WO2003070990A1

WO2003070990A1 - Method for deep decarburisation of steel melts

Info

Publication number: WO2003070990A1
Application number: PCT/EP2003/001799
Authority: WO
Inventors: Eric Perrin; Francois Stovenot; Christian Schrade
Original assignee: Vai Fuchs Gmbh
Priority date: 2002-02-22
Filing date: 2003-02-21
Publication date: 2003-08-28
Also published as: US20050109161A1; CN1650035A; AU2003210336A1; KR100889073B1; EP1476584A1; JP2005517812A; BRPI0307897A2; KR20040091653A

Abstract

A method for decarburisation of molten steel in a RH unit is characterised in that, beginning with the rise in CO released into the vacuum container (2) due to the natural decarburisation as a result of the oxygen dissolved in the steel bath and present in the system, a calculated amount of oxygen is introduced, dependent on the starting carbon content and the starting oxygen content of the melt, whereby a starting pressure for the beginning of the blowing process is calculated depending on the starting carbon content.

Description

Process for the decarburization of molten steel

Description

The invention relates to a method for decarburizing molten steel in an RH system, in which the steel is circulated from a vessel into a vacuum container placed under vacuum and back into the vessel and by means of a distance from the bath surface of the in the vacuum container Blast lance in the adjacent steel bath is blown with oxygen or an oxygen-containing gas onto the steel bath.

Such an RH system is shown in Fig. 1 of the drawing and consists of a vessel 1, on which a vacuum container 2 is placed to carry out the decarburization process, which dips into the steel in the vessel with two dip tubes 8 extending from its bottom. The vacuum container 2 is connected at its nozzle 3 to a vacuum pump, not shown, so that due to the vacuum set in the vacuum container 2 in this way, a circulation of the steel from the vessel 1 into the vacuum container 2 and from this back into the vessel 1. This circulation can be supported by introducing an inert gas such as argon into one of the dip tubes 8 via an injection device 4. An oxygen jet 6 is blown onto the bath surface 7 of the steel bath in the vacuum container 2 via a blowing lance 5 which is arranged to be movable in the vacuum container 2.

This oxygen blowing in the context of the RH process can be useful for various reasons. A first reason is that the oxygen content dissolved in a batch of melt in the vessel is not sufficient to bring about the required decarburization in a natural way; the invention described below is based on this fact. Further reasons can be that the temperature of the batch is too low, so that by "refreshing" the batch with oxygen, more oxygen is taken up than is required for decarburization, this additionally taken up oxygen usually being set off via aluminum, which leads to an increase in temperature leads, or that decarburization should be accelerated by oxygen bubbles even in the event that there is sufficient oxygen for natural decarburization.

The usual pale processes are set in such a way that an unspecified amount of oxygen is blown onto the bath surface at a predetermined, fixed vacuum pressure until the required degree of decarburization of the batch is reached. The runs Blow process usually with an excess of oxygen.

A method of the generic type mentioned at the outset is described, for example, in EP 0 347 884 B1; in the known method, the blowing of oxygen into the vacuum container of the RH system is used essentially to minimize the thermal losses of the decarburized steel in the vacuum container by the fact that the CO released by the decarburization is also greater than that following the decarburization phase the excess oxygen supplied is afterburned, so that the heat thus obtained can be used for the process. The blowing in of oxygen or an oxygen-containing gas, in particular for afterburning, is> 5% of the boundary conditions (CO + C0 ₂ )

Exhaust gas quantity and C0 ₂ / (C0 + C0 ₂ ) ≥ 30% defined. In this respect, the oxygen bubbles used in the known method are not matched to the metallurgical processes and requirements.

Since today's metallurgical requirements focus on low end carbon contents of the decarburized steel melts, which typically lie between 10 and 15 ppm at the end of decarburization, the object of the invention is to provide a process for decarburizing molten steel with which low end carbon contents can be achieved in the steel, and with which the need for oxygen to be injected can be set lower. This object is achieved, including advantageous refinements and developments of the invention, from the content of the patent claims, which follow this description.

The invention provides in detail that, starting with the increase in CO due to natural decarburization due to the oxygen dissolved in the steel bath and present in the system, there is a CO in the vacuum container based on a ratio of

Initial carbon content to initial oxygen content in the batch to be decarburized according to the formula

Calculated oxygen requirements for decarburization according to the formula

calculated amount of oxygen is blown in, a starting pressure P stm which is established in the vacuum container being _determined as a function of the initial carbon _content C "" of the steel melt for the beginning of the blowing process according to the formula

P _sι „„ = «- _w - + b - c, _m + c where mean

O _o , amount of oxygen to be injected (only)

0 _Dec oxygen fraction required for decarburization

(ppm) O _εndDe oxygen content present in the batch ^{at the} end of the decarburization process (ppm) C _ιm initial carbon _{content of} the steel melt (ppm)

O _ιm initial _{oxygen content of} the steel melt (ppm)

G _ch Batch weight (kg) p _θ2 Oxygen _density factor = 1.428 Kg / Nm ³

The invention takes into account the knowledge that due to the natural decarburization, oxygen should be introduced into the molten steel as early as possible, in order to react to the carbon dissolved in the molten steel to CO and from the melt with a sufficient supply of oxygen in the molten steel escape in gaseous form and thus increase the efficiency of oxygen blowing.

