KR100844794B1 - A method for refining with high purity of austenitic stainless steel - Google Patents

Abstract

The present invention relates to a method for high-cleaning refining of austenitic stainless steel that minimizes the number of inclusions in molten steel transferred to cast iron through temperature control in an AOD (Argon Oxygen Decarburization) refining furnace after tapping an electric furnace. The present invention is an electric arc furnace (EAF)-Argon Oxygen Decarburization (AOD)-Ladle Treatment (LT)-Tundish-Continuous casting process of 10 to 30% of chromium ( Cr), 5-15% nickel (Ni), 0.005% or less of titanium (Ti) and 0.1% or less of carbon (C) in the refining method of austenitic stainless steel, the method of the refining furnace (AOD) The temperature of the molten steel is adjusted to (temperature of the molten steel to tap the electric furnace +50 ℃) or less, and the molten steel temperature at the time of the component adjustment (LT) (temperature of the molten steel to tap the refining furnace -100 ℃) or more It characterized in that the step of adjusting to. With this configuration, it is possible to improve the cleanliness of the stainless steel due to the reduction of surface defects due to inclusions.

High Purity, Refinery, AOD, Component Control, LT, Temperature

Description

A method for refining with high purity of austenitic stainless steel}

1 is a schematic diagram showing a general stainless steel manufacturing process.

2A is a view showing the shape of hard inclusions in stainless steel;

Figure 2b is a view showing the cold-rolled coil surface defects due to the hard inclusions.

Figure 3 is a graph showing the oxygen concentration in the steel according to the process of the stainless steel produced by the refining method of the present invention compared with the prior art.

Figure 4 is a graph showing the number of inclusions in the steel for each process of the stainless steel produced by the refining method of the present invention compared with the prior art.

5 is a graph showing a defect index of the stainless steel produced by the refining method of the present invention in comparison with the prior art.

The present invention relates to a method for high clean refining of austenitic stainless steel, more specifically, after decarburization and deoxidation in an AOD (Argon Oxygen Decarburization) refining furnace after melting of an electric furnace, and molten steel temperature in refining casting ladle The present invention relates to a method for high clean refining of austenitic stainless steel that can improve the cleanliness of stainless steel by minimizing the number of inclusions of molten steel transferred to casting ladles through adjustment and casting temperature control.

In general, the surface quality of the austenitic stainless steel is very important. Among the factors influencing the surface defects, the composition and the number of the high melting point nonmetallic inclusions are a big problem. That is, when inclusions remain on the surface of the product, it may damage the surface or cause cracks. However, since non-metallic inclusions inevitably occur through processes such as deoxidation of molten steel and input of ferroalloy for temperature control, the occurrence of inclusions should be minimized.

Hereinafter, a stainless steel manufacturing process will be described in detail with reference to the accompanying drawings.

1 is a schematic diagram showing a general stainless steel manufacturing process, Figure 2a is a view showing the shape of the hard inclusions of stainless steel, Figure 2b is a view showing the cold-rolled coil surface defects by the hard inclusions.

Referring to FIG. 1, the molten metal generated by melting in an electric furnace, that is, an electric furnace molten metal is tapped into the charging ladles, and the charging ladles are tilted to remove a portion of the slag floating on the upper portion of the molten metal. Then, the molten metal furnace is removed into the refining furnace in which the remaining slag is removed from the exhaust site.

The molten steel causes the loss of molten steel by oxidizing valuable metals such as chromium and iron during decarburization in an argon oxygen decarburization (AOD) refining furnace, and produces chromium oxide because oxygen is blown into the molten steel to remove carbon. To reduce this, silicon oxide (FeSi) is added as a deoxidizer together with a basic flux composed mainly of quicklime (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, referring to FIGS. 2A to 2B, silicon and oxygen react to form silicon oxide (SiO ₂ ) by the reaction of [Scheme 1] below, and furthermore, silicon Is used as a reducing agent, because aluminum is contained in the silicon alloy, when the concentration of aluminum in the molten steel is a certain value or more, the aluminum oxide (Al ₂ O ₃ ) by the reaction of magnesium aluminate-based alumina or [Scheme 2] There is a problem that the inclusion of the high melting point inclusion inevitably exist in the molten steel.

[Si] + 2 [O] = SiO ₂

2 [Al] + 3 [O] = Al ₂ O ₃

In addition, the temperature of the molten steel drops more than 100 ℃ during the refining process and the transfer from the refining furnace to the casting ladle. At this time, aluminum and silicon in the molten steel decrease in solubility by lowering the molten steel and combine with oxygen in the molten steel. Since the oxidation reaction is accelerated, silicon oxide and alumina inclusions are produced. That is, the greater the drop in molten steel temperature, the higher the number of high melting point inclusions. As the temperature of the molten steel transferred to the casting lathes continues to drop until it solidifies in the casting process, the oxidation reaction of aluminum causes a change in the composition of the inclusions, increasing the concentration of alumina in the inclusions.

