WO2021214513A1

WO2021214513A1 - Method for determining the remaining service life of an argon injected slide gates

Info

Publication number: WO2021214513A1
Application number: PCT/IB2020/053717
Authority: WO
Inventors: Michael MASLEID; Bruce R FORMAN; Bernard O CHUKWULEBE
Original assignee: Arcelormittal
Priority date: 2020-04-20
Filing date: 2020-04-20
Publication date: 2021-10-28

Abstract

A method for determining the remaining service of an argon injected slide gate. An argon injected slide gate is installed. A constant argon injection flow rate Qb* and a maximum allowed total argon flow rated Qa* are selected. The maximum allowable escaping argon flow rate Qe* is calculated using Qe*=Qa*-Qb*. Next, the present escaping flow rate conductance Ge' of said argon injected slide gate is determined using either 1) the Steel Pressure Change Event Method; or 2) the Argon Flow Rate Change Event Method. The present total argon pressure Pa' is measured. The present escaping flow rate Qe' is calculated using: Qe=Pa'xGe'. A data set of multiple Qe' vs time datapoints are collected. The data set is mathematically fitted to a curve. The curve is used to predict the time until the maximum escaping argon flow rate Qe* will be reached.

Description

METHOD FOR DETERMINING THE REMAINING SERVICE LIFE OF AN ARGON INJECTED SLIDE GATES

FIELD OF THE INVENTION

The present invention relates to continuous steel casting and more specifically to inert gas injected slide gates used in continuous steel casting. Most specifically the invention relates to method for determining the remaining service life of an argon injected slide gates.

BACKGROUND OF THE INVENTION

In steelmaking operations, a slide gate is used to control the flow of liquid steel through a nozzle arrangement that drains the molten liquid steel from a metallurgical vessel. It is well known in the art that when inert gas is injected into the discharge passageway of the slide gate, the injected inert gas will reduce plugging or build-up that clogs the passageway. Continuing advancements in the art have led to the use of porous, gas permeable nozzles and slide gate plates that are able to deliver a continuous or intermittent inert gas flow to the discharge passageway where the delivered gas provides a gas barrier between the passageway surface and the liquid metal being drained. Such porous nozzles and slide gate plates are disclosed in U.S. Pat. No. 5,431 ,374 incorporated herein in its entirety by reference.

Referring to columns 1 and 2, the 374 patent discloses, although it is not certain, it is believed the inert gas flows through the porous nozzle walls, and advantageously forms a fluid film over the surface of the bore within the nozzle that prevents the liquid metal from making direct contact with the inner surface forming the bore. By insulating the bore surface from the liquid metal, the fluid film of gas prevents the small amounts of alumina that are present in such steel from sticking to and accumulating on the surface of the nozzle bore. The 374 reference also teaches that such alumina plugging will occur within the bore of a slide gate if an inert gas barrier is not provided. Therefore, as clearly taught in the art, for example, U.S. Pat. Nos. 4,756,452, 5,137,189, 5,284,278, and 5,431 ,374, inert gas barriers are used throughout the steelmaking industry to prevent alumina plugging within the discharge passageway that drains liquid steel from a tundish into the caster mold portion of a continuous caster.

Additionally, the 374 patent also discloses that in order to provide a proper inert gas barrier, the pressure of the inert gas must be maintained at a level sufficient to overcome the considerable back-pressure that the draining liquid steel product applies against the surface of the bore, and ideally, the gas pressure should be just enough to form the desired film or barrier. It is well accepted that injecting inert gas into a slide gate discharge passageway does reduce the plugging phenomenon but metering the actual gas flow to the discharge-opening has long been a problem. Leaks in the gas delivery system are a repeating and continuous problem, and the measured amount of incoming gas flow is often different from the actual gas flow delivered to the liquid metal draining through the slide gate. Such gas delivery system leaks can occur in any one of the numerous pipefitting connections along the gas feed line extending between the inert gas supply and the slide gate mechanism. Additionally, some leaks are dynamic in that they develop in the slide gate plates during casting operations as taught in U.S. Pat. No. 4,555,094. Historical information at our continuous casting operations shows that in many instances, no inert gas is delivered to the slide gate discharge passageway when the control gage readings show that the inert gas flow through the gas feed line is normal. The currently employed constant pressure or constant flow-based control methods that are used to deliver inert gas to a slide gate mechanism cannot compensate for dynamic leaks, flow resistance changes, or unknown pressure drops, and therefore, they are ineffective for maintaining a target threshold gas flow within the discharge passageway. Consequently, the state-of-the-art inert gas delivery systems often fail to shield the bore surface from the liquid metal as taught in U.S. Pat. No. 5,431 ,374.

