Crack-Like Defects on Steel Billet Surface
Steel Billet is a semi-finished product which is widely used as a raw material in metal processing. The quality of the steel billet is critical to the quality and performance of the metal part after the final processing. Billet surface crack-like defects can not only lead to a decrease in the product yield of metal parts but also cause safety concerns such as product failure or cause problems during downstream processing.
Steel billet crack-like defects usually occur on billet surfaces during hot rolling, cold rolling, and heating processes. In hot rolling, the billet surface crack-like defects are mainly caused by the large difference between the center temperature of the billet and the billet surface temperature. The large difference in temperature causes the surface of the billet to expand and contract at a faster rate than the center. This difference in the rate of expansion and contraction can cause the surface of the billet to crack. The billet surface crack-like defects during cold rolling are mainly caused by inadequate lubrication, poor mill rolls alignment, and insufficient break-down passes. A lack of proper lubrication of the workpiece leads to increased friction which can cause the surface of the billet to crack. Poorly aligned mill rolls can also increase the friction between the billet and the rolls, leading to surface cracking. Inadequate break-down passes reduce the amount of plastic deformation of the billet, reducing its resistance to cracking. During the heating process, the billet surface crack-like defects are mainly caused by the sudden cooling of the billet in the heating chamber, oxygen-induced surface oxidation, and the presence of non-metallic inclusions in the billet. Sudden cooling of the billet in the heating chamber Read the bottom of the billets causes a rapid thermal gradient, which can cause cracks in the surface layer. Oxygen-induced surface oxidation of the billet results in the formation of oxides which have a higher coefficient of thermal expansion compared to the steel. The oxides then form cracks as they expand during cooling. Non-metallic inclusions or other impurities in the billet can also form cracks in the surface due to the lower plasticity of these impurities compared to the steel.
To reduce the incidence of billet surface crack-like defects, the process design and operating conditions should be optimized to minimize the potential for such defects. During the hot rolling process, it is important to ensure that the temperature of the center and the surface of the billet is as close as possible so that the differential expansion is minimized. During the cold rolling process, the lubrication should be increased and the mill rolls should be better aligned to reduce friction and the number of break-down passes should be increased to improve plastic deformation of the billet. During the heating process it is important to reduce cooling rates as much as possible, as well as to clean the billets and reduce the level of surface oxygen and non-metallic inclusions.
In conclusion, steel billet crack-like defects are surface defects that can occur during hot and cold rolling, as well as heating. These defects can lead to lower product yields and safety issues. To reduce the incidence of these defects, the process design and operating conditions should be optimized accordingly.
