**1. Introduction**

Hall-Petch relationship was found in 1951 [1] and 1953 [2]. Since this finding, the grain-size refinement has become one of the key development topics to improve properties of metals and alloys. High-pressure torsion (HPT) is one of the early technologies to refine microstructure for aluminum or copper [3]. A disk of these material is confined in a cylinder and subjected to torsion by rotating punch under high vertical pressure. As the strains increased during torsions in a disk, the grain size of these materials becomes smaller; then rounding by four times increases its hardness together with homogeneity of grain size [4]. However, the method is only applicable for relatively soft metallic alloys. It is difficult to apply to hard metals such as carbon steels or stainless steels. In addition, since the strains during a torsion is different in the radial direction of disk, the grain size remains larger than that in outer diameter; the rotation is not enough. Hence, this method might be suitable to create hard surface around the edge of products like a miller disk in the office copy machine; its application to bulk work materials is limited by cost

and production process. The continuous production process, like rolling, stamping, or cutting by lathe in the production area, need proper and stable bulk materials. For the part productions of medical, IT, or automobile industries, stable shapes of bulk material are important to produce precise parts with high accuracy and lower cost. Especially, minimally invasive medical treatment needs smaller devices and higher strength. Thus, wire and coil shapes are preferable for these parts industries. Often, the improvements of material need changing the chemical composition, but it usually needs approval from medical regulations in each country. The approval waist a lot of investments and time. Thus, the technology of improvement without changing the chemical composition was evitable. Fine-grained stainless steel (FGSS) is highlighted as the most reliable work materials to fabricate the miniature automotive parts as well as the fine medical components.

In the present chapter, various manufacturing methods are employed to investigate each workability of FGSS materials; e.g., mechanical and lase machining, metal forming, diffusion bonding, and surface treatment after explanation on their mechanical properties and microstructure. The superiority of FGSS to this integrated manufacturing is a key to utilize FGSS to fabricate the miniature mechanical parts and medical fine components.
