Showing papers on "TRIP steel published in 2013"

On the Selection of the Optimal Intercritical Annealing Temperature for Medium Mn TRIP Steel

Abstract: A model is proposed to predict the room temperature austenite volume fraction as a function of the intercritical annealing temperature for medium Mn transformation-induced plasticity steel. The model takes into account the influence of the austenite composition on the martensite transformation kinetics and the influence of the intercritical annealing temperature dependence of the austenite grain size on the martensite start temperature. A maximum room temperature austenite volume fraction was obtained at a specific intercritical annealing temperature T M. Ultrafine-grained ferrite and austenite were observed in samples intercritically annealed below the T M temperature. The microstructure contained a large volume fraction of athermal martensite in samples annealed at an intercritical temperature higher than the T M temperature.

Strain rate dependent tensile behavior of advanced high strength steels: Experiment and constitutive modeling

Abstract: High strain rate tensile tests were conducted for three advanced high strength steels: DP780, DP980 and TRIP780. A high strain rate tensile test machine was used for applying the strain rate ranging from 0.1/s to 500/s. Details of the measured stress–strain responses were comparatively analyzed for the DP780 and TRIP780 steels which show similar microstructural feature and ultimate tensile strength, but different strengthening mechanisms. The experimental observations included: usual strain rate dependent plastic flow stress behavior in terms of the yield stress (YS), the ultimate tensile strength (UTS), the uniform elongation (UE) and the total elongation (TE) which were observed for the three materials. But, higher strain hardening rate at early plastic strain under quasi-static condition than that of some increased strain rates was featured for TRIP780 steel, which might result from more active transformation during deformation with lower velocity. The uniform elongation that explains the onset of instability and the total elongation were larger in case of TRIP steel than the DP steel for the whole strain rate range, but interestingly the fracture strain measured by the reduction of area (RA) method showed that the TRIP steel has lower values than DP steel. The fractographs using scanning electron microscopy (SEM) at the fractured surfaces were analyzed to relate measured fracture strain and the microstructural difference of the two materials during the process of fracture under various strain rates. Finally, constitutive modeling for the plastic flow stresses under various strain rates was provided in this study. The proposed constitutive law could represent both Hollomon-like and Voce-like hardening laws and the ratio between the two hardening types was efficiently controlled as a function of strain rate. The new strength model was validated successfully under various strain rates for several grades of steels such as mild steels, DP780, TRIP780, DP980 steels.

δ TRIP steel

Abstract: A combination of neural networks and genetic algorithms has been used to design a TRIP assisted steel in which the silicon concentration is kept low. In this context, the steel has a novel microstructure consisting of δ ferrite dendrites and a residual phase which at high temperatures is austenite. This austenite can, with appropriate heat treatment, evolve into a mixture of bainitic ferrite and carbon enriched retained austenite. The steel has been manufactured and tested to reveal a tensile strength of ∼ 1 GPa and a uniform elongation of 23%.

Constitutive Modeling of the Mechanical Properties of V-added Medium Manganese TRIP Steel

Abstract: In this study, medium Mn transformation-induced plasticity steel with the composition Fe-0.08 pct C-6.15 pct Mn-1.5 pct Si-2.0 pct Al-0.08 pct V was investigated. After intercritical annealing at 1013 K (740 °C), the steel contained coarse-grained ferrite and two ultrafine-grained (UFG) phases: ferrite and retained austenite. The material did not deform by localized Luders band propagation: it did not suffer from this major problem as most UFG steels do. Localization of plastic flow was shown to be suppressed because of a combination of factors, including a bimodal grain size distribution, a multiphase microstructure, the presence of nanosized vanadium carbide precipitates, and the occurrence of the deformation-induced martensitic transformation of retained austenite. A constitutive model incorporating these effects was developed. The model was used to identify the factors which can lead to a further improvement of the mechanical properties of the UFG medium Mn TRIP steels.

Microstructural evolution and mechanical properties of hot-rolled 11% manganese TRIP steel

Abstract: Mechanical behaviors of transformation induced plasticity (TRIP) steels largely depend on the amount and stability of austenite. In this investigation, a large volume fraction of austenite (>65%) was produced in a hot-rolled Fe-11Mn-3.8Al-0.18C TRIP steel by solution treatment in the temperature range of 750–800 °C for 1 h. The hot-rolled alloy exhibited an excellent combination of total elongation of 35–40% and ultimate tensile strength of 880–1100 MPa and this was found to have a similar or higher level of tensile properties compared with other TRIP steels. In the meantime, less cold-rolling work or annealing time was required in the present work. The outstanding properties of the experimental steel were mainly attributed to the enhanced TRIP effect due to the large fraction of austenite. It is shown that the morphology played a more significant role than orientation in the stability of austenite.

Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion

Abstract: The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0–5 wt% Mn, 0–2 wt% Si, constant 13.5 wt% Cr and 0.45 wt% C) and seven martensite tempering conditions (300–500 °C, 0–30 min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35 vol%) leading to a martensitic–austenitic TRIP steel with optimum mechanical properties (1.5 GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion.

Work Hardening Behavior in Steel with Multiple TRIP Mechanisms

Abstract: Transformation-induced plasticity (TRIP) behavior was studied in steel with the composition Fe-0.07C-2.85Si-15.3Mn-2.4Al-0.017N that exhibited two TRIP mechanisms. The initial microstructure consisted of both e- and α-martensites with 27 pct retained austenite. TRIP behavior in the first 5 pct strain was predominately austenite transforming to e-martensite (Stage I), but upon saturation of Stage I, the e-martensite transformed to α-martensite (Stage II). Alloy segregation also affected the TRIP behavior with alloy-rich regions producing TRIP just prior to necking. This behavior was explained by first-principles calculations which revealed that aluminum significantly affected the stacking fault energy in Fe-Mn-Al-C steels by decreasing the unstable stacking fault energy and promoting easy nucleation of e-martensite. The addition of aluminum also raised the intrinsic stacking fault energy and caused the e-martensite to be unstable and transform to α-martensite under further deformation. The two-stage TRIP behavior produced a high strain hardening exponent of 1.4 and led to an ultimate tensile strength of 1165 MPa and elongation to failure of 35 pct.

Ultrafine grained high-alloyed austenitic TRIP steel

Abstract: High-alloyed cast TRIP steel showing pronounced martensitic phase transformation during plastic deformation like tensile, compressive or cyclic loading exhibits concurrently high strength and high ductility. Further increase in the yield strength/ultimate tensile strength can be realized by a smaller austenitic grain structure. An ultrafine grain size of the austenite can be realized among others by well situated thermo-mechanically controlled processing like combination of cold rolling and subsequent specific heat treatment. Cast plates of high-alloyed TRIP steel were cold rolled to different deformation degrees. Subsequently, heat treatment experiments at different annealing temperatures and annealing times were performed. The microstructures after heat treatment were investigated by X-ray diffraction and scanning electron microscopy. The obtained grain size of the reverted austenite was determined by EBSD measurements. Finally, tensile tests on reverted austenitic steel specimens were performed in order to determine the influence of the grain size on the mechanical properties. The results show that plates of a high-alloyed cast TRIP steel can be cold rolled up to 90% of thickness reduction leading to high amount of α′-martensite (80%). Specific heat treatment of 90% cold rolled TRIP steel results in ultrafine grained reverted austenitic microstructure with mean grain size of about 1 μm. Tensile tests of heat treated steel specimens revealed an enormous increase in the yield strength up to 1000 MPa. The ultrafine grained austenitic steels still show the TRIP effect at a grain size of 1 μm.

Improved ductility of a transformation-induced-plasticity steel by nanoscale austenite lamellae

Abstract: TRIP (transformation-induced-plasticity) steel with a chemical composition of 0.19C–0.30Si–1.76Mn–1.52Al (weight percentage, wt%) have been treated by intercritical annealing and austempering process. The microstructures of the obtained samples consist of the ferrite, the bainite and the retained austenite phase. The volume fractions of the bainite and the retained austenite gradually increase with increasing the temperature of the intercritical annealing. Consequently, significantly different mechanical properties have been observed. The sample annealed at 820 °C (for 120 s) and partitioned at 400 °C (for 300 s) has the best combination of ultimate tensile strength (UTS, ∼682 MPa) and elongation to failure (∼70%) with about 26% of bainitic ferrite plates and 17% retained austenite in its microstructure. The retained austenite has a lamella morphology with 100‒300 nm in thickness and 2‒5 µm in length. On the contrary, the sample annealed at the same temperature without the partitioning process yields much lower UTS and elongation to failure.

On the effect of austenite stability on high cycle fatigue of TRIP 700 steel

Abstract: The high cycle fatigue behavior and the role of austenite stability on fatigue performance of low-alloy TRIP steel 700 have been experimentally investigated. The material was subjected to heat treatment in order to produce microstructures with different initial retained austenite volume fraction and austenite stability. High cycle fatigue tests were carried out to determine the S–N fatigue curve while austenite stability was measured by implementing a special technique for determination of M s σ temperature. The effect of austenite stability on fatigue behavior was assessed by measurements of volume fraction austenite before and after fatigue testing. The fatigue results indicated that austenite stability influences fatigue performance of TRIP steel in the high cycle regime, especially at high cyclic stresses.

