Study On Technology Of Composite Casting Large Steel Ingot With Inner Core Billet

The solidification speed and nucleation rate can be significantly improved by absorbing the latent heat during the solidification of molten metal when a rolled or forged core material of the low temperature is preseted in the mould.This leads to the significant refinement of solidification structures.This paper chooses 20# as the core,ordinary steel rod billet of Q235 steel for smelting material,the reduction or acid pickling core billet is placed in a protective atmosphere in the die preheating temperature of500-850℃,followed by the medium frequency induction furnace melting molten steel will be poured into the mold,and then cooling to room temperature,finally get the product ingot.The cooling ingot was cut by metal band saw,and the macro morphology of the profile and the interface between the core and liquid were observed under different experimental conditions.The solidification structure and core billet structure were observed by using optical microscope and scanning electron microscope,and the evolution law of microstructure and core liquid interface was carried out.At the same time,in order to observe the melt crystallization phenomenon during the solidification process,a clad casting experiment was carried out using sodium thiosulfate as solidification medium and using solid sodium thiosulfate round bar as core material.The effect of low temperature core bar on the solidification process,solidification rate and structures of sodium thiosulfate melt was investigated.The experimental results show that the solidification rate of the melt was significantly improved by the addition of solid core bar.The solidification rate reached6.80 g/min under the condition of a core bar and the solidification rate is only 4.60 g/min without core bar after 30 min.The solidification rate of the two cases is approximately equal to 4.44 g/min at 53 min.At this point,the solidification percentage of the melt reaches 62% with core bar and the solidification percentage is only 37%.Additionally,the average grain size of the solidification structures decreases from 5.33 mm to 2.66 mm and the central shrinkage reduce by 5.4% due to the core bar cast in ingot,so the core quality of ingot is improved obviously.In the actual casting experiment,under the conditions of the preheating temperature of 850℃,casting temperature of 1550℃,core billet 1.75 kg,molten steel 25 kg,mold filling ability good composite ingot was casted.The ingot casting microstructure to equiaxed crystal,core microstructure for overheating widmanstatten structure,core liquid interface of composite is better,there are only minor diameter of about 5 μm holes in the interface.At the same time,the upper part of the core billet composite interface composite is better than the lower,upper interface transition layer width is about 480 μm,the transition layer is a core blank and liquid steel mixing organization.The core billet has a forced cooling effect on the melt and increases the nucleation rate of the melt.The nucleation rate near the core is higher than that of other positions,and the fine and uniform cast microstructure is formed.The average grain size of the solidification center is refined from 215.37 μm to 31.64 μm.At the same time,increasing the preheating temperature of core billet or reducing the proportion of core quality will reduce the cooling capacity of core and increase the grain size of as cast microstructure.The composite interface of hydrogen reduction core ingot is not smooth,and the reduction interlayer exists in the local position.The interlayer thickness is 200μm,and the bonding state of the composite interface is different at different positions.The composite effect of the upper part of the core billet is better than the lower part.In contrast,the composite interface of the pickling core ingot is relatively flat,and the interface state of the core billet tends to be uniform.The fluidity of molten steel has a great influence on the interface composition of the ingot,and the fusion interface is easy to form at the location of large fluidity,and only a few holes with a diameter of about 4 m are formed at the interface.In contrast,the small flow position is easy to form diffusion composite interface,and there is a small gap between 12μm in diameter and many tiny holes in the interface.One of the important measures for the interface equilibrium temperature of casting composite state,when the equilibrium temperature is 1418.52 ℃,melt mixing region interface width about 300μm,mixed melting zone contains very few hole size is about 2μm;when the equilibrium temperature is 1347.02 ℃,the interface there is no mixing zone,interface the uniform distribution of many holes,the maximum pore size of up to 8μm;when the equilibrium temperature is 1269.56℃,the interface part of the diffusion bonding,combined with holes,the unbound fraction with porosity,maximum pore size of up to 60μm.