Showing papers on "TRIP steel published in 2020"

Effect of matrix structures on TRIP effect and mechanical properties of low-C low-Si Al-added hot-rolled TRIP steels

Abstract: We applied different hot-rolling direct quenching and partitioning (HDQ&P) processes to a low-C low-Si Al-added steel and obtained eight TRIP-assisted steels with different matrix structures, viz, martensite, ferrite/bainite, ferrite/martensite and ferrite/bainite/martensite. The microstructures were characterized using SEM, TEM and XRD. The mechanical properties were investigated by means of uniaxial tensile tests. Quasi in-situ tensile tests in combination with EBSD and microscopic digital image correlation (μ-DIC) analyses were performed on microstructure regions containing ferrite, martensite and retained austenite to investigate the TRIP effect and composite effect. Considering both the chemical and strain partitioning among the structure components, we analyzed the specific influence of the matrix structures on the austenite stabilization, martensitic transformation of retained austenite and ductility of the material. The results show that the TRIP steel with a martensitic matrix exhibits high ultimate tensile strength (UTS) and yield ratio reaching up to 1200 MPa and 0.87, with the product of strength and elongation (PSE) of about 18000 MPa%. The introduction of 8%–25% (in area fraction) of ferrite leads to a decrease of the yield strength in 100–200 MPa, but without significant reduction of the UTS. The increase of the ferrite fraction to 30%–35% results in an obvious decrease of the UTS and yield ratio to about 950 MPa and 0.6. Retained austenite, with the amount of 14 vol %, was stabilized in the TRIP steel with a martensitic matrix. The introduction of ferrite (8%–35% in area fraction) and granular bainite can promote the carbon partitioning, thus enhancing the stabilization of retained austenite. The TRIP steel with a martensitic matrix exhibits a slight martensitic transformation of retained austenite because of the low deformability of the martensite. For TRIP steel with a matrix composed of ferrite and martensite, the low deformation compatibility of the soft and hard structure components also leads to a week martensitic transformation of retained austenite. The introduction of granular bainite can effectively improve the deformation uniformity and enhance the martensitic transformation during deformation. The TRIP effect and the composite effect of matrix structures jointly control the ductility of the TRIP steels. To optimize the ductility, we not only need to enhance the TRIP effect but also to improve the deformation compatibility of the matrix structures by tuning the structure components and their strength differences.

Development of an Advanced Ultrahigh Strength TRIP Steel and Evaluation of Its Unique Strain Hardening Behavior

Abstract: A novel ferrite-53% austenite medium-Mn transformation induced plasticity (TRIP) steel with extraordinary combination of 1420 MPa tensile strength and 27% elongation was produced by a simple designed thermomechanical treatment. The deformation and strain hardening behavior of this material were studied in detail using field emission scanning electron microscopy and electron backscatter diffraction examinations of different strained specimens. Exclusive strain hardening behavior observed in this new TRIP steel was found to be related to the severe grain refinement to less than 200 nm and austenite grain disintegration within the first 1% of applied strain. Moreover, it was shown that the deformation of highly unstable retained austenite benefits the mechanical properties, since it could promote the severe grain refinement and subsequent uniform strain distribution within the microstructure of the TRIP steel. Finally, based on the strain hardening behavior analysis a new and simple measurable criterion was introduced as a reliable replacement of traditional austenite stability parameter or the K value.

Hybrid parts produced by deposition of 18Ni300 maraging steel via selective laser melting on forged and heat treated advanced high strength steel

Abstract: Long production times, the associated high costs of the products and product size limitations belong among current issues of selective laser melting (SLM) technology. Hybrid products containing small and complex-shaped parts deposited by SLM on the forged, rolled or hot stamped semi-products could offer a practical solution to these limitations. Cylindrical hybrid parts were additively manufactured by depositing 18Ni300 maraging steel on the cylindrical semi-products of CMnAlNb low-alloy advanced high strength steel (AHSS). The AHSS was used either in forged and air cooled condition or after heat treatments typically used for inducing the TRIP (transformation induced plasticity) effect. Various post-build heat treatments of the hybrid parts were performed. The mechanical properties of the hybrid parts were determined by hardness measurement across the interface and by a tensile test of the dissimilar joints. All tensile samples fractured in the high-strength steel side, several millimetres from the interface. Microstructure analysis of both materials and the interface region was carried out using light and scanning electron microscopes. The hybrid parts had the ultimate tensile strengths of 840−940 MPa, with total elongations of 12–19 %. The best combination of tensile strength and elongation was obtained with two-step heat treatment of the TRIP steel prior to additive manufacturing with no post-build heat treatment of the hybrid part.

