Fold line defect refers to the fold line formed when the corner of the billet is turned over to the surface during rolling.It is one of the important defects in the production of wide and thick plate. It is found statistically that: there is a high probability of side defects in wide specification ship plate steel, and the distance between the defect position and the side is large, which is easy to cause the side defects still exist after the steel plate is cut online.
The measures to reduce and control the defect of folding line at the edge of the plate are as follows:
Purity control of molten steel
By optimizing the smelting process to reduce steel [O], strengthen the refining and protective pouring operations to control the nitrogen content of molten steel, to control the metal oxide inclusion content and Ni, V, Ti nitride and carbon nitride precipitation.
On the one hand, the chamfer mould changes the shape of the billet, on the other hand, it improves the uniformity of the surface temperature of the billet and the billet after the heating furnace, reduces the rate of decrease of the temperature of the corner, so that the folding line defect of the broad and thick plate can be controlled.
Control the cooling intensity of continuous casting billet
The spray at the center and the edge of the secondary cooling zone in the fan-shaped section under different width sections of the continuous casting billet was controlled separately to further reduce the temperature difference between the surface center and the corner position of the continuous casting billet, so as to improve the surface temperature of the corner position of the billet in the process of drawing rectification, so as to control the grain boundary precipitation on the surface of the billet.
Optimize rolling mode
The larger the steel plate broadening degree is, the larger the roll quantity of the edge corner of the steel plate is, and the corresponding edge crack defect is closer to the center of the steel plate.Therefore, wide continuous casting billet is used to produce wide width steel plate as far as possible to reduce the width expansion ratio of the steel plate so as to reduce the uneven deformation degree of the edge of the wide steel plate in the rolling process.
improve the slab heating uniformity
The slab heating process was optimized to promote the dissolution of the second phase particles and reduce the temperature difference between the top and bottom of the slab as far as possible.It can effectively reduce the difference of deformation resistance between the upper and lower surface of the rolled piece, thus reducing the difference of deformation degree between the upper and lower surface of the rolled piece, and reducing the uneven deformation of the edge of the rolled piece.
increase the amount of rolling down
In the process of rolling, if the amount of underpressure is not large enough, the deformation will be concentrated on the surface of the steel plate, resulting in uneven deformation of the steel plate.In order to improve the degree of plate deformation and reduce the uneven deformation of the rolling process, the amount of track pressure drop should be increased appropriately.