Transformation-induced Plasticity Steel Research Articles

Microstructure and formability performance of fiber laser welded 1.2 GPa grade hot-rolled TRIP steel joints

Laser welding of 1.2 GPa hot-rolled transformation induced plasticity (TRIP) steels with thickness of 1.9 mm was carried out to study its microstructure, mechanical properties and deformation behavior. Defect-free joints with complete penetration were prepared under the applied welding parameters. It was found that the fusion zone (FZ) consisted of full martensite of high microhardness (489 HV). Compared with the base material (BM) (436 HV), the subcritical heat affected zone (SCHAZ) contained a narrow softening zone with 62 HV microhardness drop due to tempering of martensite. The laser welded joint achieved 96.6% joint efficiency comparing to the BM. The formability of welded specimen depended on the weld position. As the weld offset increased to 6 mm, the formability approached 95.7% of the BM. The deformation was small in FZ and was primarily concentrated in BM. The martensite in FZ effectively restrained transverse deformation in welded joint, such that main cracks occurred and propagated in the BM. Stretch-flange-formability in terms of hole-expansion ratio of weld specimens was degraded compared with the BM (33.3%). Premature failure appeared at joint with low punch force because complete martensite exhibited low ductility in the FZ.

Relationship between crystallographic orientation, microstructure characteristic and mechanical properties in cold-rolled 3.5Mn TRIP steel

The relationship between crystallographic orientation, microstructure characteristic and mechanical properties in 3.5Mn transformation induced plasticity (TRIP) steel that subjected to different cold rolling reduction and annealing time was elucidated. Before the hot-rolled plates were cold rolled to different thicknesses, large-fractioned Mn-enriched cementite particles dispersed in ferrite matrix were deliberately prepared to enhance Mn-partitioning and improve austenite stability. The evolution of microstructure was performed by electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD), and the mechanical properties of the intercritical annealed samples were evaluated by uniaxial tensile test at room temperature. The results show that the cementite dissolution, ferrite recrystallization and austenite formation are strongly coupled and interdependent, resulting in a complex ultra-fine microstructure composed of retained austenite, ferrite and cementite in the intercritical annealed samples. Moreover, the Schmid factor (SF) distribution within the range of 0.25–0.5 was studied to understand its correlation with the crystallographic orientation and mechanical property of the test steel. It is concluded that the mechanical properties of 3.5Mn TRIP steel can be improved when the annealing period, orientation and austenite stability are optimized.

Effect of prior annealing on the microstructure and mechanical properties of thermo-mechanically treated TRIP steel

Low-carbon low-alloyed high strength steel 0.2C-1.5Al-0.5Si-2Mn-0.06Nb (in weight %) was forged into bars and annealed at 1050 °C with either 2-hour hold and furnace cooling to room temperature or with 20-min hold and controlled cooling by 1 °C/s. These various holds and coolings created two different initial microstructures, banded ferritic-pearlitic one for the long hold and bainitic-ferritic for the shorter hold and quicker cooling. Samples with those microstructures then underwent the same thermo-mechanical treatment (TMT) typical for TRIP (transformation induced plasticity) steels. Very different microstructures and mechanical properties were obtained for both initial states even after the application of the same TMT schedules. Light and scanning electron microscopy was used to characterize obtained microstructures and tensile tests were applied for the determination of mechanical properties.

Strain-hardening behavior and mechanisms of a lamellar-structured low-alloy TRIP steel

The excellent properties of transformation-induced plasticity (TRIP) steels with a lamellar structure are generally claimed to mechanically stabilize retained austenite (RA) grains, enabling the TRIP effect to persist up to higher strains. Nevertheless, the present work finds that nearly no RA grains are transformed into martensite at the strain range of 0.10–0.14 in lamellar-structured specimens. Furthermore, the strain-hardening exponent remains nearly unchanged. To explain the underlying mechanisms of this strain-hardening behavior, the block/packet/prior austenite grain boundaries of the hierarchically lamellar-structured TRIP steel were reconstructed. Moreover, the evolutions of full-field strain distribution and back stress hardening with straining were examined through the in situ micro digital image correlation technique and cyclic loading–unloading–reloading tensile tests. Results show that the block structure controls the microstructural deformation behavior of lamellar-structured specimens. Within a block, ferrite and bainite layers could deform synergistically. However, next to block boundaries, ferrite layers are susceptible to a large strain gradient due to the different deformation tendencies of adjacent blocks. Compared to conventional polygonal-structured specimens, the more refined ferrite grains of lamellar-structure specimens reduce the strength difference between ferrite and bainite layers. The strains are more uniformly partitioned among various constituents, thus delaying the occurrence of tensile necking. Back stress hardening dominates the strain-hardening in conventional polygonal-structured specimens, facilitating the increase of flow stress but decreasing the toughness. In lamellar-structured specimens, combined moderate back stress and dislocation hardening should be responsible for the persistent strain-hardening exponent after the strains exceed 0.10.

