The application of non-quenched/tempered (NQ/T) steel

The application of non-quenched/tempered (NQ/T) steel in the high-grade hot-rolling seamless steel oil-well tubes is a new field. The tube manufacture process mainly consists of the following thermo-mechanical stages: piercing, tube-rolling, controlled cooling, reheating, stretch-reduction-diameter and cooling. However, no systematical research has been reported on the microstructure evolution and the strengthening/toughening of the high-grade hot-rolling seamless steel oil-well tubes produced by medium carbon microalloyed steel. In this dissertation, based on a real manufacture process of N80 grade hot-rolling NQ/T seamless steel oil-well tubes, the microstructure development, precipitation behavior and ferrite grain refinement in steel 33Mn2V during the above process were investigated. The precipitation and dissolution in steel 33Mn2V were studied through both the numerical calculation and the stress relaxation method. The precipitation-temperature-time curve was determined in austenite range of 600°C~950°C using the stress relaxation method on a Gleeble 2000 thermal/dynamic simulator. The curve is double C-shaped, with minimum incubation time of about 4.5 seconds at 850°C and about 3.8 seconds at 650°C, respectively. The obtained results provide important evidences for better understanding of the precipitation behavior in steel 33Mn2V at different stages during the real manufacture process of N80 grade hot-rolling NQ/T seamless steel oil-well tubes. Combined with various research methods, the microstructure evolution in steel 33Mn2V under different processing conditions was studied. The results have shown that the average austenite grain size in steel 33Mn2V would almost always continuously increase in the tube manufacture process till just before the stretch-reduction-diameter deformation. It is not consistent with the assumption that the austenite grains are gradually refined both by double dynamic recrystallization during the piercing, tube-rolling process, and the “in-line-normalization” produced when the tubes cooled down to Ar1 and reheating. The most effective microstructure refinement is produced during the austenite decomposition after stretch-reduction-diameter deformation. It has been found that the stop temperature (Ts) of controlled cooling following tube-rolling, the composition homogeneity of tube billet, and the cooling rate after stretch-reduction-diameter have strong influence on the final microstructure. Two values of Ts were used in our experiments: 850°C and 600°C. The industrial technical routines using Ts = 600°C, which is still above Ar3, would finally lead to microstructure with finer ferrite and pearlite, while the technical routines using Ts = 850°C were likely to result in microstructure consisting of ferrite, pearlite and bainite. The reason is that finer C-rich carbonitrides would form in the intermediate cooling process down to 600°C and in the reheating process, until the austenite decomposition starts in the final cooling process. The lowering of both carbon and alloying content in austenite and the uniformly distributed fine precipitation particles in austenite promote ferrite formation, especially inside austenite grains. This is quite different from that resulted from the austenite refinement principle. Based on the experimental results obtained from both the laboratory simulation and the industrial experiment, a microstructure refinement mechanism controlled by austenite decomposition characteristics resulted by vanadium precipitation during the processes after tube-rolling was firstly proposed and verified. The microstructure examination and mechanical tests of hot-rolled seamless tube samples obtained from industrial investigation were quantitatively analyzed. It is easily found that bainite (B) and B-like constituents in the microstructure are extremely harmful to the impact toughness of the steel, and that the microstructure bearing high volume fraction of ferrite (F) and bearing no B and B-like constituents almost always leads to high impact energy at 0°C. These results grain again support the microstructure refinement mechanism proposed in this work.

Keyword: hot-rolling seamless oil-well tubes; non-quenched/tempered (NQ/T); precipitation; microstructure evolution; microalloying