Low-alloy Transformation-induced Plasticity Steels Research Articles

Application of a sparse mixed regression method to design the optimal composition and heat treatment conditions for transformation-induced plasticity steel with high strength and large elongation

The chemical compositions and heat treatment conditions for obtaining low-alloyed transformation-induced plasticity (TRIP) steel were designed using a sparse mixed regression method (SMRM) that automatically and spontaneously selected several regression models. The input data concerning the elemental compositions (Fe, C, Si, Mn, Cr, Ni) and austempering conditions were collected from published reports. The SMRM-based analysis of these data established three models, which each proposed conditions for the alloy and process for obtaining high strength and large elongation. Experimental results confirmed that all three sets of conditions provided bainite with retained austenite, which was necessary for the low-alloyed TRIP steel. The tensile tests revealed that all three cases exhibited strengths >1.6 GPa and elongations >11%. Evaluating the metallurgical parameters highlighted two different design concepts, in one of which key factor is to reduce the Mn content, which is different from most of the conventional approach for preparing low-alloyed TRIP steel.

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.

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.

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.

In Situ Observation for Deformation-Induced Martensite Transformation (DIMT) during Tensile Deformation of 304 Stainless Steel Using Neutron Diffraction. PART I: Mechanical Response

304 stainless steel is one of the most common stainless steels due to its excellent corrosion resistance and mechanical properties. Typically, a good balance between ductility and strength derives from deformation-induced martensite transformation (DIMT), but this mechanism has not been fully explained. In this study, we conducted in situ neutron diffraction measurements during the tensile deformation of commercial 304 stainless steel (at room temperature) by means of a Time-Of-Flight type neutron diffractometer, iMATERIA (BL20), at J-PARC MLF (Japan Proton Accelerator Research Complex, Materials and Life Science Experimental Facility), Japan. The fractions of α′-(BCC) and ε-(HCP) martensite were quantitatively determined by Rietveld-texture analysis, as well as the anisotropic microstrains. The strain hardening behavior corresponded well to the microstrain development in the austenite phase. Hence, the authors concluded that the existence of martensite was not a direct cause of hardening, because the dominant austenite phase strengthened to equivalent values as in the martensite phase. Moreover, the transformation-induced plasticity (TRIP) mechanism in austenitic steels is different from that of low-alloy bainitic TRIP steels.

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.

Effect of Retained Austenite and Non-Metallic Inclusions on the Mechanical Properties of Resistance Spot Welding Nuggets of Low-Alloy TRIP Steels

The combination of high strength and formability of transformation induced plasticity (TRIP) steels is interesting for the automotive industry. However, the poor weldability limits its industrial application. This paper shows the results of six low-alloy TRIP steels with different chemical composition which were studied in order to correlate retained austenite (RA) and non-metallic inclusions (NMI) with their resistance spot welded zones to their joints’ final mechanical properties. RA volume fractions were quantified by X-ray microdiffraction (µSXRD) while the magnetic saturation technique was used to quantify NMI contents. Microstructural characterization and NMI of the base metals and spot welds were assessed using scanning electron microscopy (SEM). Weld nuggets macrostructures were identified using optical microscopy (OM). The lap-shear tensile test was used to determine the final mechanical properties of the welded joints. It was found that NMI content in the fusion zone (FZ) was higher than those in the base metal and heat affected zone (HAZ). Whereas, traces of RA were found in the HAZ of highly alloyed TRIP steels. Lap-shear tensile test results showed that mechanical properties of spot welds were affected by NMI contents, but in a major way by the decomposition of RA in the FZ and HAZ.

