Zhongqiu Liu

Transient Asymmetric Flow and Bubble Transport Inside a Slab Continuous-Casting Mold

A one third scale water model experiment was conducted to observe the asymmetric flow and vortexing flow inside a slab continuous-casting mold. Dye-injection experiment was used to show the evolution of the transient flow pattern in the liquid pool without and with gas injection. The spread of the dye was not symmetric about the central plane. The flow pattern inside the mold was not stationary. The black sesames were injected into water to visualize the vortexing flow pattern on the top surface. The changes of shape and location of single vortex and two vortices with time had been observed during experiments. Plant ultrasonic testing (UT) of slabs was used to analyze the slab defects distribution, which indicated that the defects are intermittent and asymmetric. A mathematical model has been developed to analyze the time-dependent flow using the realistic geometries, which includes the submerged entry nozzle (SEN), actual mold, and part of the secondary cooling zone. The transient turbulent flow of molten steel inside the mold has been simulated using the large eddy simulation computational approach. Simulation results agree acceptably well with the water model experimentally observed and plant UT results. The oscillating motions of jet and the turbulence naturally promote the asymmetric flow even without the effects of slide gate nozzle or the existence of clogs inside the SEN. The periodic behavior of transient fluid flow in the mold is identified and characterized. The vortexing flow is resulted from asymmetric flow in the liquid pool. The vortices are located at the low-velocity side adjacent to the SEN, and the positions and sizes are different. Finally, the model is applied to investigate the influence of bubble size and casting speed on the time-dependent bubble distribution and removal fraction from the top surface inside the mold.

Effects of carbon and niobium on microstructure and properties for Ti bearing steels

Abstract The objective of the study described here is to elucidate the effect of carbon and niobium on the microstructure, precipitation behaviour, and mechanical properties of 0·09C–0·11Ti (%) steel and 0·05C–0·025Nb–0·11Ti (%) steel under ultra fast cooling condition. The strengthening mechanisms are also discussed. The ferrite grains size and the size of precipitates in Ti and Nb–Ti steels were measured respectively. The mechanical properties obtained in Ti steel were similar to Nb–Ti steel with yield stress >700 MPa, elongation >20%, and good low temperature impact toughness. The study underscores that addition of higher carbon content by 0·04% under controlled rolling and ultra fast cooling conditions, we can achieve similar strength in the absence of micro-alloying element, niobium.

Modeling of Dynamic Recrystallization and Flow Stress of Nb‐Bearing Steels

The dynamic recrystallization (DRX) and flow stress of Nb‐bearing steels were investigated by means of isothermal single compression testing at temperatures of 850‐105° and at constant strain rate from 0.1 to 20s‐1 using a Gleeble 3800 thermo‐mechanical simulator in order to model the DRX processes and predict the flow stress during plate rolling. On the basis of the measured flow stress, a new model of DRX kinetics was proposed to calculate the volume fraction of dynamically recrystallized grains, which was a function of processing parameters such as deformation temperature, strain, strain rate, the initial austenite grain size and Nb content. The effect of deformation conditions was quantified by the Zener‐Hollomon parameter, in which the activation energy of deformation was expressed as a power function of Nb content. The critical strain was determined by using the method proposed by Jonas and co‐workers. It is shown that the ratio of the critical strain to the peak strain decreases with increasing Nb content, from which an empirical equation was developed. In addition, the influence of Nb content and deformation conditions on the steady state grain size was determined by fitting the experimental results to a linear relationship. Finally, the flow stress of Nb bearing steels was accurately predicted using a one‐internal‐variable evolution equation by taking Nb content as a parameter and including the influence of DRX. The comparison between the experimental and theoretical results confirmed that the modeling had a good accuracy to predict flow stresses during hot deformation.

