JP2729458B2 - Melting method of low nitrogen steel using electric furnace molten 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 low-nitrogen steel using molten steel in an electric furnace. More specifically, the present invention relates to a method for denitrifying molten steel in an electric furnace in a secondary smelting facility to a level almost equivalent to converter steel. The present invention also relates to a method for producing a low nitrogen molten steel suitable for preventing cracking in continuous casting and hot rolling, softening products and improving aging.

[0002]

2. Description of the Related Art In recent years, the importance of the steelmaking method using an electric furnace has been re-recognized in Japan and overseas from the viewpoint of energy saving and resource recycling, and the ratio of the electric furnace to the crude steel production has been increasing due to various circumstances. . On the other hand, in terms of quality, electric furnace steel is restricted by the expansion of steel grades mainly for two reasons: the Trump element content such as Cu, Sn, and Cr is higher than the converter steel, and the nitrogen content is high. Is currently imposed. The point of the above-mentioned trump element is omitted because it is outside the scope of the present invention, but when the nitrogen content is high, the hot ductility is reduced as seen in, for example, aluminum nitride brittleness, and the inside of the slab in continuous casting is reduced. It causes cracking, surface cracking, and surface flaws in hot rolling. Further, in terms of product quality, there are disadvantages such as deterioration in tensile strength and aging properties of the ultra-low carbon ultra-soft wire, increase in yield strength of cold rolled thin sheets manufactured by the continuous annealing method, and deterioration in deep drawability. Therefore, in order to reduce the nitrogen content of electric furnace steel,
Injection of argon gas from hollow electrodes and forming slag operation in electric furnaces, undeoxidized tapping and argon gas sealing tapping in eccentric bottom tapping, and RH and V for low sulfur molten steel
Measures such as vacuum degassing and oxygen gas blowing in OD degassing equipment, and prevention of air intrusion by means of a nozzle and argon gas seal between a ladle and a tundish or between a tundish and a mold have been tested and have been put to practical use. . However, it is difficult to say that a significant portion of the above-mentioned elemental technologies have matured to date, and as a result, 0.3
The nitrogen level of the electric furnace steel of the medium carbon steel and the ultra low carbon steel of not more than% C is considerably different from that of the converter steel.

[0003]

As one of the above-mentioned elemental technologies, there is an oxygen gas blowing method under reduced pressure. This method was originally developed for the purpose of improving the decarburization speed of ultra-low carbon steel and the rate of decarburization of stainless steel and carbon steel melted in a converter.
It is a method that has been put to practical use, and it has been clarified that high-carbon steels such as 0.8% C to 1.0% C have a denitrification effect as a secondary effect. However, for medium to very low carbon steels of 0.3% C or less, the denitrification effect varies, and the cause of the variation is not clear. On the other hand, according to spraying of oxide particles onto molten steel under reduced pressure, it has been discovered that 0.3% to 0.8% carbon steel has a remarkable denitrification effect and has been put to practical use. However, for 0.3% C or less, the denitrification effect is not clear.

Japanese Unexamined Patent Publication No. Sho 60-184819 discloses that a crude molten steel of 0.1% C or more in a steelmaking furnace is decarbonized by blowing oxygen gas upward under reduced pressure. A method for producing low-nitrogen steel that promotes nitriding has been proposed . And in the embodiment of the 250 ton RH equipment,
Despite the large oxygen gas flow rate of 12 Nm ³ / ton of molten steel / hour to 36 Nm ³ / ton of molten steel / hour,
A remarkable denitrification effect is described in which the nitrogen content can be reduced to 10-14 ppm in 15-30 minutes. As described above, the reason why the oxygen gas flow rate is different from the optimum range of the present invention described later is that, as shown in Example 8 in Table 1, the pressure in the vacuum chamber is set at an acid feed rate of 4,500 Nm ³ / hour. Significantly higher than in the present invention
It must be assumed that equipment having an exhaust capacity of 7 Torr was used. Further, in Example 2, the carbon content of the molten steel after top blowing was 0.01 at an oxygen gas flow rate of 4,500 Nm ³ / hour at a vacuum chamber pressure of 6 Torr.
%, And the silicon content is 0.08%. But,
When compared with the relationship diagram between the carbon content and the silicon content in the acid supply shown in FIG. 3 described below, there is a mysterious point that the silicon content is too high with respect to the carbon content. Also, Japanese Patent Application Laid-Open No. 63-186818 discloses that in order to reduce nitrogen to a concentration of 35 ppm or less, oxygen gas is blown into molten steel under reduced pressure to maintain the oxygen concentration at 140 ppm or less, and as necessary. A denitrification method in which a carbonized material is added to molten steel so as to maintain the carbon concentration at 0.05% or more is described. In this example, oxygen gas is blown intermittently or continuously in a small amount to denitrify while maintaining the oxygen concentration at the above level. However, looking at the example,
When oxygen gas is intermittently blown at a flow rate of 4.8 Nm ³ / ton of molten steel / hour, 74 ppm of nitrogen before denitrification becomes
After 45 minutes, the nitrogen is denitrified to 24 ppm. However, when oxygen gas is continuously blown at a flow rate of 4.8 Nm ³ / ton of molten steel / hour, nitrogen after oxygen gas blowing is reduced to only 40 ppm to 50 ppm. It was noted that it did not exist.
The reason why the continuous blowing at an oxygen gas flow rate of 4.8 Nm ³ / ton of molten steel / hour was that the denitrification did not proceed so much despite being within the range of the optimum oxygen gas flow rate of the denitrification period in the present invention described below. Since the circulation argon gas flow rate is only 1,200 Nl / min for 300 tons of molten steel, the circulation cycle time of molten steel is considerably longer than that of the present invention. It is presumed that it was higher than Toll or lower. Further, as described later, in order to promote a more low to denitrification an average value of the amount of dissolved oxygen in between de窒期previously blown acid <br/> containing gas to the addition of pressurized carbonaceous material into the molten steel Should be done. In addition, it takes about 45 minutes after the oxygen gas is blown until the nitrogen concentration in the molten steel becomes 25 ppm or less. Therefore, if RH or VOD processing is performed thereafter, there is a problem that the secondary refining time becomes longer as a whole.

[0005] The present invention has been made in view of the above-mentioned problems, and its object is to target not only low-carbon steel of 0.3% C or less but also ultra-low-carbon steel of 0.006% C. Aiming to improve and stabilize the denitrification effect of oxygen gas blowing treatment under reduced pressure of electric furnace molten steel, specifically, clarify oxygen gas blowing conditions and shorten denitrification time during denitrification It is another object of the present invention to provide a method for realizing rapid decarburization after denitrification so that low-nitrogen steel close to converter steel can be melted from electric furnace steel.

[0006]

SUMMARY OF THE INVENTION The present invention is applied to a method for smelting low-nitrogen steel in which molten steel smelted by an electric furnace is denitrified by secondary refining. Its features are: 0.0
The carbon content of the product should be at least 0,5% or less.
The carbon content of the molten steel is adjusted so as to be 15% or more. Then, when oxygen gas blowing treatment under reduced pressure of molten steel at RH or VOD, 0.10%
During the denitrification period until decarbonization of 0.25% of carbon, the oxygen gas flow rate was set to 2.5 Nm ³ / ton of molten steel · hour to 10 N
The denitrification is performed at a low flow rate of m ³ / ton of molten steel / hour, preferably 3.5 Nm ³ / ton of molten steel / hour to 7 Nm ³ / ton of molten steel / hour. In the rapid decarburization period for shortening the subsequent RH or VOD treatment time, the oxygen gas flow rate is reduced to 10 Nm ³ / ton of molten steel to 4 to 4 times until the carbon content of the product is nearly reached.
Rapid decarburization at a high flow rate of 0 Nm ³ / ton of molten steel.

