JP2005325389A - Method for refining molten iron - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a refining method by which desiliconization and dephosphorization blowing can be efficiently and stably performed when extra-low phosphorus molten steel is refined. <P>SOLUTION: The method for refining molten iron is characterized in that, when molten iron discharged from a blast furnace is subjected to desiliconization, dephosphorization and decarburization refining using a converter type vessel, oxygen blowing is performed in the condition that L/L<SB>0</SB>defined by the ratio between the depth L<SB>0</SB>(m) of a steel bath and the depth L (m) of a recess in the steel bath formed by a top-blown oxygen jet lies in the range of 0.5 to 0.9 in the period of the desiliconization blowing; after the completion of the desiliconization blowing period, dephosphorization blowing is performed in such a manner that either or both of the flow rate of the top-blown oxygen and a lance gap (the distance between the end of a lance and the molten metal surface) are controlled for blocking the top-blown oxygen by slag thereby preventing the direct contact of the top-blown oxygen with the molten iron; thereafter, the converter type vessel is tilted to perform slag exhaust treatment, and successively, the decarburization blowing is performed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a refining method in which hot metal is desiliconized and dephosphorized using a converter type vessel, and further decarburized.

  As hot metal desiliconization and dephosphorization methods, conventionally, a flux injection method using a torpedo car (Patent Document 1), dephosphorization on one side using two converters, and decarburization on the other side are performed. There is a method (Patent Document 2) and the like.

  However, in these methods, for example, in the method of Patent Document 1, it is necessary to discharge after the desiliconization treatment due to the capacity limitation of the container, and the supply rate of oxygen and flux to be blown in order to avoid slopping is limited. There was a problem of extended processing time. Further, even in the method found in Patent Document 2, when the desiliconization and dephosphorization time is limited due to time restrictions such as decarburization time, there is a problem that it is difficult to melt ultra-low phosphorus steel. there were.

JP-A-5-5114 Japanese Unexamined Patent Publication No. 63-195210

  That is, in the case of the method shown in Patent Document 1, extension of the processing time becomes a problem when stable operation is performed. In addition, in the method shown in Patent Document 2, an extremely low phosphorus steel solution is produced. In this case, there is a problem that it is difficult to reach the target phosphorus concentration within a predetermined refining time.

  Accordingly, an object of the present invention is to enable stable melting of ultra-low phosphorus steel without extending the processing time.

  In order to solve the problems of the prior art, in the method of refining hot metal using a converter type vessel, the top blowing oxygen is blocked by slag and sprayed so as not to contact the hot metal surface. Japanese Laid-Open Patent Publication No. 2002-322507 is disclosed as a method for increasing the oxygen potential (activity) at the slag / metal interface without excessively increasing the iron oxide concentration and greatly improving the dephosphorization efficiency.

  As a result of intensive studies on desiliconization and dephosphorization using a converter-type vessel, the present inventors have recently promoted the slag hatching at the end of the desiliconization period, thereby improving the slag / metal interface. It was found that the oxygen potential could be further improved. The present invention has been made based on this finding.

The gist is as follows.
(1) When hot metal discharged from the blast furnace is desiliconized and dephosphorized / decarburized using a converter vessel, the blowing acid condition during the desiliconization blowing period is the steel bath depth L ₀ (m L / L ₀ defined by the ratio of the depth L (m) of the steel bath formed by the top blown oxygen jet is in the range of 0.5 to 0.9, After the smelting period, dephosphorization blowing is controlled by controlling one or both of the top blowing oxygen flow rate and the lance gap (distance between the lance tip and the molten metal surface) so that the top blowing oxygen is blocked by the slag and does not directly contact the molten iron. A method of refining hot metal, characterized in that after smelting, the converter vessel is tilted to perform exhaust treatment, followed by decarburization blowing.
(2) The hot metal refining method according to (1), wherein the R value of the formula (1) is in the range of 10 to 30 during the desiliconization blowing process.
R = W · [% Si] / So ₂ (1)
So ₂ : Oxygen amount required for the desiliconization blowing period (Nm ³ )
W: amount of molten iron charged (ton)
[% Si]: Si concentration (mass%) in hot metal before desiliconization
(3) The method for refining hot metal as described in (1) or (2), wherein a solid oxygen compound is added during a desiliconization blowing period or an initial stage of dephosphorization blowing.

  According to the present invention, when producing ultra-low phosphorus molten steel, stable ultra-low phosphorus steel can be produced with little variation in blown phosphorus concentration.

  The present invention promotes slag hatching at the end of the desiliconization blowing slag, so that in the subsequent dephosphorization blowing slag, the blown oxygen is blocked by slag and sprayed so as not to contact the hot metal surface. It is based on the fact that the oxygen potential at the slag / metal interface can be increased without excessively increasing the iron oxide concentration therein.

  In general, it is known that when the molten iron discharged from the blast furnace is subjected to desiliconization and dephosphorization, the desiliconization reaction proceeds first and then the dephosphorization reaction occurs. However, the present inventors have conducted various experiments, and even if the same blocking condition is achieved in the dephosphorization blowing period after the desiliconization blowing period, the slag is not liquified (hatch). In some cases, it was found that the permeation of oxygen in the slag was insufficient at the initial stage of dephosphorization, that is, a sufficient interface oxygen potential improvement effect could not be obtained.