Conversion between non-isothermal and isothermal transformation kinetics of γ to α for C–Mn and Nb microalloyed steels

Abstract In the present paper, austenite γ to ferrite α transformation kinetics of steels at different cooling rates has been measured by thermal dilatation. The relationship between lnln[1/(1 − X)] and ln(|C R|) (C R: cooling rate) was plotted using the data at early stage of cooling, which well fitted a linear relationship for the calculation of exponential values of n in the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation for isothermal kinetics using the method developed by Rios. The values of k in the JMAK equation obtained with the Rios method, however, have led to big discrepancies when the isothermal equations were used to predict the transformation kinetics during cooling. By assuming a Gaussian function of temperature, k was calculated using an optimisation method based on the rule of additivity. The isothermal transformation model was used to predict the transformation kinetics during cooling, showing good agreement with the measured data. It has been proved that even though the rule of additivity has to be relaxed to take into account the effects of cooling rates, precise conversion between non-isothermal and isothermal kinetics can still be realised.

Eulerian two-phase modeling of cavitation for high-speed UUV using different turbulence models

The cavitation will occur when the speed of UUV(Unmanned Underwater Vehicle) exceeds the threshold. The research of cavitation is significant for the high-speed UUV research. Two different turbulence models are respectively used to study the time-averaged and instantaneous cavitation two-phase flows in the present work. The Euler-Euler approach is used to describe the phase equations of the water and vapor. The Schnerr and Sauer model is used to describe the mass transfer due to the cavitation. Firstly, the time-averaged cavitation flow in a cylindrical model is simulated using the renormalization-group(RNG) k-ε turbulence model. Good quantitative agreement with experimental data is obtained both for the vapor volume fraction and static pressure of different cavitation numbers. Next, transient computation is performed using the large eddy simulation(LES) with Smagorinsky subgrid scale model(SGS) to study the time-dependent cavitation flow of a hydrofoil model. The predicted transient cavitation behavior and cavities shedding in the hydrofoil model agree well with experimental observations. Thus, the LES can be effectively used for studying the cavity shedding and the instantaneous cavitation flow.

Precipitation characteristics during isothermal γ to α transformation and resultant hardness in low carbon vanadium–titanium bearing steel

The effects of isothermal holding time on precipitation behaviour and resultant hardness were investigated in detail. Of particular note, with prolonged isothermal holding time at 650, 700 and 750° C, the hardness peak moves to a higher value, nearly remains unchanged, and to a lower value, respectively. It has been demonstrated that the nanometer–sized particles can nucleate at migrating γ/α interface for both rapid or slow transformation kinetics, and the changes in hardness are mainly related to precipitation behaviour. Regardless of isothermal holding time, the hardness increases as the transformation temperature decreases due to particle size and particle spacing refinement.

Correlation between ductile-to-brittle transition behaviour and twinning in ferritic stainless steel

Abstract In this work, the 18Cr–2Mo ferritic stainless steel was treated with and without warm rolling at 573 K with 66.5% reduction after conventional hot rolling process. It was shown that ductile-to-brittle transition behaviour could be closely related to deformation twinning and parameters affecting critical temperature for twinning would also inevitably affect ductile-to-brittle transition temperature. This correlation between ductile-to-brittle transition behaviour and twinning was in good agreement with the Cottrell–Petch model. A lowered transition temperature and an improved toughness after introducing warm rolling process and corresponding annealing process could be mainly explained in terms of refining the recrystallised grains and reducing the volume fraction of grains having orientations favourable for twinning, which decrease the critical temperature at which twins form.

Artificial Neural Network (ANN) Modeling for the Energy Absorption of Hot‐Rolled Plates in Charpy Impact Tests

In the present paper, a modeling for the energy absorption (CVN) at room temperature of hot‐rolled plates in Charpy V‐notch impact tests was investigated, in which an BP(Back Propagation) ANN (Artificial Neural Network) model with three layers was developed to take into considerations chemical compositions, processing parameters, yield strength, tensile strength and product thickness. The measured or predicted strength values have been used to predict the energy absorption in Charpy impact tests, both showing good agreements with the measured values. In order to compare the precision of the neural‐network methods in predicting CVN, linear regression analysis was performed by using the same data. Also, analyses were made for the effects of alloying elements, microstructure and processing parameters on CVN using ANN model, being consistent with the metallurgical rules. It concluded that the absorbed energy in Charpy impact tests for given steel compositions, processing parameters, strengths and plate thickness can be predicted by using the modeling.