The high-temperature grain-growth kinetics in delta-ferrite and austenite is investigated. The delta-ferrite growth kinetics was observed directly on a model alloy that contained 2.5 wt pct aluminum in order to stabilize delta-ferrite down to room temperature. The gamma graingrowth kinetics was by etching the former austenite grain boundaries in a precipitate-free variant of APIX60 steel. At high temperatures and in the absence of precipitation, the growth kinetics in both delta-ferrite and austenite appeared to follow a simple parabolic growth law. The findings are applied to the problem of grain-size control during the process of thin-slab casting direct rolling (TSCDR).

Medium-to-high carbon hot-rolled steel strip was developed on a thin slab casting direct strip production complex at Essar Steel Algoma Inc. to meet different specifications and special customer requirements. The effects of chemistry, casting and hot rolling process parameters on microstructure and mechanical properties were studied. Soft core reduction, secondary cooling, casting speed, as well as hot rolling and coiling temperature were optimised to achieve the required mechanical properties and other requirements such as ultrasonic test after electric resistance welding, wear resistance and corrosion resistance. These new developed medium-tohigh carbon steel grades have extensive applications in oil and gas industries, general manufacturing, construction, automotive and mining equipment.

The kinetics of abnormal grain growth in austenite was examined in microalloyed steel containing Nb and Ti additions. Abnormal grain growth was triggered by the dissolution of microalloyed carbonitrides during reheating. The progress of abnormal grain growth was determined at 1200, 1300 and 1400uC. Post-abnormal grain growth was defined as the grain growth process which takes place once all of the original fine grains have been consumed by the abnormal grains. The post-abnormal grain growth kinetics appeared to follow a simple parabolic law. The parabolic constant needed to describe post-abnormal grain growth in the present alloy was three times larger than that needed to describe normal grain growth in a similar alloy without Nb and Ti additions. The possible origins of this effect are briefly discussed.On a examiné la ciné tique de croissance anormale de grain dans l'austé nite d'acier micro-allié contenant des additions de Nb et de Ti. La croissance anormale de grain é tait dé clenché e par la dissolution des carbonitrures micro-allié s lors du ré chauffage. On a dé terminé le progrè s de la croissance anormale de grain à 1200uC, 1300uC et 1400uC. La croissance post-anormale de grain é tait dé finie comme le processus de croissance de grain qui a lieu une fois que tous les grains fins originaux ont é té consommé s par les grains anormaux. La ciné tique de la croissance postanormale de grain semble suivre une simple loi parabolique. La constante parabolique né cessaire pour dé crire la croissance post-anormale de s grain dans le pré sent alliage é tait trois fois plus grande que celle qui est né cessaire pour dé crire la croissance normale de grain dans un alliage semblable sans additions de Nb et de Ti. On discute briè vement de l'origine possible de cet effet.

Thin-slab cast direct-rolling (TSCDR) has become a major process for flat-rolled production. However, the elimination of slab reheating and limited number of thermomechanical deformation passes leave fewer opportunities for austenite grain refinement, resulting in some large grains persisting in the final microstructure. In order to achieve excellent ductile to brittle transition temperature (DBTT) and drop weight tear test (DWTT) properties in thicker gauge high-strength low-alloy products, it is necessary to control austenite grain coarsening prior to the onset of thermomechanical processing. This contribution proposes a suite of methods to refine the austenite grain from both theoretical and practical perspectives, including: increasing cooling rate during casting, liquid core reduction, increasing austenite nucleation sites during the delta-ferrite to austenite phase transformation, controlling holding furnace temperature and time to avoid austenite coarsening, and producing a new alloy with two-phase pinning to arrest grain coarsening. These methodologies can not only refine austenite grain size in the slab center, but also improve the slab homogeneity.

The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer in a Fe-Al-C alloy. The results showed that the austenite phase nucleated along the delta grain boundaries. The transformed austenite morphology changed from cellular to Widmansta¨tten pattern when the holding temperature decreased from 1398 K to 1123 K (1125°C to 850°C). Full partitioning of the substitutional alloying elements was observed and the spacing of the austenite plates was controlled by the diffusing distance of the substitutional elements. Interestingly, growth of the austenite front was controlled by the long-range diffusion of carbon from the center of the delta grains to the growing front. Deformation of the delta phase enhanced the nucleation of austenite at existing grain boundaries and newly formed subgrain boundaries.