Zheng-you Tang

Formability of TRIP/TWIP Steel Containing Manganese of 18. 8%

The mechanical property and forming limit experiments were carried out on a high manganese TRIP/TWIP steels with manganese of 18.8%. And the forming limit diagram was obtained. This high manganese steel shows outstanding mechanical properties combining high strength with good formability. Its limit plane strain corresponding to the lowest point in the plain strain condition is about 38 %. The effects of conventional mechanical property on the limit plane strain were analyzed. With increasing the content of Mn, the limit plane strain increases, indicating that TWIP effect is advantageous to the increase of formability. Combining with three classical models and volume invariable principle, a FLD model was built, which is suitable for high manganese steels very well.

Effects of temperature and strain rate on flow behavior and microstructural evolution of super duplex stainless steel under hot deformation

Hot compression tests were carried out in the temperature range of 1223—1473 K and strain rate range of 0.01—30 s–1 to investigate the flow behavior and microstructural evolution of super duplex stainless steel 2507 (SDSS2507). It is found that most of the flow curves exhibit a characteristic of dynamic recrystallization (DRX) and the flow stress increases with the decrease of temperature and the increase of strain rate. The apparent activation energy Q of SDSS2507 with varying true strain and strain rate is determined. As the strain increases, the value of Q declines in different ways with varying strain rate. The microstructural evolution characteristics and the strain partition between the two constituent phases are significantly affected by the Zener-Hollomon parameter (Z). At a lower InZ, dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX) of the ferrite dominate the softening mechanism during the compression. At this time, steady state deformation takes place at the last stage of deformation. In contrast, a higher InZ will facilitate the plastic deformation of the austenite and then activate the discontinuous dynamic recrystallization (DDRX) of the austenite, which leads to a continuous decline of the flow stress even at the last deformation stage together with CDRX of the ferrite.

Effect of baking process on microstructures and mechanical properties of low silicon TRIP steel sheet with niobium

After 2% predeformation, the baking treatment with different schedule was carried out for low silicon TRIP steel sheet with niobium. The effects of baking temperature and time on microstructures and mechanical properties were investigated. The results showed that with increasing the baking temperature and time, the volume fraction of retained austenite decreases, and the volume fraction of tempered martensite increases; as baking temperature ranges from 80 °C to 170 °C, the bake-hardening (BH) value increases obviously, while from 170 °C to 230 °C, the variation of BH value is very slight; as baking time ranges from 2 min to 20 min, the BH value increases significantly, while the BH value decreases when baking time exceeds 20 min. So that when the baking temperature is 170 °C and the baking time is 20 min, the low silicon TRIP steel sheet exhibits good bake-hardening behavior, and the highest BH value is above 70 MPa.

Effect of Thermomechanical Processing on Microstructures of TRIP Steel

In order to control retained austenite, the effect of hot deformation in the intercritical region on the microstructure of hot-rolled transformation-induced plasticity (TRIP) steel was studied on a Gleeble 1500 hot simulator. Compressive strains varying in amounts from 0 to 60% were imposed in.the intercritical region, and effects on the formation of polygonal ferrite, carbidefree bainite and retained austenite were determined. With increasing the hot deformation amount and the ferrite content and decreasing the carbide-free bainite content, the volume fraction of retained austenite decreases. Increased dislocation density, grain refinement of ferrite and carbon enrichment are the main factors which control retained austenite stability.

Effect of prestrain on microstructures and properties of Si-Al-Mn TRIP steel sheet with niobium

Paint baking treatment was carried out in a silicon oil bath at 170 °C for 20 min for Si-Al-Mn TRIP Steel sheet with different prestrains, and effect of prestrain on microstructures and properties was studied before and after baking. The results show that with the increasing of prestrain amount during prestraining and baking, the volume fraction of retained austenite decreases, and the volume fraction of martensite and bainite increases as well as yield strength increases; as prestrain ranges from 0 to 4%, the baking-hardening (BH) value increases; while the prestrain ranges from 4% to 16%, the BH value decreases; when the prestrain amount is 4%, the highest BH value is about 70 MPa for Si-Al-Mn TRIP steel sheet with niobium, which displays excellent baking-hardening behavior.

Effect of isothermal bainitic processing on microstructures and mechanical properties of novel Mo and Nb microalloyed TRIP steel

Abstract A transformation induced plasticity (TRIP) assisted steel with a new composition of C–Mn–Si–Al–Cu–Mo–Nb was designed in order to achieve significant improvements in both strength and ductility. In this study, computational thermodynamics calculations and a dilatometric method were used to develop a predictive methodology to determine the processing parameters in order to reach the desired amount of ferrite and retained austenite (RA) fractions during a conventional two-stage heat treatment. The heat treatment at the bainitic temperature affects RA content and carbon concentration in the γ phase. The effect of isothermal temperature on the microstructures and the mechanical properties of the investigated steel were investigated, and the mechanical stability of RA was evaluated using the Ludwigson and Burger relation. Direct experimental evidence by electron probe microanalysis confirms the distribution of alloying elements in different phases, especially C. Most of the C existed in the RA and small M/A islands. Observation by TEM reveals that the multiple additions of Mo and Nb not only refine the final microstructure but also make fully fine and dispersive (Nb,Mo)C carbide precipitate in ferrite grains. The samples annealed at 830°C and isothermally treated at 440°C demonstrated a significant improvement in the ultimate tensile strength (1080 MPa) with good uniform elongation (23%), as compared to conventional TRIP steels.

A study on precipitation kinetics of sigma phase in a hot-rolled super duplex stainless steel during isothermal aging based on the Johnson–Mehl–Avrami model

Isothermal aging treatment of 2507 super duplex stainless steel (SDSS2507) was conducted at 850°C after solution treatment at 1150°C. The characteristics of sigma (σ) precipitation kinetics in SDSS2507 were discussed and an improved JMA (Johnson–Mehl–Avrami) model had been developed. The results show that, the precipitation mechanism of σ phase in hot-rolled SDSS2507 was quite different to other ordinary DSSs. It can be found that σ phase precipitated via two mechanisms in this experiment. (1) The eutectoid decomposition δ → σ + γ2. It dominated the precipitation kinetics of σ phase in aging 0–25 min, and at this time the volume fraction of σ phase increased rapidly. (2) After 25 min, the precipitation of σ phase absolutely relied on the transformation of γ → σ which is controlled by the diffusion of Mo and Ni in γ. In this period, the precipitation rate of σ phase was significantly decreased. In this case, the separate modeling on the basis of each transformation mechanism was adopted and then an improved JMA model was developed for the precipitation kinetics of σ phase in the whole aging process. As a result, a good agreement between the experimental data and this developed JMA model can be obtained.

Partial Inverse Grain Size Dependence of Strength in High Mn Steels Microalloyed With Nb

Two steels without and with Nb addition were chosen to investigate the effects of Nb on the microstructures and the mechanical properties of Fe-Mn-Al-Si steels. The results revealed that Nb refined the grains markedly and both TRIP and TWIP effects occurred during deformation process. The Nb containing steel possesses higher yield strength and much lower tensile strength, the latter being explained by the suppression of TRIP effect due to the increase of stacking fault energy. This indicates grain refining is secondary for strengthening of steels when TRIP or TWIP effect exists during the deformation of low carbon and high Mn steels.