The challenge for heavy steel plates: uniform properties and flatness
Thermomechanically rolled steel plates are manufactured in a four-step process, which includes slab heating, rolling, accelerated cooling and levelling. The process step of accelerated cooling in particular is decisive in adjusting the mechanical/technological properties of the plates. Their strength and ductility can be adjusted by varying the cooling rate and the cooling stop temperature. The main challenge in process engineering is to achieve well defined mechanical properties and a high degree of flatness prior to levelling.
Numeric modelling replaces experimental optimisation
The large number of influencing parameters to be considered in optimising the cooling process necessitates large-scale experiments and correspondingly high costs. This is where virtual methods come in, since varying the process parameters on the computer is significantly easier. The challenge is to develop computer models that describe the production step both qualitatively and quantitatively as detailed as possible, while at the same time reducing computation times in order to enable parameter studies.
The special challenge in the case of accelerated cooling of heavy steel plates was to provide an efficient numerical description of the mechanical behaviour of the entire structure during the cooling process. This behaviour is primarily characterised by a phase transformation (lattice transformation), which depends on a range of factors such as the microstructure prior to accelerated cooling and the cooling rate of the steel plate. The phase transformation not only transforms austenite into more stable product phases but also leads to an increase in volume. Another phenomenon associated with phase transformation is the occurrence of transformation-induced plasticity (TRIP). The physically correct description of all these phenomena is an essential prerequisite for reliable virtual process simulations.
No matter how thick and strong – the steel plates must be as flat as possible
A comprehensive 3D finite element model and a computationally efficient 1D model for real-time simulation of the accelerated cooling step were developed by Montanuniversitaet Leoben and the Materials Center Leoben in close cooperation with voestalpine Grobblech GmbH. This model provided important findings on the evolution of plate geometry and mechanical properties during the cooling process and led to a significant reduction in process development times.
Impact
The simulation models enable a more reliable adjustment of the desired properties within narrow tolerances, which is an essential quality criterion and thus represents a key factor in enhancing the competitiveness of voestalpine Grobblech GmbH.