This has been realized by having so-called functionally-gradated structure that the cutting edge is hard, and the hardness is gradually decreased to the core, which makes compressive residual stress generate at the cutting edge.
これは刃先は硬く、芯に向かうと硬さが徐々に下がるいわゆる傾斜機能構造を持つことで圧縮残留応力を刃先に発生させることで実現されている。