The investigations on the microstructure delivered details on the different behavior of the four investigated low-C steels regarding their microstructure–property relationship. Several peculiarities of the mechanical properties have been explained through the grain size and structure by means of EBSD measurements. A view on the distribution of the misorientation angles (Fig.
5) gave further insight into the microstructure. Augmented peaks were observed at angles of 6.5° and 59.5° and peaks with weaker appearance at 17° and 53° for all investigated specimens. These peaks correspond to variants of the Kurdjumov–Sachs orientation relationship of packet, sub-block and block boundaries [
8,
48,
49]. It was observed that the fraction of the misorientation at 6.5° of steels 2 and 4 is increased during re-austenitization compared to the direct-quenched variants. Due to the fact that steel 3 does not exhibit this phenomenon but rather shows an unchanged distribution at 6.5° between DQ and RQ, this difference can be attributed to the pancaking of the PAGs due to the MAE of steels 2 and 4. After RQ, the equiaxed PAGs of steels 2 and 4 contribute to an increase in the low-angle grain boundaries. The PAGs of steel 3 are already equiaxed after DQ, so that a change of the frequency does not occur. Furthermore, also the high-angle grain boundaries at 59.5° show an increase after re-austenitization. Nevertheless, these differences are too little to contribute to an explication for a remaining anisotropy.
Consequently, there is still a missing link of the remaining anisotropy which in case of steels 3 and 4 cannot be eliminated completely, and therefore, measurements of the texture were taken. EBSD reaches its limit if the detected microstructure is very small as for the present case [
50]. For a proper texture analysis, at least 10.000 grains are suggested to be measured [
51]. The step size needs to be reduced immensely when detecting microstructures with high grain boundary densities, as it is the case for martensitic steels. This would result in an extremely high effort for textures analysis with EBSD. The XRD measurements, however, provided the orientation distributions over the whole sample for the relevant sheet position of 1.5 mm below the surface. The orientations of interest and their distributions are highlighted in Figs.
6 and
7. All steels investigated contain a decisive fraction of the {001} <110> component, especially steels 1–3 after DQ from the rolling heat. This orientation arises from the recrystallized austenite cube texture [
52,
53] and thus is sparely present in steel 4 which is alloyed with strong recrystallization retarding elements (Nb) preventing a recrystallization prior to transformation. The {001} <110> texture is known to be unfavorable for impact toughness and provokes delamination and crack propagation [
52,
54]. However, a clear link to the
AV/
T diagram in Fig.
3 cannot be made. Despite a decrease in the intensity of {001} <110> (Fig.
7), the Charpy impact toughness of steels 1 and 2 decreases too and already falls after NRQ below 50 J at a temperature of 0 °C. These values are even lower than the values measured for steel 3, although a higher toughness is expected for steel 4 according the diametrical trend of strength and ductility. The prevailing components in steel 3 after DQ are {111} <112>, {112} <110> and {112} <131> which emerge from a deformed austenite [
29]. The latter ranks as a very stable orientation for higher toughness toward the rolling direction [
31,
33,
52‐
54]. Furthermore, all steels contain significant fractions of {554} <225> arising as well from a strongly deformed
γ grain [
20,
52]. This component is inherited to RQ and even present after NRQ. The relatively high proportions of the {001} sheet plane component in the DQ condition of steels 1–3 might explain differences between
L and
T. This is confirmed by the fact that a reduction in the {001} <110> partitions reduces the impact toughness anisotropy after RQ. However, the question remains, why an almost levelled-out texture distribution as it exists in steel 3 after re-austenitization (Figs.
6 and
7) still delivers small but present differences between
L and
T. And in contrast to that, steel 4 which exhibits a significant amount of defined texture components even after HRQ exhibits excellent Charpy impact values in each condition (DQ and RQ) with a relatively small anisotropy. Consequently, a clear link between a remaining anisotropy and prevailing textures thus cannot be made with certainty. Furthermore, a strict distinction whether a mechanical anisotropy can be attributed to an elongated γ grain, texture fractions or nonmetallic inclusions [
23] is not possible. In a previous publication [
10], it was found that variances in the FRT can even affect the properties of
Q +
T steels, contrary to the assumption that the FRT has no effect on the properties of steels which are later re-austenitized and quenched. The present investigations confirm these findings, in specific, that the relation of FRT and
TNR not only plays a major role during TMP but rather influences the mechanical properties through the inheritance of microstructural characteristics. Although the usage of MAE provokes through a highly deformed
γ grain a mechanical anisotropy, this does not mean simultaneously that the microstructural cancellations of these peculiarities are the key to a mechanical isotropy. Rather it can be observed that the higher the pancaking is, the more intense is the inheritance of the properties of the prior process steps.