The Effect of Multiple Deformations on the Formation of Ultrafine Grained Steels

Abstract

A C-Mn-Nb-Ti steel was deformed by hot torsion to study ultrafine ferrite formation through dynamic strain-induced transformation (DSIT) in conjunction with air cooling. A systematic study was carried out first to evaluate the effect of deformation temperature and prior austenite grain size on the critical strain for ultrafine ferrite formation (e C,UFF ) through single-pass deformation. Then, multiple deformations in the nonrecrystallization region were used to study the effect of thermomechanical parameters (i.e., strain, deformation temperature, etc.) on e C,UFF . The multiple deformations in the nonrecrystallization region significantly reduced e C,UFF , although the total equivalent strain for a given thermomechanical condition was higher than that required in single-pass deformation. The current study on a Ni-30Fe austenitic model alloy revealed that laminar microband structures were the key intragranular defects in the austenite for nucleation of ferrite during the hot torsion test. The microbands were refined and overall misorientation angle distribution increased with a decrease in the deformation temperature for a given thermomechanical processing condition. For nonisothermal multipass deformation, there was some contribution to the formation of high-angle microband boundaries from strains at higher temperature, although the strains were not completely additive.

A Descriptive Model for the Formation of Ultrafine Grained Steels

“…For example, decreasing the temperature has been shown 10) to increase the rate of generation of internal defects in the model alloy ( Fig. 1) and some recent work where there has been an ultrafine ferrite formed through static transformation was from an austenite that had been deformed at 570°C and then reheated to 750°C to allow transformation on a structure with numerous high angle intragranular defects.…”

“…Then during deformation the growth of the ferrite that forms first must be controlled so that there is sufficient time/strain for the introduction of the intragranular defects. It may be possible to partially build-up this defect structure through controlled rolling above the transformation start temperature, 10) but this also appears to have limited potential. It is clear that some form of dynamic adjustment is occurring during deformation, although at the lower strain rates it also appears that too much growth can take place before this occurs leading to very large grains.…”

Grain Refinement in Dual-Phase Steels

“…This enhanced ferrite nucleation results in ultrafine ferrite (UFF) grains. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Yada et al [9] claimed for the first time that 2-to-3-lm UFF can be formed in a C-Mn steel by deforming in the temperature range from Ar 3 + 50°C to Ar 3 + 100°C, when reductions of more than 50 pct have been applied in less than 1 second. There is evidence now that ferrite is formed dynamically from austenite and not during the post-deformation treatment.…”

Ferrite Grain Size Distributions in Ultra-Fine-Grained High-Strength Low-Alloy Steel After Controlled Thermomechanical Deformation

“…[17][18][19][20] An additional benefit of shifting the deformation temperature to higher values is the further softening of the steel, which makes it easy to roll. Grain boundary (GB) pinning from fine-strain-induced Nb precipitates can also retard the grain growth, subsequent to DSIT.…”