Mechanical Properties Of Transformation-induced Plasticity Steel Research Articles

Effects of quenching and partitioning process on mechanical properties of TRIP780 steel

A transformation-induced plasticity (TRIP) steel was applied to test its mechanical properties in quenching and partitioning (Q&P) process. A series of Q&P experiments followed by tensile tests, Charpy impact tests, fracture morphology analyses and microstructure observations were conducted. The experimental results showed that the Q&P treatment could increase the mechanical properties of TRIP steel evidently. The strength of tested TRIP780 after Q&P process reaches more than 1500 MPa with elongation of 17.8%, which is obviously greater than that of 22MnB5 after hot stamping. The microstructure observations indicate that the good combination of high strength and plasticity of TRIP steel after Q&P process is attributed to the multi-phase microstructure of hard martensite matrix and soft retained austenite.

Effect of Thermo-Mechanical Treatment with Various Annealing Hold Conditions on Al Alloyed TRIP Steel

Heat and thermo-mechanical treatment of TRIP (transformation induced plasticity) steels conventionally containes an annealing hold in the temperature region of bainitic transformation to obtain microstructure suitable for utilization of TRIP effect. The hold ensures stabilization of retained austenite and formation of sufficient amount of bainite.The conditions of this hold play an important role in obtaining good combination of high strength and total elongation of TRIP steel. This article deals with the effect of temperature and duration of a bainitic hold on the final microstructure and mechanical properties of TRIP steel processed by thermo-mechanical treatment. The holds in the interval of 350 - 500 °C were used with hold times of 25 - 1200 s. Tensile strengths of 815-976 MPa were obtained in combination with total elongations of 27-45%. The best combinations of tensile strengths around 850 MPa with total elongations above 40% were achieved for the holds at 450 °C.

Effect of two-step cooling process on mechanical properties of TRIP steel with high performance

A novel two-step cooling experiment was established to simulate the slow cooling process of continuous annealing production line for transformation-induced plasticity (TRIP) steel. The microstructures and mechanical properties of TRIP steel soaked at 700 °C for different time were investigated by tensile test, scanning electron microscopy, X-ray diffraction, and thermodynamic and kinetic calculation. It is shown that the steel soaked for 15 s exhibits the optimal product of strength and elongation (PSE > 30,000 MPa%) due to the transformation of austenite to proeutectoid ferrite, which delays the bainite transformation and improves the stability of retained austenite. In addition, the mechanical properties of TRIP steel soaked over 30 s are much lower, resulting from the precipitation of cementite, which decreases the stability of retained austenite and weakens the TRIP effect.

The Effect of Isothermal Hold Temperature on Microstructure and Mechanical Properties of TRIP Steel

TRIP (transformation induced plasticity) steels are low alloyed low carbon steels with complex microstructures consisting of ferrite, bainite and retained austenite. This complex microstructure provides them with excellent strength to ductility balance, making them a member of advanced high strength steels (AHSS) group. Suitable microstructure can be obtained by either heat or thermo-mechanical treatment. A hold in bainite transformation region is an integral part of any form of commercial TRIP steel processing route, as it enables formation of sufficient volume fraction of bainite and also stabilization of retained austenite in the final microstructure. Various bainitic hold temperatures ranging from 350 °C to 500 °C were tested within thermo-mechanical treatment of 0.2C-1.5Mn-0.6S-1.5Al steel and the final microstructures were evaluated with regard to the suitability to TRIP effect and achieved mechanical properties. The microstructures were analyzed by scanning electron microscopy and mechanical properties measured by tensile test.

Microstructures and mechanical properties of TRIP steel produced by strip casting simulated in the laboratory

Conventional transformation induced plasticity (TRIP) steel (0.17C-1.52Si-1.61Mn-0.03Al, wt%) was produced via strip casting technology simulated in the laboratory. Effects of holding temperature, holding time and cooling rate on ferrite formation were studied via analysis of the continuous cooling transformation diagram obtained here. A typical microstructure for conventional TRIP steels consisting of ~ 0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite was obtained. However, coarse prior austenite grain size of ~80µm led to large polygonal ferrite grain size of ~17µm, coarse second phase regions of ~21µm size, small amount of retained austenite (0.02–0.045) and the presence of Widmanstätten ferrite. Optimisation of the microstructure-property relationship was reached via a variation in the isothermal bainite transformation temperature. The highest retained austenite fraction of 0.045±0.003 with medium carbon content of 1.23±0.01wt% was obtained after holding at 400°C, resulting in the highest ultimate tensile strength of 590±35MPa and largest total elongation of 0.27±0.05. The presence of TRIP effect in the studied steel was revealed through the analysis of strain hardening exponent and modified Crussard–Jaoul model. Effect of processing parameters on retained austenite retention and stress-strain behaviour was discussed.

