KR101268667B1 - Method for refining of high purity stainless steel - Google Patents

Abstract

The present invention relates to a method for refining stainless steel that can reduce surface defects of a product by securing a dehydration reaction while suppressing generation of high melting point slag. In the refining method for producing a high clean stainless steel according to the present invention, the first step of refining molten steel in the AOD refining furnace; Primary tapping the molten steel; Removing slag formed on the molten steel first tapped; Secondary refining of the molten steel from which the slag has been removed; And step of tapping the molten steel secondly, including controlling the tapping temperature in the first tapping step of the molten steel, and controlling the oxidation amount and CaO input amount of Si in the secondary refining step. By this configuration, it is possible to reduce the surface defects of the product to produce high clean steel.

Description

Method for refining of high purity stainless steel

The present invention relates to a method for refining high clean stainless steel, and more particularly, to a method for refining high clean stainless steel that can reduce surface defects.

In general, steelmaking, including the refining process of stainless steel, consists of a furnace-argon oxygen decarburization (AOD) refining, ladle, tundish-continuous casting process. At this time, in the electric furnace, the scrap is melted, and in the refining furnace, molten steel containing impurities from the electric furnace is charged and refined.

In the refining furnace, in addition to removing carbon, impurities such as silicon, phosphorus and sulfur are also removed. In other words, in the refining furnace, decarburization and oxidation are performed by blowing oxygen at high pressure to remove impurities, and Si and Ar gases, which are deoxidizers, are supplied to reduce impurities and control as target components.

At this time, quicklime is added as a deliming agent, and quicklime reacts with SiO ₂ which is an oxide of Fe-Si added for reduction to generate CaO—SiO ₂ compounds. The melting point of the slag is high together with a high basicity, which causes a scratch on the surface during the rolling process.

Therefore, the production of high clean steel should be controlled with a low base to prevent surface defects. However, if the low base is controlled, the desulfurization reaction may be weakened and the final product may not be used as a product because the final product may be out of the target ingredient range. Accordingly, there is a need for a method for securing a degasser while lowering the melting point of slag.

The present invention is to control the tapping temperature, SiO ₂ generation criteria and the basicity rise portion in the AOD refining furnace to ensure the deflow reaction while preventing the generation of high melting point slag, refinement of high-purity stainless steel that can reduce the surface defects of the product It is an object to provide a method.

In the refining method of high-clean stainless steel according to the present invention, the first step of refining molten steel in the AOD refining furnace; Primary tapping the molten steel; Removing slag formed on the molten steel first tapped; Secondary refining of the molten steel from which the slag has been removed; And step of tapping the molten steel secondly, including controlling the tapping temperature in the first tapping step of the molten steel, and controlling the oxidation amount and CaO input amount of Si in the secondary refining step.

At this time, the tapping temperature in the first tapping step of the molten steel may be 1600 ℃ to 1670 ℃.

In addition, the amount of oxidation of Si in the secondary refining step may be 0.55% to 0.75%.

In addition, the final target base of the slag in the secondary refining step may be in the range of 1.0 ~ 1.3.

In addition, in the secondary refining step, the CaO amount may be added by calculating the case where the basicity is in the range of 0.7 to 1.0.

According to the present invention, by preventing the generation of high melting point slag, while ensuring the deflow reaction, it is possible to reduce the surface defects of the product to produce high clean steel.

1 is a view showing a manufacturing process of high-clean stainless steel according to the present invention.
2 is a graph showing the correlation between the primary refining basicity and the dehydration reaction.
3 is a graph showing the correlation between the secondary refining basicity and the melting point of slag.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to embodiments of the present invention and other details necessary for those skilled in the art to understand the present invention with reference to the accompanying drawings. However, the present invention may be embodied in various different forms within the scope of the claims, and thus the embodiments described below are merely exemplary, regardless of expression.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. In addition, the thickness or size of each layer in the drawings may be exaggerated for convenience and clarity and may be different from the actual layer thickness or size.

1 is a view showing a manufacturing process of high-clean stainless steel according to the present invention.

Referring to Figure 1, the production of high-purity stainless steel according to the present invention is produced by slab through molten steel after ladle, tundish after two AOD refining.

In general steel manufacturing, the molten steel is finished in one refining process in the refining furnace. However, in order to manufacture high-purity steel, a high base is also performed in the first refining process of the refining furnace and undergoes slag removal. Later in the secondary refining process, the low base is also subjected to a complex process of lowering the slag melting point.

