Jens Dierdorf

Effect of the Interdependence of Cold Rolling Strategies and Subsequent Punching on Magnetic Properties of NO Steel Sheets

Nowadays, optimization of non-oriented (NO) electrical steels toward lower iron-loss, improved, and isotropic magnetizability is critical to the improvement of rotating electrical machines. The whole production process chain adjusts the microstructure evolution, e.g., grain size and crystallographic texture, determining the magnetic properties. In particular, the interdependence of raw material properties and the resulting mechanical stress distribution during final assembly, e.g., punching, leading to magnetic property deterioration is crucial for the optimization of NO steel properties of rotating machines. This paper studies the effect of different cold rolling strategies, annealing treatments, and sheet metal blanking (punching) regarding microstructure evolution, magnetic properties, and deterioration.

On the effect of material processing: microstructural and magnetic properties of electrical steel sheets

This paper presents both the effect of cutting on the material behavior of a typical used NGO electrical steel grade M230-30A as well as a study of the effect of annealing temperature after cold rolling on microstructure and magnetic properties beginning with an industrial hot rolled 2.4 wt.% Silicon steel of 2.0mm thickness. Modifications in the local mechanical properties due to the cutting process are investigated in detail. A quantitative analysis of the impact of material degradation for non-oriented electrical steels applied in traction drives is presented. In order to consider the large speed range of drives in automotive applications and the presence of higher harmonics, this analysis is conducted for a wide range of frequencies and magnetic polarizations. Nanoindentation is used to analyze the effect of strain from cutting on the hardness near the surface. A major conclusion is that it is indispensable to take into account influences due to material processing on magnetic materials properties during the design process of electrical machines.

Investigation on Hardening and Softening Behavior of Steel after Rapid Strain Rate Changes

The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

Impact of the interaction of material production and mechanical processing on the magnetic properties of non-oriented electrical steel

Thin laminations of non-grain oriented (NO) electrical steels form the magnetic core of rotating electrical machines. The magnetic properties of these laminations are therefore key elements for the efficiency of electric drives and need to be fully utilized. Ideally, high magnetization and low losses are realized over the entire polarization and frequency spectrum at reasonable production and processing costs. However, such an ideal material does not exist and thus, achievable magnetic properties need to be deduced from the respective application requirements. Parameters of the electrical steel such as lamination thickness, microstructure and texture affect the magnetic properties as well as their polarization and frequency dependence. These structural features represent possibilities to actively alter the magnetic properties, e.g., magnetization curve, magnetic loss or frequency dependence. This paper studies the influence of production and processing on the resulting magnetic properties of a 2.4 wt% Si ...

Model for texture evolution in cold rolling of 2.4 wt.-% Si non-oriented electrical steel

Iron loss and limited magnetic flux density are constraints for NGO electrical steel used in highly efficient electrical machinery cores. The most important factors that affect these properties are the final microstructure and the texture of the NGO steel. Reviewing the whole process chain, cold rolling plays an important role because the recrystallization and grain growth during the final heat treatment can be strongly affected by the stored energy and microstructure of cold rolling, and some texture characteristics can be inherited as well. Therefore, texture evolution during cold rolling of NGO steel is worth a detailed investigation. In this paper, texture evolution in cold rolling of non-oriented (NGO) electrical steel is simulated with a crystal plasticity finite element method (CPFEM) model. In previous work, a CPFEM model has been implemented for simulating the texture evolution with periodic boundary conditions and a phenomenological constitutive law. In a first step the microstructure in the cor...

Influence of transient strain rates on material flow stress and microstructure evolution

A comprehensive knowledge about the material flow stress is a key parameter for a reliable design of hot forming processes using Finite Element (FE) software codes. Due to the microstructure evolution caused by the interaction of hardening and softening phenomena that take place during hot forming operations, the material flow stress is influenced by strain rate and temperature. While transient strain rates and temperatures typically characterize the industrial forming processes, the flow curves used in FE simulations are normally determined at arbitrary constant temperatures and strain rates. To calculate the flow stress evolution in between the measured strain rates, FE programs use linear interpolation. Hence, the material relaxation behavior caused by the microstructure evolution during transient strain rates is not considered. Previous investigations by various authors have shown that for a rapid strain rate change by one order of magnitude significant deviations between measured flow stress and line...

Integrated Computational Materials and Production Engineering (ICMPE)

The research area “Integrative Computational Materials and Production Engineering” is based on the partial integration of individual models areas within separated simulation platforms with the objective of further development and integration into a single comprehensive ICMPE (Integrative Computational Materials and Production Engineering) platform that combines materials and machining simulation with factory and production planning. In order to realize an operational platform concept, the AixViPMaP has been implemented. AixViPMaP serves as a technology platform for the knowledge-driven design, implementation and improvement of complicated process chains for materials in high-value components. This allows manufacturing related influences to be considered during production in order to optimize process performance and materials properties. The extension and application of the AixViPMaP platform towards production modeling in the sense of an ICMPE based on one holistic system integrates production related models with all material-related models into a single, unified concept. Advanced test cases are under examination to validate and assess this new integrated approach (e.g., new alloys for large gears for the wind industry).