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The main objective of the work was the assessment of the feasibility of the industrial production of hot strips for ERW pipes for 'sour service', by the thin slab technology. To this aim, hot strips made of a reference C-Mn steel and two Nb microalloyed steels were manufactured using the thin slab technology. A complete quality assessment in terms of non-metallic inclusions, microstructures, mechanical properties, segregation levels, HIC and SSC resistance was carried out and results were compared with those from conventionally processed coils. Some thermal cycles were applied to reproduce the coarse austenite structure formed in the HAl of ERW pipes and to evaluate the effect of in-Iine heat treatments on microstructure refinement in order to retain toughness.

As a general conclusion, it can be stated that API X52-X60 grade ERW strips with thickness up to 10 mm, exhibiting excellent HIC and SSC resistance, can be produced by the thin slab and direct rolling route. A careful metallurgical design is, however, needed, as well as a suitable thermomechanical rolling schedule in order to achieve good toughness levels and to prevent the formation of highly segregated bands (Mn and P are the key elements), leading to poor HIC performance. Although tensile properties seem rather insensitive to the strain, a total reduction greater than 60 % is required during finishing to guarantee an adequate toughness after direct rolling of thin slabs. In this respect, mathematical models which predict the austenite evolution during thin slab rolling proved to be a very suitable tool for defining the optimised hot rolling schedules and in understanding the re-crystallisation mechanisms which operate.

Based on the results of laboratory simulations, low HIC and SSC susceptibility can also be expected in the weld joints of the ERW pipes subjected to a local heat treatment.

Additional information

Authors: ANELI ET AL, CSM, Via di Castel Romano, Rome (IT)
Bibliographic Reference: EUR 19383 EN (2000) 113pp, EUR: 17.50
Availability: Available from OOPEC L-2985 Luxembourg
ISBN: ISBN: 92-828-9526-2
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