JP3327062B2 - Melting method of ultra-low carbon / ultra low sulfur steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ultra-low carbon and ultra-low sulfur steel having low inclusions and high cleanliness using an RH vacuum degassing system.

[0002]

2. Description of the Related Art In recent years, demands for improving the electromagnetic properties of silicon steel sheets have become strict, and ultra-low-carbon and ultra-low sulfur steels with few inclusions and high cleanliness (this is referred to as ultra-low carbon / ultra-low sulfur steel). ) Is required.

[0003] As a method for melting ultra-low carbon / ultra-low sulfur steel, in order to increase productivity, vacuum decarburization is performed using RH vacuum degassing equipment, and then molten steel in a degassing vacuum tank or rising reflux molten steel in a riser pipe. A desulfurization flux by blowing a powdery desulfurization flux (this is referred to as an RH desulfurization method.
No. 011) is common.

In this RH desulfurization method, vacuum decarburization treatment (first step) is performed on undeoxidized molten steel (molten steel containing excess dissolved oxygen (free oxygen)), and then the molten steel in a degassing vacuum chamber is subjected to vacuum decarburization. In this method, a metal Al (a deoxidizing agent) is added to the molten steel to deoxidize the molten steel (second step), and then desulfurize (third step).

[0005] In the desulfurization step, unless it is in a reduced state (usually 1 ppm or less) in which dissolved oxygen hardly exists in molten steel,
Desulfurization reaction does not proceed. Therefore, in the deoxidation process, not only remove excess dissolved oxygen (deoxidize molten steel),
Soluble Al in molten steel (usually about 0.03-0.05%)
Is added in an amount sufficient to cause the presence of Al.

[0006]

However, even if the desulfurization treatment is performed on such molten steel in a reduced state,
The ladle slag remains in a peroxide state with a high oxygen potential containing FeO and MnO at the time of converter steel tapping.

[0007] Therefore, during the desulfurization process, the slag in the ladle caused by ring diverted Ar gas - by the swinging between the metal, the soluble Al in the slag and the molten steel peroxide might undergo reaction, a large amount of Al ₂ the O ₃ is generated.

The Al ₂ O ₃ generated at this stage remains as Al ₂ O ₃ inclusions in the cast slab because the time from the end of the RH treatment to the casting is short and the floating time is not long enough. I do.

Therefore, in the conventional RH desulfurization method, extremely low carbon
Even if the production of extremely low sulfur steel is achieved, the total oxygen content of the slab (total oxygen content is the amount of dissolved oxygen dissolved in the steel and the amount of oxides such as Al ₂ O _{3 in the} steel) It is difficult to smelt a highly clean steel having sufficiently reduced T [O] (for example, T [O] ≤ 15 ppm or less), which is the total amount of oxygen to be added.

[0010] The present invention has been proposed to solve the above-mentioned problems of the prior art, and when the ultra-low carbon / ultra-low sulfur steel is melted by the RH desulfurization method, there are few inclusions and cleanliness. It is an object of the present invention to provide a method for producing ultra-low carbon / ultra-low sulfur steel having a high melting point.

[0011]

Means for Solving the Problems The first invention of the present application is an R
Using H vacuum degassing equipment, vacuum decarburization, deoxidation treatment, a method of melting the refining to very low coal-very low硫鋼in the order of the desulfurization process, burnt lime and metal before the deacidification Al
4000Nl the advance previously added on the slag in the ladle, after deacidification ends, the riser during the ascent reflux molten steel reflux /
refluxing molten steel Nde write blowing an inert gas at a higher flow rate min
Then , the molten steel surface and slag in the ladle are swung by the stirring energy of the molten steel due to the reflux , and this swing causes
Simultaneously when the slag in the ladle is reduced by the metal Al
Dissolve the calcined lime in the slag in the ladle
This is a method for producing ultra-low carbon / ultra-low sulfur steel by desulfurizing molten steel .

First, the undeoxidized molten steel is vacuum decarburized under reduced pressure to a predetermined ultra-low carbon concentration, and then a deoxidizing agent such as metal Al is added to the degassing vacuum tank to deoxidize. At this time, the molten steel is made to yield as soluble Al in a reduced state in which almost no dissolved oxygen (free oxygen) is present in the molten steel.

