Abstract
In-situ Small-Angle Neutron Scattering (SANS) is used to determine the time evolution of the chemical composition of precipitates at 650 °C and 700 °C in three micro-alloyed steels with different vanadium (V) and carbon (C) concentrations. Precipitates with a distribution of substoichiometric carbon-to-metal ratios are measured in all steels. The precipitates are initially metastable with a high iron (Fe) content, which is gradually being substituted by vanadium during isothermal annealing. Eventually a plateau in the composition of the precipitate phase is reached. Faster changes in the precipitate chemical composition are observed at the higher temperature in all steels because of the faster vanadium diffusion at 700 °C. At both temperatures, the addition of more vanadium and more carbon to the steel has an accelerating effect on the evolution of the precipitate composition as a result of a higher driving force for precipitation. Addition of vanadium to the nominal composition of the steel leads to more vanadium rich precipitates, with less iron and a smaller carbon-to-metal ratio. Atom Probe Tomography (APT) shows the presence of precipitates with a distribution of carbon-to-metal ratios, ranging from 0.75 to 1, after 10 h of annealing at 650 °C or 700 °C in all steels. These experimental results are coupled to ThermoCalc equilibrium calculations and literature findings to support the Small-Angle Neutron Scattering results.
Highlights
High-performance steels with high strength, ductility and stretch flange-ability are required nowadays in lightweight automotive parts for low fuel consumption, reduced CO2 emission and little use of raw materials [1]
The major advantages of performing in-situ Small-Angle Neutron Scattering (SANS) measurements which we demonstrate in this work, are: 1) the SANS signal of the precipitates is free from interference from the dislocations of the martensite and 2) an optimum background can be measured of the matrix without the presence of precipitates at temperatures where all precipitates are dissolved in the matrix
All nuclear and magnetic differential scattering cross-sections of the LCLV, LCHV and HCHV steels obtained by in-situ SANS during annealing at 650°C and at 700 °C are presented in the supplementary material of this paper (Fig. S2)
Summary
High-performance steels with high strength, ductility and stretch flange-ability are required nowadays in lightweight automotive parts for low fuel consumption, reduced CO2 emission and little use of raw materials [1]. Nano-steels have attracted both industrial and technological interest due to their high potential to meet these demands [2,3,4,5,6,7,8]. Their outstanding mechanical properties arise from the combination of a ferritic matrix with nanosized precipitates. The ferritic phase offers high ductility while a substantial degree of strengthening originates from the presence of precipitates. Vanadium carbide precipitates are well known for precipitation strengthening [5,9], much research has been conducted on their effect on the mechanical properties of
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