Abstract
This study evaluated the microstructure, martensite morphology, and mechanical properties of API X60 dual-phase pipeline steel, aiming to understand the relationship between microstructure and mechanical properties. Following annealing at various intercritical temperatures (740, 760, 780, and 810 °C), X60 dual-phase steel with varying martensite volume fractions was produced. To create distinct martensite morphologies, three treatments were developed. Due to the Intermediate Quenching (IQ) treatment, fine and fibrous martensite morphology was formed, which was evenly distributed throughout the ferrite matrix. The Direct Quenching (DQ) treatment showed dispersed martensite islands within a ferritic matrix. However, the martensite and ferrite morphologies produced by the Step Quenching (SQ) treatment were blocky and banded. Results revealed that increasing the amount of martensite improves both yield strength and ultimate tensile strength while decreasing ductility. This result indicated that adjusting heat treatment parameters can optimize X60 (DP) steel for strength-critical applications. According to the results of the experiment, API X60 (DP) steel with a finely distributed microstructure has higher tensile strength than steel with other microstructures.

Key words
microstructure, mechanical properties, intercritical annealing temperature, X60 dual-phase steel, work hardening, tensile strength

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