Blue Paper Steel called as Ao-gami

Blue paper steel called as Ao-gami is Shiro-gami to which Chromium and Tungsten have been added.There are three types of Ao-gami: No. 1, No. 2 and super, with super having the highest carbon content.

Steel is classified as stainless steel if it contains more than a certain amount of chromium. The reason why chromium is added to Ao-gami is to increase the ease of hardening by quenching and to make it harder. Tungsten is added to increase wear resistance so that the surface is not worn away by friction or grinding. Ao-gami is more expensive than Ki-gami and Shiro-gami, not only because of the steel itself, but also the manufacturing process. Ao-gami is often regarded as the best quality, combining hardness and resistance to chipping, but its wear-resistant construction also makes it difficult to sharpen.

(Source: How to select, use, grow for knives and whetstones. Shibatashoten Co., Ltd.)