Effect of Ce on the cleanliness, microstructure and mechanical properties of high strength low alloy steel Q690E in industrial production process

Abstract

In order to improve the strength and toughness of Q690E steel sheets, the effect of rare earth element Ce on the strength and toughness of Q690E steel was studied by means of transmission electron microscopy, scanning electron microscopy, and metallographic microscope. The results showed that the addition of Ce in steel limited the combination of S with Mn and Ca, transformed Al₂O₃ inclusion into spherical CeAlO₃ inclusion, and modified the precipitate form of some composite inclusions of TiN and sulfide oxides into TiN precipitation alone. The inclusions were spheroidizing. The size of inclusions was decreased from 3–5 μm to 1–2 μm, and the distribution was dispersed. Ce played a role in purifying molten steel through desulphurization and deoxidization. Meanwhile, the addition of Ce in steel effectively increased the nucleation particles in the liquid phase, improved the nucleation rate, enlarged the equiaxed grain refinement area, and limited the development of columnar crystals. The average grain size of slab decreased from 45.76 to 35.25 μm, and the proportion of large grain size (> 50 μm) decreased from 40.41% to 23.74%. The macrostructural examination of slab was improved from B0.5 to C2.0, which realized the refinement of the solidified structure and reduced the banded structure of hot rolled plate. In addition, due to the inheritance of refined structure in the upstream, the recrystallization of deformed austenite and the growth of grain after recrystallization were restrained, and a refined tempered sorbite structure was obtained. When rare earth element Ce was added, the width of the martensite lath bundle was narrowed from about 500 nm to about 200 nm, which realized a remarkable grain refinement strengthening and toughening effect. Mechanical properties such as tensile, yield, and low-temperature impact toughness were significantly improved.

Keywords

rare earth / HSLA steel / inclusion / structure / grain refinement strengthening

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