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

Over many decades, significant efforts have been made to improve the strength-elongation product of advanced high strength steels (AHSSs) by creating tailored multi-phase microstructures. Successive solid-state phase transformations for steels with a well selected chemical composition turned out to be the key instrument in the realisation of such microstructures. In this contribution, we first provide a brief review of the desired microstructures for Transformation-induced plasticity (TRIP), Carbide-free Bainitic (CFB), Quenching & Partitioning (Q&P) and Medium Manganese steels followed by comprehensive discussions on the phase transformations to be used in their creation. The implications for the steel composition to be selected are addressed too. As the presence of the right amount and type of metastable retained austenite (RA) is of crucial importance for the mechanical performance of these AHSSs, special attention is paid to the important role of successive solid-state phase transformations in creating the desired fraction and composition of RA by suitable element partitioning (in particular C and Mn). This critical partitioning not only takes place during final cooling (austenite decomposition) but also during the back transformation (austenite reversion) during reheating. This review aims to be more than just descriptive of the various findings, but to present them from a coherent thermodynamic / thermo-kinetic perspective, such that it provides the academic and industrial community with a rather complete conceptual and theoretical framework to accelerate the further development of this important class of steels. The detailed stepwise treatment makes the review relevant not only for experts but also metallurgists entering the field.

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Fundamentals and application of solid-state phase transformations for advanced high strength steels containing metastable retained austenite

We have studied here the carbon partitioning during continuous cooling at different cooling rates using dilatometer. An interesting phase transition was observed involving multiple stages of transformation during dynamic partitioning and a mechanism was proposed to explain the observations. Two morphologies of nano-sized retained austenite were observed, film-like and block-type. It was proposed that the amount of retained austenite was stable in a certain range of cooling rate. When the cooling rate was between 0.05 °C/s and 1 °C/s, the carbon partitioning was adequate and ~ 11 to 14% retained austenite was obtained. With increase in cooling rate from 5 and 10 °C/s, carbon partitioning was not adequate, which resulted in decrease in film-like austenite. The austenite adjacent to ferrite had two different transformation products, M/A (martensite-austenite island) and twin martensite, based on the degree of carbon transfer from ferrite. However, macro-hardness test showed that cooling rate had little effect on hardness and the hardness was between 398 HV and 407 HV at cooling rate of ~ 0.05 to 10 ℃/s. Additionally, samples cooled at low cooling rate indicated continuous TRIP effect and excellent combination of high strength 1030 MPa and high elongation of ~ 25% was obtained in the plate cooled at 0.1 ℃/s. On the basis of results, a strategy was proposed to obtain high performance hot rolled Q&P steels. The study confirmed the viability of implementing quenching and partitioning process in the hot rolling production line.

A novel phase transition behavior during dynamic partitioning and analysis of retained austenite in quenched and partitioned steels

Microstructure evolution and enhanced performance of a novel Nb-Mo microalloyed medium Mn alloy fabricated by low-temperature rolling and warm stamping

Advanced high strength steels for the auto industry are in high demand regarding the development of new type of steels, reliable and economic welding techniques, etc. In this study, the third generation advanced high strength steels, cold-rolled quenching and partitioning steels (QP980), were butt-welded using fiber laser to evaluate the microstructure, mechanical properties and formability of joint. The fusion zone composed of lath martensite with high microhardness (499 HV). Martensite tempering and carbide precipitation were found at the sub-critical heat affected zone, resulting in a slight decrease of microhardness (~ 37 HV drop). The presence of joint had no effect on tensile strength and all welded samples failed at the base metal with 98.8% joint efficiency. The formability of welded blanks in Erichsen cupping test was dependent on the weld line position. The formability ratio was only 68.08% of base metal when weld line offset was 0 mm, and increased as the weld line moving away from the blank center because more deformation was transferred to base metal. Stretch-flange-formability of the joint in hole expanding test was inferior to that of base metal due to the high volume fraction of coarse martensite in fusion zone.

Microstructural evolution and deformation behavior of fiber laser welded QP980 steel joint

A C-Mn-Si steel with lean microalloy elements was treated by a series of quenching and partitioning (Q&P) processes after a full or intercritical austenitization (hereinafter referred as F-QP and I-QP process, respectively). The discrepancy in microstructure and mechanical properties of the samples under these two heat treatment processes was investigated. The results indicate that the austenite grains are mainly film-like in the F-QP samples, while blocky austenite grains account for the majority in the I-QP samples. And the later contains a larger amount of retained austenite than the former. In addition, the reason why the austenite fraction decreases with the increasing of partitioning time after the peak value can be related to the bainite transformation due to the dynamic change of phase region in the continue cooling transformation (CCT) diagram of untransformed austenite with carbon enrichment. The I-QP samples show higher product of strength and elongation (PSE) but relatively lower strength, while the F-QP samples are just the reverse. Notably, the one-step I-QP samples show an excellent strength–ductility balance, of which the tensile strength and total elongation are 1140–1280 MPa and 17–22%, respectively. The exponential function and Olson-Cohen (O-C) model were utilized to fit the variation of retained austenite fraction with strain. By comparison, retained austenite in the I-QP sample is more sensitive to the increasing strain, i.e., lower mechanical stability.

Comparative study on microstructure and mechanical properties of a C-Mn-Si steel treated by quenching and partitioning (Q&P) processes after a full and intercritical austenitization

Lizhong Liu

Papers

Comparative study on microstructure and mechanical properties of a C-Mn-Si steel treated by quenching and partitioning (Q&P) processes after a full and intercritical austenitization