Materials Science Forum Vols. 706-709

Abstract: The current work presents the numerical analysis of solid-state transformation kinetics relating to conventional welding of ferritic steels, with the aim of predicting the constituent phases in both the fusion zone and the heat affected zone (HAZ) of the weldment. The analysis begins with predictions of isothermal transformation kinetics using thermodynamic principles, such that the chemical composition of the parent metal is the sole user-defined input. The data is then converted to anisothermal transformation kinetics using the Scheil-Avrami additive rule, including the effects of peak temperature and austenite grain growth. Subroutines developed for the Abaqus finite element package use the semi-empirical approach described to predict phase transformations in SA508 Gr.3 Cl.1 steel. To study the effect of the cooling rates and the ability of the current model to predict the final microstructure, two weld samples were subjected to autogenous beam TIG welds under a fast (TG5-F, 5.00 mm/s) and slow (TG5-S, 1.25 mm/s) torch speed. Model validation is carried out by direct comparison with microstructural observations and hardness measurements (via nanoindentation) of the fusion and heat affected zones in both welds. Excellent agreement between the measured and predicted hardness has been found for both weld samples. Additionally, it is shown that the correct identification of the partial austenisation region is a crucial input parameter.

Abstract: Avoiding recrystallization of austenite in hot strip rolling of steels is highly important for enhancing mechanical properties of hot rolled products, as well as for the products undergoing cold rolling and annealing or coating. Recrystallization can only be avoided if its incubation time is longer that the time intervals characteristic for a particular hot rolling process. The present work focuses on computation of incubation time t_inc for static recrystallization using laboratory hot deformation data and on extrapolation the results to industrial conditions. The computations are done based on application of critical conditions for initiation of dynamic recrystallization to the static case. No-recrystallization temperature in hot strip rolling is determined by setting t_inc equal to interpass time. Simulations allow for prediction of the onset of austenite static or metadynamic recrystallization after individual rolling passes during industrial hot rolling and evaluation of the effects of strip thickness, rolling speeds, etc.

Abstract: The kinematics of the wetting front, i.e., the history of the locus of the boundary between the vapor blanket and the nucleate boiling area is of outmost importance in quenching operations. In this investigation, the effect of water temperature on the wetting front kinematics was studied in forced convective quenching experiments of conical-end cylindrical probes. Three values of water temperature (30, 45 and 60 °C) were studied for a free-stream water velocity of 0.2 m/s. The wetting front kinematics was characterized from: 1) the measured thermal response at three longitudinal positions inside the probe, near the probe surface, and 2) high-speed video-recordings of the events that took place at the probe surface upon quenching. From the video-recordings, the position of the wetting front as a function of time was determined to estimate the wetting front velocity from a linear regression. The wetting front velocity was constant for a given experiment and increased as the water temperature decreased. The bubble size decreased and the frequency of bubble formation increased as the water temperature decreased. Using the measured thermal responses, the surface heat flux history was estimated, solving the corresponding inverse heat conduction problem. A regression analysis was applied to relate the maximum (critical) surface heat flux and the water temperature as a first step towards modeling the surface heat flux history as a function of water temperature.

Abstract: In this paper, finite element models of the strip shape during cold rolling of ultra thin strip in both symmetrical and asymmetrical rolling cases have been successfully developed, and the strip shape such as the thickness distribution along the strip width has been obtained. The strip shape and edge drop are discussed under both symmetrical and asymmetrical rolling conditions. Simulation results show that the asymmetrical rolling can reduce strip edge drop dramatically. The work roll edge curve also affects strip shape significantly. The developed finite element model has been verified with the experimental values.

Abstract: A mathematical model of the horizontal centrifugal casting process based on the cast cylindrical coordinates for stainless steel pipe has been developed. Thermal boundary conditions, including the radiative and convective heat transfer conditions, have been taken into consideration. The model equation was solved numerically using non-uniform cylindrical grids and the finite differential method (FDM). Several parameters of casting process such as melt superheat, preheating temperature of mold, thermal conductivity of coating and flux powder, which affect the temperature distributions in both cast pipe and mold and the changed positions of melt solidifying at last are investigated. In order to verify the results of simulation, calculated temperatures were compared with the experimental data.

