TECHNICAL FIELD OF THE INVENTION

This invention relates generally to a method of continuous casting and more particularly to a method of performing flying tundish operation in a bloom caster producing continuously cast blooms.

BACKGROUND AND THE PRIOR ART

Continuous casting is performed by eliminating a number of energy and yield inefficient processes like ingot casting and intermediate rolling into semis before final rolling in to final product. Tundish changing operation between sequences is a time consuming process and delays the production. Some of the prior arts in the related field of invention are as follows:

JP 4361860 provides a control device for molten steel surface level in a mold in a continuous caster, by which the molten steel surface level in the mold can be stably controlled even in the case of inputting disturbance. In the control device for molten steel surface level in the continuous caster, an identified signal generator always gives periodical signal with the degree exerting no adverse influence on the cast slab to a command value for stopper opening degree obtained based on the estimated flow rate gain K. By this method, as even in the case the control system is stable, the estimation operation of the above flow rate gain K, etc., is performed, even in the case of inputting the disturbance, the molten steel surface level can be stably controlled based on the flow rate gain K at this time. As the above flow rate gain K, etc., is estimated with the method of least squares in on-line, the optimum value for each is calculated. Therefore, by using the above flow rate gain K, etc., feed forward control can be executed.

US6539273 (B1) provides a method for automatically controlling operation of a continuous casting plant wherein molten steel is fed from a ladle to a tundish distributor having controlled outlets opening into a continuous caster maintaining a level of cast molten steel and in which the molten steel solidifies into a slab, said method comprising the steps of (a) measuring a temperature Tdist of the molten steel in said distributor; (b) detecting a temperature loss in the molten steel over a residence time of said molten steel between said distributor and said level; (c) determining an equivalent liquid us temperature as a limiting equivalent temperature for the molten steel in said distributer based on said temperature loss and for achieving at said level in said caster a limiting liquid us temperature T liq for a given continuous casting speed; (d) determining a succession of isotherms of the equivalent liquid us temperature in said distributor differing from said limiting equivalent temperature by a certain number of degrees for respective casting speeds assigned to the equivalent liquid us temperatures of said succession; and (e) controlling an actual continuous casting speed of the slab to match one of said respective casting speeds to maintain an actual measured temperature T dist in said distributor within an isotherm window between the isotherms determined in step (d) and having as its lowest limit the equivalent liquid us temperature.

CA 2313233 (A1) provides a method of automatically controlling operation of a continuous casting plant wherein molten steel is fed from a ladle to a tundish distributor having controlled outlets opening into a continuous caster maintaining a level of cast molten steel and in which the molten steel solidifies into a slab, said method comprising the steps of: (a) measuring a temperature T dist of the molten steel in said distributor; (b) detecting a temperature loss in the molten steel over a residence time of said molten steel between said distributor and said level; (c) determining an equivalent liquidus temperature T of the steel in said distributor and a plurality of isotherms T thereof; and (d) controlling a continuous casting speed of the slab so that the temperature T dist measured in step (a) is maintained within an isotherm window T 4. The method defined in claim 3 wherein said graph is displayed dynamically as a function of at least one of the parameters: residence time of the steel in the distributor, heating of the distributor from a temperature of 1200[deg.]C to an equilibrium temperature with the steel of about 1500[deg.]C, and distributor insulation. < T dist < T e liq.+Y[deg.]C, where Z is greater than X and Y, X~Y, and X, Y and Z are integers.

