| Aiming at realizing the endless rolling of billet to promote the well matching of production capacity between continuous casting and rolling,or increasing the casting speed to reduce costs simply,the ultra-high casting-speed continuous casting(UHCSCC)of billet is imperative.However,the large increase of casting speed will make thickness of solidified shell too thin at the mold exit,which could cause raising the potential risk of breakouts.As a result,there is limit in the further improvement of casting speed as well.Therefore,the technical core of billet UHCSCC is the high-efficiency heat transfer in the mold region.In addition,the flow features of molten steel in the mold will also change at ultra-high casting speed obviously,which has an impact on the high-efficiency heat transfer,slag entrapment and uniformity of shell thickness.Based on this,the systematic research on flow of molten steel,high-efficiency heat transfer between molten steel and mold as well as its cooling structure,high-temperature stress and strain of the strand and copper tubes,and inner cavity structures in the mold region at ultra-high casting speed was conducted in this paper,which can provide the necessary foundation of theory and technique to achieve the UHCSCC of billet.Therefore,this work studied about the flow behavior of molten steel in billet UHCSCC mold,and the flow features of molten steel under high flux were clarified.The influence of parameters such as copper tube’s thickness,length,fillet radius,water gap’s width and cooling water velocity on the high-efficiency heat transfer was investigated as well in the UHCSCC billet mold.Besides that,high-efficiency heat transfer behavior of five types of water-cooling structure molds with forced flow of cooling water(namely,water slot,water draft,combined water tube and longitudinal water slot,combined water tube and transverse water slot,combined water tube and water draft)and two-stage mold(water tube mold and spraying mold)under the ultra-high casting speed was illustrated in this paper.The high-temperature mechanical behavior of the strand of two types of steel grade(hypo-peritectic steel and medium carbon steel)in the UHCSCC mold was demonstrated,then it could fugure out that the thermal deformation law of copper tube along the casting direction and circumferential direction.Additionally,the inner cavity taper of two kinds of billet UHCSCC mold without air gap was determined based on shrinkage of the strand and thermal deformation of copper tube.The main results in this paper are shown as follows:(1)The mathematical model of molten steel’s flow and heat transfer in the mold at UHCSCC was established to study simulation and calculation method of fluid flow as well as heat transfer in the mold coupled with different computational domains based on accuracies.Considering with solidification phemomenon,a hydraulic simulation platform of billet continuous casting was adopted to investigate fluid flow in the mold through physical simulation at ultra-high casting speed.Then numerical simulation results were verified and analyzed comprehensively.It is found that the multiphase mathematical model of fluid flow and heat transfer coupled with molten steel-liquid slag-air can effectively solve the problem of obvious overflow of liquid slag under ultra-high casting speed and has the highest accuracy.The flow features of molten steel in the mold at ultra-high casting speed,and differences between it and the conventional high-efficiency continuous casting had been clarified.It could be determined that appropriate parameters of submerged entry nozzle(SEN)(180mm in immersion depth and 50 mm in inner diameter under casting speeds of 5.0～6.5m/min)under high flux.After that,influences of the properties of mold flux on the flow behavior of molten steel were figured out.Meanwhile,results also show that compared with 3.0 m/min in casting speed,the impacting depth of molten steel,the maximum surface velocity and the maximum amplitude of level fluctuation in the mold at the ultra-high casting speed of 6.5 m/min change significantly,which increase by 33.88%,58.42% and 136% respectively.In order to ensure the relatively stable liquid level in the UHCSCC mold,it is recommended that the ranges of three indicators are about790～810 mm,0.0391～0.0461 m/s and-1.13～1.75 mm respectively.The liquid slag’s thickness has the most significant effect on level fluctuation,and followed by viscosity.High viscosity mold powder should be used for the billet UHCSCC under ensuring its lubrication effect.(2)A mathematical model of fluid flow and heat transfer consisting of molten steel,mold flux film,copper tube and cooling water was established,and its accuracy was verified.Based on this model,the difference of heat transfer between ultra-high casting speed and conventional casting speed in the water tube mold was clarified,and influences of parameters such as copper tube’s length,fillet radius etc.on high-efficiency heat transfer in UHCSCC mold region was revealed.It is found that the shell thickness at the mold exit decreased by 50.66% and the heat flux increased by11.49% on average at 6.5 m/min in ultra-high casting speed compared with 3.0 m/min.The mold length affects its high-efficiency heat transfer greatly.The total heat and the shell thickness of mold exit increase by 19% and 9.21% on average with every 100 mm extension.While influences of copper tube’s thickness,cooling water velocity and water gap’s width are decrease gradually.On the basis of the requirement of circumferential uniformity of shell thickness,the appropriate fillet radius is about 10 mm.The maximum casting speed that the mold with a length of 1000 mm(clearance section of 100 mm)can reach is about 7.57 m/min.