A Descriptive Model for the Formation of Ultrafine Grained Steels

Abstract

This paper presents a descriptive model to explain the mechanisms involved in the development of ultrafine grained structure in steels through dynamic strain induced transformation. The model considers the microstructural evolution during and after deformation as well as the role of different process variables. A key factor is the competition between nucleation and growth, where it is shown that many potential nuclei can be lost under certain conditions leading to a mixed or coarser grain size.

Multiscale microstructure engineering of steels

“…This was based on the heat evolution from transformation, which makes it difficult to maintain high levels of undercooling during continuous cooling after deformation. The current authors, however, have argued that it is more a geometric effect 22 with the spacing of the nucleation sites determining the grain size in the first instance, although there is also clearly an effect of the concurrent deformation. Further evidence for this was found recently in statically formed ferrite from austenite worked at a low temperature.…”

Beneficial influence of an intercritically rolled recovered ferritic matrix on the mechanical properties of TRIP-assisted multiphase steels

“…bainite and retained austenite, are present. It is worth noting that some equiaxed grains can also be found in the matrix of specimen MP0730 due to the formation of strain-induced ferrite [26]. Fig.…”

Refinement of Ferrite Grain Size near the Ultrafine Range by Multipass, Thermomechanical Compression

“…An increase in A r3 due to the c-grain refinement and strain accumulation inside c may result in the formation of coarse a grains by ST. [48] Deformation heating may also contribute to the formation of coarse a grains by retarding DSIT kinetics. [26,49] 5. Finishing deformation in single vs multiple passes For the same T def [1103 K (830°C)] and CR (1 K/s), a flow curve of the multipass deformed sample (MP-6) closely followed the flow curve obtained after single-pass deformation (SP-1) (Figure 3(d)).…”

“…In this context, it is important to mention that besides DSIT, dynamic recovery (a-DRV) and dynamic recrystallization of ferrite (a-DRX) play a crucial role in the formation of UFF structure. [49] Due to the high stacking fault energy of a, substructures developed by a-DRV at small strain increase the fraction of low-angle boundaries (LABs), which absorb dislocations and gradually transform into high-angle boundaries (HABs) with the increase in strain. [4,21,25,26] The preceding mechanism, known as continuous dynamic recrystallization (Cont-DRX), is characterized by grain subdivision, pronounced DRV, and even shortrange grain boundary migration to develop equiaxed UFF in the final microstructure.…”

“…[4,21,25,26] The preceding mechanism, known as continuous dynamic recrystallization (Cont-DRX), is characterized by grain subdivision, pronounced DRV, and even shortrange grain boundary migration to develop equiaxed UFF in the final microstructure. [4,21,[25][26][27][28][29]49] On the other side, there is also evidence of conventional DRX (Conv-DRX) of heavily deformed a, characterized by the nucleation and growth of strain-free a grains. [50][51][52] DSIT a grains may refine by Cont-DRX or Conv-DRX upon subsequent deformation.…”