AU2003210336A1

AU2003210336A1 - Method for deep decarburisation of steel melts

Info

Publication number: AU2003210336A1
Application number: AU2003210336A
Authority: AU
Inventors: Eric Perrin; Christian Schrade; Francois Stovenot
Original assignee: USINOR GROUP; VAI Fuchs GmbH; Institut de Recherches de la Siderurgie Francaise IRSID
Current assignee: Primetals Technologies Germany GmbH
Priority date: 2002-02-22
Filing date: 2003-02-21
Publication date: 2003-09-09
Also published as: WO2003070990A1; KR20040091653A; US20050109161A1; CN1650035A; JP2005517812A; EP1476584A1; KR100889073B1; BRPI0307897A2

Description

Verification of Translation Applicant: VAI FUCHS GMBH Application No: PCT/EPO3/01799 Lodgement Date: Februar 21, 2003 I, Patent Attorney Dr. Karl-Ernst Miller, Turmstral3e 22, 40878 Ratingen, Germany declare that the attached English language translation of the specification of International Patent Application PCT/EPO3/01799 is a true and correct translation of the attached copy of that Application in the German language. Dated: this 6th day of September 2004 Signature of Translator: Method For Deep Decarburization Of Steel Melts Description The invention relates to a method for decarburizing molten steel in an RH unit in which the steel is circulated out of a container into a vacuum container that has been placed under a vacuum and is returned into the container and by which oxygen or an oxygen containing gas is blown onto the steel bath by means of blowing lances aimed at the steel bath contained in the vacuum container, the blowing lances being at a spacing from the top surface of the bath. An RH unit of this type is shown in Figure 1 and comprises a container 1 on which a vacuum container 2 is disposed for carrying out the decarburization process, the vacuum container 2 dipping into the steel retained in the container via two submersion pipes 8 extending from the floor of the vacuum container. The vacuum container 2 is connected at its branch stub 3 with a not-illustrated vacuum pump so that, as a result of the vacuum thus produced in the vacuum container 2, a circulation of the steel is set up from the container 1 into the vacuum container 2 and back again into the container 1. This circulation can be supported by the introduction of a noble or inert.gas such as argon via an injection device 4 into one of the submersion tubes 8. A blow lance 5 moveable from its disposition within the vacuum container 2 blows an oxygen spray 6 onto the bath top surface 7 of the steel bath present in the vacuum container 2. This blowing in of oxygen in the context of the RH process can be expedient for a variety of reasons. A first reason is that the oxygen content that is dissolved in a charge of a melt retained in the container is not sufficient to produce the required decarburization in a natural way; the hereinafter described invention proceeds from this factual understanding. Further reasons can be found in that the temperature of the charge is too low so that a "surplus freshening" of the charge with oxygen results in more oxygen than is needed for the decarburization, whereby this additionally retained oxygen is typically bound together via aluminum, leading to a temperature increase, or that the decarburization should be accelerated, even in the event, as well, that sufficient oxygen for naturally occurring decarburization is available. The typical blow processes are therefore so regulated that, with respect to a predetermined, fixedly set vacuum pressure, a not further specified amount of oxygen is blown onto the bath top surface until the required decarburization grade of the charge is reached. In this context, the blow process is typically executed with a surplus of oxygen. A process of the above-noted state of the art type is described in an exemplary manner in EP 0 347 884 BI: in this known process, the blowing in of oxygen into the vacuum container of the RH unit is regulated principally to minimize the thermal loss of the steel subjected to decarburization in the vacuum container in a manner such that the CO that is released on account of the supplementary oxygen surplus introduced in the above noted manner as well in connection with the decarburization phase, is combusted in a post combustion process, whereby the heat thus produced is useable in the course of the process. The blowing in of oxygen or, respectively, an oxygen-containing gas, is, in particular, via post combustion, defined by the limiting conditions (CO + CO 2 ) 5% of the exhaust gas amount as well as CO2 / (CO + CO2) > 30%. Insofar as this is concerned, the oxygen blowing in the context of the known process is not specified as a function of the metallurgical processes and the requirements. Since low final carbon contents of the molten steel melts stand in the foreground of current metallurgical requirements, these final carbon contents being, at the end of the decarburization, typically between 10 to 15 ppm, the invention provides a solution to the challenge of providing a process for decarburization of molten steel by which reduced final carbon contents in the steel are achievable and by which the requirement for the oxygen to be blown in can be regulated to a lower level. The solution of this challenge is disclosed in the advantageous embodiments and further configurations of the invention as set forth in the subject matter of the patent claims, which follow at the end of this description.