Abstract: Tons of numerical simulation results are obtained by numerical simulation of casting and heat treatment process. However, these results are not utilized sufficiently, most are wasted. The aim, to predict defects of castings, such as shrinkage, hot tearing, deformation and etc, need further processing of the simulated results. Therefore, the criteria should be established on the processing of the directly simulated results. What kind of more results can be acquired from these calculated data This is an interesting question. In this paper, schematic analysis is carried out for further processing of results, result mining (RM) is proposed and its meaning is explained. Meanwhile, some case studies are presented.

Abstract: The paper presents the results of the theoretical analysis of the asymmetric plate rolling process conducted in the plate finishing mill. The purpose of the work was to determine the influence of working rolls velocity asymmetry on the value and direction of the bending radius of strip flowing out from the deformation zone. The variable parameters of the process were: rotational speed asymmetry factor, a_v ; strip shape factor, h₀/D; and cross-sectional area reduction ε. For the theoretical study, a commercial computer program, FORGE 2008^®, was employed. Working rolls of a diameter of 1100 mm and a constant lower working roll rotational speed of n = 80 rpm were assumed for the study. The asymmetric rolling process was achieved by varying the rotational speed of the upper roll, which was lower than that of the lower roll. The range of variation of the roll rotational speed asymmetry factor, a_v, was 1.01÷1.20. On the basis of the conducted theoretical analysis, the influence of the speed asymmetry factor (a_v = 1.00÷1.20) on plate curvature upon exit from the deformation zone was determined, and the distributions of strip velocity.

Abstract: The work presents a control method for on-line adjustment of mechanical properties of as rolled steels produced at hot strip mills. The key idea of the method is a probabilistic causal (Bayesian) network which represents in a form of a directed acyclic graph the joint probability distribution of mechanical properties, chemical composition and temperature–strain parameters acting during hot rolling. As a slab moves along the mill the distribution is used for continuous recalculating the posterior probability of all mechanical properties conditioned by chemical composition and all other process parameters which become known to the moment of recalculating. Finally, when a strip is just before the finishing group we evaluate the probability distribution of finishing rolling temperature and coil temperature given the strip has the target mechanical properties. It generates new setups for these temperatures A pilot version of the method has been just implemented at CSP–line at Vyksa, Russia, United Metallurgical Company’s steel production site The adjustment is realized through appropriate correction of finish rolling temperature or/and coiling temperature setups of the mill automatic control system after the last chemical analysis of the current heat is made at the start of casting. Only “cautious” corrections of the temperatures are permissible so far (deviation from predefined level not more than ±30 degrees for each temperature) and the main aim of them is to set off the influence of chemistry variations on mechanical properties scatter of a given steel grade. The results of using the algorithm show that even these limited but interconnected actions reduce approximately twice the standard deviation of the mechanical properties inside a steel grade.

Abstract: Tailor Rolled Blank (TRB) and Longitudinal Profile plate (LP plate) are both new products for materials saving. TRB produced by cold rolling can partially substitute for Tailor Welded Blank (TWB) achieving lightweight in vehicles. LP plate produced by hot rolling can be utilized in shipbuilding, bridge manufacturing and constructions, realizing structures lightweight. This paper stressed the common point of TRB and LP plate: Variable Gauge Rolling (VGR). Basic equations of VGR were deduced, a velocity relationship which assures the shape and dimension of transition region was provided, a FEM simulation of VGR was performed, VGR experiments were carried out and TRB samples with a thickness ratio 1:2 were obtained. All above achievements have established a foundation for VGR’s industrial application.

Abstract: The paper deals with the microscale heat transfer problems. In particular, the ultrafast laser heating of thin metal film is considered. The problem is described by so-called two-temperature models consisting of two equations concerning the electron and lattice temperatures. Energy equations are supplemented by two additional ones determining the dependencies between electrons (phonons) heat flux and electrons (phonons) temperature gradient. According to the form of above dependencies one obtains the parabolic or hyperbolic heat transfer models discussed here. The problems have been solved using the finite difference method. In the final part of the paper the results of computations and the comparison of solutions obtained are presented.