There’s literally everything in scrap metal, aren’t there compounds that will worsen the quality of the steel produced?
In: Chemistry
The scrap metal certainly contains purer steel than the iron ore that virgin steel is made from. Refinement is called refinement for a reason and scrap metal requires far less of it.
The steel has already been processed once so the formula is, you might say, already built in. But because scrap comes from a number of sources, meaning it might be a mix of types of steel, the formula can be tweeked by introducing additives like nickle, sulfur or carbon just as in the manufacture of the original product from scratch. Impurities like oils and dirt are either burned off or rise to the surface in the form of ‘slag’ that is removed from the crucible before pouring.
While metling it down you can burn off some of the impurities by adding other bits that will react with those and leave “good” metal behind. You can also keep adding various other bits to make the type of steel you really need.
Making little test-pieces from the melted steel is quite easy – you can then confirm composition, strengths, hardness, toughness and any other properties you deem important.
If the steel you have there doesn’t match your requirements then you can add more iron, nickel, copper, silicon or manganese to bring the composition closer to your target.
Think of it as making lemonade – if the mix is too sweet, you add lemon juice. If its too sour, you add sugar and if its too potent you add water till it dilutes the other parts.
They actually use a recipe like you would when baking a cake. Instead of say so much flour and so much butter, they will use so much of scrap #1 and so much of scrap #2, and so on. A yard may have 10 or so different scraps from different origins, each of which the plant knows the rough chemistry for based on what was in it’s previous life. There is actually a very lucrative business sorting junk yard scrap for steel plants.
After the initial scrap is melted they take molten samples and test the chemistry. They can then add small amounts of specific raw materials to trim the chemistry.
You can ballpark the purity of metals based in their melting point, and if you know where the scrap was taken from (like which brand of car, or which brand of fridge) you can sometimes use the knowledge of how those companies make their steel znd what they mix into it
I work in the steel industry, and have been for nearly 20 years. This includes the melting and casting operations.
We can guesstimate what impurities are in the various types of scrap by the source of the metal. Light-gauge metal gets shredded, and so do cars – that means that *shred* will usually contain a high percentage of copper (from electric motors and wiring). *Heavy melt* consists of thicker sections (rails, I-beams, railway wheels, plate sections from ships and barges and bridges) that are more likely to have higher silicon and manganese and *pig iron* or *cast iron* is going to have a lot more carbon and silicon (note: North American scrap is usually further categorised). During loading of the scrap buckets prior to charging into the furnace, a blend of scrap types is chosen depending on what grade of steel is being made, as well as taking into account scrap density, price (shred is ususally cheaper than heavy melt) and other operational factors. For example, if the meltshop knows low-copper grades are scheduled, the scrap yard will be asked to load a low-copper scrap blend.
During and after melting, oxygen is blown into the steel to burn out some impurities (aluminium, silicon, manganese, carbon, phosphorous, sulphur) so that when the furnace is ready to be empited (tapped), the liquid steel ends up with about 0.1% carbon, 0.1% silicon and 0.1% manganese. This can be confirmed by taking a sample of the steel and analysing it on a spectrometer – usually takes no more than 5 minutes to get a result. The spectrometer will also report on other elements like chromium, copper, nickel and molybdenum – these elements can make the steel too strong for cold working, or (in the case of copper) make it too weak at high temperatures to be rolled down to size. The problem is, these four elements can’t be removed with oxygen (which is why they’re known as *residuals*), so if there’s too much of them in the steel, then the steel has to be diverted to a grade which can handle them, or diluted or tipped out. Normally if the scrap has been sorted, picked and loaded correctly, then the level of residuals is low.
Edit: spelling and clarity, plus a video link that briefly shows and explains sampling and spectrometry
[https://www.youtube.com/watch?v=ou9vNsVhk8M](https://www.youtube.com/watch?v=ou9vNsVhk8M)
If I was ELI5, I would say the steel is like pancake batter, but you’re not sure who made it or what’s in it.
So you make a small pancake and taste it. From there you can add water, milk, eggs, salt, flour or if you want them to be super special, chocolate chips! Make another small one, taste it, and if it’s good, make the rest of the batter into pancakes!
Same with the steel. Test a little bit, then add whatever kinds of other steels or elements you need to make the steel perfect for what you need it for!
They melt scrap metal and other ores to make a steel mixture. Then they analyze that mixture and add compounds to produce the mix that they want. If the scrap was low in chromium, they add more. If it was high in carbon or nickel then dilute it with more iron or pull the excess out chemically.
Only when the mix is right to they make something out of it.
I worked as a trad blacksmith in uk for 30 years, one problem was that cheaper imported ‘mild’ steel would often not be “mild’ at all, but would harden when cooled, which caused big problems as for example, when the ‘hapenny snub’ end of a sroll would fracture and fly off as you worked the other end — as a result of adding one too many mercadees to the furness in mauritania or wherever it was made
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