JPH07268440A - Deoxidizing method of molten steel - Google Patents

Abstract

PURPOSE:To perform deoxidation without causing the formation of large amounts of alumina and to obtain a clean steel reduced in the surface defects of a product by performing preliminary deoxidation to a specific dissolved oxygen quantity with Mg when the quantity of dissolved oxygen is large, in a method for deoxidizing a molten steel of specific composition. CONSTITUTION:A molten steel, having a cast slab composition containing, by weight, <=0.06% C, <=0.05% Si, 0.1-0.4% Mn, and 0.02-0.1% Al, is deoxidized. In this method, after the completion of decarburization of the molten steel in a refining furnace, a metal Mg containing substance is added or blown through a bottom blowing tuyere to reduce the quantity of dissolved oxygen in the molten steel to 200-500ppm. Then, one or more elements among Al, Ti, Zr, V, and Nb are added to this molten steel. The effect of using Mg can be maintained at high level when the quantity of dissolved oxygen is large, and the flowability of slag is reduced when prelimirary deoxidation products are absorbed by the slag. By this method, the flowing-out of ladle slag at the last stage of pouring can be reduced and also the surface flaw of a product can be reduced over the whole of cast slab. As a result, even a cast slab of the last stage can be used for high-grade purposes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for deoxidizing molten steel after completion of decarburizing and refining.

[0002]

2. Description of the Related Art Steel types having high C as steel composition, Si, Mn
In the case of a steel type that permits C, the oxygen concentration is reduced to about 100 ppm or less by utilizing the vacuum deoxidizing effect of C or pre-deoxidizing with Si or Mn, and finally deoxidizing with a strong deoxidizing agent such as Al. It is acidified to produce steel with few inclusions.
However, in the production of steel such as aluminum-killed steel sheet for working, which has low carbon, low Si and Mn concentrations, and high Al concentration, pre-deoxidation by C, Si and Mn can hardly be performed, and therefore, at the time of completion of decarburization refining. Almost all of the oxygen in the steel
It was deoxidized and removed with 1 or Ti. At this time, in order to promote the floating removal of inclusions in the steel, some Al or Ti
In some cases, it may be devised such that it is added at the time of tapping to perform preliminary deoxidation, and final deoxidation is performed during secondary refining. However, in the deoxidation of Al, a large amount of alumina is generated in the steel, and a part of the alumina remains, causing defects such as surface defects of the product. In addition, the alumina floated and separated from the steel mixes with the ladle slag to increase the fluidity of the slag, and a large amount of slag mixes with the molten steel at the end of the injection from the ladle to contaminate the molten steel, causing surface defects such as product surface defects. It was the cause of the defect.

The method of deoxidizing molten steel with Mg has hitherto been used in the final deoxidation process in which the oxygen concentration in the steel is already low, such as controlling the composition of inclusions in the steel. However, since the utilization efficiency of Mg is low, it has not been used as a technique for removing a large amount of oxygen from molten steel.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a preliminary deoxidation method for deoxidizing molten steel without producing a large amount of alumina and for producing a clean steel with few surface defects of the product. It is intended.

[0005]

As a result of various studies on the methods for solving these problems, the present inventors have found that the utilization efficiency of Mg can be maintained high when the oxygen concentration in the steel is high, and the preliminary deoxidation formation. It has been found that the fluidity of slag decreases when an object is absorbed by slag, and the present invention has been completed based on this finding.

That is, the gist of the present invention is that the composition of the cast slab is C: 0.06% or less by weight% and Si:
0.05% or less, Mn: 0.1 to 0.4%, Al: 0.
In the deoxidation method of molten steel of 02 to 0.1%, after the decarburization of the molten steel is completed in the refining furnace, (1) the metal Mg-containing material is added to the molten steel to reduce the dissolved oxygen amount in the molten steel to 500 ppm or less. Lower than 200ppm, then Al, Ti,
A method for deoxidizing molten steel, which comprises adding one or more of Zr, V, and Nb, and (2) blowing a hydrogen-containing gas instead of adding the metal Mg-containing material to the molten steel. A method for deoxidizing molten steel according to the above (1) is characterized.

The details of the present invention will be described below together with the operation.

