| Defects in steel ingots such as shrinkage porosities,macro-segregations,inclusions and so on are bottleneck problems which restrict the improvement of steel ingots.The formation mechanisms of these defects are still unclear,which are topics of significant interest in the research field of casting and solidification.In order to improve the quality and yield of chrome molybdenum die steel ingots,investigations were carried out on heat transfer characteristic during solidification of steel ingot,formation and control of shrinkage porosities and macro-segregations,control of key processes of ingot casting such as teeming stream protection technique using argon shroud,design of entrance nozzle structure and heat preservation of hot top.A new concept of "effective transient thermal resistance" was proposed based on analysis of heat transfer characteristic during solidification of steel ingots,which was used to dig out the restrictive steps of heat transfer?RSHT?at different stages of solidification of steel ingots.It was found that at early-stage of solidification,heat transfers through the mould and ingot/mould interface are the RSHT.At mid-stage of solidification,heat transfer section of the outer wall of mould is the RSHT and solidification can be controlled by different cooling conditions.At late-stage of solidification,the heat transfer section of the solidified shell is the RSHT and solidification is hard to control.Besides,variations of pouring temperature and mould's thickness do not change the RSHT during solidification of steel ingots.Influence of cooling conditions such as air cooling,forced air cooling,air mist cooling and water cooling on cooling rate,solidification time and shrinkage porosity of 19 ton steel ingot was investigated via numerical simulation.In order to predict shrinkage porosities in steel ingot accurately,shrinkage porosity criterion G2·L-5/3?where G is temperature gradient and L is cooling rate?was put forward with consideration of effect dendrite arm spacing on permeability.Using criterion G2·L-5/3,an optimization of 5.5 ton steel ingot design was carried out which can eliminate shrinkage porosities.Influence of casting parameters on shrinkage porosities in 40 ton steel ingot was investigated by numerical simulation.Dimensionless factors were put forward to evaluate the effect of casting parameters on shrinkage porosities correctly.Based on continuum model,mathematical model of macro-segregation in steel ingot was carried out.Influence of hot top heat preservation on positive segregation in ingot top was investigated with combination of experiments in steel plant.It was found that improve hot top heat preservation can lessen positive segregation in ingot top effectively.With decreasing thermal conductivity of insulation board from 0.447 W·m-1·K-1 to 0.163 W·m·K-1,positive segregation in 19 ton steel ingot top was lessened to a great extent.Besides,the yield of steel ingot was improved by 4 percentage points.Argon shroud protection devices used during ingot casting were designed and investigated via experiments.Mechanism of air entrapment during Teeming was analyzed by numerical simulation.It was found that argon shroud protection device seated on top of central trumpet can prevent air from being involved into the central trumpet.Effects of entrance nozzle structure on mould powder entrapment during early stage of filling of 19 ton steel ingot were investigated.Fluid of molten steel and heat transfer at early stage of filling of 19 ton ingot were calculated.It was found the risk of mould powder entrapment can be greatly decreased when the bottom-end interior diameter of entrance nozzle is larger than 90 mm.Through further investigation of formation and control of defects in steel ingots such as shrinkage porosities,macro-segregations,inclusions and so on,theoretical basis and practical guidance for optimization of casting parameters to improve quality and yield of steel ingots were obtained. |
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