KR100388239B1 - Method for producing low sulfer, low carbon steel using eaf-vtd process - Google Patents

Abstract

According to the present invention, the molten steel produced in the electric furnace is tapped into a ladle, and then the ladle is charged into a vacuum chamber before being sent to the casting operation, and the carbon in the steel is firstly removed by using a vacuum tank degassing apparatus. The present invention relates to a method for producing low sulfur and low carbon steel by an electric furnace-vacuum degassing method for removing sulfur.

The present invention provides a first step of adjusting the content of FeO + MnO present in the slag 18 in the ladle 12 to 6-20% by weight, and the slag 18 formed in the first step is a ladle. (12) Decarburization is performed so that the bottom portion 16a generated by the mixed layer of the gas and molten steel 16 supplied from the bottom becomes 35-80% of the horizontal cross section of the ladle 12, and the bottom portion 16a has an area of When it is 10% or less, deoxidizer and slag 18 are added to the molten steel 16 which has completed the decarburization reaction and the slag 18 is subjected to stirring, and the concentration of FeO + MnO in the slag 18 is stirred. 1.5 wt% or less, characterized in that the third step of desulfurization by controlling the composition of the slag 18 so that the value of the CaO / Al ₂ O ₃ ratio is 1.2-2.5.

Description

Low sulfur, low carbon steel manufacturing method by electric furnace-vacuum degassing method {METHOD FOR PRODUCING LOW SULFER, LOW CARBON STEEL USING EAF-VTD PROCESS}

According to the present invention, the molten steel produced in an electric furnace is tapped into a ladle, and then the ladle is charged into a vacuum tank before being cast into a casting operation, and the carbon in the steel is firstly removed by using a vacuum tank degassing apparatus, and secondly, The present invention relates to a method for producing low sulfur and low carbon steel by an electric furnace-vacuum degassing method for removing sulfur.

In general, the quality, performance and properties of steels depend greatly on the chemical composition of the steel. Steel having a high carbon concentration has a high strength, but is low in ductility and brittle, so it is difficult to be used in applications requiring high workability.

Therefore, low carbon concentration values of 0.015% by weight or less are required for steels requiring high workability.

On the other hand, sulfur in steel causes hot brittleness and forms MnS, which is a non-metallic inclusion, which causes various defects and anisotropy. Thus, sulfur is removed to the lowest possible level, and a concentration of 0.01% by weight or less is usually required for this purpose.

The production of steel materials under a mass production system is produced by molten steel in a molten state, then cast into a predetermined shape to be solidified and then processed through rolling or the like.

As a method of manufacturing molten steel in a molten state, it is roughly classified into a furnace- converter method and an electric furnace method.

Looking at the sulfur concentration in the steel, in the electric furnace method is manufacturing molten steel by melting the scrap metal is not only sensitive to the situation of the price of the product and supply of scrap materials, but also has a problem that the concentration of impurities in the steel depending on the quality of scrap.

In order to control the sulfur concentration in the steel, it is difficult to manufacture low-sulfur steel unless the sulfur concentration contained in the scrap metal is known in advance and only high-quality materials are selected and used.

However, it is difficult to secure a low sulfur concentration of less than 0.01% by electricity alone because it is difficult to select such high quality scrap.

Therefore, in order to manufacture low sulfur steel of 0.01% or less by using molten steel of an electric arc furnace, molten steel extruded from an electric furnace is contacted with slag and treated to absorb sulfur toward the slag.

Many reports and patents have been published for slag with high sulfur absorption ability, and in any case, it is known that the oxidation degree should be low in order to maintain a reducing atmosphere.

However, the degree of oxidation of slag increases as the amount of lower oxides such as iron oxides (FeO, Fe ₂ O _3, etc.) and manganese oxide (MnO) increases. Should be lowered.

In addition, desulfurization is possible to the expected level only if the slag amount is kept above a certain amount.

The carbon concentration in the electric furnace is related to the oxygen concentration, as shown in the following equation, and tends to decrease as the oxygen concentration in the steel increases.

[C] + [O] → CO (g) --------------- (1)

Since the oxygen concentration in the steel can not have more than saturation, there is a limit to the decarburization reaction in the electric furnace.

This reaction takes place at atmospheric pressure and under these conditions the limit of carbon concentration in the steel is known to be less than about 0.015% by weight. Therefore, in order to produce steel lower than this decarburization limit, a separate depressurization treatment must be performed after the electric furnace.

In the pressure reducing equipment, the ladles containing molten steel, such as RH and DH, are in the air and are vacuum-refined by moving into the vacuum tank through the pipe, and the molten steel and the ladles, such as VTD (Vaccum Tank Degassing or VOD), in the vacuum tank. It is divided into charging type and processing the whole under reduced pressure or vacuum condition.

