S Vandeputte

Static strain aging behavior of ultra low carbon bake hardening steel

A detailed study of static strain aging in ultra low carbon (ULC) steel has not yet been reported. Therefore, the present study was carried out to gain an understanding of the aging kinetics in a ULC steel with a total carbon content of 20 ppm. The influence of dislocation density on the aging process was also taken into account. The kinetics of the aging were determined by means of the measurement of strength properties rather than solute concentration as it was experienced that quantitative estimation of such low amount of carbon during aging course would be too difficult with the existing diagnostic tools.

Effect of dislocation density on the low temperature aging behavior of an ultra low carbon bake hardening steel

Abstract Strain aging was studied in an ultra low carbon (ULC) steel with a total carbon content of 20 ppm (wt.%) in order to identify the process stages and mechanism of bake hardening in this type of steel. The effects of dislocation density, varied by means of uniaxial tensile prestraining (1–10%) on the aging kinetics were investigated within an aging temperature range of 50–170°C. The aging was evaluated by means of strength measurements and the determination of interstitial carbon content after aging using a piezoelectric composite oscillator operating at 40 kHz. The interaction between interstitial carbon and dislocations was examined through amplitude dependent internal friction measurements. The influence of dislocation density on the aging behavior have been discussed with reference to the kinetics and mechanism of the aging process.

Kinetics of strain aging in bake hardening ultra low carbon steel—a comparison with low carbon steel

The kinetics of the static strain aging process have been analyzed in a vacuum-degassed ultra low carbon bake hardenable (ULC BH) steel with a total carbon content of 20 wt.ppm through measurement of the strength properties. The influence of prestrain and free interstitial carbon content has been studied. The kinetic results were compared with those of a BH low carbon (LC) steel. In the derivation of the time exponent and the activation energy, only the first stage of aging was considered. It was observed that, at all prestrain levels and matrix solute carbon contents, the initial aging process in the ULC steel obeyed the t2/3 kinetic law and the kinetics were not influenced by the changes in dislocation structure due to prestrain and the dissolved carbon content. In comparison, the aging process and the kinetics in the LC steel were found to be significantly influenced by the amount of prestrain. The presence of carbide particles in LC steels can modify the aging kinetics.

Dilatometric study of the effect of soluble boron on the continuous and isothermal austenite decomposition in 0.15C-1.6Mn steel

Abstract The effect of soluble boron on the phase transformations during either cooling and/or isothermal holding has been studied by means of dilatometry. Significant differences in the transformation behaviour were found for all austenite phase transformation reactions. In particular, the morphologies of ferrite and bainite were strongly affected by B alloying. The kinetics was studied in detail for the austenite decomposition reactions. Soluble B was found to be very effective in suppressing the carbide formation. It was also found to interact with the Mn partitioning to the austenite. As a result, the presence of Mn-rich regions in the final microstructures decreased strongly the Ac! temperature during reheating. Isothermal transformations in the 450–660°C temperature range showed that the incubation times for ferrite and pearlite formations were increased. The soluble B was found to affect strongly the nucleation rate. The growth kinetics was slower due to a solute drag effect caused by the partition...

Effect of grain size on the static strain aging of a ULC-bake hardening steel

Bake hardenability in ultra low carbon bake hardening (ULC-BH) steels is primarily influenced by the (i) solute carbon content in the matrix, (ii) dislocation density and (iii) grain boundaries. Grain boundaries in fine-grained steels significantly change the dislocation distribution and carbon-concentration profiles from grain interior to the grain boundary and correspondingly, the bake hardening response may change. The present study examines the effect of different grain sizes (66 – 15 lm) on the bake hardening behavior of a ULC-BH steel (total carbon content 24 wt.ppm) as a function of tensile strain (2 – 5 %) and aging temperatures (50 – 170 °C). The carbon concentration profiles across the grains of different sizes have been evaluated both through modeling calculations and experimental measurements. Results showed that the bake hardening decreases with decreasing grain size in general, but there is a critical fine grain size corresponding to a specific amount of tensile strain, at which the bake hardening increases significantly due to the contribution of grain boundary carbon and increased dislocation density adjacent to the grain boundaries.