Medium-manganese Transformation-induced Plasticity Research Articles

Properties of medium-manganese steel processed by laser powder bed fusion: The effect of microstructure in as-built and intercritically annealed state on energy absorption during tensile and impact tests

The presented research was aimed at applying laser powder bed fusion (LPBF) technology and TRIP (Transformation Induced Plasticity) steel with medium Mn content to produce thin-walled parts with enhanced crashworthiness. It was expected that the simultaneous use of the excellent combination of strength and ductility of the new type of medium-Mn steel and the advantages of LPBF technology would allow the development of an alternative to the conventional method of manufacturing the energy absorbing components. The energy absorption capability of LPBF-processed Fe-0.15C-7.5Mn-1.8Al steel was evaluated based on its deformation behavior under monotonic and impact loading. Tensile and Charpy V-notch specimens were fabricated by LPBF with layer thicknesses of 30 and 60 μm, using processing windows set established according to the criteria of low porosity (<0.3%) and good surface quality. Specimens were tested in as-built state and after intercritical annealing at a constant temperature (670 °C), but for different holding times and cooling rates to obtain a large fraction (30–50 vol%) of retained austenite capable of strain-induced martensitic transformation. Based on multiscale microstructure examinations (OM, SEM, EBSD, XRD, EDS) the relationships between the morphology of LPBF-specific microstructures before and after intercritical annealing, as well as between their behavior in tensile and impact test, were analyzed and discussed. For both layer thicknesses, the best combination of strength, elongation and impact toughness was obtained after annealing at 670 °C for 6 h followed by argon cooling. LPBF technology has been shown to be suitable for the fabrication of thin-walled energy-absorbing components from medium-manganese TRIP steel with the potential to reduce fabrication time by doubling the layer thickness compared to standard.

Temperature Effects on Tensile Deformation Behavior of a Medium Manganese TRIP Steel and a Quenched and Partitioned Steel

Third-generation advanced high-strength steels (AHSS) containing metastable retained austenite are being developed for the structural components of vehicles to reduce vehicle weight and improve crash performance. The goal of this work was to compare the effect of temperature on austenite stability and tensile mechanical properties of two steels, a quenched and partitioned (Q&amp;P) steel with a martensite and retained austenite microstructure, and a medium manganese transformation-induced plasticity (TRIP) steel with a ferrite and retained austenite microstructure. Quasi-static tensile tests were performed at temperatures between −10 and 85 °C for the Q&amp;P steel (0.28C-2.56Mn-1.56Si in wt.%), and between −10 and 115 °C for the medium manganese TRIP steel (0.14C-7.14Mn-0.23Si in wt.%). X-ray diffraction measurements as a function of strain were performed from interrupted tensile tests at all test temperatures. For the medium manganese TRIP steel, austenite stability increased significantly, serrated flow behavior changed, and tensile strength and elongation changed significantly with increasing temperature. For the Q&amp;P steel, flow stress was mostly insensitive to temperature, uniform elongation decreased with increasing temperature, and austenite stability increased with increasing temperature. The Olson–Cohen model for the austenite-to-martensite transformation as a function of strain showed good agreement for the medium manganese TRIP steel data and fit most of the Q&amp;P steel data above 1% strain.

Microstructure and dynamic tensile characteristics of dissimilar fiber laser welded advanced high strength steels

The desire for production of anti-crash weight-optimized modern vehicles has resulted in the development of tailor welded blanks (TWBs) of 3rd-generation advanced high strength steels (3G-AHSSs). Therefore, understanding crash response (dynamic loading: strain rate of 1–100 s−1) of TWBs is essential for deploying new classes of AHSSs. The present work investigates dissimilar fiber laser joints between press-hardened 22MnB5 steel - medium manganese transformation induced plasticity (MMn-TRIP), and TRIP1100 steels. Lack of mixing within the fusion zone has been evidenced in the MMn-TRIP and TRIP1100 sides of the joints through retention of considerable austenite within the structure and the non-uniform hardness distribution, respectively. It was found that the inter-critical heat-affected zone of the MMn-TRIP side of the joints with a large volume fraction of austenite is responsible for the initiation of Lüders bands (discontinuous yield). The detailed digital image correlation technique demonstrated that the rapid decomposition of martensite in the heat-affected zone of the 22MnB5, and TRIP1100 sides of the joints localizes the plastic flow. Although the strength of the dissimilar joints showed positive strain rate sensitivity, however, under the higher applied strain rates elongation to fracture shows a transition from positive to negative strain rate sensitivity as a consequence of the suppression of the TRIP effect.