That is, looking at the refining process of high-purity stainless steel, first refine the molten steel in the AOD refining furnace, and then make the first tap. Then, the slag formed on the first molten steel is removed. Thereafter, the molten steel from which slag is removed is refined for the second time and then the second steel tapping is performed.

As such, in the conventional method of refining high-purity stainless steel, there is no temperature control standard when the first tapping is completed after completion of the first high base refining process. As a result, there is a problem that less SiO ₂ may be generated. In addition, there is no standard for generating slag for purifying impurities during the second refining process and a standard for generating SiO ₂ to satisfy the basicity criteria. In addition, there is no control criterion for the increase in the basicity according to the increase in the CaO content due to contamination during the basicity calculation process for controlling the basicity in the secondary refining process of the refining furnace.

Here, looking at the process in the AOD refining furnace, the molten metal prepared by dissolving scrap and raw materials in the electric furnace in a ladle is transferred to the refining furnace. Thereafter, the decarburization step is carried out by oxygen blowing, a deoxidation step of removing oxygen from slag and steel by adding a deoxidizer, and a flux for securing degasser and slag fluidity for degassing are added to remove sulfur in the steel.

At this time, it is easy to increase the CaO content to improve the desulfurization efficiency, but the melting point of the slag also increases as the CaO content increases. As a result, not only the liquid fraction which contributes to desulfurization is reduced, but also the slag fluidity is deteriorated, so that the dewatering efficiency is reduced.

Accordingly, in the manufacture of high-clean stainless steel, it is necessary to consider to satisfy both the degassing efficiency and the low melting point slag at the same time.

Therefore, in order to produce high-purity stainless steel according to the present invention, two refining operations are performed in an AOD refining furnace, controlling the tapping temperature of the molten steel after the first refining, and controlling the oxidation amount and CaO input of Si during the second refining. do.

First, although the tapping temperature of the molten steel after the first refining was conventionally 1680 ° C, in the present invention, the temperature is controlled to 1600 ° C to 1670 ° C.

If the tapping temperature of the molten steel after the primary refining is less than 1600 ℃, there is a problem that the temperature is too low, the amount of SiO ₂ generated is excessive. When the tapping temperature of the molten steel exceeds 1670 ° C after the first refining, the temperature during the second refining process after slag removal is too high to reduce the amount of SiO ₂ generated, or the amount of coolant or CaO is increased, making it difficult to control the basicity. do.

In addition, the amount of Si oxidation may be controlled to 0.55% to 0.75% in the secondary refining step.

This is to set the SiO ₂ generation criteria for satisfying the slag generation amount of impurity purification slag or more and the basicity of 1.0 ~ 1.3. Here, Si is a temperature increasing agent, is introduced for reduction during refining, and after the completion of the secondary refining, the Si component in the steel is oxidized in order to control the slag to a low base and generate heat. At this time, by controlling the oxidation amount of Si to 0.55% to 0.75%, the basicity (CaO / SiO ₂ ) can be controlled in the range of 1.0 to 1.3.

In addition, in the second refining step, the CaO input amount can be calculated by setting the target base degree in the range of 0.7 to 1.0. In other words, the basicity after the second refining is in the range of 1.0 to 1.3, but the target basicity is lowered to the range of 0.7 to 1.0 in consideration of the increase in basicity caused by the increase of CaO due to contamination during refining.

Contaminated CaO may be generated during refining. First, it may be generated by adding CaO activator, fluorite (CaO ₂ ). In addition, contaminated CaO may be generated due to the influence of CaO eluted from refining furnace refractory dolonite and MgO-CaO.

In addition, since the contaminated CaO causes an increase in basicity (CaO / SiO ₂ ), it is necessary to lower the target basicity when calculating the CaO input amount for controlling the low basicity (1.0 to 1.3). That is, after the second refining, the target base salt of slag is in the range of 1.0 to 1.3, but the target base salt in calculating CaO input can be set in the range of 0.7 to 1.0.

As a result, the surface quality of the product may be improved by suppressing the generation of high melting point slag, which causes surface defects in manufacturing high-quality high-clean stainless steel.

2 is a graph showing the correlation between the primary refining basicity and the deflow reaction.

Referring to Figure 2, it can be seen the correlation between the basicity of the slag and the deflow reaction. In the first refining process, quicklime is added as quicklime (CaO), and the quicklime reacts with SiO ₂ , which is an oxide of Fe-Si, which is added for reduction, to generate CaO-SiO ₂ -based slag. Here, CaO / SiO ₂ is referred to as basicity, and when the basicity is high, the melting point of the slag is increased, which causes scratches on the surface of the product after the rolling process.

