JP2016079469A - Desulfurization method for molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a desulfurization method for molten steel capable of improving a desulfurization reaction efficiency by warping up-blown desulfurization flux into molten steel without using CaFor injection.SOLUTION: Provided is a desulfurization method for molten steel where, using an RH type vacuum degassing treatment apparatus having a vacuum tank, a molten steel increasing immersion tube and a molten steel lowering immersion tube, desulfurizer powder is up-blown from a lance installed at the upper part at the inside of the vacuum tank, in a recycling gas blown into the molten steel increasing immersion tube, the total flow rate (G(Nl/t min)) of the recycling gas introduced from a recycling gas blowing tuyere provided at the side face on the side of the molten steel lowering immersion tube of the molten steel increasing immersion tube and the total flow rate (G(Nl/t min)) of the recycling gas introduced from the recycling gas blowing tuyere provided at the side face on the side opposite to the molten steel lowering immersion tube side of the molten steel increasing immersion tube are controlled to the ranges of formula (1): 1.2≤G/G≤3.2(1).SELECTED DRAWING: Figure 2

Description

  In the present invention, the present invention relates to the treatment of molten steel in steel refining by using a RH-type vacuum degassing treatment device and spraying and adding desulfurization flux to the molten steel from a lance installed above in the vacuum tank. The present invention also relates to a desulfurization method for molten steel that can improve the desulfurization efficiency and reduce the amount of desulfurization flux used.

  Sulfur (S) in the steel material affects many properties of the steel material such as corrosion resistance and weldability. In steel materials used for line pipes and gas tanks, it is required to further reduce the S concentration. In particular, in an extremely low sulfur steel that requires an S concentration of 0.0010% or less, molten steel desulfurization is required in the secondary refining process after the converter treatment. Here, in secondary refining, RH vacuum degassing is mainly performed, and it is effective to improve desulfurization efficiency in RH vacuum degassing.

  The desulfurization method in the RH type vacuum degassing process includes the addition of desulfurization flux from the alloy addition hole into the vacuum tank, the addition of desulfurization flux from the lance installed in the upper part of the vacuum tank, the riser lower part, the riser pipe, or the vacuum tank A method of injecting desulfurized flux into the inside is mentioned. In any method, in order to improve the desulfurization efficiency, it is effective to increase the reaction interfacial area between the desulfurization flux and the molten steel and further increase the reaction time by entraining the desulfurization flux in the molten steel. It is.

In Patent Document 1, a desulfurization flux having a composition containing CaO and CaF ₂ as main components and having a volume ratio of raw material particles having a particle size of 75 μm or less is 70% or more is used as a flux blowing rate (kg / min) / A desulfurization method is proposed in which a molten steel ring flow rate (t / min) is blown under a condition of 3 kg / t or less. When this method is used, the molten steel and CaO that hardly wets are melted by CaF _2, so that the molten steel gets wet and is easily caught in the molten steel, so that the desulfurization reaction efficiency is improved. Moreover, the particle diameter which is easy to generate | occur | produce, and the blowing speed | rate which can suppress the reduction | decrease of the reaction interface area by aggregation of the desulfurization flux particle | grains after being included are proposed. However, this method requires the use of CaF _2, and the slag after the RH vacuum degassing treatment cannot be effectively used for roadbed materials or the like due to environmental load. In addition, CaF ₂ causes refractory melts to become violent, increasing costs required for refractory repair.

  Patent Document 2 proposes a method in which a fine powder desulfurizing agent is blown into a molten steel ascending flow together with a carrier gas from a desulfurizing agent blowing lance opened at a lower center of a dip pipe that guides the ascending flow of molten steel. When this method is used, the desulfurization flux particles are directly blown into the molten steel so that desulfurization is promoted. However, in this method, the lance for blowing the desulfurization flux into the molten steel must be immersed, and in addition to causing lance melting, a space for immersing the lance is required. The

JP-A-8-269533 JP 58-037112

Thus, it is conventionally known that the efficiency of the desulfurization reaction is improved if the desulfurization flux is entrained in the molten steel. However, as a method of entrainment, CaF ₂ is mixed in the flux as described in Patent Document 1 to make the flux composition easy to get wet with the molten steel, or directly injected into the molten steel as described in Patent Document 2. Is necessary. However, the use of CaF ₂ is restricted due to adverse effects on the environment. Moreover, when performing injection, in addition to requiring an injection facility, refractory melts and iron loss due to an increase in splash in which the scattering direction is not controlled is caused.

Therefore, the present invention provides a molten steel desulfurization method that can improve the desulfurization reaction efficiency by entraining the desulfurized flux blown up in the molten steel without using CaF ₂ or injection. Let it be an issue.

  In the RH-type vacuum degassing treatment, most of the desulfurization flux blown and added to the molten steel in the vacuum chamber is landed on the surface of the molten steel, then flows on the surface of the molten steel by RH recirculation, and is carried directly above the downcomer. In order to accelerate the desulfurization reaction, it is necessary to entrain this desulfurization flux in the molten steel.

  In order to entrain the desulfurization flux in the molten steel, it is preferable to cover the molten steel from above the desulfurization flux that has landed on the surface of the molten steel. However, in order to cover the molten steel with powder in the RH vacuum chamber, it is very difficult to mechanically spray the molten steel.

Therefore, we studied the control of the swelled shape of the surface of the molten steel directly above the riser, and covering the powder with splash splashed from the surface. In order to efficiently cover the powder with the splash, it is necessary to increase the ratio of the splash scattered toward the center of the vacuum chamber. For this purpose, as shown in FIG. 1, the total flow rate of the reflux gas introduced from the tuyere provided on the side surface on the downcomer pipe side among all tuyere provided on the riser pipe (hereinafter referred to as G _D ). the, if the downcomer side (and later G _N) opposite freewheeling introduced through tuyeres provided in the side surface of the total gas flow rate and high flow than molten steel to be formed immediately above the riser in the vacuum chamber It is thought that the top of the swell rises toward the center of the vacuum chamber, and the amount of splash splashed toward the center increases. However, when the flow rate ratio of G _D and G _N is close to 1, the desulfurization reaction efficiency improvement without amount splash large scattered toward the center is considered to be small. Further, in the case where the flow rate ratio of G _D and G _N is too large, will be unevenly distributed recirculated gas bubbles in the riser, recirculation flow is reduced, the desulfurization reaction efficiency also considered degraded.

Thus, considered in the flow rate of G _D and G _N is an appropriate range, revealed by the test flow rate ratio. As a result, the gist of the present invention can be summarized as follows.

(1) A desulfurization method for molten steel using a RH-type vacuum degassing apparatus having a vacuum tank, a molten steel ascending dip pipe and a molten steel descending dip pipe, and blowing desulfurization flux upward from a lance installed in the upper part of the vacuum tank. the recirculated gas blown into the molten steel increases dip tube, the total flow rate (G D _(Nl of recirculated gas that is introduced from the recirculating gas blowing tuyere provided on the side surface of the molten steel falling dip tube side of the molten steel increases immersion tube / (T · min))) and the total flow rate of the reflux gas introduced from the reflux gas blowing tuyere provided on the side of the molten steel rising dip tube opposite to the molten steel descending dip tube side ( _GN (Nl / (t · min))) is in the range of the formula (1).

1.2 ≦ G _D / G _N ≦ 3.2 (1)
(2) by changing the recirculated gas flow rate to be introduced from the circulating gas blowing tuyere independently wherein said G _D and the G _N in the above (1), characterized in that the range of the equation (1) Desulfurization method for molten steel.

(3) The number of the recirculation gas blowing tuyere provided on the side of the molten steel descending dip tube side of the molten steel rising dip tube is the same as the reflux gas flow rate introduced into any of the circulating gas blowing tuyere, the ratio of blade number of units of the recirculation gas blowing tuyere provided on the side surface opposite to the molten steel falling dip tube side of the molten steel increases the dip tube by a 1.2 to 3.2, the said G _{D GN is made} into the range of said (1) Formula, The desulfurization method of the molten steel as described in said (1) characterized by the above-mentioned.

