Effects of hot rolling conditions and scale breaking on the pickling performance of hot-rolled steel strip

Significant structural differences can be found in oxide scales formed on hot-rolled strip processed under different hot rolling conditions and at different locations in the coils. The variations in these oxide scale structures give rise to different pickling requirements, hence necessitating the Pickle Line to be operated at a lower speed should an adverse scale structure be present on part of the coil. Generally, this has a negative impact on the Pickle Line throughput. It was found, at the BHP Western Port Works, that for coils with a finishing temperature of 910 °C and coiling temperature of 720 °C, a scale structure which was extremely difficult to be removed by pickling was developed on the strip edges (known as edge scale), resulting in a significant reduction in the process speed for complete pickling. This paper discusses recent laboratory results and plant trials which aimed at optimizing the hot rolling conditions to produce favourable scale structure that could be pickled readily. Conditions leading to the formation of various scale structures are outlined. Successful implementation of the change in the operating practice at the hot strip mill, which resulted in doubling the processing speed at the pickle line for certain steel grades, is described. The effects of scale breaking are also discussed.