Blue Steel (Aogami, 青紙)
What it is
Blue steel, aogami, is white steel with chromium and tungsten added — Hitachi's higher-performance carbon steel, named for its blue paper wrapping. The alloying buys longer edge retention and greater toughness at a small cost in ultimate keenness and ease of sharpening, making blue the choice of cooks who want carbon-steel cutting with less frequent sharpening.
The science & materials
Adding chromium and tungsten (and, in Blue Super, vanadium and molybdenum) to the white-steel base creates hard, wear-resistant carbides distributed through the steel. These carbides resist abrasion, so a blue-steel edge lasts noticeably longer than white, and the alloying also improves toughness slightly (the edge is a touch more chip-resistant). The trade is that those same carbides make the steel marginally harder to sharpen and its ultimate apex very slightly less fine than pure white. Blue is still a reactive carbon steel — it patinas and rusts like white. Grades run Blue #2 (common, balanced), Blue #1 (higher carbon), and Aogami Super (Blue Super) — the top tier, with the highest carbon and most alloying, reaching ~64–65 HRC and offering the best edge retention of the Hitachi carbon family.
How it's used
Like white, blue steel is used as a laminated core in single- and double-bevel knives and sharpened on water stones, with full reactive-steel care. Aogami Super in particular is a favorite for high-performance gyuto where long edge life is wanted in a carbon steel.
Regional & cultural traditions
Blue steel sits alongside white as the other pillar of the Hitachi carbon tradition; the white-versus-blue choice is one of the defining debates of carbon-knife enthusiasm. Many revered Sakai and rural Japanese makers specialize in one or the other.
Cultural & historical context
Blue steel represents the 20th-century refinement of the traditional carbon steel — the same heritage as white, upgraded with modern alloying for working cooks who needed an edge to last a full service.
Reference notes
Cross-link to White Steel (Shirogami), The Hardness–Toughness Tradeoff, Reactive vs. Stainless, and Gyuto (a common blue-steel knife).
When to use
Choose blue steel over white when you want carbon-steel cut quality but longer edge retention and a bit more toughness — fewer trips to the stones. Choose Aogami Super specifically when you want the longest-lasting carbon edge available and don't mind the harder sharpening. Choose white instead if maximum keenness and easiest sharpening outweigh edge life.
What goes wrong
Still reactive (rusts if neglected), still chips under hard use, and slightly more stubborn on the stones than white — owners expecting white-steel sharpening ease are sometimes surprised. Aogami Super's high hardness rewards good technique and punishes abuse.