The 18Ni, 17Ni-0.2C and low carbon (SM490) steels were quenched after austenitization to obtain full martansite structure. Then specimens were heated rapidly up to various temperatures below As followed by compression with various strain rates. During hot deformation, some specimens show dynamic recrystallization, which is well summarized by using Zener-Hollomon (Z) parameter. The Z-values corresponding to the occurrence of dynamic recrystallization depend on chemical compositions of mantensite steels and are much higher than those for IF steel. The dynamic recrystallization observed is so-called continuous recrystallization which is different from discontinous recrystallization in the IF steel; subgrains change to grains with high angle boundaries during deformation. The grain size changes with Z-value and sub-micron sized grains were obtained in the 17Ni-0.2C steel.
