Microstructure Of TRIP Steel Research Articles

Observation of Retained γ Grains in TRIP Steels Using SEM-FIB/EBSD Method and Examination of Stability Evaluation Method

Morphologies of retained γ grains in TRIP steels are a very important factor for understanding the relationship between the microstructure and mechanical properties of TRIP steels. We have investigated a serial sectioning technique using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) and a focused ion beam (FIB) to reconstruct 3D microstructure of TRIP steels. In this paper, FIB fabrication condition dependences of reconstructed 3D structures are presented in TRIP steels and γ-grains stabilities are discussed in terms of ion beam effects. Retained γ-grains are not recognized by EBSD map from a FIB fabricated surface using normal ion accelerating voltage of 30 kV, because γ grains are easily transformed into martensite due to ion irradiation. It was found that the transformation was suppressed by using a low acceleration voltage of 7 kV or less, and the three-dimensional morphology could be observed using low acceleration voltage serial sectioning technique. It becomes possible to evaluate morphology of each phase and we proposed evaluation method of γ stabilities by ion irradiation transformation.

Multiphase steel microstructure and properties optimisation through a new heat treatment process

The aim of this work was to develop a new heat treatment technology, which allowed an ultrafine-grained multiphase microstructure in 35CrSiMn5-5-4 structural steel to be obtained. This microstructure was produced by the heat treatment, usually applied to TRIP steels, but with an additional pre-heat treatment step. It was shown that the additional pre-heat treatment resulted in a radical change of the microstructure which became more homogenous and much more fine-grained in comparison with the conventional TRIP steel microstructure. TEM observations revealed the microstructure composed of a mixture of ultra-fined grains of ferrite, bainite and retained austenite. Owing to the pre-heat treatment, 35CrSiMn5-5-4 steel gains higher strength and plasticity parameters in comparison to the steel after the treatment without the preliminary heat treatment.

Effect of strain rate on mechanical properties and microstructure of TRIP steel

TRIP effect is one of the important features for TRIP steel. A Nb-Mo microalloyed TRIP steel was developed by employing two different soaking time during annealing, and tensile tests at three different strain rate for specimens after annealing were performed. The microstructures and the volume fraction of retained martensite were examined and analyzed. The result indicated that TRIP steel showed significantly different behaviour at different strain rates. The ultimate tensile strength and total elongation decreased with the strain rate increasing. The dynamic strain aging effect might occur at low strain rate. With shorter soaking time during annealing, microstructure with ultra-refined grain size could be achieved. Fracture surfaces were different at different strain rate. As strain rate increases, the fraction of retained austenite to martensite transformation at failure point decreases which leads to the difference of mechanical properties.

Influence of chemical composition and parameters of heat treatment on the mechanical properties and microstructure of TRIP steels

Good mechanical properties of steels, in which appropriate heat treatment can produce mixed hardening structures, make them candidates for a broad range of applications, namely in the automotive industry. TRIP steels (in which transformation-induced plasticity operates) with a carbon content of approximately 0.2 % are one of such steels. Heat treatment of these steels comprises two stages. The first involve heating to the intercritical region between the A1 and A3. It is followed by cooling to a bainitic transformation temperature and holding. The resulting mixed microstructure consist of ferrite, bainite and retained austenite. Thanks to the presence of ferrite and retained austenite, the ultimate strength and elongation can reach 1500 MPa and 25-40 %, respectively. The experiments presented in this paper were performed on two steels which chemistries were specially adjusted to support formation of TRIP microstructure. The main difference between them was the level of chromium. Intercritical annealing was carried out on both steels. Aspects of interest included mainly the effect of the cooling rate above the bainitic transformation temperature and the holding time on mechanical properties and final microstructure. The heat treatment led to microstructures ferrite, bainite and retained austenite. The strength was under 1100 MPa and elongation reached 28 %.

