Effects of sulphur on hot ductility of niobium containing low carbon steels during low strain rate deformation

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The effects of sulphur on hot ductility of niobium steels, in which cracking susceptibility on the continuously cast slab surface is highest, have been studied by means of hot tensile testing with particular emphasis on the segregation of sulphur atoms to the matrix/grain boundary Nb(C,N) precipitate interfaces. When low manganese niobium steels are solution treated at high temperature and then deformed at temperatures ranging from the low temperature γ to the γ/α duplex phase regions, two troughs appear in the ductility versus strain rate curve, accompanied by intergranular fracture of γ, at strain rates of ~1 s−1 and 10−3−10−4 s−1. The loss of ductility at high and low strain rates is caused by dynamic precipitation of iron rich (Fe,Mn)S and Nb(C,N) particles, respectively, both within γ grains and on the γ grain boundaries. The dependence on sulphur content is obvious at high strain rates, but it is found that the loss of ductility owing to Nb(C,N) precipitation is also reduced by decreasing the sulphur content to less than 10 ppm. This can be explained by the reduced segregation of sulphur atoms to the grain boundary Nb(C,N) precipitate/matrix interfaces, leading to suppressed decohesion and consequent nucleation of microvoids which result in ductile intergranular fracture of γ.MST/1425