Microstructure Of Transformation Induced Plasticity Steel Research Articles

Effect of multiphase microstructure on fatigue crack propagation behavior in TRIP-assisted steels

The effect of various multiphase microstructures on the fatigue properties of intercritical annealing, quenching and partitioning (I&Q&P) steel, transformation-induced plasticity (TRIP)-assisted annealed martensite (TAM) steel and TRIP steel was investigated by S-N curve, fatigue crack growth (FCG) rate and in-situ scanning electron microscopy tests. The fatigue limits of I&Q&P, TAM and TRIP steels, determined by a derivation of the Stromeyer relationship, are 670 MPa, 770 MPa and 795 MPa, respectively. The region II of FCG curve was fitted by Paris and exponential models, and the result indicates that I&Q&P steel has the highest FCG rate, followed by TAM steel and TRIP steel. In I&Q&P steel, fatigue crack mainly propagates along the interfaces between ferrite and martensite or ferrite and martensite and austenite islands, resulting in the presence of many secondary cracks; in TAM steel, the fatigue crack propagation (FCP) can be retarded when the crack passes through the mixed microstructure containing annealed martensitic laths and film-like retained austenite at an angle (not parallel); in TRIP steel, the crack branching and interlocking can hinder the FCP effectively, benefitting from bainite densely distributing in ferrite matrix as strengthening phase and the two phases having almost identical grain size. Therefore, the multiphase microstructure of TRIP steel has the best effect on retarding FCP among the three steels. The findings are essential for the optimization of multiphase microstructure in designing a steel with excellent fatigue properties.

Microstructure and mechanical properties of strip cast TRIP steel subjected to thermo-mechanical simulation

Instead of hot rolling and cold rolling followed by annealing, strip casting is a more economic and environmentally friendly way to produce transformation-induced plasticity (TRIP) steels. According to industrial practice of strip casting, rapid cooling in this work was achieved using a dip tester, and a Gleeble 3500 thermo-mechanical simulator was used to carry out the processing route. A typical microstructure of TRIP steels, which included ~0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite, was obtained. The effects of deformation (0.41 reduction) above non-recrystallisation temperature, isothermal bainite transformation temperature and the size of second phase region on microstructure and mechanical properties were studied. The steel isothermally transformed at 400°C had the best combination of ultimate tensile strength (UTS) and total elongation (TE), whether deformation was applied or not. The deformation resulted in the improvement of mechanical properties after holding at 400°C: the UTS increased from 590 to 696MPa and TE decreased from 0.27 only to 0.26. It was predominantly ascribed to grain size refinement and dislocation strengthening. The studied TRIP steel had comparable mechanical properties with TRIP 690 produced commercially.

Influence of the Initial Microstructure on the Heat Treatment Response and Tensile Properties of TRIP-Assisted Steel

Microstructure evolution and mechanical properties were investigated in transformation-induced plasticity (TRIP) steel having a different initial microstructure. Compared with the cold-rolled structure that evolves into a typical microstructure of TRIP steel, the martensitic initial structure produces a more lath-type microstructure as the fraction of retained austenite increases in the initial microstructure. The interlath austenite after heat treatment contributes to improving the tensile properties by the enhanced stability and the refinement of the matrix phase.

The effect of primary thermo-mechanical treatment on TRIP steel microstructure and mechanical properties

The initial microstructure of transformation-induced plasticity (TRIP) steel was found to be an important factor in determining the final microstructure and mechanical properties and their sensitivity to TRIP heat treatment. The presence of banded structure rendered inhomogeneous distribution of retained austenite and resulted in inferior and anisotropic mechanical properties.

The Formation of Complex Microstructures After Different Deformation Modes in Advanced High-Strength Steels

The microstructure of transformation induced plasticity (TRIP) and dual phase (DP) multiphase steels after stamping of an industrial component at different strain levels was investigated using transmission electron microscopy. The TRIP steel microstructure showed a more complex dislocation substructure of ferrite at different strain levels than DP steel. The deformation microstructure of the stamped parts was compared to the deformation microstructure in these complex steels for different “equivalent” tensile strains. It was found that the microstructures are similar only at high levels of strain (>10 pct) for both steels.