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

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

Application of color etching to study the microstructure of TRIP steel after laser remelting

TRIP type steels have a multi-phase structure, which includes such phases as: aus-tenite, bainite, ferrite and martensite. The presence of so many co-existing phases creates difficulties in their accurate identification. One of the methods used to identify the components of the microstructure is color metallography. Methods of color met-allography in contrary to some methods of microstructure identification (e.g. TEM, EBSD) are simple to use, cheap and not very time-consuming. However, there are still no detailed recommendations on the use of this method. The paper examines the pos- sibilities of application of colored etching methods, to distinguish the components of the microstructure of the as-received material and the welds of the TRIP type steel. Light microscopy methods were used for the study. The obtained results allow for a qualitative distinction of individual components of the microstructure.

A comparative study between conventional and hybrid friction stir welding of a TRIP steel

The microstructure and mechanical properties of the conventional and hybrid friction stir welded joints of a TRIP steel were investigated. For the aim of hybrid welding, oxy acetylene flame assisted process was conducted. Optical microscopy, x-ray diffraction, and microhardness test were used to evaluate the microstructure and mechanical properties. The results showed that the stir zone composed of mainly the martensite and retained austenite. The volume fraction of the retained austenite of the base metal i.e. 9.84% was decreased to the 5.73% and 5.09, respectively at the stir zone of the conventional and hybrid joints. Indeed, the retained austenite in the TRIP steel microstructure was transformed to martensite under the thermal cycle and deformation induced by rotational tool. However, hybrid operation induced more martensite formation in the microstructure because it caused higher peak temperature. As a result, the microhardness profiles of the hybrid weldment showed the higher hardness in the stir zone in compare with that of conventional process.

Multiscale Computer Simulation of Drawing with Statistical Representation of TRIP Steel Microstructure

The drawing of TRIP steel rod is analyzed. Such steel offers a range of performance benefits. For the case where the microstructure of TRIP steel must be taken into account, problems with traditional process design based on finite-element computer simulation are noted. By means of multiscale simulation, the microstructure of the steel and the dynamic structural and phase transformations (the TRIP effect) may be taken into account. The proposed approach to multiscale modeling is tested for the drawing of TRIP 700 steel in a traditional system. The possibility of controlling the distribution of the steel’s properties by means of the process parameters is assessed. By the proposed method, the deformational interaction of the microstructural elements of TRIP steel may be studied, and the computer resources required by the model may be dramatically decreased.

Modeling Bainite Transformation and Retained Austenite in Hot Rolled TRIP Steel by Instantaneous Carbon Enrichment

The rate model inherited from Leblond and Devaux's concept has been applied to multiphase transformations in hot‐rolled TRIP steel. The phase fraction is collected from different laboratory scale process simulations, both multi‐pass rolling for repeated recrystallization and single‐pass cycles. The latter is also investigated by in situ synchrotron X‐ray diffraction (XRD) to acquire instantaneous data for the phase fraction and carbon content. The different cycles allow different austenite conditioning inputs for the model to enlarge the range of thermomechanical parameters. Bainite transformation model is a function of the instantaneous average carbon content in the remaining austenite, which is measured from the shift in the 1‐dimensional diffraction peaks. The modeling can map different process windows for a specific required microstructure of TRIP steel. The bainite formation is remarkably favored by low carbon content. But, the promotion of ferrite formation in the former step increases the carbon content dramatically and yields smaller bainite fraction that consequently larger fraction of retained austenite is allowed. A modified Koistinen–Marburger equation has been established from the in situ XRD data to model the martensite formation during the final cooling to reveal the stability of austenite. Above an average carbon content of 1.475 mass%, the retained austenite becomes stable.

