1. Acquiring a low yield ratio well synchronized with enhanced low-temperature toughness in 550MPa grade bridge steels through intercritical quenching treatment
- Author
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Lu Zhu, Bo Gao, Jun Wang, Runnong Chen, Yinpeng Wang, Chunhui Jiang, Yanguang Cao, and Zhaodong Li
- Subjects
High-performance bridge steel ,Intercritical quenching ,Ferrite ,Bainite ,Yield ratio ,Low-temperature toughness ,Mining engineering. Metallurgy ,TN1-997 - Abstract
With the development of bridge steel towards higher strength, higher requirements have been put forward for its yield ratio, low-temperature toughness, and other properties. This study focuses on 550 MPa grade high performance steel for bridge construction to explore microstructure control strategies that achieve high strength, low yield ratio, and high low-temperature toughness. Firstly, a multiphase structure consisting mainly of ferrite and bainite was obtained in the experimental steel through thermo-mechanical control process (TMCP) + intercritical quenching and tempering (IQT) process. Compared with the steel treated with TMCP + tempering (T) processes, the yield ratio decreased from 0.82 to 0.77 while ensuring a yield strength of ∼600 MPa. The yield ratio of the experimental steel is related to the microstructural parameters b (morphology and content of ferrite and bainite) and the strain-hardening index N of the steel. IQT treatment can not only introduce soft ferrite but also change the morphology of microstructure, resulting in a decrease in ln(b/N2) value and thus a decrease in yield ratio. Further, by increasing the cooling rate after intercritical quenching, the bainite morphology changes from granular to lath. The −40 °C V-notch impact energy of the tested steel increases from 236 J to 287 J. Compared to steel treated by TMCP + T, the ductile-brittle transition temperature (DBTT) of steel treated by IQT with faster cooling rate decreases from −54.5 °C to −91 °C. The improvement of low-temperature impact toughness is attributed to the weakening of variant selection, which increases the density of high-angle grain boundaries and significantly enhances crack propagation resistance. In addition, the existence of film-like retained austenite in lath bainite further enhances the low-temperature toughness.
- Published
- 2025
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