By using the present invention, without the use of CaF ₂ in the desulfurization flux, also without blowing directly to the molten steel by injection, it is possible to involve efficiently in the molten steel desulfurization flux. As a result, the desulfurization reaction efficiency of the desulfurization flux is dramatically improved, and the addition amount of the desulfurization flux can be reduced. As a result, productivity can be improved by shortening the RH type vacuum degassing processing time, and the environmental load can be reduced because the slag discharge amount is also reduced.

FIG. 1 is a schematic view of an RH vacuum degassing apparatus viewed from the dip tube side. Figure 2 is a diagram showing the effect of flow ratio of G _D and G _N on the S concentration after desulfurization flux addition.

  A mode for carrying out the present invention will be described. In the present invention, the case where RH vacuum degassing is performed after the converter treatment is taken as an example, but the atmospheric pressure is used for the purpose of adjusting the alloy components and slag reforming between the converter treatment and the RH vacuum degassing treatment. You may perform the ladle refining process by the inert gas blowing below.

(1) Vacuum degassing process After the converter process, the molten steel is put into a ladle. The ladle is transferred to the RH type vacuum degassing apparatus, and vacuum processing is started.

  The RH reflux degassing apparatus used in the present invention will be described with reference to FIG. FIG. 1 is a schematic view of the RH vacuum degassing apparatus 1 as viewed from the dip tube side. The RH reflux degassing apparatus 1 used in the present invention includes, for example, as shown in FIG. 1, a vacuum chamber 2, a molten steel ascending pipe (hereinafter referred to as “rising pipe”) 3, and a molten steel descending dip pipe (hereinafter referred to as “ 4). The ascending pipe 3 and the descending pipe 4 are connected to the vacuum chamber 2. A part of the ascending pipe 3 and the descending pipe 4 is immersed in the molten steel in the ladle. A reflux gas blowing tuyere (hereinafter referred to as “tuyere”) 3 c is provided on a side surface of the ascending pipe 3. The side surface of the ascending pipe 3 includes a side surface on the downcomer pipe 4 side (hereinafter referred to as “downcomer side surface”) 3a and a side surface of the ascending pipe 3 opposite to the downcomer tube 4 side (hereinafter referred to as “opposite side surface”). It is composed of 3b. Although FIG. 1 shows a diagram in which four tuyere 3c are provided at equal intervals as an example, the present invention is not limited to this. When such an RH-type reflux degassing apparatus 1 is used, the molten steel in the ladle rises when the vacuum chamber 1 is evacuated at a predetermined degree of vacuum and the reflux gas is blown from the tuyere 3c. The tube 3 is raised. Then, after vacuum processing in the vacuum chamber 1, the downcomer 4 is lowered and returned to the ladle.

  In the present invention, the “side surface of the molten steel ascending dip tube on the molten steel descending dip tube side” (down tube side surface) 3a means that the RH reflux degassing apparatus 1 is connected from the ascending tube 3 side as shown in FIG. When viewed, a separation line (hereinafter referred to as “separation line”) 3e that intersects the line 3 (hereinafter referred to as “center line”) 5 perpendicularly to the center 3d of the ascending pipe 3 and the center 4a of the downfall pipe 4 is shown. The side surface represented by the outer periphery on the downcomer pipe 4 side of the outer periphery of the cross section of the ascending pipe 3 is separated. In the present invention, the “side surface of the molten steel ascending dip tube opposite to the molten steel descending dip tube side” (opposite side surface) 3b is an RH type reflux degassing apparatus 1 as shown in FIG. When viewed from the side 3, the separation line (hereinafter referred to as "separation line") that intersects the line 3 (hereinafter referred to as "center line") 5 perpendicularly connecting the center 3d of the ascending pipe 3 and the center 4a of the descending pipe 4 is referred to as "separation line". .) Of the separation of the outer periphery of the cross section of the riser tube 3 in 3e, the side surface represented by the outer periphery on the side opposite to the downcomer tube 4 side.

In the RH type vacuum degassing apparatus 1, in addition to desulfurization by top blowing, component adjustment by alloy addition may be performed. Furthermore, although the refining process such as the temperature rise and decarburization of the molten steel by blowing on O ₂ is performed, it is not necessary to fix the order of the processes.