The above product has a carbon content of 0.05%
In the case of from 0.30% to 0.30%, the adjustment of the carbon content of the molten steel should be at least 0.10% or more higher than the carbon content of the product. In the aforementioned denitrification period, R
It is preferable to keep the pressure in the vacuum chamber of H or VOD at 5 Torr or less. At this time, it is preferable that the pressure in the vacuum chamber is reduced to 5 Torr or less within 5 minutes from the start of evacuation, and thereafter, the denitrification is started. Further, as the operating conditions at the above-mentioned RH, the molten steel circulation cycle time (minute) defined by the ladle molten steel amount (ton) ÷ reflux velocity (ton / minute) is as follows:

(Equation 2) It is convenient if the calculation is performed based on the recirculation velocity (ton / min) expressed by the following expression to be 1.3 minutes or less.

In the rapid decarburization period, RH or V
The pressure in the vacuum chamber of the OD may be maintained at 50 Torr or less. In addition, a solid oxygen source such as a mill scale can be added to the RH or VOD vacuum chamber. The adjustment of the carbon content of the molten steel to be higher than the carbon content of the product may be performed by adding a carbon material in a ladle refining furnace or adding a carbon material in a vacuum tank before the denitrification period in RH or VOD. It is preferable to add a desulfurizing agent to the ladle refining furnace or add a desulfurizing agent to the vacuum tank before the denitrification period of RH or VOD so that the sulfur content of the molten steel is 0.010% or less. For the product target value-
When the carbon content becomes 0.03% to + 0.04%, the supply of acid to the molten steel is stopped, and the carburized material which becomes 0.04% or less with respect to the molten steel amount in the ladle is subjected to RH or VOD vacuum. Add into tank. Then, the amount of dissolved oxygen in the molten steel may be reduced by performing self-decarburization by carbon and dissolved oxygen in the molten steel under reduced pressure, and thereafter, the deoxidation degree and the composition of the molten steel may be adjusted. The oxygen gas blowing treatment may be performed by a lance vertically inserted from above the RH. In a ladle refining furnace, reduce the silicon content of molten steel to 0.0
It is adjusted to 5% to 0.30%, and the molten steel is blown with oxygen gas under reduced pressure. When oxygen gas blowing under reduced pressure is performed by RH, nitrogen gas is used as a gas for purging the top lance, a gas for purging the observation window in the tank, a gas for purging the alloy inlet, and a gas for recovering the pressure of the alloy addition hopper. Is preferred.

[0009]

[Action] Molten steel is produced by an electric furnace. The molten steel is desulfurized and carburized while being transferred to a ladle refining furnace, and the carbon content of the molten steel is at least 0.15% of the carbon content of the product.
Adjust so that it is higher. This is to carry out a certain amount of carburization once, contrary to the purpose of obtaining low carbon steel, and it is easy for the denitrification of molten steel to proceed due to the intense generation of carbon monoxide gas accompanying decarburization. To secure decarburization costs. Adjustment of the carburization for securing the decarburization cost is performed in a vacuum tank before the denitrification period in the RH or VOD by adding the carbon material in the ladle refining furnace. Sulfur and dissolved oxygen are surface active elements that inhibit the above-mentioned denitrification reaction, and dissolved oxygen has an inhibitory action about twice and half as much as that of sulfur. For this reason, when it is desired to promote the denitrification reaction, the dissolved oxygen and sulfur contents of the molten steel are reduced to 0.010% or less. In addition, in the ladle refining furnace, in order to reduce the dissolved oxygen amount of molten steel to increase the distribution ratio of sulfur,
Adjust the silicon content of the molten steel to 0.05% to 0.30%. The dissolved oxygen content of the molten steel during oxygen blowing under reduced pressure decreases as the carbon content of the molten steel increases, as the flow rate of oxygen gas decreases, and as the internal pressure of the RH or VOD vacuum tank decreases. So, for example, RH, VOD
At least 0.10% to 0.2% during oxygen gas blowing treatment under reduced pressure in degassing equipment such as
In the period until 5% of carbon is decarburized, for example, the oxygen gas flow rate through a lance vertically inserted from above the RH is reduced to 2.5%.
It Nm ³ / ton of the molten steel without the time-and low flow rate during 10 Nm ³ / ton of the molten steel-and maintained below only 5 torr possible intracisternal pressure within 5 minutes from the exhaust start, maintaining a low dissolved oxygen of the molten steel The denitrification is advanced by decarburizing while obtaining low nitrogen steel. The operating conditions at RH in this case are: molten steel amount of ladle (tons) / reflux velocity (tons / minute)
The molten steel reflux cycle time (minutes) specified by

(Equation 3) When calculated based on the recirculation velocity (ton / min) expressed by the following equation, it is set to be 1.3 minutes or less. When performing the above oxygen gas blowing under reduced pressure at RH,
Instead of argon gas for suppressing the amount of air entering the vacuum chamber, nitrogen gas is used instead of the top lance purging gas.
If the gas is used as a gas for purging the observation window in the tank, a gas for purging the alloy inlet, or a gas for recompressing the alloy-added hopper, the cost can be reduced.

As described above, even at a low oxygen gas flow rate and a low tank pressure, the dissolved oxygen content of the molten steel increases almost in inverse proportion to the carbon content of the molten steel, and the carbon content of the molten steel becomes about 0.05%.
Below this, the amount of dissolved oxygen increases sharply and denitrification hardly progresses. Therefore, when it is necessary to avoid extending the RH or VOD processing time, after the nitrogen content of the molten steel has decreased, the pressure in the vacuum chamber of RH or VOD is first maintained at 50 Torr or less, and the mill is placed in the vacuum chamber. A solid oxygen source such as a scale is added to improve the decarburization rate. Then, the flow rate of oxygen gas is set to 10 Nm ³ /
Increase the flow rate of molten steel ton-hour to 40 Nm ³ / molten steel ton-hour and decarbonize rapidly to near the carbon content of the product. The acid supply is stopped when the carbon content of the molten steel becomes such that the carbon content of the product becomes −0.03% to + 0.04% with respect to the target value. Then, a carburizing material that is 0.04% or less with respect to the amount of molten steel in the ladle is added to the RH or VOD vacuum tank, and self-decarburization is performed by carbon and dissolved oxygen in the molten steel under reduced pressure. After reducing the amount of dissolved oxygen in the molten steel, the degree of deoxidation and the composition of the molten steel are adjusted.

On the other hand, the carbon content of the product is 0.05%
If it is from 0.30% to 0.30%, the adjustment of the carbon content of the molten steel is made at least 0.10% or more higher than the carbon content of the product. 0.05% range of carbon content of products
The case described above and the following 0.05% to 0.3%
And the lower limit of the carburized amount is 0.1%.
The difference between 5% and 0.10% is because denitrification hardly proceeds when the carbon content of the molten steel is about 0.05% or less.

[0012]

According to the present invention, low-oxygen gas is produced by lowering the pressure of the medium carbon molten steel in the tank in order to carry out decarburization while reducing the dissolved oxygen content of the molten steel in the oxygen gas blowing treatment under reduced pressure. By setting the flow rate, low-nitrogen steel having a nitrogen content close to that of the converter steel can be produced from electric furnace steel. This is because the carbon content of the molten steel is adjusted so that the carbon content of the product is at least 0.15% or more higher than the carbon content when the carbon content of the product is 0.05% or less. 0.05% to 0.30%.
Achieved with a carburizing adjustment of 10% or more. As described above, in any case, it has been impossible to manufacture in the electric furnace because of the high nitrogen content in the past. For example, extremely mild steel wire, cold-rolled steel sheet for deep drawing, steel sheet for high-tensile pipeline, high-strength steel sheet Steel grades such as plates can also be manufactured with electric furnace steel.

