The effect of the cast structure on the evolution of texture during processing of AISI 433 ferritic stainless steel

Abstract

Ferritic stainless steels (FSSs) are gaining popularity because they are cheaper than austenitic stainless steels (ASS) since they contain almost no nickel. Amongst these FSSs, there is a relatively new grade of steel designated AISI 433 i.e. 430 with added Aluminium (Al) and is produced by Columbus Stainless Steel in South Africa. However, these FSSs are inclined to develop surface-roughness defects during cold rolling or forming operations. The surface roughness defects manifest as series of ridges and valleys that run parallel to the former rolling direction (RD) and spread across the transverse direction (TD) of rolled steel sheet. These defects can be ridging or roping, depending on the type of surface defects. When a majority of ridges have a length of more than a millimetre along the RD as elements of surface roughness, the surface defects are known as ridging, but if they are shorter than a millimetre the term roping is used. The ridging or roping phenomenon is well known in FSS sheets and a lot of work has been done to improve resistance to their formation. The ridges, with a few micrometres amplitude, adversely affect the aesthetics and surface quality of the deep drawn components. The manufacturer is then obliged to perform time-consuming and costly processes such as grinding and polishing operations to remove the ridges in order to restore the smooth and shiny surface of the deep drawn articles. These further processes are objectionable to the manufacturers due to increased costs of production. It has been shown that forming properties (ridging or roping resistance and formability) are anisotropic, governed by the crystallographic texture and topological arrangement of the preferred grain orientation when a sheet of steel is uniaxial tensile strained along the RD. The homogeneous γ-fibre texture components distribution promotes good forming properties, while heterogeneous texture components distribution with predominance of Cube, Goss and α-fibre components impairs these forming properties. It is well established that an initial columnar cast structure (ICCS) promotes cube or α-fibre texture while an initial equiaxed cast structure (IECS), promotes γ-fibre texture components. If both structures are present, they promote inhomogeneous texture, which leads to roping or ridging. However, in this work, the cast structures were mechanically separated and processed separately but under the same conditions. This was done in order to see if the results would be different from mixed cast structures. As expected, it has been confirmed that IECS alone enhanced the γ-fibre texture and led to good surface roughness resistance while ICCS alone resulted in inhomogeneous texture components and impaired resistance to surface roughening. In other words, it was not the mixed as cast structure but the presence of columnar structure with cube texture that causes texture inhomogeneity in the final product that impairs resistance to surface roughening. Although at sub-micron level i.e. insignificant roughness, the surface roughness resistance of IECS sample was still better by 30% over an ICCS sample, i.e. the measured Ra-values were found to be 0.52±0.047 μm and 0.74±0.134 μm for IECS and ICCS respectively. The industrially processed FSS 430 with both structures, which was used as a benchmark, exhibited intermediate properties, i.e. the Ra-values were found to be 0.58±0.037 μm. No significant difference in yield strength, UTS and elongation were observed in this steel from both the IECS and ICCS steel strips. Although the effect of Al addition on texture and surface roughness resistance in 433 was not part of this study, the results suggested that it may have a significant influence on texture and the subsequent surface roughness. It is well known that fine AlN particles promote selective growth of favourable (111)-oriented grains during hot rolling and annealing processes, resulting in strong γ-fibre texture components. Therefore, it was not surprising that there was insignificant difference in surface roughness resistance between ICCS and IECS in Al containing 433. This is recommended for further investigation.