FIG. 1 is a schematic diagram of a vacuum tank degassing apparatus (VTD), in which a porous plug 14 of refractory material is installed at the bottom of a ladle 12 charged in a vacuum tank 10, thereby argon. When the molten steel 16 is agitated by blowing a gas such as nitrogen or the like, decarburization reaction as in formula (1) occurs on the molten steel 16 surface.

At this time, since the pressure in the vacuum tank 10 is less than 1 atm, decarburization may proceed to a lower level in the electric furnace.

However, the VTD process is difficult to determine when decarburization is completed firstly, and secondly, the slag 18 on the molten steel 16 swells with the molten steel 16 and overflows out of the ladle 12 so that subsequent work is impossible. There is a problem.

In addition, when the FeO + MnO in the slag 18 is lowered in order to desulfurize for the purpose of manufacturing low sulfur steel, the molten steel 16 is inevitably deoxidized at the same time, which is why the carbon concentration of 0.015% by weight or less is required. In the vacuum treatment applied in the production of the same steel, there is a problem that the decarburization reaction efficiency is lowered, or it is difficult to achieve a carbon concentration level below a certain level.

In the figure, reference numeral 19 is an argon bubble.

The present invention has been made in order to solve the above problems, by controlling the vacuum treatment and slag composition in a vacuum tank degassing apparatus such as VTD and the like to the molten steel produced in the electric furnace having an electric carbon having 0.015% by weight or less, 0.01% by weight of sulfur The purpose is to provide a low sulfur, low carbon steel manufacturing method by the furnace-vacuum degassing method.

1 is a schematic diagram of a VTD (vacuum tank degassing apparatus),

2 is a graph showing the effect of FeO + MnO on the decarburization rate constant K,

3A is a front schematic view showing the molten steel / slag behavior at the time of blowing low gas;

3B is a top view of FIG. 3A,

4 is a graph showing the relationship between the size of the bottom and the decarburization progress rate,

5 is a graph showing the effect of the sum of iron oxide and manganese oxide in the slag on the desulfurization rate,

6 is a graph showing the relationship between the CaO / Al ₂ O ₃ ratio in the slag and the desulfurization rate.

<Description of Symbols for Main Parts of Drawings>

10: vacuum tank 12: ladle

14: forrus plug 16: molten steel

16a: Bad Butt 18: Slag

In order to achieve the above object, the first step of adjusting the content of FeO + MnO present in the slag in the ladle to 6-20% by weight, and the slag formed in the first step is the gas supplied from the ladle bottom and Decarburizing is performed so that the bottom part formed by the mixed layer of molten steel is 35-80% of the horizontal cross section of the ladle, and when the area of the bottom part is 10% or less, the second step of terminating the decarburization reaction is performed. in the quick lime to a deoxidizer, slag in the molten steel, respectively, and stirred to control the composition of the slag FeO + MnO concentration of 1.5 wt% or less, the value of CaO / Al ₂ O ₃ ratio in the slag such that from 1.2 to 2.5 to carry out desulfurization Provided is a low sulfur, low carbon steel production method by an electric furnace-vacuum degassing method comprising a third step.

Hereinafter, the present invention will be described in detail with reference to Examples.

2 to 6 show one embodiment of a low sulfur, low carbon steel manufacturing method by the electric furnace-vacuum tank degassing method according to the present invention, the low sulfur, low carbon steel manufacturing method according to the slag in the ladle 12 The first step of adjusting the content of FeO + MnO present in (18) to 6-20% by weight, and the slag 18 formed in the first step is supplied with gas and molten steel from the bottom of the ladle 12 Decarburization is carried out so that the bottom part 16a generated by the mixed layer of (16) becomes 35-80% of the horizontal section of the ladle 12, and when the area of the bottom part 16a becomes 10% or less, the decarburization reaction is terminated. Deoxidizer and slag 18 were added to the molten steel 16 after the second step and the second step, and quicklime was added to the slag 18, followed by stirring. The FeO + MnO concentration in the slag 18 was 1.5% by weight or less, and CaO / Al ₂ O. The third step is to control the composition of the slag 18 so that the value of the ₃ ratio is 1.2-2.5, and desulfurization is performed. .

The reason for numerical limitation in the said invention is demonstrated.

If the content of FeO + MnO in the ladle slag is less than 6% by weight, the decarburization rate is lowered. If the content of FeO + MnO is more than 20% by weight, it is not expected to increase the decarburization rate. In order to reduce the concentration of FeO + MnO in the slag for the implementation of the process is disadvantageous, the content of FeO + MnO is limited to 6-20% by weight.

