A finite element analysis of the rolling load in the skin-pass of steel sheets

PEDRO MALAQUIAS ARAÚJO STEMLER ; NATHALIA DUARTE SOUZA ALVARENGA E SANTOS ; PEDRO HENRIQUE RODRIGUES PEREIRA ; FRANCO DE CASTRO BUBANI ; MARIA TERESA PAULINO AGUILAR ; PAULO CETLIN

Belo Horizonte

Simposio; International Symposium on Solid Mechanics - MECSOL 2015; 2015

UFMG - Universidade Federal de Minas Gerais

Low carbon steel sheets are produced massively and commonly display yield points in tensile testing, which are connected to deformation heterogeneities along the test specimen, known as Lüders Bands. These cause an unacceptable surface appearance in products manufactured with these sheets and employing sheet forming techniques. This problem is usually eliminated through a light cold rolling pass (?skin-pass?), imposing a thickness reduction of about 1 - 2.0% on the material as a last step in its industrial manufacturing. The skin pass also corrects sheet shape problems and affects its superficial topography, which is important for the subsequent finish processing of the sheet, such as painting procedures. The rolling load during the skin-pass is an important controlling aspect in the process, and has been measured experimentally and simulated numerically through the Finite Element Method (FEM) for the case of materials which do not display yield points in tensile testing, and where no Lüders Bands are present in the rolled material. A good correspondence has been found between the measured rolling loads and those predicted by the numerical simulations for small roll diameters; for large roll diameters there is also a good correspondence, but only for high friction coefficients between the rolls and the materials. The present paper analyzes numerically the rolling load in the skin pass through the Finite Element Method, but for the case where the material to be rolled exhibits a yield point in tensile testing and thus displays extensive Lüders Bands formation in the skin-pass. It is shown that for this situation the loads exhibit oscillations corresponding to the successive nucleation of the Lüders Bands during rolling, which are absent when the rolling does not lead to the presence of Lüders Bands in the material. The nucleation of the bands depends on the sheet thickness and roll diameters, and the magnitude of these oscillations depends on the geometry of the Lüders Bands along the sheet width. When the bands are not perpendicular to the rolling direction, the load oscillations undergo a pronounced decrease. The results from the 2D numerical model also show that, once a steady-state is reached, the nucleation and propagation of the Lüders Bands is a periodic phenomenon. This means that the final geometry of the bands is repeated along the sheet at a well-defined periodicity. The rolling load oscillations are also periodic, reflecting the regularity of the nucleation and propagation of the bands. This periodicity makes the Fourier Transform a powerful tool to analyze the phenomenon directly in the frequency domain. The Fourier Transform of the rolling load (1D) was calculated and the frequency peaks observed were successfully correlated with the geometry and the propagation of the Lüders Bands.