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The production of semi-finished flat products usually starts from ingot-cast or continuous cast material which can contain a variety of casting defects, ranging from voids to blowholes as well as metallic and non-metallic inclusions. These defects can reduce either the formability of the semi-finished product or the durability of the final product significantly. In addition, unfavorable load paths during hot rolling can lead to hydrostatic tensile stresses in the slab center preventing the closure of existing voids and blowholes. In fact, these defects can act as initiation points for further damage evolution or even failure. Current models available in the literature only cover either the closure of voids or the hot pressure bonding of the internal void surfaces. Moreover, these models do not consider the effect of simultaneous changes of the microstructure, e.g. due to recrystallization. Hence, a comprehensive analysis of the different possibilities for a damage-controlled production of semi-finished flat products appears to be impossible with present models.

The long-term goal of this project is the damage-controlled layout of the rolling process in order to produce semi-finished flat products with a known accumulated damage. Through adjustment of the rolling parameters (roll diameter, pass reduction, rolling velocity, rolling temperature, etc.) this accumulated damage shall be reduced. For this purpose, a fundamental understanding of the formation, evolution, and elimination of damage is vital. Within the CRC 188 project A04 provides semi-finished flat products with known damage and works on the review and further enhancement of current models for damage evolution. In the first funding period the main focus of this project is the void elimination, consisting of void closure and hot pressure bonding, during hot flat rolling of the dual phase steel HCT780X. FE simulations are used to investigate the influence of the process parameters on the load path and evaluate the correlation between load path and void elimination. Void closure and hot pressure bonding criteria from literature will be reviewed, evaluated, and enhanced for the determined load paths. Finally, multi-pass rolling strategies will be developed resulting in different accumulated damage conditions when going from the same initial to the same final geometry. Cold-rolled and heat-treated sheets with dual phase microstructure will be produced within project A04 and passed on to the subsequent processes of bending and deep drawing within projects A05 and A06.

Aim of the second funding period is the manipulation of damage evolution and the dual phase microstructure (grain size, distribution, and morphology of the martensite phase) through an adjustment of the cold rolling process and the subsequent heat treatment. Concepts for a damage-resistant dual phase microstructure for cold forming will be developed within project B05. By means of process modeling of the entire process chain of hot rolling, cold rolling, and final heat treatment in the third funding period the accumulated damage is to be reduced during the production of sheet metal. Furthermore, the transferability of the developed methods will be tested by applying them to a different material.

Project leader
Prof. Dr.-Ing. Gerhard Hirt
Institute of Metal Forming (IBF), RWTH Aachen University

Project coordinator
Conrad Liebsch M. Sc.
Institute of Metal Forming (IBF), RWTH Aachen University