JP7052585B2 - Dephosphorization method for molten steel - Google Patents

Description

The present invention relates to a molten steel dephosphorization treatment method for carrying out a dephosphorization treatment of a molten steel having a high carbon concentration by an RH type vacuum smelting furnace.

Phosphorus in steel products is a harmful element that causes non-uniformity of material properties due to segregation, brittle fracture at low temperatures, and a decrease in hardness and strength of thin plate products. Dephosphorization is performed to the specified value or less by the treatment. The most economical dephosphorization treatment is the reaction in which phosphorus in the molten iron is absorbed and removed as phosphoric acid in the CaO-based slag refined slag, and it is decarburized by oxidative blowing in a basic converter. Removal by phosphorus reaction has been carried out. In addition, various refining techniques are used, such as pre-phosphorus dephosphorization by pre-treatment with hot metal before decarburization in a converter and dephosphorization treatment at the ladle stage.

When the oxidative dephosphorization reaction to such molten iron is allowed to proceed, the carbon in the molten iron is oxidized at the same time. On the other hand, steel products used for high-grade bearing steel and wire rods with excellent characteristics include products with a high carbon concentration of 1% by mass or more in terms of carbon concentration, and phosphorus is contained in slag by an oxidation reaction. It is difficult to secure a high oxygen potential represented by total Fe in the slag when removing the slag. Therefore, for example, decarburization is performed until the iron oxide concentration in the slag, which increases at the end of the converter blowing, increases, the oxygen potential in the slag is increased, and then sufficient dephosphorization is performed. Means such as adding a large amount of carbon may be used in the ladle refining stage. Such a method is generally called a blow-down coal addition method. However, the damage to the refractory of the converter due to the blow-down blowing becomes large, the heat loss occurs due to the addition of a large amount of carbonaceous material, and the use of a large amount of high-purity carbon source leads to an extremely serious cost increase. Or something.

In order to avoid the operation called "blow-down carbonization method" as described above, at present, a large amount of flux is added in advance at the stage of hot metal pretreatment to dephosphorize in advance, or it is relatively high in a converter. A soft blow operation is carried out to increase the iron oxide concentration in the slag during blowing in the carbon region. However, in order to meet the growing needs for high carbon and low phosphorus steel products in the future, it is required to present new and efficient operation technology.

As an effective means for dephosphorization of high carbon molten steel, in order to increase the oxygen potential in the slag during decarburization, a CaO source such as limestone and iron oxide are premelted or sintered as an additive flux for excellent slag. Means using low melting point calcium ferrite are used. In the converter blowing, the operation as described in Patent Document 1 is proposed, and in the ladle refining (RH method), the operation as described in Patent Document 2 is proposed, and a large amount of carbon is proposed in the subsequent process. It is becoming possible to operate without the addition of.

However, the low melting point calcium ferrite sintered product produced by applying heat treatment in advance is essentially that the dephosphorizing agent itself is expensive because it involves heat energy and operating cost for its production. With challenges. Further, since calcium ferrite is reduced by carbon in the converter, there is a problem that it is difficult to secure sufficient oxygen potential necessary for dephosphorization of high carbon molten steel. Furthermore, stirring is insufficient in ladle dephosphorization refining, and the oxygen potential at the interface between molten steel with low oxygen potential (oxygen activity) in molten iron and slag with high oxygen concentration is secured at a high level suitable for dephosphorization. There is also the problem of not being able to do it.

Further, as another method, for example, a method as shown in Patent Document 3 has been proposed. In this method, in order to produce a high carbon ultra-low phosphorus steel having a C concentration of 1.2% by mass and a P concentration of less than 0.01% by mass, the molten steel in the carbon concentration region of 0.7% by mass or less can be produced. A dephosphorization treatment using a high-cost CaC ₂ flux is performed under pressurized conditions, and then carbon is added to a predetermined concentration. However, even in this method, the equipment cost is extremely high, the flux and auxiliary material costs are high, and the productivity is low.

Japanese Unexamined Patent Publication No. 2010-1536 Japanese Patent Application Laid-Open No. 2015-232157 Japanese Unexamined Patent Publication No. 62-107017

Tatsuro Kuwahara et al .: Iron and Steel, 73 (1987), S176.

In order to meet the further increase in demand for low-phosphorus and high-carbon steel products due to user demands in recent years, it is strongly desired to develop a manufacturing technology that is cheaper than before. Therefore, it is an extremely important issue to present a technology that can be manufactured at low cost as an alternative technology to the means using expensive calcium ferrite or CaC ₂ .

In view of the above-mentioned problems, the present invention does not impose a heavy load in the hot metal pretreatment stage or the converter blowing stage, suppresses a large amount of energy and operating cost in advance, and is efficient due to CaO-based dephosphorization slag in the RH process. It is an object of the present invention to provide a method for dephosphorizing molten steel having a high carbon concentration.

The present inventors focused on molten steel dephosphorization technology using the RH degassing method, and continuously CO-boiled (hereinafter referred to as internal degassing) while maintaining a high oxygen potential in the vacuum chamber of the RH type vacuum slag refining furnace. By generating charcoal), the surface oxygen potential between the slag and the molten steel is maintained under favorable conditions for the progress of the dephosphorization reaction, while an emulsion state between the slag and the molten steel is generated to promote the dephosphorization reaction. Furthermore, by discharging the dephosphorized slag in the vacuum chamber before deoxidizing or adding the alloy, it is possible to prevent the phosphorus concentration of the molten steel from increasing due to dephosphorization, and efficiently dephosphorize the molten steel with a high carbon concentration. I found that I could do it.

