**2. Texture formation by intense rolling**

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 the rolled AISI304 sheet.

#### **2.1 Rolling procedure**

*Electron Crystallography*

shearing process in metal forming.

Angle Misorientation) mapping.

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

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

*Three types of crystallographic evolution during metal forming and plasma nitriding. (a) Intense rolling,* 

*(b) embossing and piercing, and (c) low temperature plasma nitriding.*

**56**

**Figure 1.**

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 and piercing to be discussed in later.

#### **2.2 Microstructure evolution by intense rolling**

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

#### **2.3 Controlled crystallographic structure by intense rolling**

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

#### **Figure 2.**

*Intense rolling process with high reduction in thickness. (a) Illustration on the rolling process in multi-steps for reduction of thickness by 90% and (b) rolled AISI304 sheet.*

textures are classified into two zones. One is thin and long full-martensitic textures. The shorter and dotted textures are corresponding to the retained austenitic textures.

These textured crystallographic structures are further controlled by the low temperature plasma nitriding as well as the heat treatment to be discussed in later.

#### **Figure 3.**

*Variation of XRD diagrams from the original AISI304 sheet before rolling to AISI304 after intense rolling and furthermore to rolled AISI304 sheet after low temperature plasma nitriding.*

#### **Figure 4.**

*EBSD analysis on the cross-section of rolled AISI304 sheet. (a) IPF mapping, (b) KAM distribution, and (c) phase mapping.*

**59**

**Figure 5.**

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

inversely transformed together with reduction of grain sizes [4, 7].

1.5 μm became nearly the same as D = 7.5 μm as depicted in **Figure 5b** and **c**.

Fine piercing is an essential process in metal forming for accurate drilling of holes and for fine blanking. In the last decade, metastable austenitic stainless steel type AISI304 with fine grains has been developed by [18–20]; the effects of fine crystallographic structure on the elasto-plastic deformation have been closely studied in [21–23]. **Figure 5** shows three metastable austenitic stainless steel AISI304 sheets with the thickness of 100 μm, where the grain size was reduced by rolling process from the normal-grained sheet with the average grain size (D) of 7.5 μm. Two fine-grained AISI304 sheets were yielded to have D = 3.0 and 1.5 μm, respectively, by reverse transformation of the strain-induced martensitic phase. In this thermo-mechanical treatment, near-fully martensitic grains in **Figure 4c** are

Their IPF maps analyzed by EBSD were shown on the cross section in the sheet width direction. When D = 7.5 μm, most of grains have a preferred orientation to [111] direction, as shown in **Figure 5a**. The crystallographic structure for D = 3 and

These AISI304 sheets with different grain sizes were pierced by CNC stamper under the same conditions; e.g., the punch diameter was 100 μm, the die diameter, 110 μm, and the clearance, 5% of sheet thickness. **Figure 6** compares IPF mapping as well as phase mapping on the cross-section of punched hole among three AISI304 sheets with the different grain size by D = 1.5, 3.0, and 7.5 μm. As commonly seen in **Figure 6a**–**c**, most of grains along the side surface of hole are distorted and refined in size. The shearing of AISI304 sheet by piercing leaves the process-affected zones along the shearing plane. The phase mapping in **Figure 5d**–**f** reveals that these process-affected zones are just corresponding to the martensitic phase [24]. That is, the original austenitic matrix to AISI304 sheets is forced to transform to martensite by the shearing strain during the piercing process. This strain-induced martensitic transformation is a non-diffusive shear transformation; each grain in the affected-

*IPF map and intensity map in the ND direction analyzed by EBSD. (a) Normal-grained AISI304 sheet with F = 7.5 μm, (b) fine-grained AISI304 sheet with D = 3.0 μm, and (c) fine-grained sheet with D = 1.5 μm.*

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

**3. Crystallographic change by piercing**

**3.1 Microstructure evolution by piercing**

process zone massively makes transformation [25, 26].

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