KR101786972B1 - The refining method for improving cleanness of molten stainless steel - Google Patents

Abstract

The present invention relates to a refining method of high-cleaned stainless steel which can reduce the occurrence of cracks during product processing by controlling the Al concentration in molten steel and the basicity of VOD slag in a refining method of stainless steel containing VOD process. According to the present invention, it is possible to prevent precipitation of high-melting-point spinel inclusions, thereby reducing the occurrence of cracks during product processing, and to produce high-quality austenitic stainless steels of high quality free from defects.

Description

TECHNICAL FIELD [0001] The present invention relates to a refining method of a high-cleaned stainless steel molten steel,

More particularly, the present invention relates to a method for refining high-cleaned stainless steel by controlling the VOD slag basicity and the concentration of aluminum (Al) in molten steel in the VOD refining process of stainless steel, And more particularly, to a method for improving the cleanliness of stainless steel which can produce silicon austenitic stainless steels.

Generally, a stainless steel is a molten metal produced by melting in an electric arc furnace (EAF), that is, an electric furnace, is introduced into the loading ladle, and a portion of the slag floating on the upper portion of the molten metal . Thereafter, the remaining molten slag is removed from the discharge site, and the furnace is put into a refining furnace (Argon Oxygen Decarburization (AOD)), followed by decarburization and deoxidization, and then transferred to a main shaft and cast.

In this process, silicon (Si) is added to remove oxygen in the molten steel in the AOD process, and Si remaining in the deoxidation is dissolved in the stainless steel to be present in a certain amount.

The stainless steel having a high Si content as described above is rolled in an ultra-thin application of 0.3 mm or less, and then used in a bellows form through molding. As shown in FIG. 1, the high-melting-point spinel-type nonmetallic inclusions present in the product used for such a purpose cause cracking during the molding of the product, so that if the refractory and solidification process of the molten steel can not remove it, There is a problem that the product characteristics are deteriorated.

In order to improve the hot workability of the high-Si austenitic stainless steels as described above, the concentration of S should be controlled to 10 ppm or less. For this purpose, in order to reduce Cr ₂ O ₃ formed during decarburization refining of stainless steel, Is essential. A slag-based is a _{CaO-SiO 2 -Al 2 O 3} -MgO-CaF 2 that is formed when, to form inclusions in the molten steel remaining in the slag to be suspended upon stirring the molten steel. These inclusions vary in composition depending on the composition of the slag and the content of deoxidized elements in the molten steel. If the inclusions are not properly controlled, the spinel inclusions having a high melting point are formed at the casting temperature. As a result, clogging of the nozzles and drop- And the cleanliness of the molten steel is deteriorated. Further, since the high-alloy austenitic stainless steels have a low casting temperature, it is difficult to suppress the formation of spinel having a high melting point at the casting temperature.

The invention for improving the cleanliness of conventional austenitic stainless steels has been mainly concerned with the inhibition of spinel formation on Si deoxidized steel. 1) Al source reduction, 2) slag basicity (CaO / SiO ₂ ) and composition (Al ₂ O ₃ and MgO content) control. Since the high-Si-containing low-S austenitic stainless steels of the present invention are subjected to Al deoxidation, it is difficult to apply the spinel reducing technique at the time of Si deoxidation of the above-mentioned conventional inventions.

Japanese Laid-Open Patent Publication No. 1998-158720 (Jun. 16, 1998) Korean Patent Publication No. 10-2004-0056706 (Jul. 1, 2004)

The present invention provides a refining method of high-clean stainless steel which can lower the ratio of high-melting-point spinel inclusions in inclusions by controlling the slag composition and controlling the molten steel component in the VOD refining process in the production of high Si austenitic stainless steels .

The method for refining high-clean stainless steel according to an embodiment of the present invention is a method for refining stainless steel for producing high-Si austenitic stainless steel through an AOD process and a VOD process, ) Can be satisfied.

(1) 0.8 + 20 * Al - 0.7 * B? 1.0

Here, Al is the aluminum concentration (%) in the molten steel, and B is the basicity of the VOD slag expressed by CaO / Al ₂ O ₃ .

Also, the aluminum concentration in the molten steel is 0.07% or less, and the basicity (B) of the VOD slag may be 1.6 to 2.2.

In addition, the basicity (B) of the VOD slag may be 1.7 to 2.1.

Further, the AOD slag may be removed from the refining furnace of the AOD process, and the slag may be prepared and refined in the VOD process.

The high-Si austenitic stainless steel may contain 18 to 22% of chromium (Cr), 7 to 15% of nickel (Ni), 1.5 to 2.5% of silicon (Si) It may contain impurities.

The refining method of the high-cleaned stainless steel according to the embodiment of the present invention can prevent precipitation of the high-melting-point spinel inclusions, thereby reducing the occurrence of cracks during product processing, thereby producing high-quality austenitic stainless steels of high quality without defects .

