JPH10310818A - Method for refining stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To soften slag in a vessel, to facilitate the sampling and to keep decarburizing oxygen efficiency to high level by charging CaO at a specific wt. ratio to an aluminum for raising the temp., added before decarburizing by blowing oxygen in finish refining of a stainless steel having a specific Cr concn. or higher. SOLUTION: Into the molten steel 2 in a ladle 1, containing <=5% chromium concn. tapped from a converter, a straight barrel cylindrical immersion tube 4 arranged at the lower part of the vessel, is dipped and the pressure in the immersion tube 4 is reduced and also, Ar gas 9 for stirring, as inert gas, is supplied from a porous plug 3 arranged at the bottom part of the ladle 1. Further, Al is added into the vessel and the oxygen is blown with the oxygen jet 6 from an oxygen-blown lance 5 to execute the raising of the molten steel temp. and the decarburize-refining. Then, to Al for raising the temp. (WAl : kg), further, CaO corresponding to 0.8-4.0 WAl (kg), is charged. By this method, calcium aluminate compound is formed and liquid phase ratio of the slag is improved and the oxygen efficiency for decarburizing is raised.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining stainless steel by blowing acid decarburization under reduced pressure.

[0002]

2. Description of the Related Art Generally, a VOD method is widely known as a method for finishing and decarburizing stainless steel. However, the VOD method is a method in which the entire ladle is put in a vacuum vessel, or a method in which a lid is placed on the upper part of the ladle to make the whole ladle vacuum, so that the structure of the sampling device is inevitably complicated, and therefore, There is a problem that the components and temperatures during operation are unclear due to the problem that the sampling success rate is low and the sampling success rate is low, and the variation of the blow stop operation is large. In addition, as a method of blowing acid decarburization refining under vacuum, RH
The -OB method is widely known. However, RH-O
When the method B is applied to the finishing refining of stainless steel, when oxidization of chromium proceeds during the decarburization of the blowing acid, the oxidized chromium oxide (Cr ₂ O ₃ ) in the tank flows out of the tank from the downcomer pipe, Because of the high melting point of Cr ₂ O ₃ , the slag on the ladle solidifies, making it difficult to perform operations, such as difficulty in sampling, and the Cr ₂ O ₃ that once flows out of the tank is decarbonized. Since it does not contribute to the reaction at all, there has been a problem that the decarboxylation efficiency necessarily decreases. Further, JP-A-61-
In Japanese Patent No. 37912, molten steel in a ladle is sucked up into a vacuum tank through a cylindrical dip tube, and an inert gas is blown into the ladle under the dip tube through a blow tube from below, and A method is disclosed in which an oxidizing gas is blown to the surface of molten steel in a vacuum chamber through an upper lance, but in this method as well as in the RH-OB method, Cr ₂ O ₃ flows out of the tank to cause the oxidizing gas to flow out of the ladle. There were problems such as deterioration of sampling performance and reduction of decarbonation efficiency due to solidification of slag.

[0003]

DISCLOSURE OF THE INVENTION The present invention relates to the problems of equipment that the VOD method has, such as deterioration of operability due to difficulty in sampling due to the difficulty of sampling, the RH-OB method,
Further, extremely efficient spraying can be achieved without causing problems such as deterioration of operability and deterioration of decarbonation efficiency due to outflow of solidified Cr ₂ O _{3 from} JP-A-61-37912. An object of the present invention is to provide a refining method that enables acid decarburization.

[0004]

According to the present invention, there is provided a semiconductor device comprising:
The refining method of stainless steel described in the above is a method of immersing a straight cylindrical immersion pipe provided at the lower part of a tank into molten steel in a ladle having a chromium concentration of 5% or more, which is tapped from a converter, The pressure in the dip tube was reduced, and then aluminum (A
l) is added to perform blowing acid, so that in the refining method of stainless steel in which the temperature of the molten steel is raised and the decarburization is refined, the aluminum (W _Al ; kg) for raising the heat is further
CaO equivalent to 0.8 W _{Al to} 4.0 W _Al (kg) is charged to perform the decarburization refining. Thus, the calcium aluminate compound (12CaO ·
7Al ₂ O ₃ ) can improve the liquid phase ratio of the slag. The method for refining stainless steel according to claim 2 is the method for refining stainless steel according to claim 1, wherein the immersion depth of the immersion tube in the molten steel during the heating period is in a range of 200 to 400 mm. In the decarburization period after the end of the heating, the immersion depth of the immersion pipe in the molten steel is set in the range of 500 to 700 mm to perform the decarburization refining with acid. This promotes the discharge of the calcium aluminate-based slag during the heating period, and facilitates the sampling operation. Further, in the subsequent decarburization period, the residual calcium aluminate in the tank and Cr ₂ O
Suspension of ₃ promotes entrainment of the slag in the tank into the steel bath and promotes the reduction of Cr ₂ O ₃ by carbon in the steel.

