/pdf/journal-de-physique-febbraio-marzo-e-aprile-1895-1jh9b3fj7b.pdf

Journal de Physique: Febbraio, Marzo e Aprile 1895

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

Disclosed is a high-strength steel sheet having excellent processability and a tensile strength of 980 MPa or more. The high-strength steel sheet has the following composition (in mass%): C: 0.1 to 0.3% inclusive, Si: 2.0% or less, Mn: 0.5 to 3.0% inclusive, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less, with the remainder being Fe and unavoidable impurities. The steel structure of the high-strength steel sheet comprises 50% or more of martensite, 50% or less (including 0%) of ferrite, 10% or less (including 0%) of bainite, and 10% or less (including 0%) of residual austenite. In the high-strength steel sheet, the half width in the degree distribution of the nano-hardness obtained by the measurement of the hardness distribution of the martensite is 2.0 GPa or more, and the steel sheet has a tensile strength of 980 MPa or more.

High strength steel sheet and method for manufacturing the same

CMnSi steel grades with carbon contents ranging from 0.2 to 0.3 wt% and manganese contents of 3 and 5 wt% were Quenched and Partitioned (Q&P). Tensile properties were assessed and retained austenite fractions measured. Intercritically annealed and fully austenitized conditions were studied. The best combinations of tensile strength and total elongation obtained in the 0.2C–3Mn–1.6Si grade after intercritical annealing were associated with strength levels in the 1000–1200 MPa range and total elongations ranging from 14 to 20%. Optimum properties were obtained in the 0.3C–3Mn–1.6Si steel after full austenitization with tensile strength levels ranging from 1450 to 1700 MPa and total elongations ranging from 11 to 18%. The 0.2C–3Mn–1.6Si fully austenitized samples also exhibited remarkable strength/ductility combinations albeit at lower strength levels of 1200–1450 MPa UTS with 9–15% total elongation indicating the effectiveness of the manganese addition to develop novel property combinations.

Effect of Carbon and Manganese on the Quenching and Partitioning Response of CMnSi Steels

Quenching and partitioning (QP property advancements continue to be made through research on this emerging technology. Early investigations [1] also proposed a corresponding thermodynamic model for Q&P steel and its heat treatment, which is now referred to as constrained carbon equilibrium [3]. Since first proposed in 2003, Q&P steel has gained interest for its potential to enhance properties of strength and ductility with compositions similar to transformationinduced plasticity (TRIP) steel and has been proposed as a third-generation automotive steel (Fig. 1) [4]. Many researchers [5–17] have investigated the relationship between properties and microstructures of Q&P steels subjected to various heat treatments and showed that the ultrahigh strength of Q&P steel results from martensite laths, while its good ductility is attributed to TRIP-assisted behavior of retained austenite during deformation. De Moor et al. [14] examined the stability of retained austenite and showed that the TRIP effect occurs in Q&P steels, thereby effectively contributing to the significant strain hardening. Santofimia et al. [15, 16] and Takahama et al. [17] analyzed microstructural evolution during annealing Editor’s Note The following is a preview chapter from the upcoming volume Steel Heat Treating Fundamentals and Processes, Volume 4A, ASM Handbook, Jon Dossett and George Totten, editors. The volume is scheduled for publication later this year.

https://link.springer.com/content/pdf/10.1007%2Fs13632-013-0082-8.pdf

Quenching and Partitioning Steel Heat Treatment

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

A new thermal processing route, Quench and Partitioning (Q&P), has been proposed recently as a way to produce high-strength martensitic steels with good ductility or toughness. It consists of quenching a steel to a suitable temperature and subsequently reheating it to a higher temperature, the so called partitioning temperature. During the latter heat treatment, carbon is rejected from a carbon supersaturated martensitic microstructure into the remaining austenite, stabilizing the latter. The usual formation of carbides on tempering of martensite is suppressed by the presence of Si and/or Al. In the present contribution, the Q&P process is studied by means of dilatometry and differential scanning calorimetry (DSC). The effect of Si and Al on tempering is studied and the implications for Q&P processing are adressed.

https://biblio.ugent.be/publication/338369/file/455941.pdf

S. Lacroix

Papers

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

Dilatometric Study of the Quench and Partitioning Process