06-12-2012, 12:00 PM
Thermo-mechanical treatment of the C-Mn steel with Nb, Ti, V and B microadditions
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Introduction
The thermo-mechanical treatment, integrating the hot working
of steel with the direct quenching of the products at the end of
rolling temperature or after their preceding holding at this
temperature for a certain time, is an energy-saving steel-making
technology. In comparison with the classic heat treatment, boiling
down to quenching and tempering, the steel products made from
the toughening steels using the thermo-mechanical methods,
require tempering only [1-4].
Rolling with the controlled recrystallization features the typical
thermo-mechanical treatment of plates and long profile sections,
consisting in the multi-pass rolling of the charge within the correct
temperature range, whereas, in intervals between the consecutive
passes, total recrystallization of austenite should take place, and
after completing the rolling, before quenching, one should provide
conditions to forming the 50% fraction of the recrystallized
austenite.
Experimental procedure
The investigations were made using the imported heat
treatable steel, delivered as 40 mm thick plate. The steel
containing 0.17% C, 1.37% Mn, 0.26% Si, 0.012% P, 0.001% S,
0.24% Cr, 0.48% Mo, 0.05% Ni, 0.019% V, 0.004% Ti, 0.025%
Nb, 0.06% Al, 0.002% B, and 0.004% N, is destined for
manufacturing the weldable heavy plates with the yield point
exceeding 960 MPa and the fracture appearance transition
temperature T45J equal to –40°C.
Effect of deformation temperature was investigated within the
temperature range of 1100 to 900°C by twisting the specimens in
the SETERAM-7N torsional plastometer with the rate of 3 s-1. The
plastically deformed austenite recrystallization kinetics was also
determined in the same temperature range with = 0.2 at the deformation
rate 3 s-1, i.e., in conditions close to rolling of heavy plates.
Investigation results of the austenite strain strengthening
kinetics decay were employed for determining the total austenite
recrystallization time tR and the time t0.5 necessary for developing
the 50% fraction of the recrystallized phase ! as a function of
temperature. These investigations made it also possible to work
out conditions of the thermo-mechanical treatment – rolling with
the controlled recrystallization – of the flat bar with the 200 x 40
mm transverse section to the 15 mm thick plate in five passes
within the temperature range of 1000 to 900°C with decrements of
50°C. Pass reductions of 20% were applied in the first three
passes, and 15% in the last two ones.
Conclusion
The investigations carried out revealed that the process of
plastic deformation of steel in the temperature range from 1100
to 900 °C in conditions close to heavy plate rolling is controlled
by the dynamic recovery flow, where the strain hardening decay
of the austenite between the successive deformation cycles
occurs through static recovery and static recrystallization. The
nonuniform plastic metal flow is a reason for origination of the
deformation bands (shearing) in the plastically deformed
austenite, spreading in the direction of the load, disappearing
during the static recrystallization of the austenite.
The time of the total recrystallization tR of the plastically
deformed austenite of = 0.2 with a rate of 3 s-1 at the
temperature of 1100 °C is 50 sec, and increases to about 300 sec
at the temperature of 900 °C. However, time t0.5 - necessary to
develop 50% fraction of the recrystallized austenite after plastic
deformation of steel at the temperature of 1100 °C, determined
basing on the strain hardening decay kinetics - is 9 sec and
increases to 60 sec at the temperature of 900 °C.