EFFECT OF CARBON CONTENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF COLD-ROLLED C-Mn-Al-Si TRIP STEEL

The low-alloyed transformation induced plasticity(TRIP) steels demonstrate an improved combination of strength and ductility,and have became a promising candidate for the application of automotive bodies to reduce the weight without the loss of crash-worthiness.The typical microstructure of TRIP steels consists of the ferrite matrix and a dispersion of bainite.niartensite and the retained austenite.The existence of an amount of metastable retained austenite is responsible for the improved mechanical properties,resulted from the enhanced strain hardening capabilities of TRIP steels due to the strain-induced martensitic transformation during straining.The carbon content is considered as an important factor that influences the amount and stability of the retained austenite. In the present work,two cold-rolled C Mn Al-Si TRIP steels with different carbon contents, (0.1%and 0.2%,mass fraction) were fabricated by intercritical annealing and isothermal transformation. The microstructures and the mechanical behaviors of the used steels were investigated by OM,XRD and uniaxial tensile tests at room temperature.The results indicated that with the same isothermal transformation time at 400℃.the steels with high carbon content obtained lower fraction of bainite and larger fraction of martensite,and demonstrated higher strength and larger elongation than those of steels with low carbon content.The excellent ductility of steels with high carbon content was mainly attributed to its strong TRIP effect during deformation,resulted from the larger fraction of retained austenite as well as the higher carbon content of retained asutenite in the multiphase microstructure.The value of the product of tensile strength and total elongation,representing the combination of strength and ductility of steels,was increased linearly with the increase of the value of the product of volume fraction and carbon content of retained austenite,which could be used to characterize the TRIP effect.Variation of the formation rate of strain-induced martensite was similar to that of the incremental strain hardening exponent with strain during deformation,further proved the important role of TRIP effect in influencing the strain hardening capabilities of TRIP steels.

Improvement of the Hydrogen Microprint Technique on AHSS Steels

Preferential hydrogen diffusion paths in low-carbon ferritic steel and transformation-induced plasticity (TRIP) steel characteristic of new advanced high strength sheet steels (AHSS) were evaluated via the hydrogen microprint technique (HMT). As a consequence of hydrogen diffusion and evolution from the sample after charging, the technique involves reduction of AgBr emulsion applied to the surface of a charged sample to produce discrete Ag grains which identify, via scanning electron microscope (SEM) imaging, locations where hydrogen evolved. Enhancements to the results are revealed through a newly developed image-before-etch HMT method, consisting of SEM imaging before and then again after etching. The modified technique was implemented to facilitate direct comparison of silver grains with the features of an etched multi-phase TRIP steel microstructure. Results from SEM imaging demonstrated the effectiveness of the image-before-etch method to avoid artifacts and to positively identify hydrogen evolution sites in AHSS TRIP steel microstructures.

Influence of isothermal bainitic processing on the mechanical properties and microstructure characterization of TRIP steel

The mechanical properties of transformation induced plasticity (TRIP) steel are strongly affected by the conditions of isothermal bainitic processing. The multiphase microstructure of TRIP steel under different conditions of isothermal bainitic processing was investigated using OM, SEM, XRD and TEM. The volume fraction of retained austenite and the carbon content in austenite were determined quantitatively using X-ray diffraction patterns. The relationship between mechanical properties and isothermal bainitic processing parameters was investigated. The stability of retained austenite was analyzed by the volume fraction of retained austenite and the carbon content in retained austenite. The experimental results show that the multiphase microstructure consists of ferrite, bainite and metastable retained austenite. To obtain good mechanical properties, the optimal conditions of isothermal bainitic temperature and holding time are 410-430°C and 180-240 s, respectively. After isothermal bainitic processing under the optimal conditions, the corresponding volume fraction of retained austenite is 5vol%-15vol%, which can provide enough retained austenite and plastic stability for austenite with high carbon content.

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.

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.

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.

Low-cycle fatigue-hardening of TRIP-aided dual-phase steels: Yasuki, S.-I., Sugimoto, K.-I., Kobayashi, M. and Hashimoto, S.-I. J. Japan Inst. Met. Dec. 1990 54, (12), 1350–1357 (in Japanese)

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.