Mechanical behavior and microstructure evolution during steady-state dynamic recrystallization in the austenitic steel 800H

doi:10.1016/j.msea.2008.11.035

Materials Science and Engineering: A

Volume 506, Issues 1–2, 25 April 2009, Pages 101-110

https://doi.org/10.1016/j.msea.2008.11.035 Get rights and content

Abstract

Investigations of steady-state dynamic recrystallization (DRX) were carried out on an austenitic steel alloy 800H. The influence of strain rate and temperature on the mechanical behavior, microstructure development and texture evolution were analyzed. Strain rate and temperature change experiments during steady-state deformation revealed characteristic interdependencies of flow stress, hardening rate, and grain size. The grain size sensitivity of the flow stress was found to scale with the characteristic length scale of the deformed structure. Based on these observations a new model is proposed that relates the process of DRX to an interaction of mobile grain boundaries with deformation-induced subboundaries.

Introduction

Despite extensive research during the past decades the physical mechanisms of dynamic recrystallization (DRX) are not yet fully understood. The subject is not only of academic interest but also of immense importance for industrial processing, e.g. hot rolling: DRX offers a powerful tool for microstructure control and, therefore, a useful way for the optimization of the sheet properties. Furthermore, advances in through-process computer simulation require precise experimental data and physics-based models of the material behavior for each step of the complex process chain (hot rolling, cold rolling, annealing, …) for reliable predictions of microstructure and texture evolution.

Early models of DRX were based on concepts assuming a simple superposition of deformation and (static) recrystallization [1], [2], [3]. While the essential features of microstructure and flow behavior could be reproduced, e.g. the development of grain size, the apparent differences to static recrystallization – like the nucleation mechanisms and the steady-state behavior – could not be accounted for.

The current study focused on the steady-state regime of the flow curve. Particular attention was paid to the microstructure and texture development as well as to the transient behavior during strain path changes.

Section snippets

Experimental procedure

The material used in this investigation was the austenitic steel X10NiCrAlTi3220, also referred to as alloy 800H. Its exact chemical composition is given in Table 1. Cylindrical samples of 5 mm diameter and 7.7 mm height were machined from the statically recrystallized material using Rastegaev geometry [4] with BN as lubricant. Hot Compression tests were performed with true strain rates of $10^{- 4} s^{- 1} \leq \dot{ε} \leq 10^{- 1} s^{- 1}$ at temperatures of 1000 °C ≤ T ≤ 1200 °C. For texture and microstructure analysis the samples

Plastic flow behavior

The characteristic stress–strain curve of a material undergoing DRX exhibits either a single maximum (single-peak behavior) or an oscillating shape (multiple-peak behavior) depending on the deformation parameters, i.e. temperature and strain rate [6]. Fig. 1 shows the typical flow curves of the austenitic steel 800H in the temperature range 1000 °C ≤ T ≤ 1200 °C and at true strain rates of $10^{- 4} s^{- 1} \leq \dot{ε} \leq 10^{- 1} s^{- 1}$ . With increasing temperature T or decreasing strain rate $\dot{ε}$ the flow curves are shifted to

Constant deformation conditions

The most prominent point of the curve is the maximum which is characterized by the peak stress σ_p and the peak strain ɛ_p as a result of deformation-induced hardening superimposed by softening due to DRX. The maximum of the flow curves – and likewise the onset of DRX – is shifted to lower stress and strain values with increasing temperature and decreasing strain rate (Fig. 1). However, the onset of DRX in a polycrystalline material cannot be directly extracted from the flow curve.

In the

Summary and conclusions

Dynamic recrystallization was investigated under constant and transient deformation conditions in the austenitic steel 800H. Microstructure and texture development were studied to elucidate the physical mechanisms underlying the evolution and maintenance of the steady state regime.

1.
Depending on deformation conditions single-peak and multiple-peak flow curves were observed, but invariably grain refinement occurred. With increasing temperature and decreasing strain rate the flow stress maximum as

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Citation Excerpt :
One reason for the strong dependency of nucleation on the local deformation state is the competition of multiple nucleation mechanisms. In low carbon steels with medium stacking fault energy, for example, discontinuous DRX [14] can nucleate either from grain boundary bulging [4,5] or development of deformation substructure into nuclei [15,16]. To describe the connection between microstructural state and nucleation, a multitude of nucleation criteria have been developed [17–19].
Predicting microstructure and (micro-)texture evolution during thermo-mechanical processing requires the combined simulation of plastic deformation and recrystallization. Here, a simulation approach based on the coupling of a full-field dislocation density based crystal plasticity model and a cellular automaton model is presented. A regridding/remeshing procedure is used to transfer data between the deformed mesh of the large-strain crystal plasticity model and the regular grid of the cellular automaton. Moreover, a physics based nucleation criterion has been developed based on dislocation density difference and changes in orientation due to deformation. The developed framework is used to study meta-dynamic recrystallization during double-hit compression tests and multi-stand rolling in high-resolution representative volume elements. These simulations reveal a good agreement with experimental results in terms of texture evolution, mechanical behaviour and growth kinetics, while enabling insights regarding the effect of nucleation on kinetics and crystallographic texture evolution.

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Mechanical behavior and microstructure evolution during steady-state dynamic recrystallization in the austenitic steel 800H

Abstract

Introduction

Section snippets

Experimental procedure

Plastic flow behavior

Constant deformation conditions

Summary and conclusions

Acta Metall.

Acta Metall.

Scripta Mater.

Acta Metall.

Scripta Mater.

Acta Metall.

Acta Metall.

Acta Mater.

Acta Mater.

Mater. Sci. Eng. A

Scripta Metall.