On the effect of Mn-content on the strength-ductility balance in Ni-free high N transformation induced plasticity steels

The present work deals with the room temperature microstructure-mechanical properties relationship in Ni-free high N transformation induced plasticity steels. The tensile properties of the alloy with lower Mn content of 4%, and thus lower austenite fraction and austenite stability, was mainly controlled through sequential (extended) transformation route of γ→ε→α′. The extended route resulted in a higher strain hardening rate and lower ductility value. In addition, the formation of thermally-induced martensite before plastic deformation reduced the capacity of strain partitioning and load transition between the austenite and ferrite. In contrast, the tensile properties of the alloy holding higher Mn content of 8% with higher austenite fraction and stability was mainly governed by twin-aided transformation route of γ→γtwinned→α′. It was found that the higher strain compatibility between austenite and ferrite resulted in a substantially high uniform elongation and moderate strain hardening rate.

Effect of Silica Content on Mechanical and Microstructure Behaviour of Resistance Spot Welded Advanced Automotive TRIP Steels

This research investigates the effect of SiC wt% on mechanical and microstructure behavior of transformation induced plasticity (TRIP) 780 steels by resistance spot welding. The resistance spot welded samples were characterized for their properties such as hardness, tensile shear, scanning electron microscope, X-ray diffraction, ductility ratio and elongation. Results showed that the width of the nugget was closely associated with shear failure of the spot welds. X-ray diffraction analysis illustrated that the weld steels chemical composition improved in the heat-affected zones and retained austenite detected due to the influence of Si and C. Sample 4 microstructure exposed the equiaxed dimple and finer dendrites in the fusion zone. It also exhibited maximum force and fracture energy. The amount of both displacement and energy absorption has improved by 51 %. Nano hardness was significantly decreased in the fusion zone of sample 4 due to the interface among micro alloying elements and the formation of nonmetallic presences that affected the TRIP steel hardness. Hardness of the casted TRIP steels is 15.5 % higher than base TRIP 780 steel. Low ductility ratios were observed in steel 4 than the other weld steels due to higher tensile shear strength (TSS) and cross-tension strength (CTS) results. Cross-tension strength and tensile shear strength of TRIP steel 5 has 29.8 %, 17.8 % higher TRIP steel 1. Fracture analysis exhibited ductile fracture with dimples and dendrites in the TRIP steels surface. The spot-welded samples mechanical properties are correlated to chemical elements, mainly Si existing in casted TRIP steels through the cooling phase of the resistance spot welding process.

Isothermal Holding Treatment of a Transformation-Induced Plasticity Steel for Obtaining Ultrahigh Strength and High Plasticity

The determining significance of isothermal holding on microstructure and mechanical properties of a transformation-induced plasticity steel (Fe-1.67Mn-1.32Al-0.55Si-0.47C) was studied with multiple techniques including x-ray diffraction, scanning electron microscopy and transmission electron microscopy. The objective was to design an optimal isothermal holding treatment for medium-carbon TRIP steels with ultrahigh strength and high elongation. A critical analysis of the experimental observations is presented. After isothermal holding treatment, the microstructure mainly consisted of ferrite, bainite, retained austenite, and a small amount of martensite-austenite island. The volume fraction of RA first increased from 28 to 32% with the increase of temperature from 380 to 420 °C, and then decreased dramatically to a minimum value of 23% with further increasing temperature to 450 °C. However, carbon content in RA decreased from 1.42 to 1.18% with the increase of temperature. A tensile strength of 1250 MPa and a maximum elongation of 55% were obtained at 420 °C because of the optimal combination of RA and carbon content. The highest yield strength of 660 MPa was obtained at 380 °C and the highest tensile strength of 1480 MPa was obtained at 450 °C, respectively. Less stable RA transformed to martensite, while RA with a high stability was retained during tensile straining.