If the pressure in the vacuum chamber of RH or VOD is maintained at 5 Torr or less during the denitrification period, carbon monoxide gas bubbles from inside the bulk molten steel are generated from a deeper position,
The time required for bubbles to escape to the molten steel surface is longer.
In addition, the expansion during bubble floating becomes more remarkable due to the lower pressure, and the nitrogen gas partial pressure in the bubble becomes lower, so that the incorporation of molten steel nitrogen into the bubble further progresses, which is expected from the decrease in the dissolved oxygen amount. As described above, the effect of greatly improving the denitrification rate is obtained. If the pressure in the vacuum chamber is reduced to 5 torr or less within 5 minutes from the start of evacuation and then denitrification is started, the evacuation time to 5 torr is short and the time required for the entire RH treatment is reduced. Can be Further, the amount of nitrogen absorption from the atmosphere in the tank during the pressure reduction from the atmospheric pressure to about 100 Torr can be reduced as much as possible. Molten steel circulation cycle time (min) as operating condition at RH is

(Equation 4) If the time is calculated to be 1.3 minutes or less when calculated based on the reflux velocity (ton / minute) expressed by the following equation, it is possible to prevent an increase in the required time of the denitrification period.

If the pressure in the vacuum chamber of RH or VOD is maintained at 50 Torr or less after denitrification, decarbonization can be performed rapidly with a high flow rate of oxygen gas to near the carbon content of the product. Alternatively, the VOD processing time can be shortened. If a solid oxygen source such as a mill scale is added to the RH or VOD vacuum chamber during the above-mentioned rapid decarburization period, the supply amount of oxygen can be increased and the time can be reduced. By adjusting the carbon content of the molten steel to be higher than the carbon content of the product by adding carbon material in a ladle refining furnace or adding carbon material in a vacuum tank before the denitrification period in RH or VOD, the molten steel is dissolved. The amount of oxygen can be reduced, and the denitrification effect can be increased. If the desulfurizing agent is added to the ladle refining furnace or the desulfurizing agent is added to the vacuum tank before the denitrification period of RH or VOD to reduce the sulfur content of molten steel to 0.010% or less, Sulfur, which delays the nitrogen reaction, is reduced, and denitrification is promoted.

When the carbon content of the product reaches −0.03% to + 0.04% with respect to the target value, the supply of acid to the molten steel is stopped. High and stable agent yield. The amount of primary deoxidation products formed is also minimized. Therefore, it is easy to adjust the degree of deoxidation and the components of the molten steel thereafter, and it is easy to set the target of the content of all the components, the degree of deoxidation and the temperature of the molten steel at the end of the RH or VOD treatment. On the other hand, after stopping the acid supply, the carburized material which becomes 0.04% or less based on the amount of molten steel in the ladle is RH
Alternatively, when added into a VOD vacuum chamber and subjected to self-decarburization by carbon and dissolved oxygen in the molten steel under reduced pressure, the amount of dissolved oxygen in the molten steel is reduced, and thereafter, the degree of deoxidation and the composition of the molten steel are adjusted. Can be.

If the oxygen gas blowing treatment is performed by a lance vertically inserted from above the RH, no significant difference from the semi-dip nozzle on denitrification is recognized, and the secondary combustion rate is rather increased. Due to the effect of the temperature rise, the metal deposition in the vacuum chamber can be significantly reduced. 0.05% silicon content in molten steel in ladle refining furnace
To adjust to 0.30%, if the oxygen gas blowing process under a reduced pressure of molten steel, mainly using CaO-CaF ₂ based as desulfurizing agent in the ladle refining furnace, intensive metal Al system such as Al ash Without using Si deoxidation, the amount of dissolved oxygen in the molten steel for increasing the distribution ratio of sulfur can be reduced. When oxygen gas blowing under reduced pressure is performed by RH, as described above, if the exhaust capacity at each level of the tank pressure is increased, nitrogen gas is supplied to the top lance purging gas and the observation window for monitoring the inside of the tank. Even if it is used as a purge gas, a purge gas at the alloy inlet, or a gas for recompressing an alloy-added hopper, the partial pressure of nitrogen gas in the tank during acid supply and self-decarburization is kept low, and Can be suppressed. As a result, inexpensive nitrogen gas can be replaced by expensive argon gas.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for producing low-nitrogen steel using an electric furnace molten steel according to the present invention will be described below in detail. The present invention is, roughly speaking, a method of preliminarily carburizing molten steel in an electric furnace to increase the carbon content, and in the oxygen gas blowing treatment under reduced pressure, keeping the amount of dissolved oxygen in the molten steel low, The denitrification rate in the above was increased. The present invention is applied in combination with other known techniques for promoting denitrification and preventing nitrogen absorption. In the smelting method of low-nitrogen steel that denitrifies molten steel smelted by an electric furnace by secondary refining,
In the case of a product having a carbon content of 0.05% (weight ratio) or less, the carbon content of the molten steel is adjusted so as to be at least 0.15% or more higher than the carbon content. In the case of a product having a carbon content of 0.05% to 0.30%, the carbon content of the molten steel is adjusted so as to be at least 0.10% or more higher than the carbon content. In any case, contrary to the purpose of obtaining low carbon steel, it is to carry out a certain amount of carburization once, and the carbon monoxide gas generated intensely by decarburization contains By taking in nitrogen, a decarburization allowance for facilitating the progress of denitrification is secured. In addition, the range of the carbon content of the product is set to 0.
0.05% or less and 0.05% to 0.30%
And the lower limit of the carburized amount, such as 0.15% and 0.10%, is that when the carbon content of molten steel is less than about 0.05%, Because it does not progress. That is, when the target carbon content of the former product is set to, for example, 0.03%, the target carbon content is set to, for example, 0.1%.
17% is carburized, and 0.12% from 0.20% to 0.08% is decarburized by oxygen blowing, during which denitrification is performed. On the other hand, in the latter case, when the target carbon content of the product is, for example, 0.08%, for example, 0.12% is carburized, and the carbon content is reduced from 0.20% to 0.08%.
Up to 0.12% is decarburized by oxygen blowing, and then denitrification is attempted. From this example, when the target carbon content of the product is 0.05% or less, it is difficult to secure the decarburization allowance in the high coal area for denitrification unless the carburization amount is increased. When the content is 0.05% to 0.30%, it can be seen that the decarburization allowance for denitrification can be secured without increasing the carburizing amount.

By the way, even if the decarburization allowance is equal to 0.12%, the carbon content of the molten steel is 0.05%.
% Or less and, for example, in the range of 0.20% to 0.25%, the dissolved oxygen content of the molten steel in the former is extremely high, so that the rate at which nitrogen of the molten steel is taken into the carbon monoxide gas is small. Denitrification hardly progresses. Conversely, in the latter, the dissolved oxygen content of the molten steel is low,
Denitrification is known to progress significantly. As described above, the denitrification effect increases as the decarbonization is started from a higher value of the carbon content of the molten steel, but it takes a longer time to decarbonize to the carbon content of the product. R in time according to the cycle time in the early 50 minutes range of recent electric furnace operation
The balance point for performing H or VOD treatment and achieving a denitrification effect is suitably about 0.30% or less as a total decarburization amount. The adjustment to make the carbon content of the molten steel higher than the carbon content of the product is performed by the ladle refining furnace, RH or V
It is good to do before oxygen blowing of OD. That is, it is carried out by adding carbonaceous material in a ladle refining furnace or adding carbonaceous material in a vacuum tank before the denitrification period in RH degassing equipment or VOD decarburizing equipment. In addition, in terms of carburizing efficiency, it is better to add the RH in the subsequent RH tank than to the ladle refining furnace. If oxygen blowing advances and the carbon content of the molten steel once decreases, then if carburizing is performed, the dissolved oxygen content of the molten steel must be higher than when carburizing before oxygen blowing. This is because the denitrification effect is inferior.

