Influence of C and Mn on hot ductility behaviour of steel and its relationship to transverse cracking in continuous casting

The hot ductility behaviour of plain C–Mn steels, with C levels of ≤0·3%C, with heat treatments given to provide a coarse grain size and straining at a strain rate of ∼10⁻³ s⁻¹ to simulate the conditions that are applicable to the straightening operation during continuous casting, has been reviewed. For steels with a Mn level of 1·4%, the trough generally extends from the Ar₃ to the Ae₃. In the trough, deformation induced ferrite forms but only as a thin film surrounding the austenite grain boundaries, which is not able to spread into the interior. The difference between the Ae₃ and Ar₃ becomes less as the transformation temperatures increase so that the width of the trough decreases. The minimum reduction of area value, i.e. the depth of the trough, is then not influenced by the C level. However, if the transformation temperatures are raised sufficiently by reducing the C and Mn levels, so that deformation induced ferrite can form readily below the Ae₃, then the depth of the trough becomes less, so that at very low Mn and C levels, there is no trough and ductility is excellent. The use of the hot ductility curve in predicting the likelihood of transverse cracking occurring in the commercial straightening operation is discussed, and it is concluded that since both deformation induced ferrite in sufficient amounts to improve ductility and dynamic recrystallisation are not generally likely to occur because the strain during the unbending operation is so low (∼2%), the curves have to be used with great caution and with an understanding of their limitations.