CA2497489 (A1) provides a computer-implemented method for online monitoring of transient operations in a continuous caster and predicting an impending transient-cast breakout or other process abnormality, comprising the following steps: retrieving historical process data of a plurality of selected process variables during a pre-defined transient operation duration, the resulting historical process data covering most of normal transient operation regions and being used to construct a modeling dataset; dividing modeling data in each transient operation into two stages, and synchronizing the modeling data separately with respect to a set of synchronization scales pre-defined by casting speed and casting strand length, respectively, in said two stages to define a series of observations forming a synchronized modeling dataset;
performing a multi-way principal component analysis (MPCA) on said synchronized modeling dataset to develop a multivariate statistical model to benchmark normal transient operations; and calculating a loading matrix P, and values of principal components for each transient operation in the modeling dataset; computing test statistics at each observation over a predefined transient operation duration, for each transient operation in the modeling dataset, based on the multivariate statistical model; determining control limits for said test statistics and their contributions from each selected process variable; acquiring on-line process data of each selected process variable from a pre-defined start point to a current time t, in a new transient operation to be monitored;
synchronizing the acquired online data based on the synchronization scales defined in the two stages, and predicting future process trajectories for the rest of said transient operation duration, namely, from said current time t to a pre-defined end point for said transient operation duration to create complete process trajectories; computing test statistics based on the multivariate statistical model for the resulting complete process trajectories of said new transient operation;
comparing the test statistics computed from said new transient operation to their corresponding control limits; and generating at least one indication signal indicating whether said new transient operation is statistically different from its historical benchmark of normal operation in a continuous casting process.

Although readily present the prior art fails to provide an efficient method of flying tundish operation. Also the prior arts require lots of manual operation. The present invention therefore provides an efficient method of flying tundish operation consuming less time and at the same time it is more accurate providing for an efficient mass production.

OBJECTS OF THE INVENTION

A basic object of the present invention is to overcome the disadvantages/drawbacks of the known art.

Another object of the present invention is to provide a methodology of improving production from continuous bloom caster.

Other object of the present invention provides for a method of performing flying tundish operation in a bloom caster producing continuously cast blooms.

Other object of the present invention is to provide for movement of stopper mechanism controlled through a PLC having a pre installed startup curve providing appropriate ramping of stopper mechanism thereby avoiding mould overflow.

Yet other object of the present invention provides for performing a continuous secondary cooling using an air and water butterfly valve.

These and other advantages of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

There is provided a continuous bloom caster.

According to the present invention, there is provided method of performing flying tundish operation in a bloom caster producing continuously cast blooms , said method comprising the following steps of selecting flying tundish mode in a HMI; operating strand close button thereby closing stopper mechanism; selecting cast mode as fly tundish mode; readying a second tundish for casting position on a tundish heating position; carrying said second tundish to a casting position on a tundish car; stopping ladle filling old tundish; removing old tundish from casting position; placing said second tundish in casting position; opening ladle to fill a new tundish; detecting minimum specified level of molten metal in mould through length tracking sensor connected to PLC and confirming physically using a measuring rod; opening stopper mechanism of said second tundish and pouring liquid metal slowly in a mold through a sub entry nozzle thereby joining to top of old strand; restarting caster and gradually increasing casting speed thereby withdrawing joined new strand with old strand; performing a continuous secondary cooling using an air and water butterfly valve; and directing said strand forming blooms to cutting torches; wherein movement of said stopper mechanism controlled through a PLC having a pre installed startup curve providing appropriate ramping of stopper mechanism thereby avoiding mould overflow.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig 1 illustrates the graph of increase of monthly production of bloom with flying tundish operation.

Fig. 2 illustrates increase of heats per day monthly.

Fig. 3 illustrates increase of actual casting hours with flying tundish operation.

Fig. 4 illustrates decrease in cast end (CE) to cast start (CS) with flying tundish operation.

Fig. 5 illustrates an overview of stopper rod mechanism automation scheme.

Fig. 6 illustrates a stopper rod close loop control system.

Fig. 7 illustrates method steps used in flying tundish operation.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.

Reference is first invited to Fig. 1 to Fig. 4 where the graphs are provided which shows the advantages of implementing flying tundish operation.

Fig. 5 shows an overview of stopper rod mechanism automation scheme.

Fig. 6 shows a stopper rod close loop control system.

Fig. 7 shows method steps used in flying tundish operation.

The invention therefore provides a method of performing flying tundish operation in a bloom caster producing continuously cast blooms.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly in the present invention a flying tundish change happens when the old tundish is removed and a new tundish is placed in casting position. The strand is restarted using existing casting strand of particular section in the mold as the starter dummy bar. Liquid metal from the new tundish flows into the mold and welds to the top of the old strand. The caster is then restarted and the joined new strand with the old strand is withdrawn by slowly increasing casting speed. The practice saves considerable amount of time. Many shops are using the flying tundish practice to extend caster sequences to increase the casting time with respect to total caster available time. Some shops run two weeks or longer without any casting stop.

