EFFECT OF ROLLING PARAMETERS ON THE LOW-TEMPERATURE TOUGHNESS AND MICROSTRUCTURE OF HIGH-STRENGTH LINEPIPE STEEL

In recent years, large-diameter pipe producers around the world have witnessed a growing interest to develop gas fields in arctic environments in order to fulfill the energy demand. High strength linepipe grades are attractive for economic reasons, because they offer the benefit of a reduced wall thickness at a given operating pressure. Excellent low-temperature toughness of the material is essential under these conditions. Modern high strength heavy plates used in the production of UOE pipes are produced by thermomechanical rolling followed by accelerated cooling (TMCP). The combination of high strength and high toughness of these steels is a result of the bainitic microstructure and is strongly influenced by the processing parameters. For this reason, the relationship between rolling and cooling parameters of heavy plate production, the low temperature toughness and the microstructure is at the center of attention of the development efforts at Salzgitter Mannesmann Forschung (SLNIF) in collaboration Salzgitter Mannesmann Grobblech (SMGB). It has been shown previously that a variation of the processing parameters has a direct influence on the microstructure and correlates with mechanical properties that are accessible via small-scale tests. Modern characterization methods such as scanning electron microscopy in combination with electron backscatter diffraction have broadened our understanding of the underlying mechanisms and have helped to define processing conditions for the production of heavy plates with optimized low-temperature toughness in small scale tests. Within the present paper, the results of a recent laboratory investigation of the effect of a systematic variation of rolling parameters on the microstructure and low-temperature toughness of as-rolled and pre-strained Charpy specimens are discussed. In these trials, final rolling temperatures above the onset of the ferrite-austenite transformation and cooling stop temperatures above the martensite start temperature were selected. The microstructure of the plates was investigated by scanning electron microscopy and electron backscatter diffraction. In a series of Charpy tests in a specific temperature range, it was found that plate material in the as-rolled condition is not strongly sensitive to variations of the selected processing parameters, whereas pre-straining the Charpy specimens made it possible to assess the potential of individual processing concepts particularly with regard to low-temperature toughness. In addition to Charpy testing, the toughness was also quantified via instrumented drop-weight tear (DWT) testing. By comparing total energy values from regular pressed-notch DWT-test specimens to J-integral values determined in drop weight testing of pre-fatigued DWT-test specimens, the impact of variations of specimen type on material tearing resistance is shown.