Patent Application
20110174457
The present invention relates to an optimization procedure for fabricating steel and more particularly, the present invention relates to a process for optimizing the unit operations involved in steel fabrication.
Conventionally, in the steel making industry, the amount of steel tapped from steel making vessels into the ladle is generally measured with the aid of a weight scale or load cell. It is desirable to maximize the amount of steel tapped from the steel making vessel in order to increase productivity. Tapping, unfortunately, is encumbered since the amount of steel tapped into the ladle is limited by how much slag carry-over that occurs in the ladle during tapping. As is known, the greater the volume of slag carry-over, the greater the impact on the ladle refractory integrity resulting in not only reduced ladle refractory life, but also low alloy recovery with the increased chance of recontamination of the steel during casting. Certain grades of steel require minimum slag carry-over from the furnace to avoid subsequent contamination during refining and casting.
Slag carryover into the ladle occurs due to vortex reaction or slag entrainment as the ferrostatic head decreases during tapping. Compared to liquid steel, liquid electric arc furnace slag has a distinctive grey colour, providing the ability to determine when a significant amount of slag is being tapped into the ladle. Most steel companies rely on the judgment of the operator to determine the end of tapping by visual assessment of the colour difference between the steel and the slag. In some steel companies the visual evaluation has been replaced with the slag detection system, examples of which include the Amepa TDS Technology, the Vesuvius Radar System and slag detection system developed by the Land Corporation. The purpose of the slag detection system is to determine the time to end tapping based on the different emissivity of the steel and slag or difference in vibration frequency between steel and slag. While this system presents an opportunity for a good management of slag carry-over, it does not provide a quantitative evaluation of the slag carry-over, furnace heel and tap steel. Estimation of the slag carry-over weight and ladle freeboard is often done during treatment of the steel in the ladle by visual assessment. If any measurement is required, it is usually done with the aid of the dip-test technique. This testing involves the use of a steel rod immersed into the metal bath for a short period of time, enough to burn off the rod at the steel/slag interface while the slag solidifies on the portion of the rod in the slag layer. By measuring the depth of the adhered slag, the volume and weight of the slag are estimated. The dip-test technique has been improved upon by other techniques developed and patented by Heraeus Electro-Nite and Nupro Corporation.
Although the dip-test process and other subsequent techniques developed by Heraeus Electro-Nite and Nupro Corporation were useful techniques, they can only be used in the ladle once tapping has been completed and the slag fluidized in the ladle. Accordingly, these techniques do not offer the option of controlling or determining the amounts of slag carry-over and furnace heel.
Tata Steel Limited developed a new procedure for detecting the percentage of slag in the steel ladle at BOF. The procedure is predicated on differentiation between different metals' emissivity. An infrared camera is used for the thermal image of the object. The slag monitoring and detection system (SMDS) can identify the steel-slag transition under varying operating conditions.
SMDS is operable in real time. The metal stream image is captured by the infrared digital camera and the thermal image of the tapping stream is displayed for the operator. Analysis of the stream image is continuous in order to ascertain the percentage of slag. Once detected, and using the permitted slag percentage value, logical algorithms are executed. The main principle of the detection system is the difference between the emissivity of the slag and steel.
In other developments, in U.S. Pat. No. 6,166,681, issued Dec. 26, 2000, to Meszaros et al., there is disclosed a radar measuring process for determining the level of slag on molten steel as well as its thickness. The primary source of energy is microwave signals and the process essentially includes the transmission and reception of two microwave signals against the material to be measured. There is an analysis of the time of flight data from the individual signals and that a subsequent calculation of the difference between the distances to determine the thickness of the material.
In the internet website, www.engineeringtalk.com, there is a discussion of a Land Instruments Company apparatus. The article is entitled “Slag Detection System Improved Steel Quality”. In the article there is discussion of the thermal imaging system devised by Land Instruments where a thermal imager is employed to measure infrared energy emitted from the tapping system. The system capitalizes on the fact that slag produces a significantly brighter thermal image than steel at the same temperature and accordingly, any difference between the two is safe to view from a distance.