The flying tundish change practice starts with having a second tundish ready for casting position on the tundish car. When the metal level in the new tundish reaches a minimum specified limit for normal casting after opening of the new ladle, metal filling in the mould will start with opening of stopper rod in tundish. A new secondary cooling water pattern is to be adjusted during flying operation. A flying tundish change is considered successful when the entire restart strands have made it through the cutting torches.

Normally, flying operation is done in the following steps:
a) Before flying of tundish, control of stopper is shifted from auto metal feeding to manual feeding in case of automatic mould level control system.
b) The casting speed is reduced to zero from operating casting speed
c) The metal level from the bottom of the mould is kept at lower level compared to level maintained during normal casting through visual observation.
d) After the completion of casting in a tundish, the tundish is shifted by tundish car and flying of next tundish is done from tundish heating position to normal casting position.
e) After metal pouring from ladle to tundish up to a specific metal height in tundish, metal is poured slowly in mould to a particular level through sub-entry nozzle by lifting of stopper manually. Sufficient time is kept for metal filling in the mould for joining of newly formed shell with the old strand shell. It is very critical as improper shell joining may lead to break-out from the joint below mould.
f) Speed is increased very slowly to normal casting speed

The bloom caster in the present invention is having automatic stopper rod control without having any provision of manual control. Manual control of stopper is essential requirement for flying tundish operation. In addition to this, operation of flying tundish in this caster is complicated due to the requirement of strand shell joining of the previously cast shell with the newly formed shell while casting with the new tundish, as the starting speed with the new tundish is to be kept at very high in the range of 1.0 to 1.2 m/min.. The flying is also difficult due to very small size of the cast bloom with associated very low mould filling time of approximately 14 seconds. In normal flying operation, casting speed ramping starts from zero speed to normal speed over a period of time, which provides sufficient time for shell formation and joining.

In view of above, an innovative technique has been conceptualized for flying tundish operation. The flying tundish operation has reduced the average cast end to cast start time. This has resulted into increase in caster productivity i.e. increase the number of heats/day or number of heats/month. The practice has been implemented in full potential.

The objective could be realized through:

1. Development of capability of flying tundish operation for sequence casting in highly automated small size bloom caster having no manual stopper rod control

2. Development of a automated actuation of stopper rod for a new curve for mould filling to avoid metal over flow in mould

3. Development of a new mould oscillation mechanism during flying operation

4. Development of a mould level positioning method during flying physically as well as through PLC using HMI screen

5. Design of casting close & start mode through PLC during flying operation

6. Design of a secondary cooling methodology during flying operation

7. Design of a technique of welding of shells of old strand with new strand

8. Automatic tracking of bloom for synchronisation with bloom cutting and bloom marking

The salient features of the innovative techniques are as follows:

a) Welding/joining of newly formed shell of new strand with the shell of old strand after mould filling of metal with new tundish

- No external anchoring for shell joining is used and bonding/welding/joining is done by continuous solidification of new steel into old metal pool in mould cavity. To ensure smooth joining of old and new shell, excess mould powder is blown away with a high pressure air jet after the end of casting with old tundish.
- Start-up lubrication during flying: After the removal of mould powder (used for mould lubrication for casting) by high pressure air jet, pre-lubrication/start-up lubrication is provided with synthetic oil

b) Casting close & start mode in automation mode:

- Generation of HMI (Humane Machine Interface) screen and requisite change in PLC (programmable logic controller)
- Operation of strand close button to close stopper mechanism by MHI screen through PLC
- Selection of cast mode to remain as fly tundish mode
- Introduction of no FNC (Flying Nozzle Cutter) operation as strand stop button not operated. In normal casting, FNC operation is actuated through PLC once automatically once the casting is over. This safe guard is provided in normal operation to avoid metal over flow in mould

c) Mould level meniscus positioning during flying tundish operation:

- Positioning of meniscus (metal level in mould) at 250mm from bottom of the mould through length tracking in PLC
- Physical confirmation of exact positioning of meniscus at 250mm from bottom of the mould using a measuring rod

d) Mode of Oscillation during flying operation:

