  Low-alloy steels are constituted of twenty-four variants of lath martensite
that exhibit gradients of orientations from Kurdjumov-Sachs (KS) to
Nishiyama-Wassermann (NW). They are structured into four packets on each of the
common close-packed plane {111}fcc// {110}bcc; and each packet is composed of
three blocks constituted by pairs of low-misoriented variants. The habit planes
reported in literature for this type of martensite are {557}fcc, but it is not
clear whether they correspond to the laths or to the blocks. In this paper, we
present crystallographic calculations proving that the average of the two KS
distortions associated with the variants in a block is exactly a NW distortion.
A new method of averaging distortion matrices was introduced for this purpose.
It is also shown that the {575}fcc planes are let untilted by this NW
distortion, and are thus good theoretical candidates for the observed habit
planes. The predicted {575}fcc planes, however, do not contain any of the
common close-packed directions of the two variants in the block, which is in
apparent contradiction with the current view. In order to clarify this point,
some Electron BackScatter Diffraction (EBSD) maps were acquired on different
low-carbon steels; the prior austenitic grains were automatically reconstructed
and the traces of the habit planes predicted by the different models were
analyzed and compared to the morphologies. This experimental work shows that
{575}fcc planes are the habit plane of the blocks, and that the habit plane of
one block often dominates the others, which impedes to discriminate the
different models. The advantages of our model are its simplicity, the absence
of fitting parameters, and the symmetric role played by the variants the
blocks.
