JP2005517812A - Method for deep decarburization of molten steel - Google Patents

Abstract

The method of decarburizing the molten steel in the RH apparatus starts the rise in the vacuum vessel (2) of CO released for natural decarburization by oxygen dissolved in the steel bath and the initial stage of the molten metal. An oxygen amount calculated in relation to the carbon content and the initial oxygen content is blown, and for the start of the blowing process, a starting pressure calculated in relation to the initial carbon content is determined.

Description

Detailed Description of the Invention

  The present invention is a method for decarburizing molten steel in an RH apparatus, wherein the circulating steel is led from the vessel to a vacuum vessel in a vacuum and returned to the vessel, and the surface of the steel bath in the vacuum vessel Relates to the oxygen or oxygen gas containing gas being sprayed onto the steel bath by means of a blowing lance directed at a distance to the steel bath.

  Such an RH device is shown in FIG. 1 and consists of a container 1, and in order to carry out the decarburization process, a vacuum container 2 is placed on this container 1, and two immersion pipes 8 exiting from the bottom thereof are provided. The liquid steel in the container 1 is immersed. Since the connecting tube piece 3 of the vacuum vessel 2 is connected to a vacuum pump (not shown), because of the vacuum set in the vacuum vessel 2, the vessel 1 is changed from the vacuum vessel 2 to the vacuum vessel 2 and from the vacuum vessel 2 to the vessel 1 Circulation of molten steel to is performed. This circulation is assisted by introducing an inert gas, such as argon, into one of the immersion tubes 8 via the injection device 4. An oxygen jet 6 is blown to a bath surface 7 of a steel bath in the vacuum vessel 2 through a blower lance 5 provided movably in the vacuum vessel 2.

  This oxygen blowing in the range of the RH process is suitable for the purpose for various reasons. The first reason is that the oxygen content dissolved in the molten steel charge in the vessel is not sufficient to produce the necessary decarburization in a natural manner. Under this circumstance, the present invention described below starts. Another reason is that the temperature of the charge is too low so that the “over-refining” of the charge with oxygen accepts more oxygen than needed for decarburization, and this additional accepted oxygen is usually Bonding through aluminum, which causes a temperature rise, or even when there is sufficient oxygen for spontaneous decarburization, oxygen bubbles promote decarburization.

  The normal blowing process is such that, at a fixed vacuum pressure, which is fixedly set, a less detailed amount of oxygen is blown onto the bath surface until the required degree of decarburization of the charge is obtained. To be done. The blowing process is generally performed with excess oxygen.

The first type of method is described in German Patent Application Publication No. 0347884. In this known process, the CO released by the decarburization is recombusted via the excess oxygen supplied as described above following the decarburization stage and the resulting heat is utilized for the process. Being possible, oxygen is blown into the vacuum vessel of the RH unit so that the heat loss of the steel decarburized in the vacuum vessel is minimized. In particular, the injection of oxygen or oxygen-containing gas for recombustion is defined by the boundary conditions (CO + CO ₂ ) ≧ 5% and CO ₂ / (CO + CO ₂ ) ≧ 30% of the waste gas amount. In that respect, the oxygen blowing used within the known methods is not adapted to the metallurgical process and requirements.

  Under current metallurgical requirements, the low final carbon content of decarburized bath steel, typically between 10 and 15 ppm at the end of decarburization, occupies an important position and therefore forms the basis of the present invention. The problem being addressed is to present a method for decarburization of molten steel that results in a slight final carbon content in the steel and that the required amount of oxygen to be injected is set lower.

  Solutions to this problem will become apparent from the claims that follow the detailed description of the invention, including the advantageous configurations and developments of the present invention.

に従って脱炭のために式

に従って計算すべき酸素必要量を手掛かりとして計算される酸素量が吹込まれ、吹込み過程の開始のため、真空容器に現われる開始圧力（Ｐ_{ｓｔａｒｔ}）が、溶鋼の初期炭素含有量（Ｃ_ｉｎｉ）に関係して、式

により求められ、
廃ガス量の境界条件（ＣＯ＋ＣＯ_２）＝５％及びＣＯ_２／ＣＯ＋ＣＯ_２＝３０％に達した際吹込み過程が終了せしめられ、ここで

Ｏ_Ｄｅｃ は脱炭に必要な酸素成分（ｐｐｍ）、
Ｏ_{Ｅｎｄ Ｄｅｃ}は脱炭過程の終わりに装入物中に存在する酸素成分（ｐｐｍ）
Ｃ_ｉｎｉ は溶鋼の初期炭素含有量（ｐｐｍ）、
Ｏ_ｉｎｉ は溶鋼の初期酸素含有量（ｐｐｍ）、
Ｇ_ｃｈ は装入物の重量（ｋｇ）、

