Influence of titanium on hot ductility of as cast steels

Abstract

The influence on hot ductility ofa small addition (0·02%) of titanium to C-Mn-Al-Nb steel and 0·05jO·15%C-Mn-AI steels has been examined over the temperature range 700-1100°C and at a strain rate of 5 x 10-3 S -1. The tensile samples used were tested after melting, solidifying, and cooling directly to test temperature. The y grain sizes at test temperature were coarse and were independent of titanium or carbon content. In niobium containing steel, little influence of titanium was observed when the cooling rate to test temperature was 100 K min -1, but a marked improvement of hot ductility was noted at a slower cooling rate of 25 K min -1. Examination of precipitate distributions suggests that at 25 K min -1, the niobium can precipitate out at high temperatures on the coarse TiN particles leaving less niobium available for precipitating during deformation in the temperature range 800-1000°C. In C-Mn-AI steels cooled at 60 K min-1 a very slight improvement of hot ductility due to the presence of titanium occurred, independent of carbon content. The improvement is considered to be due to titanium removing nitrogenfrom solution and preventing precipitation of the more detrimental AIN. Increasing the carbon content shifted the hot ductility trough to lower temperatures and increased its width. Both these observations are explained in terms of the change of transformation temperature which occurs with carbon content. The limitations of the hot ductility test in simulating the conditions present during the straightening operation in vertical continuous casting operations are discussed.MSTj 1368

Hot ductility of TWIP steels

“…Cardoso et al [71] have also shown that an increase in the aluminum level in C-Mn-Al steel extends the ductility trough to higher temperatures and this is related to the aluminum nitride which has precipitated out, delaying the onset of dynamic recrystallisation. In this work the [Al] x [N] value is 4.25 x 10 -4 which is above the level quoted earlier [79]. AlN when it precipitates out of solution is particularly damaging to ductility and there is increasing evidence [122,123] that it can precipitate out from the melt as a very thin film, which covers the austenite grain surfaces.…”

“…The greatest improvement in ductility that has been found so far on adding Ti to as-cast steel, is when slow cooling to the test temperature has been used [79]. Whereas cooling an Nb containing steel at 100 o C/min to the test temperature only gave a small improvement in 44 ductility when Ti was present, reducing the cooling rate to 25 o C/min resulted in a significant benefit, Fig.2.22.…”

“…If compositions and cooling conditions are carefully chosen so that a coarse precipitate size is obtained, then good ductility can be achieved in Nb-Ti steels. The picture is complicated further still by cooling rate effects; at relatively slow cooling rate of 25 o C/min, Ti can give a large improvement to the hot ductility of Nb steels, again due to the formation of coarse precipitates [79]. …”

“…A considerable amount of research has been conducted to improve the hot ductility of Nbcontaining steel, mostly by addition of alloying elements such as Ti [78,79,131], Ca [99], Zr and Y [81]. B has been considered as another possible element likely to enhance the hot ductility in similar grades.…”

“…The stoichiometric ratio for TiN is not recommended unless very low N levels are present (≤0.004%) as it gives rise to the maximum precipitation volume fraction and the finest particle size, Fig.2.64 [78]. However, from the large amount of laboratory work [78,79,149] carried out on as-cast C-Mn-Al-Ti steels there have been only two cases in which Ti containing steels have given superior hot ductility to Ti free steels of otherwise similar composition. This occurred when a relatively slow cooling rate to the test temperature of 60 o C/min was used and when the steels in question (0.05%C and 0.15%C) had a high N content.…”