Sulfur and dissolved oxygen are surface active elements that inhibit this denitrification reaction, and dissolved oxygen has an inhibitory action about twice and half that of sulfur. For this reason, when it is desired to promote the denitrification reaction, it is necessary to reduce the dissolved oxygen content of the molten steel and the sulfur content described below. As described above, sulfur in molten steel is a harmful component that slows the denitrification reaction rate, and the sulfur content of molten steel is determined by adding a desulfurizing agent to a ladle refining furnace or a vacuum tank during the denitrification period of RH or VOD. Is desirably set to 0.010% or less. In addition, as an addition point,
It may be simply put into molten steel, or may be by injection or blasting which is usually performed. After adjusting the carbon content of the molten steel, oxygen gas blowing is started at RH or VOD, and the flow rate of the oxygen gas is reduced under reduced pressure of the molten steel until 0.10% to 0.25% of carbon is decarbonized. period, at the time of the oxygen gas to no time 2.5 Nm ³ / ton of the molten steel, the flow rate 10 Nm ³ / ton of the molten steel-preferably 3.
It is no time 5Nm ³ / ton of the molten steel, decarburization as a low flow rate at the time of 7Nm ³ / molten steel ton. That is, as shown in FIG. 1A, the denitrification rate of molten steel during oxygen blowing under reduced pressure is higher as the carbon content of the molten steel is higher, the flow rate of oxygen gas is lower, and As shown in (b), the lower the internal pressure of the RH or VOD vacuum tank, the higher the pressure. FIG. 1 schematically shows a result of FIG. 11 described later, which is used to complete the present invention.

In this way, 0.10% to 0.2%
Until 5% of carbon is decarburized, the amount of dissolved oxygen in the molten steel is low and denitrification remarkably proceeds as described above. Hence,
If the decarburization cost is 0.10% or less, the denitrification effect is insufficient.
A sufficient denitrification effect can be obtained with a maximum decarburization cost of 0.25%.
Of course, the higher the oxygen gas flow rate, the higher the dissolved oxygen content of the molten steel, so the same denitrification effect cannot be obtained unless a larger decarburization allowance is taken, but from the higher carbon content, that is, the lower dissolved oxygen content. There is also an advantage that you can start. On the other hand, if the flow rate of oxygen gas is increased, the amount of exhaust gas such as carbon monoxide gas increases. Therefore, when there is a restriction on the exhaust facility capacity, the pressure in the tank increases, and as described later, the amount of dissolved oxygen increases and the It is disadvantageous for nitriding. The preferred oxygen gas flow rate range is 3.5 Nm ³ / ton of molten steel · hour to 7 N
m ³ / ton of molten steel · hour is the optimum range in the equipment used in the test of the inventors, and if the exhaust capacity at a pressure of 5 Torr or less is increased, the upper limit of the appropriate flow rate is 1
It is possible up to 0 Nm ³ / ton of molten steel.

The amount of dissolved oxygen in the oxygen gas blowing period is, to a first approximation, inversely proportional to the carbon content of the molten steel, and the dissolved oxygen equilibrium with the partial pressure of carbon monoxide gas in the tank in a term substantially proportional to the oxygen gas flow rate. This is a value obtained by adding the amount of oxygen. this is,
When the oxygen gas flow rate is kept constant, the supply rate of oxygen and the consumption rate of oxygen by decarburization become equal to maintain a dynamic equilibrium. In order to maintain the dynamic equilibrium, the decarburization rate represented by the following approximate expression (1) is required. It is considered that it must be increased in proportion to the oxygen gas flow rate.

(Equation 5) Therefore, the lower the pressure in the tank, the lower the dissolved oxygen concentration of the molten steel, which is considered to be advantageous for denitrification. Therefore,
The test was conducted by changing the pressure in the tank during the denitrification period to three intermediate levels of 5 Torr, 50 Torr, and 5 Torr to 50 Torr. By reducing the pressure in the tank to 5 Torr, the amount of dissolved oxygen in the molten steel was predicted as expected. Decreases, and an apparent denitrification rate constant K _N (1 /%
・ Minutes) was found to be improved.

(Equation 6)

The reason why the denitrification rate is improved more than expected from the decrease in the amount of dissolved oxygen is that carbon monoxide gas bubbles from inside the bulk molten steel are generated from a deeper position by lowering the pressure, and the bubbles are formed in the molten steel. Since the time required to separate from the surface is longer, and the expansion during bubble floating is more pronounced due to the lower pressure and the nitrogen gas partial pressure in the bubble is lower, the incorporation of molten steel nitrogen into the bubble is higher. Probably because it advanced. That is, maintaining the pressure in the RH or VOD tank at 5 Torr or less during the denitrification period is one of the extremely desirable conditions, and it is advantageous for denitrification to keep the pressure as low as possible as long as the exhaust capacity permits. . On the other hand, in the RH treatment, it is advantageous to make the difference between the concentrations of carbon, nitrogen and dissolved oxygen in the molten steel in the tank and that in the ladle as small as possible in order to promote denitrification. And easily estimated from equation (2). Therefore, the amount of molten steel in the ladle (ton) ト ン
It is one of the highly desirable conditions throughout the denitrification period and the rapid decarburization period that the cycle time of the molten steel reflux defined by the molten steel reflux rate (ton / min) be 1.3 minutes or less.

When the molten steel reflux cycle time is 1.3 minutes or more, the same degree of denitrification effect cannot be obtained unless the oxygen gas flow rate in the denitrification period is reduced or the decarburization amount is not increased. This also increases the processing time. The above reflux velocity W
(Ton / min) is calculated by the following equation.

(Equation 7) There are various formulas for calculating the recirculation velocity, but the above formula is Ono's formula, and the above-mentioned 1.3 minutes is based on the calculation based on the formula. Therefore, even when the reflux rate of a different value is calculated by using another equation, it is sufficient that the time is 1.3 minutes or less when applied to the above equation. For reference, the following is calculated by using the equation (3) under the following conditions.

(Equation 8) Assuming that the molten steel amount is 94 tons, the molten steel circulation cycle time is 1.26 minutes.

It is also important to rapidly exhaust the gas when starting the RH or VOD process. It is preferable to start the denitrification period after reducing the pressure in the vacuum chamber to 5 Torr or less within 5 minutes from the start of the exhaust. If the evacuation time to 5 Torr is extended, not only is the entire RH treatment time prolonged, but also the time required to absorb nitrogen from the atmosphere in the tank while the pressure is reduced from the atmospheric pressure to about 100 Torr is not preferable. However, in the case of starting decompression treatment using undeoxidized molten steel containing no denitrifying element of Si or Al as raw steel (molten steel before RH treatment), the dissolved oxygen in the molten steel reacts with carbon to cause severe boiling. , The exhaust speed must be reduced. If it is necessary to shorten the RH or VOD processing time, the oxygen gas flow rate is set to 10 Nm ³ / ton of molten steel to 40 Nm after the denitrification period is completed.
³ / Tons of molten steel hourly and decarbonize rapidly to near the carbon content of the product. In the rapid decarburization period, the pressure in the vacuum chamber of RH or VOD is maintained at 50 Torr or less.