The invention uses the approach that the required for the decarburization of a batch taking into account oxygen suppliers such as ladle slag or steel bears adhering in the vacuum container for the oxygen required per unit weight of the batch is known as the ratio of the initial carbon content of the melt to the initial oxygen content of the melt sizes can be traced back and is therefore available as a calculation basis for the amount of oxygen to be blown in, so that the total amount of oxygen required (0 _En d _D eci) taking into account the size (G _C ) of the batch to be decarburized and the final oxygen content to be set in the batch at the end of the blowing process In the case of conventional processes, a final oxygen content of between 200 ppm and 400 ppm, on average of 300 ppm in the batch, is generally set, and if the final oxygen content is less than 200 ppm, the decarburization process is unnecessarily prolonged because too little oxygen If the final oxygen content is above 400 ppm, the need for deoxidizing agents to bind the oxygen in the melt, in particular aluminum, increases significantly, since the one that is still in the melt at the end of the decarburization process Oxygen ge must be bound. If the final oxygen content is too high, deoxidizing agents lead to quality problems which have a disruptive effect when the melt is poured.

According to the invention, however, not only is the need for oxygen to be blown in determined, at the same time the beginning of the blowing process is determined as a starting pressure to be determined as a function of the initial carbon content of the melt in such a way that the amount of oxygen to be blown in is introduced until the mentioned afterburning has been reached and the decarburization by means of oxygen blowing before starting one under Excess oxygen occurring after combustion is completed.

In detail, the first step for the implementation of the method according to the invention for each batch of a melt to be decarburized is the calculation of the amount of oxygen to be injected as a function of the

Initial carbon content and the initial oxygen content in the melt. The amount of oxygen to be injected for the decarburization of a batch also depends on plant-specific conditions, since the oxygen requirement required for the decarburization is already partially covered by process-related oxygen sources such as steel bears adhering to the vacuum container or existing ladle slag. Since RH systems usually process melts with a largely identical composition depending on the product groups, there are no substantial deviations during the operation of an RH system, so that the system-specific conditions can be determined by carrying out test series including the acquisition of measurement data and in one have the footing function recorded.

In the course of the test series, batches with an initial carbon _content C _in and an initial oxygen content 0 in to be _recorded are to be decarburized by inflating oxygen, the oxygen content of the melt being determined on a sample taken shortly before the decarburization _process and the _amount actually blown up to this point Amount of oxygen to be recorded. As a reference for the comparative recording The desired final oxygen content, for example, 300 ppm, is determined for the individual sample batches, whereby depending on the deviation of the actual oxygen content measured in the drawn sample from the reference value (300 ppm) upwards or downwards, the actually inflated oxygen quantity determined by measurement is to be converted or corrected for a blow quantity Q _actual (only) based on the final oxygen content used as a reference quantity. The above-mentioned results are transferred to a coordinate system in which the ratio C _ιr _ _Σ / 0_- _r _- is plotted on the abscissa and the associated oxygen _quantity Q _zst which is actually blown in, possibly converted by correction, is plotted on the ordinate. At least 10 attempts should be made to obtain the required accuracy.

The curve to be laid in the coordinate system through the measurement points obtained can be mathematically calculated using a polynomial equation of the type

y = ax ² + bx + c

describe, in the present case because of the parameters recorded in the coordinate system y = 0 _Dsz (ppm) and x = C _ιnι / O _ιnι mean. The coefficients a, b, c of the polynomial equation express the extent to which additional oxygen has to be taken into account during the decarburization carried out under vacuum by means of oxygen-related or process-related sources when determining the actual oxygen demand. FIG. 3 shows a corresponding exemplary embodiment for determining the polynomial equation associated with an RH system, 8 test batches having been subjected to a decarburization process on the RH system on which it is based. The graphical representation of the measurement results shown in FIG. 3 leads to the polynomial describing the curve laid through the reference points

y = - 35.046 x ² + 294 x - 230.37

where the coefficients are given in the dimension (ppm), so that the oxygen _requirement 0 _Dec for this system results in D _ec = 22.0, 1 [ppm]

Applied to an embodiment for a 300 tonne batch of a melt with an initial carbon _content C _ini = 400 ppm, an initial _{oxygen content} Ο _inι = 307 ppm and a final oxygen content to be set 0 _En d; == = 300 ppm

