KR101758499B1 - Aod refining method of high-copper stainless molten steel - Google Patents

Abstract

According to an aspect of the present invention, there is provided a method of manufacturing an austenitic stainless steel comprising: preparing an austenitic stainless steel having an Cu content of 2.5% or more in an electric furnace; Adjusting the carbon concentration of the molten steel to 2.0 to 2.5 wt% and introducing the molten steel into an AOD (Argon Oxygen Decarburization) refining furnace; And injecting oxygen (O ₂ ) and argon (Ar) into the molten steel and decarburizing the molten steel; And a reducing-desulfurizing step of blowing argon (Ar) into the decarburized molten steel to refine the AOD of the high-Cu-containing stainless steel molten steel.

Description

AOD REFINING METHOD OF HIGH-COPPER STAINLESS MOLTEN STEEL "

The present invention relates to an AOD refining method of high Cu-containing stainless steel molten steel.

Since stainless steel is manufactured by strip casting method, it has an advantage that the manufacturing cost is reduced as compared with the slab-making method, and it has an advantage of suppressing the precipitation phase due to rapid solidification and also having excellent internal quality of the slab by micro- As a result, the demand is increasing.

In recent years, high-Cu-containing stainless steels having improved yieldability and improved formability have been developed by incorporating high Cu into stainless steels. When the yield strength of stainless steel is reduced, springback phenomenon is reduced and workability such as bending is increased, which is an advantage to replace the use of Cu material piping in electronic products such as air conditioner piping.

On the other hand, in order to improve the workability of the final product, stainless steel species should be managed with low carbon (C) and nitrogen (N) in the molten steel. By controlling the carbon (C) and nitrogen (N) at a low level, the yield strength of the stainless steel can be reduced and the formability can be improved.

In the case of ferritic stainless steels passing through VOC (Vaccum Oxygen Decarburization), since the atmosphere is controlled by a low partial pressure by vacuum equipment, decarburization efficiency by O ₂ , Ar blowing is improved, and atmospheric atmosphere cutoff and nitrogen control are possible. Therefore, carbon and nitrogen content in steel can be minimized in ferritic stainless steels.

On the other hand, the austenitic stainless steels passing only through AOD (Argon Oxygen Decarburization) have a problem that it is difficult to reduce carbon and nitrogen.

Therefore, in order to further lower the yield strength and improve the formability of the high Cu-containing stainless steel, there is a need to develop an AOD refining method for reducing carbon and nitrogen.

Patent Document: Korean Patent Application No. 2009-0128466

One aspect of the present invention is to provide an AOD refining method of high Cu-containing stainless steel molten steel. And more particularly to an AOD refining method capable of reducing carbon and nitrogen of austenitic stainless steel by AOD refining.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

According to an aspect of the present invention, there is provided a method of manufacturing an austenitic stainless steel comprising: preparing an austenitic stainless steel having an Cu content of 2.5% or more in an electric furnace; Adjusting the carbon concentration of the molten steel to 2.0 to 2.5 wt% and introducing the molten steel into an AOD (Argon Oxygen Decarburization) refining furnace; And injecting oxygen (O ₂ ) and argon (Ar) into the molten steel and decarburizing the molten steel; And a reducing-desulfurizing step of blowing argon (Ar) into the decarburized molten steel to refine the AOD of the high-Cu-containing stainless steel molten steel.

At this time, the reduction-denitrification can be performed under the condition that the flow rate of Ar is 50 to 55 Nm ³ / min.

Further, the decarburization step may be performed by gradually decreasing the flow rate of the oxygen and gradually increasing the flow rate of the argon.

On the other hand, the nitrogen concentration of the molten steel after the reduction-desulfurization step may be 75 ppm or less.

The composition of the molten steel after the reduction-desulfurization step is 0.003 to 0.01% of C, 0.2 to 0.7% of Si, 1.0 to 5.0% of Mn, 2.5 to 8.0% of Cu, 0.03% or less of P, S: not more than 0.02%, Cr: 16 to 18%, Ni: 7 to 9%, Mo: 0.001 to 0.200%, N: not more than 75 ppm by weight, and other Fe and other unavoidable impurities.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof can be understood in more detail with reference to the following specific embodiments.

According to the present invention, it is possible to provide an AOD refining method capable of reducing carbon and nitrogen in austenitic stainless steel by AOD refining.

