Abstract
A cold-rolled Ti-V high strength low alloy (HSLA) steel was isothermally annealed at 650 °C and 700 °C for different times. A unique combination of techniques including visible light microscopy (VLM), transmission electron microscopy (TEM), matrix dissolution, small angle neutron scattering (SANS) and hardness measurement has been employed to investigate the evolution of microstructure, hardness and precipitate composition, size and volume fraction. Results show that recrystallization is completed after annealing 8 h at 650 °C and 30 min at 700 °C. Three types of precipitates were identified: large Ti(C,N), medium-size (Ti,V)(C,N) and small (Ti,V)C. The Ti/(Ti+V) atomic ratio in the (Ti,V)C precipitates decreases with increasing radius in the 1–15 nm range, which can be explained by the initial nucleation of a TiC-rich core. The average size of the (Ti,V)C precipitates increases, whereas the number density decreases during annealing. The volume fractions of the three types of precipitates were separately determined by the matrix dissolution method. The volume fractions of (Ti,V)C precipitates obtained by matrix dissolution are comparable even slightly more accurate than those obtained by SANS. The hardness first increases and then decreases when annealing at both temperatures, which can be correlated well with the observed microstructural and precipitate evolution.
Highlights
To reduce energy consumption and CO2 emission in the past few decades, there has been an increasing demand for lightweight vehicles [1]
The main active strengthening mechanisms in high strength low alloy (HSLA) steels are due to grain refinement and precipitation hardening, both of which are achieved by nanoscale precipitates containing e.g. niobium (Nb), titanium (Ti), vanadium (V), molybdenum (Mo) individually or in certain combinations [4,5,6,7]
Nano-scale precipitates can be quantitatively characterized by TEMbased imaging, transmission electron microscopy (TEM)-energy dispersive X-ray spectroscopy (EDS), small angle neutron scattering (SANS), matrix dissolution and atom probe tomography (APT)
Summary
To reduce energy consumption and CO2 emission in the past few decades, there has been an increasing demand for lightweight vehicles [1]. The main active strengthening mechanisms in HSLA steels are due to grain refinement and precipitation hardening, both of which are achieved by nanoscale precipitates containing e.g. niobium (Nb), titanium (Ti), vanadium (V), molybdenum (Mo) individually or in certain combinations [4,5,6,7]. These carbides increase strength by precipitation hardening [4,5,8], and retard recrystallization, leading to smaller grain sizes [9,10]
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