Multiphase microstructure regulation and its influence on the mechanical properties of EH500-grade ultraheavy plate steel for marine engineering

Abstract

With the increasing demand for large-scale, lightweight, and highly safe ocean transportation vessels and drilling equipment, as well as the growing need for environmentally friendly steels, the demand for high-strength, high-toughness, and weldable thick steel plates for ships and ocean engineering is becoming increasingly significant. However, as the strength and thickness of steel plates increase, the low-temperature toughness of the center of high-strength and extra-thick steel plates has become a significant challenge. Owing to low rolling reduction and central segregation in ultra-heavy plate steel, the poor low-temperature toughness of the central region poses a major challenge that limits the application of high-strength ultra-heavy steels. This work systematically investigated the effects of a two-step intercritical heat treatment on regulating the multiphase microstructure and properties of 100 mm EH500 marine engineering steel with severe central segregation. The results showed that after intercritical annealing in the 740 ℃ two-phase region, the experimental steel exhibited a yield strength of 540 MPa and a tensile strength of 869 MPa. However, the elongation and low-temperature toughness at −40 ℃ were relatively low, at only 5.1% and 14 J, respectively. Subsequent tempering at 600 ℃, 660 ℃, and 680 ℃ did not significantly alter the yield strength of the experimental steel, which remained within the range of 528 MPa to 551 MPa, while the tensile strength decreased to between 687 MPa and 730 MPa. The elongation and low-temperature toughness of the experimental steel initially increased and then decreased with the tempering temperature. At a tempering temperature of 660 ℃, the plasticity and toughness were optimized, with an elongation of 30.6% and a Charpy impact energy of 163 J at −40 ℃. Microstructure characterization results indicated that the experimental steel annealed at 740 ℃ consisted of intercritical ferrite (IF) and martensite (M). After further tempering at 600 ℃, a multi-phase microstructure comprising IF and tempered martensite (TM) with fine carbides was obtained. At a tempering temperature of 660℃, the microstructure of the experimental steel consisted of IF, TM, and fine retained austenite (RA). The RA content in the central segregation region was significantly higher than that in the matrix, resulting in a significant improvement in the plastic toughness of the experimental steel. With the further increase in the tempering temperature to 680 ℃, the experimental steel realized an IF and TM structure, with a small proportion of RA and a significant fraction of martensite/austenite (M/A) constituents in the central segregation zone. The large fraction of M/A constituents could substantially deteriorate the plasticity and toughness of the experimental steel.

Keywords

• ultraheavy steel plate
• intercritical heat treatment
• retained austenite
• central segregation
• low-temperature toughness