High strength low alloy (HSLA) steels Introduction High

 

High strength low alloy (HSLA) steels

Introduction

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High strength low alloy (HSLA) steels are low
carbon steels that are designed for automotive, aeronautical, agricultural,
rail and domestics applications. The low carbon steel can be microalloyed with
chromium (Cr), Nickel (Ni), molybdenum (Mo), niobium (Nb), Titanium (Ti) and
vanadium (V) to enhance its mechanical properties. HSLA steels have excellent mechanical
properties such high strength, toughness, formability, weldability, fatigue,
and low impact temperature unlike other steels. V, Nb and Ti are effective in
refining grain sizes which improves strength while corrosion resistance is
improved by the addition of Ni and Cr. In addition to the composition, annealing
treatment, quenching time, tempering, normalizing, hot rolling, cold rolling
and surface treatment are also used for further improvement of the mechanical
and chemical properties of the steel. The resulting microstructure could consist
acicular-ferrite, pearlite –reduced, polygonal ferrite, granular bainite (Shao
et. al., 2012). The specific design and process selection discussed later will
illustrate how properties and microstructures are developed, the advantages and
the disadvantages.

 

Effects
of Microalloying Elements

 

            Precipitation
strengthening and grain refinement are the mechanisms by which HSLA steel are
strengthened with the addition of microalloying elements. A higher yield
strength is the result of an increase of finer grain sizes which inhibit
lattice movement. Likewise, the addition of other elements can create more
dislocation which makes it deformation difficult. The addition of V increases
the strength by forming compounds with elements in the steel. When V is
microalloyed with a low carbon steel, it has the tendency to be coupled with
carbonitrides (a carbon (C) and nitrogen (N) compound). The precipitation of
the V and carbonitiride compound (VCN) in ferrite improves the strength of HSLA
(Sta?ko et. al., 2006). Ollilainen et. al. (2003) confirmed presence of VCN
precipitates, pearlite, and proeutectoid ferrite in a V-HSLA using a
transmission electron microscopy (TEM). An energy dispersive spectroscopy (EDS)
revealed that VCN precipitates were mostly located between ferrite/ferrite or
ferrite/pearlite grain boundaries. A low C and high N concentration in HSLA-V
increased hardness more than the high C low N V-HSLA (Karmakar et. al., 2017).  VC precipitates from the excess C in a high C
low N V-HSLA decrease the strength because carbide particles are more brittle
than nitride. The hardness value for HSLA with V is 334HV, harder than the 38MnSi6
steel without V (Ollilainen
et.al., 2003). A study by Show et. al (2010) investigating the effects of
microalloying elements V and Ti in Nb-HSLA also found that fine VCN
precipitated in Nb-HSLA during cooling (Show et. al., 2010).

            Nb
is another microalloying element that also contributes to the reduction of
grain size and increase in the strength. Similar to V, Nb has a strong affinity
for C and N to form Nb-carbonitrides (NbCN) precipitates within grain boundaries.

Evidence of Nb-based precipitates were found in a Nb-steel after cool
deformation process (Fatehi et. al., 2010). There is a strong correlation
between the precipitation and shear stress according Ashby-Orowan’s stress
equation (Altuna et. al., 2012). Based on the equation, the reduction in the
precipitate size increases the shear stress. A high shear stress requires a
strong force and energy to cause any deformation. Nb-HSLA can be tensile
strength of 410-510 MPa (Ganta Design, 2017).  Altuna et. al. (2012) concluded that strength
is also dependent on the additional free Nb in steel solid solution before
phase transformation.

)              Ashby-Orowan’s stress
equation

= stress from precipitation

x= average diameter of
precipitates

fv=  volume fraction of the particles

Other microalloying elements such as chromium
(Cr), nickel (Ni), and molybdenum (Mo), titanium (Ti) have. mitigating
corrosion, formability and weldability. Ti alloys are desirable for their high
strength and high toughness but Ti micro addition too have deleterious effects
when combined with other element. Ti has a stronger affinity for N than V.

Characterisation of a Ti-Nb-V HSLA steel revealed large evidence of Ti-N compound
and absence of fine grains less than 100nm in comparison to HSLA alloy without
Ti (Show et. al., 2010). Without fine precipitate small enough to fit along the grain boundaries to cause pinning and
reduce grain growth, grains grow bigger resulting in strength reduction. Mo
steel has fine precipitates as small as several nanometers and is more
responsive to 20-30mins of aging time after cool deformation (Fatehi, 2010).

 

Microstructural Control

 

The crystallization
of Fe solid solution from high temperature to room temperature can produce a
variation of steel microstructures. The variation of the steel structure is the
result of the composition of C and microalloying elements. One known category
of HSLA steels composed of 0.022 wt% C- 0.76 wt% C microalloyed steel decomposes
from austenite into an acicular ferrite as it is cooled above the eutectoid
temperature. An acicular ferritic steel is a nucleated widmanstatten ferritic
steel (Ricks et. al., 1982). The transformation from the austenite to the
acicular ferrite follows the figure 1 which depicts an example of 0.022 wt% C-
0.76 wt% C hypoeutectoid phase transformation. 
The phase transformation requires low driving force so the
transformation begins at a low temperature to provide sufficient time for the
redistribution of atoms and nucleation. The grain boundaries are able to adjust
their position to achieve a metastable equilibrium. When the alloy is cooled
past the solidus line, small ferrite nucleus catalyzes at the grain boundaries.