That is, the gist of the present invention is as follows.
(1) A method for dephosphorizing molten steel, which is dephosphorized before deoxidizing and adjusting the alloy.
C: Molten steel containing 0.7 to 2.0% by mass was placed in a pan and depressurized in an RH type vacuum smelting furnace, and CaO, SiO ₂ , Al ₂ O ₃ , t. The total of Fe, MgO, MnO and P ₂ O ₅ is 85% by mass or more, and the conditions specified by the formulas (1) to (4) are satisfied, and the viscosity η in the range specified by the formula (5) at the time of dissolution. When adding a dephosphorizing agent that becomes (cP)
A carrier with oxygen gas or iron oxide from a top-blown lance installed in a vacuum chamber with the molten steel recirculated with the average molten steel flow velocity Um of the descending immersion pipe set to 0.5 m / s or more and 1.0 m / s or less. The dephosphorizing agent is added at a CaO mass of 0.03 kg / t or more while the gas is sprayed onto the molten steel under the condition that the collision flow velocity U specified by the equations (6) to (8) is 15 m / s or more.
After starting the addition, the numerical ranges of the average molten steel flow velocity Um and the collision flow velocity U are maintained for 4 minutes or more, and the oxygen gas or iron oxide is supplied in an oxygen mass of 0.003 kg / t or more, and then the generated slag is removed. A method for dephosphorizing molten steel, which comprises discharging phosphorus slag from the vacuum chamber through the descending dip tube.
70.0% by mass ≧ (% CaO) ≧ 40.0% by mass ・ ・ ・ ・ (1)
(% CaO) / (% SiO ₂ ) ≧ 1.5 ・ ・ ・ ・ (2)
(% CaO) / (% Al ₂ O ₃ ) ≧ 2.0 ・ ・ ・ ・ (3)
(% CaF ₂ ) <3.0% by mass ... (4)
100 ≤ η (cP) ≤ 250 ... (5)
U = Lp × M ₁ / Lo × Co ・ ・ ・ ・ (6)
Lp = 0.8 × M ₁ × (5.88 + 1.54M ₁ ² ) Ds ・ ・ ・ ・ (7)
M ₁ = (2 / (K-1) x Pn ^{((K-1) / K)} -1) ^1/2 ... (8)
Here, (% X) represents the ratio (mass%) of X in the dephosphorizing agent.
η (cP) = 1.2 (% CaO) +7.3 (% SiO ₂ ) +0.8 (% Al _{2O 3} ) +0.5 (% t.Fe) +0.3 (% MgO) +0.2 ( % MnO) +0.2 (% P ₂ O ₅ ).
Lp: Potential core length (mm), M ₁ : Representative Mach number (-), Lo: Lance height from the molten steel surface in the vacuum chamber (mm), Co: Representative sound velocity (340 m / s), Ds : Nozzle equivalent diameter (mm) of lance throat, Pn: Nozzle back pressure (kg / cm ² ), K: Gas specific heat ratio (-).

According to the present invention, the dephosphorization reaction due to the dephosphorization slag in the vacuum chamber can be efficiently promoted even for high carbon molten steel by the RH degassing method, and the adverse effects of deoxidation and dephosphorization after alloy addition are adversely affected. Can be suppressed to a low level. Therefore, it is possible to reduce operational obstacles such as the addition of a large amount of carbon in the manufacturing process of the molten steel stage of high carbon steel products, which could not be carried out in the past, and to carry out inexpensive and efficient dephosphorization of high carbon-containing steel. It becomes possible.

It is a figure for demonstrating the structure of the RH type vacuum smelting furnace in the embodiment of this invention. It is a figure which shows the relationship between the slag drop diameter in molten steel and the terminal velocity. It is a figure which shows the correlation between the experimental value of viscosity at 1600 degreeC, and the estimation formula.

Hereinafter, the present invention will be described with reference to the drawings.
FIG. 1 is a diagram for explaining the structure of the RH type vacuum smelting furnace in the embodiment of the present invention. The apparatus shown in FIG. 1 is an RH type vacuum smelting furnace having an oxygen top blowing function and a powder top blowing function using a carrier gas such as an inert gas, and the top blowing lance 1 is water-cooled with little operational failure due to lance breakage or the like. The structure is desirable.

FIG. 1 shows a state during the dephosphorization treatment, in which the high carbon molten steel 3 is housed in the ladle 2, and the ladle slag 4 is present above the high carbon molten steel 3. A dephosphorizing agent 6 for dephosphorization, which is a CaO system, is added to the vacuum chamber 5 from the upper hopper 7, and the dephosphorizing slag 8 is present in a state of being slagged in the high temperature vacuum chamber 5. Further, the vacuum chamber 5 is provided with an ascending dipping tube 12 (inner diameter D _u ) and a descending dipping tube 13 (inner diameter D _d ), and is inserted into the high carbon molten steel 3 to a position deeper than the layer of the ladle slag 4. Has been done. Further, the rising immersion pipe 12 is provided with a recirculation gas tuyere 11 for blowing the recirculation gas 10. The high carbon molten steel 3 contains carbon after converter blowing in the previous step and impurities such as phosphorus and dissolved oxygen. By reducing the pressure in the vacuum chamber 5, the molten steel is sucked upward. The molten steel is circulated by the recirculation gas 10.

This embodiment is intended for a product having a C concentration of 0.7% by mass or more and 2.0% by mass or less and mainly manufactured by a steelmaking process using continuous casting, and is converted as described above. The main purpose is to solve the operational problems caused by the large amount of carbonaceous material added after decarburization / dephosphorization treatment in the furnace and the problems that an expensive dephosphorizing agent such as calcium ferrite must be used. Therefore, even if some charcoal for adjusting the components is allowed after the dephosphorization treatment in the RH type vacuum smelting furnace, the decrease in C concentration in the molten steel due to the dephosphorization treatment in the RH type vacuum smelting furnace is only recovered. From this, the C concentration in the molten steel before the start of the dephosphorization treatment according to the present embodiment, that is, immediately before the dephosphorization agent according to the present embodiment is put into the vacuum chamber, is the C concentration of the product manufactured from the target molten steel. The concentration standard is 0.7 to 2.0% by mass.

