HSLA Steels – From “Bumper Component Welding State-of-the-Art Survey”
by D. W. Dickinosn
AISI Bumper Project Group,Dec31, 2000
The mechanism of grain boundary
pinning and precipitation
strengthening is illustrated1 for a
niobium containing microalloyed
low alloy steel. When the
temperature of the material is less
than the dissolution temperature of
the precipitates that are present in
the sheet this mechanism will apply.
The dissolution temperature for
various precipitates and
combinations of precipitates will be
considered later as these may have
some effect on grain growth,
particularly in welded situations,
but for this example we consider niobium carbides and carbonitrides, a relatively stable
precipitate.
At the initial time, fine precipitates are relatively uniformly distributed throughout the
matrix. As a newly
created sweeping grain
boundary moves as grain
growth starts, the
precipitates are segregated
to the boundary at time t1.
These precipitates
combine to form
precipitate sufficient in
size to pin the boundary
and prevent further grain
coarsening at time t2.
To produce this effect,
however, control must be
exercised over the hot rolling mill processing. The measure and type of this control is
illustrated2 in this figure. In conventional hot rolling, the strip is heated into the austenite
1
Palmiere, E., “Precipitation Phenomena in Microalloyed Steels”, Microalloying ’95 Conf. Proc.
1995
2
Muschenborn et al “Recent Developments in Physical Metallurgy and processing Technology of
Microalloyed Flat Rolled Steels”, Microalloying ’95 Conf Proc, 1995
region as illustrated in the diagram to the far left. Rolling deformation takes place fully
while the material is in the austenite phase and conventional dynamic recrystalization
occurs. When micoalloying elements are added to the strip, the temperature for nonrecryataliztion and grain growth raises, and the two phase temperature range over which
ferrite and austenite are stable is lowered. This opens a processing window in which
controlled rolling practice can occur as illustrated in the three rolling temperature curves
on the right. In normalized rolling, all
deformation occurs at relatively high
temperatures, but for thermomechanical
controlled rolling the deformation takes place
within this temperature-processing window.
This results in the grain strengthening
described above.
As previously mentioned, the stability of the
precipitate has an effect on the properties
obtain and on the welding response of that
material. The solubility product of several
precipitates as a function of temperature is
presented in this figure. From these curves we
can see that the most stable precipitate is TiN
and it remains stable to very high
temperatures. On the other hand, the least
stable is VC. Thus the rolling window mentioned above will be over a larger temperature
range for the TiN micoralloyed product. Other factors such as total strength and
weldability also needs to be considered, however, when selecting microalloy addition
routes.
In addition to the precipitate type, the strain during rolling and the temperature at which
that strain occurs must also be considered. The compound combination3 of these factors
is illustrated in the diagram presented below.
3
DeArdo, “Modern Thermomechanical Processing of Microalloyed Steel” Microalloying ‘95 Conf
Proc. 1995
Strain and Ppt’s
Control
Recrystallization
& Grain Size
Grain Coarsening
Temperature
• Increased Hot Deformation
Lowers RXN Temp
• More Stable Ppt inhibit
grain coarsening at higher
temps
The graph in the upper left hand section of this diagram relates the type of grain
deformation that is expected as a function of the deformation strain and the temperature
at which this strain occurs. For high deformation temperatures, the resulting grain
structure is recrystallized austenite. If an increased amount of deformation occurs, the
energy for recrystalization increases and a lower critical temperature will still result in
equiaxed-recrystallized grains. In the region of slightly lower deformation temperatures,
only partial recrystalization occurs and the resulting structure is partially recrystalized
fine grains mixed with elongated somewhat larger worked grains. At low temperatures of
deformation, no recrystallization occurs and the grains will be elongated worked grains.
For those materials in the intermediate range, the presence of grain boundary pinning
precipitates will likewise retard the recrystallization and grain growth as observed in the
diagram in the upper right hand side of this figure. The result of these factors is
illustrated in the 3 dimensional plot for a carbon, manganese, niobium low alloy steel
(similar plots are available for other steel compositions). The results considering all steel
combinations show that increasing the hot deformation lowers the recrystallization
temperature and that the more stable precipitates inhibit recrystallization and grain
coarsening at higher temperatures. Thus the rolling deformation and temperature
selection must be made for each precipitate forming alloy composition, thus the term
“controlled rolling” unique for each alloy.
In summary, the structure present in the controlled rolling practice4 are illustrated in the
next figure. With conventional air-cooling after rolling, the resulting structures are
represented by the top 3 cooling curves. In normalizing rolling, the deformation takes
place in region I (the recrystallization region) and the austenite grains recrystallize with
grain size somewhat dependent on precipitate pinning action if stable pinning precipitates
are present. With controlled rolling where deformation takes place in region II (the nonrecrystallization region), deformed austenite grains are produced. With controlled rolling
where deformation takes place in region III (the austenite plus ferrite region), both
deformed austenite and deformed ferrite grains occur. On finish of cooling to room
temperatures the final structures are represented by the sketches b’, c’, and d’. The
material deformed in the recrystallized region has fairly large ferrite grain size (b’); The
material deformed in the non-recrystallized region have finer grain ferrite (c’); and the
material deformed in the dual phase region has the retained deformed ferrite with new
fine grains of ferrite forming from the deformed austenite. In general, the lower the final
deformation temperature, the finer the resulting ferrite grains and the higher the strength
of the resulting product.
This diagram also represents structures obtained by accelerated cooling from the hot
rolling by run-off table accelerated cooling devices. These structures represented by (c’
AC) and (d’AC) show a finer grain structure yet.
Tanaka T., “Science and Technology of Hot Rolling Process of Steel”, Microallying ’95 Conf. Proc.,
1995
4