In US patent number 6,660,220 ('220) the present inventor provided for a dynamic control system capable of delivering an inert gas at a target threshold gas flow rate to the discharge passageway in a slide gate draining a liquid metal product. The system was also capable of measuring inert gas flow resistance to determine an amount of plugging that occurs within the discharge opening passageway that drains liquid metal from a metallurgical vessel.

In addition, the '220 patent provided a mathematical model that provides on-line evaluation and dynamic control of the inert gas delivery system so that a consistent inert gas flow is maintained to prevent or reduce plugging within the discharge opening passageway that drains liquid metal from a metallurgical vessel. The dynamic control system maintains the inert gas at a constant target threshold flow rate sufficient to prevent or reduce plugging within the discharge opening, and the dynamic control system includes a gas feed line extending between an inert gas supply and the slide gate discharge passageway, a gas flow regulator, a pressure gauge; and a gas feed flow control system that detects an amount of incoming inert gas flow lost through leaks in the system and adjusts the gas flow regulator in response to the detected amount of incoming gas flow loss so that the adjusted incoming gas flow continues to deliver the target inert gas flow rate to the discharge passageway.

However, the '220 system/process only deals with inert gas losses up to the entry of the slide gate valve and does nothing to determine the portion of the inert gas that is actually injected to the steel versus that which is lost due to leaks in the slide gate itself. Improper flow rate of inert gas into the steel can cause issues such as: the re-oxidation of liquid steel (to little argon); blister type defects in slabs (excessive argon); plugging of the flow control system limiting casting time; and excessive plugging and wash-out during casting, that can cause entrainment of non-metallic inclusions. Furthermore, as casting time increases, the ratio of the inert gas injected into the steel versus that lost due to leaks changes. The slide gate warps and the leaks increase until virtually no inert gas is being injected into the steel. For a number of applications, it would be useful to be able to determine how much gas is injected into the steel versus how much is leaked from the system. The present invention provides a process to determine these flow rates on-demand in real-time.

One such application is to determine the remaining service life of an argon injected slide gate. The present invention provides a method for determining the remaining service life of an argon injected slide gate.

SUMMARY OF THE INVENTION One aspect of the invention is a method for determining the remaining service life of an argon injected slide gate. Initially an argon injected slide gate controlling the flow of liquid steel through a nozzle is installed. A constant argon injection flow rate Qb* is selected. A maximum allowed total argon flow rated Qa* is also selected. The maximum allowable escaping argon flow rate Qe* is calculated using the formula Qe* = Qa* - Qb*.

Next, the present escaping flow rate conductance Ge' of said argon injected slide gate is determined using either the Steel Pressure Change Event Method or the Argon Flow Rate Change Event Method. Then, the present total argon pressure Pa' is measured. Now, the present value of the escaping flow rate Qe' may be calculated using the formula Qe' = Pa' × Ge'.

Once Qe' is known the value and the time since the argon injected slide gate was installed may be recorded as a data point. Multiple Qe' vs time datapoints are collected to form a data set. The data set is then mathematically fitting to a curve. The curve can then be used to determine the time until the maximum escaping argon flow rate Qe* will be reached, which is the expected remaining service life of the argon injected slide gates

BRIEF DESCRIPTION OF THE IMAGES

Figure 1 depicts a schematic view of a continuous steel casting system;

Figure 2 depicts a schematic view of the tundish and slide gate of a continuous steel casting system; Figure 3 depicts a schematic of the section of the steel casting apparatus focusing on the slide gate;

Figure 4 depicts an electrical circuit model which is analogous to the slide gate of the casting system;

Figure 5 is a simplified circuit solving the circuit Figure 4;

Figure 6 plots the argon pressure (Pa) and steel head pressure (Ps) vs the casting time during a steel pressure change event;

