Study of the strain reversal effect on the recrystallization and strain-induced precipitation in a Nb-microalloyed steel

Models representing material behaviour are now an essential component of the development process for rolled products. Although models based on physical parameters are being proposed, most current models employ empirical equations, which assume that the deformation can be characterized by the strain rate, temperature and the equivalent plastic strain. However, deformation in a flat product rolling pass involves a partial reversal of shear strain, and in long product and section rolling there are more complex changes in strain path in sequential passes. This paper briefly reviews the mapping of strain paths and their effects on the micromechanics of deformation and the resulting flow stress. The influence of in–grain heterogeneity of strain is discussed in relation to the development of dislocation structures and their effects on texture evolution and subsequent recrystallization behaviour. The effects on recrystallization kinetics and resulting grain size are sufficiently large to lead to significant errors in modelling the local behaviour in multipass rolling, if strain–path effects are not considered.

The effect of strain path on material behaviour during hot rolling of FCC metals - Discussion

A 316L austenitic stainless steel was subjected to hot cyclic torsion tests to the same total accumulative von Mises equivalent strain of 2.0with different amounts of reversals in order to study the effect of strain path on dynamic recrystallisation (DRX) behaviour. Both the macroscopic flow stress and corresponding microstructural analyses revealed that there is a strong influence of applied strain path on the DRX kinetics. In the case of higher deformation per pass and less strain path reversals, the combination of three effects, i.e. higher stored energy, enhanced grain boundary serration and bulging through increased grain boundary area and more developed strain-induced high angle boundaries together facilitated DRX. On the contrary, deformation of more strain path reversals combined with lower strain per pass lead to a complete suppression of DRX even at the high accumulative strain of 2.0. Proper understanding of the strain path effects on hot deformation behaviour of steels is a key factor in control and optimisation of the microstructure and properties of semi-final and final products.

On the interactions between strain path reversal and dynamic recrystallisation in 316L stainless steel studied by hottorsion

Multipass torsion tests were performed on an Nb-microalloyed steel at decreasing temperatures so as to analyze the effect of strain reversal – in the strain accumulation region – on the γ → α transformation kinetics. Strain reversal involved a significant delay in the start of the transformation as well as a microstructural transient compared to monotonic strain conditions. However, after several passes with no reversal performed within the inter-critical temperature region, both types of tests led to similar microstructures.

Effect of the strain reversal on austenite–ferrite phase transformation in a Nb-microalloyed steel

High-resolution electron backscatter diffraction has been used to study the effects of strain reversal on the evolution of microbands in commercial purity aluminium alloy AA1200. Deformation was carried out using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300 degrees C and strain rate of 1 s(-1) to a total equivalent tensile strain of 0.5. In both cases, microbands were found in the majority of grains examined with many having microband walls with more than one orientation. For the forward/forward condition, the microband clusters were centred around -20 degrees and +45 degrees to the equivalent tensile stress axis, whereas for material subjected to a strain reversal, the clusters were at -65 degrees and -45 degrees . There was no evidence of microbands that were formed in the forward deformation step in the reversed material, which would suggest that a strain of 0.25 is sufficient to dissolve any microstructure history generated by the first step. Furthermore, the microbands within the strain-reversed material had a reduction in misorientation compared with the lineally strained material, suggesting that these microbands only formed at the onset of the second deformation step. This confirms that microband formation is complex and sensitive to strain path; however, it is still unclear to what extent microband formation is dependent on strain path history compared with the instantaneous deformation mode.

An analysis of microband orientation in a commercial purity aluminium alloy subjected to forward and reverse torsion using Electron Backscatter Diffraction (EBSD)

In the present work, a Nb microalloyed steel has been hot worked in torsion mode following multipass deformation sequences. Some of these sequences involve a reversion of the strain. The effect of this reversion on the mechanical response of the steel on reloading after the pre-straining has been analysed. For pre-strains within the strain hardening deformation range, after a transient, the normal flow curve shape corresponding to austenite exhibiting a single peak dynamic recrystallisation is reached. However, the new position of the peak indicates that the dynamic recrystallisation is clearly delayed by the reversal. Deformation sequences including holding times after different strain paths have been used to study the effect of the reversal on the static softening and the strain induced Nb(C,N) precipitation. A complex relationship between the kinetics of both processes and the strain path is observed.

R Bartolomé

Papers

Flow stress behaviour, static recrystallisation and precipitation kinetics in a Nb-microalloyed steel after a strain reversal