The Influence of Composition on the Hot Ductility of Steels and to the Problem of Transverse Cracking

A review of the literature has been made, concentrating on the influence of N, V, Ti and the residuals, Cu, Sn, S and P, on the hot ductility of steels. Nitrogen is generally detrimental to ductility in Al containing and microalloyed steels; to avoid transverse cracking the N levels should be kept as low as possible. When Ti additions are made to low N, C-Mn-Al steels, (0.005 % N) the best ductility is likely to be given by a high Ti : N ratio of 4-5 : 1. For high N steels (0.01 % N), a low Ti level (0.01 %) is recommended to reduce the volume fraction of Ti containing particles, but allow precipitation to occur at high temperatures. In addition a low soluble Al level is needed to prevent the excess N from combining to form AlN. For C-Mn-Nb-Al steels, similar recommendations can be made with regard to adding Ti. However, the presence of Nb and Al appears to have little influence on the ductility, since these elements coarsen the Ti containing precipitates. Furthermore, once the Ti has combined with the N any remaining N now seems beneficial, possibly because high N contents encourage precipitation at higher temperatures. V as a microalloying addition gives better hot ductility than Nb and the product of the V and N has to approach 1.2×10-3, e.g. 0.1 % V and 0.012 % N before ductility deteriorates to that normally shown with a 0.03 % Nb addition. S levels should be kept low to reduce the detrimental precipitation of fine sulphides at the γ boundaries. Care must be taken with Ca treatment as if the steel is too "clean", none of the Nb may be able to precipitate out prior to straightening. Residual levels of Cu> 0.15% and Sn are detrimental to surface quality and encourage transverse cracking. It is found that Cu is deleterious to ductility, but Ni can be added to compensate for this loss. It is recommended that the amount of Ni added should be greater than the residual Cu level to be sure of avoiding problems, (Ni : Cu ratio nearer to 1.5 to 2). Phosphorus although potentially beneficial to ductility can not be recommended unless its segregation during solidification can be reduced.