JPS58207316A - Production of low nitrogen and low hydrogen steel having high cleanliness - Google Patents

Abstract

PURPOSE:To produce titled steel rationally within a single reaction vessel by using molten iron which is beforehand dephosphorized and desiliconized, and providing a hood for evacuation to a converter having the function of blowing gases from the furnace bottom and having a freely slidable opening/closing device in a tap hole. CONSTITUTION:Molten steel which is beforehand dephosphorized and desiliconized is charged into a converter, and <=10kg of lime/ton of molten metal is charged therein. While a gas is blown at <=0.5Nm<3>/min/ton of molten iron, the molten iron is decarburized by blowing oxygen under the atmospheric pressure down to have a target content of C. The inside of the converter is evacuated and maintained under the reduced pressure, and the molten iron is subjected to CO deoxidization and dehydrogenation. An alloy is added to the molten iron to deoxidize the same and to control the target component value roughly. The inside of the converter is again evacuated and maintained under the reduced pressure and the molten iron is desulfurized and dehydrogenated. While the molten iron in the converter is kept shielded from the atmosphere, the molten iron is tapped into a ladle. The molten iron tapped into a ladle is stirred with a gas so that the desulfurization and flotation of inclusions are accomplished.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing low nitrogen, low hydrogen, high cleanliness steel.

-Requirements for the various properties of steel have become significantly stricter in recent years, and simple processes such as the combination of pre-desulfurization of hot metal and converter operation cannot meet strict requirements such as reducing the phosphorus and sulfur content in steel and the amount of inclusions. After steel is tapped in a converter, low-phosphorus, low-sulfur, and low-hydrogen steel with a small amount of inclusions is produced by combining a secondary refining furnace with a solution heating means such as an arc heating device and a vacuum treatment operation. . That is, by the method described above, S
=20~301)pm, P=50~100ppm
.

H2=1.5~2.0 ppm, N2=40~50
ppm, total oxygen content of 15-20 ppm.

However, in order to achieve sufficient dephosphorization in the converter, a so-called double blowing method is used. The disadvantage of using a refining vessel was that the temperature drop was large. Therefore, since the cost of nI would be extremely high, the development of a rational manufacturing method was awaited.

Furthermore, there has been a strong desire to reduce nitrogen and hydrogen in steel from the standpoint of improving material quality and saving alloying elements.

The present invention was made in view of this situation.

The feature is that when melting low-nitrogen, low-hydrogen high-cleanliness steel using an oxygen-blown converter, which is equipped with gas bottom blowing and vacuum holding functions as well as tapping hole opening and closing functions, The desulfurized hot metal is charged into an oxygen-blown converter and the flow rate is 10kj per ton of molten metal. The process of charging the following quicklime and decarburizing it to the target carbon content by blowing acid under normal pressure while blowing gas of 0.5 Nm3 or less per minute per ton of molten metal from the bottom of the furnace, and depressurizing the inside of the converter. a step of holding and deoxidizing and dehydrogenating CO; a step of deoxidizing the molten metal and roughly adjusting the target component values by adding an alloy to the converter; and a step of depressurizing the inside of the converter again and desulfurizing it. , a step of dehydrogenation, a step of separating the molten metal in the converter from the atmosphere and tapping it into a ladle, and agitating the molten metal thus tapped into the ladle to desulfurize and float and separate inclusions. The present invention provides a method for producing low nitrogen, low hydrogen and high cleanliness steel, comprising the steps of:

Now, the blowing method using a converter equipped with an argon blowing device at the bottom of the furnace has been developed by the West German company Wittan as a VOD (Vacuum Oxygen Deca).
It has been proposed as a method for producing stainless steel in a converter as an alternative to the 70 process.

However, this method utilizes vacuum to decarburize steel containing Cr, and aims to achieve preferential decarburization while suppressing oxidation of Cr, and is characterized by blowing acid under vacuum, so-called The refining principle of VOD is directly applied to a converter. Therefore, this is essentially different from the present invention, in which acid blowing is carried out under normal pressure until the target carbon value is reached, and then vacuum reduction treatment is performed.

