Continuous cast slab or billet surface shows irregular pits, which are mostly lateral pits, as well as vertical pits. Depression and more in austenitic stainless steel (Cr18Ni9 type) and a low-carbon steel (carbon 0.10 to 0.15%) of the slab surface. Depression is easy to produce in the lateral surface of the slab width or wide angle portion of any part of the face. Severe depression crack, crack the surface of the deep penetration of steel who appeared scarring, and even re-skin breakout. The reason slab surface depressions produced is multifaceted. Since the early stages of solidification solidified shell “too cold” local billet housing contraction caused by large, uneven growth will lead to the formation of bad housing depression; Billet steel shell longitudinal shrinkage and static pressure caused by the interaction between the re-bend when casting to produce a lateral depression; Ling change occurs (see off the square and diamond variations) of billet near obtuse angle near the longitudinal recesses (grooves) and longitudinal ends of the slab width surface pits are in the mold caused by uneven cooling; recessed generation also concerned with the composition of the steel, austenitic stainless steel and low carbon steel in the mold shell growth uneven easy to produce pits; high-performance stainless steel thermal expansion and thermal conductivity low prone to depression; Mr. low Carbon solidified immediately after the change of austenite into ferrite volume contraction when large steel this ingredient tendency to form pits maximum. Meanwhile, the incidence of depression is also affected by the continuous casting conditions: Performance as inappropriate mold powder, the inner mold level fluctuations, changes in casting speed and other factors. Straightening roller sticky metallic foreign body, producing a periodic distribution of pits (indentations) on the causes of slab surface.
Solutions are: mold wall practice “slow cooling”, reducing the surface area near the mold heat flux intensity, stagnant growth and solidified shell, can be used mold wall plating or so-called “hot top mold” to promote blank shell growth uniformity; mold level fluctuation minimal (less than 3mm), using the best performance fluxes, flux viscosity adjustment in accordance casting speed, to ensure uniform filling; the slag layer of appropriate thickness, stability slag film control appropriately high superheat and casting speed; the use of reasonable submerged nozzle geometries and reasonable depth of insertion, avoid liquid vortex and others have appeared in favor of shell uniform growth, as well as pay attention to clean up the straightening rollers, can inhibit pits are formed.
Post time: Sep-05-2019