In an article from Millennium Steel 2k4 there is a discussion article entitled: “The Ultra Slag Droplet Detector”. In the article there is a discussion of an electromagnetic slag detection process which takes advantage of the fact that slag droplets are often entrained in the steel stream at some time before the main slag flow begins. This gives rise to discreet slag droplets which vary in frequency and the time over a wide spectrum. This is generally attributable to the formation of the vortex characteristic electromagnetic fingerprint.
In further attempts to minimize slag carryover, Stofanak et al, in U.S. Pat. No. 6,197,086, issued Mar. 6, 2001, described a system and process for minimizing slag carryover during the production of steel. In this patent, there is discussed a system which includes an infrared imaging or detecting apparatus primarily used to image the tap stream. As it is known, the stream transmits energy indicative of whether molten steel and/or slag is in the stream at any given time. A grey scale analysis is conducted on the pixels viewed from the tap stream to determine the number of steel pixels and the number of slag pixels in the stream at any one point. When the ratio or percentage of slag pixels exceeds a predetermined amount, an alarm is actuated to cause the operator to cease tapping. This system avoids the use of the visual detection system by the operator with a view to minimizing the slag carryover.
In further attempts to avoid the use of operator judgment in reducing slag carryover, Koffron, in U.S. Pat. No. 6,280,499, issued Aug. 28, 2001, teaches a yield metal pouring system. Essentially, the patent is directed to the adjustment of the angle of the tilt of the furnace to an optimum angle in order to effect minimal slag entrainment in the liquid pouring through the tap. The optimum angle is calculated as a function of the furnace geometry and historical data of the furnace lining wear for the amount of metal residual within the metal furnace.
In U.S. Pat. No. 6,562,285, issued May 13, 2003 to Demysh, there is disclosed a process and apparatus for detecting slag carryover. In the process and apparatus taught, digital images of the molten metal steam are obtained and stored with identification of areas of similarity on the basis of texture and intensity. A subset of these areas is defined with a further comparison of at least one selected property of the subset against a predetermined parameter. An output signal is generated on the basis of comparison and the output signal is generally indicative of the presence of or lack of presence of slag. An advantage to the system and the process is that the invention does not require the use of expensive optical equipment and can be effected with inexpensive means.
It would be desirable to have an improved process of optimizing the unit operations involved in steel fabrication and in particular such a process that provides in-situ quantitative evaluation of the slag carry-over weight, tapped steel weight, heel in the steel in the steel making vessel, ladle freeboard together with a warning system for the operators to cue the end of tapping.
The present invention is directed to providing elegant process control and optimization of the unit operations noted above.
One object of the present invention is to provide an improved steel making process by optimizing the unit operations involved in the fabrication of the steel.
A further object of one embodiment of the present invention is to provide a process of optimizing steel fabrication, comprising the steps of:
As a particularly convenient advantage, the processes set forth herein provide management and optimization of the treatment of the steel and ladle and metallic charge design in the electric arc furnace.
By incorporating the methodology of this development, a number of significant advantages are realized. As an example, precise determination of steel weight is achievable which, in turn, will result in optimization of alloy and steel mechanical properties.
Further, the ability to control tap weight, slag carryover and furnace heel translates into cost savings resulting from increased alloy recovery (Al, Ti, Mn, Si . . . ). Perhaps one of the most advantageous features relates to the fact that the use of scales at tap is unnecessary. This eliminates the cost of buying and maintaining scales which are inherent in the prior art techniques.
A further object of one embodiment of the present invention is to provide a process for controlling and determining the amount of slag carryover in steel fabrication during tapping, comprising:
The practice of the protocol described herein facilitates vastly improved control of EAF charge weights, furnace heels, refining in the ladle and cleanliness of steel and the casting process.
Ancillary benefits also include the reduction of off specs and downgraded/scrapped heats and improved ladle life.
A further object of one embodiment of the present invention is to provide a process of quantitatively determining at least one of slag carryover, furnace heel, ladle freeboard and tapped steel for optimizing steel fabrication, comprising:
A further object of one embodiment of the present invention is to provide a process for optimizing the fabrication of steel, comprising:
providing a computational model for estimating the weight of steel in a furnace used in the process;
providing the model with required furnace heel or tap weight, slag line life, ladle number and ladle life;
determining with the model required freeboard for the ladle;
communicating freeboard value to a freeboard detecting means;
tapping the steel;
measuring the freeboard during tapping with the freeboard detecting means;
comparing measured freeboard with the model required freeboard;
terminating tapping when compared values are equivalent; and
computing the weight of tapped steel, carryover and furnace heel, whereby the process quantitatively determines requisite values for the process for optimization.