Computer Simulation of Transformation during TRIP Steel Rod Drawing

Axis pins, shafts and other rod-like parts are some of the main components of units and machines. They are manufactured from steel rods ranging from 7 to 60 mm in diameter after single-pass drawing. Unalloyed carbon steel grades are generally used to produce these items. TRIP steels application is advantageous in terms of achieving new properties when manufacturing steel rods. The initial billet size, the need to take account a transformation in TRIP steel microstructure and a great number of analysed technological conditions make it challenging to apply new materials into rod drawing process. The research aim is to explore the stress-strain state during rod drawing of steel TRIP700. Modified multiscale computer simulation method has been applied. The simulation method takes into account transformation of retained austenite into martensite during plastic deformation. Decreasing of the computational resource intensity and calculation time has been achieved by application of concept of Statistically Similar Representative Volume Element (SSRVE). Comparative analysis of rod drawing micromodels with and without the TRIP-effect simulation has been performed. The analysis showed that a values of equivalent strains in the deformation zone of a TRIP-ignored micromodel was three-four times lower than in a TRIP-factored micromodel. The analysis of simulation results has revealed that, due to a wide contact area with adjacent grains and interaction between microstructure elements, more intensive martensitic transformation occurred within larger grains of retained austenite. The micromodel shown that position and orientation of grains in the deformed TRIP steel microstructure are some of the factors that predetermine transformation of retained austenite. On the basis of simulation results recommendations on preparation of rod drawing conditions have been developed. Developed technological conditions provides ability to obtain: high-plasticity characteristics and high potential strain-hardening capability during exploitation of a future part; maximum strengthening throughout the cross-section of the rod after drawing; high strain-hardening of the surface layer only. The using of SSRVE concept reduced a number of elements within the micromodel in 20 times, while it lowered the calculation time in 16 times.

Formation of Medium Carbon TRIP Steel Microstructure During Annealing in the Intercritical Temperature Range

Abstract The paper presents the results of research conducted on austenite formation in the microstructure of 41MnSi6-5 TRIP steel during annealing in the intercritical temperature range. The influence of the annealing temperature on the volume fraction of retained austenite in the microstructure of the investigated steel after water quenching was also determined.Based on the results of a dilatometric analysis and metallographic investigation it was noted that the pearlite-to-austenite transformation does not occur at a constant temperature, which is referred to as Ac1, but rather within some, possible to determine, temperature range which is bounded by the values Ac1s and Ac1f.Moreover, through X-ray analysis, it was stated that the largest amount of retained austenite remained in the samples which were annealed at the lowest temperatures in the Ac1s-Ac1f range prior to quenching. Increasing the annealing temperature to a two-phase a+g (ferrite + austenite) range, resulted in a decrease of the volume fraction of retained austenite.It was also found that during annealing in Ac1s÷Ac1f temperature range, austenite is also formed from ferrite simultaneously. This could be the reason for the decrease the carbon content in the formed austenite and consequently the decrease in the volume fraction of retained austenite in the microstructure of the investigated steel, which was quenched after having reached temperatures higher than Ac1s + 30°C.

TRIP steel microstructure visualized by slow and very slow electrons

The aim of the present paper is to demonstrate the ability of the scanning low-energy electron microscopy to visualize the transformed induced plasticity steel microstructure with extremely high sensitivity. Using the retarding mode in the scanning electron microscope, the high contrast between the individual phases has been obtained, which enables us to differentiate the retained austenite and the other phases. The sets of the micrographs have been collected from the sample at a wide range of landing energies of primary electrons from 50 eV to 10 keV and the dependence of the contrast between the phases on the landing energy has been calculated. Upon a comparison of these contrast curves, the optimal conditions for achieving of maximum contrast have been established.

In situ neutron diffraction investigation of the collaborative deformation–transformation mechanism in TRIP-assisted steels at room and elevated temperatures

The deformation behaviour of two transformation induced plasticity (TRIP)-assisted steels with slightly different microstructures due to different thermo-mechanically controlled processing (TMCP) was investigated by the in situ neutron diffraction technique during tensile straining at room temperature and two elevated (50 and 100 °C) temperatures. The essential feature of the TRIP deformation mechanism was found to be significant stress redistribution at the yield point. The applied tensile load is redistributed within the complex TRIP-steel microstructure in such a way that the retained austenite bears a significantly larger load than the ferrite–bainite α-matrix. The macroscopic yielding of the steel then takes place through the simultaneous cooperative activity of the austenite-to-martensite transformation in the austenite phase and plastic deformation in the α-matrix. It is concluded that, although its volume fraction is small, the martensitically transforming retained austenite phase dispersed within the α-matrix governs the plastic deformation of TRIP-assisted steels.