(3)The heat transfer behavior of mold region under five kinds of water cooling structures based on forced flow of cooling water at the ultra-high casting speed was studied via using the established mathematical model of fluid flow and heat transfer.An idea proposed in this work is adopting high-efficiency heat transfer and cooling’s two-stage mold(water tube mold+spraying mold)in UHCSCC.Then shell thickness at the exit and maximum casting speed of molds with different cooling structures could be calculated.The results show that the heat transfer capacity of the mold combined water tube and water draft is the best among several water cooling structures based on the forced flow of cooling water.Its total heat increases by 5.58%,and the shell thickness at the mold exit and the maximum casting speed could reach 6.38 mm and 8.08 m/min.Based on the water tube mold with a length of 1000 mm,the shell thickness at the mold exit increases from 7.41 mm to 10.32 mm and the maximum casting speed raises from11.15 m/min to 13.68 m/min when the length of spraying mold is extended from 20% to60%.The integrated mold has obvious effect on further improving the casting speed and achieving the UHCSCC of billet.(4)Based on the study of fluid flow and heat transfer model,the high-temperature stress-strain model of the strand in the mold at the ultra-high casting speed was set up to investigate influences of casting speed and superheat on the high-temperature stress,strain and shrinkage of the strand of two typical types of steel grades,hypo-peritectic steel and medium carbon steel.With the increase of casting speed,there is a decreased trend in stress and strain of the strand,the closer to the mold exit,the more obvious the trend is.Compared with 3.0 m/min in casting speed,the maximum stress and equivalent strain of the strand at the mold exit under the ultra-high casting speed of 6.5 m/min decrease by 63.01% and 18.58% respectively.As the casting speed goes up,the crack possibility of the strand in the mold is reduced.Under the ultra-high casting speed,the maximum shrinkage of the strand corner is 41.84% lower than that at the conventional casting speed.At the same casting speed,the shrinkage of the strand corner is about three times than that in the symmetrical center.According to comparison with hypo-peritectic steel,the maximum stress of the medium carbon steel strand at the casting speed of 6.5 m/min is 17.78% higher,and the strain and shrinkage are 41.88%and 17.07% lower.It is more sensitive for hypo-peritectic steel for the change of csting speed.The superheat of molten steel has less effect on the high-temperature mechanical behavior of the strand.(5)The high-temperature stress-strain model of the billet mold’s copper tube was built to reveal that the thermal deformation law of mold copper tube of hypo-peritectic steel and medium carbon steel without air gap at the ultra-high casting speed.Results demonstrate that the thermal deformation of copper tube increases firstly then decreases,and rises again near the exit along the casting direction.Besides that,the thermal deformation of copper tube’s corner is the largest in the circumferential direction.The maximum thermal deformation of mold copper tube of hypo-peritectic steel at 6.5m/min is 0.267 mm,which is 0.009 mm lower than that at 3.0 m/min and 0.015 mm higher than that of medium carbon steel at 6.5 m/min.If there is no air gap,the thermal deformation of copper tube is less affected by the change of casting speed and the difference of steel grades.(6)Based on the superposition of the data of the strand’s shrinkage and copper tube’s thermal deformation,the change rule of the continuous casting mold inner cavity’s surface shape of two typical steel grades without air gap under different casting speeds and other process parameters was studied to resolve tapers of two mold types of molds under the ultra-high casting speed(on the basis of 6.5 m/min in casting speed),and the reasonability of mold taper was verified.In order to facilitate processing,several characteristic points were selected along the mold circumference,and the change curve of each characteristic point’s taper on the casting direction was obtained.It could find out that the casting speed has a great influence on the structure of the inner cavity whose shrinkage will decrease with the increase of casting speed,while superheat has little effect on the shape of the inner cavity.The taper of hypo-peritectic steel mold is larger than that of medium carbon steel’s as on the casting speed,and the biggest difference between the both occurs at the mold exit(1000 mm long),which is 30.77%.The maximum taper of hypo-peritectic steel’s and medium carbon steel’s mold is about25 mm below the meniscus of molten steel,which is 3.71%/m and 3.55%/m respectively.There is a significant difference in mold taper between the ultra-high casting speed(6.5 m/min)and the conventional casting speed(3.0 m/min).The maximum difference ratios of hypo-peritectic steel and medium carbon steel are 52.48%and 28.57% respectively.In summary,research results prove that for ensuring the high-efficiency heat transfer in the mold at ultra-high casting speed and reducing the generation of air gap as much as possible,two kinds of taper’s molds of hypo-peritectic steel and medium carbon steel are recommended for the billet UHCSCC.The mold taper determined at the ultra-high casting speed is not suitable for the conventional high-efficiency continuous casting,or the ultra-high casting-speed billet caster is not appropriate for long-term operation at the low casting speed. |
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