Numerical Simulation Of Continuous Casting Process And Solidification Structure Under External Field Of 9Ni Steel

9Ni steel is widely used in liquefied natural gas(LNG)storage tank and transportation ship,because of its high strength,excellent low temperature toughness and good welding performance.However,some quality problems are usually caused by the high viscosity of liquid steel and the strong sensitivity of slab crack.It can be avoided by controlling the cooling of continuous casting process,with cooled uniformly and effectively.The numerical simulation of continuous casting process can effectively demonstrate the variation of temperature and the evolution of thermal stress of the casting system.It can provide reliable basis for analyzing the causes of defects,to realize the process optimization,and to improve the performance of slab.Some 9Ni steels,which were produced by the vertical-arc slab caster in a certain steel plant,appeared crack and macro segregation.It proved that the defects were caused in the continuous casting process.In order to improve the quality of continuous casting billet,the numerical simulation were used to calculate the temperature field and the distribution of stress field in the continuous casting process,analyzing the causes of quality defects,and providing theory support to improve the continuous casting process.The results showed that for the 150mm×3000mm slab,the casting speed is 1.05m/min,the slab shell thickness at the crystallizer exit is about 12mm,and the local temperature inequality was presenting in the second cooling zone.Along the casting direction 5.72m,the center temperature of slab reached to the liquids temperature.Along the casting direction of 7.86m,slab was completely solidified.The thermal stress analysis results showed that there was a great stress concentration area at the corner of 1/2 casting direction and the position of 1/4 plate width,especially in the 1/4 plate width there was a large shear stress as well,both the two positions trending to produce cracks.Meanwhile the temperature was low.Therefore,it is recommended that the cooling between adjacent sectors takes a gentle transition mode,and take thermal insulation for edge,to reduce the possible growth of cracks.The casting model of continuous casting process is simulated in this paper.By applying static magnetic field,pulse current and electric-magnet compound field on the process of solidification of 9Ni steel,and under simulate conditions of continuous casting,the effect of external field on solidification structure is analyzed.The results show that without the action of no external field and single static magnetic field,the solidified structure of the ingot has a well developed dendritic structure.Under the action of single pulse current,the dendritic crystal has limited crushing and a oblique direction.And with the pulse frequency increases,the degree of crushing reduced.The dendrites were broken to a large extent under the condition of electromagnetic vibration(static magnetic field+pulsed current),and the equiaxed fairly of the dendritic crystal became obvious.With the increase of frequency or the decrease of duty ratio,the improving effect of solidification structure decreased.When the frequency of static magnetic field is 20Hz and the duty ratio is 15%,the solidification structure shows the best performance.