Transformation-induced Plasticity Steel Research Articles

Processing of 17Cr7Mn6Ni TRIP Steel Powder by Extrusion at Room Temperature and Pressureless Sintering

Transformation‐induced plasticity (TRIP) steels are known for their outstanding strength and their excellent deformation properties, which are necessary for the improvement of occupant safety elements in air, rail and motor vehicles. One precondition for the realization of the TRIP effect is the creation of a metastable austenitic microstructure through the addition of a number of alloying elements. The great affinity of some alloying elements for oxygen implies the formation of highly stable oxides during thermal processing. In the current study, an extrusion process (derived from the processing of ceramics) with subsequent debinding and pressureless sintering is used to manufacture compact strands from prealloyed and gas‐atomized 17Cr7Mn6Ni TRIP steel powder. The influence of both the debinding temperature and sintering atmosphere on the oxide particle content in the final bulk product are investigated by X‐ray diffraction (XRD) analysis and quantitative metallography. Furthermore, X‐ray photoelectron spectroscopy (XPS) analysis in association with temperature‐programmed reduction (TPR) experiments, thermogravimetric (TG) measurements and mass/infrared spectroscopy (MS/IRS) serve to monitor the changes in the steel powder surface composition and the effectiveness of hydrogen as a reducing agent.

Fusion Weldabilities of Advanced High Manganese Steels: A Review

A large amount of manganese has been added to next-generation advanced high strength steels for automotive applications. The increased manganese content changes the microstructural and mechanical characteristics by varying both the stacking fault energy and the austenite stability, but it is known to deteriorate weldability. The current review provides a general strategy to address several problems in fusion welds of advanced high manganese steels from the viewpoint of microstructural transitions, plasticity mechanism, and mechanical properties based on the welding metallurgy. The importance of research on the weldabilities of advanced Mn steels was highlighted. Among the representative types of advanced manganese steels, this review focuses on high Mn twinning-induced plasticity steel, high Mn austenitic lightweight steel, and medium Mn transformation-induced plasticity steel. Specifically, this review suggests fundamental concepts for designing each steel with improved weld characteristics by presenting many research results on the correlations between the microstructure and mechanical properties of welds and heat-affected zones for these three different steel groups.

Effect of welding speed on microstructure and mechanical properties of laser-welded transformation induced plasticity (TRIP) steels

The weldability of 0.28C–2.0Mn–0.93Al–0.97Si (wt.%) transformation induced plasticity (TRIP) steels was investigated using a 2.5 kW CO2 laser at the welding speeds of 2, 2.5 and 3 m/min. The welded joints were characterized in terms of hardness, tensile properties and microstructure. High-quality welded joints of TRIP steels with the carbon equivalent of 0.7 were obtained. Lower loss of ductility, nearly unvaried hardness of the fusion zone (FZ) and tensile strength equal to the base metal were observed with increasing welding speed. Lath martensite and lower bainite formed in FZ and the microstructure of FZ varied little with welding speed. Weld thermal simulations of heat-affected zone (HAZ) were carried out using a quenching dilatometer, and the microstructures of dilatometric samples revealed the carbon diffusion-controlled transformations in HAZ. The microstructure distribution of HAZ could be influenced by the welding speed due to the significant temperature gradient over the narrow HAZ.

Microstructural and Mechanical Characterization of Square‐Celled TRIP Steel Honeycomb Structures Produced by Electron Beam Melting

The powder‐bed additive manufacturing (AM) technology electron beam melting (EBM) is suitable for the production of metallic components with complex geometries. Therefore, it is an appropriate method of generating lightweight structures to save material and energy, which is of high importance for the transport industry. The microstructure of additive manufactured materials is usually characterized by columnar grains shape, oriented along the building direction. Therefore, the mechanical properties are strongly affected by anisotropy. Herein, the synthesis and the microstructure of CrMnNi transformation‐induced plasticity (TRIP) steel by EBM is examined. This steel is well suited for AM due to the formation of fine‐grained microstructure instead of columnar one through multiple‐phase transformations. Square‐celled honeycomb structures are produced and improved to generate damage‐tolerant struts. The removal of slightly sintered powder from the cavities of square‐celled structures is quantified. Out‐of‐plane compression tests at quasistatic loading rates show that the strength of honeycomb structures rises with increasing energy input during EBM due to lower material porosity. Therefore, an enhanced mechanical energy absorption capacity is achieved which is favorable for the production of cellular structures.