The standard of the C concentration of the product manufactured from the target molten steel is 2.0% by mass or less, and the C concentration in the molten steel before the start of the dephosphorization treatment according to the present embodiment is 2.0% by mass or less. For the reason. That is, if an attempt is made to manufacture a product from molten iron having a C concentration of more than 2.0% by mass, there are problems such as cracking in the continuous casting process and breakage when processing the final product from the slab. Because.

At this time, when the C concentration in the molten steel is 0.7% by mass or more, an oxygen source for generating internal decarburization is present as dissolved oxygen in the molten steel in the vacuum chamber when the pressure is not tens of atmospheres or more. It is not possible. Therefore, in the present embodiment, gaseous oxygen is sprayed from the top-blown lance 1 with an oxygen jet 16 to generate iron oxide in the high-carbon molten steel 3, and the iron oxide is suspended as a suspended slag 15 inside the high-carbon molten steel 3, or above. Iron oxide such as iron ore is sprayed from the blow lance 1 with a carrier gas such as argon to infiltrate into the molten steel and suspend it as suspended slag 15, and CO bubbles 14 are generated by internal decarburization shown in the following equation (9). It is an important point of this embodiment to make it.
(Fe _x O) _{in metal} + [C] → Fe + CO ↑ ・ ・ ・ ・ (9)

Here, (Fe _x O) _{in metal} indicates iron oxide existing inside the high carbon molten steel below the slag metal interface in the vacuum chamber, and is reduced by carbon inside the molten steel to cause a slag metal reaction. Internal decarburization occurs due to the required interfacial disturbance. Then, the dephosphorized slag is emulsified by internal decarburization to expand the interfacial reaction, and at the same time, it promotes mass transfer of iron oxide, which enhances the oxygen potential of the slag, to the interface. As a result, the interfacial oxygen potential can be maintained at a high level, and the dephosphorization reaction proceeds effectively.

Since the present invention aims to dephosphorize a molten steel having a C concentration of 0.7% by mass or more and which is unlikely to cause internal decarburization, the molten steel before the start of the dephosphorization according to the present invention is aimed at. The medium C concentration is 0.7% by mass or more.

The dephosphorizer added in the vacuum chamber is desired to have a high CaO activity for increasing the dephosphorization efficiency. At this time, a simple substance such as a compound, a mixture, or a premelt product may be added. The composition of the dephosphorizing agent in the vacuum chamber is 70.0% by mass ≧ (% CaO) ≧ 40.0% by mass (formula (1)), (% CaO) / (% SiO ₂ ) ≧ 1.5. It is necessary to satisfy the requirements of (Equation (2)) and (% CaO) / (% Al ₂ O ₃ ) ≧ 2.0 (Equation (3)). In addition, (% X) represents the ratio (mass%) of X in the dephosphorizing agent.

Here, the reason for satisfying the requirements of the equations (2) and (3) is that in the slag having a high CaO concentration, the presence of SiO ₂ and Al ₂ O ₃ has the effect of increasing the melting ratio of CaO. On the other hand, SiO ₂ and Al ₂ O ₃ also have an effect of lowering the activity of CaO. Therefore, the lower limit is set by the equations (2) and (3) in consideration of the balance between the melting ratio and the maintenance of the activity of CaO. The upper limit of (% CaO) / (% SiO ₂ ) and (% CaO) / (% Al ₂ O ₃ ) is not specified, but the upper limit of the CaO concentration is set to 70% by mass in the formula (1). The upper limit of them is also set naturally. The melting ratio of CaO does not have to be 100%, and the liquid being treated is placed in a slag state in which the solid phase CaO is suspended under conditions that do not affect the proper maintenance of the viscosity of the liquid phase slag in the molten slag. Maintaining a high CaO activity of the phase slag is one of the effective means for increasing the dephosphorization efficiency.

However, when the CaO compounded as a dephosphorizing agent exceeds 70% by mass, the CaO activity of the slag slag can be maintained at a high level, but P is absorbed into the slag due to poor slagging due to the unslagged CaO. Is hindered. Therefore, in the dephosphorizing agent, CaO is 70% by mass or less.

Further, CaF ₂ , which is conventionally known as a raw material for a dephosphorizing agent, has a remarkable effect of lowering the viscosity, and the slag during the dephosphorizing treatment becomes finer, so that the discharge of the dephosphorizing slag is promoted. It is not necessary to add CaF ₂ to the dephosphorizer, but CaF ₂ may be inevitably mixed when using recycled slag or the like as the dephosphorizer. Even in such a case, the CaF ₂ content in the dephosphorizing agent needs to be less than 3% by mass (formula (4)). When quantifying CaF ₂ in the dephosphorizing agent, it can be calculated and evaluated assuming that Ca is present in an equal amount molar ratio with respect to the F content obtained by chemical analysis.

It is necessary to add 0.03 kg / t or more of the dephosphorizing agent satisfying the above conditions in terms of CaO mass. This is because if the addition amount is less than 0.03 kg / t, the dephosphorization effect may not be clearly obtained. When carrying out the present invention, it is said that the effect is recognized if the dephosphorization rate {(before treatment [P] -after treatment [P]) × 100 / before treatment [P]} is not obtained at 10% or more. Is hard to say. The amount of 0.03 kg / t in terms of CaO mass is 3 CaO · P ₂ O ₅ in the slag with about 10% of the P concentration in the ultra-low P steel with a P concentration of 0.01 mass% in the assembled state in the liquid phase. Corresponds to the amount required to be absorbed. On the other hand, the upper limit of the CaO mass to be added does not need to be determined from the dephosphorization reaction surface. However, from the viewpoint of wear of the refractory of the RH type vacuum smelting furnace due to the dephosphorization treatment in the RH type vacuum smelting furnace, matching with the continuous casting operation, etc. There is an upper limit.