1 is a schematic view showing a general stainless steel manufacturing process.
2 is a graph showing the change in yield strength according to the content of C + N in austenitic stainless steel (304J1).
3 is a graph showing a change in the critical carbon concentration according to the partial pressure of CO in a stainless steel having a Cr content of 18%.
4 is a schematic diagram showing a general denitrification reaction mechanism.
FIG. 5 is a graph comparing the concentration of nitrogen in the AOD of the AOD according to the results of applying the present invention and the conventional method to actual refining.
FIG. 6 is a graph comparing the carbon concentration of the AODs emitted by the main difference according to the results of applying the present invention and the conventional method to actual refining.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

The present inventors have recognized that there is a problem that it is difficult to reduce carbon and nitrogen in an austenitic stainless steel passing through only AOD (Argon Oxygen Decarburization), and have studied to solve this problem.

As a result, it has been confirmed that the carbon and nitrogen concentrations in the molten steel can be efficiently reduced by appropriately controlling the conditions of the AOD process, and the present invention has been accomplished.

FIG. 1 is a schematic diagram showing a general stainless steel manufacturing process. A molten metal produced by melting in an electric arc furnace (EAF), that is, an electric furnace melt is introduced into the loading ladle, the loading ladle is tilted, Remove some of the slag that is floating. Then, the remaining furnace slag is removed from the discharge site, and the furnace furnace is charged into the refining furnace. In order to remove carbon from the argon oxygen decarburization (AOD), the molten steel is decarburized by blowing oxygen and argon gas into the molten steel, and at the same time, the chromium and iron oxides produced together are reduced. In the ladle treatment (LT) process, B / B (Bottom Bubbling) is performed to adjust fine components, homogenize the molten steel and improve the quality of the molten steel.

Carbon (C) and nitrogen (N) in the molten steel must be managed at a low level in order to improve the workability of the final product. When the yield strength of stainless steel is reduced, the springback phenomenon is reduced and the workability such as bending is increased. Thus, there is an advantage of replacing various applications in electronic products such as air conditioner piping. Especially, the influence of nitrogen (N) on the softening of the material is greater, and therefore, management of nitrogen is important.

FIG. 2 shows the change in yield strength according to the C + N content in austenitic stainless steel (304J1). When the C + N content is reduced by 100 ppm, the yield strength tends to decrease by 6 ~ 7 MPa. When the yield strength is maintained at about 200 MPa or less, the material is further softened and can be applied to a portion requiring high-smoothing properties such as air-conditioner piping. In particular, carbon needs to be introduced after sufficient decarburization in the AOD process, after nitrogen has been sufficiently removed through the process of promoting denitrification and adsorption during the process.

KA4 to KA7 represent sample numbers. KA6 and KA7 are values obtained through AOD (Argon Oxygen Decarburization) process, and KA4 and KA5 are values obtained through VAC (Vaccum Oxygen Decarburization) process. It can be seen that the AOD process has a limit in reducing the concentration of C + N.

Hereinafter, an AOD refining method for high Cu-containing stainless steel according to one aspect of the present invention will be described in detail.

According to an aspect of the present invention, there is provided an AOD refining method for a high-Cu-containing stainless steel molten steel, comprising: preparing an austenitic stainless steel molten steel having a Cu content of 2.5% or more in an electric furnace; Adjusting the carbon concentration of the molten steel to 2.0 to 2.5 wt% and introducing the molten steel into an AOD (Argon Oxygen Decarburization) refining furnace; (O2) and argon (Ar) are blown into the molten steel and then decarburized; And reducing-desulfurizing the decarbonized molten steel by blowing argon (Ar).

Molten steel  Injection step

An austenitic stainless steel having a Cu content of 2.5% or more in an electric furnace is prepared, the carbon concentration of the molten steel is adjusted to 2.0 to 2.5 wt%, and the mixture is introduced into an AOD (Argon Oxygen Decarburization) refining furnace.

When an austenitic stainless steel having a Cu content of 2.5% or more is used, a high-Cu-containing stainless steel having a low yield strength and excellent formability can be produced. By applying the AOD refining method of the present invention to the molten steel to lower carbon (C) and nitrogen (N), the yield strength of the final product can be further reduced and the formability can be further improved.

If the Cu content in the molten steel is less than 2.5 wt%, the effect of improving the formability due to the addition of Cu may not be sufficient. On the other hand, the upper limit of the Cu content is not particularly limited, but if it is contained in an amount exceeding 8% by weight, Cu segregation is caused in the casting, and the segregation part is present almost in the Cu component alone. To cause a cracking crack in the cast steel, so that the upper limit may be 8 wt%.