- Continuation of mould oscillation at minimum oscillation frequency ( 58 cycle/minute) during meniscus static position during flying in order to avoid shell sticking of newly formed shell with the mould
- Continuation of mould oscillation at minimum oscillation frequency ( 58 cpm) during meniscus movement from static position to mould level position
- Actuation of original mould oscillation frequency as per the speed-superheat chart ( provided with caster for normal operation) after the start of casting

e) Mould cooling during flying tundish operation:

- Mould cooling is continued as per the section wise normal cooling regime to avoid any mould distortion. Closing/reduction of mould cooling will lead to permanent deformation of mould copper tubes

f) Secondary spray cooling mode during flying tundish operation:

- Closing of secondary spray cooling water during flying tundish operation as pinch roll speed zero at meniscus static position. This is done to reduce excessive cooling of the old strand in the secondary cooling zones of the caster. Excessive cooling of strand will lead to strand stuck-up due to excessive pressure experienced by pinch/withdrawal roll. Continuation of secondary cooling water during flying will lead to blasting in the mould during metal filling with the new tundish as water will infiltrate from strand mould gap of the old strand from the loop 1 cooling zone of secondary cooling
- Secondary cooling of loop 1 will start from 30 seconds from the start of metal filling from the new tundish.
- The secondary cooling of subsequent zones will start by length tracking of strand through PLC as per the casting speeds.

g) Tracking of bloom during flying operation:

- Tracking of bloom to be defined for flying tundish mode in PLC to ensure proper operation of torch and billet marking machine

h) Actuation of stopper rod:

- New start up curve for stopper rod movement during mould filling to avoid mould overflow is as below:

Step Stopper Position Reference Ramping of Stopper Rod Time Reference
1 21/19mm 10mm/sec. 0 second
2 11mm 50mm/sec 3 second
3 9mm 10mm/sec 5 second
4 9mm 10mm/sec. 10 second
5 10.5mm 10mm/sec. 14 second
6 8.5mm 50mm/sec Start of casting

i) Tundish operation during flying:

- Old tundish is removed via tundish car after the completion of the casting
- Continuation of heating of new tundish till mould level (meniscus) positioning from 250mm from the bottom of the mould completed.
- Heating of new tundish stopped, new tundish placed in casting position, nozzle centering done
- Filling of metal in new tundish done from the new ladle and filling of metal in mould after a specific height of metal obtained
- All the steps above to be completed within in 5 minutes

To achieve Tundish changeover activity in shortest possible time several changes were done in automation system of Bloom caster. All these changes are enlisted below-

1. First change was made to inform the automation system whether we are removing tundish car 1 or 2 from casting position to have normal casting closure or flying tundish operation. For this Flying tundish selection soft button was introduced in HMI terminal.

2. If Flying tundish mode is not selected then after operating casting stop button in caster pulpit stopper will close & withdrawal , oscillator, secondary cooling system, heat tracking, material tracking & other equipments will follow the algorithm defined as per the tailout mode.

3. If Flying tundish mode is selected in HMI then after operating casting stop button in caster pulpit stopper will close as usual but withdrawal, oscillator, secondary cooling system ,heat tracking ,material tracking (for marking machine) will follow the newly defined algorithm as per Flying tundish mode.
• Withdrawal will stop automatically when metal in mould comes down at pre defined threshold level as per the feedback of mould level sensor in casting mode. Flexibility was given to caster to select preparation mode to adjust the position of steel bath miniscus in mould as per requirement. Withdrawal will wait for fresh start as per mould level threshold trigger.
• Oscillator will keep on oscillating at minimum frequency of 58 opm & defined stroke for that particular section while strand is in stopped condition . It will continue to oscillate till fresh Tundish car is taken in cast position & cast auto mode is selected by operator. After cast mode activation oscillator will stop & wait for fresh start as per mould level threshold trigger.
• Secondary cooling system air & water butterfly valve will close when metal in mould comes down below threshold level & it will open when fresh start is triggered with threshold mould level
• Heat tracking software will track the heat after flying tundish changeover as sequence heat & accordingly heat data is sent to material tracking software.
• Material tracking software will receive new heat data & as per physical position of bloom in discharge area material ID will be generated & sent to marking machine PLC for marking on bloom face.
• Material tracking software in pawl collecting area is tuned to continue material tracking after flying tundish operation.

