Kinetics of Delta-Ferrite to Austenite Phase Transformation in a Two-Phase Fe-Al-C Alloy
- PDF / 2,817,338 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 59 Downloads / 229 Views
INTRODUCTION
BECAUSE of their good combination of strength and toughness, microalloyed (MA) steels are used in a wide range of applications including oil and natural gas transmission pipelines, offshore equipment, automotive components, and machinery.[1–4] In order to achieve good toughness (low ductile-to-brittle transition temperature), weldability, formability, and corrosion resistance, the carbon content of most MA steels is maintained at ~0.03 to 0.08 wt pct.[5,6] When the carbon content is lower than 0.08 wt pct, MA steels would solidify as delta-ferrite. Fast diffusion in the delta phase leads to rapid homogenization of the as-cast microstructure. Upon further cooling, the delta-ferrite transforms to austenite, and at yet lower temperatures, austenite transforms to alpha-ferrite. While the theory of solidification of MA steels[7–10] and the kinetics of austenite to alpha-ferrite phase transformation[11–14] were extensively studied and developed, comparatively little information is available concerning the kinetics of the delta-ferrite to austenite transformation and its effect on the grain size. The reason for this is that the final alpha-ferrite grain size is largely controlled by the parent austenite grain size.[11,12,15] A smaller austenite grain size will lead to refinement of the alpha ferrite grain size. In modern MA steel production processes such as thin slab casting direct rolling (TSCDR), the opportunities for control of the microstructure through thermomechanical TIHE ZHOU, Postdoctoral Fellow, and HATEM S. ZUROB, Assistant Professor, are with the Steel Research Centre, McMaster University, Hamilton, ON L8S 4L7, Canada. Contact e-mail: [email protected] ELACHMI ESSADIQI and BENOIT VOYZELLE, Research Scientists, are with the CANMET Materials Research Laboratory, Ottawa, ON K1A 0G1, Canada. Printed by permission of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2011. Manuscript submitted January 24, 2011. Article published online June 3, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
are limited by the small number of deformation passes available. Previous study shows that the delta-ferrite and austenite grain size (before entering the homogenization furnace) is nonuniform with large grains being present inside the slab center.[16] It is often observed that during TSCDR, thermomechanical processing can reduce the average austenite grain size, but cannot eliminate the nonuniformity; a small number of large austenite grains persist in the microstructure.[17,18] Therefore, understanding and controlling the deltaferrite to austenite phase transformation becomes increasingly important to obtain fine and uniform austenite grains and, subsequently, final alpha-ferrite grain size. The main difficulty in studying this transformation (d fi c) is that any untransformed delta-ferrite would transform on quenching, making it impossible to observe the progress of the transformation in quenched specimens. A second obstacle is that the d fi c transformation temperature is very
Data Loading...
