The continuous casting technology has steadily developed in the last decades. Beside process related modernizations, steel product quality requirements increased as well. Massive damages can occur in rolled plates, even if the defects in the casted semi-products are negligible. The predominant defects are cracks, which are generated on the surface and inside the product. They are mainly induced due to stresses and strains during continuous casting and rolling.
All these stresses occur even in state-of-the-art continuous casting plants and can only be minimized, but not prevented. A minimization of surface damage is an important aspect during the production process of continuous cast slabs, since the deletion of these defects is associated with high costs. An economical production of high-quality material therefore requires an optimal production process and hence require better knowledge of mechanisms that lead to crack initiation. Reasons for the formation of surface cracks are very complex. Different scenarios occurring at the production machines cannot be generalized and crack formation cannot be prevented, even though the main influence factors with a damaging potential are known. Due to these facts, the knowledge regarding crack formation mechanisms and the corresponding influence factors must be further enlarged.
Objectives and Motivation
- Identification of the correlations between different influence factors and the surface crack formation considering the cracking potential of certain steel grades
- Determination of the precipitation behavior during continuous casting and rolling by means of simulations and experiments and implementation of models for grain boundary segregation and pore nucleation
- Experimental verification of the thermomechanical influence factors regarding their effects on the high temperature ductility of micro-alloyed steel grades
- Understanding the influence of micro and nano structure on crack formation
The methodology planned within the current Project 2.3 consists of theoretical literature studies regarding the crack formation mechanisms during continuous casting and rolling, thermomechanical calculations and experimental lab-scale trials with accompanied microscopic investigations. The IMC-B test (In Situ Material Characterization - Bending) will be used for an experimental examination of surface crack tendency under near continuous casting conditions). This experiment allows the deformation of in-situ casted and cooled samples with defined strains and strain rates. In addition to this, thermogravimetric analysis will be executed to quantify the influence of a selective grain boundary oxidation.
Beside this, precipitation formation and micro structure development will be simulated by experiments using the GLEEBLE system and the Bähr Quenching Dilatometer. The obtained data should validate simulations being executed within the project. Additionally, heat treatment trials represent another part of the planned methodology under usage of the hot tensile test apparatus BETA-250-5. The goal during these activities is to quantify the high-temperature ductility of specifically chosen steel grades.
The numerical calculations consist of thermomechanical modelling (Finite Element Simulations) as well as simulations of precipitation formation and grain size development (i.e. prediction of pore nucleation). Among others, microscopic investigations will be done via transmission electron microscopy to characterize of small and fine distributed precipitations. During this, size distribution and the chemical compositions of the precipitations will be determined, if possible. To explore phase transitions and grain growth, high-temperature Laser Scanning Confocal Microscopy (HT-LSCM) will be applied.
Results and application
It is expected that critical experimental parameters for surface crack formation are generated, and based on that, defect risk indices should be defined. Beside this, the effect of special mould designs (e.g. mould geometry) and cooling conditions (temperatures, holding periods) on defect formation should be quantified.
Regarding the simulation of precipitation formation and grain size development, a method should be developed based on a combination of different models to predict the ductility behavior. This should be the starting point to identify the main significant parameters leading to surface near cracks during continuous casting and rolling. Finally, a method can be derived from that to quantify the crack tendency by means of thermomechanical treatment of laboratory samples. In addition to this, a contribution is expected regarding the identification of the relevant mechanisms, which lead to cracks during continuous casting depending on heat treatment and steel quality.