Emmanuel De Moor

Retained Austenite Stabilization Through Solute Partitioning During Intercritical Annealing in C-, Mn-, Al-, Si-, and Cr-Alloyed Steels

Abstract Retained austenite fractions, predicted to be stable at room temperature assuming ortho-equilibrium solute distribution during intercritical annealing, were calculated for “medium-Mn” steels with varying Mn, C, Al, Si, and Cr additions using SSOL 2 and TCFE 7 Thermo-Calc® databases. While Mn additions increase retained austenite levels, increased C levels are not predicted to greatly impact austenite fractions. Additions of Si reduce the levels, whereas opposing trends are predicted for Al additions by the employed Thermo-Calc® databases. Chromium significantly reduces the dependence of retained austenite fraction on annealing temperature. Alloying effects are explained through four critical phase transformation temperatures.

Tempering Response of Bainitic and Martensitic Microstructures

The tempering response of fully martensitic microstructures has been well characterized. However, bainitic microstructures may also be found in quenched industrial materials and the present study investigates the tempering response of fully martensitic and fully bainitic microstructures. Specific thermal cycles were developed to generate both microstructures in a boron added 0.17 wt pct carbon steel. The tempering response was assessed through dilatometry and microstructural characterization was conducted using scanning electron microscopy and Mossbauer spectroscopy. The dilatometric analysis of the tempering response of the martensitic microstructures provided information about retained austenite decomposition and cementite precipitation whereas bainitic microstructures showed a less sensitive dilatometric response during tempering likely due to the low amount of carbon in solution and absence of retained austenite as measured by Mossbauer spectroscopy.

Nb-Microalloying in Next-Generation Flat-Rolled Steels: An Overview

Extensive efforts are underway worldwide to develop new steels with substantial fractions of retained austenite, for lightweight automobile manufacturing and other applications requiring improved combinations of strength and formability. It is likely that microalloying can provide product enhancements in these emerging products, such as Q&P, TBF, medium-Mn TRIP, etc. and this paper examines the expected behavior of niobium using inferences based on published AHSS literature and principles of Nb microalloying. Some benefits of Nb in terms of microstructure refinement and precipitation strengthening have been reported. The potential influences of Nb are complex due to the sensitivity of Nb dissolution and precipitation to chemical composition and processing; differences in the expected role of Nb are pointed out with respect to different product forms produced via hot-rolling or annealing after cold-rolling, and microstructures with or without substantial quantities of primary ferrite. Some issues that warrant further examination are identified, as a deep understanding of Nb microalloying and other fundamental behaviors will be needed to optimize the performance of these next-generation steels.

Microalloyed High Carbon Wire Steels

Increased demands for weight reduction in a variety of structures and applications are stimulating the development of wire products with increased strength. Boron, Vanadium, and Niobium microalloying of high carbon wire steels will be discussed, in particular alloying effects on austenite decomposition and pearlite transformation kinetics. Tensile properties and hardness data are presented and correlations with microstructure and processing are discussed.