Below, each requirement prescribed | regulated by this invention is explained in full detail.
(2) When the relationship vacuum treatment with G D _/ G _N ratio and after the desulfurization treatment [S], the desulfurization flux is blown toward the vacuum chamber from central lance. In the present invention, in order to increase the desulfurization efficiency by applying the splash to the desulfurization flux, it is necessary to increase the ratio of the splash scattered to the center side of the vacuum chamber. To achieve this, the total flow rate G _D of recirculated gas to be introduced from the tuyere 3c provided on the downcomer side face 3a is recirculated gas total flow introduced from the tuyere 3c provided on the opposite side surface 3b G _N If the flow rate is higher than that, the top of the swell of the molten steel formed immediately above the riser 3 in the vacuum chamber 2 is biased toward the center of the vacuum chamber 2, and the amount of splash scattered toward the center of the vacuum chamber 2 increases. I think that. Thereby, in this invention, the molten steel scattered by the splash can be put on the desulfurization flux mainly supplied to the vacuum tank 2 center part, and it becomes possible to aim at the efficiency of desulfurization. However, when the flow rate ratio of G _D and G _N is close to 1, the desulfurization reaction efficiency improvement without amount splash large scattered toward the center is considered to be small. Further, in the case where the flow rate ratio of G _D and G _N is too large, will be unevenly distributed recirculated gas bubbles in the riser, recirculation flow is reduced, the desulfurization reaction efficiency also considered degraded. The present inventors performed the following tests in order to obtain a flow rate optimum G _D and G _N.

  The test method is as follows. C concentration is 0.010 to 0.30%, Si concentration is 0.020 to 0.50%, Mn concentration is 0.20 to 1.50%, Al concentration is 0.020 to 0.16%, S concentration When refining 300 tons of molten steel with 0.0028 to 0.0032% using the RH type vacuum degassing apparatus 1, the main component is CaO from the upper blow lance installed in the upper part of the vacuum chamber 2 during the process. The desulfurizing flux was added to the surface of the molten steel in the vacuum chamber 2 by spraying. The amount of desulfurization flux added was standardized at 5 kg / t per ton of molten steel. The desulfurization flux spraying rate was 0.3 to 1.1 kg / (t · min) per ton of molten steel. The desulfurization flux mainly composed of CaO referred to here is a powder having a maximum particle diameter of 2.85 mm or less and a CaO concentration in the flux of 90% or more.

  Under such desulfurization flux spraying conditions, the desulfurization flux was continuously sprayed onto the molten steel together with Ar gas for about 10 minutes. After the spraying was completed, a molten steel reflux was further applied for 5 minutes, a molten steel sample was taken, and the contained S concentration was analyzed to obtain S% after the treatment according to the present invention. In the present specification, “%” of the unit representing the concentration is used in the meaning of “% by mass” unless otherwise specified.

Test for obtaining the flow rate of G _D and G _N are on such assumptions, it was further carried out at a gas flow rate change condition and feather talkative change condition. The gas flow rate changing condition is that the circulating gas blowing tuyere 3c in the ascending pipe 3 is installed at equal intervals with respect to the outer periphery of the cross section of the ascending pipe 3, and from the six tuyere 3c provided on the down pipe side surface 3a. The recirculating gas to be introduced and the recirculating gas introduced from the six tuyere 3c provided on the opposite side surface 3b are independently produced by the tuyere 3c on the downcomer side surface 3a and the tuyere 3c on the opposite side surface 3b. by controlling a condition for changing the flow ratio of _{G D} and _{G N} in the range of _G D _{/ G} N = _0.73~4.0.

The conditions for changing the tuyere number are the same for all the tuyere tuyere diameters, the flow gas flow rate to be introduced is made equal, and the number of tuyere provided on the downcomer side surface 3a is set on the side surface 3b opposite to the tuyere number. This is a condition in which the number of tuyere of the tuyere 3c is changed within a range of G _D / G _N = 0.71 to 5.0. In other words, this flow rate ratio is the ratio of the number of tuyere, specifically, 5/7/7 to 10 (number of tuyere on downcomer side surface 3a) / (number of tuyere on opposite side surface 3b). / Equivalent to two pieces.

