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

During the continuous casting process of the bloom,centre shrinkage cavity is one of the main internal defects that affect the mechanical properties of materials,heavy reduction will make a significance in welding the shrinkage cavity of the bloom.Based on the continuous casting bloom of 360 mm×450 mm and 320 mm×410 mm practical production in a domestic steel plant,the 45#steel and U78CrV steel bloom are studied respectively.The three-dimensional thermo mechanical flow and thermal mechanical coupled models were established by numerical simulation.Firstly,according to the formation of the centre shrinkage porosity of the bloom,the shrinkage radius and the optimal corrected Niyama critical value were calculated as well as the different casting speed and different superheat degree on the shrinkage cavity formation rule were studied.During the bloom reduction process,analyzed the variations of bloom temperature and the central porosity welding behavior under multi-roll reduction.Single-roll reduction under different reduction amount and different reduction position for bloom welding shrinkage porosity evolution.The main conclusions are as follows:(1)During the process of bloom solidification,the shrinkage porosity radius of 45#steel is about 1.58 mm,and Ny*=45 is the critical value to form central shrinkage,while the shrinkage cavity radius of U78CrV steel is about 1.1 mm,and the critical value of central shrinkage is Ny*=35.(2)Whether 45#steel or U78CrV steel,with the increase of casting speed and molten steel superheat,the trend of central shrinkage cavity of bloom has also increased,but the influence of casting speed on shrinkage cavity is more obvious.(3)The temperature of 45#bloom corner is faster than that of other locations.The wide center temperature of bloom drops rapidly in the mold and drops slowly in the secondary cooling zone and air cooling zone,temperature appears to rise in the junctures of these zones.The center temperature of bloom drops rapidly after the solidification latent heat is fully released.(4)In the process of multi-roll heavy reduction,whether it is 45#steel or U78CrV steel,with the increase of reduction amount,the center shrinkage cavity gradually from the initial round into oval,then further along the long axis of the oval compression for irregular geometric shape,finally center shrinkage cavity gradually be welded into a seam under the convex roll heavy reduction.The welding efficiency of U78CrV steel is higher than that of 45#steel,the former reaches 98.5%and the latter reaches 83.21%.(5)In the process of single-roll heavy reduction,in terms of reduction amount,with the increase of the reduction amount of straightening machine,bloom center shrinkage porosity gradually under the trend.As far as the reduction position is concerned,6#straightening machine of center porosity welding capability is stronger than that of 7#straightening machine.In terms of steel,the welding efficiency of U78CrV steel is higher than that of 45 steel.At the position of the 6#withdrawal straightener,the closing rate of the shrinkage cavity of U78CrV steel is 98%,and the closing rate of 45#steel shrinkage is 81%.At the position of the 7#withdrawal straightener,the closing rate of the shrinkage cavity of U78CrV steel is 82.6%and the closing rate of 45#steel is 71%.