Mechanical behavior of multi-stage heat-treated HSLA steel based on examinations of microstructural evolution

Abstract

Abstract In the current work, multi-stage heat treatment involving quenching (Q), intercritical treatment or lamellarization (L), and tempering (T) is applied in high-strength low-alloy (HSLA) steel to study the microstructural evolution by multi-scale characterizations, kinetics of austenite reversion and corresponding mechanical behavior are further investigated. The as-quenched (AQ) and QT specimens possess high strength but low ductility and especially low toughness, whereas the QL and QLT specimens exhibit a significantly improved toughness. Introducing L-treatment leads to austenite reversion with the formation of globular γG grains on high angle grain boundaries (HAGBs) and acicular γA grains on lath boundaries during L-annealing, both of which exhibit low stability and further transform to the corresponding globular marteniste (GM) and acicular martensite (AM) upon L-quenching. The Averami exponent n for austenite reversion based on JMAK analysis varies in the range of 0.285 and 0.329 with an activation energy of 293 kJ/mol, indicating the austenite reversion corresponds to a diffusional process that controlled by competition between solute drag and solute diffusion. Mechanical properties are studied in terms of microstructural characteristics involving volume fraction of martensite, dimension and morphology of intercritical ferrite, interactions of dislocation and nano-scale carbides, which are responsible for the variation in the strain hardening behavior. The L-treatment process can produce a significant grain refinement effect via microstructural modification, refined QL-martensite particles with high density HAGBs are obtained, thus improve the comprehensive performance especially impact toughness of the steel.