Modelling of austenite decomposition of hot-rolled plain carbon steels under complex cooling conditions

Abstract

The purpose of the present work is to develop a mathematical model allowing the simultaneous prediction of both transformation product portions and mean ferrite grain size from the same common principles as a result of austenite decomposition during continuous cooling of plain carbon steels. The transformation products considered specifically are polygonal ferrite and pearlite. The model is based on the classical equations of nucleation‐growth theory and also contains some empirical parameters. The chemical driving forces for nucleation and composition of elements at the phase interfaces are derived from thermodynamic analysis. Three modes of ferrite nucleation are taken into account that correspond to the nucleation on the austenite grain corners, edges and faces. The model considers the reduction of the nucleation sites due to the occupation of austenite grain boundary surface by ferrite grains. Pearlite transformation starts at the γ/α interface and suppresses further ferrite grain growth. The parameters related to ferrite reaction were determined on the basis of a series of austenite transformation kinetic curves and grain size measurements for a steel with the composition 0.084%C‐0.58%Mn‐0.02%Si obtained by dilatometric technique for cooling rates from 0.032 to 2.5 K/s. The parameters related to pearlite reaction were determined on the basis of the data for a steel with 0.66%C. After determination of the model parameters the model was applied to complex cooling conditions of the run‐out table of the hot strip mill at Voest‐Alpine Stahl Linz GmbH. Predicted ferrite grain size appeared to be 1.2 −1.3 times smaller than the observed one. With regard to experimental data on grain growth in iron, it was suggested that the underestimation of grain size is due to additional ferrite grain growth occurring after the coiling of the steel sheet. Taking that into account provided satisfactory agreement with observed values.

Modeling of austenite decomposition in plain carbon steels during hot rolling

“…Using the values obtained for plain carbon steels [8], n = 3.5, k = 6.0 × 10 8 and Q = 160 kJ/mol, the additional growth of ferrite grains in coiling procedure was calculated.…”

Computer Simulation of Microstructure Evolution in Low Carbon Sheet Steels

“…Particularly in the last decade, the emphasis has shifted to modelling the austenite decomposition on the run-out table. 31,[33][34][35][36][37] The complexity of the phase transformations in steels as well as of heat transfer of jet impingement boiling 38,39) may have led to this delayed emphasis in developing microstructure models for run-out table cooling. It is the microstructure resulting from the austenite decomposition and, thus, run-out table processing that primarily determine hot band properties.…”

Modelling the Temperature Distribution along the Length of Strip during Hot Rolling Process

“…In equations (18), (19), and (20), s; is average deformation resistance, 11 is conversion efficiency and assumed to be 0.9.…”

“…Furthermore, most of the work was relied on FEM, but a simple and fast on-line temperature prediction was never presented. Although the prediction system of microstructure and mechanical properties could have applied the through-temperature variation model, for commercial reason, these researchers did not present the details [19,20]. In the present paper, based on the heat conductive theory and steel rolling theory, thermal models are developed and validated at an actual hot strip mill.…”