Figure 7 is a plot of Ps (x-axis) vs. Pa (y-axis) a steel pressure change event caused by a ladle change;

Figure 8 is a plot of argon pressure (Pa) and argon flow (Qa) vs time during an argon flow change event; and

Figure 9 is a plot of Qa vs Pa during the Argon flow event.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 depicts a schematic view of a continuous steel casting system which streams molten steel from a ladle 1 through a ladle slide gate valve 4 to a tundish 2, and through a tundish slide gate valve 5 and a submerged entry nozzle (SEN) 6 into a mold 3. Figure 2 depicts a schematic view of the tundish 2 streaming steel through the upper nozzle 7, through the slide gate 5, through the SEN 6 and into the mold 3. In steel casting, Argon injection is used to reduce the rate of oxygen diffusion into to molten steel. It is also injected into molten steel to provide stirring and to improve steel cleanliness by removing or controlling the amount, size, and distribution of inclusions incorporated into the steel being cast. The argon is supplied to the steel through a porous material. In Figure 2 the porous material is in the shape of a ring 8 in the top plate 10 of the slide gate 5.

Figure 3 depicts a schematic of the section of the steel casting apparatus focusing on the slide gate 5. Argon is injected into the slide gate 5 through an injection system as described in the '220 patent. The argon is injected into the porous member 8 and is injected into the steel. Some of the argon percolates through the porous ceramic 8 and is injected into the steel flowing into the mold 3. The rest of the argon may find another path into the steel and bubble out into the tundish 2. Some of the argon leaks 12 back out through gaps and cracks in the assembly. This helps reduce the amount of oxygen leaking in.

Direct measurement or observation of argon injection is difficult. Using the present invention, it is possible to determine the fractions of argon leaking to atmosphere, being injected into the steel flow, and bubbling into the tundish by combining information obtained during the casting process.

Measurements used by the present inventive method include the time-based values of tundish weight, cast speed, mold level and mold width that are used to calculate steel head pressure and steel flow pressure. The argon panel provides argon pressure and argon flow. The estimator is based on three independent combinations of pressures and flow.

In one aspect of the present inventive method, constants include the height of the steel level above the argon injection point, the density of the steel at the operating temperature and the acceleration due to gravity. The present inventive method includes a step to reconstruct measurements as simultaneous values taken at equal time intervals. In this implementation sparse measurements are supplied at the nearest second of a measurement. A symmetric Gaussian window is used to sum available values near a given second, and another symmetric Gaussian window is used to sum existence (1 or 0) of the available value near the given second. The average, the first sum divided by the second sum, provides an estimate of the value as though the measurement was taken at the given second. In another implementation, where measurements are supplied with their measurement time stamp, spline interpolation methods can be used.

The present invention uses an analogy between linear flow through porous media (Darcy's Law) and an equivalent linear electrical circuit. The flow path can be represented in three components. Some argon escapes to the atmosphere. Some is injected into the steel flow. Some bubbles into the steel column. The analogous electrical model is shown in Figure 4. In that model: Vs represents ferrostatic pressure at the bore hole; Vf represents pressure drop from steel flow through the bore hole; Vb represents the pressure difference between the bore hole and an upper surface of the porous ceramic.

In the electrical analog, la corresponds to argon flow, Va corresponds to argon gauge pressure, Vs corresponds to the ferrostatic pressure in the bore hole, and Vb refers to the ferrostatic pressure at the top of the insert. The resistance, Re, is the resistance to flow out of the bottom of the insert including the resistance to flow through the space between the top plate and the throttle plate (i.e. resistance to flow to the atmosphere). The resistance, Ri, is the resistance to flow through the insert into the bore hole. The resistance, Rb, is the resistance to flow through the insert to the upper surface and up into the funnel. Ri and Rb are both resistances to flow into the steel. The steel flow through the bore hole will cause a pressure drop represented by Vf. Argon flow into the injection point may be purged to the atmosphere or may be injected into the steel against the backpressure produced by the height of the steel above the injection point. The flow of steel past the injection point causes a pressure drop, so the actual pressure at the injection point is slightly reduced. Argon bubbling out above the injection point must work against the back pressure due to the height of the steel at the point of bubbling. Argon can flow into the steel but cannot flow backwards (this is represented by diodes). Solving the circuit in figure 4 yields the circuit in figure 5.