Strain Hardening and Strain Rate Sensitivity Behaviors of Advanced High Strength Steels

Abstract: The mechanical properties of commercial dual phase (DP), transformation induced plasticity (TRIP), and high strength low alloy (HSLA-340) steel sheets are investigated and compared at various strain rates ranging from 0. 0017 to 0.17 s−1 at ambient temperature. TRIP steel outperforms the other two materials, having comparable ductility and twice as large strength relative to DP steel. TRIP has larger strength and much larger ductility than HSLA-340. The exceuent ductility of TRIP800 is due to its high strain hardening capability, which promotes stable plastic deformation. It is observed that the strain hardening rate in TRIP800 does not decrease to zero at failure, as common in most materials in which failure is preceded by necking.

Microstructure Defects Contributing to the Energy Absorption in CrMnNi TRIP Steels

Abstract: Microstructure defects control the TRIP effect and/or the TWIP effect and contribute significantly to the absorption of deformation energy in plastically deformed austenitic CrMnNi steels. In this study, the propagation and interaction of dislocations, stacking faults and twins connected with the formation of Lomer-Cottrell locks, stacking fault tetrahedra, dislocation clusters, deformation bands, microtwins with high-energy incoherent twin boundaries and the nucleation of α′- martensite in the areas of the high local lattice strain due to the fluctuation of the stacking fault density and the lattice shearing, were analysed in the CrMnNi TRIP steel after different deformation extents via transmission electron microscope with high resolution and via scanning electron microscope.

Microstructural Analysis of a Thermomechanically Processed Si-Al TRIP Steel Characterized by EBSD and X-Ray Techniques

Abstract: The work focuses on the analysis of microstructural features of retained austenite in a thermomechanically processed Si-Al TRIP-type steel microalloyed with Nb and Ti. Austenite amount was determined using XRD and EBSD. Combined methods of LM, SEM and EBSD were applied to reveal the morphology, grain size and distribution of structural constituents. It is possible to retain 14% of  phase enriched in C to about 1.14 wt.%. Retained austenite is uniformly located as blocky grains with a diameter up to 4.5 m in a fine-grained ferritic matrix or between bainitic ferrite laths as thin layers. Special crystallographic relationships between bainitic ferrite and retained austenite were identified on the basis of the analysis of misorientation angles and image quality values.

Development and characterisation of model TRIP steel microstructures

Abstract: The fundamental issue in the understanding and development of TRIP steels is the behaviour and control of the retained austenite during deformation. In this study the microstructural factors that influence retained austenite stability were isolated in four model microstructures designed for this purpose using a standard Si-TRIP steel composition. Detailed characterisation revealed that the microstructures represented a wide range of size, shape and surrounding phase for the retained austenite. Deduction of the evolution of the microstructures during processing completed the understanding of the microstructural differences for the four cases. The results of tensile tests showed that the microstructures had a corresponding wide range of mechanical properties, where YS and UTS inversely correlated with the scale of the features of the surrounding phase. In three of the microstructures the incremental work hardening exponent reached a constant, which is characteristic of the TRIP effect, the value of which increased with larger, more equiaxed martensite/austenite features and with lower carbon concentration in the retained austenite. These same three microstructures had high values of strength x ductility indicating a useful TRIP effect.

On the Low Temperature Strain Aging of Bainite in the TRIP Steel

Abstract: The aging behavior of a thermomechanically processed Mo-Al-Nb transformation-induced plasticity steel with ultrafine microstructure was investigated using transmission electron microscopy and atom probe tomography (APT). Strain aging at 73 K (200 °C) for 1800 seconds led to a significant bake-hardening response (up to 222 MPa). Moreover, aging for 1800 seconds at room temperature after 4 pct pre-strain also revealed a bake-hardening response (~60 MPa). The experimental results showed the formation of carbon Cottrell atmospheres around dislocations and the formation of carbon clusters/fine carbides in the bainitic ferrite during aging. It is proposed that this is associated with the high dislocation density of bainitic ferrite with formation of a complex dislocation substructure after pre-straining and its high average carbon content (~0.35 at. pct). The segregation of carbon and substitutional elements such as Mn and Mo to the retained austenite/bainitic ferrite interface during aging was observed by APT. This segregation is likely to be the preliminary stage for Mo-C particles’ formation. The aging after pre-straining also induced the decomposition of retained austenite with formation of ferrite and carbides.

