Study Of The Interfacial Heat Transfer Behavior And Surface Quality Control In Strip Casting Process

Strip casting is a near-net shape manufacturing technology,which has been one of the most promising new-generation steel-making manufacturing technologies in the 21^stcentury,due to its advantages of intelligence,energy saving,and emission reduction.However,strip casting is still not widely used in the steelmaking process.Only a few companies,such as Nucor in America and Sha-steel group in China,have successfully established commercialized strip casting production lines,which could only achieve stable production on basic steel grades.The serious surface quality problem of the productions is the fundamental reason hindering the development of strip casting technology.Moreover,the surface quality of the products is closely related to the interfacial heat transfer behavior,initial microstructure,and special element distribution.Thus,in this dissertation,the effect of multiple factors on interfacial heat transfer behavior between molten steel and cooling substrate,and special elements distribution in sub-rapid solidified samples and its influence on surface quality are studied progressively.Firstly,the droplet solidification technique is used to study the effect of the naturally deposited film on interfacial heat transfer behavior.The results show that,with the deposition of the film,the interfacial heat transfer increases firstly due to a better interfacial contact condition and gets down as the increasing thermal resistance of the naturally deposited film.Furthermore,the increase of Mn and Si contents leads to a faster film deposition rate,which is mainly reflected in the enhancement of particle size and thickness.The naturally deposited film of high Mn and Si samples faster dominates the interfacial thermal resistance and makes the transition point of heat transfer behavior occur earlier.Secondly,the droplet solidification technique is used to study the effect of coating properties（roughness and thickness）on interfacial heat transfer behavior.The results show that,with the increasing coating thickness,the interfacial heat transfer decreases as the raising interfacial heat resistance.While the increasing coating roughness will lead to the enlargement of the gas gap and thus increase the interfacial thermal resistance.Meanwhile,the increasing coating roughness will also improve the profile suitability between the molten sample and the cooling substrate,which increases the real interfacial contact area.The interfacial heat transfer behavior is significantly suppressed and then sightly promoted with the increasing average coating roughness.Then,the high phosphorus content samples and high hydrogen content samples are observed and analyzed respectively.The dip tester and high-temperature bending process are used to simulate the strip casting production process of high phosphorus content samples and high hydrogen content samples.The crack mechanisms and improvement methods of high phosphorus content samples and high hydrogen content samples are detected.For high phosphorus content samples,phosphorus element segregates at the original austenite grain boundary and inter-dendrite positions during the sub-rapid solidification process.The phosphorus segregation will decrease the solidus temperature and delay the solidification at the corresponding position.Thus,a certain amount of unsolidified phase will exist during the high-temperature bending process,which becomes the cracking inducer.The study proposes to improve the interfacial heat transfer by reducing the coating roughness and coating thickness and to reduce the bending force by setting the pinch rollers.For high hydrogen content samples,during the sub-rapid solidification process,the supersaturated hydrogen will escape and form a certain number of pores and depressions inside and on the surface of the steel sample,which reduces the internal bonding force and deteriorates the heat transfer uniformity and finally leads to cracking.The study proposes to extend the degassing time and reduce the moisture introduction from refractory materials and supplementary materials in the steelmaking stage,which could reduce the hydrogen content in the molten steel and thus improve the surface quality.Finally,the dip tester is used to study the improvement of surface quality of high copper content samples through strip casting technology.The results show that the copper element is evenly distributed the in sub-rapid solidified sample without significant segregation.After high-temperature treatment,only a few tiny copper oxide particles are found in the oxide layer of the sub-rapid solidified sample.Thus,the sub-rapid solidified sample has a relatively great high-temperature deformation compared with the standard solidified sample due to the improved copper behavior.The sub-rapid solidified samples are more difficult to severe surface cracks after hot rolling,which indicates that the sub-rapid solidified sample has a better resistance to thermal embrittlement.This dissertation aims to provide the theoretical basis for the sub-rapid solidification process and interfacial heat transfer behavior and to propose a corresponding solution for solving surface quality problems of strip casting products.This dissertation helps to speed up the development of twin-roll strip casting technology in China and promote the commercialization of the strip casting process.Ultimately,it will help China to achieve important national strategic goals and greatly enhance the competitiveness of China’s steel industry.120 figures,23 tables,171 references.