Technique Optimization Of Convex Roll Reduction At Solidification End For Bloom Continuous Casting Of Steels

Strand reduction at the solidification end of steel continuous casting is an advanced technology to improve the internal quality of the casting product,especially for decreasing the center segregation and porosity of high carbon steels.However,the conventional soft reduction cannot fully eliminate the center segregation and center porosity of the strand,especially for blooms with large section.In recent years,some researchers have been studying heavy reduction technology at the continuous casting solidification end,but no common conclusion has been obtained.This paper takes high carbon steel GCr 15 as the main research steel which usually has serious center defects.The solidification and reduction deformation behaviors of the bloom were calculated through mathematical simulation.The inner quality problems of casting bloom under conventional flat roll reduction were analyzed.Then experimental studies on flat roll reduction after solidification and convex roll reduction at the solidification end were conducted.The influences of convex roll surface profile and reduction parameters on the deformation and internal quality of bloom are analyzed to optimize the convex roll reduction process.Finally,the convex roll reduction technology was applied to other steels production in a special steel plant in China.The following main conclusions are obtained.(1)Compared to the shell of the bloom at the solidification end,the size of mushy zone is small.Accordingly,the reduction force should be large when using flat roll reduction.Both the inner and outer arc side shells become thinner after flat roll reduction,and thus the mushy zone is compressed by the inner arc side,while moving to outer side.However,for convex roll reduction,there is no obvious deformation on the outer arc side,and the mushy zone is compressed due to the deformation of inner side shell,thus the reduction efficiency is improved.The solidification end moves about 0.5m away from mould meniscus if casting speed increases 0.01m/min.Thus,the size of mushy zone will become large for a certain withdraw-straightening unit,and the reduction efficiency will be improved correspondingly.The reduction efficiency of low carbon steels is much lower than that of high carbon steels if the reduction process is applied at the same center solid fraction because of the smaller mushy zone of low carbon steels.(2)The results of flat roll reduction after complete solidification for bloom SA-213 T12 show that the initial size of the shrinkage cavities can be reduced by increasing the reduction rate,and the shrinkage cavities can be further reduced if the bloom is rolled after solidification.The maximum size of shrinkage cavities was decreased by 37.5%when soft reduction rate increases from 0.38mm/min to 0.66mm/min,while the the number of shrinkage cavities almost did not change.The size of the remaining shrinkage cavities is reduced from 8 mm to 2 mm,and the number is reduced by 50%if the bloom was rolled after complete solidification.The simulation results with elastic-plastic model revealed that the effect is 22.5%higher for solidification reduction than hot rolling under the same deformation.However,there is no significant difference in relative size h/d0(the ratio of the shrinkage cavity thickness after reduction to before reduction)for different initial sizes of shrinkage cavities under solidification reduction.To further decrease the shrinkage cavity,it is advisable to increase the reduction rate to reduce the initial size of shrinkage cavities,or to increase the reduction quantity after solidification.(3)The force of flat roll reduction is mainly concentrated in the areas with low temperature on both sides of the bloom,while that of convex roll reduction is concentrated in the center area with high temperature.The deformation zone occurs in the middle high temperature area when using convex roll,and the maximum reduction quantity for GCr15 can reach more than 20mm under single roll reduction.The average reduction force is decreased from 500kN/mm with flat roll to 80kN/mm with convex roll.Meanwhile,the ratio of reduction quantity to broadening quantity increases from 2.0 to 4.4,showing that the reduction efficiency of unsolidified core is significantly improved.The center segregation index of carbon is reduced from 1.49-1.90 to 0.95～1.19 with heavy reduction,and negative segregation may occur.The center shrinkage cavity is also clearly decreased,and its maximum size is remarkably reduced from 4.5mm to 2.8mm.Being different from soft reduction which usually applied at fs=0.3～0.7,the center segregation can also be eliminated even the reduction position delayed to fs=0.8～0.9 with heavy reduction.(4)The shape of the convex roll has a significant effect on the deformation and internal quality of the bloom.The ratio of reduction quantity to broadening quantity with convex roll B is 69.1%higher than that with convex roll A because of narrower reduction region,and the average reduction force is decreased from 85.5kN/mm with roll A to 73kN/mm with roll B.Convex roll A is more beneficial to the elimination of V segregation and the decrease of center porosity,while convex roll B is more beneficial to the decrease of shrinkage cavities and center segregation.The center segregation index of carbon with roll B is reduced from 1.16 to 1.02.(5)The reduction location of convex roll also has obvious influence on the reduction effect.Molten steel is more easily squeezed out when heavy reduction is applied at center solid fraction of fs=0.76.Compared with the heavy reduction at fs=0.88,the center carbon segregation index is reduced from 1.09 to 0.95.The center porosity is reduced from 1.58 to 1.0,and V segregation and center shrinkage cavities are almost completely eliminated.There is little influence on the internal quality of bloom when applying flat roll reduction after convex roll reduction,because it can only cause plastic deformation of the solidified shell and thus reduce the reduction efficiency.(6)The application results of convex roll reduction technology show that excellent metallurgical effects are obtained.The quality of continuous casting bloom is significantly improved.The center segregation index of carbon is reduced to 0.93 for low carbon steel.The V segregation is almost completely eliminated for medium carbon steel.The shrinkage cavities are also eliminated for medium and low carbon steels.The center crack of high alloy steels such as 1Cr13,2Cr13 and H13 is significantly reduced form grade 3.0 to 2.0 or even eliminated.The hardenability of gear steel is improved,and the hardenability bandwidth can be controlled within 3HRC.The qualification rate of ultrasonic detection of large-diameter rolled bars of high alloy steels is increased.The purpose of producing large-size rolling materials with a small compression ratio is achieved.The rolled bars with diameter of 230mm were successfully produced with the compression ratio as low as 4.48.