Influence of undercooling thermal cycle on hot ductility of C–Mn–Al–Ti and C–Mn–Al–Nb–Ti steels

Abstract

The influence of an undercooling thermal cycle during cooling after melting on the hot ductility of C–Mn–Al and C–Mn–Al–Nb steels with titanium additions in the range 0–0·015% has been examined. Tensile specimens were melted and cooled at 100 K min−1 to test temperatures in the range 1000–800°C, and strained to failure using a strain rate of 3 × 10−3 s−1. The hot ductility curves obtained from this procedure were then compared with curves obtained when a 100 K undercooling step was introduced before tensile testing; this cooling programme simulated more accurately the cooling conditions during continuous casting. Titanium additions were found to impair the hot ductility, with niobium containing steels giving the worse ductility. When undercooling was applied to the titanium free, C–Mn–Al containing steels, greater amounts of AlN could form, and hence ductility was worse. Increasing the titanium level in these C–Mn–Al steels reduced the adverse influence of undercooling on hot ductility, as more of the nitrogen was taken out of solution as TiN. When only one thermal cycle was introduced, TiN precipitation was shown to have a more detrimental effect on the hot ductility than AlN precipitation. However, whereas increasing the number of thermal cycles would not lead to any further TiN precipitation, this was not so for AlN, since precipitation was sluggish, and, provided that free nitrogen was available, further precipitation could occur giving rise to worse ductility. For the niobium containing titanium free steel, undercooling gave worse ductility because the lower temperature allowed a finer precipitation of Nb(CN) to occur. Increasing the titanium level in these niobium containing steels again caused the ductility to deteriorate, but this time there was a small improvement in ductility on undercooling for the lower temperature range. Although titanium has commercially been found to be a beneficial element to add to reduce transverse cracking and improve surface quality on continuous casting, laboratory work has again shown that ductility is impaired even with an undercooling step. This difference in behaviour can be accounted for by more intense segregation occurring on solidification in commercial casts compared with laboratory casts, thus favouring coarser precipitation of TiN and hence better ductility.