In addition, the present applicant has previously filed a separate application for a converter steel manufacturing method in which hot metal that has been previously dephosphorized and desulfurized is decarburized while introducing a steel bath stirring gas from the bottom of the furnace. This process allows smelting in the converter without using almost any quicklime, which suppresses the occurrence of slopping, reduces the gap between the converter hood and the furnace mouth, and provides high decarburization efficiency with low carbonation. Because it is a trench, it is possible to increase the amount of CO2 recovered with a low combustion rate, and it also enables heavy-duty operation, and only a small amount of quicklime can be used, so the amount of water charged can be reduced. Hydrogen and nitrogen contents can be lowered during blow-off. Furthermore, with regard to the pickup of hydrogen, nitrogen, etc. during tapping, the so-called sealed tapping method (separate application pending), which isolates the tapping flow from the atmosphere, makes it possible to produce low-nitrogen, low-hydrogen steel. be. In addition, the amount of quicklime used has been increased to 9/To with the goal of melting extremely low-temperature steel.
When dephosphorizing is carried out more actively in the converter by increasing the amount of n-8 steel, it is necessary to remove most of the slag from the converter before entering the next reduction refining process.

In contrast, in the present invention, when refining hot metal that has been previously dephosphorized and desulfurized with a CaO or Na2CO3-based refining agent while blowing gas for bath stirring from the bottom of the converter furnace,
The amount of quicklime used is limited to a small amount, less than 10 kg/Ton-8teel, and decarburized to the target carbon amount by blowing acid under atmospheric pressure. Next, the hood is connected to a vacuum system to maintain the inside of the converter under reduced pressure, and while argon gas is blown into the converter from the bottom of the furnace, deoxidation by carbon and simultaneous dehydrogenation and denitrification are carried out.

[1)-[o%]<o, o'oo4s Tsu#)P in the bath
After reducing C8 to 0.2 atm or less, the alloy was added to the target steel under reduced pressure or atmospheric pressure with the central value of the target component set to 0. After that, the molten metal was stirred again under reduced pressure.2.・
“Noise slag is reduced to (FeO+MnO) in the slag.
After reducing the content/' amount to less than 1%, continuing desulfurization and dehydrogenation, and maintaining the furnace pressure at 10 Torr or less for 5 minutes or more, introduce argon gas etc. into the furnace to return the furnace pressure to atmospheric conditions. .

The furnace body is tilted to a nearly horizontal state with the exit steel hole closed except for the hood, and a lid is provided that penetrates through the molten steel communication section.
In addition, the molten steel communication section and the tapping hole of the converter (
The steel is pressed into contact with the nozzle of the tapping hole opening/closing device, or is degassed by a screen curtain, and the tap is tapped (detached steel).

In the present invention, a gas such as argon is continuously flowed from the bottom of the converter during the melting period in an amount of 0.5 Nm 3 or less.

In the next step of the present invention, the amount of quicklime used is less than filOiw per ton of molten metal, preferably 3 to 1oyu/Ton-
The amount of slag limited to 8te*1 is necessary and sufficient for rounding to prevent an increase in the amount of fumes generated during decarburization, and at the same time to ensure the amount of slag necessary to perform slag coating to protect the furnace refractories. It is. Also, the amount of slag is 10 kg/To
If it exceeds n-8 tsel, a large amount of water will enter the converter along with quicklime, which is not preferable for low-hydrogen steel melting, and the amount of slag to be reduced during reduction refining will increase. For example, the amount of added alloy such as At increases, which is extremely undesirable.

Further, in the present invention, in order to increase the desulfurization rate during reduction refining, it is desirable that the basicity (CmO/S i O2) of the slag is 4 or more. Further, it is desirable to use quicklime with as low a moisture content as possible. For example, in place of quicklime, it is desirable to use limestone or calcium ferrite with a low moisture content from the viewpoint of hydrogen reduction.