  Therefore, as a method for more efficiently contributing to the dephosphorization reaction, the interfacial oxygen potential increased during the dephosphorization blowing period, the present inventors optimized the amount of oxygen during the desiliconization blowing period, Hatching can be promoted remarkably, so that in the subsequent dephosphorization blowing stage, the top blowing oxygen is shut off by slag and sprayed so as not to contact the hot metal surface without excessively increasing the iron oxide concentration in the slag. It was found that the oxygen potential at the slag / metal interface can be increased.

Here, as conditions for promoting the slag hatching at the end of the desiliconization blowing process, the steel bath depth (L ₀ ) and the depth of the steel bath dent (L) formed by the top-blown oxygen jet are It is important that L / L ₀ defined by the ratio is in the range of 0.5 to 0.9. (See Figure 1)

  With this method, after effectively utilizing the interfacial oxygen potential in the dephosphorization blowing process, once desulfurization is performed (hereinafter, sometimes referred to as intermediate desulfurization). Since the total amount of phosphorus input when performing can be suppressed, it is possible to produce extremely low phosphorus steel. Here, it is preferable that the waste rate of the intermediate waste (the amount of waste / the amount of slag generated in the furnace × 100) is high because molten steel having a lower phosphorus concentration can be produced.

  In the present invention, the desiliconization blowing period is for the purpose of reducing the silicon in the hot metal before the treatment, and the top blown oxygen jet is directly applied to the steel bath over a period of 1 minute to 4 minutes from the start of blowing. It is the period of the blowing form which supplies oxygen so that it may collide. In addition, the dephosphorization blowing period is the amount of oxygen sufficient to reduce the mass concentration of phosphorus in the hot metal before treatment to below the mass concentration of phosphorus that should be targeted. This is a period during which oxygen is supplied in a blown form blocked by slag. Here, the mass concentration of silicon and phosphorus in the hot metal can be exemplified by a method based on cantback analysis, for example.

Next, the steel bath depth L ₀ (m) and the dent depth L (m) of the steel bath formed by the top blowing oxygen jet, which are the blowing acid conditions during the desiliconization blowing process, are known methods. Although not particularly limited, for example, the following method is recommended.

First, the steel bath depth L ₀ (m) can be obtained by the following equation (2).
_{L 0 = W / ρ M /} (πD 2/4) × 1000 formula (2)
here,
W: Mass of molten iron (kg)
ρ _M : Hot metal density (= 6900 kg / m ³ )
D: Diameter of smelting vessel (m)

Moreover, about the dent depth L (m) of the steel bath formed by a top blowing oxygen jet, it can obtain | require by the following formula | equation (3) and Formula (4).
L = L _h · exp (−0.78 _h / L _h ) Formula (3)
L _h = 63 · (Fo ₂ / d) ^2/3 formula (4)
here,
h: Distance between lance and hot water surface (lance gap: m)
L _h : Depth depth when h = 0 (m)
Fo ₂ : Upflow oxygen flow rate per nozzle (Nm ³ / h)
d: Throat diameter of lance nozzle (mm)
It is.

Therefore, (L / L ₀ ) is calculated from the above L and L ₀ .

Here, when L / L ₀ is less than 0.5, the stirring force of the slag due to the top blowing during the desiliconization period becomes insufficient, resulting in a delay in hatching of the slag. If hard blow is performed such that / L ₀ exceeds 0.9, the decarburization reaction is promoted and the iron oxide concentration in the slag is reduced. As a result, even if the blocking condition is achieved, This is because it becomes difficult to increase the oxygen potential during the phosphorus treatment.

  In addition, the specific condition for preventing the top blowing oxygen from directly contacting the molten iron, which is the blowing acid condition during the dephosphorization blowing process, can be determined by a known method and is not particularly limited. The following methods are recommended.

The conditions (Ls <Ls ₀ ) are set so that the slag recess depth Ls by the oxygen jet calculated by the following formulas (5) and (6) is less than the slag layer thickness Ls ₀ . (See Figure 1)
Ls = L _h ′ · exp (−0.78 _h / L _h ′) Formula (5)
L _h ′ = 63 · (ρ _S / ρ _M ) ⁻¹ / ³ · (Fo ₂ / d) ^2/3 formula (6)
here,
ρ _S : Bulk density of slag (= 1500 kg / m ³ )
The slag layer thickness Ls ₀ is calculated by the following equation (7).
_{Ls 0 = W s / ρ S} / (πD 2/4) × 1000 formula (7)
here,
W _s : charging slag mass (kg)
It is.

