Study On The Dehydrogenation Behavior Of A New Vacuum Casting Process With Liquid Drops Passing Through Refining Slags For Heavy Steel Ingot

With the development of industrial technology,the proportion of heavy steel ingot in aerospace,shipbuilding,electric power,iron and steel,petrochemical,transportation,military,heavy machinery,industrial equipment,oil pipelines and chemical industry increases,and these industries require a higher quality and better metallurgical mechanical properties of the heavy steel ingot overtime.As a result,the technology of making heavy steel ingot and its quality reflects the level of a country's industrial and the strength of national defense.Because of its large size,varieties of quality problems are easily produced during the process of large steel ingot casting.Among the different quality problems,a small content of hydrogen can cause ingot in formation of flakes,hydrogen embrittlement,crack,bubbles and pinhole defects,which restricting the quality grade of steel ingot seriously.The hydrogen content should be reduced to less than 2 × 10-6 to avoid the problems caused by hydrogen by vacuum dehydrogenation.This paper uses a new heavy-ingot-making process which makes molten steel fall through a filter device at the outlet of the ladle nozzle to form spherical molten steel drops,So as to achieve a series of refining effects when the drops fall into the casting ingot mold,and pass through refining slags.This paper presents a new model of vacuum dehydrogenation,which makes hydrogen atoms diffuse outwards when the drops fall into the vacuum environment with the low hydrogen partial pressure,so the hydrogen content in molten steel can be reduced effectively.In this paper,the thermodynamics and kinetics of diffusion of the hydrogen atoms have been studied,and the numerical calculation of the phenomenon has been carried out,the following conclusions are obtained:(1)The solubility of hydrogen in molten steel comfort to the Sieverts' law of dissolution,which means the content of hydrogen is proportional to the square root of hydrogen partial pressure,at the condition of this paper describe,the hydrogen dissolution coefficient is 0.0025;(2)The rate of dehydrogenation decreases as the time that the liquid drops increase,analyze the mass transfer process,hydrogen atoms diffuse outwards,and the diffusion process is carried out in time,so the rate of dehydrogenation positively correlated with the fall time;With the size of drops decreased,the rate of dehydrogenation increased significantly,when the drops just pass through the drops-generation filter,the difference value of hydrogen concentration in center and at surface of drops is a constant number,thus the larger size of droplet means the smaller concentration gradient of hydrogen.According to Fick's law,the diffusion rate of hydrogen is proportional to the mass concentration gradient,thus the dehydrogenation rate is negatively correlated with drops size.The dehydrogenation rate increases with the increase of the vacuum degree,according to the Sieverts' law of dissolution,hydrogen solubility decreases with the increase of the vacuum degree,with decrease of the hydrogen partial pressure.For a certain amount of hydrogen,the higher vacuum degree means higher dehydrogenation rate.(3)Based on practical production data,the model is well validated with the renew time of 0.0035s.