Austenite formation kinetics in two high-strength experimental microalloyed steels with different initial microstructures comprising bainite–martensite and ferrite–martensite/austenite microconstituents was studied during continuous heating by dilatometric analysis. Austenite formation occurred in two steps: (1) carbide dissolution and precipitation and (2) transformation of residual ferrite to austenite. Dilatometric analysis was used to determine the critical temperatures of austenite formation and continuous heating transformation diagrams for heating rates ranging from 0.03°C•s−1 to 0.67°C•s−1. The austenite volume fraction was fitted using the Johnson–Mehl–Avrami–Kolmogorov equation to determine the kinetic parameters k and n as functions of the heating rate. Both n and k parameters increased with increasing heating rate, which suggests an increase in the nucleation and growth rates of austenite. The activation energy of austenite formation was determined by the Kissinger method. Two activation energies were associated with each of the two austenite formation steps. In the first step, the austenite growth rate was controlled by carbon diffusion from carbide dissolution and precipitation; in the second step, it was controlled by the dissolution of residual ferrite to austenite.

Effect of initial microstructure on austenite formation kinetics in high-strength experimental microalloyed steels

Microstructure and mechanical properties of microalloyed steel forgings manufactured from cross-wedge-rolled preforms

ACMOD-2D : A heat transfer model for the simulation of the cooling of wire rod

In this investigation an algorithm is given to simulate the flow stress curves for multi-pass hot deformation processes simply from the data obtained from single-pass hot torsion test. Using the obtained multi-pass curves the value of no-recrystallization temperature (Tnr) is calculated. This method is examined on a Nb-microalloyed steel. Maximum error in calculation of Tnr with this method and the values obtained from empirical tests was 1.5% which is a very low value. The predicted results are found to be in accord with the experimental data.

L.L. Teoh

Papers

Thermo-mechanical processing and microstructure of microalloyed steel bar and wire rod products

Technological developments in near-net-shape casting for mini-steelmills