Effect of Manganese Segregation on Fine-grained Ferrite Structure in Low-carbon Steel Slabs

Abstract

Using a 100 mm thick continuously cast slab with a chemical composition of 0.10C-0.16Si-0.58Mn-0.01P-0.003S (mass%), we have clarified the effect of the manganese segregation on the transformed ferrite structure when the slab was reheated to make austenite grains equi-axed and smaller and followed by the Grain Refinement Process (GRP) of heavy compression and subsequent controlled cooling. Samples from which the Mn segregation was eliminated were also prepared for comparison. The resultant microstructure was examined for compressive strains between 0.7 and 2.1.When the austenite grain size is 160 or 120 mm prior to GRP, the ferrite grain size of the samples with the Mn segregation is smaller at the given strain than that of the samples from which the Mn segregation was eliminated. The Mn band formed by the heavy compression is thought to act as a barrier in the form of a relatively stable austenite against the ferrite growth.Moreover, we found a unique equation to predict the transformed ferrite grain size in the samples with the Mn segregation. Namely, the ferrite grain size can be accounted for according to a parameter, ES (effective spacing): a mean spacing that considers both the pancake austenite grain boundary and the Mn band. We also suggest that the smaller Mn band spacing may make the ferrite structure finer even when the austenite grain size is large.KEY WORDS: cast slab; low carbon steel; heavy deformation; TMCP; HDR; HCR; phase transformation; grain boundary; ferrite grain; austenite grain; grain size; Mn segregation. Type 2 is the homogenized form of Type 1 without forging. The micro-segregation disappears by heating for several hours at an elevated temperature. The austenite grain size is large. This type is not always utilized practically but is important from a scientific viewpoint.For Types 1 and 2 microstructures, the recrystallization process has been developed and essentially combined with GRP to obtain fine austenite grains. The addition of Nb and Ti has been adopted for this purpose especially for Type 1. [7][8][9][10][11] Type 3 is the reheated form of Type 1 that is cooled once in the fully ferritic temperature regime. The micro-segregation remains but the austenite structure becomes equi-axed and smaller. The austenite grain size is between 20 and 300 mm.Type 4 is somehow classical. First the solidified structure is destroyed by some deformation at an elevated temperature, then it is homogenized and normalized. Figure 2 illustrates the difference between Types 3 and 4 microstructures prior to GRP. Micro-segregation does not exist, and the austenite grain size is mostly as small as that of Type 3. Torizuka et al. reported that the size of ferrite grains transformed from the heavily deformed austenite depends on the thickness of the compressed austenite grain for a Si-Mn steel with a Type 4 microstructure. 12,13) For Types 3 and 4 microstructures, a fine austenite grain size can be easily obtained by austenitizing or reheating the steel for a relatively short period in a lower austenitic ...

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