High and Very-High Strength Steels under harsh conditions of temperature and loading

Forni, Daniele and Briccola, Deborah and Dotta, Matteo and Tesio, Nicoletta and Cadoni, Ezio (2023) High and Very-High Strength Steels under harsh conditions of temperature and loading. Procedia Structural Integrity, 47. pp. 348-353.

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A comprehensive understanding of the plastic deformation of material at high strain rates and elevated temperature is required in steel structure design, particularly in special cases (e.g. off-shore, large span bridges, high-rise buildings). High Strength Steel (HSS) and Very High Strength Steel (VHSS) are being used in increasingly modern and advanced structures. When compared to traditional grade steel, they have superior mechanical properties, which result in weight savings and a lower life cycle cost (minor costs for construction and maintenance). To produce these steels, they have to be quenched and tempered. It produces differentiation between materials with martensitic microstructures on the outside and bainitic-ferritic microstructures on the inside. Due to their differentiated microstructures, these steels behave as functional graded materials. Therefore, understanding the mechanical response of homogeneous parts at various strain rates and temperatures is fundamental. The deformation mechanisms of these steels under mentioned combined harsh conditions are complex and depend on the micro-structural features of the material. At high strain rates and elevated temperatures, plastic deformation can occur through mechanisms such as dislocation slip, dynamic plasticity, and dynamic strain ageing. The competition between these deformation mechanisms can lead to a significant change in the mechanical behaviour of these advanced steels, particularly in the strain rate and temperature regimes where these mechanisms are active. In this paper, we will review the recent state of knowledge developed at DynaMat Laboratory on the mechanical behaviour of HSS and VHSS under combined conditions of elevated temperature and high strain rate, including its macro-structural features, deformation mechanisms, and constitutive modelling. The tests were carried out in a wide range of temperature (20÷900°C) and strain rate (10-3÷103 1/s) by means of a Split Hopkinson Tensile Bar equipped with a water-cooled induction heating system on round specimens having a diameter of 3 mm and gauge length of 5 mm. Both the core and the peripheral parts of the section of two slabs 40 mm thick of S690QL and S960QL steels were investigated.

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