**2. Mechanical properties and microstructure of FGSS**

There are only two ways to produce FGSS in the production place in 2019. One method is oval-square rolling, which has been developing by Torizuka and Muramatsu since 1998 as a suitable means to produce the bar and wire [5]. **Figure 1** shows the change of work cross section during oval-square rolling process. Steel or stainless steel coil with ϕ6.0 mm is used as a starting work material. Grains are decreased by heating and severe plastic deformation with use of oval and square shaping rolls. The high strains at the center of coil drive dynamic recrystallization from 20 to less than 3 μm across the all cross section. This process continues till coil diameter reaches ϕ3.0 mm.

**Figure 2** compares the IPF (inverse pole figure) of AISI316L by Electron Back Scattered Diffraction Pattern (EBSD) before and after this oval-square rolling. There are wide range of grain size before the process and these are austenite (**Figure 2(A)**), then the average grain size becomes 1.3 μm (**Figure 2(B)**).

The tensile strength of initial work ranges from 520 to 580 MPa. This is increased up to 980–1150 MPa by the grain reduction by 1/10 just after the Hole-Petch relationship. To be noticed, this strengthening also accompanies with work hardening and processing-induced transformation to martensite. This difference in strengthening mechanism between work hardening and grain-size reduction must be distinguished carefully.

Inverse phase transformation method is also cost effective method to produce the thin FGSS sheets [6, 7]. The grain-size reduction is driven by the inverse phase transformation from martensite by work hardening to austenite phase. Austenitic stainless steels of type AISI304 were employed as a work. The work materials were manufactured in a heat batch processing. **Table 1** shows the chemical composition of the material.

**5**

**Figure 3.**

*Integrated Manufacturing of Fine-Grained Stainless Steels for Industries and Medicals*

The normal grain stainless steel was formed to reduce the plate thickness from 3 to 0.2 mm in rolling with heat treatment. **Figure 3(A)** shows the microstructure, which is observed by EBSD. *θ* is the crystal orientation angle; and *α*′ and *γ* are the crystal phase. The marten-sites structures are observed and the grain sizes are

*Picture of grain condition by EBSD. (A) Left: normal grain stainless steel. The average grain size is 9.8 μm and* 

*Picture of grain condition by EBSD. (A) Left: before oval-square rolling. The average grain size is 14.7 μm* 

Carbon (C) 0.06 Silicone (Si) 0.4 Manganese (Mn) 1.09 Phosphorus (P) 0.03 Sulfur (S) 0.004 Nickel (Ni) 8.03 Chromium (Cr) 18.02

**Composition %**

ranged from 2 to 20 μm, where the average grain size is 9.10 μm.

*(B) right: FGSS by reverse phase transformation. The average grain size is 1.52 μm.*

*DOI: http://dx.doi.org/10.5772/intechopen.89754*

*and (B) right: after the process. It becomes 1.3 μm.*

*Chemical composition of the specimen.*

**Figure 2.**

**Table 1.**

**Figure 1.** *Illustration on the wire fabrication by the oval-square rolling process.*

*Integrated Manufacturing of Fine-Grained Stainless Steels for Industries and Medicals DOI: http://dx.doi.org/10.5772/intechopen.89754*

#### **Figure 2.**

*Engineering Steels and High Entropy-Alloys*

parts and medical fine components.

automotive parts as well as the fine medical components.

**2. Mechanical properties and microstructure of FGSS**

*Illustration on the wire fabrication by the oval-square rolling process.*

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

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

There are only two ways to produce FGSS in the production place in 2019. One method is oval-square rolling, which has been developing by Torizuka and Muramatsu since 1998 as a suitable means to produce the bar and wire [5]. **Figure 1** shows the change of work cross section during oval-square rolling process. Steel or stainless steel coil with ϕ6.0 mm is used as a starting work material. Grains are decreased by heating and severe plastic deformation with use of oval and square shaping rolls. The high strains at the center of coil drive dynamic recrystallization from 20 to less than 3 μm across the all cross section. This process continues till coil diameter reaches ϕ3.0 mm. **Figure 2** compares the IPF (inverse pole figure) of AISI316L by Electron Back Scattered Diffraction Pattern (EBSD) before and after this oval-square rolling. There are wide range of grain size before the process and these are austenite (**Figure 2(A)**), then the average grain size becomes 1.3 μm (**Figure 2(B)**).

The tensile strength of initial work ranges from 520 to 580 MPa. This is increased up to 980–1150 MPa by the grain reduction by 1/10 just after the Hole-Petch relationship. To be noticed, this strengthening also accompanies with work hardening and processing-induced transformation to martensite. This difference in strengthening mechanism between work hardening and grain-size reduction must be distinguished carefully. Inverse phase transformation method is also cost effective method to produce the thin FGSS sheets [6, 7]. The grain-size reduction is driven by the inverse phase transformation from martensite by work hardening to austenite phase. Austenitic stainless steels of type AISI304 were employed as a work. The work materials were manufactured in a heat batch processing. **Table 1** shows the chemical composition

**4**

**Figure 1.**

of the material.

*Picture of grain condition by EBSD. (A) Left: before oval-square rolling. The average grain size is 14.7 μm and (B) right: after the process. It becomes 1.3 μm.*


#### **Table 1.**

*Chemical composition of the specimen.*

The normal grain stainless steel was formed to reduce the plate thickness from 3 to 0.2 mm in rolling with heat treatment. **Figure 3(A)** shows the microstructure, which is observed by EBSD. *θ* is the crystal orientation angle; and *α*′ and *γ* are the crystal phase. The marten-sites structures are observed and the grain sizes are ranged from 2 to 20 μm, where the average grain size is 9.10 μm.

#### **Figure 3.**

*Picture of grain condition by EBSD. (A) Left: normal grain stainless steel. The average grain size is 9.8 μm and (B) right: FGSS by reverse phase transformation. The average grain size is 1.52 μm.*


**Table 2.**

*Mechanical properties.*

The fine-grained stainless steel was formed with repeating plastic deformation and reverse phase transformation. **Figure 3(B)** shows the microstructure, where the average grain size is 1.52 μm.

Although the grain size was different among specimens, their ultimate tensile strength became around 900 MPa, irrespective of the grain size.

**Table 2** shows the mechanical properties of the normal stainless steel and FGSS. Here, the mechanical properties are similar to each other. The hardness and tensile strength is controlled by work handing for normal grain, and by decreasing the grain size by reverse phase transportation for FGSS. These different hardening methods for stainless steel should be distinguished in terms of microprocessing.

As a summary, FGSS has refined microstructure with the average grain size of 1.5 μm and higher ultimate strength than 900 MPa. Its hardening process is governed by the reverse phase transformation in different from the work hardening in the normally grained AISI304.
