EFFECT OF INTERPASS TIME ON AUSTENITE GRAIN-REFINEMENT BY MEANS OF DYNAMIC RECRYSTALLIZATION OF AUSTENITE

A 0.06 pct C-0.3 pct Mn and a 0.07 pct C-0.6 pct Mn-0.028 pct Nb steel were deformed in torsion at a constant strain rate of 2/s. Two schedules were used. In schedule A, seven roughing passes executed between 1260-degrees-C and 1130-degrees-C were followed by a single large finishing pass with a strain of 3.5 at constant temperatures between 1010-degrees-C and 840-degrees-C. The time between roughing and finishing was 200 seconds. In schedule B, the seven roughing passes were followed by 10 finishing passes, again applied isothermally, with strains of 0.3 and interpass times of 0.6, 2, and 10 seconds. The results indicate that for the Nb steel, low rolling temperatures (870-degrees-C) and strains above 2 are required for complete dynamic recrystallization, which results in austenite grain sizes under 6-mu-m. Cooled at a rate of 10-degrees-C/s, the dynamically recrystallized austenite grain structures transform into ferrite with grain sizes under 4-mu-m. Extrapolations from the present data suggest that at industrial strain rates and cooling rates, ferrite grain sizes under 2-mu-m should be achieved.