Verformung und Bruch von modernen, hochfesten Multiphasenstählen

Translated title of the contribution: The deformation and fracture behaviour of advanced, high-strength multiphase steels

Marianne Kapp

Research output: ThesisDoctoral Thesis

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Abstract

Reduced vehicle weight means less fuel consumption and, therefore, reduced CO2 emissions. For this reason there is a strong demand in the automotive industry for thinner sheets. A reduction of the sheet cross-section is only possible by using higher strength steels. At the same time ductility and formability must not be deteriorated in order to fulfill the strict safety regulations and enable complex forming procedures. These demanding requirements have been realized by the development of new material concepts based on multiphase steels. A key point was the development of dual phase (DP) steels. They consist of ferrite and martensite, i.e. mechanically very different phases (soft/hard). DP-steels have a very good combination of high elongation and high strength in the tensile test, however, cracks are often nucleated during forming operations, especially at small radii and in the crash test. Complex phase (CP) steels consist of multiple phases with similar mechanical properties, whereby bainite or martensite are used as the matrix phase. Compared to DP-steels they show slightly lower uniform elongation at the same strength, but significantly higher resistance against cracking. A clarification of the reasons for the discrepancy of the deformation and fracture behavior of these two steel types is of vital interest for steel producers and automotive industry. The aim of this work was to develop a thorough understanding of the relationship between microstructure and formability in order to enable the design of an improved microstructure in terms of an optimum combination of ductility, strength and formability. The investigation focuses on two steels, DP 1000 and CP 1000, which have exactly the same chemical composition and exhibit very similar tensile strengths of 1000 MPa. In addition, a DP steel with lower strength (DP 600) and a CP steel with higher strength (CP 1200) are examined. The steels were characterized by using a combination of two methods: fracture mechanics tests and the method of local deformation analysis. The fracture behavior was analyzed by conducting especially developed tests for thin sheets, and the critical J-integral and the critical crack opening displacement were determined. The fracture surfaces were analyzed in the scanning electron microscope (SEM), e.g. in order to find the dominant void initiation sites. The local deformation behavior was analyzed by conducting in-situ tensile and in-situ fracture mechanics tests in the SEM, with subsequent digital image correlation and the evaluation of the distribution of the local in-plane strains. The results of the local deformation analyses demonstrate that, caused by the specific soft/hard-microstructure, the local strains in DP-steels are distinctively stronger concentrated than in CP-steels. Local strains reach values up to a multifold value of the global strain. These highest-strained sites ("hot spots") have a significant influence on the subsequent void formation and damage behavior. "Hot spots" are found frequently in the DP-steels and rarely in the CP-steels. The lower degree of strain concentration and the low density of "hot spots" in CP-steels are responsible for the pronounced resilience to cracking.
Translated title of the contributionThe deformation and fracture behaviour of advanced, high-strength multiphase steels
Original languageGerman
QualificationDr.mont.
Supervisors/Advisors
  • Kolednik, Otmar, Assessor A (internal)
  • Clemens, Helmut, Assessor B (internal)
Publication statusPublished - 2011

Bibliographical note

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Keywords

  • Multiphase Steels
  • dual phase steels
  • complex phase steels
  • fracture toughness
  • local deformation analysis
  • digital image correlation

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