In order to remove nitrogen dissolved in the steel during refining of the AOD, the ability to discharge nitrogen gas to the top of the ladle must be improved. A typical denitrification reaction mechanism can be represented as shown in FIG.

1) Each of the nitrogen atoms in the liquid phase moves in the unspecific direction, but as a whole, it moves to the interface. 2) Nitrogen atoms moved to the interface are adsorbed to the interface. 3) Nitrogen atoms adsorbed at the interface collide with each other. 4) The collided nitrogen atoms become nitrogen molecules (N ₂ ) and move from the liquid-phase interface to the gas-phase interface. 5) N _{2, which} has moved to the interface of the gas layer, becomes a gas and moves to the vapor phase.

Is generally the 4) step is the rate limiting step is the nitrogen molecule (N ₂₎ is produced in the denitrification reaction, especially in a low nitrogen concentration, so the speed is the rate limiting step of nitrogen atoms move to the reaction interface to a molecule generated CO, CO _2, or Ar bubbles act to move nitrogen atoms to the reaction interface, thereby promoting the generation of molecules.

In order to allow CO, CO _2, or Ar bubbles to transfer nitrogen atoms to the reaction interface, the amount of CO (g) generated by blowing oxygen through the tyuere tube at the bottom of the top of the decontamination process should be increased.

The CO gas generated by the chemical reaction in the steel floats at the interface, causing the nitrogen atoms in the molten steel to be adsorbed to the CO gas and causing the gas flow to flow to the interface, resulting in the transfer of nitrogen atoms to the interfacial layer, . For this, the carbon concentration of molten steel is adjusted to 2.0 ~ 2.5 wt%, which is higher than the existing value, so that the amount of CO gas can be increased by injecting into the AOD (Argon Oxygen Decarburization) refining furnace.

If the carbon concentration is less than 2.0 wt%, the amount of CO gas generated is insufficient, and the nitrogen reducing effect may be insufficient. On the other hand, when the carbon concentration is more than 2.5% by weight, the carbon concentration becomes too high at the time of tapping, thereby increasing the decarbonization time during refining, resulting in an increase in product production cost and a decrease in productivity. In addition, the yield strength of the final product is increased. Therefore, the carbon concentration is preferably 2.0 to 2.5% by weight.

As the carbon concentration in the electric furnace is increased, a large amount of CO gas can be generated as described later, and the initial carbon concentration of the refining furnace is lowered to [C] by O ₂ injected through the upper lance at the top of the refining furnace, Decarburization proceeds to the critical carbon concentration in each step by direct decarburization due to direct decarburization and Cr ₂ O ₃ + 3 [C] = 2 [Cr] + 3CO (g) When the carbon concentration reaches the decarbonization efficiency, the oxygen concentration is adjusted to increase the decarburization efficiency to lower the carbon concentration to the next level, gradually decreasing the carbon concentration. , The flow rate of oxygen is gradually decreased, and the flow rate of argon is gradually increased to progress decarburization, thereby effectively removing carbon while suppressing the oxidation of chromium. Therefore, It is possible to lower the dose.

Decanter  step

Oxygen (O ₂ ) and argon (Ar) are blown into the molten steel and decarburized.

In stainless steel, decarburization refining means decarburization effectively while suppressing the oxidation of Cr, which has a large oxidizing power and is expensive. Generally, in the decarburization reaction of direct molten steel containing [C] + [O] = CO (g) and molten steel containing Cr, the oxygen introduced into the molten steel first oxidizes Cr, and decarburization proceeds with the oxide as the medium . The specific reaction proceeds according to the following relationship.

[C] + [O] = CO (g)

2 [Cr] + 3 [O ] = Cr 2 O 3

Cr ₂ O ₃ + 3 [C] = 2 [Cr] + 3CO (g)

1OG K = -40,990 / T + 25.83

(Where a _i is the activity of the i component, K is the equilibrium constant, and Pco is the partial pressure of the CO gas).

At this time, the decarburization step can be performed by gradually decreasing the flow rate of the oxygen and gradually increasing the flow rate of the argon.

