inclusions, shows that the regulation of aluminum in the raw material is very difficult in the actual stainless steel manufacturing process, and the manufacturing cost increases, adding calcium (Ca). Since the method has a problem of generating a melt loss of the alumina-based refractory, it is important to find the optimum addition amount in consideration of the yield error. Japanese Patent Laid-Open Nos. 03-267312, 10-158720, and French Patent EI7603020603 for preventing the formation of high melting point inclusions are difficult to suppress the formation of high melting point inclusions and control only the concentration ratio of calcium and oxygen in molten steel. In this case, the possibility of inclusion shape control is low.

Therefore, the present invention is an invention designed to solve the above-mentioned conventional problems, in the manufacturing method of the austenitic stainless steel, for the molten steel transferred to the casting casting after decarburization and deoxidation process, while making the inclusions harmless By separating and collecting as much floating as possible, it is intended to provide a technology that can improve the cleanliness of stainless steel. In particular, it ensures stable cleanliness of molten steel and at the same time suppresses the occurrence of high melting point inclusions through the refining technology according to the present invention, thereby improving the ductility of inclusions when processing stainless steel, and reducing the number of inclusions as much as possible due to inclusions. It is an object of the invention to provide a method for producing austenitic stainless steel that can reduce surface defects.

In the method for producing austenitic stainless steel according to the present invention, the basicity of the slag is adjusted to 1.5 to 1.9 in the AOD refining furnace, the alumina concentration of the slag is 3% or less, and the deoxidizer concentration is adjusted to 0.4 to 0.5%. Include. It also includes reducing the inclusions in the molten steel in the casting ladle.

Here, the basicity of slag in the AOD refining furnace is adjusted by adding fluorite.

As the refractory material of the casting ladle, it is preferable that the metal zone uses dolomite, and the freeboard uses magcarbon (MgO-C).

On the other hand, the step of reducing the inclusions in the molten steel in the casting ladle is to adjust the basicity of the slag in the casting ladle to 1.5 ~ 1.9 and to perform the stirring for at least 3 minutes with a stirring strength of 5 to 7 bar and the casting ladle Adding a wire made of FeCaSi, and injecting argon gas from the bottom of the casting ladle, and performing stirring for 15-20 minutes at 3.5-4 bar.

Here, the basicity of slag in the casting ladle is adjusted by adding wollastonite and fluorspar.

Further, the method may further include controlling the casting temperature to be 40-50 ° C. higher than the melting temperature of the stainless steel while setting the casting standby time of the cast ladle to which the FeCaSi wire is added to 20 minutes or more.

As described above, the present invention adjusts the slag basicity after reduction with AOD refining, injects wollastonite and fluorspar during refining of the casting ladle, and adjusts the stirring strength, time, molten steel temperature, etc. It can suppress occurrence. In addition, the present invention has the effect of improving the ductility of inclusions during stainless steel processing, significantly reducing the number of inclusions to improve the surface quality of the final product.

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

First, the manufacturing method of the present invention can be applied to any steel type in the case of manufacturing austenitic stainless steel, but preferably 10 to 30% of chromium (Cr) and 5 to 15% of nickel ( It is very useful to manufacture stainless steel containing Ni) and containing carbon (C) having a concentration of 300 ppm or less.

2 is a view briefly showing a manufacturing process of stainless steel according to the present invention. Referring to FIG. 2, austenitic stainless steel (18.5% Cr) is manufactured through an electric furnace (1) -AOD refining furnace (2) -cast ladle (3) treatment-continuous casting process (4). . First, scrap and ferrochrome (FeCr) are used as raw materials to dissolve in a 90 ton electric furnace 1, and decarburization and refining using an argon-oxygen mixed gas is performed in the AOD refining furnace 2. At this time, in order to reduce the concentration of alumina in the inclusions, fluorite (CaF ₂ ) is added to adjust the slag basicity (a ratio of calcium oxide (CaO) and silicon oxide (SiO ₂ )) of the AOD refining furnace ₂ to 1.5 to 1.9. do.