A further object of one embodiment of the present invention is to provide a process for optimizing the fabrication of steel, comprising:
i) geometrically profiling a ladle used in the fabrication to determine weight of tapped steel, slag carryover and furnace heel;
ii) quantitatively determining the requisite alloy addition for predetermined steel product specifications; and
iii) controlling casting liquid steel with properties inherent from i) and ii), and steps i) through iii) being conducted in sequence and based on quantitative information derived from each precursory step, whereby the process quantitatively determines requisite values for the process for optimization.
A platform for the technology discussed herein is the dip-test process known to those skilled in the metallurgical field.
Similar numerals used in the drawings denote similar elements.
The apparatus includes a ladle 10, steel rod 12 which rod 12 includes a tip portion 14. The rod also includes a support plate or other means 16 for resting on the ladle 12. Disposed within the ladle 10 is the steel (liquid) represented by numeral 18. The slag or contaminant layer resting thereon is denoted by numeral 20 and freeboard area denoted by numeral 22.
In greater detail, the volume of ladle increases with ladle life and slag line life. The slag line region 24 consists of 12 courses of 7″ bricks 30 and the barrel region 26 consists of 17 courses of 7″ thick brick. The bottom region 28 consists of one course of 9″ thick bricks 30. To account for increase in volume due to refractory brick wear with ladle life, the wear rate for each course of bricks in each region is established based on historical records. Each course of refractory is considered a cone. By factoring in the wear rate the volume of each frustum of cone can be calculated with increasing lives of the slag line and barrel regions. As a result an algorithm was created for modeling ladle profile.
The established ladle profile provided the basis for calculating ladle volume, steel weight and slag weight for given heights of steel and slag. Using a modified dip-test technique, the apparatus for which is shown in
The dependence of the slag carryover on the furnace heel presents an opportunity to minimize slag carryover at tap if the level of the steel can be established while tapping into the ladle. If the level of steel can be determined in-situ, then with the given information on ladle slag line and barrel life the model for ladle profiling will calculate the weight of steel in the ladle and amount of slag carryover with rising steel level in the ladle. It was determined that the level of steel in the ladle can be determined with the aid of a radar system as depicted in
Referring to
For a given tap weight and ladle, the protocol as discussed herein results in the calculation of the required freeboard and this value is sent to the radar system 38. As steel 18 is being tapped into ladle 10, the radar system 38 will detect the relative position of the liquid steel, the top 40 thereof in the ladle 10 to the ladle rim 42 and determine the distance from the ladle rim 42 to the top 40 of the steel in the ladle 10. The radar system 38 compares the distance to the freeboard height determined by the protocol discussed herein. When the distance determined by the radar system 38 is equivalent to the freeboard height, the radar system 38 sounds an alarm (not shown) to the furnace operator (not shown) to end tapping. The radar system 38 continues to monitor the level of steel 18 in the ladle 10 until no further change is registered. The system communicates the final freeboard height back to the protocol which then recalculates the weights of steel slag and furnace heel.
Ultimately, with this technique, the weights of slag carryover, steel and furnace heel can be determined and used in steel making optimization, specifically charge scrap weight, and ladle alloying and refining optimization as depicted in
Once tapping is complete and the tapped weights of steel and slag have been calculated, the ladle optimization model immediately calculates the bulk amounts of various alloying agents to be added into the steel. Given the high Oxygen potential of the slag and steel, it is necessary to avoid early additions of certain alloying agents that have high Oxygen potential until the steel is reasonably de-oxidized to ensure good recovery. Therefore, the first alloying agents to be added are Aluminum, Manganese alloys and Silicon alloys. In general, the model recommendation for alloy addition follows a certain order.
In general, the ladle optimization process can be divided into five major sub-processes: 1) de-oxidation; 2) alloying; 3) inclusion modification; 4) Phosphorous content prediction; and 5) ladle slag composition prediction.
