Investigations On Mold Corner Structures And Strand Friction And Stress Behaviors In Slab Continuous Casting Mold

Transverse corner crack is one of the typical problems in conventional continuous casting,especially for microalloyed steels.The chamfered mold can improve the heat transfer and stress around the strand corner effectively,reducing the corner crack initiation in the mold and the corner crack propagation at the bending and straightening segments in the secondary cooling zone(SCZ).The change of the mold corner structures not only changes the fluid flow and heat transfer behaviors of the molten steel directly around the strand corner in the mold,but also affects the variation rule of the friction stress between the strand surface and the hot-face of copper plates,as well as the stress distribution features around the strand corner.The chamfered structures also have genetic effect on the evolution of the temperature and stress around the strand corner in the SCZ.Therefore,the research on the fluid flow and heat transfer of the molten steel in the mold,the friction between the strand surface and the hot-face of copper plates,and the stress behaviors of the strand in the mold and SCZ,can optimize the parameters of the mold corner structures,then lay the theoretical and technical foundation for reducing the transverse corner cracks significantly.Four types of slab molds with different corner structures,including the right-angle,big-chamfered,multi-chamfered,and fillet molds,are investigated in this thesis.Firstly,the fluid flow,heat transfer,and inclusion motion behaviors in the mold with different corner structures are studied by numerical simulation.Then the friction coefficients of mold slag at different temperatures are measured experimentally.Based on the strand temperature field and friction coefficients of mold slag,the change rule of the friction stress in the mold with different corner structures is revealed through the research.Based on the distributions of the strand temperature and friction stress,the effect of mold corner structures and oscillation marks on the stress distribution around the strand corner is discussed.Finally,according to the temperature field at the mold exit,the genetic rule of the mold corner structures on the evolution of temperature and stress around the strand corner in the SCZ is analyzed by mathematical modeling.Thus the effect mechanism of the mold corner structures on the transverse corner cracks can be illustrated through the above research.The main results are as follows:(1)A three-dimensional(3D)fluid flow and heat transfer slab-mold model coupled with the mold oscillation is employed to investigate the effect of the four mold corner structures on the fluid flow and heat transfer of the molten steel and the inclusion motion behaviors.Results show that the chamfered structures can increase the flow velocity of the molten steel around the strand corner significiantly,which is helpful to increase the corner temperature.However,this will lead to the result that the number of inclusion particles in the corner regions of the big-chamfered,multi-chamfered,and fillet mold is 20.7%,19.7%,and 32.4%more than that in the right-angle mold.The corner temperature of the big-chamfered,multi-chamfered,and fillet strands at the mold exit is increased from 1261.2 K to 1412.7 K,1420.8 K,and 1450.7 K compared with that of the right-angle strand.The temperature fluctuation of the meniscus resulted from the mold oscillation reaches 4.6 K.The temperature in the trough with the non-sinusoid waveform is slightly higher than that with the sinusoid waveform,which is helpful to reduce the depth of oscillation marks.The chamfered structures will not result in the too thin shell thickness around the corner region.The variation of the shell thickness around the fillet-strand corner is the most uniform,which is beneficial to reducing the stress concentration.(2)The friction coefficients of two different types of mold slags(crystalline slag and glassy slag)at different temperatures are measured experimentally,and the equivalent friction coefficient of the liquid slag is measured by using an edible oil,laying the foundation for studying the friction behavior around the mold corner.It has been discovered that the surface softening of slag during rising temperature will increase the friction coefficient,while the surface sintering at high temperature will decrease the friction coefficient significantly.From 25? to 600? then to 800?,the friction coefficient of the crystalline slag increases from 0.98 to 1.52,then decreases to0.75 with the increasing temperature.As for the glassy slag,the friction coefficient at25?,200?,and 400? is 0.82,1.14,and 1.08,respectively.The equivalent friction coefficient of the liquid slag is around 0.17.(3)A friction model with the mold oscillation is built to study the effect rule of the mold corner structures on the variation of friction stress on the surface of the stand and copper plates.The chamfered mold is capable of effectively reducing the area of the air gap.On the strand surface,the friction stress on the wide face increases exponentially from 200 Pa to ~0.45×10~6 Pa along the casting direction,while that on the narrow face is lower than 5400 Pa.Around the air gap,the very thin liquid slag layer leads to the friction stress peak of ~4×10~6 Pa.The friction stress at the corner of the right-angle,big-chamfered,multi-chamfered,and fillet strands decreases in that order.The friction stress distribution around the strand corner in the chamfered molds is more uniform than that in the right-angle mold.On the hot-face of copper plates,the mold friction stress on the wide face reaches a maximum of 22396 Pa at the mold exit,while the mold friction stress distribution on the narrow face is similar to that of the strand.The chamfered strands around the corner is in closer contact with the copper plates than the right-angle strand.At the strand corner,the maximum tensile stress and compressive stress under the non-sinusoidal oscillation is 22.7%lower and 42.3%higher than that under the sinusoidal oscillation,respectively,which is helpful to reduce the formation of transverse corner cracks.(4)A 3D thermo-elastoplastic mold model coupled with the nonuniform friction stress and a local stress model including the oscillation marks are proposed to illustrate the effect rule of the mold corner structures on the stress distribution of the strand.The thermal stress plays a dominant role in the shell stress.The stress concentrates in the corner edge and the region that is 10 mm?60 mm from the corner edge.The stress peak of the right-angle,big-chamfered,multi-chamfered,and fillet strands is 26.78 MPa,24.43 MPa,24.13 MPa,and 23.51 MPa,respectively.The friction stress increases the corner stress around the air gap,leading to the result that the positions of the stress peak are the same as those of the friction stress peak.The chamfered structures decrease the stress gradient around the strand corner.The fillet mold is the most effective.The maximum stress in the troughs of oscillation marks of the four types of strands is 24.5MPa,22.7 MPa,21.0 MPa,and 20.2 MPa,respectively.(5)A two-dimensional(2D)slice heat transfer model of the strand transverse section is built to analyze the genetic effect of the mold corner structures on the temperature variation around the strand corner.In the whole SCZ,the corner temperatures of the three types of chamfered strands are 80K higher than that of the right-angle strand on average.At the straightening segment,the corner temperatures of the chamfered strands are more than 100K higher than that of the right-angle strand,all avoiding the low-ductility zone effectively.The corner temperatures of the multi-chamfered and fillet strands are 20 K higher than that of the big-chamfered strand and the temperature gradient around the strand corner is smaller,helping reduce the thermal stress concentration.(6)A 3D thermo-elastoplastic model coupling the strand and rollers at the bending and straightening segments is established to illustrate the genetic rule of the mold corner structures on the stress evolution around the strand corner and the formation tendency of transverse corner cracks in the SCZ.During the bending process,the maximum stress around the strand corner increases from 24 MPa to 43 MPa.In the outer arc,the stress peak of the big-chamfered,multi-chamfered,and fillet strands is 10.5%,20.6%,and 22.4% smaller than that of the right-angle strand on average.During the straightening process,the maximum stress around the strand corner increases from 48 MPa to 78 MPa.In the inner arc,the stress peak of the big-chamfered,multi-chamfered,and fillet strands is 0.1%,2.9%,and 2.2% smaller than that of the right-angle strand on average.The chamfered structures can reduce but cannot eliminate the stress concentration around the strand corner.And the reduction effect is enhanced with the increase of the included angle.