Orientation dependence of transformation induced plasticity in high carbon bainitic steel

The crystallographic orientation dependence of the transformation induced plasticity (TRIP) in the high carbon bainitic steel was studied. 0.6% C-2.0% Si-1% Cr steels were austenitized at two conditions to obtain different size of prior-austenite grains. Subsequently, they were austempered to evolve the bainitic structure composed mainly of bainitic ferrite and retained austenite. The characteristic of this sample is that it has much larger amount of carbon content compared with that of the conventional low alloyed TRIP steels. The bainitic steels with different sizes of mean prior-austenite grain show high strength with large ductility. The measurement of electron back scattering diffraction (EBSD) of the samples both before and after the tensile test clarified that much of the retained austenite transformed to ferrite while the austenite grains having the tensile orientation nearly parallel to <111> kept remaining after the fracture. This change indicates that the retained austenite with its tensile axis parallel near to <001> is preferential for the deformation-induced martensitic transformation. This behavior was found in the samples with different prior-austenite grain sizes.

Powder Forging of in Axial and Radial Direction Graded Components of TRIP-Matrix-Composite

Powder metallurgy is one way of producing complex, graded structures that could allow material systems to be produced with properties tailored to individual applications. However, powder metallurgy requires that the semi-finished products are very similar to the final component. It is much more economical to produce simple semi-finished products and then combine them by powder forging and simultaneous compaction than forming complex components with the desired graded structure. However, it is absolutely necessary that the graded structure of the semi-finished products is maintained during the forming process. In this study, pre-sintered cylindrical semi-finished products, consisting of axially graded as well as radially graded components, were produced by powder forging at 1100 °C. The microstructures, densities and mechanical properties of the final components were investigated to verify the effectiveness of the process route. It was observed that the components formed solid structures after compaction, in which the reinforcing ZrO2 particles were fully integrated into the transformation-induced plasticity steel matrix.

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.

Effect of Type-B liquid metal embrittlement cracks on high-cycle fatigue properties of spot-welded 1180 TRIP steel

This study examines the effect of Type-B liquid metal embrittlement (LME) cracks on high-cycle fatigue resistance of a spot-welded plate of transformation-induced plasticity (TRIP) steel. Tensile shear and cross tension were employed to simulate a spot-welded component subjected to a complex state of stress. Interestingly, neither the existence nor the depth of Type-B LME cracks changed the high-cycle fatigue resistance of the spot-welded samples under the two deformation modes tested. The finite-element analysis revealed the formation of maximum local stress at the sample notch, which acted as a crack-initiation site. It should be noted that this site was not changed even upon adding a Type-B LME crack to the simulation model.

Microstructure and Mechanical Properties of a Lamellar-Structured Low-Alloy TRIP Steel

In this paper, we report a lamellar-structured low-alloy transformation-induced plasticity (TRIP) steel; the microstructure of the steel consists of alternate lamellae of intercritical ferrite and reverted austenite on microscale, with the latter consisting of bainitic ferrite laths and retained austenite films on nanoscale. Such a microstructure was produced by a heat treatment process similar to that for producing conventional TRIP-assisted steels, i.e. intercritical annealing followed by austempering. Nevertheless, quenched martensite rather than a mixture of ferrite and pearlite was used as the starting structure for intercritical annealing to form austenite, and the resulting austenite was then transformed to bainite by austempering treatment. This steel exhibits much enhanced strength-ductility combinations as compared with those conventional polygonal-structured low-alloy TRIP steels.

The Influence of the Drawing Process on the Mechanical Properties of TRIP Steel Wires with 0.4% C Content.

