1. Dual Phase Steels
Producing a new high strength steels without reducing the formability or increasing costs

2. Iran University of Science and Technology (found in 1929)
Iran Ministry of Industries & Business
National Automotive Manufacture (found in 1962)

3. General Characteristics
The idea of producing dual phase steel grades originated in 1970s.
Efforts have been made to produce lightweight vehicles without losing structural integrity and crash resistance.
The reduction of weight in order to limit fuel consumption and gas emissions
Dual-phase (DP) steels are low-carbon low-alloy materials with 20–30 vol.% hard phase (matrensite or bainite) in a ductile ferrite matrix
Soft ferritic network provides good ductility; while, hard particles play the load bearing role
This microstructure, in fact, shows some kind of metallic composite

4. Heating to intercritical temperature Quenching to water or salt bath
Production Methods
Intercritical annealing is the only way to produce dual phase steel.
α+ϒ A3 A1
Heating to intercritical temperature
Quenching to water or salt bath
Intercritical area
Fe-C phase diagram

5. Martensite Martensite Ferrite Ferrite
SEM (left) and Optical (right) microstructure image of a martensitic dual phase steel

6. Bainite Bainite Ferrite Ferrite
5 μm
SEM (left) and Optical (right) microstructure image of a bainiticdual phase steel

7. Stress-Strain curve of a dual phase and a HSLA steel
Mechanical Properties
Continuous yielding behavior and high elongation (good formability) are important features of dual phase steels
They combine high strength and good formability at low production costs
Stress-Strain curve of a dual phase and a HSLA steel

8. σ = kεn (Hollomon's equation)
Mechanical Properties
Two stage of work (strain) hardening (unique properties of dual phase steels)
High work (strain) hardening exponent at the beginning of plastic deformation
σ = kεn (Hollomon's equation)
Work hardening exponent (higher n means easier plastic deformation)
ln σ = n ln ε + ln k (Hollomon's equation in logaritmic form)
Can be determined by plotting ln σ Vs ln ε
n is an indicator of formability of a metal. A metal with high value of n is preferred for processes which involve plastic deformation

9. ln σ (MPa)
ln ε
ln σ (MPa)
ln ε
ln–ln plot of stress vs. strain for dual phase steel. Martensitic (left) and bainitic (right) at different volue fraction of hard phase

10. Mechanical Properties
Crash energy absorption
Capacity and fatigue strength
Low UTS/YS ratio (around 0.5)
Good uniform elongation
Spring back and bendability
DP steels do not age at room temperature

11. Application Widely used for automotive applications
These steels are used in such applications as body panels, wheels, rocker reinforcements, bumpers, fasteners, shock towers and door intrusion beams.
In particular the automotive industry has demanded steel grades:
With high tensile elongation to ensure formability,
With high tensile strength to establish fatigue and crash resistance,
Low alloy content to ensure weldability without influencing production cost

12. Automotive industry applications of the DP steels

13. σDP = VF σF + VHP σHP (general law of composite)
Effects of production parameters
σDP = VF σF + VHP σHP (general law of composite)
σDP Special parameter of dual phase steel
σF Special parameter of ferrite
VF volume fraction of ferrite
σHP Special parameter of dual phase steel hard phase (martensite or baonite)
VHP volume fraction of hard phase (martensite or baonite)
With any change in right side parameters, the properties of dual phase steel will change

14. UTS (left) and total elongation (right) as a function of hard phase volume fraction in DP steels

15. Work hardening exponent as a function of volume fraction of bainite (left) and martensite (right)

16. Effect of intercritical annealing temperature and time
Increasing annealing temperature will increase the volume fraction of hard phase
Increasing soaking time also results in increasing volume fraction of hard phase
On the other hand increasing annealing temperature can decrease the hardenability of austenite that results in softer martensite or bainite
Chemical composition, cooling rate, cooling media, ferrite grain and primary microstructure can also have effect on final properties of dual phase steels

17. Conclusion
Dual phase steel was designed in order to reduce the weight of used steel without loosing its benefits
Ferritic and bainitic are two kind of dual phase steels.
Continues yielding and having two different stage of work hardening are unique mechanical properties of dual phase steels.
These linds of steels mostly used in automotive industries
Bainitc dual phase steel has lower strength but has better formability and weldability
Overall properties of these steel is related to volume fraction of and characteristics of hard phase according to general law of composite

18. Go as far as you can see, and when you get there you'll see further (Persian proverb)
Thanks !
Vai tão longe que você pode ver, e quando você chegar lá, você vai ver ainda mais (provérbio persa)
Obrigado !