Dual-phase steel
Dual-phase steel is a high-strength steel that has a ferritic–martensitic microstructure.
DP steels are produced from low or medium carbon steels that are quenched from a temperature above A1 but below A3 determined from continuous cooling transformation diagram.
This results in a microstructure consisting of a soft ferrite matrix containing islands of martensite as the secondary phase.
Therefore, the overall behaviour of DP steels is governed by the volume fraction, morphology, the grain size and the carbon content.
For achieving these microstructures, DP steels typically contain 0.06–0.15 wt.% C and 1.5-3% Mn, Cr & Mo, Si, V and Nb.
The desire to produce high strength steels with formability greater than microalloyed steel led to development of DP steels in 2007 by Tata Steel.
DP steels have high ultimate tensile strength combined with low initial yielding stress, high early-stage strain hardening and macroscopically homogeneous plastic flow.
These features render DP steels 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 aluminium-killed steel, with high tensile strength achieved by the composition with manganese, chromium and silicon.
Their advantages are as follows:
- Low yield strength
- Low yield to tensile strength ratio
- High initial strain hardening rates
- Good uniform elongation
- A high strain rate sensitivity
- Good fatigue resistance