Control of sheet surface defects and deep drawing properties in final strip production steps

The objectives of the project were to understand and control sheet surface defects coming from Lüders bands in finishing lines, to guarantee the strip quality by optimising process parameters and to develop and validate industrial procedures to set the optimum temper rolling degree in such a way that plastic bending deformations induced by rolls and by bending caused by coiling are compensated. During skin pass rolling, the additional plastic deformation by bending influences the mechanical properties of the strips in a similar way to the skin pass rolling process itself. To achieve the most favourable mechanical properties, it is necessary to take into account the deformations caused by plastic bending when the optimum degree of rolling is specified. A completely different behaviour between bending and temper rolling was observed in terms of generation of Lüders effects. The origin and expansion of Lüders bands is not prevented but rather favoured by the 'free' strip surface in the strip bending process. A finite element method, associated with a physically based description of the constitutive behaviour of steel, allows simulation of both bending and temper rolling processes and prediction of the skin pass elongation necessary to suppress the yield point elongation (YPEL). A numerical machining procedure of flat tensile specimens was developed in numerically temper rolled steel sheet. Good agreement was found between experimental and calculated evolution of YPEL as a function of the bending elongation and temper rolling reduction. Some improvements could be made to be still more representative of industrial conditions but a numerical procedure such as this could be useful for building master charts and efficiently replace exhaustive and expensive experimental work to determine the optimal skin pass reduction by also taking into account deflecting rolls and plastic bending deformations. Special attention was devoted to relate defect appearance to the microstructure and to the mechanical properties and ageing properties in relation to the deformation modes (skin pass and bending) and the processing parameters used for suppression of YPEL. Results observed in a literature study were globally confirmed by testing at the laboratory scale.