In Situ Observation of Bainite Transformation and Simultaneous Carbon Enrichment in Austenite in Low-Alloyed TRIP Steel Using Time-of-Flight Neutron Diffraction Techniques

An in situ neutron diffraction experiment during austempering of low-alloyed transformation-induced plasticity steel, Fe-1.48Si-1.52Mn-0.15C, in wt pct was conducted. In this study, time-of-flight neutron diffractometer with a large detector coverage, iMATERIA at J-PARC MLF, was employed. The phase fraction and carbon concentration in austenite could be quantitatively determined with a time resolution 1 minute although considerable textures existed for both ferrite and austenite. The carbon concentration in austenite during austempering was found to be inhomogeneous, resulting in a bimodal concentration distribution. The low-carbon region was consumed by bainite transformation whereas the high-carbon austenite slightly increased and even survived the final cooling to room temperature, forming a retained austenite. The rate of bainite transformation was affected by the state prior to the start of austempering. Consequently, different morphological features of the retained austenite were formed. More block-shaped austenite was observed in the case of slower bainite transformation, and it was determined that film-shaped austenite could also exist. The average carbon concentration was similar to that of high-carbon austenite during austempering. Hence, the film and block shapes of the retained austenite do not necessarily reflect the difference in carbon concentration.

Probing the Evolution of Retained Austenite in TRIP Steel During Strain-Induced Transformation: A Multitechnique Investigation

X-ray diffraction analysis, magnetic force microscopy, and the saturation magnetization method have been employed to study the evolution of the percentage and size of retained austenite (RA) particles during strain-induced transformation in a transformation-induced plasticity (TRIP) steel. A low-alloy TRIP-700 steel with nominal composition Fe-0.2C-0.34Si-1.99Mn-1Al (mass%) was subjected to interrupted tensile testing at strain levels of 0–22% and the microstructure subsequently studied. The results of the three experimental techniques were in very good agreement regarding the estimated austenite volume fraction and its evolution with strain. Furthermore, this multitechnique approach revealed that the average particle size of RA reduced as the applied strain was increased, suggesting that larger particles are less stable and more susceptible to strain-induced phase transformation. Such experimentally determined evolution of the austenite size with strain could serve as an input to kinetic models that aim to predict the strain-induced transformation in low-alloy TRIP steels.

Observation of deformation twinning and martensitic transformation during nanoindentation of a transformation-induced plasticity steel

For the first time, deformation twinning and martensitic transformation were observed in retained austenite in a low-alloyed transformation-induced plasticity steel using nanoindentation in conjunction with electron backscattering diffraction and transmission electron microscopy. Dislocation glide, martensite formation and deformation twinning were correlated to pop-ins and deviation from linearity in the load-displacement curve. Deformation twinning was found to enhance the stability of retained austenite. This observation furthers our understanding of RA stability during straining of low-alloyed multiphase TRIP steel.

Impact properties of low-alloy transformation-induced plasticity-steels with different matrix

The effects of matrix types on Charpy impact properties were investigated in Fe–0.2%C–1.5%Si–1.5%Mn (mass%) transformation-induced plasticity steels. The steels with annealed martensite and bainitic ferrite matrix possessed higher upper shelf Charpy impact absorbed energy than the steel with polygonal ferrite and martensitic matrix. In addition, the low ductile–brittle fracture appearance transition temperatures were achieved in annealed martensite and martensite types in comparison with those of other steels.

Micro and macro-mechanical behavior of a transformation-induced plasticity steel developed by thermomechanical processing followed by quenching and partitioning

A low-alloyed transformation induced plasticity steel was subjected to thermomechanical processing (TMP) followed by quenching and partitioning treatment to achieve a desired combination of the strength and ductility. The developed microstructures were precisely analyzed, and the mechanical responses of individual micro-constituents were studied by nanoindentation using a reliable scanning probe microscopy. The results indicate that the characteristics of the constituent phases (i.e., the lath martensite, blocky fresh martensite and the retained austenite) were dictated by the prior TMP. The occurrence of dynamic recrystallization and the formation of equiaxed-shape fine austenite grains during TMP would provide fast diffusion track for carbon to diffuse through the untransformed austenite. The carbon partitioning from martensite to the surrounding austenite ensures the austenite stabilization and was identified as the major factor for martensite softening at micro-scale level. The room temperature mechanical properties were studied via shear punch and tensile testing methods. The obtained superior mechanical properties, the ultimate tensile stress of ~1400MPa and shear elongation to fracture of ~19% were justified considering the proper work hardening behavior of the material.