The dephosphorization treatment time (the time from the start of addition of the dephosphorization agent to the start of discharge of the dephosphorization slag after dephosphorization) should be 4 minutes or more in order to clearly obtain the dephosphorization effect according to the present invention. is necessary. On the other hand, the dephosphorization treatment time is preferably 25 minutes or less from the viewpoint of wear of the refractory of the RH type vacuum smelting furnace, matching with the continuous casting operation, and the like. Considering the dephosphorization treatment time of 25 minutes, it can be said that the upper limit of the CaO mass of the dephosphorizing agent to be added is preferably 30 kg / t in light of the stable transport capacity of the carrier gas.

In high carbon molten steel, the dissolved oxygen in the molten steel has a low concentration that is insufficient for the dephosphorization reaction. However, in the dephosphorization process in the hot metal pretreatment by the converter, the dissolved oxygen in the hot metal with a C concentration of about 4% by mass is desorbed with high oxygen potential by strong stirring accompanying blowing against the extremely small amount of hot metal. It is well known to those skilled in the art that dephosphorization proceeds efficiently by promoting mass transfer of iron oxide in phosphorus slag and increasing the oxygen potential at the interface. For this reason, even in the dephosphorization treatment for high carbon molten steel, it is necessary for the efficient dephosphorization treatment to stir strongly in the vacuum chamber by generating the internal decarburization of the above-mentioned formula (9). This can be easily explained by a similar reaction mechanism. In order to generate this internal decarburization, the collision pressure of the oxygen jet is increased, the oxygen jet breaks through the slag layer formed by the dephosphorizing agent, and then iron oxide produced by the reaction between molten steel and oxygen is used. It is necessary to once penetrate the inside of the molten steel. Alternatively, it is necessary that the iron oxide ejected by the carrier gas breaks through the slag layer and then the iron oxide penetrates into the molten steel. Then, iron oxide is reduced by carbon inside the molten steel to form a fire point region. For that purpose, it is necessary to continuously or intermittently maintain the collision flow velocity U on the molten steel surface calculated by the equations (6) to (8) at 15 m / s or more.
U = Lp × M ₁ / Lo × Co ・ ・ ・ ・ (6)
Lp = 0.8 × M ₁ × (5.88 + 1.54M ₁ ² ) Ds ・ ・ ・ ・ (7)
M ₁ = (2 / (K-1) x Pn ^{((K-1) / K)} -1) ^1/2 ... (8)

Here, Lp: potential core length (mm), M ₁ : representative Mach number (-), Lo: lance height from the molten steel surface in the vacuum chamber (mm), Co: representative sound velocity (340 m / s), Ds: Nozzle equivalent diameter (mm) of the lance throat portion, Pn: Nozzle back pressure (kg / cm ² ), K: Gas specific heat ratio (−). Further, when calculating the collision flow velocity U, in the case of a laval nozzle, the nozzle back pressure Pn indicates the pressure of the throat portion, and in the case of a tapered nozzle or a straight nozzle, the nozzle back pressure Pn indicates the absolute pressure at the nozzle tip portion. Further, the nozzle equivalent diameter Ds refers to the diameter of the portion, but in the case of a shape that is not a perfect circle, it is defined by the equivalent diameter when the cross-sectional area is equivalent to a perfect circle.

The gas specific heat ratio K (-), which is a physical property value, varies depending on the gas type, but the gas used for iron making may be 1.4 for oxygen, nitrogen, hydrogen, and air, and argon or helium is used. In the case, 1.7 is an appropriate value, and when carbon dioxide and water vapor are used, 1.3 is an appropriate value.

The pressure in the vacuum chamber, the exhaust gas component, and the exhaust gas flow rate are continuously monitored by the exhaust gas measuring device 9 installed in the exhaust hole in the vacuum chamber. Therefore, it is possible to know the suction height of the molten steel in the pan exposed to atmospheric pressure by the pressure in the vacuum chamber, the lance height Lo can be controlled by the difference in lance height, and the nozzle equivalent diameter Ds is determined. The collision flow velocity U of the gas in the vacuum chamber can be controlled by selecting the nozzle back pressure Pn of the top blowing lance. In addition, the occurrence of internal decarburization can be confirmed by observing the inside of the vacuum chamber with a monitor, and the degree of vacuum, exhaust gas flow rate, and CO and CO ₂ components in the exhaust gas should be continuously analyzed with an exhaust gas measuring instrument. Therefore, the rate of CO gas generation can be monitored. In general, when carbon of 5 ppm / min or more is generated from the molten steel, it can be determined that internal decarburization having sufficient stirring power for the present invention has occurred.

As the oxygen source supplied from the top-blown lance to generate internal decarburization, there are those containing a large amount of iron oxide such as scale, iron ore, and dust in addition to gaseous oxygen gas. When spraying iron oxide such as iron ore, it is sprayed together with a carrier gas such as argon or nitrogen. Since the present invention targets high-carbon molten steel having a low concentration of dissolved oxygen, the collision flow velocity U of the sprayed gas needs to be 15 m / s or more.

Further, in the case of supplying solid iron oxide such as iron ore, if the maximum particle size exceeds 2.00 mm, it becomes difficult to transport iron oxide by the carrier gas. On the other hand, if the maximum particle size is less than 0.01 mm, iron oxide is less likely to penetrate into the molten steel even if the collision flow velocity U of the carrier gas is 15 m / s or more. Therefore, the maximum particle size is preferably 0.01 mm or more and 2.00 mm or less. The type of carrier gas is not particularly limited, but argon gas, nitrogen gas, or the like can be used. If cost is more important, it is preferable to use nitrogen gas.

