JP3293023B2 - Vacuum blowing method for 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 refining molten steel by a vacuum refining apparatus, and more particularly, to a method using a vacuum refining apparatus having a straight-body immersion pipe and blowing oxygen from a lance inserted into a vacuum chamber to improve the efficiency. The present invention relates to a method for vacuum blowing acid of molten steel which enables decarburization refining to an extremely low carbon region.

[0002]

2. Description of the Related Art As a method of decarburizing ultra-low carbon molten steel under reduced pressure,
RH and DH are widely used. However, when smelting ultra-low carbon steel at RH, a large amount of splash is generated, and the metal adheres to the vacuum tank, causing a great hindrance to the operation, and the metal with a high carbon concentration becomes more difficult. The carbon pick-up by re-melting causes a serious problem that the decarburization rate in an extremely low carbon region is significantly reduced. Also, higher oxygen concentration is advantageous for decarburization,
If the blower carbon in the converter is lowered to increase the oxygen concentration, the blower temperature will increase and the cost of the converter refractory will increase. Is needed.

To cope with these problems, Japanese Patent Application Laid-Open No. 2-775
No. 18 discloses a method of utilizing a secondary combustion reaction by blowing oxygen gas from above a predetermined distance from a molten steel surface so as to reach a pressure within a proper range of the surface level. Also, Japanese Patent Application Laid-Open No. 2-54714 discloses a method in which oxygen is blown from a water-cooled top blow lance by RH.

However, in the case of RH, two immersion tubes are provided in a vacuum chamber. Since the vacuum chamber has the bottom of the vacuum chamber inevitably, it is necessary to blow the top-blown oxygen to such an extent that it does not reach the bottom of the tank. is there. Further, since there is no effect even if oxygen is blown until the molten steel starts to be recirculated, as described in JP-A-4-176812, the degree of vacuum is set to a higher vacuum than about 200 Torr. Unless
No oxygen is blown. Further, at such a degree of vacuum, the top-blown oxygen gas directly collides with the tank bottom refractory, so that the tank bottom refractory is severely melted and damaged.

[0005] As described above, the problems of oxygen overblowing at RH can be summarized as follows. (1) Since oxygen cannot be blown unless the degree of vacuum is higher than about 200 Torr, decarburization is achieved by increasing the oxygen concentration. In addition to being most effectively promoted, oxygen is not sprayed during the early stages of the process, when the carbon concentration is high and most dislikes the adhesion of molten steel to the furnace wall as ingot. (2) Even if the degree of vacuum is high and the degree of vacuum is small, since the distance between the molten steel surface and the tank bottom is small, the velocity of the jet of top-blown oxygen to the molten steel is reduced by the dynamic pressure of the gas. The cavity (cavity) cannot be deepened (hard blow), and the utilization efficiency of oxygen is low. Conversely, since the cavity is shallow (soft blow), secondary combustion in which the CO gas generated by the decarburization reaction is oxidized to CO _{2 in the} space is likely to occur, but the heating of the molten steel is poor and the temperature of the molten steel is compensated. In this case, the temperature of the refractory is too high, and when the purpose is to prevent metal adhesion by heating the refractory, there is a problem that the temperature of the molten steel cannot be increased. In addition, since the conventional top-blowing lance is designed under conditions that show proper expansion behavior under the normally used oxygen flow rate, lance operation secondary pressure , and atmospheric pressure, the atmospheric pressure changes during processing like a vacuum refining furnace. In this case, hard blow is performed under high vacuum conditions, and soft blow is performed under low vacuum conditions. When the atmospheric pressure is changed, there is a problem that acid transfer cannot be performed under appropriate conditions.

The present inventors have disclosed in JP-A-6-116624 a vacuum refining furnace in which gas is stirred from a deep position and the inside of the tank is decompressed using a large-diameter straight-body immersion tank. This is based on the finding that it is very effective to effectively agitate the surface of molten steel exposed to a vacuum to increase its substantial surface area. By widening the bubble activated surface, which is a region floating on the surface of molten steel, extremely efficient decarburization is enabled. However, in this method, decarburization proceeds without stagnation of the decarburization rate to an extremely low carbon concentration, and there is little splash. However, in the case of long-term operation, sticking of the metal becomes a problem.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a method disclosed in
In the technique of acid supply from a top-blowing lance at RH disclosed in Japanese Patent Application Laid-Open No. 77518, JP-A-2-54714, and JP-A-4-176812, since the tank bottom is provided, the acid is supplied in a low vacuum range. There is a problem in that the acid cannot be used and hard blow cannot be performed even at a high vacuum degree, so that the utilization efficiency of oxygen is low. In addition, with the method disclosed in Japanese Patent Application Laid-Open No. 6-116624 alone, metal deposits may occur when operating for a long time. In addition to solving the problem that the conventional top-blowing lance does not allow acid transfer under the appropriate conditions when the atmospheric pressure changes, the efficient The purpose is to provide a decarburization refining technology up to the low carbon region.