In the work, the results of the research concerned with the TRIP (Transformation Induced Plasticity) steel wire drawing process in experimental and theoretical ways are shown. The wire drawing process tests on the experimental way were conducted in both laboratories as well as industrial conditions, with the use of two drawing speeds (1.6 and 6 m/s) and two drawing schemes (low and high single reductions). The mechanical properties of wires drawn with high drawing speed equal to 6 m/s showed higher values of mechanical properties for wires drawn with low single reductions than for wires drawn with high single reductions. Such a phenomenon contradicts the theory of drawing wires from steel with a ferritic-pearlitic structure and must be related to TRIP structure and the presence of retained austenite in it, which is transformed into martensite during the deformation process. In order to explain this phenomenon, the theoretical wire drawing process analysis was conducted with the use of the Drawing 2D program based on the finite element method. On the base of the simulation, a large increase in temperature was found on the surface for wires drawn with high drawing speed and low single reductions, which can cause the blocking of transformation retained austenite into martensite and thus a decrease in Rm. To confirm this thesis, further studies will include tests of the amount of retained austenite in wires obtained during experimental tests.

Comparison of high strength steels with different aluminium and manganese contents using dilatometry

In this paper, the properties of three low carbon steels containing different percentage of manganese (1.5 or 3 wt%) and aluminium (1.5 or 2 wt %) were tested using dilatometric analysis and metallog-raphy. The steels had chemical compositions typical for TRIP (Transformation induced plasticity) steels and the two steels with an increased manganese content already belonged to the third genera-tion of AHS (advanced high strength) steels. The dilatometric measurement was employed not only to determine the characteristic transformation temperatures but also to simulate the heat treatment procedure commonly used for TRIP steels. Resulting microstructures obtained through this simula-tion were evaluated and compared. The two main alloying elements were proved to have a significant effect on the transition temperatures and thus on the processability and overall resulting properties of the material.

Fracture assessment in dual phase and transformation-induced plasticity steels during 3-point bending

The goal of this study was to examine the fracture behavior of advanced high strength steel (AHSS) sheets in bending. For this purpose, 3-point bending (3 PB) tests were conducted on DP980 and TRIP1180 sheet samples. The test specimens were prepared in different orientations with respect to the rolling direction of the sheets. The onset of fracture and fracture strains were determined through digital image correlation technique. The fracture regions of the deformed specimens were observed at different magnifications to understand the crack initiation mechanism. Apart from fracture characterization, a finite element study was performed for both steels using an Abaqus/Explicit solver. The parameters required for the constitutive equations were obtained based on mechanical characterizations carried out on these sheets in a previous work. Finally, an uncoupled fracture model was adopted to predict the fracture location and onset of cracking during the 3 PB test. The predicted results were found in reasonable agreement with the experimental data when the fracture model was calibrated using an appropriate set of mechanical tests.

Lean duplex TRIP steel: Role of ferrite in the texture development, plastic anisotropy, martensitic transformation kinetics, and stress partitioning

Constitutive micromechanical behavior of transformation induced plasticity (TRIP) stainless steel (SS) alloys was investigated using high-energy synchrotron x-ray diffraction (S-XRD) and in-situ neutron diffraction techniques. First, four different steel alloys were designed and produced: (1) a metastable austenitic TRIP SS, (2) a stable austenitic SS, which is a stable counterpart of the TRIP SS, (3) a lean duplex TRIP SS with ferrite and metastable austenite phases, and (4) a lean duplex stable SS, which is a stable counterpart of duplex TRIP SS. Then, effects of chemical composition, microstructure, and texture on the plastic anisotropy, martensitic transformation kinetics, and residual stress concentration during a tensile deformation were investigated. The results show that the plastic anisotropy, governed by the initial microstructure and texture, has insignificant effect on macroscopic tensile properties and martensitic phase transformation kinetics despite different R-values observed along different loading directions. On the other hand, the interplay between stress partitioning among constituent phases and martensitic phase transformation plays a critical role in the micromechanics of plastic deformation, and, consequently, determines the distributions of in-situ martensite fraction and residual stresses. The phase stress partitioning in the TRIP alloy clearly shows that a large tensile residual stress of 1.8 GPa can concentrate on the martensite phase with 30% plastic strain. In contrast, the introduction of the tensile load-sharing ferrite phase in the cost-effective lean duplex TRIP alloy significantly reduces the tensile residual stress concentration in the martensite phase, which could improve the formability of high-strength TRIP steels.