Microstructure Characterization and Mechanical Properties of TRIP-aided Steel under Rapid Heating for Different Holding Time

In order to develop a comprehensive understanding about the effect of different holding time under rapid heating on the microstructural evolution and mechanical properties of transformation-induced plasticity (TRIP) steel, continuous annealing process simulations were performed using a thermal system with resistance heating method. The morphology and distribution of all phases present in the microstructure and the mechanical properties of TRIP steel were revealed. It appeared that the final tensile strength of the TRIP steel increased and retained austenite carbon content decreased with increasing holding time. An overlap between ferrite recrystallization and austenitization was observed during intercritical holding. In addition, the work hardening of the samples was evaluated by calculating the instantaneous n value as a function of the true strain. The difference in work hardening behavior corresponds to the rate of the retained austenite transformation during straining, which can be attributed to the carbon content and the morphology of the retained austenite.

Prediction of mechanical properties of TRIP590 steel with strain rate effect

As a typical advanced high strength steel, transformation induced plasticity (TRIP) steel can improve the manufacturability, safety and lightweight of automobiles due to its high strength and good ductility, which result from the transformation of austenite into the martensite during the deformation process. In this paper, the TRIP effect is studied to learn about its influence on the mechanical properties of TRIP steels at different strain rates. The prediction has shown that: hardening rate with TRIP effect decreases with increasing strain rate; strain hardening exponent with TRIP effect increases almost linearly with increasing plastic strain and increases with increasing strain rate; necking strain with TRIP effect is higher than that without TRIP effect, which decreases with increasing strain rate.

The Effect of Texture on the Stability of Retained Austenite in Al-Alloyed TRIP Steels

ABSTRACTSteels with transformation induced plasticity (TRIP) offer an excellent combination of high strength and ductility. The transformation of meta-stable austenite into martensite during straining leads to strong local hardening and prevents early localization of strain. Therefore, the mechanical properties of TRIP steels, including the damage resistance depend to a significant extent on the stability of retained austenite. The aim of this study was to evaluate the effect of texture on the stability of retained austenite. In order to compare the changes in both tension and compression the steel was deformed by a micro 3-point-bending device. The texture development upon bending was followed by electron backscatter diffraction (EBSD) technique. Based on a simple analysis using the relation between face centered cube (FCC) and body centered cube (BCC) shear geometries theoretically expected changes of texture components due to deformation are proposed. Using the results of this analysis the observed changes of the austenite texture due to deformation could be distinguished from those due to transformation, by comparing the experimental results with the theoretically expected behavior. From this comparison, austenite grains with “Brass (B) {011} <211>” and “Goss (G) {110} <100>” texture components were found to transform into martensite much easier than differently oriented grains.

Influence of Al on Microstructure and Mechanical Behavior of Cr-Containing Transformation-Induced Plasticity Steel

Chromium in transformation-induced plasticity (TRIP) steel is known to have a detrimental effect on the mechanical properties by increasing the hardenability of austenite introduced during intercritical heat treatment. In this study, it is suggested that an Al addition can counterbalance the effect of Cr by encouraging ferrite formation during fast cooling and austempering. This contributes to securing the thermal stability of austenite and to retrieving the excellent mechanical properties of TRIP steel even with the addition of Cr.

Investigation of Forming Conditions on Microstructure Development and Mechanical Stability of Trip Steel

Experimental simulations of thermomechanical processing (TM) using press forging of Si-Mn TRIP (transformation induced plasticity) steel were performed. In order to rationalize the retained austenite (RA) volume fraction, six TM schedules were employed at experiment where different austenite conditioning was considered. The various multiphase structure characteristics were obtained after of TM processing, with different volume fraction of ferrite, bainite and RA. The modification of structural characteristics influenced the mechanical properties of TRIP steel. The present work also focuses on monitoring of RA transformation during mechanical incremental straining using in situ neutron diffraction technique. This method is used to characterize the kinetics of RA transformation and its stability during the straining.

