A DFG-Funded and Industrial Improved FEM Package for Rolling of Sections and Tubes 

 
FEM model can be an independent software package without other source code; it can also be a package further developed based on existing source code such as MARC, ABAQUS, etc. FEM Model for the rolling of complcated sections or tubes, usually requires thermomechanical elastic-plastic model. Therefore, an independent software package developed from scratch often has weaknesses, including simplification of the algorithm (such as non elastic-plastic), or less user-friendliness in interface for data input / output (e.g. mesh for the steel, grove and roll). The model described here is based on the further-development of source code package MARC. This model of development lasted six years with nearly 10 participants. The work was led by the developer currently works for Metal Pass, for five years, on model development, model validation, and development of a simplified model. In addition, it involved one year of the validation work for the simplified model, completed by another colleague (now in charge of SMS model development). Total development costs were about half a million US dollars. The model was then under more than ten years of further development and optimization, including the joint development with the source code supplier MARC Corp. The model is characterized by delicate modeling, optimization for high precision and fast operation.

The technology package for this model includes followings:

1. An entire set of input program for MARC source code preparation. It is to apply thermomechanical elastic-plastic model, based on Lagrange algorithm (full scale simulation, rather than only the deformation zone). It includes the following details of the technology and data.

2. Set of mesh creation and optimization technology. As the cross-section shape is very complicated, how to use the best design of the mesh to improve accuracy and save time, involves high technology. This part of the technology includes mesh design, mesh initial creation, and remesh during computing process, and so on. This technology designs and optimizes mesh in each pass, based on the final rolled shape. The purpose is for achieving the best operation accuracy with the same computing cost. Some modelers optimize the mesh from initial shape, and some even uses automatically generated mesh; those are undesirable because it cannot reach the best results.

3. Physical property parameters, mostly temperature-dependent and for high temperature. The data is different for different steel grade. This package includes all the key physical property parameters in temperature dependence for the rolling temperatures. The number of data follows the list of the steel grades and the rolling temperature range. Metal Pass has a complete metal high temperature property database; database contents are from more than twenty years of data collection worldwide and the related modeling of the material properties.

4. Mechanical property parameters, such as flow stress, Young's modulus, poisson ratio, and so on. These parameters change with temperature at high temperatures, and the data is different for different steel grade. Flow stress also depends on strain, strain rate, temperature, and grain size and so on.

5. Boundary condition data or prediction model. Prior to FEM model, a set of models to predict rolling process boundary conditions, need to be established. For example, the heat transfer coefficient between the roll and the metal varies in the range 2500-12500W / m ^ 2K based on a series of factors. They are more complex when the cooling water is applied. Even in the air, the heat transfer coefficient also depends on the relative speed with air, and so on. These require accurate model to calculate.

6. Temperature processing in the multi-pass rolling deformation zone. Temperature should not be treated as uniform, even in the first pass. As the steel moves from the reheating furnace to the roll gap, its temperature over the cross-section is uneven. Usually the metal has large deformation in the high temperature zone and small deformation in the low temperature region. With an error in the initial temperature field, the cross-section shape will not be right.

7. The definition, input and output of rolling process parameters. FEM source code such as MARC, does not involve rolling-specific parameters, such as the pass reduction, rolling speed, rolling force and torque, and so on. Those rolling-specific parameters must be defined in and entered through the user subroutine. Then the calculated results can be displayed and graphically illustrated by the MARC Post-Processor MENTAT. There are some parameters, such as the shape of projective deformation zone can also be calculated and displayed.

8. High-end features, e.g. microstructure simulation. If there exist mechanical property uniformity problems of the product , FEM can calculate and display the distribution of the internal microstructure parameters, such as recrystallization percentage ratio, grain size distribution over the cross-section, and so on. The result of  this part of the work was initially achieved by a Metal Pass expert while working in Morgan under cooperation with MARC source code development team, and later enhanced through large scale investigations.

9. A set of coordinated parameters to make the model run smoothly and achieve high prediction accuracy. Examples are such as the best mesh sizes and the related steps (Time Steps), and so on. This part of the work needs most trials-and-errors for the developer in the initial modeling stage for rolling processes, and is very complicated. FEM simulation for hot rolling of sections and tubes is extremely difficult. Model parameters in all conditions, if not well coordinated, even with a little inconsistence, would cause the program to terminate before the completion of the calculation. Inexperienced rolling process modelers, even with experience of general FEM modeling, often need six months to a year of the tuning work, to achieve the stage that the model can run smoothly to the natural completion (rather than accident interrupts) and to have right results.

10. A series of Optimization measures are provided that make the model both time-saving and with high accuracy. For example, if the part of the steel is not in the deformation zone, its mesh is allowed to be rough (big in size). We have a number of optimization measures, which can be provided according to the needs of the users.

11. Automatic generation of model input parameters. FEM source code such as the MARC requires an input file, which contains all of the input data. Typically, after user generates mesh, the source code allows the user to manually enter various boundary conditions, material data and the processing steps (Time Steps), and so on. However, those data can be automatically inserted by an external package supplied by Metal Pass. In addition, the meshing parameters used to optimize the mesh can also be optimized by the external package. The external package, through its own user interface and database, can greatly reduce the difficulty and time of modeling work.

12. Integration into a roll pass design program. If the external package serves as a part of a roll pass design program package, the roll pass design can be performed based on the metal deformation knowledge supplied by the FEM simulation. It then takes place on a technical leap forward. This would be the start point to develop a roll pass design package based on the FEM simulation. A series of optimization measures, such as the modern intelligent automation technology like self learning, etc., will be added to this roll pass design package. For the efficiency during learning, a simplified FEM model, developed by Metal Pass, would be also used. The simplified model requires only 0.5% calculation time of the full model.

Each modeling approach and experience vary, and the quality of the results obtained differ greatly. If a model is not accurate, it would lose its significance in guiding the design, rather it causes misleading. The FEM model package introduced here allows an inexperienced modelers to create quality model in a short time with sufficiently accurate results. In addition, this FEM package may provide interface with some roll pass design programs, so that the quality of the roll pass design reaches very high technical level.


See Development Note