After cold deformation and continuous annealing, the steel plate is coated with a uniform zinc layer on the surface, so as to obtain the steel plate with good corrosion resistance, i.e. galvanized steel plate, which is widely used in construction, home
appliances, automobile and other industries. Due to its complex production process and various processes, galvanized sheet may produce a variety of quality defects, affecting the appearance and user use.
In this paper, the common defects of galvanized sheet, such as slip line, zinc flow pattern and uneven zinc flower, are analyzed, and the field solutions are given from the practical point of view.
01 slip line
1. Morphological characteristics.
In the subsequent processing, the line like dark lines appear on the board surface, sometimes with bulge and obvious touch, sometimes not clearly visible.
2. Cause analysis.
In the actual production process, the slip line is mainly caused
by the following reasons:
(1) The setting of flatness rate is unreasonable. After a certain amount of flat deformation, a large number of movable dislocations are produced in the annealed steel plate, which make the dislocations get rid of the binding of solution C and
N atoms, and the deformation can continue under a small stress, thus eliminating the yield platform.
Under a certain flatness, the yield strength decreases with the improvement of flatness until it reaches the lowest point, at which time the yield
platform disappears; with the further improvement of flatness, the yield strength increases again, the work hardening increases continuously, the deformation ability decreases, and the work performance decreases.
(2) Natural aging. After annealing
and flattening, the yield platform of the galvanized steel plate has been eliminated. After being placed at room temperature for a long time, the C and N atoms spread around the dislocation again to form "Coriolis air mass", which leads to the re emergence
of the yield platform.
(3) Artificial aging. The yield platform of galvanized steel sheet which needs to be painted and baked or coated and heated will reappear at high temperature.
3. Improvement measures.
(1) Reasonable setting of
flatness can not only eliminate the yield platform, but also avoid the excessive increase of yield strength and decrease the molding performance of materials.
(2) Compared with continuous annealing, the speed of heating and cooling is slow (generally
20% / H40 ~ C / h), and the holding time is long. After annealing, a large amount of C in steel is precipitated in the form of FEC, and N atoms are fully precipitated in the form of A1N, which greatly reduces the number of solid solution atoms in steel.
(3) Adopting advanced continuous annealing technology, such as R-OA technology, through rapid cooling and low-temperature cooling, the driving force of FEC nucleation and growth is improved, the solid solution C is precipitated in the form of FEC,
and the aging resistance is increased.
(4) Hot rolling can heat the slab at a lower temperature, at the same time, it can promote the precipitation and growth of A1N, Fe, C, and effectively reduce the content of solid solution N in the steel plate
by cooperating with the warm coiling process of the new high-tech products of high-74-china.
(5) Using ultra-low carbon interstitial free atomic steel, the steel contains very low amount of C and n elements. At the same time, strong carbon nitride
forming elements such as Ti and Nb can further fix the C and N atoms, which can completely solve the aging problem.