The fine non-metallic inclusions thus produced are suspended as fine solid particles inside the molten steel at 1600-1700 ° C., so that the aggregation and growth between the particles are difficult, and they do not float to the upper portion of the molten steel by buoyancy and remain in the molten steel. These inclusions react with slag incorporated into molten steel when tapping in AOD refining furnaces, transforming into large inclusions and ultimately adversely affecting product surface quality and ultimately affecting product surface quality and high melting point spinel inclusions. The shape of the precipitated phase is combined. Various techniques have been applied to control the number, size, and the like of such inclusions.

Referring to the published documents, Korean Patent Publication No. 2002-0022275, Korean Patent Publication No. 2001-0063536, and Japanese Patent Publication No. 195-188861 disclose the inclusions in a manner of preventing the inclusion of already created inclusions or suppressing the alumina inclusions. It is not related to suppression of development. In addition, Korean Patent Publication No. 2004-0056706 discloses a method of using a dolomite ladle to reduce the concentration of alumina in inclusions, but this method is also difficult to reduce the number of inclusions in molten steel. In addition, Japanese Patent Laid-Open Publication No. 19991-267312, Japanese Patent Laid-Open Publication No. 1998-158720, and French Patent EI7603020603 regulate the production of slag basicity and the concentration of alumina and magnesia in the slag, thereby producing high melting point inclusions. The present invention relates to a method of preventing an interference component, and the above-described conventional techniques do not provide a method of reducing the number of inclusions.

Therefore, the present invention is an invention designed to solve the above-mentioned conventional problems, the object of the present invention is the inclusion of molten steel transferred to the casting after the decarburization and deoxidation process in the Argon Oxygen Decarburization (AOD) refining after the electric furnace tapping The present invention provides a method for high clean refining of austenitic stainless steel that can minimize the number.

In order to achieve the above object, according to the present invention, the high-cleaning refining method of the austenitic stainless steel is an electric arc furnace (EAF) -refining furnace (AOD: Argon Oxygen Decarburization) -component adjustment (LT: Ladle Treatment) ) -Tundish-10-30% chromium (Cr), 5-15% nickel (Ni), 0.005% titanium (Ti) and 0.1% carbon (C) In the refining method of austenitic stainless steel comprising a, the temperature of the molten steel in the refining furnace (AOD) is adjusted to (temperature +50 ℃ of the molten steel for tapping the electric furnace) or less, the component adjustment (LT ) The molten steel temperature is characterized in that the step of adjusting to (the temperature of the molten steel to the tapping-refinery -100 ℃) or more.

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

(Example)

In the manufacturing process of the austenitic stainless steel through the electric furnace-AOD refining furnace-casting ladle-continuous casting process described in FIG. 1, scrap and ferrochrome (FeCr) were used as raw materials, and melted in a 90 ton electric furnace, Is adjusted to 1580 ~ 1600 ℃. The decarburization refining with argon-oxygen mixed gas is performed in the AOD refining furnace. 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. Before moving to the casting ladle, adjust the temperature at the time of tapping to 1630 ~ 1650 ℃, and remove some of the slag from the upper part of molten steel to 1.5 ~ 2.0% of the molten steel and blow argon gas from the bottom of the casting ladle. Final component and temperature adjustment are performed while stirring.

When the stirring operation was completed, the temperature of molten steel was adjusted to maintain 1530 ~ 1550 ° C, and the casting temperature was controlled to be 40-50 ° C higher than the melting temperature of stainless steel, and slab slabs of stainless steel were manufactured by continuous casting. The surface was examined under a microscope to determine the number of inclusions on the surface, and this was expressed as a cleanliness index of stainless steel divided into levels between 0 and 10.

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

ingredient C Si Mn Al Ca Cr Ni N P S % 0.1 or less 0.30-0.70 1.00-1.50 Less than 0.0003 0.001 or less 17.0 ~ 20.0 5.0 ~ 10.0 0.05 or less 0.03 or less 0.01 or less

In addition, Table 2 shows the experimental conditions and the cleanliness index accordingly.

division Conduct number Experimental condition Cleanliness Index (0 ~ 10) Furnace tapping temperature (℃) AOD tapping temperature (℃) Molten steel temperature after LT (℃) Casting temperature (℃) Prior art One 1550 1680 1530 1483 5.0 2 1543 1700 1524 1479 8.6 3 1578 1695 1518 1487 5.5 4 1566 1678 1510 1480 6.7 5 1555 1667 1506 1485 9.9 The present invention One 1583 1631 1535 1495 0.4 2 1585 1630 1533 1503 0.8 3 1596 1628 1530 1503 1.0 4 1588 1625 1528 1495 2.2 5 1581 1620 1525 1493 0.8

As shown in [Table 2], the molten steel temperature in the refining furnace (AOD) controls the temperature in the refining furnace decarburization process after the tapping of the electric furnace to (electric furnace tapping temperature + 50 ° C) or lower, and adjusts the component ( LT) The molten steel temperature can be seen that exhibits a significantly higher cleanliness index compared to the prior art by controlling the temperature after the component and the temperature adjustment in the cast ladle after the tapping of the refining furnace (above the tapping temperature-100 ℃). .