[0008]

[Operation] Molten steel contains a large amount of dissolved oxygen at the end of decarburization refining. The amount depends on the refining method and carbon content, but 400 ppm to 1000 for low carbon steel with a carbon concentration of 0.1% or less.
It reaches even more than ppm. In the case of steel types that allow Si and Mn as the steel composition, by pre-deoxidizing with Si and Mn, the oxygen concentration is reduced to about 100 ppm or less, and then finally deoxidizing with a strong deoxidizing agent such as Al to form inclusions. Manufactures less steel. However, in the conventional technology, the composition of the product steel material is C
0.06% or less, Si 0.05% or less, Mn 0.1
0.4%, the steel type which cannot be pre-deoxidized by C, Si, Mn also depended on Al for pre-deoxidation. In the case of Al deoxidized steel, the amount of Al is 0.
02% or more is necessary, and 0.1% or less is usually sufficient.

Deoxidation with Al produces Al ₂ O ₃ . Al ₂ O ₃ increases the fluidity of CaO-rich ladle slag. Therefore, if a large amount of Al ₂ O ₃ is mixed in the ladle slag, the proportion of the molten portion in the ladle slag increases, and the outflow amount of the ladle slag at the final stage of pouring increases. Since the ladle slag contains a large amount of oxidizing components (FeO, MnO, SiO ₂ ), the molten portion is increased and the molten steel is further oxidized to increase the amount of Al ₂ O ₃ produced. Further, the ladle slag that has flown out and is entrained in the molten steel reacts with the molten steel to generate new Al ₂
Generates O ₃ . Since these slags that have been caught and newly formed Al ₂ O ₃ are not floated and removed for a short period of time, they are likely to remain in molten steel and cause defects in steel products.

Deoxidation with Mg produces MgO. M
gO reduces the fluidity of ladle slag originating from converter slag
Reduce. Therefore, by pre-deoxidation with Mg,
Suppressed outflow of ladle slag by MgO floated and separated from
To be done. Since Mg hardly remains in the steel, the composition of the steel,
Does not affect the material. Increase the amount of MgO produced
It sufficiently reduces the fluidity of the rug, and it is Al₂O ₃Raw
In order to exert the effect of preliminary deoxidation by sufficiently reducing the amount of composition
If the initial oxygen content is as high as 800 ppm or more, M
Addition of g reduces the amount of dissolved oxygen in molten steel to 500ppm or less
Can be deoxidized to an initial oxygen content of 500-800
When ppm is low, the dissolved oxygen content in molten steel is 400pp
It is necessary to deoxidize to m or less.

In deoxidizing molten steel, the dissolved oxygen concentration is at least 100 pp in order to improve the material properties and surface properties of the steel.
It is necessary to reduce it to m or less. However, if Mg alone is used to deoxidize to a required oxygen amount, the utilization efficiency of Mg sharply decreases at a low acidity concentration, so a large amount of Mg needs to be added, resulting in an increase in cost and a decrease in temperature of molten steel. It will be so large that it is difficult in practice. Therefore, it is practical that the dissolved oxygen amount after preliminary deoxidation with Mg is 200 ppm or more.

As the metal Mg-containing material, Mg powder, a mixture of Mg powder and oxide powder such as CaO, and a Mg-containing alloy such as Ni-Mg alloy are used. Since Mg has a boiling point lower than the temperature of molten steel, it is extremely easily vaporized. Therefore, as a method of adding the metal Mg-containing material to the molten steel, a method of adding it into the furnace from above, which is a normal deoxidizing agent addition method, and a method of adding the metal Mg-containing material to the molten steel and entraining it in the molten steel are also applicable. The method of blowing in is efficient. The blowing position is preferably such that the bottom of the converter bottom where the depth of molten steel is ensured, or the blowing into the molten steel through a blowing pipe installed immediately below the lower end of the rising pipe of the RH. Here, immediately below the lower end of the rising pipe is defined as a lower part within 1 m from the lower end of the rising pipe.

Further, Mg can be blown as powder or granules using various existing ladle powder blowing equipment. When the converter bottom blowing tuyere is used, if oxygen is present in the tube, it is replaced with an inert gas and then Mg is blown as an inert gas as a carrier gas. When blowing through a blowing pipe installed just below the lower end of the rising pipe of the RH, an inert gas is also used as the carrier gas, but the advantage is that the reaction distance between Mg and molten steel can be lengthened and the utilization efficiency of Mg improves. is there.

