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Processing and microstructure design of medium-manganese low-density steels for automotive and transportation applications
Scuseria, Tomás
Scuseria, Tomás
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2024
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2025-10-04
Abstract
With the automotive industry under pressure to reduce greenhouse gas emissions, vehicle lightweighting has become a pivotal technology. In addition to sheet downgauging of advanced high-strength steels (AHSS) used in vehicle body structures, mass savings can be achieved by reducing density through alloying. Low-density steels encompass a broad class of Fe-Mn-Al-C-based alloys with decreased densities (–1.3 % per wt% Al). With medium-Mn (3–12 wt%) and Al (3.5–10 wt%) additions, they display duplex (austenite + ferrite) or multiphase (+ martensite, carbides) microstructures with an excellent balance of mechanical properties and decreased alloying costs relative to austenitic grades. A wide range of properties can be tuned through intercritical annealing (IA), enabled by numerous strengthening and strain hardening mechanisms essential for crash performance.
Despite this, reduced strengths and damage tolerances have been observed when these steels are processed using laboratory-scale methods that mimic industrial cold- or hot-stamping lines, requiring significant modifications or the development of new processing routes to enhance specific strengths. Al additions promote banded δ-ferrite and increased austenite stabilities, limiting use of as-quenched martensite and transformation-/twinning-induced plasticity (TRIP/TWIP) in metastable austenite. Looking deeper, this complex metallurgy offers interesting pathways to address these challenges. In this work, novel processing and microstructure design of steels with 7–9Mn, 4–6Al, 0.2–0.3C and 0–1Si and Cr (wt%) was explored with intent to promote strength-ductility synergies geared towards enhanced crash resistance and formability.
Initial efforts were concentrated in cold-rolling and IA, finding microstructures and deformation behaviors were sensitive to IA parameters, as elevated temperatures and shorter isothermal hold times promoted optimal properties, characterized using electron backscatter diffraction (EBSD) and quasistatic tensile testing. Additionally, the use of rapid heating rates up to 300 °C/s and omission of the hold step were investigated, observing improved tensile properties attributed to a rapid austenite growth mechanism concurrent with partially recrystallized and refined microstructures.
To further reduce the austenite stability and promote as-quenched martensite, increased IA temperatures are required (e.g., >950 °C), which can promote low yield strengths and early fracture. Cold quenching treatments (–20 or –196 °C) in conjunction with tempering at 180 °C were investigated on specimens previously IA at 950 °C to optimize martensitic microstructures. Tensile testing revealed increases to yield strength (e.g., +341 MPa) with minimal ductility losses (e.g., 4) after cold quenching treatments, attributed to the depletion of unstable austenite in favor of hard martensite.
Large fractions of soft δ-ferrite can limit strengthening pathways, and thus warm-rolling deformation was proposed as a method of dislocation-strengthening while retaining large austenite fractions. An exceptional strength-ductility combination (1380 MPa, 16.5% elongation) was achieved after warm rolling at 400 °C, attributed to the dislocation-rich, deformed martensitic and δ-ferrite. The latter specimen also displayed continuous, uncommon to warm-rolled steels, which served to shed some light on elusive mechanisms responsible for yield point elongation in steels.
Lastly, limited data on application properties such as formability is available. Taking select specimens, plastic anisotropy ratios were calculated in tensile specimens oriented 0, 45, and 90° to rolling directions, finding r-values common to AHSS, and direction-dependent tensile properties intimately tied to the microstructural banding. Notably, increased yield strengths and brittle fracture were noted at 90° to the rolling and banding directions. Overall, this work demonstrates potential in improving the crashworthiness and formability of low-density steels.
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