Hot ductility and its relationship to transverse cracking in steels
The influence of the Ara on the hot ductility of steels was examined. Tensile samples were heated to 1603K, cooled at 60Kmin" and tested in the temperature range 1323-873K at a strain rate of 3x10-3S-1. The Ara was altered by variations in C, (0.1 to 0.15%C), Mn, (0.6 to 1.4%Mn), Si (0.1 to 0.5%Si) and P, (0.001 to 0.025%P). Elements that raise the Ara e. g. Si and P were found to lead to recovery in ductility at slightly higher temperatures at the low temperature end of the trough. However, reducing C levels to <0.1C had the most pronounced influence on hot ductility since ductility was then observed to recover just below the Ae3 and the trough was very narrow. The lower C level allowed large amounts of deformation-induced ferrite (equilibrium amounts) to form, ferrite having excellent ductility. mThe influence of both S in solution and elongated sulphides on hot ductility was also examined. Although elongated sulphides encouraged ferrite formation, this was a relatively minor improvement. In these steels, low C levels (<0.1%) i. e. high Ae3 temperatures, ensured ductility recovered rapidly just below the Ae3 obscuring the effect of S. Similar behaviour was noted with as-cast steel, increasing the S level from 0.004 to 0.019% had little influence on the hot ductility. Increasing the cooling rate after solidification, from conventional continuous casting to that pertaining to thin slab casting, results in worse ductility. This can be ascribed to finer precipitation and/or a finer inclusion distribution at the y grain boundaries. A model was devised for predicting the hot ductility curve for simple plain C-Mn steels. However, this model has limited application and further development is needed to take into account the influence of grain boundary sliding on the reduction of area values. Columnar grains were found to be very detrimental to hot ductility and to avoid transverse cracking, it is suggested magnetic stirring be used in the mould to break up the columnar structure.