DP steels contain islands of hard martensitic second phase in a ferritic matrix. Steel strength improves when the volume fraction of hard second phases increases. The formation of DP steels involves transformation of some austenite to ferrite by controlled cooling, followed by rapid cooling to transform the rest of the austenite structure to the martensite structure.
Hot-rolled DP steel products are fabricated from the austenite phase; whereas hot-dip coated and continuously annealed cold-rolled DP steel products are from the two- phase ferrite plus austenite phase. The resulting material may also contain trace amounts of other phases such as retained austenite and bainite.
Based on the process route and composition, steels with a microstructure consisting of substantial amounts of bainite can also be fabricated to meet the requirement of improved crack resistance on a stretched edge. A schematic microstructure of the DP steel consisting of ferrite and islands of martensite is illustrated in Figure 1.
DP steels have superior ductility because of the continuous soft ferrite phase, and exhibit high initial work-hardening rate (n-value) during deformation due to concentration of strain in the lower-strength ferrite phase in the vicinity of the islands of martensite. Figure 2 shows a photomicrograph depicting the martensite and ferrite components.
Figure 1. Schematic shows islands of martensite in a matrix of ferrite.
Figure 2. Photomicrograph of DP steel.
Key Properties of DP Steels
DP steels have a unique combination of outstanding elongation and work hardening rate, providing them superior ultimate tensile strengths when compared to traditional steels of same yield strength. The comparison of the engineering stress-strain graph for the DP steel and HSLA steel of same yield strength is illustrated in Figure 3. The DP steel has better TS/YS ratio, higher ultimate tensile strength, and superior initial work hardening rate when compared to the HSLA of same yield strength.
Figure 3. The DP 350/600 with higher TS than the HSLA 350/450.
Bake hardening effect is a key advantage of the DP steel and other AHSS over traditional higher strength steels. The improved yield strength as a result of elevated temperature aging subsequent to prestraining is referred as the bake hardening effect. Thermal history of an AHSS and the extent of forming strain for the specific chemistry determines the level of the bake hardening effect in the steel. Figure 4 shows the tensile strength-elongation graph of the DP steels.
Figure 4. The tensile strength-elongation graph of the DP steels
Martensite can be formed in DP steels at practical cooling rates as a result of improving the hardenability of the steel. Hardenability can also be increased by adding nickel, vanadium, molybdenum, chromium, and manganese individually or in combination. Like phosphorus and silicon, the martensite structure is strengthened as a ferrite solute strengthener by carbon.
Carefully balanced additions of these elements yield unique mechanical properties as well as good resistance spot welding capability. Nevertheless, the welding practice may need to be adjusted to weld higher strength grades to themselves (DP 700/1000 and above). The following table lists the current production grades of DP steels and the corresponding example automotive applications:
||Roof outer, door outer, body side outer, package tray, floor panel
||Floor panel, hood outer, body side outer, cowl, fender, floor reinforcements
||Body side inner, quarter panel inner, rear rails, rear shock reinforcements
||Safety cage components (B-pillar, floor panel tunnel, engine cradle, front sub-frame package tray, shotgun, seat),
Download the Advanced High-Strength Steels Applications Guidelines free here, to learn more about the metallurgy, forming and joining of these new steels.
This information has been sourced, reviewed and adapted from materials provided by WorldAutoSteel (World Auto Steel).
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