Materials Science Forum Vols. 704-705

Abstract: Based on the actual production data of ASP (Angang Strip Production) hot strip rolling line, mechanical properties of thin gauge X70 pipeline steel were simulated by BP neural network method. Recursive functions were used to verify the mechanical properties which calculated by BP neural network. Based on predicted mechanical properties with high precision, BP neural network and Genetic Algorithm (GA) were combined to establish the temperature schedule of X70 pipeline steel during ASP hot strip rolling. It is shown that there are four important temperatures during ASP hot strip rolling, such as rough rolling temperature, refine start rolling temperature, refine finish rolling temperature and coiling temperature. Temperature difference of adjacent stages and temperature of former stage is a linear function relationship. For a given mechanical properties, deviations between simulated temperature and actual temperature are within ±10°C. This method can be used to produce different strips with the same compositions but different strengths by regulating suitable temperature schedule, so it is effective to resolve conflicts during hot strip rolling.

Abstract: The local wall thinning defect is very normal on pipes in power plants, which may result in stress redistribution of the pipes during the service process at elevated temperature. For the purpose of understanding the stress redistribution and strain accumulation of pipes with local wall thinning affected by load properties under creep condition, three groups of models were calculated, using three-dimensional elastic-plastic finite element analyses (FEA) based on FEA codes ABAQUS. In this study, the pipes has an identical defect of local wall thinning, the load properties and values are changed. Three groups of load properties, considering here, were monotonic internal pressure, monotonic moment and both internal pressure and moment, respectively. The numerical simulation conducted on P91 full-scale steel pipes at 625°C, with local wall thinning located at the inner surface. Then, von Mises stress and creep strain of pipes after 100,000h could be obtained corresponding to different models. Based on the analysis, the figures of creep stress and strain varying with load properties were plotted. Then, the stress and strain of pipes with local wall thinning affected by load properties were discussed. The results indicate that creep stress and creep strain increase with load properties. The variation laws have been summarized. The research results can provide the possibility on safety assessment and structure integrity analysis of the pipe with local wall thinning at high temperature effectively.

Abstract: Crack initiation, propagation and microfracture processes of B-class shipbuilding steel have been investigated by using an in-situ tensile stage installed inside a scanning electron microscope chamber, meanwhile the secondary crack propagation at low temperature brittle fracture has also been studied. It is revealed that micro cracks always nucleated at the notch of specimens due to the stress concentration and then propagate along the interface of ferrite-pearlite. The plastic deformation of polygonal ferrite occurred in the loading process, the cracks propagated as “Z” morphology in the matrix. In the low temperature brittle fracture zone, the secondary crack propagated through the ferrite matrix in the manner of transgranular crack. When the secondary crack propagated to pearlite region, the intragranular crack and transgranular crack were observed in the pearlite region .

Abstract: The control of welding distortion during assembly process is very important. At present, there are mainly two numerical simulation methods for the welding distortion which are thermo-elastic-plastic finite element method and its simplified approach of inherent strains. At first, taking T-joint as an example, the welding distortions were computed using two methods mentioned above. Based on thermo-elastic-plastic finite element method, welding process was simulated by life and death element, moving heat source and transient state thermal field etc. Then, the welding distortion was calculated by inherent strains method after thermo-elastic-plastic computation. It was concluded from the comparison that the simulation results by using the two methods are consistent. Therefore, the inherent strains method can be conveniently and economically applied to prediction of structural welding distortion in engineering. Applied the inherent strains method, welding deformation was predicted for the bogie frame side beam of high speed train. According to the deformation results from the finite element analysis, the welding deformation of the bogie frame side beam was lager than the tolerance of quality and in reasonable agreement with the experimentally determined distortion values. The work in this paper indicated that the inherent strains method was effective to predict the welding deformation so as to control the welding quality in large complex structures, such as the bogie frame of railway vehicle. Key words: welding distortion; thermo-elastic-plastic method; inherent strains approach; numerical simulation; bogie frame;

Abstract: In asymmetric roll bonding, two metal sheets with dissimilar properties are subject to a rolling step to create a bimetal strip. The paper reviews earlier work in this field which generally uses the upper-bound or finite element methods. The use of the slab method appears simpler than the former, but allows identifying critical aspects of process and model which were not addressed before. To obtain a complete set of equations, the bonding problem must be analysed not only from the cinematic viewpoint, but also from a mechanical one. After defining mechanical equilibrium, one additional cinematic equation is needed to solve the problem, which is obtained by minimising the plastic work in the zone where both sheets deform plastically but are not yet bonded. The new model has the advantage over earlier approaches in that it not only predicts the final thickness of both sheets but also permits to determine whether the bonding is possible under given process conditions. Keywords: Roll Bonding, Cladding, Modelling, Adhesion, Slab method.

