  Martensitic transformation in steels is responsible for their very high
strength and has thus been studied for more than one century since the first
works of Martens. However, there is not yet simple physical theory. A rigorous
classification of the crystallographic subgroupoids (packets) of the KS
variants and the continuity between the KS, NW and Pitsch variants are
introduced to represent the crystallographic intricacy associated to the
martensite transformation. From this analysis, a new simple theory based on
Pitsch distortion is proposed. The distortion respects the hard sphere packing
of the iron atoms and implies the existence of a neutral line along the close
packed directions. Its principal strains are 0, -5.8 and +15.5 %, well below
the +12, +12 and -20 % values of the Bain distortion. Martensite variants
nucleate by Pitsch distortion in an austenitic matrix continuously deformed by
the transformation. The martensite variants grow by the same Pitsch distortion;
they are locally in Pitsch orientation, and therefore are gradually oriented
inside the deformation field of austenite leading to the continuum of
orientations including KS and NW. Many observations reported in literature can
now be re-interpreted.