As stated above the amount of Aluminum or Silicon required for de-oxidation is dependent on the weights of tapped steel and slag, the Oxygen potential of the slag, the Oxygen potential of the steel and the required contents of the elements in the steel. From the slag analysis at tap, chemistry analysis of the steel and the tap weights of steel and slag, the total amount of Oxygen available for reaction is determined. The available Oxygen from the slag carryover for reaction with the Aluminum or Silicon is calculated from the FeO, SO2, MnO, P2O5 and TiO2 components of the slag. Of the oxide components of the slag only FeO, SO2, MnO, P2O5 and TiO2 are considered to contribute significantly to the total Oxygen available for reaction with the de-oxidants. If for example the concentrations of FeO, SO2, MnO, P2O5 and TiO2 are a %, b %, c %, d % and e % with molecular weights designated as MFeO, MSiO
From the weight of steel and dissolved Oxygen in steel at tap, the Oxygen available for reaction with Aluminum and Silicon is calculated. Let Z % designate the percentage of Oxygen in steel and let Wsteel be the weight of tapped steel in kg, then the available Oxygen in kg is
Aluminum and silicon react with Oxygen as per the following thermodynamic equilibrium reactions:
2[Al]+3[O]═(Al2O3) (3)
[Si]+2[O]═(SiO2) (4)
From expression (3), 2 atoms of Aluminum combine with 3 atoms of Oxygen. Based on mass balance, Aluminum content in Aluminum addition agent and recovery coefficient of Aluminum (R Aluminum %), from (1) and (2), we can determine the amount of Aluminum addition agent required to fully de-oxidize the steel and slag as follows:
Where AAluminum is the atomic weight of Aluminum and CAluminum% is the concentration of Aluminum in Aluminum addition agent.
Similar expression is derived for Silicon-deoxidized steel. The dissolved Oxygen in steel at equilibrium is considered negligible.
Taking into account the required dissolved Aluminum in steel, designated as X %, the total amount of Aluminum addition agent required for addition can be calculated as follows:
If Silicon is used for de-oxidation similar expression as in equation (6) is established.
Generally, some Aluminum is added at tap except for some specialized products where additions of Aluminum and other alloying agents at tap are not required. Where Aluminum addition is required at tap the model will recommend adding ⅓ of the total amount of Aluminum determined for complete de-oxidation of steel at tap with the remainder being added gradually during refining in the ladle.
Coefficient of recovery for each element from its primary alloying agent was established (see Tables 1 and 2). The concentration of each element in steel is affected by the residual level of that element in other alloying agents and this is taken into consideration in calculating the resulting concentration of each element in steel.
Complete chemical analyses were performed on various alloying agents and consumables from different suppliers. The chemical compositions of the alloying agents and consumables were compiled into a data base for the model. By referencing the compositions of the alloying agents and specified chemistry limits of the steel being manufactured, the model will first compute the respective amounts of alloying agents required to achieve the minimum content for each element in the specification. The model recommends a combination of alloying agents such that residual element pickups are maximized or minimized whatever the required case may be. The selection of a combination of alloying agents is also based on minimizing cost. For example, for products where the Carbon content required is 0.04% maximum, to ensure that Carbon pickup is kept to a minimum, the model will not recommend trim amounts for alloys which will result in a significant Carbon pickup (i.e. where Carbon pickup is not required, the model will recommend the use of LcMn instead of McMn). Another example is where there is a need to trim for Silicon and Manganese, the model will recommend the optimum amount of SiMn required to minimize the amount of LcMn or McMn for cost savings. The model will forecast the pickups from different alloying agents and will provide the expected total percentage of each element. Further, based on actual chemical analysis the model will compute how much additional trim is required for its concentration to reach the mid-point between the minimum and maximum specified values in the metallurgical practice. The model will recommend the combination of trim amount of different alloys required to ensure that the sum of certain elements are within specification (i.e. Carbon equivalent number, Titanium to Nitrogen Ratio, Ideal Diameter (DI) value etc.). For example, the model will recommend how much and which element (s) should be trimmed further to ensure that the value specified for carbon equivalent is met while individual contents of respective elements still remain within specified ranges.
Once the concentrations of all elements except Calcium and Sulfur are within specified ranges the model will then recommend that CaSi or FeCa treatment be performed. The Sulfur level of the bath will have to reach a certain percentage prior to the model recommending the amount of CaSi or FeCa required for treatment. Example, for Sulfur practice of 0.001 or 0.002%, the model will recommend that CaSi (FeCa) treatment be performed once the Sulfur level of the bath is less than or equal to 0.003% etc.