をそれぞれ意味する。In particular, according to the invention, the charge to be decarburized is initiated in the vacuum vessel by starting the rise of CO released in the vacuum bath due to natural decarburization by oxygen dissolved in the steel bath. Based on the ratio of initial carbon content to initial oxygen content in

According to the formula for decarburization

The oxygen amount calculated using the required oxygen amount to be calculated as a clue is injected, and the start pressure (P _start ) appearing in the vacuum vessel is changed to the initial carbon content (C _ini ) of the molten steel for the start of the injection process. Related, expression

Sought by
When the boundary conditions (CO + CO ₂ ) = 5% of waste gas amount and CO ₂ / CO + CO ₂ = 30% are reached, the blowing process is terminated,

O _Dec is the oxygen component (ppm) required for decarburization,
O _{End Dec} is the oxygen content (ppm) present in the charge at the end of the decarburization process.
_Cini is the initial carbon content (ppm) of the molten steel,
O _ini is the initial oxygen content (ppm) of the molten steel,
G _ch is the weight of the charge (kg),

Means each.

  The present invention enables natural decarburization to allow oxygen to enter the molten steel as early as possible, and when the carbon content in the molten steel is still high, reacting with the carbon dissolved in the molten steel by sufficient supply of oxygen. CO is taken into consideration, and the recognition that it escapes from the molten steel in a gaseous state, thereby increasing the efficiency of oxygen bubbles is taken into consideration.

The required amount of oxygen per unit weight of the charge is known in view of the oxygen source such as steel ladle adhering to the ladle slag or vacuum vessel for decarburization of the charge. The amount of charge to be decarburized (GCH) as explained by the ratio of the initial carbon content of the molten steel to the initial oxygen content of the molten steel as a quantity and thus can be used as a basis for the calculation of the amount of oxygen to be injected And the final oxygen content (O _{End Dec} ) to be set in the charge at the end of the blowing process, the total amount of oxygen required can be determined and blown. In the usual way, a final oxygen content of 200 ppm and 400 ppm, on average 300 ppm on average is set in the charge. At a final oxygen content of less than 200 ppm, too little oxygen is available for effective subsequent decarburization, thus unnecessarily lengthening the decarburization process. If the final oxygen content is above 400 ppm, the required amount of deoxidizer to bind oxygen, particularly in the molten aluminum, is significantly increased. This is because the oxygen still present in the melt must be combined at the end of the decarburization process. If the final oxygen content is too high, the required deoxidizer causes quality problems that are detrimental when casting the melt.

  However, according to the present invention, not only is the required amount of oxygen to be blown determined, but at the same time, the start of the blow process is determined as the starting pressure to be defined in relation to the initial carbon content of the melt. The amount of oxygen to be introduced is introduced until recombustion is reached, so that the decarburization with oxygen bubbles has ended before the start of recombustion starting with excess oxygen.

  Specifically, in order to carry out the method according to the invention, as a first step, for each charge of the melt to be decarburized, the calculation of the amount of oxygen to be blown is the initial carbon content in the melt. And in relation to the initial oxygen content. The amount of oxygen to be injected for decarburization of the charge is also related to the conditions specific to the device. This is because the oxygen requirement required for decarburization is already partly covered by oxygen sources due to processes such as steel scrap adhering to the vacuum vessel or ladle slag present. . Since the RH apparatus normally processes molten metal having substantially the same composition depending on the product group, there is no substantial difference during the operation of the RH apparatus, so the apparatus-specific state involves the detection of measurement data. It is detected through the execution of a test sequence and detected in an approximation function based on it.

Within the scope of the test sequence, charge with an initial oxygen content O _ini should initial carbon content C _ini and detection to be detected, it is decarburized by blowing oxygen, removal immediately before the end of the decarburization process For the sample to be detected, the oxygen content of the melt is detected and the amount of oxygen actually sprayed up to this point is determined. As a reference quantity for the comparative detection of individual sample charges, a desired oxygen content, for example at a height of 300 ppm, is defined, and the actual oxygen content measured in the removed sample is the reference quantity ( Depending on whether it is shifted upward or downward from 300 ppm), the actual amount of sprayed oxygen obtained by measurement is converted into the amount of blown Q _ist (Nm ³ ) related to the final oxygen content based on the reference amount. Or modified. The results of the is transferred to the coordinate system, the abscissa in this coordinate system, the ratio C _{ini /} O _ini is taken on the ordinate, is actually blown rare oxygen amount Q _ist to be converted by the modifying some cases Be taken. In order to obtain the required accuracy, at least 10 experiments must be performed.

The curve shown in the coordinate system by the obtained measurement points is expressed mathematically by the following polynomial,
y = ax ² + bx + c
In this case, because of the parameters held in the coordinate system, y = O _Dec (ppm) and x = C _ini / O _ini . The coefficients a, b, c of the polynomial are taken into account when determining the actual oxygen requirement through any oxygen source based on the device or process during the decarburization performed in vacuum. Indicates what should be done.