The effect of martensite banding on the mechanical properties and formability of TRIP steels

Transformation Induced Plasticity (TRIP) steels possess high strength and great formability. In the current research work, the effect of banded martensite microstructure onto the mechanical properties and formability behaviour of low-alloyed TRIP steels was investigated. The results showed that the TRIP steel accumulated a larger strain prior to fracture when martensite appeared in form of bands.

Effect of Partial Replacement of Si with Al on the Microstructures and Mechanical Properties of 1000 MPa TRIP Steels

Two newly synthesized C-Mn-Si-Mo-Nb transformation-induced plasticity (TRIP) steels with and without Al addition were designed in order to achieve significant improvements in the mechanical properties. The effect of substitution of Si by Al on tensile properties and the microstructure of cold-rolled C-Mn-Si TRIP steel was investigated under different heat treatments. It was shown that a complex ultrafine microstructure composed of different phases was formed and two types of morphology for ferrite were detected (equiaxial and polygonal). The distribution of alloying elements was observed by using electron probe microanalysis. It was clear that C was concentrated in the retained austenite (RA) and small M/A (austenite/martensite) islands. The Al addition facilitated the formation of polygonal ferrite and increased the stability of the RA. The strain-hardening behavior was studied in detail. All the investigated specimens showed a very high strain-hardening exponent (instantaneous n) but their strain dependence was different. For the C-Mn-Si-Mo-Nb TRIP steel, the maximum n value was achieved when the strain was only about 0.04, while the n value of the Al substituted TRIP steel increased gradually until strains in the range of 0.07-0.10 were reached and the maximum value was achieved. As a result, the elongations of the steel with Al addition increased considerably without obvious deterioration of strength. It was the first time to find microtwinned martensite located between ferrite and bainitic ferrite after tensile deformation in the low alloy TRIP steel with Al.

Position-dependent shear-induced austenite–martensite transformation in double-notched TRIP and dual-phase steel samples

While earlier studies on transformation-induced-plasticity (TRIP) steels focused on the determination of the austenite-to-martensite decomposition in uniform deformation or thermal fields, the current research focuses on the determination of the local retained austenite-to-martensite transformation behaviour in an inhomogeneous yet carefully controlled shear-loaded region of double-notched TRIP and dual-phase (DP) steel samples. A detailed powder analysis has been performed to simultaneously monitor the evolution of the phase fraction and the changes in average carbon concentration of metastable austenite together with the local strain components in the constituent phases as a function of the macroscopic stress and location with respect to the shear band. The metastable retained austenite shows a mechanically induced martensitic transformation in the localized shear zone, which is accompanied by an increase in average carbon concentration of the remaining austenite due to a preferred transformation of the austenite grains with the lowest carbon concentration. At the later deformation stages the geometry of the shear test samples results in the development of an additional tensile component. The experimental strain field within the probed sample area is in good agreement with finite element calculations. The strain development observed in the low-alloyed TRIP steel with metastable austenite is compared with that of steels with the same chemical composition containing either no austenite (a DP grade) or stable retained austenite (a TRIP grade produced at a long bainitic holding time). The transformation of metastable austenite under shear is a complex interplay between the local microstructure and the evolving strain fields.

The Effect of Intercritical Annealing on the Microstructure and Mechanical Properties in a High Strength TRIP Steel with C‐Partitioning

Direct experimental evidence confirms the partitioning of C to austenite during intercritical annealing is the intrinsic factor that affects the stability of the retained austenite for a newly developed high strength transformation‐induced plasticity (TRIP) steel. The volume fraction of retained austenite and C concentration decreases with increasing intercritical annealing time. Few retained austenite grains and a number of M/A islands were observed in the sample at 830°C annealed for 30 min. When the sample annealed at 830°C for 0.5 min and then isothermally treated at 440°C for 3 min, the investigated low alloy TRIP steel achieved the tensile strength higher than 1000 MPa, the total elongation of 21.8% and the n‐value of 0.32.