An Analysis of Geometrical Orientation of Martensite in TRIP Steel by Using Fourier Transformation Image Analysis

Due to a strain-induced martensitic transformation (SIMT), TRIP (transformation-induced plasticity) steel possesses favorable mechanical properties such as high strength, ductility and toughness. However, since the SIMT depends on temperature, strain rate and stress state, the desired mechanical properties are obtained only in a fairly restricted working environment. On the other hand, a spatial distribution of geometrical characteristics of martensitic phase, including size, shape, and orientation against the loading axis, are heterogeneous at a microstructure of TRIP steel as observed in micrographs. It can be thought that the mechanical properties of TRIP steel manifest strong dependences on the geometrical inhomogeneity and spatial configuration of martensite. If these geometrical characteristics can be controlled, the enhancement of the mechanical properties of TRIP steel might be expected. Hence, it is necessary to evaluate the geometrical characteristics in TRIP steel to describe and control its deformation behavior. Here, in order to study the geometrical orientation from micrographs, the Fourier transformation image analysis (FTIA) is employed. After the suitable magnification of the micrographs for the FTIA and a spatial frequency are discussed, the dependences of plastic strain, temperature and stress states on the geometrical orientation are investigated by using the FTIA.

Relationship of microstructure and mechanical properties of TRIP-aided steel processed by press forging

Abstract The main emphasis of this study has been placed on thermomechanical processing simulations performed using press forging. In order to develop a comprehensive understanding of the effect of large deformation of bulk specimens, introduced in the austenite recrystallized, nonrecrystallized and intercritical temperature regions, on the microstructure evolution and mechanical properties, the C–Si–Mn transformation induced plasticity (TRIP)-aided steel was used for experimental. The choice of applied strain and thermal parameters had strong impact on austenite transformation kinetics and obtained final multiphase structure, which clearly modified the mechanical properties of TRIP steel. To rationalize the retained austenite volume fraction employing different thermomechanical schedules, the effect of austenite conditioning on ferrite transformation was considered at the first step and the subsequent isothermal bainite transformation treatment was optimized in the second step. The microstructures of treated samples were characterized by optical metallography and scanning electron microscopy. The tensile tests were conducted to characterize the development of TRIP effect with respect to volume fraction of retained austenite in multiphase structure. The volume fraction of retained austenite was measured using X-ray and neutron diffraction methods. The specific design of thermomechanical schedule with varying of strain and temperature values affected multiphase structure development in TRIP bulk steel and yielded in good combination of high tensile strength over 900 MPa and elongation over 20% nearly at all experimental schedules. The complex relationship among the volume fraction of the retained austenite, the morphology and distribution of phases present in the microstructure, and mechanical properties of TRIP steel were revealed. These findings suggested that the intercritical treatment conditions prior austenite transformation appeared to have decisive effect on steel refinement and forming of convenient multiphase structure, which should provide both, the high strength and sufficient ductility of bulky products.

Vanadium Precipitation During Intercritical Annealing in Cold Rolled TRIP Steels

Flat carbon steel manufacturers are currently developing new TRansformation Induced Plasticity (TRIP) steels as a response to strong demands for vehicle lightening and security reinforcement from the automobile sector. Compared to conventional high strength steels these advanced grades exhibit a very favourable compromise between strength and ductility and can therefore be produced in thinner, lighter gauge strips with equivalent functional properties. The excellent mechanical properties of TRIP steels are attributed to the high strain hardening coefficient generated by the progressive transformation of metastable retained austenite to martensite during plastic deformation. Further improvements in mechanical properties can be obtained by microalloying, especially with vanadium and nitrogen additions. In this paper we discuss the rather complex evolution of vanadium carbonitride V(C, N) precipitation during continuous annealing of cold rolled strip. Transmission electron microscopy (TEM) and selective chemical dissolution are used to characterise the precipitation state during interrupted intercritical annealing cycles. The experimental results are compared with calculations made using a recent kinetic precipitation model. We show that reasonable agreement can be achieved using a simple uncoupled model, however a complete description of the precipitation sequence during continuous annealing will require fully coupled kinetic models describing the interactions between cementite dissolution, the ferrite to austenite transformation and V(C, N) precipitation.

