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

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Synopsis A mathematical method is presented to quantify solute redistribution during solidification with a/' transformation; new methods to solve the equation of diffusion numerically and to estimate the equilibrium phase diagram of multi-component low alloyed steel are described. Thermal analyses of low alloyed steels were made to verify the mathematical method described. It has been shown that the unequilibrium phase diagram of low alloyed steel for a given cooling rate can be predicted by this method.

Effect of Bon Microstructures of Hot Bands Figure 3 showsoptical microstructures of hot bands. Steels A, B and C showedpolygonal grain structure. Steel D showedthe mixture of polygonal grain structure and a small fraction of quasi-acicular ferrite. The effect of B content on the average grain diameter of hot bands is shown in Fig. 4 (Tf) were derived from the hardness change as shown in Fig. 7. T, andTf almost corresponded to the optical microstructure change. Figure 8 showsthe effect of Bon recrystallization temperature. Both T* and Tf rose with increasing B