High-energy X-ray diffraction study on the temperature-dependent mechanical stability of retained austenite in low-alloyed TRIP steels

The stability of the retained austenite has been studied in situ in low-alloyed transformation-induced-plasticity (TRIP) steels using high-energy X-ray diffraction during tensile tests at variable temperatures down to 153 K. A detailed powder diffraction analysis has been performed to probe the austenite-to-martensite transformation by characterizing the evolution of the phase fraction, load partitioning and texture of the constituent phases simultaneously. Our results show that at lower temperatures the mechanically induced austenite transformation is significantly enhanced and extends over a wider deformation range, resulting in a higher elongation at fracture. Low carbon content grains transform first, leading to an initial increase in average carbon concentration of the remaining austenite. Later the carbon content saturates while the austenite still continues to transform. In the elastic regime the probed { h k l} planes develop different strains reflecting the elastic anisotropy of the constituent phases. The observed texture evolution indicates that the austenite grains oriented with the {2 0 0} plane along the loading direction are transformed preferentially as they show the highest resolved shear stress. For increasing degrees of plastic deformation the combined preferential transformation and grain rotation results in the standard deformation texture for austenite with the {1 1 1} component along the loading direction. The mechanical stability of retained austenite in TRIP steel is found to be a complex interplay between carbon concentration in the austenite, grain orientation, load partitioning and temperature.

In-situ synchrotron diffraction and digital image correlation technique for characterizations of retained austenite stability in low-alloyed transformation induced plasticity steel

Direct measurement and quantification of phase transformation in a low-alloyed transformation induced plasticity steels depending on the tensile load as well as determination of the real true stress and true strain values were carried out in-situ using high energy synchrotron radiation . Digital image correlation technique was used to quantify more precisely the true strain values. The aim of the work was to obtain a better understanding of the phase transformation of commercial low-alloyed transformation induced plasticity steel depending on the true strain and true stress values.

The effect of aluminium and phosphorus on the stability of individual austenite grains in TRIP steels

We have performed in situ synchrotron X-ray diffraction experiments to assess the influence of aluminium and phosphorus on the austenite stability in low-alloyed transformation-induced plasticity steels during the high-temperature bainitic holding and the subsequent martensitic transformation during cooling to temperatures between room temperature and 100 K. Although the addition of aluminium increases the chemical driving force for the formation of bainitic ferrite plates significantly, the phosphorus exerts a larger influence on the bainitic transformation kinetics. Consequently, the addition of phosphorus leads to a higher degree of carbon enrichment and a narrower grain volume distribution of the metastable austenite. The stability of the individual austenite grains with respect to their martensitic transformation depends on both the local carbon content and the grain volume for austenite grains smaller than 20 μm 3. The presence of aluminium and phosphorus further stabilizes the austenite grains.

Simulation of mechanical behavior of multiphase TRIP steel taking account of transformation-induced plasticity

Due to the effect of transformation-induced plasticity, multiphase low alloy TRIP steel exhibits an enhanced combination of strength and ductility. Moreover, volume fractions of the constituents in TRIP steel vary during its plastic deformation. In this paper, a constitutive model for mechanical behavior of multiphase TRIP steel is presented. In the model, TRIP steel microstructure is decomposed into four individual constituents: austenite, martensite, bainitic ferrite and ferrite. Mechanical behavior of each individual phase is described by using physically-based model. On the basis of introducing transformation-induced plasticity (TRIP) strain into decomposition of total strain, stress–strain relation of multiphase composite is obtained by using mixture rule and Iso-W hypothesis. Kinetics of strain-induced martensitic transformation is described by a generalized form of Olson–Cohen model, which takes into account temperature and stress state. Moreover, a new method to describe evolving grain size of retained austenite due to martensitic transformation is developed. On the basis of presented model, mechanical behavior of multiphase steel is simulated by using finite element method. Parameters of the model are calibrated for different mixture laws used in calculation. The simulated results have a good agreement with those observed in experiments.