• PDF / 1,809,332 Bytes
• 10 Pages / 598 x 778 pts Page_size
• 51 Downloads / 327 Views

DOWNLOAD

REPORT

---

I.

INTRODUCTION

DURING recent years, microstructure engineering in hot strip mills has gained significant attention, with the goal being to develop a predictive tool which quantitatively links the processing parameters to the properties of the hot-rolled steel product. For low-carbon steels, mechanical properties such as strength and toughness can be directly related to the ferrite grain size developed as a result of the thermomechanical treatment. This ferrite grain size is the product of the anstenite decomposition taking place on the run-out table. The austenite microstructure obtained after rolling and the cooling conditions determine the kinetics of the austenite-to-ferrite transformation. Phase transformations in steels have been investigated more extensively than those in any other material, and many articles have been published since the 1930s. However, the characteristics of the entire transformation process are not yet fully understood. In the absence of retained strain, ferrite predominantly nucleates at the austenite grain boundaries. In the 1980s, Aaronson and co-workers performed pioneering studies to shed some light on the mechanisms of this type of ferrite nucleationY -4] Subsequent ferrite growth is usually discussed as a process governed by carbon diffusion in austenite. I5,6] Experimental results, however, indicate a solute draglike effect from alloying elements such as Mn.tT.81Frequently, semiempirical relationships, based on the Avrami equation, tg~ are proposed to quantify the effect of chemistry and austenite grain size on the transformation kineticsY ~ ~1Employing additivity prin-

M. MILITZER, Research Associate, R. PANDI, Ph.D. Student, and E.B. HAWBOLT, Professor, are with The Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 124. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled "Atomistic Mechanisms of Nucleation and Growth in Solids," organized in honor of H.I. Aaronson's 70th Anniversary and given October 3-5, 1994, in Rosemont, Illinois. METALLURGICALAND MATERIALSTRANSACTIONS A

ciples, which consider the equilibrium portion of transformed fraction at each temperature,tin31 this approach was successfully applied to account for the transformation kinetics during continuous cooling, v~ But to gain confidence in predicting the austenite-to-ferrite transformation kinetics under industrial conditions, a more fundamental model is required. The present article deals with the austenite-to-ferrite transformation in two commercial low-carbon, plain-carbon steels under cooling conditions similar to those obtained on the run-out table of a hot strip mill. A phenomenological model is proposed to describe nucleation and growth of ferrite from a more fundamental point of view. II.

EXPERIMENTAL

A. Materials and Tests To determine the kinetics of decomposition of austenite, dilatometric measurements have been performed on the tubular specimens (8-mm diameter and 1-mm wall thickness) of two