Improved work hardening of a medium carbon-TRIP steel by partial decomposition of retained austenite

In this study, the microstructure and mechanical properties of the 0.56 wt % C transformation-induced plasticity (TRIP) steel are investigated by conducting tensile tests at a wide span of deformation temperatures (25–400 °C) and strain rates (0.001–1 s−1). The target is to explore the stability of retained austenite (RA) at various temperatures and strain rates, and the influence of the RA stability on the TRIP effect. Tensile strength decreases with the increasing deformation temperature at a constant strain rate. When temperature changes from 200 °C to 300 °C, tensile strength increases from 1154 ± 10 MPa to 1227 ± 10 MPa. Interestingly, at 300 °C the total elongation at a strain rate of 0.01s−1 can reach 90%, which is twice than that at 25 °C. Moreover, the deformation induced precipitation of carbides reduces the stability of RA, promoting the martensitic transformation. This process counteracts the suppressed TRIP effect with the increase of temperatures. The newly formed precipitates effectively increase work hardening. Thus the TRIP effect and precipitation strengthening jointly lead to excellent mechanical properties of the steel at 300 °C. After deformation, the volume fraction of RA increases with the increasing strain rate at constant temperatures, suggesting that the increasing strain rate suppresses the transformation from RA to martensite.

The Effect of Preliminary Thermomechanical Processing on the Kinetics of Localized Plasticity Autowaves in Trip Steel

The kinetics of the martensitic transformation fronts in transformation-induced plasticity (TRIP) steel was studied in relation to preliminary thermomechanical treatment using the digital image correlation method. It was found that warm rolling of steel to 40–63% reduction significantly increases the stress of the onset of strain-induced phase transformation and changes the loading curve stages. The strain-induced phase transformation in TRIP steel occurring through the formation of Lüders and Portevin–Le Chatelier bands is shown to be an autowave process of localized plasticity. The austenite → martensite transformation at the elastic-plastic transition occurs in the form of several switching localized plasticity autowaves. At the jerky flow stage, excitation autowaves of localized plasticity are generated and propagate repeatedly until the strain-induced austenite → martensite transformation is completed. It is shown for the first time that the sources of excitation autowaves in the material are the sites of nucleation or annihilation of switching autowaves.

Evidence of Martensitic Transformation in Fe-Mn-Al Steel Similar to Maraging Steel

The Fe-Mn-Al steels claim a low density, and some fall into the category of transformation-induced plasticity (TRIP) steel. In Fe-Mn-Al TRIP steel development, phase transformations play an important role. Herein, the martensitic transformation of an Fe-16.7 Mn-3.4 Al ternary alloy (wt pct) was experimentally discovered, whose equilibrium phases are a single phase of austenite at 1373 K and dual phases of ferrite and austenite at low temperature. Ferritic lath martensite forms in the prior austenite grains after cooling from 1373 K under various cooling rates via quenching, air cooling, and furnace cooling. The formation mechanism of the ferritic lath martensite is different from that of traditional ferritic lath martensite in steel and quite similar to that in maraging steel. A slight strain energy coupled with a small temperature gradient can lead to the formation of ferritic lath martensite in the Fe-Mn-Al alloy after cooling from high temperature. It is also found that micro-twins exist in the ferritic lath martensite.

Application of response surface methodology for weld strength prediction in FSSWed TRIP steel joints

The effect of friction stir spot welding (FSSW) parameters on microstructure and mechanical properties of transformation-induced plasticity (TRIP) steel joints was investigated. The microstructure of the base metal and welding zone (stir zone, thermomechanically affected zone, and heat-affected zone) was examined using OM, SEM, EBSD, and XRD. The results of fractography of joints show that the microstructure and size of bonding ligament are the main parameters affected on the weld strength. The influence of FSSW parameters such as rotational speed, dwell time, and plunge depth on the strength of the TRIP steel joints was investigated using response surface methodology and considering bonding ligament of joints. The results of analysis of variance (ANOVA) show that the mathematical model was in good agreement with actual amounts of joints strength. Analysis of variance also determined that the plunge depth was a more effective parameter than other parameters. By optimization of the parameters, it was found that the maximum strength (9.42 kN) can be achieved by choosing of the rotational speed at 1218 rpm, dwell time at 1.65 s, and plunge depth at 0.19 mm.