Yoshida Hamono is located in Saga Prefecture, a very rural area on the island of Kyushu. Like many rural blacksmiths, they specialize in making agricultural tools such as sickles and hoes, but they are nationally renowned for their incredible kitchen knives. The first generation of the Yoshida family was, like many others, blacksmiths. After World War II, they decided to open a small workshop to produce knives and other steel tools, and in 1971, they expanded to large-scale production to meet growing demand from all over Japan. They invested early on in modern machinery that made large-scale production possible, which is one of the reasons they are able to coat their own steel, rather than purchasing it pre-rolled. Because Saga is not a major knife production center and there aren't many other craftsmen, they built a facility capable of handling the entire knife-making process from start to finish, whereas most knife makers outsource their steel rolling and sharpening.
When HAP 40 first became available, Osamu Yoshida quickly purchased the steel and tried it out. He was amazed at how incredibly sharp the steel could become and how long it would hold its edge, so he began making kitchen knives using this super steel. Although it is a difficult steel to handle, he feels a greater sense of accomplishment when forging with it than with other steels.
In addition to laminating their own steel, they also sub-zero heat treat their knives to purify the steel, allowing the knife to perform and sharpen better. This is done after tempering, by treating the knives with liquid nitrogen to remove excess retained impurities that haven't been converted to martensite. This prevents the knife from bending over time by "stabilizing" its structure. While this is scientific mumbo jumbo to most, it's a big deal in the knifemaking world.

The secret of this knife lies in its hard core made of a somewhat exotic Hitachi HAP-40 steel, which falls into the category of modern and technologically advanced steels. HAP-40 is thin enough to sharpen very well, and knives made from it retain their edge 3 to 5 times longer than traditional knives. An interesting fact is that, since it is a powdered steel, it has a very low chromium content (about 4%) and can react like a high-carbon steel under specific circumstances.
This steel is heat-treated to an incredible 68 HRC, but can still be sharpened relatively easily on whetstones. It is extremely durable and therefore less likely to chip along the edge than the steel used in traditional knives.