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Metals 2017, 7(2), 65; doi:10.3390/met7020065

Evaluating Strengthening and Impact Toughness Mechanisms for Ferritic and Bainitic Microstructures in Nb, Nb-Mo and Ti-Mo Microalloyed Steels

CEIT and TECNUN, University of Navarra, 20018 San Sebastian, Basque Country, Spain
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Academic Editor: Carlos Garcia-Mateo
Received: 10 February 2017 / Revised: 17 February 2017 / Accepted: 17 February 2017 / Published: 22 February 2017
(This article belongs to the Special Issue Bainite and Martensite: Developments and Challenges)
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Abstract

Low carbon microalloyed steels show interesting commercial possibilities by combining different “micro”-alloying elements when high strength and low temperature toughness properties are required. Depending on the elements chosen for the chemistry design, the mechanisms controlling the strengths and toughness may differ. In this paper, a detailed characterization of the microstructural features of three different microalloyed steels, Nb, Nb-Mo and Ti-Mo, is described using mainly the electron backscattered diffraction technique (EBSD) as well as transmission electron microscopy (TEM). The contribution of different strengthening mechanisms to yield strength and impact toughness is evaluated, and its relative weight is computed for different coiling temperatures. Grain refinement is shown to be the most effective mechanism for controlling both mechanical properties. As yield strength increases, the relative contribution of precipitation strengthening increases, and this factor is especially important in the Ti-Mo microalloyed steel where different combinations of interphase and random precipitation are detected depending on the coiling temperature. In addition to average grain size values, microstructural heterogeneity is considered in order to propose a new equation for predicting ductile–brittle transition temperature (DBTT). This equation considers the wide range of microstructures analyzed as well as the increase in the transition temperature related to precipitation strengthening. View Full-Text
Keywords: microalloyed steels; niobium; molybdenum; titanium; mechanical properties; yield strength; impact toughness; modeling; microstructure; EBSD microalloyed steels; niobium; molybdenum; titanium; mechanical properties; yield strength; impact toughness; modeling; microstructure; EBSD
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Larzabal, G.; Isasti, N.; Rodriguez-Ibabe, J.M.; Uranga, P. Evaluating Strengthening and Impact Toughness Mechanisms for Ferritic and Bainitic Microstructures in Nb, Nb-Mo and Ti-Mo Microalloyed Steels. Metals 2017, 7, 65.

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