The proeutectoid ferrite begins to thicken and elongate as the alloy is cooled
further past the solidus line to the eutectoid line. phase. Below the
hypoeutectoid line, the thick slabs of ferrite continue to grow rapidly along
the grain boundary around the austenitic grains called allotriomorphic ferrite.

             Figure 1. Phase transformation of
a hypoeutectoid steel (Cochrane, 2017)

 

Small particles or
inclusions in the new allotriomophic ferrite slabs, the region with the red
line indicator in figure 1, becomes a nucleation sites for acicular
ferrites.  Small particles can be
microalloying elements or inclusions. As the alloy continues to undercool, the
particles or inclusions develops into plates growing from the grain boundaries
into the parent austenite grains. Shao et. al. (2017) observed similar
plate-like matrix microstructure which can be seen in figure 2a from Nb-HSLA
steel transformed from 1050 °C. There was no evidence of the widmanstatten
ferrites in ferrite islands in figure 1b when the temperature was raised to
1200 °C because of Nb dissolution present in the alloy at higher temperature
(Shao et. al., 2017). The insoluble Nb was concluded to be the contributing
factor in the formation of acicular ferrite when it was present in figure 2a. There
would not be a nucleation of nuclei for acicular ferrite without the the
presence of soluble particles or inclusions below the proeutectoid temperature.

Ricks et. al. (1982), also discovered the inclusions in weld metal had a strong
impact the nucleation of ferrite by playing the role of an inert particle and
decreasing the energy barrier needed to nucleate. Acicular ferrites and their
properties can be controlled and enhanced by multiple process treatment. Some
processes by which the acicular ferrites were obtained include intercritical
heat treatment, step quenching/normalizing and thermo-mechanical controlled
processing (TMCP). TMCP increase the yield strength, toughness and fraction of
acicular ferrite in steels (Shao et. al., 2017).  The yield strength, and toughness is what
makes HSLA steels highly sought in the fabrication of pipelines.

 

Figure 2. Microstructure of Nb-HSLA Steel quenched from (a) 1050 °C,
(b) 1200 °C (Shao et. al., 2017)

 

Contrast to the
acicular ferrite, polygonal ferrite, pearlite and bainite form when an alloy with
0.08 wt % C cooled below the eutectoid temperature. At high supercooling, nucleus
along the grain boundary in the austenite begins to nucleate rapidly into
nodules. The nodules grow together growing into pearlite lamella after the
allow is further cooled past the eutectoid temperature as seen in figure
3.  Pearlite steel has alternating
lamellar spacing of ferrite and cementite. 
A slower cooling rate to 600 °C coiling temperature yields a
ferrite-pearlite microstructure. A faster cooling rate of 15K/s to about 300 °C
to 400 °C coiling temperature forms a baintic microstructure. But HSLA with
ferrite-pearlite has higher strength than bainite (Altuna et. al., 2012). A
higher undercooling than used in pearlite formation results in bainite
microstructure between 350-550°C. Unlike pearlite, bainite compromises of needles
or plates of ferrite. The resultant bainite microstructure is influenced by
amount of undercooling. Faster cooling rate increases the Gibbs free energy needed
the transformation reaction. An example of ferrite-bainite steels is typically
used for car-body parts, wheels, gear boxes, cross beams and longitudinal beams
because it has excellent formability and high tensile strength (Ganta Design,
2017).

Figure 3.

Phase transformation of a eutectoid steel (Cochrane, 2017)

 

 

Effects of Process Treatment

 

            Process treatment is a
method of microstructural control to improve material properties. Process like
cold rolling, hot rolling, annealing etc. can make grain sizes finer for a
stronger material or coarser grains resulting in a brittle material. Different processes
can have varying mechanical effects on the HSLA steel composition, rate of
treatment and temperature. To list a few, cold rolling, hot rolling, annealing,
direct quenching and step quenching are examples of processes studied and can influence
the properties of HSLA.

            A metal that is cold
rolled experiences strain as it compressed. 
Mechanical and microstructural changes are introduced into material when
it is cold worked. Elongated grains are often characteristic of cold rolling. The
process increases the amount of dislocation in the material which directly
influences the mechanical properties. Annealing relieves the strain from the
cold working. The newly strain relieved sites begin to nucleate, recrystallize
and grow new finer grains. For example, strain was reduced by 86.8% in V-HSLA
after annealing for 12hours. A cold rolled V- HSLA steels was confirmed to have
elongated grains along the direction of the roller and the grains transformed in
equiaxed grains after annealing (Liu et. al., 2016c). The longer annealing time
of 720 minutes resulted a more uniform equiaxed and less traces of deformed
grains than when annealed for 60minutes (Liu et. al., 2016c). A cold rolled
Nb-Ti HSLA has a tensile strength of 470-590MPa and a hardness value of 151-183
HV( Ganta Design, 2017).

