Microstructures and Mechanical Properties of Tough Ductile Ultrahigh-Strength Steels Processed through Direct Quenching and Partitioning

Abstract:

Using a novel TMR-DQP processing route, two ultrahigh-strength steels have been developed with yield strengths up to 1100 MPa combined with good uniform and total elongations and low-temperature impact toughness. Processing involved thermomechanically controlled rolling including significant reductions below the recrystallization stop temperature (RST), subsequent direct quenching to desired quench stop temperatures between M_s and M_f and finally partitioning of carbon from the supersaturated martensite to the untransformed austenite in a furnace at the quench stop temperature. Samples were cooled slowly in the furnace over 50 hours to simulate the cooling of coiled strips on industrial hot strip mills. The approach used was to utilize a suitable 0.3C steel composition based on high silicon and/or aluminium contents. Detailed metallographic studies using LOM, FESEM-EBSD, TEM and XRD showed that the desired martensite-austenite microstructures were achieved. The advantage of strained austenite in respect of refinement of martensite packets/blocks was clearly evident. Austenite was finely divided between martensite laths and only an insignificant amount of austenite existed as pools. The fine lath martensite structure with narrow interlath retained austenite films enabled the achievement of excellent combinations of mechanical properties. Promising results in respect of microstructures and mechanical properties indicate that there are possibilities for developing tough ductile structural and abrasion-resistant steels through the TMR-DQP route.

• Periodical: Materials Science Forum (Volume 941)
• Pages: 468-473
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.941.468
• Online since: December 2018
• Authors: Pekka K. Kantanen*, Mahesh Chandra Somani, David A. Porter, Jukka I. Kömi, Devesh K. Misra
• Keywords: Direct Quenching, EBSD, Martensite, Partitioning, Retained Austenite, TEM, Thermomechanical Processing, TMR-DQP, Ultrahigh-Strength Steel, X-Ray Diffraction

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