Referring to FIG. 3, which is a graph showing the change in the critical carbon concentration according to the partial pressure of CO in stainless steel having a Cr content of 18%, the critical carbon concentration is expressed as a function of the CO partial pressure, the activity (Cr concentration) . As can be seen from FIG. 3, as the temperature is higher, the partial pressure of CO is lower, and the lower the chromium concentration, the lower the critical carbon concentration.

That is, as the carbon is removed from the molten steel, the equilibrium chromium concentration decreases and chromium in the molten steel can not exist above the equilibrium concentration, so the chromium is oxidized at a high rate. When the temperature is raised or the CO partial pressure is decreased, the carbon concentration at the same chromium concentration is lowered, so that the carbon can be effectively removed while suppressing the oxidation of chromium.

Therefore, the method of lowering the CO partial pressure maximizes the efficiency of the CO gas reaction by appropriately controlling the amount of O 2 reacting with the carbon through the proper control of the O ₂ and Ar gases, .

The flow rate of oxygen in the decarburization step is gradually reduced, and the flow rate of argon is gradually increased to carry out the decarburization, whereby the CO partial pressure can be effectively lowered. As a result, the carbon concentration which equilibrates at the same chromium concentration is lowered, and carbon can be effectively removed while suppressing the oxidation of chromium.

For example, carbon can be effectively removed while suppressing the oxidation of chromium by progressively reducing the flow rate of oxygen in the first to fifth stages of the decarburization step and gradually increasing the flow rate of argon, as described below.

First stage of blowing: O ₂ : Ar = 150 Nm ³ / min: 20 Nm ³ / min, critical carbon concentration: 0.35 wt%

Second stage of blowing O ₂ : Ar = 60 Nm ³ / min: 20 Nm ³ / min, critical carbon concentration: 0.20 wt%

Blowing step 3 ^{_{O 2: Ar = 45Nm 3 /}} min: 45Nm 3 / min, the critical carbon concentration of 0.10% by weight

Blowing step 4 ^{_{O 2: Ar = 20Nm 3 /}} min: 60Nm 3 / min, the critical carbon concentration of 0.05% by weight

5 steps of blowing O ₂ : Ar = 12 Nm ³ / min: 48 Nm ³ / min, critical carbon concentration: 0.01 wt%

In addition, the amount of inert gas must be increased to lower the Pco according to the [C]% during blowing, although inert gas may be used, but N ₂ may be used as long as there is no defect in quality. However, in the present invention, since the yield strength and the moldability and workability must be decreased, C + N should be reduced as much as possible, so it is preferable to use Ar as the inert gas.

On the other hand, the initial flow rate of oxygen is more preferably 140 to 170 Nm ³ / min.

If the initial flow rate of the oxygen is less than 140 Nm ³ / min, the decarburization efficiency may deteriorate. If the initial flow rate exceeds 170 Nm ³ / min, oxygen may remain in the molten steel to generate oxides (inclusions).

restoration- Desorption  step

Argon (Ar) is blown into the decarburized molten steel to perform reduction-degassing. And to reduce the chromium and iron oxides generated in the decarburization step again.

At this time, the reduction-denitrification can be performed under the condition that the flow rate of Ar is 50 to 55 Nm ³ / min. The Ar gas in the reduction-desulfurization also serves to transfer the nitrogen atom to the reaction interface as described above.

By controlling the flow rate of Ar to 50 to 55 Nm ³ / min, which is higher than the existing flow rate, nitrogen atoms are adsorbed on the surface of the Ar bubbles, which is an inert gas, to move the nitrogen atoms to the reaction interface, .

When the flow rate of Ar is less than 50 Nm < ³ > / min, the nitrogen reducing effect is insufficient.

As described above, the Ar injection is a principle in which the nitrogen molecules contained in the molten steel are easily adsorbed to the Ar bubbles while the AOD is injected through the pipe, and the nitrogen is removed by the upward flow. When the Ar flow rate is excessively high Since the flow of bubbles coming from the bottom is high, the molten steel and the slag covering the molten steel may be exposed to the atmosphere to cause the adsorption phenomenon that the nitrogen of the atmosphere is dissolved in the steel. Therefore, the upper limit of the Ar flow rate is preferably 55 Nm ³ / min.

Further, the nitrogen concentration of the molten steel after the reduction-desulfurization step may be 75 ppm or less. More preferably 70 ppm or less, and even more preferably 65 ppm or less.