When the lower part of the slab is 35-80% of the horizontal cross section of the ladle, when the slag is less than 35%, the slag overflows and swells, and when the amount exceeds 80%, the amount of gas blown through the ladle bottom is too high. Swelling occurs, so limit to 35-80%.

The decarburization reaction was terminated when the area of the slag was 10% or less because the progress of the decarburization reaction was excellent at 95% or more.

The reason why the FeO + MnO concentration in the slag was 1.5% by weight or less and the value of the CaO / Al ₂ O ₃ ratio was 1.2-2.5 is because a relatively good desulfurization rate of 70% or more in such a composition range can be obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples.

If the FeO + MnO concentration in the slag 18 in the ladle 12 is too low in the first step, as shown in FIG. 2, the decarburization rate is lowered during the decompression treatment, thereby lowering the vacuum treatment efficiency.

The decarburization rate equation is generally expressed as the following equation (2).

[C] = [C] o exp (-Kt) --------------- (2)

Where [C] o is the initial carbon concentration, t is the reaction time, [C] is the carbon concentration at time t, and K is the decarburization rate constant.

The larger the K, the faster the decarburization reaction, and the shorter the treatment time.

In FIG. 2, it can be seen that when the value of FeO + MnO is 6 wt% or less, the K value starts to decrease. Therefore, in order to keep FeO + MnO low within a range where the decarburization rate is not lowered, the lower limit of FeO + MnO is limited to 6% in the first step of the present invention.

On the other hand, when the content of FeO + MnO exceeds 20% by weight it can be seen that as shown in Figure 2 to maintain a horizontal state without increasing the decarburization rate constant K.

This means that an increase in the decarburization rate cannot be expected, and the concentration of FeO + MnO in the slag should be lowered for desulfurization in the third process described later. When the FeO + MnO content exceeds 20% by weight, the FeO + Implementing a process for lowering the concentration of MnO is disadvantageous (see FIG. 5).

Next, the second step of controlling the decarburization reaction and determining the completion time will be described in detail. When the slag 18 is prepared as in the first step, since the slag 18 has a specific gravity smaller than that of the molten steel 16, the slag 18 is positioned above the molten steel 16 as shown in FIG. 3A.

In this state, if the vacuum tank 10 is depressurized, the molten steel 16 and the slag 18 swell in a moment by the gas generated in the molten steel 16 layer, overflowing the ladle 12 and causing damage to the apparatus. Alternatively, the ladle 12 and the bottom of the vacuum tank 10 are fused so that subsequent work is impossible.

Until now, in order to solve such a problem, efforts have been made to keep the gas blowing amount in the molten steel 16 extremely low, to change the composition of the slag 18 or to remove the slag 18 in advance.

However, these efforts have resulted in additional time and subsidiary consumption or lowering the molten steel 16 temperature.

In the present invention, while trying to solve such a problem, when argon or nitrogen gas is introduced through the porous porous plug 14 installed at the bottom of the ladle 12 before the pressure reduction process in the vacuum tank 10 to FIG. As shown, a mixed layer of gas and molten steel 16 pushes up the upper slag 18 to form a molten steel 16a through which the molten steel 16 can directly contact the gas phase in the vacuum tank 10. The pressure in the vacuum tank 10 is lowered while maintaining the bottom portion 16a at 35-80% of the overall cross-sectional distance of the ladle 12, that is, A / (A + B) = 0.35-0.8 in FIG. 3B. It was confirmed that the mask slag 18 and molten steel 16 did not overflow out of the ladle 12.

The present invention intends to use this fact in the decarburization treatment operation, and at this time, as the pressure in the vacuum tank 10 decreases, the turret 16a becomes larger and larger, so the flow rate of the low odor gas must be reduced.

Since the lower portion 16a has a different brightness (brightness) than the portion covered with the slag 18, the size of the lower portion 16a can be measured by using an image analyzer which is widely used industrially.

<Example 1>

Table 1 summarizes whether or not the overflow phenomenon of the slag 18 according to the size of the measured turret 16a. As shown in the table, the overflow phenomenon of the slag 18 did not occur in the size of the bottom part 16a in the range of 35-80%. When the size of the lower part 16a is less than 35%, the slag 18 overflows and swells. When the size of the lower part 16a becomes more than 80%, the amount of gas blown through the bottom of the ladle 12 is increased. It has been observed that the overflow phenomenon due to the swelling of the slag 18 may be somewhat too large.

