Analysis of Orange Peel Defects on Hot-Dip Galvanized High Strength Low Alloy Steel

The surface of hot-dip galvanized high strength low alloy (HSLA) steel easily occurs orange peel in the deformation process. On the other hand, the defects possess a specific directivity and sits at approximately a 45-degree angle to the sheet steel rolling direction. The microstructures and properties of steel specimens with the orange peel defects and the normal were analyzed, which results showed that their microstructures consist of ferrite and few granular pearlite. The yield point elongation of the HSLA steel resulted in the orange peel defects on the surface of sheet stamping and it is associated with skin rolling and stretch rolling process. Further studied on the fine microstructures by means of SEM and electron back scatter diffraction (EBSD) techniques, which was apparent for the defect steel that the orange peel defects were resulted from weak favorable {111} texture might be the key factors aggravating the formation of orange peel defects. It can be concluded that the formation of Cottrell atmospheres caused the yield point elongation by the interaction between dislocation and diffusive solute atoms as basic reason and the directivity of the orange peel defects was related with the LUDERS slip forming. The yield point elongation can be eliminated to avoid the orange peel defects beyond to 1.8% skin-rolling and stretch rolling method with an appropriate annealing technology.

Influence of the Forming Operations on the Yield Stress Measured on Pipe

The yield stress of pipes is measured among purposes to assess the resistance of the pipe to the internal pressure of the fluid. Unfortunately, it is not possible to sample a straight specimen in the hoop direction, and therefore the pipe has to be flattened prior to testing. In the present investigation, different materials were tested in tensile–compression mode in order to provide data for a kinematic hardening model. Based on this experimental data set, a model was built to take into account several features of the material behaviour (presence of yield point elongation, strain hardening, etc…) and the processing route (longitudinal or spiral seam weld, expander, hydrotest…). Pipe production was also sampled at different moments (base material, after leveling, after pipe forming, after hydrotest). The testing program included tensile testing and ring expansion tests. The results show that the model gives a prediction in good correspondence with the experimental results. The model also reproduces several experimental facts, like for example the presence of a yield point elongation on the base material and its absence on the flattened pipe sample. Finally, the model is compared with an industrial database containing different steel grades (from grade B to X80) and different ratios of wall thickness over diameter (t/OD) ratios. The difference of yield stress between coil and pipe is predicted on this database with an accuracy of 20MPa.