The powder metallurgy HSS steel HAP40 manufactured by Hitachi can be hardened to between 64 and 68 HRC, although Gihei limits the figure to 65-66 HRC.

The goal of the Gihei owner,
Atsushi Hosokawa has been to preserve the outstanding characteristics of Japanese knives, like high sharpness, hardness and ease of sharpening, but to work on some of the admitted disadvantages, such as the relatively brittle edge and susceptibility to rust so that a more "ideal" knife is developed - with all the positive characteristics retained and the disadvantages minimized.
Powder Steel - HAP40
Uber steel - Gihei Zuika - ZDP189 here
As far as we know, these knives were made for us in this format and the normally available HAP40 western style handle has no bolster and the only other Japanese handled models have a charred chestnut handle, with a resin ferrule and don't look as good (I.O.H.O.)
Gyuto 200mm - Gihei HAP40
Gihei HAP40 blade grind
Gyuto 170mm - Gihei HAP40
Santoku 165mm - Gihei HAP40
Nakiri 165mm - Gihei HAP40
HAP40 from Hitachi.

HAP40 is fine-grained enough to sharpen very well and rates 64 - 68 on the Rockwell Hardness Test (HRC) - even harder than the traditional carbon steels used for knives. Contrary to other HSS steels, it can be relatively easily sharpened on normal waterstones, if not quite as easily as traditional steels. Knives from this steel keep their sharpness 3 to 5 times longer than traditional knives. HAP40 is extremely tough, and so not as likely to chip along the edge as the steel used in traditional knives.

The knives are manufactured in three layers, HAP40 is used only in the "Hagane" or middle layer, that becomes the cutting edge. The use of Hitachi stainless steel grade SUS405 for the "Jigane", or the two outside laminates, solves the rust problem elegantly.

These hand-forged knives are provided with a magnolia d-shape handle with a water buffalo horn ferrule. Gihei considers this knife, with its traditional form, and high-tech materials and performance, to be close to the ideal he has been looking for.

Hosokawa san recommends Shapton-Stones to sharpen these knives. Any other Japanese water stone will do the job, but they work slower with this steel.  

See Shapton Stones here.
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Fine Japanese chef's knives to Australia since 2007