Influence of the Quenching and Partitioning Process on the Transformation Kinetics and Hardness in a Lean Medium Manganese TRIP Steel

The quenching and partitioning (Q&amp;P) process of lean medium Mn steels is a novel approach for producing ultra-high strength and good formable steels. First, the steel is fully austenitized, followed by quenching to a specific quenching temperature (TQ) in order to adjust an appropriate amount of initial martensite (α’initial). Subsequently, the steel is reheated to a partitioning temperature (TP) in order to ensure C-partitioning from α’initial to remaining austenite (γremain) and thus retained austenite (RA) stabilization. After isothermal holding, the steel is quenched to room temperature (RT), in order to achieve a martensitic-austenitic microstructure, where the meta-stable RA undergoes the strain-induced martensitic transformation by the so-called transformation induced plasticity (TRIP) effect. This paper systematically investigates the influence of the Q&amp;P process on the isothermal bainitic transformation (IBT) kinetics in a 0.2C-4.5Mn-1.3Al lean medium Mn steel by means of dilatometry. Therefore, the Q&amp;P annealing approach was precisely compared to the TRIP-aided bainitic ferrite (TBF) process, where the samples were directly quenched to the temperature of the IBT after full austenitization. The results indicated an accelerated IBT for the Q&amp;P samples, caused by the formation of α’initial during quenching below the martensite start (MS) temperature. Furthermore, a significant influence of the annealing parameters, such as TQ and TP, was observed with regard to the transformation behavior. For further characterization, light optical microscopy (LOM) and scanning electron microscopy (SEM) were applied, showing a microstructure consisting of a martensitic-bainitic matrix with finely distributed RA islands. Saturation magnetization method (SMM) was used to determine the amount of RA, which was primarily depending on TQ. Furthermore, the hardness according to Vickers revealed a remarkable impact of the annealing parameters, such as TQ and TP, on the predicted mechanical properties.

Strain Rate Effect on Tensile Flow Behavior and Anisotropy of a Medium-Manganese TRIP Steel

The dependence of the plastic anisotropy on the nominal strain rate for a medium-manganese (10 wt.% Mn) transformation-induced plasticity (TRIP) steel with initial austenite volume fraction of 66% (balance ferrite) has been investigated. The material exhibited yield point elongation, propagative instabilities during hardening, and austenite transformation to α′-martensite either directly or through e-martensite. Uniaxial strain rates within the range of 0.005–500 s−1 along the 0°, 45°, and 90° orientations were selected based upon their relevance to automotive applications. The plastic anisotropy (r) and normal anisotropy (rn) indices corresponding to each direction and strain rate were determined using strain fields obtained from stereo digital image correlation systems that enabled both quasistatic and dynamic measurements. The results provide evidence of significant, orientation-dependent strain rate effects on both the flow stress and the evolution of r and rn with strain. This has implications not only for material performance during forming but also for the development of future strain-rate-dependent anisotropic yield criteria. Since tensile data alone for the subject medium-manganese TRIP steel do not satisfactorily determine the microstructural mechanisms responsible for the macroscopic-scale behavior observed on tensile testing, additional tests that must supplement the mechanical test results presented herein are discussed.

The significant impact of pre-strain on the structure-mechanical properties relationship in cold-rolled medium manganese TRIP steel

We elucidate here the impact of pre-strain on the structure on mechanical properties of cold-rolled Fe-0.2C-5.5Mn-1.0Al (wt%) transformation induced plasticity (TRIP) steel. The steel was intercritically hardened at 650°C and exhibited excellent combination of total elongation (TEL) of 45.2%, ultimate tensile strength (UTS) of 1100MPa, and UTS×TEL of 50.2GPa%. Pre-strain increased both yield strength (YS) and ultimate tensile strength (UTS), while it decreased total elongation (TEL). Product of strength and elongation (PSE) was similar with 10% pre-strain (50.4GPa%) and 0% pre-strain (50.2GPa%). However, the 10% pre-strain had lower yield-to-tensile ratio. The increase in YS is attributed to the increased dislocation density and transformed martensite, while the continuous increase in UTS was associated with increase in the fraction of transformed martensite and enhanced work-hardening rate. On the other hand, the increase in the average stability of retained austenite, increased density of dislocations in ferrite and increased volume fraction of transformed martensite induced by pre-strain led to decreased ductility. The desired mechanical properties can be obtained by optimizing the amount of pre-strain.