C with a resulting ratio —— = 1.30: im

0 _Dec = - 35.046 • 1.3 ² + 294 • 1.3 - 230.37 [ppm] ° Dec = ⁹² > ⁶ PP ^m

Accordingly, the oxygen requirement O _α results in _ (92.6ppm + 3 OOppm) ^■ 300000 kg ~ ° ^{2 ~} 1.428 kgl Nw ³

O = 82.48 Nw ³

If the need for decarburization determined according to the invention is compared with the actual amount of oxygen introduced according to FIG. 3 for C _ini / 0 _ln i = 1.3, there is a significant reduction. Corresponding to the example explained above, the need for the oxygen to be introduced via the blowing lance as a function of the analysis values C _in 1 and 0 _{ini of} the analysis of each individual batch to be decarburized can thus be achieved for a retracted RH system which has been tested with regard to the polynomial equation to be used Determine the melt.

Another essential step for the invention is that is to be started with the injection of the calculated quantity of oxygen as soon as a calculated function of the initial carbon content in the melt limit pressure P = _sta rt is reached. At the beginning of the pressure drop, a naturally occurring secondary combustion occurs initially due to a low CO volume in the exhaust gas and relatively high proportions of oxygen from residual air and leaks, even without the separate blowing in of oxygen, the subsequent combustion rate being due to the further course of the process of rising CO emissions, however, is decreasing. In this respect, the start pressure P _s ta _r t for the release of the blowing process is largely dependent on the initial carbon content in the melt.

There is also a dependency on system-specific parameters for the starting pressure. Similar to the procedure for determining the oxygen requirement for decarburization, the system-specific influence on the execution of test series including the acquisition of measurement data must first be determined with regard to the inventive determination of the starting pressure for the blowing process and recorded in the form of a nutritional function based thereon.

In the course of the test series, batches with a different initial carbon _content C _{ιnι can be observed} when the CO output is _reduced , the highest CO exchange (CO peak) in Nm ³ and the pressure P associated with this CO output in mbar being recorded are. These measurement results are graphically transferred to a coordinate system in which the initial carbon _content C _{ιnι is plotted} on the abscissa, the pressure P is plotted on the left ordinate and the CO emissions are plotted on the right ordinate. Again, 10 attempts should be carried out in order to obtain the required accuracy.

The curve to be made in the coordinate system through the measurement points obtained can in turn be mathematically calculated using a polynomial equation of the type y = cix ¹ + bx + c

describe, in the present case y - P _slan and x = C _im mean. The coefficients a, b, c of the polynomial equation in turn take into account the plant-specific parameters that determine the CO peak.

FIG. 4 shows the procedure explained above using an exemplary embodiment, with a total of 12 batches being driven. The curve to be drawn through the measuring points leads to the polynomial equation

y = - 0.0002 x ² + 0.2159 x + 118.01

Applied to the exemplary embodiment with an initial carbon content C _in i = 400 ppm, the associated starting pressure is determined

p _{s rt} = -0.0002 • 400 ² + 0.2159 • 400 + 118.01 P _aβrt = 172.4 mbar

At this starting pressure, the introduction of the amount of oxygen calculated to be 82.48 N ^{3 must} be started in order to complete the blowing process before the afterburning occurs.

In practice, a delay between the start of the cycle and the start of inflation must be taken into account when determining the start pressure for the release of the blowing process. This period spans triggering the automated process, lance travel and switching from inert gas operation to oxygen blowing operation. Depending on the system design, time delays of up to 45 seconds can occur. Insofar as this time span can also be expressed as the difference in pressure at the start of the cycle and at the start of oxygen _blowing , this differential _pressure must be taken into account when determining P _{3 tare} .

The duration of the blowing process must be checked in addition to the determination of the oxygen demand by monitoring the afterburning, since a limit value for the end of the blowing process is a post-combustion rate of 30%, above which the blowing of oxygen cannot be observed in view of the metallurgy and the decarburization rate is more efficient, but as in the prior art would only serve to minimize the thermal losses of the decarburized steel in the vacuum container. Initially, the oxygen blowing during the decarburization phase leads to a partial afterburning of the CO released from the steel bath, even if the other operating parameters are optimally set. The post-combustion rate during oxygen blowing is directly related to the release of CO from the melt. The ratio of the afterburning increases more or less depending on the decarburization rate, whereby it is obvious that the afterburning increases during the oxygen blowing if the decarburization rate and thus the CO release decrease. The optimal operating range for blowing in the oxygen therefore begins when the Limit pressure P _s tart / the optimal operating range is extended with a higher initial carbon content of the melt. In particular, the monitoring of the vacuum pressure present in the vacuum container can serve as an indication, in particular at the end of the oxygen blowing, in order to maintain an optimal output of the oxygen with a low post-combustion. In fact, the pressure level within the vacuum container is related to the amount of gas released at a defined suction power of the vacuum pump, the CO content in the exhaust gas during the treatment process being dependent on

Initial carbon content of the melt is dependent. With identical oxygen blowing conditions, the post-combustion rate at a given pressure is therefore higher, the lower the vacuum pressure within the vacuum container. This is understandable because the pressure drop runs with the decreasing CO development.

