Research On Soft Reduction Theory And Internal Quality Improvement For Continuous Casting Slab

Soft reduction at the solidification end is an effective means to improve internal quality defects such as segregation and porosity of the continuously cast strand,and has become an essential process for high quality steel.However,in the practical application of a steel company in China,there are still problems such as unreasonable reduction position,reduction effect,and incomplete improvement of center defects.In addition,unreasonable reduction position and reduction amount can deteriorate the internal quality of the strand,even damage the equipment,reducing the product yield and causing the economic damage to the plant.Therefore,accurate control of the reduction parameters,and their reasonable matching with the pouring parameters are the key to ensure the reduction effect.The present paper focused on the soft reduction process of high-strength alloy steel slabs,carried out detailed investigations to design and optimize the key technical parameters such as reduction position and reduction amount.The reduction position is determined by the solid fraction in the slab center.Therefore,it is necessary to develop a solidification heat transfer model which meets the actual conditions of continuous casting production and can predict the solidification process of the slab accurately.The amount of reduction should fully compensate the solidification shrinkage of the mushy zone,and avoid inducing the intergranular crack of the slab solidification front due to the excessive reduction.So the reasonable reduction and its implementation effect should be determined in consideration of slab deformation behavior in the mushy zone.For these respects,the 230-250 mm × 1650-1950 mm slab continuous casting machine in this steel plant is concerned and the high-strength pipeline steel is used as the objective in this paper.Firstly,the high-temperature thermal/mechanical parameters of the high-strength alloy steel and the heat transfer boundary conditions corresponding to the casting process were carried out.Then,a three-dimensional solidification shrinkage model and a thermo-mechanical soft reduction model for the continuously cast slab were established to analyze the reduction position,reduction amount,reduction effect and optimize the key process parameters.The three-segment-reduction and two-segment-reduction schemes were carried out under the same reduction amount condition.On this basis,industrial tests were carried out to investigate the effectiveness and feasibility of the optimization scheme.(1)Experimental tests for High temperature thermal and mechanical parameters and inverse computation for heat transfer coefficientsGleeble 3500 thermal simulator was used to determine the high temperature thermoplastic curves of two steel grades,X80(C:0.045wt%)and KAH32H(C:0.105 wt%),through a series of tensile tests at different test temperatures and deformation conditions.It is clear that the third brittle temperature zones of X80 and KAH32H are 750-880 and 780-890?,respectively.The micro segregation model was established to accurately reveal the solidification process of X80 and KAH32H.So,their zero strength temperature(ZST),zero ductility temperature(ZDT)and the thermal property parameters in the high temperature zone were determined.The ZST and ZDT of X80 are 1500 and 1468?,respectively.And,the ZST and ZDT of KAH32H are 1513 and 1497?,respectively.The physical parameters of these two steels exhibit same patterns in solid and liquid phases,however due to the difference of C content,differ in the mushy zone.In this paper,surface temperatures of pipeline steel slab at different cooling zones were measured by FLIR infrared thermal imager.Using the measured temperature as the target temperature,the differential method and adaptive genetic algorithm were employed to establish the inverse-calculation model for the slab solidification heat transfer to obtain the slab heat transfer coefficient of different cooling zones,and modify the relationship between the spraying water flux W density and the heat transfer coefficient h of each zone.The coefficient A in h=A*W0.55(1-0.007TW)was determined.Under the test conditions,the heat transfer coefficient decreases along the casing direction due to the increase of the solidified shell thickness.In addition,the surface temperature were not in the third brittle temperature zone,the comer temperature was low,but the temperature difference of the same surface temperature is<150?,which confirms with the temperature drop rule.