Structure dependence of the TRIP phenomenon in Si–Mn bulk steel

Experimental simulations of thermomechanical processing (TM) using press forging of cylindrical specimens of C–Si–Mn TRIP (transformation induced plasticity) steel were performed. The aim was to understand the effect of the hot deformation introduced in the austenite recrystallization, nonrecrystallization and intercritical temperature regions on the austenite conditioning and microstructure evolution of the studied steel. The microstructures resulting from a variety of TM processing were characterized by light and scanning electron microscopy. Volume fraction of retained austenite was evaluated by neutron diffraction. It was found that the choice of applied strain and temperature of deformation prior transformation had strong impact on transformation kinetics, final multiphase (ferrite, bainite and retained austenite) structure characteristics and mechanical properties of TRIP steel. A good combination of strength and ductility was achieved in the material with refined microstructure and sufficient volume fraction of retained austenite.

Strain-induced transformation of retained austenite in low-carbon low-silicon TRIP steel containing aluminum and vanadium

Effects of intercritical annealing temperature on the microstructure and mechanical properties in low-carbon low-silicon TRIP (transformation-induced plasticity) steel containing aluminum and vanadium were investigated. The highest volume fraction of retained austenite of samples annealed at 1053 K was approximately 18%, while the product of strength and elongation of samples annealed at 1033 K was up to 22,000 MPa%. In addition, the retained austenite transformation kinetics were measured by tensile test and fitted by the function y = 1 − e − Ax . The correlation between the experimental and fitting results was good. A in the function might be a more effective factor to measure the mechanical properties of TRIP steel than the volume fraction of retained austenite.

Multiscale computational simulation of deformation behavior of TRIP steel with growth of martensitic particles in unit cell by asymptotic homogenization method

Due to the strain-induced martensitic transformation (SIMT), the strength, ductility and toughness of TRIP (transformation-induced plasticity) steel are enhanced. Such favorable mechanical properties of TRIP steel can be realized by the appropriate combination of the SIMT behavior and deformation behavior of both austenite and martensite. However, since the SIMT and deformation behavior of both phases depend strongly on the conditions of the working environment such as temperature, strain rate and stress system, the desired mechanical properties are obtained only in a fairly restricted working environment. On the other hand, martensitic particles of various sizes, shapes and orientations can be observed and are distributed spatially in the microstructure of TRIP steel, therefore it is possible that the mechanical properties of TRIP steel strongly manifest a dependence on the geometrical inhomogeneity and configuration of martensite. If the geometrical configuration and inhomogeneity of martensite can be controlled, we can expect to enhance the mechanical properties of TRIP steel. Here, in order to investigate the effects of the geometrical configuration and inhomogeneity of martensite in the austenitic matrix on the macroscopic deformation behavior of TRIP steel, the computational simulation of the deformation behavior of TRIP steels with the growth of an ellipsoidal martensitic particle embedded in the center of the austenitic unit cell, is performed using the homogenization technique.

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure.

Microstructure and mechanical properties of C–Si–Mn(–Nb) TRIP steels after simulated thermomechanical processing

Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.

Identification of Constitutive Equation for TRIP Steel and Its Application to Improve Mechanical Properties

Due to the strain-induced martensitic transformation during plastic deformation, a transformation-induced plasticity (TRIP) steel possesses favorable mechanical properties such as high strength, ductility and toughness. Since its favorable mechanical properties are realized under restricted circumstances, the prediction and control of deformation processes are indispensable to determine the expected mechanical properties of TRIP steel. However, it is very difficult to obtain TRIP steel that only has the martensitic structure before deformation, and the stress-strain relationship of each phase at various temperatures cannot be measured experimentally. Therefore, the identification of the parameters in the constitutive equations for TRIP steel is necessary. Here, a method employing finite-element simulation and the nonlinear least-squares method under constraint conditions was proposed to identify the constitutive parameters of TRIP steel from the true stress-plastic strain and the volume fraction of martensite-plastic strain relations obtained by the uniaxial tensile test and computational simulations. Then, the application of this procedure employing virtual data of an ideal TRIP steel that possesses highly favorable mechanical properties is discussed to investigate the improvement of the mechanical properties of TRIP steel.

K-0335 均質化法に基づくTRIP鋼の変形挙動の数値シミュレーション(G01-4 変形解析)(G01 計算力学部門一般講演)

The transformation-induced plasticity (TRIP) steel possesses such favorable mechanical properties as tenacity and strength due to strain-induced martensite transformation. This excellent mechanical properties of TRIP steel depend on inhomogenious morphology of martensite in microscopic region. However, to evaluate the effect of microscopic morphology of martensite on its mechanical properties, it is difficult to express microscopically its heterogeneous deformation behavior by a conventional FEM. Here, the computational simulation of deformation behavior of TRIP steel is performed by recently developed homogenization method.