[0005] In blowing acid decarburization refining of stainless steel, chromium in the bath is once oxidized by oxygen blown into the steel bath to form chromium oxide (Cr ₂ O ₃ ), and then this Cr ₂ O ₃ is formed. ₃ is known to proceed by being reduced by oxygen in the steel. This Cr ₂ O ₃ is an oxide having a high melting point, and therefore the presence of the Cr ₂ O ₃ significantly lowers the liquid phase ratio of the slag. In the method of immersing a straight cylindrical cylindrical immersion pipe in molten steel in a ladle and then reducing the pressure in the immersion pipe to perform a bleaching acid decarburization refining as in the present method, the Cr once formed in the tank is used. _{When 2} O ₃ is discharged out of the tank and is taken into the slag on the ladle, the slag on the ladle remarkably solidifies, thereby making sampling work difficult. It becomes unclear and hinders the determination of the blowing stop. Furthermore, if this Cr ₂ O ₃ remains in a solidified state in the tank, some C
As described above, although r ₂ O ₃ is entrained in the bath and discharged out of the tank, most of the r ₂ O ₃ remains floating on the molten steel in the vacuum tank, and therefore there is little contact with the carbon in the molten steel. Causes significant chromium oxidation loss.

The present inventors have conducted various experiments in order to solve these problems, and softened the slag in the tank to promote the entrainment of Cr ₂ O ₃ -rich slag into the molten steel. A method for improving the reduction efficiency of slag and preventing solidification of slag after discharge from the tank was found. The present invention has been made based on these findings. As a specific method, a heat-up Al amount (W _Al ; k) in an Al heat-up period performed before blowing acid decarburization.
g) to 0.8 W _{Al to} 4.0 W _Al (kg).
aO is to be introduced. This is an Al oxide (Al ₂
O ₃ ) itself is an oxide having a high melting point, but is reacted with CaO to form a calcium aluminate compound (12C).
aO.7Al ₂ O ₃ ) can reduce the melting point. Thereby, Cr ₂ O ₃ is appropriately suspended in the molten calcium aluminate compound, and entrainment into the bath is promoted, whereby Cr ₂ O ₃ is promoted.
_The chance of contact between ₂ O ₃ and carbon in steel increases, resulting in Cr
_It is possible to maintain the reduction efficiency of ₂ O ₃ , that is, the decarbonation efficiency at a high level. Furthermore, even if these molten compounds are discharged out of the tank, solidification of the ladle slag is prevented, which is extremely effective for sampling work. Here, the amount of CaO to be added at the time of raising the temperature of Al includes the amount of heat-raising Al (W _Al ;
It is preferable that the amount is 0.8 W _{Al to} 4.0 W _Al (kg) corresponding to the weight of (kg). This is because the amount of CaO added is 0.8
If it is less than W _Al (kg), 12CaO · 7Al ₂ O ₃
Is insufficient, and a single phase of Al ₂ O _3, which is a high-melting oxide, precipitates in large quantities, resulting in insufficient melting of the slag in the tank. Conversely, 4.0 W of CaO
_{If the} amount added exceeds _Al (kg), the amount of slag in the tank itself increases, so that the spraying of the oxygen jet on the steel bath surface is hindered, and as a result, the efficiency of decarbonation decreases. It is.

The immersion depth of the immersion tube in the molten steel during the heating period is preferably in the range of 200 to 400 mm. This is because Al ₂ O ₃ and Ca
This is for promoting the generation of calcium aluminate by appropriately contacting O, and when the immersion depth is less than 200 mm, the contact time of Al ₂ O ₃ with CaO in the tank is short,
Before the calcium aluminate is produced, it is discharged out of the tube. As a result, the slag on the ladle is solidified, and the sampling property is deteriorated.
If the immersion depth exceeds 00 mm, the residence time of the calcium aluminate in the immersion pipe is long, which promotes the erosion of the refractory in the immersion pipe, and the residual in the tank during the subsequent decarburization period. Due to excess slag,
This is because the efficiency of decarboxylation is reduced due to the inhibition of the blowing acid jet from reaching the molten steel. Furthermore, in the decarburization period, it is desirable to set the immersion depth to 500 to 700 mm. This is because the Cr ₂ O ₃ in the decarburization period is retained in the tank for a longer time to promote reduction by reaction with carbon in the steel, and when the immersion depth is less than 500 mm,
The discharge of slag entrained in steel is promoted,
This is because the reduction of Cr ₂ O ₃ by carbon in the steel does not occur, resulting in a decrease in decarbonation efficiency.
If the immersion depth is made deeper than 0 mm,
Although the retention of ₂ O ₃ is promoted, the circulation of the molten steel in the vacuum chamber and the ladle is deteriorated, so that the supply of carbon to be used for the reduction reaction is hindered. This is because the elementary efficiency is reduced.