Thereafter, 4000 Nl / min of an inert gas for circulation such as Ar gas is introduced into the ascending reflux molten steel from a tuyere provided on the riser or from a lance or the like disposed directly below the riser and having a tip facing upward. By blowing in the above, the molten steel is recirculated between the degassing vacuum tank and the ladle, and the molten steel in the degassing vacuum tank is vigorously stirred. This stirring energy is transmitted from the downcomer pipe to the molten steel which recirculates into the ladle and floats, and swings the molten steel surface and the slag in the ladle up and down.

By this rocking, calcined lime added to the ladle slag before the deoxidizing treatment is dissolved in the ladle slag, the slag basicity is increased, and the reaction between the slag and metal is promoted. The molten steel is desulfurized.

[0015] Similarly, by swinging, the ladle slag
The added metal Al reduces the ladle slag in the peroxide state.
Detoxify before desulfurization. As a result, during desulfurization
Al in the molten steel is oxidized by the rocking that occurs
And therefore Al_TwoO _ThreeCan not cause
No.

Further, the inert gas for reflux is 4000 Nl /
The rocking energy generated by blowing more than min is much higher than the stirring energy by the conventional ladle desulfurization method (a method in which a lance is immersed in molten steel in a ladle and a desulfurization flux is blown from the tip of the lance to desulfurize). Ladle slag does not get caught in the molten steel. Therefore, it becomes possible to produce ultra-low carbon / ultra-low sulfur steel having few inclusions and high cleanliness.

Inert gas for reflux is 4000 Nl / min
In the following, the swing width is small and the swing energy is insufficient.
Insufficient dissolution of calcined lime increases viscosity of slag
And the desulfurization reaction does not proceed. In addition, metal Al uses ladle slag
Al cannot be reduced sufficiently_TwoO _ThreeCause
Melts ultra-low carbon / ultra low sulfur steel with low inclusions and high cleanliness.
Cannot be made.

On the other hand, when metal Al is added to the slag in the ladle after the deoxidizing treatment, the reduction of the ladle slag is not completed at the start of desulfurization. Therefore, during the desulfurization treatment, the soluble Al
Is oxidized to generate Al ₂ O ₃ , and it is not possible to melt ultra-low carbon / ultra low sulfur steel having few inclusions and high cleanliness.

The second invention of the present application is an RH vacuum degassing facility.
Refining in the order of vacuum decarburization, deoxidation, and desulfurization
A method for melting the ultra-low carbon-very low硫鋼to the metallic Al by deacidification before possible to add beforehand on the slag in the ladle, after deacidification completed, reflux the riser It increased circulating 2000 NL / min or more flow in the molten steel Nde write blowing powdery CaO-CaF ₂ based desulfurizing flux 50 kg / min or faster with blowing an inert gas into molten steel to
And the stirring energy of the molten steel by the reflux
Ri to oscillate the molten steel surface and the slag in the ladle, the swing
The slag in the ladle is not reduced by the metal Al
A method for producing ultra-low carbon / ultra-low sulfur steel, the method comprising desulfurizing molten steel with the desulfurization flux .

2,000 Nl / min of inert gas for reflux
With the above blowing, the molten steel is recirculated and stirred in the degassing vacuum chamber.
Powdered CaO-C blown into ascending molten steel
aF _TwoThe system desulfurization flux is dissolved in the molten steel to be stirred,
Desulfurizes molten steel.

At this time, the CaF in the desulfurization flux
₂ is a gas (CaF
_{2. When} heated, fluorine gas, SiF ₄ gas, AlF ₃
It is considered to be thermally decomposed into gas and the like. ), The molten steel is also stirred, so that the stirring energy is added.

The inventors set the flow rate of the inert gas for reflux at 20.
While blowing over 00Nl / min, CaO-CaF
_{When the} system ₂ desulfurization flux is blown into the ascending reflux molten steel at a speed of 50 kg / min or more, the inert gas flow rate for reflux increases by 4%.
It has been found through experiments that the same swing as in the case of blowing 000 Nl / min or more is given to the molten metal surface and slag in the ladle.

According to the above findings, by the same action as in the first invention, the metal Al added before the deoxidizing treatment is reduced in the oxidized slag by the rocking and is harmless before the desulfurizing treatment. Therefore, no Al ₂ O ₃ is generated during the desulfurization treatment, and it is possible to produce ultra-low carbon / ultra-low sulfur steel having few inclusions and high cleanliness.

