Correlation between the Microstructure and Mechanical Properties of Intercritically Annealed Advanced High-Strength Steels

Olli Oja

Research output: Book/ReportDoctoral thesisMonograph

Abstract

Automotive steels are undergoing constant research and development. The automotive industry has inspired the development of advanced high strength steels in recent decades by targeting a reduction in automotive body weight. In order to develop novel high-strength and formable steels for this purpose, the present work investigates the metallurgy and properties of silicon- and aluminium-alloyed, niobium- and vanadium-microalloyed steels by performing intercritical annealing and austempering as well as quenching and partitioning heat treatment. Materials were characterized by studying the microstructure and mechanical properties with the emphasis on local ductility.

The heat treatments investigated herein produced microstructures containing ferrite, bainitic ferrite and martensite with various amounts of metastable retained austenite, which led to differences in material properties. The retained austenite morphologies were investigated using electron backscatter diffraction (the EBSD method). As a novel feature, retained austenite was found in steel containing low carbon (0.08 – 0.10 wt.%) that had been processed by quenching and partitioning.

The present work investigated ten different chemical compositions which were utilized to produce four concepts for a 980 – 1180 MPa class steel. Depending on the heat treatment and processing, the strength-elongation combination showed local or global formability type mechanical properties. The local-global formability was studied by performing fracture strain measurements on the tensile test specimens. As a summary, this work defines the formability diagram of a large number of differently treated experimental steels.

The results showed that intercritical quenching and partitioning heat treatment typically led to a microstructure with local formability properties, while intercritical annealing and austempering produced more of a global type of formability. By adjusting the chemical composition and heat treatment, it was also possible to achieve a combination of these formability properties, which is useful in an advanced high strength and formable steel. These combined properties are especially required when automotive industry utilizes modern steels for more energy-efficient vehicles.
Original languageEnglish
Place of PublicationTampere
PublisherTampere University
ISBN (Electronic)978-952-03-2455-1
ISBN (Print)978-952-03-2454-4
Publication statusPublished - 2022
Publication typeG4 Doctoral dissertation (monograph)

Publication series

NameTampere University Dissertations - Tampereen yliopiston väitöskirjat
Volume623
ISSN (Print)2489-9860
ISSN (Electronic)2490-0028

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