Dual Phase Steels
Microstructure
•
This class is
characterized by a tensile strength value of approximately 550 MPa (80 ksi) and
by a microstructure consisting of about 20% hard martensite particles dispersed
in a soft ductile ferrite matrix.
Properties
•
These steels
have a low yield strength and continuous yielding behavior; therefore they form
just like low-strength steel, but they can also provide high strength in the
finished component because of their rapid work-hardening rate.
•
Typical
as-shipped yield strength is 310 to 345 MPa (45 to 50 ksi) and tensile strength
550 MPa (80 ksi) .
•
A higher total
elongation than other HSLA steels of similar strength.
Production
•
Dual-phase
steels can be produced from low-carbon steels in three ways:
Ø Intercritical austenitization of carbon-manganese
steels followed by rapid cooling
Ø Hot rolling with ferrite formers such as silicon and
transformation-delaying elements such as chromium, manganese, and/or molybdenum
Ø Continuous annealing of cold-rolled carbon-manganese
steel followed by quenching and tempering
Composition
•
In general,
these steels have a carbon content of less than 0.1%, which ensures that they
can be spot welded.
•
Manganese in
amounts of 1 to 1.5% is added to ensure sufficient hardenability so that
martensite is formed upon rapid cooling.
•
Chromium and
molybdenum have also been added in amounts that are usually under 0.6%.
•
Silicon is added
to provide solid solution hardening.
•
Small amounts of
microalloying additions, such as vanadium, niobium, and titanium, may be added
to provide precipitation hardening and/or grain size control.
•
Nitrogen may be
added to intensify the precipitation-hardening effects of vanadium.
