Dual Phase Steels– From “Bumper Component Welding State-of-the-Art Survey”
by D.W. Dickinson
AISI Bumper Project Group,Dec31, 2000
Dual Phase steels1 are a group of steels, often based on the low alloy steel composition
Two phase region of the iron-iron
carbide diagram
Typical Ferrite Martensite
Microstructure in Dual Phase
Steels
and rolling practices, which have been given a specific heat treatment which takes the
material back into the dual phase (austenite plus ferrite) phase region (see figure) for a
time sufficient to establish the two phases in equilibrium. On cooling, the austenite
transforms to martensite islands in a matrix of low carbon ferrite. A typical
microstructure is presented in figure. The ferrite promotes ductility while the martensite
promotes strength.
There are a number of processing routes by which this dual phase structure can be
attained. Some variation in the chemistry is required for these various routes as
illustrated in this table. The techniques include box annealing, continuous annealing and
strip produced directly from as rolled cooling.
Chemistry of Dual Phase Steels produced by various Processing Techniques.
1
ASM Handbook, Vol 1, 1990
A3
Temperature
In the box annealing processing
technique, coils of strip are loaded
into an annealing furnace and
subjected to a thermal cycle of
heating into the two phase region for
a fairly long period of time as
illustrated in this figure, after which,
the coils are cooled to room
temperature and shipped as dual
phase product.
A1
Box
Continuous
Time
In the continuous annealed product,
Schematic Illustration of Box Annealed
the coil is unwound and traversed
and Continuous Annealed Dual Phase
through a continuous annealing
Steel.
furnace line, recoiled and cooled to
room temperature. The temperature, line speed, run off table configuration and coiling
conditions control the time temperature cycle, but it is a shorter time period than that for
box annealing.
TEMPERATURE
The as rolled dual phase materials2 attain their properties directly from the hot rolling
procedure by control of chemistry and rolling; and post-roll cooling and coiling
techniques to attain the dual phase properties. As seen in this figure, lowering the coiling
Ferrite - Pearlite
650 C
450 C
250 C
Coil Temp
Ferrite –Bainite –
Martensite
Ferrite - Martensite
TIME
Effect of Coiling Temperature to
Produce as Rolled Dual Phase Steel
Ductility (Hole Expansion) vs.
Tensile strength of As Rolled Dual
Phase Steels
temperature sequentially produces a microstructure change ranging from ferrite-pearlite
(P), to farrite-bainite-martensite (B), to ferrite martensite (M) as the coiling temperature
is reduced. The tensile and ductility variations of this effect are seen in figure for a series
of four as rolled dual phase alloys. Note that as the alloy content increases (moving from
2
Hulka, K “Relationships between heat treatment conditions, microstructure and properties of
Niobium microalloyed TRIP steel”, 41st MWSP Conf Proc. 1999
alloy 1 to 4 on this figure), the
tensile strength increases and
the nominal ductility decreases.
The properties of the dual phase
steels in comparison to other
steels is illustrated in figure.
Note that they have higher
tensile strengths than that of the
structural quality solid solution
hardened steels, and better
ductility than that of the low
alloy precipitation hardened
(HSLA) type steels. Thus they
find applications where both
moderately high strength and
good forming applications are
required.
Comparison of Dual Phase and other Steels.