Study On The High Temperature Creep Characteristics Of Continuous Casting Slab And Continuous Bending And Straightening Theory

It plays a significant role of efficient continuous casting technology for steel production efficiency improvement,resource consumption decrease and carbon emissions reduction.As one of the approaches to achieve efficient continuous casting,continuous bending and straightening technology of continuous casting slab greatly affects the quality of steel production.Most layout curves of the available continuous casters only make the slabs deform in the sections of bending and straightening and there is no effective deformation happened during the long basic arc segment,which easily leads to the bending and straightening cracks and is unfavourable for drawing speed improvement,but resulting the bad slab quality and low production efficiency.High temperature creep deformation is usually regarded as harmful deformation to effect the material service performance,which is also obvious and inevitable during continuous casting processing.If creep deformation of the continuous casting slab could be utilized to accomplish the bending and straightening deformation of itself,then the goals of seeking the advantages and avoiding the harms of creep deformation,and the aims of improving productivity and production quality could be achieved ultimately.A low alloy steel is chosen as the research subject,elevated temperature mechanical properties and tensile creep tests are firstly carried out.According to the results of mechanical properties tests,the material has good hot ductility at 1000~oC～1150~oC,then the tensile creep test is conducted at 1050~oC～1200~oC,the stress levels near but below the yield strengths under the test temperatures are set as the creep stresses.Based on the creep tests,the creep curves and steady-state creep rates under different temperature regimes and stress levels are obtained,and the creep constitutive equations are also fitted.Secondly,the influence mechanism of high temperature creep deformation on microstructure is studied by characterizing the original microstructure of the steel,the microstructure after creep deformation,and the microstructure at the same elevated temperature but without creep deformation.Corresponding to different regions in the continuous casting slab,the microstructures under low temperature&high stress and high temperature&low stress after creep fracture are characterized by scanning electron microscopy and transmission electron microscopy.It is found that the low angle boundaries and dislocation density will have a positive effect on high temperature creep resistance.The obstruction effect of dislocation substructure on dislocation movement is also conducive to improve the creep resistance.Furthermore,the method of utilizing the high temperature creep characteristics to accomplish the continuous bending and straightening deformation of the casting slab is brought up on the basis of the relationship between the creep strain rate and the slab strain rate during continuous casting slab bending and straightening processing.When the strain rate of a position inside the solidifying slab is lower than the creep strain rate under the corresponding temperature and stress level below the yield strength,then the slab bending and straightening deformation could be accomplished by its creep deformation.The relationship between continuous casting slab strain rate and the curvature changing rate of the continuous bending and straightening curve is deduced,and the design and calculation method of creep bending and straightening curve is also given.A smoothly connected combined straightening curve by four segments of involutes and a segment of optimized cubic curve is designed based on the existing small continuous casting straightening equipment in the laboratory.The straightening deformation is simulated and verified by the means of finite element simulation and physics experiment,which illustrates the validity of the continuous bending and straightening method by utilizing high temperature creep characteristics.At last,the layout curve of Ansteel R9300 continuous caster is redesigned by utilizing the high temperature creep to accomplish the continuous bending and straightening method.The original basic arc segment is removed,the curve with sinusoidal curvature is smoothly connected as the bending section with the quadratic even polynomial curve as the straightening section to form a new layout curve of whole process continuous bending and straightening.The sinusoidal curvature-quartic continuous bending and straightening curve significantly prolongs the length of bending section and straightening section,so as to decrease the strain rate of continuous casting slab.By the comparison with steady-state creep rate,it is found that the bending and straightening deformation of the continuous casting slab could be totally completed by creep deformation in the good hot ductility area;furthermore,most creep deformation could also be finished by bending and straightening deformation in the area of bad hot ductility,the possibility of bending and straightening cracks could be reduced,then the quality of casting slab could be improved.In this paper,the bending and straightening deformation of continuous casting slab is completed by utilizing the high temperature creep characteristics,and the design method of whole process continuous bending and straightening model is proposed,which both lay a foundation for effectively avoiding the bending and straightening cracks in continuous casting slab and provides a reference for the design of new continuous caster.