Voltage is replaced by pressure. Resistance is replaced by its reciprocal, conductance. Argon flow is in cubic centimeters per second (cm³/s), pressure is in megapascals (MPa), and conductance is in (cm³/s / MPa). The network solution for argon pressure is:

Here, Pa is the argon pressure as measured. Qa is the argon flow controlled to a set point. Ps is the calculated ferrostatic head pressure where argon is injected into the steel flow. Pf is the calculated pressure reduction due to steel flow past where argon is injected into that steel flow. Pb is the difference between ferrostatic pressure at the high flow injection point and the ferrostatic pressure at the bubbling site (next to the bore hole for the top plate geometry). Argon cannot flow out of the steel. The backflow conditions can be implemented as:

(2) Gb = 0 when (Ps - Pb) ≥ Pa

Gi = 0 when (Ps - Pf) ≥ Pa

The height of steel in the tundish (in centimeters cm) can be calculated from the tundish geometry and the density and weight of the steel in the tundish. The distance from the bottom of the tundish to the injection point can be measured. The pressure at the injection point (Ps) is the product of steel density (in g/cm³) times the total height of steel above the injection point times the acceleration due to gravity.

The position of the bubbling leak relative to the injection point is not known. As an initial assumption the bubbling leak is above the injection point and remains constant. The pressure (loss) due to bubbling distance (Pb) is the product of steel density (in g/cm³) times the distance above the injection point times the acceleration due to gravity.

The pressure (loss) due to laminar steel flow (Pf) can be determined using the flow volume (cast speed times mold thickness times mold width). Divide this by the cross-sectional area at the injection point to get the flow velocity. Bernoulli's equation gives the pressure due to steel flow as one half the velocity squared times the steel density.

The components of the argon flow are determined by observing measurements taken during the casting process. Events, such as ladle changes, steel flow changes, and argon flow changes are detected. Each type of event is processed to provide solutions to parts of the analogous circuit. A ladle change event stops flow of steel into the tundish but does not stop the continuous casting process. The tundish weight changes as the tundish empties. The level of steel in the tundish drops. Therefore, the ferrostatic head pressure at the bottom of the tundish also drops.

A speed change and mold size change can result in a change in flow rate out of the tundish. The change in flow through the injection point changes the ferrostatic pressure loss due to the flow velocity.

A change in argon flow reduces the pressure drop caused by argon flow into the steel or through purge paths into the atmosphere.

The steel flow event (a change in the product of cast speed times mold size) will cause the plot of Pa against Pf to have a slope equal to the derivative of:

The Pa intercept (at Pf = 0) will be

It follows that:

The steel pressure change event will cause the plot of Pa against Ps, while weight is changing, to have a slope equal to the derivative of:

The Pa intercept (at Ps = 0) will be at:

It follows that:

The argon flow change event will cause a plot of Pa against Qa, while weight is not changing, to have a slope equal to the derivative of:

The Pa intercept (at Qa = 0) will be at:

It follows that:

These equations apply for small changes in argon pressure, where argon pressure remains greater the ferrostatic head pressure. Large changes in argon pressure require a piece wise linear solution, since argon flow into steel is not reversible.

Examples

STEEL PRESSURE CHANGE EVENT

Figure 6 plots the argon pressure (Pa) and steel head pressure (Ps) vs the casting time during a ladle change event. The ladle change event produces a distinct dip in argon pressure (as steel pressure drops) when argon pressure is greater than steel pressure. Argon pressure is measured at the argon panel. Argon is supplied to a top plate. Steel flow pressure is not used for this calculation due to turbulent flow at the bore hole. During the event, the mean argon flow is 200.4 cm³/s. Figure 7 is a plot of Ps (x-axis) vs. Pa (y-axis) during ladle change as the level of steel in the tundish is falling A and then subsequently refilling B. The falling curve A follows the equation y = 0.3513x + 0.1366 MPa. Using this falling edge trend line with equation (4) from above, and wherein:

Setting Gi = 0 for the top plate and solving for Gb with Pb = 0, then using the argon pressure and steel head pressure after the event to calculate argon injected as bubbles from the top plate. Solving for Gb using the mean value for argon flow:

Now we can calculate argon injection (as bubbles into the steel column) after the event using argon and steel pressure just after the event where Pa = 0.1741 MPa and

Ps - 0.109 MPa.

