We study the microstructural change by heavy deformation after reheating the cast slabs of the 0.1mass% carbon steel.

	C Si Mn P S 0.11 0.16 0.58 0.01 0.003
Fig.1 Three kinds of austenite grain structures : (a) as-cast slab, (b) cast and reheated slab, (c) hot rolled and reheated plate. Top : Schematics, Bottom : light micrographs.	Table 1. Chemical compositions of the steels used (mass%).

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Fig.2 Relationship between compression strain and ferrite grain size (deformed temperature : 1093K) .	Fig.3 Variation of ferrite structure by strain (e) for two austenite grain sizes (dg) in HPH1 steel. Specimens were compressed by the given strain at 1093K at a strain rate of 10/s and then cooled at 10K/s.

(1) Figure 2 indicates the change of the ferrite grain size by the strain. When the austenite grain size is small, 20mm, the fine ferrite grain size that is smaller than 5mm can be obtained within the strain range investigated for the Type 2 structure as well as for the Type 3 one. When the austenite grain size is large, 300mm, the ferrite grain size becomes smaller with an increase in the strain and reaches 5mm at the heaviest strain.

(2) Figure 3 represents a variation of the ferrite grain structure of the HPH1 steel. When the austenite grain size is large, the Widmanstatten ferrites form clearly but disappear at the heaviest strain. On the other hand, no Widmanstatten ferrite forms when the austenite grain size is small. Eventually the fine-grained polygonal ferrite structure forms regardless of the austenite grain size at the heaviest strain

We can conclude that the fine ferrite grain structure can be obtained by a heavy deformation for the Type 2 structure as well as for the Type 3 one.