Softening Behavior of 18.7Cr-1.0Ni-5.8Mn-0.2N Low Nickel-Type Duplex Stainless Steel During Hot Compression Deformation Under Large Strain
NI Ke, YANG Yinhui(), CAO Jianchun, WANG Liuhang, LIU Zehui, QIAN Hao
School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
Cite this article:
NI Ke, YANG Yinhui, CAO Jianchun, WANG Liuhang, LIU Zehui, QIAN Hao. Softening Behavior of 18.7Cr-1.0Ni-5.8Mn-0.2N Low Nickel-Type Duplex Stainless Steel During Hot Compression Deformation Under Large Strain. Acta Metall Sin, 2021, 57(2): 224-236.
It is difficult to form the precipitated phase of duplex stainless steel (DSS) with low nickel content, and a low Cr content of 18.7% (mass fraction) during hot working. However, the stability of the austenite phase changes by substituting Mn for Ni, which can increase the difference between the softening mechanisms of the two phases under high temperature and deformation conditions. Under a temperature and strain rate of 1123-1423 K and 0.1-10 s-1, respectively, the large thermal compression deformation behavior (70%) of 18.7Cr-1.0Ni-5.8Mn-0.2N DSS was investigated. The thermal deformation microstructures were analyzed by OM, SEM, and EBSD. The results show that the dynamic recrystallization (DRX) of the ferrite phase mainly occurred at a lower deformation temperature of 1123 K, and that the degree of grain refinement increased, and the degree of grain inhomogeneity decreased with an increase in strain rate. The strain rate was observed to have a large impact on the ferrite phase DRX, while the austenite phase DRX was more sensitive to deformation temperature. The ferrite phase underwent continuous dynamic recrystallization (CDRX) with the transition from low-angle to high-angle grain boundaries under deformation at 1223 K and 10 s-1, while the austenite phase was dominated by discontinuous dynamic recrystallization (DDRX) deformed at 1323 K and 0.1 s-1. DDRX can be easily induced by increasing the temperature at a low strain rate, while CDRX can be induced at a higher strain rate. The crystal orientation of the austenite phase is mainly characterized by the recrystallization texture of the (001) and (111) planes at higher temperatures and lower strain rates. In the ferrite phase, there is a competitive relationship between the recrystallization texture of the (001) and (111) planes. The critical stress (strain) was obtained by data fitting and its relationship with the peak stress (strain) was determined. As the strain increased, the flow instability domain of hot compression decreased, and the stability zone gradually moved toward higher temperature and strain rate. Furthermore, the optimum hot working conditions, 1323-1423 K and 0.01-6.05 s-1, were obtained.
Fig.2 True stress-true strain curves of duplex stainless steel samples deformed at 1123-1423 K (—strain rate)
Fig.3 OM images of duplex stainless steel samples deformed at different conditions and solution state (The ellipses in Figs.3e, g, h, j, and k exhibit the recrystallized nucleation structures)
Fig.4 SEM images of duplex stainless steel samples deformed under 1123 and 1223 K (The ellipses in Figs.4c~h exhibit the recrystallized nucleation structures)
Fig.5 Relationship between average ferrite grain size and strain rate of duplex stainless steel samples deformed at 1123 K
Fig.7 Number fraction variations of HAGB with the strain rate and temperature in two phases and the sum of two phases
Fig.6 Grain boundary diagrams of duplex stainless steel samples under deformation condition of 1223 K and 0.01-10 s-1, and 0.1 s-1 and 1123-1423 K (The blue lines indicate the high angle grain boundaries (HAGBs, 15°-180°), the green lines indicate the low angle grain boundaries (LAGBs, 2°-15°))
Fig.8 Orientation distribution maps of the composition phases of duplex stainless steel samples under deformation conditions of 1223 K and 0.01-10 s-1, and 0.1 s-1 and 1123-1423 K (RD—rolling direction)
Fig.9 The inverse pole figures (IPFs) of duplex stainless steel samples deformed at 0.1 s-1 and 1123-1423 K
Fig.10 The relationships between strain hardening rate (θ) and stress (σ) of duplex stainless steel samples deformed at 10 s-1 and 1323 K (a), and 0.01 s-1 and 1123-1423 K (b) (σs—steady stress, σc—critical stress, σp—peak stress)
Fig.11 The relationships between σc (a), εc (b) and temperature (T) at different strain rates (εc—critical strain)
Fig.12 Relationships of σc-σp (a) and εc-εp (b) for duplex stainless steel samples (εp—peak strain, r2—linear correlation)
Fig.13 3D-energy dissipation (a) and flow instability (b) diagrams of duplex stainless steel samples deformed at 1123-1423 K and 0.01-10 s-1, and hot processing map at ε=1.2 true strain (c)
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