Showing papers by "John G. Speer published in 2017"

Influence of Temperature and Grain Size on Austenite Stability in Medium Manganese Steels

Abstract: With an aim to elucidate the influence of temperature and grain size on austenite stability, a commercial cold-rolled 7Mn steel was annealed at 893 K (620 °C) for times varying between 3 minutes and 96 hours to develop different grain sizes. The austenite fraction after 3 minutes was 34.7 vol pct, and at longer times was around 40 pct. An elongated microstructure was retained after shorter annealing times while other conditions exhibited equiaxed ferrite and austenite grains. All conditions exhibit similar temperature dependence of mechanical properties. With increasing test temperature, the yield and tensile strength decrease gradually, while the uniform and total elongation increase, followed by an abrupt drop in strength and ductility at 393 K (120 °C). The Olson–Cohen model was applied to fit the transformed austenite fractions for strained tensile samples, measured by means of XRD. The fit results indicate that the parameters α and β decrease with increasing test temperature, consistent with increased austenite stability. The 7Mn steels exhibit a distinct temperature dependence of the work hardening rate. Optimized austenite stability provides continuous work hardening in the temperature range of 298 K to 353 K (25 °C to 80 °C). The yield and tensile strengths have a strong dependence on grain size, although grain size variations have less effect on uniform and total elongation.

Combined Intercritical Annealing and Q&P Processing of Medium Mn Steel

Abstract: The microstructure and mechanical properties of intercritically annealed medium Mn steel are dependent on the selection of the intercritical annealing (IA) temperature. While the yield strength (YS) decreases with increasing IA temperature, the ultimate tensile strength increases with increasing IA temperature. Strain aging phenomena, both static and dynamic, are also often observed. The present contribution shows that, by combining IA with the quench and partitioning processing of the intercritical austenite, it is possible to obtain non-aging mechanical properties which combine a high YS with an ultra-high tensile strength. These properties are particularly suitable for automotive parts related to passenger safety.

Bainitic and quenching and partitioning steels

Abstract: This chapter reviews bainitic and quenched and partitioned steels as two potential approaches to produce the so-called third-generation advanced high-strength sheet steels. Both approaches result in fine microstructures containing retained austenite stabilized by interstitial solute carbon enrichment which decreases the martensite transformation start temperature. This chapter provides a review of thermal processing and alloying strategies pursued for both microstructural concepts. Tensile properties are reviewed and a brief discussion of local formability, in particular hole expansion testing, is included. In addition, mechanisms that may compete with solute enrichment of austenite are discussed.

Comparison of Ab-initio Solute-Boundary Binding Energies and Experimental Recrystallization Data in Austenite for Solute Nb and Other Elements

Abstract: Niobium is a critical alloying addition in modern high strength steels, employed at “microalloy” concentrations in the hundredths of mass percent. Nb was first incorporated into industrial heats of steel in 19581) and has since been studied extensively.2–12) Many of the studies have focused on the effects of Nb as a precipitate strengthening addition,2,3) but it is also known to have significant solute drag effects.3–11) The suppression of austenite recrystallization in particular has enabled the important class of HSLA (high strength low-alloy) steels. Nb is known to segregate to both austenite and ferrite grain boundaries4) as a result of a binding energy between the grain boundary and Nb.4–6) This energy has been investigated for ferrite both experimentally4,5) and computationally.6) Maruyama et al. calculated the segregation energy of solute Nb to α-Fe (ferrite) grain boundaries from measurements of the excess Nb at grain boundaries and found a segregation energy of 38 kJ/ mol.5) Jin et al. used density functional theory calculations to determine the solute segregation energy for Nb, and other elements, for a symmetric ∑5 (013) grain boundary and reported an average interaction energy of 39 kJ/mol.6) Felfer et al. observed Nb enrichment at prior austenite grain boundaries (PAGBs) using atom probe tomography,4) and Comparison of Ab-initio Solute-Boundary Binding Energies and Experimental Recrystallization Data in Austenite for Solute Nb and Other Elements

Microstructural Evolution in Microalloyed Steels During Thermomechanical Rod Rolling

Abstract: Steel rods are hot-rolled at high strains and strain rates with a subsequent controlled cooling process to influence the microstructure. The microstructure and mechanical properties of the hot-rolled rods are controlled to produce high-strength fasteners in the cold heading process without subsequent heat treatment. In the present study, simulations of rod rolling by torsional deformation and controlled time-temperature schedules were conducted to examine the effects of thermomechanical processing parameters and microalloying additions on the microstructure evolution and mechanical properties of low-carbon steel rods. Transformation and precipitation behaviors during the thermomechanical process were investigated and related to the increased strength in the steel rods.

Modified hot-dip galvanize coatings with low liquidus temperature, methods of making and using the same

Abstract: The present invention relates to a coated substrate material. The substrate can be a steel alloy and the microstructure in the steel alloy can be altered with a low temperature coating process. The present invention also relates to a method to coat the substrate at the low temperature. The present invention also relates to a coating wherein the melting temperature of the coating is reduced with a dopant compared to the melting temperature without the dopant. The present invention also relates to a method to make the coating with the dopant.