The internal oxidation of the above formula (9) is an oxidation reaction of C in molten steel by supplying an oxygen source. In addition, the amount of oxygen supplied is important for oxidizing P in molten steel. In order to clearly obtain the dephosphorization effect when the present invention is carried out, as described above, a dephosphorizing agent is added at a CaO mass of 0.03 kg / t or more, and internal decarburization at 5 ppm / min is maintained for 4 minutes. As the oxygen mass required for this, it is necessary to supply oxygen gas or iron oxide in an oxygen mass of 0.003 kg / t or more. The upper limit of the oxygen supply amount is not particularly set, but as described above, the dephosphorization treatment time is preferably 25 minutes or less, so that the oxygen supply amount during the dephosphorization treatment is preferably 6 kg / t or less.

Further important in this operation is to make the dephosphorized slag into a stirred suspension state by internal decarburization in the vacuum chamber and allow it to stay as a suspended slag for a long time, and the dephosphorizing agent added in the vacuum chamber. It is important to suppress the proportion of dephosphorized slag that has melted and suspended in the molten steel in an emulsion state and is discharged from the descending dip tube during the dephosphorization treatment.

Regarding the emulsion phenomenon of the two-liquid phase, it is well known that the droplets produced become smaller when the viscosity of the floating phase having a small specific density is low. For example, when CaF ₂ is added, the viscosity of the dephosphorized slag decreases, the slag droplets become finer, and the specific surface area increases. Here, when the particle size of the slag droplets becomes finer, the residence time in the molten steel becomes longer because the ascent rate is small. As described above, there is a great effect in promoting the reaction in terms of both the expansion of the specific surface area and the prolongation of the residence time. However, in the embodiment of the present invention, when small size slag droplets are dispersed in the vacuum chamber, they are discharged from the vacuum chamber in a short time by the molten steel flow of the descending immersion tube, and then the ladle slag. Slag droplets are absorbed inside and effective dephosphorization cannot be performed.

Here, the floating speed (terminal velocity) of the slag droplets in the molten steel when the dephosphorized slag is dispersed is calculated by Newton's formula shown in the following formula (10).
Uf = {(4/3 (ρ _m -ρ _s ) g · d) / (0.5 · ρ _m )} ^1/2 ... (10)
Uf: Terminal velocity (m / s), d: Slag drop diameter (m), g: Gravitational acceleration (9.8 m / s ² ), ρ _m : Molten steel specific gravity (about 7000 kg / m ³ ), ρ _s : Slag specific gravity (Approximately 3000 kg / m ³ )

On the other hand, the ring flow rate of the RH type vacuum smelting furnace can be easily calculated based on the specified formula for the ring flow rate described in Non-Patent Document 1 (formula (11) below).
Q = 11.4 × G ^1/4 × D ^4/3 × {ln (P ₁ / P ₂ )} ・ ・ ・ ・ ・ (11)
Q: Recirculation flow rate (t / min), G: Rising tube recirculation gas flow rate (Nl / min), D: Immersion tube inner diameter (m), P ₁ : Recirculation gas blowing pressure (atmospheric pressure in normal operation), P ₂ : Pressure inside the vacuum chamber

Therefore, it can be seen that the average descending flow velocity Um (m / s) of the descending immersion pipe is calculated by the following equation (12).
U _m = 1000 ・ Q / {60 ・ ρm ・ π (D / 2) ² } ・ ・ ・ ・ ・ ・ (12)

According to the equation (12), the average descending flow velocity U _m is defined based on the ring flow rate Q obtained by the equation (11), and the range thereof is 0.5 m / s or more and 1.0 m / s or less. The reason why the average descending flow velocity U _m is limited to this range is as follows. That is, the dip tube diameter of the RH type vacuum smelting furnace currently industrially applied is about 0.4 to 0.7 m, and the degree of vacuum during the smelting process accompanied by the generation of CO bubbles is 20 to 2 kPa. .. Further, the flow rate of the circulating gas passing through the immersion pipe is about 500 to 3000 (Nl / min). From this range, the average descending flow velocity Um calculated by the equations (11) and (12) is in the range of about 0.5 to 1.0 m / s.

Therefore, when the terminal velocity of the slag droplet defined by the equation (10) is less than 0.5 m / s, the dephosphorizing agent is added into the vacuum chamber to slag and emulsify the slag droplet for a short time. It is not possible to carry out effective dephosphorization treatment because it is discharged from the descending immersion tube.

FIG. 2 is a diagram showing the relationship between the slag drop diameter d (m) and the terminal velocity Uf (m / s) calculated by the equation (10). As shown in FIG. 2, when the slag drop diameter is 0.02 m or less, the terminal velocity Uf is less than 0.5 m / s, so that the effective dephosphorization treatment as described above cannot be carried out.

On the other hand, when the slag drop diameter exceeds 0.07 m, the terminal velocity Uf exceeds 1.0 m / s. In this case, since the slag droplets are likely to float, it is extremely difficult to discharge the slag from the vacuum chamber after the dephosphorization treatment within the range of normal operation such as an increase in the recirculation gas flow rate or an operation for increasing the degree of vacuum. become. Therefore, slag remains in the vacuum chamber after the dephosphorization treatment, and phosphorus oxidized and absorbed in the form of P ₂ O ₅ is reduced in the residual slag by the strongly deoxidizing element by deoxidizing or adding an alloy to adjust the composition. May be restored. Therefore, it is not possible to stably produce a predetermined low-phosphorus steel product.

The size of the slag droplets when suspended in a vacuum chamber is most affected by the slag viscosity. Therefore, the present inventors conducted a basic experimental investigation of slag viscosity. As an experimental method, the viscosity at 1600 ° C. was investigated by a rotary viscosity measuring method using a dephosphorizing agent in the range shown in Table 1 in which the composition was investigated in advance.

