Surface cracking mechanism of continuously cast lovv carbon lovv alloy steel slabsThe present state of understanding of surface cracking in low C low alloy steel slabs in the continuous casting (CC) and direct rolling (DR) processes is outlined. Hot cracking of the CC slab surface can be explained in terms of carbide and/or nitride precipitation behaviour. In addition to'}' grain boundary precipitation, the matrix strengthening owing to dynamic precipitation and the existence of softer layers along the boundaries such as grain boundary allotriomorphs of ferrite or precipitate free zones play animportant role in intergranular ductile fracture. The origin of hot cracking during the DR process lies also in the precipitation of carbides and/or nitrides, and is not related to the severe embrittlement caused by a similar mechanism with dynamic precipitation of sulphides, which is observed usually in the high strain rate deformation after reheating at higher temperatures. Furthermore, a well known effect of C on hot cracking susceptibility in both CC and DR processes, attaining a maximum in the range 0·10-0·15 wt.-%C, isfound to arise mainlyfrom '}' grain growth during solidification in the mould. Some methods to prevent surface cracking are also discussed.MST/1226
SynopsisIn order to understand the effect of surface roughness of CC slabs such as oscillation mark on the surface cracking, hot deformation of some low carbon low alloy and austenitic stainless steels has been investigated by means of hot tensile tests using the specimens with round notch at temperatures from 800 to 1100°C at average strain rates from 10-4 to 10-1 s-1. Although the total elongation of the parallel portion decreased with strengthening by the notches, the effects were markedly reduced by lowering either deformation temperature or the average strain rate or by JVb addition. This can be explained in terms of dynamic precipitation behavior of carbonitrides such as NbC and/or Alisi, i.e., the precipitation can be suppressed by the increase of true strain rate which arises from local straining in the notched region, resulting in the ductility improvement. The ductility loss due to the notch is determined by the depth and does not depend on the initial sharpness, since the shape can easily change in the early stage of deformation. Therefore, control of the oscillation mark depth is the most important to prevent surface cracking of CC slabs.
SynopsisA V-shaped segregation, found in a sulfur print of longitudinal strand section, has been macroscopically investigated. A simple model, using polyethylene particles of 2 mm diameter, has been prepared to simulate formation of the V-shaped segregation.The results obtained are summarized as follows :(1) The V-shaped segregation is observed only in an equiaxed crystal zone very periodically along the casting direction.(2) The frequency of the V-shaped segregation becomes smaller, while its density becomes larger with increase in the thickness of the equiaxed zone.(3) The mechanism of formation of the V-shaped segregation may be explained as follows; The enriched liquid between the equiaxed crystals is sucked and flows downwards and is accumulated along the planes which are made by the forcible movement of the equiaxed crystals piling in the end of the solidification regions toward the strand center.(4) The periodicity of the V-shaped segregation is quantitatively explained by the rheological approach assuming the equiaxed crystals piling in the end of the solidification regions as cohesive particles.
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