Replacement of silicon by aluminum with the aid of vanadium for galvanized TRIP steel

Abstract: Silicon existence in conventional low carbon TRIP steel is essential for inhibiting cementite precipitation, despite that it tends to obstruct the hot-dip galvanizing process. Aluminum is considered the mostly preferable substituent of silicon for such purpose. However, it has a counter effect on the strength/ductility balance of TRIP steel as a result of ferrite grain growth. Thereby, this study aimed to completely replace silicon by aluminum in the presence of vanadium in order to enhance the coatability of such grade of steel. Microstructure of the studied steel was examined by using optical microscope and FE-SEM. XRD analysis and tent-etching technique were performed to study the fractions and morphology of the retained austenite. Moreover, the influence of the selected parameters on the mechanical properties were investigated. Showcased results promoted the significant potential of aluminum and vanadium as a substituent to the conventional silicon-based TRIP steel. This substitution has a great influence on promoting the retained austenite stability and hence the mechanical performance of TRIP steel.

Investigating the local deformation and transformation behavior of sintered X3CrMnNi16-7-6 TRIP steel using a calibrated crystal plasticity-based numerical simulation model

Abstract: In this study, DAMASK was used to model and elucidate the microstructural deformation behavior of sintered X3CrMnNi16-7-6 TRIP steel. The recently developed TRIP-TWIP material model was us...

Role of the crystallographic texture in anisotropic mechanical properties of a newly-developed hot-rolled TRIP steel

Abstract: In general, hot-rolled steels have higher strength but lower ductility in the transverse direction. In this work, the effects of the crystallographic texture on the anisotropic tensile properties of a new hot-rolled TRIP steel, named DF-TRIP steel, were investigated by electron backscatter diffraction. The tensile tests showed that the hot-rolled steel exhibited nearly the same yield strength but different TRIP effects when loaded along the rolling direction and transverse direction. The low anisotropy in yield strength was explained by the negligible difference in rolling texture in the ferrite due to strain-induced dynamic ferrite transformation. However, a lower Taylor factor of the austenite was found along the rolling direction, leading to early onset of slip and thus providing nucleation sites for strain-induced martensitic transformation. This new hot-rolled steel shows higher ultimate strength and better total uniform elongation in the rolling direction.

Modeling the Local Deformation and Transformation Behavior of Cast X8CrMnNi16-6-6 TRIP Steel and 10% Mg-PSZ Composite Using a Continuum Mechanics-Based Crystal Plasticity Model

Abstract: A Transformation-Induced Plasticity (TRIP) steel matrix reinforced with magnesium-partially stabilized zirconia (Mg-PSZ) particles depicts a superior energy absorbing capacity during deformation. In this research, the TRIP/TWIP material model already developed in the framework of the Dusseldorf Advanced Material Simulation Kit (DAMASK) is tuned for X8CrMnNi16-6-6 TRIP steel and 10% Mg-PSZ composite. A new method is explained to more accurately tune this material model by comparing the stress/strain, transformation, twinning, and dislocation glide obtained from simulations with respective experimental acoustic emission measurements. The optimized model with slight modification is assigned to the steel matrix in 10% Mg-PSZ composite material. In the simulation model, zirconia particles are assigned elastic properties with a perfect ceramic/matrix interface. Local deformation, transformation, and the twinning behavior of the steel matrix due to quasi-static tensile load were analyzed. The comparison of the simulation results with acoustic emission data shows good correlation and helps correlate acoustic events with physical attributes. The tuned material models are used to run full phase simulations using 2D Electron Backscatter Diffraction (EBSD) data from steel and 10% Mg-PSZ zirconia composites. Form these simulations, dislocation glide, martensitic transformation, stress evolution, and dislocation pinning in different stages of deformation are qualitatively discussed for the steel matrix and ceramic inclusions.

