DP steels consist of a ferritic matrix containing a hard martensitic second phase in the form of islands. Increasing the volume fraction of hard second phases generally increases the strength. DP (ferrite plus martensite) steels are produced by controlled cooling from the austenite phase (in hot-rolled products) or from the two-phase ferrite plus austenite phase (for continuously annealed cold-rolled and hot-dip coated products) to transform some austenite to ferrite before a rapid cooling transforms the remaining austenite to martensite.
Depending on the composition and process route, hot-rolled steels requiring enhanced capability to resist stretching on a blanked edge (as typically measured by hole expansion capacity) can have a microstructure containing significant quantities of bainite.
The following figure shows a schematic microstructure of DP steel, which contains ferrite plus islands of martensite. The soft ferrite phase is generally continuous, giving these steels excellent ductility. When these steels deform, strain is concentrated in the lower-strength ferrite phase surrounding the islands of martensite, creating the unique high work-hardening rate exhibited by these steels.
The work hardening rate plus excellent elongation creates DP steels with much higher ultimate tensile strengths than conventional steels of similar yield strength. The next figure compares the engineering stress-strain curve for HSLA steel to a DP steel curve of similar yield strength. The DP steel exhibits higher initial work hardening rate, higher ultimate tensile strength, and lower YS/TS ratio than the similar yield strength HSLA. Additional engineering and true stress-strain curves for DP steel grades are located in the AHSS Application Guidelines Image Gallery, Figure 2-9B.
DP and other AHSS also have a bake hardening effect that is an important benefit compared to conventional higher strength steels. The bake hardening effect is the increase in yield strength resulting from elevated temperature aging (created by the curing temperature of paint bake ovens) after prestraining (generated by the work hardening due to deformation during stamping or other manufacturing process). The extent of the bake hardening effect in AHSS depends on the specific chemistry and thermal histories of the steels. Additional bake hardening information is located in AHSS Application Guidelines Image Gallery, Section 2.C.1.g.
In DP steels, carbon enables the formation of martensite at practical cooling rates by increasing the hardenability of the steel. Manganese, chromium, molybdenum, vanadium, and nickel, added individually or in combination, also help increase hardenability. Carbon also strengthens the martensite as a ferrite solute strengthener, as do silicon and phosphorus. These additions are carefully balanced, not only to produce unique mechanical properties, but also to maintain the generally good resistance spot welding capability. However, when welding the highest strength grade (DP 700/1000) to itself, the spot weldability may require adjustments to the welding practice.