Study On The Evolution Of Solidification Shrinkage Porosity At Continuous Casting Bloom During Heavy Reduction And Rolling Process

Central segregation,shrinkage and porosity are one of the main internal quality defects of continuous casting blooms,which seriously restricts the high-efficiency and stable preparation of large-size rolled products under high-quality continuous casting billet and low rolling compression ratio.At present,the solidification end heavy reduction technology is one of the important technical means to significantly improve the above internal quality defects,which makes full use of the temperature difference of the outer cold end heat of the solidified end of the casting stream.By applying large mechanical compression deformation in the direction of slab thickness,the effective transfer of the compression amount to the center of the casting slab could be realized,which significantly improves the central segregation,shrinkage and porosity defects of the casting slab.The solidification end heavy reduction technology lays a foundation for the production of high-quality large-size rolled materials under the condition of low rolling compression ratio.In this paper,the 320 mm×425 mm of GCr15 and 42CrMo steel blooms produced by a 7#bloom caster in a domestic steel mill are studied.Combined with numerical calculation and field test,the roll shape design,the billet under different casting and rolling conditions under the heavy reduction,and the evolution of shrinkage cavities under the heavy reduction are systematic studied,on and subsequent rolling processes under casting and rolling conditions,which provides theoretical guidance for equipment and process design and optimization of continuous casting blooms caster under heavy reduction.The content of this paper and the results obtained are as followed:(1)The 320 mm × 425 mm section bloom is designed for continuous casting with a gradient curvature convex roll type structural parameters:the length of flat roll is 130 mm,height of projection is 25 mm,the gradient curvature length is 85 mm;(2)In the process of continuous casting bloom under heavy reduction,the rolling type,steel type,single roll and multi-roll pressure have an effect on the welding shrinkage rate of the center shrinkage hole of the slab.When the reduction amount is 20 mm,shrinkage welding ratio of the convex roller is higher than that of the conventional flat roller by 2.0%,and the pressure required for the convex roller is 43.4%lower than that of the conventional flat roller.Shrinkage welding ratio of 42CrMo is higher that of GCr15 by 1.4%,and shrinkage welding ratio of single roll reduction is 3.05%higher than that of multi-roller pressure.(3)Based on the designed gradient curvature convex roll applied in heavy reduction,the shrinkage and porosity defects in the center of the bloom are significantly improved.The maximum value of the center segregation index of the GCr15 steel bloom center is reduced from 1.09 to 1.05,and the center looseness width is reduced from 110 to 85 mm.The maximum value of the center segregation index of 42CrMo steel is reduced from 1.17 to 1.10,and the central looseness width is reduced from 75 to 50 mm.(4)During the rolling process,the central spherical shrinkage cavity of the slab gradually evolves into an ellipsoidal shape.The central shrinkage hole has a large deformation amount in the pressing direction and has a certain widening trend.(5)During the rolling process,the depth of the inner arc side groove of the concave blank after the heavy pressure of the convex roller is not significantly increased,and the groove is basically eliminated after the third pass rolling.After 9 passes rolling,the change rate of the cross-sectional area of the concave billet is 65.55%,which is 6.2%higher than that of the conventional cast slab.