ECCC-51板坯中心偏析的预测

1 1 2 3 3
Óbuda University, Banki Donat Faculty of Mechanical and Security Engineering, Department of Material Science and Technology, Nepszinhaz u. 8. Budapest 1081 Hungary 2 Bay Zoltan Institute for Material Science and Technology, Fehervari ut 130, Budapest, 1116, Hungary 3 ISD Dunaferr Co. Ltd. Vasmű tér 1-3, Dunaujváros, 2400, Hungary Abstract: In the light of experiences and of literature it can be stated that solidification shrinkage porosity accompanying crystallization also contributes to the formation of centerline segregation. In many cases, micropores formed during the crystallization process can be observed in the centerline of the hot rolled products too. From this aspect, the size of the cluster of the shrinkage pores formed during the solidification of the slab and the conditions under which such porosity is developed are of primary importance. This is discussed, on the basis of a comprehensive analysis of statistical data. An important conclusion drawn from this analysis is that after a certain solid/liquid ratio has been reached in the section of the strand which only consists of mushy, practically no more liquid will be supplied. A mathematical model was developed to estimate the flow rate and direction and of the expected porosity in the center part of the slab. Calculations show that centerline segregation is basically affected, at a given composition and cooling technology by the setting, deformation and eccentricity of the supporting rolls. Bulging of the strand between the supporting rolls can also play role. Key words: slab casting, centerline segregation, macrosegregation, porosity, mushy, modeling
Düsseldorf, 27 June – 1 July 2011
Control of solidification structures and management of defects
Session 12
1
Prediction of Centerline Segregation of CC Slabs
Mihaly REGER , Balazs VERO , Zsolt CSEPELI , Robert JOZSA
2. Introduction of the LMI (Liquid Motion Intensity) model
The main conclusion of an extended statistical analysis performed earlier [4-5] is that the changes in the level of centerline segregation cannot be explained by the primary measurements and technological parameters of casting or by the direct data obtained from the heat transfer modeling of continuously cast strand not even under steady-state casting conditions. Not even the technological data contain those data types which can directly be linked to the value of centerline segregation. It seems evident that we have to take into account the fluid flow inside the mushy accompanying solidification and the parameters which describe the liquid supply compensating for solidification shrinkage. The problem is that in a real industrial case it is very difficult to monitor the fluid flow in the mushy zone taking into account the deformation of strand due to ferrostatic pressure and due to the actual values of roll gaps. The core idea of the LMI
பைடு நூலகம்
part of the slab will have a different chemical composition compared to the average composition and/or it will contain shrinkage holes. According to the industrial and laboratory experiences and to theoretical considerations [1-3], the segregation level (including the carbon content) can hardly be decreased by heat treatment. It follows from the above mentioned statements that the only way to reduce the centerline segregation level is a better control of solidification process during casting.
1. Introduction
The slab continuous casting technology is aimed at producing cast product with a proper chemical composition, geometry and surface quality, without any external or internal defects. One of the most unpredictable defects of the slabs is centerline segregation, which has a negative effect on further processing of the slabs and on the possible uses of the final products. Since the full complexity and the most common problem areas of continuous casting are typically present in the formation of centerline segregation, in this paper we are focusing on the formation of this defect and on research investigating possible ways of how to reduce such defects. The centerline segregation in slabs develops in a complex way; it is connected partly to the macrosegregation and partly to the shrinkage of solidifying melt which are influenced by a lot of technological features. According to literature data and to our results, the shrinkage porosity and deformation of slab in the last stage of solidification play the main role in the formation of centerline segregation. The volume change due to solidification and the deformation of slab shell influence the liquid movement in the mushy zone as well. Among the casting parameters the supporting roll settings in the mushy area is the most important influencing factor concerning the formation of centerline segregation, but all of the casting parameters can affect this process (chemical composition, superheat, casting rate, secondary cooling system, rigidity of the casting machine, roll bearings, etc.) to some extent. As a result of these processes the centerline segregated