A Model for Predicting the Austenite Grain Size at the Surface of Continuously-Cast Slabs

Abstract

During the continuous-casting process, retarded cooling of the strand surface below oscillation marks and surface depressions results in the formation of coarse austenite grains. These coarse grains have proven to dramatically reduce the ductility of steel within the second ductility trough, and thus increase the risk of surface crack formation. In addition to the thermal history the composition of the steel, in particular the content of carbon and precipitation-forming elements, plays a decisive role in the development of the austenite grain size. The present work addresses the development and validation of an experimental and numerical model for predicting the austenite grain size in the continuous-casting process. In a first step, the previous austenite grain size on the surface of slabs was determined by metallographic examinations for several slabs with various carbon content. Next, a solidification experiment was adjusted in order to simulate the cooling conditions in the mold of a slab caster, but also to suppress the precipitation of nitrides and carbo-nitrides by subsequent accelerated cooling. Thus, it was possible to study the influence of steel composition on austenite grain growth at temperatures close to the solidus temperature, unaffected by precipitates. The results of both the plant and laboratory experiments point to a maximum austenite grain size with a carbon content of approximately 0.17 mass pct.The parameters of a grain size prediction model were fitted to the results of the experiment. The resultant model was coupled with a precipitation model and then applied to the slab casting process. The measured and calculated grain size values at the surface and immediately below the surface of the slabs agree very closely. The model was finally applied to calculate the grain growth in the center of a virtual oscillation mark under simplified assumptions. Although only the surface temperature in the mold diverges significantly from the original solution, the difference of the initial cooling conditions results in an increase of the final grain size by up to 40 pct.

Austenite Grain Growth in Peritectic Solidified Carbon Steels Analyzed by Phase-Field Simulation

“…Therefore, when the time evolution of grain size at a fixed position, y, is considered, the grain size discontinuously changes from its initial value (short axis diameter of FCG) to ( ) Therefore, when V T is very close to but less than V critical , the discontinuous growth is basically dominant but the continuous growth from FCG partly occurs, which should hinder the competitive growth between CCG at the growth front described by the first term of Eq. [7] and should result in d S smaller than the value predicted by Eq. Since the mobility of the grain boundary at the growth front should be affected by a low temperature region when G is high, we assumed that γ T ′ = T γ − G<r'> in this calculation.…”

“…These dependencies have 13 not been revealed in early works on as-cast γ grains during casting. [5][6][7][8] In the following section, these dependencies of d S are discussed based on a theory of grain growth. …”

“…[11] The result is plotted with respect to the distance from the mold wall in Figure 9 samples. Also, we employed T γ = 1751 K, [19] M 0 = 4.0×10 -3 m 2 s -1 [7] and Q = 177 kJ mol -1 [7] for 0.2C steel. The calculated values of V critical are shown in Figure 9(a).…”

“…In the early works, [5][6][7][8] the description and prediction of as-cast γ grain size have been attempted based on the assumption that the CCG form by continuous grain growth during cooling below T γ . One of the important findings in our previous experimental study [11] is that the short axis diameter of CCG increases only near FCRB, viz., near T γ and it does not virtually change during cooling below T γ .…”

Experimental Verification of a Critical Condition for the Formation of As-Cast Coarse Columnar Austenite Grain Structure in a Hyperperitectic Carbon Steel

“…The closed square plots with the solid line indicate V CCG and its vertical axis corresponds to the right side one. We used M(T γ * ) = 2.2×10 -8 m 2 /s for the carbon steel of our focus [21] and ξ = 0.45 which was estimated by considering the effect of liquid phase on the motion of FCRB [13] . V CCG does not change significantly (1).…”