The amount of argon escaping can be found from the derivative. Solve for Ge:

Calculate the argon escaping based on the argon pressure after the event:

The calculated sum of argon injected (as bubbles) and argon escaping after the event is 199.2 cm³/s which is slightly smaller than the amount, 200.4 cm³/s, of argon supplied. The event was chosen at a time when the caster was stable with respect to argon escape.

ARGON FLOW CHANGE EVENT

The argon flow change event produces a very large drop in argon pressure, but the argon pressure drops below steel pressure, and will require a piecewise linear solution. Figure 8 is a plot of argon pressure Pa and argon flow Qa vs time as an SEN is changed. During the event, the steel head pressure is 0.1091 MPa. Figure 9 is a plot of Qa (x-axis) vs. Pa (y-axis) during the event Argon pressure rises as argon flow overcomes leak (purge) resistance. Once argon pressure exceeds steel pressure, argon pressure rises more slowly because resistance has a parallel path. Argon flow begins to bubble into the steel. The two different regimes can be seen in Figure 9. One flow regime has a fitted line having a slope of 9.0 × 10^-4 and an intercept of 0. The other regime has a fitted line having a slope 5.75 × 10^-4 and an intercept of 0.0345. For the first segment Gi = 0 and Gb = 0. Although we have solved for this segment, it should be noted that this segment will give no useful information on argon injection into the steel because all argon is escaping (none is being injected, i.e. Pa < Ps):

Solving for the second segment where Gb ≠ 0 we can use equation (5):

Now solve for Ge:

Argon escape during the event is changing continuously as Pa × Ge. Now we calculate argon injection (as bubbles into the steel column) and argon escaping after the flow event using argon and steel pressure just after the event where Pa=0.1537 MPa, Ps=0.1091 MPa, then we compare that to Qa=200.4 [cm³/s]:

Qb = (0.1537 - 0.1091) × 612.1 = 27.3 [cm³/s]

Qe=0.1537 × 1127 = 173.2 [cm³/s]

Qa = Qb + Qe = 200.5 [cm³/s]

The following is a summary of the herein above described methods to calculate the volume of argon injected into the steel and volume of argon escaping in a system using an argon injected slide gate to control the flow of liquid steel through a nozzle. The argon injected slide gate having an argon injection point. Steel Pressure Change Event Method

Herein after this method will also be known as the “Steel Pressure Change Event Method”.

The method includes the initial step of creating a steel pressure change event by changing the steel pressure above the slide gate with respect to time. The method also includes the steps of measuring the argon pressure (Pa) vs time and calculating the steel pressure (Ps) vs time during the steel pressure change event, wherein the steel pressure is calculated by multiplying the height of the steel above the argon injection point multiplied by the density of the steel. The method further includes the step of measuring the average argon flow rate (Qa') during the steel pressure change event. The method also includes the step of plotting the steel pressure on the x-axis of a graph versus argon pressure on the y-axis during the steel pressure change event (Ps-Pa plot). Once the graph is plotted the method calls for fitting a line to the Ps-Pa plot. The line has the general formula y=Mx + B, wherein M is the slope of the line and B is the y- intercept of the line and then measuring the slope M and y-intercept B of the line.

Next, the method calls for calculating the argon injection bubbling conductance Gb using the formula: Gb = (M/B) × Qa'. The method also calls for measuring the argon pressure (Pa') and calculating the steel pressure (Ps') immediately after the steel pressure change event. Then the steel injection argon flow rate Qb is calculated using the formula: Qb = (Pa' - Ps') × Gb and the argon escaping conductance Ge is calculated using the formula: Ge = (Gb/M) - Gb. Finally, the argon escape flow rate Qe using the formula: Qe = Pa' × Ge. Argon Flow Rate Change Method

Herein after this method will also be known as the “Argon Flow Rate Change Event Method”.

The method includes the initial step of creating an argon flow change event by changing the argon flow rate into the slide gate with respect to time. The method also includes the step of measuring the argon pressure (Pa) vs time and measuring the argon flow rate (Qa) vs time during the argon flow change event. Also, the method includes calculating the average steel pressure (Ps) during the argon flow change event. Also, the steel pressure (Ps') is calculated at the end of the argon flow change event.

