Formation Mechanism of Slab Transverse Corner Crack and Technological Development of the Chamfered Mold in a Slab Continuous Caster for Typical Micro-alloyed Steels

The formation mechanism of slab transverse corner cracks in typical niobium, vanadium, titanium micro-alloyed steels was investigated by the theoretical models and industrial experimental trials. The chamfered mold, which is used to minimize the cracks, was developed. A numerical simulation of the fluid flow, heat transfer and macro-solidification in the conventional mold and the chamfered mold, together with a finite element stress-strain model in the straightening process of both molds, were performed to analyze the relative influence of the conventional model and chamfered mold with different chamfer shapes (including the chamfer angle and the chamfer length) on the fluid flow, temperature distribution and solidification, and the stress and strain in the slab corner. Then the mould copper plate with an optimum chamfer shape is designed on the basis of the numerical results and is applied in industrial tests. The effect of the chamfered mold on the slab corner quality of the typical micro-alloyed steels is analyzed. The following conclusion can be drawn:(1) The results from the"lying slab"experiment show that there is no crack in the mold and the vertical segment below the mold. However, at the location of 3270mm from the meniscus, where the slab bending starts after the 710mm from the vertical segment, the transverse corner cracks frequently occur in the outside curved surface of the slab. Therefore, the cracks are formed in the slab bending process.(2) The predicted results from numerical simulation of fluid flow, heat transfer and macro-solidification in the conventional mold and the chamfered mold show that the variations of the chamfer shapes don't significantly affect the overall flow pattern near the SEN in both molds, but change the flow features near the meniscus corner. With the increase of the chamfer angle, the flow separation location near the meniscus corner is closer to the narrow face of the slab. The fluid flow near the intersection of the width face and narrow face and its impingement on the slab corner are also stronger. With the increase of the chamfer length, it is found that the flow near the corner becomes intensive. At the mold exit, the increased chamfer angle leads to an approximately linear increase of the slab surface temperature, but it also causes the strong flow near the corner. As far as the chamfer length is concerned, very small length (e.g. L=10mm) can lead to the significant increase of the temperature near the slab corner. As the chamfer length increases fromL=60mmto L=80mm, the temperature of the slab corner increased slightly while the flow near the chamfered corner of the slab obviously enhances and the thickness of solidified shell becomes thinner. Therefore, in order to optimize the design of the chamfer angle and length, it is necessary to comprehensively consider their effects on the flow, heat transfer and solidification on the slab corner.(3) The calculated results from the finite element stress-strain analysis in the straightening process show that when the straightening (pressing) velocity is constant, the slab temperature (700℃~1000℃) has little effect on the equivalent stress and strain on the cross sections in the slab. Equivalent stress are concentrated within the incline position in the chamfer, from the corner about 15mm~33mm; When the straightening temperature is above 900℃, the range of maximum equivalent stress within the sloped drops significantly, which reduces the possibility of occurrence of cracks. The chamfer angle has great impact on the tangential stress-strain near the slab edges and corners. At the same slope width,if the chamfer angle are chosen as 30o and 45o , the tangential stress-strain on the slab edges and the corners is least, only 40% to 46% of rectangular slabs with the same cross-sectional area. The chamfer angle of 30°is better than 45°. Chamfer length on the part of the slab edges and corners have great impact on the tangential stress-strain. If chamfer angle(30°) on the part of the slab corners were constant, when the chamfer length is controlled between 65mm~85mm, the tangential stress-strain on the part of the slab edges and corners were least. It is only 40% of conventional slabs with the same cross-sectional area if the slop width is 75mm.(4) Industrial test results show that the copper narrow plate with the chamfer shape can be used for Shougang Jingtang's slab production. It hasn't significant effect on the level fluctuations and mold withdrawal resistance. The service life of the mold plate is up to 236 furnaces. The slab corner temperature in the chamfered mold with the large chamfer increases by about 100℃, compared to the conventional molds. This increase corner temperature improves the high-temperature ductility of the slabs in the top bending section and the straightening section, and thus it is helpful for controlling of the transverse corner crack in the micro-alloy steel slabs. Using the chamfered mold for the production of X65, L290, SPA-H and other micro-alloy steel slabs, the slab transverse corner cracks have been reduced significantly, about 80% or more in comparison with those in the conventional mold.