Third-generation Advanced High-strength Steels Research Articles

Behavior of ultrafast annealed advanced high strength steels under static and dynamic conditions

Ultrafast annealing (UFA) is considered a feasible alternative processing route for third-generation advanced high strength steels (AHSSs). In fact, significant grain refinement occurs in low carbon steels (0.1 wt%) when heating rates (example, 1000 K/s) considerably higher than the conventional heating rates are applied. Additionally, high heating rates result in multiphase microstructures containing ferrite, retained austenite, and martensite/bainite. The mixture of structural constituents is induced by carbon gradients resulting from the short time available for diffusion during ultrafast annealing of the steel. Quasi-static and high strain rate tensile tests, with strain rates ranging from 0.0033 s−1 to 600 s−1, revealed that the tensile strength of both conventional and UFA samples increases with increasing strain rates, whereas the strain to fracture decreases. However, compared with the conventionally heat-treated samples (heating rate: 10 K/s), the UFA samples exhibited less ductility deterioration at high strain rates. The samples were investigated using light optical microscopy, scanning electron microscopy, electron backscatter diffraction (EBSD), and X-ray diffraction. The results indicated that the improved tensile properties of the UFA steels (compared with those of the conventional steels) resulted mainly from the presence of retained austenite (up to 5%) in conjunction with a lower carbon martensite/bainite fraction. Owing to adiabatic heating associated with dynamic conditions, the stability of retained austenite shifted to higher strain values than those encountered under static conditions. The TRIP effect of the UFA samples appeared at higher strain values than in the conventionally heat-treated samples. The delayed transformation is considered essential for realizing improved preservation of the deformation capacity and, in turn, improved crashworthiness.

Mechanical property development of a 0.15C–6Mn–2Al–1Si third-generation advanced high strength steel using continuous galvanizing heat treatments

The effects of starting microstructure and intercritical annealing (IA) temperature on the mechanical properties of a prototype 0.15C–6Mn-1.5Al–1Si (wt%) third-generation medium manganese advanced high strength steel (3G AHSS) were determined. The general 3G AHSS mechanical property targets UTS × TE of 24,000–40,000 MPa% were met after a continuous galvanizing line (CGL)-compatible IA treatments at 710 °C and 740 °C for 120 s from an 80% martensite – 20% ferrite starting microstructure due to the as-annealed microstructure having large volume fractions of stable retained austenite. The deformation-induced retained austenite to martensite transformation – or TRansformation-induced plasticity (TRIP) – kinetics were a strong function of IA temperature and resulting retained austenite stability. In particular, the 665 °C intercritical anneal produced retained austenite which only partially transformed during deformation and the 740 °C intercritical anneal produced retained austenite which exhausted the TRIP effect at significantly lower strains than those observed for the 710 °C treatment. It was also shown that significant deformation-induced mechanical twinning occurred in the retained austenite, aiding in sustaining the work hardening rate. Overall, it was determined that medium-Mn 3G AHSS can be produced using CGL-compatible thermal cycles, which holds promise for cost-effective production of these alloys.

Austenite Stability and Strain Hardening in C-Mn-Si Quenching and Partitioning Steels

Quenching and partitioning (Q&P) processing of third-generation advanced high strength steels generates multiphase microstructures containing metastable retained austenite. Deformation-induced martensitic transformation of retained austenite improves strength and ductility by increasing instantaneous strain hardening rates. This paper explores the influence of martensitic transformation and strain hardening on tensile performance. Tensile tests were performed on steels with nominally similar compositions and microstructures (11.3 to 12.6 vol. pct retained austenite and 16.7 to 23.4 vol. pct ferrite) at 980 and 1180 MPa ultimate tensile strength levels. For each steel, tensile performance was generally consistent along different orientations in the sheet relative to the rolling direction, but a greater amount of austenite transformation occurred during uniform elongation along the rolling direction. Neither the amount of retained austenite prior to straining nor the total amount of retained austenite transformed during straining could be directly correlated to tensile performance. It is proposed that stability of retained austenite, rather than austenite volume fraction, greatly influences strain hardening rate, and thus controls strength and ductility. If true, this suggests that tailoring austenite stability is critical for optimizing the forming response and crash performance of quenched and partitioned grades.