Therefore, in the manufacture of high-quality high-clean steel, the slag should be controlled with a low base to prevent surface defects. However, controlling the slag to a low base also weakens the degassing reaction, making it difficult to manufacture the product in the target component range.

Therefore, in the first refining of the present invention, the basicity is set relatively high at 1.8 to 2.0, and the dehydration reaction is performed. As shown in Figure 2, it can be seen that the deflow reaction proceeded smoothly in the 1.8 ~ 2.0 range of the primary refining basicity. That is, it can be seen that more sulfur is removed than the second refining basicity section of the range 1.0-1.3.

3 is a graph showing the correlation between the secondary refining basicity and the melting point of slag.

Referring to FIG. 3, the slag melting point in the range of 1.0 to 1.3, which is the second refining titration range, is lowest. That is, the melting point according to the slag basicity is v-shaped, the melting point decreases as the basicity gradually increases, and the basicity starts to increase again in the range of 1.0 to 1.3. As such, the slag melting point has the lowest basicity in the range of 1.0 to 1.3, and the slag of low melting point can be obtained by controlling the secondary refining basicity in the range of 1.0 to 1.3.

As can be seen in Figures 2 and 3, in the present invention, in order to ensure the deflow reaction while lowering the melting point of the slag, refining is performed twice. At this time, in the first refining process, it is possible to secure an appropriate deflow reaction at a relatively high basicity (1.8 to 2.0), and in the second refining process, the melting point of slag can be lowered at a low basicity (1.0 to 1.3).

Hereinafter, the present invention will be described through examples.

(Example)

In the manufacturing process of STS 304 steel through 1st furnace AOD refining, 2nd AOD refining, ladle-continuous casting, scrap, ferro chromium (Fe-Cr) is used as raw material and dissolved in electric furnace, and oxygen-Ar in AOD refining furnace Two times decarburization refining was performed using the mixed gas. After decarburization and refining, quicklime and fluorspar were added together with Si in order to reduce and recover the oxidized chromium, and argon was blown into the ladle after reduction refining. In the ladle, final components and temperature adjustment were performed while stirring with Ar gas. Thereafter, the casting was cast into a cast steel having a width of 1200 mm and a thickness of 200 mm by continuous casting, and the cast steel was manufactured into a cold rolled steel sheet having a diameter of 0.6 mm through a hot rolling process and a cold rolling process.

Table 1 shows the components of the STS 304 steel used in the examples.

C Si Mn P S Cr Ni N 0.055 0.40 1.10 0.033 0.0060 18.25 8.04 0.035

In the manufacture of existing high-purity steel, there was no temperature control standard when tapping after primary refining, and there was no SiO ₂ generation standard to meet slag generation criteria and basicity criteria (1.0 ~ 1.3) during the secondary refining process. In addition, there was no control criterion for the increase in basicity due to the increase of CaO due to contamination during the basicity calculation process for the basicity control for the secondary refining process.

However, when manufacturing the high clean steel according to the present invention having the components as shown in Table 1, the tapping temperature of the molten steel after the primary refining is controlled, and the amount of oxidation and CaO of Si during the second refining is controlled. As a result, it is possible to obtain high-purity steel that does not cause surface defects even after the rolling process in manufacturing stainless steel.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention.

The scope of the present invention is defined by the following claims. The scope of the present invention is not limited to the description of the specification, and all variations and modifications falling within the scope of the claims are included in the scope of the present invention.

Claims (5)

In the refining method for producing high clean stainless steel,
Primary refining of the molten steel in an AOD refining furnace;
Primary tapping the molten steel;
Removing slag formed on the molten steel first tapped;
Secondary refining of the molten steel from which the slag has been removed; And
Including the second step of tapping the molten steel; including,
Controlling the tapping temperature in the primary tapping step of the molten steel, controlling the oxidation amount and CaO input amount of Si in the secondary refining step,
In the first tapping step of the molten steel tapping temperature is 1600 ℃ to 1670 ℃ refining method of high clean stainless steel. delete The method of claim 1,
Si oxidation amount in the secondary refining step is 0.55% ~ 0.75% refining method of high clean stainless steel. The method of claim 1,
The final target base of the slag in the secondary refining step is a method of refining high-purity stainless steel in the range of 1.0 ~ 1.3. 5. The method of claim 4,
CaO injection in the secondary refining step is a method of refining high-purity stainless steel to calculate the case where the basicity is in the range of 0.7 ~ 1.0.

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