The invention provides, in the details thereof, that beginning with the increase of the CO, which is available in the vacuum container after having been released in the steel bath on account of natural decarburization, oxygen is blown in, the amount of blown in oxygen having been calculated, by taking into account, on the basis of the ratio of the starting carbon content to the starting oxygen content in the charge that is to be subjected to decarburization as specified by the formula ODec = a (Cin/Oini) 2 +b "(Cn/Oini) + c, the amount of oxygen required for decarburization as calculated by the formula Q 02 = [ODec + OEnd Dec] - GCh p 02 , whereby, for the beginning of the blow process, a starting pressure Pstart to be set up in the vacuum container is specified as function of the starting carbon content Cinoof the steel melt in accordance with the formula Pstart = a - Cini 2 + b - Cini + c whereby the formula terms have the following meanings: Q 02 oxygen amount to be blown in (Nm 3 ) ODec for the oxygen portion required for the decarburization (ppm) OEnd Dec oxygen amount available in the charge at the end of the decarburization process (ppm) Cini starting carbon content of the steel melt (ppm) Oin; starting oxygen content of the steel melt (ppm) Gch weight of the charge (kilograms) p 02 thickness factor of oxygen = 1.428 Kg/Nm 3 The invention takes into consideration the recognition that, due to the natural decarburization, the earliest possible introduction of oxygen into the steel melt should be effected in order, in connection with still high carbon contents in the steel melt, to effect the reaction of carbon to form CO and the removal in gas form from the melt, dissolved in the steel bath, due to a sufficient offering of oxygen, as a consequence of which the efficiency of the oxygen blowing in is increased. The invention exploits, in this manner, the approach that the amount of oxygen required per unit weight of the charge, which takes into consideration the oxygen delivery mediums such as casting slags or steel bars retained in the vacuum container, can be tied in a unit-specific manner into the ratio of the starting carbon content of the melt to the starting oxygen content of the melt as known parameters and this amount of oxygen required per unit weight of the charge can thereby be available as the computational background for the oxygen amount to be blown in, so that the required total amount of oxygen, which takes into account the size GCh of the charge to be subjected to decarburization as well as the final oxygen content OEnd Dec that is to be set up at the end of the blow process, can be determined and blown in. In the usual processes, generally, a final oxygen content in the charge of between 200 ppm and 400 ppm, on average 300 ppm, is set up. In connection with a final oxygen content of less than 200 ppm, the decarburization process is unnecessarily prolonged because too little oxygen is available in order to perform an efficient further decarburization. If the final oxygen content lies above 400 ppm, the requirement for de-oxidation means for binding the oxygen in the melt, especially aluminum, sharply increases, as the oxygen that still exists at the end of the decarburization process in the melt must be bound or combined. In connection with a final oxygen content which is too high, the required de-oxidation means leads to quality problems that can have a disruptive effect upon pouring of the melt. In accordance with the invention, however, not only the requirement for the oxygen to be blown in is determined but, at the same time, the starting pressure that is to be determined for the beginning of the blow process as a function of the starting carbon content of the melt is set such that the amount of oxygen to be blown in until reaching the requested post combustion, is introduced and, thus, to this extent, the decarburization that occurs by virtue of the oxygen blowing in before the beginning of a post combustion event that itself takes place in an oxygen surplus situation, is concluded. With regard to the details, in order to carry out the inventive method as well as being the first step for each charge of a melt to be subjected to decarburization, the calculation of the amount of oxygen to be blown in is determined as a function of the starting carbon content and the starting oxygen content of the melt. The oxygen amount to be blown in for the decarburization of a charge depends, as well, upon unit-specific circumstances, as the oxygen requirement necessary for the decarburization is, in part, taken care of via the process-dictated oxygen sources such as, for example, the steel bars retained in the vacuum container or available casting slags. As the typical RH units, for each respective product group, process melts with a substantially similar composition, no substantial deviations occur during the operation of an RH unit, so that the unit-specific conditions can be determined via the execution of test series that comprise the capture of measurement data, and can be captured in an approximation operation based thereon. In the context of the test series, the charges having a starting carbon content Cin; that is to be captured and a starting oxygen content Oinr that is to be captured are subjected to decarburization via the blowing in of oxygen, whereby, via a probe withdrawn shortly before the end of the decarburization process, the oxygen content of the melt and the actual amount of oxygen that has been blown in up to this time point can be captured. In order to serve as reference metrics for the comparable values captured by the individual probe, the desired final oxygen content is established at, for example, the amount of 300 ppm, whereby, with respect to each deviation above or below from the reference metric (300 ppm) of the actually measured oxygen content measured by the probe, the measured actual blown in oxygen amount to be converted into or, respectively, to be corrected into, a blown in amount Qist(Nm 3 ) takes into account an end oxygen amount established as a reference metric. The above-noted results are transmitted or converted into a coordinate system in which the abscissa sets forth the relationship CnJO;,,ii and the ordinate sets forth the actual blown in oxygen amount Qist or, as the occasion arises, the oxygen amount corrected via conversion. At least 10 attempts should be performed in order to obtain the required precision. The curve that is to be determined via the received measurement points in the coordinate system can be expressed mathematically in the form of a polynomial equation y = ax 2 + bx + c whereby, in the above-mentioned event, because of fixedly established parameters in the coordinate system, y = ODec (ppm) and X = Cini/Oini. The coefficients a, b, and c of the polynomial equation thus highlight the extent to which additional oxygen delivered via unit- or, respectively, process-, dictated oxygen sources are, in connection with decarburization conducted under vacuum condition, to be taken into account in establishing the actual oxygen requirement.