Microstructural evolution and mechanical properties of Ni-containing light-weight medium-Mn TRIP steel processed by intercritical annealing

Abstract: Ni was deliberately added into a light-weight medium-Mn TRIP steel for further improving comprehensive mechanical properties through the precipitation of NiAl intermetallics. Ni addition results in the precipitation of 2–140 nm sized NiAl particles at phase boundaries and the interior of both austenite and ferrite, and more easily inside ferrite. Additionally, Ni addition benefits not only the increase of austenite fraction, ranging from 43% to 57%, but also the improvement of austenite stability because of the partition behavior of Ni from ferrite to austenite during annealing. Similar to Ni-free light-weight medium-Mn TRIP steels, martensitic transformation (TRIP effect) governed by austenite content and stability is the main deformation mechanism in the experimental steel. However, due to Ni addition, NiAl particles can increase yield strength through the precipitation strengthening. And the occurrence of twinning-induced plasticity (TWIP) effect has been confirmed, which contributes to the ductility. The present steel annealed at 680 °C demonstrates high yield strength of 855 MPa, ultimate tensile strength of 961 MPa and superior total elongation of 53%, which is ascribed to the combined action of precipitation strengthening, strong TRIP effect and minor TWIP effect.

Influence of strain rate and strain at temperature on TRIP effect in a metastable austenitic stainless steel

Abstract: Metastable TRIP (TRansformation-Induced Plasticity) steel strips with a microstructure comprising ferrite, retained austenite and martensite were subjected to tensile tests at different strain rates of 10−5s−1, 10−4s−1, 10−3s−1 and temperatures 27 °C, 50 °C and 100 °C. The TRIP steel specimens exhibited very high tensile strengths (up to ~1700 MPa) in tests carried out at room temperature, which is attributed to the TRIP effect. Thermal stability of the phases present in the TRIP steel in the as-received condition was assessed by in-situ high temperature XRD and DSC (Differential Scanning Calorimetry) analysis to determine the retained austenite decomposition temperature. Microstructural analysis by XRD, optical microscopy and electron microscopy was used before and after the tensile tests to identify the features present at micro- and nano-scales, which are responsible for the observed mechanical properties. Macrographs taken from the surface of the specimens after the tensile tests showed the presence of Luders bands and their occurrence was concurrent with the TRIP effect. The effect of strain rate and temperature on the TRIP effect and the associated mechanical response are discussed in detail.

Effect of Compositional Variation Induced by EBM Processing on Deformation Behavior and Phase Stability of Austenitic Cr-Mn-Ni TRIP Steel

Abstract: Electron beam melting (EBM) is an established powder bed-based additive manufacturing process for the fabrication of complex-shaped metallic components. For metastable austenitic Cr-Mn-Ni TRIP steel, the formation of a homogeneous fine-grained microstructure and outstanding damage tolerance have been reported. However, depending on the process parameters, a certain fraction of Mn evaporates. This can have a significant impact on deformation mechanisms as well as kinetics, as was previously shown for as-cast material. Production of chemically graded and, thus, mechanically tailored parts can allow for further advances in terms of freedom of design. The current study presents results on the characterization of the deformation and strain-hardening behavior of chemically tailored Cr-Mn-Ni TRIP steel processed by EBM. Specimens were manufactured with distinct scan strategies, resulting in varying Mn contents, and subsequently tensile tested. Microstructure evolution has been thoroughly examined. Starting from one initial powder, an appropriate scan strategy can be applied to purposefully evaporate Mn and, therefore, adjust strain hardening as well as martensite formation kinetics and ultimate tensile strength.

