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

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 degrees C to 5.39 +/- 0.05%% at 650 degrees C, and the high reversed austenite content at 650 degrees 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 degrees 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 similar to 750.5 MPa.

Automated summary

We present a new low-carbon ultra-high strength seamless tube steel with an alloy design strategy, rolling, and heat treatment processes. Based on the reverse transformation of austenite, the microstructure and mechanical properties of the experimental steel in different states were investigated. The co-precipitation behavior and the evolution of secondary martensite were also examined. The experimental steel achieved a maximum yield strength of 1483.5 +/- 3.5 MPa after aging at 550 degrees C for 1 h.