Mechanical properties and microstructure of TRIP steels produced using TSCR process

C-Si-Mn TRIP steels were produced using the thin slab casting and rolling (TSCR) process under simulation in laboratory. The results of tensile tests show that the yield strength, tensile strength, and the total elongation of the experimental TRIP steels are 430 MPa, 610 MPa, and 28.4%, respectively. Optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to identify the microstructures of the TRIP steels. The final microstructures consist of ferrite, bainite, and retained austenite. The results of quantitative color metallography show that the fraction of the retained austenite is about 5.8%.

Detailed study of the transformation mechanisms in ferrous TRIP aided steels

Development of TRIP aided ferrous alloys is one answer to the demand for weight decrease in the automotive industry. The microstructure of hot rolled and cold rolled TRIP steels is quite complex and the optimisation of such steel products requires a detailed understanding of the mechanisms of phase transformation, during thermomechanical treatment as well as during mechanical testing or metal forming.We present in this paper the results obtained at Irsid concerning the study of austenite stabilisation through bainitic transformation during thermal treatment and its transformation into martensite during mechanical testing.First of all, the characterisation methods are presented. An effort has to be put on this point due to the refinement of the microstructure of TRIP steels, especially the size of austenite and martensite islands. Carbon replicas for the observation by means of transmission electron microscopy (TEM) are used to analyse the morphological features of the microstructure ‐ nature of the constituents, size and shape ‐ and the composition of cementite particles present in the steels. The mean value for this carbon content in retained austenite is deduced from X‐ray diffraction measurements.Then the kinetics of bainitic transformation are discussed as well as cementite precipitation. The typical composition of the steel studied is 0.5 % C, 1.5 % Mn. The use of 0.5 % C steels facilitates the study of bainitic transformation by avoiding the ferrite formation usually occurring in TRIP steels. Cementite nucleation appears at the ferrite/austenite interface without any partitionning of substitutional elements. To satisfy thermodynamic equilibrium at the interface, the silicon content on the cementite side is very low and high on the austenite side. Then, carbon diffusion towards austenite is delayed and, as a consequence, cementite growth is also delayed. As the diffusion kinetics are low at 400 °C, cementite keeps this “non partitioned” composition, even after 3 hours holding. At 500 °C, diffusion kinetics are higher and cementite composition approaches that predicted by equilibrium.Finally, the stability of retained austenite during mechanical testing is studied. Before and after mechanical testing the morphological characteristics of the microstructure (austenite island size and elongation) are analysed by TEM replicas and image analysis. There is a high density of very small austenite islands but they represent only a small fraction of the total retained austenite. These results confirm and quantify the size effect on austenite stabilisation during deformation.

201 予ひずみを受けたTRIP型複合組織鋼板の疲労信頼性評価

Pre-straining effects on fatigue strength of TRIP-aided multiple phase steel was studied from microstructural viewpoint in order to evaluate fatigue reliability of mechanical product after plastic working process. Effects of pre-strain were examined by comparing two types of specimen: one was cut from 10% pre-strained TRIP steel sheet, the other was untreated virgin material. The pre-strained material ensured a 20% improvement in the fatigue strength over the virgin material. Both of pre-strained and virgin specimens were broken with ductile fracture appearance as well as uniaxial tensile fracture. Difference of fatigue strength between two types of materials was closely related to the deformation behavior during cyclic loading. The improvement of fatigue strength could be explained in the following factors associated with the microstructure of TRIP steel: (a) connected configuration of hard phase in microstructure, (b) γ-phase transformation induced plasticity by pre-straining, and (c) origination of compressive residual stress in α-phase by pre-straining.