PSI - Issue 79

Giuseppe Macoretta et al. / Procedia Structural Integrity 79 (2026) 508–516

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3. Results and discussion

3.1. Microstructural investigation

The microstructure of the as-received QP1180 was formed by a fine martensitic matrix containing ferritic islands and occasional regions of RA. Fig. 2 reports the microstructure observed sectioning the sample with a plane parallel, Fig. 2a), and orthogonal, Fig. 2 b), to the sheet surface, where the white arrow points out the sheet rolling direction. Moreover, no significant influence of the rolling direction on the detailed microstructure was detected at the scale observable by optical microscopy. An almost isotropic martensitic matrix can be observed. This microstructure is consistent with findings reported in the recent literature for as-received QP1180, [6]. The microstructure was found to be quite uniform along the sheet thickness, apart from the limited segregation bands present in the middle region of the sheet. This evidence was confirmed by the microhardness profiles, which turned out to be quite uniform and with no significant drop in the sheet's middle region, as shown in Fig. 3 a) where a representative hardness profile is reported. The microstructure in the regions surrounding the indentations was quite similar. The mean and the standard deviation values calculated among the measured profiles turned out to be 416 HV0.1 and 9 HV0.1, respectively.

Fig. 2. Microstructure of the as-received material, observed on a plane parallel a) and orthogonal b) to the sheet surface. White arrows point out the sheet rolling direction.

Fig. 3. Vickers microhardness profile along the specimen thickness, measured on samples perpendicular to the sheet plane a), example of an indentation on the polished and etched material b).