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DP steels

DP steels: Dual-phase steel
Definition:Multiphase steel grade consisting of a soft ferritic matrix with small hard martensitic islands of the volume fraction equal to 10-30 %.
Explanation:DP steels show a combination of the low yield strength due to the soft ferrite phase with the high tensile strength resulting from the hard martensite. Such property provides a compromise between strength and formability. Main alloying elements are manganese and silicon, amd sometimes, aluminum or chromium are also used. The alloying elements reduce the critical cooling rate for the pearlite and bainite formation, and foster the martensite formation after cooling. Owing to their high tensile strength, DP steels have a low ratio of yield stress to tensile strength and a high strain hardening. They show continuous yielding even in the non-skin passed state, which can be explained by the free dislocations at the ferrite/martensite phase boundary. These dislocations are called geometrically necessary dislocations, which compensate the volume change from the austenite to martensite transformation.

The hard martensite particles may cause a deterioration of material behavior during fracture formation in hole expansion tests or at bending close radii.
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Diagram:

Typical dualphase microstructure; ferrite is bright, martensite islands appear dark.

Typical chemical composition in wt% (all are maximum values unless mentioned).

Typical dualphase microstructure; ferrite appears in a bright contrast, whereas martensite islands show a dark contrast.
SFB-Link:DP steels belong to the early developed multiphase steels (advanced high strength steels (→AHSS) 1. Generation) having special composite-type strain hardening characteristics which can serve as a reference steel within the SFB.