Development of oxide scales in low carbon steel wire rods

E. BRANDALEZE; M. AVALOS; M. MELIA

Darmstadt

Congreso; Materials Science and Engineering Congress 2018; 2018

FEMS - TMS

The low carbon rods obtained from commercial hot rolling are the raw material for thedrawing processing of wires. The oxide scales developed during this process promote theloss of metallic performance and compromises the subsequent processes of the wires. Forthat reason the reduction or elimination of oxide scales is a problem of interest andconsiderable effort is made to solve it. The usual thermo mechanical deformationprocesses for manufacturing low carbon rods include an on line cooling stage. Thetemperature and the cooling rate of the process determine not only the thicknesses of thescales but also the oxide phases present in the scale namely Wüstite (FeO, cubic),Magnetite (Fe3O4, cubic) and Hematite (Fe2O3, trigonal)According to the nature of the oxide scales, different methodologies are employed toremove them. As a consequence, the characterization of the scales, including themeasurement of layer thicknesses and identification of the different oxides in a short timeis decisive to modify processing parameters or to select the methodology for appropriatedescaling. The conventional techniques to analyze the oxide scales are optical andscanning electron microscopy. Those studies have been approached with relative successbecause the problem is complex: differences in composition are difficult to detect: it is noteasy to distinguish between Magnetite and Hematite; and the similarities in crystalstructure made difficult to distinguish between Wüstite and Magnetite. However, with thecombined use of techniques such as EDS (Energy Dispersive X-ray spectroscopy) andEBSD (Electron Backscatter Diffraction), it should be possible to recognize the differentphases, and also characterize microstructural aspects such as grain size, morphology andgrain boundaries.The main goal of this work is to analyze the characteristics of the oxides developed in lowcarbon wire rods using EDS and EBSD to set up an on line characterization tool. In thiswork SAE 1008 wire rods of 6.4 mm in diameter were treated at two different temperaturesat the Stelmor conveyor. Microestructural aspects, like thickness and composition of theconstituent phases of scales and interfaces between the grain boundaries of the substrateand the oxides, are considered. The results are also correlated with a thermodynamicstudy of the formation of oxides contemplating the cooling conditions applied in theindustry.