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

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

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

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

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

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

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

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

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

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

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

Automotive industry applications of the DP steels

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

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

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

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

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

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 !