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Achieving an Excellent Hole Expansion Behavior in Complex Phase Steels by Characteristic Distribution of Martensite-Austenite Constituents
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China.
Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China.
Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China.
Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China.
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2025 (English)In: Acta Metallurgica Sinica, ISSN 0412-1961, Vol. 61, no 5, p. 674-686Article in journal (Refereed) Published
Abstract [en]

Complex phase steels (CP steels) exhibit an excellent hole expansion performance due to the subtle hardness difference between different kinds of microstructures, in which the high-hardness martensite-austenite (MA) constituents are the critical structure type. The present study aims to improve the hole expansion property by constructing the continuously distributed MA constituents along the rolling direction at the thickness center. Microstructures and hole expansion behavior were investigated using laser confocal microscopy, scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and hole expansion tests. Microstructure characterization results indicate that after improvements, the MA constituents were aggregated at the thickness center in a continuous distribution along the rolling direction with a long axis of approximately 1.25 mu m, and an average distance of less than 1.0 mu m. Hole expansion behavior analysis reveals that the fracture initiated at the edge of the base steel, shown in a mixed ductile-quasi-cleavage fracture. The fracture mode changes to pronounced necking ductile fracture induced by void aggregation at the thickness center of the advanced steel. Micro-hardness quantification of the plastic damage on the punching edge shows that the advanced steel exhibits the highest hardening at the thickness center with a 41% hardness increase over the pre-punching, higher than the 31% hardening in the maximum hardening burr zone of the base steel. The hole expansion ratio of the advanced steel suffering serious punching damage was approximately 43%, higher than that of the base steel (34%). Quasi-in-situ interrupted hole expansion test indicates that on the thickness center of the advanced steel, the circumferential cracks formed through the multiple void interaction mechanism which promotes the stress release. In the matrix, pit-like damage is in a void coalescence mechanism. Both mechanisms lead to mechanical instability and eventual failure. The position of the damaged particles at the hole edge had a decisive impact on the fracture mode.

Place, publisher, year, edition, pages
SCIENCE PRESS , 2025. Vol. 61, no 5, p. 674-686
Keywords [en]
complex phase steel, martensite-austenite (MA) constituent, hole expansion ratio, fracture
National Category
Applied Mechanics Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-365278DOI: 10.11900/0412.1961.2024.00285ISI: 001477387800002Scopus ID: 2-s2.0-105003895106OAI: oai:DiVA.org:kth-365278DiVA, id: diva2:1973897
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QC 20250620

Available from: 2025-06-20 Created: 2025-06-20 Last updated: 2025-06-20Bibliographically approved

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Mu, Wangzhong

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