The method also includes the step of plotting the argon flow on the x-axis of a graph versus argon pressure on the y-axis during the argon flow change event (Qa-Pa plot) and fitting a line to the Qa-Pa plot. The line has the general formula y=Mx + B, wherein M is the slope of the line and B is the y-intercept of the line. The next step is to measure the slope M and y-intercept B of the line.

Next the argon injection bubbling conductance Gb is calculated using the formula: Gb = B/(M*Ps'). The next step is measuring the argon pressure (Pa') and calculating the steel pressure (Ps') immediately after the argon flow change event.

Next the argon injection flow rate Qb is calculated using the formula: Qb = (Pa' - Ps')*Gb. The argon escaping conductance Ge is calculated using the formula: Ge = (1/M) - Gb; and the argon escape flow rate Qe is calculated using the formula: Qe = Pa'* Ge.

Determining the Remaining Service Life

As the casting campaign runs its course, the argon injected gas gate begins to deteriorate. Eventually, the gas gate must be replaced, but there is no set service life of the slide gates and no way to estimate when the slide gate should be replaced.

The present invention is a method of determining the remaining service of such a slide gate. The method begins with two set assumptions. The first assumption is that the argon injection flow into the steel Qb will be constant. That is, Qb will be set to the optimal flow rate Qb* for the particular steel being cast.

The second assumption results from the fact that there is a maximum total argon flow Qa available is to the argon injected slide gate. The assumption is that some maximum allowed total argon flow Qa* is chosen for the slide gate. Qa* may be any flow rate up to and including Qa. The value of Qa* may be chosen for any reason be it physical practicality, economic considerations, and the like.

Qb* and Qa* are now known constants. Thus, as the gas gate deteriorates, the escaping flow rate of argon Qe will increase. The maximum allowable escaping flow rate Qe* can be calculated by the equation Qe*=Qa* - Qs*.

The method includes determining the present argon escaping flow rate conductance Ge' of the argon injected slide gate using either the Steel Pressure Change Event Method or the Argon Flow Rate Change Event Method. Then the present escaping argon flow Qe' is calculated by the formula Qe' = Pa' × Ge', wherein the term Pa' is the present value of argon pressure, which can be measured. The present value of the is escaping argon flow Qe' is saved along with the time since the gate's installation as a data point. The value of the is escaping argon flow Qe' is determined at numerous times to create a data set. The data set is plotted as Qe' versus time to form a curve. The curve is then fitted to a mathematical curve.

Generally, as time goes on the value of the is escaping argon flow Qe' increases and, at some point, Qe' reaches the value of Qe*, the service life endpoint of the sliding gate.

The mathematical curve can be used to project the time at which Qe* will be reached and the slide gate will be at its service life endpoint. Once this time is known, preparations may be made in advance to replace the gate.

Claims

We Claim:

1. A method for determining the remaining service life of an argon injected slide gate, the method comprising: a) installing an argon injected slide gate controlling the flow of liquid steel through a nozzle; b) selecting an argon injection flow rate Qb*; c) selecting a maximum allowed total argon flow rated Qa* d) determining the maximum escaping argon flow rate Qe*, wherein Qe* = Qa* - Qb*; e) determining the present escaping flow rate conductance Ge' of said argon injected slide gate using either the Steel Pressure Change Event Method or the Argon Flow Rate Change Event Method; f) measuring the present total argon pressure Pa'; g) calculating the present escaping flow Qe', wherein Qe' = Pa' × Ge'; h) recording the present escaping flow rate Qe' and the time since said argon injected slide gate was installed as a data point; i) repeating steps e-h a plurality of times to form a data set; j) mathematically fitting said data set to a curve; k) solving said curve to determine the time until said maximum escaping argon flow rate Qe* will be reached.

2. The method of claim 1 , wherein said step of determining the present escaping flow rate conductance Ge' of said argon injected slide gate uses the Steel Pressure Change Event Method.

3. The method of claim 1 , wherein said step of determining the present escaping flow rate conductance Ge' of said argon injected slide gate uses the Argon Flow Rate Change Event Method.