Materials and Processes for the Third-generation Advanced High-strength Steels

The third-generation advanced high-strength steels (AHSS 3. Gen.) have been developed in order to meet the requirements of the automotive industry for weight reduction, improved fuel efficiency, and CO2 mitigation. The alloy design of AHSS 3. Gen., their microstructure characteristics, mechanical properties, potential applications as well as the need for thorough process parameter control are discussed in this paper. The cold-rolled AHSS 3. Gen. can be differentiated as quenching and partitioning (Q&P) steels and medium-Mn steels (MMnS). Specific to these materials are the occurrence of different phases with typical sizes on the nm-scale, the elemental partitioning between these phases and the impact of the metastable austenite phase on the mechanical properties by means of transformation-induced-plasticity (TRIP) as well as of twinning-induced-plasticity (TWIP) effects. The requirements for the process design and the new annealing concepts are emphasized. Furthermore, the industrial feasibility and challenges of production of cold-rolled high-strength steels are discussed. In particular, the development of new furnace technologies is introduced for modern steels with complex microstructures.

The Evaluation of Laser Weldability of the Third-Generation Advanced High Strength Steel

To meet the demands of vehicular safety and greenhouse gas emission reduction, the automotive industry is increasingly using advanced high strength steels (AHSS) in the production of the components. With the development of the new generation of AHSS, it is essential to study their behavior towards manufacturing processes used in the automotive industry. For this purpose, the welding capability of newly developed third-generation Gen3 980T steel was investigated using the Nd:YAG (Neodymium:Yittrium Aluminum Garnet) laser-welding with different parameter conditions. The analysis was made by uniaxial tensile tests, micro-hardness, Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The criteria used to evaluate the quality of the weld were the distance between the fracture and the weld bead and the surface finish. A relationship between the quality of the weld and the energy density was observed, expressed by a partial penetration for values below the optimal, and by irregularities in the weld bead and a high number of spatters for the values above the optimal.

Effects of Hot Band Annealing on the Mechanical Properties of Cold-Rolled and Intercritically Annealed Medium Manganese Steels

The effects of intercritical annealing, before and after cold rolling, on retained austenite volume fractions, austenite stability, and tensile properties of a 7.39Mn-0.14C-1.55Al-0.2Si (wt pct) medium Mn sheet steel were evaluated. The extent of hot band annealing at 650 °C prior to cold rolling is shown to be an important parameter to consider in the development of processing histories leading to high strength–ductility combinations required for third-generation advanced high-strength steels.

Analysis of Hot- and Cold-Rolled Loads in Medium-Mn TRIP Steels

The purpose of this work is to investigate the hot- and cold-rolling requirements to produce third-generation advanced high-strength steels (AHSS). Therefore, five medium-Mn (10 to 14 wt pct Mn) alloys that exhibit transformation-induced plasticity (TRIP) were compared to a commercially produced grade of AISI 1018 using hot- and cold-rolling experiments. Experimental data collected from a STANAT instrumented rolling mill was utilized to measure force and torque during hot- and cold-rolling. Experimental data were processed by a 1D analytical model, based on Orowan model, to determine rolling pressure. It was determined that pressures required to hot-roll TRIP alloys are 1.4 to 1.8 times greater than pressures for rolling AISI 1018 steel. Cold rolling of the medium-Mn TRIP steels was found to be 1.5 to 2.8 times greater than the AISI 1018 steel. Mechanical and microstructural characterization was also performed and the variation in rolling pressure was related to the starting microstructural constituents, and alloys containing greater starting quantities of e-martensite in the microstructure had higher flow stresses at equivalent rolling strains during cold rolling.

Design of Fully Austenitic Medium Manganese Steels

Due to their higher ferrite potential compared to high Mn twinning-induced plasticity (TWIP) steels, medium Mn steels usually exhibit austenitic-ferritic microstructures, which makes them suitable for third-generation advanced high-strength steel applications. Nevertheless, the strain hardening characteristics of medium Mn steels are inferior to those of fully austenitic high Mn steels. The present work introduces alloy design strategies to obtain fully austenitic medium Mn steels capable of the TWIP effect. To achieve a fully austenitic microstructure, the martensite start temperature is reduced by raising the C concentration to above 1 mass-%, which in turn facilitates the formation of cementite. The formation of cementite during cooling from austenitization temperature is counteracted by alloying with Al. Microstructural examination of slowly-cooled Fe−Mn−Al−C and Fe−Mn−C steels indicated that Al changes the morphology of intergranular cementite from plate-shaped to equiaxed.

