  The identification of orientation relationships (ORs) plays a crucial r\^ole
in the understanding of solid phase transformations. In steels, the most common
models of ORs are the ones by Nishiyama-Wassermann (NW) and Kurdjumov-Sachs
(KS). The defining feature of these and other OR models is the matching of
directions and planes in the parent face-centred cubic $\gamma$-phase to ones
in the product body-centred cubic/tetragonal $\alpha/\alpha'$-phase.
  In this paper a novel method that identifies transformation strains with ORs
is introduced and used to develop a new strain-based approach to phase
transformation models in steels. Using this approach, it is shown that the
transformation strains that leave a close packed plane in the $\gamma$-phase
and a close packed direction within that plane unrotated are precisely those
giving rise to the NW and KS ORs when a cubic product phase is considered.
Further, it is outlined how, by choosing different pairs of unrotated planes
and directions, other common ORs such as the ones by Pitsch (PT) and
Greninger-Troiano (GT) can be derived.
  One of the advantages of our approach is that it leads to a natural
generalisation of the NW, KS and other ORs for different ratios of
tetragonality $r$ of the product bct $\alpha'$-phase. These generalised ORs
predict a sharpening of the transformation textures with increasing
tetragonality and are thus in qualitative agreement with experiments on steels
with varying alloy concentration.