The purpose of this rapid decarburization stage is to decarburize in as short a time as possible, and since denitrification cannot be expected so much, the higher the oxygen gas flow rate, the better. However, when the oxygen gas flow rate is too high, when oxygen gas is blown in with the top lance, there is a problem that the erosion of the refractory on the bottom surface and the side surface increases due to the collision of the oxygen jet with the bottom surface and the side surface of the vacuum chamber. . On the other hand, when oxygen gas is blown with a semi-dip nozzle (a nozzle at a height immersed in molten steel during oxygen gas blowing embedded in the lower side of the vacuum chamber), the lower side of the vacuum chamber opposite to the nozzle is blown. The upper limit of the flow rate of oxygen gas is restricted due to an increase in the erosion of the refractory. The oxygen gas flow rate can be increased as the inner diameter of the vacuum chamber is increased. However, the overall upper limit of the oxygen gas flow rate in the rapid decarburization period is 40 Nm ³ / ton of molten steel. further,
When it is necessary to shorten the time of the rapid decarburization period in the degassing equipment of RH or VOD, if the reduction of the molten steel temperature is acceptable, add a solid oxygen source such as mill scale or iron ore into the vacuum chamber. The decarburization speed may be improved by doing. Further, the reason why the pressure in the vacuum chamber during the rapid decarburization period is desirably 50 Torr or less is that a slight denitrification also proceeds during this period, and the degree of deoxidation after the end of self-decarburization after rapid decarburization. In order to reduce the amount of deoxidizer at the time of adjustment, the pressure in the tank should be as low as possible in order to reduce the amount of dissolved oxygen in the molten steel. However, considering the exhaust capacity (kg / h) at each pressure level in the vacuum tank, This is because, in reality, the compromise must be reduced to 50 Torr or less.

At the end of the RH or VOD treatment, it is naturally necessary to properly adjust the contents of all components, the degree of deoxidation and the temperature of the molten steel to the target. Therefore, it is desirable that the dissolved oxygen amount of the molten steel immediately before adjusting these before the end of the treatment is as low as possible so as to stabilize the yield of the deoxidizing agent at a high level and minimize the amount of the generated primary deoxidizing products. . Therefore, in the rapid decarburization period, when the carbon content reaches -0.03% to + 0.04% with respect to the target value of the product, the supply of acid to the molten steel is stopped. Carburizing material that is 0.04% or less of the amount of molten steel in the ladle is added to the RH or VOD vacuum tank, and if possible, under a high vacuum of 1 Torr or less, self-decarburization due to carbon and dissolved oxygen in the molten steel. , The amount of dissolved oxygen in the molten steel is preferably reduced. The addition of the carburizing agent is used, for example, when producing ultra-low carbon steel with a carbon content of 0.003%, when the dissolved oxygen content at the end of rapid decarburization is extremely high compared to the carbon content. This is because the amount of dissolved oxygen hardly decreases even if decarburization is performed, and thus the amount of dissolved oxygen is reduced by self-decarburization by supplying a carbon source. When the dissolved oxygen is reduced by adding a deoxidizing agent such as Al, A
This is because a deoxidized product such as l ₂ O ₃ is generated in excess, and it is necessary to make the reflux time for floating the deoxidized product longer.

The nozzle used for oxygen blowing is provided by a top lance which can be inserted vertically from the upper part of the vacuum chamber and can move up and down like a lance of a top-blowing converter.
Oxygen gas may be blown from above the molten steel in the vacuum chamber, or during the oxygen gas blowing on the side of the lower vacuum chamber, oxygen is introduced into the molten steel obliquely from above by a semi-dip nozzle embedded at a height immersed in the molten steel. It is also possible to inject gas. Before starting the test, the top lance imagined that the area of the hot spot where the oxygen gas jet collides with the molten steel was smaller than that of the semi-deep nozzle. If the ratio of decarburization at the flash point to the total decarburization amount, including decarburization from the molten steel bulk, is high, the denitrification reaction rate at the fire point where the dissolved oxygen concentration is extremely high is slow. This is because the lance was considered to be more disadvantageous for denitrification than the semi-dip nozzle. However, as a result of the test, no significant difference was observed between the Tofrance and the semi-dip nozzle on denitrification. In addition, if a top lance is used as is well known, the secondary combustion rate Pc [= CO ₂ % / (CO%
+ CO ₂ %)], the effect of the temperature rise due to the secondary combustion, and the effect that the metal ingot in the vacuum chamber is significantly reduced is considered. It is determined that the use of a lance is advantageous.

In general, in the denitrification treatment of molten steel in RH, VOD, and the like, it is a common sense to minimize the amount of air entering a vacuum chamber. Certainly, when air enters from the immersion tube itself or from the joint between the immersion tube and the bottom of the vacuum chamber, nitrogen gas enters the molten steel bulk and undergoes nitrogen absorption. Therefore, it is common sense to use argon gas for all kinds of purging in a vacuum chamber as well as for circulating molten steel. However, since the unit price of the argon gas is high and the amount of the gas used is large, the purging gas of the top lance, the purging gas of the observation window for monitoring the inside of the chamber attached to the upper part of the vacuum chamber, the alloy, and the alloy are tried for cost reduction. The gas for purging the iron inlet and the gas for recompressing the alloy-added hopper were all switched from argon gas to crude nitrogen gas (containing about 1% oxygen gas). According to the test results, not only the denitrification behavior during the period when a large amount of carbon monoxide gas is generated from the molten steel by blowing oxygen gas, but also the transition of the nitrogen content of the molten steel after stopping the oxygen gas blowing No significant difference was observed before and after switching to nitrogen gas. It is probable that nitrogen gas absorption did not occur as a result of maintaining a low nitrogen gas partial pressure in the tank during acid feeding and self-decarburization by increasing the exhaust capacity at each tank pressure level. That is, it is understood that if the exhaust capacity at each level of the tank pressure is increased, the nitrogen gas is replaced with expensive argon gas as various purging gases. Incidentally, it is known that the equilibrium value of the nitrogen content in the molten steel is 10 ppm when the nitrogen gas partial pressure is about 0.4 Torr, and 20 ppm when the nitrogen gas partial pressure is about 1.5 Torr.

The composition of the molten steel after ladle refining and before the RH treatment includes not only Si killed steel but also a carbon content of 0.00%.
Adjustment was made so that silicon of 0.05% to 0.30% was also previously contained in the element steel of the ultra-soft wire steel having a silicon content of about 3% and a silicon content of about 0.015%. This mainly using CaO-CaF ₂ based as desulfurizing agent in the ladle refining furnace, allowed heavy metals Al system such as Al ash as a means of reducing the dissolved oxygen amount of the molten steel to increase the distribution ratio of sulfur Rather, the use of Si deoxidation. By the way, the above 0.30
If it is less than about%, it will be sufficient not only for extremely soft wires but also for thick plates.

As described above, the carbon content is approximately 0.5.
In the region of 10% or more, the amount of dissolved oxygen in the molten steel depends on the pressure in the tank, the carbon content of the molten steel, and the oxygen gas flow rate.
Under the condition of m ³ / ton of molten steel / hour or less, if the silicon content of the molten steel is about 0.30% or less, the amount of desiliconization accompanying oxygen gas blowing is small. The ratio of the amount of desiliconization to the amount of decarburization decreases as the oxygen gas flow rate decreases and the pressure in the tank decreases. That is, when it is desired to smelt low-nitrogen steel having a carbon content of 0.1% or more and a silicon content of about 0.2% as a product component target, silicon is contained in the raw steel before the RH treatment. Even so, there is almost no desiliconization loss. When it is desired to obtain an Al-Si killed steel product, the amount of dissolved oxygen in the molten steel is reduced by the above-described self-decarburization, and then the Al in the tank may be added. Thus, the carbon content of the molten steel is about 0.1
% Or more, preferential decarburization in oxygen gas blowing is recognized, but as the carbon content of the molten steel decreases to about 0.1% or less and the dissolved oxygen content of the molten steel increases to about 80 ppm or more, the amount of desiliconization with respect to the decarburization amount The ratio gradually increases. As the carbon content of the molten steel decreases, the dissolved oxygen content of the molten steel seems to be more dependent on the silicon content of the molten steel. If an attempt is made to desiliconize to a silicon content of about 0.015%, the dissolved oxygen content becomes about 400 ppm or more, and the carbon content of the molten steel also decreases to 0.01%. In order to obtain a product having such a low silicon content, it is not advisable to increase the silicon content before the RH treatment in terms of loss of silicon and oxygen gas required for desiliconization. However, from the viewpoint of the desulfurization efficiency in the ladle refining furnace, it is advantageous to include at least 0.05% of silicon in the molten steel after the ladle refining in view of the relative price of Al and silicon.

