Analysis of the Reasons for the Tearing of Strips of High-Strength Electrical Steels in Tandem Cold Rolling

High-strength non-oriented electro-technical steels with a low thickness possess excellent isotropy of electromagnetic and mechanical properties which is highly required in the production of high-efficiency electric motors. The manufacturing process of this type of steel includes very important and technologically complex routes such as hot rolling, cold rolling, temper rolling, or final heat treatment. The final thickness is responsible for the decrease in eddy-current losses and is effectively achieved during cold rolling by the tandem rolling mill. Industrial production of thin sheets of high-strength silicon steels in high-speed tandem rolling mills is a rather demanding technological operation due to the increased material brittleness that is mainly caused by the intensive solid solution and deformation strengthening processes, making the dislocation motion more complex. The main objective of this work was to investigate the distribution of local mechanical strains through the thickness of high silicon steel hot bands, generated during the cold rolling. The experimental samples were analysed by means of electron back-scattered diffraction and scanning electron microscopy. From the performed analyses, the correlation between the material workability and the nucleation of cracks causing the observed steel strip failure during the tandem cold rolling was characterized. Specifically, the microstructural, textural, misorientation, and fractographic analyses clearly show that the investigated hot band was characterized by a bimodal distribution of ferrite grains and the formation of intergranular cracks took place only between the grains with recrystallized and deformed structures.

Influence of specimen geometry and strain rate on elongation in tensile testing of packaging steel

Packaging steel is characterized by low thickness (0.1 mm – 0.5 mm) and ferritic microstructure resulting from low carbon contents. In combination with continuous annealing processes and temper rolling, this results in only little elongation observed in tensile tests. However, as in real forming processes much higher deformation occurs, it is important to receive true stress-true strain data up to a highest possible level e.g. to characterize material for finite element analysis. Therefore, tensile tests with three different measuring lengths (80 mm, 50 mm, 20 mm) were conducted for the packaging steel TH415. Likewise, the testing speed was reduced to investigate the possibility to receive more elongation under the condition of a constant stress level. The results revealed a significant increase in elongation when using smaller tensile test geometries. As well, the reduction in testing speed leads to much higher elongation while showing only little strain rate influence. While for the 80 mm geometry and standard speed no homogenous forming condition could be reached due to early failure before Lüders strain, this could be improved by using smaller testing specimens and a lower strain rate. Combining the influence of strain rate and geometry a significant increase of more than ten percentage points in elongation was reached.

Local Strain Measurement in Tensile Test for an Optimized Characterization of Packaging Steel for Finite Element Analysis

The continuous development of packaging steels for thickness reduction processes requires an advanced process design. This process is increasingly supported by finite element analysis to simplify tool construction and material selection purposes. Therefore, the fundamental basis is always the precise material characterization of packaging steel commonly based on tensile tests to determine flow curve and Lankford coefficients. However, due to strong temper rolling and the occurrence of slip bands, most packaging steels just show little elongation in tensile test. Therefore, a method of Paul et al. to determine the flow curve with digital image correlation (DIC) methods in the necking zone was applied in this work to meet the requirements of packaging steel. For the use of anisotropic yield functions, it is necessary to determine Lankford coefficients. Thus, a new method is proposed to measure Lankford coefficients locally with a DIC system in tensile test, also in case that no homogenous forming condition is reached. With the presented approaches the packaging steel TH415 was characterized. In order to validate the developed methods, a demonstrator was simulated with anisotropic yield function Yld2000-2d . The comparison between simulation and experiment showed clear improvements in simulation accuracy when using the newly presented methods for packaging steel.