Dual phase steels are microstructurally composed of 75-85% ferrite with the remainder being martensite, bainite, and retained austenite. They are processed through thermomechanical treatments to achieve better formability than ferrite-pearlite steels of similar strength. Dual phase steels work harden rapidly at low strains, have low yield strength but high ultimate tensile strength. They were initially developed in the 1960s but further improved in the 1970s for automotive applications requiring increased strength and fuel efficiency. Processing methods like continuous annealing, batch annealing, and as-rolled techniques are used to control the microstructure and resulting mechanical properties.

1. Dual Phase Steels
Overview
Evan Sanders

2. Dual Phase Steels
● Microstructure
○ 75-85 vol% ferrite
○ Remainder mixture of martensite, lower bainite,
retained austenite
○ Usually consists of more than 2 phases
● Essentially just a low carbon steel
thermomechanically processed for better
formability than ferrite-pearlite steels of
similar tensile strength

4. Stress-Strain Behavior
● Characteristically different from HSLA (High
Strength Low Alloy) or plain carbon steels
○ Continuous Stress-Strain curve with no yield point
elongation
○ Work harden rapidly at low strains
○ Low yield strength
○ High UTS
○ Strength-Ductility data falls on separate curve

7. Development
● Ferrite-Martensite steels developed by
British Iron and Steel Research Association
(BISRA, UK) and Inland Steel Corporation
(ISC, US) in mid 1960s
○ Focus was for producing steels with tinplate
○ Neither group focused on improved formability
● Development for formability triggered in
1970s by conflicting demands in automotive
industry for decreased weight for fuel
economy and increased weight to meet
safety standards
○ Matsuoka & Yammamori, and Hayami and

8. Processing Methods
● Before processing, starting steel consists of
a ferrite matrix with grain boundary iron
carbides and small islands of pearlite
● 3 types of processing methods to produce
dual phase steel
○ Continuous annealed
○ Batch annealed
○ As-rolled

9. Continuous annealed method
● Rapid heating above the critical temperature
● Short time holding at that temperature
● Cooling below the martensitic start
temperature
● Some processes also include a short time
tempering above 500 degrees Celsius
● Rate of heating is far less critical than the
heating temperature
● Faster cooling required for steels with lower
hardenability

11. Batch annealed
● Used with high alloy content and high
hardenability
● Very slow cooling (days)

12. As-Rolled
● Steel composition chosen such that 80-90%
of the steel is transformed to ferrite after the
final roll pass in normal conventional hot
rolling and before entering the coiler
● Remaining 10-20% does not transform until
slow cooling in the coiler
● This method possible with steels that
express certain characteristics in their
continuous cooling transformation diagrams

15. Deformation behavior
● Typically stress strain behavior is not
satisfied for dual phase steels
● 2 proposed methods for changes in
deformation behavior
○ n i(j)=[log(σj)-log(σj-1)]/ [log(εj)-log(εj-1)]
○ σ=σo+Bε^m
○ Where σ is the true stress, σo us the true yield
stress, B and m are constants, and j=1 to L, where L
is the number of segments in the curve

19. Deformation behavior (cont)
● The shear and volume change
accompanying the austenite to martensite
transformation upon cooling from above the
critical temperature produce numerous free
mobile dislocations in the surrounding ferrite
matrix
○ Upon application of the load, free dislocations move
with stresses much less than that required to move
restrained dislocations as commonly found in ferrite-
pearlite steels, so dual phase steels yield plastic flow
at lower stresses of equivalent tensile strength
○ Magnitude of work hardening in dual phase steels at
low strains too large to be explained by dislocation

20. Deformation behavior (cont)
● Martensite is the principal load bearing
constituent
○ Volume percent of martensite and steel strength are
linearly related
○ Carbon content is also important though, and
separate linear relationships exist
○ Martensite strength can be increased by decreasing
its particle size

23. Transformation Mechanisms
● Continuous annealed
○ Upon heating the steel above the critical
temperature,islands of carbon-rich, nonequilibrium
austenite form at the carbide locations.
■ Heating temp determines volume fraction of
austenite and carbon content that can exist
○ Carbon migration

24. Transformation Mechanisms (cont)
● Batch annealed
○ Similar to those observed during continuous
annealing
■ However, grain size and substructure are
characteristic of slower cooling rates