2,000 Nl / min of inert gas for reflux
Above or desulfurization flux blowing speed is 50kg
If one of the conditions of / min or more is not satisfied, the swing energy is insufficient, and the metal Al cannot sufficiently reduce the ladle slag. Therefore, during the desulfurization treatment, A
Since l ₂ O ₃ is generated, it is not possible to melt ultra-low carbon / ultra-low sulfur steel having few inclusions and high cleanliness.

Further, in the second invention in which desulfurization flux is blown and the desulfurization treatment is performed, a higher desulfurization rate can be obtained as compared with a case where calcined lime is added to ladle slag and desulfurized.
It becomes possible to produce extremely low sulfur steel having a low ultimate [S] value.

[0026]

【Example】

Confirmation test (1): Fig. 1 shows calcined lime and metal Al on slag
The molten steel in the ladle is charged with Ar gas for reflux into the riser from the lance with the tip facing upward located in the RH vacuum degassing facility. This shows the situation where extremely low sulfur steel is being produced.

Here, 1 is a ladle, 2 is molten steel, 3 is a ladle slag, 4 is an ascending pipe, 5 is a descending pipe, 6 is a degassing vacuum tank, 7
Denotes an exhaust duct, 8 denotes an alloy input port, 9 denotes a reflux gas blowing pipe, and 10 denotes an auxiliary material input hopper.

The ladle 1 contains 250 tons of undeoxidized molten steel 2 which has been refined by a converter.
Ladle slag 3 in the oxidized state at the time of converter tapping is approximately 1 in weight.
~ 1.5 tons.

The pretreated hot metal subjected to desulfurization and desiliconization is charged into a converter, and in the converter blowing, decarburization smelting is mainly performed, and a carbon concentration (hereinafter referred to as [C]) as a molten steel component at the end point of the converter. Written as
Was adjusted to 160 to 200 ppm, and the sulfur concentration (hereinafter referred to as [S]) was adjusted to about 30 ppm.

The recirculation gas injection pipe 9 is a lance having a metal pipe covered with a refractory on the surface thereof, the tip of which faces upward, and is inserted into the molten steel 2 from the side of the riser pipe 4. A gas discharge hole is provided at the tip of the lance, and the gas discharge hole is disposed immediately below the rising pipe 4 and substantially at the center. Accordingly, a predetermined reflux Ar gas flow rate was blown into the riser pipe 4 (in the ascending reflux molten steel) from the gas discharge holes, and the molten steel 2 was refluxed between the ladle 1 and the degassing vacuum tank 6.

There are two auxiliary material input hoppers 10 (one is shown in the figure) beside the degassing vacuum chamber 6, each containing calcined lime powder and metallic Al (particle diameter 5 to 10 mm). 2 can be thrown.

Therefore, in the confirmation test (1), metal Al was used as a slag modifier for reducing FeO and MnO in the ladle slag 3, and calcined lime powder was used for increasing the basicity of the ladle slag. Used as a slag component regulator.

Table 1 shows the smelting conditions and results of ultra-low carbon / ultra-low sulfur steel in Examples 1 to 8 of the confirmation test (1).

[0034]

[Table 1]

In Examples 1 to 5, 1 ton of calcined lime powder and 200 kg of metal Al were put on the slag in the ladle 1 immediately after the start of the converter.

In Examples 1 to 5, during the decarburization treatment, a predetermined Ar gas flow rate for recirculation was blown from the recirculation gas injection pipe 9 to reduce [C] to 15 ppm or less (after RH decarburization) under reduced pressure. Vacuum decarburized.

Thereafter, 300 kg of metallic Al was uniformly added to the molten steel in the vacuum chamber 6 from the alloy inlet 8 to deoxidize the molten steel.
Blowing was performed at a rate of 5500 Nl / min for 8 to 10 minutes (referred to as reflux time) to perform desulfurization treatment. Note that the soluble Al concentration in the molten steel during the desulfurization treatment was 0.030% or more, and the dissolved oxygen concentration was 1 ppm or less.

By blowing the Ar gas for reflux in the above range, the molten steel in the vacuum chamber 6 is vigorously stirred, and this stirring energy is transmitted to the molten steel circulating in the ladle 1, and the ladle 1
Molten steel 2 and ladle slag 3 in the vertical direction
m rocked.

As a result of this rocking, 1 ton of calcined lime powder was dissolved in the ladle slag 3 and the desulfurization reaction proceeded. As a result, [S] at the end of RH was reduced to 20 ppm or less (desulfurization rate was 3%).
8-53%).

