Austenite Grain Growth and the Surface Quality of Continuously Cast Steel

Abstract

Austenite grain growth does not only play an important role in determining the mechanical properties of steel, but certain surface defects encountered in the continuous casting industry have also been attributed to the formation of large austenite grains. Earlier research has seen innovative experimentation, the development of metallographic techniques to determine austenite grain size and the building of mathematical models to simulate the conditions pertaining to austenite grain growth during the continuous casting of steel. Oscillation marks and depressions in the meniscus region of the continuously casting mold lead to retarded cooling of the strand surface, which in turn results in the formation of coarse austenite grains, but little is known about the mechanism and rate of formation of these large austenite grains. Relevant earlier research will be briefly reviewed to put into context our recent in situ observations of the delta-ferrite to austenite phase transition. We have confirmed earlier evidence that very large delta-ferrite grains are formed very quickly in the single-phase region and that these large delta-ferrite grains are transformed to large austenite grains at low cooling rates. At the higher cooling rates relevant to the early stages of the solidification of steel in a continuously cast mold, deltaferrite transforms to austenite by an apparently massive type of transformation mechanism. Large austenite grains then form very quickly from this massive type of microstructure and on further cooling, austenite transforms to thin ferrite allotriomorphs on austenite grain boundaries, followed by Widmanstätten plate growth, with almost no regard to the cooling rate. This observation is important because it is now well established that the presence of a thin ferrite film on austenite grain boundaries is the main cause of reduction in hot ductility. Moreover, this reduction in ductility is exacerbated by the presence of large austenite grains.

Hot Ductility Behavior of a Peritectic Steel during Continuous Casting

“…Different studies are available for continuous casting simulation in the Gleeble machines [2,3,[14][15][16]. Studies performed on this subject are generally related to the effect of strain rate, cooling rate, holding period before tensile test, austenite grain size and chemical composition on ductility.…”

“…Transverse surface cracks seen on the continuous casting slabs have a great importance in the continuous casting process defects [1][2][3][4][5][6][7][8]. Crack-free slab production is an important key to success.…”

Grain Growth and β to α Transformation Behavior of a β‐Solidifying TiAl Alloy

“…At this temperature exclusively b phase is present, in agreement with findings of Schwaighofer et al [9] The grain boundaries are clearly delineated as shown in the inset of Figure 2. [20] A square of about 2 mm side length, with a homogeneous temperature distribution, was mapped in preselected time intervals in order to gain statistically relevant data of grain size. a grain boundary groove that was formed at an earlier stage will remain visible as a groove even though grain growth had occurred and the new grain boundaries are located in different areas).…”

Influence of Ti(C,N) precipitates on austenite growth of micro-alloyed steel during continuous casting

“…When the initial austenite grain is infinitely small, the driving force will be infinite, which is not allowed in the mathematical model. The initial temperature of austenite transformation is set to 1,440 ℃ in the model [5] , and the initial austenite grain size is set to 100 μm, which is in the range of primary dendrite arm spacing.…”

“…However, the hot ductility of micro-alloyed steels remains very poor during continuous casting, which leads to the formation of transverse cracks during the straightening process [3,4] . The influence factors of hot ductility in the third brittle zone include the size of austenite grain, carbonitride precipitation and pro-eutectoid ferrite film [5,6] . Research concerning the impact of Ti(C,N) precipitation on the growth of austenite mainly focuses on the hot Abstract: Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting.…”