For each Sulfur practice and depending on the Sulfur level of the bath the model computes how much CaSi (we will consider only CaSi henceforth) is required for complete treatment. In calculating the amount of CaSi required the model accounts for the dissolved Calcium in steel, Calcium fade (due to re-oxidation), reaction with the oxygen in slag and steel, reaction with Sulfur and recovery coefficient of Calcium, RCalcium (its value is less than or equal to 1 but it varies depending on the Calcium-bearing alloy).The effect of Oxygen from slag and steel were accounted for by establishing a relationship between the Aluminum content in steel and total Oxygen based on de-oxidation equilibrium by Aluminum (see Table 2). The actual Oxygen levels are several times greater than the equilibrium oxygen levels as equilibrium is never truly achieved in metallurgical systems. In this work the actual oxygen was considered to be about 15 times the equilibrium values.
Any Oxygen present in the steel is considered to be present in the form of oxides and for Aluminum-de-oxidized steel the Oxygen is tied down in the form of alumina. A completely modified alumina inclusion will have a chemical formula of 12CaO.7Al2O3. Therefore, if the concentration of Oxygen is designated as N %, based on mass balance the amount of CaSi required to modify the alumina completely is calculated as:
Where RCalcium, a value less than or equal to 1, is the recovery coefficient of Calcium from and CCalcium is the % of Calcium in the Calcium-bearing alloy.
The amount of CaSi required to modify the sulphide is calculated based on the following thermodynamic equilibrium reaction:
[Ca]+[S]=(CaS) (8)
The amount of CaSi required for modification of dissolved Sulfur depends on whether a complete or partial modification is desired. In general, the amount is determined as follows:
Where Y is the concentration of Sulfur in steel in %.
The amount of CaSi required for maximum solubility (SCalcium) of Calcium in steel (about 0.0035%) is calculated as follows:
The sum of the amounts of the CaSi for all the three cases above will be the total amount required to be added for complete inclusion modifications.
There are three major sources that contribute to the total Phosphorous content of the steel: a) carryover slag from the EAF, b) Phosphorous content in steel at tap and 3) Phosphorous contents in alloying agents. In a completely de-oxidized steel the P2O5 (Phosphorous is assumed to exist in slag as P2O5) component of the slag is completely de-oxidized reverting all the Phosphorous to the steel. The amount of Phosphorous in % resulting from carryover slag with P2O5 content of PO % is calculated as follows:
The Phosphorous contents in the alloying agents are also completed dissolved in the steel. By adding up all the Phosphorous contributions from the three sources above the model is able to determine the total Phosphorous content of the steel. The Phosphorous contents of the steel determined as described above are very close to measured values.
By considering composition of slag carryover from the EAF, composition and amount of desulphurizing powder added into the ladle and composition and wear rate of the ladle refractory, the model computes the composition of ladle slag based on mass balance.
For example, the % of alumina was determined by considering 1) how much alumina is produced from de-oxidation of the slag and steel, 2) % alumina in the de-sulphurizing mix and amount of alumina dissolving in steel from the ladle refractory.
% CaO was determined base on three sources: 1) CaO in EAF slag carryover, Ca % in desulphurizing mix and 3) CaO resulting from CaSi (FeCa) treatment.
% MgO was determined based on three sources: 1) MgO from EAF slag carryover, MgO from desulphuring mix and 3) MgO from the ladle refractory.
% SiO2 was determined based on three sources: 1) Partial or full de-oxidation by Silicon from alloys, 2) SiO2 from the refractory and 3) SiO2 from the desulphurizing mix.
% S was determined based on two sources: 1) Sulfur from EAF slag carryover and 2) Sulfur from the steel. It is well known that some Sulfur is lost to the atmosphere via reaction with Oxygen but this loss was not accounted for in this calculation.
% TiO2 was determined was based on two sources: 1) Formation of TiO2 by reaction of some of the Ti in the FeTi with oxygen in the slag or air and 2) TiO2 from the ladle refractory.
% FeO in the slag was considered to be from the ladle refractory only. The FeO dissolving from the refractory is considered impossible to reduce as the FeO is considered to be present in spinel form.
