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. Due to the production process, small amount of other phases (Bainite and Retained Austenite) may be present.
Depending on the composition and process route, steels requiring enhanced capability to resist cracking on a stretched edge (as typically measured by hole expansion capacity) can have a microstructure containing significant quantities of bainite.
Figure 2-2 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 initial work-hardening rate (n-value) exhibited by these steels. Figure 2-3 is an actual photomicrograph showing the ferrite and martensite constituents. The work hardening rate plus excellent elongation creates DP steels with much higher ultimate tensile strengths than conventional steels of similar yield strength. Figure 2-4 compares the engineering stress-strain curve for HSLA steel to a DP steel curve of similar yield strength.
Figure 2-2: Schematic shows islands of martensite in a matrix of ferrite.
Figure 2-3: Photomicrograph of DP steel.
Figure 2-4: The DP 350/600 with higher TS than the HSLA 350/450
The DP steel exhibits higher initial work hardening rate, higher ultimate tensile strength, and higher TS/YS ratio than a similar yield strength HSLA. Additional engineering and true stress-strain curves for DP steel grades are located in Figure 2-5 in the Advanced High-Strength Steels Application Guidelines.
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 an adequate amount of forming strain for the specific chemistry and thermal history of the steel. Additional bake hardening information is located in Section 3.B.1.i. of the Advanced High-Strength Steels Application Guidelines.
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 higher strength grades (DP 700/1000 and above) to themselves, the spot weldability may require adjustments to the welding practice. Current production grades of DP steels and example automotive applications:
|DP 300/500||Roof outer, door outer, body side outer, package tray, floor panel|
|DP 350/600||Floor panel, hood outer, body side outer, cowl, fender, floor reinforcements|
|DP 500/800||Body side inner, quarter panel inner, rear rails, rear shock reinforcements|
|DP 600/980||Safety cage components (B-pillar, floor panel tunnel, engine cradle, front sub-frame package tray, shotgun, seat),|
|DP 700/1000||Roof rails|
|DP 800/1180||B-pillar upper|