As a result of statistical processing of the obtained experimental values and composition, it was found that the viscosity η (cP) can be quantitatively evaluated by the following formula (13) except for the synthetic briquette containing 5% by mass of CaF ₂ . rice field. In the formula (13), (% X) means the ratio (mass%) of X in the dephosphorizing agent, and t. Fe represents the sum of the mass% of divalent and trivalent iron oxide contained as iron oxide. Further, as a result of investigating the influence of unavoidable impurities by adding titanium oxide, chromium oxide, etc., the components (CaO, SiO ₂ , Al ₂ O ₃ , t.Fe, MgO, described in the formula (13), It was also confirmed that if the total of MnO and P ₂ O ₅ ) is 85% by mass or more, the error to the calculated viscosity value given by the impurities is less than 7%, and there is no problem in practical use.
η (cP) = 1.2 (% CaO) +7.3 (% SiO ₂ ) +0.8 (% Al _{2O 3} ) +0.5 (% t.Fe) +0.3 (% MgO) +0.2 ( % MnO) +0.2 (% P ₂ O ₅ ) ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (13)

FIG. 3 shows the correlation between the viscosity measured experimentally at 1600 ° C. and the value calculated by Eq. (13). As shown in FIG. 3, it can be seen that the dephosphorizing agents other than the CaF ₂ -containing products shown in Table 1 can reproduce the viscosity of the slag well.

In order to carry out the above operation industrially in the RH type vacuum smelting furnace, as described above, the main distribution of the slag drop diameter of the suspended slag in the vacuum chamber should be in the range of 0.02 to 0.07 m. There is a need to. The characteristics of the refined slag satisfying the conditions were investigated by an actual RH experiment for verifying the effect of the present invention, which will be described later. In this experiment, the surface slag around the descending immersion tube was crushed and removed with a metal rod, and the state of the slag droplets rising from the descending immersion tube was photographed and observed with a high-speed camera. Then, as a result of comparing and evaluating the image analysis data obtained by the high-speed camera and the viscous η (cP) defined by substituting the composition of the added dephosphorizing agent into the equation (13), the viscous η (cP) is obtained. When it was 100 cP or more and 250 cP or less, it was possible to efficiently perform the dephosphorization treatment and discharge the slag from the vacuum chamber after the dephosphorization treatment was completed.

As described above, since the present embodiment targets high carbon molten steel having a low dissolved oxygen activity, the composition of CaO-based dephosphorized slag satisfies the range of the above-mentioned formulas (1) to (4). Is required. Further, in order to allow the slag droplets to stay as suspended slag for a long time, it is required that the viscous η (cP) satisfies the range of the formula (5) when the dephosphorizing agent is further dissolved. Therefore, in the embodiment of the present invention, it is important to understand the composition of the dephosphorized slag in the slag vacuum chamber.

It is difficult to collect the dephosphorized slag in the vacuum chamber during the dephosphorization process. Therefore, it is convenient to calculate the composition of dephosphorized slag from the composition of the dephosphorizing agent and the mass balance of the amount of iron oxide generated from the oxygen source added from the top-blown lance. When conducting a dephosphorization test on a trial basis, a metal plate for collecting adhesion is installed on the top blown lance to collect slag scattered with internal decarburization, or the ring flow rate is temporarily increased. It is possible to confirm the slag directly by component analysis, for example, by collecting the slag discharged from the descending immersion tube and floating from the vicinity of the descending immersion tube with an immersion type sampler. However, as a result of investigation in the present invention, the composition did not change between the composition of the added dephosphorizing agent and the composition of the dephosphorizing agent (dephosphorus slag) spilled within the range of variation in the analytical values. It has been confirmed that the composition of the dephosphorizing slag can be regarded as the composition of the dephosphorizing agent in the compounding composition.

Further, the composition of the dephosphorizing agent to be added does not have to be a mixture having the same composition, and the dephosphorizing agent is added in advance in a vacuum chamber so as to maintain the specified composition during the dephosphorizing treatment, which is a feature of the present invention. Then, it may be mixed and squeezed in a vacuum chamber. Further, if necessary, a method such as adding a dephosphorizing agent component into the vacuum chamber during the dephosphorization treatment may be used. Either method can enjoy the effect of the invention without any problem.

The predetermined dephosphorization treatment may be performed according to a general operation standard, such as analyzing a sample to determine the end point or determining the treatment time according to the operation conditions. After this dephosphorization treatment, even if there are treatment standards such as dehydrogenation, before adding the deoxidizing elements aluminum, silicon, manganese, etc., the circulation speed is increased and the refined slag in the vacuum chamber is dipped down. It is necessary to drain it from the pipe and float it on the molten steel in the ladle. As a means for that, the flow rate of the recirculation gas is increased or the degree of vacuum is increased to increase the recirculation speed. In addition, in order to confirm the discharge treatment, the surface solidified slag around the descending immersion pipe in the vacuum chamber is crushed, and the situation where the slag starts to float on the molten steel surface around the descending immersion pipe and then ends is captured with a camera. It is common to check visually and set it as an operating standard.

When deoxidation is carried out without carrying out this slag discharge, the phosphorus separated and removed as P ₂ O ₅ in the slag in the vacuum chamber is reduced and rephosphorused, so that the effect of the present invention can be obtained. not.

Examples of the present invention will be described. The conditions described here are examples of one condition adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this one condition example. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

In order to confirm the effect of the present invention, a 290t / ch RH type vacuum smelting furnace was used, the initial temperature of the molten steel was about 1600 ° C, and the pre-treatment C concentration was the standard of the C concentration of the target product of the target molten steel. If the C concentration standard is 1.0 to 1.2% by mass, the pre-treatment C concentration is 1.0 to 1.2% by mass, and the C concentration standard is 1.8 to 2. For 0% by mass, the pre-treatment C concentration was 1.8 to 2.0% by mass. For all steel types, the standard of P concentration of the target product is 0.01% by mass or less. Therefore, the P concentration before the treatment was set to about 0.025% by mass by the hot metal dephosphorization treatment and the converter decarburization / dephosphorization treatment. The other main components of each steel type were Si: less than 0.05% by mass, Mn: less than 0.05% by mass, and S: less than 0.01% by mass both before and after the treatment.