            Hot rolling, on the other
hand, has deleterious effects on V-HSLA. Data collected by Liu et. al. (2016a) revealed
that hot rolling impacts the transformation of HSLA as seen in figure 4. The
degree of strain placed on the steel increased the start and final
transformation temperatures during cooling (Liu et. al., 2016a). It allows the
transformation process to occur at higher temperature and quickly.  They also observed that the amount of
polygonal ferrite increased and acicular ferrite and bainite decreased with
increasing strain. At the same time, vicker hardness values decreased with
increasing strain (Liu et. al., 2016a). The resulting microstructure and
hardness values for the steel as the strain increase are evidence that acicular
ferrite and bainite improves strength. However, the high strain rates limit the
dislocation recovery time, resulting in increased nucleation sites and finer
grains (Liu et. al., 2016a). Hot rolled Nb-Ti HSLA steel has tensile strength
between 410-510MPa and a hardness value of 134-162 HV. Contrast to the cold
rolled HSLA previously mentioned, the hot rolled steel is lower in both tensile
strength and hardness.

 

Figure 4. The effect of increasing strain on the start and final
transformation temperature (Liu et. al., 2016a)

            Quenching has also been
documented to have an impact of the phase transformation of HSLA steels. Traditionally,
HSLA steels are heat treated using the reheat quench and tempering (RQT) process to strengthen and toughen the steels (Dhua and Sen, 2011; Xiao et. al.,
2010). The steel is
air cooled after rolling, reheated, quenched then tempered in RQT
process. Direct quench and
tempering (DQT) process, step quenching, tempering (SQT) and intercritical quench and temper
(IQT) can improve the steel’s mechanical properties more than the process of RQT. Unlike RQT, DQT is
involves rapid cooling without reheating. With DQT, a Nb-Cu HSLA steel
increased yield strength to 902 MPa because of the increase of densely
dislocated fine grains (Dhua and Sen, 2011).  Both studies by Dhua and Sen (2011) and Xiao et. al.

(2010) documented martensitic lath
microstructure in Nb-HSLA and Nb-Ti-V HSLA respectively. SQT is a process that involves cooling to an intercritical temperature, the rapid cooling and
tempering (Liu et. al., 2016b). SQT can produced a mixture of polygonal ferrite, pearlite and martensitic lath microstructure and significantly increased the formability of
steel while steels treated using the IQT produces martensitic lath and “stripe-like” ferrite for a tougher
steel (Liu et. al., 2016b).

            Some of the heat treatment
process are only advantageous if the are treated at the proper temperatures.

The improvement of the properties can easily be reversed at the tempering step
in the DQT process because of the temperature. Xiao et al. (2010) observed a
decrease in strength due to the NbC precipitate coarsening as the tempering
temperatures increase. Likewise, increases austenization temperature negatively impacts toughness
((Liu et. al., 2016b).

 

Conclusion
 

The
design of HSLA steel with desired properties are strongly based on the
selection of particular process and temperature. Mechanical properties such high strength, toughness, formability, and weldability
which defines HSLA steels are design with the combination of microaddition impurity
element and heat process treatments. The alloying elements from Mn, Nb, Ti, V, to
Mo combine to create steels with varying degree of each property. The micro
element addition enhances the mechanical properties by precipitation hardening.

Nb, Ti and V are coupled with carbonitrides combine along the grain boundaries to cause pinning and inhibit grain growth
resulting in strength increase. The addition of Mo also promotes the growth of fine
precipitates several nanometers. However, the presence and amount of Ti, C, or
N in a steel with Nb and V can reduce formation of fine precipitates and impact
strength and toughness.

The steels composed
of 0.022 wt% C- 0.76 wt% C microalloyed steel decomposes from austenite into pearlite
and slabs of proeutectoid ferrites it is cooled above the eutectoid
temperature. Small particles or inclusions in the new proeutectoid ferrite
slabs becomes a nucleation sites for acicular ferrites. However above 1200 °C, there
was no evidence of the widmanstatten ferrites in ferrite slabs. The formation
of pearlitic, bainitic or martensitic microstructure from an alloy with 0.08 wt
% C is dependent on the magnitude and rate of undercooling to the cooling. The
final method often completing the method of microstructural control to improve material properties is the process
treatment. cold rolling, hot rolling, annealing, direct quenching and step
quenching are a few examples of processes used to manipulate the properties of
HSLA. Cold rolled HSLA steels are have stronger and harder than hot rolled HSLA
steels. SQT, and DQT are better than the historical RQT process because they
can increase steel formability and toughness. Precipitate coarsening can be
prevent using SQT and DQT by maintaining low tempering temperatures.