In the present invention, the slag basicity of the AOD refining furnace (2) is limited to 1.5 to 1.9, and when the basicity is 1.9 or more, magnesium aluminate-based spinel (MgAl ₂ O ₄ ) inclusions in the inclusions increase rapidly and the fluidity of the slag is poor. This is because it is difficult to use residual slag when bubbling. In addition, when the slag basicity is 1.5 or less, the sulfur removing ability of the slag is remarkably reduced, which lowers the hot work cracking factor and the surface quality of the stainless steel. Therefore, in the present invention, the slag basicity of the AOD refining furnace 2 is adjusted to 1.5-1.9 to facilitate the cohesion of fine slag particles floating in the molten steel after tapping in the AOD refining furnace 2, thereby eliminating inclusions by floating separation. It is smooth.

At this time, the concentration of alumina in the slag is 3% or less to prevent the inclusion of high melting point inclusions, and to prevent the inclusions increase due to the increase in the amount of residual oxygen, the concentration of the deoxidizer is adjusted to 0.4 ~ 0.5%.

In order to reduce and recover the oxidized chromium after decarburization, quicklime and fluorite are added together with silicon, and argon gas is blown to reduce refining and then transferred to the casting ladle (3).

In the casting ladle (3) to remove the slag of the upper molten steel to 1.0 ~ 2.0% of the molten steel weight and by adding wollastonite and fluorspar (CaF ₂ ) to adjust the slag basicity to 1.5 ~ 1.9. At this time, the fluorspar (CaF ₂ ) is injected from the top to ensure the fluidity of the slag. Preferably, 4-6 g / TS is charged.

After that, the steel was stirred for 3 minutes with the stirring strength of 5 bar or more. After carrying out the steel stirring, a wire made of FeCaSi is added to the casting ladle 3, and the argon gas is blown for 15 to 20 minutes to be 3.5 to 4 bar from the lower portion of the casting ladle 3, and the molten steel stirring time is 15 to 20. Let's raise the inclusions enough in minutes. In this case, when the molten steel stirring strength is 4 bar or more in pressure units, the pressure is increased, so inclusion is problematic, so that the molten steel stirring strength is adjusted to 3.5-4 bar so that sufficient separation of the inclusions is performed.

And final component and temperature adjustment are performed, stirring. Casting wait time is 20 minutes or more to ensure the floating time of the fine inclusions, while controlling the casting temperature is 40 ~ 50 ℃ higher than the melting temperature (1450 ℃) of stainless steel to prevent the inclusion precipitates. In this way, high cleanliness of stainless steel is ensured.

On the other hand, as the refractory material of the casting ladle (3), dolomite is used as a main material, and a portion of the free board after removal is made of MgO-C material to reduce the adhesion of slag. .

When alumina-based oxide is used as the refractory material of the casting ladle 3, the refractory reacts with the slag to increase the concentration of alumina (Al ₂ O ₃ ) in the slag, and increase the concentration of aluminum in the molten steel, resulting in the inclusion of inclusions. It is preferable to use dolomite because it increases the alumina concentration. When dolomite is used as the refractory of the casting ladle (3), the service life is long, and there is an economic effect. On the other hand, dolomite casting ladle is characterized in that the slag is coated on the wall even after casting, it is essential to ensure the fluidity of the slag (low melting point). For this purpose, it is preferable to lower the slag basicity of the AOD refining furnace _{2 and} to add an appropriate amount of CaF ₂ . The free board, which does not react directly with molten steel, has the effect of reducing the adhesion of slag while suppressing Al ₂ O ₃ -MgO-based spinel crystallization by using mag carbon (MgO-C) refractory.

Thereafter, a cold cast steel sheet made of stainless steel is produced through the continuous casting process 4, hot rolling and cold rolling.

Table 1 shows the components of the austenitic stainless steels used in the examples of the present invention.

Cr (wt.%) C N Si S 17.0-19.0 ~ 0.070 ~ 0.050 0.3 to 0.6 ~ 0.010

As shown in Table 1, the components of the molten steel used in the examples of the present invention are by weight, Cr 17.0-19.0%, C 0.07% or less, N 0.05% or less, Si 0.3-0.6%, S 0.01% or less. Include.