Shows the relationship between the flow rate ratio of G _D and G _N on S concentration in Figure 2 after the addition spraying desulfurization flux. From 0.73 at a gas flow rate change condition, from 0.71 feathers talkative change _condition, G after the desulfurization treatment as D / _{G N} is increased [S] is rapidly _decreased, G D / _{G N} 1.2 In this case, [S] was reduced to 0.001% after the desulfurization treatment. _Then, G D / _{G N} is after the desulfurization treatment at 1.4 [S] is reduced to 0.0006% and _0.0005%, after the desulfurization treatment to G D / _{G N} is 3.0 [S ] Was maintained at 0.0007% or less. _Then, next to 0.001% after desulfurization [S] at G D / _{G N} is _3.2, the desulfurization efficiency was worse as G D / _{G N} further increases.

Thus, to the extent the flow rate ratio of _G D _{/ G} N = _1.2~3.2 of _{G D} and _{G N,} S concentration after desulfurization flux addition desulfurization reaction to a low concentration of 0.0010% or less Progressed. To the extent the flow rate ratio of _G D _{/ G} N = _1.4~3.0 of G _D and _{G N,} S concentration after desulfurization flux addition further desulfurization reaction proceeded to low concentrations of less 0.0007%. This is because the desulfurization flux sprayed and added from the lance was efficiently covered with the splash generated at the molten steel bulge just above the riser pipe. Although G D _/ G _N is S concentration after the desulfurization treatment is less than 1.2 becomes a high concentration exceeding 0.0010%, which is the position of the vertex of the molten steel raised part can be directly above riser riser This is because the amount of splash scattered near the center and scattered in the center of the vacuum chamber did not increase. When G _D / G _N exceeds 3.2, the S concentration after desulfurization treatment increased to a high concentration exceeding 0.0010%, but this caused uneven circulation gas bubbles in the ascending pipe, This is because the ring flow rate of RH has become low.

  As described above, in the present invention, the recirculation gas flow rate derived from the tuyere is controlled so as to satisfy the expression (1) by changing the recirculation gas flow rate introduced from the tuyere or by changing the number of tuyere. It was found that desulfurization was realized.

(3) Tuyere position As shown in FIG. 1, the tuyere 3c is provided on the downcomer side surface 3a and the opposite side surface 3b.

  When the flow rate of the recirculation gas introduced from the tuyere is changed to satisfy the formula (1), the tuyere position is preferably arranged on the outer periphery of the cross section of the rising pipe 3 at equal intervals. To do. As a result, the flow rate of the reflux gas can be easily adjusted and the amount of splash scattered in the central direction of the vacuum chamber can be increased.

  Further, when the number of tuyere is changed to satisfy the formula (1), as a preferred mode of the tuyere position, as shown in FIG. 1. The downcomer side surface 3a is a portion indicated by a fan-shaped arc having a central angle α expressed by the equation (2). The two conditions that the bisector of the center angle α is the center line 5 are satisfied. When the tuyere 3c is arranged so as to satisfy these conditions, the tuyere is arranged without being greatly separated from the center line 5, so that even if the tuyere are not equally spaced, the splash is placed on the center side of the vacuum chamber 1. It becomes possible to scatter. As a more preferable aspect, the tuyere is arranged at equal intervals on the arc.

α (°) = 180 × (n / (n + 1)) (2)
In the formula (2), n is the number of tuyere provided on the downcomer side surface 3a.

(4) Control mode of recirculation gas flow rate (4-1) Control of recirculation gas flow rate introduced from tuyere In the present invention, G _D / G is controlled by independently controlling the recirculation gas flow rate introduced from the tuyere. You may control so that it may become the range of _N = 1.2-3.2. For example, the flow rate of the reflux gas may be controlled independently by two systems of all tuyere 3c on the downcomer side surface 3a and all tuyere 3c on the opposite side surface 3b, and all tuyere are independent systems. The reflux gas flow rate may be controlled by

  In addition, it is desirable that the reflux gas flow rate is 4.5 to 14.0 Nl / (t · min) per ton of molten steel as a total flow rate. When the reflux gas flow rate is excessively low, the circulation flow rate is decreased, and adverse effects other than the desulfurization reaction such as component adjustment, degassing, and purification may be increased. If the reflux gas flow rate is excessively high, bubbles of the reflux gas blown into the molten steel may collide and coalesce, so-called blow-through may occur, and the effect of increasing the circulation flow rate may not be obtained.