(2)Study on solidification and shrinkage behaviors of high strength pipeline steel and influence mechanisms of reduction zoneStarting from the solidification mechanism of slab,based on the thermal-elastic-plasticity theory,the three-dimensional thermo-mechanical finite element model of continuously cast slab was established with MARC finite element commercial software.With the heat transfer coefficient,high temperature thermal physical and mechanical parameters obtained at last step,the solidification characteristics and heat shrinkage behavior of the cast slab during continuous casting were studied.The influences of key process parameters such as casting speed and superheat on the reduction and solidification shrinkage of the reduction zone were revealed.The research results show that the length of the reduction zone increases by 0.26m for each casting speed increment of 0.1m/min;the length of the reduction zone increases by 0.2 m for each slab thickness increment of 10 mm.When the superheat changes from 10? to 30?,the length of reduction zone is basically the same.Before the complete solidification,the change of superheat and slab thickness has little effect on the amount of heat shrinkage.After the complete solidification,the amount of heat shrinkage decreases as the superheat increases.In addition,the amount of heat shrinkage decreases with increasing the slab thickness and the casting speed.The casting speed increases by each 0.1m/min,and the average shrinkage of the wide-face center is reduced by 0.03mm.Compared with superheat and slab thickness,the casting speed is the main factor affecting the reduction position,length of reduction zone and solidification shrinkage.(3)Analysis of the slab deformation behavior and reduction parameters based on thermo-mechanical coupling simulationIn this paper,a three-dimensional thermo-mechanical coupled finite element model for high-pressure pipeline steel was established to analyze the stress-strain behavior of the slab in mushy zone during the reduction process,to reveal the influence mechanisms of the slab size and continuous casting condition on the reduction rate,reduction velocity,and reduction efficiency.The results show that during the implementation of the reduction,the largest total strain is on the surface of the slab,not the solidification front.At the same casting speed,when the reduction amount is 4.2,4.6 and 4.8 mm,the reduction velocity and the reduction rate decrease as the reduction amount decreases.When the reduction amount is less than 3 mm,the reduction efficiency is more influenced by the reduction amount.As the casting speed is 1.2 m/min,and the reduction efficiency tends to be stable after the reduction amount is increased to 4.5 mm.Under the same reduction amount,with the casing speed increasing,the reduction gradient is decreasing,but the reduction rate does not change,the reduction completion time is shortened.At the same casting speed and reduction amount,as the thickness increasing,the reduction rate and velocity are decreasing,and the reduction completing time is prolonged.Based on the above analysis,combined with the analysis of the solidification frontier strain during reduction,it is concluded that the thickness of the high-strength pipeline steel slab is 230 mm,the width is 1650 mm,and the casting speed is 1.2 m/min,the reduction should be less than 5 mm.(5)Industrial experiment and evaluation of new reduction modeBased on the calculation and analysis of the reduction parameters above,compared with the original roll gap table,the main reason for the poor quality of the segregation of the continuous casting slab is that the reduction position is forward.So optimization scheme I for the three-segment reduction mode and optimization scheme ? for the two-segment mode are proposed with the reduction position moved back.In scheme ?,"8-10 segment" reduction mode is changed from the original "7?9 segment" reduction.The scheme II is "9?10 segment"reduction,a two-stage reduction mode.Industrial tests were carried out for different steel grades and different casting speeds,and the reduction effect of the original scheme and the new schemes.Through the tests of pipeline steel and ship plate steel,the quality of segregation of the reduction position is improved compared with the original scheme,and the two-stage reduction mode of scheme ? can significantly improve the segregation quality.The quality level of the slab is improved from the original B1.0.to B0.5,and can effectively reduce the center porosity.Compared with the two-segment reduction mode at deferent casting speeds and reduction amount,the center quality of the slab at the casting speed of 1.2 m/min and the reduction amount of 4.5 mm,is the best.Finally,the reduction should starts at the solid fraction of 0.3 and end at the solid fraction of 0.8 for the high-strength pipeline steel and ship plate steel.And,the optimum casting speed is 1.2 m/min,and the reduction amount is 4.5 mm,and reduction position is "9?10 segment".The continuous production statistics show that the slab with a quality level of B0.5 has approximately reached 90%.The optimized reduction mode effectively solves the problem of poor inner quality of high-strength alloy steel in the plant.