Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. The steels possess a single α′ martensite phase in hot and cold rolled states and multiphases after intercritical annealing. Many studies have focused on investigating the influences of chemical composition and annealing conditions on the microstructure, particularly the grain size and retained γ (γR), and on the tensile properties. The steels exhibit high strength and good ductility due to transformation induced plasticity occurring in γR, whose volume fraction is approximately 0·2–0·4. The present review summarises the important results of previous studies about the effects of both intercritical annealing conditions and alloying elements on the microstructure and tensile properties of medium Mn steels.

Current opinion in medium manganese steel

A novel heat-treating process, quench and partitioning (Q&P), has been proposed as a fundamentally new way to produce martensitic microstructures containing retained austenite. The two-step process hypothesizes carbon enrichment of the austenite by decarburization of the martensite. Significant amounts of retained austenite have been measured in the final microstructure, although evidence for transition carbide formation in the martensite also exists. The mechanical properties obtained via Q&P are reported for a CMnAlSiP steel after intercritical annealing for A50 specimens. Tensile strength/total elongation combinations, ranging from 800 MPa/>25 pct to 900 MPa/20 pct to 1000 MPa/10 pct, indicate that Q&P is a viable way to produce high strength steel grades with good ductility. The instantaneous strain hardening of Q&P steels shows a significant dependence on the partitioning conditions applied. Lower partitioning temperature (PT) leads to continuously decreasing instantaneous n-values with strain, similar to the strain hardening behavior observed for dual-phase (DP) steels, whereas higher PTs for the same partitioning time increase the strain hardening significantly. After an initial increase, the observed n-values remain high up to considerable amounts of strain, resulting in similar strain hardening behavior observed for austempered transformation-induced plasticity (TRIP) grades. Assessment of the mechanical stability of the retained austenite indicates that the TRIP effect is effectively contributing to the increased strain hardening as function of strain.

Effect of Retained Austenite Stabilized via Quench and Partitioning on the Strain Hardening of Martensitic Steels

The transformation-induced plasticity (TRIP) in advanced high-strength steels (AHSS) is reviewed, where the main concepts and the recent progress in the processing and properties of AHSS are introduced. The metastable austenitic stainless and multiphase TRIP-assisted steels, as well as the more recent third generation AHSS grades, namely the medium-Mn and quenching and partitioning (Q&P) steels, are critically discussed. These steels utilize the TRIP effect and the enhanced work-hardening rate through the transformation of (retained) austenite in their microstructures to martensite during plastic deformation for the improvement of strength-ductility balance, which make them especially suitable for the automotive industry to be used in the lightweight car body for addressing the safety, fuel consumption, and air pollution issues. The kinetics of strain-induced martensitic transformation (SIMT) as well as the effects of chemical composition, grain size, deformation temperature, strain rate, and deformation mode on the austenite stability are reviewed. The effects of holding temperature and time during the isothermal bainitic transformation (IBT) in TRIP-aided steels, during the austenite-reverted-transformation (ART) annealing in medium-Mn steels, and during the quenching and partitioning steps in the Q&P steels are critically discussed towards enhancement of the amount of retained austenite and optimization of strength-ductility trade off. The alternative thermomechanical processing routes as well as the modified grades such as δ-TRIP and quenching-partitioning-tempering steels are also introduced.

Transformation-induced plasticity (TRIP) in advanced steels: A review

Thermomechanical processing of advanced high strength steels

Extensive research efforts are underway globally to develop new steel microstructure concepts for high-strength sheet products, driven largely by the need for lightweight automotive structures in support of designs to enhance occupant safety and energy efficiency. One promising approach, involving the quenching and partitioning (Q&P) process, was introduced in the predecessor to this paper series, Austenite Formation and Decomposition, 2003.[1] Development of the Q&P process has continued through to the present, and the current status is highlighted in this article, along with some alternative approaches that are also receiving attention. Special emphasis is placed on the synthesis and interpretation of the fundamental phase transformation responses, perspectives related to alloying and processing, and the resulting microstructure and properties. Key mechanistic issues are discussed, including carbide formation and suppression, migration of the martensite/austenite interface, carbon partitioning, and partitioning kinetics.

Analysis of Microstructure Evolution in Quenching and Partitioning Automotive Sheet Steel

A novel concept for the heat treatment of martensite, different to customary quenching and tempering, is described. This involves quenching to below the martensite-start temperature and directly ageing, either at, or above, the initial quench temperature. If competing reactions, principally carbide precipitation, are suppressed by appropriate alloying, the carbon partitions from the supersaturated martensite phase to the untransformed austenite phase, thereby increasing the stability of the residual austenite upon subsequent cooling to room temperature. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering, and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Another approach that has been used to produce austenite-containing microstructures is by alloying to suppress carbide precipitation during the formation of bainitic structures, and interesting comparisons can be made between the two approaches. Moreover, formation of carbide-free bainite during the Q&P partitioning treatment may be a reaction competing for carbon, although this could also be used constructively as an additional stage of Q&P partitioning to form part of the final microstructure. Amongst the ferrous alloys examined so far are medium carbon bar steels and low carbon formable TRIP-assisted sheet steels.