Abstract: The microscopic phase field approach was applied for modeling the early precipitation process of Ni₇₅Al_xV_25-x alloy. Without any a prior assumption, this model can be used to simulate the temporal evolution of arbitrary morphologies and microstructures on atomic scale. Through the simulated atomic pictures, calculated order parameters and volume fraction of the θ (Ni₃V) and γ′ (Ni₃Al) ordered phases, Ni₇₅Al_xV_25-x alloys with Al composition of 0.05, 0.053 and 0.055 (atom fraction) were studied. Results show: For these alloys, θ and γ′ precipitated at the same time. With the increase of Al content, the amount of γ′ phase is increasing and that of θ phase is decreasing; the precipitation characteristic of γ′ phase transforms from Non-Classical Nucleation and Growth (NCNG) to Congruent Ordering + Spinodal Decomposition (CO+SD) gradually, otherwise, the precipitation characteristic of θ phase transforms from Congruent Ordering + Spinodal Decomposition (CO+SD) to Non-Classical Nucleation and Growth (NCNG) mechanism gradually. Both θ and γ′ has undergone the transition process of mixture precipitation mechanism with the characteristic of both NCNG and CO+SD mechanism. No incontinuous transition of precipitation mechanism has been found.

Abstract: The dendrite growth process was simulated with the Phase Field Model coupled with the fluctuation. The effect of fluctuation intensity on the dendrite morphology and the thermal fluctuation together with the phase field fluctuation on the forming of side branches were investigated. Result shows that with the decrease of thermal fluctuation amplitude, the furcation of dendrites tip also decreased, transverse dendrites become stronger and lengthways dendrites becomes degenerate, Doublon structure disappeared, finally a quite symmetrical dendrites structure formed. Thermal fluctuation can result in the unsteadiness of dendrites side branches, it is also the main reason of forming side branches, yet phase field fluctuation has little contribution to the side branches, it is usually ignored in the calculation; when the value of F_u is appropriate, the thermal noise can lead the side branches, but cannot change the steady behavior of the dendrites’ tip.

Abstract: It is essential to consider the proper position and volume of riser during the design of feeding system. In this paper, we are presenting a new method of designing optimal riser in the steel casting processes. A technique of slitting arbitrarily 3D entity model by STL (Stereolithography) was used to obtain accurate values of the partial modulus of casting, and then a mathematical model of the process of the riser design was optimized by a genetic algorithm (GA); with the help of the CAE system, which has an ability to calculate automatically and verify the validity of the optimized results, we will pursue the goal of obtaining the desired riser with optimal size and distribution but without causing any defect in casting. Thus, by combining the numerical optimization with the traditional riser design, our method proposed here will be more practical and reliable. In the end, we give an example to demonstrate the feasibility of our optimized approach in the riser design. Keywords：Riser design, Genetic Algorithm, casting optimization, STL

Abstract: Nozzle layout has an important role in fulfilling the operational and production requirements. The nozzle arrangements in Ⅱ zone of secondary cooling of a round billet continuous caster in a steel plant are simulated and the water flow density distribution at different range of air pressure and water pressure are analyzed. The result is, the spray zone of two adjacent nozzles which are mutual vertical to each other exist great wrap area，which cannot be eliminated when the air pressure and water pressure are decreased. According to solidification and heat transfer theory, with the steel performance parameter and the accurate boundary conditions, two dimensional heating transfer mathematical model was established, which based on finite difference method and the mind of setting slices on the slab along the casting direction. The surface temperature distribution of round billets in Ⅱ zone of secondary cooling are computed and the temperature curve of different stagger angles of nozzle pairs in consecutive rows are compared. The results show that the temperature at the center of nozzle and spray wrap are all located at the wave trough of surface circumferential temperature curve, the temperature fluctuation of temperature curve that the stagger angle is 22.5° is smaller than when it is 45°. Eliminate the effects of non-uniform surface temperature distribution on the strand quality, thus reducing the defects in the finished product.

Abstract: Stretch forming is one of the primary methods in skin forming process. Uniform strain distribution and springback are main factors which affect the precision of air skin. In the article the stretch forming process based on S-skin was analyzed. Firstly the parameters ranges of the loading trajectory were designed through the analytic method. Secondly the initial loading trajectory was optimized through finite element numerical simulation. The optimization processes was performed through FET software integrated with the optimization arithmetic. The motion parameters of jaw and machine’s instructions were selected as design variables. Optimization mathematics model was set up which objective is to reduce springback and improve the strain distributes uniform degree. During optimization the maximum main strain and thickness thinning rate of elements were restricted in permissive range. The forming degree of each stage was rational distributed, and the reasonable loading trajectory was founded. The result shows that the reasonable loading trajectory is including pre-stretch, wrap, press and after stretch. After optimization the strain distributes uniformly and the maximum main strain is between 3％～5％. The maximum stretching rate which appears in the shoulders area is less than 6%. In the concave area in which the insufficiency deforming can be occurred easily the strain achieves about 3%, and the deformation is enough. After optimization the unloading springback is decreased distinctly. The average springback of all elements is 0.47mm which reduces 30% compare with before optimization. The result meets the manufacture requirement.