Effect of Silicon and Continuous Annealing Process on the Microstructure, Mechanical Properties, and Hydrogen Embrittlement of DP1500 Steel

Dual-phase (DP) steels are widely used in automotive structures due to their excellent strength-ductility balance. This study examines how silicon content and continuous annealing parameters affect the microstructure, mechanical properties, and hydrogen embrittlement (HE) behavior of DP1500 steel. Two steels, 05DP (0.5% Si) and 15DP (1.5% Si), were processed under annealing temperatures of 800-850 °C and over-aging temperatures of 240-300 °C. Higher annealing temperatures increased austenite formation and produced more martensite after cooling, leading to higher strength but reduced ductility at 850 °C due to martensite coarsening. Increasing the over-aging temperature coarsened carbides and reduced strength yet stabilized retained austenite and improved ductility through the TRIP effect. An increase in silicon content suppressed carbide precipitation, promoted carbon enrichment in austenite, refined the ferrite-martensite structure, and significantly enhanced both strength and elongation. Consequently, 15DP steel exhibited superior mechanical properties compared to 05DP steel, exhibiting 90-100 MPa higher tensile strength (+6.2-7.0%), 55-65 MPa higher yield strength (+5.3-6.2%), and 1.4-1.8 percentage points higher total elongation (+10-14%), resulting in a 16-20% increase in the strength-ductility balance (Rm × A). However, due to the relatively high hydrogen embrittlement susceptibility of fresh martensite formed either by the TRIP effect during deformation or after over-aging, 15DP steel did not exhibit substantially improved HE resistance despite its higher retained austenite fraction.