Dynamic recrystallization in 18%Cr ferritic steel

Polycrystals of 18% Cr ferritic stainless steel were hot-compressed at vari ous strain rates and various temperatures, and immediately cooled by water. Equiaxed grains surrounded by clearly etched boundaries were observed in t he specimens deformed at higher temperatures or lower strain rates. The det ailed microstructural observations by optical microscopy and TEM and the cr ystallographic determinations by TEM/Kikuchi-line analysis and SEM/EBSP ana lysis showed that these equiaxed grains are the dynamically recrystallized grains with both large angle grain boundaries and dislocation substructures . In spite of the occurrence of dynamic recrystallization, a large drop in stress was not observed in stress-strain curves. At a constant strain, the occurrence of dynamic recrystallization was generally determined by Zener-H ollomon parameter (Z). For example, it appeared under the condition of Z<10 (14) s(-1) at a true strain of 0.8. The dynamically recrystallized grains w ere very much larger than that of the subgrains formed under the same defor mation condition, which suggests that the dynamic recrystallization occurre d by nucleation and growth mechanism. Such characteristics of dynamic recry stallization of ferrite is quite similar to that in the IF steel previously reported. On the other hand, the Z<10(14)s(-1) rule did not stand at 700 d egrees C, at which a number of carbides precipitated at initial grain bound aries and pinned the boundaries. It was also clarified that the size of the dynamically recrystallized grains is considerablly smaller than that of th e grains statically or meta-dynamically recrystallized during cooling after the same hot-deformation condition.