The compositions of the ladle slag determined as described above are very close to measured values. An example of comparison is shown in Table 6.
Tables 3 to 6 are typical examples of values computed by the model. As shown in
Control of Steel Flow from One Vessel to Another Vessel During Continuous Casting
Once the required chemical composition and reasonable degree of refining have been attained in the ladle at the ladle metallurgy station, the ladle is delivered to the caster for casting.
At the start of casting when the first ladle is opened, the tundish 50 is filled to a certain level prior to establishing steel flow into the mould 54. Once flow to the mould 54 is initiated, the mould 54 is filled with steel to a certain level before the strand is gradually withdrawn to give a sufficient time for solidification of the steel on the dummy bar claw (not shown). Once a steady state casting has been attained and the levels of the steel in the tundish 50 and mould 54 have reached their maximum set points, the flow rate of steel from the ladle 42 to the tundish 50 and from tundish 50 to the mould 54 have to be equal to each other as well as the amount of steel being cast per unit time to prevent overflow of steel from the vessels and maintain continuity of casting. At ladle interchange time (the time between switching an empty ladle and opening a new ladle with steel) the level of the steel in the tundish 50 drops, but both from operational and quality stand points the level of steel in the mould 54 must not be allowed to drop. The time elapsed between closing one ladle and opening the other is usually short, typically only a couple of minutes, such that the tundish 50 is not completely drained by the time a flow of steel from the new ladle to the tundish 50 is established. However, where this time is extended due to operational issues, the cast speed and flow rate of steel from the tundish 50 to the mould 54 are adjusted as required. As an example, levels of the steel in the tundish 50 and mould 54 and the flow of steel from one vessel to the other are controlled as follows:
The mould level control system uses an integrated control system that meters the flow of liquid metal from the tundish 50 to the mould 54. The system combines a three-plate tundish gate system, Berthold mould level measuring system, PLC and a hydraulic system (all not shown).
The Berthold mould level gauge, as is known to those skilled, is essentially a highly sensitive radiation unit which, in conjunction with a radioactive rod source, monitors the level of molten metal in the mould and provides a proportional output signal to the Interstop PLC, the latter device being known to those skilled. This integrated control system is used to control the interaction of the middle plate cylinder, operator station and the algorithm that solves the variance between the actual mould level and the mold level set-point.
The tundish level control system meters the flow of liquid metal from the ladle to the tundish. The system uses a Vesuvius LV 80 slide gate with a two plate system to accomplish the control of this steel flow.
The tundish level control system consists of:
There are other patented techniques, like electromagnetic processes, using thermal signal from thermocouples etc, for mould level control and tundish level control. Each of these techniques has its advantages and disadvantages. The disadvantage of all of these techniques is that they are not well integrated to form a unified system to monitor and control steel flow during casting. The other disadvantage of using radiation rod source is the danger associated with mishandling and over exposure of employees to radiation.
The basis for the current invention is to create an integrated, unified and comprehensive system for the control of liquid steel levels, flow of steel from one vessel to the other and optimization of casting process. Such system will be easily incorporated into the existing PLC control logic for interaction with the hydraulic system for adjusting slide gate and casting speed.
A radar system 38 will be used to monitor the level of steel in the tundish 50 and in the mould 54. The radar system will determine any deviation of steel level in the mould from the required set point level. An on-line model which computes the volume/weight of steel in the tundish and mould at different steel levels based on the geometries of tundish and mould operates in conjunction with the radar system. The radar system which monitors the steel level in the mould communicates any deviation from the set point level range to the model. Using the geometry of the mould the model in turn computes the required increase or decrease in the weight of steel due to the deviation from the set point level. This calculated value is communicated through the gate control circuit to the hydraulic cylinder which adjust the gate accordingly.
It is possible that the flow rate from the tundish to the mould is not adjusted even though the slide gate opening adjustment was performed as required. This can occur due to plugged ports in the submerged entry nozzle or some other reasons. Whatever the case may be, the cast speed needs to be adjusted accordingly. To correct this situation, the model monitors the level in the mould and if it detects a continuous drop or increase in the level for the next five seconds it will communicate to the cast speed regulation system to slow down or increase the speed of casting to a value sufficient to bring mould level back to the required level. The ability of the system reacting quickly to correct cast speed gives the operator time to make a decision with respect to aborting the cast if the problem persists. Once the required mould level is attained the cast speed is adjusted to match the current flow rate from the tundish.
