Effect of Grain Misorientation and Martensitic Transformation on Surface Roughening Behavior in Thin Austenitic Stainless Steel Foils

Stainless steel thin foils have unlimited applications in the field of microforming industries, making them attractive for use in industrial society. The problem hindering their use is the different mechanical properties that exist between thick and ultra-thin metals with the same plastic deformation characteristics. In this study, we compared the martensitic phase transformation (MPT) effect in SUS 304 and SUS 316, which was clarified in samples of the same grain size (Dg). The correlation between the MPT, grain misorientation (GM), and surface roughening behavior of SUS 304 and 316 thin metal foils was investigated using a uniaxial tensile test, which was repeated five times, with constant strain increments of 1.5%. Phase transformations such as MPT and GM were investigated using scanning electron microscopy/electron backscattered diffraction (SEM-EBSD). The results show that surface roughening increased proportionally in both the SUS 304 and SUS 316 thin foils with a coarse grain size (Dg of 9.0 µm). Surface roughening increased to a greater extent in the coarse-grained SUS 304 and 316 thin metal foils compared with the fine-grained (Dg 1.5 µm) samples. The SEM-EBSD results show that the grain strength of the coarse-grained SUS 316 thin metal foil was less inhomogeneous than that of the coarse-grained SUS 304 thin metal foil. The surface roughness ratio of the coarse-grained SUS 304 was higher than that of the coarse-grained SUS 316. The inhomogeneous grain strength of the fine-grained SUS 304 was similar to that of the fine-grained SUS 316. The surface roughness ratio of the fine-grained SUS 304 was similar to that of the fine-grained SUS 316. The MPT demonstrated a huge effect on the surface roughening behavior of the SUS 304 and SUS 316 samples with different Dg.

Grain Size (Dg); Grain Misorientation (GM); Martensitic Phase Transformation (MPT)

    Austenitic stainless steel foils have a wide range of applications in many industries, such as the electric power, electronics, biomedicals, nuclear, and food. Because of the high demand for microparts, austenitic stainless steel foils have received a great deal of attention (Aziz and Yang, 2020). The uniaxial tensile test induces a martensitic phase transformation (MPT) in stainless steel. The martensite phase volume fraction (Mf) increases proportionally after plastic deformation is applied to austenitic stainless steel foils (Engel and Eckstein, 2002; Xue et al., 2010). The effects of MPT on stainless steel include an increase in strength and a decrease in toughness (Milad et al., 2008; Jha et al., 2008). In previous studies, the Mf occurred in a strip of stainless steel after plastic deformation (Xue et al., 2010; Qin and Xia, 2020; Suryadi et al., 2020). MPT nucleation originates from the shear band intersection in stainless steel after plastic deformation (Tomita and Iwamoto,1995). Until recently, investigations focusing on surface roughness behavior in thin metal foils with an FCC structure have been rare, highlighting the need for further study in this area (Fauzun et al., 2011; Zhang et al., 2017; Dewi et al., 2020). The most important factor affecting formability in thin or sheet metal is surface roughening and not voids (Cheng et al., 2017). Surface roughness depends on the grain size (Dg), and an increase in the Dg leads to a decrease in the ratio of thickness to Dg; thus, there is a need to investigate the surface roughness behavior of thin metal foils with a Dg of less than 10 µm (Yoshida, 2014). The deformation of different individual grains affects the surface roughening behavior of sheet metal (Ichiro et al., 2001; Citrawati et al., 2020). In a weak grain, deformation affects the surface roughening behavior of thin metal foils and sheet metal (Furushima et al., 2013). Without annealing, coarse grain (Dg = 9.0 µm) deformation causes a proportional increase in surface roughness during uniaxial tests, whereas in fine grain (Dg 3.0 and 1.5 µm) deformation, the surface roughness increase is not proportional. The MPT has been shown to affect the surface roughening behavior of coarse-grained SUS 304 (Aziz and Yang, 2020). The ductility of SUS 304 and SUS 316 could be enhanced via annealing treatment (Shuro et al., 2010).

Based on the findings above, the effect of MPT on the surface roughening behavior of SUS 304 and SUS 316 thin metal foils with different and lower grain sizes still needs to be clarified. Until recently, a limited amount of research has investigated the effect of MPT and grain misorientation (GM) on surface roughening behavior and its associated mechanism.

The objectives of this research were to investigate the effects of MPT and GM on the surface roughening behavior of SUS 304 and SUS 316 thin foils with different grain sizes (Dg) of 1.5 µm and 9.0 µm. To achieve these objectives, thin metal foils were subjected to a uniaxial tensile test. The uniaxial tensile test was carried out five times, and the surface roughness behavior was measured at every step. After performing the uniaxial tensile test five times, SEM-EBSD was used to analyze the phase mapping and the GM behavior. 

       This research investigated the effect of the MPT and GM on the surface roughening behavior of SUS 304 and SUS 316 with the same grain size (Dg) after annealing treatment. The following conclusions can be drawn from the five-stage tensile tests and the SEM-EBSD investigation:

The effect of the MPT on the Ra was greater than that of the GM in the coarse-grained samples. However, the increase in the Ra was greater for the coarse-grained SUS 304 than it was for the fine-grained SUS 304 because the slip band intersection was lower in the coarse-grained sample than it was in the fine-grained specimen, resulting in a lower MPT in the coarse-grained material.

The inhomogeneous grain strength of the coarse-grained SUS 304 was higher than that of the whole material. The Ra of the coarse-grained SUS 304 was higher than that of both the coarse- and fine-grained SUS 316 thin foil.

The grain strength of the coarse-grained SUS 316 was more inhomogeneous than that of the fine-grained SUS 316, resulting in a greater increase in the Ra in the coarse-grained sample. The inhomogeneity of the higher grain strength of the coarse-grained SUS 316 thin foil was affected by its lower GM compared with the fine-grained SUS 316.

The fine-grained SUS 304 and SUS 316 had similar Ra behaviors because of their similar grain strength homogeneity.

    The authors would like to express their sincere gratitude and thanks to Komatsuseiki Kosakusho Co. Ltd., especially Tomoaki Yoshino San and Yohei Suzuki San, for providing the samples. The authors are also grateful to Assistant Professor Oshima; Sota, Tokyo Metropolitan University, for lending the Shimadzu Tensile Machine to them.

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