1 is a photograph showing cracks during processing of a product by a high melting point inclusion.
2 is a graph showing the Al ₂ O ₃ concentration in inclusions according to VOD slag basicity.
3 is a graph showing the Al ₂ O ₃ concentration in the inclusion depending on the Al concentration in molten steel.
4 is a graph showing the spinel ratio in inclusions according to the spinel effect index.

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.

The present invention can be more preferably applied to a manufacturing method of stainless steel through an electric furnace process (EAF) - AOD refining process - VOD refining process - ladle process process (LT) - tundish process - continuous casting process. The present invention is characterized in that it further comprises a vacuum oxygen decarburization (VOD) process in the refining process.

As the content of Al ₂ O _{3 in} stainless steel molten steel inclusions increases, precipitation of spinel (MgAl ₂ O ₄ ), which is a high melting point inclusion, is facilitated and defects may occur on the surface of stainless steel. The present invention provides a stainless steel refining method capable of improving the cleanliness by controlling the content of Al ₂ O ₃ in inclusions in order to prevent precipitation of spinel (MgAl ₂ O ₄ ) which is a high melting point inclusion.

The method for refining high-cleaned stainless steel according to an embodiment of the present invention is a method for refining stainless steel for producing high-Si austenitic stainless steel through an AOD process and a VOD process. In the VOD process, %) And slag basicity (B) satisfy the following formula (1).

(1) 0.8 + 20 * Al - 0.7 * B? 1.0

Depending on the intended use of the stainless steel, production of stainless steel containing Si up to several percent by weight may be required. In the present invention, the high Si austenitic stainless steel is subjected to decarburization in the refining furnace to a target carbon concentration, The chromium oxide is recovered and deoxidized by addition of Al instead of Si in the AOD refining process for deoxidation and desulfurization of molten steel. The reason for deoxidation using Al in the AOD process is to form CaO-Al ₂ O ₃ slag having excellent dewatering ability by performing deoxidation with Al in the AOD process and to increase desulfurizing ability by adding Al with high deoxidization power .

After the deoxidation and desulfurization are completed, the AOD slag can be excluded from the refining furnace after simultaneously feeding molten steel and slag into the ladle. If the slag is not excluded, the S concentration is increased by returning from the slag to the molten steel in the subsequent VOD process. Additional slag preparation can be performed in a vacuum refining facility (VOD) to control inclusions in molten steel. For this purpose, the slag can be prepared by adding additional calcium carbonate in the VOD process and adding calcium oxide (CaO), fluorite (CaF ₂ ) and Al. Thus, the S concentration can be controlled to 10 ppm or less by eliminating the AOD slag and preparing slag in the VOD process.

As a result of the thermodynamic study, in order to suppress the formation of a high melting point spinel (MgAl ₂ O ₄ ) inclusions at the casting temperature (1,460 to 1,480 ° C.), Al ₂ O ₃ , SiO ₂ and MgO content in the inclusions must satisfy the following formula .

(2) 0.079 *% Al ₂ O ₃ + 0.044 *% MgO + 0.12 *% SiO ₂ - 3.64 <1.0

The inclusions after VOD scouring originate from the fact that the VOD scouring slag is suspended in the molten steel, and thereafter the reaction between the deoxidized element and oxygen is added due to the falling of the molten steel temperature, thereby changing the composition. Accordingly, the concentration of Al ₂ O ₃ in the inclusions is inversely proportional to the basicity (CaO / Al ₂ O ₃ ) of the VOD slag.

Also, inclusions formed from the suspended VOD slag temperature drops during thermodynamics with a decrease in the equilibrium dissolved oxygen due to the equilibrium of the inclusions to form an additional Al ₂ O ₃ through the Al and reaction of the molten steel, Al ₂ O ₃ concentration over time Is increased.

In the VOD process, the MgO content in the slag has a constant range of 8 to 12% due to the reaction between the slag and the slag line refractory of the ladle, and the SiO ₂ concentration in the slag has a constant range of 3 to 8% The factors influencing the spinel formation of inclusions are mainly Al ₂ O ₃ concentrations in inclusions.

As mentioned above, since the basicity (B) of the VOD slag and the Al concentration (%) in the molten steel are influenced simultaneously by the Al ₂ O ₃ concentration of the inclusions, the spinel effect index which affects the formation of the high melting point spinel (3) below.

(3) 0.8 + 20 * Al - 0.7 * B

It is aimed to control the basicity of the slag and the Al concentration in the molten steel by controlling the amount of Al input during the addition of slag in the VOD process so as to keep the spinel effect index at 1.0 or less. For this purpose, it is preferable to adjust the basicity of the VOD slag represented by CaO / Al ₂ O ₃ to the range of 1.7 to 2.1. If the basicity is lower than 1.7, desulfurization ability, which is a required characteristic of the steel, may be lowered, so that the S concentration in the molten steel may not be lowered to 10 ppm or less. In addition, when the basicity is higher than 2.1, the solid-phase ratio of the slag increases, and the raw materials to be supplied during the component adjustment may remain on the slag, and the component hit ratio may decrease.

