Causes of surface defects of 4J36 invar alloy cold rolled coil

The thermal scratch defects on the surface of 4J36 invar alloy cold rolled coil were analyzed by means of macroscopic observation, scanning electron microscopy and energy spectrum analysis,metallographic examination,X-ray diffraction analysis. The results show that the thermal scratch defect on the surface of cold rolled coil came from the residual oxide of hot rolled coil. The oxides such as Fe₃O₄ and FeO in the inner oxide layer on the surface of the hot rolled coil were island-like distributed in FeNi₃. After pickling, FeNi₃ was coral-like, which made the oxides at the bottom unable to be pickled clean. After cold rolling, the residual oxides at the bottom were exposed to the surface, forming thermal scratch defects.