A systematic experimental study has been conducted on ferrite recrystallization and intercritical austenite formation for two low-carbon steels with chemical compositions typically used for dual-phase and transformation-induced plasticity (TRIP) steels. Different initial heating rates, holding temperatures, and times were applied to the materials to examine the ferrite recrystallization and austenite formation kinetics. An Avrami model was developed to describe the isothermal ferrite recrystallization behavior and was applied successfully to the nonisothermal conditions. It was found that the initial heating rate affects the isothermal austenite formation kinetics for both the hot-rolled and cold-rolled materials albeit the effect is more pronounced for the cold-rolled material. This can be attributed to the interaction between the ferrite recrystallization and austenite formation processes. Furthermore, it was found that the distribution of austenite phase is also affected by the ferrite recrystallization process. When ferrite recrystallization is completed before the austenite formation (i.e., under sufficiently slow heating rate conditions), austenite is to a large extent randomly distributed in the ferrite matrix. On the other hand, incomplete recrystallization of ferrite due to higher heating rates leads to the formation of banded austenite grains. It is proposed that this observation is characteristic of simultaneous recrystallization and austenite formation where moving ferrite grain boundaries do not provide suitable sites for austenite nucleation.

The precipitation behavior of a commercial high-strength low-alloy (HSLA) steel microalloyed with 0.086 wt pct Nb and 0.047 wt pct Ti has been investigated using transmission electron microscopy (TEM) and mechanical testing. The emphasis of this study is to compare an industrially hot-rolled steel and samples from a laboratory hot torsion machine simulation. From TEM observations, the Ti and Nb containing precipitates could be grouped according to their size and shape. The precipitates in order of size were found to be cubic TiN particles with sizes in the range of 1 m, grain boundary precipitates with diameters of approximately 10 nm, and very fine spherical or needleshaped precipitates with sizes on the order of 1 nm. The needlelike precipitates were found on dislocations in ferrite and constituted the dominant population in terms of density. Thus, they appear to be responsible for the precipitation strengthening observed in this steel. Aging tests were carried out at 650 ЊC to evaluate the precipitate strengthening kinetics in detail. The strengthening mechanisms can be described with a nonlinear superposition of dislocation and precipitation hardening. The mechanical properties of torsion-simulated material and as-coiled industrial material are similar; however, there are some microstructural differences that can be attributed to the somewhat different processing routes in the laboratory as compared to hot strip rolling.

Precipitation kinetics and strengthening have been investigated for a Fe-0.8wt%Cu alloy. Microstructure evolution during aging at 500°C has been studied by a combination of Transmission Electron Microscopy and Small-Angle X-ray Scattering to provide information on the nature and location of the precipitates as well as a quantitative estimate of their size and volume fraction. The associated mechanical properties have been studied by hardness and tensile tests.The precipitation kinetics measured in this study are fully compatible with results reported for alloys with higher Cu levels. Nucleation of Cu precipitates is promoted by the presence of dislocations whereas coarsening rates in the later stages of aging appear to be not affected by fast diffusion paths along dislocations.The strength of individual precipitates increases with precipitate size based on the analysis of the mechanical test results. However, the strength of the largest precipitates observed remains approximately half of the strength required for the Orowan by-passing mechanism. The Russell-Brown model for modulus strengthening has successfully been applied to the current data.Study of the plastic behavior shows that the maximum initial hardening rate is related to the highest strength of the material. This unusual result may be explained by a dynamic strained-induced phase transformation of the precipitates from the bcc to the 9R structure. Consequently, the hardening potential of Fe-Cu alloys is associated with good plastic properties close to peak strength thereby indicating the excellent potential of copper as hardening element for the development of novel high strength interstitial free (IF) steels.KEY WORDS: iron-copper alloy; precipitation kinetics; precipitation strengthening; strain hardening; smallangle X-ray Scattering.formation to the 9R structure has been initiated. 10,11,19) The strengthening is usually described by employing the approach of Russell and Brown which is based on modulus strengthening. 20) This approach allows for the prediction of mechanical properties during an aging treatment assuming that the strength of the interaction between the dislocation and the precipitate increases with particle size. An alternative approach has recently been proposed by Osamura et al.,19) assuming that the hardening during the initial stage is controlled by coherency strains. The decrease in strength after the peak strength is attributed to the loss of coherency of the precipitates. Currently available experimental data do not allow to give preference to one of these two models.The effect of precipitation, coupled with the evolution of solid solution content, on the overall work hardening behavior is still poorly understood. Current knowledge derives from the pioneering contribution of Hornbogen et al. 21) where a few curves of initial work hardening rates are presented. More generally, a limited theoretical framework including the effect of bypassed precipitates has been proposed recently by Estrin,22) but the complete picture is still ...

The microstructural evolution during hot-strip rolling has been investigated in four commercial highstrength low-alloy (HSLA) steels and compared to that of a plain, low-carbon steel. The recrystallization rates decrease as the Nb microalloying content increases, leading to an increased potential to accumulate retained strain during the final rolling passes. The final microstructure and properties of the hot band primarily depend on the austenite decomposition and precipitation during run-out table cooling and coiling. A combined transformation-ferrite-grain-size model, which was developed for plain, lowcarbon steels, can be applied to HSLA steels with some minor modifications. The effect of rolling under no-recrystallization conditions (controlled rolling) on the transformation kinetics and ferrite grain refinement has been evaluated for the Nb-containing steels. Precipitation of carbides, nitrides, and/or carbonitrides takes place primarily during coiling, and particle coarsening controls the associated strengthening effect. The microstructural model has been verified by comparison to structures produced in industrial coil samples.

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