Evaluation of factors influencing the lath martensitic deformation behavior of multi-layered steels

Herein, the factors influencing the deformation behavior of lath martensite along with the crystallographic features, we used multilayered steels (MLS) containing martensitic steel and transformation-induced plasticity (TRIP) steel. Since martensite has an extremely strong and low ductility, ductile TRIP steels are coupled with the Martensitic steels to increase overall the plasticity of MLS. The influence factors were divided into three types such as the size of prior austenite grains (PAGs), the alignment of blocks with a tensile axis, and position of block (e.g., vicinity or center of PAG) around the blocks, and analyzed by matching microstructure and mechanical properties. The martensitic features up to an engineering strain of 20% were observed by means of an ex-situ electron backscatter diffraction (EBSD), compared with the operative slip systems of each influencing factor. The operative slip system was derived from an activation priority of the slip systems during deformation with the classification in two types such as in-lath-plane and out-of-lath plane systems. A fine block located near the PAG boundary with an initial shape parallel to the tensile axis is significantly deformed because of an activated multiple slip system, matching with the result by means of the crystal plasticity simulations.

Superior mechanical properties in high-Ni transformation induced plasticity steels after friction stir welding

The high-Ni TRIP steel plates were butt joined by friction stir welded. For this aim, the rotational and traverse speeds were set at 600rpm-100 mm-min−1 and 950rpm-50 mm-min−1. The optical microscopy, X-Ray diffraction (XRD, orientation image microscopy, and tensile test were employed for the characterization of the joints. The results showed that friction stir welding replaced the large grains of the base metal with fine and equiaxed grains by continuous dynamic recrystallization (CDRX). In addition, friction stir welding lead to the stabilization of the austenite. The lower heat input condition caused finer grain sizes. Using lower heat inputs resulted in more retained austenite by means of both work hardening and finer grain size mechanisms. The friction stir welded joints showed superior UTS and elongation of 1130 MPa-115 % and 1050 MPa-76 % respectively for the low and high heat input conditions compared to that of the base metal i.e., 880 MPa-60 % due to stabilization of austenite and formation of more transformation induced plasticity (TRIP) effect. The low heat input joint showed superior tensile properties thanks to more austenite stabilization.

Investigation of the microstructure, mechanical properties and tensile behavior of a low carbon nickel-manganese dual-phase transformationinduced plasticity steel by heavy warm rolling

A dual phase (martensite–austenite) low carbon nickel-manganese transformation-induced plasticity (TRIP) steel was fabricated by heavily warm rolling (HWR), and the effect of annealing on the phase fraction, mechanical properties and tensile deformation behavior of the heavily warm rolled (HWRed) steel was investigated. The results showed that the reverse transformation of γ-austenite from α′-martensite occurs and that the γ-austenite volume fraction (VA) decreases from 91% to 55% as the annealing temperature increases from 400 °C to 800 °C, respectively. The HWRed steel annealed at 400 °C exhibits a high strength-high ductility combination with yield strength of 706 MPa, ultimate tensile strength (UTS) of 1573 MPa, total elongation (TEL) of 21.6%, and the product of the strength and elongation (PSE: UTS×TEL) is 34 GPa%. These excellent mechanical properties are principally attributed to the formation of a large volume fraction of austenite (γ) by the reverse transformation and subsequent TRIP effect during tensile deformation. It was found that the HWRed and annealed steels exhibit a special tensile behavior with a large yielding strain followed by pronounced strain hardening. The tensile curve can be readily divided into three obviously different stages. The strain-induced martensite (SIM) transformation (γ -α′) occurs in the early yielding deformation stage and in the intermediate rapidly hardening deformation stage, indicating that the TRIP effect dominates the process of these two stages. However, the retained γ-austenite remains very stable, and no TRIP effect is observed in the final hardening deformation stage. The load-unload reload (LUR) test was performed to evaluate the back stress (σb) hardening effect during tensile testing. It is believed that the pronounced strain hardening behavior after yielding is mainly associated with the σb enhancement induced by the strain partitioning between the soft retained γ-austenite and the hard α′-martensite due to the SIM transformation during tensile deformation.

Enhancing both strength and ductility of low-alloy transformation-induced plasticity steel via hierarchical lamellar structure

In this paper, martensite-to-austenite reversion during intercritical annealing was applied to a conventional low-alloy transformation-induced plasticity steel to form reverted austenite, which was followed by austempering to form bainite, and hence the resulting microstructures and mechanical properties were investigated. The obtained microstructure is composed of alternating lamellae of intercritical ferrite and reverted austenite on microscale, with the latter consisting of bainitic ferrite laths and retained austenite films on nanoscale. This unique microstructure exhibits excellent strength–ductility combinations, which surpass those of conventional polygonal blocky-structured counterparts, providing a strategy for developing high-performance structural materials at low production cost.

