SynopsisThe effect of C on hot ductility of low alloy steels has been studied in view of surface cracking of continuously cast (CC) slabs. As the ductility was not affected by C content in hot tensile test of the reheated specimens, the well-known C dependency of surface cracking susceptibility in CC slabs can be ascribed to the microstructural change during the solidification process. Austenite grain size of as-cast materials was found to depend largely on C content, i.e., the maximum grain size in O.1ON0.15 % C region. This can be explained by the higher austenite formation temperature in these C region. Austenite grain growth rapidly occurred after the complete transformation or solidification into r phase, as the strong pinning effect of the second phase such as o-ferrite or liquid phase on r grain boundary migration was relieved. Carbon dependency of r grain size became more marked with increasing cooling rate up to that of ordinary continuous casting.Such coarsening of r grains enhanced intergranular fracture, resulting in ductility loss inversely proportional to the r grain size. Uneven surface solidification in the mold due to the peritectic reaction will produce much coarse r structure because of the local delay of cooling. Surface cracking susceptibility will also be largely accelerated by this mechanism. Carbon range where surface cracking susceptibility was the largest varied with the chemical compositions. This shift can be explained in terms of the effect o f alloying elements such as Mn on the peritectic composition.
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路10-0路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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