| Long products mainly refer to steel with wire,bar and products.With the progress and development of society,the demand for long products is increasing and the quality requirement as well.For long products,most of the quality defects come from continuous casting(solidification)process.Due to the influence of solidification characteristics of molten steel and continuous casting process parameters,the internal quality defects of billet such as porosity,shrinkage cavity and centre segregation are inevitable.The internal defects will be inherited to the subsequent rolling process and even to the finished material,which will reduce the uniformity and compactness of the steel and deteriorate the performance of steel,such as cord steel drawing fracture,bearing steel and heavy rail steel fatigue life reducing,etc.Based on it,in order to improve the internal quality of billet for long products,a series of studies were carried out to improve the internal defects of billet for long products in this paper.The solidification structure of billet for long products was studied under static water distribution method.The experimental results showed that:Under different casting speed conditions,the Secondary Dendrite Arm Spacing(SDAS)at the same position of billet was different,which resulted in unstable solidification structure of billet and high fluctuation of centre segregation of billet.The relationship among the SDAS,the temperature gradient G and solidification rate R at different positions in each secondary cooling zone was found out by combining experimental detection and simulation calculation.The SDAS can be characterized by solidification index M·(GR)m.Therefore,a method of secondary cooling water quantity control based on solidification structure control was proposed in this paper.The stable control of SDAS at each position was realized,which ensured the stability of the centre segregation of billet.The standard deviation of the centre segregation index of 180 mm ×180 mm section SWRH82B and 160 mm×160 mm section GCr15 decreased from 0.127 to 0.04 and 0.103 to 0.037 respectively.The most important parameter in the reduction process at the solidification end is the solidification end position.In this paper,low melting point metal(lead)was used as tracer to determine the solidification end position of billet.The time which takes for the tracer to settle to the end of billet penetration(the end of the lead trace)is critical.By establishing the mathematical model of multiphase flow of lead liquid in the core of billet,the settling time of lead liquid in the core of billet was calculated,and the movement length of billet after adding tracer was obtained to correct the length of lead trace.The solidification end position was determined for each typical section and typical steel of billet,so as to correct the solidification end position in the dynamic water distribution and the reduction model.The method laid a foundation for the reduction implementation of billet.In addition,based on this method,the influence of M-EMS and superheat on lead trace length of 180 mm×180 mm section SWRH82B billet was determined for the first time.After applying the M-EMS(350A,4Hz),the lead trace length was shortened by 0.70 m.With the superheat increasing by 20℃,the lead trace length increased by 0.30 m.The internal crack sensitivity caused by reduction of different steel billet was studied.The results showed that high carbon steel billet had weak internal crack sensitivity caused by reduction,while high carbon alloy steel billet(heavy rail steel,bearing steel)had strong internal crack sensitivity caused by reduction.The evolution law of internal cracks caused by reduction in billet was studied by finite element mathematical model of rolling.It was found that the compression ratio of the bearing steel billet was 4.52 and the internal cracks could be healed by rolling.However,the stress variables of the heavy rail steel billet were inconsistent at each point in the rolling process,and the internal cracks couldn’t be completely healed by rolling.In addition,a thermodynamic coupling mathematical model was established for the reduction process of billet,and it was found that when the heavy rail steel billet was reducted 5 mm at the position where the central solid phase ratio was less than 0.40,there was a risk of internal cracks in the heavy rail steel billet.The internal crack caused by reduction of heavy rail steel(U71Mn)was optimized experimentally.The research results showed that the internal crack could be alleviated by using the multi-roll mode and reducing the reduction amount of single roll.The macro segregation model of billet was established,and the centre segregation index of billet was calculated under different sections,types of steel and casting speed.The centre segregation index of 160 mm×250 mm heavy rail steel billet with reduction was predicted,and the optimized reduction range of billet was put forward.It pointed out the direction of the control centre segregation of billet under reduction test.According to the crack sensitivity of different types steel billet,the reduction technology for steel classification was proposed.The experiments of different types steel billets were carried out by using the reduction technology for steel classification.Bearing steel billet adopted multi-roll reduction mode,combined with the characteristics of large rolling compression ratio,allowing a small amount of internal cracks in bearing steel billet,it was easier to improve the centre segregation,and the centre segregation index of billet can reach about 1.06.The heavy rail steel billet adopted multi-roll reduction mode,and reduced the reduction amount of single roll,which can not only effectively improve the centre segregation index of billet,but also avoided the appearance of internal cracks under the reduction,and the centre segregation index could be controlled at about 1.05.The centre segregation index of high carbon steel billet can be controlled at about 1.07 by using single roll or double roll reduction mode. |
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