Nanoscale Solute Partitioning and Carbide Precipitation in a Multiphase TRIP Steel Analyzed by Atom Probe Tomography

Abstract

Nanoscale solute partitioning across multiple constituent phases in a 980grade quenched and partitioned (Q&P) steel was analyzed using atom probe tomography (APT). The Q&P process was used to increase the C content in the retained austenite phase thereby improving its stability under plastic straining. Significant carbon enrichment of austenite was measured with decreased levels of C in martensite and almost depleted C content in ferrite, supporting the C partitioning mechanism in the literature. The APT analysis of retained austenite surrounded by martensite demonstrated a higher amount of C content compared with retained austenite surrounded by the ferrite phase. Lath and discrete carbide particle precipitation was also observed inside martensite colonies, tying up C and reducing the total amount of C available for austenite stabilization. In addition, the partitioning of Mn and other minor elements was quantitatively investigated by correlating APT and SEM-EBSD. These techniques provide a robust methodology for analyzing nanoscale compositional partitioning in multiphase steels, TRIP steels in particular, which can be used to better explain their microstructure-mechanical property relationships.

Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

“…Such predictions are obviously contrary to experiments [256]. Also, the substantial redistribution of substitutional alloying elements across the interface as detected using 3DAPT [158,161,162,164,181,[252][253][254] cannot be explained by the QP-PE model. Later Dai et al [54] applied the QP-LE model to design the RA in Q&P steels.…”

“…However, as shown in Fig. 19a and b[254], a nanoscale redistribution of substitutional alloying elements across the martensite/austenite interface during partitioning has been detected by several research groups using 3DAPT [158,161,162,164,181,[252][253][254]. Seo et al [161,162] argued that the substitutional alloying elements do not redistribute until carbon has fully partitioned fully from martensite into austenite under the CCE condition.…”

“…Given that the crystal structure of hexagonal ε transition carbide is quite similar to that of orthorhombic η transition carbide, it is usually difficult to distinguish them from each other. In some cases, cementite could also be observed in Q&P steels [162,181,253,280,282], especially for high carbon steels [252], after partitioning at higher temperatures [162,280] or at longer partitioning times [280].…”

Quasi-Situ Characterization of Retained Austenite Orientation in Quenching and Partitioning Steel via Uniaxial Tensile Tests

“…Table 1 shows the main chemical composition (in mass fraction) of the investigated material. Carbon and manganese were added to stabilize austenite [ 13 ], and silicon was used to inhibit the formation of cementite [ 14 ]. The investigated steel was produced by a two-step Q&P process [ 15 ].…”

Quasi-Situ Characterization of Deformation in Low-Carbon Steel with Equiaxed and Lamellar Microstructure Treated by the Quenching and Partitioning Process

“…The carbon atoms must pass through the austenite at the edge of the martensitic block that may exist and the ferrite to reach the equiaxed austenite, leading to less carbon enrichment in the equiaxed RA grains than the film-like one. Devaraj et al [30] visually characterized that the carbon content of lamellar RA inside the martensite is higher than the equiaxed type by using three-dimensional atom probe analysis. It is the difference in carbon content that makes several types of austenite have different stabilities.…”