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Evolution of Ladle Treatment - Steel Making - Lecture Notes, Study notes of Metallurgy

Jiwaji UniversityMetallurgy

The major points which I found very informative are:Evolution of Ladle Treatment, Basics of Gas Stirring, Mixing Time, Gas Injection Rates, Choice of Stirring Energy, Ladle Metallurgy, Secondary Steelmaking, Injection Metallurgy, Synthetic Slag Practice

Typology: Study notes

2012/2013

Uploaded on 04/20/2013

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Lecture 21: Ev olutio n of ladle Treatm ent an d Requi rements
Content s:
Pream b l e
What modif ications a re requ ired?
Basics of gas stirring
Mixing time
Gas inj ection rat es
Choice of stirring energy
Key words: Ladle met allurgy , Secondar y steelmak ing, in jectio n metallurgy, synthetic slag practice
Pre amb le
In s teelma king, ladles are employed to transfer molten steel from BOF/EAF to ingot casting or
continu ous casting. It has been realized that ladles can be used ve ry effe ctively as a reactor which can
perform any of th e followin g functions:
• To desu lphuriz e molten steel tapped from BOF/EAF
• To homo genize molten ste el to m inimiz e gradi ents i n concentr ation and tempera ture a nd to
attain desired te eming temperature .
• To deox idize molt en steel
• To improve cleanl iness o f stee l by re moving inc lusions
• To e nginee r the i nclusions so as to alleviate t heir harmf ul effe cts on mechan ical pr operti es of
steel
• To add alloying e lements
• To remove dissolved gases
The effectiveness of each of th e function re quires modifying t he ladle i n terms of molten steel flow, and
extra heating facility etc.
What modificati ons ar e required?
Ladle i s a cy lindric al refractory lined vessel and aspect ratio of the bath v aries between 0. 8 and 0 .9. This
means t hat the ba th is deep .
Bath ag itation wo uld be req uired to carry-out the functi ons effect ively. At high tempe rature, bath can
be agit ated eithe r by an in ert gas or by induction . One h as to determi ne the amo unt of stirri ng gas and
locatio n of t he inje ction of g as in the lad le. The gas can either be injected through the nozzle o r porous
plug s. Loc ation o f the injection elements is a n important issue . Injectio n elements could be located
either axis-symmetric or asymmetric to the center of the ladle.
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Lecture 21: Evolution of ladle Treatment and Requirements Contents: Preamble What modifications are required? Basics of gas stirring Mixing time Gas injection rates Choice of stirring energy Key words: Ladle metallurgy, Secondary steelmaking, injection metallurgy, synthetic slag practice Preamble In steelmaking, ladles are employed to transfer molten steel from BOF/EAF to ingot casting or continuous casting. It has been realized that ladles can be used very effectively as a reactor which can perform any of the following functions:

  • To desulphurize molten steel tapped from BOF/EAF
  • To homogenize molten steel to minimize gradients in concentration and temperature and to attain desired teeming temperature.
  • To deoxidize molten steel
  • To improve cleanliness of steel by removing inclusions
  • To engineer the inclusions so as to alleviate their harmful effects on mechanical properties of steel
  • To add alloying elements
  • To remove dissolved gases The effectiveness of each of the function requires modifying the ladle in terms of molten steel flow, and extra heating facility etc. What modifications are required? Ladle is a cylindrical refractory lined vessel and aspect ratio of the bath varies between 0.8 and 0.9. This means that the bath is deep. Bath agitation would be required to carry-out the functions effectively. At high temperature, bath can be agitated either by an inert gas or by induction. One has to determine the amount of stirring gas and location of the injection of gas in the ladle. The gas can either be injected through the nozzle or porous plugs. Location of the injection elements is an important issue. Injection elements could be located either axis-symmetric or asymmetric to the center of the ladle.

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Enough free board height in the ladle must be available to accommodate the quantity of slag required for refining and to absorb inclusions. Additional heating may be required to keep the molten steel to the teeming temperature. This can be achieved either by tapping steel at slightly higher temperature or to provide addition heating arrangement in the ladle itself. In many situations it is required to inject the slag forming materials either for refining or for inclusion engineering. In this case suitable injection device must also be available. Above all, the most important would be the selection of refractory (see lecture 9 and 10) to meet the refining requirements. Refractory materials for injection elements and their fixing must also be considered. Basics of gas stirring Argon is usually bubbled into the molten steel covered with slag either through the top lance or through a porous plug fitted at the bottom. A plume of gas rises upwards when gas is injected through the bottom. The stirring homogenizes bath composition and temperature. With centrally placed nozzle at the bottom, stirring action is small near the bottom of the ladle. An asymmetrically placed bubble plume gives velocities near the bottom which are greater than for symmetrically placed nozzle. The liquid flow in the ladle occurs via bulk motion of the metal. Very roughly, characteristic velocity vc for gas bubbling is vc = � Qg H^2 V � 1 / 3 (1) Where Q is gas flow rate� m^3 s �, g is� m s^2 �, H is bath height (m) and V is bath volume (m (^3) ). For gas injection at 50 l/min in a ladle of bath diameter 3.5m and bath height 3.5m, equation 1 calculates 0.14 m/s as the gas bubbling velocity. At velocities greater than 0.3 m/s at the slag/metal interface slag droplets may be entrained by the metal flowing along the interface and into the melt. For gas stirring recirculation rate of molten steel is of interest. The mass flux in (tons/s) of entrained steel passing through the top section of the bubble plume can be calculated by the following semi- empirical equation: Ṁ = 13. 3 (H + 0. 8 ) �ln � 1 + H

  1. 48
  1. 5 × Q^0.^381 (2) Consider a ladle with bath height 3m and Q=650 l/min (1 atm and 273K). We can calculateṀ =
  2. 5 tons /s. Increasing the flow rate to 800 l/min increases Ṁ to 10.3 tons/s.

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Choice of stirring energy Correct stirring is of utmost importance. It should be known when to stir what, and how vigorously. Vigorous stirring would be required for slag/metal reaction such as desulphurization. Inclusions removal would require weak stirring. Bath homogenization would also require weak stirring. Vigorous mixing of metal and slag is achieved with gas stirring. Less disturbance at the slag/metal interface can be obtained by induction stirring. Slag carry-over from BOF/EAF must be avoided. Vigorous stirring in the neighborhood of slag/metal phase boundary activates interfacial mass transfer, leading to reduction of slag by deoxidizing elements, reversion of phosphorus, and oxygen and nitrogen pick up from the atmosphere.

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