[0008]

Means for Solving the Problems The inventors of the present invention set various conditions based on a system in which molten steel is sucked into a cylindrical dip tube and an inert gas is introduced from a gas injection hole provided at the bottom of the ladle. A test was performed with a change, but it was not possible to perform decarburization to a stable extremely low carbon region. This is because the splash with a high carbon concentration scattered immediately after the start of the decarburization treatment adheres to the refractory wall surface and dissolves at the end of the decarburization period when the carbon concentration is low to supply carbon. Therefore,
Although the top blowing lance was introduced, the lance
It has been found that by adopting a lance design that properly considers the effects of the working pressure and the degree of vacuum, it is possible to supply acid under appropriate conditions even when the atmospheric pressure changes.

That is, the gist of the present invention is as follows. (1) Vacuum degassing device with a degree of vacuum of 10 to 400 Torr
In the range of r, oxygen gas is 3-1 to 1 to 5 m above the molten steel surface by using the gas supply upper blowing lance inserted from above.
8 nm ³ / A (Hr · ton) refining method of molten steel supplied at a flow rate of, at the time of operation the lance operation secondary pressure P _o (kg
f / cm ² · G) is the lance design secondary pressure P _s (kgf / c)
m ² · G), while changing the oxygen gas flow rate during operation by changing it in the range of 0.7 to 2.5 times , the following (1)
The parameter u calculated by the formula is in the range of 0.5 to 2.
The distance from the lance tip to the molten steel surface (Lance height:
Vacuum吹酸method molten steel and controlling the G _o (mm)).

[0010] In _{u = H / G o ...... (} 1) (1) equation, H is a jet core length, the following
It is obtained from equation (2). In H = f (X) × M s × (4.2 + 1.1M s 2) × d t ...... (2) (2) equation, f (X) is the following formula (3) from the calculated et al
It is.

[0011]

(Equation 2)

In the equation (3), X is given by the following equation (4)
Desired. X = (P _o + Q _o / 730) / (P _s + Q _o / 730) (4 ) In the equation (2), M _s is a design discharge Mach number.
It is obtained from the expression (5). M _s = 2.24 × [{(P _s + Q _s / 730) / (Q _s / 730)} ^2/7 -1] ^1/2 (5) where H is the jet core length (mm) ), _Go is Lance
Height (mm), P _o is the lance operation secondary pressure (kgf / cm
² · G), P _s is the lance design secondary pressure (kgf / cm ² ·
G), Q _o is operating vacuum (Torr), Q _s design vacuum
Degree (Torr), _dt is the nozzle throat of the top blowing lance
Part diameter (mm). (2) In the above (1), the reference vacuum degree is set to Q _B (Torr
r), if the reference lance height was set to G _B (mm), operating vacuum Q _o (Torr) in the lance height G _o (m
m) is controlled according to the following equation (6) .

[0013] _{G o = α × G B ×} (Q B / Q o) 1/2 ...... (6) where α takes a value of 0.8~1.2, Q _B is 40-2
00 (Torr), G _B is 1000~3000 (mm)
It is desirable that (3) A vacuum degassing device that immerses a part of the molten steel surface in the ladle with a straight-body immersion tube connected to the lower part of the vacuum tank, using an upper blowing lance for gas supply inserted from above the vacuum tank. The vacuum blowing acid method for molten steel according to the above (1) or (2) , characterized in that:

Here, the diameter of the nozzle throat portion (d _t : mm) of the upper blowing lance and the diameter of the nozzle outlet portion (d _e : m)
m) is desirably determined by the following equation. _{d t = (1.27 × S t} ) 1/2 S t = F s /{(0.581×n×(P s + Q s / 730)} F s = f s × W d e = (1. ^{_{27 × S e) 1/2 S e}} = S t × {(1 + 0.2M s 2) /1.2} 3 / M s M s = 2.24 × _{[{(P s + Q s /} 730) / ( ^{_{Q s / 730)} 2/}} 7 -1 ] ^1/2 where, S _t is the nozzle throat cross-sectional area (mm ^2), F _s
Is the designed acid supply rate (Nm ³ / Hr), n is the number of nozzles, P _s
Is a lance design secondary pressure of 3 to 13 kgf / cm ² · G, Q
_s is the design vacuum degree (Torr), f _s is the ton of molten steel
5 to 20 (Nm ³ / (Hr · to
n)), W is amount of molten steel (ton), S _e is the nozzle exit section
The product (mm ² ), M _s is the design discharge Mach number.

