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

Ivan Petryshynets,Ladislav Falat,František Kováč

doi:10.3390/ma14237124

Ivan Petryshynets, Ladislav Falat + Show 1 more

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https://doi.org/10.3390/ma14237124

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Abstract

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.

Highlights

Published: 23 November 2021Fe-Si electrical steels play an important role in the generation and distribution of electricity [1] as well as in electrical appliances and devices on the consumer side [2]
The samples were taken from the experimental hot band after its rupture during the cold rolling by a 5-stand tandem cold rolling mill
In order to investigate the reason for the tearing of the high strength silicon hot band, the samples were taken from the vicinity of the failure edge of the band, see Figure 1

Summary

Introduction

Published: 23 November 2021Fe-Si electrical steels play an important role in the generation and distribution of electricity [1] as well as in electrical appliances and devices on the consumer side [2]. Variations of silicon steels grades differ with respect to their magnetic properties, sheet thickness, mechanical properties, the value of electrical resistivity, and the type of coating. The development of new electrical motors requires that the rotor of the motor operate at very high rotary speeds, i.e., rounds per minute (RPM). The design conditions for this situation of very high rotation require that the steel components of the rotor in the form of electrical steel laminations should possess a sufficiently high yield strength to resist the resulting high centrifugal forces. The requirement of high yield strength is the additional essential pre-condition for the electrical steel material used in considered heavy-duty electro-motors as the ordinary material requirements rely on the selection of the steel grades fulfilling only the electro-magnetic properties.

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