Study of Microstructure Evolution in a Medium Carbon Microalloyed Steel Used for

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-linenormalization” produced when the tubes cooled down to Ar1 and reheating. The most effective
microstructure refinement is produced during the austenite decomposition after stretchreduction-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
- (3) -
= 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.