What is the cause of yellow spot of hot rolled steel plate?

Hot rolled steel plate usually refers to the use of hydrochloric acid to clean the hot rolled surface of the product after the iron oxide.
After pickling, the substrate is in direct contact with the environment, unlike the surface of cold-rolled annealed plate, which usually has a dense oxide layer to play a certain protective role, so the surface of hot-rolled steel plate is very prone to oxidized macular defects.

The cause of macula on the surface of pickling products is very complex, which may occur in the production, packaging, storage, transportation and one processing of steel mills. Therefore, in the analysis of macula defects, it is necessary to mobilize the information of each link for comprehensive analysis, so as to make an accurate judgment.
Many technicians like to analyze the elemental composition of the macula to see if there is any residual chlorine to determine the cause.