The effects of the present invention will be described below with reference to examples. Molten steel produced by an electric furnace was tapped by an eccentric bottom tapping method, carburized and desulfurized in a ladle refining furnace, and then denitrified and decarburized by RH. The amount of molten steel in the ladle is about 90 to 94 tons, and the specifications and operating conditions of the RH degassing equipment are as follows.
m, the flow rate of argon gas for reflux is 1,000 Nl /
Min, exhaust capacity is 0.5 Torr to 5 Torr,
2,000k at 0kg / h, 5 to 60 torr
g / h, 3000k at 60 to 200 torr
g / time, oxygen gas when the maximum 1,500 nm ³ / the top lance single-hole Laval nozzle, there is a maximum 500 Nm ³ / oxygen-flow capacity when the nozzle of the two semi dipping method. Incidentally, the exhaust capacity per ton of molten steel in the ladle at 0.5 Torr to 5 Torr that this RH equipment has is as follows:
It is the best in the country as far as the literature can tell. The molten steel composition after ladle refining is generally in the following range.

(Equation 9)

FIGS. 2 and 3 show the relationship between the carbon content and the silicon content of the molten steel during oxygen gas blowing, that is, during the supply of oxygen, in the region where the carbon content of the molten steel is 0.18% or less. Data of the same heat (for one pot) is connected as a solid line, a dashed line, and a dashed line, respectively, and is represented separately for each heat. In the case where the pressure in the tank in FIG. 2 is around 50 Torr, as the oxygen gas flow rate is higher and the silicon content of the molten steel is higher, the ratio of the amount of desiliconization to the amount of decarbonization increases from the region of higher carbon content. Has begun. That is, when comparing 500 Nm ³ / h with 1,200 Nm ³ / h, in the latter, the slope of the desiliconization accompanying decarburization becomes stronger from the place where the carbon content is high, and the 1,200 Nm ³ / h
It is understood that the slope of the desiliconization accompanying the decarburization is larger in the case of the dashed-dotted line having a higher silicon content than in the case of the solid line in the case of m ³ / hour from the place where the carbon content is higher. Of course, the tendency that the ratio of the amount of desiliconization to the amount of decarburization gradually increases as the carbon content of the molten steel decreases is common to all data. When the pressure in the tank is about 5 Torr in FIG. 3, the ratio of the amount of desiliconization to the amount of decarburization starts to increase sharply from the lower carbon content region as compared with FIG. 2 in which the oxygen gas flow rate is 500 Nm ³ / hour. . These are understood from the relationship between the carbon content of the molten steel during the acid supply and the dissolved oxygen amount described below. Incidentally, the fact that the symbols in the figure, which are connected by the same kind of line with the same heat, change in the middle means that the oxygen gas flow rate is changed at that time. Since the flow rate of oxygen gas described in FIG. 2 and subsequent figures is Nm ³ / hour, as described above, when the amount of molten steel in the ladle is converted to Nm ³ / ton of molten steel / hour based on about 90 tons, the following is obtained. Become like

(Equation 10) Incidentally, as mentioned in the section of the prior art, JP-A-60-18
In Example 2 of Table 1 of Japanese Patent No. 4619, in a vacuum chamber pressure of 6 Torr, the carbon content of the molten steel after the top blowing at an oxygen gas flow rate of 4,500 Nm ³ / hour is 0.01%, and the silicon content is 0.01%. The amount is stated as 0.08%. If this is converted into the above-mentioned 90 ton RH equipment, it is about 1,620 Nm ³
Per hour, and the impression that the silicon content is too high relative to the carbon content is inevitable when compared with FIG. 3 described above.

FIGS. 4 and 5 show the relationship between the carbon content and the dissolved oxygen content of the molten steel during the acid supply. 0.05% C or less is omitted because the amount of dissolved oxygen increases rapidly. Here, the relative level of the dissolved oxygen amount seems to be reliable, but the absolute value of the dissolved oxygen amount varies depending on the oxygen probe maker, so it cannot always be said that it is reliable. Referring to FIG. 4 at 50 Torr, the difference between the amount of dissolved oxygen at the CO equilibrium at 50 Torr and the measured amount of dissolved oxygen increases roughly in proportion to the amount of acid transport, and is inversely proportional to the carbon content. And then decrease. However, the reason why the dissolved oxygen amount of the molten steel does not increase so rapidly in the section where the carbon content of the molten steel is 0.10% to 0.05% when the oxygen gas flow rate is 1,200 Nm ³ / hour can be seen in FIG. As described above, it is considered that the ratio of the amount of oxygen consumed for desiliconization in this range of the carbon content increases rapidly and the decarburization rate gradually decreases. Compared to FIG. 4 at 5 Torr with FIG. 4 at 50 Torr, an oxygen gas flow rate of 500 Nm ³ / hour clearly
Thor has lower dissolved oxygen content, and the effect of the tank pressure is apparent.

FIG. 6 shows an example of the results of a denitrification test of an ultra-low carbon / ultra-low silicon steel. Four minutes after the start of the RH treatment, acid supply at 500 Nm ³ / hour was started with the semi-dip nozzle, and after 19.5 minutes, the flow rate was increased to 900 m ³ / hour, and after 21 minutes, acid supply at 900 Nm ³ / hour by the top lance was also started. 27.
After 5 minutes, the acid supply of the top lance was stopped, the acid supply of the semi-dip nozzle was stopped after 28.5 minutes, and the decarburization was carried out until 35.5 minutes. Nitrogen content is 21p after 19.5 minutes
pm, it decreased to 20 ppm after 35.5 minutes. At this time, the carbon content and the silicon content were reduced to 0.006% and 0.010%, which are the target values of the mild steel. In this test, the pressure Pr in the vacuum chamber after 8 minutes was maintained at about 5 Torr or less.

Nitrogen gas is used for various purging of the vacuum tank, and the sulfur content of the molten steel before the treatment is 0.023%.
FIG. 7 shows an example of a test result in which a carbon material was added to the tank before acid supply. 4.5 minutes after the start of the treatment, carbonaceous material was added to the tank and carburized.
0 Nm ³ / hour of acid transfer was started, and after 19 minutes 900 Nm ³
/ Hour while to stop by raising the flow rate after 30 minutes, 500Nm by the top lance ³ / time of the oxygen-flow-from after 19 minutes 2
It goes until 9.5 minutes later. In addition, 2 in C in the figure
The portions to which the numbers 195 in 9 and Pr are attached do not fit in the figure, and are displayed at lower positions. Although the nitrogen content drops to 24 ppm after 20.6 minutes, denitrification is hardly observed during the rapid decarburization period after the oxygen gas flow rate was increased. This equipment
It has been confirmed that the uniform mixing time of molten steel under operating conditions is less than 1 minute when Cu is added to the tank. It seems that uniform mixing takes several minutes. The pressure Pr in the tank after the start of the acid transfer was between 50 Torr and 5 Torr. In any case, it was confirmed that nitrogen gas for purging could be used, and even if the sulfur content of the molten steel being treated was 0.020% or more, denitrification could be achieved up to 24 ppm.