Effects of Cold Rolling Reduction on Retained Austenite Fraction and Mechanical Properties of High-Si TRIP Steel

Abstract: Transformation-induced plasticity-aided steel [TRIP steel (0. 4C-1. 5Si-1. 5Mn)] was rolled at room temperature to different thickness reductions (0, 4%, 10%, 20%, 40%, and 60%). The mechanical properties, microstructure and austenite fractions of the rolled samples were measured by tensile test, electron back scattered diffraction (EBSD) and X-ray diffraction (XRD) for each rolling. The deformation behavior was studied based on the analysis of the mechanical properties and microstructure of steel after tensile deformation, aiming at understanding the effects of cold rolling reduction on the decay behavior of the austenite and the change of mechanical properties of the TRIP steels. It was found that increasing rolling reduction increases the yield stress gradually but decreases the total elongation significantly. It is very interesting that after 10% rolling reduction the yield stress is about 1000 MPa but still with 20% total elongation, which gives an excellent combination of yield strength and ductility. Based on the XRD results, it was revealed that in both rolling and tension the austenite volume fraction monotonically decayed with the increase of rolling strain, but the decaying rate is faster in tension than in rolling, which may be ascribed to the higher temperature in rolled specimens than in the tensioned ones during deformation. Experimental results and theoretical reasoning indicate that the decreasing trend of austenite volume fraction with strain can be formulated by a unique equation.

Microstructure Evolution during Friction Stir Spot Welding of TRIP steel

Abstract: Transformation Induced Plasticity (TRIP) steels have been developed for automotive applications due to the excellent high strength and formability. The microstructure of TRIP steels is a complex mixture of various microstructural constituents; ferrite, bainite, martensite and retained austenite. The TRIP effect is activated under the influence of an external load, thereby leading to a martensitic transformation of the retained austenite. This transformation induced plasticity contributes to the excellent mechanical properties of this class of steels and provides high tensile strength without deteriorating the uniform elongation. The unique deformation properties can be exploited in automotive applications for crash resistant parts due to the high energy absorption, thus improving passenger safety. Furthermore, the high strength and good formability permits the application of thinner sheet material and thereby reduced weight of the vehicles. One of the limitations for the wide application of TRIP steel is associated with joining, since so far no method has succeeded in joining TRIP steel, without comprising the steel properties. In this study, the potential of joining TRIP steel with Friction Stir Spot Welding (FSSW) is investigated. The aim of the study is to assess whether high quality welds can be produced and, in particular, to obtain an understanding of the microstructural changes during welding. The microstructure of the welded samples was investigated by means of reflected light microscopy, scanning electron microscopy and electron backscatter diffraction. Microhardness measurements and lab-shear tests completed the investigations of the welded samples and allow evaluation of the quality of the welds as seen from a practical point of view. Selected samples were also investigated by X-ray diffraction. The complementary use of the various characterization techniques allowed subdivision of the microstructure in the weld in different zones: two thermo-mechanically affected zones (TMAZs), and two heat-affected zones (HAZs). The dual behavior of the microstructure in the

Ultra Fine-Grained Advanced High Strength Steel Sheet Having Superior Formability

Abstract: A cold rolled, annealed TRIP steel sheet which has a composition including (in wt. %): C: 0.1-0.3; Mn: 4-10, Al: 0.05-5, Si: 0.05-5; and Nb: 0.008-0.1, the remainder being iron and inevitable residuals. The cold rolled sheet has an ultimate tensile strength of at least 1000 MPa, and a total elongation of at least 15%. The cold rolled sheet may have at least 20% retained austenite in its microstructure and may have greater than 50% lath-type annealed ferrite structure. The cold rolled sheet may have an ultra fine grain size of less than 5 micron for the retained austenite and ferrite.

Neural network modelling of flow stress and mechanical properties for hot strip rolling of TRIP steel using efficient learning algorithm

Abstract: Transformation induced plasticity (TRIP) steels exhibit excellent strength and ductility and can be engineered to provide excellent formability for manufacturing complex parts. In this study, a data driven multi-input multi-output multilayer perceptron based neural network model has been developed to predict the flow stress, yield strength, ultimate tensile strength and elongation as a function of composition and thermomechanical processing parameters for strip rolling of TRIP steels. The input parameters in this generalised regression artificial neural network (ANN) model are steel chemistry, cooling rate and finish roll temperature. The network training architecture has been optimised using the Broyden–Fletcher–Goldfarb–Shanno algorithm to minimise the network training error within few training cycles. The algorithm facilitates a faster convergence of network training and testing errors. There has been an excellent agreement between the ANN model predictions and the target (measured) values for ...