Liquid zinc penetration induced intergranular brittle cracking in resistance spot welding of galvannealed advanced high strength steel

Abstract: This study aims to explore the mode of Zn transportation and the failure mechanism of cracking associated with liquid metal embrittlement (LME) using fractography as the key technique. A three-point bend test was performed on a TRIP steel resistance spot weld to open the LME crack in the form of a free fracture surface for the fractographic investigation. The presence of liquid Zn on the fracture surface was revealed by the Fe-Zn phase transformation and the spike-like morphology of the residual Zn, confirming that the mode of Zn transportation in LME cracks was liquid penetration through the austenite grain boundary. In addition, the fractography of the bend test samples and electron backscattered diffraction of the cracks revealed the failure mode of the LME crack as a complete intergranular brittle fracture without the generation of any microplasticity. Thus, the underlying failure mechanism of cracking in Zn-LME can be explained by the Stoloff-Johnson-Westwood-Kamdar brittle fracture model induced by the decohesion of interatomic bonds. Overall, a dramatic reduction in the interatomic bond strength by lowering the surface energy of the grain boundary with liquid Zn penetration causes the decohesion-induced intergranular brittle cracking.

Achieving extraordinary strength and ductility in TRIP steels through stabilization of austenite up to 99.8 % by friction stir welding

Abstract: Friction stir welding caused formation of fine-equiaxed grains and stabilization of austenite phase up to 99.8% (promotion of TRIP effect) in the high-Ni TRIP steel. Ultimate tensile strength and elongation were increased from 980 MPa-61% to 1270 MPa-121%, which has been never reported before.

Effects of in-situ post-weld heat treatment on the microstructure and mechanical properties of the coarse-grained heat-affected zone in a resistance spot weld in medium Mn TRIP steel

Abstract: The tensile properties of the coarse-grained heat-affected zone (CGHAZ) play an important role in the cross-tension strength of resistance spot weldment of medium Mn transformation-induced plasticity (TRIP) steel. In this paper, an in-situ post-weld heat treatment is proposed as a way to achieve remarkable improvement of the ductility of the CGHAZ. Using four different weld thermal cycles to simulate the heat treatment, thermo-mechanically simulated specimens are prepared and used for mechanical testing. The experimental results show that the enhancement in ductility is based on the prevention of intergranular brittle fracture at the prior-austenite grain boundary because of the tempering of martensite. The tempering of the CGHAZ is affected by both the amount of martensite present immediately before reheating and the peak tempering temperature. Along with existing theory on tempering kinetics, the phenomenon is discussed in terms of carbon diffusion causing nanoscale austenite reversion and carbide precipitation.

Effects of hot rolling and homogenisation treatment on low alloy steel ingot

Abstract: The effects of hot rolling and homogenisation treatment on the microstructure and mechanical properties of low-alloy transformation-induced plasticity (TRIP) steel were studied. With increasing the...

Analysis and design of a three-phase TRIP steel microstructure for enhanced fracture resistance

Abstract: The goal of this paper is to predict how the properties of the constituent phases and microstructure of a transformation induced plasticity steel influence its fracture resistance. The steel selected for study was a three-phase quenched and partitioned (QP) sheet steel comprised of 50% ferrite, 42% martensite and 8% retained austenite (RA) with $$\sim $$ 980 MPa tensile strength. Experiments show that ductile fracture in the steel involves nucleation, growth, and coalescence of micron-scale voids. Accordingly, the failure process is modeled at the microstructure scale by idealizing the individual phases of the steel using elastic-viscoplastic constitutive relations that account for the loss of strength resulting from cavitation, as well as the effects of transformation of metastable RA to martensite. The flow behavior of the phases and the transformation kinetics of RA are calculated by homogenizing the microscale calibrated crystal plasticity constitutive relations from a previous study (Srivastava in J Mech Phys Solids 78:46–69, 2015) while the damage parameters are determined by void cell model calculations. The microstructure-scale simulations are used to compute the fracture and instability loci for the steel, which are used to calibrate the GISSMO (Generalized Incremental Stress State Dependent Damage Model) (Andrade in Int J Fract 200:127–150, 2016). The microstructure-informed GISSMO model for QP980 is found to predict fracture strains within 18% of experimental measurements of ligament-type test specimens. Finally, a series of virtual steel microstructures are analyzed to determine the influence of the phase volume fractions on the fracture resistance of the steel. Two candidate microstructures are identified that exhibit increased engineering fracture strains ($$>57$$%) without significantly compromising (within 6%) the tensile strength when compared to the baseline QP980.