になる。FIG. 3 shows a suitable embodiment for determining a polynomial belonging to an RH unit, in which eight test charges have undergone a decarburization process in the underlying RH unit. The measurement result graph shown in FIG. 3 is a polynomial that draws a curve passing through the reference point.
y = -35.046x ² + 294x - 230.37
Here, since the coefficients are shown in the dimension of ppm, the oxygen requirement O _Dec of this apparatus is

become.

When applied to an example of a charge comprising 300 t of molten metal having an initial carbon content C _ini = 400 ppm, an initial oxygen content O _ini = 307 ppm and a final oxygen content to be set O _{End Dec} = 300 ppm, in the ratio _C ini _/ O ini = 1.30 obtained results ^{_{O Dec = 35.046 · 1.3 2 +294}} · 1.3-230.37 [ppm]
O _Dec = 92.6 ppm
Is obtained.

When the required amount of decarburization required by the present invention is compared with the actual amount of oxygen entered for C _ini / O _ini = 1.3 according to FIG. Thus, corresponding to the example described above, for an RH unit to be tested for the polynomial to be applied, the required amount of oxygen to be introduced through the blow lance is the analysis of each charge to be decarburized of the melt. It is obtained in relation to the analysis values _Cini and _Oini .

Another important step for the present invention is to start blowing the calculated amount of oxygen when the critical pressure calculated in relation to the initial carbon content in the melt = P _start is reached. At the beginning of the pressure drop, due to the low CO generation in the soot, and the high oxygen fraction from the residual air and leaking gas, the first spontaneous recombustion takes place without another oxygen blow, The reburn rate during the subsequent process decreases due to increased CO generation. In that respect, the starting pressure P _start for the start of the blowing process is considerably related to the initial carbon content in the melt.

  The starting pressure is also related to the parameters specific to the device. As with the method used to define the oxygen requirement required for decarburization, the device-specific effect of the determination of the starting pressure for the blowing process according to the present invention is first to include the detection of measurement data. It is determined through the execution of the sequence and detected in the form of an approximation function based on it.

Within the scope of the test sequence, charge with different initial carbon content _{C ini} was observed that CO emissions during the pressure drop, the highest CO switch (CO-Peak) is detected in Nm3, and this The pressure P corresponding to the CO emission amount is detected by mbar. These measurement results are transferred to a coordinate system in a graph, where the initial carbon content _Cini is taken on the abscissa, the pressure P is taken on the left ordinate, and the CO emission amount is taken on the right ordinate. It has been taken. Ten experiments were performed again to obtain the required accuracy.

The curve passing through the measurement points obtained in the coordinate system is mathematically represented by the following polynomial again.
y = ax ² + bx + c
In this case, y = P _start and x = C _ini are meant.
The coefficients a, b, and c of the polynomial take into account the parameters specific to the apparatus that determines CO-Peak again.

In FIG. 4, the method described above is illustrated by way of example, with a total of 12 charges being used. The following polynomial is obtained from the curve passing through the measurement points.
y = −0.0002x ² + 0.2159x + 118.01

When applied to an example with an initial carbon content C _ini = 400 ppm, the corresponding starting pressure is determined.
P _start = −0.0002 · 400 ² + 0.2159 · 400 + 118.01
P _start = 172.4 mbar

At this starting pressure, the introduction of the oxygen amount calculated as 82.48 Nm ³ must be started in order to terminate the blowing process before recombustion takes place.

In the actual reaction, the delay between the start of the cycle and the start of spraying must be taken into account when defining the starting pressure for the start of the spraying process. This time interval includes the initiation of an automated process, lance transfer, and switching from protective gas operation to oxygen blowing operation. In this case, for example, a time delay of up to 45 seconds may occur depending on the configuration of the apparatus. As long as this time interval is also expressed as the pressure difference at the beginning of the cycle and at the beginning of the oxygen spray, this pressure difference should be taken into account when defining _Pstart .

The duration of the blowing process should be monitored by reburn monitoring in addition to the requirement for oxygen requirements. This is because, as a limit value for the end of the blowing process, a reburning rate of 30% should be maintained. Above this reburning rate, oxygen spraying is seen in terms of metallurgy and decarburization rate. Is no longer effective and only serves to reduce the heat loss of the decarburized steel in the vacuum vessel as in the prior art. First, oxygen injection during the decarburization stage causes partial recombustion of CO released from the steel bath, even if other operating parameters are set optimally. The reburning rate during oxygen blowing is directly related to the CO emission from the melt. It is clear that the rate of reburning increases or decreases depending on the decarburization rate, and if the decarburization rate and thus the CO emission decreases, the reburning increases during oxygen blowing. Thus, the optimum operating range for oxygen injection begins when the critical pressure _Pstart is reached, where the optimum operating range extends with the higher initial carbon content of the melt. In particular, monitoring of the vacuum pressure generated in the vacuum vessel serves as an indication to maintain optimal production of oxygen with a slight reburn, especially at the end of oxygen blowing. In fact, the pressure level in the vacuum vessel is related to the amount of gas released at a given suction capacity of the vacuum pump, and the CO content in the waste during the process is equal to the initial carbon content of the melt. Involved. Therefore, under the same oxygen blowing conditions, the reburning rate at a given pressure is higher as the vacuum pressure in the vacuum vessel appears lower. This can be seen by the pressure drop occurring with less CO generation.