Microstructure and mechanical properties of fibre laser welded medium manganese TRIP steel

Fibre laser welds of Fe-0.15C-10Mn-1.5Al medium-Mn transformation induced plasticity (TRIP) steel and its dissimilar combination with high strength low alloy (HSLA) and dual-phase (DP980) steel was assessed with respect to microstructure, microhardness, formability, and tensile properties. The fusion zone (FZ) of the medium-Mn TRIP steel weldment was consist of predominantly martensite with some interdendritic austenite due to high manganese concentration which stabilizes austenite. The FZ of dissimilar combinations to HSLA and DP980 steel consisted of mostly martensite. Microhardness profiles of the medium-Mn TRIP steel showed no indications of detrimental heat-affected zone (HAZ) softening despite the high base metal (BM) hardness (396HV). Laser welds containing medium-Mn TRIP displayed similar FZ hardness values (413–438HV). Failure of medium-Mn TRIP laser welds under uniaxial tension occurred adjacent to the FZ where localized strain accumulated. The tensile results showed consistent failure in the weaker BM of dissimilar joints at high joint efficiency (95–100%) with respect to the weaker material of the joint. Biaxial stretch formability of TRIP-TRIP similar welds was determined to be severely limited by the brittle FZ. Strain accumulation in the BM of HSLA and DP980 steel in dissimilar combinations improved formability of laser welded blanks containing medium-Mn TRIP steel.

Effect of two-step intercritical annealing on microstructure and mechanical properties of hot-rolled medium manganese TRIP steel containing δ-ferrite

The microstructure-properties relationship, work-hardening behavior and retained austenite stability have been systematically investigated in a hot-rolled medium manganese transformation-induced-plasticity (TRIP) steel containing δ-ferrite subjected to one-step and two-step intercritical annealing. The steel exhibited tensile strength of 752MPa and total elongation of 52.7% for one-step intercritical annealing at 740°C, tensile strength of 954MPa and total elongation of 39.2% in the case of intercritical quenching at 800°C and annealing at 740°C. The austenite obtained by two-step annealing mostly consists of refined lath structures and increased fraction of block-type particles existing at various kinds of sites, which is highly distinguished from those characterized by long lath morphology and small amounts of granular shape in one-step annealed samples. In spite of a higher C and Mn content in austenite and finer austenite laths, two-step annealing can lead to an active and continuous TRIP effect provided by a mixed blocky and lath-type austenitic structure with lower stability, contributing to a higher UTS. In contrast, one-step annealing gave rise to a less active but sustained TRIP effect given by the dominant lath-like austenite having higher stability, leading to a very high elongation. The further precipitation of vanadium carbides and the presence of both dislocation substructure and fine equiaxed grain in ferrite matrix facilitate the increase of yield strength after double annealing.

Deformation behavior in cold-rolled medium-manganese TRIP steel and effect of pre-strain on the Lüders bands

Deformation behavior was studied in cold-rolled 0.2C-1.6Al-6.1Mn-Fe transformation-induced plasticity (TRIP) steel subjected to intercritical annealing. The steel intercritically hardened at 650℃ exhibited excellent mechanical properties, and the excellent ductility was primarily associated with the discontinuous TRIP effect. Moreover at 650℃, the formation of Lüders bands was associated with TRIP effect and cooperative dislocation glide. The length of Lüders strain was gradually reduced with increasing pre-strain, and was eventually eliminated when the pre-strain was increased to 10%. The increased average stability of retained austenite and increased dislocation density in ferrite induced by pre-strain was responsible for decrease and ultimate elimination of Lüders bands. While in steel intercritically annealed at 600℃, ferrite and austenite was predominantly deformed, which was responsible for poor work hardening rate and inferior tensile properties.