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High-resolution characterization of the martensite-austenite constituent in a carbide-free bainitic steel

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Carbide-free bainitic steels represent attractive candidates for the automotive industry to meet the conflicting requirements of increasing safety and decreasing vehicle weight. Their fine-grained multiphase microstructure enables a well-balanced combination of high strength and ductility. To identify and characterize all phases in dependence of the isothermal transformation temperature, metallographic and high-resolution techniques are applied. The stability of the individual phases is evaluated based on their C concentration determined by atom probe tomography and, subsequently, linked to the mechanical properties.

Steel sheets of Fe-0.2C-2Mn-0.2Si-0.03Ti-0.003B (m%) for the automotive industry were isothermally heat-treated, comprising austenitizing and subsequent isothermal annealing at temperatures between 300°C and 500°C. As a consequence, microstructures ranging from granular bainite over lower bainite to auto-tempered and untempered martensite were obtained. In tensile, hole expansion and bending tests, the performances in different forming conditions were compared and the changes of microstructure and texture were studied by complementary electron backscatter diffraction (EBSD) analyses. Samples with granular bainitic microstructures exhibited high total elongations but lower hole expansion ratios; in subsequent EBSD and texture analyses, evidence for inhomogeneous deformation was found. In contrast, the lath-like bainitic/martensitic microstructure showed higher strength and lower elongation to fracture. This results in a reduced bendability, but also in a high tolerance against damage induced by the shearing of edges, and, thus, allows homogeneous deformation to higher strains in the hole expansion test.

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KurzfassungWährend des Stranggießprozesses treten in der erstarrten Strangschale der Brammen thermisch und mechanisch induzierte Spannungen und Dehnungen auf, die bei ungünstiger Prozessführung gepaart mit dem spezifischen Werkstoffverhalten zu Rissen führen können. Eine wichtige Phase im Prozessablauf ist die γ-α-Umwandlung, welche die Duktilität des Werkstoffes stark verringert und die Rissgefahr erhöht. Zusätzlich sollten sich laut Literatur feine Aluminiumnitridausscheidungen negativ auf Hochtemperatureigenschaften der Stähle auswirken. Ziel der Arbeit ist es, die möglichen Auswirkungen von Aluminiumnitrid-Ausscheidungen für Baustähle im Legierungsbereich um 0,2 Masse-% Kohlenstoff anhand von Heißzugversuchen zu untersuchen.Die Ergebnisse der Heißzugversuche in Verbindung mit den mikrostrukturellen Untersuchungen ergaben jedoch, dass unterschiedliche Abkühlbedingungen keinen signifikanten Einfluss auf die Hochtemperaturduktilität haben.