It is also possible to enclose Mg in the hollow wire and apply the wire addition technique to add Mg to the ladle and tundish. In this case, there is a problem that a wire manufacturing cost is added. Dissolved oxygen in molten steel is 500 ppm or less and 200 ppm or less by any of these methods.
Pre-deoxidize up to the above. Most of the deoxidized products are floated up and removed by casting and absorbed by the slag, which contributes to a decrease in the fluidity of the slag. Although a small amount of the deoxidized product remains in the cast slab, MgO is difficult to accumulate and does not easily form a large inclusion, so that it does not cause a surface flaw of the product.

After preliminary deoxidation with Mg, Al, Ti, Z
Add one or more deoxidizers of r, V, Nb
By doing so, final deoxidation and component adjustment are performed. these
The deoxidizing agent is added by the usual method, that is, by adding it upward in the ladle.
Addition, addition in RH vacuum tank, injection of dipping pipe in ladle, tandy
Upward addition in the ash is applied. Deoxidation with hydrogen
The deoxidation product is water vapor, and it does not remain in the molten steel.
And, it doesn't shift even during slag. In this sense hydrogen is
It is an ideal deoxidizer. However, the deoxidizing power of hydrogen was weak.
Therefore, until the final deoxidation, or low dissolved oxygen close to the final deoxidation
A huge amount of hydrogen is required to deoxidize to the concentration
Yes, not practical. High efficiency of hydrogen utilization in the present invention
Since hydrogen is used only in the high dissolved oxygen concentration range,
It has the advantage of not requiring hydrogen. Amount of hydrogen blown
Is 3 Nm per ton of molten steel³Is less than the effect of deoxidation is insufficient
And 12 Nm per ton of molten steel ³Hydrogen over
The use efficiency of is reduced.

Conventionally, hydrocarbon gas such as propane gas and natural gas has been blown into the converter as a coolant for bottom-blown oxygen in the converter. However, since these hydrocarbon gases contain carbon, they cannot be used when oxygen is not blown in at the same time, especially when the molten steel has a low carbon concentration because the carbon concentration in the steel increases. As the hydrogen of the present invention, industrial hydrogen gas, partially oxidized reformed gas such as natural gas or propane gas is used. Since the partially oxidized reformed gas contains hydrogen and CO as main components, it has an advantage that the carbon concentration in the molten steel does not increase.

After the completion of decarburization refining, hydrogen gas is blown from the bottom blowing tuyere. Cooling gas passages and oxygen gas passages are used as the tuyere passages. When using the cooling gas passage, the cooling gas may be switched to the hydrogen gas. When the oxygen gas flow path is used, the oxygen gas in the tube is replaced with an inert gas such as argon and then switched to hydrogen gas. The unreacted hydrogen gas may be recovered by the exhaust gas recovery device of the converter and used as fuel, so it is not wasted. Deoxidation by hydrogen in the converter also reduces the iron oxide concentration in the slag. Therefore, since the fluidity of the converter slag is reduced, the amount of slag that flows out to the ladle at the time of tapping is reduced, and the oxidizing ability of the slag is also reduced, so molten steel is also removed by deoxidation in the ladle. It has the advantage of being less contaminated. Further, there is also an advantage that the iron yield is improved by reducing the iron oxide in the slag.

[0018]

【Example】

[Example 1] C: 0.0 for production of cold-rolled steel sheet for processing
4 to 0.06%, Si: 0.01%, Mn: 0.16 to
At 0.21%, the initial oxygen before starting RH treatment is about 600 pp
After the RH decarburization treatment of the molten steel of m, 800 to 1000 g of Mg powder per ton of molten steel and Ar as a carrier gas were blown through a blowing pipe installed immediately 500 mm below the lower end of the rising pipe of the RH. The amount of dissolved oxygen is about 60 after RH decarburization.
It went from 0 ppm to 200-230 ppm. Then
450 to 500 g of Al was added per ton of molten steel, the mixture was refluxed for 3 minutes, and the RH treatment was finished (Nos. 1 and 2). A slab was manufactured by ordinary continuous casting through a tundish, and a cold-rolled Al-killed steel plate for processing was manufactured by continuous hot rolling and continuous cold rolling. As a comparison, when a small amount of Mg was blown (No. 3)
And a steel sheet (No. 4) manufactured by a conventional deoxidizing method that does not use Mg.