The reason why the bottom blowing made the slag less than 0.5 Nm3 per ton of molten metal per minute was due to the T, Fe (Total
It is determined from the amount of gas required to stabilize the Fe) at a low level, and from the economic point of view considering the cost, it is approximately 0.1
~0.2 Nm3/Ton-min is desirable. It is desirable to reduce the flow rate of gas at the beginning of the decarburization period and increase it at the end of the decarburization period. N
It is also possible to significantly reduce the bottom blowing gas flow rate to m3/Ton-min.

Figures 1 to 4 show examples of equipment and operational patterns used to carry out the present invention.

In Figure 1, 1 is the steel shell of the converter, 2 is the lining refractory, and 3
indicates a steel bath, 4 indicates a slag, and 5 indicates a top blowing lance for blowing acid. Reference numeral 6 is a motherboard for introducing a stirring gas such as Ar provided near the bottom of the furnace, and 6a is a special blowing plug or a gas blowing plug made of refractory material. Reference numeral 7 denotes a tapping hole opening/closing device which is slidably provided in the tapping hole in order to bring the tapped steel into a more perfect condition. 8 is a hood for exhaust gas, and 9 is a pedestal for installing the hood connected to the vacuum system.

Fig. 2 shows a vacuum hood 10 having an alloy addition hole A'-12, and if an O-ring 11 or the like is used, the hood 10 can be in airtight contact with the pedestal 9 of the converter elongation to facilitate reduced pressure operation. make it easier The vacuum hood 10 is connected via a conduit to an exhaust system (not shown). It is desirable to keep the tapping hole opening/closing device 7 in a closed state during vacuum and pressure reduction operations.

In addition, the degree of vacuum and bottom blowing during depressurization are due to the progress of deoxidation and decarburization by the C + O → CO reaction, and when the degree of vacuum and bottom blowing reaches 0.00048 [Cchi] and [Ochi]. ,
After adding a predetermined amount of alloy, the pressure in the furnace is increased again to 10
By keeping the temperature below Torr for 5 minutes or more and reducing the slag 4 so that (FeO + Mn0) in the slag ≦1,
The molten steel 3 is desulfurized and further dehydrogenated.

Thereafter, the exhaust speed is reduced and the pressure inside the furnace is returned to normal pressure by argon blown into the bottom of the furnace or argon blown through a pipe attached to the hood in advance, and then the hood 10 is removed from the furnace body.

FIG. 3 shows an example of a vented steel tapping process, in which the tap hole opening fitting fi 17 is closed and the converter is lowered to a horizontal position, and then a pot lid 15 and a molten steel communication pipe 16 are installed. After pressing the ladle 14, which has been replaced with Ar in advance, in a state where it can be communicated with the tapping hole (the lower surface of the tapping hole opening/closing device 7) via the molten steel communication pipe 16, the tapping hole opening/closing device 7 is operated to open. To tap steel. 13
is ladle lining, and 17 is molten steel. At this time as well, it is desirable to maintain the interior of the ladle in an Ar atmosphere by blowing argon gas into the ladle 14 through the argon gas supply pipes 18 and 19.

In this case as well, it is possible to add the necessary alloy from the alloy chute 20, but this may also be done in the next finishing step.

FIG. 4 shows the finishing process of the present invention, in which gas is poured into the molten steel 17 covered with reduced slag 22 that has partially flowed out from the converter through the gas supply pipe 18 at a rate of 100 to 20 ONL/min.
While blowing in a small amount of argon gas, inclusions are floated and removed, and if necessary, alloy is added from the alloy sheet 20 to adjust the composition of the steel. Addition using sulfide system,
It is also possible to control the morphology of At203 clusters. Note that 21 is a sliding opening/closing device, and 23 is a lid of the molten steel communication pipe 16.

As described above, the present invention uses hot metal that has been dephosphorized and desiliconized in advance, and is installed in a converter that has a gas injection function from the bottom of the furnace and has a loading capacity that can be slid freely into the tapping hole. By attaching a hood, it is possible to produce low-nitrogen, low-hydrogen, high-cleanliness steel in a very rational manner within a single reaction vessel.

In the ingot making and continuous casting processes, the steel produced according to the present invention must be cast without air.

