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.
A study has been made of the distribution of phosphate chains obtained in phosphate glasses by quenching melts with compositions corresponding approximately to the mean anions P3O 3.0 and P4O 9 5. The cations present were Na+, Lif, Ca2f and Zn2f singly, and Na++ Li+, Ca2+ + LI+, Ca2++Zn2+, Na++Zn2+ in pairs. The glasses were dissolved in water and the distributions determined chromatographically ; the validity of the techniques were checked by measurements on crystalline phosphates.The distributions obtained were always narrower than the ideal Flory distributions, the width increasing from Na+ to Zn2+ in the above order. The divergence from the Flory distributions can be expressed in terms of the phosphate ion equilibrium products, i.e.,
A theoretically based model was developed by using numerical integration methods on a multiparticle system to predict the dissolution and growth kinetics of nitrides and carbides in steels undergoing heat treatment. This model takes fully into account the equilibrium thermodynamic properties of the systems, the local equilibrium at the interface, curvature effects, and diffusion along the grain boundary. Dissolution and coarsening are being treated as one continuous, simultaneous process. In the present work, the model is applied to study the dissolution and coarsening behavior of aluminum nitride (AlN) in Al-killed low-carbon steels. Theoretically predicted particle-size distributions are in good agreement with the experimental measurements.
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