Effect of start cooling temperature on microstructure, crystallographic orientation and ductile-to-brittle transition behavior of high strength steel

Abstract

The ductile-to-brittle transition temperature (DBTT) of high strength steels can be optimized by tailoring microstructure and crystallographic orientation characteristics, where the start cooling temperature plays a key role. In this work, X70 steels with different start cooling temperatures were prepared through thermo-mechanical control process. The quasi-polygonal ferrite (QF), granular bainite (GB), bainitic ferrite (BF) and martensite-austenite constituents were formed at the start cooling temperatures of 780 °C (C1), 740 °C (C2) and 700 °C (C3). As start cooling temperature decreased, the amount of GB decreased, the microstructure of QF and BF increased. Microstructure characteristics of the three samples, such as high-angle grain boundaries (HAGBs), MA constituents and crystallographic orientation, also varied with the start cooling temperatures. C2 sample had the lowest DBTT value (−86 °C) for its highest fraction of HAGBs, highest content of <110> oriented grains and lowest content of <001> oriented grains parallel to TD. The high density of {332} <113> and low density of rotated cube {001} <110> textures also contributed to the best impact toughness of C2 sample. In addition, a modified model was used in this paper to quantitatively predict the approximate DBTT value of steels.

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