On the likely origin of the Gold Dust Defect in the production line of industrial ferritic stainless steel

The “Gold Dust Defect” that sometimes appears on the surface of the AISI 430 ferritic stainless steel has been related to an improper recrystallization of the material. Hot-rolling, a decisive step in the production process, seems the step where this defect is induced and temperature during this treatment plays a key role. To trace back the origin of this defect, the microstructural evolution of a sample of an AISI 430 was monitored at the different stages in the production process of the steel. In particular, a detailed study combining electron backscatter diffraction with other techniques, such as optical microscopy and scanning electron microscopy has been performed. The whole set of results evidences that after hot-rolling and subsequent annealing, the material contains a large amount of martensite in the center of the cross-section due to a fast cooling of the austenite originated during rolling. In addition, the surface shows a top-layer made up by unrecrystallized subgrains separated from the matrix by an accumulation of chromium carbides. After pickling, the top-layer of subgrains semi-detaches from the surface forming flakes which cannot recrystallize. Most of these flakes do not peel off during the subsequent stages but survive until the end of the production process, forming Gold Dust Defect flakes.