FIG. 8 shows an example of the test results of the medium carbon / Si killed steel. Four minutes after the start of the treatment, carbon material was added to the tank,
After 5.5 minutes, the top lance started feeding 500 Nm ³ / hour of acid, and stopped after 27.5 minutes. Further, acid feeding at 500 Nm ³ / hour by the semi-dip nozzle was performed for 2 minutes after 32.5 minutes. In this heat, the oxygen gas flow rate was not increased during the acid feeding. The nitrogen content is 22 after 24 minutes.
ppm, but the subsequent denitrification is slight. Starting acid supply at a higher carbon content than in FIG.
FIG. 9 shows an example of a test result obtained by continuing acid supply for a long time in the middle carbon region. Six minutes after the start of the treatment, carbonaceous material was added into the tank,
500 minutes from the minute to 40 minutes with the semi-dip nozzle
Acid transfer at Nm ³ / hour was continued for 30 minutes. The pressure Pr in the tank during acid feeding is about 50 Torr, but the nitrogen content is 22 ppm 24 minutes after the start of the treatment, and 12 pp after 40 minutes.
m.

FIG. 10 shows an example of a test result in which acid supply was started at a high carbon content as in FIG. 9 and acid supply was continued for a long time in a medium carbon region. Seven minutes after the start of the treatment, a carbon material was added into the tank, and acid supply at 500 Nm ³ / hour was continued for 30 minutes by a semi-dip nozzle from 11 minutes to 41 minutes. The pressure in the tank is 10 Torr or less except during a period when the pressure temporarily rises due to inadequate operation of the exhaust system. Nitrogen content is Si
It dropped to 40 ppm as early as 11 minutes after the start of the treatment by killed steel degassing, and 23 ppm after 18.5 minutes.
After 34.5 minutes, it decreased to 12 ppm. Although the influence of the dissolved oxygen content of the molten steel was not mentioned, the lower the dissolved oxygen content, the better the denitrification, and the denitrification seemed to be stagnant when the dissolved oxygen content was 80 ppm to 100 ppm or more. The dissolved oxygen amount decreases as the carbon content increases and the oxygen gas flow rate decreases and as the vacuum chamber pressure decreases. 6 to 10 show the tendency of denitrification accompanying decarburization, the oxygen gas flow rate described in Japanese Patent Application Laid-Open No.
0 in terms of tons RH equipment, from about 1,100 nm ³ / no time becomes 3,300Nm ³ / hour, to no 315 nm ³ / time is optimal oxygen-flow range of de窒期the present invention 630Nm
^The flow rate greatly exceeds ³ / hour. Nevertheless, 10 to 14 ppm in 15 to 30 minutes
The remarkable denitrification effect in the above-mentioned publication that the nitrogen content is reduced to the nitrogen content of the present invention is presumed to be due to the use of equipment having an evacuation capacity that is remarkably higher than that of the present invention, such as a vacuum chamber pressure of 1.7 Torr. Is done. Therefore, it is understood that according to the present invention, no excessive exhaust capacity is required.

Here, the abscissa represents the carbon content of the molten steel at the start of acid supply, the ordinate represents the apparent denitrification rate constant K _N ^′, and the stratification by the tank pressure and the oxygen gas flow rate. FIG.
Shown in The sulfur content of the molten steel is also indicated. In addition, since the equilibrium nitrogen content [% N] _e in the equation (2) was difficult to estimate, K _N ^′ was determined by the following equation (4), assuming that [% N] _e = 0.

[Equation 11] Since data after the time when denitrification stagnated in the evaluation of K _N ^' were cut off, K _N ^' has some arbitrary parts. FIG. 11 clearly shows that the higher the carbon content at the start of the acid supply and the lower the flow rate of the oxygen gas, the higher the K _N ^′ is in the data at around 50 Torr in the tank pressure. Further, when the oxygen gas flow rate is 500 Nm ³ / hour, K _N ^′ is highest when the pressure in the tank is around 5 Torr, and is next to 5 Torr to 50 Torr,
It is also clear that the pressure around 50 Torr is the lowest, indicating that the influence of the pressure in the tank is large. When the pressure in the tank is 5 Torr and the oxygen gas flow rate is 500 Nm ³ / hour, if the carbon content at the start of acid supply is set to 0.20% or more, it is easily estimated that K _N ^′ becomes 25 or more. Where it is. Further, the sulfur content of the molten steel is reduced from 100 ppm or less to 200 p.
When it rises above pm, K _N ^′ has fallen slightly.

The target of the nitrogen content of the molten steel after the RH treatment naturally varies depending on the type of steel and the destination.
ppm or 25 ppm, and assuming that the nitrogen content at the start of acid supply is 100 ppm or 80 ppm, K _N ^′ and the time required for denitrification are calculated by simple division using equation (4). FIG. By the way, in this test, no special measures were taken to promote denitrification and prevent nitrogen absorption in the operation of electric furnaces, tapping and ladle refining furnaces, so heat with a nitrogen content of around 80 ppm before RH treatment was used. There were many. Assuming that the nitrogen content before the start of acid supply is 1
The nitrogen content after the 00 ppm denitrification treatment was set to 20 ppm,
The carbon content before the start of acid supply is 0.193%, the oxygen gas flow rate is 500 Nm ³ , the pressure in the tank is around 5 Torr, and K _N ^′ is 2
If the data of 4.6 (1 /% · min) is adopted from FIG. 11, the required denitrification time is 16.3 minutes from FIG. 50
Acid is fed at 0 Nm ³ / hr for 16.3 minutes and the amount of molten steel in the ladle is 90
In the case of tons, the decarbonation efficiency is 0.10% C or more and 90
If a value close to the lower limit of the test result is adopted as 75% for% and 0.10% C or less, the decarburization amount during this period is 0.137%. The remaining 0.056% of carbon, 0.05% of silicon and 0.13% of Mn were converted to 1,200N with an oxygen utilization efficiency of 75%.
Assuming that oxygen is removed at an oxygen gas flow rate of m ³ / hour and that the amount of dissolved oxygen increases by 650 ppm in an extremely low carbon / ultra low Si region,
The required amount of oxygen gas is 197 Nm ³ , and the required time for the rapid decarburization period is 9.9 minutes.

Here, the oxygen utilization efficiency is the ratio of the amount of oxygen consumed to the amount of oxygen gas sent. In this case, C is CO, Si is SiO ₂ and Mn is MnO.
And an increase in the amount of dissolved oxygen is added to this. The reason for assuming the removal of Mn of 0.13% is that R in the case of melting extremely low carbon and extremely low silicon steel is used.
The test results of the difference between the Mn content before H treatment and the Mn content after melting are adopted. After all, the acid supply starts 5 minutes after the start of the RH treatment, and if it takes 16.3 minutes for the denitrification period and 9.9 minutes for the rapid decarburization period, the total is 31.2 minutes. The total RH treatment time, including the time required for self-decarburization, component / deoxidation / temperature control, and stirring for flotation separation of the primary deoxidation product, can be matched with the cycle time of the electric furnace in the early 50 minutes. it can. This example is an example of melting an ultra-low carbon / ultra-low silicon steel that requires the longest time for processing, and de-siliconization and Mn-elimination when melting an Si-killed steel having a higher carbon content. Therefore, the amount of oxygen required for increasing the amount of dissolved oxygen can be reduced, so that the RH treatment time can be shortened. R
Assuming that the H treatment time is not shortened, the lower nitrogen content can be achieved by initiating a lower oxygen gas flow rate acid feed from a higher carbon content and a longer denitrification period at a lower dissolved oxygen content. 9 and 10
As shown in FIG.

As can be seen from the above description, the present invention relates to the oxygen gas blowing treatment of molten steel under reduced pressure such as RH without employing any special denitrification technology or nitrification prevention technology in electric furnaces and ladle refining. By combining existing technologies for preventing nitrogen absorption in continuous casting and continuous casting, it becomes possible to produce low-nitrogen steel having a nitrogen content close to that of converter steel as electric furnace steel. Therefore, steel types that could not be produced in an electric furnace because of their high nitrogen content, such as extremely mild steel wire rods, cold-rolled steel sheets for deep drawing produced by a continuous annealing process, and heat treatment for high-tensile line pipes Rolled steel sheets and the like can also be manufactured from electric furnace steel.
In the case of medium carbon steel or high carbon steel of 0.3% C or more, denitrification by spraying oxide particles to molten steel under reduced pressure has been put to practical use. If low-nitrogen steel of medium carbon steel or ultra-low carbon steel of C or lower is to be melted, a remarkable denitrification effect is exhibited.

