The relationship between microstructure, crystallographic orientation, and fracture behavior in a high strength ferrous alloy

Abstract

The study described here focuses on toughening and crack-arrest mechanism in a 560 MPa microalloyed pipeline strip steel processed by combination of thermo-mechanical controlled processing and ultra-fast cooling (TMCP-UFC) and compared with the strip processed at low cooling rate and high cooling interrupt temperature (TMCP-LC). Furthermore, delamination mechanisms involved in drop weight tear test (DWTT) were also studied from the perspective of microstructure, crystallographic orientation, and stress field ahead of the crack. The TMCP-UFC processed strip was primarily composed of acicular ferrite (AF), bainitic ferrite (BF), and small-sized martensite/austenite (M/A) constituent, and the microstructure was homogeneous across the thickness. While the TMCP-LC processed strip mainly consisted of quasi-polygonal ferrite (QPF), degenerate pearlite (DP), and small fraction of BF. The TMCP-UFC processed strip exhibited excellent low-temperature toughness with upper shelf energy (USE) of ∼302 J and transition temperature (TT) of ∼−75 °C in relation to TMCP-LC processed strip with USE of ∼273 J and TT of ∼−30 °C. Significant consumption of crack-propagation energy during ductile fracture and smaller effective grain size of ∼2.4 μm were responsible for higher USE and lower TT of TMCP-UFC processed strip compared to TMCP-LC processed strip with effective grain size of ∼4.0 μm. The smaller effective grain size with respect to TMCP-UFC processed strip contributed to excellent crack-arrest property by deflecting the propagated crack. In the TMPC-UFC processed strip, delamination occurred along the crystallographic cleavage plane of {001} under plane strain condition at mid-thickness. In contrast, besides delamination mechanism along the cleavage plane, delamination also occurred along the macro-segregation band under plane strain condition at mid-thickness in the TMCP-LC processed strip, which led to severe delamination.