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Steel Production

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The quality of a steel is also impacted by the way it’s produced. There are basically three production methods of steel:

 

  • “Conventional” metallurgy - continuous casting and ingot casting

  • Powder metallurgy

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Regardless of the method, the production process at the beginning is identical and resembles the cooking of a soup:

 

  1. Scrap metal is poured into a container and molten (Boil water)

  2. Alloys are added to the steel (Add ingredients)

  3. Everything is mixed well (Stir)

  4. Samples are taken from the mix for a chemical analysis (Taste)

  5. Depending on the result, further elements are added to reach the desired chemical composition of the material (Season)

  6. Another mixing (Stir)

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The production starts to differ from here:

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Continuous casting

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  • The liquid steel is poured down a chute and cast into a strip of steel

  • The strips are hot/cold rolled into the desired width and length

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Continuous casting is the most common and inexpensive way to produce steel.

Ingot casting

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  • Liquid steel is poured into a mold to an obelisk like shape (ingot)

  • The ingot is pounded (for knife steel) into strips

  • The strips are hot/cold rolled into the desired shape

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In ingot casting, the impurities usually segregate to the top and bottom of the ingot and are cut off. Furthermore, the pounding of the hot ingot into a flat shape (for knives) has a positive impact on the microstructure, with a reduced size and a more even distribution of carbides. Both steps make ingot cast steel slightly more expensive than continuous casting, which is why it's less common for strip steel.

Powder metallurgy

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  • The liquid melt is atomized into small particles (powder steel) with Nitrogen gas and filled into metal containers

  • The powder steel is put into a hot isostatic press, where the powder is compacted with heat and pressure into to a fully dense block

  • The blocks are compacted and hot / cold rolled into the desired shape

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Powder metallurgy produces the highest quality steel, with very fine and very evenly distributed carbides. But production costs are significantly higher than in both continuous and ingot casting.

Which is now the best way to produce knife steel?

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It very much depends on the desired properties of a steel. With low alloy steel / low carbon content (<0.6%), you won’t see much benefit from the more expensive production methods like ingot casting or powder metal.

The more highly alloyed steel becomes and the higher the carbon content is, the larger the carbide size becomes. With middle-alloyed steels (carbon content between 0.6% and 0.8%), ingot casting can help to improve the microstructure. 

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Once the carbon content rises (above 0.8%, e.g. VG10 – see picture), the size of the carbides increases significantly. Moreover, it’s impossible to avoid large primary carbides, resulting in a coarse microstructure with large, unevenly spread carbides (in red circles in pic. below).

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So for the hardest knife with high carbon content, there is little choice to using powder steel like SG2 or ZDP189, which even at the hardness of HRC 66 will have reasonably small, evenly dispersed carbides, despite of a very high carbon volume (see picture below).

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