Evolution of crystal orientation and inner surface roughness of SUS316L stainless-steel tubes in hollow sinking

The inner surface roughness of drawn stainless steel microtubes strongly affects their performance in applications such as medical devices. In hollow sinking without an inner tool, the wall-thickness change controlled by the drawing speed ratio significantly influences inner-surface roughening; however, the link between grain-scale orientation evolution and roughness remains unclear. In this study, the inner-surface roughness and crystal orientation of SUS316L stainless steel tubes (outer diameter 2.0 mm, wall thickness 0.1 mm) were examined to clarify the relationship between intragranular orientation development and surface roughening during hollow sinking. Tubes were drawn at speed ratios that produced both wall-thickening and wall-thinning conditions, and the grain orientation evolution was quantified using GOS and Taylor factors. The results show that intragranular misorientation increases during drawing regardless of whether the wall thickens or thins, but this increase is much more pronounced under wall-thickening conditions. Under wall-thickening, the average GOS level required to trigger a rapid increase in inner-surface roughness is reached at a smaller equivalent strain, at which stage the intragranular spread of the Taylor factor becomes large, indicating strongly heterogeneous deformation within individual grains and leading to accelerated roughening. In contrast, under wall-thinning, intragranular variations in the Taylor factor are strongly suppressed, promoting more homogeneous deformation and mitigating the development of inner-surface roughness. These findings demonstrate that controlling wall reduction via the drawing speed ratio is an effective strategy to suppress inner-surface roughening by reducing intragranular heterogeneity in deformation resistance.