Effects of Retained Austenite Volume Fraction, Morphology, and Carbon Content on Strength and Ductility of Nanostructured TRIP-assisted Steels

Journal Article · Mon Jun 01 00:00:00 EDT 2015 · Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing, 636:551-564

DOI:https://doi.org/10.1016/j.msea.2015.04.030· OSTI ID:1194286

Shen, Yongfeng; Qiu, LN; Sun, Xin; Zuo, Liang; Liaw, Peter K.; Raabe, Dierk

With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C-0.30Si-1.76Mn-1.52Al (weight percent, wt.%). After intercritical annealing and bainitic holding, a combination ultimate tensile strength (UTS) of 1,100 MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800 MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governingmicrostructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1194286
• Report Number(s): PNNL-SA-110616; VT0505000
• Journal Information: Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing, 636:551-564, Journal Name: Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing, 636:551-564
• Country of Publication: United States
• Language: English