Abstract
The dynamic recrystallization behavior of ultra-high strength boron-microalloyed steels optionally alloyed with niobium and molybdenum is analyzed in this paper. Multipass torsion tests were performed to simulate plate rolling conditions followed by direct quenching. The influence of alloy composition on the transformed microstructure was evaluated by means of EBSD, thereby characterizing the morphology of the austenite grain morphology after roughing and finishing passes. The results indicated that for Nb-microalloyed steel, partial dynamic recrystallization occurred and resulted in local clusters of fine-sized equiaxed grains dispersed within the pancaked austenitic structure. A recrystallized austenite fraction appeared and transformed into softer phase constituents after direct quenching. The addition of Mo was shown to be an effective means of suppressing dynamic recrystallization. This effect of molybdenum in addition to its established hardenability effects hence safeguards the formation of fully martensitic microstructures, particularly in direct quenching processes. Additionally, the circumstances initiating dynamic recrystallization were studied in more detail, and the interference of the various alloying elements with the observed phenomena and the potential consequences of dynamic recrystallization before quenching are discussed.
Highlights
Publisher’s Note: MDPI stays neutralUltra-high strength steel with a martensitic microstructure is the preferred material for structural applications requiring an extreme load-bearing capacity or superior wear resistance
Martensitic steels are traditionally produced by conventional quenching (CQ), where the steel is reheated from ambient temperature back into austenite before quenching
electron backscattered diffraction (EBSD) analysis was performed in the same specific locations corresponding to the optical images
Summary
Ultra-high strength steel with a martensitic microstructure is the preferred material for structural applications requiring an extreme load-bearing capacity or superior wear resistance. Direct quenching (DQ) is an increasingly often practiced variant for processing ultra-high strength steel that enables cost and capacity optimization in steel mills [1]. The martensite substructure originating from the DQ process develops within a pancaked austenite microstructure [2]. The microstructural homogeneity of austenite before quenching is related to recrystallization phenomena occurring along the entire austenite hot working process. An inhomogeneous prior austenite microstructure is detrimental for the toughness and () ductile-to-brittle transition temperature of the quenched steel [2,3]. Microstructural heterogeneity in austenite can be generated at different stages during the hot working with regard to jurisdictional claims in published maps and institutional affiliations
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