Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C-Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200uC, the samples were cooled to 1100uC, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800uC for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800uC. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850uC.

Phase transformations contributing to the properties of modern steels

“…The deformation adds to the driving force for transformation, and for this reason, anything which retards the recrystallisation of austenite, for example microalloying with niobium, facilitates this process [87]. This results in extensive intragranular nucleation of (idiomorphic) ferrite.…”

Microstructure Evolution of HSLA Pipeline Steels after Hot Uniaxial Compression

“…These ultra-fine grains are supposed to be strain-induced ferrite during hot uniaxial compression. It has been recognized that a relatively low deformation temperature leads to dynamic strain-induced transformation (DSIT) [ 14 , 15 , 16 ]. It is generally acknowledged that severe deformation near the A r3 temperature results in the formation of the ultra-fine ferrite grains, namely DSIT.…”

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

“…The DRX of c, therefore, is unable to form UFF grains after the transformation. As T def decreases to just above A r3 (i.e., T def £ 1103 K (830°C)), c cannot recrystallize dynamically and the large accumulated strain will promote DSIT, [14][15][16][17][18][19][20][21][22][23][24] resulting in the formation of UFF grains (Figures 7(d) through (g) and Figures 7(j) through (o)). The largest grain size for those samples hardly exceeds 10 lm (Figure 8(b)), with 2-to 3-lm average grain size.…”

“…[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.…”

“…Grain boundary (GB) pinning from fine-strain-induced Nb precipitates can also retard the grain growth, subsequent to DSIT. [17][18][19][20] Pre-existing microalloy precipitates (undissolved during soaking) are inactive in retarding c-recrystallization and preventing the a-grain growth. [1] Therefore, to achieve fine-scale (strain-induced) microalloy precipitation during deformation, complete dissolution of pre-existing microalloy precipitates is required at the soaking stage.…”