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

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 Düsseldorf 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.

Initiation of hydrogen induced cracks at secondary phase particles

The goal of this work is to propose a general mechanism for hydrogen induced crack initiation in steels based on a microstructural study of multiple steel grades. Four types of steels with strongly varying microstructures are studied for this purpose, i.e. ultra low carbon (ULC) steel, TRIP (transformation induced plasticity) steel, Fe-C-Ti generic alloy, and pressure vessel steel. A strong dependency of the initiation of hydrogen induced cracks on the microstructural features in the materials is observed. By use of SEM-EBSD characterization, initiation is found to always occur at the hard secondary phase particles in the materials.

Design of an effective heat treatment involving intercritical hardening for high-strength–high elongation of 0.2C–1.5Al–(6–8.5)Mn-Fe TRIP steels: microstructural evolution and deformation behaviour

ABSTRACT We propose an effective heat treatment involving a combination of intercritical hardening and tempering to obtain high strength–high ductility in hot-rolled 0.2C–1.5Al–(6–8.5)Mn–Fe transformation-induced plasticity (TRIP) steels. An excellent combination of high ultimate tensile strength of 1045–1380 MPa and total elongation of 34–39% was obtained when the steels were subjected to intercritical hardening at 630–650 °C and tempered at 200 °C. Intercritical hardening impacted the co-existence of austenite, ferrite and martensite, such that the deformation behaviour varied with the Mn content. The excellent properties of the steels were attributed to cumulative contribution of enhanced TRIP effect of austenite and ferrite and martensite constituents. The discontinuous TRIP effect during tensile deformation involves stress relaxation and led to consequent enhancement of ductility.

Effects of Al content on the high-temperature mechanical properties of Al-TRIP steel

ABSTRACT As-cast transformation-induced plasticity (TRIP) steels with two different aluminium contents (2.80 and 6.75 wt-%) were subjected to investigate the effect of aluminium content on the tensile strength and hot ductility. As the Al content increases from 2.80 to 6.75 wt-%, the third brittle zone of the steel is shifted from 950–750°C to 800–750°C. The 6.75 wt-% Al-TRIP steel is a typical δ-TRIP steel that contains 44.5 wt-% δ-ferrite compared with no δ-ferrite in the 2.80 wt-% Al-TRIP steel. The solidification behaviour of these two types of TRIP steel was compared by calculating equilibrium phase diagrams. The 6.75 wt-% Al-TRIP steel was found to have better ductility than the 2.80 wt-% Al-TRIP steel below 1100°C.

Weldability and mechanical behavior of laser-welded TRIP 750 steel sheets

Transformation-induced plasticity steels have been developed and widely applied in the automotive and aerospace industries. They exhibit ductility and mechanical strength associated with a high formability due to their complex microstructure of bainite-associated pro-eutectoid ferrite and significant retained austenite fractions. The weldability of these steels is limited by the high content of alloying elements in the composition, causing the thermal cycle to modify the carefully designed microstructure, which in turn generates unsatisfactory weld mechanical properties. Laser welding has a relatively low thermal input, and, therefore, a narrow heat-affected zone is obtained. As known, the literature had not been definitively reported the microstructural features of the fusion and the heat-affected zones after laser welding of TRIP steels in conjunction with their mechanical behavior. The aim of the present work is to characterize the microstructure and mechanical behavior of laser-welded TRIP steel after uniaxial tensile and Erichsen formability tests. The coupons of TRIP 750 steel sheets were subjected to different laser welding conditions in order to analyze their impact on the microstructure, hardness, and mechanical strength of the material. After some preliminary tests, the laser power was fixed at 900 W and the weld speed fixed at 50 mm/s as the best choice of operating parameters. Under these conditions, the fusion zone was almost completely martensitic, while the heat-affected zone had a mixture of ferrite and martensite. The martensite transformation is corroborated by finite elements analyses as the cooling rate was 4200 °C/s for material at martensite start temperature. The average hardness of the fusion zone was 530 HV and the heat-affected zone was 550 HV, compared with 270 HV of the base material. In terms of mechanical behavior, the tensile strength of the welded coupons was found to reach 740 MPa and the ductility reached 22% in uniform deformation. The Erichsen index for the welded sheets attained 15 mm for a load of 48.5 kN, similar with the non-welded base material. Both in the case of the uniaxial tensile testing and in the Erichsen testing, the fracture occurred in the base material away from the weld, showing a good toughness of the welded component.