The composition of the molten steel after the reduction-desulfurization step is 0.003 to 0.01% of C, 0.2 to 0.7% of Si, 1.0 to 5.0% of Mn, 2.5 to 8.0% of Cu, 0.03% or less of P, S: not more than 0.02%, Cu: 2.5 to 8.0%, Cr: 16 to 18%, Ni: 7 to 9%, Mo: 0.001 to 0.200%, N: not more than 75 ppm by weight, and other Fe and other unavoidable impurities .

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example  One)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.003 to 0.01% of C, 0.2 to 0.7% of Si, 1.0 to 5.0% of Mn, 2.5 to 8.0% of Cu, 0.03% or less of P, AOD refining was carried out to obtain molten steel containing 7 to 9% of Ni, 0.001 to 0.200% of Mo, 75 wt% or less of N, and the balance of Fe and other unavoidable impurities. The austenitic stainless steels having a Cu content of 2.5% or more were subjected to AOD refining by applying a carbon concentration and a reducing-desulfurizing Ar flow rate.

The ADO nitrogen and carbon concentrations were measured and reported in Table 1 below.

division AOD input molten steel carbon concentration
(weight%) Reduction - Ar flow rate of desulfurization (Nm ³ / min) AOD Nitrogen concentration
(Ppm by weight) AOD carbon concentration
(Ppm by weight) Comparative Example 1 1.2 45 83 68 Comparative Example 2 1.4 45 82 65 Inventory 1 2.0 45 71 55 Inventory 2 2.5 45 66 52 Inventory 3 2.5 55 53 49

It can be confirmed that Inventive Examples 1 to 3 satisfying the control conditions of the present invention have a lower nitrogen concentration and carbon concentration in the AOD than in Comparative Examples 1 and 2. [

( Example  2)

In order to compare the method of the present invention with the existing method, the results of applying this method to actual refining are shown in FIG. 5 and FIG. In the week 1 to 7, the carbon concentration of the molten steel was set to 1.4 wt%, the flow rate of the reducing-desulfurating Ar was set to 45 Nm ³ / min, the carbon concentration was set to 2.5 wt% And the flow rate was 55 Nm ³ / min.

The nitrogen concentration and the carbon concentration of 70 ppm or more and the carbon concentration of 60 ppm or more were shown in the first to seventh parking lots to which the conventional refining method was applied.

( Example  3)

Further, in order to confirm the change with the initial O 2 concentration in the decarburization step, further experiments were conducted under the conditions shown in Table 2 below, and the carbon concentration in the first stage of AOD blowing is shown in Table 2 below. However, the initial carbon concentration of the AOD feed molten steel was made equal to 2.0 wt%.

division Stage 1
AOD molten steel injection
O ₂ flow rate (Nm ³ / min) Stage 1
AOD molten steel injection
Ar flow rate (Nm ³ / min) Step 1 of AOD
Carbon concentration
(weight%) Comparative Example 1 130 10 0.41 Comparative Example 2 135 10 0.40 Inventory 1 140 10 0.38 Inventory 2 150 10 0.36 Inventory 3 150 20 0.35

As can be seen in Table 2, when the initial O ₂ flow rate in the decarburization step, that is, the first-stage AOD molten steel injection oxygen flow rate is 140 Nm ³ / min or more, the decarburization effect is further increased.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. It will be possible.

Claims (6)

Preparing an austenitic stainless steel having a Cu content of 2.5% or more in an electric furnace;
Adjusting the carbon concentration of the molten steel to 2.0 to 2.5 wt% and introducing the molten steel into an AOD (Argon Oxygen Decarburization) refining furnace;
And injecting oxygen (O ₂ ) and argon (Ar) into the molten steel and decarburizing the molten steel; And
And a reducing-desulfurizing step of blowing argon (Ar) into the decarbonized molten steel at a flow rate of 50 to 55 Nm ³ / min,
Wherein the decarburization step is performed by gradually decreasing the flow rate of oxygen and gradually increasing the flow rate of argon, wherein the initial flow rate of oxygen is 140 to 170 Nm < ³ > / min,
The composition of the molten steel after the reduction-desulfurization step is 0.003 to 0.01% of C, 0.2 to 0.7% of Si, 1.0 to 5.0% of Mn, 2.5 to 8.0% of Cu, 0.03% or less of P, 0.02% or less of Cr, 16 to 18% of Cr, 7 to 9% of Ni, 0.001 to 0.200% of Mo, 75 wt% or less of N and the balance of Fe and other unavoidable impurities. AOD refining method.
delete delete delete delete delete

Images