Research And Application Of Soft Reduction Model For Dynamic Soft Reduction In Continuous Casting

Dynamic soft reduction technology has become one of the best methods to improve center segregation and porosity of continuous casting products, and is an important indication of modern conticaster. Based on national technology innovation projects "Research, development and application of dynamic soft reduction technology for slab conticaster" and "Process control technology research of dynamic soft reduction for bloom conticaster", research of a key process control model-soft reduction model was processed. The main contents and findings are as follows:(1) Reduction gradient model of soft reductionBased on the analysis of quality flow in soft reduction process, a theoretical model of reduction gradient, suitable for any rectangular continuous casting products, was derived. The impact of arbitrary rectangular section, solidification contraction, thermal shrinking, deformation on reduction gradient can well be described by the formula as follows: Where:VRG is for the reduction gradient,VSZ for the speed, Xsuf for width of casting product, Ysuf for thickness of casting product, x for the direction of coordinates along width, y for the direction of coordinates along thickness, z for the direction of coordinates along casting direction,ρL andρs for steel liquid and solid phase density respectively. The equivalent densityρ, is from theρLfL+ρs(1-fL), fL is for liquid fraction.(2) Methods for solving reduction gradient modelBased on the actual soft reduction process, a method to solve reduction gradient model was proposed. That overlooks the impact of deformation on the solid-liquid phase sectional shape, and calculates reduction gradient with the solid-liquid phase density variations. The simplified model of reduction model is described by the formula as follows:The impact of deformation on the reduction gradient is calculated by reduction efficiency model.(3) Model building of reduction efficiency modelBased on the theoretical analysis of the deformation process, an accurate calculation formula of reduction efficiency model was derived. That formula can calculate the reduction efficiency with arbitrary shape, namely:Where:77 is for the reduction efficiency,ΔAi for the reduced area of liquid core,ΔAH for the reduced surface area of casting products.(4) Numerical analysis of reduction gradientA finite element numerical method of solving heat transfer was adapted to calculate reduction gradient, combined with different speeds, steel grades and section sizes. The results show that:●Reduction rate and reduction gradient decrease linearly along the casting strand for continuous casting slab and bloom.●The average reduction gradient decreases linearly as the casting speed increases linearly.●The range of reduction rate does not change with the change of speed.●Steel grade has little effect on reduction gradient and reduction rate for continuous casting slab and bloom. Reduction gradient has little change with different grades.●Reduction gradient is influnced heavily by thickness. The greater thickness, the less reduction gradient at the inlet and outlet of soft reduction zone.●Thickness has little effect on average reduction gradient of slab. And the average reduction gradient decreases as thickness increases.(5) Numerical analysis of reduction efficiencyWith actual casting conditions of a factory, the process of soft reduction was simulated by a 3D finite element numerical calculation. And reduction efficiency was calculated with different reduction amount, liquid core thickness, steel grade and section size. The results show that:●As reduction amount is less than 2.3 mm, reduction efficiency of slab increases with the increase of reduction amount. When the reduction amount is bigger than 2.3 mm, reduction efficiency of slab keeps a certain value with the increase of reduction amount.●Reduction efficiency of bloom with reduction amount of less than 0.6 mm, increases with increase of reduction amount. As reduction amount is between 0.6 mm and 4.7mm, reduction efficiency of bloom decreases with increase of reduction amount. When reduction amount is bigger than 4.7 mm, reduction efficiency is independent with the increase of reduction amount.●Reduction efficiency of slab increases linearly as thickness increases linearly, and the reduction efficiency of bloom increases linearly with the linear increase of 2nd power of thickness.●Steel grade has little effect on reduction efficiency.●Reduction efficiency of slab is affected mainly by thickness, compared to width, and increases linearly as thickness increases linearly.●Reduction efficiency of bloom increases linearly with the linear increase of width or linear decrease of thickness.(6) Application model of soft reduction modelThe soft reduction model was applied on a slab caster with good results. The ratios of class C of 0.5 for centre segregation, centre porosity, angle crack are 100%,98%,88.2% respectively for all testing steel grades. And the ratio of class C of 0.5 for middle crack and triangle crack is up to 96%.