  The total number of recirculating gas blowing tuyere is preferably 4-25. When the number of tuyere is excessively small, the circulating gas bubbles blown into the molten steel are likely to be unevenly distributed in the riser pipe, and the circulating flow rate may be reduced. When the number of tuyere is excessively large, before the bubble reaches the center of the riser, it collides with and merges with the bubble blown from the adjacent tuyere, resulting in an increase in the ring flow rate. There is a case. The number is preferably 8-16.

  When the flow rate of the reflux gas introduced from the tuyere is controlled, the ratio of the number of tuyere on the downcomer side surface 3a and the number of tuyere on the opposite side surface 3b is not particularly limited. Even when the number of tuyeres on the downcomer side surface 3a is smaller than the number of tuyeres on the opposite side surface 3b, the effect of the present invention is exhibited as long as the reflux gas flow rate introduced from the tuyeres on the downcomer side surface 3a is large. it can.

(4-2) Control of recirculation gas flow rate by the number of tuyere In the present invention, the recirculation gas flow rate derived from all tuyere with the same tuyere diameter is made the same, and the number of tuyere on the downcomer side is opposite to that on the side opposite to the downcomer. By setting the numerical ratio to 1.2 to 3.2, the expression (1) may be satisfied. That is, (the number of tuyere on the downcomer side surface) / (number of tuyere on the opposite side surface) = 3/2, 4/3, etc. to 19/6. Preferably, the ratio of the number of tuyere may be in the range of 1.4 to 3.0, for example, 6/4, 7/5 to 12/4, etc.

  The total flow rate of the reflux gas flow in the case of such control is the same as in the case of (4-1). The number of tuyere is preferably 5 to 25, and more preferably 10 to 16.

(5) Pressure in the vacuum chamber The pressure in the vacuum chamber is preferably 67 to 13300 Pa. Furthermore, 133-6670 Pa is desirable. If the pressure is too low, the flow rate of the ring increases, while the splash at the time of bursting of the bubbles blown as the reflux gas becomes intense, and the degree of adverse effects such as refractory erosion may increase. If the pressure is too high, the flow rate of the ring decreases, and adverse effects other than the desulfurization reaction such as component adjustment, degassing, and purification accompanying the addition of the alloy may become strong.

(6) Desulfurization flux The composition of the desulfurization flux is mainly CaO. The flux mainly composed of CaO here has a concentration of pure CaO of 80% or more, and the balance may contain oxides such as Al ₂ O ₃ and MgO in order to lower the melting point of the flux. An alloy powder such as CaSi may be included for the acid. Furthermore, an impurity component inevitably mixed may be contained. When the concentration of pure CaO is less than 80%, the flux refining reaction efficiency may be reduced.

  The maximum particle size of the desulfurization flux is preferably 2.85 mm or less in order to widen the interface area between the molten steel and the desulfurization flux particles, and more preferably 1.0 mm or less, particularly preferably 0.15 mm in order to further increase the desulfurization efficiency. It is as follows.

  The amount of desulfurization flux added is preferably 1.0 to 11.0 kg / t per ton of molten steel. If it is less than 1.0 kg / t, the amount of the desulfurization flux is too small, and the S concentration after the addition of the desulfurization flux may not be reduced to the target concentration. If it exceeds 11.0 kg / t, the effect of desulfurization will be saturated, and the flux particles entrained in the molten steel will remain in the molten steel even after the RH vacuum degassing treatment. By remaining in the steel material, the steel material characteristics may be deteriorated.

  The feed rate of the desulfurization flux is preferably 0.20 to 1.20 kg / (t · min) per ton of molten steel. If it is less than 0.20 kg / (t · min), the supply time of the desulfurization flux may be prolonged. When the flow rate exceeds 1.20 kg / (t · min), the flow rate of the flux particles ejected from the top blowing nozzle does not increase sufficiently, and the arrival rate to the molten steel becomes low due to dust collection loss in vacuum exhaust. There is.

