Spray cooling pattern and microthermomechanical rigidity criterion for improving inner quality in continuously cast steel slabs

Abstract

A new concept has been developed to assess, and a new technique developed to improve, the microthermomechanical rigidity of the solidifying shell in the secondary cooling zones during continuous casting of steel. This is to maximise the coherent solid shell resistance 'I c ' against thermometallurgical and mechanical stresses at different levels from meniscus. The idea behind this concept is to optimise the degree of homogeneity of the cooling pattern between a pair of rolls. The effect of the degree of homogeneity of the cooling pattern between a pair of rolls 'HDCP', in different spray cooling zones on the different types of centreline segregates has been investigated. A number of plant trials with two different water cooling patterns between a pair of rolls have been performed. The metallographic investigation contains examinations and measurements of the level of centreline segregates of collected samples. A one dimensional (1D) transient finite difference mathematical model of thermal, solidification, solid shell resistance and cooling conditions has been developed to test the different effects of spray patterns on the microthermomechanical rigidity concept and therefore on the mechanism of centreline macrosegregation level. The results indicate that the increase in the degree of homogeneity of the cooling conditions is proportional to the increase in slab internal microquality. The results show that the degree of homogeneity of the cooling pattern affects centreline macrosegregation significantly. This depends significantly on the degree of homogeneity of cooling pattern between a pair of rolls, its location from the meniscus and, generally, on the increase microthermomechanical rigidity concept.

Macrosegregation Quality Criteria and Mechanical Soft Reduction for Central Quality Problems in Continuous Casting of Steel

“…In the case of central region, C segregation ratios of upper and lower sides follow the same trend whereas the clear difference is observed between S segregation ratio patterns of different slab sides. This is due to a difference in interdendritic strain hypotheses between lower and upper slab sides and in partition coefficients of C and S [38]. The coupled effect of these factors controls the macrosegregation formation and its distributions.…”

“…C and S segregates behave the same trend in the upper slab side where positive segregations of C and S increase by different rates until the slab centerline. This is because the solid shell starts to bulge in and pushes the mushy dendrites in the direction of slab center as shown in the flow pattern A in Figure 5 [17] [38]. This draws the solute liquid from above hotter region in the liquid pool into mushy zone just below the liquidus isotherm [38] [39].…”

“…This is because the solid shell starts to bulge in and pushes the mushy dendrites in the direction of slab center as shown in the flow pattern A in Figure 5 [17] [38]. This draws the solute liquid from above hotter region in the liquid pool into mushy zone just below the liquidus isotherm [38] [39]. This is results in a steep S segregation and small C segregation in the centerline zone of upper slab side illustrated in Figure 4(a).…”

“…The uniform behaviors of C and S fluctuations result from the reasons mentioned above in the analysis of first group. In addition to these reasons, the constant casting speed leads to increase the homogeneity degree of heat transfer between the rolls and reduces the levels of segregations fluctuations of C and S [38] [44] [45].…”

“…This results from decreasing in the thermo-mechanical resistance of slab solid shell associated with declining of growth rate of slab solid shell resistance related to the increasing in casting speed [16] [17] [40]. This results in a high sensitivity to the thermo-mechanical stresses generated from different which affects directly the interdendritic solute liquid and therefore, affects the fluctuation segregation levels of different alloying elements [38] [40].…”