What is killed steel?

Fully killed is the term to describe de-oxidised steel.

After the steel is made, it is then poured into the continuous caster to make a long slab of steel. Think of a sausage maker – molten steel goes in at the top and rectangular slab comes out the bottom. This is called casting.

During casting, small carbon monoxide bubbles can form between the steel grains if the oxygen is not removed. If you’ve ever painted a door and seen bubbles in the paint once you apply it you’ll recognise the similarities. To stop these bubbles appearing you paint slower, but in steel you add certain elements to the steel as you define the metallurgy. For most steels to achieve this effect – de-oxidisation – you add Silicon or Aluminium or both.

killed steelThe impact is quite dramatic. A slab of steel is full of little steel grains. If you imagine a whole room full of golf balls you can also imagine the space between them. You don’t want the space, so one way of reducing it is to make the golf balls smaller. This is in effect what the Aluminium and Silicon do to the grains in a slab of steel.

If it isn’t killed then as the slab cools you can see little bubbles forming on the surface as the CO bubbles out.

The grains become smaller, reducing the spaces and it is then called fine grained steel – improving the microstructure and thus, its strength. This is of great use for Structural and Pressure Vessels, indeed anywhere where you require improved strength.

Generally if a steel has Silicon content of more than 0.10% then it is considered to be killed – and ASME requirements for pressure vessels generally require any steel with a carbon content of more than 0.24% to be killed

As as to why it is called fully killed? Well when you compare a fresh non-killed slab and a fully killed one – well one has bubbles coming out and the other is dead. Hence, killing.

So now you know this you can tell just from reading the title of EN10028-3 “Weldable Fine Grained Steels, Normalised” what some of the chemical composition is going to look like.

Hope this helps and have another amazing week.

Comments

  1. Hello.

    Very informative content. In clear way you explaining what is killed steel. I have a question regarding the issue. In the Internet can find materials which stated that carbon steel can be divided within two broad categories, namly Alluminium killed-steel, 0,03% Si.
    Meanwhile I spoke with some steel supplier and he said that the steel can have >0,03% Si content and it’s killed by aluminum. Is it possible to acknowledge based upon chemical analysis, that particular steel is killed by the Si or Al? Why rimming steel according to ISO 10025-2 is not permitted?

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