Microstructural and mechanical characterization of high-alloy quenching and partitioning TRIP steel manufactured by electron beam melting

Abstract: The applicability of electron beam melting (EBM) as a powder-bed based process of additive manufacturing was examined for a high-alloy transformation-induced plasticity (TRIP) steel developed, in particular, for quenching and partitioning (Q&P) treatment. The chemical composition of the EBM manufactured material was determined to investigate the effect of EBM process on interstitial alloying elements such as carbon and nitrogen, which are relevant for the partitioning step. In Q&P state, tensile tests were performed and compared to the as-built as well as solution annealed state. In addition, these investigations were supplemented by microstructural characterization using electron backscatter diffraction. Compared to the as-built state, the tensile samples of Q&P state show a significant increase in yield strength of up to 1100 MPa with no significant decrease in ductility. Furthermore, it is shown that despite large defects resulting from the manufacturing process, material with extraordinarily high strength and good ductility was achieved. Finally, the EBM process on a high-alloy Q&P steel led to a fine-grained and texture-free martensitic microstructure, which develops due to solid-solid-phase transformations at elevated temperatures.

Investigation on the effect of power and velocity of laser beam welding on the butt weld joint on TRIP steel

Abstract: In this study, the characterization of laser weld joint on transformation-induced plasticity (TRIP) steel sheets coupled with Nd:YAG laser welding was investigated, and the influence of welding conditions like angle of weld, power of laser, and welding speed on the strength of the joint was measured. The microstructure, tensile behavior, and microhardness of TRIP laser-welded sheets were examined in detail. The power was maintained constant, i.e., 1800 W, and by relatively varying the velocity from 25 to 30 mm/s, the strength of the joint increased drastically to 11%. In contrast, by varying the power, the effect of velocity was reduced; however, the point performance was enormously stable. Finally, the microhardness behavior of the heat-affected zone and fusion zone was investigated and discussed.

Effect of two-step bainite treatment on the morphology and texture of retained austenite and mechanical properties of austenitizing pretreated transformation-induced plasticity steel

Abstract: In this paper, the experimental steel was treated with an austenitizing pretreatment followed by a two-step bainite treatment. The volume fraction of the retained austenite was significantly increased at room temperature, and moreover, film-like retained austenite accounts for more than 85% of the total retained austenite. The grain size of the blocky retained austenite was controlled to be less than 2 μm. The texture of the retained austenite has changed significantly: The retained austenite exhibited strong {hkl} textures, in which the retained austenite had a lower Schmidt factor when subjected to uniaxial stretching parallel to the rolling direction; its stability was greatly improved. The one-step bainite treatment based on an austenitizing pretreatment can effectively improve the mechanical properties of TRIP steel by an increase of more than 20% compared with the conventional TRIP heat treatment process. However, the two-step bainite treatment based on an austenitizing pretreatment process applied herein can further significantly improve the comprehensive mechanical properties of the experimental steel compared with the one-step bainite treatment based on an austenitizing pretreatment process, which has increased by 23.8% on average. When the two-step bainite treatment process is 500 °C × 20 s + 420 °C × 220 s, the experimental steel exhibits optimal comprehensive mechanical properties with an ultimate tensile strength of 885 MPa, an elongation of 32.2%, and a product of the strength and elongation of 28,947 MPa%.

Dilatometric Study of Phase Transformations in 5 Mn Steel Subjected to Different Heat Treatments.

