University of Oulu

Pallaspuro, S., Mehtonen, S., Kömi, J., Zhang, Z., Porter, D. (2019) Effects of local grain size and inclusions on the low-temperature toughness of low-carbon as-quenched martensite. Materials Science and Engineering: A, 743, 611-622. doi:10.1016/j.msea.2018.11.105

Effects of local grain size and inclusions on the low-temperature toughness of low-carbon as-quenched martensite

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Author: Pallaspuro, Sakari1,2; Mehtonen, Saara3; Kömi, Jukka1;
Organizations: 1Materials and Production Engineering, Centre for Advanced Steels Research, University of Oulu, Finland
2Department of Structural Engineering, Faculty of Engineering Science and Technology, NTNU, Norway
3SSAB, Rautaruukintie 155, PO box 93, 92101 Raahe, Finland
Format: article
Version: accepted version
Access: embargoed
Persistent link: http://urn.fi/urn:nbn:fi-fe201902286581
Language: English
Published: Elsevier, 2019
Publish Date: 2020-11-23
Description:

Abstract

The segregation of alloying elements that occurs during the solidification of steel leads to microscale and macroscale microstructural heterogeneity that can cause anomalous mechanical behaviour. The centreline macrosegregation of a cast and its increased inclusion content are usually considered to be particularly detrimental in the case of conventional structural steels. Samples from centreline and off-centreline positions in a single continuously cast slab of an ultrahigh-strength steel were subjected to hot rolling, reheating and water quenching to 12 mm thick fully martensitic plates to explore the differences in mechanical properties between a homogeneous clean matrix and a heterogeneous inclusion-rich centreline. Despite the presence of strong macrosegregation and a high inclusion content, the centreline material has a significantly better, i.e. a 15 °C lower, fracture toughness reference temperature T0. However, neither the 28 J Charpy V impact toughness transition temperature T28J nor the tensile properties show notable differences. The inclusion rich heterogeneous material achieves its unexpected toughness properties despite the higher hardness of the centreline and an abundance of large inclusions. Thorough microstructural characterisation shows that the centreline enrichment of alloying elements and impurities leads to a profound refinement in the local grain size, which more than compensates for the expected detrimental effects of the inclusions and the harder microstructure. The results have practical importance regarding the levels of macrosegregation and inclusion contents that can be tolerated by ultrahigh-strength steels.

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Series: Materials science & engineering. A, Structural materials: properties, microstructure and processing
ISSN: 0921-5093
ISSN-E: 1873-4936
ISSN-L: 0921-5093
Volume: 743
Pages: 611 - 622
DOI: 10.1016/j.msea.2018.11.105
OADOI: https://oadoi.org/10.1016/j.msea.2018.11.105
Type of Publication: A1 Journal article – refereed
Field of Science: 216 Materials engineering
Subjects:
T0
Funding: The authors gratefully acknowledge the funding provided through the BSA programme of DIMECC Ltd. from the Finnish Funding Agency for Innovation (Tekes) and the participating companies, and from the Academy of Finland (311934). Dr. Pallaspuro would also like to thank the Association of Finnish Steel and Metal Producers, the Finnish Foundation for Technology Promotion (TES) and the University of Oulu Graduate School for financially supporting the visit to NTNU.
Copyright information: © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
  https://creativecommons.org/licenses/by-nc-nd/4.0/