Analysis of the methods to calculate austenite static recrystallization volume fraction
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Double deformation technique is often used to study static recrystallization (SRX) behavior of materials, and SRX volume fraction can be calculated by the analysis of the stress–strain curve from this technique. The suggested methods to analyze the curve are different, which may lead to different results or even large deviation. In the present research, the methods were discussed based on the stress–strain curves from double deformation compression tests. The results showed that 2% offset method agrees well with 5% total strain method and they can give a better evaluation of SRX volume fraction for the tested steel in the absence of precipitates. While the results from 2% offset or 5% total strain method deviate from the ones from area measurement method when precipitation occurs. Therefore, area measurement method is more suitable when precipitates play a role in SRX process. However, the strain should be large enough to perform this method.
I. INTRODUCTION
The development of new steels or forming processes involves many works on microstructure evolution including the processes of dynamic recovery or recrystallization, static recovery, or recrystallization, grain growth, etc. Modeling for these processes is an effective approach to predict microstructure evolution. Therefore, many works aim to find the most suitable models for the processes. As the key to modeling static recrystallization (SRX) is to determine SRX volume fraction, the microstructure observation or double deformation technique has been developed for this purpose. Since the former one is tedious and time consuming, the latter one has been widely used. However, there are many methods such as 0.2% offset, 2% offset, 5% total strain, back extrapolation, mean flow stress, and area measurement methods available to analyze the stress–strain curves resulting from the double deformation technique. And different methods may lead to different results. Some researchers find that 0.2% offset method can estimate SRX softening fraction of a Nb microalloyed steel very well.1 While it has been revealed in Ref. 2 that the softening fraction determined by using 0.2% offset method consists of both recovery and recrystallization softening and SRX actually occurs when the softening fraction reaches 0.2. It is found in Ref. 3 that the SRX volume fraction for a low carbon steel determined by using 2% offset method is consistent with metallographic examination and that determined by Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.217
using 0.2% offset method is much larger. Consequently, some researchers suggest the 2% offset method to analyze the data from double deformation test to exclude the recovery. Moreover, other researchers propose the 5% total strain method to measure the degree of recrystallization.4 It is reported elsewhere5 that the result from 0.2% offset method is consistent with that from mean flow stress method and the value from back extrapolation method is lower. However,
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