A corresponding embodiment for the computation of the polynomial equation for an RH unit is shown in Figure 3, whereby, in the base-conformed RH unit, 8 test charges have been subjected to a decarburization process. The graphical depiction of the measurement results shown in Figure 3 lead to the following polynomial expression that is described by the curve laid out in accordance with the reference points y = -35.046 x 2 + 294 x - 230.37 whereby the coefficients are respectively given in the dimension (ppm) so that the oxygen requirement ODecfOr this unit is as follows ODec = -35.046 (Cini/Oini)2 + 294 (Cin/Oini) - 230.37 [ppm] As applied to an example of a 300 t charge of a melt with a starting carbon content Cin = 400 ppm, a starting oxygen content O;n; = 307 ppm, and having a final oxygen content OEnd Dec to be established as a final oxygen content OEnd Dec = 300 ppm, the following obtains with respect to a ratio Cin/Oini = 1.30 ODec = -35.046 - 1.32 + 294 - 1.3 - 230.37 [ppm] ODec= 9 2

.

6 ppm It thus follows that the oxygen requirement Q 02 is determined as Q 02 = (92.6ppm + 300ppm) - 300000 kq 1.428kg / Nm 3 Q 02 = 0.03926% - 300000 kq 1.428kg / Nm 3 Q 02 = 82.48 Nm 3 If one compares the requirement determined in accordance with the invention for the decarburization with the introduced actual oxygen amount shown in Figure 3 for C;,lOini = 1.3, one can see a clear reduction. In accordance with the herein above set-forth example, it is possible, with respect to a run-in and, in consideration of the respective polynomial equation to be used, tested RH unit, to determine the requirement for oxygen to be introduced via the blow lances as a function of the analysis values Cini and Oi,,n derived from the analysis of each respective individual charge of a melt that is to be subjected to decarburization. A further important step for the invention is comprised in the fact that the blowing in of the calculated oxygen amount should begin as soon as a limit pressure = Pstat, that has been calculated as a function of the starting carbon content of the melt, has been reached. At the beginning of the lowering of the pressure, there necessarily comes about a naturally occurring post combustion due to a lower CO output in the exhaust gas and, relative thereto, a high portion of oxygen coming from the remainder air and leaks even without any special blowing in of oxygen, whereby the post combustion rate in the further course of the performance of the process is, however, due to the reason of the increasing CO output, a decreasing value. To this extent, the starting pressure Pstart for the release of the blow process is decidedly dependent upon the starting carbon content in the melt. As well, with respect to the starting pressure, there is an interdependence of unit-specific parameters. In a manner similar to the determination of the oxygen requirement necessary for the decarburization, initially, the unit-specific influence is, in addition to the inventive determination of the starting pressure for the blow process, to be determined via the running of test series comprising the capture of measurement data and is to be captured in the form of an approximation operation based upon such captured measurement data.