In addition, in this case, the desulfurization rate is low in the desulfurization process required for the subsequent work, there is a disadvantage that the manufacture of the low sulfur steel pursued by the present invention is not easy.

Table 1 Slag Overflow and Desulfurization Rate in Subsequent Work according to Slender Size

Locust Size (%) Slag overflow occurs Desulfurization rate in subsequent work (%) 20 × - 30 △ 80 35 ○ 60-80 50 ○ 60 80 ○ 75 85 △ 35 90 △ 40

○: No overflow, △: Swell slightly, ×: Overflow

Even if the pressure in the vacuum tank 10 is reduced while adjusting the size of the bottom portion 16a as described above, the size of the bottom portion 16a suddenly decreases after a certain time, and the size of the bottom portion of the ladle 12 is reduced. When the size is less than 10%, it can be determined that the decarburization reaction is completed.

The decarburization reaction can be explained by the above formula (1), and the carbon concentration at the completion of the decarburization reaction depends on the oxygen concentration in the steel ([wt% O]), the pressure in the vacuum chamber (P), the temperature, and the like.

In theory, the carbon concentration in steel at the completion of the decarburization reaction for molten steel of 1,600 ° C is [wt% C] f.

[wt% C] f = 0.0024 × P ÷ [wt% O] --------------- (3)

Is displayed.

Therefore, the decarburization reaction progress rate can be defined as follows.

% Decarburization rate = [wt% C] ÷ [wt% C] f × 100 ----- (4)

Where [wt% C] is the carbon concentration in the steel at any point in time.

The relationship between the size of the bottom part 16a measured during the decarburization reaction and the decarburization reaction progress rate is shown in FIG. 4. As shown in the figure, the decarburization reaction progresses to 95% or more when the size of the bottom portion 16a is 10% or less.

This means that the decarburization reaction is almost complete under the given conditions. Therefore, when the size of the bottom part 16a is 10% or less, it may be determined that the decarburization reaction is completed.

As described above, since the decarburization completion time can be accurately determined, as shown in FIG. 4, the reproducibility of the decarburization operation is improved, and the time required for the decarburization operation can be obtained.

Next, the third step process will be described in detail.

The desulfurization ability of slag depends on its composition, in particular, the lower the concentration of lower oxides such as FeO and MnO, the better, and the better the ratio of calcium oxide and silicon oxide / aluminum oxide.

In order to find out the effective slag conditions for refining molten steel in this slag composition, experiments were conducted at 1,600 ° C. using a vacuum induction furnace.

<Example 2>

First, 30 kg of ultra low carbon steel was dissolved, followed by deoxidation of molten steel with 0.15 kg of aluminum and a slag preparation such as quicklime. After confirming that the slag aid was completely melted, 0.15 kg of aluminum was further added and stirred to sample molten steel at a time point of 20 minutes, and the desulfurization rate was examined according to the following Equation (5).

Desulfurization rate (%) = (concentration before reaction-concentration after reaction) ÷ (concentration before reaction) × 100 ----- (5)

6 and 7 show the relationship between the desulfurization rate and FeO + MnO and CaO / Al ₂ O ₃ in the slag.

As shown in the figure, it can be seen that the desulfurization rate of 70% or more is obtained at FeO + MnO concentration of 1,5 wt% or less and CaO / Al ₂ O ₃ ratio of 1.2-2.5.

That is, if the sulfur concentration in the molten steel before the treatment was 0.03% or less, it was possible to secure a sulfur concentration of 0.01% or less after the treatment.

As described above, according to the present invention by using the furnace-VTD process to prevent the slag overflow phenomenon during the VTD treatment, accurately determine the decarburization completion point while reducing the treatment time, the carbon concentration in the steel is 0.015% by weight or less, There is an effect that can produce a steel having a sulfur concentration of 0.01% by weight or less.

Claims (1)

A first step of adjusting the content of FeO + MnO present in the slag 18 in the ladle 12 to 6-20% by weight, In the first step, the slag 18 is formed by the mixed layer of the gas and molten steel 16 supplied from the bottom of the ladle 12, and the bottom part 16a is 35-80% of the horizontal cross section of the ladle 12. Decarburization is carried out so that the second step of terminating the decarburization reaction when the area of the turret 16a is 10% or less; The deoxidizer and slag 18 were added to the molten steel 16 after the second step, and quicklime was added to the slag 18, followed by stirring. The FeO + MnO concentration in the slag 18 was 1.5 wt% or less, and the value of the CaO / Al ₂ O ₃ ratio was increased. A low-sulfur, low-carbon steel production method according to the furnace-vacuum tank degassing method, characterized by comprising a third step of desulfurization by controlling the composition of the slag (18) to be 1.2-2.5.