The inner diameter of the dipping tube of the RH type vacuum smelting furnace used in the experiment was 0.6 m for both the ascending dipping tube and the descending dipping tube. Then, the immersion tube was immersed in the molten steel contained in the ladle and evacuated to bring the pressure in the vacuum chamber to 20 kPa. Further, 1000 Nl / min of Ar gas was blown as a circulating gas from the tuyere of the ascending immersion tube, and the average descending flow velocity Um of the descending immersion tube was adjusted to 0.57 m / s. Then, a massive dephosphorizing agent having a size of about 100 mm was added all at once from the upper hopper, but in order to understand the effect of the change in CaO activity due to the change in the components in the dephosphorizing agent, based on the compounding components. The dephosphorization treatment was carried out with a unified CaO content of 600 kg / ch (2.1 kg / t). Immediately before adding the dephosphorizing agent, oxygen gas is blown from the top blown lance, or nitrogen gas is used as the carrier gas, and the maximum particle size is 0.1 to 0.2 mm on a mill scale (main component Fe ₂ O ₃ ). The spraying was carried out, oxygen was started to be supplied under the spraying conditions described later, immediately after that, the dephosphorizing agent was collectively added, and then 0.70 kg / t of oxygen was supplied in 12 minutes. During the treatment time, it was confirmed by the exhaust gas monitor that the occurrence of CO boiling could be maintained.

The top blow lance used a water-cooled single-hole Laval nozzle with a throat diameter of 18 mm. The basic conditions for spraying were pure oxygen and nitrogen gas containing mill scale, the lance back pressure Pn was an absolute pressure of 4 kg / cm ² , and the gas spraying speed at that time was 710 Nm ³ / h. When the nitrogen gas containing the mill scale was sprayed, the nitrogen flow rate of the carrier gas was the same and the amount of oxygen in the mill scale was equal to that of pure oxygen.

The basic condition was that the lance height Lo was 1500 mm, and the collision flow velocity U at this time was 23 m / s. When the spraying speed was changed, the lance height Lo was changed and controlled, but when the lance height Lo was 2300 mm or less (collision flow velocity U was 15 m / s or more), the decarburization speed at the time of oxygen spraying was about 40 ppm /. A constant value was shown in min. On the other hand, when the lance height was higher than that at the same back pressure, the decarburization rate dropped sharply to about 3 ppm / min. This indicates that the collision velocity U of the oxygen jet became smaller and the internal decarburization accompanied by the formation of a fire point did not occur due to the soft blow.

After the above dephosphorization treatment is completed, the flow rate of Ar gas, which is a recirculating gas, is increased to 2000 Nl / min, the degree of vacuum is set to 0.20 kPa, and the average descending flow velocity Um of the descending immersion tube is 1.13 m / s. Under control, dephosphorization slag was discharged through the descending immersion tube. At this time, the slag around the descending immersion pipe was crushed and removed in advance, and the slag droplets rising from the molten steel surface were monitored on a monitor image to confirm that the discharge was completed. After that, aluminum was added for deoxidation, and the component changes before and after the treatment were compared. After the experiment, the molten steel was made into a slab product by a continuous casting process after performing the predetermined alloy adjustment again.

In the above-mentioned appropriate particle size distribution of slag droplets and the calculated viscosity η (cP), the component change of the dephosphorized slag is within the error range from 0 to 2 minutes from the start of internal decarburization in advance. I confirmed that it was calculated correctly. In addition, the dephosphorization slag discharge treatment was carried out 1.5 minutes after the start of internal decarburization, and the particle size distribution was obtained from the analysis results of the image data at that time. The change in iron oxide concentration as a slag component due to gaseous oxygen blown by top blowing to maintain internal decarburization or mill scale was within the analytical error range of a series of experiments. From this observation test result, it was confirmed that when the viscosity η (cP) was less than 100 cP, a large amount of dephosphorized slag was discharged in a size of less than 0.02 m when internal decarburization occurred. Furthermore, when the viscosity η (cP) exceeds 250 cP, many slags remain in the vacuum chamber even when the average descending flow velocity Um exceeds 1.0 m / s, and only a part of the size is close to 0.07 m. It was confirmed that it was discharged with a large slag drop. In addition, all the samples shown in Table 2 below are CaO, SiO ₂ , Al ₂ O ₃ , t. The total of Fe, MgO, MnO and P ₂ O ₅ was 85% by mass or more. Table 2 shows a list of experimental results.

In Examples 1 and 2, a converter slag was used as the de-neighboring agent, and dephosphorization treatment was carried out using gaseous oxygen and scale, respectively, in order to generate internal decarburization. In both cases, good results were obtained with a P concentration, which is the target value, well below 0.01% by mass.
On the other hand, Comparative Example 1 shows the result when a converter slag was used as the dephosphorizing agent but oxygen was not supplied. The monitored decarburization rate was about 1 ppm / min, and dephosphorization was also poor. This is because the slag stirring effect by internal decarburization, which is a requirement of the present invention, could not be obtained.

Comparative Example 2 uses a dephosphorizing agent containing a large amount of CaF ₂ , and is subjected to dephosphorization treatment by spraying gaseous oxygen in order to generate internal decarburization. As described above, since the CaF ₂ -containing slag cannot be evaluated by the estimation formula (13), the viscosity values in the table are the measured values by the rotational viscometer. In this example, a large amount of slag outflow was observed from the vicinity of the descending immersion tube at the initial stage of the dephosphorization treatment, and the dephosphorization was also poor. This is because the dephosphorization slag in the vacuum chamber is discharged at the initial stage of the dephosphorization treatment due to the low viscosity, resulting in poor dephosphorization.

