INFLUENCE OF THERMOMECHANICAL TREATMENTS ON THE MECHANICAL PROPERTIES AND THE MICROSTRUCTURE OF LEAN CHEMISTRY HSLA-100 STEEL PLATES

摘要

The influence of thermomechanical treatments on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76 , Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 pct) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, DQT plates of Nb-Cu heat exhibited good strength (YS～900 MPa) and low-temperature impact toughness (average: 74 J at -85 °C); the CVN impact energies were however, marginally lower than conventional HSLA-100 steel. In Nb-Cu-B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. The mechanical properties of the DQT-plates were superior to RQT-plates. Transmission electron microscopy revealed that smaller martensite inter-lath spacing in DQT plates resulted in their superior strength and impact toughness.