Renbo Song

Hot Deformation and Dynamic Recrystallization Behavior of Austenite-Based Low-Density Fe–Mn–Al–C Steel

The hot deformation and dynamic recrystallization (DRX) behavior of austenite-based Fe–27Mn–11.5Al–0.95C steel with a density of 6.55xa0gxa0cm−3 were investigated by compressive deformation at the temperature range of 900–1150xa0°C and strain rate of 0.01–10xa0s−1. Typical DRX behavior was observed under chosen deformation conditions and yield-point-elongation-like effect caused by DRX of δ-ferrite. The flow stress characteristics were determined by DRX of the δ-ferrite at early stage and the austenite at later stage, respectively. On the basis of hyperbolic sine function and linear fitting, the calculated thermal activation energy for the experimental steel was 294.204xa0kJxa0mol−1. The occurrence of DRX for both the austenite and the δ-ferrite was estimated and plotted by related Zener–Hollomon equations. A DRX kinetic model of the steel was established by flow stress and peak strain without considering dynamic recovery and δ-ferrite DRX. The effects of deformation temperature and strain rate on DRX volume fraction were discussed in detail. Increasing deformation temperature or strain rate contributes to DRX of both the austenite and the δ-ferrite, whereas a lower strain rate leads to the austenite grains growth and the δ-ferrite evolution, from banded to island-like structure.

Effects of Temperature and Strain Rate on Solid-/Liquid-Phase Flow Behavior of 9Cr18 Steel During Thixoforging

Abstract A thixoforging process of the 9Cr18 steel was conducted in a designed setup, and a kind of multi-diameter component was fabricated. The effects of the forming temperature and the strain rate on the solid-/liquid-phase flow behavior were discussed. The results showed that functional gradient properties of the 9Cr18 steel could be obtained after thixoforging. Changes of microstructure along radial direction could be obtained. Solid austenite was retained after fast cooling, and the liquid film enriched in alloying elements was extruded outside to form a dendrite skin layer. As temperature increased, more molten liquid formed during thixoforging. A heterogeneous flow phenomenon was activated as free liquid channels were formed. The macro-separation of solid and liquid phases was critical for the formation of functional gradient properties. Above 1300xa0°C, full dendrite skin layer could be formed. The strain rate affected the thixotropic property via influencing the deformation time of thixoforging. In the presence of lower strain rates, there was more time for the flow of liquid metal, which was the key to the extension of the thixotropic stage. High temperatures and low strain rates contributed to the formation of full skin layer for the designed specimen. The average thickness of skin layer for current specimen could be over 1000xa0μm when thixoforged at 1340xa0°C and under a strain rate of 0.02xa0s−1.

Effects of Solution Temperature on Microstructure and Properties of Fe–Mn–Al–C Ferrite-Based Steel

The effects of solution temperature of 700–950 °C on microstructure and properties of Fe–8Mn–6Al–0.2C ferrite-based steel were investigated. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) were used to study the morphology and composition of microstructure. The tensile test at room temperature was used to detect mechanical properties. The results showed that the as-hot-rolled steel exhibited a complex microstructure, consisting of banded ferrite, martensite and small amount of retained austenite. With the increase of solution temperature, δ-ferrite solidified from the liquid phase and existed during subsequent heat treatment at all the temperatures, while the other phases varied from M5C2, α-ferrite, austenite and martensite. At 900 °C, dual phase microstructure with ferrite and island of austenite was obtained. With increasing solution temperature, the ultimate tensile strength (UTS) decreased at first and then increased after reaching the minimum at 850 °C, while the total elongation (TE) increased firstly and declined after the peak value of 44.5% at 850 °C. Therefore, the as-hot-rolled experimental steel heat treated at 900 °C had an excellent combination of tensile strength and elongation, with UTS of 846.4 MPa, TE of 32%, and UTS × TE of 27.1 GPa%. The remarkable mechanical properties, both strength and elongation, were attributed to the relative fraction and stability of austenite. And the partition of elements, especially for Mn, had great effect on the austenite stability.