Comparative Example 3 uses a highly viscous dephosphorizing agent (viscosity η (cP)> 250 cP at the time of dissolution). In this example, no slag outflow was observed from the vicinity of the descending immersion tube during the slag discharge treatment after the dephosphorization treatment, and the P concentration after the RH treatment was also high, which was unacceptable. This is because when a deoxidizing agent is added to the vacuum chamber after the dephosphorization treatment, dephosphorization occurs and low-phosphorus molten steel cannot be produced.

Comparative Example 4 is an experimental result in which silica sand was added to the decarburized slag as a dephosphorizing agent and dephosphorized under the condition of (% CaO) / (% SiO ₂ ) <1.5. Further, Comparative Example 5 is an experimental result in which alumina was added to the decarburized slag and the dephosphorization treatment was performed under the condition that the dephosphorizing agent was (% CaO) / (% Al ₂ O ₃ ) <2.0. Both silica sand and alumina used as additives used granules of about 1 mm, and both of them could not be dephosphorized to the target level due to the action of lowering the CaO activity.

Examples 3 and 4 are experimental results obtained by dephosphorizing the same silica sand and alumina addition amounts as in Comparative Examples 4 and 5 so as to satisfy the conditions of the formulas (2) and (3), respectively. be. It was confirmed that these can be dephosphorized at the target level within the specified range.

In Example 5 and Comparative Example 6, the lance height Lo is changed to change the oxygen collision flow velocity U, respectively. As described above, in Example 5 under the condition of the collision flow velocity U = 15 m / s, which is within the specified range of the present invention, the reaction promoting action by internal decarburization was obtained, so that the dephosphorization treatment could be carried out satisfactorily. .. On the other hand, in Comparative Example 6 (collision flow velocity U = 14 m / s), it was detected that the occurrence of internal decarburization did not occur even by the exhaust gas measurement, and the dephosphorization treatment was also poor.

Example 6 is an experiment conducted on a high carbon steel grade having a higher C concentration than other examples. The effectiveness of the present invention was confirmed even in the case of high carbon molten steel having a C concentration of nearly 2% by mass.
Comparative Example 7 is the result of performing the dephosphorization treatment with the initial C concentration set to 0.65% by mass. The dephosphorization efficiency was almost the same as that of other high carbon molten steels. However, in order to produce molten steel for products with a C concentration above a predetermined standard, it was necessary to add a large amount of high-purity carbonaceous material in an amount exceeding 0.4% by mass, which is very costly. It was confirmed that this was the case.

According to the present invention, it is possible to efficiently carry out low phosphorus reduction of high carbon steel, which has required high cost operation such as adding a large amount of carbonaceous material after decarburizing to a low concentration. Therefore, its industrial utility value is extremely high.

1: Top blown lance 2: Ladle 3: High carbon molten steel 4: Ladle slag 5: Vacuum tank 6: Smelter 7: Upper hopper 8: Dephosphorization slag 9: Exhaust gas measuring instrument 10: Recirculation gas 11: Recirculation gas Tuft 12: Ascending immersion tube 13: Downward immersion tube 14: CO bubble 15: Suspended slag 16: Oxygen jet

Claims (1)

It is a dephosphorization treatment method for molten steel that dephosphorization treatment is performed before deoxidation and alloy adjustment.
C: Molten steel containing 0.7 to 2.0% by mass was placed in a pan and depressurized in an RH type vacuum smelting furnace, and CaO, SiO ₂ , Al ₂ O ₃ , t. The total of Fe, MgO, MnO and P ₂ O ₅ is 85% by mass or more, and the conditions specified by the formulas (1) to (4) are satisfied, and the viscosity η in the range specified by the formula (5) at the time of dissolution. When adding a dephosphorizing agent that becomes (cP)
A carrier with oxygen gas or iron oxide from a top-blown lance installed in a vacuum chamber with the molten steel recirculated with the average molten steel flow velocity Um of the descending immersion pipe set to 0.5 m / s or more and 1.0 m / s or less. The dephosphorizing agent is added at a CaO mass of 0.03 kg / t or more while the gas is sprayed onto the molten steel under the condition that the collision flow velocity U specified by the equations (6) to (8) is 15 m / s or more.
After starting the addition, the numerical ranges of the average molten steel flow rate Um and the collision flow rate U were maintained for 4 minutes or more, and the oxygen gas or iron oxide was supplied in an oxygen mass of 0.003 kg / t or more, and then produced. A method for dephosphorizing molten steel, which comprises discharging dephosphorizing slag from the vacuum chamber through the descending dip tube.
70.0% by mass ≧ (% CaO) ≧ 40.0% by mass ・ ・ ・ ・ (1)
(% CaO) / (% SiO ₂ ) ≧ 1.5 ・ ・ ・ ・ (2)
(% CaO) / (% Al ₂ O ₃ ) ≧ 2.0 ・ ・ ・ ・ (3)
(% CaF ₂ ) <3.0% by mass ... (4)
100 ≤ η (cP) ≤ 250 ... (5)
U = Lp × M ₁ / Lo × Co ・ ・ ・ ・ (6)
Lp = 0.8 × M ₁ × (5.88 + 1.54M ₁ ² ) Ds ・ ・ ・ ・ (7)
M ₁ = (2 / (K-1) x Pn ^{((K-1) / K)} -1) ^1/2 ... (8)
Here, (% X) represents the ratio (mass%) of X in the dephosphorizing agent.
η (cP) = 1.2 (% CaO) +7.3 (% SiO ₂ ) +0.8 (% Al _{2O 3} ) +0.5 (% t.Fe) +0.3 (% MgO) +0.2 ( % MnO) +0.2 (% P ₂ O ₅ ).
Lp: Potential core length (mm), M ₁ : Representative Mach number (-), Lo: Lance height from the molten steel surface in the vacuum chamber (mm), Co: Representative sound velocity (340 m / s), Ds : Nozzle equivalent diameter (mm) of lance throat, Pn: Nozzle back pressure (kg / cm ² ), K: Gas specific heat ratio (-).

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