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The modern high strength steel plates have an excellent combination of strength and toughness based on micro-alloying and complex microstructure. Retaining this combination of properties in the weld zone is a major challenge for applications in high-demanding structural construction. This work investigates the weldability of three different modern high strength steel plates, with a thickness of 8 mm. Two of the test materials were produced by a thermo-mechanically controlled process (TMCP) and one by a quenching and tempering method (Q&T). Two-passes MAG (metal active gas) welding was used with four different heat inputs. The tests implemented on all the materials included tensile, hardness profiles (HV5), Charpy-V impact toughness tests, and microstructure analysis using scanning electron microscope (SEM). For one of the TMCP steels, some extended tests were conducted to define how the tensile properties change along the weld line. These tests included tensile tests with digital image correlation (DIC), and 3-point bending tests. The most notable differences in mechanical properties of the welds between the materials were observed in Charpy-V impact toughness tests, mostly at the vicinity of the fusion line, with the Q&T steel more prone to embrittlement of the heat affected zone (HAZ) than the TMCP steels. Microstructural analysis revealed carbide concentration combined with coarse bainitic structures in HAZ of Q&T steel, explaining the more severe embrittlement. During the tensile tests, the DIC measurements have shown a strain localization in the softest region of the HAZ. Increasing the heat input resulted in earlier localization of the strain and less maximum strength. The tensile properties along the weld line were investigated in all welding conditions, and the results emphasize relevant and systematic differences of the yield strength at the transient zones near the start and end of the weld compared with the intermediate stationary domain.

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Zn and ZnFe coated 22MnB5 and 34MnB5 steels were subjected to the direct press hardening process in order to investigate the influence of steel composition on the resulting phase structures.Microstructures were characterized using advanced methods of microscopy. In addition, X-ray diffraction, glow discharge optical emission spectroscopy and thermodynamic calculations with Thermo-Calc® were carried out to support the analysis. The results indicate that the steel composition has a clear effect on the phase development within coating and interface regions.Whereas the behavior of the 22MnB5 was comparable to earlier investigations, a clearly nonconventional behavior of the 34MnB5 was observed: the formation of martensitic micro constituents, designated here as α'-Fe(Zn), were identified after die-quenching. The regions of the α'-Fe(Zn) formed mainly in vicinity of steel/coating interface and were emerged into the steel by sharing martensitic morphology with the base steel. The thermodynamic calculations suggest that carbon is effective in stabilizing the γ-Fe(Zn) phase, which enables the formation of the α'-Fe(Zn) in fast cooling. Therefore, the higher initial C content of the 34MnB5 may result in the kinetic stabilization of the γ-Fe(Zn) as the inter-diffusion between Zn and Fe occurs during annealing. Simultaneously occurring carbon partitioning from α-Fe(Zn) to γ-Fe(Zn) could explain a clearly increased C content of the coating/steel interface as well as higher Zn contents in the α'-Fe(Zn) phase compared to 22MnB5. Actually, the present study shows that the same phenomenon occurs also in 22MnB5 steels, but in a much smaller scale. In Zn and ZnFe coated 34MnB5, the thickness of the α'-Fe(Zn) layer was increased with longer annealing times and at higher temperatures. The morphology of the α'-Fe(Zn) layer resembled plate-like martensite and can be assumed to be brittle. Regarding this, the formation of α'-Fe(Zn) interface layer needs to be taken into account in press hardening of 34MnB5 steels.