Example After P and S removed hot metal with P=O1009 and S=0.007 is charged into a converter in which the present invention is implemented, 0.0.
Ca0 equivalent to #5 per ton of hot metal while blowing argon at 02 to 0.10 Nm3/ton-min.

Decarburization was carried out using MgO equivalent to 0.54 and iron oxide equivalent to about 20 kf, and decarburization was carried out to, for example, C=0.101. After that, with the tapping hole closed, the hood at the top of the furnace was operated to connect it to the vacuum exhaust system, and from the bottom of the furnace approximately 0.0 mm was removed. OIN
The furnace pressure is kept below 10 Torr for about 10 minutes while blowing in m3/ton-min of argon, and the decarburization reaction of the steel bath is used to reduce oxygen, hydrogen, and nitrogen in the steel bath, and to reduce the amount of oxygen in the slag. FeO and MnO were partially reduced. After interrupting the depressurization operation and introducing argon into the furnace to bring it to normal pressure, alloys are added to target the center value of the components to deoxidize the molten steel, and the furnace is depressurized again to eliminate FeO in the slag. , MnO, etc. are removed, S is removed, and dehydrogenation is continued. Reduce the furnace pressure to less than 1QTorr by approx.
After holding for 0 minutes, argon is introduced into the furnace again to return to normal pressure. Thereafter, the steel is tapped with air, and during tapping, a part of the slag in the converter flows out into the ladle, and the tapping is completed.

By blowing a small amount of argon at 100 NJ3/min from the bottom of the ladle to promote flotation and separation of suspended inclusions, H (1.5 ppm), N <30 ppm
.

T, 0(20ppm, P(0,010%, S(
0.003mm high-purity, high-cleanliness steel was melted. The molten steel produced in this manner is solidified as a steel ingot or continuously cast slab in an atmosphere of argon or the like in an aerated state.

The present invention, configured as described above, makes it possible to efficiently perform decarburization refining under normal pressure, vacuum refining, and reduction refining in a single refining vessel, and is particularly capable of performing decarburization refining under normal pressure, vacuum refining, and reduction refining in combination with the degassing method. By doing so, we can suppress the hydrogen and nitrogen build-up during steel tapping, and realize the production of low-nitrogen, low-hydrogen, high-cleanliness steel, which was considered extremely difficult until now.The effect of contributing to quality improvement is extremely high. big.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of a converter during decarburization refining under atmospheric pressure, Figure 2 is an explanatory diagram when decarburization is completed, vacuum treatment and reduction refining are carried out, and Figure 3 is an explanatory diagram of the decarburization process. Explanatory diagram showing an example, 4th
The figure is an explanatory diagram of the final adjustment of the steel composition by flotation of inclusions or addition of alloys, and the addition of special elements to control the morphology of inclusions after steel tapping is completed. be.
, 6.1' 2 Patent Applicant Nippon Steel Corporation No. 11 Certain Twice

Claims (1)

[Claims] When producing low-nitrogen, low-hydrogen, high-cleanliness steel using a nitrogen-blown converter equipped with gas bottom blowing and vacuum holding equipment as well as the ability to open and close the tapping hole, the molten metal that has been dephosphorized and desulfurized in advance is used. 10 per ton of molten metal when inserted into an oxygen-blown converter
A process of charging lime of 1 kg or less and decarburizing the molten metal per ton per minute (15 Nm3 or less) from the bottom of the furnace under normal pressure to decarburize it to the target carbon content; a step of deoxidizing and dehydrogenating CO by maintaining the molten metal under reduced pressure; a step of deoxidizing the molten metal and roughly adjusting the target component values by adding an alloy to the converter; and depressurizing the inside of the converter again. A process of holding, desulfurizing and dehydrogenating the molten metal, a process of separating the molten metal in the converter from the atmosphere and tapping it into a ladle, and agitating the molten metal thus tapped into the ladle to remove sulfurization and inclusions. A method for producing low-nitrogen, low-hydrogen high-cleanliness steel, which comprises a process of flotation separation.