[Brief description of the drawings]

FIG. 1 schematically shows the denitrification effect according to the present invention. FIG. 1 (a) shows the denitrification with respect to the carbon content at the start of acid supply when the pressure inside the tank is kept constant and the oxygen gas flow rate is changed. (B) is a graph showing the change in the denitrification rate with respect to the carbon content at the start of acid supply when the oxygen gas flow rate is kept constant and the pressure inside the vacuum chamber is varied, in a graph showing the change in the rate.

FIG. 2 is a graph of test results showing the relationship between the carbon content and the silicon content of molten steel during acid supply at a tank pressure around 50 Torr.

FIG. 3 is a graph of test results showing the relationship between the carbon content and the silicon content of molten steel during acid supply at around 5 torr in the tank.

FIG. 4 is a graph of test results showing the relationship between the carbon content of molten steel and the amount of dissolved oxygen during acid supply at around 50 torr in the tank.

FIG. 5 is a graph of test results showing the relationship between the carbon content of molten steel and the amount of dissolved oxygen during acid supply at a tank pressure around 5 Torr.

FIG. 6 is a graph showing an example of a result of a denitrification test of an ultra low carbon ultra low silicon steel.

FIG. 7 is a graph showing an example of a result of a denitrification test of an extremely low-carbon low-silicon steel.

FIG. 8 is a graph showing an example of the results of a denitrification test of medium-course Si-killed steel.

FIG. 9 is a graph showing an example of the results of a denitrification test on medium-coal Si-killed steel.

FIG. 10 is a graph showing an example of a denitrification test result of medium-course Si-killed steel.

FIG. 11 is a graph of test results showing the relationship between the carbon content of molten steel at the start of acid supply and the apparent denitrification rate constant.

FIG. 12 is a graph of a calculation result showing a relationship between an apparent denitrification rate constant and a time required for denitrification.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toru Otsuyama 1-1-2 Nishijima, Nishiyodogawa-ku, Osaka-shi, Osaka Inside the Joint Works of Osaka Works (72) Inventor Daigo Kaneko, Nishiyodogawa-ku, Osaka-shi 1-2-1, Nishijima, Joint Works, Osaka Works (72) Inventor Go Go Honda 1-2-1, Nishijima, Nishiyodogawa-ku, Osaka-shi, Osaka, Japan Joint Works, Osaka Works (56) References JP Hei 5-195041 (JP, A) JP-A-60-1884619 (JP, A) JP-B-56-27576 (JP, B2)

Claims (13)

(57) [Claims] 1. A low-nitrogen steel smelting method in which molten steel smelted by an electric furnace is denitrified by secondary refining, wherein the carbon content of the product to be 0.05% or less is at least 0.15% or more. The carbon content of the molten steel is adjusted so as to obtain an oxygen gas blowing treatment under a reduced pressure of RH or VOD.
De窒期of from 10% to 0.25% of carbon up to decarburization, during the oxygen gas to no time 2.5 Nm ³ / ton of the molten steel, the flow rate 10 Nm ³ / ton of the molten steel-preferably 3.5 Nm ³ /
During the rapid decarburization period to reduce the RH or VOD treatment time, the oxygen gas flow rate was reduced to near the carbon content of the product, by denitrification at a low flow rate of molten steel ton / hour or 7 Nm ³ / molten steel ton / hour. To 10 Nm ³ / ton of molten steel / hour or 4
A method for producing low-nitrogen steel using electric furnace molten steel, characterized by rapid decarburization at a high flow rate of 0 Nm ³ / ton of molten steel / hour. 2. A method for refining low-nitrogen steel in which molten steel produced by an electric furnace is denitrified by secondary refining, wherein the carbon content of the product is at least 0. The carbon content of the molten steel is adjusted so as to be at least 10% higher, and then 0.10% to 0.25% of carbon is decarburized during the oxygen gas blowing treatment of the molten steel under reduced pressure by RH or VOD. de窒期until the to no time 2.5 Nm ³ / ton of the molten steel, the oxygen gas flow rate during 10 Nm ³ / ton of the molten steel-preferably 3.5 Nm ³ / to no time ton of the molten steel, during 7 Nm ³ / ton of the molten steel, Denitrification at low flow rate and subsequent RH or V
The rapid decarburization period for reducing the OD treatment time, to rapid decarburization as a high flow rate at 40 Nm ³ / ton of the molten steel, to no time 10 Nm ³ / ton of the molten steel, the oxygen gas flow rate until the carbon content close to the product A method for producing low-nitrogen steel using electric furnace molten steel. 3. In the denitrification period, the RH or VO
3. The method of claim 1, wherein the pressure in the vacuum chamber of D is maintained at 5 Torr or less. 4. The method according to claim 3, wherein the pressure in the vacuum chamber is reduced to 5 Torr or less within 5 minutes from the start of evacuation, and then denitrification is started. Low nitrogen steel melting method. 5. The operating condition at the RH is as follows: molten steel recirculation cycle time (minute) defined by ladle molten steel amount (ton) ÷ reflux velocity (ton / min). 3. The electric furnace molten steel according to claim 1 or 2, wherein when calculated based on the recirculation velocity (ton / min) expressed by the following expression, it is set to 1.3 minutes or less. Low nitrogen steel melting method. 6. The electric furnace molten steel according to claim 1, wherein the pressure in the vacuum chamber of the RH or VOD is maintained at 50 Torr or less during the rapid decarburization period. Melting method of low nitrogen steel used. 7. The method according to claim 7, wherein RH or VOD is applied during the rapid decarburization period.
3. The method of claim 1 or 2, wherein a solid oxygen source such as a mill scale is added to the vacuum chamber. 8. The adjustment to make the carbon content of the molten steel higher than the carbon content of the product is performed by adding a carbon material in a ladle refining furnace or adding a carbon material in a vacuum tank before the denitrification period in RH or VOD. A method for producing low-nitrogen steel using the electric furnace molten steel according to claim 1 or 2. 9. A desulfurizing agent is added to a ladle refining furnace, or
The electric furnace molten steel according to claim 1 or 2, wherein a desulfurizing agent is added to the vacuum tank before the denitrification period of RH or VOD to reduce the sulfur content of the molten steel to 0.010% or less. Of low-nitrogen steel using smelting. 10. When the carbon content of the product reaches −0.03% to + 0.04% with respect to the target value, the supply of acid to the molten steel is stopped, and the amount of molten steel in the ladle is reduced to 0.04%. % Or less of carbonized material in a vacuum chamber of RH or VOD, and self-decarburization by the carbon and dissolved oxygen in the molten steel under reduced pressure to reduce the amount of dissolved oxygen in the molten steel. The method for producing low-nitrogen steel using molten steel in an electric furnace according to claim 1 or 2, wherein the degree of deoxidation and the composition of the steel are adjusted. 11. The oxygen gas blowing process comprises the steps of:
3. The method according to claim 1, wherein the lance is vertically inserted from above H. 12. A steel ladle refining furnace to reduce the silicon content of molten steel to 0.1.
The method for producing low nitrogen steel using the electric furnace molten steel according to claim 1 or 2, wherein the oxygen steel is blown under reduced pressure by adjusting the molten steel to 0.05% to 0.30%. . 13. The oxygen gas blowing under reduced pressure is performed by RH
Nitrogen gas, purge gas for top lance,
3. The electric furnace molten steel according to claim 1, wherein the molten steel is used as a gas for purging an observation window in the tank, a gas for purging an alloy inlet, and a gas for recompressing an alloy-added hopper. Melting method of low nitrogen steel.