As the upper blowing lance, a water-cooled upper blowing lance having 1 to 4 holes is preferable. The present invention is based on the design vacuum degree.
10~400Tor degree of vacuum Q _o at the time of operation for the Q _s
This provides an upper blowing lance that can be hard blown even if it changes to r. The present inventors have studied in detail the characteristics of jets ejected from various lance nozzles under conditions where the degree of vacuum is greatly changed, and as a result, have reached the following facts.

Regardless of the degree of vacuum, the gas ejected from the nozzle passes through a potential core region where the flow velocity does not decrease, and reaches a characteristic attenuation region where the flow velocity decreases in inverse proportion to the first power of the distance from the nozzle tip. Take. If the enhanced degree of vacuum in the same nozzle, the jet core length hardly attenuated because entrainment of surrounding gas is reduced
(The nozzle tip at the position where the Mach number on the center axis of the jet becomes 1
(H: mm), but the behavior in the characteristic attenuation region is not affected, and the effect of the degree of vacuum is expressed only by the jet core length .

The effect of the degree of vacuum does not change even if the lance design (throat diameter, outlet diameter) is changed. Based on these three findings, the velocity U (m / s) on the jet center axis at the position of the distance Y (mm) from the nozzle tip is expressed by Expression (7) ,
The jet core length (H: mm) is represented by equations (2) to (4) .

U = 320 · H / Y (7) H = f (X) × M _s × (4.2 + 1.1 M _s ² ) × d _t (2)

[0019]

(Equation 3)

X = (P _o + Q _o / 730) / (P _s + Q _o / 730) (4) Accordingly, the behavior of the jet flowing from any nozzle is
It can be estimated under any vacuum condition. The inventor has
Based on this equation, the change in U when the lance secondary pressure was significantly changed under vacuum was measured. As a result, as shown in FIG. 1, U hardly changed even when the lance secondary pressure was greatly changed. It was found that there was a condition, and that this condition hardly depended on the degree of vacuum. From this, when blowing acid refining is performed by the vacuum degassing device, the secondary pressure P of the lance operation during operation
_o (kgf / cm ² · G) is the secondary pressure P _s (k
gf / cm ² · G), the flow rate to the molten metal surface hardly changed even when the oxygen gas flow rate was changed during operation, and the hard blow was maintained. It is possible to feed the acid at an optional flow rate as needed.

Here, when P _o / P _s during operation is smaller than 0.7, the jet flow is greatly attenuated, resulting in soft blow, which lowers the efficiency of heating the molten steel, and when P _o / P _s is larger than 2.5. In this case, since the inertia of the jet is dominant over the energy loss due to inappropriate expansion at the nozzle outlet, excessive hard blow results in operational difficulties such as generation of splash.

In the present invention, the degree of vacuum is 10 to 400 T.
It is necessary that oxygen gas is supplied at a flow rate of ³ to 18 Nm ³ / (Hr · ton) from 1 to 5 m above the molten steel surface in the range of orr. The degree of vacuum is 10 Torr
When the vacuum is higher than that, the jet flow attenuation is small and hard blow is unavoidable. When the vacuum is lower than 400 Torr, the jet flow is large and it is difficult to avoid soft blow. In addition, when the distance between the lance and the molten steel surface is smaller than 1 m, there is a problem that the lance life is short due to a large amount of radiant heat transfer to the lance. There is a problem that erosion is large. The oxygen gas is ³ Nm ³ / (H
When the value is smaller than (r · ton), the effect of using oxygen does not appear because a sufficient amount of heat cannot be obtained, and 18 Nm ³ /
If it is larger than (Hr · ton), the heat input is too large, causing refractory melting.

Further, according to the above equation, the flow velocity U (m / m) on the center axis of the jet at a distance Y (mm) from the nozzle tip.
s) is obtained, but if the lance height is controlled so that the parameter u based on this value is in the range of 0.5 to 2, more efficient blowing acid can be achieved. u is a parameter obtained by dividing U by 320 m / s, which is the sound speed under atmospheric pressure, and indicates the jet strength at the time of reaching the steel bath surface. As shown in FIG. 2, when u is smaller than 0.5, the jet intensity is too weak, so that CO burns to CO ₂ in the space, that is, so-called secondary combustion occurs violently, and the exhaust gas temperature rises. If the refractory is significantly melted and u is greater than 2, there is a problem that the jet strength is too high and a strong splash is generated.

