(1) Metal oxide inclusions brought in by poor deoxidation of liquid steel;
(2) The condition of complete deoxidization is:choose dry and clean charge,add ferro manganese first after melting,then add ferrosilicon deoxidization,then add calcium silicon deoxidization,then stop power for 2min,then add aluminum final deoxidization after insulation casting.Immediately after pouring,add wood chips or waste wax to recover the box for sealing and cooling.
In the case of incomplete deoxidation,if the lid seal cooling,because of a long cooling time,the presence of large amounts of oxygen in the liquid steel has sufficient reaction time,the formation of a large number of pitting is inevitable.In the case of complete deoxidation,cap cooling ensures that no external oxygen is absorbed into the molten steel,avoiding secondary oxidation of the molten steel and preventing the formation of pitting points.
(1) When the shell roasting is not penetrating,a small amount of gas will be generated at the high temperature of pouring,which will promote the occurrence of interface reaction and form pitting.
(2) The increase of impurities in refractory materials,especially the high content of Fe2O3,will directly lead to its participation shell and liquid steel interface reaction,oxygen into the liquid steel to cause secondary oxidation,thus increasing the trend of pitting formation.
(1) The most important factor affecting the generation of pitting is the quality of molten steel;(i.e.whether deoxidation and scum are complete)
(2) Operation shall be carried out in strict accordance with the smelting process.
According to the bottom slag→melting cover→pre-deoxidization after melting→deoxidization after power failure→deoxidization before pouring,this process is carried out.
(3) the choice of deoxidizer to achieve not only the steel fully deoxidized,and can make the oxide formed after deoxidized low melting point,easy to aggregate and the purpose of floating.The final amount of aluminum deoxidizer must be strictly controlled,too much aluminum will promote the formation of pitting.(The residual amount of aluminum should be controlled between 0.015-0.02,which is ideal.If it is too low,it is easy to produce stomata and pitting points.If it is too high,white and black points will occur after processing.)
(4) steel should be very clean,and should not use too much return charge and steel raw inclusion increase,melting process should try to prevent the surface of liquid steel exposed time,to prevent Cr,Fe,Si element oxidation.
(5) because of pouring type shell temperature about 800℃to 900℃above,pouring the molten steel temperature is about 1600℃or so,the contact on the surface of the casting temperature is too high for instant metal not solidified,especially thick wall parts,the oxygen from the atmosphere through shell reaction with the metal surface,the formation of oxides,and the inclusions in molten steel gathered to form a hemp dot,so big when pouring,to the addition of wood(or charcoal),insulation module after the photo,immediately join waste wax block cover tank insulation,this is one of the important measures to prevent pitting.
(6) The roasting temperature of the shell shall be no less than 1180℃and the holding time shall be no less than 45min.When the shell is not completely burned through,there will be a small amount of gas during pouring,and it will participate in the interface reaction between liquid steel and shell,bringing oxygen into liquid steel and causing secondary oxidation.As carbon steel has a high content of C and does not contain Cr,when cooled in the air,the interface reaction can only form a decarburized layer without pitting.If there are pitting points,then the shell is not burned.
(7) Refractory materials,especially surface refractory,in addition to the principal component should meet the requirements,the impurity content dominated by Fe2O3 must be low.Excessively high Fe2O3 content will aggravate the oxidation atmosphere of the interface reaction and lead to the generation of pitting.
(1) Pitting is the aggregation of complex oxidized inclusions of iron,chromium,silicon and aluminum in molten steel on the surface of casting.
(2) The main measure to prevent the occurrence of pitting is to fully and completely deoxidize in smelting,and make the deoxidized product float easily;Secondary oxidation of the casting surface should be prevented during cooling of the mold.
(3) Strictly implement the roasting process of shell.
(4) Effective measures to prevent casting pitting:purchase graphite sand(petroleum coke particles)for the transition layer shell.
For more than ten years,we have been improving the surface quality of the castings(surface pitting)by adding graphite powder to the coating of the back layer.Through long-term practical experience,the effect of adding graphite electrode(carbonizing agent)into the transition layer sand of shell making is also remarkable.