Figure 3 is a graph showing the oxygen concentration in the steel for each process of the stainless steel produced by the refining method of the present invention compared with the prior art.

As shown in Figure 3, in the case of the prior art it can be seen that the change in the oxygen concentration is severe with the progress of the process is higher than the present invention. Therefore, in the conventional case, since silicon oxide (SiO ₂ ) may be generated due to the high oxygen concentration during the entire process during deoxidation through the addition of silicon, if the oxygen concentration is low while the oxygen concentration is constant, as in the present invention, It is possible to reduce the occurrence of melting point inclusions.

And, Figure 4 is a graph showing the number of inclusions in the steel for each process of the stainless steel produced by the refining method of the present invention compared with the prior art.

Referring to Figure 4, as described above, the number of inclusions in proportion to the oxygen concentration is also a lot of ups and downs in the prior art, while in the present invention it can be seen that the number of inclusions gradually decreases as the process proceeds. .

As a result, when comparing the embodiment of the present invention and the example of the prior art of Table 2, it can be seen that the cleanliness index is significantly improved. The cleanliness index, which was an average of 7.1, was drastically lowered to an average of 1.0, indicating that the cleanliness of stainless steel was improved by 60%. 5 is a graph showing the defect index of the stainless steel produced by the refining method of the present invention in comparison with the prior art, the results of the above [Table 2] is shown as a graph in Figure 5, from the inclusions in the molten steel according to the present invention The validity of the mitigation effect can be demonstrated.

The number of inclusions in the molten steel that has the greatest impact on the cleanliness of the molten steel depends on the temperature after the AOD operation, the temperature during the composition adjustment and the casting temperature. This is because the number of inclusions in the molten steel is proportional to the oxygen content in the molten steel, and the oxygen content in the molten steel is proportional to the molten steel temperature.

Therefore, the present invention comprises a method of adjusting the molten steel temperature of the refining furnace AOD and a method of adjusting the temperature at the time of the component adjustment LT in order to reduce the number of inclusions.

In the present invention, a method of adjusting the temperature of the molten steel of the refining furnace (AOD) is to control the temperature rise in the refining furnace (AOD) decarburization process after the tapping of the electric furnace to an electric tapping temperature of + 50 ° C. or less. The temperature of the refinery tapping furnace is limited to (electric furnace tapping temperature + 50 ℃), because when the temperature is higher than that, the oxygen content in the molten steel increases and the number of inclusions in the steel increases rapidly. In addition, when the molten steel temperature after the refining operation is set to 1700 ° C or higher, the melting loss of the refractory becomes severe and worsens the cleanliness of the molten steel.

On the other hand, after the transfer to the casting ladle in the refining furnace, the alloying iron for the component adjustment and the molten steel should be adjusted to more than (AOD tapping temperature-100 ℃) during the stirring time, it is possible to minimize the occurrence of inclusions in the molten steel. In addition, it is necessary to control the casting temperature higher than the melting temperature of stainless steel 40 ~ 50 ℃ to ensure high cleanliness of stainless steel.

That is, the step of adjusting the molten steel temperature of the refining furnace (AOD) is to control the temperature rise in the refining furnace (AOD) decarburization process after the electric furnace tapping (electric furnace tapping temperature + 50 ℃) or less, and during the component adjustment (LT) The step of adjusting the temperature is to adjust the temperature after the component and the temperature adjustment in the casting ladle after the tapping (refining tapping temperature-100 ℃) or more, by controlling the casting temperature 40 ~ 50 ℃ higher than the melting temperature of the stainless steel, molten steel By minimizing the occurrence of heavy inclusions, it is possible to prevent surface defects caused by inclusions when processing stainless steel products.

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.

As described above, according to the present invention, the temperature after the decarburization and deoxidation in the AOD (Argon Oxygen Decarburization) refining furnace after the electric furnace tapping and the molten steel temperature during casting ladle refining, casting temperature control, etc. By minimizing the number of inclusions of molten steel transferred to the stools, it is possible not only to improve the ductility of the inclusions in stainless steel processing, but also to improve the cleanliness of the stainless steels due to the reduction of surface defects caused by the inclusions.

Claims (3)

Electric Furnace (EAF)-Refinery Furnace (AOD)-Composition Control (LT)-Tundish-Continuous casting process, in weight percent, 10-30% chromium (Cr), 5-15% nickel In the refining method of an austenitic stainless steel containing (Ni), 0.005% or less of titanium (Ti) and 0.1% or less of carbon (C), The temperature of the molten steel in the refining furnace (AOD) is adjusted to (temperature of the molten steel for tapping the electric furnace +50 ℃) or less, and the molten steel temperature during the component adjustment (LT) (melted steel for tapping the refining furnace The temperature of -100 ℃) is a step of adjusting the high clean refining method of the austenitic stainless steel. delete delete

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