University of Oulu

Hannula, J.; Porter, D.; Kaijalainen, A.; Somani, M.; Kömi, J. Optimization of Niobium Content in Direct Quenched High-Strength Steels. Metals 2020, 10, 807.

Optimization of niobium content in direct quenched high-strength steels

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Author: Hannula, Jaakko1; Porter, David1; Kaijalainen, Antti1;
Organizations: 1Materials and Mechanical Engineering, Faculty of Technology, University of Oulu, P.O. Box 4200, 90014 Oulu, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 7.3 MB)
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Language: English
Published: Multidisciplinary Digital Publishing Institute, 2020
Publish Date: 2020-06-22


This paper focuses on understanding the effect of niobium content on the phase transformation behavior and resultant mechanical properties of thermomechanically rolled and direct-quenched low carbon steels containing 0.08 wt.% carbon. Investigated steels contained three different levels of niobium: 0, 0.02 and 0.05 wt.%. The continuous cooling transformation (CCT) diagrams covering cooling rates in the range 3–96 °C/s constructed based on the dilatometer studies showed only a minor effects of Nb on the phase transformation characteristics. In addition, experiments were performed for reheating and soaking the slabs at 1050–1200 °C and the results revealed that for these low-carbon steels, Nb failed to prevent the austenite grain growth during slab reheating. In the case of hot rolling trials, two different finish rolling temperatures of 820 °C and 920 °C were used to obtain different levels of pancaking in the austenite prior to direct quenching. The resultant microstructures were essentially mixtures of autotempered martensite and lower bainite imparting yield strengths in the range 940–1070 MPa. The lower finish rolling temperature enabled better combinations of strength and toughness in all the cases, predominantly due to a higher degree of pancaking in the austenite. The optimum level of Nb in the steel was ascertained to be 0.02 wt.%, which resulted not only in marginally higher strength but also without any significant loss of impact toughness.

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Series: Metals
ISSN: 2075-4701
ISSN-E: 2075-4701
ISSN-L: 2075-4701
Volume: 10
Issue: 6
Article number: 807
DOI: 10.3390/met10060807
Type of Publication: A1 Journal article – refereed
Field of Science: 216 Materials engineering
Funding: This research was funded by SSAB Europe Oy.
Copyright information: © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (