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Title

EFFECT OF AUSTENITE GRAIN SIZE ON THE BAINITIC TRANSFORMATION IN A HOT-ROLLED LOW CARBON MICROALLOYED STEEL

Authorship

Cui, Junjun; Zhu, Wenting; Chen, Zhenye; Chen, Liqing

Cui, Junjun

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Zhu, Wenting

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Chen, Zhenye

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Chen, Liqing

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DOI

10.5151/9785-9785-32310

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Abstract

In this presentation, a low carbon high strength steel microalloyed with Nb and Ti was processed by thermo-mechanical controlled process (TMCP) containing two stages of controlled rolling. The austenite with various grain sizes was obtained by austenitizing this hot-rolled steel at different temperatures. And the effect of austenite grain size on bainitic transformation was investigated in a dilatometer to reveal microstructural change. The results showed that the microstructure of the hot-rolled steel was mainly composed of granular bainite, lath-like bainite, and martenite/austenite constituents (M-A constituents). With increasing the austenitizing temperature, or increasing the austenite grain size, the microstructure was changed from granular bainite to lath bainite. When the steel was reheated to a lower temperature of 860 ºC, a relatively higher start temperature (Bs) was measured for bainitic transformation. This was mainly caused by the formation of solute-depleted region in the austenite and the promotion of the granular bainitic nucleation. The highest reaction rate occurred at austenitizing temperature of 1000 ºC with fine grain size, while the coarse austenite grain can lead to a sharp increase in the amount of bainite. An analysis was made on the kinetics of bainite transformation which was controlled by the volume diffusion of C atom.

Keywords

hot-rolled low carbon steel; bainite transformation; austenite grain size; austenitizing temperature