**1. Introduction**

Most of metals and metallic alloys have crystalline structure, intrinsic to each material property. This crystalline structure is classified by several items; e.g., single and poly-crystals, grain size, grain boundary characteristics, crystallographic orientation, and so forth [1]. These items are controllable by mechanical and chemical interaction with internally and externally straining [2]. For examples, a single crystal changes itself to polycrystalline state by introduction of dislocations with sufficiently high density [3]. The original grains are much refined by intense rolling [4] and by high shear straining [5]. The initial grain boundaries are also tunable by shuffling process through their interaction with dislocations [6]. That is, the crystalline structure is tailored by metal forming and materials processing to have preferable grain size, crystallographic orientations, and grain boundaries [7–9].

Let us consider how to control the crystallographic structure of metals and metallic alloys by the technology of plasticity in metal forming and surface treatment. **Figure 1** depicts three case studies on the crystallographic structure change by rolling, shearing, and nitrogen supersaturation. Even when the initial grains are equiaxial, some of them are forced to partially align along the rolling direction. The skew distortion by intense rolling drives to shear the grains with spin-rotation as shown in **Figure 1a**. In case of embossing and piercing the sheet metals, the grains are distorted to plastically flow along the shearing plane with grain size refinement as depicted in **Figure 1b**. In the low temperature plasma nitriding, the plastic straining is induced by the nitrogen supersaturation into metallic lattices to form the slip-line system as shown in **Figure 1c**. Without externally applied stresses, the slip-line network is formed from the surface to the depth together with the nitrogen interstitial diffusion. In particular, the low temperature plasma nitriding process [10–15] works as a powerful means to demonstrate that austenitic and martensitic stainless steel substrates are hardened and modified to have two-phase structure with the average grain size of 0.1 μm. These previous studies proved that grain size as well as crystallographic structure should be significantly controlled by the materials processing other than the shearing process in metal forming.

In the present chapter, the crystallographic structure evolution of stainless steels during the rolling, the piercing, and the plasma nitriding at 623 K for 14.4 ks is first described to deduce the mechanism of microstructure evolution during metal forming and materials processing. Next, the uniaxial loading test of plasma nitrided work at 623 K is performed to investigate the possibility of further microstructure evolution during posterior metal forming. Through these experiments, the effect of the interstitial element concentration as well as the plastic straining on the crystallographic evolution is discussed to search for the materials science model to describe the interaction between the interstitial mobility and the plastic straining. In the following, EBSD (Electron Back Scattering Diffraction) is employed to make crystallographic analyses. This technique is based on the automatic analysis of the Kikuchi pattern by the excitation of the electron beam on the surface of the sample in SEM (Scanning Electron Microscope) [16]. Among several analytical tools in EBSD, the crystallographic orientation for each grain is described by IPF (Inverse Pole Figure) and the strain induced phase transformation is also analyzed by phase mapping. In addition, the equivalent plastic strain distribution is estimated by KAM (Kernel Angle Misorientation) mapping.

**Figure 1.**

*Three types of crystallographic evolution during metal forming and plasma nitriding. (a) Intense rolling, (b) embossing and piercing, and (c) low temperature plasma nitriding.*

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**Figure 2.**

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing*

Intense rolling with heat treatment has been utilized to fabricate the finegrained stainless steel plates and sheets [4, 7, 17]. In the following, AISI304 sheet was employed as a work to reduce its thickness from 10 mm down to 1 mm by intense rolling. EBSD analysis was used to describe the crystallographic change in

A typical rolling system was illustrated in **Figure 2a**. AISI304 sheet with the initial thickness of 10 mm was compressed and sheared between two work rolls in a single reduction. Since the reduction of thickness was 10% in this single rolling, nine steps were utilized to reduce the thickness down to 1 mm through this intense rolling. **Figure 2b** shows the rolled sheet by 90% reduction in thickness. This rolling is effective to reduce the average grain size of stainless steel works for embossing

The original AISI304 sheet was characterized by three high intensity peaks in XRD analysis. As depicted in **Figure 3**, three peaks were detected in correspondence to γ (111), γ (200), and γ (220) planes in the austenitic phase. This microstructure changes to nearly full-martensitic phase; as also depicted in **Figure 3**, three martensitic peaks were detected as α' (211), α' (200), and α' (110) besides for γ (220) and γ (111). This proves that original austenitic grains massively transform to martensitic ones in AISI304 sheets during the intense rolling. These martensitic grains can be inversely transformed back to austenite by heat treatment. This technique is

In addition to the strain-induced phase transformation in **Figure 3**, the intense rolling has much influence on the crystallographic structure in AISI304. **Figure 4** depicts the inverse pole figure mapping, the KAM distribution, and the phase mapping on the cross-section of rolled AISI304 sheet, analyzed by EBSD. As stated before, the average grain size is reduced down to 1 μm in most of AISI304, since the plastic strains are applied to these regions as shown in **Figure 4a** and **b**. Low plastic strained regions in **Figure 4b** corresponds to assembly of larger grains laterally aligned in the rolling direction. Through comparison between **Figure 4b** and **c**, these

*Intense rolling process with high reduction in thickness. (a) Illustration on the rolling process in multi-steps for* 

*DOI: http://dx.doi.org/10.5772/intechopen.91281*

the rolled AISI304 sheet.

and piercing to be discussed in later.

**2.2 Microstructure evolution by intense rolling**

useful to reduce the average grain sizes to be stated later.

*reduction of thickness by 90% and (b) rolled AISI304 sheet.*

**2.3 Controlled crystallographic structure by intense rolling**

**2.1 Rolling procedure**

**2. Texture formation by intense rolling**

*Micro-/Nano-Structuring in Stainless Steels by Metal Forming and Materials Processing DOI: http://dx.doi.org/10.5772/intechopen.91281*
