Formation mechanism of CuNiAl-rich multi-structured precipitation and its effect on mechanical properties for ultra-high strength low carbon steel obtained via direct quenching and tempering process

Ultra-high low carbon baintic steel strengthened by CuNiAl-rich multistructured particles was studied in this work. The influence of direct quenching and tempering (DQ-T) process on microstructural transformation and precipitation behavior of CuNiAl-rich particles were investigated. The high-density dislocations formed in DQ process provide a large number of nucleation sites for the CuNiAl-rich particles. According to the results of phase field method simulation, the coarsening mechanisms of CuNiAl-rich particles included merger mechanism and Ostwald maturation mechanism. The formation of 9R–B2 core-shell structural CuNiAl-rich particles was owing to the inverse concentration gradient diffusion of Cu between particles with different Cu content. The yield strength was 1228 MPa, because of the co-precipitation of 9R–B2 core-shell structural particles with ∼12.5 nm and B2-ordered particles with ∼3.4 nm. The impact absorbed energy was 44.2J at −40 °C. The small grain size contervailed for toughness reduction from co-precipitation.