(7) Lance shape The shape of the lance nozzle that blows up the desulfurization flux may be laval, spike, or straight, but supersonic jets such as laval and spike can be obtained to reduce dust collection loss due to vacuum evacuation of the flux. A nozzle is desirable.

(8) Al concentration in molten steel When adding desulfurization flux, the Al concentration in molten steel is desirably 0.020% to 0.20%. Al is a strong deoxidizing element and has the effect of reducing the S concentration in equilibrium with the desulfurization flux. Therefore, when the Al concentration is less than 0.020%, the S concentration reached by the flux addition may not reach less than 0.0010%. If the Al concentration exceeds 0.20% and becomes high, the deoxidation effect due to Al will be saturated, and the S concentration reducing effect balanced with the flux will also be saturated.

  300 tons of molten steel decarburized in a converter was put into a ladle. At the time of steel removal, the molten steel was transferred to the RH vacuum degassing apparatus together with the ladle, and then the vacuum treatment was started. The pressure in the vacuum chamber was 133 to 4000 Pa.

  Metal Al was added after the start of processing, and the Al concentration was adjusted to 0.035 to 0.14%. After adjusting the Al concentration, a total of 6.0 kg / t of vacuum desulfurization flux is supplied from the lance installed above the vacuum tank at a supply rate of 0.6 to 1.0 kg / (t · min) per ton of molten steel. It was sprayed and added to the inner molten steel.

Table 1 shows the S concentration before and after the addition of the desulfurization flux and the treatment conditions. By controlling the range of _G D / _{G N} where, as shown in Table 1 the present invention is defined, S concentration after desulfurization flux addition was reduced to 0.0004 to 0.0007%. As a control method of G _D / G _N , either the flow rate of the reflux gas introduced into the tuyere belonging to each region or the number of tuyere belonging to each region is changed. Similarly, the desulfurization reaction can be promoted.

On the other hand, test no. Since 5, G _{D /} G _N splash of the vacuum chamber central portion below the 1.2 was insufficient, it was not possible to reduce the S concentration. Further, in Test No. 6~8 G _{D /} G _N exceeds 3.2, since the ring flow RH unevenly recirculated gas bubbles rise tract has become the low, it is possible to reduce the S concentration There wasn't.

  DESCRIPTION OF SYMBOLS 1 RH-type vacuum degassing apparatus, 2 vacuum tank, 3 riser pipe, 3a Side face on the downcomer pipe side of the riser pipe (downcomer pipe side face), 3b Side face on the opposite side of the downcomer pipe side of the riser pipe (opposite side face) 3c Reflux gas injection tuyere (tuyere), 3d center of riser, 3e center separation imaginary line (imaginary line) at the outer periphery of the cross section of riser, 4 downcomer, 4a center of downcomer, 5 of riser A line (center line) connecting the center and the center of the downcomer

Claims (3)

A desulfurization method for molten steel using a RH-type vacuum degassing apparatus having a vacuum tank, a molten steel ascending dip pipe, and a molten steel descending dip pipe, wherein the desulfurization flux is blown up from a lance installed above the vacuum tank,
The total flow rate of the reflux gas introduced from the reflux gas blowing tuyere provided on the side surface of the molten steel ascending dip tube on the molten steel descending dip tube side (G _D (Nl / (T · min))) and the total flow rate of the reflux gas introduced from the reflux gas blowing tuyere provided on the side of the molten steel rising dip tube opposite to the molten steel descending dip tube side (G _N ( Nl / (t · min))) is in the range of the formula (1).
1.2 ≦ G _D / G _N ≦ 3.2 (1) By changing the recirculated gas flow rate to be introduced from the circulating gas blowing tuyere independently, desulfurization of the molten steel according to claim 1, characterized in that said G _D in the range of the said G _N (1) formula Method. The reflux gas flow rate introduced into any of the reflux gas blowing tuyere is the same, the number of tuyere of reflux gas blowing tuyere provided on the side of the molten steel descending dip tube side of the molten steel descending dip tube, and the molten steel rising dip the ratio of blade number of units of the recirculation gas blowing tuyere provided on the side surface opposite to the molten steel falling dip tube side of the tube by a 1.2 to 3.2, the G _D and the G _N 2. The molten steel desulfurization method according to claim 1, wherein the range is within the range of the formula (1).

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