SK Material/Yellow Paper Steel called as Ki-gami

SK material, which contains slightly more impurities such as sulphur and phosphorous, is often used for comparatively low-priced knives. The higher the impurity content, the easier it is to ‘harden’ the knife in the manufacturing process, and the less skill is required of the forge smith. As a result, knives can be manufactured cheaply, but impurities can lead to embrittlement.

Yellow paper steel called as Ki-gami is a steel material based on this SK material, to which iron sand of approximately 50% purity is added. Although it contains fewer impurities than SK material, it still has impurities and is therefore often used for household knives. The only one currently manufactured is Ki-gami No. 2.

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