The hot rolling of austenitic stainless steels in Steckel Mills introduces particular characteristics to the development of oxides scales and surface structures. In this work, the formation of oxide structures during multipass hot rolling of 302 steel was studied under different sets of processing parameters in a laboratory system designed for the simulation of the Steckel process. The resulting surface structures were characterized by a set of complementary techniques involving scanning electron microscopy, energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and glow discharge optical spectroscopy (GDOS). The surface analysis revealed two alternative surface structures: one consisting in a thin protective oxide layer rich in Cr 2 O 3 and the other consisting in thick complex structures containing several successive nonprotective oxide scale and metal layers resulting from a cyclic oxidation pattern involving stages of protective oxidation, chemical breakaway, and duplex oxidation. The critical condition that determined the activation of one mechanism or the other was identified associated with the parabolic rate constant for Cr 2 O 3 growth and the diffusivity of Cr in the alloy. The effects of changes in temperature, deformation, and furnace atmosphere are discussed. Alternatives for controlling scale development are presented.

Precipitation strengthening by fine Nb-rich particles represents an important element on the design of low carbon high strength steels. This is typically obtained on steel strips by thermal holding at temperatures above 600°C following the austenite to ferrite transformation. These conditions are beneficial to obtain a large precipitation of small Nb-rich precipitates. On the other hand, precipitation at lower temperatures, in a phase already hard, such as bainite, has been scarcely studied. In this work, the precipitation phenomena occurring during isothermal treatments following the austenite to bainite transformation at 450°C are investigated. For this purpose, two Nb-alloyed low carbon steels with and without silicon are studied and the evolution of the microstructure is determined by the use of transmission electron microscopy and followed by hardness measurements. The presence of a hardness peak is not detected until long isothermal times (150h). Preliminary atom probe tomography (APT) characterization provides insight on the possible presence of fine NbC precipitates at the peak hardness treatment. A comparison with a Nb-free alloy indicates a significant hardening effect of niobium on the bainitic structure.

The development of oxide scales on hot-rolled austenitic stainless steels under conditions imposed by the industrial Steckel Mill operation introduces particular characteristics that impact downstream on the surface quality of the hot-rolled strip product. In this research, the development of these particular surface structures were studied on 302 austenitic stainless steel by means of laboratory process simulation involving mechanical deformation in a multipass hot rolling schedule and long interpass time inside equalizing furnaces. Surface analysis using a set of complementary techniques that included field emission gun scanning electron microscopy (FEG-SEM), energydispersive spectroscopy (EDS), X-ray diffraction (XRD), and glow discharge optical spectroscopy (GDOS) provided a full characterization of the resulting surface structures. These structures consisted in multiple oxide scale and metal layers that were developed after each rolling pass and heavily modified by the following rolling operation establishing a single repetitive mechanism of surface development. The mechanism of formation of these complex multilayer surface structures has been fully determined as being associated with a cyclic oxidation pattern involving successive stages of protective oxidation, chemical breakaway, and the progress of duplex oxidation.

No abstract