In the context of the test series, the CO outputs of the charges with a varying starting carbon content Cirni during pressure lowering are to be observed, whereby the highest CO exchange (CO-peak) is captured in Nm 3 and the pressure P associated with this CO output is captured in mbar. These measurement results are graphically converted into a coordinate system in which the abscissa shows the start carbon content C;;, the left ordinate shows the pressure P, and a right ordinate shows the CO output. Again, approximately 10 attempts should be performed in order to achieve the required precision. The curve to be laid out in correspondence with the received measurement points in the coordinate system can be mathematically expressed with a polynomial expression of the type y= ax 2 + bx + c whereby, in the above-noted event, y = Pstartand x = Cini. The coefficients a, b, and c of the polynomial equation take into account as well unit-specific parameters that dictate the CO peak. A further example is shown in Figure 4 of the above-noted process course, whereby, in total, 12 charges have been run. The curve to be laid out based upon the thus-obtained measurement points yields the polynomial equation y= - 0.0002X 2 + 0.2159 x +118.01 Using an example of a starting carbon content C;,,;= 400 ppm, the determination of the associated starting pressure yields Pstart = -0.0002 4002 + 0.2159 -400 + 118.01 Pstart =1 7 2 .4 mbar In connection with this starting pressure, the introduction of the oxygen amount calculated as 82.48 Nm 3 must be started in order to conclude the blowing in process before the post combustion. In the practical conversion, an enlargement between the beginning of the cycle and the beginning of the blowing in of oxygen must be taken into consideration in connection with the establishment of the starting pressure for the release of the blow process. This time period comprises the release of the automatic process, the running in of the lances, and the switching over from the protective gas operation to the oxygen blowing in operation. In accordance with the respective unit configuration, the time delays can comprise, for example, up to 45 seconds. To the extent that these time delays can also be expressed as the difference in pressures between the cycle beginning and the beginning of the oxygen blowing in, this pressure difference should be taken into account in determining the Pstat. In addition to determining the oxygen requirement, the duration of the blow process is to be controlled via a monitoring of the post combustion, as the limit value for the end of the blow process is to be maintained at a post combustion rate of 30%, above which the blowing in of oxygen is no longer efficient in view of the metallurgical properties and the decarburization rate, whereby such blowing in of oxygen would, as occurs in the state of the art process, serve only to minimize the thermal loss of the steel subjected to decarburization in the vacuum container. Initially, the blowing in of oxygen during the decarburization phase even with an optimal setting of the usual operational parameters, leads to a partial post combustion of the CO released from the steel bath. The post combustion rate during the blowing in of oxygen is directly tied together to the release of CO from the melt. The relationship of the post combustion increases with respect to the decarburization rate more or less, whereby it is disclosed that, during blowing in of oxygen, the post combustion increases in the event that the decarburization rate and, consequently, the release of CO, drops. The optimal operation range for the blowing in of oxygen begins, in this connection, with the reaching of the limit pressure Pstart; in this manner, the optimal operation range is extended with a higher starting carbon content of the melt. In connection with details thereof, the monitoring of the vacuum pressure occurring in the vacuum container, especially at the end of the blowing in of the oxygen, can serve as an indication in order to maintain an optimal introduction of the oxygen with a minimized post combustion. In actuality, the pressure level within the vacuum container is dependent upon a defined suction performance of the vacuum pump together with the amount of the released gasses, whereby the CO content in the exhaust gas during the treatment process is dependent upon the starting carbon content of the melt. With respect to identical oxygen blow conditions, the post combustion rate at a predetermined pressure is respectively higher, the lower the vacuum pressure within the vacuum container is set. This can be understood in that the lowering of the pressure tracks the increasingly reducing CO development. The efficiency of the oxygen blow in process is further dependent upon the introduction of oxygen into the steel melt as, in contrast to the state of the art, wherein the post combustion of the freely released CO is effected above the level of the bath, instead the oxygen blown into the steel melt should be dissolved in order to react with the carbon dissolved in the bath. The oxygen output - that is, the relation of the oxygen dissolved in the melt bath to the oxygen blown onto the bath top surface - is strongly dependent, in individual cases, upon the level of the position of the blow lance in the RH container, the free top surface of the level of the melt - that is, the diameter of the RH container - and the circulation rate of the melt through the RH container. Generally, this oxygen output can be set at approximately 80 to 90 % so that the oxygen amount Q 02 that has been calculated for the practical conversion of the melt in the blow process can be increased, taking into account the noted oxygen outputs. The oxygen outputs are also influenced via the configuration of the blow jets which impact the bath top surface within a small top surface area with a compact oxygen spray at a high velocity so that the oxygen can penetrate sufficiently deeply during the movement of the steel melt through its circulation. In this connection, in accordance with an embodiment of the invention, it is provided that the blow jets, which are configured as converging-diverging jets, produce a supersonic blow stream that, ideally, remains in the shape of a thin cylinder until contact with the bath top surface and cannot be fanned out.

