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Large diameter spiral welded pipes are produced from hot rolled coil. The forming of a spiral pipe out of a coil is a sequence of cold deformation steps which are: decoiling, levelling and 3-roll forming (followed by seam welding). Obviously the material experiences a quite complex deformation history since several strain reversals occur during the different steps. A further complexity is that the strain history will even vary along the thickness as it mainly concerns bending deformation. It is therefore not at all surprising that the mechanical properties on pipe and coil are different. The steel manufacturer is able to control the production of the steel within well-defined process limits. Consequently he can guarantee the properties of his product, i.e. the coil. However, the spiral pipe manufacturer only has limited possibilities to control the steel properties but eventually he is responsible for the properties of his product, i.e. the pipe. A detailed understanding of how spiral pipe forming affects the mechanical properties would definitely help steel mills to specify and target coil strength to ensure the final pipe strength. Therefore an experimental study was launched in which a 4-point bending setup was used to reproduce the different forming steps on lab scale. The mechanical properties were measured at intermediate process steps, i.e. on coil, after levelling, after pipe forming and after subsequent flattening. The last step was included because, in practice, the mechanical properties along the pipe transverse direction are typically measured using flattened tensile samples, i.e. after introduction of an additional cold deformation step with strain reversal. The advantages of this experimental approach are twofold: first, one has full control and knowledge on the deformations introduced during the different steps. Second, the typical statistical variation of mechanical properties from coil to coil or even within one coil is far less pronounced as all samples are taken within a relatively short distance from each other. For a more detailed understanding of the experimental study, an efficient Finite Element model to simulate spiral pipe forming was developed in Abaqus. A nonlinear kinematic-isotropic hardening law was applied to describe the material behavior. In this way it was possible to capture both yield point elongation and the well-known Bauschinger phenomenon. This paper summarizes numerical and experimental results for a 16mm thick X70 grade, where different production parameters (leveller settings, ratio of wall thickness to outer diameter) were considered. |
