One thing to keep in mind is that this refers to the "potential" of steelmaking, not the actual practices used every day. There is very little steel that is produced to these levels. Air melted steel cannot achieve < 20 ppm N-- this requires a secondary treatment in a Vacuum Degasser, etc. It is impressive to consider that in a little over 30 years, detrimental elements like S, P, N, and O have been reduced by extraordinary amounts.
yes this reduction of residuals over 30 years is not very important, but a lilte reduction of residuals like H or N can improve the safety of steel parts.
@ KENVLACH
Yes but i believe the author would like to say reducing the amount of carbone in the very low carbon steel..like austenitic stainless steel
stanislasdz,
Thanks for posting the figure. Didn't mean to sound critical. I favor lowering S in SS for corrosion resistance, but a small bit may help in welding, and machinists love the crappy 303.
Cheers.
No, that data is not representative of typical recycled steel. The exact numbers depend on end-use of the product, but I would estimate that typical recycled steels have the following values:
P = 0.020-0.030
S = 0.025-0.035
N > 120 ppm
O > 20 ppm
As you can see by the Oxygen values in the chart, today's typical production (using only an electric arc furnace and a ladle metallurgy furnace, no vacuum degassing) still does not reach the values from 1970's. This is why I mentioned previously that the chart is attempting to demonstrate steelmaking improvements that are "possible", not necessarily commonplace.
Hey, answer a question for a ME that doesn't need to know but wonders how it is done. Watching the magnets load all kinds and shapes of scrap into the electric arc furnace from the yard of a recycle steel plant (I was there in association with the back end-ductwork, fans, baghouses, etc) I wondered how the alloying elements were controlled.
Can anybody give me a tutorial on that?
rmw
PS: end product at the steel mill in question was I Beams.
Of course the incoming scrap can have uncontrolled alloying elements (like Ni, Cr, Mo, Sn, etc.). Oxygen can be blown into the melted steel to oxidize the alloying elements, and they float to the top as slag. A small slug of the melt is collected and solidified, then has its chemical composition analyzed with a spectrometer. The melt can be adjusted with intentional alloying additions using high quality master alloys.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
rmw,I will try and reply to your question in a simple way.Sorry if you find it rather long.
> Arc furnace is a big time energy guzzler.
> Also it has no restrictions on the kind of inputs charged.
> However, the scrap density and quality is always monitored.
> Light scrap like machine shop generated turnings and borings are popular.
> Shredded automobile scrap (properly pressed and baled) is added,but the percentage addition is restricted as this is a major source of tramp elements.
> Heavy melting scrap
> Lastly melting shop and plant rejects and returns.
The scrap should permit the graphite electrodes to strike an arc and penetrate. Along with the charge limestone is added to form a slag. It is in this slag metal interface that the refining takes place. The first step is to create an oxidising slag. This is done by blowing oxygen into the molten bath and adding limestone .Elements like carbon and Phosphorous are removed. Then comes the reducing slag when coke and limestone,magnesite and calcium fluoride (fluospar) are added to the bath. this reduces the oxygen level, and removes the Sulphur .
Lastly after the reducing slag has been removed a refining slag is prepared . Any carbon pickup ,manganese and silicon additions are done now. If the chemistry calls for any further additions of Ni,Cr or Mo these are done. Finally aluminium is added to kill the bath.
One of the greatest advantages of arc furnace melting is the ability to refine and control,carbon, Sulpur and Phosphorous contents,win important elements from slag without any loss and produce a metal low in gases. This makes it largely popular over the induction furnaces where refining is not possible.
Yes you can still have tramp elements which have not got blown away during oxidation,but their presence is reduced..
Graphite electrode consumption and refractory costs are the two other factors in addition to the power costs which affect the viability.
You may refer to the link below for any further reading.
The point to also remember is that if a metal is redily soluble in Fe and not easily oxidized (Cu, Ni, Co) then you can't remove it.
This is a very interesting process. In the oxidation pahse a lot of metal that you want (Fe, Cr, Mo). These oxides are trapped in the slag and when the slag chemistry is shifted to more reducing condition the metals go back into the melt.
This gets a lot more complex when you are dealing with higly alloyed products like tool steels or stainless grades.
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