ADVANTAGES:

Technical/Technological: The innovative technique of flying tundish operation in highly automated bloom caster having small sections without manual stopper rod operation has been implemented in bloom caster.

Economic: The operation of flying tundish led to improvement in productivity of the caster in terms casting of number of heats/day or number of heats/month (tonnage) by reducing the cast end to cast start time and increasing the total casting hours.

Impact in industry: The innovative technique of flying tundish operation in highly automated small section size caster without manual stopper rod operation has the capability to improve the boom caster productivity.

The average bloom caster production has gone up from ~45,000 to ~ 54000 ton/month (Figure-1) with the introduction of flying tundish operation. The number of heats/day has been increased from 11-12.5 heats to 14.5-15 heats (Figure-2). The average casting hours/month has been increased from 450-520 hours/month to 570-620 hours/month (Figure-3) with significant reduction of cast end to cast start time from 2.6-3.2 hours to 1.6-1.75 hours

Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the embodiments herein with modifications. However, all such modifications are deemed to be within the scope of the claims.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the embodiments described herein and all the statements of the scope of the embodiments which as a matter of language might be said to fall there between.

WE CLAIM:

1. A method of performing flying tundish operation in a bloom caster producing continuously cast blooms, said method comprising the following steps:

selecting flying tundish mode in a HMI;
operating strand close button thereby closing stopper mechanism;
selecting cast mode as fly tundish mode;
readying a second tundish for casting position on a tundish heating position;
carrying said second tundish to a casting position on a tundish car;
stopping ladle filling old tundish;
removing old tundish from casting position;
placing said second tundish in casting position;
opening ladle to fill a new tundish;
detecting minimum specified level of molten metal in said mould through length tracking sensor connected to PLC and confirming physically using a measuring rod;
opening stopper mechanism of said second tundish and pouring liquid metal slowly in a mold through a sub entry nozzle thereby joining to top of old strand;
restarting caster and gradually increasing casting speed thereby withdrawing joined new strand with old strand;
performing a secondary cooling using an air and water butterfly valve; and
directing said strand forming blooms to cutting torches;

wherein movement of said stopper mechanism controlled through a PLC having a pre installed startup curve providing appropriate ramping of stopper mechanism thereby avoiding mould overflow.

2. Method as claimed in claim 1 further comprising a step wherein said blooms are automatically tracked for synchronization with bloom cutting and bloom marking.

3. Method as claimed in claim 1 wherein said joining between old and new strand is achieved by gradual continuous solidification of said molten metal.

4. Method as claimed in claimed in claim 3 wherein excess mould powder is blown away with a high pressure air jet after end of casting with old tundish.

5. Method as claimed in claim 1 further comprising a step of providing start up lubrication with synthetic oil thereby preventing sticking of molten metal.

6. Method as claimed in claim 1 wherein a minimum oscillation frequency of approximately 58 cycles per minute maintained during meniscus movement from static position to mould level position.

7. Method as claimed in claim 6 further comprising the step of actuating original mould oscillation frequency after start of casting.

8. Method as claimed in claim 1 wherein said secondary cooling is substantially a spray cooling with water and air.

9. Method as claimed in claim 1 wherein during said flying tundish operation said secondary cooling is closed thereby preventing excessive cooling of old strand.

10. Method as claimed in claim 1 wherein said secondary cooling is controlled through said PLC by length tracking of strands through sensor means.

11. A method of performing flying tundish operation in a bloom caster producing continuously cast blooms as herein described and illustrated with respect to accompanying drawings.

ABSTRACT

This invention relates generally to a method of continuous casting and more particularly to a method of performing flying tundish operation in a bloom caster having no provision of manual control stopper. Method comprising the following steps of selecting flying tundish mode in a HMI; operating strand close button thereby closing stopper mechanism; selecting cast mode as fly tundish mode; readying a second tundish for casting position on a tundish heating position; carrying said second tundish to a casting position on a tundish car etc. The operation of flying tundish led to improvement in productivity of the caster in terms casting of number of heats/day or number of heats/month (tonnage) by reducing the cast end to cast start time and increasing the total casting hours.