Incidentally, in an actual process, the degree of vacuum during operation is not always constant but fluctuates. Since the jet behavior under vacuum is greatly affected by the degree of vacuum, such fluctuations have a considerable effect. According to the detailed examination of the jet characteristics under vacuum by the present inventors, no matter what the relationship between the throat diameter and the outlet diameter of the nozzle, the flow rate U ₇₆₀ on the jet center axis under atmospheric pressure and the vacuum In the jet below P
There is the following relationship between the on- axis flow velocity _UP and the following equation. Further, there is an inverse relationship between the lance height and the maximum jet velocity, regardless of the nozzle shape and the degree of vacuum.

U _P = 29 × U ₇₆₀ / √P These items cannot be estimated from the results of the conventional investigation of the jet characteristics under atmospheric pressure, and the pressure reduction under the reduced pressure, which has been completely unknown until now. This shows that the jet behavior at the can be controlled by a relatively simple function. Based on this finding, the reference vacuum Q _B (Torr), if the reference lance height was set to G _B (mm), operating vacuum Q _o (Tor
If the lance height G _o (mm) is controlled in accordance with the following equation in r), the blowing acid can always be blown with a certain range of jet strength without being affected by the above-mentioned fluctuation of the degree of vacuum.

[0026] _{G o = α × G B ×} (Q B / Q o) 1/2 ...... (6) where, α is from 0.8 to 1.2, if α is less than 0.8 In this case, there is a problem that the soft blow is excessive, and when it is larger than 1.2, the hard blow is excessive. The top-blowing method of the present invention is most effective when applied to a vacuum degassing apparatus as shown in FIG. 3, in which a straight body type immersion pipe connected to the lower part of a vacuum tank is immersed in a part of the molten steel surface in a ladle. It is. This is because the effect of the ladle slag can be eliminated by partially evacuating only the inside of the immersion tube, and the molten steel head is raised and the area where the gas blown from the lower part floats on the surface is raised. This is because there is an advantage that can be taken widely. In such a case, fine molten steel particles generated when the stirring gas breaks on the surface are scattered into the space, and the steel bath is heated to a high temperature by being heated by the heat generated by the oxygen blowing in the space. Since the mechanism of returning to the above works effectively, high heat arrival efficiency becomes possible. In contrast, in the case of the so-called tank degassing method, in which the entire ladle is vacuumed, the effects of the ladle slag cannot be excluded, and the heat generated by the slag in the upper space is transferred to the steel bath. Since it becomes a heat insulating layer, the heating efficiency does not become sufficiently high. In addition, when stirring gas is blown only from a narrow riser pipe such as RH, the gas floats intensively in a narrow area, and molten steel particles generated when bubbles are broken on the surface are scattered at high speed into the space. However, there is a problem that it does not act as a heat transfer medium and adheres to the refractory as a mere splash.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Example] Comparative Example 1 shown in Table 1 is a result of using an 8-ton scale vacuum ladle refining apparatus, in which the entire ladle is placed in a vacuum chamber. The pressure was reduced, a water-cooled top-blowing lance was inserted into the vacuum chamber from above, and oxygen gas was blown upward. Undeoxidized steel having a carbon concentration of 250 to 450 ppm before the treatment was used as the molten steel.

Here, the rate of increase of the molten steel temperature per hour was measured and defined as a rate of heat rise V (° C./min). The heat transfer efficiency η (%) was evaluated as an increase in the sensible heat of the molten steel with respect to the total calorific value when all the oxygen blown into CO ₂ . Here, Q _o is operating vacuum (Torr), G _o lance height (mm), f _o is per ton of the molten steel operating oxygen-flow-rate (N
^{m 3 / (Hr · ton)} ), P o / P s 2 primary pressure is lance operation at the time of operation ^{_{P o (kgf / cm 2 ·}} G) and the lance design secondary pressure ^{_{P s (kgf / cm 2 ·}} G) And the ratio.

Comparative Example- 2 shown in Table 2 was carried out in a vacuum refining furnace having the shape shown in FIG. As shown in FIG. 3, the vacuum chamber 1 has a straight-body immersion pipe 6 immersed in molten steel 2 in a lower ladle 3, and a top-blowing water-cooling lance 4 is inserted from above the vacuum chamber 1. And
The top blown water cooling lance 4 is gripped by a lance gripping device 7 provided on the canopy of the vacuum chamber 1 and has an appropriate lance.
Elevation control is performed to maintain the distance between the molten steel surfaces. Ar gas is blown eccentrically from the porous brick 8 at the bottom of the ladle 3 into the straight-body immersion pipe 6, and the molten steel 2 rises with the gas along one side wall in the straight-body immersion pipe 6, and from the other side. It descends and circulates in the ladle 3 and the inside of the straight body type immersion pipe 6.