The control of casting is monitored from the beginning of cast to the end and must follow the following sequence of events.
At the start of casting a sequence of heats, once the mould radar monitoring system detects that a flow of steel has been initiated to the mould, it starts determining the change in steel height. At the start of the sequence, the tundish slide gates always have the bores in the upper and lower plates aligned to some extent (assume 82% alignment). At the maximum bore alignment of 82% the model will determine the maximum flow rate and store the flow rate as the maximum allowable flow rate for the sequence of heats. Based on the diameter of the bore the model computes the distance the plate has to move in the opposite direction for the bores in the plates to be completely misaligned in order to end the flow of steel through the gate. With this analysis, the model will understand the relation of the position of the lower plate to the flow rate of steel from the tundish to the mould and use this logic to determine how the plate should be displaced to increase or decrease steel flow rate.
Once the flow of steel into the mould has been detected, the model begins recording level change in the steel in the tundish communicated by the radar system. At the start of a sequence and whenever a new ladle is opened the bores in the lower and upper plates are aligned 100% to ensure that the tundish is filled up within the shortest time possible. The model will compute the volume change per unit time in the tundish and it will record the sum of the flow rates to the tundish and mould as equal to the total amount flow rate through the ladle slide gate at 100% open. This value is stored by the model in its memory as the maximum flow rate possible which will be used in determining the direction of displacement of the hydraulic cylinder to compensate for level change in the tundish during steady state casting.
Once the level of the steel in the mould reaches 95% of the specified height in the mould the mathematical model will start decreasing the alignment of the lower and upper plates while ensuring that once the steel level reaches 100% of the specified level the flow rate from the tundish to the mould is equal to the casting through put.
Once the steel level in the tundish reaches 95% of the specified height in the tundish the mathematical model will start decreasing the alignment of the bores in the lower and upper plates while ensuring that once the steel level reaches 100% of the specified level the flow rate from the ladle to the tundish is equal to the flow rate from the tundish to the mould.
Steady state is considered achieved once both the tundish level and mould level have attained 100% of their specified levels.
a) As soon as steel is detected in the mould the model will start computing the volume of the steel left in the ladle as follows:
b) Along with estimating the remaining weight of steel in the ladle, the model estimates how long it will take to empty each ladle as follows:
c) The total cast time for each heat can be calculated as follows;
d) The calculated weight of steel in the ladle as in the equation (10) is used to determine when to arm the slag detection system, which determines when to shut the ladle off due to entrainment of slag into the tundish during casting. An audible alarm can be used to alert the operators to arm the system or aiming of the system can be done automatically.
e) The model determines that the ladle slide gate is closed if the backward displacement of the hydraulic cylinder is equal to or greater than the diameter of the lower plate.
f) Once the model determines that the ladle has been closed based on the backward displacement of the hydraulic cylinder, communication between the tundish radar system and the model is discontinued until a new ladle is opened. However, communication between the model and mould radar system continues until the tundish slide gate is closed.
It should be noted that using the radar system in conjunction with geometrical profiling of any vessel or reservoir similar analysis outlined in this invention can be employed for calculating volumes or weight of any liquid and controlling the flow from one vessel to the other for any system. It is equally important to note that controlling steel flow from vessel to vessel during continuous casting and profiling of the vessels as described in this invention can be integrated with other level monitoring systems beside a radar system. The overall advantage of this system is that its response to deviations from steel level in the tundish and mould will be instantaneous and highly accurate.
With accurate determination of the EAF heel the amount of metallic scrap required to be charged into the furnace as specified by the production schedule can be managed better to prevent over or under charging of the furnace. Generally, the furnace operators eyeball the level of heel in the furnace and estimate how much steel is left in the furnace. A gross misjudgement can result in under or over charging of the furnace. The consequences of misjudgement includes yield loss due to loss of steel through the slag door, increased power consumption due to exposure of arc to atmosphere, excessive nitrogen pickup from the atmosphere, increase refractory wear rate, and low productivity. Accurate determination of furnace heel will ensure consistent bath level, high productivity and good process control.
Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto. The entire disclosures of all references recited above are incorporated herein by reference.
