Elsevier

Materials & Design

Volume 188, March 2020, 108484
Materials & Design

Unravelling the mechanical behaviour of advanced multiphase steels isothermally obtained below Ms

https://doi.org/10.1016/j.matdes.2020.108484Get rights and content
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Highlights

  • Holding time effect on microstructure-properties relationship of a low-C steel isothermally treated below Ms is discussed.

  • The mechanical behaviour of below-Ms multiphase steels is primarily affected by the distinct tempering degree of martensite.

  • The bainitic character of the isothermal product phase formed below Ms is confirmed by the analysis of Kocks-Mecking plots.

  • Multiphase steels obtained below Ms exhibit comparable properties to conventional steels after shorter processing routes.

Abstract

The initial formation of athermal martensite was proven to accelerate the subsequent bainite formation kinetics during isothermal holdings below the martensite-start temperature (Ms). The presence of prior athermal martensite (PAM) within the phase mixture is expected to modify the overall mechanical response of these newly-designed multiphase steels. Differences stem not only from the balance of product phases, but also from the effect of tempering of the PAM with variations in the applied holding time. This work investigates the effect of tempering time on the mechanical behaviour of the PAM and, as consequence, on the overall mechanical response of these microstructures. Results show that, for short holding times (several minutes), PAM yields similar to as-quenched martensite while, for longer holding times, its yielding behaviour becomes comparable to the one exhibited by typical tempered martensite. Furthermore, the use of Kocks-Mecking curves for the analysis of the mechanical performance confirms the bainitic character of the product phase isothermally formed below Ms. Tailoring the bainitic-martensitic microstructure with variations of the holding time below Ms enables to obtain advanced multiphase steels with comparable mechanical properties to those exhibited by conventional bainitic steels, but in shorter processing times due to the acceleration of bainite formation.

Keywords

Multiphase steels
Microstructure-properties
Prior athermal martensite
Tempering
Bainite formation
Yielding behaviour

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