In Situ Local Measurement of Austenite Mechanical Stability and Transformation Behavior in Third-Generation Advanced High-Strength Steels

Austenite mechanical stability, i.e., retained austenite volume fraction (RAVF) variation with strain, and transformation behavior were investigated for two third-generation advanced high-strength steels (3GAHSS) under quasi-static uniaxial tension: a 1200 grade, two-phase medium Mn (10 wt pct) TRIP steel, and a 980 grade, three-phase TRIP steel produced with a quenching and partitioning heat treatment. The medium Mn (10 wt pct) TRIP steel deforms inhomogeneously via propagative instabilities (Luders and Portevin Le Châtelier-like bands), while the 980 grade TRIP steel deforms homogenously up to necking. The dramatically different deformation behaviors of these steels required the development of a new in situ experimental technique that couples volumetric synchrotron X-ray diffraction measurement of RAVF with surface strain measurement using stereo digital image correlation over the beam impingement area. Measurement results with the new technique are compared to those from a more conventional approach wherein strains are measured over the entire gage region, while RAVF measurement is the same as that in the new technique. A determination is made as to the appropriateness of the different measurement techniques in measuring the transformation behaviors for steels with homogeneous and inhomogeneous deformation behaviors. Extension of the new in situ technique to the measurement of austenite transformation under different deformation modes and to higher strain rates is discussed.

Validation of GISSMO Model for Fracture Prediction of a Third-Generation Advanced High-Strength Steel

Validation of GISSMO Model for Fracture Prediction of a Third-Generation Advanced High-Strength Steel

Development of a Lightweight Third-Generation Advanced High-Strength Steel (3GAHSS) Vehicle Body Structure

Development of a Lightweight Third-Generation Advanced High-Strength Steel (3GAHSS) Vehicle Body Structure

Simulation of the Growth of Austenite from As-Quenched Martensite in Medium Mn Steels

As part of an ongoing development of third-generation advanced high-strength steels with acceptable cost, austenite reversion treatment of medium Mn steels becomes attractive because it can give rise to a microstructure of fine mixture of ferrite and austenite, leading to both high strength and large elongation. The growth of austenite during intercritical annealing is crucial for the final properties, primarily because it determines the fraction, composition, and phase stability of austenite. In the present work, the growth of austenite from as-quenched lath martensite in medium Mn steels has been simulated using the DICTRA software package. Cementite is added into the simulations based on experimental observations. Two types of systems (cells) are used, representing, respectively, (1) austenite and cementite forming apart from each other, and (2) austenite forming on the cementite/martensite interface. An interfacial dissipation energy has also been added to take into account a finite interface mobility. The simulations using the first type of setup with an addition of interfacial dissipation energy are able to reproduce the observed austenite growth in medium Mn steels reasonably well.

Automotive Industry Partnership Paves the Way for Advanced High-Strength Steel

Abstract Scientists and engineers working on behalf of the U.S. automotive industry are nearing completion of a multiyear effort to accelerate the incorporation of advanced high-strength steels in American-made cars and trucks. This article reports on work to develop and validate an integrated computational materials engineering (ICME) model optimized for third-generation advanced high-strength steels.

Relationship between Microstructure and Mechanical Properties in Q&P-Steels

Third-Generation advanced high strength steels are being developed with the goal of reducing the body-in-white weight while simultaneously increasing passenger safety. This requires not only the expected increase in strength and elongation, but also improved local formability. Optimizing elongation and formability were often contradictory goals in dual-phase steel developments. Recent results have shown that so-called "quench and partitioning" (Q&P) concepts can satisfy both requirements [1]. Many Q&P-concepts have been studied at thyssenkrupp Steel Europe. Thorough investigation of the microstructure has revealed relationships between features such as the amount, morphology and chemical stability of the retained austenite and the obtained mechanical properties. An evaluation of the lattice strain by means of electron-back-scattering-diffraction has also yielded a correlation to the obtained formability. The aim of this work is to present the interconnection between these microstructural features and propose hypotheses for the explanation of how these features influence the macroscopically observed properties.

Ultrahigh Ductility, High-Carbon Martensitic Steel

Based on the proposed design idea of the anti-transformation-induced plasticity effect, both the additions of the Nb element and pretreatment of the normalization process as a novel quenching–partitioning–tempering (Q–P–T) were designed for Fe-0.63C-1.52Mn-1.49Si-0.62Cr-0.036Nb hot-rolled steel. This high-carbon Q–P–T martensitic steel exhibits a tensile strength of 1890 MPa and elongation of 29 pct accompanied by the excellent product of tensile and elongation of 55 GPa pct. The origin of ultrahigh ductility for high-carbon Q–P–T martensitic steel is revealed from two aspects: one is the softening of martensitic matrix due to both the depletion of carbon in the matensitic matrix during the Q–P–T process by partitioning of carbon from supersaturated martensite to retained austenite and the reduction of the dislocation density in a martensitic matrix by dislocation absorption by retained austenite effect during deformation, which significantly enhances the deformation ability of martensitic matrix; another is the high mechanical stability of considerable carbon-enriched retained austenite, which effectively reduces the formation of brittle twin-type martensite. This work verifies the correctness of the design idea of the anti-TRIP effect and makes the third-generation advanced high-strength steels extend to the field of high-carbon steels from low- and medium-carbon steels.