Dual Phase Steels

Lecture 17

Dual Phase Steels
1. Introduction
- Dual phase steels were developed by Hayami et al.
- Dual steel structures consist of islands of martensite or
lower bainite in ferrite (seeFig.1)

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Fig.1 Islands of martensite

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Dual phase steel

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Stress – strain behavior

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Strength vs martensite volume

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Elongation vs tensile strength

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2.Manufacturing of Dual Phase Steels

 Hot rolled → cold rolled → continuously annealed at
temperature in (α+γ) region (around 760 ℃) → cooled so
that islands of γ transform to martensite or lower bainite
Types of dual phase steels
Type

Annealing

Cooling

Alloy System

Low yield ratio
Steels

Batch

Furnace

High Mn

Continuous

Gas jet

Low C-High Mn

Gas jet+waterquench+tempering

Si-Mn
Mn-Cr

W.Q+tempering

Low C
Si-P

High bake
hardening steels

Continuous

- Low yield ratio steels need slow cooling, thus needs high alloying
elements enhancinghardenability of austenite
- High bake hardenable steels need rapid cooling, thus low alloy is
enough
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 Cooling rate (R) is closely related with alloying elements
log R ≥ 5.36 – 2.36 Mn equivalentMn equivalent = Mn + 0.45 Si + 1.15 Cr + 2 P
3. Mechanical properties
- high tensile strength (665 MPa), low yield strength (370
MPa), high elongation (22 %)
- strain hardening, bake hardening,and aging resistance are good.
- low r value → deep drawability is bad
☞ D.P. steels are applied for the automotive parts
requiring high stretchability

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4.
-

Transformation
annealed attemperature in (α+γ) region
production of martensite particles in matrix ferrite
creation of residual stress in ferrite surrounding each
particle of martensite
- volume fraction of martensiteincreases strength

α ’ volume has linear relationship
with tensile strength, but this
relationship may not always be
valid because tensile strength is
depends on C content of α ’

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γ to...