Dual-phase steel (DPS) is a high-strength steel that has a ferrite and martensitic microstructure. DPS starts as a low or medium carbon steel and is quenched from a temperature above A1 but below A3 on a continuous cooling transformation diagram. This results in a microstructure consisting of a soft ferrite matrix containing islands of martensite as the secondary phase (martensite increases the tensile strength). Therefore, the overall behavior of DPS is governed by the volume fraction, morpology (size, aspect ratio, interconnectivity, etc.), the grain size and the carbon content. For achieving these microstructures, DPS typically contain 0.06–0.15 wt.% C and 1.5-3% Mn (the former strengthens the martensite, and the latter causes solid solution strengthening in ferrite, while both stabilize the austenite), Cr & Mo (to retard pearlite or bainite formation), Si (to promote ferrite transformation), V and Nb (for precipitation strengthening and microstructure refinement). The desire to produce high strength steels with formability greater than microalloyed steel led the development of DPS in the 1970s.
DPS have high ultimate tensile strength (UTS, enabled by the martensite) combined with low initial yielding stress (provided by the ferrite phase), high early-stage strain hardening and macroscopically homogeneous plastic flow (enabled through the absence of Lüders effects). These features render DPS ideal materials for automotive-related sheet forming operations.
The steel melt is produced in an oxygen top blowing process in the converter, and undergoes an alloy treatment in the secondary metallurgy phase. The product is aluminum-killed steel, with high tensile strength achieved by the composition with manganese, chromium and silicon.
- Low yield strength
- Low yield to tensile strength ratio (yield strength / tensile strength = 0.5)
- High initial strain hardening rates
- Good uniform elongation
- A high strain rate sensitivity (the faster it is crushed the more energy it absorbs)
- Good fatigue resistance
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