Here, the supply amount of the inert gas to be used for the molten steel stirring is 3.3 to 4.7 NL /
(Min.t), 1.7 to 6.0 NL / (min.
t) is desirable. The reason for this is, as in the above, to ensure an appropriate reaction time between Al ₂ O ₃ and CaO during the heating period, and from the viewpoint of the retention of Cr ₂ O ₃ and the circulation balance of the molten steel during the decarburization period. Even if the immersion depth of the immersion tube is kept within the above-mentioned appropriate range, if the gas flow rate during the heating period is less than 3.3 NL / (min.t), a large amount of slag itself will remain in the tank. The decarburization efficiency is reduced during the decarburization period due to the residual carbon dioxide, and at a gas flow rate exceeding 4.7 NL / (min · t), sampling performance is deteriorated due to discharge outside the tank before calcium aluminate generation. Is a problem. Furthermore, if the gas flow rate is less than 1.7 NL / (min.t) during the decarburization period, the reduction of decarboxylation efficiency due to insufficient reflux becomes a problem, and 6.0 NL.
If the gas flow rate exceeds / (minute · t), the early discharge of the generated Cr ₂ O ₃ becomes a problem.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention. Here, FIG. 1 is a schematic structural view of a vacuum refining apparatus can be suitably used refining method of the stainless steel according to an embodiment of the present invention, FIG 2 is W _CAO / W _Al
FIG. 3 shows the relationship between the immersion depth and the decarboxylation efficiency during the heating period, and FIG. 4 shows the relationship between the immersion depth and the decarbonation efficiency during the decarburization period. FIG. 5 is a diagram showing the relationship between the flow rate of the bottom blown Ar gas and the decarbonation efficiency during the heating period, and FIG. 6 is a diagram showing the relationship between the flow rate of the bottom blown Ar gas and the decarbonation efficiency during the decarburization period.

As shown in FIG. 1, a straight cylindrical immersion pipe 4 provided at the lower part of a tank is immersed in molten steel 2 in a ladle 1 having a chromium concentration of 5% or more and produced from a converter. After doing
The pressure in the immersion tube 4 is reduced, and the Ar gas for stirring, which is an inert gas, is passed through the porous plug 3 provided at the bottom of the ladle 1.
A gas (bottom blown Ar gas) 9 is supplied. Further, aluminum (Al) is added into the tank, and blowing acid is performed by blowing oxygen from the blowing acid lance 5 with an oxygen jet 6, thereby raising the temperature of the molten steel and performing decarburization refining. Here, aluminum for temperature heat; to (W _Al kg), further, 0.8W _Al ~4.0W _Al
(Kg) of CaO is supplied. As a result, a calcium aluminate compound is formed, the liquid phase ratio of the slag is improved, and as shown in FIG. 2, the decarbonation efficiency is increased. Further, by setting the immersion depth of the immersion pipe 4 in the molten steel 2 during the heating period to be in the range of 200 to 400 mm,
The oxide and CaO are appropriately contacted to promote the formation of calcium aluminate, and as shown in FIG. 3, the decarboxylation efficiency is improved. In the decarburization period after the completion of the heating, the immersion pipe 4 is immersed in the molten steel 2 at a depth of 500 to 700 mm to promote the reduction of Cr ₂ O ₃ by reaction with carbon in the steel. As shown in FIG. 4, the decarboxylation efficiency is maintained at a high level. Further, A for stirring used for molten steel stirring
As shown in FIG. 5 and FIG. 6, the supply amount of the r gas 9 is 3.3 to 4.7 NL / (min · t) in the heating period, and
The decarboxylation efficiency is increased to 1.7 to 6.0 NL / (minute · t). In addition, in FIG. 1, the code | symbol 7 shows the slag in a tank, and the code | symbol 8 shows a slag on a ladle.