The efficiency of oxygen bubbles is further related to the introduction of oxygen into the molten steel. This is because the injected oxygen is dissolved in the molten steel to react with the carbon dissolved in the molten steel, rather than recombusting the released CO above the bath surface as in the prior art. It is because it makes it. The ratio of oxygen generation or oxygen dissolved in the steel bath to oxygen blown to the bath surface is determined in each case by the height of the blowing lance position in the RH vessel, the free surface of the melt surface and hence the diameter of the RH vessel, And the circulation rate of the molten metal through the RH vessel. In general, this oxygen production starts at about 80-90%, so it is calculated in the actual reaction for the blowing process.

  Oxygen production is also affected by the configuration of the blowing nozzle that applies a compact oxygen jet to the small surface at a high velocity, so that oxygen penetrates deeply into the molten steel that is kept in motion by circulation. For this reason, according to the embodiment of the present invention, a supersonic blowing jet is generated in the blowing nozzle configured as a Laval nozzle, and is ideally kept in a thin cylindrical shape until it hits the bath surface. It does not spread. The spacing of the blowing nozzle from the steel bath surface is also set appropriately and is kept between 2.5 m and 5.5 m within a known range.

  In order to be able to adapt the geometry of the nozzle at least in part to the changes in the parameters for the blowing process, the use of a blowing nozzle whose shape can be changed by means of a movable adjustment cone is considered. 2 is shown schematically. When the adjusting cone 11 is in the position indicated by position Pot1, this means a fully open nozzle section 12, in which the Laval nozzle 10 operates according to a defined design point. When the adjustment nozzle 11 is at the position Pot2, the shape of the nozzle is designed to be extremely small back pressure. However, if the inlet pressure is unchanged, the flow rate through the nozzle will decrease.

  The features of the subject matter disclosed in the claims, the detailed description of the invention, the abstract and the drawings, whether alone or in any combination, are important for implementing the invention in various embodiments.

  RH apparatus is shown.   The blowing nozzle is shown.   The change of the oxygen blowing amount is shown with respect to the ratio between the initial carbon content and the initial oxygen content.   The change in pressure with respect to the initial carbon content is shown.

Claims (4)

A method of decarburizing molten steel in an RH apparatus, in which circulating steel is led from a vessel (1) to a vacuum vessel (2) in a vacuum and returned to the vessel (1), and a vacuum vessel (2 In the case where oxygen or oxygen gas-containing gas is blown into the steel bath by the blowing lance (5) directed at a distance from the bath surface of the steel bath in (3), An increase in CO released due to natural decarburization due to oxygen present in the vacuum vessel is initiated in the vacuum vessel and is calculated based on the ratio of initial carbon content to initial oxygen content in the charge to be decarburized

According to the formula for decarburization

The oxygen amount calculated using the required oxygen amount to be calculated as a clue is injected, and the start pressure (P _start ) appearing in the vacuum vessel is changed to the initial carbon content (C _ini ) of the molten steel for the start of the injection process. Related, expression

Sought by
When the boundary conditions (CO + CO ₂ ) = 5% of waste gas amount and CO ₂ / CO + CO ₂ = 30% are reached, the blowing process is terminated,

O _Dec is the oxygen component (ppm) required for decarburization,
O _{End Dec} is the oxygen content (ppm) present in the charge at the end of the decarburization process.
_Cini is the initial carbon content (ppm) of the molten steel,
O _ini is the initial oxygen content (ppm) of the molten steel,
G _ch is the weight of the charge (kg),

A decarburization method characterized by meaning each.   2. The steel bath surface present in the vacuum vessel (2) can be subjected to a compact oxygen blowing jet (6) flowing out at a high velocity from the blowing nozzle (10). The method described.   3. A method according to claim 2, characterized in that the blowing speed generated in the blowing nozzle (10) is set to supersonic speed.   A blowing nozzle (10) having a variable operating range by the movement of the adjusting cone (11) is used, and with a slight back pressure in the vacuum vessel, the blowing jet generated by the blowing nozzle (10) is superfluous. 4. A method according to claim 2 or 3, characterized in that sonic flow is maintained.

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