L. F. Hou

Development in fundamental research on TWIP steel used in automobile industry

Abstract This paper tracks the progress in research regarding the use of twinning induced plasticity (TWIP) steel in the automobile industry. The chemical composition of TWIP steel ensures that it has stable austenite and proper stacking fault energy at room temperature, allowing the main deformation mechanism (twinning) to work. The effects of alloying elements on the microstructure and deformation mechanism of TWIP steel are explained in detail, and their properties deformed under static and dynamic conditions are examined. The TWIP steel deformed at a low strain rate shows higher total elongation and strength. When the TWIP steel deforms under dynamic strain conditions, the stress, microhardness and the work hardening rate, all increase along with the increase in strain and the strain rate. The twin characteristics of TWIP steels deformed at various strain rates vary also and the twins generated under a high strain rate exhibit thinner widths and smaller interspaces compared with those formed under a low strain rate. It has also been observed that multisystem twins are able to generate and develop together. The mechanisms of toughening and strengthening in TWIP steels are noted. Finally, some potential application fields have been found for the promising material.

High-frequency corrosion fatigue behavior of AZ31 magnesium alloy in different environments

The high-frequency corrosion fatigue behavior of extruded AZ31 magnesium alloy was investigated in different environments: air, gear oil and 3.5% NaCl solution. Compared with that in air, the corrosion fatigue limits were degraded by approximately 3.52% and 58.91%, respectively, and the corrosion fatigue lives were shortened by about 13.43% and 89.36%, respectively. In the same environment, the high-frequency fatigue limits are all higher than those tested at low frequency. The specimen geometrical shape plays a certain factor on stage characteristics of S–N curves. Compared with that of arc transition specimens, the stress sensitivity of line transition specimens is only reflected in a relatively high cycle fatigue life region. Different environments influence the corrosion failure kinetics processes (the crack initiation mechanism), but do not change the fatigue fracture mechanism of the alloy, and the higher loading frequency only accelerate the crack nucleation processes.

Effect of temperature on impact properties and microstructural evolution of twinning induced plasticity steel

Abstract The Charpy V notch impact toughness of twinning induced plasticity steels was investigated at 20, −50, −80, −110 and −196°C. The fracture morphologies were analysed by scanning electron microscopy. The microstructural characteristic near the fractures is investigated by optical microscopy, X-ray diffraction and transmission electron microscopy. The impacted twinning induced plasticity steels exhibit better impact toughness due to the simultaneous generation of deformation twins and martensite, which hinders crack propagation and does not display ductile to brittle transition. The ruptures exhibit cleavage characteristic, and traces of deformation twins can be observed in the cleavage facets, so the cracks should propagate through the deformation twin, martensite and grain. The martensite transformation occurs because of the intensive high rate force with rapidly changing direction. The nucleation and growth of deformation twins are accomplished by the pile-up of dislocations at the positions of stress concentration, and the stacking fault grows through the moving and vanishing dislocation.

Evolution of intergranular corrosion resistance for HR3C heat-resistant austenitic stainless steel at elevated temperature

ABSTRACT An attempt has been made to investigate the evolution of intergranular corrosion (IGC) resistance of HR3C heat-resistant austenitic stainless steel. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests were conducted to evaluate the IGC resistance of HR3C steel. The results show that the side peaks can be observed in active–passive region of DL-EPR curves due to inhomogeneous distribution of the chromium dissolved in matrix. The variation of the degree of sensitisation (DOS) during sensitising at 800°C can be divided into three regions (sensitisation dominant region, desensitisation dominant region and equilibrium region). When HR3C steel is sensitised at 800°C for over 24 h, the equilibrium state in DOS can be reached. Simultaneously, the IGC morphologies after DL-EPR tests can correlate well with the DOS. On the basis of the equilibrium state in DOS, proper heat treatments and corrosion protection measures can be taken to avoid IGC of HR3C steel.

Effect of the heat-treatment temperature on the mechanical properties and microstructural evolution of cold-rolled twinning-induced plasticity steel

TWIP steels with 70% cold-rolled reduction were heated at 500, 600, 700, 800, 900, 1000, and 1100 °C. Then, the properties before and after heating were examined through tensile and hardness experiments. The microstructures were also analyzed by optical microscopy and transmission electron microscopy. The relationship between the properties and microstructure was examined as well. Finally, the evolution process of cold-rolled microstructures during heating was discussed in detail. Moreover, some conclusions can be drawn, and the heating evolution characteristics are described.