Nitrogen pick up is maximum during tapping. After end of blow that is in bath sample, it varies in the range 20-30 ppm while after tapping it may increase upto 70-80 ppm. How can we control it? As some Value added has nitrogen limitation.
Hi :) ,
I don’t know if you already got an answer but there are some things that can be done to limit N-pick up during tapping.
1) Delay the deoxidation during tapping (especially with Al-killed steels). By doing this you minimize the time that the steel is in a state of low oxygen potential state during tap. The tap stream will draw the surrounding air (79 vol% N2) into the melt. If the steel is killed, the risk of dissolution of N is relatively higher. Although, the timing of adding the killing agent has to be weighed against the addition of alloying material during the tap; a too late addition of the alloying material increases the risk of poor dissolution into the steel, which can negatively affect the material yield.
2) Semi-kill the steel with FeSi. By using FeSi as killing agent the dissolved oxygen will not go very low (Maybe to around 60-70 ppm O) but the risk of getting a carbon boil due to the reaction between carbon, from the alloying material, and dissolved oxygen is also minimized. The downside is that the created top slag will be quite rich in SiO2 which can limit its sulfur refining capabilities (can be solved by using CaF2 but it is a very expensive and non-environmental friendly material) and the steel is still required to be fully killed later on.
3) Using inert gas purging from the bottom. By injecting Ar gas from the bottom of the ladle the Ar-gas may push away the surrounding air and does limit the amoun if air being drawn down by the tapping stream into the melt thus limiting the contact between air bubbles and the steel.
4) Nitrogen control of the alloying material. Usually there are a lot of nitrogen in the carbon, Ferrochromium and manganese alloying materials. By using purer sources, you will minimize the N-increase from the dissolution of those materials. This can be a quite expensive alternative.
I hope this was of use for you! :)
Good luck!
Best regards
Yousef
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