Microstructural origins of the toughness contrast between low-carbon Nb–Ti–B bainitic and higher-carbon Cr–Ni–Mo martensitic 900 MPa-Grade HSLA steels

Two 900 MPa-grade steels with distinct alloying strategies were processed under the same TMCP and tempered to comparable strength levels to construct a comparison platform: a Nb-Ti-B microalloyed lower-carbon (LC) steel (0.08 wt.% C) and a higher-carbon Cr-Ni (MC) steel (0.18 wt.% C). At comparable yield strengths (967 vs. 1025 MPa), LC exhibited a lower yield ratio (0.83 vs. 0.97), more than doubled crack initiation energy at -40 °C ( W i = 38.7 vs. 16.0 J), and a DBTT 67 °C lower than MC steel. This advantage persisted under triaxial constraint up to η = 1.43, with true fracture strain holding at ε f = 0.74-0.82 in LC vs. 0.50-0.54 in MC. Microstructural analysis links these differences to a multiscale chain associated with the combined low-carbon Nb-Ti-B route: an elevated T nr (1040 vs. 905 °C) expands the non-recrystallization rolling window, drives strain-induced Nb(C,N) precipitation on pre-existing TiN, and accumulates 71% higher S v , refining the prior-austenite grain size by 41% (8.6 vs. 14.6 μm). Within this refined austenite, strong K-S variant selection produces bainitic packets approaching the prior-austenite grain dimension, so that the cleavage barrier scale collapses onto the prior-austenite grain itself. This bainitic matrix carries a lower as-transformed dislocation density (KAM 0.94° vs. 1.10°), providing work-hardening capacity that manifests as crack-tip blunting under constraint and during dynamic loading. Together with Ti-Nb nano-precipitates (5-50 nm) that suppress coarse interlath carbides, this PAG-variant-bainite triad reproduces the macroscopic toughness advantage of LC steel and provides design criteria for high-toughness 900 MPa-grade HSLA steels.