Dual-phase steel is a popular material in the automobile industry, and in many instances is being used to replace more traditional HSLA steel. Dual-phase steel is not only lighter in weight than traditional steel, but also has a greater ability to absorb crash energy and resist fatigue. Today, dual-phase steel is used in such applications as rocker reinforcements, bumpers and door intrusion beams.
Dual-phase steel is formulated with a mixture of two substances. Ferrite matrix, a soft iron alloy, provides the soft phase. Martensite, a steel crystalline, provides the hard phase.
Strength and Weight
Dual-phase steel hardens rapidly during work. At strain levels of only 2 to 3 percent, strength increases from 21 to 31 kilopounds per square inch (ksi). When compared to traditional HSLA steel, dual-phase steel shows a weight reduction of up to 25 percent.
Yield Point Elongation
Yield point elongation refers to the difference between the elongation at the start and at the finish of a material. Yielding is the area in which increased strain occurs with no increase in stress. Dual-phase steel exhibits virtually no yield point elongation.
Forming and Welding
Dual-phase steel performs predictably in stamping, with a low incidence of instability and kinking. This is due to the fast hardening rate and absence of yield point elongation. The steel also meets automobile welding requirements.
A forming limit diagram (FLD) is a plot of the maximum strain that sheet metal can sustain. FLD curves in dual-phase steel are calculated in the same manner as other steels. Dual-phase steel can withstand strain more effectively than traditional HSLA steel.
Springback and Bendability
Springback is the elastic recovery that a sheet metal possesses. Because dual-phase steel is highly stable with rapid hardening and no yield point elongation, springback with this material is easier to control than in traditional HSLA steel. The material is also highly bendable.
Dual-phase steel possesses a high capacity for bake hardening, which increases the strength of the steel by approximately 5 to 10 ksi.
Ultimate Tensile Strength
Ultimate tensile strength (UTS) of dual-phase steel is higher than traditional steel, at 72 to 175 ksi.
Dual-phase steel does not age at room temperature.
Crash Energy Absorption
Compared to traditional HSLA steel, dual-phase steel has a higher capacity to absorb crash energy.