The use of X-ray diffraction for phase identification of press hardened steels

The numbers of hot stamped components have been steadily increasing in the automotive industry. Press hardened steels are generally used in hot stamping process since at the end of the process, the steel may achieve a tensile strength about 1500 MPa. The hot stamping process consists to heat the steel blank at total austenitization temperature and to transfer the blank into the press tooling for shaping and fast cooling to fully martensitic transformation. The transfer of the blank from the furnace to the press might promote at some extent, steel oxidation. The application of metallic coatings avoids this hindrance. The AlSi has been the most applied coating on steel. In parallel, alternative coatings such as Zn-Ni have been developed. It is known that the heating causes chemical elements diffusion, which results in intermetallic formation between the elements presents in the coating and in the substrate. This study had the objective of characterizing hot stamped and coated 25MnB5 steel samples with Al-Si and Zn-Ni, with X rays diffraction (XRD) technique. Some literature suggested phases such as Zn-Fe, Zn-Ni-Fe or Al-Fe-Si were not seen on XRD results. Nonetheless, X-ray diffraction detected the presence of ZnO and α-Fe for Zn-Ni coated steel plate and Al5Fe2, AlFe3 and α-Fe for the Al-Si coated steel plate.


VI Workshop of Applied Crystallography to
Materials Science and Engineering

Introduction
The

Materials
The samples were removed from the B -pillar inner, from a commercial and experimental parts of the 22Mn5B steel plate 1.2 mm thick.Two series of samples were taken: Sample I: Zn-Ni coated (20x20 mm 2 ).Sample II: Al-Si coated (20x20 mm 2 ).Due to the shape of the part, it was not possible to obtain totally flat samples.

Methods
X-ray diffraction was undertaken at Bragg-Brentano geometry, with Cu anode.The angle of scanning was (2): 4°-70°.intermetallic, however this phase was not observed in the present study [5].

Results and discussion
Fig. 3 shows the result of X-ray diffraction pattern for sample II, Al-Si coating.(peak B) was identified, but information about this phase was not found in literature.

Conclusion
In respect to sample I, Zn-Ni coated steel plate, ZnO was observed by X-ray diffraction as expected.
However, phases composed by Zn-Fe or Zn-Fe-Ni were not identified.
The corresponding X-ray diffraction patter of sample II, Al-Si coated steel plate, shows the phases Al5Fe2, AlFe3 and AlMn8.4,but the reported phases composed by Al-Fe-Si were not identified.
Even so X-rays diffraction is an important technique for characterization of crystalline materials, mainly during product development.
application of hot stamped components in automotive industry is steadily increasing.It is a strategic product to help the automakers to achieve the INOVAR -AUTO targets regarding fuel consumption and emissions.The aim is to produce lighter and safer vehicles, and then reduces the CO2 emissions[1].Press hardened steels (PHS) are boron-manganese steels.They are usually used in hot stamping process achieving after processing a tensile strength around 1500 MPa.Moreover, the spring back effect is not seen in the shaping process as a consequence of steel chemical composition combined with high temperatures[2];[3] [4].The hot stamping process consists in heating a steel blank at total austenitization temperatures and to transfer the blank into the press tooling for forming and fast cooling to fully martensitic transformation, as shown in Fig.1.At the beginning of the process the steel has around 600 MPa of tensile strength; at the end it increases to 1500 MPa.The blank transfer from the furnace to the press might promote some steel oxidation.In order to avoid this phenomenon, coatings are applied on the blank[5];[6].The Al-Si coating has been the most applied on PHS by hot dip.The bath consists in 10 % in mass of silicon in aluminum.However, it is a patented material developed by ArcelorMittal known as USIBOR.Thus, alternative coatings zinc base, have been developed.Zn-Ni is one of alternative to Al-Si; it is an electrodeposited coating, which consists in around 15 % in mass of nickel[5];[6].It is known that the heating causes chemical elements diffusion that results in intermetallics formation amongst the elements presents in the coating and base metal[5].The present study had the objective of characterizing the formed phases in samples coated with Al-Si and Zn-Ni after hot stamping, through X-ray diffraction technique (XRD).

Fig. 2
Fig. 2 shows the result of X ray diffraction pattern for sample I, Zn-Ni coated.

Figure 2 .
Figure 2. X-ray diffraction pattern of Zn-Ni coated plate showing the presence of ZnO and α-Fe.

Figure 1 .
Figure 1.X-ray diffraction pattern of Al-Si coating showing the presence of Al5Fe2 (peak A) AlFe3 (peak E) and α-Fe.