Evolution And Control Of Inclusions During Refining Process For Ce-containing Q690D High Strength Steel

Q690D high strength steel is a kind of low alloyed high strength steel widely used in engineering machinery and equipment.The deoxidation products Al2O3 of aluminum deoxidation,will seriously affect the quality of steel.Ce treatment in RH refining process,can effectively modify the composition,shape and size of Al2O3 inclusions,to generate the composition of controlled,diffuse distribution,small size of rare earth inclusions,and to improve the overall performance of steel.It can improve the overall performance of steel.In this thesis,the effect of rare-earth Ce and calcium treatment on inclusions in high strength steel is investigated,and the suitable rare-earth alloying and calcium treatment processes for the production of Ce-containing Q690D high strength steel are clarified.The study on the number and size distribution of inclusions in the current calcium treatment process of Ce-containing Q690D high strength steel found that Ce treatment in the RH vacuum process,followed by calcium treatment after breaking the vacuum,would form a composite Ca-Mg-Al-O-RE inclusions,increasing the size of inclusions,and the number density and proportion of inclusions from 4 to 5 μm and above 5 μm have increased.The number density of inclusions above 4 μm increased from 16 to 34 inclusions/mm2 and the proportion increased from 6%to 16%.Based on this,it is proposed that when the T.O content of the molten steel is lower than 0.002 wt%,the LF calcium treatment is proposed to generate low melting point liquid inclusions with Ca content ranging from 0.0006 to 0.0018 wt%,to promote the floating of inclusions and eliminate the calcium treatment after RH to reduce the size of inclusions in the molten steel.In order to clarify the effect of calcium treatment on inclusions in Cecontaining Q690D high strength steel,the evolution of inclusions containing Ce was analyzed by laboratory experiments,and the relationship between the order of Ca and Ce addition on the formation of inclusions was studied to clarify the mechanism of inclusions evolution in Ca-Ce composite treatment.The results show that under the conditions of 0.003 wt%S and 0.0012 wt%T.O,with the increase of Ce content,the sequence of modified Al2O3 is Al2O3→CeAlO3→Ce2O3→Ce2O2S→CeS.Ca cannot change the type of rare earth inclusions,and Ce can modify the liquid calcium aluminate formed after calcium treatment into rare earthcalcium aluminate composite inclusions.The effect of adding RE to the RH refining process and no calcium treatment after RH on the clogging of the submerged entry nozzle(SEN)was investigated.The anatomical study of the SEN for the production of Ce-containing high strength steel revealed that the distance from the inner wall of the SEN can be divided into three reaction product layers,MgO-Al2O3→CeAlO3 and CaO-Al2O3→CeAlO3 and CaO.CeAlO3 can aggregate and sinter with other inclusions and block the SEN.The agglomeration of inclusions grows into large clusters,which is an important cause of clogging formation at the SEN during the pouring of Ce-containing high strength steels.By controlling the formation of Ce-O-S inclusions in the molten steel,it is not easy to form large clusters of inclusions and solve the clogging in the SEN of Ce-containing high strength steel.Combined with industrial production conditions,the optimal conditions for the formation of Ce-O-S in Q690D steel were determined:T.O content range from 0.001 to 0.0015 wt%,S content range from 0.001 to 0.003 wt%,and Ce addition range from 0.0045 to 0.0055 wt%.In order to achieve the control of fine rare earth inclusions in Q690D high strength steel,two different rare earth Ce addition methods of feeding rare earth wire and adding rare earth alloy and two different rare earth Ce addition amounts of 0.286 kg/t and 0.572 kg/t Ce-Fe alloy were carried out for rare earth alloying process tests.The results show that,after RH refining,before soft blowing and stirring,feeding rare earth wire solution,the Ce yield at RH is 72.3%;RH vacuum treatment process,adding rare earth alloy,the rare earth Ce yield at breakout is 87.5%.Adding Ce-Fe alloy during RH process,the molten steel in the vacuum chamber continuous circulation flow,rare earth and other elements in the steel and inclusions fully collision reaction.Compared with feeding rare earth wire way,the steel is significantly purified and the rare earth inclusions are evenly dispersed.The proportion of inclusions of 1～2 μm size increased from 48%to 65%with the addition of 0.572 kg/t rare earth alloy,compared to 0.286 kg/t rare earth alloy addition.