Cause analysis of iron oxide scale in steel rolling

　　In the surface quality of hot rolled products, the pressing defect of iron oxide scale is a headache in the product quality of many factories. After pickling, the iron oxide scale pressed into the surface leaves small pits with different depths at the defect, especially the pressing of primary and secondary iron oxide scales. After pickling, there are often uncleared iron oxide scale particles at the rough pit bottom, which seriously affects the surface quality of cold rolled plates in the subsequent process, resulting in the decline of product quality, It is bound to affect economic benefits. Therefore, it is necessary to analyze the causes of iron oxide skin pressing defects, put forward preventive measures, and provide judgment basis for relevant improvement work.

　　The furnace scale formed by the billet surface and high temperature furnace gas is called primary scale; During the rolling process, the iron oxide scale on the surface will fall off, and the hot metal surface will contact with water and air to form a new iron oxide scale, which is called the secondary iron oxide scale.
 

　　Causes of primary iron oxide scale pressing

　　1. Heating reasons

　　(1) high heating temperature and long heating time; ⑵ the atmosphere in the furnace is not good, and the air supply and inlet volume is too large; (3) negative pressure is formed in the furnace and cold air is inhaled; (4) the heating temperature in the furnace is too much lower than the minimum temperature specified in the regulations.

　　During the heating process, if one or more of the above conditions occur, during tapping and rolling, the iron oxide scale will stick to the billet and steel plate, which is not easy to be removed, thus forming a primary iron oxide scale pressing defect.

　　2. Reasons for descaling equipment

　　(1) insufficient high pressure water pressure; ⑵ the nozzle is severely worn and has small capacity; (3) high pressure water nozzle is blocked; (4) the high-pressure water fails to be sprayed onto the surface of the billet; (5) the descaling nozzle (nozzle angle) is improperly assembled; (6) poor spraying distance; (7) improper descaling sequence and insufficient equipment input.

　　3. Influence of slab chemical composition, such as excessive sulfur, silicon and aluminum

　　4. Improper production command

　　(1) when the rolling mill is shut down due to fault, the production commander fails to timely notify the heating worker of the shutdown time; (2) after the production fault is eliminated, the heating temperature is not notified in advance; (3) when the descaling capacity decreases due to the failure of descaling equipment, the production personnel shall continue to produce.
 

　　Reason for rolling schedule

　　Because the slab is arranged behind the steel with slow rolling speed or high heating temperature, or between the two steel with high heating temperature, the furnace scale increases and is difficult to remove at high temperature.

　　Causes of secondary iron oxide scale pressing

　　The main causes of secondary iron oxide scale are high start rolling temperature, improper descaling sequence, and descaling equipment (descaling box) for finish rolling and rough rolling.

　　1. causes of roll wear and iron oxide scale pressing

　　⑴ during the rolling process, the oxide film on the roll surface peels off and is rolled into the strip steel surface;

　　⑵ the rough roll behind stripping has a similar furrow effect on the surface of the strip steel, which promotes the formation of iron oxide scale on the surface of the strip steel itself;

　　(3) there is no iron oxide scale removal device between each or part of the rolling mills of the finishing mill.