Improvement of cold-rolled products by computer-assisted metallurgical modelling

Uncoated, electrolytically coated and hot-dip galvanized thin sheet grades are high value added products of great importance in the steel industry. Soft thin sheet grades based on aluminum-killed low or extra low carbon and interstitial free compositions have the highest market share. Moreover, high-strength grades such as bake hardening, high-strength IF, rephosphorized and microalloyed grades are based on almost the same compositions. Despite the excellent level of knowledge of these grades, there is still an additional demand for improvement of the mechanical properties and the reduction of the variation in the mechanical properties, both over strip length and width as well as with respect to variations from coil to coil. One possibility to meet this challenging requirement is a profound understanding of the impact of the different production parameters on the mechanical properties and an appropriate adjustment of the production conditions. A very promising way to improve the knowledge of the process is rigorous modeling based on physical-metallurgical models. In the past such models were successfully developed for the hot rolling process and the essential outcome was a drastic increase of the knowledge of the hot rolling process and the possibility to calculate the resulting microstructure and mechanical properties. Moreover, at present first attempts are being made to integrate these models in the plant to adjust the processing conditions. This "in line" adjustment will result in a significant improvement of the uniformity of the mechanical properties. Therefore, the modeling of the cold rolling, annealing and temper rolling steps for thin sheet grades can result in a similarly big impact on the understanding of the process and it can be expected that the integration of the models in the plant will provide an easier way to improve the uniformity of the mechanical properties. Such an overall model can be used for the calculation of the mechanical properties of thin sheet material and therefore the reduction of the mechanical testing for the quality control. Additionally, such a system will be a valuable tool in steel development or in the designing of production parameters. The impact of changes in the production conditions or the clarification of the causes of manufacturing problems can be investigated in "what-happens-if-tests" without expensive plant trials. The aim of the project is therefore the development of an overall metallurgical-based model for the calculation of the microstructure and the mechanical properties of batch annealed, continuously annealed and hot-dip galvanized interstitial free and extra and low carbon based killed-killed steel grades. The input parameters for this model are the microstructure in the ashot rolled material and the processing parameters during cold rolling, annealing and temper rolling. To successfully reach this goal, the overall model was divided into different submodels with well defined interfaces. The submodels are: modeling of cold rolling influence of cold rolling on the microstructure in cold rolled condition modeling of precipitation, Ostwald ripening and dissolution of AlN and (Ti,Nb)(C,N) modeling of the dissolution of cementite and the formation of austenite during heating and soaking modeling of recrystallization and grain growth modeling of the precipitation of FexC during cooling and the transformation of the austenite temper rolling and mechanical properties During the project an overall model linking the different submodels was developed. The layout for the overall program concentrates on an open structure and the comprehensibility of the data exchange between the different submodels. In the resulting overall program the addition of new parameters, modification of models and even the implementation of new models is possible without problems. However, the overall program is not trimmed to high calculation speed.