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Hot Stamping of Ultra-High-Strength Steels: Surface Modification to Improve the Adhesion of Oxide Scales
Luleå University of Technology, Department of Engineering Sciences and Mathematics.
2019 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Hot stamping of low-carbon boron-alloyed steel is commonly used in the automotive industry in the manufacturing of safety components, as it allows for the production of complex parts with negligible dimensional and shape distortion, ultra-high strength and reduced weight. Nevertheless, high-temperature surface oxidation of steel poses a considerable challenge due to low steel-oxide adhesion.

Multiple strategies to dispose of thermally grown oxide films are already in use at the industrial scale. In the present study, a novel surface pretreatment using phosphoric acid has been extensively investigated, as previous results showed a significant increase in the steel-oxide scale adhesion. For this purpose, laboratory scale etching and hot stamping trials were carried out on multiple 22MnB5 steel strip samples under controlled conditions. As-received materials were extensively investigated by means of Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDS), Electron Backscatter Diffraction (EBSD) and Glow Discharge Optical Emission Spectroscopy (GDOES) to correlate the etching response to microstructural and compositional features.

The present study proved that roughening of rolled surfaces does not occur because of intergranular etching as previously thought. Although the actual mechanism could not be identified, sub-surface cracks propagating to as deep as 5.62±1.29μm were concluded to play a major role, as well as their opening by the action of the acid bath. The origin of these cracks remains uncertain and further investigations are therefore of interest. In some cases, improved adhesion was achieved on rather flat surfaces containing dispersed (Fe,Mn,Cr)3C microparticles. A chemical effect is thus considered to exist between these precipitates and the oxide scale. Further investigations with this regard are also of interest, as a combination of mechanical and chemical anchoring might lead to a highly efficient and robust pretreatment.

Place, publisher, year, edition, pages
2019. , p. 54
Keywords [en]
Hot stamping, press-hardening, boron steel, high-temperature oxidation, surface treatment
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:ltu:diva-73302OAI: oai:DiVA.org:ltu-73302DiVA, id: diva2:1298884
External cooperation
Gestamp HardTech AB; SSAB
Educational program
Materials Engineering, master's level
Supervisors
Available from: 2019-03-28 Created: 2019-03-25 Last updated: 2019-03-28Bibliographically approved

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CiteExportLink to record
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Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
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Language
  • de-DE
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