Table 2 shows the defect index of the surface of the cold rolled steel sheet according to the prior art and the present invention, and visually inspects the surface of the cold rolled steel sheet to investigate the incidence of defects caused by inclusions on the surface of the cold rolled steel sheet, and classifies it into a level between 0 and 10. It is represented by the defect index of one stainless steel.

division number Experimental condition Defective Index (1-10) Casting Ladle Refractory Refining furnace slag basicity Molten steel temperature after refining (℃) Whether to use LT slag Wollastonite input (kg) Fluorite input (kg) Casting wait time (min) Casting temperature (℃)  Prior art One Alumina 1.9 1680 not used 0 0 18 1483 5.0 2 Alumina 1.9 1650 not used 0 0 15 1479 8.6 3 Alumina 1.9 1677 not used 0 0 17 1487 5.5 4 Alumina 1.9 1670 not used 0 0 17 1480 6.7 5 Alumina 1.9 1648 not used 0 0 15 1485 9.9  The present invention One Dolomite 1.75 1705 use 400 60 23 1495 0.4 2 Dolomite 1.7 1730 use 400 60 25 1503 0.8 3 Dolomite 1.8 1705 use 400 60 30 1503 1.0 4 Dolomite 1.75 1710 use 400 60 25 1495 2.2 5 Dolomite 1.75 1705 use 400 60 24 1493 0.8

Comparing the embodiment of the present invention and the prior art, it can be seen that the defect index in the stainless steel according to the present invention is significantly improved. In the present invention, the defect index, which was an average of 7.1, was significantly lowered to 1.0 in the present invention, indicating that the cleanliness of the stainless steel was improved by about 60%. The results of Table 2 are shown graphically in FIG. As shown in FIG. 3, the variation in defect index between the cold rolled coils is also greatly reduced, which is very advantageous in terms of operation management.

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.

It is a figure which shows the shape of the hard inclusions of stainless steel.

1B is a view showing a defect of a cold rolled coil surface due to hard inclusions.

2 is a view briefly showing a manufacturing process of stainless steel.

Figure 3 is a graph comparing the defect index of the stainless steel according to the embodiment of the present invention and the prior art.

Claims (6)

Adjusting the basicity of the slag to 1.5 to 1.9 in an AOD refining furnace, adjusting the alumina concentration to 3% or less in the slag, and adjusting the deoxidizer concentration to 0.4 to 0.5%; And Reducing the inclusions in the molten steel in the casting ladle, Reducing the inclusions, the first step of adjusting the basicity of the slag in the casting ladle to 1.5 ~ 1.9 and performing the first stirring with a stirring strength of 5 ~ 7 bar; Adding a wire made of FeCaSi to the casting ladle; And Injecting an argon gas from the bottom of the casting ladle, and performing a second agitation with a stirring strength of 3.5 ~ 4bar; Method of producing austenitic stainless steel comprising a. The method of claim 1, The slag basicity of the slag in the AOD refining furnace is a method for producing austenitic stainless steel, characterized in that by adjusting the input. The method of claim 1, Refractory of the cast ladle Metal zone (Metal zone) using dolomite, Freeboard (Freeboard) is a method of manufacturing austenitic stainless steel, characterized in that using the carbon (MgO-C). The method of claim 1, The first stirring step is carried out for at least 3 minutes, the second stirring step is 15 to 20 minutes of the manufacturing method of the austenitic stainless steel. The method of claim 1, The slag basicity of the slag in the casting ladle manufacturing method characterized in that the addition of wollastonite and fluorite is adjusted. The method of claim 1, Reducing the inclusions in the molten steel in the casting ladle, The method of manufacturing the austenitic stainless steel, characterized in that it further comprises the step of controlling the casting temperature of 40 ~ 50 ℃ higher than the melting temperature of the stainless steel while the casting wait time of the cast ladle added to the FeCaSi wire 20 minutes or more .

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