However, the desulfurization rate is [(RH decarburized [S] -R
At the end of H [S]) / after decarburization of RH [S]] x 100%.

Similarly, this swing causes ladle slag 3
Was reduced and rendered harmless by 200 kg of metallic Al added to the slag, so that during the desulfurization treatment, the soluble Al in the molten steel was not oxidized, and T [O]
Stably became 15 ppm or less.

As a result of continuous casting of the molten steel thus produced, the slab components after casting were [C] ≦ 20 ppm and [S]
≦ 20ppm ultra low carbon / ultra low sulfur steel and T [O] ≦ 1
5 ppm of inclusions and high cleanliness steel could be melted.

In Examples 6 to 8, calcined lime powder and metal A
The case where l was added to the ladle slag 3 was before the deoxidation treatment in the RH vacuum degassing equipment, and the other test conditions were the same as in Examples 1 to 5.

As a result, as in Examples 1 to 5,
At the end of RH and the slab components are [C] ≦ 20 ppm,
[S] ≦ 20ppm, T [O] ≦ 15ppm
Extremely low sulfur steel and high cleanliness steel could be produced stably.

In Comparative Examples 1 to 5, the reflux Ar gas flow rate in the desulfurization step was less than 4000 Nl / min (20
(In the range of 00 to 3800 Nl / min), and other test conditions are the same as those in Examples 1 to 8. Table 2 shows the test conditions and melting results.

[0046]

[Table 2]

In Comparative Examples 1 to 5, extremely low carbon steel having a content of [C] ≦ 20 ppm could be melted in the slab component at the end of RH and after casting. T [O] ≦ 15 ppm could not be secured.

The reason is that Ar for reflux in the desulfurization step
Since the gas flow rate was less than 4000 Nl / min, the swing width was small and the slag of calcined lime powder was insufficient, and the viscosity of the slag was increased. As a result, the desulfurization reaction did not proceed, and the desulfurization rate was low (7 to 33%).

At the same time, it is probable that T [O] ≦ 15 ppm could not be obtained because the ladle slag 3 was insufficiently reduced by the metal Al added on the slag and a part of the molten steel was oxidized during the desulfurization treatment. .

FIG. 2 shows the transition of components from the time of tapping from the converter to the cast slab in Examples 1 to 8.

FIG. 3 shows the results of an investigation on the influence of the reflux Ar gas flow rate on the cast slab T [O] after casting in the desulfurization step in the confirmation test (1). From this, A for diversion
If the r gas flow rate of 4000 Nl / min or more is not satisfied, it can be seen that slab T [O] ≦ 15 ppm cannot be secured.

Confirmation test (2) : In this test, 200 kg of metal Al was placed on the ladle slag before deoxidation after vacuum decarburization to [C] ≦ 15 ppm or less as in confirmation test (1). After being charged and deoxidized, an Ar gas for reflux is supplied at a flow rate of 2,000 Nl / min or more and a powdery desulfurization flux is blown at a rate of 50 kg / min or more from a tuyere provided in the riser to obtain extremely low carbon / electrode. Low sulfur steel was produced. FIG. 4 illustrates this situation.

The composition of the desulfurization flux is CaO-20 wt.
% CaF ₂ was used, and a uniform unit of 4 kg / ton was blown together with a carrier gas (Ar gas).

As shown in FIG. 4, in the confirmation test (2), the desulfurization flux and the circulating Ar gas are blown together from the riser tuyere, and the carrier gas also acts as a part of the circulating gas for the molten steel. Therefore, the flow rate of the Ar gas for recirculation in this confirmation test includes the flow rate of the Ar gas as the carrier gas.

Table 3 shows the test conditions and the results of smelting in the example of the confirmation test (2).

[0056]

[Table 3]

Examples 9 to 13 are cases in which metallic Al was added to the ladle slag immediately after the start of the converter and the blowing rate of the desulfurization flux was 50 kg / min. In Examples 14 to 16, metal Al was added from the auxiliary material input hopper 10 before the deoxidation treatment by RH vacuum degassing, and the blowing rate of the desulfurization flux was 70 kg / min. In each case, the desulfurization rate was as high as 73 to 88% as compared with the confirmation test (1).

As a result, the components at the end of RH and the slab components were [C] ≦ 20 ppm, [S] ≦ 10 ppm, T [O]
Ultra-low carbon / ultra-low sulfur steel of ≦ 15 ppm and highly clean steel could be stably melted.

