Vanadium - Transforming Possibilities

Microstructure and Properties of Non-quenched/Tempered Seamless Tubes Made of Medium Carbon V-Microalloyed Steel

Abstract:

Hot rolling non-quenched and tempered (NQT) tube manufacturing process consists of a series of sub-processes such as billet-heating, piercing, tube-rolling, intermediate cooling, reheating, stretch-reduction-diameter, and final cooling. Both industrial experiments of production of seamless tubes using a medium carbon vanadium microalloyed steel and corresponding laboratory compression simulation with a Gleeble thermal/dynamic simulator were carried out. First, it is found that the most effective microstructure refinement did not occur in austenite state, but mainly in austenite decomposition following the stretch-reduction-diameter process, and the intragranular ferrite formation should be very useful for effective refinement of the whole microstructure. Second, more than half of precipitates in final tubes were formed in stretch-reduction-diameter and final cooling processes. Third, the results show that proper selection of intermediate cooling stop temperature T(stop) is of vital importance in order to achieve desired microstructure and optimized combination of strength and toughness, though no austenite ferrite transformation occur during the intermediate cooling and reheating. Compared with that using T(stop)=850C which may lead to harmful bainite and bainite-like constituents, the NQT tube manufacture process using T(stop)=600C results to finer microstructure and much larger amount of intra-granular ferrite, much larger fraction and finer particle size of precipitates in final tube products.

Keywords:

vanadium microalloyed steels; hot rolling: precipitation: intra-granular ferrite

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