Study on microstructural evolution and constitutive modeling for hot deformation behavior of a low-carbon RAFM steel
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The constitutive equation was established based on the consideration of strain compensation to describe the hot deformation behavior of low carbon reduced activation ferritic/martensitic (RAFM) steels at the temperatures of 850–1050 °C and the strain rates of 0.01–10 s1. The result indicates that the flow stress is increased with the increase of strain rate but decreased with increase of deformation temperature. During the hot deformation process, the increase of temperature is beneficial to attain the complete dynamic recrystallization (DRX). However, excessively high temperature leads to grow up of dynamic recrystallized grain. Higher strain rate leads to finer recrystallized grains. The material constants (a, n, A) and deformation activation energy (Q) are calculated by the regression analysis. The increase of strain caused the decrease of Q, indicating the DRX occurred more easily. In addition, the developed constitutive equation could accurately predict the hot deformation behavior of the low carbon RAFM steel.
I. INTRODUCTION
Reduced activation ferritic/martensitic (RAFM) steels are regarded as the preferred structural material in the fusion energy systems owing to their low activation and favorable behavior in high dose fusion reactor environment.1–4 In the past several decades, a large number of RAFM steels have been developed in many countries, such as F82H steel in Japan, EUROFER 97 steel in Europe, 9Cr2WVTa steel in USA, the China Low Activation Martensitic (CLAM) steel in China.5–7 Carbon content is important to control the precipitation behavior of carbonitrides and mechanical properties of RAFM steels. Fine MN replaces MC, precipitating densely along boundaries in the steels containing carbon content 0.002%, which improves the precipitation strengthening.8 Nanometre-size MX carbonitride particles are achieved for the martensitic 9% Cr steel containing 0.049% nitrogen by reducing carbon concentration to 0.002% and their low coarsening rate at increased temperatures gives rise to excellent creep strength at 650 °C.9 Hot forming is more suitable for fabricating some components of blanket structure because of the highstrength and low plasticity of RAFM steel. Therefore, the understanding of hot deformation behavior of RAFM steels has great benefit for designers to conduct hot rolling and forging. During the hot deformation process, Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.77
dynamic recrystallization plays the main role in the microstructure evolution of the steels.10 The flow behavior is significant indicator to evaluate the applied load in the actual process of hot deformation. In metallurgy, constitutive equations incorporating the effects of forming temperature and strain rate can be achieved from the data of hot deformation experiment, which can describe the high temperature flow behavior of steels.11,12 So far, the high temperature flow behavior of different steels has been investigated. Zhang et al.13 investigated the
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