Abstract
The austenite decomposition phase transformation in a low-carbon dual-phase (DP) steel is studied as a function of inter-critical annealing parameters: annealing time, annealing temperature, and cooling rate. The austenization kinetics are obtained by dilatometric analyses, water-quenched metallography, and thermodynamic calculations, and the austenite formed at different inter-critical annealing temperatures influences the subsequent formation of secondary phases during continuous cooling. Results indicate that the austenite volume fraction increases with both annealing temperature and time; although their effect on the final microstructure and mechanical properties lessens when the cooling rate decreases to the slow cooling regime below air cooling. Interestingly, the yield strength (YS) values of samples cooled at ∼1 °C/s with sand cooling, are 75∼100 MPa greater than the corresponding values at a rate of ∼6 °C/s with air-cooling. The results indicate that the yield strength is positively correlated with yield-point elongation but negatively correlated with the ultimate tensile strength. The effects of the cooling rate on the mechanical properties can be explained by the formation of Cottrell atmospheres of carbon together with the increase of the pearlite volume fraction. Based on the secondary phase components and strength evolutions, there are four possible outcomes for the different cooling rates: (I) martensite (M); (II) ferrite-martensite (FM); (III) coexistence of martensite and pearlite (MP); and (IV) ferrite-pearlite (FP). Our results reveal that the microstructure-property correlation can be illustrated schematically as a function of cooling rates and can thereby direct the heat-treatment parameters required to obtain desirable mechanical properties.
Original language | English |
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Article number | 145801 |
Journal | Materials Science and Engineering: A |
Volume | 888 |
DOIs | |
State | Published - Nov 17 2023 |
Externally published | Yes |
Funding
This work was supported financially by A. O. Smith Corporation , Milwaukee, WI. This work made use of the MatCI Facility at Northwestern University, which receives support from the MRSEC Program ( NSF DMR- 1720139 ) of the Materials Research Center at Northwestern University , and the Northwestern University NUCAPT center , which received support from the NSF-MRI ( DMR-0420532 ) and ONR-DURIP ( N00014-0400798 , N00014-0610539 , N00014-0910781 , N00014-1712870 ) programs, and support from the MRSEC program ( NSF DMR-1720139 ) at the Materials Research Center, the SHyNE Resource ( NSF ECCS-2025633 ), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University . This work also made use of the EPIC facility of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource ( NSF ECCS-2025633 ); the MRSEC program ( NSF DMR-1720139 ) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. QQR also acknowledges Wei Zhang, Eddie Pfeifer, Ying Lu, and Jennifer Semple at the Ohio State University, for helping with the Gleeble testing and data analyses.
Keywords
- Dilatometry
- Dual-phase steels
- Inter-critical annealing
- Microstructure-property correlations
- Thermo-Calc calculations