Effect of Different Descaling Processes on Hot-dip Galvanizing Quality of Hot Rolled Strip

The oxide scale on the surface of the hot rolled strip must be removed before hot-dip galvanizing, and this descaling process can also affect the surface quality of the galvanized plate. Acid pickling is currently the most widely used descaling process, but the acid mist discharge during this process seriously pollutes the environment. In the process of seeking a new environmentally friendly descaling technology, Eco-Pickled Surface (EPS) has emerged, but its application in continuous hot rolled strip plating production has certain difficulties. The work aims to study the effect and mechanism of four different descaling processes, namely acid pickling, EPS, EPS+plus and dry shot blasting, on the surface and hot-dip galvanizing quality of Q235B hot rolled strip. The surface roughness of Q235B steel plate after descaling process was measured by roughness profilometer and the surface morphology and oxide scale residue were analyzed by scanning electron microscopy. Hot-dip galvanizing experiment was carried out, the zinc consumption was calculated, and the zinc layer adhesion was evaluated by bending experiment. Scanning electron microscopy was used to observe the surface morphology of the zinc layer, and the surface morphology and composition analysis of the inhibition layer between the zinc layer and the substrate interface were observed and analyzed. Electron probe microanalyzer was used to analyze the cross-sectional morphology and composition distribution of the zinc layer. Also, EPS and EPS+plus treatment were carried out on the EPS production line of Angang Steel Group Limited built by Jingu Co., Ltd. and the galvanized layer quality of the product was evaluated by mechanics, bending and cupping tests. The results showed that the surface roughness of the steel plate after pickling, EPS, EPS+plus and dry shot blasting was 1.444 2, 3.330 5, 2.396 4 and 4.338 8 μm respectively. There was obvious oxide scale residue on the surface of dry shot blasting treated sample. No residual oxide scale was found on the surface of acid pickling treated sample and EPS treated sample, but there were obvious acid-washed corrosion grain boundaries on the surface of the acid pickling treated sample. Although there was only a very small amount of residual oxide scale found on the surface of EPS+plus treated sample, the adverse effect on the galvanizing quality can be ignored. The zinc layer adhesion of the acid pickling treated sample was rated as grade 1, but the flatness of the interface between the zinc layer and the substrate was poor, and the zinc consumption was also more, which was 176.2 g/m2. The quality of the zinc layer of the dry shot blasting treated sample was poor, with obvious cracks and dezincification, which was rated as grade 3. During the hot-dip galvanizing process of the steel plates treated by EPS and EPS+plus process, a continuous and denser inhibition layer was formed between the substrate and the zinc layer. The adhesion of zinc layer was rated as grade 1, and the zinc consumption was less, which was 148.3 and 123.5 g/m2, respectively. The quality of galvanized steel plate treated by EPS and EPS+plus process is better than that of pickling and dry shot blasting process treated samples, and the EPS and EPS+plus process can fully meet the galvanizing quality requirements of hot rolled strip in actual production. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.