[0011]

EXAMPLES Examples were carried out using a 150-ton vacuum refining apparatus. In the converter, [% C] is 0.6 to 0.7%,
After smelting molten steel containing 16.0 to 16.5% of [% Cr], heat was raised and blowing acid decarburization was performed using a vacuum refining apparatus having the shape shown in FIG. In this case, the acid transfer rate is 2% during the period from the heating period to the decarburizing period [% C] = 0.15%.
3.3 Nm ³ / (ht) constant, then [% C]
= 0.05%, while performing acid feed rate control to gradually reduce the acid feed rate at a constant frequency up to 10.5 Nm ³ / (ht), blowing at [% C] = 0.05% stopped.

[0012]

[Table 1]

[0013]

[Table 2]

Tables 1 and 2 show examples of the present invention together with comparative examples. Cases 1 to 12 are examples according to the present invention. On the other hand, in case 13, since the W _CAO / W _Al ratio was less than 0.8, the formation of calcium aluminate was not promoted, and the slag remained in a solid state. Poor decarbonation efficiency. Case 14 is C
The amount of slag becomes large due to excess aO,
As a result, decarburization inhibition of the oxygen jet occurs during the decarburization period. Cases 15 and 16 show that the immersion depth during the heating period was 2
This is the case where it is less than 00 mm and more than 400 mm. If it is less than 200 mm, the sampling property is poor and the decarboxylation efficiency in the decarburization stage is low. On the other hand, if it exceeds 400 mm, although the sampling property is good, there is a problem of a decrease in decarbonation efficiency (inhibition of decarburization by covering) due to insufficient discharge of slag in the tank. Case 17,
No. 18 has an immersion depth of less than 500 mm and 700
mm. 500m
When the diameter is less than m, slag solidification (sampling property deterioration) and decarbonation efficiency decrease due to early outflow of slag rich in Cr ₂ O ₃ are observed, and when it exceeds 700 mm, it is caused by deterioration of molten steel circulation. The reduced decarbonation efficiency becomes a problem. Further, in Examples 19 and 20, the bottom blown Ar flow rate during the heating period was less than 3.3 NL / (min.t) and 4.7 NL.
/(Min.t), 3.3 NL
If it is less than /(min.t), the decarbonation efficiency decreases due to the slag remaining in the large amount tank, and if it exceeds 4.7 NL / (min.t), the sampling property deteriorates due to insufficient production of calcium aluminate. Cases 21 and 22 are cases in which the bottom blown Ar flow rate during the decarburization period is less than 1.7 NL / (min.t) and more than 6.0 NL / (min.t), respectively, and is 1.7 NL, respectively. / (Min · t) is less than the reflux limit, and 6.0 NL / (min · t) is greater than the generated Cr ₂ O.
₃ ) Decrease in decarbonation efficiency due to early extravasation.

[0015]

According to the present invention, it is possible to control the chromium oxidation loss at the time of raising the temperature of Al and during the decarburization of the blowing acid, and to perform the refining of the bleaching acid decarburized stainless steel having a very small unit of reducing material.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a vacuum refining apparatus that can suitably use a method for refining stainless steel according to one embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the W _CAO / W _Al ratio and the efficiency of decarbonation.

FIG. 3 is a graph showing the relationship between the immersion depth during the heating period and the efficiency of decarbonation.

FIG. 4 is a diagram showing the relationship between the immersion depth in the decarburization period and the decarboxylation efficiency.

FIG. 5 is a graph showing the relationship between the flow rate of bottom-blown Ar gas and the efficiency of decarbonation during the heating period.

FIG. 6 is a diagram showing the relationship between the flow rate of bottom-blown Ar gas and the efficiency of decarbonation during the decarburization period.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Ladle 2 Molten steel 3 Porous plug 4 Immersion pipe 5 Blowing acid lance 6 Oxygen jet 7 Slag in tank 8 Slag on ladle 9 Ar gas for stirring

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Kanno 1-1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Nippon Steel Corporation Yawata Works

Claims (2)

[Claims] 1. A straight cylindrical cylindrical immersion pipe provided at the lower part of a tank is immersed in molten steel in a ladle having a chromium concentration of 5% or more, which is produced from a converter. In the method for refining stainless steel, in which the pressure is reduced and then aluminum (Al) is added into the tank and the acid is blown out, thereby raising the temperature of the molten steel and performing decarburization refining, the aluminum (W _Al ; kg), CaO equivalent to 0.8 W _{Al to} 4.0 W _Al (kg) is further added, and blowing acid decarburization refining is performed. 2. The immersion depth of the immersion pipe in the molten steel during the heat-up period is set in a range of 200 to 400 mm, and in the decarburization period after the completion of the heat-up, the immersion pipe is immersed in the molten steel. 2. The method for refining stainless steel according to claim 1, wherein the blasting of the stainless steel is performed by setting the immersion depth of the steel to 500 to 700 mm.