Quenching and partitioning martensite-a novel steel heat treatment

The microstructure following a new martensite heat treatment has been examined, principally by high-resolution microanalytical transmission electron microscopy and by atom probe tomography. The new process involves quenching to a temperature between the martensite-start (Ms) and martensite-finish (Mf) temperatures, followed by ageing either at or above, the initial quench temperature, whereupon carbon can partition from the supersaturated martensite phase to the untransformed austenite phase. Thus the treatment has been termed ‘Quenching and Partitioning’ (Q&P). The carbon must be protected from competing reactions, primarily carbide precipitation, during the first quench and partitioning steps, thus enabling the untransformed austenite to be enriched in carbon and largely stabilised against further decomposition to martensite upon final quenching to room temperature. This microstructural objective is almost directly opposed to conventional quenching and tempering of martensite, which seeks to eliminate retained austenite and where carbon supersaturation is relieved by carbide precipitation. This study focuses upon a steel composition representative of a TRIP-assisted sheet steel. The Q&P microstructure is characterised, paying particular attention to the prospect for controlling or suppressing carbide precipitation by alloying, through examination of the carbide precipitation that occurs.

Microstructural Features of 'Quenching and Partitioning': A New Martensitic Steel Heat Treatment

The Quenching and Partitioning (Q&P) process is a steel heat treatment to create microstructures with retained austenite [1,2]. It involves quenching austenite to between the martensite start and finish temperatures, followed by a partitioning treatment to enrich the untransformed austenite with carbon, thereby stabilizing it to room temperature. Carbon partitioning to austenite, however, is competitive with carbide precipitation [2] and so Si and/or Al alloying is suggested and employed in order to suppress cementite formation to protect carbon. However, previous studies show that alloying is ineffective against transitional epsilon carbide precipitation [2]. Consequently, it is important to understand the effects of Si and Al on tempering behaviour relevant to a Q&P treatment of steel. Microanalytical electron microscopy has thus been used to determine the compositions of epsilon carbide and cementite and the epsilon to cementite transition during tempering of quenched Si- and Al-containing high-purity Fe-C alloys.

Martensite tempering behaviour relevant to the quenching and partitioning process

The Quenching and Partitioning (QP exceptionally high carbon concentration of austenite fraction, which is decoupled from austenite decomposition as compared with carbon partitioning during the bainite transformation; fast processing limited only by the rapid diffusion of carbon.

Microstructural characterisation of steel heat-treated by the novel quenching and partitioning process

We presented a theoretical study for the structural, mechanical, and thermophysical properties of the precipitates in 2xxx series aluminum alloy by applying the widely used density functional theory of Perdew-Burke-Ernzerhof (PBE). The results indicated that the most thermodynamically stable structure refers to the Al3Zr phase in regardless of its different polymorphs, while the formation enthalpy of Al5Cu2Mg8Si6 is only -0.02 eV (close to zero) indicating its metastable nature. The universal anisotropy index of AU follows the trend of: Al2Cu > Al2CuMg ≈ Al3Zr_D022 ≈ Al20Cu2Mn3 > Al3Fe ≈ Al6Mn > Al3Zr_D023 ≈ Al3Zr_L12 > Al7Cu2Fe > Al3Fe2Si. The thermal expansion coefficients (TECs) were calculated based on Quasi harmonic approximation (QHA); Al2CuMg shows the highest linear thermal expansion coefficient (LTEC), followed by Al3Fe, Al2Cu, Al3Zr_L12 and others, while Al3Zr_D022 is the lowest one. The calculated data of three Al3Zr polymorphs follow the order of L12 > D023 > D022, all of them show much lower LTEC than Al substance. For multi-phase aluminum alloys, when the expansion coefficient of the precipitates is quite different from the matrix, it may cause a relatively large internal stress, or even produce cracks under actual service conditions. Therefore, it is necessary to discuss the heat misfit degree during the material design. The discrepancy between a-Al and Al2CuMg is the smallest, which may decrease the heat misfit degree between them and improve the thermal shock resistant behaviors.

/pdf/theoretical-prediction-of-structural-mechanical-and-3ap97n90.pdf

K. He

Papers

Quenching and partitioning martensite-a novel steel heat treatment

Microstructural Features of 'Quenching and Partitioning': A New Martensitic Steel Heat Treatment

Martensite tempering behaviour relevant to the quenching and partitioning process

Microstructural characterisation of steel heat-treated by the novel quenching and partitioning process

Theoretical Prediction of Structural, Mechanical, and Thermophysical Properties of the Precipitates in 2xxx Series Aluminum Alloy