Evolution of microstructure and mechanical properties along the thickness direction of 500 MPa HSLA steel heavy plates

Abstract

High strength heavy plates with large thickness are demanded for the construction of large-span bridge projects. However, there is a lack of understanding on the microstructure evolution and mechanical properties through the entire thickness of heavy plates with yield strength over 500 MPa, especially manufactured by the thermo-mechanical control processing (TMCP) and tempering (T) process. In this research, the microstructure and mechanical property evolution through the thickness was investigated for the 85 mm thick, 500 MPa grade heavy steel plate manufactured by TMCP + T. The results show that the microstructure throughout the thickness predominantly consists of granular bainite (GB) and polygonal ferrite (PF). The yield strength and tensile strength at 1/8t, 1/4t, 3/8t and 1/2t (t, the thickness) are 659.4 MPa, 520.6 MPa, 500.3 MPa and 494.0Mpa, and 739.2 MPa, 642.9 MPa, 627.8 MPa and 625.2 MPa, respectively. Yield and tensile strength both significantly decline from 1/8t to 1/2t. The strengthening mechanisms at different thickness were analyzed, and grain refinement has the most significant contribution. In addition, the mechanical properties of ferrite/bainite heterostructure were analyzed by Cyclic load-unload-reload (LUR) tests. The hetero-deformation induced (HDI) stress increases from 1/8t to 1/2t. At 1/8t, HDI hardening is higher but occurs over a narrower strain range compared to that at 1/2t. The impact energy at −40 °C for 1/8t, 1/4t, 3/8t and 1/2t are 182.7 J, 213.0 J, 242.2 J and 253.7 J, respectively. It was discerned that PF undergoes plastic deformation and absorbs a large amount of energy, constituting the primary rationale for the enhanced impact toughness at 1/2t compared to 1/8t. Through this research, not only the microstructure and mechanical property evolution through the thickness was revealed, but also the ferrite/bainite microstructure was found a promising heterostructure to obtain high strength, high toughness and low yield ratio for heavy plates.