Surface flaws of the product cold rolled coil were visually inspected to determine the length of the rejected portion. Table 1 shows the inspection results of the product cold rolled sheet together with deoxidizing conditions.

[0020]

[Table 1]

Example 2 The carbon concentration after completion of RH was 20.
After the RH decarburization treatment, Mg powder was added to the ultra-low carbon steel of less than 1000 ppm and oxygen concentration of about 1000 ppm at 100 per ton
The carrier was blown with 0 to 1200 g of Ar as a carrier gas through a blower pipe installed 200 mm directly below the lower end of the RH rising pipe. Next, Al is added to 600 to 6 per ton of molten steel.
50 g and 530 g of Ti per ton of molten steel were charged and refluxed for 3 minutes, and then the RH treatment was completed. Slabs were manufactured by continuous casting through a tundish, and cold-rolled steel sheets for deep drawing were manufactured by continuous hot rolling and continuous cold rolling (Nos. 1 and 2). For comparison, when a small amount of Mg was blown into the same steel type (No. 3)
And a steel sheet (No. 4) manufactured by a conventional deoxidizing method that does not use Mg.

The surface flaws of the cold rolled product coil were visually inspected to determine the length of the rejected portion. Table 2 shows the inspection results of the product cold rolled sheet together with deoxidizing conditions.

[0023]

[Table 2]

[Embodiment 3] 92% of hot metal in an upper bottom blowing converter,
Using 8% scrap as the main raw material, a normal oxygen blowing operation was performed, and decarburization was completed when the blowout carbon was 0.05%. After that, the reformed gas of the natural gas is fed from the bottom blown tuyere to 5.2 to 7.
After blowing 5 Nm ³ / t, at the time of tapping and after RH secondary refining, A
1 deoxidized to produce low carbon Al killed steel (No. 1,
2). The composition of the reformed gas is H ₂ : 74 vol%, CO: 2
It was 1 vol% and N ₂ : ₂ vol%. For comparison,
A test was conducted in which hydrogen was blown at 0.8 Nm ³ / t after blowing (No. 3), and production was performed by a conventional operating method in which no hydrogen was blown (No. 4). Molten steel is continuously cast into slabs,
A cold rolled coil was obtained through the same operating conditions of continuous hot rolling, continuous cold rolling and continuous annealing. The surface flaws of the cold rolled coil of the product were visually inspected, and the length of the rejected portion was obtained. Table 3 shows the inspection results of the product cold rolled sheet together with the hydrogen gas blowing conditions of the converter.

[0025]

[Table 3]

[0026]

EFFECTS OF THE INVENTION According to the present invention, the outflow of ladle slag at the final stage of pouring is reduced and the surface flaws of the product are reduced over the entire slab, so that the slab at the final stage can also be used for high-grade applications and the product yield is improved. .

Claims (5)

[Claims] 1. The composition of the cast slab is C: 0.0% by weight.
6% or less, Si: 0.05% or less, Mn: 0.1 to 0.
4%, Al: 0.02-0.1%, in the deoxidizing method of molten steel, after completion of decarburization of molten steel in a refining furnace, metal Mg
The dissolved oxygen content in the molten steel is adjusted to 500 by adding the inclusions to the molten steel.
ppm to 200 ppm or more, then A
A method for deoxidizing molten steel, which comprises adding one or more of 1, Ti, Zr, V and Nb. 2. The deoxidizing method for molten steel according to claim 1, wherein the metallic Mg-containing material is blown into the molten steel instead of adding the metallic Mg-containing material to the molten steel. 3. The method for deoxidizing molten steel according to claim 2, wherein the metal Mg-containing material is blown into the molten steel through the bottom blowing tuyere of the converter. 4. The method for deoxidizing molten steel according to claim 2, wherein the metal Mg-containing material is blown into the molten steel through a blowing pipe installed just below the lower end of the rising pipe of the RH in the ladle. 5. The molten steel according to claim 1, wherein instead of adding the metal Mg-containing material to the molten steel, a hydrogen-containing gas is blown into the molten steel after decarburization at ³ Nm ³ or more and 12 Nm ³ or less per ton of molten steel. Deoxidation method.