Meanwhile, the stainless steel produced through the refining method of the high-cleaned stainless steel according to the present invention is high-Si austenitic stainless steel, preferably containing 18 to 22% of chromium (Cr), nickel (Ni) : 7 to 15%, silicon (Si): 1.5 to 2.5%, the balance Fe and other unavoidable impurities.

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.

Example

Stainless steel was subjected to Al deoxidation in a VOD refining furnace, and CaO and CaF ₂ were added thereto to prepare VOD slag. Samples were collected to analyze VOD slag components. The molten steel subjected to deoxidation and composition adjustment as described above was manufactured as a cast through a tundish process and a continuous casting process. Specimens were sampled for inclusion analysis and analyzed by SEM-EDS. Table 1 shows the slag basicity (CaO / Al ₂ O ₃ ), Al concentration (%) in molten steel and spinel ratio in inclusions according to the spinel effect index.

division number VOD slag basicity (CaO / Al ₂ O ₃ ) Al in molten steel
(%) Al ₂ O ₃ average concentration (%) in the inclusion inclusion Ratio of casting spinel inclusions (%) Spinel effect indicator Final product machining crack Example One 1.64 0.063 35 13 0.91 X 2 1.75 0.071 36 18 1.00 X 3 2.18 0.052 26 5 0.31 X 4 1.9 0.031 23 2 0.09 X 5 1.85 0.043 27 3 0.37 X 6 1.8 0.062 33 7 0.78 X 7 2.1 0.055 29 9 0.43 X Comparative Example One 1.58 0.084 41 78 1.37 O 2 1.42 0.089 44 85 1.59 O 3 1.72 0.084 41 65 1.28 O 4 1.69 0.095 43 86 1.52 O

* O: End product product cracking

* X: No final product processing crack

In Table 1 and FIG. 4, the spinel ratio in the inclusion according to the spinel effect index is shown. As the spinel effect index represented by the formula (3) increases, the spinel ratio in the inclusions increases. Further, as can be seen from the evaluation results of the final product machining cracks in Table 1, when the Al concentration in the molten steel was 0.071% or less, the spinel effect index was 1.0 or less and stable processing quality was obtained. When the spinel effect index exceeds 1.0, the final product cracks occurred in the form of bundles.

2 shows the concentration (%) of Al ₂ O ₃ in the slip inclusion according to the VOD slag basicity (CaO / Al ₂ O ₃ ) of the examples and comparative examples according to Table 1. As described above, as the basicity of the slag increases, the concentration of Al ₂ O ₃ in inclusions decreases, indicating that the slurry has an inversely proportional relationship.

_{3 shows} the concentration (%) of Al ₂ O ₃ in the slip inclusion depending on the Al concentration (%) in the molten steel of Examples and Comparative Examples according to Table 1. As described above, as the Al content in the molten steel increases, additional Al ₂ O ₃ is formed, and Al ₂ O ₃ And the concentration was increased.

Table 2 shows the desulfurizing activity and activity index according to VOD slag basicity.

number Slag basicity (CaO / Al ₂ O ₃₎ S ≤ 10ppm hit ratio (%) in molten steel Operability Comparative Example 5 1.3 to 1.5 84% Good Comparative Example 6 1.5 to 1.7 96% Good Example 8 1.7-1.9 100% Good Example 9 1.9 to 2.1 100% Good Comparative Example 7 2.1 to 2.3 100% Bad

As shown in Table 2, when the basicity of the slag is low, there is a problem that the desulfurizing ability, which is a property required for improving the hot workability, is lowered. When the basicity of the slag is high, There was a problem. Therefore, it is preferable that the basicity of the VOD slag is controlled in the range of 1.7 to 2.1.

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)

AOD process - A refining method of stainless steel for producing a high Si austenitic stainless steel through a VOD process,
The VOD process satisfies the following formula (1)
(1) 0.8 + 20 * Al - 0.7 * B? 1.0
(Al is the aluminum concentration (%) in the molten steel, and B is the basicity of the VOD slag expressed by CaO / Al ₂ O ₃ )
The aluminum concentration in the molten steel is 0.07% or less,
Wherein the VOD slag has a basicity of 1.6 to 2.2. delete delete The method according to claim 1,
And the basicity (B) of the VOD slag is 1.7 to 2.1. The method according to claim 1,
The AOD slag is removed from the refining furnace of the AOD process,
In the VOD process, slag is prepared and refined,
Wherein the S concentration in the molten steel is 10 ppm or less. The method according to claim 1,
The high Si austenitic stainless steel is characterized by containing 18 to 22% of chromium (Cr), 7 to 15% of nickel (Ni), 1.5 to 2.5% of silicon (Si), balance Fe and other unavoidable impurities Wherein the refining method comprises:

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