Abstract: The work presents results of phase transformation kinetics of hot-rolled 5% Mn steel subjected to different heat treatments. Three different schedules were introduced: isothermal holding in a bainite region, coiling simulation and intercritical annealing. The evolution of microstructure components was investigated using dilatometric and metallographic analyses. According to obtained results, the medium-Mn steel exhibits high resistance for γ/α transformation during the bainite transformation and coiling simulation (upon cooling from the austenite region). During 5 h isothermal holding, no bainite and/or ferrite formation was detected. This results in the formation of martensite upon cooling to room temperature. Differently, when the steel was subjected to the intercritical annealing at 720 and 700 °C (upon heating from room temperature), a final microstructure consisted of ferrite, martensite and retained austenite. At 700 °C, no fresh martensite formation was detected upon cooling to room temperature. This means that the austenite was enriched in carbon during the intercritical annealing step enough to keep its thermal stability.

The determining role of pre-annealing on Mn partitioning behavior in medium-Mn-TRIP steel: experimental and numerical simulation

Abstract: The influence of the pre-annealing on Mn partitioning behavior was investigated for hot-rolled Fe–0.095C–6.93Mn–1Al–1.07Si medium-Mn-TRIP steel. Unlike the fully martensitic microstructure before one-step intercritical annealing, a mixed microstructure of preexisting ferrite, austenite and martensite was obtained prior to the final annealing in two-step annealed steels. The microstructure observation showed that lath-type austenite existed in the martensitic matrix, blocky and granular austenite occurred at the boundaries of primary austenite/preexisting ferrite or martensite/preexisting ferrite after two-step annealing. Based on the numerical simulations of the second annealing process, the preexisting austenite further grew rapidly without nucleation, while some secondary austenite nucleated at the martensitic lath boundaries and then grew into martensite at a relatively sluggish rate. Moreover, significant Mn partitioning with a special “bimodal distribution” feature was detected in the preexisting austenite, resulting in the formation of martensite/austenite constituents. Compared with the case of one-step heat treatment, an excellent combination of strength (1145 MPa) and ductility (39%) was obtained in the steel with pre-annealing, which was ascribed to the occurrence of martensite/austenite islands and positive TRIP effect over a wide strain range.

Processing and Properties of Medium-Mn TRIP Steel to Obtain a Two-Stage TRIP Behavior

Abstract: Two medium-Mn steels of nominal composition 015C-20Si-105Mn-(075, 15) Al-004Nb-balFe (wt pct) were processed to produce sub-micron grain sizes of 065 and 080 μm Mechanical testing was performed in three successive conditions: hot rolled, intercritically annealed, and cold rolled with subsequent intercritical annealing Intercritical annealing was performed at 923 K for 20 hours Electron-backscattered diffraction and X-ray diffraction were utilized to characterize the microstructure, consisting of α-ferrite, α-martensite, e-martensite, and γ-austenite Microstructural constituents were tracked during tensile deformation and it was found that both steels exhibited two-stage TRIP with γ-austenite martensitically transforming first to e-martensite and as strain increased e-martensite transformed to α-martensite

Effect of Deformation Temperature on the Mechanical Behavior and Stability of Retained Austenite in TRIP-Assisted Medium-C Multiphase Steel.

Abstract: The temperature-dependent microstructural evolution and corresponding mechanical stability of retained austenite in medium-C TRIP-assisted 0.43C-1.45Mn-0.98Si-1Al-0.033Nb-0.01Ti steel obtained by thermomechanical processing was investigated using static tensile tests and microstructural studies. The light microscopy, image analysis, XRD diffraction and the Jaoul–Crussard analysis were applied to reveal relationships between microstructure and mechanical properties. Specimens were deformed in the static tensile tests in a temperature range of −20–140 °C. It was found that an increase in deformation temperature resulted in the reduced intensity of the TRIP effect due to the higher stability of retained austenite. An increase in the retained austenite stability along with a smaller grain size and a change from its blocky morphology to thin layers was also indicated. The impact of strengthening mechanisms at different temperatures was analyzed. The best combination of strength and ductility was obtained in the samples deformed at 20 and 60 °C, which is associated with the moderate work hardening in this temperature range. The Jaoul–Crussard analysis showed much less strengthening during the second phase of deformation at 100 and 140 °C due to the high stability of retained austenite. The higher C content in the investigated TRIP steel resulted in substantial volume fractions of retained austenite stable after completing deformation.