Furthermore, as shown in FIG. 2, the relationship between the amount of oxygen So ₂ (Nm ³ ) required for the desiliconization blowing period, the amount of molten iron W (ton), and the Si concentration (% by mass) in hot metal ([% Si]) In this case, it is desirable that the R value of R = W · [% Si] / So _{2 be} in the range of 10 to 30. This is because the R value exceeds 30 because of the lack of oxygen required for the desiliconization blowing period, a large amount of Si remains in the molten iron even during the dephosphorization blowing period. This is because it becomes difficult to secure the target phosphorus concentration (mass%) ([% P]) in the hot metal within the blowing time. Moreover, if the above-mentioned blowing is performed under the condition that the R value is less than 10, excessive blowing is performed under the hard blow condition, and at the end of the desiliconization blowing period, the desiliconization reaction no longer occurs and the decarburization reaction occurs. As a result, the iron oxide concentration in the slag does not rise and the progress of hatching is delayed. As a result, the effect of improving the oxygen potential during the dephosphorization blowing process cannot be obtained. This is because it becomes difficult to secure the fluidity of the slag at the time of intermediate evacuation, and the intermediate evacuation rate is significantly reduced.

  More preferably, it is desirable to add a solid oxygen compound in the desiliconization blowing period or in the initial stage of the dephosphorization blowing process (that is, after the desiliconization blowing period). This is because it is possible to achieve hatching immediately after the desiliconization blowing period ends and shift to the dephosphorization blowing period, and to increase the oxygen potential in the slag more efficiently. Examples of the solid oxygen compound include iron ore and mill scale.

Continuous treatment of hot metal desiliconization, dephosphorization, and decarburization was performed using a 250-ton converter. The hot metal conditions before treatment were carbon concentration of 4.1 to 4.5% by mass, silicon concentration: 0.1 to 0.5% by mass, phosphorus concentration: 0.09 to 0.12% by mass, and initial hot metal temperature. The range was 1250 to 1330 ° C., and the top blowing acid rate in the desiliconization / phosphorus removal treatment was 35000 to 4000 Nm ³ / hr during the desiliconization blowing process. At the time of blowing, both lance nozzle throat diameter is 58mmφ lance, lance gap is 1.5 ~ 1.8m, L / L ₀ at this time is 0.6 ~ 0.7, The value of R indicated by (1) was in the range of 15-20. Next, after completion of the desiliconization blowing period, the lance gap is adjusted to 3.5 m, and further, the upper blowing acid speed is set to 15000 Nm ³ / hr and the dephosphorization blowing period is started, and Ls / Ls ₀ = 0.7 As a condition, iron ore was added as a solid oxygen compound of 1.5 tons at the same time as changing the blowing acid rate to 15000 Nm ³ / hr.

The conditions of the comparative example were as follows: L / L ₀ = 0.2 and R = 35 by setting the blowing acid rate during the desiliconization blowing period to 35000 Nm ³ / hr and the lance gap to 2.4 m. . In addition, iron ore, which is a solid oxygen compound, was not added.

The operating conditions during the dephosphorization blowing period were the same except for the addition of iron ore. In addition, the operating conditions in the decarburization blowing after the intermediate waste are the same in the present invention and the comparative example, the top blowing acid speed in the decarburization blowing is 38000 Nm ³ / h, and the lance gap is 1.2 m. The condition was L / L ₀ = 0.7.

  Table 1 shows the mass concentration ([% P]) of phosphorus in hot metal after 100-charge average dephosphorization refining of each of the examples and comparative examples according to the present invention, the intermediate waste rate (the amount of waste / the amount of slag generated in the furnace × 100), and the mass concentration of reached phosphorus after decarburization is shown in comparison.

  According to the present invention, the phosphorus concentration after dephosphorization is reduced and the intermediate rejection rate is improved, so that it is clear that the ultimate phosphorus concentration after decarburization treatment is also reduced.

The schematic diagram which shows the condition of the jet, slag, and hot metal in a converter type container at the time of hot metal dephosphorization and decarburization refining. The figure which shows the relationship between the parameter | index R value at the time of desiliconization blowing, and the ultimate phosphorus density | concentration after dephosphorization.

Claims (3)

When hot metal discharged from the blast furnace is desiliconized and dephosphorized / decarburized using a converter-type vessel, the blowing acid conditions during the desiliconization blowing period are the same as the steel bath depth L ₀ (m). L / L ₀ defined by the ratio of the dent depth L (m) of the steel bath formed by the blown oxygen jet is in the range of 0.5 to 0.9, and then the end of the desiliconization blowing process Dephosphorization blowing is performed by controlling either one or both of the top blowing oxygen flow rate and the lance gap (distance between the lance tip and the molten metal surface) so that the top blowing oxygen is blocked by slag and does not directly contact the molten iron. After that, the refining method for hot metal is characterized in that after the converter type vessel is tilted, the waste metal treatment is performed and the decarburization blowing is subsequently performed. The hot metal refining method according to claim 1, wherein the R value of the formula (1) is in the range of 10 to 30 during the desiliconization blowing process.
R = W · [% Si] / So ₂ (1)
So ₂ : Oxygen amount required for desiliconization blowing (Nm ³ )
W: amount of molten iron charged (ton)
[% Si]: Si concentration (mass%) in hot metal before desiliconization   The method for refining hot metal according to claim 1 or 2, wherein a solid oxygen compound is added during a desiliconization blowing period or an initial stage of dephosphorization blowing.

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