In Comparative Examples 6 to 9, the blowing speed was 5
0 to 70 kg / min, but the reflux Ar gas flow rate is less than 2000 Nl / min. In Comparative Examples 10 to 13, the reflux Ar gas flow rate was 2000 Nl / mi.
ni or more, but the blowing speed is less than 50 kg / min.

Table 4 shows the test conditions and melting results in these comparative examples.

[0061]

[Table 4]

In these cases, as shown in Table 4, a high desulfurization rate of about 80% was obtained, and the components at the end of RH and the slab components were extremely low carbon and extremely low in carbon content of [C] ≦ 20 ppm and [S] ≦ 20 ppm. Although a low sulfur steel was obtained, a highly clean steel with T [O] ≦ 15 ppm could not be melted.

[0063]

According to the present invention, in the RH vacuum degassing equipment,
Calcined lime and metal A on ladle slag before deoxidation
l, and inert gas for reflux in RH vacuum degassing equipment
Flow rate 4000Nl / min or more, or RH vacuum degassing
Metal Al on ladle slag before deoxidation treatment
2,000 Nl / min or more and CaO-CaF
_TwoSystem desulfurization flux at a speed of 50 kg / min or more
By injecting into ascending molten steel of H vacuum degassing equipment
To make the ladle slag harmless.
Highly clean ultra-low carbon / ultra-low sulfur steel can be stably melted.

[Brief description of the drawings]

FIG. 1 is a view showing a state in which ultra-low carbon / ultra-low sulfur steel is melted in a confirmation test (1).

FIG. 2 is a diagram showing a change in components from converter tapping to cast slabs in Examples 1 to 8.

FIG. 3 is a diagram showing the effect of the reflux Ar gas flow rate on a slab T [O] after casting in a desulfurization step in confirmation test (1).

FIG. 4 is a view showing a situation in which ultra-low carbon / ultra-low sulfur steel is melted in confirmation test (2).

[Description of Signs] 1 Ladle 2 Molten steel 3 Ladle slag 4 Ascending pipe 6 Degassing vacuum tank 9 Recirculation gas injection pipe 10 Sub-material input hopper

Continuation of the front page (72) Inventor Junichi Fukumi 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-6-299229 (JP, A) JP-A-6-240338 (JP, A) JP-A-5-214426 (JP, A) JP-A-59-208011 (JP, A) JP-A-59-56515 (JP, A) JP-A-58-113313 (JP, A) 58-37112 (JP, A) 2-38458 (JP, U)

Claims (2)

(57) [Claims] 1. A method for refining ultra-low carbon / ultra-low sulfur steel by refining in the order of vacuum decarburizing treatment, deoxidizing treatment, and desulfurizing treatment using an RH vacuum degassing equipment, wherein the deoxidizing treatment is performed before the deoxidizing treatment. Contact with the burnt lime and metal Al was added in advance to the slag in the ladle
Can, after deacidification ends, riser refluxed elevated circulating molten steel Nde <br/> write blowing an inert gas at a higher flow rate of 4000Nl / min in the molten steel to reflux and stirred energy of the molten steel by the recirculation
The molten steel surface and the slag in the ladle are rocked by the lug, and the slag in the ladle is changed to the metal Al by the rocking.
At the same time as reducing, the calcined lime was dissolved in the slag in the ladle.
A method for producing ultra-low carbon / ultra-low sulfur steel, wherein the molten steel is desulfurized by the slag . 2. A method for refining ultra-low carbon / ultra-low sulfur steel by refining in the order of vacuum decarburization treatment, deoxidation treatment, and desulfurization treatment using an RH vacuum degassing equipment, wherein the decarbonization treatment is performed until before the deoxidation treatment. 2000N in the metal Al possible to add beforehand on the slag in the ladle, after deacidification ends, the riser during the ascent reflux molten steel circulates
Inert an inert gas at a flow rate of 1 / min or more and
CaO-CaF ₂ in powder form at a speed of 0 kg / min or more
Nde write blow the system desulfurization flux to reflux of molten steel, this ring
The molten steel surface and slag in the ladle are swung by the stirring energy of the molten steel by the flow, and the swing in the ladle is caused by this swing.
While reducing the lag with the metal Al, the desulfurization flash
A method for producing ultra-low carbon / ultra-low sulfur steel, which comprises desulfurizing molten steel by using heat treatment.