Study on the pickling mechanism based on structural differences of oxide scale in steel grades

To address the synergistic control challenge of pickling efficiency and surface quality of hot rolled strip, this study selects three typical steel grades i.e.low carbon aluminum killed steel, carbon manganese steel, and cold rolled high strength steel,and systematically investigates the correlation between the oxide scale structure on the strip surface and its pickling behavior. The results indicate that the oxide scale on low carbon aluminum killed steel is primarily composed of a loose and cracked eutectoid structure (Fe+Fe₃O₄), which can be efficiently removed within 60 s, yielding a bright and defectfree surface. In contrast, the oxide scale on carbon manganese steel mainly consists of dense proeutectoid Fe₃O₄, requiring 180 s for complete pickling, and tends to leave residual scale that causes surface darkening. For cold rolled high strength steel, slow cooling leads to the precipitation of Fe particles and the formation of a 15 μm thickness intergranular oxidation layer, extending the pickling time to 400 s. This is attributed to the Fe particles hindering acid penetration and the intergranular oxidation accelerating grain boundary corrosion, while residual Cl^- may induce surface yellowing. The findings clarify the critical influence of oxide scale structural differences on pickling kinetics, providing a theoretical basis for developing differentiated pickling processes.