Effects of austenite conditioning on microstructural development and interphase precipitation in titanium-molybdenum microalloyed steels, The

Abstract

A challenge exists in the automotive industry to develop new, hot-rolled, microalloyed steels offering a balance of high tensile strength and superior stretch-flange formability. The steel industry has responded by developing ferritic steels strengthened with extensive nanometer-scale precipitation. The single-phase ferritic matrix eliminates hard constituents and imparts superior stretch-flange formability, while high strengths are derived from nanometer-scale precipitates. In this context, interphase precipitation has regained substantial academic and industrial interest. Interphase precipitates repeatedly nucleate at the austenite (γ)/ferrite (α) interface during γ → α decomposition resulting in densely packed sheets of precipitates. The precipitation strengthening due to interphase precipitation in some recent steel designs has been estimated to be over 300 MPa, which is two or three times higher than the precipitation strengthening obtained in more conventional microalloyed steels. This study investigated the influence of austenite strain accumulation on γ → α kinetics, microstructural development, and interphase precipitation within polygonal ferrite using a low carbon, titanium-molybdenum microalloyed steel. Deformation dilatometry was performed to study γ → α kinetics after different levels of austenite strain accumulation and associated microalloy precipitation. Austenite conditioning was performed above and below the non recrystallization temperature (Tnr). Greater austenite strain accumulation resulted in accelerated γ → α kinetics. Microalloy precipitation within polygonal ferrite was investigated with transmission electron microscopy (TEM) using specimens constructed from regions exhibiting incoherent γ (martensite)/α interfaces with focused ion beam (FIB) techniques. Interphase precipitation was observed after both austenite conditioning simulations, where relatively slower γ → α kinetics resulted in a finer interphase precipitation sheet spacing compared to faster γ → α kinetics. Multipass hot torsion testing was performed to study the influence of extensive differences in austenite strain accumulation on microstructural development and associated microalloy precipitation. Again, austenite conditioning was performed above and below the Tnr. The amount of imposed true strain was between approximately 2.76 and 4.97, where three radial positions were investigated. A metallographic technique was used to quantify austenite strain accumulation. Austenite conditioning above the Tnr resulted in negligible austenite strain accumulation, while austenite conditioning below the Tnr accumulated roughly 0.85 true strain. Extensive austenite strain accumulation resulted in enhanced austenite recrystallization and substantial refinement in prior austenite grain size. Isothermal holding was performed after austenite conditioning to simulate coiling. Extensive austenite strain accumulation was required to achieve fine, homogeneous microstructures of polygonal ferrite desired for the application of this steel, avoiding small amounts of hard, secondary phase constituents. Microalloy precipitation was investigated with TEM, where specimens were constructed from polygonal ferrite grains that exhibited a near <100>α grain normal using FIB techniques so that two of the three possible Baker-Nutting orientation relationship variants could be imaged within a given specimen. Interphase precipitation was inferred after transformation following austenite conditioning above the Tnr (based on variant selection) and directly observed after transformation following austenite conditioning below the Tnr. Greater austenite strain accumulation resulted in a less regular interphase precipitation morphology (i.e. incomplete sheets, localized variations in sheet spacing, and relatively few sheets). Tested hole-expansion samples of industrially produced material were examined to relate microstructure and precipitation behavior with hole expansion performance. The samples exhibited a variation in hole-expansion performance despite being taken from the same respective hot-rolled coil. Fractography, uniaxial tensile testing, and multiple microstructural analysis techniques were used to investigate the samples. Differences in microalloy precipitation were likely not responsible for the variation in measured hole-expansion performance, while small differences in hard, secondary phase constituents and non-metallic inclusions may have contributed to the reported variation.