The efficiency of the oxygen blowing also depends on the introduction of the oxygen into the molten steel, since it is not, as in the prior art, that the released CO is caused above the bath level, but the blown-in oxygen in the molten steel is to be dissolved in order to be able to react with the Melt to react with dissolved carbon. The oxygen output, i.e. the ratio of the oxygen dissolved in the melt bath to the oxygen blown onto the bath surface depends in individual cases essentially on the level of the

Blow lance position in the RH vessel, from the free surface of the melt level, i.e. the diameter of the RH Vessel and the circulation rate of the melt through the RH vessel. In general, this oxygen output can be set at about 80 to 90%, so that in the practical implementation for the blowing process the calculated oxygen quantity O _{α has} to be increased accordingly, taking into account the aforementioned oxygen output.

The oxygen output is also influenced by the formation of the blow nozzle, which is to act on the bath surface with a compact oxygen jet at high speed on a small surface so that the oxygen penetrates sufficiently deeply into the molten steel which is kept in motion by the circulation. For this purpose, it is provided according to an embodiment of the invention that a blowing jet having a supersonic speed is generated in the blowing nozzle designed as a Laval nozzle, which ideally remains as a slim cylinder until it hits the bath surface and does not fan out. The distance between the blow nozzle and the steel bath level must also be set accordingly, which is between 2.5 meters and 5.5 meters in the known frame.

In order to enable at least some adaptation of the nozzle geometry to a change in the parameters for the pale process, the use of a blowing nozzle which is changeable in its configuration by means of a displaceable adjusting cone is provided, which is shown in a schematic illustration in FIG. 2 of the drawing. If the adjusting cone 11 is in the position indicated by position 1, this means a fully open nozzle cross-section 12 with which the Laval nozzle 10 operates in accordance with the defined design point. Is the Setting cone 11 in position 2, the nozzle geometry is designed for a significantly lower back pressure; however, the throughput through the nozzle will decrease if the admission pressure remains constant.

The features of the subject matter of these documents disclosed in the above description, the patent claims, the abstract and the drawing can be essential individually or in any combination with one another for realizing the invention in its various embodiments.

Claims

P a t e n t a n s r u c h e

Process for the decarburization of molten steel in an RH plant, in which the steel is circulated from a vessel (1) into a vacuum container (2) placed under vacuum and back into the vessel (1) and by means of a distance from it Bath surface (7) of the blowing lance (5) directed into the steel tank in the vacuum tank (2), oxygen or an oxygen-containing gas is blown onto the steel bath, characterized in that starting with the increase in the decarburization due to natural decay in the steel bath and existing in the system Oxygen released CO in the vacuum container using a formula based on the ratio of the initial carbon content to the initial oxygen content in the batch to be decarburized

Calculated oxygen requirements for decarburization according to the formula yJpec ⁺ ^ End Decl ^' ^ Ch

Q _n _ =

Po, calculated amount of oxygen is blown in, for the start of the blowing process, a starting pressure P _start which is established in the vacuum container as a function of the initial carbon _content C _{mi of} the steel melt is determined according to the formula a C, in “+ b • C i, m“ + c

and the blowing process is ended when the boundary conditions (CO + C0 ₂ ) = 5% of the exhaust gas amount and C0 ₂ / CO + C0 ₂ = 30% are reached, where Q ₀ means oxygen amount to be blown in (Nm ³ )

0 _Dec oxygen fraction required for decarburization

(ppm) 0 _EndDec at the end of the decarburization process oxygen content present in the batch (ppm) C _jm initial carbon content of the steel melt (ppm)

O _ιm initial _{oxygen content of} the steel melt (ppm)

G _Ch Batch weight (kg) p _θ2 Oxygen _density factor = 1.428 Kg / Nm ³

Method according to claim 1, characterized in that the bath surface (7) of the steel bath in the vacuum container (2) is acted upon by a compact oxygen blowing jet (6) emerging from the blowing nozzle (10) at high speed. A method according to claim 2, characterized in that the blowing speed generated in the blowing nozzle (10) is set to supersonic speed.

Method according to Claim 2 or 3, characterized in that a blowing nozzle (10) with a working range which is variable by displacing an adjusting cone (11) is used in such a way that the supersonic flow of the blowing jet generated by the blowing nozzle (10) is maintained with a lower back pressure in the vacuum container ,