I get this kind of question a lot and just ended up posting this in a different forum to help clarify. I thought some people here might enjoy this quick rundown of the steels many often talk about here:
Here is how the hitachi paper steels work out: JIS SK steel is the cheapest and Aogami Super is the most expensive (that would be blue super)... JIS SK Steel (SK4, SK5, etc.) is a simple carbon steel with moderate levels of sulfur and phosphorus in it. Yellow steel is a more pure version of the SK Steel. Yellow 3 has the least carbon, 2 has more carbon, and so on. White steel is a more pure (less sulfur and phosphorus) version of yellow steel. White #3 has the least carbon, white #2 has a bit more, and white #1 has the most carbon. Blue steel is white steel with added chromium and tungsten. Blue #2 has the same amount of carbon as white #2, and blue #1 the same as white #1. Blue super has a bit more carbon, chromium, and tungsten than blue #1.
When comparing the steels normally found in kitchen knives, this is how it breaks down... within white steels, white #1 is the most brittle but has the best edge retention and can hold the keenest edge, while white #3 has the greatest toughness and resists chipping. Blue steels will not take as good of an edge as white steels across the board. However, blue steel has greater edge retention. Blue #2 has the greatest toughness, while blue super has the best edge retention at the expense of often being brittle and not taking quite as keen of an edge (larger carbides due to the alloying elements).
Take this for what it is... a general guideline. Mileage may vary depending on the maker/heat treatment.
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