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
3.
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
5.
6.
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
10.
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
13.
14.
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
16.
17.
18.
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
21.
22.
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