Correspondingly, the spacing of the blow jets from the steel bath level is also to be controlled, this spacing being, in a given known context, between 2.5 meters and 5.5 meters. In order to at least make possible a certain accommodation of the jet geometry to variation of a parameter for the blow process, a blow jet with a working area that is variable via displacement of a position cone is provided in accordance with the invention as is schematically illustrated in Figure 2. If the position cone 11 is disposed in the position 1, this means that a fully open jet cross section 12 is available with which the converging-diverging jet 10 operates according to the defined construction point. If the position cone 11 is in the position 2, the jet geometry is adjusted for a substantially reduced counter-pressure; in any event, the throughput via the jet is reduced, if the predetermined pressure remains constant. The features of the subject matter of this case as set forth in the herein above description, the patent claims, the summary, and the drawings, can be important individually or in desired combinations with one another in order to realize the invention in its various forms.

Claims

1. A method for decarburizing molten steel in an RH unit in which the steel is circulated out of a container (1) into a vacuum container (2) that has been placed under a vacuum and is returned into the container (1) and by which oxygen or an oxygen containing gas is blown onto the steel bath by means of blowing lances (5) aimed at the steel bath present in the vacuum container (2), the blowing lances being at a spacing from the top surface (7) of the bath, characterized in that, beginning with the increase of the CO, which is available in the vacuum container after having been freely released in the steel bath on account of natural decarburization, oxygen is blown in, the amount of blown in oxygen having been calculated, by taking into account, on the basis of the ratio of the starting carbon content to the starting oxygen content in the charge that is to be subjected to decarburization as specified by the formula ODec = a (Cin/Oini)2 + b -(Cin/Oini) + c, the amount of oxygen required for decarburization as calculated by the formula Q 02 = LQDec ± OEnd Dec1 GCh p 02, whereby, for the beginning of the blow process, a starting pressure Pstart to be set up in the vacuum container is specified as function of the starting carbon content Ciniof the steel melt in accordance with the formula Pstart = a - Cini 2 + b - Cini + c and whereby the blowing in of oxygen is concluded upon the reaching of the limiting conditions (CO + CO2) = 5% of the exhaust gas amount as well as CO2 / (CO + CO2) = 30%, whereby ODec = a i(Cn/Oin) 2 + b -(Cin/Oini) + C Q 02= _oDec + OEnd Del Gc, P 02 P o2 Pstart = a - Cin 2 + b - Cini + c Q o02 oxygen amount to be blown in (Nm 3 ) ODec for the oxygen portion required for the decarburization OEnd Dec oxygen amount available in the charge at the end of the decarburization process (ppm) Cini starting carbon content of the steel melt (ppm) Oini starting oxygen content of the steel melt (ppm) GCh weight of the charge (kilograms) p 02 thickness factor of oxygen = 1.428 Kg/ Nm 3

2. A method according to claim 1, characterized in that the bath top surface (7) of the steel bath present in the vacuum container (2) is impacted with a compact oxygen blow stream (6) that exits the blow jet (10)with a high velocity.

3. A method according to claim 2, characterized in that the blow velocity produced in the blow jet (10) is set at an ultrasonic velocity.

4. A method according to claim 2 or 3, characterized in that a blow jet (10) with a working area that is variable via displacement of a position cone (11) is configured such that the ultrasonic flow of the blow stream produced in the blow jet (10) is maintained in the event of reduced counter pressure in the vacuum container.