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

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 Lüders 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.

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

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.

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

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.

Analysis of temperature and stress-strain fields during laser beam welding of a TRIP steel

Transformation Induced Plasticity (TRIP) steels belong to the group of Advanced High Strength Steels (AHSS) that are characterized by good strength-strain combination and formability required for special applications in the automotive industry. The excellent combination of strength, ductility and formability of TRIP steels is achieved by a careful control of microstructure development in the process of their production. The microstructure of multiphase TRIP steels typically consists of ferrite, carbide-free bainite and metastable retained austenite, which can be transformed into martensite by plastic deformation. Upon crash, this feature allows the TRIP steels to absorb more energy, ensuring greater passenger safety. In the automotive industry, TRIP steels are mainly joined by resistance spot welding, laser or electron beam welding. Generally, the thermal cycle of a fusion welding process destroys the sophistically designed microstructure of these steels in fusion and heat-affected zones resulting in deterioration of mechanical properties of the weld joint. Negative consequences of the welding process can be eliminated using proper welding parameters. The paper deals with numerical simulation and analysis of the temperature and stress-strain fields developed during the laser beam welding of a CMnSiNb TRIP steel sheets with the thickness of 2 mm. Simulation model takes into account non-linear temperature and phase dependent material properties. The heat input during the laser beam welding is modelled using the conical volumetric heat source. The optimal welding parameters for production of butt joints of CMnSiNb TRIP steel sheets using the TruDisk 4002 disc laser with the maximum power of 2 kW are designed.

Current challenges and opportunities in microstructure-related properties of Advanced High Strength Steels

This is a viewpoint paper on recent progress in the understanding of the microstructure-property relations of Advanced High Strength Steels (AHSS). These alloys constitute a class of high strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite or martensite matrix or of duplex or even multiphase mixtures of these constituents, often enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure-property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual phase steels, complex phase steels, transformation induced plasticity steels, twinning induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation and twinning effects. Recent developments on micro-alloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modelling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation.

Transformations During Intercritical Annealing and Their Implications for Microstructure and Mechanical Properties of Medium Mn Transformation-Induced Plasticity Steel in Continuous Annealing Line

Phase transformation during intercritical annealing (IA) and the relationship between microstructure and mechanical properties of the intercritical annealed 0.2C-5MnTRIP steels were investigated using a combined method of dilatometry, scanning electron microscope, transmission electron microscope, x-ray diffraction, and tensile testing. With the increase in IA temperature and holding time, the fraction of reversed austenite increases while the chemical stability of reversed austenite decreases, leading to the transformation of reversed austenite to martensite in the cooling stage. IA temperature is found to have a greater influence on the stability of austenite than IA time. The microstructures mainly comprised of ferrite and retained austenite, with granular carbide and martensite also observed in some specimens. The carbide precipitates formed during hot rolling remain after cold rolling and annealing process and gradually dissolve at higher IA temperature or with longer IA time. The optimal IA parameter, i.e., 650 °C for 15 min, contributes to a good product of tensile strength and elongation (997 MPa × 29.25%) due to an excellent TRIP effect. The present work gives a clear insight in controlling the mechanical properties of medium Mn TRIP steel in the continuous annealing line by a reasonable design of the IA process.

Ductile–Brittle Variation Phenomenon and a Special Transformation-Induced Plasticity Effect in NbTi-NiTi Composite

In order to study the transformation-induced plasticity (TRIP) effect in a NiTi shape-memory alloy composite, an in situ NbTi-NiTi composite was prepared by vacuum arc melting, hot forging and wire drawing. An unusual ductile–brittle variation phenomenon was observed by means of a series of tensile tests. The composite was brittle in the absence of transformation at 200 °C, and the fracture strain was about 2%. With lowering the tensile temperature, the composite became ductile when stress-induced martensitic transformation occurred, and the elongation increased to about 18% (a ninefold increase) during tensile test at room temperature. This ductile–brittle variation phenomenon indicates that the stress-induced martensitic transformation of NiTi alloy helps to improve the ductility of the composite, which is just the exhibition of TRIP effect in the NbTi-NiTi composite. Different from conventional TRIP steels, the TRIP behavior in NiTi induces only an increase in the elongation, but not any increase in fracture strength.