Effect of precipitation evolution of NiAl and Cu nanoparticles on strengthening mechanism of low carbon ultra-high strength seamless tube steel

Abstract

We report an alloy design strategy, rolling and heat treatment processes for a new low-carbon ultra-high strength seamless tube steel. The evolution of multiphase microstructure and mechanical properties of experimental steel in the rolled, quenched, and quenched-tempered states has been investigated based on the reverse transformation of austenite correlated with the transformation of secondary martensite and co-precipitation behavior. Microstructure observation results show that the microstructure of rolled state is composed of 88.1 ± 0.1% lath bainite (LB) and 11.9 ± 0.1%% granular bainite (GB), the quenched state is composed of 82.1 ± 1.1%% lath martensite (LM) and 17.9 ± 1.1% GB, while the QT steel is composed of tempered martensite (TM) and GB, and the reversed austenite content increases from 0.66 ± 0.03%% at 500 °C to 5.39 ± 0.05%% at 650 °C, and the high reversed austenite content at 650 °C is the basis of secondary martensitic transformation. The co-precipitation sequence of nanoparticles observed by high-resolution transmission electron microscope (HRTEM) is “supersaturated solid solution → B2 NiAl + bcc Cu → B2 NiAl + bcc Cu + B2 core-9R shell → B2 NiAl + fcc Cu”. Aged at 550 °C for 1 h, the experimental steel obtained a maximum yield strength of 1483.5 ± 3.5 MPa, which was attributed to the combined effect of shear and Orowan strengthening mechanisms, and the maximum strengthening increment reached ∼750.5 MPa.