Example 3 shown in Table 4 is an example of operation performed using the lance shown in Table 3 and the apparatus shown in FIG. Here, f _s is the designed acid feed rate per ton of molten steel (Nm ³ / (Hr · ton)), n is the number of nozzles, P _s is the lance design secondary pressure (kgf / cm ² ·
G), Q _s is the design vacuum degree (Torr), d _t is the top lance nozzle throat diameter (mm), d _e is the nozzle exit diameter (mm).

[0031] Examples are shown in Table 5 -4, an operation example of the G _o was conducted using an apparatus shown in the lance and 3 shown in Table 3 in a proper range. Here, Q _B is the reference vacuum (T
orr), G _B is the reference lance height (mm), Q _o is the operating vacuum (Torr), G _o lance height (mm).

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

[Table 5]

[0037]

According to the present invention, it is possible to supply oxygen with high decarburization efficiency and no sticking of metal in the high carbon concentration region at the initial stage of treatment. As a result, it became possible to heat the Al with high thermal efficiency.

[Brief description of the drawings]

FIG. 1 is a view showing experimental results of a relationship between P _o / P _s and U, where P _s is a lance design secondary pressure (kgf / cm ² · G),
P _o lance operation secondary pressure ^{(kgf / cm 2 · G)} , U denotes the central axis upstream speed of the jet.

FIG. 2 is a diagram showing an experimental result of a relationship between a parameter u, an exhaust gas temperature, and a splash generation state.

FIG. 3 is a schematic diagram showing an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum tank 2 Molten steel 3 Ladle 4 Top blown water cooling lance 5 Gas jet 6 Straight body type immersion pipe 7 Lance gripping device 8 Porous brick

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21C 7/10

Claims (3)

(57) [Claims] 1. A vacuum degassing apparatus, wherein the degree of vacuum is 10 to 40.
This is a method for refining molten steel in which oxygen gas is supplied at a flow rate of ³ to 18 Nm ³ / (Hr · ton) from 1 to 5 m above the molten steel surface using a gas supply upper blowing lance inserted from above in the range of 0 Torr. Lance operation secondary pressure P during operation
^{_{o (kgf / cm 2 · G}} ) the lance design secondary pressure P _s (k
gf / cm ² · G) in the range of 0.7 to 2.5 times, while changing the oxygen gas flow rate during operation ,
The parameter u calculated by the expression (1) is in the range of 0.5 to 2.
Distance from the lance tip to the molten steel surface so that
(Height: G _o (mm)) . In _{u = H / G o ...... (} 1) (1) equation, H is a jet core length, the following
It is obtained from equation (2). In H = f (X) × M s × (4.2 + 1.1M s 2) × d t ...... (2) (2) equation, f (X) is the following formula (3) from the calculated et al
It is. (Equation 1) In the equation (3), X is obtained from the following equation (4). X = (P _o + Q _o / 730) / (P _s + Q _o / 730) (4 ) In the equation (2), M _s is a design discharge Mach number.
It is obtained from the expression (5). M _s = 2.24 × [{(P _s + Q _s / 730) / (Q _s / 730)} ^2/7 −1] ^1/2 (5) where H is the jet core length (mm) ), _Go is Lance
Height (mm), P _o is the lance operation secondary pressure (kgf / cm
² · G), P _s is the lance design secondary pressure (kgf / cm ² ·
G), Q _o is operating vacuum (Torr), Q _s design vacuum
Degree (Torr), _dt is the nozzle throat of the top blowing lance
Part diameter (mm). 2. The method according to claim 1,Standard vacuum degree is Q
_B (Torr), reference lance heightG _B (Mm)
In case, the operating vacuum degree Q_oLance height at (Torr)
G _o (Mm)Equation (6) belowSpecially to control according to
Vacuum blowing acid method for molten steel.G _o = Α × G _B × (Q _B / Q _o ) ^1/2 ...... (6) Here, α takes a value of 0.8 to 1.2. 3. A vacuum degassing apparatus for immersing a part of the surface of molten steel in a ladle with a straight-body immersion pipe connected to a lower part of a vacuum tank,
Vacuum vessel according to claim 1 or 2 molten steel vacuum吹酸method wherein the use of lance on for the inserted gas supplied from above the.