Dedicated to Professor Dr. PAVEL LuKAC on the occasion of his 60th birthdayA very fine-grained Al-Zn-Mg-Cu-Zr alloy prepared using high energy gas atomization was studied. The alloy exhibits structural superplasticity at temperatures between 700 and 800 K at strain rates of the order lo-' 5s'. Its deformation behaviour may be described by the concept of a threshold stress for grain boundary sliding. A continuous recrystallization takes place during superplastic straining. The gradual increase of the mean misorientation angle between neighbouring grains is believed to contribute to the development of grain boundary sliding and to the dynamic softening. Untersucht wurde eine sehr feinkornige Al-Zn-Mg-Cu-ZrLegierung, die durch hochenergetische Gasatomisierung hergestellt wurde. Diese Legierung ist superplastisch bei Temperaturen zwischen 700 und 800 K und bei Verformungsraten in der GroBenordnung lO-'s-'. Ihre superplastische Verformung kann mit dem Konzept einer Schwellspannung fur die Korngrenzgleitung erklart werden. Kontinuierliche Rekristallisation tritt wahrend der superplastischen Verformung auf. Das Anwachsen des mittleren Misorientationswinkels zwischen den Nachbarkornern kann zur Entwicklung der Korngrenzgleitung und zu dynamischer Entfestigung fiihren.

The HTS (Helical Seam Two Step) manufacturing process is used for the production of HSAW-pipes for high pressure service since 1987. The production process is split into pipe forming combined with continuous tack welding followed by submerged arc welding. The process has been continuously developed further. Details of this modern technology are described and qualitative and economical advantages over the conventional process are explained. The benefits of the existing network with steel plant and hot rolling mill as well as the quality controls, process automatisation and new NDT-equipment used in the pipe production are highlighted. Statistics on mechanical, chemical and dimensional properties of HTS pipes are presented to furnish proof of the uniform and well balanced profile of this product. Results from hot and cold bending on site, field weldability trials and tests related to the safety of spiral pipes are discussed. Results and experiences with the application of three-layer coatings (especially HDPE-high density polyethylene) on spiral pipes are also presented. Actual examples for the use and application of spiral welded pipes in international gas and oil projects are presented and an outlook on future development trends is given.

The paper discusses the development and processing of hot rolled X80 coil material and its conversion into thick-walled helical seam welded pipes. Microstructure, texture and mechanical properties of strips and pipes produced are characterized and compared. High strength characteristics and good deformability as a result of the fine homogenous mainly bainitic microstructure have been determined. Stress strain characteristics and the response to cold deformation during pipe forming have been investigated. Correlations between strip and pipe properties are described and have been used as a data basis for FEM simulations of the pipe forming process. The real pipe behavior has been investigated by means of burst tests performed on 48″ and 42″ pipe sections with 18.9mm wall thickness. The results achieved have been compared with results for other pipe grades, dimensions and types of pipe. An outlook will be given on future material and process development steps and use of X80 HSAW-pipes produced.

One of the major priorities of the research and development department of the Salzgitter Group is placed on the product and process optimisation of both approved conventional steel grades for line pipe application and novel high strength alloying concepts. With respect to reduced wall-thicknesses and higher operating pressures for gas transportation pipelines, the requirements for hot wide strip material are steadily increasing. Material strength can be increased either by grain refinement of the ferritic-pearlitic phase in combination with precipitation hardening or by replacing the ferrite-pearlite by an intermediate microstructure, so-called acicular ferrite. A low carbon content supports the formation of the microstructure desired and results in an improvement of the carbon equivalent. This acicular ferrite does not only raise the material strength but also improve low temperature toughness, weldability and workability. New processing routes based on the demands of novel alloying concepts have been developed in order to achieve the intermediate microstructure. In the following paper, two different alloying and processing strategies and the resulting mechanical properties and microstructures are described and illustrated.